Over the many years I have taught electrification basics for the Wisconsin Public Utilities Institute’s (WPUI) Energy Utilities Basics course, I dive deeply into heat pumps and their impact on the grid. I explain the messy chart in Figure 1 like a palm reader, except I have engineering behind my assertions rather than mystical conjectures.
Cold-Climate, Air-Source Heat Pump Performance
As outdoor temperatures fall below zero Fahrenheit, as they do from the panhandle of Idaho and Wyoming across the country to Maine and Massachusetts, cold-climate air source heat pump (CCHP) performance and capacity dive while heating loads increase. The chart shows that measured heat pump efficiency drops below a coefficient of performance (COP) of 2.0, down to 1.0, and even lower.
Figure 1 Cold Climate Heat Pump Performance
The mathematical definition of COP is presented in Equation 1, where:
- Btu/hr is measured using airflow and temperature increase across the heat pump
- kWinput is the measured power consumption of the heat pump
Equation 1 Heat Pump Coefficient of Performance
In English, COP is the ratio of heat delivered divided by the energy required to deliver it.
Energy System Efficiency
For reference, the COP of an electric stove, hair dryer, and toaster is 1.0, 100%. It sounds good, but it’s awful. It’s the equivalent of 35% efficiency compared to 95% for a natural gas furnace. Why? Because 35% is the efficiency of a thermal power plant fueled by coal, nuclear, or natural gas using a gas turbine. The efficiency of a combined-cycle natural gas plant is around 65%, which is why I am a proponent and not a puritan.
Renewables are infinitely efficient because they require no fuel, but their efficiency is indeterminate, zero divided by zero, at night when the wind isn’t blowing. Since renewables are indeterminate, they deserve to be kicked to the curb. Renewables with any type of storage will never reliably provide heat through a season in the region of the country described above. That is the point. Are you listening, New York and Massachusetts?
Climate Zone Applicability
The next slide in my WPUI Energy Utility Basics delivery is shown in Figure 2. The narrative is that I would not recommend cold climate heat pumps alone, with no backup from natural gas or even propane, north of the mid-point of the green, Zone 4. Why? Tremendous peak load on the grid.
Heating is unlike air conditioning, which people can get by for a few hours without. The temperature difference between indoors to outdoors during cooling is only 30-40 degrees. For heating, it can be 90 degrees. Furthermore, as Texans learned during Uri a few years ago, water freezes around 32 degrees, or sometimes as low as 24 degrees. When that happens, the damage is not something homeowners want to endure. And, notice Texas’ climate for heating in Figure 2!
Figure 2 Heat Pump Climate Zones
Grid Impacts of CCHPs
Grid Impacts of CCHPsFortunately, The Wall Street Journal’s Climate & Energy email roundup of recent news tipped me off to a McKinsey study that analyzed cold-climate heat pump impacts on the grid in a fully electrified heating scenario. In Figure 3, McKinsey shows that peak loads in Vermont and Massachusetts (Massachusetts…where have I heard Massachusetts?) will quadruple under a fully electrified cold-climate heat pump scenario. This bodes well for woodstove sales but not well for trees.
McKinsey’s analysis notes results are without demand management measures. As a walking museum of extreme cold, I can tell you there are practically no options for demand management in brutally cold conditions. Nothing exposes weakness and breaks things like extreme cold. Cold climate heat pumps run balls out with no option for curtailment since they can – at best – maintain temperature in extreme cold conditions even without curtailment. I have one so I speak from direct experience.
Figure 3 McKinsey Electrified Heating Analysis
Boring DERs
Boring DERsTo The Wall Street Journal’s credit, they call holes in the ground grid investments. The grid investment is a ground-source heat pump, aka “geothermal” heat pump, using the massive earth below and its relatively stable temperature and heat sink for space heating. I’ve been espousing geothermal heat pumps (GHPs) over CCHPs for every deep blue state in the Figure 3 McKinsey cartoon for years – here, here, and here, for example.
Ground-source heat pumps were all the rage early in my efficiency career in the late 1990s. Every school district in Iowa wanted them – successfully promoted by utilities as being cool. They were almost as popular as solar panels. What happened? Do they think CCHPs are just as efficient?
Peak Load and Transmission Avoidance
The answer to that question is referenced by the same Wall Street Journal article which points to a Brattle report on electrifying New England heating. The peak-load impact of GHPs versus CCHPs is a 17% increase versus 94%. Which would you rather pay for?
Figure 4 Peak Load Increase Air Source v Ground Source
If you want to know what minimizing waste and maximizing value looks like, it’s shown in Figure 4 via the load factor. Customers get 61% of generating and delivery capacity with GHP adoption versus a measly 38% for CCHP adoption.
And what about distributed energy resources (DERs)? These holes in the ground are DERs, which avoid excessive generating capacity and transmission and distribution upgrades.
Speaking of avoided transmission, this Oak Ridge National Laboratory calculates GHPs will avoid the construction of 65.3 TW-miles of transmission lines compared to CCHPs. What in tarnation is a TW-mile? We’re learning together. That’s what we do here. A TW (terawatt) is 1,000 gigawatts. A gigawatt is the typical capacity of a commercial nuclear power plant. Therefore, 65 TW-miles will transmit a nuclear power plant’s power 65,000 miles.
The report notes that the average transmission line carries 1.5 GW. Therefore, in an electrified heating scenario, GHPs would eliminate the need for 43,500 miles of transmission compared to CCHPs. That is 16 trips across our continent from sea to shining sea.
To quote myself from 2021, “I’m just a Schmendrick with a web browser and a keyboard.” States need to drive policy per the McKinsey report to include benefits from the ORNL report.
[1]https://www.aceee.org/files/proceedings/2016/data/papers/1_836.pdf
