How can CO2 actually help us keep the planet cooler? When it’s the refrigerant in a heat pump! And these heat pumps can warm your house while they heat your water. All while being more than twice as energy efficient as their competitors, at least according to some calculations.1 So, where has this nearly 150 year-old tech been all my life and why is it only getting noticed now?

We’ve done this a few times before, so I’ll keep it brief. Heat pumps use a refrigerant to move heat around your home, warming it or cooling it as desired. Because they’re just moving heat around, heat pumps are much more energy efficient than normal, fossil fuel-based devices. Burning stuff isn’t always the best option … who knew?2 However, most heat pumps and similar devices, like air conditioners, used to use chlorofluorocarbons (CFCs) or hydrochlorofluorocarbons (HCFCs) as the refrigerant, because these chemicals are really well suited to absorbing and dumping heat on command. However, we later learned they were also super well suited to destroying the ozone layer, so they’ve been slowly phased out or banned since the 1990s. But good news, there’s a lot of alternative refrigerants out there. One of which is CO2, which has some interesting advantages.

The nature of CO2 means that a heat pump can operate much more efficiently in both high temperature ranges and at low ambient temperatures. We’ll take a look at the physics later, but for now just know that CO2’s thermophysical properties means it takes less energy to move heat around.3 In essence, it can make CO2 heat pumps holistically more efficient than their peers. For instance, in the case of domestic water heaters where you want hot water around 90°C, CO2 can be more efficient as a heat transfer fluid.3 Heating water is hard to do, so this is a notable benefit. Some of the larger CO2 heat pumps are designed for massive resorts, including lots of fancy pools. So when I say ‘domestic’ I mean the larger, luxury-apartment-building end of domestic. Not just a single household.

Heat pumps are good, and CO2 heat pumps might be even better, but there’s a way to push this design even further. But how and why? Well, there’s a company that caught my eye because of what they’re doing with CO2 heat pumps and heat storage. This tech is very interesting because it exemplifies the benefits of combining underappreciated hardware with smart software. It could prove to be a great example of the future of heat pump tech.

The company, Harvest Thermal, has built upon an existing heat pump product from Eco2 called SANCO2.4 You can probably guess by the name of the product, but the SANCO2 uses CO2 as the fluid in their heat pump. Harvest Thermal is adding two elements to the heat pump setup: 1) a large water tank for hot water, and 2) a smart management system called the “smart pod.” And those two elements are really important for why this performs so well and saves so much money.

Space heaters like heat pumps are generally measured by their coefficient of performance (COP) values. That’s a ratio of the amount of heat energy moved versus the amount of electricity used to do so. Like moving 2 kW of heat with 1kW of electricity is a COP of 2. The higher the COP, the more cooling or heating you get per kW of energy spent. For reference your average air source heat pump can hit a COP of 3, while a gas burner can’t even break a COP of 1. But Eco2’s SANCO2 heat pump can hit a COP from 2.6 all the way up to 5.5 depending on the ambient temperature.5 So we’re off to a good start, but Harvest Thermal’s smart pod and water tank, used as a thermal battery, can push the energy cost savings even further.

Climate control devices tend to draw the most power when people get up in the morning and when they get home from work. Since everyone is using their climate control devices around the same time, that demand can drive up electricity prices. It creates what’s known as the duck curve. The mornings and evenings have the highest costs, while the middle of the day tends to have the cheapest rates when there’s abundant, cheap solar electricity being created. But Harvest Thermal is breaking apart how and when a heat pump has to run. The system can run the heat pump in the middle of the day during the lowest part of the duck curve, store that heat in the water, then in the evening run the hot water through an air exchanger to warm the air in the home. It’s the combo of a high efficiency CO2 heat pump with the thermal battery plus smart energy management that unlocks all the benefits.6 And you can use the hot water as … hot water … in the house too. Two birds, one hot water thermal battery.4

Between the CO2 and this smart optimization, Harvest Thermal claims they can cut your bills by 45% versus a standard gas heater.7 Like other heat pumps, it can cost a little more upfront. But also like other heat pumps, there are programs and rebates in place to help make it more affordable.8 Harvest Thermal says that even without incentives, the system can offset the upfront costs in about 5-10 years.

