Patent Application
20110088684
The present invention generally relates to a solar concentrator. In particular the present invention relates to a low-cost system dimensioned for supplying energy to a household. The rays of the sun are reflected and concentrated to a point focus, and the heat concentrated in said point is used to generate energy to supply the house. A self-supplying household may be taken off the electrical supply grid, contributing both to the household finances and environmental conservation.
The problem of energy supply at a reasonable cost has never been completely solved. As the worldwide demand for energy increases exponentially, there is a heavy burden placed on traditional sources of energy. The spiraling cost of energy in recent years adversely affects the household economy. Therefore, alternate sources of energy, e.g., solar power, have become increasingly attractive in recent times. The option of local generation is even more attractive, as it eliminates the cost associated to distribution and supports an autonomous supply model, with the obvious economic advantage arising from it.
Over the years, solar energy research has helped develop systems that have improved efficiency and are more economical. However, a dearth of information, materials, complexity, and manufacturing skills remain an impediment to large-scale production and utilization of this abundantly available energy source for household supply.
The typical solar concentrators can be classified according to several aspects. The ones relevant for the purpose of the present description are the kind of focusing employed (point or line), positional adjustability of the various reflectors involved in the concentration process (fixed or tracking devices) and characteristics such as the positioning of the heat absorber element.
Before addressing the Prior Art relevant for the present invention, it is important to understand certain conditions associated with the problem it solves. One of them is the energy requirements. As the straightforward calculation below indicates, in order to yield the amount of energy required for the supply of a household (about 20 kWh/day on average), a solar concentrator system must be able to collect the solar radiation incident over a surface of about 36 m2. Such an area (for instance 6 m×6 m) is typically available at the roof of an average sized house, conveniently exposed to regular, direct sun incidence. The amount of energy provided by the sun is on average 1 Kwh per square meter under ideal conditions (clear day, direct incidence, etc.), which means that the typical household could easily collect 36 Kwh. Assuming an average 5 hours of direct sun incidence per day, the roof of an average sized house could generate 180 kWh/day. Even considering an efficiency of a mere 20% along the conversion from the incident light to electricity, the resulting 36 kWh/day would still suffice to supply an average sized house.
Several examples of solar energy concentrators are found in the prior art. These feature several inconvenient aspects, such as complexity and cost. Furthermore these designs do not easily lend themselves to installation in the scale contemplated for supplying a household. For example, the structural weight of even a small-sized, movable dish reflector complicates its deployment atop of a house roof. It is also vulnerable to wind damage. Scale reduction seriously limits the amount of energy such kind of concentrator may yield.
Based on the end application, different types of solar concentrators are employed to achieve optimum results. In the specific scope of the present invention—highly efficient concentration of solar light to a point focus to generate energy for supplying a standard household—the state of the art solar concentrators' performance is less than optimal. For instance, central receiver-type concentrators are typically employed in large scale applications for electricity generation. These require vast real-estate for proper deployment and are thus not economical for small and medium-scale applications. Continuous surfaced parabolic dish concentrators entail limitations such as the prohibitive costs associated with compound and complex reflector curves and expensive mirror substrates.
Over the years attempts have been made to design and construct solar concentrators that provide point focus (high solar concentration) with minimum complexity and cost. U.S. Pat. No. 5,374,317 (Lamb et al.) discloses a multiple reflector concentrator solar electric power system. In this system, the sun's rays first reach a plane of individual primary reflectors (which may be flat or curved). The primary reflectors then reflect the solar radiation to the location of secondary reflectors (which may again be flat or curved) and are then passed through to an energy converting component. The system disclosed by Lamb et al. uses a large number of components other than primary and secondary reflectors such as tertiary reflectors, optional cover plates, and heat dissipation components particularly suited for solar power generation, resulting in a system that is complex and expensive for household-scale use.
U.S. Pat. No. 6,530,369 (Yogev et al.) also describes a system comprising two reflectors that are successively arranged along an optical path of the system so that the first of the two reflectors reflects the radiation towards the second reflector. The concentrated radiation from the second reflector is directed to a solar receiver. However, the second reflector is realized as a tower reflector. The system disclosed by Yogev is not very efficient regarding solar energy conversion, requiring vast real-estate for proper deployment, which associated with its complexity makes it uneconomical for small and medium-scale household use.
International Patent Publication No. WO 2005/022047 A2 (Shifman) discloses a solar energy utilization unit comprising a solar radiation concentrating component and a solar energy receiving component. The concentrating component comprises a continuous, concave primary reflector and a convex secondary reflector, for concentrating incident solar radiation and redirecting the concentrated radiation into the receiving component. However, the primary reflectors are dish-shaped and require high precision curved surfaces for obtaining proper concentration effects. The requirement of a continuous (or even the alternatively recited multiple array of dishes) primary reflector dish entails limitations regarding structural durability of the component, expensive construction and maintenance costs and (in the case of multiple primary dishes) the need of multiple tracking mechanisms, increasing the costs even further.
