Solar oven questions and answers

I received the following mail from a reader, Vinitha, and with her permission, I would like to publish it here, together with my reply.

Before I do that though, I would like to state that I am not an expert - I am just someone who enjoys using her solar oven, whenever and however she can.  If recounting my trials, tribulations and successes mean that someone, somewhere else in the world decides to make or buy a solar oven in order to prepare their meals, then I have attained the goal of this blog - caring enough to share what I know to help make this a better planet and to try and reverse the damage that we are causing, through our waste / garbage and our extravagant squandering of its' precious resources.

Vinitha wrote:

"I live in Bangalore India and we have no dealers for solar ovens here. I am using a solar oven that I built out of wood, glass and a mirror as a reflector. The insides are lined with heavy duty aluminium foil. The temp inside the oven shows only about 80 degree C. Food gets cooked. I have baked cakes, roast beef, rice, veggies. Can you please advise how I can improve it? Is my temp meter recording the air temp inside the oven. When you say yours reaches 325 degrees do you mean the air temp inside the oven gets to that temp?"

She also included a photo of her solar oven:

My reply to her was as follows:

Thank you for your e-mail. It was a lovely surprise to hear from India.  Thanks for also sending a photo of your solar oven – it helps me understand your problem.  It looks a brilliant effort at a solar oven.  Well done.

When I first started solar cooking (a good couple of years ago) I initially used a “Coleman’s” cooler box, with a piece of 4mm glass as the lid. This worked – sort of – but it didn’t get terribly hot. It was not ideal!  (And I was worried about the fumes from the cooler box insulation - I had no idea what the insulation was made of.  I made sure that I cooked in vessels that were tightly sealed in that homemade solar oven.)

Then I bought a simple blow mold solar oven, which had no reflectors and the lid was a very flimsy plastic. The heat produced also wasn’t the greatest – the hottest I recorded was 105oC.  It did the job - even if that was only cooking rice and vegetables.  But it was limited in what it could cook.  Certainly not bread LOL

I finally purchased the model I have now, and it is with this model that I am getting the best results.  I think the reasons that I am successful are as follows:

My kitchen helper LOL

1 The box is solidly constructed, with well insulated sides.

2 The lid is a piece of double glazed glass, which seals against the bottom oven section with a strip of what looks like oven seal, (i.e. able to withstand high temperatures and similar to that which is used on an oven door). I think one of the surfaces within the double glazed glass also has a film, which helps to attract / retain the heat.

In this photo I propped open the glass lid to show
the oven door seal on the inner surface

3 The reflector is adjustable – so that I can aim the ever changing reflected light into the centre of the oven.

4 The base of the oven is black – to attract / absorb the UV rays / heat.

Please bear in mind also, that the strength of UV rays decide the temperature you will attain in your oven. Ideally, the UV rays should be higher than 6 – ideally 7. You can find your levels at this link: http://www.intellicast.com/local/weather.aspx?location=INXX0012 It would seem that your UV levels, even in your winter, are higher than ours at the moment .

If your thermometer is placed inside your oven, then, yes, that is your oven temperature. I place my thermometer on the floor of the oven, right where the cooking vessels go.

I have done a search for solar ovens in India, and have come up with the following links:

http://www.tradeindia.com/manufacturers/indianmanufacturers/solar-oven.html

http://www.tulsisolarcooker.com/Why%20Tulsi%20Solar%20Cooker.htm

I am thrilled to hear that you are able to cook cakes, roast beef, rice, veggies, etc – but please be careful with meat and poultry – in order to kill the bacteria which forms on all raw meat, the pre-heated (oven and cooking vessel)  temperature should not be lower than 120oC. Rice and veggies don’t have that limitation, but they should still be cooked at a minimum temperature of 80oC - 100oC is boiling point.

Then I have also had quite a few people asking me how hot should the weather be, in order to use a solar oven.  Cooking in a solar oven is not, as far as I am aware, dependent on the outside temperature, but more due to the UV levels - the higher the levels, the better your solar oven will work - with a UV of 7 being an ideal starting point.

Think of a solar oven as a small greenhouse.  A greenhouse will heat up in winter when the outside temperature is cold or freezing, similarly the interior of a solar oven will also be much warmer than the ambient temperature.  How hot will it be on a clear day, with snow on the ground - I'm afraid I can't answer that, as unfortunately I am not fortunate to have that amount of snow in winter.  However I have read somewhere that someone baked bread in their solar oven, in the northern hemisphere, under those conditions :-)

If you do a Google search on UV levels for your country / area you will find a link easily.  I did enter a couple of Google links in reply to a question from Mr H on my Ultimate Solar Bread Recipe posting.

Finally, I will, within the next few days, be posting a link to a .pdf file which will give an introduction to cooking in a solar oven. Hopefully, you’ll find that helpful.