WO2016037156A1

WO2016037156A1 - System and device for capturing solar energy in windows

Info

Publication number: WO2016037156A1
Application number: PCT/US2015/048733
Authority: WO
Inventors: Monica COOK; Said Al-Hallaj
Original assignee: Sun Phocus Technologies, Inc.
Priority date: 2014-09-04
Filing date: 2015-09-04
Publication date: 2016-03-10

Abstract

A system and window wherein the system is implemented is provided for capturing solar light in photovoltaic cells in a window using optical elements. The solar energy window including a double paned window with a first and a second pane, the first pane having a first surface and a second surface, a plurality of bifacial photovoltaic ("TV") cells on the first surface of the first pane, the PV cells are on a bottom third of the first pane, and the PV cells have a backside for absorbing light rays. Further, a plurality of optical elements are included on the second surface of the first pane, the optical elements are on the top two thirds of the first pane, and the optical elements are in the shape of microfacets configured to allow incident light rays to experience total internal reflection ("TIR").

Description

A SYSTEM AND A DEVICE FOR CAPTURING SOLAR ENERGY IN WINDOWS

Field of the Invention

The present invention relates to a system and a device for capturing solar energy in the window. More specifically, the present invention relates to a more efficient system for capturing solar energy and concentrating the same onto photovoltaic ("PV") cells in windows, and a window of implementing the same.

Background

As the impacts of global warming become apparent, the demand for reliable renewable energy sources will increase. Solar energy is a proven technology that has been available for decades and will undoubtedly be a part of the energy portfolio going forward. Further, solar technologies offer the opportunity to produce energy in a distributed manner that is close to the point of use. This has implications for national security as well as energy efficiency. While urban areas are high demand areas, the opportunity to deploy photovoltaic technology in traditional installations is limited. The rooftop surface area is relatively small and often houses the building mechanicals or architectural elements. Additionally, there are seldom large areas of open land in urban areas sufficient to install a useful number of photovoltaic modules.

In addition to a large surface area, the installation site must be free even of minimal shadows. Because solar materials are wired in series, a small shadow will affect the current flow of the entire unit. Because the rooftops of buildings house the mechanical or architectural elements, the area available for a shadow-free installation is minimal.

A typical solar installation includes significant costs for items that are ancillary to the performance of the cells themselves. By incorporating the solar materials into the building materials themselves, a significant cost savings can be expected as compared to a traditional installation. Therefore, photovoltaics have been integrated into the building materials common in urban architecture. Building integration also has the added benefit of sharing the system costs with the expenditure for the building materials themselves. However, it remains that an installation including solar technology which is integrated into the building materials that is effective and efficient is needed. There remain many problems in the effectiveness and efficiency of the solar cell itself. Many variables play into the effectiveness of the solar cells including structure, configuration, and location, among others.

In addition, the performance of the systems as a whole is effected by various additional parameters not only including the electrical specification of the photovoltaic material itself, but also the location of the building site along with the optical

characteristics, the primary utility supply of the area, as well as the utility cost of can have a significant impact in the overall cost of the system.

For example, a window-based photovoltaic installation will likely be installed at a non-ideal angle. This is a complication, because solar materials perform ideally when they are perpendicular to the rays of the sun. The daily and seasonal changes in the position of the sun provide a moving target. To compensate for this, sophisticated PV systems are often mounted on brackets that mechanically move every day to align the solar cells with the sun. Less complex photovoltaic systems, such as those mounted to a roof-top, are often tilted toward the sun. Most commonly, the system is aligned toward the south with a tilt that is equal to the latitude of the installation site. In the case of a window system, however, there will be few times when the window's orientation will be anything other than perpendicular to the ground. For example, the latitude of Chicago, IL is 41 .88^°N, so a typical window-based solar system would be 48.12^° off of the alignment that would be most useful for the photovoltaic materials.

Therefore, it is necessary to provide a system and device that effectively captures solar energy but is universally effective no matter the time of year used.

Summary of the Invention

The system and device for the capture of solar energy in windows provides a solar energy window having a double paned window with a first and a second pane wherein the first pane has a first surface and a second surface; a plurality of bifacial photovoltaic ("PV") cells on the first surface of the first pane, wherein the PV cells are on a bottom third of the first pane, and wherein the PV cells have a backside for absorbing light rays; and a plurality of optical elements on the second surface of the first pane, and wherein the optical elements are on the top two thirds of the first pane, and wherein the optical elements are in the shape of microfacets configured to allow incident light rays to experience total internal reflection ("TIR").

