The instantaneous efficiency of a solar collector is universally expressed by the following equation:

Qu

(Ac G )

According to the most utilized technical standards for solar flat-plate collectors, e. g. ASHRAE 96-1980 (RA 1989) standard [1], the forgoing instantaneous efficiency can be expressed (as an approximation to the actual values) as a quadratic function of the variable Z = (Ti-Ta)/ G as follows:

П = T|max — B Z — C Z (2)

Here r|max is the maximum efficiency, which corresponds to the ideal non-losses case, and B and C are coefficients, which depend on how big, are the conduction, convection and radiation thermal losses. These values are obtained by an experimental procedure described in the standard. For a good commercial flat-plate collector qo is about 0,70 ; A is about 3,5 and B is near 12,0. These values can be entered to the computer code developed to simulate the performance of the system. Ti is the fluid temperature in the entrance of the collector and Ta is the ambient temperature.

The total (global) irradiance G is a quasi-chaotic function of time due to the presence of clouds, dust and other non-deterministic factors which affect its value in a given instant. However, for a very clear day, G can be approximate by a deterministic function, taken into account the latitude, solar hour, day of the year, etcetera. In order to simplify the modeling the following very simple approximation [3] is employed:

G (t) = Gm ( sin 12 [ n (t — ta) / N ]) (3)

Here N is the length of a given day, ta is the dawning time and Gm is the maximum irradiance at this day. Obviously, N, ta and Gm varies day after day, but in the equinoxes N is 12 hours, ta is about 6 o’clock a. m. (solar time) and Gm is almost 1000W/m2.

The ambient temperature reach its minimum value just before dawning, it increases as the Sun rises and has its maximum value about two hours after noon, approximately. The temperature begins to descend till the dawn of the morning after. At the afternoon twilight the ambient temperature is generally higher than in the dawn due to the infrared radiation of the earth surface during the night, which cause a cooling effect. The ambient temperature is registered daily in meteorological stations, so its profile is easy to obtain for most important cities. This profile can be modeled by a curve adjusted with cubic splines from meteorological data. Here a few data is needed to create a profile for a given day. Other approximation is by means of a polynomial function as follows:

Ta (t) = c0 + c1 t + c212 + c313 +… cn tn (4)

Now it is possible to model not only the instantaneous efficiency of a flat-plate solar collector, but the useful heat in a given day of the year and the mean thermal efficiency

that day, among many other interesting parameters. This way, the instantaneous thermal efficiency, the useful energy gathered (per m2) in a day and the daily mean efficiency can be evaluate by the following formulae:

Qu = f П (t) G (t )k (t) dt

ta

where the integration limits correspond to the dawning instant ta and the Sun set hour ta+N, both in solar time. Of course, these limits vary from place to place and day by day in a deterministic and well-known way [2]. The angle modifier function is there:

where 0, is the incidence angle of beam solar rays with the normal to the cover of the collectors, which is a trigonometric function of time t that takes into account the reflection losses at this cover [3]. The coefficient of the parenthesis term is experimentally obtained and has a value of about 0,19 for a typical flat collector.

The forgoing formulae can be easily inserted in a computer code to determine the mean efficiencies and the gathered energy in one hour intervals of time, for inlet temperatures of 30, 60, 90, 120 and 140° C, for example.

It was observed that in the first and in the final hours of solar energy collection, the thermal efficiencies is too small to contribute to the energy gathering, so it is proposed to sacrifice these ineffective periods by setting mirrors which shadow the collector absorbers when the efficiency is too low, but increase the area of collection when the thermal efficiency of the collector is greater. The mirrors form a multi-compound solar concentrator, which is described below.

Investigations confirmed that a circulation system had the largest influence on the loss of heat through distribution. This leads, when permanently in use, (which in practice is often the case) to increased use of energy, and can in decentralized systems of one and two family houses mean a doubling of energy use and around a halving of the original planned solar fraction. Through the use of technical devices such as clock timers, thermostats, current control switches and keypads, energy savings can be achieved; however, a heating concept without any circulatory system and an appropriately short distribution system offers the best solution.

