Smart Grd Analyss of Centralzed Coolng for an Urban Communty Francne Vera, Jéssca Henrques, Larssa Soares, Leandro Rezende, Moses Soares Martns, Ramundo R. Melo Neto and Donald J. Chmelews* Department of Chemcal and Bologcal Engneerng, Illnos Insttute of Technology, Chcago, IL * Correspondng author 312-567-3537, chmelews@t.edu Ths project wll nvestgate three dfferent methods to provde energy to operate the coolng system for a new communty that wll have about 1, resdents n an area formed only by resdental buldngs. Frst case we wll study a decentralzed system, each home has ts own resdental electrc chller to provde coolng. Centralzed systems wll be analyzed, the frst one we wll have a bg chller for coolng. An absorpton chller wll be used to produce the coolng and t wll use a natural gas combned cycle NGCC power plant to produce the waste heat that t needs and to generate energy. In addton, by usng smart grd analyss and technology t wll be decded when t wll be proftable to produce energy or turn the power plant off whle the energy producton does not provde revenue. Fnally, another centralzed system case, a smlar analyss wll be mplemented, however a thermal energy storage TES wll be added to the frst centralzed system case. I. INTRODUCTION Energy consumpton of buldngs are responsble for 4% of the global prmary energy usage. The majorty of energy consumpton of buldngs s caused by the operaton of heatng,
coolng and ar ventlaton systems Km, 214. Durng the summer, the coolng system operates almost all day long removng heat from buldngs to mantan the nsde envronment at a comfortable temperature. These thermal systems can be classfed nto two broad categores, centralzed and decentralzed. Centralzed ar condtonng systems serve multple spaces from one base locaton. These typcally use chlled water as a coolng medum and use extensve ductwor for ar dstrbuton. Decentralzed ar condtonng systems typcally serve a sngle or small spaces from a locaton wthn or drectly adjacent to the space Batha, 212. The followng fgures llustrate the centralzed and decentralzed system. Fgure 1 Centralzed System Fgure 2 Decentralzed System To desgn and estmate costs of a coolng system, t s necessary to now the coolng load, whch s the amount of heat that needs to be removed from the buldngs see Fgure 3. Loong at the graphc of fg.4, t s possble to see that the coolng load wll vary durng dfferent perods of the day due Fgure 3 Dfferent forms of heat. to dsturbances provded by dfferent heat sources.
Fgure 4: Tme-seres of day-ahead electrcty prces and coolng load for a sngle house. The second curve on fgure 4, llustrates the oscllaton of the prce of electrcty durng the day. Durng the day, we need more coolng consequently the demand of electrcty wll ncrease, through that we can correlate t wth the graphc of the prce of electrcty, observng that when the coolng s ncreased the prce wll also be hgh. The same concept s appled when there s a decrease n the demand. Those fluctuatons n the demand and prce of electrcty wll be the basc concept of the smart grd analyses, n whch wll be consder the cost of electrcty to fnd
out what source of coolng must be used. For example, when the prce of electrcty s hgh, we wll use the absorpton chller snce t requres just heat as source of energy, and when prce of electrcty s low we can use the electrcal chller. Ths stuaton wll be analyzed n three dfferent cases that wll be exposed through ths report. The project s gong to focus on two manly chllers technologes that wll be useful to study solutons for ths problem, the electrcal and the absorpton chllers. The electrcal chller uses electrcty as source of energy, ths electrcty wll be purchased from the grd, and ts prce oscllate durng the day accordng to the demand. The absorpton chller uses waste heat as a fuel. Therefore, f a source of waste heat s avalable and the prce of electrcty s hgh a good deal would be usng absorpton chller nstead of the electrcal chller. Many ways to provde heat to the absorpton chller exst, however the one that s gong to be studed for ths project wll be the Natural Gas Combned Cycle Power Plant NGCC. Combned cycle plants are bult around one or more combuston turbnes, essentally the same technology used n jet engnes. The combuston turbne s fred by natural gas to rotate a turbne and produce electrcty. The hot exhaust gases from the combuston turbne are captured and used to produce steam, whch drves another generator to produce more electrcty. By convertng the waste heat from the combuston turbne nto useful electrcty the combned cycle acheves very hgh effcency, comparng a sngle cycle to a combned cycle, the effcency can be about 1% hgher Kaplan, 28. Assemblng two dfferent thermodynamc cycles can provde more than a hgh effcency, t also reduce fuel costs. Common cycle used nowadays are the Brayton gas turbne n whch natural gas s burned, resultng n the producton of heat used to generate steam n the Ranne
