Low-temperature thermal solar systems, which have been described until now and in which the energy transfer from the storage place to its utilization is realized by fluids moved by pumps and ventilators, are also called active systems. By the
expression ‘passive heating systems’ we generally mean all applications where the thermal hygrometric well-being conditions are obtained only by solar energy which is used without employing any conventional heating systems requiring elec­tricity or fuel. In merely passive systems, even the heat distribution and removal are realized by natural the phenomenon of conduction, convection or radiation, rather than using forced systems. Passive heating systems require the installation of wide glazed surfaces for solar energy interception and also structures with high thermal capacity storage function.

The efficiency of these systems is limited to the width of the glazed area which has to be correctly oriented, to the efficiency of the thermal storage realized by the walls and inner floors and eventually to the stored heat distribution towards the building parts characterized by scarce solar radiation. Currently, the realization of passive solar heating systems capable of guaranteeing the comfort conditions required in every inner room of a building seems to be an impossible goal both in cold climate zones and in mild climate zones such as Italy. However, its contribu­tion to the reduction of the yearly heating requirement could be relevant. Passive solar heating systems can be classified on the basis of the mechanism of energy transfer towards the heated room as follows [1, 3, 4]:

• Direct gain systems

• Indirect gain systems

• Isolated gain systems

The solar constant Ics is the average energy radiated by the Sun per time unit on a unitary surface situated outside the Earth’s atmosphere and perpendicular to the Sun’s rays. It measures 1367 W/m2 . Considering the atmospheric phenomenon of absorption and diffusion and the Sun’s inclination above the horizon, on the Earth’s soil the solar constant reaches a maximum of 1000 W/m2 (radiation on land, at midday, during a clear sky day) [1, 4].

The total power radiated by the Sun can be calculated as follows:

P = 4kRJIcs = 4n(150 • 109)21367 = 3.8 • 1026 W (1)

where Rm is the average distance between the Earth and the Sun [2].

The Earth intercepts only 1.73 ■ 1017 W of that power. Owing to nuclear reac­tions, a mass of 4.27 ■ 109 kg can be destroyed in a second; thus, nearly 0.0067% of the solar mass will be lost in a billion years [1].

Once we know the power emitted by the Sun, it is easy to calculate the heat produced internally per unit of solar volume:

q = P/(4/ 3)nR3 = 3.8 • 1026 /(4/ 3)n(7.25 • 108 )3 = 0.24W/ m3 (2)

where R (= 7.25 ■ 108 m) is the solar ray.

The quantity calculated above is a particularly low value considering that, for example, the human body’s heat production per unit volume is roughly 1400 W/m3 [2].

This system uses reflecting panels which have a parabolic shape and track the Sun by rotating around two orthogonal axes. These panels also concentrate solar radiation towards a receiver which is installed at the focal point. High tempera­ture heat (>650°C) is normally transferred to a fluid (helium or sodium vapour) and is then used in a motor, which is located above the receiver (see Fig. 95), where mechanical or electrical energy is directly produced. For economic reasons, concentrator dimensions do not exceed a diameter of 15 m, limiting its power to about 25-30 kWe. With a row of these collectors, it is possible to realize sys­tems of any size and power. An interesting application of parabolic dish collec­tors is the one which regards electrical energy production for small communities which are decentralized and distant. These systems have a conversion efficiency which is more than 30% (the highest efficiency among the currently existent solar technologies) [45, 50,51]. This technology has now reached the industrial phase, mostly due to the research which has been developed in Europe, in the USA and in Australia.

Among the described technologies, this system is the one which has the highest electrical energy production cost (in 2004 costs were about 1 €/kW h); neverthe­less, it is interesting for the prospects it offers concerning the drop in this cost [50]. The cost for the construction of a solar thermal electrical system which uses para­bolic dish collectors in 2004 was about 7100-3700 €/kWe with a forecast for the medium term of about 2000-1200 €/kWe.

