Pumps
The pump model used by Polysun is based on the pump curve that regulates the flow rate foreseen in the circuit. In one circuit only a single pump can be employed. When not in operation, the pump blocks the flow.
The pumps can be subdivided into various types, according to which mechanism defines the flow rate.
- The flow rate is established by means of circuit pressure drop. The flow rate is calculated in correspondence to the pump curve that regulates the flow rate in the circuit.
- Fixed flow rate: the flow in the circuit is regulated by a pre-established flow rate.
- Controlled flow rate: a controller manages the pump flow rate.
Solar energy systems in which the collector is installed on the roof and the tank is installed in the basement require a circulation pump. Otherwise, due to the force of gravity, the heated fluid would remain above in the collector and the cold fluid below in the tank. The systems in which the collector is mounted below the tank can do without a pump. Such natural circulation (thermosiphon) systems require only a non return valve. In addition to saving on a pump, that is also tied to costs for electrical energy, such systems also present an inherent physical regulation, that in the presence of more intense solar irradiation allows the fluid to circulate more rapidly.
Often the collector is found above the tank, therefore forced circulation of the fluid is indispensable. The operation of the pump can therefore require considerable energy consumption. If a solar energy system works for 2000-3000 hours per year (in one year there are 8760 hours) the absorption of energy by the pump can in itself represent a certain percentage of the solar energy obtained. In well-planned large systems the consumption of energy by the pump is nevertheless lower than one percent of the obtained solar energy.
The specific flow rate is a measurement adapted to the definition of flow rate as directly linked to the maximum increase in temperature (heating of fluid by means of the collector). For example heating the fluid from 20 to 60°C is possible in the presence of full irradiation, high external temperatures and a low flow rate. At higher flow rates there is a corresponding lower temperature increase. The two techniques are respectively referred to as “Low-Flow” (10-20 1/m2/h) and “High-Flow” (30-40 1/m2/h).
The efficiency of the pump depends on among other things, the loss of pressure in the collector loop and on the collector area, but with these, pump efficiency increases linearly much less. A part of the heat lost by the pump (25-90%) is passed to the fluid and goes in favour of the thermal balance of the solar energy system. Still, this gain of energy is valued with scepticism because the consumed electrical energy is expensive and produced in an ecologically sustainable manner.