Evaluation of transient characteristics of medium temperature solar thermal systems utilizing thermal stratification

Abstract

System level transient moving boundary models are interesting in the context of advanced control of the solar thermal systems. However, development of control-oriented heat exchanger models faces the key challenges that these models should remain both reasonably accurate and mathematically tractable. In this paper, dynamic moving boundary model is developed for the medium temperature (∼200 °C) solar thermal power plant using a novel concept based on auxiliary pressurized water storage tank to enhance overall thermal efficiency, where the time varying pressure on the subcooled organic Rankine cycle working fluid (refrigerant) at the two-phase heat exchanger inlet is considered as the sole independent variable and all other thermodynamic variables are assumed to be dependent on the specified pressure. Modified idealized stratification model is adopted to analysis the thermal stratification in the storage tank. Small sinusoidal variation about the steady-state pressure level is considered to investigate its effect on the heat exchanger tube wall temperature and the length of different flow regimes. Additionally, the model incorporates a provision to capture the effect of transient changes in the bulk temperature of the heat transfer fluid (commercial thermic oil) on the moving phase change boundaries in the working fluid side of the heat exchanger. At an indicative solar radiation level of 450 W/m2, the primary energy transfer at the solar collector is found to be enhanced by about 11% by using an auxiliary pressurized water storage tank, compared to the case where commercial thermic oil is used in the collector loop. The overall thermal efficiency of the solar thermal system for a peak power level of 840 kWt is estimated as 20.75%.