Design Guidelines: HVAC Simulation Guidebook, Volume I

July 8, 2012
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The HVAC Simulation Guidebook Volume I, 

HVAC Simulation Guidebook Coverfirst published in 2007, is one of the most popular Energy Design Resources design guidelines. This second edition (2012) has been updated to cite new research and reflect recent changes in the California Building Energy Efficiency Standards (Title 24 2008). It contains three chapters with step-by-step instructions on how to simulate the following advanced HVAC technologies:

Underfloor Air Distribution
Underfloor air distribution (UFAD) and thermal displacement ventilation (TDV) have become increasingly common in commercial new construction because they are energy-efficient, enhance indoor air quality, and increase flexibility for space reconfiguration. However, conflicting opinions exist concerning the benefits of UFAD and TDV. This often leads to inappropriate analysis and unrealistic customer expectations. There are many different notions regarding the energy efficiency of UFAD and TDV systems, with some people claiming that these systems save little or no energy, while others suggest that they can cut HVAC energy usage by fifty percent or more. To help the energy modeler evaluate the energy benefits of UFAD and TDV, this simulation guidebook identifies the key characteristics that distinguish UFAD and TDV systems from traditional overhead systems and presents a logical, engineering-based method for analyzing UFAD and TDV with DOE-2-based simulation programs.

The second edition includes a new section on modeling UFAD systems with EnergyPro 5 and DOE-2.1e.

Energy Efficient Chillers
Advances in heat transfer surface technology, digital control, and variable frequency drives have resulted in chillers that are much more efficient at part load and low lift conditions than those available ten years ago. For example, many chillers equipped with Variable Frequency Drives (VSDs) perform up to three times better at 30-50% load when chilled water supply temperature is raised and entering condenser water temperature is lowered.
At present, VSDs are only available on centrifugal chillers. To achieve any savings, condenser water temperature must be lowered on centrifugal chillers with VSDs. This is due to the fact that these chillers operate with both inlet vanes and VSDs to achieve both capacity reduction and to keep out of surge. If the entering condenser water temperature is kept high (high chiller lift), the capacity control is entirely with the inlet vanes, and the chiller will be less efficient than the same chiller without a VSD due to the drive losses.

DOE-2-based simulation programs have the capability to accurately model the chiller performance if the programmer specifies appropriate performance curves. However, this approach is often overlooked by building simulation programmers, who opt to use default chiller performance curves rather than develop curves calibrated for the specific chillers under investigation. This significantly limits the effectiveness of the energy model as a tool for chiller selection and optimization. By developing chiller performance curves to match the performance of the specific chillers being modeled, energy modelers can accurately reflect the product capabilities of each chiller, and avoid the over or underestimation of savings that commonly occurs with default curves.

Accordingly, this simulation guidebook addresses the following topics to present strategies for modeling customized chiller curves in DOE-2-based simulation programs:

  • Chiller curves used to define chiller performance data in DOE-2
  • Two methods for developing chiller curves and implementing them into the DOE-2 model
  • Manufacturer's data necessary to generate chiller curves.

Advanced Control Sequences
The recent widespread use of digital controls in building construction has greatly expanded the opportunities for optimizing building efficiency. Using digital controls provides more accurate sensing of data and enhances flexibility for modifying control logic. However, relatively few buildings that use digital control technologies attain their full potential for costeffectively minimizing energy demand and consumption. Common problems that prevent the use of efficient digital control technologies include:

  • Misinformation regarding the risks and benefits of the technology
  • An inadequate understanding of the energy and cost benefits associated with these strategies
  • Complete ignorance regarding the availability of such strategies.

Energy models that accurately demonstrate the operating cost benefits of these technologies can present decision makers with compelling reasons for implementing the strategies into the project design. Accordingly, this simulation guidebook highlights the following digital control strategies and sequences that may improve efficient operation of water-side systems. The guidebook also provides a guide for modeling each technology in EnergyPro, native DOE-2.1e, and eQUEST.

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