Storing wind power by direct thermal energy conversion and thermal energy storage

This study explores the possibility of integrating thermal energy storage with a wind turbine, aiming to maximize wind energy within the limits of Betz law for later use. The model developed in this study provides a steady power output, determining the efficiency and output of such a system. The main tasks of storage include buffering during transient weather conditions, dispatch ability, improving the annual capacity factor of the power plant, even distribution of electricity, and achieving full load operation of the steam cycle at high efficiency. The major requirements of a storage system are high energy density of the storage material, good heat transfer between the heat transfer fluid and the storage medium, mechanical and chemical stability of the storage material, chemical compatibility of the heat transfer fluid, heat exchanger, and storage medium, reversibility of charging/discharging cycles, low thermal losses, and ease of control. Thermal energy storage systems can be categorized into three main approaches: sensible heat storage, latent heat storage of phase change materials (PCM), and thermochemical storage by reversible chemical reactions. Sensible heat storage involves using a storage medium that is the same as the liquid heat transfer fluid in either or both of the two flow loops, allowing for direct thermal storage. Indirect thermal storage systems, on the other hand, involve a separate storage medium from the collector heat transfer fluid, incorporating some form of heat exchange arrangement. Solar Energy Generating Systems (SEGS) in California, with a combined capacity of 354 MW, is the second-largest solar thermal energy facility in the world. The SEGS-1 facility employs a two-tank mineral oil storage system with a capacity of 115 MWh, while another facility uses a two-tank molten salt storage system with a capacity of 105 MWh. Phase change materials (PCM) have also been discussed for thermal energy storage, offering high storage density due to their high phase-transition enthalpy. However, their low thermal conductivities require relative heat transfer enhancement technologies. The thermal energy storage system modeled in this work uses the two-tank-direct configuration, where the heat transfer fluid (HTF) also acts as the energy storage medium. This system eliminates the need for an additional heat exchanger to transfer heat from the collection HTF to the storage medium. The stored energy in the hot tank is delivered to the load by pumping the HTF through the boiler, generating saturated steam. The output power is represented by the flow rate of the saturated steam generated in the boiler. In a wind turbine, the kinetic energy in the wind turns the propeller-like blades around a rotor, which spins a generator to create electricity. The maximum theoretical value of the coefficient of performance is 0.593, determined by the Betz limit. However, the concept used in this study involves using the mechanical work of the rotor instead of converting it into electricity, converting the rotating energy to thermal energy at the top of the tower directly. This thermal energy can be stored as sensible heat in the thermal energy storage system, transferred by the heat transfer fluid to produce steam that drives the turbine generator when required. The performance coefficient Cp of the turbine is the mechanical output power of the turbine divided by wind power and a function of wind speed, rotational speed, and pitch angle (beta). By calculating rotational velocity, torque, and work done can be determined. The work done is given by W = Iαω^2, where I is the moment of inertia, and ω is the rotational speed. For simplicity, the shape of the turbine blade has been considered a cylindrical rod, with the actual shape varying among different manufacturers. The next step is to convert mechanical work into heat using a heat generator, which has a lighter weight and lower cost than an electric generator. The mechanical equivalent of heat states that motion and heat are mutually interchangeable, and that in every case, a given amount of work would generate the same amount of heat, provided the work done is totally converted to heat energy. By the first law of thermodynamics, ΔU = Q + W, for no change in internal energy, all the work done on a system will be converted to heat.