History of Solar Panel Development

Domestic solar systems have been developed over the course of more than a hundred years under the influence of local plumbing practices and the developing national regulations that have foilowed to ensure safe and efficient practice. All of the systems that are currently used have their own strengths and weaknesses, there is no perfect solar system configuration, just good systems templates that can be adapted to work efficiently through proper site assessment, design and installation.

The collector in its various designs, must capture the sun's radiation and transfer it to the solar loop fluid, for a period of maybe 20-25 years while installed in an exposed location. The solar loop must transfer the energy gained with minimum heat losses into the storage cylinder. The storage cylinder should retain this heat until it is required.

Most Common Solar Plumbing Arrangements

• Direct Systems
• Indirect Pressurised
• Drainback
• Thermosyphon
• Warm Air

Looking at each in turn;

Direct Systems

In these systems, the hot water that is used at the taps is circulated through the collector to be heated directly by the sun, without any intermediate heat exchanger. This approach minimises maintenance as it obviates the need for changing the solar loop fluid every 5 to 8 years and simplifies the installation. The systems can be operated off-grid on DC electricity provided by a small solar photovoltaic PV panel due to the low energy demand of DC pumps. Since pure water has a higher heat capacity than a glycol-water mix and there is no heat exchanger between the solar loop and stored hot water, the efficiency of the energy transfer from the collector to cylinder is increased. If the original cylinder is compliant with current standards for insulation, then retro-fit installation is a fairly straightforward process that can be completed in a day at lower cost to the customer. However this type of system is can only be installed in an area with a low limescale reading (less than 160ppm). Higher water hardness will cause the panel to "fur-up" with limescale unless a water softner is used. Solartwin are the most known and successful company pushing this technology.

Pressurised Indirect

The majority of the European market has traditionally been occupied by pressurised indirect solar thermal systems. In this case the solar loop is fully-filled with a mix of glycol and water in sufficient proportions to prevent freezing. The system is pressurised during commissiioning and must include an expansion vessel to contain both the normal thermal expansion of the glycol-water mix and the expansion due to the formation of steam in the collector during stagnation. Additionally the system must have a pressure-relief valve to safeguard against overpressure. The system is termed indirect as it contains a heat exchanger to separate the glycol-water mix of the solar loop from the domestic hot water. The heat exchanger is normally a large area coil placed in the lower section of the domestic cylinder (an internal heat exchanger). However it may also be placed outside the cylinder (an external heat exchanger) as either a plate heat exchanger or an annular heat exchanger as in the Willis Solasyphon.

Drainback

This system design has its origins in the Dutch Water regulations, which originally did not permit the only the single skin of the solar heat exchanger to separate the glycol-water mix of the solar loop from the domestic hot water (considered as drinking water). The heat transfer medium adopted in the solar loop was plain water and this necessitated changes to be made to the system design to prevent freezing or boiling. The solution adopted was to allow the water to drain back to a holding reservoir when heat was not required from the collector. Once the demand for heat returned, the controller would operate the pump to fill the loop and collect heat as a normal system would. Once the pump switches off the water drains back under gravity (all pipes must have an adequate fall). There is therefore no water to freeze in the collectors in cold weather, nor to boil in hot weather when the full contents of the cylinder have been brought up to temperature.

Thermosyphon

Thermosyphon systems in global terms represent the most common solar system type. In warmer countries around the Mediterraean, in Africa, Australia and particularly China, thermosyphon systems would be regarded as the norm. In this arrangement the storage cylinder is mounted above the collector (generally on the roof attached to the top of the collector) so that solar heated water will circulate from the collector to the cylinder by natural convection (natural circulation). As long as freezing is not a problem and ambient temperature is reasonably high, the sytem can work quite efficiently without an external electrical supply. Provision must be made for coping with thermal expansion in sustained hot weather and this is normally provided by a simple pressure relief valve. These systems are not generally suitable to our climate due to the risk of freezing and the substantial heat losses that would occur from the exposed cylinder in our windier and cooler conditions. They also impose a substantial roof load that would require additional and general costly roof-strengthening.

Warm-air systems

Air has been employed in a variety of solar systems as an alternative to liquids to transfer heat from the collectors in the roof to a heat exchanger for either domestic hot water heating or possibly also to preheat ventilation air to assist with space heating. System design must make a careful assessment of the air flows within the ducting to ensure effective distribution and also to minimise the cost of running the electrical fans as to transport a given amount of energy requires much more energy than moving a liquid with a pump.