Performance investigation of a new geothermal combined cooling, heating and power system

Performance investigation of a new geothermal combined cooling, heating and power system

Energy Conversion and Management 208 (2020) 112591 Contents lists available at ScienceDirect Energy Conversion and Management journal homepage: www...

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Energy Conversion and Management 208 (2020) 112591

Contents lists available at ScienceDirect

Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman

Performance investigation of a new geothermal combined cooling, heating and power system

T



Jianyong Wang , Chenxing Ren, Yaonan Gao, Haifeng Chen, Jixian Dong Department of Power Engineering, College of Mechanical and Electrical Engineering, Shaanxi University of Science and Technology, Xi’an 710021, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Geothermal energy Combined cooling Heating and power system Ammonia-water absorption refrigeration cycle Exergy loss analysis Thermodynamic parameter analysis

In this paper, a new geothermal combined cooling, heating and power system that integrates flash power cycle and ammonia-water absorption refrigeration cycle, is proposed to supply electricity, refrigerant water and domestic hot water simultaneously to users. In the system, the refrigeration cycle serves as the bottom cycle of the power cycle by further utilizing the exhausted geothermal water from the flasher of the power cycle, meanwhile all waste heat of the power and refrigeration cycles is recovered for supplying heat, thus effectively improving the energy conversion efficiency of whole system. This paper establishes detailed mathematical models of the proposed system and conducts a valid model validation. Then a preliminary design condition of the system is given and the results show that the exergy efficiency of system could reach 43.69% under the condition of 170 ℃ geothermal water. An exergy loss analysis is carried out based on the design condition, demonstrating that the maximal exergy destruction exists in the condenser of flash cycle, accounting for 48.53% of the total exergy destruction of the system; the components used for separating or mixing fluids including rectification column, absorber and flasher, occupying 17.68%, 9.02% and 9.30% respectively, are prone to generate exergy destructions. Finally a thermodynamic parameter analysis, in order to assess the effects of seven key parameters on the system performance, is performed. The results show that there are an optimal flash pressure (about 300 kPa) and an optimal generator temperature (about 120 ℃) respectively that could make the exergy efficiency of system maximal. Within some scopes, lower turbine back pressure and rectification column pressure, higher ammonia concentration of ammonia-strong solution, bring about higher exergy efficiency of system. Additionally the evaporation pressure and the reflux ratio of rectifier just make little difference on the exergy efficiency of system.

1. Introduction With the large-scale consumption of fossil fuels worldwide, more and more countries are turning attention to the development and utilization of renewable energy. Geothermal energy, as a kind of renewable energy source that has the advantages of wide distribution, cleanness and direct utilization, is attracting strong attention from the energy consumers. In 2011, The UN intergovernmental panel on climate change (IPCC) published an analysis report [1], which indicates that geothermal energy is the second clean energy source after solar energy in terms of technological exploitation potential. As a result, many researchers, especially those in the countries with rich geothermal resources, have started to invest huge amounts of energy in geothermal development and utilization. Firstly, the geothermal resources are widely applied to power generation. Up till now, the geothermal power generation technology has



been evolved from the initial geothermal steam expansion directly and flash cycle to the current binary power generation cycles based on organic Rankine cycle (ORC), Kalina cycle, etc. and some combined power generation system. In recent years, there are still much works on the geothermal power generation. Yousefi et al. [2] compared two geothermal power generation systems including the single and double flash systems with two district heating applications of geothermal energy. Surindra et al. [3] compared the performance of geothermal ORC power generation system with different working fluids under two geothermal temperature. Kharseh et al. [4] investigated the optimal working parameters of a geothermal ORC power generation system to maximize the exploitation of the geothermal resource. Wang et al. [5] conducted thermodynamic analysis and optimization for a combined geothermal power generation system composed of flash cycle and Kalina cycle. Mosaffa and Zareei [6] carried out thermoeconomic analysis for a combined geothermal power generation system that couples flash

Corresponding author. E-mail address: [email protected] (J. Wang).

https://doi.org/10.1016/j.enconman.2020.112591 Received 26 November 2019; Received in revised form 4 February 2020; Accepted 7 February 2020 0196-8904/ © 2020 Elsevier Ltd. All rights reserved.

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Nomenclature E h I m P Q RR s T W x X

cool cooling capacity dhw domestic hot water exg exergy gw geothermal water heat heating capacity in input liq liquid net net power output out output rw refrigerant water s isentropic strg ammonia-strong solution thm thermal vap vapor weak ammonia-weak solution 0 ambient state 1–9, a1–a9, b1-b2, c1–c4 state point

Exergy, kW Specific enthalpy, kJ·kg−1 Exergy destruction/loss, kW Mass flow rate, kg·s−1 Pressure, kPa Energy input/output, kW Reflux ratio of rectifier Specific entropy, kJ·kg−1K−1 Temperature, ℃ Power output/consumption, kW Quality Ammonia concentration, %

Greek symbol η

Efficiency

Subscript ammo

ammonia

cycle and organic flash cycle. Manente et al. [7] proposed an integrated flash-binary power plant and a two-phase binary power plant for the geothermal resources with high content of non-condensable gases. In order to improve the utilization efficiency of geothermal resources, the development and utilization of geothermal energy is extended from the simple geothermal power generation to generating multiple kinds of energy simultaneously including power, cooling and heating [8]. Some multi-generation systems, which were often studied in recent years, employed geothermal resources as the heat source, including combined cooling and power (CCP) system, combined heating and power (CHP) system, combined cooling, heating and power (CCHP) system. Parikhani et al. [9] made a comprehensive analysis on a novel geothermal CCP system that couples an absorption power system and an absorption refrigeration system. Zhao et al. [10] proposed a new CCP system that integrates a flash-binary power generation system with a bottom CCP subsystem combining an ORC and an ejector refrigeration cycle and performed exergoeconomic analysis on the system. Ghaebi et al. [11] carried out single- and multi-objective optimizations to get the optimum design variables of a geothermal CCP system based on Kalina and ejector refrigeration cycles. Heberle and Brüggemann [12] conducted working fluid selection for two ORC-based geothermal CHP systems, including series and parallel circuits of ORC and heating generation system. Habka and Ajib [13] investigated the operation characteristics of two configurations of geothermal CHP systems, which are the parallel and integration of ORC and heating system. Van Erdeweghe et al. [14] conducted comparative study for two geothermal CHP systems that considers the series and parallel configurations of an ORC and a district heating system. Fiaschi et al. [15] considered an innovative ORC layout for heating and power generation from geothermal resources. Li et al. [16] analyzed the series and parallel circuit geothermal CHP and oil recovery combined system. Wang et al. [17] carried out thermodynamic analysis for a new CHP system using geothermal resources that integrates a superheated Kalina cycle and an ammonia-water compression heat pump cycle. Zare [18] made a comparative thermodynamic analysis for two different designs of geothermal CCHP systems, which integrate the ORC and Kalina cycle respectively with a LiBr/water absorption chiller and a water heater. Pastor-Martinez et al. [19] assessed and compared several different configurations of CCHP systems, which considered variations of series and parallel coupling schemes along with different alternatives of ORC and absorption cooling machines.

