For several decades, our industry has applied closed-loop, water-source heat pump systems in commercial and institutional buildings. These systems can provide excellent energy performance when the building’s internal heat gains match the building’s heat losses or loads on a real-time basis, simply moving heat from where it is not needed to where it is required. However, the application of thermal energy storage in these systems has generally occurred in only a small portion of buildings such as the occasional water storage tank that served as a diurnal “thermal flywheel” to hold surplus heat from a daytime cooling cycle to provide the basis for nighttime heat requirements being the most common example. The polar shift towards net-zero building energy use requires us to rethink any application where we might be discarding energy that has potential to do additional beneficial work.
As the application of geothermal heat pump technology has evolved, the design engineer has an entirely new set of opportunities to “time shift” energy on both a daily and seasonal basis. This is a major change in thinking for most engineers, requiring an adjustment in their design process from typically focusing on peak loads during design days and instead considering the benefits of harvesting, storing, and distributing energy between multiple loads, sources, and sinks.
Large buildings with diverse uses often have opportunities to move energy; waste heat from air conditioning can serve as a preheat energy source for domestic hot water, etc. If these buildings are connected via some form of an energy-sharing network or “virtual central energy plant,” the opportunities grow enormously and the overall net cost of the needed infrastructure begins to drop. At a recent ASHRAE Conference, an engineer1 presented a case study of a college campus with a common geothermal heat-pump loop interconnecting buildings that totaled nearly 1,500 tons of peak cooling load, yet due to the energy sharing and diversity of the campus, the energy loads were being handled by a geothermal earth heat exchanger that was sized for only 300 tons.
Real-world examples such as this compel the building design team to consider the larger picture beyond their standalone project. This is a challenge in today’s marketplace where we parse large, complex projects into manageable smaller “bits,” often losing the opportunities for energy sharing and cost reduction available when we think in macro terms. We typically separate our mechanical systems by function such as chilled water generation, hot water generation, domestic hot water generation, etc., and design them independently instead of looking for synergistic relationships and opportunities between those systems. Some examples of attempting to take a larger view in both corporate and campus settings follow.
SELF-LEARNING GEOTHERMAL HEAT EXCHANGERS – EXAMPLE PROJECT #1
Example project #1 is a 344,000-sq-ft corporate center being constructed in Michigan for a large food service company. In addition to traditional building functions such as office space and data processing areas, this building houses a large commercial kitchen for traditional food preparation as well as the development of new products. The project includes a snow-melting system at the major entries to improve employee access and safety. These last two items provide an excellent heat sink for much of the excess thermal energy generated by the earlier-noted operations.
The central energy plant consists of several heat recovery chillers totaling 760 tons, as well as two 250-ton custom rooftop units serving a large underfloor air distribution system. What makes this project unique is the integration of dry-cooler sections in the rooftop units, as well as a predictive thermal-management system to control the heat flux to/from the geothermal earth heat exchanger, the dry coolers (a heat rejection option or “sink”), and the snow-melt system (a “discretionary” heating load).
The advanced control technology allows the geothermal heat exchanger to be an intelligent system component instead of merely “pipe in the ground.” Controls track real-time heat flux and then project the heat exchanger performance well into the future allowing corrective action to be automatically applied weeks or months before an overheated or overcooled geothermal heatexchanger situation arises. This control and pump package (Figure 1) is constructed off site in an ISO-9001 facility and arrives at the site pre-wired, with controls installed and programmed and hydronic components flow tested — all significantly reducing construction time, control commissioning time, and subsequently, project risks.
This application of intelligent, self-adapting predictive controls as well as innovative deep-earth directional boring in lieu of some of the vertical geothermal bores allowed the geothermal heat exchanger to be significantly reduced in size and first cost — in this case from $1,500,000 to $1,150,000 while still affording all of the positive benefits of geothermal heat pump technology. Tracking real-time HVAC loads in/out of the geothermal earth heat exchanger and then comparing these loads to design loads enables the system to become even more intelligent over time as the history of actual performance is documented. Instead of the traditional project where controls function best at their first day of operation and then degrade over time, this system actually becomes “smarter” about managing thermal assets and then leveraging them for beneficial use.