Development of a Molten-Salt Thermocline Thermal Storage System for Parabolic Trough Plants

Thermal storage improves the dispatchability and marketability of parabolic trough power plants allowing them to produce electricity on demand independent of solar collection. One such thermal storage system, a thermocline, uses a single tank containing a fluid with a thermal gradient running vertically through the tank, where hotter fluid (lower density) is at the top of the tank and colder fluid is at the base of the tank. The thermal gradient separates the two temperature potentials. A lowcost filler material provides the bulk of the thermal capacitance of the thermal storage, prevents convective mixing, and reduces the amount of fluid required. In this paper, development of a thermocline system that uses molten-nitrate salt as the heat transfer fluid is described and compared to a two-tank molten salt system. Results of isothermal and thermal cycling tests on candidate materials and salt safety tests are presented as well as results from a small pilot-scale (2.3 MWh) thermocline. NOMENCLATURE A = cross sectional area of bed, m Cp = specific heat of fluid (f) or bed (b), J/kg K D = particle diameter, m h = heat transfer coefficient between bed & fluid, W/mK hv = 6 h(1-ε)α/D = volumetric heat transfer coefficient, W/m K T = temperature of fluid (f) or bed (b), C y = vertical dimension, m α = surface area shape factor ε = void fraction of bed ρ = density of fluid (f) or bed (b), kg/m τ = time, s INTRODUCTION Thermal storage for parabolic-trough power plants offers the potential to dispatch electricity without fossil fuel backup. The value of the electricity is often higher during peak periods. A plant that can meet the peak demand independent of the solar resource has the potential to generate more revenue. The Solar Electric Generating Station (SEGS) I plant had a two-tank thermal storage system that used Caloria heat transfer fluid. This fluid was pumped through the collector field. The storage tanks were conventional atmospheric tanks. Caloria is a liquid at atmospheric pressures when its temperature is below about 315oC. The SEGS I thermal storage system operated at temperatures between about 175oC and 295oC. The hot Caloria oil was pumped through a steam generator to make saturated steam. Fossil fuel was used to superheat the steam to generate electricity. The Solar One central receiver pilot plant also used Caloria, but in a thermocline tank. A thermocline tank is one that uses a single tank to store thermal energy. A thermal gradient separates the hot from the cold fluid. A low-cost filler material is used to displace the higher-cost liquid. The filler material as well as buoyant forces help to maintain the thermal gradient. When the system is charged, cold fluid is drawn from the bottom, heated as it passes through a heat exchanger (heated with the receiver heat transfer fluid) and returns to the top of the tank. When the tank is discharged, hot fluid is drawn from the top, cooled as it passes through a heat exchanger (to transfer heat for steam generation), and returns to the bottom of the tank. In the Solar One thermocline storage system, steam from the receiver passed through a heat exchanger, which heated the Caloria oil drawn from the bottom of the tank. The heated Caloria returned to the top of the tank where it transferred heat to the rock and sand (the filler material). During a discharge cycle, hot Caloria was pumped from the top of the tank, through a steam generator that made low pressure