Energy, exergy and environmental analysis of a hybrid combined cooling heating and power system utilizing biomass and solar energy

Energy Conversion and Management 124 (2016) 566–577 Contents lists available at ScienceDirect Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman Energy, exergy and environmental analysis of a hybrid combined cooling heating and power system utilizing biomass and solar energy Jiangjiang Wang ⇑, Ying Yang School of Energy, Power and Mechanical Engineering, North China Electric Power University, Baoding, Hebei Province 071003, China a r t i c l e i n f o Article history: Received 23 May 2016 Received in revised form July 2016 Accepted 21 July 2016 Keywords: Combined cooling heating and power (CCHP) system Biomass energy Solar energy Thermodynamics analysis Energy complementarity a b s t r a c t A hybrid combined cooling heating and power (CCHP) system driven by biomass and solar energy is proposed, and their complementarity to enhance the system’s energy efficiency is analyzed and shown The CCHP system is primarily composed of a biomass gasification sub-system, solar evacuated collector, internal combustion engine and dual-source powered mixed-effect absorption chiller The product gas produced by the gasifier drives the internal combustion engine to generate power, and the waste heat after generation is utilized to produce cooling and heating with the collected heat from the solar collectors Under the design conditions, the thermodynamic performances under variable external conditions and energy ratios are investigated and analyzed The results indicate that the primary energy ratio and the exergy efficiency are 57.9% and 16.1%, respectively, and the carbon emission reduction ratio is about 95.7%, at the design condition The complementarity analysis between the biomass and solar energy shows that the biomass subsystem makes a greater contribution to the total system primary energy ratio and exergy efficiency than the contributions from the solar subsystem, and the participation of solar energy is conducive to the system emission reduction Ó 2016 Elsevier Ltd All rights reserved Introduction Distributed energy systems (DES) are becoming one of the more attractive options worldwide because of their high overall efficiency, low greenhouse gas emissions, high reliability and other features [1] A DES, which includes combined heating and power (CHP) system, combined cooling, heating and power (CCHP) system, and distributed renewable energy technologies can realize a cascading utilization of energy The advance and development of DES has promoted various studies on their technology [2], system configuration [3,4], performance evaluation [5], and optimization [6,7], and most of the studies have concentrated on establishing optimal DES to achieve favorable costs, energy savings and emission reductions In particular, renewable energy resources are sustainable alternatives to natural gas for driving traditional CHP/CCHP systems [8], which has gradually become a topic of intense study Focusing on the energy sources in DES, hybrid DES combine renewable energy resources and fossil resources to decrease greenhouse gas emissions and simultaneously accommodate instabilities in renewable energy The literature on hybrid DES discusses different forms of ⇑ Corresponding author E-mail address: jiangjiang3330@sina.com (J Wang) http://dx.doi.org/10.1016/j.enconman.2016.07.059 0196-8904/Ó 2016 Elsevier Ltd All rights reserved complementary energy, for example hybrid wind/photovoltaic energy systems [9], multicomponent systems, including photovoltaic panels, wind generators and biomass gasification plants [10], hybrid geothermal-solar systems [11], hybrid solar and chemical looping combustion systems [12], CCHP systems based on cofiring natural and biomass gasification gases [13], solar-biomass hybrid air-conditioning systems [14] and hybrid polygeneration systems that utilize biomass fuel and solar power [15] Among the renewable energy resources, biomass and solar energy currently have attracted considerable attention from academics and researchers for their green environmental protection and inexhaustibility advantages Moreover, biomass is a stable energy resource that can produce continuous power and simultaneously reduce carbon dioxide (CO2) emissions To date, only a few studies have been conducted to explore hybrid system driven by biomass and solar energy, especially in terms of analyzing the hybrid proportion of energy resources and showing how they can provide complementary sources of energy Wang et al [13] analyzed the influence of different mixture ratios of natural gas and biogas on thermodynamic performance and exergoeconomic cost and discussed the complementary performances of biomass and natural gas Hashim et al [16] used the concept of an IBS (Integrated Biomass Solar) town and developed a hybrid solar and biomass plant, which, however, focused on the complementarity of J Wang, Y Yang / Energy Conversion and Management 124 (2016) 566–577 567 Nomenclature CCHP CHP COP CO2 DES HHV HX IBS ICE LHV RMSE combined cooling heating and power combined heating and power coefficient of performance carbon dioxide distributed energy system higher heating value heat exchanger integrated biomass solar internal combustion engine lower heating value root mean square error Symbols A E EX HHV LHV _ m P Q T area (m2) electricity (kW) exergy (kW) higher heating value (MJ kgÀ1 or MJ NmÀ3) lower heating value (MJ kgÀ1 or MJ NmÀ3) mass flow rate (kg/s) pressure (kPa) energy(kW) temperature (K) the electrical supply rather than a system thermodynamics analysis Academics and specialists have conducted a variety of relevant solar and biomass energy research, including the optimal design of a hybrid solar-assisted biomass energy system for heating [17], a hybrid solar and biomass energy complementary system for power generation [18], a hybrid solar and chemical looping combustion system for solar thermal energy storage [12] and a study of a hybrid solar-biomass air-conditioning system for cooling [14] However, that research has primarily focused only on particular energy