The present invention relates to a low ratio solar concentrating device and system and, more particularly to a device and system in which a concentration ratio varies as a function of solar declination angle. Further, the invention relates to a method of design and manufacture of such device and system.
Photovoltaic (PV) panels are becoming increasingly important as a source of renewable energy and low carbon energy generation. There is a trend to incorporation of PV panels with building structures, so called Building Integrated PV (BIPV). In particular there is a desire to incorporate PV panels within windows so that a window could perform the function of a window and at the same time generate energy. A problem with this approach is that there is a trade-off between the requirement for light to pass through the window and at the same time generate electricity. Nevertheless, panels of this type already exist. Problems with such panels include they generally have a low transmittance and they create visible artifacts that affect the primary performance as a window. The reason for the low transmittance is usually that PV windows are made by making strips of PV cell with gaps between, and in order to maximize the power generated the ratio of PV cell area to gap area is made small.
What a smart PV window needs is a variable concentration ratio so that the light can be guided more to the solar cell when it is needed, i.e., around noon when the sun is high and irradiance is high, and allow light to pass through the window at other times. As used herein, the concentration ratio is defined as the ratio of light absorbed by PV cells (also known as solar cells) to light passing through the concentrator. Some prior art exists that attempts to solve this problem by making the power generation vary as a function of the solar declination angle (incident angle of the sun on the window). Whilst the solutions described below achieve this function, they only partially meet the requirements. They, however, also have the problem of being difficult and expensive to manufacture due to the design features that make the ideas unpractical.
US patent 2009/0255568 A1 (Morgan Solar Inc., Oct. 15, 2009) explains a system in which a plurality of solar cells are fabricated on ridged surfaces of the substrate such that the light impinging a PV window at the pre-determined viewing angle is directed and concentrated to solar cells with an incident angle dependent concentration ratio. The issues associated with this system include difficulties of solar fabrication on the ridged surface, and poor see-through quality when used for window type applications.
US patent 2008/0257403 (R. Edmonds, Oct. 23, 2008) suggests an idea to fabricate solar cell strips incorporated within the body of a window glass such that the active area of the solar cell is nearly perpendicular to the glass surface. This design does provide an incident angle dependent performance. It does not, however, concentrate the light. It is also difficult to fabricate the solar cell within the substrate.
A first aspect of the invention present invention provides an incident angle dependent transparent solar concentrator including: at plurality of PV cells arranged on a substrate; a plurality of light redirecting elements (e.g., slits) in a substrate wherein the refractive index of a substance in the light redirecting elements is lower than that of the substrate, the plurality of light redirecting elements aligned with the plurality of PV cells.
When light is incident normal to the substrate containing the light redirecting elements some of the light is absorbed by the PV cells and other light passes through the substrate. When light is incident non-normal to the substrate then some of the light that would have passed through the structure is totally internally reflected (TIR) by the plurality of light redirecting elements and is absorbed by the plurality of PV cells. In this way, proportionally more light is absorbed by the plurality of PV cells as the incident angle increases, up to a maximum value determined by the physical parameters of the device.
The plurality of light redirecting elements may be arranged such that they do not penetrate completely through the substrate in which they exist.
The plurality of light redirecting elements may be arranged such that there is one light redirecting element aligned with one PV cell.
The plurality of light redirecting elements may be fabricated in the same substrate on which the plurality of PV cells is fabricated.
The plurality of light redirecting elements may be fabricated in a different substrate to that on which the PV cells are fabricated.
The plurality of light redirecting elements may include air.
The plurality of light redirecting elements may include a material that has a refractive index which is different but lower than the substrate in which they exist.
The plurality of light redirecting elements may be made with the sides of the light redirecting elements being non-parallel.
The substrates containing the plurality of light redirecting elements and plurality of PV cells may be laminated between other substrates so as to provide environmental protection from damage, humidity and UV radiation.
According to a different aspect of the invention, the plurality of light redirecting elements may be arranged such that there is more than one light redirecting element arranged to align with one PV cell, and the plurality of light redirecting elements do not penetrate completely through the substrate in which they exist.
According to a different aspect of the invention, the plurality of PV cells may include more than one type of PV cell, in order to receive different wavelengths of radiation.
