|Boiling Point No. 27 - April 1992 (ITDG - ITDG, 1992, 40 p.)|
Update from BP25 by Stephen Fisher - Project Manager, Micro-Hydro Programme, ITDG
The heat storage cooker has been developed by ITDG in Nepal in collaboration with a UK company, Dulas Engineering. The main aims of the work are to investigate materials and processes which enable cooking to be undertaken with reliable, low-cost heat storage devices using electricity generated at micro-hydro installations. This will assist in smoothing energy demand peaks and raising load factors at micro-hydro schemes. Work on establishing the cost, acceptability, safety and performance constraints is designed to make heat storage cookers a realistic alternative to current cooking methods.
The air transfer heat storage cooker involves the retention of energy in a container of small (6mm) stones. This heat store measures 280 mm in diameter and 280 mm deep and forms the core of the cooker. Insulation is provided by a 200 mm thick layer of rice husk ash contained within an outer shell of wood or clay. Overall efficiency of energy transfer for the cooker is around 25%.
Heat is extracted for cooking by switching on a low power (35 watt) electric fan to blow air from outside the cooker through a pipe and up through the heat store from bottom to top. The outlet pipe leaves the top of the heat store and passes down to below the bottom of the store before flowing to the cooking hob. This 'trap' arrangement prevents air escaping from the store through convection. At the hob, heat is transferred to the cooking vessel by directing the hot air through a number of small holes or nozzles of l.5mm diameter in a flat plate. Two hobs are supplied - a boiling or frying ring containing 125 holes and a simmering ring of 20 holes. Air temperatures of over 300°C are maintained for a total of around 2 hours per day enabling, for example, water to be boiled at an average rate of 1 litre in 10 minutes.
The design of the cooker has been governed by social science work carried out in Nepal. In order that access is widened to as many households as possible, the charging of the heat store (that period when the element is switched on) has been designed to fit a typical Nepali daily cooking pattern. Households with a connection to the electricity supply of only 250 watts charge the heat store for around 18 hours each day, using a toggle switch to divert that electricity to a separate circuit when either the lights and/ or the cooking fan are required.
Households having a connection of greater power, for example 500 watts, are able to charge the heat store constantly.
In technical terms the design has proved to be successful. Manufacture of the first cookers began in Kathmandu in November 1991, where use of local components and processes has enabled further development by the three manufacturers involved in the work. For example, the Ceramics Promotion Project in Bhaktapur is investigating the performance of clay components to allow production of the cooker in rural areas and at lower cost. At present only the rice husk ash and outer container are available in rural areas, other components being transported from Kathmandu. Other manufacturers are looking at combinations of stainless and galvanised steel. The target cost of production is Nepalese Rs 2,000 (US $47).
Fig 1 Air Transfer Heat Storage Cooker
A total of 20 cookers is being installed in the villages of Ghandruk and Salleri between February and April 1992 with the cooperation of local people and village organisations. The process of manufacture and installation involves using the initial reactions of householders to the cooker to influence the manufacture of following batches, so that "user-led" modifications are included from the outset. Comprehensive field testing and monitoring of the cookers will continue until January 1993.
Heat storage technology has the potential to offer a number of benefits to people living in rural communities. Where electricity is supplied from micro-hydro schemes which sell power in watts at a flat rate rather than energy through a kW-hour meter, users do not incur an additional cost for use of the cooker. Pressure on forest resources and time spent collecting firewood are likely to be reduced. The cooker itself is easy to use and is smokeless. The use of air as the heat transfer medium allows a range of cooking vessels to be used, including dented or damaged pans, without any significant change in performance. Heat storage technology has the potential to be scaled up for commercial activities such as crop drying end paper making.