1、VOLUME 10, NUMBER 3 HVAC&R RESEARCH JULY 2004231EDITORIALFuel Cells for Stationary PowerThe Promise behind the Press ReleasesGregory S. JacksonFew, if any, other technologies in the energy conversion industry have received more publicattention in recent years than fuel cells. This public interest ha
2、s been largely driven by the pushto develop fuel cells for automobiles and, to a lesser extent, for handheld portable applicationssuch as cell phones. For such applications, the focus has been on the development of low-tem-perature proton-exchange membrane (PEM) fuel cell technology, which relies on
3、 a hydratedpolymer membrane to transport H+or H3O+ions for the electrochemical reaction of hydrogenwith oxygen in air. The low-temperature operation of the PEM fuel cell makes it attractive forapplications requiring rapid start-up, such as passenger automobiles, but the need for low tem-peratures to
4、 maintain membrane hydration creates problems with minimal flexibility in fuelcomposition/purity and with reliance on expensive precious metal catalysts. In spite of the pub-lic focus on PEM fuel cells for automobiles, it is quite possible that the benefits of fuel cell tech-nology over the next ten
5、 years may be realized not as much in the arena of automotive PEM fuelcells (which experts claim are still more than a decade away from mass commercial production)as in the application of other higher-temperature fuel cell technology for stationary power appli-cations. The potential widespread imple
6、mentation of combined cooling, heating, and powerapplications makes the continued development and refinement of high-temperature fuel cells,such as solid oxide and molten carbonate fuel cells, a critical path for providing reliable, clean,and relatively quiet power with the necessary fuel flexibilit
7、y for wide-scale implementation.While automotive PEM fuel cells have been grabbing the public media headlines andeye-catching press releases, the continued improvement in commercial potential, particularly forsolid oxide fuel cells (SOFCs) and, to a lesser extent, molten carbonate fuel cells (MCFCs)
8、, maybring the first large-scale industrial benefit from fuel cells. This benefit may in large part bedriven by the implementation of combined cooling, heating, and power (CHP) for buildings andindustrial use, which, in turn, may arise with the portent of rising fuel costs and the correspond-ing pus
9、h for improved energy efficiency. SOFCs, which are now under development by numer-ous companies both large and small, rely on the high-temperature transport of O2-ions through adense oxide ceramic membrane to allow for the electrochemical oxidation of a fuel (which maybe hydrogen, syngas, natural ga
10、s, or even perhaps LPG). The high-temperature operation andexhaust (600C-900C), which are detrimental for the rapid start-up requirements in the automo-tive application, are beneficial in the stationary application, where waste heat recovery can beused to drive an auxiliary turbine for additional po
11、wer, an absorption chiller or desiccant systemfor cooling/air conditioning and/or any heating system. The potential CHP combined with thevery high thermal efficiencies of SOFC plants (50%), their fuel flexibility, and their quiet oper-ation make them a prime player in the changing electric power ind
12、ustry.Although less popular and flexible than the simpler SOFCs, MCFC technology has also beendeveloped for stationary applications and is being introduced with a broad potential for CHP.MCFCs rely on the transport of carbonate ions across a molten carbonate membrane at around650C to facilitate the
13、electrochemical oxidation of natural gas or a hydrogen-rich reformatestream. MCFCs are showing good durability in field demonstrations, but the material demands 2004. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in HVAC&R Research, Vol.
