How optimised cooling can improve Power Usage Effectiveness (PUE)
Ways to improve Power Usage Effectiveness or PUE as it has become better known as has become an important target for most Data Centre Managers in recent years.
Power Usage Effectiveness (PUE) is a popular metric by which a Data Centre can measure how efficiently it uses its energy. PUE is a ratio that compares the total amount of energy a Data Centre facility uses (cooling, lighting etc.) against the energy used by the computing IT equipment itself. The formula looks like this:
PUE = Total Facility Energy / IT Equipment Energy
The PUE metric has been around for 10 years, having been introduced in 2006 by The Green Grid. The metric has become the standard way of measuring energy efficiency within Data Centres as it enables companies to calculate their energy efficiency over a period of time and see how seasonal changes, as well as the improvements they make, effects their PUE. This in turn helps them determine if additional changes still need to be made. PUE has also become a useful marketing tool to help win new clients by advertising how efficient the Data Centre is.
The ideal PUE is 1.0, but there are a number of factors that can affect the PUE and means every Data Centre will be different. For instance a Data Centre operating in a cold climate, for example Alaska, will require less need for a large cooling system than one located in a warm climate. As cooling systems account for about 30% of the energy used within a Data Centre optimising the cooling system for maximum efficiency can improve PUE and help enable energy cost savings.
A lot of Data Centres are over cooled, do not follow basic hot/cold aisle configurations, or suffer from other inefficiencies that result in bypass airflow. Bypass airflow is any cooled air that fails to pass through the IT equipment, and reducing this can help make the cooling of the Data Centre more efficient.
The first steps to reducing bypass airflow is to look at the Data Centre’s layout. Most Data Centres with a raised floor environment are laid out in a hot aisle / cold aisle configuration. In this configuration it is important that air delivery grilles are only placed within the cold aisles, and that any in hot aisles are closed off or replaced with solid tiles. The next area to look at is to prevent the cooled air escaping through cable openings. Unsealed cable openings can add up to a lot of open floor area, for instance 200 120mm x 150mm openings is the equivalent of having 10 floors tiles removed. One of the best methods to seal these openings is through the use of KoldLok raised floor grommets, a range of brush grommets in various shapes and sizes especially designed to seal cable openings whilst still allowing the cables to pass through.
Other areas to look at include how the cool air is delivered through the floor grilles within the cold aisle. In tests carried out by the Uptime Institute it was discovered that only about 28% of air coming through flat-bottomed floor grilles actually passes through the servers, and in some cases the speed of the air flowing under the grilles can in fact cause a negative airflow, leading to air being sucked back into the plenum. Triad floor grilles help to improve this situation through their specially designed fins and hole patterns that enable greater dissipation of the air coming through the tile.
Preventing the mixing of hot and cold airflows is another way of improving airflow efficiency. First of all any unused rack space should be sealed off through the use of blanking panels. This prevents hot exhaust air from circulating back to the front of the racks where it can mix with the cold air. Other gaps in the front of racks should also be sealed for instances the gaps between the sides of the racks and the mounting rails, these gaps can be sealed with RackSEAL air barrier foam tape.
A more structural way of preventing hot & cold air from mixing is through the use of an aisle containment system. Cold Aisle Containment encloses the cold aisle whilst Hot Aisle Containment contains the hot aisle and directs the hot exhaust air back to the CRAC units, usually through ducting in the ceiling plenum. There are many forms of aisle containment solutions available including AisleLok Modular Containment which provides a flexible, out of the box containment solution consisting of Rack Top Baffles and Bi-directional swing or sliding doors.
Adopting each of these steps will enable alterations to the cooling system to be made and through increasing the set points on the CRAC units it will provide improvements in energy efficiency that can help improve PUE.
In order to carry out the second half of the equation you need to know how much power the IT equipment is using. There are many power monitoring solutions on the market including intelligent PDUs, sensors that clip onto existing wiring to full wireless power monitoring systems. All of these systems can be used to determine either how much power an individual rack is consuming or can go deeper and monitor how much energy each individual piece of equipment is using, the later will help identify which pieces of equipment are using more energy and may need to be replaced with more efficient models.
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