a) Temperature Gradient in the oven

First of all, characterization of the empty oven has been done with one and two reflectors: the temperatures profile in the oven, in the direction of the height during the day, is observed and plotted (see curves fig. 2).

140

P 120

100

ш

80

60 (D

40 20 0

—♦—536 W/m2 -■—854W/m2 —A—866W/m2

Temperature versus height curves indicate that the points are almost at the same temperature.

b) Temperature rise in the oven

Two experiments have been made using one reflector and two reflectors to see the influence on the temperature rise.

The two reflectors experiment data shows that the temperature rise up to 156°C in the oven depending on the solar radiation and with one reflector, the temperature rises up to 130°C (see curves Fig.3 & 4).

Fig.4.

Fig.3

From these tests, it results that the reflectors have great influence in the cooking capacity of the oven.

Eun-Chul Kang, Jin-Soo Kim, and Euy-Joon Lee, Korea Institute of Energy Research(KIER), 71-2 Jang-Dong Yusong-Gu, Daejeon 305-343, South Korea

M. Masaood Hashmi and I. A. Qazi, Pakistan Council of Renewable Energy Technologies (PCRET), Plot No. 25, Sector H-9, Islamabad, Pakistan

Solar Air Heating is a simple and inexpensive technology that results in reduced energy consumption and lower operating costs associated with fresh air ventilation requirements (Seidermann, 1997) in a building. This device, while drawing the fresh air from outside the building, preheats it by letting it pass through tiny holes in a dark — colored thermally conductive surface that has been heated absorbing by the thermal component of the sun’s radiation. Obviously, such devices are usually mounted on the side of a building that receives the most sunlight (e. g. the south wall, in the Northern hemisphere). Compared to other types of solar heat collectors, the unglazed perforated cladding is a cost effective, virtually maintenance free, room-heating solution. The well-known and commercially available solar air heating systems called the solar wall, has been the subject of several studies by Hollick (1994,1998), Kokko and McClenahan (1994), Gunnewiek (1996), and van Decker (2001). As indicated above, the basic physical system of the solar air heating collector is one where suction is applied to a heated perforated plate that is placed as a facade on the south facing external wall of a room. The outside air is drawn straight from ambient through the entire surface of the perforated blackened plate. The intimate heat transfer, between the plate and the sucked air, keeps the plate temperature low, minimizing the radiant loss (Gerald W. E and Van Decker). Such a system is usually subject to the natural buffeting and turbulence of the wind. The cooling effect of the wind would obviously depend, among other things, on the ambient temperature and the wind velocity. In order to quantify this effect and to develop a mathematical model for it we studied the efficiency of a system under the laboratory conditions.

An integrated collector/storage solar water heater was designed based on the principles of optics and heat transfer. The system has two horizontal cylindrical tanks made of galvanized — iron (0.8 mm thick), painted matt black on the exterior surface, with a capacity of 61.8 litres each. One of the tanks is located in the upper part while the other tank is located in the lower part of the system. Half of the upper tank was insulated, and a clear glass cover was fitted directly above the lower tank to allow incoming solar radiation reach the tanks. The transparent cover was inclined at 16° to the horizontal to optimize solar radiation collection at the test site, Malawi Polytechnic (15o 48′ S, 35o 02′ E), and the aperture size was 1.1 m2. Further, a stationary parabolic concentrating reflector with focal line along the axis of upper tank, was fitted below the tanks. Hard board and waste cotton were used as insulation materials. The waste cotton was sandwiched between a galvanized-iron sheet case (on the

outside) and hard board inside, with an aluminium foil forming the inner most layer of the bottom and vertical faces of the system. A schematic view of the system cross-section is presented in Fig. 1. The whole system weighed about 65.1 kg, with empty tanks, and the design details are presented in Table 1.

Fig. 1: Schematic presentation of an integrated collector storage solar water heater showing its cross-section (Not drawn to scale).

