# Storing Energy as Heat in Vegetable Oil

• TheSlovakEngineer
In summary, the blogger calculated that he needs 185.4 liters of vegetable oil to substitute a 6.4kWh battery if he had the same energy conversion (Tesla says 90%) and energy loss over time (no idea about this value). The range is 100°C to 180°C.f

#### TheSlovakEngineer

Hello, I am writing a blog article about storing solar energy as heat. Can someone credible check this?

I calculated I need 185.4 liters of vegetable oil to substitute a 6.4kWh battery if I had the same energy conversion (Tesla says 90%) and energy loss over time (no idea about this value) as batteries. The range is 100°C to 180°C. Is this correct?

It just seems really weird that people want to use batteries for solar installations if there was such an alternative.

Also, any information about this topic would be awesome. I found this. I want to try to find out if it is plausible to consider such a system with a low-temperature differential sterling engine. I want to find out how it could compare to today's solar installations.

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"if I had the same energy conversion (Tesla says 90%) "

If you use the solar energy to heat up the vegetable oil, how would you propose to get this energy out again at high efficiency? Your low-temperature differential Stirling engine will be limited to thermodynamic efficiency, which, depending on the temperature differential, will probably be on the order of 20%. Suppose the low temperature reservoir is room temperature (273K) and the high temperature reservoir is 100C (373K), then the maximum thermodynamic efficiency would be 27%.

TheSlovakEngineer
"if I had the same energy conversion (Tesla says 90%) "

If you use the solar energy to heat up the vegetable oil, how would you propose to get this energy out again at high efficiency? Your low-temperature differential Stirling engine will be limited to thermodynamic efficiency, which, depending on the temperature differential, will probably be on the order of 20%. Suppose the low temperature reservoir is room temperature (273K) and the high temperature reservoir is 100C (373K), then the maximum thermodynamic efficiency would be 27%.
Yes, I am counting on getting a low efficiency of the energy stored and having to change the size of the reservoir accordingly. I forgot to change to Kelvin and calculated the efficiency with Celsius. Now it looks very improbable, that that contraption works during the whole night.

Water is a great (and cheap) medium for heat storage due to its high specific capacity. What is the reason to use cooking oil?

"if I had the same energy conversion (Tesla says 90%) "

If you use the solar energy to heat up the vegetable oil, how would you propose to get this energy out again at high efficiency? Your low-temperature differential Stirling engine will be limited to thermodynamic efficiency, which, depending on the temperature differential, will probably be on the order of 20%. Suppose the low temperature reservoir is room temperature (273K) and the high temperature reservoir is 100C (373K), then the maximum thermodynamic efficiency would be 27%.
Well I saw a huge tank in the original they showed. I got 832 litres for 20% efficiency. The temperature goes up to 180°C. Still seems like it could be done

Is that 180F?

Water is a great (and cheap) medium for heat storage due to its high specific capacity. What is the reason to use cooking oil?
I think he used cooking oil because it does not boil above 100°C and you can go very low tech. At least from what I understand water would have to be pressurized etc. I am asking here, you tell me.

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Is that 180F?
No, operating temperature is 100°C to 180°C.

Is there a "low tech" solution to converting the internal energy of the oil into electrical energy?

Is that 180F?
Do you think that it is reasonable to expect such a system to work, or do you see some problems that can not be solved with it?

Is there a "low tech" solution to converting the internal energy of the oil into electrical energy?
Well, I think that his Stirling engine could be reasonably easy to manufacture as opposed to one that needs to be perfectly tight so the gas it operates with does not escape. Do you see a problem with this solution? What is it?

Well, I was not sure what you consider "low tech". I have seen many models of Stirling engines. Are there real engines (maybe 1 kW power) easily available?
This site claims that they are the only ones to provide something similar, after they invested 200 million euros.
http://www.okofen-e.com/en/engine/
Does not look like a do-it-yourself project. :)
But that was in 2010.

Yes, I know them. I was going to include them in the article. http://www.microgen-engine.com/buy-engage/
But they do not do any energy storage with heat as far as I understand. I think it would be useless because that is not a low-temperature Sterling. Or am I wrong? I am not looking for a do-it-yourself. Rather trying to find out what credible people think about this one.

Well I saw a huge tank in the original they showed. I got 832 litres for 20% efficiency. The temperature goes up to 180°C. Still seems like it could be done

Have you thought about safety? That much hot oil could kill several people. What precautions do you plan to guarantee that people and the oil never come in contact with each other?

Have you thought about safety? That much hot oil could kill several people. What precautions do you plan to guarantee that people and the oil never come in contact with each other?
Well, I guess there would have to be some dump loops to keep it from overheating, but I have seen those in systems with water. It could get dangerous though. But I am asking rather than advocating. The whole article is just supposed to be an exploration of whether it can be done, how it would look and maybe try to give a few estimates of the costs.

