The rational justification for a low-temperature thermal system derives from eco­nomic and environmental reasons. The reduction of environmental pollution and the saving of energy which can be obtained using solar energy represent a solid advantage for the community.

The Sun can normally give us 80-95% of the hot water that we use daily to wash our hands, take a shower, do the dishes or also wash clothes, if we manage to connect the solar system to the dishwasher’s or washing machine’s hot water entrance. If the products are of good quality and the system is well sized, the outcomes are very good even in northern Italy. While in southern Italy the out­comes are excellent in winter also, in northern Italy we can achieve good results only if one more panel is installed or if a capacious tank is provided to make up for the lack of sun during cloudy days, since a good tank keeps water at a constant temperature for a few days. It is not easy to know the exact saving that could be obtained from a sanitary water solar system, as it depends a lot on the habits of families. So it depends on how much water is used for personal hygiene or for dishwashing and also on the kind of central-heating boiler or water heater the family possesses. Using an approximate calculation, which is sufficiently reli­able, we can indicate a yearly methane saving of 100-180 m3 per person, with a yearly lack of carbon dioxide emission (the main cause of greenhouse effect) of 230-400 kg per person. To these quantities we have to add the wastes generated by the accumulation of the central-heating boiler and water heater, for example, with a simple pilot flame or with intermittent running of the water heater to keep the temperature of a huge quantity of water constant during the more unthinkable hours of the day and night. These methane wastes will be 150-200 m3 per year and they have to be added to the real savings calculated on the basis of the number of persons in a family. The methane cost per m3 or the cost for the fuel used by our own central-heating boiler has to be taken into consideration to calculate cor­rectly the saving in conformity with taxes, the additional tax when exceding the permitted use of energy, the IVA, the inflation, the rise which will take place in the next few years, etc. Since solar systems are capable of producing more than 90% of the hot water requirements between April and October, the central-heating boiler or the water heater could be turned off during this period, resulting in a significant reduction of wastes.

It is important to determine the regeneration time of the investment, which may be important to justify a solar system installation. The number of years taken to regain the investment can be obtained by dividing the cost paid by the yearly max­imum money saving obtained by the use if sanitary hot water produced by solar energy. On average, if we talk about a single family system, the hot water cost alone will be about 1500-2600 €, amortizable within 3-5 years, while the sys­tem’s useful technical life span is about 20 years and the maintenance costs are 1% of the system’s original cost.

A traditional water heater (electrical or methane one) never regains its cost because there is always a bill cost, whereas the more a solar energy system is used, the more it is convenient.

Only when the use of water is reduced, it is more convenient to have a small electrical or gas water heater [16, 18].

3.1 Concentratiing solar power technology: clean energy for power tenability

Energy availability has always been an essential component of human civilization. In the last 150 years, the yearly average rate of world energy consumption has grown by about 2.3%. The energy requirement of human beings, mostly satis­fied by fossil fuel, has grown so much that it has overcome sum of the thermal energy coming from the Earth’s core and from the ties induced by the Sun and the Moon. The endogenous energy of the Earth has been more than doubled by human activities. However, it is important to underline that the overall human energy con­sumption is only 1/10,000 of the energy received on the Earth from the Sun. Solar radiation, despite its scarce density, remains the most rich and clean energy source on the terrestrial surface [45-47].

This statement, together with the exhaustion of fossil fuels and the growing environmental risks, leads us to seriously consider solar energy as one of the most important candidates for the planet’s energy tenability project.

The greatest part of the solar source’s potential can be found in the so-called ‘sun belt’, which is the area of the planet that receives the highest quantity of solar radiation, as shown in Fig. 81. In particular, Northern Africa and the Middle East have large areas with a high level of solar radiation which is suitable for the installation of a large number of solar thermal systems as they cannot be used in any other way. These countries spontaneously stand out as candidates for the intensive development of solar energy [45].

From the recent MED-CSP research, commissioned by the German Ministry of Environment Policies and carried out at the Aerospatiale Centre DLR, it has

. •V’V-V-V — ‘• V-V’V-Y

• • •*.* • • •: ■: • V • .* •: • • * •: •’••••

ж

-•

Smtahiliiy for the implemcntatioii of solar concentration

Мй excellent = tnod

Figure 81: World map of solar radiation underlining the sun belt.

been shown that the solar energy potential available in the countries on the Mediterranean coast is much larger than the actual electrical energy consump­tion of the area which includes Southern Europe, the Middle East and Northern Africa. Besides the DLR and other German organizations, other centres of research such as NERC (Jordan), CNRST (Morocco), NREA (Egypt) and NEAL (Algeria) contributed to the development of this study. This project shows that a common interest could link European countries (energy importers and technol­ogy exporters) with the Northern Africa countries (owners of fossil fuel resources and solar energy who will see a significant growth in their energy consumption in the next few years) [45, 48]. To exploit this huge solar energy potential, the concentrating solar power (CSP) technology is very useful. This technology exploits solar radiation, concentrating it using mirrors for electricity production and also for the realization of high-temperature chemical processes, for example, the production of hydrogen. A more detailed description of CSP technology is given in par. 4.4. This technology can be considered as a competitor of the photovoltaic technology, which is already common and is growing in Europe (actually, this is only partially true). On this matter, two aspects have to be considered: first, the photovoltaic technology exploits both the direct radiation and the spread (diffuse) radiation and so it is also suitable in areas such as Northern Europe where direct solar radiation is scarce; second, it is fit for many different applications (from a few watts of a mobile phone’s solar battery char­ger, to the megawatts of dedicated solar systems, passing through a few or tens of kilowatts used in many applications in the residential or civil field). CSP tech­nology, on the other hand, exploits only the direct radiation and lends itself poorly, excepting in a particular situation or only in thermal applications, to the realization of small dimension systems. As regards systems that have a capacity of about or more than a megawatt and that are used in areas with strong direct radiation, CSP technology allows achieving a cost for electrical energy produc­tion which is much less than that for the energy produced using photovoltaic technology, and this advantage, in this specific case, is destined to last for a long time unless there is a radical technological improvement in the photovoltaic field [45, 48].

Considering only the European part of the Mediterranean area, we can observe a sort of integration between the two technologies: the photovoltaic technology is installed where there is lesser direct radiation and in applications which require from a few kilowatts to a hundred kilowatts of power. On the other hand, CSP technology is installed in areas with stronger direct radiation and in medium-large power systems (starting from a megawatts of power). It is also possible to think of a situation where Europe increases its own consumption of ‘green electricity’ by taking it both from the different renewal sources available in the area and from the solar energy imported from the most suitable regions.

To highlight the significance of this project, which has got many prospects, it is necessary to point out that the density of the solar energy received in the Mediterra­nean southern coast and its territorial characteristics allow to cut the solar energy production costs, which are then invested to produce solar energy in the southern

Figure 82: Map of the solar direct radiation in the Mediterranean area: lighter spots indicate regions with the strongest radiation.

European area; moreover, large areas which cannot in any case be used for agricultural purposes are available on the coasts of Northern Africa and the Middle Eastt. Since the cost for high-tension and continuous line electricity transmission for a distance of about 1000 km, of which 100 km is by submarine cable, is about 0.7-1.5 c$/kW h, it is not illogical to think of realizing in those regions — inside a wider project of social and economic integration — solar energy production capable of satisfying the growing requirement for electricity in Northern Africa and also a part of Europe [45-47].

With the regard to this matter it is important to underline how, despite mere energy considerations, the so-called ‘Mediterranean electric belt’ has been studied for a year, which will shortly allow the complete electrical interconnection between the Mediterranean countries and the European electric net.

Considering what we have just stated, it is clear that the wide exploitation of the solar source in the Mediterranean area is a very important topic with a high politi­cal and economic agenda since it has important consequences in terms of bringing about the integration of the northern and southern parts of the world and also in terms of development of pacific relationships.

As regards future prospects, the direct production of hydrogen, mainly by CSP technology, will allow the sun belt to increase the production of energy. Presently, a few Southern European areas, Spain in particular and also southern Italy, have a fairly good potential, which allows them to exploit the CSP technologies to increase the renewal quantity of electricity production.

Spain in particular is in a favourable situation because of both the presence of a huge energy potential and the remarkable experience it has with experimental activities developed since 1981 at the Plataforma Solar De Almeira.

As regards Italy, currently, there are no accurate studies on the energy potential which can be exploited using the CSP technology. The MED-CSP study [48] estimates that the ‘technical exploitable’ potential is about 88 TW h/year while the ‘economically exploitable’ potential is about 5TW h/year; actually, these figures refer to very approximate values. Despite this, it is clear that the principal aim of

using the CSP technology is to obtain advantages, in addition to economical advantages, by the exploitation of the energy potential in the areas that are rich in the solar source. This is more justified if we consider the case of Germany which has been pursuing the development of this technology for many years although its economically exploitable energy potential is almost non-existent. Since the primary source is free, it is also important to underline that the total turnover related to energy production from solar energy benefits those who realize this and take care of the production systems; those who own the know-how are then designated to exploit the biggest part of the business connected with solar energy production [45].

The direct solar energy received at a certain time interval on an oriented surface situated on the Earth is given by the expression:

rt0 +At

Eb = J Ibn(t )cos idt (36)

Jt0

where At (time interval) may vary (an hour, a day, a month, etc.) and Ibn(0 stands for the normal direct radiation.

As a rule, it is not possible to use this equation to calculate Eb since Ibn(t) depends on local atmospheric conditions which cannot be known in advance. It is possible, instead, to calculate that quantity per surface on soil during clear sky days using one of the models described in par. 5.

It is often useful to calculate using eqn (36) the radiation received during the day on a horizontal surface placed outside the atmosphere, which is equal to:

Г h

Hex = Г Icse(t)sen a dt (37)

J ha

where ha and ht are, respectively, the hour angles at dawn and sunset. Through a few passages we get:

Hex = 24/nIcs[1 + 0.0033cos(2n«/365)]

• (cos L cos d sen ha + hasen L send) (38)

where ha, in the last term, should be expressed in radians. The daily extraterrestrial radiation calculated using eqn (38) is expressed in watt-hour per square metres (W-h/m2) [1].