According to a different aspect of the invention, the plurality of light redirecting elements are not perpendicular to the substrate in which they exist.
According to a different aspect of the invention, the plurality of light redirecting elements may be of a different depth in the substrate in which they exist, dependent on the position along the substrate.
According to a different aspect of the invention, the plurality of light redirecting elements may be fabricated from both sides of the substrate in which they exist.
The plurality of light redirecting elements on one side of the substrate may by aligned with the plurality of light redirecting elements on the opposite side of the substrate.
According to a different aspect of the invention, the interfaces between the plurality of light redirecting elements and the substrate are different with one interface comprising an optically flat interface and the other comprising a rough interface.
According to a different aspect of the invention, the incident angle solar concentrator can include part of a window.
According to one aspect of the invention, a transparent solar concentrator includes: a first light transmissive substrate; a plurality of solar cells for receiving solar energy and converting the solar energy into electrical energy, the plurality of solar cells positioned relative to the first substrate; a plurality of light redirecting elements arranged in the first light transmissive substrate, each of the plurality of light redirecting elements configured to direct light incident on a first side of the first light transmissive substrate to a respective one of the plurality of solar cells on an opposite side of the first light transmissive substrate.
According to one aspect of the invention, the first light transmissive substrate has a first refractive index, and the plurality of light redirecting elements have a second refractive index, the second refractive index being less than the first refractive index.
According to one aspect of the invention, each of the plurality of light redirecting elements include a strip or groove arranged in the first light transmissive substrate, the strip or groove filled with a medium having a refractive index corresponding to the second refractive index.
According to one aspect of the invention, the medium is air.
According to one aspect of the invention, the plurality of solar cells are formed as a plurality of photovoltaic strips, each strip spaced apart from an adjacent strip by a predetermined distance.
According to one aspect of the invention, each light redirecting element is aligned with a respective one of the photovoltaic strips.
According to one aspect of the invention, the transparent solar concentrator further includes a second light transmissive substrate, and the plurality of light redirecting elements are formed in the first light transmissive substrate, and the plurality of solar cells are positioned relative to the second light transmissive substrate.
According to one aspect of the invention, the plurality of light redirecting elements do not penetrate completely through the first light transmissive substrate.
According to one aspect of the invention, the plurality of light redirecting elements include a first part having a reflecting surface and a second part having a reflecting surface, wherein the reflecting surface of the first part is offset from the reflecting surface of the second part.
According to one aspect of the invention, the plurality of light redirecting elements have an upper surface and a lower surface, and the upper and lower surfaces are non-parallel to each other.
According to one aspect of the invention, at least two light redirecting elements are assigned to a respective one of the plurality of solar cells
According to one aspect of the invention, the plurality of solar cells include a first type of solar cell configured to convert light having a first range of wavelengths into electrical energy, and a second type of solar cell configured to convert light having a second range of wavelengths into electrical energy, the second range different from the first range.
According to one aspect of the invention, a reflecting surface of the plurality of light redirecting elements is not perpendicular to an outside light-receiving face of the first light transmissive substrate.
According to one aspect of the invention, the plurality of light redirecting elements include first and second light redirecting elements, the first light redirecting element extending into the first light transmissive substrate to a first depth, and the second light redirecting element extending into the at least one substrate to a second depth, wherein he first and second depths are different from one another.
According to one aspect of the invention, the first and second depths correspond to a location of the respective light redirecting element within the first light transmissive substrate.
According to one aspect of the invention, at least one surface of the light redirecting element includes an optically flat surface and another surface of the light redirecting element includes an optically rough surface.
According to one aspect of the invention, the transparent solar concentrator further includes first and second outer light transmissive substrates, wherein the first light transmissive substrate is arranged between the first and second outer light transmissive substrates.
According to one aspect of the invention, a window system includes: a first outer light transmissive substrate and a second outer light transmissive substrate; and a transparent solar concentrator as described herein, wherein the solar concentrator is arranged between the first and second outer light transmissive substrates.
According to one aspect of the invention, the plurality of solar cells are patterned to provide an image.
