32 Energy Efficient Air Conditioning
Demystifying the cooling landscape The cooling landscape is a complex one, driven by the need to balance capital and
operational budgets, while maximising system reliability. Airedale International’s new product development director, George Hannah gives an overview.
IN ORDER TO arrive at a fair comparison, Airedale International has conducted in- depth analysis of the multiple configurations available – from mechanical cooling through to full fresh air free cooling where PUEs (power usage effectiveness) of near unity can be achieved.
For the sake of simplicity, these were narrowed down to nine system types ranging from traditional PAC or CRAC (computer room air conditioning) systems with external condenser delivering refrigerant-based mechanical (DX) cooling, through four chiller-based configurations with and without free cooling and adiabatic (or evaporative) options, to four air handling unit (AHU) configurations with and without adiabatic cooling.
Factors for consideration The initial capital investment is just one part of the equation. Operational costs, including energy and ongoing servicing, need to be factored in, in order to arrive at the ‘true’ lifetime cost of any cooling solution. Additional considerations may also affect the purchase decision. For example, internal and/or external footprint may be limited, calling for high density solutions in the first case and limiting opportunities for more energy-efficient external AHUs in the latter example; similarly noise and vibration levels invariably need to be taken into account. Carbon footprint objectives may mean that more weight is given to products that qualify for capital allowances under the Enhanced Capital Allowance (ECA) scheme or which use low global warming potential (GWP) refrigerants such as R1234ze.
Resilience will of course be a key consideration. Robust systems, combined with advanced controls logic, will be critical factors in data centre tier classification. Dual or concurrent cooling capability,
ACR News February 2014
which ensures there is a back- up if the primary source of cooling fails to cope with the heat load, N+1 fan redundancy and back-up power capability also deliver all-important cooling plant resilience.
The efficiency toolbox With advanced design techniques such as
computational fluid dynamics (CFD) applied throughout, the surface area for heat exchange can be increased and air flow improved. Incorporating the latest energy efficient components such as EC (electronically commutated) fans and compressors with permanent magnet motors can also reduce energy consumption significantly, delivering up to 70% better efficiency at part-load than AC equivalents.
Intelligent control logic also ensures system performance is optimised, reducing power draw and allowing visibility of cooling plant performance.
The findings
For each of the nine system types, Airedale’s analysis compares annualised PUE and footprint, and provides informed calculations on the total cost of ownership (TCO) over a five-year period. Energy represents by far the biggest cost as far as traditional refrigerant and water-based cooling systems are concerned. Not surprisingly, water and air- based cooling systems that offer free cooling and adiabatic or evaporative cooling performed the best in terms of both PUE and TCO. In addition, based on estimates that one kilowatt of power saved for every hour of continuous operation could generate savings of £876 a year (equivalent to over four tonnes
of CO2), the potential savings from operational budget are significant.
Despite lower initial capital costs, the total cost of ownership of the DX PAC system over the five-year
period used as the basis for comparison was more than 40% higher than the average of the systems compared. Total cooling footprint was also significantly higher than any of the other eight systems. Annualised PUE was, similarly, the highest.
Chilled water (CW) PAC systems require the lowest total footprint although they necessitate both indoor and outdoor space. Annualised PUE on the non-free cooling configurations was just 0.01 below that of the DX PAC system and significantly higher than both the free cooling CW equivalents and AHUs. Despite having below average capital costs of the nine systems, non- free cooling CW configurations had the highest TCO of all nine systems. Capital costs of the free cooling CW configurations, albeit slightly above the average of the nine systems, resulted in significantly lower TCO over five years (between 13% and 19%).
While having a larger
external footprint, air handling units save valuable internal space. Annualised PUE across all four AHU configurations compared was the lowest, at around 1.21-1.22, resulting in the lowest annual operational costs. Direct AHUs benefit from near average capital costs and significantly lower total costs of ownership when compared with all systems but
are not generally suited to data centre applications due to concerns over contamination. In contrast, despite higher initial capital costs, indirect AHUs combine the benefits of low PUE and low TCO to deliver a highly energy-efficient alternative for data centres and computer rooms that reduces the dependency on back-up mechanical cooling.
Energy saving benefits The analysis conducted by Airedale is intended as a form of balanced quantification, based on lab and field test data, for determining which solution best suits which application. Capital costs, while competitive in the marketplace, are indicative and serve as a basis for consistent comparison of each system configuration. What is clear is that free cooling options can and do save vast amounts of energy, particularly when room temperatures are high. Concurrent free cooling operates when the temperature difference between the ambient air and hot return water is as small as 1°C.
Incorporating adiabatic cooling increases free cooling potential still further. In more challenging environments, an optional mechanical cooling module will ‘top-up’ the cooling capacity with a partial DX supplementary cooling section.
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