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.