A First and Second Thermodynamics Law Analysis of a Hydrogen-Fueled Microgas Turbine for Combined Heat and Power Generation

ABSTRACT As is well known, the increasing energy demand requires an efficient use of conventional energy sources, as well as the development of renewable technologies. The distributed generation systems entail significant benefits in terms of
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  Francisco Toja-Silva CIEMAT,Av. Complutense, 40,Madrid 28040, Spaine-mail: frantojasilva@yahoo.es Antonio Rovira E.T.S. Ingenieros Industriales – UNED,C/Juan del Rosal, 12,Madrid 28040, Spaine-mail: rovira@ind.uned.es A First and SecondThermodynamics Law Analysisof a Hydrogen-FueledMicrogas Turbine for CombinedHeat and Power Generation  As is well known, the increasing energy demand requires an efficient use of conventionalenergy sources, as well as the development of renewable technologies. The distributed generation systems entail significant benefits in terms of efficiency, emission reduction,availability and economy consequences. Renewable energy technologies are fed by inter-mittent resources. This feature makes the energy storage an important issue in order toimprove the management or to enlarge annual operation of the facility. The use of hydro-gen as an energy vector may satisfy this requirement and; at the same time, it introducesadditional advantages in terms of energy efficiency and emissions reduction. This work  presents an analysis based on the first and second thermodynamics law to investigate theefficiency of a hydrogen/oxygen-fueled gas turbine, which produces both electrical and thermal energy (cogeneration). A 20 kWe, microgas turbine is proposed to supply thebase load demand of a residential area. The results show that the proposed facility isappropriate when the thermal energy demand is significant. We obtain an exergy effi-ciency of 45.7  %  and an energy efficiency of 89.4 %  regarding the lower heating value(LHV) of hydrogen. This high energy efficiency remains on the use of the liquid water effluent and the condensation heat. The main sources of irreversibility are analyzed and the effect of the design parameters on the energy and exergy efficiencies is discussed. [DOI: 10.1115/1.4025321]  Keywords: exergy, hydrogen, cogeneration, CHP, combined heat and power, microgasturbine, second law analysis 1 Introduction According to the estimations of the International EnergyAgency (IEA) [1], the primary energy demand in the world willgrow by more than one-third over the period 2010–2035. Thisdemand should not be covered with conventional energy genera-tion technologies because of the limited primary resources, therestriction on the pollutant and greenhouse gas emissions, etc. Inorder to satisfy this demand, the rational use of the energy as wellas the development of renewable resources is required, whichreduce the dependence on the fossil fuels and also the greenhousegas emissions.The need for increasing the weight of the renewable resourcesin the energy mix is evidenced; for example, in the exponentialgrowth of the world installed power of both photovoltaic andwind energy [2]. On the other side, the adoption of distributedenergy generation brings significant benefits in efficiency terms(regarding loss reduction in energy transportation), CO 2  emissionreduction, energy resource availability, and economy impulse.Renewable energy sources usually come from intermittentresources, especially solar and wind, so the energy is generatedwhen the resource is available (sun and wind), but not when thedemand rises. Thus, there is a need for energy storage whenthe generation exceeds the demand (off-peak hours). Furthermore,the energy storage allows a guaranteed supply when the demandis higher than the primary generation (peak hours). For example,Hadjipaschalis [3] presents a comparative analysis of differentenergy storage technologies where it is shown that hydrogen, asan energy vector, is the reversible