As ever more significant volumes of renewable power enter the power grid, it is obviously a great benefit for the drive to a low carbon future. This growth of renewable energy puts a significant financial strain on the traditional operation of natural gas-fired power plants. These power plants are crucial to the security of electricity supply and grid stability, producing electricity when the sun is not shining, or the wind is not blowing and topping up the grid during periods of peak demand.
One solution to enable these power plants to continue to operate efficiently is to reduce their carbon footprint by burning a blend of green hydrogen and natural gas while coupling them with an electrolyzer to produce the green hydrogen when there is renewable power to spare. The benefits of this mode of operation is that it ensures that the power plant is flexible and future proof. As the renewable input grows and continues to diminish the role of traditional power plants, it can transition over to produce hydrogen and increase the percentage in the fuel it burns until it reaches 100 percent. This fuel flexibility enables efficient optimisation and operation of the plant.
Therefore, such a combined-cycle gas power plant coupled with a proton exchange membrane (PEM) electrolyzer and hydrogen storage is an integral step on the path for the full-scale commercialisation of the hydrogen value chain. The current vision is a combined cycle hydrogen power plant that would utilise renewable energy or spare capacity to generate green hydrogen using PEM electrolyzer technology.
Pathway to a hydrogen future
It is also possible to upgrade existing units to operate on a hydrogen co-firing mix by the following steps: First, the combustion system of the gas turbine needs to be upgraded for a higher percentage of hydrogen burn before incorporating a natural gas and hydrogen fuel blending station and ensuring all other relevant systems of the plant are hydrogen capable as well. Next, to produce the hydrogen on-site, a Silyzer 300 modular PEM electrolyzer can be added along with a hydrogen pipeline or storage solution.
The Silyzer 300 is a powerful electrolyzer in the double-digit megawatt range by Siemens Energy. Its modular design enables economies of scale effects to minimise investment costs for large-scale industrial electrolysis plants. The optimised solution results in low hydrogen production costs due to high plant efficiency and availability.
Part of the future development is to increase efficiency, reduce costs and scale up the electrolyzer portfolio. In June 2020, Germany became one of the first countries to publish a national hydrogen strategy (Nationale Wasserstoffstrategie). The strategy states that hydrogen is another critical technology for reducing overall emissions. The plan, as the project title suggests, is to field electrolyzer capacity in the gigawatt (GW) range. At present, the Silyzer 300 is a 17.5MW electrolyzer. The Silyzer is fully modular so the output can easily be increased by combining additional electrolyzers.
Aside from the use of hydrogen there are other steps that can be taken to improve the system’s overall efficiency. The plant performance is optimized with the Siemens Energy Omnivise intelligent control system. Finally, an optional heat recovery system with heat pumps and thermal storage can utilise the waste heat and increase the overall plant efficiency.
As a combined cycle plant, it already utilises the waste heat from the gas turbine, but additional efficiencies can be gained by also using the waste heat produced by the electrolyzers. Using a high-temperature heat pump the waste heat can be fed into district heating systems which only require input heat in the range of 150C.
Making sound economic sense
The system that we have based our example on uses a Siemens Energy SGT5-9000HL gas turbine that can already burn a fuel mix containing up to 50 percent hydrogen. This turbine can be paired with 9 x Silyzer 300 electrolyzers and large scale H2 storage.
For our calculations we have taken what we consider a typical day with optimum weather. Over the 24-hour period the electricity price varies dramatically based on demand and the availability of renewable energy. In the hours from midnight, the electricity price is €70/MWh, it then peaks at around €100/MWh in the morning and evening high -demand periods, dropping as low as €50/MWh at noon. In the future, as the volumes of solar power increase, the variation over the 24-hour period would be even more dramatic. With optimum weather conditions and the sun shining brightly, the price does not climb above €10/MWh between 9 am and 5 pm, and even dips into negative figures for five hours during the day, reaching a low of -€40 in the early afternoon. During these nine hours, the average electricity price is -6.8 euros/MWh.
Not an ideal scenario for a power plant operator. However, if that power plant was coupled with electrolyzers to produce hydrogen, the picture is far more advantageous. During the nine hours of low prices, the power plant could be shut down and power from the grid could be diverted to the electrolyzers to produce green hydrogen, which could be stored and used for co-firing in the gas turbines later saving on fuel, increasing grid earnings through the provision of primary frequency control by both the power plant and the electrolyzer, and reducing CO2 emissions.
This changes the revenue outlook dramatically. Under the assumption that the natural gas price is €8/MMBTU, and the current rate of CO2 tax is €55 over the day, there is a total daily savings of €45,401. This comprises €23,431 saving on fuel, an extra €11,425 in grid earnings and a reduction of CO2 tax of €10,545. Aside from the financial benefit, there is also improved environmental performance with a reduction in CO2 emissions of over 190 tons a day.
There are other options for using this spare energy, the most feasible being electricity storage of which there are various options including batteries, flywheels, and supercapacitors. While the storage of excess energy in a battery storage solution would be a more efficient and cost saving alternative the size of battery storage required to handle the amount of energy over the day, weeks and months when the energy price is attractive would not be practical. Batteries are an ideal option to top up the grid at times of peak power, but it is not feasible for providing GW-sized electricity to the grid for six or more hours per day. When handling electricity storage at that volume the most realistic option is chemical storage utilising hydrogen.
This solution is not only relevant for large-scale power plants as it was presented for the SGT-9000HL turbine. The optimised standard integration of hydrogen production, storage, re-electrification, and heat recovery can be adapted to power plants of all sizes. A system using the SGT-400 gas turbine can cater for outputs as low as 10 MW. Particularly in Europe, which has smaller district heating systems, this size can still be effective. It also complements the growing trend toward smaller, decentralised power generation.
