In heat pump systems, energy is rejected to a hot reservoir at the desire temperature while using as little energy as possible.As promised in the previous issue, the discussion on heat pumping continues. Since we want to have a more in-depth look at several of the more challenging aspects of heat pumps in commercial and industrial settings in future issues, we are going to take a quick step back and go through a few of the fundamentals.
Most of us were probably taught the second law of thermodynamics in several of its forms at some point in our lives. Maybe this lesson was done in school or maybe it was an outcome of some life experience. I remember vividly one particular instance of putting my thumb into a car lighter as a young boy (it had such cool-looking red circles; how could I resist?). As soon as my thumb touched that glowing red heating element, the energy from the hot element moved into my thumb and burnt my skin badly.
Figure 1: Heat will spontaneously move from hot to cold and not the other way around.As I jumped out of the car and put my thumb in a puddle, I was certainly not thinking as Rudolf Clausius did in 1854 when he was laying a foundation for the second law of thermodynamics — that “heat can never pass from a colder to a warmer body without some other change, connected therewith, occurring at the same time.” Yet, I certainly knew that the “hot” moved from the lighter into the “cold” of my thumb. The fact that energy transfers as heat from hot to cold is intuitive to most people – the warm drink doth not make the ice cube colder. Figure 1 best represents this energy transfer.
Transferring energy from cold to hot takes some other change, connected therewith, occurring simultaneously. The way to do this is to have an engine or machine that can force the heat to move in the other direction; probably the most common type of machine we have that works in this way is the vapour compression refrigeration cycle we talk about all the time. Figure 2 represents the energy movement with a refrigeration system added. Engines and machines require some form of energy to accomplish this effect.
Figure 2: A machine, like a vapour compression refrigeration system, can move energy from cold to hot.Balanced system
You may also remember that conservation of energy is necessary for any closed system. In our case here, the machine in Figure 2 cannot magically produce or destroy energy. This means that the sum of the energy coming in from the cold reservoir and the energy being used to run the machine must equal the energy leaving into the hot reservoir. In other words:
If we substitute a simple vapour compression system into our “machine,” we end up with Figure 3. This theory is now approaching something we have discussed many times here before: the amount of heat we need to reject in our condenser is equal to the amount of energy we put in our compressor plus the heat we absorbed in the evaporator.
A refrigeration system and a heat pump are conceptually the same thing; the differences lay in which portion of the heat transfer we really care about. In a refrigeration system, we want to optimize the energy absorbed from a cold reservoir at some desired temperature using as little energy as possible. In a heat pump system, we want to optimize the energy rejected to a hot reservoir at some desired temperature using as little energy as possible. These desires can lead to different challenges and applications of technology to enhance refrigeration and heat pump systems differently. And of course, nothing is stopping us from wanting both the heat rejection and the cooling effect at the same time.
Figure 3: A system that moves heat from cold to hot using vapour compression.Heat pump dryers
The application of heat pump technology is increasing and there is a drive to push the technology in an increasing number of applications and in colder climates. Three main applications that are currently standard fare in residential applications:
- Space heating
- Water heating
- Clothes drying
Products using heat pumps are widely available for each of these applications. In the last issue, we discussed some of the challenges with residential space heating applications and, particularly, how the outdoor climate affects their performance. Just as we did with the space heating, we need to pay attention to details in order to understand which type of system is good for a specific application.
Heat pump clothes dryers are expensive to buy and repair but they operate more efficiently than their resistance heating element. We will be going through the rationale behind a lot of the energy-saving associated with using heat pumps in an upcoming issue but in simple terms, a heat pump dryer uses about 50 per cent less energy per load of laundry compared to a standard resistance electrical model.
Venting for heat pumps
This analysis is complicated because you must add the effects the dryer has on the house. For example, a normal clothes drier requires that air from the house is exhausted outside which means that you have to bring in fresh air to your house to replace this exhaust. A heat pump dryer doesn’t have a vent outside so there is not the same need to bring in and heat/cool outside air.
However, as we have learned about refrigeration systems, our condenser must reject both the energy put into the compressor and the energy absorbed in the evaporator. This means that a refrigeration system running in a closed room (i.e., put a fridge in a sealed room and leave the fridge door open) will always heat up the room.
One of the ways that evaporators absorb energy is by condensing water in the air, which is obviously the main purpose of the evaporator in a heat pump dryer. If you put electricity into a heat pump clothes dryer and you remove water onto the evaporator while cooling the air, there must be a net heat energy input into your home. In the winter this would help with heating costs and in the summer, it would add to the air conditioning load. Whether this technology makes sense depends on how much laundry you do and how much your electricity costs. For high laundry volumes in areas with expensive electricity, this technology becomes quite appealing. And if you have no way to install a vent it’s a no-brainer.
You may be wondering why we spent so long talking about clothes dryers in a refrigeration article. Well, in short, this is the exact same thing we can do with commercial and industrial heat pumps in large commercial laundries, indoor agricultural facilities, ice rinks and many other industrial processes. As we continue to move forward with decarbonization, there will be more and more applications that use these types of systems. In the next issue, we will talk about water heating with heat pumps.