WO2021148701A1

WO2021148701A1 - Linear-focus solar collector with horseshoe-shaped open receiver

Info

Publication number: WO2021148701A1
Application number: PCT/ES2021/070040
Authority: WO
Inventors: Juan José SERRANO AGUILERA; Luis Parras Anguita
Original assignee: Universidad De Málaga
Priority date: 2020-01-22
Filing date: 2021-01-22
Publication date: 2021-07-29
Also published as: GB2604529A; ES2844999A1; GB2604529B; ES2844999B2; GB202207874D0

Abstract

Disclosed is a linear-focus solar collector with a horseshoe-shaped open receiver, which is a new solar collector concept for linear-focus solar thermal systems that allows the design of a more robust receiver that is durable under operating conditions at moderate temperate (<300ºC). A concave metal absorber, the upper front side of which is covered by a thermal insulating material, encloses a cavity with air at atmospheric pressure. The opening of the cavity is limited with a piece of glass. This piece of glass can be partially or fully closed, allowing the existence of a stratification chamber that limits convective losses. In addition to thermal functionality, the glass piece is optically active, improving the interception index of a slit through which the rays access the metallic absorber.

Description

DESCRIPTION

SOLAR COLLECTOR WITH LINEAR SPOTLIGHT WITH OPEN RECEIVER IN THE SHAPE OF

HORSESHOE

TECHNICAL SECTOR

The present invention corresponds to the technical field of concentrating solar energy, specifically to the technology of solar thermal receivers with a linear focus, providing a new receiver design for parabolic trough collectors in a low operating temperature range.

BACKGROUND OF THE INVENTION

In the context of the solar thermal industry, parabolic trough technology is the one that arouses the most commercial interest due to its technological maturity and the reduced risk to be assumed by investors in its implementation. Of all the elements that any sensor consists of, the receiver is the most critical element, not only because of the technical complexity involved in its manufacture, but also because it is one of the most vulnerable and expensive elements. Since the introduction of the first SEGS plants in California in the 1980s, the concept of receiver tube used by the industry has not changed substantially. The first patents where this concept is described appeared in the 70s and 80s, this is the case, for example, of document US4432343, which describes a parabolic trough sensor formed by a reflector with a parabolic section whose receiver is formed by a tubular metal absorber (with a selective coating, eg black chrome) surrounded by a glass cover concentric to the previous one. This invention also details that vacuum conditions must exist between the absorber and the glass cover in addition to using expansion bellows to compensate for the different degrees of thermal expansion between the metal and the glass.

Since then, most of the registered patents have proposed partial modifications, but all of them based on the concept of a cylindrical tubular receiver. In document ES2125828 an absorber tube with slits was proposed on the walls to favor the thermal transfer coefficient between the metal wall and the heat transfer fluid. For its part, the invention described in W02007076578 adds an insulating cover in the upper area of the absorber (upper half where it does not receive concentrated radiation) to reduce thermal losses. Configurations such as that referred to in DE10033240 have also been proposed where the absorber tube and the cylindrical glass cover are not concentric. The geometry of the glass cover can also be modified. According to DE10305428, the partial modification of the cylindrical section geometry by means of slits in the glass cover improves the interception rate of the receiver.

On the other hand, documents US2007034204 and US2008087277 propose alternative solutions to the classical connection between the absorber tube and the glass cover by means of a metallic bellows.

Although the state of the art abounds with inventions based on circular section tubular absorbers, there are some inventions that have already introduced the concept of cavity receiver in the context of parabolic trough collectors. Documents US20130192226 and WO2015089273 highlight the advantages of using a tubular receiver, which is covered in its upper part with thermal insulation and comprising a cavity in its lower part. This cavity can be closed in its lower part by a simple glass closure whose purpose is to seal the existing cavity to promote stratification. According to its authors, to reduce thermal losses, it is very important that the emitting surface of the metallic absorber is the minimum possible, despite being in a cavity. For this reason, the lateral surfaces of said cavity are not part of the body of the metallic absorber. In a previous document, US20100043779, a receiver with a concave cavity absorber is proposed, but this cavity is surrounded by three elements: concave absorber tube, thermal insulator and simple glass closure that is passive from the optical point of view (see figures 6 and 7 of the aforementioned document). On the other hand, document US1661473 also proposes a receiver with a cavity-shaped metal absorber to which a conventional solid lens can be attached at its lower aperture.

