Make an Automatic Evaporative Cooling System

This project follows the Engineering Design Process. Confirm with your teacher if this is acceptable for your project, and review the steps before you begin.

This procedure is divided into four sections:

1. Assemble your circuit: you will build your circuit on a breadboard.
2. Test your circuit: you will upload code to your Arduino and make sure that your circuit works properly.
3. Build an enclosure: you will cut holes in a cardboard box to serve as a model building for your evaporative cooling system.
4. Test your evaporative cooling system: you will test your system and try to find ways to improve its performance. 

Assemble Your Circuit

Assemble your circuit on a breadboard, as shown in Figures 3 and 4. Orient the breadboard so the writing is upright when facing you. You can also access a Tinkercad Circuits simulation here. Note: depending on where you bought your breadboard, the left-right orientation of the positive (+) and negative (-) buses on the breadboard may be reversed from what is shown in the figures. 

1. Put the MOSFET (labeled NMOS in Figure 3) in the breadboard with its three pins in separate rows. The large metal tab on the back of the MOSFET should face to your left, and the writing on the front should face to your right. 
   1. Connect the MOSFET's gate pin (the leftmost pin when the MOSFET is facing you) to Arduino pin 10.
   2. Connect the MOSFET's source pin (the rightmost pin when the MOSFET is facing you) to the breadboard ground bus.
   3. Connect the breadboard ground bus to one of the Arduino's GND pins. 
2. Connect your cooling fan (represented by a DC motor in the diagrams).
   1. Connect the negative (black) wire to the MOSFET's drain pin (the middle pin).
   2. Connect the positive (red) wire to the breadboard's power bus.
   3. Connect the breadboard's power bus to the Arduino's 5V pin.
3. Connect the TMP36 temperature sensor (labeled TMP in Figure 3). Put the sensor in the breadboard with each pin in its own row. The rounded side of the sensor should face to your left, and the flat side with writing on it should face to your right. Connect the sensor's pins directly to the corresponding Arduino pins with jumper wires - do not use intermediate breadboard connections.
   1. Connect pin 1 (the leftmost pin when the writing on the sensor is facing you) directly to the Arduino's 3.3V pin.
   2. Connect pin 2 (the middle pin) directly to Arduino in A0.
   3. Connect pin 3 (the rightmost pin) directly to a separate Arduino GND pin, not the breadboard ground bus. 

Image Credit: Ben Finio / Science Buddies

Figure 3. Breadboard diagram for model evaporative cooling system. The DC motor represents a cooling fan.

Image Credit: Ben Finio / Science Buddies

Figure 4. Circuit schematic.

Test Your Circuit

1. Download evaporative_cooling.ino. Read through the commented code so you understand how it works. Note: if you are using an Arduino UNO R3 or equivalent, you will need to change lines 25 and 29. See comments in the code. 
2. Upload the code to your Arduino.
3. Open the serial monitor in the Arduino IDE (Tools → Serial monitor). You should see the analog-to-digital (ADC) converter reading, the equivalent voltage in both volts and millivolts, and the temperature in both °C and °F.
4. It is normal for the readings to fluctuate slightly. If the readings do not change at all or something seems wrong (for example, the voltage is always zero), double check that your temperature sensor is wired correctly.  
5. Gently pinch your fingers around the black plastic part of the temperature sensor. Watch the serial monitor to make sure the temperature goes up.
6. By default, the fan should automatically turn on when the temperature exceeds 80°F. It should turn off when you release the sensor and the temperature drops back below 80°F. 
   1. If your sensor does not get hot enough to exceed 80°F, try lowering the thresh value (the setpoint temperature at which the fan should turn on) and re-uploading your code.
   2. If your fan still does not turn on, double-check that the MOSFET is wired properly.

Build an Enclosure

As shown in Figures 5-8, cut holes in a cardboard box to serve as an enclosure for your experiment. The box acts as a model for the building or room you want to cool. Make sure your Arduino, breadboard, and small tray that will hold water all fit under the box before you continue. You will need to cut four holes:

1. An observation window so you can see inside the box (Figure 5). Cover this window with clear tape or plastic wrap so air cannot get through.
2. One for the cooling fan (Figure 6). Trace the outline of the fan on the side of the box and then carefully cut it out with a knife. The fit should be snug so you can press the fan into the hole without it falling out. Use tape to hold the fan in place if necessary. Make sure the fan is pointed such that it will blow air into the box, not suck air out.
3. An exit hole for air on the opposite side of the box from the fan (Figure 7).
4. A small hole for the Arduino's USB cable to pass through (Figure 8).
5. Mount a heat lamp directly above the box (Figure 9).

Image Credit: Ben Finio / Science Buddies

Figure 5. Front view of box with observation window. 

Image Credit: Ben Finio / Science Buddies

Figure 6. Side of box with hole for mounting cooling fan.

Image Credit: Ben Finio / Science Buddies

Figure 7. Opposite side of box with vent hole for air to escape. 

Image Credit: Ben Finio / Science Buddies

Figure 8. Box tilted up to show the Arduino, breadboard, and small tray that fit inside. 

Image Credit: Ben Finio / Science Buddies

Figure 9. Heat lamp mounted above box.

Test Your Evaporative Cooling System

Once you have everything set up, you can test your model evaporative cooling system and try to optimize the design to cool the interior of the box as much as possible. A data table like Table 1 may be useful for recording your data (you can add more rows as needed).

1. Set up your experiment in an area where the ambient temperature will stay as constant as possible (for example, do not put it in front of a window where it may be exposed to direct sunlight, or near a heating or cooling vent). Set up a separate external thermometer to measure ambient temperature in the room. The thermometer should not be under the heat lamp. Monitor the ambient temperature periodically throughout your experiment and record any changes.
2. Unplug one of your cooling fan's power wires so it will not run for now.
3. Place the cardboard box over your Arduino. Watch the serial monitor for a few minutes to make sure the temperature is stable. Write down this steady-state temperature. It should be close to the ambient temperature in the room.
4. Turn the heat lamp on. Watch the temperature change in the serial monitor. Wait for the temperature to stop changing (remember that small fluctuations are normal), then write down the new steady-state temperature with the lamp on.
5. Based on your measurements in steps 3 and 4, you may need to adjust the thresh variable in the code. Halfway between ambient temperature and the temperature with the heat lamp on is a good place to start.
6. Test your system without any water. Reconnect the power wire for your cooling fan. Watch the temperature in the serial monitor. 
   1. Does the fan turn on automatically when the temperature exceeds your threshold value?
   2. Does the temperature start dropping when the fan runs?
   3. Does the temperature get low enough for the fan to turn off? If not, what is the new steady-state temperature?
7. Add a tray of water in front of the fan and repeat step 6. If the temperature gets low enough for the fan to turn off automatically, can you decrease the thresh variable and try again?
8. Fold a piece of paper into a zig-zag shape (Figure 8) and place it into the tray of water. The paper will soak up the water, increasing the surface area available for evaporation. Repeat step 6. Does the box get colder? How low can you push the thresh variable before the fan will no longer turn off?
9. Now, try to optimize your design. Can you experiment with different paper "fin" designs? How do the shape and surface area of the paper affect cooling? What about airflow patterns through the box? Can you change the size or location of the air exit holes? Does this have any impact? There are many more things you can try - see the Variations section for more ideas.