Solar cooking saving the environment by avoiding deforestation
1(8)
Solar cooking saving the environment by avoiding
deforestation
Food preparation and water disinfection vital parts of the
programme
Sven A. Svennberg, professor em. (operation and maintenance)
Solvägen 10 135 50 Tyresö, tel +468712 4058;
e-mail sbg@glocalnet.net
Date:2006-10-27
Summary
All Eastern Africa is suffering from deforestation caused by wood fires for cooking and the use of
wood for house-building and also forests taken down in order to arrange land for growing crops.
Insolation power is in the order of 1 kW per square-metre. A solar cooker with an area of about 0,4
squaremetres (could be the lid of a barrel) can catch 1 kWh in less than 3 hours. This is sufficient
for preparing a dinner meal. Even if there are clouds a solar panel can absorb solar energy in the
form of diffuse sky solar radiation.
The storage of heated food can take place in an insulated (double walled) cardboard-box. The
storage time can be prolonged if a heated stone or a heated piece of scrap iron is put into the box.
A mound of stones having a volume of about one cubic metre can be used as an over-night energy
storage if it is covered by an insulating layer.
Water disinfection. To-day water is normally taken from rivers or ponds or collected from rain.
Very often the water is contaminated and needs both filtration and disinfection. Filtering can be
made through a piece of tightly woven cloth. The killing of bacteria can be performed by heating the
water to more than
80 °C (175 °F).
Proposed products, such as sun panels, storage systems for heat and cold, cleaning and disinfection
of potable water have been developed on an ideal basis.
Information on earlier work regarding those products has been taken from own knowledge of
products and their use in actual energy saving areas and by searching at Internet.. Knowledge of the
condition actual in Eastern Africa (especially the areas around Lake Victoria) is known through the
work within the Swedish Church help organization
Project leaders are:
Sven A. Svennberg, professor em. at KTH in operation and maintenance of buildings, supply
systems, and installations. Göran Frisk, Captain in the Swedish Navy, organizer of activities to help
people in the Victoria lake region.
Development costs including travels, material, and information leaflets as well as an informative
report and a training compendium have been calculated to 1,3 MSEK (about 200 000 US$) .
The work is to be carried out on a consulting basis through Ramas Teknik AB, seated in Tyresö,
reg no SE-556173-8724.
An over-view of the contents of this project description follows next page.
D:\533571560.doc
1 (8)
Solar cooking saving the environment by avoiding deforestation
2(8)
Environmental background
1)
2)
3)
4)
5)
6)
Deforestation shall be avoided
To-days food preparation using open wood or charcoal fire shall be avoided
Food preparation shall be possible during day-time and meals served after sun down
Solar energy shall be used because it is easily available also during rain periods
Solar cookers (and fryers) are to be produced locally using low price material
Storage of prepared hot food shall be possible for some hours (also during night) in canteens
and accumulating low-price storage facilities.
7) Disinfection of potable water (by heating to more than 80 °C, 175 °F) shall be possible using
the same equipment as for food preparation.
Technical background
1) Solar power data is available in research literature and in Internet, se appendix 1), practical
knowledge is normally sufficient.
2) Solar cookers can be produced as plane or parabolic using aluminum foil or any other
reflecting material (i.e. stainless steel) as a mirror material,.
se example in appendix 2.
3) Storage canteens can be made in the form of double wall boxes from card-board,
see example in appendix 3
4) Accumulating storage material can be stone or steel. The accumulating ability is presented in
appendix 4.
.
2. Parabola – Calculation of parabolic solar
Solar cookers in practice
cookers See appendix 9
1) Plane solar cooker
2) Parabolic solar cooker
3) Spherical solar cooker
4) Foldable solar cooker for easy
transportation
3. Girdo: Solar heating of buildings in
Scandinavia.
Swedish Building Research Board R108:1978
Appendices
1. Solar power and solar energy data for
Eastern Africa
2. Solar cooker design
3. Design of a temperature keeping insulated
storage
4. Accumulating ability of some materials
5. Raw material for food preparation in
developing countries
6. Disinfection of potable water
7. Sponsors
8. Development costs
9. Design of a parabolic reflector
10. Food preparation time, example
Literature
1. Brown-Isfält: Insolation and solar shading
Swedish Building Research Board R19:1974
D:\533571560.doc
2 (8)
4. Isfält: Light transmission through glass.
Royal Institute of Technology, Dep of heating
and ventilation KTH U&V Medd 37 1974
5. Andersson: Field measurement on solar
panels.
Royal Institute of Technology, Dep of heating
and ventilation KTH U&V Medd 179:1980
Internet addresses
http://www.solarcooking.org/ (Foldable Family
Panel, Solar Cooking Review)
http://www.knowledgehound.com/topics/solarc
oo.htm
http://www.sunspot.org.uk/Solar.htm
http://www.kaapeli.fi/~tep/projektit/index.html
Solar cooking saving the environment by avoiding deforestation
3(8)
1. Deforestion shall be avoided
All Eastern Africa is suffering from deforestation caused by wood fires for cooking and the
use of wood for house-building and also forests taken down in order to arrange land for
growing crops. The increase in wood material is large but not sufficient in those areas where
people live. Forests are good water preservation areas and trees diminish the land erosion and
improve the local climate. As a result earlier fertile land is transformed into desert. Thus there
will be necessary to re-establish the forests in order to prevent formation of deserts and reform
desert areas into fertile land.
