WO2013132297A1

WO2013132297A1 - A photovoltaic device

Info

Publication number: WO2013132297A1
Application number: PCT/IB2012/054374
Authority: WO
Inventors: Siu Chung Tam; Moh Kim WONG
Original assignee: Individual
Current assignee: Individual
Priority date: 2012-03-08
Filing date: 2012-08-27
Publication date: 2013-09-12
Anticipated expiration: 2014-09-08

Description

A PHOTOVOLTAIC DEVICE

FIELD OF INVENTION

The present invention relates broadly to a device for producing a voltage when exposed to radiant energy (especially light). More specifically, the present invention relates to a device adapted for converting solar radiation including direct sunlight and scattered sunlight into electricity.

BACKGROUND OF INVENTION

Previously, concentration/concentrator photo voltaics (CPV) system has gained traction in recent years, owing to the commercial availability of affordable terrestrial CPV solar cells and modules, and the recognition that the long-term cost per installed peak watt ($/Wp) or cost per generated kilowatt-hour ($/kWh) will be lower than prevailing solar photovoltaics (PV) system, which are mainly based on crystalline silicon. However, while CPV system has potential economic benefits, they can only operate efficiently under direct sunlight. CPV system cannot operate effectively in cloudy or aerosol-infected environments, where scattered sunlight cannot be focused onto the CPV receiver. Also, for the traditional reflective CPV systems that works with only primary optics, the CPV receiver is required to be positioned at a very high level from the ground in order to receive the maximum radiation from the reflector, as normally the height of the CPV receiver is positioned at or near the effective focal length of the reflector.

One problem to be solved is to improve the efficiency of existing photovoltaic devices in order to maximize the electrical output by capturing the maximum solar radiation. Another problem to be solved is to reduce the size of existing photovoltaic devices based on area of estate (land/floor/roof) used per kilowatt-hour generated. SUMMARY OF INVENTION

The present invention provides A photovoltaic device comprising a focusing means adapted for focusing solar radiation to a receiving means; the receiving means adapted for receiving the solar radiation from the focusing means comprising a concentrator photovoltaic receiver and the focusing means comprising a photovoltaic member adapted for converting the solar radiation into electricity positioned on top of the receiving means.

Typically, the photovoltaic member is a lens.

Typically, the photovoltaic member is a Fresnel lens.

Typically, the photovoltaic member is a diffractive optical element.

Typically, the photovoltaic member is made of silicon (Si).

Typically, the focusing means further comprises a lens made of a transparent material, the photovoltaic member is mounted to a side of the lens..

Typically, the lens is a Fresnel lens.

Typically, the lens is made of polymethylmethacrylate (PMMA).

Typically, the photovoltaic member is made of silicon (Si).

Typically, the photovoltaic member is made of copper indium gallium selenide (CIGS).

Typically, the focusing means further comprises an optical filter adapted to allow a portion of the solar spectrum with wavelength with equal to or longer than 700 nm to pass through.

Typically, the optical filter is mounted to a side of the focusing means facing the receiving means.

Typically, the optical filter comprises at least one layer of dielectric material.

Typically, the at least one layer optical filter comprises alternate high and low refractive index dielectric materials.

Typically, the high refractive index material is zirconium oxide.

Typically, the low refractive index material is silicon dioxide. Typically, a homogenizer for homogenizing the solar radiation is positioned on top of the receiving means.

Typically, the receiving means further comprises a second photovoltaic member which is mounted on a face of the receiving means towards a ground.

DESCRIPTION OF THE DRAWINGS

This and other objects, features and advantages of the present invention will become apparent upon reading of the following detailed descriptions and drawings, in which:

Figure 1 shows a typical CPV design;

Figure 2 shows a classical refractive CPV design;

Figure 3 shows an illustration for Figure 2;

Figure 4 shows a schematic diagram of a first embodiment of the present invention;

Figure 5 shows a schematic diagram of the first embodiment of the present invention with a heat sink and a homogenizer;

Figure 6 shows a structural diagram of the first embodiment of the present invention;

Figure 7 shows a structural diagram of the first embodiment of the present invention by using a PV Fresnel lens;

Figure 8 shows a structural diagram of the first embodiment of the present invention by using a PV Fresnel lens without filter;

Figure 9 shows a structural diagram of a second embodiment of the present invention;

