Building a solar cooker which can store heat

In summary, a professor from MIT designed a solar cooker which used lithium nitrate to store the heat. He suggests using water instead of something else, using a fresnel lens to focus the sunlight, and lining the container with refractory bricks for insulation.
  • #1
PraAnan
77
1
Hi everyone,

I'm currently trying to build a solar cooker which can also be used during the night / the next day so I was wondering if I can get some advice about how it can be done, especially the material that I can use to store the heat.

I saw an article that said a professor from MIT had designed a similar grill which used lithium nitrate to store the heat and I came across a couple of articles about using olive or soybean oil.

Not sure if I did it the correct way but I also looked at substances which have a high heat of fusion and came across sodium acetate, paraffin wax and glycerol.

Thanks for reading.
 
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  • #2
Well. What I had in mind isn't exactly what you asked for but seems easy enough do to: Use the solar cooker to heat some something say water. Store that hot water inside thermal or pressure cooker, then you can use it to cook during night either the water or the heat given off by the water, but you'd probably have to do some rigging to your cooker if you aren't cooking with the water. Just an idea.
 
  • #3
PraAnan said:
I saw an article that said a professor from MIT had designed a similar grill which used lithium nitrate
Well yeah.
Just make sure you look at the Material Safety Data Sheet of any substance you may feel is a good choice.
Lithium Nitrate could be problematic for a number reasons.
http://www.lbdassociates.com/libr/MSDS/Lithium Nitrate.htm

PraAnan said:
a high heat of fusion
Probably the way to go as one consideration.

PraAnan said:
sodium acetate
There is the trihydrate, and there is the anhydrous.
Anahydrous has a much higher melting temperature ( 324 C ) than the trihydrate ( 58 C ).
https://en.wikipedia.org/wiki/Sodium_acetate
 
  • #4
SIQI said:
Well. What I had in mind isn't exactly what you asked for but seems easy enough do to: Use the solar cooker to heat some something say water. Store that hot water inside thermal or pressure cooker, then you can use it to cook during night either the water or the heat given off by the water, but you'd probably have to do some rigging to your cooker if you aren't cooking with the water. Just an idea.

I didn't consider water because of it's low boiling temperature and I didn't want to go through the hassle of dealing with super-heated water.

256bits said:
Well yeah.
Just make sure you look at the Material Safety Data Sheet of any substance you may feel is a good choice.
Lithium Nitrate could be problematic for a number reasons.
http://www.lbdassociates.com/libr/MSDS/Lithium Nitrate.htmProbably the way to go as one consideration.There is the trihydrate, and there is the anhydrous.
Anahydrous has a much higher melting temperature ( 324 C ) than the trihydrate ( 58 C ).
https://en.wikipedia.org/wiki/Sodium_acetate

Yeah lithium nitrate plus some of the other substances are a bit too dangerous for me to consider using them.

When it comes to sodium acetate I ruled out the trihydrate form because of it's 122C boiling point but do you think the melting temperature of the anhydrous form is too high for solar storage?

What do you think of paraffin wax, it's cheap and easily obtainable, it seems to have a low melting point and a high enough boiling point so I wouldn't have to worry about it but I do wonder about it's flammability at that temperature even if it's in an insulated and sealed container.

Do you have any idea on alternative substances that could be used?
 
  • #5
What temperatures are you planning for? Rupture of a tank full of hot paraffin could kill someone. We don't like to discuss dangerous topics here on PF.

So independent of the chemical agent, can you tell us how you plan to make this project safe?
 
  • #6
PraAnan said:
What do you think of paraffin wax, it's cheap and easily obtainable, it seems to have a low melting point and a high enough boiling point so I wouldn't have to worry about it but I do wonder about it's flammability at that temperature even if it's in an insulated and sealed container
Which is why this post
anorlunda said:
So independent of the chemical agent, can you tell us how you plan to make this project safe?
 
  • #7
Flamability is not the only risk, the boiling point of paraffin is 370C.
 
  • #8
anorlunda said:
What temperatures are you planning for? Rupture of a tank full of hot paraffin could kill someone. We don't like to discuss dangerous topics here on PF.

So independent of the chemical agent, can you tell us how you plan to make this project safe?

The project itself is pretty simple. I have a large fresnel lens (wanted to use a mirrored satellite dish but I already have a lens) and I will use it to focus the sunlight into a metal container (with a temperature monitor) which will be lined with refractory bricks for insulation. I intend to have the container on wheels so it can be moved easily rather than having to carry it. The idea is to heat the heat storage material during the day and then wheel it into the 'cooking area' during the evening.

anorlunda said:
Flamability is not the only risk, the boiling point of paraffin is 370C.

My fresnel lens has an area of less than 1m squared, so even if you assume that the lens is focusing 1kW of solar energy into the metal container with the paraffin wax then it would take more than an hour for 5kg of wax to go from 30C to 370C (4.13MJ / 1146Wh). I'll be monitoring the temperature so I can just 'turn off' the lens once the temperature goes over 300C right?

If you think paraffin wax is too dangerous what are some safer substances? There is sand but it's heat capacity is about 3 times less and I'm not sure about how good it'll be at transferring heat to a cooking pan as it's a solid. There is molten salt (potassium and sodium nitrate mix) but that has a pretty high melting temperature of 220C and not easy to get hold of.

As a side note, with regards to the heat storage substance do you think it would be better to post in the chemistry section?
 
  • #9
I appreciate your efforts to consider safety. However, your purpose is a cooker. Anything hot enough to cook is hot enough to burn people. Yet you are asking strangers on the Internet for advice.

We cannot know the entire situation, regardless how detailed your description might be. I.e. we have neither all variables nor enough statistical nor mechanical data, which a responsible judgement would require.

We don't know what country you are in and therefore we can't know what safety codes and regulations might apply to your situation, if any. Your posts do not mention your own knowledge of applicable safety codes.

The best advise we can give is that you should consult a local engineer who is familiar with your local regulations and who can inspect the overall safety of your project.

Good luck. You have some answers to your original question. This thread is closed.
 
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1. How does a solar cooker store heat?

A solar cooker can store heat through the use of thermal mass materials, such as rocks or water, which can absorb and retain heat energy from the sun. These materials are placed inside the cooker and act as a heat sink, allowing the cooker to maintain a consistent temperature even when the sun is not shining.

2. What is the best material to use for thermal mass in a solar cooker?

The best material for thermal mass in a solar cooker is one that has a high heat capacity, meaning it can absorb and retain a large amount of heat energy. Some commonly used materials include rocks, sand, and water. The specific material used will depend on the design and size of the solar cooker.

3. How long can a solar cooker store heat?

The length of time a solar cooker can store heat will vary depending on factors such as the amount of thermal mass used, the outside temperature, and the type of cooking being done. Generally, a well-insulated solar cooker with a sufficient amount of thermal mass can store heat for several hours.

4. Can a solar cooker store heat overnight?

While some solar cookers may be able to retain some heat overnight, they are not typically designed to store heat for such a long period of time. It is important to use the stored heat within a few hours for optimal cooking efficiency.

5. What are the benefits of using a solar cooker with heat storage?

Using a solar cooker with heat storage can provide several benefits, including the ability to cook food even when the sun is not shining, increased cooking efficiency, and cost savings on energy bills. It is also a more environmentally friendly way of cooking, as it utilizes renewable energy from the sun.

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