14、10, No. 3, July 2004. For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission.232 HVAC&R RESEARCHof confining molten carbonate membrane make this technology suited for larger applicationsw
15、here electrical power requirements are 100 kW. All the same, with overall thermal efficien-cies near 50% before waste heat recovery, the MCFC fuel cells may also play a role inlarger-scale CHP applications where reliable clean power on site is a must.Obviously, in light of the limited commercializat
16、ion of SOFC and MCFC technology to date,outstanding technical challenges remain before these fuel cell technologies can be implementedon a large scale and impact the distributed power generation industry. To a large extent, thesechallenges can be traced back to material durability and system cost. W
17、ith the higher tempera-tures, SOFCs and MCFCs gain fuel flexibility over and against PEM fuel cells and thereby elim-inate many of the needs for complex fuel pre-processors to convert natural gas or some otherfuel into a relatively clean H2-rich stream. Secondly, the high temperatures eliminate the
18、needfor precious metal catalysts. However, the high temperatures do present new challenges interms of the reliability of ceramic/metal interfaces required for the combined ionic and elec-tronic conductivities in SOFCs and the durability of conducting materials in contact with themolten carbonate sal
19、ts in MCFCs. These material issues are further complicated with the directuse of hydrocarbon fuels where improved system designs are sought to provide stable long-termoperation for several hundreds to thousands of hours. Further systems integration research forCHP applications will need to be forthc
20、oming as well. Progress in research and development on all of these fronts for high-temperature fuel celltechnology continues rapidly on a yearly basis. Continued support from both the U.S. and othergovernments indicate that the prospects for these fuel cells in stationary applications for distrib-u
21、ted power generation and, more specifically, CHP are tremendous. In light of the possible ris-ing gas costs, the high efficiencies and the flexibility of these fuel cells should make them primecandidates for numerous applications. In light of that, it is critical that the various originalequipment m
22、anufacturers (OEMs) in both the fuel cell and HVAC&R industry consider howintegration of high-temperature fuel cells may be optimized. 2004. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in HVAC&R Research, Vol. 10, No. 3, July 2004. For
23、personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission.VOLUME 10, NUMBER 3 HVAC&R RESEARCH JULY 2004233REVIEW ARTICLEScrew Compressors inRefrigeration and Air ConditioningNikola Stoic, Ph.D.R
24、ecent advances in the techniques for manufacturing vital parts, such as rotors and bearings,have enabled improvements to be made to screw compressors that were difficult to imagine onlya few years ago. This has inevitably influenced the prospects for these machines in refrigerationand air-conditioni
25、ng applications. Some aspects of this are presented, together with well knownbut not always fully appreciated principles of rational screw compressor design. Despite theimprovements already achieved, there is a continuing need to make refrigeration compressorsoperate more quietly, reliably, and with
26、 higher efficiencies over a long service life. This can bestbe done, with the minimum of experimental development, by consideration of all of the relevantvariables that affect their operation at the design stage. An evolving means of doing this is toinclude all of these factors simultaneously in mul
27、tivariable optimization procedures, and exam-ples of these are included. Several innovative ideas, some of them not necessarily new but onlyrecently considered or put into practice, are reviewed, and it is shown how the scope for suchmachines can be widened by performing more than one function withi
28、n a single pair of rotors,such as two-stage compression or combining compression with expansion. INTRODUCTIONCompressors used for industrial, commercial, and domestic applications consume approxi-mately 17% of the worlds electrical power output. The majority of these are of the positive dis-placemen
29、t type, of which the present world production rate is in excess of 200 million units peryear. Most of these are required for compressed air and refrigeration systems. Although recipro-cating compressors still dominate this market, many other types have a substantial share of it.Among these, screw co
30、mpressors have a growing role, especially where the power requirementis high and machine sizes are relatively large. Apart from their use in refrigeration and air-conditioning systems, a significant number ofscrew compressors are used in building engineering, food processing, and pharmaceutical indu
31、s-tries and for metallurgical and pneumatic transport applications. Screw compressors are essentially simple volumetric machines in which the moving parts alloperate with pure rotational motion. This enables them to operate at higher speeds with lesswear than most other types of compressors. Consequ
32、ently, they are up to five times lighter thantheir reciprocating counterparts of the same capacity and have a nearly ten times longer operat-ing life between overhauls. Furthermore, their internal geometry is such that they have a negligi-ble clearance volume, and leakage paths within them decrease
