CSI System Choices: Glycol, Drainback, & High Sierra Drainback

 

To obtain the CSI Rebate, a system must be freeze-proof. Active Direct systems, those with pressurized city water relying on recirculating freeze protection, are NOT allowed. Here are the 3 types of systems most commonly applying for Rebate:

 

GLYCOL SYSTEMS: These systems have Propylene Glycol in the collector loop, transferring heat by external double-wall heat exchanger to glass-lined tanks of potable water, requiring two pumps which run concurrently (both relatively small). There also must be an Expansion Tank to allow for expansion and contraction of the fluid. These systems are commonly used where daytime temperatures are frequently below freezing in the winter: Germany, Denver, etc. They are freeze-proof, but NOT overheat proof. Should there ever be an overabundance of solar heat that collects before it is utilized, the pump turns off and the collectors bake, reaching temperatures of up to 400 degrees. This breaks down the glycol, which eventually will turn acidic and begin to eat the copper of the piping and the panels. Depending on the frequency of overheating, the Glycol, of which there will be around 60 gallons, will need to be changed every 3 to 10 years…at a cost of $2,000 to $3,000. I can tell you that in Los Angeles today, there is probably no one but our company that is ready to go change out a big commercial 60 gallon glycol system. Nonetheless, this is NOT our system of choice, as we do NOT like required maintenance as a matter of principle. These systems also incorporate glass-lined potable water storage tanks…which last approximately 15 to 20 years. They each have anodes that must be inspected and replaced at intervals of around 5 years for hundreds of dollars each. Other parts that are known to fail in glycol systems are expansion tanks, check valves and air vents.

 

STANDARD DRAINBACK: In a Closed-Loop Drainback system, the collectors have a sealed, closed loop of water and air designed in such a way that whenever the pump is off, the water drains out of the panels by gravity and into a reservoir holding around 2 gallons per panel. When the pump turns on, the water pushes the air out of the panel and into the top of the reservoir. Then water flows through this reservoir with the air floating on top of it, and down through a heat exchanger where heat is transferred to the potable water tanks. This heat exchanger can be single-wall (more efficient than double-wall) because the fluid is plain non-toxic water. Again there are two pumps: one on the collector loop and one on the potable loop. Both are relatively small…slightly smaller than in glycol systems. These systems also incorporate glass-lined potable water storage tanks…which last approximately 15 to 20 years. They each have anodes that must be inspected and replaced at intervals of around 5 years for hundreds of dollars each. The water never needs to be changed, and there are no expansion tanks, air vents or check valves to fail. These systems are considerably better than Glycol systems for lack of maintenance and simplicity.

 

HIGH SIERRA DRAINBACK: A High Sierra Drainback Tank is a rectangular high-tech polypropylene plastic tank with a Lifetime Warranty. Its rectangular shape allows for much wiser use of space. The tank holds unpressurized plain water that is vented (not completely sealed). Immersed in this vat are Type “K” Wolverine “Load-side” Heat Exchangers (copper tubing with fins on it to result in phenomenal heat exchange efficiency) through which the city water travels on its way to the backup gas hot water storage tank(s). At rest, the collectors are empty. When the pump turns on, the water is lifted into the panels by two series pumps that push the air in the panels down into the tank. Upon completion of that process, one of the pumps turns off, and the other smaller pump has the siphon effect to continue collecting solar hot water in the High Sierra solar tank. When the pump turns off for any reason, the water drains out of the panels and back into the tank by gravity…which NEVER fails. An empty solar panel cannot freeze or boil, so no harm is done. The manufacturer provides a Lifetime Warranty, no strings attached, and they have NO corrosive linings, no dielectric unions, no anodes, and no Glycol Fluid to dispose of…it is all just water. In small systems, these fine tanks can be more costly than conventional glass-lined tanks…but in large ones, they are significantly less expensive. This is our preferred system if we have a good tank location with adequate access (door widths, etc.).

 

ROOFTOP DRAINBACK CONFIGURATION OPTION: With either High Sierra or Standard Drainback systems above, we have developed an option we call Rooftop Drainback (RTDB for short). In drainback system projects where the lift is more than 3 stories or where any sound from the system is to be avoided (high-end residential), we install slanted 4” copper insulated piping sections BELOW the solar panels on the roof as the drainback tank. The pump can be located down at the solar tank(s), but is only lifting 6’ to the top of the panels, not 4 or more stories. This eliminates the need for two pumps, or for a large pump to overcome the “head.”