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Utilization of the waste heat from internal combustion engines
Resources, Conservation and Recycling, 2 (1989) 297-304
297
Elsevier Science Publishers B.V./Pergamon Press plc - - Printed in The Netherlands
Short C o m m u n i c a t i o n
U t i l i z a t i o n of the Waste Heat from Internal Combustion E n g i n e s S. SUBRAMANYAM Kakatiya Institute of Technology & Science, Warangal - 506 015 (India)
(Received January 9, 1989; accepted in revised form March 28, 1989)
ABSTRACT
Subramanyam, S., 1989. Utilization of the waste heat from internal combustion engines. Resources Conserv. Recycl., 2" 297-304. Cooking devices could be evaluated for their performance using solar cookers as a bench mark. Accordingly, any device rejecting heat at 100°C and 80 W could be used for cooking. On this criteria diesel engine exhaust which rejects heat at 135-400 °C and 3.8 kW is an excellent source. Methods of designing cookers to meet different end-purposes are discussed. Feasibility of cooking different types of food under exhaust gas temperature regimes is indicated.
INTRODUCTION Cooking is possible using several sources of energy. Solar cookers are well u n d e r s t o o d devices a n d t h e y can be used as a c o n v e n i e n t b e n c h m a r k for evalu a t i n g o t h e r sources a n d devices. A n y source which supplies e n e r g y at a similar or b e t t e r quality a n d q u a n t i t y can be c o n s i d e r e d as a suitable source for exploitation. T e m p e r a t u r e m e a s u r e m e n t s inside a solar cooker indicate t h a t rarely do p a r t s (including air) r e a c h 200 ° C. T h e c o o k e r itself b e c o m e s active at a b o u t 90 ° C. If the inside t e m p e r a t u r e reaches 130 ° C, it could be said to be working well. T h e r e f o r e , a n y h e a t source r e a c h i n g a t e m p e r a t u r e of 120 ° should be c o n s i d e r e d feasible. If a source has a t e m p e r a t u r e of a r o u n d 120°C, the principle m o d e s of h e a t t r a n s f e r would be c o n d u c t i o n a n d convection. U n d e r n o r m a l conditions, t h e air t e m p e r a t u r e would r e a c h a r o u n d 100°C which should be sufficient to induce cooking. T h e rate of h e a t flow has a n i m p o r t a n t b e a r i n g on the time required for cooking. T h e s h o r t e r t h e t i m e required, t h e greater will be the a t t r a c t i o n of the oven. A solar c o o k e r could have a m a x i m u m i n p u t rate of a r o u n d 2 k W / m 2 of a p e r t u r e u n d e r c o n d i t i o n s of m a x i m u m solar e l e v a t i o n a n d with suitable mirror boosters. S u c h favorable c o n d i t i o n s last only a few days a year. E v e n t h e n ,
0921-3449/89/$03.50
© 1989 Elsevier Science Publishers B.V./Pergamon Press plc
298
the favorable conditions last only for a short time, around noon. Considering the mean rate of energy input over a year and during its span of effective availability (ca. 6 h / d a y ) , it is not likely to be more t h a n 0.6 k W / m 2. The solar cookers commercially manufactured are of sizes 0.1-0.25 m 2. Therefore, it can be concluded that they operate on an input of 60-70 W at the minimum. Thus, any source that rejects heat at the rate of 80 W or more can be useful for cooking food. HEAT SOURCES
There are a number of devices that reject heat at the rates and temperature mentioned above. All internal combustion engines utilize heat at rather low levels of efficiency, of around 30%. Nearly equal amounts are lost through the cylinder head and exhaust. The temperature of the cooling water in a watercooled engine is about 65 °C so does not meet the criterion for utilization derived above. On the other hand, the heat carried out by the exhaust stream is about 135-430 °C and, therefore, a good source for cooking. The range of temperature measured in the laboratory (while testing commercial engines) is indicated in Table 1. TABLE1 Extract from observations of performance test on a commercial 3.8 kW diesel engine Output (kW)
Exhaust gas temperature (°C)
Remarks
0 0.933 1.865 2.978 3.780
135 180 235 330 430
Engine idling (3.8 kW 1500 rpm stationery diesel engine )
QUANTUM OF HEAT AVAILABLE
The amount of energy available for utilization can be estimated. Consider the smallest size among the engines, an output of 3.7 kW (5 Bhp). Such petrol engines are not used in stationary applications in India as the cost of generating power through t h e m is 4 to 5 times that of a diesel engine. The smallest petrol engines of 20-25 cm 3 displacement are used in knapsack sprayers and 50-60 cm 3 engines in mopeds. A new trend has been introduced of generator sets to provide stand-by power during load shedding in summer months. Since the power shut down is a regular and programmed event, these small generators are a reliable source of thermal energy. The portable generators produce power at ratings of 0.6-3.6 kW or more.