DETAILED DESCRIPTION OF THE INVENTION

Concentration/concentrator photovoltaics ("CPV") systems have gained traction in recent years, owing to the commercial availability of affordable terrestrial CPV solar cells and modules, and the recognition that the long-term cost per generated kilowatt-hour ($/kWh). i.e. the levelized cost of energy or LCOE, will be lower than prevailing solar photovoltaics ("PV") systems, which are mainly based on crystalline silicon. However, while CPV systems have potential economic benefits, they can only operate efficiently under direct sunlight. CPV systems cannot operate effectively in cloudy or aerosol-infected environments, where scattered sunlight cannot be focused onto the CPV receiver. Therefore, it is viable and advantageous to have an integrated CPV and PV system to drive down unit cost in terms of cost per installed watt ($/Wp) and $/kWh, and simultaneously unit land use in terms of hectares per peak megawatt (ha/MWp) and hectares per generated megawatt-hour (ha/MWh). Furthermore, as far as it is economical to do so, waste heat from cooling the CPV receiver may be utilized to provide warm water for utility use, generate electricity through heat engines, or otherwise in order to maximize the use of the incident solar energy. Typically, if a CPV or PV system simultaneously generates warm water for utility or process applications, the system is referred to as a combined heat and power (CHP) system.

A pure CPV system typically comprises the following key subsystems: CPV receiver, concentration optics, 2-axis tracker, and balance of system.

The CPV receiver consists of a packaged module or an extended array, which is typically made of high-temperature high-efficiency PV solar cell material such as metamorphic or lattice-matched triple-junction GaInP/Ga(In)As/Ge. As sunlight is concentrated onto the surface of the CPV material, surface temperatures may reach up to 1,000°C and efficient cooling must be maintained to reduce junction temperatures. CPV materials are required to operate at low junction temperatures to yield high quantum efficiencies and prolong working lives. While silicon or other solar cell photovoltaic materials may be used, multi-junction CPV material systems are usually chosen for their optimized high efficiencies over the entire useful spectrum of the solar radiation. The disadvantages of choosing multi-junction GaAs-based material systems are the relatively higher cost and higher indirect carbon emission per unit area compared to silicon or other solar cell photovoltaic materials.

The concentration optics should have as high a concentration ratio as possible, to reduce the cost of CPV solar cell material per unit area of sunlight footprint. This is important as the cost of the CPV solar cell receiver accounts for a significant percentage of the system cost. Today, concentration ratio between 500X - 1200X is commercially attainable. Optical configurations may be based on refractive, reflective, or diffractive design, frequently coupled with beam homogenizers prior to incidence onto the CPV receivers.

The electro-mechanical 2-axis tracker working in conjunction with a sun sensor is used to align the aperture of the solar concentrator normal to incident solar radiation at all times. The two axes are the elevation axis and the azimuth axis, respectively. Common designs involve the use of gears and drive motors, to provide tracking movements of approximately 90° in elevation and 180° in azimuth. For refractive and diffractive designs, the tracker frame has to bear the weight of the CPV modules. For reflective designs, the tracker frame has to support the reflecting elements (e.g. curved mirrors, plane mirrors) against a conic surface (e.g. paraboloidal, spherical, ellipsoidal), so that the incident solar radiation will be reflected towards the CPV receiver. The CPV receiver may be located at the primary focus (e.g. Newton design) or secondary focus of the reflective layout (e.g. Cassegrain design), and has to be supported by a spider structure fixed onto the tracker frame. Sturdiness and durability of the 2-axis tracker are important as the entire tracker and its load have to be designed to last for at least 20 years and withstand wind speeds typically in excess of 140 km/h.

Depending on the system design, balance of system (BOS) consists of DC combiners, inverters, AC combiners, grid-tied connectors and control, sun sensor, sun tracking electronics, sun irradiance monitor, anemometer, global positioning system (GPS), wireless transceiver for remote monitoring and control, lightning conductors, de-icing device, heat sink / cooler, water tank, etc.

A pure PV system typically comprises the following key subsystems: PV panel and balance of system.

The PV receiver is usually made into the shape of flat panels mounted on to structures facing the general direction of the mid-day sun. Technically, a PV panel operates at one sun (IX) solar conditions, though there are designs to use reflectors for low concentration ratios for higher efficiency, or trackers to increase its total energy output. Common PV materials include monocrystalline silicon (m-Si), polycrystalline silicon (p-Si), amorphous silicon (a-Si), cadmium telluride (CdTe), copper indium diselenide (CIS), copper indium gallium diselenide (CIGS), dye-sensitized polymers, and others. Among these PV materials, varying degree of response to scattered solar radiation has been studied. Good candidates to operate efficiently under scattered radiation include a-Si, CIS, and CIGS.