supply products and rarely concentrated on the combined supply of cooling, heating and power Based on those considerations, the present study is motivated to explore this issue The main aim of this work is to propose a hybrid CCHP system that is driven by biomass and solar energy and to explore the complementarity of biomass and solar energy on the energy efficiency Four complementary conditions are discussed, variable solar irradiation, variable power loads, variable biomass input and variable solar energy input The first two conditions are studied under single variable, respectively, and the subsequent two variables are conducted simultaneously to analyze all combinations of biomass and solar energy input In addition, to evaluate the specific influences of biomass and solar subsystems, we propose the concept of subsystem contribution in energy efficiency Therefore, thermodynamic models of a hybrid CCHP system were constructed and validated; those models used existing technologies of solar heat collection, biomass gasification, absorption refrigeration and power generation Performances under varying operating conditions were then analyzed, and the system thermodynamic performance, including the primary energy ratios and exergy efficiencies under different energy proportions are discussed to determine the energy efficiency enhancement mechanism in the hybrid CCHP system The hybrid CCHP system offers several advantageous features, including (1) combined two kinds of renewable energy which was environmentally friendly, (2) reduced the consumption of fossil energy, (3) revealed the complementarity of biomass and solar energy that benefit for the system optimization This hybrid CCHP system can be innovatory in combined application of biomass and solar energy, especially suitable for remote areas where there are sufficient crops and solar energy z g mass fraction (dimensionless) efficiency Subscripts b biomass bio biomass c cooling ch chemical e electricity ee energy ex exergy exh exhaust f fuel h heating hw hot water jw jacket water n nominal p pump s solar sol solar w water rw refrigeration water v variable System description The flowchart of a hybrid CCHP system driven by biomass and solar energy is shown in Fig 1; the system is composed of a biomass gasification subsystem, solar photothermal collection subsystem, internal combustion engine (ICE) power subsystem and waste heat utilization subsystem Biomass material is first gasified in the downdraft gasifier, and then its product gas is sent to be further cooled in cyclone and purified in spray scrubbers, respectively Subsequently, the product gas fuel drives the ICE to generate power During this process, the heat exchanger (HX-01) is employed to recover the sensible heat from the product gas exiting the gasifier to produce domestic hot water The solar evacuated collectors are used to collect solar photothermal energy to produce mesothermal hot water, the outlet temperature of which is designed to match the outlet temperature of the jacket water from the ICE at approximately 85 °C The mixture of jacket water and solar hot water cooperates with the exhaust gas from the ICE, which has a temperature of approximately 460 °C, is fed to a dual-source powered mixed-effect LiBrH2O absorption chiller to produce chilled water After releasing heat in absorption chiller, the outlet exhaust gas still has a temperature of approximately 170 °C, and the heat exchanger (HX-02) is therefore used to recover the waste heat to preheat the cool water Regarding the hot water part, the outlet temperature, which is approximately 70 °C, is split into two streams that return to the collector and jacket, respectively, for the next cycle Moreover, when the temperature of the cooled jacket water cannot meet the requirement on engine cooling, it can be further cooled in cooling tower 01 Consequently, the system creates three products: electricity, chilled water and hot water Furthermore, the absorption chiller can be used as a heat exchanger to produce hot water, and two products, electricity and hot water, are generated For later analysis, the base design parameters are shown in Table Thermodynamic model The thermodynamic models (biomass gasification, ICE, solar evacuated collectors, dual-source absorption chiller and heat 568 J Wang, Y Yang / Energy Conversion and Management 124 (2016) 566–577 Fig Schematic of a hybrid CCHP system driven by biomass and solar energy Table Base design parameters Parameter Value HX-01 outlet temperature (°C) Chilled water temperature (°C) Hot water temperature (°C) Cooling water temperature (°C) Jacket water temperature (°C) Cool water temperature (°C) Chiller exhaust gas temperature HX-02 exhaust gas temperature Efficiency of gas-water heat exchanger 200 (state 5) 7/14 (states 22/21) 60 (state 31) 32/36 (states 27/28) 70/85(states 13/12) 25 (state 29) 170 (state 19) 120 (state 20) 0.90 exchanger) are presented in this section The biomass gasification, ICE and heat exchanger were constructed following [13], and they are briefly introduced 3.1 Biomass gasification In the literature, there are several studies on various models of biomass gasification [19–22] The thermochemical equilibrium model for biomass air gasification in [13] contains pyrolysis and gasification modules and considers the residual tar and char, which were simulated and validated for biomass air gasification preferably Using that biomass gasification model, wood chips as a biomass material (Table 2) are gasified to produce the product gas in Table In which, the characteristics of wood chips and the gasification process are assumed to be constant, and the property of the product gas is considered stable The exergy of biomass can be calculated as [23]: EX biomass ¼ mbiomass ð1 À zMoisture À zAsh Þ Â bLHV biomass þzMoisture  exch;water þ zAsh  exch;Ash 1:044 þ 0:0160ðzH =zC Þ À 0:3493ðzO =zC Þ Â½1 þ 0:0531ðzH =zC ފ þ 0:0493ðzN =zC Þ b¼ À 0:4124ðzO =zC Þ ! and ð1Þ ! ð2Þ where mbiomass is the mass flow rate of biomass, zMoisture, zAsh, zH, zC, zO and zN are the mass fraction of moisture, ash, hydrogen, carbon, oxygen and nitrogen of biomass, respectively