■ Due to the longer circulatory system used in centralized systems and the fixed regulations set down by the DVGW regarding Legionnaire’s Disease, it is not recommended to omit the appropriate protection measures. In any case, the additional energy requirements are not a critical factor due to the steady use of the heating system over day and night, and losses can be limited further by the use of the above technical devices. These findings apply also to conventional energy systems, and are not dependent on a solar heating unit.

■ Further optimization opportunities were provided by the system set up. In many systems, the upper storage temperature limit was set too low at 55 — 65 °C, which causes a reduction in the possible solar energy yield. With the decentralized systems in Gelsenkirchen, the energy yields could be improved on average by 25% simply by setting the temperature limit to 85°C, and with the centralized systems by at least 2.6%. The conditions for this are that the circulation always passes through the thermal hot water system mixer and not only through the latter part of the freshwater storage tank. Only under these conditions can both energy-saving advantages be enjoyed, and sufficient protection against scalding be at all times guaranteed.

Comparing the decentralized and centralized systems

■ The decentralized system showed a 4 to 10 increase in percentage points of the solar fraction in comparison to the centralized system, when in optimal use. In any case the decentralized system demonstrated higher deviations due to the individual differences in the use of the system. Decentralized systems are much more sensitive to these changes in consumption than centralized units, as the latter benefit from many individual users at differing times of day, and a more straightened demand profile. This leads to the conclusion that in practice centralized systems will frequently offer energetic advantages in comparison with non-optimized systems installed in single
family dwellings. This is also supported by the higher performance from the usually better serviced units of a centralized system by a company with the appropriate technical experience. Finally, changes to the installation of the hot water system by non-specialists are usually thus avoided, which often occur in private decentralized units.

■ The economic comparison of the systems has shown that central heating units for several terraced houses with a solar hot water system have proved to offer clear cost advantages in comparison with single family units.

The design factor is one of the most important criteria for the solar funnel. Generally Line Concentrated Solar Cooker does not require any tracking mechanism. In order to prevent tracking mechanism in solar funnel, the design is in such a manner that the maximum sunrays are impinging on the reflecting surface of the Line Concentrated Solar Cooker all the time. This line concentrated solar cooker has been designed in such a way that minimum of at least 35 percent of the area of the cooker is exposed to the sunlight by 8:00 AM and maximum by 100 percentage by noon. The design based on this criterion gave us a good result leading to heating effect from morning to evening. As the time goes on increasing the area covered on
the Line Concentrated Solar Cooker by the sunlight increases up to afternoon. As the sun sets the area covered on the cooker, by the sunlight starts to decrease thus decreasing the effect of the cooker by evening. It has been calculated that for a family of four members, at least half meter square of area (minimum of 35% area of the cooker) has to be exposed to the sunlight all the time. That is starting from 8:00 AM to 4:00 PM. Hence a Solar Cooker was designed with the total area of 1.66 square meter out of which 0.585 square meter area will be exposed to the sunlight by 8:00 AM and gradually increasing by noon. The top diameter, bottom diameter, height of the Solar Cooker was found to be 138 cm, 43 cm, and 33 cm respectively. In order to find the area exposed to the sunlight at different times, the following data like latitude, longitude, declination, hour angle, altitude angle and incident angle were calculated for each hour for concern latitude and longitude. For the geological location where the experiment is to be conducted, the latitude and longitude were found to be 10.8 0 north and 78.28 0 east respectively. The declination angle was found as 5 = 11.403 0. The corresponding Altitude angle, Hour angle, Incident angle for different time period were calculated and tabulated as follows:

From the above tabulation, it can be found that the altitude angle goes on increasing from morning 8:00 AM to noon and then decreases. Similarly, the incident angle of the solar rays on the solar cooker decreases up to noon and then increases. This ultimately indicates that concentration of the sunlight is increasing until noon and then decreasing. The following table shows the percentage of area exposed to the sun at various time starting from morning 8:00AM to evening 5:00PM.

This area of exposure to the sun was determined theoretically. But in order to prove that the values which got theoretically as calculated above is correct, the area exposure to the sun was determined practically by keeping the Line Concentrated Solar Cooker exposed to the sun. On conducting this experiment, the values correlated with the actual theoretical values approximately. The values got on conducting practical experiment is as follows:

When the above two graph were merged together, all the points from both the graph were fitting to the curve approximately.

