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In Chapter 3 we considered the Green metrics that allow the energy efficiency to be measured and we also saw that several competing schemes are coming to the market. We have also considered the resilience models and also the market dynamics and likely build costs. Green data centres are a market sector that is becoming a mainstream expectation within all new data centre projects and some companies are starting to claim completely carbon neutral operations. We can consider carbon neutrality from three viewpoints:

• Running the data centre as efficiently as possible • Using non-fossil fuel energy sources

• Entering into a carbon offset scheme

The easiest way to reduce energy consumption is to design and run the data centre as efficiently as possible. This may seem self evident but there are two drawbacks;

• Maximum efficiency does not equate to highest reliability. Redundant power and HVAC systems will not usually be running at peak efficiency because they are often very lightly loaded

• Nearly every energy saving device means a higher capital cost and the cost versus return of many products currently available in the market needs careful consideration

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Figure 17: Typical energy overheads in a data centre The following is a list of energy saving ideas

Low cost

1. Organise the racks in a hot aisle/cold aisle format

2. Use blanking plates in racks to prevent hot and cold air mixing

3. Use brush strips and grommets to prevent cold air following cable paths

4. Ensure raised floor areas are sealed and not leaking away expensive chilled air 5. Run temperatures slightly hotter and with wider humidity bands (ASHRAE 200820)

Medium cost

6. Ensure the building is insulated to at least Part L Building regulations to keep the warm air out and the chilled air in and minimises solar thermal gain

7. Use virtualised, high performance servers to lower IT power overheads 8. Use enclosed cold or hot aisle racking systems

9. Use higher efficiency UPS (Uninterruptible Power Supplies) such as transformerless UPS and load them to their 40-80% optimum efficiency band

10. Load all three phase power systems to within 5% balance to minimise neutral conductor heating and harmonics

11. Buy IT equipment with high efficiency power supplies and with high power factors 12. Use power factor correction and voltage conditioning equipment for the whole site 13. Use simple airside economisers where possible to lower HVAC costs

High cost

14. Instead of a battery-backed UPS use a rotary kinetic energy storage system 15. Use hot air return plenums

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16. Use centralised chilled water systems rather than distributed DX HVAC units 17. Use more sophisticated air and water economisers, e.g. cooling towers 18. Use ‘Dry-cooler’ HVAC heat exchange equipment

19. Large scale heat exchangers

The ideas mentioned in ‘Low’ and ‘Medium’ costs are so cost effective that it would be foolish not to incorporate them into data centres. However one cannot presume that they all are currently used, especially in old data centres, due to lack of knowledge of these processes by data centres managers and builders.

Two ideas from the medium cost section that we would like to highlight are: enclosed cold aisle racking and air economisers.

Enclosed cold aisle

The traditional method of rack layout in a computer room is hot aisle/cold aisle. This is considered very efficient as air is delivered to the front air intakes of the IT equipment via grids in the cold aisle and removed from the rear of the equipment in the hot aisles. Cold air and the hot return air are thus prevented from mixing to a large extent which would be a great source of inefficiency.

The enclosed cold aisle idea takes this one step further by putting a roof over the cold aisle and doors at each end to totally enclose it. The cold air thus delivered in to the enclosed area, via the floor tiles, has nowhere to go but through the hot IT equipment. This method is believed to raise the efficiency of cold air actually used from;

• Poor layout in a whole volume cooling approach 30% • Well designed and installed hot aisle/cold aisle layout 50-75%

• Enclosed cold aisle 90%

Figure 18: Enclosed cold aisle layout

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Air side economiser

In the cooler latitudes it makes sense to make more use of the cool air naturally available. At the latitude of the UK the air temperature is below 200 for about 70% of the year. An air side economiser will dump the hot air from the computer room outside when the external temperature is below about 200 C and replace it with free cool air from the outside. When the external air temperature goes above 200 C then the economiser turns off and the system operates in a conventional air conditioning cooling manner.

If we presume that an air economiser will subsidise the air conditioning by 50% for 70% of the year and air conditioning on average takes 35% of the electricity bill then we would expect a reduction of 0.5 x 0.7 x 0.35 or about 12% off the data centre electricity bill.

The 60 rack model we have discussed would have an electricity load of about 600 kW or 5,256,000 kW.hrs/pa. At £0.09 per unit this would cost about £473,000 per year in electricity. A 12% saving would mean a saving of about £57,000 p.a. In the British climate we believe air side economisers to be a very cost effective addition.

Figure 18 shows an air side economiser installation at Cheshire County Council designed by Capitoline. This 80-rack installation added air-side economisers for about £40k capital investment and is expected to achieve payback of less than three years.

The air economiser must be used in conjunction with filtered and monitored outside air and be controlled by a Building Management System, BMS, in order to be truly effective.

