Life Cycle Exergy Analysis of Wind Power

Transcription

1 Lfe Cycle Exergy Analyss of Wnd Power G. Wall Department of Culture, Energy and Envronment Unversty of Gotland Cramérgatan 3, SE , Vsby, Sweden Emal: Abstract Exergy concepts and exergy based methods offer an nsght to the understandng of sustanable energy engneerng. The utlzaton of energy and other resources by applyng physcal concept as exergy and exergy based methods and the value of these tools n the desgn are presented, n partcular Lfe Cycle Exergy Analyss (LCEA). LCEA s appled to a typcal wnd power plant. Ths brngs a new approach and nsght to the engneerng condtons for a sustanable development that s further elaborated. The mportance of ntroducng ths new knowledge nto present engneerng educaton and practces s argued for. Nomencalture E exergy... J E ndrect exergy ndrect nput... J E n exergy nput... J E n exergy power nput... W E out exergy output... J E net,pr exergy net of product... J E pr exergy of product... J E pr exergy power of product... W E tot total exergy... J E tr transt exergy... J E waste exergy of waste... J H enthalpy... J, j, k, l unt, 1, 2,... P 0 pressure of the envronment... Pa Q heat (thermal energy n transt)... J S entropy... J K -1 S tot entropy of the total system,.e. the system and the envronment... J K -1 t tme... s t 0 tme when a project starts, e.g. the frst steps to buld a power plant... s t close tme when an operaton closes, e.g. a power plant close down... s t lfe tme when a project fnally closes,.e. after complete restoraton to orgnal state... s t payback tme when a payback stuaton s reached... s t start tme when an operaton starts... s T temperature... K T 0 temperature of the envronment.. K U nternal energy... J V volume... m 3 µ 0 chemcal potental of substance n ts envronmental state... J mol -1 Introducton Exergy s a sutable scentfc concept n the work towards sustanable development. Exergy accountng of the use of energy and materal resources provdes mportant knowledge on how effectve and balanced a socety s n the matter of conservng nature s captal. Ths knowledge can dentfy areas n whch techncal and other mprovements should be undertaken, and ndcate the prortes, whch should be assgned to conservaton measures, effcency mprovements and optmzatons. Thus, exergy concept and tools are essental to the creaton of a new engneerng paradgm towards sustanable development. 1

2 Exergy The exergy concept orgnates from works of Carnot [1], Gbbs [2], Rant [3] and Trbus [4] and the hstory s well documented [5]. Exergy of a system s [6], [7] E = U + PV 0 T0S µ 0n (1) where U, V, S, and n denote extensve parameters of the system (energy, volume, entropy, and the number of moles of dfferent chemcal materals ) and P 0, T 0, and μ 0 are ntensve parameters of the envronment (pressure, temperature, and chemcal potental). Analogously, the exergy of a flow can be wrtten as: E = H T0 S µ 0n (2) where H s the enthalpy. All processes nvolve the converson and spendng of exergy, thus hgh effcency s of utmost mportance. Ths mples that the exergy use s well managed and that effectve tools are appled. Presently, an excellent onlne web tool for calculatng exergy of chemcal substance s also avalable [8]. Energy s always n balance, however, for real processes exergy s never n balance due to rreversbltes,.e. exergy destructon that s related to the entropy producton by tot tot tot En Eout = T0 S = ( En Eout ) > 0 (3) tot tot tot where S s the total entropy ncrease, E n s the total exergy nput, E out s the total exergy output, and ( E n Eout ) s the exergy destructon n process. The exergy loss,.e. destructon and waste, ndcates possble process mprovements. In general tackle the bggest loss frst approach s not always approprate snce every part of the system depends on each other, so that an mprovement n one part may cause ncreased losses n other parts. As such, the total losses n the modfed process may n fact be equal or even larger, than n the orgnal process confguraton. Also, the use of renewable and non-renewable resources must be consdered. Therefore, the problem needs a more careful approach. Exergy analyss In engneerng, flow dagrams are often used to descrbe the energy or exergy flows through a process. Fg. 1 shows a typcal thermal power staton, ts man components and roughly the man energy and exergy flows of the plant. Ths dagram shows where the man energy and exergy losses occur n the process, and also whether exergy s destroyed from rreversbltes or whether t s emtted as waste to the envronment. In the energy flow dagram energy s always conserved, the waste heat carres the largest amount of energy nto the envronment, far more than s carred by the exhaust gases. However, n the exergy flow dagram the temperature of the waste heat s close to ambent so the exergy becomes much less. The exergy of the exhaust gas and the waste heat are comparable. Fg. 2 llustrates the energy and exergy flows of an ol furnace, an electrc heater, an electrc heat pump and a combned power and heat plant,.e. a cogeneraton plant. The produced heat s used for space heatng. In the ol furnace the energy effcency s assumed to be typcally about 85%, losses beng due manly to the hot exhaust gases. The exergy effcency s very low, about 4%, because the temperature dfference s 2

