HEAT EXCHANGER FOR STEAM GENERATION FOR A SOLAR THERMAL POWER PLANT

The invention relates to a heat exchanger for generating a steam flow for a solar thermal power plant.

Factors such as, for example, an increased economic and political awareness for the environment and the increased cost and growing scarcity of fossil fuels have led to a rethinking in the area of power generation. New technologies have led to the increased utilization of regenerative wind and solar energy. In particular solar thermal installations with parabolic fluted collectors have meanwhile established themselves in large industrial applications so that installations have already been put into operation in the USA and Europe, and further large installations will be added in the near future.

In solar thermal power plants with parabolic fluted collectors, the sunlight is concentrated by means of parabolic reflectors on the absorber pipes so that the thermal oil found in the absorber pipes is heated to a temperature of approximately 400° C. Thermal energy is drawn from the thermal oil with the help of heat exchangers and transferred to water for the purpose of evaporation so that the steam generated thereby drives a turbine for power generation in a connected steam power plant in the conventional fashion. Heat exchangers with U-shaped pipe bundles in which the separation of the vaporous water from the fluid phase occurs in a casing region above the pipe bundle, which is created from a constructional viewpoint by an expansion of the diameter of the casing, are conventionally used for steam generation.

It has been shown that a separation of the steam in the same casing by means of an expansion of the diameter of the casing is disadvantageous in solar thermal power plants and their characteristic cyclic operating mode. The expanded casing diameter requires an enlargement in the casing wall thicknesses, which has a disadvantageous effect on the thermoelasticity of the heat exchangers, which means that the maximally permissible temperature gradients during the start-up and the alternating load operation of the power plant are reduced. Accordingly, the availability of the power plant decreases while the risk of material fatigue increases.

The invention is therefore based on the object of providing a heat exchanger for generating steam for a solar thermal power plant which reduces or overcomes the aforementioned disadvantages in the state of the art.

This object is achieved by the subject matter of independent claim 1. The dependent claims are directed to advantageous embodiments of the invention.

The heat exchanger in accordance with the invention for generating a steam flow for a solar thermal power plant comprises a casing for receiving a casing-side fluid and pipes extending inside the casing for a pipe-side fluid. The heat is transmitted via the pipes from the pipe-side fluid to the casing-side fluid, wherein the pipe-side fluid is a thermal oil or salt and the casing-side fluid is water.

The diameter of the casing can be reduced considerably with the help of the heat exchanger in accordance with the present invention. The use of headers instead of sectional pipe elements reduces the mechanically required wall thicknesses even further. As a result, the maximally permissible temperature gradients during the start-up and alternating load operations can be increased considerably, which leads to a greater thermoelasticity and availability of the power plant. The increased thermal elasticity further increases operational reliability, as the risk of material fatigue and thermal cracks is reduced considerably.

The heat exchanger preferably comprises a fluid inlet conduit which is connected to an entrance opening for the casing-side fluid and encloses at least a part of the pipes in such a manner that the fluid inlet conduit is adapted as a preheater and/or flow director for the casing-side fluid entering the casing. In accordance with this embodiment of the invention, the cold water entering the heat exchanger casing first passes through this fluid inlet conduit before it mixes with the already heated water or water-steam mixture in the heat exchanger. This way, an integrated preheater section is formed, which proves to be advantageous from a thermodynamic and fluidic point of view. Moreover, the fluid inlet conduit serves as a flow director.

In a further embodiment of the invention, the fluid inlet conduit encloses approximately ⅛ of the surfaces of the pipes. The fluid inlet conduit is preferably constructed in the shape of a box and encloses a part of the heat-emitting pipe surfaces. The fluid inlet conduit can also be configured in the shape of a cylinder. The ratio of the pipe surface enclosed by the fluid inlet conduit to the entire pipe surface in the heat exchanger is ⅛. This value can be adjusted in accordance with the respective application.

The heat exchanger further preferably comprises a fluid outlet conduit which is arranged in the region of an outlet opening for the casing-side fluid in such a manner that the fluid outlet conduit is adapted as a flow director and/or water separator for the casing-side fluid exiting the casing. This ensures a directed flow of the steam exiting the heat exchanger. Furthermore, the fluid outlet conduit can comprise elements which are used for better water or droplet separation.

Preferably, the pipes in the heat exchanger casing are configured as a U-shaped pipe bundle. This way, a large surface area for heat transmission or steam generation and the longest possible dwelling time of the heat-emitting thermal oil in the heat exchanger are provided in a compact manner. The pipes can also extend in a meandering fashion. The dimension and arrangement of the pipe bundle can be correspondingly designed in an optimal fashion that is adapted to the respective application.

In a preferred embodiment, the heat exchanger in accordance with the invention comprises a steam drum which is arranged above the heat exchanger and which is coupled to the heat exchanger by riser pipes and downpipes. The steam generated in the heat exchanger reaches the steam drum via riser pipes, from where it is removed for further use or superheating. The condensate can be carried off from the steam drum via downpipes and guided back to the heat exchanger. The arrangement of the steam drum above the heat exchanger allows a natural circulation. Depending on the application, it is also possible to provide a forced circulation by means of a pump.