Besides power, cooling and heating, there are some other products that can be generated simultaneously in the process of utilization of geothermal resources, such as hydrogen, water and so on. Akrami et al. [20] made a comprehensive analysis for a geothermal multi-generation system composed of ORC, domestic water heater, absorption refrigeration cycle and proton exchange membrane (PEM) electrolyzer to generate cooling, heating, power and hydrogen simultaneously. Using geothermal water as heat source and LNG as heat sink, Emadi and Mahmoudimehr [21] proposed a multi-generation system that couples a cascade of two ORC, an absorption refrigeration cycle, a domestic water heat exchanger and a PEM electrolyzer. Ebadollahi et al. [22] proposed a multi-generation system by combining an ORC, an ejector refrigeration cycle, an LNG power generation system and a PEM electrolyzer system to generate the same products with the last system. Mohammadi and Mehrpooya [23] made energy and exergy analysis for a combined desalination and CCHP system driven by geothermal energy, which couples a flash power generation system and a CCP system proposed in reference [24]. He et al. [25,26] proposed two geothermal power and water production combined systems, of which one couples an organic Rankine cycle (ORC) and a desalination unit, and the other integrates a flash cycle and a desalination unit. Calise et al. [27] used geothermal energy for thermal drying of wastewater sludge and electricity supply of the whole wastewater treatment. Han et al. [28] investigated an ammonia-water absorption cycle long-distance cooling and heating system by using geothermal resources. It is known that the temperature of geothermal fluids is not very high generally, resulting in low energy conversion efficiency of the connected thermodynamic systems. Therefore, in order to enhance the energy conversion efficiency of thermodynamic systems, some other heat sources are proposed to combine with the geothermal resources to drive the thermodynamic systems, such as solar energy, biomass energy and so on. Rostamzadeh et al. [29,30] considered combining geothermal energy and biomass energy for multi-generation including power, cooling, heating, hydrogen and freshwater generation. They conducted 4E (Energy, exergy, exergoeconomic and environmental) analysis for the complex system. Zhang et al. [31] proposed a new CCHP system based on biomass energy, geothermal energy and natural gas, in which the geothermal energy mainly provides energy for the heat pump subsystem. Ciani Bassetti et al. [32] studied a hybrid geothermal-solar organic Rankine cycle power generation system. Bonyadi et al. [33] conducted techno-economic and exergy analysis for a 2

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absorber is abundant, but these heat is generally not recovered to use due to its lower temperature, resulting in a huge waste of energy. In addition, the ARC in Fig. 1 has another characteristic that the solution heat exchanger (SHX) is equipped to recover the waste heat of the fluid exhausted from the generator, thus alleviating the heating load of the generator and the cooling load of the absorber. The ARC is often employed as a bottom cycle to recover the waste heat of a top cycle, so there is a question that how to use the ARC to recycle the waste heat of the top cycle as efficiently as possible. In this paper, a method is proposed to improve the energy recovery efficiency of the ARC. Firstly, the operating parameters of the ARC is regulated to enlarge the temperature difference of the released heat in the condenser and the absorber, i.e. making the average temperature of releasing heat process in the condenser less than that in the absorber. Therefore a stream of cool fluid could be set to absorb the waste heat of the condenser and the absorber to generate warm fluid by flowing through these two heat exchangers successively. Although the temperature of the obtained warm fluid is lower, this heating process could be the preheating parts before some other heating processes. Secondly, the SHX is omitted so that more energy could be absorbed by the generator from the waste heat of the top cycle and the absorber could also release more energy for the heating process, which not only improves the energy utilization efficiency of the whole combined cycle, but also reduces the number of heat transfer devices. The schematic diagram of the modified ARC with waste heat recovery function in the P-T coordinates is shown in Fig. 2.

combined power generation system coupling steam Rankine cycle and ORC driven by solar and geothermal energy. Calise et al. [34] designed a hybrid solar-geothermal poly-generation system to supply electricity, heating, cooling and fresh water for a small community. Islam and Dincer [35] also developed a combined solar and geothermal energybased multi-generation system to provide power, heating, cooling and drying for users. As can be seen from the literature review, most of the proposed CCHP systems driven by geothermal resources are mainly some comprehensive systems that integrate multiple single-output subsystems. These systems are characterized by a large number of devices and complex structures, resulting in large heat loss in the systems and low energy conversion efficiency of the systems. In this paper, a new geothermal CCHP system with simple structure, coupling flash power cycle and ammonia-water absorption refrigeration cycle, is proposed to supply electricity, refrigerant water and domestic hot water simultaneously to users. In the proposed system, all of the waste heat of the flash power cycle and the absorption refrigeration cycle is recovered for supplying heat, efficiently reducing the energy loss from the system to the environment. In this paper, the mathematical models of the proposed system are established in detail, and then an exergy loss analysis and a thermodynamic parameter analysis are carried out to investigate the exergy destruction/loss distribution in the system and the effects of key parameters on the system performance based on a preliminary design condition, respectively. 2. System proposal and description

2.2. Proposal and description of new system

In this section, the way to recover the waste heat of absorption refrigeration cycle is discussed firstly, then a new geothermal combined cooling, heating and power (Geo-CCHP) system is proposed and described in detail.

