Using the sun to power data centers

Adding a large photovoltaic array to the roof of a data center raises its profile, and adds a reliable source of clean power.

By Jack Pouchet Emerson Network Power, St. Louis December 16, 2009

Emerson ‘s new 35,000-sq-ft corporate data center showcases innovative power, cooling, and monitoring technologies. Located at the company’s St. Louis headquarters, the data center consolidates its global network of more than 100 data centers worldwide into four facilities.
The facility’s energy-saving design minimizes building footprint and includes daylighting features. However, the energy-saving technology receiving the most attention is the solar photovoltaic (PV) array located on the facility’s roof. The 7,800-sq-ft PV array is among the largest rooftop PV arrays used by a data center—the largest in Missouri at the time of the facility’s opening. The array’s more than 550 solar panels provide up to 100 kW of power to support the data center’s IT load.


Several factors must be considered before equipping a data center with a PV array. Applicability of PV for data centers, type of PV, and impact on facility design are among the concerns that must be resolved upfront. Here are answers to some of the major questions that Emerson considered:

Is a PV array ideal for use with any data center? There are two main factors at play here. The first is the type of data center being considered. Tier levels and total power requirements play important roles in deciding if PV is right for the data center. Solar power is not available 24×7, and even during daylight hours, it may not be available at 100% of rated power. One simple rule of thumb is the lower the total site power requirement and the IT power density, the more likely that PV power will carry the load.

The second factor is the location of the data center. Certain states are ideal for PV arrays in terms of amount of sunshine, as well as the availability of utility and state rebates and incentives. For example, states such as New Jersey, Hawaii, Oregon, and California have attractive utility and state rebates and incentives.

Another location factor is the type of weather experienced in the area. For example, there are limitations to the amount of wind force that an array can handle. Since thunderstorms and tornados are possibilities in the St. Louis area, Emerson’s PV array was engineered to survive winds of up to 90 mph.

What type of PV array is best to use with a data center? There are two basic types of PV arrays: fixed and tracking. In a tracking array, the array panels are mounted on one or more pedestals so they can track the sun’s path across the sky.

PV array size and location are important factors when considering alternative power for data centers. Tilt, roof penetrations, and roof loading are added concerns if arrays are roof mounted.

Tracking arrays add to overall site footprint because each pedestal must be under a certain size so the shadow does not interfere with an adjacent pedestal’s array. They also can be more costly than a fixed-mount array. However, tracking arrays can offer higher overall efficiency, often reducing the total number of modules required. Tracking arrays increase the amount of delivered power over the course of the day, and they tend to be ground-mounted due to their weight. They require a clear, unobstructed line of sight to the sun’s path. They also have requirements for earthquake, tornado, hurricane, and lightning damages. Because tracking arrays have moving parts, they require regular maintenance and inspections.

Fixed-mount arrays do not follow the path of the sun. They can be placed on the ground, located on the facility roof, or integrated into architectural elements such as parking structures, walkways, and even window covers. In some cases, fixed-mount arrays can be adjusted seasonally for tilt, which is the angle at which they face the sun. Making two or four seasonal adjustments a year may add 5% to the array’s total kWh production. However, this is best done on lower-power arrays as the complexity, costs, and manpower required on arrays over 10 kW can negate the increase in energy production.

Emerson’s PV array is a fixed-mount array at an optimized year-round tilt and is integrated into the total building design.

How will the use of a PV array affect the data center design? A PV array also introduces a number of design considerations that might not be possible with every data center. When designing a data center that will use a PV array, it important to remember that the array must be properly positioned to receive the maximum amount of sunlight. This may mean a change in the original site plan to account for shadows from other buildings, trees, power/utility poles, or other aspects of the surrounding area. One also must take into consideration plans for the surrounding area to ensure no future obstructions will be placed in or around the solar array.

The array also should be placed in close proximity to the data center because its output is dc. In a typical installation, dc power is brought back to one or more central power inverters to produce ac. The shorter the combined dc power runs, the better the overall efficiency. Care must also be taken to ensure that the additional power produced by the array does not interfere with the facility’s critical power and emergency backup power systems.

It is also important to factor in the location and size of the array. Will the array be placed on the roof of the data center or on land next to the facility? On average, 3 to 5 acres per MW is needed for solar arrays.

While roof and building-mounted arrays maximize the total site footprint available to collect sunlight, with Tier III or Tier IV data centers, it is important to consider the potential for and consequences of roof penetrations and roof loadings associated with the PV array. Emerson’s data center uses a purpose-built structure to hold the array that spans the working data center floor area with no penetrations through or contact with the roof. This is an especially important design consideration in areas that receive a fair amount of rainfall. Environmental factors such as earthquakes, tornadoes, hurricanes, and lightning strikes can potentially affect decisions surrounding the use of PV arrays in data center designs.

