Solar Water Heating--Revised & Expanded Edition. Bob Ramlow
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been around long enough to have a proven track record, it has a couple of drawbacks. The corrugation that makes it flexible creates increased surface area inside the tubing and results in significantly more resistance to flow, or friction head. In some cases this may require the use of a larger pump and cause increased electricity operating costs. Be sure to factor in the friction losses when selecting and sizing the pump. At this point it is also more expensive, although much of this is offset with savings in labor. Overall, it is still a good product and a welcome addition to the solar professional’s toolkit.
Because of its flexibility, corrugated stainless tubing should never be used on a drainback system. The dips between the stainless links and sagging between pipe hangers will create pockets where solar fluid can accumulate. In a pressurized system, it may be necessary to flush or blow out the solar loop when replacing the fluid to ensure that all of the old liquid has been removed from the pipes.
Pipe Insulation
Pipe insulation comes in a wide variety of materials and specifications. Most kinds will not withstand the temperatures experienced in a solar loop; most plastic or rubber pipe insulation will melt right off. Only a few kinds of pipe insulation will work, and these are made specifically for high-temperature situations. We recommend that insulation should have a minimum continuous temperature rating of at least 250°F on the hot pipe coming off the collector to the heat exchanger, and a rating of at least 180°F on any other pipes on the solar loop. The material it is made of is not critical as long as the temperature rating meets the minimum standards. This will ensure that it retains its insulating value.
Fiberglass pipe insulation is great for all interior pipe runs but is unacceptable for exterior runs or buried runs because it will soak up moisture, become saturated and lose its insulation value. Some lower-temperature rubber and EPDM types will work, but unless intended for high temperature and exterior applications, they will eventually become hard and brittle and can wear away, especially when exposed to the elements.
It is always best to place a jacketing material on exterior insulated pipes. It is a tough world out there, and exterior insulation is subjected to UV radiation, rain, freezing conditions, insects and birds. Installing a jacket on all exterior pipe runs will greatly prolong the life of the insulation. Aluminum jacketing has proven to last the longest, but there are some new vinyl types that appear to be promising. PVC jacketing is also commonly used, but we have seen several issues with longevity in exterior applications, especially on the joints and fittings.
Solar Fluids
The best solar fluid is plain water. All other fluids are less efficient at transferring and holding heat. Because water is the best heat transfer fluid, many solar water heating system designs use it as the heat transfer fluid. The good and bad traits of the different types of systems will be discussed in Chapter 5.
Pressurized antifreeze systems use some type of fluid that will withstand freezing conditions. Many different fluids have been tried. Some examples are propylene glycol/water mix, ethylene glycol/water mix, synthetic oil and silicone oil. Of all the examples, only the propylene glycol/water mix is acceptable. The ethylene glycol/water mix is toxic; it will also deteriorate very quickly and damage your system. The synthetic and silicone oils require specialized components within the solar loop because the oils will dissolve all rubber and plastic components and seals, and they are just poor conductors of heat.
Propylene glycol/water mixes are the industry standard today for heat transfer fluids in solar water heaters. Propylene glycol is essentially nontoxic and biodegradable and is a relatively stable product, even at high temperatures. Be aware, though, that not all propylene glycols are created equal. These glycols come in various formulas and you need to carefully choose the appropriate formulation. You want to choose a product that is formulated to withstand at least 350° F of continuous usage without breaking down. Most major manufacturers make a propylene glycol product that is rated up to 370° F. Make sure you select the correct blend. In addition, many glycol products also have additives mixed with the glycol to help stabilize the product, inhibit corrosion and make it more compatible with system components.
You never use 100 percent glycol as your solar fluid; always mix it with a certain proportion of water. You want to have the highest percentage of water in the mix as possible and still have the protection you need for your particular climate. Dilution charts provided with the product show the necessary concentration of glycol in the mix to protect the system at various temperatures. Figure 4.8 is an example of a dilution chart.
These dilution charts show two types of protection: freeze and burst. As glycol/ water mixes cool, the fluid becomes thicker and harder to pump. At a certain point ice crystals start to form in the solution, and the fluid will have a jellylike consistency but will continue to protect the piping from bursting. The freeze temperature on the chart shows the temperature at which the ice crystals start to form. The burst temperature on the chart is the temperature at which the fluid will begin to freeze solid and will burst a rigid pipe. For instance, a high-quality glycol at a 50:50 mix of water and glycol will provide freeze protection down to–34°F and burst protection down to–70°F. When choosing a dilution for your climate, you should choose one that is at least 10°F below the lowest temperature ever recorded at your location. Note that when the temperature gets near the freezing point of your dilution, the solar water heating system will stop working, but you will still be protected from bursting pipes to a much lower temperature. The leanest mix of propylene glycol you should consider in the mix is 20 percent glycol. The richest mix of propylene glycol you should consider in the mix is 60 percent glycol. Remember to always use the leanest percentage of glycol that you can get away with and still have the protection you need for your climate.
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