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In most drain back solar water heating systems the pump is located just outside the tank and near the bottom of the tank. A U-tube line starts inside the tank near the bottom, goes up over the side of the tank, then drops down to the pump inlet.
When the collectors drain down, the U-tube stays full of water, and keeps the pump primed with water for the next startup. This works fine.
But, if its not possible for the pump to be located at a level near the bottom of the tank (for example its a buried tank), then the scheme shown below might be workable. It uses a priming tank located at the pump that basically supplies water for the next startup and also sucks water up the suction pipe as it empties.
I have never heard from anyone who has used this, so no guarantees that it will work. The ideas is from a handbook on centrifugal pumps by Karassik and Tullo.
Thanks to BBP and the Yahoo SimplySolar group for finding this.
Please let me know if you try this and it works (or doesn't work).
Comment from Michael:
"Nearly all "self priming centrifugal" pumps have the same kind of chamber ahead of their suction inlet. Hayward SuperPump is a good example. The priming chamber most certainly works. Any pool pump with the suction strainer on the end looks this way. Once the chamber is filled, the pump will be able to move air out of the suction pipe. (Hayward SuperPump, Pentair Intelliflo, Sta-Rite Duraglass, etc all look the same as your diagram)"
Single Tank Priming Chamber
A single-chamber primer is a tank with a bottom outlet that
is level with the pump suction nozzle and directly connected to it. An inlet at the top of
the tank connects with the suction line (Figure 1). The size of the tank must be such that
the volume contained between the top of the outlet and the bottom of the inlet is approximately
three times the volume of the suction pipe. When the pump is shut down, the liquid
in the suction line may leak out, but the liquid in the tank below the suction inlet
cannot run back to the supply. When the pump is started, it will pump this entrapped liquid
out of the priming chamber, creating a vacuum in the tank. The atmospheric pressure
on the supply will force the liquid up the suction line into the priming chamber
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