Talk title: The View from Denmark: Large-Scale Heat Pumps & District Heating
Speaker: Thomas Dam Leth, Sweco Denmark
Event: CIBSE Scotland Conference 2026: Heating the Future, 27 May 2026
Video: https://youtu.be/7MbpKdTXhPo

[00:00] A brief introduction. This is me. My name is Thomas. I'm a senior specialist, as we heard before. I've got a few years of experience working with industrial design of heat pumps as well as refrigeration plants, and I've been working quite some time as well with utilization of waste heat, or in a nicer term, excess heat. It sounds a bit better. So why are we looking at large...

[00:25] Large-scale heat pumps now? Well, in Denmark, it's because we have a very large focus on electrifying the district heating grid. And I presume it's the same here in Scotland, for instance, right? Or anywhere else in the UK, or anywhere else in Europe, basically. And in Denmark now, we have currently installed approximately 700 megawatts of heat pumps. And within 10 years, it is predicted that we are going to be installing up to 4000 megawatts of electrical heat pumps.

[00:59] It took a while for me to let that sink in because that's a lot. It's multiplied by approximately six. And how do you drive this? We do it by installing large heat pumps. Currently in Denmark, in the northern part, there's been installed a 170 megawatt heat pump utilizing seawater as energy source. And...

[01:24] Have a lot of the utility companies in Denmark committing to installing a large amount of heat pumps as well within the next 10 years. This also means that heat pumps are shifting now from being like what you can call a complementary load to the power plants to actually becoming the base load of the district heating system. This also means that we have a greater interconnection with the heating sector and that also enables us to use it or use the heat pumps for grid stabilization as well. So stabilizing the electrical grid basically. Then we also look into utilizing multiple different energy sources. There are some that we have abundance of and of course we also have excess heat which is like basically a dependent energy source. And then I have an old colleague of mine who had this nice phrase that says we look to use for chimneys for waste heat now we look for cooling surfaces. And that's basically also the fact now because now we are trying to utilize excess heat from for instance refrigeration plants all over the country basically.

[02:30] If we look at this into the big picture then we have some main design drivers for when we are designing our heat pumps. Of course the first and most important one is the energy source. Where do we get the energy from? There's a lot of different opportunities. The next is of course the operating profile. How are we expecting our heat pump to operate? Is it going to be operating at base load? Do we need to have some part load flexibility in it as well?

[03:00] What is the purpose of this heat pump in the system? And it's no joke or anything but the more operating hours we can get for a heat pump the more efficient it becomes and the better the business case becomes basically. So I'm going to give you a key takeaway on this as well but like the major one is maximize your operating hours for your heat pump. That is, give you the best return of investment basically. Then of course we also have the existing district heating systems that we need to implement our heat pumps in and that we need to optimize as well to suit the heat pump most beneficially. So we need to have the best conditions for heat pumps and one of the big design drivers in this is basically the return temperature because a lot of the heat pumps are sensitive to return temperatures especially in terms of performance. So we need to optimize the performance and there's ways to get around this. There's both going out to the consumers and regulating their district heating systems but there's also ways you can do it with the heat

[04:04] Pumps as well. For some instances we see someone doing what we call mechanical stop cooling of the return temperature as well which is a nice way to improve the efficiency of some heat pumps. So if I move on to the energy sources as I talk about there's a lot I have chosen to categorize them into which is the independent energy sources which could be seawater, air, geothermal. So basically what you can say is the energy sources that we have abundance of.

[04:35] I mean like we have plenty of air in the room here, we have plenty of air everywhere around us so that's a potential energy source that we can have at all times and similarly with seawater. But loosely the independent energy sources have a very low temperature or source temperature which is of course affecting the performance of the heat pumps. So we need to have a very low temperature or source temperature which is of course affecting the performance of the heat pumps. Then we have the dependent energy sources. This is usually what we classify as being excess heat.

[05:01] That could be from data centers, the industry or wastewater as well. And the difference here is that this is usually relying on some sort of process that varies during the day, during the week, during the month, during the year. So this means that we do not always have this energy source available. However it's usually the case that the energy source from the dependent energy sources is significantly higher or the temperature is, which means that we usually get a significantly better COP. But what we need to focus on is of course the availability of it, the temperature levels and the annual operating hours that we can provide for a heat pump in order to increase our business case.

[05:47] Oh, there we go. It is a heavy Excel curve. But when we look into excess heat, when does it make sense basically? Well, what we need to look into is of course the operating hours of our excess heat. And we need to differentiate that from the peak capacities that we can obtain in the system. Then we need to look into how do we want to utilize this? Can we use it internally at the site where we get the excess heat? Or can we use it externally? So basically sell it to the district heating grid. And there's no joke, that is, it's even better to use it internally. No coincidence that the return of investment is of course driven, as I also talked before, by the utilization rate of this excess heat. And then usually, actually, when we look at like a distribution curve, or some call it something else, but here we can actually say that this could often be the case that this is defining the size of the heat pump as well.

