Focus on Concentrated Solar PV. Rehnu
REhnu was founded as an Limited Liability Company (LLC) in early 2009 to bring the solar concentrating technology developed at the University of Arizona into production. (1) REhnu holds an exclusive license to commercialize this patent-pending technology.
REhnu (pronounced “renew”) derives its name from the formula E=hν or E=hnu which relates the energy E of a photon to the photon’s frequency, given by the Greek letter ν, or “nu”, and Planck’s constant h. The R preceding E in our name signifies the REnewable nature of energy derived from light.
The company logo is inspired by Richard Feynman’s (2) iconic diagram of a photon (wavy line) transferring its energy to an electron.
About the Technology
REhnu’s dish reflectors are made from float glass, the same type manufactured in huge volume for windows, fire-polished on both front and back sides. The glass is produced in the factory as flat sheets, 3.3 m square. In a process developed by the University of Arizona, each sheet is heated and molded in one operation into a deep and precisely shaped paraboloid. Once cooled, a reflective film of silver is applied to the back surface, similar to a bathroom mirror. With rolled edges for stiffening, the finished reflector surface is 3.1 m x 3.1 m.
The company predicts thats these reflectors will be robust and have excellent chemical stability, based on decades of experience with large solar thermal plants in the Mojave desert.
Desert Tested:
These solar plants use trough reflectors made of the same back-silvered glass, in the form of cylindrically bent segments. After 20 years of exposure to the sun, they have retained their original high reflectivity of 93.5%, and only 1 in 300 (0.3%) trough reflector panels has been lost each year to wind and hail.
The Reflective Dish
The accuracy of the reflector dish molding technology has been measured by tests of a 3.0m diameter reflector, shown below. It was made from back-silvered segments replicated by the same technology being developed for full-size single-piece dishes. The point spread function (PSF) of the reflector was measured by pointing at the full moon and measuring the lunar image by photometry. The data show 99% of all the focused energy lying within a circle of 1.5° diameter. This is small enough to ensure that all this energy is relayed in the receiver to the cells, because of the broad angular tolerance provided by the ball lens optical system
Steps to Commercialization, Manufacturing and Sales
As a first step in commercializing the dish manufacturing process, REhnu plans a glass shaping and silver coating facility to operate on a 3 hour cycle time. Operated at maximum capacity this will yield 3,000 dishes a year. Each dish will power a 2.5 kW receiver, for 7 MW of new generating capacity a year. For gigawatt scale production levels, the shaping and silvering steps will be built into a dedicated float glass factory, which will produce one square meter of reflector per second, enough for 6 gigawatts per year.
More Technical Details for the Engineers and Techies out there
Evenly Distributing the Energy
REhnu’s unique receiver optics take concentrated sunlight energy and apportion it evenly among 36 triple-junction cells. The optics comprise a ball lens and a concave array of optical funnels, as shown in the photograph of a prototype made at the University of Arizona.
Sunlight from the reflector dish (arrows) comes to a highly concentrated focus near the center of the ball lens. On emerging from the ball, the light forms a concave image of the dish at ~400x concentration. Here, the array of optical funnels constructed to match the size and shape of the image captures all of the light. The light is further concentrated by passing through the funnels, emerging at 1200x geometric concentration. Immediately behind the funnels are the triple-junction photovoltaic cells,
The funnels are sized so that all the cells receive the same power. This ensures that all cells generate the same electrical current, as required for efficient electrical connection. Because the funnel entrances lie at the dish image, the uniformity of illumination is preserved even when there are tracking errors, as caused by wind gusts. Another powerful feature of the optical system is that all the collected light is directed toward the active areas of one cell or another. No sunlight is wasted on the light-insensitive cell edges or gaps.
Triple-junction photovoltaic cells
Regular photovoltaic cells have a single photoelectric junction and can only capture a small fraction of the energy of solar photons to make electricity. Triple-junction cells have three different junctions which separately capture the energy of different wavelengths of light.
The result is more than double the conversion efficiency of single-junction cells, such as silicon. Triple-junction cells with 35% conversion efficiency under field operating conditions are manufactured commercially by technology that is already mature. More advanced cells with conversion efficiency >40% will likely be commercially available within a few years.