ech,water and ech,Ash are the chemical exergy of water and ash, in this paper, they are 1300 and kJ/kmol, respectively 3.2 Internal combustion engine An appropriate model for an ICE will provide realistic estimates of performance/efficiency maps for both electrical power output 569 J Wang, Y Yang / Energy Conversion and Management 124 (2016) 566–577 Table Properties of biomass material and product gas [13] Fuel Parameters Wood chips Proximate analysis (wt%) Volatile 70.2 Fixed carbon 28.3 Ash (zAsh) 1.5 Moisture (zMoisture) 15 Ultimate analysis (wt%) C (zC) 51.2 H (zH) 6.1 O (zO) 40.9 N (zN) 0.3 HHV (MJ/kg) 20.43 LHV (MJ/kg) 19.09 Composition analysis (mol%) CO 12.05 H2 12.32 CO2 15.21 LHV (MJ/Nm3) 3.12 Product gas and useful thermal output for various capacities of engines Different modified methods have been proposed to predict engine performance [24,25] Furthermore, the characteristics of an ICE driven by fuel with lower heating value are dramatically different from those of a traditional ICE model [26] By modeling the error between the actual fuel and design fuel, Wang et al developed an ICE model driven by the product gas instead of natural gas [13] and is the model adopted in this paper For ICE which driven by lower heating value fuel, the generation efficiency (ge) can be calculated as [24,27]:  ge ¼ 0:102  LHV f 0:0563 þ 0:897  28:08ðNÃe Þ LHV NG ð3Þ where LHVf and LHVNG are the lower heating value of product gas and natural gas, respectively Ne⁄ is the nominal generation capacity of ICE, which is 1.1 times of practical generating volume Similarly, the outlet temperature of exhaust gas from the ICE can be expressed as [24]: T ex   LHV f ¼ 0:025 þ 0:974 LHV NG h i   10À5 ðNÃe Þ À 0:0707NÃe þ 758:33 ð4Þ In terms of the recovery efficiency of exhaust gas (gr, exh) and jacket water (gr, jw), assume the practical recuperated heat equals Heat pipe condenser CH4 1.1 to the nominal recuperated heat, the gr, lated as: gr;exh N2 59.33 exh À Á Q exh mexh hexh;i À hexh;o ¼ ¼ Qf Qf gr;jw ¼ À Á Q jw mjw cp;jw T jw;o À T jw;i ¼ Qf Qf and gr, jw can be calcu- ð5Þ ð6Þ where Qf is the energy of input fuel, mexh and mjw are the mass flow rate of exhaust gas and jacket water, respectively hexh,i and hexh,o are the empathy of inlet and outlet exhaust gas of absorption chiller, respectively cp,jw is the average specific heat of jacket water and the Tjw,o and Tjw,i are the outlet and inlet temperature of cooling water of jacket, respectively 3.3 Solar evacuated tube collector Among the various forms of solar collectors on the market, the advantages of an evacuated tube collector lie in its simple installation and high operating temperature and thermal efficiency, especially given the wide adaptability in solar irradiance, and is more appropriate for DES in various types of regions The detailed structure of the solar evacuated tube collector is shown in Fig [28] The collector features a heat pipe (a highly efficient thermal conductor) placed inside a vacuum-sealed tube (evacuated tube) The heat Manifold Fluid flow Evacuated tube Absorber plate Heat pipe evaporator Cross-sectional detail Fig Diagram of the evacuated tube collector [29] 570 J Wang, Y Yang / Energy Conversion and Management 124 (2016) 566–577 pipe, which is a sealed copper pipe, is then attached to a black copper fin that fills the tube (absorber plate) The liquid–vapor phase change materials (water) that are used to transfer heat undergo an evaporating–condensing cycle in the heat pipe In that cycle, the liquid is evaporated by the solar heat, and the vapor then rises to the heat sink region, condensing and releasing its latent heat Subsequently, the condensed fluid falls back to the bottom of the heat pipe, and the process repeats In the heat exchange process, the metal tip projects into a heat exchanger (manifold), and, when the working fluid (water) flows through the manifold, it will pick up heat from the tubes and gives off its heat in the next procedure 3.3.1 Heat transfer analysis The instantaneous efficiency of the evacuated tube collector can be expressed as [28]: g¼ ! AP Ti À Ta ; F R ðsaÞe À U L As G where AP and As are the absorber plate area and insolation area, respectively (m2) (sa)e is the efficient fraction of the incident solar energy ultimately absorbed by the absorber plate (s is the transmissivity, and a is the absorptivity), and Ti and Ta are the inlet water flow and ambient temperatures, respectively (K) G is the irradiance of the total solar radiation on the horizontal surface FR is the heat removal factor which is concerned with the collector structure, the detail formulas can be found in [30] UL is the overall heat loss coefficient (W/(m2ÁK)), and the calculation method is as follows The thermal analysis of collector is based on heat transfer theory, which considers the convective heat transfer between the collector and ambient fluid, radiative transfer between the absorber plate and the ambient fluid and the heat transfer of the twophase flow in the slender heat pipe In order to simplify the heat transfer of evacuated tube collector, generally the basically assumptions are adopted as follows: (1) Ignore the convective heat transfer and heat conduction between the rare air and tube wall in evacuated tube (2) Ignore the contact thermal resistance between the absorber plate and the evaporation section of heat pipe, and between the manifold wall and condensation section of heat pipe (3) Assume the heat transfer from heat pipe to the ambient keeps exclusively radial transfer way (4) Assume the water in heat pipe, ambient air around the tube wall are both under steady flow (5) Ignore the micro heat transfer with regard to wick conduction or other wispy liquid form, include but not limits to vapor-liquid and liquid-vapor interface thermal resistance, coefficient of internal thermal resistance into the evaporator To distinctly illustrate the heat transfer relationship, a thermal network is given, as is shown in Fig Where Tp, Tg is the temperature of absorber plate and glass tube, respectively (K); Ub is the convective heat transfer coefficient between the manifold insulating layer and ambient, hp–g is the radiation heat transfer coefficient between the absorber plate and glass tube inside wall, and hr,g–a and hc,g–a are radiation heat transfer coefficient and convective heat transfer coefficient between the glass tube outer wall and ambient, respectively Qu and QL are useful energy can obtain and heat losses of the collector, respectively Then, the overall heat loss coefficient can be calculated by the following equations set [28,30]: UL ¼ Ut þ Ub Ut ¼ hpÀg þ ð8Þ hgÀa Fig The thermal network of evacuated tube collector ð7Þ !