storage technology with highestspecific energy.There are a lot of methods to obtain molecular hydrogen (frommetals, hydrocarbons, etc.). Among them, water electrolysisbecomes the most suitable in the case of using renewable energysources, such as photovoltaic or wind. Accordingly to Zeng andZhang [4], water electrolysis is the best method to obtain hydro-gen. It is simpler than other technologies, although less efficient.Inefficiency is mainly due to the heat rejection to the environment.Other authors [5] show that, when obtained from renewable sour-ces, emissions are lower than when obtained from hydrocarbons,although the cost is higher. Challenges for this technology are theefficiency enhancement, durability and safety reinforcement(regarding the hydrogen use, because of its high diffusion and itshigh flammability) [4].Hydrogen storage is an issue with an intense research activity,as it is a key aspect to the development of this technology [6,7]. The most commonly used method is the pressurized gas one.Although the density is not a significant storage problem in thecase of stationary applications, the pressurized gas technology hasthe inconveniences of low density and high cost for very highpressures. The use of liquid hydrogen has a higher cost, and theuse of metallic hydrides requires still a large amount of energy.The physisorption on nanotubes/nanofibers and the cryoadsorptionon superactivated carbon micropores are promising to have alow cost in the future [6]. However, neither absorption nor adsorp-tion materials developed until now meet all the required propertiessuch as high storage capacity, low reaction enthalpy, fast Contributed by the Combustion and Fuels Committee of ASME for publication inthe J OURNAL OF  E NGINEERING FOR  G AS  T URBINES AND  P OWER . Manuscript receivedDecember 11, 2012; final manuscript received August 21, 2013; published onlineOctober 25, 2013. Assoc. Editor: Kalyan Annamalai. Journal of Engineering for Gas Turbines and Power  FEBRUARY 2014, Vol. 136  / 021501-1Copyright V C  2014 by ASME Downloaded From: http://gasturbinespower.asmedigitalcollection.asme.org/ on 01/17/2014 Terms of Use: http://asme.org/terms  kinetics, high number of life cycles, low desorption temperature,etc. [8].Regarding the energy exploitation of the hydrogen, the objec-tive is to use it in the most efficient way; i.e., intending to recover the energy supplied for its generation or to minimize the exergyloss in the different processes. In this way, the combined heat andpower generation (CHP) is the best option, because it allows theproduction of high quality energy (electricity), as well as recover-ing part of the wasted heat produced in the electrical energy gen-eration process. The main hydrogen-fueled CHP technologies arethe fuel cells and cogeneration turbines.To evaluate the convenience of the use of a system, energy andexergy analysis are needed. There are a lot of works in the techni-cal literature that analyze facilities based on gas turbines, mainlyfocusing on combined cycles [9 – 11], cogeneration [12 – 15] and trigeneration [16] hydrocarbon fueled.Hydrogen-fueled turbines are in a lower grade of maturity.Some authors have analyzed systems partially fueled by hydrogen(mixed with hydrocarbons, mainly with natural gas) [17] or sys-tems exclusively hydrogen-fueled. For example, Jericha et al. [18]propose a high temperature steam cycle fueled with hydrogen.Kato and Nomura [19] presents an experimental facility based ona hydrogen gas turbine (simple Brayton cycle) and they carriedout an energy analysis of it. Jin and Ishida [20] analyzed a verysimilar configuration as the presented in this paper making use of exergy tools, but only from the point of view of electricity produc-tion. Finally, Xiaodan et al. [21] analyzed a more complex config-uration also aimed to electricity production. Hydrogen-fueledcombined cycles have been also proposed by other authors[22,23]. The contribution of the present work is to conceive