Patent Application
20160365730
This invention is in the field of photovoltaic cells and in particular in the field of multi-junction, multi-element photovoltaic cells.
In the normal operation of a solar panel, photons from sunlight strike the photovoltaic cells of the solar panel and are absorbed by semiconducting materials, such as silicon. The energy from the absorbed photons excites electrons from their respective atomic or molecular orbitals. Once excited, an electron can either dissipate the energy as heat and return to its orbital or travel through the cell until it reaches an electrode. Current flows through the material in response to the electric potential generated by the photons striking the photovoltaic cells, and this electric current, and its associated energy, is captured. Thus, an array of solar cells in the solar panel converts solar energy from sunlight into a usable amount of direct current (DC) electricity. The energy of the direct current can be stored in batteries, capacitors or other energy storage processes, or the direct current can be converted to alternating current.
Traditional photovoltaic cells are commonly composed of doped silicon and then depositing metallic contacts on the top and bottom. The doping is normally applied to a thin layer on the top of the cell, producing a pn-junction with a particular bandgap energy, Eg. Photons that irradiate the top of the solar cell are either reflected or transmitted into the cell. Transmitted photons have the potential to give their energy hv to an electron if hv≧Eg, generating an electron-hole pair. Electrons and holes are accelerated towards their respective n-doped and p-doped regions (up and down, respectively). The resulting current Ig is called the generation photocurrent.
At present, the most common type of photovoltaic cells are single layer, single junction cells constructed of crystalline silicon, or thin film cells. Based upon present technology, typical solar cells are made from semiconductors wafers which commonly range between 180 to 240 micrometers thick.
Solar cells are often encapsulated as a module. Photovoltaic modules often have a sheet of glass on the sun-facing side, allowing light to pass while protecting the semiconductor wafers. Solar cells are usually connected in series in modules, creating an additive voltage. Connecting cells in parallel yields a higher current; however, problems such as shadow effects can shut down the weaker, less illuminated parallel string of a number of series connected cells causing substantial power loss and possible damage because of the reverse bias applied to the shadowed cells by their illuminated partners. Strings of series cells are usually handled independently and not connected in parallel, though individual power boxes are often supplied for each module, and are connected in parallel. Although modules can be interconnected to create an array with the desired peak DC voltage and loading current capacity, using independent MPPTs (maximum power point trackers) is preferable. Otherwise, shunt diodes can reduce shadowing power loss in arrays with series/parallel connected cells.
Traditional single-junction cells have a maximum theoretical efficiency of 34%. The theoretical performance of a solar cell was first studied in depth in the 1960's, and is today known as the Shockley-Queisser limit. The limit describes several loss mechanisms that are inherent to any solar cell design. Under the Shockley-Queisser limit, the efficiency of a single-junction solar cell, under unconcentrated sunlight, cannot exceed 34%. The first of the losses taken into consideration in the Shockley-Quiesser limit, is the blackbody radiation loss, a loss mechanism that affects any material object above absolute zero. In the case of solar cells at standard temperature and pressure, this loss accounts for about 7% of the power. The second is an effect known as “recombination”, where the electrons created by the photoelectric effect meet the electron holes left behind by previous excitations. In silicon, this accounts for another 10% of the power. However, the dominant loss mechanism is the inability for a solar cell to extract all of the power in the photon, and the associated problem that it cannot extract any power at all from certain photons. This is due to the fact that the electrons must have enough energy to overcome the bandgap of the material.
If the photon has less energy than the bandgap, it is not collected at all. This is a major consideration for conventional solar cells, which are not sensitive to most of the infrared spectrum, although that represents almost half of the power coming from the sun. Conversely, photons with more energy than the bandgap, say blue light, initially eject an electron to a state high above the bandgap, but this extra energy is lost through collisions in a process known as “relaxation”. This lost energy turns into heat in the cell, which has the side-effect of further increasing blackbody losses.
Combining all of these factors, the maximum efficiency for a single bandgap material, such as a conventional silicon cell, is about 34%. That is, 66% of the energy in the sunlight hitting the cell will be lost. Practical concerns, notably reflection off the front surface or the metal terminals, further reduce the actual efficiency. It is reported that single p-n junction crystalline silicon devices are now approaching the theoretical limiting power efficiency of 33.7%, noted as the Shockley-Queisser limit in 1961. Modern high-quality cells commercially available have an efficiency of about 25%.
Multi-junction solar cells are solar cells with multiple pn junctions made of different semiconductor materials. Each material's pn junction will produce electric current in response to a different wavelength of light. A multi-junction cell solar cell will produce electric current at multiple wavelengths of light, increasing the conversion efficiency of the no-cost solar light power to usable electric power.
A multi-junction cell, however, can exceed that limit. Theoretically, with an infinite number of junctions, multi-junction cell efficiency would be 87% under highly concentrated sunlight. Maximum demonstrated efficiencies of multi-junction cells have demonstrated performance over 43%. Commercial, two layer, photovoltaic cells are available with 30% efficiency under one-sun illumination, and 40% under concentrated sunlight. However, these two layer cells have a higher price to performance ratio than the traditional single layer silicon cells, which has limited their commercialization to date. The potential for commercialization of multi-layer cells is more promising for concentrated photovoltaics (CPV). It is reported that in September 2013, a solar cell achieved a new record with 44.7 percent efficiency, as demonstrated by the German Fraunhofer Institute for Solar Energy Systems. Photovoltaic cells made from multiple materials have multiple bandgaps, and will respond to multiple light wavelengths. Hence, some of the energy that would otherwise be lost to relaxation as described above, can be captured and converted. Following an analysis similar to that performed for single bandgap devices, it can be concluded that the perfect bandgaps for a two-gap device are at 1.1 eV and 1.8 eV.
