Analysis of a feasible trigeneration system taking solar energy and biomass as co-feeds

Energy Conversion and Management 122 (2016) 74–84 Contents lists available at ScienceDirect Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman Analysis of a feasible trigeneration system taking solar energy and biomass as co-feeds Xiaofeng Zhang, Hongqiang Li ⇑, Lifang Liu, Rong Zeng, Guoqiang Zhang ⇑ College of Civil Engineering, National Center for International Research Collaboration in Building Safety and Environment, Hunan University, Changsha 410082, PR China a r t i c l e i n f o Article history: Received 13 January 2016 Received in revised form 20 May 2016 Accepted 22 May 2016 Keywords: Biomass gasification Solar energy Internal combustion engine Trigeneration system System integrating a b s t r a c t The trigeneration systems are widely used owing to high efficiency, low greenhouse gas emission and high reliability Especially, those trigeneration systems taking renewable energy as primary input are paid more and more attention This paper presents a feasible trigeneration system, which realizes biomass and solar energy integrating effective utilization according to energy cascade utilization and energy level upgrading of chemical reaction principle In the proposed system, the solar energy with mid-andlow temperature converted to the chemical energy of bio-gas through gasification process, then the bio-gas will be taken as the fuel for internal combustion engine (ICE) to generate electricity The jacket water as a byproduct generated from ICE is utilized in a liquid desiccant unit for providing desiccant capacity The flue gas is transported into an absorption chiller and heat exchanger consequently, supplying chilled water and domestic hot water The thermodynamic performance of the trigeneration system was investigated by the help of Aspen plus The results indicate that the overall energy efficiency and the electrical efficiency of the proposed system in case study are 77.4% and 17.8%, respectively The introduction of solar energy decreases the consumption of biomass, and the solar thermal energy input fraction is 8.6% In addition, the primary energy saving ratio and annual total cost saving ratio compared with the separated generation system are 16.7% and 25.9%, respectively Ó 2016 Elsevier Ltd All rights reserved Introduction Recently, fossil fuels have been the main primary energy in the worldwide However a series of serious problems have occurred due to over utilization of fossil fuels, such as CO2 emission, climate change and ecological balance disruption Therefore, various renewable energy resources are drawn increased attention for their environmental advantages, especially solar energy and biomass energy, have been widely used as a result of their unique advantages, such as cleanliness, safety, abundant reserves and so on [1–3] For solar energy utilization, mid-and-low solar thermal utilization technology obtains the widespread attention for its good thermal performance and economy The solar energy can not only be used as heating driving resource, such as evaporation and recuperation processes, but also can be used for chemical processes, like decomposition and reforming Modi et al [4] compared the thermodynamic performance of the Kalina cycle for a central receiver solar thermal power with direct steam generation and a Rankine cycle, and emphasized that Kalina cycle showed a clear advantage ⇑ Corresponding authors E-mail addresses: lhq@hnu.edu.cn (H Li), gqzhang@hnu.edu.cn (G Zhang) http://dx.doi.org/10.1016/j.enconman.2016.05.063 0196-8904/Ó 2016 Elsevier Ltd All rights reserved when heat input was primarily from a two-tank molten-salt storage and Rankine cycle showed better performance than Kalina cycle when the heat input was only from the solar receiver Calise et al [5] designed and simulated a novel prototype of a kWe solar power plant, mainly consisting of flat-plate evacuated solar collectors and a small Organic Rankine Cycle (ORC) to evaluate the energy and economic performance of the system At the same time, many researchers have investigated the possible of thermochemical utilization of solar energy Steinfeld [6] summarized and reviewed the current research on thermo-chemical production of hydrogen by solar energy Hong et al [7] analyzed the performance of a new solar thermal power cycle combined with middle-temperature solar thermal energy and methanol decomposition and concluded that the novel system was more competitive compared with conventional power system Xu et al [8] developed a novel combined cooling heating and power system integrated with mid-and-low temperature solar energy thermo-chemical process and the methanol decomposition, and presented an energy and exergy analysis to investigate the performance of the system Zhang et al [9,10] proposed a solar-assisted methane chemically recuperated gas turbine system, which converted the low temperature solar heat into vapor latent heat and then via the reforming reactions to the syngas chemical energy Liu et al [11] studied a X Zhang et al / Energy Conversion and Management 122 (2016) 74–84 75 Nomenclature Abbreviation CCHP combined cooling, heating and power CGE cold gas efficiency CHP combined heating and power COP coefficient of performance CSP concentrated solar power ER equivalence ratio FT Fischer–Tropsch GHG greenhouse gas HHV higher heating value LHV lower heating value HX heat exchanger ICE internal combustion engine ORC organic rankine cycle PESR primary energy saving ratio SBR steam/biomass ratio VCC vapor compression cycle Symbols ATC ATCSR C CGE COP EX F annual total cost (Yuan) annual total cost saving ratio (%) cost (Yuan) cold gas efficiency (%) coefficient of performance exergy (kW) solar thermal energy input fraction (%) hydrogen production with the integration of methanol steam reforming