A Power Shave and a Cost Cut: New Hybrid System Aims to Save Data Centers on Energy Usage

Jun 26, 2017  |  Energy

On the topic of energy efficiency in large data centers, it actually may be possible to have the best of both worlds … at least when it comes to methods used to help them respond to energy demand fluctuations.

That’s because a group of Ohio State University researchers have designed a new cost-efficient energy storage solution, which they estimate could save data centers 26 percent more than traditional technologies used in the “power-shaving” process. Power shaving works by adjusting a data center’s power demand through back-up energy storage devices. Previous research has shown it can greatly reduce energy costs for data centers by limiting the drain on power and cooling infrastructures.

In spite of lowering the energy bill, the two methods for using energy storage devices for power shaving – uninterruptible power supply (UPS) system batteries and supercapacitors – can still bring additional costs to data centers that reduce overall savings.

Using UPS batteries to adjust the output of data centers can cause the batteries to burn out and reduce their lifetime, as they are designed for power emergency situations only. Despite this, these batteries are reliable and widely used.

Supercapacitors are high-powered capacitors that have a lower internal resistance and higher charging cycling capabilities than uninterruptible power supply batteries. However, supercapacitors cost more than UPS batteries and have a higher rate of losing stored electrical charge.

To tackle this problem, the researchers designed HyStore, which aims to take the best parts of each method and combine them into one energy storage system. It integrates a hybrid supercapacitor bank with UPS batteries to protect battery lifetime and reduce the energy loss caused by a supercapacitor’s self-discharging.

In a data center deploying the HyStore system, the power grid transfers energy through power distribution units (PDUs) that route to server racks. The server racks then run the energy through server power supply units (PSUs) to internal components such as processors and the hybrid energy storage. When the server power exceeds its capacity or the power of one rack exceeds its limit, the hybrid energy storage discharges to reduce the power loads on the PSU and PDU. When both the server power and rack power stay below their respective capacities, the remaining budget can be utilized to charge the hybrid energy storage.

Cost Comparison between power shaving methods using either battery or supercapacitors and HyStore

 

In testing, the HyStore prototype achieved 26 percent more savings on total cost of ownership than previous power shaving solutions that employ just one type of energy storage device.

For a 10-megawatt data center over a 10-year period, the researchers estimate that the savings can be converted to $3 million in total cost of ownership, through lower energy bills and allowing the data center to deploy more servers without needing to upgrade its power infrastructure. Beyond this, while there are factors that can impact savings, the researchers expect those savings to scale in proportion to data center power demand.

The team continues to modify the model and scale high power demand testing scenarios. In addition to testing, the researchers are working to coordinate HyStore with dynamic voltage and frequency scaling (DVFS) for power capping, which would lead to additional power savings.

Power capping would allow the HyStore system to enforce power limits when the stored energy is inadequate or unavailable. Incorporating DVFS allows the system to use different algorithms to cap data center energy usage in both voltage and frequency.

“With HyStore, power shaving helps reshape the power demand curve so that data centers can host more servers within the capacity of their given power infrastructure,” said Wenli Zheng, a researcher from Ohio State University. “When we coordinate HyStore with DVFS for power capping, we not only reshape the power demand curve, but also rigorously enforce the power limit at runtime to guarantee safety, especially when the stored energy is unavailable.”

The proposed HyStore design with Power Capping

 

When finalized, the HyStore storage system could have a significant impact on companies in the cloud computing industry, especially for IT companies running their own data centers.

Synergy Research Group forecasts the number of large data centers is to grow from 300 to 400 in two years. With some data centers already consuming over 150 megawatts of power1 and the prospect of continued growth, this power shaving system could have an enormous effect on the future of data center savings.

Learn more about power shaving in IEEE Xplore.

1 “Most Power Consumed (MW).” World’s Top Data Centers, World’s Top Data Centers, worldstopdatacenters.com/power/. Accessed 12 June 2017.
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