Technical Description of the Stockton College Geothermal HVAC Retrofit

Transcription

1 Technical Description of the Stockton College Geothermal HVAC Retrofit Harold E. Taylor *, Lynn F. Stiles and William Hemphill December 1997 ABSTRACT The need to replace old, worn out and obsolete Heating, Ventilating and Air Conditioning (HVAC) units led Stockton College to seek a study of current technologies which might be used. The study indicated that a ground coupled water source heat pump system would pay for its extra initial costs in about 3 ½ years. Hence, a new geothermal HVAC retrofit system was designed. It is a closed loop system of 400 wells each 425 feet deep with a U-tube of 11/4 inch high density polyethylene pipe inserted in each well. The entire well field is buried under a 4 acre parking lot. This well field is coupled through a primary supply loop and 5 separate secondary circulation loops to 61 roof mounted heat pumps to provide the heating and cooling for about 350,000 square feet of the 440,000 square foot original academic complex of buildings. A newer building of approximately 40,000 square feet with 58 smaller heat pumps was added to the system in Energy savings and cost estimates are summarized and the specifications of the new geothermal HVAC system are presented. 1. INTRODUCTION AND OVERVIEW Stockton College is a public, undergraduate college of arts and sciences located in the Pine Lands of Southern New Jersey, 12 miles northwest of Atlantic City. The main academic building complex consists of 14 wings connected by an enclosed gallery containing approximately 440,000 square feet of floor space. The Arts & Sciences building with about 40,000 square feet of additional space was added to the system in The college first opened in 1971 and in 5 years grew to its present size of over 5000 students and 200 faculty. The original buildings were designed and built prior to the energy shortages of the mid 1970s and thus are not as energy efficient as they might be. These buildings house the classrooms, laboratories, studios computing facilities and offices for faculty administration and staff. In addition they include a gym, library, pool, theater and cafeteria. The campus buildings were constructed in three stages: Phase 1 (wings A through D including CC) was completed in 1971, Phase 2 (wings E through H) was completed in 1973 and Phase 3 (wings I through M) was completed in By 1990, the Heating, Ventilating and Air Conditioning (HVAC) units manufactured by Nesbitt and Lennox were reaching the end of their

2 useful life. Nesbitt, the company who supplied the units in Phase 1, was no longer in business and Lennox no longer manufactured the multi zone units. Changes were needed. The engineering firm of Vinokur-Pace was retained to study the situation. The Vinokur-Pace study compared the costs and projected energy savings of both a direct replacement of the existing units and a replacement with ground-coupled water-source heat pumps. Their projections showed that a geothermal system would cost about $1.2 million more to install, but that it would be expected to save $330,000 per year in energy costs. Thus, the simple pay back of the extra costs of the geothermal system would be about 3.5 years. Furthermore, because the reduced peak electrical demand would reduce the need for Atlantic Electric Company to install new generators, the utility offered an $800/Ton rebate for installing the geothermal system. On the 1600 ton system, this rebate would just cover the incremental initial costs of the geothermal system. With additional funding from the State of New Jersey obtained in 1992, a contract was awarded for the retrofit and the geothermal system was installed in The new heat pumps were lifted to the rooftops and the old units removed in a day and a half by helicopter during the 1993 Christmas break and in mid-january 1994 when classes began, the new geothermal system was turned on. The Geothermal system consisted of replacement rooftop water source heat pump units coupled to a closed loop system of 400 wells each 425 feet deep located under a 4 acre parking lot. 2. THE COLLEGE ACADEMIC BUILDINGS The 14 wings of the academic complex are labeled alphabetically from A through M with a small CC wing located opposite C-wing. These wings are connected by an enclosed, 2-level gallery. I-wing (the gym) and L-wing (the pool) are not cooled and M-wing (a 550 seat theater) has a separate 175 ton Westinghouse chiller using 2 cooling towers. The wings all have two levels except E (the library) and F (housing many of the science labs) which have 3 levels. Typically each level of each wing is conditioned by two multi zone roof mounted units. Energy usage for 1990 was 9,205,200 kwh of electricity and 40,543,000 cubic feet of gas. These translate into 106,780 million BTU of electricity and 41,759 million BTU of gas. The costs were $6.81/million BTU for electricity and $5.28 per million BTU for gas or an average of $6.38/million BTU for gas and electricity combined. 3. PRE-RETROFIT EQUIPMENT The original HVAC equipment consisted of multi-zone combination gas-fired heaters and electric compressor type coolers. There were 70 units with a total cooling capacity of 20,962,000 BTU/hr or 1,747 Tons. These units were mounted on the roofs of the wings they served. Table 1 is a summary of the total capacities/ratings for the original units by building. The 18 Phase 1 units have a total cooling capacity of 5,814,000 BTU/hr or 485 Tons. The 30 Phase 2 units have a total cooling capacity of 10,451,000 BTU/hr or 871 Tons. The 22 Phase 3 units had a total cooling capacity of 4,690,000 BTU/hr or 391 Tons. Detailed ratings of individual units are given in Appendix 1.

