SHI Solar Thermal System Design & Components 3 — Questions and Answers
Question 1: When sizing a solar thermal system for domestic hot water, what is the commonly used rule of thumb for collector area per person?
- 2–4 sq ft (0.2–0.4 m²) per person
- 8–12 sq ft (0.7–1.1 m²) per person (Correct answer)
- 20–25 sq ft (1.9–2.3 m²) per person
- 35–40 sq ft (3.3–3.7 m²) per person
Correct answer: 8–12 sq ft (0.7–1.1 m²) per person
A common sizing guideline is approximately 8–12 sq ft of collector area per person in a household for domestic hot water in average US climates.
Question 2: What type of heat transfer fluid is most commonly used in pressurized closed-loop solar thermal systems in cold climates?
- Distilled water
- Propylene glycol/water mixture (Correct answer)
- Mineral oil
- Ethylene glycol undiluted
Correct answer: Propylene glycol/water mixture
Propylene glycol mixed with water is the preferred antifreeze in solar thermal systems because it is non-toxic, providing freeze and burst protection without hazard to potable water if the heat exchanger leaks.
Question 3: In a thermosiphon solar water heater, what drives fluid circulation without a pump?
- Wind pressure on the collector
- Density difference between hot and cold fluid creating natural convection (Correct answer)
- Capillary action in small-diameter tubing
- An aspirator valve in the return line
Correct answer: Density difference between hot and cold fluid creating natural convection
Thermosiphon systems rely on the principle that heated fluid becomes less dense and rises naturally, creating a convective loop that circulates fluid without mechanical pumping.
Question 4: What is 'stagnation' in the context of solar thermal collectors?
- Zero fluid flow due to a blocked strainer
- The condition where the collector reaches maximum temperature with no fluid flowing (Correct answer)
- Sediment settling in the storage tank
- Loss of vacuum in an evacuated tube
Correct answer: The condition where the collector reaches maximum temperature with no fluid flowing
Stagnation occurs when pump flow stops (power outage or thermostat satisfaction) while solar radiation continues, causing collectors to reach very high temperatures that can degrade fluid and system components.
Question 5: Which pipe material is generally NOT recommended for solar thermal collector loop piping due to potential degradation at high temperatures?
- Type L copper
- Standard PEX (cross-linked polyethylene) (Correct answer)
- CPVC with appropriate pressure rating
- Stainless steel
Correct answer: Standard PEX (cross-linked polyethylene)
Standard PEX can soften and fail at the high stagnation temperatures (above 200°F/93°C) reached in solar collector loops; high-temperature PEX-a may be acceptable at reduced pressures but must be verified.
Question 6: What is the function of an air separator (air eliminator) in a solar thermal system?
- To keep oxygen away from the collectors to prevent corrosion
- To remove dissolved and entrained air from the circulating fluid, preventing air locks (Correct answer)
- To vent the storage tank when filling
- To regulate glycol concentration in the fluid
Correct answer: To remove dissolved and entrained air from the circulating fluid, preventing air locks
Air separators remove air bubbles from the circulating heat transfer fluid, preventing air locks that reduce or stop flow and cause noise and reduced efficiency.
Question 7: In a solar thermal system, what is the purpose of a mixing valve (tempering valve) on the storage tank outlet?
- To blend collector and storage fluids for optimal efficiency
- To limit hot water delivery temperature to prevent scalding at fixtures (Correct answer)
- To control collector fluid concentration
- To balance flow between multiple storage tanks
Correct answer: To limit hot water delivery temperature to prevent scalding at fixtures
A thermostatic mixing valve blends stored solar-heated water with cold water to deliver a safe, scald-preventing temperature (typically 120°F/49°C) to fixtures even when tank temperatures are much higher.
SHI Solar Thermal System Design & Components 3
When sizing a solar thermal system for domestic hot water, what is the commonly used rule of thumb for collector area per person?