Comment by zdragnar | Hacker News Reader

Comment by zdragnar | Hacker News Reader

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Ah, I missed the pipes bit. Yes, that is exactly how the mass storage heaters I am familiar with work- pipes run through clay, which does need to be fully dried before you heat it to avoid cracking.

I did overlook the 600*C bit at first- straw would definitely be a bad idea, lol. I still would guess that the additional cost of clay would be worth it. Clay is both more conductive AND has better capacity- the air between the grains of sand is an insulator, and has significantly lower storage capacity than the grains of sand themselves.

Another option some people use if they don't want a permanent installation (or just need occasional portability) is large stones with gravel for fill. It's quite a bit less efficient than clay, but the larger size of the gravel and rocks inside make it a better option than sand.

I imagine they have done the math on the cost of the sand and the efficiency they are getting out of it, it just goes against everything I have ever learned about thermal mass storage from the permie / rocket heater mass storage community.

Yep.

I have not seen a picture of the filler but they said its the cheapest sand they could get (probably even free).

So i assume it is mostly rocks and gravel anyway.

Not sure about the math here, but it is surely cheaper to build a bigger tank with crappy sand than a smaller one with more expensive clay.

I would say rocket heaters have more constraints on space while for a large industrial tank you would not really care about it being large and heavy.

From an efficiency standpoint you will not run into many problems, since the efficiency of heat based systems is based on the thermal loss.

Thermal loss occurs on the walls and in processing (like the rest heat in the air venting out of the system).

The process losses are independent of your storage material and often unavoidable.

And since those are large installations, the mass to wall ratio is really really good (square–cube law).

If you make them big enough not much insulation is required and the overall efficiency is high.

>… but they said its the cheapest sand they could get (probably even free).

For a one time capex on an industrial installation, what would it matter? As long as the material is not liquid gold, it seems like the material is a drop in the bucket for everything else you have to maintain (personal, pumps, generators, etc).

> Not sure about the math here, but it is surely cheaper to build a bigger tank with crappy sand than a smaller one with more expensive clay.

It's also interesting that the sand is heated with energy generated from wind turbines and solar panels, which means that as long as enough energy is being thrown into the thermal capacitor, it will heat up with little cost. From this point onward, it's an economics problem. Either money is thrown at the efficiency of these heating capacitors, which brings at most small gains, or money is thrown in building additional storage capacity for cheap along with increasing how much energy is fed into the system. Would it be more cost effective in replacing sand with more efficient materials, or spend the money in, say, another cheap thermal energy source?

> Either money is thrown at the efficiency of these heating capacitors, which brings at most small gains,

It really depends on whether the air in the system is a closed loop or not while they are building up the heat in the device. Sand is, at best, going to be about 1/3 as efficient as clay in an open system (hot air goes in, travels through some pipes, then comes back out as waste exhaust). This number is based on experiments with rocket mass heaters I've seen. It could actually be worse- I honestly can't remember if the 1/3 number was for sand, or the large rock (think bowling ball sized) + gravel mix, which was more efficient than sand.

In this scenario, since your input energy is basically free, more storage capacity = more money, and switching to clay could triple your effective capacity.

If, on the other hand, air is blown in a closed loop, you really only risk burning out your fan and resistive heating elements faster (they don't get cooled down by fresh air). In this scenario, 100% of the energy eventually transfers into the sand, so you're really only losing money if you need to spend more on land for setting up additional units.

> In this scenario, since your input energy is basically free, more storage capacity = more money, and switching to clay could triple your effective capacity.

That's all fine and dandy, but the whole point is that projects are evaluated in terms of money and not joules. This means that if a material like clay is , as you claim, 3 times more efficient but ends up costing twice or more to replace and more to maintain, you are in reality looking at a prospective best-case scenario that lies somewhere significant losses or irrelevant gains.

Meanwhile, building the exact same sand-based prototype right next to the original one would literally double the capacity at a far smaller cost.

People want to get warm, not discuss whether clay or sand pack as much heat. Half the world could not care less about what goes into AA batteries.

Projects are evaluated in financial and economical terms. It matters nothing if there's a theoretical possibility that some other material or design is more efficient, if it's savings are irrelevant. The world rides Volkswagens, not Formula 1 cars.

What makes you think any part of this conversation is theoretical? I'm speaking with the experiences of people who do this for a living.

A few tons of clay are not doing to cost more than the benefit from an operating lifetime of triple returns.

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