Heat transfer fluids: Safe through the winter

Tue, 01/08/2017 - 14:32

The technician should check the status of the heat transfer fluid once per year during maintenance of the solar power system. (Photo: Sentinel)

Heat transfer fluids with frost protection are an important component in solar power systems and geothermal heat pump systems. Temperature resistance plays a major role for solar power systems, and for heat pumps, the potential water hazard from leaks is just as important.

Jens-Peter Meyer

While solar power systems and heat pumps complement each other very well, both technologies also require a heat transfer fluid that is protected against freezing. In the case of solar power systems, there is the additional challenge of ensuring that the heat transfer fluid can withstand high temperatures. For most products, manufacturers use monopropylene glycol (MPG) as a component for frost protection in the aqueous solutions. The substance, which boils at just under 190° C, is considered to be stable at a constant temperature of 170° C, and can briefly be subjected to higher temperatures. Glycols that boil at higher temperatures are an alternative for solar power systems that are subjected to particularly high stagnation temperatures, such as systems with vacuum tube collectors. Some manufacturers use the substance di-propylene glycol, which boils at about 230° C, others, like Climalife, use 1,3-propanediol, which boils at 213° C. With these frost protection solutions, solar power systems are expected to withstand short-term temperatures of up to 260° C, as the manufacturer Aqua-Concept states for its Coracon Sol 5 HF. None of these glycols are considered harmful to health, and are grouped in water hazard class 1. Di-propylene glycol is also added to cosmetic products. The 1,3-propanediol that Climalife uses in its products is obtained from renewable raw materials. Other manufacturers also offer this particularly environmentally friendly option.

Even though modern products are designed for high temperatures, heat transfer fluids should generally undergo thermal stress as briefly as possible in order to achieve a long service life. This is the only way to prevent so-called glycol cracking, which in extreme cases can lead to lumpy residues that clog the system. For this reason, solar power systems should be designed to empty well (see SW&W 10/2015, page 46). With these systems, evaporation of the heat transfer fluid as stagnation sets in causes the fluid to be quickly pressed out of the collectors and then protected.

It is not just the temperature resistance of glycol that is important. Also important are the additives to the heat transfer fluid for preventing corrosion of the pipes and for neutralising the acidic degradation products of glycol. If these additives remain as solid residues in the collectors during evaporation, they will be exposed to the full force of the solar energy. Additives that evaporate together with the other constituents are better protected. Independent tests carried out by the SPF Institute of Solar Technology provide the best substantiation of these products' properties. For example, Clariant has had its new product Antifrogen Solar intensively tested by the SPF with regard to the requirements of solar power systems. Antifrogen Solar is a heat transfer fluid based on monopropylene glycol and is available as a prepared mixture or as a concentrate.

Hot heat transfer fluids also have a different effect on solar circulation system seals than hot water does. For example, the manufacturer Tyforop Chemie states in the Tyfocor LS data sheet that the seals made of EPDM 281, AFM 34, or Centellen 3820, which are often used in solar power systems, are resistant. The suitability of the pressure equalization vessel membrane should always be checked, because the properties of the membrane not only depend on the outlet rubber, but also on the respective production conditions.

Alternatives to glycol

The Greenway® Neo Solar fluid from Climalife secures a longer working life of solar thermal installations. (Photo: Climalife)

The temperature resistance is not important for the heat transfer fluids of geothermal heat pumps, because the ground collector circuit does not become warmer than 30° C. Thus, glycols that boil at higher temperatures are not necessary for this application. The manufacturers often use monopropylene glycol for heat pumps. Monoethylene glycol (MEG) is the alternative in this case. Although MEG is harmful to humans, there is no difference to MPG because it is also a water hazard class 1 substance. In addition to glycol-containing fluids, manufacturers also offer heat transfer fluids based on ethanol, alkali carbonates, and alkali formates. The German Working Group on water issues of the Federal States and the Federal Government (LAWA) has formulated water management recommendations for geothermal probes and geothermal heat collectors, and publishes a table of water hazard class 1 products that can be used in geothermal heating circuits. LAWA does not list potassium carbonate because it is "very easily soluble in water and has a strongly alkaline reaction." Furthermore, the soil's natural potassium content is quite low. Ethanol also does not fulfil the LAWA recommendation criteria. However, it is comparable to the LAWA recommendations with regard to water hazard potential, which is why there are two fluids based on ethanol in the LAWA list.

Ronald Klukas, manager at Aqua-Concept, definitely sees advantages to ethanol and salt solutions: "These fluids have good heat transfer values and low viscosities." On the other hand, Markus Hafner, branch manager of GHC Gerling, Holz & Co. Handels GmbH, has a more critical view of ethanol: "It often makes no sense to use ethanol-based heat transfer fluids, because it generally entails high investments for explosion protection. Some breweries are currently using these fluids. Other applications are rare." Aqueous salt solutions are not the preferred choice for geothermal probes or geothermal heat collectors. They are generally used when there is a requirement for temperatures below -30° C. A further advantage of the salt solutions is that they are not combustible. "However, care must be taken to ensure that the installation does not undergo continuous oxygenation, and that the fluid has good corrosion inhibitors," said Hafner.

The new fluid does not endanger groundwater

Klukas has identified another aspect of ethanol: it is possible to use bio-ethanols from renewable raw materials, "which are very favourable in terms of the overall CO2 assessment due to the low CO2 emissions during the manufacturing process." This also applies to organic glycols from renewable raw materials. Aqua-Concept is currently working to make their products and production CO2-neutral. The company has also launched a new product this year, Coracon Geko W, which is not hazardous for water. "Coracon Geko W is of particular use if the authorities have prohibited products with a water hazard class of 1 or higher," Klukas stated. Gernot Krakat, responsible for the sale of heat transfer fluids at Fragol, confirms that there are "more frequent inquiries about fluids for use in water protection areas".

