Solar Concentrators

Solar Concentrators

• Concentrating collectors use curved/ plane mirrors to concentrate sunlight

• Several hundred times the normal solar intensity is usually targeted to achieve temperature up to 400- 500 deg C.

• Higher concentration can be achieved for temperatures about 2500 deg C

• Primarily in commercial and industrial applications.

• They function only when direct radiation is available, hence require tracking. Concentration cannot be used with diffused radiation

• Some designs concentrate solar energy onto a focal point, while others concentrate the sun's rays along a thin line called the focal line.

• A heat-transfer fluid flows through the receiver and absorbs heat.

• Concentrators are most practical in areas of high insolation close to the equator and in the desert.

Concentrating configurations

• 1-Dimensional – Concentrating Solar Water

Heater/Cooker (CSWH).

• 2-Dimensional {Concentrating Solar power (CSP)} – Tracking Parabolic Trough

Concentrators (PTC) – Compound Parabolic

Concentrators (CPC) – Inverted Absorber Compound

Parabolic Concentrators

• 3-Dimensional – Tracking Parabolic Dish

Concentrators (PDC) – Heliostat fields – Solar Pumped lasers

Concentration ratio is given ideally by (Area of receiver)/ (Area of absorber)

Parabolic Trough systems

• Parabolic trough collectors use trough-shaped reflectors that concentrate sunlight on a receiver tube running along the reflector's focal line, achieving much higher temperatures than flat-plate or evacuated-tube collectors.

• These systems usually include a mechanical control system that keeps the trough reflector pointed at the sun throughout the day.

• Evacuated tube collectors are advantageously used with these concentrators

• Parabolic-trough concentrating systems can provide hot water and steam, and are generally used in commercial and industrial applications.

Tracking Line Axis Parabolic Trough Concentrator (2-D)

PTC Collectors track the sun continuously so any ray entering the concentrator parallel to the axis will, either after reflection or directly, intercept the tubular absorber. Less

diffuse radiation is absorbed by PTC collectors

Line Axis Parabolic trough concentrators

• Fluid in collector heated up to 400°C

• > 400 MWe Running in California since 1980’s.

• Hybridised with natural gas for times when solar energy is not available (25%)

• 2million m2 parabolic collectors globally.

• Fluid often “synthetic thermal oil” used to superheat steam

• Efficiency suffers slightly as large surface area of collector has long fluid runs.

Compound Parabolic Concentrating Collectors

• Compound parabolic concentrating collectors (CPCCs) use mirrored surfaces to concentrate the sun's energy on an absorber called a receiver, similar to parabolic trough collectors.

• CPCCs achieve moderate concentration and moderately high temperatures but, unlike parabolic trough collectors, they can collect both direct and diffuse sunlight and don't require an automated sun-tracking system.

• CPCCs are being investigated for use in commercial applications where higher temperatures are required.

Line Axis Compound Parabolic Trough Concentrator (2-D)

CPC collectors do not track the sun but through their geometry via multiple reflections absorb more diffuse radiation.

Tracking Parabolic Dish Concentrator (3-D)

Heat removal pipe

Absorber cavity, Cu-Zr alloy, black chrome plated

Multi-mirror concentrators

Multi-reflector Parabolic Dish collector

• Larger dishes often comprised of many smaller reflective elements.

• Cheaper than making a single reflector surface.

• Often used to power Stirling heat engines.

Stationary Multi-reflector PDC with tracking point absorber

• Auroville Solar Dish, India.

– Reflective dish focuses heat

– Captures enough energy to cook 700 to 2000 meals per day

Heliostat Reflector Concentrator

Accuracy & cleanliness are very important!

Summary of all collector types

Stationary (Non-tracking) Solar Thermal Energy Collectors

Tracking Solar Thermal Energy Collectors

Schematic of a solar-thermal conversion system

Eurotrough Reflector

The focus pipe has flexible pipes on the ends to allow rotary motion to track the sun, while the absorber pipe is kept at the heat focus

Layout

• Each pair of tubes forms a loop from the central manifolds

• The power block is centrally located to minimize thermal losses

• Oil is heated to ~500°C

Solar Power Towers

• Solar power towers were developed at Sandia, south of Albuquerque, New Mexico

– Salt was melted by the focused heat

– If the sun was blocked, the salt could harden in the pipes, requiring torches to get it flowing again

– The heliostat mirrors are about ten feet square and mounted on the azimuth-elevation mounts (like a radar antenna mount) about 10 feet off the ground

• The larger version with a full circle array, Solar One, was built near Barstow, CA

– Water is used, but the system is being modified for molten salt

Solar Two Heliostat, Barstow

• 10 MW Power Plant with 1000x concentration ratio.

• Each heliostat is separately driven to focus its beam on the receiver

• Molten Nitrate Salt. – Properties like water > 240°C

melting point. – Heated to 565°C and kept in hot

salt storage until needed.

• Circulated through steam generator on demand to produce electricity.

• Good long term storage capacity. • Pilot for 50MW “off-the-shelf”

Power stations in Spain.

Solar CSP Summary

Concentrating Solar Power and Sun Lab Sandia National Laboratories http://www.energylan.sandia.gov/sunlab/overview.htm