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Marine and Hydrokinetic Power Generation and Power Plants                   281


            PTOS, a pressure accumulator is often used to store energy [40]. In addition, most of the power
              converters presently used in MHK generators are AC–DC–AC systems, and  placing the energy stor-
            age on the DC bus is a very viable option. Another example of energy storage is  presented in [40, 41]
            as an indirect flywheel connected to the system, thus storing the energy as kinetic energy. In [42],
            energy storage is achieved using a superconducting  magnetic energy  storage system.

            11.4.2  CEC Generator
            A CEC generator is usually smoother than a WEC generator. The ocean current is known to be very
            constant over time. Similarly, for a run-of-the river CEC generator, the flow is relatively constant.
            However, the tide changes direction every 6 h; thus, for a tidal current generator, the power genera-
            tion varies within a longer time constant (6 h), and a longer-term energy storage may be beneficial
            for this type of generation.
              For a CEC generator, the best place to install energy storage is at the POI at the substation
            transformer onshore. In addition, the size is much larger, the infrastructure cost is cheaper, and the
            environmental impact is much easier to manage if it is built onshore.

            11.5  PROBLEMS AND EXERCISES

            MHK renewable energy has a large diversity in its implementation. The purpose of the problems and
            exercise in this section is to help readers understand the basics of the typical issues faced by MHK
            engineers in understanding MHK generators, MHK generation, and supporting MHK infrastructure.

            11.5.1  WEC Generator

            A useful ocean wave is affected by wind speed, the depth and surface characteristics of the ocean,
            and gravity. The ripple effect in the form of the ocean wave far from the exciting point is called the
            “swell.” Although ocean waves may be excited by a tsunami or storm, the normal, day-to-day, some-
            what predictable waves excited by normal wind speeds are of interest for conversion into electrical
            energy. Based on linear wave theory and assuming that the waves are in deep water, the power across
            each meter of wave front associated with a uniform wave with height H (m) and wavelength λ (m) is

                                                        2
                                                  .
                                             P w = 05ρg H s λ                         (11.1)
            where
              P  is the power density (kW/m) of the wave front
               w
              ρ is the specific density of the water (kg/m )
                                                3
              H  is the significant wave height (m)
                s
                                     2
              g is the specific gravity (m/s )
              Example Problem 11.1

              At one site, the average significant wave height H = 2.5 m and the water density ρ = 1 kg/m . Compute
                                                                                 3
              the power density across the wave front for this particular site if the specific gravity is given as 9.81 m/s
                                                                                        2
              and the wavelength is λ = 9 m.
              Solution

              The power density is computed as
                                                            .
                                                  25 *
                                      P w = 05.* 9.81 *.  2  9  = 275 9 kW          (11.2)
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