Molten Nitrate Salt Development for Thermal Energy Storage in Parabolic Trough Solar Power Systems

Abstract

Thermal energy storage can enhance the utility of parabolic trough solar power plants by providing the ability to match electrical output to peak demand periods. An important component of thermal energy storage system optimization is selecting the working fluid used as the storage media and/or heat transfer fluid. Large quantities of the working fluid are required for power plants at the scale of 100-MW, so maximizing heat transfer fluid performance while minimizing material cost is important. This paper reports recent developments of multi-component molten salt formulations consisting of common alkali nitrate and alkaline earth nitrate salts that have advantageous properties for applications as heat transfer fluids in parabolic trough systems. A primary disadvantage of molten salt heat transfer fluids is relatively high freeze-onset temperature compared to organic heat transfer oil. Experimental results are reported for formulations of inorganic molten salt mixtures that display freeze-onset temperatures below 100°C. In addition to phase-change behavior, several properties of these molten salts that significantly affect their suitability as thermal energy storage fluids were evaluated, including chemical stability and viscosity. These alternative molten salts have demonstrated chemical stability in the presence of air up to approximately 500°C in laboratory testing and display chemical equilibrium behavior similar to Solar Salt. The capability to operate at temperatures up to 500°C may allow an increase in maximum temperature operating capability vs. organic fluids in existing trough systems and will enable increased power cycle efficiency. Experimental measurements of viscosity were performed from near the freeze-onset temperature to about 200°C. Viscosities can exceed 100 cP at the lowest temperature but are less than 10 cP in the primary temperature range at which the mixtures would be used in a thermal energy storage system. Quantitative cost figures of constituent salts and blends are not currently available, although, these molten salt mixtures are expected to be inexpensive compared to synthetic organic heat transfer fluids. Experiments are in progress to confirm that the corrosion behavior of readily available alloys is satisfactory for long-term use.

Heat Transfer Fluids

“…As nitrate is formed, the mixture shifts away from the eutectic point elevating the freezing point such that insoluble products may form in the heat transfer fluid which clog the valves and pipes [51]. An inert atmosphere can prevent this problem since the nitrite/nitrate reaction is governed by the partial pressure of oxygen in contact with the molten salts.…”

“…In a simple binary mixture system, a particular composition of the two components (i.e., eutectic point) will minimize the freezing point (or liquidus temperature Recent work has focused on extending this concept by investigating multi-component salt mixtures. In such systems, the phase diagram can be significantly more complex and harder to predict [51]; hence, a combinatorial experimental approach has been adopted to determine the composition with the lowest melting point.…”

Technological assessment of different solar-biomass systems for hybrid power generation in Europe

“…After generating the steam the hot molten salt loses heat and therefore the salt is returned from the steam generator to the cold tank where it is stored and eventually reheated in the receiver [77]. The Insulation of heat is very important for both hot and cold heat storage tank.…”

New operating strategies for molten salt in line focusing solar fields - Daily drainage and solar receiver preheating

“…The main motivation for the use of molten salt instead of thermal oil in line focusing solar power plants is the increase of the operating temperature, which on the one hand increases the power plant efficiency and on the other hand increases the efficiency of the thermal energy storage [1,2]. For solar power towers the so called solar salt (60%NaNO3, 40%KNO3) has prevailed [7,8] due to its high temperature resistance and low costs [9].…”

“…These salts however have several disadvantages compared to solar salt: Salt mixtures containing Lithium are relatively expensive and therefore uneconomic for large scale applications with storage tanks. Hitec salt needs a nitrogen blanketing and according to the manufacturer Halotechnics it can only be used up to 454°C [9,10] for long period operation. Hitec XL can be used up to 500°C [1].…”