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.


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.

Figure 1: BIPV uses PV arrays installed as an element of a building such as the Exelon Pavilions in Chicago. Courtesy: Environmental Systems Design

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” at the end of this article).

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.

Figure 2: Chicago's Exelon Pavilions incorporated PV modules into their four-wall curtain wall systems. Courtesy: Environmental Systems Design

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.

Generating green power with wind turbines

There is an enormous amount of energy available from the wind. It has been estimated that wind energy resources are greater than five times the world’s energy usage. Electric wind turbines extract the kinetic energy of the available wind resources and convert it to electrical energy. The energy extracted by a wind turbine can be determined by the following equation:

Wind Energy = ½(ρ A v3 t)

where: ρ = density of the air; A = swept area of the turbine rotor; v3 = wind velocity, and t = time

From the above relationship, it is apparent that the factors having the greatest influence on the turbine energy output are rotor swept area and wind velocity. Wind profiles show that wind velocity increases logarithmically with height above the surface. Therefore, a turbine located 196 ft high will be exposed to a much higher velocity than the same turbine located 33 ft high. Turbine swept area is a function of the rotor diameter. So, turbines with large rotors located high above the ground will capture a greater portion of the available wind energy compared to small wind turbines located nearer to the ground.

Figure 3: A black covering was applied over silver bus bars that connect the PV cells and reflective glass replaced the standard matte glass to create the black minimalist appearance of the curtain wall at the Exelon Pavilions. Courtesy: Environmental Systems Design

Another factor influencing wind turbine performance is the roughness of the surrounding area. Turbine rotors are most efficient when subjected to a smooth laminar wind profile, as surface roughness affects the wind profile. For example, large open bodies of water have a very low roughness length, typically 0.008 in. Flat terrain with low vegetation has a roughness length around 1.18 in. Cities and areas around buildings have chaotic wind profiles with a roughness length of around 6.56 ft.

In consideration of the aforementioned points, it does not make sense to install large turbines near building sites. The turbulence caused by the building itself will cause the turbine to operate at less than optimal capacity. The significant investment required for a large turbine installation causes turbines to be installed away from cities and buildings in remote areas with minimal ground obstructions. Small turbines with small rotors operate with the same constraints as large turbines. However, because the scale is changed, small turbines can be installed on or near buildings more economically.

Other turbine designs are available that tolerate the turbulent wind pattern found on and around buildings (see “The Margot and Harold Schiff residences, Chicago”). Vertical axis wind turbines are available in sizes from less than 1 kW to more than 50 kW. Designs vary, but many small wind turbines contain dc generators connected to an inverter to convert the generator dc voltage to ac voltage compatible with the electrical distribution system similar to a PV inverter.

Selling green power

Per the Public Utilities Regulatory Act of 1978, utilities must buy back power from qualified facilities at an avoided rate. A qualified facility as determined by the Federal Energy Regulating Commission can be either a small power production facility of 80 MW or less, which uses a renewable energy source such as hydro, wind, solar or biomass, waste, or geothermal sources, or a cogeneration facility that produces useful thermal energy in addition to electricity.

Figure 4: A wind turbine system sprawling the length of the roof is designed to supplement the electrical needs of the Margot and Harold Schiff residences building in Chicago. Courtesy: Environmental Systems Design

An avoided rate or cost is the price that the utility would pay for energy if not provided by the qualified facility. This rate is typically much less than what a utility customer pays, as it does not include the added costs associated with the transmission and distribution of power. In order for the utility to purchase power from the customer at a different rate than it sells it, two customer meters are generally required.

For very small production facilities (e.g., renewable producers of 2,000 kW or less for Commonwealth Edison Co.), some utilities will allow net metering where the utility company will credit the customer for power that it provides to the utility at the same rate as is purchased by the customer. In this case, a single meter at the customer’s location runs in both directions depending on whether power is being provided by the utility to the customer or vice versa.

To protect the utility power distribution infrastructure and avoid disturbances to nearby customers, the interconnection between the utility and the customer’s power production facility must include monitoring and control equipment. This will ensure that the power produced by the facility is compatible with the utility and disconnects from the utility when the locally generated power is out of tolerance or in an emergency condition, such as a system fault or utility outage in the surrounding area.

Industry standard interconnection criteria are published by IEEE in Standard 1547, but utilities generally have their own standards, especially for interfacing with larger production facilities. The extent and complexity of the interconnection protection system depends on the size of the power production facility relative to the capacity of the utility infrastructure at the point of connection.

Special protective equipment may not be required for a small power production facility as long as the facility is provided with integral protection compliant with industry standards. For example, in the Midwest region, ComEd requires no special external protective devices for inverters up to 50 kVA as long as they are installed per IEEE 929 standards, are listed under UL 1741, and are utility interactive (non-islanding) with no adjustable setpoints.

At the other extreme, large systems require extensive and sophisticated protection equipment at the utility interconnection. Protective monitoring and control includes protection from over/under voltage, over/under frequency, de-synchronization and, in some cases, control from remote equipment in the utility substation.