At our latitudes, low-temperature solar thermal systems are more common than the medium and high temperature ones (especially among private users, who want to save money on their energy bills). They are usually used [1-5, 17]:

• to heat sanitary water for domestic, hotel and hospital use;

• to heat water for showers (bathing establishments, camping, etc.);

• to heat rooms;

• to heat water used in processes at a low temperature;

• to dry foodstuffs;

• to refresh rooms (although it is still too expensive).

A solar thermal system always works in the same way, although there may be slight changes according to its application and use. First, there are a few solar collectors which absorb large quantities of sunlight and then convert it into heat.

The collectors are crossed by a fluid (thermal vector fluid) which removes the absorbed heat; this fluid, crossing the so-called solar circuit, arrives at an accumu­lator which stores the large quantities of thermal energy to be used in the future when there will be a real need [17].

We will now analyse the low-temperature solar thermal system starting with its most important element: the collector.

The predicted development of CSP applications follows on after 20 years of the development of the Aeolian applications; a possible trend could be the world achiev­ing 5000 MW by 2015. The overall ‘portfolio’ of the CSP systems planned at different levels in the world accounts for about 1562 MW; adding to it both the predicted 28 MW coming from the Italian Archimedes Project (described in par. 4.5.3) and the portfolio of the Global Environment Facility (GEF, an independent financial orga­nization connected with the World Bank and also with the Environment Project of the United Nations; it was founded in 1991 to help developing countries with proj­ects and programmes aimed at protecting the world environment) projects which are currently foreseen will add another 130 MWe, which results in a potential world portfolio for the short to medium term of over 1700 MW.

Of this 1700 MW, 300 MW is considered to be surely realized.

The factors which have resulted in a reduction of the levelled electrical energy costs produced by these systems, valued by the GEF, are shown in Fig. 83; it has been forecast that the levelled energy cost (LEC) will be reduced from the current 16 c$/kW h to about 6 c$/kW h by the year 2025, also reaching by that date the predicted cost for the fossil fuel systems. Other organizations have predicted even lower costs (till 3.5 c$/kW h).

The fulfilment of these developmental forecasts will mostly depend on the polit­ical and economic situation in the next few years. However, it is clear that the knowledge and diffusion of CSP technology is currently at the same stage as that of the Aeolian ones during the mid-1980s; at that time, nobody would have bet on the Aeolian energy; instead, we have currently reached only in Europe an installed Aeolian power of 34,000 MW [45, 49].

The hour angles which correspond to the different positions of the Sun in the sky concern the true solar time and not the conventional time measured by a clock. However, true solar time is determined by the Sun’s position in the sky: for example, when the Sun is on the meridian of a certain place, the hour angle is nil because of the solar midday; instead, the clock will indicate a different time.

To convert clock time to true solar time, first, it is necessary to correct the lon­gitude difference between the local meridian and the standard time zone meridian, considering that any grade of longitude difference corresponds to a four-minute correction; second, because the angle speed of the Earth is not constant during the year, it varies positively or negatively as regards the average conventional value of 360/24 grad/h, we have to correct it using the equation of time (ET).

We use the relation [1]:

True solar time = Conventional time + 4′ (Longitude of the local meridian — Longitude of the standard time zone meridian) + ET

If we apply legal time, the conventional hour must not be the legal one, but the one for the standard time zone meridian. Longitudes are considered to be positive if they are eastward from Greenwich. As regards Italy, the datum meridian is at 15° east of Greenwich and it passes by the volcano Etna. Figure 10 shows the ET trends during the year.

Some values of ET are shown in Table 2.

Table 2: Values of the equation of time during the year. Date ET Date ET Months Days Min. Sec. Months Days Min. Sec. Jan. 1 -3 14 Jul. 1 -3 31 13 -8 27 13 -5 30 29 -13 5 29 -6 23 Feb. 1 -13 34 Aug. 1 -6 17 13 -14 20 13 -4 57 29 -13 19 29 -1 10 Mar. 1 -12 38 Sep. 1 0 15 13 -9 49 13 3 45 29 -5 7 29 9 22 Apr. 1 -4 12 Oct. 1 10 1 13 0 47 13 13 30 29 2 33 29 16 10 May 1 2 50 Nov. 1 16 21 13 3 44 13 15 47 29 2 51 29 11 59 Jun. 1 2 27 Dec. 1 11 16 13 0 18 13 6 12 29 -3 7 29 -1 39

Flat plate collector Flat plate collector

with transparent insulating

-oooooooooo-

Unglazed collectors,, ^. „

* Vacuum tube collectors

Figure 1: Sections of different kinds of collectors.

Currently, there are four principal types of solar collectors which have been studied to get the best ratio between costs and benefits according to the different conditions for their application and their possible uses [1-6, 12, 17]:

• flat plate collectors (very common, medium cost, versatile);

• vacuum tube collectors or evacuated tube collectors (high efficiency, more expensive, but useful during any time of the year);

• unglazed collectors or pool collectors (only for use in the hot season, generally for swimming pools or bathing establishments, very economical);

• integrated storage collectors (useful in mild climate zones, they decrease the cost of the solar system).

2.2.1.1 Flat plate collectors Let us analyse the working principle of a generic flat plate solar collector used to heat a fluid. Every device included in this category aims to convert the maximum part of the electromagnetic energy received with the solar radiation into thermal energy, which is available to the users. To serve this purpose, we exploit and strengthen the capacity of certain materials, for example, metals such as copper and alloys such as steel, to warm up fast when exposed to solar radiation and to release the stored heat very easily. The most important ele­ment of solar panels is the absorber plate, which has the above-mentioned char­acteristics; this plate is crossed by tubes through which the fluid that has to be warmed up flows.

insulating material

Figure 2: Structure of a flat plate collector.

All the mechanisms for exchanging heat from the plate-tube elements to ele­ments which are not the fluid have to be minimized or reconverted to transfer the greatest quantity of the solar radiation received to the fluid.

For this reason, the posterior part of the plate, the part which is not exposed, and its side parts are lined with insulating materials, and the temperature inside the collector is also kept at its highest level, thanks to one or more covering transparent plates [5].

Let us now see the collector’s working in detail.

The solar panels in Fig. 7 have a structure that comprises a rigid container case insulated on the inside. A transparent cover reduces energy losses to the outer side of the collector and favours penetration of the received radiation which is inter­cepted by a black metal plate situated below (intercepting or absorber plate) [2].

Figure 7: Section of a flat plate collector.

The thermal vector fluid flows inside the pipes, which are in contact with the metal surface, and removes the absorbed heat. Sometimes, the pipes through which the fluid flows are welded on the plate; however, most of the time, canalizations are made as shown in Fig. 8, that is, using the roll bond method, which comes from
refrigerating technology. (This method consists of welding two plates by hot-rolling; but before this, the worm-pipe design — to allow the flow of the thermal vector fluid — has to be printed on one of these plates by the silk-screen process.) Canalizations are normally able to resist pressures of 6-7 bar, although some collectors can guarantee a resistance of pressures even up to 10 bar.

Figure 8: Pipes through which the thermal vector fluid flows, obtained using the roll bond technology between two welded metal plates.

Usually, the most commonly used thermal vector fluid is a mixture of water and propylenic glycol (non-toxic and a good antifreeze), but depending on the applica­tion either water alone (which poses two major problems, namely lime scale accu­mulation and a very high freezing point) or simple saline solutions can also be used. The hydraulic circuit of the panel is depicted in Fig. 9.

The most important requirements of a thermal vector fluid are:

• high density and high specific heat (to use pipes of small dimensions);

• it should not corrode the walls of the circuit;

• chemical inertia and stability at a temperature of less than 100°C;

• restrained hardness to limit lime scale accumulation;

• low freezing point;

• low viscosity;

• it must not be toxic (in the case of sanitary hot water supply). As regards room heating, since the requirement of non-toxicity is not essential, a mixture of water and ethylic glycol (which has a thermal capacity higher than that of propylic glycol) can also be used.

If we are going to use water mixtures, it is important to prevent the freezing of these mixtures using antifreeze solutions. As a matter of fact, when the days or nights are very cold and there is lack of solar radiation, the liquid may freeze and in the process it expands and may break the collectors or the solar circuit [2, 5, 8].

As we will see in par. 2.2.1.6, there are some collectors which use air as the thermal vector fluid instead of a liquid.

The intercepting plate (or absorber plate) should be made of a metal that has a low thermal resistance. For this reason, the most commonly used plates are made of copper (the best ones), aluminium (the next best ones) and steel. The absorber plate is covered on the outside by a dark finish coating (as we will see later, because of the kind of finish there can be selective or non-selective surface panels). When solar radiation hits the absorber plate, the radiation is almost completely absorbed, while only a small part is reflected. The absorbed radiation produces heat which is transferred by the sheet to the copper pipe through which the thermal vector fluid flows; finally, this fluid absorbs the heat. The quantity of the reflected radiation has to be restricted as much as possible because it has the same characteristics as the received radiation and it is unsuitable that a large part of the radiation is returned to the atmosphere.

Figure 10: ‘Front-back’ view of an absorber plate.

Steel plates, besides having a low thermal conductivity, have a high thermal capacity and hence they are less efficient in exploiting the thermal transitories which are connected to the passing of the clouds. If we use aluminium plates, we have to insert dielectric joints inside the hydraulic circuit, which generally has copper elements, to avoid corrosion due to the creation of copper-aluminium piles [2, 8].

There are two kinds of plate panels [2, 5, 8, 17, 21]:

• Those with a non-selective surface: The absorber plate surface is treated with dark mat paints. These kinds of paints reduce the losses due to reflection and increase the plate’s ability to absorb the wavelengths of solar radiation. This panel is recommended for holiday houses, as hot water is used only in summer and it takes care of the supply of sanitary hot water. If there is a good quality tank, then 100% supply of sanitary hot water can be easily achieved.

• Those with a selective surface: The heat absorber is potentiated by a surface which allows the panels to combine the non-selective surface characteristics (reduction of reflected radiation losses and high ability to absorb wavelengths of solar radiation) with a low emissivity for wavelengths that characterize the infrared radiation, which is characteristic of a body at a temperature of nearly 100°C. As a matter of fact, although the plate can count on the covering opacity as regards infrared radiation (which remains inside the collector, see Fig. 11), it is important that the absorber plate emits the smallest quantity of energy to the atmosphere by radiation, since a part of the absorbed energy is dispersed in any case (e. g. one of the flows in Fig. 11 represents the quantity of energy absorbed by the covering and then emitted outside). This kind of panel is much more efficient and expensive, but it can be used during all 12 months of the year. Table 1 lists a few examples of selective surfaces.

