Patent Application
20170200848
The present invention relates generally to the concentration or collimation of light, and especially to photovoltaic solar energy.
High concentration photovoltaic solar concentrators need to be pointed directly at the sun to achieve maximum efficiency. Even slight errors in tracking will degrade their performance, and large errors will result in a failed system. As the trackers used are not perfect, it is therefore important for the concentrator to have a good “acceptance angle”. The acceptance angle is the tracking angular deviation from a perfect alignment with the Sun for which the concentrator is still providing most of its expected power. This is usually rated at 90% of the power generated when the concentrator is in perfect alignment with the Sun.
The higher the acceptance angle: the more relaxed the tolerance for the tracker's accuracy for pointing at the Sun; the more relaxed the tolerances for the assembly of the components in a solar concentrator module; and the more relaxed the tolerances for the assembly of an array of modules onto the tracker. Also, the higher the acceptance angle, the less susceptible the system is to loss of power when the modules on the tracker flutter, from winds or a sag of the modules (due to gravity). This issue is especially a concern for systems having a very large array of modules on a tracker.
All tracking, alignment, assembling and manufacturing tolerances can be expressed in terms of the acceptance angle that they consume. It is important that these tolerances don't exhaust the available acceptance angle because in this case, any extra tolerance will cause a loss of efficiency. This is why it is very important to have an acceptance angle budget as big as possible.
There are different types of optical architectures used for high concentration photovoltaic (HCPV) concentrators. Each type has a unique relationship between acceptance angle and concentration. This relationship is referred to as the Concentration Acceptance-Angle Product (CAP), given as:
CAP=√{square root over (Cg)}×sin α
Where, Cg is the concentration and ±α is the acceptance angle. This acceptance angle is the available budget. The practical CPV array will have a smaller acceptance angle for tracking errors because in general most of the budget has been consumed in manufacturing tolerances. The CAP is almost constant for a given optical architecture, no matter the particular Cg used. Then, maximizing a is the key to the success of any HCPV system, especially when it is scaled from a single module to an array of modules on a tracker.
HCPV systems are designed for high solar concentration. This allows the use of smaller solar cells to achieve the same power output. The type of solar cells used in HCPV systems, typically high-performance multi junction cells, are one of the most expensive components in the system, and reducing the size of the solar cell and/or the number of solar cells in the system helps to reduce cost. Maintaining a high acceptance angle at these higher concentrations is only possible for architectures having a high CAP.
Optical systems that only have one optical element, such as a single lens or a single reflector, have a low CAP. Adding a secondary optical element (SOE) increases the CAP. Depending on the design, the SOE can also facilitate cell electrical and moisture isolation, which can help simplify assembly and, thereby, reduce costs.
The solar cells used in HCPV systems have very good efficiency at converting sunlight into electricity, some in the low 40% range. This still means that the majority of the sunlight is converted to heat, and dissipating this heat is very important because the cooler the solar cells, the more efficient the system, and the better the system is at dissipating heat the higher its efficiency. A novel approach to solving this issue is to use the cover glass of the module for heat dissipation. The means for doing this is taught in above-mentioned U.S. patent application Ser. No. 12/957,826 and WO 2011/066286, by several of the same inventors as the present invention.
This approach uses small solar cells and is designed such that the heat dissipation is sufficient for efficient operation. Although glass is not a good thermal conductor it can be used. The key is to spread the heat using small cells, each one of them attached to a small heat spreader, which is attached to the cover glass. The heat spreading capability of cells does not scale with area resulting in the perimeter heat conduction becoming a limiting factor for large cells. Heat production is proportional to the area (length squared), but the perimeter over which heat must flow is only proportional to length. The ratio of the perimeter to the area of a cell decreases as the size of the cell increases so the heat flow rate is inversely proportional to length. This is well known in the CPV industry where at least 2 companies (Soitec http://www.soitec.com/en/products-and-services/solar-cpv/ and Semprius (http://www.semprius.com/) manufacture CPV modules with small cells. The main drawback to using small cells is the increase in the number of manufacturing operations per unit of module aperture area compared to the traditional approach. For this reason, an automatic manufacturing process becomes necessary for small cells. Fortunately, the microelectronics industry has already developed such equipment (pick-and-place equipment, for instance). Some others, such as Semprius, employ micro-transfer printing (http:/ /www.semprius.com/tech_micro-transfer.htm) that is specific to the CPV industry. The use of a flat glass substrate for these automatic processes is common.
