Läs mer om fjärrmvärmens spännande framtid

Läs mer om fjärrmvärmens spännande framtid

Future of District Heating Part 1: The Past

Today, at the beginning of the 2020s, district heating is the most popular form of heating in Finland: more than half of Finns live in a district-heated house. District heating is also a common way to heat homes around the world. District heating thrives especially in cities where the district heating system is most efficient. Although various district heating systems have been in use for almost two centuries, the principle of district heating has remained largely the same: thermal energy produced in a district heating plant is transferred along a pipeline network to properties in need of heat.

This blog series takes you on a time travel from the early days of district heating to the present day, without forgetting the development that has taken place over the years. We also elaborate a bit on what the life of a heater, i.e. an ordinary city dweller, has been like during different generations of district heating. The first part of the blog series takes a look at history: how district heating systems came to be and how they evolved through 1880s to 1970s. The second part deals with the third generation of district heating, which is still the current generation of district heating. The third part focuses in fourth-generation district heating (4GDH), the era of which is beginning in the 2020s.

Urbanization and the prevalence of district heating go hand in hand: it is no coincidence that in the era of industrialization, as people moved to cities, the need for a reliable heating system was also increasing. For urban living, district heating is an effortless, reliable and affordable way to keep your home warm also in the 2020s.

neljännen sukupolven kaukolämpö

District heating's development has been divided into generations

Three generations of district heating

The evolution of district heating has traditionally been divided into three generations: first-generation systems with hot steam as the heat transfer medium saw the light of day in the late 19th century. Second-generation district heating systems switched to using water as a heat transfer medium. The golden age of these systems dates back to the 1930s and 1960s. The third generation of district heating, the so-called Scandinavian district heating, became more common in the 1970s and is gradually making way to the fourth generation of district heating.

District Heating revolutionizes urban heating

Throughout the history, there have been many systems resembling district heating, for example the water transfer systems that were used to heat spas in the Roman Empire. However, the first actual district heating systems were invented in the United States in the 19th century.

American Birdsill Holly is considered to be the inventor of the commercial district heating system. Holly was a mechanical engineer and inventor who, after inventing a fire hydrant system, turned his attention to urban heating. Could the heating of urban buildings be implemented in a more energy efficient way? At that time, the common practice was for each property to have its own heating boiler. Holly’s goal was to create a system where heat would be produced centrally in one place, making heating more economical and energy efficient. Environmental issues probably weren’t emphasized as much as today, but still, this new invention would improve urban air quality by replacing thousands of small boilers with one large heating plant.

Holly’s invention took off and district heating networks began to be built at a rapid pace: Manhattan built its own district heating network in the 1880s, while Europe’s first district heating network was commissioned in Germany in the early 1890s. First-generation district heating systems from these times are still in use in New York and Paris!

kaukolämpö new york

Built in the 19th century, New York's district heating network is still operating at full steam

Full steam ahead

In the 19th century, first-generation district heating worked very much in the same way as current systems, with a few exceptions, of course. Heat was produced centrally in district heating plants by burning coal and urban waste. Thus, district heating solved two problems at once: homes were heated more energy-efficiently and waste was conveniently disposed of at the same time. In a way, district heating was the predecessor of modern circular economy!

The new district heating system did not work completely without problems: from the user's point of view, the steam system is quite noisy, expensive to build and maintain. Transfer of steam requires large pipes, and large pipes require large holes to be dug in the ground. However, district heating represented the state-of-the-art technology of its time: hot steam passing through pipes dug underground transferred the heat generated by a district heating plant to the homes of city dwellers in an unprecedented way.

High pressure - high risks

However, due to the high pressure and high temperature, steam-transmitting district heating pipes are quite prone to explosions and other disturbances. For example, in 2007, a district heating pipe that exploded in Manhattan during the rush hour caused extensive damage to properties, injuring 45 people and killing one person. Mild disadvantages also include steam bursts, like the ones seen in American films, which are caused by the evaporation of cold water from the surface of a hot district heating pipe.

