Q4Power Generation Sources
Question
Q.4. Explain in detail flat plate and concentrating solar collectors with suitable diagrams.
Answer
Flat plate collectors use a blackened absorber plate under a glass cover to capture non-concentrated solar radiation for low-temperature applications (water/space heating up to ~80°C), while concentrating collectors use mirrors/lenses to focus sunlight onto a small receiver, achieving much higher temperatures for power generation and industrial process heat.
Solar thermal collectors absorb incident solar radiation and convert it into heat, transferred to a working fluid (usually water, air or oil). They are broadly classified into non-concentrating (flat plate) and concentrating collectors, depending on whether the collector area equals the absorber area or is larger (using optical concentration).
Flat Plate Collector
A flat plate collector (FPC) consists of: (1) a blackened absorber plate (usually copper or aluminum, coated with a selective black coating for high absorptivity and low emissivity) with bonded fluid-carrying tubes/risers; (2) one or two transparent glass covers on top, spaced above the absorber, which transmit incoming shortwave solar radiation but trap the re-radiated longwave infrared heat (greenhouse effect), and also reduce convective heat loss to ambient air; (3) insulation (glass wool/polyurethane foam) on the back and sides to minimize conductive heat loss; and (4) a metal casing to house and protect the assembly.
Solar radiation passes through the glass cover(s) and is absorbed by the blackened plate, heating the fluid circulating through the attached tubes by conduction. Since the collector does not track the sun and accepts both direct and diffuse radiation without concentration, it can only reach moderate temperatures (typically 40-80°C), making it suitable for domestic/commercial water heating, space heating, solar drying and low-temperature industrial process heat. FPCs are simple, low-cost, require no tracking mechanism, and can utilize diffuse (cloudy-sky) radiation as well as direct radiation.
Concentrating Collector
A concentrating collector uses reflective (mirrors) or refractive (lenses) optical elements to focus a large area of incident solar radiation onto a much smaller receiver/absorber area, thereby achieving a concentration ratio (collector aperture area / receiver area) that can range from a few times up to several thousand times. Because the same amount of solar energy is delivered onto a smaller absorber surface, concentrating collectors achieve substantially higher temperatures (from 100°C up to over 1000°C for high-concentration systems) than flat plate collectors, but require accurate tracking of the sun's position (single-axis or dual-axis tracking) since only direct-beam radiation can be effectively concentrated, and diffuse radiation is largely wasted.
Common types include: parabolic trough collectors (long, curved trough-shaped mirrors focusing sunlight onto a linear receiver tube running along the focal line, used widely in commercial Concentrated Solar Power (CSP) plants), parabolic dish collectors (a dish-shaped mirror focuses sunlight onto a single focal point, often with a Stirling engine or small turbine mounted at the focus, achieving very high concentration ratios and temperatures), and solar power tower/central receiver systems (a large field of individually sun-tracking flat mirrors, called heliostats, reflect sunlight onto a receiver mounted atop a central tower, achieving the highest concentration ratios and temperatures, sufficient to drive a conventional steam Rankine cycle for utility-scale electricity generation).
In summary, flat plate collectors are simple, low-temperature, non-tracking devices suited to heating applications, whereas concentrating collectors are more complex, higher-cost, sun-tracking systems capable of delivering the high temperatures needed for efficient thermal power generation and high-temperature industrial processes.
Concentration ratio and efficiency: the concentration ratio C is defined as the aperture (collector) area divided by the receiver (absorber) area, C = A_a/A_r. Higher concentration ratios permit higher stagnation temperatures at the receiver because, for a given receiver, radiative heat loss (proportional to receiver area and to the fourth power of its temperature by the Stefan-Boltzmann law) is reduced relative to the much larger useful collection area, so the useful thermal efficiency of a concentrating collector, unlike a flat plate collector, can actually improve as operating temperature rises, up to the point where optical/tracking losses dominate. This is why parabolic trough plants achieve receiver-fluid temperatures around 390°C and central-tower plants can exceed 550°C, whereas a flat plate collector plateaus around 80-100°C regardless of design refinements, since it has a concentration ratio of essentially 1.