Experimental Results of a Solar Cooker with Heat Storage Maria Eugênia Vieira da Silva, Klemens Shwarzer*, and Marelo Riardo Queiroz Medeiros. Universidade Federal do Ceara, Laboratório de Energia Solar Apliada Bloo 714, Campus do Pii s/n, Pii, Fortaleza 619-8 BRAZIL E-mail: eugenia@les.uf.br *Ingenieurbüro für Energie und Umwelttehnik Tuhbleih 12, Juelih, GEMANY Abstrat This researh artile determines the experimental effiieny of a solar ooker with storage system. This system is omposed of flat plate olletors, a ooking unity, ontrol valves and a heat storage tank. In its operation, the working fluid is heated in the solar olletors and moves in a thermal siphon iruit. The ontrol valves diret the fluid flux to the ooking unity or to the storage tank for later use. In the experiments, the gathered data are measured by preision instruments and are stored in a data-logger. These data are used to determinate the heat sensible and boiling thermal effiienies. The results show the good system performane, with the sensible effiieny varying from,34 to,38. The results also show that when the same tests are made with the heat stored into the tank, the heating time intervals for diret and indiret ooking are very losed. The latent effiieny varied around,3. 1. INTRODUCTION The use of solar energy, friendly to the environment and eonomial in various appliations, in residential and industrial proesses is beoming a reality in various ountries. In spite of that, the destrution of forests without ontrol has been ourring in other regions. Wood burning has been widely used for ooking in many plaes, suh as in Afria, beause it is the only affordable and/or available energy soure. Food proessing is a pratial solar energy appliation that an be used in regions with limited vegetation resoures and with good solar radiation intensity. The working fluid temperature, in good quality flat plate solar olletors, an reah values around 2 o C, whih makes possible the use of these olletors in solar ooking systems. The purpose of this work is to present the experimental results, the thermal effiienies, and the operational experiene with a solar ooking system with heat storage, tested in the ity of Fortaleza, loated in the Northeast oast of Brazil. The original system was developed by a private ity firma in Germany and in the last years it has been further adjusted and tested by two researh institutions, one in Germany and the other in Brazil. Various systems, with and without heat storage, have been installed in different ountries, suh as Germany, India, South Afria, Mali, Chile, and Argentina, among others (Shwarzer and Krings, 1996; Vieira et al., 1997). The addition of a baking oven and other omponents, operational adjustments and speifiation of the working fluid, and the identifiation of new appliation are among the themes being urrently studied. 1.1 System Desription A photo of the solar ooking system is shown in Figure 1. The photo shows the flat plate olletors, the ooking unit with the storage tank inside, and a baking oven. In operation, the working fluid is heated up in the solar olletors and flows in a thermal siphon iruit due to the differene in density between the hot and ooler parts of the system. As the hot fluid leaves the solar olletor, ontrol valves diret the flow either to the ooking unit or to the top of the storage tank. After use, the ooler fluid returns to the olletor inlet. The most important advantages of this system are the possibility of indoors ooking, very RIO 2 - World Climate & Energy Event, January 6-11, 22 89
safe operation when ompared to the more popular butterfly ookers, and the unneessary use of an external eletrial soure to pump the working fluid though the system. The disadvantage is the higher first ost when ompared with the low ost onentrator ookers. The system shown in Figure 1 is installed in Fortaleza and its basis harateristis are: high quality flat plate olletors (4m 2 ); one storage tank (5 L); one ooking unit with 3 pots and 1 oven; 5 ontrol valves. Piture 1. Photo of the solar ooker tested in Fortaleza - outdoors model with heat storage tank and oven. 2. APPROACH To study the thermal performane of the solar ooking system, experimental measurements and analytial alulation have been made. The measured values are: the global solar radiation intensity on the tilted olletor plane, wind speed, ambient temperature, and temperature in various points of the system. The experimental data are read and stored in data logger. The thermal sensible and boiling effiienies are determined using the definition of the sensible and boiling powers, as defined below. 2.1 Analytial Approah The thermal sensible effiieny, Equation 1, is defined as the ratio of the energy used to sensibly heat a ertain mass of water ontained in a pot of the ooker from the ambient temperature to 95 o C and the global solar energy inident on the time interval. This end temperature value is used to avoid the unertainly in the start of boiling. mw. p. DTamb-95 η 95 = (1) t A G.dt In this equation, m w is the mass of the water in [kg], p is the speifi heat at onstant pressure in [J/(kg.K)], T is the temperature differene in [C], and t is the time elapsed in the heating proess in [s], 9 RIO 2 - World Climate & Energy Event, January 6-11, 22
