Installation of thermal solar energy

How Does a Forced Circulation Solar System Work?

A forced circulation solar system is a solar thermal installation in which the water circulates within the circuit driven by a pump.

How does a forced circulation solar system work?

Unlike solar installations with a thermosyphon, forced circulation water heating systems do not move the hot water to the highest point of the closed circuit. Instead, the pump makes water go down from the solar collectors to the accumulation tank.

On many occasions, installing solar thermal energy equipment to obtain sanitary hot water by thermosyphon is not feasible since the solar collectors often are above the accumulator.

This renewable energy system is a must-have in underfloor solar water heating.

In these types of solar thermal systems, the water that circulates between the solar collectors and the storage tank cannot do so by natural convection since the hottest water is already at its highest point. Instead, it will require a conventional water pump and, therefore, an external electrical power source.

The forced systems are always indirect, except for pool heating, where the proper impulsion system for filtering the water in the glass can be used.

Using an external energy source can no longer be considered a passive solar energy system by harnessing solar energy.

Advantages of the Forced Circulation System

These solar domestic hot water systems have the following advantages:

  • First, the structure of the house does not determine its location. Therefore, the hot water accumulator can be inside the building, and outside it will only be necessary to install the solar panels.

  • Better protection of the accumulator: The possibility of placing the accumulator inside the house allows you to install a larger accumulator; it is better insulated. By not being exposed to the weather, it will improve its durability.

  • Better thermal efficiency: the solar heat efficiency to heat the water is higher than in the thermal solar system with thermosyphon because it can be optimized to obtain the highest thermal performance.

Disadvantages of the Forced Circulation System

On the other hand, this type of system has certain drawbacks:

  • The system requires the installation of a water pump to allow the water to circulate. The presence of the pump implies an increase in the cost of maintenance since more elements appear with the possibility of suffering breakdowns.

  • We need to have an electricity point to be able to use electrical energy.

  • The overall energy efficiency is lower than with the thermosyphon system. Although the thermal efficiency is higher, it is necessary to add the electricity consumption for the operation of the water pump.

Essential Elements of a Solar System with Forced Circulation

Solar thermal energy installations with forced circulation have the following components:

Solar Collectors

Solar collectors heat transfer fluid using solar radiation. Since the fluid circulation through the collector is forced in these assemblies, the pressure losses (resistance to the passage of water) are not an important limitation in terms of their value since they can be compensated when choosing the water pump. What will be vital is to know the pressure drops of selecting the optimal circulating pump.

In forced circulation installations, vertical or horizontal solar collectors can be used indistinctly depending on integration criteria, although vertical ones present better performance.

Hot Actuation Accumulator

The accumulator is a tank where heated water is stored. This type of assembly allows several variants regarding the accumulator since the circulator will pass the water through the heat exchanger that it incorporates.

Differential Thermostat

Differential thermostats control the pump so that it only drives the water at times when there may be energy gain. Thus, avoiding energy waste at night hours or when the solar radiation received is insufficient.

The differential thermostat continuously compares the collector and storage temperatures by switching the pump on or off depending on the higher temperature.

Security Elements

It is mandatory to install a safety valve in each group or row of solar panels to protect the solar collector from overpressures.

On the other hand, the primary circuit will be equipped with a security group that will consist of at least:

  • an expansion vessel

  • a safety valve

  • a pressure gauge.

Likewise, these facilities will incorporate active protection systems against low temperatures (frost) or high temperatures (overheating).

Circulation Group

The circulation group allows fluid circulation through the circuit; generally, it is a pump.

The circulation group is generally located in the lower part of the tank (cold water inlet), towards the lower part of the collectors. In addition, it will have the following elements:

  • Electromechanical circulator for closed circuits, of the type used in heating circuits.

  • Non-return valve to avoid uncontrolled reverse circulations.

  • Flow regulator, which will allow adjusting the flow of the circuit.

  • Filter, which will guarantee the durability of the circuit elements.


Published: September 22, 2015
Last review: September 21, 2021