Can Wind Energy Be Captured in New York City? Case Study on Urban Wind based on a Feasibility Study by Orange Line Studio Spark 101 Educator Resource Copyright 2013
Defining Key Concepts What is wind power? How do you quantify wind power? How do you capture wind energy? Wind is the movement of air flowing around the earth due to differing weather conditions. As the sun heats the earth, there are temperature variations due to the uneven heating and cooling of the land, oceans, and atmosphere. These temperature variations create higher and lower air pressure areas. As warm air rises in one area, cooler dense air flows in to fill the space it had occupied, and this flow or movement of the air is what we call wind. Moving air molecules have kinetic energy, which is the energy created whenever mass is in motion. This kinetic energy gives wind the capacity to perform work. We measure this work in terms of Watts. With the right technology, the wind s kinetic energy can be captured and converted into other forms of energy, such as electricity or mechanical power. The conversion of the wind s energy into another form is how we can use wind power. Wind power is measured in terms of wind power density, or Watts per Square Meter. Watts are a unit of work, and the number of Watts available in one square meter of wind varies depending on wind speed and air density. Wind speed is extremely important in determining the amount of energy a wind turbine can convert to electricity. Wind speed has a cubic relationship to wind power, so strong winds have exponentially more power than mild winds. In fact, with each 10% increase of wind speed, the power available increases by 33%. For example, wind at 15 mph is almost twice as powerful as wind at 12 mph, doubling the electricity output of a wind turbine. At sea level, wind at a 5.5 meters/second (12 mph) will contain about 100 Watts of power per square meter. At higher altitudes, where air is less dense, there will be fewer watts per square meter in the same speed wind. Wind power is captured using a rotor to convert the kinetic energy in the moving air molecules into mechanical (rotational) energy. The amount of energy that can be captured by a rotor and converted depends on the size of the rotor, the height of the rotor above and away from air turbulence, and the efficiency of the turbine. Rotors can take a variety of shapes, but the most efficient rotors resemble propellers and use airfoils to turn a driveshaft. Since turbulence from obstacles reduces wind speeds, rotors are mounted on towers as high above turbulence as possible.
Defining Key Concepts 1 wind is fluid 2 3 turbulence reduces power higher wind speeds = lots more power How is wind power converted to electricity? Where will you find wind? Key Concepts Windmills and wind turbines obtain their power input by using the force of the wind to turn a rotor, converting the kinetic energy in the wind into rotational energy. A windmill uses this rotational energy to perform mechanical work, as in running a water pump or turning a millstone to grind grain. A wind turbine makes electricity by using a rotor mounted on a driveshaft to turn a generator. A generator is a device that forces the flow of electrons by charging a magnetic field around a coil of wires, thus converting the rotational energy of the rotor into electricity. More wind can be collected with a larger rotor by providing more surface area and greater turning forces. These greater turning forces are able to turn a larger generator faster to produce more electricity. Similarly, higher wind speed and more consistently flowing winds will also turn the rotor faster. To evaluate a site for a wind turbine installation, the available local wind resource must be determined first. Key factors include the average annual wind speed, consistency and prevailing direction of the wind, and the impact of local geography and obstacles on air flow. The windiest areas of the US are along the coasts and through the Great Plains, with isolated areas on mountain ridges and passes, or in geographies that naturally funnel wind to higher speeds. Obstacles to the wind, such as buildings, trees, rock formations, etc., create turbulence and can decrease wind speeds significantly. The turbulent zone downwind of an obstacle may extend a distance from three to ten times the height of the obstacle. When manufacturers or wind farm developers calculate the energy production for wind turbines, they must take into account all nearby obstacles. 1. Wind is fluid. It will bend around, before and after obstacles, such as buildings, trees, mountains, and wind turbines. 2. Turbulence, like eddies in a stream, robs the wind of speed, and speed is power. 3. Higher wind speeds mean exponentially more power. 4. Conversely, lower wind speeds mean exponentially less power. 5. Higher wind speeds occur where there is less turbulence, and this is usually at higher altitudes.
