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.

1. 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: (H. Li), (G. Zhang).
/>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 6 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 6 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 2 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.

2. System flowsheet description
The flowsheet of the suggested system is shown in Fig. 1. 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. 1. Flowsheet of a trigeneration system with solar energy and biomass coupling utilization.

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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, 1 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 2
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 (%)

9

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 4
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 4.

The main chemical reactions that occurred in the biomass gasification process are presented in Table 1.
In this study, rice husk is selected as the biomass material.
Table 2 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 1
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 3
Key operating parameters of system.

3. 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


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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. 2. 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 6 2Þ
1 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. 2. Simulation and analysis procedures of the proposed system.


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X. Zhang et al. / Energy Conversion and Management 122 (2016) 74–84

PESR ¼ 1 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 Â

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 Â

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 1

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 5
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

–
–
–
–
–
–
–


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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. 4 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.

4. 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 6 ER 6 0.5 and 0 6 SBR 6 4.0.
Fig. 3 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. 6 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. 5 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. 3. Effect of SBR on the gasification temperature with various ERs.

50
N2

40
30
H2
CO2

20
10

CO
CH4

0
0.0

0.5

1.0

1.5

2.0

Steam/Biomass Ratio
Fig. 4. Effect of SBR on the bio-gas composition (ER = 0.4).


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X. Zhang et al. / Energy Conversion and Management 122 (2016) 74–84

90

8

Bio-gas Yield (Nm3/kg)

7

ER=0.4
ER=0.5

88

Overall Energy Efficiency (%)

ER=0.35
ER=0.45

6
5
4
3
2
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. 7. Effect of SBR on the overall energy efficiency with various ERs.

Fig. 5. 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

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Steam/Biomass Ratio

Fig. 6. Effect of SBR on the cold gas efficiency of bio-gas with various ERs.

Fig. 8. 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 0 to 4.0 at solar collector temperature
of 350 °C. In addition, it can be seen from Fig. 7 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. 7 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. 8 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. 9. Primary energy saving ratio (PESR)
has been calculated to assess the performance between trigeneration system and conventional separated generation system. Fig. 9
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. 9 that the PESR decreases
from 19.5% to 15.5% with increase in SBR from 0 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 0. 5. And PESR decreases to a constant


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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
0.0

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
0.0

1.5

Fig. 11. Effect of SBR on the system efficiency (ER = 0.4).

Fig. 9. 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).


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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
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

6

3x106

2x106

1x106

0

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.
5. 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 0 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.


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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).
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