Multi-junction cells are constructed by layering the different materials on top of each other, shortest wavelengths on the “top” and increasing through the body of the cell. As the photons have to pass through the cell to reach the proper layer to be absorbed, transparent conductors need to be used to collect the electrons being generated at each layer.
For a typical MJ solar cell, there are six important types of layers: pn junctions, back surface field (BSF) layers, window layers, tunnel junctions, anti-reflective coating and metallic contacts. Producing a two layer, “tandem” cell is not an easy task, largely due to the thinness of the materials and the difficulties extracting the current between the layers. The easy solution is to use two mechanically separate thin film solar cells and then wire them together separately outside the cell. This technique is widely used by amorphous silicon solar cells.
The more difficult solution is the “monolithically integrated” cell, where the cell consists of a number of layers that are mechanically and electrically connected. These cells are much more difficult to produce because the electrical characteristics of each layer have to be carefully matched. In particular, the photocurrents generated in each layer need to be matched, otherwise electrons will be absorbed between layers. This limits their construction to certain materials, best met by the III-V semiconductors. Since each sub-cell is connected electrically in series, the same current flows through each junction. The materials are ordered with decreasing bandgaps, Eg, allowing sub-bandgap light (hc/λ<e·Eg) to transmit to the lower sub-cells. Therefore, suitable bandgaps must be chosen such that the design spectrum will balance the current generation in each of the sub-cells, achieving current matching. The maximum conversion efficiency for every junction as a function of the wavelength is directly related to the number of photons available for conversion into photocurrent.
The majority of multi-junction cells that have been produced to date use three layers. These cells require the use of semiconductors that can be tuned to specific frequencies. Commonly utilized materials include germanium for the bottom and gallium arsenide compounds for the middle and the top-cell. The layers must be electrically optimal for high performance. For example, materials with favorable characteristics for use in a three layer cell are: InGaP for the top sub-cell (Eg=1.8-1.9 eV), InGaAs for the middle sub-cell (Eg=1.4 eV), and Germanium for the bottom sub-cell (Eg=0.67 eV). The use of Ge is mainly due to its lattice constant, robustness, low cost, abundance, and ease of production. Current efficiencies for commercial InGaP/GaAs/Ge cells approach 40% under concentrated sunlight. Lab cells (partly using additional junctions between the GaAs and Ge junction) have demonstrated efficiencies above 40%.
Dual junction cells can be made on Gallium arsenide wafers. Alloys of Indium gallium phosphide in the range In0.5Ga0.5P through In0.53Ga0.47P serve as the high bandgap alloy. This alloy range provides for the ability to have bandgaps in the range of 1.92 eV to 1.87 eV. The lower GaAs junction has a bandgap of 1.42 eV.
The metallic contacts are low-resistivity electrodes that make contact with the semiconductor layers. They are often aluminum and provide an electrical connection to a load or other parts of a solar cell array. They are usually positioned on two sides of the cell, and on the back face so that shadowing of the lighting surface is reduced.
Anti-reflective (AR) coating is generally composed of several layers in the case of MJ solar cells. The top AR layer has usually a NaOH surface texturation with several pyramids in order to increase the transmission coefficient T, the trapping of the light in the material (because photons cannot easily get out the MJ structure due to pyramids) and therefore, the path length of photons in the material. On the one hand, the thickness of each AR layer is chosen to get destructive interferences. Therefore, the reflection coefficient R decreases to 1%. On the other hand, the thickness of each AR layer is also chosen to minimize the reflectance at wavelengths for which the photocurrent is the lowest. Consequently, this maximizes JSC by matching currents of the three subcells. Because the current generated by the bottom cell may be greater than the currents generated by the other cells, the thickness of AR layers is adjusted so that the infrared (IR) transmission (which corresponds to the bottom cell) is degraded while the ultraviolet transmission (which corresponds to the top cell) is upgraded. Particularly, an AR coating is very important at low wavelengths because, without it, T would be strongly reduced to 70%.
For maximum efficiency, each subcell should be operated at its optimal J-V parameters, which are not necessarily equal for each subcell. If they are different, the total current through the solar cell is the lowest of the three. By approximation, it results in the same relationship for the short-circuit current of the MJ solar cell: JSC=mm (JSC1, JSC2, JSC3) where JSCi(λ) is the short-circuit current density at a given wavelength λ for the subcell i.
The theoretical efficiency of MJ solar cells is 86.8% for an infinite number of pn junctions, implying that more junctions increase efficiency. The maximum theoretical efficiency is 37, 50, 56, 72% for 1, 2, 3, 36 pn junctions, respectively, with the number of junctions increasing exponentially to achieve equal efficiency increments. The exponential relationship implies that as the cell approaches the limit of efficiency, the increase cost and complexity grow rapidly. Decreasing the thickness of the top cell increases the transmission coefficient T.