U.S. Pat. No. 8,000,018 (by several of the inventors of the present invention) describes HCPV systems using Fresnel-Köhler (FK) architecture. FK designs typically have a better acceptance angle and produce more uniform illumination onto the solar cell than traditional Fresnel-based optical designs.
In above-mentioned US 2010/0126566 there is taught the concept of “sky splitting”, which is a solar photovoltaic system that can efficiently handle both direct and diffuse solar radiation. As some sunlight is diffused by the earth's atmosphere and clouds, not all of the solar radiation can be focused onto the solar cell. The approach is to have two separate types of solar cells, one for high concentration (such as a triple junction cell) and the other to convert diffuse radiation using low cost, low concentration, solar cells. The ratio of the areas of the two cells and their position relative to each other is designed for maximum performance. Also, a portion of the circumsolar radiation not intercepted by the high concentration cells will be handled by the low cost cells, which are the larger in area of the two. This approach is useful in that it can work in a wide variety of climate types, from locations with a high number of sunshine hours per year to those with more cloudy conditions.
In above-mentioned US patent application 2010/0269885, there is taught the concept of “spectral splitting”. In this approach a fraction of the solar spectrum received from the Sun that would have been received by the multi-junction cell is redirected to a second cell, which is a single junction cell. Typically, the wavelengths chosen to be redirected are those which balance the current in the triple junction cell, while producing a combined efficiency of the system which is higher than the multi junction cell itself. A component of a “spectral splitting” system is a special type of spectrally selective filter, which is typically a multi-layer thin film. Various filter designs are possible such as the “L-shape” filters of that patent application as well as band-pass and minus filters. In some filter designs it is useful to divide the energy of certain wavelengths between the multi-junction and single-junction cell, whereas in others sharp cutoff band-pass or minus filters are required. Or the design can be a combination of these two, which is exemplified by the “L” type filter. Also, that application teaches that the incidence angles on the spectral filter ideally should be less than 35 degrees, the lower the better, to reduce the problem of “angle shift”.
A common problem for most CPV architectures is that they have a deep profile. A thin profile is very desirable because there is less material used to make the modules and they require less space for shipping to the site of the power plant. The smaller profile also makes the modules easier to handle. All of these greatly reduce the cost of manufacturing and installation of the system.
Most HCPV systems need careful alignment between the primary optical element (POE) and the secondary optical element (SOE). It would be desirable to have a simple construction that requires little, or no, special alignment between the front of the module and the rear of the module. This lowers the cost of manufacture.
It is, in general, very desirable to have a small SOE. Nevertheless, when the cell is very small, as in the Semprius system, a small SOE may be prohibitive because it is difficult to manufacture and to attach to the solar cell. This can limit the SOEs available in this case to spheres, which can be manufactured with very low cost procedures, but which may not be the most efficient optically and do not provide the other benefits of an SOE.
A problem of small SOEs is that their cost does not necessarily scale with size. Consider an array of devices, each having a PV cell of area Ac and a geometrical concentration of Cg. Then, each device has an entry aperture area of AcCg, and within one unit of total area there will be 1/(AcCg) concentrator devices. If the cost of one SOE is cSOE, then the total cost of the SOEs contained in one unit of area is cSOE/(AcCg). For small cells, the SOE cost per unit of entry aperture increases dramatically when Ac decreases. This is because the cost of placing the SOE is almost constant with size and the cost of manufacturing a glass SOE stays fairly constant even as the amount of material decreases. To help solve this problem, the SOE could be made of injection molded silicone, which for small optics, would have reduced manufacturing costs compared to glass. But, although injected silicone can have lower costs for small sizes, again its cost does not scale, and the cost of placing it remains the same, so this is still not the best solution for very small cells. It would be desirable to lower the cost associated with placement of small cells and their manufacture.