3 not utlzed when the temperature s decreased, to a low of about 20 C, as a comfortable ndoor clmate. Fg. 1. Energy and exergy flow of a thermal power plant. Fg. 2. Energy and exergy flows through typcal some energy systems. Electrc heatng by short-crcutng n electrc resstors has an energy effcency of 100%, by defnton of energy conservaton. The energy effcency of an electrc heat pump s not lmted to 100%. If the heat orgnatng from the envronment s gnored n the calculaton of the effcency, the converson of electrcal energy nto ndoor heat can be well over 100%, e.g. 300% as n Fg. 2. The exergy flow dagram of the heat pump looks qute dfferent. The exergy effcency for an electrc heater s about 5% and for the heat pump, 15%. In Fg. 1 the energy and exergy effcences are the same because for the nflow of fuels and the outflow of electrcty both energy and exergy s almost equal. For a combned power and heat plant,.e. a cogeneraton plant (Fg. 2) the exergy effcency s about the same as for a thermal power plant (Fg. 1). Ths can be better understood from the exergy dagrams. The man exergy loss occurs n the converson of fuel nto heat n the boler. Snce ths converson s practcally the same n both the condensng and the combned power plants, the total exergy effcency wll be the same,.e. about 40%. However, t may be noted that the power that s nstead converted nto heat corresponds to a heat pump wth a coeffcent of performance (COP) of about 10. Thus, f there s a heatng need a cogeneraton plant s far superor to a condensng power plant. The maxmum energy effcency of an deal converson process may be over 100%, dependng on the defnton of effcency. The exergy effcency, however, can never exceed 100%. Lfe Cycle Exergy Analyss To estmate the total exergy nput that s used n a producton process t s necessary to take all the dfferent nflows of exergy to the process nto account. Ths type of budgetng s often termed Exergy Analyss [6] & [7], Exergy Process Analyss, see Fg. 3, or Cumulatve Exergy Consumpton [9], and focuses on a partcular process or sequence of processes for makng a specfc fnal commodty or servce. It evaluates the total exergy use by summng the contrbutons from all the ndvdual nputs, n a more or less detaled descrpton of the producton chan. 3

4 Envronmentally orented Lfe Cycle Analyss or Assessment (LCA) are common to analyze envronmental problems assocated wth the producton, use and dsposal or recyclng of products or product systems, see Fg. 4. Every product s assumed to be dvded nto these three lfe processes, or as t s sometmes named from cradle to grave. Fg. 3. Levels of an exergy process analyss. Fg. 4. The lfe cycle from cradle to grave. Fg. 5. Man steps of a LCA. For every lfe process the total nflow and outflow of energy and materal s computed, thus, LCA s smlar to Exergy Analyss. In general Exergy Analyss and LCA have been developed separately even though they are strongly lnked. Ths nventory of energy and materal balances s then put nto a framework as descrbed n Fg. 5. Four stages n the LCA can be dstngushed: (1) Objectves and boundares, (2) Inventory, (3) Envronmental mpact, and (4) Measures. These four man parts of an LCA are ndcated by boxes, and the procedure s shown by arrows. Green arrows show the basc steps and red arrows ndcate sutable next steps, n order to further mprove the analyss. Fg. 6. LCEA of a fossl fueled power plant. In LCA the envronmental burdens are assocated wth a product, process, or actvty by dentfyng and quantfyng energy and materals used, and wastes released to the 4

