Tools for Sustainable Energy Engineering

Transcription

1 Tools for Sustanable Energy Engneerng Göran Wall Department of Culture, Energy and Envronment, Gotland Unversty, Vsby, Sweden Tel: +46(0) , Fax: +46(0) 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 and optmzaton are presented, n partcular Lfe Cycle Exergy Analyss (LCEA). Optmzaton methods ncorporatng both exergy and economc condtons are also presented. 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. Keywords: Renewable energy, Energy Polcy, Energy engneerng, Sustanable development, Educaton. Nomenclature 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 Introducton Exergy s a well establshed scentfc concept sutable n the work towards sustanable development. Exergy accountng of the use of energy and materal resources provdes unque knowledge on how effectve a process s n utlzng physcal resources. 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. 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] 2323

2 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 ( E E ) tot tot tot En Eout = T0 S = n out > 0 (3) where output, and tot E n s the total exergy nput, E s the exergy destructon n sub process. tot S s the total entropy ncrease, ( n out E ) tot E out s the total exergy 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 nonrenewable resources must be consdered. Therefore, the problem needs a more careful approach. 3. Exergy dagrams 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 2324

3 low, about 4%, because the temperature dfference s 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 snce both energy and exergy s almost equal for the nflow of fuels and the outflow of electrcty. 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). 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%. 4. 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. 2325

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. 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 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 exergy supply systems 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 2326

5 of operaton, and fnally the clean up or destructon stage. These tme perods are analogous to the three steps of the lfe cycle of a product n an LCA. The exergy nput orgnatng from non renewable resources used for constructon, mantenance and clean up we call ndrect exergy E ndrect. Indrect exergy nput orgnatng from renewable recourses are not accounted for. When a power plant s put nto operaton, t starts to delver a product, e.g. electrcty wth E pr, by convertng the drect exergy power nput n exergy power E. 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. 6. LCEA of a fossl fueled power plant. Fg. 7. LCEA of a wnd power plant. In the frst case, the system s not sustanable, snce we use exergy orgnatng from a nonsustanable 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 tstart E pr tlfe ( t) dt = E ( t) dt = E 0 ndrect ndrect (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. 4). 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. For modern wnd power plants ths tme s less than one year [11]. 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. 2327

6 LCEA s very mportant n the desgn of sustanable systems, especally n the desgn of renewable energy systems. Assume 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 renewable energy systems n order to certfy a sustanable resource use. 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 the 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 non-renewable resources 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. 5. Exergy and economcs Exergy measures the physcal value of an energy resource. Thus, t relates to the economc value, whch reflects ts usefulness. Ths makes exergy a valuable energy polcy tool. In order to encourage the use of sustanable resources and to mprove resource use, an exergy tax could be ntroduced. The use of non-renewable resources and ts waste should be taxed by the amount of exergy t accounts for, snce ths s related to depleton of resources and an envronmental mpact. In addton to ths, toxcty and other ndrect envronmental effects must also be consdered. In the case of rreversble envronmental damage, a tax s not sutable, nstead restrctons must be consdered. Eventually, ths should also be the case for the depleton of assets from future generatons. At least t ndcates a moral dlemma. A system could be regarded as a part of two dfferent envronments, the physcal and the economc envronment. The physcal envronment s descrbed by pressure P 0, temperature T 0, and a set of chemcal potentals μ 0 of the approprate substances, and the economc envronment by a set of reference prces of goods and nterest rates. These two envronments are connected by cost relatons,.e. cost as a functon of physcal quanttes (Fg. 8). Wth the system embedded n the physcal envronment, for each component there are mass and energy balances needed to defne the performance of the system. In addton, these balances descrbe the physcal behavor of the system. 2328