Triple-junction cells are more difficult to manufacture than silicon cells and cost much more per unit area, but when used with highly concentrated light, cost much less per unit power generated. When sold in the quantity needed to generate several megawatts, the cost is projected to soon reach $5/cm2, which at 1200x concentration and 30% module conversion efficiency, works out to $0.16/watt. This is one sixth the power conversion cost of even the cheapest photovoltaic panels with single-junction cells used without concentration.
Triple-junction cells must be well cooled when operated at high concentration, both to minimize the stresses of thermal cycling with clouds, and to achieve maximum output. The power produced by triple-junction cells is reduced by 1.35% for every 10°C rise in temperature, placing a premium on thermal management, which brings us to thermal management.
How do they do it?
Thermal management—Active Cooling with no water consumption
To illustrate the power of concentrated light at the dish focus, Rehnu engineers replaced the ball lens with a ¼-inch-thick steel plate and melted a quarter-sized hole in seconds. Even with highest efficiency cells available today, most of the sunlight energy still goes to heating the cells. REhnu’s receivers use a closed-loop system to actively remove this heat. Liquid coolant is circulated in a closed loop behind the cells and through a fan-cooled radiator, as in an automobile. While this process is more complex than the simple aluminum heat sinks used in conventional CPV at lower concentration, it uses much less aluminum, only in the radiator which weighs 10 kg/kW. The sealed cooling system has no consumption of water.
The pumps and radiator fans are designed to be very mechanically efficient, so their power consumption is more than offset by the extra cell power that results from efficient cooling.
The basic modular unit of REhnu’s system consists of a 2×4 array of eight dish reflectors and receivers mounted in a spaceframe structure and pointed at the sun. The total collecting area carried by one unit is 73 m2. The above photo shows a prototype space frame module at the Steward Observatory Solar Lab on the University of Arizona campus, awaiting installation of mirrors and receivers. Square tarps have been installed in place of the eight dish reflectors, for pointing stability tests in wind. The regular square array of the shadows shows that the structure is oriented to the sun.
The lightweight spaceframe structure of a module is optimized for high stiffness and extremely low mass. The structure consists mostly of large cube frames, each holding in coalignment a dish and receiver.
The spaceframe also serves as the complete azimuth structure for sun tracking. It includes two stiff, widely spaced nodes near its center of gravity that support the elevation bearing, and a cut-away allowing for elevation motion. In this way, the added motion for 2-axis sun tracking requires only a pedestal with a vertical axis azimuth bearing. The spaceframe acts structurally as a beam with a large hexagonal cross section. This design achieves, with a minimum of steel, the high stiffness needed to avoid bending under gravity and the high strength needed to withstand very high wind gusts.
Low mass per watt of output power
The two-axis tracker can easily become the most expensive single item of any high-concentration photovoltaic system. Hence, the overriding consideration in REhnu’s tracker design is to minimize its mass and cost per kilowatt of output power, while constraining the energy consumed and carbon emitted during manufacture. On this basis, the entire structure is made of lightweight high strength low alloy (HSLA) steel, configured in the lightest possible structure. The total mass of steel in the spaceframe, pedestal, and foundation of the REhnu design amounts to 100 kg/kW of peak power. This is less than most two-axis trackers and single-axis solar thermal trough systems. It is also less than for most wind power turbines.
Turning well clear of the ground
The spaceframe structure sits 8 feet above the ground, leaving vegetation and wildlife largely undisturbed. The high clearance also minimizes soiling of the reflectors. The pedestal foundation is built with a minimum of steel and no concrete, disturbing only a small ground area.
Power Generation
An end-to-end test was made using a partial paraboloidal reflector with 4 rectangular, back-silvered segments, as illustrated. The reflector area is 22% of a full 3.1 m square paraboloidal dish.
In the test receiver, the image of the 4 segments formed by the ball lens falls onto four pairs of adjacent optical funnels, instead of the full set of 36. Behind the 8 funnels are eight 15 mm square cells, totaling 3 square inches, operated at the full geometric concentration of 1200x.