Russian patent 2,496,181 shows a compound optical architecture with a planar mirror that folds the rays of a Fresnel primary back to a PV cell either directly or through a secondary. Several optical architectures are described, with all of them employing a standard circular symmetric Fresnel lens. In
It would be desirable if there was a compound folded concentrating photovoltaic system that addressed some or all of the above limitations of the prior art, and could be easily configured to add sky splitting and spectral splitting apparatus in the factory or in the field.
Embodiments of the present disclosure provide a module, especially a high concentration photovoltaic solar power module, referred to as a “Cool Cover Fresnel” or CCF. The optical architecture of the CCF is compound, inasmuch as a portion of the rays in the system are folded by a mirror or spectral filter located near the rear of the module and redirected toward the front of the module. This approach reduces the depth of the module by approximately half, compared with a module in which a Fresnel POE at the front of the module focuses the light directly onto an SOE and PV cell at the back of the module. The position of a multi-junction cell is inside the front cover just below a heat spreader, which is attached to the front cover. Surrounding the multi-junction cell is a solid dielectric secondary lens, which in the preferred embodiments has four-fold symmetry to work in conjunction with a four-fold primary Fresnel lens, as taught in U.S. Pat. No. 8,000,018, to achieve Köhler integration of solar radiation onto a front-located multi-junction cell. The mounting position of SOE and multi junction cell components is similar to what is taught in U.S. Ser. No. 12/957,826 and WO 2011/066286, an important difference being that in the present modules, the POE is also a lens attached to the front cover, not a mirror at the rear of the module. The basic construction of the POE is a method known as “silicone-on-glass” (SOG). With this method the Fresnel lens can made of silicone that is constructed by various means onto a sheet of glass. The use of a glass outer surface has advantages, as it protects the other components in areas of high moisture and areas of wind-blown sand. And the SOE and POE are molded at the same time as one piece.
In one embodiment, a high concentration photovoltaic device comprises a Fresnel lens having a front side and a back side, a mirror behind the Fresnel lens and facing the Fresnel lens, a secondary lens unitary with the Fresnel lens and facing the mirror, and a photovoltaic cell in front of the secondary lens and facing the mirror through the secondary lens. Two optical elements of the device form a Köhler integrator between a remote source in front of the Fresnel lens and the photovoltaic cell as a target.
The unitary Fresnel lens and secondary lens may be formed on the back of a cover plate. The cover plate may then be glass, and the unitary Fresnel lens and secondary lens may be of plastic molded onto the cover plate. The photovoltaic cell may then be embedded in the plastic between the secondary lens and the cover plate.
The device may further comprise a heat spreader between the photovoltaic cell and the cover plate, in thermal contact with the photovoltaic cell and the cover plate.
The heat spreader may further comprise arms radiating from the photovoltaic cell, the arms being in contact with a back side of the cover plate along the length of the arms.
6. The device may further comprise a second heat spreader on a front side of the cover plate, the second heat spreader having arms in contact with the back side of the cover plate along the length of the arms, the arms of the second heat spreader being aligned in front of the arms of the first heat spreader and the second heat spreader being separated from the first heat spreader by the cover plate, so that at least some of the heat from the first heat spreader is conducted to the second heat spreader through the cover plate, is conducted radially outwards on the front side of the cover plate by the arms of the second heat spreader, and is returned to the cover plate by the second heat spreader for dissipation into the external environment.
The device may further comprise a third lens in front of the mirror. Any two of the Fresnel lens, the secondary lens, and the third lens may then form the Köhler integrator.
The mirror may be mounted tiltably relative to the Fresnel lens.
The mirror may be a frequency selective partially transmissive mirror, and the device may then further comprise a second photovoltaic cell behind the mirror. The photovoltaic cell in front of the secondary lens may then be a multi-junction photovoltaic cell, and the second photovoltaic cell may then be a single-junction photovoltaic cell.
The frequency selective partially transmissive mirror may be a band-pass mirror comprising a long-pass mirror and a short-pass mirror, which may be formed one on each side of a sheet of glass or other substrate.
The long-pass mirror may be partially transmissive at wavelengths longer than the pass-band of the band-pass mirror, and/or the short-pass mirror may be partially transmissive at wavelengths shorter than the pass-band of the band-pass mirror, and the two mirrors may then be matched so that at least some wavelengths outside the pass-band at which each mirror is partially transmissive are wavelengths at which the other mirror is substantially completely reflective.
