Use greenhouse effect to cook

YOU could be cooking for free this summer in a scaled-down greenhouse with shiny walls and a cookpot inside.

Free hot water and electricity are well-known benefits of solar power – but you can also do your cooking and even cool the house.

With the summer warming up I plan to use the heat of the sun and some basic physics – plus time – to produce simple delicious dishes.

There are a few types of solar cooker but the simplest is the box cooker or heat-trap box which really is a scaled-down greenhouse or cold frame: a box with a glass lid.

It is used by many thousands of the poorest people in India for basic cooking. You can buy one or make it yourself – it will cost next to nothing and works brilliantly.

There are many designs on the internet but, boiled down, they are all boxes – cardboard will do, lined with tinfoil or similar shiny reflective material and a lid, preferably glass.

It works because the inside of the box is heated as sunlight enters through the glass top and heats up the cooking pot. Thermal energy is then radiated inside the box but cannot get out of the glass lid and the box gets hotter.

At its simplest, that is all a box cooker is. You can improve the efficiency by insulating the walls, by getting more sunlight to reflect into the box or by putting a tinfoil-lined layer underneath your pot and painting it matt black to absorb more heat.

Paint a pot matte black on the outside with non-toxic paint and use it for cooking. Set it in the “oven”, cut your food into small pieces so it cooks quicker and if you start in the early morning it will be ready by lunchtime.

Obviously you will need to experiment – and avoid foods like chicken until you are confident the process is thorough - but you will be surprised at what you can cook.

The box cooker cooks at moderate to high temperatures so heat is more gentle and there is no need to keep opening up to check as you are simply letting heat escape. Instead, simply turn the cooker to give it maximum sun.

Having enjoyed our solar lunch it might be nice to sit somewhere cool to digest the contents. You can paint your roof white as light-coloured objects reflect heat – but neighbours will object so the next best thing is to dispel the heat using a solar fan.

Using a solar photovoltaic (PV) panel they give most cooling on the days with the most sun. Mount it on the wall or desired location and fit some vents but once done it is done. They are automatic, should last a lifetime and are not vastly expensive. You can also buy ceiling fans operating the same but pushing warm air downwards in winter (sun permitting).

The real cheap way is to close shutters, curtains, windows and doors – it makes a big difference.

This column is written by Marc Asker, the head of EcoPower, a renewable energies company he set up in 2007. He has been involved in the industry since 2003.