Energy and Environment practice questions

Answer: C) 34%

• AIncorrect. That's inverted.
• BIncorrect. That's the loss fraction.
• CCorrect. 3,412/10,000.
• DIncorrect. Recheck: 3,412/10,000.

Answer: A) 400 kg CO₂

• ACorrect. 1,000 × 0.4.
• BIncorrect. Multiply, don't divide.
• CIncorrect. Recheck the arithmetic.
• DIncorrect. Apply the emission factor.

Answer: B) The highest kW demand in the billing period

• AIncorrect. That's the energy charge.
• BCorrect. Peak shaving lowers them.
• CIncorrect. That isn't the basis.
• DIncorrect. That isn't the basis.

Answer: C) Sunlight directly into electricity

• AIncorrect. That's a wind turbine.
• BIncorrect. That's concentrating solar power.
• CCorrect. Through the photovoltaic effect in semiconductors.
• DIncorrect. It converts sunlight.

Answer: A) A prioritized list of energy conservation measures with estimated costs and savings

A comprehensive energy audit looks at how a facility uses energy and identifies cost-effective conservation measures, usually as a prioritized list with estimated costs and savings. Emissions inventories, site assessments and stack tests are separate studies with different goals.

• ACorrect. An energy audit (evaluation) finds where energy is used and identifies conservation measures, usually ranked by cost-effectiveness, so the owner can decide which to implement.
• BIncorrect. A GHG inventory adds up emissions for reporting. It does not evaluate where energy can be saved or what measures to take.
• CIncorrect. A Phase I environmental site assessment looks for contamination liability during property transactions, not energy use.
• DIncorrect. Stack testing shows whether the plant complies with its air emission limits. It does not identify energy-saving opportunities.

Answer: A) 51% of full power

• ACorrect. 0.8³ = 0.512.
• BIncorrect. Power isn't linear in speed.
• CIncorrect. That's a square.
• DIncorrect. That isn't the calculation.

Answer: C) 1,668 kW

• AIncorrect. Include ρg.
• BIncorrect. Apply the 85% efficiency.
• CCorrect. P = ηρgQH = 0.85(1,000)(9.81)(10)(20) W.
• DIncorrect. Multiply by efficiency, don't divide.

Answer: B) Combined heat and power, also called cogeneration of energy

Making electricity from digester biogas and reusing the engine heat to warm the digesters is combined heat and power (cogeneration). It makes much more use of the fuel's energy than generating power alone.

• AIncorrect. Peak shaving means running generation or storage only during peak demand periods to cut demand charges. It says nothing about using the engines' heat for digester heating.
• BCorrect. One fuel (biogas) produces both electricity and useful thermal energy (digester heating). That is combined heat and power, and it gives much higher overall fuel efficiency than generating power alone.
• CIncorrect. Sequestration means capturing CO2 and storing it long term. Nothing in this arrangement captures or stores CO2.
• DIncorrect. Demand response means cutting or shifting loads when the utility asks. It does not describe using engine heat for a process.

Answer: C) Improves the power factor

• AIncorrect. That's a rectifier.
• BIncorrect. They reduce reactive current.
• CCorrect. They supply reactive power locally.
• DIncorrect. Speed depends on frequency.

Answer: B) 754 kW

Power available in the wind is P = ½ρAv³, where A is the swept area πr². With r = 20 m and v = 10 m/s, P ≈ 0.5 × 1.2 × 1,257 × 1,000 ≈ 754 kW.

• AIncorrect. This uses v² instead of v³. Kinetic energy flux through the rotor depends on the cube of wind speed: ½ × 1.2 × 1,257 × 10² ≈ 75 kW.
• BCorrect. A = π(20 m)² ≈ 1,257 m². P = ½ρAv³ = 0.5 × 1.2 × 1,257 × 10³ ≈ 754,000 W ≈ 754 kW.
• CIncorrect. This leaves out the ½ in the kinetic-energy flux: 1.2 × 1,257 × 1,000 ≈ 1,508 kW.
• DIncorrect. This uses the 40 m diameter as the radius, so A = π(40)² ≈ 5,027 m² and P ≈ 3,016 kW.