In 2010, Dynalene was approached by Therma-Hexx LLC of Portsmouth, N.H., to provide a coolant for their thermaPAVER® product, which uses solar thermal energy to heat swimming pools, cool pavers and walkways, domestic hot water, and snow melt systems (Fig. 1). The goal set out by Therma-Hexx LLC for the heat transfer fluid was to offer freeze protection and low viscosity at low temperatures while having good thermo-physical properties. The coolant would also require good thermal stability and low corrosion rates at higher temperatures.
Typical propylene-glycol-based or ethylene-glycol-based heat transfer fluids with phosphate inhibitors could be used in most situations where freeze protection and corrosion resistance would be important. The major component in the ThermaPAVER® heat transfer loop was aluminum, however, and the surface area of the aluminum that comes in contact with the coolant was very high.
Aluminum could cause problems in systems with typical inhibitors because of pH issues. For systems using steel, copper or brass, a higher pH of 8.5 to 10 is recommended. If aluminum is used in that pH range, the corrosion rate will be high and aluminum hydroxide will form. As aluminum hydroxide forms, the pH will increase, which causes even more corrosion. If the system is below 50°C, a simple pH adjustment can be done to stabilize the coolant and stop corrosion. Above 50°C, adjusting the pH will not reduce the potential for corrosion at higher temperature but will cause severe corrosion in aluminum components.
Finding the Answer
Dynalene separated the issue into two parts. The first part was to choose a base coolant that would demonstrate good thermo-physical properties, low viscosity at low temperatures and high durability at higher temperatures. The second part was to develop an inhibitor package that would offer excellent corrosion protection and pH buffering in an aluminum system cycling through a large temperature range.
In identifying the proper base coolant, Dynalene had a number of options to choose from but narrowed it down to using propylene glycol, ethylene glycol and a corn-derived glycol called BioGlycol. Even though ethylene glycol has good thermo-physical properties, it was ruled out due to its toxicity. Propylene glycol is safe, but the product was not as efficient as ethylene glycol or BioGlycol due to its higher viscosity at lower temperatures. BioGlycol offered a safe product with better viscosity than propylene glycol at lower temperatures, making it easier to pump under cold conditions. BioGlycol also offered better thermal stability than ethylene glycol or propylene glycol at higher temperatures.
The corrosion inhibitor package must have a good reserve alkalinity for pH buffering, and the ability to mitigate the acids that form as glycols degrade at higher temperatures like a phosphate-based inhibitor package (e.g., Dynalene PE-1). In addition, it should have the ability to protect the aluminum that is in contact with the heat transfer fluid. To get the best results, Dynalene decided to test multiple inhibitors at various concentrations to optimize the coolant for the high surface area of the aluminum in the system. Electrochemical corrosion tests were performed over a period of 96 hours using a Gamry pontentiostat.
After consideration of the properties of the different glycol-based fluids, BioGlycol was chosen due to its non-toxicity, durability and lower viscosity at lower temperatures.
For the electrochemical testing, Dynalene PE-1 inhibitor package and the aluminum specific Dynalene V1 inhibitor package were tested. The results concluded that the Dynalene V1 inhibitor package offered superior protection for the aluminum (Table 1). As Table 1 illustrates, the corrosion rates are similar at 25°C, but as the temperature increases to 60°C the Dynalene V1 demonstrates a lower corrosion rate than Dynalene PE-1.
A product containing both Dynalene PE-1 and Dynalene V1 inhibitors proved to be a viable option for Therma-Hexx LLC’s ThermaPAVER® product. The presence of Dynalene V1 inhibitor prevented the corrosion of the aluminum. Dynalene PE-1 stabilized the pH at the higher temperatures up to 60°C at which the fluid was used. The final heat transfer fluid, a combination of BioGlycol, Dynalene PE-1 and Dynalene V1, offered safe and reliable performance while maintaining a very low corrosion rate.