Preferably, the steam drum comprises a fresh water inlet. This way, a separate inlet opening for the casing-side fluid (water) on the heat-exchanger side can be dispensed with. The water to be heated reaches the steam drum in accordance with this embodiment via the fresh water inlet and further via the downpipes to the heat exchanger. The invention is explained below in greater detail by reference to the schematic drawings, wherein:

FIG. 1 shows a side view of a first embodiment of the invention;

FIG. 2 shows a front view of the first embodiment of FIG. 1;

FIG. 3 shows a sectional view along the line A-A of FIG. 1;

FIG. 4 shows a side view of a second embodiment of the invention;

FIG. 5 shows a front view of the second embodiment of FIG. 4;

FIG. 6 shows a sectional view along the line B-B of FIG. 4;

FIG. 7 shows a side view of a third embodiment of the invention, and

FIG. 8 shows a front view of the third embodiment of FIG. 7.

FIGS. 1 to 3 show a first embodiment of the heat exchanger 1 in accordance with the invention. The heat exchanger 1, which is positioned here horizontally, comprises a casing 10 for receiving a casing-side fluid (water) and is erected on a support structure 11. Pipes 20 are arranged within the casing 10, the axes of symmetry of which are shown by means of broken lines. This is a pipe bundle with pipes 20 bent in a meandering manner. The hot, heat-emitting fluid, thermal oil, enters the heat exchanger 1 at a temperature of approximately 400° C. and a pressure of approximately 20 bar via the oil inlet nozzle 21 and is directed by means of a distributor 23 into the individual pipes 20 of the pipe bundle. After having flowed through the pipes 18, the thermal oil leaves the heat exchanger 1 at a temperature of approximately 300° C. and a pressure of approximately 16 bar via a header 24 and via an oil outlet nozzle 22 and is re-fed to the absorber pipes of the parabolic fluted collectors (not shown).

The water to be heated enters with a temperature of approximately 300° C. and a pressure of approximately 110 bar through the water inlet nozzle 12 or into the heat exchanger 1. The cold water first flows into a fluid inlet conduit 14 via an inlet opening 13. The fluid inlet conduit 14 is designed here in the shape of an angular box and comprises a rectangular opening 14′ so that the water is necessarily directed upon entrance in the direction of the arrow 15 and only comes into contact with already heated water or water-steam mixture after passing through the opening 14′. The fluid inlet conduit 14 thus serves to direct the flow of the cold water and to preheat the same. The fluid inlet conduit 14 encloses a part of the pipes 20 directing the heat-emitting thermal oil so that forced convection occurs within the conduit 14. It has proven that the ratio of the surface area of the pipes 20 enclosed by the fluid inlet conduit 14 to the total surface area of the pipes 20 in the heat exchanger 1 is ideally approximately ⅛.

By means of the transmission of the heat from the thermal oil to the water, steam is formed in the heat exchanger 1 so that there is a mixture of water and steam there, the steam rising in the direction of the steam drum 30 on account of the difference in density and the water being found predominantly in the floor region of the heat exchanger 1. The steam makes its way into the riser pipes 31 via the openings 32 which are preferably in the vertically upper region of the heat exchanger 1, and further into the steam drum 30. The steam is removed from there via the connection 35 and used further. A further heat exchanger (not shown) for superheating the steam is preferably connected. The condensate in the steam drum 30 is re-fed to the heat exchanger 1 via the downpipes 33 and the openings 34. The steam drawn from the steam drum 30 has on average a temperature of approximately 380° C. and a pressure of approximately 108 bar.

FIGS. 4 to 6 show a second embodiment of the invention. The essential difference from the first embodiment illustrated above is that the heat exchanger 1 does not comprise a separate water inlet nozzle. Instead, the heat exchanger 1 is supplied with fresh water via the downpipes 33 and the openings 34. For this purpose, the steam drum 30 comprises a fresh water inlet 36. The production costs can thereby be reduced because a separate water connection is no longer required. It is also possible to dispense with a fluid inlet conduit 14 because the preheating of cold water has already taken place in a separate preheater.

FIGS. 7 and 8 show a third embodiment of the invention. This embodiment is similar to the first embodiment (FIGS. 1 to 3) in principle. The essential difference is that the pipes 20′ are configured as a U-shaped pipe bundle. As a result, the thermal oil enters the pipes 20′ via the lateral oil inlet nozzles 21 in the direction of arrow 25 via the sectional pipe element 27, gives off heat to the water and leaves the heat exchanger 1 in the direction of arrow 26 via the oil outlet nozzle 22. The water to be evaporated enters the heat exchanger casing 10 via the water inlet nozzle 12 and flows through the fluid inlet conduit 14, wherein the position of the water inlet nozzle 12 and thus also of the fluid inlet conduit 14 is changed in comparison with the first embodiment. Preferably, the fluid inlet conduit 14 is positioned in the region of the outlet of the thermal oil.

Temperatures and pressures of the fluid in the heat exchanger can vary depending on the location or size of the power plant.