A Calculation Model for a Thermodynamic Analysis of Solar Plants with Parabolic Collectors Cooled by Air Evolving in an Open Joule-Brayton Cycle

The paper presents a model able to carry out a thermodynamic analysis and evaluation of the annual performance of solar plants provided with cylindrical parabolic collectors cooled by atmospheric air. The air is taken from the outside, compressed in an interrefrigerated compressor, in order to reduce the compression work, sent to the field of solar collectors for its heating, then sent to the turbine connected to the electrical generator and then discharged. Before discharging the air to the ambient, it is convenient to recover a part of its enthalpy, preheating the outcoming air from the compressor in a heat exchanger-regenerator before being channelled to the solar field.The plant can be operated with variable flow rate and constant outlet temperature from the solar field, or with constant flow rate and outlet temperature variable with solar irradiance: both modes of operating the plant are discussed and analysed in the paper. The thermodynamic air cycle is an open-type Joule-Brayton cycle.In the paper a variant of the plant configuration is also studied, with the reheating of the fluid in the collectors after an initial expansion in the first stage of the turbine; after the reheating, the fluid completes its expansion in the second stage of the turbine. In some circumstances this type of plant performs better than the first.The plant proposed is particularly simple, is able to compete well with other more complex plants operating with different heat transfer fluids and is also attractive from an economic point of view.Calculation results are reported for plants located in some Italian and foreign places, in terms of annual electricity production, average efficiencies of collectors, turbine and whole plant.