Why Improving The Cold Weather Performance of Heat Pumps Can Help Achieve Decarbonization Goals
- Feb 9, 2022 8:40 pm GMT
According to a survey conducted at the end of 2021 by S&P Global Market Intelligence, 26 of the largest U.S. utilities have set net-zero greenhouse gas emissions targets for 2050.
Ten of the companies surveyed pledged to reach net-zero goals even earlier and significant pressure is building for all utilities to accelerate their pace of decarbonization. For example, this past October the Institutional Investors Group on Climate Change, which represents investors with over $60 trillion under management, released a report calling on utilities to achieve net-zero carbon emissions by 2035.
Achieving ambitious utility and government decarbonization targets requires an in-depth and expansive examination of how energy is used, including to heat and cool homes. For example, space conditioning and water heating currently account for 70 percent of the primary energy consumed in American homes. As power generation continues to decarbonize – between 2006 and 2020, power plant emissions dropped 40 percent – interest in the expanded deployment of highly efficient heat pumps as a tool to lower greenhouse gas emissions has increased.
Heat pumps are a uniquely powerful decarbonization tool
Heat pumps are not a new technology. Air-source heat pumps extract heat from the air (other heat pumps take heat from the ground) to provide warmth or to cool a home or commercial building. Because they simply transfer existing heat from the air, heat pumps are far more efficient than conventional electric resistance heaters or furnaces that generate heat by burning oil or gas.
According to the U.S. Department of Energy (DOE) air-source heat pumps can provide as much as three times more heat energy to a home than the electrical energy they use. Not only does this lead to lower carbon emissions, it can also save homeowners a significant amount of money. The non-profit Northeast Energy Efficiency Partnerships estimates that an air-source heat pump can save consumers about $1,000 annually compared to an oil furnace.
The advantages of air-source heat pumps has led to their widespread and growing use both in the U.S. and around the world. According to the International Energy Agency (IEA), 180 million heat pumps (not all of them air-source) were in use around the globe in 2020. The IEA also points out that air-source heat pumps “dominate” global heat pump sales for new buildings and that annual U.S. shipments of the technology rose from 2.3 million in 2015 to 3.4 million in 2020.
Cold weather challenges traditional heat pump technology
But the popularity of air-source heat pumps in the U.S. has traditionally been geographically limited to areas with mild climates, especially the southeast. That’s because traditional air-source heat pump technology has not performed well when temperatures plummet. For example, a conventional air-source heat pump utilizes a single-speed compressor that turns on and off depending on when heating and cooling is needed. EPRI lab and field research has demonstrated that both the heating capacity and efficiency of single-speed air-source heat pumps decline as outdoor temperatures fall.
At 10 degrees Fahrenheit outdoors, for instance, a single-speed heat pump operates at less than 50 percent of its maximum capacity, compared to 100 percent at 50 degrees Fahrenheit. Because air-source heat pumps can’t cost effectively satisfy the heating needs of a home when the outdoor temperature falls, they often require supplemental heating sources, like gas or electric resistance heating. For homeowners with air-source heat pumps, the need for supplemental heating can increase energy costs. And for utilities that have many air-source heat pumps operating in their service territory, their use during times of extreme cold can lead to a spike in peak load demand.
Newer technologies perform better in cold temperatures – at a cost
More advanced types of air-source heat pump technology perform better in cold climates. Variable speed heat pumps can increase the speed of the compressor in order to elevate the pump’s heating capacity as outdoor temperatures fall, which means that they can operate at over 65 percent of their maximum capacity when temperatures reach 10 degrees Fahrenheit. More advanced variable speed technology, known as low-ambient heat pumps, perform even better, providing over 75 percent of their maximum capacity at 10 degrees.
EPRI research has demonstrated that newer heat pump technologies can keep homes warm in frigid conditions – often without any supplemental heat. For instance, as part of field tests that took place during 2021’s polar vortex, EPRI found that variable speed heat pumps installed at several sites in Nebraska were able to meet an entire home’s heating needs without any supplemental heat at 0 degrees Fahrenheit.
While these advances are important, they come at a significant cost. “These advanced heat pumps are available in the marketplace,” said Don Shirey, a senior project manager for EPRI’s Customer Technologies program and project manager of the heat pump demonstration project in Nebraska. “However, their cost can be 60 to 100 percent higher than conventional single-speed heat pumps, depending on the manufacturer.”
