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How Germany’s renewable energy supply targets can be achieved

In 2015, almost a third of Germany's electricity came from wind, sun and biomass. We need to continue reducing CO2 emissions to become even less dependent on fossil fuels such as coal and petroleum, and thus make electricity generation even more climate friendly. Baden-Württemberg has set an ambitious target for the shift in direction from nuclear and fossil fuels to renewable energy sources: reducing current energy consumption by 50% and replacing 80% of the energy used with energy generated from renewable sources, thus reducing greenhouse gas emissions by 90%.

Dr. Müller-Sämann in front of a field of Miscanthus. © Müller-Sämann private

BIOPRO spoke with Dr. Karl Müller-Sämann from the Agentur für Nachhaltige Nutzung von Agrarlandschaften (Agency for Sustainable Management of Agricultural Landscape) in Freiburg about the implementation of these goals. Dr. Müller-Sämann, along with colleagues in a multidisciplinary working group, has developed a novel energy concept that involves growing Miscanthus instead of corn and rape on German fields to mitigate the power fluctuations of solar and wind power systems.

Why can't we just expand wind and solar power systems, which are already firmly established?

In 2015, Germany met 12% of its electricity demand with wind power and 6% with photovoltaics. An increase to 70% is possible, as energy from wind and sun is available in almost unlimited quantities. However, the energy produced with wind and solar power systems has the key disadvantage that it is not controllable and undergoes huge fluctuations. Supply and demand are often far apart. Wind turbines and solar cells produce no energy on misty and windless days. However, on windy and sunny days they produce excess energy, which in turn leads to negative prices, and electricity producers pay consumers to buy their electricity. This, of course, isn't good business.

What can be done to solve the problems associated with the intermittent nature of wind and solar energy?

The ideal solution would be to store the surplus energy. However, inexpensive batteries with appropriate energy storage capacities are not likely to be available in the foreseeable future.

So a buffer is required, an energy source that can compensate for fluctuations in wind and solar energy. This energy source must be controllable and be able to match supply and demand within seconds. At the moment, we use fossil energy as a balancing energy, which is harmful to the climate. It would be much better to use biomass, which is storable and can also supply electricity on demand with the appropriate technology in place.

Biomass is already being used in energy production. In 2015, around 8% of the primary energy came from biomass. Why are corn and rape unsuitable as energy buffers?

Electricity supplied through the grid is a cheap product. Electricity produced from coal and nuclear power plants only costs about 3-6 cents per kilowatt hour (kWh). In comparison, electricity produced from biomethane is too expensive to be used as a balancing energy. It costs 25 cents per kWh and can only be used in combination with fossil-based natural gas. Inexpensive balancing energy can be produced from biomass if the right plant species and a specifically adapted method for the plant is used.

What kind of plant do you have in mind?

Silvergrass, also known as Miscanthus x giganteus. It comes from East Asia, can grow up to four metres high and is the most efficient energy plant currently known. It grows in the same places as corn. At present, only one variety of Miscanthus is grown in Europe. However, the University of Hohenheim is investigating new genotypes in a project called OPTIMISC aimed at securing the positive properties of the energy plant in the long term.

What are the advantages of Miscanthus?

It has a long list of advantages: it is a rather frugal crop, requires little fertiliser, produces high yields and grows well on less fertile soils. No pests are known, which means that complex plant protection measures are not required. It is a perennial crop that can be grown and harvested for at least 20 years. The plant’s energy efficiency - that is, the energy you get when you deduct the energy that you need for ploughing, fertilising, general care and harvest - is much better than that of rape and corn. Miscanthus is superior to both rape and corn in all aspects mentioned.

And how can Miscanthus be used for energy generation?

Dried Miscanthus stems are harvested in spring. © ANNA Agency, Müller-Sämann

Miscanthus is typically harvested in spring, when the plant stems are dry and new shoots have not yet emerged. This eliminates the need for complex drying processes. The plants are shredded with a corn shredder and subsequently stored. We have called the method for producing energy from giant miscanthus TREMIS, Top REnewables from MIScanthus. It involves thermal gasification, i.e. the gasification of shredded Miscanthus grass at a temperature of 750°C in the presence of oxygen and water vapour. This procedure results in a carbon monoxide-hydrogen mixture that drives a gas engine that then generates electricity. The system allows engine waste heat to be used in various ways: for heating in winter, and for generating electricity via a downstream steam turbine in summer. We can make liquid fertilisers from the waste gases emitted from the gasification process. The temperature is kept at the right level to ensure that all nutrients such as phosphate, potassium and magnesium can be used as fertilisers.

How much power can be produced with a power plant of this kind?

TREMIS has the potential to be used in concert with wind and solar energy. An individual 5 MW power plant produces around 20,000 MWh of electricity in around 4000 operating hours, which is enough to provide a stable supply of power to around 10,000 households. For optimal use as a balancing energy, 100 power plants would need to be combined into a virtual power station supplying 500 MW. Power generation and value generation are decentralised, which means that farmers and the public benefit from the system at the location where the grass is grown and the power plants are operated. The idea behind the concept is that biomass does not have the energy density of coal. Therefore, transporting it long distances is not cost-effective. 

How many hectares does a 5 MW power plant need?

We need around 500 hectares of arable land per power plant. We would need more than twice this area to produce the same amount of power with corn.

And how can the systems be used as energy buffers?

Each TREMIS system runs at full load, which is the most economical way of doing it. Power demand is controlled by the number of systems operated, which can be switched on and off according to demand. If a lot of electricity is required, all systems will be running; if only small quantities of electricity are required, then only a few systems will be running.

Can your concept provide all the balancing energy needed?

No, this is unrealistic. We do not have an unlimited amount of biomass since we have only limited agricultural land, currently around 12 million ha. At present, 2.5 million ha are used to grow rape and corn and therefore we would be competing with land use for food and animal feed. As a general rule, I would therefore say that we should not produce normal power with bioenergy if this power could also be produced by wind and solar power stations. We would need to grow Miscanthus on around 3 million ha in order to cover all the demand for balancing power. A realistic amount of land that could be used for growing Miscanthus is one million ha. This would cover around 30 percent of the balancing energy required.

And the remaining 60 percent?

A large proportion can be provided by increasing the efficiency of industrial processes, something that Baden-Württemberg is working on, but also changing old household appliances for new ones in the private sector. The rest needs to be covered by other energy sources such as hydropower.

It sounds like a well thought-out concept. How far have you come?

We are currently looking for investors to help us build the first demonstration plant and we have developed a step-by-step plan. If we stay on schedule, we could start serial production of our TREMIS plants in 2025.

How much will the pilot plant cost?

Approximately eight million euros for the first plant, and then later when we have started serial production, 5 million euros per plant

How much will one KWh of energy cost using Miscanthus?

According to current calculations, we believe that one kWh will cost around 10 cents.

Schematic set-up of a TREMIS balancing power plant unit.
Schematic set-up of a TREMIS balancing power plant unit. © TREMIS work group, Müller-Sämann
Website address: https://www.biooekonomie-bw.de/en/articles/news/how-germanys-renewable-energy-supply-targets-can-be-achieved