Harnessing the renewable powers that be: Understanding the pros and cons of generating and selling green power enables engineers to make better renewable energy choices.

Consulting-Specifying Engineer
Dec 15, 2011

The green building industry, concern over global warming, and the rising cost of fossil fuels have driven recent interest in renewable energy, making relevant products and systems more readily available and easier to use. Unlike finite energy sources like fossil fuels, renewable sources like solar, wind, biogas, biomass, and geothermal are for the most part inexhaustible, provided they are used at the rate at which they are naturally replenished.

As the renewable energy industry has grown, a new submarket has developed: the buying and selling of green power. Green power can be purchased and sold in different ways. Depending on the state, local electric utilities may offer green power options directly from its grid. Some areas even allow the buyer to choose from competitive suppliers. Renewable energy certificates (RECs), which represent a unit of renewable energy equal to 1 MWh, may be an option as well.

Sources of renewable energy currently being used in building construction projects include solar thermal, solar photovoltaic (PV), wind, biogas, biomass, and geothermal. Solar PV and wind turbines will be considered in this article.

Generating green power with PV

Energy from the sun strikes the earth’s surface with an average power density of about 1,000 W/sq meter. PV modules convert the sun’s energy directly to electrical energy through the photoelectric effect. When photons strike materials like silicon, they are absorbed, causing the material to release electrons. Conducting material collects the electrons, resulting in a small electric current, which is magnified to a more usable level by connecting the solar cells in a series-parallel arrangement. The highest performing commercially available PV modules convert the sun’s energy to electricity with an efficiency of around 20%. The most common PV systems are the ground array and the building integrated PV (BIPV).

A ground array is exactly what it sounds like: an array of PV modules installed on a structure independent from a building. For construction projects with available real estate, PV arrays can be installed on the ground, any flat surface, or a hillside, and even elevated to provide shade as in the case of a parking lot carport.

BIPV uses arrays installed as an element of a building (see Figure 1). BIPV arrays can be incorporated into building rooftops and walls or installed as shading devices like window awnings. Glass surfaces can also serve as BIPV sources through the use of PV glass (see “Exelon Pavilions, Chicago”).

Commercially available PV modules include mono-crystalline silicon, poly-crystalline silicon, and amorphous (thin film) technology with conversion efficiencies of approximately 6% to 19% with a power density as high as approximately 13 W/sq ft. Mono-crystalline cells are black and are usually more efficient than blue-tinted poly-crystalline cells. Costs for both PV module types of typical efficiency are around $4/W, not including installation or incentives.

The dc produced by PV modules is converted to ac by an inverter. Inverters range in size from small models capable of serving one PV module up to large 3-phase units with ratings higher than 800 kVA. The inverter converts the dc input power to ac matching the voltage, phase, and frequency of the electrical distribution system to which it is connected. The inverter also incorporates the system controls and safety features required to shut down the PV system in the event of a grid failure or building distribution system failure. Inverters used in the U.S. are designed and tested to the requirements of UL Standard 1741 and IEEE Standard 929. PV system dc voltages are limited to 600 V in the U.S. while voltages up to 1,000 V are common outside the U.S.

Power generated by the PV system is generally consumed by the facility to which it is connected. However, instances can arise where power generated by the PV system cannot be used by the facility. In this case, power is typically allowed to flow from the PV system to the utility’s electrical grid. For this reason, net metering is required, which has the ability to measure and record power flow in either direction (incoming or outgoing).

Local regulations dictate pricing for the power purchased and sold. Pricing varies widely between countries, regions, and states, greatly affecting the economics of PV systems.
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