论文igu2006pdfigu2006paperpaperadd9851

1、23rd World Gas Conference, Amsterdam 2006Engine-driven desiccant-assisted hybrid air-conditioning systemC. K. Qin ChinaG. Schmitz GermanyABSTRACTDespite that in China gas cooling is becoming more attractive, absorption chiller has not been successfully utilized due to its lower efficiency and compar

2、atively higher operation cost. The initial cost of engine-driven chiller seems to be an obstacle to its popularity. The gas/electricity price ratio in big cities does not favor gas cooling.In order to explore the margin within which gas cooling may defeat its electric counterpart, a hybrid air-condi

3、tioning system incorporating engine-driven chiller and desiccant dehumidification was configured out and experimentally tested. Sensible load was undertaken by chilled water from engine- driven chiller, while moisture load was undertaken by recovered waste heat. A test rig was set up to measure the

4、performance of engine-driven chiller and dehumidification capacity. Comparison between theoretically predicted and measured values was presented. The waste heat recovered from engine- cooling system and exhaust was experimentally determined. Also measured was the performance improvement of chiller w

5、ith higher temperature chilled water being capable of only sensible cooling.Economic benefits of hybrid air-conditioning system over conventional electric chiller were calculated for a reference building, on the basis of experimental data. The result reveals more than 30% savings on operation cost c

6、an be achieved.The work in this paper will help promote efficient utilization of natural gas in small commercial building air-conditioning systems.TABLE OF CONTENTSAbstract1.Introduction1. Overview of gas cooling and desiccant dehumidification1.1 Gas cooling options1.2 Desiccant dehumidification and

7、 moisture load1.3 Engine-driven hybrid air-conditioning system (EDHAS)2. Exprimental facilities2.1 Engine-driven chiller2.2 Waste heat recovery2.3 Desiccant wheel system3. Experimental results3.1 Desiccant wheel performance3.2 Engine performance measurement3.3 COP changing with chilled water tempera

8、ture (CWT)3.4 Waste heat recovery changing with supply water temperature (SWT)4. Economic benefits analyses5. Conclusions and outlookAbstract ReferencesList of TablesTable 1Dehumidification capacity comparison: measured vs predicted Table 2Engine-driven chiller system performance under variable spee

9、dsList of FiguresFigure 1Engine-driven chiller system (including waste heat recovery) Figure 2Illustration chart of humidity measurementFigure 3aHourly sensible load and moisture load of reference building Figure 3bSchematic illustration of hybrid air-conditioning systemFigure 4Hourly heating/coolin

10、g capacity required in hybrid air-conditioning system1. IntroductionIn China gas cooling is becoming increasingly attractive. With the rapid development of economy and improvement of living standard, more and more buildings and families are equipped with air- conditioning (AC) most of which are elec

11、tricity-driven vapor-compression (VC) systems due to simplicity and lower initial cost. The excessive utilization of electricity for AC resulted in a severe shortage of peak-electricity. It was estimated that more than one-third of peak electricity consumption (18,000 MW) of Shanghai in the summer o

12、f 2005 was attributable to AC systems1. With the adjustment of Chinas energy strategy, natural gas will play more important role in national framework. The commercial operation of Great-Gas-Project, a 5000km-long pipeline with annual transmission capacity 12*109m3, together with the constructions of

13、 several LNG terminals along east coastal regions, will bring natural gas consumption up to 12% among primary energy by the end of 20202.Gas cooling can be helpful by moving some of summer load from overloaded electricity systems to underused gas systems. Although encouraging policies from gas utili

14、ties help promotion of gas cooling systems, the most important issue influencing popularity is the cost-effectiveness resulting from energy efficiency. In order to explore the margin within which gas cooling may defeat its electric counterpart, a series of research have been carried out in Tongji Un

15、iversity, Shanghai, China. An engine-driven AC system incorporating desiccant dehumidification was configured and experimentally tested. Energy consumptions and operation costs were calculated on the basis of measured performance data.2 Overview of Gas Cooling and Desiccant Dehumidification2.1 Gas c