In the parabolic dish collectors, the thermal vector fluid can reach temperatures which can be even higher than 1000°C, and at such high temperatures it is also possible to produce hydrogen by the dissociation of water. In prospect, this is the most important reason for the interest shown in this technology: in Europe, since 2002 the hydrogen economy has become one of the mainstays of the EU sustain­able energy policy, acknowledging the uniqueness of hydrogen both as a clean fuel and as a high efficiency energy vector [45, 53, 56].

The hourly values of global radiation received on a horizontal surface Hh can be divided into diffuse components Dh and direct components Bh by Liu and Jordan’s method using the following expressions:

Dh/Hh = 1 — 0.09k if k < 0.22

Dh/Hh = 0.9511 — 0.1604k + 4.388k2

— 16.638k3 + 12.336k4 if 0.22 < k < 0.8 (46)

Dh/Hh= 0.165 ifk > 0.8

where k is the hourly clearness index defined by the ratio between the hourly global energy Hh received on a horizontal plane and the hourly energy received on a horizontal plane Hh, ex situated outside the atmosphere.

k=Hh/Hh>ex (47)

Hh, ex can be calculated using the equation:

Hh, ex = /J1 + 0.033cos(2rc«/365)]

• (cos L cos d cos h + senL send) (48)

The hour angle h is calculated at the centre of the considered hour or using the exact equation:

Hhex = 12/n/cs[1 + 0.033cos(2n«/365)]

• {cos L cos d(sen h1 — sen h2) + (h1 — h2)sen L sen d} (49)

h1 and h2 are the hour angles at the extremities of the said hour; in eqns (48) and (49) Hh, ex is expressed in W-h/m2.

Once we have obtained the value of the hourly global radiation Hh received on a horizontal plane, we can calculate the hourly diffuse radiation Dh. The hourly direct radiation on a horizontal plane is calculated by difference:

Bh=Hh-Dh (50)

The hourly global radiation received on an inclined surface turns out to be:

Eh=Rb Bh+Rd Dh+Rr(Bh+Dh) (51)

In this case, Rb is calculated at the centre of the said hour [1].

The direct gain system is the most common and simplest solution for a passive solar heating system: solar radiation enters the room through a glazed surface and directly warms it (Fig. 69). So, the living space works as a solar collector, but it must have the means and structures that are capable of absorbing and storing the intercepted thermal energy to keep the internal air temperature constant as much as possible. In this way, the daily overheating and the excessive decrease

in night temperature can be reduced. A direct gain system needs a wide glazed surface oriented southward to allow transfer of the winter solar radiation through direct communication with the living space. The southward orientation generally allows intercepting the greatest quantity of solar energy during winter, whereas in summer since the sun is very high, the transmitted radiation is less and it can be minimized by a suitably proportioned horizontal object (overhang). The choice of window components is very important in planning the solar heating system. Windows with a high heat transmission coefficient are preferred to maximize the quantity of intercepted radiation when the radiation is very poor and also to restrict heat losses.

A wide glazed surface oriented southward can cause overheating inside the house during the day and excessive inner air temperature fluctuations when there is no direct solar radiation (during the night or when the sky is cloudy). To solve these problems, it is important to use a thermal mass that is connected with the walls and floors whose surfaces and thermal capacities are well proportioned and also well positioned to intercept solar radiation and store thermal energy. During the day, the heat produced by the intercepted radiation is not completely released into the room; it is partially stored and released later after a delay of a few hours to stabilize the air temperature of the house. The storage thermal mass is generally made of masonry. Masonry materials for thermal storage are characterized by a high thermal capacity: cement blocks, concrete, bricks, stone, etc. To restrict dis­persion of stored energy inside the thermal mass, the brickwork walls are insulated on the outside, while floors are realized with a perimeter or an extrados insulation [1, 3,4].

Since the Earth’s orbit around the Sun is elliptical, the distance between them varies during the year, causing a ±3.3% fluctuation of the extraterrestrial radiation (Fig. 3). This radiation can be roughly calculated for every day of the year using the following equation:

where n(t) is the progressive number of the day of the year [1].

Currently, the most concrete application that CSP plants find use in is the production of electricity.

Also, in the medium to brief term, it is predicted that this will be their main application. Similar to all the other forms of renewable energy that have been introduced recently, to affirm themselves, the CSP plants must face the hard competition as regards the energy generation costs.