In this paper, the ARC is employed to recover the waste heat of a single-stage geothermal flash power cycle, thus constituting a geothermal combined cooling, heating and power (Geo-CCHP) system, which is illustrated in Fig. 3. It can be seen that although this system is mainly comprised of a power cycle and a refrigeration cycle, the system can not only produce power and cooling, but also generate heating by means of recover the waste heat of the power and refrigeration cycles. In this paper, the ARC of the Geo-CCHP system adopts ammoniawater as the working fluid in order to achieve cooler refrigeration temperature. As is known that the boiling point of ammonia is close to that of water, so the refrigerant ammonia vapor produced in the generator would be mixed with much steam, which has negative influence

2.1. Waste heat recovery of absorption refrigeration cycle Fig. 1 shows the schematic diagram of an absorption refrigeration cycle (ARC) in a pressure-temperature (P-T) coordinates. As can be seen in the ARC, the generator and the evaporator absorb heat from the heat source and the source needing refrigeration respectively, while the condenser and the absorber release heat to the environment. As is known, the amount of the released heat from the condenser and the

Fig. 1. Schematic diagram of absorption refrigeration cycle (ARC) in P-T coordinates. 3

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P

cool fluid

Pc=Pg

Qc

Qg

condenser

generator

valve

Pe=Pa

valve

evaporator

absorber

Qe

Qa

Te

Tc

pump

warm fluid

T Tg

Ta

Fig. 2. Schematic diagram of the modified ARC with waste heat recovery function in P-T coordinates.

In the geothermal water loop, the geothermal water is firstly extracted from the underground and is sent to a flasher (FLS), where the water is throttled to a lower pressure to generate two-phase fluids, and then the two-phase fluids is separated to steam and liquid. The steam is sent to a steam turbine (TUB) to generate power, whereas the liquid is delivered to the generator (GEN) of a rectification column (RCL) as the heat source for the rectification process. After releasing heat, the liquid is throttled by a valve (VAL-I) to reduce pressure and then mix with the turbine exhaust. The mixed fluid continues to enter a condenser (CND-I) to release heat and finally is reinjected to the underground. In the ammonia-water loop, a stream of ammonia-strong solution is

on the refrigeration performance. Therefore, a rectifier is added to the generator to generate highly purified ammonia as the refrigerant and here the rectifier and the generator compose the rectification column. Additionally the reflux fluid of the rectifier is provided by the parts of the condensed ammonia fluid at the outlet of the condenser. If the system is viewed from the aspects of working fluids, the system contains four loops, including the loops of geothermal water (red lines), ammonia-water (black lines), domestic hot water (blue lines) and refrigerant water (green lines). It is noted that the loops of domestic hot water and refrigerant water are open loops because the parts of the loops that connect users are omitted in this paper.

3 CND-II

FLS

c1

2

1

RTF

c2

a1

a3

a2

a8

RCL

5 GEN PUP

7

VAL-I

VAL-II

CND-I

c3

ABS

b1

EVP b2 Users

Domestic hot water

Ammonia-water

Refrigerant water

CND GEN RTF

Condenser Generator Rectifi Rectifier f er

EVP PUP TUB

Evaporator Evap a orator Pump Turb Turbine r ine

Fig. 3. Geothermal combined cooling, heating and power (Geo-CCHP) system. 4

c4

Users

9

Geothermal water

Absorber Absorb r er Flasher Rectification Rectifi f cation column Valve

8

c3 a6

c2

ABS FLS RCL VAL

a5

a9

a7

Geothermal Well

4

VAL-III

6

a4

G

TUB TUB

Geothermal Well

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The process of the geothermal water flowing through the flasher can be regarded as the geothermal water flowing through a valve and a separator in series. Therefore, the geothermal water is throttled to a lower pressure to generate two-phase mixtures firstly and then the mixtures are separated to saturated vapor and saturated liquid. The mathematical expressions of the processes are shown as follows.

delivered into the rectification column (RCL), then undergoes a series of volatilization, stripping and rectification processes, and finally generates ammonia vapor in the rectifier (RTF) and ammonia-weak solution in the generator (GEN). The ammonia vapor is condensed to liquid in a condenser (CND-II) and then is divided into two streams. One stream is sent back to the rectifier (RTF) as the required reflux fluid for the rectification process. The other stream passes through a throttle valve (VAL-III) to drop its pressure and generate low-temperature twophase ammonia. The low-temperature ammonia acting as refrigerant goes into an evaporator (EVP) to absorb heat to produce refrigerant water of 5 ℃, and afterwards the formed ammonia vapor is delivered to an absorber (ABS). The ammonia-weak solution from the generator (GEN) is throttled through a valve (VAL-II) and then also goes into the absorber (ABS). The ammonia vapor is absorbed by the ammonia-weak solution in the absorber (ABS) and meantime the mixed solution (i.e. ammonia-strong solution) is cooled to liquid. Finally the ammoniastrong solution is pressurized to a high pressure by a pump (PUP) and is transported back to the rectification column (RCL), completing the ammonia-water absorption refrigeration cycle. As for the domestic hot water loop, firstly the water under ambient temperature working as cooling water enters the condenser (CND-II) to absorb heat from the ammonia vapor, then it goes into the absorber (ABS) to absorb the released heat of absorption process, and finally the water enters the condenser (CND-I) to continue enhancing temperature to 70 ℃ by absorbing the remnant heat of the geothermal water before reinjected to the underground. With regards to the refrigerant water loop, the water under ambient temperature is directly cooled to 5 ℃ by the low-temperature ammonia in the evaporator (EVP), thus generating refrigerant water used for central air-conditioning or others.

h1 = h2

(1)

mgw = m vap + mliq

(2)

mgw h2 = m vap h3 + mliq h5

(3)

where mgw is the mass flow rate of the geothermal water; mvap is the mass flow rate of the steam produced from the flasher; mliq is the mass flow rate of the liquid geothermal water exhausted from the flasher. (2) Turbine (TUB) In the turbine, the steam is expanded to generate work, and an isentropic efficiency is employed to describe the non-isentropic expansion process.

ηTUB =

h3 − h4 h3 − h4,s

(4)

The power output of the turbine is expressed as

WTUB = m vap (h3 − h4 )

(5)

(3) Rectification column (RCL) For the rectification column, in order to obtain the thermodynamic states of the inlets and outlets, this paper focuses on the fluids (ammonia-water and geothermal water) and the heat that gets in or out of the rectification column, and neglects the details of the rectification process. In the rectification column, the geothermal water provides heat in the generator; the ammonia-strong solution goes into the middle of the rectification column; the pure ammonia vapor (99.98%) and the ammonia-weak solution is generated in the top and bottom of the rectification column (i.e. the rectifier and the generator) respectively; additionally some of the condensed ammonia liquid is sent back to the rectifier as the reflux fluid for the rectification purpose. The laws of mass and energy conservation are applied and the mathematical expressions can be given as follows.

