Solar Energy: Your Questions and Answers


SolarUK: Glossary

Recirculation - pumped secondary

Pipework carrying secondary hot water.

Recirculation or pumped secondary systems are a loop of pipework which travels round the building, carrying ‘secondary’ (hot) water. This loop is circulated by a pump and is designed to avoid long dead legs in large buildings which occur due to the water in the pipe cooling between periods of draw-off. These dead legs can lead to bacteria growth in the pipe work and water wastage as water is run away to drains before hot water is delivered to the taps. A ‘recirculation loop’ maintains the temperature of the hot water supply pipe work as if it were an extension of the cylinder volume, and minimises the stretch of pipe between hot water and the taps. The secondary circuit is drawn from the top of the cylinder and returned to a port on the side of the cylinder within the top third. If this port is lower, the recirculation will have a de-stratifying effect on the cylinder volume. All pipes carrying hot water to outlets are drawn from this loop.

‘Drain back’ and ‘sealed’ primary solar circuit principles: definitions

When the temperature drops below zero, sealed solar systems either run the circulation pump (active) to draw heat from the cylinder through the pipe work and collectors to avoid freezing, or are left stationary with anti-freeze inhibitors (passive) providing protection to the fluid in the entire system (including external parts).

Drain back/drain down systems drain the water from the collectors into a tank when the circulating pump is not running; this also avoids freezing.

Solar systems use a temperature differential controller to measure and react to temperature differences, according to a pre-determined logic, which results in relays which power pumps and valves being switched on and off in the appropriate circumstances.

Other systems also use temperature differential controllers to circulate fluid within them to avoid freezing. However it is common to dose all primary systems to protect against corrosion as well as frost damage. The reason secondary systems are not dosed is that the concentration would continually be diminished through draw-off and also some of the dosing chemicals (inhibitors) are potentially harmful to the environment.

More information can be found here: Solar Thermal Differential Controllers

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Other Associated glossary terms : Commercial

Solar Thermal
The collection of the sun's energy as heat rather than converting it to electricity.

Vacuum Tube Solar Collector
This is a technology that uses a glass tube, think of a long thermos flask to insulate the solar collector and ensure that effectively all solar energy is retained.

Feed in Tariff
A method to reward the producer of renewable energy (electricity) by paying a fixed amount of money for each unit of electricity generated.

Recirculation - pumped secondary
Pipework carrying secondary hot water.

Sequential heating arrangement
The heating that occurs as the solar thermal system heats the water before the conventional heat source provides any necessary additional heating.

Heat store
Stores the suns energy as heat captured by the collector.

Refers to any period when the heat transfer fluid is not circulating, even though the solar panel is receiving the suns energy.

Primary and secondary systems
This refers to the water travelling round the boiler and the water in the cylinder which is heated by it.

Stratification / de-stratification
The layering of hot water on colder water.

Expansion vessel
A trapped volume of pressurised air or gas inside a steel vessel to allow for the expansion that accompanies temperature increases.

Buffer vessel
A tank storing heated water in anticipation of variable demand requirements.

Pre-feed cylinder
A cylinder placed between the cold water feed and the existing hot water cylinder.

Any pipe with several outlets to or from other pipes, or a junction of pipes.

A water storage vessel, which has the capability to increase the temperature of the water contained within it.

The controller compares the different temperatures at specific locations in the solar systems circuit.

Circulation module / Pump module
Operates the heat transfer system.

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