Responsible Energy Production and Use
You plug your cell phone into the outlet and it charges in an hour. That electricity was produced at a power plant, traveled along high-voltage lines, and before that came from a source: gas, wind, water, the Sun, uranium. None of it is free or perfect. Every form of energy production has a cost, a limited efficiency, and an environmental footprint that can be measured. By the end of this page you will calculate how much CO₂ is behind one charge of your phone.
Where electricity comes from
In Mexico, the electricity that reaches your home through the CFE grid does not come from a single source. Some of it comes from gas plants, some from dams, some from wind farms in the Isthmus of Tehuantepec or solar plants in Sonora, and so on. That mix changes with the time of day, the season of the year, and the country's decisions.
In the exercises we will use this approximate practice mix. The real one changes every year and does not match it exactly.
The Mexican electricity mix for one year can be approximated as follows: wind power $25\ \%$, nuclear power $20\ \%$, solar power $17\ \%$, combined cycle gas $15\ \%$,…
The Mexican electricity mix for one year can be approximated as follows: wind power $25\ \%$, nuclear power $20\ \%$, solar power $17\ \%$, combined cycle gas $15\ \%$, hydro power $12\ \%$ and other sources $11\ \%$, as shown in the graph.
- a) Considering wind, solar and hydro power as renewables, what percentage of Mexican electricity was renewable?
- b) Check that all the percentages in the mix add up to $100\ \%$.
Classify these energy sources as renewable or non-renewable, and justify your choice in each case: wind, coal, uranium, Sun, natural gas and biomass.
Classify these energy sources as renewable or non-renewable, and justify your choice in each case: wind, coal, uranium, Sun, natural gas and biomass.
Mexico's annual electricity demand is about $250\ \text{TWh}$. With last year's approximate mix (wind power $25\ \%$, solar power $17\ \%$): a) How many $\text{TWh}$…
Mexico's annual electricity demand is about $250\ \text{TWh}$. With last year's approximate mix (wind power $25\ \%$, solar power $17\ \%$):
- a) How many $\text{TWh}$ were produced by wind power?
- b) How many $\text{TWh}$ were produced by solar power?
- c) How many $\text{TWh}$ did both add up to together?
Efficiency: no transformation is perfect
When a power plant, a motor, or a light bulb transforms energy, only part of it comes out as useful energy. The rest is dissipated, almost always as heat. Energy does not disappear (it is conserved), but it is converted into a form that is no longer useful to us.
That is why the energy efficiency label on appliances matters. In Mexico you see it on refrigerators, air conditioners, and other appliances (NOM-ENER and the FIDE seal). In the exercises we will use a scale of classes from A (the most efficient) to G (the least efficient). A class A appliance uses less energy to provide the same service.
The same goes for light bulbs. A $10\ \text{W}$ LED bulb gives the same light as a $60\ \text{W}$ incandescent one, so it converts much more of the energy it consumes into light. To compare energy use, remember $E = P \cdot t$, with $P$ in kW and $t$ in hours, to get kWh.
A $10\ \text{W}$ LED light bulb gives the same light as a $60\ \text{W}$ incandescent bulb, as shown in the figure. a) Calculate the energy each consumes if they are…
A $10\ \text{W}$ LED light bulb gives the same light as a $60\ \text{W}$ incandescent bulb, as shown in the figure.
- a) Calculate the energy each consumes if they are switched on for $5\ \text{h}$, in $\text{kWh}$.
- b) Which of the two is more efficient? Explain using your results.
Household appliances carry an energy rating label with classes from A to G, as shown in the figure. a) What does it mean if one refrigerator is class A and another is…
Household appliances carry an energy rating label with classes from A to G, as shown in the figure.
- a) What does it mean if one refrigerator is class A and another is class G?
- b) Explain why it is worth looking at this label before buying, even if the class A one is more expensive.
In an entrance hall, $10$ incandescent $60\ \text{W}$ light bulbs are replaced with $9\ \text{W}$ LEDs. They are switched on for $4\ \text{h}$ a day for $365$ days a…
In an entrance hall, $10$ incandescent $60\ \text{W}$ light bulbs are replaced with $9\ \text{W}$ LEDs. They are switched on for $4\ \text{h}$ a day for $365$ days a year.
- a) How many $\text{kWh}$ are saved in a year? Round to one decimal place.
