WO2008012390A1

WO2008012390A1 - Solar-powered boiler

Info

Publication number: WO2008012390A1
Application number: PCT/ES2007/000462
Authority: WO
Inventors: José María MARTÍNEZ-VAL PEÑALOSA; Alberto ABÁNADES VELASCO
Original assignee: Universidad Politécnica de Madrid
Priority date: 2006-07-28
Filing date: 2007-07-26
Publication date: 2008-01-31
Also published as: ES2272194A1

Abstract

The invention relates to a solar-powered boiler. According to the invention, collectors that absorb concentrated reflected sunlight are positioned along the height and width of a central tower in a field of heliostats, such that said collectors occupy most of the active facade of the tower. The invention is intended for the efficient boiling of the thermodynamic fluid flowing through the panels, with smaller exergetic losses, thereby producing superheated steam at the outlet of the tower. The assembly of tubes includes the recirculation of the liquid phase of the fluid.

Description

Boiler of solar energy. Technical sector

The invention is framed in the field of solar energy utilization in order to heat materials, typically fluids, to very high temperature, possibly with phase change, particularly boiling.

Within this sector, the invention falls within the scope of installations based on concentration of sunlight through optical reflection systems, to focus it on a more or less wide area in which the desired heating occurs.

In turn, within this type of installation, the invention is located in systems with one, or several, central towers, in which the heating zones are located, with concentrated sunlight proceeding from the reflection of several conveniently orientable mirrors , which are deployed in what is called the heliostat field.

Finally, within this type of installation, the invention focuses on the geometric and physical arrangement of the absorbing collectors of concentrated sunlight, and therefore also affects, or conditions, the heliostat orientation system along of the diurnal and seasonal evolution of the sun. State of the art

For more than a century (see US historical patent No. 608755, granted to HFCottle in 1898 on an "Apparatus for storing and using solar heat") various configurations of a central receiver have been proposed in which to concentrate an important irradiation of sunlight in order to obtain very high temperatures.

An especially significant patent in this field was granted in 1975 in the United States to D.E. Anderson (US Patent 3,924,604) in which a central tower housed in its upper part a receiving volume (very much in Cottle's original idea) in which the rays reflected by a set of heliostats with an adequate inclination with respect to the position of the sun at every moment. Although in that patent the tower occupied a central position to the field, an obvious generalization of the idea could shift the location of the tower towards the southern end of the heliostat field (in the northern hemisphere) in order to obtain angles of incidence and reflection more easily manageable by solar tracking systems.

In fact, a simplification of the solar tracking system and the mechanical orientation of heliostats can be found in US Patent 4,136,674, granted to ALKorr in 1977. In this patent, the tower occupied a central position to the heliostat field, arranged circularly around the tower, with fixed inclination, so that only an azimuthal turn (in the horizontal plane) was performed in function of the sun's hourly evolution. Since the heliostats have a fixed inclination, the incidence, on the tower, of the reflected rays, would not always occur at the same height; but the rays would affect the high part of the tower when the sun was low, and on the low part when the sun was high. For this reason, the entire tower was covered with absorbed tubes of concentrated solar radiation, in such a way that at any moment there was a part of the tower that acted as an active collecting element, and the rest would be useless for that purpose. Obviously, a major drawback of that invention was that the heating surface was much smaller than the total cooling surface (thermal losses) by convection and radiation from the entire tower lining.

The control of the inclination of the heliostats to obtain high concentration values of sunlight can be done by various methods, such as that of the patent PCTYIL96 / 00018, of A. Yogrev and V. Krupkin, in which it seeks to maximize energy concentrated in a small receiver volume, which acts as a true solar oven.

This solar oven idea is the common and dominant in all concentration tower configurations, such as those existing in the Almería Solar Platform (CIEMAT, Spain) in the National Solar Thermal Test Facility of the Sandia National Laboratory of Albuquerque (New Mexico, USA), in the Barstow Center (California) belonging to the US DOE, as well as the prototype thermoelectric power plant under construction in Seville (SOLUCAR, Abengoa). The adoption of a small surface focusing solar reactor focused on the reflection of the heliostat field is understandable from the purpose of having a high temperature solar furnace, which serves as a heat source of a thermodynamic cycle. However, it does not seem the most appropriate solution for optimal use of exergy available in concentrated sunlight, when it is desired to be used to produce electricity by means of a thermodynamic cycle with boiling and condensation, which is the most usual route.

Explanation of the invention.

