Thermal energy, which is more commonly known as heat, is a form of energy. It is measured in joules. Thermal energy is an internal energy for a given system. Thermal energy is the cause for the temperature of a system.Every system having a temperature above absolute zero has a positive thermal energy.hermal energy is a form of random energy, which is not able to do work, when the whole system is considered. Higher the thermal energy of a system higher will be the randomness of the system. Thermal energy can be converted in to mechanical energy using a heat engine.