Tankless Water Heater
A water heater that heats water before it is directly distributed for end use as required; a demand water heater.

Task Lighting

Any light source designed specifically to direct light a task or work performed by a person or machine.

Temperature Coefficient (of a solar photovoltaic cell)

The amount that the voltage, current, and/or power output of a solar cell changes due to a change in the cell temperature.

Temperature Humidity Index

An index that combines sensible temperature and air humidity to arrive at a number that closely responds to the effective temperature; used to relate temperature and humidity to levels of comfort.

Temperature/Pressure Relief Valve

A component of a water heating system that opens at a designated temperature or pressure to prevent a possible tank, radiator, or delivery pipe rupture.

Temperature Zones

Individual rooms or zones in a building where temperature is controlled separately from other rooms or zones.

Tempering Valve

A valve used to mix heated water with cold in a heating system to provide a desired water temperature for end use.

Tennessee Valley Authority (TVA)

A federal agency established in 1933 to develop the Tennessee river valley region of the southeastern U.S., and which is now nation’s largest power producer.

Termite Shield

A construction element that inhibits termites from entering building foundations and walls.


A unit of heat containing 100,000 British thermal units (Btu).

Thermal Balance Point

The point or outdoor temperature where the heating capacity of a heat pump matches the heating requirements of a building.

Thermal Capacitance

The ability of a material to absorb and store heat for use later.

Thermal Efficiency

A measure of the efficiency of converting a fuel to energy and useful work; useful work and energy output divided by higher heating value of input fuel times 100 (for percent).

Thermal Energy

The energy developed through the use of heat energy.

Thermal Energy Storage

The storage of heat energy during power provider off-peak times at night, for use during the next day without incurring daytime peak electric rates.

Thermal Envelope Houses

An architectural design (also known as the double envelope house), sometimes called a “house-within-a-house,” that employs a double envelope with a continuous airspace of at least 6 to 12 inches on the north wall, south wall, roof, and floor, achieved by building inner and outer walls, a crawl space or sub-basement below the floor, and a shallow attic space below the weather roof. The east and west walls are single, conventional walls. A buffer zone of solar-heated, circulating air warms the inner envelope of the house. The south-facing airspace may double as a sunspace or greenhouse.

Thermal Mass

Materials that store heat.

Thermal Storage Walls (Masonry or Water)

A thermal storage wall is a south-facing wall that is glazed on the outside. Solar heat strikes the glazing and is absorbed into the wall, which conducts the heat into the room over time. The walls are at least 8 in thick. Generally, the thicker the wall, the less the indoor temperature fluctuates.

Thermal Resistance (R-Value)

This designates the resistance of a material to heat conduction. The greater the R-value the larger the number.


A device consisting of two dissimilar conductors with their ends connected together. When the two junctions are at different temperatures, a small voltage is generated.

Thermodynamic Cycle

An idealized process in which a working fluid (water, air, ammonia, etc) successively changes its state (from a liquid to a gas and back to a liquid) for the purpose of producing useful work or energy, or transferring energy.


A study of the transformation of energy from one form to another, and its practical application. (see Law(s) of Thermodynamics above).

Thermoelectric Conversion

The conversion of heat into electricity by the use of thermocouples.


A building energy auditing technique for locating areas of low insulation in a building envelope by means of a thermographic scanner.

Thermophotovoltaic Cell

A device where sunlight concentrated onto a absorber heats it to a high temperature, and the thermal radiation emitted by the absorber is used as the energy source for a photovoltaic cell that is designed to maximize conversion efficiency at the wavelength of the thermal radiation.


A large number of thermocouples connected in series.

Thermosiphon System

This passive solar hot water system consists relies on warm water rising, a phenomenon known as natural convection, to circulate water through the collectors and to the tank. In this type of installation, the tank must be above the collector. As water in the collector heats, it becomes lighter and rises naturally into the tank above. Meanwhile, cooler water in the tank flows down pipes to the bottom of the collector, causing circulation throughout the system. The storage tank is attached to the top of the collector so that thermosiphoning can occur.


The natural, convective movement of air or water due to differences in temperature. In solar passive design a thermosyphon collector can be constructed and attached to a house to deliver heat to the home by the continuous pattern of the convective loop (or thermosyphon).


A device used to control temperatures; used to control the operation of heating and cooling devices by turning the device on or off when a specified temperature is reached.


A layer of semiconductor material, such as copper indium diselenide or gallium arsenide, a few microns or less in thickness, used to make solar photovoltaic cells.

Three-phase Current

Alternating current in which three separate pulses are present, identical in frequency and voltage, but separated 120 degrees in phase.

Tidal Power

The power available from the rise and fall of ocean tides. A tidal power plant works on the principal of a dam or barrage that captures water in a basin at the peak of a tidal flow, then directs the water through a hydroelectric turbine as the tide ebbs.

