Transcritical–Transcritical Cascade CO2 Heat Pump with Expansion Work Recovery: A Thermodynamic Analysis

Abstract

High-temperature heat pumps are a promising pathway for electrifying industrial process heat, but their application to large-temperature-glide duties remains technically challenging. Milk powder spray drying is one such application, requiring air heating to approximately 200 °C while offering only low-to-medium-temperature waste heat sources. Transcritical CO2 heat pumps are attractive for this duty because the sensible cooling profile of supercritical CO2 can be matched to the large temperature glide of air heating. However, the high operating pressures required in transcritical CO2 cycles lead to substantial expansion losses, creating a potential opportunity for expansion work recovery. This study evaluates ejector- and expander-based expansion work recovery in high-temperature transcritical–transcritical CO2 heat pump cycles for spray dryer air heating. Baseline and modified cycle configurations were modelled using steady-state thermodynamic analysis and compared using heating coefficient of performance, maximum achievable sink temperature, component-level exergy destruction, and discharge-pressure sensitivity. Under the investigated conditions and assumed component efficiencies, expanders improved the COP of all evaluated cycles. TTX-2 achieved a COP of 2.35, 5.4% above its corresponding TT-2 baseline. TT-2 and the external benchmark TT-4 each achieved a COP of 2.23 at 150 bar for the investigated duty. The comparison with TT-4 is a benchmark comparison, not an evaluation of a TT-4 recovery variant. The improvement was modest, and an upper-bound break-even expander cost of approximately 150 EUR/kW of delivered heat was estimated for the most favourable expander case. Ejector cycles reduced expansion losses in some cases but did not provide a clear cycle-level COP improvement because they altered compressor pressure ratios, gas-cooler outlet conditions, and cascade heat transfer performance. These findings apply to the investigated spray dryer duty and demonstrate that reducing expansion exergy destruction alone is insufficient to guarantee improved whole-cycle performance under the stated operating and component-efficiency assumptions.