Large-scale urban renewable electricity schemes – Integration and interfacing aspects

Large-scale renewable electricity schemes (RES) for urban areas are investigated here with emphasis on RES integration and interfacing with the energy system to limit the outflow of RES electricity outside the city boundaries. Different electricity-to-thermal energy strategies to supply also heating and cooling demands were investigated to enable higher than usual RES shares through converting surplus renewable electricity into thermal energy which often constitute a major part of the end-use energy in a city. Case examples from a southern climate (Shanghai) with distributed PV schemes and a northern city (Helsinki) with off-shore seaside wind power were analyzed. The results from the Shanghai case indicate that when using thermal storage options, 2-folding of the PV capacity from the self-use limit of PV could be possible. With optimal electricity-to-thermal energy strategies and energy management, a 40–65% solar fraction of yearly electricity could be possible. In the Helsinki climate with wind power coinciding with the seasonal load peak, RES may satisfy beyond 30% of all energy and beyond 70% of all electricity demand annually. The size of thermal storage needed in the above cases is between 10 and 100 GW h. A higher RES use leads to higher spatial power flows and power variability in the city which requires more sophisticated control strategies and flexible auxiliary power options to handle transmission bottlenecks.