Goal
Generate steam from water using ambient sunlight without optical concentration for applications such as desalination, water heating, wastewater treatment, and sterilization.
Problem
High cost and complexity of conventional solar steam systems that rely on mirrors or lenses for optical concentration.
Concept Summary
A low-tech floating structure consisting of a sponge-like porous carbon/graphite absorber coated with a spectrally-selective metallic film, a copper heat-spreading layer, and an insulating bubble-wrap covering. Sunlight is absorbed, heat is localized and trapped, and capillary action draws water to a heated channel where it vaporizes at 100 deg C.
Detailed Description
The device uses a porous carbon/graphite sponge that draws water upward by capillarity. A thin copper sheet coated with a spectrally-selective absorber (visible-absorbing, infrared-reflecting) captures solar radiation and conducts heat to a single evaporation channel. The whole assembly floats on a thermally insulating foam and is wrapped in clear bubble-wrap, which lets light in while reducing convective heat loss. Experiments showed the structure could heat water to its boiling point and convert ~20 % of incident sunlight into steam under ambient, even overcast, conditions. Earlier versions achieved 85 % conversion under 10-sun simulated illumination.
Principles
- Solar thermal absorption
- Thermal concentration and localization
- Capillary water transport
- Convective heat loss reduction
- Spectral selectivity (visible absorption, infrared reflection)
Scientific Domains
Materials
- Graphite
- Carbon foam
- Copper sheet
- Spectrally-selective metallic coating
- Polyethylene bubble-wrap
- Thermally insulating foam (e.g., polystyrene)
- Porous carbon sponge
Mechanisms of Action
- Solar photon absorption by spectrally-selective coating
- Heat spreading through copper layer
- Insulation via bubble-wrap and low-conductivity foam
- Capillary rise of water through porous sponge
- Phase change to steam at heated channel
Energy Sources
Applications
- Desalination
- Residential water heating
- Wastewater treatment
- Medical tool sterilization
Claimed Performance
Steam generation at 100 deg C under ambient sunlight; ~20 % solar-to-steam conversion efficiency; earlier prototype achieved 85 % conversion under simulated 10-sun illumination.
Experimental Evidence
The authors placed the solar sponge on a roof and on a floating platform, observing water heating to boiling temperature and steam production even on overcast days. Efficiency was measured as 20 % of incident sunlight converted to steam.
Limitations
- Short operational lifespan (1-2 years before replacement)
- Performance drops with low solar intensity
- Convective heat loss still present despite insulation
- Scalability of bubble-wrap insulation for large-area devices