Using a wind farm without wind

What happens to wind farm infrastructure when there is no wind?

Virtually all wind farms are designed for nominal power: The wind turbine (WTG) transformer, cabling, roads and substation (BoP) are all designed for a wind speed of 12 m/s.  However, especially in low wind regions, this might only occur for a few hours of the year, while the rest of the time, the wind speeds are lower. For example: I have installed a number of 3.45 MW WTGs including the necessary infrastructure. This system is setup for nominal wind speeds. However, with a typical net capacity factor of about 30%, the WTG setup (WTG and BoP) is underused by 70% of its annual capacity.

As engineers, it’s our job to turn environmental challenges like this into opportunities for our clients. In this case, I’d suggest a solution using an attached Photovoltaic plant.

A wind-solar hybrid uses synergy effects

Why not oversize the installed power on site directly at the source and create a wind-solar hybrid system at each WTG?  In this way, you could utilize the capacity of your infrastructure to a higher level.

The idea is to build a small Photovoltaic (PV) plant next to each wind turbine. The PV plant would be directly connected to the transformer of the WTG. Also, the cabling from the WTG to the substation as well as the substation itself can be used without harming or changing the WTG design. Hence, the PV plant investment is reduced to the panels, the mounting structure and the inverters. The new hybrid system would optimize the use of the whole wind farm infrastructure. The land required is also often available, or can be acquired.

PV investment savings of 30%

In cases where the land is flat, approx. 30% of PV investment cost can be saved by using the described infrastructure of the WTG for a PV plant. Under strong winds, the wind turbine needs the full capacity of the grid connection infrastructure; energy produced by the PV plant may have to be dumped. However, when the wind speed is low, which happens for approx. 70% of the time as in our example, this leads to a successful solar combination and a unification on my renewable energy feed-in.

Add-on of renewable energy production 

The tropics are the ideal location for a wind-solar hybrid. Solar radiation is intense and high wind speeds are rare as we can often observe with high k-factor (k>3). This means that the frequency of wind speed groups is concentrated in a small band around the average wind speed (normally 5-7 m/s). To maintain an economic approach, the wind farm can be oversized by installing supplemental additional PV capacity.

Consider West-Africa. The wind statistics show a wind speed of below or equal to 9 m/s about 94% of the daytime. In our example above, the 3.45 MW WTG would generate 75% of its nominal power. Hence, at each location of a WTG/PV plant, I have 800 kW capacity available for each PV plant generating at 94% of the yearly daytime. Each PV plant connected to a wind turbine provides an additional yield of about 976 MWh/a. This includes the dumping of 5.0% (number based on the solar intensity 1.44 Wh/Wp/a for that region). For this example, I’d estimate an additional spec. investment of 500-600 USD/kWp as hybrid system –  a sound economic value for smaller projects.

Worthy of consideration

Surely, 11.4% additional PV energy yield in the described case may not convince or impress everyone, but it certainly is worthy of consideration. Further options considering short term battery storage and/or short term overload of the infrastructure could be linked here and would increase the benefit and reduce the waste energy.