Solar Cooking Seminar

Solar Cooking Seminar – Course notes. 6^th March 2005; 4^th March 2006 & 10^th March 2007– Engineers Without Borders Energy Course at Imperial College, London.

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Hello, my name is Quinton Stowell, I am Product Development Manager of a Company called Nordson UV Ltd and I am here today by kind invitation to talk about Solar Cooking.

1. Overview of the sessions.

Before I start has anyone had any experience of Solar Cooking? Solar Cookers are devices that convert Solar Radiation into usable heat energy for cooking purposes.

The session is broken up into 2 main parts. The first 50 minutes is going through the:

¨ Theory and application of solar energy and solar cooking.

¨ We will be looking at the comparison between Solar and wood/gas.

¨ Design features of Solar Cookers

¨ Case Study in Peru

¨ Successful initiatives.

This will be followed by a practical session where we will make some solar cookers, and weather permitting we can cook something! In the practical sessions you will be divided into 3 groups of 6 or 7 to each make a cooker.

2. Who am I and how I got involved.

I did a degree in Mechanical Engineering at Southampton University, and since then I have been working for a company that designs and manufacturers UV curing systems for printing inks and coatings. This is an example of a UV lamphead that I have designed….

3. Links with Peru.

In my spare time I also run a youth group at our local church for 14 to 18 year olds, and it was at one of these events that a seed was sown. Our Church has strong links with a small charity in Peru called the Peru Children’s Trust, and one of their workers was presenting the work that they do and some of the issues that they have. The families that they work with are extremely poor, and live in the shanty towns of Huancayo. Huancayo is in the Andes at 11,000 feet, and they get plenty of sunshine. They also have very little money to buy gas or get hold of wood. “Could they use the sun to cook with?” was the question that was asked.

4. Experience in related field.

Having spent over 10 years designing reflector systems for UV lamp boxes I had plenty of understanding about the physics associated with the collection of radiation energy, so off I went with my invention hat on. I had never heard of Solar Cooking before, so I didn’t know if it was possible or effective.

5. Overview of work.

To summarise what I have done since then, which I will go through in more detail is:

a. Literature and internet research on existing technologies

b. Discussions with Peru Children’s Trust

c. Made 3 different prototypes

d. Went to Peru and organised a workshop whereby 10 of the children (12 to 16 year olds) each made their own solar cooker over a 2 day period.

e. Designed a web site with plans and links. www.sunspot.org.uk

f. Numerous enquiries and interest.

g. This is my first seminar, and I am fully supportive of the principles behind Engineers without Borders.

6. Why is solar cooking important?

The ‘Manual for solar box cookers’ published by Technology for Life, Finland quotes: -

"About 2000 million people, over one-third of the population of the world, are daily dependent on firewood as the source of their cooking and heating energy. They live in the tropics, in the most favourable areas for the use of solar energy. Every year the cutting of firewood results in the loss of 20,000 - 25,000 km² of tropical forests (UNEP).

The use of solar cookers also brings with in important health benefits. Diseases spread through contaminated water cause 80% of the illnesses in the world (WHO). Heating water to the pasteurisation temperature of about 60 ⁰C destroys disease organisms. This temperature is easily achieved with solar cookers. Acute respiratory infections (ARI) are the cause of death for millions of children in the world each year. The large majority of these casualties occur in the developing countries as a result of polluted indoor air due to cooking over open fires in houses without chimneys or ventilation. This problem could be greatly reduced by using solar cookers, which are, of course, completely smokeless."

7. How much energy is in the sun?

This is the all important question. How much energy is available from the sun, and how big does the cooker have to be to be useful.

The Solar constant is a value of 1367 W/m². This is the sum of the UV, Visible and Infrared radiated energy that is available.

This picture describes the absorption that occurs as the radiation goes through the earth’s atmosphere. So the highest intensity that you will get, on a clear day, with low air pollution is 83% of 1367W/m², which is 1134W/m². For simple engineering figures it is best to stick to 1kW/m².

Below is a graph of actual measurements on a sunny day and cloudy day in California. The figures are from the vertical direction, so it is possible to get closer to the 1000W/m² if the solar collector is directed towards the sun.

8. Theory behind solar cooking.

Here is a picture for a solar power station in California where they use parabolic reflectors to heat a fluid to very high temperatures which is then used in a steam turbine.

So we have 1kW/m² of usable energy. It is fairly low grade energy, so all solar collectors are designed to either collect and retain the solar energy, or focus the solar energy into a small area of high intensity.

Regarding cookers, the two main designs are the parabolic cooker, which was the design I chose, and the box type cooker.

9. Solar cookers versus wood burners or gas.

Before we get carried away and think that Solar Cooking is the ultimate we should just compare the options:

Wood Burners or Gas	Solar Cookers
Disadvantages	Advantages
Takes time to collect wood, and is using up natural resources.	Solar energy is free
Smoke filled rooms = ARI (Acute Respiratory Infections)	Smoke free cooking
Advantages	Disadvantages
Cook any time of the day/night and when cloudy.	Only cook when it is sunny.
Gas is instantaneous and controllable.	Box cookers take time to heat up. Parabolic cookers need frequent adjustment.
Cook summer and winter.	Need to store cooker in rainy season.

10. Essential design features of the parabolic type.

On the physics side, you start off with 1000W/m², which you have to concentrate into a very small area to make it useful. The reflector material is important. We chose reflective aluminium with 85% reflectivity and good optical clarity. The accuracy of the reflector is important to ensure you don’t miss the cooking vessel. The colour of the cooking vessel is important to absorb the radiation and transfer it into heat.