- b) If the price is $3.20\ \text{\$/kWh}$, how many pesos are saved per year?
Installed power and capacity factor
A plant does not always produce its maximum power. A wind farm only gives its maximum when the wind blows hard, and a solar panel does not produce at night. That is why we distinguish between maximum power (or installed power) and average power.
The same idea works for small installations. If a region receives, for example, $1{,}400\ \text{kWh}$ per year for every kW of installed panels, panels totaling $1.6\ \text{kW}$ produce $1.6 \times 1{,}400 = 2{,}240\ \text{kWh}$ per year. Compare that figure with the household's consumption to see how much energy is left over or missing.
A wind farm has a maximum capacity of $100\ \text{MW}$, but the wind does not always blow at the same speed, so its capacity factor (average percentage of maximum…
A wind farm has a maximum capacity of $100\ \text{MW}$, but the wind does not always blow at the same speed, so its capacity factor (average percentage of maximum capacity) is $30\ \%$, as shown in the graph.
- a) What is its average power output?
- b) How much energy does it produce in a day, in $\text{MWh}$?
$6$ solar panels rated at $450\ \text{W}$ each are installed on the roof of a house. In that area, each installed kilowatt produces about $1500\ \text{kWh}$ per year. a)…
$6$ solar panels rated at $450\ \text{W}$ each are installed on the roof of a house. In that area, each installed kilowatt produces about $1500\ \text{kWh}$ per year.
- a) How many kilowatts of power capacity do the panels add up to?
- b) How many $\text{kWh}$ do they produce per year?
- c) The household consumes $3600\ \text{kWh}$ per year: do the panels cover that consumption? How much energy is left over or short?
A nuclear power plant with a capacity of $1000\ \text{MW}$ operates for $8000\ \text{h}$ per year. a) How much electrical energy does it produce in a year, in…
A nuclear power plant with a capacity of $1000\ \text{MW}$ operates for $8000\ \text{h}$ per year.
- a) How much electrical energy does it produce in a year, in $\text{MWh}$? Express it in $\text{TWh}$ as well.
- b) If that same energy were produced by burning gas, which emits $0.45\ \text{kg}$ of $\text{CO}_2$ per $\text{kWh}$, how many metric tons of $\text{CO}_2$ would be emitted? Nuclear power emits virtually no $\text{CO}_2$ when operating: how many metric tons does it avoid per year compared to gas?
- c) State two advantages and two drawbacks of nuclear energy, and finish by giving your reasoned opinion on whether it should be part of the electricity mix.
The carbon footprint: how much CO₂ is behind each use
A tailpipe puffs out visible smoke. An electric car moves along silently and without smoke. Even so, charging its battery has a footprint if the electricity was produced by burning fuels. How is it calculated?
When a family has several sources of emissions (electricity, car, gas), each item is calculated separately and then added together. To find out how much a measure reduces the footprint, calculate the footprint before and after, and subtract.
Producing $1\ \text{kWh}$ of electricity in Mexico emits an average of about $0.15\ \text{kg}$ of $\text{CO}2$ into the atmosphere. If a household consumes…
Producing $1\ \text{kWh}$ of electricity in Mexico emits an average of about $0.15\ \text{kg}$ of $\text{CO}_2$ into the atmosphere. If a household consumes $300\ \text{kWh}$ in a month, how many kilograms of $\text{CO}_2$ correspond to that consumption?
Burning one liter of gas emits about $2.3\ \text{kg}$ of $\text{CO}2$. If a car uses up a tank of $40\ \text{L}$, how many kilograms of $\text{CO}2$ has it emitted?
Burning one liter of gas emits about $2.3\ \text{kg}$ of $\text{CO}_2$. If a car uses up a tank of $40\ \text{L}$, how many kilograms of $\text{CO}_2$ has it emitted?
A household consumes $3200\ \text{kWh}$ of electricity per year and the emission factor of the Mexican mix is $0.15\ \text{kg}$ of $\text{CO}2$ per $\text{kWh}$. a)…
A household consumes $3200\ \text{kWh}$ of electricity per year and the emission factor of the Mexican mix is $0.15\ \text{kg}$ of $\text{CO}_2$ per $\text{kWh}$.
- a) Calculate the kilograms of $\text{CO}_2$ corresponding to that annual consumption.
- b) Express it in metric tons.
Comparing and reducing
The same method solves more complete situations. You calculate the energy or fuel consumed in a year (for example, $\dfrac{\text{L}}{100\ \text{km}} \times \dfrac{\text{km per year}}{100}$), then the $\text{CO}_2$ for each item, and finally you add or compare them. To say how many times more one option emits than another, divide one emission by the other. For a savings of a given percentage, multiply the consumption by that percentage.
A gas car consumes $6\ \text{L}$ per $100\ \text{km}$ and travels $12000\ \text{km}$ per year. Burning one liter of gas emits $2.3\ \text{kg}$ of $\text{CO}2$. a) How…
A gas car consumes $6\ \text{L}$ per $100\ \text{km}$ and travels $12000\ \text{km}$ per year. Burning one liter of gas emits $2.3\ \text{kg}$ of $\text{CO}_2$.
- a) How many liters does it consume in a year?
- b) How many kilograms of $\text{CO}_2$ does it emit in a year? Express the result in metric tons as well, rounding to one decimal place.
For a trip of $400\ \text{km}$, two options are compared per person: Gas car consuming $6\ \text{L}$ per $100\ \text{km}$ with a single occupant ($2.3\ \text{kg}$ of…
For a trip of $400\ \text{km}$, two options are compared per person:
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Gas car consuming $6\ \text{L}$ per $100\ \text{km}$ with a single occupant ($2.3\ \text{kg}$ of $\text{CO}_2$ per liter).
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Electric train consuming $12\ \text{kWh}$ per passenger on that trip ($0.15\ \text{kg}$ of $\text{CO}_2$ per $\text{kWh}$).
- a) Calculate the $\text{CO}_2$ emissions for the trip in each option.
- b) How many times more does the car emit than the train in this case? Round to one decimal place.
A detached house consumes $8000\ \text{kWh}$ of natural gas per year for heating. The emission factor of natural gas is $0.20\ \text{kg}$ of $\text{CO}2$ per…
A detached house consumes $8000\ \text{kWh}$ of natural gas per year for heating. The emission factor of natural gas is $0.20\ \text{kg}$ of $\text{CO}_2$ per $\text{kWh}$.
- a) How many kilograms of $\text{CO}_2$ does that heating emit per year?
- b) If good insulation reduces consumption by $25\ \%$, how many kilograms of $\text{CO}_2$ are prevented from being emitted per year?
A gas car ($6\ \text{L}$ per $100\ \text{km}$; $2.3\ \text{kg}$ of $\text{CO}2$ per liter) is compared with an electric car ($16\ \text{kWh}$ per $100\ \text{km}$;…
A gas car ($6\ \text{L}$ per $100\ \text{km}$; $2.3\ \text{kg}$ of $\text{CO}_2$ per liter) is compared with an electric car ($16\ \text{kWh}$ per $100\ \text{km}$; $0.15\ \text{kg}$ of $\text{CO}_2$ per $\text{kWh}$ with the current Mexican mix).
The graph shows the cumulative $\text{CO}_2$ for each car depending on the distance driven:
- a) Calculate the kilograms of $\text{CO}_2$ per $100\ \text{km}$ for each car.
- b) How many kilograms does each one emit per year if they cover $15000\ \text{km}$?
- c) Manufacturing the electric car's battery emits about $6000\ \text{kg}$ of $\text{CO}_2$ more than manufacturing the gas car. Using the emissions savings per $100\ \text{km}$, calculate after how many kilometers the electric car offsets the manufacturing of its battery. Round to the nearest thousand.
- d) Using these results, write a short paragraph arguing when an electric car is worthwhile from an emissions point of view.
A family wants to reduce their carbon footprint. Their current situation: Electricity: $3800\ \text{kWh}$ per year ($0.15\ \text{kg}$ of $\text{CO}2$ per $\text{kWh}$).…
A family wants to reduce their carbon footprint. Their current situation:
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Electricity: $3800\ \text{kWh}$ per year ($0.15\ \text{kg}$ of $\text{CO}_2$ per $\text{kWh}$).
-
Gas car: $12000\ \text{km}$ per year with a consumption of $6.5\ \text{L}$ per $100\ \text{km}$ ($2.3\ \text{kg}$ of $\text{CO}_2$ per liter).
- a) Calculate their current annual footprint by adding both items, in kilograms of $\text{CO}_2$.
- b) They implement two measures: habit changes and LED lighting that save $700\ \text{kWh}$ per year, and carpooling, which halves the annual kilometers. Calculate the new annual footprint.
- c) By what percentage have they reduced their footprint? Round to one decimal place and comment on which item contributed most to the reduction.
An electric car consumes $16\ \text{kWh}$ per $100\ \text{km}$. Its footprint depends on how the electricity was generated (emission factors: current Mexican mix…
An electric car consumes $16\ \text{kWh}$ per $100\ \text{km}$. Its footprint depends on how the electricity was generated (emission factors: current Mexican mix $0.15\ \text{kg}$ of $\text{CO}_2$ per $\text{kWh}$; coal-fired electricity $0.9\ \text{kg}$ of $\text{CO}_2$ per $\text{kWh}$). A comparable gas car emits $13.8\ \text{kg}$ of $\text{CO}_2$ per $100\ \text{km}$.
- a) Calculate the kilograms of $\text{CO}_2$ per $100\ \text{km}$ for the electric car using the current mix and using coal-fired electricity.
- b) Compare the three cases: with which electricity source does the electric car emit even more than the gas car?
- c) Using these data, write a short argument in favor of the electric car and another one that qualifies this conclusion, mentioning the role of the electricity mix.
The carbon cycle and the greenhouse effect
Why does $\text{CO}_2$ matter so much? Because it changes the energy balance of the entire Earth.
Carbon circulates among the atmosphere, the oceans, plants, animals, and the soil. Plants take it from the atmosphere as $\text{CO}_2$ during photosynthesis. Living things return it when they breathe and when they decompose. Fossil fuels are carbon that stayed stored underground for millions of years. When they are burned, that carbon goes into the atmosphere all at once as $\text{CO}_2$.
The energy that drives the planet comes in as solar radiation. The Earth absorbs it, heats up, and, to keep its temperature stable, sends energy back to space as infrared radiation. A system in equilibrium emits as much energy as it receives.
Gases such as $\text{CO}_2$ and methane absorb part of that infrared radiation and send it back toward the surface. This greenhouse effect is natural and necessary: without it, the Earth would be much colder. The problem is that burning fossil fuels increases the amount of these gases. More energy is trapped and the system readjusts to a higher average temperature.
Environmental impact and the energy transition
No way of producing electricity is free of impact. It is a matter of comparing economic, social, and environmental costs and benefits.
| Source | Advantages | Disadvantages |
|---|---|---|
| Fossil fuels (gas, coal, oil) | Produce energy when needed; infrastructure already exists | Emit $\text{CO}_2$ and pollutants; they run out |
| Nuclear | Almost no $\text{CO}_2$ emitted during operation; a lot of energy from little fuel | Long-lived radioactive waste; risk of accidents; high cost; limited uranium |
| Solar photovoltaic | No gases emitted when generating; abundant resource (Sonora, Coahuila) | Only produces with light; takes up land; panel manufacturing; needs storage |
| Wind | No gases emitted when generating; wind in the Isthmus of Tehuantepec | Depends on the wind; takes up land; affects birds and landscape |
| Hydroelectric | Large capacity and water storage (Chicoasén, El Cajón) | Changes rivers and ecosystems; displaces populations; depends on rainfall |
The energy transition is the gradual shift toward a mix with more renewables and fewer fossil fuels. It has practical challenges. The Sun and the wind are intermittent, so energy must be stored (batteries, pumped-storage reservoirs) and transmission grids must be strengthened. In the meantime, gas is often used as a backup to fill the gaps.
To supply a population with panels alone, you have to calculate the daily energy it consumes and divide it by what each square meter of panel produces. You will see that hectares of land are needed. That is a concrete way to see that renewables also cost land and materials.
Name two advantages and two disadvantages of solar photovoltaic energy as a way of producing electricity, and briefly explain each one.
Name two advantages and two disadvantages of solar photovoltaic energy as a way of producing electricity, and briefly explain each one.
In the approximate Mexican electricity mix, combined cycle gas power plants produce $15\ \%$ of an annual demand of $250\ \text{TWh}$ and emit $0.45\ \text{kg}$ of…
In the approximate Mexican electricity mix, combined cycle gas power plants produce $15\ \%$ of an annual demand of $250\ \text{TWh}$ and emit $0.45\ \text{kg}$ of $\text{CO}_2$ per $\text{kWh}$. The chart summarizes the mix data, present and future:
- a) How many $\text{TWh}$ does gas produce today?
- b) In a future scenario, wind power rises from $25\ \%$ to $35\ \%$ and gas falls from $15\ \%$ to $5\ \%$, with demand remaining constant. How many $\text{TWh}$ of gas are no longer produced?
- c) How many metric tons of $\text{CO}_2$ would be avoided per year with this change? Express the answer in scientific notation.
- d) Gas is usually used as backup when there is no wind or sun. Explain what technical problem would need to be solved to reduce it to a minimum and name a possible solution.
A village of $5000$ households wants to power itself using solar energy alone. Each household consumes on average $10\ \text{kWh}$ per day, and the panels in that area…
A village of $5000$ households wants to power itself using solar energy alone. Each household consumes on average $10\ \text{kWh}$ per day, and the panels in that area produce about $0.25\ \text{kWh}$ per day for every $\text{m}^2$ installed. The diagram shows the flow of energy between the panels, the households and the batteries:
- a) How much energy does the village consume per day, in $\text{MWh}$?
- b) How many $\text{m}^2$ of panels would be needed? Express the answer in hectares ($1\ \text{ha} = 10000\ \text{m}^2$) as well.
- c) If $40\ \%$ of the consumption occurs at night, how much energy, in $\text{MWh}$, should the village's batteries store each day?
- d) Outline two technical or land-use challenges of supplying a village using only solar power, and a possible solution for each.
COP of a heat pump
A heat pump (for example, an air conditioner that also heats) does not burn anything to produce heat. It moves heat from outside to inside the house, just as a refrigerator removes heat from its interior and sends it into the kitchen.
With $\text{COP} = 3$, every $1\ \text{kWh}$ of electricity moves $3\ \text{kWh}$ of heat. If a pump delivers $6\ \text{kWh}$ of heat and its COP is $3$, it used $6/3 = 2\ \text{kWh}$.
Energy savings and payback of an investment
An LED bulb costs more than an incandescent one, but it uses much less. How long does it take to recover the extra cost through what you save on your electric bill? The same goes for solar panels or insulating a house.
To get the annual savings, you often first need to calculate energy: the kWh no longer used per year multiplied by the price per kWh, or the percentage by which spending is reduced.
A solar panel installation costs $\$4000$ and saves $\$500$ per year on the electricity bill. The graph shows the investment and cumulative savings: How many years does…
A solar panel installation costs $\$4000$ and saves $\$500$ per year on the electricity bill. The graph shows the investment and cumulative savings:
How many years does it take to pay for itself, that is, to recover the money invested through the savings?
A solar panel installation costs $\$100\,000$ and produces $4000\ \text{kWh}$ per year that the family no longer buys at $3.20\ \text{\$/kWh}$. a) How many pesos does it…
A solar panel installation costs $\$100\,000$ and produces $4000\ \text{kWh}$ per year that the family no longer buys at $3.20\ \text{\$/kWh}$.
- a) How many pesos does it save per year?
- b) How many years does it take to pay for itself? Round to one decimal place.
- c) If a grant covers $30\ \%$ of the installation, what is the new payback period?
- d) The panels last about $25\ \text{years}$. How much money will have been saved in total over that time (without a grant)? Comment on whether the investment is worthwhile.
A home consumes $9000\ \text{kWh}$ of gas per year for heating, at $1.40\ \text{\$/kWh}$. Insulating the facade costs $\$60\,000$ and reduces consumption by $30\ \%$. a)…
A home consumes $9000\ \text{kWh}$ of gas per year for heating, at $1.40\ \text{\$/kWh}$. Insulating the facade costs $\$60\,000$ and reduces consumption by $30\ \%$.
- a) How much does heating cost per year before insulating?
- b) How many pesos does the insulation save each year?
- c) What is the payback period for the investment? Round to one decimal place.
- d) If a government grant covers $40\ \%$ of the insulation cost, what would be the new payback period? Comment on how grants influence energy savings decisions.
Back to the beginning: charging a cell phone battery uses about $0.015\ \text{kWh}$. With the factor of $0.45\ \text{kg}\ \text{CO}_2/\text{kWh}$, each full charge corresponds to $0.015 \times 0.45 = 0.00675\ \text{kg}$, that is, about $6.75\ \text{g}$ of $\text{CO}_2$ at the power plants. It is little per charge, but it is multiplied by millions of cell phones.