CO2 Heat Pump Performance

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CO2 Heat Pump Performance ( co2-heat-pump-performance )

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A Comparative Study of CO2 Heat Pump Performance for Combined Space and Hot Water Heating Abstract Eivind Brodal* and Steve Jackson UiT-The Arctic University of Norway Heat pumps used for combined space and hot water heating are often used in modern energy- efficient buildings. Performance depends on both system design (exchanger sizes, compressor efficiency, etc.) and operating conditions (inlet water temperature, ratio between space and water heating, etc.). Designs using both CO2 and HFCs, such as R410A, are available, but the relative performance of these is not extensively studied. This article presents performance results based on a system model developed in MATLAB where exchangers are modelled using fixed temperature pinches and pressure drops. Ejectors and compressors are modelled using defined efficiencies. Operating parameters are optimized using the Genetic Algorithm for a set of sensitivity studies. The results show that CO2 can outperform R410A when the ratio of space to water heating is below 0.6 – 1.0, feedwater is below 20 Β°C (as in norther Europe) and heat exchangers are designed with temperature pinches below 10 K. Keywords Integrated heat pump; R744; R410A; Ejector; Optimization; Pinch point * Corresponding author. Tel.: +47 77660364. E-mail: eivind.brodal@uit.no Nomenclature Subscripts comp Compressor crit Critical ejec Ejector is Isentropic rec Recovered refrig Refrigerant COP Coefficient of performance [-] DHW Domestic hot water HFC Hydrofluorocarbons 𝑔𝑔 Inequality constraints function GWP Global warming potential -1 h Specificenthalpy[kJkg ] π‘˜π‘˜ Penalty factor LMTD Log mean temperature difference LNG Liquefied natural gas -1 π‘šπ‘šΜ‡ Mass flow rate [kg s ] NTU Number of transfer units 𝑃𝑃 Duty [kW] 𝑝𝑝 Pressure [bar] Δ𝑝𝑝ex Pressure drop in heat exchangers π‘žπ‘ž Penalty function π‘Ÿπ‘Ÿheat Heating ratio (Space to DHW heating) [-] 𝑅𝑅refrig Refrigerant (CO2 or R410A) 𝑇𝑇 Temperature [Β°C] 𝑇𝑇�a Temperature at point β€œa” in Figure 1 [Β°C] 𝑇𝑇 Average evaporation temperature [Β°C] evap 𝐗𝐗 Key design variables 1

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