6. Conclusion and future work
Through the use of a global optimization technique we compared the operating schedules of a WDS system minimizing the operating cost alone and minimizing the operating cost while participating in different demand response schemes in the UK. Through this analysis we show that for a wide range of electricity tariffs and water demands there exist demand response mechanisms which allow the WDS to provide demand response and reduce its cost and provide response energy at low GHG emissions. These new results should encourage water utilities to investigate the potential to schedule for demand response provision from their WDS. To be viable demand response from a WDS there would be no need for a no minimum response duration. Unlike competing STOR and demand response providers, demand response from a WDS does not have shut down (start up) costs that need to be recouped by a minimum STOR operation, which makes potential demand response provision from WDS particularly valuable to National Grid [9],
STOR when tendering only for the first window can be provided at little extra cost to the WDS compared to regular optimal operation, as the highest price tariff times can be excluded from the provision period. Due to the specific requirements this poses, it may be necessary to provide STOR service through an aggregation or combine the STOR provision with another energy asset. When tendering to both windows the maximum price of the electricity tariff, if charged during the operation window, limits the financial viability of STOR provision.
The provision of response energy in an DR event is shown to have limited additional cost and GHG emissions. The operation scheduling during and after the event was performed using the same optimal scheduling techniques used to obtain the global optimal operating schedule. Through optimization with a receding time horizon considering the uncertainty of future events occur¬ring to allow repeated provision of demand response could further reduce the cost of providing DR through a better schedule.
The environmental impact is dependent on a range of factors, but demand response from WDS can often be provided at very low carbon intensity per unit of response energy provided. The faster responding services - FFR and FCDM - provide small amounts of energy potentially leading to worse carbon intensity of the energy provided, however the custom nature of FCDM may enable the inclusion of such services with small changes in scheduling and thus small changes in GHG emissions. Shorter response events have lower carbon intensities as the originally optimal schedule is only perturbed a little.
Electrical power distribution losses and life cycle emissions were ignored in this analysis since they are similar for the different technologies considered, for which we assume the infrastructure to be already in place. With the additional grid regulation services that come with the introduction of more renewable to the grid, the usage of already built WDS to regulate demand could provide significant reductions in GHG emissions compared to newly built infrastructure.
The WDS was modeled using quasi steady state modeling. For the provision of FFR the power reduction ramping speed constraints need to be analyses. This will require a more detailed analysis of the transient response of the network [52]. Further work could consider the difference in business case of upgraded surge protection devices to enable faster pump ramp rates without causing pressure induced failures in the pipes. Upgraded pump controls or battery systems to enable a gradual shut down may also be considered [53].