The following article was published in ASHRAE Journal, September 2006. Copyright 2006 American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. It is presented for educational purposes only. This article may not be copied and/or distributed electronically or in paper form without permission of ASHRAE. Hybrid Geothermal Heat Pumps for Beachfront Hotel By Al Barfield, Member ASHRAE Hybrid geothermal heat pump systems as described in the 2003 ASHRAE Handbook HVAC Applications, are a variation of ground-coupled geothermal systems where cooling-dominated building loads are present or where there may be restricted availability of loop field area. The hybrid geothermal system can use a closed-circuit fluid cooler tower to provide for a modified, groundcoupled geothermal heat pump system installation. In most applications, the hybrid geothermal configuration is a loop field grid of vertical boreholes with small diameter, high-density polyethylene U-bend tubes inserted from top to bottom in the boreholes. The loop field is piped in parallel with a fluid cooler. The fluid cooler is used when loop peaking heat rejection requires more capacity than the installed ground loop can provide.
Because only a few of the hybrid geothermal system components are exposed to the harsh oceanic environment, long-term, expensive component replacement and the system s life-cycle cost is reduced substantially. A hybrid geothermal heat pump system has some advantages over a full ground-coupled heat pump system. Hybrid systems can reduce the first cost of a geothermal installation since the closed-loop fluid cooler capacity is less expensive than the same heat rejection capacity in a closed-loop ground-coupled grid. Additionally, a hybrid system typically is designed to meet a building s peak heating load with the ground-coupled loop heat extraction peak requirements. This eliminates a supplemental heat-source boiler from the system. The advantage of a low pumping energy, closed-loop system also is attainable with the hybrid system using a fluid cooler. Often, in a cooling-dominated building, a closed-loop watersource heat pump system can be readily changed to a boilerless system with the addition of a limited number of ground-coupled loops on a space constrained site that does not accommodate a full ground-coupled geothermal heat rejection installation. However, proper extended range heat pump equipment must be used with this change. A hybrid geothermal heat pump hotel installation has some disadvantages. The performance data that follows defines the fluid cooler component consumption and maintenance disadvantages for this geothermal installation. System Selection The resort-style hotel discussed here is located on a barrier island on the Florida Gulf Coast. It is a five-story, 80,145 ft 2 (7445 m 2 ), 117 room facility with large public area and meeting facilities, and a large dining area with a limited breakfast service kitchen, but no in-house restaurant. Additional amenities include a health club, two large outdoor heated pools, one outdoor heated spa and a large, shallow children s pool that can be heated in winter or cooled during summer. Selecting an HVAC system for a beachfront barrier island hotel was a balance of many issues. High priority was given to equipment maintenance, replacement life and performance in the harsh humid and salt air environment. The hotel owners had experienced rapid equipment deterioration at another hotel property they operated on this same island. Additional priorities were guest comfort as outlined in ANSI/ASHRAE Standard 55-2004, Thermal Environmental Conditions for Human Occupancy, appropriate outside air ventilation levels for acceptable indoor air quality to conform to ANSI/ASHRAE Standard 62-2004, Ventilation for Acceptable Indoor Air Quality, and to maintain good humidity control within the hotel. Year-round indoor humidity control presents a large challenge when more than 7,500 cfm (3540 L/s) of outside air is to be constantly provided into the building. Based on the requirement of quality individual room comfort, with good ventilation, temperature and humidity control, the HVAC system selection was narrowed to a four-pipe, central plant chiller and boiler system or a geothermal ground-coupled