Thermally efficient improved wood-burning metal cooking stove with water-heating arrangement for cold climates in India

by Rajeev Aggarwal, Assistant Professor, Department of Basic Sciences and Dr. O. P. Sharma, Joint director (training), Directorate of Extension Education, of the Dr. Y.S. Parmar University of Horticulture and Forestry, Nauni, Solan 173 230 HP, India.

Introduction

People in the temperate wet zone of the Himachal Pradesh region of India (altitude 1840-2200m) experience annual temperatures ranging from 29.4°C to -10°C. This zone constitutes about a third of the total geographical area of the state. In this region about 140cm of rainfall is recorded annually, mainly during the period July-September. The average snowfall is 150cm per annum and it occurs from November to March, accompanied by ground frosts. Thus space and water heating, as well as cooking, are major requirements and fuel consumption is nearly double the average used in northern India. As power failures are frequent during the winter, wood and coal-burning metal stoves are used in high altitude regions of this zone, and three-pot mud stoves are used in the lower parts.

Generally, no wood is purchased by the rural people in these areas, and between three to six hours are spent daily per household collecting fuel. Usually this fuel is collected by women, who collect fuelwood, agro-residues, bushes, trees and pruning wood to meet their requirements. As agricultural residues are in short supply, forests in the state have been degraded, as most of the pressure for fuel has fallen on them.

Needs assessment

In order to assess the basic requirements of people in the region, a survey was carried out by the Technical Back-up Support Unit (TBSU) at Y.S. Parmar University, Solan. This was set up by the Ministry of Non-conventional Energy Sources (MNES) under a national programme on improved cooking stoves. The surveys revealed that:

· A three-pot mud cooking stove without chimney is used in the lower parts of the region. It is made of stones, clay and cow dung, or occasionally just of stone.

· A metal stove, with or without a water heating section and fitted with a chimney is used in the upper parts of the region. It is made by local artisans.

· Space heating is a major requirement during the winter, so the firebox is kept large and, in the case of mud stoves, dug 5-6 cm into the floor, so that the burnt charcoal and ash can remain in the firebox for a longer period. The whole family sits around the cooking stove in winter.

· Stove size is variable between houses.

· Both metal and mud stoves are without grates or air passages.

· In some places the top of the firebox of the mud stoves is omitted or removed, especially in Brahmin families, because of religious beliefs. The cooking efficiency of these stoves is between 8-13% and this modification also causes health hazards.

Characteristics of the improved cooking stove

Based on the requirements of the communities in this area, a thermally efficient wood-burning metal cooking stove has been developed (Figure 1). Of the total available heat energy, about 19% is used in cooking, water heating accounts for around 40% and space heating uses up about 30%. The hot water tank fitted inside the stove has a capacity of 30 litres. The water temperature rises to 60°C above the ambient temperature and the room temperature is increased by 8°C.

Figure 1: Water heating three pot stove

The sizes of the pot holes and the distances between them allow large pots to be used. The grate is made up of 16 SWG sheet, provided with small holes for air circulation and with a slight upward slope to increase the heat reaching the second and third pot. A sliding plate is provided below the grate to collect ash. A slope from the second and third pot-holes to the chimney reduces the heat loss through the chimney (see Figure 1).

A water tank with a capacity of thirty litres is situated inside the cooking stove adjacent to the combustion chamber. Usually, a steam pipe is fitted from this tank (Figure 2). It is of equal length to the chimney, with a cowl at the top of the steam pipe and chimney to prevent the rainwater coming into the pipes. However, the steam pipe is not shown in the model illustrated (Figure 1). The chimney and the steam outlet pipe radiate heat into the room, thus increasing room temperature. The stove has an inlet at the top for cold water, and a tap for hot water, as shown in Figure 1.

Thermal performance test

The MNES water boiling test was followed to calculate the cooking and water heating efficiencies of the stove. The fuelwood used was eucalyptus, with a calorific value of 3800kcal/hour and it burnt at 1.3kg/hour. The space heating efficiency was calculated by calculating the heat gain and heat loss due to convection, conduction and radiation. The temperature variations were recorded by digital temperature indicators. The duration of the test was one hour.

Results and discussion

Table 1: Results of the water boiling tests

	Pot number
	1st	2nd	3rd	Total
Efficiency (%)	11.76	4.52	2.42	18.7
Power output	0.68	0.26	0.14	1.08

The low efficiency recorded is because the water situated in the tank adjacent to the fire uses part of the heat produced, and this useful application of heat is not shown in the standard test results. The power output rating of the stove is 1.08kW, which is equivalent to the power output of a gas burner. Figure 3 shows that the temperature in the water tank increased from 12°C to 40°C within 30 minutes after the stove was lit, and a temperature increase of 60°C was recorded after completion of the test. Throughout the test, the ambient temperature varied between 10.5°C and 11°C. The water heating efficiency of the stove was found to be about 40%.

The 30 litres of water heated to 72°C can be used for washing and bathing. In addition to providing water heating and cooking, the room temperature increased from 9°C to 17°C within one hour, consuming only 1.3kg of wood. The space heating efficiency has been calculated to be about 30%, making the total thermal efficiency of the stove up to nearly 90%.

Stove maintenance

It is essential with this stove that the soot from the chimney is removed each week, otherwise the pipe will become choked and the stove will not function properly.

Figure 2: Schematic diagram of water-heating three pot stove

Figure 3: Temperature increase in room and water tank due to stove

Stove cost and lifespan

The cost of this stove is estimated to be RS 700, of which MNES will provide a subsidy of Rs. 250. The model is in great demand, not only in the mountainous regions, but also in the lower hilly areas of the state. The demand for this stove is estimated to be about 0.1 million families in Himachal Pradesh. Rural people already spend Rs. 500 to Rs. 1500 on cooking and heating stoves in the local market. The life of the stove is about 10 years.

Conclusions

This stove, which meets the basic requirements of cooking, water heating and space heating utilizes the fuelwood very efficiently. This model has reduced fuelwood consumption from 5kg/hr to 1.3 kg/hr, thereby reducing the burden on forests. The model has been approved by the state level testing an certifying committee constituted by MNES new Delhi, and is likely to be approved by MNES for its propagation at a subsidized rate, not only in Himachal Pradesh but also in other hilly areas of the country. Those manufacturers who are already fabricating high altitude metal stoves in the state have started its production.

Improvements being carried out at present aim to improve the cooking efficiency of this stove to above 20%, which will make cooking much faster.