| Boiling Point No. 25 - August 1991 |
by Rod Edwards, Assistant Programrne Manager, MicroHydro Programrne, ITDG
Domestic cooking is one use to which electricity from micro-hydro schemes may be put and which has become increasingly popular in recent years. Why is this?
There are probably three basic reasons. Firstly, traditional fuels such as fuelwood are rapidly becoming scarcer and more expensive in many areas, and householders often have to travel considerable distances to gather enough for their needs. Secondly, cooking with electricity offers benefits to health, as it can replace fires that fill houses with smoke and cause many respiratory and other illnesses, particularly in children. The third reason is to do with the increase of fixed-tariff charging to consumers connected to micro-hydro schemes. In this system, each consumer is charged a fixed rate each month, and limiting devices are used to ensure that a maximum consumption is not exceeded. Thus a consumer in Nepal might pay, say 10 Rupees per month for a 200 watt supply. Generally, this amount is only consumed for a few hours at night, for lighting, with little demand during the rest of the day. Meanwhile, the consumer is having to pay for fuel for cooking, or devote time to collecting it. The benefit of being able to use the 200 watts that have already been paid for are obvious.
There is one major drawback to using electricity from micro hydro schemes for cooking. A simple meal for four people needs, on average, about 1 kilowatt-hour of energy to cook it, and generally people in a community tend to cook at about the same time. This often means that a micro-hydro unit would not be able to satisfy the peak demand at popular cooking times in the morning and evening.
Consider a 10 kilowatt micro-hydro unit. If all the consumers were able to draw 200 watts on a fixed tariff, the plant could provide electricity for around 50 consumers. If however, everyone decided to cook at the same time (which in feet is surprisingly likely), the demand would be around 50 kilowatts - well in excess of the capacity of the plant.
Now, if we suppose that the consumer only uses the 200 watts available for 6 hours a day (again, a typical figure), there will be 18 x 200 watt hours of energy not being used, which has been paid for. If this could be stored, there would be 3.6 kilowatt hours (forgetting efficiency for a moment) available for other purposes, which would go a long way towards satisfying an average family's cooking needs, without exceeding the capacity of the plant.
The economic storage of electricity is notoriously difficult, so an alternative method of storing this energy is needed. As the energy is required in the form of heat for cooking, one approach is to store the energy directly in this form. This is the basis of Heat Storage Cooking.
In recent years, two devices have been developed that make use of this concept. The first is the BIJULI DEKCHI or Low Wattage Cooker, developed in Nepal by Development Consulting Services, and now manufactured commercially in Nepal. The Bijuli Dekchi consists of an aluminium pan (dekchis), welded inside a slightly larger one, leaving an air space between them. Flat elements are attached to the underside of the inner pan, and temperature is controlled by a thermostat. Figure 1 shows the basic construction of the Bijuli Dekchi.
A typical use of the Bijuli Dekchi is to provide hot water for cooking. It is filled with water and switched on by the user when spare electricity is available, for instance just before going to bed. Because of the good contact between the element and the water, maximum temperature is reached in an hour or so, depending on the capacity, and the thermostat maintains that temperature until the water is required. To prevent the pan boiling dry, the water temperature is limited to about 80° C.
In addition to hot water, the Bijuli Dekchi can be used to cook a variety of food, such as rice or porridge, but because of the good contact between the element and the pan, care must be taken to avoid localised overheating, and burning of the food. It cannot, however be used for frying, where a minimum temperature of 160°C is required.
The Bijuli Dekchi is currently made in a range of capacities, from I litre to over 10 litres.
The second altemative, the Heat-Storage Cooker, is currently being developed by the Intermediate Technology Development Group, in collaboration with Kathmandu Metal Industries, and a UK company, Dulas Engineering. In this approach, heat from a low wattage element is stored in a well insulated block of cast iron or similar material, and can be used when required for any cooking process. This is not a new concept, having been widely used in Europe for some years, but the problem with these devices is their cost.
Figure 2 shows the basic concept of a Heat Storage Cooker. A low wattage element is inserted in a block of cast iron, which is insulated externally.
The block should have a mass of around 35 kg, depending on the thermal properties of the material. Rice husk ash provides an excellent, cheap, readily-available insulating material. A well-insulated lid is important, to allow access to the top of the block which provides the cooking surface. It has been found that a design that allows the lid to be closed during cooking greatly improves efficiency.
The cooker should ideally always be connected to the supply, and some form of load diversion is required, to ensure that available energy is always diverted to the cooker. The cooker will reach a steady state temperature of around 400° C some 24 hours from cold, the temperature dropping to around 250°C after cooking a meal. Full temperature can be recovered in about 4 hours.
In an effort to reduce costs, alternative materials and designs are being investigated. These include the use of concrete for the storage core, and blowing air over a store of hot stones. One of the main disadvantages of the HSC is its weight (up to 150 kg), which makes transport to remote communities difficult. However, by building the cooker into the floor of the kitchen, it will be possible to keep the maximum weight of any component to 35 kg, and also reduce costs.
Both of these concepts can be applied to other uses apart from domestic cooking. Small commercial activities such as bakeries, crop drying or milk processing can be made possible by storing energy from micro-hydro schemes, which otherwise could not satisfy the peak demand.
We hope to publish details of the improved designs in our December edition - No. 26.