Cavity receivers are an excellent alternative to solar thermal systems for process heat. The concept of receiver currently used is formed by a metallic absorber tube, through which the heat transfer fluid circulates, generally thermal oil. A glass cover surrounds the absorber so that air can be extracted from the space between them, forming a chamber or vacuum ring. Maintaining this level of vacuum is essential to limit thermal losses. Due to the geometry and the configuration of the parabolic mirrors, only part of the perimeter of the absorber tube receives concentrated solar radiation, which can be of the order of a few tens of kW / m ² compared to the little radiation that the part receives. not exposed. Depending on the internal cooling of the absorber (characteristics of the thermal fluid and its flow rate), thermal stress may appear on its wall.

The conventional design, with a vacuum ring, is designed to operate with temperatures greater than 300 ° C in the heat transfer fluid. This entails an increase in the complexity of the design, which entails a considerable increase in production costs, as well as a penalty in their durability due to some inherent aspects of their design. This means that they are still expensive and not very robust, since their optical and thermal properties tend to degrade over time. The following technical drawbacks stand out:

(i) The vacuum ring is sealed with a solder between the glass and the metal at the two ends of the receiver (the length of which is usually 4.06 m). This weld joins the end of the glass cover with a metal bellows whose mission is to compensate for the different coefficients of thermal expansion between the metal of the absorber and the cover glass. Any failure or leakage, both in that weld and in the bellows, causes the loss of vacuum in the receiver, which results in a drastic reduction in its thermal performance.

(ii) Concentrated solar radiation falls on the outer surface of the absorber. This surface is the one that reaches the highest temperature in the entire receiver and emits thermal radiation towards the outside. This is the reason why a selective coating is needed that increases the absorption of incident solar radiation and in turn reduces the emission of infrared radiation from said surface.

(iii) When the thermal fluid is synthetic oil, H2 molecules are generated due to the temperatures reached by said fluid. Diffusion of these molecules through the absorber wall to the vacuum ring degrades the vacuum levels themselves. It is a relatively slow process, but over the first few years of operation this phenomenon can significantly limit the life of the receiver.

For these reasons, the failure rate in receivers has direct effects on the operation and maintenance costs of parabolic trough plants (CCPs). Any of these problems implies the replacement of the entire receiver, having a direct impact on the levelized cost of electricity production from these plants.

Therefore, more robust receivers are necessary, which without requiring the most vulnerable and expensive elements existing in current systems, allow to guarantee acceptable levels of thermal and optical losses. The receiver concept described here proposes a suitable design for systems with moderate operating temperatures thanks to the optical system that fulfills a double function: reducing thermal losses through a protected stratification chamber and increasing the concentration factor on the slit of opening.

SUMMARY OF THE INVENTION

The present invention describes a new collector with a linear focus (figure 2) analogous to the CCPs currently existing in electricity generation plants. By means of a new geometry of the primary reflector and a new cavity receiver, an alternative is provided for systems that operate in a moderate temperature range (<300 ° C), as is the case of solar thermal systems that provide process heat.

Unlike the state of the art, the present invention presents a reflector plus receiver assembly with a cavity, below which is a piece of glass that provides the following advantages:

(i) Thermal: It favors the stratification of the air inside the cavity and protects it against external air currents or due to the inclination of the receiver itself.

