Page 293 - Renewable Energy Devices and System with Simulations in MATLAB and ANSYS

280 Renewable Energy Devices and Systems with Simulations in MATLAB and ANSYS ®
®

11.4 ENERGY STORAGE
The topic of energy storage is very important in renewable energy generation because the
resources continuously vary over time. Because renewable energy generation requires high-
energy capture as well as the capability to maintain the reliability of the power system, energy
storage has been implemented in many big renewable generation projects. On a positive note,
research and energy storage is also driven by the large electrical transportation sector, which
has been very successful commercially. A comprehensive review on energy storage can be
found in [38].
In this section, the energy storage for MHK generation is discussed.

11.4.1 WEC Generator
Because the nature of a wave fluctuates over time, the output of a WEC generator can be expected
to follow the same characteristics.
Figure 11.14 shows the numerical simulated instantaneous power, maximum absorbed power,
rated power, and mean power of a two-body floating-point absorber WEC system in irregu-
lar waves modeled using a time-domain numerical model [39]. Because of the oscillatory and
irregular nature of waves, WEC components must be designed to handle loads (e.g., torques,
forces, and powers) that are many times greater than the average load. Converting the peak power
requires that the PTOS be designed to process the peak power, but WEC-generated power must
be smoothed before it is introduced to the grid so that high power spikes are eliminated; thus, the
utilization of the power converter becomes very low. Here, it is easy to see that energy storage can
be very useful to allow processing average power instead of peak power. If designed correctly, a
significant saving in the capital cost can be achieved along with the overall impact of lowering
the cost of energy.
The implementation of energy storage can be in many different forms; however, to reduce the
power converter size, the implementation must be at the closest point to the generator. For a hydraulic

1600 Instantaneous power
Maximum power
1400
Rated power
Mean power
1200 Maximum power (P max )
1000
Power (kW) 800

Rated power (P
600 Rated power (P r ratedd ))
ate
400
Mean power of the given sea state
200
0
1000 1050 1100 1150 1200 1250
Time (s)

FIGURE 11.14 The instantaneous power, maximum absorbed power, rated power, and mean power for the
RM3 floating-point absorber WEC system in irregular waves with a significant wave height of 4.25 m and a
peak period of 10.7 s. (Courtesy of NREL, Denver, CO.)