The solar cooker with the above dimensions looks as follows:

43 cm

◄——————— ►

The CLON concentrator follows, in principle, from the CPC concentrator. From the construction point-of-view (fig. 1) it has similarly placed parts like absorber, reflector and tansparent cover, but the reflector is intended to be from flat mirrors instead of parabolic curvatures. Like the CPC, CLON has two compund mirrors as well, when each of the mirror has its „focus" at the opposite corner, the intersection of the opposite mirror with absorber.

It is clear that system consist of flat-plate mirrors cannot have a focus. But assuming that the solar radiation should be reflected to area between points F1 and F2 (see fig. 2), functionality similar to CPC will be satisfied. Character of irradiation distribution on the plane of absorber (i. e. between Fi and F2) will not be interesting in that case because it will certainly differ from CPC. On the other hand we can expect that using flat mirrors, the distribution of irradiance will be more uniform than that formed by CPC and this may be useful for some applications (i. e. for use with solar cells where bad uniformity may lead to flow of compensation currents). For the CLON concentrator we will require:

1. concentration of all the radiation within angular range (-©A; ©A)

2. concentration with only one reflection on mirrors within the range (-©A; ©A).

In general, CPC concentrator redirects sun rays to absorber after one or more reflections. Especially rays comming under angles close to 0° are reflected to lower parts of concentrator where again a reflection acts. This way the optical efficiency is decreasing as it is, first of all, given by reflectivity of mirrors.

Properties of CLON can be compared to CPC type of concentrator. Similar properties are following:

• geometry: concentrator consist of a couple of compound mirrors, receiver is placed between lower rim of mirrors and upper transparent cover between top rim points.

• Solar radiation in the angular range (-©A; ©A) is concentrated

• Concentrator can be truncated

• The same construction of receiver can be used

• The same tracking requirements

Differences in comparison to CPC can be listed as follows:

• Concentrator’s forming curve is much simplier — linear, what leads to use of simple and inexpensive mirrors

• Irradiation of receiver is more uniform

• Lower optical losses due to single-reflection principle

• Lower concentration factor at the same acceptance angle (but for the same acceptance angle CPC has bigger size, therefore must be truncated and its concentration factor C decreases)

A — transparent cover, B — concentrator (reflector), C — receiver
1 — transparent cover, 2 — mirrors, 3 — receiver, 4 — selective layer, 5 — tubes with heat transfer media, 6 —

thermal insulation

Louise Jivan Shah & Simon Furbo
Department of Civil Engineering, Technical University of Denmark

Building 118
DK-2800 Kgs. Lyngby
Denmark

E-mail: lis@bvg. dtu. dk

Introduction

A new collector design based on parallel-connected double glass evacuated tubes has previously been investigated theoretically and experimentally (Shah, L. J.

& Furbo, S. (2004)). The tubes were annuluses with closed ends and the outside of the inner glass wall was treated with a selective coating. The collector fluid was floating inside the inner tube where also another closed tube was inserted so less collector fluid was needed.

The collector design made utilization of solar radiations from all directions possible. Fig. 1 shows the design of the evacuated tubes and the principle of the tube connection.

The investigations resulted in a validated collector model that could calculate the yearly thermal performance of the collector based on hourly weather data. The advantages of the model were that shadows, the solar radiation and the incidence angle modifier for Fig. 1: Design of the each tube were precisely determined for all solar evacuated tubes (top)and positions, including solar positions on the “back” of the the tubes connected to a collector. However, the model could be improved solar panel (bottom). further as the model was only valid for vertically tilted

pipes and as the model was not developed for a commonly used simulation program.

In the present paper, the theory is further developed so it can simulate solar collector panels of any tilt and based on the theory a new TRNSYS (Klein, S. A. et al. (1996). ) collector type is developed. This model is validated with the measurements from outdoor experiments.

TRNSYS simulations of the yearly thermal performance of a solar heating plant based on the evacuated solar collectors are carried out and among other things it is investigated how the distance between tubes and the collector tilt influences the yearly thermal performance. The calculations are carried out for two locations: Copenhagen, Denmark, lat. 56°N, and Uummannaq. Greenland, lat. 71°N.