The mirror may be smaller in area than the Fresnel lens, and the device may then further comprise an additional photovoltaic cell behind an outer part of the Fresnel lens outside the mirror, operative in use to generate electricity from light incident from directions other than directly in front of the Fresnel lens.
The CCF embodiments taught herein employ Köhler integration while some add additional “spectrum splitting” and/or “sky splitting” functionality. However, other possible optical architectures can used as well and will be evident to those skilled in the art once the principles taught herein are fully understood.
A “Köhler integrator” is a device in which a first optical element images a light source onto a second optical element, and the second optical element images the first optical element onto a target. In one ideal configuration, not usually achievable, the image of the source exactly coincides in shape, size, and position with the second optical element, and the image of the first optical element exactly coincides in shape, size, and position with the target. In a solar photovoltaic module, the “source” is typically either the sun, or a disk defined by the “acceptance angle” of the module, centered on the sun and including an allowance for tracking errors, as discussed above. The “target” is then typically the active entry surface of the actual photovoltaic cell. However, in some embodiments with more complex optics, either the “source” or the “target” may be an intermediate image at which the light is transferred from or to another optical element. Either or both of the optical elements of the Köhler integrator may be, for example, a mirror, a lens, or one refractive surface of a thick lens or other transparent body. The Köhler integrator may include additional “relay” or “intermediate” optical elements between the first and second optical elements that form the actual Köhler integrator.
Because most HCPV systems have a separate POE and SOE, each of these also need separate holding fixtures. In some cases a number of POEs are manufactured into a single panel, and this does help. It is highly beneficial to combine the POE and SOE into a single panel and have the POE and SOE fabricated at the same time, making additional alignment unnecessary. This improves the performance of the system and also reduces manufacturing costs. This is particularly important when the cells are small, because in this case in the prior art approach, the number of concentrators per unit of entry aperture area is high and since all the manufacturing operations are proportional to the number of concentrators, the number of parts becomes a huge problem. By having the secondary and primary lenses manufactured as one piece this mitigates the problem of the prior art.
The SOE in the present devices also facilitates cell electrical and moisture isolation. It is desirable that the SOE completely encapsulate the solar cell and the electrical connection to the solar cell. And when the solar cell is behind a plate of glass, the entire electrical system can then be well protected. This approach is also advantageous because the cell encapsulation can be handled at the same time as the molding of the one-piece Fresnel lens POE and SOE. This has a big advantage over the prior art.
In a first embodiment, which does not employ “spectrum splitting” and “sky splitting”, there is a front cover made of glass and components proximate to it comprising: a heat spreader (and related components as taught in U.S. Ser. No. 12/957,826 and WO 2011/066286), a multi junction cell (and related electrical components) and a four-fold secondary refractive lens molded as one with a four-fold primary lens. To the rear of the module in this embodiment there is a centrally located mirror on a substrate, which folds rays from the primary lens to the secondary refractive lens. The mirror needs to cover only a central area of the substrate, about half the width of the primary lens.
In a second embodiment, sky splitting apparatus are added to the first embodiment by adding PV cells onto the region of the substrate which is not covered by the mirror. In a third embodiment, spectral splitting apparatus are added to the first embodiment by replacing its centrally located mirror with a single-junction PV cell covered by a spectral selective filter. The latter element sends one fraction of the radiation received from the primary lens to the single-junction PV cell below it (by transmission) and the remaining fraction to the refractive secondary lens above it (by reflection). In a fourth embodiment, both sky splitting and spectrum splitting apparatus are added to the first embodiment by replacing its mirror with the sky the splitting apparatus of embodiment 2 and the spectral splitting apparatus of embodiment 3.
In a fifth embodiment, a tracking adjustment function is added to any of the previous embodiments by allowing the mirror to have an adjustable tilt. In a sixth embodiment there is added to the first embodiment, a secondary Fresnel lens in front of the mirror.
An optional spherical glass ball can be molded into a region of the secondary for all the above embodiments to improve transmission compared to an all silicone secondary. This can be achieved using standard molding techniques so that the ball (manufactured in volume) is placed as an insert in the mold in position with respect to the refractive lens and primary features and the silicone will fill the gaps in between the ball and the secondary cavity.