and middle-temperature solar thermal energy based on experiments The research results showed that the chemical conversion of methanol could reach levels higher than 90% and the maximum hydrogen yield per mole of methanol was 2.65– 2.90 mol Biomass is the plant material derived from the photosynthesis between CO2, water and sunlight to produce carbohydrates [12,13], thus it is renewable and carbon–neutral resource Biomass has some other advantages such as abundant in resources, widely distributed, environmental friendly One of the most potential technology of biomass utilization is gasification, by which biomass can be transformed into bio-gas The bio-gas can be used as a feedstock for the production of chemicals or power [14–17] In order to realize biomass gasification, gasifying agent like air, steam, or oxygen will be required As the most availability and economy gasifying agent, air is widely used in demonstration or commercial scale biomass gasification [15,18,19] However, in this way, due to the introduction of nitrogen, the bio-gas has a low heating value The use of oxygen is not economical owing to the high cost of oxygen production, although it can increase the bio-gas heating value Gasification with steam can produce bio-gas with a heating value of 10–14 MJ/Nm3 However, this process is an endothermic reaction, which needs extra heat to sustain the gasification reaction To summarize, air–steam gasification process may be a better way to realize gasification The combustion reaction provides the required heat for gasification, which is termed as auto-thermal process Combined cooling, heating and power (CCHP) system combines distributed power generation with thermally activated equipments to meet the cooling, heating and power needs for users It has been used worldwide because of its high efficiency, low greenhouse gas (GHG) emission and high reliability [20–22] In recent years, combined heat and power (CHP) systems based on biomass and solar energy have been widely concerned [2,23,24] Pablo et al [25] HHV I LHV m N p Q T V W g higher heating value (MJ/Nm3) interest rate (%) lower heating value (MJ/Nm3) mass flow rate (kg/h) installed capacity (kW) service life (year) heat (kW) temperature (°C) volume flow rate (Nm3/h) electricity (kW) efficiency (%) Subscripts b biomass c cooling capacity d domestic hot water de desiccant capacity el electricity ex exergy g bio-gas i/j the number of equipment M annual maintenance sep separated generation system sol solar th thermal heat tri trigeneration system modeled and optimized a biomass steam gasification system, which include two main parts: solar assisted steam production part and micro gas turbine power generation part The solar collector generates high temperature steam (800–1200 °C) as the gasifier agent The research results showed that, the overall system performance can be improved by such an integrating way Tanaka et al [26] presented a hybrid power generation system coupling biomass gasification and concentrated solar collecting processes, the generated bio-gas was taken as fuel in a gas turbine in a further way Utilizing the molten-nitrate salt as heat carrier to absorb the heat from the receiver in molten salt heat storage system, the heat is used for producing steam for Rankine cycle and is converted to electricity Ravaghi-Ardebili et al [27] investigated the efficiency of biomass gasification process on low temperature condition, which coupled with a Concentrated Solar Power (CSP) plant As a heated working fluid molten salt produced the steam ($410 °C) to participate in the gasification reaction Angrisani et al [28] presented a new concept solar-biomass cogeneration system using a Stirling engine for the combined production of the heat and electric power As a biomass combustion chamber, the fluidized bed simultaneously absorbed the heat concentrated from the solar collector The Stirling engine converted the heat collected in the fluidized bed into mechanical and then electrical power In addition to the combined heating and power system integrated with biomass and solar energy, some studies have also investigated producing synthetic fuels in polygeneration systems Bai et al [29] investigated the thermodynamic analysis and the economic performances of a solar-driven biomass gasification polygeneration system for the methanol production and the power generation The solar-biomass gasifier produced raw bio-gas through absorbing the solar thermal energy reflected by heliostats The purified bio-gas was used for the methanol production as syngas, while the un-reacted syngas would be used for power generation And the results indicated that the energy and exergy 76 X Zhang et al / Energy Conversion and Management 122 (2016) 74–84 efficiency of the proposed system approximately reached to 56.09% and 54.86%, respectively Hertwich et al [30] presented a new concept of producing synfuel from biomass using concentrating solar energy, which contained main parts: steam gasifier, reverse water gas shift, hydrocarbon synthesis, heat recovery and steam generation, and solar power system The molten-salt provided the high temperature heat for gasification, which was obtained from solar power system, and the H2 for reverse water gas shift reaction was generated by electrolyzing water driven by solar power And they modeled the production of methanol in the proposed system compared with the traditional system only using biomass or coal as a fuel Guo et al [31] studied the energetic and environmental performance of the solar hybrid coal and biomass to liquid system integrated with a solar hybrid dual fluidized bed gasifier, the olivine was used as bed material in the gasifier to transfer the heat from combustion reactor and/or solar receiver to gasification reactor, and using storage units to compensate the influence of solar radiation