Alternative formate salts as low temperature secondary fluids.

Abstract

Potassium organic salts like formates and acetates are known as environmentally friendly secondary fluids. The most important advantages of salts compared to aqueous solutions of alcohols and glycols are good thermophysical properties, low toxicity and non-flammability. The purpose of this work is to investigate properties of different formate salt based secondary fluids like: lithium formate, ammonium formate and sodium formate at low temperatures in order to propose new blends of formate salts that could be used as secondary fluids for low temperature applications. Studies showed that different alkali metal ions like sodium, lithium or ammonium are affecting the solubility level, freezing point, thermal conductivity, dynamic viscosity and specific heat capacity in different way. Among examined formate salts, ammonium formate showed the best performance by giving the lowest freezing point as well as the highest specific heat capacity and highest thermal conductivity and similar dynamic viscosity. Lithium formate salts had the highest dynamic viscosity among all samples and despite high specific heat capacity and thermal conductivity values these salts cannot be recommended for low temperature applications. 30 wt-% sodium formate and 36 wt-% lithium formate were recrystallizing at lower temperatures than 0 °C when fast cooling rate was applied. Thus, the possible application for these salts is rather limited to higher temperatures only. As seen, different type of cation group in the formate salt can result in different properties, thus, further studies to investigate ammonium formate as well as different acetate salts are recommended.