[06:44] Because if we take this example, this is from a large Danish shopping center, some might know where it is? Most likely not. But here you can see the rate of return, the return of investment, is of course driven, as I also told before, by see the cooling load profile as well as the heating load profile and the ambient temperature I agree it would have been quite a lot easier if it was red and blue and green perhaps but now it's just three different shades of gray and that's the swiggle temperature colors or you can say but in the middle you see the dark green area is basically the cooling load we have here and it looks like there's quite a lot basically but we have to consider that at a shopping mall people are not there during the night so we don't need any cooling during the night that they're there during the day and often it varies as well and the temperature outside is of course of utmost importance to how much cooling we need for our system so if we put that into distribution curve we can see here that's this is then the waste heat we have from the cooling plant we can see we have a very high peak capacity at approximately four megawatts

[07:47] However it's only approximately two and a half percent at the time then it doesn't make sense to make a heat pump that's four megawatts because we only utilize it for two and a half percent of the year so what we look into is then defining a span to where we can see like this area or this many hours we can get from our heat pump and then from that we can decide the capacity of the necessary heat pump for this exact example here it actually ended up being only a small heat pump for internal use and it was only approximately 150 kilowatts but then the return of investment was really good yeah if we look into refrigerants there are different types of refrigerants I myself primarily work with natural refrigerants of course also atfos but in my opinion a lot of people also perhaps here is more fond of course the natural refrigerants so we have like three

[08:47] Large off with three major you could say we have ammonia which is very well known co2 as well and then we have hydrocarbons as well and hydrocarbons could be propane isobutane and so forth and you can say there's different types of solutions when using the different types of refrigerants and especially for the large scale heat pumps we usually look into either one stage or two stage solution so basically that's where you have if you have a two-stage solution is where you have two compressors coupled in series on the refrigeration side and we usually use that for ammonia where we have high temperature lifts and for co2 and hydrocarbons for the very large heat pump they're actually using only a single stage solution and that means they are able to produce quite a high temperature lift on those compressors that they're using and common for all of them is that they're actually now able to supply 90% of the heat pump and they're actually able to supply 90% of the heat pump and they're

[09:47] Actually able to supply 90% of the heat pump and they're actually able to supply 90% of the heat pump five degrees of supply temperature and this is a game changer for a lot of the old district heating grids because there we have perhaps small pipes which needs a quite high temperature in order to get all of the efficiency out to the system so this is a game changer as well some of the heat pumps are also able to produce plus hundred degrees it of course provides you with some challenges for your system there are some new pressures that need to be taken into consideration but it's a it's a very large development and then of course always some key advantages and challenges I'm not going to go too much into it but I think one of the major things that we look into when you design refrigeration plants or heat pumps is basically like the safety around it because for instance you have ammonia it's toxic you have hydrocarbons which is flammable I've learned that the right term is but let's be frank it's explosive so there are some things you need to take into consideration during

[10:47] Atex and there you have the british en standard en3 80 or 78 which is a very nice standard to look into and one that you need to look into when you're designing heat pumps and refrigeration plants so what about technologies for these large scale heat pumps we usually operate when we're in this capacity range we usually operate when we're in this capacity range we usually operate when we're in this capacity range with three core technologies we have the very well-renowned piston compressors that's been there for many centuries basically but it's been there for at least 125 50 years for refrigeration alone the thing about that is that of course since it's been here so long it's very efficient as well there's been a lot of development on it however the capacity is not necessarily that high currently on a single compressor for a piston we can generate approximately four to 25 megawatts of heat capacity depending on the source temperature and then we also unfortunately have quite say low service intervals on the compressors as well which of course has an effect

[11:53] On your opex of your system if we move a step up in capacity we have the screw compressors screw compressors is well renounced in the oil and gas industry prior to being used for refrigeration and now we use it here as well and for screw compressors we are now we are now also able to achieve quite high capacities, between 3 to 20 megawatt of heat capacity, depending on the source temperature.

[12:20] And for screw compressors, we also have significantly longer service intervals compared to our pistos compressors. So that means that they are more beneficial in terms of OPEX of the system. And lastly, this is what you can say, the new kind of block in terms of natural refrigerants. That is the centrifugal and turbo compressors. These are the compressors used when we acquire quite a lot of capacity.

