Patent Application
20140352762
This invention relates generally to luminescent solar collectors and building integrated photovoltaic windows.
Luminescent Solar Collectors (LSCs) are beneficial for capturing solar energy for conversion to electrical power. An LSC has a sheet containing a fluorescent material that absorbs solar radiation from the sun after which it emits photons to longer wavelengths through the process of photoluminescence or fluorescence. The light, or photons, that are emitted through this process are waveguided (via total internal reflection) down a sheet that is coupled to a photovoltaic cell or solar cell that converts the light to electrical power. Current approaches of LSCs focus on maximizing the power conversion efficiency of the LSC with little regard to the application of this technology as building integrated PV windows for greenhouses and related structures where plant growth is important.
Adjusting the spectrum, or color, of light is known to be benefitial to certain plant functions like vegetative growth, flowering and fruiting.
Accordingly, there is a need in the art for luminescent solar collectors which are can produce power with no harm to plant growth.
Disclosed, in various embodiments, are luminescent solar collectors which have an absorption and optical designed for both plant growth and power production for applications involving plant growth under windows having LSCs, including greenhouses, atriums, solariums, skylights and agricultural covers. For example, the relative absorption of the luminescent sheet in the blue/green/red portions of the spectrum is determined specifically to not degrade plant growth.
In an exemplary embodiment, the luminescent solar collector has a luminescent sheet and light energy converter. The sheet can include or is a polymer material containing a fluorescent material dispersed therein. The fluorescent material absorbs greater than 40% of the solar photons between 500 and 600 nm, absorbs less than 70% of the solar photons between 410 and 490 nm, and absorbs less than 40% of the solar photons between 620 and 680 nm. This ratio of absorption in each band is chosen for optimum photosynthesis and plant growth. The polymer layer is designed to transmit the radiated light to the light energy converter and wherein the light energy converter is optically coupled o the luminescent sheet. The luminescent sheet may be further attached to an additional glass, acrylic, or polycarbonate-based substrate in such a manner that the luminescent light is optically coupled to the substrate. The absorption of the luminescent sheet is controlled by the choice of luminescent dye and the concentration. Luminescent sheets that absorb too much light in the bands specified above will harm the plant growth. Sheets that absorb too little light in the above bands will benefit little from power generation.
In other embodiments, the fluorescent material dilution in the polymer material, measured in weight percent of fluorescent material by weight polymer, multiplied by the thickness of the luminescent sheet, measured in millimeters, is between 0.005 to 0.05 to achieve an optical density (absorption) in the range specified above.
In further embodiments, the fluorescent material is selected as a fluorescent dye, conjugated polymer, or a quantum dot wherein the fluorescent dye is based on perylene, terrylene or rhodamine, the conjugated polymer is a polyfluorene, polythiophene, or polyphenylenevinylene, and the quantum dot is comprised of CdTe, CdS, CdSe, PbS, PbSe, GaAs, InN, InP, Si or Ge and the light energy converter is a photovoltaic comprised of silicon, gallium arsenide, copper indium gallium selenide, or cadmium telluride as the active absorbing layer.
In other embodiments, the front active face of the light-energy converter (PV cell) is attached parallel to the surface of the luminescent sheet and the back face is encapsulated with an additional polymer layer or attached to the structural frame of the greenhouse. The active area of the light converter is between 5% to 25% of the active area of the luminescent sheet.
In other embodiments, an addition sheet or sheets of an IR-emitting material, a diffuser, and/or and IR-absorber/reflector are added to further improve efficiency and plant growth while reducing cooling costs.
The luminescent energy-conversion greenhouse of the present disclosure is described herein with reference to exemplary embodiments. Modifications and alternations will occur to others upon reading and understanding the description. It is intended that the exemplary embodiments be constructed as including all such modifications and alternation insofar as they come within the scope of the invention or the equivalents thereof. Exemplary embodiments of the invention can be summarized, without any limitation, according to the following statements.
In one example, the invention pertains to a luminescent solar collector having a absorption optimized for plant growth and electrical power generation with a luminescent sheet and a light energy converter. The luminescent sheet comprises a polymer material containing single or multiple fluorescent material(s) dispersed therein, wherein the fluorescent material(s) absorbs and emits light that is ideal for plant growth with greater than 50% of the solar photons between 500 and 600 nm, absorbs less than 70% of the solar photons between 410 and 490 nm, and absorbs less than 50% of the solar photons between 620 and 680 nm, and wherein the polymer layer is designed to transmit the radiated light to the light energy converter. A light energy converter can be optically coupled to the luminescent sheet.
In another example, one could have a luminescent solar collector, wherein the luminescent sheet is also optically connected to a substrate that is largely transparent between 400 and 700 nm.
In yet another example, one could have a luminescent solar collector, wherein the polymer material is comprised of a material containing poly (alkyl methacrylates), polycarbonate, or a derivative, or combination thereof.
In yet another example, one could have a luminescent solar collector, wherein the fluorescent material emits at least 50% of the radiated photons with wavelengths between 600 and 690 nm.
