Novel Ionic Liquid Thermal Storage for Solar Thermal Electric Power Systems

Feasibility of ionic liquids as liquid thermal storage media and heat transfer fluids in a solar thermal power plant was investigated. Many ionic liquids such as [C4min][PF6], [C8mim][PF6], [C4min][bistrifluromethane sulflonimide], [C4min][BF4], [C8mim][BF4], and [C4min][ bistrifluromethane sulflonimide] were synthesized and characterized using thermogravimetric analysis (TGA), differential scanning calorimeter (DSC), nuclear magnetic resonance (NMR), viscometry, and some other methods. Properties such as decomposition temperature, melting point, viscosity, density, heat capacity, and thermal expansion coefficient were measured. The calculated storage density for [C8mim][PF6] is 378 MJ/m 3 when the inlet and outlet field temperatures are 210C and 390C. For a single ionic liquid, [C4mim][BF4], the liquid temperature range is from –75C to 459C. It is found that ionic liquids have advantages of high density, wide liquid temperature range, low viscosity, high chemical stability, non-volatility, high heat capacity, and high storage density. Based on our experimental results, it is concluded that ionic liquids could be excellent liquid thermal storage media and heat transfer fluids in solar thermal power plant. NOMENCLATURE Cp Heat capacity, J/(kg K) E Storage density, MJ/m Pr Prandlt number Tin Inlet temperature, C Tout Outlet temperature, C Tdec Decomposition temperature, C Tmp Melting point temperature, C k Thermal conductivity, W/(m K) μ Viscosity, Pa S ρ Density, kg/m INTRODUCTION Solar energy is the most abundant energy source on the earth. Compared with other energy from oil, coal and nuclear reaction, solar energy is clean and in unlimited supply. Solar energy usually is collected and transferred to thermal energy to heat room and water, or generate electricity. Energy transformation of solar energy thermal energy electricity is the most important application of solar energy. Because the solar energy availability depends on time, weather condition, and latitude, and the electricity demand varies with time, the energy originally from solar energy needs to be stored. This energy can be stored as thermal energy or electricity, but storage in thermal energy is considered the more economic method. Currently thermal oil and molten salt are used as liquid storage media. The main problems for oil media are the low decomposition temperature (e.g. 300C) and for molten salt media it is its high melting point (e.g. 220C). The low decomposing temperature limits the energy storage and high melting point can cause molten salt freezing in evening or cold weather, resulting in high operating costs. The rate of solar energy intercepted by the earth is about 5,000 times greater than the sum of all other energy sources, but less than 0.5 percent is represented in the kinetic energy of the wind, waves and in photosynthetic storage in plants. The amount of the solar energy intercepted by earth is only one thousandth of one million of the total released energy in the sun. A wide variety of equipment is readily available to capture solar energy and use it for space and water heating, and for electricity generating. The three major components for solar thermal energy utilization systems are the solar energy collector, the energy storage system, and the steam