Gas-Fired Chiller-Heaters

Transcription

1 Direct gas-fired double-effect chiller-heater. Courtesy of York International. Gas-Fired Chiller-Heaters as a Central Plant Alternative for Small Office Buildings By ROGER M. THIES, JDB Engineering, Inc., York, Pa., and WILLIAM BAHNFLETH, PhD, PE, Dept. of Architectural Engineering, The Pennsylvania State University, University Park, Pa. William Bahnfleth is a member of HPAC s Board of Consulting and Contributing Editors. Gas absorption chillerheaters have been applied successfully in large projects where use of multiple chillers is feasible; however, small office buildings present part-load design difficulties. This article shows how these application problems can be overcome by a variety of means. Packaged absorption chillers activated by steam or hot have been used for many years in a variety of settings. A common application for absorption chillers is to provide cooling to a campus served by a district steam system. In this environment, absorption refrigeration reduces consumption of electricity and increases the opportunity for cogeneration. Such systems require external combustion or heat-recovery boilers to provide steam to the chillers. More recently, gas-fired doubleeffect absorption chillers have entered the market. These machines are commonly referred to as chiller-heaters because they can produce both chilled and hot. Chiller-heaters are an attractive alternative to electric motor-driven vapor compression equipment when the cost of electricity is high relative to natural gas. In addition, the hot output of a chiller-heater can reduce January 1998 HPAC /Piping/AirConditioning 103

2 or eliminate conventional boiler capacity requirements. In a favorable application, chiller-heaters can provide cooling production cost reductions while also saving heating plant space and cost. A chiller-heater has three distinct operating modes: coolingonly, heating-only, and simultaneous cooling and heating. Fig. 1 schematically depicts operation in these three modes. In cooling-only mode, a chillerheater functions as a conventional double-effect absorption chiller with the gas-fired burner as the heat source for the cooling process. Typical chilled supply temperatures for a chiller-heater range from 40 to 44 F. In heating-only mode, the condenser section of the chiller-heater is shut down, and the evaporator functions as a condenser. Refrigerant vapor produced in the directfired primary condenses on the evaporator tube bundle to make hot. Available hot supply temperatures range from 140 F to as high as 180 F. In simultaneous heating and cooling mode, the evaporator and condenser sections perform their normal refrigeration cycle functions. Simultaneous heat production occurs in a supplemental heat exchanger that condenses a fraction of the hot refrigerant vapor produced in the primary. The remaining thermal input drives the cooling process. As in heating-only mode, the supplemental heating process can provide hot at temperatures ranging from 140 to 180 F. In simultaneous heating and cooling mode, heating capacity is produced at the expense of cooling output. The total heating and cooling outputs that can be produced depend on the total energy input to the system. At 100 percent of maximum energy input, 30 to 100 percent of peak cooling output can be delivered. Depending upon cooling output, up to 50 percent of peak heating capacity can be produced. The relationship between cooling Simultaneous heat exchanger () Simultaneous heat exchanger () Simultaneous heat exchanger () High-temperature Gas-fired burner High-temperature Gas-fired burner Cooling-only mode Low-temperature Simultaneous heating and cooling mode High-temperature Gas-fired burner -only mode Low-temperature Low-temperature Evaporator Absorber Evaporator Absorber Evaporator Absorber Chilled Chilled 1 Chiller-heater operating modes. (Dashed lines indicate not in operation.) 104 HPAC /Piping/AirConditioning January 1998

