Patent Application
20110308571
Traditional approaches to generate electricity have focused on using fossil fuels, such as coal, oil, and natural gas. More recently, for environmental and other reasons, attention has focused on renewable energy sources. Such renewable energy sources include wind, geothermal, and solar. With respect to solar energy in particular, a solar cell is used to convert energy from the sun into electrical energy.
As noted in the background section, a solar cell is used to convert energy from the sun into electrical energy. While solar energy is gaining traction as an energy source from which to generate electricity, it has so far failed to achieve widespread adoption on the same order that fossil fuel energy sources have. One reason why this is the case is because generating electrical energy from solar energy remains expensive, in part because of the inefficiencies and the manufacturing cost of solar cells.
Embodiments of the disclosure provide for a solar cell that is more efficient than some types of conventional solar cells and that is less expensive to manufacture than other types of conventional solar cells of similar efficiency. A solar cell of an embodiment of the disclosure includes a metal frame to which at least two plastic sheets that are parabolic in shape are mounted. A first plastic sheet is dichroic, and reflects light within a first wavelength range towards a first photovoltaic (PV) mechanism, and transmits light outside this wavelength range towards a second plastic sheet. The second plastic sheet is reflective, and reflects light towards a second PV mechanism.
The parabolic shape of the plastic sheets concentrates the amount of solar energy that is directed towards the PV mechanisms. Efficiency of the solar cell is also increased by the use of a metal frame, which ensures an efficient thermal path of the solar energy. The coefficient of thermal expansion (CTE) of the plastic sheets closely matches the CTE of the metal frame, so that the solar cell has maximum efficiency even when the solar cell experiences a change in temperature. Furthermore, the PV mechanisms are tuned to different solar energy bands to maximize the amount of electrical energy generated by the solar cell as a whole.
The plastic sheets can be manipulated to achieve their parabolic shape. In one type of manipulation process, the plastic sheets are bent and held to the desired shape, whereas in another type of manipulation process, the plastic sheet undergoes thermal forming to realize the desired shape. These types of processes have been found to increase cost linearly with the size of the sheets. In contrast, using injection molding to form the parabolic shape has been found to increase the cost cubically with the size of the sheet. Utilizing plastic in lieu of glass or another material for the sheets also decreases the manufacturing cost of the solar cell.
The light assembly 100 thus includes the frame 102 and the plastic sheets 104 and 106. The frame 102 is metal, such as magnesium. The sheets 104 and 106 are plastic, such as polyethylene naphthalate (PEN), and may have a thickness of 125 micron. The plastic sheets 104 and 106 are mounted to the frame 102. The plastic sheet 106 is mounted to the frame 102 below the plastic sheet 104. The CTE of the plastic sheets 104 and 106 closely matches the CTE of the frame 102. This means that the CTE of the plastic sheets 104 and 106 is similar or identical to the CTE of the frame 102. In one embodiment, the CTE of the plastic sheets 104 and 106 is within plus-or-minus 25% percent (or another threshold) of the CTE of the frame 102, which is about five parts-per-million (ppm) per ° C.
The sheets 104 and 106 are described herein as being plastic sheets. However, more generally, the sheets 104 and 106 may be a material other than plastic. Examples of such other materials include metal, such as polished metal foil, as well as combinations of metal and plastic, such as metalized plastic.
The surface of the plastic sheet 104 is dichroic, which means that the plastic sheet 104 both reflects and transmits light. By comparison, the surface of the plastic sheet 106 is reflective, such that the plastic sheet 106 just reflects light. The plastic sheet 104 is adapted to reflect light having a wavelength range, such as the blue to shorter red wavelengths in the visible light spectrum, and to transmit light outside the wavelength range. As such, the plastic sheet 104 is a high-reflectance and low-wavelength band filter, whereas the plastic sheet 106 is a high-reflectance reflector.
The plastic sheets 104 and 106 have a parabolic shape. The frame 102 thus has a parabolic shape as well where the plastic sheets 104 and 106 are adjacent to the frame 102, in correspondence with the parabolic shape of the sheets 104 and 106. The parabolic shape of the plastic sheets 104 and 106 serves to focus and thus concentrate the light reflected by the sheets 104 and 106. For example, the plastic sheet 104 may focus its reflected light along a first line, and the plastic sheet 106 may focus its reflected light along a second line below the first line.
