The performance of a low cost clay solar cooker

RenewableEnergy Vol. 1, No. 5/6, pp. 617-621, 1991 Printed in Great Britain.

0960-1481/91 $3.00+.00 © t991 Pergamon Press pie

THE P E R F O R M A N C E OF A LOW COST CLAY SOLAR COOKER M. A. AL-SAAD a n d B. A. JUBRAN Department of Mechanical Engineering, University of Jordan, Amman, Jordan

(Received 24 May 1991 ; accepted6 September 1991) Abstract--This paper reports the development of a low cost clay solar cooker. The main features of this cooker are that it is made from cheap, locally available materials, and needs no skilled labour. One of the new design features of the solar cooker is the replacement of the absorber plate with locally available black stones. The effects of using the black stones instead of the absorber plate resulted in a solar cooker capable of storing solar energy, hence making late cooking possible.

INTRODUCTION

CONSTRUCTION AND DESIGN

Jordan is a developing non-oil producing country importing most of its energy needs in the form of petroleum and its products. Jordan's energy resources are limited to oil shale and solar energy, both of which are unlikely to contribute much to the future supply of energy given the present technologies [1]. Solar energy is regarded as the main renewable energy resource in Jordan. Its intensity (in the horizontal plane) is substantial and estimated to be about 19,872 kJ/m2/day [2]. The only commercial utilization of solar energy in Jordan pertains to the use of solar water heaters, used mostly for providing domestic hot water. The most popular solar cooker, the box-type cooker with solar radiation entering the cooker through the transparent cover, has been thoroughly investigated [3-6]. Several attempts were made to evaluate and investigate the utilization of box-type solar cooking in Jordan [7, 8]. A1-Saad and Jubran [7] conducted an experimental investigation to study the performance of two versions of a portable double glazing box type solar cooker manufactured from local materials. The results obtained were very encouraging, especially when reflectors were used, giving a maximum water temperature of 100°C in the cooker. This temperature was maintained for more than 3.5 h, which means it can permit cooking in the late afternoon. Jubran and AI-Saad [8] developed a computer simulation to study the various parameters affecting the optimum design of a solar box-type cooker under Jordanian climate. The aim of the paper is to investigate experimentally the performance characteristics of a low cost, simple design, home made clay cooker made from locally available materials.

The main features of the present cooker is that it must be made from very cheap material and its construction should not require trained labour. The material selected was clay which consists of a mixture of natural deposits formed by weathering of certain rocks. Clay is abundant in Jordan [9] and possesses a certain degree of plasticity when wet, and is strong when it is dry. Two versions of the clay flat plate box-type cooker were constructed. The main difference in the design between the two versions ofthe cooker was that in one version the clay cooker was equipped with a metallic absorber plate while in the other one the absorber plate was replaced by black stones. At one stage of the experimental programme a reflector was added to the cooker. The reflector was added to increase the temperature, and consequently decrease boiling time. A preliminary study was conducted to obtain the optimum inclination angles of the reflector. It was found that at low angles of the sun, especially in winter, the optimum angle was 80 ° from the horizontal of the box, while at high angles of the sun the optimum angle was found to be 120 ° from the horizontal of the box. The dimensions of the reflector were taken to be the same as the area of the transparent glass cover of the cooker to make use of all incident rays, and reflects maximum flux. Figure 1 shows a schematic diagram of the cooker. The cooker is made of a single glazing construction with the outer wall made of clay mixed with sawdust. A specimen of this mixture was tested in the laboratories of the Royal Scientific Society (RSS), Amman, Jordan, to find its thermal conductivity-0.425 W/m °C. The wall thickness of the edges and 617

618

M. A. AL-SAADand B. A. JUBRAN heat transfer coefficient of the cooker (U0) could be found. The thermal analysis used to find U0 from the stagnation test (Fig. 2) is given below. As the steady state of the unloaded solar cooker is reached, the heat balance of the cooker requires that the useful heat collected, Q,, equals the heat absorbed, Qa, minus the heat losses to the surroundings, Q~, according to the equation :

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(1)

Q, = Q a - Q I .