The system and device, according to the principles of the present invention, is related to a window having a first external surface, an internal second surface, and photovoltaic ("PV") material placed on the inner second surface to capture solar energy.

The system and device according to the principles of the present invention is also related to a system and device wherein placement of the non-transparent PV cells on the second surface provides that the cells are protected from the external environment as well from any possible tampering from the building interior.

The system and device according to the present invention is further related to a system and device having a plurality of optical elements interspersed between the PV cells, are preferably in a geometric shape, are located in the high priority viewing/glass area of the window, generally the top two thirds of the window, to promote total internal reflection ("TIR"), not possible with a window using smooth glass on the second surface.

The system and device according to the present invention is even related to a system and device further having a reflective element to angle the solar energy onto the PV cell(s).

There has thus been outlined, rather broadly, the more important features of the invention in order that the detailed description thereof that follows may be better understood, and in order that the present contribution of the art may be better

appreciated.

Numerous objects, features and advantages of the present invention will be readily apparent to those of ordinary skill in the art upon reading of the following detailed description of presently preferred, but nonetheless illustrative, embodiments of the present invention when taken in conjunction with the accompanying drawings. In this respect, before explaining the current embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced and carried out in various ways. Also, it is to be understood that the

phraseology and terminology employed herein are for the purpose of description and should not be regarded as limiting.

As such, those skilled in the art will appreciate that the conception, upon which this disclosure is based, may readily be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention.

Brief Description of the Drawings

Figure 1 illustrates a front view of an embodiment of the device in accordance with the present invention;

Figure 2 illustrates a cross sectional view of an embodiment of the device in accordance with the principles of the present invention;

Figure 3 illustrates a cross sectional view of an embodiment of the device in context in accordance with the principles of the present invention;

Figure 4 illustrates an exploded view of A of the embodiment of the device illustrated in Figure 3 in accordance with the principles of the present invention;

Figure 5 illustrates a comparison between an ordinary window and an embodiment of the device in accordance with the principles of the present invention;

Figure 6 illustrates an embodiment of the device in accordance with the principles of the present invention;

Figure 7 illustrates a schematic of the affect of facet angle on internal reflection of an embodiment of the system in accordance with the principles of the present invention; Figure 8 illustrates an embodiment showing the effect of a 25.4° facet of an optical element of the system on internal reflection of the solar energy in accordance with the principles of the present invention;

Figure 9 illustrates various facet design embodiments of the system and device in accordance with the principles of the present invention;

Figure 10 illustrates an embodiment of a reflective element in the system and device in accordance with the principles of the present invention; and

Figure 1 1 illustrates the geometry of distance travelled in an embodiment of the system and device in accordance with the principles of the present invention.

Detailed Description of the Drawings

The following detailed embodiments presented herein are for illustrative purposes. That is, these detailed embodiments are intended to be exemplary of the present invention for the purposes of providing and aiding a person skilled in the pertinent art to readily understand how to make and use of the present invention.

Accordingly, the detailed discussion herein of one or more embodiments is not intended, nor is to be construed, to limit the metes and bounds of the patent protection afforded the present invention, in which the scope of patent protection is intended to be defined by the claims and their equivalents thereof. Therefore, embodiments not specifically addressed herein, such as adaptations, variations, modifications, and equivalent arrangements, should be and are considered to be implicitly disclosed by the illustrative embodiments and claims described herein and therefore fall within the scope of the present invention.

Further, it should be understood that, although steps of various the claimed method may be shown and described as being in a sequence or temporal order, the steps of any such method are not limited to being carried out in any particular sequence or order, absent an indication otherwise. That is, the claimed method steps are to be considered to be capable of being carried out in any sequential combination or permutation order while still falling within the scope of the present invention. Additionally, it is important to note that each term used herein refers to that which a person skilled in the relevant art would understand such term to mean based on the contextual use of such term herein. To the extent that the meaning of a term used herein, as understood by the person skilled in the relevant art based on the contextual use of such term, differs in any way from any particular dictionary definition of such term, it is intended that the meaning of the term as understood by the person skilled in the relevant art should prevail.

Furthermore, a person skilled in the art of reading claimed inventions should understand that "a" and "an" each generally denotes "at least one," but does not exclude a plurality unless the contextual use dictates otherwise. And that the term "or" denotes "at least one of the items," but does not exclude a plurality of items of the list.

Figure 1 illustrates an embodiment of the device which is a front cross-sectional view of a double-glazed, commercial window 2. For ease of viewing, the window framing 4 has been simplified. The window 2 has been divided into two sections, a first section of a glass area 12 and a second section of a photovoltaic area 14.