3. Mathematical model and performance criteria The mathematical models of the Geo-CCHP system are built on the basis of the laws of energy and mass conservation. At the beginning, some reasonable assumptions are made to simplify the theoretical models of the components in the system, which are shown as follows. (1) Each state point reaches a steady state in the system. (2) There is no pressure drop in heat exchangers, flasher, rectification column and connection pipes. (3) The heat loss of the components, i.e. the heat transfer between components and environment, is neglected. (4) The vapor and the liquid produced in the flasher (FLS) are saturated vapor and saturated liquid, respectively. (5) The ammonia at the outlet of rectifier (RTF) and the ammonia-weak solution at the outlet of generator (GEN) are saturated vapor and saturated liquid, respectively. (6) The refrigerant ammonia at the outlet of condenser (CND-II) and evaporator (EVP) are saturated liquid and saturated vapor, respectively. (7) The fluids flowing through the valves are isenthalpic depressurization process. (8) The generated ammonia vapor in the rectifier of rectification column is assumed to be the ammonia-water mixture with a concentration of 99.98%. (9) The turbine and the pump have a prescribed isentropic efficiency, respectively.

mstrg + mammo ·RR = mammo + m weak

(6)

mstrg Xstrg + mammo ·RR·Xammo = mammo Xammo + m weak Xweak

(7)

mliq h5 + mstrg ha8 + mammo ·RR·ha2 = mliq h6 + mammo ha1 + m weak ha4 (8) where mstrg is the mass flow rate of the ammonia-strong solution into the rectification column; mammo is the mass flow rate of the produced ammonia vapor out of the rectifier; mweak is the mass flow rate of the produced ammonia-weak solution out of the generator; RR is the reflux ratio; Xstrg, Xammo and Xweak are the ammonia concentration of the ammonia-strong solution, the ammonia vapor/liquid and the ammoniaweak solution respectively. (4) Absorber (ABS)

As shown in Fig. 3, the Geo-CCHP system is composed of three heat exchangers, a flasher, a rectification column, an absorber, a turbine, a pump and three valves. Based on the above assumptions and the laws of mass and energy conservation, the mathematical model of each component of the system can be described as below.

In the absorber, the ammonia-weak solution absorbs the ammonia vapor and meanwhile releases heat to the domestic hot water. Finally the ammonia-strong solution is re-formed and sent to the rectification column. Besides the equations that abide by the law of mass conservation (Eqs. (6) and (7)) are used, an equation that abides by the law of energy conservation is needed, which can be expressed as follows.

(1) Flasher (FLS)

mdhw hc2 + mammo (1 − RR) ha6 + m weak ha9 = mdhw h c3 + mstrg ha7 5

(9)

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where mdhw is the mass flow rate of the domestic hot water.

only from the geothermal water but also from the refrigerant water in the evaporator (EVP). In consequence, if using the traditional thermal efficiency to estimate the performance of the system, the thermal efficiency of Geo-CCHP system would exceed 100%, which seems illogical. Therefore, in order to assess the utilization degree of the geothermal water by the Geo-CCHP system objectively, the exergy efficiency on the basis of the second law of thermodynamics is selected as the performance indicator. The exergy efficiency of the Geo-CCHP system can be expressed as

(5) Pump (PUP) In the pump, the ammonia-strong solution is boosted to a highpressure state, and an isentropic efficiency is used to describe the nonisentropic process.

ηPUP =

ha8,s − ha7 ha8 − ha7

(10)

ηexg =

The power consumption of pump is given by

WPUP = mstrg (ha8 − ha7)

(24)

(11)

(6) Heat exchangers When establishing the thermodynamic models of heat exchangers, the law of energy conservation is mainly used. In this paper, all of the heat exchangers employs the counter-flow type heat exchangers. It should be noted that when calculating the heat transfer process, the terminal temperature difference of heat exchangers should not be less than 5 ℃ to avoid overlarge heat transfer areas. Evaporator (EVP)

mammo (1−RR)(ha6 − ha5) = mrw (hb1 − hb2)

Wnet + Eheat + Ecool Ein

Condensers (CND-I and CND-II) (13)

mammo (ha1 − ha3) = mdhw (hc2 − hc1)

(14)

(25)

Ecool = Eb2 − Eb1

(26)

Ein = E1

(27)

where Eheat and Ecool are the exergy of heating capacity and cooling capacity of the Geo-CCHP system respectively; Ein is the exergy input to the system, which is identified as the exergy of the geothermal water extracted from the underground in this paper. According to the definition, the exergy of a state point is the maximum useful work possible during a process that brings the state point into equilibrium with the environment. When calculating the exergy of steady flow, only the physical exergy is taken into account, whereas the kinetic exergy, potential exergy and chemical exergy of the fluids are ignored. The exergy of steady flow at a certain state point can be denoted by

(12)

mgw (h8 − h9) = mdhw (h c4 − h c3)

Eheat = Ec4 − Ec1

where mrw is the mass flow rate of the refrigerant water.

E = m [(h − h 0 ) − T0 (s − s0 )]

(7) Valves (VAL-I, VAL-II and VAL-III)

where T0 is the ambient temperature; h0 and s0 are the specific enthalpy and entropy of fluids under the conditions of ambient temperature and pressure. The exergy analysis on the basis of exergy concept would show the exergy distribution of components in the system from the aspects of exergy utilization, consumption and loss, and consequently point out some feasible directions to improve the energy conversion efficiency of the thermodynamic system. This paper will conduct an exergy analysis for the Geo-CCHP system. Firstly the exergy balance equations should be established for each component of the system, which can be expressed as the following equation in general terms.

As to the valves, isenthalpic processes are assumed and they are denoted by

h6 = h 7

(15)

ha4 = ha9

(16)

ha3 = ha5

(17)

Moreover, the process of mixing two streams of fluids to one stream (point 8) complies with the law of energy conservation.

m vap h4 + mliq h7 = mgw h8

EQ +

(18)

In this paper, the Geo-CCHP system converts geothermal energy to power, cooling and heating energy. Generally the thermal efficiency on the basis of the first law of thermodynamics is often used to assess the performance of thermodynamic systems. For the Geo-CCHP system, the thermal efficiency can be written as

ηthm =

Wnet + Q heat + Qcool Qin

(20)

Q heat = mdhw (hc4 − hc1)

(21)

Qcool = mrw (hb1 − hb2)

(22)

Qin = mgw (h1 − h 9)

(23)

(29)

where EQ is the heat exergy input to the component from the environment; ∑Ein is the total amount of fluid exergy into the component; W is the power output/consumption of the component, which is positive for power output and negative for power consumption; ∑Eout is the total amount of fluid exergy out of the component; I is the exergy destruction of the component. The assumptions that are made before establishing the mathematical models stipulate that the heat transfer between the components and the environment is neglected, which indicates that the term EQ of each component equals zero in the Geo-CCHP system. Therefore, the exergy destruction of each component of the Geo-CCHP system can be obtained by the general formula of the exergy balance equations, which are shown in Table 1.

