In this article we will discuss about:- 1. Introduction to Solar Cooker 2. Non-Acceptability of Present Solar Cookers 3. Design Proposal 4. System Design 5. Conclusion.
Introduction to Solar Cooker:
Efforts to design and introduce solar cookers have for many years concentrated on two main types the cooker with a plane collector (box type) and cooker with special/parabolic mirrors of average precision. The cost of second type is too high to be used economically in rural areas. The Government of India is trying to introduce box-type solar cookers with financial subsidies but with poor success.
Eighty per cent of the basic energy needs of rural India are the provision of cooking energy, which amounts to 0.5 to 1 kWh/person/day with a peak power of 0.2 to 1.2 kW per capita. It is mostly (for 83% of rural household) met from traditional fuels like fire-wood, agricultural wastes and cow-dung cakes.
Cooking with fire-wood and cow-dung cakes poses a number of problems for the users and for the society as a whole. The serious implications of deforestation and subsequent soil erosion, pollution and lack of recycling of the natural fertilisers have a significant effect on the overall productivity of these areas. Therefore, it becomes essential to find out an alternate source of cooking fuel.
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India is bestowed with abundant supply of solar energy and hence it can be a very promising source of cooking energy. Large scale solar cooking technology must be attempted to alleviate the demand for cooking fuel. Other non- conventional energy sources such as bio-gas would be best used to increase the industrial and agricultural productivity of rural areas, while allowing solar energy to provide the bulk of cooking requirements.
Non-Acceptability of Present Solar Cookers:
Despite Government efforts to introduce box-type solar cookers under subsidy programmes, they have failed to find a place in kitchens of the rural household contrary to the expectations. The beneficiaries’ do not accept solar cookers in daily cooking and their possession has made no impact on their conventional fuel consumption pattern.
i. Socio-Economic Factors:
Among the socio-economic factors inhibiting the large scale utilization of solar cookers, initial investment cost is one obstacle to the purchase of solar cooker. Families in rural areas can gather wood and cow-dung for fuel for Electrical India, Mumbai conventional cooking. Furthermore, solar cooking involves alteration in the household routine, since the housewife must prepare the evening meals during day time, well before the traditional time, which increases the additional problem of keeping the food hot.
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Secondly the housewife must remain in the hot sun while cooking. On rainy days or when there is insufficient sunshine, conventional fuels have to be used. There is lack of privacy as cooking has to be carried out in the open.
ii. Technical Drawbacks:
The technical drawbacks of the present box-type solar cooker patronised by the Government may be summarised as follows:
(a) The temperatures obtained in the cooking vessel (food) are less than 100°C. It does not ensure hygienic cooking as at some stage of cooking cycle, the temperature of the food must exceed 100°C to kill bacteria.
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(b) It takes more time than conventional cooking as it is only a low-temperature, low-energy flux device.
(c) Solar cooking cannot be employed for all modes of cooking processes. For example, chapati making and frying which are very important cooking processes in most part of the country especially north India cannot be carried out in a box-type solar cooker. The practice of conventional cooking is very different in India and is a big hinderance to the social acceptability of solar cooking. Solar cooking is very slow and time consuming and some foods have to be pre-cooked before keeping them in the cooker box for final cooking.
(d) There is no provision for storage or heat energy Cooking has to be carried out while the sun is shining which is an odd time for preparing dinner and breakfast. Also on rainy days and cloudy days when sufficient insolation is not available, auxiliary fuel has to be used.
(e) Cooking has to be carried out in the open without privacy and in the sun which is very inconvenient to the housewife. The glare of the mirror/glasses can be injurious to the eyes of the housewife and the accompanying children.
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iii. Study of M.S. University Baroda:
A study was conducted by Dr. (Mrs.) Rachel Georage, Reader, Home Management Department, Faculty of Home Science, M.S. University, Baroda, recently on reasons for non-acceptability of solar cookers and her findings are reported as follows:
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The figures indicate the per cent response to an opinion poll conducted in rural areas of Gujarat.
The effectiveness of solar cooker can be enhanced by the design of a system which can overcome the above technical and socio-economical problems. The cooker has to be coupled to a heat storage system. It must be capable of storing sufficient heat at a temperature high enough to permit effective cooking later on. It is also desirable to be able to regulate the rate of heat release for cooking.
