ADVANTAGES OF USING THE HYBRID GEOTHERMAL OPTION
J. B. Singh, PE*Gustav Foster, Jr. PE** *J and P Engineers, P.A.
14 Kory Drive
Kendall Park, NJ 08824 USA phone: 732-940-7994
fax: 732-940-5528 **Conectiv Solutions LLC
1240 South Spring Road Vineland, NJ 08361USA
phone: 609-696-0880 fax: 609-569-1357
ABSTRACT
Most applications of geothermal technology for commercial buildings are limited to full dependence on the ground soil temperatures for 100% of the heating and cooling energy. Although the advantages of lowest energy and maintenance costs favor this approach, first costs and space limitations may prohibit a full geothermal installation. In some states, such as New Jersey, restrictive regulations mandating minimum bore size, grouting materials, wage rates and method of heat exchange greatly increase the cost of the system. Frequently the initial cost may place the project beyond budget, and in some instances the drilling conditions may preclude the use of a large conventional closed loop bore field.
The paper explores first cost savings resulting from using hybrid geothermal design and two examples of actual hybrid design. One site is an office building in operation and the other is a large school project in design in the Atlantic City area.
1. OFFICE BUILDING
The Paragon Center building is an office condominium located in Allentown, Pennsylvania. It is an 7,432 m2 (80,000 ft2), 4-story building. The initial budget for the system, set at $950,000, was developed in anticipation of construction of a 100% geothermal closed loop system The project was to use a closed loop geothermal system composed of 55 bore holes, 152 m (500 ft) deep to supply 703 kW (200 tons) of heating and cooling capacity.
Although the initial test bore indicated no problems to a depth of 152 m (500 ft), subsequent drilling at the rear of the property resulted in collapse of the bore at approximately 48.8 m (160 ft). Two additional bores were attempted in the same area, but casing advance shoes could not hold up below 33.5 m (110 ft). The reason for the bore failures was high water flow in a limestone strata. A static water level of 11.9 m (39 ft) was observed 24 hours after drilling. To continue deeper would have required casings which would have exceeded the project budget. Because of the presence of these drilling conditions, the site did not lend itself to a deep bore field. A decision was made to abandon the idea of a 100% geothermal system and to use a closed circuit cooler, to assist in building cooling at time near summer design conditions. The installation was made with 88 bores, each 38 m (125 ft) for 281 kW (80 tons) of cooling while the closed circuit cooler was sized to provide the remaining 422 kW (120 tons) of capacity. The following table provides a comparison on cost to install various systems.
Table 1
Cost of Various Options Office Building Dollars Standard CLWSHP System Full Geothermal 55 bores @ 152 m (500 ft) (not cased) Full Geothermal 240 bores @ 33.5 m (110 ft) (cased) Hybrid System 88 bores @ 33.5 m (110 ft) (cased) Base Cost 620,000 620,000 620,000 620,000 Permits 3,100 3,500 4,500 3,700 Excavations 0 15,000 55,000 28,000
Misc. concrete, fence 30,000 20,000 20,000 30,000
Closed Circuit cooler 91,300 0 69,000 69,500
Extra Electric 6,000 0 0 5,400
Boreholes & Piping 0 219,500 417,900 177,200
Boiler 20,000 0 0 0 TOTAL 770,400 878,000 1,117,400 933,800 Loop cost, $/kW ($/ton) 312 (1,098) 594 (2090) 630 (2,215)
Note that the borehole and looping costs more than double because of the need to use casings. If casing had not been necessary, the hybrid system cost would have been less than the 100% geothermal system even considering a higher drilling cost because of a smaller number of bores. At one point during the design process, it was contemplated abandoning the concept of a geothermal system entirely because of well field problems. In this example of a hybrid system, the design allowed the project to stay within the proposed construction budget despite the conditions at the site which required casing and shallow bores.
The use of a closed circuit cooler increases energy costs. However, other areas of energy control more than offset the increased cost of employing the closed circuit cooler. Incorporated in the project was sensible heat recovery, adjustable speed drives on the pumps and monitoring equipment. By the middle of 1994, Dr. Steven Kavanaugh was brought onto the project as an Electric Power Research Institute (EPRI) consultant. Dr. Kavanaugh reinforced the need for reduction of pumping energy, from 22.3 kW (30 hp) to 11.2 kW (15 hp), and also the need to use variable frequency drives for the pumps. After more than two full years of operation, the project operating energy cost is less than $0.0929/m2 ($1.00/ft2) at an average electric cost of $0.08 annually, including demand charges.
