Decarbonization of Building Energy Systems

For many years, ASHRAE members have had the responsibility of controlling the temperature in buildings, transportation, and refrigeration through the design and construction of HVAC systems. Additionally, members should consider the impact of greenhouse gas emissions when designing a new system or retrofitting an existing one. Moving forward, it will be necessary to decarbonize building energy systems. 

In order to achieve this decarbonization, it is necessary to determine the production of carbon dioxide (CO2) in typical buildings. For fossil fuels, the amount of CO2 can be determined from the amount of fuel consumed. The carbon intensity for typical fossil fuels is shown in Table 1. For example, the amount of CO2 produced by burning natural gas is 117 lb CO2 per 1 million Btu. 

The use of electricity also involves emission of CO2 since most electric grids use fossil fuels, such as coal and natural gas in generating plants. Different electric utilities around the country produce varying amounts of CO2 for each kilowatt-hour (kWh) of electricity based on the mix of plants, including coal, natural gas, oil, nuclear, hydroelectric, wind, solar, and other fuels. The average carbon intensity for electricity in the U.S. is currently 1 lb CO2/kWh. Information about different intensities of CO2/kWh for different areas is required. The carbon intensity of electric grids in different states and Washington, D.C., is given in Table 2, listed from lowest to highest. 

By using the actual or projected energy use of a building and the data in Tables 1 and 2, it is possible to evaluate the carbon emissions for any building with various energy systems in different states.

Note that the median state is Mississippi, with a carbon intensity of 0.941 lb CO2/kWh. This is very close to the national average of 1 lb CO2/kWh. Twenty-five states have lower electric carbon intensity, and the other 25 have higher electric carbon intensities.

By looking at Table 2, a number of observations are made. Vermont has virtually zero carbon emissions per kWh. Therefore, a building in Vermont using electricity as the only fuel will be virtually carbon free. On the other end of the spectrum, Wyoming and West Virginia have approximately 2 lb CO2/kWh, which is twice the national average. It is important to minimize electrical use in those states. Trade-offs between electric and fossil fuel thermal energy systems should be carefully evaluated.

All energy conservation measures will reduce carbon emissions. Requirements in the most recent version of ANSI/ASHRAE/IES Standard 90.1 should be observed, and additional energy conservation measures should be considered. Wherever possible, Energy Star appliances and HVAC equipment should be used. Since the carbon intensity of electricity varies so widely from state to state, the relative benefits in carbon emission reduction will vary significantly from state to state.

Current goals should be to reduce CO2 emissions of any building to the lowest levels possible.

In buildings, lights, computers, air-conditioning compressors, fans, and pumps all use electricity as their energy source. Heating, service-hot-water, humidification, and cooking appliances may use electricity or fossil fuels as the primary energy source.

The Commercial Building Energy Consumption Survey (CBECS) uses energy use intensity (EUI) as a measure of energy use. EUI is a number equal to thousands of Btu used in a year, divided by the square footage of the building. EUI for most buildings ranges from 34 kBtu/ft2·yr to 283 kBtu/ft2·yr. For example, the typical EUI of a hospital, as of 2012, is 231.1 kBtu/ft2·yr. CBECS further gives a breakdown of energy use, again in kBtu/ft2, for various energy uses, including heating, cooling, ventilation fans, lighting, and other uses (Table 3a). Since the heating, hot-water, and cooking energy use is broken out in CBECS, it is possible to look at the electric EUI and the thermal EUI of typical buildings. Heating, service hot water, and cooking are usually the main thermal energy uses. The electric and thermal EUIs are shown in Table 3b.

Energy analysis software often gives estimates of total lb of CO2 produced by a prospective or existing building. The EPA Portfolio Manager website gives total greenhouse gas equivalent emission intensity in kg CO2/ft2 for a reported building. The EPA Portfolio Manager can also give a breakdown of thermal, or direct, carbon emission intensity and electric, or indirect, carbon emission intensity. This article will use a number referred to as carbon emission intensity (CEI), expressed as lb CO2/ft2.

Where natural gas or another fossil fuel is the source of heat, one can derive a thermal CEI number similar to the EUI. This number will be lb of CO2 produced per year per ft2 of the building. Many residential and commercial buildings are heated by natural gas. As examples, the CEI of buildings assuming an electric fuel intensity of 0.5 lb, 1 lb, and 1.5 lb CO2/kWh, and assuming use of natural gas as the thermal fuel, are shown in Table 4. The CEI for typical buildings varies from 4.6 lb CO2/ft2 to 72.4 lb CO2/ft2.