Growing crops in a forest area is favourable thanks to the humidity keeping ability and the
shadowing function of the trees.
.
A good example on the improvement achieved by forest plantation is Teneriffa, where the
climate to-day is favourable following a good planning (royal ukase). In the middle of the 19th
century. Earlier the island was almost a moon landscape caused by deforestation.
2. To-day’s cooking over open fire shall be avoided
A family of 6-8 persons burns per week about 50 kg charcoal (roughly 350 kWh) or the
equivalent energy amount in the form of non-refined wood. Thus the family uses about 50
kWh per day for cooking. In an industrialized land the same family size would use less than
10 kWh for that purpose.. In Eastern Africa there is an unlimited solar energy supply.
Insolation power is in the order of 1 kW per square-metre. A solar cooker with a diametre of
0,6 metres will produce about 0,2 kW and give energy enough for cooking 3 kg of food in less
than two hours.
3. Food preparation shall be possible during day-time and the meal be eaten
after sun-set.
A solar cooker with an area of about 0,4 squaremetres (could be the lid of a barrel) can catch
1 kWh in less than 3 hours. This is sufficient for preparing a dinner meal. Even if there are
clouds a solar panel can absorb solar energy in the form of diffuse sky solar radiation. The
diffuse solar power will be in the order of 20-25 % of the direct solar power (Pleijel:
I=244*sin h watts, where h = solar altitude; page 52 in Isfält, ref 1). Solar cookers should
preferably be designed and placed so as to avoid the cooling influence of wind (use a windshed).
The storage of heated food can take place in an insulated (double walled) cardboard-box. The
storage time can be prolonged if a heated stone or a heated piece of scrap iron is put into the
box.
4. Storage of heated food during night
A mound of stones having a volume of about one cubic metre can be used as an over-night
energy storage if it is covered by an insulating layer. Using this facility will make possible a
hot meal in the morning without burning wood or charcoal. A storage at a temperature higher
than 60 °C (140 °F) will effectively prevent the growth of bacteria. See appendices 3 and 4.
5. Disinfection of potable water
To-day water is normally taken from rivers or collected from rain. Very often the water is
contaminated and needs both filtration and disinfection. Filtering can be made through a piece
of tightly woven cloth. Communities could arrange tanks where the water pollutant particles
can settle during a day or more. The killing of bacteria can be performed by heating the water
to more than
D:\533571560.doc
3 (8)
Solar cooking saving the environment by avoiding deforestation
4(8)
80 °C (175 °F). The prepared potable water should be stored in a vessel under a tight lid
Cooling of water can be performed by surrounding the water vessel with a wetted piece of
cloth, thus giving evaporative cooling, which can be improved by putting the vessel in a
shadowy and windy place See appendix 7.
6. Solar cookers produced on the spot using low price material
a) Concentrating sun panel
A concentrating sun panel can be used for both cooking and frying. An ideal concentrating
sun panel has the form of a parboloide, see appendix 9. A sun panel being of part of a sphere
in practice can give an almost as effective result. A lid of a barrel can easily be formed to a
sun panel. After the forming it has to be furnished with a layer of aluminum foil to become an
effective reflector.
The sun panel (the mirror) has to be furnished with a stand for the cooking vessel. See
drawing in appendix 2.
An equation for the calculation of paraboloides is given in appendix 9.
The sun panel shall be placed in a free area where it can easily be directed towards the sun
and protected from wind. If a barrel lid is being used for the production of paraboloidic
mirrors the cylindric part of the barrel can be used for making two or three linear solar panels
useful for heating and frying large pieces of meat or the heating of a roasting pan.
b) Plane sun panel
A plane sun panel reflects sun without concentrating it to a point or line. In spite of this fact it
can often be used for the cooking of food. A plane sun panel has the ability to utilize diffuse
solar irradiation (for instance during rain periods), which a concentrating solar panel can not.
Normally the preparation time will be longer with this type of sun panel.
The sun panel shall be placed in a free area where it can easily be directed towards the sun.
A better design is a solar cooker in the form of a cut-down pyramid placed with the wider part
upwards. This type of cooker is produced from four pieces of plane elements clad with
aluminium foil, in order to form a sort of cauldron where the cooking pan can be placed. The
sides of the pyramid should have an inclination of about 45°. The pan will be protected from
wind. Thus the solar cooker will have a better efficiency. Proposed measures are given in
appendix 2.
c) Foldable sun panel having a partly concentrating function
As a consequence of the lower efficiency a plane sun panel shall have a larger area than that
of a concentrating sun panel. By using a number of inclined elements a concentration can be
achieved and the efficiency be improved. Such a design is presented in appendix 2.