The purified syngas was fed into a Fischer–Tropsch (FT) reactor to produce FT liquid, and the un-reacted gas was burned to generate power in the gas turbine At the same time, some researchers have studied the combined cooling, heating and power (CCHP) system integrated with biomass and solar energy Karellas et al [32] investigated the thermodynamic and economic analysis of a trigeneration system using biomass and solar energy, which consisted of an Organic Rankine Cycle (ORC) and a vapor compression cycle (VCC) Khalid et al [33] reported that the energy and exergy analysis of an integrated multigeneration system using biomass and solar energy It contained two Rankine and gas turbine cycles, as well as an absorption cooling cycle Biomass combustion drove Gas turbine cycles to produce electrical power and the oil heated by concentrated solar collector provided Rankine cycle and absorption cooling cycle with thermal energy They concluded that system efficiency had an obvious improvement compared with a single renewable energy source The literature survey on biomass and solar-driven trigener- ation system indicates that the trigeneration system is mostly integrated with biomass combustion and Organic Rankine Cycle, while the research focusing on biomass gasification and Otto Cycle integrated trigeneration system which driven by biomass and solar energy is relatively fewer In this paper, a small-medium trigeneration system coupled with biomass gasification and solar thermal process is suggested and discussed In the proposed system, the mid-and-low temperature solar thermal energy is transformed into the chemical energy of bio-gas by gasification process, utilizing the sensible heat of biogas to produce a part of domestic hot water The internal combustion engine (ICE) is driven by the bio-gas to generate electricity Then, the flue gas is sent to absorption chiller and heat exchanger consequently to generate chilled water and domestic hot water The jacket water derived from ICE is utilized in a liquid desiccant unit for dehumidification So as to evaluate the system performance, the thermodynamic and economic performances of the trigeneration system are studied Several key system integrating parameters are investigated, including equivalence ratio (ER), steam/biomass ratio (SBR), air preheating temperature, solar collector temperature and fuel price System flowsheet description The flowsheet of the suggested system is shown in Fig The system consists of three main parts: (1) air–steam biomass gasification and purification subsystem, which contains a fluidized bed gasifier, a biomass preheater, a cyclone separator, an air splitter and heat exchangers (HX-1 and HX-2); (2) steam generation subsystem, which contains a parabolic trough solar collector and a pump; (3) internal combustion engine power generation subsystem, which contains an internal combustion engine, a LiBr–H2O absorption chiller, a liquid desiccant unit and a heat exchanger (HX-3) Fig Flowsheet of a trigeneration system with solar energy and biomass coupling utilization 77 X Zhang et al / Energy Conversion and Management 122 (2016) 74–84 The grinded biomass material (stream 1) is preheated by air (stream 8, 200 °C) in preheater, and reducing the biomass moisture to about 10% Then the biomass material (stream 2) is fed into a fluidized bed gasifier after preheated in the biomass preheater The preheated air (stream 8, 200 °C) and steam (stream 12, 350 °C) generated from solar collector are fed into the gasifier with biomass (stream 2) The high temperature bio-gas (stream 4) after removed the ash and char is fed into the heat exchangers (HX-1 and HX-2) Utilizing the sensible heat of bio-gas to preheat the air (stream 6, 25 °C, bar) and produce domestic hot water (stream 27, 80 °C) Then, as the fuel, the purified bio-gas (stream 13) is fed into the internal combustion engine for electricity generation The jacket water (stream 18) from the engine is used to provide low temperature waste heat for the liquid desiccant unit, and then the unit supplies dehumidified air (stream 20) to customers The LiBr–H2O absorption chiller is driven by waste heat from ICE flue gas (stream 15), in which provides cooling for users After transferring the heat to domestic hot water (80 °C) in the heat exchanger (HX-3), the exhausted gas (stream 17) is released to the atmosphere at a temperature of 120 °C Table Characteristics of biomass material The process is isothermal and steady state There is no pressure loss in the gasifier Biomass particles are of uniform size and temperature The bio-gas consists of H2, CO, CO2, CH4, H2O, and tar formation is disregarded  Char only contains carbon and ash, and ash is used to be inert material  The sulfur and nitrogen go to H2S and NH3 respectively Heat of reaction (kJ/mol) Carbon partial combustion Carbon combustion Hydrogen partial combustion Boudouard Methanation Water gas CO shift Steam-methane reforming H2S formation NH3 formation C + 0.5O2 M CO C + O2 M CO2 H2 + 0.5O2 M H2O C + CO2 M 2CO C + 2H2 M CH4 C + H2O M CO + H2 CO + H2O M CO2 + H2 CH4 + H2O M CO + 3H2 S + H2 M H2S 0.5N2 + 1.5H2 M NH3 À111 À393 À242 +172 À75 +131 À41 +206 – – Item Value Item Value Gasification temperature (°C) Solar collector temperature (°C) Compression ratio of ICE 890 350 0.1 60 Item Gasification pressure (MPa) Solar collector efficiency (%) Parameter Equipment investment cost (Yuan/kW)a Gasification subsystemb Gas ICE Absorption chiller Electric chiller Boiler Solar collectorc Gas–water HX Water–water HX Liquid desiccant unit a Reaction equation 39.78 4.97 40.02 0.46 0.20 14.144 Table The economic parameters of system [35–38] Value 2500 4800 1200 970 375 4525 400 210 1200 Economic Interest rate (%) 6.15 Service life (year) 20 Maintenance cost ratiod (%) 2.5 Operating hourse (h) 2000 Fuel cost Biomass (Yuan/ton) Natural gas (Yuan/kW h) Electricity (Yuan/kW h) 350 0.194 0.936 1US$ = 6.12 Yuan (RMB) The gasification subsystem includes the gasifier and the gas conditioning, the former accounts for 95% of the investment, and the latter accounts for 5% of the investment c The initial investment cost of the solar collector field includes the solar collector, the related equipment investment and the solar collector land The cost of solar collector and related equipment is 1225 Yuan/m2; the area of solar collector land is three times that of the solar collector, and the cost of solar collector land is 225 Yuan/m2 d The maintenance cost ratio is the ratio of the maintenance cost to the investment cost e The annual operating hours of the trigeneration system is determined by the solar collector subsystem, according to [29], the annual operating hours of solar collector subsystem is 2000 h b Reaction name Ultimate analysis (%, dry basis) Carbon (C) Hydrogen (H) Oxygen (O) Nitrogen (N) Sulfur (S) HHV (MJ/kg) feed rate, mbiomass = 1400 kg/h; the air equivalence ratio, ER = 0.4; the steam/biomass ratio, SBR = 0.4) The main parameters are listed in Table 3, and the initial investment costs and parameters are presented in Table The main chemical reactions that occurred in the biomass gasification process are presented in Table In this study, rice husk is selected as the biomass material Table shows biomass material characteristics used in the simulation process [34] To analyze the thermodynamic performance of the trigeneration system, a case study is investigated (the biomass Table Gasification reactions of biomass 70.36 15.07 14.57 14.43 Flue gas pressure of ICE 0.12 (MPa) Flue gas temperature of ICE 450 Jacket water temperature of 87 (°C) ICE (°C) Mechanical efficiency of pump 99 Isentropic efficiency of pump 75 (%) (%) COP of absorption chiller 1.2 COP of liquid desiccant unit 0.8 Node temperature difference 20 Node temperature difference 20 of HX-1/2 (°C) of HX-3 (°C) 3.1 Assumptions     Value (%) Proximate analysis (%, dry basis) Volatile matter Fixed carbon Ash Moisture Table Key operating parameters of system System thermal performance calculation To further analyze the thermodynamic performance of the trigeneration system, the Aspen Plus process model simulator is used The selections of key process equations are as follows: the Peng– Robinson thermodynamic model is selected in compression, combustion, expansion and other processes of bio-gas and air The STEAM-TA thermodynamic model is selected in water and steam generating processes Selecting the thermodynamic equilibrium model for the biomass gasification process And the following assumptions are considered in modeling the fluidized bed gasifier gasification process: Character 78 X Zhang et al / Energy Conversion and Management 122 (2016) 74–84 To analyze the thermodynamic and economic performances and the influences of the related parameters, the simulation and analysis procedures are shown in Fig The inputs conditions consist of system assumptions, biomass characteristics, key operating and economic parameters By mean of the Aspen Plus simulator, the thermodynamic performances including energy and exergy analysis are calculated At the same time, the equipment capacity of different components can also be obtained by Aspen Plus, which contributes to computing the economic indicators including annual total cost and annual total cost saving ratio Moreover, the effects of relevant parameters on the proposed system performances can also be analyzed through the simulation Qb is the biomass energy input of the trigeneration system, kW; mb is the mass flow rate of biomass, kg/h; Qsol is the solar energy absorbed by steam generation subsystem, kW; LHVb is the lower heating value of biomass, kJ/kg; the lower heating value is calculated as [39]: LHV b ¼ HHV b À 21:978 H where HHVb is the higher heating value of biomass, MJ/kg; H is the percentage of hydrogen in the biomass material, % Besides the overall energy efficiency, the exergy efficiency of trigeneration system is defined as: W þ EX d þ EX c þ EX de  100% EX sol þ EX b W þ EX d þ EX c þ EX de ¼  100% EX sol þ b Á mb Á LHV b gex ¼ 3.2 Performance evaluation criteria In the proposed system, the overall energy efficiency is selected as an evaluation indicator of the thermodynamic performance of trigeneration system, which can be defined as: W þ Q c þ Q d þ Q de  100% Q b þ Q sol W þ Q c þ Q d þ Q de  100% ¼ mb Á LHV b þ Q sol g¼ ð1Þ Furthermore, the electrical efficiency has been calculated as: gel ¼ W  100% LHV b Á mb þ Q sol ð2Þ where W is the electricity generation of the trigeneration system, kW; Qc is the cooling generation of the trigeneration system, kW; Qd is domestic hot water generation of the trigeneration system, kW; Qde is the desiccant capacity of the trigeneration system, kW; ð3Þ ð4Þ where EXd is the domestic hot water exergy of the system, kW; EXc is the cooling exergy of the system, EXde is the desiccant exergy of the system, kW; EXsol is the solar thermal exergy of the system; EXb is the biomass exergy of the system; b is the multiplication factor, which can be calculated as [40]: b¼ 1:044 þ 0:0160ðH=CÞ À 0:3493ðO=CÞð1 þ 0:0531ðH=CÞÞ þ 0:0493ðN=CÞ ðO=C 2Þ À 0:4124ðO=CÞ ð5Þ where C, H, O, N are the mass fraction of carbon, hydrogen, oxygen and nitrogen of biomass in ultimate analysis, respectively The primary energy saving ratio (PESR) is selected to compare the performance between trigeneration system and separated generation system with the same products The primary energy saving ratio can be defined as: Fig Simulation and analysis procedures of the proposed system 79 X Zhang et al / Energy Conversion and Management 122 (2016) 74–84 PESR ¼ À Q b þ Q sol  100% Q sep Q b þ Q sol ¼1À W gsep;el c þQ de þ COPQsep;c Ág sep;el þ g Qd  100% ð6Þ sep;th where Qsep is the fuel consumption of the separated