Further, the results are compared to the calculated thermal performance of the solar heating plant based on traditional flat plate collectors.

The advantage of the chosen solar collector (figure 8) for small systems is its resistance against corrosive liquids. Its absorber consists of selectively coated glass tubes and silicone header tubes. Therefore the absorber is resistant to hot sea water and the collector can directly be implemented into the feed circuit of the MD-module. No additional collector loop and heat exchanger is necessary. An aluminium zigzag-reflector is mounted behind the glass tubes to utilise the whole aperture area for the collection of solar radiation. This kind of collector was developed in our institute for the SODESA-project and investigated during a one year field test at a desalination plant in Gran Canaria from summer 1999 to summer 2000 (Hermann 2002). The tests with respect to performance and long term resistance were successful, identified week points were improved for future collectors. The graph in figure 9 shows the collector efficiency curve. The range of operation for MD is between AT/I of 0.06 and 0.09 (K m2)/W. The collector efficiency in this range is between 53% and 40%.

Small scale test system

A compact experimental desalination system as sketched in figure 10 consisting of the MD module, a corrosion free solar collector, a pump and a temperature hysteresis controller was installed on the outdoor test site of our institute. Sensors for temperatures, volume flow and solar insolation were integrated for the monitoring of the operational parameters.

A PV-power supply was not integrated but all electrical parts were supplied by the grid.

Since the energy for the distillation process is almost independent from the salt concentration, the system was operated with drinking water to avoid trouble with corrosion at auxiliary components.

The results from the experimental investigations (figure 11) showed that the handling of the system is quite easy and long term operation periods without maintenance are possible. The performance of the system is shown as an example for one day in June 2002 in the diagram of figure 7. The system starts operation at 10:15 h when the solar insolation was in the range of 700 W/m2. The feed flow is manually adjusted at about 225 l/h. The maximum evaporator inlet temperature rises up to 90°C. At the same time the maximum of distillate production reached 15 l / h. The total amount of distillate gained on that day was about 81 litres. The maximum of distillate gain during the test period of summer 2002 was about 130 l/d under the meteorological conditions at Freiburg in central Europe.

System simulations

For system simulation calculations an empirical simulation model of the MD-module was developed which is based on its measured performance data. The model was implemented into the simulation program for thermal systems ColSim (Wittwer 1999). The system design consists of one MD-module with 7m2 membrane area connected to a 6 m2 SODESA-collector, a pump and a distillate storage.

One-day and annual simulation calculations for three different locations, (Eilat in Israel, Muscat in Oman and Palma de Majorca in Spain) were carried out using weather data sets of these locations. It can be seen from the graph in figure 12 that for example in Eilat a maximum distillate output of 28 l per day and m2 collector area (equal to total amount of 161 l/d) can be gained on a day with good weather conditions during summer. The minimum production rate is in the range of 11 l/d and square meter collector area (equal to total amount of 63 l/d) in December. Two different control strategies were investigated (Wieghaus 2002).

Since the most common small scale solar desalination systems in the third world countries are solar stills a briefly comparison between the simulated performance of a MD-system and the performance of a simple solar still (V. Janisch, 1995) is drawn in figure 13. The used insolation data for the performance calculations were averaged from the weather data sets of Eilat. Since the MD-system is modular and each module has a maximum distillate capacity in the range of 150l/d, the number of modules rise step by step (the graph represents these steps since the system performance rises non-linear when a new module is attached). The comparison shows that the simulated MD-system has a 4.5 times higher distillate output.

The development of small stand-alone operating desalination systems is an important task to provide people in rural remote areas with clean portable water. The fact that the lack of drinkable water often corresponds with a high solar insolation speaks for the use of solar energy as the driving force for a water treatment system. Membrane distillation is a process with several advantages regarding the integration into a solar thermally driven desalination system. Simulation calculations for such systems with module characteristics derived from several experimental investigations were carried out for different potential installation locations. The simulation results show that a very simple compact system with a collector area less than 6m2 and without heat storage can distill 120 to 160 l of water during a day in summer in a southern country. Experimental investigations on a testing system are currently carried out at Fraunhofer ISE. New MD-modules will be developed aiming at a higher GOR value and a lower pressure drop.