(19)

Wnet = WTUB − WPUP

∑ Ein = W + ∑ Eout + I

(28)

4. Results and discussion

where Wnet, Qheat and Qcool are net power output, heating capacity and cooling capacity of the Geo-CCHP system respectively; Qin is the heat absorption of system from the geothermal water. As can be seen from the schematic diagram of the Geo-CCHP system (Fig. 3) that the heat that should have been exhausted to the environment in the condenser II (CND-II) and absorber (ABS) is recovered to heat the domestic water, and moreover the system absorbs heat not

In this section, the mathematical models of the proposed system in the last section are validated firstly, then the preliminary design results of the system are given in several tables, and finally an exergy loss analysis and a thermodynamic parameter analysis are conducted to assess the system performance. 6

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4.2. Preliminary design results

Table 1 Exergy destruction of each component in Geo-CCHP system. Component

Exergy destruction

Flasher (FLS) Turbine (TUB) Rectification column (RCL) Absorber (ABS) Evaporator (EVP) Condenser-I (CND-I) Condenser-I (CND-II) Pump (PUP) Valve-I (VAL-I) Valve-II (VAL-II) Valve-III (VAL-III)

IFLS = E1 − E3 − E5 ITUB = E3 − E4 − WTUB IRCL = Ea8 + Ea2 + E5 − Ea1 − Ea4 − E6 IABS = Ea6 + Ea9 + Ec2 − Ea7 − Ec3 IEVP = Ea5 + Eb1 − Ea6 − Eb2 ICND - I = E8 + Ec3 − E9 − Ec4 ICND - II = Ea1 + Ec1 − Ea2 − Ea3 − Ec2 IPUP = Ea7 + WPUP − Ea8 IVAL - I = E6 − E7 IVAL - II = Ea4 − Ea9 IVAL - III = Ea3 − Ea5

In this section, a numerical simulation calculation is conducted for the preliminary design condition of the Geo-CCHP system to demonstrate the feasibility of the proposed system. The MATLAB software is selected as the calculation platform and the REFPROP [38] software are called to obtain the thermophysical properties of the fluids in the system. The specific setting conditions of the simulation are displayed in Table 4. The thermodynamic parameters of each state point and the corresponding thermodynamic performance of the Geo-CCHP system at the simulation conditions are listed in Tables 5 and 6, respectively. It can be seen from Table 6 that the thermal efficiency of Geo-CCHP system is more than 100% as predicted, reaching 110.68%, which indicates that the Geo-CCHP system not only transforms all the heat absorbed from the geothermal water to useful energy, but also utilizes the heat absorbed in the evaporator to further increase the energy conversion efficiency of the system. Although the thermal efficiency exceeds 100%, the overall energy conversion of the system conforms to the law of energy conservation (Qin + Qcool = Wnet + Qheat). Since the thermal efficiency could not measure the actual energy utilization degree of the geothermal water, the following thermodynamic parameter analysis would employ the exergy efficiency as the performance indicator of the Geo-CCHP system.

4.1. Model validation In this paper, the model validation is conducted by comparing the data obtained from the mathematical models and program codes of this paper with the data in the published references. The Geo-CCHP system consists of a flash cycle and an absorption refrigeration cycle, so the model validation is carried out for the two cycles respectively. For the flash cycle, some thermodynamic properties of the state points 1–5 of the Geo-CCHP in Fig. 3 are checked by comparing with the data in the reference [36] under the same boundary conditions. For the absorption refrigeration cycle, the data of an ARC used as a bottom cycle of a supercritical CO2 Brayton cycle in the reference [37] is selected for the model validation. It is worth noting that the ARC in Ref. [37] has one more solution heat exchanger than the ARC of the Geo-CCHP system in this paper, and moreover the ARC in Ref. [37] doesn’t consider the reflux fluid from the condenser to the rectifier, so the schematic diagram and the mathematical models of the ARC in this paper are modified slightly when doing the model validation. The schematic diagram of the ARC for model validation is shown in Fig. 4. Tables 2 and 3 present the data comparison of this paper and the reference for model validation of the flash cycle and the absorption refrigeration cycle respectively. It can be seen from the tables that the data obtained from this work is highly consistent with the data in Ref. [36] and [37], which demonstrates the accuracy of the mathematical models and program codes in this paper.

Generator

Rectifier

4.3. Exergy loss analysis An exergy loss analysis is carried out based on the simulation results of the preliminary design condition listed in Table 5 to show the exergy destruction/loss distribution in the Geo-CCHP system. As is shown in Fig. 5, the maximal exergy destruction exists in the condensers, reaching 50.76% of the total exergy loss of the Geo-CCHP system, of which condenser I and condenser II occupy 48.53% and 2.23% respectively. The large exergy destruction in condenser I is mainly caused by large heat transfer temperature difference and large mass flow rate of fluids during the process of heat exchange. Utilizing high-efficiency heat exchangers to decrease temperature difference of heat transfer is a helpful method to reduce the exergy destructions in the heat exchangers. The exergy destructions in the rectification column, absorber and flasher also make up a large portion of the total exergy destructions of the Geo-CCHP system, accounting for 17.68%, 9.02% and 8.30% respectively, which indicates that the components used for separating or mixing fluids are prone to generate exergy destructions. The exergy destructions in other components as well as the exergy loss of the

1

Condenser 2

7

8

Solution Heat Exchanger

Valve

6

9 Valve

Pump

10

3

5 4

Absorber

Fig. 4. Absorption refrigeration cycle for model validation. 7

Evaporator

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Table 2 Data comparison between this paper and Ref. [36] for model validation of flash cycle. State Point

1 2 3 4 5

T/℃

h/kJ·kg−1

P/kPa

m/kg·s−1

This paper

Ref. [36]

Error/%

This paper

Ref. [36]

Error/%

This paper

Ref. [36]

Error/%

This paper

Ref. [36]

Error/%

230.00 162.98 162.98 98.58 162.98

230 163 163 98.58 163

0 0.01 0.01 0 0.01

2797.09 666.50 666.50 96.40 666.50

2795 666.5 666.5 96.4 666.5

−0.07 0 0 0 0

990.19 990.19 2760.67 2531.05 688.43

990 990 2761 2531 688.7

−0.02 −0.02 0.01 −0.00 0.04

1.0000 1.0000 0.1456 0.1456 0.8544

1 1 0.1454 0.1454 0.8546

0 0 −0.14 −0.14 0.02

reinjected geothermal water are relatively much less than the exergy destructions of the above-mentioned components and little improvement space is left to decrease these exergy destructions.