À1 ð9Þ Ab Ap ð10Þ   2rðT p þ T g Þ T 2p þ T 2g   ¼ 2Ap 1 ep þ Ag eg À ð11Þ Ub ¼ hpÀg tb kb þh Á c;bÀa hgÀa ¼ hr;gÀa þ hc;gÀa ð12Þ   hr;gÀa ¼ eg rðT g þ T a Þ T 2g þ T 2a ð13Þ hc;gÀa ¼ Nu Á ka Dg ð14Þ Ag hgÀa ðT p À T a Þ ¼ Ap hpÀg ðT P À T g Þ ð15Þ where Ut is the gross heat transfer coefficient between tube and ambient, tb, kb, Ab are thickness, heat conduction coefficient, and area of manifold insulating layer Generally choose fiberglass as insulation material, the thickness is about 0.05 m and the heat conduction coefficient is about 0.048 W/(mÁK) at normal temperature [30] hc,b–a is the convective heat transfer coefficient between the manifold insulating layer and ambient, the typical value range is 1.5–2.0 W/(m2ÁK), thus this paper choose the average value 1.75 In addition, to ensure the accuracy of the analysis under dynamic conditions, the heat transfer coefficients of the evaporation section and condensation section in the heat pipe can be determined [31]: ! 0:23  0:4 0:3 0:2  q0:65 kl c0:7 Psat q l pl g he ¼ 0:32 and 0:4 0:1 Pa pDe le q0:25 v hlv ll À hc ¼ 0:943 Á ql ql À qv gk3l hlv ll ðT v À T c ÞLc ð16Þ !0:25 ð17Þ where ql, kl, ll and cpl are the density (kg/m3), conductivity (W/ (mÁK)), viscosity (kg/(mÁs)) and specific heat (kJ/(kgÁK)), respectively, of liquid water g (m/s2) is the local acceleration of gravity, q (W/m2) is the heat flux of the evaporation section, and qv and Tv are the density and temperature of the vapor, respectively hlv (kJ/kg) is the latent heat of vaporization, and Psat and Pa (kPa) are the pressures of the saturated vapor and ambient, respectively Tc and Lc are the temperature and length of the condensation section, respectively J Wang, Y Yang / Energy Conversion and Management 124 (2016) 566–577 3.3.2 Simulation and validation The detailed structural parameter values of the collector are shown in Table Based on these initialization parameters and certain external conditions, the simulation can be performed using the EES software package And the performances in each transient point can be fitted in a linear curve Due to the restriction of experimental platform in this research group, this paper identifies the simulation through the experiment which operated by the solar energy research institute of Beijing The experimental data and external environmental statics can be found in Ref [30] A comparison of the experimental results and the simulated linear curve of the instantaneous efficiency is shown in Fig The instantaneous efficiency of the collector is the ratio of useful heat absorbed by the work fluid to the solar energy which project on the lighting surfaces under a certain approximated steady-state condition, as is shown in formula (5) The comparison indicates that the trend of the simulated linear curve is consistent with the distribution of the experimental data In addition, the root mean square error (RMSE) is less than 5%, which confirms the accuracy of the collector model 3.4 Absorption chiller Because there are two streams of waste heat from the ICE, the exhaust gas and jacket water, the dual-source powered mixedeffect LiBr-H2O chiller shown in Fig was adopted [29] The primary parts of the dual-source powered absorption chiller include a high pressure generator (HG), two low pressure generators (LG1 and LG2a condenser, throttle, evaporator, absorber, low temperature exchanger (LX) and a high temperature exchanger (HX) The dual-sources (exhaust gas and hot water) are sent into HG and LG1, respectively, to evaporate the liquid refrigerant The 571 refrigerant vapor from HG then flows into the LG2, releasing its partial condensing heat to generate the third part of refrigerant vapor The all-refrigerant vapor from HG, LG1 and LG2 finally flows into the condenser, where it is cooled and condensed by the cooling water After a throttling process, the low-temperature and lowpressure liquid refrigerant fall into the evaporator In the evaporator, by absorbing heat from the chilled water, the liquid refrigerant evaporates and produces a continuous cooling output The new refrigerant vapor will be absorbed by the strong solution in the absorber and is then pumped into HG and LG1 for the next cycle During the desorption and absorption, HX and LX recover heat by transferring it from the high temperature strong solution to the low temperature weak solution In terms that there are complex influence factors would impact on the performance, there proposes some commonly adopted assumptions about the refrigeration process: (1) Ignore the heat losses in each components and pressure losses between each connection lines; assume the pressure difference between the absorption and evaporation is 0.05 kPa and the pressure of condenser is equal to that of low generator (2) The simulation and analysis are both under steady state, the LiBr-H2O solution are in steady state during the cycle (3) The outlet refrigerant steam of evaporator, outlet refrigerant liquid of condenser, the outlet weak solution of absorber and the outlet solution of high generator and low generator are all statured (4) Ignore the power consumption of solution pump (5) All the heat transfer unit are regarded as countercurrent heat exchange mode, and use the logarithmic mean temperature difference in heat-transfer calculation During the cycle of LiBr-H2O solution, there are continuous energy and matter enter and leave in each components The energy balance, mass balance and solute equilibrium can be summarized as [29]: Table Design parameters of solar evacuated tube collector [30] Parameters Values Absorber plate area per tube AP Insolation