WO2014207269A1 - Solar receiver with gaseous heat-transfer fluid - Google Patents

Abstract

The invention relates to a solar receiver with gaseous heat-transfer fluid, of the type that is arranged in a cavity of a solar tower. The receiver is formed from at least two connected panels, where each panel comprises at least two passages, a passage being an assembly of tubes wherein the gas circulates in the same direction, and where, inside the first panel, the number of tubes of each passage is reduced by between 5 and 10% compared to the previous passage, and where the following panel, connected in series with the first panel, starts with a first passage which contains the same number of tubes as the last passage of the first panel but where the diameter of the tubes is between 5 and 10% less compared to the tubes of the last passage of the first panel. In the subsequent passages, the diameter of the tube is maintained constant along the panel, and the number of tubes of each passage is reduced by between 5 and 10% compared to the previous passage in said panel.

Description

 SOLAR RECEIVER WITH GASEOUS COLLECTION FLUID

Technical sector of the invention

 The present invention is part of the solar energy sector, specifically referring to a solar cavity receiver, with heat transfer gas at high temperatures.

 Background of the invention

 Although solar radiation is a thermal source of high temperature and high energy in origin, its use in the conditions of the flow that reaches the earth's surface destroys practically all its potential to become work, due to the drastic reduction of temperature available in the fluid. For this reason, use is made in thermoelectric solar power plants of optical concentration systems, which allow to achieve higher flow densities and thereby higher temperatures.

Currently there are mainly three different technologies developed for use in solar plants called: central receiver, parabolic trough collectors and Stirling discs. All of them make use only of the direct component of solar radiation, which forces them to have solar tracking devices:

1. Central receiver (3D) systems use large surface mirrors (for example 40-125 m ² per unit or even higher surfaces) called heliostats, which are equipped with a control system to reflect direct solar radiation on a central receiver located at the top of a tower. In this technology, concentrated solar radiation heats a fluid in the receiver at temperatures of up to 1000 ° C, whose thermal energy can then be used for electricity generation. The solar tower concentration receivers can be placed in a cavity located in the upper part of the tower structure, whose cavity usually includes insulation that helps reduce thermal losses. The configuration must allow the incident power to exceed in magnitude the losses that occur due to radiation and convection.

2. In parabolic trough collectors (2D), direct solar radiation is reflected by parabolic trough mirrors that concentrate it in a receiver or absorber tube through which a fluid that heats as a result of solar radiation circulates concentrated that affects it at maximum temperatures of 400 ° C. In this way, solar radiation is converted into thermal energy that is subsequently used to generate electricity using a Rankine cycle of steam water.

A variation of this technology is the linear fresnel concentration systems, in which the parabolic mirror is replaced by a fresnel discretization with mirrors of smaller dimensions that can be already flat or have a slight curvature in its axial axis, and that by means of the Control of its axial orientation allows concentrating solar radiation on the absorber tube, which in this type of applications usually remains fixed.

3. Stirling parabolic discs (3D) systems use a surface of mirrors mounted on a parable of revolution that reflect and concentrate the sun's rays in a specific spot, where the receiver in which the working fluid is heated is located. a Stirling engine that, in turn, drives a small electric generator. The technology of central receiver using a gaseous heat transfer fluid, such as C0 ₂ , has not been used so far in any commercial or demonstration plant in the world, although there are several patents in this regard that we will set forth below. C0 ₂ has been used in solar applications in parabolic trough concentrators, but in these the concentration ratios, efficiencies, stresses in tubes and materials are radically different from those of a central receiver in a tower.

EP1930587 describes a tower concentration system with solar salt receiver, which includes a supercritical C0 ₂ turbine operating at 550 ° C.

The use of a subcritical (low pressure) or supercritical (high pressure) C0 ₂ cycle with the fluid at very high temperatures may allow the implementation of more efficient thermodynamic cycles in plants (both in turbines and in systems of storage). While in highly optimized steam cycles, the thermodynamic cycle efficiencies barely exceed 45%, efficiencies above 55% are estimated for thermodynamic cycles with air or C0 ₂ at very high temperatures.

The difficulty of solar technology for the production of C0 ₂ at a very high temperature lies in the demanding thermomechanical conditions at which the receiver is operated. The walls of the receiver tubes are continuously subjected to thermal cycles between the ambient temperature, the C0 ₂ temperature at the entrance of this receiver (typically 200 to 300 ° C), and the wall temperature necessary for heating the C0 ₂ to, for example, 800 ° C (in that case it would reach more than 1000 ° C wall temperature).