generation system, kW; gsep,el is the electrical efficiency of the separate power plant, %; COPsep,c is the coefficient of performance (COP) of electrical refrigerator and dehumidification unit; gsep,th is the thermal efficiency of a boiler, % In order to compare the trigeneration system with the separated generation system on the condition of same products, the performance parameters of the separated generation system are as follows: the electrical efficiency of the separate power plant is 33%, the COP of electrical refrigerator and dehumidification unit is 3.0, the thermal efficiency of a boiler is 85% The introduction of solar energy decreases the consumption of biomass material, in order to determine the effect of solar energy in the trigeneration system, the solar thermal energy input fraction has been calculated as: F sol Q sol Q sol ¼  100% ¼  100% Q b þ Q sol LHV b Á mb þ Q sol ð7Þ where Fsol is the solar thermal energy input fraction, %; Qsol is the solar energy absorbed by steam generation subsystem, kW The cold gas efficiency of the gasification process is defined as the ratio of the energy of bio-gas to that of biomass material: CGE ¼ LHV g Á V g  100% LHV b Á mb ð8Þ where CGE is the cold gas efficiency, %; LHVg is the lower heating value of bio-gas, kJ/Nm3; Vg is the volume flow rate of bio-gas in the standard state, Nm3/h; mb is the mass flow rate of biomass, kg/h; LHVb is the lower heating value of biomass, kJ/kg The annual total cost of the proposed system consists of three parts: annual initial capital cost, maintenance cost and operation cost Both the initial capital cost and maintenance cost are function of equipment capacities The annual total cost of the trigeneration system can be calculated as: ATC tri ¼ R  X Ni C i þ C tri;M þ Q b C b ð9Þ And the annual total cost of the separated generation system can be calculated as: ATC sep ¼ R  X À Á Nj C j þ C sep;M þ WC e þ Q gas C gas ð10Þ where N and C are the installed capacity and the investment cost of the equipment respectively (kW and Yuan/kW); i and j are the number of equipments of trigeneration and separated generation system respectively; Ctri,M and Csep,M are the annual maintenance costs of trigeneration and separated generation system respectively, Yuan Qb and Cb are the annual consumption and price of the biomass respectively (kg and Yuan/ton); Qgas is the natural gas consumed by the boiler of separated generation system, kg; Ce and Cgas are the energy charges of electricity and natural gas respectively, Yuan/kW h The capital recovery factor, R, can be defined as: R¼ Ið1 þ IÞp ð1 þ IÞp À ð11Þ where I is the interest rate, %; and superscript p is the service life of the equipment, year The annual total cost saving ratio (ATCSR) is used as economic criterion to compare the performance between the trigeneration system and separated generation system It can be calculated as: ATCSR ¼ ATC sep À ATC tri ATC tri ¼1À ATC sep ATC sep ð12Þ where ATCsep is the annual total cost of the separated generation system, Yuan; ATCtri is the annual total cost of the trigeneration system, Yuan 3.3 System performance calculation results In the case system, the relevant parameters are as follows: air equivalence ratio (ER): 0.4, steam/biomass ratio (SBR): 0.4, gasification temperature: 890 °C, gasification pressure: 0.1 MPa As we can see from Table 5, the input, output and system performance are listed For the trigeneration system, in the case of input 5076 kW biomass energy, it consumes extra solar energy of 477 kW to provide the steam for biomass gasification process The input of solar energy reduces the consumption of biomass, which makes the solar thermal energy fraction reaches to 8.6% With the same products, the trigeneration system saves more primary energy than separated generation system, the primary energy saving ratio reaches to 16.7% And the overall energy efficiency is 77.4% by utilizing biomass energy and solar energy Through Aspen Plus simulation, it can calculate the inputs and outputs exergy of the trigeneration system, which contributes to determining the exergy efficiency of the proposed system The total exergy efficiency of the proposed system is 19.2%, which is approximately 9.8% higher than the separated system (17.3%) The heating sources of absorption chiller and liquid desiccant unit in the proposed system are from waste heat of ICE, while the Table Calculation results of trigeneration system Item Trigeneration system Separate system Input Fossil fuel (kW) Biomass energy (kW) Solar heat (kW) – 5076 477 6667 – – Output Electricity (kW) Domestic hot water (kW) Cooling generation (kW) Desiccant capacity (kW) 987 1988 843 482 987 1988 843 482 System performance Electrical efficiency (%) Cold gas efficiency (%) Solar thermal energy input fraction (%) Overall energy efficiency (%) Total exergy efficiency (%) Primary energy saving ratio (%) Annual total cost saving ratio (%) 17.8 59.3 8.6 77.4 19.2 16.7 25.9 – – – – – – – 80 X Zhang et al / Energy Conversion and Management 122 (2016) 74–84 energy source of electrical refrigerator is from high grade electricity From the perspective of the waste heat utilization, in spite of the lower energy grade of flue gas and jacket water, the absorption refrigeration and liquid desiccant technology make full use of the waste heat And these measures also improve the exergy efficiency of the proposed system Moreover, the equipment capacity of the components could be determined in the case study The initial capital cost of system can be calculated by the equipment investment cost in Table 4, thus the operation cost and maintenance cost can be obtained by the economic formula subsequently Finally, the annual total cost and annual total cost saving ratio are determined based on the above results In the case study, it shows that the annual initial capital cost of the