Water Heating
Friday, September 22, 2006
Solar water heating
The solar water-heating movement has weathered bad press over the years, caused by the provision of government subsidies early in the 1980s for installations, many of which failed prematurely because the industry was not fully fledged or experienced in the technology. There is no longer an incentive system to help homeowners install solar systems (though there is talk of bringing one back) and in the intervening 20 years the technology and the service has improved vastly.
The biggest hurdle to installing a solar hot-water system is the upfront cost. Once installed, it can provide 50-80% of the water-heating requirements for the average family without adverse effects on the environment.
Solar systems are reckoned to give at least 20 years’ service with little maintenance. Financially, most break even in between eight and 12 years.
If solar water heating is used in conjunction with mains water heating it is advisable to check ‘flat rate’ versus ‘controlled rate’ mains supply options with your electricity supplier, as different electricity tariffs will affect savings.
A tempering valve must be included in the system to meet regulations governing water temperature at the point of supply.
How Do Solar Panels Work?
Solar systems consist of a collector panel (1 sq m per person) connected by insulated water pipes to a storage cylinder. The collector panel is usually mounted on the north face of the roof and angled to catch the sun: higher angles collect more winter sun, lower angles more summer sun.
The storage cylinder (300-litres) can be mounted externally above the collector panel, in which case the water circulates naturally as it heats; or within the roof space or internal hot-water cupboard, with a temperature-sensitive electric pump (which requires a mains connection) controlling circulation.
Systems can use either low- or mains-pressure. Mains-pressure cylinders must be either stainless steel or ceramic-lined steel; low-pressure systems can be copper. Most modern cylinders are suitable, older models will need to be replaced.
Because sunshine is variable, cylinders are usually equipped with backup electric element(s) or gas booster, and possibly (with low-pressure only) a wood stove wetback for the sun-starved winter months.
An indoor temperature gauge helps users to plan best usage of solar-heated water and a tempering valve allows cold water to be added if the water overheats.
High-quality design and skilled installment are essential. If installed correctly, little maintenance is needed, though panels can be damaged by winter freeze, so must have inbuilt protection in frost zones.
Wetbacks
Wetbacks can be attached to coal ranges and open, closed or freestanding fireplaces. A wetback is essentially a water jacket or pipe, usually made of copper, which circulates water between the fire and hot water cylinder. Obviously, the hot-water cylinder and fireplace need to be in close proximity – preferably no more than 4 m apart.
Wetbacks may be fitted only to open-vented systems to eliminate the risk of explosion. For this reason they are usually installed in low-pressure hot water systems with conventionally vented cylinders. (Wetbacks can be used in mains- or high-pressure water systems but such installations must have an additional, separately vented water circuit, which makes the installation more complex and expensive.)
Wetbacks also create very high water temperatures, high enough to boil water, with attendant risks to users. As with all hot-water systems, a tempering valve must be included in the system to meet regulations governing water temperature at the point of supply (see above).
Unlimited water temperatures also pose a risk of damage to water pipes – especially non-copper pipes – and appliances that draw hot water.
Depending on the source of fuel, a wetback can be a stand-alone water heater or an energy efficient backup to a mains- or solar-powered hot water supply. Wood-fired wetbacks can also heat water economically for use by a central heating system, in which water is pumped to fixed radiators throughout the house, or through pipes buried in the floor.
Wetbacks are worth considering if you use a solid -fuel burner for space heating or cooking; if you have a reliable and cheap source of sustainable firewood; or if you live in a location where other energy sources are expensive, limited or intermittent.
Hot-water Heat Pumps
Heat-pump technology can be used to heat or cool both air and water. Heat-pump water heaters are uncommon in New Zealand but because they use much less energy than the amount of heat they move, they are an efficient form of water heating. Ideally, they are suited to buildings in mild or cool climates with large, steady hot-water requirements, such as families of four or more members.
They work by extracting ‘free’ heat from the surrounding air and transferring it to the water in the tank. The amount of electrical energy required to capture and transfer the heat is generally less than half the amount of heat, which is finally transferred to the water. This means that for the same electrical input, a heat pump will heat more than twice the amount of water as that available from a standard resistance water heater, which makes it a very efficient system of water heating.
The electricity to run the heat pump can be supplied by solar photovoltaic panels, making it a cheaper and more self-reliant form of water heating. Other options involve the use of heat-recovery units to capture the warm exhaust of air conditioning systems, which would otherwise be wasted. Alternatively, the cool air expelled by the heat pump can be used to cool and dehumidify indoor air in summer.
Hot-water heat pumps can be fitted as an adjunct to an existing water cylinder (providing it is serviceable) or installed as a complete system with an inbuilt tank. The payback period of five to 10 years depends on the type of system purchased, its ‘coefficient of performance’ and the amount of hot water used.