The heat absorbed by the cooker is given by the relation : Fig. 1. Schematic diagram of the clay solar cooker.

Q. = rlopAcH.

the bottom of the cooker are 16 and 34 cm, respectively. The wall was built from the clay bricks which were manufactured using a wooden mould, allowed to dry, and then put into an oven at 100°C for 3 days in order to remove moisture. The glazing is ordinary window glass of 6 mm thickness. The absorber plate is made of copper sheet of 1 mm thickness and is painted black. To increase the simplicity of the cooker the absorber plate was removed and replaced by black stones. The pot is made of galvanized steel with a glass cover to allow for radiation to heat the black painted bottom of the pot and consequently provide more heat to the pot. The cooking pot is of square shape (0.19 x 0.19 m), 80 mm in depth and thermally bonded to the absorber plate.

Where r/op is the optical efficiency,Ac is the collection area of the cooker and His the incident solar radiation intensity. The heat loss from the cooker is expressed

EXPERIMENTAL The clay box-type solar cooker with the two versions of the cooking pots were tested when the absorber plate was in place and when it was replaced by the black stones. In order to assess the performance of the cooker, several tests were conducted to heat and boil specified amounts of water and corn oil. The tests were conducted on clear days in the months of April-May 1990. Copper--constantan thermocouples were placed at appropriate locations in the cooker. Temperature was measured using a multi-point digital electronic thermometer (Microprocessor Thermometer 6200). A Kipp and Zonen Pyranometer with a calibration factor of 11.49 #V/W/m 2 and a solar integrator were used to record the solar radiation intensity. The temperatures and the total insolation on the horizontal surface were recorded at regular 30-min time intervals. RESULTS AND DISCUSSION A preliminary test was conducted to test the solar cooker at a stagnation condition so that the overall

(2)

as •

(3)

Q, = UoAc(Tp - T®).

Where Tp and T~ are the absorber plate and ambient air temperatures, respectively. Using eqs (2) and (3) into eq. (I), the useful heat gain becomes : Q, = A¢[r/opn- Uo(TpT~)].

(4)

The stagnation condition of the solar cooker occurs when the useful heat gained is equal to zero, i.e. Q, = 0. Substituting Qu = 0 into eq. (4), the useful gain equation for the stagnation condition reduces to : ~/opH = Uo(Tp - To~).

(5)

Knowing the stagnation and the ambient temperatures together with the average insolation on a horizontal surface one could find the overall heat 130

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transfer coefficient, Uo from eq. (5). For this solar cooker U0 was found to be 6.6 W/m 2 °C. It is interesting to note that in an early work by the present authors on a traditional solar cooker for utilization in Jordan [7] it was found that when the main body of the solar cooker was made of a wooden casing filled with insolation material of similar geometry, the U0 obtained was equal to 5.1 W/m 2°, which is close to that obtained for the clay cooker where no insulation material was used. The performance of the solar cooker was investigated using a boiling test with 1 kg of water together with a heating experiment using 1 kg of corn oil. The temperature distribution over 6 h for the solar cooker when the water boiling test of 1 kg is used is shown in Fig. 3. No reflectors are used in this test. The figure shows the temperature variations of the absorber plate, inside air temperature, water temperature, ambient temperature and the solar radiation. A maximum temperature of water of 78°C occurs at 13:30 local time. The various temperature distributions are shown in Fig. 4 when the reflectors were used. The maximum temperature of the water was increased around the same local time to 85°C. Similar increases were observed for average absorber plate temperature and inside air temperature. Similar tests involving the heating of 1 kg of corn oil were conducted with and without the reflector in place during typical sunny days in April, and the results are plotted in Figs 5 and 6. The aim of these tests is to investigate the performance of the solar cooker with cooking oil which has a smaller convective heat transfer coefficient than water, and to

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Fig. 3. Temperature distribution of the solar cooker without a reflector--using 1 kg of water.