M. Meirf, J. Rekstadf, E. Svasand*

fUniversity of Oslo, Department of Physics, P. O. Box 1048, Blindern, N-0316 Oslo, Norway Tel.: +47 228-56469, Fax: +47 228-56422, E-Mail: mmeir@fys. uio. no *Agricultural University of Norway, Department of Mathematical Sciences and Technology, P. O. Box 5003, 1432 As, Norway

Abstract — The present paper studies the performance of fagade integrated solar collectors with absorbers of polymeric materials. Absorbers with different colours have been integrated in a timber-frame wall with and without ventilated cavity. The temperature and the relative humidity have been measured in several layers of the wall construction behind the integrated collectors. The objective was to determine whether non-ventilated integration is possible in a wooden construction under Nordic climate. It was found that omitting the ventilated cavity did not represent a risk for high relative humidity and condensation inside the wall construction. The integrated collector fagade improved the U-value by more than 15% without that additional thermal insulation was added to the wall. Simulations have shown that fagade integration can be a way to overcome the mismatch between the availability of solar energy and the heating demand.

1. Introduction

Traditionally, solar collectors have been mounted on roofs. In the last decade, a considerable market growth was observed for combined solar systems for domestic hot water preparation and space heating (solar combisystems) in the middle and north of Europe. Requiring larger collector areas, building-integrated collector installations become a natural choice for solar combisystems. At high latitudes, as in Norway, the integration into the fagade represents an obvious alternative due to the low declination of the sun during the heating season from the middle of September to the end of April. Further, by introducing coloured absorbers, the fagade integration opens new opportunities for building planners and architects. Coloured fagade collectors can be seen as multi­functional building modules, providing energy, new possibilities of fagade design and surface protection for the building. Currently, considerable attention is given to fagade integration of solar collectors in several European countries, where metal absorbers, building physics and coloured coatings with spectral selectivity are subject of investigation (Tripanagnostopoulos, 2000; Bergmann and WeiR, 2002; Bergman and Muller, 2003; WeiR, 2003). The present work studies non-selective, polymeric solar collectors produced by the Norwegian company Solarnor. The study was carried out within the REBUS project (Competitive Solar Heating Systems for Residential Buildings, http://energi. fysikk. uio. no/rebus), a collaboration between Danish, Latvian, Swedish and Norwegian research institutes and companies, partially financed by Nordic Energy Research.

Examples of realized and planned collector fagades in Sweden and Norway (Source: Solarnor).

The temperature (T) and relative humidity (RH) in the layers of the collector fapade were measured since June 2003. The sensors were placed behind the collector field with — and without ventilated cavity at two different vertical positions (15 cm/ 95 cm and 140 cm measured from the lower end of the collector field) and in two different layers of the wall (see Fig. 2b and c). The temperature and the RH-signals were recorded by two temperature and voltage loggers, NI 4351 by National Instruments, and LabVIEW software. The instrument has ±0.12 K RTD-accuracy. The temperatures were measured by Pt-100 sensors and the relative humidity by absorption based humidity sensors from Honeywell, series HIH3610, with an accuracy of ±2%.

The layers of the standard timber-frame wall and the collector wall without ventilated cavity are compared in Table 1 for the present test house. The static U-value is 0.31 W/(m2K) for the regular timber-frame wall and 0.26 W/(m2K) for the collector wall without ventilated cavity. This represents an improvement of 16% without adding any additional thermal insulation.

The collector fapade will during cold periods have a higher temperature than a wall with standard exterior cladding (passive solar heating during solar system standstill). The temperature gradient will therefore be smaller than for a regular wall and provide a further reduction of the building’s thermal losses.

