A PV pumped glycol system is absolutely freeze proof, uses no grid electricity, infinitely adjusts flow thru the collector in perfect synchronicity to the heat in the collector, has no electronic controls to malfunction, only needs one small pump, is zero carbon, and is easier for a homeowner to install, as there is no concern about improper drainage freezing and damaging the system. The COP (Coefficient of Performance) is infinite on a PV pumped system, well above any system that requires grid energy. Full disclaimer- I am just a homeowner who has studied the options and lived with my system for 17+ years, I do not sell or install systems for a living, so I have no vested interest in any particular product.
I wanted my system to be totally freeze proof, off grid, and as simple as possible, so I chose PV pumped closed loop, which requires one small pump, which can be PV powered, while drain back needs two larger ones, which are grid powered. Differential controllers, and sensors are not part of the PV pumped closed loop system, these parts can be problematic. In a PV pumped system the sun is your control. There is a simple elegance to having the PV panel directly wired to the circulating pump, no further controls needed.
The sunlight reaching the collector panel varies exactly as the sunlight reaching the PV panel, so, if they are properly sized, as the sun sets, or clouds move in, the pump slows at the same rate the heat slows to the collector. The collector does not hold heat well, it's designed to transfer the heat to the small amount of fluid in the lines, which heats up quickly.
Just moving the water
around a filled loop does not require much power or much energy, (because
the water falling down one side
of the loop lifts the water up the other) but pumping the water up an empty loop is a sizable load on the systems energy usage, so
either the number of times that happens, or the energy demand of each time, or both, should be reduced, if possible.
Closed loop antifreeze systems provide the most reliable protection from freezing. These systems circulate an antifreeze solution through the collectors and a heat exchanger. Propylene glycol is the most common antifreeze solution. Unlike ethylene glycol (used in automobile radiators), propylene glycol is not toxic. Closed loop systems like this are quite common, whether they be for solar domestic hot water, radiant floor heating, or hydronic baseboard heating. Despite the additional parts and fittings, they have a high degree of reliability, and are well understood by heating contractors.
My years experience living with a PV pumped closed loop system tells me that it works well just as it is, remember, sensors, controllers, microprocessors, and their failures were a large reason for the demise of many SDHW systems in the 70`s and 80`s. Let's keep it simple, and reliable. Let the sun be your control.
You can reach Doug at: dougkalmer AT gmail DOT com (replace AT with @, and DOT with a period)
Nov 26, 2007
Just as a note: There have been some very dynamic discussions in Internet forums about the pros and cons of the PV powered pump system that Doug likes very much vs the AC pump powered controlled by a differential controller. My own 2 cents is that both systems work well and efficiently and have a long track records of success. The same is true about antifreeze systems vs drain back systems. You can do a little digging, and come to your own conclusions about which appeals to you more. A good place to start is the Home Power Basics articles. Gary