Investigation of a Transcritical CO2 Heat Pump in a District Heating Network with Excess Heat from a Power-to-X Facility

Abstract

Specialet undersøger ydeevnen og den økonomiske gennemførlighed ved at integrere en transkritisk CO₂-varmepumpe i Aalborgs fjernvarmenet, med fokus på at udnytte overskudsvarme fra et fremtidigt Power-to-X-anlæg. En stationær model af én 44,25 MW enhed viste en COP på 3,35, mens den dynamiske model, som tager højde for sæsonvariationer og transiente forhold, gav en lavere COP på 2,33. Inklusionen af en lagdelt varmtvandsbeholder forbedrede systemets fleksibilitet og øgede effektiviteten en smule. En skal-og-rør varmeveksler blev modelleret til at forvarme havvand, der ledes ind i fordamperen, ved hjælp af overskudsvarme. Dette hævede indløbstemperaturen med kun mellem 0,6 og 2,3 K og forbedrede COP'en marginalt fra 0,0256 til 0,0431. Den økonomiske analyse viste, at kapitalomkostningen for varmeveksleren blev estimeret til €14,7 millioner og ikke kunne retfærdiggøres af de driftsbesparelser, der var forbundet med den øgede COP. Nutidsværdiberegninger over 20 år viste, at systemet uden varmeveksleren var mere økonomisk rentabelt. Konklusionen er, at selvom den transkritiske CO₂-varmepumpe er teknisk effektiv og miljømæssigt fordelagtig, er integrationen af overskudsvarme via en stor varmeveksler i øjeblikket ikke omkostningseffektiv. Studiet anbefaler yderligere optimering og udforskning af alternative konfigurationer for at forbedre både ydeevne og økonomisk levedygtighed.

The thesis investigates the performance and economic feasibility of integrating a transcritical CO$_2$ heat pump into Aalborg’s district heating network, with a focus on utilising excess heat from a future Power-to-X facility. A steady-state model of one 44.25 MW module showed a COP of 3.35, while the dynamic model, which accounts for seasonal variations and transient behaviour, yielded a lower COP of 2.33. The inclusion of a stratified hot water storage tank improved system flexibility and slightly increased efficiency. A shell-and-tube heat exchanger was modelled to preheat seawater entering the evaporator using excess heat. This raised the inlet temperature only between 0.6 and 2.3 K and improved COP marginally from 0.0256 to 0.0431. The economic analysis revealed that the capital cost of the heat exchanger was estimated at $\EUR$14.7M and was not justified by the operational savings linked to the increased COP. Present worth value calculations over 20 years showed that the system without the heat exchanger was more economically viable. In conclusion, while the transcritical CO$_2$ heat pump is technically effective and environmentally beneficial, the integration of excess heat via a large-scale heat exchanger is not currently cost-effective. The study recommends further optimisation and exploration of alternative configurations to improve both performance and economic viability

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A master programme thesis from Aalborg University

Investigation of a Transcritical CO2 Heat Pump in a District Heating Network with Excess Heat from a Power-to-X Facility

[Undersøgelse af en trans kritisk CO2-varmepumpe i et fjernvarmenetværk med overskudsvarme fra et Power-to-X-anlæg]