Note: This is the third article of a three-part series covering plastics in solar energy (photovoltaic) (1) trends, (2) material/process advances and (3) applications.
Financial incentives, government renewable energy targets and technology cost reductions remain the three forces driving the adoption of solar power. Photovoltaic (PV) energy devices produce electricity that requires little or no maintenance, causes no pollution and does not deplete natural resources.
Let's take a look at a few new solar energy plastic application developments.
To start with, the most promising lens for concentrator PV (CPV) applications is the Fresnel lens, which uses a miniature sawtooth design to focus incoming light. Compared to conventional bulky lenses, the Fresnel lens is much thinner, larger and flatter, and it captures more oblique light from a light source.
Fresnel lens design allows construction of lenses of large aperture and short focal length without the mass and volume of material required by a lens of conventional design. When the teeth run in straight rows, the lenses act as line-focusing concentrators. When the teeth are arranged in concentric circles, light is focused at a central point. However, no lens can transmit 100 percent of the incident light. The best that lenses can transmit is 90 to 95 percent, and in practice, most transmit less.
Arzon Solar, powered by Amonix technology, is using economical but effective acrylic polymethyl methacrylate (PMMA) Fresnel lenses to collect sunlight, concentrating it up to 500 times onto small and highly efficient advanced PV solar cells. Arzon Solar has over 70 percent market share in the CPV sector.
Arzon Solar
Cross-section of a saw tooth Fresnel lens (left), cross-section of a conventional plano-convex lens (center) of equivalent power, and concentrator optics acrylic Fresnel lens (right).
Kuraray is supplying Arzon Solar with its commercialized PMMA concentrating lens. The Kuraray material lens features:
- Excellent light, water resistance using improved PMMA resin
- High degree of precision and light-concentrating efficiency achieved based on precision molding technology
Evonik has also developed a grade of its PMMA (Plexiglas Solar), suitable for the demands of CPV applications. Acrylic supplier Evonik and one of its customers, Concentrator Optics, now offer a guarantee of light transmission for solar PMMA lenses for at least 20 years.
The latest version, Plexiglas Solar OZ023, is adapted to the absorption spectrum of solar cells blocking sunlight below 350 nanometers, which damages the solar cells and cannot be converted into electricity anyway. At wavelengths of 350-400 nanometers, however, it allows more high-energy photons to pass through than other transparent plastics, thereby increasing the electricity yield of the solar materials.
Evonik also has launched a complete acrylic lens package, Plexiglas Solar Pre-Fab lens panels, designed to provide access to CPV optics while also saving customers costs in advance and several months of manufacturing time.
Plexiglas Solar Pre-Fab acrylic lens panels are combined with a secondary optical element (SOE) to create a complete optical train package for assembly directly into CPV modules. The prefab package allows customers to bypass buying the tooling device needed to create the product, which is expensive and requires a time-consuming installation process of several months.
Next, the use of rail and road right-of-ways for solar panel installation offers the advantage that it does not require the development of unused and potentially environmentally sensitive lands. Also, unlike other large PV installations, new transmission corridors across environmentally-sensitive land would not be required to bring power to consumers in urban areas.
The roof of a 2-mile tunnel for a high-speed Belgium rail line has been fitted with 16,000 solar panels by solar PV development company Enfinity. The installation, which took one year to complete, uses a special ballast tile structure that eliminated the need for rooftop perforations.
The installation, covering a 50,000-square-meter surface area, is generating approximately 3,300 megawatt hours (MWh) of electricity per year. Energy from the project, the first of its kind in Europe, is being used to power railway infrastructure such as the signaling, lighting and heating of railway stations etc.
The U.S. Federal Highway Administration (FHA) has awarded Solar Roadways a Phase II SBIR contract for $750,000 over two years. The money will be used to create a fully functional prototype solar parking lot for testing. Solar panels developed for road surface applications consist of three basic layers:
- Road surface layer — acrylate elastomer-based, translucent and high strength, it is rough enough to provide great traction, yet still passes sunlight through to the solar collector cells embedded within, along with LEDs and a heating element. Capable of handling heavy loads under the worst of conditions, it is weatherproof, protecting the electronics layer beneath it.
- Electronics layer — contains a microprocessor epoxy board with support circuitry for sensing loads on the surface and controlling a heating element sandwiched in polyethylene terephthalate (PET) sheet layers. The on-board microprocessor controls lighting, communications, monitoring, etc. With a communications device every 12 feet, the Solar Roadway is an intelligent highway system.
- Base plate layer — while the electronics layer collects energy from the sun, it is the base plate layer that distributes power. Weatherproof, nitrile rubber-based, it protects the electronics layer above it.
Finally, let's review research in solution processed near transparent organic PV. A disadvantage of applying solar cells to windows is that the cells absorb visible light, reducing the light within the room and the ability of the human eye to see the exterior through the window.
An ideal photoactive layer material for visibly transparent organic solar cells needs to harvest most of the photons from ultraviolet (UV) and near-infrared (NIR) wavelengths in the solar spectrum, while the photons in the visible range should be transmitted to enhance the cells transparency to the human eye.
UCLA researchers have created a new solution: a processable organic PV polymer, near-infrared photoactive polyester that is nearly transparent and more durable/malleable than silicon at about 0.1 microns thick. The high-performance transparent PVs are achieved by combining polymeric PV materials sensitive to NIR light but highly transparent to visible light, together with solution-processed high performance AgNW (silver nanowire)-based composite transparent conductors.
UCLA-YangYang Laboratory
AgNW-based composite transparent conductor with organic solar cell polymer absorption profiles.
As high power conversion efficiency (PCE) is strongly dependent on the fraction of photons absorbed, there is often a compromise between captured photons and polymeric film transparency that limits materials development for visibly transparent PSCs.
For example, poly(3-hexylthiophene) (P3HT):[6,6]-phenyl-C61 butyric acid methyl ester (PCBM) blend is the most commonly used photoactive layer material in visibly semitransparent organic solar cells. However, due to its efficient photon harvesting in the visible wavelength region, P3HT:PCBM devices often have low optical transparency.
The transparent conductor is also a key factor in performance of visibly transparent organic solar cells. An ideal transparent conductor for visibly transparent organic solar cells must simultaneously have high transparency and low resistance together with ease of processing and effective charge collecting.
Both visible light transparency and PCE are addressed simultaneously. The technology demonstrates an economical solution processed, highly transparent solar cell. About 6 percent of solar energy (equivalent to one-third of the available IR spectrum) is converted into electricity, in comparison to 11-12 percent conversion from commercial organic PV.
Researchers expect to be able increase solar energy conversion to 10 percent in the next 3-5 years. A maximum transmission of approximately 66 percent at 550 nanometers has been achieved. Typical window glass is 96 percent transparent to visible light.
