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Comment by apatheticonion | Hacker News Reader

I was interested in trying to make a DIY thermal battery as a hobby experiment. Other than using thermal energy directly, I couldn't find a way to effectively convert the heat energy to electrical energy.

Peltier modules can be used to generate electricity, but they are crazy inefficient.

An efficient steam turbine is largely inaccessible to hobbiests and I am scared of steam/pressure. Though I did look at repurposing a car turbo for this purpose. There were additional issues with regulating the amount of heat you wanted to extract (load matching) and recycling waste heat.

I wondered if it was possible to use a Sterling engine, but you can't buy anything other than very small toys online and I don't have the facilities to machine my own.

Haha, would love to get something working, but I suppose I'm not smart enough to figure out an effective way to get that heat back out as usable/controlled electricity.

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The answer in almost all electrical production boils down to spinning a turbine with steam (or wind). Nuclear does it, all the fossil fuels do it and ultimately heat batteries do it too. The alternative is photovoltaic or directly nuclear to electron production and then storage with chemical batteries or massive capacitors.

Most of our electrical production is based on a solution found several hundred years ago, we just made it really big and worked out how to control the heating and pressure of the steam well.

You missed thermoelectric generators that uses the Seebeck effect to generate a current between two temperature differentials. It's terribly inefficient, unfortunately.

Non-steam turbines have been operated (e.g. https://en.wikipedia.org/wiki/Mercury_vapour_turbine), but… steam is just so much easier to work with.

Come on, mercury vapour sounds like sooooo much fun! Where's your sense of adventure?

> An efficient steam turbine is largely inaccessible to hobbiests and I am scared of steam/pressure.

Thermal electricity generation really benefits from scale and extremes. The Carnot efficiency is proportional to the temperature differential between hot and cold. Even so-called "low quality" heat from a standard nuclear rector design is far hotter than anybody should deal with at home and it only gets ~1/3 efficiency. And dealing with small turbines is really inefficient too.

This is where batteries and solar really shine. They scale so well, and are extremely economical and electrically efficient.

Heat storage works well when you get beyond the scale of individual homes, but it's hard to make it work. I'd love to see something related to heat pumps in the future for homes, but district heating, such as could be accomplished by converting natural gas systems to heat delivery, are probably required for it to make sense.

Yeah, sadly, it seems almost impossible to get anything higher than 30% efficiency (theoretically with a Stirling engine, if you can find one, haha) out of a thermal battery without extreme pressures and temperatures.

Back-of-the-napkin math felt promising. A 1kg block of sand heated to 500 degrees Celsius should contain about 100Wh of electricity. Scaling that capacity up is easy, as it's just about adding sand or temperature (+ an effective method of transporting heat across the sand - maybe sand + used motor oil?).

Assuming 80% efficiency, tariff arbitrage (buy electricity during off-peak hours and use it during high-price hours) would pay off very quickly. In my area (Australia) it would be a matter of months - but the low real-world efficiency and lack of parts make it impossible.

It could work for heating during winter, though perhaps an AC/heatpump with the condenser a couple metres underground would be better value for money.

Heat storage can work for individual homes on the shorter scale. If you heat your home with in-floor heating (lower temperature requirements) you can have ~1-2m3 buffer tank that you heat up during the night and then use the stored heat during the day to heat your home. Works very well.

This project is for district heating, not producing electricity.

In general it is true that low-grade heat is difficult to convert to electricity, and there isn't any existing mass-market device that does it. You'll have to make your own, which involves learning to machine and responding to your perfectly reasonable fear of steam and pressure with proven safety measures.

Every couple of years I look around to see if anyone is selling sterling cycle engines in the 5-10 hp range, I always find a couple neat projects but nowhere can you just buy an engine.

I assume that because there is no current market for small sterling generators nobody wants invest in tooling to make one and because there are no small sterling generators there is no market for them.

In the articles case the end use of energy is household heating, so there is no need to convert back to electricity. The whole beauty of thermal energy storage that the end use of energy in many use cases is.. heat: heating buildings, cooking, industrial heating (from food processing to iron smelting), producing steam, etc.

If you need to use heating in a cold climate, you could use your stored energy to heat the radiator of a heat pump, which would then be drastically more efficient than using normal air on the radiator.

There's a video of people doing this on YouTube. They use the ground as their heat source. https://youtu.be/s-41UF02vrU

https://en.wikipedia.org/wiki/Thermoelectric_generator

Seebeck generator, generally. Peltier goes the opposite way. But basically the same thing.

If your hot source is really hot, thermophotovoltaic (https://en.wikipedia.org/wiki/Thermophotovoltaic_energy_conv...) makes sense and can offer much better efficiency...

LFP is so cheap that small-scale thermal battery makes not sense for electricity generation. Even in big scale (like OP) it mostly makes sense for heat, e.g. district heating systems, industry process heat, etc.

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