Internatonal Journal of Energy Engneerng (IJEE) Nov. 01, Vol. Iss. 4, PP. 137-14 Solar-Coal ybrd Thermal Power Generaton an Effcent Way to Use Solar Energy n Chna ongjuan ou *, Jan Mao, Yongpng Yang, Na Luo Bejng Key Laboratory of Safe and Clean Energy Technology, North Chna Electrc Power Unversty Bejng, 1006, Chna * hongjuanhou@ncepu.edu.cn Abstract- To solve the problem of hgh ntal nvestment and low thermal performance for solar alone thermal power plant, solar/fossl fuels hybrd power system has become a trend of solar thermal power generaton n recent years. Chna s rch n coal and solar energy. At present, coal s the man resource n generatng electrcty n Chna. Therefore, solar ntegrated wth conventonal coal-fred power generaton cycles s consdered the best way n Chna. As an opton for easy operaton and control flexblty, solar aded feedwater heatng of a coal-fred power generaton system s dscussed and analyzed n ths paper. Compared to the common hybrd power system, the man feature of ths generaton system s, the thermal ol carryng solar energy replaces the extracton steam to heat the feed water and the steam thus saved can contnue to do work. Because the solar heat does not enter the turbne, the effcency (of solar to power) s not lmted by the temperature of the solar heat. The performance of the ntegrated system wth dfferent replacements s analyzed based on an example and the mpact of solar collector areas and DNI (drect normal rradaton) on the performance of the generaton system s dscussed as well. The results show that the new ntegrated system not only contrbutes to ncreasng the effcency of the conventonal power staton and reducng ts emsson of greenhouse gases, but also ncreases the effcency of solar to electrcty; further LEC s also reduced consderably compared wth solar only thermal power system. owever, the results also ndcate that the replacement type, solar collector areas and DNI have great nfluence on the generaton system. Keywords- Solar Energy; Coal-Fred Thermal Power Generaton; ybrd; Performance I. INTRODUCTION Nowadays, electrcty s manly generated by consumng fossl fuel, whch has serous negatve mpacts on our envronment. As a clean, free and non-depletng source, solar thermal power s recevng more and more attenton. Snce solar energy has relatvely low ntensty, nstablty and perodcty, generally speakng, ts utlzaton s costly. In order to make use of solar energy more effectvely, many solutons of solar and fossl fuel hybrd power generaton system have been put forward by scholars [1-5]. Studes on solar ntegrated wth coal-fred conventonal power plant stll stay on the stage of the desgn of ntegraton [5-10]. Chna s rch n coal and solar energy. At present, coal s the man resource n generatng electrcty n Chna. Therefore, solar power ntegrated wth conventonal coalfred power generaton cycles s consdered the best way n Chna. Solar energy has been proposed wth varous utlzatons n conventonal steam power plants lke feedwater heatng, superheatng/reheatng of steam and ar preheatng [5-10]. In these combned approaches, as an opton for easy operaton and control flexblty, solar aded feedwater heatng of a coal-fred power generaton system s dscussed n ths paper. In the ntegrated system, thermal ol carryng solar energy replaces the extracton steam to heat the feed water and the steam thus saved can contnue to generate work. Comparng to the exstng systems, the new ntegrated system reduces exergy losses n regeneratve heater and acheves energy cascade utlzaton. II. TE MODE OF SOLAR-COAL YBRID TERMAL POWER GENERATION SYSTEM In both solar thermal power system and conventonal power system, heat s always taken as the energy carrer. In the ntegrated system, the subsystems of solar collectors and conventonal coal-fred power generaton are coupled by heat. In the conventonal coal-fred power system, the endothermc process of workng flud water can be dvded nto three stages: preheated, vaporzed and superheated, and ts temperature ranges from tens degree to hundreds of degree. Ths provdes a very dverse way for the ntegraton of solar energy and conventon power system. Among the numerous ntegratons, applyng the thermal ol carryng solar energy n regeneratve system of conventon coal fred power staton s relatvely easy and feasble. In conventonal power system, n order to reduce the temperature dfference of boler heat absorpton and heat transfer, the extracton