¨ Size of reflector

¨ Reflectivity of reflector

¨ Accuracy of reflector

¨ Emissivity of cooking vessel

Thermal power to cooking vessel = (initial usable power) x (area of reflector) x (reflectivity of aluminium) x (efficiency of reflector) x (emissivity of cooking vessel)

Q = 1000 x 1 x 0.85 x 0.95 x 0.9 = 727 W.

Thermal energy to boil 1 litre of water = (mass of water - kg) x (specific heat capacity of water – J/kg^oC) x (temperature rise – ^oC)

Q = 1 x 4180 x (100-20) = 334400 Joules

Time taken to boil 1 litre of water = 334400/727 = 460 seconds = 7 minutes, 40 seconds.

This calculation ignores the secondary heat losses as the cooking vessel gets hotter. In reality it takes closer to 10 minutes to boil a litre of water. The focus is so good, that it is easy to ignite a piece of paper. On the units we made in Peru we measured up to 420 deg C at the focal point, in fact this was how we judged them. Perhaps we could do the same later, weather permitting!

11. Essential design features of box type cookers.

The box cooker works along the lines of the greenhouse. The solar radiation passes through the glass, and then gets absorbed by a black surface. The black surface gets hot and radiates long wave IR which is then trapped inside the box. In this design the critical factors are:

¨ Area of top surface, and additional reflectors.

¨ Transmission of radiation through the glass/plastic.

¨ Emissivity of the cooking vessel.

¨ Insulation of the box to retain the heat.

It is possible to get temperatures up to 80-100 deg C. There appears to be some good ones available and being used in South Africa and India by an organisation called SUNSTOVE.

Key SUNSTOVE materials are:

l) A low cost housing which is blow molded from 75% recycled plastic bottles and 25% virgin material. The case is cheap, strong, light, UV resistant, weather proof and allows the aluminum sheet (walls and bottom) to be snapped into place over the insulations

2) Sloped internal walls and bottom are made from a scrap aluminum off-set (lithograph) printing plate which can be cleaned.

3) Wall insulation from fibreglass blanket, and bottom insulation of high density fibreglass to support the pots of food.

4) A clear cover of glass, plastic sheet or plastic film. In Africa "medium impact acrylic" was chosen because of low cost, availability and strength

Further details at www.sungravity.com

12. Advantages and disadvantages of various types of cooker.

Parabolic Cooker	Box Cooker
Disadvantages	Advantages
Needs frequent adjustment	Adjust once an hour.
More likely to be dazzled by reflected light.	Much less reflected light.
Unstable	Very stable

Advantages	Disadvantages
Very high temperatures. More like a BBQ or Grill	Only 80 - 100 deg C. Cooking takes much longer.
Easy to see it cooking quickly.	Not too sure it is working!

13. Trip to Peru.

With my success with the parabolic type and failure with the box type I decided that the design to go for was the parabolic type.

I had discussions with the charity in Peru and fortunately they had many workshop tools, and said that the raw materials could be bought locally, except the reflective aluminium. So this meant that we had to take it out in our suit case. This was one of the main reasons for choosing the size of the cooker!

The charity hosted ‘fun days’ for the children and young people, and it was decide that 10 of the children 12 to 16 years old would get to make one each and keep it. We bought all the materials before, which cost £30 for 10 of them, and made up one for them to copy. The work was split up into three sections:

a) Woodwork to make the box enclosure.

b) Cardboard to make the internal ribs.

c) Metalwork to make the grill, and the reflector.

Our plan is to make a couple of these later.

14. How did it go, and what did we learn?

¨ The workshops went well, and it was amazing how keen the young people were to get involved once they saw water boiling on our prototype.

¨ The language barrier was not really an issue either. I can’t speak any Spanish, and they couldn’t speak any English, so it was all done via signs and demonstrations.

¨ It was also a good way of getting the young people to feel valued. They all learnt new skills which is good.

On the negative side there were a few things.

¨ The cookers we made were only 700 x 560mm = 0.4m². The cooker is OK for ¾ litre of water, but not much more. Most of the families have +5 children, so it was too small. I should have taken larger sheets.

¨ The families of the young people are very poor. Many of them live in houses which are about the same size as a single garage. The house is split into 2 rooms. A bedroom, with 2 beds for all 7 people, and a kitchen. If we had have made the cookers 1m x 1m, they would not have had anywhere to store it!

¨ Many of their houses are damp, and in the rainy season they became flimsy and inefficient.

15. Years later?

The Peru Children’s Trust’s primary aim is in supporting the +100 children in their schooling. As such there has not been the local support to push anything forward. Also I think that the climate is not hot enough for long enough. Having said that one is still in regular use. Also the trust has set up a training centre for teaching the young people a trade such as welding or carpentry.

16. What makes an initiative successful?

Choosing the right design for the right location.

¨ In hindsight I would have said that the parabolic design is not ideal for the very, very poor. It is good for barbeques in Australia. This one retails at £750! à

¨ You really need over 4kW per sq. meter per day. They will never take off in the UK! They have been successful in India, China and South Africa. This lady lives in the Mongolian Gobi Desert and uses her parabolic cooker for boiling water and cooking rice.

¨ Design must be easy to use, easy to adjust and robust. It also needs to perform well and cook efficiently.

¨ Being made from locally available materials is very important.