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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?
Along with founding the Resilient Design Institute in 2012, Alex is founder of BuildingGreen, Inc. To keep up with his latest articles and musings, you can sign up for his Twitter feed.
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Alex, thank you sharing the insight. You are spot on with the need for localized cogeneration, trigeneration of maybe even quadgeneration. At Sangha we are starting a journey to develop and model a vision of ultra small multi-generation systems that will interlink with your neighbors and allow for heat, backup power, grid interconnection and cloud based control. While not technically feasible now this approach with site specific biomass collection and grid connection could possible provide another alternative to centralized power plants where piping hot water to 1000 homes miles away would be impossible. The northern Europeans use the model you describe in the article with great success but that is also due to housing density – which is a major barrier in the USA. Let’s stay in touch and spread the word.
Sangha’s vision is site net positive power, water and waste – a very high bar but worth trying to jump to! I like the phrase “let’s make the grid the backup;)”
Thanks Tod,
Very glad to learn about Sangha. Really interesting model. -Alex
Alex-I assume it will be a number of years before enough of us are producing our own power and heating/cooling our homes with the power we make such that the grid’s peak power need will shift from hot days in July to cold days in January. Right now, PV probably is a net benefit to the grid, since it reduces that July peak. By the time we hit a crossover point, I suspect we’ll be looking at some other technology that, like air source heat pumps, changes the amount of energy we need and when we need it.
Stephen, this is absolutely the case in most places. Solar-electric is very well matched to air conditioning loads and electricity use in commercial buildings. But we need to be thinking ahead. I hear of more and more people in northern New England, and even a local school, planning to replace oil boilers with air-source heat pumps. Some of our northern New England utilities are already winter-peaking today, so we need to pay attention to this issue and be ready with some solutions, for we will surely experience push-back on renewable portfolio requirements and net metering.
Alex,
Great to stumble across your post on the Building Energy 15 site as I am looking through the agenda.
Last winter, NYS got thrown into a winter peak in January, the first I can remember. This will likely occur again as so much of Buffalo area is electric heat, relying on Niagara Falls production.
SUNY Plattsburgh is also a dominant electric customer, with very low electric rates as part of their community electric use agreement. Part of the discussion of co-gen to curb winter peak situations down the road will need to be pricing structures. Right now there is not even a financial incentive to conserve electricity at SUNY Plattsburgh (thought the campus staff is still working very hard toward efficiency). In fact they are rewarded with a lower price per kWh for using more than a certain threshold amount. I suspect community or campus co-gen would not get even a little nibble!
Hi Alex,
Are you familiar with the SunPump made in BC? It looks like a promising improvement to air source heat pumps. Here is a link:
http://www.qmata.ca/qproducts.html
I had heard someone mention this product, but hadn’t looked into it. Thanks much. -Alex
So far we have not seen a general use hybrid (PV/thermal) panel due to the low grade heat produced by the panels (120 degrees +/-) and the need for 180+ water in most residential radiant systems. Though the concept is quite exciting. Any news from Sunpumps on real product or real implementations?
A US company called Solar Logic in New Mexico is doing some great work on solar thermal modeling, design and implementation http://www.solarlogicllc.com/
The Solar Logic system allows for multiple heat sources and loads – so fits with cogeneration on site or off site.
There are multiple residential cogeneration systems now available: Yanmar, Marathon, Honda to name a few. Price points are very high but the path is clear. You’ll be able to install cogeneration in your basement within this decade. I started on the path to become a Yanmar dealer but had to put that on the back burner for now. California has a very short number of heating days so lower cost solutions are key.
Alex, this is right on target, and timely. I think that, in Scandinavia, where district heat now heats large portions of indoor space, and where the low-density alternative has been heat pumps (Sweden has a very high penetration of heat pumps), the rationale has always been the relative costs, but you’ve made a significant contribution with your point about the need to supplement electrical supply and offset thermal demand during winter peaking, when solar is at its ebb. Biomass/WHR CHP is the high-density renewable thermal alternative to low-density heat pumps, but it can also be the cold-months electrical complement to summertime large-scale solar PV.
One of the issues in reducing peaks by moving to district heating is cooling. If you just supply hot water, then commercial buildings still need on site systems for cooling. Once they spend the money for a cooling system, the incremental cost to make that a heat pump that also supplies heating is very little, so it is tempting to use that system for heating also. In Stockholm they are supplying chilled water as well as hot water, and this approach may solve this issue. (Also, it was my impression that buildings either have to or are very strongly encouraged to hook up to the district heating/cooling — can we be so cooperative in the US?) The heat from the CHP plant can be used in absorption chilling, creating a use for the heat in summer. They are also using large scale heat pumps to supply some of their district heating in all but coldest months (when their HP technology can’t make the water hot enough) using sewage treatment plant effluent as the heat source. This seems like a good strategy for times in these colder months when wind power is available, in order to be able to shift to more “pure” renewable than biomass. I worry about the long term sustainability of biomass, particularly in future years when the demand may become quite strong.
Amory Lovins long ago made the point that energy supply should be matched in quality to end-use needs, and I think his observation still applies. Heating and cooling require low-quality thermal energy, and can be supplied using thermal systems — which may provide an alternate solution to the problem you’ve identified.
A couple examples of this: The 2007 University of Cincinnati Solar Decathlon house (on which I worked) applied this logic, and used evacuated tubes to supply both heating AND cooling. Heating was relatively straightforward (we used Warmboard for in-floor radiant); cooling was supplied with a small-scale heat-driven absorption chiller. Another example is a Net Positive energy residence I worked on at William McDonough + Partners: thanks to good passive design, we completely eliminated the cooling system, and used solar thermal + radiant floor for heating. Evacuated tubes are far more efficient at capturing the sun’s energy than PVs, and seem to me to be a good match for heating.