steam s normally used to preheat feed water enterng the boler, whch has a large sum of exergy loss. To reduce the exergy loss, the thermal ol carryng solar energy replaces the extracton steam to heat the feed water and the steam thus saved can contnue to generate work. Fg.1 s a schematc dagram of the ntegrated system, n whch the replacement type can be adjusted. The workng temperature of ol depends on the temperature of the feed water, whch s heated commonly at a lower temperature than that heated n solar only thermal power system. The feed water s heated n the ol-water exchanger by thermal ol carryng solar energy. Owng to the nstablty of solar energy, the heat energy collected by the solar feld s also nstable. Therefore, the control system needs to adjust the flow rate of feed water to match the heat collected by the solar feld and thus to make the output temperature of feed water meet the desgn requrements. - 137 -
Internatonal Journal of Energy Engneerng (IJEE) Nov. 01, Vol. Iss. 4, PP. 137-14 Boler Turbne Condenser The heat losses of the solar feld nclude two parts: the absorbng tube heat losses and ppng system heat losses. Both of them can be obtaned from the correspondng emprcal formula. For absorbng tube heat loss per unt aperture area, t can be calculated wth the followng the emprcal formula [1]. Pump Regenerator Ol-water exchanger loss, CE 3 3 4 4 3 3 a1( T 0 T ) a( T 0 T ) a3( T 0 T ) b1 ( T 0 T ) a0( T0 T ) + + + + Ib, n[ b0 ( T0 T ) + ] = 3 4 3 ( T0 T )W (4) The emprcal constants a0, a1, a, a3, b0, b1 have been determned durng collector tests. Solar feld For the heat loss of ppng per unt aperture area, t s accounted for by the followng emprcal equaton [1] : III. Fg. 1 A schematc dagram of the solar-coal hybrd thermal power generaton system ANALYSIS OF TE YBRID POWER GENERATION SYSTEM A. Solar Feld In ths paper, the solar feld model s based on a steady state. The heat capacty of the heat transfer flud, absorbng tubes and connectng ppes are neglected. LS- parabolc trough collectors are chosen for ths solar feld. The thermal output of a solar feld depends on the absorbed solar radaton ncdent on the collector reduced by the losses of the solar feld. The drect normal rradance (DNI) projected on the collector area can be expressed by: Ld = xib, n cosθ A c (1) Solar radaton absorbed by the recever tubes can be expressed as follows: = Ld k η 0 absorbed () Where, k s the ncdence angle modfer. It can be calculated wth the angle of ncdent θ n degrees and two emprcal constants c1 andc 1. θ θ k = max 1 c1 c, 0 cosθ cosθ η0 s the solar feld effcency consderng the losses due to optcs and mperfectons. Accordng to the lterature [1-13] η =0.7133 ncludng cleanlness correcton factors. 0 1 Dudley et al.[11] determned the parameters c 1 = 0.000884 / 1, C = 0.00005369 / (1 ) The parameter a 0 =-9.463033, a 1 =0.309616, a =-0.001386833, a 3 =0.0000069943, b 0 =0.0764961, b 1 =0.000000118818 are gven by Angela M. Patnode [1]. (3) 7 3 0.01693 T 0.0001683 T 6.78 T, = + 10 loss ppng Where, T [ C] s the dfference between the average feld temperature and the ambent ar temperature. Thus, the net energy collected by the heat transfer flud ol over the feld s: ( loss, CE + loss ppng ) A c collected absorbed, B. Power Generaton (5) = (6) Applyng the thermal ol carryng solar energy to replace the extracton steam to heat the feed water and the steam thus saved can contnue to generate work. Accordng to the necessary electrcal load, the system can operate at a fuelsavng (fuel and emsson reducton whle keepng the same generatng capacty) or power boostng mode (addtonal power generaton wth the same fuel consumpton). In the analyss, the power booster operaton mode was chosen for the hybrd power system [3]. Equvalence enthalpy descendng analyss method [14] s used to calculate the extra work generated by the saved extracton steam. The amount of the extracton steam replaced by the heat collected by solar feld s calculated as: collected x = (7) q The extra work generated by the saved extracton steam s expressed as: ΔE = x (8) Where, s the extra work generated by the 1 kg saved extracton steam stage. Accordng to the Lterature [14], f the extracton steam enters an open feed water heater, s expressed by: h hn 1 = ( ) r= 1 τ r q r r (9) - 138 -
