The solar circuit is the connection between the solar collectors and the storage tank. A solar circuit can be made of copper or stainless steel pipes. To restrict heat losses, the pipes which connect the collectors to the tank have to be short and insulated to their maximum. Stainless steel pipes have a surface which is less smooth than that of copper pipes and so they cause bigger load loss. In open systems, it is better to use tubes which have a smooth inner surface (e. g. cop­per) to prevent encrustments. When we choose the circuit components, it is very important to consider the kind of fluid they are going to contain; actually, anti­freeze solutions or products used in the swimming pool impose the utilization of components (taps and fittings, sets) which do not corrode when they come into contact with certain chemical substances. The good processing of a solar system strongly depends on how the solar circuit’s insulation has been carried out. A suf­ficient insulation layer and also a good insulation execution without interruptions or escapes are necessary. This also applies to the circuit’s elbows. Concerning the choice of the insulating materials, it is important to take into consideration their
resistance to high temperatures; for very short periods, the temperature inside the solar circuit’s tubes can reach more than 200°C. Moreover, the insulation should be able to resis atmospheric agents and ultraviolet rays. Suitable materials could be insulating mineral fibres or insulating materials such as Aeroflex and Armaflex HT. On the outside, the insulation can be protected by tube-coverings made of a steel or zinc-plated layer.

In the solar circuit, besides the pipes, there are devices which are necessary to guarantee the fluid’s motion and the security on the basis of its assemblage and utilization (pumps, buffer vessels, expansion vessels, security valves, air discharge valves, etc.). Generally, all the basic hydraulic components offered are already pre-set by a great majority of the manufacturers. Also, the control instruments (manometers and thermometers) are pre-set [5, 6, 9, 17].

Let us now see the elements which characterize the solar circuit, with reference to systems with a forced circulation, because of its complexity. It has been seen

that in the natural circulation system the storage tank can be directly heated up by the natural circulation or by a heat exchanger. Moreover, there is no device which is able to actively regulate the solar circuit [6].

Pump As in the centralized heating systems, in the solar systems, there are ‘lots’ and ‘returns’. The pipe in which the hot thermal vector fluid flows from the col­lector to the storage tank is called ‘lot’, whereas the ‘return’ is the pipe with the colder fluid which flows from the storage tank to the collector. The pump must be installed on the return line, with the motor’s shaft in the horizontal direction. The pump must not be insulated [5, 6, 33].

Figure 55: Circulation pump.

Non-return valve (or restraint valve) When collectors are installed in an ele­vated position than the storage tank and the fluid inside the tank has a higher temperature than the fluid which flows inside the collectors (especially during the night), the temperature difference between the hot fluid inside the exchanger (in a lower position) and the cold fluid inside the collectors (in a higher position) would start a natural circulation inside the primary circuit causing dispersion of the heat stored in the tank during the day.

Figure 56: Non-return valve.

To avoid this phenomenon, it is necessary to install a non-return valve between the pump and the collector; this valve has to be well proportioned so as to prevent its opening by the sole thrust force of the thermal vector fluid. In this way, the storage tank will not be cooled by the collector when the pump is not running [5, 9].

Regulating power unit The regulating unit of a thermal solar system controls the running of the circulation pump to exploit the solar energy to its fullest. Often, we talk about simple electronic power units based on the temperature difference. This kind of power unit installed in standard systems (collectors on the roof and storage tanks in cellars) are provided with two temperature sensors. The first sen­sor is installed inside the collector, at the hottest point of the solar circuit, and the second sensor is installed inside the tank, connected with the heat exchanger of the solar circuit.

Figure 57: Electronic power unit.

The temperature values detected by the sensors are compared by a control device: the pump is run by a relay when the intervention temperature is reached. The correct definition of the intervention temperature comes from different factors. Generally, the longer the circuit pipes are, the larger is the temperature difference or the delay in the intervention. To make the pump work, standard directions sug­gest that the temperature difference between the solar collectors and the storage tank should be between 5°C and 8°C. Instead, the pump switches off when the temperature difference reaches 3°C. It is also possible to insert a third optional sensor which measures the temperature of the upper part of the storage tank [33].

Temperature sensors The efficiency of the pump’s intervention mostly depends on the position of the thermal sensors. Each collector’s sensor is positioned on the storage pipe or directly on the absorber (the part of the collector which absorbs solar radiation), near the lot’s exit. However, the thermal sensor has to record the absorber temperature and communicate it to the regulating power unit, even in a stalemate situation, i. e. when the pump is not working.