U.S. Pat. No. 3,118,437 (Hunt) discloses a system of two reflective surfaces or two sets of reflective surfaces facing each other in an arrangement that causes all rays striking the first reflective surface to converge onto a substantially one point or limited area. Although Hunt discusses a system of two reflective surfaces, wherein the sun light is redirected from the first to the second, the practical embodiments of Hunt's reflective system are complex and require elaborate infrastructure. Among other complications, Hunt's primary reflector is mobile (not stationary), requiring carriages and tracks.
International Patent Publication No. WO 2008/046187 A1 (Kinley) discloses a solar concentrating system comprising primary and secondary reflectors to concentrate the solar light to a point focus. Each of the reflectors as taught by Kinley is parabolic in one dimension only (i.e. single-curved mirrors), and his invention was conceived for instance for melting metals in a foundry furnace. The system disclosed by Kinley would not be able to efficiently solve the problems addressed by the present invention, especially generating enough energy to supply a regular household Kinley's primary reflector—which could be construed as analogous to the present invention's primary concentrating reflector—is a continuous surface. In order to yield the amount of energy required for the supply of a household (about 20 kWh/day on average), a primary reflector as taught by Kinley would need to be very large. In order to collect the required 20 kWh/day, the collection area must be about 400 sqft. Such a bulky primary reflector would be susceptible to wind and weather damage, complicate the structural requirements for the house's roof and entail comparatively high building and operational costs. All these problems would be compounded by Kinley's teaching that the primary reflector is movable, not stationary. As illustrated in Kinley's
U.S. Pat. No. 4,131,336 (Frosch) discloses a solar concentrating system comprising a stationary primary reflector in the shape of a trough that concentrates the solar light to a linear focus along a line occupied by a movably supported collector containing a working fluid. Compared to the system disclosed in the present invention, it is expensive, bulky and not much efficient. As will be detailed below, the present invention features point focus, is compact and highly efficient. The system disclosed by Frosch relies on a line focus, which yields lower thermal efficiency than a point focus. The proportion between the primary reflector area and the area of the absorber/collector is small, thus preventing the working fluid in the collector from reaching as high a temperature as the compact absorber disclosed by the present invention, which in turn entails lower thermal efficiency. Furthermore, the line focused concentrator is underpowered: For the same amount of light-collecting real-estate, the system according to the present invention yields more heat. Frosch's movably supported collector is bulkier than the compact absorber of the present invention, being therefore susceptible to much higher heat loss through such an extensive surface. Finally, both Frosch's collector and the present invention's absorber are encapsulated in a vacuum shell; however Frosch's movably supported collector is bigger and therefore its manufacturing cost is that much more expensive.
The system disclosed in Japanese Patent No. JP59097460A (Horigome) would not be able to efficiently solve the problems addressed by the present invention, especially generating the required amount of energy at a low manufacturing and operational cost. JP59097460A discloses a solar concentrating system comprising a stationary, continuous parabolic primary reflector that concentrates the solar light to a point focus on a movable receiver that moves along the extension of an arcuate guide rail that tilts to position the receiver. A flexible optical fiber cable conveys the light originating from the primary reflector to an absorber element. Just like in Kinley's WO 2008/046187 A1 discussed hereinabove, Horigome's primary reflector—which could be construed as analogous to the present invention's primary concentrating reflector—is a continuous surface. The amount of energy needed for supplying the house would require Horigome's primary reflector to be rather large, therefore complicating the structural requirements for the house's roof and entailing comparatively high building and operational costs. Horigome discloses a flexible optical fiber cable for conveying the light originating from the primary concentrating reflector to an absorber element. However the mechanism to orient such light into the optical fiber cable (for example a waveguide) would render the system complex and expensive. The flexible optical fiber cable itself is expensive and fragile. Furthermore it limits both the distance and physical positioning of the absorber element.
There is accordingly a need for an improved solar concentrating system that overcomes the limitations associated with using complex construction requiring high degree of skills. Moreover, there is a need for an improved solar concentrating system wherein the prohibitive costs associated with manufacture and deployment of a traditional solar concentrating system are minimized thereby making it attractive for use by small and medium scale household use.
It is therefore an object of the present invention to disclose a small-scale, dimensionally-adaptable solar concentrator system featuring high energy conversion efficiency, providing point focus (high solar concentration) with low building and operational costs.