(ii) Optics: The walls of this cavity can be used to (by means of glass walls of variable thickness) redirect the rays so that the width of the opening slit of the metal absorber can be reduced. BRIEF DESCRIPTION OF THE FIGURES

To complement the description that is being made and in order to help a better understanding of the characteristics of the invention, a set of drawings is attached as an integral part of said description, in which, with an illustrative and non-limiting nature, the following has been represented. following:

Figure 1. Graph with general representation of the radial pattern that follows both the inner r _inner edge (θ) and the outer r _outer edge (θ) of the wall of the glass piece.

Figure 2. Shows a cross section of the collector assembly including the primary reflector and the receiver.

Figure 3. Shows the cross section of the horseshoe receiver with a partially closed glass piece.

Figure 4. Shows the cross section of the horseshoe receiver with integral closure glass piece.

DESCRIPTION OF THE INVENTION

The solar collector of the invention comprises a primary reflector and a receiver. The primary reflector comprises two regions represented in figure 2. The central region of the reflector (1) has a parabolic section, characterized by a focal length f and half-width of the aperture plane. The other region that makes up the ends of the reflector receptor (2) is a modified parabola, defined in the interval x∈ (w _p , w). This is the solution to an ordinary first-order differential equation, whose solution (y) depends on two parameters: a, of the same order as the opening of the receiver slit and δ, of the same order as the half-angle of the solar cone (4.65 -10 ³ rad), where the origin of coordinates is at the focal point of the parabola (4):

The initial point of the solution that describes the outer region of the primary reflector is determined by the transition point between the two regions, where x = w _p , so that y (w _p ) = -f + w2 _p / 4 f. The resulting reflector is a linear focus mirror with a total aperture width w.

The horseshoe-shaped open receiver that incorporates the solar collector (3), takes advantage of the upward action of buoyancy forces in natural convection processes. According to Figures 3 and 4, the receiver is located on the focal point of the reflector (4) and comprises three main elements:

(i) The metallic absorber (5), through which the heat transfer fluid (6) circulates. The internal surface of the absorber, with a concave geometry, covers most of the perimeter of the cavity (7). The horseshoe shape of the absorber is defined, among other parameters, by the width of the opening slit (d). The interior surface of the metal absorber that surrounds the cavity may be treated with a selective coating, although the absorbance and emittance values of this absorber surface are not as critical as in conventional absorber tube designs, since the slit is the only surface. where radiant losses can occur.

(ii) The thermal insulation (8) reduces to negligible levels any type of thermal loss through the outer perimeter of the metallic absorber (5). This insulation is covered and protected from external agents by a protective casing (9). In addition, a fraction of the lower contour of the insulation (8) is protected by a piece with selective coating (11) whose function, in addition to protecting the insulation (8), is to absorb the energy from the residual fraction of rays that do not cross. the slit of the metal absorber (5) of width d.

(iii) The piece of glass (10) has the double function of redirecting the solar rays concentrated by the primary reflector and favoring the stratification of the air in the cavity (7) even when the solar collector is tilted or there is an outside wind, reducing the thermal losses by convection. This piece of glass can be partially closed (formed by two symmetrical pieces)

(figure 3) or integral closure (figure 4). Based on this configuration, the cavity is open to the outside (partially closing glass part) or closed to the outside (integral closing glass part). In either case, the air pressure in the cavity is the same as outside. The general description of the geometry of this piece of glass involves the determination of its two contours in polar coordinates:

The glass piece (10) consists of a concave wall of several sections, of variable thickness, in the shape of an inverted arc facing the focal point of the reflector and the opening of the receiver. Both the internal and external contours of this wall follow a geometric pattern that can be defined in polar coordinates r {6) with the origin of the coordinate system being the focal point of the primary reflector (4) and the angle Q defined as positive in clockwise with angular origin in the horizontal direction (see figures 3 and 4). Based on Figure 1, this pattern is determined by a continuous and differentiable piecewise function defined on the interval

1. Stroke 1, corresponding to Region I (see figures 1, 3 and 4) and defined in the interval

passing through the end points of the interval

(0, R ₀ ,), (θ _a1 , R _a ). This piece of the function is defined by a polynomial with increasing trend (R _a > R ₀ ).