2. Experimentation

2.1 System mounting

The integrated collector/storage solar water heater was mounted on a horizontal concrete roof top (about 6 m above the ground), and it faced north at the Malawi Polytechnic (15° 48′ S, 35° 02′ E) in Blantyre, Malawi. The tanks were externally connected with insulated 12.7mm — diameter hose pipes : a) parallel to each other (P-connection), b) in series with one insulated

hose pipe from the top part of the lower tank to the bottom part of the upper tank (S1-tank interconnection), and c) in series with two insulated hose pipes of which one pipe linked the bottom part of the lower tank to the bottom part of the upper tank while the other pipe linked the top part of the lower tank to the top part of the upper tank (S2-interconnection). Outlets from the tanks were bent down to form U-shaped tubes before rising up into the expansion tank (E-tank) to avert back-flow of cold water from the tubes into the collector-storage tanks, during the periods of low insolation or at night. In addition, the arm of the U-tube adjacent to the collector-storage tank was insulated up to the lowest part of the U-section. Details of the experimental set up are shown in Fig. 2.

in the tanks. The ambient temperature was monitored by using a minimum and maximum mercury-in-glass thermometer placed in a room with louvered glass. The louvers were kept open to allow free circulation of air. Wind velocity was measured by using a Casella low — speed air meter (N 1462) while the intensity of global solar radiation was measured by a Kipp & Zonen pyranometer (CM 6B) mounted in the plane of the inclination of the transparent cover, and connected to a Kipp & Zonen solar integrator (CC 14). The water heating process was monitored from 06:00 to 17:00 hrs each day, and hot water was stored from 17:00 to 06:hrs the next day. These experiments were conducted between September and December 2003.

3.3 Data analysis

The overall mean temperatures of water at the beginning (Toi) and end (Toe) of the solar collection processes were used to calculate the total amount of heat (Qw) absorbed by water when the amount solar energy (QR) falls on the aperture of the system surface (Aa), within a period of time t = ti to t = te. In this study, the mean daily collection efficiency (pc) was calculated as follows (Tripanagnostopoulos et al., 2002):

TOC o "1-5" h z Pc = Qw/Qr (1a)

Qw = MCpw (T0e — T0i), (1b)

Qr = Aa (He — Hi) (1c)

where Cpw = specific heat capacity of water at constant pressure,

Hi = total radiation received per unit area from sunrise to t= ti and He = total radiation received per unit area from sunrise to t = te,

M = mass of water, Toe = 0.25 (TLbe + TLte +Tube + Tute), and

T0i = °.25 (TLbi + TLti +Tubi+ Tuti).

The efficiency of heat retention (pr) is given by (Smyth et al., 2003):

hr = (Tof — Tan)/( Toe-Tan) (2)

This is an overall system efficiency of heat retention.

In Korneuburg, Austria a newly constructed two-family passive house was chosen as one of the pilot systems. The house will be referred to as "System Korneuburg” in the paper. The pilot system for Germany is located in Lahntal near Marburg and is a retrofitting project. This system is called "System Lahntal” in the paper. Both systems have been realised with direct integrated fagade collectors, which means that there is no ventilated air gap between collector backside and the adjacent wall.

01

Calculations for both systems have been made regarding the moisture and heat transfer through the wall constructions to find out whether the wall constructions are suitable for an integration of non-ventilated fagade collectors or not. Also the question of a connection of the collectors onto the wall with as few thermal bridges as possible has been investigated and answered. To evaluate the calculations, the relative humidities and temperatures of the ambient and within the wall constructions have been monitored and analysed.

M. Quispe, J. Cadafalch, G. Van Der Graaf, and A. Oliva

Centre Tecndlogic de Transferencia de Calor (CTTC)

Lab. de Termotecnia i Energetica Universitat Politecnica de Catalunya (UPC) labtie@labtie. mmt. upc. es, www. cttc. upc. edu

A numerical and experimental investigation has been undertaken to study the natu­ral convection in large, rectangular and inclined air cavities with parallel slats inside. This phenomenology is present in the technology of transparent insulation for so­lar thermal applications. Computations of three different configurations have already been performed: (1) parallel slats in contact with the two isothermal walls, (2) parallel slats in contact only with the front isothermal, and (3) parallel slats separated from the isothermal walls. Taking advantage of the spatial periodic behaviour of the phenom­ena, configurations (2) and (3) have both been solved using reduced computational domains with periodic boundary conditions. To do so, a mathematical formulation of the spatial periodicity of the different variables (velocity, temperature and pres­sure) has been proposed. The validity of these mathematical formulation has been assessed by comparison of the numerical solutions on reduced domains to the nu­merical solutions obtained from the computation of the whole air cavity. Numerical results of the studies (1) and (2) have been compared to experimental and numeri­cal correlations from the literature. Numerical results of the studies (3) have been validated by comparison to experimental data obtained from an ad-hoc experimental set-up equipped by a Digital Particle Image Velocimetry (DPIV). Details will be given about how the quality of the numerical data has been assessed by means of a post­processing verification procedure applied to all the numerical solutions.

