Experimental study of multipurpose solar hot box at Freiburg, Germany

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Vol. 12, No. 1, pp. 1-20, 1997 © 1997 Elsevier Science Ltd All rights reserved. Printed in Great Britain 0960-1481/97 $17.00 + 0.00

Renewable Energy ,

Pergamon PII : S0960-1481

(97)00014-1

EXPERIMENTAL S T U D Y OF MULTIPURPOSE SOLAR HOT BOX AT FREIBURG, G E R M A N Y SHYAM S. N A N D W A N I * International Institute of Theoretical and Applied Physics, Iowa State University, Ames, Iowa, U.S.A.

and

JOSEF STEINHART, H. M. HENNING, M. ROMMEL and V. WITTWER Fraunhofer Institut fur Solare Energiesysteme, Oltmannsstrasse 5, Freiburg, D 79100, Germany

(Received 20 December 1996; accepted 12 February 1997) Abstract--With the aim to test and compare some properties of materials and c o m m o n geometries that are used for designing solar cookers, water heaters, etc. we have made a solar hot box with two similar compartments. In the present study this hot box has been used for, (a) comparing the behavior of a metallic slab filled with a phase change material for short term heat storage, with a conventional absorbing sheet, (b) the use of a selectively coated, as compared to a normal black painted, cooking pot, and (c) for finding the overall heat loss coefficient and thermal capacity of the box. Experiments with the solar hot box will yield valuable information on solar systems that are to be constructed. Besides its use for research this multipurpose device has been used both to pasteurize up to 14-16 1 of water and for cooking. © 1997 Elsevier Science Ltd.

INTRODUCTION The functioning of any solar system depends, besides on the climatic conditions, on the design and the materials used. Depending on the need and place of use, sometimes cheap materials and simple designs are made in developing countries ; or possibly expensive and sophisticated designs in the case of developed countries. It is worth designing the components appropriately, either theoretically or experimentally, before the complete system is made. This theoretical simulation is not always possible due to the various parameters

*Permanent address after 23 January 1997: Laboratorio de Energia Solar, Departamento de Fisica, Univresidad Nacional, Heredia, Costa Rica.

2

S.S. NANDWANI et al.

involved. Thus [1-4], working in different aspects of solar energy, we thought of designing and making a simple multipurpose hot box, which has been used for : (a) Measuring the overall heat loss coefficient of the box, (b) measuring the thermal heat capacity of the box, (c) comparing the behavior of a metallic slab filled with a phase change material for short term heat storage, in contrast to the use of normal absorbing sheet, (d) comparing the behavior of a cooking pot pasted with Maxorb selective foil on top (lid) and around (perimeters), in contrast to the normal pot painted black, (e) pasteurizing of water in metallic and glass containers, (f) cooking of food. CONSTRUCTION Figure 1 shows the schematic drawing of the device. It is an insulated wooden box of external dimensions 84 cm x 43 cm x 52 cm. It is divided into two equal identically insulated compartments. On the top of the box there are two normal window glasses which are inclined to an angle of 35 °. Although the latitude of Freiburg is 48 °, this slope of 35 ° was used as it is the better inclination during the possible period of use (summer). The main components of the system are : (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11)

Two window normal glasses, each 0.4 cm thick and separated by 2 cm, wood for the outer box, 1.2 cm thick, bottom glass wool, 4 cm thick, lateral glass wool, 2.5 cm thick, wood, 0.2 cm thick, normal cooking aluminum foil, wooden frame for the two glasses, absorbing sheet (33 cm x 35 cm) of two kinds (to be explained later), reflecting paper pasted on a 0.4-cm-thin wooden sheet, doors for taking in and out the load, wooden strip (2 cm wide) with various holes to change the angle of the reflector (N-S adjustment), (12) four wheels for easy carrying and orienting the box towards the sun (E-W adjustment). Figure 2(a) shows the model as it was constructed. The area of black absorbing surface of each compartment (box) is 0.1155 m 2. We have tested the two boxes using a normal galvanized iron sheet, which was bent all around by 1 cm, in the form of a tray. This tray is painted normal matt black on the sunfacing side. On the other hand, for the heat storage study we have made a metallic slab of black iron (mild steel) 35 cm x 33 cm x 1 cm with a small hole to fill the phase change material. Figure 2(b) shows the normal galvanized iron tray and black iron slab. HEAT STORAGE MATERIAL

