District Heating
What is it, and why do we need more of it?
There is a growing consensus that heating our homes and hot water with natural gas, oil and coal is unsustainable, as it is one of the biggest contributors to greenhouse gas emissions: buildings are responsible for around 30% of global energy demand, with around half of that attributable to space and water heating. But while there is agreement on the need for change, opinions vary widely on what we should replace our CO2-spewing boilers with.
The obvious alternative is electric heating, but traditional resistance heaters use a lot of power, and if everyone were to go over to using them, it would electricity demand would go through the roof. Which would, in turn, create a need for massive grid upgrades, as well as for lots of additional electricity generation. In the short term, fossil fuel consumption at thermal power plants would rise, negating many of the benefits of switching to electricity. And in most countries, it would be much more expensive to heat buildings like this.
Air-source heat pumps are therefore rightly touted as a much better solution: by taking heat from the ambient air (even at sub-zero temperatures) they can provide up to 5 times as much heat as the electricity they consume. This makes them cheaper to run and means they have a much smaller impact on overall electricity demand. However, they are not without issues. This is particularly true in cities, where space limitations, planning laws and other regulations, conservation and heritage considerations and difficult neighbours may make it difficult to install them in practice. Many of these barriers can and should be overcome, but in the meantime, what are the other options?
Wood burners can provide heat and even hot water, and over the very long run they are arguably almost carbon neutral, but unfortunately they have some major downsides. Chief amongst those is the fact that they pollute the indoor environment and local area, including with particle pollution (often referred to as PM2.5/PM10), carbon monoxide, nitrogen oxides and volatile organic compounds. These pollutants can contribute to serious illnesses like cardiovascular disease and lung cancer, particularly for the people using the wood burners, but also for everyone else in their communities.
Which leaves district heating, which is ideally suited to densely populated areas and solves almost all of the problems associated with heat pumps. So why is so little known about district heating, and why is it so completely ignored in the political and public debate in many countries?
I don’t have a good answer to that, but in this post I’ll try to explain a bit about what it is, how it works, what the advantages are and where it is being adopted.
What is district heating?
Let’s start with the basics. District heating systems, or heat networks, supply heat from a central heat source to end users through a network of underground hot water pipes. Consumers can then use the hot water for space heating and domestic hot water.
Big district heating systems may cover an entire town, but networks can also just cover a few buildings or homes, with the latter sometimes referred to as “local heating”. They’re also a great option for campuses and large institutions like hospitals and schools.
In a district heating system, there is normally a primary network and a secondary one. The primary network brings hot water (at 60 °C or higher) from the central heat source – the “energy centre” – to the customers’ buildings, while the secondary source brings the hot water at lower temperatures to wherever individual consumers need it – their radiators, hot water taps, etc. Between the primary and secondary networks you have heat exchangers which change the flow temperature of the water. In the case of a block of flats, this may be located in the plant room in the basement. Individual flats may also have a further, smaller, heat exchanger for their domestic hot water. This reduces heat losses and means you need fewer pipes, but it is not essential.
After the hot water has been used, or the heat has been extracted from it, the remaining water returns to the energy centre at a temperature of around 25-30 C, before being heated back up to the flow temperature. By increasing the distance that the hot water is transported, you increase the potential for heat losses, but because you are supplying many customers through well-insulated pipes, this is not a huge problem – in a typical system heat losses are only around 5-10 percent.
What are the benefits of district heating?
When thinking about the benefits of district heating, it makes sense to distinguish between the benefits to consumers and the benefits to society. Starting with the advantages for end users, it means that individual homes and buildings don’t need to have their own boiler, reducing the risk of fire and explosion, as well as the danger of carbon monoxide buildup. It also saves space and eliminates the noise associated with boilers. And in the same way as with a combination boiler, you have hot water on demand at all times.
There are also potential financial benefits – as well as the cost per kWh often being lower than for other sources, maintenance is cheaper and you avoid the risk of suddenly having to replace your boiler. And if you decide to go electric for your cooking (if you haven’t already), then you can cut yourself off the natural gas network and save on standing charges.
As you may have noticed, this turns some of the principal objections to heat pumps on their head: instead of needing more space-taking and expensive equipment, you need less, and whereas people are concerned about the noise of heat pumps, district heating is quieter than a traditional boiler.
From the point of view of society, the benefits are also significant. First and foremost, district heating has the potential to be a greener, more climate-friendly solution. Some current systems generate heat with large oil or gas boilers, which may be slightly more efficient to run than lots of small boilers in individual homes, but that is clearly not the way forward. A gas-fired combined heat and power (CHP) plant is a great improvement, as it uses the waste heat from generating electricity to supply the heat for the district heating network, resulting in overall fuel savings of up to 30%.
But when the aim is Net Zero, modern district heating networks need to do even better. Fortunately, there are many ways they can do this, because their scale enables solutions that would not make practical or financial sense for individual homes or businesses. For example, you can harness the waste heat from industrial processes, extract relatively low-temperature heat from rivers, wastewater and the ground using heat pumps, use energy from waste or biogas, and harness renewable energy sources like wind and solar, either directly or via the power grid. Many district heat networks use small amounts of fossil fuels to meet peak loads, but provided that they do this relatively infrequently, they are still vastly less polluting than gas- or oil-fired boilers.
Climate change is also increasingly making it necessary to cool buildings in summer, even in countries where it was not traditionally required. District heating networks can do that very efficiently, for example through so-called free cooling, which involves taking relatively cold water from rivers or the sea and using it to cool buildings.
Denmark leads the way
If you live in one of the many countries where district heating is virtually non-existent, you may be thinking “If it’s so great, why isn’t anyone using it?” That’s understandable, but in fact district heating is a major source of heating and hot water in a number of countries, including some of the coldest ones, where the need for heating is greatest.
For example, in Denmark 63% of all homes are connected to a district heating network for their space heating and hot water. Although Denmark has a long history of combined heat and power plants and district heating, it was really after the oil crisis in the 1970s that it went all in on improving energy efficiency and increasing energy independence. District heating is principally used in cities, and the Copenhagen district heating system supplies 250,000 households with 17 petajoules (PJ) of energy each year. The world’s biggest district heating network, it meets 98% of the city’s heating needs, using 85% renewable energy. However, even much smaller towns and villages down to 500 households may be supplied by district heating. Most of the renewable energy for Denmark’s district heating networks comes from biomass CHP plants, but the share from electric boilers and heat pumps is on the rise.
Other countries where district heating is popular include Sweden (where it supplies 93% of all dwellings in multi-family residential properties), Iceland, the Baltic states, Norway and Germany.
Final thoughts
The “problems” with heat pumps, which are in any case greatly overstated, are most acute in densely populated urban areas where they may be objections – either from planning authorities or residents – against outdoor units that are perceived as being unsightly or noisy. It may also be relatively costly to replace radiators with fan coils or larger radiators that work at a lower temperature, and are therefore more efficient with a heat pump.
This is precisely where district heating is most efficient – where many end users are packed close together, which reduces the cost of building and maintaining the infrastructure and keeps heat losses to a minimum.
In rural areas, where people live further apart and have more space, it is relatively straightforward to install air-source heat pumps, and it is more practicable to consider even more efficient options like ground-source heat pumps.
Heat pumps and district heating networks are therefore highly complementary solutions, and they will both be essential tools in countries’ efforts to decarbonise space heating and hot water.