The heat pump has become a common fixture in many parts of modern life. We now have reverse-cycle air conditioning, heat pump hot water systems, and even heat pump dryers. These home appliances have all been marketed as upgrades over simpler technologies from the past, and offer improved efficiency and performance for a somewhat-higher purchase price.
Heat pumps aren’t just for the home, though. They’re becoming an increasingly important part of major public works projects, as utility providers try to do ever more with ever less energy in an attempt to save the planet. These days, heat pumps are getting bigger, and will be doing ever grander things in years to come.
Magical Efficiency
The heat pump is a particularly attractive tool because it has a near-mystical property that virtually no other machine does. It is capable of delivering more heat energy than the amount of electricity fed into it, appearing to effectively have an efficiency greater than unity. We’re told that thermodynamic laws mean that we can never get more energy out than we put in. If you put 1 kW of electrical energy into a resistive heating element, which is near 100% efficient, you should get almost 1 kW of heat out of it, but never a hair more than that. But with a heat pump, you could get 1.5 kW, or even 2 kW for your humble 1 kW input. The trick is that the heat pump is not actually a magical device that can multiply energy out of nothing. Instead, the heat pump’s trick is that it’s not turning your 1 kW input into heat energy. It’s using 1 kW of energy to move heat from one place to another. If you’re running a heat pump-based HVAC system to cool your home, for example, it might use 2 kW of electricity to pump 3 to 4 kW of heat from your lounge room and dissipate it outdoors. Since the outdoors doesn’t change much in temperature when you pump out the heat from your home, you can keep doing this pretty much all day. You can even reverse the flow if your heat pump system allows it, instead pumping heat from the outdoors into your home. This works well until temperatures get so low that there isn’t enough heat left in the outdoors to appreciably warm your house up.
The heat pump achieves the feat of making heat go where we want it to go via the use of refrigerant. Specifically, refrigerant enters the compressor as a low pressure and low temperature vapor. It exits as a gas at high temperature and high pressure, and is then passed through a series of condenser coils. As it passes through, it releases heat to the surrounding environment and reduces in temperature, condensing into a liquid. From there, the liquid, still under high pressure, passes through an expansion valve, which rapidly lowers the pressure and drops the temperature further. The liquid is now cold, and passes through an evaporator coil where it picks up heat from the surroundings and turns back into a low-pressure, low-temperature vapor to start the cycle again as it heads back to the compressor. This system runs your fridge, your car’s air conditioner, and is used in so many other applications where it’s desirable to make something colder or hotter as efficiently as possible. You just choose which direction you want to pump the heat and design the system accordingly. Air conditioners and fridges pump heat out of a confined space, heaters and dryers pump it in, and so on. It’s heat pumps all the way down!
Bigger Applications
Thus far, you’ve probably used many a heat pump in your daily life, whether it be for heating, cooling, or drying clothes. However, there is a new push to build ever-larger heat pumps to work on the municipal scale, rather than simply serving individual households. The hope is to make utilities more energy efficient, and thus cheaper and greener in turn, by taking advantage of the efficiency gains offered by the magic of the heat pump.
One such project is taking place just off the River Rhine in Germany. A pair of massive heat pump units are being constructed by MVV Energie, each with a capacity of 82.5 megawatts. They will deliver heat to a total of 40,000 homes via a district heating system, and will be constructed on the site of a former coal power plant. Each pump will effectively draw energy out of the massive watery heat battery that is the River Rhine, and use it to warm homes in the local area. Thankfully, the river’s capacity is large enough that drawing all that heat out of the river should only affect temperatures of the water by around 0.1 C.
The Rhine project builds upon a previous effort to install a large heat-pump heating system in Mannheim, in partnership with Siemens Energy. That installation draws 7 megawatts of electricity to supply 20 megawatts of heating to the local district heating grid. Installed in 2023, it supplies the heating needs of 3,500 local households.
A similar project is underway in Denmark, which will supply 177 megawatts of heat to homes in Aalborg. The installation of four 44 megawatt MAN Technology heat pumps will be hooked up to the existing district heating system, which is also supported by other sources including waste heat from a local cement factory. The benefit of using smaller individual units is that it allows some of the pumps to be shut down when heating demand is lower, as winter passes through autumn into summer.
What makes these projects special is their sheer scale. Rather than being measured in the kilowatt scale like home appliances, they’re measured in the many tens of megawatts, delivering heating to entire neighborhoods instead of single homes. As it turns out, heat pumps work just fine at large scales—you just need to build them out of bigger components. Bigger compressors, bigger expansion valves, and bigger condensors and evaporators—all of these combine to let you pump enormous amounts of heat from one place to another. As utilities around the world seek ever greater efficiency in new projects, heat pumps will likely grow larger and be deployed ever more widely, seeking to take advantage of the free heat on offer in the earth, water, and air around us. After all, there’s no point dumping energy into making heat when you can just move some that’s already there!
