Thermal energy storage (TES) with phase change materials (PCM) in solar power plants (CSP). Concept and plant performance

TLDR: In this article, a cascade type phase change materials (PCM) energy storage system for solar thermal electricity plants with its technical assessment is presented, using four buckets with the PCM organized based on melting temperature and the latent energy of the materials.

About: This article is published in Applied Energy.The article was published on 2019-11-15 and is currently open access. It has received 122 citations till now. The article focuses on the topics: Energy storage & Solar power.

Figures

Figure 3. Cost per kJ of each PCM considered 17

Figure 2. Charging and discharging orientations of a PCM system comprised of pre-fabricated heat 2  exchangers 3  4  The system dimensions are largely governed by the thermal properties of the individual PCMs 5  and the pinch points of the individual buckets during charge and discharge. This is compounded 6  by the transient nature of the PCM freezing and melting on the tube surfaces. Overall this causes 7  the system to be hard to size correctly without iteration. Instead the system is sized iteratively to 8  achieve as stable an outlet temperature as possible at the correct outlet temperature and storage 9

Figure 8. Baseline plant configuration with indirect two-tank molten salt TES 2

Figure 17. Results of a “good” day, May 25th: (a) Gross electrical output, (b) net electrical output, (c) dumped 1  energy, and (d) parasitic losses. 2  3  Using the same data, several values can be calculated that help compare the two systems. These 4  are the TES utilization and efficiency. In general, they represent the amount the theoretical energy 5  density that is used and the amount of energy that is recovered once it has been put into the storage 6  system. PCM utilization is calculated using the equation below and represents the total energy 7  stored in the storage system divided by the maximum potential energy that might be put into the 8  storage system if the system was taken from the design outlet temperature of the plant, 293°C, to 9  the design solar field outlet temperature, 393°C: 10  11

Table 1. Technical description of materials used in the system [19] 2

Figure 16. Insolation for both May 25th and August 4th 2  3  Figure 17 shows the gross and net output of the system. The effects of charging feed-water heater 4  deactivation is clearly visible in the PCM system as the turbine is made to run in an overpower 5  mode during the charging hours of the day. In future phases of this project this overpower time 6  will be reduced by implementing a control strategy that shifts this excess power to the storage 7  system. This will mean a gross output closer to 140 MW during charging and a larger storage 8  output. Also apparent is the transient nature of the PCM system. As the system moves from liquid 9  to solid the effect on the performance is clearly evident. Also of interest on a daily basis are the 10  dumped energy and parasitic losses of the system. 11  12  For the representative “good” day, the PCM plant performs similarly to the baseline. The main 13  difference occurs in the overpowering of the turbine, the lower output during discharge, the 14  reduced dumped energy and the higher parasitic losses. The lower output during discharge is 15  largely due to the lower outlet temperature from the storage system then from the two tank system. 16  The lower dumped energy is largely due to the low outlet temperature from the turbine during 17  charge and the fact that the PCM system can be partially charge for the entirety of the day. The 18  higher parasitic losses during charging result from the higher mass flow in the field necessitated 19  by the solar field; allowing the PCM system to be charged for more of the day. 20  21