The development of new collectors in the operating temperature range of 80° to 250°C is only one objective of the Task 33/4. In order to achieve the goal to integrate solar heat into industrial processes work is carried out in the following four Subtasks:

• Subtask A: Process Heat Survey and Dissemination of Task Results (Lead Country Spain, Aigualsol, Mr. Hans Schweiger). The main objectives are to provide a comprehensive description of the potential and the state-of-the-art of solar heat for industrial process. This includes the evaluation of completed research programs, of projects realised and the study of ongoing developments in

this field, as well as carrying out economic analyses.

• Subtask B: Investigation of Industrial Energy Systems (Lead Country Austria, JOINTS, Mr. Uwe Begander). The main objectives are to identify applications and the corresponding temperature levels of the processes and/or the energy utility system suitable for solar energy and also to investigate and develop integrated solutions considering solar thermal, waste heat recovery and improvements in the processes and energy utility systems.

• Subtask C: Collectors and Components (Lead Country Germany, Fraunhofer ISE, Mr. Matthias Rommel). The main objectives are to develop, improve and optimise collectors, components and systems with a potential for integration in industrial processes with a temperature level up to 250°C.

• Subtask D: System integration and Demonstration (Lead Country Germany, DLR, Mr. Klaus Hennecke). The main objectives are to initiate pilot projects covering a broad variety of technologies in suitable applications representing a significant part of industrial process heat consumers (in terms of size, temperature levels, heat transfer media, load patterns, etc.).

The operating agent of Task 33/4 is Werner Weiss, AEE-INTEC, Austria. The kick-off meeting took place in December 2003 in Gleisdorf (Austria). The second task meeting was carried out in Brussels (29 — 31 March 2004) and the next meeting is planned for the first week of October 2004 in Mexico. The Task is scheduled up to 2007. For more information and in case of interest in participation from solar thermal companies or research institutions see http:/www. iea-ship. org.

The lower accuracy of irradiance sensors compared to a pyranometer originates from several influences. The deviation is depending on the intensity of the irradiance, the ambient temperature, the angle of incidence and spectral effects due to the limited spectral response of the sensors. Using little measurement effort, the project intended to find cost- effective ways to incorporate the temperature and irradiance dependency in an adequate mathematical model. Due to a delayed start of the measurement, the testing was carried out at a small range of low temperatures from 0 to 16 °C. For this reason, only an irradiance-calibration incorporating the dependency of the deviation on the intensity of irradiance was accomplished at the current state.

Using the values measured by the pyranometer CM21 as a reference, for all sensors calibration functions were calculated through a regression analysis of higher order. All values with G(CM21) > 10 W/m2 recorded during the 8-days period were included in the calculation. The ascertained functions were applied to the measurements and the calculations that had been undertaken to compare the sensors were repeated. After that, a comparison of the statistical results before and after the calibration was carried out to quantify the effectiveness of the irradiance-calibration. The analysis showed that the application of the calibration functions led to a significant reduction of the relative deviation for irradiance sums as well as for instantaneous values.

The relative deviation of the irradiance sums between sensors and pyranometer could be reduced below 3 % for all sensors (see Figure 3).

Also the quality of the instantaneous measurements could be improved. For the day with clear sky as well as for the day with clouded sky, apart from 2 exceptions, the mean values of the relative deviation between sensors and pyranometer were reduced (Figure 4). As expected, the irradiance-calibration proved to be not the appropriate tool to reduce the variance of the measurements. On the day with clear sky, the standard error of just 2 sensors could be reduced, while for the other 6 sensors it slightly increased or remained the same. On the day with clouded sky the calibration led to a small reduction of the standard error for 6 of the 10 sensors. Figure 5 illustrates the trend of one sensor before and after the calibration compared to the trend of the CM21 pyranometer on day with clear sky.

Figure 4: Relative deviation on the day with clear sky before and after the

irradiance-calibration

Conclusions

The results presented in this paper assign a good standard of quality to most of the tested irradiance sensors. The deviations of the instantaneous measurements are within a tolerable range for the common applications in thermal solar systems for the bigger part of the tested devices. The relative deviations of partly far below 5 % and the insignificant offset during the 8-day analysis show, that some of the irradiance sensors may be used for measuring irradiance sums in a monitoring of thermal solar systems.

