Renewable energy will be the world’s fastest-growing source of electricity generation over the next two decades, although it will still make up a relatively minor portion of the global energy supply, according to the Energy Information Administration. The majority of that increase will come from the use of wind power and water power, or hydropower. The Obama Administration advocates a policy that would require 25% of United States electricity demand be met by renewable energy by 2025.
But wood, coal, and oil continue to be the primary fuels that provide energy for the majority of human activities. Alternative technologies that utilize the natural energy sources of the Earth — such as wind, water, and the Sun — continue to make progress.
Building a Better Wind Turbine
The U.S. now has with the world’s largest installed base of wind power, according to the World Wind Energy Association. More than 8,300 megawatts of wind power were installed in 2008, expanding the nation’s total windpower- generating capacity by 50 percent in a single year. As part of the economic stimulus plan signed in February, President Obama extended tax credits for wind and increased the amount the government will spend on those credits by 30 percent.
Wind turbines convert the kinetic energy of the wind into mechanical energy, and then into electricity. A turbine is generally made up of rotor blades, a gearbox, and generator. Because wind speeds fluctuate, operating the generator and the turbine in the most efficient way is difficult. Engineers at Purdue University and Sandia National Laboratories have developed a technique that uses sensors and computational software to constantly monitor forces exerted on wind turbine blades, which are made primarily of fiberglass and balsa wood.
The engineers embedded sensors called uniaxial and triaxial accelerometers inside a wind turbine blade as the blade was being built. The sensors measure two types of acceleration. One type, dynamic acceleration, results from gusting winds, while the other, called static acceleration, results from gravity and the steady background winds. Accurate measurement of both forms of acceleration is essential to estimate forces exerted on the blades. Sensor data in a smart system might be used to better control the turbine speed by automatically adjusting the blade pitch, while also commanding the generator to take corrective steps.
Hydropower: Utilizing the Ocean’s Waves
Of the several forms of hydropower, wave and tidal power are rising in popularity. Wave motion might yield more energy than tides, and its feasibility has been particularly investigated in Scotland and in the UK. Wave and tidal power plant developer Aquamarine Power of Edinburg, Scotland plans to develop 1 gigawatt of wave and tidal power by 2020.
Aquamarine’s Oyster® wave power device is meant to be deployed at short depths of 10-12 meters, and is designed to capture the energy found in amplified surge forces in near-shore ocean waves. The system consists of a simple oscillating wave surge converter, or pump, fitted with double-acting water pistons. Each passing wave activates the pump, which delivers high-pressure water via a subsea pipeline to the shore. Its offshore component is a simple, highly reliable mechanical flap with minimal submerged moving parts. Onshore, the high-pressure water is converted to electrical power using conventional hydroelectric generators. Any excess energy is spilled over the top of the device’s flap — its rotational capacity allowing it to duck under the waves. Oyster is unique in that it starts generating electricity in almost calm sea conditions and can continue generating during storms.
Another company developing wave energy technology is the Wiltshire, UKbased Checkmate Seaenergy. Their Anaconda wave energy converter is a 200-meter-long, water-filled distensible rubber tube that is anchored to the ocean’s floor. The device is moored at the bow so that it faces waves head-on, and floats just beneath the ocean’s surface in waters up to 50 meters deep. Anaconda is squeezed by passing waves, which form bulges in the tube and travel down its length, gathering energy from the wave as it goes. The bulge wave then hits a hydraulic turbine at the stern, creating electricity. Since An - aconda is mainly made of rubber, it offers a durable and cost-effective addition to wave energy technologies.
A New Solar Concentrator
The U.S. solar energy industry grew about 9 percent in 2008, but the recession has cut demand for some solar installations. Solar energy projects will now be getting a financial boost of $117.6 million from the American Reinvestment and Recovery Act, which aims to accelerate commercialization of clean solar energy. Solar energy can be converted to electricity in a couple of ways. Photovoltaic (PV) devices, or solar cells, change sunlight directly into electricity, whereas solar power plants indirectly generate electricity: the heat from solar thermal collectors is used to heat a fluid, which produces steam that is used to power a generator.
