REP Geothermal and Hydropower 5 — Questions and Answers
Question 1: What is the typical round-trip efficiency range for pumped-storage hydropower systems?
- 30–50%
- 70–85% (Correct answer)
- 90–95%
- 99–100%
Correct answer: 70–85%
Pumped-storage hydropower achieves round-trip efficiencies of approximately 70–85%, accounting for losses in pumping, penstock friction, turbine/generator conversion, and transformer stages.
Question 2: Which U.S. state leads in installed geothermal electricity generating capacity?
- Hawaii
- Nevada
- California (Correct answer)
- Idaho
Correct answer: California
California leads the U.S. in geothermal capacity, largely due to The Geysers in Sonoma and Lake counties, the world's largest complex of geothermal power plants operating since 1960.
Question 3: A hydropower project's 'hydraulic head' decreases by 20% due to drought-reduced reservoir levels. Assuming constant efficiency, how does this affect power output?
- Power decreases by approximately 20% (Correct answer)
- Power decreases by approximately 40%
- Power decreases by approximately 10%
- Power is unaffected because flow rate increases to compensate
Correct answer: Power decreases by approximately 20%
Since power output is directly proportional to hydraulic head (P = ρgQHη), a 20% reduction in head causes approximately a 20% reduction in power output, assuming flow rate and efficiency remain constant.
Question 4: What does the term 'minimum environmental flow' (or 'minimum instream flow') refer to in hydropower project management?
- The minimum water temperature required for downstream aquatic species survival
- The legally required minimum discharge that must be released downstream to sustain ecosystem functions (Correct answer)
- The minimum flow rate needed to keep the turbine operating above its cavitation threshold
- The minimum reservoir volume required to maintain dam structural stability
Correct answer: The legally required minimum discharge that must be released downstream to sustain ecosystem functions
Minimum environmental flow is the regulated minimum discharge that hydropower operators must release downstream to protect aquatic habitat, riparian vegetation, and water quality below the dam.
Question 5: In ground-source heat pump systems, what is the 'coefficient of performance' (COP) measuring?
- The ratio of heat energy delivered to electrical energy consumed by the heat pump (Correct answer)
- The depth of ground loop required per kilowatt of heating capacity
- The percentage of geothermal energy converted to electricity
- The thermal conductivity of soil compared to a standard benchmark
Correct answer: The ratio of heat energy delivered to electrical energy consumed by the heat pump
COP is the ratio of useful heat output (or cooling effect) to the electrical energy input consumed by the heat pump compressor, with typical ground-source systems achieving COPs of 3–5.
Question 6: Which phenomenon, sometimes observed near geothermal injection wells, has raised regulatory concerns about EGS development in urban areas?
- Magnetic field interference with utility infrastructure
- Induced seismicity from fluid injection into subsurface rock (Correct answer)
- Groundwater contamination from dissolved heavy metals
- Surface subsidence from reservoir pressure depletion
Correct answer: Induced seismicity from fluid injection into subsurface rock
High-pressure fluid injection in EGS projects can reactivate pre-existing faults, triggering induced seismicity that may be felt at the surface, a key regulatory and public acceptance concern.
Question 7: What distinguishes a 'tidal barrage' hydropower system from a 'tidal stream' or 'in-stream tidal' system?
- Tidal barrages use submerged turbines in open water while tidal stream systems use dams across estuaries
- Tidal barrages dam an estuary to create a head difference between tides, while tidal stream systems use turbines in free-flowing tidal currents (Correct answer)
- Tidal barrages generate electricity only during incoming tides; tidal stream systems operate during both tides
- Tidal barrages require offshore platforms while tidal stream systems are always onshore
Correct answer: Tidal barrages dam an estuary to create a head difference between tides, while tidal stream systems use turbines in free-flowing tidal currents
Tidal barrages are dam-like structures built across estuaries that trap water to create hydraulic head for turbine generation, while tidal stream systems place turbines directly in fast-flowing tidal channels without impoundment.
REP Geothermal and Hydropower 5
What is the typical round-trip efficiency range for pumped-storage hydropower systems?