passive

Temperature Swings

Now that the Concrete Chimney Experiment is built, let’s take a look at what should be going on inside! To understand if the Optimal Tuning Strategy is cooling and ventilating the space within the chimney, we compare four temperature signals. Quick reminder, the Optimal Tuning Strategy refers to the set of mathematical scaling rules that proportion thermal mass and buoyancy ventilation to act together in a natural feedback loop. The Thermal Mass and Buoyancy Ventilation Research Project team prefer nicknames, typing their project name is enough work.

Let’s take a look at these four temperature signals which identify how effectively the Optimal Tuning Strategy is operating with the Concrete Chimney Experiment. The temperature signals are; exterior air temperature, interior air temperature, thermal mass surface temperature, and thermal mass interior temperature. These temperatures are taken within the chimney using GreenTeg sensors. The exterior air temperature is the temperature of the air outside, like the temperature you read on a forecast. The interior air temperature is the temperature of the air within the chimney, like the temperature you read on your thermostat in your home. The thermal mass surface temperature measures the temperature of the surface of the concrete panel. This surface interacts with the interior air. The thermal mass interior temperature is the temperature inside the mass. We can use a melting ice cube to understand the difference in the thermal mass temperatures. When an ice cube melts, the surface melts first while the center of the cube remains frozen. So, the surface and interior temperatures of the thermal mass can differ just as the outdoor and indoor temperatures can.

Theses four temperature signals describe if the thermal mass is absorbing and offloading heat from the air which should, in turn, drive conveyance ventilation cycles. The times of day the mass is absorbing and offloading heat should be relatively consistent day-to-day due to the diurnal cycle. The diurnal cycle is the variation between a high temperature and a low temperature that occurs during the same day. In other words, for most days the temperature rises until a peak typically in the afternoon and then falls again until reaching a low before the sun begins to rise again.

Each day the cycle repeats. Though the time of day of the high and low can vary. Here, you can see the diurnal cycle for a typical summer day in Hale County. We can then normalize that temperature swing into a Sin Wave for mathematical analysis. This is the exterior air temperature.

To see how all these temperatures should compare to each other throughout the day we can look at this graph. Notice the axis of temperature and time are simplified radially, but we are still looking at a full day with a typical temperature swing. This graph represents an Optimally Tuned space where the proportions of thermal mass and buoyancy ventilation are ideally balanced. The solid black line represents the exterior air temperature. The dotted gray line represents the interior air temperature. The solid orange line represents the thermal mass interior temperature. The dotted orange line represents the thermal mass surface temperature. As you can see, the interior air temperature is never hotter or cooler than the exterior air temperature. It is dampened by the thermal mass absorbing and offloading heat from the interior space. The thermal mass surface and interior temperatures show the mass warming by the absorption of heat from the air and cooling when the heat releases back into space. When the thermal mass and buoyancy ventilation proportions are not balanced the graph looks drastically different.

On the left, you see what it would be like if there were a lot more ventilation and a lot less thermal mass. Too much ventilation causes the interior environment to act just like the exterior environment and there is not enough thermal mass to affect the space. This would be like being in a tent. On the right, you see what it would be like if there were a lot less ventilation and a lot more thermal mass. Too little ventilation does not bring in enough heat for the thermal mass to absorb. The thermal mass is also so large it takes too much heat to fill up, which means it takes longer for the mass to start offloading it into the space. This is like being in a cave.

temperature signal graph
Temperature signal data graphed to compare to ideal Optimal

Finally, here is some of the data we have pulled from our sensors so far! Although the Concrete Chimney Experiment is definitely damping the temperature within the space the thermal mass temperatures are essentially the same. This means we may not have enough ventilation, not enough heat being brought in with be absorbed and offloaded. We are working on getting airflow sensors to see if this could be the case. The team is also recalculating the size needed for the ventilation openings.

If you stuck around until the end of this one, big thanks! Here’s a picture of Cory’s kittens Rosemary and Dijon to ease your mind. As always, we will be back soon with more rural science so stay tuned…. optimally tuned.

Chimney Cricket!

