Thermodynamic and Exergy Analysis of a Parabolic Dish-Driven Transcritical CO2 Pumped Thermal Storage System for Combined Heat and Power

Abstract

Abstract

Rankine cycle CO2 pumped thermal energy storage (R-CPTES) offers high-density, emission-free grid storage, but existing designs are limited by modest turbine inlet temperatures and produce electricity only, leaving their thermal potential unused. This paper introduces a Rankine CO2 storage cycle driven by a high-concentration parabolic dish collector (PDC) and configured solely for combined heat and power, representing a combination of point focus solar energy with CO2 pumped thermal storage that has received limited attention in the literature. During discharge, the dish superheats the working fluid and raises the high temperature turbine inlet from 456 °C to 500 °C, boosting net power. A heating recovery exchanger placed ahead of the second regenerator then extracts useful heat from the turbine exhaust for district or process supply, without the absorption refrigeration subsystem used in comparable cooling inclusive designs. The aim is to characterise this system through energy, exergy, and parametric analysis. A closed, pinch-consistent model is developed under steady-state assumptions using the Span–Wagner equation of state, with the discharge low pressure, discharge mass flow rate, and PDC outlet temperature varied independently and jointly at a fixed 10 MPa high-pressure boundary. The analysis reveals a power-versus-heat trade-off governed by the discharge pressure and bounded by physical limits rather than interior optima, shows that the solar superheat is a prerequisite for cogeneration, and identifies the system as heat-transfer destruction dominated, with the latent cold storage the largest single source of irreversibility. At the design point the system delivers 16.1 MW of power and 2.5 MW of heat, attaining a storage round-trip efficiency of 73.2% (electricity-only), a solar-inclusive electrical efficiency of 58%, an energy utilization factor of 67%, and an overall exergy efficiency of 61.3%. A preliminary economic assessment gives a levelised cost of storage of 0.10–0.18 $/kWh, competitive with comparable CO2 storage systems. The proposed system thus provides a simple, fossil-free cogeneration solution for high-DNI regions based on a modular, point focus solar configuration.