Repowering is the process of replacing older power stations with newer ones that either have a greater nameplate capacity or more efficiency which results in a net increase of power generated.

The replacement reserve (also known as contingency reserve) is reserve power provided by generators that require a longer start-up time (typically thirty to sixty minutes).

A web tool developed by Imperial College London and ETH Zürich that shows the estimated amount of energy that could be generated by wind or solar farms at any location.

Renewable Natural Gas, also known as biomethane, is a biogas which has been upgraded to a quality similar to fossil natural gas and having a methane concentration of 90% or greater.

The Renewable Fuels Association (RFA) represents the ethanol industry promoting policies, regulations, and research and development initiatives that will lead to the increased production and use of ethanol fuel.

Renewable fuels are fuels produced from renewable resources they can include fuels that are synthesized from renewable energy sources, such as wind and solar.

Renewable energy law is a particular kind of energy law, and relates primarily to the transactional legal and policy issues that surround the development, implementation, and commercialization of renewable sources of energy, such as solar, wind, geothermal and tidal.

A renewable energy credit is a certificate corresponding to the environmental attributes of energy produced from renewable sources such as wind or solar.

Renewable energy commercialization involves the deployment of three generations of renewable energy technologies dating back more than 100 years.

The Renewable Energy and Energy Efficiency Partnership (REEEP) is a Vienna-based international organisation that advances markets for renewable energy and energy efficiency with a particular emphasis on the emerging markets and developing countries.

An energy source that comes from naturally existing resources that are easily replenished or in constant supply.

A measure of the generating capacity that can be acheived from renewable energy sources.

A think tank and a multistakeholder governance group which is focused on renewable energy policy.

A relief valve or pressure relief valve (PRV) is a type of safety valve used to control or limit the pressure in a system.

Reliability engineering is a sub-discipline of systems engineering that emphasizes the ability of equipment to function without failure.

A relay is an electrically operated switch.

Relative volatility is a measure comparing the vapor pressures of the components in a liquid mixture of chemicals.

Relative density, or specific gravity, is the ratio of the density of a substance to the density of a given reference material.

Reid vapor pressure is a common measure of the volatility of gasoline and other petroleum products.

Regulation and licensure in engineering is established by various jurisdictions of the world to encourage public welfare, safety, well-being and other interests of the general public and to define the licensure process through which an engineer becomes authorized to practice engineering and/or provide engineering professional services to the public.