a hydrogen-fueled cogeneration cycle (not only focused on the elec-tricity production), in which the liquid effluent (combustion prod-uct) is exploited, for increasing the energy efficiency.The objective of this paper is to devise both energy and exergyefficiencies of a microgas turbine fed with hydrogen and oxygen,that has the purpose of generating mechanical power and hotwater to cogenerate.The facility, whose objective is to serve as a base load supplyof both thermal and electrical demands of a residential area, isdescribed in detail in Sec. 2, and its operation conditions are alsodiscussed. Section 3 presents the developed methodology to calcu-late the fluid properties and devices. In Sec. 4, the simulationresults are presented and discussed. Finally, Sec. 5 presents theconclusions. 2 Description of the Facility In this work, a CHP microgas turbine fed with hydrogen andoxygen is analyzed. These gases may be previously generatedfrom intermittent renewable energies, such photovoltaic and windpower.Since the system is designed to operate in steady regime, it maywork as a base load generator. When the electrical demandexceeds the base load generation, the power grid is used to supplythe peak loads. Besides, the electricity surplus may be injectedinto the power grid. To ensure the exploitation of the whole gener-ated thermal energy, the facility may be designed accordingly tothe thermal demand. The generated thermal energy may satisfythe base load, and the rest of the demand may be satisfied by anadditional thermal generation. As an example of application, thefacility can be designed to satisfy the base load of some residentialbuildings in Norway, where Pedersen et al. [24] have been done ademand analysis. The cost-effectiveness may be analyzed byconsidering specific economic conditions.Figure 1 shows a diagram of the proposed facility. It consists of a Brayton cycle fed with hydrogen as a combustible and oxygenas an oxidant. Both hydrogen and oxygen are compressed anddirected to a combustion chamber, where the combustion reactiontakes place. Since the temperature reached in stoichiometrichydrogen-oxygen combustion is dramatically high, liquid water isalso added to the combustion chamber. The product of the com-bustion reaction is steam, which is expanded in a turbine produc-ing mechanical work that, in turns, is used to generate electricpower by means of a generator coupled to the turbine shaft andalso for driving the oxygen and hydrogen compressors. The steamat the turbine outlet is directed to a heat recuperator, which hasthe function of preheating the water injected into the combustionchamber. At the outlet of the heat recuperator, the steam is con-ducted to the condenser, where its latent heat is used to producehot water. At the condenser outlet, part of the condensate is re-circulated to be injected into the combustion chamber. This water is previously pumped to the heat recuperator to preheat it. Therest of the condensate can be exploited, either preheating the localwater distribution or being mixed with the hot water outlet flow.Figure 2 shows the temperature-specific entropy diagram of thethermodynamic cycle. The points 1-2-3-4 describe a simple Bray-ton cycle. Notice that there are two points 2 in the figure, one thatcorresponds to the oxygen compressor outlet and the other to thehydrogen compressor outlet. As said before, in the recuperator (stage 4–5) the steam transfers heat to the water that is directed tothe combustion chamber. Cooling between the points 5 and 6 isused to produce hot water.The hot water generated may be managed by the regulation anddistribution system of the buildings. The outlet flows at the points12 and 10 are introduced in that system at 80  C, considering theresults presented in Sec. 4. There is a recirculation flow 11extracted from that system. The temperature at the point 11 canoscillate in practice, although 25  C are considered to be consist-ent with the reference temperature used in the exergy analysis(death state). 