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Ph.D. Research In STBE Leads To Improved Ground Source Heat Pumps Procedures

Ph.D. Research In STBE Leads To Improved Ground Source Heat Pumps Procedures

A doctoral student’s research that produced a simple model and complete data set that helps better predict undisturbed ground temperatures in more than 4,000 locations worldwide is featured in the latest edition of the Science and Technology for the Built Environment.

Lu Xing’s Ph.D. research at Oklahoma State University on ground source heat pump procedure resulted in a three-part series that expanded the worldwide data set to improve the design and energy calculation procedure for ground source heat pumps.

Including Xing’s three-part series, the latest STBE has 10 technical papers that expand the science of the HVAC&R industry through specific contributions in ventilation, air conditioners, ground source heat pumps, and thermal comfort.  ASHRAE members have free online access to STBE using their existing ASHRAE.org login credentials.

Xing and one of her professors, Jeff Spitler, addressed a few aspects of Xing’s research including how it will help industry practitioners.

  1. What is the significance of the three-part “Prediction of undisturbed ground temperature using analytical and numerical modeling” series?
    Ground temperatures are needed for building heating load calculations, building energy analyses, and design of ground heat exchangers used with ground source heat pump systems. Yet local measurements are rarely available, and the ASHRAE Handbooks rely on methods originally developed in the 1800s combined with maps originally developed more than 50 years ago. The maps are hard to read and only apply to North America. In this series of papers, we provide updated methodology that gives better accuracy and applies worldwide.
    In addition, the existing methods give ground temperatures that apply for “typical years.”  We developed and validated a method for “extreme years”—it’s described in the third paper.


  1. What lessons, facts, and/or guidance can an engineer working in the field take away from this series?
    First and foremost, the end result is a simple model and complete data set that can be used to predict undisturbed ground temperatures for 4,112 sites worldwide. Temperatures are predicted as a function of depth and day of the year for either typical years or extreme years. These temperatures can be used for design heating load calculations, building energy calculations, design of vertical and horizontal ground heat exchangers for ground source heat pump systems, and design and energy analysis of district heating and cooling systems.  One limitation that an engineer should be aware of is that the predicted ground temperatures are for undisturbed (i.e. green field) sites.  Ground temperatures for urban locations will tend to be warmer, and our study didn’t extend that far.


  1. Were there any surprises or unforeseen challenges for you when writing this series?
    The biggest challenge was to develop procedures that worked for all types of climates.  We found prediction of ground temperatures for warm and temperate climates was relatively “easy.”  However, climates that are arid or seasonally arid were a challenge for the plant evapotranspiration model. This challenge was overcome by adopting a climate-dependent vegetation density in the evapotranspiration model. Climates that have heavy snow cover can also be problematic—the timing of the snow fall can make a significant difference. Typical weather year files seldom have good precipitation data and the timing varies from year to year.  A heuristic model was developed that correlated snowfall to outdoor temperature and, for purposes of predicting ground temperatures, it gives adequate results.

Science and Technology for the Built Environment

ASHRAE members have free online access to Science and Technology for the Built Environment. Read the August edition. August’s issue explores recent advances on heat and mass transfer in refrigeration and air-conditioning systems.


  • Falling film evaporation on a thermal spray metal coated vertical corrugated plate conduits
  • Condensation of superheated R-134a and R-437A inside a vertical tube
  • Design optimization and validation of high-performance heat exchangers using approximation assisted optimization and additive manufacturing
  • Evaluation of governing heat and mass transfer resistance in membrane-based energy recovery ventilators with internal support structures
  • Enhancement of R-1234ze(Z) pool boiling heat transfer on horizontal titanium tubes for high-temperature heat pumps
  • Saturated R-134a flow boiling inside a 4.3 mm inner diameter microfin tube
  • Defrosting performance on hydrophilic, hydrophobic, and micro-patterned gradient heat transfer surfaces
  • Nanolubricants flow boiling heat transfer enhancement in a microfin tube evaporator—IRG0021
  • Design and numerical parametric study of a compact air-cooled heat exchanger
  • Continuous versus pulsating flow boiling. Experimental comparison, visualization, and statistical analysis
  • Minimization of electricity demand and cost for multi-zone buildings: Part I, Modeling and validation
  • Experimental evaluation of transcritical CO2 refrigeration with mechanical subcooling
  • Algorithm for explicit solution to the three parameter linear change-point regression model
  • Energy use predictions with machine learning during architectural concept design
  • Building-scale experimental validation of a new model for walls with phase change materials
  • Influences of building information modeling (BIM) on oil, gas, and petrochemical firms