HERS Renewable Energy Systems 3 — Questions and Answers
Question 1: What is the 'solar fraction' in the context of a solar water heating system?
- The percentage of roof area covered by solar collectors
- The proportion of hot water heating load met by solar energy (Correct answer)
- The ratio of collector area to storage tank volume
- The percentage of time solar radiation exceeds design threshold
Correct answer: The proportion of hot water heating load met by solar energy
Solar fraction represents the fraction of the total water heating load provided by the solar system, with the remainder supplied by a backup conventional heater.
Question 2: In a solar thermal domestic hot water system, what component prevents reverse thermosiphoning that could drain heat from the storage tank at night?
- Expansion tank
- Tempering valve
- Check valve or differential controller (Correct answer)
- Pressure relief valve
Correct answer: Check valve or differential controller
A check valve or differential controller prevents reverse flow when the collector is cooler than the tank, which would cause the tank to lose heat.
Question 3: What distinguishes an active solar thermal system from a passive solar thermal system?
- Active systems use evacuated tube collectors while passive use flat-plate collectors
- Active systems use pumps and controls to move the heat transfer fluid (Correct answer)
- Active systems store energy in phase-change materials
- Active systems only function in climates with more than 300 sunny days per year
Correct answer: Active systems use pumps and controls to move the heat transfer fluid
Active solar thermal systems rely on pumps, fans, and controls to circulate the heat transfer fluid, whereas passive systems use natural convection or thermosiphon principles.
Question 4: Which metric is used to compare the environmental impact of different renewable energy technologies over their full lifecycle?
- Energy payback period
- Carbon intensity (gCO2/kWh) (Correct answer)
- Capacity factor
- Levelized Cost of Energy (LCOE)
Correct answer: Carbon intensity (gCO2/kWh)
Carbon intensity (grams of CO2 per kWh) measures lifecycle greenhouse gas emissions per unit of electricity generated, enabling environmental comparison across technologies.
Question 5: A home energy rater is evaluating a small wind turbine installation. Which site factor is MOST critical to assess first?
- Turbine tower height options
- Average annual wind speed at hub height (Correct answer)
- Turbine blade material and noise rating
- Local utility net metering policy
Correct answer: Average annual wind speed at hub height
Wind power output scales with the cube of wind speed, making average annual wind speed at hub height the most critical factor determining whether a turbine is economically viable.
Question 6: What is the purpose of a 'combiner box' in a solar PV array?
- Combines AC output from multiple inverters into a single feed
- Combines multiple PV string circuits into a single DC output to the inverter (Correct answer)
- Combines solar and battery storage inputs for the inverter
- Combines monitoring signals from multiple panels
Correct answer: Combines multiple PV string circuits into a single DC output to the inverter
A combiner box aggregates the DC output from multiple PV strings, providing overcurrent protection for each string before sending combined DC power to the inverter.
Question 7: In HERS rating software, which solar PV input parameter most directly affects the modeled annual energy production?
- Panel brand and model number
- System DC capacity (kW) and local solar irradiance (Correct answer)
- Inverter manufacturer warranty period
- Wire gauge used in the installation
Correct answer: System DC capacity (kW) and local solar irradiance
System DC capacity combined with location-specific solar irradiance data (from TMY files) are the primary drivers of modeled annual PV energy output.
HERS Renewable Energy Systems 3
What is the 'solar fraction' in the context of a solar water heating system?