Figure 19: Air economiser installed at Cheshire County Council. Having the CRAC units placed directly beside the exterior walls makes the air ducting very simple to implement University of East Anglia  Doc ref CPTL 9750‐09  © Capitoline LLP 2010  Data centre report UEA 09‐001  Issue 005   16‐3‐10  BJE      Page 34 of 49 

Figure 20: Improvements in the DCiE at an American data centre after installing air economisers (Digital Realty)

The ideas mentioned under the ‘High cost’ heading need much more careful consideration when considering return on investment. It is worth explaining them in more detail.

Kinetic energy storage systems. Most ICT installation have a battery backed UPS.

These can be very inefficient due to the need to take the AC input and convert it to DC to charge the batteries, and then take it back to AC again. Efficiencies of around 88% are common. An alternative method is to use the mains electricity to drive a motor which then drives a generator. The motor-generator combination makes a much more efficient filter, around 97%, and the energy storage component comes from using a large rotating mass, i.e. a flywheel, between the motor and generator. The rotating mass has kinetic energy and will keep driving the generator for several tens of seconds should the input power fail completely and so give time for a standby diesel generator to start. This is of course a simplification of how they work and there are many variants on this theme. They are expensive capital items to buy and they seem to be most popular in the 1 MW+ class of datacenter where the 97% efficiency represents such a huge saving in electricity that they are justified. The downside is that the backup time is measured in only tens of seconds so that the diesel generators must be ready to fire up at the first invitation.

Figure 21: UPS Efficiency17

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Hot air return plenums

The most common way of supplying cold air into a computer room is to pump it into a raised floor plenum zone where it can be delivered directly to the front of the computer racks by grids in the floor. Usually the hot air makes its way back to the top of the Computer Room Air Conditioning (CRAC) unit by natural convection.

It would be more economic to capture the hot air directly above the racks by building a hot air return plenum, like a suspended ceiling, to capture the hot air and deliver it directly back to the top of the CRAC units. A hot air return plenum would also make an air economiser system easier to implement. The depth of the return plenum needs to be at least the same as the under floor delivery plenum which is typically 600 mm. Variations on this model include:

• Chimneys on the back of racks that deliver the hot air directly into the hot return plenum

• Hot aisle containment systems that enclose the hot aisle to facilitate delivery of that air into the return plenum

igure 22: Racks delivering hot air via ‘chimneys’ into a ceiling return plenum F

entralised Chilled Water cooling systems

Europe the most common form of air conditioning system is known as the DX or

C

In

Direct Expansion system. DX systems work by allowing a refrigerant to evaporate in a coil (jn the CRAC unit within the computer room) and by so doing removes heat from the air flowing past it. The DX CRAC unit is then connected by pipes to a corresponding condenser unit outside where the refrigerant condenses and delivers its heat energy to the ambient external air.

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These are popular in the smaller system due to lower capital costs, simplicity of design and the fact that every unit is independent of any other so the system becomes very resilient.

As installations get larger though it becomes more efficient to have a centralised chiller plant that produces large volume of cold water at about 60 C. This cold water is pumped into the same CRAC units and the heat removal mechanism is now the absorption of heat by the cold water.

Cooling systems are measured by their coefficient of performance, CoP, which is also referred to as the Energy Efficiency Ratio, EER. EER is easily understood as it is the number of kilowatts of electricity required to produce a kilowatt of cooling capacity. It appears that the theoretical minimum is about 33%, i.e. 3 kW of cooling requires about 1 kW of electricity to produce it. The EER will decline as the ambient temperature increases as more work is required to lose heat across a smaller thermal gradient. Research has shown17 that on average, due to poor layout, leaking air, blocked filters etc. that the average is more like 60%. It can be as low as 166%, i.e. 3 kW of cooling has taken 5 kW of electricity to produce. A well designed water-based cooling system should achieve an EER of 50% whereas a well designed DX system will probably only achieve an EER of about 66%.

One must be careful with a centralised chilled water system that a single point of failure is being built in. At least two chillers are required to give an N+1 layout and all essential piping, valves and pumps must be duplicated. This of course raises the capital cost. Another advantage of chilled water system is that high density, water cooled racks can be implemented if a source of chilled water is available.

Water economisers

If one is using chilled water then there are other ways of cooling the warm return water other than passing it back through an energy intensive chiller unit.

In a cooler climate the water can first be passed through a radiator so that it will lose some of its heat naturally to the environment and present less load to the chiller when it finally reaches it. A larger version of this is the cooling tower where there is an evaporative cooling effect as well. A cooling tower will of course consume water and steps to remove the risk of Legionnaire’s Disease must also be implemented.

Dry Cooler

Conventional air conditioning relies on a compressor to raise the temperature of the refrigerant gas to a point, usually in excess of 500 C, so that it can lose its heat to the cooler ambient air. However if that air is already cool then it may be possible to switch off the compressor, which is the main energy-hungry ingredient of air conditioning, so that the refrigerant can lose some of its heat naturally to the environment via a separate radiator assembly, the so called ‘dry cooler’. This method appears to work when the ambient temperature goes below about 140 C. The disadvantage is the higher capital cost of the dry cooler equipment.

