Solar Heat Pumps

Earlier I said there were 2 types of solar hot water system, the panel based systems and the heat pump systems. I've gone into panel based systems in quite some detail (and probably bored the pants off you - sorry!). So now it is the heat pumps' turn for the limelight.

The previous few pages have shown that panel-based solar hot water systems have many variations of design and operation and can be quite complex beasts. If you prefer to keep things simple, then heat pumps are definitely worth considering.

Heat pumps are dead simple. Get an air conditioner, run it in heating mode, and use the heat it blows out to heat a well insulated cylinder of water.

That's all there is to it folks!

Here's a diagram that shows the operation in a bit more detail, including how the heat is transferred to the water by compressed refrigerant that flows through a coil inside the hot water cylinder:

How Heat Pump Hot Water Works

Now - whilst people may argue that it is not true solar, I'd say they are wrong. That's because a heat pump transfers the heat that is already in the surrounding air and 'pumps' it into the water. This heat's original source is, of course, the sun. So I'd argue that a heat pump is solar powered with an electric boost.

A heat pump is driven with electricity, but is about 5 times more efficient than a conventional 'kettle' style electric heater. In fact the CSIRO calculate that in some Australian climate zones it can use less electricity over a year than a solar panel hot water system with an electric boost.

Here are the pros and cons of a heat pump system over a 'conventional' solar hot water system:

PROS:

  • No roof mounted hardware - leaving all your roof free for Solar PV panels.
  • Easy to protect from hail and storm damage
  • Doesn't require direct sunlight - so it will work in all weathers.
  • No danger of overheating in summer.
  • Relatively simple (i.e. about 50% cheaper!) installation than panel based systems.
  • Doesn't matter when you run them - so you can take advantage of off peak electricity tariffs

CONS:

  • The compressor can be noisy.
  • Work most efficiently in warm-hot climates. i.e. they cost more to run in cold climates.
  • They may use ozone-depleting CFCs in their coils.
  • Lack of panels means the neighbours won't know you have a solar powered hot water system (less green street-cred)!

When are heat pumps the better choice?

I would argue that if you have reticulated gas, live in climate zones 2 or 3 (see below), and you have plenty of unshaded north facing roof space (and don't need it for Solar PV) and you don't get regular heavy cloud cover, then a gas boosted panel based system is usually the more efficient option.

Heat pumps are generally a better bet in zone 1 as this zone is cloudy and typically without reticulated gas. Also panels (particularly evacuated tubes) are more vulnerable to storm and cyclone damage.

If you live in zones 3 or 4 and don't have reticulated gas, then a heat pump will probably save you more money by heating your water more efficiently than electric boosted solar panels. The reason being that in these zones, a panel based system will typically heat 50%-65% of your water with the sun (the rest electrically with a kettle element).

A good solar heat pump in these areas should heat 65%-75% of water with the sun (when compared to a conventional electric heating element).

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