A flexible alternating current transmission system (FACTS) is a system composed of static equipment used for the alternating current (AC) transmission of electrical energy.

Flat-plate collectors consist of an enclosure containing a dark-colored absorber plate with fluid circulation passageways, and a transparent cover to allow transmission of solar energy into the enclosure.

A flat engine, also known as a horizontally opposed engine, is a piston engine where the cylinders are located on either side of a central crankshaft.

Steam produced when the pressure on a geothermal liquid is reduced.

Flash steam plants take high-pressure hot water from deep inside the earth and convert it to steam to drive generator turbines. When the steam cools, it condenses to water and is injected back into the ground to be used again.

The flash point of a volatile material is the lowest temperature at which its vapors ignite if given an ignition source.

Flash evaporation (or partial evaporation) is the partial vapor that occurs when a saturated liquid stream undergoes a reduction in pressure by passing through a throttling valve or other throttling device.

Flammable materials are combustible materials that ignite easily at ambient temperatures.

A set (array) of solar power collectors that do not pivot to follow the track of the sun in the sky.

A turbine designed to operate at near constant speed.

A fixed platform is a type of offshore platform used for the extraction of petroleum or gas.

Thin metal plates are stacked with a small spacing between plates.

Fixed foundation offshore wind turbines have fixed foundations underwater, and are installed in relatively shallow waters of up to 50 to 60 metres (160 to 200 ft).

Fixed costs, also known as indirect costs or overhead costs, are business expenses that are not dependent on the level of goods or services produced by the business.

A fission fragment reactor is a nuclear reactor that generates electricity by decelerating an ion beam of fission byproducts instead of using nuclear reactions to generate heat

Fission chain reactions occur because of interactions between neutrons and fissile isotopes.

Fissile material is material capable of sustaining a nuclear fission chain reaction.

The Fischer–Tropsch process is used to produce synfuels from gasified biomass. While biodiesel and bio-ethanol production so far only use parts of a plant biomass to liquid production can gasify and utilize the entire plant.

First generation biofuels are made from sugar-starch feedstocks (e.g., sugarcane and corn) and edible oil feedstocks (e.g., rapessed and soybean oil), which are generally converted into bioethanol and biodiesel, respectively.

Firm services, also called uninterruptible services, are services, such as electricity and natural gas supplies, that are intended to be available at all times during a period covered by an agreement.