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Thermodynamic solar panels

thermo panel on wallA single thermodynamic solar panel mounted on a vertical wall can supply all the hot water for a typical 2-3 person household.

Imagine heating your house and all your hot water from solar panels alone. 'You must be kidding', you say, but no, that is the claim made for thermodynamic solar panels. Portuguese company Energie have been making them for 30 years and these are now available in the UK. Another company, Hyrax Solar, assembles thermodynamic systems in the UK.

The panels work on a different principle from conventional solar thermal panels (and neither do they produce electricity - that's solar PV panels!). One or two panels are sufficient for a domestic hot water system, and multiple panels can supply heat for underfloor or skirting space heating.

Like a 'fridge in reverse'

The heat is derived from slender black panels that can be mounted on a roof or wall, not necessarily south-facing, and form part of a system that heats water, either in a cylinder for baths, showers, etc. or for low-temperature space heating. The system works essentially as a refrigerator in reverse: a refrigerant fluid circulates through veins in the thermodynamic panel, which absorbs heat from the surrounding air (or even rain and snow), as well as radiation from direct sunlight. The fluid vaporizes to a hot gas and is piped to a compressor, which raises the pressure and heats it further before passing it through a heat exchanger. Here it transfers its heat to the water in the cylinder, and then goes through an expansion valve, thereby losing pressure and reverting to a liquid at a temperature of something like minus 26°.thermodynamic panel diag

The British-made Little Magic Thermodynamic Box system consists of an external aluminium panel, integral heat exchanger, pump, and hot-water cylinder. Refrigerant liquid absorbs heat from the panel and is transformed into a gas, which is compressed and changes back to a liquid. The transferred heat raises the temperature of water in the cylinder to 55°C. Image courtesy of Magic Thermodynamic Box.

All-weather 24/7 operation

Such a system is similar to an air source heat pump, but relies on the natural circulation of air over the large surface area of the panels to extract heat from the atmosphere, instead of a fan to draw air through the collector of an external unit. Heat is absorbed through both sides of the panel, instead of just the outer face as with solar thermal or solar PV, and the panels work in virtually all weather conditions and also at night, even down to outside temperatures as low as minus 10°. However, they will absorb energy more efficiently in warm weather and when the sun shines directly on them. Water in the tank can be heated to 55° throughout the year, with potential savings on energy bills of up to 80%, it is claimed.

Advantages

  • require less electricity to run than an air source heat pump;
  • no noisy external fan
  • panels will work day and night in most weather conditions;
  • can be installed in various locations, mounted on a wall, flat roof or on the ground, but better in direct sunlight if possible;
  • coated aluminium panels are relatively light and thin, each weighing 8 kg and measuring 200 x 80 x 2 cm
  • no corrosion and virtually no maintenance needed
  • low-carbon technology and could qualify for the Renewable Heat Incentive (but see note below)

Drawbacks

  • supplementary immersion heater required to recharge hot water tank rapidly, or to supply hot water at temperatures over 55°C, especially in winter, and to combat legionnaire's disease if water is not drawn off for prolonged periods;
  • work best with low-temperature heating systems, e.g. underfloor heating or oversized radiators;
  • noise from compressor - similar to that from a refrigerator;
  • takes several hours to heat 200 litres of water from cold
  • unlikely to be cost-effective compared to mains gas

Costs

A single-panel system suitable for supplying a 200 litre hot water cylinder for a three-person household will typically cost in the region of £4500 to £6000 installed, although this depends on factors such as how easily the existing hot water system can be adapted. Households with more people will require an additional panel, which will of course increase the cost. It pays to shop around for several quotes.

The electricity to run the system is estimated to cost 30-35p/day, a saving of around 80% compared to an electric immersion heater. In fact, to heat 200 litres water from cold (at, say, 15°) to 55° will cost about 135 pence using electricity, but only around 47 pence using gas.

Performance data

Preliminary results of an independent test of a Hyrax panel undertaken by Narec Distributed Energy found a coefficient of performance (COP) generally in the range 1.5-2.5. This means that the unit delivered 1.5 to 2.5 units of heat energy for each unit of electrical energy required to run the system. On the face of it this compares unfavourably with air-source heat pumps, which usually exceed 2.5. However, the COP is reduced by the relatively high output temperature (55C) needed for supplying hot water, rather than space heating. Also, the system is expected to perform better in higher summer temperatures than in winter. Running costs would be reduced considerably if the system was using electricity generated on site by solar PV, making it an attractive option for properties off the gas grid.

Thermodynamic panels, MCS and the RHI

Thermodynamic panels were originally registered under the Microgeneration Certification Scheme (MCS) in 2013, but subsequently deregistered because they had been wrongly categorised. Subsequently a new catergory, Solar-Assisted Heat Pumps, was created for them, and some manufacturers are undergoing recertification of these products under revised criteria. Currently MCS only covers systems that provide domestic hot water, not space heating.

However, panels will need to achieve an average COP value of 2.5 or greater over a full heating season to be considered for inclusion in the Renewable Heat Incentive (RHI) scheme. A decision on this has still to be taken by the government.

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