New Heat Pump Designs and Applications: Cold-Climate Heat Pumps, Capacitor-Based Heat Pump Research, and Heat Pumps for EVs That Can Reduce Winter Range Loss

 

     Heat pumps have some serious limitations, but that appears to be changing as new models come on the market that can operate efficiently at lower temperatures. I have an air-source heat pump in my house. It is great for air conditioning. My cost for A/C is barely noticeable. However, with an instantaneous nearly 30% rise in power costs for my region and provider and my own difficulty making money lately, I can really feel all power cost increases as an acute pain in the wallet. I have had to decrease my winter thermostat to 62 degrees F and increase my summer thermostat to 78 degrees F. 

     Another thing of which I am acutely aware is that below an outside temperature of about 10-15 degrees F for a length of time, my heat pump works inefficiently. I have to supplement with other electric heat. I will likely struggle with this over the next few weeks as high temps in the 20s and low temps in the single digits are forecast for several days ahead. To be honest, it is no fun. One strategy that helps is to raise the thermostat during the early evening to build up heat in the house so it will remain longer. As predicted, my heat pump is indeed saving me over $500 per year in power costs compared to my previous electric furnace. At an installation cost of $5000 in late 2018, I calculate that after this winter I will have saved over $3000 compared to my previous furnace and by 2027 or 2028, my installation costs will have paid for the system with 2-10 years or more of operating life remaining. Lucky for me, when I eventually need a new one, it will be one with much better low-temperature performance.

 

 

Cold-Climate Heat Pumps

 

     Fortunately, there are new air-source heat pump models that are much more efficient in lower temperatures. Electrek reported at the end of 2022 that Johnson Controls developed an air-source heat pump prototype that can operate well at temperatures as low as -20 degrees F. This was developed as part of the DOE’s Residential Cold Climate Heat Pump Challenge. The DOE is aiming to commercialize some of these models soon, putting them on the market in 2024. Other companies including Carrier, Trane, Lennox, Rheem, and Bosch are participating. Trane developed a prototype that can work at -23 degrees F. More recently, companies Daikin, Midea, and Johnson Controls were added. Those who buy can also take advantage of the Energy Efficient Tax Credit 25C tax credits.

     The DOE writes in the Cold-Climate Heat Pump (CCHP) Technology Challenge factsheet: “Space conditioning and water heating consume over 40% of the nation’s primary energy and are a major source of greenhouse gas (GHG) emissions. Electric heat pumps (HPs), which extract heat from the air and ground, are an efficient alternative to fuel-fired space conditioning and water heating equipment. However, the performance of conventional HPs declines in colder climates, which have high space heating demands. In recent years, HVAC manufacturers have developed specialized cold-climate heat pumps (CCHPs) which incorporate advanced designs to operate with greater capacity and efficiency at low outdoor temperatures (below 32°F).” Space conditioning accounts for about 46% of all building greenhouse gas emissions, 42% of all building energy bills, and 56% of all household energy bills.

     The DOE effort involves two types: increasing the efficiency of models operating at 5 degrees F and models with successful operation at -15 degrees F. The DOE reported in June 2022 that American heat pump manufacturer Lennox International became the first to develop a next-generation electric heat pump as part of the challenge. It was reported that the Lennox “prototype delivers 100% heating at 5°F at double the efficiency, and 70% to 80% heating at -5°F and -10°F. DOE’s Oak Ridge National Laboratory validated the performance and efficiency of Lennox’s prototype.”

     Here is a good explanation from an article in CNET for why heat pumps slow down and work less efficiently in cold weather:

 

“The heat pump is cycling refrigerant through coils outside of your home, trying to pick up what thermal energy is available in the air. When it gets too cold, water in the air starts to condense on the coils and then freeze around them, blocking the refrigerant from being able to absorb that energy.”

 

“Heat pumps are equipped with defrosting technology to keep this from happening, but that usually means the pump's energy is being spent trying to warm those coils up rather than warming up your house. At a certain point, if it's too cold, it becomes a losing battle.”

 

At a certain point of coldness, this becomes all that the heat pump is doing.

