The present invention relates to a novel type of solar steam generators.

Among the various technologies for the solar energy use, the steam generators with mirrors conveying the radiation on the steam generator exchange panels is well known. A scheme of this technology, wherein a steam generator 13 is irradiated through reflecting mirrors 11, is illustrated on Fig. 1. 12 represents the support of 13. This figure represents a schematic view of the solar steam generator with the field of mirrors .

A commonly used type of steam generator having a steam drum producing subcritical steam, that means having a pressure lower than the critical pressure of about 220 bar, is represented in Fig. 2.

In this kind of steam generators, 21 indicates the steam generating section GV, 22 means the superheating section SH, the steam drum 23 is represented in the figure over the section 21. In this kind of steam generators, liquid sub- cooled water is fed into a section of exchange panels (GV 21 section) ; the water evaporates, due to the power reflected by the reflecting mirrors. In the evaporating GV section 21, there is the contemporaneous and simultaneous presence of liquid water and steam wherein the liquid component generally represents the largest fraction of the mixture for avoiding an excessive heating of the exchange tubes componing the panels of GV section 21. The two-phase steam-vapour mixture from the GV section 21 is conveyed into the steam drum 23, wherein the steam is separated from the saturated water. The latter is conveyed into the evaporating section GV 21, the steam separated in the steam drum 23 is conveyed to the superheating section SH 22. A drawback of this kind of steam generators due to the large size and, consequently, the high thickness of the steam drum 23 brings to a limitation of load variations. This implies the need to maintain the steam drum 23 pressurized (and thus heated) during the night downtime, or during the non production of steam, for example due to bad weather. As a matter of fact, when the mirror irradiation is missing, the steam generator exchanging panels (of section GV 21 and SH 22) quickly cool the whole system, comprising water and the whole metal mass of the tubes forming the panels, headers and connecting tubes among the panels, etc.. This is due to the extended panel surface which makes the steam generator a large radiator.

Therefore, quick temperature increments in the walls having a high thickness of the steam drum 23 during the described transients cause a rapid consumption of the steam drum life, and also of the other metal components, due to oligocyclic fatigue.

The need was therefore felt to have available a kind of steam generators capable to overcome the above mentioned limitations of load variations (and thus heating) of the prior art steam generators above described.

It has been surprisingly and unexpectedly found by the Applicant a kind of steam generator allowing to overcome the above described inconveniences.

It is an object of the present invention once through solar steam generators, i.e. without a steam drum, comprising at least an evaporating section GV wherein the water entering at the liquid state becomes steam, the latter is then conveyed into at least one superheating section SH to be superheated up to the inlet conditions for steam turbines or other processes requiring superheated steam at high temperature and pressure. Optionally the steam returning from a turbine section or from other processes can be resuperheated in one or more re- superheating sections (RH sections) .

Optionally one or more desuperheaters can be present, i.e. mixers of the main steam flows, (to be superheated or resuperheated) with liquid water or steam at a temperature lower than that of the main steam flows. The desuperheaters can be placed among the panels of the GV and/or SH and/or re- superheating sections and/or among them. The desuperheater function is to control the temperature of the steam at GV, SH and resuperheating outlet sections.

An example of representation of once through solar steam generators according to the present invention is shown in Fig. 3, wherein only one evaporating section GV 31 and one superheating SH section 32 are shown. The turbine or other processes (as defined above) are not shown in this Figure.

In another embodiment SH section 32 can be placed above GV section 31 or at the side of section 31. In case two SH sections be present, these sections can be placed on different sides of sections GV 31.

The once through solar steam generators of the present invention can be subcritical or supercritical.

Fig. 4 reports the water diagram H-p-T of a subcritical once through steam generator. Fig. 5 shows the water diagram H-p-T of a supercritical once through steam generator.

