1、AN-04-7-1 Experimental Verification of an Absorption Chiller for BCHP Applications Vikas Patnaik, Ph.D. Member ASHRAE ABSTRACT A single-effect water-lithium bromide absorption chiller has been designed and tested for low-grade waste-heat utili- zation. The nominal capaciv of the chiller is 90 tons,
2、and it is to be driven by water heated by both the exhaust stream and jacketwaterofa 615-kWnaturalgas reciprocatingengne. Lab performance of the stand-alone chiller has matched expecta- tions. The test data were in reasonable agreement with model predictions and arepresented in this papel: The succe
3、ss of this cost-optimized design further substantiates the promise of thermally activated cooling equipment integrated with distrib- utedpower generation sources to meet building energy needs holistically, with a single energy-eficient and environmentally friendly package. INTRODUCTION The Californi
4、a energy crisis of 2000 spurred a consider- able amount of development activity in the distributed power generation area. Gas engines, microturbines, and fuel cells have been at the center of this activity, as the need for reliable power and/or grid independence became evident. These devices are als
5、o being promoted to reduce the need for addi- tional central-station peaking power plants. As would be expected, however, they come at a first cost premium, which can range from $1 O00 to $4000/kW (Ellis and Gunes 2002). At the same time, operating efficiencies have remained in the vicinity of those
6、 of the large, centralized power plants, even after the transmission and distribution losses are taken into account. This is particularly true of smaller reciprocating engine and microturbine generating sets (25% to 35% electri- cal efficiency), while fuel cells promise higher efficiencies (-SO%), a
7、lbeit at the higher cost premiums ($3000-$40001 kW). The concept of cooling, heating, and power (CHP, previ- ously used to describe the subset of combined heat and power) is not new but has certainly seen a remergence in the last of couple of years. The prefix “buildings” has been added to the latte
8、r (BCHP) to emphasize the specific application and, more recently, these combined localized systems have been referred to as integrated energy systems (IES). The goal, regardless of the abbreviation, is to improve system efficiencies or source fuel utilization by availing of the low-grade heat that
9、is a by- product of the power generation process for heating andior cooling duty. Fuel utilization efficiencies as high as 80% have been reported (Adamson 2002). The resulting savings in oper- ating cost, relative to a conventional piece-wise system, are then viewed against the first cost, and simpl
10、e paybacks under four years have been anticipated (LeMar 2002). This is particularly important from a marketing perspec- tive, for both the distributed-generation and the thermal equip- ment provider. This is because, by themselves, a microturbine manufacturer and an absorption chiller manufacturer,
11、 for example, would find it difficult to compete with a utility and an electric chiller manufacturer, respectively, as the provider of low-cost power and cooling. Last, but by no means least, the higher (fossil) fuel utilization rates result in reduced emissions of CO, the greenhouse gas with over 5
12、5% contribution to global warming (Houghton et al. 1990). This work presents a first step toward the commercial development of such an integrated energy system: the design and testing of an absorption chiller that can utilize relatively low-temperature heat such as that from a heat recovery unit tha
13、t taps into the exhaust stream and jacket water of an engine. Vikas Patnaik is a project leader in the Thermal Systems Department of Engineering Technology, Trane, La Crosse, Wisc 02004 ASHRAE. 503 CHILLER DESIGN The hot water temperature suitable to drive a standard indirect-fired single-effect abs
14、orption chiller is about 270F (leaving the generator at -230F). An entering temperature lower than this would result in derating the capacity for a given flow rate. Conversely, if a certain cooling capacity is desired at an entering hot water temperature of, say, 200“F, a signifi- cantly oversized s
15、tandard chiller would have to be employed. In the authors experience, this can be by as much as 50% for a leaving temperature of -190F. Low-source-temperature firing of the absorption chiller necessitates a different design concept. With a lower mean temperature difference available in the generator
16、, a higher overall thermal conductance or UA value (U being the overall heat transfer coefficient, A the heat transfer surface area) is required here. A preliminary study of heat (and mass) transfer component performance was carried out, comparing the present conceptual (BCHP) design to that of a st
17、andard prod- uct offering. Table 1 lists the increases in UA, going from the latter (270F water-fired) to the former (207F water-fired): From Table 1, it is apparent that the generator needs more than a doubling of size for the given tube and flow rates per tube. This enormous boost notwithstanding,