In addition to its loudness and operational risks, the problem with the steam system is also its poor energy efficiency: although relatively little energy is needed to move the steam, the network losses of hot steam are really high. Network loss refers to the energy convected to the surrounding ground during the transfer of steam.

Despite the risks and costs associated with operation, the benefits of district heating were so great that building the systems was profitable. The late 19th and early 20th centuries were the golden age of first-generation district heating systems. District heating systems were built all around the world at the time, but they were particularly popular in the United States. The first district heating network in the Nordic countries was completed in Denmark at the turn of the 19th and 20th centuries.

kaukolämpö tanska frederiksberg

The Nordic countries' first district heating plant was built in Frederikseberg, Denmark in early 20th century

How did the first generation of district heating change the world?

The industrial revolution of the 19th century revolutionized people’s lives in unprecedented ways. Part of district heating in this revolution was to simplify and facilitate urban heating. Instead of each property taking care of its own heating by using logs or coal in its own heating boiler, the heat could be produced in a combined heat and power plant. At the same time, air quality in urban centers was also improved.

Technological developments eventually led to the ability of district heating systems to use water as a heat transfer medium, replacing steam. As water-using systems became more common, the second generation of district heating began.

Second generation: from steam to liquid

As technologies evolved at a rapid pace in the early 20th century, the renewal of the district heating system also took place. Among other things, the development of pumping technology made it possible to change the heat transfer medium from steam to pressurized water. At the same time, the size of district heating networks could be increased thanks to more efficient pumps. Construction of the first second-generation district heating systems began in the 1930s.

The transition from steam to water was a major leap forward for district heating. The benefits of water include improved energy efficiency, a system that is easier to build and maintain, the ability to make more efficient use of return heat, and a more reliable way to measure customers' energy consumption. Second-generation district heating systems were also more reliable and safer than steam systems.

Massive pipelines shape the landscape

Unlike first-generation district heating systems, pipelines for second-generation district heating networks were often built above ground. These massive pipelines were often made on site, which was laborious, time consuming and expensive. The networks usually consisted of two large pipes, one carrying hot water to the buildings and the other carrying cooler return water back to the power plant. The pipes were usually insulated with mineral wool or polyurethane, which helped to reduce the heat loss in the network. On the other hand, pipes built on the ground were susceptible to damage caused by weather and other external factors, so their energy efficiency tended to decline over time.

toisen sukupolven kaukolämpö

Second generation district heating pipeline built above ground

Water-circulating district heating networks were often closed, ie the water circulating in the property’s heating network was separated from the water in the district heating network. In this case, the thermal energy of the district heating water is transferred to the use of the houses by heat exchangers. In some Soviet countries, open district heating networks were built, where the district heating water flows directly for the use of houses.

Cogeneration of electricity and heat begins

During the second generation of district heating, combined heat and power plants (also known as CHP) were also developed. These cogeneration plants revolutionized energy production by significantly improving plant efficiency. Simply put, the CHP plant operates in a way that the steam resulting from the combustion of fuel is first utilized by a turbine in the production of electricity and finally fed as thermal energy to the district heating network. The lower temperatures in the district heating network also enabled more efficient utilization of new fuels. The use of water also made it possible to store energy more efficiently. The result of this development can be seen today, for example, in the form of district heating batteries.

During the second generation of district heating, energy plants continued to use waste and coal as fuel, but the use of oil in energy production also began to become more common.

How did the second generation of district heating change the world?

The transition of district heating networks from steam to water also had clear advantages for the heater: heating was safer and more reliable than before. Life was also a bit more comfortable when the banging and rumbling of steam systems went down in history, thanks to the new technology. It was possible for more and more people to connect to district heating as technological developments made it possible to build bigger networks.

The development of CHP production was also of great importance. Instead of two separate plants, only one plant was needed to produce electricity and heat, so the costs of electricity and heat were kept low. While environmental issues may not have been at the top of the priority list during the second generation of district heating, transitioning from two smoke-puffing chimneys to one improved urban air quality even further.

