THERMAL ACTUATION

By Odera Umeugoji

Within technology, actuation can be defined as the action of causing a machine or device to operate. Therefore, thermal actuation refers to the operation of a device due to an alteration in the temperature. This temperature change can result from the environment, although it is more often attributed to joule heating (the passage of electric current through a conductor induces a temperature increase). Specifically, thermal actuators convert thermal energy to mechanical energy – they cause [linear] motion.

how does it work?

Thermal actuation is used widely in everyday life. One of its usual applications is in wax motors.

The crux of thermal actuation is thermal expansion. In this phenomenon, the material is subjected to heat energy, which contributes to increasing the overall kinetic energy of individual atoms in the substance, causing more movement and vibrations amongst the them. This pushes all the atoms further away from each other (more distance between neighbouring atoms), so the body of material enlarges overall.

In the context of wax motors, wax is the material which expands. It is thermally sensitive, meaning it undergoes a physical or chemical change when it is exposed to heat (its volume can increase by 5–20% when heated). In thermal actuators, the expansive material used should be thermally sensitive for optimum function – a greater expansion will result in greater motion for the device.

When the wax expands, it exerts a force on a diaphragm, causing it to move in the direction of the expansion. As the diaphragm extends upwards, the piston follows, as it is pushed to its desired distance. The temperature increase has led to motion.

The opposite process also occurs – when the wax cools, it contracts (thermal contraction). The diaphragm lowers, so the piston does too.

The distance through which the device (e.g. piston) moves is dependent on the temperature supplied and the variety of the wax, both of which can be controlled to suit an appliance.

applications

Thermal actuation’s bilateral nature renders thermal actuators useful for thermostats, particularly in cars. These can adequately sense temperature changes within the vehicle and regulate them accordingly.

When the temperature increases and the expansion material expands, the piston moves. When in use, cars radiate heat outwards, which induces a reduction in the temperature. This causes the piston to lower again as the expansive material contracts.

It can be gathered from this that thermal actuators are able to play a significant role in temperature regulation in general, hence their use in pool-heater regulation and fuel cell temperature regulation, as well as several other thermoregulatory applications.

the future

This technological process is commonly used and its uses will most likely diversify. This can be ascribed to its advantages, which include the durability of thermal actuators, due to the pressures and vibrations they are able to withstand, and also the range of sizes they exist in.

The size range is particularly useful – emerging now is the use of thermal actuators in the micro (1 x 10-6 m) and nano (1 x 10 -9 m) sizes. Actuators of this miniscule size are often required in the field of robotics, which will undoubtedly be an important subset of technology in the near future. Thermal actuation is very beneficial and will continue to increase in utility as time progresses.