As data center operators take actions to reduce or neutralize the impact of capacity growth on climate change, they’ll face a refrigerant challenge similar to what occurred with chlorofluorocarbons (CFCs). The issue then was ozone depletion, and the solution was the transition to hydrofluorocarbons (HFCs).
Now, alternatives to HFCs are required due to global warming concerns. The UN Kigali Amendment to the Montreal Protocol calls for an 80-85 percent reduction in the use of HFC refrigerants by the end of 2040 due to the high global warming potential (GWP) of these refrigerants.
As operators increase their focus on sustainability while continuing to expand capacity, many are looking to accelerate the transition away from HFCs as part of their sustainability strategy.
By doing so they take an important step in lowering the Total Equivalent Warming Impact (TEWI) of data center cooling systems. TEWI captures the total carbon emissions of a cooling technology by combining the direct and indirect effect of a system through its operating cycle.
By using refrigerants with a low GWP in cooling systems that are highly energy efficient, operators can drive the TEWI of cooling systems to new lows. Chilled water systems represent an ideal solution for accomplishing this goal as they are effective at reducing both direct and indirect emissions.
How chilled water systems reduce direct emissions
The direct component of TEWI relates to the impact on global warming of a refrigerant leak. HFCs have a relatively high GWP (above 1500), while newer hydrofluoroolefins (HFO) and HFO blends offer a much lower GWP. R1234ze, for example, has a GWP close to zero.
Chilled water systems facilitate the transition to these new refrigerants by minimizing the risks associated with the flammability of some HFO refrigerants because the chiller is installed outside the data center.
Chilled water systems are also effective at reducing the risk of leaks because systems are installed with a ready-to-use refrigerant circuit that is tested for leaks at the factory, tested again after installation, and often includes monitoring capabilities that can shut down the system if a leak occurs.
Together, the ability to isolate low-GWP refrigerants from IT systems and the reduced risk of refrigerant leaks enables chilled water systems to minimize direct emissions.
How chilled water systems reduce indirect emissions
The other half of the TEWI calculation is indirect emissions, or the emissions created by the energy used by the cooling system when carbon-based energy sources are used.
The most common metric for evaluating cooling system efficiency is partial power usage effectiveness (pPUE).
This represents the ratio between the energy used by the IT load plus that used by the cooling system divided by the energy used by the IT load. A pPUE of 1 represents a data center in which the cooling system uses no energy.
Today’s chilled water systems can support pPUE values below 1.1 in cities like London. This is enabled through various optimization strategies such as raising air and water temperatures, implementing system-level controls and using an adiabatic system to extend the use of free cooling.
The Vertiv white paper, ‘How chilled water systems meet data center availability and sustainability goals’, provides more detail on optimization strategies and includes simulations showing the impact of various strategies.
When water is used to improve energy efficiency, controls should also be employed to prevent water from being wasted. As a result, chilled water systems are able to balance energy and water efficiency to achieve a low pPUE and low water usage effectiveness (WUE).
Applicable for raised floor and non-raised floor environments
Chilled water systems have proven effective in both raised floor and non-raised floor data centers, which is particularly important today as much of the capacity being added is in the form of large, non-raised floor data centers.
Raised floor data centers typically use perimeter chilled water CRAH units, but non-raised floor data centers require a different approach due to the absence of the underfloor plenum that helps balance air pressures across rows.
In non-raised floor data centers, perimeter chilled water systems designed specifically for that environment are used in concert with thermal wall units with designs that are based on air handling units, improving air distribution and reducing the risk of negative pressures at the front of racks.
Airflow control based on pressure measurements across the larger space of the data center is more challenging than in raised floor environments but implementing a control strategy based on temperature differential (Delta T) reduces reliance on pressure measurements and enables more precise and efficient thermal management.
The future of data center thermal management
For data center operators planning to expand capacity, chilled water systems provide the ideal solution to support sustainability and availability goals.
With effective use of low-GWP refrigerants, minimal risk of leaking, and a low pPUE and WUE, chilled water systems provide low impact thermal management for raised floor and non-raised floor data centers.
For more information, read the Vertiv white paper, ‘How chilled water systems meet data center availability and sustainability goals’, or contact Vertiv today.
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