The growing use of cold-climate heat pumps to heat net-zero-energy, solar-powered buildings will shift electricity peaks to the winter—we need to confront this issue
For some time I’ve been promoting the use of air-source heat pumps for heating well-insulated homes and light-commercial buildings, and generating the electricity for that heat using a solar array—in a net-metered, net-zero-energy arrangement. This is what we’re doing in our newly-remodeled, Dummerston, Vermont home.
With or without solar, air-source heat pumps are growing rapidly in popularity. I must be getting several queries a week from friends, neighbors, and total strangers about air-source heat pumps—often called cold-climate heat pumps or minisplit heat pumps:
“Will this work for my house, which isn’t as well insulated as yours?”
“Can I get one of these heat pumps that has significantly greater output to replace by 100,000 Btu/hour oil boiler?”
I love the idea that this is a shift away from the combustion of fossil fuels that spew carbon dioxide into the atmosphere—even if the electricity is currently being generated with coal- or gas-fired power plants, having electricity as the medium of heat generation will make the later conversion to renewable generation easier.
Shifting electrical peaks to the winter months
The problem with all this is that as more people switch to electric heat, even if they do so with solar power in net-zero-energy buildings, our utility companies (in cold climates) will increasingly shift to winter peaks. And during the winter months, solar energy usually isn’t satisfying the needs of our net-zero-energy buildings, because our electric heat is drawing more power from the grid than our solar systems are delivering.
This isn’t too be a problem today, but as solar begins accounting for a significant percentage of our electricity generation, it does become significant. Some small, municipal utilities in Vermont are already pushing 15% of their generation from renewables.
The really bad news about this is that, on a macro-scale, utility companies—and probably public utility commissions—will increasingly be questioning or seeking repeal of net-metering rules that have made possible the system that I and many others in the green building community have long been advancing. Our home energy system works because the utility company is letting me generate more power than I need during the summer (and buying that power) and letting me pull more power than I’m producing in the winter when there isn’t as much sunlight and my electricity use (for heat) is far greater.
But if utility companies have to build expensive new capacity to meet winter electrical demand, their enthusiasm for net metering of solar systems will quickly wane, and so will their support of shifting to air-source heat pumps for heating.
A solution in combined heat and power
One answer to this conundrum is actually fairly straightforward—not easy, but straightforward: We need to shift to combined heat and power for utility-scale power production. And we need to convert our more densely populated communities to district heating systems.
Combined heat and power (CHP) is also referred to as cogeneration, and the idea has been around for a long time, and it is widely used in northern Europe. With CHP, the waste heat that is produced with all thermoelectric power plants is captured and productively used.
To provide a little more background: thermoelectric power plants burn something like coal or natural gas or carry out nuclear reactions to generate heat, and that heat is used to produce high-pressure steam, which is used to spin turbines that generate electricity. Most thermoelectric power plants operate at only about 35% efficiency, meaning that of the primary energy going into the plant, only about a third of it is utilized; the rest is lost as waste heat.
Today’s best combined-cycle gas power plants operate at about 60% efficiency—much better, but still throwing away four out of every ten units of primary energy. The rest of that energy is a waste product that itself becomes a problem as thermal pollution.
In the nuclear power plant ten miles from me, much of the waste heat is dumped into the Connecticut River. Thermoelectric power plants also evaporate billions of gallons of water in cooling towers to deal with that unwanted heat—water that is increasingly precious in many parts of the country.
In cold climates, the increased use of CHP would balance the demand for electricity (for heating) with distributed heat via piped hot water. District heat works where the building density is great enough to justify the cost of burying insulated piping to distribute hot water.
Renewable energy sources for CHP plants
Further, these CHP systems should be designed to rely, to the extent possible, on renewable energy sources. Here in Vermont, that means wood-chip plants, such as those that are widely used in Scandinavian countries. In the Midwestern grain belt, the fuel source might be waste agricultural fiber, such as wheat straw. In the southeast, we might see dedicated plantations of fast-grown biomass crops, such as coppiced poplar or willow.
Operating CHP plants based on thermal demand
CHP plants can either be operated based on the demand for electricity and deal with the waste heat as a byproduct, or they can be operated to meet thermal demands and feed electricity into the grid more as a byproduct. In the vision presented here, the recommendation is to operate CHP plants in northern climates to satisfy heating demand first, and have the electricity produced in the process fluctuate accordingly.
Such a configuration is sometimes—confusingly, in my opinion—referred to as thermal-following. With such an operating plan, there is flexibility to ramp up a plant to meet electrical demand when needed—though doing so can sacrifice overall efficiency is there isn’t a way to productively use the co-generated heat.
Moving toward a renewable energy economy
So, my bottom-line recommendation is to convert from gas or oil heat to electricity in low-density homes and light commercial buildings—installing solar so that they operate at net-zero-energy on an annual basis—and shift to renewably powered district heating in urban and densely populated towns. Maintain—and increase—incentives to encourage the installation of solar-electric systems, including net-metering and group-net-metering provisions. (With group net metering, one can own a solar system located somewhere else within the utility territory.)
Solar-electric systems make the most sense where buildings are more spread out and only one or two stories tall (so that solar systems sized to meet annual electricity needs can be installed on roofs, keeping more land open for agriculture), which very well compliments district heat that requires density and is particularly appropriate for attached homes.
Operation of these CHP plants will be greatest in the winter months—at least in cold climates—and the electricity so produced can provide the extra electricity needed to power the air-source heat pumps installed in more widely spaced buildings where district heat is impractical. This strategy will solve the conundrum that net-metered solar-electric systems generate more electricity in the summer than winter.
CHP, renewables, and resilience
From the standpoint of resilience, a shift to this sort of renewables-based energy system will create a more resilient, distributed power grid with a more diverse generation sources. And the energy performance of building needed to achieve net-zero-energy performance will achieve the sort of passive survivability that I’ve long been calling for—ensuring that livable temperatures will be maintained in buildings that lose power for an extended period of time.
With the challenges our society is facing through climate change and increased risks of power outages, we need to look for integrated solutions. I think this is would be a smart approach—marrying CHP plants that are operated based on the need for their heat output with distributed, rooftop solar systems in less densely populated areas that produce more electricity in the summer.
Does this make sense?