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Solar Thermodynamic Water Heating

Solar Thermodynamic Heaters: the Sustainable future of Hot Water?

17th October 2020

As part of a recent project, fourLINK have been looking at the relatively little-known solar thermodynamic water heaters, which can be used for creating domestic hot water and powering underfloor heating with minimal environmental impact. Otherwise known as "solar assisted heat pumps", they are different to the rooftop mounted solar water heaters which are often seen on rooftops alongside photovoltaic panels.


For domestic purposes, it is claimed by the leading manufacturers that the cost of creating solar hot water from a thermodynamic system is comparable to running a small fridge, with the added advantage that the system does not need warm and sunny days to operate. A single panel can provide enough hot water for a family of 5, 24/7, 365 days a year, and coupled with a photo voltaic panel the system can run off-grid during daylight hours.


A thermodynamic solar panel is similar to an air source heat pump but instead of an external large fan unit it uses a black, flat metal plate collector which can be either externally wall or roof mounted. Rather than relying directly on the sun’s rays to fall on the plate to make the system work -  like a photovoltaic panel does - the thermodynamic plate relies on the ambient air and air moving across its surface to raise the temperature of the inert refrigerant coolant running through the veins of the panel. The more exposed the panel to the elements, the more absorption of heat from the elements. The system operates effectively with rain, wind, and even snow, day and night.


The refrigerant has an incredibly low boiling point and turns to a gas at minus -26°c (-14.8°f) meaning that any exposure to the external environment in most habitable locations heats the liquid. Back inside the building, the heated gas pipes are fed into a compressor, which increases the temperature further. The super-heated refrigerant gas pipes coil around the hot water tank and exchange their heat with the tanked cold water, creating water temperatures of over 55°c (131°f). Having transferred its energy and heated the cold water, the refrigerant gas then cools back to a liquid and the closed cycle is repeated.      


The system works both day and most importantly at night, and has been successfully installed and tested in countries such as Scotland, where the winters are long, dark and cold. In warmer climates, the system also effectively works to ensure the target temperature of the domestic hot water is maintained, switching on the system to top up the hot water when a tap or shower is opened to draw off hot water.


The system can also be used to heat spaces via underfloor heating, which typically has a lower water heating temperature requirement than wall mounted radiators. A solid screed floor will require an underfloor water temperature of between 30°c - 45°c (86°f - 113°f) and a timber suspended floor, or floating floors a slightly higher temperature between 40°c - 60°c (104°f - 140°f).


The solar thermodynamic heater is a closed system which requires little or no maintenance. The compressor is the main component likely to fail first, requiring repair or replacement.


As the technology develops further with more efficient systems, we should be seeing more integration of this technology into the back of Photovoltaic panels, to pull heat from the PV’s, to use more of the PV's power and to create substantially more green buildings.