3 Methodology This section presents the models used to simulate the devicesand to calculate the fluid properties. Sections 3.1, 3.2, and 3.3describe the mass, energy and exergy balances applied to the dif-ferent components. The equation system and the method to solveit are described in Sec. 3.4. Different figures of merit for assessingthe system efficiency are also commented. 3.1 Mass Balances.  The different circulating mass flows arethe hydrogen mass flow (  _ m  H  ), the oxygen mass flow (  _ m O ), thewater process mass flow (  _ m wp ), the recirculation water mass flow(  _ m wr  ), the outlet water mass flow (  _ m out ), and the refrigerationwater mass flow (  _ m hw ). All of them are related due to mass conser-vation at the combustor and the water extraction point _ m wp  ¼  _ m  H   þ  _ m O  þ  _ m wr   (1) _ m wp  ¼  _ m wr   þ  _ m out  (2) 3.2 Energy Balances.  The facility has two compressors, cor-responding to the hydrogen and the oxygen streams. Compressionis assumed to be adiabatic but not reversible. The pressure ratio,the isentropic efficiency and the mechanical efficiency are shownin Table 1. The thermodynamic properties for the gasses wereobtained from Refs. [25,26]. The power to drive the compressor is _ W  C  ¼  _ m gas h 2    h 1 g mC (3)Both hydrogen and oxygen are introduced into the combustionchamber. The mixture burns and its temperature increases up to adetermined value (Table 2). The first law applied to a genericcombustion chamber yields to _ m O h O 2  þ  _ m  H  h  H  2  ¼  _ m 3 h 3    _ m  H  HHV g cc  (4)where the considered enthalpies are the sensible and latent ones(the enthalpy of all the species are zero at 25  C and 1atm) while 021501-2 /   Vol. 136, FEBRUARY 2014  Transactions of the ASME Downloaded From: http://gasturbinespower.asmedigitalcollection.asme.org/ on 01/17/2014 Terms of Use: http://asme.org/terms  the enthalpies of formation are included within the higher heatingvalue (HHV) of the hydrogen.Since a certain amount of water is added in order to control thetemperature of the products, the energy balance results in _ m O h O 2  þ  _ m  H  h  H  2  þ  _ m 9 h 9  ¼  _ m 3 h 3    _ m  H  HHV g cc  (5)The generated steam is directed to the turbine. The isentropic effi-ciency of the process and the mechanical efficiency are shown inTable 1. Properties for the water were obtained from Refs.[25,26]. The power produced by the turbine is _ W  T   ¼  _ m 3 ð h 3    h 4 Þ g mT   (6)After the turbine, the steam goes to the recuperator, which heatsup the water that will be injected into the combustion chamber.The energy balance in the recuperator is _ Q rec  ¼  _ m 4 ð h 4    h 5 Þ ¼  _ m 8 ð h 9    h 8 Þ  (7)The steam coming from the heat recuperator is firstly cooled andthen condensed. The released heat is used to produce hot water.The condensation pressure is a design data (Table 2). The heattransferred in this process may be calculated as follows: _ Q cond  ¼  _ m 5 ð h 5    h 6 Þ ¼  _ m 11 ð h 12    h 11 Þ  (8)Finally, the pump power is calculated assuming water as an idealliquid and using the isentropic and mechanical efficiencies shownin Table 1. The power consumption is _ W   p  ¼  _ m 7 ð  P 2    P cond Þ qg sp g mp ¼  _ m 7 ð h 8    h 7 Þ  (9) 3.3 Exergy Balances.  Once the thermodynamic variables andthe mass flow rates are known, the second law of thermodynamicscan be applied to each element of the cycle, in order to quantifythe irreversibilities.The general formulation of the exergy balance applied to anopen steady-flow system, which exchanges heat with a heat sourceat temperature  T  r   and transfers shaft work and generates someirreversibility, is Fig. 1 Diagram of the CHP microgas turbine fed with hydrogen and oxygen Journal of Engineering for Gas Turbines and Power  FEBRUARY 2014, Vol. 136  / 021501-3 Downloaded From: http://gasturbinespower.asmedigitalcollection.asme.org/ on 01/17/2014 Terms of Use: http://asme.org/terms  _  E out    _  E in  ¼  _  E Q   _ W     _  I   (10)where  _ W   is the shaft power, and the outlet and inlet exergy flowsthat includes both the physical and chemical components (as theformation enthalpy and entropy are considered) are _  E out  ¼ X out _ m out e out  (11) _  E in  ¼ X in _ m in e in  (12)The exergy associated to the heat exchange is _  E Q ¼ X r  _ Q r   1    T  amb T  r     (13)The irreversibilities at the component are _  I   ¼ X i T  amb  _ r i  (14) _ r i  being the entropy generation.As the flows at the compressors, turbine and pump follow anadiabatic process (  _ Q  ¼  0), Eq. (10) yields _  E out    _  E in  ¼   _ W     _  I   (15)In the combustion chamber, the mechanical power is  _ W   ¼  0. Con-sidering the exergy content of the heat losses as a part of the irre-versibilities, the equation results in _  I   ¼  _  E in    _  E out  ¼ ð  _  H  in    _  H  out Þ   T  amb ð  _ S in    _ S out Þ  (16)The term in enthalpies (including the enthalpies of formation) isbalanced with Eq. (5) except for the heat losses: _ m  H  HHV ð 1    g cc Þ . Therefore, irreversibility may be assessed asfollows: _  I   ¼  T  amb ð  _ m 3 s 3    _ m 9 s 9    _ m O s O 2    _ m  H  s  H  2 Þ þ  _ m  H  HHV ð 1    g cc Þ (17)In Eq. (17), the entropy srcin for all the species are coincident(zero at 25  C and 1bar), except for the entropy of formation of the water (entropy of formation for oxygen and hydrogen arenull). The entropy of formation of the water (srcin of the entropyof the water) may be assessed with the following equation: D  s o f   ¼ D  h o f    D  g o f  T  amb (18)The resulting value is added to the sensible entropy of the water.For the heat exchangers (recuperator and condenser), shaftwork is not generated and external losses are neglected. Thus,Eq. (10) yields _  E out    _  E in  ¼  _  I   (19) 3.4 Resolution of the Problem.  The start datum of the prob-lem is the electricity demand (  _  D e ), which corresponds with the netshaft power and is assumed to have the value of 20kW, _  D e  ¼  _ W  T     _ W  CO    _ W  CH     _ W   p  (20)The combustion reaction takes place under stoichiometric propor-tion conditions  H  2  þ  1 = 2  O 2  !  H  2 O  (21)Taking into account the molar mass of the involved species, therelationships between the reagents mass flow rates are8  _ m  H   ¼  _ m O  (22)9  _ m  H   ¼  _ m wp    _ m wr   (23) Fig. 2 Diagram T-s of the thermodynamic cycle of theproposed facilityTable 1 Design parameters of the devices and fluid Parameter Value Reference g sC  0.87 [9] g mC  0.95 [9] g cc  0.97 [21] g sT   0.9 [9] g mT   0.95 [9] g sp  0.758 [12] g mp  0.75 [19]LHV 120.9MJ kg   1 [27]HHV 142.9MJ kg   1 [27] Table 2 Facility design parameters and reference source Variable Value Reference h  H  1  T   H  1  ¼  298 : 15K [21] h o 1  T  o 1  ¼  298 : 15K [21] h 3  T  3  ¼  1700K [21] h cond  ¼  h 6  ¼  h 7  ¼  h 10  T  cond  ¼  T  satliq ð  P cond Þ h 9  T  9  ¼  T  satliq ð  P 2 Þ h 11  T  11  ¼  25  C h 12  T  12  ¼  80  C  P 1  1bar  ¼ 101300Pa [21]  P 2  30bar  ¼ 3001300Pa [21]  P cond  50000Pa [21] 021501-4 /   Vol. 136, FEBRUARY 2014  Transactions of the ASME Downloaded From: http://gasturbinespower.asmedigitalcollection.asme.org/ on 01/17/2014 Terms of Use: http://asme.org/terms  _ m O 9 = 8  ¼  _ m wp    _ m wr   (24)Replacing Eqs. (22) and (23) into Eq. (5), it results that _ m  H   ¼  A  _ m wp  (25)where  A  is  A  ¼  h 9  þ  h 3 8 h O2  þ  h H2  þ  9 h 9  þ  HHV g cc (26)Replacing Eqs. (3), (6), and (9) into Eq. (20), we get the following: _  D e  ¼  _ m wp ð h 3    h 4 Þ g mT     _ m O h O 2    h O 1 g mC   _ m  H  h  H  2    h  H  1 g mC   _ m wr  ð  P 2    P cond Þ qg sp g mp (27)Rearranging Eqs. (23), (24), and (27) we obtain _  D e  ¼  _ m wp  B    _ m  H  C  (28)  B  and  C  being  B  ¼ ð h 3    h 4 Þ g mT    ð  p 2    p cond Þ qg sp g mp (29) C  ¼  8 ð h O 2    h O 1 Þ g mC þ  h  H  2    h  H  1 g mC   9 ð  P 2    P cond Þ qg sp g mp (30)Replacing Eq. (25) into Eq. (28) yields to _ m wp  ¼ _  D e  