1. — Introduction

In recent years there has been a great deal of interest in understanding the natural con­vection of air in cavities with parallel slats inside for transparent insulation technology pur­poses, in particular for their use in solar thermal system applications. The air-filled enclosure is divided by these parallel slats into a large number of cells. Due to the reduced dimensions of each cell, in comparison to the single enclosure, the amount of viscous forces acting on the air in each cell is increased. Because of the high number of cells, the computer memory and the calculation time are clearly penalized when numerical simulations tools are used. With periodic boundary conditions solved in reduced domains (a few cells), these problems can be avoided.

J0rgen M. Schultz, Simon Furbo. Department of Civil Engineering, Technical University of Denmark, Brovej, Building 118, DK-2800 Kgs. Lyngby, Denmark. Email: js@byg. dtu. dk Fax: +45 45 88 32 82

Introduction

Solar heating systems for combined domestic hot water and space heating has a large potential especially in low energy houses where it is possible to take full advantage of low temperature heating systems. If a building integrated heating system is used — e. g. floor heating — the supply temperature (and the the return temperature) would only be a few degrees above room temperature due to the very low heating demand and the large heat transfer surface area.

One of the objectives in a newly started IEA Task 32 project is to investigate and develop improved thermal storages for combined solar systems through further improvement of water based storages and in parallel to investigate the potential of using storage designs with phase change materials, PCM.

The advantage of phase change materials is that large amounts of energy can be stored without temperature increase when the material is going from solid to liquid form (Fig. 1). Keeping the temperature as low as possible is an efficient way to reduce the heat loss from the storage. Furthermore, the PCM storage might be smaller than the equivalent water storage as more energy can be stored per volume. If the PCM further has the possibility of a stable super cooling, i. e. the material is able to cool down below its freezing point (Tfusion) and still be liquid, the possibility exist for a storage with a very low heat loss. When energy is needed from the storage the solidification is activated and the temperature rises almost instantly to the melting point.

The work within the IEA Task 32 project focuses on the phase change material Sodium Acetate with xanthan rubber. This material melts at 58 °C, which means that low temperature heating systems could make full use of such a storage system. Energy to a large extent can be withdrawn even when the storage is in its super cooled phase without activation of the phase change.

This paper presents an initial simulation model of a PCM storage for implementation in TRNSYS 15 [1] as well as the first test results achieved with the model.

Sodium acetate with xanthan rubber

For the moment only one material, Sodium Acetate with Xanthan rubber, is considered for the PCM storage. Sodium Acetate has a melting point of 58 °C and a heat of fusion capacity of 265 kJ/kg. Addition of xanthan rubber to the hydrate makes it very stable when super cooled [2].

Fig. 1 shows that the PCM storage compared to water has a slightly lower storage capacity in the solid phase below the melting point of 58 °C, but when the sodium acetate begins to melt the heat storage capacity increases dramatically due to the heat of fusion. It is also seen that the amount of energy stored at a temperature of 58 °C is about twice the amount of stored energy in traditional water storage even if this was heated to near 100 °C. This shows one of the advantages of a PCM storage: A very large amount of energy can be stored at a moderate temperature.

Figure 1 also shows the advantage of super cooling as the storage can be allowed to cool down to room temperature and still contain large amounts of latent energy (the dotted thick line in figure 1). If the storage has reached a temperature equal to the room temperature no further heat losses occur before the phase change is activated. When the super cooled PCM is activated the temperature increases almost instantly to 58 °C. However, some of
the heat of fusion is used to heating up the PCM to the melting point as indicated with the dashed arrow in figure 1

One of the critical questions is how to activate the phase change in the super cooled material. One method is to make contact between the super cooled material and a solid crystal of the same material. This method is however not feasible in case of thermal storages. Other methods are to apply a sudden force on the solution e. g. mechanically or acoustically [3].

The question on how to activate the super cooled phase change material has not been considered so far in the project and for the energetic potential evaluation it is anticipated that the PCM can be activated on demand.