[12:46] And these are also the ones that's been used for a lot of the large heat pump projects in Denmark currently. So what we have with them is that we can go up to approximately 50 megawatt of heat capacity on one single compressor. And we're using this for actually both CO2 and hydrocarbons as well. They have very high capacity, as I said, but they also have a significantly lower efficiency compared to the other two compressor types.

[13:11] But I think that in terms of the future, it's most likely going to change because they are, of course, doing a lot of R&D work on these types of compressors in order to raise the efficiency of them. Then they also have very long service intervals. Have in mind that a lot of these centrifugal compressors are hermetic because they were used in the oil and gas industry to be placed at the bottom of the sea in order to move gas. And you don't really want to go to the bottom of the sea to perform service each year, or at least hourly.

[13:41] So you want to get the best out of them. But there are some other things that are very important. When designing heat pumps, there are, of course, some design pitfalls. I'm not going to lie, we have had a lot of trouble in Denmark because the development of heat pump systems went really quick. So let's say we learned along the way. And I think that's the same for a lot of other countries. So there are some things you need to take into consideration when designing your heat pumps. And one of them is, for instance, temperature and flow fluctuations.

[14:09] These are actually one of them. The most, let's say, heat pump killing things that you can have. There's actually a lot of suppliers that have limits on how much your temperature or your flow can vary it. And you will be surprised on how low it actually is. Usually it's between one to three Kelvin per minute in temperature fluctuations, which is not very much, especially if you're operating with, for instance, excess heat and process systems that can vary quite rapidly.

[14:37] So therefore, it's a good thing to think this into the solution from the start. And a way to do so could be by implementing buffer tanks, for instance, and also like correcting your hydraulic systems as well and be sure that these are dimensioned correctly. Then you also have external effects that could affect your heat pump. And similarly, you also have, of course, the amount of start-stop frequencies that you're doing on your heat pump.

[15:06] The more you start and stop your heat, the more wear it will acquire. So that's, of course, also necessary to take into consideration. An example of external effects, especially for air-to-water heat pumps, is, of course, air recirculation. So air recirculation is basically what happens when you cool down your air in order to take out the energy for your heat pump. Then if you don't have any wind or you have like layout issue, basically if you have buildings right next to your energy absorbers, what can happen is that, the air that it just cooled actually re-enters the evaporators of your heat pump.

[15:43] And what happens then is basically that your source temperature drops. And when that drops, the performance of the heat pump drops as well. So this is one of the performance killers for the air source heat pump. So often it's a very good idea and often a cheap solution to provide a CFD analysis of your system before installing it so you can get the optimal layout for your heat pump. If we look at it from a strategic perspective, as we mentioned, COP and uptime are basically what pays your heat pump.

[16:16] So what we need to take into consideration is also, of course, what is the heat pump purpose? Is it purposely being the baseload of your systems or does it need to have flexible operations as well? If it's a baseload, then it might be a good idea to consider to have some sort of redundancy. So instead of installing one heat pump with a hundred percent capacity, a good idea could maybe be to install two 50 percent capacity heat pumps because then you're able to produce heat even if your heat pump crashes or if you have some service that you need to perform on your heat pump.

[16:51] So redundancy is often a good thing to take into consideration. Then we have all of the integration with electrical boilers and buffer tanks. In Denmark, heat pumps have been widely used in terms of stabilizing the electrical grid and in combination with electrical boilers and buffer tanks. That is very, let's say, a very stable solution for your heat pump system. And they say a solution where you can actually provide a really good amount of money on your heat pump compared to just running the heat pump itself.

[17:23] They've done that in the northern part of Denmark for the last heat pump project, I said, and they have installed, I think it's approximately 90 megawatt of electrical boiler and four buffer tanks with a collected volume of 200,000 cubic meters. So that's a lot. And then we also look a bit into the startup time of the heat pump in terms of like this grid stabilization. This has been a large focus in Denmark in order to have the heat pumps to start up quicker.

[17:50] But that, of course, has some challenges along the way.

[17:55] If I am to give you a few key takeaways from this, it's something as simple as good planning provides good results. That is at least like the overall theme of this. It is, for instance, that what we need to do is that we need to design our heat pumps also for the real operation, not just the normal data. The example was, of course, the excess heat that we need to operate. We need to design it for like the actual system that we're going to be having.

[18:25] Similarly, we need to maximize for operating hours. The more operating hours, the better the business case. And then, of course, good COP is irrelevant without uptime. And basically that what we see is that we try to perform, we try to maximize performance of the heat pump, and that often leads to very complex systems which result in a lot of breakdowns and then we have low uptime. So that's pretty much a bad thing.

[18:53] I'm not going to go through this, but this is just a planner's guide I've made from beginning to end of a heat pump. You're going to be able to see that in the slide deck that will be presented.