Energy matching and optimization analysis of waste to energy CCHP (combined cooling, heating and power) system with exergy and energy level

Energy xxx (2014) 1e14

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Energy matching and optimization analysis of waste to energy CCHP (combined cooling, heating and power) system with exergy and energy level Penghui Gao a, d, *, Yanjun Dai b, YenWah Tong c, Pengwei Dong d a

School of Architecture and Civil Engineering, China University of Mining and Technology, Xuzhou 221116, China School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China Department of Chemical & Biomolecular Engineering, National University of Singapore, 4 Engineering Drive, Singapore 117576, Singapore d Environmental Research Institute, National University of Singapore, Singapore 117576, Singapore b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 7 July 2014 Received in revised form 23 October 2014 Accepted 16 November 2014 Available online xxx

CCHP (combined cooling, heating and power) system as a poly-generation technology has received an increasing attention in field of small scale power systems for applications ranging from residence to utilities. It will also play an important role in waste to energy application for megacities. However, how to evaluate and manage energy utilization of CCHP scientifically remains unclear. In this paper, energy level and exergy analysis are implemented on energy conversion processes to reveal the variation of energy amount and quality in the operation of CCHP system. Moreover, based on the energy level analysis, the methodology of energy matching and optimization for the CCHP system is proposed. By this method, the operational parameters of CCHP system can be deduced to obtain an efficient performance and proper energy utilization. It will be beneficial to understand and operate the CCHP system, and to provide a guiding principle of the energy conversion and management for the CCHP system. © 2014 Elsevier Ltd. All rights reserved.

Keywords: CCHP (combined cooling heating and power) Energy level Energy matching

1. Introduction The consumption habits of modern lifestyles are causing a worldwide waste problem and how to dispose the waste has become a major concern to achieve a sustainable society with minimum environmental impact. The present waste recycling processes include thermal conversions (incineration, gasification, etc.), bio-chemical conversions (anaerobic digestion, composting, vermicomposting) and chemical conversions (transesterification and vegetable oils). Among them, the incineration, gasification and anaerobic digestion are widely taken in practical application. Their energy conversion processes are shown in Fig. 1. As shown in Fig. 1, for waste incineration incineration system, heat is provided by incineration firstly, then the heat can produce steam in waste heat boiler and the steam go through a steam turbine to generate power. The exhaust steam after turbine can

* Corresponding author. School of Architecture and Civil Engineering, China University of Mining and Technology, Xuzhou 221116, China. Tel.: þ86 (516) 83882193; fax: þ86 (516) 83885478. E-mail addresses: [email protected], [email protected] (P. Gao).

provide heating and cooling so as to recover energy. For CCHP (combined cooling, heating and power) system, combustible gas like CH4, H2 and other C2Hx, can be obtained by gasification and anaerobic digestion firstly. Then the fuel gas is transported into the CCHP system to provide electricity, cooling, and heating. The advantages and disadvantages of incineration, gasification and anaerobic digestion are presented in Table 1. In general, the low heat value of combustible gas from gasification and/or digestion is higher than raw material in incineration. Since the raw state of feedstock for incineration varies a lot, it requires many treatment processes like drying and melting, which will lead to heat loss. Gasification and anaerobic digestion can provide more value-added products for other downstream process while their fuel gas product can be used for power generation. The multi-staged utilization of gasification and anaerobic digestion can improve the whole efficiency and reduce environmental impact. Combined cooling, heating and Power (CCHP) systems which also referred as trigeneration system [1] are integrated energy systems producing electricity, cooling, and heating simultaneously. Due to cascade utilization of energy, the overall fuel energy utilization efficiency of CCHP system can be much higher than that of conventional independent energy supply systems. CCHP, as a

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Nomenclature E

h EX

U N G H S R C V Y T

g k

u w q 4

l

M

amount of energy (J) efficiency amount of exergy (J) energy level value mole number standard free enthalpy (J/kg) enthalpy (J/kg) entropy (J/K) molar gas constant specific heat (kJ/(N m3 K) volume fraction (N m3/N m3dry gas) heat value (kJ/N m3) temperature (K) compression ratio adiabatic exponent temperature ratio work (J) heat (J) heat transfer efficiency difference value consumption (N m3/s)