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Yes, I know them. I was going to include them in the article. http://www.microgen-engine.com/buy-engage/
But they do not do any energy storage with heat as far as I understand. I think it would be useless because that is not a low-temperature Sterling. Or am I wrong? I am not looking for a do-it-yourself. Rather trying to find out what credible people think about this one.
I am not saying to use their motor. It was just what I found, and it did not look low tech to me.
Do you have another provider of Sterling engine with the required parameters?
In my opinion the question should be not "can it be done" but rather "it is worth doing it"? Obviously you can do it if you don't care about efficiency, safety and availability of the components.
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The Moon landing can be done. But it is not done too often.

Have you thought about safety? That much hot oil could kill several people. What precautions do you plan to guarantee that people and the oil never come in contact with each other?
You could make french fries and donuts during the experiment.
Water would get my vote any day for the safety reason.
Alternatively, why not use 'hot rocks' as in thermal night store heaters and use an alternative circulating fluid as is done in some motorcycles and other engines?

I am not saying to use their motor. It was just what I found, and it did not look low tech to me.
Do you have another provider of Sterling engine with the required parameters?
In my opinion the question should be not "can it be done" but rather "it is worth doing it"? Obviously you can do it if you don't care about efficiency, safety and availability of the components.
.
The Moon landing can be done. But it is not done too often.
Well, I think the answer would be something like this: https://www.sciencealert.com/austra...t-a-world-record-for-solar-thermal-efficiency But I think this is more exciting.

It just seems really weird that people want to use batteries for solar installations if there was such an alternative.

I worked as a solar power plant developer for 6 years and the problem always came down to efficiency, cost, and maintenance. Then there is the almighty cost vs benefit analysis.

There were 4 main types of power storage that we explored: batteries, pumped water (pump water uphill and let if flow down at night), compressed air (pumping air into tanks then using it later to turn a generator), and kinetic storage (spinning massive fly wheels to use their kinetic energy later).

The best efficiency I remember came from batteries at around 96%, and batteries also require very little maintenance. The problem always came down to the little bit of money gained by selling more power during peek times barely made the investment worth the trouble.

You might have a good idea here but you have to conciser the upfront cost to install a system like this, the cost of maintaining the system for the life of your panels (20-30 years), the efficiency (power in vs power out), and the monetary benefit you will get from using that power during the night.

nasu, Asymptotic, TheSlovakEngineer and 1 other person
You could make french fries and donuts during the experiment.
Water would get my vote any day for the safety reason.
Alternatively, why not use 'hot rocks' as in thermal night store heaters and use an alternative circulating fluid as is done in some motorcycles and other engines?
"For the purposes of this article, I will calculate heat storage with vegetable oil, but there are far better chemicals to store heat in." This is what I wrote even before I came here. I know there are better alternatives, but I wanted to explore this since all systems with heat storage were commercial and I was wondering why we do not even attempt to do something for homes.

It just seems really weird that people want to use batteries for solar installations if there was such an alternative.

How do you charge your electric car using a tank of hot oil?

As I recall Stirling engines of >1HP are quite big.

TheSlovakEngineer
Heat engines are inefficient when the temperature gradient is small. You might be able to use an expansion engine with a low boiling point working fluid, such Freon or propane.

TheSlovakEngineer
Thank you all for the responses. Nice input.

The biggest problem is the low efficiency. There's a reason solar thermal plants use heat reservoirs like molten salt at several thousand degrees rather than some kind of working fluid at only a few hundred - you need very large temperature differences between your hot and cold reservoirs to make a heat engine efficient enough to be worthwhile.

TheSlovakEngineer
The biggest problem is the low efficiency. There's a reason solar thermal plants use heat reservoirs like molten salt at several thousand degrees rather than some kind of working fluid at only a few hundred - you need very large temperature differences between your hot and cold reservoirs to make a heat engine efficient enough to be worthwhile.
Yes, thank you. I was planning on considering these things in the article. I actually expect this particular solution not to be practical. I just wanted to check if there was something I did not consider that would make the solution absolutely impossible. I want to advocate storing solar as heat in general mainly because a lot of times, heat is exactly what you need when it comes to using energy and in those cases, I think a photovoltaic system comes short and thermal systems are much more efficient. So if we could make them efficient enough at producing energy, they might be the better option.

The biggest problem is the low efficiency. There's a reason solar thermal plants use heat reservoirs like molten salt at several thousand degrees rather than some kind of working fluid at only a few hundred - you need very large temperature differences between your hot and cold reservoirs to make a heat engine efficient enough to be worthwhile.
In my head I had something like this: https://dlsc.ca/index.htm. The beauty of internet... You have a concept in mind and find someone already working on it. I was thinkig how to do this and produce electricity as well.

The beauty of internet...
It's a beauty that should be treated with a lot of care. Information transfer is fast but the information is not all 24 carat!

It's a beauty that should be treated with a lot of care. Information transfer is fast but the information is not all 24 carat!
Well yes... That's why it's usefull to do the math so one gets an idea about what it's really like. But this is a Canadian governement project... I have not looked at it in detail yet. Do you think it's bs?

I will look at it when I have time...

Another caution, vegetable oil will gradually degrade when heat-cycled, give all sorts of nasty, intractable deposits in your plumbing...

sophiecentaur