299 The power carried away by the exhaust gases is nearly equal to the mechanical power delivered through the piston. Therefore, 0.7-3.6 kW of energy can be available from exhausts of petrol engines and 3.7 kW or more from small diesel engines. Normally, the small petrol driven generators are in commercial establishments in the cities and diesel engines in the rural areas. Estimates of the number of small stationary diesel engines in Indian villages are greater than 450 000. Most of them are used for pumping water and are operated usually for periods ranging from 1-3 hours, depending on the availability of water in the well. The heat energy available from these engine exhausts is a superior source compared to solar energy. In fact, the heat from the exhaust of the smallest diesel engine at 3.6 kW is far greater than the requirements of an ordinary household for cooking and water heating. A diesel engine working for 2 hours should be capable for meeting the energy requirements of at least 2 or 3 rural families. UTILIZATIONOF WASTEHEAT The temperatures at which heat is rejected by engines is ideal for cooking, water heating, drying, etc. It could also be stored in a buffer so as to de-link the operation of the engine from the utilization of the waste heat. Simple (and effective ) or sophisticated devices could be built to utilize the heat. The usual factors of technological complexity and cost would determine the usefulness, adaptability and popularity. HEAT UTILIZATIONDEVICES - OVENS The oven to be coupled to a 3.7 kW (5 Bh) diesel engine is shown in Fig. 1. It is a well insulated and reasonably air tight box with interior dimensions 600 L × 400 D × 600 H mm 3. A silencer-radiator is located near the back wall. The radiator is mounted on a suitable vibration damping mount (not shown in the figure) and provided with fins to improve heat transfer. The silencer is of the labyrinth type, the inner details of which are shown schematically. The radiator is shielded by a welded mesh enclosure to prevent accidental burns. In the remainder of the area, suitable grill type shelving is provided to support cooking vessels. It is essential that all sides, including the bottom of the vessels, be contacted by the hot circulating air. The vessels should preferably be of thin-walled aluminum and have a firm fitting lid. Wide, shallow vessels are preferred to tall, narrow ones. Stainless steel, which has a conductivity 1/ 10 of aluminum, should be avoided.
300 OUTLET
HIGHTEMPFLEXIBLE HOSE
OIE
"~
ENGINE HEAT TRANSFER
~
~
II
-4 I
'NS INSULATION ~
'
II_ .
~
DOOR
\1 SHELVESFORVESSELS MADEOFVVELDMESH
~
LET
EXHAUST GASINLET
SECTIONALELEVATION
E
RIBS SECTIONALPLAN
Fig. 1. Exhaust heat oven. INSULATION Contrary to conventional practice, two stages of insulation could be used instead of a single layer of material. At present, expanded polystyrene (EP) is the least expensive and most effective. It has a drawback of a low melting point of around 150°C. To overcome this, a thin layer of insulation that can withstand higher temperatures should be placed next to the contact face. At the outer face of such insulation, the temperature would have dropped to levels below 150°C. A thick layer of EP (25-35 mm) could be added to prevent further heat losses. If otherwise acceptable, the inner cladding of the oven can be asbestos cement plain sheets and insulation of a layer of EP and a small air gap of 3 mm between the two materials could be used.
