MAILBOX: PV-Direct Water Heating Vs. Solar Thermal

Intermediate
A Willis style immersion heater is plumbed in parallel with a standard water heater to provide a small amount of hot water quickly, while also contributing to the main tank temperature.

In Mailbox HP179, Hugh Piggott cited some advantages that make PV-powered water heating preferable to solar thermal technology. Here are a few more:

  • Reliability—There’s no heat-transfer fluid that can leak or deteriorate; and no circulating pump means no moving parts that can fail.
  • Simplicity—The systems are easier to install, test, and commission. A tank water heater having one ordinary and one solar immersion element is no big deal, compared to a special “solar tank” with its necessary heat exchanger.
  • Autonomy—Many solar thermal systems need grid electricity for the pump controller and the circulation pump. PV systems can be entirely self-powered—ideal for those in developing countries, as well as “off-gridders.”
  • Layout flexibility—It’s easier and cheaper to run cables than plumbing. There are no airlock worries, either.
  • Guaranteed energy harvest—Lower solar input in early spring, late autumn, and winter, can compromise a solar thermal system—will the collectors achieve the necessary temperature to input heat to the tank? With PV, if there’s any energy available, it’ll be harvested. Plus, PV modules are more efficient in cold weather.
  • High-temperature capability—Even if the available PV power is low, it can directly heat the hot water supply tank; there’s no need for a preheat tank.
  • Instant delivery—By using a Willis-type external immersion heater arrangement (popular in Northern Ireland, but universally applicable), hot water can be drawn almost as soon as your PV array starts generating a surplus.
  • Greater end-use flexibility—With a PV-heat arrangement, you aren’t just restricted to heating water. With a little creativity and electrical know-how, you can use conventional switches and relays to readily swap between water heating, background space heating (very useful in spring and autumn), or greenhouse soil heating. You can also cook with PV power—think of the deforestation, the fuel-collecting time, and the smoke-related illnesses this could prevent. Meanwhile, even Scots, Alaskans, or Canadians can—using a conventional AC hot plate fed from a dedicated “PV surplus” socket, on sunny days—be bulk-bottling fruit or juice, jam-making, or brewing beer in large batches, and enjoying full tanks of hot water!

Christopher Jessop • Pembrokeshire, West Wales, UK

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