Buildings consume over 70% of the US electricity supply, and this is why energy efficiency measures have such a high potential to reduce the environmental impact of modern cities. Traditionally, electricity has been generated at power stations and then delivered to buildings using power lines and transformers. However, modern generation technologies like photovoltaic panels and natural gas microturbines allow electricity production at the point of use.

There is a wide variety of electricity generation methods, each with advantages and limitations. Generation technologies can be classified in many ways:

• Energy source used to produce electricity

• Renewable or nonrenewable

• Production of greenhouse gas emissions

• Dispatchable or variable

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Energy Sources Used in Electricity Production

A basic principle of physics is that energy cannot be created or destroyed, it simply changes form. Since electricity is a form of energy, generation systems can only produce it by consuming another form. The following are some of the most common energy sources:

• Chemical energy in fossil fuels: Their combustion releases heat, which can be used to drive a turbine and generator, producing electricity.

• Kinetic and potential energy in water: Water can be stored in a reservoir, and then used to drive a turbine and generator. There are also hydroelectric power plant designs that don’t use a reservoir, and instead they extract energy from a river in motion. There are also experimental technologies that produce electricity from waves or tides.

• Kinetic energy in the wind: Wind turbines produce rotational motion, driving a generator and producing electricity.

• Chemical energy in organic waste (biomass): Organic waste can be processed into various types of biofuels. These can be used for turbines just like fossil fuels.

• Solar radiation: Photovoltaic panels can produce electricity from the energy contained in sunlight. Unlike the previous examples, solar panels don’t have moving parts and their maintenance is very simple.

• Underground heat (geothermal): By drilling in a suitable location, it is possible to drive a turbine and generator with underground heat.

• Nuclear energy: Atoms of radioactive elements such as uranium and plutonium can be split to release large amounts of heat, which is then used to power a steam turbine.

The price of electricity is strongly dependent on the resource used to produce it. Also note that the availability of energy sources changes by region, and electricity costs are increased if a resource must be transported from remote locations.

Renewable and Nonrenewable Electricity Sources

The media often uses the concept of renewable generation to describe clean energy sources, but there is an important difference between both terms. Of course, many energy sources are both clean and renewable.

An energy source is called renewable if the current consumption rate does not limit availability in the future. Solar generation is renewable, for example, since adding more photovoltaic arrays to buildings does not limit the future availability of sunlight.

• Even if all rooftops in the world were covered with solar panels, the amount of solar radiation reaching the planet would stay constant.

• Wind power is also renewable, since natural wind patterns continue to exist with or without wind turbines.

Nonrenewable energy sources are finite, and using more of them today leaves less available for the future. Fossil fuels are the best-known example, since their formation takes millions of years and they cannot be replenished fast enough to keep up with human consumption.

Greenhouse Gas Emissions from Electricity Generation

Many renewable sources are clean, but this is not always the case. For example, biomass power is considered renewable because it uses biofuels processed from organic waste. However, the combustion of these fuels releases greenhouse gases, even when their source is renewable.

All generation systems have an environmental impact due to equipment manufacturing and power plant construction. However, polluting sources continue releasing emissions during operation, while clean sources produce little or no emissions after construction.

Electricity sources that produce emissions also vary in terms of their carbon footprint. For example, natural gas and coal both produce emissions, but the emissions per kilowatt hour are more than twice as high when coal is used.

Dispatchable and Variable Electricity Generation

The concept of dispatchable generation is the least known among the general public, but for utility companies it is vital to keep a reliable power supply. A dispatchable power source in one that can be relied upon to supply electricity on demand. This excludes solar and wind power, since they use inputs that cannot be controlled. On the other hand, hydroelectric power plants can store water to deliver electricity at any time, and they are considered dispatchable.

If a power grid relies too much on variable energy sources, the electricity supply can become unstable because production is constantly fluctuating. Thus, utility companies normally rely on a mix of sources: wind and solar power are variable, but they offer a very low cost per kilowatt-hour, while dispatchable sources are used to balance generation and consumption.

An innovative concept is using large-scale energy storage to complement solar and wind power, turning them into dispatchable sources. Although the cost of batteries is still a barrier, it is decreasing fast and the concept has already been deployed successfully in places with expensive electricity.

Deploying Electricity Generation Systems in Buildings

Since every property is unique, the first step to deploy on-site generation is getting an inspection by a qualified energy consultant. For example, a rooftop that is constantly covered by the shadows of neighboring building is not suitable for solar power. Ideally, on-site generation should be complemented with energy efficiency - powering a building completely with on-site generation is easier and less expensive when electricity consumption has been optimized first.