301 PERFORMANCE
Assuming the following, the performance of the oven can be predicted assuming: - the engine is fully loaded and performing at an output of 3.7 kW; - specific fuel consumption is 150 g kW -1 h - l ; - oven efficiency is 0.25; - heat carried out by the exhaust gases-- 30% of input. Under these conditions, the oven should be capable of cooking 3.5 to 4.0 kg of rice per hour after an initial warm-up period of about 15 minutes. This capacity represents a high level of performance as the cooked rice would be sufficient for about 25 or 30 persons for a single meal. Performance evaluation of other types of cooking are a little more difficult. It can be roughly assumed that the 1 h stagnation temperature would be 2540 ° C less than the exhaust gas temperature. Baking can also be considered the most critical of the cooking processes. On these bases, the parameters in Table 2 can be predicted. TABLE2 Exhaust temperature and cooking feasibility Exhaust gas temperature ( ° C)
Types of food cooked/baked
150
Rice, milk products, eggs, vegetables
200
Pulses, fish and other white meats, cakes, pastries
250
Red meats of thin sections, biscuits, candied sweets
>1300
Meat sections of all sizes, bread, and anything else
EXOTIC DESIGNS
The conventional design described could be used as a base for developing exotic designs aimed at modifying ordinary devices to serve several different end uses. Usually, the following will be the basis for such developmental work: improving performance, extending the range of operation, carrying out special tasks, making the device appealing to the users by adopting aesthetic or user convenience criteria. A few approaches of such designs are suggested here. An impression has been given so far that the conventional expansion type silencer is introduced directly into the oven. By implication, the material of construction of such a silencer (i.e., mild steel) is also adopted. This, no doubt, would provide a cheap and workable device. However, change of both material and shape should improve the performance significantly. Consider the data of
302 TABLE 3 Heat conductionof metals Material
Thermal conductivityat different temperatures ( W / m -I K -~)
100°C 200°C 300°C 400°C Mild steel 0.5% carbon 51.9 Pure aluminum 206.0 Pure copper 379.0
48.5 45.0 41.5 214.6 228.5 249.2 373.9 368.7 363.6
Table 3 [ 1 ]. Perusal of Table 3 indicates that copper would be the ideal material. Further, when operated at temperatures around 300-400 ° C, it would acquire a thin layer (several ~tm thick) of cupric oxide. By virtue of its black color, a cupric oxide surface is also an excellent radiator. The density of copper is almost three times that of aluminum and costs about 1.5 times as much as one made of aluminum. An aluminum radiator would perform below the levels of one of copper but is better than mild steel by a factor of four. The performance of the aluminum radiator could be further marginally improved by giving its external face a thin coat of suitable black coating. Thus, it is seen that in cases where volume of material used is small, and cost of fabrication is the major cost, copper should be the preferred material. This criterion is satisfied by ovens attached to small petrol driven generator sets. In their case, there is also an operational need for higher efficiency as the total available heat would be small. In the case of an oven attached to a diesel engine, perhaps aluminum would be a better choice.
OVENWITH INTERNALBUFFER STORAGE A buffer storage is a device which evens out the fluctuations in supply and demand. Excess supplies of heat will be dumped in the storage and drawn when demand exceeds supply. Such a buffer storage of heat could, to an extent, delink the operation cycle of the diesel engines and period of cooking. Heat from an engine operating intermittently could also be used effectively. The heat storage could be in the form of various low temperature melting salt combinations suggested in the solar energy literature [2]. The heat transfer fluid could be either ethylene glycol or any of the heat transfer or thermal fluids. The thermal fluid circulates between the silencer and heat storage mass by convection. A possible system is shown in Fig. 2.
