• LPSP is lower when both the battery and hydrogen storage are larger.
• EE is lower when both the battery and hydrogen storage are larger. The over-dimensioned hydrogen storage imply that the electrolyser power and the battery capacity are the limiting factors for storing energy.
• ACS is mainly dependent on the size of the electrolyser and the fuel cell, due to higher investment costs and shorter replacement times. The cost of battery system and the cost of replacement of fuel cell and electrolyser seem to cancel each other out.
• Loss ratio is similar, 56-57%, for all simulations in the performance analysis, mainly due to the low round-trip efficiency of the hydrogen storage system, 33%. The losses are slightly lower when the battery capacity is large and the hydrogen system small.
• Runtimes, FC and EL, decrease when the battery capacity increase. The variation for the fuel cell, 20-28%, is larger than for the electrolyser, 58-60%.
• Number of starts, EL, decrease when the battery capacity increases.
• Number of starts, FC, increase when the battery capacity increases, the opposite trend compared to the electrolyser.
• Battery cycles decrease when the battery capacity increases.
9.2.2 Control and operation
Control at different levels in the system is required to achieve an overall effective grid operation and a functioning coordination between energy storage systems. Different controllers handle different parts of the coordination. A PCS can manage primary and secondary control, while the
i.e. energy management, and is responsible for the overall management including forecasting and optimising power set points.
The operation strategy is formed in line with the objectives established by the stakeholders of the project. These can be technical, economic and environmental. An operation strategy can be rule-based or based on optimisation algorithms. It can be valid on different time scales, which have to take different aspects, like dynamics and transients, into account.
The control strategy of this thesis connect the battery and fuel cell operation very closely. Changing the power level at which the fuel cell charges the battery to a lower level than maximum power could have profitable impacts on the fuel cell operation and lifetime. The SOCB limits,
which strongly govern the energy management of the system, are essential and how they are set affect the sizing of the system. The energy management strategy of this thesis is rule-based. For implementing a real HESS system it might be good to explore the concept of using optimisation algorithms and forecasts, thus including a long-term energy management. Hence, economic and environmental objectives can be accounted for to a higher extent and the operation of electrolyser and fuel cell could be enhanced and optimised, as unnecessary starts and stops could be avoided.
9.2.3 Sizing and design
The sizing and design of a microgrid with renewable energy sources and a hybrid energy storage system can be achieved through evaluating performance indicators reflecting the objectives established for the grid operation. In addition, physical and technical system constraints should be defined. These often include maximum and minimum charge and discharge power levels, and limits of state of charge of the storage components in the system. How the control strategy is implemented has a large impact on the operation of the components and thus their sizing. For the simulations conducted in the thesis the ratio between fuel cell and electrolyser maximum power is kept constant, at 1:2. It could be interesting to vary this ratio since it might adapt the storage system to the rest of the Simris grid in a better way. It was necessary to increase the power production of the system to be able to analyse the behaviour of the storage system. This was done by adding a second wind turbine, identical to the prevailing turbine in Simris. An alternative measure could be to increase the solar power production, which would probably have large impacts on the results.
9.2.4 Performance evaluation
For evaluating the technical performance, LPSP, Loss of power supply probability, is a common measurement to evaluate grid reliability. EE, excess of energy, could be interesting to consider in the future, but as the market situation is today it is not considered viable to dimension the system based on this indicator. Loss ratio can be used to evaluate the energy efficiency of the system. The runtimes and number of starts and stops for the fuel cell and electrolyser can give indications of the individual component operation, rate of degradation and expected lifetimes. Battery cycles can be used to study the operation of the battery, but a more accurate method than the one used in this thesis would be required to draw any quantitative conclusions. For evaluating the economic peformance, ACS, annualised cost of system, is a common measure. In this thesis, this indicator only comprise the costs of the storage systems to make an economic
comparison between different storage configurations. To attain a quantitative measure of the entire system cost, a more comprehensive cost analysis would have to be conducted which takes all system components into account.
To design a microgrid while allowing an LPSP higher than zero can be questioned, but a system where this has to be zero would likely have to be over-dimensioned. Solutions to avoid LPSP higher than zero could be to utilise DSR, demand-side response, or to connect a portable reserve generator if the critical periods are known. The loss ratio of the system investigated in the thesis, due mainly to conversion losses, would be smaller if the hydrogen storage capacity was smaller and thus allowed less energy to enter the hydrogen system. The system could be self sufficient with a smaller storage, but more energy would be lost as EE. The economic viability of hydrogen storage systems of today depends on what surroundings and alternative power supplies the microgrid has access to. If more systems are implemented and investigated the learning curve might improve. If a system perspective is applied and for example heat could be recovered from the fuel cell, hence improving the economic viability of the solution.
9.2.5 Concluding remarks
Hybrid energy storage systems (HESS) comprising a hydrogen and a battery storage add flexibility to microgrids with a high share of renewable energy sources, such as wind and solar. The grid reliability, the main indicator for technical viability of the solution, increases with an increase in size of the energy storage systems i.e. the energy capacity of the battery and the power capacity of the electrolyser and fuel cell. The technical performance is also evaluated with regards to power losses, which is constantly very high, about 57 %, for all twenty-four simulations of different storage configurations. This is due to a large energy conversion loss in the electrolyser and the fuel cell resulting from low conversion efficiencies for these processes. This required an additional electricity production unit, a wind turbine, to achieve energy balance of the system and to conduct the simulations. Thus, the applicability of the proposed HESS in Simris can be questioned. Furthermore, the hydrogen storage system has a negative impact on the economic viability of the HESS solution due to a high capital cost of the electrolyser and fuel cell. In combination with the short lifetimes of these components, resulting in high replacement costs, these components constitute a dominant share of the total annual cost of the system.
Hence, the opinion of the authors is that the technical and economic viability of a hybrid energy storage system (HESS) with a hydrogen and a battery storage, must be evaluated specifically for each case. The system performance must be compared to other prevailing flexibility options and alternative costs to properly evaluate the applicability and suitability of the proposed solution.