The largest geothermal heating and cooling project in the US northeast, this system is designed to use up to 20 standing-column wells averaging approximately 1,500 feet deep to provide all of the heating and cooling load for an entire city block in New York City. The block is occupied by the General Theological Seminary of the Episcopal Church. The seminary occupies the block bounded by 9th and 10th Avenues and by West 20th and West 21st Streets in Manhattan. The facility includes classrooms, a library, faculty and administrative offices, dormitories, food services, a conference center, and a large chapel. Several geothermal heat-exchange wells are currently in operation and more are under construction.
Hatch Mott MacDonald was asked by the primary consultant to help overcome difficulties involving the proximity of a city water tunnel, dealing with NYC agencies, and well drilling. HMM’s role quickly expanded, as our hydrogeologic and water well construction expertise was sought. Of particular concern was the interpretation of aquifer testing results and the prospect for increasing the thermal exchange potential of the wells being installed for the project.
Optimally, the principal medium of geothermal exchange is the ground water that flows through the fractures surrounding the well. Geothermal wells installed in less transmissive formations must rely on the thermal conductivity and thermal capacity of the bedrock itself, which requires much more depth.
A well’s yield is used to gauge its geothermal potential. As the wells were installed and tested, it became evident that the primary consultant’s pre-drilling estimate of geothermal potential of the average well had been overly optimistic. This was based upon an incomplete understanding of the local hydrogeology. HMM provided timely advice to GTS about the prospect for increasing the well yield and avoided wasting resources on an expensive development plan proposed by the driller that would have had little effect.
Ultimately, the average well depth needed to be greater than originally planned. HMM’s experience with problems encountered in deep drilling projects in Manhattan was invaluable for ensuring that the wells were straight and plumb, thereby avoiding impacts to deep infrastructure, including the NYC Water Tunnel No. 3.
HMM also made good use of our close working relationship with the United States Geological Survey. Through our contacts, we were able to access their enormous database of boring logs, the experience of their field geologists, and their superb borehole-imaging instrumentation and make these resources available to the benefit of our client.
The geothermal heating system has been providing cooling and heating for the Desmond Tutu Education Center and other buildings at the seminary since February 2008.