According to a certain aspect of the present invention, there is disclosed an apparatus comprising a primary concentrating reflector made up of multiple reflecting surfaces that are stationary with respect to earth and laid over a two-dimensional flat plane surface, all of said multiple reflecting surfaces cooperating to redirect the incident solar radiation towards a small primary target area. A secondary redirecting reflector is positioned near said small primary target area for redirecting the light concentrated by the primary concentrating reflector towards a remote absorber that is fixed with respect to earth, said secondary reflector presenting a reflective surface that is convex in design and being selectively movable above the flat plane of the primary reflector in two orthogonal dimensions that are both parallel to said flat plane. The secondary reflector is ball-pivotally connected to a mobile element that moves according to solar tracking data for allowing the secondary reflector to keep its concentrated light output pointed towards the stationary remote absorber while the movement of the sun across the sky causes the area of concentration of the light output by the primary reflector to change position.
The above as well as additional features and advantages of the present invention will become apparent in the following written detailed description.
A more complete understanding of the present invention may be had by reference to the following detailed description when taken in conjunction with the accompanying drawings, wherein:
a is a perspective view of a particular embodiment of the present invention illustrating one of the isolated modules of the primary reflector of
a is an elevation view illustrating a certain position of the elongate arm (5) that causes the concentrated solar light to travel a certain distance;
b is similar to
The invention will now be described with reference to the figures. The figures are intended to be illustrative rather than limiting and are included herewith to facilitate the explanation of the invention.
The following description requires the previous definition of the concepts of primary and secondary reflectors, as well as the heat absorber element. The primary concentrating reflector is herein defined as the element responsible for the initial deviation of the incident sun rays, concentrating them towards a designated target. The secondary redirecting reflector is herein defined as the element responsible for redirecting the radiation deflected by the primary concentrating reflector towards a designated target. The absorber element is herein defined as an element that receives the concentrated radiation, increasing its temperature as a result. Furthermore, in the context of the present description, the term spherical pivot joint or ball pivot joint is intended to encompass any mechanical arrangement that allows rotation around a given axis “Z” that is orthogonal to the supporting surface as well as independent pivoting around any second axis that is orthogonal to said axis “Z”. A schematic example of such articulation is illustrated on
The basic outline of the system as illustrated in
Description of the Primary Concentrator
As discussed previously in the Background section, a solar concentrator system capable of supplying an average household must be able to collect and concentrate the solar radiation incident over a surface of about 36 m2. It is not practical to deploy the required primary reflective surface of 6 m×6 m in one continuous parabolic mirror atop of a house. Such a choice would entail high manufacturing and maintenance costs of the mirror itself, require structural reinforcement of the standard house roof and also bring about the risk of wind and weather damage.
In order to curb the inconveniences listed above, the present invention replaces such a movable, continuous reflector surface by an equivalent, multiple-section reflective arrangement that can be easily harmonized with the ground, the roof or other suitable surface of a building. Just like the continuous reflector surface it is designed to replace, it concentrates the solar radiation onto a small area. However the primary concentrating reflector array (1) of the present invention is made up of several individual facets (2) arranged in a two-dimensional field and forming concentric annular sections (i.e. similar to a heliostat), wherein each of the individual facets points towards a target area, such that the facets combined concentrate the incident solar radiation onto said target area. Examples of the primary concentrating reflector array (1) as seen from above can be seen on
The primary concentrating reflector elements are distributed in a geometric pattern which optical performance is similar to that of a single and continuous concave mirror, forming a stationary array wherein several individual reflecting elements collaborate to concentrate the incident light onto a small target area. The effect obtained is analogous to that of a Fresnel lens.
It is well known in the art that solar light can be concentrated by means of a continuous-surfaced parabolic reflector that continually adjusts its position according to that of the sun. Said continuous adjustment is termed solar tracking. As already indicated in the previous paragraphs, the present invention replaces said sun-tracking, continuous parabolic surface by an array of fixed reflective surfaces. It is possible to achieve many of the results normally obtained with a sun-tracking, continuous-surface parabolic reflector by using several small segments of the parabola. In other words, the present invention segments the continuous parabolic reflective surface into a saw tooth-like reflecting surface. Incoming parallel light is brought to a substantially narrow area of focus for most angles of incidence of the sunlight to the array of reflecting surfaces, although the fact that the reflecting array is fixed causes the location and size of said area of focus to vary with the solar incidence angle. This positional variation is addressed by the present invention's movable deployment of a secondary reflecting surface, which will be described further down. As for the arrangement of the primary reflector, the segments or facets are brought down to lie on a common plane. All segments or facets typically have a common height, although on an alternative embodiment they may present different heights. Because a shallow reflecting surface is desired from the standpoint of economy in construction and maintenance, the maximum height of the reflecting facets is relatively small, for instance about 12″. Preferably, the reflecting facets raise between 4″ and 15″ from the supporting surface.
