ANALYSIS OF ENERGY EFFICIENCY OF CHILLERS OPERATION AT NIGHT

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1 , Vlue 2, Nuber, p.8-22, 200 ANALYSIS OF ENERGY EFFICIENCY OF CHILLERS OERATION AT NIGHT Y.A. Li, M.Z. Yu and G.L. Xu Shandng Institute f Architecture and Engineering, Ji nan, 25004,.R. China (Received 6 February 200; Accepted 7 May 200) ABSTRACT This paper is based n actual easureent weather data f recent decade in Ji nan,.r. China, and prfile and characteristics f utdr air wet bulb teperature are analyzed. It is fund that utdr air wet bulb teperature has an effect n the perfrance f chillers. Results shw that the cling capacity is increased by 9.0 percent, cnsuptin pwer is reduced by 7.2 percent, and cefficient f perfrance is stepped-up by 3.5 percent when chillers perate at night. Last suer, the perfrance f chillers in a htel building in Ji nan was easured. Measureent results shwed gd agreeent with thery calculatin results.. INTRODUCTION Refrigeratin is defined as the prcess f extracting heat fr a lwer-teperature heat surce, and transferring it t a higher-teperature heat sink. Refrigeratin aintains the teperature f the heat surce belw that f its surrundings while transferring the extracted heat, and any required energy input, t a heat sink. A refrigeratin syste is a cbinatin f cpnents and equipent cnnected in a sequential rder t prduce the refrigeratin effect. The refrigeratin systes cnly used fr air cnditining is a chiller f vapr cpressin. In chillers, cpressrs activate the refrigerant by cpressing it t a higher pressure and higher teperature level after it has prduced its refrigeratin effect. The cpressed refrigerant transfers its heat t the sink and is cndensed t liquid fr. This liquid refrigerant is then thrttled t a lw-pressure, lw-teperature vapr t prduce refrigerating effect during evapratin. The cndenser is a water-cled cndenser in which the cling water is used t reve cndensing heat fr the refrigerant, and cling water is the recirculating water fr the cling twer which is evaprative cling twer in the research prject. The chillers run in the day in the traditinal air cnditining systes. The perfrance f chillers will change when chillers perate at night. 2. DAILY WET BULB TEMERATURE ROFILE Inlet teperature f cling water has a significant effect n the perfrance f chillers and it depends n the wet bulb teperature f utdr air. In general, utlet teperature f cling twer, that is, inlet teperature f chillers is equal t the abient air wet bulb teperature plus 3~5 C. Measured hurly wet bulb teperature f recent decade in Ji nan in August has been analyzed and studied based n standard eterlgical year []. Fig. and Table shw that the average wet bulb teperature f utdr air is 2.5 C at night, it is 23.2 C in the day and at night is.7 C less than that in the day. In ther wrds, cling water utlet teperature f cling twers, cndensatin teperature f refrigeratrs at night is.7 C less than that in the day. Teperature p ( ( ) C) tie (hur) Fig. : Daily wet bulb teperature prfile in Ji nan 8

2 Table : Wet bulb teperature in Ji nan August ( C) hur date day night Teperature ( ) Teperature ( C) 28 day 27 night Tie (day) Fig. 2: Cparisn f wet bulb teperature 3. ANALYSIS OF ENERGY EFFICIENCY A htel building in Ji nan has a grss space f (8-stry). The ttal cnditined space is The building s peak air-cnditining lad is 636 kw. istn cpressrs in this building are used and the refrigerant is R22. Theral calculatin f ne chiller is carried ut in the day and at night respectively. With the aid f the pressure-enthalpy diagra, cling capacity ay be expressed as: 9

3 Φ Φ = V R q v = η V h q v () = η V h q /V Factrs that influence the η f the cpressr include: clearance vlue and cpressin rati η v. Bth factrs affect the vlue f reexpansin gas trapped in the clearance vlue. 2 ηv = c (2) Cefficient f heat effect η t. When vapr refrigerant enters the cpressr, heat absrbed by the vapr results in a heating effect that increases the specify vlue f the refrigerant and, therefre, the V R value. η = T / T (3) t k Cefficient f leakage η l. Refrigerant leaks thrugh the gap and the clearance acrss the high- and lwpressure sides f the cpressr, such as the clearance between the pistn rising and the cylinder in a reciprcating cpressr. η l = 0.98 (4) Cefficient f thrttle η p. There is resistance when vapr refrigerant runs thrugh suctin and exhaust valves. + c η p = (5) η By using Eqs. (2)~(5) η = η v v t l p 2 + c = c 0.98 Tk ηv + c = 0.98 c v η T k (6) By using Eqs. () and (6) + c q = 0.98 c V v η Tk V (7) h The fllwing equatin will be used fr accunt shaft pwer. e = M = M R R W W th th /( ηi ) + bt Tk (8) The effectiveness f refrigeratin cycles, r cefficient f perfrance (CO), is ne paraeter that affects the efficiency and energy cnsuptin f the chiller. The CO f a refrigeratin cycle using a specific refrigerant depends ainly upn the isentrpic wrk input t the cpressr at a given cndensing and evaprating pressure differential, as well as the refrigeratin effect prduced. The CO f chillers is als defined as: CO = Φ e (9) Experiental fitting frulas f calculatin cling capacity and shaft pwer are derived fr references [2-6] in rder t siplify the calculatin. Φ 0 = a exp[b (t + b 2 ) 2 + b 3 (t k + b 4 ) 2 + b 5 (t t k +b 6 ) 2 ] (0) e = a 2 exp[c (t + c 2 ) 2 + c 3 (t k + c 4 ) 2 + c 5 (t t k +c 6 ) 2 ] () Since refrigerant in the chillers is R22, these cefficients are respectively as fllws: a = , b = , b 2 = , b 3 = , b 4 = , b 5 = , b 6 = a 2 = , c = , c 2 = , c 3 = , c 4 = , c 5 = , c 6 = It is ver-siplified t assue that the cndensing teperature drps by the sae aunt as the wet bulb teperature in Fig is the rating cnditin in the day, the cndensatin teperature is 40 C and the evapratin teperature is 5 C is the rating cnditin at night, the evapratin teperature is still 5 C, and the cndensatin teperature is 38.3 C which is.7 C less than that in the day. 20

