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| Fig. 1: Heat Pump Diagram (Source: Wikimedia Commons) - This figure dangles. - RBL |
Natural gas boilers have long dominated residential space heating in the United States, but growing concerns about greenhouse gas emissions have prompted investigation of alternative technologies. The International Energy Agency reports that more than one-sixth of global natural gas consumption goes toward heating buildings, representing a substantial opportunity for decarbonization. Air-source heat pumps (ASHP) present a thermodynamically efficient alternative that can reduce both energy consumption and carbon emissions while providing both heating and cooling capabilities. According to the IEA, widespread deployment could decrease worldwide carbon dioxide emissions by a minimum of 500 million metric tons annually by 2030. [1]
Heat pumps operate on a four-stage cycle that consists of compression, condensation, expansion, and evaporation. (See Fig. 1.) During this process, a refrigerant changes phase as it captures thermal energy from a heat source, in the case of air-source residential heat pumps this is the air outside. The refrigerant then goes through the reverse phase transition as it delivers this energy to a heat sink, which would be the air indoors. A four-way valve enables the refrigerant flow to be reversed, allowing the system to extract heat from interior spaces and reject it outdoors for space cooling. This reversibility makes heat pumps versatile year-round climate control systems. [2]
Since heat pumps rely mostly on heat transfer rather than heat generation, they are more efficient than conventional heating from gas boilers and electric heaters. A heat pump's performance is quantified through the coefficient of performance (COP), which is the energy output over the input energy, and a typical residential heat pump will have a COP of around 4. This means that the heat pumps energy output is four times as much as the energy used to operate it. [1]
The potential for heat pumps to reduce carbon emissions is substantial. Analysis by Wilson et al. quantifies the residential impact in the United States: complete conversion to ASHPs would decrease annual carbon dioxide equivalent emissions by 2.5 to 4.4 metric tons per household over the equipment's 16-year service life, with results varying by heat pump type and electrical grid composition. When scaled across the entire U.S. residential sector, this translates to annual emission reductions of 330 to 590 million metric tonsequivalent to 36% to 64% of 2020 residential sector emissions and 5% to 9% of total national emissions. [3]
Economic analysis by Wilson et al. reveals that ASHPs are financially viable without government subsidies for approximately 59% of U.S. households, representing roughly 65 million homes. In addition, nationwide adoption would yield substantial energy savings, reducing on-site energy consumption by an estimated 3.8 to 6.2 exajoules annually (equivalent to 3.6 to 5.9 quadrillion BTUs). [3]
Despite promising benefits, several economic barriers impede widespread heat pump adoption. Heat pumps designed for complete electrification of space heating typically cost more to install than a combined air conditioner and natural gas furnace system. Large heat pumps are needed to meet large heating loads, which often requires upgrading electrical wiring and, in some cases, electrical panels. Additionally, contractors lacking heat pump installation experience may charge premium rates to offset perceived risks associated with unfamiliar technology. [3]
Data published by the IEA shows that the levelized cost of heat pumps is close to that of a traditional HVAC system in the United States, despite heat pumps have a much higher energy efficiency. In 2021 they found the levelized cost of heating and cooling with a heat pump to be around $86 per MWh, with capital and installation costs contributing $30.6 per MWh. This is less than the cost of heating and cooling with a gas furnace and AC, which was found to be $99 per MWh. However, people are still deterred by the upfront costs. Having to pay a large sum for the heat pump and its installation can be a large financial burden, and many would not be motivated to switch to a heat pump without subsidies or incentives. [1]
Furthermore, there are non-cost barriers that are slowing the adoption of heat pumps. Many people are not informed about how heat pumps could save energy and money. Others are deterred by the inconvenience of installing a new appliance. This inconvenience is made worse by building codes and a shortage of qualified installers. [2]
Heat pumps are an efficient method of residential space heating and cooling that have the potential to substantially reduce energy consumption and greenhouse gas emissions. However, upfront installation costs remain the primary obstacle to mass adoption. Government subsidies and incentive programs can help overcome initial cost barriers, enabling homeowners to realize long-term operational savings. [2]
© Kimia Sattary. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] "The Future of Heat Pumps," International Energy Agency, 2022.
[2] Z. Wang et al., "State of the Art on Heat Pumps for Residential Buildings," Buildings 11, 350, (2021).
[3] E. J. H. Wilson et al., "Heat Pumps For All? Distributions of the Costs and Benefits of Residential Air-Source Heat Pumps in the United States," Joule 8, 1000 (2024).