There are many advantages for CCF. What follows is a short list of some of them for embodiments without sky splitting and spectrum splitting. Those familiar in the art will know of other advantages.
1) With the small SOE used in the CCF, the SOE and POE are both manufactured with the SOG process at the same time. By molding them together the SOE costs very little extra. In the prior art the POE and SOE are made separately, increasing the cost of manufacture.
2) As the POE and SOE are molded at the same time they almost perfectly aligned. This overcomes a major issue of the prior art, as it difficult to line up accurately a POE and SOE, which in many cases are not even mounted on the same plane, so they have many degrees more of freedom to be out of alignment (X,Y,Z and rotational).
3) Because the POE and SOE are made of the same material and located on the same surface, the system avoids the mismatch in the coefficients of thermal expansion (CTE) that can affect other systems.
4) The SOE and POE alignment depends only on the parallelism of the glass substrate of the POE and the mirror.
5) The cell and heat spreader can be placed on the glass substrate using a very economical “pick-and-place” method.
6) As the cell is small, the heat spreader can also be small, and still have enough heat transfer to the glass to keep cell temperature within its desirable temperature range. In fact the heat spreader size can in many cases be limited to the projected area of the SOE, so that it does not cause any additional loss of useful light collection area.
7) The invention allows the module to be vertically very compact, about half the height of other HCPV systems with a similar cell size and concentration.
There are also advantages of the basic CCF configuration when used in conjunction with sky splitting and spectrum splitting features of this disclosure. A partial list follows:
1) The spectrally selective reflector is planar and can be designed to operate with air on one side and a solid dielectric on the other or with both sides in air. Those with both sides in air can be easily replaced with new ones in the field. It occupies only about ¼ of the POE area, which implies a cost advantage with respect to systems which use a selective reflector of larger area, and operates only at about 4x concentration, which implies low risk of degradation.
2) The angles of incidence of the ray bundle from the POE onto the spectrally selective reflector are typically all less than or equal to 25 degrees. The angle is determined by the f-number of the Fresnel POE, which is limited by other considerations. The prior art spectrum-splitting HCPV systems of other designs typically have angles of incidence on the spectrally selective element of 35 to 45 degrees or even much higher, which are much more difficult to design and manufacture.
3) A hybrid CCF system with sky and spectral splitting is easy to implement in the present devices, because the lower-concentration photovoltaic cells for both apparatus can reside on the same plane, and are in a sub-assembly separate from the primary and secondary lenses and the high-concentration PV cell. This allows for optimization of systems for a wide variety of climates as different type of cells can be easily swapped for others to match the climate conditions and design goals.
4) The hybrid CCF systems will be able to achieve higher efficiencies compared to prior art designs based on solar concentration systems using only multi junction cells. First, a properly designed CCF with a high efficiency multi-junction cell and added spectrum splitting hardware (cell and spectrally selective reflector) can achieve a higher efficiency than one without this hardware for sunny climates (on the order of a 10% increase). The additional sky splitting functionality will further boost the performance for a wide variety of climate types compared to prior art solar concentrator PV systems, broadening the systems' commercial viability.
Other embodiments also provide the mentioned heat spreaders and/or frequency selective filters independently of the other mentioned novel features, in other forms of photovoltaic concentrator or elsewhere.
The above and other aspects, features and advantages of the present invention will be apparent from the following more particular description thereof, presented in conjunction with the following drawings wherein:
A better understanding of various features and advantages of the present invention will be obtained by reference to the following detailed description and accompanying drawings, which set forth illustrative embodiments in which certain principles of the invention are utilized.
Reference is now made to
The solar cells used for HCPV cannot convert the entire solar spectrum into electricity. With present day “multi-junction” (MJ) cells some of the spectrum is under-utilized. By adding a single junction solar cell that is designed for this unused spectrum, more of the solar spectrum can be converted to electricity.
The CCF architecture is much simpler than some of the earlier approaches taught in the aforementioned inventors' previous applications. The new approach can easily handle both sky splitting and spectrum splitting in the same module, as exemplified by the embodiments of
The rotation movement can be centered near the center of the mirror. The rotation center could also be more than a single point so that both rotation and displacement movements are combined to compensate for off-axis focal length variations. The movement could also be only a displacement parallel to the optical axis of the system that compensates focal length variations of the optical system.