The components for balance of system (BOS) are essentially the same as those required by CPV systems.

The present invention of a first embodiment essentially combines a concentrator photovoltaics module with a photovoltaics module to form an integrated CPV-PV module. This is made possible using a PV material that not only converts incident direct and scattered solar radiation into electricity, but provides focusing functionality and/or filtering functionality to the direct normal irradiance (DNI). The design works for refractive, reflective, catadioptric, or diffractive designs.

A typical CPV design with reflective optics comprises curved or plane mirrors which are supported by a web-like structure fixed on to a spherical or paraboloidal frame to reflect the incident solar radiation onto the CPV array detector located at approximately the focal plane of the sphere/paraboloid. The mirror coating can be deposited on the surface facing the sun (front coating) or away from the sun (back coating). Metal or dielectric coatings with enhanced reflectance or abrasion resistance properties may be used. The substrate may be made of a wide range of materials such as metal, composites, ceramics, or glass. Tens to hundreds of reflector elements are clamped on to the curved frame to form the concentration dish. A beam homogenizer may be placed near the CPV receiver to render the distribution of the collected solar radiation more uniform, thereby avoiding the formation of hot spots thus prolonging service life. The CPV receiver is typically cooled actively by circulating water or passively by heat sinks. The process heat can also be used to drive a heat engine to extract more electricity output, or be used for utility heating in industrial/residential settings. The building blocks of a reflective CPV system are illustrated in Figure 1. It is a pure CPV system as no PV elements are used.

A variation of the reflective system is the catadioptric design, whereby a front element acts as a protective window that may carry some focusing power. The primary mirror then reflects the incident radiation onto the HCPV receiver that is located substantially at the focal plane of the reflector as in a Newton configuration. A secondary mirror may also be used to reflect the converging solar radiation on to the HCPV receiver located substantially at the secondary focus as in a Cassegrain configuration. The secondary mirror may also be formed at the back of the front element.

Figure 2 shows a typical CPV design with refractive optics. A refractive element converges the incident direct solar radiation onto the CPV detector located at substantially the focal plane of the lens. A homogenizer placed in front of the CPV detector is frequently used to improve on collection efficiency due to chromatic dispersion, and prevent hot spots from forming on to the surface of the solar cell. Concentration ratio ranging from 500 times to 1200 times can be attained typically. The CPV receiver is typically cooled passively by heat sinks or actively by system is illustrated in Figure 4.

The two key components of a refractive CPV system are (a) Refractive element - The function of the refractive element is to converge solar radiation ranging typically from 300nm-1800nm at high concentration ratio on to the solar cell. The refractive element typically comprises a Fresnel lens made of a polymer such as PMMA, PC or PET. The choice of a Fresnel lens is to reduce the weight of the converging element, but the refractive element may also be made out of glass in the form of a microlens array. Another common configuration is to use silicone on glass, which has the advantage of being robust against abrasion, (b) Solar cell - The solar cell for HCPV application is typically made up of a multi-junction compound semiconductor material system such as triple-junction III-V GalnP/GalnAs/Ge, double-junction material system, or single junction material system such as crystalline silicon. Quantum efficiencies for production volumes can reach beyond 40% for 3-J GaAs solar cells, and around 20% for crystalline silicon. High cell efficiency is paramount in reducing cost both in terms of $/Wp or $/kWh.

The present invention relates to the maximization of electrical output from a refractive or catadioptric CPV system by incorporating an active PV device within the system to attain high total efficiency. Alternatively speaking, the innovation can be regarded as enhancing a two-axis tracked PV system with CPV capability.

Figure 3 shows the schematic diagram of the first embodiment of the present invention. In this embodiment, the solar radiation enters to the refractive element or the focusing means and then enters into CPV receiver or receiving means by the refraction solar radiation through the focusing means. Further, the focusing means of the first embodiment of the PV device design of the present invention is shown in Figure 4. The configuration, geometry, and descriptions are not exclusive and can be modified by people skilled in the trade.