proposed system is larger than the separated generation system, while the operation cost is obviously lower than the separated generation system The annual total cost saving ratio (ATCSR) is approximately 25.9% compared with the separated generation system These results indicate that, the novel trigeneration system with the combination of renewable energy can improve the overall energy efficiency of system and provide various products for customers ature increases with the increase in the equivalence ratio As we know, steam gasification requires sufficient heat for endothermic gasification reaction The higher air flow rate contributes to generating more combustion heat, which is favorable to steam gasification reaction When keeping equivalence ratio constant, the endothermic reaction of water–gas and steam-methane reforming are strengthened with the increase in the steam flow rate, then leading to the decrease in gasification temperature 4.2 Effect of SBR on the bio-gas composition Fig shows the variation of bio-gas composition as a function of the SBR over the range of 0–4.0 With the increase in steam/biomass ratio, the content of N2 and CO decrease gradually, and H2 and CO2 content increase gradually However, the variation of CH4 content is not obvious, though the trend is decreasing With the increase in steam flow rate, the reaction of water–gas and CO shift is enhanced, which consumes more steam and CO and produces more H2 and CO2 Although keeping the equivalence ratio constant, the mole of combustible gas increases Therefore the N2 content introduced by the air is diluted in the bio-gas And the reaction of steam-methane reforming is strengthened with the increase in steam flow rate, which decreases the CH4 content Discussion 4.3 Effect of SBR on the bio-gas yield with various ERs In order to know better about the novel system, air equivalence ratio (ER), steam/biomass ratio (SBR), air preheating temperature, solar collector temperature and fuel price are selected as key operating parameters to analyze the performance of the proposed system 4.1 Effect of SBR on the gasification temperature with various ERs Gasification temperature is critical for air–steam gasification process Both air flow rate and steam flow rate have an effect on gasification temperature in the adiabatic condition In this study, the gasification temperature is varied from 700 °C to 1000 °C And the performance analysis is performed in the range of 0.35 ER 0.5 and SBR 4.0 Fig illustrates the effects of steam/biomass ratio (SBR) and equivalence ratio (ER) on the gasification temperature It can be seen that the high equivalence ratio and low steam/biomass ratio favor the increase in gasification temperature With the increase in SBR, the gasification temperature decreases In addition, the equivalence ratio has a significant effect as the gasification temper- ER=0.35 ER=0.4 ER=0.45 ER=0.5 1000 950 900 850 800 750 700 650 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Steam/Biomass Ratio 3.5 4.4 Effect of SBR on the cold gas efficiency with various ERs Cold gas efficiency is an important indicator to evaluate the performance of the gasifier Fig presents the cold gas efficiency (CGE) at different steam/biomass ratios and equivalence ratios of the gasification process Either the increase in the steam/biomass ratio or the equivalence ratio leads to the decrease in cold gas effi- Bio-gas Composition (mol %, dry basis ) Gasification Temperature ( ) 1050 Fig depicts the effect of steam/biomass ratio on bio-gas yield at different ER With the increase in steam flow rate, the reaction of water gas and CO shift is enhanced, which promotes the yield of bio-gas As shown in Fig 5, the bio-gas yield increases significantly with the increase in steam/biomass ratio For example, when keeping equivalence ratio at 0.35, the bio-gas yield increases from 2.22 to 3.45 While increases from 3.88 to 7.52 at ER of 0.5 Moreover, due to the introduction of N2 in the air, the gas yield enhances However, the increase in bio-gas yield is not obvious with the increase in ER For example, the bio-gas yield increases from 3.45 to 3.88 at SBR of 1.0 4.0 Fig Effect of SBR on the gasification temperature with various ERs 50 N2 40 30 H2 CO2 20 10 CO CH4 0.0 0.5 1.0 1.5 2.0 Steam/Biomass Ratio Fig Effect of SBR on the bio-gas composition (ER = 0.4) 81 X Zhang et al / Energy Conversion and Management 122 (2016) 74–84 90 Bio-gas Yield (Nm3/kg) ER=0.4 ER=0.5 88 Overall Energy Efficiency (%) ER=0.35 ER=0.45 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 84 82 80 78 76 74 72 0.5 1.0 Solar Thermal Energy Input Fraction (%) 70 ER=0.35 ER=0.4 ER=0.45 ER=0.5 CGE (%) 55 50 45 40 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 2.0 2.5 3.0 3.5 4.0 Fig Effect of SBR on the overall energy efficiency with various ERs Fig Effect of SBR on the bio-gas yield with various ERs 60 1.5 Steam/Biomass Ratio Steam/Biomass Ratio 65 ER=0.4 ER=0.5 86 70 0.0 4.0 ER=0.35 ER=0.45 4.0 Steam/Biomass Ratio Tsol=150 50 Tsol=250 Tsol=350 40 30 20 10 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Steam/Biomass Ratio Fig Effect of SBR on the cold gas efficiency of bio-gas with various ERs Fig Effect of SBR on the solar thermal energy input fraction for various solar collector temperatures ciency The cold gas efficiency experiences a obvious reduction when the equivalence ratio is increased For example, the cold gas efficiency decreases from 63.7% to 45.8% with the increase in ER from 0.35 to 0.5, when keeping the steam/biomass ratio at 1.0 Similarly, with the increase in steam flow rate, the cold gas efficiency decreases rapidly at low value of ER, then decreases slowly at high value of ER As can be seen from Fig 6, the cold gas efficiency varies from 67.0% to 64.1% at ER of 0.35, and varies from 45.8% to 43.8% at ER of 0.5 biomass ratio at different solar