A wax motor is a linear actuator device that converts thermal energy into mechanical energy by exploiting the phase-change behaviour of waxes. During melting, wax typically expands in volume by 5–20% . A wide range of waxes can be used in wax motors, ranging from highly refined hydrocarbons to waxes extracted from vegetable matter. Specific examples include paraffin waxes in the straight-chain n-alkanes series. These melt and solidify over a well-defined and narrow temperature range. Design The principal components of a wax motor are: An enclosed volume of wax A plunger or stroke-rod to convert the thermo-hydraulic force from the wax into a useful mechanical output A source of heat such as: Electric current; typically a PTC thermistor, that heats the wax Solar radiation; e.g. greenhouse vents Combustion heat; e.g. excess heat from internal combustion engines Ambient heat A sink to reject heat energy such as: Convection to cooler ambient air Peltier effect device arranged to transfer heat energy away When the heat source is energized, the wax block is heated and it expands, driving the plunger outwards by volume displacement. When the heat source is removed, the wax block contracts as it cools and the wax solidifies. For the plunger to withdraw, a biasing force is usually required to overcome the mechanical resistance of seals that contain the liquid wax. The biasing force is typically 20% to 30% of the operating force and often provided by a mechanical spring or gravity-fed dead weight applied externally into the wax motor . Depending on the particular application, wax motors potentially have advantages over magnetic solenoids: They provide a large hydraulic force from the expansion of the wax in the order of 4000 N (corresponding to roughly 400 kg or 900 lb at standard gravity) . Both the application and the release of the wax motor is not instantaneous, but rather, smooth and gentle. Because the wax motor is a resistive load rather than an inductive load, wax motors controlled by TRIACs do not require snubber circuits. Wax motors can be operated entirely passively by exploiting ambient sources of energy. Given that a variety of melting-points are possible for the wax used inside the motor, one can be selected to match the range of ambient operating temperatures in a given application. In this way the wax can be melted and solidified within this range by the transfer of thermal energy. When co-located