Puzzled by “near 100% efficient” for resistive heating. Is it really only “near”? I thought it was exactly 100%. The only way I can imagine for the energy to do non-heating work is inductive coupling. Like a forced-air electric furnace is gonna have a heating element inside a metal box / duct, and the 60Hz AC running through the element will have some interaction with the enclosure. But won’t that interaction just induce eddy currents that generate heat in the enclosure? Curious what other effects there could be…
And as for heat pumps becoming ineffective at low temperatures…it’s not really a question of having enough heat outside. The coldest winter’s air is still about 240K…just an enormous amount of heat still present. The trouble isn’t that the environment isn’t warm enough, but that it condenses all of the moisture out of the air and forms a layer of ice on top of the coils, isolating it from the environment. It does make me curious how cold the inside layer of that ice will become once it’s isolated…
The thing i always keep in mind is that as these things scale, the natural reservoir that you’re heating or cooling really does start to look shockingly finite..
“near 100%” because you lose a few percent of the electrical power between the location you generate it (or meter it) and the location you convert it to heat.
But it’s only ‘lost’ as heat -> as we’re looking for heat then it’s not lost at all, just perhaps not providing heat where it’s wanted.
It is not 100%. Some is converted into magnetic flux.
About 2-6% in the usual case, up to 10-15% if you have to transmit longer distance.
For actual non-heat losses, there is the small amount of ULF radio energy that gets emitted by power lines.
Even that won’t stop the heat pump from working. What does is the fact that the working fluid, such as R-22, boils at −40.7 °C so the heat pump stops working entirely when the ambient temperature reaches about 40 below zero, C or F. Other refrigerants have different boiling points, some higher, some lower. Propane goes down to -42 C.
The difference in temperature between the outside air and the boiling refrigerant directly determines the rate of heat flow into the system. It’s like Ohm’s law: temperature difference is the voltage that drives current through whatever resistance you have, which could be frosted coils or not. When the difference is small, very little heat gets in anyways and there’s little or nothing to pump, so the coefficient of performance approaches 1. At some point the heat leaking out through the tubing exceeds the amount of heat you can pump from the boiling refrigerant, so the efficiency of the system drops below 1. This typically happens when the outdoor temperature drops below -20…-25 C.
It becomes slightly above the boiling point of the refrigerant. Commonly around -40…-50 C.
Everything is an antenna, even a heating element, so at 50Hz/60Hz a minuscule amount of energy radiates away in a different part of the electromatic spectrum than the desired Infrared.
using 1 kW of energy
Lewin, a kilowatt is not a unit of energy. It’s a unit of power.
Language is important, especially if you want to be a writer.
So you get a water temperature drop of 0.1K per 40k people. The river Rhine is home to at least 50 million people. That gives us a drop of 125 Kelvin if every home is heated by heat pumps. I really would like to see how that plays out. :evilgrin:
If the minimum water temperature in the Rhine averages 4.7°C in January, it would mean that the entire river would freeze over at the heating load of about 2 million people.
But fortunately, the latent heat capacity of freezing is 80 times greater, so what you could do is deliberately freeze the water into blocks and then release those blocks to float down to the sea.
It’s also a viable idea at home. If your landlord doesn’t let you install an air source heatpump, you can get a freezer and load it up with buckets of water from the tap. Then just keep throwing the ice blocks out the window…
Nice. I’d stand the refrigerator in the doorway, with the fridge open to the outside in winter, open to the inside in summer.
I realize you’re joking, but I’d just do a “swamp cooler”. You fill a bath tub with hot or cold water (depending on your temperature goal) and blow a fan over it. When the water gets near room temperature, drain it and repeat.
There’s plenty of heat left. It’s nowhere near absolute zero. It’s just your heat pump that gets less and less effective with increasing temperature difference between outside and inside. Eventually it’s going to get so bad that the heat pump starts leaking heat backwards and heating the outdoors with the heat from your house.
More critical to the lower limit of an air-sourced heatpump is the efficiency of thermal transfer between the coils and the outside air at lower temperatures. In normal operation, the fan sucks air through the radiator vanes that the coils are embedded in, which provides plenty of opportunity to transfer energy between large volumes of air and the working fluid. However, when the temperature drops, the moisture in the air starts to freeze on the vanes, which blocks the airflow between them. Yes, there’s still thermal transfer between the working fluid and the air, but it’s now severely reduced because the air the fan is moving is no longer traveling quickly through the vanes and past the coil directly, but mostly just lingering in the vicinity of the block of ice that happens to contain the coil.
This is why ground- or water-sourced heatpumps can be far more efficient in otherwise extreme environments, without needing such fancy refrigerants: they can rely on a very specific and very narrow temperature range that they need to interface with, and are immune to the vagaries of things like moisture freezing on the coils and curtailing the thermal transfer.
How are they delivering the heat? Is this via steam or hot water loops, or does it send out refrigerant gas to be compressed and condensed at people’s homes and liquid refrigerant in the summer?
The linked article provides the answer.
Ah, I see it’s circulating hot water. Some European heat pump projects have been using heat pumps to make steam, which isn’t a very efficient way to use them.
So you aren’t allowed to say sh1t, but you are allowed to say Siemens? What the fudge?
Cities should be mandating that data-centres (AI boo hiss) capture the heat from their equipment and feed it into municipal heating systems.
When it’s to hot to use the heat for heating, pump the heat into the ground where it can be recovered in the winter with ground source heating.