A geothermal system forms the backbone of a well-designed and versatile HVAC system in the Bloomberg Center building on Roosevelt Island in NYC.
Seven years ago, Mayor Michael Bloomberg requested proposals from universities for a new or expanded engineering and applied-sciences campus in New York City. Though competition was stiff, with the likes of Stanford, Columbia, and Carnegie Mellon submitting proposals, Cornell Univ., Ithaca, NY, was awarded the project.
Listen to Jay Egg and editorial director Gary L. Parr provide more detail about the Cornell Bloomberg Center geothermal system.
The Bloomberg Center, the education facility on the 12-acre Cornell Tech campus on Roosevelt Island in NYC, opened in July 2017, welcoming its first 250 students. The Center is an elegant mix of design features that include radiant floor heating, dual-purpose modular chiller/heat-pumps, and active chilled-beam cooling, all of which are connected to a decoupled ground loop. A goal for the building is to earn LEED Platinum status and net-zero performance.
GI Energy, New York, was chosen to develop the earth-loop design. They have completed numerous projects around the world, and are currently engaged in the geothermal design for the new Google campus outside San Francisco.
Cornell’s geothermal system uses a vertical-loop design consisting of 80, 400-ft.-deep vertical boreholes into which closed loop HDPE exchangers have been inserted. Normally, the boreholes are filled with bentonite grout to enhance heat transfer and seal the borehole. In this case, the boreholes were drilled into solid rock with a water-bearing fracture zone below 300 ft. through which ground water flows. Groundwater movement through the fracture zone enhances the ability of the HDPE exchangers to extract and reject heat as needed for the 360-ton HVAC system.
Designs such as this are actually fairly common in northern European locales such as Sweden and Norway (countries that are actively and extensively using geothermal heating and cooling). This unusual, but favorable, application shows the importance of understanding variables in earth-loop design. As an example, the city of Drammen, Norway, uses a central river-source geothermal system to heat and cool its 225 commercial buildings, as well as 65,000 residences.
Could the Cornell project have done something similar to what was done in Drammen, especially since it’s surrounded by water? Yes, that was an option, but NYC authorities having jurisdiction, and decision-makers across the country for that matter, do not yet have a sufficient comfort level with placing heat exchangers in rivers. The education required to reach that comfort level is why the New York state government is devoting considerable resources to geothermal-system design education and workforce development.
In New York’s Capitol City of Albany, the Empire State Plaza uses a geothermal surface-water exchange system that was constructed more than 50 years ago. The Hudson River serves as a geothermal heat sink for the massive chillers in that city’s central-energy plant, dramatically increasing the exchange efficiency with the cool river water, and eliminating the need for cooling towers and the related maintenance, noise, and replacement costs.
Room for Growth
The Bloomberg Center’s geothermal exchange design provides considerable room for growth as occupancy at the Center grows to its maximum load of 3,000 students over the next several years. For example, while the vertical loops have plenty of exchange capacity as designed, there is an annulus pumping system that can be engaged, increasing capacity by encouraging increased flow through the annulus of the boreholes. (The annulus is the space between the borehole wall and the exchange piping, usually filled with bentonite grout. Since no grout was used in this design, it naturally fills with cool ground water.) Intuitively, separate pumps are engaged by controls that monitor the borehole temperatures. When called for, ground water is pumped from the annulus of boreholes, promoting greater advective heat transfer.
The ground-loop system is decoupled from the condenser water loop inside the building, allowing separate operation of the condenser-water pumps for the chillers. Unlike air-sourced air conditioners and heat pumps, water- or geothermal-sourced systems have the ability to exchange rejected energy from one area and/or process it to another.
For example, south-facing building spaces will need cooling most of the winter while some of the other spaces that don’t have natural solar and internal heat gains will need heating. The “waste heat” from cooling the southern exposures is piped to spaces that need heat, dramatically increasing overall system efficiency. This is called “thermal-advantage” load sharing.
In this symbiotic operation, the Center’s condenser water loop may be running at a stated capacity, such that there is no need to extract or reject heat to/from the 55 F earth. Because of the decoupled earth loop, the loop pumps may remain idle for extended periods of time. During this type of operation, heat-pump efficiency, normally at a 4 or 5 COP (coefficient of performance) is dynamically increased.
It’s important to remember that geothermal HVAC systems are fundamentally hydronic, meaning that the heating (and cooling) energy is distributed within pipes filled with water. This provides a level of thermal-energy control and energy efficiency that is not available with air-sourced and forced-air distribution systems. Chilled-beam systems handle return air and sensible loads inside the conditioned space, reducing the volume of supply air, resulting in a significant decrease in the amount of distribution materials (ductwork) and installation labor requirements. Reducing space required for ductwork can yield construction savings and allow for greater ceiling heights and/or reducing overall building height. A dedicated outdoor air system (DOAS) handles the remaining ventilation and latent-load needs of the building and occupants.
The creature comforts in Cornell’s Bloomberg Center are impressive. Radiant heating on the ground floor is a welcome feature walking in the front doors on a cold winter day. While all of the spaces have piped heating and chilled water available for simultaneous operation (useful for effective dehumidification on muggy summer days) of variable-air-volume fan-coils (VAVs), the wide-open spaces employ “chilled-beam” technology.
The increase in overall comfort provided by chilled beams results from decreases in noise, draft conditions, and temperature inconsistencies. Typical air-distribution systems may produce sound levels in the range of NC 35 to 40, while chilled-beam systems operate with sound levels less than 20 NC.
Some of the other features in the facility include rain-water storage, automatic window shades, a green (planted) roof and, of course, photovoltaic modules covering most of the building. A walk along the roof-top reveals openings in the PV panels in strategic positions to allow natural daylight into the building through strategically placed skylights. Mayor Bloomberg wanted a forum to implement his “Applied Sciences NYC Initiative,” designed to transform New York’s innovation economy and Cornell’s Bloomberg Center is helping to make that a reality.
Jay Egg is a geothermal consultant, writer, and owner of EggGeothermal, Kissimmee, FL. Egg has co-authored two textbooks on geothermal HVAC systems published by McGraw-Hill Professional. He can be reached at firstname.lastname@example.org.
This article and accompanying podcast are part of our multi-month coverage of geothermal technology in commercial facilities in collaboration with the International Ground Source Heat Pump Association, Stillwater, OK (igshpa.org). Click here for more information.