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Fig. 4. Temperature distribution of the solar cooker with reflector--using 1 kg of water. determine the maximum oil temperature when heated in the cooker. Figure 5 indicates that the maximum oil temperature is 98°C and is reached at 14 : 00 local time. The maximum temperature was increased to 120°C when the reflector is used and this occurs at an earlier time of about 13 : 00 local time. As was mentioned earlier the main objective of the present work was to develop and test a low cost solar cooker. One of the steps taken to achieve that goal was to ,replace the absorber plate of the solar cooker with black stones. These black stones are abundant in Jordan. The results obtained when the black stones are in place shown in Fig. 7. Comparing the tern1200

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M. A. AL-SAADand B. A. JUBRAN 160

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stones than the absorber plate to store the energy collected during sunny hours.

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Fig. 6. Temperature distribution of the solar cooker with rettector--using l kg of oil. perature distributions of the absorber plate used in the previous tests, Figs 3-6, with that in Fig. 7, indicates clearly that while for the absorber plate there was always a maximum temperature reached at around 13:00 h local time, the black stone temperature continues to increase with the increase in time. The oil temperature distribution when the stone is used shows similar trends to that obtained for the temperature of the stones, i.e. the temperature of the oil is increasing with the increase in time. The importance of this is that it will enable cooking late in the day. This could be explained by the higher potential of the black :1200

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A simple economical analysis was conducted to evaluate the total cost of the various versions investigated in the present work. The main versions are basically the clay solar cooker with a copper absorber plate and the clay solar cooker with black stones. The material cost of each component of the cooker was found to be as follows : 20.0 J.D for the copper sheet (J.D = U.S.$1.54), glass 4.0 J.D, mirror 4.0 J.D, labour cost 4.0 J.D and others 2.0 J.D. The total of the copper absorber plate solar cooker with reflector is 34.0 J.D. If the absorber plate were replaced with the black stones (assuming the cost of the black stones is 1.0 J.D) the cost will be reduced by almost 19.0 J.D leaving the total cost at 15.0 J.D, this represents a 60% reduction in the cost of the cooker. One has to point out that the low cost of the clay solar cookers is achieved due to the very low cost of the clay used in building the cooker, no use of insulation, and in the last version it is the use of black stones instead of the absorber plate which reduces the cost significantly. Another important feature of this cooker is that it needs no skilled labour for construction and it can be made from local materials available in rural areas with little effort.

CONCLUSIONS The present investigation was carried out to develop, construct and test a low cost clay solar cooker. The main outcome of the present work was that a solar cooker can be built using clay with the absorber plate replaced by locally available black stones. This clay solar cooker needs no skilled labor and can be constructed and used by people in rural areas in Jordan with very little effort. The use of black stones enables solar energy to be stored in the cooker which in turn enables cooking late in the day.

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REFERENCES

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Fig. 7. Temperature distribution of the solar cooker when black stones are used.

1. Jordan Eneryy Sector Study. Report No. 40412-JD, Feb-

ruary (I 983). 2. M.A. Alsaad, Solar radiation map for Jordan. Solar Wind Technol. 7, 267-275 (1990). 3. A. M. A. Khalifa, M. M. A. Taha and M. Akyurt, An energy thrifty solar cooker--the Mina oven. Solar Wind Tech. 1, 81-91 (1984). 4. M. G. Osman, Solar energy for cooking in Egypt, in Solar Energy: International Proyress, Vol. 4, pp. 1768-1795. Pergamon Press, Oxford (1980).

A low cost clay solar cooker 5. M. Telkes, Solar cooking ovens. Solar Energy, 3, 1-11 (1959). 6. T. E. Bowman, Solar Cookers: Test Results and New Designs. Simp. Int. de Ing, San Salvador (1979). 7. M. A. AI-Saad, B. A. Jubran, Utilization of solar energy for cooking in Jordan. Journal of University of Halab, Syria, accepted 1990.

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8. B. A. Jubran, M. A. AI-Saad, Parametric study of a box type solar cooker. J. Energy Cony. Mgmt., accepted (1990). 9. F. Sarraf, Clay and clay bricks in Jordan, previous studies and existing situations. Report published by the Royal Scientific Society, Building and Research Center, Amman (1982).