--- title: "Flat plate collector and solar water heater" subject: "Energy Resources" chapter: 2 key: "semester_6/energy_resources:flat-plate-collector-construction-water-heater" source: https://www.bctengineer.com/q/semester_6/energy_resources/flat-plate-collector-construction-water-heater site: BCT Engineering --- # Flat plate collector and solar water heater Energy Resources · Chapter 2: Solar energy - Asked in: year not given, 6 marks - Video: [flat plate collector | flat plate solar collector | flat plate collector in renewable energy |solar](https://www.youtube.com/watch?v=JWH-whj7kgk) — Mind of Engineer - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/energy_resources/flat-plate-collector-construction-water-heater - Whole chapter: https://www.bctengineer.com/md/semester_6/energy_resources/chapter-2 ## Question Describe the construction and working of a liquid flat plate collector. Explain how a thermosyphon solar water heater works, with a sketch. ## Short answer A flat plate collector has a black absorber plate with tubes, a glass cover, and back/side insulation. Solar radiation heats the plate, warming water in the tubes. In a thermosyphon system, the storage tank is placed above the collector; hot water rises to the tank top while cold water descends, driving natural circulation without a pump. ## Answer A **flat plate collector** (FPC) absorbs both beam and diffuse radiation on a flat surface and transfers the heat to a fluid, without concentrating it. It is used for temperatures up to about 80–100 °C. #### Construction - **Absorber plate:** a copper or aluminium sheet with tubes (risers) attached to it, coated with a black or selective coating (high absorptance, low emittance). - **Transparent cover:** one or two glass sheets; they pass short-wave solar radiation but block long-wave re-radiation from the plate (greenhouse effect) and reduce convection loss. - **Insulation:** glass wool or polyurethane (50–75 mm) at the back and sides. - **Casing (box):** aluminium or galvanised steel, weatherproof. - **Headers:** connect the riser tubes to the inlet and outlet pipes. ```text sun rays \ \ \ ===== glass cover ====== air gap ~~~ absorber plate + tubes ~~~ ///// insulation ////////////// |________ casing ____________| ``` #### Working Radiation passes through the glass and heats the plate. Water in the tubes takes up this heat and leaves hotter. The useful heat is given by the Hottel-Whillier-Bliss equation $Q_u = A_c F_R\,[S - U_L (T_i - T_a)]$, and the efficiency is $\eta = Q_u/(A_c G_T)$. #### Thermosyphon water heater ```text hot water to taps ^ cold mains +-------|-------+ -------------->| storage tank | | (hot at top) | +---------------+ hot ^ | cold water | | water rises | v falls +-----------------+ | flat plate | | collector | +-----------------+ (collector placed below the tank) ``` The storage tank is placed above the collector. Water heated in the collector becomes less dense and rises to the top of the tank, while the colder, denser water from the bottom of the tank flows down to the collector inlet. This natural circulation (thermosyphon) needs no pump and runs only while the sun shines. The bottom of the tank should be at least 0.3–0.5 m above the top of the collector, and the pipes must be short and sloped, without air traps. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the function of the transparent cover? A. It passes short-wave solar radiation but blocks long-wave re-radiation and reduces convection loss B. It absorbs solar radiation to heat the water C. It insulates the back of the collector D. It carries the water to the tank Answer: A. The greenhouse effect traps heat in the collector. (Key point: Glass cover) 2. Why is the absorber plate given a black or selective coating? A. Low absorptance and high emittance B. To reflect the beam radiation C. To make the plate transparent D. High absorptance and low emittance Answer: D. It absorbs strongly and loses little by re-radiation. (Key point: Absorber coating) 3. How does water circulate in a thermosyphon heater? A. A pump drives the water B. Hot water falls due to higher density C. Heated water is less dense and rises to the tank while cooler dense water flows down to the collector D. Water flows by pressure from the mains only Answer: C. Natural circulation needs no pump; the tank must be above the collector. (Key point: Thermosyphon principle) 4. The useful heat of the collector is given by... A. Qu = Ac FR [S + UL (Ti - Ta)] B. Qu = Ac FR [S - UL (Ti - Ta)] C. Qu = S/(Ac UL) D. Qu = G_T / Ac Answer: B. Absorbed radiation minus loss, scaled by the removal factor. (Key point: HWB equation) ## Related in Energy Resources - [Flat plate collector useful heat and outlet temperature](https://www.bctengineer.com/question.md?key=semester_6%2Fenergy_resources%3Aflat-plate-collector-efficiency-numerical) — Energy Resources, chapter 2 - [Solar concentrators and concentration ratio](https://www.bctengineer.com/question.md?key=semester_6%2Fenergy_resources%3Asolar-concentrators-types-concentration-ratio) — Energy Resources, chapter 2 ## Related in other subjects - [Short note: Solar water heater](https://www.bctengineer.com/question.md?key=semester_7%2Fenergy_environment_and_society%3Asolar-water-heater) — Energy Environment and Society, chapter 3 - [Types of solar collector](https://www.bctengineer.com/question.md?key=semester_7%2Fenergy_environment_and_society%3Asolar-collector-types) — Energy Environment and Society, chapter 3 - [Regenerative scheme with open feed water heater](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Aregenerative-open-heater-layout) — Power Plant Equipment, chapter 7 - [Jet on flat plate, force and efficiency](https://www.bctengineer.com/question.md?key=semester_3%2Ffluid_mechanics%3Ajet-on-flat-plate-efficiency) — Fluid Mechanics, chapter 7