- A mini gas turbine complements the ESI Platform at PSI
Renewable energy sources are to be expanded in Switzerland and to replace nuclear power. That was the decision of the federal government in its Energy Strategy 2050. At the Paul Scherrer Institute PSI, researchers are investigating technologies that improve the utilisation of new forms of energy through efficient further use of surplus electricity, for example, from wind and solar plants. Hydrogen plays an important role here. It can be produced by means of power-to-gas technology from excess solar power and used in fuel cells or injected and stored – though currently only in a very small amount – in the existing natural gas network. Through a further step, it can also be converted to methane, the main constituent of natural gas. PSI researchers are now using a mini gas turbine to test how the hydrogen content in the natural gas mixture can be increased.
With the ESI Platform at the Paul Scherrer Institute PSI, researchers are seeking solutions for Switzerland's energy future. ESI stands for Energy System Integration. The platform makes it possible for specialists to test different alternative energy systems, alone or in combination, in close proximity. The researchers are not working directly with wind turbines or solar cells at ESI, but they are devising solutions that could allow surplus electricity from wind and photovoltaics to be reused and stored as efficiently as possible. That is why PSI has added another component to its ESI Platform: a miniature gas turbine. This converts methane, which can be produced for example from biomass, to electricity and heat. This is particularly useful when solar cells and wind turbines are providing only small amounts of power.
Methane is the main constituent of the fossil natural gas that's been used up to now. Therefore, the infrastructure of the natural gas networks is designed for methane. If we, at the ESI Platform, use methane from alternative energy sources, we can maintain the existing infrastructure and make the change more rapidly, explains chemical engineer Peter Jansohn, who is responsible for the ESI Platform.
Balancing current fluctuations
In Switzerland, the renewable energy sources wind and sun are only sporadically available: High demand for energy doesn't always happen to arise when a strong wind is blowing or the sun is shining. So at times of high wind or sunshine, excess energy that isn't immediately needed anywhere has to be stored. Today this is already possible to some extent in pumped hydro storage power plants, where surplus electricity is used to pump water back into the upper reservoirs. With the increasing proportion of wind and solar power, however, the existing pumped hydro storage power plants are reaching their limits and can no longer absorb all current peaks.
Another storage option is hydrogen. The gas can be produced through power-to-gas technology in a so-called electrolyser, which breaks water down into oxygen and hydrogen. This hydrogen, obtained in an environmentally friendly way, can either be used as fuel in fuel cells or, when mixed in with natural gas, drive turbines that feed electricity back into the grid at the desired time. But at present this technology still has limits. In Switzerland, no more than two percent hydrogen is permitted to be added to natural gas. This is because hydrogen burns differently, is highly reactive, and can lead to material problems in gas turbines.
Methane as an energy source
One alternative is therefore further processing of the hydrogen: Together with carbon, it forms methane. This can be added to natural gas with practically no limits. Furthermore, methane can be obtained directly from biomass, available practically everywhere as organic waste: either in a dry form such as wood chips or in aqueous solutions, as in manure and sewage sludge. The big advantage of methane: It is easier to store than hydrogen, since for the same amount of energy, methane requires only one-third of the storage volume needed for hydrogen. However, the additional step needed to obtain methane from hydrogen costs energy and thus reduces the efficiency of the overall process.
Hot exhaust gas drives turbines and produces hot water
To generate power from methane without the system losing more energy, you need efficient gas turbines that deliver not only electricity, but also hot exhaust gases for heat production, Jansohn says. That is precisely what the new device on the ESI Platform does. The incoming air needed for combustion is compressed and preheated by the warm exhaust from the turbine. Then it is mixed with the methane as warm combustion air. The hot gas produced through combustion alone is responsible for driving the entire machine, the chemical engineer explains. The hot gas moves the turbine wheel and the compressor wheel connected with it, which compresses the air. Fixed on a single shaft, the two wheels are directly coupled to the alternator that produces the electrical energy. The electricity generated in this way flows either into the power grid or back into other subsystems of ESI, where it is then available for other processes. With that, the power-to-gas-to-power cycle is closed – that is, the demand-based conversion of electricity into gas and back into electricity.
Exploiting the exhaust heat already inside the turbine increases its electrical efficiency by 30 percent. In addition, the residual heat also serves PSI: Via an additional heat exchanger, the turbine contributes in small measure to the institute's hot water supply. If residual heat can be used to produce hot water, the overall efficiency increases to an impressive 80 percent.
Small and compact
The miniature gas turbine in its housing is no bigger than a wardrobe, delivers an electrical output of 100 kilowatts, and perfectly complements the other systems of ESI. We set ourselves a goal of testing all subsystems of ESI on a 100-kilowatt scale, so that the power demand of a small neighbourhood with around five to seven single-family houses could be met, Jansohn says. This approach fits in with the current trend to decentralise the power supply. The mini gas turbine is a commercial product and a typical cogeneration system that can be used in hotel facilities, housing estates, or campgrounds to supply the local infrastructure with power and heat. Jansohn and his team at ESI are testing the small gas turbine at PSI under research conditions. For this they have a clear mission: to improve the efficiency of new systems driven by renewable energy.
More hydrogen in natural gas
The chemical engineer is especially interested in the chemical aspect of the energy supply. This always leads him back to hydrogen. In the future, hydrogen can be obtained relatively easily. If we can use more of it in the natural gas mix, we do not need a further conversion step, says the researcher. His earlier studies with other gas appliances have shown that up to 20 percent hydrogen can be added to methane without damaging the material due to local overheating. Therefore he now wants to find out, together with project leader Dominik Ebi and his team, how much hydrogen the small gas turbine will tolerate and how, with a higher hydrogen content, it will respond to electricity demand peaks. Since hydrogen is highly reactive and burns very quickly, the turbine might respond even better to rapid load changes, the scientists hope. A look into the past shows that the natural gas network could be made fit for high proportions of hydrogen: Until about 50 years ago, urban gas was widespread in Switzerland and contained up to 50 percent hydrogen.
Text: Sabine Goldhahn
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