Internatonal Journal of Energy Engneerng (IJEE) Nov. 01, Vol. Iss. 4, PP. 137-14 If the extracton steam enters a closed feed water heater s expressed by: γ 1 ( 1 1) q = h h 1 1 (10) Defnng the effcency of solar to electrcty wth the Equaton (11), the results wth dfferent replacement types are shown n Table 3. E η se = (11) Ld The annual performance can be obtaned by solvng the above equatons at every tme step. In ths paper, snce only hourly meteorologcal data are avalable, so the followng calculatons are based on ths workng tme ntervals. C. Economcs In order to show the economcal effect of the new ntegrated system, the levelzed electrcty costs (LEC) and the statc payback tme are calculated. The LEC s calculated as follows: LEC = ( CC AF ) + O & M + FUEL C E annual (1) Where CC s the total captal cost; AF, the annual factor; O & M, the annual operatng and mantenance cost; FUEL, fuel cost; C, other recepts; E annual, annual net electrcty generaton n kwh. In ths study, the manly related data used are shown n the Table 1. TABLE I MAINLY RELATED DATA USED IN TIS STUDY solar felds ( /m ) 000 O & M ( /m ) 50 standard coal cost ( / ton) 850 Cost of land Ol-water exchanger Not consdered Not consdered Solar electrc power prce ( / kwh) 1.09 auxlary power percent of solar system 8% nterest rate 6% lfetme (year) 5 annual factor 0.078 IV. AN EXAMPLE In ths secton, a solar energy ntegrated wth a 15 MW coal fred conventonal power generaton unt s taken as an example to analyze. The smplfed scheme of the orgnal generaton unt s shown n Fg.. The specfc standard coal consumpton of electrcty generatng s 333.6 g/kwh. The other related parameters of the orgnal generaton unt are shown n Table. boler extracton steam 7# 6# pump 5# 4# 3# # 1# regenerator Fg. Smplfed scheme of the 15MW coal-fred conventonal power unt TABLE II MAINLY RELATED DATA USED IN TIS STUDY EXTRACTION STEAM PARAMETERS OF TE UNIT ANNUAL PERFORMANCE WIT DIFFERENT REPLACEMENT TYPES Pressure Mpa Temperature Enthalpy kj/kg Flow t/h Water Enthalpy at Regeneratve Beater outlet kj/kg prmary steam 13.4 550 3467 395 Extracton steam No.7 3.64 375.4 3164 17.39 1039.6 Extracton steam No.6.55 331 3083 45.53 946. Extracton steam No.5 0.77 394 356 6.56 667.4 Extracton steam No.4 0.46 36 310 14.9 609. Extracton steam No.3 0.5 55 980 19.7 509.1 Extracton steam No. 0.07 135 749 15.18 357.9 Extracton steam No.1 0.016 55 541 9.33 1.7 exhaust steam 0.005 33 40 66.4 136.3 condenser In the hybrd thermal power system, solar energy collected by a solar feld wth 4710 m aperture area replaces the extracton steam to heat the feed water and the saved steam contnue to generate work. The power plant locaton s set n Lhasa, Chna. The hourly meteorologcal data are used n the followng calculatons. In order to smplfy the problem, we only take solar energy replacng the extracton steam No. 7 as an example to llustrate. The same method can also be appled n other replacements. Based on the typcal year meteorologcal data, daly DNI, thermal output of the solar feld and the solar power generaton for extracton steam No. 7 replaced by solar energy are shown n Fg. 3. The annual performance wth dfferent replacement types are lsted n Table 3. - 139 -
Internatonal Journal of Energy Engneerng (IJEE) Nov. 01, Vol. Iss. 4, PP. 137-14 DNI, collected, E 7 (MJ/(m day)) 50 40 30 0 10 DNI collected E 7 0 0 50 100 150 00 50 300 350 400 day (n) Fg. 3 Daly DNI, thermal output of solar feld, solar power generaton for extracton steam No.7 replaced by solar energy TABLE III ANNUAL PERFORMANCE WIT DIFFERENT REPLACEMENT TYPES The Extracton Steam Replaced Solar Collector Workng Temperature ( ) No.1 No. No. 3 No. 4 No. 5 No. 6 No. 7 5 79 115 143 16 199.5 40 Solar-to-Electrc Effcency 0.07 0.069 0.116 0.138 0.149 0.180 0.189 Annual Thermal Output of Solar Feld (kwh/m /year) Annual Power Generaton (*10 5 kwh/year) 1510 1505 1498 1490 1484 1467 1440 3.59 9.18 15.5 18.5 19.9 4.1 5.3 LEC( / kwh).71 1.06 0.63 0.53 0.49 0.40 0.38 Payback (year) 75.7 13.8 7.1 5.8 5.4 4.3 4.1 *solar feld aperture area s 4710 m It can be seen from Table 3, for the same solar aperture area, dfferent replacements result n dfferent performances. The hgher grade extracton steam s replaced, the hgher workng temperature of solar feld s needed, and thermal output of solar feld decreases when workng temperature rses. As the loss of thermal output s not enough to offset the extra work generated by the saved extracton steam,the LEC and payback also decreases. From ths we can conclude