3 Table 1: Original HVAC Units Building Supply CFM Fresh CFM Fans kw Compressors kw Cooling 10 6 BTU/hr Phase 1 137, , Phase 2 239,115 62, , Phase 3 164,049 69, Totals 540, , , Energy Source Electricity Gas Quantity Used 9,205, ,433 Units kwh CCF Energy Used 10 6 BTU 106,780 41,759 Table 2: Energy Consumption in1990 Cost Energy Cost $ $/10 6 BTU 727, , These units were installed between 1971 and 1975 and thus were nearing the end of their useful lifetime in the 1990s. Parts could no longer be obtained for these units and they were becoming increasingly difficult to keep in proper operating order. Thus in 1991, a study of possible replacement options was commissioned by the college. [Vin92] 4. THE GEOTHERMAL RETROFIT The Vinokur-Pace engineering firm made cost savings and energy savings estimates for both a direct replacement of the existing units with current technology new units and with the watersource heat-pumps. Their estimates were based on 1990 energy use figures which are shown in Table 2. For this calculation an overall COP of 3.4 was assumed for the electrical usage so that 1 kwh of electricity yielded 11,600 BTU of useful energy. Also the energy content of the natural gas was assumed to be 103,000 BTU/CCF. Projected energy usage and costs, based on rates in effect on in 1991 are shown in Table3. Projected installed cost of the direct replacement system was $3,567,493. For the geothermal system, the projected installed cost was $4,964,594 but the electric utility company offered rebates of nominally $800 per installed ton which came to an actual rebate of $960,000 making the net installed cost of the geothermal system $4,004,594 or $437,101 more than the direct replacement system. Comparing the projected energy costs for the Geothermal system with the Direct Replacement system shows a savings of $126,047 per year. Thus the simple pay back time for the incremental cost of the geothermal system would be: 437,101 / 126, years. Table 3: Projections of Energy Use and Cost Electrical Gas Total System kwh Cost ($) Therms Cost ($) Cost ($) Existing 8,427, , , , ,401 Direct Replacement 7,112, , ,919 87, ,136 Geothermal 6,372, ,296 50,996 27, ,089

4 Table 4: New HVAC Units: Building opened December 1996 Supply Min Water Heat A & S Building Size Tons Flow GPM Reject BTU/hr Absorb BTU/hr Fan/pump kw Air ,662,233 1,634, Water , , Totals ,110,770 1,918, ROOFTOP MULTI-ZONE HEAT PUMPS The geothermal HVAC system installed at Stockton is shown conceptually in Figure1. The heat pumps are mounted on the roof of the academic buildings and connected to the well field under a large parking lot by piping. The specifications of the new (replacement) rooftop heat pumps are shown in Tables A, A and A. Note that the unit for the CC-wing (Unit No. RSZ 18) was not replaced and therefore does not appear in this table of new units. In Phase 1, (Table A) the 17 units have a total nominal size of 435~tons or 5,220,000 BTU/hr. In Phase 2 (Table A) there are 30 units with a total nominal size of 755 tons or 9,060,000 BTU/hr. In Phase 3, the 8 old, heating-only units in the Gym (Wing-I; units 80-E, 80-F, 80-G & 80-H) and Pool (Wing-L; units 80-A, 80-B, 80-C & 80-D) were not replaced. The specifications of the units replaced in Phase 3 are given in Table A. There are 14 units with a total nominal size of 280 tons or 3,360,000 BTU/hr. Thus the total nominal size of the 61 replacement units is 1,470 tons or 17,640,000 BTU/hr or 5.17 Megawatts. The system was designed to accommodate a new building in December The 57 units on this building added an additional 242 tons bringing the total load on the well field to 1712 tons (2.2 x 10 6 BTU/hr or 5.91 MW). 4.2 THE WELL FIELD Once the heating and cooling load is determined, the size of the well field can be determined. The well field for the Stockton geothermal HVAC system was designed by Howard Alderson of his Alderson Associates engineering firm. The well field consists of 400 wells, each 425 feet deep as shown in Figure 2 with a U-tube if 1 ¼ inch high density polyethylene pipe inserted in each well. The wells are grouted with bentonite to prevent water from one aquifer getting into another. Twenty wells are fed from one 4-inch diameter lateral in a `reverse return' configuration to equalize the pressure drop (and presumably the flow) through each of the 20 wells. The 20 4-inch laterals come together into a 16-inch diameter manifold in a small cinder block building located on the east side of the well field. This manifold house is the only visible feature of this large well field since the wells and the laterals are all buried under a 4 acre parking lot.

5 The heat pumps are fed by a pair of 16-inch diameter pipes (the supply and return pipes) of the main loop coming out of the manifold house and going to the pumps in the F-wing of the buildings on which the heat pumps are mounted. This primary loop provides a flow of up to 4000 gallons per minute of water supply through the U-tubes in Figure 1: Conceptual view of the Stockton geothermal HVAC system. Also shown are the hydro geological layers in which the well field is implanted.

