DATA CENTRE COOLING
Uncovering the hidden costs of cooling
STULZ UK’s Johnathan Attwood warns that environmental regulation is increasing the cost of conventional cooling approaches, widely used by the data centre sector. So how can operators reduce their OPEX and improve their green credentials?
T
here is a need for education in the data centre sector on the different cooling options available and their implications for
operating expenditure (OPEX), energy efficiency, and environmental impact. Many operators are familiar with Direct Expansion (DX) systems or chilled water type systems, but there is a need for greater awareness of new approaches that can help deliver on long-term sustainability goals. The market is advancing at a pace with new
technologies such as hybrid cooling, which has the potential to significantly reduce energy consumption and the impact of data centres on the environment. With cooling accounting for up to 40% of the energy used in data centres, there is a need to understand the hidden costs associated with the various technologies and how they compare.
DX cooling During the past 15-20 years, DX has become the dominant cooling solution and, until recently, this was driven by the fact that it was considered cheap and simple to install. DX units offer a number of benefits, such as good levels of cooling, coupled with a low footprint. As these systems are based on indirect cooling, there is also no danger of introducing contaminants from outside into the data centre. However, DX systems are less desirable from an energy efficiency perspective as well as their dramatic instability, in terms of refrigerant pricing, meaning that these units are no longer the low-cost
30 option that they once were.
Chilled water cooling Chilled water systems have also come to the fore in recent years. However, hybrid solutions offer the advantage of having a lower refrigerant charge per unit – despite the fact that these operate using a similar process to a chiller. Stulz UK’s largest hybrid solution, for example, uses 7.6kg of refrigerant per circuit (with a twin circuit). An equivalent, conventional chiller system will use around 40% more refrigerant on a 1:1 basis. With a hybrid unit, the capacity is spread over multiple units, but the risk of losing large quantities of refrigerant in the event of a failure is significantly reduced.
Hybrid cooling
Hybrid cooling combines the reliability and control of a DX system, with the energy saving benefits of a free cooling system. In warmer months, when the external ambient temperature is above 20°C, the Stulz GE system operates as a water-cooled DX system and the refrigeration compressor rejects heat into the water circuit via a plate heat exchange (PHX) condenser. The water is pumped to an air blast cooler where it is cooled, and the heat rejected into air.
In cooler months, below 20°C external ambient
temperature, the system automatically switches to partial free cooling mode known as “mix mode”. In this mode, the water is directed through both
proportionally controlled valves and enables proportional free cooling and water-cooled DX cooling to work together, with the dry cooler fans being used to cool the water to the desired level to achieve the required cooling capacity. In the winter months, dependant on water temperature and/or heat load demands, the water can be used in ‘free cooling mode’.
Mix mode cooling
Mix mode cooling is the point between free cooling and DX. This means the ambient air outside is cold enough to pre-cool the water provided to the hybrid unit but not quite cold enough to lower the temperature fully, to the required parameters. (Typically this is three-six degrees below the internal setpoint – dependant on system efficiency.) This ‘sweet spot’ requires an element of mechanical cooling as a ‘top up’ to meet the load within a room. This mix mode cooling typically makes up between 50%-68% of the potential operation mode of a system.
Data centre operators can extend the free cooling and mix mode potential of their hybrid equipment by raising the return air temperatures. Potentially, it is possible to increase the return air from 24°C to 27°C, resulting in additional cost savings of up to 40% per year. This hybrid approach drastically reduces power consumption over traditional DX systems and hence data centres can achieve significant cost savings.
November 2019
www.acr-news.com
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