The storage tank’s sensor has to be placed at the same height as the heat exchanger and it can be an immersion sensor or a contact sensor. The sensor for the auxiliary heating communicates to the power unit when this intervention is necessary and has to be put at the same height as its respective heat exchanger [33].

Figure 58: On the left, the processing of the auxiliary heating can be seen; on the right, the storage tank is heated by the solar circuit’s exchanger.

Expansion vessel It absorbs the thermal vector fluid’s expansion. The size of this component depends on the fluid quantity inside the circuit, since the vessel must be able to contain the fluid’s dilatation between 4°C and 90°C.

Figure 59: Expansion vessels.

Let us assume a system with a 100-l tank and with collectors and pipes which can contain a water volume of 20 l. The expansion vessel’s volume must be able to absorb the dilation of 120 l of water which occurs at the just said temperatures. The pipes which connect the expansion vessel to the system must not be insulated [5, 6, 17, 33].

Security valve Security valves protect the system when the pressure increases because of various reasons, such as superheating. Such circumstances might arise when the circulation pump is broken or is not working due to a power black-out. So the fluid’s temperature inside the collectors and other circuit com­ponents may increase until the formation of steam which is then released by the security valve.

Figure 60: Security valve.

The valve must not operate during the system’s normal processing and therefore it has to be set at a higher pressure than the maximum pressure of the circuit. For example, it is chosen as a pressure of 600 kPa (6 bar) if the circuit’s pressure is set at 550 kPa (5.5 bar) [5, 17].

‘Jolly’ valve To avoid air storage inside the pipe and thereby the reduction of fluid delivery and thermal exchange, there must be vent-holes in the upper parts of the circuit.

Figure 61: ‘Jolly’ valves.

These vent-holes (jolly valves) can work automatically or manually. In the open circuit systems, the jolly valve should be left open since air continually enters [5, 33].

Flow regulating valves Especially for medium — and large-sized systems, these valves are inserted in every collector’s row to balance the flows inside the different branches of the circuit. In this way, uniform performance from different parts of the system is guaranteed [5].

Intercepting valves The function of these valves is to interrupt the flow and insulate certain circuit elements (such as valves or pumps), when maintenance is needed or when there are security problems. They are installed at the upper and lower part of each system’s element [5].

Emptying taps Manual emptying taps are installed at different circuit points. Generally, there is one in every collector’s row. To allow the gradual emptying of the fluid contained inside the circuit, it is necessary to ‘fix’ an emptying tap between two intercepting valves [5].

Three-port valves Three-port valves allow combining two flows (mixing valves) or separating a flux into two parts (diverting valves) [5].

The pump, the non-return valve, the expansion vessel and the security valve are offered in the market as a ‘pumps and security’ pre-set group. The expansion ves­sel and the security valve have to be installed in any case to avoid interruption between them and the collector [6].

temperature sensors

Jolly valve

solar

^.collector

insulation

during the summer and use it in the winter (seasonal storage), but as we have already stated in point 3 of par. 2.2.2.2, the technological solutions based on this principle are not convenient for limited domestic use because of its cost and logis­tics. Currently, the most common storage systems are the ones which allow the storage of the heat efficiently for a day or two. Storage tanks can be classified depending on their final utilization and the kind of insulation used [5, 9].

Table 4: Classification of storage tanks. Type Pressurized tank Non-pressurized tank Drinkable water storage tank Stainless steel Enamelled steel Plastic-covered steel Buffer Storage tank Steel Plastic Combined storage tank Steel/Stainless steel Steel/enamelled steel

2.2.4.1 Storage tank materials Pressurized tanks are made of stainless steel, enamelled steel or plastic-covered steel. Stainless steel tanks are lighter and last longer, but they are much more expensive than enamelled steel tanks. However, stainless steel is easily corroded by water with a high chlorine content. To reduce the corrosion risks, this kind of tank is generally provided with a magnesium anode which has to be replaced periodically. Non-porous plastic-covered steel tanks are also available in the market and they cost less than the other tanks. However, this storage tank cannot withstand temperatures higher than 80°C. Plastic-covered tanks are characterized, in fact, by a lower reliability level compared with other constructive typologies [9].

2.2.4.2 Sanitary water storage tank Figure 64 shows the storage tank installed in standard solar systems. In this storage device, there are generally two heat exchangers: the solar exchanger, which allows the thermal exchange between the thermal vector fluid inside the solar system and the fluid inside the tank, and the additional exchanger, which allows heat transfer from the integrative heating system (e. g. a central-heating boiler) to the fluid stored inside the tank.