B    AC  (31)The values of the design parameters of the devices and fluid (  H  2 )are shown in Table 1. Table 2 shows the values of the facility design parameters and the reference source.Since the stream flow 10 (in Fig. 1) can be exploited to generatethermal energy, the total thermal energy obtained is _ Q  ¼  _ m hw ð h 12    h 11 Þð Þ þ  _ m 10 ð h 10    h 11 Þð Þ  (32)The energy efficiency of the cogeneration system can beexpressed with respect to the LHV or with respect to the HHV of hydrogen, by g LHVcog  ¼ _  D e  þ  _ Q _ m  H  LHV (33) g HHVcog  ¼ _  D e  þ  _ Q _ m  H  HHV (34)Since thermal and electrical energy have different qualities, theequivalent electric efficiency with respect to the LHV can be cal-culated by means of the following expression: g LHVeq  ¼ _  D e ð  _ m  H  LHV Þ   _ Q g t  conv (35)where  g t  conv  is the efficiency of a conventional thermal energygeneration facility. It is assumed a mean average value of  g t  conv  ¼  0 : 85 since the energy efficiency of a conventional water heater takes values ranging between 0.75 and 0.95 [28]. Noticethat the thermal energy is subtracted from the maximumexploitable energy,  _ m  H  LHV, because the thermal energy isassumed to be generated by a conventional water heater. Thus, theequivalent electric efficiency can be directly compared with other technologies of electrical energy generation.Likewise, in Eq. (35), the equivalent electric efficiency withrespect to the HHV is g HHV eq  ¼ _  D e ð  _ m  H  HHV Þ   _ Q g t  convHHV (36)where  g t  convHHV  is the efficiency of a conventional thermal energygenerator with respect to HHV. This value can be found from theconventional efficiency by means of the following expression: g t  convHHV  ¼  g t  conv LHVHHV    (37)The exergy efficiency may be calculated as below: g ex  ¼ _  D e  þ  _  E 10  þ  _  E 12 _  D e  þ  _  E 10  þ  _  E 12  þ P  _  I  ¼ _  D e  þ  _  E 10  þ  _  E 12 _  D e  þ  _  E 10  þ  _  E 12  þ  _  I  CH   þ  _  I  cO  þ  _  I  T   þ  _  I  cc  þ  _  I  rec  þ  _  I  cond  þ  _  I   p (38)where  _  E 10  and  _  E 12  are the sensible exergy of the warm water stream obtained at the points 10 and 12, respectively. Notice thatthe up right hand side term of Eq. (39) represents the total recov-ered exergy, while the down right hand side represents the totalexpended exergy throughout the process. 4 Results and Discussion Table 3 shows the main design parameters obtained for the pro-posed facility, and Table 4 shows the value of temperatures, pres-sures, enthalpies and entropies at each point. Figure 3 shows theirreversibility generated at each component of the facility andFig. 4 presents the Sankey diagram.Very high cogeneration energy efficiencies are obtained for theproposed configuration (89.4% and 75.6% with respect to bothLHV and HHV, respectively), due to the exploitation of the con-densation heat and the process outlet flow to generate hot water.The equivalent electric efficiencies are even higher (100.5% and85.1% with respect to LHV and HHV, respectively) because theefficiency of a conventional water heater is lower than the effi-ciency of the thermal energy generation process of the proposedfacility. Notice that the equivalent electric efficiencies may behigher than the cogeneration efficiency, and can even exceed100%.The exergy efficiency is moderated-low (45.7%), because ahigh rate of the exploited energy is in form of heat at low tempera-ture, which has low quality. The heat-to-electricity rate is 2.17. Table 3 Main design parameters of the proposed facility Parameter Value _ Q  43.36kW g LHVcog  89.36% g HHVcog  75.60% g LHVeq  100.54% g HHVeq  85.06% g ex  45.69% T  9  506.30K T  11  298.15K T  cond  354.14K  P cond  50000Pa Journal of Engineering for Gas Turbines and Power  FEBRUARY 2014, Vol. 136  / 021501-5 Downloaded From: http://gasturbinespower.asmedigitalcollection.asme.org/ on 01/17/2014 Terms of Use: http://asme.org/terms
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