3 capacity and supplementary heating capacity is inverse. When 100 percent of the design cooling capacity is required, no supplementary heating capacity is available. If 30 percent of the peak cooling capacity is required, approximately 50 percent of the unit s peak heating capacity is available to the system. Chiller-heaters have been applied successfully in many locations across the U.S. One notable installation is the Philadelphia Convention Center, a system showcased in a technical tour at the 1997 ASHRAE Winter Meeting. The convention center uses four chiller-heaters ranging in capacity from 1000 to 1500 tons to meet the entire heating and cooling loads of the building. However, chiller-heaters are relatively new to the market, and many engineers lack direct experience with their design, operation, and maintenance. This is not surprising in view of market statistics. While annual sales of chiller-heaters are increasing steadily, they account for less than 3 percent of the packaged chiller market. In 1996, sales of direct-fired absorption chillerheaters totaled approximately 140 while more than 6000 conventional vapor compression and absorption chillers were sold. Through a detailed case study, this article explores the use of a chiller-heater system as the central heating and cooling plant for a small office building. Design of the case study system takes into account current limitations of chillerheater performance that may not be widely known to design engineers. The chiller-heater system is compared with two, more-conventional central plant alternatives: an electric motor-driven air-cooled reciprocating chiller with a gasfired boiler plant and an electric motor-driven -cooled centrifugal chiller with a gas-fired boiler plant. Economic merits of these alternatives are evaluated through simple payback and life-cycle cost comparisons. Although this case study deals with a particular set of design conditions, it illustrates the generic application issues that arise when chiller-heaters are applied in small facilities. Application considerations Several factors influence the feasibility of chiller-heaters as a central plant alternative for a small office building. One of these is the type of secondary system served. An ideal secondary system has a load profile that requires simultaneous heating and cooling at rates that can be met by a chiller-heater and a secondary system that can utilize the supplementary heat of a chillerheater for reheat. Variable air volume (VAV) systems with hot January 1998 HPAC /Piping/AirConditioning 105

4 reheat are ideal for chillerheater application because the relatively small hot capacity needed for reheat can be produced in the supplementary heat exchanger, eliminating the need for summer boiler operation. A second, critical factor is the relationship between the minimum cooling capacity of the chillerheater and the minimum facility heating and cooling loads. Most chiller-heaters available today cannot operate below approximately 30 percent of their design cooling or heating load. For example, a chiller-heater serving a small office building with a design load of 200 tons would typically be incapable of operating at a capacity of less than 60 tons in either simultaneous or cooling-only mode. In heating-only mode, low heating loads can be met by cycling the burner of the chiller-heater. However, this mode of control reduces heating mode efficiency relative to continuous operation and may adversely affect system performance. A third consideration is the expected distribution of simultaneous heating and cooling loads. For most chiller-heaters, the cooling load controls gas burner operation during simultaneous cooling and heating operation. Consequently, the amount of heat that can be reclaimed from the cooling process is limited by cooling demand. If the heating load frequently exceeds the supplementary heating capacity of the chiller-heater at times when both heating and cooling are needed, a supplementary source of heat is required. Yet another potential problem is that there may be times when only heating is required, but the chillerheater is in simultaneous cooling and heating mode. This may occur, for example, when cooling loads are being met by outdoor air economizer operation part of the time and by the absorption chiller at other times. Chiller-heaters cannot be switched instantaneously from heating mode to simultaneous cooling and heating operating mode due to the cool-down time required by the evaporator tube bundles. Thus, the cost of having the chiller-heater available for cooling duty is that full heating output is not available on demand. Case study The case study building is a 46,000 sq ft structure in northern Ohio that formerly housed a department store but was converted into an office complex in The renovated building has a design cooling load of 190 tons and a design heating load of 2000 MBtuh. The secondary system providing conditioned air to the facility is VAV with two 29,000 cfm air-handling units. Both air-handling 106 HPAC /Piping/AirConditioning January 1998

