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

Energy, exergy and environmental analysis of a hybrid combined coolingheating and power system utilizing biomass and solar energy Jiangjiang Wang⇑, Ying Yang School of Energy, Power and

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 8 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 collec-tors 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

1 Introduction

Distributed energy systems (DES) are becoming one of the more

attractive options worldwide because of their high overall

effi-ciency, 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)

sys-tem, 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

emis-sion reductions

In particular, renewable energy resources are sustainable

alter-natives 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

emis-sions and simultaneously accommodate instabilities in renewable

energy The literature on hybrid DES discusses different forms of

complementary energy, for example hybrid wind/photovoltaic energy systems [9], multicomponent systems, including photo-voltaic panels, wind generators and biomass gasification plants

[10], hybrid geothermal-solar systems[11], hybrid solar and chem-ical looping combustion systems[12], CCHP systems based on co-firing 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 aca-demics and researchers for their green environmental protection and inexhaustibility advantages Moreover, biomass is a stable energy resource that can produce continuous power and simulta-neously 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] ana-lyzed 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 (Inte-grated Biomass Solar) town and developed a hybrid solar and bio-mass plant, which, however, focused on the complementarity of

http://dx.doi.org/10.1016/j.enconman.2016.07.059

0196-8904/Ó 2016 Elsevier Ltd All rights reserved.

⇑Corresponding author.

E-mail address: jiangjiang3330@sina.com (J Wang).

Contents lists available atScienceDirect

Energy Conversion and Management

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / e n c o n m a n

the electrical supply rather than a system thermodynamics

analy-sis 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

considera-tions, 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

com-plementarity of biomass and solar energy on the energy efficiency

Four complementary conditions are discussed, variable solar

irra-diation, variable power loads, variable biomass input and variable

solar energy input The first two conditions are studied under

sin-gle 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

influ-ences of biomass and solar subsystems, we propose the concept

of subsystem contribution in energy efficiency Therefore,

thermo-dynamic 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

condi-tions were then analyzed, and the system thermodynamic

perfor-mance, 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

advan-tageous features, including (1) combined two kinds of renewable

energy which was environmentally friendly, (2) reduced the

con-sumption of fossil energy, (3) revealed the complementarity of

bio-mass and solar energy that benefit for the system optimization

This hybrid CCHP system can be innovatory in combined

applica-tion of biomass and solar energy, especially suitable for remote

areas where there are sufficient crops and solar energy

2 System description The flowchart of a hybrid CCHP system driven by biomass and solar energy is shown inFig 1; the system is composed of a bio-mass gasification subsystem, solar photothermal collection subsys-tem, 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 approxi-mately 460°C, is fed to a dual-source powered mixed-effect

LiBr-H2O 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 there-fore 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 col-lector and jacket, respectively, for the next cycle Moreover, when the temperature of the cooled jacket water cannot meet the require-ment 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 analy-sis, the base design parameters are shown inTable 1

3 Thermodynamic model The thermodynamic models (biomass gasification, ICE, solar evacuated collectors, dual-source absorption chiller and heat

Nomenclature

CCHP combined cooling heating and power

CHP combined heating and power

COP coefficient of performance

CO2 carbon dioxide

DES distributed energy system

HHV higher heating value

HX heat exchanger

IBS integrated biomass solar

ICE internal combustion engine

LHV lower heating value

RMSE root mean square error

Symbols

A area (m2)

E electricity (kW)

HHV higher heating value (MJ kg
1or MJ Nm
3)

LHV lower heating value (MJ kg
1or MJ Nm
3)

_m mass flow rate (kg/s)

P pressure (kPa)

T temperature (K)

z mass fraction (dimensionless)

Subscripts

e electricity

jw jacket water

rw refrigeration water

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 2 In which, the characteristics of wood chips and the gasification process are assumed to be constant, and the prop-erty of the product gas is considered stable

The exergy of biomass can be calculated as[23]:

EXbiomass¼ mbiomass

ð1
zMoisture
zAshÞ  bLHVbiomass

þzMoisture exch ;waterþ zAsh exch ;Ash

b ¼

1:044 þ 0:0160ðzH=zCÞ
0:3493ðzO=zCÞ

½1 þ 0:0531ðzH=zCÞ þ 0:0493ðzN=zCÞ

1
0:4124ðzO=zCÞ

ð2Þ

where mbiomassis the mass flow rate of biomass, zMoisture, zAsh, zH, zC,

zOand zNare the mass fraction of moisture, ash, hydrogen, carbon, oxygen and nitrogen of biomass, respectively ech,waterand ech,Ashare the chemical exergy of water and ash, in this paper, they are 1300 and 0 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

Fig 1 Schematic of a hybrid CCHP system driven by biomass and solar energy.

