Our project uses thermoelectric coolers to reduce the ambient temperatures of 30°C in hot, humid Singapore, to 24°C cool air which can then be blown at the person to cool a localised spot. Under this section, we discuss the principles and theory behind thermoelectric cooling and radiative cooling.

Thermoelectric Coolers (TECs)

Thermoelectric coolers (TECs) operate according to the Peltier effect.

The Peltier effect creates a temperature difference by transferring heat between two electrical junctions. A voltage applied across two conductors joined together creates an electrical current. When current flows through the junctions of the conductors, heat is removed at one side and heat is deposited at the other. This creates a cooling effect on one side and a heating effect on the opposite side. Attaching a heat sink to the hot side allows for rapid dissipation of heat, allowing the hot side of the TEC to remain close to ambient temperature. On the other hand, the cold side is able to reach temperatures below room temperature.

Within the TEC plate, there are two unique semiconductors, one n-type and one p-type. These are selected due to the difference in electron densities. The alternating n-type and p-type semiconductor pillars are placed in series to one another in an electrical series. They are then joined with a thermally conducting plate on either side. When a DC current is supplied, the side with the cooling plate absorbs heat, which is then transported by the semiconductor to one side of the device. The side with the heating plate rejects heat, which is then also transported by the semiconductor to the other side of the device, therefore creating a temperature difference.

Fig. 1: How a thermoelectric cooler works

Radiative Cooling

Radiative cooling is the process through which a body loses heat via radiation.

Using water to remove the rejected heat from the hot side of the Peltier plate, the heat from the hot water can then be removed from the set-up via radiative cooling. A fan is used to move air through the radiator, transferring heat away from the radiator and hence the water. Water in the hot reservoir also acts as a heat sink due to the high specific capacity of water. This cools the hot side of the Peltier, allowing temperatures of the hot side to be lowered. On the other hand, water can be cooled using the cold side of the Peltier and passed through a radiator. Moving air is then cooled as it passes through the radiator and is blown towards the user.

Using radiators with larger surface area implies that heat can be dissipated faster on the hot side, and air can be correspondingly cooled to a larger degree on the cold side. A larger volume of water is used for the hot reservoir while the amount of water for the cold side is minimised. This amplifies the Peltier effect, allowing hot air leaving the system to be close to ambient temperatures while cold air blown towards the user to be below room temperature.