303 FRYING OF FOODS Moving up the ladder of affluence from the poverty line, the ratio of fried foods to the total increases. The strident and persistent rise in the demand for cooking oils is an indicator of improvement in life styles in India. One of the major criticisms of the solar cooking devices is their inability to cook by frying. This drawback can be overcome in the new oven provided with buffer heat storage. A slight design modification could enable the heat stored in the buffer to be used for frying. Details of the modifications to buffer storage required for cooking are shown in Fig. 3. A cavity is made in the top of the buffer storage to accommodate the frying pan which should fit snugly into the cavity to improve heat transfer. Air pockets, however thin or small, are to be avoided. Introduction of a thin layer of heat transfer oil between the cavity and pan could solve the contact problem. \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ ~--~
OVEN ENCLOSURE "IN5 POINT TURE
i.)i .... "
...... i ii ii
INCORPORATINGFLUIDPASSAGES SHADEDAREAACTSAS A FIN TO ASSIST HEATTRANSFER
Fig. 2. Bufferheat storagesystem. INSULATEDLID TO [LOSE CAVITY ,
~
U
_
S
E
_
TRANSFER ~r~ BUFFERHEAT STORAGESILENCER
Fig. 3. Fry pan adapter.
FRYINGPAN
CCOMOOATE
304 The introduction of oil, however, may not be desirable as accidental spilling may contaminate food as well as being a likely cause of burns.
O T H E R APPLICATIONS OF HEAT SOURCES Because the heat from engine exhaust is likely to be much greater than the demand for cooking, other applications could also be thought off:water heating, drying of agricultural or other produce, or space heating in winter in northern latitudes.There are possibilitiesfor industrial uses, as well. The economic criteriathat justifythe need for developing and using devices such as described here must include that the opportunity costs of the incremental availabilityof energy is much higher in periods of shortage than in other periods.
REFERENCES 1 C.P.Kodandar~mRn and S.Subramanyan.Heat and Mass TransferData Book.WileyEastern Ltd,New Delhi,1975,6 pp. 2 A.B.Meineland P.M. Meinel.AppliedSolarEnergy.An Introduction.AddisonWesley,Reading,MA., 1976.
297
Elsevier Science Publishers B.V./Pergamon Press plc - - Printed in The Netherlands
Short C o m m u n i c a t i o n
U t i l i z a t i o n of the Waste Heat from Internal Combustion E n g i n e s S. SUBRAMANYAM Kakatiya Institute of Technology & Science, Warangal - 506 015 (India)
(Received January 9, 1989; accepted in revised form March 28, 1989)
ABSTRACT
Subramanyam, S., 1989. Utilization of the waste heat from internal combustion engines. Resources Conserv. Recycl., 2" 297-304. Cooking devices could be evaluated for their performance using solar cookers as a bench mark. Accordingly, any device rejecting heat at 100°C and 80 W could be used for cooking. On this criteria diesel engine exhaust which rejects heat at 135-400 °C and 3.8 kW is an excellent source. Methods of designing cookers to meet different end-purposes are discussed. Feasibility of cooking different types of food under exhaust gas temperature regimes is indicated.