area per tube As Diameter of glass tube Dg Out diameter of evaporation section of heat pipe De Out diameter of condensation section of heat pipe Dc Length of condensation section Lc Coating absorptivity a Coating emissivity ep The transmissivity of glass tube s The emissivity of glass tube eg _ Working fluid mass flow rate m The number of tubes g 0.175 m2 0.19 m2 100 mm mm 14 mm 60 mm 0.92 0.08 0.9 0.88 0.03 kg/s X X mi À mi wi À Qþ X mo ¼ X mi hi À mo wo ¼ X mo ho ¼ ð18Þ ð19Þ ð20Þ The simulation was developed using EES commercial version 6.883-3D, which utilizes simple programming rules and comprehensive thermophysical property functions for classical thermodynamic calculations, is especially applicable for the modeling of cyclic process and helps researchers Meanwhile, thermophysical properties of the LiBr-H2O solution can be directly obtained from the EES software The average relative errors between the simulated parameters and the reference values in [29] are shown in Table It can be seen that the average relative errors of the enthalpy, concentration, pressure and temperature in a cycle are small The average relative error of the mass flow rate appears to be somewhat larger, but considering its small magnitude, which varies from 0.0034 to 0.248 kg/s, the average relative error is acceptable, which validates the veracity of the absorption chiller model By simulating, the COP of absorption chiller is obtained as 1.058 while slightly higher than the reference value which is 0.9402 To evaluate the overall energetic performance of the chiller, the coefficient of performance (COP) can be calculated as COP ¼ Fig A comparison of the experimental and simulated values X Qc ; Q exh þ Q hw þ W p ð21Þ 572 J Wang, Y Yang / Energy Conversion and Management 124 (2016) 566–577 Fig Schematic of the dual-source powered absorption chiller Table Average relative error of each parameter Average relative error m (kg/s) 12.71% h (kJ/kg) 0.08% W (%) 0.80% P (kPa) 0.07% T (°C) 0.57% Ignoring the power consumption of the solution pump, the COP depends strongly on the ratio of the hot water heat to the exhaust gas heat Based on the view of reference [29], an analysis was conducted on the influence of the heating proportion on the COP and cooling output, as shown in Fig It can be seen that the COP is approximately 1.20, and the cooling output reached a maximum when the heat inputs of the hot water and exhaust gas were the same, which presents a consistent conclusion with the reference simulation The scene which hot water heat to the exhaust gas heat ratio was 1:1.0 was assumed to be the base work condition If the hot water or exhaust gas inputs decrease relative to the base work condition, both cooling outputs decrease However, the COP always increases with a decreasing ratio of hot water and exhaust gas, and, moreover, an increase in the exhaust gas causes the COP to increase more quickly than seen for the hot water This indicates that an increase in the proportion of the exhaust gas has a greater contribution to the COP than that of hot water Performance evaluation criteria The energy efficiency (gee) and exergy efficiency (gee) were employed to evaluate the thermodynamic performance of the hybrid CCHP system The systematic energy efficiency, gee, is Fig Influence of the proportion of heating sources on the COP and cooling output where Qc is the produced cooling (kW), Qexh and Qhw are the exhausted heat from the exhaust gas and hot water (kW), respectively, and Wp is the power consumption of the pump (kW) gee ¼ E þ Qc þ Qh Q biomass þ Q solar ð22Þ To measure the contributions of the solar and biomass subsystems, the subsystem primary energy ratios, ge,sol and ge,bio, are gee;sol ¼ Q c;sol =Q solar and ð23Þ 573 J Wang, Y Yang / Energy Conversion and Management 124 (2016) 566–577 À Á gee;bio ¼ E þ Q c;bio þ Q h =Q biomass ; ð24Þ where E (kW) is the electrical output, Qc (kW) is the total cooling output, and Qc,sol and Qc,bio are the two parts of the total cooling output from the solar subsystem and biomass subsystem, respectively Qh (kW) is the hot water output, Qbiomass (kW) is the biomass energy input, and Qsolar (kW) is the solar energy input Qsolar and Qbiomass comprise the overall energy input, and the proportion of the two parts will influence the system energy efficiency The system exergy efficiency, gex, is gex ¼     À Q c þ À TT Q h   h : EX biomass þ À TT Q solar T0 T rw Eþ ð25Þ sol Similarly, the subsystem exergy efficiencies, gex,sol and gex,bio, are gex;sol ¼ 0   T0 T0 Q solar and À Q c;sol 1À T rw T sol  gex;bio ¼ E þ ð26Þ   !0  T0 T0 Qh EX biomass ; À Q c;bio þ À T rw Th  ð27Þ where T0 is the reference ambient temperature, EXbiomass is the exergy of the biomass input, and T rw ; T h and T sol are the mean temperatures of the refrigerated water, domestic hot water and solar collector, respectively Likewise, the proportion of the solar energy input to biomass energy input affects the exergy efficiency The reference state in the exergy analysis was defined as 101.325 kPa and 25 °C Since the biomass and solar energy are pollution free, the hybrid system which integrated these two resources in this paper may show a significant performance in carbon emission In order to evaluate the systematic environmental effects, carbon emission reduction ratio (CERR) is introduced The reference system is a typical biomass-fired Organic Rankine Cycle-CCHP system for the same products which produces electricity by the turbine through organic rankine cycle utilizing the biomass combustion heat and recovers the condensing heat for heating and cooling In order to obtain the primary energy consumption of ORC-CCHP system, coefficient of performance (gT) is selected as the evaluation criteria, when organic medium is R245fa and the ejector coefficient of which is at its maximum, the coefficient of performance is 0.53 [32] Due to the consumed primary energy of these two system are the same, here only needs to compare the biomass consumption when calculates CERR Therefore, CERR can be calculated as: EþQ c þQ h CERR ¼ gT gb À Q biomass ð28Þ EþQ c þQ h gT gb where gT is the performance coefficient of