Air-source heat pumps also suffer from a belief among potential customers that they simply aren’t up to the task of keeping a home warm when it’s cold outside. “There’s been this perception that heat pumps just couldn’t keep up with winter and indoor conditions are less comfortable than for gas furnace heating,” Shirey said. “There has been a customer perception barrier that has kept more from being used in colder locations.”
DOE’s Cold Climate Heat Pump Challenge
In order to leverage the full decarbonization potential of air-source heat pumps, then, it is necessary to both improve their performance in cold climates and reduce their upfront cost. “Many people in the U.S. live in a cold climate,” said Micah Sweeney, a senior research engineer at EPRI. “Interest in improving heat pumps is being driven by efficient electrification to reduce greenhouse gas emissions. These technologies have to go toe-to-toe with gas furnaces and electric resistance, perform better in cold climates, and be available at a lower cost.”
Last May, DOE Secretary Jennifer Granholm announced the Cold Climate Heat Pump Technology Challenge, which is targeted at establishing a new technology specification for air-source heat pump performance in cold weather. The challenge brings together six leading heating, ventilating and air conditioning (HVAC) manufacturers along with DOE, Natural Resources Canada, the U.S. Environmental Protection Agency (EPA), utilities and others to work together to overcome technical and market barriers that stand in the way of commercializing affordable and high-performance cold climate air-source heat pumps.
More specifically, the challenge sets out precise performance requirements that heat pumps must meet. “They want 100 percent heating capacity at 5 degrees Fahrenheit outdoors, which is the same capacity the heat pump would provide at 47 degrees,” Shirey said. In addition, the challenge also encourages the development of a heat pump able to perform well at -15 degrees.
The challenge also includes energy efficiency specifications. One basic measure of heat pump efficiency is the coefficient of performance, or COP. At the most basic level, COP is the ratio of the amount of useful heat a heat pump can produce given a certain amount of electricity input.
For example, a COP of 2 means that a heat pump delivers 2 units of heat energy for every single unit of electricity it uses. For the purposes of this challenge, heat pumps must achieve a minimum COP of between 2.1 and 2.4 at 5 degrees Fahrenheit. By contrast, the COP of electrical resistance heating is slightly below 1. The challenge also requires the use of low GWP (global warming potential) refrigerants and provide for grid interactivity so that heat pumps can participate in demand response programs.
If manufacturers build heat pumps that meet the challenge’s specifications, the new products would exceed the cold climate performance of any pumps available on the market today. The aim of the challenge is to commercialize better performing, more affordable air-source heat pumps by 2024.
EPRI and Oak Ridge National Laboratory collaborate on new technology
Separately, in August 2021 the DOE awarded a three-year grant to EPRI to develop a new, more energy efficient heat pump system, in coordination with Oak Ridge National Laboratory (ORNL). The new system will combine elements of a thermoelectric heat pump with a conventional air-source version to better perform during extreme cold weather.
This technology is developed through DOE’s Building Efficiency Frontiers and Innovation Technologies (BENEFIT) program. Sreenidhi Krishnamoorthy, a senior research engineer at EPRI and principal investigator on the project, says that the new hybrid heat pump technology has the potential for significant energy savings, increased affordability, and improved comfort for occupants of residential and commercial buildings. “This is a potentially transformative technology to evolve building energy efficiency and reduce carbon emissions,” Krishnamoorthy said. EPRI will provide initial modeling and analysis of baseline cooling/heating equipment, predict the performance of the thermoelectric heat pump, and field test full-scale prototypes.
“ORNL is responsible for developing and lab testing the prototype units, but EPRI will help with reviewing and analyzing the measured data, and assist with refining the design before the prototypes are completed,” Shirey said. “Then EPRI is responsible for completing field demonstrations. We’re going to install the prototypes in occupied homes and see how they perform in a real application, both in terms of efficiency but also in terms of customer satisfaction.”
The potential for decarbonization using this new technology is significant. If installed throughout the nation, the new heat pump system could reduce U.S. carbon emissions by 65 million metric tons by 2040. That would be the equivalent of eliminating all of the carbon emissions produced by California’s automobiles.
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