2. Piece 2, corresponding to the transition between region I and II (T _a in figure 1). It is a spline (cubic polynomial) that must pass through the initial and final points of the interval: (θ _a1 , R) _a , (θ _a2 , R _a ) in addition to ensuring that the function is differentiable at θ = θ _a1 and θ _a2 .

3. Piece 3, corresponding to region II (see figures 1, 3 and 4) and defined in the interval that passes through the extreme points of the interval:

(θ _a2 , R _a ), (θ _b1 , R _b ). This piece of the function is defined by a polynomial with decreasing trend (R _a > R _b ).

4. Piece 4, corresponding to the transition between region II and III (T _b in figure 1). It is a spline (cubic polynomial) that must pass through the starting and ending points of the interval in addition to ensuring

let the function be derivative at θ = θ _b1 and θ _b2 .

5. Piece 5, corresponding to region III (see figures 1 and 4) and defined in the interval that passes through the extreme points of the interval:

Due to the tendency in regions I and II of the internal and external contours of the glass piece, a convergent lens with a curved axis is configured whose function is to redirect the rays that fall more obliquely on the entrance slit to the cavity absorber (5). The embodiment of the invention with a piece of glass with partial closure (figure 3) is only defined in regions I and II, that is, between pieces 1 and 3, while the embodiment with a piece of glass with an integral closure (figure 4 ) encompasses the three regions and the five pieces. The proposed receiver concept has the following advantages:

(i) It is a more robust design, since, as it does not have a vacuum chamber, the levels of thermal losses are not affected over the years of operation as the vacuum level degrades. By operating in a lower temperature range than conventional receivers, the vacuum ring and all its associated complexities can be dispensed with.

(ii) The application of a high efficiency selective coating on the inner surface of the metallic absorber (5) can be avoided. Note that the slit of width d is the only critical area through which there are losses by infrared radiation. The width of this opening slit is significantly less than the perimeter of a conventional absorber.

(iii) In case of breakage in any of the three elements of the receiver, it is not necessary to replace the entire receiver, but only the affected element. This also makes any repairs easier.

(iv) The thermal insulation material (eg rock wool) means that there is no heat flow through the upper contour of the metal absorber (5), and therefore contributes to reducing the temperature difference between the fluid and the upper wall thereof, limiting the thermal stress to which the absorber is subjected. This limits the thermal gradients in the metal wall.

(v) No type of bellows would be needed, since the elements are embedded, This allows the sliding between the upper face of the glass piece and the rest of the elements to compensate for the different coefficients of expansion.

(vi) The active heat exchange surface between the metallic absorber and the fluid (in relation to the passage section) can be greater, since the geometry of the absorber can be modified without affecting the width d of the opening slit.

(vii) It is much easier to install sensors to monitor the temperature of the metallic absorber at different points through the upper thermal insulation.

(viii) In the case of the partially closed piece of glass (figure 3), the two symmetrical parts located on the sides redirect or refract the rays coming from the outer region of the reflector This set of rays, which strike more obliquely based on the horseshoe-shaped receiver and come from a more distant reflector region, they are the most likely to move away from the focal point. On the other hand, the rays coming from the center of the reflector have a less dispersed distribution and do not need to be redirected. To these last central rays (carriers of a majority fraction of the energy) is added the advantage that they do not suffer optical losses as they do not have to cross the double air-glass border.

PREFERRED EMBODIMENT OF THE INVENTION

Below, specific embodiments of the invention are shown, without these embodiments implying a limitation as to what an expert will understand as the scope of the invention. Each one of them aims to show the two most representative variants that are derived from the present invention.

Figure 2 shows the horseshoe-shaped open receiver and primary reflector assembly. The parabolic or central region of the reflector (1) has a focal length (f = 2.1 m) greater than the conventional LS-3 reflector model with a half-width of w _p = 2.40 m. The external region of the reflector (2) is the solution to the aforementioned equation first-order ordinary differential where a = 0.03 m, and δ = 5.5-10 ^-3 rad, so that the total half-width of the reflector aperture plane is w = 2.88 m, which is equivalent to the total width of the collector LS-3. The aforementioned reflector geometry is compatible with the two open and closed receiver models described above and shown in Figures 3 and 4.