5 units have dry-bulb economizer controls. The actual cooling plant installed in the case study building is a 200-ton air-cooled reciprocating chiller with a nominal rating of 1.35 KW per ton. It provides 480 gpm of 44 F to cooling coils located in the air-handling units. The heating plant comprises two 1200-MBtuh natural gas-fired boilers that provide hot to air-handling unit heating coils, to VAV box reheat coils, and to a small amount of finned tube radiation and cabinet heater loads in the atrium, perimeter offices, and mechanical room. This analysis considers several central plant alternatives as if they Unmet hours, percent were options in a new design i.e., demolition costs, salvage value, and other factors associated with a retrofit project are not addressed. The alternatives ultimately subjected to detailed analysis were developed through preliminary studies. The issues of load versus capacity outlined above proved critical in this application and had a strong effect on the final list of technically feasible solutions. Alternatives Two conventional plant alternatives were compared with an alternative featuring chiller-heaters. The first conventional alternative was the plant actually installed in the building and described above. Apr May Jun Jul Aug Sept Oct Unmet cooling hours Unmet heating hours 2 Percentage of monthly hours with unmet heating or cooling loads for a single chiller-heater plant. Supplemental heat exchanger 60-ton chillerheater only Cooling only Supplemental 750 MBtuh boiler only 200-ton chillerheater Airhandling units Perimeter and reheat loads The second was a plant identical to the existing one except that the chiller used in this case was an electric motor-driven -cooled centrifugal chiller with a nominal energy consumption of 0.58 KW per ton. The initial approach taken in developing a chiller-heater alternative was to assume that the entire heating and cooling load of the facility could be satisfied by a single chiller-heater. To ensure the feasibility of this approach, we conducted a preliminary analysis to investigate low cooling load and combined cooling and heating load scenarios. An hourly energy analysis was conducted using a commercial load program, and a careful comparison of loads with equipment capacity was made. Fig. 2 summarizes the results of this analysis. It was found that for off-peak months such as April, May, September, and October, the proposed chiller-heater could not provide the cooling and/or heating capacity required for the majority of operating hours. The percentage of total operating hours during which either cooling or heating loads were not fully met by the chiller-heater is shown for each month of the typical simulated year during which unmet loads occurred. During swing months requiring both heating and cooling, problems arose because of heating capacity limitations during simultaneous heating and cooling operation. Unmet cooling loads were distributed throughout the year due to the occurrence of cooling loads below the minimum capacity limit of the chiller-heater. For example, of the 182 hr in April during which cooling was required, 158 hr had loads below the minimum chillerheater output of 60 tons. Of the 476 hr during which heating was required, unmet loads occurred during 472 hr because the heating capacity of the chiller-heater in simultaneous heating and cooling mode was exceeded. Similar results were found in the other months of the year. 3 Single chiller-heater system and the modified alternative with two chillerheaters and a supplementary boiler (indicated by dashed lines). continued on page 110 January 1998 HPAC /Piping/AirConditioning 107

6 continued from page 107 System performance with one chiller-heater was more satisfactory during months when heatingonly mode predominated. As described previously, heating loads could be met by cycling the burner when the load fell below 30 percent of design; however, the efficiency of the chiller-heater operating at heating part load was roughly 5 percent lower than the existing gas boiler plant, which had an efficiency rating of 80 percent. It was clear, at this point, that a single chiller-heater would not be a feasible solution for this facility. However, when load conditions fell within the range of acceptable chiller-heater operating parameters, considerable operating cost savings were predicted relative to the existing central plant. Rather than simply discard the chillerheater alternative, we decided to modify the chiller-heater plant concept in two ways. First, to be able to meet low cooling loads, a second 60-ton chillerheater was added to the plant to cover loads smaller than the minimum capacity of the larger (200- ton) machine. The size of the larger machine was not reduced by an amount equal to the capacity of the smaller machine because the cost of this 260-ton combination was less than the cost of 200 tons divided between a 140-ton chillerheater and a 60-ton chiller-heater. The addition of the smaller chillerheater eliminated low cooling load problems but did not address the problem of capacity limitations during simultaneous cooling and heating operation. To do so required the further addition of a supplemental 750-MBtuh gas-fired boiler sized to meet 30 percent of the design heating load and the largest heating load anticipated during simultaneous cooling and heating mode. The single chillerheater system and the modified alternative with two chiller-heaters and a supplementary boiler are shown schematically in Fig. 3. The use of a supplemental boiler Billing demand, KW Jan Feb Mar Apr May Jun Jul Aug Sept Oct Nov Dec Existing plant Centrifugal chiller plant Chiller-heater plant 4 Monthly billing demand comparison. in a small chiller-heater plant has several advantages over a chillerheater-only system. The first is system redundancy. A supplemental boiler sized to handle 30 percent of the design heating load can maintain adequate building temperature should the chiller-heater be unavailable for any reason. The second advantage is that low heating loads can be met by continuous firing of the more efficient supplementary boiler instead of by chiller-heater burner cycling. At higher percentages of the design load, a chiller-heater can operate with the efficiency of a condensing boiler. In simultaneous heating and cooling mode, the supplementary heat exchanger of the chillerheater and the supplementary boiler function efficiently in series, as shown in Fig. 3. The supplementary boiler meets loads both less than the chiller-heater s minimum capacity and greater than its full capacity. An approach to elimination of part-load problems that was not pursued in this case is the use of dual-temperature thermal storage. Stored capacity could be used to meet peak heating loads during the shortfalls that can occur during simultaneous cooling and heating mode and to meet heating or cooling loads falling below minimum equipment capacity. In a small installation such as the one under consideration, the cost of this option would be prohibitive by comparison to the alternative chosen, and space for storage tanks was not available. A further option not evaluated was the use of a hybrid cooling plant combining a large chiller-heater with a small electric vapor compression chiller and a supplemental boiler. Electric, gas rate structures Realistic electric and gas rates are key to the accurate evaluation of operating costs. Case study gas and electric costs were based upon prevailing local rates and escalated for life-cycle cost purposes using U.S. Dept. of Commerce projections. The main features of the electric rate for the site are a monthly demand charge of $6.09 per KW of highest peak daily demand. Energy charges are assessed according to a load factor block rate as follows: First 500 KWH, $ per KWH. Balance to 165 KWH per KW of billing demand, $ per KWH. Next 85 KWH per KW of billing demand, $ per KWH. Over 250 KWH per KW of billing demand, $ per KWH. In addition, there is a flat monthly service charge of $15.88 added to the electric bill. In comparison, the cost of natural gas is $ per thousand cu ft plus a monthly service charge of $6.50. Operating costs Annual energy consumption for each system was estimated using models that took into account power consumed by chillers, pumps, air-handling unit fans, and cooling tower fans. It was also necessary to calculate non-hvac energy consumption because of its effect on the incremental cost of HVAC energy. Monthly electric demand charges for each alternative are shown in Fig. 4. The gas-fired chiller-heater plant has a flat demand profile rela- 110 HPAC /Piping/AirConditioning January 1998

