The polymeric material of the present absorbers opens the possibility to choose the colour as a property of the bulk material. In the present studies, the absorbers were coloured by a commercial non-selective alkyd-based paint, Bengalack Emaljelack 90 by Jotun A/S, which revealed suitable adhesion and no visible changes during the relatively short test period. The absorptance for the unpainted black absorber and for the colours green, blue, red and white were measured with an spectrophotometer model 746 from Optronic Laboratory, USA owned by the Solar Energy Research Centre at Hogskalan Dalarna in Borlange, Sweden. The absorptance spectra are shown in Fig. 8 for the wavelength range of 350 nm — 2500 nm. The weighted solar absorptance (Duffie and Beckmann, 1991) of the various colours is listed in Table 2. The uncertainty of the absorptance measurements is approximately 1%. Except for the colour red, all colours were applied on the absorbers and the thermal performance was investigated.

The polymeric absorber is part of a drain-back system. A description and comparison of the most conventional solar heating systems in Europe is given in (Weiss, 2003), and for the polymeric material in (Meir, 2003).

For the fapade integration of drain-back collectors, the location of the heat storage tank determines normally to which level over ground level the fapade can be covered by solar collectors. A complete draining of the solar system is — in the most simple system design — secured when the lower end of the collector fapade is above the storage level. This limitation is normally given in one — storey buildings without basement where the technical room is commonly placed.

In the present test set-up, the performance of the collector fapade with and without ventilated cavity was studied separately. The hydraulic scheme is shown in Fig. 9, details are given in Table 3. Further, following properties of the solar thermal system are specific: During solar system standstill, the absorbers are filled with air and the heat carrier (water) is drained to the storage. For the flow of the heat carrier, several collector modules are connected in parallel. The absorber and the collector cover consist of polymeric materials (modified NORYL blend, polycarbonate) and are sheets with an internal twin-/multi-wall structure. Due to the properties of polymeric materials, the system design avoids high operation and stagnation temperatures.

Table 3. Dimensions of the studied test system

01

After the discussion of angle-dependent efficiencies of the light guide and optical fibers and by considering the light intercept factor at the focus, the final transmitted solar power Pout was calculated using equation (2):

Pout =Ъл, (Фі )vg (Фі )Vf {Фі )Щ (Фі) (2)

7=0

where: rjc is the concentrator reflectivity (=0.85), гц (фі) is the angle-dependent intercept factor of the angle transformer light guide, pf (фі) is the transmission efficiency for optical fibers, I is the solar insolation, typically, 800W/m2, Д(ф^ is the ring area of the parabolic mirror between (фі) and (фі+і), and щ (фі) the transmission efficiency of the angle transformer light guide.

The theoretical analyses were confirmed by the following experimental results.

The obtained results were affected both by the irradiance fluctuations in time and mirror misalignment. The maximum measured power on the output end of the optical fiber bundle was 184W, with an efficiency of 50% and a 2,63W/mm2. The experimental results were obtained in a clear day of February of 2003. Stronger solar power can be expected in the summer time in Lisbon.

The new term ‘medium temperature collectors’ is used to address collectors with operating temperatures in the range of 80°C to 250°C. The aim is to develop collectors that are suitable for applications in this temperature range in which up to now only very limited experience exists. What are the activities that are investigated in the Task 33/4? To give a short overview, three categories may be introduced:

• improved flat plate collectors

• low concentration collectors (flat plate or vacuum tube collectors)

• small parabolic trough collectors.

Improved flat plate collectors

Especially for applications in the temperature range of 80 to 120°C there exists a number of possibilities to improve flat plate collectors such that they become suitable for those applications. In order to achieve this, it is necessary to reduce the collector heat losses without sacrificing too much of the optical performance at the same time. To mention some possibilities: hermetically sealed collectors with inert gas fillings, double glazed flat plate collectors, low concentration collectors using reflectors, or even vacuum flat plate collectors.

Figure 1 shows estimated efficiency curves of single, double and triple glazed flat plate collectors when newly developed anti-reflection glazings (‘AR-glass’) are used.

First of all, it is important to point out that there is a big potential for improvement of flat plate collectors if the normal solar glass is replaced by AR-glass. This improvement is independent of the operating temperature (comparison standard flat — plate collector to 1AR). Secondly, it is interesting to note with respect to double glazings that the 2AR collector reaches the same no-value as the standard flat-plate collector. Therefore, the complete efficiency curve of the 2AR-collector is above the standard flat-plate collector. Especially for higher operating temperatures the advantage of 2AR collectors is promisingly large. At AT/G=0.1 (K m2)/W the efficiency of the 2AR collector (in Fig. 1) is better than the standard flat-plate collector by more than 33% (relative)! These results from estimated efficiency curves have been verified by experiments and measured efficiency curves, by Fraunhofer ISE /1/. This temperature range of 80° to 150° is important for new fields of applications such as solar cooling and air conditioning, sea water desalination and other process heat applications like cleaning, washing, dying etc. Figure 2 shows the collector field of a solar driven desalination system. Fraunhofer ISE presently develops a desalination technology based on membrane distillation which is especially suitable the operating conditions for solar thermal driven desalination application /2/. Pilot plants will be
installed in Jordan, Egypt, Morocco and Spain within the frame of projects funded by the EC. It is also considered to use 2AR collectors in some of these plants.

