AP Environmental Science

## Learning Outcomes

- Explain how **net energy yield** affects energy-resource choices
- Distinguish **energy quality** from energy quantity and efficiency
- Identify how **externalities** and external costs change energy decisions
- Evaluate energy sources using availability, reliability, infrastructure, and environmental impact
- Apply simple calculations involving net energy yield, efficiency, and capacity factor
- Compare market cost with broader social and environmental cost
- Use decision-factor language clearly in AP Environmental Science responses

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand energy use decision factors, with a focus on the following syllabus points:

- Know that energy-resource decisions are based on more than price alone
- Identify **net energy yield**, **energy quality**, **resource availability**, **environmental impact**, and **external costs** as key decision factors
- Explain how a source with a low market price can still have high social and environmental costs
- Understand the difference between **dispatchable** and **intermittent** energy sources
- Evaluate how geography, infrastructure, technology, and policy influence energy choices
- Apply simple calculations for net energy yield, efficiency, and capacity factor
- Distinguish cost measures like direct price from broader tools such as cost-benefit analysis
- Understand that global energy demand is rising fastest in developing regions, especially in Asia

## Test Your Knowledge

_Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision._

1. What is the difference between **net energy yield** and **efficiency**?
2. Why might coal appear cheap but still be a poor energy choice overall?
3. What makes electricity a high-quality form of energy?
4. How does an **intermittent** energy source differ from a **dispatchable** one?
5. Name two non-economic factors that affect energy-resource decisions.

## Introduction

Energy decisions are rarely simple. A country, business, or household does not choose an energy source by asking only, “Which one costs less right now?” Instead, good decisions depend on several linked factors: how much useful energy a source actually delivers, how reliable it is, where it is available, what infrastructure already exists, and what environmental damage it causes.

This matters a lot in AP Environmental Science because the exam often asks you to compare or evaluate energy options. A strong answer must show trade-offs. For example, an energy source may be abundant and cheap at the point of sale, but still create serious air pollution, climate impacts, or habitat damage that are not included in its market price.

> **Key Term: energy use decision factors**
> The physical, economic, social, and environmental considerations used to judge whether one energy source is preferable to another.

## Introduction to the main decision factors

### Net energy yield and why it matters

One of the most useful starting points is **net energy yield**. This asks a basic question: after all the energy needed to find, extract, process, and transport a fuel is counted, how much useful energy is left?

> **Key Term: net energy yield**
> The amount of useful energy obtained from a resource minus the energy used to find, extract, process, and transport it.

If a fuel requires almost as much energy to obtain as it provides, it is not a strong long-term choice, even if people can still use it. That is why conventional oil has generally been more attractive than harder-to-extract sources such as tar sands or oil shale. The deeper, dirtier, or more energy-intensive the extraction, the lower the net energy yield tends to be.

This is also why natural gas and coal were often defended historically: they were seen as concentrated fuels with relatively high net energy yield and existing infrastructure. By contrast, some unconventional fossil fuels have a lower net benefit because much more energy must be spent on drilling, heating, processing, pumping, or cleanup.

A simple way to think about it is this:

- High net energy yield means a large surplus of useful energy
- Low net energy yield means much of the energy gain is “used up” getting the resource

Worked example:

A fuel source produces **1000 J** of usable energy.
It takes **250 J** to locate, extract, process, and deliver it.

$$
\text{Net energy yield} = \text{energy produced} - \text{energy used}
$$

$$
\text{Net energy yield} = 1000\text{ J} - 250\text{ J} = 750\text{ J}
$$

So the net energy yield is **750 J**.

A related measure is energy return on investment, or **EROI**:

$$
\text{EROI} = \frac{\text{energy returned}}{\text{energy invested}}
$$

$$
\text{EROI} = \frac{1000}{250} = 4
$$

So the EROI is **4:1**.

A higher EROI usually means a more attractive resource.

Common student errors:

- Confusing net energy yield with efficiency
- Forgetting to include transport and processing energy
- Assuming a large total energy reserve automatically means a high net yield
  > **Test Tip:** If an FRQ asks you to justify an energy choice, mention not just how much energy a source contains, but how much energy is left after extraction and processing.

### Energy quality, efficiency, and usefulness

Students often mix up **energy quality** and energy quantity. They are not the same. Quantity means how much energy exists. Quality means how useful that energy is for doing work.

> **Key Term: energy quality**
> The ability of a form of energy to do useful work, based on how concentrated, convenient, and versatile it is.