In yet another example, one could have a luminescent solar collector, wherein the percentage of solar photons absorbed between 410 nm and 490 nm or between 620 nm and 680 nm is less than the percentage of solar photons absorbed between 500 and 600 nm to optimize plant growth.
In yet another example, one could have a luminescent solar collector, wherein the concentration of the fluorescent dye in the polymer material, measured in weight percent, multiplied by the thickness of the sheet, measured in millimeters, is between 0.005 to 0.05.
In yet another example, one could have a luminescent solar collector, wherein the photoactive surface of the light energy converter in mounted approximately parallel to the plane of the luminescent sheet.
In yet another example, one could have a luminescent solar collector, wherein the back surface of the light energy converter in mounted on a supportive frame.
In yet another example, one could have a luminescent solar collector, where the percentage of active area of the light energy converter to the active area of the luminescent sheet is between 5% and 35%.
In yet another example, one could have a luminescent solar collector, wherein the light energy converter is silicon, gallium arsenide, copper indium gallium selenide or cadmium telluride photovoltaic.
In yet another example, one could have a luminescent solar collector, wherein an additional transparent sheet is added behind the light-energy converter for purposes of protection.
In yet another example, one could have a luminescent solar collector, wherein a second luminescent sheet is added that contains a fluorescent material which absorb less than 50% of the solar photons between 620 and 680 nm, and wherein the luminescent sheet is optically coupled to the light energy converter.
In yet another example, one could have a luminescent solar collector, wherein the luminescent sheet is textured so that transmitted light is diffuse.
In still another example, one could have a luminescent solar collector, wherein additional single or multiple non-luminescent sheets are added that contain a light diffuser, an IR-absorber, a IR-reflector, or combination thereof.
The present invention together with its objectives and advantages will be understood by reading the following description in conjunction with the drawings, in which:
Device Structure
The LSC device diagram described here is shown in
The Luminescent Sheet and Impact on Power Efficiency and Plant Activity
The ideal fluorescent material for the luminescent sheet has of a fluorescent dye with a quantum yield greater than 50% and emits a majority of its photons between 600 and 690 nm, where chloropyll a and b are most active. The fluorescent dye is also chosen to minimize overlap between the absorption spectra and fluorescence spectra as well as to minimize the absorption of light that is absorbed by chloropyll a and b (between 410 and 490 nm and between 620 and 680 nm) while maximizing the light absorption in the remaining portions of the solar spectrum (i.e. 380 to 410 nm, 490 to 620 nm, and 680 nm to 780 nm). Red-emitting materials from perylene and rhodamine family meet many of these criteria. In particular, the series of red-emitting Lumogen dyes, including LR305, contains the more promising candidates for this application; however, there are other materials, including those yet to be discovered, that could result in better overall performance. As shown in
The dye can be diluted into the polymer host to maximize the photoluminescence efficiency or quantum yield. The polymer host is chosen to be largely transparent to the PAR spectrum (i.e. 380 to 780 nm) and to be chemically compatible with the fluorescent material. For solution deposited films, the polymer and fluorescent material should have a compatible solvent. Many fluorescent dyes undergo photoluminescence quenching at concentrations above 0.5% in the polymer host. We observe an optimal range for the luminescent dye Lumogen 305 between 0.2% and 0.001%, which depends both on the absorption coefficient of the dye and the thickness of the luminescent sheet. Typically, the luminescent dye is added to the polymer material to maximize the surface photoluminescence. To harvest as much of the solar photons as possible, this concentration results in a peak absorption above 90%. However, such high absorption can result in reduction in the photosynthetic activity in plants. The impact on plant photosynthesis is shown in
The typical upper, lower and near optimal absorptions for the luminescent Lumogen 305 dye to optimize both power production and plant growth is shown in
Overall, we determine that the concentration of the fluorescent dye in the polymer material, measured in weight percent, multiplied by the thickness of the sheet, measured in millimeters, should be between 0.005 to 0.05 for most fluorescent materials, although a fluorescent material that is engineered with anomalous high or low absorption coefficient may fall outside this range. Furthermore, the percentage of absorption of blue photons (410 to 490 nm) should be less than 70%, the percentage of absorption of green photons (500 nm to 600 nm) should be greater than 50%, the percentage of absorption of red photons (620 nm to 680 nm) should be less than 50%, and that overall, the percentage of absorption of the blue or red photons should be less than the absorption of green photons, as defined above. Optimal films may typically have blue absorption less than 50%, green absorption above 70% and red absorption below 10%. Here, we define the percentage of photons absorbed as the number of photons absorbed by the luminescent sheet over the spectral range indicated divided by the total number of solar photons incident on the luminescent sheet over the spectral range indicated, converted to percentage. Finally UV stabilizers and oxygen/H2O scavengers can be added to the luminescent sheet to improve photoluminescence stability.
While the results presented here focus on fluorescent materials that are small molecule organics, this should not be construed as limiting. We have also shown (Sholin) that quantum dot and semiconducting polymers can be used as luminescent materials for this application. In particular, polyspiro red has a similar absorption/emission to LR305 and a larger Stokes-shift, making it a possible suitable replacement material. We also note that the fluorescent material may include a combination of one or more fluorescent materials that have different absorption but have a majority of their emission over a similar wavelength, namely between 600 to 690 nm.
