Carbon-neutral steam supply for a chemical plant: Thermoeconomic analysis of the integration of a thermal storage unit to a high-temperature heat pump using CO2
Ensuring a CO2-neutral supply of process heat is critical for advancing the energy transition in industrial sectors. A promising approach is the integration of high-temperature heat pumps, powered by renewable electricity, to generate process steam at elevated pressure levels, which is widely used in the chemical industry. An ultra-high temperature heat pump concept with supercritical CO2 as the working medium, designed for the supply of a generic chemical plant, was developed as part of the project CO2NEICHEM. At this stage, the inclusion of a thermal storage unit, consisting of one hot and one cold tank, and its effect on the part load behaviour of the heat pump is to be investigated. The heat pump represents a transcritical reverse Brayton cycle with an internal heat exchanger and an expansion turbine. For the analysis of the part load behaviour, characteristic curves are used for the turbomachinery and the heat exchangers, while the part load itself is controlled by the heat flow at the heat source side. Within this study, it is shown that a high relative COP (COPPartLoad/COPDesign) can be maintained with and without the inclusion of the storage unit, with a corresponding shift in the pressure levels at inlet and outlet of the compressor. Moreover, it is shown how different part load operation conditions and a variation of the design point influence the results and technical limitations. The storage unit is integrated to be able to decouple the electricity consumption from the heat generation, so the effect of high electricity prices and the peak load demands can be moderated. To investigate the economic benefit, a thermo-economic evaluation is performed based on the LCOH to later allow the integration of the system in different configurations into an energy system analysis with the determination of relevant parameters like operational and capital costs of the overall system.