heat pump system. Contractor estimates showed the geothermal ground-coupled system cost less than the central plant system. Additionally, a central plant would have added building mechanical area square footage and expense to the project. Available area for a loop field grid became a challenge after the contractor declined to place loops under a deep parking lot drainage matrix to be installed under the street-side, guest parking area. This complication required that loop field installation be made only on available beachside hotel property. The resulting 98 borehole field was approximately the equivalent of a 100 ton (352 kw) heat rejection capacity. Building diversified cooling load was less than 150 tons (530 kw). Given these parameters, the contractor selected a 150 ton (530 kw) closed-loop evaporative fluid cooler to parallel the ground-coupled loop field and formed a hybrid geothermal system. Diversified building heating load required less than 330 kbtu/h (97 kw) of heat extraction, thus considerable additional diversified loop heat extraction was available for hotel pool heating and water heating applications. Hotel System Features The geothermal system installation evolved with a number of notable features. First, the loop field was set up in parallel with the 150 ton (530 kw) fluid cooler, which offered some considerable heat rejection control and redundancy. Second, the primary domestic water heaters were three 64 kbtu/h (18.8 kw) waterto-water geothermal heat pumps. Third, the swimming pool and spa heating equipment consisted of two 390 kbtu/h (114 kw) water-to-water geothermal heat pumps. Fourth, the hotel s five ice machines were water-cooled units tied to the geothermal loop. Finally, more than 300 tons (1055 kw) of room unitary, ducted geothermal heat pumps, public area unitary heat pump units, non-public area geothermal heat pumps and two rooftop, 100% About the Author Al Barfield is a marketing principal at Gulf Power Company in Pensacola, Fla. S e p t e m b e r 2 0 0 6 A S H R A E J o u r n a l 4 9
Geo. Hot Geo. Geo. Geo. FF Cooling Cooling Avg. Building Domestic Water OA Main, Pool, Ice Tower Towers Return Total Water Use Units (2) Loop Spa Machine Pump, Use Loop Calendar Heating Pumping Heating Fan Temp. Month May 2003 2,673 kwh 45,333 gal 11,849 kwh 24,622 kwh 3,304 kwh 149 kwh 9,548 kwh 90,149 gal 83.9 F 156,930 kwh June 2003 3,129 kwh 66,501 gal 13,181 kwh 23,716 kwh 1,421 kwh 218 kwh 8,398 kwh 89,118 gal 89.3 F 129,898 kwh July 2003 3,439 kwh 77,576 gal 13,951 kwh 23,618 kwh 1,528 kwh 301 kwh 6,969 kwh 91,343 gal 92.5 F 185,721 kwh Aug. 2003 2,622 kwh 59,663 gal 14,500 kwh 23,879 kwh 1,069 kwh 230 kwh 7,950 kwh 104,552 gal 93.6 F 188,721 kwh Sept. 2003 1,725 kwh 36,215 gal 12,689 kwh 23,772 kwh 2,842 kwh 139 kwh 5,134 kwh 77,291 gal 93.7 F 168,456 kwh Oct. 2003 2,197 kwh 43,290 gal 8,749 kwh 24,640 kwh 6,810 kwh 132 kwh 1,062 kwh 17,994 gal 94.1 F 149,590 kwh Nov. 2003 2,221 kwh 37,867 gal 9,534 kwh 23,869 kwh 8,992 kwh 90 kwh 881 kwh 11,563 gal 90.3 F 138,009 kwh Dec. 2003 2,425 kwh 30,122 gal 13,455 kwh 25,786 kwh 15,041 kwh 59 kwh 0 kwh 646 gal 76.0 F 133,420 kwh Jan. 2004 2,511 kwh 29,289 gal 7,305 kwh 24,905 kwh 17,181 kwh 57 kwh 0 kwh 125 gal 72.9 F 120,108 kwh Feb. 2004 3,221 kwh 42,325 gal 7,122 kwh 23,374 kwh 15,817 kwh 70 kwh 0 kwh 13 gal 69.0 F 115,140 kwh March 2004 3,979 kwh 66,294 gal 3,286 kwh 24,764 kwh 10,873 kwh 134 kwh 0 kwh 0 gal 78.8 F 120,773 kwh April 2004 3,708 kwh 65,485 gal 4,147 kwh 23,980 kwh 8,867 kwh 162 kwh 165 kwh 2 gal 85.3 F 130,950 kwh 12 Month Total 33,850 kwh 599,960 gal 119,768 kwh 290,925 kwh 93,745 kwh 1,741 kwh 40,107 kwh 482,796 gal 84.95 F 1,737,716 kwh Table 1: Hybrid geothermal heat pump hotel performance. outside air, modified geothermal heat pumps were attached to the main hybrid geothermal ground-coupled loop system. Additional energy-efficient items used were compact fluorescent lighting and ozone laundry process washing system. Hybrid Geothermal System Performance The hotel opened to the public on the first weekend in July 2002. Energy use, water consumption, loop temperatures and indoor space conditions were monitored during a one-year period from May 2003 through April 2004. Preconditioned, zone neutral temperature outside air is