distributed generation technology, can reduce electricity use from the grid by using low emission fuels such as natural gas, anaerobic digestion gas and thermal syngas [2]. Moreover, CCHP systems can improve the stability and power quality considerably by regulating the peak-loads; for micro-grid with distributed energy systems, CCHP systems can meet various energy demands from diverse users by adjusting operating modes. It will also play an important role in waste to energy application for megacities. Now, combining with anaerobic digestion and/or gasification, CCHP may be the most popular waste to energy method for most megacities for its advantages, such as flexible layout, energy saving, less impact on environment and etc. In order to efficiently dispatch the energy generated by CCHP, many researches have been conducted to provide optimization strategies for CCHP system in several aspects, such as exergy, economy, environment, optimization theory, etc. Exergy analysis, which is developed from the first and second laws of thermodynamics, is used to evaluate the performance of energy systems. Ghaebi et al. [3] analyzed CCHP system from exergetic and thermoeconomic points of view. Khaliq and Kumar [4] studied on the performance of the trigeneration system with combustion gas turbine from point of the first law and second law. Their results showed that performance evaluation of the trigeneration system based on the first law alone is not adequate, and the second law must be included to obtain more meaningful evaluation. Schicktanz et al. [5] analyzed the operational planning of CCHP system from energetic and economic issues. Knizley et al. [6] studied the performance of CCHP system with dual power generation units. Ebrahimi and Keshavarz's work [7] gave the performance of a residential micro-combined CCHP system according to the multi-criteria sizing function. Jabbari et al. [8] presented a method to design and optimize CCHP system using pinch technology. Ahmadi et al. [9] analyzed a novel integrated multigeneration system using thermoeconomic modeling and multi-objective optimization. Kong's work [10] used a basic linear programming model to determine the optimum energy combination for CCHP system. Kitagawa et al. [11] developed an MINLP (mixed-integer non-liner programming) model using particle swarm optimization for optimal operational planning of a

Subscripts th thermal process ex exergy f medium l low value g gas p pressure in inlet out outlet c cooling h heating 0 standard state E evaporator G generator fg flue gas fgh heat through direct flue gas ther gas combustion in engine ei inlet of the evaporator eo outlet of the evaporator hi inlet of heat recovery boiler ho outlet of heat recovery boiler

cogeneration system, but they only focused on the analysis of optimization method. Wu et al. [12] used a multi-objective optimal operation strategy to analyze the micro-CCHP system. Their results showed that optimal operation strategy changed with load conditions in energy saving optimization. Jing et al. [13] developed a multi-objective optimization method based on life cycle assessment, in which several objectives were combined into a single objective by weighted method. Kavvadias et al. [14] proposed a multi-objective optimization method based on indicators of economic, exergetic and environmental performance. In addition, the energy model was extended to the environmental assessment by defining an equivalent CO2 emission reduction (CO2ER) indicator in the literatures [15,16]. Some of the studies [17e19] are about the integrated optimal planning of the supply chain, in which the whole process from the original energy supplies to the final customers is considered. However, the second law of thermodynamics mainly deals with energy quality and determines the maximum amount of work from an energy resource. In this regard, exergetic optimization reveals minimizing thermodynamic inefficiency through energy systems. It is well known that different energy has different energy levels, which means their energy quality is different. For example, electricity has higher energy quality while heat with lower temperature is a lower quality of energy. It is important to efficiently and scientifically dispatch the energy for the CCHP system. However, exergetic analysis cannot completely solve the energy matching problem between supply and demand. Chen et al. [20] applied energy level to analyze the energy conversion process of CCHP system under the rated and part-load conditions. Nevertheless, how to optimize the energy matching for the CCHP system was not given. The objective of this paper is to present a methodology of energy matching and optimization for the CCHP system based on the energy level and exergy analysis between supply and demand. The thermodynamics model of CCHP system is presented, and the energy level values of inlet gas, flue gas, cooling user and heating user are calculated. Based on energy demand of every part, temperature requirement of exhausted flue gas and certain exergy analysis of

Please cite this article in press as: Gao P, et al., Energy matching and optimization analysis of waste to energy CCHP (combined cooling, heating and power) system with exergy and energy level, Energy (2014), http://dx.doi.org/10.1016/j.energy.2014.11.050

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3

waste incineration cogeneration systems electricity power

general waste

heating kitchen waste

waste heat boiler

incineration

steam turbine

cooling

CCHP systems by gas

waste tyre

electricity power

shell waste

heating

anaerobic digestion

gas engine

cooling

straw waste gasification

sludge waste Fig. 1. Waste to energy process in use.

100% of efficient. The sketch of exergy utilization is shown in Fig. 3, and exergy efficiency can be obtained as

the system, the performance of CCHP system with different energy levels is analyzed and the optimal operational zone is determined. Compared with other study, the analysis using energy level and exergy theory in our work, provides a new method for energy use and management in the CCHP system. It is beneficial to understand scientifically and operate efficiently the CCHP system for cascade utilization of energy.

