A solar still purifies water using nothing but sunlight. It traps solar energy to evaporate contaminated or salty water, then collects the clean vapor as it condenses back into liquid, leaving behind salt, bacteria, heavy metals, and other contaminants. The concept is simple enough to build in a backyard or a survival situation, yet the same physics powers larger desalination systems in arid regions around the world.
The Basic Process: Evaporation and Condensation
A solar still works in four stages, all driven by the sun’s heat. First, sunlight passes through a transparent cover, usually glass, and strikes a dark-colored basin underneath. That basin absorbs the solar energy and transfers heat to the water sitting inside it. As the water warms, the space between the water surface and the glass cover acts like a miniature greenhouse, trapping heat and raising temperatures further.
Once the water is hot enough, it begins to evaporate. This is where the purification happens: when water molecules turn to vapor, they leave behind dissolved salts, heavy metals, bacteria, and other contaminants. The vapor rises and meets the cooler glass cover overhead. Because the glass is exposed to outside air, it stays cooler than the water below, causing the vapor to condense into droplets on the underside of the cover. Those droplets slide down the tilted glass surface, following gravity into a collection channel that drains into a separate container. What you collect is distilled water.
Of the three ways energy moves from the hot water to the cover (convection, radiation, and evaporation), evaporation transfers the most energy. That’s why the system works as a purifier: the dominant energy pathway is the one that separates clean water from everything dissolved in it.
How Much Water a Solar Still Produces
Output depends heavily on sunlight intensity, ambient temperature, and still design. A conventional basin-type solar still typically produces around 3.9 liters per square meter per day under decent sun. Enhanced designs that store heat more effectively can push that to about 5.4 liters per square meter per day. That’s enough for roughly one person’s drinking needs from a still about the size of a card table.
Production peaks in the early afternoon when solar radiation is strongest, and it drops to nearly zero at night unless the still has some form of thermal storage built in. On cloudy days, output can fall by half or more. In practical terms, a solar still is not a high-volume water source. It’s a reliable, low-cost one, best suited for situations where energy infrastructure is unavailable and water demand is modest.
Key Components and Materials
Every solar still has the same core parts: a basin, an absorber surface, a transparent cover, and a collection channel. The basin holds the contaminated water. The absorber, typically a dark-colored plate made of stainless steel or painted metal, sits at the bottom to soak up sunlight. A good absorber captures about 90% of the solar energy that reaches it. Basin water temperatures can reach 60 to 70°C (140 to 158°F) on a sunny day, depending on design.
The cover is almost always glass. It needs to be transparent enough to let sunlight through, cool enough on the outside to promote condensation, and tilted at the right angle so droplets slide down rather than dripping back into the basin. Research on optimal tilt angles suggests matching the cover angle roughly to your latitude. For a location around 31° north, for example, the ideal fixed angle is close to 22°. Adjusting the tilt monthly rather than keeping it fixed year-round can squeeze out about 2.5% more water over a year.
The collection trough runs along the lower edge of the glass, catching condensed water and channeling it to a storage container. It needs to be sealed well so distilled water doesn’t leak or get recontaminated.
Design Variations
The simplest and most common design is the single-basin still: a shallow tray of water under a single sloped glass panel. It’s cheap, easy to build, and requires no moving parts or electricity. But its efficiency is limited because a lot of absorbed heat gets lost through the basin walls and back into the ground.
Wick-type stills replace the open basin with a fabric or mesh material that draws water upward through capillary action, spreading it into a thin film. This thin layer heats up faster and evaporates more readily than a deep pool of water. Wick-type designs have shown about 30% higher productivity compared to conventional basin stills. Adding fins to the absorber surface, which increases the area available to absorb heat, has boosted output by as much as 45%.
Multi-effect stills stack multiple evaporation and condensation stages on top of each other. The heat released when vapor condenses in one stage warms the next stage above it, recycling energy that a single-basin still would waste. Pyramid-shaped stills use multiple angled glass panels to capture sunlight from different directions throughout the day, reducing the need to orient the still perfectly.
The Survival Pit Still
You may have seen the survival version: a hole dug in the ground with a plastic sheet stretched over it, a rock in the center to create a low point, and a container underneath to catch drips. Moisture from the soil and any plant material placed in the pit evaporates, condenses on the underside of the plastic, and trickles into the cup.
It works on the same physics as a glass-covered basin still, but the output is dramatically lower. In desert conditions, a pit still averages only about 240 milliliters (8 ounces) per day. A person at rest in a desert needs roughly 3.8 liters per day. Several days of collection from a pit still may not even replace the water you lost digging it. As an emergency technique it’s better than nothing, but it’s not a realistic sole water source.
Water Quality
Because distillation separates water from virtually everything dissolved or suspended in it, solar stills produce water with very low contamination. The process removes salt, making it effective for desalination of seawater or brackish groundwater. It also eliminates bacteria and other microorganisms, since they can’t travel in the vapor phase. Studies combining solar distillation with photocatalytic materials have achieved 99.99% microbial disinfection. Heavy metal removal rates of up to 98% and organic contaminant removal around 90% have been reported in enhanced systems.
The distilled water itself is essentially pure, which means it also lacks minerals your body normally gets from drinking water. For long-term use, some people add a small amount of mineral salts back into the collected water, though this is mainly a concern if a solar still is your only water source for extended periods.
Maintenance and Long-Term Challenges
Solar stills have no pumps, no filters, and no fuel costs, which makes them attractive for off-grid use. But they’re not maintenance-free. The most persistent issue is salt scaling. As water evaporates from the basin, dissolved minerals concentrate and eventually form a crusty layer on the absorber surface. This scale reduces heat absorption and lowers output over time. In high-salinity areas, regular scraping and cleaning of the basin is necessary.
The glass cover also needs periodic cleaning. Dust, mineral deposits from condensation, and algae growth (biofouling) can reduce the amount of sunlight reaching the basin. There are no widely standardized guidelines for how often to clean or replace components, which makes long-term cost estimates unreliable. UV exposure gradually degrades plastic covers, rubber seals, and some advanced materials like nanocoatings. Metal components can oxidize. For a homebuilt still, expect to inspect and clean it at least every few weeks, more often if you’re distilling heavily mineralized water.
What Affects Efficiency
Several factors determine how well a solar still performs. Solar radiation is the biggest variable: more sun means more evaporation. Wind speed matters too, since wind cools the glass cover and increases the temperature difference between the water and the cover, which actually helps condensation. Ambient temperature and humidity play a role as well. High humidity reduces the evaporation rate because the air inside the still is already moisture-laden.
Design choices that improve efficiency include reducing the gap between the water surface and the glass cover (which keeps the humid air closer to the condensing surface), using thermal storage materials in the basin to retain heat after sunset, and painting or coating the absorber surface matte black to maximize solar absorption. Even small modifications can make a measurable difference. A sliced absorber plate with 90 small fins increased basin water temperature by nearly 10°C compared to a flat plate in one study, simply by increasing the surface area exposed to water.