6 Figure 2: Diagram showing the layout of the well field with the monitoring wells. The wells are separated by at least 15 feet.

7 the wells. This is distributed to the heat pumps throughout the conditioned space in 5 separate secondary loops. The wells go through three different aquifers and the intervening clay layers which confine them as shown schematically in Figure 3. The Upper Cohansey Aquifer begins near the surface and extends down to 80 feet. Then there is a 28 foot thick confining layer followed by the Lower Cohansey Aquifer from 108 to 160 feet down. Transition layers surround a 12 foot thick confining bed from 160 to 315 feet in depth. Then there is the Rio Grande water bearing zone from 315 to 351 feet, below which there is a thick confining bed to beyond the bottom of the wells. The water bearing layers are expected to have a higher specific heat and a higher thermal conductivity. In addition the flow of the water will help in the heat transfer process. Four hundred wells each 425 feet deep gives 170,000 lineal feet of well for heat exchange purposes. This translates into 106 feet per ton of cooling. The pipe for this closed loop system is actually twice as long since each well contains a U-tube. Furthermore the 4-inch laterals provide an additional 19,200 feet of pipe in contact with the ground for additional heat exchange. Though this is only about 11% additional length, the 4-inch pipe has over three times the surface area of an equal length of 1 ¼-inch pipe or about 1.5 times the area of the double 1 ¼-inch pipe of the U-tubes in the wells. The heat exchange of the laterals thus adds about 15% to the heat exchange capacity of the well field. The designers expected this well field configuration to maintain the temperature of the water supplied to the heat pumps between 40 F and 90 F under the loads produced by the college buildings. 5. CONCLUSIONS This system was put into operation on January 19, During the first six months of operation the pumps were running wide open, 24 hours a day. In June of 1994 the variable frequency drives for the pumps were made operational and since then the pumping power has been only what is needed for the daily load. In addition, a sophisticated energy management system (EMS) is being developed on site by the Trane Manufacturing Company to control the system. This EMS monitors over 2000 points - temperatures, flow rates (or pressures) valve or damper positions etc. - and adjusts the system accordingly. As the EMS is completed and optimal operating strategies are developed additional energy savings and conditioning comfort are anticipated. Also, a monitoring program has been set up to measure the performance of this system. It is to be described in a forthcoming article. Some results were reported at the EPRI meeting in December 1995 [TEP +95]. Additional papers are being prepared for publication. Project reports are available [SEP +94a], [SEP+94b] and [SEP +95] as is the EPRI meeting paper.

8 Figure 3: Geologic layers that are penetrated by the 425 foot deep wells.

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17 REFERENCES [SEP+94a] Lynn Stiles, Claude Epstein, Sipra Pal, Louise Sowers, and Harold Taylor. Monitoring and analysis for the electrical demand, energy flow and well field parameters of the geothermal heat pump installation at the Richard Stockton College of New Jersey. Technical report, The Richard Stockton College of New Jersey, Faculty of Natural Science and Math, Stockton College, Pomona, NJ 08240, Jun Six Month Progress report to Atlantic Electric, Electric Power Research Institute, Sandia National Laboratory and South Jersey Transportation Company. [SEP+94b] Lynn Stiles, Claude Epstein, Sipra Pal, Louise Sowers, Harold Taylor, William Hemphill, and Joseph Sowers. Monitoring and analysis for the electrical demand, energy flow and well field parameters of the geothermal heat pump installation at the Richard Stockton College of New Jersey. Technical report, The Richard Stockton College of New Jersey, Faculty of Natural Science and Math, Stockton College, Pomona, NJ 08240, Dec Annual 1994 Report to Atlantic Electric, Electric Power Research Institute, Sandia National Laboratory and South Jersey Transportation Company. [SEP+95] Lynn Stiles, Claude Epstein, Sipra Pal, Louise Sowers, and Harold Taylor. Monitoring and analysis of the geothermal heat pump installation at Stockton College (NJ). Technical report, The Richard Stockton College of New Jersey, Faculty of Natural Science and Math, Stockton College, Pomona, NJ 08240, Jul January - June 1995 report to Atlantic Electric, Electric Power Research Institute, Sandia National Laboratory and South Jersey Transportation Company. [TEP+95] Harold Taylor, Claude Epstein, Sipra Pal, Louise Sowers, and Lynn Stiles. Monitoring and analysis for the electrical demand, energy flow and will field parameters of the geothermal heat pump installation at the Richard Stockton College of New Jersey. In Proceedings of the 1995 Conference on Meeting Customer Needs with Heat Pumps, St. Louis, MO, December Electric Power Research Institute. [Vin92] Vinokur-Pace Engineering Services, Inc., 135 Old York Road, Jenkintown, PA Geothermal Energy System, March Report to Stockton College and the New Jersey Division of Building Control (DBC I0082).