Optimisation of size and control of grid-connected storage under real time electricity pricing conditions

In this paper, a storage system has been considered to be added to a private electricity facility in order to reduce the electricity bill. This kind of system could make sense with a time-of-use (TOU) tariff (with two or three periods of different electricity price) or a real-time pricing (RTP) tariff (which can vary hourly depending on wholesale market), where each day, electricity is bought from the AC grid during off-peak hours (at low price) to store energy, and during on-peak hours (at high price), the storage supplies the whole load or a part, avoiding the purchase of expensive electricity from the AC grid. A new optimisation model to obtain the best storage size and control of the charge-discharge process under RTP tariff has been developed, minimising the net present cost of the system. This methodology allows the electricity consumer to evaluate the profitability of adding storage. A case study of battery storage with a hypothetical hourly RTP tariff based on the Spanish electricity market pool of 2013 is shown. Considering the cost of batteries per kW h cycled expected in the near future (0.045 €/kW hcycled), even the optimal storage size with the optimal control is not profitable compared to the system without storage. A much lower battery cost (0.0225 €/kW hcycled) would be needed so that the storage system could be economically profitable. Considering a stretched RTP hourly pricing tariff (hourly prices higher than the average of each day multiplied by 1.3 and the lower ones by 0.7), the storage system with the costs expected in the near future (0.045 €/kW hcycled) is profitable. We conclude that an hourly tariff that takes the base of the Spanish electricity market pool is not adequate to stimulate this technology, as the difference between maximum and minimum prices during the day is too low to compensate the capital cost plus replacement, operation, and maintenance of the storage system. In other countries, with much higher difference between minimum and maximum electricity price of the day, present battery cost could make it profitable.