INTRODUCTION Cooking is possible using several sources of energy. Solar cookers are well u n d e r s t o o d devices a n d t h e y can be used as a c o n v e n i e n t b e n c h m a r k for evalu a t i n g o t h e r sources a n d devices. A n y source which supplies e n e r g y at a similar or b e t t e r quality a n d q u a n t i t y can be c o n s i d e r e d as a suitable source for exploitation. T e m p e r a t u r e m e a s u r e m e n t s inside a solar cooker indicate t h a t rarely do p a r t s (including air) r e a c h 200 ° C. T h e c o o k e r itself b e c o m e s active at a b o u t 90 ° C. If the inside t e m p e r a t u r e reaches 130 ° C, it could be said to be working well. T h e r e f o r e , a n y h e a t source r e a c h i n g a t e m p e r a t u r e of 120 ° should be c o n s i d e r e d feasible. If a source has a t e m p e r a t u r e of a r o u n d 120°C, the principle m o d e s of h e a t t r a n s f e r would be c o n d u c t i o n a n d convection. U n d e r n o r m a l conditions, t h e air t e m p e r a t u r e would r e a c h a r o u n d 100°C which should be sufficient to induce cooking. T h e rate of h e a t flow has a n i m p o r t a n t b e a r i n g on the time required for cooking. T h e s h o r t e r t h e t i m e required, t h e greater will be the a t t r a c t i o n of the oven. A solar c o o k e r could have a m a x i m u m i n p u t rate of a r o u n d 2 k W / m 2 of a p e r t u r e u n d e r c o n d i t i o n s of m a x i m u m solar e l e v a t i o n a n d with suitable mirror boosters. S u c h favorable c o n d i t i o n s last only a few days a year. E v e n t h e n ,
0921-3449/89/$03.50
© 1989 Elsevier Science Publishers B.V./Pergamon Press plc
298
the favorable conditions last only for a short time, around noon. Considering the mean rate of energy input over a year and during its span of effective availability (ca. 6 h / d a y ) , it is not likely to be more t h a n 0.6 k W / m 2. The solar cookers commercially manufactured are of sizes 0.1-0.25 m 2. Therefore, it can be concluded that they operate on an input of 60-70 W at the minimum. Thus, any source that rejects heat at the rate of 80 W or more can be useful for cooking food. HEAT SOURCES
There are a number of devices that reject heat at the rates and temperature mentioned above. All internal combustion engines utilize heat at rather low levels of efficiency, of around 30%. Nearly equal amounts are lost through the cylinder head and exhaust. The temperature of the cooling water in a watercooled engine is about 65 °C so does not meet the criterion for utilization derived above. On the other hand, the heat carried out by the exhaust stream is about 135-430 °C and, therefore, a good source for cooking. The range of temperature measured in the laboratory (while testing commercial engines) is indicated in Table 1. TABLE1 Extract from observations of performance test on a commercial 3.8 kW diesel engine Output (kW)
Exhaust gas temperature (°C)
Remarks
0 0.933 1.865 2.978 3.780
135 180 235 330 430
Engine idling (3.8 kW 1500 rpm stationery diesel engine )
QUANTUM OF HEAT AVAILABLE
The amount of energy available for utilization can be estimated. Consider the smallest size among the engines, an output of 3.7 kW (5 Bhp). Such petrol engines are not used in stationary applications in India as the cost of generating power through t h e m is 4 to 5 times that of a diesel engine. The smallest petrol engines of 20-25 cm 3 displacement are used in knapsack sprayers and 50-60 cm 3 engines in mopeds. A new trend has been introduced of generator sets to provide stand-by power during load shedding in summer months. Since the power shut down is a regular and programmed event, these small generators are a reliable source of thermal energy. The portable generators produce power at ratings of 0.6-3.6 kW or more.
299 The power carried away by the exhaust gases is nearly equal to the mechanical power delivered through the piston. Therefore, 0.7-3.6 kW of energy can be available from exhausts of petrol engines and 3.7 kW or more from small diesel engines. Normally, the small petrol driven generators are in commercial establishments in the cities and diesel engines in the rural areas. Estimates of the number of small stationary diesel engines in Indian villages are greater than 450 000. Most of them are used for pumping water and are operated usually for periods ranging from 1-3 hours, depending on the availability of water in the well. The heat energy available from these engine exhausts is a superior source compared to solar energy. In fact, the heat from the exhaust of the smallest diesel engine at 3.6 kW is far greater than the requirements of an ordinary household for cooking and water heating. A diesel engine working for 2 hours should be capable for meeting the energy requirements of at least 2 or 3 rural families. UTILIZATIONOF WASTEHEAT The temperatures at which heat is rejected by engines is ideal for cooking, water heating, drying, etc. It could also be stored in a buffer so as to de-link the operation of the engine from the utilization of the waste heat. Simple (and effective ) or sophisticated devices could be built to utilize the heat. The usual factors of technological complexity and cost would determine the usefulness, adaptability and popularity. HEAT UTILIZATIONDEVICES - OVENS The oven to be coupled to a 3.7 kW (5 Bh) diesel engine is shown in Fig. 1. It is a well insulated and reasonably air tight box with interior dimensions 600 L × 400 D × 600 H mm 3. A silencer-radiator is located near the back wall. The radiator is mounted on a suitable vibration damping mount (not shown in the figure) and provided with fins to improve heat transfer. The silencer is of the labyrinth type, the inner details of which are shown schematically. The radiator is shielded by a welded mesh enclosure to prevent accidental burns. In the remainder of the area, suitable grill type shelving is provided to support cooking vessels. It is essential that all sides, including the bottom of the vessels, be contacted by the hot circulating air. The vessels should preferably be of thin-walled aluminum and have a firm fitting lid. Wide, shallow vessels are preferred to tall, narrow ones. Stainless steel, which has a conductivity 1/ 10 of aluminum, should be avoided.