P P EXproduct ðEXsource  EXloss Þ P hex ¼ P ¼ EXsource EXsource

2. Concept of energy level and description of CCHP system

0 < hex < 1

2.1. Energy level

Energy conversion efficiency reveals how to reduce the energy loss in the process of the energy conversion, in which the quality of energy is not distinguished. Exergy efficiency stresses the importance of energy quality, but it does not solve the matching problem between the supply and the user. In order to achieve the energy matching and energy loss reducing, energy level is defined as

Energy conversion efficiency shows the conversion rate of total energy quantity; which is a dimensionless performance indicator of a device that uses thermal energy. In general, it is the ratio between the useful output and the input of a system, in energy terms. The sketch of energy conversion efficiency is shown in Fig. 2, and energy conversion efficiency can be obtained as

hth

Eout ¼ Ein

(1)

According to the first law of thermodynamics, the energy output cannot exceed the input, so,

0  hth  1

According to the second law of thermodynamics, the exergy efficiency is

U¼

EX E

(4)

(5)

The values of energy level can denote whether the quality of energy that is good or bad. The sketch of energy level between the supply and the user is shown in Fig. 4. From Fig. 4, the energy level of supply energy and the user are presented as follows respectively,

(2)

Exergy is the maximum theoretical work which can be obtained from an energy source. Exergy efficiency shows the conversion rate of total useful energy quantity, which presents the performance of a device that outputs useful energy. From the second law of thermodynamics, it can be demonstrated that no system can ever reach

(3)

U1 ¼

EX1 E1

(6)

U2 ¼

EX2 E2

(7)

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Table 1 Advantages and disadvantages of incineration, gasification and anaerobic digestion. Method of waste to energy

Technology

Advantages

Disadvantages

Incineration

Waste incineration cogeneration systems (overall efficiency is less than 70%)

I. Most suitable for high calorific value waste; II. Units with high throughput and continuous feed can be set up; III. Thermal energy for power generation or direct heating.

I. Least suited for aqueous, high moisture content, low calorific value and chlorinated waste; II. Toxic metal concentration in ash, particulate emission SOx, NOx, chlorinated compounds, ranging from HCI to dioxins; III. High capital and costs; IV. Unsuitable for arrangement in cities.

Gasification

Combined cooling, heating and power (CCHP) systems (overall efficiency is about 70e90%);

I. Production of fuel gas, which can be used for various purposes; II. Pollution control superior as compared to incineration. I. No power requirement for sieving and turning of waste pile; II. Enclosed system enables trapping the gas produced for use; III. Free from bad odor, rodent and fly menace, visible pollution and social resistance; Compact design needs less land area; IV. Net positive environmental gains.

I. Net energy recovery may suffer in waste with excessive moisture.

Anaerobic digestion

2.2. CCHP system Energy and Environmental Sustainability Solutions for Megacities (E2S2) is a major collaboration program between Shanghai Jiao Tong University and National University of Singapore. It aims to develop a platform to facilitate the understanding, design, and implementation of infrastructure and sustainability solutions for coupled problems in future cities. In this program, one major focus is to design and optimize an efficient CCHP system driven by biogas, which is from hybrid biological and chemical waste-to energy system. Fig. 5 illustrates a diagram of the CCHP system used in this program, which can produce power, heating and cooling simultaneously. The proposed system consists of a thermal cracking unit, an anaerobic digestion unit, gas distribute unit, gas engine, heat exchanger, heat recovery boiler, absorption refrigeration unit, hot water storage tank and other related components. As is shown in Fig. 5, the gas from thermal cracking unit and/or anaerobic digestion unit flows into gas engine through gas distribute unit. Gas composition can be adjusted according to need from gas engine. The biogas combusts and releases much heat, generating power in gas engine. Flue gas, from gas engine, flows into heat recovery boiler. From this diagram, one part of heat from heat recovery boiler is used to drive the absorption refrigeration unit. This unit uses a mixture of lithium bromideewater as working

I. Unsuitable for waste containing less organic matter.

fluid to absorb the heat from exhaust gas and provides cooling load for users. The other part of heat from heat recovery boiler and refrigeration unit is used for heating. Monitoring and control system can collect signals from different spots and issue commands to different units like thermal cracking unit, anaerobic digestion unit, gas distribute unit, gas engine and the absorption refrigeration unit according to the users' need. The technical specification of main equipments in Fig. 5 is presented in Table 2. The operation process of the CCHP system can be simplified into the structure as in Fig. 6. As shown in Fig. 6, there are mainly four energy levels for the whole system listed as energy level of gas (EL1), energy level of exhaust flue gas (EL2), energy level of

system

EXproduct

EXsource

EXloss Fig. 3. Sketch of exergy utilization.

system

Eout

Ein

Eloss Fig. 2. Sketch of energy conversion.

Fig. 4. Sketch of energy level.

