Solar Geyser Technology Explained
Thanks to our geographical location South Africa experiences an abundant supply of annual sunshine, putting us in an ideal position to take advantage of this great renewable energy source.
The two solar radiation maps opposite give us a visual representation of this annual solar radiation, both in average daylight hours and in megajoules per square metre. Solar water heating systems allow us to harness this solar energy for use in our homes and businesses in a very efficient, yet simple, way.
Several different types and configurations of solar systems are available on the market but all have two key components, the solar collector and the storage tank or geyser. The solar collector (commonly referred to as the panel) and the geyser can be configured as either passive or active systems. Both of these configurations can then be further defined as direct or indirect systems, indirect systems generally only being required in frost prone regions.
From a solar collector perspective there are two distinct types of collector available on today's market, the flat plate collector and the evacuated tube collector. Evacuated tube collectors are a newer technology compared to the older flat plate technology but these have been in the field for nearly three decades now, offering proven performance, reliability and longevity.
Please read on below for further details on solar components and configurations, we've tried to keep it as simple as possible but are always happy to provide more technical detail during our site visits.
Flat Plate and Evacuated Tube Solar Collectors
A flat plate collector comprises a series of vertical copper tubes (risers) joined top and bottom to two larger diameter, horizontal copper pipes (headers), all of which is enclosed in an insulated, rectangular box covered with a tempered glass panel.
Flat absorption plates, connected to the risers, absorb solar energy from the sun creating heat which is transferred to the copper risers. The risers in turn heat up water which circulates through them from the geyser and back again.
The diagram opposite illustrates a cross section of a flat plate collector, including it's main components.
An evacuated tube (also known as a vacuum tube) collector comprises an array of vertical, evacuated tubes each of which fits into a horizontal manifold located at the top of the collector. Each tube is made of up of an outer and inner glass wall within which sits a copper heat pipe.
During manufacture a vacuum is created between the two glass walls of the tube. Being a poor conductor of heat, the vacuum ensures that nearly 100% of the solar radiation absorbed by the tube is contained inside the tube, unable to dissipate back into the environment. The trapped heat is then transferred via fluid in the heat pipes to the manifold where it heats up water circulated from the geyser through the manifold and back to the geyser.
The two diagrams below illustrate a cross section of an evacuated tube and an evacuated tube array.
Evacuated Tube Array Cross Section
Evacuated Tube Cross Section
Direct and Indirect Solar Configurations
A direct or open loop configuration is one in which the potable water held in the geyser circulates directly from the tank to the solar collector and back again, in effect the water is heated directly as it passes through the solar collector.
An indirect or closed loop configuration is one in which the fluid that flows to the solar collector is not the actual geyser water itself but rather a glycol mixture, contained within a closed loop, running from the geyser to the collector and back again.
The water in the geyser is heated indirectly as the glycol mixture returns from the solar collector and passes through a heat exchanger (or coil) located inside the geyser.
As mentioned earlier, indirect systems are generally only required in frost prone areas where water in a direct system would be likely to freeze, expand and burst the external copper pipes in the solar system. Indirect systems should also be considered in rural areas where poor water quality can, over time, cause damage to the solar collector.
Passive and Active Solar Configurations
Passive and active configurations describe to us how the water in a solar system is circulated between the solar collector and the geyser.
A passive or thermosiphon configuration is the most basic of the two where the geyser is located above the solar collector, either externally on the roof or on a raised internal frame inside the roof space.
Hot water from the top of the collector rises naturally back into geyser while the denser, colder water from the geyser flows to the bottom of the collector to be heated. This is called the thermosiphon effect and can be seen in the diagram opposite.
An active or pumped configuration is one in which the geyser is located below the solar collector, usually inside the roof space where a conventional geyser would be located. In these systems, a small, energy efficient circulation pump is used to pump the water around the solar system from the geyser to the collector and back again.
Pumped systems are technically more advanced than passive systems, giving the homeowner more control and interaction with their solar system through an easy to use digital, solar controller interface. The diagram opposite illustrates a standard pumped system configuration.
Summary
The system features and characteristics described above are summed up in the comparison table below.
In our opinion a pumped, direct, evacuated tube system offers the best all year round solar performance from a collector efficiency viewpoint. Different weather conditions and installation/maintenance considerations are also taken into account in the comparison table.