In this study we have used a solid-liquid phase transition material, for the short term heat storage. It is a high density polyethylene (HDPE, high density polyethylene spheres of

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approximately 3 m m diameter) made by the c o m p a n y Vestolen G M B H in Germany. Its trade name is Vestolen A6016 where each digit represents some of its characteristics. We call this material (PCM1). These polyethylenes are used numerously for, e.g. chairs, ducts and bags, etc. To measure the temperature of P C M 1 in the slab, a thermowell system has been made with a small copper tube and is fixed at the center of the slab.

4

S.S. N A N D W A N I et al.

Fig. 2(a). Actual hot box constructed and studied in the present work. (b). The normal galvanized iron tray and black iron slab.

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EXPERIMENTAL STUDY The transition temperatures (melting, the solidification) of Vestolen A6016 only, and the specific heat in the solid and liquid phases, and latent heat of fusion have been measured using a differential scanning calorimeter by Mr Olivier Coudevylle at the Fraunhofer Institute. F r o m this the following properties are derived : • • • • •

A melting temperature of 126°C, a solidification temperature of 122°C, a heat of fusion of 235 kJ/kg (56 kCal/kg), a specific heat in the solid phase of 2.2 kJ/kg°C (0.524 kCal/kg°C), a specific heat in the liquid phase of 2.8 kJ/kg°C (0.667 kCal/kg°C).

However, we thought of checking only its transition temperature by a different means. The experiment was done on 15 June 1996. Approximately 560 g of PCM1 contained in one metallic container was kept in an electric oven which was controlled by the computer. The oven and PCM1 temperatures were recorded continuously. In order to have more reliable data, two thermocouple sensors Tsl and Ts2 were tied together and were kept at the same place in the PCM1. The electric oven was switched off after 3 h, when its temperature reached 200°C, but with the door of the oven opened. The measured temperature T vs time t curve is shown in Fig. 3. One can clearly see a fiat portion in both heating/melting (at about 128°C) and during cooling/solidification (at about 125°C). This can only be explained by assuming that more heat is required during phase change, both

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S.S. NANDWANI et al.

6

from solid-liquid (storing heat), as well as from liquid-solid (releasing heat). Furthermore, two sensors, Tsl and Ts2, reported exactly the same reading. In other words, both the melting and solidification transitions are clearly observed. A similar experiment was performed with the actual slab, which will be explained later. Based on the energy to be stored for the short term, we planned to use 1 kg of PCM 1 and 100 g of copper shavings in the slab, in order to increase the heat transfer rate from PCM 1 to the top of the metallic slab. The material PCM 1 has a thermal conductivity of 0.45 W/m°K, compared to 387 for copper. The size of the slab was made accordingly. However, while filling the PCM 1, we found it impossible to fill this quantity in the slab, due to its spherical shape and high viscosity. We tried to fill it, even by heating the slab at 200°C in an electric oven. Being highly viscous, we could only fill it with 584 g of PCM1. Due to this we added only 35 g of copper shavings. As the heat capacity of the absorbing sheet (the tray, as well as the PCM 1 slab) will be used in theoretical analysis, it will be useful in calculating this. (a) Normal galvanized iron tray: actual weight = 0.755 kg, thus the heat capacity ( m C ) t = 0.755 kg x 0.4368 kJ/kg°C = 329.8 J/°C = (0.0785 kCal/°C) (b) Heat storage slab : in addition to the properties already mentioned, the heat storage material PCM 1 has the following properties : • density, 960 kg/m 3, • thermal conductivity, 0.45 W/m°K. Now we will calculate the heat capacity of the whole slab, below the transition temperature (the sensible heat storage) and just above transition temperature (sensible and latent heat storage)" (i) Up to the transition temperature (say 125°C), heat capacity of mild steel (black i r o n ) , nci = 1.76 kg × 0.437 kJ/kg°C = 768 J/°C (=0.183 kCal/°C) (ii) the heat capacity (sensible) of PCM1 HCp = 0.584 kg × 2.2 kJ/kg°C = 1285.3 J/°C ( = 0.306 kCal/°C) (iii) the heat capacity of copper shavings, HCs = 0.035 kg × 0.382 kJ/kg°C = 13.38 J/°C ( = 0.00318 kCal/°C). Thus, the total" (a) Below the transition temperature" the heat capacity of slab = 2066 kJ/°C the weight of slab = 2.38 kg the specific heat of the slab (rnC)s = 868.2 J/kg°C ( = 0.2068 kCal/kg°C) (b) Just above transition temperature" the heat capacity of slab = 2066 J/°C + 0.584 kg × 235 kJ/kg = 2.066 k J / ° C + 137.2 kJ. ACTUAL PERFORMANCE OF HOT BOXES

