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Drainback vs closed loop

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  • Doug Kalmer
    There are pros and cons to each system, neither is the best in every situation. For example, many installations have the collectors out in the yard, where
    Message 1 of 1 , Apr 27 10:29 AM
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       There are pros and cons to each system, neither is the best in every situation. For example, many installations have the collectors out in the yard, where drainback is difficult if not impossible. Some collectors are not plumbed for drainback. Drainback requires more pumping power to repeatedly lift water to the collectors, which almost all the time means grid powered pumps. Since such pumps are performing more work, they wear faster than circulation pumps, as do the contactors controlling them. If a cloud bank passes over a drainback, it drains the fluid down, and then needs to add the energy back again when the sky clears, this can occur many times during the day. PV pumped systems will just reduce the pump speed to closely match heat gained in the panel, never requiring the considerable energy to lift a column of fluid. Being grid powered, drainback has parasitic losses requiring large differential settings compared to PV pumped. Most drainback systems are not set up for variable flow, PV pumping naturally provides variable flow. Because of these last two points, PV pumped is the most efficient. Don't take it from me, read what the experts have to say-
      From Homepower Issue #126, Aug, Sept 2008 Article titled "Under control-solar water heating- SHW controllers" Page 60
      "Parasitic loss occurs when energy is consumed or lost in order for a system to make more energy. <snip> In grid powered SHW systems, the pumps and controllers take a certain amount of electrical energy to operate. <snip> If the parasitic losses are greater than the solar energy being put into the SHW system, a net loss of energy results. In PV-direct DC systems, the pump energy losses are inconsequential because the energy from the PV module is there regardless, and at no cost. But in AC systems, a wider temperature differential is used to ensure that the parasitic losses are not greater than the solar energy gained. <snip> Since there is no utility generated parasitic power consumed with a PV powered DC differential controller, the turn off differential is lower-sometimes zero is appropriate if the pipe losses are negligible."
      This makes it clear that PV pumped SHW systems are more efficient because they have a zero degree differential, gaining solar heat whenever collector temps are above tank temps, while AC powered system differentials range as high as 18*F, to overcome losses caused by carbon based grid powered pumps. PV pumped systems also have the ability to moderate flow according to the amount of sun received, not just on or off like AC systems.Doug
      Subject: Most efficient solar water heater

      Homepower Issue #121, Oct. and Nov. 2007 "Pick the right pump" article by Chuck Marken states on Page 86-
      "Using a utility powered AC pump for your solar water heating system will give you a COP between 12 and 25, this is an excellent value compared to electric water heaters, which have a COP of 1. But the COP will never be as good as a DC PV powered SHW system. DC hot water circulation pumps have a higher COP than AC pumps because there is no traditional energy input if a PV module powers the system. If you use a solar-electric module to power the pump, your COP is infinite- you`re not adding any energy input. The sun provides it all, and you get something for nothing after the initial investment. PV powered systems are immune to utility outages."
      "PV powered DC pumps are normally the optimal choice for a solar heating system except in high-head drainback and very large antifreeze systems."
      No more electric bills, now the electric company pays me every month.

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