Guide to the Video 1 Define the Problem 2 Identify the Problem Constraints This video (www.spark101.org) examines a proposed renewable energy project in New York City. The client was interested in exploring the possibility of generating electricity by installing wind turbines on a urban rooftop to power a new LED sign. Two sets of constraints were identified as critical to understanding how to solve the problem:: 1. What is wind energy and how does it work? (See Defining Key Concepts ) 2. How does wind energy affect our site? (See below) Challenges to determining the feasibility of this project were: Gathering local wind data - available wind data is generally for regional areas. Little data exists on local wind. Predicting wind patterns in an urban environment - buildings and other obstructions block and redirect wind, creating airflows which are difficult to visualize. Providing the client with a quick study, at a low cost. Various disciplines were involved to solve this problem, including architecture, engineering, and construction; use of CAD (Computer Aided Design) and 3D modeling; and a study of weather data. Local Prevailing Wind Directions Data on wind direction and annual average wind speeds for the location is extremely useful for deciding where to site wind turbines. Having wind coming from the right direction and at speeds that are fast enough to make enough electricity are crucial factors to validating any wind project. However, gathering wind data for each specific site, especially in urban areas, is challenging because of the time and cost required. Complexities of Wind in Urban Areas The high density of buildings in urban areas creates wind turbulence and unusual air flow. a.) Tall buildings in the way reduce or block wind flow b.) Air funneling between buildings can accelerate wind speeds; also, bodies of water, such as rivers and bays, often create conditions that are conducive to air flow. (Many cities are built by water.) 1 2 3 4 5 Local Obstructions The proposed building site had adjacent taller buildings on two sides. These would block wind coming from certain directions. The building site was also a few blocks from the river this could offer access to higher wind speeds coming from the direction of the water. Additionally, wind funneling between a set taller buildings by the river could cause a wind tunnel effect and increase wind speeds before reaching the proposed site. Selecting the Best Turbine Location Potential locations for the wind turbines were identified on different sites on top of the building. The large roof area (half a city block) provided five possible installation sites that were either closer to or farther from adjacent buildings. 0:00 0:56 2:45 5:44 6:07 8:04
Guide to the Video 3 Define and Collect Data 4 Evaluate the Solutions To begin solving the problem, we used several standard wind industry tools to gather information and to determine baseline data points. However, the unusual complexity of this project (i.e., being on a roof in a city) required adapting tools from other industries to fill in missing data and offer a more complete evaluation. Wind Data - Wind Rose Diagram The basic tool to assess the local wind resource is a wind rose. This is a diagram that shows historical data on wind direction and wind speeds at a given location throughout the year. Wind data is often available at a regional level and rarely accounts for local site specific anomalies. Building Site Survey This survey involved measuring and documenting the proposed building to understand its location, construction, and roof conditions. Local Area Site Survey In order to better understand the local environment, we constructed a 3-D model, using CAD or Computer Aided Design, to map the city buildings around the proposed project site (a 12 block area). This included measuring each building s foot-print, height, and distance from the proposed site. Computational Fluid Dynamics - CFD This predicted air flow modeling study was created to help visualize the predicted movement of wind around the proposed site. CFD software uses related wind data and building dimensions from 3D-modeling to calculate the flow of air over a site and around adjacent obstacles. Site Evaluation Matrix Based on a matrix developed from analysis of all the data, the CFD software showed slices of wind movement at different heights and from varying directions. This computer-generated visualization helped to identify the best site for the proposed installation of the small wind turbines. 1 2 3 4 5 The completed site assessment for this wind project had to answer several questions. Were our initial hypotheses corroborated by the data? Would the project meet the client s requirements? What could be done to increase the chances of success for this project? The initial assessment of the project suggested that minimum wind speeds existed to justify a wind turbine installation on the building. However, after performing the CFD study of the area, it was clear that only two proposed locations were far enough away from air turbulence to be effective for wind turbines. To improve the potential success of this project, we considered different types of wind turbines and the addition of taller towers. We also looked to research in other disciplines to find ways to increase the power generation from the limited group of turbines. Research related to biology and bio-propulsion, two seemingly unrelated disciplines, presented a tested possibility for improving our project. 0:00 0:56 2:45 5:44 6:07 8:04
Thinking Forward 5 Expanding Our Solutions Collaboration of ideas, research studies, and strategies for solving problems can often bring about innovations. This proposed wind energy project had already taken input from various specialties: wind energy technology, architecture, weather data, CAD and 3-D modeling, and Computational Fluid Dynamics. Where else could we find inspiration to make this a more successful project? In recent years, bio-inspired research, or biomimicry, has influenced wind power technology. One concept, incorporating bumps on propeller blades to mimic the nodules on whale fins, has shown increases in blade efficiency. In 2010, biophysicist Dr. John Dabiri, director of the Biological Propulsion Lab at the California Institute of Technology, applied his research using a different approach. Principles learned from studying how fish move together were used to test increases in energy produced by an array of vertical-axis wind turbines (with rotors spinning around the pole, like an egg-beater.) 1 2 3 4 5 Could wind turbines generate more electricity by taking cues from schooling fish? Dr. Dabiri s research data and field tests have shown that a tighter than normal arrangement of multiple vertical wind turbines increases the total power generation of the array when compared to conventionally spaced turbines. (See Resources for video link.) This concept could potentially improve the rooftop wind power project by allowing more turbines to be placed within the same roof space to produce more electricity. 0:00 0:56 2:45 5:44 6:07 8:04
Resources Spark101 The Spark101 Video: Can Wind Energy be Captured in New York City? http://www.spark101.org/engineering/ CALTECH / Biological Propulsion Laboratory / Wind Turbine Testing Caltech Field Laboratory for Optimized Wind Energy (FLOWE) YouTube Interview: Biophysicist John Dabiri MacArthur Fellow Award winner 2010 www.dabiri.caltech.edu/research/ wind-energy.html www.youtube.com/watch?v=x2audol niaq&feature=player_embedded More on Biomimicry and Wind Technology Several links related to research in wind turbine blade design inspired by whales. http://www.thenakedscientists. com/html/content/interviews/interview/1282/ US Department of Energy How Does A Wind Turbine Work? is one of several pages offered on the Dept. of Energy s Wind Program. See several images, animations, and videos on this page. http://energy.gov/eere/wind/how-dowind-turbines-work Danish Wind Industry Association Follow the Windpower Wiki on this website for reference information on wind power. http://www.stle.org/assets/news/document/techbeat_tlt_12-08.pdf http://www.windpower.org/en/knowledge/windpower_wiki.html NC Learn Complete lesson plan (grade 9-12) on wind energy, Who Has Seen the Wind? Harnessing alternative energy, by Linda Schmalbeck, North Carolina School of Science and Math. See video referenced at top of page Roping the Wind in Texas. www.learnnc.org/lp/ pages/6307?ref=search