Taking a normal window into consideration, at no time will a window using smooth glass on an inside surface of the window experience total internal reflection ("TIR"). In Figure 2 is illustrated the photovoltaic system 10 which more effectively utilizes photovoltaic cells 14 for window applications, by providing bifacial photovoltaic cells ("BFPV") 14 installed into a window 2 along with optical elements 16, thereby providing a system 10 for concentrating solar energy onto the back surface of the cells 14 and inducing total internal reflection. The bifacial nature of the cells 14 improves the overall output of the photovoltaic materials. The backside is illuminated by optical elements 16 in the form of micro-facets which are also incorporated into the window structure, which allow the incident light to experience TIR through the various seasonal incident angles of the sun. The geometry of the micro-facet will also allow TIR to occur at each subsequent surface of the glass, which will allow the window glass to function as a wave-guide, thus redirecting solar energy to the back side of the cell 14. In order to allow the window 2 to function as a window 2, it is important to keep the non-transparent cells 14 out of the high priority areas of the window 2. According to window manufacturer's, the bottom third of a window 2 is of relatively low value from the view-glass perspective. Often, office furniture is placed so as to obscure this portion of the window 2. Even if furniture is not positioned in front of this section of the window 2, inhabitants of the office are seldom aligned to see out of this portion of the window 2, even while seated.

In this embodiment, the solar technology is incorporated into approximately the lower third of the window 2, in a low priority viewing/glass area.

Using solar windows, such as those illustrated in Figures 1 and 2, for installations removes the need for bracketry as the solar technology of the system will be supported by the window itself, therefore reducing the cost by at least that much. In addition, some of the glass that is necessary in the fabrication of the solar module, will be provided by the window glass, allowing further savings. In addition, using windows as in Figures 1 and 2 provide for a large shadow-free surface area in, for example, the fagade of a high- rise building as compared to that of a rooftop.

Figure 3 illustrates photovoltaic ("PV") cell placement on an embodiment of the device in a cross sectional view. The PV cells 14 are shown on an inside surface 18 of the glass window structure. This surface 18 is encapsulated and completely protected by the outside environmental elements. This is useful, as it will keep the cells 14 protected from the external environment, as well as inhabitants of the office space.

To allow the glass to collect and deliver solar energy to the back side of the embedded PV cells 14, geometrical optical elements 16 are interspersed between the PV cells 14. The optical elements 16 are micro-facets that alter the incident angle upon which the sun's energy is incident.

Figure 4 illustrates the expanded region of the device in Figure 3 designated A, wherein the strategically positioned facet of the optical elements 16 will promote TIR, trapping the light energy in the glass until it encounters the back side surface 22 of the PV cells 14. For instance, the seasonal and maximum angles of direct solar radiation may be 24.5^° and 71 .5^°, respectively (using Chicago Illinois, U.S.A. by way of example). Additionally, using the refractive indices for air (N1 = 1 .0000) and optical glass (N2 = 1 .5185) Snell's law can be applied to calculate the angle of the light energy incident on surface 2 of the structure. For example,

Winter Solstice: Θ2 = arcsin 15.8°

Summer Solstice: Θ2 = arcsi = 38.6°

Further, the critical angle (0c) for the system can be calculated, above which total internal reflection (TIR) will occur. Note that in this application of Snell's law, the second medium of the boundary is air, not glass as above. Therefore, the following calculation, N1 = 1 .5185 for the glass and N2 = 1 .0000

9c = arcsin

Nl rl.OOOOl

9c = arcsin

Ll.5185.

9c = 41.19°

At no time will a window using smooth glass on Surface 2 experience TIR. Even at the steepest incident angle of 38.6^°, the solar energy is well below the critical angle of 41 .19^°. Because of this, the refracted rays that enter the glass will simply pass through the window and enter the office space. However, knowing the range of incident solar angles on Surface 2, along with the critical angle for the glass allows a facet design that will promote TIR.

See for example Figure 5, illustrating an embodiment of the system 10 of the invention illustrating ray trace 36 entering smooth glass 38 on the left and ray trace 36 entering faceted glass 40 on the right. A simple ray fan can be directed at each of the surfaces 38 and 40. The width of the ray trace 36 is drawn to approximately the steepest and most shallow incident angle. As illustrated in Figure 5, the smooth surface 38 does not allow the TIR to occur, as expected based on Snell's law, and regardless of incident angle, therefore allowing the light energy to escape to the other side of the glass. The faceted surface 40 on the other hand, does allow TIR to occur, allowing the rays 36 to travel the vertical length of the glass 40. By using these facets in an optical element 16, incident solar energy can be directed to the back surface of the PV cell 14. The angle of the rays in this example are in the plane of the surface, more illustrative of the noon sun.

