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Optimization of cooling-dominated hybrid ground-coupled heat pump systems.

Article Excerpt
INTRODUCTION

Ground-coupled heat pump systems can save up to 50% of the energy that would otherwise be used by conventional heating and cooling systems (EERE 2007). As a result, the number of ground-coupled heat pump systems installed in the U.S. has grown dramatically over the last several decades. However, ground-coupled heat pumps are still a secondary design choice for most applications, in large part due to the high first cost associated with the ground-coupled heat exchanger. In order to allow ground-coupled heat pump technology to capture a larger portion of the heating and cooling market, innovations are required to improve the economics of this technology, particularly for cooling-dominated climates and buildings.

One such innovation is the hybrid ground-coupled heat pump system. Hybrid ground-coupled heat pump systems (HyGCHPs) interface conventional ground-coupled heat pump (GCHP) equipment with supplemental heat rejection or extraction systems. In a cooling-dominated climate (e.g., the southern U.S.), the supplemental heat-rejection device is operated during the cooling season in order to reduce the cooling load on the ground heat exchanger (GHX). The presence of the supplemental heat-rejection device allows the installation of a smaller, less expensive GHX that experiences a more balanced load (i.e., the heat rejection to the ground during the cooling season is more closely matched to the heat extraction during heating season). The more balanced load leads to more efficient heat pump operation by mitigating increases in ground temperature that otherwise occur over time. The benefits of HyGCHP systems in cooling-dominated climates were demonstrated previously on a case-by-case basis either through instrumentation or simulation of installed systems. However, the proper design and control of an HyGCHP system in general is not intuitive due to the interactions between the building load, the components, the climate, and the ground.

This paper presents a systematic and comprehensive design study in which a physics-based TRNSYS model (Klein et al. 2006) of an HyGCHP system is integrated with an optimization engine. The optimization engine varies the system design (i.e., the physical size of the equipment) and the system control strategy (i.e., the algorithm used to control the operation of the equipment) in order to minimize the life-cycle cost (LCC) associated with owning and operating the system. The model utilizes subhourly time steps and simulates twenty years of operation. It is, therefore, capable of resolving the small time-scale effects associated with changes in the building load and control of the equipment, as well as the longer time-scale effects associated with seasonal changes in the climate and changes in the ground temperature that build up over several years due to unbalance between the cooling and heating loads. The simulation/optimization tool is applied to a range of buildings and climates and the results show that an optimally designed HyGCHP system is economically attractive under most conditions. The lifecycle savings (LCS) associated with the lower first cost of the GHX and more efficient operation of the heat pump during its life more than offset the cost of buying and operating the supplemental device in most situations. The characteristics (equipment size and control strategy) of an optimally designed HyGCHP system are examined and used to generate a set of approximate design guidelines that can be applied to a given building and climate to arrive at a near-optimal HyGCHP design.

Hybrid Systems in Literature

The ground heat exchanger model is the critical component in the simulation because it must provide accurate predictions of the behavior of the GHX for short time-scale disturbances associated with changes in its operation over a single day, as well as long time-scale changes associated with the unbalance in the cooling and heating load that occur over a single year. Further, the GHX model must capture the impact of bore-to-bore interactions and the variation in the ground temperature both very near and very far from the bore hole. In order to enable efficient optimization, the GHX model must also be computationally efficient. This project relies on the duct ground heat storage model (DST) of vertical borehole GHXs that was developed at the University of Lund (Hell-strom 1989). Hellstrom's model builds on previous Swedish research in ground heat storage and was implemented in TRNSYS by Pahud et al. (1996). Validations of the model were since done using experimental data, including work by Shonder et al. (1999) and McDowell and Thornton (2008).

Several papers presented the details of actual, installed hybrid systems (Wrobel 2004; Phetteplace and Sullivan 1998). A few studies also used simulation tools to model (Ramamoorthy et al. 2000) and optimize hybrid systems. One example is the optimization of the hybrid ground-coupled system installed in an administrative building at Fort Polk in Louisiana (TESS 2005). The model discussed in this paper is based in part on the model developed for the Fort Polk case study. However, much of the equipment used in the HyGCHP model developed for the Fort Polk case study was of fixed design because the simulation considered a single climate/ building combination. In order to accomplish optimization over a range of buildings and climates, it is necessary to develop much more general models capable of simulating a broad range of designs.

The only general methodology that has been developed for the design of HyGCHP systems is discussed by Kavanaugh (1998) for cooling-dominated climates. One primary premise of Kavanaugh's method is that the GHX in a cooling-dominated hybrid system should be sized to just meet the peak heating load. The results of this project generally agree with that basic premise. Two additional studies in the literature focused on identifying the most optimal control strategies for hybrid ground-coupled systems. A study by Yavuzturk and Spitler (2000) examined several general control strategies applied to hybrid systems in order to identify the optimal choice. The previously mentioned Fort Polk study (TESS 2005) also studied several control strategies and identified the one that resulted in the lowest LCC. These two control studies both conclude that the same general control strategy was optimal; this strategy is used for the simulation presented in this paper, though the specific setpoints used to implement the strategy are modified and optimized for each climate/building combination.

HYBRID MODEL

The objective of this project is to identify the optimal equipment and control methodology for specified building/ climate combinations under a set of economic conditions. The effects of changing building loads, ambient conditions, and energy cost throughout a day suggest that an energy simulation with subhourly time resolution is required in order to obtain meaningful results. Additionally, the annual unbalance in the load and its effect on the ground temperature will substantially affect the long-term performance of the system; therefore, a multiyear simulation covering the life of the system is required. TRNSYS was selected as the most appropriate simulation tool to meet all these needs, as well as for the ability to accurately model a GHX. The hybrid system model created for this project integrates several component models according to their energy and mass flows; these components and their interaction are described in more detail below.

Simulation Strategy

The HyGCHP model developed for this project utilizes simplifying assumptions that are necessary in order to allow the consideration of a wide range of equipment sizes, buildings, and climates while remaining computationally efficient--characteristics necessary to accomplish the optimization exercises that are the focus of this project. First, the current hybrid study utilizes building models that are independent of the simulation of the HyGCHP system itself by requiring that the HyGCHP system meet the building load at all times. This methodology of decoupling the building simulation from the heating and cooling equipment was previously adopted and justified in the Fort Polk case study (TESS 2005). The savings in computational time afforded by this assumption are considerable.

Secondly, the hybrid configuration examined in this paper always places the cooling tower upstream of, and in series with, the GHX, as shown in Figure 1. This configuration is not arbitrary. The Fort Polk study (TESS 2005) examined several configurations, as well as different control strategies and supplemental devices for the cooling-dominated administrative building that was being designed, and found that a series configuration always resulted in a lower LCC than a parallel configuration. Based on this observation, the HyGCHP model is always configured with the supplemental device in series with the ground-coupled heat exchanger. However, only the rated amount of fluid is pumped through the cooling tower; the remainder of the flow bypasses the cooling tower. The placement of the tower upstream of the GHX is based on the fact that the cooling tower is more expensive to operate than the GHX. The cooling tower requires the operation of fans that consume more energy than the pumps required to operate the GHX. Therefore, when the tower is operating, it should do so with the largest possible temperature difference...

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