The applied irradiance-calibration proved to have a significant impact mainly on the long­term measurement and on the relative deviations between sensors and pyranometer of instantaneous measurements. For reducing the variance of the deviation, a stand alone irradiance-calibration proved not to be the appropriate tool. Future research activities will focus on the development of models that feature a further improvement of accuracy of instantaneous measurements by incorporating the influencing factors ambient temperature, angle of incidence and spectral dependency.

After that empty test, a mutton carcass containing couscous has been introduced in the oven and the following parameters measurements were carried out:

— inside temperature,

— absorber temperature,

— meat weight and temperature

— ambient temperature, wind velocity, solar radiation (global radiation+ beam radiation reflected onto the oven)

3.3 Computation of the efficiency in load

To compute the efficiency of the oven let us consider the following equations:

Pi =Pu +Ps +Pi

i) Estimation of Pi:

The incident power is given by the solar radiation falling directly on the glass and the reflecting power from the reflectors.

P = T(g, +pGt )

ii) Estimation of Pu

Pu =^Mmeat*Cmeat*(iTmeat—’Tamb )

Pbot, om ={Tp — Tamb k, + ^ / Ik,

The sides lost are given by the conduction in the glass wool and the wood board

Usides —1/{ei/ki+ei/ki )

iii) Estimation of the stored heat Ps

It is constituted by the heat of the air in the oven and the heat in the stones

Ps =Ms, ones*Cstones*(Tstone^Tamb }±Mair*Cair*(Tair-Tamb )

iv) Estimation of the heat lost p

P —Ptop +Pbottom + Psides Ptop ~Ut*(Tp — Tamb )

Value of Ut for flat-plate collectors is given by Duffie and Beckman in Solar Engineering Thermal Processes.

The efficiency is given:

Eff ~Pu / P

The calculated efficiency of the oven is given by the following curve.

3.4 Discussion

The Test with the loaded oven has been run during a typical day of Sahelian region: fairly sunny, important diffuse radiation (~ 20%), a little windy, ambient temperature varying from 20-22 C° in the morning to 40-42 C°, at the beginning of the afternoon.

As we can see, the efficiency is very low, particularly during the early hours, when the inertia of the system has to be overcome. In fact it is not very satisfactory as a whole. An other prototype, with a new design in order to improve the performances has been constructed and is being experimented. For example, by reducing the height of the oven, convection losses are reduced (see curve Fig.7)

и

c

a)

=5

re

Ф

Q.

E

ф

Hours

—•—(Tp-Tc) New —■—(Tp-Tc) old

Fig.7

When solar radiation strikes a surface, a portion of it is transmitted or absorbed, while the rest is reflected. Solar wall system absorbs solar radiations with high absorbency panel, and heats the plenum by conduction and convection. When the suction fan is operated, fresh air is introduced to the plenum through the tiny holes on the panel. The air thus absorbs the heat stored in the plenum. The efficiency of energy transformation of solar panel is expressed by the following Eq.1.

П = pm *Cp (T0ut — Tn)/AI (1)

In this equation m* is the mass airflow rate Cp. is the specific heat of air at constant pressure, Tout is the plenum temperature, Tin is the ambient temperature, A is the cross-sectional area of the solar collector in m2 and I is irradiance in W/m2.

Replacing m by AV in Eq.1 we get Eq.2 n = pVsCp(Tou, — Tn)/1 (2)

where Vs means the velocity of air which goes through a hole of the panel and it is expressed the flow rate in m3/sec. The values for air density (p) and specific heat under constant pressure (Cp.) used in our work were p =1.167 kg/m3and Cp=1007J/Kg-k respectively. The efficiency of the solar panel was calculated by using Eq.2, at different airflow rates under no­wind, low-wind and high-wind conditions as mentioned earlier.