Light concentrators increase efficiencies and reduce the cost/efficiency ratio. The three types of light concentrators in use are refractive lenses like Fresnel lenses, reflective dishes (parabolic or cassegraine), and light guide optics. Thanks to these devices, light arriving on a large surface can be concentrated on a smaller cell. The intensity concentration ratio (or “suns”) is the average intensity of the focused light divided by 0.1 W/cm2. If its value is X then the MJ current becomes X higher under concentrated illumination.
Using concentrations on the order of 500 to 1000, meaning that a 1 cm2 cell can use the light collected from 0.1 m2 (as 1 m2 equal 10000 cm2), produces the highest efficiencies seen to date. Three-layer cells are fundamentally limited to 63%, but existing commercial prototypes have already demonstrated over 40%. These cells capture about ⅔ of their theoretical maximum performance, so assuming the same is true for a non-concentrated version of the same design, one might expect a three-layer cell of 30% efficiency. This is not enough of an advantage over traditional silicon designs to make up for their extra production costs. For this reason, almost all multi-junction cell research for terrestrial use is dedicated to concentrator systems, normally using mirrors or Fresnel lenses.
Using a concentrator also has the added benefit that the number of cells needed to cover a given amount of ground area is greatly reduced. A conventional system covering 1 m2 would require 625 16 cm2 cells, but for a concentrator system only a single cell is needed, along with a concentrator. The argument for concentrated multi-junction cells has been that the high cost of the cells themselves would be more than offset by the reduction in total number of cells. However, the downside of the concentrator approach is that efficiency drops off very quickly under lower lighting conditions. In order to maximize its advantage over traditional cells and thus be cost competitive, the concentrator system has to track the sun as it moves to keep the light focused on the cell and maintain maximum efficiency as long as possible. This requires a solar tracker system, which increases yield, but also cost.
Measurements on MJ solar cells are usually made in laboratory, using light concentrators (this is often not the case for the other cells) and under standard test conditions (STCs). STCs prescribe, for terrestrial applications, the AM1.5 spectrum as the reference. This air mass (AM) corresponds to a fixed position of the sun in the sky of 48° and a fixed power of 833 W/m2. Therefore, spectral variations of incident light and environmental parameters are not taken into account under STC.
Consequently, performance of MJ solar cells in terrestrial environment is inferior to that achieved in laboratory. Moreover, MJ solar cells are designed such that currents are matched under STC, but not necessarily under field conditions. One can use QE(λ) to compare performances of different technologies, but QE(λ) contains no information on the matching of currents of subcells. An important comparison point is rather the output power per unit area generated with the same incident light.
The environment in space is quite different. Because there is no atmosphere, the solar spectrum is different (AM0). The cells have a poor current match due to a greater photon flux of photons above 1.87 eV vs. those between 1.87 eV and 1.42 eV. This results in too little current in the GaAs junction, and hampers the overall efficiency since the InGaP junction operates below MPP current and the GaAs junction operates above MPP current. To improve current match, the InGaP layer is intentionally thinned to allow additional photons to penetrate to the lower GaAs layer.
In terrestrial concentrating applications, the scatter of blue light by the atmosphere reduces the photon flux above 1.87 eV, better balancing the junction currents. Radiation particles that are no longer filtered can cause damage the cell. There are two kinds of damage: ionization and atomic displacement. Still, MJ cells offer higher radiation resistance, higher efficiency and a lower temperature coefficient.
As has been discussed above, the type of photovoltaic cell that is referred to as a “monolithically integrated” cell, which consists of two or more photovoltaic layers that are physically and electrically connected, is that the electrical characteristics of each layer have to be matched because the current will and must be the same through each layer. Accordingly, the photocurrent, that is the current generated by the electromagnetic radiation absorbed in each of the respective layers, has to be equal, or electrons will be absorbed between the layers resulting in a loss of efficiency. Each layer must then be designed to produce the same photocurrent for the electromagnetic radiation absorbed within its layer bandgap as each of the other layers will for its respective layer bandgap. For example, for a typical three layer, multi-junction photovoltaic cell, the top layer may be designed to absorb a portion of the ultraviolet spectrum and perhaps a portion of the visible light spectrum, and to pass the remaining wave lengths of visible light and infrared. A second layer, may be designed to absorb the visible light passed by the top layer and to pass infrared. A third layer may be designed to absorb as much of the shorter wavelength infrared spectrum as practical. Each of the three layers must be comprised of material specifically selected and must be designed to produce as close as possible the same photocurrent as the other two layers for the radiation absorbed in its layer bandgap.
