Renewable Energy Glossary

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HAWT

A horizontal-axis wind turbine (HAWT) is a wind generator that spins on a horizontal axis; this is the most common wind generator configuration.

HCFCs

Hydrochlorofluorocarbons (HCFCs) are gaseous compounds that contribute to stratospheric ozone depletion.

Head

The drop in elevation between the water level at the intake and the level of the turbine or tailrace determines the "static head" of pressure at the turbine. "Dynamic head" during operation will be lower than this due to friction loss in the pipe and fittings. Head can be expressed in units of height (e.g. feet) or units of pressure (e.g. PSI).

Head Loss

Lost energy due to friction. For a given size of pipe, friction loss rises rapidly with increased flow. (It varies roughly with the square of the flow.) There will also be friction losses in belt drive systems.

Header

The portion of a collector that provides the inlet and/or outlet for the primary heat transfer fluid.

Headrace

A flume or channel that feeds water into a hydro turbine.

Heat Exchanger

A device that is used to transfer heat between fluids or gases through an intervening surface.

Heat Gain

The amount of heat gained by a space from all its sources, including occupants, lights, equipment, and solar radiation.

Heat Loss

A decrease in the amount of heat contained in a space, resulting from heat flow through floors, walls, doors, windows, roofs, and other building envelope components.

Heat Recovery Ventilator

A mechanical ventilation unit that is used as an air-to-air heat exchanger removing heat from interior, exhaust air and transferring it to the incoming, exterior air stream. Typically HRVs are used in cold, dry climates. These units are known for being efficient and reducing energy losses.

Heat Sink

A medium or container to which heat flows. Thermal mass walls and floors in a passive solar home act as a heat sink during the day. In addition, air conditioner and heat pumps use the outdoor air or the ground as a heat sink when they are being used to cool air.

Heat Transfer

Heat is transferred from one substance or location to another by three methods--radiation, convection, and conduction. The sun's rays are a good example of radiation; warm air rising is heat movement by convection; and touching a hot iron or frying pan with your hand is heat transfer by conduction.

Heat Transfer Fluid

Any fluid circulating through a solar collector or array.

Hertz

The number of cycles per second when describing a sine wave curve. Most AC electrical systems are either 50 or 60 Hertz, meaning they have 50 or 60 cycles per second.

HEVs

Hybrid electric vehicles (HEVs) are powered by a combined gasoline and electric drive train. Generally referred to as a "hybrid," HEVs include the popular Toyota Prius and Honda Insight.

High-Head Site

There is no strict definition of "high head," but even systems with as little as 20 feet of head may be considered high head for a domestic hydro system. A site with higher head can produce as much power as a low-head site while using less flow, but needs a different type of turbine. Pelton turbines are popular for high-head sites.

Homerun

The positive and negative lengths of wire that run from both ends of a source circuit to the next component in a PV system.

Horizontal-Axis Wind Turbine

A wind generator that spins on a horizontal axis; this is the most common wind generator configuration.

HRV

A heat recovery ventilator (HRV) is a mechanical ventilation unit that is used as an air-to-air heat exchanger removing heat from interior, exhaust air and transferring it to the incoming, exterior air stream. Typically HRVs are used in cold, dry climates.

HVAC

The term heating, ventilation, and air conditioning (HVAC) refers to all mechanical systems that heat, ventilate, and cool a building.

Hybrid Electric Vehicle

Often simply called a "hybrid," a vehicle powered by a combination of gasoline- and electric-powered drive train in three basic configurations: "parallel," "series," or "split" hybrids. The electric motor can either share the load, drive the vehicle with the gas motor as generator, or combine the two.

Hybrid Systems

Systems that have more than one source of energy generation, such as a PV array and a wind turbine.

Hydro-Electricity

Any electricity that is generated by the flow of water.

Hydronic Heating System

A type of heating system where water is heated in a solar collector or boiler, and is pumped to heat exchangers or radiators in rooms. Radiant floor systems have a grid of tubing laid out in the floor for distributing the heat. The temperature of the space is controlled by regulating the flow and/or temperature of the circulating water.

Hypermiling

A term coined in 2004 by Wayne Gerdes of CleanMPG.com that refers to driving techniques and practices that can increase the fuel economy of any vehicle by as much as 50%.

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