Description of the PCM storage model

The solar system under consideration is outlined in Fig. 2. The system consists of a solar collector, a domestic hot water tank and the PCM storage. The use of two separate storages is due to the idea of extensive use of the super cooling effect of the PCM storage, which would be impossible if a combined storage for domestic hot water and space heating is used. The system is designed to give priority to the domestic hot water tank.

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The PCM storage design for the first investigation is made without any thoughts on economy or practical problems as the first objective is to evaluate the potential of using a PCM storage compared to traditional water storages. If full benefit of the super cooling effect with respect to reduced heat loss should be achieved a multi- sectioned storage design is needed. By sub-dividing the storage into many separate layers or sections it will be possible only to activate the phase change in the storage volume needed to match the energy demand, and this will be the only part of the storage that will be heated up to the PCM melting point. This has been the main idea behind the design outlined in figure 3.

A first draft of a TRNSYS type model has been developed. The model subdivides the simulation time step in smaller time steps in order to achieve a sufficient accuracy. The following takes place in each of the small time steps (Fig. 4).

Based on the input parameters the most favourable section of the storage is chosen for storage of solar energy or — in case of a space heating demand — heating of the solar fluid to cover the space heating demand. The strategy is always to minimise the storage mean temperature and to avoid activation of phase change in a super cooled section as long as possible.

In each time step the transfer of energy between the solar fluid and the PCM storage is calculated. Next the heat loss to the surroundings is calculated and the final temperature of each section is found. This first model does not take internal heat exchange between the different storage sections into account.

The inputs, parameters and outputs are shown in table 1.

Each section is simulated as a lumped model, i. e. the section is supposed to have a uniform temperature. Figure 5 shows the model of one section.

Of special interest is the STATUS parameter, which is the measure of the state of the PCM material. If the storage section is liquid STATUS equals 1 and if the storage section is solid STATUS equals 0. When a fully solid PCM layer reaches the melting point continuous supply of energy will make the PCM begin to melt and a mixture of solid and liquid PCM material will be present. In the simulation model this is registered in the STATUS parameter, which increases proportional to the fraction of melted PCM from a value of 0 to the value of 1, when an energy amount equal to the heat of fusion has been supplied to the storage section.

Fig. 5 Lumped model of one section of the PCM storage

Preliminary simulation results

The model has been implemented in TRNSYS 15. Data for the main components is shown in table 2. For the reason of simplicity and the fact that the model is purely theoretical no pipes have been modelled, i. e. no pipe heat loss. The flow in the solar collector loop is constant all year round. If the outlet temperature from the solar collector is lower than the supply temperature to the collector + 10 K the controlled valve bypasses the solar collector. If the fluid temperature is higher than the bottom temperature of the domestic hot water tank the fluid is lead through the DHW tank before it reaches the PCM storage. Hourly values of the space heating demand is read from a file generated with a detailed building simulation program tsbi3 [4]. The building is a single-family low energy house with an annual space heating demand of approximately 15 kWh/m2 (2000 kWh/year) corresponding to the Passive House concept.

Figure 6 shows the temperature evaluation and the status of the PCM storage for a theoretical example with almost no heat loss from the storage (U = 0.001 W/m2K). The example has been chosen for illustrative purpose only.

It is clearly seen that during the late winter and early spring the storage is charged and discharged several times without fully melting of the PCM material (the temperature does not exceed the melting point of 58 °C).

When the space heating demand becomes close to zero in the late spring the storage is charged and the status becomes equal to 1 meaning that the storage is fully liquid. In the autumn the storage is discharged and the status drops from 1 to zero with only a small plateau at 58 °C.

Fig. 7 shows the same system but with a U-value for PCM storage insulation of 0.66 W/m2K. The system is not supposed to be optimised. The annual results are shown in table 3

Conclusion

Phase Change Materials (PCM) for heat storage in combined solar systems offers the possibility of reducing the storage size compared to traditional water storages. Of special interest is the use in combination with low energy houses with low temperature heating systems. If the PCM further allows for stable super cooling the possibility exists for a storage with very low heat losses.

As part of the IEA Task 32 project "Advanced storage concepts for solar thermal systems in low energy buildings” a first draft of a TRNSYS type model of a PCM storage has been developed.

The first simulations with the model have proven the functionality of the model and reasonable overall results are obtained. On a detailed scale some irregularities are observed with respect to unexpected temperature drops (1 — 2 K) during periods with no space heating demand.

