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.

All you need is Sun. All you need is Sopelia.

In solar thermal energy systems, heat exchanger is in charge of transmitting the heat energy collected by solar collectors to medium that needs to be heated.

Depending on type of heat transfer system used, they can be classified into:

• Direct: Domestic hot water for consumption circulates through primary circuit and, therefore, will circulate through collectors. This system is suitable for small systems located in areas where there is no freezing danger. The trend is towards the restriction of its use, not being admitted in several countries.

• Indirect: Domestic hot water for final consumption circulates only through secondary circuit, which means that heat transfer liquid only flows through the primary circuit and is never in contact with domestic hot water. In this case, an exchanger is needed to pass the heat collected in first to second circuit.

The selected exchanger will withstand the maximum working pressure of the system.

According to section HE-4 of Spanish CTE:

– In case of an independent heat exchanger, the minimum power of heat exchanger P will be determined for working conditions in day central hours, assuming a solar radiation of 1,000 W / m2 and a performance of solar energy conversion to heat of 50 %, fulfilling the condition:

P = 500. A

Being:
P = minimum power of the exchanger [W]
A = the collector area [m2].

– In case of an exchanger incorporated into the accumulator, the ratio between useful exchange surface and total collection surface shall not be less than 0.15.

In each of water inlet and outlet pipes of the heat exchanger, a shut-off valve will be installed next to the corresponding sleeve.

The heat exchangers used in sanitary water circuits will be made of stainless steel or copper.

The design head loss in the heat exchanger shall not exceed 3 m / ac, both in primary and in secondary circuit.

Solar exchangers type:

Plate heat exchanger: This type of heat exchanger is made up of a series of corrugated metal plates, joined together in a frame by pressure and sealed by a gasket. Plates form a series of interconnected corridors through which working fluids circulate. These fluids are powered by pumps.

In order to choose correct plate heat exchanger for the system, it is necessary to consult the manufacturer’s guidelines. However, it is recommended that the thermal power to be transferred (in Kw) is equal to 2/3 of the collecting surface (in m2).

Double wrap exchanger: this system consists of a tank in which the secondary fluid (hot water) is accumulated and which has a double wall through which heat transfer fluid circulates, giving heat to domestic hot water.

Exchanger’s operating conditions dictate the choice of its material, which is usually carbon steel or alloy steels. Minimum exchange surface must be between 1/4 and 1/3 of useful collectors surface. However, there is a geometric limit to its use, which is given by housing dimensions. For a certain range of measurements, exchange surface can become less than a quarter collector surface. For volumes greater than 750 liters, the necessary exchange surface (which is the accumulator wall) is increasing and could result in very high accumulators for which it would be necessary to have a suitable machine room.

Coil exchanger: is made up of a tube that is submerged in a tank where the secondary fluid accumulates. The primary or heat transfer fluid circulates inside the tube, giving heat to the secondary fluid.

According tube shape they are distinguished:

• Helical coil exchanger. The spiral wound tube that carries heat transfer fluid is submerged inside accumulator at the bottom.

• Tube bundle coil exchanger. They are commonly used to obtain ACS. Primary fluid circulates through several tubes, not one as in the helical. Liquid flows inside coil by forced circulation, while outside the fluid in contact with coil is renewed by natural circulation.

To know if a coil heat exchanger is suitable for use in solar applications, its minimum exchange surface must be between 1/4 and 1/3 of collectors useful surface.

The exchange surface of a helical coil or tube bundle will be the lateral surface of a cylinder based on outer section of the tube used and by height total length of the same. With this criterion it will be easy to size a tubular exchanger.

Some recommendations:
– The coil must be placed in the lowest part of accumulator.
– If it is helical, distance between turns should be equal to 2 times outer diameter of the tube.
– If we use antifreeze in a proportion of up to 30%, exchange surface must be increased by 10%.

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

All you need is Sun. All you need is Sopelia.

Accumulator is responsible for storing the thermal energy generated by the solar collectors.

It is essential in solar systems since periods of solar radiation and energy transfer do not usually correspond to periods in which hot water consumption takes place.

Storing energy using hot water is cheap, easy to handle, has a high heat capacity and is at the same time the consumption element in case of DHW (domestic hot water).

Accumulator type depends on the application: domestic hot water, air conditioning, heating or industrial use.

Most common are:

– Domestic hot water accumulators: they must be able to withstand high levels of pressure and expected working temperatures, not suffer deterioration due to corrosion phenomena and compulsorily comply with requirements for storing drinking water.
They are generally offered with capacities of 100 to 5,000 liters of accumulation.

– Inertia accumulators: they are used as a heat accumulator for heating systems or for large DHW installations. They fulfill the function of buffer for heat or cold storage. They act as hydraulic memory between heat production and release.
They are generally offered with capacities of 500 to 5,000 liters of accumulation.

– Combined accumulators: they combine accumulation of DHW and accumulation of heating.
In the same accumulator, for example, 175 liters of DHW accumulation and 600 liters of heating accumulation are combined.
They are generally offered with capacities from 175 to 250 liters for DHW accumulation and 500 to 2,000 liters for heating accumulation.

The most used materials accumulators’ construction are:

– Steel: it needs internal treatments based on epoxy or vitrified to avoid corrosion.

– Stainless steel: it is without a doubt the best material.

– Galvanized steel: accumulation temperature must not exceed 65º C.

– Reinforced fiberglass: resists corrosion, weighs little and is easy to maintain, but withstands low temperatures (60º C maximum).

– Plastics: it has similar qualities to fiberglass.

– Aluminum: it is not advisable due to corrosion problems.

In addition to interior treatments, accumulators incorporate corrosion protection devices.

One of the problems caused by corrosion is that rust and sediments favor the legionella development.
It is essential to avoid it by building accumulators with noble materials such as some type of stainless steel and / or combination of some inner lining and a cathodic protection system.

Accumulators are usually cylindrical in shape and have a vertical dimension greater than horizontal one to favor thermal stratification of the inside water.
Hottest water from top will be located in the extraction zone towards consumption or towards conventional support system. Coldest water is in the lower part of the tank, which will be from where it will be pushed towards solar collectors.
In this way, we operate the collectors at the minimum possible temperature, increasing their performance.

Accumulation volume size depends mainly on three factors:

1 • Installed collectors surface

As a general criterion for DHW, an accumulation volume between 50-100 liters per m2 of solar collector is recommended.
Higher values do not lead to a significant increase in solar energy use, and accumulator cost increases.
In contrast, smaller sizes increase the temperature, thus decreasing collectors’ efficiency.
For small DHW systems production, solar tank capacity should be equal to daily hot water consumption.

2 • Operating temperature

This will determine type of stratification device, as well as insulator thickness to be used, depending on maximum losses that are admissible considered.

3 • Offset between collection – storage and consumption

Accumulation volume will be a function of lag between collection – storage and consumption period, which can be:

* Coincidence between collection period and consumption period (case of preheating a boiler in a continuous process).
In this case, accumulator specific volume will be 35-40 liters / m2.

* Offsets between collection and consumption not exceeding 24 hours (heating of sanitary water in multi-family homes, hotels, etc.).
In this case, volume will be 60-90 liters / m2.

* Offset between usual and periodic collection and consumption for more than 24 hours and less than 72 hours (heating of domestic hot water in industrial processes, etc.).
In this case, volume will be 75-100 liters / m2.

* Offsets between collection and consumption greater than 72 hours (heating of sanitary water in second home, on weekends.).
In this case, volume will be determined by balancing energy losses and gains and insulation optimizing.

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

All you need is Sun. All you need is Sopelia.