Glide to efficiency
Thermodynamic and techno-economic evaluation of high-temperature heat pumps for industrial sensible heat upgrading with temperature glides
J.C. Stok (TU Delft - Mechanical Engineering)
S.A. Klein – Mentor (TU Delft - Mechanical Engineering)
J.L.B. van Reisen – Mentor (TU Delft - Applied Sciences)
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Abstract
Industrial process heat remains strongly dependent on fossil fuels. High-temperature heat pumps can reduce electricity demand and direct emissions by upgrading available waste heat, but their performance depends strongly on the temperature profiles of the heat source and heat sink. This study evaluates high-temperature heat pumps that absorb and reject heat over a temperature glide for industrial sensible-heat upgrading. Vapor-compression heat pumps (VCHPs), reversed-Brayton heat pumps (RBHPs), and an absorption-compression heat pump (ACHP) are compared for three heating scenarios with heat sink outlet temperatures of 150 or 200 C using a 100 C heat source.
Steady-state thermodynamic models were developed and validated for the VCHP and RBHP. Pure fluids and binary mixtures were optimized and evaluated through Pareto-based multi-objective screening and working-fluid family analysis. Thermodynamic performance was subsequently combined with environmental, safety, and practical criteria to select favorable working fluids. Preliminary compressor and heat-exchanger sizing, equipment-cost correlations, and levelized-cost-of-heat calculations were used to evaluate technical and economic performance at heating duties of 1 MW and 10 MW. Vendor data were used for the ACHP.
The VCHP achieved the highest and most consistent thermodynamic performance. Subcritical operation with a zeotropic mixture provided the largest benefit when the heat sink outlet temperature was 150 C and both the heat source and heat sink had a 50 K temperature glide. The zeotropic mixture advantage decreased at higher heat sink temperatures, where transcritical operation made pure fluids and low-glide mixtures more competitive. The RBHP performed most favorably when the heat-source-to-heat-sink temperature glide ratio matched its sensible heat-transfer profile, but generally had a lower COP and required larger high-pressure heat exchangers. Its internal heat exchanger provided a significant benefit mainly for the scenario with the largest heat-source-to-heat-sink temperature gap.
Equipment feasibility and economics improved substantially with scale. At 1 MW, several compressor and RBHP turbomachinery configurations required small, high-speed, or high-temperature designs with uncertain practical feasibility. At 10 MW, larger dimensions and estimated compressor efficiencies near or above 80% made centrifugal turbomachinery more credible and reduced specific investment costs. The VCHP generally achieved the lowest levelized cost of heat, while all evaluated heat pumps remained less expensive than direct electric heating under the assumed conditions. At 10 MW, the heat pumps were also competitive with gas-fired heating in most scenarios. Overall, temperature-glide heat pumps are most effective when the cycle, working fluid, process-temperature profiles, and heating duty are selected together.