Building decarbonization is one of the most important ways to cut pollution while creating jobs and aiming to deliver carbon neutral performance. Building in the U.S. account for 40% of carbon emissions. Eight percent of that is from electricity use and the remainders is from the combustion of fossil fuels for heating and other uses at the building. Many States, utilities, and large corporations are moving to get electricity from clean, carbon neutral sources. In fact, Many municipalities throughout the United States are taking action to reduce their carbon emissions.
Duke Energy, one of the largest investor owned utilities in the U.S. has committed to having 100% of its energy from carbon neutral sources by 2050.
This article explores heat pumps as one way building decarbonization can transform heating, ventilation and air conditioning (HVAC) systems. It is clear that electricity is becoming the carbon free choice of energy for many in the U.S. An all electric building coupled with a renewable or carbon-free source of electricity is considered to be decarbonized in its operation. Building HVAC Systems, water heating and even cooking will be transformed by the move to decarbonized buildings.
In addition to operational carbon, buildings have embodied carbon. While both embodied and operational carbon emissions must be reduced in a decarbonization effort, embodied carbon is a one-time carbon emission during the construction process.
The ease of implementing building decarbonizaton is greatly dependent on the local electricity grid. A building owner is able to make the decision to eliminate on-site fossil fuel use and add photovoltaic(PV) panels to their building.
Many electric grids are still heavily reliant on fossil fuels with over 62% of utility-scale electricity generation originating from fossil fuels in the U.S. Over the last decade improvements in power plant efficiency and the switch to more affordable natural gas instead of coal have reduced carbon emissions of electricity.
In areas that rely heavily on fossil fuel-based electricity, building owners have the option of installing on-site solar systems. By purchasing carbon credits, an individual or company is essentially financing carbon reduction projects to offset their own emissions.
In many states, the grid is currently a mix of carbon-based electricity and renewables. In this scenario, even if there is no gas consumption in the building, a building will never be truly carbon neutral without further interventions.
Net Zero energy buildings that use photovoltaics to offset all electrical energy consumption can easily decarbonize if all of the building systems are electric. Luckily many electric grids are moving in the direction of providing 100% carbon-free electricity.
Electric Heating Systems
Heat pumps are the main devices to achieve the goal to decarbonize buildings. Electric heater systems are highly efficient, allowing them to reach target temperatures faster and using less energy. Current typical HVAC systems can be replaced with alternative systems that use heat pumps for heating instead of gas. Heat pumps use the refrigeration cycle to transfer heat energy from one space to another. In some cases boilers are replaced with heat pumps. Qualities of electric heat exchangers and electric process heaters have seen many improvements over the past decade.
The market for heat pumps is growing in the U.S., and new options are coming on the market. Many of the new heat pump systems are coming from Asia and Europe.
In a typical variable air volume (VAV) reheat system in a large building, hot water is provided by a central gas boiler. Air-source heat pumps are the most common alternative to boilers.
Variable Refrigerant Flow(VRF) systems are large-scale ductless HVAC system that can perform at a high capacity. They have gained traction in recent years given the capital cost savings of using the same compressor for both heating and cooling instead of a separate boiler and cooling compressor.
Many commercial buildings, particularly retail, use single zone packaged units with natural gas heating and compressor-based cooling.
Gas furnaces are by far the most common residential heating system in the U.S., particularly in colder climates. The cooling system usually uses the same duct system as the furnace. By purchasing a heat pump that can provide both heating and cooling, homeowners can avoid the cost of two systems. In residential applications in which no central air system exists or needs to be installed, a ductless mini-split system can be an excellent alternative.
Heat Pumps for Domestic Hot Water
The Domestic hot water heat pump uses outdoor air to heat hot water supply. Unlike standard air-to-water heat pumps, this particular heat pump only provides domestic hot water for showers and sinks. This system is an ideal choice for homes with or without space heating. An opportunity also exists to replace more conventional domestic hot water (DHW) heaters with heat pumps. In fact, the residential heat pump water heater (HPWH) market is already far more developed than the heat pump market for space heating.
Residential domestic hot water heaters are the best opportunity for installing a heat pump. The Market for residential HPWH has been active for years and many options are available. Where as, in larger domestic hot water systems, a central hot water heater can be replaced with larger air-source heat pumps. Some jurisdictions require that Domestic Hot Water (DHW) systems are separated from space heating water with a double-wall heat exchanger. Smaller multifamily buildings can use residential heat pump water heaters piped together or separately for each unit.
But in Commercial buildings, a heat pump water heater may not necessarily be the best choice. If a commercial building includes a kitchen, however, a heat pump water is usually the best choice.
A limiting factor of air-source heat pumps is their operation at low ambient temperatures. In Colder climates, heat pump applications have historically been limited to more expensive ground-source heat pumps to ensure they would be able to operate during the extreme winter cold.
Costs
Costs are always a factor in building systems decisions. Heat pump systems that provide only heating generally are more expensive than their natural gas counterparts. The rapid growth in heat pumps means that the costs will most likely drop as further economies of scale are reached. Stand-alone air-source heat pumps that provide only heating or hot water are currently more expensive than their gas equivalents.
All-electric buildings have been around for a long time. The difference today is that buildings can also be efficient and use carbon-free sources of electricity. Affordable renewable energy and heat pumps are two critical components that are making decarbonized buildings possible today.
Energy costs of electrified buildings are roughly equivalent to gas buildings, but their carbon emissions are lower. Like electric Cars, electric buildings using heat pumps have lower emissions than their fossil fuel cousins in all but the most carbon- intensive electrical grids.
Conclusion
Manufacturers now supply numerous heat pump options that can replace gas heating systems. These range from stand-alone heat pump water heaters to large centralized air-source heat pumps or heat recovery chillers. These systems use a wide variety of refrigerants including lower GWP refrigerants such as R-32 and R-744 (CO2).
Heat Pumps lower greenhouse gas emissions in all but the most carbon-intensive electrical grids. Heat pumps generate less carbon emissions even when powered by natural gas-generated electricity.
Capital costs for heat pump systems, in which the compressor provides both heating and cooling, are in line with systems that have separate cooling and natural gas heating. Where stand-alone heat pumps replace a gas boiler, capital costs are higher. The market for heat pumps, especially VRF systems, is growing rapidly.
We have seen and expect to see strong momentum in the use of heat pump systems. The costs of heat pumps will most likely drop as sales increase.
References
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5. EPA. Undated. " Renewable Energy Certificates (RECs). U.S. Environmental Protection Agency.
6. EIA. 2020. "How Much Carbon Dioxide is produced when Different Fuels are Burned? U.S. Energy Information Administration.