collector temperature The solar thermal energy input fraction can be reached up to 48.1% when steam/biomass varies from to 4.0 at solar collector temperature of 350 °C In addition, it can be seen from Fig that with the increase in ER, the overall energy efficiency increases For example, the overall energy efficiency increases from 77.2% to 82.3% with the increase in ER from 0.35 to 0.5, when keeping the steam/biomass ratio at 1.0 4.6 Effect of SBR on the primary energy saving ratio with various ERs 4.5 Effect of SBR on the overall energy efficiency and solar thermal energy input fraction The curves presented in Fig show the variation of overall energy efficiency at different SBR and ER The increase in steam/ biomass ratio causes the increase in overall energy efficiency on account of solar thermal energy input Obviously, the high steam flow rate requires more solar thermal energy input Because the solar collector provides the heat to raise the temperature of steam, therefore the consumption of biomass material could be reduced Solar thermal energy input fraction is selected to evaluate the contribution of solar thermal energy As shown in Fig the solar thermal energy input fraction increases with the increase in steam/ The effect of SBR on the primary energy saving ratio (PESR) at different ER is shown in Fig Primary energy saving ratio (PESR) has been calculated to assess the performance between trigeneration system and conventional separated generation system Fig presents that PESR decreases with the increase in SBR and ER When the gasification process operates at a lower steam and air flow rate, the PESR drops obviously with the increase in steam/biomass ratio, but decreases slowly with the increase in steam/biomass ratio As can be seen from Fig that the PESR decreases from 19.5% to 15.5% with increase in SBR from to 1.0 at ER of 0.35, however decreases from 13.7% to 12.9% with increase in SBR from 1.0 to 4.0 at ER of And PESR decreases to a constant 82 X Zhang et al / Energy Conversion and Management 122 (2016) 74–84 20 Thermal efficiency Electrical efficiency Overall energy efficiency 100 ER=0.35 ER=0.45 19 ER=0.4 ER=0.5 90 80 18 Efficiency (%) PESR (%) 70 17 16 15 14 60 50 40 30 20 13 10 12 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 0.0 4.0 0.5 Steam/Biomass Ratio value of 12.9% when the SBR increases from 2.5 to 4.0 at ER of 0.5 The results show that the proposed system has an apparent advantage to the separated generation system, especially for saving the fossil fuels 4.7 Effect of SBR on the system products and efficiency Figs 10 and 11 describe the distribution of system products and efficiency at different SBR respectively The results in Fig 10 indicate that SBR has a significant influence on domestic hot water generation As shown in Fig 5, the bio-gas yield increases with the increase in SBR when the solar collector temperature is maintained at 350 °C, therefore increasing the sensible heat of bio-gas and the domestic hot water obtained by heat exchanger (HX-2) Besides that, the electricity, cooling generation and desiccant capacity decrease with the increase in SBR, but not obviously Due to the decrease in lower heating value of bio-gas, the input energy of ICE goes down while the bio-gas yield increases with the increase in steam flow rate Fig 11 shows the system performance for various SBRs at ER of 0.4 The electrical efficiency decreases with the increase in SBR, as it can be seen from Fig 10, domestic hot water increases significantly compared with other products, consequently increasing the thermal efficiency with the increase in SBR However the bio-gas yield increases with the increase in steam flow rate, the LHV of bio-gas is reduced, which decreases the electrical efficiency The trigeneration system uses air and steam as gasification agent, and the air preheating temperature has a significant impact on the overall energy efficiency Fig 12 represents the variation of overall energy efficiency with the air preheating temperature at different ER As shown in Fig 12, the higher temperature of air improves the gasification performance more The overall energy efficiency increases from 77.8% to 82.8% with increase in air preheating temperature from 100 °C to 500 °C at ER of 0.4 Moreover, the overall energy efficiency increases with the increase in ER For example, the overall energy efficiency increases from 77.2% to 82.3% with increase in ER from 0.35 to 0.5 at air preheating temperature of 200 °C 4.9 Effect of solar collector temperature on the overall energy efficiency with various ERs As mentioned above, the solar collector temperature has an important effect on the overall energy efficiency similarly Fig 13 illustrates the overall energy efficiency with solar collector temperature at different ER The solar collector provides heat with Overall Energy Efficiency (%) System Products (kW) Electricity Domestic hot water Cooling generation Desiccant capacity 3000 2000 1000 1.0 1.5 2.0 ER=0.35 ER=0.4 ER=0.45 ER=0.5 85 80 75 70 0.5 2.0 4.8 Effect of air preheating temperature on the overall energy efficiency with various ERs 90 0.0 1.5 Fig 11 Effect of SBR on the system efficiency (ER = 0.4) Fig Effect of SBR on the primary energy saving ratio with various ERs 4000 1.0 Steam/Biomass Ratio 100 200 300 400 500 Air Preheating Temperature ( ) Steam/Biomass Ratio Fig 10 Effect of SBR on the system products (ER = 0.4) Fig 12 Effect of air preheating temperature on the overall energy efficiency at different ER (SBR = 1.0) 83 X Zhang et al / Energy Conversion and Management 122 (2016) 74–84 ER=0.35 ER=0.45 60 ER=0.4 ER=0.5 Biomass Natural gas Electricity 50 ATCSR (%) Overall Energy Efficiency(%) 85 80 40 30 20 10 75 100 150 200 250 300 350 400 450 Solar Collector Temperature( ) 0.4 0.6 0.8 1.0 1.2 1.4 1.6 The Change Multiple of Price Fig 13 Effect of solar collector