18、 the absorber and condenser enhancement requirements are not altogether insig- nificant. Thus, the reallocation of tube surface area involves the condenser, evaporator, and absorber as well. A detailed analysis of low-temperature-fired absorption from a systems standpoint can be found in the work of
19、 Goodhear et al. (2002). An optimized chiller design would be one that yields the lowest first cost within certain operating constraints. Variable elements such as the cost of tubes (which depends on number, length, etc.) and shell material cost (which depends on the girth of the bundle, steel plate
20、 thickness, etc.) compose the first cost, e.g., chiller cost that varies with bundle size and aspect ratio. Minimizing this cost via bundle manipulation, recogniz- ing the interdependence (nonlinearity) of headmass exchanger performance, is, thus, the objective function. The operating constraints co
21、nsist of equality constraints, such as capacity and crystallization margin, and inequality constraints, such as pressure drops. A comprehensive optimi- zation algorithm based on a modified dynamic programming technique was developed a few years ago and has been used since in the design of various st
22、andard chillers (Patnaik 2001). The same routine was applied to the given problem with the lower heat source temperature, and a very different tube surface area distribution was obtained between the component bundles. The source-related operating conditions for the design optimization are shown in F
23、igure 1. The hot water entering the chiller is at 207“F, and it leaves at 190F. For lower entering temperatures, the cost advantage of the optimized design relative to a standard product grows. This can be seen in Figure 2. As would be expected, the resulting generator bundle is much larger than tha
24、t of the standard single-effect chiller for a given tube diameter. Commensurate with the bigger tube Component Absorber Evaporator Condenser Generator YO Increase 36% 8% 31% 115% SILENCLR I HEAT RECOVERY 35% chiller IO% 3501. chiller I ENGINE II I J, oil-cooler CiW“il I ZOP Figure 1 Schematic of env
25、ironment for which absorption chiller has been designed. - 180 190 zw 210 no 230 ao 250 280 no 280 hwi Input (ha-mratoring) bymeratwe. F Figure 2 Relationship between chiller jrst cost and heat input temperature. count would be a greater number of passes to keep the inside water velocities (and, hen
26、ce, heat transfer coefficients) at the desired level. A second benefit of the greater number of passes is the migration from crossflow to a more counterflow heat exchange (Bowman et al. 1940). Thus, the heat transferred in the generator is given by 504 ASHRAE Transactions: Symposia Q UA . FAT, , (1)
27、 where ATuvg is the counterflow log-mean-temperature differ- ence and the correction factor F approaches unity. In addition to the augmented generator, the chiller employs a highly effective solution heat exchanger and novel solution flow handling/plumbing. LAB TESTING The testing carried out in thi
28、s work deals only with the optimized absorption chiller. The hot water supplied to the unit was controlled to simulate that from the heat recovery unit of the future integrated package (engine + chiller with new system controls). The actual package will be put together and tested in the next phase o
29、f the project, later this year. The absorption chiller was fabricated as a special manu- facturing order. Figure 3 shows an isometric view of the lab chiller. Installation in the lab included a steam-to-water heat exchanger and associated piping. Accurate control of the entering hot water state (tem
30、perature, flow rate) was accom- plished through two-piece pneumatic and three-way butterfly “energy“ valves in the steam and (return) water lines, respec- tively. A schematic of the test loop with envelope temperatures is shown in Figure 4. The test plan for this chiller was designed to cover the ty
31、pical operating map for air-conditioning applications. This consisted of chilled water (supply) temperatures ranging from 42F to 55 OF, and tower water (entering) temperatures from 70F to 100 OF, with standard operating conditions at 44F and 85“F, respectively. The water (specific) flow rates were s
32、et at the ARI value standard for single-effect absorption, i.e., 2.4 and 3.6 GPWton, respectively, with the hot water flowing at its design rate of -1.9 GPWton. Part-load tests were also conducted, albeit at the standard conditions only. It should be noted that the objective of this testing was prim
33、arily proof-of-concept. The chiller was not intended to be a production prototype; further design improvements resulting Figure 3 Front isometric view of 90-ton lab chillel: from the outcome of the tests-and subsequent qualifica- tion-would be necessary to bring it to that status. RESULTS AND DISCUS
