Ground source heat pumps (GSHPs) offer an efficient method for cooling and heating buildings, reducing energy usage and operating cost. In hot, arid regions such as Texas and the southwest United States, building load imbalance towards cooling causes design and performance challenges to GSHP systems in residential and commercial building applications. An integrated building load and GSHP model is developed in this thesis to test approaches to reduce GSHP cost, to properly size ground heat exchanger (GHEX) installations and to offer methods to improve GSHP performance in commercial buildings. The integrated model is comprised of a three-story office building, heat pumps, air handling system and a GHEX. These component models were integrated in the Matlab® Simulink® modeling environment, which allows for easy model modification and expansion. The building-load model was developed in HAMBASE, which simulates the thermal and hygric response of each zone in the building to external weather and internal loads. The building-load model was validated using the ASHRAE 140-2007 Standard Method of Test and with results from EnergyPlus. The heat pump model was developed as a performance map, based on data commonly provided by heat pump manufacturers. This approach allows for easy expansion of the number and type of heat pump models supported. The GHEX model was developed at Oklahoma State University and is based on Eskilson’s g-function model of vertical borehole operation. The GHEX model accurately represents the interaction between boreholes and the ground temperature response over short and long time-intervals. The GHEX model uses GLHEPRO files for parameter inputs. Long time-interval simulations of the integrated model are provided to assess the sensitivity of the GSHP system to various model parameters. These studies show that: small changes in the total GHEX length reduce system cost with minimal impact on performance; increased borehole spacing improves system performance with no additional cost; supplemental heat rejection reduces installation costs and improves system performance; industry-recommended design cutoff temperatures properly size the GHEX system; and, while cooling is the greatest contributor to operating cost in the southwest and southcentral United States, heating is the limiting design case for GHEX sizing. / text

Ground source heat pumps (GSHP) have the ability to significantly reduce the energy required to heat and cool buildings. Historically, deployment of GSHP's in the cooling-dominated Texas and Southwest region has been significantly less than in other regions of the United States. The long term technical and economic viability of GSHPs in arid regions such as Texas has been questioned due to failures of ground loop heat pump systems by early adopters. A proposed solution is to include a supplemental heat rejection (SHR) device to help offset the unbalanced ground loads. An integrated building load-ground source heat pump model is developed in this thesis and is designed to be a test bed for potential SHR devices. The model consists of discrete component models that can be mixed and matched to represent various types of buildings and ground source heat pumps. One of the unique features of the integrated model is the use of the Simulink/Matlab environment. This environment allows the user to develop component models that take advantage of the built-in functionality of Matlab and Simulink. Another unique feature is the full coupling of the building load, heat pump, and ground loop at every time step. The building load, heat pump, and ground loop models were chosen to allow for short time step simulations, which allows for a range of dynamic response times to be modeled and for different heat pump/SHR control methods to be explored. The integrated model can be used on any computer that has the Matlab and Simulink software. The building load model used, called HAMBASE, can model both residential and commercial buildings. HAMBASE was validated using the ASHRAE 140-2007 standard. The heat pump model uses readily available data provided by GSHP manufacturers to accurately model operation across a wide range of input conditions. The vertical borehole ground loop model, developed at Oklahoma State University, is based on Eskillson's g-function model, but included a one-dimensional numerical model to calculate the short term thermal response of the borehole and ground. The ground loop model utilizes GLHEPRO, a ground loop sizing and simulation tool, to create the required parameter files. Using the integrated building load-ground source heat pump model, a model of a single family house with a ground source heat pump was developed. The house model was validated by the results from eQuest and GELHPRO. A series of sensitivity studies were completed to determine dominant factors affecting the use of GSHPs in Texas and the Southwest regions of the United States. The results show that the life of a vertical borehole can be significantly extended/cut short if the ground parameters are properly/not properly designed prior to ground loop sizing. / text

Ground source heat pumps (GSHP) are efficient alternatives to air source heat pumps to provide heating and cooling for conditioned buildings. GSHPs are widely deployed in the midwest and eastern regions of the United States but less so in Texas and the southwest regions whose climates are described as being semi-arid. In these semi-arid regions, building loads are typically cooling dominated so the unbalance in energy loads to the ground, coupled with less conductive soil, cause the ground temperature to increase over time if the ground loop is not properly sized. To address this ground heating problem especially in commercial building applications, GSHPs are coupled with supplemental heat recovery/rejection (SHR) systems that remove heat from the water before it is circulated back into the ground loops. These hybrid ground source heat pump systems are designed to reduce ground heating and to lower the initial costs by requiring less number of or shallower boreholes to be drilled. This thesis provides detailed analyses of different SHR systems coupled to GSHPs specifically for residential buildings. The systems are analyzed and sized for a 2100 ft2 residential house, using Austin, Texas weather data and ground conditions. The SHR systems investigated are described by two heat rejection strategies: 1) reject heat directly from the water before it enters the ground loops and 2) reject heat from the refrigerant loop of the vapor compression cycle (VCC) of the heat pump so less heat is transferred to the water loop at the condenser of the VCC. The SHR systems analyzed in this thesis are cooling towers, optimized VCC, expanded desuperheaters and thermosyphons. The cooling towers focus on the direct heat rejection from the water loop. The VCC, desuperheater, and thermosyphon systems focus on minimizing the amount of heat rejected by the VCC refrigerant to the water loop. In each case, a detailed description of the model is presented, a parametric analysis is provided to determine the amounts of heat that can be rejected from the water loop for various cases of operation, and the practical feasibility of implementation is discussed. An economic analysis is also provided to determine the cost effectiveness of each method. / text