In Fig. 4 the transformation from subcooled water into steam in section GV 31 of Fig. 3 takes place through the liquid water-steam phase transition. In section SH 32 of Fig. 3 the steam is superheated. By subcooled water it is meant liquid water at a temperature lower than the saturation one at that fixed pressure. In practice, in Fig. 4, the subcooled water is represented by each point of the area 41 of the diagram below the evaporation area (42) up to the critical pressure p of about 220 bar. In fact, in the area 42, water liq- uid and saturated steam are contemporaneously and simultaneously present. Of course, above the area 42 and up to the pressure p of about 220 bar (area 43 in Fig. 4) only the steam is present. In Fig. 4 as an example it is represented a line between points 44 and 45 indicating the transformation of liquid water into steam in section GV 31 of Fig. 3. In Fig. 4 it is represented as an example a line between points 45 and 46 indicating the superheated steam in section SH 32 of Fig. 3. Fig. 4 is only a schematic representation of the subcritical steam generator. Generally, ^' point 44 is shifted more on the right with respect to point 45, inside area 41, or above the critical pressure of about 220 bar, for taking into account the pressure losses occurring in the system. For the same reason also point 45 could be represented more on the right, always inside area 43, with respect to point 46.

In Fig. 5 the H-p-T diagram of the transformation above the water critical pressure (about 220 bar) is represented. By starting from subcooled water, i.e. below the critical temperature of about 374 ⁰ C, there is no phase transition from the liquid state to steam, but there is a pseudo evaporation outside the two-phase mixture area. The pseudo evaporation is thus characterized by the absence of two-phase mixture (transformation in section GV 31 of Fig. 3) . This part is represented as an example by a line between points 54 and 55 in Fig. 5. In section SH 32 of Fig. 3 the steam is superheated. This portion is represented as an example by a line between points 55 and 56 in Fig. 5. As in Fig. 4, also in Fig. 5 points 54 and 55 could be shifted on the right in the H-p-T diagram, point 54 with respect to point 55 and point 55 with respect to point 56, for considering the pressure losses in the system.

More specifically, as said, the subcritical steam generators have in the tubes exchange surfaces of sections GV 31 of Fig. 3 the presence of liquid water-steam two-phase mixture .

Depending on the pressure, enthalpy, mass flow-rate, thermal flux and on the tube geometrical characteristics, internally smooth or ribbed tubes, (preferably with internal helicoidal ribbing) , can be used, in particular in GV sections. The tube arrangement can be with vertical tubes (Fig. 15) or inclined tubes (Fig. 16) .

The sections (GV, SH and RH) comprise one or more panels. The latter comprise tubes between at least one inlet and at least one outlet header. Preferably the tubes inside a single panel are operated in parallel.

GV panels can be arranged in parallel or in series, as it will be described more in detail hereinafter.

When the GV panels are arranged in series, problems of a bad distribution of liquid water and steam in the tubes and in GV sections can arise due to the presence of the steam liquid-water two-phase mixture.

In these cases, in order to obviate this inconvenience, it is preferable to use an once through steam generator with steam generating GV sections, each of them operating in supercritical conditions. In fact, in this embodiment there is no problem of presence of liquid water-steam two-phase mixture, as at pressures over about 220 bar the water heated in the tubes is continuously transformed from the pseudo liquid state to the pseudo steam state. For pseudo liquid state it is meant supercritical water having high density of the order of the subcritical water at the same temperature. For pseudo steam state it is meant supercritical steam having low density of the order of the subcritical steam at the same temperature. In Fig. 5 one operates on the right of the evaporation area (pressure p above >220 bar) wherein the two-phase mixture is not present. The supercritical configuration can be achieved with the panel arrangement, comprising the exchange tubes, in series or in parallel. As said in the case of subcritical steam generators, smooth or ribbed tubes with vertical (Fig. 15) or inclined (Fig. 16) arrangement can be used.

Panels can be operated in series or in parallel inside the GV sections. In the latter configuration (in parallel) flow-rate at the GV inlet is divided in the whole panels. On the contrary in the configuration of the panels in series, the flow-rate is not divided among the single panels.

The panels of GV sections are operated, preferably, in series for the above mentioned grounds.

The panel arrangement in series, is preferably the one illustrated in Fig. 6. In this representation the inlets are all in the lower portion of the panels, as shown in the figure.

Another embodiment of the arrangement in series is the one wherein in the first panel there is the inlet from the bottom; the outlet of the first panel is sent to the upper portion of the second panel; the outlet in the lower portion of the second panel is sent to the lower part of the third panel. Then the described cycle starts again. This arrange ^¬ ment is preferably the one represented in Fig. 7.