The plastic sheet 104 has tabs 108A and 108B, collectively referred to as the tabs 108, at its ends, and the plastic sheet 106 likewise has tabs 110A and 110B, collectively referred to as the tabs 110, at its ends. The tabs 108 and 110 increase the stiffness of the plastic sheets 104 and 106, respectively, which permits the shape of the sheets 104 and 106 to be maintained when the sheets 104 and 106 are supported by the frame 102 just at the edges of the sheets 104 and 106. The plastic sheets 104 and 106 are thus mounted to the frame 102 at the tabs 108 and 110, and/or by a suitable adhesive along the edges of contact of the sheets 104 and 106 with the frame 102.
The PV mechanism 304 receives light reflected by the plastic sheet 104 to convert this light into electrical energy, and the PV mechanism 306 receives light reflected by the plastic sheet 106 to convert this light into electrical energy. As noted above, the plastic sheet 104 may reflect the shorter, blue to red wavelengths of light in the visible light spectrum. As such, the PV mechanism 304 is optimized to absorb this light.
By comparison, the plastic sheet 106 reflects the other, longer wavelengths of light outside the visible light spectrum, by virtue of these other wavelengths being transmitted through the plastic sheet 104 to the plastic sheet 106. As such, the PV mechanism 306 is optimized to absorb this light. Therefore, in the parlance of solar cells, the PV mechanism 304 is said to be a mid-E PV cell having a middle energy gap, and the PV mechanism 306 is said to be a low-E PV cell having a low energy gap.
The plastic sheet 104, in a flattened state, is coated so that the sheet 104 has a dichroic surface (504). For example, the plastic sheet 104 may be coated with a series of dielectric layers so that the sheet 104 has a dichroic surface. The opposite surface of the sheet 104 may be coated to minimize reflections of longer wavelengths that are not optimally collected by PV mechanism 304, permitting them to pass to the sheet 106 for redirection to the PV mechanism 306 for optimal collection by the PV mechanism 306. The plastic sheet 106, also in a flattened state, is coated so that the sheet 106 has a reflective surface (506). The plastic sheet 106 may also be coated with a series of dielectric layers, or metal layers and dielectric layers, so that the sheet 106 has a reflective surface. Coating the plastic sheets 104 and 106 in their flattened state provides for greater accuracy in the coating process, as compared to coating the plastic sheets 104 and 106 after they have been placed in their parabolic state.
The plastic sheets 104 and 106, while still flattened, are cut to desired sizes. The plastic sheets 104 and 106 are then manipulated, such as by bending and holding and/or by thermal forming as described above, so that the sheets 104 and 106 have a parabolic shape and the tabs 108 and 110 (510). As such, the plastic sheets 104 and 106 are generally rigid and not flexible. Once the plastic sheets 104 and 106 have been manipulated into their parabolic shape, the sheets 104 and 106 remain in this shape.
With respect to thermal forming in particular, thermal forming the plastic sheets 104 and 106 so that the sheets 104 and 106 have a parabolic shape can be achieved by raising the temperature of the sheets 104 and 106 above their glass transition temperatures while holding the sheets 104 and 106 in a tool of the desired parabolic shape. The plastic sheets 104 and 106 are then permitted to cool below their glass transition temperatures while in the desired parabolic shape. This process imparts the desired unstressed parabolic form on the plastic sheets 104 and 106 so that the sheets 104 and 106 will hold their parabolic shape after removal from the tool. During such thermal forming, the plastic sheets 104 and 106 are not stretched, so that the coatings on the sheets 104 and 106 still maintain their optical performance characteristics relative to their flat coated shape. The plastic sheets 104 and 106 can be mounted to the frame 102 at their tabs 108 and 110 (512), such as by employing screws. The plastic sheets 104 and 106 may further be secured to the frame 102 via a suitable adhesive being applied to the edges of the frame 102 at which the sheets 104 and 106 make contact. Part 512 concludes the method 500 as to manufacture of the light assembly 100 of
Where the light assembly 100 of
It is noted that embodiments of the disclosure have been substantially described in relation to a solar cell that converts solar energy to electrical energy. However, the light assembly that has been described can be used for purposes other than functioning as a solar cell. In general, the light assembly includes a frame and at least two plastic sheets as has been described. Light can enter the assembly so that it first impinges the first plastic sheet and then is transmitted through to the second plastic sheet, or so that it first impinges the second plastic sheet and then is reflected to the first plastic sheet.
The invention that is the subject of this patent application was made with Government support under Subcontract No. CW135971, under Prime Contract No. HR0011-07-9-0005, through the Defense Advanced Research Projects Agency (DARPA). The Government has certain rights in this invention.