 The difficulties encountered in demonstration receivers that have operated with other gases at high temperature are:

· Lack of controllability of the system, especially in transitory conditions, passing clouds, etc., mainly due to the poor thermal properties of gases that act as heat transfer fluid.

 In this type of receptors the most frequent structural failure is the appearance of cracks.

The thermal stress due to the large temperature differences causes cracks in the junction between the pipes and the manifolds that are used to configure the different fluid passages through the panels.

 Problems if working at high pressures (for example, in the case of cycles of

C0 ₂ supercritical), which requires greater tube wall thicknesses, which when transferring high power densities to the heat transfer fluid, necessarily implies high thermal gradients.

 • Problems due to the low efficiencies achieved in this type of receivers due to the high metal temperatures reached. Of the energy that reaches these receptors only slightly more than half is absorbed by the fluid.

· Problems of distribution of radiative flux density peaks to homogenize the sunspot on the receiver. The point of the heliostats to the receiver must be much more careful than in the receivers with steam or liquid due to the low densities of the gases that act as heat transfer fluid. Low densities have a worse cooling and higher metal temperatures.

The invention that follows, therefore, seeks to take advantage of the use of a high temperature gaseous fluid in solar tower receivers, through a receiver configuration that solves existing risks in conventional receivers, achieving greater control of the plant and thus favoring the stability and durability of this and its components. Furthermore, said configuration allows to minimize the loss of pressure or pressure of the gaseous fluid and increase the efficiency in the absorption of solar energy. Description of the invention

The present invention relates to a solar receiver, of which they are located in a cavity of a solar tower, through which a gas circulates at low pressures (subcritical) or high pressures (supercritical), said gas being the heat transfer fluid. Preferably, in the present invention, C0 ₂ is used as a heat transfer fluid, although use with other gases such as air and helium is not ruled out.

The receiver is composed of at least two panels connected to each other: at least one first panel through which the heat transfer fluid enters and at least another panel through which the heat transfer fluid exits. In the same receiver there can be several panels through which the cold fluid enters and several panels through which the hot fluid exits.

The panels in turn are formed by tubes through which the heat transfer fluid circulates. Each panel comprises at least two steps. By step means a set of tubes in which the circulation of the fluid occurs in the same direction. Each step will have a certain number of tubes and with a certain diameter depending on the panel.

 The first panels (those through which the heat transfer fluid enters) have at least two steps. In the second step the number of tubes will be reduced between 5 and 10% with respect to the number of tubes of the immediately previous step, following the same reduction criteria in the case of having more than two steps per panel and until Finish the number of steps of this one.

 In the next panel, second panel adjacent to the first, the first step will start with the same number of tubes as the last step of the first panel, but reducing the diameter of the tubes between 5 and 10% with respect to the diameter of the tubes from the previous panel. As the steps progressed, the constant tube diameter is maintained along the panel, and the number of tubes of the second and subsequent passages is decreased according to the above criteria, that is, between 5 and 10% with respect to the number of tubes from the previous step.

As already mentioned, one of the main problems of using a gaseous heat transfer fluid such as CO ₂ in the receiver is the controllability of the receiver during transients as well as the high temperatures of the metal from which the receiver tubes are made.

In order not to exceed the limit temperatures of the materials in the receiver, a compromise must be reached between adequate cooling of the receiver (fluid acceleration), and the minimum loss of load or possible pressure loss. This last factor is also critical in the case of using a fluid Gas-type heater such as C0 ₂ , since the cycle requires, before the turbine entry, a compressor whose electrical consumption is high, which is why the lower the loss of load in the receiver, the lower It will be the work done by the compressor and minor self-consumption.

By means of the described configuration (reduction of the number of pipes between passages and decrease of the diameter of the pipes between panels) it is possible to keep the cooling of the receiver tubes constant since if the area of passage of the gaseous fluid is reduced (since it decreases the number of tubes) the speed of the tube is increased and, therefore, its cooling capacity is increased, thereby compensating for the effect of loss of cooling capacity that occurs as it is getting hotter and hotter. This avoids very high temperatures in some areas of the receiver that could damage the material. In the same way it happens when the diameter of the tubes decreases since, when maintaining the flow and decreasing the area (flow = area x speed), it is the speed of the fluid that increases, thus increasing the cooling.

