I am working with a small team on a university project to build a greenhouse heat sink. This will pass warm air at the top of the greenhouse down through a chamber underground filled with a material to absorb and store the warm air. We have two prototype greenhouses; one will act as a control for baseline measurements and the other will have the heat sink.
I have built several temperature sensors and loggers for the final prototype, but some preliminary tests are being made on various materials:
- Granite chips between 15-25mm, irregular shape
- Tempered glass fractured into small pieces 7-15mm approx., atleast 2 sides are flat
- Concrete fragments 30-80mm, irregular shape - test not completed
These were placed in a 5 L box. The box has a small fan and piping at the bottom to blow air into the chamber and release the air through a number of 6 mm holes in the pipe at the base of the box. The top of the box is sealed except for a vent which has the same diameter as the tube with the fan. A PT1000 temperature sensor is also inserted into the centre of each material to capture measurements every second. Here is an image of the test box:
The free air space was calculated on a smaller sample of both materials to give a rough figure of 42% for the granite and 43% for the glass. Two tests were then performed on the granite and then the glass:
- Both cooled outside for a several hours to about 5.5 °C, then brought into the room and left for 1 hour with the fan on. The temperature was recorded as the material warmed up to room temperature.
- After the first test the materials were then placed in a freezer and cooled down to -20 °C, the temperature was recorded again.
As can be seen below, the glass exhibits a lag in both data sets, warming up and cooling down, after which the temperature change becomes more linear. Whereas the granite shows a more linear change in temperature throughout.
Glass Warming (x-axis seconds, y-axis temperature)
Glass Cooling (x-axis seconds, y-axis temperature)
Granite Warming (x-axis seconds, y-axis temperature)
Granite Cooling (x-axis seconds, y-axis temperature)
We are discussing the results at the moment and I am interested in expert opinions of the data we collected. The data is interesting and we are interpreting it correctly. Specifically:
- The shape of the glass fragments allows for a more interlocking shape, which could restrict airflow more, but wouldn't this still have a more linear temperature change?
- Could the glass data be due to minor thermal expansion changes in the material?
- The glass has a lower thermal conductivity rating than the granite, is this the reason for the lag?