The presence of a growing market for ‘green energy’, which calculates a form of economic incentive, allows to overcome, in favourable cases and taking into consideration the growing cost of fossil fuel energy, the competition gap compared to traditional products.

To acquire market quotations it is necessary to reduce the production costs and to improve the market value of the energy produced. As for the cost reduction, two main modalities can be adopted: the reduction in the specific costs of investment and the increment of production efficiency. The improve­ment in the market value can be achieved by making the electrical energy pro­duction less dependent on the solar source variability. The introduction of a storage system or the use of an integrated solar-combustible fossil system is indispensable.

It must be stressed that reduction in the investment costs, improvement in the efficiency and independence from the solar source variability are contrast­ing aspects. Achieving a winning compromise in the course of time is very necessary, but it will necessarily leave space for specific innovations (as it hap­pens in the automobile technology or, in a more persistent way, in the Aeolian technology) [45].

As regards Italy, there are medium-height solar radiation regimes with a big varia­tion between northern and southern regions. Keeping in mind that the parameters required to determine univocally the position of an intercepting surface are the surface’s inclination and its azimuth orientation, we now list the principal sources for the retrieval of radiation data.

‘La radiazione globale al suolo in Italia’ [10], a paper edited by ENEA, is with­out any doubt the bibliographic source which supplies, on a national level, the most detailed information about the average global radiation (i. e. the one which includes direct, diffuse and reflected components) received on a square metre of a horizontal surface per month and year. The same kind of information, which refers to a smaller number of Italian places, is also provided by UNI ISO 10349 international norms.

‘L’atlante europeo della Radiazione Solare’ [11] is without any doubt one of the most authoritative sources for the valuation of solar radiation received in a certain period of time on a surface which is exposed in any manner. This atlas gathers all the data supplied by national metrological offices. These data, gathered in maps and tables, are the result of a 10-year study. The atlas is divided into two volumes: the first takes into consideration the horizontal surfaces and the second the inclined sur­faces. The first volume reports, for every Italian place we consider, the values of the daily average radiation expressed in W-h/m2 or in kW-h/m2. Every place is charac­terized by its latitude, longitude and height above sea level.

As regards the design of solar panels, the results of the second volume appear to be much more interesting than the first. As a matter of fact, solar panels are usually arranged with a certain inclination on a horizontal plane, and the second volume reports the values of the daily average radiation (global and diffuse) per month and year for different positions of the intercepting surface. However, in the European Atlas of Solar Radiation, only the principal cities of each country are mapped.

To obtain the values of radiation received on variously oriented and inclined sur­faces, there are a few algorithms (which can be easily found in the currently avail­able design software) among which the most well known and used is that of Liu and Jordan which is discussed in par. 11, 12 and 13. The steps to get a correct extrapo­lation of the data for a surface which is positioned in any manner, starting from the values for a horizontal surface, are outlined in the UNI 8477 norms (first part) [4, 5].

As regards indirect gain systems, solar radiation does not directly enter the room that has to be heated up, but it falls on a thermal mass which is placed between the Sun and the living space. The solar energy absorbed by that mass is converted into thermal energy and then distributed inside the room in different ways. By the position of the thermal mass, we can distinguish two kinds of indirect systems: solar walls where the thermal mass is contained inside a wall and roof-ponds where the thermal mass is put on the roof of the room which is to be heated up. Indirect gain systems need a wide glazed surface oriented southward and the ther­mal mass used for storing the absorbed energy is placed behind it at a distance of at least 10 cm.

The storage is normally made of brickwork or water (with the latter put inside metallic, plastic or concrete waterproof containers) [1, 3, 4].

2.3.2.1 Brickwork solar walls Solar radiation received by a solar wall which is painted with a dark colour is absorbed causing a superficial heating up (Fig. 70). This heat, which is like a temperature damped wave, is transferred by con­duction to the wall’s inner surface and from there it spreads all over the room by radiation and convection. The delay and the temperature wave damping depend on the storage material and thickness. Dispersion of stored heat towards the outside is resticted by the insulation created by the air space between the glazed surface and the solar wall.