Tilt Angle (of a Solar Collector or Module)

The angle at which a solar collector or module is set to face the sun relative to a horizontal position. The tilt angle can be set or adjusted to maximize seasonal or annual energy collection.

Time-of-Use (TOU) Rates

The pricing of electricity based on the estimated cost of electricity during a particular time block. Time-of-use rates are usually divided into three or four time blocks per twenty-four hour period (on-peak, mid-peak, off-peak and sometimes super off-peak) and by seasons of the year (summer and winter). Real-time pricing differs from TOU rates in that it is based on actual (as opposed to forecasted) prices which may fluctuate many times a day and are weather-sensitive, rather than varying with a fixed schedule.


A device that can be set to automatically turn appliances (lights) off and on at set times.

Timer (Water Heater)

This device can automatically turn the heater off at night and on in the morning.

Tip Speed Ratio

In reference to a wind energy conversion device’s blades, the difference between the rotational speed of the tip of the blade and the actual velocity of the wind.

Ton (of Air Conditioning)

A unit of air cooling capacity; 12,000 Btu per hour.


A means to increase the thermal efficiency of a steam electric generating system by increasing temperatures and interposing a device, such as a gas turbine, between the heat source and the conventional steam-turbine generator to convert some of the additional heat energy into electricity.

Torque (Motor)

The turning or twisting force generated by an electrical motor in order for it to operate.

Total Harmonic Distortion

The measure of closeness in shape between a waveform and it’s fundamental component.

Total Heat

The sum of the sensible and latent heat in a substance or fluid above a base point, usually 32 degrees Fahrenheit.

Total Incident Radiation

The total radiation incident on a specific surface area over a time interval.

Total Internal Reflection

The trapping of light by refraction and reflection at critical angles inside a semiconductor device so that it cannot escape the device and must be eventually absorbed by the semiconductor.

Tracking Solar Array

A solar energy array that follows the path of the sun to maximize the solar radiation incident on the PV surface. The two most common orientations are (1) one axis where the array tracks the sun east to west and (2) two-axis tracking where the array points directly at the sun at all times. Tracking arrays use both the direct and diffuse sunlight. Two-axis tracking arrays capture the maximum possible daily energy.

Trailing Edge

The part of a wind energy conversion device blade, or airfoil, that is the last to contact the wind.


An electromagnetic device that changes the voltage of alternating current electricity. It consists of an induction coil having a primary and secondary winding and a closed iron core.


The process of sending or moving electricity from one point to another; usually defines that part of an electric power provider’s electric power lines from the power plant buss to the last transformer before the customer’s connection.

Transmission and Distribution Losses

The losses that result from inherent resistance in electrical conductors and transformation inefficiencies in distribution transformers in a transmission and distribution network.

Transmission Lines

Transmit high-voltage electricity from the transformer to the electric distribution system.

Traveling Grate

A furnace grate that moves fuel through the combustion chamber.


An architectural feature used to shade exterior walls; usually made of a lattice of metal or wood; often covered by vines to provide additional summertime shading.

Trickle (Solar) Collector

A type of solar thermal collector in which a heat transfer fluid drips out of header pipe at the top of the collector, runs down the collector absorber and into a tray at the bottom where it drains to a storage tank.

Triple Pane (Window)

This represents three layers of glazing in a window with an airspace between the middle glass and the exterior and interior panes.

Trombe Wall

A wall with high thermal mass used to store solar energy passively in a solar home. The wall absorbs solar energy and transfers it to the space behind the wall by means of radiation and by convection currents moving through spaces under, in front of, and on top of the wall.

True Power

The actual power rating that is developed by a motor before losses occur.

True South

The direction, at any point on the earth that is geographically in the northern hemisphere, facing toward the South Pole of the earth. Essentially a line extending from the point on the horizon to the highest point that the sun reaches on any day (solar noon) in the sky.

Tube (Fluorescent Light)

A fluorescent lamp that has a tubular shape.


A type of solar thermal collector where the heat transfer fluid flows through tubes formed in the absorber plate.

Tube-Type Collector

A type of solar thermal collector that has tubes (pipes) that the heat transfer fluid flows through that are connected to a flat absorber plate.

Tungsten Halogen Lamp

A type of incandescent lamp that contains a halogen gas in the bulb, which reduces the filament evaporation rate increasing the lamp life. The high operating temperature and need for special fixtures limits their use to commercial applications and for use in projector lamps and spotlights.


A device for converting the flow of a fluid (air, steam, water, or hot gases) into mechanical motion.

Turn Down Ratio

The ratio of a boiler’s or gasifier’s maximum output to its minimum output.

Two-Tank Solar System

A solar thermal system that has one tank for storing solar heated water to preheat the water in a conventional water heater.

Two-Axis Tracking

A solar array tracking system capable of rotating independently about two axes (e.g., vertical and horizontal).