The difficulty in matching the photocurrent produced by each layer of a multi-layer photovoltaic cell is further complicated by the variations in the power distribution over the overall operating spectrum range, the “overall bandgap,” for which energy is intended to be absorbed by the photovoltaic cell. Significant variations in the power distribution among the ultraviolet, visible light, and infrared spectrums occur with variations in the time of day, season, latitude, altitude, and cloud cover. Even variations in atmospheric pressure and humidity may significantly affect the power distribution over the overall bandgap. Photovoltaic layer material selection and layer design for a sea level, equatorial, frequent cloud cover, and high humidity application, may be poorly suited for a high latitude, high altitude, clear sky, and low humidity application. Further, a photovoltaic cell with material and layer design selected to optimize efficiency during a particular season, may result in a substantially reduced efficiency during other seasons, when the power distribution within the overall bandgap will be substantially different. Still further, a photovoltaic cell which has its layer material and layer design selections made based upon a particular time of day, i.e. optimized based upon the power distribution within the overall bandgap during a particular time of day, for example solar noon, may result in substantially diminished efficiency during other times of day, particularly the early morning and later afternoon hours. In each operating condition, the photocurrent produced by the least productive photovoltaic layer will determine the cell output photocurrent, and hence the power output and efficiency of the photovoltaic cell.
One solution to this problem of photocurrent differential between layers is to physically and electrically isolate the photovoltaic layers and to combine the current from each layer outside the photovoltaic cell. This is referred to as an amorphous photovoltaic cell.
Another inherent problem affecting the efficiency of a photovoltaic cell, or a photovoltaic layer of a multi-layer cell is related to the output voltage range of the layer. Referring to
The formula for the instantaneous power (P) generated by the photovoltaic cell may be determined by the formula P=I*V. The I and V values at which the maximum power Pmax 79 is generated are I=Imp 81 and V=Vmp 77 respectively. I is at its maximum I0v 78 when the voltage in the circuit to which the photovoltaic cell layer is being discharged is zero. In the case of a photovoltaic cell layer producing charge that is stored in a capacitor, the maximum current occurs when there is no charge on the capacitor. As the V to which the photovoltaic cell layer is subjected by the capacitor charging circuit, increases above Vmp, the current produced by the photovoltaic cell layer decreases rapidly and goes to zero at the voltage reaches the shut-off voltage (Vs). During any time period that V in the circuit to which the photovoltaic cell layer or the photovoltaic array as a whole, is discharging current, exceeds Vmp, the efficiency of the photovoltaic cell layer or photovoltaic array as a whole will be significantly diminished.
Energy from a photovoltaic system is generally stored in batteries for later use or converted to an AC current for discharge to an electrical grid. If the energy is to be stored in a battery, the voltage for the photovoltaic system will have to be adjusted to exceed the transient voltage of the battery. Since the maximum voltage output of a photovoltaic cell or a photovoltaic cell layer of a multi-layer photovoltaic cell, is typically on the order of 0.5 volts, the voltage must be stepped up before the energy can be stored in a battery system. Similarly, if the energy generated by a photovoltaic cell is to be discharged to an electrical grid system, which may be operated at 240 volts, 480 volts, or much higher voltages, the voltages must be stepped up to a voltage exceeding the minimum voltage required by an inverter which will invert the DC current to a pulsed AC current. Various filters may be used to impose a sinusoidal wave form on the AC.
To maximize the efficiency of a multi-layer photovoltaic cell, the problem of mismatched photocurrent of the photovoltaic layers of a monolithically integrated photovoltaic cell, and the problem of the photocurrent being reduced as the discharge voltage increases on the photovoltaic cell, must be addressed.
For purposes of this application, including but not limited to, the Summary of the Invention, the Brief Description of the Drawings, the Detailed Description, the Claims, and the Abstract, the term “photovoltaic layer” shall be defined to include a layer of material having the characteristic and ability to receive and absorb electromagnetic radiation and to generate a current, namely a photocurrent, through the absorption of the electromagnetic radiation; the term “photovoltaic element” shall be defined to include a photovoltaic layer and one or more other functional layers or components, such as window layers, anti-reflective coatings, conduction layers, or metallic contacts; and the term “multi-element photovoltaic cell” shall be defined to include a photovoltaic cell having two or more photovoltaic elements. The term “electromagnetic radiation” includes particularly the ultraviolet, visible light and infrared spectrums respectively.
An objective of the present invention is to provide a multi-element photovoltaic cell having a photovoltaic controller, which provides for the continuous production of current by each of the irradiated photovoltaic cells of a photovoltaic array regardless of the level of irradiation.
A further objective of the device and method of the present invention is to provide for the continuous production of current by a photovoltaic cell by avoiding increasing the voltage of the discharge circuit, at each of the photovoltaic cells, above Vmp.
A further objective of the present invention is to provide for the continuous and optimized production of energy by each of the photovoltaic elements of a photovoltaic cell assembly while simultaneously stepping up the voltage of an aggregate current discharged by the photovoltaic cell assembly to a level required for discharge to an inverter or to a DC battery storage system, or both.
A further objective of the present invention is to provide for the continuous and optimized production of energy by each of the photovoltaic cells of a photovoltaic array while simultaneously stepping up the voltage of an aggregate current discharged by the full photovoltaic array to a level required for discharge to an inverter or to a DC battery storage system, or both.
The multi-element photovoltaic cell of the present invention may have two or more photovoltaic elements with an isolation layer interposed between all contiguous photovoltaic elements. Also, regardless of the number of photovoltaic elements, each photovoltaic element will have an element front conductor and an element rear conductor which are in electrical contact with the photovoltaic layer of the photovoltaic element. There may also be an anti-reflective coating at the element front of each photovoltaic element. A rear insulation layer may electronically isolate the multi-element photovoltaic cell from its environment.
