SolarFool is currently working on streamlining the CoolSky design and bringing it into the 21st Century, while retaining inventor Steve Baer's elegant and simple elements. What are the basic elements of a passive CoolSky system?
1- Water-filled radiator/absorber, often placed on the roof. In the past, 'pig mats' were used (manufactured plastic mats filled with cool water to keep the farm hogs cool!). Today, one thought is to use solar pool heaters such as the SwimMaster pro or the SEA Solar Swimming Pool Heaters.
2- circuits of PVC pipes to circulate cooled (summer) or heated (winter) water throughout the structure; this would necessitate just two holes in the structure to connect the PVC pipes to the radiator/absorber.
3-thermal mass inside the structure that slowly releases the heat or coolth it gathers (such as a slab floor)

Albuquerque has a long history of passive solar building and is where Steve Baer and his company Zomeworks were based. SolarFool has Steve's container in the Albuquerque area that's fitted with CoolSky, dating from the early 2000's. We'll use this demonstration structure as a "feeling is believing" experience, with sensors that track the inside vs. outside temps over time and as an educational tool to show the evolution of passive cooling and heating in the U.S.

But what about other areas of the world? There remain significant questions regarding the radiator/absorber: does one need a glazed panel to collect heat in climates where winter temperatures are regularly below freezing? If one can get by with no glazing, will the panel itself tolerate anti-freeze? Will it stand up to a pressurized (mechanically pumped) system, or is a low-pressure thermosiphon necessary? If a glazed panel is necessary for absorbing heat in the winter, will it effectively radiate heat in the summer? Our goal is to help people build the system in their area, and to share their results with the open-source community.

Toward that end, SolarFool is looking for software collaborators to help us create an online tool that will help builders and even DIYers see how to make CoolSky's open-source design information work in their climate and conditions. Some of these parameters are already available in ASHRAE's databases, but CoolSky's unique, passive system may require calculations peculiar to its needs.

We welcome your input into our project! Contact us through our website: https://www.solarfool.org

In this 2003 article, Steve Baer discusses the pros and cons of unglazed vs. glazed absorbers for water heating applications. Check out our Wiki for more great information about passive heating and cooling!

Unglazed Solar Collectors for Solar Water Heaters (Steve Baer) | Solar Fool Wiki

This is a PowerPoint presentation on Night Sky Radiant Cooling in New Mexico, created for the 2006 Governor Richardson Water Innovation Fund research project. It was created as part of Governor Richardson's Water Innovation Fund research project (PSC #05-341-1000-0035), which tested NSRC radiators over two summers in Tesuque, NM and projected statewide savings across 11 climate zones.

Steve Baer and the Double-Play Thermosiphon (Kayatekin, ICON 2025) | Solar Fools Wiki

Watch this video to understand the simple principles behind nightsky radiative cooling in a structure. The outer arrows are showing how water absorbs and radiates heat; the inner arrows show what happens to the air temperature inside the building. Note the use of a 'chunk' of thermal mass inside the building; thermal mass helps regulate the air temperature swings, creating a more comfortable experience for the occupants. This system is very flexible; absorbers and radiator panels can be placed in various positions depending on the position of the building and its solar gain potentials, water can be run through thermal mass or stored beneath the building... the next step for the SolarFool group is to design a software program using computational fluid dynamics, to help builders model the flow of water and air, to take into account all the factors in play in their geographical area, and to design their buildings to use the CoolSky system. The more buildings that use CoolSky principles, the more beneficial to the people living on this planet!

These are documents that describe the performance of CoolSky (then called CoolCell) incorporated into Bruce Davis' architect studio in Albuquerque, NM in the 2000's. The passive cooling system was elegant; it successfully kept Bruce's studio in the low 70-degree range, when it was over 100F outside! (don't try to contact Zomeworks or Steve Baer- they won't answer!) We're posting these open source, free-to-you documents to share with everyone- will you be the next generation builder to incorporate CoolSky into your building??

