Pressurized Solar Thermal System Pumps

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One of the advantages of a pressurized solar hot water system is that the solar loop is always full. This means that the pump has no static head pressure to overcome (just friction head), so low-head, slow-starting pumps can be used effectively. And those pumps can be run on power from a PV array, eliminating your grid-tied power costs for operation.

In fact, a PV-powered pump is recommended for pressurized systems because its operation matches the solar fluid's needs. First off, the pump operates whenever the sun shines. This means that the solar fluid is always in motion when it's being heated, instead of lying idle, which extends its life.

The pump also changes speed as the sun provides more energy: more sun means faster pumping. Since the solar fluid will heat up quicker with more sun, it needs to be pumped faster, so that matches perfectly.

However, there are some situations that could cause problems and which need to be considered:

  • In installations where there is no load for an extended period (water heating in a holiday home, for long vacation periods, etc.) and where the climate is hot, a drainback system should be considered to cope with potentially hazardous situations.
  • In installations using corrugated stainless steel pipes, friction head is a lot higher and the pump must be carefully sized to match. If it's not powerful enough, solar fluid will collect in the corrugations and deteriorate.
  • The PV panel must be situated where there is absolutely no shade. Even a tiny bit of shade affects the panel's output, which makes pumping erratic.
  • If your region experiences heavy snowfalls or long periods of snowy weather, it's worth making the PV panel adjustable (and accessible). Turn it vertical at the start of the cold season, so that it doesn't accumulate snow which would block power generation, then turn it back to its normal latitude tilt angle when the danger has passed.

Many pressurized solar hot water installations that use a DC-powered pump also use a differential temperature controller. This adds readouts for system monitoring, but it's best to disable the high-limit function to ensure that the solar fluid always circulates if the sun is shining.

You'll also need to check your solar fluid more frequently if you install the controller, as a power-out when the sun is shining would stop circulation and allow the fluid to stagnate and deteriorate in the loop.