Demystifying Heat Pumps
By Kristof Irwin, P.E., M. Eng.
Heat pumps are so common in our lives that we ordinarily fail to notice them. That said, if you live indoors chances are that you interact with a heat pump every day. This is because that refrigerator in your kitchen is a heat pump. Do you trust it to do its job reliably? Generally speaking, refrigerators are not all that exciting. They just do their jobs day after day. Like all machines, they can and do fail over time but overall your refrigerator is solid. In my own home I’ve replaced the dishwasher twice but still have the same fridge keeping my food fresh and ice frozen after 20 years. This reliability is due to the fact that at the core of your refrigerator are simple, reliable, inexorable laws of physics.
The fact that heat pumps remain a mystery is a mystery. Perhaps it’s a cloaked sales strategy to keep us just slightly confused, so that we put more trust in sales rhetoric than we otherwise would. Or perhaps it’s because we’re somewhat intimidated to try to understand them. Regardless, over time it’s become clear to me that many decision makers are not sure what a heat pump is, how they work, and whether they trust them or not.
This is a shame and, increasingly due to the need to reduce carbon emissions, a rather pressing area of societal confusion to clear up. The goal of this series is to explain what heat pumps are, why they matter and how they do what they do. After that we are then ready to go over a few of the more important types of heat pumps used in our homes to keep our families warm in the winter and cool in the summer. The underlying goal of this series is to support clear thinking in order to help us make good decisions both as homeowners and industry professionals.
Why Heat Pumps Matter
Heat pumps matter because they are able to harvest otherwise unusable or waste heat in our environment and refine it into high quality thermal energy. By otherwise unusable or waste heat in our environment, I'm referring to the thermal energy contained in the air, ground water or the earth itself. These energy sources are capable of being used to heat or cool our homes and buildings, create hot water, and keep our food cold and fresh. We unlock these energy sources using electrical energy to run compressors, pumps and fans.
You don't get something for nothing with a heat pump, but you can get more energy out than you put in. And not just barely more out than in, it is common for us to get two to fives times more useful work out of our heat pumps than we put in. This is one core reason why heat pumps matter.
Contrast this to the false economy of fuels that we burn and use up forever. When we run our natural gas furnace or water heater the best we can do is get all of the energy out that we put in; in that case we'd be 100% energy efficient. Operating near 100% efficiency sounds great, until you remember that heat pumps routinely operate at site energy efficiencies in the 200% to 500+% range. High site COPs are important because of the energy losses involved in source energy generation and distribution. When these Source-Site energy losses are accounted for, only heat pumps are able to remain above a COP of one (1). COPs below one tell us that we are either needlessly using up fuels forever or taking a net loss on the transaction, or both. Table 1 below lists common Site COPs and also Source COPs that have been adjusted by US average site-source multipliers..
An unfortunate reality is that when we burn a fuel we are not only using the sky like a free public sewer, we are also destroying a high-grade heat source. Thinking a layer deeper, we are destroying this high-grade heat source in order to meet a low-grade heating demand. For example, in the case of natural gas water heaters or furnaces, we use a flame (a powerful exothermic oxidation reaction) that exists at a temperature of around 3500F to simply heat water to 120F or air to 70F. This huge temperature mismatch situation gets only slightly better when we cook with gas to heat our food to around 400F. Yet the phrase “cooking with gas” is considered an accolade. How does that make sense? How are we doing as consumers of technologies for our homes? Are we paying attention and making sound decisions?
Future generations may go looking for high-quality, lightweight, portable, energy-dense, high temperature, process-grade fuel sources only to learn that we burned them all up to satisfy stationary low-grade, low temperature applications. This is the textbook definition of an exergy inefficient process. That is not a misspelling, we are all familiar with energy and the related term exergy is one that needs to become just as familiar.
Exergy is important because it represents the capacity to do useful work. Work in this context means the capacity to make things move. One could make the argument that the nature of fuels to be compact and lightweight relative to their energy density makes them undeniably useful for transportation applications. Or further that switching from coal powered electrical generation to natural gas is a step forward and good use of the resource. Still in both cases burning fuels, long term, there are clearly better options.
Exergy is what we waste by destroying it forever, when we burn a flame at 3500F to meet the need of heating needs of our homes. Exergy may tend to sound nerdy, complicated and intimidating but it's a simple concept. If you would not use an excavator to plant your tomatoes, or a fire hose to rinse your dishes, or a flame thrower to light your campfire, then you can relate to exergy efficiency.
Beyond either energy or exergy efficiency is the profoundly significant fact that we need to stop, or substantially and rapidly reduce, the burning carbon emitting stuff to run our global economy. It's impossible to overstate the urgency of this fact. It's like we're all part of a massive carbon burning stampede and no one understands why anymore, only that it's been normal for a long time; ever since we invented fire. We burn fuels so chronically and habitually that it goes invisible – at least psychologically; the impacts of carbon in the atmosphere are plain to see with just a little attention to the world around us.
Fortunately we, as a global society, are also engaged in a starting new stampede. This one is an inexorable by-industry, for-profit, market transition toward basing our global economies with clean renewable energy and distributed energy storage. There are several promising electrons-to-molecules research efforts aimed at carbon free, even carbon-negative, fuel production using excess renewable energy. The common feature to all these scenarios is that our economies rely solely on electrical energy and the tie in here being that heat pumps run on electricity. Heat pumps can cool us off in the summer, warm us up in the winter, heat or cool water for us to drink, swim or bathe in; they can also dry our clothes and keep our food fresh and frozen. Now is the time to pay attention to heat pumps in our everyday lives.
This post is meant to be a conceptual primer. Re-read if necessary and we’ll talk more about the graphic below, as well as a lot more detail on heat pumps in part 2. Thanks for reading!
