Sunday, April 22, 2012

Solar hot water or heat pump?


With the growing price of electricity and increased awareness of climate change, increasing numbers of people are turning to solar hot water or heat pumps.

The way solar heating works is obvious: you have an energy collector on the roof that picks up energy from the sun, and heats your water. A few details may be less obvious, like why you still get hot water after dark with minimal electricity use, and how it can work on a cloudy day.

A good solar system can heat the water in the tank to up to 95°C, and reduces the temperature at the tap by using a tempering valve, which mixes in cold water to keep your hot tap water down to a safer level. You store energy in effect by heating the water more than you need to when the sun is out. What then about when the sun’s obscured by cloud? Some energy obviously still gets through because cloudy days are mostly warmer than cloudy nights. A good modern solar hot water system can at least maintain temperatures on all but the cloudiest days, and even increase the temperature if it falls as a result of using hot water. Wikipedia has a detailed discussion of different types of heat collectors, and their advantages and disadvantages.

What of heat pumps then?

A heat pump is based on a law of physics: the ideal gas law, which says that temperature is a direct function of pressure, if nothing else changes. One effect of this is that if you increase the pressure of a gas it gets hotter; another is if you drop its pressure it gets cooler. The picture on the right illustrates the principle of a heat pump. If you have a fixed amount of gas and pump it in a fixed-sized container from one side to the other, with a valve that can be closed to allow the pressure to be high on one side but not the other, you make the high-pressure side hot. If you then vent some of the gas to the low pressure side, while keeping the pressure lower than on the other side, it cools. You can use this principle to make a fridge or air conditioner, by routing the high-pressure side of the container through a radiator on the outside of the space you need to cool, and let it cool a bit, before releasing it at lower pressure to the cold (low pressure) side of the system.

A heat pump (or for that matter, a reverse-cycle air-conditioner working as a heater) reverses the role of the two sides. You use the hot side and radiate out the cool side to the outside world.

The key to understanding the whole thing is that the side you don’t want (the hot side of a fridge or aircon, the cold side of a hot water system or heater) has to be able to get rid of its unwanted temperature differential to the outside environment. A fridge or aircon does this simply by radiating away heat; a heat pump may need to be more aggressive in getting rid of unwanted cold with a fan.

A heat pump is a far more efficient process for heating than using electrical resistance to create heat.


How much more efficient?

When I used to live in Australia, I had the benefit of a dual electricity meter, with a separate one for water heating because we had the option to pay a lower off-peak rate for electricity. I was able to observe the benefit of changing to a heat pump directly (picture at  left, note the fan to expel cold air; some models are split in two parts, with the heat pump machinery separate from the tank): a drop of 80% in electricity usage for hot water.

That sounds dramatic, but remember it’s 80% of only one component of electricity usage, and this was in a two-person household with relatively light water use. The benefits you see may vary depending not only on your usage pattern but the efficiency of the system (for example, if the heat exchanger is not well sited, you will get a lesser saving). Solar systems with electric backup reportedly save 90% of hot water electricity usage.

Which type?

So, the bottom line: should you get solar hot water or a heat pump?

It depends. A heat pump, while slower than a traditional resistance heater to fully reheat a large water tank, can top up lost heat overnight even when there is no sunshine. A good design can take heat out of the air at a temperature as low as -10°C. If you use a lot of water overnight, you may find this better overall than a solar system – though a solar system with a bigger tank may still come out ahead. The best case for a heat pump is when you don’t have a roof on which to install the solar collector (e.g., if you are in a ground floor flat). In Brisbane, I installed a heat pump because I wanted my roof space for solar power – we could feed our solar power into the grid and earn a premium price of almost 3 times the retail price for excess electricity, not yet an option here.

For my house here, I decided to go for a solar collector because I have plenty of roof space – and because it works even if the electricity is out. So far, it’s been pretty good. It took a few days (including some that weren’t too sunny) to build up to full heat but since then the water has been pretty hot even on cool cloudy days. The only problem I’ve had with mine is that the installer promised that a fibreglass resin odour in the water would dissipate quickly, but it lingered for months. It’s hard to compare before and after electricity bills because I wasn’t in the house long enough before installing it, and other electricity use may have changed, so I am not going to make any specific claims.

Thinking long term

As the price of electricity goes up, solar hot water or a heat pump should become standard in new dwellings and when replacing existing systems. It’s great that solar hot water is being installed free of charge in poorer communities (even if the target of 3,000 houses in Makana looks low). Many of these households would never have been able to afford hot water otherwise: even if they could scrape together the money for a system, they would battle to find the cost of the extra electricity. And if we are serious about climate change, we need to make it easy for those who already have a low carbon footprint to stay that way without being disadvantaged.