Liquefied Petroleum Gas (LPG) is the predominant cooking fuel used in urban areas of India as cooking fuel. Although many cooking devices based on solar energy exists, they still lack the convenience provided by the LPG stove for Indian cooking methods. In box-type solar cookers, cooking happens outdoor at a slow rate. Hotbox ovens are limited to cooking during clear sky periods. Parabolic cookers are relatively effective but the cooking rate cannot be controlled. In a country blessed with abundant sunshine, solar energy can contribute considerably towards meeting domestic cooking energy requirements if it could be more user friendly.
Although systems and products exist based on the conversion of solar energy into electrical energy, the project aims to propose an alternative approach in the use of solar energy as a cooking fuel in Indian households by creating usable product systems.
To store the heat to be able to carry out the cooking during any time of the day, pressurized water vessel, phase change material, and pebble-bed thermal energy storage (TES) systems have been proposed and taken to the prototyping and product stages by various people and organizations. Split-system solar cooking devices, which has its solar collector outdoors and the cooking unit inside the kitchen, with heat pipes transferring the energy between the two, have been taken to the proof of concept stage. The project thus aims to explore the possibilities and bring forward a plausible application of the technology of storage and transfer of solar energy for cooking purposes in Indian kitchens.
Major design considerations for the efficiency of the system:
1. The diameter of the transport pipe is exclusive to the system.
2. The type of heat storage plays a very important part of the design of the cooking unit. Due to the technical complexity and high costs involved, 'sensible heat storage medium' (oil-based) is chosen for the system.
2. For the collector loop- the optimal flow rate is not fixed for a given system. Hence one has to vary the flow rate accordingly in order to extract optimal energy from the sun.
3. For the heat storage tank- the fluid should be taken from the top of the storage tank which has the highest temperature due to density stratification.
4. For load side loop- Power transferred to the load can be controlled by varying the mass flow rate in the load side loop.
5. For the heat exchanger(stove)- The contact area of the pipes carrying hot oil should be maximized with the body to be heated and the diameter of the pipe should be consistent to avoid change in oil pressure.
A schematic diagram of the system proposed is shown in the figure. The design of the cooking unit to be kept in the kitchen is further explored. The proposed system will have an ‘in and out’ oil pipe installed in the kitchen over which the cooking unit can be fixed.
Major observations regarding the user:
1. Kitchens usually have very little space. Another device as big as the LPG stove would be tough to accommodate in the kitchen.
2. Traditional cooking methods, Cooking time, cleaning, and maintenance are the major considerations. Users have no issues or hassles with an LPG connection in the apartment. They find it very affordable too. Induction cooktop, rice cooker, and microwave are the major devices used other than LPG.
3. Dry vegetables, deep frying, etc leaves brown marks in the utensils which are hard to clean.
4. Dal (lentils), rajma (kidney beans), red meat, etc needs deep vessel with a lid or a pressure cooker.
5. Kadhai (cauldron/wok) has to be given support while cooking.
6. Cooking dry vegetables need a rounded bottom for even heating.
7. Dosa tawa (griddle) and idli maker are separate but rarely used.
8. Baked vegetables and baked fish is sometimes made in the microwave in borosil dishes.
The initial ideations were done based on the proposed system of heat storage and transfer. As mentioned earlier, the ‘in and out’ oil pipes in the kitchen area to be connected with the cooking unit (heat exchanger). The ideations fall into three main categories:
1. Tubes circulating hot oil are immersed in the liquid medium. The analogy of an electric rod heater.
2. Tubes circulating hot oil integrated with the vessel in which food has to be cooked.
3. Tubes circulation hot oil heating the vessel containing food through direct contact (conduction)
The ideations were further combined and distilled into three major concepts keeping in mind the usability of the product systems.
It was realized that all three concepts have their own advantages and disadvantages. Flexibility in the usage of vessels/utensils is an important factor as the system will be used as a substitute for an LPG stove. Concept 3 doesn’t provide any possibility to use existing vessels/utensils but can be ideal for cookers/pressure cookers. It was realized in the user study that pressure cookers are an integral part of Indian kitchens.
A system containing all the three concepts can suffice for all the common vessels/utensils in the Indian kitchen. Furthermore, modularity in units can be incorporated into a design where users can choose the type according to their requirements.
The project aims to bring new technologies and research forefront and show their practical application. Like photovoltaic cooking, the technology of solar heat transfer and storage has the potential to become common ground. Although, to create usable product systems for Indian kitchens, complete reliance on solar power shall not be appropriate at the nascent stage of the applications of this technology.
The system is hybrid consisting of solar-powered cooking units as well as cooking units powered by LPG or electricity (induction plate) can be plausible for Indian kitchens. Solar-powered units will reduce the use of conventional energy whereas LPG and induction plate can serve as secondary sources to compensate for the unpredictability of solar cooking.
The height of the cooking unit with respect to the oil storage tank is variable in an apartment typology. This makes the system with an internally fixed pump impractical. Therefore, a system with an externally fixed pump depending upon the height of the cooking system with respect to the storage tank is proposed. Moreover, to make the system independent of the externally supplied energy, the pump and other electrical devices are powered by photovoltaic panels installed on the apartment terrace.
The system was also becoming restrictive in terms of transportation of the hot oil. Factors such as insulation, servicing, and repairing and safety calls for a dedicated shaft for the pipelines. Like electrical and plumbing shafts, it is hypothesized for the project that future apartment buildings will be having a provision for the transportation of hot oil.
As the technology is in the initial stage. A hybrid system at this point might hinder the workflow of the cooking process due to the inconsistency posed by the solar-based system as well as the lack of swift heat transfer in many of the utensils.
A small independent solar cooktop in a kitchen corner that provides an alternative cooking option depending upon the user convenience of time, cooking method, and vessel/utensil is proposed. The system will have a single cooktop for flat bottom vessels.
