TECHNICAL FIELD

The present invention relates to a joint for concentration solar plants .

BACKGROUND ART

Concentration solar plants (CSP) comprise a plurality of parabolic mirrors (parabolic troughs) to concentrate solar energy on respective receiving pipes arranged in the focuses of the mirrors, through which a heat transfer fluid flows .

The high temperatures reached by the heat transfer fluid allow using the accumulated heat to produce through a heat exchanger supersaturated steam that can, for instance, rotate a turbine and produce electric energy.

In order to increase the efficiency of the plants, the mirrors are configured so as to rotate following the apparent motion of the sun. Therefore, during the day, the receiving pipes rotate integrally with the parabolic mirrors and are mutually connected by means of fixed ground pipes .

It is therefore necessary to provide fluid-tight joints interposed between the rotating pipes and the fixed pipes. Flexible pipes, also allowing the thermal expansion of the receiving pipes, are generally used for this purpose. However, these pipes suffer from fatigue resistance limits, since they are subjected to cyclic bending and torsion stresses and to high temperatures.

The efficiency of the CSP plant is closely linked to the maximum temperature reached by the heat transfer fluid. Therefore, the use of substances that can be stable at high temperatures is of interest.

The use of diathermic oil as a heat transfer fluid sets an upper limit to the operating temperatures, which may not exceed 400°C.

Substances that can withstand operating temperatures of 600°C have been tested with the aim of increasing the efficiency of the plants. Some of these substances, e.g. mixtures of molten salts such as nitrates and nitrites, also have further advantages with respect to diathermic oil, because they are not flammable and pollutant.

However, these substances have relatively high solidification temperatures if compared to diathermic oil, with values that may even reach 240 °C. On the one hand, the use of these substances allows increasing the energy efficiency of the plant, but on the other hand it poses further technical requirements, such as the need to prevent the solidification of the heat transfer fluid, which beside forming inclusions in the pipe can damage its structure during the solid residues melting step due to a simultaneous volume increase from the solid phase to the liquid phase.

For these reasons, the pipe is traversed by a low voltage and high amperage current (about 400 A) to maintain a temperature able to prevent the solidification of the fluid by means of the Joule effect.

DISCLOSURE OF INVENTION

The object of the present invention is to provide a connection unit between the fixed and the movable pipes of a CSP plant, which allows solving the aforesaid problems.

The above object is achieved by a joint according to claim 1.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, it will now be described a preferred embodiment as a non- limiting example and with reference to the accompanying drawings, in which:

Figure 1 is a perspective view of a connection unit made according to the present invention;

Figure 2 is a side view of the unit of Figure 1 in a partial section;

Figure 3 is a partially sectioned perspective view of the unit of Figure 1, with parts removed for clarity's sake; Figure 4 is a section along the line IV-IV of Figure 2; Figure 5 is an enlargement of the detail X of Figure 3; and Figure 6 is a perspective view of a second embodiment of the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

With reference to Figures 1 and 2, the reference number 1 indicates as a whole a connection unit for sealingly connecting two coaxial, mutually rotating, pipes 2, 3 (partially shown) . In particular, the pipe 2 can be formed by a fixed ground pipe; the pipe 3 can be formed by a connection pipe with the receiving pipe of a parabolic mirror of a CSP system (not shown) employing a heat transfer fluid at a high temperature of solidification, e.g. NaN03+KN03. The pipe 3 is preferably a flexible pipe. The connection unit essentially comprises:

- a first end pipe 4 to be rigidly and sealingly connected to the pipe 2,

- a first intermediate pipe 5,

- an insulating joint 6 connecting the first end pipe 4 to the first intermediate pipe 5 and axially interposed between them,

- a second pipe 7, and

- a rotary joint 11 sealingly connecting the first intermediate pipe 5 to the second pipe 7.

The connection between the unit 1 and the pipe 3 is conveniently achieved by means of a second end pipe 8 and a conductive joint 9 connecting the second intermediate pipe 7 to the second end pipe 8 and axially interposed between them.

The pipes 4, 5, 7, 8 are made of a conductive metallic material and are coaxial to each other and to the rotary joint 11.

The insulating joint 6 comprises a flange 15 fixed to the first end pipe 4, a flange 16 fixed to the first intermediate pipe 5 and an electrically insulating spacer 17 (e.g. mica) interposed between the two facing flanges 15, 16. The flange 15 comprises a radial conductive appendage 19, whose function will be explained below. The flanges 15, 16 are conveniently welded to the respective pipes 4, 5, and are removably connected, preferably by means of a plurality of bolts 20.

The conductive joint 9, similar to the insulating joint 6 but with a conductive gasket, comprises a flange 25 fixed to the second pipe 7 and a flange 26 fixed to the second end pipe 8. The flange 25 comprises a radial conductive appendage 27, whose function will be explained below. The flanges 25, 26 are conveniently welded to the respective pipes 7, 8, and are removably connected, preferably by means of a plurality of bolts 28.

