Solar hot water units
Solar hot water units are available
in different configurations:
Flat plate collectors on the roof
Evacuated tube collectors on the roof
Roof mounted storage tank
Ground mounted storage tank
Different back-up heating
sources, e.g. gas or electricDirect systems where water
flows through the collectorsIndirect systems where the collectors
are filled with a heat transfer agent
(normally glycol), which in turn heats
stored water at a heat exchanger
Features of Solar Units
Reduce energy bills associated
with water heating by 70% or moreReduced use of fossil fuels and
the release of greenhouse gases
into the atmosphereCurrently attract large rebates from
Federal and State GovernmentsRequire electric or gas boost
system for back-up on days
when the sunlight is limitedSome components can have
comparatively longer warranty
periods than conventional water heatersHigher purchase price than
standard electric and gas water
heater (before rebates)Installation is more expensive
Not all homes are suitable for solar
as the collectors need to be mounted
on north facing roof areas with
limited shading and a specific pitch
Positioning panels and tubes
Solar collectors require sunshine (light) rather than hot temperatures to operate effectively. However, the performance will also depend on the temperature of the water coming into the unit. This is why solar units are more effective in warmer weather.
North facing roof is optimum.
Up to 45° either side of north is OK.
Must follow manufacturers’
recommendations—usually 15m
between tank and solar panels.Ensure that trees or buildings
do not shade the collectors.Flat panels—best solar
input is from 9am to 3pmMinimum 10° inclination
(this allows self cleaning)Maximum 45° inclination
Allow at least 0.5m from
the gutter for maintenance.
Frost Protection
Solar water heaters from some manufacturers require the installation of anti-frost valves in areas prone to frost to protect against freezing water damaging panels and pipe work.
Not all units use anti-frost valves—most rely on sensors and their controllers to send heated water back into the collectors when the temperatures drop below a set point.
The following map supplied by Dux shows those parts of
Australia requiring anti-frost valves on Dux solar units.
Flat Panel Solar Hot Water Unit
Pumped or split systems have solar panels on the roof but the tank is located at ground level (or elsewhere in the building). Hot water is pumped from the panels to the tank.
Boosters are needed in solar hot-water systems to keep up the hot water supply when there’s not enough sunlight to do the job. Boosters can be either electric- or gas-powered, and are usually incorporated in the water tank.
Open Circuit (Direct) System
An open circuit system has the potable water passing through the solar collectors. Open circuit is better suited for areas not subject to harsh water conditions.
Closed Circuit (Indirect) System
Glycol (anti-freeze) passes through the collectors instead of water in a closed circuit system. Closed circuit heating systems are particularly suitable for hard water areas where water would leave sedimentary deposits in the panels.
Flat Panel Solar Hot Water—
Thermosiphon Systems
Thermosiphon systems have both the collector panels and the storage tank mounted on the roof. The liquid in the panels circulates into the tank via the thermosiphon effect (as water heats up, it becomes lighter and rises into the tank). In warm climates, the panels can heat water directly. However, in frost-prone areas, the water can freeze and damage the panels, so frost-tolerant panels which use a heat-exchange fluid with a freezing point lower than water are used. The panels heat the fluid, which then heats the water in the tank.
Evacuated tube systems
Work by copper pipes encased in glass being heated by sunlight and transferring that heat to a manifold which in turn heats water passing through
Tubes are able to passively track the sun throughout the day. Round shape, means less reflection and therefore greater efficiency
Larger surface area to collect heat
compared to the same area of flat panelsTubes are inherently frost protected
as no water flows through the
tubes themselves.No risk of glycol leaking from the system
Easier to install without use of heavy lifting equipment
Replacement of damaged tubes
easier and cheaper than flat panelsModular configuration allows the
system to work even if some tubes
malfunction. However, this would
decrease the efficiency of the system.
