Results of the background work and simulations

Table 2 provides results of the excess energy investigation for different power protection topologies and example configurations illustrated in Figure 5. The last column shows that as the redundancy level increases from nonredundant systems (N) to parallel and redundant systems (2N+1), also the excess energy in the battery systems increases. This is natural, as the amount of hardware (UPSs and battery systems) increases significantly as the redundancy level increases, but the load and autonomy requirements remain the same. The table shows that the 2N+1 example configuration has a total amount of 1333 kWh of designed energy capacity, while the 10 min autonomy requirement (in normal conditions) with the full 3 MW can be met with 500 kWh, as shown by the equation below:

500 kWh 3000 kW=1

6h = 10 min (6)

Table 2. Excess energy amounts inherent to different UPS system topologies (examples).

Design

Topology Number of 1 MW

UPSs Total amount of energy in

the battery systems [kWh] Excess energy in the battery systems [kWh]

N 3 500 0

N+1 4 667 167

2N 6 1000 500

2N+1 8 1333 833

A simulation was run to identify the total energy requirement of the FCR-D activation events and the state of charge behavior of the individual battery systems. The system configuration under investigation was the N+1 configuration, made up of four 1 MW UPS systems according to Figure 5. The simulations were performed with the current Nordic FCR-D requirements and with year 2015 frequency data from the Nordic power system and data from the UK power system.

Figure 7 shows the output charts based on the data provided by the simulation program.

The total energy requirement from the simulations is illustrated in Charts A (Nordic) and B (UK). The 167 kWh limit in the figures is the previously identified amount of excess energy capacity in the N+1 example case. Charts C (Nordic) and D (UK) illustrate the state of charge behaviors of the most stressed UPS batteries from the same simulations.

The 42 kWh limit in the figures is the amount of excess energy per battery systems (167 kWh divided by four).

The results show that while there were some more energy-demanding events during the year 2015, the average energy demand of the reserve operations was limited, and even the highest demand peaks remained well below the 167 kWh limit; however, the simulation results also show that the limits for the individual battery systems were met several times during the simulated year 2015.

Figure 7. Simulation results for a N+1 (4 x 1 MW) UPS system, total discharged energy amounts with the year 2015 frequency data from the Nordic (A) and UK (B) power systems, and the state of charge profiles of the most stressed UPS batteries Nordic (C) and UK (D).

Further simulations were performed with different price limits and aggregate sizes to investigate their effect on the number of cycles that the battery system is subjected to.

The price limits, aggregate sizes, and results (number of cycles for the most stressed battery systems) used in the simulations are gathered in Table 3.

The number of cycles was compared with the cyclic life expectancy characteristics of a commonly used UPS battery type (Figure 6). The depth of discharge (DoD) was limited to 42 kWh, which corresponds to 25% of the 167 kWh limit. Figure 6 shows that the cyclic life expectancy of a typical UPS battery with the DoD of 25% is 700 cycles.

Depending on the local grid stability, in the normal usage, the UPS batteries may encounter a few longer discharges and several shorter ones during a year of operation.

This can be assumed to equal ten cycles with the DoD of 25%. The service lifetime expectancy of UPS battery systems normally ranges from seven to eight years. This means that with the expected ten effective annual cycles, the accumulated cyclic usage of UPS batteries (in good grid conditions) is expected to be in the range of ~80 cycles (DoD 25%).

As a result, the UPS batteries can be expected to handle up to 600 cycles of demand response usage in addition to the cycles resulting from the primary operation. This means that the annual (additional) usage should not exceed 75 to 85 cycles (calculated with the seven- to eight-year life expectancy). The colors in the cells of Table 3 illustrate the combinations where the usage stays within the boundaries and is expected not to affect the lifetime of the battery systems. The last column of the table shows the relative income, which is calculated by summing the revenue for all the hours when the market price was above the price limit. The results show that by imposing a price limit, the wear and tear could be controlled while maintaining a significant portion of the market revenue.

Table 3. Simulated number of charge/discharge cycles within one year, with different aggregate sizes, minimum bid prices, and the effect of minimum bid prices on the relative income.

Price limit Aggregate size:

4 UPSs Aggregate size:

20 UPSs Aggregate size:

50 UPSs Relative income [%]

No limit 205 cycles 130 cycles 122 cycles 100 5€/MW 140 cycles 86 cycles 81 cycles 94 10€/MW 72 cycles 45 cycles 42 cycles 83 15€/MW 58 cycles 37 cycles 34 cycles 79

As a reference, Table 4 provides the calculated revenue estimations for a 1 MW of FCR-D reserve in the hourly markets during the years 2015 and 2018. The revenue estimations have been calculated by

𝑅 _, = 𝛼 ∗ 𝛽 ∗ 8760h ∗ 𝑃 , (7)

where 𝛼 is the bid acceptance rate (100%), 𝛽 is the asset availability (95%), and 𝑃 is the average market price. The full bid acceptance rate is used, as no price limit will be imposed on the bids.

Table 4. Calculated market income potentials for a 1MW FCR-D reserve unit (Fingrid, 2017).

Year Average market price [€/MW/h] Market income for a 1MW FCR-D reserve unit[k€]

2015 14.43 120

2016 5.15 43

2017 3.39 28

2018 5.31 44