5 envronment. Secondly one must assess the mpact on the envronment, of those energy and materal uses and releases. Thus t s dvded nto several steps (Fg. 5). The multdmensonal approach of LCA causes large problems when t comes to comparng dfferent substances, and general agreements are crucal. Ths problem s avoded f exergy s used as a common quantty, whch s done n Lfe Cycle Exergy Analyss (LCEA) [10]. In ths method we dstngush between renewable and non renewable resources. The total exergy use over tme s also consdered. These knds of analyses are of mportance n order to develop sustanable supply systems of exergy n socety. The exergy flow through a supply system, such as a power plant, usually conssts of three separate stages over tme (Fg. 6). At frst, we have the constructon stage where exergy s used to buld a plant and put t nto operaton. Durng ths tme, 0 t t start, exergy s spent of whch some s accumulated or stored n materals, e.g. n metals etc. Secondly we have the mantenance of the system durng tme of operaton, and fnally the clean up stage. These tme perods are analogous to the three steps of the lfe cycle of a product n an LCA. The exergy nput used for constructon, mantenance and clean up we call ndrect exergy E ndrect and we assume ths orgnates from non renewable resources. When a power plant s put nto operaton, t starts to delver a product, e.g. electrcty wth exergy power E pr, by convertng the drect exergy power nput E n nto demanded energy forms, e.g. electrcty. In Fg. 6 the drect exergy s a non-renewable resource, e.g. fossl fuel and n Fg. 7 the drect exergy s a renewable resource, e.g. wnd. Fg. 7. LCEA of a wnd power plant. In the frst case, the system s not sustanable, snce we use exergy orgnatng from a non-sustanable resource. We wll never reach a stuaton where the total exergy nput wll be pad back, smply because the stuaton s powered by a depleton of resources, we have E pr <E n +E ndrect. In the second case, nstead, at tme t=t payback the produced exergy that orgnates from a natural flow has compensated for the ndrect exergy nput, see Fg. 7,.e. tpay back tlfe E pr ( t) dt = E ndrect ( t) dt = Endrect tstart 0 (4) Snce the exergy nput orgnates from a renewable resource we may not account for t. By regardng renewable resources as free then after t=t payback there wll be a net exergy output from the plant, whch wll contnue untl t s closed down, at t=t close. Then, exergy has to be used to clean up and restore the envronment, whch accounts for the last part of the ndrect exergy nput,.e., E ndrect, whch s already accounted for (Eq. 8). By consderng the total lfe cycle of the plant the net produced exergy becomes E net,pr =E pr -E ndrect. These areas representng exerges are ndcated n Fg. 7. Assume that, at tme t=0, the producton of a wnd power plant starts and at tme t=t start t s completed and put nto operaton. At that tme, a large amount of exergy has been used n the constructon of the plant, whch s ndcated by the area of E ndrect between t=0 and t=t start Then the plant starts to produce electrcty, whch s ndcated 5

6 n Fg. 7 by the upper curve E pr =E ndrect +E net,pr. At t=t payback the exergy used for constructon, mantenance and clean up has been pad back. For modern wnd power plants ths tme s only some months. Then the system has a net output of exergy untl t s closed down, whch for a wnd power staton may last for decades. Thus, these dagrams could be used to show f a power supply system s sustanable. LCEA s very mportant n the desgn of sustanable systems, especally n the desgn of renewable energy systems. Take a solar panel, made of manly alumnum and glass that s used for the producton of hot water for household use,.e. about 60 C. Then, t s not obvous that the exergy beng spent n the producton of ths unt ever wll be pad back durng ts use,.e., t mght be a msuse of resources rather than a sustanable resource use. The producton of alumnum and glass requre a lot of exergy as electrcty and hgh temperature heat or several hundred degrees Celsus, whereas the solar panel delvers small amounts of exergy as low temperature heat. LCEA must therefore be carred out as a natural part of the desgn of sustanable systems n order to avod ths knd of msuse. Another case to nvestgate s the producton of bofuels n order to replace fossl fuels n the transport sector. Ths may not necessarly be sustanable snce the producton process uses a large amount of fossl fuels, drectly for machnery or ndrectly as fertlzers, rrgaton and pestcdes. Thus, t may well turn out to be better to use the fossl fuels n the transport sector drectly nstead. Ths wll be well descrbed by a LCEA. Sustanable engneerng could be defned as systems whch make use of renewable resources n such a way that the nput of non-renewable resources wll be pad back durng ts lfe tme,.e. E pr >E n +E ndrect. In order to be truly sustanable the used deposts must also be completely restored or, even better, not used at all. Thus, by usng LCEA and dstngushng between renewable and non-renewable resources we have an operatonal method to defne sustanable engneerng. LCEA dagrams are of partcular mportance n the plannng of large scale renewable energy systems of multple plants. Intally, ths system wll consume most of ts supply wthn ts own constructons phase. However, some tme after completon t wll delver at full capacty. Thus, the energy supply over tme s heavly affected by nternal system dynamcs. LCEA Appled to Typcal Wnd Power Systems There are numerous publcatons on lfe cycle analyss of dfferent wnd power systems. The energy use for constructon, operaton and destructon are often presented. Schlesner [11] analyzed energy and emssons of the producton and manufacturng of materals for an offshore and land based wnd farm. The offshore wnd farm conssted of 10 unts of 500 kw turbnes or 5 MW and a land based wnd farm 18 of 500 kw or 9 MW. The prmary energes used n the producton and dsposal of materals amount to about 44 and 47 TJ for the offshore and land based farm respectvely. Wth yearly electrcty producton of about 45 and 71 TJ ths mply about 1 and 0.7 years payback tme respectvely. Assume that the constructon accounts for 97%, operaton for 1% and deconstructon for 2% of the total exergy nput and that constructon and deconstructon last for one year each then we wll have the followng LCEA dagram for a lfe tme of 20 years, see Fg. 8. We see that the total exergy nput s mnor to the produced electrcal exergy and the exergy nput for operaton and deconstructon s hardly vsble. Thus, the LCEA shows that ths wnd power system s truly sustanable. 6