In our studies, a pyranometer was used to measure global solar intensity on a horizontal surface, thermocouples were used to measure temperatures, and a data logger was used to record and store the measured data. All the times referred to in the text are CET or Central European Time.

Multipurpose solar hot box

7

As mentioned already, we have used the box for many purposes and can mention these one by one : The overall heat loss coefficient and thermal heat capacity o f the solar hot box

One can calculate these parameters theoretically by knowing the types of materials used, their dimensions and properties. However, these data are not always available. The other standard method for the calculation of overall heat loss coefficient is through the measured cooling curve of the box with time, in the absence of solar intensity. We will analyze this aspect. For a body at an initial temperature of T~, its final temperature Tf (t), after an interval of time t, during which the input solar energy H (or more precisely, absorbed energy Qa), remains constant, is given by the standard equation : Zf(t) = Ta -+-O a / f r - [Qa/UL - (Ti - Ta)] × e x p ( - Z),

(1)

where Ta is the ambient temperature (°C), Q a is the absorbed solar radiation (W/m2), UL is the overall heat loss coefficient (W/m2°C) and Z = Ac x Ur x t/(mC)o

(la)

where Ac is the absorbing area (m 2) and (mC)o is the heat capacity of the hot box (J/°C). Alternatively, if a hot body at a temperature Ti is left at an ambient temperature and in the absence of solar radiation ( H or Q~ = 0), then the final temperature Tr (t) will be given by (from eq. 1), Tr(t) = Ta + (Ti - - T,)exp(--Z).

(2)

UL = -- [(mC)o/Act] × ln[(Tr-- Ta)/(Ti -- Ta)].

(3)

F r o m eq. 2,

Thus, to calculate UL, one needs to know the heat capacity of the box which is, again, unknown in most cases. On the other hand, to solve two unknown variables (UL and mC), one needs two equations. It is here where we make use of two identical boxes, except the absorbing sheet. In our design, we have two hot boxes (O and S), with the same basic components. One box (O) has a normal absorbing tray, and the heat capacity of this box is (mC)o and its overall heat loss coefficient is UL (both unknown). In the second box (S), instead of a normal absorbing tray we have the slab. Although the heat capacity of this box (S) is (mC)s (again unknown), we can assume this second box (S) has the same overall heat loss coefficient UL because all the bottom, edge and top insulating materials are the same. In order to perform the heat loss experiment, firstly both boxes are heated with solar energy, and then they are allowed to cool in a constant ambient temperature with no solar intensity. We simultaneously measured fall in plate temperature with time, in both boxes. Let T~0, Tf0 and T,0 be the measured initial, final plate and ambient temperatures in the box O, and T~s, Tfs and Tas be the corresponding plate and ambient temperatures in the box S, in the same interval of time. We know all of these six parameters. Now, from eq. (3), (mC)o = - (UL × A~T)/ln[(Tfo -- Ta0)/(Ti0 - - Ta0)]

(4a)

8

S.S. NANDWANI et al. ( m C ) s = -- ( U L × A d ) l l n [ ( T f s - T a s ) l ( T i s - Tas)].

(4b)

As the box has either tray or slab, thus we can write various heat capacities as, ( m C ) s = [ ( m C ) o - (mf)t] + ( m f ) s

(5)

where ( m C ) t is the heat capacity of only the tray (known) and (mC)s is that of only the slab (known). By putting the values of eq. (4a) and (b) into eqn. (5), and with simple manipulation, it can be shown that, UI. = [ ( m C ) s - (mC)t] x (In O x In S ) / [ A ~ x t x (In S - In O)]

(6)

where 0 = (Tro- Tao)/(Tio- T.o)

(6a)

S = (Tfs -- T , s ) / ( T i s - T~s).