Table 1: Examples of selective surfaces. Coating Substrate Solar absorption (a) Infrared emissivity (e) Black nickel on nickel Steel 0.95 0.07 Black chrome on nickel Steel 0.95 0.09 Black chromium Copper 0.95 0.14 Black chromium Steel 0.91 0.07 Iron oxide Steel 0.85 0.08 Manganese oxide Aluminium 0.70 0.08 TiNOX Copper 0.95 0.04

Non-selective surface panels have a higher infrared emissivity (on average e=0.85) than selective surface panels, resulting in higher leakage of useful energy. The best material used to construct an absorber plate is a thin copper sheet lined with a TiNOX selective material (a titanium and quartz covering released in the market in 1995; it does not contain either chrome or nickel). As one can observe from Table 1, besides a high level of solar absorption, TiNOX has the lowest emissivity per wavelength of infrared radiation.

The container case should provide compactness and mechanical solidity to the collector and should also protect the inner elements from dirt and atmospheric agents. The container case should be perfectly waterproof to prevent humidity from
entering te collector; otherwise, the humidity that enters evaporates as soon as it comes into contact with the hot plate and if the outside temperature is low, it con­denses against the inside face of the glazing reducing its transparency. Moreover, humidity can raise the thermal conductivity of fibrous materials (such as wool, polyurethane, polyester wool or stone wool) which are used for internal insulation of the panels. The container case is made of stainless steel (generally, zinc-plated or a pre-treated one), anodized aluminium or, more rarely, fibreglass [2, 5].

To increase the penetration of the radiation received to its maximum and to restrict the energy losses to the atmosphere, the collector’s covering has to be trans­parent to the wavelengths of the solar radiation (on average 0.2-0.5 pm) and, at the same time, it has to be opaque to the infrared radiation which comes from the pipes and plate taken together while their temperatures increase gradually (wavelengths higher than 4 pm). Glass meets these requirements best especially if it is treated to get more resistance and transparency (generally, two sheets of tempered, prismatic and antireflection glass are used). Nevertheless, because of the fragility of glass and its weight, sheets of plastic materials (such as polycarbonate) are preferred to glass. Glass provides an additional level of security because if it breaks, it breaks into very small but not sharp parts, thereby reducing the risks of accidents.

The transparent covering is the Achilles’ heels as regards the thermal losses to the atmosphere; it is the only surface that cannot be insulated in a proper way [5].

The panel is insulated to avoid conductive losses towards its back and sides and thanks to the insulation they are negligible. Moreover, to create microscopic air spaces (which are good barriers to heat transmission), the different utilizable materials used (polyurethane, polyester wool, fibreglass or stone wool) are always characterized by a porous or alveolar structure. To fight humidity, insulating materials are often cov­ered with a very thin aluminium sheet, which, at the same time, reflects towards the absorber plate, the energy that it receives from the same plate by radiation [5].

Figure 11 shows the thermal flows inside a collector.

Compared to conductive losses (the choice of a good insulating material can effectively restrict the losses towards the back and the sides of collector), the losses due to convective motions in the air space between the absorber plate and the transparent covering are difficult to contain. This causes damage to the collec­tor’s performance, especially in places where the temperatures are low during most part of the year. When air comes into contact with the plate, it warms up quickly and tends to move up transferring a substantial part of its heat to the cover, which, as it is made of non-thermal insulated materials, then allows the heat to follow its natural course towards the environment which has a lower temperature (i. e. the outside).

To restrict this heat loss, we can use two glazings: the still air space (or better the insulating gas space) between the two glazings forms an efficient barrier against the escape of heat. However, in panels with double glazing, the flow of received radiation decreases because the limit angle q, the angle above which glass becomes reflecting, is small.

Another solution would be to use a transparent surface made of alveolar poly­carbonate, but, although it is more light, handy and cheap, its optical properties tend to deteriorate more quickly than that of glass [2, 5].

Collector’s efficiency The collector’s efficiency is determined by the ratio of the energy acquired from the thermal vector fluid and the energy received on the collector’s surface at a certain time unit:

h = 9in

Actually, the collector’s efficiency is an index of the device’s capacity to exploit the available solar source to meet the users’ requirements. The higher the collector’s efficiency, the larger is the percentage of usable received energy [5].

Efficiency depends on:

• temperature and radiation outer conditions: when room temperature decreases, the collector holds back the heat with more difficulty;

• thermal vector fluid temperature: the greater the difference between the tem­perature of the pipes and that of the fluid, the quicker and the more efficient is the heat exchange;

• the structural characteristics of the collector: the materials chosen, the optical characteristics of the covering, the absorber plate, the kind of connection be­tween the plate and the pipes, all of which are indicate that the collector’s effi­ciency depends on the its ability to restrict the different outward losses that are always present.

Since the analytical formula for the collector’s efficiency is very complex, con­structors, installers and design engineers tend to use the practical formula:

h = A — B-AT * AT * = (Tfm — TJIG

where A is the factor which, given that it is constant, sums up the material’s optical characteristics and represents the maximum radiation power which the fluid can actually reach; B represents the collector’s ability to hold back the acquired/ received heat; as for A it is given that B has a constant value; G is the global radiation received on 1 m2 of intercepting surface in a time unit (W/m2); Tfm is the average temperature of the thermal fluid which flows inside the collector; Ta is the ambient temperature.

Figure 12: The efficiency curve for a flat plate solar collector.

To guarantee a rigorous comparison between two devices, the collectors’ effi­ciency curve is one of the documents that should be certified by the application of ISO 9806-1 standards (glazed collectors) and those of ISO 9806-3 (uncovered col­lectors, which will be analysed later). The simplified formula given above allows us to easily represent the efficiency curve, which then becomes a simple straight line (Fig. 12) [1, 5, 9].

Once we have described the different kinds of collectors available on the market, we will be able both to compare the different efficiency curves and to understand their meanings. However, by now it is only possible to compare the efficiency of a flat panel with a copper absorber lined by TiNOX selective mate­rials with the one of a flat panel with a black painted copper absorber (Fig. 13).

The graph in Fig. 13 shows the efficiencies of the two collectors (blue line for the selective collector and black line for the non-selective collector) while the outside temperature decreases. If during the warmer periods of the year the two efficiencies almost coincide, during the colder period the efficiency of the selective collector’s is nearly three times higher than that of the black painted collector [18].

Hydraulic connection plans for solar panels The most used hydraulic connection plans for solar panels are shown in Fig. 14.

Figure 13: Comparison between the efficiency of a flat panel with a copper absorber lined by TiNOX selective materials (line with squares) and that of a flat panel with a black painted copper absorber (line with circles).

Figure 14: Hydraulic connections for solar panels: (a) parallel; (b) parallel with inverse return; (c) series, to a maximum of five panels; (d) series/ parallel, to a maximum of five panels.

These connection systems can be described as follows [2]:

• in the parallel connection, panels work with equal sending and return temperatures;

• in the parallel with inverse return connection, longer pipes are required;

• in the series connection, temperature gradually increases and load losses are bigger, which results in a higher final temperature;

• the series/parallel connection is the cheapest as regards its realization, so it is generally used in small solar systems that have only two panels.

Example of solar panel which is currently commercially available We will now analyse some of its features to show the best solutions [18]:

glass

EPDM rubber si icon

aluminium

clips

outer anodized a uminium case

fibreglass insulation

Figure 15: Section of a flat solar panel which is currently on the market.

• Outer anodized aluminium case, possibly black coloured, it is aesthetically very attractive and it has great corrosion strength that lasts a few years.

• Back and side fibreglass insulation of no less than 4 cm thickness, at least as regards the back part, while the upper part is lined by an aluminium sheet; an excellent absorber insulation to the ambient.

• EPDM rubber (thermal polymer ethylene/propylene/diene) and silicon contour to guarantee the waterproofing of the upper part which is more exposed to the rain.

• Low iron tempered solar glass to avoid danger if it accidentally breaks and to guarantee the best transparency to solar rays.

• Black painted copper pipes to get good solar energy absorption even from the elements which connect the selective absorber to the rest of the solar system.

• Ultrasonic welding to guarantee good thermal energy transfer from the absorber to the pipes through which the water that has to be heated flows.

• A TiNOX coated copper thin absorber to obtain the best conversion of almost all the solar ray frequencies into heat and to decrease the reflected light to its maximum as otherwise it would not be exploited.

• At least a 5-year warranty on manufacturing defects.

• A European agency certification which certifies its production, the quality of its materials and the producer’s earnestness.

The glazed flat collector is the most common and well known on the market

because of its versatility. It can be used in different ways and in different

working conditions. As regards sanitary hot water production, this collector is the most used. This collector is know for its excellent cost/performance ratio; it lasts at least 20 years and is capable of supplying hot water from 30°C to 90°C [1, 2, 5, 17].

Working conditions:

• Every kind of place (latitudes and altitudes)

• Any time of the year

Possible uses:

• Small solar systems installed to produce sanitary hot water for domestic uses

• Medium-large systems installed for domestic heating or to produce sanitary hot water for several users

• Solar systems installed to produce low temperature heat for industrial use

To meet the requirements of some specific users or to manage in the climatic con­ditions of certain places, specific kinds of collectors have been put on the market. We will now describe these in the next few paragraphs.

2.2.1.2 Unglazed collectors Unglazed collectors are widely diffuse and are known for their hot season uses. These collectors do not have a transparent covering, an outside insulation and a container case. In these collectors, the absorber body is made up of a sum of tubes which could be obtained by the extrusion of some plastic materials (polypropylene, neoprene or PVC). The thermal vector fluid flows in these tubes. These tubes are produced in a modular sheaf, which is one metre wide and has a variable length, and are connected at their ends with pipes of the same materials as the collector tubes. The collector is only composed of the absorber plate [4, 5].

Figure 16: Unglazed collector.

