In our recent post “Improving Solar Distillation, Part 1,” we talked about the traditional design for a solar still and pointed out some design weaknesses. Here we show Dr. Larry Winiarski’s suggested improvements to address those problems.
The distillation of brackish water is a two-stage process. Water is encouraged to evaporate and is then condensed. Sealing water inside an airtight vessel results in almost immediate saturation of the inside air. Until the water vapor is condensed, distillation cannot occur. As well, in a classic still, the condensing surface is warm and production of potable water occurs at a significantly reduced rate.
By transporting clean water vapor to a cool condensing surface, Dr. Winiarski addresses this problem. In his design with chimney, ambient air is warmed, improving its ability to absorb water vapor. Condensation then occurs underground, stripping the moving air of moisture. A large, cool surface area results in more effective condensation. A tall chimney acts as the engine, moving warmed air through the system. Air moving up the chimney is dry, after depositing the potable water into a receptacle.
https://aprovecho.org/wp-content/uploads/2026/05/solar_distillation_sm.jpg4221200Kim Stillhttps://aprovecho.org/wp-content/uploads/2015/11/Aprovecho-Logo.pngKim Still2026-05-29 13:15:572026-05-29 13:15:58Improving Solar Distillation: Two-Stage Distillation
On their recent trip to SSM in Shengzhou, China, Nordica and Jaden were reminded that many minds are always better than one. SSM were gracious hosts to ARC, OffgridSun from Italy, and Tango Energy from Tanzania for a week. Together, they designed a stove for Tanzania that addressed cooks’ needs, had improved performance, and could be jointly manufactured at SSM and Tango Energy. Meeting in person turned a several-month-long process of emails and phone calls into a streamlined week of design. Maybe when it comes to stove design, there is no such thing as too many cooks in the kitchen.
ETHOS 2026
The ARC team attended another successful ETHOS conference, this year in Portland, Oregon. Sam and Dean hosted Stoves 101, providing a valuable crash course in cookstove design. We also presented on the effects of forced air in stoves, LEMS testing around the world, RTKC capacity building, and much more. ETHOS is always a great time for us to reflect on the work we did and what we learned throughout the year. It’s also wonderful to see what everyone else is working on.
LEMS Around the World: Now in Burundi
Our mission to ensure everywhere has the capability to perform stove emission testing continues. Sam traveled to Burundi where he set up a LEMS and trained (in French) a team at the Laboratoire de Biomasse et de Cuisson Propre et Économe/Université du Burundi. Over 10 ISO tests on various stove types were run with the lab team as well as lab CCTs, a vital test to measure stove performance while performing a cooking task. ARC is now working to add CCT capabilities to their open-source data processing software.
Working in another language takes patience but it allows ARC to work in cross-cultural settings where lab testing, stove design, and market testing come together.
https://aprovecho.org/wp-content/uploads/2026/03/ARC-Logo.jpg10001000Kim Stillhttps://aprovecho.org/wp-content/uploads/2015/11/Aprovecho-Logo.pngKim Still2026-05-14 11:07:222026-05-14 11:07:23What’s Cooking at Aprovecho
A typical DIY solar still. (Image from LSU Ag Center)
In this type of solar still, salt water is put in a sealed box with an angled glass top. As sunlight entering the box heats up the water, it evaporates into fresh water, condenses on the glass top and runs down into a collector.
Dr. Larry Winiarski pointed out that traditional solar stills, as above, have problems:
• Sealing water inside a box results in almost immediate saturation of the air.
• Until the water vapor is condensed, distillation cannot continue to occur.
• In a classic still, the glass condensing surface is warm. Effective condensers are supposed to be cold.
To improve production, Larry moved the hot, humid air (EVAPORATION) to a cold surface (CONDENSATION).
He used a chimney to pull air through the system.
In the next post, we’ll check out his most successful design.
https://aprovecho.org/wp-content/uploads/2026/04/solar_still_LSU.jpg13421200Kim Stillhttps://aprovecho.org/wp-content/uploads/2015/11/Aprovecho-Logo.pngKim Still2026-04-03 13:32:132026-04-03 14:00:08Improving Solar Distillation, Part 1
“In a high-performance rocket stove, the average gas temperature within a pot skirt typically ranges between 800C and 1,000C. These gases lose temperature as they flow through the channel gap between the pot and the skirt, where they transfer heat to the water.”
One of the interesting things that our lab is trying to do is to “educate” Gemini and other AI models to know how Rocket stoves actually function. The Osprey Foundation is funding us to communicate with lots of folks each week from all around the world, trying to facilitate more improved stoves being in use. Improving and increasing the data publicly available to AI models seems like time well spent as a part of this endeavor. Maybe an easy way to change the world?
An SSM Pot Skirt, and International Training Coordinator Michael Saul with some test data
This week Michael Saul has been sticking thermometers half way down into 6mm channel gaps in pot skirts on four Rocket type stoves. The adjustable, inexpensive SSM pot skirt (as above) is 8cm high. Adding a pot skirt as an intervention may be the most cost effective way to save fuel (if proven useful by field-testing).