Another possible embodiment is the one using in a combined way the arrangement in series, preferably of Fig. 6, with the above described arrangement and preferably reported in Fig. 7. Preferred possible exemplifying representations are reported in Figs. 8a, 8b and 8c which represent various preferred exemplifying ways to combine the panels of Fig. 6 with the panels of Fig. 7.

An alternative arrangement to the one in series is represented by the arrangement of the panels in parallel. The preferred representation is the one reported in Fig. 9, wherein the inlets of the single panels are all in the lower portion. An alternative representation of Fig. 9 (not shown) could consider the inlets all in the upper portion.

The embodiments reported in Figs. 6, 7, 8a-8c, 9 can be applied to both supercritical and subcritical steam generators .

With the arrangement in series, preferably the one of Fig. 6, there are higher pressure losses and lower temperature differences between inlet and outlet from the panels than in the arrangement in parallel, for example the one of Fig. 9, the exchange tubes in GV sections, which divide the flow-rate, being equal. In the arrangement in series the panel materials are less stressed and therefore also less performing materials can be used as they are not subjected to high temperature variations during the steam generator function.

The arrangement in parallel is characterized by lower pressure losses in the circuit than in the arrangement in series, and by large temperature differences between the panel inlet and outlet. In this embodiment the materials are more stressed, wherefore more performing heat-resistance materials are to be used. As regards the pressure losses, which are reduced in the arrangement in parallel of Fig. 9, it is to be noted that these losses can cause unbalance problems of the flow rate in the tubes. Therefore these losses can be balanced by pressure losses concentrated at the inlet of the exchange tubes and/or panels to equalize the flow rates. This is carried out with orifices at the tube/panel inlet. However it is to be noticed that these concentrated pressure losses are not very effective at low loads.

In both the panel arrangement in parallel and in series, the arrangements with inlets of the single panels from the bottom are preferred in order to take advantage of the flow- rate self-regulation principle through the hydraulic static head in the single panels. The solar steam generator of the invention can preferably be operated according to the following ways:

• constant pressure (in solar steam generator) sliding temperature (at the inlet of the turbine or of other processes) ;

• sliding pressure (in solar steam generator) constant temperature (at the inlet of the turbine or of other processes) ;

• constant pressure (in solar steam generator) constant temperature (at the inlet of the turbine or of other processes) .

In the constant pressure sliding temperature operating way the superheated steam at the outlet of the solar steam generator is at the same pressure and temperature for the different loads. This steam conveyed to the turbine or to other processes is brought to the characteristics required by the turbine (or processes) through a lamination as regards the steam pressure. The steam temperature is regulated through the steam generator: the lamination leads to a pressure lowering and also to a temperature lowering down to the desired value.

In the sliding pressure constant temperature operating way the steam outlets the solar steam generator already at the pressure and at the temperatures required by the turbine or by other processes. These conditions of the steam at the outlet of the steam generator can be obtained by operating at supercritical pressures to pass the evaporation area wherein the two-phase mixture is present, then lamination is carried out in one or more steps in the steam generator, as described hereinafter, and at that point the steam is superheated to bring it to the temperature value required by the turbine or by other processes. An alternative is to operate the whole steam generator also under subcritical pressure conditions up to the temperature values required by the turbine or by other processes.

A particularly preferred solar steam generator according to the present invention is described hereinafter. It is represented in Fig. 10, wherein two panels 101 and 102 of the GV section (31 of Fig. 3) and one panel 103 of the SH section (32 of Fig. 3) are reported. 104 represents a recircle pump with related recircle circuit to the inlet panel of the GV section. Of course more panels of 101 and 102 type, variously interconnected, or 103 can be present. Also panel 102 can be absent. Valve 105 is used to open or to exclude the line connecting the outlet from the GV section of panel 102 to the flash tank 108. Valve 106 is a lamination valve used during the load variations to maintain the GV section at supercritical conditions and the SH section in sliding pressure to carry out the steam lamination inside the steam generator. More valves 106 can be used, which can be placed also inside the SH section, i.e. among two or more sections 103. Valve 107 is used to open or to exclude the line connecting the outlet from the SH section (32 of Fig. 3) to the flash tank 108. In Fig. 10, 109 represents the line leading to the tur ^¬ bine or to other processes and 110 represents the feeding water line.

