Chris Petrone, Virginia Sea Grant, Virginia Institute of Marine Science
But what about renewable or sustainable energy sources? Using water, the sun, the wind, even trash, people are harnessing renewable energy all around us and most of the time it goes on unnoticed. From simple ideas, like a water mill, to more sophisticated, like the solar panel, the technology is available to shift a lot of our energy demand to more environmentally-friendly sources. By harnessing natural processes, we decrease the dependency on other countries and the destruction of our own back yard for fossil fuels.
Another source of renewable energy that is gaining focus and popularity is wave energy, utilizing the ocean's waves to create electricity. If you have ever surfed, boogie-boarded, or just swam in the ocean, you know how much power can be unleashed when a wave crashes. What about those swells outside the surf zone, can they produce as much energy? The answer is an emphatic yes! Scientists have figured out ways to harness energy from the rise and fall of the water as a wave moves past.
Most people never think twice about the electricity that powers the lights in their house, the computer, the TV, or the hot water heater, until that is, they hit the switch and nothing happens. Regardless of whether it comes from a coal-fired power plant, a hydroelectric power plant, or a nuclear power plant, electricity is produced pretty much the same way. Electricity comes from the spinning of a generator, typically powered by steam, which is produced by the burning of coal or oil, or the splitting of atoms.
Once the electricity is produced, it is moved through a power grid. After traveling mile after mile, and being stepped-down several times, it is fed into your house and is available at the flick of a switch.
The electricity that a power plant produces, and the customer consumes, is measured in kilowatt hours (kWh). In 2001, electricity consumption by 107 million U.S. households totaled 1.140 trillion kWh (U.S. Department of Energy), or approximately10,000 kWh of electricity per household. For comparison, the sun radiates approximately 174,423,000,000,000 kW of energy to the Earth per hour!
When designing a new power plant, in addition to the important decision of what type it should be, other factors must be taken into consideration. Distance to the consumer and the power grid are very important determinants of the location of the new plant. After all, if the cost of producing the electricity and getting it to the consumer is higher than the price the plant receives for the power, it will not be in business for very long. It is widely accepted that in order for a power plant to be economically viable, it needs to produce electricity at a cost at or below $0.10/kWh. This is now the goal of those exploring alternative energy sources and increasingly, scientists and engineers are reaching that goal.
Water waves can be generated any number of ways &emdash; a rock being thrown in the water, a passing boat, underwater earthquakes (tsunamis) &emdash; but the most common wave generating force is wind.
Every wave, regardless of whether it is a sound wave, light wave, or ocean wave, has the same components. The wave characteristics we'll be using in this data activity are wave height and wave period. (Click here for an interactive demonstration.) As waves move into shallow water or encounter obstacles, their forward direction will change. This plays a large part in the placement of an effective wave energy power plant.
It should be obvious that taller waves will produce more electricity than shorter ones, but what about their frequency? As waves move, the water itself is not moving forward, but instead, in a circular motion caused by the forward motion of the energy. Therefore, as a deep water wave becomes taller, it also becomes longer. Longer waves travel faster; however, the wave frequency actually decreases. These tall, lower frequency waves produce the most electricity.
So where are the best waves to harness for electricity? As it turns out, just about any wave is capable of producing at least some electricity, however, the very best places to use wave energy technologies occur outside the United States. Places such as the United Kingdom and the west coast of Europe, southern Chile, South Africa, the west coast of Australia, and New Zealand have the potential to produce electricity that far exceeds the world's demand. However, despite smaller wave power levels, the New England and northwestern coasts of the United States still possess characteristics that make them feasible sites for wave-generated electricity production.
Types of Wave Energy Technologies
Although there are many designs currently available, there are four general forms of wave energy technologies. Two are stationary or land-based and two are floating or sea-based. The two we will focus on are the LIMPET (Land Installed Marine Powered Energy Transformer) system, and the floating hinged contour device, specifically the Pelamis system. The LIMPET system found in Islay, Scotland is strictly an experiment in alternative energy sources. This 20 meter long energy converter built along the shoreline produces electricity as waves flow in and out of the system forcing air past the turbines. The Pelamis system is a floating structure of hinged cylinders. As waves cause the hinges to flex, hydraulic rams pump high-pressure fluid through the motors turning the generators and producing electricity. The Pelamis system is used in areas with a depth of 50 - 100 meters.
Using ocean observing system (OOS) buoys from all over the coastal United States, we will evaluate the feasibility of wave energy as a practical alternative energy source. With real time data from the OOS buoys, we will calculate the amount of electricity that could be produced by either the LIMPET (land-based) or Pelamis (floating) systems at certain locations along the US coastline.
Begin by printing the wave energy activity chart. Access the wave data from the following ocean observing system buoys and fill in the missing wave height, dominant wave period and site depth values on the chart.
Use the following conversions, equations, and graph to find the LIMPET Amount and the Pelamis Power Yield. These values are the amount of electricity that these alternative systems could potentially produce at each particular location. Inshore locations will use the LIMPET system and offshore stations will use the Pelamis system. After completing the handout, compare your answers with the Bridge's sample answers and answer the discussion questions.
J = 0.5 x (Hs)2 x Tp
LIMPET Amount = J x 20Where:
J is the wave energy flux (kW/m)
Hs is the significant wave height (meters)
Tp is the dominant wave period (seconds)
Tpow = 0.857 x Tpeak