1. Experiment

A solar collector, with a corrugated steel absorber sheet (Table1. and Fig.1), with perforations covering 1% of its surface was selected for experimental purposes. Its area was 1m2 with a duct of diameter 0.06m which was connected by a fan used to vent out heated air for outlet temperature (plenum temperature) measurements (see Fig.2). Measuring duct airflow rate, ambient and plenum temperatures, and the solar irradiance monitored the collector performance. For the experimental work, the duct airflow rate was measured with a digital blade anemometer, the temperature conditions were measured with a Type T thermocouple (Omega. co USA).

The primary components of the indoor test facility system included, (a) a solar simulator, (b) an air suction system for drawing air through holes of the absorber plate at different known rates, and (c) an open-ended wind deflector to allow the wind to flow over the absorber plate. A model M11 Kipp and Zonen (Holland made) calibrated pyranometer connected with a high-precision millivoltmeter (see Fig.3) was installed near the collector at a distance of 2m from the artificial irradiance source. An irradiance level of about 300W/m2 was delivered to

the entire test plate by the sun-simulator, which was consisted of an Osram halogen light bulb with highly reflective metal laminate. The irradiance level was uniform over the plate’s test area, excluding the part with in 0.01m of the plate edge. The experiment was designed to measure the efficiency dependent wind conditions under different suction velocities.

As can be seen from the defining equation for efficiency Eq.2, determining the efficiency requires only the measurement of two temperatures. Four thermocouples were thus fixed at different locations on the test plate, so that the average plenum temperature Tp could be measured. The temperature of the approaching wind was measured inside the duct at the top center of the plate. After a steady state was reached, measurements were made for different wind velocities and suction airflow rates.

In the S2-tank interconnection configuration, the system converted 20 to 41 % of the intercepted solar radiation to useful thermal energy (for wind speed v < 2.6 m s-1) and retained 28.7 to 39.7 % of this energy (Table 3). The average efficiency of heat retention (pr) was 34.7 % which compares well with the findings of Chaurasia and Twidell (2001) who report an average value of 38.9 % for an ICS system with a transparent insulation material (TIM) fitted between the absorber plate and glass cover.

SHAPE * MERGEFORMAT

5. Conclusion

A solar water heater of the integrated variety has been designed, constructed and tested. The system has two horizontal cylindrical tanks: one in the lower part while the other tank is in the upper part of the system. Test results show that the thermal performance of the ICS system is most satisfactory when the two tanks are interconnected with two pipes: one pipe connected from the top part of the lower tank to the top part of the upper tank while the other pipe is connected from the bottom part of the lower tank to the bottom part of the upper tank. In this mode of operation, the heater converted 20.4 to 40.7 % of solar radiation to thermal energy, and stored 28.7 to 39.7 % of the collected heat for use the next morning.

Nomenclature

Aa aperture surface area of the system (m2)

Cpw specific heat capacity of water at constant pressure (J kg-1 oC-1)

Hi total solar radiation received by a unit surface area from sunrise to time t = t| (J m-2)

He total solar radiation received by a unit surface area from sunrise to time t = te (J m-2)

M mass of water (kg)

Qr amount of solar energy intercepted by system (J)

Qw amount of heat absorbed by water (J)

Tb temperature of water at the bottom part of a tank (oC)

Tt temperature of water at the top part of a tank (oC)

T mean temperature of water at the beginning of the solar collection process (oC)

Te mean temperature of water at the end of the solar collection process (oC)

T0 overall mean temperature of water (oC)

^c mean daily solar collection efficiency

^r mean efficiency of heat retention

Subscripts

an ambient air at night

c collection

e end

f final

i initial

L lower tank

u upper tank

The wall construction consists from the inside to the outside of a gypsum plaster, expanded clay bricks (Leca-bricks), cellulose insulation and the fagade collectors. The four collector modules have been mounted with a backside of oriented strand boards (OSB), the insulation of the collector is mineral wool. The absorbers of the collectors are coated with black solar varnish and selective collector coatings in blue, green and grey. The black solar varnish is a standard coating for absorbers, whereas the coloured coatings have been developed in the project.

