As federal, state and local governments look for ways to reduce carbon emissions and build more sustainable communities to combat the effects of climate change, they have turned their attention to buildings. According to the Department of Energy (DOE), buildings are responsible for about 40% of the United States’ energy consumption and greenhouse gas emissions. It is clear that in order to achieve the goals of the Paris Climate Agreement and limit global warming, decarbonizing buildings needs to be at the forefront of climate policy.
With an increasing number of states and municipalities enacting bans on new natural gas appliances and requiring buildings to reduce their energy use and carbon emissions, the focus is on adopting electric heat sources, with rebates available from local jurisdictions on heat pump purchases and tax credits through the Inflation Reduction Act.
But decarbonizing commercial heating, ventilation and air conditioning (HVAC) systems is a challenge. With only about 2% of U.S. building stock turning over each year, electrification of commercial heating will require retrofits in most buildings. The boiler systems most buildings use tend to last for many years, making building owners reluctant to replace them, and in the northern regions of the U.S., many buildings are hesitant to install heat pumps due to concerns about performance in cold temperatures. But collaboration between the public and private sectors has produced new technologies that enable the electrification of commercial heating in colder climates and provide a significant return on investment.
Cold Climate Heat Pumps
In colder climates, the requirements for heating are greater than the cooling requirements in the summer months. In summer, an air conditioning system might have to bring a building from 95 F to 70 F. In winter, the heating system might have to keep the building warm with outside temperatures at 20 F, meaning the heating system will need to produce twice as much heat. Because of this, when ambient temperatures drop below freezing, traditional heat pumps usually require an auxiliary heating system such as a natural gas system to ensure sufficient heat. While this is effective, it also results in the release of greenhouse gases into the atmosphere.
In 2021, the DOE launched the Cold Climate Heat Pump (CCHP) Technology Challenge, partnering with over 30 OEMs, utilities and state agencies to develop heat pumps that deliver 100% of heating capacity at temperatures as low as 5 F without the use of auxiliary heat. This challenge created incentives and public education campaigns to increase heat pump adoption, resulting in 23 cold climate heat pump prototypes. The next phase is the installation and monitoring of their performance in various cold climate locations in the U.S. and Canada. Early results indicate they have reduced energy consumption by 15-20%.
Heat pumps are inherently more efficient than fossil fuel sources. When gas or other fossil fuels are combusted, their efficiency is measured in annualized fuel utilization efficiency (AFUE). The most efficient furnaces and boilers can achieve 98%, which means, at best, 98% of the energy is converted into heating (many are around 80%). Heat pumps are measured in coefficient of performance (COP), and the DOE’s CCHP challenge required a COP of 2.1 at 5 F. 2.1 can be seen as 210% compared to 98%, at most, from a furnace or boiler. For every 1 kilowatt (kW) of power a heat pump consumes, it will produce 2.1 kW of heat. Heat pumps are even more efficient at higher ambient temperatures and in some applications can exceed a COP of 4 most of the year.
The Technology
Variable speed compressors allow vapor compression systems to match the load based on the season and occupancy conditions. In cold climates, as well as more temperate climates where the temperature is just below the freezing mark, vapor compression systems will have a higher load for heating than cooling. This means that variable speed compressors will have their accompanying variable speed drives (VSDs) run at higher speeds for heating than cooling.
The lower the outside temperature, the less warm air there is for the heat pump to pull from. This means the heat pump’s compressor has to work harder to transfer the heat indoors. So, in addition to increasing the heating requirements, whether the energy source is gas or electric, lower outdoor temperatures also lower the effectiveness of a heat pump to meet those heating requirements. A feature called vapor injection can allow compressors to operate more efficiently in heating mode by delivering more mass flow to the condenser. This is accomplished by adding additional mass flow through an intermediate stage of the compressor where gas can be compressed at a lower compression ratio. Having this feature makes a system’s condensing unit more complicated because the vapor injection requires an economizer. This economizer will include an extra heat exchanger, an additional expansion valve and additional accompanying valves and controls.
As temperatures drop below zero, the act of compression generates so much internal heat that it can challenge the motor’s ability to safely operate. In this case, compressors can be injected with liquid to allow them to continue to run with very low evaporator temperatures but still produce very high condensing temperatures. This is particularly important with heat pumps that produce hot water to replace a boiler. Liquid injection can be done with a dedicated valve injecting liquid directly to the scroll. It can also be done via overfeeding a vapor injection system so the economizer will deliver refrigerant that has not been fully evaporated to the compressor’s intermediate stage. This type of injection is what we refer to as “wet injection.”
Coordinating the variable speed compressors and various injection systems creates new control challenges with cold climate heat pumps.
The Challenges of Installation
Installing a cold climate heat pump is similar to the process of a traditional heat pump. Contractors need to consider building layout, heating load and existing wiring and voltage to ensure the right size heat pumps are installed and the system is designed for optimal performance and energy use. Cold climate heat pumps may be slightly larger than traditional heat pumps, so contractors need to make sure they size the wiring appropriately.
Infrastructure Considerations
The transition to electric heat in areas where natural gas furnaces are the standard will result in increased demand on the electrical grid. As heat pump adoption increases, the infrastructure needs to be improved in order to support the demand. Current electric generation and transmission cannot produce or carry enough kilowatts per hour (kWh) to replace all of the heating British Thermal Units (BTUs) provided by gas. A gradual rollout and transition to cold climate heat pumps will give municipalities time to make the necessary upgrades that will enable a shift to sustainable HVAC systems. This will include greater transmission capacity and potentially greater renewable power sources (wind, solar, etc.) so the source of natural gas consumption does not simply switch from buildings to power generation facilities. However, due to heat pumps having a COP advantage on furnaces and boilers, even if more gas is consumed in the short run to generate electricity, less natural gas would still be consumed overall, resulting in lower carbon emissions.
As pressure to reduce carbon emissions and energy consumption intensifies, cold climate heat pumps will provide a path to full electrification in HVAC. With continued public-private sector collaboration, infrastructure upgrades and adoption incentives, the shift to cleaner heat sources in the commercial building sector will become a reality.