Carnot Batteries as a Power-to-X-to-Power Alternative : A Study on Optimization of Thermal Energy Storage Usage
Abstract: The implementation of renewable energy sources continues the rise throughout the world as a means to combat the threats of climate change. This does however bring with it problems within the power sector. The intermittent nature of both solar and wind power means that the reliability in production that comes with many fossil-based power sources could be heavy reduced. With the addition of energy storage, the intermittency can be reduced as it allows for production to be shifted to better meet the demand. This thesis aims to evaluate how well the Carnot battery system can enable a more stable production from a wind farm as well as how it can be used to increase the economics of the same wind farm. Two Carnot battery systems have been simulated, one utilizing a molten salt thermal energy storage (TES) and one utilizing a packed bed TES. A wind farm in Jänschwalde, Germany, has been used as a reference for the power production which charges the storage unit. The wind farm was scaled up to 200MW. The storage was discharged through a steam cycle. The models were built in MATLAB and uses CoolProp for fluid properties. Two operational strategies have then been applied to the system. The first aimed to achieve base load like production, a certain power output target is set and the stability of the system, defined as production within ± 10 % of the target, is calculated based on the storage size. Optimization of the system was done by varying both the storage size and output target to find the best configuration, with the highest stability and levelized cost of energy ratio. The second strategy aims to increase the monetary gain of a set system with 6 hours of storage and a steam cycle with an electric capacity of 50 MW. The optimization was done by varying the price limits of electricity at which the TES would be charged and discharged and finding the combination that yielded the highest income. This optimization was done on a daily basis over the whole modelled time period. The results show that it is possible to achieve 83 % stability when utilizing a packed bed TES with a capacity of 22 hours and a target output of 70 MW, while the utilization of a molten salts TES with 18hours capacity and a target output of 80 MW achieved a stability of 77 %, the levelized cost of energy of the two storage systems were 32.30 and 40.74 €/MWh respectively. The addition of 6 hours of storage and a 50 MW steam cycle could increase the yearly system income by 16.4 % and 10.9 % for the molten salt and packed bed systems respectively when following the economic operational strategy. Additionally, by running the optimization on a daily basis instead of a weekly basis the added income was significantly higher for both systems. Finally, it was concluded that base load production was not achieved by either system as the stability did not reach 90 %. However, by lowering the target output and allowing some curtailment to lower the overproduction it would be possible to achieve by both systems. The packed bed technology is recommended for the base load application as the lower capital costs are favourable. When optimizing the economic strategy, the heat price plays an important role as to which technology is preferable as the molten salt technology can better utilize the ability to sell heat as it can charge and discharge simultaneously. Therefore, in systems that sell heat when discharged the molten salt storage is recommended while if no heat can be sold the packed bed is favourable due to the lower investment costs.
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