Technological marvels have been a recurring storyline in these essays, ranging from extraordinarily sensitive sensors that yield an accurate measurement of the Earth’s energy balance to huge physical, deployable barriers that hold back rising seas and protect historic cities. And then there are the scores of satellite technologies that allow us to see how the world is changing in real time.
But some marvels are so much a part of our everyday life that we may not really appreciate them. Here’s one – how do we extract heat from cold air, or use hot air to cool a room?
This little miracle describes what a common, if noisy, room air conditioner does, or what is accomplished by the aptly named heat pump.
So how does this little everyday miracle happen? Can we link that window unit to global issues like the ozone hole, greenhouse gas emissions and the price of oil? And are there ways to improve the efficiency of these technologies and reduce the environmental footprint of heating and cooling? Big questions from a little box.
Room air conditioners are indeed noisy machines, and the major source of that noise, far out-roaring the fan that delivers the cooled air into your room, is the compressor. It lies at the heart of the miracle by driving a process called phase change.
Phase change in this case is the transition of a material (called a refrigerant) from liquid to gas and back again; processes we know as evaporation and condensation. You can watch these happen as you hold a cold glass of water on a hot muggy day. Condensation causes water vapor in the humid air to convert to those liquid water droplets that form on the glass. This process releases energy, transferring heat from the air to your glass and water! At the same time, some of the water you were planning to drink will disappear through evaporation. That requires or absorbs energy, cooling the liquid in your glass.
Evaporation and condensation can be driven by pressure as well as temperature and humidity, and all three are used in your window A/C. Here is how it works.
You turn the unit on, the compressor swings into (noisy) action, and your electric meter starts to spin more quickly! The refrigerant begins as a low temperature gas under low pressure (upper left corner of this figure). That noisy compressor increases the pressure and the temperature of the gas, which then flows through pipes in the heat-exchanging grill on the outdoor side of your unit, where it condenses into a liquid, releasing heat to the grill. A fan draws warm outside air over the grill, producing hot air that is vented outside.
The liquid is still under pressure until it passes through the expansion valve. This reduces the pressure, causing further cooling. This cold liquid passes through pipes in another heat exchanger on the indoor side of your window unit. A fan draws warm, inside air past the pipes in this unit, causing the refrigerant to evaporate back to a gas, absorbing heat from the air passing over the pipes in the process. The resulting cooled air is blown into the room, and the refrigerant is ready to start another cycle.
In a heat pump, the parts are the same, but the cycle is basically reversed. The compressed, hot, high pressure gas is pumped to a heat exchanger inside the house to heat the room. The cooler, low pressure liquid is pumped to the outdoor heat exchanger where it is evaporated, absorbing heat from the outdoor air.
Most modern installations are reversible heat pumps which can switch from producing warm air from cold to producing cold air from warm!
And gone are those noisy window A/C units! A much quieter compressor/heat exchanger system is placed outside the building. This heats or cools a liquid that is pumped into the building and run through the indoor, wall-mounted heat exchangers called "splits" with fans that blow heated or cooled air out into the room.
But wait - if it is below zero (Fahrenheit) outside, how do you use that cold air to evaporate the refrigerant? How do you absorb heat from super-cold outside air? Good question! The refrigerant has to evaporate at very cold temperatures. Current systems can use refrigerants that will change from liquid to gas at temperatures as low as -50 degrees Fahrenheit!
However, the amount of energy required for either heating or cooling by these systems is directly related to the difference in temperature inside and outside the house. You may have noticed that the compressor, the noisiest part a window unit, has to run more frequently (and uses more electricity) on a very hot day. It is harder to pump heat into air that is already hot! Same for drawing heat from very cold air.
Efficiency of either process is captured in a number called the Coefficient of Performance (COP). This is the amount of heating or cooling achieved divided by the amount of electric energy used. If you use a stand-alone electric heater, the resistance to the electric current makes that bar inside glow red hot. Heat is transferred either to the air as it warms and moves past the bar or as it is radiated into the room. The COP of this is about 1 - all the energy used is converted to heat.
Heat pumps routinely achieve COP values of 3 to 5, meaning that they deliver the same amount of heat as that stand alone heater would, but using much less electricity.
For a short period in the 1960s, when it was thought that nuclear power would make electricity "too cheap to meter," some houses here in New Hampshire were built with baseboard resistance electric heating. That quickly became the most expensive way to heat a house, and most of those older homes have been converted at least partially to oil or propane.
But that may be changing. I was recently up in Northern New Hampshire, where I heard that a spike in the price of heating oil (up to more than $7 a gallon!) was causing a scramble for new, highly efficient heat pump/splits systems. Folks up there know how to calculate heating costs, so we might take that as a sign that heat pumps can be cost effective even in very cold areas.
And as the climate warms, these same heat pumps might be used more frequently on the cooling cycle, even up north.