Table 4 Setting conditions for numerical simulation of Geo-CCHP system.

4.4. Thermodynamic parameter analysis As is shown in the schematic diagram of the Geo-CCHP system (Fig. 3) that the flash pressure in the flasher directly affects the thermodynamic parameters of the vapor and liquid produced in the flasher, which will impact the power output of turbine and the performance of the ammonia-water absorption refrigeration cycle. The turbine back pressure influences the enthalpy drop of turbine and the turbine outlet temperature, directly affecting the turbine power output and the system heating capacity. The rectification column pressure has significant effects on the mass flow rate and temperature of the ammonia vapor produced in the rectifier, which directly affect the system cooling capacity. The generator temperature in the rectification column also influences the performance of the whole ammonia-water absorption refrigeration cycle. The evaporation pressure of ammonia in the evaporator has great effects on the system cooling capacity directly. The ammonia concentration of ammonia-strong solution has important effects on the whole system performance. Additionally the reflux ratio of rectifier also has influences on the system cooling capacity by affecting the mass flow rate of ammonia into the evaporator. As a consequence, a detailed thermodynamic parameter analysis is conducted to investigate the effects of the above-mentioned seven key thermodynamic parameters on the performance of the Geo-CCHP system based on the operation conditions simulated in the last section. It is worth noting that only the studied parameter varies during the process of thermodynamic parameter analysis, while the other parameters maintain unchanged.

Term

Value

Unit

Geothermal water temperature Geothermal water pressure Geothermal water mass flow rate Ambient temperature Domestic hot water temperature Refrigerant water temperature Flasher pressure Turbine back pressure Rectification column pressure Generator temperature Evaporation pressure Ammonia concentration of ammonia-strong solution Isentropic efficiency of turbine Isentropic efficiency of pump Minimal temperature difference of heat exchangers

170 900 30 25 70 5 300 100 1500 100 400 45 80 70 5

°C kPa kg·s−1 °C °C °C kPa kPa kPa °C kPa % % % °C

the flasher increases from 111.35 ℃ to 155.46 ℃. The mass flow rate of the saturated vapor drops from 3.40 kg·s−1 to 0.91 kg·s−1 and the mass flow rate of the saturated liquid rises from 26.60 kg·s−1 to 29.09 kg·s−1 as a result. The enthalpy drop of turbine increases because of the increased turbine inlet pressure and temperature. Therefore in Fig. 6(b), the power output of turbine presents a trend of rising from 186.95 kW firstly, reaching 326.86 kW and then falling to 210.36 kW under the influence of the decreased mass flow rate of saturated vapor and the increased enthalpy drop of turbine, indicating that an optimal flash pressure about 300 kPa can be reached to yield the maximal power output of turbine. The saturated liquid produced in the flasher with increased temperature and mass flow rate drives more ammonia-strong solution (from 1.53 kg·s−1 to 13.43 kg·s−1) available working in the ammonia-water absorption refrigeration cycle, hence the ammonia vapor generated in the rectification column gains in mass flow rate from 0.21 kg·s−1 to 1.85 kg·s−1, bringing about an increase in the mass flow rate of refrigerant water from 2.21 kg·s−1 to 19.42 kg·s−1, which can be seen in Fig. 6(c). The power consumption of pump increases from 2.82 kW to 24.77 kW (Fig. 6(b)) on account of the increased mass flow rate of ammonia-strong solution. Since the power output of turbine is

4.4.1. Flash pressure Fig. 6 shows the effect of the flash pressure on the system performance. As can be seen, the flash pressure mainly affects the performance of the geothermal flash power cycle and then indirectly affects the performance of the ammonia-water absorption refrigeration cycle further. In Fig. 6(a), with the increase of flash pressure from 150 kPa to 550 kPa, the temperature of the saturated vapor and liquid produced in

Table 3 Data comparison between this paper and Ref. [37] for model validation of absorption refrigeration cycle. State Point

1 2 3 4 5 6 7 8 9 10

T/℃

h/kJ·kg−1

P/kPa

X/%

This paper

Ref. [37]

Error/%

This paper

Ref. [37]

Error/%

This paper

Ref. [37]

Error/%

This paper

Ref. [37]

Error/%

109.36 33.00 0.00 0.00 33.00 33.16 72.58 109.36 48.40 48.55

109.36 33 0 0 33 33.08 72.57 109.36 48.34 48.49

0 0 0 0 0 −0.24 −0.01 0 −0.12 −0.12

1274.89 1274.89 429.38 429.38 429.38 1274.89 1274.89 1274.89 1274.89 429.38

1275 1275 429 429 429 1275 1275 1275 1275 429

0.01 0.01 −0.09 −0.09 −0.09 0.01 0.01 0.01 0.01 −0.09

1841.45 499.47 499.47 1605.39 70.46 71.84 274.51 386.61 106.16 106.16

1841.45 499.47 499.47 1605.39 70.46 71.50 274.36 386.61 105.90 105.90

0 0 0 0 0 −0.47 −0.05 0 −0.24 −0.24

100.00 100.00 100.00 100.00 50.38 50.38 50.38 31.33 31.33 31.33

100 100 100 100 50.38 50.38 50.38 31.33 31.33 31.33

0 0 0 0 0 0 0 0 0 0

8

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much more than the power consumption of pump, the net power output of system presents a similar variation with the power output of turbine (i.e. increases from 184.13 kW to 313.55 kW, and then decreases to 185.60 kW), as shown in Fig. 6(b). It can be seen in Table 5 that the domestic hot water obtains its majority of temperature rise in the condenser I, which indicates that the geothermal water provides much energy for the domestic hot water. As the flash pressure increases, more geothermal energy is converted to power output of turbine or used to heat the ammonia-water in the rectification column, so the part of geothermal energy for heating the domestic hot water drops. In order to maintain the final temperature of domestic hot water, the mass flow rate of domestic hot water has to decreases from 93.04 kg·s−1 to 87.89 kg·s−1, as presented in Fig. 6(c). In Fig. 6(d), the exergy of heating capacity of the Geo-CCHP system decreases from 1202.33 kW to 1135.72 kW, while the exergy of cooling capacity increases from 6.52 kW to 57.20 kW, due to the decreased mass flow rate of domestic hot water and the increased mass flow rate of refrigerant water. According to the definition in this paper, the exergy input to system keeps unchanged. The exergy efficiency of the Geo-CCHP system shows a trend of increasing from 40.37% to 43.69% firstly and then decreasing to 39.95% by calculation.

