3.2. Pump operation cost
Hydraulic balance
The pressure delivered by a pump (i.e. the piezoelectric head difference across it) can be described by a set of linear constraints that
3.4. Mass balance at network nodes
3.5. Demand response cost function and constraints
5. Results and discussion
We investigate three aspects of demand response from WDS, how optimal pump operations change to enable the provision of demand response before and during a DR event, requirements needed for the provision of DR through pump scheduling to be financially viable and the environmental aspects of DR from WDS and how it compares to other alternative response energy provision technologies.
The results and discussion are separated into sections focusing on the financial viability of providing DR from WDS in Section 5.1, the GHG emissions associated with the provision of DR in Section 5.2, and the optimal scheduling for DR provision and DR events in Section 5.3.
Financial viability
Fig. 7 shows the volume formed by the combinations of price ratios, reward size and pump utilization rate that are financially viable to provide a FFR or FCDM service from the Van Zyl Network when assuming a maximum event duration of 30 min. It shows that a high pump utilization rate, high reward and low price ratio benefit the financial viability of the fast response schemes. It also
demonstrates that the ratio of the maximum and minimum prices of the electricity pattern as well as the pump utilization rate have the strongest effect on the financial viability of the DR service. The reward level on the other hand has a lower impact on the financial viability. Demand response through FFR provides the highest amount of yearly revenue for committed power while requiring the least amount of energy provision. The revenue primarily stems from the nomination and availability payments for the power capacity provided and to a lesser extend from the energy provision. Assuming a WDS can fulfill the technical requirements with regards to size and pump switching speeds, it could provide a good opportunity for a profitable committed demand response provision if the pump utilization rate and the electricity tariffs are moderately high. Otherwise, a bespoke FCDM agreement where the peak hours of the contract are spared out could provide a viable alternative.
The financial viability of STOR services is explored with an annual reward of £25,000/MW, which represents the lower bound estimate of the revenue from availability payments alone, based on approximately 3500 h of availability per year and E7-9/MW availability payments as detailed in Table. The optimal power level to commit to be available for STOR provision in both availability windows is shown in Fig. 8. Fig. 9 shows the same for provision in the first availability window only. The two figures show that for lower pump utilization rates no STOR service is viable as the optimum power is 0 kw. For a small range of pump utilization rates and electricity peak price ratios, STOR provision from the booster pump with 89 kw is the optimal power for DR while for a large range of pump utilization rates J la pump from the main pump station with 178 kw can be committed to STOR provision. With a reward of £25,000/MW, the additional revenue from DR provision, if viable, can be up to 4.6% of the normal operating cost.
The cost of scheduling for STOR relative to normal operations not only depends on the pump utilization ratio and maximum price of the electricity tariff, but also on which availability windows STOR is offered in. If only the first STOR window is used, a wider range of conditions and a larger capacity of pump power can be committed to the provision of demand response. On the other hand, also providing STOR services in the second STOR window as well reduces the range of financially viable options as the peak electricity price becomes more relevant.
The cost of providing response energy in a STOR event are estimated by modeling a wide range of events in a Monte Carlo simulation, as shown in Fig. 10. Table 4 shows that the cost of energy when rescheduling due to an event reduces the income generated