The first element front conductor must be optically transparent, including the ultraviolet and infrared wavelengths within the overall bandgap, and must be electrically conductive. It should have a high transmissivity for the portion of the electromagnetic radiation spectrum to be used for power generation, namely the portion of the incident solar radiation having a wavelength falling within the overall bandgap radiation for the photovoltaic cell, which includes the bandgap radiation falling within the element bandgap for each photovoltaic element.
Regardless of the number of photovoltaic elements, each element front conductor and each element rear conductor should have a high transmissivity for the transient bandgap radiation that it must pass to the following photovoltaic elements, with the exception of the last rear conductor of the last photovoltaic element.
Whether the photovoltaic cell is a two element cell, a three element cell, or has more than three elements, for a preferred embodiment of the photovoltaic cell assembly of the present invention, the current from a respective photovoltaic element which is generated as incident solar radiation irradiates the photovoltaic cell, flows independently of the other photovoltaic elements to at least one element capacitor.
Solar radiation, which may include visible light and portions of the ultraviolet spectrum and the infrared spectrum, is incident to the photovoltaic cell assembly which is comprised of a plurality of photovoltaic elements. The photons of the solar radiation strike the absorption medium in each of the photovoltaic elements, thereby resulting in the release of electrons by the absorption medium of the photovoltaic elements. The resultant current will continue so long as the receiving voltage of the receiving circuit is less than the shut-down voltage, and so long as the photovoltaic cell is being irradiated by incident solar radiation.
For certain preferred embodiments of the photovoltaic cell of the present invention a transparent conducting film, such as a transparent conducting oxide may be used for the element transparent conductors. Carbon nanotube networks graphene, or polymer networks are examples of materials that may also be used for one or more of the transparent conductors. Other materials of that type may be known to persons of skill in the art and other similar materials may likely be the result of future technological development.
An alternative to providing a transparent conducting film for the element conductors is to provide an element front conduction zone and an element rear conduction zone which are integral with the element. These respective conduction zones can be provided through a high or increased level of doping of the conduction zones thereby transforming the semiconductor material into a conduction zone. Isolation layers interposed between contiguous photovoltaic elements must a high transmissivity rate for the radiation that must be passed to the following photovoltaic elements, and must be electrically non-conductive.
A preferred embodiment of the photovoltaic controller circuit includes a photovoltaic controller, a capacitor network comprised of a plurality of capacitor banks. Each photovoltaic element is electrically connected to a capacitor bank by a capacitor charge circuit. Each of the capacitor banks may comprise a plurality of element capacitors. A capacitor voltage sensor may be connected to each element capacitor and continuously or frequently monitor the voltage across the capacitor.
The process of the selective and sequential charging and discharging of the respective element capacitors of each photovoltaic cell may thus be controlled by the photovoltaic controller, based upon the voltage monitored by the voltage sensors. The photovoltaic controller may cycle between the element capacitors based upon the level of irradiation of the photovoltaic cell, the resultant current production of the photovoltaic cell, and the voltages across the element capacitors as measured by the voltage sensors.
The capacitor switches as controlled by the photovoltaic controller can provide for the photovoltaic elements of the photovoltaic cell, to be connected in parallel and with selected element of other cells to equalize the voltage before they are switched to discharge in series, providing for stepping up the voltage. If not equalized, the lowest voltage differential would determine the current.
The output circuit provides for each photovoltaic element to be connected in series or parallel. For a preferred embodiment, the capacitor switches may be operated by the photovoltaic controller to provide for each group of photovoltaic element capacitors that are to be discharged in series, to be first connected in parallel to provide for equalization of the voltage on each capacitor. This prevents the lowest voltage capacitor in the series from limiting the current when the capacitors are switched to discharging in series.
For capacitors connected in series, the current is the same at all points in the interconnecting circuit, and thus the current is the same to and from each capacitor. If independently charged capacitors are switched to series connection, the capacitor with the least voltage differential between the cathode and the anode at the time of the initial switching to a series configuration will determine the current flow from the series of capacitors. Further, the total charge discharged from the capacitor series, will be limited to the total charge stored in the capacitor of the series with the least total charge. Therefore, in order to maximize the discharge current and total charge discharged from independently charged capacitors of the same characteristics and capacitance, and hence the total energy discharged, the capacitors should be connected in parallel, immediately before connecting them in series, for equalization of the voltage and charge stored on each of the capacitors.
It is anticipated, based upon current technology, that the capacitors, switches, voltage sensors, and circuit connections between these components, will be components of an integrated circuit in which the photovoltaic cells are imbedded. The utilization of additional capacitors, switches and voltage sensors for embodiments with a larger number of capacitors for each photovoltaic cell, would certainly increase the cost of the photovoltaic controller of the present invention.
For preferred embodiments, the photovoltaic controller may receive continuous voltage measurements, or voltage measurements made at intervals, from the voltage sensors of each photovoltaic cell of a photovoltaic array, and use the voltage data to control the switches so as to attempt to optimize the output power production for the photovoltaic array, while providing for connecting the discharge output of each photovoltaic cell to the output circuit. The current flowing from each photovoltaic element, and each photovoltaic cell may also be measured continuously, or at intervals, by a cell current sensor, and the current data transmitted to the photovoltaic controller. This current data may be used, along with the voltage data, by the photovoltaic controller to attempt to optimize the output power production for each photovoltaic cell and for a photovoltaic array as a whole.