The focusing means of the first embodiment of the present invention comprises a first broadband anti-reflection (AR) coating facing the sun being deposited on to the tempered glass cover. The tempered glass provides environmental protection such as resistance against hail impact and sand storm abrasion. The coating has to be hard, and may optionally be designed to be overlaid with an abrasion resistant hard coating such as diamond-like carbon or DLC.

Additionally or alternatively, the focusing means comprises a second broadband anti-reflection coating which is used to reduced reflection losses of solar radiation from the tempered glass cover to the PV lens or the photovoltaic member. As shown in Figure 6, the PV lens 11 or the photovoltaic member 11 is made of a PV material that may be shaped into a lens to focus the incident solar radiation as commonly found in a refractive system, or carries practically zero power as commonly found in a catadioptric system. The solar radiation will first pass to the PV lens 11 and a portion of the solar radiation will be converted into electricity through the PV lens 11. The material is typically chosen to respond to the first spectrum of the solar radiation (e.g. short wavelength range of the solar radiation comprising the ultraviolet, visible, and near infrared bands), for example, crystalline silicon. An optical filter 12 or a long-wave pass optical coated can be coated or mounted to a side of the lens 11. Preferably, a homogenizer 13 can be positioned above the receiving means 14 or the CPV receiver 14 in order to receive the homogenized solar radiation from the lens 11 for the purpose to increase the efficiency of the photovoltaic device. Alternatively, as shown in Figures 7 and 8, the optical filter 12 can be mounted to a surface of the lens 11 and shaping may commonly take the form of a focusing Fresnel lens, a low-power aspheric lens, or a diffractive lens.

Additionally or alternatively, bus bars are laid or a conductive coating may be deposited on top of the PV lens for collection of holes or photoelectrons. Advantageously, as shown in Figures 6, 7 and 8, a long-pass edge filter 12 or an optical filter 12 is deposited on the bottom surface of the PV lens. The long-pass edge filter 12 or an optical filter 12 serves two purposes: (a) reflect the first spectrum of the solar radiation back through the PV medium to generate more photoelectron-hole pairs, and (b) allows high transmission of the rest of the solar radiation to propagate through the rest of the optics onto a CPV receiver or the receiving means. By judiciously choosing the PV material, geometry of the PV lens, secondary optics inclusive of the beam homogenizer, and the material system for the CPV receiver or the receiving means, high photovoltaic yield from the PV lens and simultaneous high photovoltaic yield from the CPV receiver can be obtained when there is direct sunlight. In the absence of direct sunlight, the 2-axis tracked PV material is still able to generate a significant amount of PV electricity from Mie-scattered and Rayleigh-scattered solar radiation.

Regarding the performance of an ideal long-pass edge filter 12 or an optical filter 12, the experimental result shows that the solar radiation with wavelength less than the cutoff wavelength, in the case shown being 700 nm, will be totally reflected by the edge filter, and solar radiation with wavelength longer than the cutoff wavelength will transmit totally through the edge filter. Most modern day multilayer dielectric coatings can be designed and fabricated with reflectance close to the ideal performance. As the PV material interacts only with the first spectrum of the solar radiation, typically between 300 nm and 800 nm waveband or 300 nm and 700 nm waveband, heating effect is reduced, and PV yield drop will be lower than that without the long-pass edge filter. If the thermal effect is still a problem, a heat conducting layer such as diamond like carbon film, may be deposited at the back of the long-pass edge filter to diffuse the heat.

Additionally or alternatively, the CPV receiver 14 or the receiving means 14 generates electricity from focused solar radiation that passes through the PV lens, the long-pass edge filter or the optical filter, and secondary optics before reaching the CPV receiver 14 or the receiving means 14. Figure 5 illustrates the composition of the solar spectrum that passes through a refractive design. Solar concentration ratios can typically reach between 500 times and 1200 times, and may go even higher as long as the CPV material system and the bonding agents can withstand the relatively higher temperatures.

Further, the CPV receiver 14 or the receiving means 14 may be made up of a triple junction material system, a double junction material system, or even a single junction PV material. Additional solar energy in the form of heat can also be harvested from behind the CPV solar cell.

Typically, a double junction structure (e.g. GalnAs/Ge) may have higher overall efficiency than a multi-junction structure because of lower requirement on current density balancing. The cost of production will also be lower owing to a simpler epitaxial growth process, better uniformity, and higher manufacturing yield.