According to one aspect of the invention, a method for creating a solar concentrator, includes: arranging a plurality of solar cells relative to a light transmissive substrate; forming a plurality of light redirecting elements in the light transmissive substrate, wherein respective ones of the plurality of light redirecting elements are positioned relative to respective ones of the plurality of solar cells so as to direct light incident on a first side of the light transmissive substrate to a respective one of the plurality of solar cells on an opposite side of the light transmissive substrate.
In accordance with the present invention, it is possible to simply make an incident angle solar concentrator in which the concentration ratio of the concentrator increases as the angle of incidence increases from a normal direction in one way, and decreases as the angle of incidence increases negatively from the normal direction in the other way.
The device and system in accordance with the present invention have good potential for application to BIPV (Building Integrated PV). At the time when the sun is low, i.e., early morning and evening time, and especially in the winter, more light passes through the PV window and illuminates the interior of a building. This is the time when most light is needed in a building. In the middle of the day when the sun is high and irradiance rises, there will be more light absorbed by the PV cell. This will generate more electricity that would be possible with no incident angle concentration. In addition, there is less solar radiation entering the interior of the building and therefore solar gain is less; this will lower the cooling requirements of the building resulting in significant energy saving.
The device and method in accordance with the present invention can also be used for mobile devices that are (partially) powered by PV, in that the mobile devices do not need to be fully covered by PV cells but will still generate enough power to trickle charge a battery.
a is an exemplary 3D schematic drawing of the concept in accordance with the present invention.
b is a simulation result of the optical efficiency vs. incident angle for a device in accordance with the present invention. The optical efficiency is defined as the percentage of the incident light hitting the solar cell.
a and 4b are exemplary schematic drawings of a ray trace result for an embodiment in accordance with the present invention that utilizes a group of small air slits instead of a single long air slit.
a and 6b are exemplary schematic drawings of the PV window cross section with tapered shaped air slits in accordance with the present invention.
a and 7b are exemplary schematic drawings of the PV window cross section with tilted air slits in accordance with the present invention.
a is an exemplary schematic drawing showing the tracing results of an assembly of the optical element and the solar cell that is fabricated on a separated substrate in which the solar cell is patterned to show an image in accordance with the present invention.
b schematically shows an exemplary image produced by the device of
Total internal reflection (TIR) is an optical phenomenon that occurs when a ray of light strikes a medium boundary from higher refractive index media to lower refractive index media at an angle larger than a particular critical angle with respect to the normal to the surface. When TIR occurs, no light can pass through boundary and all of the light is reflected. The critical angle is the angle of incidence above which the total internal reflection occurs.
Most conventional see-through PV windows, such as the PV window shown in
In accordance with the present invention and as shown in
As used herein, a light redirecting element is a device that alters a direction of light incident on the light redirection element. The light redirection element is preferably formed via strips or grooves formed in the substrate 2, and can be filled with air or other media to provide a relatively lower refractive index so as to achieve total internal reflection. Thus, the solar concentrator can include a substrate 2 that has a first refractive index and light redirecting elements 4 that have a second refractive index, where the second refractive index is less than the first refractive index.
a is a 3D schematic drawing of a concept in accordance with the present invention, and
The slits 4 do not need to penetrate all the way through the substrate on which they are formed, and this is shown
The simulation result shown in
In many cases of window applications, privacy is quite important. People in a room appreciate more sunlight entering the room or generating more electricity from the solar cell, but they do not want people outside the building to see inside. A privacy feature is illustrated in
One of the challenges in the manufacture of PV windows is how to form the slits 4 with high aspect ratio, the ratio of h/w shown in
When a high aspect ratio of the slit 4 is needed it may go beyond the limit of the current molding ability.
The optical feature can be any of the other shapes that are described in above embodiments. The gap between the elements can also be filled by transparent glue such as resin to reduce the surface reflection loss and gain the mechanical performance.
a and 16b illustrate a feature wherein images may be displayed on the interior of a see-through PV window in accordance with the present invention. In this embodiment, on the side which faces ‘inside the building’, it is possible to create a decoration pattern 12 on the areas aligned with the solar cells 3 by either patterning the solar cells 3 in the desired manner, or coating the solar cells 3 with a reflective or absorbing coating on the correct side.
1. Building Integrated PV (BIPV) field.
2. Solar powered mobile device.
3. Green houses.
4. Conservatories and sun roves.