New heat pump tech for cold temperatures: cascade configuration

[Will Prowse]

Just found out about cascade heat pumps:

In cold temperatures, single stage heat pumps have horrible efficiency and can be near impossible to use during the winter with off-grid solar. The colder it gets, the less efficient they become. Usually when it is cold, sunshine is reduced, making it more problematic.

Different refrigerants work better at different temperatures. Most heat pumps are designed to work best for the majority of temps that people are exposed to, but not really optimized for cold temps specifically.

So this company is using multiple refrigerant loops (and different refrigerants rated for use in different temperatures) in cascade configuration (like putting them in series, or daisy chaining then):


This allows one loop to exchange heat to the next one. This will increase efficiency of the entire system in cold temperatures.

Has anyone here tried these heat pumps? I wonder what the efficiency is with normal temps. Maybe it can switch to single stage heat pump mode instead for the majority of operation. Then switch to cascade heat pump mode when the temp falls to a certain threshold.

With this configuration you have two compressors and two line sets, so perhaps higher chance of mechanical failure with the moving parts over time.

 

[WorldwideDave]

Interesting design. Thanks for sharing.

 

[SparkyJJO]

Seems like a good idea on paper, especially if as you suggest they can make it run just the one stage during normal temp ranges and then only kick on the second stage when it gets really cold.

I installed a "2 stage" AC/heat pump at my old place but it wasn't actual stages like that. It was just two speeds I suppose. Then it had a 2 stage propane burner which when I was living there I'd have it switch to propane around 30 degrees outdoor temp as it became cheaper to run at below that temp (though I tested it down to 22 F on heat pump only mode and it worked just fine).

 

[upnorthandpersonal]

Combined CoP is typically slightly over 2. You need twice the power input, and you likely have double the chance of failure since it's basically two heat pumps.

Here in Finland we tend to just use R32/R290 refrigerant heat pumps with EVI (enhanced vapor injection), and they do fine for winter.

impossible to use during the winter with off-grid solar

Yeah, that they are here - no solar to speak of during winter.

 

[Rednecktek]

They work. I occasionally have shipping containers that run at -60c and the second compressor/loop just cools the first loop. The implementation looks like a total kludge but it sure works...

As long as there are no leaks because they won't give us spare R123.

 

[Backyard]

Sounds like a ground source heat pump would be a better option in the cold.

How much power does a two stage cascade heat pump draw? How does this compare to plane ol, electric elements?

 

[Skypower]

Good topic Will
Looks like it’s rare, real and really expensive.

 

[upnorthandpersonal]

How does this compare to plane ol, electric elements?

As long as our CoP is over 1, you gain energy-wise.

 

[NotTooBright]

How does the second stage condenser not freeze over? Heat pump efficiency drops off a cliff below freezing because they have to periodically reverse refrigerant flow to defrost the condenser, which requires energy but is counter-productive to heat generation. Are these systems that much more efficient that they still maintain the COP even with these defrost cycles?

 

[BerndL]

As I understand it the Mitsubishi EVI system is basically a "stage and a half" thermodynamically. It's got the same internal counterflow heat exchanger that this does, and it injects the medium pressure gas halfway up the compressor instead of having two separate compressors.

I wonder how much this system actually saves compared to that. None of these trash clickbait propaganda YouTubers ever show a COP/temp chart...

Notice a trick they used in the video. "-33F degree air, all the way up to 77F, at a cop of 2.3"
Heat pumps need to get the outlet temp much higher than 77, so that's not a real comparison.

Edit, I was wrong, he actually did show a cop chart at one point.
But you can see the -33F performance touted is actually cop ~1.2 vs 2.3. 20% better than a baseboard heater is not a great return on investment for something so expensive!

Think I'll stick with a Mitsubishi. I want to overlay their performance curve on this one, see what the difference is.

 

[bolhuijo]

While it may seem that doubling up the compressors would make it less efficient, the thermodynamics math says otherwise. (Now when theory meets practice, things get uglier ) Here's a relevant page from my thermo textbook for those who love T-s diagrams:

 

[Backyard]

While it may seem that doubling up the compressors would make it less efficient, the thermodynamics math says otherwise. (Now when theory meets practice, things get uglier ) Here's a relevant page from my thermo textbook for those who love T-s diagrams:
View attachment 292882

I'd guess it would be similar power draw and COP as a single stage system with a higher compression ratio. Cascade being used because compressors operating at such ratios are not common and more expensive then two compressors working together.