Overall, the transition to second-generation district heating and its development during the 20th century was a huge leap towards a district heating system like today. The golden age of second-generation district heating systems dates back to the 1930s and 1970s, after which third-generation solutions began to take over the industry.

The next two parts of the series focus on the 3rd generation and 4th generation of district heating.



Future of District Heating Part 2: The Present

Today, at the beginning of the 2020s, district heating is the most popular form of heating in Finland: more than half of Finns live in a district-heated house. District heating is also a common way to heat homes around the world. District heating thrives especially in cities where the district heating system is most efficient. Although various district heating systems have been in use for almost 200 years, the principle of district heating has remained largely the same: thermal energy produced in a district heating plant is transferred along a pipeline network to properties in the need of heat.

In the first part of the blog series, we traveled back in time to the 19th century, when first-generation district heating systems were invented and started gaining popularity. You can read the article here . This post moves forward in time, presenting the third-generation of district heating.

Third generation of district heating: energy efficiency and the environment as priorities

It’s the 1970s. After being invented in the early 20th century, the second-generation of district heating has revolutionized urban heating and in the ever-urbanizing world, the need for district heating continues to grow. At the same time, the requirements for the district heating network are even greater. Gradually, it started to become apparent that the second-generation technology was outdated.  District heating needed to be more efficient regarding energy consumption, construction and maintenance. The time of third generation district heating had come.

The most visible change between second and third generation district heating systems is that district heating disappeared from view! Instead of pipelines being built above ground, the development of technology and materials made it possible to install factory-made, insulated pipes underground. Substations’ components, such as heat exchangers, also took up even less space. Like the second generation system, water acts as the heat transfer medium, but the temperatures in the third generation networks are considerably lower. Lower temperatures play a huge role in the efficiency of the entire district heating system, but we will return to these benefits a little later.

kaukolämpö district heating animation
Third generation district heating system animated
Animation: Laura Toffetti, DensityDesign Research Lab

At the end of the day, the manufacturing of prefabricated pipes and fittings was the most significant thing that separated third-generation district heating system from its predecessors. Factory-made, property-specific substations were also presented in the 1970s. Thanks to prefabricated parts, construction costs for district heating systems were reduced and construction sites were completed much faster than before. Instead of building all the parts of the system on site, the prefabricated “building blocks” could be assembled with less effort.

Third-generation district heating is sometimes referred to as Scandinavian district heating, as companies specializing in district heating systems began to appear especially in the Nordic countries. Today, Finnish district heating companies are world-class experts whose professional skills are highly appreciated around the world.

HögforsGST UNIS kaukolämpö valmistus manufacturing
Unis substation being manufactured at HögforsGST's factory


New energy sources and technologies

For more than a hundred years, district heating had been produced exclusively with fossil fuels such as coal and oil. During the third generation of district heating, new fuels and forms of energy began to be imported alongside fossil fuels: biomass, peat, solar energy, waste heat and various industrial heat pump solutions.

CHP plants also got greener: in addition to coal and peat, wood chips and other biomass were used in the combined production of electricity and heat. There were, and still are, large regional differences in the consumption of different fuels due to the availability of biomass, for instance.

As temperatures have been reduced as a result of technological development, the energy efficiency of the whole system has improved. Lower temperatures have also made it possible to connect new technologies and energy sources to district heating networks. For example, the efficiency of flue gas scrubbers that clean flue gases from power plants and at the same time recover waste heat, is improved when the return temperature of district heating is lower. The operating costs of power plants are reduced when the water pumped into the network can be up to tens of degrees cooler than before. At the same time, heat loss through the network is also lower.

District cooling is here to stay

During the third generation of district heating, the utilization of district cooling also began. Like district heating, district cooling technology was first developed in the late 19th century, but its real breakthrough had to wait for almost a hundred years.

District cooling basically works in the opposite way to district heating: when district heating transfers thermal energy in and out of the house, the energy entering in the district cooling is cold and the return water is warm. District cooling is particularly well suited for cooling larger properties in the city. The popularity of district cooling has grown significantly in the recent years.

district cooling kaukokylmä högforsgst

Prefabricated district cooling substation at SEB Bank in Stockholm, Sweden

The advantages of district cooling are its environmental friendliness and reliability. The source of energy for district cooling is usually a large heat pump plant or so called free cooling, where energy is obtained directly from lake or sea water, for instance.

Environmental issues gain more importance

It can be said that in the third generation, district heating has developed into a more flexible entity than before, also in terms of fuels and the forms of energy. In previous generations, heat was produced exclusively by burning fossil fuels in traditional district heating plants, but the rapid development of heat pumps over the last century, for example, has made it possible to utilize many previously inaccessible energy sources for heating.

For various reasons, district heating’s CO2 emissions have roughly been cut to half during the third generation. Energy-efficient production and distribution saves money, but it also benefits the environment. Awareness of climate change and the serious problems it causes has also created pressure to further reduce district heating emissions and to seek alternative energy sources to replace fossil fuels.

As a curiosity, it should be mentioned that the change in attitude can be noticed while reading between the lines. Today, the District Heating Statistics published by the Finnish Energy Industry Association also accurately list the specific emissions from district heating production. The 75-page statistics published in 2005, however, does not mention emissions at all.


How did the third generation of district heating change the world?

Although heat is one of the most basic basic human neccesities, district heating became less visible during its third generation. Huge pipeline structures disappeared underground, power plants “specializing” in district heating switched more often to CHP power plants, and the system became more reliable. Today, heating is taken for granted by many city dwellers.

However, the district heating industry has quietly done a massive service to reduce the carbon footprint of their customers. The carbon footprint has been reduced by introducing lower-emission energy sources, developing heat transfer equipment and systems to minimize energy loss. As energy meters have developed, it has also been possible to provide more accurate and reliable information on end-user energy consumption.

So can it be said that district heating has succeeded in making itself invisible by operating smoothly and without interruption throughout the year? The beauty is in the eye of the beholder, but what is certain is that district heating is the absolute backbone of modern urban heating.

Third generation's era is coming to an end

The curtain is closing for third generation of district heating, as the fourth generation is ready to take its place in the 2020s. The reasons for this are partly the same as in the previous two generational changes: the third generation system is no longer efficient enough.

The generational change has been pending for quite some time, as fourth-generation district heating solutions have been emerging into the market at a growing pace in the recent years. However, the final push for change has been given by the fight against climate change. Heating is one of the largest single sources of greenhouse gas emissions in modern cities, so it is put under particular pressure to reduce emissions.

The needs of customers also largely determine the direction in which district heating should develop in the future. As competing forms of heating gain more and more ground, district heating companies must  evolve from commodity suppliers to more comprehensive service providers. The possibilities brought by the Internet, such as remote measurements, remote control systems and the collection of large amounts of data, will be a key part of the district heating system of the future.

The fourth generation period is about to begin and is expected to last at least until the 2050s. What this era will bring, is impossible to fully predict at this point in time. The core of fourth-generation district heating is discussed in the next part of this series.



Future of District Heating Part 3: The Future

Today, at the beginning of the 2020s, district heating is the most popular form of heating in Finland: more than half of Finns live in a district-heated house. District heating is also a common way to heat homes around the world. District heating thrives especially in cities where the district heating system is most efficient. Although various district heating systems have been in use for almost 200 years, the principle of district heating has remained largely the same: thermal energy produced in a district heating plant is transferred along a pipeline network to properties in the need of heat.

In the first two parts of the blog series discussed the development of district heating systems from the 19th century to present day. As mentioned before, the development of district heating can be divided into generations. We are currently living in a turning point where third generation district heating is giving way to fourth generation. The reasons leading to the generational change have been discussed in our previous blog.

4GDH - open, clean and flexible

First-generation district heating revolutionized urban heating, while during the second generation the heat transfer medium changed from steam to water. During the third generation the construction of district heating systems was revolutionized due to prefabricated parts and components. Fourth generation district heating, or 4GDH, will change district heating even more dramatically. At the heart of the change are eg. energy sources and the way in which the network is utilized.

Please note that the blog’s examples apply mostly to the Finnish district heating networks.

högforsgst 4GDH city

The last push for generational change has been given by the fight against climate change. The use of coal and other fossil fuels has become unprofitable over time and their use has been severely restricted. For example, Finland will ban the use of coal in energy production from 2029, so the energy sector will have to come up with new solutions quite rapidly. The transition to fourth generation district heating is one part of this process.

The district heating network is the energy route of the future

Fourth-generation district heating is more flexible, cleaner and more rapidly evolving than its predecessors.

- One could say that the very concept of district heating is changing. The district heating networks of the future are energy routes to which several different energy producers and consumers can be connected, ponders Åke Vikstedt, who is an energy industry specialist at HögforsGST.

- I agree. Addressing district heating as a ”black-as-coal” form of heating is more or less outdated information. Nowadays, the heat for the district heating network can be produced in countless different ways, of which coal is in the minority and its role is growing smaller as we speak, says Antti Hartman, Managing Director of HögforsGST.

So what does all of this actually mean? Scientists who have studied the development of district heating have listed five properties that describe fourth-generation district heating:

1. Ability to supply low-temperature district heating for space heating and domestic hot water (DHW) to existing buildings, energy-renovated existing buildings and new low-energy buildings.

2. Ability to distribute heat in networks with low grid losses.

3. Ability to recycle heat from low-temperature sources and integrate renewable heat sources such as solar and geothermal heat.

4. Ability to be an integrated part of smart energy systems (i.e. integrated smart electricity, gas, fluid and thermal grids) including being an integrated part of 4th Generation District Cooling systems.

5. Ability to ensure suitable planning, cost and motivation structures in relation to the operation as well as to strategic investments related to the transformation into future sustainable energy systems.

But how do these things work in practice? Let's find out!

Lower temperatures – the core of 4GDH

The water circulating in the (Finnish) district heating networks is currently around 90 degrees celcius. The temperature depends a lot on the need for heating energy: in the wintertime, the district heating water has to be hotter and in warmer weather and warmer climates the temperature can be a bit lower. The temperature of the water going from the district heating plant to the properties varies between 65 and 115 degrees, while the temperatures of the return network are often between 40 and 60 degrees.

The goal of fourth-generation district heating is to lower these network temperatures so that the average flow temperature can be reduced closer to 65 degrees and the return temperature to around 25 degrees. Lowering both flow and flow temperatures has a major impact on the energy efficiency of the entire district heating system and, at the same time, on costs and emissions.

- Thanks to modern heat transfer technology, the reduction of network temperatures can be done gradually without major additional investments. The current third-generation district heating networks can  be upgraded to meet fourth-generation requirements bit-by-bit, Hartman says.

Simply put: thanks to more efficient heat exchangers, it is no longer necessary to heat the flow water to a boil in order to keep the properties warm. The more temperatures are lowered, the more money is also saved.

Illustration of how a 4GDH system works. The illustration is a part of a 4GDH themed video, which is available here

Lowering temperatures provides the following benefits:

  • The flow in the network is reduced, which reduces pumping costs
  • The heat transmission capacity of the network is increased
  • The heat loss of the network is reduced
  • The efficiency of combined heat and power (CHP) is improved
  • The efficiency of industrial heat pumps and flue gas scrubbers is improved

Thus, lowering the temperatures improves the efficiency of the district heating network at many points. Lowering temperatures is, in fact, such a significant part of a functioning fourth generation district heating system that it deserves its own blog post in the future!

Waste heat recovery is energy being recycled

A quite significant change in fourth-generation district heating compared to the previous generations is the diversity of energy sources. In fact, the operating principle of district heating is almost reversed: customers become co-producers of district heat!

- In a fourth-generation district heating system, the role of the customer can be more than just the final consumer of heat. In the future, for example, industrial plants or even local businesses can act as small producers of district heating by selling their own waste heat to the district heating network, says Vikstedt.

In Oulu, Finland, S-Market Kaukovainio's waste heat is being used to heat nearby apartment buildings. Image: Oulun Energia


- The district heating system of the past was one-way only: energy was transferred through a “straight pipe” from the power plant to the customers. Now the relationship between customers and energy utilities is deepening when some customers suddenly become heat producers! If anything, then this is a great example of circular economy, Vikstedt continues.

No more burning

District heating has almost always been produced by burning coal, oil, waste, wood chips or other combustible materials. Incineration is an efficient way to produce energy, but its problem is the emissions from combustion. The main problems are carbon dioxide and various small particles reducing the air quality. The use of fossil fuels has been both a cost-effective and energy-efficient way of producing heat and electricity, so the threshold for giving them up has been high. Now the situation is changing.

- The use of coal, and fossil fuels in general, has been profitable so far, so there has been no incentive to let go of it. However, the situation is changing as environmental awareness increases and the use of fossil fuels is being restricted. Now the energy industry is looking for alternative energy sources, Hartman says.

There is not a single form of energy that could replace coal completely. Lower-emission fuels, such as biomass and biogas, are reasonable options. Other potential options are waste heat recovery, geoenergy and industrial heat pump plants, for instace. The role of incineration in the future is likely to be a reliable source of energy at the time of peak demand. The goal is that the basic demand can be met by other energy sources.

Smart energy grids extend across the city

A more complex district heating network will not work without an intelligent control system. Smart technology improves the energy efficiency, reliability and money of networks.

- In many ways, 4GDH is a smarter entity than its predecessors. For example, smart substations installed in individual properties work to benefit both parties: residents receive accurate information about their energy consumption, system operation and they can also control their heating remotely. At the same time, the power plant receives a huge amount of data on the operation of the network, Hartman lists.

Vikstedt also believes that smart solutions will become more common in the future.

- District heating networks are transforming into open energy routes, which are maintained and controlled by, for example, energy companies. As the number of stakeholders increases and the network expands, more advanced technology is also needed to provide up-to-date information on the operation of the network. The modern district heating substation is also a data substation!

Towards a cleaner future

Intelligence, energy efficiency, new energy sources and communality. The deep core of fourth-generation district heating consists of these things. Together with growing environmental awareness these characteristics create a powerful weapon against climate change. According to Hartman, the generational change of district heating does not come too early.

- Heating residential buildings produces about a quarter of Finland's carbon dioxide emissions. Emissions have been reduced quite a lot over time, but more needs to be done. The fourth generation of district heating solutions will cater to this need.

- Economic benefits and the well-being of the environment are often in conflict with each other, but in the development of district heating the situation is the opposite. Improving energy efficiency saves huge amounts of money while reducing emissions at the same time, Hartman states.


HögforsGST's 4GDH solutions

HögforsGST already offers multiple fourth-generation district heating solutions that benefit both individual properties and the power plants that maintain the district heating network. HybridHEAT is an exceptionally energy-efficient heating solution that, as the name implies, is able to utilize several different heat sources. HybridHEAT can use, for example, district heating, ground heat and exhaust air heat recovery as a source of energy. The operation of a hybrid system is comparable to a hybrid vehicle that alternates between electricity and fuel. HybridHEAT alternates between different energy sources in a similar manner.

HybridHEAT is controlled by the Fiksu Control System. Fiksu's role in utilizing various 4GDH solutions is important, as it can be used to remotely control the heating system and collect information about its operation. Fiksu improves the energy efficiency of an individual property and at the same time provides the power plant with information that allows them to optimize their operations and save on costs.

However, our work is not over yet, and in the coming months we will release several 4GDH solutions that can improve the energy efficiency of district heating systems and reduce the heating bill of a district heater.

Although the blog series has reached its end, our 4GDH journey is just beginning. In future blog articles, we will address subjects like heat recovery, energy renovations and, of course, fourth-generation district heating. Follow us on our social media channels to stay up to date!


Watch a video about the 4th Generation District Heating system!