7 Fg. 8. LCEA of an offshore wnd power farm and a land based farm. In a study for Tawan the lfecycle stages of a planned land based wnd turbne farm of three dfferent turbnes are analyzed [12]. Three turbnes are nvestgated Vestas (V-47), Enercon (E-40), and Vestas (V-66), see Table 1.The turbne manufacturng, foundaton constructon, as well as operaton and dsposal of the wnd power systems are consdered. Energy nput (GJ) Vestas (V-47) Enercon (E-40) Vestas (V-66) Manufacturng of turbne 3,486 3,202 3,898 Constructon of foundaton 3,226 2,964 3,608 Installaton of the system Operaton & generaton Destructon & clean up Total energy nput 23,547 Total electrcty generated 1,640,670 Table 1. Lfecycle nventory of energy nput of three demo wnd power systems From these data we can set up an LCEA by assumng that the energy nput can be regarded as exergy snce the energy orgnates mostly from fuels and electrcty, see Fg. 9. Fg. 9. LCEA of a wnd power plant. In ths case we see that constructon of foundaton and nstallaton of the system accounts for hgh exergy power nput n the fnal state of constructon. However, as before the total exergy nput s pad back n only a few months by the produced electrcal exergy. 7

8 Conclusons Exergy s an excellent concept to descrbe the utlzaton of energy and materal resources n systems. The combnaton of LCA wth exergy and dstngush between non renewable and renewable resources to form LCEA offers a unque effcent method to analyze the level of sustanablty for energy systems. The applcaton of LCEA to dfferent wnd power systems gves an excellent vsualzaton of the exergy flows nvolved. Thus, n these cases t becomes obvous that the exergy nput s well pad back and that the systems wll be net producers of exergy. LCEA s strongly recommended as a sutable tool n the analyss of level of sustanablty for energy systems and should be part of every energy engneerng currcula. Acknowledgements The permsson to use my work for the UNESCO s Encyclopeda of Lfe Support Systems [13] for ths paper s hereby gratefully acknowledged. References [1] S. Carnot, Réflectons sur la pusance motrce du feu et sur les machnes propres a développer cette pussance. 1824, R. Fox, Pars: Bacheler, [2] J.W. Gbbs, A Method of Geometrcal Representaton of the Thermodynamc Propertes of Substances by Means of Surfaces. Trans. Conn. Acad, vol. II, pp , [3] Z. Rant, Exerge, en neues Wort für technsche Arbetsfähgket. (Exergy, a New Word for Techncal Avalable Work). Forschungen m Ingeneurwesen, vol. 22, pp , [4] M. Trbus, Thermostatcs and Thermodynamcs. New York: Van Nostrand, [5] E. Scubba, and G. Wall, A Bref Commented Hstory of Exergy from the Begnnngs to 2004 Int. J. of Thermodynamcs, vol. 10, pp.1-26, [6] G. Wall, Exergy a Useful Concept wthn Resource Accountng, Report No , Insttute of Theoretcal Physcs, Göteborg, [7] G. Wall, Exergy a Useful Concept Ph.D. thess, Chalmers Unversty of Technology, Göteborg, Sweden, [8] The Exergoecologcal Portal, [9] Szargut J, Morrs D. Cumulatve exergy consumpton. Energy Research, 1987, vol. 11, pp [10] M. Gong and G. Wall, On Exergy and Sustanable Development Part 2: Indcators and methods, Exergy, an Internatonal Journal, 1 (2001) [11] L. Schlesner, Lfe cycle assessment of a wnd farm and related externaltes. Renewable Energy, 20 (2000) [12] Y.-M. Lee, Y.-E. Tzeng, and C.-. Su, Lfe Cycle Assessment of Wnd Power Utlzaton n Tawan. Insttute of Natural Resource Management, Natonal Tape Unversty, Tawan [13] EOLSS, Encyclopeda of Lfe Support Systems. 8