provide a virtually endless supply of energy:
• Wind turbines sited in locations with strong, constant wind produce electricity by the slow rotation of three long blades. Turbines are typically installed on hilltops or open plains, with each tower rising 200 feet into the air. Wind turbine output ranges from 500kWto 3 MWeach, and installations range from a single turbine up to several dozen in one array. In the U.S., windpower has occasionally encountered opposition from people who dislike the visual impact of turbine towers; however, in Europe, people tend to find wind turbines aesthetically pleasing.
• Hydroelectric power plants harness the energy in falling water to turn turbines. Large hydro plants are built as dams in major rivers, and can have thousands of megawatts of generation capacity. But flooding for large hydro projects may displace native peoples, and the dams disrupt river ecosystems. Small or “run of the river” plants (less than 10 MW) simply use flowing river water to turn turbines, and don’t require a dam.
• Solar thermal power plants use large arrays of parabolic mirrors to concentrate sunlight more than 80-fold to heat water into steam that can turn turbines.
• Photovoltaic (PV) systems are often casually referred to as “solar,” but operate very differently from solar thermal plants, using heat from sunlight to excite electrons and create a current. The energy-intensive manufacturing process for PVproducts offsets some of the energy savings from PV(and drives the capital cost up quite high).
• Geothermal power plants pump water deep into the earth, where volcanic heat turns it to steam that can run turbines back on the surface. Injection wells and effluent ponds scar the landscape to some degree; however, in Iceland, which relies 100% on geothermal power, effluent ponds have been turned into high-end health spas.
• Tidal barrages generate electricity in much the same way as hydroelectric dams, and can be as large as 250MW. As the tide in an estuary ebbs, flowing water turns underwater turbines. But similar to hydro plants, these underwater dams and turbines disrupt the aquatic ecosystems.
• Biomass plants operate by burning wood chips, sugar cane bagasse, corn cobs, or other organic waste to heat water and run steam turbines, and are often built adjacent to farming areas or paper plants where combustible organic byproducts are produced. Plant material can also be made into ethanol, an oil that can be burned just like petroleum. Biomass is composed primarily of carbon, and thus produces a lot of carbon dioxide and particulates when combusted. However, since the fuel material would have been burned or decomposed anyway without generating electricity, these plants reduce net pollution in the system.
• Landfill gas is methane gas produced by decaying organic material. This natural gas can be captured as fuel for a power plant, rather than left to vent to the atmosphere. While methane is a fossil fuel and produces carbon dioxide during combustion, capturing and utilizing it offsets an equivalent volume of mined gas that would otherwise have been combusted.
People debate about which of the above technologies truly merit the label“renewable.” Purists focus primarily on windpower, which is not only at present the most cost-effective renewable technology, but also the one that many agree has the least net impact on the environment.
Renewable resources supply just around 5% of world energy usage. Why not more? First of all, most energy consumption is for transportation and heating, while renewable technologies are focused on electricity generation. In other words, renewables supply a healthy percentage (about 20%) of world electricity needs (primarily from hydroelectric plants), but the world’s electricity needs are only one part of total energy consumption. Secondly, many renewable electricity generation technologies are more expensive than fossil fuel generation. Even in cases where renewables are cost competitive with non-renewables, new technologies have a difficult time displacing entrenched standards: regulations can create an unfair disadvantage, and the public can be skeptical and disapproving.
Hydrogen is in some ways the ultimate renewable fuel, if only we could figure out how to produce it without using up as much energy as the hydrogen in turn produces for us. For this reason, hydrogen doesn’t typically appear on a list of renewable energy sources: it’s an output of energy use. Once hydrogen is produced, it can be used as a power plant fuel, or used to power fuel cells or combustion engines in cars.
While hydrogen is the most abundant element in the universe (90% of all atoms are hydrogen), it doesn’t exist in its elemental state on our planet. To access hydrogen as a fuel, we must first split it out of water or organic matter through one of two ways.
1. If we use fossil fuels to generate electricity to electrolyze water, or steam to reform natural gas into hydrogen, we simply displace pollution from the point where hydrogen is used to the point where it is produced. Using non-renewable fossil fuel to produce hydrogen makes the resulting hydrogen a non-renewable resource as well.
2. On the other hand, if we use renewable resources to generate electricity to produce hydrogen and then use the hydrogen as fuel, then the entire system is emission-free and renewable.
The United States government’s energy policy has recently focused prominently on the hydrogen-powered car. However, the much-criticized American “drive here, pollute elsewhere” policy envisions using domestic coal reserves to produce electricity to make hydrogen, liberating the U.S., in part, from imported oil. If we create hydrogen using coal, we don’t reduce
pollution – we only shift it from the roads to the power plants. In contrast, Europe has chosen to dramatically increase renewable resources in its electricity supply over the coming years. Renewably-generated electricity will produce hydrogen, which can be both a clean car fuel as well as an energy storage mechanism, enabling the electric grid to function with a high
percentage of wind and solar generation.