Better calculations of energy in moving water
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In rivers, straits and oceans around the world there is flowing water which makes up a renewable energy source with large potential. In a new thesis on marine current power at Uppsala University, Emilia Lalander shows that currently available water data are valuable for estimating the movement speed of water and thereby the potential energy resource available in a particular area.
A growing branch within renewable energy research deals with transforming kinetic energy in flowing waters into electricity. These can be either tidal currents or flowing water in rivers. When extracting energy directly from water currents, a technique is used which is similar to today’s wind turbines: a turbine, which is powered by the flowing water, connected to a generator.
Marine current power is an ongoing research project at the Division of Electricity at Uppsala University. The work presented in Emilia Lalander’s thesis is part of that project.
The goal of the project is to develop a method as simple as possible for extracting energy from flowing water. During 2012 a full-scale prototype was finalised, a verticle axis turbine connected directly to a generator, and in March 2013 it was launched in river Dalälven by the small town Söderfors.
For a system extracting kinteic energy, the speed of the current is of great importance. The energy in the water is proportional to the cube of the speed. The focus of the study has been to measure and simulate speeds in rivers and tidal currents, as well as studying the effects of energy extraction from water currents.
Numerical models verified by water speed measurements are a well-tried method for estimating the energy resource available in a particular area. In the large-scale resource estimations made, for instance across whole countries, general descriptions of the speed have been used. Actual speed measurements have not been carried out to verify these methods. The estimations are therefore very rough, resulting in large uncertainty surrounding the results. For example, in Norway the available tidal current power has been estimated to be between 2 and 17 TWh, depending on how many places are taken into account.
Even if these numbers indicate a significant energy potential, the results are very uncertain, mainly due to the lack of data on water speeds.
“The thesis work showed that good estimations of speed for rivers can be obtained by using available data on discharge. For tidal currents, the speed can be estimated by using data from nautical charts and data on tidal water height”, says Emilia Lalander.
The thesis shows that regulated rivers, non-regulated rivers and tidal currents are all interesting from an energy extraction point of view since a high yield is possible to obtain, provided that the turbine is sized for highest efficiency close to the average speed of the water flow, rather than the maximum speed.