Table 5 Thermodynamic parameters of each state point of Geo-CCHP system. State

T/℃

P/kPa

h/kJ·kg−1

s/kJ·kg−1K−1

x

X/%

m/kg·s−1

1 2 3 4 5 6 7 8 9 a1 a2 a3 a4 a5 a6 a7 a8 a9 c1 c2 c3 c4 b1 b2

170.00 133.52 133.52 99.61 133.52 105.00 99.61 99.61 41.30 41.56 38.71 38.71 100.00 −1.87 8.25 37.08 37.29 65.30 25.00 27.94 36.30 70.00 25.00 5.00

900.00 300.00 300.00 100.00 300.00 300.00 100.00 100.00 100.00 1500.00 1500.00 1500.00 1500.00 400.00 400.00 400.00 1500.00 400.00 101.30 101.30 101.30 101.30 101.30 101.30

719.14 719.14 2724.88 2575.42 561.43 440.41 440.41 596.05 173.07 1643.85 528.42 528.42 353.84 528.42 1630.57 58.28 60.13 353.84 104.92 117.21 152.17 293.12 104.92 21.12

2.04 2.06 6.99 7.09 1.67 1.36 1.36 1.78 0.59 5.68 2.10 2.10 1.79 2.17 6.22 0.96 0.97 1.83 0.37 0.41 0.52 0.96 0.37 0.08

0.00 0.07 1.00 0.96 0.00 0.00 0.01 0.08 0.00 1.00 0.00 0.00 0.00 0.15 1.00 0.00 0.00 0.11 / / / / / /

/ / / / / / / / / 99.98 99.98 99.98 38.21 99.98 99.98 45.00 45.00 38.21

30.00 30.00 2.19 2.19 27.81 27.81 27.81 30.00 30.00 0.99 0.20 0.79 6.43 0.79 0.79 7.22 7.22 6.43 90.03 90.03 90.03 90.03 10.44 10.44

/ / / / /

4.4.2. Turbine back pressure Fig. 7 shows the effect of the turbine back pressure on the system performance. The turbine back pressure has no influence on the mass flow rate of the saturated vapor and liquid produced in the flasher. The enthalpy drop of turbine decreases from 260.19 kJ·kg−1 to 87.94 kJ·kg−1 with the increase of the turbine back pressure from 40 kPa to 160 kPa, so the power output of turbine drops from 569.04 kW to 192.31 kW, as shown in Fig. 7(a). Since the mass flow rate and temperature of the saturated liquid produced in the flasher keeps unchanged, the ammonia-water absorption refrigeration cycle works at the preliminary operation conditions all the time, resulting in the power consumption of pump and the mass flow rate of refrigerant water remaining constant at 13.32 kW and 10.44 kg·s−1 respectively, as presented in Fig. 7(a) and (b) respectively. As the turbine back pressure increases, the net power output of system presents a same trend with the power output of turbine (i.e. decreasing from 555.73 kW to 179.00 kW), as show in Fig. 7(a). The increased turbine back pressure leads to an increase in the temperature of turbine exhaust, so the geothermal water could provide more energy in the condenser I, which

Table 6 Performance of Geo-CCHP system at simulation conditions. Term

Value

Unit

Wtb Wp Wnet Qheat Eheat Qcool Ecool Qin Ein ηthm ηexg

326.86 13.32 313.54 16943.59 1163.39 874.91 30.76 16382.22 3450.74 110.68 43.69

kW kW kW kW kW kW kW kW kW % %

2.72%

0.20%

9.02%

3.36% 17.68%

50.76% CND-I 48.53% CND-II 2.23%

8.30%

2.77% 5.19% Flasher Pump Condenser

Rectification column Evaporator Valve

Turbine Absorber Reinjected geothermal water

Fig. 5. Exergy destruction/loss distribution in Geo-CCHP system. 9

Energy Conversion and Management 208 (2020) 112591

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160

350

30 28

150

300

26

250

TFLS

6

mvap

120

mliq

4

110 100 100

200

(a)

300

400

500

W/kW

130

-1

24

m/kg·s

TFLS/kPa

140

200

WTUB 150

WPUP

40

Wnet

2

20

0 600

0 100

PFLS/kPa

200

300

(b)

500

600

PFLS/kPa

1300

100

16

400

48

95

85

25 20

200

Eheat

40

Ecool

15

4

100

10

/%

42

exg

mrw

1100

E /kW

80

mdhw

8

44

-1

mammo

mdhw, mrw/kg·s

mstrg, mammo/kg·s

-1

mstrg

46

1200

90

12

exg

38

5 0 100

200

(c)

300

400

500

0

0 600

100

200

300

(d)

PFLS/kPa

400

36 600

500

PFLS/kPa

Fig. 6. Effect of flash pressure on system performance.

constant at 30.76 kW. Because the variation range of the net power output of system is much larger than that of the exergy of heating and cooling capacity of system, the exergy efficiency of system decreases from 50.19% to 40.08%, as similar as the net power output of system.

means more domestic hot water could be obtained (i.e. increasing from 88.63 kg·s−1 to 90.80 kg·s−1), as shown in Fig. 7(b). In Fig. 7(c), the exergy of heating capacity of the Geo-CCHP system increases from 1145.35 kW to 1173.37 kW, and the exergy of cooling capacity keeps

95

10

52

1200

WTUB 8

Wnet

Ecool

1180

exg

85

200

mdhw

15

mrw

10

100

5

10 0

0

0 40

(a)

60

80

100 120 140 160

PTUB,back/kPa

46

1160

44 1140 42 40

2

20

48

exg

80

mammo 4

50

-1

mstrg

6

E /kW

300

mdhw, mrw/kg·s

mstrg, mammo/kg·s

-1

400

W/kW

Eheat

90

WPUP

500

40

(b)

60

80

40

30

38

20

100 120 140 160

PTUB,back/kPa

Fig. 7. Effect of turbine back pressure on system performance. 10

/%

600

40

(c)

60

80

100 120 140 160

PTUB,back/kPa

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system, as presented in Fig. 9(a). The mass flow rate of refrigerant water shows a similar variation (increasing from 7.00 kg·s−1 to 10.44 kg·s−1 firstly, and then decreasing to 4.10 kg·s−1) with the mass flow rate of ammonia vapor (Fig. 9(b)). Since the saturated liquid from the flasher releases less heat in the generator of rectification column, the geothermal water into the condenser I could provide more heat for the domestic hot water and as a result the mass flow rate of domestic hot water increases from 86.62 kg·s−1 to 92.34 kg·s−1 (Fig. 9(b)). In Fig. 9(c), the exergy of heating capacity rises from 1119.34 kW to 1193.26 kW and the exergy of cooling capacity rises from 20.61 kW to 30.76 kW firstly and then drops to 12.07 kW. The exergy efficiency of system presents a variation of rising from 41.74% to 44.33% firstly and shows a trend of falling. It is noting that the generator temperature of the maximal exergy efficiency is different from that of the maximal cooling capacity.