7 Electric energy charge, $ 12,000 10,000 8,000 6,000 4,000 2,000 0 Jan Feb Mar Apr May Jun Jul Aug Sept Oct Nov Dec Existing plant Centrifugal chiller plant Chiller-heater plant 5 Monthly electric energy charge comparison. tive to the characteristically summer-peaking electric chiller alternatives. The annual demand charge savings achieved by the chiller-heater plant are approximately $7000 relative to the existing system and $2500 relative to the -cooled centrifugal chiller plant. Fig. 5 shows monthly electric energy charges for each system. Both the electric $120,000 $140,000 centrifugal chiller and $100,000 the chiller-heater, which substitutes gas $ 80,000 consumption for electric $ 60,000 $ 40,000 $ 20,000 $ 0 consumption, have much lower energy charges than the existing plant. A portion of these savings is due to shifts in load factor blocks caused by changes in billing demand. Total energy charges gas and electric are shown in Fig. 5. On the basis of energy cost only, the chiller-heater plant alternative is the best performer, saving nearly $30,000 per year relative to the building s existing system. However, energy cost alone does not give a complete picture of system cost; maintenance and capital costs must also be considered. $124,100 Existing plant $101,300 Centrifugal chiller plant 6 Annual energy cost comparison. $94,000 maintenance expenses at a fixed rate must allow for the possibility of catastrophic events such as crystallization of lithium bromide in the chiller-heater. If crystallization occurs, a lengthy and costly maintenance procedure is required to bring the chillerheater back to its original operating condition. Contract costs must reflect the risk of this major expense in the same way that the cost of an insurance policy reflects the average cost of a claim. The system design engineer can help reduce the risk of crystallization by providing emergency power to the chiller-heater and circulator pumps. This will help to eliminate one mechanism by which lithium bromide can concentrate to the point that crystallization will occur. For purposes of life-cycle cost and payback analysis, we estimated in this study that the maintenance cost for the two chiller-heaters and their cooling towers, the supplemental boiler, and various pumps would be approximately $18,000 per year. For the existing central plant, we estimated that the owner would incur an $8000 annual expense and that the -cooled centrifugal chiller plant would have a $9000 annual maintenance cost. These figures include annual mainte- Chillerheater plant Maintenance costs Annual maintenance costs for absorption chillers are higher than for electric motor-driven vapor compression equipment, based on annual contract maintenance rates. One factor in the higher cost of gas-fired absorption chiller maintenance is the relatively small number of service technicians. Additionally, service contracts covering all foreseeable $250,000 $200,000 $150,000 $100,000 $50,000 $0 $82,750 Existing system $120,000 Centrifugal chiller plant $228,000 Chillerheater plant $2,250,000 $2,200,000 $2,150,000 $2,100,000 $2,050,000 $2,000,000 $1,950,000 $1,900,000 $1,850,000 $1,800,000 $1,750,000 $2,236,095 Existing system Life-cycle cost $1,936, Centrifugal chiller plant Simple payback 7.24 $2,057,467 Chillerheater plant Years 7 Capital cost comparison. 8 Life-cycle cost and simple payback comparison. January 1998 HPAC /Piping/AirConditioning 111