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heat output

hot water storage tank

5

cold output

heat from condenser

absorption refrigeration unit

feed water

monitoring and control system heat recovery boiler

heat exchanger 1

heat exchanger 2 flue gas dust collector

G

flue gas gas engine

electric generator

gas distribution unit solid waste CO, H2,CH4

solid waste

anaerobic digestion unit

thermal cracking unit

material flow

signal feedback control network

Fig. 5. Operation diagram of the CCHP based on waste to energy.

consumption refrigerating capacity (EL3) and energy level of consumption heat (EL4) respectively. In order to achieve scientifically and efficiently energy utilization for the CCHP system, related analysis is carried out and the optimization principle is proposed based on the energy level and exergy analysis.

3.1. Energy level analysis

EX0 ¼

3.1.1. Energy level value of biogas Biogas is composed of different combustible gas. The exergy of biogas is mainly chemical exergy. The exergy can be obtained by formula (8) [21]

X

Ni $EX0i  NO2 $EX0O2

DG0f ¼ DHf0  T0 $DS0f

(9)

The benchmark exergy of oxygen and the different products can be obtained by formula (10)

3. Methodology of energy level analysis and optimization

EX0f ¼ DG0f þ

the products; DG0f is the standard free enthalpy of gas combustion reaction, which is deduced by formula (9)

1 P RT ln 0 N 0 P0i

(10)

where N is the atomic number of elemental molecule, R is molar gas constant; P0i is the partial pressure of ground state air. If the exergy of oxygen and the exergy of products are ignored, then the exergy of biogas is presented as follows:

(8)

where, NO2 , Ni are the mole number of oxygen and different products; EX0O2 and EX0i are the benchmark exergy of oxygen and

EX0f ¼ DG0f ¼ DHf0  T0 $DS0f

(11)

The exergy change can be obtained by formula (12)

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Table 2 Technical specification of main equipments. Main equipments

Technical capacity

Size

Other

Gas engine Anaerobic digestion unit Thermal cracking unit Gas distribution unit

Rated power output 1.0 MW Rated gas production 650 m3/h Rated gas production 1500 m3/h Distribution capacity 3500 m3/h, distribution pressure 0.2e0.4 MPa Amount of dust recovery 100 kg/day, maximum throughput 2000 m3/h Heat recovery rate 73%, operation in ordinary pressure Rated refrigerating capacity 280.0 kW, temperature of heat source 80e130  C Available capacity of tank 15 m3 Heat exchange efficiency 96%, thermal power 2000 kW

6000 mm  1800 mm  2000 mm ∅5500 mm  6000 mm 5000 mm  3500 mm  1800 mm 2000 mm  1500 mm  1200 mm

Net weight 3300 kg 3 groups, material is enamel Material is stainless steel With explosion-proof requirements

1200 mm  600 mm  1500 mm

Operation temperature is less than 90  C

2800 mm  3000 mm  3300 mm

Inlet gas temperature is less than 360  C

5000 mm  2300 mm  3000 mm

Net weight 21,000 kg

3000 mm  4000 mm  2000 mm 3000 mm  2500 mm  1500 mm

Material is stainless steel Operation temperature is less than 600  C

Dust collector Heat recovery boiler Absorption refrigeration system Hot water storage tank Heat exchanger 1 & heat exchanger 2

      Tg DG0f ¼ Hg  Hg0  T0 Sg  S0g ¼ Cpg Tg  T0  T0 ln T0

(12)

For biogas combustion process, the total energy can be deemed as the heat amount of exhaust gas, which is deduced by formula (13)

 g E ¼ Cp Tg  T0

(13)

where Tg is deemed as the combustion temperature; T0 is the environment temperature of standard state. Biogas combustion temperature is obtained by formula (14)

Tther ¼

Yl VRO2 CRO2 þ VH2 O CH2 O þ VN2 CN2

(14)

where CRO2 , CH2 O , CN2 are the average pressure volume specific heat of three atomic gases, steam and nitrogen respectively; VRO2 is volume content of three atomic gases (N m3/N m3 dry gas), VH2 O is steam volume content in theoretical flue gas (N m3/N m3dry gas), VN2 is nitrogen volume fraction in theoretical flue gas (N m3/N m3 dry gas); Yl is low heat value of biogas (kJ/N m3). Based on formulas (5), (12) and (13), energy level value of biogas can be obtained by formula (15)

Ug ¼

EX0f ¼ E

g

Cp



  T Tg  T0  T0 ln T0g  Cpg Tg

 T0



Ufg ¼ 1 

T0 T ln in Tin  Tout Tout

where Tin is the input temperature of flue gas; Tout is the output temperature of flue gas. 3.1.3. Energy level of consumption refrigerating capacity For the refrigeration part, the exergy of cooling capacity can be deduced as

 EXc ¼ Qc

T0 1 Tc

(17)

Then, the energy level value of refrigeration can be obtained by formula (17)

Uc ¼

EXc T0 ¼ 1 Qc Tc

Tg T0 ln Tg  T0 T0

(18)

(15) Uh ¼

EXh T ¼1 0 Qh Th

(19)

heating

refrigeration

EL3

gas

EL4

absorption refrigeration unit

EL1

E

(16)

3.1.4. Energy level of consumption heat According to the second law of thermodynamics, energy level of consumption heat can be obtained by formula (19)

 ¼1

3.1.2. Energy level of flue gas in heat exchanger Same as energy level value of biogas, the energy level value of flue gas can be deduced as follows:

heat exchanger

gas engine

EL2

exhaust flue gas

Thi

Tho heat recovery boiler

Fig. 6. Schematic diagram of the CCHP system.