Electricity is a classic high-quality energy form. It is concentrated, easy to move through wires, and can power lights, computers, motors, and heating systems. Gasoline and natural gas are also high-quality because they are concentrated and easy to use. Low-temperature heat is lower-quality because it is more dispersed and harder to convert into useful work.

This links directly to the **Second Law of Thermodynamics**. Each time energy changes form, some becomes less useful, often as waste heat. So even if a source contains a lot of total energy, the useful part may be limited.

Do not confuse this with **efficiency**.

> **Key Term: efficiency**
> The fraction of input energy converted into useful output energy, usually expressed as a percentage.

Worked example:

A power plant receives **5000 J** of input energy and delivers **2000 J** of useful electrical energy.

$$
\text{Efficiency} = \frac{\text{useful output}}{\text{total input}} \times 100
$$

$$
\text{Efficiency} = \frac{2000}{5000} \times 100 = 40\%
$$

So the plant is **40% efficient**.

This does not tell you net energy yield. It only tells you how much input was converted into useful output at that stage.

Important distinction:

- **Energy quality** = how useful the energy form is
- **Efficiency** = how much input becomes useful output
- **Net energy yield** = how much energy is left after obtaining the resource

A source can be high-quality but still environmentally damaging. Gasoline is high-quality energy, but burning it releases carbon dioxide and air pollutants. A source can also be renewable and clean during use, but still less reliable in some contexts.

## Economic and system-level decision factors

### Cost, externalities, and the limits of market price

One of the biggest mistakes in energy evaluation is to assume the market price includes all costs. Often, it does not.

> **Key Term: externalities**
> Costs or benefits of an energy source that are not included in its market price and are experienced by other people or natural systemss.

For energy decisions, externalities usually mean negative external costs such as:

- air pollution
- climate change
- habitat destruction
- water contamination
- cleanup costs
- human health damage
  > **Key Term: external costs**
  > Negative externalities such as environmental damage or health harms caused by an energy source but not paid directly by the producer or consumer.

Coal is a clear example. Its direct price may seem low, especially where mining and power infrastructure already exist. But when coal is burned, it releases large amounts of carbon dioxide, sulfur dioxide, nitrogen oxides, particulates, and trace metals. These contribute to climate change, acid deposition, respiratory disease, and other health problems. Those harms may be paid by the public through healthcare costs, reduced crop yields, environmental cleanup, and habitat destruction loss.

So the **private cost** of coal may be lower than the **social cost**.

This is why APES questions often reward students who explain that an energy source can be “cheap” only because many costs are shifted onto society.

A **cost-benefit analysis** tries to account for this.

> **Key Term: cost-benefit analysis**
> A comparison of the total expected costs and total expected benefits of a decision, ideally including environmental and social effects.

This matters because some energy benefits are easy to price, like electricity sales or jobs. Other effects are harder to measure, like cleaner air, lower asthma rates, or preserved habitats. That is why different groups may reach different conclusions.

Worked example idea:

Suppose a coal plant produces low-cost electricity, but nearby communities face higher asthma treatment costs and local streams are harmed by pollution. The electricity bill may look cheap, but the total social cost is much higher than the private cost.

> **Test Tip:** In compare or evaluate questions, use phrases like “market price,” “external cost,” and “social cost.” This shows you understand why economic decisions in environmental science are not just sticker-price decisions.

### Reliability, dispatchability, intermittency, and capacity factor

Cost and environmental impact are not enough. An energy source must also provide power when people need it.

> **Key Term: dispatchable energy source**
> An energy source that can be turned on or adjusted more easily to meet demand when needed.

Coal, natural gas, and many hydroelectric and nuclear systems are generally more dispatchable than wind or solar. That does not make them automatically better, but it does matter for grid reliability.

> **Key Term: intermittent energy source**
> An energy source whose output depends on variable natural conditions such as sunlight or wind.

Solar output falls at night and during cloudy periods. Wind output changes with wind speed. These sources can still be excellent choices, especially because they reduce air pollution and greenhouse gas emissions during operation, but grids may need storage, backup generation, or transmission upgrades to use them effectively.

A related idea is **capacity factor**.

> **Key Term: capacity factor**
> The fraction of maximum possible output that a power source actually produces over time.

Worked example:

A wind farm could produce **100 MWh** in a certain time period if it ran at full capacity the whole time.
It actually produces **35 MWh**.

$$
\text{Capacity factor} = \frac{\text{actual output}}{\text{maximum possible output}}
$$

$$
\text{Capacity factor} = \frac{35}{100} = 0.35
$$

So the capacity factor is **35%**.

A lower capacity factor does not mean the source is useless. It means the source does not operate at full output all the time. This is common for intermittent renewables.

A stronger answer compares sources fairly:

- Wind and solar may have low emissions and falling costs
- Natural gas may be more dispatchable and easier to ramp up
- Hydroelectric may be reliable where geography allows it
- Coal may provide steady generation but has high pollution costs

Students often treat all electricity sources as interchangeable because they all generate kWh. On the exam, that is too simple. Timing and reliability matter.

## Broader evaluation of energy choices in global context

### Geography, infrastructure, and real-world trade-offs

Even if an energy source looks ideal on paper, geography may limit it. This is why APES stresses that real energy decisions depend on location and resource availability.

> **Key Term: resource availability**
> The extent to which an energy source is physically present and accessible in a particular location.

Hydroelectric power needs moving water and suitable river systems. Geothermal energy is strongest where underground heat is accessible, often near tectonic activity. Solar works best where sunlight is strong and consistent. Wind is best in regions with steady wind patterns. Coal, oil, and natural gas are unevenly distributed because they depend on geologic history.

Infrastructure also matters. Fossil fuels have often remained dominant because pipelines, refineries, power plants, and engines are already built around them. Switching systems can be expensive and politically difficult, even when cleaner options exist.

This helps explain global energy trends. World energy consumption is projected to rise strongly, with much of the growth occurring in developing regions, especially Asia. Industrial sectors, including manufacturing, mining, and refining, account for a very large share of energy use. Electricity is one of the fastest-growing end-use forms of energy. In such situations, countries often balance several pressures at once:

- rising demand
- affordability
- energy security
- job concerns
- pollution control
- existing infrastructure
- public acceptance

An APES evaluation should therefore avoid one-word conclusions like “solar is best” or “coal is cheapest.” Better reasoning sounds like this:

On one hand, solar has low operating emissions and uses an essentially inexhaustible source. On the other hand, it is intermittent and may require storage or backup. Its value depends on sunlight, grid structure, land availability, and the presence of alternative sources.

Enrichment only: some analysts use measures like **levelized cost of electricity (LCOE)** and **levelized avoided cost of electricity (LACE)**. LCOE estimates average lifetime generation cost per unit of electricity, while LACE estimates the value of that electricity to the grid. These can be useful, but for APES the key lesson is simple: cost alone is limited, because timing, intermittency, and grid value matter too.

### How to structure an evaluative APES answer

When an FRQ asks you to **evaluate**, **justify**, or **compare**, do not just list pros and cons. Build a judgement.

A good structure is:

- identify the factor
- explain how it affects the decision
- connect it to the specific source
- weigh it against another factor
- finish with a conditional judgement

For example:

A natural gas plant may be preferred over coal in the short term because it produces less sulfur dioxide, particulates, and carbon dioxide per unit of energy input. However, methane leakage during extraction and transport remains a major climate concern. Therefore, natural gas may be a lower-emission transition fuel, but it is not a complete long-term solution.

Strong evidence to cite:

- net energy yield
- reliability or dispatchability
- pollution released during use
- habitat or water impacts
- existing infrastructure
- resource availability in that location

Weak evaluation usually sounds generic:
“It depends on many factors.”

Strong evaluation says what those factors are and why they change the conclusion.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Energy decisions depend on physical, economic, and environmental trade-offs, not just price
- **Net energy yield** measures useful energy gained after subtracting energy used to obtain the resource
- A higher **EROI** usually makes an energy source more attractive
- **Energy quality** means how useful an energy form is for doing work; electricity is high-quality energy
- **Efficiency** is different from net energy yield and measures useful output divided by total input
- **Externalities** are costs or benefits not included in market price
- Fossil fuels can appear cheap because health and environmental damages are often excluded from the private cost
- **Cost-benefit analysis** should include social and environmental effects, not just direct financial cost
- **Dispatchable** sources are easier to turn on when needed, while **intermittent** sources depend on natural conditions
- **Capacity factor** helps describe how much output a source actually produces compared with its maximum possible output
- Geography, resource availability, infrastructure, and policy all influence energy choices
- Strong APES evaluation answers explain trade-offs and support conclusions with specific decision factors

## Key Terms and Concepts

- energy use decision factors
- net energy yield
- energy quality
- efficiency
- externalities
- external costs
- cost-benefit analysis
- dispatchable energy source
- intermittent energy source
- capacity factor
- resource availability


Global Energy Consumption - Energy use decision factors