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igure 23: Energy consumption of an 1100 kW chiller with and without a dry cooler. F

(Climaveneta)

Large scale heat exchangers

e have discussed traditional air conditioning as a way of removing heat from a

nce again high capital cost is the main drawback to this and similar systems.

igure 24: ‘Kyoto-cooling’ W

computer room and also mentioned air side economisers whereby hot air may dumped outside (or re-used more profitably elsewhere) and cool external air is taken in rather than producing it in a chiller. Another method is to allow the hot air in the computer room to lose its heat to the outside via an intermediate heat exchanger. One such method is marketed under the name of ‘kyoto-cooling’. This rather tongue-in-cheek marketing name describes a large, two metre diameter, aluminum wheel that is placed half in and half out of the computer room roof. As it slowly rotates the bottom half picks up the heat of the room whilst the top half loses its heat to the cooler outside air. This will be effective from ambient temperatures below the mid twenties Celsius.

O

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8

The carbon footprint of a data centre

we look at the 140 rack model described in Appendix 3, and if each rack is rated at 7 kW

• IT load = 140 x 4 = 560 kW

at 18% = 101 kW

l load = 356 kW hich adds up to 1017 kW or approximately one megawatt of load.

he floor area of this example building is 1008 square metres so the power density is about

here are only two ways to improve on this; use less energy by employing the techniques

ther sources of energy include tricity

r, CHP, plants

bsorption and adsorption cooling is a method of producing a cooling effect from a waste

he University of East Anglia is probably unique in having an alternative source of electricity

he biomass gasifier is currently being built and being commissioned. There has been a

will have a capacity of 1.4MWe and 2MW thermal with the possibility of another similar

he current loading for the site is about 5MW and the existing infrastructure can go up to

can provide adsorption chilling to produce chilled water at about 2 degrees C. It is not If

but running at an average of 4 kW each then we can estimate the power consumption of the data centre to be

• Power and other overheads

• HVAC load assuming cooling is 35% of the tota W

T

1 kW per square metre and if running continuously over a year then the energy consumption is 8760 kW.hrs/pa. Using the government’s18 figures for grid derived energy of 0.442 Kg.CO2/kW.hr leads to 3872 KgCO2 m-2/pa. In terms of building energy efficiency this figure would not fair very well on a certificate. Data centres typically take more than fifty times the energy, on a square metre basis, than a typical office.

T

described in chapter 7 and/or use a less fossil carbon intensive source of energy. O

• Wind turbines • Photovoltaic elec

• Combined Heat and Powe • Biomass

• Absorption cooling A

heat source. T

currently being constructed on site which will provide a much greener source of electricity and a potential of efficient cooling by use of some of the waste heat generated.

T

technical glitch with the wood chip feeder which has delayed the process. The current estimate is that the plant will be operational in March 2010.19

It

sized gasifier being located in the building once the first one is working. The building has been built for two gasifiers.

T

about 8MW we believe. So there should be room for a 1.1MW data centre. It

known what cooling capacity would be available for a data centre project though the general thinking is that the present chiller is under utilised. A new one can be added so there should be plenty of capacity.

Whether the electrical and thermal capacity of the gasifier plant can be used for new data centre project either on or off site is a political and economic question beyond the scope of this report. Even if only partly used the marketing impact of it would be significant.

Replacing the government’s grid derived CO2 figure of 0.422 with the biomass figure of 0.025 kg.CO2/kW.hr is a 94% reduction and would give our 1000 m2 building example above a CO2 figure of 229 KgCO2 m-2/pa.

The above calculations have used the CO2 figure of 0.422 which is still the current figure in the Building Regulations Part L. However a more up-to-date figure of 0.537 could be used instead which would give even greater levels of saving. This figure comes from the DEFRA- recommended 2009 edition of the MTP Carbon Dioxide Emission factors for UK energy Use document. 21

It remains unclear at this stage whether or not the carbon reduction secured for electricity generation would be able to be passed on to other consumers. This is because UEA has yet to decide if it is going to sell the Renewable Obligation Certificates (ROCs) to a third party. Should it do so then the carbon reduction from the electricity generated by biomass gasification will be accrued elsewhere (by the purchaser of the ROCs) rather than by UEA. This would have an impact on the ability of UEA to market the data centre as green, though the potential reduction in carbon emissions from cooling would still be intact.

The Carbon Reduction Commitment - Energy Efficiency Scheme comes into force in April 2010. UEA's operations fall under this legislation which means that UEA has to report its carbon footprint on an annual basis and purchase carbon allowances to cover the predicted carbon footprint for the coming years. The price of carbon allowances is fixed at £12 per tonne for the first two years and then is subject to auction. Analysts expectations are that the carbon price will rise under auction to between £30-£60 per tonne. Constructing a new data centre on site would increase UEA's carbon footprint thus increasing the amount of and cost