Mechanical: Plant Optimization
Goal: When a chilled water plant is used, all the standard design best practices apply, with a few additions. The unusual nature of a datacenter load, which is mostly independent of outside air temperature and solar loads, makes free cooling very attractive and increases the importance of efficiency over first cost. Also, the typical level of redundancy and reliability can influence the value of various design options. As an indicator of the variability of efficiencies in practice, benchmarking results [chart] of real data centers in the field found that HVAC energy use ranged from 21% to 54% of the total.
- Objective 1: Maximize the chiller system efficiency. While a whole-plant approach must be taken to achieve the most efficient chilled water plant solution, the chiller is a very large energy consumer and should be selected to minimize its energy consumption. See chart for relative role of HVAC energy use to total data center use in actual data centers.
- Strategies:
- Select Chiller for High Efficiency. Chiller selection should include a bin analysis to predict chiller energy usage over a typical weather year. The use of water-cooled chillers with an aggressive condenser water reset is critical to achieve peak system efficiency with modern chillers.
More > Cooling Plant Optimization [See Chapter 4 of the Design Guidelines Sourcebook] - Implement an Aggressive Condenser Water Reset. Chiller load is determined both by the temperature of the chilled water return and the temperature of the condenser water supply. A condenser water reset should be implemented to provide low temperature condenser water, to the extent allowed by the chiller manufacturer. Additional tower fan power required to produce lower temperature chilled water should be considered, but chiller efficiency gains typically dominate the optimization. Resetting the chilled water temperature to 5F above ambient wetbulb temperature has been found to be a simple approach that captures significant savings.
More > Cooling Plant Optimization [See Chapter 4 of the Design Guidelines Sourcebook] - Minimize Tower Fan Power and Size Towers for Close Approach. For maximum efficiency, space should be allowed for an efficient tower system utilizing a low pressure drop fan system (typically draw-through) and a close approach. The minimum flow of the tower, typically limited by the need to maintain complete wetting to avoid fouling of the media, is of interest in plants with multiple cells/towers $#8212; greater turndown ability provides greater staging flexibility. For peak efficiency, the maximum number of cells should always be used at minimum (or zero) fan speed.
More > Cooling Plant Optimization [See Chapter 4 of the Design Guidelines Sourcebook] - Use Free Cooling / Waterside Economization. Free cooling provides cooling using only the cooling tower and a heat exchanger. It is very attractive in dry climates and for facilities that have local concerns about outside air quality that may cause concern about the use of standard airside economizers.
More > Cooling Plant Optimization, Free Cooling [See Chapters 4 and 6 of the Design Guidelines Sourcebook] - Use a Medium Temperature Chilled Water Loop. The great majority of datacenter cooling is all sensible heat — there is very little dehumidification required. Chiller performance improves when higher temperature water is produced, for example a typical centrifugal chiller's efficiency is 15-25% better when producing 55F chilled water versus 42F chilled water. In addition, using medium temperature chilled water is a common method of preventing uncontrolled dehumidification while conditioning the sensible loads and significantly improves the savings available from free cooling waterside economization.
More > Cooling Plant Optimization, Free Cooling [See Chapters 4 and 6 of the Design Guidelines Sourcebook] - Use Primary Only Variable Flow Chilled Water Pumping. The flow through modern chillers can vary significantly without problem, and when combined with a simple minimum-flow bypass, there is no need for primary-secondary pumping systems. The energy savings from eliminating the constant flow primary pumps can be significant in datacenter plants since they are commonly oversized. First cost savings and potential points of failure are also reduced.
More > Cooling Plant Optimization [See Chapter 4 of the Design Guidelines Sourcebook] - Consider Thermal Storage. The predictable nature of datacenter loads makes an economic assessment of using thermal storage systems to shift peak electrical use accurate and recommended. Thermal storage typically does not reduce total energy use, but it can shift it to off peak hours. Chilled water storage is recommended to avoid the significant equipment energy penalty that occurs from ice storage, even when ice chillers only operate during cool night hours.
More > Cooling Plant Optimization [See Chapter 4 of the Design Guidelines Sourcebook] - Monitor System Efficiency. Install reliable, accurate monitoring of key plant metrics such as such kW/ton. The first cost of monitoring can be quickly recovered by identifying common efficiency problems, such as: low refrigerant charge, non-optimal compressor mapping, incorrect sensors, incorrect pumping control, etc. Efficiency monitoring provides the information needed for facilities personnel to optimize the system's energy performance during buildout and avoid efficiency decay and troubleshoot developing equipment problems over the life of the system.
More > Cooling Plant Optimization [See Chapter 4 of the Design Guidelines Sourcebook] - Rightsize the Cooling Plant. Due to the critical nature of the load and unpredictability of future IT equipment loads, datacenter cooling plants are oversized. The design should recognize that the standard operating condition will be at partload and optimize for efficiency accordingly. Consistent part-load operation dictates using well know design approaches to part load efficiency such as utilizing redundant towers to improve approach, using multiple chillers with variable speed drive, variable speed pumping throughout, chiller staging optimized for partload operation, etc.
More > Cooling Plant Optimization [See Chapter 4 of the Design Guidelines Sourcebook]
- Select Chiller for High Efficiency. Chiller selection should include a bin analysis to predict chiller energy usage over a typical weather year. The use of water-cooled chillers with an aggressive condenser water reset is critical to achieve peak system efficiency with modern chillers.
- Metric: Plant efficiency should achieve better than 0.98 kW per ton of delivered chilled water (chiller, towers, pumps included).
- Strategies:
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