Using RECs

RECs represent the nonpower aspects of renewable energy and are the means by which the environmental attributes of power generation are certified and traded. When renewable energy is delivered to the grid, there is no means by which to track it and distinguish it from the energy provided by other sources. RECs are used to establish a one-to-one correlation between a quantity of energy produced by a renewable source and a quantity of energy used elsewhere. The used energy is not necessarily the same energy that was generated renewably, but the RECs certify that the energy produced and delivered to the grid was done with renewable inputs and therefore allow transfer of their environmental attributes.

Figure 5: The wind turbine contains helix-contoured vanes. There is always a surface facing the wind. Courtesy: Environmental Systems Design

Information associated with the REC includes the type of renewable source of electricity, date generated, location and age of generator, and greenhouse gas emissions associated with the generator. End users may purchase RECs with the specific attributes they require (e.g., locally produced solar energy).

RECs are created at the point of generation in denominations of MWh of renewable energy. Power producers are paid for RECs in addition to the money they receive for the energy itself. The RECs are then bought and sold on the open market and can be traded multiple times before being bought by the ultimate owner. A REC can be purchased for use remote from the location of the renewable generator. RECs are also independent of time, which relieves the problem of scheduling delivery on the transmission grid. There is even a futures market for some RECs. They can be sold bundled with energy or sold independently of energy. After the ultimate buyer claims credit for the use of the renewable energy, the REC is permanently retired.

RECs are important tools used by utilities to establish that they meet renewable portfolio standards (RPSs) mandated by many states. RPSs are state mandates that require electric companies to have specific portions of the energy assume specific green attributes. For example, Illinois requires that by 2015, 10% of all electric energy sold in the state be renewable with at least 7.5% wind generated and 0.6% solar.

To avoid fraud, REC verification is very important. RECs are verified by two methods: an REC contract with associated audit trail or an REC tracking system. A tracking system is an electronic database typically organized to cover a geographical region and often associated with the regional transmission organization (RTO) or independent system operator (ISO) in that region. The RTO or ISO is independent of the generator owners, controlling the flow of electrical energy in and around the region, conducting real-time and day-ahead wholesale markets.

There are two markets for RECs: the compliance market and the voluntary market. Electric companies use the compliance market to obtain RECs to satisfy their RPS requirements. RECs in this market are generally traded in large quantities indicative of the size of the participants. The voluntary market is used by household or corporate energy users that are interested in buying renewable or green energy but are not mandated to do so. The scale of purchases in the voluntary market is smaller than the compliance market, and the RECs are generally less expensive.

Renewable energy is finally a reality. As the building industry creates more high-performance facilities and green power technology improves, renewable energy will become more efficient and economical, with an improved return on investment.

Tomorrow’s average building owner will find renewable energy sources within immediate reach. Along with this will come the buying and selling of green power, purchased and sold in these ways and others that we can’t even imagine today.

Eich is a vice president and electrical engineer with Environmental Systems Design. He has more than 20 years of experience in the nuclear power and commercial building industries and has designed renewable energy systems in the U.S. and internationally. Toporek is a senior vice president at Environmental Systems Design. He has more than 30 years of experience in the commercial building industry developing electrical systems for a wide variety of building types including high-rise buildings, hospitals, data centers, convention centers, hotels, museums, and cogeneration facilities.

Exelon Pavilions, Chicago

The Exelon Pavilions in Chicago’s Millennium Park are outfitted with BIPV modules on all four exterior walls (see Figures 2 and 3). Built in November 2004 as a demonstration project funded by the Exelon Company, the buildings cost $7 million and earned LEED-NC Silver. The PV arrays are among the first curtain wall installations in the U.S.

Designed to offset 10% of each tower’s annual electricity use, each building typically uses all the energy generated from its BIPV system. In the event that it generates ex-cess power, it is sold back to the utility.

The optimal angle for the BIPV modules considering Chicago’s climate and latitude is 41 deg, facing south. However, because the BIPVs are in a fixed curtain wall system, they are actually tilted at 90 deg in every direction, significantly reducing the system’s potential energy generation by 50%, from 48 kW to just 28.9 kW.

Performance data from 2008 reveals an annual average electricity use offset of 4%, ranging from 1% in December (highest energy use) to as high as 12% in April (lowest energy use). Using a rate of $0.084/kWh, the BIPV system saved approximately $1,800 in energy costs for each building between 2004 and 2008.

While this demonstration project proves that BIPVs are possible, a simple calculation reveals a return on investment period that greatly exceeds the life expectancy of the modules.

The Margot and Harold Schiff residences, Chicago

The 47,000-sq-ft Margot and Harold Schiff residences by Mercy Home in Chicago’s Near North neighborhood hosts 96 units, rented at different market rates including the subsidized federal Section 8 program and single-room subsidized occupancy for the homeless. It is the first-known installation of battery-free wind power generators in the world.

Outfitted with a curved, highly reflective skin made of steel and glass, the building features a demonstration wind turbine system sprawling the length of its roof and de-signed to minimally supplement the building’s electrical needs (see Figure 4). Locally made, the wind turbine contains helix-contoured Plexiglas vanes so there is always a surface facing the wind, allowing the turbine to function in turbulent wind conditions (see Figure 5).

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