The invention is based on using the entire height of a reception tower of sunlight reflected with concentration, so that the illuminated wall of the tower, on which the appropriate collectors are arranged, acts in a manner similar to how a Conventional vertical tube boiler, although in the conventional case the smoke-tube convection has a very important role, and in the case of this invention the thermal load is produced by the incident radiation on the active side of the tower.

The tower therefore has an elementary structural mission that is to give mechanical consistency to the structure, and to support the active elements thereof, which are described in more detail in subsequent drawings. The fundamental active elements of this invention are the sensors located at the width and height of the tower, within which the fluid to be heated circulates to boiling conditions. On top of the tower, but not in at its apex, the separation boiler of the liquid-vapor phase is located, from which the recirculation of the liquid phase falls, and from which the separated steam emerges above.

Apart from the fact that the boiler can incorporate steam separators to avoid the drag of liquid droplets, the specific aspect of this invention is that a solar superheat receiver is available in the upper part of the tower, until it is finished, so that a totally dry and superheated steam is reached, with the advantage that this reports from the point of view of the thermodynamic cycle in which the energy of said steam has to be exploited.

Although in the subsequent figures the tower is presented with a flat active facade, it could be circular or with a certain curvature, to accommodate it as efficiently as possible to the reception of the reflected sunlight.

A fundamental issue in this invention is that the reflection heliostats of sunlight are not oriented exclusively towards the upper area of the tower where the solar collector (or solar oven) is located in conventional installations, but that the heliostats focus their reflected light towards different heights of the tower, being able to adjust the monitoring control of each heliostat, or group of heliostats, to make the thermal radiation with the necessary intensity at each height influence the tower collector.

Given the cross-sectional and height dimensions that the tower can have, there is enough surface to be able to concentrate the sunlight from a given heliostat field, and achieve the best exergy conversion conditions of the initial solar radiation into superheated steam of the working fluid.

It must be taken into account that heating must occur gradually, and depending on the film coefficient of the working fluid inside the tower collectors. This will depend on the thermodynamic conditions that the fluid has at each height.

The optimum performance will be achieved by adapting the radiation intensity to the cooling conditions, to effect the transmission of energy in optimal conditions, with a minimum temperature jump. This also avoids mechanical stress problems due to strong thermal gradients.

The recirculation of the liquid phase that occurs from the boiler to the foot of the tower through the non-active part of the tower, or inside, allows the flow of fluid that rises through the active part to reach these conditions suitable for the boiling heat transmission, combining good cooling of the solar collector with the generation of the steam flow required for the power sought in the installation.

Said recirculation can be modified by means of a recirculation pump at the foot of the recirculation column, which will appropriately modify the contribution of the main fluid that comes from the thermodynamic cycle of expansion and condensation of the generated steam. Explanation of the drawings

In drawing 1, the tower, 1, which gives mechanical consistency to the assembly is shown, in which the absorbent collectors, 2, of the active part of the tower stand out. These absorbent collectors cover practically the entire facade on which the rays reflected from the heliostat field, 3.

The fluid is driven from the supply line by the pump, 4, which drives it towards the active part of the tower with its collectors, 2. Said drive is complemented with that provided with the recirculation pump, 5, located at the foot of the recirculation drop column or columns 6.

As the fluid rises inside the solar light absorption collectors, 7, it changes phase, increasing its steam title, until the arrival at the boiler, 8, where both phases coexist, which is a function of the working pressure in the boiler, which has to be in equilibrium with the admission of superheated steam, 9, in the thermodynamic cycle of energy exploitation, and also has to be in balance with the impulse overpressure given by the pumps 4 and 5, plus the loss of manometric load on the ascent.

The control of the conditions in the boiler allows monitoring of power in the demand of the thermodynamic cycle, which however is not the object of the invention, which is limited to providing the steam to be exploited thermodynamically. δ

To improve the conditions of said steam produced, in the upper part of the boiler there is a set of steam separators, 10, which return the drops of liquid down, allowing the steam to continue its ascent with practically unit title, and from from there it is superheated in the upper collector of the tower, 11, which will have its active part finishing off it, and receiving the appropriate thermal radiation to its conditions of heat absorption by dry steam.

To avoid heat losses in the recirculation of the fluid and in the steam produced, the columns and pipes of said systems should be conveniently insulated or heat-insulated, as shown in the insulating layer, 12.

The tower may have its proper structural foundation, 13, just like heliostats, but this does not specifically pertain to the invention, which focuses on the arrangement of solar panels across the active wall of the tower.

Said panels may have a different type of configuration, particularly with regard to the channeling of the ascending fluid, which can go through independent pipes, or in a wide conduit that covers the entire cross section available for the passage of the fluid.