cycle and also provdes heat to the absorpton chller. The motvaton for usng a power plant s that the context of day-ahead electrcty prce for a deregulate energy maret can be used to change the system expenses nto revenues. The power plant wll produce electrcty that wll be sold to the grd whenever the rescaled prce of fuel natural gas s lower than the prce of electrcty and the waste heat produced by the energy generaton process wll be used to operate the absorpton chller. Usng those concepts a dagram of the frst centralzed case of study can be drawn: Fgure 5: Decentralzed Coolng System wth NGCC power plant dagram From ths model, usng smart grd technology, the system s gong to use the absorpton
chller to produce coolng when the prce of electrcty s hgh and durng the nght, when t s low, the system purchase energy from the grd and wll use the electrcal chller to produce coolng. To mprove the coolng system, a thermal energy storage TES can be used to stoc energy produced when the prce of electrcty s hgh see fgure 6. Thermal energy storage has been wdely used n the commercal sector snce the 198s to shft HVAC coolng load out of the pea demand perod of the day ASHRAE Journal, June 213. The TES wll help the system because t s gong to store energy when the prce to produce t durng the day s low and wll use ths energy when the prce s hgh. Fgure 6: Thermal Energy Storage Dagram
Fgure 7: Decentralzed Coolng System wth NGCC power plant and TES dagram An mportant concept that wll be used to evaluate the proftablty of the results n each case of study s the Net Present Value NPV of each case that wll be studed. For those cases the ntal cost, the annual operatng costs and mantenance expenses wll be consdered. For the decentralzed systems the captal costs are nstallng the ar condtoner for each buldng, and the only operatonal cost s purchasng electrcty from the grd. For centralzed cases the ntal costs are mplementng the power plant, the purchase of the absorpton chller and buldng the networ dstrbuton, wth the operatng cost of the power plant and the Thermal Energy Storage mplementaton when t s appled. In ths paper we present an economc analyss of three dfferent cases of coolng system, the frst case s a decentralzed system, where each buldng has ts own electrcal chller, the second case wll be a centralzed system usng an ndustral szed absorpton chller wth a power plant as source of energy, and the thrd case wll be the same as the second, but wth the addton of a
TES system. Consequently, the performance of each system was compared and valdated usng NPV analyss. II. DECENTRALIZED COOLING SYSTEM ANALYSIS As n decentralzed system each buldng has ts own refrgeraton system, the coolng load must be calculated ndvdually accordng to the constructon materals and locaton to be able to attend the coolng demand. Coolng load CL s altered by any source of heat. For nstance, the heat that s conducted from the outsde ar through walls and especally sun lght wll mae the temperature ncrease durng the day, however durng the nght, the coolng load wll decrease because the outsde temperature wll be smaller than the nsde temperature. Ths oscllaton occurs contnuously and can be estmated by adaptng the Coolng Load Temperature Dfference CLTD method proposed by Amercan Socety of Heatng, Refrgeratng, and Ar-Condtonng Engneers ASHARE and descrbed n the 1997 Fundamentals Handboo. The CLTD was developed based on tabulated data obtaned for dfferent studes by Rudoy and Duran 1975 to smplfy the coolng load calculaton to be done by hand. The CLTD tabulated values can be adjusted to most stuatons as the Fundamental Handboo ASHARE, 1997 provdes nstructons to adjust the data accordng to the maxmum and mnmum temperature of a specfc day and also accordng to the buldng locatons, orentaton and materals of constructon. An algorthm can be develop to provde a tmes-seres CL consdered any perod snce some hstorc weather data s avalable. Although ths s not the most accurate method to be consdered, t s stll used by some engneers and buldng desgners to estmate rapdly the pea load of a buldng. For desgnng and effcency purposes t s recommended to use methods that gve results more precse, such as the Transfer Functon Method ASHARE, 21.
