Effective design and operation of hybrid ground-source heat pumps: Three case studies
Highlights
► Three actual hybrid ground-source heat pump systems were studied. ► Systems were deemed cost effective compared to both ground-source-only and conventional HVAC. ► Lessons were learned regarding equipment sizing, pump operation, and cooling tower control. ► Hybrid systems saved nearly as much energy and emissions as ground-source-only systems.
Introduction
Ground-source heat pump (GSHP, or geothermal heat pump) systems have shown potential to significantly reduce energy consumption in commercial buildings in comparison to more conventional systems. Yet ground-source systems have only captured a few percent of the heating and cooling market, primarily because of the prohibitively high cost of installing the necessary ground heat exchanger (GHX), which can cost an extra $50/m2 ($5/ft2) or more (a 20–30% increase in system cost). Hybrid ground-source heat pump (HyGSHP, or simply ‘hybrid’) systems are an innovative version of GSHPs; they build on GSHP systems by utilizing conventional technology such as a cooling tower or boiler to meet a portion of the peak heating or cooling load in the ‘geothermal’ loop (see Fig. 1). Because less expensive conventional equipment displaces a portion of the expensive ground heat exchanger, HyGSHP systems have the potential to make GSHP systems substantially more cost effective, thus increasing the rate of deployment of ground-source systems in general and creating larger aggregate savings. But the added complexity of hybridizing a GSHP system has technical challenges that have slowed the growth of the hybrid approach. A lack of knowledge beyond the basic design principles remains a problem-more information is needed to improve design, control, and operation of hybrids. More data and tools are needed to study the feasibility of installing hybrid systems for their projects.
Our study was designed to meet some of those needs. We selected sites in both hot and cold climates so the sites could serve as “bookends” for applying lessons learned to a wider variety of buildings. We monitored each building for one year and used the data collected to validate models of hybrid systems, demonstrate the effectiveness of the hybrid approach, and identify lessons learned and further improvements in the design and operation of these systems. It is worth clarifying that our study focuses (primarily) on improving the economic effectiveness of heating, cooling, and ventilation (HVAC) systems. While the life-cycle economics of such systems generally favors saving energy, sometimes economic optimization comes at the price of-often minimal-increases in energy consumption. If economics are not the key concern, and a building owner has the funding to minimize energy at any cost, best practices may look somewhat different than those concluded here. But if economics are optimized, then GSHP technology can be implemented in even more buildings, saving more energy in the aggregate.
Some substantial research has been conducted on this subject already. Previous studies have presented the details of installation and operation of actual hybrid systems, generally in a case study format, with some lessons learned (for example [13], [10], and others). Some studies have also used simulation tools to model [1], [12] and optimize hybrid systems that are not yet constructed. Some theoretical studies have also considered design [6], [14] and control strategies [14], [15] for hybrids. Our research builds on this previous work by focusing on both lessons learned from actual hybrid installations (to fully account for the challenges of implementing and operating a complex system in a real building) and on theoretical (modeled) comparisons to optimized hybrid designs and other system types.
Section snippets
Data collection
Three sites in two different locations were selected for this study. They are summarized in Table 1.
Two cooling dominated hybrid ground-source systems were selected. Both are in the Las Vegas area—a hot, dry climate. The other site was a heating dominated system and was located in Madison (WI)—a cold, humid climate. Of a dozen or so hybrid installations we considered, these sites proved most applicable for research given the quality of data and typical operational characteristics of the
Modeling
One key objective of our research was to go beyond the actual operation of the building and compare its operation to other hypothetical design and control strategies, including conventional (non-GSHP) systems. These objectives required computational models of these systems to complement the collected data. Models of the HyGSHP system were constructed in TRNSYS using both off-the-shelf and custom components [8]. The GHX model used in TRNSYS is the vertical GHX with U-tube piping (using the DST
Results
Using the robust model we created of our three sites, we were able to effectively analyze hybrid system operation. Our discussion of this analysis begins with lessons learned and potential improvements to these systems, and moves on to consider the impact of hybrid systems versus other system types.
Conclusions
Our study of these three HyGSHP has demonstrated that, if implemented correctly, hybrid systems can be a cost effective method of incorporating a ground-source system into a building system. Generally, for these unbalanced building hybrids were more cost effective than ground-source-only or conventional systems. There seem to be a few areas that designers and operators of these systems need to focus on to improve operation: proper equipment sizing, design and operation of part load pumping
Acknowledgements
Generous funding was received from the US Department of Energy, Alliant Energy, and Madison Gas and Electric. The authors also thank Cary Smith, and the Organizations of all three buildings.
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