Much of the focus on data centre energy efficiency is on how the energy is used within the data centre. Yet this may not be the best way to look at creating a sustainable,
In this tip, we’ll consider the realities of energy generation and delivery and examine the impact on data centres.
The energy efficiency chain
Energy generation is subject to efficiency losses along every step of the process. The burning of fuel, the heat losses from that combustion, the efficiency of converting heat into electricity, losses in electrical transmission and distribution, as well as losses in voltage transformation and rectification from AC to DC all introduce losses. In actual practice, around 60% of the original energy produced is being lost before any IT equipment (or, in fact, any business or consumer equipment) is plugged in. The Digest of United Kingdom Energy Statistics (DUKES) comes out with an overall energy efficiency of 38.5%.
Therefore, with only 40% of the available energy being used in the data centre facility and its equipment, improving data centre energy efficiency by 10% will only have a 4% overall efficiency improvement on a total energy basis. On the other hand, improving the energy efficiency based on usage from the point of generation by 10% will result in efficiency gains throughout the whole chain, and 6% at the data centre itself – or actual gains of 50% above what would have been gained purely by messing around at the data centre.
Many new-generation plants use heat recovery as part of their operations, which does lead to higher overall thermal efficiencies, but this heat isn’t utilised in any significant manner. Some main generation plants capture the heat for use in complementary endeavours such as agricultural environments. Some power plants are even capturing the carbon dioxide (CO2) by-product and using this in complementary tasks like tomato production as a means of growing tomatoes more rapidly.
However, not all main generating plant are suitably positioned next to a suitable agricultural outlet, or even near to large conurbations of human dwellings where excess heat can be used for space and water heating. And, although generating thermal efficiency is driven higher, the energy usage efficiencies still remain low because heat and transmission losses continue to be high.
Even though improvements have been made in energy transmission, it is impossible to overcome the basic laws of physics. The longer a wire is, the more energy has to be used to overcome the resistance of the wire. The power loss equation [P (loss) = P²R/V²] shows that to minimise the energy loss, the voltage (V) needs to be as high as possible, while the resistance of the cable (R) needs to be as low as possible. However, if the voltage is pushed too high, electrical arcing can occur, so wires have to be higher off the ground and spaced further apart. To maintain tensile strength of high tension cables, it is not possible to use extremely low resistance metals (these tend to be more ductile, and, as such, will stretch and finally break). Therefore, a careful balance has to be made between the types of cable used and the voltages the energy is transmitted at.
In general, energy is created at a power station and is transformed into high-voltage alternating current (AC) -- generally above 100 kV -- for transmission. At a substation, it is transformed to a “customer” voltage, which may still be at the kV level or may be at the more normal levels used in homes and data centres of 110 V or 220 V. Once this is delivered to the data centre, it then must be transformed to the various DC voltages used by the computing equipment within the data centre itself -- for example, 3 V, 5 V and 12 V. Each step introduces losses.
Data centre energy efficiency and local energy production
Many data centres now own supplemental power-generation equipment such as diesel generators along with a growing mix of solar, wind and biomass cogeneration technologies. The same limitations that impact a commercial utility provider also affect onsite cogeneration, so although there isn’t much you can do to boost the efficiency of a regional utility provider or transmission infrastructure, there are clear strategies that can increase the efficiency of your own power-generating equipment.
The two main areas that data centre managers should consider for data centre energy efficiency are thermal efficiencies and transmission losses. Bringing the point of energy generation closer to the point of use (such as a local generator) will reduce losses. Also, you can ensure that as much thermal energy is captured from the generation process as possible. This is not just looking at electrical energy but also at the heat generated through fuel burning.
One way forward is to look at community combined heat and power (CCHP). The generating plant produces both electrical power and heat. The owner of the plant (for this article, a data centre owner), takes the electrical power it needs as the base load from the generating plant. Any excess electrical energy can be sold off to other local businesses or homes for their use.
As the data centre will then create heat through the thermal losses of electrical energy usage, this needs to be captured and used for space heating or (through the use of heat pumps) for water heating elsewhere in the business. The heat created through burning fuel in the generating plant is captured and sold to local businesses and homes, helping to offset the cost of the generating plant itself.
Tips and tactics for data centre energy efficiency
The key to generating power efficiently is to size things correctly. By building smaller plants, their overall energy efficiencies can be above 80%, which is double the energy efficiency for a basic large generating plant. Equipment costs can be more controlled. High voltage transformation is not required, because the electrical energy does not need to be transmitted over large distances. The generator must cover the base load of the company concerned (not just the data centre’s needs, but the organisation’s total needs across its associated campus), and consideration should also be given to the future power needs of the business (an often overlooked part of capacity planning).
In many cases, such excess energy can be sold back to the grid at good rates. The UK has a good feed-in tariff covering such a situation. By careful planning, the excess load calculation gives sufficient pay-back from other businesses, homes and feed-in tariffs such that the electrical energy created and used will be at least as cheap, if not cheaper, than electricity taken direct from the grid.
However, a fully available system based on system redundancy may not be financially viable. In that case, the system must have the capability to fail over to the grid should the local system fail.
CCHP can be far more sustainable than a national grid-based energy system. For instance, the University of Warwick introduced a combined heat and power (CHP) system for its campus in 2003, without any “community” aspect (i.e. no sale of excess heat or power to other interested parties. Even so, the payback was calculated at 5.5 years, based on stable 2003 energy prices). Carbon savings were calculated at 3,500 tonnes per annum. Although this is not a data centre example, it demonstrates how CHP is worth considering. And with feed-in tariffs and excess energy resale, ROI would be far faster.
Although headline figures can make it appear that electricity provided can be expensive -- and so counter to the current drive for reduced costs -- careful planning of how excess energy is used can make this a very cost-effective approach to achieve further data centre energy efficiency.
Clive Longbottom is a service director at UK analyst Quocirca Ltd. and a contributor toSearchVirtualDataCentre.co.UK.
This was first published in October 2011