From the hghest CL value s possble to defne what electrc chller wll be able to attend the coolng demand because the hghest CL wll lead the coolng capacty of the chllng system. Each chller has a feature called Coeffcent of Performance COP n ton/w that s used to measure ts effcency and for electrc chllers the COP s defned by the bellow equaton: Table 1: Paceged Compressor and Condenser Unts Electrc Chller Coolng capacty ton Total Cost $ 1.5 1,7 5 4,4 1 7,775 2 16,6 Ths coeffcent s extremely mportant because t helps to estmate the electrcty consumpton accordng to the CL obtaned. Therefore the power energy consumed by a regular electrc chller s gven by: Where ton s the coolng load for the entre communty and s n W/h. Once the CL s estmated hourly, electrcty consumed s also an hourly tme-seres result, smlarly t s nown that the electrcty prce s gven as a functon of tme based on the consumers demand, fnally the day-ahead prce s estmated by hour and the real-tme prce s
regstered by the regonal transmsson organzaton as well. When developng a model, t s expected that the varables prce and electrcty are correspondents at tme and have the same total of elements, consequently the cost for coolng a buldng s acheved by multplyng these two varables, n addton the summaton of costs gves the operatonal cost for a decentralzed CL: Consequently, the operatonal cost of the decentralzed system s bascally the cost of purchasng electrcty from the grd, and t s used to estmate the Net Present Value NPV The IC parameter corresponds to the captal cost of mplementaton of the coolng system. For the decentralzed system, the ntal cost s the cost of nstallng the ar condtoner n each resdence. The PV parameter s the present value of all operatonal costs that apples. In the frst case, the only operatonal cost s the cost of purchasng electrcty from the grd. The formula used to calculate the PV s presented below: Usng the equatons above, t s possble to defne how much each project would be worth nowadays. Example 1: Ths study analyzed a new resdental communty of 1, people that would be placed n Chcago. Consderng that Chcago has a densty of 2.58 persons per household, the communty s assumed to have 3,876 resdental buldngs. Census, 213
Weather and prce data needed were based on the years 25, 28 and 212. The hstorc weather data was obtaned on the database of Natonal Oceanc and Atmospherc Admnstraton NOAA as ths governmental organzaton has weather regsters from the majors weather statons stuated n the U.S. snce the year of 25, thus the MIDWAY Internatonal Arport- Chcago Weather staton was chosen as reference to guarantee that the smulated coolng load would have smlar results to the actual coolng load for resdental buldngs n the area of Chcago. The selecton crtera was ept whle selectng the data of hstorc electrcty prce, therefore the nformaton was collected from the regonal transmsson organzaton responsble for the Chcago power grd, the Pennsylvana, Jersey, Maryland Power Pool PJM. To estmate the coolng load t was assumed a house model that contans features smlar to a standard Chcago resdence. By usng MATLAB to run the smulaton for a perod of a year t was possble to obtan the pea load value, whch s equal to 3.89 ton, then a 5 ton electrc chller wll be able to attend the coolng demand of a sngle house and has a of 1.14 ton/w Ulloa 214, hence the energy power consumed as a functon of tme was obtaned as a tmeseres vector. It s mportant to hghlght that the heat load was neglected n ths wor,.e. whenever the coolng load was negatve, t was changed to be zero. See fgure 5.