 In order to maximize the efficiency of the receiver, two contiguous panels are arranged forming a certain angle between them in order to achieve a map of radiative flux density as homogeneous as possible and reduce thermal losses. In cases where there are more than three panels, the central panels are located away from the opening of the cavity of the solar tower and so that they are not facing parallel to the opening and the panels located at the ends are they are forming 90 ° with the opening of the cavity, in this way, the efficiency of the receiver is maximized in terms of capturing the energy from the solar field.

With this arrangement it is achieved that the energy that arrives from the solar field mostly affects the central panels of the receiver and that the part that does not, is reflected by the insulation and finally falls on the panels, because due to the arrangement of these and the use of insulation, the opening of the cavity supposes a very small area compared to them and almost all the reflected energy ends up affecting the panels through which the fluid circulates. In this way, the temperatures of the metal tubes of the panels are more equalized, increasing the efficiency of the receiver, since the panels with a higher metal temperature emit radiation around 20% of the energy they receive towards the panels. Colds that absorb it. In addition to the colder panels, as well as the insulation panels, which receive energy from the solar field that does not It has reached directly the hottest central panels, they reflect part of that energy towards the hotter panels with the effect of homogenization of flow density in the receiver and taking advantage of almost all the energy that reaches the cavity since nothing is Escape through the opening.

The plane that contains the opening of the cavity in the solar tower forms an oblique angle with respect to the vertical in order to reduce the effective area of said opening, thus reducing also the thermal losses caused by the reflected and escaping energy of the cavity, and the closest heliostats can continue to focus on the top of the receiver. If this opening were in a completely vertical plane (perpendicular to the ground), it should be larger with respect to the oblique case, so that with the same aim of the heliostats closest to the tower, the same energy will be passed. The cavity is internally coated with an insulating material that reflects the radiation. The fact of minimizing the opening thanks to an optimized geometry in the form of an ellipse (the same shape as the reflection of the energy that comes from the heliostats) improves the controllability of the receiver during the transients, since the inertia of the system is increased due to because the temperature drop in the receiver is much slower due to the fact that very little energy is emitted outwards (since the opening is in the form of an ellipse, the majority of the radiation emitted by the panels reflects the insulation of the internal face of the cavity and is again reabsorbed by the panels).

 In this type of receivers it is essential that the fluid moves at a fast speed only in the areas where it is required (areas of possible high temperature of metal, that is, where the fluid circulates at a higher temperature) because the electrical self-consumption is so high at compress it that it is necessary to have a loss of load as low as possible, which is achieved by causing the fluid to accelerate only when necessary. Hence, in the receiver configuration described above, it is considered appropriate for the fluid to travel between 12 and 17 m / s in the first or first panels (lower metal and fluid temperature zone), and travel between 20 and 25 m / s in areas with higher metal temperatures. This ensures proper cooling of the tubes in all areas while maintaining metal temperatures within the strength limits of the material.

The novelty within the panels themselves lies in the appropriate combination of the number of tubes and their diameter in each panel, as well as in the combination of different materials and / or coatings in the same receiver. Although a possible configuration for this type of receiver is to build them with the same number of tubes and the same diameter of all the tubes in each step of each panel, this configuration would not be optimal.

 In order to optimize the transfer of heat to the fluid, reduce thermal losses and minimize the cost of the receiver, the panels may consist of different materials depending on the area of the receiver. That is, not all tubes that make up the receiver are of the same material.

 In this way the materials with the appropriate thermal conductivity are chosen depending on the maximum temperature of the receiver, so that they withstand the expected stresses in each zone, and thus maximize the thermal efficiency. For the first steps of the first panel, (panel where the fluid enters the receiver) the fluid travels at relatively low speeds, so you can choose steels with chromium contents below 20% to work with cheaper materials, steps with a maximum metal temperature of 700 ° C, and with material conductivities between 25 and 37 W / mK. In the following steps of the receiver, where higher metal temperatures are present (the maximum metal temperature is in these steps of 1100 ° C), more expensive materials with a nickel base and with conductivities between 15 and 25 W / m-K will be chosen.