MacSolar (Solarc Innovative Solarprodukte GmbH), Spectron 100 (Tritec Energie GmbH), CM3 (Kipp&Zonen), Strahlungsfuhler GBS01 (Technische Alternative, Elektronische Steuerungsgerategesellschaft m. b.H), IBC-Sol-2000 (IBC Solar AG), CS10 (Resol Elektronische Regelungen GmbH), pSf2 (Prozeda GmbH), SP Lite (Kipp&Zonen),

SOZ-03 (NES Measuring Systems), Einstrahlungssensor (Hanazeder electronic GmbH)

The cost of the oven is 375 €.

Investigations have been made in order to compare the cost of the oven to the cost of wood the butcher can save.

We considered the case of one butcher who roasts two muttons a day in a traditional oven (see Fig. 8)

The amount of wood he should burn is 5. 48 € per day. In 69 days he will spend the equivalent of a solar oven cost.

More generally the following table compares daily expenses of wood consumption with gas, coal and oil for cooking.

5.

The prototype of solar oven described in this article has been presented to some butchers of the city. They have been very interested by the device. However they find that the cost of the oven is too high and also its capacity is small (only two muttons at the same time for a cooking duration of 3 to 4 hours).

But as mentioned in the introduction, Niger is facing severe desertification, so, solar energy instead of firewood utilization for cooking is worthwhile, in despite of the costs of solar equipment.

As matter of fact, the government has adopted recently a National Plan of Action and Strategy for Renewable Energies. Theses energies (mainly solar) would play during the next decades an important role in National Energy balance.

Further study in that matter, will concern cost reduction, improvement of the performances and the capacity of the solar oven.

SHAPE * MERGEFORMAT

A linear equation (Eq. 3) that has a second order dependency on the suction velocity and the wind speed was used for modeling the efficiency of the experiment.

П = A(U) + B(U )VS + C(U )V2 (A = J a,, B = І b, and C = £ c,) (3)

i=0 i=0 /=0

where in Eq.3 q is the efficiency of perforated solar collector, Vs is the suction velocity and U is the wind speed. The parameters a; , b and c were determined by a least square method to minimize deviation of calculated efficiencies from experimental ones. The obtained parameters are listed in Table.4. These parameters are only valid within the experimental region of suction velocity and wind speed. The maximum and average absolute deviation of calculated efficiency was obtained by using the parameters a;, b and c and Eq.3.

D. Manolakos, G. Makris G. Papadakis and S. Kyritsis

Agricultural University of Athens
Dept. of Agricultural Engineering, Farm Structures Laboratory
75, Iera odos street 11855 Athens, Greece
Tel: +30-210-5294033 Fax:+30-210-5294023 e-mail: dman@aua. gr

K. Bouzianas

Hellas Energy K. Bouzianas P. Moschovitis & Co
10, Saint George Square 11257 Athens Greece
Tel: +30-210-8222519 Fax: +30-210-8238314 e-mail: kostasb@compulink. gr

The research regards the development, application testing and performance evaluation of a low temperature solar organic Rankine cycle system for Reverse Osmosis (RO) desalination. Below is given a technical description of the system under development:

Thermal energy produced by the solar array evaporates the working fluid (HFC — 134a) in the evaporator surface. The super-heated vapour is driven to the expanders where the generated mechanical work produced by the Rankine cycle drives the RO unit pumps (high pressure pump, cooling water pump, feed water pump) and circulating pump. The saturated vapour at the expanders’ outlet is directed to the condenser and condensates. On the condenser surface, seawater is pre-heated and directed to the seawater reservoir. Seawater pre-heating is applied to increase the fresh water recovery ratio. The seawater tank is insulated. The use of seawater on the condenser surface decreases the temperature of “Low Temperature Reservoir” of Rankine cycle thus a better cycle efficiency is achieved. The saturated liquid at the condenser outlet is pressurised in a special pressurisation arrangement consists of two vessels and three valves, substituting a pump. The sub-cooled liquid at the pressurisation arrangement outlet is driven to the economiser. The economiser acts as working fluid pre-heater. In the economiser outlet saturated liquid is formed, which is directed to evaporator inlet and the cycle is repeated.