With the conventional FK system, the POE and SOE form the two-element Köhler lens pair. With the addition of the “2nd lens”, the Köhler lens “pair” can be any combination of the three lenses. In
In
Mirror 911 can either be a one or two-sided dichroic mirror. In
The method of designing the longpass filter of Table 1 can be summarized as follows. You start with the following seed formula: 0.73 (.75H.5L.75H)̂8 0.85(.75H.5L.75H)̂8 1.0(.75H.5L.75H)̂8 1.18(.75H.5L.75H)̂8 1.30(.75H.5L.75H)̂8, where H represents a quarter wave thickness of the high index material, in this case Tantalum Pentoxide, and the L represents a quarter wave thickness of the low index material, in this case Silicon Dioxide. The convention is that the stack is defined as from the medium (air) to the substrate (BK7 glass). The constants in the seed formula, 0.73, 0.85, 1.0, 1.18 and 1.30 can be modified as needed as can the number of terms of the (.75H.5L.75H)̂8. For example, the term with the constant 0.73 creates a high reflectance region centered at approximately 425 nm with a width of 100 nm and region of high transmittance at longer wavelengths. The next term with the constant 0.85 adds a reflectance zone centered at approximately 525 nm with a 100 nm width and region of high transmittance at longer wavelengths but with ripples going from approximately 50 to 90% transmittance below 475 nm, which is in reflectance zone relating to the 0.73 term. This lower rippled zone reinforces the reflection of the 0.73 term stack. By adjusting the constants for a number of (.75H.5L.75)̂8 terms an excellent starting long pass filter can be designed. Then one must set up the desired targets and apply optimization to reach the final design.
The targets are based on the desired 100% transmission zone, which in this case is 964 nm to 1028 nm, and the shorter wavelength region, where a 100% reflectance is desired, which in this case is 350 nm to 962 nm. Note that the targets are in 2 nm increments going from 350 nm to 1028 nm. No targets are set above 1028 nm, allowing the zone above 1028 nm to 1800 nm to have transmission ripples with spikes and troughs, which is this case may be desirable, as will be explained below. A target of 100% reflectance with a tolerance of 0.05 is set for the shorter wave band and a target of 100% transmission with a tolerance of 0.05 is set for the transmission band. The reference angle is set to 642 nm and the angle of incidence for all wavelengths is set to the mean wavelength of the bundle of rays striking the two-sided filter, which in this case is 12.5°. Also setting minimum and maximum thickness for each element in the stack is useful to make sure the stack is manufacturable. For the design in Table 2 a minimum of 20 nm and maximum of 200 nm for all layers in the stack. Optimization using standard Simplex or Conjugate Gradient or others known in the prior art arrive readily to the solution.
The method of designing the shortpass filter uses the more standard starting seed formula of (LH)̂27L, where H and L are the same two materials in the longpass stack. In this case the zone of 100% transmission is set substantially the same as the longpass filter, while the 100% reflectance zone is set to start a few nm above the end of the transmission zone and end at the longest wavelength of the design, in this case 1800 nm. The tolerance settings for the transmission and reflectance zones are 0.1. And the angle of incidence for all the targets is chosen to be the median of the bundle of rays on the filter, which as before is 12.5° . In this case the lower reflectance band starting from 350 nm is allowed to float. The optimization approaches of refinement and synthesis can be used to closely meet the target goals. In this case for the design of Table 2 the approach used was the Optimac algorithm in the software Essential Macleod by The Thin Film Center, Inc of Arizona, USA.
After the long pass and short pass designs are completed, the two can be modeled as a complete two-sided filter on a substrate. The stacks can be further refined using optimization techniques with the targets now including the full range of wavelengths, which in this case are from 350 nm to 1800 nm. Typically, this is not required. However, another approach can be used which works quite well and is very easy to implement. The approach is to make small adjustments in the reference angle so that either the shortpass or longpass filters are either moved to the left on the transmission plot (toward the shorter wavelengths) or to the right (toward the longer wavelengths). If the transmission zones for the shortpass and longpass filters are a little wider than is required, this allows for adjustment of the two positions of the curves using the reference angle. And also it allows the designer to pick the zones of desired reflectance such that undesirable spikes in one of the filters in the reflectance zone lines up with a trough in the other in the same wavelength region. This works very well for the short wavelength region of the longpass filter where the spikes are very narrow in width but not so well for the longer wavelength. Still, even in the longer wavelength region there is a reflectance boost resulting from the multiplicative effect of having two filters.
