CHP on the climate test bench

Author: Armin Müller, Energy & Management (As of April 2017) Does cogeneration help achieve climate protection goals? The answer is not as clear-cut as thought. The "sector coupling" has now entered the political discussion as a somewhat different term for cogeneration. What initially looks like a clear political commitment to CHP and local or district heating is not necessarily so on closer inspection. In politics, the term is rather associated with the idea of supplying all consumption sectors, as well as heat and transport, electrically and providing the electricity for this from renewable sources. What significance then still has the cogeneration was one of the questions discussed at the annual conference "BHKW 2017", which BHKW Consult organized in Kassel at the beginning of April. The answers to these questions differ in detail, but they also have one thing in common: the most complete possible decarbonization of our energy supply must come if we are to achieve the goals of the Paris Climate Agreement. From around the end of the 2030s, fossil fuels must then also no longer be used for combined heat and power plants.

No sector coupling without energy storage

For Michael Sterner, professor at the East Bavarian Technical University (OTH) in Regensburg, the first priority in the conversion to a new energy system is the intelligent coupling of electricity and gas grids and of storage facilities. With a mixture of heat and electricity storage plus grid expansion, the future tasks of the energy transition can be mastered: "No sector coupling without energy storage," he emphasized in Kassel. Renewable electricity would then become the primary energy for all sectors, including the heating market. In this context, grid expansion must ensure the spatial exchange of energy flows, while storage facilities must provide the temporal balance. The extent to which the two technologies must be weighted depends on the scenario chosen, Sterner explained. With a large addition of PV systems and home storage, about 1,000 km fewer power lines are needed than with an expansion focus on offshore wind. Generally speaking, however, the number of kilometers of expansion calculated in many studies and grid development plans is far from being achieved in this country, so there is a lot of catching up to do in terms of grid expansion. And the CHP? For Sterner, it continues to have an ecologically justifiable future if the fuel gas needed for its operation is produced renewably. It can be supplemented - for example in highly thermally insulated houses - by heat pumps, which, however, must also be powered by renewable electricity. Otherwise, their emissions are far too high. Both technologies, together with PV systems, storage and a control system, then lend themselves to supplying electricity and heat to residential areas and neighborhoods. "A virtual power plant is a nice business model for CHP plants," Sterner said at the Kassel CHP conference. He also believes that there is no 100 percent decarbonization without converting renewable electricity into other energy sources, i.e., "power to X."

Most CO2 has already been emitted

Volker Quaschning comes to similar conclusions, but also to a concrete end in time for the use of fossil fuels. The professor at the University of Applied Sciences (HTW) in Berlin has calculated how much CO2 mankind is still allowed to emit if it wants to limit the temperature rise, as agreed in Paris, to 1.5 °C compared to pre-industrial times. A total of 2,250 billion metric tons may be blown into the atmosphere, 1,950 billion metric tons of which have already been emitted. By 2040, Quaschning argues, the remaining budget of 300 billion metric tons will have been used up. At that point, no one would be allowed to release any more CO2 into the atmosphere. Despite major environmental efforts in this country, he sees Germany falling well short of this target. If we really want to stop emitting carbon dioxide by 2040, he says, much more drastic measures will be needed than at present. In particular, the heat supply, 74% of which (or 570 billion kWh) is currently provided by fossil fuels in Germany, would have to be switched completely to renewable energy sources. Because the use of biomass, solar thermal energy and deep geothermal energy is limited according to Quaschning's calculations, only electrical energy comes into consideration for covering this demand - after a significant increase in energy-related building renovation.

2040 marks the end of the use of fossil fuels

In terms of technical development, the scientist is focusing first on power-to-gas technology (PtG) for the production of gas from renewable electricity and then on its conversion into heat. As efficiency progresses, PtG must then be dispensed with. In numbers: If one assumes an annual heat demand of 15,000 kWh for an old building that has been renovated for energy efficiency, then 23,000 kWh of primary energy is needed annually if this is to be covered by PtG and a gas condensing boiler. If a CHP plant is installed downstream of the PtG, it requires around 38,000 kWh/a, but also produces additional electricity. With a gas heat pump, the electricity requirement used drops to 11 500 kWh/a, with an electric heat pump (i.e. then without PtG) with an annual performance factor of 3 to  5 000 kWh/a, and with one with an annual performance factor of 5 to 3 000 kWh/a. The most important measures for a sustainable heat supply are for Quaschning therefore the increase of the energetic building reorganizations to at least 3% per year, the advancement of heat pumps, the obligation to the solar energy use in new buildings and the development of a 100% regenerative power supply by 2040. At the same time, the operation of oil and gas boilers as well as fossil-fired CHP plants will end. If remaining operating times are taken into account, the last such plants may then be installed around 2030. For Quaschning, it is already true that PtG plus CHP is a combination of technologies that cannot generally be used to cover the heat demand. This is because, according to all calculations, PtG technology thereby increases electricity demand to such an extent that it could no longer be covered by renewable sources. Electricity-led CHP systems are nevertheless not yet completely on the brink of extinction. Together with a strong expansion of PV systems and electricity storage, with renewable gas, heat storage and electrically driven heat pumps, they are still a back-up system for electricity gaps when wind and sun are not available as the main energy sources. For Quaschning, cogeneration based on biomass or incorporating geothermal energy will then also still have a justification, while cogeneration based on fossil fuels or purely heat-led will no longer.