The Light-Energy Converter
The light-energy converter absorbs the luminescent light that is waveguided down the luminescent sheet using total internal reflection and converts it to electrical power. The light-energy converter is typically a photovoltaic (PV). The PV should have high quantum efficiency (>60%) between 600 and 690 nm where a majority of the fluorescent light is emitted. Many Silicon (Si)-based, Gallium Arsenide (GaAs), Cadmium Telluride (CdTe), and Copper Indium Gallium Selenide (CIGS) photovoltaics meet this criteria, as well as photovoltaic technologies which are yet to emerge as commercial products. The photovoltaic is cut into strips that can be mounted either on the edge, or perpendicular, to the luminescent sheet (the standard LSC configuration) or on the front or parallel to the luminescent sheet. For the edge mounted cells, the strips are cut at or about the thickness of the luminescent sheet. For the face mounted cells, the strips are between 2× and 20× wider than the thickness of the luminescent sheet, with thinner strips resulting in greater contributions of the luminescent sheet to the overall power efficiency. The face-mounted configuration, as depicted in
The individual strips of photovoltaic cells are wired in series or parallel with the wires coming out of the LSC package so they can be easily connected to. A typical IV curve for a greenhouse window with and without the luminescent material is shown in
Additional Polymer Films
An additional IR-emitting luminescent material may be added above or below the luminescent sheet in order to improve power efficiency and reduce heating of the greenhouse. This IR-luminescent material should have a photoluminescence quantum yield above 20%, should emit at wavelengths between 700 and 950 nm for single or polycrystalline Si light-energy converters (700 to 850 nm for other forms of Si, CdTe, CIGS, and GaAs light-energy converters) and should absorb less less than 50% of the photons between 620 nm and 680 nm to assure that these wavelengths are transmitted to the plants. The IR-emitting luminescent material must be optically coupled to the light-energy-converter and will normally be mounted below the first luminescent film so that the solar light is incident on the first luminescent film before being incident on the IR-emitting luminescent film.
A non-luminescent IR-absorbing or reflecting film may also be added in order to decrease heating of the greenhouse. This IR-reflecting film does not need to be optically coupled to either the PV cell or luminescent sheet, but may be laminated at the back of the PV cell to provide additional protection. Generally, the IR-reflecting film would be located below the luminescent sheet; however, there may be instances where the reverse configuration is desirable.
A light diffusing layer may be added within or below the luminescent sheet to provide more even lighting within the greenhouse structure. The diffusing film might contain white scattering particles or a texture in the luminescent sheet that slightly redirects light that is transmitted through the glass thus providing a more uniform light on the plants. This diffusing film may also scatter some light back to the luminescent sheet, providing an additional chance for the transmitted light to be absorbed and converted to electrical power.
The following description includes one or more device examples according to the invention, which not meant to be exclusionary of any other designs that have been described.
The 3 mm thick luminescent sheet contains polymethylmethacrylate (PMMA) with a fluorescent dye, Lumogen 305, is diluted into the sheet at a concentration of 0.006% by weight percent of Lumogen 305 in PMMA. A silicon PV cell is attached directly to the acrylic using an optical clear glue that is thermally stable above 85 C and allows for differential thermal expansion. A thin plastic sheet is laminated to the back of the substrate for protection. At 16% area of PV per area of luminescent sheet, the power efficiency is approximately 4%. The sheet absorbs less than 60% of the photons between 410 and 490 nm and less than 10% of the photons between 620 and 680 nm, and approximately 70% of the photons between 500 and 600 nm.
The 0.5 mm thick luminescent sheet contains polymethylmethacrylate (PMMA) with a fluorescent dye, Lumogen 305, diluted into the sheet at a concentration of 0.03% by weight percent of LR305 in PMMA. This film and the silicon PV cells are laminated to a glass or acrylic sheet that is 3 mm thick using EVA. A thin glass sheet is laminated with EVA to the back of the substrate for protection purposes. At 16% coverage, the power efficiency is approximately 4.5% and the sheet absorbs less than 60% of the photons between 410 and 490 nm and less than 10% of the photons between 600 and 690 nm, and approximately 70% of the photons between 500 and 600 nm.
The 0.2 mm thick luminescent sheet contains polymethylmethacrylate (PMMA) with a fluorescent dye, Lumogen 305, diluted into the sheet at a concentration of 0.1% by weight percent of Lumogen 305 in PMMA. The silicon PV cell is attached to a supporting frame, and the luminescent sheet is coupled to the silicon PV using an optical glue. At 10% coverage, the power efficiency is approximately 3% and the sheet absorbs less than 50% of the photons between 410 and 490 nm and less than 10% of the photons between 600 and 690 nm, and approximately 60% of the photons between 500 and 600 nm.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US2013/024393 | 2/1/2013 | WO | 00 | 7/15/2014 |
| Number | Date | Country | |
|---|---|---|---|
| 61594477 | Feb 2012 | US |
