HEATING AND COOLING design engineers have used pressurized glycol systems for decades to help transfer heat from boilers to the load. This method would be the standard for hydronic heating systems and many industrial warm transfer tasks. Naturally, they applied this design that will solar systems, treating the collectors as the boiler and then the tank as the receiver. If you walk into most professional engineering providers and ask them to design you a hydronic heating system OR a solar system, they will grab their manuals that show how to throw together a pressurized glycol loop.
Since it is too expensive towards fill the solar storage tank with a glycol alternative, the heat is transferred from the collector loop into the water tank through a heat exchanger. The collector fluid goes through an individual side of the exchanger and the tank water goes through the other one side. There are two pumps, one on each side with the exchanger, and controls to turn the pumps on. Glycol loops are “charged” all the time. This is good. They continue ready to run whenever the pump kicks in. When ever such a system is installed, coin vents (can turn often the screw with a dime) are installed at all the high points which is where air can accumulate and vapor lock the system. The itc procedure is to fill and pressurize the lines (maybe 15 psi) and go around to all the coin visage and burp the air out. Over the years, people have invented cunning coin vents that when dry will leak air once wet will seal. That way you don’t have to go to each one so that you can 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 goes toward the water, all is fine. If you take it out and let the caulking be deprived of water, it will leak until the caulking gets soaked again. There are a number other kinds of automatic air vents, some based on the float system seen in toilets. Safety also requires a pop-off valve towards the boiler (i. e. collectors) to relieve pressure in case the exact boiler controls go haywire. A glycol-water mix is a popular solvent for shingles and plastics, including tile carpeting / flooring. So the pop-off valves must have a pipe running to a draw to contain the liquid in the event of a failure.
Since pressure goes up and down with temperature, a clever system was develop to maintain a nominal pressure in the loop. A tank, called some sort of expansion tank, is installed in a tee in the tier. The expansion tank has a rubber membrane running all over the middle. The system fluid fills up one side and even air fills the other side. The fluid in the program can expand and contract with temperature into the improvement tank, and the air bladder will keep the pressure inside of a specified range. The air pressure is set with an air hese and tire inflator, just like a car tire. A road is used to determine the correct pressure according to the temperature of the method at the time. However , expansion tanks have a lifetime. The rubberise, rubberize (or neoprene, or whatever) bladder will someday resolve from flexing as it ages and the expansion and burden regulation benefits of the tank are lost. The system often vapor lock somewhere and the whole startup procedure is required to be repeated.
Unfortunately, solar hot water systems don’t like to play by way of the rules. They are not well-behaved. Typical HVAC glycol systems you should not go through the extreme temperatures that solar collectors do. A central heating boiler heating loop may have a maximum temperature of 140-160ºF. It never gets colder than room temperature quite building, so the maximum temperature swing from summer to winter may be 90ºF (70-160ºF).
A solar hot water technique, on the other hand, has the “boiler” sitting outside in the weather. It’s off at night where there is no sun. In the winter, the heat may go down to -40ºF (Willmar, MN). Even in the mountains for NC, winter evening temperatures can go well below focus. A solar hot water system can have a maximum temp swing as high as 260ºF (-40-220), or almost three times you wrote a typical boiler system sees. In the summer time, the photovoltaic hot water system will see its maximum temperature, which fluctuates in intensity according to the application. The most extreme case occurs when there is a truly hot day with high solar radiation, and there is little requirement the hot water. This can occur randomly on weekends, and also summer vacations, and especially on space heating systems the fact that sit idle all summer. When this scenario happens, the warmth from the collectors is not needed and the temperature builds up through to the boiling point is reached.
This same problem can occur is simply too a power failure and the pump stops. At this point, a glycol system is in big trouble. If it gets to the cooking food point, it will blow the pop-off valve. This comes the pressure in the system. The next night, there will be void in the lines and the air vents will leak fresh air in, vapor locking the system. The next day the hot glycol method has air in it. A chemical reaction occurs considering the oxygen that breaks the glycol into fatty acids, that can clog and eat the pipes if the situation simply corrected promptly. This scenario is not self-correcting. The system stops being employed, compounding the problem, and needs to be attended to. This is a progressive fail mode. The pump should never stop running during the day at a glycol system in warm weather. To avoid the over heat range problem, large glycol systems have additional equipment installed in order to dump excess heat. It usually consists of a big buff coil unit in the collector loop that kicks on when the temperature gets too high and dumps the heat to your outside world. The components include temperature controls, bypass valves, fans, and pumps. The added complexity just adds a great deal more failure modes. Heat dump systems cannot overcome capability failures, unless you add a back up generator, which can have some failure modes. At night in the winter when the collectors are freezing, the cold glycol solution will try to circulate naturally affordable the supply line, creating a thermal convection loop. Some systems have even frozen the heat exchanger in this manner, causing rupture of your cold water line. A check valve must be installed on the collector supply line to prevent fluid from flowing upwards under cold conditions.
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