Installation of thermal solar energy

Solar Thermosyphon: Types, Operation and Characteristics

Solar thermosyphon: types, operation and characteristics

Thermosiphon solar systems are solar energy equipment that present a natural circulation of the working fluid. This circulation is based on convection currents that form in fluids at different temperatures.

The thermosiphon is the physical phenomenon by which a convective circulation is established in a hydraulic circuit due to the only density difference between fluid volumes at different temperatures. The thermosiphon principle is used in some solar thermal energy systems, when the structure of the pipes allows it, that is, when the refrigerant route is on several levels and is not too long (as in the presented installation). The movement of the heat transfer fluid between the solar collector and the hot water tank is carried out solely by convection.

If we heat a tank of water from the bottom, when the bottom water is heated, it loses density and rises to the surface where it cools. Then it returns to the bottom of the container and thus a natural circulation current is generated.

This is the operating principle of a thermosyphon unit, in which it will be essential that:

  • The solar collector (heat sources) is always located at a lower level than the accumulator.
  • The primary circuit of the solar thermal installation is as short as possible and with a continuous slope that facilitates natural circulation.

How Does a Solar Thermosyphon Work?

Solar thermal energy system by radiatorThe cycle of a thermosyphon system begins at the moment that solar radiation strikes the solar collector, with values ​​greater than 200 watts/m 2 . The fluid found in the solar collectors increases in temperature. Due to this increase in temperature, the flux density varies slightly. This variation is enough for the fluid to circulate through the primary circuit to the accumulator. Cold fluid is denser and tends to go down, likewise hot fluid tends to go up. Once the fluid is in the accumulator, a heat exchange is carried out by means of a thermodynamic convection process.

The operation in the primary circuit of this renewable energy system is by thermosyphon. The usual temperature difference at the collector mouths (T2-T1) is usually 5 to 15 degrees Celsius, depending on the level of insolation.

As it heats up, the water in the accumulator is stratified by temperature, that is, the upper part is occupied by hot water and the colder water remains in the lower part. In vertical accumulators, this temperature differential can reach 15ºC. In horizontal accumulators, this differential drops to only 4-5 degrees Celsius.

The water accumulated in the storage tank increases its thermal energy and is now available to be used mainly as heating or domestic hot water.

Types of Thermosyphon: Horizontal and Vertical

Since the operation of the thermosyphon system depends on the stratification of the water in the storage tank, vertical tanks are more effective. It is also preferable to have the auxiliary heater as high up in the storage tank as possible, to heat only the top of the tank with auxiliary power when needed. This is important for three reasons:

  • Improves layering.
  • Tank heat losses increase linearly with storage temperature.
  • The solar collector operates more efficiently at a lower collector inlet temperature.

However, to reduce the overall height of the unit, horizontal tanks are often used. The performance of horizontal tank thermosyphon systems is influenced by the conduction between the high temperature auxiliary zone at the top of the tank and the solar zone and by the mixing of the flow injection points.

The performance of these systems can be improved by using separate solar and auxiliary tanks or by separating the auxiliary and preheat zones with an insulated baffle. A disadvantage of two tank or segmented tank systems is that the inlet cannot heat the auxiliary zone until there is a demand.

Basic Components of a Solar Radiator

Thermosiphon solar thermal systems have a very simple configuration with few elements. The most important elements are the solar collector and the accumulator. 

In these systems, the circulation of the water that circulates through the solar collectors is not forced. As it is not a forced circulation, it is convenient that the loss of load be minimal, that is, that the tubes that form the grill of the collector have the maximum possible diameter.

Regarding the number of solar collectors to connect, it is not recommended to connect more than 10 m 2 of collectors. The reason for not connecting too many solar collectors is to avoid loss of charge in the collection circuit and avoid a considerable reduction in the performance of the installation.

The accumulator used in equipment with indirect circuit thermosiphon operation is usually of the double envelope type. Double shell accumulators have a larger thermodynamic exchange surface with the minimum pressure drop in the circuit.

The arrangement of the storage tank will facilitate natural circulation. In this case, the best configuration would be to be able to use vertical accumulators to take advantage of the temperature stratification. However, the conditions of aesthetic integration mean that most of the units incorporate horizontal accumulators.

Another quality to consider is that the water intakes of the components of the primary circuit have a diameter similar to that of the connection pipe in order to avoid pressure losses that flow reductions represent.

It is also important that the cold water inlet is located in the lower part of the tank in order to prevent it from cooling the hot water area when new water enters.

Security Elements of a Thermosiphon Solar System

To protect the primary circuit from overpressures, it is mandatory to install a safety valve (VS) that does not have any sectioning or cut-off element that hydraulically isolates the installation.

This is the only necessary safety element in installations that work at ambient pressure. In pressurized installations it is essential to add an expansion vessel (VE) and a pressure gauge.

Due to the specific characteristics of these facilities, it is not feasible to install active protection elements against low temperatures (frost) or against high temperatures (overheating).


Published: September 22, 2015
Last review: September 16, 2019