Heat Pump Solar Hot Water Systems

Heat pumps are pretty simple to understand. 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 heating energy is transferred to the water by compressed refrigerant that flows through a coil inside the hot water cylinder:

Now – many argue that it is not true solar. I’m an electrical engineer. My worst marks at university were always in the thermodynamics class. So I’m not qualified to argue. But I will anyway!

To my brain, a heat pump transfers the energy that is already in the surrounding air and ‘pumps’ it into the water. This energy’s original source is, of course, the sun. So I’d argue that a heat pump is solar powered with an electric boost. 

Further – because of its low power draw and time-agnostic electricity demand, a heat pump is an ideal way to use solar electricity to heat water.

That’s because a heat pump is driven with electricity but is 3 to 5 times more efficient than a conventional ‘kettle’ style electric heater. In fact, the CSIRO calculates 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
• You can power them directly from your solar electricity instead of the grid

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 and reliable option (although rising gas prices have moved the needle towards heat pumps in these areas).

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 only 50%-65% of your water with the sun.

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). And if you have solar PV on your roof, then a good sparky can set the heat pump up to run mostly off your solar electricity instead of the grid.