Solar thermal power generation is currently an attractive solar electricity technology. Currently, we face an important issue of lower annual solar-to-power efficiency (approximately 10.0%) using parabolic trough technology because the direct normal irradiance instantly varies, and the solar thermal power cycle always derivates from the designed operation. Here, we investigate a middle-temperature solardriven Kalina cycle that uses a parabolic trough collector with a variable concentration ratio. From lower to higher direct normal irradiance, both the aperture area of collector and the flow process of the Kalina cycle can be changed. As a result, a much border direct normal irradiance of 100–1000 W/m2 achieves a solar-to-power efficiency of 4–20%, resulting in an annual solar-to-power efficiency of approximately 14%. Furthermore, the interactions are analyzed among direct normal irradiance, the aperture area of the collector, and the flow process of the thermal cycle. An operation method for off-design conditions is proposed to greatly improve the annual solar-to-power efficiency, offering a pathway to efficiently utilize a border range of direct normal irradiance.
Conclusions
The solar-driven Kalina cycle, having a variable concentration ratio, was investigated in this paper; this cycle has advantages of off-design operation by synergistically adjusting the aperture area of collector and the flow process. The collecting temperature is decided by direct normal irradiance ranges, and the Kalina cycle flow process can be switched to match the collecting temperature. The results show that on the autumn equinox, the variable concentration ratio pattern has the daily solar-to-power efficiency of approximately 15.2%, which is 2.8% higher than that of the design concentration ratio pattern, and the daily power output is 727.5 kW higher than that of the latter pattern. Thus, this cycle can efficiently utilize a border range of direct normal irradiance to power, providing an approach for effective solar energy use.
The results have been published on Applied Thermal Engineering 126 (2017) 997-1005.
Fig.1. The side profile of the parabolic trough collector with an adjustable mirror size.
