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NaREC's Solar Thermal Unit

Explore Renewables project leader NaREC have installed a permanent Solar Thermal Demonstrator Unit at their premises in Blyth, Northumberland. The unit is a scaled-up version of the Solar Thermal Trolley, and the data it collects is being fed live to this website for schools to use in their lessons – see the blue data feed box on the right.

System Description

NaREC’s Solar Thermal Unit heats water to a temperature of 50°C and then discharges the water, simulating the behaviour of the schools’ Solar Thermal Trolleys by collecting the infrared energy present in daylight and using it to heat water by an indirect coil fitted in the bottom of the hot water cylinder.

The system comprises the following components:

• One 2.75m2 flat plate solar thermal collector mounted on a purpose built south facing enclosure, located in the NaREC Marine Testing Facility.
• One 35 litre unvented hot water storage cylinder incorporating cold water make-up tank.
• A solar primary circuit of the fully filled pressurized variety charged with a proprietary heat transfer medium which includes anti-freeze and corrosion inhibitors.
• A solar rated pumpstation comprising a solar rated circulation pump, check valve and isolation / filling valves.
• An 18 litre solar rated expansion vessel.
• Solar rated flow setting device on the pumpstation.
• A temperature sensor mounted on the solar collector absorber plate.
• Two immersion temperature sensors in the hot water cylinder.
• A differential temperature solar controller.

Normal system operation

The solar circulation pump is automatically controlled by the differential temperature controller in response to a temperature differential between the collector and the storage cylinder.

The controller is set up so that the solar circulating pump comes on when the collector temperature is 6°C higher than the stored water temperature. When this temperature difference drops to 4°C, the solar circulation pump is automatically switched off again.

When the upper temperature in the hot water cylinder reaches 50°C, the solenoid valve opens to empty the hot water cylinder. As the cylinder empties it is automatically refilled with cold water via the integrated make-up tank. When the upper temperature of the cylinder reaches approximately 20°C, the solenoid valve closes and the system commences the heating cycle again. When the temperature in the storage cylinders reaches 50°C, the solar pump is switched off again.

In very sunny weather when there is no demand for hot water, once the cylinder is heated to 50°C the heat transfer fluid in the system will turn to vapour and be forced into the expansion vessel. The controller will prevent the circulation from restarting until the collectors have cooled below 140°C. This is a safety feature to protect the pipework and collectors from damage that could be caused by restarting the circulation in these conditions.