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Batteries Included: A Leaner and Greener Internet using Smart Batteries

The electric grid experiences the same daily peak demand issues as our freeways and the Internet with everyone wanting power at the same time during the middle of the day.  This makes providing power more expensive because extra power plants have to be built to meet this peak demand.  Servers that provide the world's Internet content are also their busiest during this peak power period.  And with the Internet consuming 2% of the world's energy and predicted to surpass the airline industry by 2020, the problem is only getting worse.

Furthermore companies that have large IT deployments are being challenged by rapid expansion and rising energy costs.  For companies like Akamai that host their IT infrastructure in third-party collocation data centers energy is typically priced based on the total supplied power in kilowatts (KW) charged at a fixed rate in $/KW for example 50KW at $200/KW per month, similar to a fixed number of minutes for a mobile phone plan.  While the supplied power is fixed, the power drawn by the servers vary with server activity which peaks and lulls daily as shown below.  Energy costs could be reduced if the server peak-power demand, and hence the supplied power, can be lowered.


Figure 1.  Daily variation of Internet traffic.  Source: www.akamai.com

One of the innovative ways Akamai is looking at mitigating this challenge and reducing network operational costs is to reduce the grid-based power supplied to its servers during peak demand by supplementing with batteries and recharging the batteries at night when most energy consumers are asleep and power production is cheap and plentiful.  A recently published research paper by Ramesh Sitaraman, Akamai Fellow and UMass professor, written in collaboration with researchers at Penn State and BBN, evaluates using smart batteries in an Internet-scale Distributed Network such as the Akamai Network. The smart batteries that are placed inside a server or within a server rack detect when a server's power draw crosses a threshold level, as it becomes busier, and then supply battery power until the server power draw drops below the threshold.  The batteries would recharge at night or whenever energy and server demand are low, as shown in Figure 2 below. 

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Figure 2.  Diagram depicting battery supplement of server power.

The effect would be that the peak power draw from the electricity grid of a cluster of servers would flatten and lessen, with peak demand shifting from the electricity grid to the batteries, as depicted in Figure 3 below using production server load data from Akamai's network. 

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Figure 3.  Variation in aggregate server power demand over a month in an Akamai New York data center. With smart batteries the required power supply drops from 432 KW to 381 KW as shown.  Data source: Akamai Technologies, Inc.

Despite the control intelligence of these batteries the chemistry is conventional lead-acid, similar to a deep-cycle marine battery.  Lead-acid batteries are fully recyclable.

The capital cost of the battery is a function of the energy storage capacity of the battery in kilowatt-hours (kWh) which determines how long the battery can charge the server.  This study uses reasonable estimates ranging from $100-300/kWh and a lifespan ranging from three to five years.  The graph below in Figure 4 shows that the batteries don't have to supply power for very long to achieve a compelling peak power reduction benefit.  Even a battery that can power a server for only 5 minutes, comparable to those batteries used in uninterruptable power supply (UPS) systems today, can provide a 7% power savings, while a larger 40-minute battery supply can save up to 14%.  Furthermore, most of the power savings are achievable with a small cycle rate of one full discharge/recharge cycle every three days which is conducive to a 5-year battery life.  Figure 4 also shows how power savings improve demonstrably as servers get better at power management.  The power proportionality factor (PPF) is the ratio of the difference between server peak and idle power and a server's peak power.  A PPF of 1.0 means the server power draw at idle is zero, which is ideal.

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Figure 4.  Percentage power savings as a function of battery size (power supply in minutes) and server power management capabilities (PPF).

While the reduction in peak power draw may not look significant the aggregate effect is quite substantial, up to 14% power savings based on today's server technology, increasing to 22% with next-generation servers that have improved power management capabilities where a server draws very little power when idle, and an upper bound of 35% power savings for maximized server power management.  The costs savings inclusive of the battery costs range from 13-34%. 

In the near term cost savings are derived from a reduction in the power supplied to each server cluster - power circuit size.  Using smart batteries to reduce peak power loads would reduce the required power circuit size and, hence, the cost of each circuit.  An alternate but equivalent way of viewing the benefit is that batteries allow more servers to be powered using the same power supply than before. A key assumption is that the batteries could be fit into an existing server rack without taking up additional data center floor space which costs additional.

Longer term as the smart grid matures and electric utilities implement time-of-day energy pricing, electricity prices will be more expensive during the day when demand is high and cheaper at night.  Batteries make even more sense with time-of-day pricing since energy can be stored during the cheap off-peak hours and discharged to servers during the expensive peak hours.  In addition, large-scale adoption of this power demand-shifting technique would increase demand for wind energy which typically has higher production capacity at night, and help slow the need to build power plants to meet peak energy demand.

Dr. Sitaraman emphasized "Our research helps establish batteries as a key part of the architecture of Internet-scale distributed networks. In addition to providing a distributed UPS function, batteries can also provide a cost reduction by decreasing the required power supply. Further, as servers become more energy-efficient and as batteries become cheaper and better, the case for batteries will only become more compelling."

Nicola Peill-Moelter is Akamai's Director of Environmental Sustainability