Battery cycles

There is a coarse trend for the battery cycles, following the size of the battery storage system, figure 30. When the battery storage system is larger the number of battery cycles decreases. A smaller battery system gives a higher number of battery cycles. However, this trend is not linear.

of battery cycles increases with a larger hydrogen storage. The largest number of cycles is 212 whereas the lowest number of cycles attained in a simulation was 144.

Figure 30: Number of battery cycles for different energy storage configurations.

7.3

Additional results

Other results from the simulations that are worth mentioning are: average grid loss and storage usage. The average grid loss remains the same for all 24 simulations of the performance analysis, with a value of about 4.7 kW.

The trend for the battery storage usage show that in a majority of the simulations the battery storage is used to a full extent. This implies that the outer limits for both maximum and minimum SOC are reached at least once during a simulation. But it should be noted that although the maximum SOC is reached in all simulations, the minimum SOC is not reached in all cases. In a couple of simulations, the difference between the lowest measured SOC level and the minimum SOC constraint, SOCBmin2, reached as much as 16.5%. This suggests a lower utilisation of the battery storage for such storage configurations.

In contrast, the hydrogen storage is never utilised to a full extent in any of the 24 simulations. Hence, the tank is never entirely discharged to its outer minimum SOC-limit. The difference between the lowest observed SOC levels and the minimum SOC limit constitutes about 11.5% of the available storage space. This trend is consistent for all simulations. In contrast, larger variations in actual- and maximum SOC levels are deduced. The results indicate a excess in storage space ranging from 3.5 to 7 % depending on storage configuration.

8

ANALYSIS

This chapter contains the analysis of the results. In the first section of this chapter the results from the performance analysis simulations are analysed. In section 2, the performance of the different storage configurations is compared to determine a nominal storage configuration. This configuration serves as a reference case for the extended analys, which is presented in section 3.

8.1

Performance analysis

As can be seen in figure 24 the LPSP is lower when both the hydrogen and battery storage systems are larger. This means that isolated microgrids comprised of larger storage systems, i.e., more storage units, are more likely to be able to cover all the loads and attain a higher grid reliability. In the figure it might seem as if the size of the hydrogen system has a higher impact on reliability than the size of the battery system. However, this is very difficult to decide. Since the units of battery system and hydrogen system were shaped independent of each other they are difficult to compare. In addition, the y- and x-axis have different units since battery storage, y-axis, is represented with regards to its energy capacity whereas the hydrogen storage, x-axis, is represented through the fuel cell and electrolyser power level.

The trends showing in the figure for LPSP are similar to those showed in figure 25 displaying the results for EE. As for LPSP, the value for EE is lower with a larger storage system. The hydrogen storage is most probably over-dimensioned in this configuration in accordance with the results of storage usage, see section 7.3. These results verify a consistent redundancy in hydrogen storage space throughout the simulations. Hence, it is mainly the maximum power of the electrolyser and the capacity of the battery that limit the ability to store any surplus electricity.

8.1.2 ACS

The annualized cost of system, figure 26, is mainly dependent on the size of electrolyser and fuel cell. This is probably due to the higher investment cost, replacement cost and shorter lifetime of these components, compared to the battery. The lifetimes of the fuel cell and electrolyser are calculated through utilising the run time measurement. A shorter run time per year entails a longer lifetime in years. As can be seen in figure 29 the run time of both the electrolyser and the fuel cell are mainly dependent on the size of the battery storage system. When the battery system is larger the run times are shorter. The consequence of this is that the costs of the battery system and the hydrogen system balance each other. When the battery system is larger the cost

and electrolyser decrease, since the lifetime increases. This could be why variations of the battery storage system size seem to have little effect on the ACS, figure 26.

8.1.3 Loss ratio

The loss ratios of all the different configurations are very similar, around 56-57%, figure 27. The major part of the losses is due to conversion losses in the electrolyser and the fuel cell, since the round-trip efficiency is very low, around 33%. The coarse trend that can be observed in the figure can be explained by diminishing conversion losses when the size of the hydrogen system is reduced. Also, the larger the battery system is, the less the hydrogen system is engaged in grid operation which results in shorter run times for the electrolyser and fuel cell respectively. This allows more energy to be stored in the battery, which has significantly higher energy conversion efficiency. Thus, the lower right corner of the diagram shows configurations with the largest losses and the upper left corner shows configurations with the lowest losses. The slight decrease of loss ratio when comparing the second row from the bottom of the diagram to the ones above and below can be related to the electrolyser run time, which shows a very similar trend, figure 29a.