Because cellulose was used as an insulation material, condensation at the backside of the collector would damage the insulation material. According to the supplier of the cellulose, small amounts of condensate are acceptable, but in any case permanent condensation has to be avoided.

Calculations with the simulation program WUFI [1], developed by the Fraunhofer Institute for Building Physics, have shown that a relative humidity of up to 90% is encountered at the backside of the collector — that means no condensation was encountered with the program. Nevertheless, it was recommended to install the system in winter but only start operation in spring, so that in winter the collector will reach higher temperatures and the wall can dry out
to the inside of the building. This reduces the risk of condensation, which could harm the insulation material.

A major positive effect of a non-ventilated fagade collector is the reduction of the effective U — value. The effective U-values of the wall are calculated from the real ambient temperature, the room temperature and the heat transfer through the wall. This value will change with the conditions and differ in most cases from the static U-value, which is used to characterise a wall and must comply with legislative regulations.

With “Instationar”, a program developed by AEE INTEC to solve transient heat transfer equations, the effective U-values of the wall construction with and without collectors have been calculated for a winter day with high (hemispherical solar irradiation in the wall plane: 4814 Wh/m2.d — primary collector loop operating) and low (hemispherical solar irradiation in the wall plane: 434 Wh/irF. d — primary collector loop not operating) solar irradiation respectively [2]. The results show the positive effect of a fagade integrated collector. The effective U-value of the wall with the collector is up to 77% lower on a winter day with high solar irradiation compared to the static U-value. An improvement of 21% can still be reached on a day with low solar irradiation. The effect of a lower effective U-value is a reduction of transmission heat losses through the wall. The following table gives an overview of the calculation results for the wall constructions with and without integrated collectors. Separate columns are used for absorbers with black solar varnish and the absorbers which are coated with the selective colours developed in the project.

Finally, the maximum temperatures which can occur during stagnation in January (incidence angle close to perpendicular) have been calculated using the program “WandMax” which was developed in the project by AEE INTEC. The results show that no temperatures occur which could harm the used building materials. A maximum temperature at the backside of the collectors of approx. 35°C was calculated.

To investigate the real hygro-thermic processes within the wall, the most important parameters have been monitored and analysed. These parameters were the outside temperature and relative humidity, temperature and relative humidity between the glass cover and the absorber, behind the collector backside, between cellulose insulation and Leca-bricks and inside the room.

The analysis of the data has shown a very 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 85% in the documented period from December 2003 to February 2004. Also the temperatures within the wall construction show good correlation with the results of all simulation programs. At the end of February, a decrease of the relative humidity at the backside of the collector can be seen. At the same time, the relative humidity between insulation and LECA-bricks increased. This could be a first indication of the drying process towards the inside of the building as it is expected.

The mounting of the collectors onto the wall has been investigated for thermal bridges causing heat losses to the outside using THERM [3].

Gypsum plaster

Leca bricks 20 cm

Collector backside OSB

Collector insulation Absorber Glass cover

Figure 6: Wall construction — System Korneuburg

Figure 6 shows the top view of a cross-section of the wall with the construction used to mount the collector. A wooden beam has been mounted to the wall with oriented strand boards at each side. At the front side a second wooden beam is used. The collector frame is connected with screws to the latter mentioned wooden beam. The gap between the OSBs is filled with an insulation material. Figure 7 is the side view of the mounting construction and illustrates the temperature distribution with an outside temperature of -12°C and an inside temperature of 20°C. The two mounting screws can be seen, but there is almost no influence on the temperature distribution.

The results have shown that the mounting construction is well optimized and no additional heat losses will occur.

