Forget mirrors. A pool of water shaped like a plano-convex lens will refract sunlight just fine — n = 1.33, no silver coating required. If we could machine a kilometer-wide "water lens" and hold its shape, could it replace a concentrated solar power plant?

The lensmaker's equation, blown up. For a plano-convex lens with one flat side, focal length is f = R / (n − 1), where R is the curved surface's radius of curvature. For water, that's f ≈ 3R. If we want a focal length of 500 m (a reasonable tower height for the receiver), we need R ≈ 165 m. A 1 km diameter lens with that curvature has a sagitta (center thickness above the edges) of s ≈ D²/(8R) ≈ 758 m. That's absurd — the "lens" would be taller than the Burj Khalifa.

So we cheat: use a Fresnel-style water lens. Concentric annular troughs, each shaped like a slice of the ideal curve, tiled across a 1 km disk. Each ring is maybe 2 m thick. Now the whole lens fits in a shallow basin — think a segmented aquaculture pond with precision-molded transparent membranes on top.

Power budget. Solar irradiance is ~1000 W/m². A 1 km disk intercepts π × (500)² × 1000 ≈ 785 MW of sunlight. Water absorbs light non-trivially in the near-IR — the absorption coefficient averaged across the solar spectrum through 2 m of water is roughly 30–40%. Call the transmission efficiency 60%, and Fresnel losses at the air/water interface another 4%. Useful throughput: ~450 MW at the focus. That's Ivanpah-scale, with no mirrors.

The focal spot is enormous. Chromatic aberration is brutal — water's refractive index varies from 1.344 (blue) to 1.331 (red), a 1% spread. Focal length varies by the same fraction: Δf ≈ 5 m. Combined with the finite angular size of the sun (0.53°), the focal spot at 500 m is roughly 500 × tan(0.27°) ≈ 2.4 m radius from solar divergence alone, and several meters from chromatic smear. Peak flux: 450 MW / (π × 3²) ≈ 16 MW/m². That's ~16,000 suns — enough to vaporize steel, easily enough to superheat a molten salt loop.

Now the deal-breakers.

• Wind. A membrane holding 2 m of water in a curved shape flexes catastrophically at 10 m/s wind. You'd need the whole thing indoored under an ETFE cushion — doubling the losses.
• Sun tracking. Refractive lenses don't gimbal. You'd need the receiver tower on rails, tracing a 500 m arc across the ground as the sun moves. Or — better — accept a fixed north-tilt and only run at solar noon ±2 hours.
• Thermal blooming. The lens itself warms unevenly, changing n and defocusing the beam. Circulating the water helps but costs pumping energy — ~5 MW to turn over the pool hourly.
• Algae. A warm, sunlit, kilometer-wide pond becomes a swamp in weeks. Requires continuous UV sterilization and filtration, or an inert fluid overlay.

The verdict. A heliostat field of the same footprint delivers ~500 MW with 92% mirror efficiency, tracks trivially, and doesn't grow algae. The water lens is engineering poetry — but PS10 wins on every axis except aesthetics.