Heat pump technology is one option for district heating applications as the world looks to decarbonise.It has been almost exactly one decade since I had my first article published in Plumbing & HVAC magazine. It was the first in a three-part series on “Refrigeration from Heat.” To celebrate this anniversary, instead of revisiting the topic, I thought it would be fitting to flip it around and start 2022 off with a series on “Heat from Refrigeration.”
We are witnessing a slow revolution and it can be summarized by one word—decarbonisation. Depending on the context, decarbonisation can mean several different things (including removing fizz from your pop according to my daughter). For our purposes, it is simply the act of reducing the amount of carbon dioxide humans release into the atmosphere for the purpose of reducing the impact to climate change.
A decarbonising world
China has released a plan that would see massive decreases in carbon emissions over the next five years, reaching peak emissions in 2030 (with an economy that is growing, this is a reduction of carbon/GDP), and carbon neutrality in 2060. Much of Europe has been moving along this path for a long time. In North America, we are learning to live with carbon taxes and increasing commitments to reach reduction targets.
If you are wondering what this has to do with refrigeration, you are not alone. Most people aren’t aware of how closely connected refrigeration is to carbon emissions. There are two perspectives to consider. First, in industries that require refrigeration such as food production, refrigeration is one of the highest energy users. In jurisdictions that use fossil fuels to generate power, the inefficiency of this refrigeration can have significant impacts. I think most would agree that this aspect is pretty obvious.
The second perspective is more “global” and more interesting. Almost all buildings, facilities, plants, and institutions require heating. Right now, in North America, and much of the world, a large portion of this heat is obtained by directly burning fossil fuels. Using heat pumps, refrigeration can actually be used to replace quite a lot of these fossil fuels.
Heat pumps
A heat pump is simply a refrigeration system that has the heat rejection component (i.e., the condenser or in the case of CO2, the gas cooler) rejecting heat to somewhere we want it. The “stuff ” we are absorbing the heat from often gets used in the nomenclature to describe the heat pump. For example, an air-source heat pump is a heat pump that takes energy from outdoor air and rejects it indoors. A ground-source heat pump does the same thing except takes the heat from the ground. Finally, a water-source heat pump extracts heat from water.
If you live in a reasonably mild climate, you are likely familiar with residential air source heat pumps. In Canada, they are common in BC’s lower mainland and are gaining popularity in the form of ductless splits in Atlantic Canada. In the prairies, they are almost unheard of. Most areas have at least a spattering of ground-source heat pumps, but they are more common in milder climates where the ground temperature is higher. Ground-source heat pumps can be expensive and if poorly designed, they have some operational challenges, but the technology has been successfully applied for decades.
For residential space heating applications, water-source heat pumps appear most in apartment-style condo buildings or apartments. They are used in conjunction with a low-temperature boiler that provides warm water to individual living units and a small heat pump takes the heat from the water and uses it to make the air warmer.
The change in heat pump capacity at different temperatures for a modern heat pump.Heat pumps have been used in commercial and residential space heating for a very long time. So, what’s changed and why might we see more of them in these types of applications?
Air-source heat pumps for space heating have a really big problem to contend with the colder the air is outside the less heat it can produce, which is exactly the opposite of what is needed because the colder it is outside, the more heat we need. This problem is precisely why they are only common in climates where it doesn’t get that cold.
You may recall from a previous article that I have an air-source heat pump installed for my house. It is a generic base model heat pump and works perfectly until it’s about -4C outside. At -10C outside, it’s useless. This is pretty typical of many heat pumps because as the air gets colder the capacity drops dramatically due to the lower density of the suction gas entering the compressor and the system has to contend with defrosting the outdoor coil more (it does this by reverting to refrigeration mode—cooling the house which it then needs to heat). The Table shows the ratings for a small relatively high-end modern heat pump. You can see that the power consumption goes down by about 13 per cent and the capacity decreases about 35 per cent going from 8C to -8C outdoor air temperature; this particular unit is rated to run only at -12C or above.
To compound all of this, even if it would run at really cold temperatures, sizing a heat pump for heat in a cold climate like mine (approximately -35C design temperature) would result in an absolutely massive unit and a huge air conditioning capacity in the summer; recall that oversized air conditioners lead to humidity control problems.
Manufacturers have recently been driving the operation of air-source heat pumps to lower extremes using, among other things, variable frequency compressor technology. One manufacturer now claims that they have been able to maintain heating capacity down to -20C (I bought one, so I’ll probably write about how it worked next winter). It looks highly likely that this trend will continue, and I think that the air-source heat pump technology will keep driving into colder climate regions.
The author’s base model heat pump can operate until it gets to be about -4C, when the unit isn’t able to produce heat.Shifting gears
Those of you following the debate about electric cars are likely asking yourself a question that always comes up in Canada. If you get all of your power from coal or natural gas, what’s the point. Of course, it’s true that carbon emissions created in your power supply affect your ability to reduce your fossil fuel consumption and decarbonise, but it’s not always so cut and dry. Furthermore, many provinces don’t get a lot of their power from fossil fuels. We are going to dive into this a bit in an upcoming issue.
Perhaps more interesting than an increase in residential heat pumping is the opportunity in the process and industrial industries for unique and innovative heat pumping solutions. Over the next few issues, we are going to continue the discussion and touch on commercial heat pumping applications, challenges with heat pumping, opportunities for industrial and process heat pumping and do a bit more of a dive into what this all could mean for our industry in Canada.