When do heat pumps reduce air pollution? Parth Vaishnav Adilla Fatimah Meghana Kavakuntala Carnegie Mellon University

Decarbonization requires a shift from natural gas to electric heat pumps https://www.flickr.com/photos/ardyiii/504375744 US DOE Damages caused by natural gas combustion Natural gas use for heating NOAA Hourly ambient temperature Electricity use by heat pump Heat pumps are air conditioners that run in reverse: they provide heating rather than cooling Most analyses—including that done by the late David MacKay here at Cambridge—consider them essential to decarbonizing the economy Heat pumps extract heat from the ambient air (or, in some cases, from the ground) and force it into interior spaces that are warmer than ambient The amount of energy required to push heat up this temperature gradient is typically several times less than the amount of heat that is delivered: the ratio of the amount of heat delivered to the work done is the coefficient of performance of the heat pump. The coefficient of performance falls with ambient temperature. What we do here is to take an estimate of the hourly natural gas use for a typical home in about 1000 different locations in the United States We then find out the hourly ambient temperature in those places, and based on data on hundreds of different models of heat pumps, calculate the coefficient of performance for those heat pumps for each hour of the year Combining how much heat you need with the coefficient of performance tells us how much electricity you need to supply to the heat pump Now, over the years, we have created and refined a dataset that tells us—for about 1000 locations in the US—how much environmental and health damage, in $, a marginal kilowatt hour of electricity demand in any given hour of the year is likely to do. We also have estimates of how much of that damage comes from increased CO2, increased particulate matter, increased Nox, and increased SO2. This is done by combining electricity system data, with air quality models, with epidemiological data, with value of statistical life. Combining all this tells me how much damage to the environment and to human health running the heat pump in each of these locations would do. I then compare this to the damage that simply burning natural gas in a furnace would do. In addition to estimating environmental and health damage, we also calculate what each of these different ways of heating would cost house holds. We know how much gas they burn in a year, and how much electricity they would use if they used heat pumps instead. And we know the prices of natural gas and electricity in each state: so we can calculate what they would pay if they used each fuel. NEEP Heat pump coeff. of performance Average / Marginal damage from electricity use MacKay, D. J. Sustainable Energy - Without the Hot Air. (UIT Cambridge Ltd., 2009). (*) (*) Siler-Evans et al. (2013); Vaishnav et al. (2017); Azevedo et al. (2017)

If the existing fuel is oil, we know that heat pump is better Blumsack, S., Brownson, J. & Witmer, L. Efficiency, economic and environmental assessment of ground source heat pumps in central Pennsylvania. in System Sciences, 2009. HICSS’09. 42nd Hawaii International Conference on 1–7 (IEEE, 2009).

With the current U.S. grid, this would save energy but increase fuel costs and pollution The first thing that we find is that a switch to heat pumps would indeed reduce energy use across the US In much of the South, it would also reduce bills. In the north, it would increase bills. Exceptions: Oregon and Washington, where electricity is exceptionally cheap. And California, where it is exceptionally expensive. More interestingly, in parts of the US where the grid still uses a fair bit of coal, heat pumpt would not reduce CO2 emissions, although it would in the rest of the country. However, the monetized benefits of the reduced CO2 emissions would be completely swamped by the monetized damages from increased emissions of what are in the US called criteria pollutants: SO2, Nox, and PM2.5

In most (but not all) places energy costs exceed environmental benefit

A *modest* CO2 price would equate energy costs for hybrid heat pumps & current fuel

A much cleaner grid would be needed to produce an environmental benefit

SO2 emissions could be cut ~79% SO2 emissions if all coal plants scrubbed 95% of their emissions Current SO2 emissions

Natural gas generation would substantially reduce pollution

Total Environmental benefit Energy cost + Environmental benefit With a nat gas grid, heat pumps would produce a total environmental benefit Total Environmental benefit CURRENT GRID NATURAL GAS GRID Energy cost + Environmental benefit CURRENT GRID NATURAL GAS GRID

Efficiency improves (energy cost + environmental benefit) Total Environmental benefit BASE DEMAND LOW DEMAND Energy cost + Environmental benefit BASE DEMAND LOW DEMAND

Increased efficiency reduces the cost-effectiveness of heat pumps 12 Increased efficiency reduces the cost-effectiveness of heat pumps BASE DEMAND + CURRENT GRID BASE DEMAND + NATURAL GAS GRID CO2 price to equate fuel costs BASE DEMAND + CURRENT GRID LOW DEMAND + CURRENT GRID This is not obvious because, in some cases, efficiency might do away with the need for space heating.

The effect of a warming climate is complex (HadGEM2-ES_rcp60_r1i1p1; current grid)

“Easy” and “hard” decarbonization is entangled: interesting to explore how Davis, S. J. et al. Net-zero emissions energy systems. Science 360, eaas9793 (2018).

Acknowledgement This work was supported by the center for Climate and Energy Decision Making (SES-0949710), through a cooperative agreement between the National Science Foundation and Carnegie Mellon University

The effect of a warming climate is complex (HadGEM2-ES_rcp60_r1i1p1; natgas grid)

Accounting for the capital cost of heat pumps Next steps Accounting for the capital cost of heat pumps Retrofit versus new build How will the map change as the climate warms? Target journal: Environmental Science & Technology The costs considered so far are all fuel costs: I have not considered the cost of actually installing heat pump versus a furnace. In many cases you might actually have to do both. What I’m working on now with Meghana is to update this analysis to consider what it would look like as the climate warms. This is a projection based on the national climate assessment: in a high-emissions scenario, Pittsburgh is expected, by the end of the century to have the same sort of climate as Alabama, and that of Kentucky by mid-century. So, we want to look at what the analysis you just saw would look like at that time. We hope to ship this to ES&T by the end of the summer. Source: Union of Concerned Scientists