(6b)

and

In our particular case, as calculated earlier, the extra sensible heat capacity, will be, (mC)s-

(mC) t = (2066-- 329.8)J/°C = 1736J/°C ;

(6c)

and Ac = 0.1155m 2. Thus UL = 15030.3 (In O × In S ) / [ t x (In S - I n O)].

(7)

Thus, from eq. (7), UL can be calculated easily. Once UL is known, then ( m C ) o and ( m C ) s can be calculated from eqs 4(a) and (b) and can be checked with eq. (5). An experiment was performed on 11 July 1996 with the normal absorbing tray in one c o m p a r t m e n t and the slab in the other compartment. The box was heated (below the transition temperature) with solar energy and then once heated, it was allowed to cool in the respective compartments, in the absence of solar radiation, but with the reflector opened. The results are shown in the Fig. 4. F r o m this graph, the temperature drop in the period of 30 rain (17.45-17.15 CET, t = 1800 s), is 0 = ( T f o - - T a o ) / ( T i o - Tao) = (57--24)/(88--24) = 0.5156

and S = (Tfs -- Tas) / (Tis -- Tas) = (74 -- 24) / (94 - 24) -- 0.7142. Thus, from eq. (7), UL = 5.71 W/m2°C. Also, from eq. (4a),

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H e a t storage slab compared with normal absorbing tray

It is well-known that solar energy can be stored, either in the form of sensible or latent heat. Although the first form is cheaper and convenient, it requires a large mass compared to storage in the second form. Furthermore, the amount of heat storage material required is proportional to the quantity of heat to be stored. In the present study, as an experiment we thought of storing heat in the form of latent heat and also for a short time ( 3 0 4 0 min). We expect that this form of storage should help in compensating for reduced solar intensity due to some clouds passing over during the cooking period. One of the materials easily available in G e r m a n y was Vestolen A6016, with a transition temperature of 126°C. Instead of making the complete cooker integrated with this heat storage material, we decided to compare the behavior of this phase change material in contrast with a conventional absorbing sheet/tray. This slab was put into one of the compartments and the normal tray, which was painted black, was kept in the other. Different types of studies were made, including some which could simulate clouds. To simulate clouds or to reduce solar intensity when required, we either disoriented the whole

10

S.S. NANDWANI

et al.

box opposite to the sun (i.e. box azimuth = sun azimuth + 180 °) or used some partly reflecting shading film. F r o m the transmittance spectrum with the wavelength measured at the institute and integrating the curve in the solar spectrum, the total transmittance was found to be 14 and 50% for two different films. In a given experiment, both boxes were covered with the same shading film. In order to measure the reduced solar intensity during the time of shading, a small frame was also made with the same shading film, just to cover the pyranometer. The different studies made were : (1) No load at all, natural clouds : 11 July 1996. The plate temperature in both the absorbing tray and slab, and the solar radiations are shown in Fig. 5. It can be observed that the maximum temperature of the normal plate is 126°C, whereas the maximum temperature of the slab is 118°C. On the other hand, the corresponding minimum temperatures observed are 87 and 97°C, respectively. As the PCM 1 did not reach phase transition temperature Ts, only sensible heat storage effects could be observed. With natural clouds, the maximum variation (immediate fall) in plate temperature observed was 25°C in the case of the normal plate and 10°C in the storage slab during the same period. In order to see the fall in temperature due to possible created clouds, during 1715-1815 CET the box was disoriented by 180 °. With this movement the normal tray temperature fell by 43°C (90-47°C), whereas in the case of slab PCM1, the temperature fell by only 32°C (92-60°C). (2) No load, simulated clouds : 23 August 1996. 11:00

