Energy efficiency series: warm rooms with less energy

Some 90 percent of total energy consumption in German homes results from heating. About two-thirds of this are accounted for by heating rooms and preparing hot water. In commerce, trade and services, approximately 60 percent of final energy is used for various heat applications.

Therefore, anyone seeking to optimise energy consumption in buildings cannot discount the heat transition. At the same time, this harbours huge potential since substantial amounts of heat generated in industrial and commercial operations as well as thermal power stations goes unused. Making better use of this heat could reduce fossil and green primary energy usage when producing thermal energy.

Heating grids make waste heat usable

Combined heat and power (CHP) generation is a principle that has been applied for decades. This involves using the exhaust heat of thermal power plants – conventional coal and gas-fired power stations – to heat buildings via a district heating network. This is also the principle applied by block-type thermal power stations to supply heat to residential areas and large buildings, albeit on a smaller scale.

According to the German Federal Office for Economic Affairs and Export Control (BAFA) CHP plants with an electric output of more than ten gigawatts (GW) were commissioned in Germany in the ten-year period from 2012 to 2021. Both electricity and heat generation are thus in line with the German government’s expansion goals. However, there are still numerous thermal power stations that do not employ the CHP method. In 2020, only 111 terawatt hours (TWh) of the 318 TWh of electricity produced by thermal power stations came from CHP plants.

Sector coupling: “Utilise all available energy”

However, usable waste heat is not only a by-product of conventional power plants. The Living Lab Energy Campus (LLEC) test laboratory at the Julich Research Centre already makes use of the exhaust heat of the facility’s central mainframe computer. Plans envisage feeding the heat generated when storing green hydrogen into the test lab’s heating network as well.

One of LLEC’s research projects consists of exploring how various electricity and heat sources and storage systems can be interconnected to optimise the energy supply of entire industrial and commercial sites and residential areas. Says project manager Stefan Kasselmann, “After all, this is the objective of sector coupling: utilising all available energy so that the whole system can work in an economically viable manner.”

Where does efficient heat come from during winter wind lulls?

In the quest to become sustainable, however, it remains to be seen how conventional power plants can run efficiently on renewable energy sources as these stations have primarily been operated with fossil fuels thus far. To date, sustainable operation of these assets has almost only been possible using biomass, which is at the bottom end of the energy efficiency scale.

Between 15 and 50 megawatt hours (MWh) of electricity can be produced using biomass on a hectare of farmland. Depending on the location and system efficiency, wind and solar farms can generate between 400 and 1,000 MWh of electricity on the same surface area.

Chemical energy carriers such as hydrogen and synthetic fuels produced using sustainable methods are thus about ten times more efficient than biomass. Depending on the fuel, production, storage and transport are energy-intensive, but, all things considered, they have an energetic surface efficiency of approximately 160 to 500 MWh/ha.

Moreover, such fuels make a contribution to energy efficiency as seasonal energy stores, because they can be produced when solar and wind power are in excess supply. This ensures that the summer energy offering does not remain unused. Chemical energy sources are thus a relatively efficient way of bridging both temporary and seasonal shortages of solar and wind power – and not just on the heat market.

Heat pumps are not only efficient in single-family homes

Green electricity can enable efficient heating, in part because heat pumps provide for much more energy efficient heating systems than conventional radiators do. Depending on their size, design and usage, modern heat pumps have efficiencies of 250 to 500 percent. This means that they extract up to five watt hours of thermal energy from one watt hour of electricity.

Heat pumps use a refrigerant to extract heat from their immediate surroundings and pass the heat on to heating systems or boilers via heat exchangers. This is why they work all the more efficiently the warmer the environment through which the refrigerant flows.

Therefore, heat pumps with refrigerants that flow through a probe extending through the ground or a large body of water are especially efficient. This is because winter temperatures in these locations are almost always above freezing. The same holds true vice-versa, if a heat pump can also function as an air conditioning system. In such applications, the ground and water are colder than the ambient air and help to cool the refrigerant. Consequently, it is particularly worthwhile to make an added investment in a probe above all at locations with significant temperature fluctuations.

Heat pumps are definitely suited to applications other than single-family homes. Residential construction companies and municipal utilities are increasingly using heat pumps for local and district heating.

Seasonal heat stores and geothermal technology

The heat pump principle and solar thermal collectors also enable usage of seasonal heat storage systems. These can be large water or brine tanks that are heated during the summer, when a large amount of energy is available. Thanks to their strong insulation, these tanks can store the heat until the winter when the heat is released for use via heat pumps and direct heat exchangers. This principle even works in combination with geothermal technology. This involves storing energy produced during the summer in rock formations in the Earth’s crust.

Even more heat can be obtained from deep geothermal energy which produces warm thermal water from the subsoil. At depths of 3,000 to 4,000 metres, this water is even hot enough to be used to generate electricity. CHP technology can increase energy efficiency in these applications as well, while accelerating amortisation of the costs of investing in the drillings.