Renewable Energy for Existing Buildings

Oct. 1, 2009

Until recently, the green building movement focused on the construction of new buildings; however, 75 to 85 percent of all buildings that exist today in urban areas will still be around in 2030. Thus, the more complicated task at hand is greening our existing building stock. Once all efforts have been made to optimize the energy efficiency of an existing building, the renewable energy technologies described here are some of the easiest to implement.

By mimicking a plant’s transformation of sunlight into energy, photovoltaic (PV) systems can be used to produce electrical energy for a building. The electricity produced is stored in batteries, or solar panels can be connected through a meter that will feed excess electricity back into the power company’s grid, “reversing” the meter reading and shrinking the utility bill. PV modules or panels range in size from 10 to 300 watts, depending on the size and number of cells. When considering the space needed for a PV system, the rule of thumb for a 12-percent efficient system is 100 square feet per kW generated.

Solar parking pavilions are another way that PV systems are being integrated into existing buildings, especially for suburban properties with large parking lots. A solar racking system for the panels creates a canopy, covering parking spaces that, in turn, can be rented for a premium.

Newer and more cost-efficient technologies, such as thin films that work by depositing extremely thin layers of semiconductor material on a support structure, require less light-absorbing material. Thin films have made it possible to see PV systems integrated into building materials, such as roof shingles. Another form – laminate roll-type thin film PV – weighs just 0.7 pounds per square foot and can be attached to a smooth roof using adhesive, keeping the building envelope intact. Thin films maintain their power conversion efficiencies better than panels during hot midday temperatures.

While PV is estimated to cost 1 percent of what it did 40 years ago, it still runs about 25 cents per kilowatt-hour, or about two to four times what most consumers pay for electricity. Maintaining PV systems by keeping them free of dust build-up and debris will improve efficiency and shorten the length of payback periods.

Solar water heating and solar tracking skylights are other ways of using solar energy to reduce energy use. For example, a warehouse can greatly reduce daytime lighting requirements and the size of roof penetrations with the help of solar tracking skylights that focus sunlight to the interior, no matter the position of the sun.

A second major source of renewable energy is wind. If open land is available, a small-scale wind turbine can be set up. The disadvantage of a small-scale wind turbine is that the level of wind power exponentially decreases as the rotor size and the distance-to-ground is reduced. Due to equipment expense, wind (like solar) does not provide a quick payback, with the average ROI being 15 years.

While data for almost every region is available for solar irradiance, wind data is more site and height specific; therefore, an extensive wind study in each potential location should be completed before purchasing turbines to get a clear picture of the potential power generation.

If a building is already conditioned by a geothermal heat pump system, it may be possible to use the area below the structure as a heat exchanger to condition the indoor air, taking advantage of the ground’s constant temperature. Wells are dug at a depth that will depend on the heat-exchange rate. According to the U.S. Environmental Protection Agency (EPA), geothermal heat pumps (GHPs) can consume 44 percent less energy than air-source heat pumps, and up to 72 percent less than electric-resistance heating with standard air-conditioning equipment.

“Make the most of what you have” is an important adage for a building owner or manager when he/she is first setting out to tackle energy management in existing buildings. While renewable energy generation is flashy and exciting, existing building commissioning or energy audits, which involve the implementation of no- and low-cost adjustments to building systems and operations, can reduce energy use by 10 to 40 percent, in addition to providing a much faster ROI. Maintaining proper system function, efficient equipment, and necessary staff will result in cost-effective implementation of renewable energy technology.

Jodi Greene is an engineer at AMEC Earth & Environmental Inc. in Lisle, IL.

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