16、ooling optionsThree types of gas cooling are commercially available3, namely absorption, gas-engine driven VC and desiccant cooling. Absorption was recognized as the origin of contemporary refrigeration technology, and has been dominating gas cooling market due to its reliability, maturity, and avai

17、lability of various chilling output. Its growth began to slow down due to comparatively lower efficiency, especially with the advent of engine-driven systems. From the 1990s, engine-driven systems began to play an important part in US, Japan etc, as a result from the progress of engine technology an

18、d correspondingly prolongation of non-stop operating duration without maintenance. Desiccant cooling which dated back to Penningtons patent in 1950s4, has been an active research field due to its characteristics of being environmentally benign. Most of desiccant cooling systems make use of heat from

19、 gas combustion to regenerate desiccant equipment therefore to undertake moisture load of AC system. For sensible load, evaporative cooling (EC, direct/indirect) has preferably been selected. Because the efficiency of EC is quite sensitive to climate condition, desiccant cooling has been limited to

20、some special applications until the middle of 1990s. As a result of continual research since 1980s to increase dehumidification capacity and to overcome time-over degrade567, etc, desiccant cooling increased to such an extent that annually installed capacity in 2001 accounted up to about 30% in nort

21、h America gas cooling market8.The successful utilization of gas cooling depends upon economic benefits. In most regions of China thegas/electricity price ratio seems not so favorable to gas cooling. Take Shanghai as an example, natural gas (calorific value 35.6MJ/m3) costs 2.1 RMB/m3; By comparison

22、electricity costs 0.61 RMB/kWh for domestic users. For commercial users electricity price varies with time and input rates. Assuming theprice of natural gas and electricity is 2.1 RMB/m3 and 0.71 RMB/kWh (time-averaged) respectively, and COP (coefficient of performance) of electricity-driven VC chil

23、lers, absorption chillers and engine-driven chillers is 4, 1.2, 1.4, respectively, corresponding operation cost for above technology is 0.175, 0.175, 0.15RMB per kWh of chilling output. Measured in terms of maintenance and initial cost comprehensively, natural gas AC equipments are not as competitiv

24、e as their electrical counterparts. By the end of 2003 installed gas cooling capacity in Shanghai accounted up to only 500MW with annual gas consumption 43*106m3, about 3% of total annual gas consumption.Many manufacturers in China begin to consider the economic utilization of waste heat from gas-en

25、gine. Some manufacturers introduced a “double-effect” chillers which produces chilled water from waste heat recovered from the engine, in addition to the chilled water from VC refrigeration. The COP can be raised to 2.5, but its price almost doubles as well, preventing further recognition by end-use

26、rs. In this paper the waste heat was recovered to regenerate desiccant wheel and then to undertake moisture load.2.2 Desiccant dehumidification and moisture loadDesiccant is a special kind of hygroscopic material which has strong affinity for moisture. All desiccants work on principle of moisture tr

27、ansfer driven by vapor pressure difference between air and desiccant. When the desiccant is cool and contains lower moisture, it attracts moisture from the air until the partial pressure of water vapor is in equilibrium with that of air. Then the desiccant must be regenerated (or reactivated), an ai

28、r stream of higher temperature (lower relative humidity) passes through desiccant to cause the adsorbed/absorbed moisture to vaporize again. In AC engineering9, desiccant wheel which provides continuous sorption and regeneration consists of several important components: matrix impregnated with desic

29、cant, driving motor, supporting structure and frames. Across flow area of slowly rotating matrix two physically separated air streams are arranged in counterflow pattern to achieve favorable heat/mass transfer results. Consequently the matrix is exposed to different airstreams alternatively within a

30、 cycle, repeating sorption and regeneration process. Viewed in an overall effect, desiccant wheel fulfills dehumidification task at the cost of heat of regeneration air.The dehumidification capacity of desiccant depends upon its sorption property, regeneration temperature, rotational speed and heat/