delivered from the rooftop heat pumps directly into each guestroom and into the common area of the hotel. The exhaust air balance is such that the building has a positive pressure. In general, conditioned areas around the hotel maintained 72 F to 74 F (22 C to 23 C) dry bulb with 45% to 50% relative humidity in cooling and 68 F to 70 F (20 C to 21 C) dry bulb with 30% to 50% relative humidity in heating. Occupants have been pleased with the indoor air quality. Data collected for a 12-month period is presented in Table 1. Items that were submetered at this hotel were: Domestic hot water gallons and geothermal water heating energy; Rooftop 100% outside air-conditioning geothermal heat pumps energy; Main geothermal closed-loop pumping energy; Geothermal pool and spa heating heat pump energy; Geothermal loop coupled first-floor ice machine energy; Closed loop evaporative fluid cooler tower energy; Water consumption by the fluid cooler tower; Average monthly return water temperatures from the loop and tower; and Total building monthly electric energy consumption. The domestic water heating configuration was set up so that three 64 kbtu/h (18.8 kw) geothermal, water-to-water heat pumps provided primary 130 F (54 C) water heating into two parallel-piped storage tanks. This stored hot water was passed to two parallel-piped 80 gallon (303 L) supplemental 36 kw electric water heaters as backup final finish and redundant water heating. Hot water was sent to the blending valve and building hot water circulation loop at 120 F (49 C). Supplemental water heating has not been necessary to maintain the primary water heating. As can be seen, the 33,850 kwh use of the geothermal water heaters amounts to an average of only 289 kwh per room per year. An annual COP of 2.38 efficiency was calculated for the geothermal water heating system. Two modified geothermal rooftop, packaged heat pumps are set up to provide 7,630 cfm (3600 L/s) of zone neutral outside air directly into guestrooms and other areas throughout the hotel. Sensors are placed in a downstream supply duct location on these 100% outside air heat pumps. The heat pump system discharges air to the zones and tracks the required performance to maintain the zone neutral supply air conditions. Even at lower ambient, below 50 F (10 C), the outside air heat pumps are kept on line for heating. Submetered energy for the two outside air units was 119,768 kwh, or about 15.7 kwh/cfm (120 MJ per L/s) annually. Main geothermal loop pumping is provided by a 40 hp (30 kw) constant speed loop pump with a standby pump. The annual energy consumed by this main loop pump was 290,925 kwh or about 16.7% of the building s annual electric use. A variable speed drive with loop water shut-off valves at the heat pumps was suggested during construction. However, budget constraints did not allow for this feature initially. If variable speed main loop pump control had been installed on this geothermal system, it is estimated that the annual energy savings would have been about 116,370 kwh. Also, the main loop pumping energy would have been only 10% of the building s annual electric use. Outdoor pool and spa heating was accomplished with two 390 kbtu/h (114 kw), water-to-water geothermal heat pumps. The pool heating temperature was held at 85 F (29 C), while the spa was maintained at 102 F (39 C). Energy consumption for this pool and spa heating was 93,745 kwh for the year. Estimated efficiency for these units in the pool and spa heating 5 0 A S H R A E J o u r n a l a s h r a e. o r g S e p t e m b e r 2 0 0 6
mode was 6.5 COP, resulting in an 86% heating cost reduction from standard pool heating equipment. Water-cooled ice machines were placed in the vending areas and in the breakfast kitchen of the hotel. These five water-cooled machines were connected to the main hybrid geothermal loop. Several advantages were achieved by this geo-loop icemaker connection. First, offensive heat rejection into the conditioned corridor vending area was avoided and sensible heat load for the building from the ice-making process was eliminated to reduce zone cooling tonnage. Second, this measure reduced hotel interior common area noise adjacent to guestrooms. Finally, watercooled ice-making equipment saves considerable