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Multipurpose solar hot box

11

The experiment was started at 0800 CET, without any load in boxes. The plate temperature and solar intensity using two pyranometers, both at a surface inclined at 45 +, were measured. The results are shown in Fig. 6. As expected, the normal plate temperature was greater than the slab temperature. Also, the slab temperature (105°C) was below the phase transition, i.e. heat was again stored only in the sensible form. Then, a shading device (about 50% transmission) was put on the box, as well as on one o f the pyranometers, so that the real reduction in solar intensity due to simulated clouds could be known. Here, one can see the lower reduction in slab temperature (3s = 1 0 5 - 98 = 7°C) as compared to the reduction in the normal plate temperature box (fn = 1 2 0 - 97 = 23°C). The shading device was removed and, again, without any load, both the plate and slab temperatures were measured until the slab temperature was above the transition temperature. At 1200 C E T the plate and slab temperatures were 142 and 131 °C, respectively. This time the heat stored in the slab is due to sensible and latent heat. To see the effects of clouds, again some shading device was put between 1200-1240 CET, both on the box, as well as on one of the pyranometers. N o w the reduction in plate temperatures are, fin = 142 - 106 ( = 36°C) and fs = 1 3 2 - 114 ( = 18°C). Here we see that the plate temperature in the normal box dropped to 106°C, whereas in the case of the slab, it dropped only to 114°C. Then, the shading device was removed (i.e. clouds have passed) and once again the shading device was kept between 14 and 16 CET. The plate temperatures can be seen in Fig. 6. One can observe a large drop in the normal plate temperature compared to that of the slab. (3) Heating of water after the box has stored some solar energy : 15 July 1996. This is something like a realistic case. First, the free solar energy is stored in PCM1 during the period when it is not in use, and then when the food is kept around 10-11 am, 08:00

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12

S.S. NANDWANI et al.

the stored heat needs to be released and combined with the instantaneous solar radiation. In this way any reduction in solar intensity due to short period clouds could be partially compensated for. In the present experiment, boxes with a normal absorbing tray and storage slab did not'have any load till 1030 CET. As shown in Fig. 7, the normal plate and PCM1 temperature reached 145 and 130°C, respectively. At 1030 CET, we opened the doors of the hot box and put 2 1 of water in two glass jars in each box, and put the temperature sensor in one of the jars into each box. Again, in this process there was some loss of heat. Although the solar intensity was not very high, the water temperature reached about 90°C in both cases. In addition, the water in the jar in the storage slab box was 5-6°C higher than in the normal plate box. Also, the drop in normal plate temperature (145-50 = 95°C) is almost twice as much as compared to the drop in plate temperature in the case of the storage slab box (130-81 = 49°C). Thus, we see that the heat storage slab has the advantage, if it has already stored some heat before keeping the load. (4) Simulated heat storage in the slab : 29 July 1996. N o w to simulate the stored heat quantitatively, especially above the phase change transition temperature, the complete slab was heated in an electric oven set at 200°C. The oven air and slab temperatures are shown in Fig. 8. Then the heated slab was taken out of the oven and kept in the solar hot box. The other c o m p a r t m e n t had nothing. Then, 1 1 of water was put in one jar and kept on the slab in the hot box. The oven temperature sensor was then put in a glass jar containing water. The box door was closed, but the reflector was kept open (to avoid any possible breakage of the glass). The temperature of the slab and 08:00

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water are again shown in Fig. 8. As the box was inside the laboratory, any increase in water temperature should only be due to the heat stored in the slab and losses. The change in the slope o f temperature vs time curve for PCM1 at about 126-128°C during heating and at 122-123°C during cooling shows the phase transition of the Vestolen material used. The water temperature increased from 24 to 49°C. Although this increment seems low, let us make a simple analysis to see what we could expect theoretically with the available data. F r o m Fig. 8 it can be seen that the slab released the heat and, thus, its temperature reduced from 170 to 65°C in 1.15 h, whereas during the same period the cold water absorbs this heat and increases its temperature. We are interested in calculating this rise in temperature. Total energy released by the slab : (a) Sensible heat released from 170 to 125°C (PCM1 is in the liquid phase)" released by iron + copper + P C M 1 =[1.76 k g x 0 . 4 3 7 k J / k g ° C + 0 . 0 3 5 x 0 . 3 8 2 + 0 . 5 8 4 kg×2.8 kJ/kg°C]x(170 125)°C = 108.8 kJ. (b) Latent heat released during phase transition = 0.584 kg × 235 kJ/kg = 137.2 kJ. (c) Sensible heat released from 125 to 65°C (PCM1 is in the solid phase)" released by iron + by copper + by P C M 1 = [1.76 kg × 0.437 kJ/kg°C+0.035 × 0.382+0.584 kg × 2.2 kJ/kg°C] × (125-65°C) = 124.0 kJ.