They have a much lower cost than the glazed panels and their installation is so simple that it can be done without the help of skilled workers.

To work well these collectors need high outside temperatures and can warm water from 10°C to 40°C, depending on their model. For this reason, these collec­tors are cut out for hot season uses (bathing establishments, seasonal hotels, camping, holiday houses, etc.).

Figure 17: Installed unglazed collectors.

The use of unglazed collectors is restricted to applications which do not require high temperatures. Since there is neither a glazed covering nor a thermal insula­tion, in case of high temperature applications the heat losses would be too big, while the efficiency would be too small. These collectors can be installed on flat roofs or on pitches. A wind protection would increase their efficiency. They last nearly 30 years [5, 9, 17].

Working conditions:

• Temperate climates

• Only hot season

Possible applications:

• To warm up water in open air swimming pools;

• In systems installed for hot season use (we talk about bathing establishments, camping, seasonal residences).

Figure 18: Unglazed panels used to warm up a swimming pool.

2.2.1.3 Vacuum tube collectors Although vacuum tube collectors (or evacuated tube collectors) are the most sophisticated and expensive technology, they allow the use of solar systems utilization during all 12 months of the year, even in a harsh climate.

Figure 19: Vacuum tube collectors.

To restrict heat dispersions which are typical in a collector and to improve the efficiency in the vacuum tube collectors, a vacuum is created between the glazed covering and the absorber plate. To completely eliminate the thermal dispersions by convection, a vacuum is created inside the tubes until a pressure less than 10-2 bar is obtained. A stronger evacuation allows avoiding the losses caused by ther­mal conduction. The losses due to radiation, as in the case of flat plate collectors, may be reduced by treating the absorber plate with selective materials. So in these ways, heat losses are considerably reduced, and even when the temperature of the absorber plate is more than 120°C, the outer surface of the tube is cold to the touch. The most important feature of the vacuum tubes which are on the Italian market is that they reach a pressure of 10-3 bar (in a few cases, air could be sucked up until a pressure of 10-5 bar is achieved) [9].

There are different constructive typologies for evacuated tube collectors. In par­ticular, evacuated tube collectors can have a flat or curved metallic sheet which crosses the glass tube horizontally and works as an absorber plate or a selective coating left on a glass bulb which is inserted inside the glass tube where the vac­uum is created. Moreover, a constructive typology does exist, characterized by an absorber plate which is a metallic cylinder put inside the two glass tubes between which the vacuum is created. Its tubular structure is particularly good to balance the stress caused by the atmospheric pressure on the outer surface of the tube [9].

An evacuated tube collector consists of a row of parallel tubes which are joined by connecting their upper ends to a gathering pipe through which the thermal vec­tor fluid flows. The tubes are then fixed by connecting their lower ends to a fitting support.

The evacuated tube collectors currently on the market are [9]:

• Evacuated tube collectors with a direct circulation system: The thermal vector fluid absorbs the heat directly circulating inside the vacuum tubes.

• Heat pipe evacuated tube collectors: The thermal vector fluid only circulates inside the gathering pipe, which connects the vacuum tubes, without entering the tubes. Each vacuum tube has a second fluid which, during its passage inside the tube, evaporates and then transfers its heat to the thermal vector fluid by condensation.

Evacuated tube collectors with a direct circulation system This constructive typology includes two different solutions. As regards the first solution (a), for which two different current applications are shown in Figs 20 and 21, the solar system consists of two coaxial tubes through which the thermal vector fluid flows. This fluid initially flows inside the inner cylinder, then when it arrives at the base of the glass bulb, it reverses its course and circulates inside the air space between the two coaxial tubes. The second solution (b) (Fig. 22) has a little pipe which longitudinally crosses the metallic cylinder, which works as an absorber plate, following a U-shaped route.

Evacuated tube collectors with a direct circulation system can be oriented south­ward at their optimum inclination with respect to the latitude of a place. Moreover, by virtue of the absorber plate’s curvature, they can be exposed horizontally too.

(A) box connection

(B) thermal insulation

(C) pipe now

(D) coaxial tube collector

(E) coaxial tube heat exchanger

(F) absorber

(G) glass vacuum tubes

Figure 20: Example of an evacuated tube collector with a direct circulation system (solution (a)).

Figure 21: Another example of an evacuated tube collector with a direct circulation system (solution (a)).

Figure 22: An evacuated tube collector with a direct circulation system (solution (b)).

The Sydney type collector (Fig. 23) is a kind of evacuated tube collector with a direct circulation system. This collector consists of two coaxial glass tubes between which a vacuum is created. The inner tube is covered by a copper sheet which is treated with carbon selective material. Inside that tube, there is an oppor­tunely shaped thin sheet which favours thermal conduction between the absorber plate and the U-shaped pipe through which the thermal vector fluid flows.

This kind of collector generally has a variable number of tubes, which depends on the supplier (from a minimum of 6 to a maximum of 21 evacuated tubes). To increase its capacity to intercept solar radiation, the collector has a few reflectors which are suitable for installation on steep roofs. The flat roof model does not have these reflectors and so it is better to install this system on a reflecting surface (e. g. gravel).

However, one of Schott’s collector models (Figs. 21 and 24) consists of three different glass tubes fitted one in another: the outer tube works as the absorber

1) Outer Tube

2) Vacuum

3) Selective copper thin sheet

4) Heat conduction thin sheet

5) U-shaped tube

6) Reflector

Figure 23: Sydney’s tube scheme.

plate and it is covered by a selective treatment and the inner tube. The thermal vector fluid flows inside the inner tube; once it reaches the base of the glass bulb, it reverses its course and warms up, circulating inside the air space between the two coaxial tubes.

The vacuum zone pressure, where air has been sucked up and an inert gas has been put in, reaches 30 mbar. A silver coloured reflecting surface is longitudinally put on the lower half of the outer tube, putting the reflector inside the outer tube, so the protection against outside agents increases. [1, 5, 9, 17].

Heat pipe evacuated tube collectors In this kind of device, the heat is exchanged by the passage of phase: the fluid, which is warmed by radiation, first evaporates and then condenses when it comes into contact with a condenser and gives back the thermal energy which it previously absorbed.

The evacuated tube has a thin flat sheet inside which is placed longitudinally and treated in a selective way. This thin flat sheet works as an absorber plate and on it there is a vacuum pipe (heat pipe) which receives the heat from the absorber plate by conduction. The heat pipe is closed and generally contains water or alco­hol which evaporates at about 25°C in vacuum conditions. The vapour goes up to the collector’s head and there it transfers its thermal energy to the thermal vector fluid which flows inside the gathering pipe. At this point, a new thin liquid film is created which comes back into the evaporation zone by gravity (see Fig. 25). To work properly, tubes have to be installed at an inclination of more than 25°.

This kind of collector has two models currently on the market: a model with a dry heat exchange and another with an immersion heat exchange.

Figure 25: Working scheme of a heat pipe evacuated tube collector.

In the first case (Fig. 26), the thermal vector fluid, which receives the thermal energy produced by condensation from the vapour, flows in a separate pipe. This pipe surrounds the condensers which are placed in series, so the thermal flux flows along the metal walls of this exchanger-pipe (Fig. 27). This constructive solution allows us to easily substitute one of the collector’s tubes when it breaks without having to empty out the solar circuit (the connection between the collectors and the energy tank).

Figure 26: Transverse section of a heat pipe evacuated tube collector with dry heat.

Figure 27: Heat exchanger (dry exchange).

In the second collector model (Figs 28-30), i. e. with an immersion heat exchange, the condensers are directly dipped in the thermal vector fluid. If a tube breaks, we need to empty at least the thermal vector fluid gathering pipe, which is in the head with the vacuum tubes [1, 5, 9, 17].

The efficiency of an evacuated tube collector is on average higher than that of a flat plate collector by virtue of the reduction of thermal dispersions which could be obtained by this system.

Figure 28: Working scheme of a heat pipe evacuated tube collector with immersion heat exchange.

1) Glass tube

2) Absorber

3) Heat pipe

4) Box collector with thermal insulation heat-resistant

5) Lid of the box collector

6) Condenser

7) Pipe with bulb immersion

8) Metal cap

9) Ring seal of the box

10) Upper guide

11) Getter to Bario

12) Lower guide

Figure 29: Components of a heat pipe evacuated tube collector with immersion heat exchange.

The advantages of an evacuated tube collector are [9]:

• It has excellent efficiency even when the temperature differences between the absorber plate and the outside are very high.

• It has a high efficiency even in case of reduced radiation conditions (e. g. in winter).

• It allows heating of the thermal vector fluid to high temperatures and so it can be used in heating systems, in room conditioning systems and also in vapour/ steam production.

• It can be easily transported to any place where it has to be installed.

• It can be oriented southward easily, even during the assemblage phase (this only refers to some products). In fact, in this phase, the tubes can be turned round to place the thin sheets, which work as the absorber plate, perpendicular to the direction of the solar radiation.

• As regards the collector with a direct circulation system, it can be directly installed on a flat roof, reducing anchorage problems and, of course, installation costs.

The limits are:

• It is more expensive than a flat plane collector.

• The heat pipes have to be installed at an inclination of more than 25°.

Working conditions:

• Places with low outside temperature or short solar radiation

• All the times of the year

Possible applications:

• Heat production at a higher temperature (e. g. industrial process heat)

• To heat sanitary water or rooms

2.2.1.4 Integrated storage collectors Integrated storage collectors are not very common, although in suitable working conditions they could offer more advan­tages than flat plate collectors. They could be an easy and interesting solution especially in places with a mild climate. These devices consist of only a single ele­ment which substitutes the absorber plate, the serpentine and the hot water storage tank. They are, for example, a series of flanked pipes which have a diameter of 10 cm, a group of pipes of similar dimensions which are created by placing two sheets face to face, a unique big pipe or a variable form tank; the most important thing is that the element in question is able to substitute the storage tank which is generally placed outside the collector [5, 9].

The water, which stands inside the collector and will be used later, absorbs the heat and uniformly diffuses it inside the collector, thanks to spontaneous convec­tive motions. To get an idea about the differences between this collector model and the traditional models, it is sufficient to underline that the water storage per square metre of an integrated storage collector’s intercepting surface reaches 80-100 l/m2 while the storage for devices with an outer storage tank reaches 0.6-2 l/m2.

