Direct Solar 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 can simplify the installation process. If the original cylinder is compliant with current standards for insulation, then retro-fit installation is a fairly straightforward process that can be completed quickly and with less disturbance at a potentially lower cost to the customer.

Particular precautions must be taken with the design of these systems to overcome the following issues.

• Freezing and Boiling
• Contamination of the water through contact with non-approved or corroded materials
• There should be minimum storage losses, so that captured heat can be used the next day (or later)
• Lime-scale build-up in hard-water areas
• Legionella control

The use of water without antifreeze requires either that the pipework in both the collector and external parts of the solar loop can accommodate expansion caused by freezing OR that the solar pump can be exercised to circulate water within the panel during freezing conditions. (use of "bought-in" energy to circuate to the panel however is counter to BRE guidelines)

Pipework in the solar loop and collector and the joints and the pump must be made from materials that will resist corrosion under conditions where freshly oxygenated water is circulated through them and be certified for contact with domestic hot water.

In hard water areas, precautions must be taken to eliminate the build-up of scale in the solar loop in operation above 60oC, which could fairly rapidly obstruct the narrow pipes in the collector.

Legionella will be dealt with in more detail later and indeed, all systems must be designed to prevent its growth. Direct systems need to ensure that all the water in the cylinder can be brought up to 60°C periodically for sterilisation purposes, particularly during the Spring and Autumn.

Examples of Direct Solar Systems

Solartwin

Solartwin markets a systems that is based on certain innovations that seek to overcome the limitations that direct use of domestic hot water in solar systems entail. The Solartwin system can be retrofitted to an existing vented hot water cylinder or additionally be configured to operate with a thermal store.

The pipework from the cylinder and within the collector absorber plate are made from silicone rubber. The controller and pump are both supplied with dc electricity from a small pv panel mounted on the thermal collector on the roof. If temperatures drop below freezing in winter, the pipework can expand to accomodate this and the pump has pressure relif valve to accomodate blockadges in the pipework on very cold but bright days.

The metal collector absorber plate has a non-selective coating to assist in the promotion of heatloss on very hot days when the cylinder has exceeded the desired level. The pump continues to run in this case so that an equilibrium temperature is reached in the collector at which the solar gains are matched by the losses to the environment (test recorded a maximum of 78°C). In case of pump failure the sytem can stll vent through the DHW cylinder.

The silicone materials are certified for use in DHW systems. The manufacturers provide recommendations on the type of hard water controls that must be applied in vulnerable areas. For legionella control, the manufacturers recommend that the cylinder is brought to 60°C for an hour every day using the auxiliary heat exchanger in the bottom of the cylinder.