In order to ensure the reliable supply of power in Germany during the energy transition and to compensate for fluctuations in renewable energies, the flexibility of thermal power stations will become extremely important in future, particularly in partial load operation. Coal power stations are already predominantly used to compensate for fluctuating renewable energies on account of their flexibility.
A study by renowned consulting firm Pöyry Management Consulting, presented by the German Coal Importer Association (VDKi), Hamburg/Germany, investigated the direct and indirect greenhouse gas emissions generated during electricity production at coal and gas power stations. Partial load operation, which is particularly important for compensating for fluctuations in renewable energy supplies, was also taken into account. Within the scope of the analysis, extensive international studies of emissions in the extraction and transport of coal and natural gas were compared and evaluated. If these indirect greenhouse gas emissions are added to those generated during electricity production in the power stations, taking into account coal and gas procurement for Germany in 2014, it was revealed that in a partial load operation scenario the direct greenhouse gas emissions from electricity production in open gas turbines is up to 76 % higher than in modern coal power stations. The difference between greenhouse gas emissions in modern coal power stations and gas-and-steam power stations, without heat extraction, drops from 36 % under full load to 30 % under partial load.
If we include the greenhouse gas emissions generated during the extraction and transport of the two energy sources, partial load electricity production in modern coal power stations, combined with the varying electricity demand for current -German power station complexes, is clearly a much more environmentally friendly alternative to open gas turbines for compensating for the fluctuating supply of renewable energies. Although these turbines are only a short-term provision for load compensation, they cause significant losses in energy efficiency and have an adverse impact on the carbon footprint. Even in the case of direct emissions, without taking into account the extraction and transport of the fuel, an open gas turbine produces up to 29 % more greenhouse gases during partial load operation than a coal power station.
In the ongoing discussion regarding the best bridge technology during the energy transition, natural gas is currently the preferred energy source among politicians and members of society due to the fact that it is purported to have a better carbon footprint. The results of the Pöyry study indicate, however, that this is based on incorrect assumptions. When it comes to achieving global climate targets in particular, indirect emissions that are generated during the extraction and transportation of the various energy sources must also be taken into account. When considered holistically, particularly in terms of partial load operation, coal is actually a better option than natural gas, which has been assumed to be more environmentally friendly.
In addition to CO2 emissions, the Pöyry analysis also considers the emissions of the greenhouse gas methane, which are produced during the production, transport and processing of both shale gas and conventionally extracted natural gas. Over a period of 100 years, methane has 28 times the global warming potential of CO2. After the world climate conference in Paris,/France when it became clear that there was an urgent need for action, Pöyry based its CO2 equivalent calculations on a period of 20 years. In this case, the global warming potential of methane is actually 84 times higher than that of CO2.
The key components in the emissions generated by electricity production are therefore the direct combustion process (CO2), the energy required for transport (CO2) and the amount of methane generated during extraction and as a result of leaks. The concentration of the greenhouse gas methane in the atmosphere has increased significantly since 2006. The Karlsruhe Institute of Technology (KIT) has recently proven that the extraction of oil and natural gas, particularly in the USA, is responsible for this. Following a leak in a large gas tank in the Aliso Canyon, California, at the start of the year, in which 77,000 t of methane was released into the atmosphere, the public became more aware of the relevance of this greenhouse gas, which is far more potent than CO2.
“As a result of the priority feed-in of renewable energy sources, fossil-fuelled power stations will increasingly take on the role of compensating for fluctuations and network stability. They will therefore be increasingly operated under partial load,” says Roland Lorenz, energy expert and Managing Director of Pöyry Management Consulting. “According to the results of the study, coal power stations are the more environmentally friendly alternative to open gas turbines in this load range.” Efficient gas-and-steam power stations generate electricity in parallel with the generation of heat, i. e. for district heating systems, and cannot therefore respond as flexibly to fluctuations as is necessary for the energy transition. In the current energy market, they are therefore built almost exclusively to meet heat requirements and not to compensate for load peaks. Only open gas turbines that are not connected to a steam process have the flexibility required for network stability during the transitional period until the development goals for wind and photovoltaic plants are met and until the storage problem has been resolved, although these do not perform as well as modern coal power stations in terms of efficiency and therefore greenhouse gas emissions.
The results of the study underline the political pressure to act: Electricity production from coal is just as suitable as electricity production from natural gas for use as a flexible bridge technology until the development goals for wind and photovoltaic plants are met and until the storage problem has been resolved. Public perception, however, is another matter. Political figures continue to advocate electricity production from natural gas, which supposedly has lower emissions. The problem is that the debate on greenhouse gases only takes into account direct emissions and the overall efficiency of highly efficient gas-and-steam power stations with power and heat diversion. It is important to distinguish, however, the type of load operation and technology required. Obviously the emissions benefits of coal power stations in partial load operation have not been considered. In order to secure the power supply in Germany in the medium term without increasing emissions while retaining the necessary flexibility in the operation of thermal power station complexes, all available energy sources need to be used economically. Fair competition must also be guaranteed between fossil fuels in order to protect consumers against further price increases. (VDKi/Si.)