energy. As reported in the manufacturer s performance specifications for air-cooled and water-cooled equipment, there is a 1.9 kwh (23%) energy advantage, per 100 lbs (45 kg) of ice production, from the water-cooled ice-making equipment over an air-cooled equivalent machine. Annual energy consumption from the first-floor ice-machine data was 1,741 kwh. Ice-making energy savings from five hotel icemakers is estimated at 2,544 kwh per year, plus the zone cooling load reduction energy. The operation of a closed-loop evaporative fluid cooler has some advantages and some penalties when used in a hybrid geothermal ground-coupled system. Monthly data collection for this hotel clarifies the energy and water consumption part of the cost to operate a fluid cooler. Annual energy use to operate the fluid cooler spray pump and fan was 40,107 kwh or about 2.3% of the total building s electrical consumption. Fluid cooler makeup water consumption was 482,796 gallons (1828 m 3 ) for the year. These energy and water consumption components are in addition to any maintenance, parts replacement or chemical treatment that may be involved with the annual total cost to operate the fluid cooler on this hybrid system. This fluid coolers operation was not required and essentially off-line for five months of the year. Table 1 contains the monthly average return loop water temperatures from the loop field and fluid cooler to the building. Although the average annual return water temperature was around 85 F (29 C), a maximum hourly temperature of 117 F (53 C) return water was experienced in August 2003, as the result of a fluid cooler malfunction. A minimum return water temperature for this hybrid geothermal loop occurred in February 2004, and was 63 F (17 C). This minimum hourly temperature was obviously at a maximum heat extraction time for the ground-coupled loop exchange and no fluid cooler usage was involved. Total building electrical consumption for the annual sub metered period was 1,737,716 kwh or about 21.7 kwh/ft 2 (2 kwh/m 2 ). The only other energy consumed in this hotel was used by three propane clothes dryers. Annual propane consumption for this hotel laundry equipment was 4,719 gallons (17.9 m 3 ). Overall annual site energy intensity for this hybrid geothermal hotel was 79 (1,000 Btu/ft 2 [11 400 kj/m 2 ]), which is 37% below the 1995 U.S. Department of Energy (CBECS) survey 1 intensity for the lodging segment, which is a national average of 125 (1,000 Btu/ft 2 [11 400 kj/m 2 ]). By comparison, another hotel in the same geographic location, but with air-source equipment, was operating at an annual energy intensity of 139 (1,000 Btu/ft 2 [11 400 kj/m 2 ]). Non-hurricane damage and maintenance on this hotel s hybrid geothermal system has been nominal. The closed evaporative fluid cooler has required bearing replacement on the fan and a pump motor replacement. There has been replacement of five of the small compressors on the guestroom unitary water-source heat pumps during the first three years of operation. However, the compressor replacement issue seems to have been abated by installation of start kits on the units experiencing failure. Low maintenance cost, characteristic of closed loop geothermal systems, 2 appears to hold true for this hybrid geothermal hotel system. Hurricane Ivan Damage On Sept. 16, 2004, a major storm surge inundated the barrier island where this hotel is located. The Gulf of Mexico pushed its way over the island, across the intercostals waterways, and slammed into the mainland with more than a 20 ft (6 m) wall of water. The hotel had a lower level parking garage under the south wing. This lower parking area was just behind some low beachfront dunes, but was at about beachfront elevation. Swimming pools were filled with sand from the onrush of the storm surge. In addition, pool heating geothermal heat pumps and pool filter systems at the garage level were submerged and filled with sand. Water also reached the first-floor level of the hotel. A heavy-duty geothermal loop connecting hose set was torn from one of the 390 kbtu/h (114 kw) pool heating heat pumps as the unit was knocked over. However, the buried loops, main headers and header risers into the first floor mechanical room remained intact. Rooftop