A- Receiver with partially closed glass piece (figure. 3): As described, the partially closed glass piece is made up of two symmetrical pieces (figure 3), so that the geometry of its contours is expressed by means of a continuous and differentiable function formed by three pieces (Regions I and II). By this criterion, the radius of the inner contour is defined, _internal r:

where θ _a1 = 0.2655 rad, θ _a2 = 0.3310 rad, θ _b1 = 1.04 rad.

The external contour is defined in an analogous way through the same function model expressed in polar coordinates:

where θ _a1 = 0.2961 rad, θ _a2 = 0.3004 rad, θ _b1 = 1.04 rad.

Thanks to the partially closing glass part, optical losses in the central rays that do not cross any air-glass border are reduced. If the operating temperature is low enough (<150 ° C), the thermal losses are less significant compared to the optics, so a design that prioritizes reducing losses is desirable. optical versus thermal.

B- Receiver with integral closure glass part (figure. 4): As described and represented in figure 4, the geometry of the integral closure glass part (10) is expressed by a continuous and differentiable function made up of 5 pieces. By this criterion, the radius of the inner contour is defined, _internal r:

where θ _a1 = 0.2037 rad, θ _a2 = 0.3433 rad, θ _b1 = 0.9684 rad and θ _b2 = 1.108 rad.

where θ _a1 = 0.2540 rad, θ _a2 = 0.3936 rad, θ _b1 = 0.9709 rad and θ _b2 = 1.1105 rad. Thanks to the configuration of the piece of glass with integral closure, the stratification of the air existing in the cavity is protected, therefore, this design is indicated for a medium range of temperatures (150-300 ° C) where the closure of the cavity it is intended to reduce thermal losses, but at the expense of higher optical losses, since all rays must cross the air-glass interface.

Claims

1. Linear focus collector comprising a primary reflector (1, 2) and a horseshoe-shaped cavity receiver (3) characterized in that: the reflector has a central region (1) with a parabolic section, with a focal length fy half-width of the aperture plane w _p and two ends (2) that have the shape of a modified parabola, with its geometry y (x) being the solution to an ordinary first-order differential equation

which depends on the parameters a, of the same order as the opening of the receiver slit), and d, of the same order as the semi-angle of the solar cone, where the origin of coordinates is at the focal point of the parabola (4); and the receiver comprises a metallic absorber (5) surrounded by a thermal insulator (8) and is delimited in its lower part by a concave piece of glass (10) facing the focal point of the reflector formed by several sections in the shape of an inverted arc of variable thickness, whose contours follow a pattern defined by continuous and differentiable piecewise polynomial functions.

2. Collector according to claim 1 where the radius of the internal contour of the glass piece (10) is defined as:

where θ _a1 = 0.2655 rad, θ _a2 = 0.3310 rad, θ _b1 = 1.04 rad; and the outer contour has a radius defined by:

where θ _a1 = 0.2961 rad, θ _a2 = 0.3004 rad, θ _b1 = 1.04 rad.

3. Collector according to claim 2 where the radius of the internal contour of the glass piece (10) is defined as:

where θ _a1 = 0.2037 rad, θ _a2 = 0.3433 rad, θ _b1 = 0.9684 rad and θ _b2 = 1.108 rad; and the outer contour has a radius defined by:

where θ _a1 = 0.2540 rad, θ _a2 = 0.3936 rad, θ _b1 = 0.9709 rad and θ _b2 = 1.1105 rad.

4. Collector according to any of the preceding claims wherein the receiver is covered by a thermal insulator (8).

5. Collector according to claim 4 wherein the thermal insulator (8) is covered by a protective casing (9) and a fraction of the lower contour of the insulation (8) is protected by a piece with selective coating (11).