H

HEAD The difference in elevation between two parts of a liquid-based system. In hydro power, the difference between a source of water and the location at which the water from that source may be used (synonym: vertical drop). With pumps, the vertical distance the pump must move the water.

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 PUMP A device typically used for heating and cooling of buildings by drawing from or dissipating into the ambient temperature of air or water. When cooling, a heat pump works like a refrigerator. When heating, it also works like a fridge, except the heat produced is used to heat a space.

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.

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.

HERTZ (Hz) Cycles per second. Generally refers to the number of complete cycles of the AC sine wave per second, or the frequency at which a radio or computer processor works.

HYDRO-ELECTRICITY Any electricity that is generated by the flow of water.

HYDROGEN FUEL CELL A device that converts hydrogen to DC electricity.

HYDROMETER A hydrometer is an instrument for measuring the density of liquids in relation to the density of water. The hydrometer is used to indicate the state of charge in lead-acid cells by measuring the specific gravity of the electrolyte.

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. Temperature of the space is controlled by regulating the flow and/or temperature of the circulating water.

HORSEPOWER A measure of the capacity to generate energy or do work. 1 horsepower = 746 watts.

HYBRID SYSTEM An energy system consisting of two or more generating subsystems, such as the combination of a wind turbine or diesel generator and a photovoltaic system.