7 If the cost relatons are known, then the physcal and economc envronments could be lnked. The cost equatons can sometmes be smplfed to a scale effect, tmes a penalty of ntensty. Then the system of lowest cost, whch s physcally feasble, can be found. Usually the mantenance and captal costs of the equpment are not lnear functons, so n many cases these costs have more complex forms. If, by some reason, t s not possble to optmze the system, then at least cost could be lnked to exergy by assumng a prce of exergy. Ths method s called Exergy Economy Accountng (EEA). Fg. 8. The system surrounded by the physcal and the economcal envronments, whch are lnked through cost relatons. Snce exergy measures the physcal value, and costs should only be assgned to commodtes of value, exergy s thus a ratonal bass for assgnng costs, both to the nteractons that a physcal system experences wth ts surroundngs and to the sources of neffcency wthn t. The exergy nput s shared between the product, and the losses,.e. destructon and waste. EEA smply means determnng the exergy flows and assgnng economc value to them. Thus, EEA does not nclude consderaton of nternal system effects. It does not descrbe how the captal nvestments n one part on the system affect exergy losses n other parts of the system. In the EEA method the exergy losses are numbers and not functons. However, ths smple type of analyss sometmes gves deas for, otherwse, not obvous mprovements, and a good start of an optmzaton procedure, n whch the exergy losses would be functons. When constructng a system, the goal s often to attan the hghest possble techncal effcency at the lowest cost, wthn the exstng techncal, economcal and legal constrans. The analyss also ncludes dfferent operatng ponts (temperatures, pressures, etc.), confguratons (components, flow charts, etc.), purpose (dual purpose, use of waste streams, etc.), and envronments (global or local envronment, new prces, etc.). Usually, the desgn and operaton of systems have many solutons, sometmes an nfnte number. By optmzng the total system, the best system under the gven condtons s found. Some of the general engneerng optmzaton methods could be appled, n order to optmze specfc desgn and operaton aspects of a system. However, selectng the best soluton among the entre set requres engneerng judgment, ntuton and crtcal analyss. Exergy Economy Optmzaton (EEO) s a method that consders how the captal nvestments n one part of the system affect other parts of the system, thus optmzng the objectve functon. The margnal cost of exergy for all parts of the system may also be calculated to fnd where exergy mprovements are best pad off. 6. Fnal words These tools must be ncorporated wth energy engneerng and polcy to develop sustanable energy systems. My own experence from educaton s a strong postve feedback from the students and parts of the educatonal establshment, e.g., the UNESCO project Encyclopeda 2329

8 of Lfe Support Systems (EOLSS) [12]. However, sometmes there s also a strong skeptcsm among the academc establshment for ths that also has to be dealt wth. Thus, tradtonal boarders between dfferent dscplnes must be removed and more of nterdscplnary studes and actvtes must be appled at both hgh school and unversty levels. More problem orented approaches and a focus on sustanable development ssues are also to be encouraged. 7. Conclusons Exergy based tools are excellent to descrbe the utlzaton of renewable energy resources and mportant wthn sustanable energy engneerng. A system that consumes the exergy resources at a faster rate than they are renewed s not sustanable. The educatonal system has a crucal role to play to ntroduce these tools n order to promote educaton for sustanable development. Ths educaton must be based on a true understandng of our physcal condtons. Exergy s a concept that offers a physcal descrpton of the lfe support systems as well as a better understandng of the use of energy and other resources n socety. Thus, exergy and descrptons based on exergy are essental for our knowledge towards sustanable development. 8. Acknowledgements The permsson to use my work for the UNESCO s Encyclopeda of Lfe Support Systems [12] 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, 1873, pp [3] Z. Rant, Exerge, en neues Wort für technsche Arbetsfähgket. (Exergy, a New Word for Techncal Avalable Work), Forschungen m Ingeneurwesen, vol. 22, 1956, 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, 2007, pp [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, vol. 11, 1987, pp [10] M. Gong and G. Wall, On Exergy and Sustanable Development Part 2: Indcators and methods, Exergy, an Internatonal Journal, vol. 1, 2001, pp [11] Schlesner L. Lfe cycle assessment of a wnd farm and related externaltes. Renewable Energy, vol. 20, 2000, pp [12] EOLSS, Encyclopeda of Lfe Support Systems