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

A schematic of the problem is shown in Fig. 1. It consist of a rectangular cavity tilted 45° and with parallel slats inside (cross — slope). The front and back boundaries are isothermal walls at and Tft, respectively ( ), and the others are adiabatic walls. The distance

between isothermal walls is L, while the height of the enclosure is H. The parallel slats are formed by multiple layers of thin glass sheets. These are described by the parameters r1, r2, r3, …(see Fig. 1a). The bottom and top air gap thicknesses are denoted by lh and 4, while £ and l give the thickness and height of each glass sheets (for all cases ),

(see Fig. 1b). The distance between glass sheets was considered uniform. The overall

aspect ratio was defined as A = H/L, while the cell aspect ratio was defined as A’ = H’/L. The average Nusselt number and the Rayleigh number are defined as:

where A is the air thermal conductivity; Q is the total rate of heat transfer between the isothermal walls; Ai is the surface area of the isothermal wall; 0 is the gravitational acceleration; and,3, p, cp and p respectively are the thermal expansion coefficient the density the specific heat at constant pressure and the dynamic viscosity, which are assumed con­stant (Prandtl number ). A thermal con­

ductivity ratio value (ДА = A„/A) of 28.6 was cho­sen, where A„ is the glass thermal conductivity.

Computations of three different configurations have been performed. Fig. 2 shows a summary of all cases studied. These are divided in two groups: computational domain whole, where, and

computational domain reduced, where the domain is limited to a cell of multiple glass sheets. In Fig. 2n and к parameters are used in order to properly define the mesh: n is the refinement level, and & is a mathematical parameter which depends on A’.

1.1 Study I: Parallel slats in contact with isother­mal walls.- The grids used were of nx(8+A;n), con­sidering values of к = 1, 1, 2 and 4 (related to A’ values of 0.5, 1, 2 and 4, respectively); n = 5, 10, 20 and 40; and Rayleigh numbers of, 104

and 105 (see Fig. 2-(I b)).

1.2 Study II: Parallel slats in contact only with cold isothermal wall.- For whole computational domains (Fig. 2—(II a)) simulations were performed using grid of nx[(

1) + ] (where rh is the slat number), considering

values of =8, 16, 32 and 64 (in Table 1 — “whole domains” shows theses cases). For reduced computa­tional domains (see Fig. 2—(II b)), the grids used were of nx(8+kn), with values of n = 10, 20, 40 and 80 (in Table 1 — “reduced domains” shows a summary of cases studied).

1.3 Study III: Parallel slats separated from the isother­mal walls.- Three configurations were considered: (i)

Symmetrical: lh = lc, (ii) Asymmetrical close to hot isothermal wall: , and (iii) Asymmetrical close to

cold isothermal wall: lh> lc.

These different configurations have been solved using whole computational domains. As for asymmetrical configurations, both have also been solved using reduced computational domains with periodic boundary conditions.

For whole computational domains, Fig. 2-(III a), the grids used were of Зпх[(гл+1)А;п-|-гл] considering values of n = 3, 6, 12 and 24; while for reduced computational domains, Fig. 2—(III b), grid of 3nx(8 + kn) were performed, with values of n = 5, 10, 20 and 40.

Table 2 shows the summary of asymmetrical cases when slats are close to hot and cold isother­mal walls, while the symmetrical cases studied are summarised in Table 3.

Heat Recovery

C. MOller/K. Schwarzer/ E. Vieira da Silva*/ C. Mertes
Solar-Institut JOlich / *Universidade Federal do Ceara
Heinrich-Mussmann-Str.5, 52428 JOlich
Tel.: (0049-2461) 99 35 42, Fax: (0049-2461) 99 35 70
E-Mail: c. mueller@sij. fh-aachen. de,

Internet: http://www. sij. fh-aachen. de

1. Introduction

During the thermal desalination of sea water, the evaporation process has a high energy demand. Around 2294 kJ/kg are required to produce just one litre of distilled water. If solar energy is used to power this process a large area is required. Due to the extensive installation required this involves high costs. To make such an idea economically viable, energy-saving desalination technology must be used. An improvement in energy efficiency is possible because of the recovery of the evaporation enthalpy in a multi-layer arrangement (see Figure 1). The main benefit of the development described here is that it is easy to use and avoids the use of moving components such as pumps and electronic controls. The unit also does not need an electricity supply and can be operated by users with little technical skills. This system should provide an economically attractive alternative to the technically demanding desalination systems commercially available, while still producing between 50 and 5000 litres of drinking water per day. The basis of the multi-layer desalination plant was researched as part of an AIF Research Project (FKZ:1708499). Using a seven-layer unit, an energy recovery level of GOR= 2,8 was achieved with a production rate of 8 kg/m2h.