2. ELEMENTARY SCHOOL BUILDING
This building is planned for expansion, in the West Atlantic City, New Jersey area and is situated on a relatively small city block 64 m x 177m (210 ft x 580 ft) or 11,315 m2 (121,800 ft2). Conditioned space within the building is 2,138 m2 (23,000 ft2) for the existing structure, 5,853 m2 (63,000 ft2) for the new addition, totaling 7,897 m2 (85,000 ft2). Enrollment is between 550 and 670 students, with 9 months a year use. A requirement of 7.079 L/s (15,000 cfm) of outside air is needed to meet indoor air quality guidelines, resulting in a total cooling requirement of 967 kW (275 tons) . Heat recovery equipment reduces this requirement to 879kW (250 tons) of loop cooling. The building is a combination one and two story masonry structure. In addition to classrooms, the school contains a multi-purpose room, library and a 195 m2 (2,100 ft2) swimming pool area with a 83.6 m2 (900 ft2). swimming pool. Electricity cost averages $0.997/kWh including demand charges. Typical individual classroom cooling requirement is 8.79kW (2.5 tons) utilizing separate outside air heat recovery, and 12.3 kW (3.5 Tons) if classroom cooling is performed without heat recovery equipment.
The initial 100% closed loop geothermal design called for 90 bores 122 m (330 ft). After several rough layouts of the Elementary School borefield were made, it was determined that available space was not sufficient to provide a 100% closed loop geothermal system. It was decided to consider a hybrid design, employing a closed circuit cooler. The hybrid design uses 66 bores 122 m (400 ft) deep to provide all the required heating and most of the cooling. The geothermal borefield provides 468 kW (133 tons) of capacity. On hot and humid days the closed circuit
cooler assists with an additional 411 kW (117 tons) of cooling. The following table provides a comparison on cost to install various systems.
Table 2
Estimated Cost of Various Options Elementary School Dollars Standard CLWSHP System Chiller, Boiler & Tower Full Geothermal 90 bores @ 122 m (400 ft) (not cased) Hybrid System 66 bores @ 122 m (400 ft) (not cased) Base Cost 1,043,300 1,211,700 894,100 846,930 Cooling Tower 50,000 50,000 0 0
Closed Circuit cooler 0 0 0 36,570
Boreholes & Piping 0 0 310,000 255,600
Boiler 25,000 25,000 0 0 TOTAL 1,118,300 1,286,700 1,204,100 1,139,100 Loop cost, $/kW ($/ton) n/a n/a 352.57 (1,240.00) 546.43 (1,921.80) Costs were calculated for construction in 1996.
Annual energy savings were estimated to be $6,118 for full geothermal and $4,500 for the hybrid geothermal, compared to the standard CLWSHP system. Maintenance savings were estimated to be $8,895 and $5,000. Based on these estimates, simple payback would be 5.7 years for the full geothermal and 2.2 years for the hybrid.
In this example, the idea to construct the system using hybrid came about from the tight footprint of the site. However, a clear economic advantage is shown by the use of hybrid geothermal. Significant first cost savings are traded for slightly higher operating and maintenance costs.
REFERENCES
1. Kavanaugh, S.P., Analysis and Development of a Design Method for Hybrid Geothermal
Heat Pumps, Draft, University of Alabama, Tuscaloosa, March 1997.
2. Kavanaugh, S.P., M. Khattar, D.R. McGraw, S.L. Rubin, T.F. Brown, Jr. and J.B. Singh, “The Geothermal Hybrid System at the Paragon Center in Allentown, PA”, EPRI, December 6, 1995.
3. Kavanaugh, S.P. and K. Rafferty, Ground Source Heat Pumps, Draft, University of Alabama, Tuscaloosa, 1997.
4. Singh, J.B., “Information on the Hybrid System”, Letter to contractor, June 1, 1996.
5. Singh, J.B., “Life Cycle Cost Analysis of Various HVAC System Options for Chelsea Heights Elementary School”, Letters to contractor, June 1, 1996.