An important consideration that HVAC designers should examine is the choice of heating fuels; that is, whether to use some form of electric heat or a fossil fuel. It is not necessarily true that the fossil fuel will produce more carbon emissions. Depending on the carbon intensity of the electricity used, the fossil fuel may be a better choice in some scenarios.

Electric Resistance Heat Compared to Natural Gas
Although electric resistance heat is less efficient than using heat-pump technology, electric resistance heat is still used in some applications, such as variable air volume systems with electric resistance reheat coils. In which states would electric resistance heat have equal or lower carbon emissions than natural gas? For a thermal load of 1 million Btu and a boiler or furnace efficiency of 80%:
Natural gas CO2 = (1,000,000/0.8) × 117 lb CO2/million Btu = 146 lb CO2
Electricity to produce 1,000,000 Btu = 1,000,000/3,412 lb CO2/kWh = 293 kWh
Equivalent electric carbon intensity = 146/293 kWh = 0.5 lb CO2/kWh

According to Table 2, 10 states would have equal or lower carbon emissions using electric resistance heat. In the other 41 states, including Washington, D.C., natural gas would be a better choice. 

Heat Pump Compared to Natural Gas
A package unit that is 5 tons or less has a 2015 International Energy Conservation Code (IECC) requirement for a heating seasonal performance factor (HSPF) of 8.0. This is an annual coefficient of performance (COP) of 2.34.

As with the example above, consider a thermal load of 1,000,000 Btu.

If an electric heat pump is used, 1,000,000/2.34 = 427,350 Btu/3,412 Btu/kWh = 125 kWh.The equivalent electric carbon intensity = 146/125 kWh = 1.168 lb CO2/kWh.

Thirty-six states would have equal or lower electric carbon intensity. However, with heat pumps it is necessary to conduct a careful analysis, including size and efficiency of the heat pump, outside temperatures, and the number of hours where the heat pump operation must be supplemented with electric resistance heat to get a true picture of the electricity used in a particular building and climate.

Of course, more efficient condensing furnaces and boilers are available, as well as better heat pumps, than the minimum efficiencies considered above.

Office Lighting Example
From Table 3a, a typical office building has an EUI of 77.8 kBtu/ft2·yr and a lighting EUI of 9.2 kBtu/ft2·yr. If lighting is reduced by 50%, the savings is 4.6 EUI, a savings of 5.9%. With natural gas as the thermal fuel and a national average electric carbon intensity of 1 lb CO2/kWh, the base case has a CEI of 19 lb CO2/ft2 (Table 4). The savings on lighting is 1.35 CEI. With the savings on lighting, the overall CEI drops to 17.65 lb CO2/ft2, a savings of 7.1%. The office example has a heating thermal CEI of 2.3 lb CO2/ft2. It would be extremely difficult to get a similar savings by conventional heating energy conservation measures or fuel switching.

Electric Vehicles
Although traditionally not part of building energy systems, some buildings and building locations are providing electric vehicle charging stations. It is important to look at the trade-offs between gasoline (or diesel) use and using electricity to operate a vehicle. Table 1 gives the carbon intensity of vehicle fuels.

An electric vehicle may get as much as 4 miles/kWh. Compared to a car that gets 40 miles/gallon (mpg) on gasoline, an electric vehicle at the national average of 1 lb CO2/kWh would produce 0.25 lb CO2/mile. The gasoline vehicle would produce 0.49 lb CO2/mile. At the national average carbon intensity, the electric car would produce only about half as much carbon as a gasoline vehicle rated at 40 mpg. However, in a state with an electric carbon intensity of 2 lb CO2/kWh, the electric car would produce 0.5 lb CO2/mile and, therefore, would produce more CO2 than a gasoline car rated at 40 mpg.

The emission of CO2 from buildings is a major source of greenhouse gases, and reducing carbon emissions is critically important. In assessing existing buildings or modeling new buildings, the CEI of the buildings should be evaluated. The carbon emissions of any building can be calculated by knowing the carbon intensity of the fuels that are used. CEI can be calculated for every building and can then be used as a comparative tool to evaluate the relative carbon emissions of any new or existing building. For many buildings in numerous states, the electricity used will be the major source of carbon emissions, even if a fossil fuel is used for thermal energy needs. Savings in CO2 emissions can be calculated for various energy-efficiency strategies. •

About the Author
John W. Roberts, P.E., HFDP, LEED BD + C,
is a senior mechanical engineer with Dewberry in Raleigh, N.C.