Table 1

Base design parameters.

HX-01 outlet temperature (°C) 200 (state 5)

Chilled water temperature (°C) 7/14 (states 22/21)

Hot water temperature (°C) 60 (state 31)

Cooling water temperature (°C) 32/36 (states 27/28)

Jacket water temperature (°C) 70/85(states 13/12)

Cool water temperature (°C) 25 (state 29)

Chiller exhaust gas temperature 170 (state 19)

HX-02 exhaust gas temperature 120 (state 20)

Efficiency of gas-water heat exchanger 0.90

and useful thermal output for various capacities of engines

Differ-ent modified methods have been proposed to predict engine

per-formance [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]:

LHVNG

þ 0:897

eÞ0 :0563 ð3Þ

where LHVfand LHVNGare 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]:

 2  10
5ðN

eÞ2

ð4Þ

In terms of the recovery efficiency of exhaust gas (gr, exh) and

jacket water (gr, jw), assume the practical recuperated heat equals

to the nominal recuperated heat, thegr, exhandgr, jwcan be calcu-lated as:

gr ;exh¼Qexh

Qf

¼mexhhexh ;i
hexh ;o

Qf

ð5Þ

gr;jw¼Qjw

Qf ¼mjwcp ;jw Tjw ;o
Tjw ;i

where Qfis the energy of input fuel, mexhand mjware the mass flow rate of exhaust gas and jacket water, respectively hexh,iand hexh,oare the empathy of inlet and outlet exhaust gas of absorption chiller, respectively cp,jwis the average specific heat of jacket water and the Tjw,oand 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 installa-tion and high operating temperature and thermal efficiency, espe-cially given the wide adaptability in solar irradiance, and is more appropriate for DES in various types of regions The detailed struc-ture of the solar evacuated tube collector is shown inFig 2 [28] The collector features a heat pipe (a highly efficient thermal conductor) placed inside a vacuum-sealed tube (evacuated tube) The heat

Fluid flow

Evacuated tube Absorber plate Heat pipe evaporator

Cross-sectional detail

Table 2

Properties of biomass material and product gas [13]

Wood chips Proximate analysis (wt%) Volatile Fixed carbon Ash (z Ash ) Moisture (z Moisture )

LHV (MJ/Nm 3

pipe, which is a sealed copper pipe, is then attached to a black

cop-per 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]:

g¼AP

As

FR ðsaÞe
UL

Ti
Ta

G

where APand Asare the absorber plate area and insolation area,

respectively (m2) (sa)eis the efficient fraction of the incident solar

energy ultimately absorbed by the absorber plate (sis the

transmis-sivity, andais the absorptivity), and Tiand Taare the inlet water

flow and ambient temperatures, respectively (K) G is the irradiance

of the total solar radiation on the horizontal surface FRis the heat

removal factor which is concerned with the collector structure,

the detail formulas can be found in[30] ULis the overall heat loss

coefficient (W/(m2K)), and the calculation method is as follows

The thermal analysis of collector is based on heat transfer

the-ory, which considers the convective heat transfer between the

col-lector and ambient fluid, radiative transfer between the absorber

plate and the ambient fluid and the heat transfer of the

two-phase 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

conduc-tion 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 inFig 3 Where Tp, Tgis the

temper-ature of absorber plate and glass tube, respectively (K); Ubis the

convective heat transfer coefficient between the manifold

insulat-ing layer and ambient, hp–gis 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 Quand QLare 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]:

p
gþh1

g
a

ð9Þ

Ub¼tb 1

k bþ 1 c;b
a

Ab

hp
g¼2rðTpþ TgÞ T2

pþ T2 g

1

epþ2A p

A g 1

eg
1

hr;g
a¼egrðTgþ TaÞ T2

gþ T2 a

ð13Þ

hc ;g
a¼Nu ka

where Ut is the gross heat transfer coefficient between tube and ambient, tb, kb, Abare 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 con-duction coefficient is about 0.048 W/(mK) at normal temperature