All well and good - so how does this relate to the climate change themes of this Substack site?
Here are two ways:
First, the conventional refrigerants compressed and expanded in window air conditioners (as well as in refrigerators and other temperature controlled devices) were chlorofluorocarbons (CFCs). These were ideal as refrigerants as they were non-toxic, non-flammable, chemically inert, and changed from gas to liquid and back over a usable range of temperatures and pressures.
Unfortunately, this ideal material was not ideal for the environment. The discovery that the chlorine in CFCs could destroy the layer of ozone in the upper atmosphere that protects living things from harmful ultraviolet radiation is one of the great environmental science stories of the 20th century. CFCs, also a powerful if still relatively rare class of greenhouse gases, were essentially banned through the Montreal Protocol first enacted in 1987. Other, hopefully less dangerous substances are now used instead.
Second, knowing that the efficiency of heat pumps is determined by the difference in temperature between the house and the outside air, are there other places to dump the unwanted heat or cold besides in the ambient air outside? Any increase in efficiency would decrease energy consumption and its carbon footprint.
Geothermal heating is gaining increased acceptance here in the north. Instead of using the outside air as the destination for waste heat or cold, geothermal systems pair a heat pump with a second system of pipes that circulates a liquid down into the ground and then back to the heat pump. A heat exchanger transfers excess heat or cold from the heat pump refrigerant to the external water pipes that then transfer it to the soil.
In this region, the ground 6 feet or more down stays a relatively constant 47 Degrees. Using a liquid at 47 degrees as the "heat" source instead of air that might be below zero increases the efficiency of the heat pump significantly. Savings on the electric bill can offset the initial costs of the pipes and pump, while also reducing the carbon footprint of the process. In effect, the soil serves as long-term storage for solar energy!
The efficiency of the cooling cycle is also greatly improved with a heat sink at 47 degrees rather than outside air at 80, 90 or more.
There are other ways to multiply the efficiency of a heat pump. You could use solar heated water instead of groundwater as your heat source. Some use the water in a swimming pool or pond. Imagine using your A/C system to heat your pool!
Or you could get really creative and add other methods for storing or generating heat.
Just as an example, the University of New Hampshire has an active research program in the use of high tunnel greenhouses (relatively permanent plastic-covered structures) to extend the growing season, or even support year-round crop production. Heating one of these through the winter can cost thousands of dollars.
A local energy innovator (Jack Bingham) worked with colleagues to append a large water storage tank to one greenhouse and install two reversible heat pumps inside. Especially in spring and fall, when a greenhouse can swing from too hot during the day to too cold at night, the heat pumps run like air conditioners during the day, exporting heated water to the tank, and then use that water as a heat source for the reversed heat pump at night, warming the greenhouse and cooling the water in the tank.
Going one step further, what if there was another, external source of heat that could be used to keep that water tank warm all winter long, so that the heat pumps in the greenhouse could work at near maximum efficiency?
An earlier essay in this series described a method for heating water by drawing air through large, fully aerated compost piles. The energy released by this aerated composting system was enough to heat several high tunnel greenhouses. Multiplying this heat source through a high COP heat pump could also multiply the number of greenhouses that could be heated.
So, we have traveled from that window air conditioning unit through reversible heat pumps for both heating and cooling, and into innovative ways to access sources of heat and cold to increase the efficiency of these units.
Still, all these systems require electricity. They are not carbon neutral unless they draw on sources of electricity that are. However, if we think of the Coefficient of Performance and ways to maximize that number, maybe we can think of these systems as ways of reducing the greenhouse gas footprint of heating and cooling significantly - and maybe of food production as well!
As an added bonus, those heat pump units are much quieter than either a standard oil furnace in the basement, or a conventional air conditioner in the window!
Sources
A description of the operational cycle in a conventional air conditioner can be found here: https://en.wikipedia.org/wiki/Vapor-compression_refrigeration
Heat pumps are described here:
https://en.wikipedia.org/wiki/Heat_pump_and_refrigeration_cycle
https://en.wikipedia.org/wiki/Heat_pump
A very informative video on reversible heat pumps, including geothermal systems, is here:
https://www.youtube.com/watch?v=QykwWs3L1W8
Coefficient of Performance (COP) is described here: https://en.wikipedia.org/wiki/Coefficient_of_performance
Information on COP for geothermal systems is here:
https://www.energystar.gov/products/energy_star_most_efficient_2020/geothermal_heat_pumps
The geothermal diagram is from:
https://www.energy.gov/energysaver/geothermal-heat-pumps
There are Wikipedia pages for CFCs, ozone hole and Montreal Protocol.
A more complete description of the potential for compost heating of greenhouses is in the epilogue to this report: https://scholars.unh.edu/faculty_pubs/921/
Photographs in the text were taken by the author.