that, for ths solar aperture area, the hgher grade extracton steam s replaced, the more annual power generaton and the better economcal effcency wll be. In order to obtan how solar feld aperture area effects on the performance of the solar-coal hybrd thermal power system, dfferent replacements wth dfferent solar feld aperture areas are calculated and the results are presented n Fg. 4. From t we can see that, when the solar aperture area s relatve small, extracton steam No. 7 replaced would obtan a hgher power generaton, solar-to-electrc effcency and lower LEC than extracton steam No. 6 replaced. owever, for the replacement type that extracton steam No. 7 replaced, when the collector aperture area ncreases to a certan degree, any further ncrease of the collector area wll make solar power effcency decrease rapdly, LEC ncreases rapdly and the growth rate of annual power Ann. Elec. Generaton n GWh/a 5 0 15 10 LEC to replace the extracton steam 7 LEC to replace the extracton steam 6 Ann. Elec. Generaton to replace the extracton steam 6 Ann. Elec. Generaton to replace the extracton steam 7 5 η se to replace η se to replace the extracton steam 7 the extracton steam 6 0 0 5 10 15 0 5 30 35 40 45 solar feld sze A c n 1000m 0.45 0.40 0.35 0.30 0.5 0.0 0.15 0.10 Mean annual effcency η se or LEC ( /kwh) Fg. 4 Performance of the solar-coal hybrd thermal power system wth dfferent replacements and collector aperture areas generaton slows down. For example, when the collector aperture area s 30000 m, the power generaton, solar-toelectrc effcency of No. 7 replaced wll be lower than that - 140 -
Internatonal Journal of Energy Engneerng (IJEE) Nov. 01, Vol. Iss. 4, PP. 137-14 of the stuaton when No. 6 replaced and LEC vce versa. The reason s that the flow of extracton steam No. 6 s larger than that of No. 7. When collector aperture area ncreases, the thermal power of solar feld wll eventually exceed the energy needed for heatng feed water n regeneratve heater. The larger the aperture area, the hgher rato of surplus energy wll be. Therefore, from Fg. 4, we TABLE IV ANNUAL PERFORMANCE UNDER DIFFERENT DNI can determne whch replacement and how bg aperture areas are the best to be consdered based on the specfc case. Snce the DNI also has a great nfluence on the performance of the ntegrated system, the annual performances wth dfferent typcal meteorologcal data are calculated and the results are lsted n Table 4. Typcal Year Extremely gh Radaton Year Extremely Low Radaton Year Solar-to-electrc effcency 0.189 0.197 0.189 Power generaton (kwh /year),57,694,786,944 1,33,917 Solar feld thermal output (kwh/m /year) 1440 1587.5 754 DNI (kwh/m /year) 845 3008 1485 LEC ( /kwh) 0.38 0.35 0.73 *The aperture area s 4710 m, extracton steam no.7 s replaced. From the table we can see that, the annual performance and LEC vary from the extremely hgh radaton year to extremely low radaton year. V. CONCLUSIONS In ths paper, a solar coal hybrd power system s presented where solar (thermal) energy as an adng heat source s ntegrated nto the conventonal coal-fred power generaton cycles. The performance of the hybrd power system s analyzed wth an example. In the hybrd power system, thermal ol carryng solar energy replaces the extracton steam to heat the feed water, the steam thus saved can contnue to generate work. Compared wth the solar only power generaton system, the system overcomes the nstablty of solar only power system and shares the good stablty of conventonal coal-fred power plant. The nfluence of the replacement, solar collector aperture area and weather condton on the hybrd power system are analyzed. The results show that, the effcency of the conventonal power staton ncreases and ts emsson of the greenhouse gases reduces. Besdes, the hybrd power system also ncreases the effcency of lght to electrcty, and LEC reduces consderably compared wth solar only thermal power plant. owever, the results also show that, dfferent replacements, solar collector areas, and DNI have great nfluence on the solar coal hybrd power system. When the collector area s small, thermal output of the solar feld wthout any surplus wth extracton steam replaced, the hgher grade extracton steam replaced, the better thermal performance and economcal effcency wll be. When collector area ncreases to a crtcal value, the system performance wll decrease rapdly wth the ncrease of the collector