Concentrated Solar Power

8 min read

Concentrated Solar Power

While flat plate collectors reach 60-90°C — enough for hot water — many applications demand higher temperatures. Concentrated solar power (CSP) uses curved reflectors to focus sunlight onto a small area, achieving temperatures of 300-1,500°C depending on the concentration ratio. This enables steam generation, metalworking (soldering, brazing, melting), water distillation, and even powering a steam engine — all with zero fuel.

Concentrating Optics

The principle is simple: a curved mirror focuses parallel sunlight (from the sun, which is effectively at infinite distance) onto a single point or line.

Parabolic Trough

A curved, trough-shaped reflector focuses sunlight onto a pipe running along the focal line:

  • Concentration ratio: 30-80x (temperature: 150-400°C)
  • Applications: Generating steam, heating oil for cooking, pasteurizing water at scale
  • Advantages: Only needs to track the sun in one axis (east-west), simpler mechanism
  • Construction: Bend sheet metal or plywood into a parabolic cross-section, cover with reflective material
  • Size: 1-3 meters wide, as long as needed (each additional meter adds more energy collection)

Parabolic Dish

A bowl-shaped reflector focuses sunlight onto a single point:

  • Concentration ratio: 100-1,000x (temperature: 300-1,500°C)
  • Applications: Metalworking (melting, soldering, brazing), high-temperature chemistry, Stirling engine drive
  • Advantages: Highest temperatures of any solar technology
  • Disadvantages: Must track the sun in two axes (azimuth and elevation). Focus is a point — useful only for small targets
  • The satellite dish conversion is the most practical post-collapse approach (see below)

Fresnel Reflector

Multiple flat mirror strips arranged to approximate a parabolic curve:

  • Concentration ratio: 10-40x
  • Advantages: Flat mirrors are easier to salvage than curved ones. Can be large-scale
  • Disadvantages: Less precise focusing, lower temperature
  • Ideal for: Community-scale water heating, pre-heating for steam generation

Concentration Ratio & Temperature

The concentration ratio determines achievable temperature:

RatioApproximate Max TemperatureReflector Type
1x (flat plate)80-100°CNo concentration
5-10x150-200°CFlat reflector boost
30-80x250-400°CParabolic trough
100-300x400-800°CSmall parabolic dish
500-1,000x800-1,500°CLarge precision dish

Building Reflectors from Salvage

Satellite Dish Conversion

The most practical high-concentration solar device you can build:

  1. Find a satellite TV dish — common on abandoned buildings. Sizes from 60 cm to 3 meters
  2. Clean the dish surface thoroughly
  3. Apply mirror tiles: Cut small mirror pieces (2-5 cm squares) from salvaged mirrors. Glue them to the dish surface with silicone adhesive or epoxy, fitting as closely as possible
  4. Alternative: Cover with reflective Mylar film (emergency blankets, balloon material) stretched smooth
  5. Find the focal point: Hold a piece of wood at increasing distances from the dish while aiming at the sun. The focal point is where the reflected light concentrates to the smallest, brightest spot
  6. Mark the focal length — this is where you will mount your target (pot, metal piece, boiler tube)

A 1.5-meter dish covered in mirror tiles achieves roughly 200-500x concentration — enough to melt aluminum (660°C), braze copper, and boil water in seconds.

Polished Sheet Metal

If mirror glass is unavailable:

  • Polish aluminum sheet with progressively finer abrasive (sandpaper, then polishing compound, then metal polish)
  • A well-polished aluminum surface reflects 80-90% of sunlight (vs 95% for glass mirror)
  • Form into a parabolic shape over a wooden or sand mold

Accuracy & Focus Testing

The tighter the focus, the higher the temperature:

  • Use a piece of white paper or wood to find the focal spot
  • A well-made dish should focus sunlight into a spot no larger than 2-5 cm diameter
  • Irregularities in the dish surface spread the focus — each misaligned tile wastes energy
  • Re-adjust tiles that are reflecting off-target. Even a 5° misalignment on one tile sends its light completely outside the focus

High-Temperature Applications

Metalworking

CSP can replace a forge for many tasks:

  • Soldering (230-350°C): Focus sunlight on the joint with a small dish. Flux and solder flow normally
  • Brazing (600-900°C): Requires a larger dish or trough. Position the work at the focal point
  • Melting aluminum (660°C): A 1.5-meter dish easily melts aluminum for casting
  • Melting copper (1,085°C): Requires a well-made 2+ meter dish or very accurate smaller dish
  • Forge welding / steel heat treatment (800-1,100°C): Achievable with precision dishes but challenging to control compared to a coal or charcoal forge

Advantage over fire-based metalworking: No fuel consumption, no smoke, no bellows needed. Can operate for hours with zero input except sunlight.

