Effect of tube size on alternative material heat exchangers for geothermal applications by numerical model

The geothermal energy is widely diffused for high-size thermoelectric applications by using high temperature sources. The low-medium temperature reservoirs are basically used for direct balneotherapy scopes despite the low temperature fields are more present worldwide than high one. Geothermal fluids at medium and low temperature could be mainly used by considering the installation of small size power plant for power only or for heat and power by using Organic Ranking Cycles technology or in alternative for direct heating and cooling application. The optimization of system parameters in geothermal plant can be a good strategy for cost reduction mitigating the economy of scale. The use of small deep sites in low-medium enthalpy could be favourited by the cost’s reduction and by the exploitation of small-size plant for energy communities solving geothermal social acceptability issue. A high cost in the geothermal plant regards the heat exchanger evaporator enables the thermal heat exchange between the working fluid (which is commonly an organic fluid for an Organic Rankine Cycle) and the geothermal fluid (supplied by the aquifer). The high cost depends on aggressivity of geothermal fluid that leads to the corrosion of steel traditionally used in the heat exchangers. The maintenance of steel heat exchangers or in alternative the investment cost of anticorrosion metal (such as titanium) influence the diffusion of geothermal small plant. This paper analyses an alternative to metallic heat exchanger for thermoelectric geothermal use. Plastic materials usage was simulated for evaporator installed in an Organic Rankine Cycle plant in order to overcome the problems of corrosion/cost and the increase of heat exchanger thermal resistance due to the fouling effect. The design and performance investigation of optimum operating configurations for the evaporator is realized by considering titanium, steel and polymers taking into account both pressure drops and economic aspects. A mathematical approach via one-dimentional model is performed by using the correlation referred to in the literature about heat transfer in single‐phase and two‐phase fluids. A sensitivity analysis will be conducted on the tubes dimensions to minimize the plastic footprint. The outcomes provide the heat transfer area for the shell and tube heat exchanger with a fixed electric power size of thermoelectric plant. The results have demonstrated that the best combination of fluid, layout and polymer leads to a cost saving of evaporator of 70% with respect to titanium on the basis of the same life cycle time.