outside air units were destroyed from wind impact, which ripped the panels off the units, allowing wind and water to damage the interior equipment. The evaporative fluid cooler tower managed to remain essentially undamaged. It is believed that lack of damage to the rooftop fluid cooler tower was due to its location adjacent to the elevator shaft penthouse. Recovery of the geothermal loop system was vital to help dry out the moisture from the hotel facility. Once owners and their contractors were allowed onto the barrier island, the storm damage could be assessed. Utility water and power were not yet restored to the island hotel when the geothermal system contractor began recovery for the system. Essentially, all that was required to reactivate the geothermal closed loop was to valve off the damaged rooftop units and the broken connecting hose from the loop to the garage level heat pump pool heater to purge the loop system. Fresh water was transported to the island hotel to flush and recharge the geothermal loop. A generator was set up to operate as an interim power supply to the hotel. Within days of the storm, the geothermal heat pumps were in operation to dehumidify the hotel interior. Because there was minimal impact on the geothermal loop, this hotel was one of the first facilities to begin moisture remediation and post hurricane recovery on the island. Since this storm, new rooftop heat pumps have been modified for 100% outside air to zone neutral air operation. Outdoor pool 5 2 A S H R A E J o u r n a l a s h r a e. o r g S e p t e m b e r 2 0 0 6
and spa heat pumps are being commissioned, and this hybrid geothermal hotel is back to full operation. With the loop flow and temperature rebalanced, indoor conditions of temperature, humidity and ventilation for acceptable indoor air quality are all at, or better than, original system performance. Summary and Discussion The annual data shows excellent results with the match of hybrid geothermal HVAC technology to this hotel site. Benefits to the owners, operators and hotel guests include occupant comfort with highly controlled temperature, humidity and indoor air quality. The data also shows the HVAC system has low maintenance cost, high energy efficiency and consistent performance and availability. Owner payback for the initial investment in this hybrid geothermal system is not addressed here because the hybrid geothermal system was at less cost than the optional HVAC system considered. As discussed previously, the energy intensity (1,000 Btu/ft 2 [11 400 kj/m 2 ]) for this hotel facility is at 37% below the 1995 DOE (CBECS) survey s average intensity for the national lodging segment. The integrity and projected longevity of this hybrid geothermal loop system was truly tested by a severe hurricane storm surge, but the system sustained minimal damage and was readily repairable. Because only a few of the hybrid geothermal system components are exposed to the harsh oceanic environment, long-term, expensive component replacement and the system s life-cycle cost is reduced substantially. Overall, the performance of this hybrid geothermal system has been outstanding. In retrospect, some system enhancements might have been incorporated with the installation. First, of course, would have been to cost effectively develop a way to have made the system a completely closed-loop earth-coupled system. The fluid cooler power savings resulting from a full geothermal design would have been 40,107 kwh of electricity and 482,796 gallons (1828 m 3 ) worth of water and sewer charges annually. Second, a main loop pump, variable speed drive enhancement could have added an annual pumping energy savings of 113,461 kwh. Finally, the addition of a master loop control management system would have enhanced the maintenance and monitoring capability of this hybrid geothermal system with an automated data reporting and alarm features. References 1. U.S. Department of Energy. 1995. Commercial Buildings Energy Consumption Survey (CBECS). 2. Cane D., and J.M. Garnet. 2000. Update on maintenance and service costs of commercial building ground-source heat pump systems. ASHRAE Transactions 106(1):399 407. Advertisement formerly in this space. S e p t e m b e r 2 0 0 6 A S H R A E J o u r n a l 5 5