For the prototype system 240 m2 of vacuum tube solar collectors will be deployed. The evaporator and condenser capacity is estimated to be about 100 kW. For these systems’ characteristics and considering a water recovery ratio of seawater RO desalination unit of 30%, the average yearly fresh water production is estimated at 1450 m3 (or 4 m3 daily).

Specific innovations of the system are:

Low temperature thermal sources can be exploited efficiently for fresh water production; solar energy is used indirectly and does not heat the seawater; the RO unit is driven by mechanical work produced from the process; the system condenser acts as sea water pre-heater and this serves a double purpose; (1) increase of feed water temperature implies higher fresh water production (2) decrease of temperature of “low temperature reservoir” of Rankine cycle implies higher cycle efficiencies.

The wall construction of this retrofit building consists from the inside to the outside of a gypsum plaster, first brick layer, air gap, second brick layer, the old plaster, EPS (extruded polystyrene) boards for insulation, and the fagade collectors. The collector has been mounted with a backside of oriented strand boards (OSB), the insulation of the collector is mineral wool. The absorbers of the three collector modules have been coated with different shades of blue colour which were developed in the project. Also in this case, the collector has a non-ventilated backside and is thus directly integrated in the fagade of the building.

Just like for "System Korneuburg”, calculations with WUFI have been made to investigate the risk of condensation which could harm the building materials. The results have shown that no harmful condensation will occur. Due to the air gap between the two brick layers which acts as a relaxation layer, this had not been expected.

Effective U-values have been calculated for the above mentioned winter days with high and low solar irradiation respectively. On a day with high solar irradiation the effective U-value of the wall construction with integrated fagade collector is approx. 86% lower than the static U — value for the wall construction. Even a day with low solar irradiation, an improvement of the

static U-value of up to 25% is possible. The following table gives an overview of the calculated static and effective U-values of the wall construction with and without integrated fagade collectors.

WandMax results show that even in summer no harmful temperatures occur behind the collector backside, even if the collector is in stagnation. The maximum temperature at the EPS boards was calculated with 46°C. In this case, the temperatures at the backside of the collector are more critical since the maximum allowable temperature for EPS is approx. 80°C.

The ambient temperature and relative humidity, temperature and relative humidity between the glass cover and the absorber and behind the collector backside have been monitored and analysed to investigate the real hygro-thermic processes within the wall.

The analysis of the data has shown a good correlation with the simulations done in advance. The relative humidity at the most critical point for condensation — the back side of the collector — has not exceeded values of 80% in the documented period from December 2003 to February 2004. Most of the time, the relative humidity at the backside of the collectors is about 50% to 60%. Short increases of the value up to 80% have occurred but the humidity dropped to usual values shortly after. The reason for the increase can partly be found in an increase of the relative humidity at the outside of the collector and the intermediate space between glass cover and absorber.

The mounting of the collectors onto the wall has been examined for thermal bridges just like for system Korneuburg. The results have shown that the mounting construction can be optimized by inserting insulation material around the mounting screws, which would otherwise be located in direct contact to ambient temperatures.

Summarizing, the data show the suitability also of this wall construction for the integration of non-ventilated fagade collectors.

Conclusions

Fagade collectors have opened a new field of application for solar thermal systems. The technical feasibility of the technology has been proved in many realised systems. Architects have begun to discover the possibilities and advantages of fagade collectors and see the need of coloured absorbers for a broad dissemination of the systems. The development of selective colours for absorbers is an essential step to construct visually attractive systems with thermal performances comparable to state-of-the-art absorber coatings. Questions of building physics have been investigated and results show that condensation will not occur with direct integrated fagade collectors.

Fields of application can be seen in newly constructed houses as well as in retrofits, but also for multi-purpose and public buildings, where direct solar irradiation onto the south fagade is not appreciated.

1.4 Governing equations.- The

fluid flow and heat transfer is as­sumed to be governed by the two dimensional Navier-Stokes equations, together with the en­ergy equation, using the following restrictions: steady state, lam­inar flow, fluid Newtonian be­haviour, Boussinesq approxima­tions, radiatively non — partic­ipating medium and negligible both heat friction and influence of pressure on temperature. This set of differential equations are represented in Eqs. 2 — 5, where ( ) are the Cartesian — coor­

dinates; T is the temperature;

T0 the reference temperature; pd the dynamic pressure; (u, v) and ( ) are and у components

of velocity and the gravitational acceleration.

The solid parts (glass sheets) are governed by the energy equation (Eq. 5) without consid­ering convective terms.