300 OUTLET
HIGHTEMPFLEXIBLE HOSE
OIE
"~
ENGINE HEAT TRANSFER
~
~
II
-4 I
'NS INSULATION ~
'
II_ .
~
DOOR
\1 SHELVESFORVESSELS MADEOFVVELDMESH
~
LET
EXHAUST GASINLET
SECTIONALELEVATION
E
RIBS SECTIONALPLAN
Fig. 1. Exhaust heat oven. INSULATION Contrary to conventional practice, two stages of insulation could be used instead of a single layer of material. At present, expanded polystyrene (EP) is the least expensive and most effective. It has a drawback of a low melting point of around 150°C. To overcome this, a thin layer of insulation that can withstand higher temperatures should be placed next to the contact face. At the outer face of such insulation, the temperature would have dropped to levels below 150°C. A thick layer of EP (25-35 mm) could be added to prevent further heat losses. If otherwise acceptable, the inner cladding of the oven can be asbestos cement plain sheets and insulation of a layer of EP and a small air gap of 3 mm between the two materials could be used.
301 PERFORMANCE
Assuming the following, the performance of the oven can be predicted assuming: - the engine is fully loaded and performing at an output of 3.7 kW; - specific fuel consumption is 150 g kW -1 h - l ; - oven efficiency is 0.25; - heat carried out by the exhaust gases-- 30% of input. Under these conditions, the oven should be capable of cooking 3.5 to 4.0 kg of rice per hour after an initial warm-up period of about 15 minutes. This capacity represents a high level of performance as the cooked rice would be sufficient for about 25 or 30 persons for a single meal. Performance evaluation of other types of cooking are a little more difficult. It can be roughly assumed that the 1 h stagnation temperature would be 2540 ° C less than the exhaust gas temperature. Baking can also be considered the most critical of the cooking processes. On these bases, the parameters in Table 2 can be predicted. TABLE2 Exhaust temperature and cooking feasibility Exhaust gas temperature ( ° C)
Types of food cooked/baked
150
Rice, milk products, eggs, vegetables
200
Pulses, fish and other white meats, cakes, pastries
250
Red meats of thin sections, biscuits, candied sweets
>1300
Meat sections of all sizes, bread, and anything else
EXOTIC DESIGNS
The conventional design described could be used as a base for developing exotic designs aimed at modifying ordinary devices to serve several different end uses. Usually, the following will be the basis for such developmental work: improving performance, extending the range of operation, carrying out special tasks, making the device appealing to the users by adopting aesthetic or user convenience criteria. A few approaches of such designs are suggested here. An impression has been given so far that the conventional expansion type silencer is introduced directly into the oven. By implication, the material of construction of such a silencer (i.e., mild steel) is also adopted. This, no doubt, would provide a cheap and workable device. However, change of both material and shape should improve the performance significantly. Consider the data of
302 TABLE 3 Heat conductionof metals Material
Thermal conductivityat different temperatures ( W / m -I K -~)
100°C 200°C 300°C 400°C Mild steel 0.5% carbon 51.9 Pure aluminum 206.0 Pure copper 379.0
48.5 45.0 41.5 214.6 228.5 249.2 373.9 368.7 363.6
Table 3 [ 1 ]. Perusal of Table 3 indicates that copper would be the ideal material. Further, when operated at temperatures around 300-400 ° C, it would acquire a thin layer (several ~tm thick) of cupric oxide. By virtue of its black color, a cupric oxide surface is also an excellent radiator. The density of copper is almost three times that of aluminum and costs about 1.5 times as much as one made of aluminum. An aluminum radiator would perform below the levels of one of copper but is better than mild steel by a factor of four. The performance of the aluminum radiator could be further marginally improved by giving its external face a thin coat of suitable black coating. Thus, it is seen that in cases where volume of material used is small, and cost of fabrication is the major cost, copper should be the preferred material. This criterion is satisfied by ovens attached to small petrol driven generator sets. In their case, there is also an operational need for higher efficiency as the total available heat would be small. In the case of an oven attached to a diesel engine, perhaps aluminum would be a better choice.