However, this kind of collector has the problem of not being able to restrict the outward heat dispersions; actually, only five of the six storage tank’s surfaces which are exposed to weather conditions are insulated. Therefore, it is clear that when weather conditions get worse, the integrated storage collector’s efficiency quickly decreases, indicating that these devices are unsuitable for sufficiently mild climates or periods of the year [5, 9].

Figure 33: Integrated storage collector.

In comparison with other collector typologies, the integrated storage collector is cheaper, compact, handy, occupies lesser room and can be installed without the help of skilled workers.

Working conditions:

• Temperate climates

• Mild periods of the year

Possible uses:

• Sanitary water heating

• Water heating for low temperature industrial processes

2.2.1.5 Spherical collectors Some thermal solar solutions could regard the sys­tem’s shape or aesthetic impact. In this case, a spherical system could be less invasive and cheaper. It is a simple integrated storage collector [17, 25].

2.2.1.6 Air collectors This is another kind of collector suitable for low tempera­ture applications. They are very similar to normal glazed panels, but in this case air is used as the vector fluid rather than water. Air can circulate between the glass and the absorber or between the absorber and the bottom of the panel. Usually, the absorber is finned to make the passage of air slower and tortuous, because air exchanges heat with more difficulty compared with water. So, we require that the air remains inside the panels longer to make it absorb the greatest quantity of heat. Since air never freezes, there is no need to use antifreeze techniques. Currently, these collectors have reached 60-70% efficiency and they have a long life-time (even over 30 years).

Figure 34: Spherical collector.

Figure 35: Air collector.

Air collectors may have different applications such as heating water or produc­ing compost using toilet emissions. However, their principal application is in solar heating for buildings. In fact, one particular example of air solar panels is the lin­ing panels which are used as a coating for normal plugging walls in industrial, commercial and residential buildings. They are not glazed but have an outer metal surface which works as an absorber and heats the air which enters the collector through micro-perforations. The air which circulates inside the air space between the panel and the wall can then be circulated inside the rooms using a proper aspi­ration system, thus contributing to heating and changing of air in the same rooms. During the summer, they help to bring down the temperature by not allowing the solar radiation to fall directly on the building’s external walls. When the fan is
switched off, fresh air enters from the lower perforations and by natural convective motion it goes out from the higher perforations, creating a continuous flow/flux which helps to maintain the wall’s temperature.

safety glass solar

solar cells

absorber

aspiration

material

output —

Figure 36: Working scheme of an air collector.

These systems are able to supply between 25% and 50% of the energy needed to heat a room [16, 17, 23, 25].

2.2.1.7 Efficiency curves in comparison Figure 37 shows the efficiency curves of the most diffuse collector typologies (see section ‘Collector’s efficiency’ under par. 2.2.1.1). These collectors are the flat plate collector (with and without a selective treat­ment), the unglazed collector and the evacuated tube collector. Taking into considera­tion the fact that they have the same thermal vector fluid temperature, the increasing in the AT* value concerns the unfavourable weather conditions (radiation and/or decreasing ambient temperature). Therefore, moving rightward along the abscissa we can see how the collector’s efficiency changes, if working conditions get worse [5].

evacuated tube со lector

flat plate collector

*-^with a selective plate

flat plate collector without

a selective treatment

unglazed

col ector

AT*

Figure 37: Comparison of the efficiency curves of different kinds of collectors.

From the efficiency curves, we can immediately observe the differences between these kinds of collectors [5, 9]:

• Since the unglazed collectors does not have a transparent covering, it has the best chance to absorb the incident radiation (the straight line meets the ordinate axis at the highest point); however, its efficiency decreases rapidly until it reaches zero in places where the other kinds of collectors have a decent efficiency.

• The flat plate collector with a selective plate has a better performance in every working condition compared with the simple flat plate collector.

• The evacuated tube collector has the most stable efficiency curve and it guaran­tees good performances even during unfavourable working conditions.

As an example, Table 2 lists some values of the A and B coefficients for some of the commercial devices available in the Italian market.

Table 2: A and B coefficient values. A B Flat plate collector without a selective treatment 0.70-0.85 5.5-7.6 Flat plate collector with a selective plate 0.75-0.85 3.5-5.8 Unglazed collector 0.80-0.86 22.0-28.0 Evacuated tube collector 0.80-0.85 2.0-3.0

2.2.2 Typologies of solar systems

The passage from the collector to the solar system requires few elements which can make the service enjoyable to users and stabilize the collector’s performance [5].

• The role of the storage tank is to supply hot water to users at any time and in any weather condition, whenever it is required; it stores the water heated by collectors in small amounts and maintains the hot water at a constant temperature until it is demanded by the users.

• The auxiliary system (specifically methane central-heating boiler or electric water heater) is needed to make up for any contingency in the solar source and for the lower solar energy availability during winter. In this way, we avoid oversizing the solar system until it becomes too expensive.

• The expansion tube is the part which is able to receive any excessive thermal expansion of the thermal vector fluid, thus avoiding the creation of dangerous overpressures.

• Safety-bolts and system check (‘jolly’ valves, intercepting valve, thermostats, etc.)

We can find other circuit elements (such as circulators, control station, etc.) only in a few kinds of system.

2.2.2.1 Nomenclature and principal applications Solar energy systems can be divided into four principal categories: first, according to the relationship between the thermal vector fluid and the service given to users; second, according to the way in which the fluid is circulated.

We talk about open systems when the fluid inside the collector is water itself, which is provided to the users once it has reached the required temperature. By contrast, we talk about closed systems if the thermal vector fluid flowing inside the collectors transfers its heat to the usable fluid (water) through an exchanger. In the latter case, we have two distinct circuits, one for the thermal vector fluid and another for the water that has to be heated up [5, 6, 17].

Observing the circulation of the thermal vector fluid inside a solar system, we must distinguish [5, 6]:

• Forced circulation system: In this case, to regulate the flux, we need to insert an automatic system which consists of a circulator with thermostats and a control station.

• Natural circulation system: In this case, the fluid’s flow inside the collector is automatically stabilized by spontaneous convective motions.

Figure 38: Forced circulation system and an example of the installation of a forced circulation system.

Figure 39: Natural circulation system and an example of the installation of a natural circulation system.

In theory, the two variables are completely independent and so it is possible to install systems having all the four possible combinations. However, the experience acquired through installations has shown that only a few possible solutions can be put into practice. The advantages of an open circuit system are the simplicity of its hydraulic circuit realization and the absence of thermal dispersions, which occur every time heat moves from one circuit to another [5].

Nevertheless, an open circuit system is not usually adopted for two reasons: (1) water can easily freeze when the temperature is below zero and (2) there could be lime scale accumulation inside the collector’s pipes. In both cases, the collector can be damaged till it becomes out of service. Because of these problems, which are difficult to control, the open circuit system has been substituted with more complex installations; however, the simplicity of the open circuit system is still exploited in these cases [5]:

• Systems whose collectors are unglazed and are used only during the hot season: These systems avoid the freezing problem and the lime scale accumulation is limited to the working temperature (not above 40-45°C).

• Integrated storage systems installed in places with a mild climate: These devices use such large quantities of water that would hardly freeze completely and, at the same time, the absence of small sized pipes does not make probable scales very dangerous (but they have to be checked in any case using softener filters).

Except for the above-mentioned applications, the closed circuit system repre­sents the widest and the most reliable solution. In this case, two different hydrau­lic circuits are involved (actually, closed systems are also called double circuit systems): the primary circuit, where only the thermal vector fluid flows, and the secondary circuit, where water, coming from the water network and assigned to users, flows. The thermal vector fluid absorbs the energy from the intercepting plate and then transfers the greatest part of that heat to the water that has to be warmed. The place where the heat transfer takes place is a very important element and it is called the exchanger. Since walls with high thermal conductivity separate the thermal vector fluid from the water, the fluid transfers heat to the cold water in proportion to the temperature difference between the two liquids. If the interface surface is large, the thermal energy exchange will also be high, especially when temperature differences may not be relevant. To satisfy the need to have a large exchanging surface and at the same time a compact device to rely on, the most common exchangers currently in use are in the form of a dipped worm-pie, a sheaf of tubes or plates.

The choice of the exchanger is really important since a good performance of this device, besides making the service quicker and more efficacious, allows the ther­mal vector fluid to return to the collectors at a highly decreased temperature, which in turn increases the collectors’ efficiency [5].

As we have just examined the differences between open and closed systems, we go on to analyse the circulation of the thermal vector fluid (either water or anti-freeze solution) inside the system.

Single circuit systems Double circuit systems Figure 40: Working scheme of a natural circulation system.

Natural circulation [5, 6, 9] (Fig. 40) exploits the spontaneous behaviour of fluids to create convective motions when there is a localized increase in the tem­perature. The systems which exploit this phenomenon can be realized using any kind of solar panel and are characterized by a storage tank which is elevated com­pared to the collector (Fig. 41); thanks to this property, the fluid in the collector, once it has heated up on coming into contact with the exposed plate, becomes less and less thick and spontaneously tends to move up towards the storage tank. In this way, it leaves enough room for the fresher fluid inside the collector.

Figure 41: Natural circulation.

Therefore, to obtain excellent performance, this kind of device regulates itself by optimizing the fluid circulation spontaneously. However, the system’s structure has some characteristics that restrain its utilization; for example, the storage tank is completely exposed to all weathers and seasonal variations. So, even if the stor­age tank is correctly and efficaciously insulated, it cannot avoid the host of energy losses when exposed to very low temperatures.

Second, since the storage tank is located above the collector, the system can have remarkably high weights especially when the intercepting surfaces are very big. In this case, the system’s weight can become a problem for the structural resistance of the roof and garret, as these are the usual places of installation.

Eventually, the aesthetic impact of the most common natural circulation collec­tors is not one of the best: their structures make them particularly showy (Fig. 42). To overcome this problem, scientists have been realized a few less showy devices which have the storage tank behind the collector, for example (Fig. 42).

If the roof is steep, the storage tank can be installed inside the building, although it stays above the collector. In this case, we will not have any energy losses or aesthetic impact problems.