14

S. S. NANDWANI et al.

Thus, the total heat released by the heated slab = 370.0 kJ. Now, part of this heat is lost to the ambient and the rest goes into the useful energy for heating water (and also the jar). Assuming that the heat loss by the box is proportional to its mean temperature (T), the total heat lost by the box, in the period of 1.15 h will be, = ULxT×Acxt = 5.71J/(sm 2 x C) × [{(170+65) x 0.5} --25 as Ta]°C x 0.1155 m 2 x 1.15 × 3600s = 252.6kJ. Useful energy --- 3 7 0 - 252.6 = 117.4 kJ, which effectively goes for heating 1 kg of water (specific heat = 4.2 kJ/kg°C) and 0.78 kg of glass jar (specific heat = 0.80 kJ/kg°C). Thus, the increment in temperature will be 117.4 kJ/(1 x 4.2+0.78 x 0.80) = 24.3°C. This is very near to 25°C which we observed experimentally. Selectively foiled pot compared to the normal painted pot Normally, to increase the solar absorptance and reduce the infrared emittance, collectors are painted with a selective coating. However, in the case of solar cookers, it has been mentioned in the literature that instead of the absorbing sheet, if the cooking pot is selectively coated, probably one gets a higher food temperature. As no systematic study has been done, we thought of using this box for comparing the performance of a cooking pot pasted with selective M A X O R B foil on top (lid) and around (the perimeters), as compared to the black painted pot. As reported by the manufacturer [5], M A X O R B is an ultra-thin nickel foil with a black surface. Its solar absorptance as calculated from the reflection spectrum curve is 0.95-0.99 and infrared emittance at 100°C is in the range of 0.08-0.11. This foil was bought 10 years ago and was used in the heat storage cooker by Rommel et al. in 1986 [2]. Since that time it was lying in the office. As it comes with a protective layer on both sides, we believe that its spectral properties were not changed. Two fingers size free space was left on the lid without selective foil, so that the pot could be handled properly without touching the foil. It should be noted that in all these studies the absorbing sheet in both the boxes was a normal black painted tray and not the slab. In one of the compartments the pot with normal black paint was used, while in the other, the pot with selective foil was used. Again, different types of studies are made with and without the load. (1) Empty pots : 22 July 1996. Both the pots were empty. The reflector and the box were adjusted constantly to reduce the shading in the box and to get maximum solar intensity. The temperatures of the plate, as well as that of the air in the pots were measured. As shown in the Fig. 9, although the absorbing plate temperatures in both boxes are nearly the same, the air temperature in the selective pot went up to 155°C, whereas in the case of normal pot it was only 138°C. At 1300 CET, the shading device was kept on the box, to see the cooling effect. The shading device (15 % transmittance) is very strong (850-170 W/m/), not like natural clouds. Thus, the fall in temperature is also very strong. Although all the temperatures dropped, however, the air in the selective foiled pot was still 5°C higher than the normal painted pot.

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16

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Multipurpose solar hot box

17

• The PCM1 in the selective pot reached above the Ts. This was observed (after finishing the experiment) from the melting of some P C M 1 in the cooking pot. • To simulate clouds, at 1830 CET, the whole box was turned away from the sun by 180 ° (with the reflector opened, as usual). In the cooling experiment, the difference of the temperatures between the two pots was almost 20-25°C. This could be due to two reasons, low emissivity of the selective pot and some extra heat stored in PCM 1 kept in the selective pot. • At 2015 CET, the reflector was closed to see the cooling effect. The temperature of P C M 1 in the selective pot was always higher than in the normal pot.

