The design and analysis of a coaxial casing heat exchanger used in geothermal energy systems

Geothermal energy has been in rich in both onshore and offshore conditions. Air can be used as working fluid at offshore platforms for geothermal energy development. However, existing models cannot present the comparison results between onshore and offshore coaxial casing heat exchanger (CCHE), between single component and multi-omponent working fluids, and between insulated and uninsulated conditions. In order to fill this gap, this paper present a more comprehensive model that can be used for simulating the performances of working fluids of various compositions in both onshore and offshore CCHE systems under both insulated or uninsulated conditions. The results show that: (a) the working fluid of air is more suitable to extract geothermal energy due to higher temperature at the effective extraction length (EEL) position. (b) For both working fluids of CO(2)and air, the TTI values will be increased by nearly three times when the CCHE system is insulated. (c) While the air extracts more geothermal energy at the section of below the EEL position, the heat flux between CCHE and earth is almost close to zero for both CO(2)and air at the section of above the EEL position. (d) The CCHE system is more effective and economical when it is onshore condition or when the length of seawater section of the CCHE system is not too long.