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Large share of solar energy imposes a higher system flexibility to resolve the increased demand/supply imbalance due to the inherent intermittency and variability of the resource. In this work, we demonstrate that the additional solar-induced flexibility requirement can be fully provided by a special kind of solar farms, namely flexible PV. These plants are able to provide ancillary services by proactive generation curtailment and storage power injection and they can be managed exactly as the secondary reserve currently used. At the current and future penetration levels, we sized the flexible PV fleet required to reduce the Italian imbalance by 36 % (with respect to its 2016 value) while keeping the curtailment at 6 % of the national PV generation. We show how this result can be achieved at an equal or lower dispatching cost than current cost (depending on the solar share). In addition, we found that a fleet composed of many flexible PV plants with different capacity randomly distributed throughout the country provides an optimal solar regulation performance. Finally, we showed that the effectiveness of the proposed imbalance mitigation strategy depends only slightly on the year-specific load, wind, PV and energy prices profiles used to size the capacity of the flexible fleet.

This article proposes two strategies for the mitigation of power imbalances and related costs resulting from increasing PV penetration onto the Italian grid. New “state of the art” solar and netload day ahead forecast models were developed and applied to real data. These strategies consist of: (1) Improving the accuracy of PV and net load power forecast and enlarging the footprint of the controlled grid area; (2) Transforming unconstrained PV plants into “flexible PV plants”: remotely controlled PV plants that can be proactively curtailed and work with cost-optimized Battery Energy Storage Systems. We demonstrate that the first strategy can effectively limit the imbalance impact when integrating a large share of PV generation, reducing imbalance volumes and costs, both at current and future solar penetration levels. We further demonstrate that the second strategy can entirely eliminate the imbalance impact of PV penetration, hence providing operational certainty to the TSO. Indeed, we show how flexible PV plants can be cost-optimally sized to set the imbalance volume at a desired target value regardless of PV installed capacity, hence allowing massive solar penetration. Finally, we show that the cost of implementing these strategies is less than the current cost of handling such imbalance impacts.