2 A is the olletor area in [ m ], and G is the instantaneous global solar radiation flux on the tilted plane 2 in [ W m ]. The thermal sensible power is the rate of sensible energy used to heat up the water, as in Equation (2). m.. T Q & w p D amb-95 h = Dt (2) To estimate the boiling effiieny and power, the numerator in the above equations are replaed by the orresponding latent heat expressions. The boiling power, Equation 3, is the rate of the energy used to boil a ertain mass of water in the elapsed time interval. m.h Q& w fg b = (3) Dt In Equation 3, h fg is the water latent heat in [J/(kg)], and t is the time elapsed in the boiling proess in [s]. The average latent effiieny is determined as the ratio of the energy used in the boiling proess to the integrated global radiation in the time interval, as expressed in Equation (4). 2.2 Experimental Measurements mw.h fg η boiling = (4) t A. G.dt To measure temperature in the ooking system, on the outside walls of the opper piping, type-k thermoouples are used. Ambient temperature is measured with a PT-1 and the wind speed with an anemometer. The global solar radiation flux on the tilted plane is measured with a pv-ell type sensor, whih has an error of 1% when the values are ompared with those from a preision pyranometer, The solar flux on the horizontal plane is measured by a preision pyranometer. The instruments are sanned every 2 seonds and the average values stored at every minute. In the experiments, 8 liters of water were used in the 1-liter pot to determine the sensible energy needed to raise the water temperature from ambient to 95 o C. Three thermoouples are use to determine the water temperature in the pot. The pot is also alibrated to allow the visual determination of the water evaporated during the boiling proess. 3. RESULTS Figure 2 shows the variation of the water temperature and the global solar flux during a sensible heating experiment. The measuring started as 12: o lok noon and lasted 32 minutes. The average value of the thermal sensible effiieny, as determined using Equation (1), using the measured data is,38. Other values of the thermal sensible effiieny presented in literature vary from,3 to,34 and this small differene an be assoiated with the different kind of oil used in this experiment. In this ase, syntheti thermal oil was used instead of vegetable oils due to problems assoiated with dissoiation at temperatures near 2 o C. The vegetable oils available were produed in pressing proess and presented this dissoiation phenomenon. The average flux of global radiation on the inlined plane was 786,1 W/m 2. The heating proess lasted 32 minutes to heat up 8 kg of water. The same proess with the heat from the storage tank lasted 37 minutes. RIO 2 - World Climate & Energy Event, January 6-11, 22 91
12 1 1 9 8 Temperature [ oc ] 8 6 4 7 6 5 4 3 Global solar flux [ W/m2 ] 2 Global solar flux Water temperature 1 4 7 13 19 26 31 36 37 Time [ min ] 2 1 Figure 2. Temperature of the water and solar global radiation flux during a sensible heating experiment Figure 3 shows the experimental data gathered during a heat harging (storage tank) experiment. This plot shows the temperature fields from sunrise to sunset time. The storage tank is 5L in size and has a spherial shape. The loation of the temperature sensors is: inlet and outlet of the solar olletors, and at two positions of the storage tank. These data are to be used in the alulation of the aumulated energy in the system and the temperature distribution in the tank. It is also shown the global solar flux. 18 16 1 Temperatura (oc) 14 12 1 8 6 8 6 4 Radiação Solar (W/m2) 4 2 Temperatura Saída do Coletor Temperatura Armazenamento 2 Temperatura Armazenamento 8 Temperatura no Retorno 2 Radiação Global Inlinada 1 2 3 4 5 6 7 Tempo (min) Figure 3. Temperature data in the system during a heat storage harging experiment (24.3.21) The results presented in these initial tests show the good performane of the solar ooker system in the onditions in Fortaleza. The thermal sensible effiieny reahed a slightly higher value than expeted beause of the adequate thermal properties of the oil used in the system. The latent effiieny was somewhat more diffiult to estimate. A new pot over was made to perform the tests and the latent effiieny value was,3 for an average global radiation on the horizontal plane of 664 W/m 2. In 92 RIO 2 - World Climate & Energy Event, January 6-11, 22
summary, the tests showed that the system an and should be used to prepare meal in institutions, shools, among others, beause of its very safe operation, apaity to ook large meals and keep them warm a whole day, and its use of a muh needed and environmentally friendly energy soure. 4. ACKNOWLEDGEMENTS The support from the Brazilian institutions SECITECE for finaning most of the equipment, and from CNPq for a researh fellowship has been muh appreiated. Also, thanks to FUNCAP (Brazil) and BMBF (Germany) for supporting travel expenses. 5. REFERENCES Vieira, M. E., Gomes, C. A. S., Teixeira, R. N. P., 1997, Thermal Effiieny of a Solar Cooking System with Storage. COBEM, São Paulo, Brazil Shwarzer, K., Krings, T., 1996, Demonstration- und Feldtest von Solarkohern mit temporärem Speiher in Indien und Mali. Shaker Verlag, Germany. RIO 2 - World Climate & Energy Event, January 6-11, 22 93