Figure 6 illustrates a broader example of the device demonstrating the travel of sunrays 36 through the system 10 during other than peak noon hours. This example utilizes approximately 100 conical rays 36 and to more accurately reflect incident angles of the sun, and therefore be more representative of the directionality of solar radiation. Also included is a receiver 42 to confirm the number of rays 36 which participated in TIR. Out of the 100 rays 36, 25 rays are shown to be incident to the bottom receiver 42.

Therefore, even for off-plane incident rays, approximately at least 25% of the simulated rays will TIR because of the prismatic elements of the optical elements 16 of the device and system.

Figure 7 illustrates the concept for the facet dimensions. In this example, Θ1 is incident upon the window normal at a relatively shallow angle. This angle is less than the critical angle of the glass, so TIR would not occur. However, if a facet is included that tilts the incident surface relative to the incident beam, the new incident angle is Θ1 '. Proper facet design allows Θ1 ' to exceed the critical angle, thus causing TIR in the system.

For the purposes of this system, in an embodiment, the tilt angle of the facet is calculated to promote TIR at 15.8^°; the shallowest of solar angles incident upon Surface 2 of the window. Given that the critical angle for the system in this example is 41 .18^°, a requisite tilt angle is calculated as follows:

41.2° - 15.8° = 25.4°

An angle of 41 .2 is used because for TIR to occur, the incident angle must exceed the critical angle. Additionally, the tilt of 25.4 is measured clockwise from the surface of a typical window i.e. a window installed perpendicular to the ground. In Figure 8 is illustrated another example of a system wherein angles are produced which are the result of a shallowest (15.8^°) and steep (38.6^°) direct solar angles. These angles are merely illustrative, are not intended to be binding, and are not drawn to scale.

A facet angle of 25.4^° will allow TIR off of its surface for both the shallowest and steepest incident angles. This is noteworthy, because it will allow TIR from this surface for the entire range of incident direct solar radiation. In addition, the angle of the light that is reflected back to Surface 1 will exceed the critical angle of the system. For the shallowest and steepest incident angles, the incident angle that is directed back to surface one is 66.6^° and 89.4^°, respectively. Recalling that 9c for the system is 41 .19^°, the light will TIR again at the boundary of Surface 1 .

This concept is illustrated in Figure 9. Some embodiments of facet designs are illustrated in Figure 9, including the original fact 24, a reduced height facet 26, an obtuse surface angle 28, and an acute surface angle 30. The appropriate facet design for each application will be easily calculated by those skilled in the art by using any software known in the art, for example ray tracing software. The goal of the

optimization will be to properly treat the various incident angles that will be encountered from the rays which have experienced TIR on previous surfaces.

Also included in the system 10 is an option to change the dimensions of the optical element 16 in order to accommodate different angles of the sun over a year and thereby optimize the efficiency of the system 10.

A further optical element 16 in the form of a reflective element 32 is considered for the bottom surface 34 of the glass. It can be seen in Figure 9 that the light rays 36 reflect off the facets of the reflective element 32 at very steep angles. For instance, at an incident angle of 38.6^°, it can be seen that the reflected ray 36 is incident on the back side of Surface 1 at an 89.4^°. It can be expected that much of the light that becomes trapped in the pain of the glass will begin to travel tangentially through the glass itself, rendering it unusable to the PV cells 14 positioned on the second surface. With this in mind, a reflective element 32 will be included to redirect light rays 36 traveling in this orientation back onto the solar materials. Figure 10 is an illustration of this concept. The exact shape of this element can also be determined using any software commonly used in the art, for example ray tracing software.

The window in which the system 10 of the present invention is optimized traps rays of light 36 in the glass of a window 2, thus allowing the ray 36 to travel through the glass to be concentrated onto the surface of a PV cell 14. An important consideration, therefore, is the loss of the light energy as it makes its way through the system 10 until it is incident onto the PV cells 14. An estimation of such loss can be made utilizing the property of internal transmissivity. This internal transmissivity of a glass is thickness dependent. Typically the transmissivity τ1 is reported for a known thickness of glass d1 . To calculate the transmissivity at a new thickness, d2, the following equation is applied:

As an example, common optical glass is considered for which the transmissivity T1 is reported as 0.9992 at a thickness of d1 = 10mm. Applying the above formula, the transmissivity at various thickness (d2) can be calculated, which is equivalent to the total distance traveled. The results are summarized in the table below.

