Simulation and Performance Evaluation of Parabolic Trough Solar Power Plants

Abstract

Nine Solar Electric Generation Systems (SEGS) built in southern California between 1984 and 1990 continue to produce 14-80 [MWe] of utility-scale electric power each from solar thermal energy input. The systems collect energy using a synthetic heat transfer fluid pumped through absorber tubes in the focal line of parabolic trough collectors. The heated fluid provides the thermal resource to drive a Rankine steam power cycle.

A model for the solar field was developed using the TRNSYS simulation program. The Rankine power cycle was separately modeled with a simultaneous equation solving software (EES). The steady-state power cycle performance was regressed in terms of the heat transfer fluid temperature, heat transfer fluid mass flow rate, and condensing pressure, and implemented in TRNSYS. TRNSYS component models for the steam condenser and cooling tower were implemented in the simulation as well. Both the solar field and power cycle models were validated with measured temperature and flow rate data from the SEGS VI plant from 1998 and 2005. The combined solar field and power cycle models have been used to evaluate effects of solar field collector degradation, flow rate control strategies, and alternative condenser designs on plant performance.

Comparisons of measured solar field outlet temperatures between 1998 and 2005 indicate some degradation in field performance. The degradation in performance over time may be attributed, in part, to loss of vacuum in the annulus surrounding the absorber tube. Another potential contributor to solar field degradation is hydrogen accumulation in the annular space; hydrogen may dissociate from the synthetic heat transfer fluid and permeate through ii the absorber tube into the annulus. The thermal losses and resultant outlet temperatures are modeled assuming 50% of collectors experience some loss of vacuum and/or hydrogen permeation. The loss in electric power from the cycle is quantified as a function of the prevalence of vacuum loss and hydrogen accumulation in the field.

The electric power output from the system at a given incident radiation depends on the system efficiency, defined as the product of the solar field efficiency and the power cycle efficiency. The solar field efficiency will decrease with increasing outlet temperature, while the power cycle efficiency will increase with increasing outlet temperature. The magnitude of these competing trends is such that the net change in system efficiency with outlet temperature is small.

The SEGS plants use induced draft cooling towers for heat rejection. Cooling towers provide an effective means of heat rejection, but require makeup water to compensate for evaporative losses. The use of air cooled condensers can reduce plant water consumption; however, system efficiency suffers with the higher condensing pressure. The optimal size of an air cooled condenser unit is evaluated, and its performance assessed and compared to that of the current condenser/cooling tower system.