However, as regards a small dimension system which is installed in a place with a mild climate, the natural circulation remains the best solution as it is simple, cheap and compact.

Natural circulation may be used in the following cases [5]:

• integrated storage solar collectors which are placed in any manner;

• monobloc solar collectors (i. e. with a storage tank fastened to the upper part of the collector);

• solar collectors installed on the ground and the storage tank (separated from the collectors) located on an elevated structure which is inside the building;

• solar collectors installed on the roof’s slope and the storage tank (separated from the collectors) placed inside the garret and located in a more elevated position than the Figure 43: Scheme of a forced circulation system.

Forced circulation [5, 6, 9] is always necessary when it is not possible to place the storage tank in a higher position than the collector. In this case, the best circu­lation for the thermal vector fluid would be one which is completely opposite to the circulation which is considered natural. Because of this, the circuit needs a few additional devices such as a circulation pump, which moves the fluid in the right direction, a non-return valve, which does not allow the reverse circulation to take place in any situation, and a control station, which automatically operates inside the circulator to regulate the fluid circulation and optimize the system’s performance.

This system is certainly complex and expensive and also requires that each of its parts is accurately proportioned. However, it gives us freedom in terms of its design and architectural integration (the storage tank is actually completely separated from the collectors) and is also suitable for any weather condition.

Forced circulation systems can be realized using any kind of collector, except­ing the integrated storage collector, since this collector also works as a storage tank.

We are almost forced to use a forced circulation system [5]:

• when we want the system to be more precisely checked and regulated;

• when the weight of the storage is more than the roof’s resistance;

• when there is no garret where we can install the storage tank, and its aesthetic impact poses a serious disadvantage;

• when, due to logistic reasons, it is not possible to realize a natural circulation.

Figure 44: Images of forced circulation systems.

The advantages of natural circulation systems are:

• the speed of thermal exchange is proportional to the temperature difference between the storage boiler and the panels;

• the circulation is self-regulated;

• there are no circulation pumps, control stations and feelers;

• quick and cheap installation;

• minimum maintenance.

The advantages of forced circulation systems are:

• architectural integration of the collectors;

• maximum flexibility of the system.

Table 3 lists the principal applications of each system in terms of its typology.

Table 3: System typologies used in various applications. Circulation System’s typology Principal applications Natural Open system Small systems used to heat sanitary water (no rigorous climate or with integrated storage collectors) Systems used only during hot season (e. g. bathing establishments or camping) Natural Closed system Systems used to heat sanitary water or to heat rooms Forced Open system Systems used only during hot seasons (bathing establishments or camping) Systems used to heat water in swimming pools Forced Closed system Small systems used to heat sanitary water for domestic use when it is not possible to put the storage tank above the collectors Systems installed to heat sanitary water which will be used by collective users Systems used to heat sanitary water and rooms Systems used to heat water in swimming pools

2.2.2.2 System-type description In this paragraph, we will offer an outline of the most common system schemes as regards the applications listed below [5, 9, 17]:

1. domestic system for sanitary hot water production;

2. big collective use system for sanitary hot water production;

3. small combined systems for sanitary hot water production and room heating;

4. system for heating swimming pools.

Another important application which has to be mentioned is room refreshing during the hot season. Some of the newest and most efficient devices which can be used for air conditioning, i. e. absorption heat pumps, require a hot thermal source to supply air for refreshing to the user. The thermal vector fluid, which flows inside the collectors, can perform this function. This application is particularly interesting since the refreshing system’s peak load of work corresponds exactly with the max­imum availability of the solar source. Actually, if there is a reduced phase displace­ment with time, we can assume that the thermal load to be taken from the outside is roughly proportional to the incident solar radiation.

Moreover, this solution is very desirable if we think about the consumption of electricity and air-conditioning costs.

As regards the heat pump which uses helium, the consumption can be limited only to the integration of solar feeding and for this reason its consumption can be lower than that of a normal heat pump. However, this solution is not so common due to the high cost of the absorption heat pumps and the need for reaching higher hot source temperatures. In fact, these temperatures have to be higher than the working temperatures of a flat plate solar collector. Recent studies have underlined how the latter problem can be overcome by the installation of an evacuated tube solar collector furnished with mirrors for the concentration of solar radiation. The economic problem has not been overcome yet, but as the dimensions of the system increase, the more its importance decreases [5].

Domestic systems for sanitary hot water production As regards sanitary hot water production, the most common combinations are:

• natural circulation (or through radiators), closed circuit;

• systems with integrated storage collectors (less common);

• forced circulation, closed circuit.

The first application (Fig. 45) is generally the preferred one.

Figure 45: Radiator collector (natural circulation) and closed circuit.

In this case, the primary circuit (where the thermal vector fluid flows) is completely inside the storage-collector system and the exchanger (generally has a dipped worm-pie configuration or an outer cover) is inside the storage tank. As stated before, the regulation of the radiation system is completely spontaneous: the thermal vector fluid, once it has warmed up inside the collector, tends to move up towards the storage where, thanks to the exchanger, it transfers heat to the water which is inside the tank. When the temperature of the fluid which is inside the col­lector becomes higher than the temperature of the fluid inside the exchanger, because of the density difference, the circulation in the primary circuit begins spontaneously. In this manner, without any outside intervention, the collector is always filled up with the thermal vector fluid which has a lower temperature (which results in the increase of the efficiency of the device) and inside the exchanger there is always the warmer fluid. Since the water which has to receive the heat stratifies upwards when temperatures are high, the exchanger is installed in the lower part of the storage system; in fact, even in this case, it is important that the exchange happens using the colder water which is inside the tank. Some closed circuit radiator collectors have (or are equipped for the insertion of) an electric resistance inside the storage tank, which is switched on by a thermostat when it is not possible to reach the required temperature. The advantage of this solution is to have also, in a single device, the traditional integrative system to supply the required service directly to the users.

The disadvantage instead is the use of electricity to produce heat at a low tem­perature, although in an integrative and limited way. As we actually know, electric­ity is a very noble and versatile kind of energy and for its production (in Italy it happens in 80% of the cases) fossil fuels are burnt at very high temperatures, los­ing almost 67% of the energy developed in the burning phase, i. e. during the thermo-electrical conversions, and during the transport to where the service is supplied.

From an economic, energetic and environmental point of view, the use of elec­tricity for the sanitary hot water production is extremely irrational. So it is preferred, where possible, to use an outer integration such as the gas boiler.

The scheme of an integrated storage collector’s system is very similar to the circuit scheme (as the one we have already seen) where a monobloc radiator col­lector is inserted. If we imagine the collector as a black box, i. e. if we observe only the inputs and the outputs without considering the inner working of the device, the two cases could be superimposable. As regards the integrated storage system, the maintenance and frequent control of the softener filter are very impor­tant since the water which is supplied to the users is the same water which passes through the collector and so it is important to reduce the lime scale accumulation which water can cause to its minimum [5].

For all domestic applications where it is not possible to place the storage tank above the collector (or the choice of an integrated storage collector is not conve­nient), it is very common to choose a forced circulation system with a closed cir­cuit. In this last case, the tank is located in any part of the building and always in a vertical position to favour the stratification of water when the temperature increases and also to reduce the mixing between the cold water that enters and the water that is ready for the users (a phenomenon which has a negative influence on the overall efficiency of the system).

Figure 46: Forced circulation system with closed circuit.

As we can observe from Fig. 46, the primary and secondary circuits as a whole are more complex and articulated compared with the radiator scheme. In this case, it is necessary to use a circulator which is run by a definite regulation power unit.

The circulator is a small centrifugal pump capable of moving the fluid inside the primary circuit. Its running has to allow excellent processing of the collectors (the fluid inside them should not be too hot) and at the same time has to allow efficient heat transmission from the primary and the secondary circuits (the temperature difference between the two circuits should allow the exchange).

To regulate the temperature difference between the two circuits, the circulator has a differential thermostat fitted for measuring the temperature difference between the thermal vector fluid which comes out from the collector and the water which is in the lower part of the storage tank. If the measured difference, for exam­ple, is over 5-8°C, the circulator moves the primary fluid, starting the heat exchange between the two circuits; as soon as the measured temperature difference is too small for thermal exchange to take place (going down to <3-4°C), the power unit switches off the pump.

The electrical power unit can also protect the system, having the possibility to control the maximum running temperature of both the collector and the storage tank. The exchanger can be inside or outside the storage tank; this last solution is preferred when the systems are big and normally plate exchangers are installed.

When the hot water storage tank is located in a place which is very near the final users, it results in a small inefficiency due to the presence of cold water inside the tubes fitted for the transfer of hot water from the storage tank. Actually, the users will have to let a few litres of cold water run before getting water at the tempera­ture they require. This problem can be solved by the predisposition of a recycling system inside the general hydraulic circuit; a small quantity of stored hot water periodically passes through the pipes which connect the storage tank to the final user to keep both the tubes and the water at nearly the required temperature. This solution involves a small loss of stored energy but it is the most commonly used technique especially in plants of medium to large size [5].

Big collective use systems for sanitary hot water production In this specific case, we have systems characterized by an absorber plate which exceed an area of 100 m2; their realization often depends on collective users such as apartment buildings, sport centres, schools, hospitals and hotels.

Figure 47: Pictures of collective use system for sanitary hot water production.

Actually, all the structures which are characterized by a relevant, continuous and concentrated demand for sanitary hot water are interested in this application. The realization of a large-sized system could be favourable in terms of cost: the system’s dimensions allow a significant reduction in the total price (because of both the ‘stair’ effect, which is connected to the components that have to be bought, and the installation costs, which has a lesser influence than the former on the final price).

The most common solution adopted is one characterized by the closed circuit and forced circulation.

In big residential buildings, solar collectors can meet 30-50% of the yearly energy requirement for sanitary hot water. In this case, we talk about ‘pre-heating’, since a complementary system which brings water to the required temperature is generally needed even during the hot season. The proportioning of the solar system and its combination with the heating system depend on the subject characteristics [5, 17].

Figure 48 shows the scheme of a solar system (with a closed circuit and forced circulation) installed to heat water in an apartment building.

Figure 48: Example of a solar system for an apartment building.