ORC-CCHP part, gb is the efficiency of biomass-fired boiler, which is 0.85 [33], and Qbiomass is the consumed biomass energy Results and discussion 5.1 System integrated design case A building with a 100 kW electricity load was used as the design condition of the hybrid CCHP system and as a case study The collected heat and outlet temperature of the evacuated collector were set to match that of the recovered jacket water Therefore, to make the outlet temperature of the solar hot water close to that of the jacket water, three collectors were connected in series Assuming that the inlet water temperature was 70 °C, using the collector model in Section 3.3.2, the outlet water temperatures of the three collectors in turn were calculated to be T 1o ¼ 76:1  C;T 2o ¼ 82:1  C, and T 3o ¼ 87:9  C Thus, the heat collection of the basic collector column could be determined Because the solar hot water heat was assumed to match the jacket water heat, the rows of the basic collector column could be determined Furthermore, the total collector area under the design condition was calculated to be 96 m2 for an 800 W/m2 solar irradiance Using the thermodynamic models in Section and the design parameters in Table 1, the results at the design work condition are summarized in Table At the design condition, when the absorption chiller was only driven by solar hot water, the calculated energy efficiency of the solar subsystem (ge,sol) was 47% Similarly, when the absorption chiller was only driven by the recovered heat from the ICE, the energy efficiency of the biomass subsystem () was calculated to be 61% Therefore, as the participation of the solar subsystem increases, the system energy efficiency decreases The solar subsystem exergy efficiency (gex,sol) and biomass subsystem exergy efficiency (gex,bio) were 9.4% and 6.22%, respectively Therefore, an increase in biomass energy or a decrease in solar energy input improved the system exergy efficiency However, in terms of the system products, an increase in the solar energy input can produce more solar hot water, which will result in a higher cooling output, while keeping the other parameters constant In general, high solar energy often occurs at noon in the hot summer when the cooling demand just reaches its maximum The adoption of a solar subsystem can decrease the demand for power for electrical refrigeration and relieve the load fluctuations on the ICE Furthermore, it reduces biomass consumption 5.2 Performance analysis with variable work conditions The system does not always run under design work condition, thus it is necessary to explore the system performance under different off-design conditions for the timely response to different external variations In this paper, variable external conditions include electricity load factor (5–100%) and solar irradiation (0– 900 W/m2) are discussed When change any one of these two variations, the other inherent system conditions and external conditions keep its design state unchanged Meanwhile, it is important to note here that this paper merely concentrated on the system Table Results at the design work condition Parameter Value Parameter Value Solar irradiance Solar collector area Mean temperature of solar collector Inlet temperature of exhaust gas Solar energy input Biomass consumption Heat of exhaust gas Heat of jacket water Heat of solar hot water 800 W/m2 96 m2 127 °C 459.7 °C 76.8 kW 613.7 kW 82.15 kW 51.29 kW 45.23 kW Cooling Heating Power System energy efficiency Solar system energy efficiency Biomass system energy efficiency Exergy efficiency Solar system exergy efficiency Biomass system exergy efficiency 197.2 kW 102.7 kW 100 kW 57.9% 47.0% 61.0% 16.1% 9.4% 16.2% 574 J Wang, Y Yang / Energy Conversion and Management 124 (2016) 566–577 performance analysis under variable conditions but not specific to any operation strategy research 5.2.1 Variable electricity load factor When the building electrical load changes, the performances of the main subsystems vary as shown in Fig The performance of the ICE can be expressed by the generation efficiency (ge), exhaust gas heat recovery efficiency (gr,exh) and jacket water heat recovery efficiency (gr,jw) The gr,exh and gr,jw remained basically unchanged, but the generation efficiency rose slightly with the addition of the electric load factor It is easy to understand that a fixed ICE often has a high efficiency at a high electric load factor and the overall heat loss ratio is essentially invariant under different electric load factors, the gr,exh and gr,jw vary slightly, and ge shows an incremental trend Moreover, it can be seen that the cooling output increases almost linearly with the electric load factor This is probably attributable to the addition of recovered heat, which is related to the increase in the electric load factor The COP rapidly increases at first then increases slowly Although a fixed ICE has an essentially constant proportion of hot water heat to exhaust gas heat, the existence of basic solar hot water heat still can induce a change in the proportion of the overall heating source, and thereby possibly produce a change in the COP At the preliminary stage in the growth of the electric load factor, an increase in the exhaust gas could greatly increase its proportion of the heating source, which is expressed as a change in the COP With further increases, because of the gradually increasing proportion of hot water, there is no incremental change in the COP The system energy efficiency and exergy efficiency change with the electrical load factor as shown in Fig Their increases are also caused by the greater contribution of the biomass subsystem At the preliminary stage, a low electricity output implies a low biomass input; although the rate of contribution can be greater than that of the solar subsystem, the biomass subsystem can only play a small part, and the values of the energy efficiency and exergy efficiency are therefore low As the electrical output increases, more biomass can take part in the overall performance, and the two parameters will therefore increase rapidly However, because of the existence of the basic solar