Little Engine That Could: The Heat Pump
By Diana Powers
The heat pump has come into focus as the key technology to decarbonize the heating sector in the energy transition. Analogously to the electric car for the transportation sector, the heat pump directly replaces a fossil-fuel burning technology that not only stops emissions at the end-use point, but is intrinsically more efficient. Both the heat pump and the electric car can become even more climate-friendly when they source their electricity from renewable energy. Both can conveniently replace their fossil-burning counterparts in a rather simple one to one, drop-in manner. They are both mature technologies, ready to manufacture and go. Both can be chosen by the individual, who is thereby empowered to play a crucial role in the energy transition. No wonder these two have become the champion technologies of their respective sectors, much as solar PV and wind power are to the electricity sector.
Heating buildings is an absolute necessity in cooler climate zones, and yet, up until recently decarbonizing the heating sector was excluded from discussions and analyses about the energy transition. According to the International Energy Agency (IEA), global heating is responsible for almost 4 gigatons of CO2 emissions annually, about 10% of the total 40 gigatons of global CO2 emissions. A successful energy transition plan is incomplete (in most countries) without a heating strategy.
Since the advent of fire, most heating has been achieved by burning. The fuels, wood, coal, oil, and natural gas, are still the main sources of home and industrial heating. Heat pumps don’t generate heat by combusting fuels, but rather they move heat from one place to another. In fact, in a counter-intuitive manner, they move heat from a colder place to a warmer place. This is analogous to a ball rolling uphill, and, therefore, it requires energy. But less energy is required to move energy from a colder to a warmer place than is needed to generate heat through combustion. In general, a heat pump is 3 to 5 times more efficient than a gas-burning boiler.
Specifically, the ordinary fossil fuel boiler has an efficiency of 80-85%, meaning that for every unit of energy put in, 80-85% of that energy becomes useful heat. The latest version of condensing gas boilers, that incorporates a heat exchanger within it, achieves 95% efficiency. While these are impressive numbers, they pale in comparison to the heat pump. Heat pumps are wonderfully efficient. A common measure for this is the coefficient of performance (COP). Heat pumps have COPs between 3 and 5, meaning for every unit of electricity put it, they produce 3 to 5 units of heat.
In addition, heat pumps don’t emit CO2. They are versatile, and can be used to heat buildings and heat water. Some are designed to be reversible, so that they can provide heating in winter and cooling in summer. Italy’s generous heat pump subsidy program promotes reversible heat pumps. Most of the US needs both heating and cooling as well. As global warming continues, countries in more northern latitudes that might not have needed cooling in the past will find it a necessity in the future.
The concept of the COP is a fuzzier concept in application on the ground than in theory. The efficiency of a heat pump is greater when it has a smaller temperature difference to span between outdoor (source) and indoor (sink) temperatures. Heat pump installation has been coupled with retrofitting of buildings, increased insulation, and double or triple glazing. Scandinavian countries with high heat pump uptake also have well-insulated buildings. Keeping the warmth in the building, and not having it pour out through the walls and windows, means the heat pump has less work to do to keep the building warm and keep its efficiency high.
The Fraunhofer Institute for Solar Energy in Germany, has created a promising and surprising innovation that would decrease the source-sink gap that the heat pump has to span. They have designed outdoor façade panels that would capture solar thermal heat, replacing the bulky and noisy outdoor heat pump. Made of ultra-high performance concrete (UHPC), the panels can be designed as architectural features. Inside the tiles have fluid-containing channels that mimic the veins of a leaf to equally cover the whole area. The fluid would absorb heat from the ambient air and from the sun’s radiation and transport it to a heat exchanger and then to the home’s heat distribution system. Such a system would seamlessly adapt to urban landscapes where space is scarce, noise is already too plentiful, and some beauty would be welcome. (Fraunhofer ISE press release, 17 April 2023)
Waste heat from industry is a great untapped source of energy in most industrialized countries. The European Union defines waste heat as “unavoidable heat or cold generated as a by-product in industrial or power generation installations, or in the tertiary sector, which would be dissipated unused into air or water….” Europe generates waste heat roughly equivalent to its heating needs. Project HEATLEAP is an EU-funded project that seeks to tap this resource in a variety of industries, using large heat pumps with COP values as high as 8. The heat can be used to service district heating networks or channeled back into the industry, or into another industry, for both cost and CO2 savings. (HEATLEAP website)
Another great untapped source of heat is wastewater (IEA, The Future of Heat Pumps). Many wastewater treatment plants are near district heating networks. Large heat pumps could capture this heat and provide a fifth of the heat needed by these networks. These projects increase the efficiency of the heat pump by lifting the temperature of the source.
Another kind of innovation has to do with the choice of the fluid, or refrigerant, that circulates in the heat pump. While heat pumps are supposed to be closed circuits, leakages can occur, especially during manufacturing and decommissioning, but also during malfunctions. In the past, chlorofluorocarbons (CFCs) were used. They were banned by the Montreal Protocol, adopted in 1987, due to their contribution to depleting the protective ozone layer around the planet. They have been replaced mainly by hydrofluorocarbons (HFCs) that are fine for the ozone layer but are potent greenhouse gases, with Global Warming Potentials (GWPs) 1000 to 2000 times greater than CO2. The search for better alternatives is on.
Propane is emerging as one of the best alternatives. With a GWP of about 3 times greater than CO2, propane is far more climate-friendly. The exponential growth of the heat pump market, that governments around the world are counting on to address the climate crisis, demands that we get this right. The only problem with propane is that it is flammable. Current heat pumps that use propane must therefore be kept outdoors.
Fraunhofer ISE and its industry collaborators have optimized the refrigeration circuit, keeping the amount of propane needed under 150 grams in a hermetically sealed circuit. At about a sixth of the amount of propane used in other heat pumps, the new optimized model can now safely be located indoors. With a COP of 4.7, the new model has become even more efficient as well. With such high efficiency, they perform well even in homes that are not well-insulated. This group is now working on a larger model for multi-family buildings.
Around the world, research and innovations are improving the already mighty heat pump. Their uptake and deployment are being promoted by powerful policies, such as the Inflation Reduction Act in the US and REPowerEU in the EU. Policies such as banning gas boilers from 2035 are sending a strong signal to manufacturers that this is the direction of the future. Programs are being designed to include training for planning and installing heat pumps within existing related training programs. Heat pumps will also be part of the solution to compensate for the variability of renewable energies. Through aggregation, many small users can shift to off-peak times of use, or digitalized to help optimize the whole energy system of the not-so-distant future.