Moreover, in the lower part of the storage tank there is a connection to the water pipes for the supply of cold water. The operating pressure inside the pressurized storage tanks is about 4-6 bar.

As regards the choice of the storage tank’s volume, we generally consider 40-100 l/m2 of flat collector surface. Concerning the proportioning of the solar system, while for the big-sized systems we refer to values that are near the lower limit of the above interval, it is vice versa for small systems. Large-sized storage

Figure 64: Storage tank installed in standard solar systems.

tanks can contain larger quantities of energy; however, this choice also involves larger heat losses and frequent starting of the integrative heating system. This hap­pens because the heating of larger quantities of water requires more energy. As regards sanitary water storage tanks, it is important to take into consideration the calcareous encrustment problem, which may form at high temperatures in the exchanger: for this reason, in the solar systems used inside houses the tank temperature should not be above 60-70°C [9].

2.2.4.3 Storage tank shape If the storage tank works properly it should have different water layers inside. The creation of these layers is possible thanks to the variation in fluid density at different temperatures. Actually, hot water which is ‘lighter’ is stored in the upper part of the tank while the ‘heavier’ cold water is stored at the bottom of the tank. This layering effect is an essential requisite for the good processing of the solar system. As soon as the hot water is requested by users, for example, for showering, the cold water flows into the tank from the pipes and mixes with the previously heated water. To limit this undesired effect and to maintain the temperature layering for as long as possible, the storage tank (generally shaped like a cylinder) should be tall and narrow.

These conditions can be realized using vertical storage tanks whose height — diameter ratio is at least 2.5:1 (in monobloc natural circulation systems the storage

Figure 65: Example of a solar storage tank.

tank is generally horizontal because of aesthetic and encumbrance reasons). Low temperatures in the lower part of the tank guarantee a high performance of the solar system even in case of insufficient radiation and low temperature of the thermal vector fluid.

Before installing a vertical tank, it is important to make sure that its height is compatible with the place where the storage system will be placed [5, 9].

2.2.4.4 The cold water inlet device in the storage tank This particular inlet device, when suitably installed in connection with the adduction pipe, weakens the strong motion of cold water flowing into the tank and limits the risk of its mixing with the warmer water layers.

2.2.4.5 Hot water collection In traditional storage tanks, hot water is collected from the upper part of the tank; because of the layers’ phenomenon and the outlet pipe being located in the upper part of the tank, we are always sure to collect the hottest water. After the collection, a part of this hot water stagnates inside the pipes getting cold. It is possible that this cold water can flow back into the upper part of the tank where it mixes with the hot water which is stored there. This causes heat dispersions of the relevant entity (see Fig. 64). To avoid this drawback, it is possible to direct the lot tube downwards making it pass inside the storage tank or outside across the insulated layer which covers the tank [9].

2.2.4.6 Heat exchangers and respective connections The solar circuit’s heat exchanger should be installed in the lower part of the storage tank to ensure that the thermal exchange occurs inside the water volume present at the bottom of the tank. The heat exchanger for the integrative heating system is generally placed in the upper part of the tank, to guarantee quick heating of the water volume at a temperature (corresponding to the daily requirements) without resulting in a tem­perature increase in the lower part of the tank, where the solar circuit’s exchanger is installed. This disposition of the exchanger ensures that the thermal exchange in the lower part of the tank, where the water is the coldest, occurs with the highest efficiency even when the solar circuit fluid does not reach the highest temperature [5, 9, 33].

2.2.4.7 Storage tank insulation The purpose of the storage tank insulation is to reduce the heat dispersions to the outside environment to its minimum. To have a storage tank insulation which efficiently limits heat losses, the following charac­teristics are needed:

• It should be 10 cm thick on the sides and 15 cm thick in the connections with the upper surface.

• It should also cover the bottom of the tank.

• It should be perfectly adherent to tank’s sides to avoid losses by convection.

• It should be made of materials which do not contain CFC and PVC and have low thermal conductivity (<0.035 W/m K).

Thermal dispersions in an insulated storage tank must be lower than 2 W/K. To limit these losses, it is very important to make sure that the thermal covering in connection with flanges and pipe fittings is hermetically sealed.

The tank linings that are currently available in the market can be flexible (expanded polyurethane foam, fibreglass, etc.), inflexible (they could be used out­side for retrofit interventions) or by direct injection with a plastic or metal cover­ing [5, 9, 33].