Green Data Center Design and Build Strategies
- By Douglas Alger.
- Sample Chapter is provided courtesy of Cisco Press.
- Date: Aug 12, 2009.
Chapter DescriptionThis chapter discusses methods for limiting the environmental impact that occurs during the construction of a Data Center through decisions concerning physical location, choice of building materials, landscaping choices and jobsite construction practices.
Data Center Configuration
The physical configuration of your Data Center—where you place it in a building and how you arrange its physical infrastructure components—provides another opportunity to make the facility more efficient. Strategies to consider include the following:
- Situating your hosting space at the center of a building rather than right against an external building wall provides some isolation from outside temperatures, for instance, so your cooling system won't have to work as hard on hot days.
- Placing cooling infrastructure near heat-producing hardware, a practice known as close-coupled cooling. Compared to traditional Data Center designs, where large air handlers attempt to cool large sections of the hosting space, close-coupled cooling requires less fan energy to project cooling where it's needed and reduces unwanted opportunities for chilled air and server exhaust to mix. This approach and the inefficiency that comes with mixing Data Center airflows are covered in Chapter 5.
- Streamlining your structured cabling design by adopting a distributed physical hierarchy. A distributed design uses significantly fewer cabling materials and improves the cooling airflow. This design and a detailed look at the reduced length of cable runs it offers are presented in Chapter 6, "Cabling Your Way to a Greener Data Center."
The outside of your Data Center building will be subjected to a variety of weather and temperatures during its lifespan. So, in addition to the green characteristics you want for other building elements—durable and preferably made from renewable or recycled content—look for external building components that can mitigate those outdoor conditions.
For instance, you can lower the temperature of your building and reduce how hard your internal cooling system must work by using surfaces that have high solar reflectance and thermal emittance. That is, they efficiently reflect sunlight and shed absorbed heat.
Both solar reflectance and thermal emittance are typically expressed as either a percentage or as a value between 0 and 1. The higher the number, the less a material absorbs and retains heat. To qualify for an Energy Star label, for example, low-slope roofs must have an initial solar reflectance of at least 0.65 and after 3 years at least 0.50. Steep slope roofs must have an initial solar reflectance of at least 0.25 and after 3 years at least 0.15.
Roofs with high-radiative properties, often called cool roofs, make your building greener not only because they conserve energy, but also because they decrease heat islands. Heat islands, where urban areas have higher temperatures than nearby rural ones, can increase peak energy demand on an electrical grid, possibly leading to brownouts or blackouts, and contribute to the creation of smog.
Heat islands are caused by the reduced quantity of trees and foliage in developed areas, airflow restrictions created by tall buildings, and exhaust heat from motor vehicles and buildings. Many cities can see a temperature difference of as much as 10 degrees Fahrenheit (5.6 degrees Celsius) above adjoining rural areas, according to the U.S. Environmental Protection Agency.
A subset of cool roofs, also known as green roofs or living roofs, employs live vegetation atop conventional roofing. In addition to the temperature-reducing benefits of other cool roofs, green roofs reduce storm-water runoff, act as additional building insulation, and are credited with nearly doubling a roofing system's lifespan by shielding the surface from sun and rain. Green walls or living walls, which apply the same mechanism to a building's vertical surfaces, can also be employed, although are much less common.
Whether applied to a roof or wall, a living surface requires careful engineering. Simply allowing ivy to grow up the side of your building does not equate to a green wall. A proper installation involves a protective membrane to prevent either moisture or plant roots from penetrating to the building, a drainage system to keep foliage from being flooded by pooled water, a soil layer to anchor plants and absorb nutrients and, of course, the vegetation itself—typically plants that are fast growing, drought tolerant, and low maintenance. The entire system needs to be lightweight so as to not pose structural problems for the roof.
External building surfaces are also, obviously, prime locations to install photovoltaic cells—devices that convert solar energy into electricity. These can include solar panels mounted on rooftops or walls or even building integrated photovoltaics, in which components are embedded within the envelope of the building. Building integrated photovoltaic systems can take the form of roofing tiles, spandrel panels (opaque glass used between floors in commercial building facades), awnings, skylights, sunshades, walls, and more.
Photovoltaics today typically generate 5 watts to 15 watts per square foot (50 watts to 150 watts per square meter) when in full sunlight. You, therefore, need from 65 square feet to 200 square feet (6 square meters to 18.6 square meters) of photovoltaics to produce one kilowatt of power.
Exactly how much energy can be harvested by a solar array varies by product, because some are more efficient than others, and by environmental conditions, including the following:
- Latitude: Various parts of the world receive more or less sun exposure than others, which affects how much solar energy can be collected.
- Climate: Overcast or stormy weather reduces the amount of sun that a photovoltaic system is exposed to. Nearby snowy surfaces can actually boost performance by reflecting more light onto a solar array, but only if the array itself isn't covered with snow.
- Orientation: Photovoltaic components should be installed to receive maximum exposure to the sun. Avoid obstructions to the system such as trees or other structures especially during peak collection hours, when the sun appears highest in the sky.
- External air quality: The more contaminants in the air, the less solar energy that reaches a solar array.
If you install a photovoltaic system and employ a cool roof on your building, clean their surfaces frequently. Any dirt or debris that covers them reduces their efficiency, reducing how much energy you collect.
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