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Subject Index - Technology-Related Terms

air-source heat pump


"Bandgap" is one word.


"Baseload" is one word when used as a noun or adjective.

black start

When used as a noun, “black start” is two words. Hyphenate “black-start” when it is used as a modifier. Avoid using as a verb if possible.

This building is capable of black start.
This building has black-start capabilities.


BOS stands for balance of systems (not system).

chemical terms

Do not use a hyphen in most chemical expressions, even when the terms are used as modifiers.

carbon dioxide levels
hydrogen ion activity

Use a hyphen after prefixes when that's the standard for certain chemical formulas.

L(+)-2, 3-butanediol

Use a hyphen to indicate mixtures or combinations.



"Cleantech" is spelled as one word. It is not hyphenated, and the "t" is not capitalized. The word "cleantech" is typically used in reference to investments in sustainable technologies, including renewable energy and energy efficiency. Don't use as a shortened form of "clean technology" in other references.

close-spaced sublimation

The term is "close-spaced sublimation," not "closed-space sublimation."




CPV (concentrating photovoltaics) uses lenses to intensify the sunlight striking PV cells, which enhances the cells' electricity production.


CSP (concentrating solar power) captures the sun’s heat and uses the thermal energy to produce electricity (e.g., via a steam turbine).

dish/engine systems

Use a slash rather than a hyphen.

enhanced geothermal system

The preferred term is "enhanced geothermal system." It is also sometimes referred to as an "engineered geothermal system."



geopressured geothermal resource

Geothermal Electric Technology Evaluation Model

geothermal heat pump

heat mining

high-performance computing

“High-performance computing” should be hyphenated. The abbreviation HPC can be used to refer to high-performance computing but should not be used to mean high-performance computer. However, the term "supercomputer" may be used in place of high-performance computer.

hybrid electric vehicle

This phrase contains no hyphens.

III-V solar cell

This term refers to a cell composed of semiconducting materials from Group III (e.g., gallium) and Group V (e.g., arsenic) elements of the periodic table.


Inverters convert direct current to alternating current.

Kalina cycle



microseismic events

multijunction solar cell

This term is preferred over "tandem solar cell."

net zero

When used as a noun, “net zero” is two words. It is also two words when referencing the Net Zero Labs program. Hyphenate “net-zero” when it is used as a modifier.

NREL is eliminating its carbon footprint through the Net Zero Labs initiative.
We are close to achieving our emissions goal of net zero.
The goal is to achieve net-zero emissions.

noncondensable gases

NREL attribution statement

The NREL attribution statement, which recognizes NREL's affiliation with the U.S. Department of Energy and the Alliance for Sustainable Energy, is required on all NREL publications. In addition, note that a period is used on the "sentence" version of the statement.

photovoltaics and photovoltaic

"Photovoltaics" is a singular noun. "Photovoltaic" is an adjective. The acronym PV can be a noun or an adjective, but do not pluralize it.

Rankine cycle

renewable portfolio standard

Only capitalize "renewable portfolio standard" when a state name precedes it.

Renewable energy certificates have been proposed under California Renewable Portfolio Standards.


Smart Grid and smart grid

Use capital letters for "Smart Grid" when referring to the overall goal or concept and lowercase letters for "smart grid" when referring to current implementations or when used as an adjective.

solar cell interfaces

Use a slash rather than a hyphen to designate solar cell interfaces or layers.


solar conversion efficiency

Define in outreach publications as "the percentage of sunlight striking a solar cell that is converted into electricity." A definition is often unnecessary in technical publications.

solar electricity

This term is interchangeable with "photovoltaic power," "PV power," or "PV electricity."

systems integrator

The correct term is "systems integrator," not "system integrator."

technical abbreviations

Unless your profession, technical field, or scientific discipline specifies something different, use the abbreviations in NREL's list of technical abbreviations or the National Institute of Science and Technology's Guide.


Hyphenate. Don't spell as one word.

zero energy building

Use the term to indicate an energy-efficient building where, on a source energy basis, the actual annual delivered energy is less than or equal to the on-site renewable exported energy. Similar terms include zero energy campus, zero energy portfolio, and zero energy community.