area. Therefore, whch replacement and how bg aperture areas to be chosen need to analyze n combnaton wth specfc case. From the result we also can see that, the thermal output of the solar feld depends on the value of DNI and ts dstrbuton. Even n the same area, LEC vares sgnfcantly from year to year. It should also be notced that, snce the analyss based on many specfc parameters, the obtaned results are only vald for the specfcally defned condtons and may vary sgnfcantly from project to project. Nomenclature A c aperture area of solar feld [m ] h enthalpy of the extracton steam [kj/kg] h n enthalpy of the exhaust steam [kj/kg] I b, n drect normal nsolaton (DNI) [W/m ] absorbed the heat absorbed by CE tube [W] Ld the drect solar radatons fall on the aperture of solar feld [W] collected energy collected by the TF over the solar feld [W] loss,ce the CE heat loss of per unt length [W/m ] loss,png the thermal losses of the ppng system n the solar feld [W/m ] q enthalpy drop of extracton steam No. [kj/kg] x row shadow factor[-] x the saved extracton steam No. [kg/s] T nlet temperature of solar feld[ ] T o outlet temperature of solar feld [ ] γ heat released by the dran n the feed water heater [kj/kg] τ enthalpy ncrease of feed water n regenerator [kj/kg] - 141 -
Internatonal Journal of Energy Engneerng (IJEE) Nov. 01, Vol. Iss. 4, PP. 137-14 θ ΔE η se ncdent angle of collector aperture[ ] the more work generated by the saved extracton steam No. [W] the effcency of solar to electrcty[-] ACKNOWLEDGMENT Ths work was supported by The Natonal gh Technology Research And Development Program of Chna (863 Program) (01AA050604); the Natonal Natural Scence Foundaton of Chna (5106049); the Fundamental Research Funds for the Central Unverstes(11MG07); Technology Development Foundaton of Suzhou Industral Park (ZXG01035). REFERENCES [1] Jurgen Dersch, Mchael Geyer, Ulf errmann, et al.. Trough ntegraton nto power plants a study on the performance and economy of ntegrated solar combned cycle systems[j]. Energy, 004, 9: 947 959. [] Peter Schwarzbozl,Rener Buck,Chem Sugarmen,et al.. Solar gas turbne systems: Desgn, cost and perspectves [J]. Solar Energy, 005. [3] Mechthld orn, ener Fuhrng, Jurgen Rhenlander. Economc analyss of ntegrated solar combned cycle power plants A sample case: The economc feasblty of an ISCCS power plant n Egypt [J]. Energy, 004, 9: 935-945. [4] Gregory J Kolb. Economc evaluaton of solar-only and hybrd power towers usng molten-salt technology [J]. Solar Energy, 1998, 6(1): 51-61. [5] Yng You, Erc J. u, Thermodynamc advantages of usng solar energy n the regeneratve Rankne power plant, Appled Thermal Energy 1999 (19): 1173 1180. [6] Erc u, YongPng Yang, Akra Nshmura, Ferd Ylmaz, Abbas Kouzan.Solar thermal aded power generaton. Appled Energy 010, 87(9): 881-885. [7] Gupta M.K., Kaushk S.C. Exergetc utlzaton of solar energy for feed water preheatng n a conventonal thermal power plant[j]. Internatonal Journal of Energy Research. 009, 33(6): 593-604. [8] Dmtyr Popov. An opton for solar thermal repowerng of fossl fuel fred power plants. Solar Energy 85 (011) 344 349. [9] Yang YongPng, Cu Ynghong, ou onjuan et al. Researeh on solar aded coal-fred Power generaton system and Performane Analyss[J]. Scence In Chna (Seres E). 008, 51(8): 111-11. [10] n Yan, Yongpng Yang et. al. Mult-pont and mult-level solar ntegraton nto a conventonal coal-fred power plant [J]. Energy and Fuels. 010, 4(7), 3733-3738. [11] Dudley, V., Kolb, G.J., Mahoney, A.R., Mancn, T.R., Matthews, C.W., Sloan, M., and Kearney, D. Test Results: SEGS LS- Solar Collector. Sanda Natonal Laboratores, SAND94-1884. December 1994. [1] Angela M. Patnode.Smulaton and performance Evaluaton of Parabolc trough solar power plants:[master paper]. Unversty of Wsconsn-Madson,006. [13] Forrstall, Russell. eat Transfer Analyss and Modelng of a Parabolc Trough Solar Recever Implemented n Engneerng Equaton Solver. Natonal Renewable Energy Laboratory, NREL/TP-550-34169. October 003. [14] Wanchao Ln.Energy Conservaton Theory of Thermal Power Plant [M].1994, X an Jaotong Unversty, X an.(in Chnese). ongjuan ou graduated from the Department of Energy and Power Engneerng, Shangha Jaotong Unversty, and got her PhD. n 006. She manly engaged n the research of solar thermal appled. At present she s a faculty of the Department of Energy and Power Engneerng, North Chna Electrc Power Unversty. - 14 -