The performance of the two-sided filter is shown in Transmittance Plot 1100 with x-axis 1103 for the wavelength range of 300 nm to 1800 nm and y-axis 1102 for transmittance in percent from 0 to 100%. There are 3 plot lines in Plot 1100: solid line 1104 representing the transmittance of the band-pass filter at 0° incidence angle, dashed line 1105 representing the transmittance of the band-pass filter at 12.5° and short long line 1106 representing the transmittance of the band-pass filter at 25°. The overall bandwidth of the high transmission range of the two-sided filter of Table 1 and 2 is around 100 nm, with the high reflectance bandwidth going from 350 nm to 920 and 1050 to 1800 nm. The filter exhibits very little incidence angle shift as can be seen in the lateral displacement going from incidence angles of 0 to 25°. The angle shift is low because the incidence angles are kept low on the filter but it is also a consequence of the design algorithm used, which is partly based on teachings in U.S. Pat. No. 7,859,754“Wideband dichroic-filter design for LED-phosphor beam-combining”.
In all of the above described configurations, if a large cover glass is used, and especially because the module can be thin, a dense honeycomb structure can be used between the glass and the mirror to provide stiffness.
Depending on the use, many of the configurations described above, especially when “sky splitting” or “rotating mirrors” is employed, have “free areas” surrounding the mirrors and within the enclosure. These free areas can be used for other purposes. A couple of examples are listed below.
The free area could be used to change the look of the CPV module. Currently, almost all CPV modules look grey. This limitation could be overcome by painting the mirrored part of the substrate which is not optically active nor has any function other than the enclosure.
The free area could be used to display an advertising logo. When the size of each concentrator unit is small, an image of the cell, with a size much bigger than a single unit aperture, can be seen when looking at the concentrator normal to the aperture (at a distance greater than a few meters). The image seen is a combination of the individual cells' images created by each concentrator unit. The angular size of this image is constant (and equal to the concentrator acceptance angle). In particular, it does not depend on the distance at which one looks at the module. This is why, the cell image occupies more and more concentrator units when we increase this distance. This effect can be used to create logos or advertisements whose size is adapted to the observer distance. The particular configuration of the CCF allows creating these images for the solid angle occupied by common observers during normal operation of the CPV array. These images are created from features printed on the free area of the substrate supporting the mirror. Additionally, we can use other effects such as the Moire Effect.
There are some disadvantages in the CCF design; however; as will be shown below, these are minimal and can be overcome by novel solutions.
The heat spreader and MJ cell block part of the incoming radiation. For an FK concentrator with Cg=1024×, and with an acceptance angle of ±1.1 deg, only 1.8% of the aperture area is blocked. This is not a significant amount and is not a major drawback.
The mirror is not a perfect reflector and some energy will be lost. All HCPV systems have optical losses. Inexpensive mirrors with efficiencies above 96% for the spectrum of interest are available. This includes conventional 2nd surface flat mirror on glass, to high reflectance solar reflective films (http:/ /solutions.3m.com/wps/portal/3M/en_US /Renewable/Energy/Product/Films/Solar_Mirror/.
A solution is to use total internal reflectors made of V grooves. In the case of FK architectures, the V grooves should be in the radial symmetry with respect to the symmetry axis of each one of the POE quadrants. The principle is taught in US Publication 2010-0002320-A1 by several of the same inventors.
For “spectrum splitting”, a dichroic or other frequency selective mirror is required and these can be expensive, especially if a custom design is needed. All-polymeric solutions are available, such as 3M Cool Mirror film, and one of these could be a good fit. 3M, and others, could also adapt an inexpensive design to fit the requirements of the new systems.
The heat load that can be adequately dissipated by the cover glass in the CFSC design is low so this design works most effectively with small solar cells. This can be seen as a disadvantage, but the combined advantages of the system have distinct advantages in many applications.