The DC power output of the 8 cells connected in series was measured at 511 watts. This is at the optimum power point of 24.4 A and 20.9 volts shown in the I-V curve. The same curve indicates an open circuit cell voltage 3.05V, and hence a cell operating temperature of 60°C, only 20°C above the high ambient air temperature. From the measured DNI of 940 W/m2, the end-to-end efficiency is calculated at 25%. These results scale up for a full receiver with 36 instead of 8 cells, powered by a full, single-piece dish, to an output close to the projected 2.5 kW at 1000 W/m2 DNI solar flux. Tests of such a receiver and dish are projected for early 2011. Efficiency improvements made later in 2011, from higher efficiency optical coatings and more efficient cells optimized for the 1200x concentration, will ensure the 2.5 kW target is met or exceeded.
Ok but how does it do in a big wind?
The power-stabilizing property of the optical concentrator system was measured by switching off the tracking drive. The output measured as the sun moved across the sky, passing through the system axis, is shown in the figure at right. We find an angle of acceptance for power >90% of its on-axis peak of ±0.6°. Such a wide tolerance is enough to maintain full power in all but the strongest wind gusts, because the spaceframe module and its support has shown to be very stiff, despite its lightweight construction. During a month of daytime solar tracking, the prototype spaceframe module loaded with tarps was measured to point within 0.1° of the sun for over 99% of the time. Thus even in a less sheltered location on a solar farm, we can expect very small loss to wind buffeting.
Lets take it a step further:
Consider this: A solar farm spread over 6 square miles of the sunniest areas of the U.S. southwest will yield 2.3–2.6 billion kW-hr/year A 1 gigawatt REhnu farm comprises 50,000 20 kW generators. Completed units are set out on the farm in a triangular grid pattern on 50′ centers. In the middle of each farm is the assembly facility.
Completed units are moved out to the field for installation along narrow access roads running between every other row of generator units. Clearance needed for installation is obtained by turning units parallel to the road. In this way, installation and reflector cleaning operations can be fully mechanized. An optimally sized single farm of gigawatt scale will cover 6 square miles, with the longest distance to transport a generator out from the assembly facility being 2.5 miles.
Annual energy yield
The annual energy yield depends on the annual average direct solar flux at normal incidence. Across most of Arizona and southern California this is between 7 and 8 kW-hr/m2/day. The long horizontal profile of REhnu’s generators minimizes self-shadowing to less than 10%, despite the farm’s closely packed layout. Thus, depending on location, the actual annual yield will be 2.3–2.6 billion kW-hr/year, about a quarter the output of a coal-fired plant running continuously at 1 GW (but with no fuel cost and no CO2 emission).
(1) – The LLC now has six members, with a broad range of technical and business experience. REhnu’s key collaboration remains with the University of Arizona Steward Observatory, where glass molding facilities and the prototype spaceframe module are located.
(2) -Richard Phillips Feynman ( /ˈfaɪnmən/; May 11, 1918 – February 15, 1988,was an American physicist known for his work in the path integral formulation of quantum mechanics, the theory of quantum electrodynamics and the physics of the superfluidity of supercooled liquid helium, as well as in particle physics (he proposed the parton model). For his contributions to the development of quantum electrodynamics, Feynman, jointly with Julian Schwinger and Sin-Itiro Tomonaga, received the Nobel Prize in Physics in 1965. He developed a widely used pictorial representation scheme for the mathematical expressions governing the behavior of subatomic particles, which later became known as Feynman diagrams. During his lifetime, Feynman became one of the best-known scientists in the world.
He assisted in the development of the atomic bomb and was a member of the panel that investigated the Space Shuttle Challenger disaster. In addition to his work in theoretical physics, Feynman has been credited with pioneering the field of quantum computing,[3] and introducing the concept of nanotechnology.He held the Richard Chace Tolman professorship in theoretical physics at the California Institute of Technology.
Tags: buy solar mirrors, cheaper solar mirrors, concentrated solar power, flat sheets, mirror, mojave desert, reflective film, richard feynman, solar plants, thermal plants
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