subsystem, a further increment will not be able to greatly influence the two parameters Therefore, the energy efficiency and exergy efficiency both increase gradually at first, and then increase slowly after reaching a certain level 5.2.2 Variable solar irradiation At a 100 kW electrical output, the variable solar irradiance mainly influences the collected solar heat and solar collection efficiency (gs), and the cooling output and COP of absorption chiller also changed, as shown in Fig When the solar irradiance was less than 100 W/m2, the solar collectors could not collect enough heat to recover the basic heat loss, and the curve of gs therefore began at a critical value of approximately 100 W/m2 It can be observed that the increasing solar irradiance improved gs, which rose quickly in the beginning, then gradually slowed at a high irradiance This is the case because at initial stages, an increment in solar irradiance can deeply promote phase-change heat transfer in the heat pipe, which will result in a holistic increment of the solar collection efficiency When the irradiance rises further, the overall heat transfer will finally reach saturation, and the solar collection efficiency therefore will remain ultimately unchanged In addition, an increase in solar irradiance will cause the absorption chiller to produce a greater cooling output, while decreasing the COP The reason is that higher solar irradiance implies a higher solar hot water output, which increases the total heat input of the absorption chiller, which naturally enhances the cooling output On the contrary, an improvement in the solar hot water leads to a higher proportion of hot water in the heating source, which weakens the contribution of the exhaust gas and then decreases the COP The variations in the system energy efficiency and exergy efficiency are shown in Fig 10, which shows that they trend downward in a similar manner This is probably due to the higher contribution of the biomass subsystem to the overall system performance relative to that of the solar subsystem Therefore, an increase in solar energy results in a decrease in energy efficiency and exergy efficiency From this perspective, if the system emphasizes increases in system energy and exergy efficiency, the assumption of solar irradiance should not be too great Fig Variation in performance with the electrical load factor Fig System energy efficiency and exergy efficiency with electrical load factor Fig Influence of solar irradiance on the COP, cooling output and solar collection efficiency 575 J Wang, Y Yang / Energy Conversion and Management 124 (2016) 566–577 0.61 Solar energy ratio=0.0 0.59 Energy efficiency 0.57 0.55 0.53 0.51 0.49 0.47 Fig 10 Influence of the solar irradiance on the energy efficiency and exergy efficiency 0.45 ð29Þ Similarly, the biomass energy ratio is defined as the ratio of the variable biomass energy input to the nominal solar energy input: Rb ¼ Q bio;v ; Q sol;n (a) 0.61 0.59 0.57 Energy efficiency Q sol;v Rs ¼ : Q bio;n Biomass energy ratio 5.3 Complementarity performance between biomass and solar energy In the overall system performance, the proportions of the solar energy and biomass energy inputs play a crucial role on the energy efficiency and exergy efficiency Herein, the solar energy and biomass energy ratios are defined to express their proportions in the entire CCHP system The solar energy ratio is defined as the ratio of the variable solar energy input to the nominal biomass energy input at the base design condition: 0.55 0.53 0.51 0.49 0.1 ð30Þ 0.47 0.45 where Rs and Rb are the solar energy and biomass energy ratios, respectively, and Qbio,n and Qsol,n are the nominal inputs of the biomass energy and solar energy at the design condition, respectively Their values can be found in Table Qsol,v and Qbio,v are the variable inputs of the biomass energy and solar energy under different conditions Regarding the detailed results the variable conditions, the biomass energy ratio varied from to 8, which corresponds to a variation in the electrical output from to 100 kW The solar energy ratio varied from to 0.6, and the solar collector area therefore varied from to 460.8 m2, which was within the maximum area of the roof of the case study building 5.3.1 First law of thermodynamics analysis The variation in energy efficiency under different conditions is shown in Fig 11 From Fig 11(a), it can be seen that under all of the solar energy ratio (Rs) conditions, as the biomass energy ratio (Rb) increased, the energy efficiency increased and then leveled Moreover, the energy efficiency leveled earlier with decreasing Rs This can be explained because a higher biomass ratio can lead to a higher electrical output and greater proximity to the design condition, which will result in a higher ge and an almost linear increase in both the cooling and heating outputs Therefore, by analyzing the composition of formula (16), it can be concluded that when the solar subsystem remains unchanged, an increase in the biomass energy ratio should improve the energy efficiency In addition, low Rs leads to a decreased solar subsystem effect, and, from formula (16), the energy efficiency will be greatly influenced by Rb Therefore, when Rs is 0, the energy efficiency is maximized and becomes constant earlier Meanwhile, at certain Rb, an increase in the solar energy ratio results in a reduced energy efficiency, and the details in the change 0.1 0.2 0.3 0.4 0.5 0.6 Solar energy ratio (b) Fig 11 Influence of the biomass energy and solar energy ratios on the energy efficiency are shown in Fig 11(b) The figure shows that the energy efficiency decreases steeply at a low solar energy ratio, then gradually changed slowly as the solar energy ratio further increased This can be explained because in the total system performance, variations in the solar energy input can only influence the cooling output, and although the COP of the absorption chiller was greater than in most cases, the energy efficiency still could not increase because of the basic biomass energy input When the Rb was high (e.g., 8), the energy efficiency was reasonably maintained at a