 

     In early 2022 Lennox introduced their variable-capacity SL25XPV cold-climate heat pump touting comfort levels in extreme cold comparable to a gas furnace. They projected cost savings of 58% per year. They also introduced their “Quantum™ Coil as a proprietary aluminum alloy designed to weather the harshest elements and provide enduring reliability in its latest heat pump. Exclusive to Lennox, this coil technology allows the unit to have longer lasting sustainability, which preserves the system performance and reduces environmental footprint.”

     Heat pumps utilize a reverse refrigerant cycle (reversing the flow of R22, R410A refrigerant) to gather available heat from outdoors and bring it indoors. Those refrigerants are HFCs that replaced CFCs that had much higher global warming potential (GWPs). HFCs are currently being replaced by refrigerants with further significantly fewer GWPs.

     Efficiency for furnaces and heat pumps is typically measured as Co-efficient of Performance (COP). Learn Metrics HVAC Systems gives some efficiency/COP specs as follows:

 

“Best mini split heat pumps can achieve a 400% efficiency (COP = 4) at 47°F.

 

Average heat pumps will achieve above 200% efficiency (COP = 2 or higher) at 20°F.

 

Best low-temperature heat pumps will achieve 188% efficiency (COP = 1.88) even at 0°F.

 

In below zero temperatures, standard heat pumps COP will fall below 1. However, new mini split heat pumps for cold weather will be able to maintain the efficiency above 100% even at -15°F.”

 

The 2^nd graph below depicts the COP values for cold-climate heat pumps as a function of temperature. Thus, the best mini-split heat pumps can have efficiency of 200% (COP =2) at 0 degrees F. The minimum operating temperature (MOT) for older heat pumps, like my LG model installed in 2018 is 5 degrees F. It is basically useless under that temperature. Better older models have MOT down to about -4 degrees F. The newer models bring MOT down to as low as -22 degrees F. However, it is important to determine at what temperature the model begins to lose efficiency. Mine begins losing efficiency at 14 degrees F, I believe.

 

Source (for both): Learn Metrics 

 

     In Europe and the UK, there have been strong pushes to adopt more heat pumps as the price of natural gas climbed in 2022. The UK has been lagging behind the EU in heat pump deployment. The EU has a goal of deploying 60 million heat pumps by 2030.

 

 Source: European Climate Foundation and the European Heat Pump Association

     Heat pumps can be outfitted to heat water as well as space, which can make water heating more economical to operate than traditional electric or gas hot water heaters.

 

 

Capacitor-Based Heat Pumps in Research for the Future

 

     Most heat pumps rely on compressed gaseous refrigerants, usually hydrofluorocarbons (HFCs) for heat exchange. As these gases are compressed they heat up and as they are uncompressed they cool down. HFCs replaced chloroflurocarbons (CFCs) which are among the world’s most potent greenhouse gases. However, HFCs are potent greenhouse gases as well. There are some hydrocarbon-based refrigerants, usually some form of propane, butane, or pentane, used as refrigerants as well.

     A new type of heat pump is being researched that has a totally different mechanism for heat exchange. This new type utilizes a capacitor that changes temperature as it's charged and discharged. It has the potential to be very efficient. Recently, researchers in Luxembourg have been working with materials that change temperature in response to electrical fields. This is known as electrocalorics. Layered capacitors that heat up when charged by an electric field and pull heat from the surrounding environment when discharged are utilized in the new solid-state model. According to Ars Technica: “For the electrocaloric device, the researchers created a multilayer capacitor using a lead/scandium/tantalum oxide material. This was crafted into a series of parallel plates with gaps in between them, which allowed fluid to flow through the device.” These experiments showed impressive results, with the potential to develop a long-lasting (potentially 30 years) electrocaloric device that has at least a 50% improvement over previous electrocaloric devices. While the experiments were impressive in determining the limits of electrocalorics, there are many problems to be worked out and this technology seems to be in very early stages.   

