Solar Cooker

Finally seeing a little sun this week inspired me to play with making a solar cooker. Here's some notes on the topic

Materials for Concentrating Sunlight

Fresnel Lenses

Cheap, A4 size, Fresnel lenses can be had for under a pound. I got one from a discount book store. They're sold to magnify text, but can also be used to concentrate solar energy.

Disadvantages: For best performance, the angle to the incoming sunlight needs to be exactly 90 degrees; They are designed for accuracy of focussing light, which isn't always the key consideration in solar cooking; My impression is that they reflect quite a bit of light. The focal distance is fixed.

Advantages: Light, cheap, sturdy. Concentrate light/heat effectively.

Ideas: The light from a lens can be reflected, using a mirror smaller than the lens, onto a target. If several lens/mirror pairs were used, this might be a useful way to concentrate a fair amount of energy on a small target. For example, 5 lenses arranged in a cross shape (all at right angles to the incoming sun) could supply energy to 5 sides of a cubic target. A back of the envelope calculation (210mm \xD7 297mm * 5) shows that this would capture light from about 0.3 of a square meter, and could concentrate (when ideally aligned) onto a target cube with edges of about 5cm.

Conclusions: Probably more useful for melting or burning things than cooking!

Old CDs

I've lots of old CDs knocking about, each is quite an effective mirror. A rough test showed that yes, if you can align them correctly, they do seem to work to concentrate energy. They were a pain to arrange for the test, requiring lots of blue tack, and wedges to keep them at the correct angles. If mounted on an appropriately curved surface, they could provide a good and pretty reflector. Maybe useful for a semi-permanent installation like a solar BBQ.

Disadvantages: Fiddly to mount. Hole in the middle and non-silvered areas make gaps in coverage.

Advantages: Like a lot of little mirrors. Effectively free. Seem relatively robust as mirrored surface is protected by plastic.

Ideas: Some sort of cheap stand which could hold the CDs in the centre and angle them would be great. Mount in a curved concrete bowl for a solar BBQ. Paste on a garden wall to provide extra light for plants.

Glass mirrors

Disadvantages: Heavy, breakable, not necessarily on hand in correct sizes.

Advantages: High reflectivity. Optically accurate which is useful to reflect light collected by another source)

Ideas: Use to reflect light from Fresnel lenses or reflectors onto a central cooker/collector. Handy to make a default "floor" reflector when experimenting.

Silver foil on corrugated cardboard

Prit-stick seems to stick kitchen foil to cardboard very well. I very quickly made 2 reflectors about 30cm by 60cm out of materials I had lying about. Corrugated cardboard folds well, without tearing, and I found that I could easily fold the reflectors down into a quite small package. This had the side-effect advantage of giving them a natural curved shape when unfolded.

Advantages: Cheap, light, flexible, high reflectivity. Easy to make in bulk. Easy to make in any required shape. Foldable. Portable. Provides some insulation if needed.

Disadvantages: Not waterproof.

Conclusion: The way of the future.

Infrared Transmission

http://www.solarstop.net/edtm/Energy_Transmission_Measurements.htm

Targets

Once you've focussed the energy coming in, you need to convert it to heat by absorbing it in a "target".

Targets are what the light and heat are aimed at. This can be directly onto food, or onto a dark pan/container. So far, I've mainly used a black pan with a black lid. This rests on top of a pyrex dish, allowing light to be targeted at it from below as well as the sides and top.

Considerations: The key one is absorption of light/heat. There's no point gathering all that energy if a lot of it is reflected away from the target. Other considerations are the retention of heat by elimination of radiation, convection and conductance. A lighter target would heat up more quickly, but is also more vulnerable to fluctuations in solar input (i.e. clouds!)

Transparent Materials to Keep Heat in

Now we've concentrated the energy and converted it to heat, we need to keep the heat in one place. Glass or similar materials can reflect Far infrared energy (radiated heat) back to the target and stop it escaping. This is the "greenhouse effect", light and near-infrared energy can get in, but the heat cannot get back out.

• Notes on Energy Transmission through Glass

Calculations

How much energy can you get from a given size of reflector

According to this map of UK insolation %EXT% each square meter of the southern half of Great Britain receives about 1000 Kilowatt Hours of solar radiation per year. Of course, there's more in the summer than the winter, the chart shows below 2kWh per day from October to March, and more than 4kWh/day in May, June and July. The other big factor is that these are averages. Some days it's sunny and some days it is cloudy, so there's a high variance.

Still, based on this we should be able to estimate just how big a collecting surface we'd need to cook successfully in the UK.

Let's start by assuming we use a collector 1meter square, and that we've getting 4kWh/m²/day (e.g. a sunny day in the warmest half of the year). Over the day, we get 4kWh coming in to the system. Say we capture half of it, that's 2kWh, and that we're cooking over a 3 hour period (1/4 of the 12 hours of daylight), which gives us 1/2kWh.

Let's put this approximation into an equation

Energy received = energy in per day * proportion retained * number of hours/12

Energy received for 3 hours on sunny day = 4kWh * 0.5 * 3/12 = 0.5kWh_

How much water can we raise to 100degrees with 1/2kWh?

According to this diagram showing energy stored by hot water %EXT%, we could raise the temperature of 10 litres of water by about 50degrees C, or 6 litres by 80degrees, or 4 litres by 100 degrees. Cooking food requires around 80 degrees (food is pasturised around 70 degrees, and water around 65). Also note that for solar cooking, we're not typically heating large amounts of water. Let's pick a target raise in temperature of 80 degrees above the ambient.

Let's check this using the equation given on the site:

energy required = specific heat capacity * temperature difference * volume of water in litres

The specific heat capacity of water is 4.2 kJ/kg per degree C. As 1kWh =3600kJ and 1kJ is 0.000277kWh, we can say that this is equal to 4.2/3600 or 0.0011666kWh/kg per degree C.

So, to raise one litre of water by 80 degrees, we need 0.0011666* 80 * 1 = 0.09333 kWh.

With 0.5kWh, we could theoretically raise 0.5 / 0.0933 = 5.3 litres of water by 80 degrees.

This assumes we're not losing any heat (e.g. through conduction) during the process, and that we're capturing about 50% of the incoming energy (a very conservative estimate). If we assume we lose half our energy during the cooking period, we can only head about 2.5 litres of water by 80 degrees.

See also: http://wiki.answers.com/Q/How_much_energy_is_needed_to_boil_water and http://www.sunspot.org.uk/ewb/

To Investigate...

What are good values for the efficiency approximations. For example, these solar cooker tests %EXT% gave efficiencies of 15% to 20%.

Doing a "Solar Survey" - 1 hour to evaluate the solar potential of a site and understand how the sun position changes by hour and month.

Absorptivity and Emissivity of a Solar Selective Surface (pdf)

Other technology to investigate

Heat pumps / heat pipes. These respectively concentrate or selectively transport heat. Could you use them to move heat from multiple sources to a cooker?

Seebeck / Peltier Thermo-Electric Cells

Peltier Cells (sold here) convert heat to electricity (or vice-versa)... someone has demonstrated using a Peltier cell to convert solar heat to electricity %EXT% (video)...and apparently the National Science Foundation is funding a version invented by the same guy who invented the "Super-soaker" water pistol!

Nicely documented home experiment using a modified solar cooker - the results weren't great though!

Stirling Engines

Drinks Can Stirling Engine

Project Websites

• Build it Solar
• Fieldlines.com
• Educational Projects -