Cooling medium for solar power system

June, 2013г.

No one will be surprised today seeing the panel of a solar power system on a roof. But it is still attracts attention. Sun is the source of free energy and heat, but so far only forward-minded people use solar power systems in our country. General public basically has no information on that account.

In this article we want to illuminate one of the integral parts of a modern solar power system, namely a cooling medium circulating in a solar power circuit.

A cooling medium in a solar power system bears the function of receiving of heat energy from the Sun and transferring it to the consumer source (WAN system, swimming pool etc.). A cooling medium has to conform with certain requirements — it must not freeze at negative outside temperatures, must be thermally stable under overheating conditions, must not be aggressive to materials of a solar power system (metals, gasket materials). All of these requirements must be fulfilled by the content of a cooling medium. Let us see what components a cooling medium consists of.
The base of a typical cooling system for a solar power system is propylene glycol. Pure propylene glycol is a thick colourless liquid without odour, sweetish by taste and greasy by touch. It is a nutritional supplement used in production of cigarettes, combined fodders, medicinal products, cosmetic products and perfumery products. That is why cooling media on the base of propylene glycol are considered to be environmentally friendly and biodegradable. Another important quality — propylene glycol mixed with water does not freeze at negative temperatures. A solar power system located on a roof or open site should not lose its operability in winter time. Pretty hard frost is quite common for our climatic zone, so a cooling medium should not freeze at temperatures up to −300С. And that requirement propylene glycol fulfills entirely.
As it follows from the foregoing, the second component of a solar power system cooling medium is water. In order not to pollute a cooling medium (and equipment) with foreign matters and not to increase aggressiveness of a cooling medium manufacturers use treated water, distilled or demineralized, from which all or most of dissolved matters removed.
Both propylene glycol and demineralized water are corrosion active substances. When cooling medium temperature increases its aggressiveness increases too by several times, so that is why a cooling medium contains corrosion inhibitors. A cooling medium in the system contacts not only with metals but with a range of other materials which it affects as well (rubber, chloroprene, polyethylene, silicone, etc.). Corrosion inhibitors not only protect metals but minimize influence of a cooling medium on other materials.

Concerning boiling temperature of a cooling medium it is known that propylene glycol starts to boil under temperature 184-1890С. But any cooling medium contains a lot of water and that’s why all propylene glycol cooling media start to boil if the temperature of 1000С is slightly exceeded. Sometimes consumers say that the description of a foreign cooling medium can contain boiling point higher than 1500C. Read the documentation carefully! This temperature is specified for the CONCENTRATE of a cooling medium which contains almost no water. But it is prohibited to use the concentrate in a circuit because it becomes very thick at the temperature of several degrees below zero and stops to be pumped. It is necessary to mix the concentrate with water in a right proportion before filling into a circuit. And that is where we have a question — how to use a cooling medium in a solar power system if its’ boiling temperature is significantly lower than required operational temperatures? Consider that a solar power system is always overpressurized. While pressure is increased, cooling medium temperature is increased too.
If pressure in a circuit is 2 bars, then a cooling medium will boil at the approximate temperature 1260С, if it is 3 bars — at approximately 1420С, 4 bars — at approximately 1500С. Standard operational condition is when a cooling medium does not boil, in other words a solar power system have to be designed in a way to prevent boiling of a cooling medium.
This can be prevented by different means:

  • to pump a cooling medium with a speed sufficient to ensure fast heat transfer;
  • to close the panel with a louvers when overheating;
  • withdraw a cooling medium to an expansion tank.

But sometimes a cooling medium boils anyway. Why is it bad?

Thermal degradation of propylene glycol following with formation of carbon oxide starts at temperatures exceeding 1700C. A cooling medium starts to get cloudy, gains dark colour and losses its operational properties. Steady dark carbon residue formed on the walls of equipment. In this case it is necessary to change a cooling medium and to clear equipment or partially change its components. So, it is critical to prevent boiling of a cooling medium or reduce boiling time to minimum in order to protect a solar power system.

Now we got acquainted with contents and properties of a solar power system cooling medium and will continue with description of two members of this group.

Tyfocor LS cooling medium manufactured by TYFOROP CHEMIE GmbX German company is one of the foreign cooling media most commonly used in a solar power equipment. Equipment often delivered assembled and filled with a foreign cooling medium. But when a customer needs to refill or change a cooling medium he often bumps into impossibility to buy this cooling medium separately — it may be absent in stock or delivered only as a part of a completed equipment. And its price is very high.

We will review the alternative option — Tepro-Solar 30P of domestic manufacturer — «STRON» company. «STRON» Limited Liability Company has been manufacturing cooling media since 2005. Three years ago Tepro-Solar trade name for solar power systems was put into production because of increased demand on solar power systems. Cooling materials are manufactured of qualitative foreign raw materials and are equal to Tyfocor LS by its characteristics. Tepro-Solar cooling medium is always in stocks of the manufacturer and its partners because of steady demand. The price for the cooling medium is significantly lower than the prices for foreign equal products because of domestic Ukrainian production, which is a big advantage to domestic customers.
The table below contains physical and chemical characteristics of cooling media described above. Data were taken from technical documentation of manufacturers.

Indicator

Tepro-Solar 30P

Tyfocor LS

Density at 200С, g/cm3

1.047-1.048

1.032 - 1.035

Kinematic viscosity at 200С, mm2/s

4.5

4.5 - 5.5

Chilling temperature, 0С

< -30

-28

Boiling temperature under atmospheric pressure, 0С

> 100

102 - 105

Water content, %

50 - 52

55 - 58

рН index

8.5 - 9.5

9.0 - 10.5

Corrosion effect on metals, g/m2 daily:    

 

 

copper

0.08

0.14

brass

0.07

0.29

steel

0.03

0.01

cast iron

0.03

0.01

aluminium

0.03

0.02

solder

0.08

0.43

Note: The results of corrosion tests on metals expressed in measurement units used in Ukraine because this tests are conducted by different (although related) methods.

Alkalinity reserve index is deliberately omitted because the differences in measurement methods and interpretation of this index will not supply the reader with any objective information to consider.
As we can see from the table, cooling media are compatible.
Terpo-Solar, as well as Tyfocor LS, have been used for a long period of time at vast amount of objects on the overall territory of Ukraine.
Finally, we want to say that you can call to ‘STRON’ LLC office at any working day, anytime. Our personnel will answer all you questions and help to resolve many accompanying tasks related to packaging, delivery, documentation and others.