Dynamic discharging performance of a latent heat thermal energy storage system based on a PID controller

Abstract

A PID controller is introduced into a latent heat thermal energy storage unit to compose a coupling system in order to control the discharging performance. Outlet temperature of the working fluid can be precisely regulated by means of its inlet velocity variation based on the PID controller. The theoretical model is built through combining heat transfer of the latent heat thermal energy storage unit with the PID controller. Experimental results are used to validate the proposed theoretical model. PID control analysis indicates that its parameters have obvious effects on performance of the coupling system. Kp of −0.02 m/(s·K), Ki of −0.15 m/(s2·K) and Kd of −0.001 m/K are further optimized according to dynamic response of the transient outlet water temperature. System parameter analysis exhibits that higher target temperature produces larger heat discharging rate, which then reduces the outlet water flow rate accordingly. The discharging rate and total discharging energy of the latent heat thermal energy storage unit decrease with augment of the target outlet temperature. Increase of PCM melting point, thermal conductivity and latent heat is favorable of elevating the working fluid velocity. The discharging rate is improved by the PCM melting point, thermal conductivity and latent heat. The latent heat thermal energy storage units can separately obtain maximum discharging rates of 257.691, 294.437 and 257.603 W at 200 min for PCM melting point of 327.15 K, PCM thermal conductivity of 0.8 W/(m·K) and PCM latent heat of 250 J/g. It is apparent that PCM melting point, thermal conductivity and latent heat are conducive to enhancing the total discharging energy. The largest discharging energy at 200 min is respectively determined as 1280.409, 1060.105 and 974.153 kJ. Temperature and phase change contours can also be substantially affected by the system parameters. In conclusion, the built coupling system is beneficial to efficiently regulate and control discharging performance of latent heat thermal energy storage units.