For a preferred embodiment, the photovoltaic controller may incorporate a digital computer and may communicate by wire or wireless with the capacitor voltage sensors to receive voltage measurements and may communicate by wire or wireless with the charge switches and the discharge switches to cause the switches to open and close as needed to manage the charging and discharging of the photovoltaic element capacitors so as to optimize the energy extraction of the photovoltaic array and to control the voltage and other characteristics of the output from the photovoltaic array so as to appropriately interface with storage, electric grid or other application for the extracted solar energy. For a preferred embodiment, the capacitor switches may be operated by the photovoltaic controller to provide for each group of photovoltaic element capacitors that are to be discharged in series, to be first connected in parallel to provide for equalization of the voltage on each capacitor. This prevents the lowest voltage capacitor in the series from limiting the current when the capacitors are switched to discharging in series.
The photovoltaic controller of the photovoltaic cell assembly of the present invention may also provide for the concurrent operation of a plurality of photovoltaic cell assemblies, such as would be present in a photovoltaic array. The photovoltaic cell assembly may receive voltage sensor signals from a plurality of other photovoltaic cell assemblies, such as for a photovoltaic array of which the photovoltaic cell assembly of the present invention is a component, and may generate charging signals and discharging switch control signals, which are directed to a plurality of photovoltaic cell assemblies of the present invention, as in a photovoltaic array. In general, the photovoltaic controller will operate element capacitor charging switches and element capacitor discharging switches for each of the photovoltaic cell assemblies so as to optimize the charge, power and total energy output of the photovoltaic array of which the photovoltaic cell assembly of the present invention is a component.
Referring first to
For photovoltaic cells 1 having more than two photovoltaic elements 3, an isolation layer 14 is interposed between all contiguous photovoltaic elements 3. Also, regardless of the number of photovoltaic elements 3, each photovoltaic element will have an element front conductor 8 and an element rear conductor 10 which are in electrical contact with the photovoltaic layer 16 of the photovoltaic element 3.
There may also be a first anti-reflective coating 22 at the first element front 24 of the first photovoltaic element 5, and a second anti-reflective coating 26 at the second element front 28 of the second photovoltaic element 7. A rear insulation layer 20 may electronically isolate the multi-element photovoltaic cell 1 from its environment.
The first element front conductor 9 must be optically transparent, including the ultraviolet and infrared wavelengths within the overall bandgap, and electrically conductive. It should have a high transmissivity for the electromagnetic radiation 23 to be used for power generation, namely the portion of the incident solar radiation 25 having a wavelength falling within the overall bandgap, i.e. the overall bandgap radiation 27 for the photovoltaic cell, which includes the first bandgap radiation 29 falling within the first element bandgap for the first photovoltaic element 5, and the second bandgap radiation 31 falling within the second element bandgap for the second photovoltaic element 7. Similarly, the second element front conductor 13 should have a high transmissivity at least for the second bandgap radiation 31 providing for the portion of the incident solar radiation 25 having a wave length which falls within the second bandgap to pass to the second photovoltaic element 7.
For certain preferred embodiments of the photovoltaic cell 1 of the present invention a transparent conducting film, such as a transparent conducting oxide, which may be indium 10 oxide, fluorine doped 10 oxide, or doped zinc oxide. Carbon nanotube networks graphene, or polymer networks are examples of materials that may be used for the first element front conductor, first element rear conductor, and second element front conductor. Other materials of that type may be used for first element front conductor, first element rear conductor, and second element front conductor may be known to persons of skill in the art and other similar materials may likely be the result of future technological development.
Referring now to
Regardless of whether a separate transparent conducting layer, or an integral conducting layer is used, the first element current I1 41 and second element current I2 43 will be produced by the photovoltaic cell 1 at voltages V145 and V247 respectively.
Referring again to
Referring now to
Referring now also to
For the embodiment shown, solar radiation 25 as shown in
Referring also to
Each of the three capacitor banks 111, the first capacitor bank 201, the second capacitor bank 203, and the third capacitor bank 205, for the preferred embodiment of the photovoltaic controller circuit 101 shown in
Referring further to
A second capacitor first charge switch 139, 239, 339 and a second capacitor second charge switch 140, 240, 340 of the capacitor charge circuit 110 are closed, and a second capacitor first discharge switch 141, 241, 341 and a second capacitor second discharge switch 142, 242, 342 are open, thereby providing for a current to flow by the capacitor discharge circuit 118 to the output circuit 143 from the first element capacitor 113 while current flows by the capacitor charge circuit 110 from the photovoltaic cell 1 to the second element capacitor 115, providing for uninterrupted current production by the photovoltaic cell 1.