Water pasteurization in metallic and glass containers It has been confirmed by different studies [6] that water could be pasteurized when heated above 65°C for 15-30 min (milk is pasteurized at 71.7°C for only 15 s or 30 min at 62.8°C). In this study, the objective was to first see how much water could be pasteurized in a hot box with the normal absorbing tray and the heat storage slab as the absorbing surface. Secondly, it was to compare the temperature, and quality of water heated in a normal black painted cooking pot and transparent glass jar. On 17 July 1996, both boxes were opened at 0750 C E T and two glass jars containing 2 1 of water were put in each box. The plate and water temperatures were measured in both boxes. The water in the normal box reached about 71°C at 1140 C E T and it was replaced immediately by another 2 1 of fresh water. On the other hand, the same temperature was reached at 1200 C E T in the box with the storage slab. Immediately, two hot jars were replaced with two new jars filled with fresh water. The process was continued. At the end of the experiment we could observe that, although the day was not very sunny, roughly 16 1 could be pasteurized with an absorption area of 0.23 m 2. In other words, approximately 70 1 of water per day could be pasteurized in a hot box of 1 m 2. Also, as expected, when the load is put in at the beginning, the normal absorbing sheet gives more useful energy compared to the storage slab. Although, at the end of the day the slab is left with more energy as compared to the thin absorbing sheet, this m a y not be useful for our purposes (e.g. pasteurization). Regarding the second objective, an experiment was performed on 21 August 1996, in collaboration with Orlando Parodi and Eng. Vosseler, at Fraunhofer ISE. Two liters of river water, but with known coliform at the beginning, were divided in two parts. One litre of this was put in a glass jar and the other litre was kept in a metallic container. In this way one could study the effect of visible and/or ultraviolet light on the quality of the water. Each container was kept in different compartments. Under these pots there was a sensor to measure the plate temperature. The global solar intensity was measured at a surface inclined at 45 ° . Both boxes had the normal absorbing tray. The experiment was started at 0900 CET, the solar intensity was not high on this day. The results are shown in Fig. 12, and can be summarized as : • In this period the water temperature in the metallic jar reached 66°C, whereas it could reach only 60~C in the glass jar. • The m a x i m u m water temperature difference was about 10°C. Regarding the quality of the water in both containers, it was analyzed by Vosseler and Parodi. Although they did not have the necessary instruments for measuring the amount

S. S. N A N D W A N I

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Cooking in each box Last, but not least, we used the multipurpose box for cooking with free solar energy. Although different meals have been and can be prepared, we just inform of one, cooked on 1 August 1996. On this day we cooked 1 kg of meat, 1 kg of potato (cut into regular pieces) and about 200 g of rice in the solar hot box. We did not do any other measurements, except enjoy the solar cooked meal. It t o o k about 2.5 h and was a sunny day. Potatoes and meat were cooked without any extra water or oil. The meat left a great deal of water (juice) during the process of heating. Although we did not use it, it could be used for cooking rice. CONCLUSIONS (1) As it can be seen, this simple hot box has been used for m a n y academic and practical purposes. (2) The m a x i m u m temperatures observed under different conditions are : N o r m a l plate 145°C Slab PCM1 132°C Air 154°C. (3) Studying the cooling curve of two different masses during the same interval of time,

Multipurpose solar hot box

19

in an identical box, can give us the idea of overall heat loss coefficient UL of the box and its thermal heat capacity. In our specific case the value of UL and the heat capacity of the box with normal absorbing sheet is found to be 5.7 W/m2°C and 1792 J/°C, respectively. (4) The heat storage material has been shown to perform better, both in the case of air and water as the load. This was due to the sensible heat, as well as the latent heat storage. Unfortunately, its advantage as latent heat storage could not be confirmed. The possible reasons could be : • due to a high transition temperature of this material P C M 1 (126°C) ; • low quantity of PCM1 (as explained we could only fill 584 g as compared to 1 kg, as planned) ; • although we measured the slab temperature more than the transition temperature, this m a y be just the surface temperature and not always the whole slab ; and • losses while opening the door. Let us see quantitatively, what we could expect from this slab containing 0.584 kg P C M 1. In other words, we will calculate the m a x i m u m time tmax during which the heat could be released during phase transition (125°C). /max = total heat stored/heat losses = HxM/UL