31、mass transfer area etc. Accompanied with moisture transferred to desiccant, a certain amount of heat (latent heat plus wetting heat) is released, preventing further sorption from happening. The higher the regeneration air temperature is and the higher the specific area of desiccant matrix is, the lo

32、wer the humidity ratio of leaving processed air will be.In traditional AC systems, air must be deeply cooled below dew point to make moisture condensing out. Usually the deeply cooled air has to be reheated to a comfortable temperature prior to delivery. This process, first deeply cooled and then re

33、heated again, wastes lot of energy on dehumidification. But it has been adopted by Chinese designers for a long time owing to its simplicity and easiness for control. If desiccant dehumidification is incorporated, this thermodynamically unreasonable process can be avoided, leading to some inherent a

34、dvantages. The humidity control independent of temperature can be realized. Fan coils (FC) distributed in air-conditioned rooms can work under “dry” conditions, preventing fungus growth and therefore improving indoor air quality (IAQ) as well. In addition, with moisture load being undertaken by desi

35、ccant, chilled water of higher temperature (or natural cooling source) can be used for sensible cooling.2.3 Engine-driven hybrid air-conditioning system (EDHAS)In 1988 Maclaine-Cross10 proposed an EDHAS configuration, where an engine-driven chiller in combination with an electric chiller accommodate

36、d a hotel. A packaged air handler including desiccant wheel provided fresh air and was powered by recovered heat from gas-engine. His EDHAS proposal was targeted at decreasing peak electricity by transferring moisture load to waste heat at reasonable price. Later Parsons11 explored the possibility o

37、f recovering waste heat from GHP (gas heat pump) to supplement heating in winter, and to enhance cooling in summer. His calculations showed that in most climate regions in US, such a hybrid system could defeat electric AC systems. From then on, many proposals have been researched on the combination

38、of engine-driven chiller or heat pump with desiccant dehumidification.One amazing climate characteristic in Shanghai and southern part of China is abnormally high humidity. The design parameter of outdoor air in Shanghai is 34, 65% RH, and the treatment of fresh air usually accounts up to 1/3 of AC

39、systems energy consumption. Under such climate conditions previous researches are not applicable if no adaptation is configured, because the maximum dehumidification capacity of presently available desiccant equipment is limited to about 810g/kg. Some other dehumidification measures should be combin

40、ed.Qin12 proposed two different EDHAS systems to accommodate the high humidity. One incorporated pre-cooling from engine-driven chiller, and the other combined desiccant dehumidification with passive dehumidification (enthalpy exchange). The initiatives of these approaches were to explore the potent

41、ial benefits compared with conventional systems.Whether the increasing complexity of EDHAS can be compensated by the improved energy efficiency depends upon following factors:(1) The dehumidification capacity of desiccant wheel;(2) When the required regeneration air temperature increases, the temper

42、ature of waste heat recovery must increase as well. The decreasing extent of available waste heat should be quantified in order to evaluate the system performance;(3) The improvement degree of chiller cooling capacity with the rising temperature of chilled water under constant cooling water temperat

43、ure.In order to answer above questions, an experiment system (as shown in Fig1) which combined a desiccant wheel and an engine-driven chiller was developed.3 Experimental facilities3.1 Engine-driven chiller 13A commercially available VC chiller with a 4FV-7 reciprocal compressor of four cylinders wa

44、s selected. An automotive-derivative petroleum engine with maximum output 32.5kW was converted to natural gas to take the place of the electric motor. First the carburetor was replaced by a proportional mixer of equal vacuum to intake natural gas and air simultaneously. In order to precisely control

45、 the throttle openness (therefore the flow rate of gas-air mixture), a leverage system driven by step-motor was developed. Lean-burn was adopted by controlling gas supply pressure. The residual O2 content in exhaust gas was always maintained above 4% to ensure complete combustion. To measure therota