When the voltage across the second element capacitor 115, as monitored by the second voltage sensor 125, 225, 325 equals or exceeds the desired target maximum voltage, the second capacitor first charge switch 139, 239, 339 and the second capacitor second charge switch 140, 240, 340 are opened, disconnecting the second element capacitor 115 from the photovoltaic cell 1, and a second capacitor first discharge switch 141, 241, 341 and a second capacitor second discharge switch 142, 242, 342 are opened, connecting the second element capacitor to the output circuit 143. The third element capacitor 117 is connected to the photovoltaic cell 1 by a third capacitor first charge switch 144, 244, 344 and a third capacitor second charge switch 145, 245, 345, again providing for the uninterrupted production of current by the photovoltaic cell. When the voltage across the third element capacitor 117, as monitored by the third voltage sensor 127, 227, 327, equals or exceeds the desired target maximum voltage, the third capacitor first charge switch 144, 244, 344 and the third capacitor second charge switch 145, 245, 345 are opened, disconnecting the third element capacitor 117 from the photovoltaic cell 1, and the third capacitor first discharge switch 146, 246, 346 and the third capacitor second discharge switch 147, 247. 347 are opened, connecting the third element capacitor to the output circuit 143. The first element capacitor 113 may be connected to the photovoltaic cell 1 and the cycle is started again.
The process of the selective and sequential charging and discharging of the respective element capacitors 119 of each photovoltaic cell 1 may thus be controlled by the photovoltaic controller 103, based upon the voltage monitored by the first voltage sensor 121, 221, 321, the second voltage sensor 123, 223, 323, and the third voltage sensor 125, 225, 325. The photovoltaic controller may cycle between the element capacitors 119 based upon the level of irradiation of the photovoltaic cell 1, the resultant current production of the photovoltaic cell 1, and the voltages across the element capacitors 119 as measured by the voltage sensors 120, namely voltage sensors 121, 221, 321, 122, 222, 223, 321, 322, 323. The photovoltaic controller 103 may control the charge switches
For a preferred embodiment, the photovoltaic controller 103 may maintain the discharge switches, from the element capacitor 119 being discharged to the output circuit 143, open until the voltage across the element capacitor 119, as measured by a voltage sensor, is zero or a selected minimum, until the charge switches for the element capacitor 119 are opened by the photovoltaic controller 103, or until such other time or occurrence as may be determined in accordance with a control algorithm.
The use of the three photovoltaic element capacitors 119, the first element capacitor 113, the second element capacitor 115, and the third element capacitor 117, for each capacitor bank 1, may provide for the availability of an adequately discharged photovoltaic capacitor for the receipt of current from the photovoltaic cell 1 when one or both of the other photovoltaic element capacitors 119 are still discharging current to the output circuit 143. However, simplified embodiments may utilize only two photovoltaic element capacitors 119 for each photovoltaic cell, and, for most applications, this may provide for adequate cycling of charging and discharging, so that the photovoltaic cell may achieve reasonably efficient and reasonably continuous production of current by the photovoltaic cell. Similarly, more complex and even more efficient embodiments may utilize four or more photovoltaic element capacitors 119 for each photovoltaic cell. While the embodiment shown in
The capacitor switches 128 as controlled by the photovoltaic controller 103 can provide for the photovoltaic elements 3 of the photovoltaic cell 1, to be connected in parallel and with selected element of other cells to equalize the voltage before they are switched to discharge in series, providing for stepping up the voltage. If not equalized, the lowest voltage differential would determine the current.
Referring again to
For a preferred embodiment, the capacitor charge switches 128 and the capacitor discharge switches 130 may be operated by the photovoltaic controller 103 to provide for each group of photovoltaic element capacitors 119 that are to be discharged in series, to be first connected in parallel to provide for equalization of the voltage on each capacitor. This prevents the lowest voltage capacitor in the series from limiting the current when the capacitors are switched to discharging in series.
For capacitors connected in series, the current is the same at all points in the interconnecting circuit, and thus the current is the same to and from each capacitor. If independently charged capacitors are switched to series connection, the capacitor with the least voltage differential between the cathode and the anode at the time of the initial switching to a series configuration will determine the current flow from the series of capacitors. Further, the total charge discharged from the capacitor series, will be limited to the total charge stored in the capacitor of the series with the least total charge. Therefore, in order to maximize the discharge current and total charge discharged from independently charged capacitors of the same characteristics and capacitance, and hence the total energy discharged, the capacitors should be connected in parallel, immediately before connecting them in series, for equalization of the voltage and charge stored on each of the capacitors. When the capacitors of uniform characteristics and capacitance are then connected in series, the voltage differential across each of the capacitors will be the same, resulting in the current, and hence the power and total energy discharged being maximized. The capacitors can ultimately all be fully discharged, if full discharge is desired for the overall operation, or will be discharged to the same level at the time that discharge is terminated by switches controlling the charging and discharging of the capacitors. If the voltage across each capacitor is V after being connected in parallel, if there are 3 capacitors, then the voltage experienced by the series of capacitors will be 3V. The current flowing from the series of capacitors will depend on the nature of the load that the series is connected to. If the load is a battery that is being recharged, then the discharge rate of the capacitor series will be dependent upon the battery charging rate.
Following are the formulas for capacitance for interconnected capacitors for a series and for a parallel configuration of capacitors.
1/Ct=1/C1+1/C2+ . . . +1/Cn Series
Ct=C+C2+ . . . +Cn Parallel
I=C dv/dt
The DC current may be input to an inverter which will generate a pulse alternating current with a voltage of approximately 120 volts AC. The inverter current output may be input to a filter which will impose a sinusoidal waveform on the AC current. Similarly, the photovoltaic controller can be programmed to control the photovoltaic capacitor switches so as to maintain a photovoltaic array output voltage within a desired target range so as to be compatible with the needed output of an attached inverter. Similarly, the voltage may be controlled by the photovoltaic controller 103 to maintain a photovoltaic array output voltage that will provide for the charging of one or more batteries attached to the output circuit 143.
