“The Hunt for the most efficient heat pump in the world”
DCcontrarian
| Posted in General Questions on
Somewhat interesting article about hobbyists who compete for the most efficient heat pump:
https://arstechnica.com/science/2024/07/the-hunt-for-the-most-efficient-heat-pump-in-the-world/
I found it a little breathless, and it kind of misses the point. Heat pump efficiency is only one piece of the puzzle, the efficiency of the whole building is the real goal.
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The article does not mention the outdoor air temperature range. In looking at heat pump data, I look at the COP vs. outdoor temperature, and focus on -20F, -8F, 4F, 25F, and 45F for Zone 6 A. Wonder where the temperature sensors are located?
Curious on how they tweak installation Details? Air sealing and insulation? For heating based climates do they orient the heat pump on the south / sunny side where ambient air temperatures may be higher? Or do you focus on efficient air flow through the outdoor unit? I look at noise of a unit (dBA) as noise maybe caused by loss of mechanical energy.
All they seem to care about is COP, so details like air sealing and a insulation are ignored. The only optimization the article offers is running the water as cool as possible, which just means big radiators.
I'd be more interested in a competition that was something like kwh per square foot per degree-day. My suspicion is it would favor small houses in moderate climates
Or kwh per occupant per degree-day.
It’s just a short article, not a textbook. I think the COPs are impressive and knowing what’s possible can help installers push for better.
What I thought was interesting:
The leader in the air-to-water category had an average COP of 5.5. The overall leader had a geothermal system with solar preheat and got 6.0.
The geothermal well cost 60,000 pounds or about $75,000. That's not a whole lot of return on investment.
I would like more real information on the how, aside from low output temps
They do mention insulation as being more important toward the end
Geothermal only makes sense if you cannot do air source for some reason
Or if you live next to old faithful
I bit more tech info
https://weltmeister.cc/en/die-weltweit-effizienteste-waermepumpe-stammt-aus-tirol/
Not a lot of detail in that story (it doesn't help that it seems to have been auto-translated into English.)
What I got out is they run at low temperatures -- like everyone -- and use propane as the refrigerant. From what I can tell we probably should be using a lot more propane as refrigerant, but it's not currently legal in the US at the necessary scale.
Then there's this:
"We have a very high heat transfer, i.e. in the transition from air to the refrigerant via an air heat exchanger. We achieve the high efficiency because we can create a special flow in the air heat exchanger’s pipes in the refrigerant-carrying components."
Not enough detail is given to assess the validity of the claim.
One might want to look at the patent EP3816543B1 which deals with the control of the expansion valve so to maximize COP. That alone will not explain it but reduces the temperature raise used in the whole system.
Thanks. OK, I read the patent. What they describe is very different from what's in the article. The refrigeration cycle involves transforming the refrigerant from gas to liquid and back again, and transferring heat at each stage. At the evaporation stage, the refrigerant is a liquid at high pressure, it is released into a lower-pressure chamber (the evaporator), which causes it to boil, absorbing heat from the environment. Liquid refrigerant is a better conductor of heat than gaseous refrigerant, so the evaporator works better if the flow of refrigerant is managed so that there is always some liquid refrigerant in the evaporator. It also means that the evaporator stays at the boiling point rather than warming up. Heat pump efficiency is all about temperature delta so that gives you a better delta. Think of boiling water in a pan, if you allow all of the water to boil off the steam coming out of the pan becomes hot and not a lot of heat goes into it.
The patent is about putting temperature and pressure sensors in the evaporator to detect whether all of the liquid has boiled off, and a valve on the evaporator to modulate the flow of liquid refrigerant.
From the patent:
"Conversely, this means that a large part of the energy can be transferred with a fraction of the heat exchanger surface of the evaporator or alternatively with a significantly higher evaporation temperature (increase in efficiency). "
This was translated from German, and I think something was lost in the translation, it should be either a higher temperature delta or a lower evaporation temperature.
Note that this also allows for a physically smaller outdoor unit, which is a nice benefit.
What's interesting is that technological advances like this are usually seen first in air-to-air heat pumps, because that's a much bigger market, at least in the US.
I imagine propane in an exterior A/W heat pump unit would be as safe as any propane appliance, probably safer.
While the efficiency numbers are great, with the average low temps in the UK [for instance] being around freezing, it is not awe inspiring.
The quantities of propane are miniscule, maybe a pound. The claim is made that the danger is greater with a compressor because pressures are greater, but once the gas is liquified pressure isn't really an issue.
"While the efficiency numbers are great, with the average low temps in the UK [for instance] being around freezing, it is not awe inspiring."
The idea behind SCOP -- seasonal COP -- is that you weight the COP by temperature to give an even playing field regardless of outdoor temperature.
The big issue I see is capacity at cold temperatures. If you look at this article: https://www.greenbuildingadvisor.com/article/three-types-of-heat-pumps
In the comments I have a chart that shows COP vs. cold weather capacity retention for three types of heat pumps. In general, it's a tradeoff -- the higher the COP, the more capacity is lost at colder temperatures. I wrote: "The Chiltrix has the highest COP across the board, but also the biggest drop-off in capacity, less than half of its rated capacity at -20C." The Mitsubishi has a lower COP across the board, but at -15C the Mitsubishi keeps 94% of its rated output. If the alternative is to use resistance heat to make up for the loss in capacity when it's cold, the Mitsubishi actually costs less to operate even though its COP is lower.