Integrating alternative power and existing electrical systems

Despite (or as a result of) an economic downturn, the use of renewable energy options has grown over the past few years. This growth has been supported by federal and state programs including federal tax credits, state renewable portfolio standards, and a federal renewable fuels standard.


This article is peer-reviewed.Learning objectives:

  • Define renewable energy technologies in use today.
  • Compare the programs in place that encourage continued development of alternative power systems.
  • Evaluate different case studies that show the use of alternative power. 

The world has witnessed a relatively steady increase of installed base numbers of renewable energy generation over the past 10 to 15 years. Strong contributing factors to this trend in the U.S. are incentive programs developed at the state and federal levels.

Figure 1: The photo shows a 275-kW rooftop photovoltaic (PV) installation at a public middle school in North Las Vegas. The system includes modules mounted on both ballasted racking systems and attached to the standing seam barrel roof over the gymnasium.Renewable energy generation technologies

There are many renewable energy-generation technologies that can be incorporated into residential- and commercial-building energy systems. Solar photovoltaic (PV) is by far the most recognizable renewable energy source (see Figure 1). Wind turbines are a close second (see Figure 2). However, these sources are only in operation during specific environmental operating parameters. For example, the fuel that drives a PV cell is light from the sun. Performance diminishes on a cloudy day, and the system shuts down at night. Geothermal, fuel cell, biomass, and hydro technologies are considered baseload generating plants, able to operate on a 24-hour basis with no reliance on weather conditions.

One advantage of PV is it can be easily installed across a wide range of projects. It can be deployed to provide a utility-scale power generation plant or scaled down to only a few modules to supplement the power needs of a single family home. In the case of fixed-tilt module systems, there is very little maintenance with no moving mechanical parts. The primary challenge, beyond availability of sunlight, is real estate. PV installations are commonly installed on roofs that are out of sight and out of mind. Solar PV cells convert sunlight to electricity in direct current (dc) voltage. The cells are comprised of a semiconductor material (such as silicon) that absorbs photons, which excites the electrons to cause a current flow. PV inverters are used to convert the dc voltage to alternating current (ac) voltage.

Figure 2: The photo shows a 63-MW wind-power project in Dry Lake, Ariz. Courtesy: U.S. Department of EnergyInstallation and maintenance of wind turbines are more complicated. This type of renewable energy-generation source is typically installed in rural areas and takes up a great deal of land. Turbines contain mechanical components and experience wear and tear, requiring regular maintenance—normally twice per year. In addition, wind turbine installations are most effective in areas where the minimum sustained wind speed is at least 12 mph.

Geothermal technology can be used to produce electricity at utility scale. However, applications in commercial and residential settings are mainly limited to heating and cooling. This technology typically uses the heat from large reservoirs of water found deep underground. Harnessing these massive heat sources to produce steam that drives large turbines, rather than boiling water with fossil fuels, reduces the emissions from steam turbine power plants dramatically, to less than 5% as compared with coal. The remaining source of emissions is due to naturally occurring dissolved gasses present in the underground reservoir.

Fuel cells produce electricity based on a chemical reaction that strips electrons from fuel containing hydrogen and directs the electrons toward oxygen through a sandwich of close-coupled bipolar plates and electrolyte (conductive fluid; see Figure 3). Electrons passing across the terminals result in electric current flow. The most common fuel used is natural gas. However, because the process does not include combustion, there are no pollutant byproducts. This technology is considered renewable only if the fuel source used is renewable. The typical byproducts are carbon dioxide, water, and heat.

Figure 3: This diagram shows how a fuel cell works. The electrons are pulled from the hydrogen through the left plate and directed to the oxygen through the right plate. The result of these electrons passing from one plate to the other is current flow. CoBiomass electricity generation uses organic waste as fuel. The fuel is burned to create steam that, similar to geothermal plants, drives large turbine engines. One example is the methane byproduct from rotting landfill waste that is normally vented or flared off to the atmosphere to prevent dangerous gas buildup beneath the soil. To produce electricity, the methane is captured, processed, and burned to produce steam. This is a renewable and carbon-neutral process, but not necessarily a clean process. Advances in pollution-control technologies reduce emissions, but do not eliminate them.

One additional renewable energy-generation technology worth mentioning is hydro. In use for more than 100 years, hydro is the most widely used renewable energy-generation technology worldwide, accounting for 70% of the world's renewable energy generated, according to REN21 (the Renewable Energy Policy Network for the 21st Century). This technology works by harnessing the power of flowing water that rotates turbines coupled with an alternator that generates electricity. This is similar to the idea behind wind power, but with a more constant source of energy. Due to complexities, cost of deployment, and the need for a steady stream of water, this technology is not widely employed for connection to residential- and commercial-building energy systems in the U.S.

Policies and incentive programs

Government programs have had significant impact on the level of adoption and increased deployment of renewable energy technologies. Two of the most widely recognized programs are the IRS' Residential Renewable Energy Tax Credit and Business Energy Investment Tax Credit. Although originally scheduled to expire at the end of 2016, these programs were amended in December 2015 to extend through to 2019 (and beyond, in some instances). Both programs provide a tax credit based on the in-service date of the renewable energy system.

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