Direct Fresh Air Free Cooling of Data Centres

Transcription

1 White Paper Introduction There have been many different cooling systems deployed in Data Centres in the past to maintain an acceptable environment for the equipment and for Data Centre operatives. The systems in use were chosen on known requirements of the design, technologies and techniques employed at the time. This has resulted in many Data Centres operating inefficiently, compared to the latest systems and techniques available for todays Data Centres. All Data Centre owners and operators require energy efficient cooling systems as they currently account for a large proportion (40% to 60%) of the energy used within the Data Centre. This paper demonstrates why direct fresh air cooling should be deployed in new and modernised Data Centres to minimise PUE, energy usage and CO2 emissions while maintaining optimum temperature requirements. Temperature 0 C Direct Fresh Air Free Cooling of Data Centres Direct Fresh Air Free Cooling Availability UK Temperatures from 2000 to 2010 obtained from the Met Office Useful Fresh Air < 27 0 C (Cold Aisle Containment Controlled) Useful Fresh Air < 17 0 C (Return Air Temp Controlled) Maximum temperature in each month over 10-years Highest minimum temperature in each month over 10-years Data Centre Cooling Systems Comparison On page 2, seven schemes are compared to demonstrate the economic and annual CO2 emissions savings that can be acheived using direct fresh air free cooling. The case is based on using 100kW nett cooling load. The seven schemes are; 1 Chilled water system with water temperatures at 7 C supply and 12 C return and Computer Room Air Conditioning (CRAC) cooling units air temperatures at 12 C supply and 22 C return. 2 Chilled water system with chiller free cooling and water and CRAC air temperatures as for Scheme 1. 3 Chilled water system with chiller free cooling, with water temperatures at 14 C supply and 19 C return and CRAC air temperatures at 17 C supply and 28 C return. 4 DX split system with CRAC, air temperatures at 12 C supply and 22 C return. 0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Figure 1: UK Average Temperature from 200 to 2010 Figure 1 illustrates the average UK temperatures, obtained from Met Office. It plots the highest minimum and maximum temperatures for each month over a 10 year period from 2000 to The graph shows useful direct free cooling at 17 C used for return air controlled cooling systems. However, if the data centre has cold aisle containment, the cooling system can be controlled to keep the containment to 27 C resulting in the Data Centre being cooled 24/7 for the whole year using just outside fresh air based on UK average temperatures. Weatherite fresh air / dx modular unit with air temperatures at 12 C supply and 22 C return. 6 Weatherite fresh air / dx modular unit with air temperatures at 17 C supply and 28 C return. 7 Weatherite fresh air / dx modular unit controlled on supply air into cold aisle containment at 27 C and 37 C return.

2 Scheme Drawings: Note: Scheme 1 is used as the base for energy factor for each scheme. 1 CHILLER EXHAUST AIR 7ºC 12ºC 2 CHILLER 6 EXHAUST AIR 7ºC 28ºC 12ºC 17ºC 3 7 CHILLER EXHAUST AIR 28ºC 14ºC 37ºC RACKS COLD AISLE RACKS 19ºC 17ºC 27ºC 4 Fresh Air Cooling Availability CONDENSING UNIT LIQUID LINE Each of the seven schemes described here either have no facility to provide free cooling (Scheme 1 & 4) or provide free cooling in one of two methods, non-direct (Scheme 2 & 3) or direct (Scheme, 6 & 7). Non-direct free cooling uses the chilled water system pumps, condenser fans and a heat exchanger to discharge heat when the ambient outside temperature is sufficiently cold. SUCTION LINE Direct free cooling can cool the data centre for the majority of the year, if not all of the year, depending on the location of the Data Centre. This is demonstrated in Figure 2 overleaf where the schemes deploying free cooling are illustrated, based on the annual temperatures at London Heathrow. 2

3 1000 % of Days Per Year Temperature (Degrees C) Figure 2: Percentage of Hours Below Temperature Point - London Heathrow Figure 2 shows the percentage of the year below a range of temperatures from -6 C to 36 C for Heathrow. It illustrates the point up to which the various free cooling systems are available. A free cooling chilled water system with chilled water at 7 C delivering air at 12 C (Scheme 2) is only operational for 10% of the year on full free cooling; however, with elevated water and air temperatures at 14 C and 17 C respectively (Scheme 3) the full free cooling availability rises to 48% of the year. Elevating the supply air temperature to 17 C (Scheme 6) allows direct free cooling to operate for 78% of the year. However, if direct free cooling is deployed into a cold aisle containment configuration (Scheme 7) with the cooling system controlling the supply air temperature and raising the air temperature to 27 C, direct free cooling will operate for 98.7% of the year based on typical London, Heathrow temperatures. Direct free cooling, supplying air at 12 C, (Scheme ) operates on free cooling for 4% of the year. 3