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where T0 is the ambient temperature, Tc and Th are the cold water temperature and the hot water temperature respectively (in this study, Tc and Th are assumed to be constant, which are respectively 278 K and 353 K).

heat recovery boiler. The definition of temperature ratio u is given by

3.2. Performance analysis of CCHP system

Based on the formula (22) and (16), the outlet temperature of flue gas from the heat recovery boiler Tho can be obtained by

Performance of CCHP system driven by biogas is presented in this section. The related data used in this model is given in Table 3. The absorption refrigeration is single-effect absorption which is similar to the approach used by ASHRAE [22]. In this study, some assumptions are employed as follows: I. Gas is completely combusted and chemical reaction reaches equilibrium state. II. Flow temperature at the inlet/outlet of every unit is constant. III. Radiation heat transfer is neglected in every unit. IV. The whole system reaches steady state. V. Heat loss of every unit is not considered.

u¼

Thi Tho

Tho ¼

h¼1

T1 1 ¼ 1  k1 T2 g

T2 ¼ T1 $gðk1Þ

Thi ¼

Table 3 Input data of CCHP system in designed condition.

Heat recovery boiler

u$T0 $logðuÞ  ðu  1Þ$ 1  Ufg

(24)

For the gas engine, the exhaust temperature T4 of gas engine is determined by

T1 $T3 T $T3 T3 ¼ 1 k1 ¼ k1 T2 T1 $g g

(25)

where T3 can be deemed as the gas combustion temperature in the gas engine which is biogas combustion temperature Tther and can be obtained by formula (14), T4 is deemed as the temperature of input flue gas Thi. Based on formulas (25) and (24), the compression ratio of gas engine can be deduced, which is presented as follows:

ð

Tther $ðu1Þ$ 1Ufg u$logðuÞ$T0

g ¼ eðlogðTther =Thi Þ=ðk1ÞÞ ¼ e

Þ

(26)

Based on formulas (14), (20), (21) and (26), output work of gas engine is obtained by

w ¼ Cv ðT3  T2 Þ$h

(27)

(21)

Adiabatic exponent k is determined by its composition. From Fig. 5, Thi is deemed as the exhaust gas temperature from gas engine and Tho is the outlet temperature of flue gas from the

Absorption refrigeration

(23)

(20)

where g ¼ v1/v2 is compression ratio, T1 and T2 are the beginning compression temperature and completed compression temperature (K), k is adiabatic exponent. Completed compression temperature can be expressed by

Gas engine

T0 $logðuÞ  ðu  1Þ$ 1  Ufg

Then,

T4 ¼ 3.2.1. Gas engine The operation process of gas engine consists of two isometric processes and two isentropic processes. In order to obtain the performance of CCHP, we consider the gas engine operates in ideal process. So, the efficiency of gas engine is

(22)

Parameters

Value

Rated power output Gas consumption Efficiency of power generation Exhaust gas temperature Rated compression ratio

1.0 MW 0.3 m3/kW h 36.5% 510  C 8

Overall heat transfer coefficient of desorber Overall heat transfer coefficient of condenser Overall heat transfer coefficient of evaporator Overall heat transfer coefficient of absorber Heat transfer efficiency Effectiveness of solution heat exchanger Inlet/outlet temperature of frozen water Concentration of strong solution in design condition Concentration of weak solution in design condition Flow ratio Rated refrigerating capacity Rated refrigerating coefficient (COP)

70 kW/K 80 kW/K 95 kW/K 75 kW/K 80% 70% 12/7  C 0.582

Exhaust gas temperature of boiler Heat transfer efficiency Heat recovery rate

105  C 83% 73%

0.556 22.38 280.0 kW 0.71

3.2.2. Absorption chiller In this study, a single-effect absorption chiller is used. Thermodynamic analysis of ARS has been carried out in literature [23]. It includes a generator, an absorber, a condenser, an evaporator, a pump, an expansion valve, a solution heat exchanger, a refrigerant heat exchanger and a solution-refrigerant heat exchanger.

qc ¼ qEV ¼ heo  hei

(28)

where hei is the refrigerant enthalpy input to the evaporator, heo the output enthalpy of refrigerant. Heat used by generator which is from heat recovery boiler, is obtained by

qGE ¼ Cv ðTin  Tout Þ$4

(29)

where f is the heat transfer efficiency. Then, the COP of the absorption refrigeration system is

COP ¼

qEV qGE

(30)

3.2.3. Heating system Heat used by the heating system can be obtained by formula (31)

qh ¼ qC þ qfgh

(31)

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where qC is the exhaust heat from the condenser of absorption refrigeration system, qfgh is the heat used by hot water through the direct flue gas.

where A is the constant which can denote the matching degree of energy.

