Solar Still Geometry for Water Yield

ISEF Category: Environmental Engineering

Subcategory: Water Resources Management  ·  Difficulty: Intermediate  ·  Setup: Home Setup  ·  Time: 1 to 2 Months

The Hook

A small change in shape can decide how much clean water a solar still makes. That matters when you need drinking water and have only sunlight, heat, and a simple setup. Your job is to find which geometry pulls the most distilled water from the same sun.

What Is It?

A solar still is a simple device that turns dirty or salty water into cleaner water by using sunlight. The sun heats the water, the water evaporates, and the vapor condenses on a cooler surface. That condensed liquid, called distillate, can be collected as fresh water.

Think of it like a tiny weather system in a box. The water is the ocean, the air inside is the sky, and the cover is the cloud ceiling where droplets form. The shape of the still changes how much sunlight it captures, how fast water evaporates, and where the droplets run off. A pyramid, a tilted roof, and a spherical design each move heat and vapor in different ways.

Your project asks a practical question. Which shape gives the most potable water for the same sunlight? You can also track insolation, which means incoming solar power, with a photodiode sensor that acts like a simple pyranometer. That lets you compare water output to light input, not just to time.

Why This Is a Good Topic

This topic works well because you can test it with simple materials, but the question still feels real. You are comparing shapes, measuring output, and connecting design choices to a human need, safe water. The data are easy to collect, yet the analysis can get deep if you compare efficiency, yield per unit light, and changes across weather conditions.

Research Questions

• How does still geometry affect distillate yield per unit area under the same outdoor sunlight conditions?
• What is the effect of pyramid, tilted, and spherical cover shapes on the rate of water collection over a full day?
• Does a higher peak condensation surface increase total distillate yield compared with a lower or flatter cover?
• To what extent does the angle of the cover change the amount of water that runs into the collection channel instead of dripping back into the basin?
• Which geometry produces the highest yield normalized by measured insolation from a photodiode sensor?
• How does the inside air temperature profile differ among the three still shapes during peak sunlight?

Basic Materials

• Clear plastic sheet or acrylic sheet for cover material.
• Shallow black basin or food-safe tray.
• Small waterproof collection cup or graduated beaker.
• Distilled water and salt for making a test feedwater.
• Weatherproof tape or silicone sealant.
• Black paint or black liner for the basin.
• Ruler or measuring tape.
• Digital kitchen scale with 1 g resolution.
• Thermometer or temperature probe.
• Photodiode sensor or light sensor module.
• Microcontroller or data logger for light readings.
• Notebook or spreadsheet for recording yield and light data.

Advanced Materials

• Glass sheets or acrylic panels cut for multiple still geometries.
• Insulated basin with controlled heating option.
• Reference pyranometer or calibrated irradiance sensor.
• Calibrated photodiode and resistor circuit.
• Data logger with temperature, humidity, and light channels.
• Thermal camera or infrared thermometer.
• Analytical balance for mass change measurements.
• Salinity meter or conductivity meter for feedwater characterization.
• Sealed collection tubing and condensate traps.
• Replicate still chambers for side-by-side comparisons.

Software & Tools

• Google Sheets: Organizes yield, light, and temperature data, then helps you graph performance by still shape.
• ImageJ: Measures surface areas and condensation patterns from photos of each still design.
• Logger Pro: Records sensor data if your school already has compatible hardware.
• Python: Lets you calculate efficiency, normalize yield by insolation, and make cleaner plots.
• GeoGebra: Helps you sketch and compare still geometry before building prototypes.

Experiment Steps

1. Choose one output metric, such as distillate yield per square meter or yield per unit insolation, and keep it the same for every design.
2. Define the three geometries you will compare, then make sure each one starts with the same basin area and feedwater amount.
3. Plan a light-measurement setup that records solar input beside each still, so you can compare designs under the same sky conditions.
4. Build controls that separate geometry effects from leaks, shading, and differences in cover material.
5. Decide how you will collect mass or volume data at each observation point and how you will turn those readings into daily yield.
6. Pre-plan your statistics, including replicate trials and a test that compares the shapes across multiple days.

Common Pitfalls

• Letting one still sit in partial shade, which makes geometry look better or worse for the wrong reason.
• Comparing designs with different basin areas, which changes yield just because one model holds more water.
• Using a loose seal, which lets vapor escape and hides the real effect of cover shape.
• Recording light only once, which misses cloud changes and makes the insolation data misleading.
• Mixing up condensed water with spilled feedwater, which inflates the distillate yield.

What Makes This Competitive

A strong version of this project does more than name a winner. It shows why one geometry wins by tying yield to light input, condensation path, and heat loss. You can push it further by normalizing output, testing multiple days, and comparing performance under different sun angles or weather conditions. If you add careful controls and stronger statistics, the project starts to look like real design research.

Project Variations

• Compare clear, dark, and reflective basin liners to see how absorber color changes yield in the same still geometry.
• Test the same three shapes with saltwater, muddy water, and dyed water to see how feedwater quality changes distillation efficiency.
• Add a cooling fin or wet outer surface to one design and compare its condensate rate with an uncooled control.

Learn More

• NOAA Solar Radiation resources: Search NOAA for pages on solar radiation, irradiance, and weather station data to understand outdoor sunlight measurements.
• NASA Earth Observatory: Search for articles on the water cycle, evaporation, and solar energy to build the background section.
• USGS Water Science School: Search for explanations of desalination, evaporation, and clean water basics.
• NIH PubMed: Search for review articles on solar still performance, desalination, and passive water treatment.
• MIT OpenCourseWare: Search for heat transfer and thermodynamics lecture notes that explain evaporation and condensation.