7. Better food preparation without deforestation
This publication promotes cooking during day-time and eating after sun-set.
The sun panel needs some service in the form redirection towards the sun in order to achieve a
good efficiency. The evening meal should preferably be taken within a couple of hours after
sun-set.
If a hot morning meal is wanted this can be achieved by storing heated food in a heat
accumulating stack, see appendix 3.
D:\533571560.doc
4 (8)
Solar cooking saving the environment by avoiding deforestation
5(8)
For the realizing is needed:
a) A sun panel with a cooking device. The solar cooker can be produced according to
drawings in appendix 2.
b) Heat storage (insulated mound of stones) for the storage of heated food during night.
The storage is produced and utilized according to description in appendix 2..
Appendix 1
Solar power and solar energy data for Eastern Africa
Solar power kW/m2 as a function of day-time
Insolation at the surface of the Earth is highest at noon and in the order of Io = 1 kW/m2
towards a surface perpendicular to the sun direction.
Other times of the day the insolation is lower owing to the fact that sun light has to pass a
longer way through the atmosphere.( I = Io/sin h kW, h= sun altitude, see Isfält 1, page 28)
The irradiation towards a sun panel is depending on the angle of attack (the angle between the
normal to the surface and the sun beam), The highest effect is achieved if the angle of
incidence is 0°, i.e. if the surface of the sun panel is perpendicular to the sun beam. If the
angle of incidence increases the achieved solar power will be reduced with a factor equal to
the cosine of the angle of incidence. In order to get as much solar power as possible the sun
panel has to be redirected from time to time in vertical as well as horizontal angle.
A sun panel shall have a near horizontal surface in the middle of the day and be inclined
towards the sun at other times of the day. If the prepared food is to be used after sun-set it is
practical to have the solar panel inclined at an angle of about 45° and redirected towards the
sun from time to time. Small divergence from the ideal direction + or – 15 degrees, equal to
one hour before and one hour after ideal direction) is not critical.
The altitude influences solar intensity, because the air mass for the sun shine to pass is
reduced att higher altitudes. For the area around Lake Victoria, being at an altitude of around
1200 metres, sun intensity is about 10 % higher than at sea level. A low moisture content of
the air gives a similar influence.
Diagram 1 Solar altitude as a function
) of latitude and day-time at equinox (March 20, Sept
90
23)
Solar altitude degreesr
80
70
60
lat it ud e
50
0
40
10
20
30
30
20
10
0
6(18)
7(17)
8(16)
9(15)
10(14)
Tim e
D:\533571560.doc
5 (8)
11(13)
12
Solar cooking saving the environment by avoiding deforestation
6(8)
Diagram 2. Solar power as a function of solar altitude
Low solar altitude causes longer way for the light to pass the atmosphere, causing
a reduced solar power
Solar power W/m2
Solar power as a function of solar altitude
1000
900
800
700
600
500
400
300
200
100
0
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90
Solar altitude
Diagram 3 Solar power to a) a horizontal and b) a vertical surface as a function of the time
of day. Near the equator the insolation is strongest towards horizontal surfaces, independantly
of the time of day. Vertical surfaces give lower solar power.
a)
b)
Influence of deviation from ideal angle of incidence
If the ideal angle of incidence is not kept at 0° the solar power will be reduced with a factor
equal to cosine of the angle of incidence. A divergency in the order of 30° can normally be
accepted, which means that the sun panel has to be adjusted every second hour. Reduction
level can be found in diagram 3a, putting the angle of incidence = 90° minus the solar
altitude. Example:
Solar altitude = 60°, angle of incidence = 90-60 =30°, relative solar power = 0,5 (power~ 500
W/m2)
D:\533571560.doc
6 (8)
Solar cooking saving the environment by avoiding deforestation
7(8)
Appendix 2
Sun panel design
A.
Concentrating sun panel
A wooden matrix can be used for series production of sun panels
using pieces of sheet metal. The sun panel can be un-symmetric
(se figure) in order to reduce material consumption without loosing
efficiency in solar power. For instance, a part (~1/3rd) of the
envelope of a barrel can be used as a linear solar panel, preferably
shaped to part of a parabola.
Sun is reflected towards a point to the left of the sun panel, where
the pot is placed.
B.
Plane sun panel composed of parallellepipedic side elements
Proposed measures
Bottom sheet
0,3 * 0,3 m
Height ca
0,5 m
4 side elements in the form of trapeziums:
Long side
1,3 m (ca)
Height
0,7 m (ca)
Short side
0,3 m
All surfaces except the bottom are to be covered with aluminum foil
1,3 m
0,3 m
D:\533571560.doc
0,7 m
7 (8)
Solar cooking saving the environment by avoiding deforestation
8(8)
C. Foldable sun panel with some concentrating action, outline sketch
Paper model ,
scale ca 1:10,
internally covered with
aluminum foil
D:\533571560.doc
8 (8)