33、in size as compression proceeds.Thus, provided that the running clearances between the rotors and between the rotors and theirhousing are small, they can maintain high volumetric and adiabatic efficiencies over a widerange of operating pressures and flows. Specialized machine tools now enable the mo
34、st complexrotor shapes to be manufactured with tolerances of the order of 5 m or less at an affordablecost. The use of these in screw compressor manufacture, together with advances in rolling ele-ment bearings in which the rotors are retained, have led to great improvements in performanceNikola Stoi
35、c is with the Centre for Positive Displacement Compressor Technology, City University, London, England. 2004. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in HVAC&R Research, Vol. 10, No. 3, July 2004. For personal use only. Additional r
36、eproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission.234 HVAC&R RESEARCHand an increasing percentage of all positive displacement compressors sold and currently inoperation to be of this type. Consequently, as pointed ou
37、t by Fleming et al. (1998) in a review ofcontemporary design, modeling, and optimization of screw compressors mainly for refrigera-tion, the development of these machines is one of the great success stories of the last quarter ofthe twentieth century, given that they were harldy used prior to that t
38、ime.Historically, Lysholm (1942) was the first to publish a work on the asymmetric screw rotorprofile with a small blow-hole area, which formed the basis for modern screw compressor pro-files. The invention of oil flooding in 1955 was a major breakthrough in the application of thescrew compressor, w
39、hich resulted in cheaper machines without synchronization gears and sealsand, consequently, their ever increasing popularity in refrigeration. However, serious work onscrew compressors started only about 30 years ago when it first became possible to manufacturerotors that met the requirements for re
40、asonable compressor efficiency. Since then, many patentsin the field of screw compressor rotor profiles have led to successful contemporary designs.Typically, Bammert (1979), Shibbie (1979), Astberg (1982), and Ohman (2000) patented theirwork in asymmetric and “low” contact force rotors. Their profi
41、les were used for a large numberof applications, starting mainly with air compressors but followed almost immediately withrefrigeration. Rinder (1987) patented rotors that have been used exclusively in refrigerationcompressors. Kasuya et al. (1983), Lee (1988), and Chia-Hsing (1995) patented rotors
42、especiallyfor refrigeration, which later spread out to other fields. The rotors patented by Stosic (1996)describe a family of rotors suitable for a wide range of uses, of which refrigeration is a majorapplication.Despite the increasing popularity of screw compressors, public awareness and understand
43、ingof their mode of operation are still limited. The earliest reference textbooks by Sakun (1960) andAmosov et al. (1977) were published in Russian. Subsequently, Rinder (1979) and Konka(1988) published books in German, while works in English came only later by ONeill (1993)and Arbon (1994). Recentl
44、y, Xing (2000) published a reference textbook in Chinese, which hasalready made a positive impact on the new screw compressor manufacturers in the Far East.Textbooks on gears give a very useful background to the understanding of the principles ofscrew rotor profiling. One of these, by Litvin (1994),
45、 has recently been used as a serious refer-ence for screw compressor rotor profiling. Only lately have more journal publications appearedon screw compressors, such as that of Fujiwara and Osada (1995), who described their seminalwork on screw compressor modeling, which has been much used for the dev
46、elopment of a vari-ety of types of compressor, including those for refrigeration systems. Currently, there are regular international conferences held on compressors. These are the Pur-due University compressor engineering conference in the U.S., the IMechE conference on com-pressors and their system
47、s in the U.K., the SRM Technical Conference in Sweden, the VDI“Schraubenkompressoren Tagung” in Germany, and the Compressor Technique conference inChina. Despite being limited to SRM licensees, the Sweden conference is well attended andmakes regular contributions to the state of the art of screw com
48、pressors, and it is, as well as theGerman conference, exclusively devoted to screw machines. All of these conferences are valu-able sources for up-to-date information on screw compressors. For example, Sangfors (1982)presented one of the first papers on the contemporary modeling of screw compressor
49、processes.Sauls wrote a series of papers between 1992 and 2002 (for example, Sauls 1994, 1998) thatoutlined important routes and suggested actions needed to produce better refrigeration screwcompressors. It is impossible to review progress on screw compressors without taking account of the workdevoted to rotor manufacturing, tooling, and machine tool development. A significant advancein this area has been the combining of simultaneous grinding with profile inspection and its cor-rection, a contemporary practice that is becoming increasingly popular (Holmes and Stephen1999).