Fig. 1: Schematic of a solar multi-layer desalination unit

In Figure 7 the influence of store insulation and UA values of the heat exchangers is shown. The highest influence on fsax, ext can be seen for the side insulation of the tank, followed by the top insulation (with lower area than the sides). The high standard variation for the insulation comes from very different variation ranges for the insulation thickness

and reciprocal dependency of heat losses of the store to the thickness of the insulation. Above 15 cm insulation of the store there is nearly no more change for all systems simulated. This is due to the short storage characteristic of all systems (maximum of fsav around 50%). For long term seasonal storage the insulation should be thicker (Streicher, 2003a).

Figure 7 Dependency of fsav, ext on specific parameter change of heat exchangers and store insulation

The insulation of the bottom of the tank is not very significant on fsax, ext, because it should be cold there anyway. Nevertheless, a little insulation should be placed at the bottom in order to avoid condensate dropping on the floor.

Figure 8 shows the dependency of fsav on the insulation for system #4. The insulation of the whole store gives an increase of fsav up to 15 cm thickness; above this value the changes are very small (ref. Figure 8 left). Top insulation is less significant than side insulation, because the top area is smaller.

Figure 8 Variation of fsav with thickness of side and bottom insulation of storage tank, example of system #4 (Bony, Pittet, 2003).

Looking for the differences of top sides and bottom insulation it can be seen, that the bottom insulation is not significant above 5 cm, because the temperature at the bottom of the tank should be low anyway (ref. Figure 8 right). Only on hydraulic layouts that also heat up the bottom of the store this value becomes more relevant.

Nearly no influence can be seen for the heat exchanger variations. Of course they should not be sized too small.

The large indoor solar simulator used for some of the measurements in this work, is shown in figure 1. The simulator is further described in Hakansson (2003 a, b) and Hakansson (2001). Seven large parabolic reflectors are used to render the light nearly parallel, which is a rare quality for solar simulators. The simulator was originally designed for simulation of

daylight for evaluations of solar shadings. However, the parallel light is an important characteristic also for evaluation of the incidence angle dependence of concentrating collectors and it is desirable to be able to adequately perform this kind of evaluations. In order to achieve correct registered values of the irradiation on the collector surface during measurements, it is important that the total irradiation is known throughout the measurement, hence also when shifting solar altitude, i. e. lifting the simulator.

The good parallel light quality of the simulator has been achieved, to some extent, at the expense of a uniform area distribution. Fewer larger lamps generating nearly parallel light are used instead of smaller, but more numerous lamps generating more diverging but more evenly distributed light. The light intensity has been measured at several locations over a central test area, perpendicularly to the simulator, and the resulting intensity distribution diagram is shown in figure 2 (Hakansson, 2003 a, b).

The light intensity peaks emanates the rim of the reflectors surrounding the lamps (Hakansson, 2003 a). When the simulator is lifted to simulate different incidence angles,
the light intensity pattern tend to move over the test area, thus possibly changing the total irradiation on the object. As the irradiation normally is measured by a single pyranometer, the results could be misleading. Earlier results from evaluations of incidence angle dependence of concentrating collectors were initially found to be poorly corresponding to similar outdoors measurements, thus indicating that the effect of the lifted simulator and the consequently moving light intensity pattern significantly altered these results. (Gajbert et al., 2003)

method for non-uniform illumination

In order to compensate for the moving light pattern an array consisting of six photodiodes connected in parallel, has been created and placed on the glazed surface of the collectors at the time of the measurement. The placement of the photodiodes divides the front area of the solar collector into smaller areas of approximately equal size with one photodiode placed in the centre of each area. With this arrangement, the total current from the photodiode array should be a rather good representation of the total irradiance on the collector surface.

The non-uniform irradiance makes it difficult to get absolute values of optical efficiency, ^0, from indoor measurements. Therefore, the calculated optical efficiencies from indoor measurements were adjusted to match the value of ^0 from outdoor measurements.