AC Refrigeration is the most common way for cooling in a fridge or a room. But this is a heavy-duty process, which requires a significant amount of electrical power and bulky electrical hardware to perform this task.
Fortunately, it’s possible to make a fridge that keeps food cool using a portable DC battery and something called the Peltier effect.
What Is the Peltier Effect?
Thermoelectric coolers or Peltier effect devices are comparatively miniature devices used for cooling or heating. Peltier devices use DC (direct current) for cooling, in contrast to conventional refrigerators or air conditioners which use AC current. The Peltier effect is a process in which, if DC current is passed through a circuit having two conductors of different materials, heat from one junction is removed, causing it to become cold, and the other junction absorbs heat, causing it to become hot.
It can also be observed that if the polarity of the current is reversed, the junction that was previously cold becomes hot; while the junction that was hot before reversing the current polarity, becomes cold.
Using Thermoelectric Peltier Devices for Temperature Control
The above phenomenon creates an opportunity to design a device that can extract and remove heat. If you place the colder junction on the hot surface or device from which heat is required to be extracted, and the hotter junction goes on a heat sink, you can keep the surface or device cool. Not only that, but you can maintain an ongoing optimum temperature for good performance.
How to Build a Mini DC Refrigerator
We will use the concept mentioned above to design a DC refrigerator. A mini refrigerator is the target design, however, it can be scaled to a medium-sized refrigerator using multiple Peltier effect devices using the same technique.
What You Will Need
The equipment needed is as follows:-
A Peltier Module (TEC1-12706) with wires attached. Two Heat Sinks. A 12 V DC battery or DC power supply. A Box (Internal should be thermally nonconductive material like wood, cardboard, or styrofoam) Two Fans Delta electronics FFB0412HN on heat sink fins for forced convection Thermal Paste Electrical Wires
The thermoelectric cooling Peltier device we are using here is a DC operating device with an optimum operating voltage of 12VDC which takes almost 6A current (72W). The operating voltage should not exceed 15 VDC. Don’t power up the Peltier device without a heat sink. If it is used even momentarily, it can exceed the maximum operating temperature of the device junction and result in performance degradation or complete failure of the device.
Step 1: Prepare the Box
Now take an empty box of thermally insulating material. For this demonstration, we used cardboard. Make a hole or cavity according to the dimensions of the Peltier device TEC1-12706 (i.e. 40mm x 40mm) for mini-refrigerator design. Place the Peltier device right on the cavity you created in the box
Step 2: Place Heat Sinks on Peltier Device
Now place heat sinks on both the junctions of the Peltier device. Each junction of the Peltier device is attached to the heat sink using highly conductive thermal paste. Thermal paste ensures a smooth heat flow between each Peltier junction and heat sink base plate, while not allowing the hot junction to get too hot and the cold junction to get too cold to damage either junction.
Heat sink increases the efficiency of the Peltier device by regularly maintaining airflow. This is because heat exchange between ambient air and the Peltier device is increased due to the large surface area offered by the heat sink for heat extraction from the inner junction and heat removal through the outer junction. For a good design, heat sinks with larger fins and greater surface area are recommended.
Step 3: Attach Fans
Now attach your choice of fan on each heat sink fin. Forced convection provided by the fan increases the airflow speed through heat sink fins, which allows for more heat extraction is possible.
The fans used in this project are 12VDC brushless (Delta electronics FFB0412HN) with airflow of 10.6 cfm. Each fan consumes less than one watt of power from the battery. However, one can use a fan with more airflow for better performance.
Take care not to connect the device junctions through a metal interface like a screw mounted on heat sinks, to avoid short-circuiting thermal junctions. This can result in significant degradation in the performance of the Peltier device refrigerator, as heat flow may get the shortest path through the metal screw, etc.
In case screws can’t be mounted due to thermal short circuit between two heat sinks, you can use glue to attach heat sinks to the box. However, it is mandatory to use heat sink paste between a heat sink and the Peltier device.
Step 4: Power Up Using 12 VDC
Now connect the junctions of the Peltier device and wires of fans with a 12 VDC battery. The battery must have the current capacity to provide a 7A DC current. A battery of 10Ah or more is sufficient for this single Peltier module with fans. Set the voltage polarity of device junctions such that the inner side of the box gets colder and the outer side gets hot, taking the heat out of the box for cooling.
Now join the wires with the battery to allow the device to take current. Observe closely the current drawn from the battery. It should be within the limits specified by the Peltier device manufacturer. Allow the system to take current for some time and observe the voltages and current for safe limits.
After a few moments, you will feel the colder air inside the box. You can add water bottles and juices to keep them cool. Now cover the box for use as a mini-fridge. For reliable operation, you can add a temperature controller to protect the Peltier device from exceeding operational limits.
You’ve Made Your Own DIY Mini Refrigerator
Now you can build yourself a mini Refrigerator using the Peltier devices and a few other simple components. Peltier effect modules can also be used for temperature control for applications like cooling in electronic devices, circuits, and systems where a low budget and lightweight cooling system is required.