The U.S. Department of Energy re - cently granted $3 million to Skyline Solar of Mountain View, CA for solar photovoltaic research. The company manufactures a system called High Gain Solar (HGS), which uses metal reflectors to concentrate the sunlight onto monocrystalline silicon solar cells for electricity production. Skyline’s HGS architecture delivers ten times more energy per gram of silicon versus flat-panel systems in sunny locations. It is built primarily out of commodity materials with globally available manufacturing processes from the PV and automotive industries, thereby making it cost effective.
Parabolic troughs are used in solar thermal plants to concentrate light on tubes, heating up a fluid inside them that is then used to drive power-generating turbines. Skyline Solar replaced those tubes with narrow solar panels, adding a heat sink to keep them from getting too hot. Each HGS array consists of a reflective parabolic trough, or reflective rack, and four rows of HGS panels along its edges. The arrays are mechanically coupled together into long columns with adjacent units sharing mounting and tracking hardware. Single-axis tracking collects the maximum amount of light throughout the day, increasing overall energy production. Multiple columns are installed side-by-side to create large solar fields.
The reflective rack provides structural support for the overall array, and its top surface is covered with a thin, durable, metallic coating encased in oxide layers. This top surface is combined with a set of prefabricated struts and ribs underneath, forming a lightweight space frame. Skyline replaces most of the silicon found in traditional PV systems with highly reflective sheets of metal on the top of its reflective rack, and the arrays are built almost entirely of recyclable metal. The HGS systems are designed for large ground-mounted solar applications from 50-100 kilowatts up into the many megawatts.
For more information, visit Skyline Solar at www.skyline-solar.com
The Question of Appearance
Solar and wind-generating stations have been criticized from an aesthetic point of view, and a company called Solar Botanic of London plans to address this issue by building artificial trees that reap both solar and wind energy. The company’s plan involves three different energy-generation technologies: photovoltaics, thermoelectrics (electricity from heat), and piezoelectrics (electricity from pressure). Solar Botanic’s trees could be installed in areas where naturally growing groups of trees would previously have been used, such as along motorways, suburban streets, and parks.
Solar Botanic’s essential technical element is the Nanoleaf, which is designed to capture the Sun’s energy in photovoltaic and thermovoltaic cells. The Nanoleaves would rely on thin-film solar cells, perhaps made of copper indium gallium selenide, to convert sunlight into an electric current. The leaf stem and twigs comprise nano-piezovoltaic material — these tiny generators produce electricity kinetic energy caused by wind or falling raindrops. As the wind blows the layers of voltaic material in the stems, twigs and branches are moved, compressed, and stretched, creating electricity.
For more information, visit SolarBotanic at www.solarbotanic.com
Works in Progress
These technologies, along with countless other advancements, are constantly evolving and undergoing further development. Skyline Solar has completed the construction of a grid-connected HGS demonstration project in San Jose, CA. The system provides power for a transportation maintenance facility serving over a million residents in Santa Clara County, CA.
In April, Aquamarine’s Oyster wave device produced electricity onshore on a full-scale test rig, proving that it can deliver electricity on a commercial scale. The output from a single pumping cylinder delivered over 170kW of electricity, demonstrating that a full-scale device, with two pumping cylinders, would deliver well in excess of the modeled output of 350kW. A commercial farm of just ten devices could provide clean renewable energy to a town of 3,000 homes.
The Purdue University blade is being tested on a research wind turbine at the U.S. Department of Agriculture’s Agriculture Research Service Laboratory in Bushland, TX. The research is ongoing, and the engineers are now pursuing the application of their system to advanced, next-generation wind turbine blades that are more curved than conventional blades.