In order to provide the desired concentration of focus, the sloping of the various facets is chosen so that no matter which facet is impinged upon by parallel light rays, the reflected rays will intersect in the narrow focusing area previously described. For each individual facet, the slope is a function of both the distance between the facet and the center of the primary reflector array and the distance between the focusing area and the common support surface to which the facets are fixed. By choosing the slope of the various facets in this manner, a primary reflecting surface that effectively replicates the concentration of a continuous-surfaced parabolic reflector, but is actually shaped as illustrated on
As the primary concentrating reflector array (1) features a mere approximation of a truly continuous and regular parabolic surface concentrator, the primary reflector's focus is not a point. It is indeed an area of focus—a ring—instead of a dimensionless point. Therefore, whenever the primary focus is discussed in the present description, it is referred to as a focus ring instead of a point.
All the facets (2) are stationary, being attached to a common support surface (3) that is itself stationary regarding the ground. The size of each of the facets (2) is such that their height above the support surface (3) is relatively small, for instance about 12″. The slope of each facet is determined according to its distance to the center of the arrangement, as can be seen on the cross-sectional view illustrated on
Several different shapes are contemplated for the individual facets (2). The reflective surface of each facet (2) may be flat such as in
In one embodiment of the present invention, the primary concentrating reflector array (1) is subdivided in multiple modules (4) that are sold as a kit. Once put together over the supporting surface (3)—for example with fasteners or adhesive—as indicated in the accompanying set of assembly instructions, the kit results in the desired primary concentrating reflector. For cases wherein the primary concentrating reflector array (1) is made up of a series of concentric, annular continuous sections such as seen on
The external appearance of an exemplary module discussed in the previous paragraph is illustrated in
The fact that all the facets (2) are stationary eliminates costly and potentially complicated tracking gear and reduces the bulk of the cost associated with the prior art solar concentrators. The relatively small surface area and low vertical dimension of each individual facet (2) eliminates potential problems with wind damage, and the small individual weight results in a load distribution pattern that eliminates the need for structural reinforcement of the support surface (3). Both of the latter aspects reduce the cost of the primary reflector. Maintenance is also simplified, as the small scale and inherent durability of the facets ensures that the array of primary concentrating reflectors can be frequently and easily cleaned by washing, spraying and many other methods known in the art, keeping a high level of optical performance. Finally, the primary concentrating reflector array (1) of the present invention features high efficiency, as it effectively concentrates most of the direct sunlight impinging on the area it occupies, yielding a high proportion between real-estate and amount of light concentrated.
Description of the Elongated Arm
According to the present invention, the solar radiation incident on the primary concentrating reflector array (1) is reflected towards a discrete point or small area, herein designated primary target point. Given the stationary disposition of the primary concentrating reflector array, the position of the primary target point changes as the sun runs its course across the sky.
In order to match both the daily and the seasonal solar movement, the elongated arm (5) of the present invention features a proximal end (P) ball-pivotally connected (i.e. by means of a spherical pivot joint) to a stationary supporting point—for example in the supporting surface (3)—and a distal end (D). This distal end (D) of the elongated arm (5) continually tracks the primary target point during its movement. A multiple set of primary actuators (6) move the elongated arm (5) according to sun track data supplied by a digital control system connected to optical sensors. This arrangement ensures that the distal end (D) of the elongated arm (5) is continually positioned close to the primary target point. The ball-pivoting of the elongated arm (5) allows the movement of the distal end (D) at least in two dimensions—for example in the direction of any of the two main axis that form the supporting surface (3), therefore allowing repositioning of the distal end (D) without moving the primary concentrating reflector (1). Besides solar tracking, the ball-pivoting of the elongated arm (5) also allows the device to cope with variations caused by possible optical imperfections in the stationary primary concentrating reflectors.
In another embodiment of the present invention, the elongated arm (5) is telescopic, such that it extends and shortens its axial length to adjust the distance between its proximal and distal ends (P and D) as required. This movement is controlled by a set of telescope actuators (13) operated by the aforementioned digital control system. Combined with the mobility afforded by the spherical pivot joint at the proximal end (P) of the arm, this telescoping action enhances the effective three-dimensional repositioning capability of the distal end (D) of the elongated arm (5) for continuous solar tracking Moreover, given that the focal distance of the primary concentrating reflector array (1) varies according to the sun's incidence angle, the telescoping of the elongated arm (5) affords crucial axial adjustability to harmonize the focal point of the primary concentrating reflector array with the tracking of the primary target point regardless of the sun's relative position.
In a further alternative embodiment of the present invention, the elongated arm (5) may be replaced by different structures that perform the same task. Examples include the arcs (15) and (16) illustrated on
In a further alternative embodiment, the proximal end of the elongated arm (5)—or the equivalent end of the alternative structures illustrated in FIGS. 8 and 9—is ball-pivotally connected (i.e. by means of a spherical pivot joint) to a supporting surface (3) that sits not in the roof of a house, being instead ball-pivotally connected to a different supporting surface such as a wall or even another building. In such case the elongated arm (5) is deployed such that it is still capable of performing the task of properly positioning its distal end (D) to track the primary target point, which is continually positioned between the sun and the primary concentrating reflector array (1).
