Underground hydrogen storage is the practice of hydrogen storage in caverns, salt domes and depleted oil/gas fields.

A process to convert coal to gas which occurs underground. Coal, and volatiles embedded in the coal (mostly methane) are partially combusted in the presence of steam, oxygen and / or air, to produce hydrogen and carbon monoxide.

Underfloor heating and cooling is a form of central heating and cooling that achieves indoor climate control for thermal comfort using hydronic or electrical heating elements embedded in a floor.

Unconventional wind turbines are those that differ significantly from the most common types in use.

A form of electromagnetic radiation with wavelength from 10 nm to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiation is present in sunlight, and constitutes about 10% of the total electromagnetic radiation output from the Sun.

Ultrasonic testing is a family of non-destructive testing techniques based on the propagation of ultrasonic waves in the object or material tested.

UltraBattery is a hybrid energy storage device invented by Australia's Commonwealth Scientific and Industrial Research Organisation (CSIRO).

Ultra-supercritical – up to 1,400 °F (760 °C) and pressure levels of 5,000 psi (340 bar)

A passive electrical device that transfers electrical energy from one electrical circuit to another, or multiple circuits.

A switch used to transfer an electrical load from one source to another. Utilised when a source has failed and can include manual or automatic switching.

In organic chemistry, transesterification is the process of exchanging the organic group R″ of an ester with the organic group R′ of an alcohol as is used in the production of biodiesel.

A trademark is a type of intellectual property consisting of a recognizable sign, design, or expression which identifies products or services of a particular source from those of others, although trademarks used to identify services are usually called service marks.

A traction network or traction power network is an electricity grid for the supply of electrified rail networks.

Traction, or tractive force, is the force used to generate motion between a body and a tangential surface, through the use of dry friction, though the use of shear force of the surface is also commonly used.

A measure of how well, or badly, a product, person or organisation has performed in the past.

Toxicity is the degree to which a chemical substance or a particular mixture of substances can damage an organism.

Toxic waste is any unwanted material in all forms that can cause harm (e.g. by being inhaled, swallowed, or absorbed through the skin).

Town gas is a general term referring to manufactured gaseous fuels produced for sale to consumers and municipalities.

The alteration in uniform flow of wind due to the presence of the tower

The sum of production and imports subtracting exports and storage changes.