Renewable Energy Glossary

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I-V Curve

Plots the relationship of current and voltage output for a PV module for specific conditions; also known as current-voltage curve.

IAQ

Indoor air quality (IAQ) refers to the quality of indoor air as it relates to the health and comfort of the occupants. Factors that affect IAQ include moisture, the off-gassing of furnishings, finishes, and dust and mold.

ICE

Internal combustion engine (ICE) is commonly used in the electric vehicle community to refer to gasoline motors. It is sometimes used in reference to the EV revolution and the obsolescence of fossil fuel-powered transportation, as in, "The ICE Age."

ICFs

Insulated concrete forms (ICFs) are hollow insulated forms made of two faces of 2" polystyrene held apart by spacers. The ICFs are stacked and the cavity created is then filled with concrete. The system remains in place after the concrete is poured, serving as the structural monolithic wall and insulation in one.

ICS Systems

Integral collector storage (ICS) systems integrate the solar collector and storage tank(s) into a single unit. ICS systems can be batch-type heaters or thermosyphon designs.

IGUs

Insulated glass units (IGUs) are two panes of glass separated by an air space and sealed to be airtight.

Immersed Coil

A heat exchanger that is contained inside of a solar storage tank.

Impulse Turbine

A hydro turbine with a runner that operates in air, driven by the kinetic energy of a jet (or jets) of water. Pelton, turgo, and crossflow turbines work on the impulse principle. Impulse turbines work at higher heads, and do not usually have draft tubes. Crossflows, however, occasionally operate within sealed casings at negative pressure.

Incandescent Light Bulb

A bulb that makes light by heating a filament wire to a high temperature until it glows. Because of this, only approximately 10% of the electricity consumed by the incandescent actually creates visible light; the rest is consumed to generate heat.

Inclinometer

A device used to measure slope or tilt angle.

Indirect Forced Circulation System

A closed system with a heat exchanger that could be configured as a drainback or antifreeze system.

Indoor Air Quality

Refers to the quality of indoor air as it relates to the health and comfort of the occupants. Factors that affect IAQ include moisture, the off-gassing of furnishings, finishes, and dust and mold.

Induction Generator

An induction motor will feed power into the grid if driven faster than its motoring speed. Coils in the motor's body produce a magnetic field that rotates with the frequency of the grid. The difference in speed between the moving rotor and the moving field (known as “slip”) induces a current in the rotor that in turn generates torque and power.

Induction Generator

An induction motor will feed power into the grid if driven faster than its motoring speed. Coils in the motor's body produce a magnetic field that rotates with the frequency of the grid. Induction generators need capacitors if they are to work in stand-alone situations (without the grid). Diversion load controllers are used to control voltage.

Infiltration

The uncontrolled leakage of air into buildings through cracks and joints in any building section, and around windows and doors of a building, caused by temperature or pressure differentials.

Input Voltage Window

The DC voltage range (minimum and maximum) accepted by an inverter from a PV array.

Insolation

Also known as irradiation, an amount of solar radiation energy on a surface over a period of time, measured in watt-hours (or kilowatt-hours) per square meter (Wh/m2 or kWh/m2). Energy is power over time, so insolation is cumulative irradiance over a given time.

Insulated Concrete Forms

Hollow insulated forms made of two faces of 2" polystyrene held apart by spacers. The ICFs are stacked and the cavity created is then filled with concrete. The system remains in place after the concrete is poured, serving as the structural monolithic wall and insulation in one.

Insulated Glass Unit

Two panes of glass separated by an air space and sealed to be airtight.

Insulation

A material with high resistance to heat flow. Good insulators have low thermal conductivity and high R-values or thermal resistance. Four major classifications of building insulation materials are batt, loose fill, reflective, and rigid.

Insulation Resistance Testing

A test that applies a high level of DC voltage, typically between a current-carrying conductor and ground, and measures resistance, which is used to evaluate the insulation integrity.

Insulators

Materials that are resistive to the flow of electrical current.

Intake

The structure that receives the water and feeds it into the penstock (pipeline). Usually incorporates screening or filtering to keep debris and aquatic life out of the system. The intake must be robust to withstand floods. In dry times of low flow, it should release some water back into the stream for ecological reasons.

Intake Screen

A typically metal screen (mesh) that allows water to enter the penstock (pipeline) without debris. The size of the apertures depends on the type of turbine. A well designed screen will be self-cleaning. This can save a great deal of maintenance work.

Integral Collector Storage System

A system that integrates the solar collector and storage tank(s) into a single unit. ICS systems can be batch type heaters or thermosyphon designs.

Inter-row Shading

Occurs in arrays with multiple rows of modules tilted up; one row will cast a shadow on the row behind it at some point in the day. Proper design layout can mitigate power losses due to this shading.

Interconnection

Refers to the policy and electrical standards for connecting a PV system to the utility grid.

International System of Units

A system of metric units of measurement, which uses various unit prefixes such as giga-, mega-, kilo-, milli-, micro-, and nano-.

Inverter

A power appliance that converts direct current (DC) electricity to alternating current (AC).

Inverter Output Circuits

The conductors between the inverter and the service equipment or another electric power production source, such as a utility in a grid-direct system.

Inverter Output Maximum Circuit Current

The rated continuous output current as specified by the manufacturer (as defined by the NEC).

Inverter Rated Maximum AC Output Power

The highest power (watts) that the inverter can provide, which in an off-grid system must be large enough to operate all loads running simultaneously.

Inverter/charger

A device that can function as both an inverter--changing DC electricity to AC electricity; and as a battery charger--- taking AC electricity and converting it to DC electricity to charge a battery bank.

Irradiance

Irradiance is the intensity (or power) of solar radiation, measured in watts per square meter (W/m2).  Power is an instantaneous value, so irradiance is the particular sunlight intensity at any given moment.  It is constantly changing throughout the day, as the sun moves across the sky, and throughout the year, as the sun moves higher or lower relative to the horizon.

Irradiance

The intensity (or power) of sunlight, measured in watts per square meter (W/m2). Power is an instantaneous value, so irradiance is the particular sunlight intensity at any given moment. Irradiance changes constantly throughout the day due to changes in the atmosphere and the sun's angle.

Irradiation

An amount of solar radiation energy on a surface over a period of time, measured in watt-hours (or kilowatt-hours) per square meter (Wh/m2 or kWh/m2); also known as insolation. Energy is power over time, so insolation is cumulative irradiance over a given time.

Isc

Short circuit current (Isc) is the current a PV cell or module produces with no resistance or loads in the circuit; this occurs at zero voltage (and thus zero power).

Islanding

A dangerous situation wherein the grid shuts down, but a distributed generation source continues to operate and provide electricity to the otherwise de-energized grid.

Isolation Valve

A shut-off valve--normally ball valves used to isolate tanks or components; an isolation valve can also be a gate valve.

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