Solar Power Potential
PV performance is largely proportional to the amount of solar radiation available at the local latitude and
climate, which may vary from the long-term average by ± 30% for monthly values and ± 10% for yearly values.
Boston’s monthly solar radiation data is shown below and accounts for the typical cloud cover. It is helpful,
in trying to quantify the direct sunlight to a site location, to compare the direct radiation to other cities
around the world. The graph below shows Boston’s average annual solar radiation compared to San Francisco, Los
Angeles, New York, Chicago and London.
Photovoltaic (PV) arrays are a robust, proven method of capturing the sun’s energy and put it to use by
converting light energy into electrical energy. Commonly known as solar cells, individual PV cells are
electricity-producing devices made of semiconductor materials.
The size of an array depends on several factors, such as the amount of sunlight available and the needs of the
consumer. The modules of the array make up the major part of a PV system, which can also include electrical
connections, mounting hardware, power-conditioning equipment, and batteries that store solar energy for use
during overcast days.
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Wind Potential
At 12.4 mph, Boston has an average wind speed 2 mph greater than the Chicago. In fact, 13 of the 20 cities
with the highest average wind speeds are located in Massachusetts. In addition, rising sea surface
temperatures suggests that more hurricane-intensity storms will continue to impact the East Coast, increasing
the potential for extremely high-speed wind gusts that will need to inform the installation of any renewable
energy technologies.
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The climate in Boston is classified as a humid continental climate (Koppen Dfa, Cfa respectively) and features
four distinctly separate seasons. Spring and fall are known to be mild but also widely varied based on wind
direction and jet stream positioning. The climate graphs in this report represent weather data collected at
Boston’s Logan International Airport by the U.S. Department of Energy from 1976-2003.
Unlike solar radiation, wind directions are much more difficult to accurately measure, track and predict.
Understanding specific micro-climatic conditions provides great challenges for sighting a small-scale turbine.
Currently, there are a variety of methodologies in the market for analyzing site wind conditions for
understanding the viability of wind power generation; these range from simple methods, like taking nearby
airport data as an estimation of wind resources at the project site, to comprehensive wind-tunnel and
computational fluid dynamics (CFD) testing based on actual data obtained on-site.
Wind turbines convert kinetic energy to mechanical energy which is then used to produce electricity. There are
two primary wind turbine technologies on the market: Vertical axis wind turbines (VAWTs) are designed to take
advantage of wind from any direction, including updraft, while horizontal-axis wind turbines (HAWTs) are
designed to face specifically into a single prevailing wind direction.
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Laboratory Assignment
Please answer the following questions based on information found in class lecture notes, reading assignments
in the Citizen Powered Energy Handbook: Community Solutions to a Global Crisis (Pahl) or other texts, review
of the Museum of Science Wind Turbine Lab presentation, or research obtained from books, journals,
local/national/regional periodicals or podcasts.
In addition, please cite any additional sources of research and show all calculations.
Short Answer Questions (5 points each)
1) List at least 5 different renewable energy sources and define each term.
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Short Answer Questions (5 points each)
2) Provide four examples of real world building projects that integrate renewable energy sources. One must
include Biomass, one must include Liquid Biofuels, one must include Wind and another must include Geothermal.
Please also include the name and location of the project, a brief description, the amount of power generated,
total cost of the project and length of time to complete the project.
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Short Answer Questions (5 points each)
3) What is the difference between distributed and centralized power? List three examples of distributed power
located in the United States.
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Short Answer Questions (5 points each)
4) Please list 10 complexities of a given site that may determine the success of a given wind projects in an
urban environment. Describe each briefly.
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Short Essay (10 points each)
5) What state is leading the way in promoting and supporting locally owned wind projects? Explain the
financial strategy that farmers in that state developed to take advantage of state policies favoring local
wind development. Could this approach work in Massachusetts?
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Short Essay (10 points each)
6) Summarize what happened with the demise of Evergreen Solar and the manufacturing of solar panels in
Massachusetts. Could anything have been done to keep them from relocating in China?
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Short Essay (10 points each)
7) Summarize the Department of Energy’s loan guarantee program for alternative energy projects. Do you think
that this program needs more restrictive oversight and performance standards to reduce the chance of defaults,
such as the Solyndra Project?
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Problem Sets
8) Calculate the Power of Wind (5 points)
The power of the wind moving toward a turbine’s rotor is calculated by this equation:
P = ½ · ? · V ³ · A
For Power in Watts,
? = density of air (kg/m3)
V is wind velocity (meters/second)
A is swept area of the rotor (square meters)
Assume standard air density of 1.225 kg/m3.
Calculate the power of the wind in watts per square meter at wind speed = 13 m/s. Round to the nearest watt.
Please show all calculations.
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Problem Sets
9) Calculate Electrical Power from Turbines (10 points)
The Skystream 3.7’s rated power is 2.4 kW at 13 m/s. That is, when the wind blows at 13 m/s, the Skystream
should produce 2400 W. The rotor diameter is 3.7 meters.
Calculate the electrical power in watts (We) the Skystream generates per square meter at this wind speed?
Round to the nearest watt. Hint: Swept area = pr2, where r is radius of the rotor. Please show all
calculations.
What percent of the wind’s power does the Skystream convert to electrical power? Please
show all calculations.
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Problem Sets
10) Calculate Power for Massachusetts Wind Turbines (15 points)
The table below lists some specifications for different wind turbines in this area*. You’ve already calculated
the Skystream data. Complete the table for all of these turbines. Please show all calculations.
TurbinekW at 13 m/sDiameter (m)Turbine We/m2Wind W/m2Turbine We / Wind WSkystream 3.72.43.7Northern Power
10092.821FL 1500150077SWT-3.6-1203600120
*NOTES:
The Skystream is one of the turbines on the Museum of Science roof. The Northern 100 is installed at the
McGlynn School in Medford. Note that 13 m/s is lower than its rated speed of 15 m/s, so the power here is less
than its rated power of 100kW. FL 1500 is similar to the MWRA’s turbine in Charlestown. Siemens 3.6-120 is
currently planned for the anticipated Cape Wind project.
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Problem Sets
11) Analyze the Power Curve (20 points)
This is the power curve for the Skystream 3.7. Since this turbine is performing to spec (to specifications, as
designed) at the Museum of Science, we’ll use its data to estimate how much energy it produces.
The table below shows a wickedly simplified wind distribution for the Museum of Science’s Skystream.
% of timeWind Speed m/sPower WeEnergy kWh38%042%416%6.53%81%10kWh/month
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For each wind speed in the table, record the power the turbine should produce. (Visually estimate it from the
power curve.) How much energy would you expect to generate in one month? Hint: Energy (kWh) = Power (W/1000) x
Time (hours). Please show all calculations.
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Compare the number in the previous example to your monthly electricity bill. What percentage of your
electricity could be provided by the Skystream, given the wind distribution?
Please show all calculations.