3.3. Assessment of the CCHP system

3.4.2. Exergy efficiency constraint When the energy level of units in the CCHP system meets requirement, the exergy efficiency is another constraint.

Performance assessment of the CCHP system is very important. The index used to evaluate the performance includes PER (primary energy ratio) and exergy efficiency.

1 > hex > B

3.3.1. Primary energy ratio In order to indicate the performance of CCHP system, parameter of PER (primary energy ratio) is used, which is defined as

W þ Qh þ Qc PER ¼ Mg  Yl

(32)

where, W e total engine electric power (kW), Qh e total heating power (kW), Qc e total refrigerating capacity (kW), Mg e gas consumption (N m3/s), Yl is the heat value of gas (kJ/N m3). 3.3.2. Exergy efficiency Exergy efficiency is the ratio of the output exergy to the input exergy for the CCHP system, which is defined as

hex ¼

EXe þ EXh þ EXc EXg

(33)

where hex is the exergy efficiency, EXe, EXh, EXc and EXg are the exergy of electricity, heating, cooling and gas respectively,

EXe ¼ w

(34)

 T EXh ¼ qh 1  0 Th

(35)

 EXc ¼ qc

T0 1 Tc

(36)

where T0 is the ambient temperature, Tc and Th are the cold water temperature and the heat water temperature respectively (in this case study, Tc and Th are assumed to be constant, which are respectively 278 K and 353 K). The exergy of gas can be written as

EXg ¼ Mg $Yl

(37)

3.4. Constraints In order to achieve energy saving and cascade utilization, the optimization principle for the CCHP system can be proposed based on the energy level, exergy and exhaust temperature requirement. The main constraints are presented as follows. 3.4.1. Energy level value constraint Energy level value is used to describe the quality of energy in the CCHP system. The difference of energy level value can denote the matching characteristic of energy between supply and user, which is given by

l ¼ U1  U2

(38)

jlj  A

(39)

(40)

where B can be determined with considering the efficiency of gas engine, the exergy efficiency of the CCHP system, the operation cost and etc. 3.4.3. Exhaust temperature requirement Corrosion of flue gas to the metal wall would happen when the temperature of exhaust flue gas is lower than the corresponding dew point temperature. As a constraint, the temperature of exhaust flue gas must be higher than 105  C. 3.5. Optimization solution The optimization process is given in Fig. 7. The solution process is as follows: (1) Give the cooling load, heating load, gas composition, heating temperature, cooling temperature, environmental temperature and etc. (2) Calculate the energy level value of gas, flue gas, consumption refrigerating capacity and consumption heat. (3) According to the energy level of flue gas, calculate the compression ratio of gas engine. (4) Calculate the PER, thermal efficiency of gas engine and exergy efficiency of CCHP system. (5) Operation parameters are determined according to whether the calculation results are satisfied with the constraints of CCHP system. Through the process in Fig. 7, the final optimal solution can be achieved. 4. Results and discussion In order to achieve energy matching and optimization for the CCHP system, we analyzed the performance of CCHP system driven by biogas. The composition of biogas is mainly CH4 and H2, which occupies 56.1%Vol and 3.0% Vol respectively. Its adiabatic exponent is 1.475, combustion temperature is 2203  C and low heat value is 20,684 kJ/N m3. The variation of exergy efficiency of CCHP system and efficiency of gas engine with energy level value of exhaust gas is shown in Fig. 8. From Fig. 8, the efficiency of gas engine decreases while exergy efficiency of CCHP system increases with energy level value of exhaust gas increasing. The reason is that when increase energy level value of exhaust gas, meaning the exhaust gas temperature increasing, and it results in the decrease of gas engine efficiency because combustion temperature is constant. However, the higher exhaust gas temperature of gas engine will be favor of absorption refrigeration system and heating system, and the total exergy efficiency of CCHP system will increase consequently. When energy level value is constant, the efficiency of gas engine will be higher at smaller temperature ratio u, while the exergy efficiency of CCHP system will be higher at bigger temperature ratio u. It is that the bigger temperature ratio u denotes the difference between Tin and Tout is larger, and Tin may be higher when Tout is constant. Tin can be deemed as the exhaust gas temperature of gas engine. So, the efficiency of gas engine will decline as the exhaust gas temperature increasing. The variation cause of exergy efficiency is on contrary with gas engine efficiency in the same energy level value. Fig. 9 shows the variation of PER of CCHP system and efficiency of gas engine with energy level value of exhaust gas. The efficiency of gas engine decreases with the energy level value increasing,

