REP Renewable Energy Technologies & Applications 5 — Questions and Answers
Question 1: What distinguishes a 'virtual power plant' (VPP) from a traditional centralized power plant?
- A VPP uses only nuclear energy distributed across small reactors
- A VPP aggregates distributed energy resources to act collectively as a single dispatchable unit (Correct answer)
- A VPP is a simulated model used only for grid planning purposes
- A VPP stores electricity in underground caverns for peak demand
Correct answer: A VPP aggregates distributed energy resources to act collectively as a single dispatchable unit
A VPP coordinates distributed assets — rooftop solar, batteries, EVs, demand response — through software to behave as a single controllable resource visible to grid operators.
Question 2: In a tidal barrage system, electricity is generated during which phase(s) of the tidal cycle?
- Only during high tide when the barrage gates are open
- Only during low tide when water drains from the basin
- During ebb flow, flood flow, or both, depending on system design (Correct answer)
- Continuously at a constant rate regardless of tidal phase
Correct answer: During ebb flow, flood flow, or both, depending on system design
Tidal barrage systems can be designed for ebb-only, flood-only, or two-way generation by controlling sluice gates and turbine operation to exploit head differences in both tidal directions.
Question 3: Which factor most directly determines the output of a wind turbine at a given wind speed?
- Tower height above sea level
- Rotor diameter (swept area) (Correct answer)
- Nacelle mass
- Generator voltage rating
Correct answer: Rotor diameter (swept area)
Power captured from wind is proportional to the swept area of the rotor (πr²), so rotor diameter is the dominant design factor determining energy capture at any given wind speed.
Question 4: What is the role of 'renewable energy certificates' (RECs) in voluntary green power markets?
- They certify the physical delivery of renewable electricity to a specific customer
- They represent the environmental attributes of one MWh of renewable generation, tradeable separately from the electricity (Correct answer)
- They are government permits required to construct renewable energy facilities
- They guarantee priority grid interconnection for renewable generators
Correct answer: They represent the environmental attributes of one MWh of renewable generation, tradeable separately from the electricity
A REC (also called SREC, I-REC, or GO in Europe) represents the non-power attributes of one MWh of renewable generation and can be sold separately from the underlying electricity.
Question 5: Which characteristic of perovskite solar cells makes them particularly attractive for next-generation PV, despite current commercialization challenges?
- They are manufactured from abundant silicon with no rare elements
- They can be solution-processed and have achieved efficiencies >25% in the lab (Correct answer)
- They have a proven 30-year outdoor degradation track record
- They require no encapsulation due to moisture resistance
Correct answer: They can be solution-processed and have achieved efficiencies >25% in the lab
Perovskite cells have achieved laboratory efficiencies exceeding 25% (and over 33% in tandem with silicon) and can be fabricated using low-cost printing processes, but face durability and lead-content challenges.
Question 6: In building-integrated photovoltaics (BIPV), what distinguishes BIPV from building-applied photovoltaics (BAPV)?
- BIPV panels are always more efficient than BAPV panels
- BIPV modules replace conventional building materials and serve a dual function (Correct answer)
- BIPV systems are only used on commercial buildings
- BIPV always requires battery storage to qualify
Correct answer: BIPV modules replace conventional building materials and serve a dual function
BIPV modules replace conventional building envelope components — roofing tiles, facades, glazing — serving both a structural/weatherproofing function and generating electricity, unlike BAPV which is added on top.
Question 7: What is the primary technical reason that integrating high percentages of variable renewable energy (VRE) into a grid requires increased flexibility resources?
- VRE plants produce only DC power incompatible with AC grids
- VRE output fluctuates with weather and does not inherently match demand patterns (Correct answer)
- VRE plants cannot provide reactive power for voltage support
- VRE generation equipment depreciates faster than conventional plants
Correct answer: VRE output fluctuates with weather and does not inherently match demand patterns
Solar and wind generation varies with irradiance and wind speed rather than following load, creating mismatches between supply and demand that require flexible resources — storage, demand response, dispatchable generation — to balance.
REP Renewable Energy Technologies & Applications 5
What distinguishes a 'virtual power plant' (VPP) from a traditional centralized power plant?