Abstract

Pivotal heat transfer components of solar thermal systems may involve single phase flow of the working fluid in some unit (e.g. single phase solar collector), whereas two phase flow of the working fluid occurs in the other units (e.g. two-phase solar collector coupled directly to the turbine, boilers and evaporators). Dynamic modelling of these systems are important to understand the heat transfer behaviour as well as to develop the system level control among many other attributes. Among many other modelling approaches, a particular class of heat exchanger model, namely the moving boundary lumped-parameter model, has emerged as an efficient and effective tool for simulating dynamic characteristics of the two-phase solar collectors and the evaporators pertinent to organic Rankine cycle (ORC) systems. These models are efficient in tracking of the continuously moving phase change boundary without the requirement of well-formulated starting solutions. From the computational results it has been found that increasing refrigerant mass flow rate through the evaporator tube, results in an increased length of the subcooled segment. In order to achieve a fixed desired superheated condition at the heat exchanger exit, the length of the superheated region also increases as the subcooled refrigerant mass flow rate is increased at the evaporator inlet. It is observed that fluctuation in the wall segment temperatures, owing to the instantaneous fluctuation in the pumping pressure, is reduced considerably as the refrigerant mass flow rate is increased. Moreover, it is also observed that the mean temperature of all the wall segments decreases with increasing refrigerant mass flow rate. Study of the transient moving boundary characteristics for different steady-state mean pressure values as well as different extent of the dynamic fluctuations will provide further insight about the feasible operating conditions and would facilitate advanced control of the solar thermal load loop.