 For the last steps of the last panel (panel through which the fluid exits) where the maximum metal temperature is 900 ° C, the materials can be made of nickel-chromium alloy with conductivities between 15 and 30 W / m K.

 On the other hand, not all panels have coatings or the same type of coating. The panels with lower metal temperature (between 300 and 700 ° C, being those through which the fluid circulates with a lower temperature), have a lower absorptivity, either because it has a coating of absorptivity between 0.3 and 0.7 or for being unpainted directly and the panels with a higher metal temperature (between 700 and 1100 ° C, for being those through which the fluid circulates with a higher temperature) have a coating of high absorptivity (greater than 0.95) put which are heated almost only with the energy that is reflected in the panels with lower metal temperature.

Brief description of the drawings

In order to help a better understanding of the features of the invention, a series of figures accompany this descriptive memory where, with a purely indicative and non-limiting nature, the following has been represented: Figure 1 shows a diagram of a four-panel solar receiver according to the present invention.

 Figure 2 shows a top view of the panels and the inner faces of the cavity that are close to them, of a six-panel solar receiver according to the present invention.

 Figure 3 shows a three-dimensional view of the cavity and panels of a four-panel receiver

 Figure 4 shows a profile of the cavity and receiver of Figure 3

In the figures, the numerical references correspond to the following parts and elements.

 1. - Cold fluid inlet to the first step of the first panel.

 2. - Hot fluid outlet from the last step of the last panel.

 3. - Steps that make up each panel.

 4. - Fluid inlet panel

5.- Fluid outlet panel

 6 - Ellipse shaped opening.

7. - Insulation inside the cavity

Description of a predetermined embodiment

 To complement the foregoing description and in order to help a better understanding of the features of the invention, a description of a preferred embodiment of the present invention will be made.

 Figure 1 represents a preferred embodiment of the receiver of the present invention. In this embodiment the receiver is formed by four contiguous panels, two of them located at the ends (4), each of which includes an inlet (1) of the heat transfer fluid and two centrals (5), each of which includes an outlet (2) of said heat transfer fluid.

 Each panel is formed, in turn, by four steps (3) or four sets of tubes through which the fluid circulates in a certain direction. This figure illustrates the direction of circulation of the fluid that enters through the arrows pointing upwards and exits, after going through several steps, through the arrows pointing downwards. The path that the fluid follows is described by the curved arrows.

It is estimated that the adequate mass flow for a subcritical C0 ₂ receiver with maximum peaks of radiative flux density between 200 and 250 kW / m2 varies between 130 and 160 Kg / m2 s and for a supercritical C0 ₂ receiver with maximum peaks of power between 300 and 350 kW / m2 varies between 2000 and 2500 Kg / m2 s what It implies configurations with internal tube diameters between 20mm and 50mm, which would mean having for each fluid passage and for every kg / s between 4 and 20 tubes. The reduction ratios of tube diameters and number of tubes, as well as the choice of the number of tubes and diameter of the same initials for the first panel will be conditioned by the objective of achieving speeds in the first step of the first panel (panel of fluid inlet, (4)) in volume at 12m / s and speeds in the last step of the last panel (fluid outlet panel, (5)) in volume of 25m / s. In a preferred embodiment, such as that shown in Figure 1, in the panels located at the ends (panels where the heat-carrying fluid enters, (4)) all the tubes would have an internal diameter of 20mm. The first step of the first panel would have 20 tubes, the next 19, the next 18 and the next 17. In the central panels (panels where the heat-carrying fluid comes out, (5)) all the tubes have an internal diameter of 19 mm. The first step of a central panel would have 17 tubes, the next 16, the next 15 and the last step would have only 14 tubes. In each step of each panel the reduction in the number of tubes is between 5 and 10%, and the percentage of area reduction (and speed increase) is greater as the fluid reaches the final steps of each panel.

 The length of the tubes that form the panels will be between 2 and 6 m (they should not be too long to avoid tensions due to displacements) and their external diameter will range between 20 and 60 mm, with thicknesses between 2 and 6mm

 The receiver tubes can be of different material or have different coating. In a preferred embodiment of the invention, one third of the total number of tubes of the first panel of the receiver (the first panel being the panel through which the fluid enters) will be austenitic steel, alloyed with up to 20% chromium, with metal temperatures maximums of up to 700 ° C and materials with conductivity between 25 and 37 W / mK. In the following steps and as we approach the central areas of the receiver where it is expected to have higher metal temperatures (maximum temperatures up to 1100 ° C), more expensive nickel-based materials with conductivities between 15 and 25 W / m K will be chosen .