Design Proposal of Solar Cooker:
In order to develop a small family size (four persons) solar cooker, a project was undertaken at Regional Engineering College, Kurukshetra. A solar cooker of hot-plate type was designed and successfully constructed. It consists of a flat plate type collector with mirror boosters. It is coupled to an oil storage insulated tank placed under the cooking platform inside the kitchen through a thermosyphon to the outside collector.
In can provide sufficient flux of heat energy at high enough temperature to a hot plate through a heat pipe. The system would be suitable for cooking of conventional foods at convenient time for breakfast, lunch and dinner inside the kitchen. It would just replace a bottled gas cooking range and would be fitted permanently. It will fit in with the age-old habits and practices of rural households and should be acceptable to the user without reluctance.
i. Design Data:
The following data has been assumed for the design of the cooker after carrying out detailed survey of heating habits, normalised menu for break-fast, lunch and dinner, cooking times and temperatures for different dishes, energy requirements and burner ratings of bottled gas cooking range. Average sunshine hours and solar radiations are taken for Kurukshetra from the handbooks.
ii. Component Sizing:
Assuming overall efficiency of 15%, specific heat of oil as 2.63 kJ/kg-K and oil density at 175°C as 0.905 kg/m3, the following sizes are worked out:
System Design of Solar Cooker:
The basic system design should respond to local climate: sunshine hours, solar radiation level, wind speed, ambient temperature and to energy demand characteristics. The functional flow diagram of the system is shown in fig. P. 7.1. It consists of the following sub-systems.
i. Solar Collection and Conversion System:
The flat plate collectors are easy to fabricate and cheaper than the concentrated type. In addition, flat plate collectors can absorb all the solar insolation, i.e., beam and diffused radiations whereas concentrated types are not sensitive to diffuse radiations.
This factor can be very important during cloudy days especially for Indian regions with long monsoon months. The problem of orientation and tracking can be easily handled in flat plate collectors. Concentrating parabolic collectors with special mirrors are not considered on economic and above mentioned technical factors.
The technology of flat plate collector is sufficiently developed for other applications such as space heating of small and large buildings, water heating, space cooling, process heat, crop drying, distillation, etc. It will be, therefore, easy to adopt it for solar cooking.
A temperature in the region of 200°C has been obtained in flat plate collector with selective coating of absorber plate and energy reception augmentation with the help of mirror boosters. In order to obtain a temperature of 475K, we can use augmented flat plates with a concentration ratio of nearly 4 and a/E ratio of 10.
In order to satisfy the conditions of concentration ratio, four booster plain mirrors are used to get the necessary augmentation. The selective coating can be either chrome black or nickel black with the properties given in Table P.7.1.
The arrangement consists of flat plate collector of 1m2 surface made from Derco Roll Bond Aluminium panels. Three panels of size 915 mm × 356 mm × 1.5 mm are arranged in parallel and housed in a properly insulated wooden box. The panels are coated with black chorme. Three window glass sheets 3 mm thick and 1000 mm × 1150 mm size are provided as top glazing to cut off losses due to re-radiation and convection from absorber plate.
Four plain mirrors of size 1068 mm × 915 mm are arranged at the four sides of the panel box with hinges to manipulate necessary tracking of the sun. The collector is installed at 30° (latitude of the site) with the horizontal. The top and bottom mirrors will need only seasonal adjustments whereas side mirrors can be adjusted every hour for optimum collection of solar radiations.
ii. Thermal Storage:
Thermal storage is used in most solar systems to provide a thermal capacitance effect. Thermal capacitance is required to damp out the fluctuations of collector energy delivery that occurs in response to short-term fluctuations in sunshine level and diurnal variations related to the 24 hours day-night cycle. In our case, we have to collect solar energy during day time only.
On a clear day the energy delivered by the solar system is approximately a sinusoidal function of time with a maximum at noon and minimum near sunrise and sunset. The demand on the thermal storage system will be variable and energy will be extracted three times a day for preparation of breakfast, lunch and dinner.
The temperature history of storage for a day will therefore, varies and condition of minimum temperature required for cooking must be met. Also on rainy and cloudy days when sufficient insolation is not available, stored heat should be available for cooking.
The amount of thermal storage is usually subject to economic constraints, since very large storage is generally prohibitively expensive. As a compromise, a storage tank of 100 litres capacity is provided which is equivalent to 3 kWh of heat at 175°C. The hot oil used as working fluid is also the sensible heat storage fluid as it has favourable heat capacity properties.