However, since Coracon Geko W does not contain frost protection, it must be ensured that the entire ground circuit and the system's supply lines will not freeze. This should not be a problem, given well designed probes and supply lines situated deep in the ground. Solar power systems can also be operated without frost protection, as is evident from the Aqua-System from Paradigma. However, during cold nights heat must be actively pumped through the solar circulation system in order to prevent freezing.

Many heat transfer fluids only contain additives classified as water hazard class 1, but some have constituents that are classified as water hazard class 2 or 3. And yet, Ronald Klukas observes that the water hazard class is not a factor in the sale, because the sale is only made based on the price. Nevertheless, he sees a trend towards products with very good biodegradability and the use of environmentally friendly glycols. Moreover, most current products only have frost protection of -8° C to -10° C. This is a great advantage for the environment, since the hazard potential is significantly lower due to the low glycol content. Since the viscosity is lower in comparison to higher frost protection solutions, a smaller circulating pump can also be used to save on electricity, depending on the system design.

The entire industry is now facing the price pressure. The assessment is that low-cost suppliers are drawing prices down with products of inferior quality. "Quality doesn't seem to have any meaning at this point. Unfortunately, the cheapest fluid wins," said Markus Hafner of GHC Gerling, Holz & Co.

Jens-Peter Meyer

“Solar thermal system with glycol fluids based on petroleum chemistry is not green”

Climalife uses a unique composition, based on a high performing glycol, for solar thermal systems. SW&E spoke to Emma Bardolph, Marketing & Business Development Manager, about heat transfer fluids for solar thermal.

S&WE: Which properties need a good heat transfer fluid for solar?

Emma Bardolph: A good heat transfer fluid for solar needs to prevent premature aging of the heat transfer due to overheating, increases the life of the installation, preserves the effectiveness, reduced maintenance and therefore generates savings to exploitation, reduced tarring, control corrosion. A good transfer fluid for solar needs at least to be derived from nature.

S&WE: How is Greenway neo Solar in contrast to other heat transfer fluids on the market?

Bardolph: Greenway® Neo Solar heat transfer fluid provides excellent performance under high temperature conditions comparing to traditional Propylene based fluids and avoids using fossil derived, petroleum based resources. Its unique composition is based on a high performing glycol, Susterra® 1.3 propanediol by DuPont Tate & Lyle Bio Products Company.

S&WE: How can you confirm the properties of Greenway neo Solar?

Bardolph: Glycol cracking occurs as heat transfer fluid sees temperature fluctuations, producing corrosive organic compounds and darkened fluid. Glycol fluids under these conditions may break down forming glycolates, a thermal decomposition product of glycol.  Susterra® propanediol, the glycol used in Greenway® Neo Solar fluids, in comparative tests with fluids refluxing at 192°C for 16 hours, has shown that it produces 85-90% fewer glycolates, than inhibited propylene glycol and ethylene glycol fluids. 

The low degradability of Greenway® New Solar fluid provides protection against periods of stagnation, slows fluid ageing, and prevents “tarring” in the systems to give installations a longer working life. When tested at over 150°C for 150 hours, Greenway® Neo Solar fluid degrades three times more slowly than a 50% concentrated solution of mono propylene glycol.

Stability at high temperatures is critical but how about the overall life of the fluid. Accelerated ageing tests on Greenway® Neo Solar fluid were run under nitrogen for 500 hours at a temperature of 150°C, stopped, then ran again for another 250 hours at 150°C and then for a further 72 hours at 200°C. After 822 hours the viscosity of the product was still 3.12 cSt and the pH of the product remained stable in a range from 8.7 to 7.99. A stable fluid means a long lasting installation that maintains its heat transfer efficiency as well as its ability to inhibit corrosion.

A thermogravimetric test (TGA) in pressurized air revealed stability of Greenway® Neo Solar fluid up to 109°C until decomposition which was defined as a weight loss of 5%. When tested under nitrogen and 10 bars of pressure this value rose to 199°C which is significantly higher when compared with mono propylene glycol -30°C which decomposes at 103°C under the same conditions.

This improved performance under pressure was confirmed with differential scanning calorimetry (DSC). The DSC illustrated that Greenway® Neo Solar fluid had a degradation temperature of 215°C under air and 290°C under nitrogen at 10 bars of pressure.  By comparison, the same mixture with a base of mono propylene glycol - 30°C under nitrogen at 10 bars of pressure degrades as low as 119°C.  With less decomposition of the fluid it significantly enhances fluid life as well as reduces the need for maintenance and therefore may generate operating savings.

S&WE: Greenway Neo Solar isn´t based on petroleum resources. Why do you use renewable raw materials?

Bardolph: When you think about solar thermal systems you think about harnessing the energy from the sun and not using petroleum based energy resources. The problem with most of the glycol fluids is that they are based on petroleum chemistry which means your environmentally friendly solar thermal system is not as “green” as you might have hoped.

S&WE: Are customers ready to pay more for high quality products like fluids from renewable raw materials?

Bardolph: More and more end users are sensible to ‘Green’ products and are ready to pay more for it as they buy organic food.

But “Green” does not matter if the performance is not there. Greenway® Neo solar heat transfer fluid provides excellent performance under high temperature conditions. Under solar stagnation conditions and high pressures, it does not decompose as rapidly as other fluids.

As it increases the lifetime of the installation the customers are ready to pay a little bit more.

With less decomposition and sludge formation it significantly reduces the need for maintenance and therefore generates operating savings while preventing corrosion in the system. This also is a saving money. At the end, the customer does not pay more!