8 nance, chemical treatment, and makeup costs. Capital costs The first costs of the three central plant systems can be seen in Fig. 7. A more detailed breakdown of costs is given in Table 1. The existing system cost is based on the air-cooled reciprocating chiller, two natural gas-fired boilers, and their associated pumps. The -cooled centrifugal chiller central plant cost includes the chiller, cooling tower, two natural gasfired boilers, and their associated pumps. The chiller-heater system cost is based on two chillerheaters, a supplemental boiler, and their associated pumps. The cost of added piping and controls required in the chiller-heater plant is also included. The cost of piping common to each system has been neglected. Both the -cooled centrifugal alternative and chiller-heater alternative are considerably more expensive than the existing plant. The chiller-heater central plant has an installed cost that is more than 2.5 times the first cost of the existing system. Life-cycle costs, simple paybacks Alternatives were evaluated using two criteria: 25-year life-cycle cost and simple payback relative to the existing system. Fig. 8 shows that the -cooled centrifugal chiller plant has a much lower simple payback and life-cycle cost than the chiller-heater plant system its life-cycle cost is lower than that of the existing system, and the payback period is less than two years. The chillerheater alternative is handicapped with respect to capital cost by the need for supplementary boiler capacity and the relatively high cost of chiller-heaters compared to electric-drive vapor compression chillers. The operating cost advantage of the chiller-heater relative to the -cooled centrifugal plant is largely negated by high maintenance costs. The lifecycle cost of the chiller-heater plant is less than that of the existing system, but because of its high capital cost (as indicated by long payback period), it probably would not be considered a feasible alternative by most owners. Consequently, the -cooled centrifugal chiller plant is the best alternative for this set of economic parameters. Conclusion Gas absorption chiller-heaters have been applied successfully in large projects where use of multiple chillers is feasible. Large facilities typically have a substantial base cooling load. If the base load is greater than 30 percent of the minimum capacity of the smallest chiller, chiller-heaters alone can be used as the building central plant. However, this study shows that a small office building presents part-load design difficulties that tend to favor the use of other technologies. TABLE 1 Detailed cost breakdown. Existing system Centrifugal chiller Chiller-heater Equipment cost, $ plant cost, $ plant cost, $ Chiller 60,000 69, ,000 Boilers 18,000 18,000 s 4,750 4,750 7,000 Cooling tower 28,000 28,000 Supplemental chiller-heater 66,000 Supplemental boiler 7,000 The engineer can overcome these application problems by a variety of means, as has been illustrated. Manufacturers, too, are addressing the problems associated with low-load operation of direct-fired chiller-heaters. A new generation of chiller-heaters that can unload down to 10 percent of design load will soon be available. If these new machines are capitalcost-competitive and perform up to expectations, the routine application of chiller-heaters in small commercial buildings may be just around the corner. HPAC The Penn State Dept. of Architectural Engineering, a unit of the College of Engineering, offers undergraduate and graduate professional education to students who desire careers in the building industry and allied fields. The five-year undergraduate Bachelor of Architectural Engineering (BAE) curriculum consists of a three-year core, in which all students pursue studies in Building Mechanical and Energy Systems, Building Structures, Building Illumination and Electrical Systems, and Construction Management, followed by two years of concentration in one of these four option areas. Coursework in the Dept. of Architecture further enhances students appreciation for the building as an integrated whole. Fifth-year AE students undertake a year-long project in which they comprehensively analyze the design of an actual building and perform a significant redesign project in their specialty area. Design documents, which the students must obtain during the summer preceding the fifth year, are frequently donated by summer employers. Faculty members serve as on-site consultants, but students also interact extensively with the project building designers, equipment suppliers, and utility representatives during the course of the year. The year culminates with presentations to a faculty jury, from which a select few are chosen for presentation to an external jury of distinguished practitioners. The fifth-year project of 1997 BAE graduate Roger Thies, described in this article, typifies the excellent work produced every year by upper level mechanical and energy systems students in the Penn State program. Further information on Penn State Architectural Engineering can be found at the department s web site engr.psu.edu/www/dept/arc/server/aetop.html. 112 HPAC /Piping/AirConditioning January 1998