It is anticipated, based upon current technology, that the capacitors, switches, voltage sensors, and circuit connections between these components, will be components of an integrated circuit in which the photovoltaic cells are imbedded. The utilization of additional capacitors, switches and voltage sensors for embodiments with a larger number of capacitors for each photovoltaic cell, would certainly increase the cost of the photovoltaic controller of the present invention.
For alternative preferred embodiments of the present invention, the interconnection between the voltage sensors 120 and the photovoltaic controller 103 may be wireless. Similarly, the interconnection between the photovoltaic controller 103 and the switches may be wireless.
For preferred embodiments, the photovoltaic controller 103 may receive continuous voltage measurements, or voltage measurements made at intervals, from the voltage sensors 120 of each photovoltaic cell 1 of a photovoltaic array, and use the voltage data to control the switches so as to attempt to optimize the output power production for the photovoltaic array, while providing for connecting the discharge output of each photovoltaic cell 1 to the output circuit 143. The current (I) flowing from each photovoltaic cell 1 may also be measured continuously, or at intervals, by a cell current sensor 171, and the current data transmitted to the photovoltaic controller 103. This current data may be used, along with the voltage data, by the photovoltaic controller 103 to attempt to optimize the output power production for the photovoltaic array.
For a preferred embodiment, the photovoltaic controller 103 may incorporate a digital computer and may communicate by wire or wireless with the capacitor voltage sensors to receive voltage measurements and may communicate by wire or wireless with the charge switches and the discharge switches to cause the switches to open and close as needed to manage the charging and discharging of the photovoltaic element capacitors 119 so as to optimize the energy extraction of the photovoltaic array and to control the voltage and other characteristics of the output from the photovoltaic array so as to appropriately interface with storage, electric grid or other application for the extracted solar energy. For a preferred embodiment, the capacitor switches 128 may be operated by the photovoltaic controller 103 to provide for each group of photovoltaic element capacitors 119 that are to be discharged in series, to be first connected in parallel to provide for equalization of the voltage on each capacitor. This prevents the lowest voltage capacitor in the series from limiting the current when the capacitors are switched to discharging in series.
Referring now to
Voltage sensor output signals 525 preferably are transmitted to the photovoltaic controller 103. The photovoltaic controller 103 transmits capacitor charge switch control signals 529 to the respective capacitor charge switches, in directing the cells switched current 511, namely the switched first element current 513 switched second element current 515 and switched third element current 517, to the plurality of photovoltaic element capacitors contained in the capacitor bank 519 in a manner so as to optimize the handling of the charge generated by each of the photovoltaic elements of the photovoltaic cell 1.
Similarly, the element capacitor voltage signal output signals 525 are utilized by the photovoltaic controller 103 to generate capacitor discharge switch signals 531, which are transmitted to the element capacitor discharge switch assembly 543. The plurality of discharge switches contained in the element capacitor discharge switch assembly 543 directs the capacitor bank discharge current 535, namely the first element discharge current 537, the second element discharge current 539, and the third element discharge current 541, and directs the cell output current 545, which may be comprised of the first element output current 547, the second element output current 549 and the third element output current 551. The output current 545, including any or all of the first element output current 547, the second element output current 549 and the third element output current 551 may be connected in parallel, series or combination thereof with other capacitors of the photovoltaic cell assembly 557, to a load, to a photovoltaic array charge storage system or to an output circuit. The photovoltaic controller 103 may provide for all or a portion of the current from of the element capacitor discharge assembly 543 to be discharged as a composite cell output current 553, which may be directed to a load, to a charge storage unit of a photovoltaic array, or to a discharge circuit. Alternatively, the output from a photovoltaic cell assembly 57 may be directed by the photovoltaic controller 103 by the use of cell discharge switches 400 as shown in
The photovoltaic controller 103 of the photovoltaic cell assembly 57 of the present invention may also provide for the concurrent operation of a plurality of photovoltaic cell assemblies 57, such as would be present in a photovoltaic array. The photovoltaic cell assembly 57 may receive voltage sensor signals 533 from a plurality of other photovoltaic cell assemblies, such as for a photovoltaic array of which the photovoltaic cell assembly 57 of the present invention is a component, and may generate charging signals 553 and discharging switch control signals 555, which are directed to a plurality of photovoltaic cell assemblies which are operated in conjunction with the photovoltaic cell assembly 57 of the present invention, as in a photovoltaic array. In general, the photovoltaic controller will operate element capacitor charging switches and element capacitor discharging switches for each of the photovoltaic cell assemblies 57 so as to optimize the charging assembly and power and total energy output of photovoltaic array or which the photovoltaic cell assembly 57 of the present invention is a part.
Referring further to
In view of the disclosures of this specification and the drawings, other embodiments and other variations and modifications of the embodiments described above will be obvious to a person skilled in the art. Therefore, the foregoing is intended to be merely illustrative of the invention and the invention is limited only by the following claims and the doctrine of equivalents.
