Dreams of power-to-gas and power-to-liquid technologies are yet to become a reality in our daily lives. So far, it has only been possible to generate gaseous or liquid synthetic fuels – with the help of electricity from renewable energy – within small pilot plants. But power-to-heat technology is a whole other story. It has been possible to turn electricity into heat for quite some time now – by using heat pumps, for example. As renewable energy contributes ever more to the electricity mix, this technology is set to become even more important as time goes on. In the latest instalment of the ‘Power-to-X’ series, en:former explains just how power-to-heat can be used to make the supply of heating greener and how it can put excess power to good use.
A kettle serves a clearly defined purpose: Water for pasta can be brought to a boil faster and coffee granules can be turned into fresh coffee in no time. But moving forward, these appliances could be used for a whole host of other applications. Now, this may come as a surprise, but the humble kettle could soon help drive the energy transition – thereby solving major challenges such as the issue of energy storage. In fact, technology to this effect is already being tested. Electricity will still be used to heat water, simply on a much larger scale. The appliances used are not your average, commercially available household appliances with a capacity of 1 to 1.5 litres, but rather massive industrial immersion heaters which are able to bring tens of thousands of litres up to the boil.
This is just one example of what is referred to as power-to-heat technology. Taken at face value, power-to-heat, also abbreviated as PtH or P2H, only refers to the generation of heat using electrical energy, i.e. electricity is converted into heat. The advantages are plentiful. For one, it is much easier to store heat than electricity. Plus, electricity from renewables can also help reduce greenhouse gas emissions in the heating sector.
This technology has actually been around for quite some time. According to German Ministry of Economics, the most efficient and important such commercially available device is in fact the downstairs heat pump found in boiler rooms, as it uses electricity to absorb the heat available from the ground, which it compresses and then uses to operate the heating system, for example. Electric heat pumps are currently being installed in around one third of all new buildings. This means that large amounts of electricity are actually already being converted into heat.
Power-to-heat technology is currently being discussed in relation to a problem that is only set to worsen moving forward: At times, wind turbines and photovoltaic systems generate more electricity than can be consumed or transported within the network. To prevent the power grid from becoming overwhelmed during these periods, the systems are shut down. This particularly applies to wind turbines in the north of Germany. This means green electricity is virtually thrown away – and that on an industrial scale.
In 2017 and again in 2018, temporary wind farm standstills resulted in the loss of more than five billion kilowatt hours (kWh) of electricity. The German Association of Energy and Water Industries BDEW estimates that the first quarter of 2019 alone, when there was a particularly strong storm, accounted for losses of around 3.2 billion kWh. As a frame of reference: Six million e-cars could have been driven for a quarter of a year with the electricity that was lost between January and March due to stationary turbines. As the popularity of renewables grows, the question of how the temporary surpluses can be put to best use is becoming more urgent. Not least because the expansion of the grid, which is so desperately necessary, is progressing slowly.
However, commercially available basement heat pumps are not suitable for such volumes of surplus electricity, according to the blog Energie-Lexikon. For they only generate heat as needed, for example when the temperature in a room drops. This means they are not really able to react to power surges. So, at the very least pilot projects prove that connected heat pumps in combination with battery storage systems can help relieve the load on the power grid.
Given that there is so much excess electricity from wind and solar power, there is a need for different solutions. This is where another form of heat supply comes in: district heating. Water is primarily heated using gas- and coal-fired power plants with combined heat and power generation, before being piped to consumers. If district heat (which is used to heat around one seventh of houses in Germany) were produced using electricity generated from renewable sources, then the need for fossil fuels could be reduced. Emissions in the heating sector would also drop.
By far the largest plant of this kind is located in Berlin. “Europe’s largest kettle”, is how Vattenfall lovingly refers to its power-to-heat plant at the Reuter CHP station. It has been up and running since mid-September and operates based on the same principle as immersion heaters, giving it a thermal output of 120 megawatts – equivalent to around 60,000 commercially available kettles. Three electrode boilers, each with a capacity of 22,000 litres, heat water to 130 degrees using electricity. This energy is then transferred via a heat exchanger to the district heating network in the north-west of Berlin. Vattenfall generates district heat for up to 30,000 households.
However, the system by no means runs 24/7. Instead, it only springs into action when there is a lot of electricity from renewables in the grid. Or rather, when the numerous wind turbines in neighbouring Brandenburg are spinning at full speed. The green electricity then replaces the need for hard coal, which is otherwise used to generate heat in the CHP plant. This makes district heating much greener. It also means that fewer renewable energy plants have to be turned off.
A number of other companies are also testing technologies that convert electricity into heat. RWE, for example, is planning to build a heat storage power plant in a coal-fired power station in the Rhenish lignite mining region in Germany, where excess electricity from wind farms will be used to heat liquid salt to up to 560 degrees Celsius. The benefit in this regard is that the hot molten salt can be easily stored in tanks. If necessary, it is then used to generate steam, which in turn drives the power plant’s turbine to generate electricity. In this way, volatile renewable power can be stored and converted as required.
In its ‘Power-to-X’ series, en:former will be looking into technologies that convert electricity into other energy sources. Articles have been published on Power-to-Gas, Power-to-Liquid and on sector coupling in general. In the coming weeks, the series will explain how these technologies can be progressed and made marketable.
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