4 Energy Usage, Cost and CO2 Emissions Per 100kW of Cooling Load The calculations used are; Direct Fresh Air Free Cooling of Data Centres Total Energy Usage = Room supply fan + Condenser fan + Water Pump + Refrigerant Compressor Annual Energy Usage = ((Room supply fan + Condenser fan + Water Pump) x 8760) + (Compressor x Compressor Run Hours) Energy Factor = Scheme X Annual Energy Usage / Scheme 1 Annual Energy Usage Where X is Scheme 2 to 7. The table below shows the calculated values for annual energy usage, energy cost and CO2 emissions per scheme. Scheme Annual Energy Usage* (kwh) Annual Energy Cost* ( ) Annual C02 Emissions* (kg of C02) Energy Factor* 1 384,000 38, , ,600 31, , ,00 17,300 94, ,00 33, , ,600 18, , ,000 11,600 63, ,300 6,800 37, * Rounded up or down for clarity The calculations used are; Annual Energy Cost = Annual CO2 Emissions = Annual Energy Usage x Cost per unit of electricity Annual Energy Usage x (Obtained from carbon trust) Assumptions used for the above tables are; Room supply fan power is consistent at 4. kw for each case. Condenser fan power is consistent at 3.0 kw for each case. KgCO2/kWh for grid electricity is Cost for a unit of electricity is 0.10 Free cooling systems are more economic that those that do not deploy free cooling but using direct free cooling will save significantly more energy and therefore reduce running cost and CO2 emissions over other systems. The use of direct free cooling into a cold aisle containment system significantly reduces the energy use and therefore the operational cost and CO2 emissions are reduced. This means that annual energy usage for every 100 kw of cooling load will be 68,300 kwh at a cost of 6,800 producing 37,300 kg of CO2 and with an energy factor of 0.18 compared to Scheme 1 that uses 384,000 kwh of energy, costing 38,400 producing 209,200 kg of CO2 per year. 4

5 Free Cooling: The Perceived Objections Objection Direct free cooling is restricted due to the temperature parameters of the Servers Response ASHRAE have introduced two new classes which widens the operating temperature limits of Data Centre up to 4 C. Many server manufacturers now state that existing equipment will operate in the new ASHRAE classes. Server manufacturers are members of the ASHRAE TC 9.9 committee who have introduced the new classes. The Green Grid have produced a Data Centre Maturity Model in their White Paper 36 that sets fresh air cooled Data Centres as Level Visionary years away. Free cooling availability is restricted where equipment manufacturers specify close control of humidity Many Data Centres, and all ETSI-standard telephone exchanges do not control humidity in UK. Of those that have historically controlled humidity, many have experimented with abandoning humidity control with no ill effects to the servers what so ever. The use of wrist straps removed the need to control humidity in the UK for operative interaction with the Data Centre equipment. Free cooling requires more frequent filter changes. Free cooling introduces more dust into the data hall and equipment Filtration up to EU/6 is readily available with the filters in circuit with the supply air during both free and mechanical cooling. However, EU4 filters are commonly used. The additional maintenance required to change the filters is minimal relative to the demonstrated power cost savings, and as a consequence is balanced by savings in maintenance and replacement of DX components. Combined with the rack inlet filters on sensitive equipment, it has been demonstrated by several authorities in both the UK and the USA that there are little or no dust deposits found on the servers nor any increase in component failure. Free cooling allows smoke and other contaminants access to the data hall This may be a concern where vehicles can park near to the fresh air inlets, or where security from vandalism or terrorism might be an issue. Most sensitive sites have structural and/or electronic security systems in place for the latter. Vehicles can present localised problems, especially on ground floor level installations but parking is kept away from the data centre walls minimising the potential of drawing in exhaust fumes. Vandalism is of limited concern, as data centre site security is high. Current Weatherite free cooling units have the option to sense particulate and/or gas entering the free cooling inlet and to close off the outside air and rapidly (less than seconds) revert temporarily to full recirculation and DX cooling mode. Depending on the structure of the Data Centre, air can be taken in and discharged at roof level, avoiding potential vandalism or the intake of fumes.

6 Free Cooling: The Advantages Operating only the main supply fans for a large percentage of the time saves energy and money whilst reducing the systems carbon footprint. Free cooling freshens the room for occupants providing higher comfort levels. Pressurisation of Data Centre is improved, ensuring effective cooling distribution to server racks. Direct Free Cooling reduces wear and tear on expensive compressors and their accessories. With non-operation of mechanical cooling plant, free cooling reduces overall noise levels. Cooling Unit Operation The unit starts in fresh air mode delivering direct fresh air via the louvre and ductwork through the EC supply fan into the floor void. The air is discharged in to the room from the floor void via the floor grilles into the room or cold aisle containment, if installed. The warm air from the Servers rises to high level and, due to the pressurisation in the room, flows to the Pressure Relief Ventilators (PRVs), where it is discharge to outside. If PRVs are not deployed the condenser fan will operate, drawing the warm air back to the cooling units and discharging warm air to outside via ductwork and louvre. If the outside air is too cold the unit will mix warm room air with the outside air to provide supply air at the required temperature to cool the Servers. As the outside temperature rises this mixing reduces to the point the unit is only supplying outside air. As the outside temperature rises further and the room conditions cannot be maintained by direct fresh air, the damper will change position to divert the outside air to the condenser coil via the condenser fan. The refrigerant systems will start. The warm air from the Servers will return to the unit via the supply fan where it is drawn across the evaporator coil, cooled and then discharged back into the floor void. Schematic Diagram of a Typical Weatherite Outside Air, Free Cooling DX Unit Condsenser Air Filters Condsenser Coil Exhaust Condsenser Fan Outside Air Sensor Outside Air Filter Switch Supply Air Sensor Supply Air Flood Switch Return Air Sensor Compressor 1 The unit constantly reviews the internal and external temperature to determine if it can operate on direct fresh air cooling. Heater Main Fan Extract Compressor 2 (where applicable) Damper Position Cooling Coil Main Air Filters Compressor 1 HP Compressor 2 HP (where applicable) Compressor 1 LP Compressor 2 LP (where applicable) For further information about our Data Centre cooling solutions please contact the sales team on , sales@weatheritegroup.com or visit our website Weatherite Manufacturing Limited. Weatherite House, Credenda Road, West Bromwich, West Midlands, B70 7JE, England Tel: Fax: sales@weatheritegroup.com issue 3 / April