Energy Cost [$/MWh] 2 15 1 5 Electrcty Natural Gas/ p 1 2 3 4 5 6 7 8 9 tme [h] 15 Coolng Load [TON] 1 5 1 2 3 4 5 6 7 8 9 tme [h] Fgure 8: Day-ahead electrcty & Coolng Load In the next step, the hstorc hourly day-ahead electrcty prce for the PJM area durng each year was multpled by the equvalent power energy data. The result s the cost of coolng each buldng as a functon of tme $/h and represents the operatonal cost, the costs of nstallaton and equpment descrbe the captal cost for the electrcal chller, fnally from these numbers the NPV method was calculated based on dfferent year and the results are presented on table 2.
Table 2: Cost analyss n mllon for the decentralzed case Year 25 28 212 Captal Cost $ 14 14 14 Operatonal Cost $ 19 19,2 12 NPV $ 33 33,2 26 It s possble to observe on fgure 4 that exsts a relaton between electrcty prce and coolng load, when the coolng demand s hgh, the coolng system wll consume more electrcty, demandng the power plants that supply the grd to ntensfy ther operaton resultng the ncrease of the energy prce. As the coolng system consumes a hgh amount of electrcty when the prce s hgh, an alternatve approach wll be consdered. III. CENTRALIZED COOLING SYSTEM ANALYSIS From fgure 5 that llustrate a centralzed system t s possble to observe that there s a balance between the coolng load demand and the coolng load provde by two dfferent types of chllers. Where and represent the coolng load provded by the electrc and absorpton chllers respectvely and s the coolng demand that comes from all the buldngs, the same varable descrbed n the prevously secton. Also n fgure 5 the energy balance s exhbted between the power plant and coolng load,
the amount of natural gas consumed s equal to the electrcty produced plus the waste heat generated n the process that feeds the absorpton chller. Where s the mass flow of fuel MMBTU/h, s the overall effcent of power plant, s the electrc energy MW/h produced by the power plant, and represents the heat consumed by the absorpton chller ton/wthermal and s the effcency of the Ranne Cycle n the power plant. It s mportant to now that all the features defned on the last two equatons are postve and have maxmum values based on equpment capacty. Addtonally, t s mportant to hghlght that,.e. the plant can be turned off, however, whle operatng, the power plant s not allowed consume less than the mnmum fuel defned. Ths statement s true f and only f: Where s the power plant state, s the on swtcher and s the off swtcher. The operatonal cost for the centralzed structure wll become dynamc allowng an optmzaton to be appled n the perspectve of smart grd. For example, the power plant wll stop produce electrcty when energy prce s to low and wll consume fuel just to produce enough heat to supply the absorpton chller. In addton, the centralzed electrc chller wll be
actvated f and only f the absorpton chller reaches ts maxmum operaton and s not able to supply the coolng demand. Then, the operatonal cost s represented by the followng three components: cost of buyng natural gas, plus the cost of purchasng electrcty when the electrc chller s runnng and m. Where s the electrcty prce as descrbed on the decentralzed system analyss and s the prce of natural gas. As the power plant wll sell energy to grd, the NPV n a long term can represent revenue f the power plant produce suffcent electrcty. Even that ths economc approach provded can be solved by a computer, t s recommended to avod ths mplementaton for a tme-seres predcton. The Economc Model Predctve Control EMPC offers a better methodology to solve ths category of smart grd problems. On the followng methodology presented, the ndex represents a predctve tme and the ndex represents the actual tme:
= max E E s s max AC AC Q Q max EC EC Q Q max load load Q Q max g g P P max S S =
max g g P P max EC EC Q Q max E E s s Where = + N 1. Ths predctve model s then used wthn the followng optmzaton problem. 1 ˆ ˆ ˆ mn 1,,, E E N P c c COP Q c s s N g e f f AC AC e E P Q s g EC f Where E s s the actual amount of energy at tme. Example 2- Centralzed System To llustrate the statements presented n ths secton and help to vsualze how the system wors, an example usng sne wave as data was done. The result are exhbted on the plots bellow.