relatively high level (no less than 55%) However, when the biomass energy ratio was reduced to 0.1, the energy efficiency declined rapidly to approximately 47% at the low solar energy ratio, and further increases in the solar energy ratio will not improve this situation It can be concluded that a higher biomass energy ratio always corresponds to higher energy efficiency, and to slow the rate in the decline of the energy efficiency caused by an increasing solar energy ratio, the biomass energy ratio should always be high in system operation 5.3.2 Second law of thermodynamics analysis The variations in exergy efficiency under the different conditions are shown in Fig 12 The exergy efficiency varies from 9% to 17%, high biomass ratio corresponds to high exergy efficiency and low solar energy ratio conducive to the velocity of exergy efficiency 576 J Wang, Y Yang / Energy Conversion and Management 124 (2016) 566–577 80% Carbon emission reduction ratio 18% Exergy efficiency Rs=0.0 15% 12% 800W 700W 600W 500W 400W 300W 200W 60% 40% 20% 0% 0% 9% 20% 40% 60% 80% 100% Electric load ratio Biomass energy ratio Fig 13 Carbon emission reduction ratios under different conditions (a) 100% Carbon emission reduction ratio Exergy efficiency 18% 15% 12% 9% 80% 60% 40% Rs=0.0 20% 0% 0.1 0.2 0.3 0.4 0.5 0.6 Solar energy ratio Biomass energy ratio (b) (a) 100% Carbon emission reduction ratio Fig 12 Influence of the biomass energy and solar energy ratios on the exergy efficiency increase It can be clearly seen that the curvilinear trend under different biomass and solar energy ratios are similar to its primary energy ratio variation tendency under the given condition that confirms the complementary performance from two different angles It is parallel to aforementioned explanations that because of the small contribution of the solar subsystem, an increase in the solar energy ratio weakens the influence of biomass subsystem and then results in a reduction of the exergy efficiency In summary, higher biomass energy ratio or lower solar energy ratio can increase the exergy efficiency 0.1 80% 60% 40% 20% 0% 5.4 Environmental analysis 0.1 0.2 0.3 0.4 0.5 0.6 Solar energy ratio Compared with the biomass-fired Organic Rankine Cycle-CCHP system, the variation in carbon emission reduction ratio under different electric ratios and solar irradiances are shown in Fig 13 It can be observed that when the solar collection area keeps constant, with the increment of solar irradiance, the hybrid system will show higher CERR Meanwhile, when electric load ratio increases, the values of CERR will diminished gradually and tends to be almost uniform at full-load status At design condition, the CERR is about 30.87%, which shows enormous advantage in reduction ability of system which integrated biomass and solar energy From the perspective of energy ratio analysis, remain the solar irradiance unchanged, the variation in carbon emission reduction (b) Fig 14 Influence of the biomass energy and solar energy ratios on the CERR ratio under different biomass and solar energy ratios are shown in Fig 14 The Fig 14 reveals that when enlarges the solar collection area, the increment of solar energy ratio can be benefit for the systematic carbon emission reduction The reason can be easily understand that certain biomass energy input corresponds to settled products of electricity, cooling and heating, when the solar energy J Wang, Y Yang / Energy Conversion and Management 124 (2016) 566–577 takes part in, the cooling will raise and then enhances the coal consumption of reference separated system and will not increase the carbon emission for the hybrid system at the same time Otherwise, in Fig 14(a), it can be observed that when the solar collection area decreases to zero, i.e Rs is 0.0, it is special that with the addition of biomass energy ratio, the CERR shows a slight upward trend This condition merely operates under the driven of biomass, the upward trend shows high biomass energy ratio is benefit for pure biomass CCHP system And when the system integrates with solar energy, the performance will converse and could improve the integral level of carbon reduction to some extent In general, when the internal engine combustion operates from part-load status to full-load, the solar irradiance will play less influence on CERR And at a certain electric load ratio, high solar energy ratio is more conducive to emission reduction than low ones Conclusions This paper investigated a hybrid CCHP system driven by biomass and solar energy and configured and integrated the submodules of a solar evacuated collector, a dual-source powered mixed-effect LiBr-H2O chiller and an ICE The system thermodynamical performances under variable electricity outputs and solar irradiances were analyzed, and the complementarity performance between the solar and biomass energy were discussed The following conclusions were reached Firstly, the biomass subsystem had a greater contribution to the total system energy efficiency and exergy efficiency than that of the solar subsystem Under the design work condition, the system energy efficiency and exergy efficiency were 57.9% and 16.1%, respectively, the energy efficiencies of the solar and biomass subsystems were 47.0% and 61.0%, respectively, and the exergy efficiencies were 9.4% and 16.2%, respectively Secondly, the analysis of the variable work conditions indicated that an increase in the electrical output corresponded to a linear increase in the cooling output and a gradual increase in the COP that improved the energy efficiency and exergy efficiency However, a high solar irradiance resulted in higher cooling output, although it decreased the COP because of the higher proportion of low-grade heat in the dual-sources chiller, which in total decreased the energy efficiency and exergy efficiency Thirdly, a high biomass energy ratio always corresponds to a higher energy efficiency, and a decrease in the solar energy ratio can cause an increase in the energy efficiency, which reaches a constant value earlier Regarding the exergy analysis, an increase in the 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