In thermodynamics, the Helmholtz free energy is a thermodynamic potential that measures the useful work obtainable from a closed thermodynamic system at a constant temperature and volume.

A double layer (DL, also called an electrical double layer, EDL) is a structure that appears on the surface of an object when it is exposed to a fluid.

Technology that utilises Heliostats that contain a mirror to redirect sunlight to a collector or target.

A plane mirror device that moves with the sun to continuously direct solar power to a receiver.

The wires are placed helically in every layer.

The blades of a Darrieus turbine can be canted into a helix, e.g. three blades and a helical twist of 60 degrees. Since the wind pulls each blade around on both the windward and leeward sides of the turbine, this feature spreads the torque evenly over the entire revolution, thus preventing destructive pulsations.

Heavy water (deuterium oxide) is a form of water that contains only deuterium rather than the common hydrogen-1 isotope that makes up most of the hydrogen in normal water.

Heavy industry is an industry that involves one or more characteristics such as large and heavy products; large and heavy equipment and facilities or complex or numerous processes.

Heavy fuel oils are blended products based on the residues from refinery distillation and cracking processes.

Heavy crude oil is highly-viscous oil that cannot easily flow to production wells under normal reservoir conditions. It is referred to as "heavy" because its density or specific gravity is higher than that of light crude oil. Heavy crude oil has been defined as any liquid petroleum with an API gravity less than 20°.

The heating value of a substance, usually a fuel or food, is the amount of heat released during the combustion of a specified amount of it.

A measure of heating efficiency for air source heat pump equipment on an annual basis, expressed as the heating energy provided to the space (Btu) divided by the electric energy consumed (Watt-hour) over the entire heating season.

A heating element converts electrical energy into heat through the process of Joule heating.

A heat transfer fluid is a gas or liquid that takes part in heat transfer by serving as an intermediary in cooling on one side of a process, transporting and storing thermal energy, and heating on another side of a process.

Heating degree day is a measurement designed to quantify the demand for energy needed to heat a building.

The heat transfer coefficient or film coefficient, or film effectiveness, in thermodynamics and in mechanics is the proportionality constant between the heat flux and the thermodynamic driving force for the flow of heat.

Heat transfer is a discipline of thermal engineering that concerns the generation, use, conversion, and exchange of thermal energy (heat) between physical systems.

A thermal reservoir, also thermal energy reservoir or thermal bath, is a thermodynamic system with a heat capacity so large that the temperature of the reservoir does not change when a reasonable amount of heat is added or extracted.

Heat recovery ventilation, also known as mechanical ventilation heat recovery, is an energy recovery ventilation system which works between two sources at different temperatures.

A energy recovery heat exchanger that recovers heat from a hot gas stream, such as a combustion turbine or other waste gas stream.