There are three main types of pumping systems or electrocirculators:

1. Alternatives
2. Rotary
3. Centrifuges

Usually used in solar thermal energy systems are centrifuges.

The electrocirculator or pump is the element of solar thermal system in charge of moving the fluid from primary circuit, or other closed circuits of the system (circuit between accumulator and external exchanger, recirculation rings for domestic hot water, heating circuits, etc.).

In the particular case of primary solar circuit, the objective of forcing this circulation is to transport the heat from solar collectors to exchanger, compensating for pressure losses (resistance to fluid movement) of different accessories that make up the circuit: pipes, valves, branches, manifolds and exchanger.

In most solar hot water production systems, circulating flows are not very important. The most widely used pumps are in-line, single-phase and small-power type.

Different materials are used to manufacture pump body depending on the circuit in which it is integrated:

– Closed circuits: cast iron is the most used material in manufacture of the hydraulic body of pumps intended for these circuits, since it is cheaper than other materials. Circulating liquid is always the same, generally water with anti-calcareous and antifreeze additives. In addition, this fluid is not for consumption, so it does not have to keep the characteristics of water unchanged.

– Open circuits: bronze and stainless steel are the most widely used materials in open circuits. The liquid that circulates is drinking water and, therefore, salts that it contains dissolved cause calcification and corrosion problems in certain materials, such as cast iron. Furthermore, having to be in contact with drinking water, the construction material of roller must keep the characteristics of water unchanged.

The behavior of the electrocirculator is represented:

P = C. p

Where:

P is the required power

C is the flow (l / sec) between two points of a pipe with pressure difference p

This means that pump power is a function of head loss and flow rate.

With these two axes manufacturer will represent it in its characteristic curve, each pump having its own characteristic curve.

With time passage, pipes acquire corrosion, so pressure drop increases. Generally the calculations are made as if there were only water in the system, while antifreeze is often added, for this reason in practice the chosen pump must be a little oversized.

The pumps usually have several speeds and manufacturer indicates this in their graphics. It is advisable to work at an intermediate speed in order to increase or decrease speed if we have fallen short or have oversized the pump respectively.

By associating two electric pumps in series, manometric height is greatly increased and flow rate is low, while if they are connected in parallel, flow rate increases greatly and pressure does little.

Pump has to counteract pressure drop only on the worst track. If circuit is balanced, one will be chosen at random.

Circuit is preceded by a filter to prevent impurities from entering the welds and rest of system into the pump. It also has a non-return valve to prevent backflow of heat transfer fluid from collector to the pump. The cutoff wrenches are used in case of pump failure to be replaced or repaired.

By operating stopcocks, we obtain delivery pressure and suction pressure on the manometer. If we subtract the results, pressure drop is obtained, which must coincide with that of the system.

At the rear electrocirculator must have a small pressure to be able to start, regulations indicate that it must be at least 2 bar or 5 bar for high temperatures.

Experience indicates that for a flat collectors system minimum necessary flow is 50 liters per hour per m2 of collecting surface if heat transfer fluid is water. If it is an antifreeze mixture, flow rate will be higher to compensate for lower capacity to transport heat. For this we must take into account relationship between antifreeze mixture Ce and water Ce.

In general, thermal flow should be at least equal to 50 kilocalories for each collector´s square meter, for each hour and for each thermal jump degree centigrade. For example: if fluid experiences a thermal jump of 5º C in collectors, minimum thermal flow will be = 50 x 5 = 250 kcal / h / m2.

When we speak of a certain flow we are referring to volume that each collector’s square meter actually passes through in time unit considered.

Once the flow has been found, head losses that this flow causes in system must be calculated, which will be the sum of head losses of each components (pipes, accessories, exchanger, etc.).

The best way to carry out calculation will always be to go to the flow-pressure characteristic curves in pump´s technical data sheet.

This content was extracted from the Solar Thermal Energy Technical-Commercial Manual and is part of Solar e-learning.

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