Still considering the structure of the elongated arm (5) and remembering the requirements imposed by cost and wind resistance, the elongated arm (5) should preferably be as short and light as possible. According to the present invention, the use of a multiple-section reflective arrangement laid on a flat surface instead of a large-sized, continuous surface reflector arrangement for the primary concentrating reflector offers another advantage: The resulting focal distance of the primary concentrating reflector array (1) is shorter that the focal distance of a continuous-surface curved reflector of equivalent area. That makes possible to use a shorter elongated arm (5) in the system disclosed by the present invention, making the arm both cheaper and sturdier.
Description of the Embodiment with the Secondary Reflector at the End of the Arm
In one embodiment of the present invention, a secondary redirecting reflector (9) is positioned at the distal end (D) of the elongated arm (5). The secondary redirecting reflector (9) is ball-pivotally connected (i.e. by means of a spherical pivot joint) to the distal end (D), with said ball-pivotal connection allowing the movement of the secondary redirecting reflector (9) at least in two dimensions, for example free rotation as well as articulation around at least one axis that is perpendicular to the aforementioned rotation axis (see example at
The purpose of said ball-pivotal connection at the distal end (D) is to allow the continual redirection of the secondary redirecting reflector (9), keeping it pointed directly to the remote radiation absorber (11) regardless of the movement of the elongated arm (5) around the ball-pivotal connection at the proximal end (P). Thus the secondary redirecting reflector (9) is able to continually redirect the concentrated radiation output by the primary concentrating reflector array (1) towards a stationary, remote radiation absorber (11) that may be positioned away from the elongated arm (5).
The continuous repositioning of the reflective surface of the secondary redirecting reflector (9) is provided by a set of secondary actuators (10). These are connected to and controlled by the very same digital control system and optical sensors that control the primary actuators (6) used to move the elongated arm (5) as already described.
Given that the beam reflected by the secondary redirecting reflector (9) is extremely concentrated and could pose a health risk, the secondary redirecting reflector (9) conveniently points in a trajectory that minimizes the risk of interference with human or animal traffic. Furthermore, the remote radiation absorber (11) targeted by said concentrated radiation is preferably positioned at a place where access is restricted, thus minimizing the risk of accident.
Description of the Progressive Focusing Concept
The system is designed to provide progressive focusing of the solar radiation towards the absorber element. Said focusing is herein described as progressive because the idea is not to obtain a point focus out of each and every reflection to which the light is submitted in its path between the primary reflector and the absorber element; the objective is that after all the consecutive reflections the focus of the resulting beam that reaches the absorber is as close to a point focus as possible.
Convexity of the Secondary Reflector
In order to continually redirect the radiation deflected by the primary concentrating reflector array (1) towards a stationary, remote radiation absorber (11) that might eventually be positioned away from the elongated arm (5), the reflective surface of the secondary redirecting reflector (9) cannot be concave—it must be either flat or convex. That is so because a concave reflector would make the focal distance too short to allow focusing on a stationary absorber that may eventually be positioned far from the secondary reflector. In other words, the primary and secondary reflectors must cooperate to provide consecutive reflections that together achieve a point focus on the radiation absorber (11). As the light rays coming from the primary concentrating reflector array (1) are inherently convergent—after all, they originate from a concentrator—a concave reflective surface in the secondary redirecting reflector (9) would make the light rays even more convergent, thus preventing the desired point focusing on the absorber when the latter lies far away from the secondary redirecting reflector (9). That explains why the secondary redirecting reflector (9) must be divergent, therefore featuring a convex reflecting surface.
Now that the elements involved are described, let us discuss certain systemic advantages originating from the design of the primary concentrating reflector according to the present invention: The primary concentrating reflector array (1) behaves like a concave lens, featuring an inherently short focal distance that is selected by criterious design and positioning of the fixed reflective elements that make it up. Said characteristic of the present invention brings along two inter-related advantages, discussed below.
The first advantage is that a short focal distance for the primary concentrating reflector array (1) eliminates the need for a long elongate arm (5)—which would be fragile and expensive when compared to a shorter arm.
The second advantage is that the ability to precisely select the focal distance of the fixed primary reflector array (1) upon designing it allows the definition of the focal point so as to position it near the distal end (D) of the elongated arm (5). The purpose of such design consideration is to allow the secondary redirecting reflector (9) to be small in size and weight. That makes both the secondary redirecting reflector (9) and its supporting elongated arm (5) cheaper.