For the last steps of the last panel (understanding the last step as the one through which the fluid comes out and the last panel, the one that contains the last step), the maximum metal temperature is 900 ° C so that the materials can be nickel -chrome of conductivities between 15 and 30 W / m K and Figure 2 shows a plan view of a configuration of the receiver with respect to the plane of the tower containing the opening of the cavity. Said receiver consists of six panels. Be check that the panels (4) located at the ends form an angle of 90 ° with respect to the plane that contains the cavity.

 Figures 3 and 4 represent perspective and profile views respectively of the receiver located in the cavity of a solar tower. In both cases, the receiver comprises four panels and the opening (6) of the cavity has an elliptical shape and is located in an oblique plane with respect to the vertical in order to reduce the effective area of said opening, thus reducing also the thermal losses caused by the reflected energy that escapes from the cavity. These figures also represent the isolation panels (7) of the receiver.

Claims

Claims  1.-Solar receiver with gaseous heat transfer fluid, of those located in a cavity of a solar tower, characterized by comprising at least two panels connected to each other: at least one first panel through which the heat transfer fluid enters and at least one last panel through which the heat transfer fluid exits and where each panel comprises at least two passages, one step being a set of tubes in which the gas circulation occurs in the same direction, and where:  - within the first panel the number of tubes of each step is between 5 and 10% lower than the number of tubes of the immediately previous step until said panel is finished, and  - within the second panel and later, the first step contains the same number of tubes as the last step of the previous panel but the diameter of the tubes is between 5 and 10% lower with respect to the last step of the previous panel, in the following steps the constant tube diameter is kept constant and equal to that of the tubes of the first step of the panel, while the number of tubes of each step is between 5 and 10% lower with respect to the immediately preceding step until this end is finished panel and where the tubes that make up the receiver are not all of the same material and / or do not all have the same coating. 2. Solar receiver according to claim 1 characterized in that for the first steps of the first panel, which reach maximum metal temperatures of 700 ° C, the materials used have thermal conductivities between 25 and 37 W / m K, the following steps of The following 1100 ° C maximum metal temperature panels use materials with thermal conductivities between 15 and 25 W / mK and in the last steps of the last panel the materials used have conductivities between 15 and 30 W / nrvK with maximum metal temperatures of up to 900 ° C  3. Solar receiver according to claim 2 characterized in that one third of the total number of tubes of the first panel of the receiver with maximum metal temperatures of 700 ° C is made of alloyed austenitic steel with up to 20% chromium, in the following steps of the following panels with a maximum metal temperature of 1100 ° C the tubes are nickel-based and in the last steps of the last panel with maximum metal temperatures of up to 900 ° C, the tubes are made of nickel-chrome materials. 4. Solar receiver according to claim 1 characterized in that the panels that reach maximum metal temperatures of 700 ° C have a coating of absorptivity between 0.3 and 0.7 or are unpainted and panels that reach metal temperatures between 700 ° C and 1100 ° C have an absorptive coating greater than 0.95.  5. Solar receiver according to claim 1 characterized in that in the case that the receiver has more than three panels connected in series, the panels located at the ends form 90 ° with the plane containing the opening of the cavity of the solar tower.  6. Solar receiver according to claim 5 characterized in that the central panels are not facing the opening of the cavity. 7. Solar receiver according to claim 1, characterized in that the gaseous heat transfer fluid is C0 ₂ or air or helium under subcritical (low pressure) or supercritical (high pressure) conditions.  8. Solar receiver according to claim 7 characterized in that the length of the tubes forming the panels will be between 2 and 6 m and the external diameter thereof will range between 20 and 60 mm, with thicknesses between 2 and 6 mm. 9. Solar receiver according to claim 7 characterized in that for a subcritical C0 ₂ receiver with maximum peaks of radiative flux density between 200 and 250 kW / m2 with mass flux that varies between 130 and 160 Kg / m2 or for a supercritical receiver of C0 ₂ with maximum power peaks between 300 and 350 kW / m2 with mass flow that varies between 2000 and 2500 Kg / m2-s the internal tube diameters vary between 20mm and 50mm and have for each fluid passage and for every kg / s between 4 and 20 tubes.