Fgure 9: Electrcty prce, fuel prce and coolng load represented by a sne wave Real electrcty prce and coolng load has a behavor smlar to a sne wave, as can be observed on fgure 9, however a smooth sgnal can mae easer the understandng of the problem. Fgure 1: Coolng produced by electrc and absorpton chllers
There are two stuatons on ths example where the centralzed electrc chller had to be operated, see fgure 1. Whenever the absorpton chller reached ts maxmum level of operaton and whenever the prce of electrcty was lower than the prce of natural gas. On ths last stuaton the power plant was swtched off because t wll not return a revenue sellng electrcty. Fgure 11: Mass flow of natural gas and power energy produced Fgure 12: State and swtcher of the power plant Fgure 11 shows the electrcty produced related to the fuel consumed by the process, whle fgure 12 show the functonalty of the power plant swtchers.
Example 3- Centralzed System- Real Data By usng the real data for energy prce and features for the communty from example 1, the centralzed system optmzaton was done. As can be observed n fgure 12, the optmzaton wored well when applyng the real data. The electrc chller only operated when t was not affordable to run the absorpton chller and the power plant produced energy only n hgh electrcty prce perods, when t was proftable. The new captal costs are descrbed n table 3 and the operatonal cost s descrbed n table 4.
Energy Cost [$/MWh] 2 15 1 5 Electrcty Natural Gas/ p Elec. Chller [TON] 6 5 4 3 2 1 45 452 454 456 458 46 462 tme [h] 45 452 454 456 458 46 462 tme [h] Coolng Load [TON] 14 12 1 8 6 4 45 452 454 456 458 46 462 tme [h] Abs. Chller [TON] 12 8 4 45 452 454 456 458 46 462 tme [h] [S] [OFF] [ON] 1.5 1.5 45 452 454 456 458 46 462 tme [h] 1.5 1.5 45 452 454 456 458 46 462 tme [h] 1.5 1.5 45 452 454 456 458 46 462 tme [h] Power Plant Fuel [mmbtuh] Power [MW] 4 3 2 1 45 452 454 456 458 46 462 tme [h] 6 5 4 3 2 1 45 452 454 456 458 46 462 tme [h] Fgure 13: Graphs for the decentralzed system.
The ntal cost of the power plant ncludes: Buldng and Structures Instrumentaton & Control Systems Accessory Electrc Plant Coolng Water System Steam Turbne Generator HRSG, Ductng and Stac Combuston Turbne/Accessores Chemcals The calculaton of the Net Present Value was made usng the data showed below: Table 3: Intal Costs of equpment. Intal Cost Value $ m More Informaton Absorpton Chller 1.4 1 chllers of 15 tons, ea costs $1. Ulloa 214 Electrcal Chller 4.2 1 bg chller that costs $44 RSMeans 211 Dstrbuton Networ 63 - Power Plant 2 Blac, 213 For the operatonal cost, three dfferent perods were analyzed. Table 4: Operatonal cost n mllon for the centralzed coolng system Year 25 28 212 Electrcal Chller $ 1.2 1.3.8 Fuel Cost$ 11 13 6.2
Electrcty sold$ 11.7 12.6 6.2 Fnal Cost: $.6 1.7.8 Usng the average of these results, the fnal value for the operatonal cost was $1.3 mllon, obtanng the followng NPV: Table 5: NPV for the centralzed coolng system Captal Cost $98.3 mllon Operatonal Cost $1.3 mllon Present Value of OC $1.9 mllon NPV $19.2 mllon IV. CENTRALIZED SYSTEM WITH THERMAL STORAGE ENERGY To mprove even more the effcency, a thermal energy storage TES can be added to the centralzed system. Ths thermal storage nvolves basc an nsulated tan that wll store chlled water from the absorpton chller. See fgure 6. Stll usng the Smart Grd concepts to mae decsons, the operatonal costs from the prevously secton can be optmzed. Whenever the power plant s not producng electrcty because of low energy prces and the coolng demand s low such the absorpton chller does not operate at maxmum rate, the thermal storage wll be charged wth chlled water. The chlled water wll be used n perods of hgh demand of coolng, when the absorpton chller reaches the maxmum operaton level, chlled water from the TES wll provde coolng to the resdences. The electrc chller wll be trggered only f the TES s exhausted and the coolng demand s stll hgh.
