Electricity consumption is a large fraction of our overall energy use.
Electricity is generated using both fossil
fuels and renewable energy sources. In this assignment, you’ll review the basics of electricity, some energy
sources, and examine electricity flow and efficiency. [40 points total]
Conceptual and quantitative problems. You will only receive credit if you show all your work and write
legibly.
I. Electricity Basics
1. ___________ is the capacity is to do work. ________________ is the rate at which work is done. [1 pt]
2. A circuit includes a 9 V battery and a copper wire of a given length and diameter. You measure the
current in the wire to be 1.5 Amperes.
a. What is the resistance of the wire? Your answer should be in Ohms. [1.5 pts]
b. With this battery and wire, how much power can be provided? Report your answer in Watts.
[1 pt]
c. What changes could you make to the wire to increase the power? [2 pts]
d. If you half the resistance of the wire, what is the power output? [1.5 pts]
e. If you double the voltage of the battery, what is the power output? [1.5 pts]
3. You have been using a 75W traditional incandescent light bulb in your desk lamp. However, you have
learned about LED light bulbs and decided to replace your inefficient light bulb with a 12W LED that
provides the same amount of light.
a. Assuming that you are using your lamp for 5 hours per day and the electricity rate is 15 cents
per kilowatt-hour, what is your annual energy cost savings? [2 pt)
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b. The incandescent bulb cost about $0.75, and the LED bulb cost about $6.00. Based on the
energy savings, does the LED bulb save you money in the long run? For more information, visit:
http://energy.gov/energysaver/lighting-choices-save-you-money. [1 pt]
II. Electricity Generation and Transmission
4. Although our small electronics like cell phones and flashlights use direct current, the electricity
coming into our homes is alternating current. What is the advantage of transporting electricity on the
large scale using AC instead of DC? [1 pt]
a. Voltage can be stepped up or down using a transformer with A/C.
b. More materials can be used as conductors with A/C.
c. Less electrical energy is lost to heat with A/C.
d. Ohm’s Law restricts D/C, but not A/C.
5. At the large scale, electricity cannot be effectively stored with current technology (though the
technology is improving https://www.eia.gov/todayinenergy/detail.php?id=40072). In other words,
the electricity generated and transmitted over the power lines needs to match the electricity that is
being used. ISO New England is an organization that carefully predicts how much energy will be used
in the New England power grid during every hour of every day. They then communicate with energy
generating companies (nuclear, solar, gas-burning power plants, etc.) to supply the most economic
electricity to the grid at a certain time. Visit the ISO New England website: http://www.iso-ne.com.
a. What is the current actual demand (MW) and fuel mix (percentages)? [1 pt]
b. Describe the daily pattern of system demand (i.e. what time of day is energy use high or low).
[1 pt]
c. At the top of the page, click on the “About Us” tab. Under “What We Do”, click on “Our Three
Critical Roles,” click on “Grid Operation.” Scroll down and read the section “Forecasting New
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England’s Electricity Use.” What three factors largely determine the hourly demand for
electricity? [1 pt]
d. Read the section “Dispatching the Power System.” Why does ISO-NE take the pulse of the
power grid every few seconds? [1 pt]
e. Energy sources can be described as dispatchable (can provide electricity on demand, can be
turned on or off relatively quickly) or non-dispatchable or intermittent (timing and/or amount
of electricity generation cannot be controlled by operators). What are the challenges in using
non-dispatchable sources (such as solar or wind) to provide all of the needed electricity for the
region? [1.5 pts]
f. At the top of the page, click on the “About Us” tab again. Under “Key Grid and Market Stats,”
click on “Electricity Use.” Scroll down to the graphs showing projected annual energy use with
and without EE (energy and efficiency improvements) and PV (photovoltaics) savings. Which
has a greater impact on energy use and summer peak demand? EE or PV? [1 pt]
g. Read the report from ISO New England on air emissions from electricity generation:
https://www.iso-ne.com/about/key-stats/air-emissions. What are the top 6 factors that have
contributed to a reduction in emissions from electricity generators? [2 pts]
III. Energy Flow
6. The Lawrence Livermore National Laboratory publishes flow charts for energy for multiple countries
and states. Visit https://flowcharts.llnl.gov/commodities/energy.
a. In 2019, the United States consumed __________ Quads of energy per year. Of this, _________ Quads
is lost as rejected or waste energy. _________ Quads of the rejected energy comes from electricity
generation. [1 pt]
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b. If the going rate for electricity is $0.15/kWh, what is the “cost” of the wasted energy from
electricity? Report your answers in trillions of dollars. [1.5 pts]
c. What is the fate of most of this “rejected energy?” Energy cannot be created or destroyed, so
where does it go? [1 pt]
7. The Lawrence Livermore National Laboratory published flow charts for both energy and carbon
(https://flowcharts.llnl.gov/commodities).
2010 2018
Energy Use CO2 Emissions Energy Use CO2 Emissions
(Quads) (million metric (Quads) (million metric
tons) tons)
Solar 0.130 0 0.949 0
Nuclear 8.40 0 8.44 0
Hydro 2.50 0 2.69 0
Wind 0.920 0 2.53 0
Geothermal 0.210 0.4 0.217 0.5
Natural Gas 25.0 1286 31 1629
Coal 21.0 1985 13.3 1259
Biomass 3.50 0 5.13 0
Petroleum 36.0 2362 36.9 2379
Total 98.0 5632 101.2 5268
a. Note that between 2010 and 2018, CO2 emissions decreased even though energy use increased.
Look closely at which energy sources and emissions increased and which decreased. Explain
the decrease in CO2 emissions based on these data and what you have learned in class. [1.5 pts]
IV. Energy Sources
8. What is the difference between a resource and a reserve? Use an example to explain your answer. [2
pts]
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9. Your uncle strongly advocates for the extensive use of coal, arguing that the energy gained by burning
coal is actually just solar energy.
a. Read pages 18-20 and note 3 on page 22 in the textbook. Describe the formation of coal (using
diagrams as needed). Is your uncle correct in his assertion that the energy in coal is solar
energy? [2 pts]
b. Based on note 3 on page 22, what is the difference between the parent material for coal and
oil? [1 pt]
V. Energy Efficiency
10. According to energycalculator.com, an average 14-15 inch laptop runs at about 60 W.
a. Assuming that every undergraduate in Harvard College (about 6,700) uses their laptop for six
hours per day, how much energy is consumed per day? Report your answer in kWh. [1 pt]
b. How much of each fuel below is needed to supply this amount of energy each day? Assume
100% efficiency. Show your work and report your answers in kilograms in the table below.
[1.5 pts]
c. How much CO2 results from the needed amount of fuel each day? Show your work and report
your answer in kilograms in the table below. [1.5 pts]
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Energy Content Carbon Footprint Daily Harvard College laptop use
Fuel
(kWh/kg) (g CO2 eq/kWh)* Fuel (kg) CO2 Footprint (kg)
Coal 8.3 900
Oil 13.0 650
Natural Gas 15.4 420
Uranium (breeder) 2.2*107 5
*http://www.geni.org/globalenergy/library/technical-articles/carbon-capture/parliamentary-office-of-science-and-technology/carbon-
footprint-of-electricity-generation/file_9270.pdf