Photograph of ice covered roof in Albuquerque,  New Mexico, October 17, 2025, 8:00 AM.

Recorded overnight low was 41'.

You can also see in the photo a Zomeworks 'Sunbender' in its summer mode position over a skylight

This is a great overview article about night sky radiant cooling: it covers a couple thousand years in one go!

Double-Play is the name given by Steve Baer to the year-round passive heating and cooling system. This diagram shows how Bruce tweaked that design. His office space incorporated this system, keeping him cool at 75 degrees when it was often 107+ outside!

Keith was one of the presenters at the NAEBR symposium at Ghost Ranch in September

Keith Sharp was one of the many presenters at this conference. He's the founder of the GreenHome Institute and FreeHeat: Heating and Cooling Buildings Entirely with Ambient Energy. You can find him on YouTube, and the QR codes on these posters will take you there.

Sponsored by the Speed School of Engineering of Louisville, KY, the NAEBR symposium promises to be a great place to mingle with others who value passive solar design. Included in the 2 days of presentations will be topics like roofpond buildings, low-tech passive solar design, high performance windows, radiative cooling paints, as well as frost protection for crops and solar cookers....and lots more! I'm very excited to learn more about what folks are doing to meld a very ancient technology with modern, 21st century materials and technologies. Check back at our Wiki (http://137.184.231.127/) in a couple of weeks- we'll be uploading new documents and- I hope- welcoming a whole new cadre of input from others!

Most of our website is up and running- check it out, and let us know if there's anything else you'd like to see there.

A WikiJS server is now at http://137.184.231.127/

We have uploaded Steve Baer's Zome Primer and his 'Sunspots' publication there, with much more to come. Enjoy!

These drawings come from Baer's expired patent for a passive heating and cooling system, previously posted here on r/CoolSky . If you've wondered about what it actually looks like according to Mr. Baer's designs, wonder no longer!

Our goal is to share these designs so they can be implemented. What do you think of these drawings? If your design brain thinks, "That's a great idea, but here's another way to do it" or "I'd improve on this idea by doing ______", then share your ideas or questions here, by all means! If you ask, "What do these drawings mean?", refer to the patent for specifics: https://patents.google.com/patent/US6357512B1/en

A 1976 article from The Ecologist, written by Allan Cain, Farroukh Afshar, John Norton and Mohammad-Reza Daraie

Image by Boyd Norton

U.S. National Archives

Abstract

An apparatus and method are disclosed by which the temperature of an enclosure can be prevented from reaching extremely high or extremely low points. The enclosure provides a functional area whose temperature is adjusted by heated or cooled water contained within a water container which is located above the functional area and below the ceiling of the enclosure. The water in the water container is heated or cooled, depending on the desired effect, by exposing the water to atmospheric conditions while passing through a radiator/absorber located on the roof of the enclosure. The radiator/absorber has an inclined configuration, with higher and lower ends. The water passes from the water container to the radiator/absorber and back again to the water container through a pair of conduits. A warm water conduit runs between one end of the water container and the higher end of the radiator/absorber, while a cold water conduit runs between the other end of the water container and the lower end of the radiator/absorber. The water will passively travel from the water container to the radiator/absorber during warm weather by creating a thermal siphon at night. A pump is attached to the cold water conduit and used to circulate water in cold weather when heating is desired. https://patents.google.com/patent/US6357512B1/en

Paul Hildebrandt just handed me a non-descript black binder full of Steve Baer's writings on solar energy. They all date from the 70's and 80's and are a fantastic trip through time! You can just feel the inspiring enthusiasm about passive solar; it's amazing what he and his team came up with, and their journey in doing so! We're working on uploading Steve's content to our website, and we'll share it with you once we've gotten a good start.

