Alternative secondary fluids
The secondary fluid is a complex mixture which also contains pH adjusting agents, antioxidants, anti-foaming agents, stabilizers, colorants, etc. These additives can stabilize and improve the fluid performance but also affects the thermophysical properties. Secondary fluid is water-based solutions of organic or inorganic substances (such as alcohols, glycols and salts) used in various indirect refrigeration systems and heat pumps. The advantages of the indirect refrigeration systems are: low refrigerant charge, low risk of refrigerant leakage, the fact that the primary refrigerant system is localized in the machine room, easy and precise control, no oil recovery problems, refrigerants like ammonia and hydrocarbons can be used; easy maintenance, no technical limitations on the distance between refrigeration and machine room, plastic pipes can be used and that it will be longer intervals between defrost periods when air cooling due to more uniform frost formation.
Unfortunately, many of the currently used secondary fluids have corrosive nature but it is possible to minimize corrosion risks by using the right inhibitors. All additives stabilize and improve performance in some respects, but affect the secondary fluid’s thermophysical properties in a way that is not well known. Additionally, new type of adsorption corrosion inhibitors belonging to the so-called "green corrosion inhibitors group" that are in 100% organic/plant based. Plants are sources of naturally occurring substances with complex molecular structures that have different chemical, biological and physical properties. These naturally occurring substances are used because they are environmentally acceptable, cost-effective and are easy to find in nature. These advantages are the reason for the use of extracts of some plants and products as corrosion inhibitors for metals. Various plant extracts that can be used as the corrosion inhibitors are generally called green corrosion inhibitors.
Ethanol based aqueous solutions are commonly used secondary fluids in the ground source heat pumps in Sweden. The available ethanol based commercial products contain up to 10% denaturating agents and no corrosion inhibitors. Measurements recently performed at KTH show that the presence of these additives in ethanol enhanced the thermophysical properties, in particular the specific heat capacity, thermal conductivity and dynamic viscosity. So far, ethanol based refrigerants have not had corrosion inhibitors. Further studies are needed to find suitable corrosion inhibitors that can be used instead of or together with denaturation products to effectively protect the system against corrosion.