In order to minimize heat loss and tank material requirements, a right circular cylinder of size 55cm diameter and 55cm high fabricated from mild steel of 2mm thickness has been provided. It can be directly placed on the concrete floor. It is properly insulated with 75mm thick glass wool sheathed in aluminum sheet. A thermometer of range 0-300°C is provided in the thermowell welded into the storage tank.
iii. Fluid Circulation:
The practical use of solar energy requires a means to transport energy from the solar collector to the storage subsystem and in turn to the load or demand points. Any fluid flow is subject to viscous and turbulent resistance to flow that must be overcome by a motive force. The most common motive force is pressure generated by a pump which requires an external source of electrical power.
The natural tendency of a less dense fluid to rise above a more dense fluid can be used to cause fluid motion through a collector. The density difference is created within the solar collector where heat is added to the liquid. This thermosyphon system does not need any pump or electrical power supply.
Since the driving force in a thermosyphon system is only a small density difference and not a pump, larger-than-normal plumbing fixtures must be used to reduce pipe friction losses. We have used 1½ NB pipes. The flow rate through a thermosyphon system is about 40 litres/m2 in bright sunshine.
After sunset, a thrmosyphon system can reverse its flow direction and lose heat to the environment during night. A gravity-type non-return valve is provided in the return cold leg of thermosyphon to avoid reverse circulation of oil. The heat loss from the piping is controlled by proper insulation with asbestos rope.
The working fluid has to be stable at 200°C and should have favourable heat transfer properties. It has to be compatible with aluminium, copper and carbon steel, the materials of construction of solar thermal system. One hundred and ten litres of oil is required for initial fill of the system. Grade Servotherm Medium of Indian Oil Corporation or Hytherm 500 of Bharat Petroleum is suitable oils.
iv. Energy Distribution and Control:
The heat pipe is relatively new device for transporting large amounts of heat with a very small temperature drop between heat source and sink. A gravity- assisted heat pipe is used to distribute heat energy from storage tank to a cooking hot plate. The evaporate section of heat pipe is 5 cm long and is inserted vertically from top of storage tank. A condenser length of 10 cm is inserted inside the hot plate.
The adiabatic length to suit site conditions is properly insulated with asbestos rope. The heat pipe is made from 32 × 2 mm copper tube of 1 m length. The working fluid is Dowtherm grade E filled at atmospheric pressure after proper evacuation of air. The boiling point of Dowtherm E is 150°C. There are no wicks inside and liquid oil flows down to evaporator section along axial grooves (2mm × 1mm deep) cut all along inside of heat pipe.
Much controls and instrumentations are not needed. In addition to a thermometer provided in the oil storage tank, a piston-type valve is provided at the top of heat pipe in the condenser section. The rotation of a small wheel controls the travel of piston in the heat pipe and length of condenser taking part in heat transfer. The heat flow rate from the storage tank through the heat pipe will be proportional to effective condenser length not cut off by piston valve.
The hot plate 200mm diameter × 125 mm height is made from copper sheet and filled with hot oil for proper heat transfer from heat pipe. It is insulated from all sides except the top cooking surface. The heat pipe has been designed for axial heat flux of 2400 × 103K cal/m2 and radial heat flux of 1200 × 103Kcal/m2.
The total cost of materials including hot oil charge and fabrication expenses has been Rs. 3000.
Conclusion to the Design of Solar Cooker:
The proposed design of hot-plate-type solar cooker overcomes most of the technical and socio-economic drawbacks of the present box-type solar cooker. It fits well with the age-old habits of conventional cooking and should be accepted by the rural population in India without any reservations. It is a permanent fixture comparable in performance to the bottled gas cooking burner.
In heat flux rates and can provide controlled temperatures required for all types of cooking processes and at suitable timing for preparation of breakfast, lunch and dinner. She can cook in all privacy of her kitchen. It is coupled to a thermal storage system and can last for a day without sunshine. The heat flux rate can be conveniently controlled by turning a small wheel suitably placed at the hot plate. There are no operating and maintenance expenses and lifelong oil fill is provided.
The prototype cost of Rs. 3000 can be substantially reduced during commercial design and manufacture. With suitable Government subsidies as applicable to other solar devices, this is an ideal design for a low cost solar cooker for adoption in rural and muffasal areas.