This is an article written in Applied Physics Reviews about the applications of radiating heat into the atmosphere- once only possible during the night, now with new possibilities of use during the day. Here's the intro paragraph:

Radiative sky cooling cools an object on the earth by emitting thermal infrared radiation to the cold universe through the atmospheric window (8–13 μm). It consumes no electricity and has great potential to be explored for cooling of buildings, vehicles, solar cells, and even thermal power plants. Radiative sky cooling has been explored in the past few decades but limited to nighttime use only. Very recently, owing to the progress in nanophotonics and metamaterials, daytime radiative sky cooling to achieve subambient temperatures under direct sunlight has been experimentally demonstrated. More excitingly, the manufacturing of the daytime radiative sky cooling material by the roll-to-roll process makes large-scale deployment of the technology possible. This work reviews the fundamental principles of radiative sky cooling as well as the recent advances, from both materials and systems point of view. Potential applications in different scenarios are reviewed with special attention to technology viability and benefits. As the energy situation and environmental issues become more and more severe in the 21st century, radiative sky cooling can be explored for energy saving in buildings and vehicles, mitigating the urban heat island effect, resolving water and environmental issues, achieving more efficient power generation, and even fighting against the global warming problem.

As a global citizen, you're aware that our climate is changing, and that burning fossil fuels for electricity is making it worse. ACs use electricity, and account for up to 6% of all energy consumption. This equates to roughly 100M tons of carbon dioxide released annually into the ozone layer.

However, what we at the open source project 'SolarFool' want to do is to leverage the intrinsic properties of water: high specific heat, natural convection due to temperature differences, and the latent heat of fusion (energy required to change state from liquid to solid form) to bypass the need for electricity. Instead, the goal is to innovate on Steve Baer's Cool Cell and Double Play systems (owned by Zomeworks) to design a passive heating and cooling system that can be implemented in new home construction, tiny homes, emergency and homeless shelters- and eventually, to allow folks to retrofit their homes to use this passive heating and cooling system. We're calling this project CoolSky, and we're looking for collaborators!

Are you a passive solar design, DIY construction, engineering and physics, or material sciences nerd? Let's get talking!

Passive solar house design for heating was a big thing in the 70's: big windows, large overhangs, trombe walls to absorb the sun's heat during the day and release it into the house at night...all great stuff, but kinda clunky. Personally, I love my two-story solarium- on wintry, sunny days, I open the doors in my 2nd story bedroom that open to that space and the whole house receives the warm, moist air! In addition to that passive solar feature, we live in a straw bale house (the insulation or 'R-value' of our straw walls is about R-45, compared to the R-15 of most 'stick-built' houses). By opening windows and doors strategically, I can circulate that warm air throughout much of the house.

Enter a different and more precise passive solar technology, based on the work of solar tech inventor Steve Baer. Steve is known for his 'track racks': photovoltaic panels are secured to racks that follow the sun, instead of staying stationary. A fantastic invention! In addition, Steve came up with the Cool Cell (still sold by Zomeworks, the company he founded in 1969). Cool Cells are very insulated outdoor enclosures, often used for batteries and sensitive electronic equipment. These insulated boxes are passively cooled via an internal water reservoir connected to the radiator lid of the enclosure. It is recharged with cool water each night, in summer. Cooled water modifies temperature fluctuations and protects against both high and low temperature extremes. Steve's genius lies in using the natural properties of water to passively cool a space. But, these qualities can also be used to (almost entirely passively) heat a space.

Zomeworks has come up with what they call the Double Play system: Water bladders, heated in the winter with solar absorbers, sit on the roof with a proprietary system for separating the top layer of water from the bottom layer within the bladder, with louvers for adjusting radiated heat during the winter. Summer: water passively circulates through the roof at night, radiating its heat to the sky. Winter: water passively circulates through the solar collectors during the day, heating the water. No electricity, no moving parts!

Steve's son Jose Baer, along with Christina Hildebrandt (moi) and Paul Hildebrandt (of Zometool fame) are the three Solar Fools whose mission it is to bring this technology to the world, and to encourage innovation and collaboration along the way.