[30] hc,b–ais the convective heat transfer coefficient between the manifold insulating layer and ambient, the typical value range is 1.5–2.0 W/(m2K), 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 evapora-tion secevapora-tion and condensaevapora-tion secevapora-tion in the heat pipe can be determined[31]:

he¼ 0:32 q0 :65

l k0l:3c0 :7

plg0 :2

q0v:25h0lv:4l0 :1

l

!

Psat

Pa

pDele

hc¼ 0:943 qlql
qvgk3lhlv

llðTv
TcÞLc

!0 :25

ð17Þ

whereql, kl,lland cpl are the density (kg/m3), conductivity (W/ (mK)), viscosity (kg/(ms)) and specific heat (kJ/(kgK)), respec-tively, of liquid water g (m/s2) is the local acceleration of gravity,

q (W/m2) is the heat flux of the evaporation section, andqv and

Tvare the density and temperature of the vapor, respectively hlv (kJ/kg) is the latent heat of vaporization, and Psatand Pa(kPa) are the pressures of the saturated vapor and ambient, respectively Tc

and Lcare the temperature and length of the condensation section, respectively

Fig 3 The thermal network of evacuated tube collector.

3.3.2 Simulation and validation

The detailed structural parameter values of the collector are

shown inTable 3

Based on these initialization parameters and certain external

conditions, the simulation can be performed using the EES

soft-ware 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 inFig 4 The instantaneous

effi-ciency of the collector is the ratio of useful heat absorbed by the

work fluid to the solar energy which project on the lighting

sur-faces 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

mixed-effect LiBr-H2O chiller shown inFig 5was adopted[29] The

pri-mary 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

tem-perature exchanger (LX) and a high temtem-perature exchanger (HX)

The dual-sources (exhaust gas and hot water) are sent into HG

and LG1, respectively, to evaporate the liquid refrigerant The

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 cool-ing water After a throttlcool-ing process, the temperature and low-pressure liquid refrigerant fall into the evaporator In the evapora-tor, 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 temper-ature 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]:

X

X

The simulation was developed using EES commercial version 6.883-3D, which utilizes simple programming rules and compre-hensive thermophysical property functions for classical thermody-namic 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 inTable 4 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 con-sidering 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

Table 3

Design parameters of solar evacuated tube collector [30]

Out diameter of evaporation section of heat pipe D e 8 mm

Out diameter of condensation section of heat pipe D c 14 mm

where Qc is the produced cooling (kW), Qexh and Qhw are the

exhausted heat from the exhaust gas and hot water (kW),

respec-tively, and Wpis the power consumption of the pump (kW)

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 con-ducted on the influence of the heating proportion on the COP and cooling output, as shown inFig 6 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

4 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

gee¼ Eþ Qcþ Qh

To measure the contributions of the solar and biomass subsystems, the subsystem primary energy ratios,ge,solandge,bio, are

gee ;sol¼ Qc ;sol=Qsolarand ð23Þ

Fig 5 Schematic of the dual-source powered absorption chiller.

Table 4

Average relative error of each parameter.

Average relative error

Fig 6 Influence of the proportion of heating sources on the COP and cooling

output.

gee ;bio¼ E þ Qc ;bioþ Qh

where E (kW) is the electrical output, Qc(kW) is the total cooling

output, and Qc,soland Qc,bioare the two parts of the total cooling

out-put 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 Qsolarand 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

gex¼Eþ

T 0

T rw
1

Qcþ 1
T 0

T h

Qh

EXbiomassþ 1
T 0

T sol

Qsolar

Similarly, the subsystem exergy efficiencies,gex,solandgex,bio, are

gex ;sol¼ T0

Trw

1

Qc ;sol 1
T0

Tsol

Qsolarand ð26Þ

gex ;bio¼ E þ T0

Trw

1

Qc ;bioþ 1
T0

Th

Qh

EXbiomass; ð27Þ

where T0 is the reference ambient temperature, EXbiomass is the

exergy of the biomass input, and Trw; Thand Tsolare the mean

tem-peratures 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

ref-erence 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

typ-ical 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

consump-tion when calculates CERR Therefore, CERR can be calculated as:

E þQ c þQ h

g T g b
Qbiomass

EþQ c þQ h

gTgb

ð28Þ

wheregTis the performance coefficient of ORC-CCHP part,gbis the

efficiency of biomass-fired boiler, which is 0.85[33], and Qbiomassis

the consumed biomass energy

5 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 col-lected 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 Section3.3.2, the outlet water temperatures of the three collectors in turn were calculated to be T1

o¼ 76:1C;T2

o¼ 82:1C, and T3o¼ 87:9C 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 col-lector area under the design condition was calculated to be 96 m2

for an 800 W/m2solar irradiance Using the thermodynamic mod-els in Section3and the design parameters inTable 1, the results at the design work condition are summarized inTable 5

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 sub-system 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 subsys-tem 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 dif-ferent off-design conditions for the timely response to difdif-ferent 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 vari-ations, the other inherent system conditions and external condi-tions keep its design state unchanged Meanwhile, it is important

to note here that this paper merely concentrated on the system

Table 5

Results at the design work condition.

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 inFig 7 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) Thegr,exhandgr,jwremained 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, thegr,exhandgr,jwvary slightly, andgeshows an

incremen-tal trend

Moreover, it can be seen that the cooling output increases

almost linearly with the electric load factor This is probably

attri-butable 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

exis-tence of basic solar hot water heat still can induce a change in the

proportion of the overall heating source, and thereby possibly

pro-duce 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

gradu-ally 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 inFig 8 Their increases are also

caused by the greater contribution of the biomass subsystem At

the preliminary stage, a low electricity output implies a low bio-mass 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 effi-ciency 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 effi-ciency (gs), and the cooling output and COP of absorption chiller also changed, as shown inFig 9 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 ofgstherefore 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 irradi-ance 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 col-lection 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 effi-ciency are shown inFig 10, which shows that they trend down-ward in a similar manner This is probably due to the higher contribution of the biomass subsystem to the overall system per-formance 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 empha-sizes increases in system energy and exergy efficiency, the assumption of solar irradiance should not be too great

Fig 9 Influence of solar irradiance on the COP, cooling output and solar collection Fig 7 Variation in performance with the electrical load factor.

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

bio-mass 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:

Rs¼QQsol;v

Similarly, the biomass energy ratio is defined as the ratio of the

variable biomass energy input to the nominal solar energy input:

Rb¼Qbio ;v

where Rsand Rb are the solar energy and biomass energy ratios,

respectively, and Qbio,nand Qsol,nare the nominal inputs of the

bio-mass energy and solar energy at the design condition, respectively

Their values can be found inTable 5 Qsol,vand Qbio,vare the variable

inputs of the biomass energy and solar energy under different

conditions

Regarding the detailed results the variable conditions, the

bio-mass energy ratio varied from 0 to 8, which corresponds to a

vari-ation in the electrical output from 1 to 100 kW The solar energy

ratio varied from 0 to 0.6, and the solar collector area therefore

var-ied from 0 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 inFig 11 FromFig 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 highergeand 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 Rsleads to a decreased solar subsystem effect, and,

from formula(16), the energy efficiency will be greatly influenced

by Rb Therefore, when Rsis 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

are shown inFig 11(b) The figure shows that the energy efficiency decreases steeply at a low solar energy ratio, then gradually chan-ged 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 1 in most cases, the energy efficiency still could not increase because

of the basic biomass energy input When the Rbwas 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 inFig 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

(a)

0.45 0.47 0.49 0.51 0.53 0.55 0.57 0.59 0.61

Biomass energy ratio

Solar energy ratio=0.0

(b)

0.45 0.47 0.49 0.51 0.53 0.55 0.57 0.59 0.61

Solar energy ratio

0.1

Fig 11 Influence of the biomass energy and solar energy ratios on the energy efficiency.

Fig 10 Influence of the solar irradiance on the energy efficiency and exergy

efficiency.