Let’s back up for a second. If CO2 heat pumps are so good, why aren’t they everywhere? And where did they even come from? Back in the early days of refrigeration, we’re talking the mid 19th century, CO2 was one of the most popular refrigerants available alongside ammonia.9 It fell out of favor because, as a refrigerant, it had to be under 20 to 30 atmospheres of pressure.9 This isn’t a tall order, but it does necessitate specialized components that can handle the increased heat and pressure. And anytime there’s specialized components there’s an increased cost, which is why CO2 was passed over in favor of ammonia and similar compounds. When CFCs and HCFCs were discovered in the early 20th century, they were hailed as miracle refrigerants and became the standard. Until, that is, we discovered this new miracle refrigerant ate away at the ozone layer. Not wanting to be fried by our own sun, starting in the 1990’s we began banning and phasing out all the CFCs and HCFCs. The race was on to find a better refrigerant.10

This leads us to the Japanese company Denso, who applied CO2 technology to automobiles after the bans. The Central Research Institute of Electric Power Industry (CRIEPI) and The Tokyo Electric Power Company (TEPCO) saw the potential for more, and asked Denso to develop a prototype CO2 air conditioner. And since Japan is an island without many resources, the resource-efficient, quick-water-heating design quickly found a home in heat pumps and water heaters throughout the country.1112 In the early 2000s, companies like Nihon Itomic developed CO2 tech for commercial and industrial use, and the model quickly spread to East Asia, Australia and Europe. It’s only been within the last few years that the Americas started to notice CO2 heat pumps and water heaters.1113

How do these things actually work? And why are they so often combo’d with water heaters? I mean, we all love a hot shower, but it seems like a hat-on-a-hat, right? Recall that by using a series of compressors we can manipulate the pressure on a refrigerant, allowing it to “pick up” heat and then “drop it” as needed.14 CO2 is easy to pressurize and is really good at absorbing heat, which makes the heat pump’s job easier. Combine that with water and you’re building out a great synergistic relationship. Water, of course, is an excellent coolant. If it’s good enough for a nuclear reactor, or more importantly, my sick gaming rig, then it’s good enough for CO2. 10 15 So after dumping its heat in the water heater, CO2 is cooled off and ready to keep on moving heat around. 16 8

CO2 also has some other little bonus benefits. It’s relatively cheap and easy to procure. Clocking in at just 1 to 2 bucks per pound, whereas most other refrigerants cost 5 to 14 dollars per pound.1718 That’s a significant cost savings. Plus, it’s non-toxic, odorless, and won’t explode. Importantly it’s also not a CFC or HCFC. Leakages, while small and rare, do happen, so anything safe we can find to replace those ozone-eaters is a good thing.16

I know what you’re about to say: “But Matt, if we’re worried about the CFC refrigerants leaking, aren’t we worried about the CO2 leaking too?” And you’re not wrong. Half the point of a heat pump is to save us from burning fossil fuels and releasing CO2 into the atmosphere. While the amount of CO2 inside a heat pump varies from system to system, it is quite marginal. CO2 is also far less harmful pound for pound than the CFC refrigerants, so leaks aren’t really a cause for concern here.16 But before you get too excited, the small amount of CO2 used in this style of heat pump also means it isn’t exactly a viable CO2 sequestration method.

How do CO2 Heat pumps compare to their standard cousins? Chemistry and physics time! A CO2 heat pump’s biggest advantage is that they operate at higher-temperatures. Exactly why they can do this is complex and has to do with CO2 being a “transcritical” or “super critical” refrigerant, meaning it doesn’t fully change phase like other refrigerants.15 Combined with CO2’s high specific heat, excellent ability at conducting heat, and how well it flows (or its low kinematic viscosity), it’s almost ideal for moving heat around as part of a heat pump.19 Almost. Remember CO2’s high temperature and pressure requirements? While helpful, financially-speaking CO2 pumps will require more advanced (and expensive) compressors and pipelines than your average heat pumps, even all these years later.9

The high temperature of CO2 heat pumps can mean that they are a little less efficient than their more standard comrades.14 CO2 heat pumps tend to have a COP just under 3, which is far better than gas heaters, but most other heat pumps (as I mentioned earlier) can often hit a COP of 3 … or better.10 Then again, most other heat pumps aren’t also heating a thermal battery, and when doing both, some calculate the CO2 thermal battery/hot water combo COP as high as 8.1 Why the vast disparity? There’s just a lot of variables in play. Local gas and electric prices, the size and shape of your building, ambient air and water temperatures at different times of the day and year, etc. This makes calculating the best bang for your buck (or pump for your penny?) very complicated very fast. This is part of why having a smart system do some of that math and optimization for you has the potential to be so impactful.

But that raised the question for me, couldn’t we do this hot water/thermal battery combo with any heat pump? Not just CO2? Perhaps. But again, water is very hard to heat, so the extra heat from the CO2 refrigerant really helps with heating water versus other common refrigerants. Think of it like a sedan versus an SUV. Technically both can drive in the city or go off-roading. But the sedan is going to struggle with rough terrain that the SUV wouldn’t even notice, while the SUV is a lot less gas efficient in a city setting.

