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> I perpetually dream of systems like this where a process might be undertaken in locations that require HVAC systems that can make use of the waste heat.

Cities are the solution there. Waste-heat from power-plants is ~200F (just under boiling), because the water needs to be steam to efficiently move a turbine. The 200F waste-water is no longer useful at turning the generators, but its still useful to pump around the city and warm up homes.

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IBM was playing around with waste-heat systems to increase the efficiency of solar panels. Solar Panels lose efficiency above 140F, which is pretty common because the sun heats up the solar panels. By running water through the panels and dropping the temperature down a bit, you end up with say, 130F water and 130F panels (increasing panel efficiency, but now you gotta deal with 130F water).

The 130F water is waste heat, but unlike New York City steamworks, its in a hot location that won't be able to use the waste-heat in a useful manner. Unfortunately, I don't think this is high enough temperature to be used for this chemical-iron process...

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That being said: Iron Fuel itself is something I haven't considered personally. But the chemical process sounds quite similar to Hydrogen Fuel / Syngas. Iron being a solid sounds like its easier to move around and store, but Hydrogen has pumping benefits (ex: hydrogen pipelines).

Chemical energy storage methodologies seem useful. There's all sorts of low-grade waste heat (130F to 200F), that's really difficult to figure out how to use. Anything, even if its like 30% or 40% efficient, will be better than the 0% efficient "Waste" that we do right now.

People have tried running water to cool solar panels. It is a bad fit. Plumbing is a source of trouble; the more fittings involved, the worse it gets.

Somebody is selling plastic frames you attach your panels to, to blow air through to cool the panels and claim the excess heat. Air ducting makes less trouble than plumbing does. The air can go through a heat exchanger attached to a heat pump to concentrate it.

Keeping the panels cool increases their efficiency enough to pay for the fan and heat pump, and makes them degrade much more slowly; and, you get the heat if you want it.

I've wondered whether you could pump heat from a solar panel into a swimming pool.

I've seen systems where heat from A/C is used to heat swimming pools. Mostly just needs an extra exchanger on the A/C coolant line that transfers the heat to the pool water. You generally run both system at the same time when it's hot out. (assuming you are fortunate enough to have a pool)

I worked for a bit on a solar powered Stirling engine to drive a refrigeration cycle for HVAC, the idea being that when the sun is shining you need your AC the most and you get all the power in the world to run it directly proportional to how much the sun is shining. So it doesn't need any further control circuitry, sun shines, pump runs. No electricity involved.

that sounds cool. did you get it working?

Stirling engines are quite tricky to get right, as I understand it. And that sounds like you'd need a good sized one, not a toy.

The basics, yes. A direct solar->Stirling path using a stationary engine and tracking mirrors worked very well, the next trick to figure out would have been to arrange for the mechanics and to scale it up a bit.

It's quite magical to see sunlight turned into motion without any intermediary.

Sounds a smart thing to try building.

It's essentially a solar powered heatpump, you could do the same thing with solar panels and a regular inverter + AC pump.

Or... Just install systems that don't have waste heat, like heat pumps.

Anytime you hear 'use waste heat for X', the thermodynamically more efficient version uses a heat pump. And with time, the thermodynamically more efficient version will also be the economically more efficient version.

Waste heat and heat pumps are not mutually exclusive. For example a data center may use heat pumps for cooling, and that exhaust heat is used along with more heat pumps to heat water in district heating circuits. There are lots of these in Europe.

There are no heat pump systems that could supply high grade heat for many (if not most) practical industrial processes done in large scales. You can’t use heat pumps to burn clinker. You can’t use them to heat aluminum for extrusion. You probably can’t even practically use it for dryers in paper mills. The technology you are asking for simply does not exist. This is not to say that it cannot exist, but it’s silly to say that it’s “just” an issue of installing heat pumps.

Would this be in a scenario where homes are producing more electricity than they need, and using the extra to reduce iron oxide? With solar getting cheaper and cheaper, definitely a possibility.

A system more similar to what we have in New England would be the opposite of what you describe: a truck drops off a load of fresh (elemental) iron, and it's oxidized over the winter to heat the house. Then, in the spring, someone comes back and removes all the rust for recycling. This plan would allow iron to replace a lot of propane and heating oil if the energy densities work out.

> Would this be in a scenario where homes are producing more electricity than they need, and using the extra to reduce iron oxide?

There's no need for this to be done at a home, since the home is on the grid.

> A system more similar to what we have in New England would be the opposite of what you describe: a truck drops off a load of fresh (elemental) iron, and it's oxidized over the winter to heat the house. Then, in the spring, someone comes back and removes all the rust for recycling. This plan would allow iron to replace a lot of propane and heating oil if the energy densities work out.

I have severe doubts that iron has as much energy density as methane or propane.

I also doubt that Iron is as easy to carry around as methane pipelines.

> I have severe doubts that iron has as much energy density as methane or propane.

The heat of reaction of elemental iron oxidizing to rust is about 1648kJ per 4 moles Iron. Propane burning is about 2220kJ per mole. Also, Iron atoms are heavier than molecules of propane (55.8g/mol vs 44.1g/mol). This means that propane has an energy density of about 50 joules/gram while Iron has about 7 Joules/gram. So no, not very energy dense on a per kilogram basis.

However, energy density is only relevant to cost of transportation. If elemental iron is stupid cheap compared to methane due to taxation of external costs associated with emitting carbon dioxide, then the additional cost to transport it would be less relevant. In addition, since it is renewable, the cost could be reduced further. But yeah, we are a long way from that since methane and propane are very cheap right now.

> then the additional cost to transport it would be less relevant

I mean, if weight is the only issue, then maybe don't transport it ever?

Its not like we try to "transport" the Bath County Pumped hydro station around (aka: the largest battery in the USA). We just let it sit in one location and convert from electricity (usually nuclear at night) into energy-storage (pump water uphill), and then convert it back later (release the water during daytime peak-electricity usage).

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If the energy "needs to be transported", then convert the energy into another form (Ethanol fuel? Syngas Kerosene? ). Each conversion loses efficiency of course, but if Iron is cheap enough to use as "energy storage", then it can be our "energy storage of last resort", since any storage is better than waste. (IE: Never turn off your solar panels. We always have "something" to dump our excess electricity into)

I was thinking on a residential basis where you would buy and recycle the iron powder. But yes, in a large commercial heat storage facility you would not need to transport the iron at all. Pretty cool!

Are you sure this is a good idea? Why not implement district heating? You can use solar collectors during sunny months and bridge winter with iron energy storage. Those solar collectors free up solar panels which means surplus electricity can be turned into iron instead.

Wouldn't plugging into district heating be a simple way to harness the thermal energy?