60 2.36 99.24%

70 2.76 99.1 1 %

80 3.15 98.99%

90 3.54 98.86%

100 3.94 98.74%

Tab e 1 - Transmissivity as a Function of Thickness

In this table is provided an example only and this invention is not intended to be limited thereto. In this example Table 1 is shown that light can travel a distance of 3.94 ft, and still have 98.74% of the light energy that was initially transmitted into the bulk material of the glass.

It is important to keep in mind that d2 is the total distance traveled, which is not the same as the height of the window. During total internal reflection, the light energy bounces from the front to the back surface of the glass. Therefore, the actual distance traveled must be calculated using geometry.

Figure 1 1 illustrates the comparison between the actual distance travelled 44 through the thickness of the window pane 50 and vertically (d2'), versus the vertical distance 48 (d2). A common thickness for commercial window glass is t = 4mm. Also, the longest distance that the light will travel to reach the PV cells 14 occurs when Θ is as at its minimum. At larger angles, the light 36 will bounce fewer times, thus yielding a shorter total distance traveled. Therefore, the maximum distance traveled by a ray of light 36 will occur when Θ = θα Because the resulting system forms a right triangle, following geometric the relation is helpful: tan (0 c) =—

By rearranging, d2' can be calculated, and the actual distance as follows: dl = t * tan (0c) dl = 3mm * tan(41.18°) d2 = 2.62mm

Similarly, calculate the actual distance traveled, d2', can be calculated using the following geometric relation: t

cos(0c) =

Solving for d2' yields the following: cos (0c) 3mm

^~ cos (41.18°) d2' = 3.98mm

The above example illustrates that whilst taking the longest possible path through the window, the light will actually travel 3.98 mm for every 2.62 mm of vertical distance. Normalized, this means that for every 1 unit of vertical distance traveled, the actual path length of the light energy will be 1 .52.

This high transmissivity, coupled with the fact that TIR is a lossless phenomenon, shows that much of the light energy that travels through the glass will be available to the PV cells 14 installed at the bottom of the window, and more specifically to the back side of the BFSM such as the PV cells 14 that are strategically interspersed between optical elements 16.

As to the manner of usage and operation of the present invention, the same should be apparent from the above description. Accordingly, no further discussion relating to the manner of usage and operation will be provided. While a preferred embodiment of the system has been described in detail, it should be apparent that modifications and variations thereto are possible, all of which fall within the true spirit and scope of the invention. With respect to the above

description then, it is to be realized that the optimum dimensional relationships for the parts of the invention, to include variations in size, materials, shape, form, function and manner of operation, assembly and use, are deemed readily apparent and obvious to one skilled in the art, and all equivalent relationships to those illustrated in the drawings and described in the specification are intended to be encompassed by the present invention.

Throughout this specification, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising" or the term "includes" or variations, thereof, or the term "having" or variations, thereof will be understood to imply the inclusion of a stated element or integer or group of elements or integers but not the exclusion of any other element or integer or group of elements or integers. In this regard, in construing the claim scope, an embodiment where one or more features is added to any of the claims is to be regarded as within the scope of the invention given that the essential features of the invention as claimed are included in such an embodiment.

Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. It is to be understood that the invention includes all such variations and modification which fall within its spirit and scope. The invention also includes all of the steps, features, compositions and compounds referred to or indicated in this specification, individually or collectively, and any and all combinations of any two or more of said steps or features.

Therefore, the foregoing is considered as illustrative only of the principles of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation shown and described, and accordingly, all suitable modifications and

equivalents may be resorted to, falling within the scope of the invention.

Claims

We claim:

1 . A solar energy window comprising:

- a double paned window with a first and a second pane, wherein the first pane

has a first surface and a second surface;

- a plurality of bifacial photovoltaic ("PV") cells on the first surface of the first pane, wherein the PV cells are on a bottom third of the first pane, and wherein the PV cells have a backside for absorbing light rays;

- a plurality of optical elements on the second surface of the first pane, and

wherein the optical elements are on the top two thirds of the first pane, and wherein the optical elements are in the shape of microfacets configured to allow incident light rays to experience total internal reflection ("TIR").

2. The window as in claim 1 wherein the optical elements are removeably affixed to the first pane.

3. The window as in claim 1 wherein the microfacets have an angle of 25.4 degrees.