Like we so often see, there isn’t a one size fits all solution to any problem. The analogy aside, Eco2 claims that the Sanco2 can offer 80% savings over more common Electric Resistance Water Heaters and 40%+ over even standard heat pump water heater (HPWH) hybrids. And that’s before we account for Harvest Thermal’s smart pod.202122232425 That’s impressive but comes with an installation price of twice what a standard HFC HVAC system would cost a household. This is partly because installing a CO2 heat pump often means replacing not just your air conditioner but also your water heater, which further drives up the price and adds very specific spatial requirements. And since the initial installation costs are some of the biggest obstacles on the road to heat pump proliferation, I can’t just handwave factors that will further increase the price. Even if they will, on a long enough timeline, likely pay for themselves.

So will CO2 heat pumps be replacing every other heat pump out there? No, again, there is no silver bullet here. Will CO2 heat pumps gain more traction in the Americas? I think it comes down to whether or not the extra costs of a CO2 heat pump outweigh its extra savings, which will vary from region to region, and house to house. But even if CO2 heat pumps prove too pricey for domestic use, there are other niches. 10 major Australian supermarkets are switching to CO2 heat pumps to keep food and people cool in the Aussie heat.26 Denmark’s Danfoss has taken it a step further, building a supermarket with a smart CO2 heat pump reminiscent of Harvest Thermal.2728 And they’re already in use across a variety of hotels and resorts. In one study they cut thermal energy consumption by an impressive 60%.29 Rather than being the one climate-control-system-slash-water-heater to rule them all, we might find CO2 heat pumps to be perfect for that larger scale niche. So, ultimately, while it’s hard to predict if CO2 pumps will take off in the Americas like they have in Europe and East Asia, their advantages mean I wouldn’t be surprised if we start seeing a lot more of these devices state-side, even if it’s not in our homes.

  1. A Heat Pump Using Carbon Dioxide as the Refrigerant ↩︎
  2. Heat Pump Sustainability Workshop ↩︎
  3. About High Temperature CO2 heat pumps ↩︎
  4. Harvest Thermal: Adding Intelligence to Hydronic Systems ↩︎
  5. Regulations and Incentives Drive U.S. Market for CO2 Hot-Water Heat Pumps, Says ECO2 Systems ↩︎
  6. Harvest Thermal Products ↩︎
  7. Harvest Thermal: One System For Space Heating, Cooling and Hot water ↩︎
  8. Harvest Thermal Savings ↩︎
  9. Carbon Dioxide (R744) – The New Refrigerant ↩︎
  10. How heat pumps of the 1800s are becoming the technology of the future ↩︎
  11. Wiki – EcoCute ↩︎
  12. CHOFU’s CO2 Heat Pump System ↩︎
  13. Nihon Itomic ↩︎
  14. The Truth About Heat Pumps ↩︎
  15. A comprehensive review and analysis on CO2 heat pump water heaters ↩︎
  16. Why CO2 is the Most Promising Refrigerant in the Cooling Industry ↩︎
  17. Transcritical CO2 Refrigeration: Basics and Benefits ↩︎
  18. Have We Reached a Tipping Point for CO2 Refrigeration Systems? ↩︎
  19. Song, Y., Cui, C., Yin, X., & Cao, F. (2022). Advanced development and application of transcritical CO2 refrigeration and heat pump technology—A review. Energy Reports, 8, 7840–7869 ↩︎
  20. SANCO₂ Water Heater Product Information ↩︎
  21. Accelerating Decarbonization with the California Load Flexibility Research and Deployment Hub ↩︎
  22. CalFlexHub Research Highlighted at 2023 ACEEE ↩︎
  23. Harvest Thermal Wins People’s Choice Award at NREL Industry Growth Forum ↩︎
  24. Harvest Thermal Wins Prestigious 2023 Edison Award™ Prize ↩︎
  25. The 10 most innovative companies with fewer than 100 employees in 2023 ↩︎
  26. Large Australian Food Retailer Switches to CO2 Heating and Hot Water with Itomic Heat Pumps ↩︎
  27. Danfoss opens supermarket of the future ↩︎
  28. Danfoss Introduces a New Algorithm for Enhanced Control of Ejectors and High-Pressure Valves in a CO2 Industrial System ↩︎
  29. S. Smitt, I. Tolstorebrov, P. Gullo, A. Pardiñas, A. Hafner, Energy use and retrofitting potential of heat pumps in cold climate hotels, Journal of Cleaner Production, Volume 298, 2021 ↩︎

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