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Comment by danimal88 | Hacker News Reader

I build controls for many heatpumps (https://flair.co/pages/mini-splits-and-window-units) and follow the space pretty closely. They are truly magical with respect to pumping heat from the cold outdoors into a warm indoor space.

That said, the efficiency gains on the energy consumption side are admirable. From a global warming standpoint however, heat pumps are actually still pretty bad because of the refrigerants.(https://www.drawdown.org/solutions/materials/refrigerant-man...) This is not an issue with more traditional electric heaters (resistive or otherwise).

The real calculus for those concerned with global warming (I understand this was not the topic of the writeup) is the net effect of these systems over their lifetime - aka, Energy Saved and its GHG effect via leveraging a heat pump vs the GHG effect from the release of the refrigerant. Perhaps the real win would be a refrigerant recovery business that is...air tight :)

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I think when I did the numbers for our heat pump, a catastrophic leak that would release all its refrigerant would result in a GHG contribution equivalent to a year of emissions from our (previous) natural gas heating. Can't find the figures now so do check before quoting me on it ;)

Alternatively, Mitsubishi has been selling a first-generation product that uses CO2 as the refrigerant (the QUHZ model). It posts a COP of 3 for producing domestic hot water (that's pretty good) - I'm not sure why they don't post the COP for room heating, maybe it's not great. But I'm curious what the second-generation product will do!

I've done some theoretical work in this area. The main problem is that most hydronic (water based) heating solutions return fairly 'hot' water after heating the space. In order to get the most energy out of your CO2 refrigerant gas, you need your other heat exchange fluid to be as cool as you can manage. A designer could choose to use a different style of radiator which would allow the water to get closer to room temp, but that is not the type of system currently being manufactured. This would also not work for in-floor heating, as you would have parts of your floor very close to room temperature, not what people expect (warm floors).

These problems really aren't specific to CO2 hydronic systems, and also appear with HFC refrigerants when heating water. CO2 is actually very good for domestic hot water production, where the hot water is not returned.

The problem is really that hydronic heating is a bad fit for heat pumps in general. You'd much rather heat air, and skip the secondary fluid loop.

I recently toured a municipal heat pump plant. It took cleaned sewage at IIRC 12C, dropped it to 6C or abouts, and the heat was pumped into the district heating network at about 90C.

The plant had 6(?) pumps, each producing 18 MW heat while pulling 6 MW electricity (and also some district cooling). Not CO2 though, each pump had 9000 kg R134a as the working fluid.

Interesting, sounds like a water to water heat pump (takes heat from sewage and pumps it into district heating). Just to dig into your example a bit: It kind of depends on what the return temperature is of the 90C heating fluid is. Based on the refrigerant and COP of 3 (18/6), its probably around 50C. If they were able to find a use for the process fluid so that it returned at 20C, they would probably have a COP closer to 5. You can still build 'relatively' efficient heat pumps for hydronic heating, but they would be significantly more efficient if we did a better job dumping all of the heat from the water (which would require a larger, more effective heat exchanger).

Edit: I guess what I'm trying to say is that most of the radiator technology we use (like the in room part of the heating system) was designed with combustion-based heating in mind. Combustion based heating sort-of works just as well if the return water is 50C or 20C, but you would get a substantial boost in heat pump hydronic heating performance if you were to redesign heat exchangers to get as much temperature out of the fluid as possible. This is an optimization problem: as you increase the size of the heat exchanger, it gets more expense, but improves performance. There is an optimum in there somewhere...

There is a such thing as a solid state heat pump which operates without refrigerants or any moving parts. They are not as efficient as regular heat pumps, though, at least currently. But maybe that could change.

https://www.researchgate.net/publication/270646164_Efficienc...

https://www.energy.gov/eere/buildings/downloads/high-efficie...

https://arpa-e.energy.gov/?q=slick-sheet-project/high-effici...

You reminded me of Lonnie Johnson's "Johnson Thermo-Electrochemical Convertor", or JTEC for short. On their webpage they say they are aiming for 85% Carnot efficiency for converting heat to electricity, and it can run in reverse to become a heat pump[0][1].

Sadly, I can find no third-party measurements of how well it performs in practice, so I guess it's still in the R&D phase. I did find a nice talk by Johnson about the project though[2].

[0] http://www.johnsonems.com/about/

[1] http://www.johnsonems.com/benefits/

[2] https://www.youtube.com/watch?v=F6FUk2E9-68

Thermoelectric Coolers (TEC) or Peltier devices.

You can buy small ones and play with them for very little money. They're used all over the place.

https://www.sparkfun.com/products/15082

Are there any that are powered by natural gas instead of electricity?

Heat pumps use actual pumps to pressurize a refrigerant. Could you use some sort of natural-gas internal combustion motor instead of an electric motor? Sure, why not. You could use the waste heat from the combustion as a heat input, but the cost of the complexity and achievable efficiency are a big question.

If I'm understanding your question correctly, yes. The term you want to search for is "Gas Absorption Heat Pump". Instead of compression, it uses heat (which can be from burning natural gas) to drive a gas (usually ammonia) out of solution. They can reach about a 1.5 COP: https://aceee.org/sites/default/files/files/pdf/conferences/....

Thanks!

So I found the GAHP-W LB but the price seems to be a state secret.

Also the Lochinvar Optimus, but again, no prices.

But thank you - I don't understand why these types of units are not standard, are there downsides I'm not aware of?

Aside from the low COP, the market is much smaller (utilitity gas availability).

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