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Possible performance of battery energy storage systems, in terms of power and energy exchanges and of the related cycling aging, is analysed in case of fast (or “enhanced”) primary frequency control supply, in the Great Britain or in the Continental Europe system. Performance and achievable profit are then analysed in case of balancingservices supply via participation in the Balancing Market in Italy.
The extensive exploitation of generation from Non-Programmable Renewable Energy Sources (NPRES) to meet demand reduces the operating hours of relevant conventional plants, which normally provide regulation reserves, in particular for frequency (and voltage) control. The power system is therefore becoming more vulnerable, also because NPRES support to ancillary service provision of is very limited in many countries. Battery Energy Storage Systems (BESS), thanks to their fast response times (power inversion in a few hundreds of ms) and their flexibility of use, could be employed to provide both traditional and innovative ancillary services. Here we analyze, in simulation, the possible supply of two different ancillary services by stand-alone BESS: an Enhanced Frequency Response (EFR), i.e. a fast primary frequency regulation service like the one recently introduced in Great Britain, for which the maximum response time allowed is 1 s (Part I); the balancing service in Italy (Part II), which is among the services dealt with in Authority Decision 300/2017/R/EEL issued by the Italian Regulatory Authority for Energy, Networks and the Environment (currently ARERA) and aiming at extending the provision of ancillary services to new suppliers, including NPRES and BESS.
In the former case, the analysis is aimed at assessing the possible technical performance of BESS in the provision of the service, in the latter it is more focused on the possible BESS payback versus their still high investment costs. Part I – Enhanced Frequency Response service The possible performance of a BESS in the provision of the EFR service is evaluated, with reference to two European synchronous areas for which frequency data are available: the GB system and the European Continental (CE) system.
In the former case, the technical requirements formulated by the GB TSO, National Grid Electricity Transmission (NGET), are considered, in the latter case these requirements are adapted to the statistical characteristics of the CE frequency. In both cases, a simple model is used which relates the dynamics of the battery State of Charge (SoC) to the power exchanges carried out; the energy exchanges, in absorption and injection separately, required from the BESS for the service are evaluated, together with the exchanges that it is able or not to carry out with respect to the requests and the possible aging due to the non-standard charge-discharge cycles undergone by the battery; performance in doing the service is computed in terms of the Service Performance Measure (SPM) indicator specified by NGET, which accounts for the distance of the power response from the required region in the power-frequency plane (that region is called EFR envelope here).
Overall, a sensitivity analysis of the results is carried out, by considering different “power-frequency EFR characteristics” compliant with the envelope, different nominal energy / nominal power ratios for the BESS (from 0.25 h to 10 h), and three battery technologies. The ability to exchange energy for the service turns out to be poor if the boundary characteristics of the envelope are considered, because they require exchanges mainly in one direction; such ability definitely improves in case the middle characteristic, which is very similar to a traditional primary regulation characteristic, is considered, and it further improves if a SoC management strategy is adopted.
In the simulated cases, however, the ability to exchange power while keeping in the envelope, i.e. the ability quantitatively translated by the SPM, can be considered to be satisfactory. Battery aging is here evaluated by employing typical curves expressing the maximum number of cycles that can be performed versus their Depth of Discharge (DoD): for the sodium-sulfur and lithium-ions technologies, which are able to withstand many low-DoD cycles, the EFR service does not seem to have a major impact on the expected useful life, but it it could cause problems to the nickel-sodium chloride technology. Finally, for the CE system results similar to those for the GB system have been obtained, from both a qualitative and a quantitative point of view, thanks to the adaptation of the amplitudes of the reference frequency bands in the envelope.
Part II – Balancing service The possible supply, via participation in the Balancing Market (BM), of the balancing service in Italy is considered. The (upward) offers and (downward) bids accepted on the BM are remunerated at the offered or bid price, i.e. according to its pay-as-bid mechanism; offers and bids are formulated on an hourly basis, but accepted on a quarter-of-an-hourly basis. Each offer consists of an amount of energy (MWh) and a unit price (€/MWh). Here we propose a preliminary strategy, in terms of prices and quantities, for a stand-alone BESS, to formulate bids and offers for the service by taking into account the timetable of the six daily sessions of the BM. The strategy favours the attempt to sell, by making offers, the energy stored in the battery, until this energy decreases so as to reach a minimum threshold: then bids are made to try to recharge the battery.
The offered and bid prices are built from the historical prices accepted on the BM, which are available for every quarter of an hour on the GME website. To simulate the offer/bid acceptance mechanism on the BM, the offered and bid prices are assumed to be “chosen” or not based on their comparison with the historical prices of the accepted offers and bids respectively. In summary, a year of power exchanges, and therefore of energy exchanges, has been simulated in relation to the acceptance of the offers and bids formulated by a BESS according to the proposed strategy, and the related costs (for energy absorption from the grid) and revenues (for injection of energy into the grid) have then been calculated, together with the costs and any revenues related to the imbalances associated with the partial or total unavailability to do the service.
Both the payback time of the investment in the BESS (PayBack Period – PBP) and the expected life of the battery against aging due to the non-standard discharge-charge cycles performed have also been evaluated. Simulations have been carried out “ex post” on the price data of the accepted historical offers, already used to derive the prices to be adopted in the offers and bids. In particular, to three price schemes have been adopted (with different upward and downward prices):
1. the seasonal averages, computed on an hourly basis, of the historical prices;
2. the hourly averages of the historical prices, with a distinction between working and non-working days;
3. the average historical prices accepted in every quarter of an hour. In order to perform a sensitivity analysis, the average prices have been varied by a fixed fraction , so as to obtain more or less favourable prices for the BESS, but also a reduction or an increase of the probability of their acceptance on the market.
The simulations for a BESS with 4 h nominal energy over nominal power ratio show that scheme 3 is much more effective than schemes 1 and 2, since it is based on “a priori” knowledge of the average price information in every ¼ h; moreover, scheme 1 has better performance than scheme 2; at present, anyway, scheme 3 is, among the three, the only one enabling to obtain PBP values which are at the same time smaller than estimated life and economically affordable. Overall, the results seem to be promising, but improvements in the bidding strategy are needed, in particular to reduce the PBP and increase the expected life. For example, improvements are expected if scheme 3 prices were better estimated as compared to what happens with schemes 1 and 2. The use of optimization techniques, moreover, in particular as for the quantities, could be considerably beneficial.
Evoluzione dei sistemi elettrici interconnessi e integrazione dei mercati (MERCATI ELETTRICI 2017)