|Boiling Point No. 37 - June 1996 : Household Energy in Emergency Situations (Intermediate Technology Development Group , 1996)|
|Non - Theme Articles|
Summarised from an article for Boiling Point by G. Beaumont Sunseed Desert Technology, Apdo. 9, 04270 Sorbas, Almeria, Espana
Designed at Sunseed Desert Technology, a British registered, charitable research centre, this ultralow cost solar cooker was built at MVUMI, Tanzania, largely from mud and straw (adobe). It is claimed to be the first truly affordable yet robust, family sized cooker designed for construction and maintenance by the users.
Intended for on-site building each cooker costs about £8 equivalent (depending on local prices). Of the 'solar cooker' type, it uses the greenhouse effect to trap solar heat inside a glazed, insulated box. Unlike other box cookers, however, it is a fixed installation with sides facing due east/west; the independent reflector is adjusted to maximise sunlight entry.
As the cookers are fairly labour intensive to build, only 10 have been made, usually in places where they would be used daily.
The cooker is a shallow, insulated, adobe box with a ground level side-opening door and a fixed, nearly horizontal, transparent cover of plastic or glass. Reflective inner walls direct sunlight onto a black metal baseplate, and extra sunlight is captured by a steel or aluminium moveable reflector panel controlled by guy ropes. Under the baseplate an adobe slab stores heat, for cloudy periods. It is capable of cooking for 10 people in clear sky conditions, assuming a daily individual requirement of 0.4 kg of dry food. The interior space is 93cm square and 20cm deep, which allows for a pot 35cm diameter x 15cm deep.
Materials currently imported (polyester window and metallized polyester reflector) cost about £4 per cooker and need yearly replacement. The cooker accepts the cheap aluminium cooking pots ('isufurias') commonly available in east Africa. Having a fixed position, it is most suitable for locations within 10 degrees of the equator as efficient tracking of the winter sun by the reflector is not convenient elsewhere.
We revised cooking procedures to take account of the local water quality. As in much of Africa. water is often contaminated with salmonella and similar species' and food once wetted should be heated quickly to 70°C, to avoid breeding conditions. The cookbook instructs that water should be preheated for I hour in the cooker before the food is put in the water; this heats the food
The base is supported over an insulating air space by crossed sticks, topped with thin layers of straw, aluminium foil and polythene. The cooker walls are insulated by a 10cm layer of tightly bound straw, wrapped in polythene to prevent wetting by steam from inside the cooker, or occasional rainfall.
The large, erect reflector is made from a thin (25 micron) sheet of metallized polyester film stretched inside a rectangular wood or cane frame braced against both twist and bending. The reflector also appears flimsy, and is exposed to the wind.
In clear conditions the cooker can bring a 4 litre load of water to cooking temp. (80°C) in 60 to 70 minutes, and cooking can be maintained to within an hour of sunset. In mid December 1994 a prototype brought 2 litres of water to over 80°C in Spain, with a maximum sun angle of 30 degrees and ambient temperature of 25°C.
Follow-up report (Visit November-December 1995)
Following pilot trials of the Sunseed ultra low cost solar cookers in Tanzania, their condition was checked to see how many of the cookers survived and-were being used for cooking.
Of the 10 cookers started, 5 were completed by the Sunseed team, I lacked only the reflector, and I lacked only the window, as the team left. 3 were completed in terms of the ground structure and window after the team left, and 3 never had reflectors made up. 1 had made little progress and no further work done at all in the absence of the team. Overall 9 cookers were completed in terms of ground structure; of these 6 survived with windows intact and in such condition that cooking could take place.
Five cookers were still in use at the time of the visits, and 3 others had been in use for heating water until the windows were broken (between Sept and Nov). Of the 5 still in use, 2 had replacement windows following incidents involving livestock. The repairs had been carried out effectively, and in one cases the replacement window was made from two pieces joined with silicon sealant.
Six reflectors had been completed, of which 5 had been in regular use. 4 of these survived in good condition at the visits, and one had the reflective panel destroyed after being caught in a gust when left out overnight. (The unused reflector is in storage). I cooker was essentially complete, but was never put into use, due to lack of black paint for the base plates.
