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Heat pumps are all the rage these days, especially in Europe, where some countries are working to ban the installation of fossil fuel stoves and boilers in favor of more environmentally friendly options, including energy-efficient heat pumps. (Furnaces heat air and distribute it through pipes throughout the house, while boilers heat water to provide hot water or steam heating.) This year, the U.S. government began offering tax incentives for installing heat pumps, which tend to cost more upfront than traditional furnaces but are much more efficient in the long run.
In the field of new energy vehicles, because the battery capacity is limited, it has also prompted the industry to turn to heat pumps. So maybe it's time to quickly learn what heat pumps mean and what they do.
What is the most common type of heat pump?
Given the recent buzz, you might be surprised to learn that you already use a heat pump - you probably have more than one in your home and more than one in your car. You just don't call them heat pumps: you use the terms "refrigerator" or "air conditioner."
In fact, these machines are heat pumps, which means they move heat from a relatively cold place to a relatively hot place. Heat flows spontaneously from hot to cold. But if you want to turn it from cold to hot, you need to "pump" it. The best analogy here is water, which flows down a hill on its own, but needs to be pumped up the hill.
When you pump the heat contained in some kind of cold storage (air, water, etc.) to the hot storage, the cold storage gets colder and the hot storage gets hotter. That's actually what your refrigerator or air conditioner is all about - it moves heat from where it's not needed to somewhere else, and you don't care if you waste a little extra heat.
How to make a practical chiller with a heat pump?
The key insight that produced heat pumps came in the early 19th century, when a number of inventors, including Jacob Perkins, realized they could cool something this way without wasting the volatile liquids that evaporated to achieve the cooling. Instead of releasing these vapors into the atmosphere, they argued, it would be better to collect them, condense them into a liquid, and reuse that liquid as a coolant.
That's what refrigerators and air conditioners are for. They evaporate liquid refrigerants and use the cold vapor to absorb heat from the inside of a refrigerator or car. Then they compress the gas, which condenses back into liquid form. This liquid is now hotter than when it started, so some of the heat it holds can easily (possibly with the help of a fan) flow into the surrounding environment - whether outdoors or elsewhere in the kitchen.
That said: you are very familiar with heat pumps; It's just that you keep referring to them as air conditioners and refrigerators.
Now let's do another thought experiment. If you have window air conditioning, you can even do it as a real experiment. Install backwards. That is, install its controls outside the window. Do this in cool, dry weather. What's going to happen?
As you'd expect, it blows cold air into your backyard and releases heat into your home. So it's still transporting heat, making your home more comfortable by heating it. Sure, it cools the air outside, but that effect becomes minimal once you're away from Windows.
You now have a heat pump to heat your home. It may not be the best heat pump, but it will work. What's more, when summer comes, you can also turn it upside down and use it as an air conditioner.
Of course, don't actually do that. If you try it, it will undoubtedly fail the first time it rains and water enters the controller. Instead, you can buy yourself a commercial "air source" heat pump that uses the same principle to heat your home.
The problem, of course, is that vodka is expensive, and you'll quickly run out of it to cool the wine. Even if you replace vodka with a cheaper rubbing alcohol, you'll soon be complaining about the expense.
Some of these devices have what are called reversing valves, which allow the same device to perform a dual role: they can pump heat from the outside in or from the inside out, providing both heat and air conditioning, as described below.
Why are heat pumps more efficient than electric heaters?
Heat pumps are more efficient than electric heaters because they do not require electricity to generate heat. The electricity used by a heat pump does generate some heat, but more importantly it pumps heat from outside into your home. The ratio of the heat released into the home to the energy sent to the electric compressor is called the coefficient of performance, or COP.
A simple electric space heater that provides all the heat generated by the electric heating element has a COP of 1. On the other hand, the COP of a heat pump can be an order of magnitude higher.
However, the COP of a heat pump is not a fixed value. It is inversely proportional to the temperature difference between the two reservoirs in which heat is pumped. That said, if you pump heat from a not-too-cold reservoir to a not-too-hot building, the COP will be a large value, which means your heat pump is very efficient at using electricity. But if you try to pump heat from an extremely cold reservoir into an already warm building, the COP value is reduced, which means efficiency suffers.
The result is what you intuitively expect: it's best to use the warmest thing you can find as an outdoor heat reservoir.
Air source heat pumps, which use outdoor air as a heat reservoir, are the worst option in this regard because the outdoor air is very cold during the winter heating season. Even better are ground source heat pumps (also known as geothermal heat pumps), because even in winter, the ground at medium depths is still quite warm.
What is the best heat source for heat pumps?
The problem with ground source heat pumps is that you need a way to access this buried reservoir of heat. If you have enough space around your home, you can dig ditches and bury a bunch of pipes at a reasonable depth, such as a few meters deep. You can then circulate a liquid (usually a mixture of water and antifreeze) through these pipes to absorb heat from the ground. Alternatively, you can drill deep holes in the ground and install pipes vertically into these holes. All of this will get expensive, though.
Another strategy available to a lucky few is to extract heat from a nearby body of water by dipping a pipe into the water at a certain depth. These are called water source heat pumps. Some heat pumps employ the more unusual strategy of extracting heat from the air leaving the building or from solar hot water.
In very cold climates, it makes sense to install a ground source heat pump if possible. This is probably why most heat pumps in Sweden (which has one of the highest number of heat pumps per capita) are of this type. But even Sweden has a large percentage of air-source heat pumps, which belies the common claim (at least in the United States) that heat pumps are only suitable for heating homes in mild climates.
So wherever you are, if you can afford the higher upfront costs, the next time you're faced with a decision about how to heat your home, consider using a heat pump instead of a traditional stove or boiler.
Post time: Oct-19-2023