× (Ts--Ts) ×Ac

= 235 kJ/kg × 0.584 kg/(5.71 J/s m2°C × 100 × 0.1155

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= 35 min. On the other hand, with the slab planned originally (1 kg of PCM1) tmax would be 60 min. Thus, next we are planning to study another P C M 2 salt having a lower transition temperature, but above the cooking range (85-90°C). However, due to its higher transition temperature, P C M 1 can probably be used better with concentrating cookers. The concentrated heat stored under direct sunlight can be released during a longer cloudy period, when these concentrators do not work at all. (5) The observed temperature increase of water due to the heat transferred by heated P C M 1 was in agreement with simple theoretical analysis. (6) Although with the P C M 1 slab, one gets a slightly lower temperature, as compared to the normal absorbing tray, it should not greatly affect cooking (except it m a y take a few more minutes), however it has its advantage too. One can cook faster at a higher plate temperature, but not if it is below 80-85°C. On the other hand, one can cook definitely although slowly, if the plate can be maintained even at 80-85°C during the cooking period. (7) Temperature in the selective pot was higher, in all three cases studied, without load, with water and even plastic granulars, P C M 1. However, due to comparatively low increase in temperature, the use of selective painted pots for practical purposes m a y not be justified. Firstly, due to some extra cost of selective foil, but also due to handling and cleaning the pots coated with selective surface. However, the concept probably m a y be used to design another model of a solar cooker in the future.

20

S.S. NANDWANI et al. (8) We think that this multipurpose hot box, in addition, can be used for many other purposes, e.g. (a) For testing/comparing two geometries of the same materials or the same geometry with different materials. (b) One can test the melting/solidification properties of different materials (PCMs, opaque, transparent insulation and heat resistant paint, etc.). (c) One can even study the effect of dirt or snow on the cover of the box/collector, etc.

If these types of boxes are used before the complete system is made, it can save many materials, e.g. an external insulated box, sealed glasses, etc. and, obviously, the labor costs. In addition to its use for researchers, this multipurpose device (area 0.11 m 2, each compartment) can pasteurize 14-16 1 of water and cook 4-6 dishes per day. We think this solar hot box would be a practical device at research centers for making absolute and comparative studies of materials, and construction geometries for solar systems, up to 140°C, and also for domestic uses, especially in developing countries. Acknowledgements--We are thankful to Mr Ingo. Vosseler and Orlando Parodi for providing river water mixed with coliforms and analyzing solar heated/treated water, and to Mrs Rochelle Dobaey of the International Institute of Theoretical and Applied Physics, Iowa State University, Ames, U.S.A. for revising the English of the manuscript. One of the authors (SSN) is grateful to the Fraunhofer Society, the Director of the Fraunhofer Institute for Solar Energy Systems, Prof. Joachim Luther and to Dr Volker Wittwer, Head of the Department of Thermal and Optical Systems, for accepting him as a Visiting Scientist for 6 months, providing all the necessary facilities and encouragement to make this short study possible, and facilitating more training on different aspects of solar energy. This complete work was carried out at Fraunhofer ISE. REFERENCES

1. Thomas, C., Henning, H. M., Nan Jia, X. and Luther, J., Parametrische Untersuchungen von Latentwarme speichern in solaren Prozel3warmesystemen. Proc. 8 Int. Sonnenforum, Berlin, 1992. 2. Rommel, M., Stahl, W. and Wittwer, V., A solar cooker based on a flat plate collector with oil storage. Sun at Work in Europe, April 1986, No. 1, 3-5. 3. Steinhart, J., Integrierende Kugeln zur Messung des winkelabhangigen direkt--hemispharischen spektralen Reflexions--und Transmissionsgrades. (Integrating sphere for the measurement of the direct hemispherical, spectral angular variation of reflectance and transmittance.) Diplomarbeit Nr. 94/80, Fachhochschule Karlsruhe, Fachbereich Maschinenbau, February 1995, 1-60. 4. Nandwani, S. S., Solar cookers--cheap technology with high ecological benefits. Ecological Econ. 1996, 17, 73-81. 5. M A X O R B solar foil, M P D Technology Ltd, England. Brochure distributed at the 1981 I S E S Solar Worm Conference, Brighton, England, 1981. 6. Metcalf, R., Solar water pasteurization and other noncooking applications of solar cookers. Proc. Second Int. Conf. Solar Cookers, Costa Rica, 12-15 July 1994, 44-50.