Overall, the survival rate of the cookers was felt to be quite respectable for such a cheap design on its first outing in the 'real world'. It is now clear that completely inexperienced rural people can use and maintain the cookers successfully if they are well motivated and trained. Cookers should not be regarded as complete until there is some kind of protection around them against animals and children. We tried to ask as many people as possible about fuel saving, but it was very difficult to get reliable answers.
Given these factors, it seems that the success rate can be very high' and each cooker may give significant fuel saving of the order of I tonne per year. Fully demonstrated cookers have been used routinely and successfully for a variety of staple dishes. They appear to be valued both for their convenience and economy, and we hope the design will soon be taken up by other agencies for larger scale dissemination.
Set up in 1986 near Almeria. southern Spain, Sunseed Desert Technology (SDT) is the research and demonstration centre of the Sunseed Trust. SDT researches into solar cooker technology as part of an overall strategy to combat desertification, and sustainably improve the living standard of poor people in decertified areas. A solar cooker was designed and tested, see Results of the Second Comparative Solar Cooker Test in Almeria.
Synopsis, Route d'Olmet, F34700 Lodeve, France, 1994. Field trials of the stove were carried out in 1995.
by Tran Quoc Giam
Vietnam is a tropical country which can make good use of a considerable amount of its solar energy potential, particularly in its south and central regions, but less so in the north because of the unsettled weather conditions.
In the past, there has been some use of solar applications, such as solar water heaters, solar cookers, solar dryers, PV-modules, etc. but in most cases, these technologies are still at the experimental stage.
Focusing systems, such as the solar reflector (parabolic) cooker, or highly sophisticated solar tower plants, should be installed mainly in the region below 17 degrees latitude because of their need for direct radiation.
The solar energy group of RERC has designed a highly efficient solar water heater (SWH) with integrated storage.
This consists of three collector cum-storage tanks which are designed as small units each of 30 litres (0.1 m x 0.1 m x 0.3m) capacity, combined to get a volume of 90 litres. Inside the tank, a wavy metal or plastic plate is linked under the absorber surface. The water heating mechanism is based on the thermosyphon effect. With solar radiation, the space and the wavy metal plate decreases which makes the heated water rise to the upper part of the tank' Normal convection circulation within the tank then occurs with the cold water from the top taking the lower space.
Since 1994, more than 40 systems of 1 m2 have been installed on an experimental basis in Hanoi and in Ho Chi Minh City. RERC aims to set up a production line for this new model of SWH sometime in the future.
Renewable Energy Research Centre (RERC), Hanoi National Institute of Technology, Hanoi, Vietnam. Reproduced from RERIC News, Vol 18 No 2/3 (June/Sept 1995)
by Washington Nyabeze
Beer brewing and bread making are rural domestic activities now increasing in Zimbabwe to supplement declining incomes from agriculture.
Both processes use wood fuel and their energy requirements are considered in this paper. Seven brewing and five baking enterprises were studied by interviews, questions and daily monitoring over six months. Their energy efficiencies, technologies and environmental impacts were examined in detail.
Beer is generally brewed domestically in a 2001 (45gal) oil drum heated on one side by an open fire.
As only about one third of the drum surface is exposed to the fire less than 50% of the heat produced from the fuel and enters the drum for utilization in the brewing process. Much of this is then lost through the un-insulated, 70% of the drum surface. Woodfuel consumption per litre of beer is between 2.0 and 3.5kg (ave 3.0) and each brew makes 200 to 200 litres.
Bread is usually baked in a double (oil) drum oven although one enterprise used a pit oven and another a Dutch oven. The oven is heated by a fire in a chamber under the oven which may have a door to control the burning rate and some have a chimney with a damper. One loaf (570g) requires 1.8 to 2.8kg of woodfuel, mostly used at the warming up stage.
Opportunities for technical improvements
The study indicated the need for technology advice to be provided to the enterprises and their readiness to accept it.