46、tional speed and torque transferred to the compressor, a torque/RPM sensor was coupled in line with the shaft.Fig.1 Engine-driven chiller system (including waste heat recovery)The allowable maximum rotational speed of the compressor is 1450rpm. With the decreasing rotational speed, the lubrication o

47、il pressure drops as well because of rotary pump involved. When the differential pressure between lubrication oil and refrigerant intake drops to less than 0.15MPa, de-energizing assembly will automatically turn on the by-pass to the compressor and stop refrigeration. The minimum rotational speed ke

48、eping compressor function was experimentally determined to be 700rpm. All experimental measurements were made within compressor rotational speed 7001450rpm, and interval was set at 100rpm. The VC chillers nominal chilling capacity was 55kW with refrigerant R22 under nominal rotational speed (1450rpm

49、), and its output could be modulated by energizing/de- energizing two cylinders, namely 0-50%-100%. With the introduction of rotational speed control, the chilling output modulation was widened. With 2 cylinders on, the modulation range was 29.5%55% when rotational speed changed from 7001400rpm. Wit

50、h 4 cylinders on, the modulation range was 60%100% for the same rotational speed changes. According to previous research13, the engine efficiency was quite lower when only 2 cylinders were energized. So all experiments in this paper were done with 4 cylinders on.Two water tanks, one for chilled wate

51、r and the other for cooling water, were built to avoid the use of cooling tower (Fig.1). Then the measurement could be made independent of weather conditions. Part of chilled water leaving evaporator was directed to cooling water tank to maintain the temperature of cooling water feeding condenser. M

52、eanwhile part of the cooling water leaving condenser was directed to chilled water tank to make the temperature of chilled water feeding evaporator satisfying requirements. In addition, certain amounts of water of room temperature were supplemented into both tanks while the same amounts drain to kee

53、p their water levels.3.2 Waste heat recovery system14Efficiency of waste heat recovery was a crucial part in hybrid AC systems. For engine cooling system, coolant (water) was directly pumped into the engine-jacket. A two-stage waste heat recovery systemwas designed to ensure recovery efficiency of e

54、xhaust heat (as shown in Fig.1). The original exhaust manifold was taken off and a jacket-type heat exchanger (HX) was installed as the first stage. Exhaust gas entered the first stage HX at ca. 500 厂 and left at 300 厂 . The second stage was a heat-pipe HX. Water from the engine-cooling system enter

55、ed the jacket-type heat exchanger and the heat-pipe exchanger in sequence, to recover heat from the exhaust gas more efficiently. Finally the flue gas temperature was reduced to 120 厂.3.3 Desiccant wheel systemLithium chloride was selected as desiccant for experimental hybrid AC system because of it

56、s lower regeneration temperature required (only 70 厂) compared with silica gel (higher than 100 厂). It is much easier to generate 80 厂 hot water from waste heat recovery at reasonable cost.An EDHAS system incorporating an enthalpy wheel and a desiccant wheel was built. Both wheels use lithium chlori

57、de as desiccant and the only difference is the rotational speed. For desiccant wheel, it is 20rph; and for enthalpy wheel, it is 10rpm.Fig2 Illustration chart of humidity measurementDehumidification capacity measurement has some intrinsic difficulties15. First, relative humidity (RH) measurement has

58、 a strong influence upon humidity ratio. Despite both RH sensor and dry/wet bulb thermometers are considered applicable, uncertainty analyses reveal that for some specific inlet conditions RH sensors of +/-3% accuracy may lead to 20% uncertainty for moisture removal capacity (MRC)16. Secondly, the c

59、ondition of higher dry-bulb temperature and lower RH should be avoided when accurately measuring humidity ratio. Accordingly a diagnostic system including 7 sets of dry/wet bulb thermometers were built (Fig.2), and all the thermometers were of Pt-100A type and calibrated to+/- 0.1厂 by repeatedly compared w