Based on the above analysis the best architecture for the CCF of the six in
The mirror 113, 203, etc., and other structures associated with the mirror are omitted from
The top heat spreader 1301 is not provided with any metallic connection through the cover plate 1303. The cover plate 1303 is uninterrupted, in the interests of mechanical integrity and weather-tightness. Surprisingly, enough heat can be conducted from the lower or back heat spreader 1302 through the glass to the top or front heat spreader 1301 for the top heat spreader to be useful. The top heat spreader can conduct the heat that it receives from the bottom heat spreader 1302 radially outwards, and can either dissipate that heat directly to the ambient environment by radiation or by conduction/convection into the atmosphere, or can return the heat to the outer surface of the glass cover plate 1303 for similar dissipation. This arrangement is valuable in some embodiments, where the thickness of the lower heat spreader 1302 (and therefore its ability to conduct heat) is limited because it is desirable to embed the lower heat spreader 1302 completely in the silicone molding of the primary lens 109a, etc., and it is desirable to keep the primary lens 109a, etc. thin, because silicone is both expensive and not perfectly transparent.
In embodiments (see
As illustrated in
The top heat spreader 1301 is not involved in the electrical circuitry, because it is isolated by the glass cover 1303, but may be identical to the bottom heat spreader 1302, so that only one silk-screening mask is needed. Because the heat is transferred vertically through the glass from the bottom heat spreader 1302, the isolating gap between the sections corresponding to the gap between sections 1302A and 1302B does not significantly detract from the performance of the heat spreader.
It will be appreciated that a heat spreader on only one surface of the glass plate 1303 may be used. However, because the width of the arms of the heat spreader may be limited, in order to avoid blocking too much of the incoming sunlight, that may require a thicker heat spreader to provide sufficient heat conduction. As noted above, there are advantages to a thin heat spreader. In particular, if a thick heat spreader, more similar to those in our earlier WO 2011/066286, is used on the underside of the glass, care may be needed to ensure that the optic is molded without distortions or bubbles.
The embodiments have been shown in the drawings with the direction from which incident light is expected to arrive upwards, and that direction has been variously referred to as “up” and “front.” These and other expressions of orientation or direction are not limiting. The HCPV devices, when used as solar concentrators, will preferably be oriented with that direction towards the sun, which depends on geographical location and time of day and year. When used for other purposes, the devices may be in other orientations. When not in use, the devices may be parked, stored, and shipped in any convenient orientation.
Various embodiments have been described, and various ways in which features of different embodiments may be combined have been mentioned. However, the skilled reader will see how other features of the described embodiments may be combined, and other ways in which the embodiments may be modified.
The preceding description of the presently contemplated best mode of practicing the invention is therefore not to be taken in a limiting sense, but is made merely for the purpose of describing the general principles of the invention. The full scope of the invention should be determined with reference to the Claims.
Priority is hereby claimed to U.S. Provisional Patent Application No. 61/997,782 filed Jun, 9, 2014, entitled Compound Fresnel Solar Concentrator, which is incorporated by reference herein in its entirety. This patent application references the following earlier U.S. patents and applications which are incorporated herein in their entirety: U.S. Pat. No. 8,000,018 issued Aug. 16, 2011 to Benitez et al for “Köhler concentrator” and related US Publication 2010/0123954 A1; U.S. patent application Ser. No. 12/957,826 filed on Nov. 23, 2010 by Miñano et al for “On-Window Solar-Cell Heat Spreader” and related PCT Publication WO 2011/066286 A2; U.S. patent application Ser. No. 12/622,664 filed on Nov. 20, 2009 by Benitez et al for “Photovoltaic Concentrator with Auxiliary Cells Collecting Diffuse Radiation” and related US Publication 2010/0126556 A1; U.S. patent application Ser. No. 12/766,298 filed on Apr. 23, 2010 by Benitez et al for “Photovoltaic Device” and related US Publication 2010/0269885; U.S. Pat. No. 8,094,393 issued Jan. 10, 2012 to Miñano and Benitez for “Reflectors Made of Linear Grooves” and related US Publication 2010/0002320 A1.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2015/034905 | 6/9/2015 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 61997782 | Jun 2014 | US |
