Alternativa etanolbaserade köldbärare med korrosionsinhibitorer i bergvärmesystem

Sammanfattning: Ground source heat pumps are very popular in the Nordic countries. The method of exchanging heat with the ground is based on the principle that a secondary fluid is circulating through a U-pipe borehole heat exchanger inserted into energy wells. Water is a very good secondary fluid, but aqueous solutions of ethanol are recommended in Sweden as a secondary fluid for such application. One reason is the fact that addition of ethanol decreases the freezing point of the fluid, which is preferred since many borehole heat exchangers in Sweden operate at the temperatures below or near the freezing point of water. Recently, the usage of adsorption type corrosion inhibitors has increased in different contexts, but no recommendation has been made regarding aqueous solutions of ethanol used in the different indirect refrigeration systems and heat pumps. These corrosion inhibitors can influence the thermophysical properties of the secondary fluid, and thereby affect the performance of the borehole heat exchanger in terms of the hydraulic and thermal performance. These not defined effects of adding corrosion inhibitors could cause uncertainties in the design and operation of the ground source heat pump. The thermophysical properties density, freezing point, dynamic viscosity, thermal conductivity and the specific heat capacity of seven aqueous solutions of ethanol (30 wt-%) with addition of different corrosion inhibitors are experimentally studied in this report. Moreover, a pure mixture of ethanol and water with the same weight concentration is studied and compared with the reference data. The suitable mixtures showing the most optimum properties in terms of low dynamic viscosity and high thermal conductivity and specific heat capacity for the borehole heat exchanger application were compared with the thermophysical properties of the secondary fluid commonly used nowadays in Sweden. The results indicate that the solutions including the corrosion inhibitors: 1H-benzotriazole and sodium gluconate indicate promising thermal properties in comparison with the pure mixture of ethanol with the same weight percentage. Furthermore, the results indicate that further studies on corrosion inhibitors added to aqueous solutions of ethanol with lower weight percentage may prove to be more effective as replacement for the denaturing agents in comparison with the field sample (20 wt-% ethanol with denaturing agents).