## 5 Conclusions  
PV power plants need to be equipped with energy storages, if output power smoothing or other production profile shaping is required. The electricity market is one case where at least big utility scale PV plants need to meet the spot market practices in one way or the other in the future. The use of an energy storage system is one potential solution to manipulate the grid feeding power. In this study, the ESS energy and power capacity requirements have been studied for 0.1, 1 and 10 MW PV plants for meeting the imbalance settlement periods of 15 and 60 min. Analysis is based on the TUT solar power research plant irradiance measurements in 2015 using 1 Hz sampling frequency to calculate spatially smoothed output powers of PV generators. For the sake of clarity, a simple ideal PV power production forecast is used as a reference for the operation of ESS and losses of the PV power plant or ESS were ignored. The energy capacity requirement for ESS was found to be directly proportional to the PV plant size so that for a typical 60 min imbalance settlement period on the electricity market the ESS needs to be able to deliver the PV plant nominal power for 0.36 h. This result is in line with earlier studies on the ESS energy capacity requirements to compensate PV power plant fluctuations with respect to different ramp rate limits. For example, the ESS energy capacity requirement for a 3%/min ramp rate limit of the PV plant nominal power has been reported to be around 0.3 h, decreasing with increasing ramp rate limit. The power capacity requirement for ESS is also directly proportional to the PV plant size being roughly 80% of the PV plant nominal power in line with other studies on irradiance fluctuations caused by cloud shading. ESS is operating most of the time only at a fraction of the nominal power. This must be considered when choosing the energy storage technology. Both requirements decrease only slightly with increasing PV plant size due to increased spatial smoothing of power fluctuations with increasing PV plant area. When ISP time is decreasing, energy capacity requirement is also decreasing. However, decreasing the ISP time does not considerably affect the power capacity requirement. The probability density distributions of the ESS energy and power capacity requirements for the studied PV generator sizes revealed the highest requirements were caused by periods of high and fast irradiance fluctuations taking place during a small portion of time of the year. The high requirements are caused by periods (days) of plentiful irradiance and power fluctuations, which might be possible to predict beforehand. This is an important result when designing an ESS controller. The required energy storage capacity could be downscaled considerably, for example, by sacrificing some produced energy during those periods to meet the market terms or by paying the cost of imbalance during corresponding settlement periods. Figure 9: (**a**) 10 MW PV generator power and the grid feeding reference power; (**b**) the corresponding power taken from the ESS and (**c**) the energy stored in ESS (state of charge) with 15 min ISP on 5 th of June 2015. Daily energy cycled through the ESS was around 11% of the annual energy production of the studied PV generators, i.e., directly proportional to the generator size. The amount of cycled energy closely follows the seasonal variation of irradiation and available PV power, having peak values in early summertime. However, a considerable portion of the produced energy passes through the ESS throughout the year. Despite using ideal forecast of the PV power production and ideal operation of the PV system in the simulations, the results were observed to be quite realistic. These results serve as a basis for more advanced studies, which should consider power losses, accuracy of real PV production forecasts, executable ESS control algorithms etc. The research is part of the project Finnish Solar Revolution covering also the costs to publish in open access.