Effective design and operation of hybrid ground-source heat pumps: Three case studies

Abstract

One innovation to ground-source heat pump (GSHP, or “geothermal”) systems is the hybrid GSHP (HyGSHP) system. A HyGSHP system can dramatically decrease the first cost of GSHP systems by using conventional technology (such as a cooling tower or a boiler) to meet a portion of the peak heating or cooling load. We monitored and analyzed three buildings employing HyGSHP systems (two cooling-dominated, one heating-dominated) to demonstrate the performance of the hybrid approach. The buildings were monitored for a year and the measured data was used to validate models of each system. Additionally, we used the models to analyze further improvements to the hybrid approach and established that it has positive impacts, both economically and environmentally. We also documented the lessons learned by those who design and operate the three systems, including discussions of equipment sizing, pump operation, and cooling tower control. Finally, we described the measured data sets and models from this work and have made them freely available for further study of hybrid 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.