A forced circulation for solar water heater is an installation in which the water circulates inside the circuit driven by a pump. With forced circulation system, the movement of water in the closed circuit is forced by a pump. This feature makes a big difference with respect to sanitary hot water installations with thermosyphon. In this way, it artificially moves the hot water not to the highest point of the closed circuit, but it does so from the solar collectors down to where the accumulator is located.
In many occasions it is not viable to install solar thermal equipment to obtain domestic hot water by thermosyphon, since often the location of the solar collectors is above the accumulator (for example, collectors to the roof and accumulator inside the housing, collector fields, etc.).
In this type of solar thermal installation the water circulating between the solar collectors and the accumulator can not do it by natural convection since the warmer water (to the solar collectors) is already at its highest point and there is no natural force that makes it displace the cold water that is already at the lowest point and is the heaviest. To achieve water circulation, a conventional water pump will be needed, which will require the contribution of an external energy source. The circulator (pump) drives the fluid (usually from the bottom of the accumulator, cold zone) towards the lower part of the solar collectors.
The forced systems are always indirect, except for the use of pool heating where the own water filtering system of the vessel can be used.
By using an external energy source, this way of harnessing solar energy can no longer be considered a passive solar energy system. If, in addition, the source of electricity does not come from a renewable energy source, it can not be considered as a completely renewable form of energy either.
Advantages and Disadvantages of Forced Circulation for Solar Water Heater
When choosing a forced circulation for solar water heater, the advantages and disadvantages associated with this system must be taken into account.
Advantages of Forced Circulation System
The main advantage of forced circulation for solar water heater is that the structure of the house does not condition for its location. In a solar thermal installation with thermosyphon it is required to put the accumulator outside since it must be higher than the solar collectors. In this case, the hot water storage tank can be inside the building and outside it will only be necessary to install the solar panels.
The possibility of placing the accumulator in the interior of the house allows you to install a larger accumulator, is better insulated and, not being exposed to the weather, will improve its durability.
The thermal efficiency is higher than in the thermal solar system with thermosyphon because when regulating the speed of the water that circulates through the solar collector it can be optimized to obtain the highest thermal performance according to the laws of thermodynamics.
Disadvantages of Forced Circulation System
On the other hand, forced circulation systems involve certain drawbacks:
The system requires the installation of a water pump to allow water circulation. The presence of the pump implies an increase in the maintenance cost since more elements appear with the possibility of suffering faults.
The overall energy efficiency of forced circulation for solar water heater is lower than with the thermosyphon system. Although before we explained that the thermal efficiency was greater, it must be taken into account that a certain amount of electric energy must be invested for the operation of the water pump.
Basic Elements of a Forced Circulation for Solar Water Heater
The forced circulation for solar water heater have the following elements:
Solar collectors: solar collectors are responsible for transforming solar radiation into thermal energy. As in these assemblies the circulation of the fluid through the collector is forced, the losses of load (resistance to the passage of water) are not an important limitation regarding its value, since they can be compensated when choosing the pump of water. What will be vital is to know the load losses of selecting the optimal circulating pump.
In forced-circulation solar thermal installations, vertical and / or horizontal solar collectors can be used interchangeably depending on integration criteria, although the vertical ones have a better performance.
Accumulator: the accumulator is a tank where the heated water is stored. This type of assembly allows the use of variant strength in terms of the accumulator, since the circulator will pass the water through the heat exchanger that incorporates it or from an external exchanger for installations with high accumulation volumes.
Control System of Forced Circulation for Solar Water Heater
In devices with forced circulation must control the pump so that only drives the water at times when it may be energy gain (a time when sunshine and therefore, the fluid temperature sensors exceeds the of the accumulator). Avoiding energy waste during night hours or in which the received solar radiation is insufficient.
The differential thermostat is the device responsible for this function. The differential thermostat continuously compares the temperatures of the collector and the accumulator by switching the pump on or off depending on which temperature is higher.
Security Elements of Forced Circulation for Solar Water Heater
To protect the solar collector from overpressure, it is mandatory to install a safety valve in each group or row of solar panels.
On the other hand, the primary circuit will be equipped with a safety group that will consist of at least: an expansion vessel, a safety valve and a pressure gauge.
Also, these facilities will incorporate active protection systems against low temperatures (frost) or against high temperatures (overheating).
The circulation group is an essential element in forced circulation for solar water heater. It is the element that allows the circulation of fluid through the circuit, usually a pump.
The circulation group will generally be located in the lower part of the accumulator (cold zone), towards the lower part of the collectors. In addition, you will have the following elements:
- Electromechanical circulator for closed circuits, of the type used in heating circuits.
- Non-return valve, to prevent uncontrolled reverse circulation.
- Flow regulator, which will allow to adjust the flow of the circuit.
- Filter, which will guarantee the durability of the circuit elements.