^ + £ = 0(2) dy

■т0)дх (3)

-T0)gv (4)

Q-2JS

The governing equations can be adimensionalized using the dimensional quantities Lref = , , , and, where is the thermal

diffusivity. The following adimensional variables are obtained: , ua = ufuref,

and. The flow structure is fully described by Rayleigh and

Prandtl numbers (Ra, Pr); by the shape of the geometry (A, A’, lh/L, lc/L and S/L); and by the thermal conductivity ratio ДА.

results demonstrate that velocities Table 2: Parameters of the mesh for study III: asym — and temperatures have a periodic metrical configurations when slats are close to hot (lh/L behaviour in direction, i. e.: = 0.2 lc/L = 0.4, and ) and cold (lh/L = 0.4, lc/L

= 0.2, and ) isothermal walls.

(6)

The dynamic pressure is characterized by:

PdJ, У) = Ра{х, y + H’)+ К„ (7)

where Жр is a constant value. The Kp value is calculated from analysis of the effects of buoyancy (Kelkar [2]). In this work, it is assumed that Kp is represented by:

(8)

where T0 is the Boussinesq temperature and the bulk temperature, which were expressed by:

In a project supported by Greenpeace International, 3 types of solar desalination plants using multi-layer heat recovery were investigated:

• 4 m2 flat plate collector with a 1 m2 evaporation surface

• 2.4 m2 parabolic reflector with a 0.66 m2 evaporation surface

• 2 m2 evacuated tube collectors with a 1 m2 evaporation surface

Figure 2 shows a diagram of the desalination module with the collector. The lowest level of the unit serves as the first level of the desalination tower and as a water tank to catch the desalinated water. The heat produced by the surface collector is introduced here. The condensate runs directly through the collectors acting as a heat transfer medium. As the collectors are situated under the condensation unit, the circulation through the solar collector is driven by normal thermal convection. The two collectors with 2 m2 of surface area each have their own circulation system. An additional reflector at the top and bottom of the collectors enlarges the surface of the aperture. Figure 3 shows the results of measurements taken using this system on a summer’s day. The temperature of the unit and the upper level is shown, as well as the production of desalinated water and the irradiation over a period of 24 hours. The ratio of condensed water to salt water was set to around 1:1.8 . The unit produced a total of 44 kg of desalinated water over the 24 hours. The total amount of solar energy collected in one day stands at around 7.23 kWh/ m2d. This means that the effectiveness of the solar energy in relation to the energy actually used (Q= Mdest*h) can be calculated to be at 98%. A simple greenhouse-type distillation unit was set up as a reference, and produced in the same time frame a total of 4.0 litres per square meter aperture. When the two units are compared, it can be seen that it is possible to obtain a production which is 2.75 times more efficient with the same energy input than in the simple system. Alternative system designs, which use evacuated tube collectors and parabolic reflectors are depicted in Figures 4 and 5.

The parabolic reflector model is always placed in an east-west alignment. The Aperture width is 1,5 m. The reflector has to be tracked over the day to keep the focal line always on the receiver. This must be done with as little effort as possible. For that reason the focal point is allowed to move over a width of 20 cm over the absorber surface during the course

of the day. This means that, depending on the geographical latitude, only one daily adjustment is required to keep the focal line in position. The absorber also holds the salt water and so serves as the first stage of the desalination tower. Heat is delivered directly and without delay to the system. In windy conditions, however, large heat losses occur as the absorber is only protected by a shallow cavity. Additionally, during night-time intense heat losses occur at the bottom surface and cool down the system fast. In particularly windy areas, the idea of protecting the absorber by a glass cover may be considered. But due to reflection on the glass, energy losses have to be taken into account. When the sun’s rays are at a low angle as many units as possible must be connected together to minimise the loss of concentrated radiation at the edges of the parabolic reflector (see Figure 4). Measurement results have shown that on the one hand the parabolic reflector system heats up very fast due to the little heat capacity and on the other hand loses heat as fast again in the night time. Overall, with a reflector aperture of 2,4 m2 the same daily output can be achieved as with a flat plate collector with 4 m2. Since a concentrating system uses only direct insolation, it depends on the climatic region if a concentrating system or a flat plate system is to be preferred.

Another solution is the utilisation of evacuated tube collectors. For a prototype 20 Sydney — type tubes were connected directly to the evaporation stage. The distillate flows driven by natural convection, directly through the tubes, carrying the heat to the evaporator. Evacuated tube collectors are currently available at low cost from Chinese manufacturers and offer an interesting economical alternative. These collectors are very insensitive to wind and work at a high efficiency even at temperatures of 100°C. However, it must be noted that with large spacing between the tubes the effective area is significantly reduced and only a limited performance is achieved. Regarding the complete collector area the optical efficiency amounts to only 40% for a system without CPC.