Most solar hot water systems use solar collectors or panels to absorb energy from the sun. Water is heated by the sun as it passes through the collectors. It then flows into an insulated storage tank for later use. In passive systems, water flows due to a thermosiphon effect between the collectors and the tank. Inactive systems, water is pumped between the collectors and the tank.
The storage tank is usually fitted with an electric, gas or solid fuel booster that heats the water when sunlight is insufficient. Some solar water heaters also have frost protection to prevent damage in frost-prone areas.
Solar collectors trap and use heat from the sun to raise the temperature of the water. There are two main types of solar collector: flat-plate and evacuated tube collectors.
Flat-plate solar collectors – These are the most common type. They are comprised of:
– An airtight box with a transparent cover.
– A dark coloured, metallic absorbing plate containing water pipes. Insulation to reduce heat loss from the back and sides of the absorber plate. One slight disadvantage of flat-plate collectors is that they only operate at maximum efficiency when the sun’s rays strike perpendicular to the flat plate. They also suffer some heat loss in cold weather.
Solar evacuated tube hot water system.
Evacuated tube solar collectors – This kind of collector consists of:
A series of transparent outer glass tubes that allow light rays to pass through with minimal reflection.
Each tube contains an inner water pipe coated with a layer that absorbs the sun’s rays, generating heat.
Water runs through this inner tube and is heated.
A vacuum (hence ‘evacuated’) exists between the outer tube and the water pipe, which acts as insulation, reducing heat loss.
Evacuated tube systems are more efficient than flat-plate systems, particularly in the cooler months and on cloudy days. This is due partly to the vacuum insulation (which minimises heat loss) and partly to the fact that the curved surface of the tubes allows the sun’s rays to strike perpendicular to the water for a greater part of the day. Evacuated tube systems weigh much less than flat-plate systems but cost significantly more. Individual tubes can be replaced in the event of damage, making long term maintenance potentially less costly. In warmer climates, such as Darwin, the additional cost of evacuated tubes is usually not warranted as a flat plate solar collector will provide most of the energy needed for water heating. Properly maintained solar thermal collectors should outlast the life of the storage tank. When the tank needs
replacing, the existing collectors can be connected to the new tank.
Frost protection for solar collectors is essential in frost prone areas. During a frost, water can freeze in the solar
collector and damage it unless preventative measures are taken. Common types of frost protection include:
Knock valves (mechanical drain down valves). These valves can be problematic as they often jam open and drain the tank, or fail to operate, causing severe damage.
Electric heating elements, which are vulnerable in the event of power failure.
Closed-circuit systems, which separate the heating fluid from the water (see illustration below). Closed circuit systems are usually the best option in frost prone areas as they ensure that water does not flow through the solar collectors and therefore cannot freeze in the collectors.
Open circuit vs closed circuit
– In an open circuit system, water flows directly through the solar collectors, into the storage tank and then through pipes into your home.
– In a closed-circuit system, a fluid other than water flows through the collectors, picks up heat from the sun, and transfers this heat to water in the storage tank through a heat exchanger. Closed-circuit systems are most commonly used for frost protection (see illustration below). A fluid with a lower freezing point than water is used to avoid ice formation in the solar collectors. It is important to choose the fluid carefully as some become ‘gluggy’ and reduce efficiency.
Some closed-circuit systems pump hot water through the collectors when temperatures approach freezing. This lowers efficiency significantly. Avoid systems with this feature.
Passive vs active systems
Passive (or thermosiphon) systems
In Passive systems (or thermosiphon systems) the tank is placed above the solar collectors so that cold water
sinks into the collectors, where it is warmed by the sun, and rises into the tank. A continuous flow of water
through the collectors is created without the need for pumps.
Passive systems come in two types:
– closed coupled or gravity feed.
In a close-coupled system, the horizontal storage tank is mounted directly above the collector on the roof. Heated water is supplied at mains pressure. This arrangement is the most cost-effective to install but efficiency is reduced in cool and cold climates by heat loss from the tank.
Additional insulation of tanks is desirable in these climates. Alternatively, tanks can be detached and moved inside the roof space, although this increases the cost. In a gravity-feed system, the storage tank is installed in the roof cavity. These systems are cheapest to purchase but household plumbing must be suitable for gravity feeding, including larger diameter pipes between the water heater and the taps. A common alternative is to use a closed circuit gravity feed system to heat mains pressure water using a heat exchanger.
Active (or pumped) systems
Inactive systems (also known as pump systems or split systems), solar panels are installed on the roof and the storage tank is located on the ground or another convenient location, that does not have to be above the solar collectors. Water (or another fluid) is pumped through the solar collectors using a small electric pump. Because active systems do not require a roof-mounted tank they have less visual impact, particularly when the solar collectors are mounted flush with the roof. However, active systems are usually more expensive to purchase and require more maintenance than passive systems.
Active systems use more energy than passive systems because extra energy is required to pump fluid around the system. There are also additional heat losses in the pipes between the tank and the solar collectors. However, if renewable energy is used to power the pump and a high level of insulation are used for the pipes and tank, active systems can reduce greenhouse gas emissions as much as a passive system. Active systems are often used for solar conversions when solar collectors are added to an existing hot water system. They may also be used when the roof can’t support a passive system.
Tanks are manufactured from stainless steel, copper or mild steel coated with vitreous enamel.
Copper-lined tanks are only suitable for low-pressure systems. The other tanks are suitable for mains pressure.
Vitreous enamel tanks are fitted with a ‘sacrificial anode’ that needs to be replaced every few years to protect against corrosion (more frequently where water quality is poor). Other tanks do not require this protection. Outdoor storage tanks can suffer frost damage and significant heat losses in cool climates. In such climates, they should be located indoors whenever possible, as part of a drying cupboard.
Solar water heaters can be gas, electric or solid fuel booster.
Electric boosters use an electric element inside the storage tank to heat water. Gas boosters use a natural gas burner to heat water either in the storage tank or more commonly as a separate
unit downstream from the storage tank. Inline gas boosters are becoming more common as they guarantee that hot water will be delivered at the desired temperature while maximising the solar contribution. Solid fuel boosters heat water through a heat exchanger, commonly known as a ‘wet back’ system.
Gas and solid fuel boosted systems produce less greenhouse gas emissions. Boosters can be manually operated or automatically controlled by a thermostat that cuts in when tank
temperatures fall below desired levels. If boosters are not appropriately designed and operated they can defeat the purpose of having a solar water heater by reducing the solar contribution.
For example, thermostat-controlled boosters located inside the tank often cut in at night, which means that when the sun rises, there is little useful heating to be done.
In well designed solar water heaters that use an electric booster inside the tank, the booster heating element will be positioned to maximise solar contribution. Hot water enters the tank at the bottom, so the element should be high up in the tank to avoid interference with hot water coming in. However, if it is too high in the tank it will not be able to heat enough water on cloudy days.
Timers can also be used to manage boosters and ensure that you get the maximum solar contribution. Talk to your supplier about the correct operation of timers.