4 Table 2: Cparisn f chillers perfrance in the day & at night Rating cnditin Calculatin Measureent Φ 0 (kw) e (kw) CO Day Night Day Night ΔΦ 0 / Φ 0 Δ e / e CO / CO ressure increased, and energy cnsuptin is reduced at night. Because energy situatin is nw very serius, s it is right tie that chillers perate at night as far as pssible. The develpent f peratin ethd is very iprtant and has a bright future. NOMENCLATURE Enthalpy Fig. 3: Cndensatin teperature vs perfrance f chillers Last suer, the cling capacity and shaft pwer f the chiller were easured. The cling capacity is knwn after the rate f flw, inlet teperature and utlet teperature f chilled water are surveyed. Shaft pwer is knwn with the help f dynaeter. Calculatin f cling capacity and shaft pwer is carried ut by using Eqs. (0) and () after evaprative and cndensing pressures are gauged because evaprating and cndensing teperature result fr evaprating and cndensing pressure. Table 2 indicates that the cling capacity is increased by 9.0 percent, shaft pwer is reduced by 7.2 percent, cefficient f perfrance is steppedup by 3.5 percent when chillers perate at night. In additin, the peratin efficiency f chillers is greatly increased since they run with peak lad, the ties f chillers stps are reduced at night. 4. ANALYSIS AND DISCUSSION Wet bulb teperature in utdr air changes hurly and average wet bulb teperature f 0 years at night is.7 C less than that in the day in Ji nan,.r. China. The wet bulb teperature is the priary paraeter that affects the perfrance f the cling twer, and s wet bulb teperature has greatly an effect n the perfrance f chillers. The cling capacity and cefficient f perfrance are t evapratin teperature, C t k cndensatin teperature, C Φ 0 cling capacity, kw e shaft pwer, kw CO cefficient f perfrance T evapratin teperature, K T k cndensatin teperature, K q ass refrigerating capacity, kjkg - V l specific vlue, 3 kg - W th wrk f cpressin, kjkg - M R ass rate f flw f refrigerant, kgs - b cnstant expansin index nuber c relative clearance vlue suctin pressure, a Δ suctin pressure difference, a, = - 2 exhaust pressure, a Δ 2 exhaust pressure difference, a, 2 = k - 2 η frictin efficiency, % evapratin pressure, a k cndensatin, a V R actual induced vlue f suctin vapr at suctin pressure, 3 V h theretical displaceent f cpressr, 3 η vluetric efficiency, % η i cpressin efficiency, % η v η t η l η p cleance vlue and cpressin rati cefficient f heating effect cefficient f leakage cefficient f thrttle REFERENCES. Y.A. Li, Study and applicatin f standard eterlgical year, Ventilatin & Dust Cntrl, N. 2 (993). 2

5 2. Z.Q. Guan, Several experiental fitting frulas in refrigerant calculatin, Refrigeratin, N. 3 (995). 3. Y. Hwang, J. Judge and R. Raderacher, Experience with refrigerant ixture, ASHRAE Transactin, art I (997). 4. J.D. Millar, G.K. Mrris and B.G. Sheltn, Leginnaires Disease: Seeking effective preventin, ASHRAE Jurnal, N. (997). 5. W. Kirsner, 3gp/tn cndenser water flw rate: Des it waste energy?, ASHRAE Jurnal, N. 2 (996). 6. ASHRAE, ASHRAE Handbk 998, Refrigeratin, ASHRAE inc., Atlanta, GA (998). 22

Model Library Industrial Utilities

Model Library Industrial Utilities Mdel Library Industrial Utilities The mdel library Industrial Utilities is a tlbx cntaining the cmmn utilities used in the pwer engineering industry: Cmpressed air generatin device Heat pump/chiller Gas