In drawing 2, a cross-section of a possible arrangement of the collector of the active part of the tower is shown, in which it would correspond to the structural part, 1, on which the collector assembly, which decomposes into the following elements: an absorbent plate, 2, of low emissivity for the predictable operating temperatures of the installation, and of very high absorbency for the typical wavelengths of solar radiation. In order to avoid convection cooling of the surrounding air, the absorbent panel would be within a vacuum area limited by a transparent window, 14. The glass of said window would be chosen in its characteristics such that it would be of high transparency for the photons. typical of sunlight, and yet, backscatter the photons characteristic of the radiation emission of the panel, 2, with temperatures characteristic of the application in question (typically hundreds of K or even 1,000 K). This greenhouse effect created within the absorption cell will minimize thermal losses from the active face of the panel.

Similarly, to reduce losses by conduction to the tower, an insulator, 15, will be available that will minimize the passage of heat from the fluid channel, 7, to the tower and its components.

A matter of particular relevance in the active part of the panel, 2, is the accommodation of the expansions that may suffer between the mounting temperature or ambient temperature and the operating temperatures, including the possible overpower conditions that are anticipated. For this, the collector of said active part will have to settle on the lateral surface of the tower or on the insulator, 15, so that dilations can be allowed, and for this the flat part must be coupled to a semi-circumferential strap, 16 , with turning point to be welded both to the collector at one end and to the surface of the tower or to the insulator, at the other end.

There could be other similar configurations, but based on channeling the fluid through tubes, as shown in drawing 3. In this case, the active manifold, 2, is composed of a set of tubes joined together by an absorbent plate, of features similar to those of the tubes, all embedded in the interior of the absorbent cell, confined between the transparent window, 14, and the rear insulating wall, 15. In this configuration it would be necessary to have a lyre to accommodate the expansion, 17, to allow transverse dilations of the tube assembly. Logically, these would have to have their height expansion accommodation, before entering the boiler.

Embodiment of the invention

The invention can be materialized on various types of tower, and for a wide variety of heliostat fields, since the requirements imposed on their control for solar tracking would be absolutely ordinary. The specific aspect of the invention would be the arrangement of the panels across the width and height of the concentration tower, in such a way that during the ascent of the working fluid through the interior of the solar collectors the partial boiling of the same took place. For this, conventional materials such as copper, aluminum or steel can be used, although the decreasing thermal conductivity in the indicated materials must be taken into account. In this sense it is essential to have a coating specific, which can also be ordinary in the current solar industry, with high absorbency for sunlight, and low emisivity for thermal radiation of the panels (corresponding to the operating temperature, several hundred K, and even 1,000 K) .

The assembly would be on the active wall of the tower, injecting the ascending fluid into the boiler, which would be mounted as a floating head on the ascending duct of the working fluid.

The invention could therefore be implemented in any of the existing towers with heliostat fields, however, requiring their refocusing to aim with the rays reflected at the appropriate height of the tower in each case, adapting the power density incident in each sector of the collecting tower, in order to maximize the exergy performance in steam production and in its reheating, for which an adequate sectorial direction of the heliostat field would have to be carried out.

Claims

1. Solar energy boiler, characterized by vertically arranging the solar light absorption collectors (2) across the width and height of the wall, flat or curved, of a tower (1) of sufficient height to enable the boiling of a working fluid that ascends through the interior of said collectors, to a boiler (8) in which the separation of the phases of the fluid is verified, descending by a suitable channeling (6) the liquid phase for its recirculation, and ascending the phase of steam for further overheating in the collectors located in the highest part of the tower (11), from which they will emerge as suitable steam for the feeding of a thermodynamic cycle.

2. Solar energy boiler according to claim 1, characterized in that the incident light on the collectors located in the tower comes from a field of heliostats with solar tracking, to reflect direct sunlight on the active wall of the tower with its collectors, in such a way that the surface density of power incident on them at various heights causes the consequent increase in enthalpy and the corresponding boiling in the first stage of passage of the fluid through the vertical collectors (2), until reaching the boiler (8) with a certain vapor title, which will be a function of the solar power received and the conditions of the fluid flow, in flow and inlet temperature.

3. Solar energy boiler, according to claim 1, characterized in that phase separation occurs in the boiler (8), the liquid phase being recirculated thanks to the pump (5) acting in the recirculation pipe (6), thus having a degree of freedom to vary the flow of ascending fluid in the first stage of manifolds (2), thus being able to have an optimal title in order to favor boiling, regardless of the steam flow rate produce, which is restored to the circuit through the condensate pump (4) from the thermodynamic cycle that is fed by the dry steam outlet (12).