34、SION The chiller produced cooling that was within AN toler- ance of the nominal capacity for standard conditions. This occurred at a COP of 0.72, which actually exceeds the prescribed 0.70 value from ASHRAE Standard 90.1 (ASHRAE 1999). All components performed satisfactorily except the condenser, wh
35、ich required some modification. The solution heat exchanger performed particularly well, attaining the effectiveness for which it was designed. The performance of the chiller was quite sensitive to the entering hot water temperature, as expected. Figure 5 shows I 1 - steam ! 1207F I I ABSORPTION CHI
36、LLER Figure4 Schematic of test loop with design input temperatures. 140 120 100 Figure 5 Efect of entering hot water temperature on chiller performance: 90-ton mode. ASHRAE Transactions: Symposia 505 140 1M 40 20 o 0.78 0.74 o.m 0.58 0.54 050 Figure 6 Effect of hot water flow rate on chiller perform
37、ance kart-load): 90-ton mode. the dramatic decline in capacity as the hot water temperature is decreased from 230F to 160F. Part-load performance was simulated by lowering the hot water flow rate while maintaining its temperature at 207F. Capacity dropped at an ever-increasing rate with flow rate (F
38、igure 6). The accompanying efficiency decrease was much more nonlinear, with little change in the 80-180 GPM range and a drastic fall between 15 and 35 GPM. The impact of decreasing water flow rate on performance via in-tube heat transfer coefficient is secondary to that via the rapidly dimin- ishin
39、g leaving hot-water temperature (and, hence, average temperature difference driving heat transfer). The testing was brought to a close with the entering hot water temperature notched up to 237”F, representing the high- est-grade engine waste heat. Nearly 132 tons of cooling were obtained at a COP of
40、 nearly 0.7. Figure 7 shows the response of the chiller to varying grades of heat in the 125-ton mode, Le., with the chilled and tower water flow rates adjusted for this nominal capacity to maintain the industry-standard 2.4 and 3.6 GPM/ton, respectively. The maximum in COP coin- cides roughly with
41、the design entering hot water temperature, Le., 207F. The chiller now awaits integration with the gas engine and heat recovery unit for testing as a packaged CHP system with a central control system. Figure 8 shows the post-testing chiller, ready for shipping. Overall dimensions have been superimpos
42、ed to illustrate its compactness. CONCLUSIONS A single-effect, water-lithium bromide chiller fired by low-temperature hot water has been designed, fabricated, and tested. The hot water loop was controlled to simulate water carrying waste heat from a gas-fired reciprocating engine. A Figure 7 Effect
43、of entering hot water temperature on chiller performance: 125-ton mode. 140” I . s.l” Figure 8 Post-testing BCHP chiller prior to shipping for system integration. crucial step in the design of the absorption chiller was the bundle optimization to utilize the low-grade heat at the lowest capital cost
44、, making it viable for future commercialization in an integrated energy system (BCHP) package. Initial testing of the chiller revealed that it came within the AN tolerance of making the 90-ton capacity at nominal operating conditions. Performance of the unit as a 125-ton chiller, using 237F engine w
45、aste heat, exceeded expectations. ACKNOWLEDGMENTS The author wishes to express his appreciation for the support received from the U.S. Department of Energy/Oak Ridge National Laboratory and the Gas Technology Institute for this project under Prime Contract No. 4000009522 and Subcontract No. PF 1 14
46、13. The funding of such efforts must be coupled with the creation of market awareness among contrac- tors and customers over integrated energy systems if proven technologies such as absorption are to contribute to sustain- able energy solutions. 506 ASHRAE Transactions: Symposia REFERENCES Adamson,
47、R. 2002, Mariah Heat Plus PowerTM Packaged CHP, Applications and Economics. Presented in the 2nd Annual DOE/CETC/CANDRA Workshop on Microtur- bine Applications at the University of Maryland, Col- lege Park, January 2002. ASHRAE. i 999. ANSI/ASHRAE Standard 90.1-1999, Energy Standard for Buildings Ex
48、cept Low-Rise Resi- dential Buildings. Atlanta: American Society of Heat- ing, Refngerating and Air-conditioning Engineers, Inc. Bowman, R.A., A.C. Mueller, and W.M. Nagle. 1940. Mean temperature difference in design. Transactions of the ASME, May 1940, pp. 283-294. Ellis, M.W., and M.B. Gunes. 2002
49、. Status of fuel cell sys- tems for combined heat and power applications in build- ings. ASHRAE Transactions 108 (1). Goodheart, K.A., S.A. Klein, and K.J. Schultz. 2002. Eco- nomic assessment of low firing temperature absorption chiller systems. ASHRAE Transactions 108( 1). Houghton, J. T., et. al. 1990. Climate change: The IPCC sci- entific assessment. LeMar, P. 2002. Integrated energy systems (IES) for build- ings: A market assessment. Final Report by Resource Dynamics Corporation for Oak Ridge National Labora- tory, Contract No. DE-AC05-000R2272.5, September 2002
