Solar thermal

Evacuated tube solar thermal systemEvacuated tube solar thermal system

Solar thermal systems are one of the best established renewable energy technologies. The most popular application is for domestic hot water but solar thermal can also be used effectively to heat swimming pools or provide heat for industrial process applications.

The panels in solar thermal systems are called collectors and there are two basic types; flat plate and evacuated tube. Evacuated tube collectors can reach higher temperatures but are less durable and offer few advantages over the simpler flat plate collectors in most applications. Both types of system work in broadly the same way: Infrared energy in sunlight is absorbed by a specially coated plate in the collector and heat is generated and exchanged into a hot water storage cylinder or into a swimming pools or industrial process using a heat exchanger.

As well as the different types of solar collector and heat exchangers, there are several different hydraulic configurations for solar thermal systems. The simplest type is the thermosyphon system where the water in the collector is heated by the sun and rises into an insulated storage tank above the collector on the roof. These systems have relatively poor thermal efficiency but are cost effective and robust, and work adequately in very sunny countries like Greece and Israel.

In Northern Europe, most systems are of the fully filled pressurised variety. An anti-freeze based heat transfer fluid is held under pressure in the primary hydraulic system which comprises the solar collector and heat exchanger or coil within a hot water storage cylinder An electrically powered circulation pump is used to indirectly transfer heat from the collector to the storage cylinder. Apart from the need to have an electrically powered pump, other hydraulic components such as expansion vessels and not return valves are required so these systems are more costly to buy, install and maintain. However, they perform very well and can be used in a wide variety of applications. A disadvantage of the sealed and pressurised system is that the heat transfer fluid degrades with overheating which tends to happen during sunny conditions when there is an inadequate load for hot water.

A third type of configuration called a drain-back system uses water as the heat transfer medium in an indirect circuit which is sealed but unpressurised. Drain-back system have a so-called drain-back vessel which is normally mounted in the roof space under the solar collectors. When its dark or when the hot water cylinder is already up to temperature, the water in the solar collector automatically returns to the drain-back vessel under gravity. This offers the distinct advantage that antifreeze chemicals are not required and problems with overheating are eliminated. Drain-back systems have not been widely adopted in the UK mainly because of a lack of familiarity with the technology but also because they only work with certain types of flat plate collectors and need careful pipework design to ensure reliable operation.

Narec DE can offer detailed independent advice about all aspects of solar thermal systems from hot water loading calculations and pre-feasibility studies through to energy yield calculations and advice about system configuration, installation, operation and maintenance.


  • Initial feasibility of renewable systems to find most appropriate solution
  • Energy output calculations
  • Energy demand calculations of domestic and commercial properties
  • Design of systems
  • Planning applications
  • Assistance with procurement of contractors
  • Assistance with registration for renewable heat incentive or equivalent.
  • Detailed R&D work on the integration of solar thermal with a range of other technologies
  • Due diligence
  • Reviews of existing and cutting edge technologies