Indirect (Pressurised) Solar Systems

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. Fluid should not be vented through this valve during normal operation, including situations where electrical power to the system is completely lost. This requirement is for the system to be designed to be 'inherently-secure'. 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.

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

• Freezing and Boiling
• There should be minimum storage losses, so that captured heat can be used the next day (or later)
• Legionella control

Normally a Water / Polyproplene Gychol mix is used to protect against freezing, sometimes however vacuum tube collectors use a mixture of water and corrosion inhibitor (without antifreeze) and rely on the panel insulation and a frost circulation function of the controller.

Pressurised systems are designed to cope with periodic high temperatures above the boiling point of water. Normally a closed-loop system is filled to about 2.5 bar guage pressure (3.5 bar absolute) at this level, the mixture tends to boil about 140 °C, after this point steam is generated within the panel, and the expansion vessel must be large enough to cope with the resulting expansion. It is also important that joints are also steam-tight, or the system can de-pressurise in hot weather. More on this in High Temperature Pipe Seals

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 control is a serious issue and will will be dealt with in more detail later. Solar/Hot water 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 Indirect Solar Systems

Viessmann Vitosol System

The Viessmann Vitrol system is a good example of a pressurised indirect system. As can be seen in the diagram, a dual coiled cylinder is used, the solar feeding the bottom of the cylinder and a normal boiler being used to provide supplemental heat at the top of the cylinder if required.