From these mplementatons, the equaton for coolng balance wll be altered. + = Where Where s the stored energy avalable and s the stored energy that wll be needed later. Example 4 On ths example, the sne wave data was used agan to mae the understandng and nterpretaton of the problem easer,.e. bascally s a complementaton of example 2.
Fgure 14: Results for the centralzed system wth TES usng replca data The thrd curve from fgure 14 llustrates the TES process, when the absorpton chller reached ts maxmum level of operaton and the prce of electrcty s hgh, the chlled water from the TES s used to help to supply the demand. The TES wll recharge when t does not worth to produce electrcty by the power plant, because of low prces of energy, therefore the electrc chller suppled the demand whle the absorpton chller recharged the TES.
Example 5 Elec. Chller [TON] 8 6 4 2 45 452 454 456 458 46 462 tme [h] 15 Abs. Chller [TON] 1 5 45 452 454 456 458 46 462 tme [h] Stored Energy [TON/h] -1-2 45 452 454 456 458 46 462 tme [h] Power Plant Fuel [mmbtuh] 35 3 25 2 15 1 5 45 452 454 456 458 46 462 tme [h] 5 4 Power [MW] 3 2 1 45 452 454 456 458 46 462 tme [h]
1.5 [S] 1.5 45 452 454 456 458 46 462 tme [h] 1.5 [OFF] 1.5 45 452 454 456 458 46 462 tme [h] 1.5 [ON] 1.5 45 452 454 456 458 46 462 tme [h] Fgure 14: Results for the centralzed system wth TES usng real data From examples 3 and 4 data and analyss s possble to understand the results for the centralzed system usng the TES and real data. It s nterestng to observe that the TES utlzaton behaved qute dfferent from the result expected, ths s due the mperfecton of data.
For the thrd case, the same data for the captal cost was used but addng the cost of $ 8 m for the Thermal Energy Storage. Table 6: Operatonal cost n mllon for the centralzed coolng system wth TES Year 25 28 212 Electrcal Chller $ 1.3 1.3.8 Fuel Cost$ 7 8.2 4.1 Electrcty sold$ 9.4 1 4.7 Fnal Cost: $ -1.1 -.5-1.4 Usng the average of these results, the fnal value for the operatonal cost was $-1.4 mllon, obtanng the followng NPV: Table 5: NPV for the centralzed coolng system Captal Cost $16.3 mllon Operatonal Cost $-1.4 mllon Present Value of OC $-14.8 mllon NPV $91.5 mllon V. CONCLUSIONS In ths wor, the smart grd optmzaton was explored to analyze three dfferent system confguratons to provde coolng for a communty. Frst t was demonstrated that the electrcty prce s correlated to the coolng load demand. By analyzng the NPV from the examples 1, 3 and 5, t can be conclude that consderng the desgnng of all equpment, the decentralzed s stll a
more sutable approach, ths occurred because the captal cost for a decentralzed s much lower than the captal cost for the centralzed system. An alternatve to ths problem s to ncrease the power capacty of the NGCC power plant, however ths s a soluton to be analyzed wsely, because by ncreasng the supply of electrcty rresponsbly, the energy maret can be affected drastcally.
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