The discharge rate is defined as the steady current in amperes (A) that can be taken from a battery of defined capacity (Ah) over a defined period(h).

Directional drilling (or slant drilling) is the practice of drilling non-vertical wellbores.

Direct-buried cable (DBC) is a kind of communications or transmissions electrical cable which is especially designed to be buried under the ground without any kind of extra covering, sheathing, or piping to protect it.

A direct plasmonic solar cell is a solar cell that converts light into electricity using plasmons as the active, photovoltaic material.

Direct insolation is the solar insolation measured at a given location on Earth with a surface element perpendicular to the Sun's rays, excluding diffuse insolation (the solar radiation that is scattered or reflected by atmospheric components in the sky).

A heat pump system in which the refrigerant is circulated in pipes buried underground.

A wind turbine whereby the rotor is directly connected to the generator.

Electrical current that flows in one direction only.

The common allotrope of elemental oxygen on Earth, O2, is generally known as oxygen, but may be called dioxygen, diatomic oxygen, molecular oxygen, or oxygen gas to distinguish it from the element itself and from the triatomic allotrope ozone, O3.

Digitization is the process of converting information into a digital (i.e. computer-readable) format.

A digital voltmeter measures an unknown input voltage by converting the voltage to a digital value and then displays the voltage in numeric form.

Diffusion is the net movement of anything from a region of higher concentration to a region of lower concentration.

A diffuser-augmented wind turbine is a wind turbine modified with a cone-shaped wind diffuser that is used to increase the efficiency of converting wind power to electrical power.

Diffuse sky radiation is solar radiation reaching the Earth's surface after having been scattered from the direct solar beam by molecules or particulates in the atmosphere.

Differential pressure is the difference in pressure between two points.

The diesel engine, is an internal combustion engine in which ignition of the fuel is caused by the elevated temperature of the air in the cylinder due to the mechanical compression; thus, the diesel engine is a so-called compression-ignition engine.

The International Energy Agency is a Paris-based autonomous intergovernmental organisation which acts as a policy adviser to its member states, as well as major emerging economies to support energy security and advance the clean energy transition worldwide.

The Geysers is the world's largest geothermal field, containing a complex of 18 geothermal power plants, drawing steam from more than 350 wells, located in the Mayacamas Mountains approximately 72 miles (116 km) north of San Francisco, California.

The first law of thermodynamics is a version of the law of conservation of energy, adapted for thermodynamic processes, distinguishing two kinds of transfer of energy, as heat and as thermodynamic work, and relating them to a function of a body's state, called Internal energy.

The Texas Interconnection is an alternating current (AC) power grid – a wide area synchronous grid – that covers most of the state of Texas.