Original Photo by Tim Foster on Unsplash

G

lobal warming

is a serious threat to the well-being of future generations and their ability to interact with this planet. While there are many industries that need to change protocols to help combat climate change, one of the most critical is the energy sector. Electricity has allowed us to develop incredible technology and connect us across the globe. However, it is also a large contributor to greenhouse gas emissions.

“According to a study conducted by the EPA in 2018, electricity production produced 26.9 of total US greenhouse gases.” [1]

Renewable energy has a large part to play in reducing this number.

There are many types of renewable energy, one being wind power. Wind power is generated by using large turbines to harness the kinetic energy of wind. Wind causes large turbines hooked up to generators to rotate and produce electricity. These turbines can be very large, standing over 80 meters high. Generally, these turbines are built in wind farms, which contain a large number of turbines close together.

In this post, we will look at wind power generation at the High Winds Project in Northern California which was conducted as part of my project for the Weather and Storms class at Stanford University. The goal of this post is to give a general sense of the relevant data involved and how it can be used to estimate wind power. I also want to show the general magnitude of numbers for things such as power generation of a typical wind farm. To give a sense of the current state of wind power in California:

In 2018, California wind projects generated 15,078 gigawatt-hours of electricity which accounted for 7.2% of all power generated . [2]

W

hat is Wind?

Did you know there are actually different types of wind? In class we learned about the four main types: geostrophic, gradient, surface along straight isobars, and surface along curved isobars. Each wind differs in the combination of forces that drive it. For example, surface winds acting along circular isobars have the coriolis, pressure gradient, friction, and apparent centrifugal force acting.

Forces for Surface Wind Along Circular Isobar [3]

Topography of an area also plays a role in the wind generated for that region. In northern California, specifically the San Francisco bay area, the terrain is mountainous and located close to the sea. Near the sea, when the land is warmer during the day you can get a land breeze, where the wind blows from the land to the ocean. Conversely, at night when the land is colder you can get a bay breeze, where the wind blows from the bay onto land. Pressure and temperature play a big role in wind formation in the atmosphere.

T

here

are two major types of wind turbines: vertical axis and horizontal axis. Vertical axis wind turbines have their axis perpendicular to the wind stream, while horizontal axis turbines axis’s are parallel to the wind stream. In general, the majority of wind turbines in production are horizontal axis.

Taking a closer look at the mechanics of horizontal axis turbine power generation, these structures harness the energy from wind via large propeller-like blades. Often there are three steel blades mounted to a large tower. Blades are mounted high up to take advantage of the faster wind speeds at higher altitudes. Wind blowing across the turbine causes low-pressure on one side of the blade, causing lift which rotates the blades. Due to the large inertial mass of the propeller blades, the shaft typically has a rotational speed of 5–20 rpms. To produce AC electricity, the shaft of the propeller blades is connected to a gearbox which increases the rotational speed up to 1800 rpm.

A study conducted by El-Ahmar et. al (2017) investigated the most critical factors for wind power generation and devised the following list: wind speed, air density, rotor swept area, and tower height [4]. Each of these components influences the power output differently. As you would expect, power generation is directly related to wind speed (i.e. larger wind speeds produce more power). In fact, the equation below is often used to measure the potential power output of a turbine. We will be using this moving forward to calculate power.