The integrated CPV-PV system is able to generate photo-electricity under direct sunlight or when it is cloudy. It solves the problem of a CPV system not generating electricity at all when there is cloud cover. Once the optimum combination of PV material system and CPV material system is attained, the total photocurrent generated by the PV lens and the CPV receiver is expected to be substantially higher than those generated by classical PV or CPV systems. The system is cost effective. The adverse effect of abrupt on-off loading on the power grid due to cloud passage is reduced. Land use per kWh generated is reduced. Shorter energy payback time (EPBT) can be achieved.

In the second embodiment of the present invention, as shown in Figure 9, in this embodiment, a PV module 21 or the photovoltaic member 21 is positioned above the lens 23 made of a transparent material, such as PMMA, SOG and glass...etc. The solar radiation first goes to the PV module 21 and converts a portion of the solar radiation into electricity. The remaining solar radiation will be focused by the lens 23 on to the receiving means 14. Alternatively, the optical filter 12 or the long-wave pass optical coating is positioned between the PV module 21 or the photovoltaic member 21 and the lens 23.

Advantageously, the present invention of the photovoltaic device comprising a lens made of a photovoltaic material with a first surface facing the sun and a second surface coated with an optical long-pass edge filter so that solar radiation with wavelength shorter than the cutoff frequency will be reflected back into the PV material and wavelength longer than the cutoff frequency will be transmitted through the lens. The PV material can be crystalline silicon, i.e. polycrystalline silicon or monocrystalline silicon. The optical filter is mounted on a surface of the lens facing towards the CPV receiver. Alternatively, the PV material can be copper indium gallium selenide (CIGS) coated on a suitable substrate such as the rear surface of the protective tempered glass panel. Alternatively, the lens 11 has focusing power as a result of curvature such as a plano-convex geometry. Alternatively, the lens 11 has focusing power as a result of concentric wedges such as a Fresnel lens geometry. Alternatively, the lens 11 has focusing power as a result of concentric rings such as a diffractive lens geometry.

While specific embodiments of the present invention has been shown and described in detail to illustrate the inventive purposes, it will be understood that the invention may be embodied otherwise without departing from said principles.

Claims

A photovoltaic device comprising:

a focusing means adapted for focusing solar radiation to a receiving means; the receiving means adapted for receiving the solar radiation from the focusing means comprising a concentrator photovoltaic receiver; and

the focusing means comprising a photovoltaic member adapted for converting the solar radiation into electricity positioned on top of the receiving means.

The photovoltaic device according to Claim 1, wherein the photovoltaic member is a lens.

The photovoltaic device according to Claim 2, wherein the photovoltaic member is a Fresnel lens.

The photovoltaic device according to Claim 2, wherein the photovoltaic member is a diffractive optical element.

The photovoltaic device according to Claim 2, wherein the photovoltaic member is made of silicon (Si).

The photovoltaic device according to Claim 1, wherein the focusing means further comprises a lens made of a transparent material, the photovoltaic member is mounted to a side of the lens..

The photovoltaic device according to Claim 6, wherein the lens is a Fresnel lens.

The photovoltaic device according to Claim 6, wherein the lens is made of polymethylmethacrylate (PMMA).

The photovoltaic device according to Claim 6, wherein the photovoltaic member is made of silicon (Si).

10. The photovoltaic device according to Claim 6, wherein the photovoltaic member is made of copper indium gallium selenide (CIGS).

11. The photovoltaic device according to any one of the preceding claims, wherein the focusing means further comprises an optical filter adapted to allow a portion of the solar spectrum with wavelength with equal to or longer than 700 nm to pass through.

12. The photovoltaic device according to Claim 11, wherein the optical filter is mounted to a side of the focusing means facing the receiving means.

13. The photovoltaic device according to Claim 11, wherein the optical filter comprises at least one layer of dielectric material.

14. The photovoltaic device according to Claim 11, wherein the at least one layer optical filter comprises alternate high and low refractive index dielectric materials.

15. The photovoltaic device according to Claim 11, wherein the high refractive index material is zirconium oxide.

16. The photovoltaic device according to Claim 11, wherein the low refractive index material is silicon dioxide.

17. The photovoltaic device according to Claim 1, wherein a homogenizer for homogenizing the solar radiation is positioned on top of the receiving means.

18. The photovoltaic device according to Claim 1, wherein the receiving means further comprises a second photovoltaic member which is mounted on a face of the receiving means towards a ground.