Verification of the Proposed Deterministic Algorithm

In the previous section, the renewable energy generation resources together with the electrolyser, hydrogen storage and fuel cell have all been sized using the proposed deterministic algorithm for the case study under consideration. To assess the suitability of the system sizes that has been calculated using the proposed deterministic method, an overall system simulation is conducted. The simulation of the sized system, for scenario E, is conducted using the component models described in appendix A. In scenario E (chapter 3), the surplus renewable energy that is not consumed by the load is absorbed by the electrolyser and converted into hydrogen gas for storage. Conversely, any energy that cannot be met by the renewable resources is supplied from the storage through the fuel cell. The results of the overall system simulation, sized by the proposed deterministic method, are shown in Table 4-5.

Item Value

Total Load Demand 1,049,507 kWh Total Wind generation 1,176,707 kWh Total PV generation 57,668 kWh Total Renewable Generation 1,234,375 kWh Renewable generation not utilised 136,749 kWh Renewable energy electrolysed 482,322 kWh Load demand supplied from fuel cell (store) 269,779 kWh Load demands not supplied 164,384 kWh Load demand supplied from renewable generation 885,083 kWh

% RES not used 11% % load demand supplied from renewable generation 84% % Load demand not satisfied 16% Average electrolyser efficiency simulated 59% Average fuel cell efficiency simulated 50% Overall storage efficiency simulated 29% Storage capacity at start 100%

Storage capacity at end 8% Storage capacity minimum 0%

Table 4-5: Performance of HRHES sized using the proposed deterministic method

The simulation results indicate that the integration of hydrogen energy storage significantly improves the utilisation of renewable energy generation by 39%. This is found from the difference between the percentage of unused renewable energy generation (i.e. constrained) when HEST has not been utilised and the unused renewable energy generation when HEST is utilised (50%-11% = 39% as taken from Table 4-3 and Table 4-5 respectively).

Additionally 84% of the load demands are met by the renewable energy sources when HEST is applied. This represents an increase of 34% from when HEST has not been applied to the energy system. Unfortunately, it is not possible to absorb all the renewable generation by the energy storage system in any of the following three circumstances:

1. When the storage tank is full and no additional hydrogen can be stored

3. When the surplus renewable energy is less than the minimum starting power of the electrolyser (20% of the name plate rating for an industrial alkaline electrolyser)

The average overall storage ‘turn-around’ efficiency, for the given system case study is found to be 29.5%. This is found by multiplying the electrolyser and fuel cell efficiencies to find the overall turn- around efficiency (59% x 50%=29.5%). The ‘turn-around’ efficiency is a measure of the losses from the energy input to the electrolyser to the energy output through the fuel cell. The value of 29.5% calculated is very low; however, a typical Proton Exchange Membrane (PEM) fuel cell can only recover the Lower Heating Value (LHV) of hydrogen gas and not the Higher Heating Value (HHV). This represents an additional efficiency penalty of approximately 16% in turn around efficiency when using HEST as described by scenario E. To mitigate the case where the demand of the load is not met by the Hybrid renewable hydrogen energy system, the demand could be either imported from the power grid, or a stand-by generator can be installed.

4.5.1 Effect of Storage Capacity at Start of Simulation

The impact of the storage state (empty and full) before starting the simulation has also been examined. Comparison results are shown in Figure 4-6. The utilisation of the renewable generation is found to be slightly improved if starting with a partially filled storage tank. However, a greater portion of the load demand remains unmet due to the storage system becoming empty at times. Therefore, it can be concluded that the best scenario for supplying the load is when the storage is full from the beginning.

Figure 4-8: Effect of energy storage state at simulation start

4.5.2 Effect of Using a Smaller Storage Tank

The simulation has also been conducted to investigate the impact of reducing the storage tank size. The results are shown in Figure 4-9. By reducing the storage system size to become 1% of the originally defined size, a renewable energy utilisation factor similar to that of having a large storage system is observed. On using a much smaller storage tank, surplus renewable energy can still be absorbed, however, and as expected, the load demands that are achievable by the overall energy system are reduced considerably from 84% to 66%. Nonetheless this is still a 7% improvement from having no storage present at all. This is found by repeating the HRHES simulation as described previously in section 4.4; however the size of hydrogen storage tank is reduced to 3,057m3 _{(1% of the}

size of storage tank given in Table 4-4).

11% 84% 16% 8% 74% 26% 8% 64% 36% 50% 59% 41% 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% % Renewable generation not used % load demand supplied from renewable generation % Load demand not satisfied

Effect of energy storage state at start

Figure 4-9: Effect of reducing storage size

In addition the storage system is utilised much more when the hydrogen storage tank is reduced in size. The impact of the storage systems size on its utilisation can be seen in Figure 4-9.

11% _10% 50% 84% 66% 59% 16% 34% 41% 0% 10% 20% 30% 40% 50% 60% 70% 80% 90%

100% store size 1% store size no storage

Reducing storage size

% Renewable generation

Figure 4-10: Impact of the storage systems size on its utilisation - storage level comparison