• research into yeast assisted brews is needed to ease the pressure of some inputs such as rapoko, sorghum and woodfuel
• recipes for more nutritious and tasty beers
• increased production per cycle with better quality control
• more fuel efficient stoves and fire management
• better heat transfer from fire to brew
• fuel switching possibilities, especially with coal.
Business skills - maximum benefits from this technology approach can only be realized if, in addition, the entrepreneurs have skills in costing, sourcing of inputs, marketing of products, record keeping and general business practice. Effective means of developing these skills through training and information supply are needed.
The following opportunities were identified:
• provision of training and information for better design and
• construction methods for baking ovens
• instruction on better use and maintenance of ovens
• better understanding of the relationship between fire management and bread quality control
• use of coal and other locally available, alternative fuels
• making the mixing of ingredients less laborious
• provision of information on alternative baking techniques
• choice of most appropriate scale of technology according to required output.
Again, maximum benefits will depend on better business skills.
Environmental considerations Brick making, beer brewing and bread making are the three largest rural industries in Zimbabwe in terms of woodfuel consumption. Estimates show that they consume about 284,000, 163,000 and 218,000 tonnes of woodfuel per year respectively (brick making is not included in this summary as it is not really a domestic activity). There are 50,000 breweries and 1,200 bakeries in Zimbabwe. Although there are more critical causes of deforestation such as land clearing for agriculture and timber extraction, deforestation itself is a problem for rural industries.
The study showed that for each activity training is urgently needed in technologies, in appropriate business skills and in concern for environmental conservation.
Training manuals on improvements to existing technologies have been prepared for each industry covered. These focused on low cost changes of methods and were for use by small scale rural entrepreneurs, artisans and extension workers and field staff from government and non-government organizations.
Sixty entrepreneurs and 30 extension workers have already been trained in the use of the manuals, through three-day training workshops. Subsequent evaluation shows that more training workshops and exchange visits between entrepreneurs are needed to allow a wider choice of technologies and to promote innovation.
Extracted from a paper by Washington Nyabeze (ITDG Energy Project Manager; Zimbabwe) presented at a Zero workshop, June 1995, entitled 'Energy and Technology Issues in Rural Industries in Zimbabwe'
by Er L C Verma
Improved cookstove (chulha) technology in India has undergone swift changes during the last decade. The improved chulha program, a 'minimum needs programme' of the Government of India, is basically for rural women, and aims to improve their living conditions and create awareness about the saving of big-fuel. It builds on the development work which has taken place in many parts of India over a much larger period.
The program, started in December 1983, is supported by 17 Technical Backup Units (TBU), established in various states of the country, for development of appropriate models of improved chulhas, based primarily on studies of actual cooking needs of the region or groups of users. The programme now has more than 40 different types of improved stoves. At the time of launching most of the models used for field propagation had dampers and baffles.
The originally recommended,
improved, two-pot, mud stove had a chimney and was fitted with dampers and baffles. Dampers are thin metallic plates, and in a twopot mud stove with chimney the front damper is put across the mouth of the fire-box. It regulates the intake of primary air for the combustion process and controls its cooling effect on the stove. The 'chimney damper' is put across the flue passage between the second pot hole and chimney pipe. It controls the flow of hot gases out through the chimney. The dampers thus play an important role in achieving higher thermal efficiency. As a result of experience, improved chulhas with dampers were rejected by the users for the following reasons:
• The dampers quickly become hot and the users fear burns if touched during cooking.
• In the case of the vertically sliding type of damper, the grooves in the mud-body of the stove become worn and do not provide smooth sliding and fixing at desired levels.
• In the case of the horizontally operating type, the metal slides frequently get separated from the body and are not refixed, so the damper can not be used.
• Damper plates are easily removable from chulha body and lost or used by children as toys.
Failure to use the dampers or incorrect use results in uncontrolled and excessive intake of air and outlet of hot gases due to the 'chimney draught' and so in turn leads to loss of useful heat, and waste of fuel.
