Shallow geothermal in Sweden – A deep penetration in the geo-energy market
In contrast to what many people may expect, shallow geothermal is bigger than deep geothermal when looking at the total installed capacity worldwide. In the series of three articles, we talk to three experts from the shallow geothermal space in Sweden, Australia and Canada to hear how the market is evolving, what the role of the subsurface is, and how technological development is making even more advances
The top three countries in terms of geothermal energy production are China, the US and Sweden. In total, these countries produced around 200 TWh in 2020. Of that figure, ground-source heat pumps account for 126 TWh, so 63 % of the total energy produced. So, it is the shallow geothermal systems that are responsible for most geo-energy produced in these places. In Sweden, it is even at 100 % of the share.
Yet, in the media, there is a lot of attention for deep geothermal drilling projects: At the end of the day, a borehole that terminates at 100 m below the surface is not as spectacular as one that makes it to 7 kilometers. A lot more geoscience is associated with deeper boreholes as well, whilst logging or well testing is something that is rarely done in the shallow drilling business. But given the amount of energy produced from ground-source heating systems, it is still very useful to also look at these projects a bit more and learn about the role of the subsurface.
What I took away from the three conversations I had, was that the costs of drilling shallow boreholes in most countries are still relatively high for many homeowners to opt for groundsource heating. That is why there is such pressure on developing technology to make drilling faster, as all three people I talked to addressed.
The geology is also of great importance and not only drives the design of every system, it is also determining the way the boreholes are being drilled. A thermally conductive lithology means that shallower boreholes will do, and groundwater flow is another major factor influencing the design of loops.
The potential for deep geothermal in Sweden is very limited, but the contrary is true for shallow geothermal
Also, because the subsurface is essentially treated as a battery, the geological setting including the heat capacity needs to be integrated into the design of the building to prevent a long-term energy imbalance. As Edward Wiarda tells us during his interview, the energy transition is about the integration of above-ground as well as below-ground factors. The shallow geothermal space is a perfect example for that.
Despite the larger upfront costs, ground-source heat pumps tend to work longer and more economically than air-source heat pumps, so in the long run it pays off. All in all, with technology advancing and more pressure and incentives to source energy for domestic heating locally, I see a bright future for shallow geothermal. And let’s not forget the statistics – shallow geothermal has already proven to be the most important player in the geothermal space!
Sweden
“The potential for deep geothermal in Sweden is very limited, but the contrary is true for shallow geothermal”, says Signhild Gehlin from the Swedish Geoenergy Center. Signhild is one of the very few people who works full-time in the shallow geothermal space, and as the only employee of the Geoenergy Center she is probably the best person to talk to about this sector. She is also the editor of a magazine on geo-energy and provides courses for those wanting to enter the industry. As such, she is very aware of everything that is happening in Sweden when it comes to ground-source heating.
HOW THE IDEA OF GROUND-SOURCE HEATING CAME ABOUT
At first, people took up the idea of storing summer heat in a borehole, which was trialled near Stockholm. “However, in the light of the conductive nature of the rocks”, Signhild continues, “the heat dissipated quickly. So that didn’t work. Then, somebody came up with the idea that if the rocks could not be used as a heat store, they could instead be used as a heat source, with a heat pump on top. It was the start of the industry.”
Why is Sweden a country that has such a high density of shallow geothermal boreholes? “Our geology, which mostly consists of old crystalline bedrock, is characterised by a high quartz content, which makes for good thermal conductors”, says Signhild. That is a good starting point. “In addition,”, she says, “Sweden has never had a gas network, so there has not been the competition with cheap gas as we tend to see in other countries.”
But it still took an oil crisis to really make people aware of the fact that energy dependency is a critical thing. It was in the late 1970’s and the decades after that shallow geothermal drilling gained in importance.
“And the costs of drilling have basically stayed the same over the last decades”, says Signhild. “It is thanks to the upscaling of the sector, the innovations when it comes to drilling and thus the time in which we can complete a borehole that the costs have been able to stay relatively that. When I started in 1995, I asked about the cost of drilling per meter: Around 20 €/m. Twenty years later it was still the same price!”
Most boreholes in Sweden are drilled using percussion hammer drilling. “That’s what basement rocks are best for; you can drill very deep, and the borehole does not collapse either. We can now drill 300 m in a single day,” Signhild adds, “which is more than deep enough for a single-family home.”
THE EXPENSIVE TOP HOLE
Drilling costs and the way projects are completed is clearly reflected in the graph below. The top line shows the average depth of all geothermal boreholes drilled in Sweden. The lower one, which indicates the wells drilled for single-family homes, starts to deviate from 2003, when single-borehole projects record a slightly lower average depth. “The reason for that”, says Signhild, “is that the cost of drilling the top hole is four to five times as much as the rest of the hole, because of the casing required for the first few meters. That’s why projects where multiple boreholes are needed, the preference is to drill fewer but deeper holes to prevent additional casing costs.”
Average depth of all shallow geothermal boreholes drilled in Sweden (thick line) versus average depth of all boreholes drilled for singe family houses. Source: Swedish Geoenergy Center
Increased energy extraction
In some ways, Sweden is already in a second cycle of ground-source heating development, thanks to advances in heat pump technology. As the ability to extract energy from the geothermal loops has increased – a Coefficient of Production (COP) of 2.5 was the norm back in the days; now many heat pumps will run on a COP of 4 – 5: Twice as much energy will be extracted from the ground”, explains Signhild.
“This means if you take more energy from the ground, your boreholes will need to be deeper in order to recharge sufficiently”, she continues. “That is why a shallow borehole of 100 m depth that was ok in 1995 won’t do nowadays. In turn, this means you will need more or deeper boreholes to retrieve the same amount of energy from the borehole again as the heat pump extracted.”
This has led to an odd development. “You can now buy a heat pump that is very high spec, but it has a button to says it works just like the old one!”, laughs Signhild. “It is absolutely not the way to go if you’d like to benefit from your new machine. It is much better to drill a second borehole instead.”
