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Comment by amelius | Hacker News Reader

Yeah but if you transfer the energy as heat then you will end up with elongated structures (pipes).

You speak theoretically but metropolitan areas in these countries all have those pipes in place and in use for the better part of a century.

Using heat for heating has many redeeming qualities. Heat is high entropy and it is not a good idea to "waste" low entropy energy to create high entropy energy. Many industrial processes run on heat and waste heat is generated everywhere. The systems are also cheap to run once in place.

That's a real issue, but this is for a district heating system which already exists and already faces this issue. And yet the district heating system is presumably practical.

Changing to a different central source of heating (i.e. storage) seems orthogonal.

Is that a problem? Pipes are not technically complicated. Is there something else I'm missing?

Larger storage structures are easier to (thermally) insulate. Because geometry.

But going with larger structures probably means aggregation (fewer of them are built, and further apart). Assuming homes to be heated are staying where they are, that requires longer pipes. Which are harder to insulate. Because geometry.

I can't help but wonder how the efficiency compares to generating electricity, running that over wires, and having that run heat pumps.

The conversion to electricity loses energy, but I assume the loss is negligible in transmission, and then modern heat pumps themselves are much more efficient.

And the average high and low in February in 26°F and 14°F according to Google, while modern heat pumps are more energy-efficient than resistive heating above around 0°F. So even around 14–26°F, the coefficient of performance should still be 2–3.

> heat pumps themselves are much more efficient.

For electricity-to-heat conversion, heap pumps are indeed much more efficient relative to resistive heating, yes. About 4 times more efficient.

In absolute terms, though - that is still only 50% of "Carnot cycle" efficiency.

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

Similarly, heat-to-electricity conversion is about 50% efficient in best case:

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

So, in your scenario (heat->electricity conversion, then transmission, then electricity->heat conversion), overall efficiency is going to be 50% * 50% = 25%, assuming no transmission losses and state-of-art conversion on both ends.

25% efficiency (a.k.a. 75% losses) is pretty generous budget to work with. I guess one can cover a small town or a city's district with heat pipes and come on top in terms of efficiency.

We've got lots of heating districts around the world to use as examples. They only make sense in really dense areas. The thermal losses and expense of maintaining them make them economically impractical for most areas other than a few core districts in urban centers... Unless you have an excess of energy that you can't sell on the grid.

Geothermal heat is also not that functional in cities, you'd need so many wells so close together that you'd most likely cool down the ground enough in winter so your efficiency tanks.

I don't understand, what am I missing? The heat pump increases efficiency by having COP 2-4 right? Assuming air to air and being in, say, Denmark.

Heat (above 100C, say, burning garbage) to electricity: 50% (theoretical best case)

Electricity to heat (around 40C): 200%-400%

Net win?

The surplus energy comes from air or ground temperatures..

Yes you cannot heat back to the temperature you started with but for underfloor heating 40C is plenty. And you can get COP 2 up to shower water of 60C as well.

Yes, this is exactly why I asked. You need to include the COP in the calculations.

I think the big cost difference is the geothermal generators to convert the heat back into electricity. More of a cost issue versus efficiency.

If the heat is stored at high temperature, but the demand (for heating buildings, say) is at lower temperature, it could make sense to generate power, then use that power to drive heat pumps. You could end up with more useful heat energy than you started with, possibly even if you didn't use the waste heat from the initial power generation cycle.

Alternately, if you are going to deliver the heat at low temperature to a district heating system, you might as use a topping cycle to extract some of the stored energy as work and use the waste heat, rather than taking the second law loss of just directly downgrading the high temperature heat to lower temperature.

High temperature storage increases the energy stored per unit of storage mass. If the heating is resistive, you might as well store at as high a temperature as is practical.

Gas-fired heat pumps have been investigated for heating buildings; they'd have a COP > 1.

I am interested if there are any cheap small scale external combustion engines available (steam? stirling? ORC?)

It can be anything between easy and impossible depending on the temperature difference. 200 C steam is easy with a commercially available turbine, but 50 C is really hard. There are things like Sterling engines that can capture waste heat but they've never really been commercially viable.

There's no way around it: We have to respect entropy.

Existing district heating systems can be large.

I live in Denmark the powerplant that heats my home is about 30km away. There are old powerplants in between that can be powered in an emergency.

Yes, building district heating systems that large is difficult and expensive, it wasn't built yesterday, more like 50 years of policies.

Pipes are competing with wires, which are much less technically complicated than pipes.

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