The rotary joint 11 (Figures 2 and 3) essentially comprises a seal 30 formed by a pair of facing sealing rings 31, 32 having an axial contact and pushed one against the other by respective metal bellows 33 34, axially interposed between each sealing ring 31, 32 and the respective intermediate pipe 5, 7.

The seal 30 is enclosed by a casing 35 comprising a bell- shaped element 36 fixed on a flange 37 of the first intermediate pipe 5 and an outer cylindrical portion 38 axially extending from the bell-shaped element 36 toward the second pipe 7.

The casing 35 also comprises a wall 39 fixed to a flange 40 of the second pipe 7 and facing the bell-shaped element 36, on the opposite side of the seal 30 with respect to this latter and inside the cylindrical portion 38.

A bearing 41 is interposed between the cylindrical portion 38 and the wall 39 to allow their relative rotation .

The casing 35 further encloses a heating element 42 clearly visible in Figures 3 and 4.

The heating element 42 comprises a C-shaped metallic heating body 43 housing the seal 30 and fixed to the bell-shaped element 36 by means of a plurality of axial screws 44 passing through the bell-shaped element (Figure 5) .

The screws 44 are electrically insulated with respect to the bell-shaped element 36 by an insulating bushing 45 and an insulating spacer 46.

The heating element 42 further comprises a pair of electric terminals formed by parallel appendages 47, 48 extending from the free ends of the heating body 43.

The appendage 47 is electrically connected to the appendage 19 of the flange 15 by means of a shaped strap 49.

The appendage 48 is electrically connected to the appendage 27 of the flange 25 by means of a flexible cable 50.

The heating body 43 is provided with a vertical through hole 51 arranged below the seal 30 (Figures 4 and 5) to allow the passage of any micro-leaks which can leave the casing 35 through a drain pipe 52 with a vertical axis passing through the cylindrical portion 38 under the heating element 42.

The unit 1 operates as follows.

The seal 30 allows a relative rotation between the pipes 2, 3 and ensures the hydraulic seal thanks to the elastic thrust of the bellows 33, 34 on the sealing rings 31, 32. A low voltage and high amperage current, which must be transmitted through the connection unit 1 from the pipe 2 to the pipe 3, flows through the plant in order to prevent the solidification of the heat transfer fluid. The current flows from the pipe 2 to the first end pipe 4 and from this, by means of the flange 15, of the appendage 19 and of the strap 48, to the heating element 42. From this, the current flows through the cable 50 and the conductive joint 9 to the second end pipe 8 and then to the pipe 3. The aforesaid elements thus form in their entirety a bypass circuit of the seal 30, to which the heating element 42 is connected in series.

The insulating joint 6 isolates the first intermediate pipe 5, and consequently the seal 30, from the first end pipe 4. Therefore, the sealing rings 31, 32 are not subjected to a passage of current and are equipotential . This avoids the formation of electric arcs between parts in relative motion.

The heating element 42, traversed by the current flowing through the plant, is heated by means of the Joule effect and heats the rotary joint 11, which would otherwise be "cold", since bypassed by the current as set forth above, and in particular the area of the seal 30 and of the bellows 33, 34, thus preventing the local solidification of the heat transfer fluid.

A closer examination of the unit 1 made according to the present invention clearly shows its advantages.

The connection unit 1 solves the aforesaid problems associated with the prior art.

In particular, the unit 1 enables the relative rotation between the pipes 2 and 3 in the presence of high temperatures and of electric current circulating in the pipes, and uses the current to heat the sealing area. It is thus prevented, on the one hand, the solidification of the transfer fluid in the sealing area, and on the other hand, the production of electric arcs between parts in relative motion. Torsion fatigue stresses on the mechanical elements of the unit are also avoided.

The unit 1 can be directly installed on the main line of the CSP system, with no further connection or thermoregulation apparatuses, thus directly intercepting the current flowing along the plant.

The unit is easy to install, requires no maintenance and is easily removable for a replacement of the components in the event of a failure.

According to the variant shown in Figure 6, the second end pipe 8 forms part of the unit 1, and is connected to the second pipe 7 through an insulating joint 54 similar to the joint 6. In this case, the flange 25 of the second pipe 7 is isolated from the flange 26 of the second end pipe 8 and the appendage 27 for connecting the cable 50 extends from the flange 26 and not from the flange 25. This solution provides a redundant electric bypass that isolates the seal 30 from the current flow even in case of a failure of the joint 6. Finally, it is clear that the connection unit 1 may be subject to modifications and variations that do not depart from the scope of protection of the invention. In particular, the joints 6, 9 and 11 may be made in a different way.

The first end pipe 4 and the second end pipe 8, if present, may be replaced by any end element securing the pipes 2 and 3 to an element of the joints 6 and 9.