Can a PV array power 100% of a data center? While data centers are good candidates for solar power, it may not be ideal for the array to power 100% of the data center. To power a data center completely, one would need a collector space about 8 to 10 times the size of the data center, which could become quite costly.

The chances are better for smaller data centers of 10 kW to 40 kW and of modest power densities. These types of sites are usually co-located with other office or manufacturing space, which increases the amount of roof space available for the PV array. In these cases, it may be possible to properly match the size of the array to provide all of the data center power during peak production daylight hours. However, it is highly recommended that data centers have a reliable 24×7 utility power source with a UPS.

Typically, dc power is directed to one or more central power inverters to produce ac. The combined dc power runs should be as short as possible to ensure overall efficiency.

A better approach is a mixed-use design with the PV array offsetting and supplementing utility power. For example, at peak output, the solar array is expected to supply about 16% of the Emerson data center power requirements.

Will using a PV array compromise data center availability? A properly designed, constructed, and commissioned PV array will not compromise the availability of a data center. In fact, it can enhance availability by providing a local, secure power source.

To ensure availability, reliability, and safety, it is critical to meet all applicable safety codes and follow proper grounding procedures for all associated equipment in the system. It is also important to provide transient voltage surge suppression to protect the system from lightning and other transients because the array and inverter introduce new conduction paths.

What is the typical return on investment associated with using a PV array? When using a PV array for data center power, the return on investment will vary based on local utility and state incentives, cost of equipment and installation, and the ability to benefit from the Federal ITC and accelerated depreciation schedules. The costs of PV panels are dropping in price, with tremendous excess manufacturing capacity in the market making it a favorable time to implement a solar PV system.

In certain areas of the country, the payback can take as long as 30 years, depending on the cost of power. Typically, the combination of incentives and offsets helps to accelerate the array payback, but this occurs over a long period of time. In current market conditions, Emerson expects it will take 20 years of operation to fully recover its investment on the solar array.

One business model that could accelerate the payback period is a power purchase agreement (PPA). Under the PPA, the data center works with a third party that supplies the solar PV system, handles the permitting information, and installs and maintains it. In return, the data center agrees to buy power from the third party at a set price for a set period of time. At end of the deal, the data center can renew the agreement or purchase the equipment. The advantage to this agreement is that there are no upfront costs for the data center. The disadvantage is that the data center could possibly pay more for electricity than a data center buying power on the market.

There are numerous other business models to consider in adding a solar component. It is important that all options are explored, including the potential for utility and/or public/private partnerships that could reduce upfront costs.

Are there specific certifications for installers of PV arrays? The North American Board of Certified Energy Practitioners (NABCEP) has created certifications and certificate programs geared toward renewable energy professionals throughout North America. It offers a certification program for solar electric installers. As of May 2009, 710 individuals have passed the NABCEP PV installer certification exam. There are also 97 NABCEP-certified solar thermal installers. The complete listing of NABCEP-certified solar PV and solar thermal installers can be viewed on the organization’s Web site ( ).

For more information on PV systems, read "Retrofitting PV and wind generation systems" by Scott Gray, LEED AP Advanced Engineering Consultants, Columbus, Ohio from the fall edition of Pure Power.

Author Information
Pouchet is director of energy initiatives for Emerson Network Power. He has more than 20 years of related experience, is an active member of the The Green Grid, and is frequently engaged with federal and state agencies in research and legislation centered on alternative and renewable energy.

The Database of State Incentives for Renewables and Efficiencies (DSIRE) is a good resource. Established in 1995 and funded by the U.S. Dept. of Energy, DSIRE ( ) is a comprehensive source of information on state, local, utility, and federal tax incentives; stimulus funding; rebates; and policies that promote renewable energy and energy efficiency. It is also a good idea to contact your local utility.


Emerson’s corporate data center in St. Louis took its first applications live in August 2009. It integrates technologies from Cisco, Dell, EMC, Sun, and Emerson Network Power.

Power usage can be scaled precisely to meet the facility’s power demands without sacrificing efficiency. The data center is equipped with three tiers of redundancy, dual utility feeds, redundant UPS protection, and on-site generators. Redundant dual-bus power capacity starts at 1,350 kW, and the UPS systems are scalable up to 4,050 kW across dual paths. Emerson anticipates the data center will have 99.982% uptime with an average of only 1.6 hours per year of unplanned downtime.

During the very early planning phase of the Emerson data center, its design team set its sights on LEED Silver Certification from the U.S. Green Building Council. Now that the data center is completed, Emerson expects it will have enough LEED points to put the facility into the Gold category because of the facility’s architectural design and efficient technology infrastructure. The company estimates that the data center will have as much as 31% in energy savings compared to a traditional enterprise data center. The PV array is part of the holistic design that contributes to anticipated achievement of LEED Gold certification.