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Long-Term Commercial GSHP Performance-Part I

©2012 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 54, no. 6, June 2012.

Steve Kavanaugh, Ph.D., Fellow ASHRAE and Josh Kavanaugh, Student Member ASHRAE

About the Authors
Steve Kavanaugh, Ph.D., is a professor emeritus of mechanical engineering and Josh Kavanaugh is a post-graduate student in the mechanical engineering department at the University of Alabama, Tuscaloosa, Ala.

Many ground source heat pump (GSHP) systems, also known as geothermal heat pumps (GHPs), have been successfully installed and operated for many years throughout the United States. However, other installations have experienced poor reliability, high energy costs, and undesirable ground loop temperatures. Some have had significant equipment replacements, have added supplemental heating, or installed fluid coolers. A few GSHPs have been abandoned. It has been suggested that a reason for abandonment may be the overheating or freezing of ground formations or groundwater due to imbalances in the amount of heat stored and removed.

An 18-month data collection and analysis project has been completed to identify common characteristics of successful GSHP systems and the prevalence of unacceptable long-term temperature change. The goal of the project is to enhance the ability of GSHPs to minimize energy consumption, electrical demand, and maintenance requirements while being cost effective and environmentally responsible.

Gathering critical information from the entire building and GSHP system enables an improved determination of the extent unacceptable GSHP performance is attributable to poor design and installation and what is a result of fundamental thermodynamic limitations. The data collection efforts were structured to gather a limited amount of the critical information for a large number of systems. This article is the first of a series (see sidebar, Page 50) that will discuss the measured results and identify the variety of applications and design options that tend to result in quality, and economically attractive GSHP systems.

 

Procedure

This project studied the long-term performance and temperature variations in systems operating in the service territories of the Southern Company and Tennessee Valley Authority (TVA). This includes sites from southern Kentucky to the Florida panhandle. To find sites with a wider variety of GSHP system types with successful long-term operation, the project was expanded to include south-central Texas and central Illinois. The approach included:

  1. Identify monitoring sites for the following:
    • Length of operation; multiple years of operation;
    • Accessibility of data;
    • Proximity to nearby sites to minimize travel;
    • 30 to 60 sites at various geographic locations with a variety of system types; and
    • New buildings with installation cost data near long-term sites.
  2. Conduct surveys (forms completed with assistance from owners, utilities, designers):
    • Building and GSHP system description performance;
    • Energy and demand from utility bills;
    • Installation costs for newer sites;
    • Comfort/IAQ/satisfaction; and
    • Maintenance personnel evaluation.
  3. Collect data including the following:
    • Temperatures: ground loop, initial ground, change with time and load;
    • Loop field description: number of bores, depth, separation, U-tube size, bore grout/fill, header arrangement and sizes, thermal property test and well logs;
    • Building details: type, size, loads, occupancy, schedules, ventilation air method;
    • Equipment description: heat pump type, capacity, pump system, interior piping, air distribution system, heat pump control method, pump control method;
    • Sufficient information to determine ENERGY STAR rating (energy consumption, number of occupants, occupancy hours, important internal loads, etc.); and
    • Installation costs for newer buildings.
  4. Provide a summary of results for each site.
  5. Summarize and compare results.
 

Results

Figure 1 is a graph of the surveyed commercial buildings with GSHP systems where sufficient information was available to obtain an ENERGY STAR rating. Buildings with ratings below 75 are not official since the EPA ENERGY STAR Buildings program does not list buildings with scores below 75. Unlike many widely used rating systems that rely on predictions of building performance, ENERGY STAR is based on actual building energy use. A simple and inexpensive procedure sums the measured annual energy input from all sources and corrects the rating for building size, type, location, occupancy, schedules, and critical internal loads. A rating of 75 qualifies for ENERGY STAR designation and indicates the normalized building source energy use is lower than 75% of equivalent buildings as determined from the Energy Information Administration Commercial Building Energy Consumption Survey (CBECS).

Twenty-two of the 35 buildings with sufficient information attained an ENERGY STAR designation. The variation in ENERGY STAR rating ranged from a low of one to a high of 100. While most of these systems performed well, this variation indicates GSHPs, in some cases, have been poorly designed and installed.

Citation: ASHRAE Journal, vol. 54, no. 6, June 2012 ©2012

 

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