Calculate the power of a solar oven

[mageflor]

Hello,

I am writing to you because I am a student and that I decided to do some work about solar ovens. Indeed, I am enthusiastic about these ovens which allow developing countries to cook without contributing to deforestation and especially not to need to go get wood which takes them an eighth of their time. In addition, it allows them to make the water drinkable so this is very beneficial in many ways.

For my project, I would like to know how we calculate the power of a parabolic oven. Is the formula good:
surface * sunshine * thermal solar yield?
On the other hand, what is the value taken for sunshine?
I have indeed contradictory information. On the site http://www.outilssolaires.com/infos/prin-ensoleil.htm
values ​​greater than 3 kW / m² are given. And yet, I was told that direct solar radiation was only 800 W / m² and the only one usable for solar ovens. What is the real answer and the real calculation?
I think promoting solar ovens is always very interesting, so I hope you will give me an answer soon ...
Goods.
Sunshine ...

[Christophe]

On the site http://www.outilssolaires.com/infos/prin-ensoleil.htm
values ​​greater than 3 kW / m² are given. And yet, I was told that direct solar radiation was only 800 W / m² and the only one usable for solar ovens. What is the real answer and the real calculation?

Hi and welcome to this forum

The values ​​given by the site you cite are correct: it is not solar power kW / m² but energy therefore in kWh / m² radiated (I suppose) over a full day. It is true that the page is confused: we go from kwh / m² daily to annual radiation. Example:

Zone A: Northern Europe, from the United Kingdom to Poland. Most of Germany with a large solar market is in this area.
- Average daily solar radiation
= 2.4 to 3.4 kWh / m2
- Average annual productivity of a solar installation = 300 to 400 kWh / m2

This is entirely consistent because 400 / 3,4 = 118 days (of full sunshine).

Here are other maps of solar radiation in France which confirm these figures.



For your project, solar ovens have already been discussed on this forum, especially:

solar-thermal / solar-kitchen-oven-t152.html
solar-thermal / tpe-study-on-the-solar-oven-t2804.html
Do not hesitate to use the search engine: search.php

[Alex]

Hello,

I am writing to you because I am a student and that I decided to do some work about solar ovens. Indeed, I am enthusiastic about these ovens which allow developing countries to cook without contributing to deforestation and especially not to need to go get wood which takes them an eighth of their time. In addition, it allows them to make the water drinkable so this is very beneficial in many ways.

For my project, I would like to know how we calculate the power of a parabolic oven. Is the formula good:
surface * sunshine * thermal solar yield?

To clarify a little, the power at the hearth is equal to the power perceived by the parabola (obtained by multiplying the surface of this one by the value of the solar flux direct expressed in W / m² (be careful, only the direct solar flux is concentrated in the solar ovens!), the whole being multiplied by what one could call the optical efficiency of the parabola (which translates the losses by absorption in the optical system of concentration (parabola even parabola + heliostat). On a good parabola, specially designed for that, the losses by absorption are of the order of 5 to 10% by reflection.
So in the end, if you have 1000W / m² of direct solar flux arriving on the dish and the optical efficiency is 90%, the power at the focus will be around 900W.

On the other hand, what is the value taken for sunshine?
I have indeed contradictory information. On the site http://www.outilssolaires.com/infos/prin-ensoleil.htm
values ​​greater than 3 kW / m² are given. And yet, I was told that direct solar radiation was only 800 W / m² and the only one usable for solar ovens. What is the real answer and the real calculation?
I think promoting solar ovens is always very interesting, so I hope you will give me an answer soon ...
Goods.
Sunshine ...

As I said earlier, the global solar flux has two components:
-the direct component: these are the rays that come directly from the sun
-the diffuse component: these are the rays which have been deflected by the atmosphere, clouds, etc.
Only the direct component is used in solar ovens. As a first approximation, you can use a direct solar flux value of 1000 W / m², but it's probably a bit optimistic (so yes, 800W / m² is a correct value ...)

[mageflor]

Thank you both Christophe and Alex for the explanations and for your speed! It allows me to better understand where all these values ​​come from.
And thank you for your clarifications, it's really good to be able to have sought-after and very complete answers to your questions!
Christophe: thank you for the cards, and also the calculations that are made below because they show how to reuse them. In fact, do you know if it is possible to find a map or you can directly find the number of Watts per square meter, and this not only for France?
Otherwise, thanks for the links on this meme forum.
Alex: thank you for the explanations of direct and diffuse light, I had not understood well until then. It seems to me in fact that for the yield, I was rather spoken of 60 to 80%. But I am not an expert, I am only repeating to you what professionals of the parabolic ovens told me ...
See you soon, and if you find a power card ... Thank you!

Do you have solar ovens yourself? It is true that it seems really interesting. As my work of studies is on that, I inquired. They sell them at iD Solaire: www.idsolaire.com and there are different types: parabolic ovens with heat concentration and box ovens, which work with the greenhouse effect. And it's frankly not expensive: 129 euros for the cheapest and 269 for the box oven. As your site Christophe said or they do the calculations on the solar panels, the main thing is not so much first the financial aspect but the fact that we participate in a real energy saving and therefore that the 'we act in favor of the safeguard of a clean planet. It is already a real investment on the part of everyone if he wants to do it himself in his daily life ... And finally, there are not so many people who are aware of the urgency of the situation, and the essential role they can play at their level: just by talking about it ...
And who knows about solar cooking? It is still very little known.
I do not know what you think of all this ... If you want to give me your opinion in any case, that interests me.

[belze1]

In my case I measure the efficiency of my oven by the following formula:

P = (CM (Tf - Ti)) / D

C: specific heat of water 4,18
M weight (in gr)
Ti: start temperature
Tf: end temperature

D: time between 2 temperatures

I use a multimeter with thermocouple to measure 1 liter of water
in a container placed on the solar cooker.

[Christophe]

Christophe: thank you for the cards, and also the calculations that are made below because they show how to reuse them. In fact, do you know if it is possible to find a map or you can directly find the number of Watts per square meter, and this not only for France?

You just have to ask


Well it is not very readable or precise (but difficult to do better on a static map worldwide)

[mageflor]

Thanks to belze 1 for the formula, this proves that you are really in practical cases! What types of solar ovens do you do? Box ovens?
Otherwise, thank you very much Christophe, for the card. This is exactly what I was looking for. Where did you find it?
Great!!
That's good, indeed, just ask ...

[Christophe]

Where did you find it?

A good moderator never reveals his sources I laugh I found it on ...Google Images

[mageflor]

Thank you Christophe. So I also searched Google Images, and typed in sunshine & power, but I couldn't find it. I would have liked to find her, but I will understand very well if you do not want to tell me.
In fact, sorry for the question that must seem silly to you, but what is a moderator? In fact, I'm not used to forums, I discovered this one for a few days.
You are the one who set up this forum?
In any case, it's really great. There are nice people to answer questions, it's really a wealth of information and relationships with people interested in the same type of phenomenal projects!

[mageflor]

Hello,

Thank you for helping me find answers regarding the formula to use in the case of a parabolic solar oven. I completely trust you on their validity and in fact I need to cite my sources. Could you tell me on the one hand where one can find the formula: sunshine * useful surface * thermal solar yield?
On the other hand, could you also tell me where we can find this famous value of 1000 W / m² which is everywhere accepted? Besides, I had been told to take 800 instead ...
So if you could tell me where I can find all of these results, I would be very grateful.
Thank you in advance!
See you soon!