Hot and bothered about heat pumps
Quite a few websites (including TreeHugger here) covered the new CO2 heat pump installation at the Alaska Sea Life Center. It’s impressive, and so was their press release with its title ASLC shifting 98% of the Center's heating needs from fossil fuel to ocean water as source heat. Many tried to explain how it worked, using the complicated description in the press release. I found these hard to understand, and thought it might be a good idea to go back to first principles and try and explain a) how a heat pump works, and b) how a CO2 heat pump is different.
© It's a heat pump!
All of us use heat pumps every day; that is what your fridge is, and your air conditioner. They pump heat from inside the box to outside. The big heat pumps work pretty much the same way:
When you use an aerosol can, it gets cold. That’s because there is a propellant, often something like propane, that is liquid under pressure but when you release the pressure, turns into a gas. In that process it absorbs what is called the heat of vaporization; it takes energy to break those bonds that hold it together as a liquid. That’s why the can gets cold.
You could probably build a very nice air conditioner or fridge around propane but it is flammable, so they invented non-flammable refrigerants that did the same thing, originally freon, (bad for the ozone layer and a serious greenhouse gas) and then its replacements like R-134a that is in the old ASLC heat pump. (still a greenhouse gas but not quite as bad).
In your fridge, the heat is sucked out of the box to evaporate the refrigerant. In the ASLC's older heat pump system, (1) 40°F sea water goes through a (2) heat exchanger to move heat to cooler fresh water with ethylene glycol antifreeze. That’s the second law of thermodynamics in action: heat moves from the hotter to cooler medium. If they just put the seawater through the heat pump it would probably freeze solid, and saltwater is corrosive.
This (3) water and antifreeze is then is sent to the (4) evaporator in the heat pump and even though the water is barely above freezing, there is enough heat in there that the evaporating refrigerant can suck it out.
The (5) warmer, low pressure vapour refrigerant is then piped to the (6) compressor. If you have ever pumped up a bike tire you find quickly that compressing a gas makes it warm. Compress it a lot and it gets hot as the molecules bump and grind together. But at these pressures, it wants to be a liquid like propane in your bbq tank. To turn into a liquid, it has to release that heat of vaporization, which it does in the (7) condenser. The heat that was gathered up out of the ocean water is now concentrated and available for use, and is transferred to the (8) building heating system loop. Then the liquid refrigerant under pressure is piped to the (9) expansion valve, which drops the pressure to the point where the liquid wants to turn into a gas, but needs that heat of vaporization, which it gets out of the glycol loop in the (4) evaporator and the process starts over again. That’s the “vapour compression cycle.”
And that’s how your heat pump (and your fridge) works; just leave off the seawater loop. Heat pumps are more efficient because they move heat rather than make it, but it still takes energy to run the water pumps and the compressors. That’s where the COP or Coefficient of Performance comes in: a COP of 4 means that it produces 4 times as much heat as straight resistance heating would for a given amount of electricity.
The variations in home heat pumps are ground source, where loops of water remove heat from the ground or dump it back in, water source, where loops are put in the water or as in Alaska, the water is pumped in and returned, or air source, like you see more commonly now in the little mini-splits.
A CO2 heat pump is a bit different. Unlike the artificial refrigerants, CO2 is not designed for this function and the heat pump has to be designed around its properties. A lot of people are rooting for CO2 as the Next Big Thing as a refrigerant because it is so much more benign environmentally. The new Alaska installation uses Myekawa water source heat pumps (we showed their “Eco-cute” unit on TreeHugger here; cute is not a description of how it looks, but short for kyūtō or “supply hot water”). The main advantage (besides the refrigerant) is that they operate at much higher temperatures and can be used to replace traditional boilers.
A key difference between the conventional heat pump and the CO2 heat pump is that CO2 pumps operate at much higher pressure. Engineer John Straube tells Alex Wilson of BuildingGreen:
“They operate at much higher pressures,” says John Straube, Ph.D., P.Eng., [formerly] of Building Science Corporation, “requiring thicker line sets, better compressors, and all-around better, heavier systems that cost a bit more and require a bit more resources.”
They are also what is called “trans-critical”- the CO2 does not condense in a condenser. Again from BuildingGreen:
Ray Cole, P.E., CEO of Axiom Engineers, a mechanical engineering firm serving northern California and headquartered in Monterey, explains the key difference of the trans-critical cycle as follows: “Unlike a conventional vapor-compression cycle, you do not have a condenser; you have a gas cooler. As you cool the gas, it changes temperature but not pressure.” As the refrigerant gas cools, according to Cole, who was involved with one of the first U.S. installations of a Mayekawa heat pump, some of it condenses into liquid and some remains a gas.
So the drawing is pretty much the same for the CO2 heat pump except the (7) condenser is this gas cooler thing. However CO2 heat pumps are “ridiculously efficient”, with a COP of as much as 8, and the output is really high temperature, delivering water at 150° to 194°. That’s why it is perfect for the hot water heating system at the Alaska Sea Life Center.
Alaska Energy mix/Public Domain
And while the ASLC may have shifted 98 percent of its heating needs from directly burning fossil fuels, it still needs electricity to run these heat pumps- I think each of the four has a 25 kW motor. Then there are the pumps that move the thousands of gallons of seawater. Given the mix of sources for Alaska, they are certainly not 98 percent off fossil fuels.
And by the way, this is why I really think we should never use the word "geothermal" to describe ground source heat pumps. The technology is pretty much exactly the same whether you are pulling the heat out of the water, the ground or out of the air. However geothermal means one specific thing: heat from the ground that is used directly, like steam in Iceland or drilling really deep for hot rock a few miles down. We really shouldn't mix the two. This has caused me a lot of trouble in the past but we should get it right.
Tip of the hat to Energy Vanguard for this explanation that got me started.