Optimal design of phase change material storage for steam production using annual simulation

A direct steam generation (DSG) solar collector can be an efficient process-heat application compared to electricity generation, and it can significantly reduce dependence on non-renewable energy sources for steam generation. Due to intermittency of the sun, thermal energy storage is required to increase the capacity factor or the yearly utilization of the system. For this study, phase change material (PCM) is used to store the heat from the steam produced in the solar collector. Sodium formate is selected as the PCM material on a cost basis. A detailed off-design analysis is carried to predict the annual performance of the system. Fins are used to enhance the thermal performance of the PCM, and an simplified method is derived to predict the required fin mass. With an increase in the number of fins, the thermal performance of the system increases significantly at design point. But the fins are expensive, so an annual simulation and techno-economic analysis is carried to calculate the optimal number of fins and other operating parameters. For the case study of Imperial, California (USA), the minimum value of the levelized cost of heat (LCOH) for steam generation is 4.2 cents/kWh(th), and the optimal value for the equivalent PCM thermal conductivity is 1.5 W/mK with 4 h of storage and a solar multiple of 2.1. A sensitivity of LCOH with the solar installation cost shows that the collector cost should be less than 220 $/m(2) for California and 103 $/m(2) for the rest of the United States, for the steam generated from DSG collector and PCM storage to be competitive with the traditionally produced steam from natural gas. Where natural gas prices are higher, steam produced from a DSG collector and PCM storage can be an economical option. For example, in Kahului, Hawaii (USA), the LCOH for steam generation from a DSG collector and PCM storage is 43% cheaper compared to natural gas steam generation, even at a current solar installation price of 300 $/m(2). The breakeven price of natural gas is 8.7 $/MMBTU for the current solar installation price; in other words, steam produced from DSG collector and PCM storage can be an economical option for locations having good solar radiation and a natural gas price greater than 8.7 $/MMBTU.