Solar thermosiphon systems are solar energy equipment that has 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 whereby a convective circulation is established in a hydraulic circuit due to the only difference in density 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 installation presented). The displacement of the heat transfer fluid, between the solar collector and the hot water tank, is done only by convection
If we heat a water tank at the bottom, when the bottom water heats up, it loses density and rises to the surface where it cools. Then it goes back to the bottom of the container and thus a natural circulation current is generated.
This is the principle of operation of a thermosiphon equipment, 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.
Operation of the thermosiphon system
The cycle of a thermosiphon system begins at the moment when solar radiation strikes the solar collector, with values greater than 200 watts / m2. The fluid found in solar collectors increases in temperature. Due to this temperature increase, the flow density varies slightly. This variation is sufficient for the fluid to circulate through the primary circuit to the accumulator. The cold fluid is denser and tends to fall, in the same way, the hot fluid tends to rise. Once the fluid is in the accumulator, a heat exchange is carried out using a thermodynamic convection process.
The operation in the primary circuit of this renewable energy system is by thermosiphon. The usual temperature difference to the collector's mouths (T2-T1) is usually 5 to 15 degrees Celsius, depending on the level of heat stroke.
As it is heated, the water in the accumulator is stratified by temperature, that is, the upper part is occupied by hot water and the lower part is the coldest water. 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 accumulator increases its thermal energy and is already available for use as heating or domestic hot water.
Horizontal thermosiphon and vertical thermosiphon
Because the operation of the thermosiphon system depends on the stratification of the water in the storage tank, the vertical tanks are more effective. It is also preferable to have the auxiliary heater as high as possible in the storage tank, to only heat the upper part of the tank with auxiliary energy when necessary. This is important for three reasons:
- Improves stratification
- The heat losses of the tank increase linearly with the 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 frequently used. The performance of the horizontal tank thermosiphon 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 mixture of the flow injection points.
The performance of these systems can be improved by using separate solar and auxiliary tanks or by separating auxiliary and preheating zones with an insulated baffle. A disadvantage of the two systems of tanks or segmented tanks is that the entrance cannot heat the auxiliary zone until there is a demand.
Basic elements of a thermosiphon system
Solar thermal systems by thermosiphon 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 water that circulates through the solar collectors is not forced. Since it is not a forced circulation, the loss of load should be minimal, that is, that the tubes that form the collector grill are of the maximum possible diameter.
As for 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 the loss of charge of the pickup circuit and avoid a considerable reduction of the installation performance.
The accumulator used in the equipment with operation by thermosiphon in indirect circuit is usually of type double envelope. The double envelope accumulators have a larger thermodynamic exchange surface with minimal loss of charge in the circuit.
The disposition of the accumulator tank will facilitate natural circulation. In this case, the best configuration would be to use vertical accumulators to take advantage of temperature stratification. However, the aesthetic integration factors make most of the equipment incorporate horizontal accumulators.
Another quality to consider is that the water intakes of the components of the primary circuit are of a diameter similar to that of the joint pipe in order to avoid the pressure losses represented by the flow reductions.
It is also important that the cold water inlet is located at the bottom of the tank in order to prevent it from cooling the hot water zone when the new water inlet occurs.
Safety elements of a solar system by thermosiphon
To protect the primary circuit from overpressure it is mandatory to install a safety valve (VS) that does not have any sectioning or cutting element that isolates hydraulically from 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 high temperatures (overheating).