Water Usage Effectiveness (WUE) is a metric developed by The Green Grid to measure data center sustainability in terms of water usage and its relation to energy consumption. WUE is the ratio between the use of water in data center systems (water loops, adiabatic towers, humidification, etc.) and the energy consumption of the IT equipment.
The formula to calculate WUE is:
WUE = Data Center Water Consumption (in liters) / IT Equipment Energy (in kilowatt hours)
The lower a data center’s WUE ratio is, the more efficient it’s use of water resources is. The average data center has a WUE of 1.8L per 1kWh. Data centers with a WUE of 0.2 L/kWh or less use less than one cup of water for every kilowatt-hour consumed by IT equipment.
How to Improve WUE
It is important to measure and improve WUE as it an indicator of a facility’s overall energy efficiency and sustainability.
Some methods that promote more effective water usage include:
- Raise temperature and humidity. By increasing the temperature and humidity in a data center, the amount of heat that needs to be dissipated by the evaporative cooling process at the cooling tower is reduced. Typically, temperatures in data centers are kept well below recommended set points while humidity is over-controlled. This practice leads to high chiller demand and excessive water use from the cooling tower system. It is highly inefficient as adjacent systems may be fighting to meet the humidity set point as one system may be dehumidifying while the other is adding humidity to the space. However, raising the temperature set point and broadening the minimum and maximum humidity allows the facility to take more advantage of free cooling strategies by leveraging a waterside economizer as opposed to the chiller and cooling tower system.
- Use recycled water. By recycling water, the amount of additional water supply needed drastically decreases. However, it is important to recognize the tradeoffs with this method as additional water treatment equipment would be required to ensure the recycled water is usable. The more equipment is used, the more energy is used, which can have an inverse effect on WUE by offsetting the gains of a reduction in the water supply. If the water is treated using acid treatments such as sulfuric, hydrochloric, or ascorbic acids, water can be reused for numerous cycles, reducing the amount of additional water needed.
- Increase cycles of concentration. Cycles of concentration is the ratio of the volume of water supplied to the cooling tower (makeup water) to the volume of water removed from the system (blowdown water). While systems typically operate at two to four cycles of concentration, six cycles are recommended for maximum concentration. Operating at six cycles reduces cooling tower makeup water requirements by 20% and cooling tower blowdown by 50%.
- Leverage DCIM software. Data Center Infrastructure Management (DCIM) software is used by data center managers to reduce their data center carbon footprint and meet corporate sustainability initiatives. DCIM software enables you to measure energy consumption so you have the data to make more intelligent decisions, get real-time charts and reports on efficiency metrics like Power Usage Effectiveness (PUE), bill back customers to facilitate more energy-efficient behaviors, avoid wasting energy by overcooling, identify power-hungry equipment that can be replaced, and intelligently consolidate and virtualize resources. By dramatically increasing efficiency with DCIM software, you will likewise improve your WUE.