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start

Initialization

Input: cooling load, heating load, gas composition, heating temperature, cooling temperature, environmental temperature

Step 1: calculation energy level value of every node in CCHP system

Calculation process

Step 2: calculation compression ratio of gas engine

Step 3: calculation PER, thermal efficiency of gas engine, exergy of CCHP system

Update constraints set

Update temperature ratio of heat recovery boiler

Constraints of CCHP system

Satisfied with constraints set No

Yes Final optimal operation solution

Fig. 7. Optimization flow chart based on energy level analysis.

while PER of CCHP system increases with energy level value increasing. The reason cause is same as the cause in Fig. 8. The variation of compression ratio of gas engine g and efficiency of gas engine h with energy level value of exhaust gas Ufg is shown in Fig. 10. From Fig. 10, the compression ratio of gas engine decreases with the energy level value of exhaust gas increasing. The reason is that the increasing energy level value of exhaust gas means increasing the exhaust gas temperature, and it results in decreasing of gas engine compression ratio according to formula (26). When energy level value is constant, the compression ratio of gas engine will be larger at smaller temperature ratio u. It is that the smaller temperature ratio u denotes the difference between Tin and Tout is smaller, and Tin may be lower when Tout is constant. So, the compression ratio of gas engine will be larger according to formula (26), as the combustion temperature in gas engine is constant.

Fig. 11 shows the variation of output work of gas engine w and efficiency of gas engine h with energy level value of exhaust gas Ufg. Firstly, the output work of gas engine increases with energy level value increasing from 0.1 to 0.6; then the output work of gas engine decreases subsequently. When input temperature and the combustion temperature in gas engine are constant, the output work of gas engine changes with the exhaust gas temperature so that there is a maximal value of output work. The variation of output work in Fig. 11 is consistent with this analysis. When energy level value is constant, the output work will be more at bigger temperature ratio u before the inflection point, and then the variation tendency will change. The reason is that the smaller temperature ratio u denotes the difference between Tin and Tout is smaller, and Tin may be smaller when Tout is constant. So, the compression ratio will increase and output work of gas engine will reduce before it reaches the inflection point when the input and the combustion temperature of gas engine are constant.

Please cite this article in press as: Gao P, et al., Energy matching and optimization analysis of waste to energy CCHP (combined cooling, heating and power) system with exergy and energy level, Energy (2014), http://dx.doi.org/10.1016/j.energy.2014.11.050

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Fig. 11. Variation of w and h with Ufg. Fig. 8. Variation of hex and h with Ufg.

Fig. 9. Variation of PER and h with Ufg.

Variation of heat input to absorption refrigeration qG and heat input to hot water qh with energy level value of exhaust gas Ufg is given in Fig. 12. From Fig. 12, the heat input to absorption refrigeration qG and heat input to hot water qh increase with the energy

Fig. 10. Variation of g and h with Ufg.

level value increasing. The cause is that the increasing energy level value of exhaust gas means increasing the exhaust gas temperature, so that the heat used by absorption refrigeration and hot water will increase. When energy level value is constant, heat input to absorption refrigeration qG and heat input to hot water qh will be larger in larger at big temperature ratio u. The reason is that the bigger temperature ratio u denotes the difference between Tin and Tout is larger, and Tin may be higher when Tout is constant. So, the exhaust gas temperature will be higher and the amount of heat used will increase. The output work variation of gas engine w with temperature ratio u and energy level value of exhaust gas Ufg is presented in Fig. 13. Fig. 13(a) is a three-dimensional map and Fig. 13(b) is a contour map. From Fig. 13, output work of engine increases with the temperature ratio u and energy level value of exhaust gas Ufg. The variation of gas engine efficiency with temperature ratio u and energy level value of exhaust gas Ufg is presented in Fig. 14. Fig. 14(a) is a three-dimensional map and Fig. 14(b) is a contour map. From Fig. 14, gas engine efficiency decreases as increasing the temperature ratio u and energy level value of exhaust gas Ufg. Comparing Fig. 13 with Fig. 14, it is obvious that as temperature ratio u and energy level value of exhaust gas Ufg increases, output work of gas engine increases while gas engine efficiency declines. So, the temperature ratio u and energy level value of exhaust gas Ufg should be determined after overall consideration. The variation of compression ratio g with temperature ratio u and energy level value of exhaust gas Ufg is presented in Fig. 15. When compression ratio g increases, the exhaust gas temperature decreases and energy level value decreases, and so does temperature ratio u. The variation of exhaust gas temperature and outlet flue gas temperature of the heat recovery boiler with temperature ratio u and energy level value of exhaust gas Ufg are shown in Fig. 16. From Fig. 16, exhaust gas temperature increases with the temperature ratio u and energy level value of exhaust gas Ufg while outlet flue gas temperature decreases on the contrary. The variation of PER with temperature ratio u and energy level value of exhaust gas Ufg is presented in Fig. 17. Fig. 17(a) is a threedimensional map and Fig. 17(b) is a contour map. From Fig. 17, PER of CCHP system increases with the temperature ratio u and energy level value of exhaust gas Ufg. The variation of exergy efficiency of CCHP system hex with temperature ratio u and energy level value of exhaust gas Ufg is presented in Fig. 18. Fig. 18(a) is a three-dimensional map and Fig. 18(b) is a contour map. From Fig. 18, exergy efficiency of CCHP