The system consists of a boiler (water heater) and a certain number of solar panels which are in proportion to the number of users (the panels must form a receiving surface that has an area of at least 2 m2 per standard family of four people). The boiler should be put on the balcony or on the thermal power unit.

Solar radiation warms the fluid which is inside the solar panels. The power unit takes the temperature of the fluid when it goes out from the solar panels and also that of the water inside the boiler. When the thermal fluid temperature exceeds the temperature of the water inside the boiler by a certain AT number (which is selected on the electronic power unit), the electronic power unit starts the exchanger and so it gives way to the thermal exchange, warming up the water inside the boiler. This situation lasts until the thermal difference is higher than the AT selected on the electrical power unit.

This system self-sufficiently supplies warm water to the users during the spring, summer and autumn months (a closed, b opened), while during the winter it ‘pre­heats’ the sanitary water (a opened, b closed), which is heated up later to the required temperature by a gas or electrical central-heating boiler. The advantages of this already tested technique are:

• Hot water distribution: The sole ring used to distribute hot water lets all the users use it immediately.

• Low installation costs for the water-supply: A single tube is used for cold water feeding and another for warm water feeding.

• System functionality: With this system typology, the non-simultaneous utilization of hot water by the users prolongs the self-sufficiency time.

The only cost of this system, which should be shared among the people living in the apartment building, is the electrical energy used by the exchanger, which is equivalent to the energy consumed by a 70 W bulb. The exchanger has to be con­nected to the electrical system of the stairwell and works for 4 or 5 hours a day. A special subtraction meter, placed in each apartment, allows calculating how much hot water each family consumes [64].

Small combined systems Many studies have indicated that the yearly thermal energy requirement for room heating is two to ten times higher than the yearly san­itary hot water requirement. The idea to move a part of this consumption from the traditional energy sources to the solar source is fascinating: although the require­ment from users is more during the part of the year with less solar radiation, the proposed solutions have become interesting even from an economic point of view (which is very important for people

Figure 49: Scheme of a combined solar system (closed cycle and forced circulation).

As regards only room heating, adopting a combined solar system will cover 10-40% of the total yearly thermal requirement. Exceeding these values will be inconvenient from a technical and economic point of view: the big active surfaces which are necessary to supply energy during winter would produce a huge energy surplus in summer, a large part of which would be wasted if there are no particular additional energy requirements.

One solution would be the so-called seasonal energy storage which exploits the heat stored in summer during the cold season. Although this solution sounds very interesting, the technological solutions which rely on this principle are not suitable for small domestic systems from both logistic and economic points of view.

Although they cannot guarantee the self-sufficiency of the system, resorting to daily storage systems shows, on the contrary, a growing improvement in the
cost-benefit ratio, even for small systems. Since during winter the best performance of a thermal solar collector can be obtained when the supplying water temperature is about 30-40°C, the room heating system cannot consist of traditional radiators. In fact, traditional radiators do not have a wide exchanging surface, so to heat the rooms it is necessary to feed them water at higher temperature. We may resort to radiant heaters (which are more suitable than radiators for the utilization of solar sources) whose surface extension should be equal, for example, to the entire floor of the house that has to be heated up (see Fig. 50).

Figure 50: Scheme of a solar source heating system.

As regards radiant devices, it is obviously possible to install different configurations. However, all of them aim to increase the exchanging surface and to guarantee the users’ thermal comfort. The radiant heater may coincide with a wall or it can be substituted by particular radiant skirting-boards which run along all the walls of the house.

Based on what we have just mentioned and the difference between the users’ requirement and the source’s availability, the installation of systems called combi, i. e. combined, is interesting if it is associated with specific characteristics of the whole building. First, an efficient thermal insulation is needed to avoid dispersion of the stored heat. Second, it is necessary to have a heating system based on radiant heaters or alternatively to realize a new system together with the restoration of the building to allow the adoption of that system. As stated, the choice of a combined solar system could be particularly favourable if the building has consistent sum­mer users. In this case, the energy surplus produced during the hot months is completely used. This results in advantages in system exploitation and also in terms of economic return of the investment [5, 9].

System for swimming pools The utilization of the solar source for heating water in a swimming pool is fascinating for several reasons: first, the temperature that has to be reached to guarantee the users’ well-being is not higher than 25-28°C. This lets the collectors work in favourable conditions as regards the efficiency. Second, it is interesting to observe that the system turns out to be particularly simple, thanks to the possibility of eliminating the storage system, substituted by the water mass contained inside the swimming pool itself (see Figs 51 and 52).

Figure 51: Solar system for heating water in a swimming pool.

Figure 52: Simplified scheme of a solar system installed to heat water in a swimming pool.

For open air swimming pools, which are mainly used during the summer, it is possible to meet almost 100% of the energy requirement using the solar source. In this case, the high ambient temperatures during the processing of the system, the low thermal vector fluid temperature and the strong radiation allows us to use the unglazed collector, which has been discussed previously (par. 2.2.1.2). The unglazed collectors are characterized by a convenient price (almost 50% less than the price of a glazed panel) and their performance is very appreciable.

More complex systems can provide for, besides heating the water in swimming pools, the requirement of sanitary hot water utilization connected to the showers. As we have already seen in the case of the combi systems, in this case also par­ticular care must be taken in the planning and regulation of the thermal priority so as to obtain the highest performances from the system (Fig. 53) [5, 9].

Figure 53: Simplified scheme of a solar system used to heat an open air swimming pool and to produce sanitary hot water.

From what we have discussed until now, it should be clear that the Italian position and interest in CSP technologies are as follows [45]:

• Pursuing the technical and industrial development of this technology to guarantee

green energy flow at a profitable price in terms of Euro-Mediterranean integration

and globalization of environmental problems.

Figure 83: Trend of the predicted costs between the years 2020 and 2025.

• To ensure in time a substantial part of the future global turnover related to green energy production in terms of R&S, that is, by the creation of new Italian enter­prises or by getting industrial orders for Italian firms.

Using a polar diagram, it is possible to visualize solar trajectories during a year at a certain place. This diagram, which is a projection of solar trajectories on a hori­zontal plane, can be obtained by the plotting the values of solar height and azimuth on a graph. These values are calculated using the eqns (5) and (6), for a certain place and as a function of the true solar time, as shown in Fig. 11.

Using this diagram, it is also possible to determine graphically the periods of time during which a surface point remains in shadow because of the obstacles which intercept solar rays. When the distance of the obstruction is large compared to the receiver’s dimensions (a solar collector, a window, etc.) it is right to consider the receiver as a punctiform one, since the shadow tends to move fast on the receiver so that it is completely in shadow or completely illuminated. To determine when the obstacle intercepts solar rays, in the diagram of solar trajectories, we have to represent the angle from the obstacle as seen from the considered

Figure 11: Diagram of solar trajectories.

West 100 50 0 50 100 East Solar azimuth, a Figure 12: Diagram of the Sun’s position.

point, plotting on it (the diagram) the azimuth and the angle height of the obstacle’s contour points.

As an alternative to the diagram of solar trajectories we can use a Cartesian diagram of the Sun’s position in which the azimuth is plotted along the horizontal axis while the altitude is plotted vertically. The Sun’s position can be read by sim­ply reading the two axes. An example of this diagram is given in Fig.12. Of course, we may use this diagram to calculate the shadows [1].

The solar circuit is the connection between the solar collectors and the storage tank. A solar circuit can be made of copper or stainless steel pipes. To restrict heat losses, the pipes which connect the collectors to the tank have to be short and insulated to their maximum. Stainless steel pipes have a surface which is less smooth than that of copper pipes and so they cause bigger load loss. In open systems, it is better to use tubes which have a smooth inner surface (e. g. cop­per) to prevent encrustments. When we choose the circuit components, it is very important to consider the kind of fluid they are going to contain; actually, anti­freeze solutions or products used in the swimming pool impose the utilization of components (taps and fittings, sets) which do not corrode when they come into contact with certain chemical substances. The good processing of a solar system strongly depends on how the solar circuit’s insulation has been carried out. A suf­ficient insulation layer and also a good insulation execution without interruptions or escapes are necessary. This also applies to the circuit’s elbows. Concerning the choice of the insulating materials, it is important to take into consideration their
resistance to high temperatures; for very short periods, the temperature inside the solar circuit’s tubes can reach more than 200°C. Moreover, the insulation should be able to resis atmospheric agents and ultraviolet rays. Suitable materials could be insulating mineral fibres or insulating materials such as Aeroflex and Armaflex HT. On the outside, the insulation can be protected by tube-coverings made of a steel or zinc-plated layer.

In the solar circuit, besides the pipes, there are devices which are necessary to guarantee the fluid’s motion and the security on the basis of its assemblage and utilization (pumps, buffer vessels, expansion vessels, security valves, air discharge valves, etc.). Generally, all the basic hydraulic components offered are already pre-set by a great majority of the manufacturers. Also, the control instruments (manometers and thermometers) are pre-set [5, 6, 9, 17].

Let us now see the elements which characterize the solar circuit, with reference to systems with a forced circulation, because of its complexity. It has been seen

that in the natural circulation system the storage tank can be directly heated up by the natural circulation or by a heat exchanger. Moreover, there is no device which is able to actively regulate the solar circuit [6].

Pump As in the centralized heating systems, in the solar systems, there are ‘lots’ and ‘returns’. The pipe in which the hot thermal vector fluid flows from the col­lector to the storage tank is called ‘lot’, whereas the ‘return’ is the pipe with the colder fluid which flows from the storage tank to the collector. The pump must be installed on the return line, with the motor’s shaft in the horizontal direction. The pump must not be insulated [5, 6, 33].

Figure 55: Circulation pump.

Non-return valve (or restraint valve) When collectors are installed in an ele­vated position than the storage tank and the fluid inside the tank has a higher temperature than the fluid which flows inside the collectors (especially during the night), the temperature difference between the hot fluid inside the exchanger (in a lower position) and the cold fluid inside the collectors (in a higher position) would start a natural circulation inside the primary circuit causing dispersion of the heat stored in the tank during the day.

Figure 56: Non-return valve.

To avoid this phenomenon, it is necessary to install a non-return valve between the pump and the collector; this valve has to be well proportioned so as to prevent its opening by the sole thrust force of the thermal vector fluid. In this way, the storage tank will not be cooled by the collector when the pump is not running [5, 9].