If the focus of the fixed primary reflector array (1) were to be far away from the distal end (D) of the elongated arm (5) as seen in
It is therefore an object of the present invention to have the fixed primary reflector array (1) designed to focus the reflected solar light to a narrow area—the focus ring previously discussed—near the surface of the secondary redirecting reflector (9), which as explained allows the latter to be small in size. As the primary reflector's focal distance is short by design, all of the individual solar rays it deflects concentrate in a small area (again by design), so that even a small-sized secondary reflector is capable of redirecting most of them.
Description of the Need for a Fluid Lens (or Adjustable Focus on the Secondary Reflector)
Now considering the requirement that the absorber be small (both to increase thermal efficiency and save cost on its thermal insulation vacuum shell) it is paramount that the secondary redirecting reflector (9) be able to provide focusing of the radiation as close as possible to a point focus, even for a stationary absorber (11) that may be positioned far away from the elongated arm arrangement. This requirement is compounded by the need to preserve such focus independently of the spherical pivoting (and in certain embodiments also the axial telescoping) of the elongated arm (5), which causes the linear distance between the secondary redirecting reflector and the targeted absorber to vary. In order to enable the secondary redirecting reflector (9) of the present invention to adjust the focus of the light beam directed to the absorber, either its degree of convexity is adjustable or it is associated with a fluid lens. Through the use of said combination of a convex secondary redirecting reflector (9) and a fluid lens (14), the present invention is capable of both redirecting and adjusting the focal length of the light coming from the primary concentrating reflector (1) that must be redirected towards the remote radiation absorber (11). In embodiments such as the one illustrated on
The presence of a fluid lens (14) in association with a flat or convex secondary redirecting reflector (9) is preferred; however a simpler convex secondary redirecting reflector (9) would also work, although possibly compromising the performance of the system due to less-than-optimal focusing of the light incident on the remote radiation absorber (11). Alternatively and as pointed out above, the preferred focusing could be achieved with any secondary redirecting reflector (9) capable of adjusting its degree of convexity. The attached
The embodiment with a secondary redirecting reflector (9) that is capable of adjusting its degree of convexity is better understood by reference to the attached
Description of the Absorber
According to the present invention, high energy density solar radiation is provided to the stationary remote radiation absorber (11), thus raising the temperature of the absorber. The high temperature attained by the absorber element is used for instance to produce steam for driving a steam turbine and generate electricity for the household. In a preferred embodiment the absorber element is encapsulated in a thermally insulating, transparent vacuum shell, for instance made of glass. The transparency of said insulating shell allows the light originating from the primary concentrating reflector array (1) to penetrate the absorber element, while the vacuum layer between the shell wall and the body of the absorber element keeps the absorber from loosing the heat it accumulates to the surrounding environment. According to the present invention, the face of the absorber element that receives the concentrated radiation is preferably small—for instance about 1′×1′ or even smaller. As explained previously, the system is designed to ensure that solar radiation that reaches the absorber element is concentrated as much as possible to a point focus, in order to increase the degree of energy concentration. Said energy concentration, progressively attained as the solar light travels through the system, reduces the heat loss in the transmission and yields higher thermal efficiency. The point focus allows the absorber element to feature small dimensions, which reduces its own heat loss rate. The material chosen for the external surface of the absorber element (11) preferably features optical properties—for instance solar absorption and output radiation—that maximize its capacity to retain heat originated from radiation incident on it. As already pointed out above, the remote radiation absorber (11) is preferably positioned at a place where access is restricted, thus minimizing the risk of accidental, possibly life-threatening contact.
Description of the Photometer
In another embodiment of the present invention, a photometer feeds data to the aforementioned digital control system that controls the primary actuators (6) and secondary actuators (10) that respectively control the elongated arm (5) and the secondary redirecting reflector (9). The digital control system is previously loaded with constant data such as the normal reflective range of each of the multiple primary reflective surfaces (1) for each possible angle of solar incidence, which allows the system to continually monitor and compare its own measured performance to the expected performance. Thus the digital control system is able to detect and correct possible imperfections in the positioning of both the elongated arm (5) and the secondary redirecting reflector (9), optimizing their position to match the energy transfer's expected performance. If the adjustments do not suffice to yield the expected performance for the detected lighting conditions, the system may assume the detection of events such as obstruction or damage to the reflective elements. Said information can be conveyed to the house owner by suitable means, such as an alert panel, to prompt for possibly required intervention (cleaning, maintenance, etc.).