The desktop experiment mock-up, “The Chimney,” is complete and already bringing in data! Here is a quick look into the making of the Thermal Mass and Buoyancy Ventilation Research Project Team’s first dive into building a scientific instrument.

Before we get into the construction of a scientific experiment in the non-scientific environment shown above, let’s go back to the Fabrication Pavilion where all the prep work was done. The team used the twelve (recently built) 1′ x 1′ concrete panels to create the four walls of the chimney. For each wall, three concrete panels were screwed to a base of foam atop OSB through the 1/2″ pex pipe that was cast into the panels.

After the four walls were completed, the team tested how they fit together. The OSB and foam base extends past the concrete panels in order for the walls to fit into one another. This also allows for continuous insulation of the concrete chimney within. As seen in the last TMBV Research Project team post, insulation is key. Therefore, the chimney sits atop 1′ of geofoam and has another 1′ geofoam hat. The ventilation PVC pipes run through this geofoam on the top and bottom and connect to the chimney’s interior chamber. This is why the chimney is lifted off the ground by the wooden base, to let air in and out the bottom ventilation pipe.

Next up we have the sensors. The sensors must make it through a foot of insulation in order to take the temperature of the chimney interior chamber air, the surface of the concrete panel, and the backside of the concrete panel. There are also sensors outside of the chimney to measure the exterior air temperature.

The interior air temperature tells the team how the thermal mass and buoyancy ventilation proportions are effecting the interior space while the panel surface and backside temperatures tell the team how efficiently the thermal mass is working. The sensor wires are encased in gasket that runs through holes in the foam, OSB wall to the outside so sensors can be charged without disassembling the whole chimney.

The sensors the Thermal Mass and Buoyancy Ventilation Research experiment used are called Green TEG sensors. They transmit data using cell service so all your data can be downloaded from online or watch your data in real-time. This is a blessing and a curse as this makes Green TEG data very convenient, however, while the Morrisette campus has great Wifi, the town of Newbern does not have great cell service. Therefore, the experiment was transported to an undisclosed carport in Greensboro, just 15 miles down AL Highway 61 where it was assembled the rest of the way.

Next up, the Thermal Mass and Buoyancy Ventilation Research Project Team will calibrate, or modify, the experiment once they see how it is performing. After that, the team can start with a wooden chimney. Thanks for tuning in!

Insulation and Other Sensations

Oh hi, didn’t see you there behind my giant block of Geofoam insulation! Let me explain. Recently, Thermal Mass and Buoyancy Ventilation Research Project Team has been designing their first experiment, the desktop scale experiment known as “the chimney,” and building a mock-up of it.

The team used the data obtained from the thermal conductivity testing in Auburn University’s material testing lab along with their test concrete panel making experience to choose which concrete mix to use. They are going with Quikcrete Pro-Finish 5000, a high strength, smooth finish mix. Next, the team poured nine new concrete panels at the adjusted thickness. The thickness of the panels increased slightly due to inputting the exact thermal properties of the concrete mix into the code of the optimal tuning application.

The desktop experiment takes the form of a 3″ x 1″ x 1″ chimney with the thermal mass panels facing the interior. The desktop experiment needs to operate in nearly ideal conditions which means eliminating as many variables as possible. It is important to remember this is a scientific experiment of an unproven theory of how an internal thermal mass can be sized for a space to control temperature and promote proper ventilation. Therefore, to eliminate the variable of heat loss or gain from the exterior to the interior, and to understand how the thermal mass panels themselves are working, the chimney needs to be highly insulated.

When you need R50 insulation, even for such a small structure, it can get expensive and big. Their creative solution to getting the proper insulative value without spending hundreds of dollars per test was combining Geofoam and Rockwool! EPS Geofoam is much like rigid insulation but is typically used for earthwork such as building up underneath highway on-ramps. It is very dense giving it more insulative value per inch. Rockwool is a rock-based mineral fiber insulation. Thankfully, Rural Studio had extra R30 from a previous donation. The Geofoam was also donated, the Breathing Wall Mass Timber team got in touch with a construction operation that had extra and transported it to Newbern. In the drawing above you can see the concrete panels screwed onto a piece of 1/2″ OSB and 2″ Geofoam which is then surrounded by 9″ of Rockwool then encased by another layer of 2″ Geofoam. This combination of materials results in R50 insulative value.

