Popular in Europe, heat pumps run air conditioning in reverse
Carnot cycle refrigeration by mechanical compressors has been around for over a century, and it’s a mature technology. Since World War II, it has been used to heat and cool structures, but with increased urgency in reducing fossil fuel use, heat pumps are under renewed scrutiny as a low-emission climate control solution. Moving heat with this technology is significantly more energy efficient than electrical resistance heating but is more expensive than natural gas boilers for space heat applications. A wholesale switch to heat pump technology will require government subsidies, carbon taxes, direct regulation or some combination of the three unless some new, lower-cost way to compress the working fluid is discovered.
Access all episodes of This Week in Engineering on engineering.com TV along with all of our other series.
* * *
Episode Transcript:
Long before environmentalism and climate action, engineers in the climate control industry mastered mechanical refrigeration. It’s now so ubiquitous that we all take it for granted, but it’s a minor mechanical miracle.
It is a basic principle of physics that the phase change of matter from solid to liquid, and liquid to gas, absorbs and releases a great deal of energy per unit mass or volume. Examples of this are almost too numerous to mention, from ice cubes in a drink to Florida orange growers watering their groves to protect against frost damage. In hot, arid climates, evaporative cooling is a low-cost way to get mild cooling. These so-called “swamp coolers” have been around since ancient Egypt and Persia.
But true air conditioning replaces open-loop evaporation of water with closed-loop evaporation of pressurized refrigerant compounds, with the resulting gas being compressed and passed through a heat exchanger called the condenser for reversion to the liquid state, creating the very effective closed-loop process that can create deep-freeze conditions.
Mechanical refrigeration is effective, but the energy required to operate that compressor, plus fans to move air past both evaporating and condensing heat exchangers, is considerable. Anyone who has felt the outside of an air conditioner knows that the heat is rejected to ambient. Run this process in reverse, and it’s possible to extract heat from ambient and move it indoors.
This is the principle of the pump, essentially a reversible air conditioner. Heat pumps have been used for space heat since before World War II and are attracting attention today because of the dual drivers of CO2 reduction and energy efficiency.
And these Carnot cycle devices can be very energy efficient.
For refrigeration, there is no cost-effective alternate technology, but for space heat there are many options. If the move to phase out combustion of fossil fuels continues, the choice will be between electrical resistance heating and heat pumps.
Resistance heating is very efficient, and almost every kilowatt of input energy is translated into heat. But moving heat is more efficient than generating heat, and according to the German Federal Office of Economic Affairs, every kilowatt hour of energy consumed in a heat pump can deliver 3 to 6 kWh equivalent of heat into the building.
The problem is cost. According to the International Energy Agency, heat pumps are two to four times more expensive than natural gas-fired equipment for the same output. Bridging the cost gap is addressed by government subsidies, taxation on fossil fuels to increase gas and oil heating costs—and in the future, legislation simply prohibiting fossil fuel-fired equipment.
But can heat pumps work everywhere? As every engineer knows, heat transfer is dependent on the temperature difference between the intercooler and ambient. In the winter, heat pumps effectively refrigerate the outdoors, and the temperature difference between the device operating in evaporator mode and external temperatures in cold climates will be low, affecting system efficiency.
The same is true in the other direction, with a heat pump effectively warming the outside air in hot climates. The physics suggest that heat pumps will be most efficient in temperate zones. In hot climates, more electricity will be needed to drive higher capacity systems, and in cold climates, some combination of fuel-fired, electrical resistance heating and heat pumps may be needed for lowest cost in space heat.
In Europe, the interruption of Russian gas supplies created a great incentive for heat pump installation, which has been seriously hampered by lack of skilled labor to install the systems.
Heat pump technology is mature, reliable and can replace the combustion of fossil fuels for heating applications. Making it cost-effective is a challenge for manufacturing engineers, and politicians.