4.4.3. Rectification column pressure Fig. 8 shows the effect of the rectification column pressure on the system performance. The rectification column has no relationship with the operation of the geothermal flash power cycle, so the power output of turbine keeps unchanged at 326.86 kW, as shown in Fig. 8(a). In Fig. 8(b), as the rectification column pressure increases from 1300 kPa to 1800 kPa, more ammonia-strong solution (from 6.43 kg·s−1 to 8.82 kg·s−1) is pumped into the rectification column to work, but the mass flow rate of the ammonia vapor generated in the rectifier decreases from 1.19 kg·s−1 to 0.56 kg·s−1 on the contrary. The power consumption of pump rises from 9.70 kW to 20.69 kW with the increase of the mass flow rate of ammonia-strong solution and the net power output of system decreases from 317.16 kW to 306.17 kW in consequence, as presented in Fig. 8(a). The cooling capacity of system drops from 1073.98 kW to 476.25 kW with the decreased mass flow rate of refrigerant water obtained (from 12.82 kg·s−1 to 5.68 kg·s−1, in Fig. 8(b)), which is on account of the decreased mass flow rate of ammonia vapor generated in the rectifier. The temperature of ammonia vapor generated in the rectifier goes up with the increase of rectification column pressure, but the ammonia vapor releases much less heat in the condenser II due to its decreased mass flow rate. Therefore, although the domestic hot water gets more heat in the absorber and condenser I, the total heat that the domestic hot water obtains still decreases, which means that the mass flow rate of domestic hot water decreases (from 90.79 kg·s−1 to 88.53 kg·s−1, in Fig. 8(b)), i.e. the heating capacity of system decreases. In Fig. 8(c), the exergy of heating capacity of the Geo-CCHP system drops from 1173.2 kW to 1143.98 kW and the exergy of cooling capacity decreases from 37.75 kW to 16.74 kW. Finally the exergy efficiency of system decreases from 44.28% to 42.51% as the rectification column pressure increases.

4.4.5. Evaporation pressure Fig. 10 shows the effect of the evaporation pressure on the system performance. It can be seen that the absorption pressure in the absorber equals to the evaporation pressure in the evaporator. As the absorption pressure increases from 350 kPa to 410 kPa, the saturation temperature of ammonia-strong solution out of the absorber increases, which signifies that the required heat absorption of per unit mass flow rate of ammonia-strong solution for rectification process in the rectification column drops, so more ammonia-strong solution is pumped into the rectification column to work (from 6.91 kg·s−1 to 7.28 kg·s−1), as shown in Fig. 10(b). The mass flow rate of ammonia vapor generated in the rectifier increases from 0.95 kg·s−1 to 1.00 kg·s−1 accordingly, bringing about more refrigerant water obtained in the evaporator (from 9.98 kg·s−1 to 10.53 kg·s−1), namely the cooling capacity of system increases. The power consumption of pump increases from 13.27 kW to 13.32 kW with the increase of the mass flow rate of ammonia-strong solution, so the net power output of system decreases from 313.60 kW to 313.55 kW on account of the constant power output of turbine, as presented in Fig. 10(a). The increased mass flow rate of ammoniastrong solution leads to that more heat could be released in the condenser II as well as the absorber. Therefore more domestic hot water is obtained (from 89.88 kg·s−1 to 90.06 kg·s−1, in Fig. 10(b)), indicating the heating capacity of system increases. In Fig. 10(c), the exergy of heating capacity of the system increases from 1161.47 kW to 1163.79 kW and the exergy of cooling capacity increases from 29.40 kW to 31.02 kW with the increase of evaporation pressure, and so does the exergy efficiency of system by calculation, i.e. increasing from 43.60% to 43.71%. To sum up, the system parameters and performance vary in very small scales with the variation of evaporation pressure, 95

10

90

320

45

1180

1160

8

44

85

280

WPUP Wnet

40

mammo mdhw mrw

4

-1

mstrg

6

80 15 10

2

20

5

0

0

0 1300 1400 1500 1600 1700 1800

(a)

PRCL/kPa

E /kW

WTUB

mdhw, mrw/kg·s

-1

mstrg, mammo/kg·s

W/kW

300

1140

Eheat

60

Ecool exg

40

42 20

41

0

1300 1400 1500 1600 1700 1800

1300 1400 1500 1600 1700 1800

(b)

(c)

PRCL/kPa

Fig. 8. Effect of rectification column pressure on system performance. 11

43

PRCL/kPa

/%

340

exg

4.4.4. Generator temperature Fig. 9 shows the effect of the generator temperature on the system performance. In Fig. 9(a), the power output of turbine keeps constant at 326.86 kW due to no effect by the increase of generator temperature. As the generator temperature increases from 90 ℃ to 120 ℃, the saturated liquid from the flasher releases much less heat in the generator of rectification column, hence the mass flow rate of ammonia-strong solution pumped into the rectification column drops in a large scale (from 14.36 kg·s−1 to 1.43 kg·s−1), whereas the mass flow rate of ammonia vapor produced in the rectifier increases from 0.66 kg·s−1 to 0.99 kg·s−1 firstly and then decreases to 0.39 kg·s−1, which are shown in Fig. 9(b). The power consumption of pump decreases from 26.49 kW to 2.64 kW due to the less ammonia-strong solution, further leading to an increase from 300.37 kW to 324.23 kW in the net power output of

Energy Conversion and Management 208 (2020) 112591

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330

16

320

14

46

1200

92

1180

90

WPUP Wnet

30

mdhw mrw

8

4

10

2

12 10

6

20

86

1140 1120

8

0 90

100

(a)

110

120

90

100

(b)

TGEN/

110

Eheat 43

Ecool

1100

exg

40

6

0

44

/%

290

mammo

10

exg

WTUB

45 1160

88

E /kW

mstrg, mammo/kg·s

-1

300

mstrg

-1

12

mdhw, mrw/kg·s

310

W/kW

94

42

30

4

20

2

10

41

120

90

100

(c)

TGEN/

110

120

TGEN/

Fig. 9. