Solar heat: an emission-free and affordable source of district heating

Climate change is currently the greatest threat to the future of humankind. All combustion produces carbon dioxide, which is a greenhouse gas, and which slows down the dissipation of the earth’s heat into space. The result is a rapid rise in global temperature with significant adverse effects.

The sun heats efficiently

The sun is our most important – almost the only – source of energy. Wind power and bioenergy also come indirectly from the sun. Utilising solar energy has the potential to reduce combustion and halt climate change.

Under the Paris Climate Agreement, the world aims for carbon neutrality by 2050 and global warming of no more than 1.5 degrees compared to pre-industrial times. Carbon neutrality means that carbon dioxide emissions are limited to the extent that they can be sequestered from the atmosphere into carbon sinks.

When we talk about solar energy, we usually think of solar electricity (PV). However, photovoltaics and solar thermal are completely different things. In a PV panel, the energy of photons from solar radiation detaches the electrons in the semiconductor material of the solar cell, which form an electric current. In a solar thermal collector, the radiant energy is absorbed into the collector coating and converted directly into heat.

In a PV panel, roughly 15 per cent of the sun’s radiant energy is converted into electrical energy, and the efficiency of the solar collector is 4-6 times better – up to 90 per cent of the radiant energy can be recovered as thermal energy. In practice, this means, for example, that the solar thermal collector produces the above-mentioned 4-6 times more energy from the same ground area as PV panels.

The world needs a lot of energy

The Intergovernmental Panel on Climate Change (IPCC) estimates that the effects of warming of up to 1.5 degrees are probably still manageable. However, staying warm to no more than 1.5 degrees is a very tough challenge and requires quite drastic and quick action.

World energy consumption in 2019 was 13,813 million tonnes of oil equivalent (Mtoe), or 160,645 terawatt hours (TWh). The annual growth is in the order of one or two per cent. Oil has the largest share of that consumption – 33 per cent – then coal at 27 per cent.

Globally, about 50 per cent of total energy consumption is related to heat used in housing and industry. A solar thermal system typically produces 300 to 600 kilowatt hours per square metre of collector per year (kWh/m2*a).

The real construction cost of megawatt solar heating plants has been in the order of 210-350€/m2, or 0.50-0.85€ per annual kilowatt-hour capacity. The price of heat has been about 40€/MWh, or 4 euro cents per kilowatt hour (see IEA SHC Task 52, Solar Thermal in Energy Supply Systems in Urban Environments). In gigawatt-class plants, the cost per square metre will certainly continue to decrease significantly.

Solar heat is a global growth market

In 2019, 22 large-scale (>500m2) solar thermal plants were commissioned for district heating production, of which 22 were in Denmark (66,800m2), six in Germany (14,700m2) and one in Latvia (21,700m2). Outside Europe, the largest growth market is China, where three plants were installed (a total of 57,386m2).

The growth of large solar thermal systems has been rapid, with global capacity growing by an average of more than 20 per cent annually between 2010 and 2019 (see Solar Heat Worldwide: Edition 2020). Compared to another major source of renewable energy, wind power, growth is clearly faster, with annual capacity growth nearly double that of wind power (approximately 12.5 per cent).

The international growth potential is significant, for example 3-11 per cent of Germany’s total heat demand, some 15-60TWh, is estimated to be able to be met by solar heat. At the very least, this would correspond to an investment potential of around €10bn.

This huge opportunity for investment, together with the development and deployment of heat storage, heat pumps and intelligent energy systems, makes solar heat a very attractive technology for the future for both companies and communities.

Helsinki wants to lead the way

The city of Helsinki wants to be carbon neutral by 2035. More than half of Helsinki’s heating energy is currently produced with coal, the use of which in energy production in Finland will be banned from May 2029.

In order to find the best solution to replace coal, Helsinki has organised the Helsinki Energy Challenge, which promises the winner a million euros. The aim is to find a solution to stop CO2 emissions in the provision of heating with as little biomass burned as possible. Registration for the competition ended at the end of September and 252 teams from 35 different countries came.

Solar heat is a viable option, together with geothermal, heat pumps and other emission-free heat sources, as well as seasonal heat storage to form a modern distributed district heating and cooling system.

The winner of the Helsinki Energy Challenge will be announced in February 2021, and several major cities such as Toronto, Vancouver, Amsterdam and Leeds have come forward as supporters of the challenge and are waiting for knowledge and solutions to use in their own climate work.

In order to cover 20 per cent of Helsinki’s annual district heating demand of 7TWh with the sun, collectors would be needed for about 3km2 and installation space so that the collectors do not shadow one another, so a total of about 8km2. The cost of implementation could be less than €500m.

The total area of Helsinki is just over 200km2 and the roof area of the buildings is over 35km2. In most cases, space is not a problem because sufficient space suitable for collectors and heat storage can be found close enough to the concentration of consumption. In Denmark, for example, there are dozens of district heating plants that make significant use of solar heat. As is well known, Denmark is a relatively small country, but even there space has been found for collectors.

It is affordable to store heat

It is characteristic of solar energy in Europe that it accumulates more in the summer season than in the winter. The need for heating, on the other hand, is up to 10 times higher in winter than in summer, so energy must be stored from summer to winter. It is known that battery stocks are still very expensive, in the order of 100-200€ per kWh.

However, the batteries store electricity! Heat storage is substantially cheaper. One cubic metre (m3) of water can easily store 50kWh of thermal energy. This only requires a temperature change of about 43 degrees Celsius (°C).

A great variety of heat stores have been built around the world, from steel tanks to underground caves and holes drilled in rock. In Denmark, for example, several seasonal stores have been built in a large pit dug into the ground, lined with insulating material such as light gravel and waterproof film. With a floating insulation cover on top, the storage is ready.

The largest pits are more than 150,000m3 and the construction cost is about 30€/m3. That is 0.60€/kWh. It is possible to store up to 10 gigawatt hours (GWh) of thermal energy in one such storage tank.

Europe leads in solar district heating

In Denmark and Sweden, as well as in Germany, Austria, Spain and Greece, large district heating plants connected to district heating have been in operation since the 1980s. The market for large plants was concentrated in Europe until 2016, after which new systems have been constructed elsewhere, above all in China.

At the end of 2019, about 400 large systems with a size of more than 350kW or 500m2 were in use. The installed collector area was 2.3 million metres squared and the capacity 1,615MW. The majority of systems are connected to district heating. Denmark is the market leader in this region, well ahead of the following, both in Europe and worldwide. China ranks second in the world with Germany second in Europe.

Denmark’s share of the above is 123 plants, with more than 1.5 million metres squared of collector area and more than 1,000MW of thermal power. The largest plant at present has a capacity of 110MW, which is produced with about 155,000m2 of collector space. This and several other large plants use a large excavated pit as a reservoir and produce about half of the area’s annual heat demand.

Savosolar has delivered some 70,000m2 of collectors to Danish plants. In addition, the company has delivered its large systems to France, Germany and Finland. The systems generate both district heat and heat for industrial processes.

Climate change requires many solutions

With renewable energy – just as in all energy supply – one clear truth is apparent: one solution cannot cover all our needs. Non-renewable energy sources include oil, coal and gas, as well as peat and nuclear power.

Renewable energy sources are currently, for example, solar, wind, geothermal, biogas and biomass.

One thing is certain, however: the final solution – perhaps through many intermediate stages – will be the sun. For the simple reason that its potential is hundreds or thousands of times greater than any of our other options.

Solar energy compared to other energy sources. Richard Perez, who drew up the chart, uses a convenient unit of energy, terawatt years, which equates to 8,766 terawatt hours. For renewables, the figures describe the annual yield, for fossils the total reserve. For the sun, the figure is radiant energy hitting open continents, assuming 65 per cent losses in the atmosphere and clouds. Source: R. Perez et al.
In a solar district heating system a large field of solar heat collectors supply heat to the local district network. The system is supported by a heating centre that generates additional heat to cover all heating needs in every situation.
A large seasonal heat store can be built into a large insulated pit. Numerous implementations have been made around the world. Denmark’s largest seasonal store has a volume of 210,000m3 and its construction cost was only €24/m3.
In Europe, approximately ten times more heat is needed in winter than in summer. From May to August, the sun covers the entire heating demand and the district heating boiler can be shut down, significantly extending its service life. Excess heat from summer can be stored in seasonal storage for winter use.