Baffles are the mounds of mud of various cross-sections put in the path of hot gases flowing through secondary pot holes. They retard the flow of hot gases under the pot and so increase the transfer of heat to the pot. This improves the thermal efficiency the chulha. Such multi-pot stoves should have improved thermal efficiency if the baffles are properly placed, used and maintained.
However, baffles are easily damaged during the course of repairs by the users. Surveys over the years have revealed that the majority of users are unable to maintain the designed form of the baffle, which becomes ineffective or detrimental to stove performance.
The foregoing description of 'improved' chulhas with dampers and baffles indicates how technologies from the laboratory may not work under field conditions. The two components which could enhance thermal efficiency were not accepted by the users for practical reasons. If such improved chulhas were to survive, development of 'darnperless' models became a necessity.
The problem was solved by the Technical Backup Units by making major changes in the cross-sections and placement of the flue passages. Air intake was controlled by reducing the size and angle of the flue passages such that secondary pots got more heat. The optimum distance between the bottom of the secondary pot and the floor of the pot seat was determined and fixed.
The improved chulha became not only simpler to make but also cheaper. Since 1989, the Government of India, under its National Programme on Improved Chulhas, has approved only damperless models for field propagation.
Improved chulhas with ceramic linings
The important parameters which determine the thermal efficiency of a chimneyless improved chulha are: design dimensions of the firebox, secondary-pot holes and flue passages. Most village stove makers and users are unable to maintain the designed size and shape of these components to the close tolerances needed.
The solution to this problem was the use of pottery (ceramic) linings which can be pre-fabricated at the village level and built into the mud stove bodies when installed in the kitchen. These can be carefully designed and accurately made, and are more resistant to wear.
Professional potters are selected and given training in making linings for the popular models of improved chulhas or those to be used in their area, and so can earn additional income from the sale of linings. Improved chulhas fitted with pottery linings not only stay with designed parameters, but also reduce repair work on the part of the user. The increase in the cost of the chulha is justified by these benefits.
'Unified' models of improved chulhas
New and more appropriate models of improved chulhas were developed to meet regional cooking needs. By 1990, the Government of India had approved more than 40 models for field propagation. A close study of various models indicated that although there were differences in designs to suit local tastes, most of the models had similar working principles.
Despite the different cooking needs and habits of rural masses in India, the traditional cooking methods are basically the same. To simplify the choice of most appropriate stove for promotion by field workers and to help production, it was felt necessary to reduce the overall numbers of models. Thus. the concept of a 'Unified' model was created.
• The model is damperless with a pottery liner in the fire-box, and secondary pot-holes and a heat recovery system.
• The fire-box has an appropriate grate for improved combustion and reduced smoke emission.
• It must be able to burn different biomass fuels available in the region with approved levels of thermal efficiency.
• The design should cater to the habitual way of cooking of the users of different states.
• The power output should be sufficient for the cooking needs of an average family.
The chimney, is an important component of an improved chulha as it extracts the products of combustion from the kitchen space and also creates a natural drought in the fire-box to provide the air needed for combustion.
However, field experience shows that the chimney-pipe is a major factor for non-adoption of improved chulhas for the following reasons:
• Most field workers are not skilled enough to make or safely install and properly seal a chimney pipe through the roof.
• Users do not clean the chimney regularly, and if broken cannot repair or replace it.
• In most of the areas of India (except cold and hilly regions) the chimney pipe used is made of asbestos-cement mixture. In some states this is easily available, but in others it needs to be transported over long distances. If the chimney pipe itself needs to be transported it is easily broken. The chimney pipe often costs more than the stove itself.
A chimneyless stove needs a completely different design to give optimum combustion and heat transfer, particularly in the combustion chamber and by addition of a grate. Some models have provision for a second pot hole for recovery of exhaust heat. In practise in the field the heat utilization may equal that of a stove with a chimney and smoke emission levels may be within safe limits. Chimneyless stoves are simpler, cheaper and easier to make and use, and so are promoted by the government and the TBUs.
Reproduced from GLOW, Vol 18, Dec 1995, a publication of the Asia Regional Cookstove Program