Steam Generation

Focusing sunlight on a water-filled tube or boiler:

  • A parabolic trough 2m x 1m focusing on a 5 cm black pipe can boil 20-40 liters per hour
  • Steam can drive a small steam engine for mechanical power or electricity generation
  • For continuous operation, a sun-tracking system is essential (see below)

Water Purification

Solar distillation at concentrated temperatures:

  • Focus on a dark vessel containing contaminated water
  • Water boils rapidly (much faster than a flat-plate solar still)
  • Condense the steam in a separate, cooled container
  • Produces pure distilled water at 5-20 liters per hour with a 2+ meter trough

Tracking Systems

Why Tracking Matters

The sun moves 15° per hour across the sky. A parabolic dish pointed at the sun at noon is 30° off-target by 2 PM — with a high-concentration system, this means zero useful output. Tracking is essential.

Manual Sun Tracking

The simplest approach:

  • Mount the reflector on a pivot (azimuth) and tilt (elevation) mechanism
  • Adjust every 15-30 minutes throughout the day
  • Adequate for cooking and small metalworking tasks that need periodic attention anyway

Gravity-Driven Clock Tracker

A mechanical tracker requiring no electricity:

  • Use a clock mechanism (salvaged from a large clock or built from gears) to rotate the reflector at 15° per hour
  • A weight-driven clock escapement provides the motion
  • Calibrate for your latitude and adjust for seasons
  • Historically used in heliostats (sun-tracking mirrors) for lighthouses and solar furnaces

Seasonal Adjustment

The sun’s elevation changes with seasons:

  • Summer solstice: Highest noon elevation (90° minus your latitude plus 23.5°)
  • Winter solstice: Lowest noon elevation (90° minus your latitude minus 23.5°)
  • Adjust the tilt of your trough or dish seasonally (every 2-4 weeks is sufficient)
  • Mark the correct tilt angles on the mount for quick adjustment

Practical Example: Solar Forge Station

A complete solar metalworking station for a community workshop:

  1. Reflector: 1.8-meter satellite dish covered in mirror tiles (salvaged from bathroom mirrors, cut with glass cutter)
  2. Mount: Steel pipe on a concrete base with azimuth bearing (lazy susan bearing from furniture) and elevation pivot (bolt hinge)
  3. Work platform: A fireproof surface (steel plate on brick) positioned at the focal point, adjustable via threaded rod for fine focus
  4. Tool rack: Tongs, flux, solder, brazing rod — all within arm’s reach of the focal point
  5. Shade screen: A sheet of plywood or metal with a small window, positioned so the operator can see the work without staring at the reflected glare

Daily workflow:

  • Morning (9-10 AM): Adjust dish for morning sun angle. Pre-heat metal stock at the edge of the focal zone (warm-up reduces thermal shock)
  • Peak hours (10 AM - 2 PM): Perform high-temperature work — brazing, aluminum casting, steel heat treatment
  • Afternoon (2-4 PM): Lower-temperature tasks — soldering, lead melting, wax melting for lost-wax casting
  • Track the sun every 20-30 minutes (manual adjustment takes 15 seconds once you learn the rhythm)

Output: On a clear day, a 1.8-meter dish provides roughly 1.5 kW of concentrated thermal power — equivalent to a small propane torch, running for free, all day.

Scaling Up: Community Solar Steam

For a Phase 4 village, multiple parabolic troughs can generate steam for mechanical power:

  • Array: 4-6 troughs, each 3m x 1.5m, focused on a single insulated pipe carrying water/steam
  • Steam temperature: 200-300°C at 5-15 bar pressure
  • Applications: Drive a small steam engine (1-5 kW mechanical), run a grain mill, power a workshop lathe, or generate electricity through a steam-driven alternator
  • Storage: Route steam through a thermal mass (rock bed or insulated water tank) for 2-4 hours of operation after sunset

This approach was demonstrated at Solar One and Solar Two power plants (California) and at Maadi solar plant (Egypt, 1913) — the technology is proven at much larger scales.

Common Mistakes

  • Using flat mirrors without curvature: Flat mirrors only achieve ~5x concentration. For metalworking temperatures, you need a parabolic curve
  • Cheap adhesive on mirror tiles: Silicone in direct sun degrades. Use high-temperature RTV silicone or epoxy rated for 200°C+
  • Ignoring eye safety: Concentrated sunlight at 500x will permanently blind you in milliseconds. Never look at the focal point. Wear welding shade 12-14 glasses when working near the focus
  • Undersized support structure: A 2-meter dish in 30 km/h wind experiences significant force. Guy the mount or weight the base heavily

See Also