What about solar systems that use a glycol antifreeze solution?
First a little history.
In the late '70s and early '80s, solar developers, including Exxon, Reynolds Aluminum, GE, Grumman Aircraft, and all the little guys like me, were trying all kinds of things to see what worked. We all knew that if you put a black surface in a box it would get hot. How to make the rest of the system work was the question. A major problem was how to keep the system from tearing itself to pieces over time.
HVAC design engineers have used pressurized glycol systems for decades to transfer heat from boilers to air handlers. This method is the standard for hydronic heating systems and many other industrial heat transfer tasks.
Naturally, they applied this design to solar systems, treating the collectors as the boiler and the tank as the receiver. When you walk into almost any professional engineering firm and ask them to design a hydronic heating system OR a solar system, they will grab their manuals that show how to assemble a pressurized glycol loop.
The design starts with methods to get the heat from the collector loop into the solar storage tank. Since it is too expensive to fill the tank with a glycol solution, a heat exchanger is used. The collector glycol fluid goes through one side of the exchanger and the tank water goes through the other side. There are two pumps, one on each side of the exchanger, and controls to turn the pumps on. A typical heat exchanger is only about 50% efficient; meaning a lot of energy collected by the collectors never makes it into the tank.
Glycol loops are full of fluid all the time. This is good (most of the time). They remain ready to run whenever the sun shines and the pump turns on.
When such a system is installed, coin vents (can turn the screw with a dime) are installed at all the high points in the piping where air can accumulate and vapor lock the system. The startup procedure is to fill and pressurize the pipes (to maybe 15 psi) and go around to all the coin vents and burp the air out. Over the years, people have invented clever air vents that when dry will leak air and when wet will seal. That way you don't have to go to each one to burp it, it will do so by itself. It is like the rope caulking used in boat hulls for thousands of years. As long as the boat stays in the water, all is fine. If you take it out and let the caulking dry out, it will leak until the caulking gets soaked again. There are many other kinds of automatic air vents, some based on the float system seen in toilets.
Safety also requires a pop-off valve near the boiler (i.e. collectors) to relieve pressure in case the system overheats on a hot summer day. A glycol-water mix is a great solvent for shingles and plastics, including tile floors, so pop-off valves should have a pipe running to a drain to contain the liquid in the event of a failure.
Since pressure goes up and down with temperature, a clever system was devised to maintain a minimum pressure in the loop. A small tank, called an expansion tank, is installed in a tee in the line. The expansion tank has a rubber membrane running across the middle. The system fluid fills up one side and air fills the other side. The fluid in the system can expand and contract with temperature into the expansion tank, and the air bladder will keep the pressure within a specified range. The air pressure is set with an air hose and tire inflator, just like a car tire. A chart is used to determine the correct pressure according to the temperature of the system at the time it is filled.
However, expansion tanks have a lifetime. The rubber (or neoprene, or whatever) bladder will someday crack from flexing as it ages and the expansion and pressure regulation benefits of the tank are lost. The system will usually vapor lock somewhere and the whole startup procedure has to be repeated.
Unfortunately, a solar system doesn't like to play by the rules. They are not well behaved. Typical boiler systems do not go through the extreme temperatures that solar collectors do. A boiler heating loop may have a maximum temperature of 140-160ºF. It never gets colder than room temperature inside a building, so the maximum temperature swing from summer to winter may be 90ºF (70-160ºF). Now consider that a solar system has the "boiler" sitting outside in the weather. It is always off at night where there is no sun. In the winter, the temperature may go down to -40ºF (Willmar, MN). Even in the mountains of NC, winter evening temperatures can go well below zero. On the other hand, glycol based solar systems can see maximum temperatures as high as 220ºF, for a 180ºF swing, which is twice what a ordinary boiler system sees.
A solar system will typically see its maximum temperature in the summer time, with variations according to the application. The most extreme case occurs when there is a very hot day with high solar radiation, and there is little need for the hot water. This can occur randomly on weekends, or summer vacations, and especially on space heating systems that sit idle all summer. When this scenario happens, the heat from the collectors is not needed and the temperature builds up until the boiling point is reached. This same problem can occur during normal operation if there is a power failure and the collector pump stops. The pump should never stop running during a hot day on a glycol system, regardless of whether the energy is needed or not. However, that will not guarantee the collectors won't boil.
If the collectors get to the boiling point, a glycol system is in big trouble. The pop-off valves will blow, dumping glycol down the drain and dropping the pressure in the system. The next night when it cools down, there will be vacuum in the lines and the air vents will leak air in, vapor locking the system. The next day the hot glycol solution has air in it. A chemical reaction occurs with the oxygen that breaks the glycol into fatty acids, which can clog and eat the pipes if the situation is not corrected promptly. This scenario is not self-correcting. The system stops working, compounding the problem, and needs to be attended to. This is a progressive failure mode. Even without boiling, the glycol solution in the collector loop will age, breaking down into acids.
For this reason, large glycol systems have additional equipment to dump excess heat. It usually consists of a big outside fan coil unit that is connected to the collector loop. The heat dump turns on when the temperature gets too high and dumps the heat to the outside air. You have to use energy to waste energy. The components include temperature controls, bypass valves, fans, and pumps. The added complexity just adds more failure modes. Heat dump systems cannot overcome power failures, unless you add a back up generator, which can have its own failure modes. Glycol systems require much more inspection and maintenance than other systems.
