May 30 2023

Should You Get a Heat Pump?

Starting around 1550 and lasting through the 1600s, England had an energy crisis. They were running out of wood, which was the main source of fuel for residential and commercial heating. England also needed a lot of wood for their massive navy – it took about 2,000 trees to build one of the larger warships. As a result they turned to coal, which has a high energy density and worked nicely for heating. It has the not-insignificant problem, however, of generating a lot of pollution, choking large cities like London in black smoke. This lasted into the 20th century, culminating with the great London smog of 1952.

The world still burns a lot of coal to generate heat, but generally not in homes. There are other options, especially where people live. We can generate heat by burning cleaner sources of fuel, like natural gas. We can also generate heat through electrical resistance, producing no pollution directly (only in the production of electricity, which is likely remote from the user). Heat can also be harvested from waste heat and pumped into buildings. Or heat can be moved from one source to another using a heat pump.

Moving to more efficient and environmentally friendly methods of producing heat is at least as important to minimizing global warming as generating clean electricity. About half of world energy is used simply to generate heat, more than any other use (generating electricity is 20% and transportation is 30%). Decarbonizing heat production is therefore arguably more important than decarbonizing either electricity production or transportation, although of course they are all important.

Two technologies are likely to make the most difference in decarbonizing heat production. The first is harvesting waste heat from energy production and other industrial processes. This requires producing energy somewhat close to where the heat is needed, which is another advantage of more distributed rather than centralized energy production. Harvesting waste heat needs to be designed up front for any installation that will generate a lot of heat. Data centers, for example, expend a lot of energy just cooling all those computers. That heat can be put to good use.

Other than redirecting waste heat, the most efficient method of heating is the heat pump. This is because heat pumps don’t generate heat, they just move it from one location to another. All that is necessary is a heat source and a heat sink that are at different temperatures, and the greater the temperature differential the more efficient the process. Heat pumps extract heat by compression moving a refrigerant through a heat source, then compressing the refrigerant to increase its temperature. The refrigerant then moves through a heat sink to extract some of the heat, it is then expanded and cooled further and run again through the heat source. The system runs on electricity, but efficient systems can provide 3-4 times as much heat energy and electrical energy they use (again, because they are moving heat, not creating it).

Heat pumps are therefore extremely energy efficient. Natural gas boilers, by contrast, are 75-85% efficient at converting the energy in the natural gas into heat. Electric heaters are essentially 100% efficient at converting electricity to heat. That is a high efficiency as a heat source, but compare that to the 400% efficiency of a heat pump. It’s no contest.

Heat pump systems can use various sources for their heat. Most homes will be appropriate for geothermal heat pumps. Pipes are buried under the ground, either horizontally or vertically, deep enough where the ground temperature is stable. Ground temperatures range from 45°F (7°C) to 75°F (21°C), which is likely to be warmer than the air in the winter and cooler in the summer. This will depend on local conditions, which will determine what kind of system is best. Also, such systems can be used for cooling as well as heating – just reverse the flow.

For buildings near a water source, either surface or below ground, the water can be the heat source. And heat pumps can also be added to factories that produce waste heat, to extract that heat for use in buildings.

In addition to residential heat pumps, increasingly towns and cities are installing district heat pumps. Homes will typically use a system in the kilowatts. These district heat pumps produce megawatts of heat. The largest individual heat pump is 30 megawatts, but there are also heat pump systems combining multiple smaller heat pumps together. These can heat thousands of homes. There is some efficiency of scale to these centralized heat pumps, plus individual homes are spared the cost of installing their own system (the one downside to a home heat pump system is that the upfront costs can be high relative to other heating systems). Combine these systems with a clean source of electricity to run them, and this can be a critical part of any decarbonization plan.

The bottom line is that heat pumps are an efficient and clean way to meet both residential and commercial heating needs, without burning fuel just to create heat. There is plenty enough heat in the system – we just need to move it around. This is especially efficient when we move heat form a location where we don’t want it (like a data center) to one where we do (like a cold building in the winter). Heat pumps can generate temperatures as high as 200C, which is useful for a lot of industrial purposes as well. Replacing our heating infrastructure to heat pumps is arguably as important as converting our energy production and transportation sectors to low carbon. Again – half of all the energy humans use is to generate heat. Not all of this can be replaced with heat pumps, but a lot of it can.


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