Trombe’s wall (Fig. 71) is different from the solar wall owing to the pres­ence of air holes in both the lower and the upper parts of the wall. In this way, the activation of a mechanism for natural circulation of air through the heated area is favoured. The warm air volume between the glazed surface and the thermal mass can reach high temperatures (about 65°C). The air holes in the upper part of the storage mass allow warm air to move up and enter the room, while the colder air which is inside the room is recalled inside the collector through the holes in the lower part of the storage mass. The openings should be located by means of dampers to prevent the reverse movement during the night [1, 3, 4].

2.3.2.2 Water wall The processing by a water wall (Fig. 72) is based on the same principle which regulates the processing by a solar wall, excepting that heat transmission through the wall also depends on thermal convection and not only on conduction. Because of the high thermal capacity of water and the inner convec­tive currents, which make it an almost isothermal heat accumulation, the system can work with a higher efficiency compared with brickwork solar walls. One of the most important problems is where to confine the liquid. Until now, bottles, tubes, watertight tanks, barrels, drums and cement walls filled with water have been used as containers (see Fig. 72) [1, 3, 4].

2.3.2.3 Roof-pond As regards roof-pond passive systems, the thermal mass is placed horizontally on the building’s roof (Fig. 73). The storage medium is water which is enclosed in small bags similar to little mattresses. They com­pletely or partially cover the roof which works as the ceiling of the rooms that are to be heated up. Water containers have to be placed in direct contact with the ceiling which sustains them to make the heat exchange between the inner room and the storage easier. During the hot season, the storage is exposed to solar radiation during the day; the intercepted energy is then transferred by conduction through the roof structure and directly exchanged by radiation from the ceiling of the room to be heated. During the night or during cloudy days, a mobile insulation mechanism covers the hot water and restricts its heat dispersion. In contrast to the systems with solar walls, systems with water walls are not always provided with a transparent cover to put on water. The use of translucent containers or glazed surfaces put on the water mirror is an efficient solution to reduce sensitive and latent (evaporation) heat losses when climate is very cold.

In places where there are high thermal ranges between day and night and where humidity is very low, the water storage on the roof can also be also for summer refresh­ing by insulating it during the day and exposing it during the night. The utilization of the water wall also presents numerous problems: besides the extra structural costs, the system does not guarantee sufficient advantages at high latitudes because of the reduced solar radiation intercepted by the horizontal plane; moreover, the stored heat can be spread by radiation only over the floor below the roof [1, 3, 4].

To determine the position of the Sun in the sky at a certain moment of the year

and in a certain place, it is necessary to define a few characteristic angles. These

angles are [1]:

• the solar height or altitude a — the angle formed by the direction of the solar rays and their projection on a horizontal plane;

• the zenithal angle — the angle formed by the solar rays and the zenith direction; this angle and a are complementary;

• the solar azimuth a, which indicates the variance of the solar rays’ projection on the horizon’s plane as regards the south; by convention, eastward orientations are negative while westward orientations are positive;

• the hour angle h, which indicates the angular distance between the Sun and its midday projection along its apparent trajectory on the celestial vault; the time angle is also equal to the angle that the Earth has to rotate to bring the Sun back above the local meridian;

• the latitude L — the angle formed by the straight line that connects the place taken into consideration and the Earth’s core and its projection on the equator’s

plane; this angle is positive in the northern hemisphere but negative in the south­ern hemisphere;

• the solar declination d — the angle formed between the solar ray and the equa­tor’s plane measured on the solar midday plane, that is, the meridian plane pass­ing by the Sun; the solar declination is positive when the Sun is above the equa­torial plane and negative when it is under the equatorial plane (Fig. 4).

The solar height a and the solar azimuth a define the instant position of the Sun [1]:

sen a = sen L sen d + cos L cos d cos h (5)

sen a = cos d sen h/cos a (6)

Solar declination d is calculated using Cooper’s equation :

d = 23.45 sen[360(284 + n)/365] (7)

where n stands for the nth day of the year. Declination depends only on the date; therefore, it is the same for all places on the planet.

The hour angle for dawn ha or sunset ht can be calculated using eqn (5), avoiding sen a, as [1]: