01 / 21

Energy
Conservation

Understand how energy flows through systems, the laws of thermodynamics, and why reducing energy waste is critical for a sustainable future.

AP Environmental Science

What is Energy?

Energy is the capacity to do work or transfer heat. In environmental science, we track how energy enters ecosystems, transforms between forms, and ultimately dissipates. Energy cannot be created or destroyed: it can only change form.

Kinetic
Energy of motion: wind, flowing water, moving molecules
Potential
Stored energy: chemical bonds in fuels, water behind a dam
Thermal
Heat energy: the random motion of atoms and molecules

Energy in Environmental Systems

Sun Producers Consumers Decomposers Heat Heat lost ~10% transferred

Why Conservation Matters

Humans consume energy at a massive scale, primarily from fossil fuels. The environmental consequences include climate change, air pollution, habitat destruction, and resource depletion. Energy conservation: using less energy to accomplish the same tasks: is one of the most effective strategies for reducing these impacts.

Climate
Burning fossil fuels releases \(\text{CO}_2\), driving global warming
Resources
Fossil fuels are finite: coal, oil, and natural gas will run out
Cost
Reduced energy use means lower utility bills and economic savings

The Laws of Thermodynamics

1
First Law: Energy cannot be created or destroyed, only transformed. The total energy of an isolated system remains constant: \(\Delta E_{\text{system}} = 0\) (for an isolated system).
2
Second Law: Every energy transformation increases the entropy (disorder) of the universe. Useful energy inevitably degrades into waste heat: \(\Delta S_{\text{universe}} > 0\).

Energy Efficiency Equation

Energy Efficiency
\(\text{Efficiency} = \dfrac{E_{\text{useful output}}}{E_{\text{total input}}} \times 100\%\)

Conservation vs. Efficiency

Conservation Efficiency
Definition Using less energy overall Getting more output per unit input
Example Turning off lights when leaving Replacing incandescent with LED
Behavioral? Yes: requires habit change No: technology-driven
Cost Free or very low Upfront investment needed
Goal Both reduce total energy consumption and environmental impact

How to Apply Conservation

1
Audit your energy use. Identify where the most energy is consumed: heating, cooling, lighting, and transportation.
2
Reduce waste behaviors. Turn off unused lights, lower thermostat settings, unplug phantom loads.
3
Upgrade to efficient technology. Install LED bulbs, Energy Star appliances, smart thermostats, and insulation.
4
Switch to renewable sources. Solar panels, wind, and geothermal reduce reliance on fossil fuels entirely.
5
Calculate savings. Use the efficiency equation to compare \(E_{\text{saved}} = E_{\text{old}} - E_{\text{new}}\) and verify improvements.

Example: LED vs. Incandescent

A household replaces a 60 W incandescent bulb (used 5 hours/day) with a 10 W LED that produces the same light. How much energy is saved per year?

1
Old usage: \(60\,\text{W} \times 5\,\text{h/day} \times 365\,\text{days} = 109{,}500\,\text{Wh} = \) 109.5 kWh/yr
2
New usage: \(10\,\text{W} \times 5\,\text{h/day} \times 365\,\text{days} = 18{,}250\,\text{Wh} = \) 18.25 kWh/yr
3
Energy saved: \(109.5 - 18.25 = \) 91.25 kWh/yr per bulb

Example: Efficiency Calculation

A coal power plant burns coal containing 1,000 MJ of chemical energy. The plant produces 350 MJ of electrical energy. What is its efficiency?

1
Identify values: \(E_{\text{input}} = 1{,}000\,\text{MJ}\), \(E_{\text{output}} = 350\,\text{MJ}\)
2
Apply the formula: \(\text{Efficiency} = \dfrac{350}{1{,}000} \times 100\% = \) 35%
3
Interpret: 65% of energy is lost as waste heat, consistent with the Second Law of Thermodynamics.

Example: Trophic Energy Loss

A grassland receives 10,000 kcal of solar energy. Using the 10% rule, how much energy is available to a tertiary consumer?

Producers: 10,000 kcal Primary: 1,000 kcal Secondary: 100 kcal Tertiary: 10 kcal 10% 10% 10%

Renewable vs. Nonrenewable

Renewable
Replenished naturally on a human timescale: solar, wind, hydroelectric, geothermal, biomass
Nonrenewable
Formed over millions of years and consumed faster than replaced: coal, oil, natural gas, nuclear (uranium)

Tracking Energy Flow

Fossil Fuel 100 units Power Plant 35 units Transmission 32 units Useful Work 10 units 65 lost 3 lost 22 lost

Strategies at Every Scale

Individual
Turn off lights, use public transit, adjust thermostats, air-dry clothes
Community
Smart grids, district heating, bike-share programs, building codes
National
CAFE standards, Energy Star programs, carbon taxes, renewable mandates

Cogeneration captures an otherwise rejected thermal stream

A fuel engine drives a blue generator beside a heat exchanger and building; separate exhaust and water pipes carry heat without mixing.Suppose CHP converts 100 kWh of fuel into 30 kWh of electricity and 50 kWh of useful heat. Separate electricity generation at 30% efficiency and heat production at 80% efficiency require 162.5 kWh for the same services. Recovered heat must match demand in temperature, place and time.

CHP: recover heat in a separate useful loop

CHP: recover heat in a separate useful loop. Suppose CHP converts 100 kWh of fuel into 30 kWh of electricity and 50 kWh of useful heat. Separate electricity generation at 30% efficiency and heat production at 80% efficiency require 162.5 kWh for the same services. Recovered heat must match demand in temperature, place and time.Suppose CHP converts 100 kWh of fuel into 30 kWh of electricity and 50 kWh of useful heat. Separate electricity generation at 30% efficiency and heat production at 80% efficiency require 162.5 kWh for the same services. Recovered heat must match demand in temperature, place and time.

Compare equal useful services, not unequal outputs

Compare equal useful services, not unequal outputs. Suppose CHP converts 100 kWh of fuel into 30 kWh of electricity and 50 kWh of useful heat. Separate electricity generation at 30% efficiency and heat production at 80% efficiency require 162.5 kWh for the same services. Recovered heat must match demand in temperature, place and time.Suppose CHP converts 100 kWh of fuel into 30 kWh of electricity and 50 kWh of useful heat. Separate electricity generation at 30% efficiency and heat production at 80% efficiency require 162.5 kWh for the same services. Recovered heat must match demand in temperature, place and time.

Common Mistakes

✕ Confusing Conservation and Efficiency
Conservation is reducing total use (behavior). Efficiency is getting more output per input (technology). The AP exam tests this distinction frequently.
✕ Ignoring the Second Law
Students sometimes claim 100% efficiency is achievable. The Second Law guarantees some energy is always lost as waste heat in any real process.
✕ Forgetting the 10% Rule
Energy at each trophic level is approximately 10% of the previous level, not 10% of the original. Be careful with cumulative calculations.

Summary Table

Concept Key Idea Example
1st Law Energy is conserved Chemical → thermal + kinetic
2nd Law Entropy always increases Waste heat from engines
Efficiency \(\frac{E_{\text{out}}}{E_{\text{in}}} \times 100\%\) Coal plant at 35%
10% Rule ~10% transferred per level 10,000 → 1,000 → 100 kcal
Conservation Reduce total use (behavior) Turning off lights
Efficiency upgrade More output per input (tech) LED replacing incandescent

Question 1 of 5

Which of the following is an example of energy conservation rather than energy efficiency?

Question 2 of 5

A natural gas power plant produces 540 MJ of electricity from 900 MJ of fuel. What is the plant's efficiency?

Question 3 of 5

If producers in an ecosystem fix 20,000 kcal of energy, approximately how much energy is available to a secondary consumer according to the 10% rule?

Question 4 of 5

The Second Law of Thermodynamics explains which of the following environmental observations?

Question 5 of 5

A family replaces five 100 W incandescent bulbs with five 15 W LEDs, each used 6 hours/day. If electricity costs $0.12/kWh, approximately how much do they save per year?