Valves 105 and 107 and the related lines to the flash tank 108 can optionally also be absent. Pump 104 with the recircle line can be absent, however it is preferably present. Furthermore, the pump 104 with its recircle line and valve 106 could be positioned in other points of the scheme reported in Fig. 10. The skilled man in the field can easily determine the most suitable position. For example the pump 104 could be placed even after the SH section. It is observed furthermore that also more pumps 104 with the related circuits can be present in different positions of the embodiment of Fig. 10, for example one after the GV section and one after the SH section.

With reference to the scheme reported in Fig. 10 the start up of the steam generator is described, preferably at supercritical conditions with multiple steps, with panels of the GV section (31 of Fig. 3) in series.

In this preferred embodiment of Fig. 10 the start up under supercritical conditions is used, the recircle to heat the panels of the GV section is used and a blow down line for swelling (swelling of the water volume due to the steam production) and water chemistry control is used.

The blow down line is also called flash tank 108 line in order to remove the water during the volume increase due to the temperature increase and to have a drainage to maintain good water purity characteristics by removing concentrated salts deposited in the tubes and connecting pipes during the period of the steam generator downtime.

During the start-up phase, see Fig. 11, the recircle starts when on the GV section (31 of Fig. 3, panels 101 and 102) there is not yet heating (Q=O) . The heating phase can take place by means of mirrors reflecting the solar radiation or by auxiliary power sources, when the solar radiation is still of low intensity. The recircle continues when the GV section starts to be heated with increasing powers (Q>0, as represented in Fig.11) .

With said recircle, panels 101 and 102 of the GV section are heated and suitable rates are maintained in the exchange tubes of panels 101 and 102 by controlling the wall temperature of the tubes. This is achieved by using in various points of the circuit temperature measurements of the re- circled flow to decrease the power when the temperature is too high or to increase the power of the heating source when the temperature is too low. In this way the material of the tubes is not subjected to high thermal stress which would compromise the characteristics thereof. The feeding water line 110 in Fig. 11 is substantially absent, except for a refilling flow rate for the control of the water chemistry, that is inlet of water with a low amount of salts, in substitution of water with very concentrated salts (as described above) .

When the water outletting the GV section 102 reaches the limit value for the project temperatures of the tubes, with the completely open lamination valves 106, also section 103 of SH which is not yet heated (Q=O), see Fig. 12, starts to be fluxed. In this way also the panels of the superheater 103 start to be heated. The water outletting the SH section 103 is conveyed to the flash tank 108 as the superheated steam as required by the turbine or by other processes does not yet come out from the SH section 103. In this embodiment of Fig. 12 the feeding water 110 is conveyed to panel 101 of the GV section. The water amount introduced in 110 corresponds to the steam sent into the SH section 103 save the drainage 108 and optional other losses.

When the panels of the SH section 103 are sufficiently hot and fluxed by the water outletting the panel 102 of the GV section, the panels of the SH section 103 (Q>0) start to be heated with the solar radiation reflected by the mirrors. By sufficiently hot and fluxed panels it is meant that the panel tubes do not suffer thermal stresses when irradiated by the solar radiation and the so produced steam is conveyed to the turbine or to other processes 109. See Fig. 13. The feeding water 110 is introduced into panel 101 of the GV section to balance the product sent to the panel 103 of the SH section.

At this point, when the panel 103 of the SH section has reached the desired temperature by irradiation of the solar mirrors, the recircle 104 is stopped and the whole flow-rate outletting the GV section 102 enters into the SH section 103 to be sent then to the turbine or to processes 109. Of course, for the mass balance the feeding water 110 is conveyed to panel 101 of the GV section, in an amount corresponding to the steam produced at the outlet of section 102 and conveyed to panel 103 of the SH section. The valves and the lines leading to the flash tank 108 are normally no more used. At this point the system is under steady conditions. See Fig. 14.

In the sliding pressure operating way, during load variations, the pressure of the SH section 32 of Fig. 3 will follow the inlet pressure into turbine or into other processes variation, while the GV section 31 will be maintained under supercritical conditions by lamination of valve 106 (Fig. 14) .

As said above, the superheated steam at the outlet of the SH section is generally conveyed into the turbine to obtain electrical power. Instead of being sent to the turbine, the superheated steam could also be sent to any industrial process requiring the use of thermal power.