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Comparisons of HVAC simulations between EnergyPlus and DOE-2.2 for data centers.

Article Excerpt
INTRODUCTION

Data centers although representing a small fraction of the total commercial building stock, consume significant amounts of electricity, 25 to 50 times of standard offices, for IT equipment and space cooling due to 24 by 7 operation and high internal loads of up to 100 Watts per square foot or more. Energy simulation tools have been used to study energy conservation measures for data centers in order to reduce energy consumption. DOE-2.1E (LBNL 1993) and DOE-2.2 (Hirsch 2007) have been two popular tools used by the data center industry. With added capabilities of central plant modeling features, DOE-2.2 is replacing DOE-2.1E for data center applications.

EnergyPlus (USDOE 2007), on the other hand, is intended to replace DOE-2 as the next generation building simulation tool. EnergyPlus inherited most of the useful features from DOE-2 and BLAST, and more significantly added new modeling capabilities far beyond DOE-2, BLAST, and other simulations tools currently available. For a detailed comparison of modeling capabilities among EnergyPlus and 19 other tools, refer to the paper (Crawley 2005) which is available on the EnergyPlus web site www.energyplus.gov.

EnergyPlus has advantages over DOE-2.2 in the simulation of energy performance of data centers:

* EnergyPlus allows user defined loads calculation time step, from one minute up to 60 minutes per time step. The systems time step is automatically adjusted, downward from the loads time step, to obtain convergence solutions. Users can set a minimum of systems time steps and a maximum of systems interactions. DOE-2.2 can only allow hourly calculations for both loads and systems. Sub-hourly time step calculations may be necessary to better model HVAC systems and controls.

* EnergyPlus does the integrated solution of loads, systems, and plant to accurately calculate their interactions within the same time step; while DOE-2.2 does the sequential calculations from loads to systems to plant without feedback within the same time step.

* EnergyPlus allows dual-setpoint humidistat with dead-band for zones to better control the zone air humidity. DOE-2.2 only allows input of the maximum relative humidity setpoint of the return air at the system level. Humidity control can be a key issue for data centers in dry or moist climates.

* EnergyPlus can model integrated water economizer at the plant level while DOE-2.2 cannot. Integrated water economizer uses two stage of cooling: first with cool condenser water from the cooling tower, then with chilled water from chillers to meet system remaining cooling loads. Integrated water economizer can be an energy efficient measure (free cooling) for data centers located in cool or dry climates.

* EnergyPlus can model underfloor air distribution (UFAD) systems with multi-node temperatures to better represent the temperature stratification of room air. DOE-2.2 assumes well-mixed uniform room air temperature. UFAD is commonly used in data centers.

* EnergyPlus allows user-defined HVAC systems with flexible connections to water, air, and steam loops. DOE-2.2 can only model predefined hardwired HVAC systems.

* EnergyPlus allows multiple HVAC systems to serve a single zone while DOE-2.2 cannot. A data center may have multiple CRAC (Computer Room Air Conditioner) units to meet the cooling loads.

On the other hand, EnergyPlus models, especially with a large number of zones, run much slower than those of DOE-2.2. For data centers modeling, a few thermal zones would be adequate, therefore the EnergyPlus run time would not be an issue. It may also take more time and effort to create Energy-Plus energy models than DOE-2.2 due to EnergyPlus' complex input data file structure. Fortunately commercial software are available and getting more powerful to help users quickly create EnergyPlus models.

EnergyPlus has been tested with several test suites...

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