Description of the Wind-Damage Prevention Aspect
In another embodiment of the present invention, a wind sensor (12) continually monitors wind speed. This wind sensor is connected to the same digital control system that controls the primary actuators (6) and secondary actuators (10) that respectively control the elongated arm (5) and the secondary redirecting reflector (9). Whenever the wind speed generates forces that threaten the structural integrity of the telescopic arm, the wind sensor (12) sends a signal to the digital control system, which in turn activates the already described set of telescope actuators (13) that retract the elongated arm (5). Once the wind sensor (12) signals that the wind conditions are no longer a threat, it sends a signal to the digital control system, which in turn activates the same set of telescope actuators (13) that redeploy the elongated arm (5) to its last operational position. In the embodiment wherein the elongated arm (5) does not telescope, it is simply laid down by the action of its primary actuators (6), redeploying to position once the wind sensor (12) signals that the wind conditions are no longer a threat.
Description of the Alternative Embodiment with an Absorber at the End of the Arm
In a simpler, alternative embodiment of the present invention illustrated at
The discrete radiation absorber (7) preferably presents an external surface that is rounded in design, such that most of the radiation output by the primary concentrating reflector array (1) impinges at the surface of the discrete radiation absorber (7) at angles approximately normal to the absorber's external surface. That maximizes the area of the absorber that is exposed to the impinging radiation.
In an alternative embodiment the discrete radiation absorber (7) may not have a rounded shape, presenting instead a receptive face where the solar radiation is to be preferably received. The discrete radiation absorber (7) positioned at the distal end (D) of the elongated arm (5) is ball-pivotally connected (i.e. by means of a spherical pivot joint) to the distal end (D), with said ball-pivotal connection allowing the movement of the discrete radiation absorber (7) at least in two dimensions, for example free rotation as well as articulation around at least one axis that is perpendicular to the aforementioned rotation axis (see example at
The purpose of said ball-pivotal connection at the distal end (D) is to allow the continual redirection of the discrete radiation absorber (7), keeping its receptive face pointed to the incoming concentrated light beam output by the primary concentrating reflector array (1) regardless of the movement of the elongated arm (5) around the ball-pivotal connection at the proximal end (P). Thus the discrete radiation absorber (7) is able to continually keep its receptive face continually oriented in the best position to absorb most of the concentrated radiation output by the primary concentrating reflector array (1) and directed to the small primary target area.
Description of the Alternative Embodiment with Safety Stoppers and Optical Barriers
Given the various degrees of freedom for the movement of the elongate arm (5) and secondary redirecting reflector (9), there is a risk that a given combination of unintended system movements and the position of the sun result in the concentrated radiation output by the secondary redirecting reflector (9) being directed to an unintended target instead of the stationary remote radiation absorber (11). For instance said unintended target could be a child playing on the ground besides the building where the system is deployed. Such unintended orientation of the concentrated solar beam to an unanticipated target is herein termed a critical misdirection. An example of a critical misdirection situation that these safety features are set to avoid is illustrated on
In order to prevent such critical misdirection, an embodiment of the present invention incorporates a series of safety devices. The normal working range of the various actuators as anticipated for that specific location where the system is deployed is limited by proximal stoppers (18) positioned around the spherical pivot joint between the proximal end (P) of the elongate arm (5) and the support surface (3). The proximal stoppers (18) are mechanically fixed to the support surface (3) and prevent the elongate arm from reaching a position that could result in a critical misdirection event. A set of distal stoppers (19) performs an equivalent task at the opposite end of the elongate arm (5). Mechanically fixed around the spherical pivot joint that connects the distal end (D) of the elongate arm (5) to the secondary redirecting reflector (9), said distal stoppers (19) prevent the secondary redirecting reflector (9) from reaching a position that could result in a critical misdirection event. Additionally, a set of optical barriers also contribute to block improperly aligned output light from leaving the secondary redirecting reflector (9). Mechanically fixed to the extension of the elongate arm (5) that supports the secondary redirecting reflector (9), said distal barriers (20) prevent the secondary redirecting reflector (9) from reaching a position that could result in a critical misdirection event.
Finally, each one of the primary actuators (6), secondary actuators (10) and telescoping actuators (13) are individually pre-wired to prevent them from positioning the device at any arrangement which would result in the concentrated beam missing the center of the intended target (the absorber 11) by more than half of its smaller dimension. Finally, a set of optical barriers (either reflectors or light-absorbing surfaces) is criteriously positioned around the secondary redirecting reflector (9) to perform the exact same task.
As an optional safety back-up, the elongate arm (5) could be automatically retracted by the action of its telescoping actuators (13) or simply laid down by the action of its primary actuators (6) whenever such risk is detected. The detection can be easily accomplished by continually comparing the actual and intended orientations of both the elongate arm (5) and the secondary redirecting reflector (9). The intended orientations can be based on data stored in the digital control system for the specific location where the system is deployed, which contains equations relating all combinations of the position of the distal end (D) of the elongate arm (5) and the positions occupied by the sun in the sky.