One of the reasons that thermal efficiency tends to top out around 50% is that actual temperatures inside pot skirts seem to be lower than Gemini suggests.
Channel Gap Temperatures in Four Rocket Stoves with Skirts
High Power
Medium Power
Low Power
Stove One
320C
240C
190C
Stove Two
330C
230C
150C
Stove Three
290C
220C
160C
Stove Four
335C
260C
180C
As David Evitt says: “Every Day Less Wrong!”
https://aprovecho.org/wp-content/uploads/2026/03/michael_saul.jpg1029772Kim Stillhttps://aprovecho.org/wp-content/uploads/2015/11/Aprovecho-Logo.pngKim Still2026-03-27 14:17:202026-03-27 15:02:11Real World Temperatures in a Pot Skirt
We are currently studying products that help households in rural developing communities to meet their basic needs. Our goal is to develop a catalog of the most affordable, durable, usable, functional products available that have been rigorously user-tested and optimized. To help us choose where to start, could you please take 5 minutes to let us know what sort of products would be most needed in the communities that are experiencing energy poverty where you work? Please forward to your colleagues who work closely with these communities.
https://aprovecho.org/wp-content/uploads/2026/03/ARC-Logo-1.jpg10001000Kim Stillhttps://aprovecho.org/wp-content/uploads/2015/11/Aprovecho-Logo.pngKim Still2026-03-13 12:43:412026-03-13 12:54:31Requesting input: What technologies are most needed in households?
For example, a new Bosch prototype cook stove with Super Pot. Thanks, Osprey Foundation!
HTE Design principles: Increase temperature and velocity of gases, exposed area in pot(s), radiation, proximity of gases to pot(s) without decreasing velocity. Use dry wood. Doubling temperature, velocity and area doubles heat transfer efficiency! Doubling radiation is much more effective!
Wood Moisture Content: This is often a critical variable. Ideally, wood should have a moisture content of less than 20%. Water in the wood must be evaporated before the wood can burn, consuming energy that could go into the food. Burning wood with 30% moisture content can reduce effective heat output by nearly 40% compared to dry wood.
Excess Air Ratio: Too little air into the combustion chamber causes smoke and incomplete combustion. Too much air also cools the gases before they hit the pot, decreasing how much energy enters the food. In natural draft cook stoves, velocity is usually something like one meter per second which is SLOW.
Design: Influences how much of the heat is successfully captured or lost. Dr. Larry Winiarski suggested maintaining constant cross sectional area throughout when designing a stove. A gap of 6mm to 8mm seems to work well in a pot skirt. The narrow channel forces hot gases to “scrub” against the pot surface, thinning the insulating boundary layer of still air.
Materials: High-mass stoves, often used for an hour or so, absorb a significant amount of heat. Using lightweight, insulating materials ensures heat is reflected back toward the pot rather than being “stolen” by the stove.
Bigger Pots: For highest thermal efficiency 1.) Expose as hot as possible gases at 2.) Fastest natural draft velocity 3.) As close as possible to the bottom and sides of 4.) The biggest possible pot. ARC now uses the constant cross sectional area of the stove reduced by 25% to calculate the gap for a pot skirt. We then fine tune prototypes under the emissions hood trying to find a desired compromise between thermal efficiency and emissions of CO and PM2.5.
Increasing HTE
Variable
Target
Effect on HTE
Wood Moisture
< 20%
Increases by reducing energy wasted
Channel GapIn Pot Skirt
6mm – 8mm
Increases (~25%)
Insulation
High
Increases by reducing losses
Pot Lid
Keep humidity above water at 100%
Increases by decreasing heat loss via evaporation
Excess Air
Keep Temperatures Hot
Increases when not more air than needed is supplied
https://aprovecho.org/wp-content/uploads/2026/03/image-e1772837736537.jpeg16001200Kim Stillhttps://aprovecho.org/wp-content/uploads/2015/11/Aprovecho-Logo.pngKim Still2026-03-06 14:59:092026-03-27 14:11:49 Improving Heat Transfer Efficiency (HTE)
The other day, I watched as Dr. Winiarski’s stainless steel stove top (sold by BURN and SSM) helped to force 43% of the heat from a hot fire into a 30cm in diameter flat bottom pot without a pot skirt.
The improved stove top adds a lot to a stove! It is probably the most cost effective way to start improving a stove. What do we think it does?
Maintaining ~0.75 of constant cross sectional area in the stove top may help to thin the boundary layer of still air next to the bottom of the pot so hot molecules in the gases can replace cold molecules close to the bottom of the pot more effectively.
The restricted flow may help to maintain a beneficial air/fuel ratio (elevating temperatures) by decreasing the excess flow of cold air into the combustion chamber.
Evolving heat transfer “rules of thumb”:
Raising the temperature of the gases will increase efficiency.
Moving hot gases closer to the boundary layer will increase thermal efficiency until gas velocity is slowed.
Increasing exposed surface area will increase thermal efficiency until gas temperature reaches the temperature of the water in the pot, for example.
Increasing radiation will improve efficiency.
