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View the documentResearch and development

Research and development

Research into integrating a wood/charcoal stove into building design

N K Bansal and M S Bhandari, Centre of Energy Studies, IIT, Hauz Khas, New Delhi - 110016, India

Abstract

An idea for integrating a cooking stove in the kitchen into the design of a building has been investigated for space heating in cold climatic conditions. The exhaust gases from the cooking stove are made to flow through a cavity wall, which acts like a chimney. The wall stores the heat during cooking hours and keeps the inside space at a comfortable temperature provided the heat loss rate from the building does not exceed 0.5 W/m2degK.

Introduction

In many regions of Nepal and India there is a need for heating round the year. This is usually achieved by using a wood stove which is also used for cooking. The usual three stone fires, have now been replaced by cleaner, more efficient cooking stoves. Although stoves of this type are designed with a chimney, most of them are not integrated into the building design. In this paper, we examine the possibility of integrating an efficient wood stove into a building, which may provide both cooking energy and the energy for space heating.

Stove design

Table 1 gives the amount of fuel needed to cook 1 kg of various foods. It is seen from the table that, theoretically, 18gm of wood per kilogramme of food cooked is required for cooking, but in practice approximately 268gm of wood is used up in the fire. Some stoves have been considered in detail to determine the quantity of energy lost during the process of combustion and cooking.

Stove efficiency

For improving the overall efficiency (PHU) of a stove, a number of factors should be considered:

Combustion efficiency: the maximum amount of energy which can be converted into heat as a percentage of the calorific value of the fuel.

Heat transfer efficiency: the maximum amount of energy which is transferred to the pot. This includes conductive, convective and radiative heat transfer processes.

Control efficiency: the mechanism which allows only as much heat to be generated as is needed to cook the food.

Pot efficiency: the characteristics of the pot which affect the proportion of heat reaching the food through the pot.

Cooking process efficiency: how efficiently the heat which reaches the food converts the raw food into cooked food. Combustion and heat transfer efficiencies are often combined for convenience and these are termed the thermal efficiency of the stove. When these are combined with the control efficiency and the pot efficiency, the efficiencies combined together are called stove efficiency.

Heat transfer processes

Heat conduction: when cooking begins, the walls of the stove are cold. With time, they warm up at a rate which is dependent on both the weight of the walls and their specific heat. Lightweight walls warm up quicker than heavier walls.

Heat transfer to the pot by convection: when a pot is being heated by hot gases leaving the fire, the factors which affect the amount of heat reaching the pot by convection are:

· the area of the pot which is in contact with the hot gases;
· the difference between the gas temperature and the temperature of the pot.

To increase heat transfer to the pot by convection three things can be done:

· the temperature of the hot gas can be increased by the choice of stove and by controlling the amount of air that enters the stove;

· increasing the area of the pot exposed to the hot gas. The pot support should be small and strong, occupying a small area and allowing the hot gas flame to rise up around the pot and contact the surface;

· the maximum heat transfer coefficient should be increased. This can be done by increasing the velocity of the hot gas.

Heat transfer to the pot by radiation: the radiative heat transfer to a pot depends on the temperature of the tire bed, the areas of the pot and the fire bed, and the distance between the two. To heat the pot more effectively by radiation the alternatives are:

· increasing the fuel bed temperature and thereby increasing the heat radiation from it;
· lowering the pot and thus reducing the distance between the pot and the fire bed;
· increasing the area of the pot 'seen' by the tire bed.


Figure 1: Hypocaust system integrated in a building


Figure 1: Hypocaust system integrated in a building

Combustion efficiency

Combustion of biomass is an extremely complex process involving chemical kinetics, heat processes, molecular diffusion and other phenomena. The most important parameters in wood combustion are the moisture content of the wood and its calorific value. Usually the calorific value per kilogram of any type of wood does not vary by any significant amount from any other, though the densities can be very different. The density does not affect the stove efficiency. Moisture content affects both the calorific value and the rate of burning very significantly.

Wood is typically composed of 80 per cent volatile material and 20 per cent fixed carbon and it is these percentages which determine the calorific value. In wood combustion, when the temperature reaches 100°C, the water is boiled out. From about 200°C, the hemicellulose begins to decompose, followed by cellulose decomposition. At about 300°C, decomposition becomes extensive when only 8- 15 per cent of cellulose and hemicellulose remains as fixed carbon and the rest is released as volatile gases. As volatiles escape the wood, they mix with oxygen in the air at about 550°C and ignite to produce a yellow flame. The flame not only radiates energy to the pot but it also maintains the combustion process. Burning of volatiles accounts for two thirds of the energy released by fire and the burning charcoal left behind accounts for the remaining third. A variety of techniques are used to increase the combustion efficiencies:

· use of grates which allow better mixing of air with fuel bed
· preheating of incoming air
· optimizing the shape of the combustion chamber
· insulating the combustion chamber

Heat of exhaust gases and building design

The escaping hot gases from a wood charcoal stove arc at a high temperature, between 300°C and 500°C These flue gases can be made to flow through a hollow wall in a 'hypocaust system', as shown in Figure 1 which is a conceptual drawing of a cooking stove integrated into the building design.

In the hollow wall, a cavity 50mm deep is created. A room of 10m2 floor area and a height of 3m, adjacent to the kitchen, has been considered. A daily variation of room temperature has been simulated and compared to the corresponding outside temperature at the same time of day. The performance of the experimental stove has been studied keeping all these parameters in mind.

To ensure good combustion the amount of air supplied for each kilogram of wood burnt should be at least 7m3. For most stoves used in developing countries, 1 kg of wood is burnt in about an hour, which corresponds to a power output of 5.5kW.

Results and Discussions

The following conclusions have been drawn from this study.

· the largest heat losses occur (1+ 42 per cent) through heat conduction into the walls of the stove

· the loss of energy into hot flue gases accounts for 22-39 per cent of the total input to wood stove

· incomplete combustion amounts to about 8 per cent

· typically half the energy entering the pot is lost in the form of steam. This is the energy lost at the point of use and mainly depends on the design of the pot.

The results of the hypocaust wall show that to provide adequate heating, it is sufficient to use the stove for 2 hours in the morning and 2 hours in the evening. The most important pan is to keep the building's envelope U-value at 0.5 W/m m20K which corresponds to a 5cm thick layer of insulation or an 80 cm thick mud wall

Table 1: Energy and theoretical amount of wood required for cooking food

Food

Sp. Heat KJ/Kg °C

Temperature change °C

Energy required for chemical reaction kJ/

Food cooking energy kJ/kg

Wood equivalent gm./kg of cooked food

Rice

1.74-1.84

80

172

330**

18

Flour

1.80-1.88

80

172

330*

18

Lentils

1.84

80

172

330*

18

Meat

2.01 -3.89

80

-

160-310

9-17

Potatoes

3.51

80

-

280

16

Vegetables

3.89

80

-

310

17

* Includes sufficient water for cooking but none for evaporation
** For wood with a calorific value of 18 MJ/ kg