Description of the Focal Performance Issues
As the primary concentrating reflector array (1) features a mere approximation of a truly continuous and regular parabolic surface concentrator, the primary reflector's focus is not a point. It is indeed an area of focus—a ring—instead of a dimensionless point. In theory, and depending on the design size of the secondary reflector (9), this primary focus ring could be wider than the secondary reflector, such that there would be a certain amount of waste—light coming from the primary reflector (1) that does not get collected by the secondary reflector (9).
From the thermal efficiency point of view, ideally the primary reflector (1) would provide a very narrow primary focus ring around the previously described primary target point (near the secondary reflector, as explained above) regardless of the sun's relative position, thus allowing the use of a small-sized secondary reflector (9). In reality, the width of said primary focus ring is narrow for some particular sun positions and not so narrow for other sun positions. As a consequence, the size of the secondary reflector (9) required to “cover” the primary focus ring area varies for different solar positions. For example, the area of the primary focus ring when the elongate arm (5) is in a vertical position as illustrated on
The various different positions occupied by the sun during the daily operational cycle require the elongate arm (5) and the secondary redirecting reflector (9) to assume several different positions in order to direct the concentrated solar output to the stationary absorber (11). Of course there are physical limits to the solar incidence angle beyond which the system is not capable of achieving such redirection. These limit-incidence angles define a perimeter beyond which the system is not capable of collecting and concentrating the incident solar radiation. Said perimeter in turn defines what is herein termed the geometric working range of the present invention. Taking the various corresponding positions occupied by the secondary redirecting reflector (9) as a geometric reference, said working range defines a hemispheric section, which is herein termed the working range hemispheric surface (21). If we consider the embodiment wherein the elongate arm (5) does not telescope, this hemispheric surface is by definition circular (i.e. has a constant radius). The geometric working range just defined herein typically contains the limits of a different kind of working range, herein termed operational working range. The latter contemplates the fact that for some solar incidence angles—most notably the very shallow ones—the amount of energy collected by the primary concentrator array (1) is much smaller than that collected during periods when the sun is higher up in the sky. Although still within the geometric working range, the solar incidence angles beyond said operational working range result in the primary concentrator (1) not collecting enough energy to justify the operation, which is why the digital control system is programmed to shut the solar concentrator off whenever it reaches the limits of said operational working range, switching back on upon re-entering the operational working range on the next solar cycle.
In contrast with the working range hemispheric surface, the hemispheric surface traveled by the primary target point during the course of the day—herein termed primary target surface (22)—may not have a circular shape, among other reasons because of the imperfectness of the primary concentrating reflector array (1) when compared to a continuous parabolic reflector. As a consequence of their different shapes, there is a distance separating the working range hemispheric surface (21) from the primary target surface (22). Said distance varies depending on the specific solar position considered, and is directly proportional to the width of the primary reflector's focus ring achieved for each particular position. Examples of both these surfaces are illustrated on
In the alternative embodiment of the present invention wherein the elongate arm (5) is telescopic and can have its length changed, the system's thermal efficiency is further increased by use of said telescoping to bring the working range hemispheric surface closer to the primary target surface, thus making the primary focus ring narrower and increasing performance for any specific solar position considered.
The focus optimization described in the previous paragraph is independent from the focus optimization achieved by criterious design of the primary reflector, which is described in the following paragraph.
For a given solar position, the narrower the primary focus achieved, the higher the thermal efficiency of the system. Instead of optimizing the shape and geometric arrangement of the fixed facets that make up the primary concentrating reflector array (1) so as to provide a very narrow ring of focus when the sun is at one specific position in detriment of the focal performance when the sun is in other positions, the present invention seeks to balance said focal performance over the whole working range of solar positions. The shape and geometric arrangement of the primary reflector's fixed facets is designed such that the average primary focus ring for all solar positions within the system's working range has its size minimized. That might result in thermal efficiency loss for some specific solar positions, but the overall average efficiency is higher. That also ensures that for any given size of the secondary redirecting reflector (9), the average amount of light coming from the primary reflector (1) and impinging on it is maximized.
The present invention provides numerous advantages over the prior art. The advantages of a point focusing, when compared to a line or surface focusing, include the ability to achieve a higher maximum solar concentration, a smaller receiver heat loss (because the receiver itself has a smaller surface and volume) and lower building and operational cost due to the generally smaller amount of materials required. The present invention also offers a small-scale, dimensionally-adaptable solar concentrator system featuring high energy conversion efficiency, providing comparatively low building and operational costs.
Although numerous characteristics and advantages of the present invention have been presented in the foregoing description, together with details of the structure and features of the invention, the disclosure is illustrative only. Those skilled in the art will appreciate the fact that the present invention is susceptible to modification including but not restricted to aspects such as shape, size and arrangement of parts as well as free and interchangeable combination between the described embodiments, without departing from the scope of fair meaning.
