Guest post by Erin McKittrick and graphics by Bretwood Higman
Home heating is both energy-intensive and unavoidable. An average house on the Kenai Peninsula is 1800 sq feet, and will use around 46 million Btu (British thermal units) of energy to heat it in a year. Producing this heat emits between 3.5 to 10.5 metric tons of CO2 equivalent, making home heating 10-27% of an average household's total carbon footprint.
Emissions per unit heat vary widely between heating methods. The highest emissions are produced by electric baseboard heat at 225 kg CO2E/MMBtu, while the lowest come from air source heat pumps at 79 kg CO2E/MMBtu. Direct fossil fuel heat comes in in the middle, and burning wood is slightly worse than burning fossil fuels. Upstream emissions (emissions produced before the fuel gets to your house or the power plant) are significant for most heating methods, especially natural gas and natural gas-powered electricity, due to the large climate impact of methane leaks.
It is possible to improve all these numbers, by reducing industrial gas leakage, improving stoves, and changing wood burning practices. However, the only heating methods that can reach near-zero emissions are the electricity-dependent ones. As the grid shifts to more renewable energy, the impact of baseboard heat and heat pumps drops with it, without the homeowner needing to change anything.
This theoretical brighter future is not intended to represent an ideal (improvements beyond these levels are quite possible), but a reasonably possible near-term future, improving changeable metrics by about half. Some of these changes are more likely than others, and some are more within local control than others.
In this theoretical brighter future, Homer Electric Association improves its emissions intensity by 50%, by adding more renewables to the system. Methane leakage from natural gas is reduced by half, as are the upstream emissions of oil and gas production. Fuel stoves and furnaces cut their inefficiency (the difference between current efficiency and 100% efficiency) in half. The COP factor of an air source heat pump improves from 2.9 to 4.
Wood is assumed to improve its emissions intensity by half. Some of this could be accomplished with more efficient stoves, but to make a significant improvement, it is necessary to stop burning trunk wood. One possibility is to burn small branches and scraps that would have rotted away quickly in the natural ecosystem. Another is to intensively manage land for wood production in a way that stores carbon as fast as it is removed (potentially by "coppicing" an alder stand and burning the branches).
Overall, improving electric grid efficiency is probably the most plausible of these scenarios. There is already broad support for renewable energy generation, and the required technology is getting more robust and affordable very quickly. Reducing emissions intensity removes an ongoing expense (fuel) for utilities, providing clear economic incentive. Improvements related to reducing waste in fossil fuel production run afoul of diminishing returns for infrastructure investment, while improvements in wood heating require either wide adoption of fastidious habits for individual wood-gatherers, or industrial-scale production and sale of a product that is currently largely hand-gathered.
This analysis assumes a timeframe of 20 years -- looking 20 years into the future for all impacts. I chose 20 years because we are rapidly entering dangerous levels of climate change, and action is imperative before the 100 year time frame used in some analyses. A longer time frame would reduce the upstream impacts of fossil fuels, because methane has a much larger warming potential over 20 years (86 times CO2) than over 100 years (34 times CO2). A longer time frame would also reduce the impact of wood burning, because a larger portion of the wood that was burnt would have rotted in that time.
I assumed an average house size of 1881 square feet: a household size of 2.58 people: and an average US carbon footprint of 15 metric as well as how much heat a house requires at a certain level of insulation. Since the Kenai Peninsula wasn't included in the calculator, I averaged the values for Dillingham (climate analog for Kenai) and Juneau (climate analog for Homer).
Electric baseboard heaters are 100% efficient in converting electricity to heat. The emissions per unit heat are calculated based on the average emissions intensity of HEAs electricity grid from 2015-2018.
Air Source Heat Pumps:
An air source heat pump moves heat from the outdoors in, even at cold temperatures (functioning like a refrigerator in reverse). Electricity is required to run it, but you get more energy out than electric energy put in, expressed by a number called the COP factor. I assumed a COP factor of 2.9 (meaning you get 290% as much energy out as electric energy is put in)
The clear winner in this analysis. Technology for "mini-split" heat pumps has improved dramatically in recent years, particularly versions that work in colder climates. They are being increasingly adopted across the state, especially in Southeast, and have been used, within Alaska, for carbon offsets.
Burning wood creates an enormous amount of CO2, which is partially offset by the amount of CO2 that would have been produced were that wood to rot in the forest. I used rotting rates for northern conifers to estimate how much carbon would have been released from a spruce log in 20 years. That number is subtracted from the stovepipe emissions. I assumed a stove with efficiency of 80% heat values for Sitka spruce and other wood stove pollutants.
Real world use of wood (burning damp wood, or damping down the stove) likely increases emissions. I assume all wood is salvaged dead wood, removed at low enough levels that its removal has a negligible effect on the forest ecosystem. Harvesting live trees or at a larger scale has more complicated impacts.
I assumed a stove with 87% efficiency.
I assumed a stove with a 90% efficiency.
I assumed a furnace with a 95% efficiency.
Methane leakage was assumed to be 2.3% of natural gas produced. This was applied to Homer Electric's use of natural gas. For natural gas consumed in the home, leakage was assumed to be 3.3%, to account for leaks in the downstream distribution system and behind consumer meters. Methane emissions were converted to CO2 equivalent based on the 20 year global warming potential of 86. Oil and gas production was assumed to have an emissions intensity equal to the US average, which was adjusted to express the portion attributable to methane to a 20 year time frame, and used for fuel oil upstream emissions. Propane was assumed to have upstream emissions averaged between fuel oil and natural gas. Salvaged wood was assumed to have negligible upstream emissions.