 

Heat Pumps for EVs Decrease Winter Range Loss

     The ideal operating temperature range for EVs with lithium-ion batteries is between 50 degrees F and 110 degrees F. In sub-freezing weather, the range of some EVs drops from 10%, which is manageable, to up to 36%, which is undesirable. Heating the inside of the car drains energy from the battery. Lithium-ion batteries of current designs are near their efficiency limits. Only by adding more batteries can the range be extended. This adds significantly to cost and weight. The added weight makes range extension by adding batteries have diminishing returns. Another way to keep from losing winter range is to add a heat pump. The superior efficiency of heat pumps makes them desirable for heating. According to an article in Top Speed: “During cold weather, a heat pump uses heat generated by the motor and battery and redirects it to the interior of the vehicle. This way, the car doesn’t have to use its battery to power a resistor and keep the passengers warm. It’s quite an elegant and simple solution if you think about it.” Colder air and snow-covered roads also produce more drag, which reduces the range a little bit as well.

     Both Hyundai and Tesla are making heat pumps a standard feature on new EVs. Hyundai also features a battery heating system. With the advantages that heat pumps provide to decrease winter range loss, it seems quite likely that they will become standard on all EV models.

 

References:

Here's Why Heat Pumps Need To Become A Standard Feature On Electric Cars. Bruno Maia. Top Speed. March 2023. Here's Why Heat Pumps Need To Become A Standard Feature On Electric Cars (msn.com)

Most Heat Pumps Slow Down in Frigid Cold. New Models Shown at CES Could Change That. Jon Reed. CNET. January 8, 2024. Most Heat Pumps Slow Down in Frigid Cold. New Models Shown at CES Could Change That (msn.com)

US companies are producing heat pumps that work below -20F. Michelle Lewis. December 26, 2022. Electrek. US companies are producing heat pumps that work below -20F (electrek.co)

Capacitor-based heat pumps see big boost in efficiency. John Timmer. Ars Technica. November 16, 2023. Capacitor-based heat pumps see big boost in efficiency | Ars Technica

High cooling performance in a double-loop electrocaloric heat pump. Junning Li, Alvar Torello, Veronika Kovacova, Uros Prah, Ashwath Aravindhan, Torsten Granzow, Tomoyasu Usui, Sakyo Hirose, and Emmanuel Defay. Science. Vol 382, Issue 6672. pp. 801-805. November 16, 2023. High cooling performance in a double-loop electrocaloric heat pump | Science and science.adi5477_sm.pdf

Heat pumps: 10 Breakthrough Technologies 2024. Casey Crownhart. January 8, 2024. MIT Technology Review. Heat pumps: 10 Breakthrough Technologies 2024 | MIT Technology Review

DOE Announces Leading Heat Pump Manufacturers Successfully Develop Next-generation Prototypes to Withstand Subfreezing Weather. U.S. Dept. of Energy. January 8, 2024. DOE Announces Leading Heat Pump Manufacturers Successfully Develop Next-generation Prototypes to Withstand Subfreezing Weather | Department of Energy

DOE Announces Breakthrough in Residential Cold Climate Heat Pump Technology. U.S. Dept. of Energy. June 17, 2022. DOE Announces Breakthrough in Residential Cold Climate Heat Pump Technology | Department of Energy

Residential Cold Climate Heat Pump Technology Challenge. U.S. Dept. of Energy. February, 2022. Residential Cold-Climate Heat Pump Technology Challenge (energy.gov)

Octopus Energy introduces new residential heat pumps. Emiliano Bellini. PV Magazine. September 18, 2023.Octopus Energy introduces new residential heat pumps – pv magazine International (pv-magazine.com)

Best Heat Pumps For Cold Climates In 2023 (Down To -22°F). Learn Metrics. Best Heat Pumps For Cold Climates In 2023 (Down To -22°F) (learnmetrics.com)

Lennox Industries Introduces New Cold Climate Heat Pump Focused On Accelerating Environmental Sustainability. Lennox. February 7, 2022. Lennox Industries Introduces New Cold Climate Heat Pump Focused On Accelerating Environmental Sustainability

Europe’s Leap to Heat Pumps: The Socio-Economic and Climate Benefits Unlocked by a Fast Heat Pump Roll-Out. European Climate Foundation and the European Heat Pump Association. April 2023. ECF-Europes-Leap-to-Heat-Pumps-Report_FINAL_April-2023.pdf (europa.eu)