temperature on the overall energy efficiency for various ERs (SBR = 1.0) steam for gasification reaction, so the temperature of steam changes with the solar collector temperature From Fig 13, the higher solar collector temperature enhances the overall energy efficiency As described above, it is also showed that the increase in ER increases the overall energy efficiency of trigeneration system 4.10 The comparison of annual total cost compositions between trigeneration and separated generation system The annual total cost compositions of the proposed and separated generation system are shown in Fig 14 It can be found that the annual initial capital cost of the trigeneration system is obviously higher than that of the separated generation system The annual initial capital cost of proposed system is approximately seven times than that of the separated generation system However, the annual operation costs between the separated generation system and trigeneration system are 3,626,740 Yuan (RMB) and 980,000 Yuan (RMB) respectively, which the separated generation system is about 3.7 times than the trigeneration system In addition, from Fig 14 it can be seen that the equipment initial capital cost of the trigeneration system mainly accounts for 51.5% of the annual total cost, and the operation cost is about 33.9% of the Trigeneration system Annual Cost (Yuan) 4x10 Separated generation system 3x106 2x106 1x106 Equipment Operation Maintenance Annual Total Cost Composition Fig 14 The comparison between trigeneration system and separated generation system in annual total cost compositions Fig 15 The effect of fuel price on the annual total cost saving ratio annual total cost in the trigeneration system For the separated generation system, the operation cost is much higher than the equipment initial capital cost, which accounts for 92.9% of the annual total cost 4.11 Effect of fuel price on ATCSR Considering the fluctuation of the market fuel price, it is imperative to analyze the variation of annual total cost saving ratio (ATCSR) at different fuel prices, such as biomass, natural gas and electricity The ATCSR sensitive analysis between the trigeneration system and separated generation system is shown in Fig 15 It can be seen from Fig 15 that the annual total cost saving ratio decreases lineally with the increase of the biomass price Due to the increase in biomass price, the operation cost of trigeneration system increases, which leads to the increase of annual total cost (ATC) subsequently Moreover, the annual total cost saving ratio increases nonlinearly with the increase of prices of natural gas and electricity The effect of electricity on the annual total cost saving ratio is greater than that of the natural gas under the same change multiple of the prices Conclusion In this study, a feasible trigeneration system coupled with biomass gasification and solar thermal process is proposed Transforming mid-and-low solar thermal energy into chemical energy of bio-gas by heating the steam indirectly, and providing fuel for internal combustion engine and exhaust heat recovery subsystem Simulation and performance analysis of the trigeneration system are performed to investigate the effects of key operating parameters on its performance The main research and conclusion are as follows: (1) Air equivalence ratio, steam/biomass ratio and air preheating temperature have a significant effect on biomass gasification reaction, and consequently affect the overall energy efficiency of the trigeneration system (2) The introduction of mid-and-low solar thermal energy in trigeneration system decreases the extra consumption of biomass, and the solar thermal energy input fraction can be reached up to 48.1% when steam/biomass varies from to 4.0 at solar collector temperature of 350 °C Similarly, the higher solar collector temperature improves the overall energy efficiency of the trigeneration system 84 X Zhang et al / Energy Conversion and Management 122 (2016) 74–84 (3) The analysis of primary energy saving ratio (PESR) achieves an advantage compared with the separated generation system, and the proposed system provides various products to meet the demand of different customers (4) In a case study (ER = 0.4, SBR = 0.4), the cold gas efficiency can reach at 59.3%, the solar thermal energy input fraction and primary energy saving ratio are 8.6%, 16.7%, respectively The overall energy efficiency and the total exergy efficiency of the trigeneration system are 77.4% and 19.2%, respectively (5) From the perspective of economic analysis, the annual total cost saving ratio (ATCSR) compared with separated generation system is about 25.9% The equipment initial capital cost of proposed system accounts for 51.5% of the annual total cost, and the operation cost is about 33.9% of the annual total cost in the proposed system (6) The efficient utilization of renewable energy has a unique advantage compared with fossil fuels And the novel trigeneration system will provide a new idea for the integration with solar energy and biomass energy Acknowledgements This study is supported by the National Natural Science Foundation Project of China (No 51541603), the International Science & Technology Cooperation Program of China (No 2014DFE70230) and the Key Project of Hunan Province (No 2011FJ1007-1) References [1] Ellabban O, Abu-Rub H, Blaabjerg F Renewable energy resources: current status, future prospects and their enabling technology Renew Sust Energy Rev 2014;39:748–64 [2] Sahoo U, Kumar R, Pant PC, et al Scope and sustainability of hybrid solar– biomass power plant with cooling, desalination in polygeneration process in India Renew Sust Energy Rev 2015;51:304–16 [3] Nzihou A, Flamant G, Stanmore B Synthetic fuels from biomass using concentrated solar energy–a review Energy 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