The future work will be concentrated on more detailed evaluations of the model and afterwards use of the model for detailed parametric studies in order to evaluate in detail the energetic potential of PCM storages compared to traditional water tanks.

References

[1] "TRNSYS 15, User Manual”. The University of Wisconsin. Madison, USA.

[2] "Report on heat storage in a solar heating system using salt hydrates”. S. Furbo & S. Svendsen. Report No. 70, Thermal Insulation Laboratory, Technical University of Denmark, July 1977.

[3] "Triggering crystallization in supercooled fluids. B. Sandnes. Department of Physics, University of Oslo, Norway. 2004.

[4] "tsbi3 User Manual”. Danish Building Research Institute, 1993.

[5] "Design Reference Year — A new Danish reference year”. J. M. Jensen & H. Lund. Report No. 281, Thermal Insulation Laboratory, Technical University of Denmark, 1995.

Figure 4 shows the various volumes that a relevant in a combistore. They are ordered by the increase of temperature from the bottom to the top of the tank. All heights in the tank are referred to as relative heights between 0 and 1.

Figure 4 Different volumes in a combistore for auxiliary heat for external (left) and internal (right) heater

In Figure 5 the dependency of fsax, ext on the relative heights of inlets/outlets and heat exchangers in the stores is shown.

Figure 5 Dependency of fsav, ext on specific parameter change of storage geometry

Most relevant seems to be the collector outlet position. As higher it becomes as less volume of the store can be heated. Additionally there may be heat load parts not delivered with heat any more. There were only two systems in the simulation were this parameter was altered. The standard variation shows, that the results differed widely. So no clear statement of this influence can be drawn.

Of less importance is the height of the boiler inlet.

For the heat sinks, the DHW heat exchanger inlet position, that often determines the not useable volume of the store is most significant followed by the position of the inlet tube for the SH system. The latter defines the preheating zone for the DHW, which represents the coldest part of the store and therefore the volume for the solar collector where its highest efficiency occurs.

The (upper) SH system outlet, that also represents the position if there is only one SH outlet, seems to be not so important, as most of the systems do not fully cover the SH demand during the heating season. Therefore the after heating volume for the DHW production is mostly not delivered by the solar collector in the heating season. In summer the SH system is not relevant.

The positions of the boiler inlet and outlet were varied for 4 systems. As the boiler outlet height defines the auxiliary volume and therefore the volume that can be preheated by the collector, this value is very significant. This volume should be kept as small as possible in order to have a high volume exclusively charged by the solar collector. The minimum value is determined by the maximum possible temperature in the auxiliary volume, the power of the heating device and its required minimum running time, and the maximum load. The higher the maximum temperature and the higher the power of the auxiliary heater the smaller can this volume be. If, on the other hand, a longer running time (i. e. wood log boiler) is required, the volume has to be big enough assuring this running time. A higher load needs also a higher auxiliary heated volume.

For the system shown in Figure 6 these effects can be clearly seen for an internal heat exchanger. fsav is increasing with decreasing auxiliary volume until the user demand can not be satisfied any more (fsi is decreasing) due to too high load or too low heating capacity of the auxiliary heater. This happens at a volume below 0.09 m3.

Helena Gajbert, Energy and Building Design, Lund University, Sweden Hakan Hakansson, Energy and Building Design, Lund University, Sweden Bjorn Karlsson, Energy and Building Design, Lund University, Sweden

By indoor measurements with steady surrounding conditions evaluations of concentrating solar collectors could be facilitated and made more systematic. In this article, a method is described of how indoor measurements of incidence angular dependence of concentrating collectors can be performed by using a large solar simulator providing nearly parallel light. The problem of spatial non-uniform irradiation from the described simulator is taken into account and compensated for by measurements of the total irradiation by several photodiodes placed in front of the test area.

Background

Concentrating solar collectors have a potential to be cost efficient in comparison to flat plate collectors due to lower investment costs (Adsten, 2002). The ability of evaluating solar collectors indoors provides independence of unwanted climate changes and facilitates the repetitiveness of the experiments. In order to enable indoor evaluation of concentrating solar collectors, a light source with parallel light is required. These kinds of light sources are not common, hence indoor evaluations are rare. A solar simulator providing nearly parallel light, and adjustable for solar angles is in use at Energy and Building Design at Lund Institute of Technology. The distribution of irradiated light from the simulator is however rather uneven. Hence, some results from previously performed evaluations of concentrating solar collectors have been somewhat difficult to interpret. (Gajbert et al., 2003)

The objective of this work is to develop a method to compensate for the non-uniform light distribution and the movements of the light pattern while using the solar simulator. The aim is to improve evaluations of incidence angle dependence of concentrating solar collectors using the simulator.