The Geofoam comes in giant 8″ x 4″ x 3″ blocks because they are typically stacked underground. So another creative solution was needed, how to cut it down to the size we need. The TMBV team did not have to think too hard on that one because their big sister research team, the Breathing Wall Mass Timber squad, had already built a hot wire cutting system for their own Geofoam needs. A copper wire was spanned at the desired height above a table and heated using cables and an external power source.

Next, the Geofoam block was slid across the table and cut through by the hot wire. Once the Geofoam is at a more manageable size it can be cut using a hack saw. Shout out to the best big sister research team ever, Fergie, Jake, Preston, and Anna, the TMBV team appreciates you!

Whew, that was a lot of insulation talk! To ease everyone’s mind here is a beautiful Newbern sunset. See you next week!

In the field trip

The Thermal Mass and Buoyancy Ventilation Research Project Team got out of Newbern last week and into the field, sawmill, and lab!

The first field trip of last week was to Charlie’s sawmill. Charlie is a retired engineer, woodworker, and long time friend of Rural Studio, having helped with the Greensboro Animal Shelter. The team met Charlie at the Animal Shelter during neckdown week, where he was leading the project to revamp the kennels.

Charlie has a “hobby mill” he has been building up over the past years. He works mainly with salvaged wood and timbers making furniture and folk art. After the team got a tour of Charlie’s sawmill, he treated them to lunch and a brief presentation on wood. Even more than lunch, Charlie has offered the team use of his sawmill. Charlie has a passion for helping others and great deal of building knowledge, the team feels very lucky to have met him! Thank you Charlie!

Next, the TMBVRP team met up with Professor David Kennedy in the material testing lab at Auburn University’s College of Mechanical Engineering to test the thermal properties of their concrete samples. These samples were made using three different concrete mixtures, high finish, fiber-reinforced and 100% Portland cement. The objective was to find the exact heat capacity, thermal conductivity, and effusivity of each mixture. Knowing the specific thermal properties will help eliminate variables in the math when evaluating how the Optimal Tuning Theory is working.

David gave the students a crash course in scientific testing procedure. When conducting such tests, everything needs to be documented. The samples were marked, 10 of each mixture, measured for thickness and diameter, and weighed. The specific volume and density were then calculated for each sample before testing. The sample was again weighed after the test had run. Everything needs to be documented!

Next, the team will analyze the data and recode the Thermal Mass and Buoyancy Ventilation proportioning application with the specific thermal conductivity results. We’ll talk to you soon!

Panel Making

This week the Thermal Mass and Buoyancy Ventilation Research Team got to use the largest skill saw they’ve ever seen and we’ll tell you why!

In the technical workshop Sal last week, the team decided to narrow the number of materials they will test throughout the experimental cycle from four to two. The lucky two will be concrete and softwood! Concrete is often used as a thermal mass material while softwood is not which will make comparing the data collected from the separate experiments all the more interesting. The Optimal Tuning Theory calls for the thermal mass to be externally insulated which allows the thermal mass material to be much thinner than a typical thermal mass. Therefore, the concrete and wood need to be panelized.

The thermal properties of wood act most efficiently as a thermal mass when the cross grain is exposed to the air. This means that panelizing the softwood is more like creating giant cutting boards. To practice this process the team used 8″ x 8″ Cypress timbers and their matching 16″ diameter skill saw leftover from the Newbern Town Hall project. The team learned that 6″ x 6″ timbers would be ideal for their project, that way they can cut the cross-grain pieces in one cut with their 16″ skill saw without having to rip down the timber.

The concrete panels are far more straightforward, build a mold, pour the concrete, let it cure. However, the team has to think about how the panels would be attached to a larger structure. To solve this they cast PVC into the panel which will allow it to be screwed into a structure.

Voila! We have much refining to do of the panel making process, but the first two turned out well. We also have here a rendering of the habitable structural with the separate concrete and wood panel rooms. Our next step is to apply what we learned working with these materials to designing and building our first experiment. Thermal Mass and Buoyancy Ventilation Research Team out.