Cornerstone Church with a roof-top solar photovoltaic system. Source: National Renewable Energy Laboratory.
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The building sector, which includes residential and commercial structures, is responsible for 39% of the total U.S energy consumption (EIA, 2019) and 21% of annual U.S. direct and indirect greenhouse gas (GHG) emissions (1.4 GtCO2e/yr) (EPA, 2019). Globally, buildings and their construction are responsible for over 33% of global final energy consumption and nearly 40% of total direct and indirect CO2 emissions globally (IEA, 2020). The U.S. Energy Information Administration (EIA) projects that global energy consumption in buildings will grow by 1.3% per year on average from present day to 2050, with non-OECD countries having five times the rate of growth compared to OECD countries (EIA, 2019). Rapid emissions reductions can be achieved in this sector through electrifying building processes, while concurrently transitioning to a power sector supplied by renewable energy sources.

The major energy demands of residential and commercial buildings are heating, appliances, cooling, lighting, and cooking. In the U.S., these processes are currently powered equally by electricity (lighting, appliances, some heating/cooling and cooking), and natural gas (space heating, water heating, space cooling, and cooking). There is a great deal of potential to rapidly decarbonize the building sector because those thermal processes currently powered with natural gas could be electrified using existing technologies.

U.S. energy use in buildings, by fuel type, in a net zero policy pathway. Source: Data from the U.S. Energy Policy Simulator 2.0.0. Energy Innovation

Electrifying Thermal Energy

Energy-star rated Heat Pump Water Heater, with its final plumbing configuration and flow meter. Source: National Renewable Energy Laboratory

Space and water heating account for nearly 50% of energy consumption in buildings. In the 2010s, 85% of residential heating loads and 65% of commercial building heating loads across the globe were satisfied by fossil fuels. Space heating is currently the largest source of non-electric demand in the U.S. building sector, accounting for approximately 70% of non-electric energy consumption in the residential sector, and 60% in the commercial sector (Deason et al. 2018). Water heating and cooking account for much of the remainder of emissions. 

An efficient alternative to fossil fuel-powered HVAC equipment for heating and cooling needs are electric air source heat pumps (ASHPs) (learn more about how they work). ASHPs are technologically mature, powered by electricity which can be decarbonized through the use of renewable power, and relatively easy to install in new construction or retrofit in existing systems. As discussed in the Efficiency Improvements section, they are more efficient than conventional systems and so are less expensive to operate, albeit typically more expensive to install than standard equipment. Future decarbonization scenarios show indicate ASHPs will be a leading heating technology in future, as opposed to other direct electrification technologies (electric space heaters or electric resistance heaters), or indirect electrification technologies (e.g., synthetic gases in gas heaters or gas pumps) (Ruhnau et al. 2019Jadun et al. 2017).

Electric ASHPs have been used in many parts of the world extensively but until recently have been scarcely implemented in areas with prolonged winters with subzero temperatures. The Department of Energy (DOE), however, has identified that the technology has transcended this cold winter limitation, making it suitable for colder regions like the Northeast and Midwest U.S. One remaining challenge of widespread adoption of ASHPs is that while the energy input can be decarbonized, the refrigerants used in ASHPs have very high Global Warming Potentials (GWPs). 

Ground source heat pumps (GSHPs) work using a similar thermodynamic principle as ASHPs but use the heat below the earth’s surface. Below ground temperatures are fairly stable and the temperature flux is sufficient to power a refrigerant cycle. GSHPs have limited implementation potential, though, owing to below-ground conditions being hyper location specific (IRENA), space constraints, and zoning barriers for GSHP installation in residential and commercial buildings. While geothermal systems costs have been historically high, there are several use cases today that show competitive economics, such as an installation at the State Capitol in the U.S. State of Michigan which has an estimated payback period of less than 7 years.

Water heating is another thermal process currently met largely with natural gas-powered equipment, but with enormous potential for electrification. Both air and ground source heat pumps can heat water two to three times more efficiently than conventional electric resistance water heaters, and can be retrofitted to work with a conventional storage water heater (DOE, 2020). Tankless or demand-type water heaters are another efficient option because they avoid the standby heat losses that storage water heaters have, and can be powered by electricity (DOE, 2020). Solar water heaters are a great option in high-sun exposure geographies, but might need to be supplemented with another type of heating source in areas with more intermittent exposure. They can be 50% more efficient than regular gas or electric water heaters and last many more years. This resource explains in detail how they work, where they work best, and other considerations. Drain-water heat recovery is a technology that recovers heat from used hot water (from showers or dishwashers, for example) for later use. It complements any heating technology and has a payback range between 2.5-7 years.

This DOE infographic provides a helpful comparison of different water heating technologies.