Heating swimming pools and sanitary water with Greenway® Neo Solar heat transfer fluid

Eklor manufactures solar heating systems that incorporate the drainback feature. Today in France, Eklor units represent more than 25,000m² of solar heating systems.

The solar heating system

Solar heating systems use sunlight to produce heat for heating sanitary water or rooms. Solar heating panels are used to provide hot water. A solar heating panel is a surface which captures solar radiation to heat up a heat-transfer fluid located underneath the panels. The heated fluid then enters a storage tank where it heats sanitary water used in taps, sinks, showers, and so on.




Heat production is carried out by the various components comprising a solar heating system. The sun is an intermittent and variable power source. In winter, it emits radiation for 2-3 hours per day. In summer, it provides up to 8 hours of sunlight.

A solar thermal collector is a sealed unit made up of glass, air, a black copper or aluminium plate, and pipes.


Drainback is a highly useful feature when it is no longer possible to store heat


Drainback is a very old technique and is standard in cold countries. The technology was brought to France to avoid overheating, as one of the problems associated with the heating system is its high power. When too much electricity is produced in a photovoltaic installation, it is sent to the grid, whereas with solar heating this excess of electricity stays within the system. This can create issues during periods when the system is not used, for instance when the hotel is closed. Solar heating is therefore only for self-consumption. It is essential to think about intermediate storage when the installation is designed in order to avoid overheating.


When the swimming pool or the tank reaches its maximum storage temperature, solar heating stops and air rises up through the collectors. Air is naturally compressible and its temperature can rise to 180°C. At this temperature, neither the solar heat-transfer fluid nor the collectors are damaged, however if the solar heat-transfer fluid remains in the panels at 180°C, it will boil and the pressure will rise. In a drainback system, this problem is mitigated as the system does not undergo this rise in pressure and the heat-transfer fluid never exceeds 100 °C.

Operation of drainback solar heating systems

This solar thermal energy production system uses gravity to automatically drain the system carrying the fluid when the system is stopped, i.e. when the heat storage levels reach maximum capacity. This process makes it easier to maintain the pipework, increases the system’s lifespan, and avoids overheating which may cause significant damage to the equipment. The absence of pressure in the system also limits the possibility of leaks.


The solar installation at the Hôtel Les Cleunes

This installation, which is used to heat both the swimming pool and the sanitary water for the rooms, comprises 57m2 of solar collectors manufactured by KBB in Germany, according to Eklor’s design incorporating a drainback function. When the hotel is full, the average quantity of water to be heated for the 40 rooms is 2,000 litres per day and the volume of the swimming pool is 150m3. Depending on the season, either the swimming pool or the water for the rooms is given priority for heating. The swimming pool’s heating is topped up by a heat pump, meaning that when solar energy no longer provides sufficient heat, the heat pump takes over. The heating of water for the rooms is topped up using a gas boiler. Previously, the swimming pool was heated by the heat pump and an electrical loop. The solar energy system is inserted upstream of the boiler, which tops up the heat as required. In summer, the top-up heating is not used. In winter, however, cold water is preheated to provide sanitary hot water. It enters the panels at 15°C and is brought up to 40°C, after which the boiler tops the temperature up to 60°C. For the swimming pool, water at 15°C is brought up to 28°C. The entire process is configured via the control system.

Fitting the system, including the solar collectors, pipework and electrical connection, took 4 weeks. For Hôtel Les Cleunes, Eklor designed a bespoke, connected solar station whose performance can be monitored remotely, and which is equipped with controls in order to guarantee total safety and specified performance levels. This compact solar station includes a tank in which the Greenway® Neo Solar heat-transfer fluid is stored, and which acts as an air collector during operation of the Wilo circulation pump.

During commissioning, the heat transfer fluid is injected, then the solar station is switched on.  The fluid circulates from the outlet to the inlet of the solar collectors, enabling the recovery of energy.

« This is a solution which causes hardly any environmental pollution. In terms of recycling, it’s important for the sector to have a product which can be recycled more easily than petroleum. For those of us working in the solar industry, it’s fantastic to have a heat-transfer fluid which is not petroleum-based. It’s a question of ideology. » Nicolas Savariaux, Chairman and Technical Director of Eklor.



Eklor has listed Greenway® Neo Solar


« After two years of testing, what surprised us in the results was that Greenway® Neo Solar did not foam at all and that it maintained a neutral odour, unlike the petroleum-based glycols which often produce a very strong odour when heated. Foaming makes it difficult to see how much liquid is left in the installation. » Nicolas Savariaux, Chairman and Technical Director of Eklor.



Installation characteristics

KBB solar thermal collectors

24 (57 m2)


40 kW

Hotel sanitary hot water

2,000 l / j

Pool sanitary hot water to keep at stable temperature

150 m3

Greenway® Neo Solar -18°C

160 litres

Drainback solar station


2 tubular heat exchangers (swimming pool and hot sanitary water)

EJ Bowman


Eklor in brief:

Founded : 2005
15 employees
1,500 solar heating installations
Turnover in 2018 : €5.4m