Whenever I think of solar glycol systems, I am reminded of the fairy tale about the little old lady who swallowed a fly .
I KNOW AN OLD LADY
Written by Rose Bonne and Alan Mills-
©1952 Peer International ( Canada) Ltd. SOCAN
I know an old lady who swallowed a fly
I don't know why she swallowed the fly
Perhaps she'll die
I know an old lady who swallowed a spider
That wriggled and jiggled and tickled inside her
She swallowed the spider to catch the fly
But I don't know why she swallowed the fly
Perhaps she'll die
I know an old lady who swallowed a bird
How absurd to swallow a bird
She swallowed the bird to catch the spider
That wriggled and jiggled and tickled inside her
She swallowed the spider to catch the fly
But I don't know why she swallowed the fly
Perhaps she'll die
. . . . . . . . . . . . . . .
In an effort to overcome the many problems of a glycol system, early researchers turned to other methods.
To overcome boiling and pressure problems with glycols, high temperature silicon oils were used. Unfortunately, they were very expensive, had poor heat transfer characteristics, and tended to leak out of soldered joints.
Others tried air as the heat transfer medium. It wonʼt boil or freeze. However, blowers and duct work to the collectors were a problem, and storing the heat from the air in a pile of rocks brought its own problems of mold and dust. You canʼt fab a rock pile and ship it to a site.
Others went back to plain water as the heat transfer fluid. It has the highest heat transfer capacity of any fluid. All others are measured against water, which is rated as 100%. Glycol is about 85%, and silicon oil is only about 20% as good as water.
Since water will freeze and boil, the idea is to drain the water from the collectors at night, so it is not there when the extreme conditions come. Early designs included air vents at the high points and heat exchangers between the collectors and storage. Some thought a vacuum breaker was required at the top to make the water drain out when the pump stopped. Others even installed a pipe between the collector supply and return lines with an electric valve to guide all the water to the return line for draining.
All these vestiges of glycol systems only caused problems. Air vents and vacuum breakers introduce fresh air (oxygen) into the water, accelerating corrosion. Ordinary air vents on tanks cause evaporation losses, which required periodic refilling (and fresh oxygen). Protecting against corrosion by lining the tank is cost prohibitive above a certain size, and subject to cracking during transport.
The way to make a drain back system efficient and durable was to rethink all the design parameters to minimize or eliminate problems. This was the origin of the GRC drain back design, used in all Holocene systems.
The original design concepts were:
1. Non pressurized operation: If the system isn't pressurized, then tank doesnʼt have to have an ASME pressure rating, which doubles or triples the tank price. Non pressurized systems donʼt need pressure relief valves, check valves, expansion tanks or other safety devices.
2. No heat exchanger in the collector loop: Tank water is pumped directly through the collectors and back to the tank. None of the collected energy is wasted going through a heat exchanger. This means maximum heat is delivered directly to the tank. Drain back systems are up to 20% more efficient than antifreeze systems.
3. Maximize delivery of heat to applications: Eliminate heat exchangers where possible (i.e., space heating, radiant slab, etc). Domestic hot water systems always require a heat exchanger between the non pressurized tank and the pressurized cold water line, but other applications may not.
4. Create a tank vent that minimizes evaporation losses: A special immersion vent was developed that prevents ordinary evaporation losses while maintaining atmospheric pressure. Occasionally, water and corrosion inhibitor need to be added, but systems may go years before this simple maintenance is needed.
5. Simple oxidation and galvanic corrosion control: A non-toxic, food grade boiler corrosion chemical was selected that scavenges oxygen from the water, prevents galvanic corrosion, and helps clean the piping lines. Lack of glycol degradation and corrosion means drain back systems last 30% longer.
6. Create a Unified Fluid Handling Systemsm: The idea is to create a single unit that contains all the working components except the collectors, is factory built, wired and ready to install. I call this the Grand Central Station approach.
7. Simple controls, no prioritization of energy among applications: All applications (DHW, space heat, pool, etc) have equal access to the energy. You never want one task to hold off energy that could be used in another task.
8. Maximum thermal energy conservation. Enclose all pumps, exchangers, and controls within the thermal insulation of the system, where feasible. Use excess heat from pumps, for example, to heat the tank. Minimize line losses by including local plumbing inside the insulation shell.
The result is a system that is the simplest possible, has the highest efficiency, is the most durable, and is the most economical to build. Many are still running after 25 years with little or no maintenance.
There are two elements to the design: These design concepts have been used to build a family of products, called Fluid Handling Systems (FHS). Thousands have been installed from New England to California.
The product consists of an insulated tank with all the controls, pumps, and exchangers built in under the insulating shell. Some classes of pumps are water cooled. They work very well inside the insulating shell.
Larger pumps that are air cooled will not work inside the thermal shell, so they are placed outside. The electrical controls are mounted in a cabinet attached to the outside of the tank. Numerous sizes and options cover the smallest to the largest solar installation. A patent was granted for the drain back product design.