Regulating power unit The regulating unit of a thermal solar system controls the running of the circulation pump to exploit the solar energy to its fullest. Often, we talk about simple electronic power units based on the temperature difference. This kind of power unit installed in standard systems (collectors on the roof and storage tanks in cellars) are provided with two temperature sensors. The first sen­sor is installed inside the collector, at the hottest point of the solar circuit, and the second sensor is installed inside the tank, connected with the heat exchanger of the solar circuit.

Figure 57: Electronic power unit.

The temperature values detected by the sensors are compared by a control device: the pump is run by a relay when the intervention temperature is reached. The correct definition of the intervention temperature comes from different factors. Generally, the longer the circuit pipes are, the larger is the temperature difference or the delay in the intervention. To make the pump work, standard directions sug­gest that the temperature difference between the solar collectors and the storage tank should be between 5°C and 8°C. Instead, the pump switches off when the temperature difference reaches 3°C. It is also possible to insert a third optional sensor which measures the temperature of the upper part of the storage tank [33].

Temperature sensors The efficiency of the pump’s intervention mostly depends on the position of the thermal sensors. Each collector’s sensor is positioned on the storage pipe or directly on the absorber (the part of the collector which absorbs solar radiation), near the lot’s exit. However, the thermal sensor has to record the absorber temperature and communicate it to the regulating power unit, even in a stalemate situation, i. e. when the pump is not working.

The storage tank’s sensor has to be placed at the same height as the heat exchanger and it can be an immersion sensor or a contact sensor. The sensor for the auxiliary heating communicates to the power unit when this intervention is necessary and has to be put at the same height as its respective heat exchanger [33].

Figure 58: On the left, the processing of the auxiliary heating can be seen; on the right, the storage tank is heated by the solar circuit’s exchanger.

Expansion vessel It absorbs the thermal vector fluid’s expansion. The size of this component depends on the fluid quantity inside the circuit, since the vessel must be able to contain the fluid’s dilatation between 4°C and 90°C.

Figure 59: Expansion vessels.

Let us assume a system with a 100-l tank and with collectors and pipes which can contain a water volume of 20 l. The expansion vessel’s volume must be able to absorb the dilation of 120 l of water which occurs at the just said temperatures. The pipes which connect the expansion vessel to the system must not be insulated [5, 6, 17, 33].

Security valve Security valves protect the system when the pressure increases because of various reasons, such as superheating. Such circumstances might arise when the circulation pump is broken or is not working due to a power black-out. So the fluid’s temperature inside the collectors and other circuit com­ponents may increase until the formation of steam which is then released by the security valve.

Figure 60: Security valve.

The valve must not operate during the system’s normal processing and therefore it has to be set at a higher pressure than the maximum pressure of the circuit. For example, it is chosen as a pressure of 600 kPa (6 bar) if the circuit’s pressure is set at 550 kPa (5.5 bar) [5, 17].

‘Jolly’ valve To avoid air storage inside the pipe and thereby the reduction of fluid delivery and thermal exchange, there must be vent-holes in the upper parts of the circuit.

Figure 61: ‘Jolly’ valves.

These vent-holes (jolly valves) can work automatically or manually. In the open circuit systems, the jolly valve should be left open since air continually enters [5, 33].

Flow regulating valves Especially for medium — and large-sized systems, these valves are inserted in every collector’s row to balance the flows inside the different branches of the circuit. In this way, uniform performance from different parts of the system is guaranteed [5].

Intercepting valves The function of these valves is to interrupt the flow and insulate certain circuit elements (such as valves or pumps), when maintenance is needed or when there are security problems. They are installed at the upper and lower part of each system’s element [5].

Emptying taps Manual emptying taps are installed at different circuit points. Generally, there is one in every collector’s row. To allow the gradual emptying of the fluid contained inside the circuit, it is necessary to ‘fix’ an emptying tap between two intercepting valves [5].

Three-port valves Three-port valves allow combining two flows (mixing valves) or separating a flux into two parts (diverting valves) [5].

The pump, the non-return valve, the expansion vessel and the security valve are offered in the market as a ‘pumps and security’ pre-set group. The expansion ves­sel and the security valve have to be installed in any case to avoid interruption between them and the collector [6].

temperature sensors

Jolly valve

solar

^.collector

insulation

during the summer and use it in the winter (seasonal storage), but as we have already stated in point 3 of par. 2.2.2.2, the technological solutions based on this principle are not convenient for limited domestic use because of its cost and logis­tics. Currently, the most common storage systems are the ones which allow the storage of the heat efficiently for a day or two. Storage tanks can be classified depending on their final utilization and the kind of insulation used [5, 9].

Table 4: Classification of storage tanks. Type Pressurized tank Non-pressurized tank Drinkable water storage tank Stainless steel Enamelled steel Plastic-covered steel Buffer Storage tank Steel Plastic Combined storage tank Steel/Stainless steel Steel/enamelled steel

2.2.4.1 Storage tank materials Pressurized tanks are made of stainless steel, enamelled steel or plastic-covered steel. Stainless steel tanks are lighter and last longer, but they are much more expensive than enamelled steel tanks. However, stainless steel is easily corroded by water with a high chlorine content. To reduce the corrosion risks, this kind of tank is generally provided with a magnesium anode which has to be replaced periodically. Non-porous plastic-covered steel tanks are also available in the market and they cost less than the other tanks. However, this storage tank cannot withstand temperatures higher than 80°C. Plastic-covered tanks are characterized, in fact, by a lower reliability level compared with other constructive typologies [9].

2.2.4.2 Sanitary water storage tank Figure 64 shows the storage tank installed in standard solar systems. In this storage device, there are generally two heat exchangers: the solar exchanger, which allows the thermal exchange between the thermal vector fluid inside the solar system and the fluid inside the tank, and the additional exchanger, which allows heat transfer from the integrative heating system (e. g. a central-heating boiler) to the fluid stored inside the tank.

Moreover, in the lower part of the storage tank there is a connection to the water pipes for the supply of cold water. The operating pressure inside the pressurized storage tanks is about 4-6 bar.

As regards the choice of the storage tank’s volume, we generally consider 40-100 l/m2 of flat collector surface. Concerning the proportioning of the solar system, while for the big-sized systems we refer to values that are near the lower limit of the above interval, it is vice versa for small systems. Large-sized storage

Figure 64: Storage tank installed in standard solar systems.

tanks can contain larger quantities of energy; however, this choice also involves larger heat losses and frequent starting of the integrative heating system. This hap­pens because the heating of larger quantities of water requires more energy. As regards sanitary water storage tanks, it is important to take into consideration the calcareous encrustment problem, which may form at high temperatures in the exchanger: for this reason, in the solar systems used inside houses the tank temperature should not be above 60-70°C [9].

2.2.4.3 Storage tank shape If the storage tank works properly it should have different water layers inside. The creation of these layers is possible thanks to the variation in fluid density at different temperatures. Actually, hot water which is ‘lighter’ is stored in the upper part of the tank while the ‘heavier’ cold water is stored at the bottom of the tank. This layering effect is an essential requisite for the good processing of the solar system. As soon as the hot water is requested by users, for example, for showering, the cold water flows into the tank from the pipes and mixes with the previously heated water. To limit this undesired effect and to maintain the temperature layering for as long as possible, the storage tank (generally shaped like a cylinder) should be tall and narrow.

These conditions can be realized using vertical storage tanks whose height — diameter ratio is at least 2.5:1 (in monobloc natural circulation systems the storage

Figure 65: Example of a solar storage tank.

tank is generally horizontal because of aesthetic and encumbrance reasons). Low temperatures in the lower part of the tank guarantee a high performance of the solar system even in case of insufficient radiation and low temperature of the thermal vector fluid.

Before installing a vertical tank, it is important to make sure that its height is compatible with the place where the storage system will be placed [5, 9].

2.2.4.4 The cold water inlet device in the storage tank This particular inlet device, when suitably installed in connection with the adduction pipe, weakens the strong motion of cold water flowing into the tank and limits the risk of its mixing with the warmer water layers.

2.2.4.5 Hot water collection In traditional storage tanks, hot water is collected from the upper part of the tank; because of the layers’ phenomenon and the outlet pipe being located in the upper part of the tank, we are always sure to collect the hottest water. After the collection, a part of this hot water stagnates inside the pipes getting cold. It is possible that this cold water can flow back into the upper part of the tank where it mixes with the hot water which is stored there. This causes heat dispersions of the relevant entity (see Fig. 64). To avoid this drawback, it is possible to direct the lot tube downwards making it pass inside the storage tank or outside across the insulated layer which covers the tank [9].

2.2.4.6 Heat exchangers and respective connections The solar circuit’s heat exchanger should be installed in the lower part of the storage tank to ensure that the thermal exchange occurs inside the water volume present at the bottom of the tank. The heat exchanger for the integrative heating system is generally placed in the upper part of the tank, to guarantee quick heating of the water volume at a temperature (corresponding to the daily requirements) without resulting in a tem­perature increase in the lower part of the tank, where the solar circuit’s exchanger is installed. This disposition of the exchanger ensures that the thermal exchange in the lower part of the tank, where the water is the coldest, occurs with the highest efficiency even when the solar circuit fluid does not reach the highest temperature [5, 9, 33].

2.2.4.7 Storage tank insulation The purpose of the storage tank insulation is to reduce the heat dispersions to the outside environment to its minimum. To have a storage tank insulation which efficiently limits heat losses, the following charac­teristics are needed:

• It should be 10 cm thick on the sides and 15 cm thick in the connections with the upper surface.

• It should also cover the bottom of the tank.

• It should be perfectly adherent to tank’s sides to avoid losses by convection.

• It should be made of materials which do not contain CFC and PVC and have low thermal conductivity (<0.035 W/m K).

Thermal dispersions in an insulated storage tank must be lower than 2 W/K. To limit these losses, it is very important to make sure that the thermal covering in connection with flanges and pipe fittings is hermetically sealed.

The tank linings that are currently available in the market can be flexible (expanded polyurethane foam, fibreglass, etc.), inflexible (they could be used out­side for retrofit interventions) or by direct injection with a plastic or metal cover­ing [5, 9, 33].