Thermodynamic Matching of a Transcritical CO2 Heat Pump for 300 °C Industrial Steam Generation Using Low-Grade Waste Heat from Compressed Air Energy Storage

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Abstract
Recovering the low-temperature heat released during compressed air energy storage (CAES) charging for industrial steam generation requires a heat pump that can provide both the required steam temperature and sufficient heating capacity. This study applies a previously reported recuperated transcritical CO2 configuration to the 11.64 MW cooling-water circuit of a 100 MW CAES plant, with water entering the evaporator at 50 °C and returning at 25 °C. The steam-generation circuit recycles saturated liquid from the flash separator and mixes it with pressurized make-up water, after which the mixture is heated at high pressure, throttled to generate steam and separated before steam superheating. At a compressor suction temperature of 97.5 °C and discharge pressure of 26.0 MPa, the system supplies 31.96 MW of heat and produces 10.79 kg s−1 of steam at 0.40 MPa and 303.33 °C, with a cycle COP of 1.533, excluding pumping. Increasing the suction temperature from 94.5 to 100.5 °C raises the discharge temperature by 7.81 K but reduces COP from 1.550 to 1.520, because the compressor power increases while the expander power decreases. Although more steam is produced, its delivery temperature decreases because the fixed superheater duty supplies less heat per kilogram. Furthermore, the baseline temperature profiles identify an internal minimum temperature difference of approximately 12 K in gas cooler B, which is smaller than either terminal difference. These findings indicate that CAES cooling water can be used for 300 °C steam generation through staged CO2 heat release, while the effects of recuperation on power consumption and heat distribution must be considered together when selecting the cycle operating conditions.

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