OVENWITH INTERNALBUFFER STORAGE A buffer storage is a device which evens out the fluctuations in supply and demand. Excess supplies of heat will be dumped in the storage and drawn when demand exceeds supply. Such a buffer storage of heat could, to an extent, delink the operation cycle of the diesel engines and period of cooking. Heat from an engine operating intermittently could also be used effectively. The heat storage could be in the form of various low temperature melting salt combinations suggested in the solar energy literature [2]. The heat transfer fluid could be either ethylene glycol or any of the heat transfer or thermal fluids. The thermal fluid circulates between the silencer and heat storage mass by convection. A possible system is shown in Fig. 2.
303 FRYING OF FOODS Moving up the ladder of affluence from the poverty line, the ratio of fried foods to the total increases. The strident and persistent rise in the demand for cooking oils is an indicator of improvement in life styles in India. One of the major criticisms of the solar cooking devices is their inability to cook by frying. This drawback can be overcome in the new oven provided with buffer heat storage. A slight design modification could enable the heat stored in the buffer to be used for frying. Details of the modifications to buffer storage required for cooking are shown in Fig. 3. A cavity is made in the top of the buffer storage to accommodate the frying pan which should fit snugly into the cavity to improve heat transfer. Air pockets, however thin or small, are to be avoided. Introduction of a thin layer of heat transfer oil between the cavity and pan could solve the contact problem. \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ ~--~
OVEN ENCLOSURE "IN5 POINT TURE
i.)i .... "
...... i ii ii
INCORPORATINGFLUIDPASSAGES SHADEDAREAACTSAS A FIN TO ASSIST HEATTRANSFER
Fig. 2. Bufferheat storagesystem. INSULATEDLID TO [LOSE CAVITY ,
~
U
_
S
E
_
TRANSFER ~r~ BUFFERHEAT STORAGESILENCER
Fig. 3. Fry pan adapter.
FRYINGPAN
CCOMOOATE
304 The introduction of oil, however, may not be desirable as accidental spilling may contaminate food as well as being a likely cause of burns.
O T H E R APPLICATIONS OF HEAT SOURCES Because the heat from engine exhaust is likely to be much greater than the demand for cooking, other applications could also be thought off:water heating, drying of agricultural or other produce, or space heating in winter in northern latitudes.There are possibilitiesfor industrial uses, as well. The economic criteriathat justifythe need for developing and using devices such as described here must include that the opportunity costs of the incremental availabilityof energy is much higher in periods of shortage than in other periods.
REFERENCES 1 C.P.Kodandar~mRn and S.Subramanyan.Heat and Mass TransferData Book.WileyEastern Ltd,New Delhi,1975,6 pp. 2 A.B.Meineland P.M. Meinel.AppliedSolarEnergy.An Introduction.AddisonWesley,Reading,MA., 1976.