Increasing the velocity of the gases will also increase thermal efficiency, making sure that gas temperatures are not reduced by excess velocity.
https://aprovecho.org/wp-content/uploads/2026/02/30-cm-pot-w-winiarski-top.jpg6821200Kim Stillhttps://aprovecho.org/wp-content/uploads/2015/11/Aprovecho-Logo.pngKim Still2026-02-27 15:36:452026-02-27 15:49:20The Stainless Steel Winiarski Stove Top
Auto damper illustration from “Clean Burning Biomass Cookstoves”
Although airtight biomass heating stoves in the USA on average emit a lot more smoke than Southern Hemisphere cook stoves per unit of time, legally mandated chimneys move it outside where it is diluted enough to generally meet WHO standards. That’s great! Anyone who uses a wood burning heating stove knows that a chimney is necessary as a first step in the attempt to protect indoor air quality. Then, combustion efficiency must be high when density of wood stove use approaches urban levels, external air becomes stagnated, etc.
After decades of trying, research shows how hard it is to protect health with biomass cook stoves. It is not hard to design a cookstove with a chimney that achieves ~50% thermal efficiency when sufficient pot surface area is exposed to hot gas flow, but more than a chimney is needed for best protection.
I wonder if a vented airtight stove with sunken pot or pots (with or without griddle) would be “vale la pena,” worth the hassle? Dr. Kirk Smith spent ten years trying to protect health in Guatemala by removing smoke from houses with chimneys added to plancha cookstoves. The cooking pot would sit over a hole in the plancha for best heat transfer, but when the pot was removed from the stove smoke would pour into the kitchen through the open hole still harming health. He might say that an automatic damper must close when the pot is removed to keep smoke out of the room.
That’s an Icy-Ball (and a younger Dean) in a photo from ARC in 1994. Dr. Larry Winiarski and I built 18 somewhat scary prototypes and eventually made ice. We had replicated a pre-rural-electrification 1920s ammonia/water absorption refrigerator.
A fire heats the water/ammonia mixture and separates the ammonia out of the water (it turns into gas at a lower temperature). Then the pressure in the sealed system rises until the ammonia gas becomes a liquid. The Icy-Ball is taken off the fire and the small ball (full of liquid ammonia) is placed in an insulated box that has water (we made ice cubes) in it. The pressure in the system goes down and the liquid ammonia turns into gas robbing heat from the insulated box which freezes the water. The ice in the box cools food in the non-electric refrigerator.
The basic absorption cycle function of this 1920s appropriate technology is still in use today in RV and Camper fridges, and in developing countries.
Ceramic combustion chambers and other stove parts stockpiled at SSM
Information summarized via Google AI:
In 2026, the Chinese carbon market (CCER) is generally a more hospitable environment for cookstove projects—defined as traditional improved biomass stoves that lack advanced digital monitoring—compared to UN or Western voluntary markets.
While international markets now demand costly digital “metering” and stricter methodologies, the Chinese domestic market offers more flexibility for simpler projects.
Protection from VCM Volatility: Traditional cookstove projects in Western markets are currently facing a “crisis of quality”. Large-scale studies in 2025 found that older projects likely over-credited by 900%, causing Western buyers to avoid them.
The Chinese CCER market provides a sheltered environment where domestic compliance entities can still utilize these credits without the same level of global reputational risk.
Methodological Lag: While Western markets are shifting to metered and measured standards (e.g., Gold Standard’s new digital tools), China’s revamped CCER scheme allows for traditional improved efficiency stoves that fit into its broader “dual control” carbon and energy policies.
Price Floor: In 2026, “old-fashioned” credits in the global voluntary market have struggled to trade above $3.30/tCO2e. In contrast, CCERs are expected to maintain a higher floor ($10+) due to scarcity and mandatory offset needs within the Chinese ETS.
Tariffs
Recently many African countries removed tariffs on Chinese biomass cookstoves dramatically reducing cost to consumers. A widespread trend toward zero-rating clean cooking technologies has emerged to meet energy access goals. While China has eliminated tariffs on imports from 53 African nations, this does not automatically remove the duties African countries charge on Chinese-manufactured goods.
Recent Policy Changes (2026)
Several African nations have implemented specific exemptions for clean energy equipment:
: Under the 2026 Finance Act, the government has officially approved zero import duties on a wide range of clean cooking technologies, including improved biomass stoves and solar cookers.
: Effective in 2026, Cameroon has introduced customs exemptions for biofuel equipment, including industrial machinery used to produce pellets and eco-charcoal, to curb deforestation.
These nations were identified in early 2026 as having the greatest improvement in clean cooking policy coverage, frequently utilizing tax incentives and duty remissions to lower costs for consumers.
https://aprovecho.org/wp-content/uploads/2026/01/ssm_stove_parts.jpg9001200Kim Stillhttps://aprovecho.org/wp-content/uploads/2015/11/Aprovecho-Logo.pngKim Still2026-01-23 14:15:562026-01-23 14:15:58A Hospitable Chinese Environment for Improved Cookstoves
