مدیریت تولید نیرو ایجاد با استفاده از باتری در مزارع بادی: تجزیه و تحلیل اقتصادی و فنی برای اسپانیا

This paper presents an hourly management method for energy generated in grid-connected wind farms using battery storage (Wind–Batteries systems). The method proposed is analysed technically and economically. Electricity generation in wind farms does not usually coincide with the electrical demand curve. If the wind-power penetration becomes high in the Spanish electrical grid, energy management will become necessary for some wind farms. A method is proposed in this paper to adjust the generation curve to the demand curve by storing electrical energy in batteries during off-peak hours (low demand) and selling stored energy to the grid during peak hours (high demand). With the results obtained and reported in this paper, for a Wind–Batteries system to be economically as profitable as a Wind-Only system, the selling price of the energy provided by the batteries during peak hours should be between 22 and 66 c€/kWh, depending on the technology and cost of the batteries. Comparison with flexible thermal generation has been performed. Additionally, the results are compared with those obtained if using hydrogen (Wind–Hydrogen system, which uses an electrolyser, hydrogen tank, and fuel cell instead of batteries), concluding that the Wind–Batteries system is both economically and energetically far more suitable.

Currently, electricity generation by means of renewable sources in Spain has established a special economic regime (Royal Decree 661/2007, 2007; Order ITC/3860/2007, 2007). This favourable economic regime has encouraged the installation of numerous wind farms and photovoltaic generators. The primary challenge with generating installations dependant on renewable resources (e.g., sun and wind) is that electricity generation cannot be fully forecasted and does not usually coincide with the demand curve. On occasion, the electricity generated by wind farms cannot be transferred to the grid. Therefore, it is necessary to stop some wind turbines, resulting in possible lost energy. In addition, there are usually large variations in electricity generated by wind farms within brief intervals of time. In order to reduce the difference existing between the generation curve and the demand curve (energy management), as well as abrupt variations in generation, a possible measure is to use some kind of energy storage means. Thus, there are various possibilities for electricity storage for energy management: water-pumping reversible hydro plants, batteries, compressed air energy storage (CAES), and hydrogen (Ibrahim et al., 2008). Amongst these, the most frequently used to store electricity is water-pumping reversible hydro plants. Electricity storage in large-size batteries is less typical (Wagner et al., 1999; Farber De Anda et al., 1999; McDowall, 2006; Kashem and Ledwich, 2007). An alternative method of energy storage coupled with wind generation is economically prudent in cases where wind turbines are connected to a weak grid or at the end of a long feeder, thus deferring the need for grid upgrade. Various battery technologies exist. The most frequently used batteries are lead acid; however, they are problematic in that the cycle life is very low (typically between 200 and 500 full equivalent cycles). In addition, their level of charge cannot be less than 40%. In addition to lead acid batteries are OPzS, which are tubular and have much longer life cycles of about 900–1200 full equivalent cycles. Other types of batteries, such as Ni-Cd, have an advantage over lead acid in that they discharge completely and do not require any maintenance; however, their cost is significantly higher (Pocock and Hamilton, 2006). In recent years, redox flow batteries have been developed (Bartolozzi, 1989; Lipman et al., 2005), using bromine as a central element (ZnBr or NaBr) or vanadium (vanadium redox batteries). These last approximately 2000 full equivalent cycles. Sodium sulphur (NaS) batteries (Oshima et al., 2005; Wen et al., 2008) have been recently developed and have a life cycle of up to 20,000 cycles, depending on the depth of discharge. Of the various battery technologies mentioned, NaS batteries are most suitable economically for energy management (Walawalkara et al., 2007). NaS batteries are environmentally benign, since the batteries are sealed and allow no emissions during operation, and more than 99% of the overall weight of the battery materials can be recycled. Only sodium must be handled as a hazard material. NaS batteries function optimally at temperatures ranging between 300 and 350 °C (Wen et al., 2008). NaS applications are shown in Kamibayashi and Tanaka (2001). Some authors propose the use of hydrogen for generated energy management (González et al., 2003; Anderson and Leach, 2004). However, as this paper will show, the low efficiency of the electricity–hydrogen–electricity conversion process (about 25–30%), together with the high associated cost, deem hydrogen unsuitable for storing energy generated by grid-connected wind farms given the current state of the technology and economics. This paper is structured as indicated below. First, the use of Wind–Batteries is justified. Then, the mathematical model used for the hourly simulation of the system is detailed, including the economic calculations performed. And finally, the study of a specific system is carried out, in various scenarios, comparing it with two systems: a Wind-Only system without energy storage and a Wind–Hydrogen system. This paper is based on previous works (Dufo-López et al., 2007; Dufo-López, 2007). GRHYSO software (Dufo-López and Bernal-Agustín, 2007) developed by the authors of this paper has been used for the simulations and economic calculations.

Different studies of the Wind–Batteries system have been carried out, comparing them to the Wind-Only system (without energy storage) and with Wind–Hydrogen systems (with energy storage in hydrogen). The storage of energy in batteries facilitates energy management; that is to say, the electric generation curve of the Wind–Batteries system has peaks and off-peak periods more similar to the demand curve than the Wind-Only system. The high efficiency of energy storage in batteries, together with the long lifespan of NaS batteries, makes Wind–Batteries systems using these batteries to be economically competitive with Wind-Only systems for selling battery energy prices of approximately 22–31 c€/kWh. This selling price of battery energy, significantly higher than the average price of the electric market, should be subsidised, taking into account the reduction of emissions and externalities as compared with flexible thermal generation. Thus, the installation of Wind–Batteries systems would be encouraged, with added advantages achieved by energy management if wind-power penetration becomes high in the Spanish electrical grid. The periods of the day during which the batteries are to be charged or discharged could also be regulated by the current legislation, taking into account that the operation of these systems, in order to obtain the maximum NPV, should be such that the batteries are charged during the off-peak periods while discharge periods should correspond to the peaks, as demonstrated in this paper. Comparing energy storage in batteries with storage in hydrogen, we see that Wind–Hydrogen systems show very high energy losses (of about 60%) given the low energetic efficiency of the electricity–hydrogen–electricity conversion. Wind–Hydrogen systems, in order to be competitive with Wind-Only, should sell the energy generated by the fuel cell at an unacceptable price of about 120 c€/kWh. It can be concluded that Wind–Batteries systems are technically suitable for energy generation management and are a great deal more suitable, both energetically and economically, than Wind–Hydrogen systems.