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Fig. 12. Variation of qG and qh with Ufg.

Fig. 13. Variation of w versus u and Ufg.

Fig. 14. Variation of h versus u and Ufg.

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Fig. 15. Variation of g versus u and Ufg.

Fig. 16. Variation of Tai, Tao versus u and Ufg.

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Fig. 17. Variation of PER versus u and Ufg.

Fig. 18. Variation of hex versus u and Ufg.

system hex increases with the temperature ratio u and energy level value of exhaust gas Ufg increasing. From Figs. 17 and 18, the variation of PER and exergy efficiency of CCHP system have the similar variation tendency, but, the value of PER is larger than that of exergy efficiency of CCHP system hex in the same conditions. (a) Three-dimensional map of hex versus u and Ufg (b) Contour map of hex versus u and Ufg.

Table 4 Optimal operation parameters and the energy level values of the CCHP system. Energy level value

Constraints

Optimal operation parameters

Ug ¼ 0.80 (EL2) Uc ¼ 0.05 (EL3) Uh ¼ 0.17 (EL4)

0:1  jlj  0:3 Tao > 378 K hex > 0.3

0.40 < Ufg < 0.50 1.4 < u < 1.6 7 < g < 11

Based on the above analysis, the optimal operation parameters of CCHP can be determined according to the energy level and constraints of the CCHP system, and the details are shown in Table 4. The optimal zones are marked by red rectangle in Figs. 13e18.

5. Conclusions In order to obtain the optimum performance of the CCHP system driven by biogas, the model of energy matching and optimization model for the CCHP system based on energy level and exergy analysis is proposed in this study. The related parameters, such as output work of gas engine, PER of CCHP system, exergy efficiency, gas engine efficiency, compression ratio of engine and etc., were analyzed in different energy level values of exhaust gas. Moreover,

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the optimal operation zone of CCHP system is given according to the energy level analysis and constraints of the CCHP system. It is beneficial to understanding and operation of CCHP system in point of energy saving and cascade utilization. The main conclusions drawn from this study are summarized as follows: (1) Compared with exergetic analysis, energy level value can denote whether the quality of energy is good or bad. Besides, energy level analysis is a comprehensive and scientific evaluation method for the energy utilization system, which can solve the energy matching problem between supply and demand. (2) Based on the energy level and exergy analysis, the optimal operation method is built and calculated. Due to the important role of exhaust flue gas from gas engine in driving the absorption refrigeration system and heating system, the energy level of exhaust gas from engine, as an important parameter, is used to analyze the whole process. The parameters, which include output work of gas engine, compression ratio, PER and exergy efficiency of CCHP system, are analyzed in different temperature ratio u and energy level value of exhaust gas Ufg to evaluate the performance of the system. (3) Based on the above analysis, the optimal operation zone of CCHP is given. In order to obtain an efficient performance and scientific energy utilization for the CCHP system, compression ratio of engine g, the temperature ratio u and energy level value of exhaust gas Ufg should be determined after overall consideration. This study will be favor to understand the optimal operation and to supply references for the system performance. The optimal analysis by energy level for the CCHP system provides another method for energy matching and optimization, which contributes to the further efficient operation of the CCHP system. Acknowledgment The work was supported by the National Research Foundation (NRF), Prime Minister's Office, Singapore under its Campus for Research Excellence and Technological Enterprise (CREATE) program, Jiangsu Overseas Research & Training Program for University Prominent Young & Middle-aged Teachers and Presidents of China and Fundamental Research Funds for the Central Universities (No. 2014QNA74). References

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Please cite this article in press as: Gao P, et al., Energy matching and optimization analysis of waste to energy CCHP (combined cooling, heating and power) system with exergy and energy level, Energy (2014), http://dx.doi.org/10.1016/j.energy.2014.11.050