A method that serves this purpose has been developed and is presented in this article. The idea is to estimate the total irradiance on the test area, based on continuous measurements with a number of photodiodes placed on the glazed surface of the solar collectors. By using this method, two different types of concentrating collectors with limited acceptance angles have been evaluated with respect to incidence angle dependence of the zero-loss efficiency and the results are presented and discussed in this article. Another method which assumes that the irradiance on the collector is proportional to the cosine of the angle of incidence is also tested. The results of these two methods are presented and discussed here.

Absorber coated with TiOxNy was cut in a hexagonal shape. The edge of the plate was hooked by small metal pin. Then, the plate was maintained at just the center of the mold. The silica sol was poured into the polyethylene mold and the plate was immersed in the sol. Thickness of the mold was 20 mm. After the gelation took place, the gel was removed from the mold and supercritical drying was done.

Analysis

Analysis of the obtained selectively solar-absorbing coatings and silica aerogel was carried out by TG-DTA (SII, TG-DTA), ESCA (Kratos, Axishs), UV-VIS (Jasco, UV-570), Emission and FT-IR (PerkinElmer,

SpectrumOne), XRD (Rigaku, RV-200)

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Results and discussion Selectively solar-absorbing coatings

TGA curve of the precursor is shown in Fig. 3. The rapid weight decreasing occurred below 200 °C by evaporation of solvent such as IPA, DMF, piperidine, and so on. Above 200 °C, the weight decreased gradually.

FT-IR spectra of selectively solar-absorbing coatings which heat-treated at 500, 600, 700 and 800 °C were measured. These spectra are shown in Fig. 4. The O-H bonds at 3300 cm-1 disappeared at 700 °C. When selectively solar-absorbing coatings was heated at 800 °C, new peak appeared

Wavenumber (cm-1)

Fig. 4 FT-IR spectra of selectively solar-absorbing coatings treated at 500 (a), 600 (b), 700 (c) and 800 °C (d).

about 700 cm-1. The peak was a signed to Ti-O-Ti stretching vibration. The result of XRD at 700 °C was shown broad hallo pattern. Then, the selectively solar-absorbing coatings heated up to 700 °C remained as amorphous state. The ESCA result showed N/O ratios of the coatings changed according to the heat treatment temperature. The N/O ratios of coatings heat-treated at 500 °C and 700°C was 0.29 and 0.94, respectively. The result was in good agreement with reduction of the O-H bonds by FT-IR measurement.

UV-VIS spectrum of selectively solar-absorbing coatings heat-treated at 700 °C was measured. The result is shown in Fig. 5. The reflectance remained below 1 % for wide rage of wavelength.

The emission spectra of the selectively solar-absorbing coatings were measured at 100 °C. The emission of the selectively solar-absorbing coatings heat-treated at 700 °C was 3%. Therefore, the selectively

solar-absorbing coatings with reflectance lower than 1% and emission of 3 %.

Important for a favourable micro-climate in a collector is the combination of ventilation rate and insulation material. The time-constant of the micro-climate in commercial collectors is much longer than the examples in table 1. A climatic cabinet was used for testing usual collectors. The ambient climate in the cabinet was set to 30°C and 95% rel. humidity (absolute humidity: 26 g/kg dry air). The collectors were pre-conditioned at 25°C and 45% rel.

humidity (absolute humidity: 9 g/kg dry air) at an inclination angle of 45°. The heat-transfer — fluid provided 40°C absorber temperature. The collector was moved into the cabinet fast and the rel. humidity in the air-gap was recorded. The results are compiled in figure 9a.

Figure 10: Frequency distribution of the humidity in the air-gap of different commercial collectors

The same collectors were exposed under simulated working conditions outdoors. The data from the measurement of the humidity in the air-gap are shown as frequency distribution

functions in figure 10. The ambient humidity peaks at high values of the relative humidity, mainly because of the night values. All collectors are drier.