Aiming at reducing the transmission loss at large angles, a novel solid-core light guide with an angular transformer input end was put forward. The angular transformer has an input diameter of D1 = 7mm and an output diameter of D2 = 10,5mm. For the input rim angle of 37o (NA1 = 0.6), the output angle of 23o (NA2 = 0.5) was calculated by using the optical invariant: NA1 D1 = NA2 D2. Due to the refractions at the input surface and the total internal reflections at inclined and polished sidewall, the input rays of large angles are transformed into the output rays of small angles. The drastic reduction in angles means that there are much less imperfect reflections along the sidewall, hence the lower transmission loss (see Fig. 5). The length of the angular transformer determines whether the relations between the input and output angles can be determined precisely by optical invariant. A long transformer gives a better approximation to the invariant. For practical reasons, the length of 12cm was chosen. Careful grinding and polishing were done to ensure high transmission efficiency.

The background on the development of solar thermal energy and the role of solar heat for industrial processes was shortly summarised in the Work Plan of the Task 33/4: It is expressed that around 60 million square meters of solar thermal collectors were installed in total by the year 2000 in the OECD countries. Until now the widespread use of solar thermal plants has focused almost exclusively on swimming pools, domestic hot water applications and space heating in the residential sector. Between 1994 and 2000 average growth rates of 18% were attained each year for these applications in Europe.

But the use of solar energy in industrial process applications is currently insignificant compared to the applications mentioned before. Most solar applications for industrial processes have been on a relatively small scale and are mostly experimental in nature. Only a few large systems are in use world-wide.

On the other hand, if one compares the energy consumption of the industrial, transportation, household and service sectors, then one can see that the industrial sector has the biggest energy consumption in the OECD countries at approximately 30%, followed closely by the transportation and household sectors.

Caused by the fact that energy is available at low cost and without limitations, industry did not care too much about energy efficiency and substitution of (fossil) fuels. The main activities in this field started in 1973 and 1979/80 following the two oil (price) crises. Later on, oil prices — and related to that the prices for natural gas and electricity — fell again. Today — even in the critical political situation in the Middle East — energy prices are low.

On the other hand, it is obvious that fossil resources are finite and alternatives have to be found for all applications, including the use in industrial applications.

The major share of the energy which is needed in commercial and industrial companies for production processes and for heating production halls, is below 250°C. The low temperature level (< 80°C) complies with the temperature level which can easily be reached with solar thermal collectors already on the market. The principles of operation of components and systems apply directly to industrial process heat applications. The unique features of these applications lie

• on the scale on which they are used,

• on the special system configurations and controls needed to meet industrial requirements,

• and on the integration of the solar energy supply system with the auxiliary energy source with respect to the industrial process itself.

For applications where temperatures up to 250°C are needed the experiences are rather limited and therefore also suitable collectors, components and systems are missing. Therefore, for these applications the development of high performance solar collectors and system components is needed.

To estimate the quality of long-term measurements of irradiance sensors, values were recorded over a period of 8 days with changing weather and sky conditions. The calculated sums of irradiation were compared to those of the CM21. During the period or recording, the CM21 measured an irradiance sum of an average 3,39 kWh per m2 and day, a quantity that is well representing the daily average of irradiation during a whole year in central Europe.

The observed relative deviation of all sensors did not exceed 10 % and for 6 sensors it was even less than 4 %. Table 4 shows the results of the analysis. The lower sums of irradiation of the sensors 9 and 10 are due to a shorter period of recording.

Off-Set

During the night-time the measured values of the devices were not absolutely zero. When this factor is ignored in a counter and the values are straight away added up, depending on the value of this off-set, the long-term measurements of irradiation might be more or less falsified. To quantify the influence of the off-set on the annual irradiance sum, the recorded values when G(device) < 0 W/m2 during the 8-days period were accumulated and extrapolated to one full year.

The results of the analysis for all sensors can be seen in Table 5. Surprisingly, the reference-pyranometer had the second highest overnight off-set of all tested devices with an annual sum of -9,8 kWh/a. For one sensor the calculation resulted in a sum of -24 kWh/a (a quantity in the range of 2 % of the annually irradiation in central Europe), while 7 sensors showed practically no off-set with an extrapolated sum of less than -1 kWh/a. To on the other hand rule out a significant positive off-set, the values of all sensors when G(CM21) < 0 W/tF were added and extrapolated to a whole year. The difference to the projected sums of the negative off-set were less than 1 KWh/a for all sensors.