As a part of this project, it was our aim to identify the barriers of wave power. One of these barriers is wave power matching the demand. So, we decided to make a study of the wave power output comparing it with wind power, because we thought wave power matches the load profile better than wind.
      Case Study
      Data and Assumptions
      Results


Case Study

For our case study we decided to look at offshore islands because we think possibilities for wave energy on a small scale are bigger.

Many remote communities and offshore islands without connection to the electrical grid depend on diesel generators. In these locations, the cost of electricity is high, so wave energy devices can be used in a fuel saving mode with diesel generators. In some of these islands, wave energy resource is high. At theses places wave energy could be competitive for electricity generation as well as for desalination and supply of fresh water.

Shetland Islands were chosen for the following reasons:
     • There are no subsea connections to the Scotland mainland system.
     • The main generation source is a diesel/gas fuelled power station (Lerwick power station).
     • Wave energy resource is high.

We carried out the study for Western Isles as well. Western Isles are connected to the mainland electrical grid, but the wave energy density is very high and we considered it would be interesting to do the analysis in two different places.

The seasonal variation of the weather affects the amount of wave energy available. One of our aims was to compare the variation of wave energy with the variation of wind energy to see if wave power output is more constant than wind output and it matches the electricity demand.


Data and Assumptions

The wave and wind data used to calculate the energy available and power output have been supplied by Oceanor. These data are from Haltenbanken in Norway. We have assumed that the wave data will be similar for Shetland and Western Islands, because the waves can travel thousand of kilometres without significant loss of energy.

The energy consumption data of Shetland and Western Isles have been supplied by Scottish and Southern Energy Plc.

Wave device characteristics: Cut in: 10 kW/m Cut out: 25 kW/m Efficiency: 24% Wave device size: 2 MW Wave power plant size: 8 MW

Wave turbine characteristics: Radius: 15.35 m Cut-in wind speed: 5 m/s Rated wind speed: 15 m/s Cut-out wind speed: 25 m/s Rated output: 600 kW at 15 m/s Wind power plant size: 8 MW Number of wind turbines: 13

Results

Complexity of waves: We have investigated the available wave and wind power on calm days and windy days. Then we found some interesting results as it can be seen from the following graphs. There is no strong correlation between wave and wind energy. Normally when there are strong winds, there will also be strong waves. However, when there are strong waves, there is not necessary to be strong winds, because waves can be generated from somewhere far away and travel thousand of kilometers without significant loss of energy. This shows that waves are complex and very difficult to be predicted.

Calm Days: Windspeed < 5 m/s	Windy Days: Windspeed > 10 m/s

---------- Available Wave Power ---------- Available Wind Power

Annual plant factors standard deviations: The normalised Annual Standard Deviation (SD) value of wave energy is lower than wind, but the difference is not as big as expected. Nevertheless, the difference between the Normalised Annual Standard Deviation value of the wave power output and wind power output from the devices is bigger. This means that the fluctuation of wave power is smaller than the fluctuation of wind power output.

The annual plant factors indicate that the energy output from wave energy converters is almost three times higher than wind energy output from wind turbines. Therefore, wave power plants are more attractive than wind farms, even though the costs may be three times higher than the wind farms.

Power Available from the Resources	Wave	Wind
Annual Average (kW/m, kw/m2)	45.22	560.26
Annual Standard Deviation	67.23	853.25
Normalised Annual SD	1.49	1.52
Power Output from Devices	Wave	Wind
Annual Average (kW)	4912	1853
Annual Standard Deviation	3428	2212
Normalised Annual SD	0.70	1.19
Annual Plant Factor (%)	61.40	23.16

Monthly energy consumption and wave/wind energy production: One of the results of our case study is the comparison of monthly energy consumption with wind and wave energy production as shown on the following bar charts. This allows us to see that wave energy matches the demand cycle better than wind energy. During the wintertime, the energy consumption is relatively high comparing to the summertime. Fortunately, there is a lot of wave energy during that period. Consequently, we can reduce the differerence between conventional fuel consumption in the wintertime and summertime, which means the reduction in fuel storage and procurement difficulties in remote areas.

Hourly variation of electricity demand and wave/wind power production: From the analysis carried out, it can be confirmed that wave power output is smoother and more reliable than wind power. There are some isolated cases, where the wave power output is not so good and wind output is almost zero, this can be seen in the last graph from the winter weeks, shown below.

During summer time, the wave and wind output are smaller due to the variation of the weather affects the amount of energy available. Nevertheless wave power is again more constant and bigger than wind output.

We can conclude that wave energy output is smoother than wind output and matches the electricity demand better.

Electricity Demand and Wave/Wind Power Production for Shetland Isles Winter Weeks

	---------- Electricity Demand ---------- Wave Power Output ---------- Wind Power Output

Electricity Demand and Wave/Wind Power Production for Shetland Isles Summer Weeks

---------- Electricity Demand ---------- Wave Power Output ---------- Wind Power Output

Electricity Demand and Wave/Wind Power Production for Western Isles Winter Weeks

	---------- Electricity Demand ---------- Wave Power Output ---------- Wind Power Output

Electricity Demand and Wave/Wind Power Production for Western Isles Summer Weeks

---------- Electricity Demand ---------- Wave Power Output ---------- Wind Power Output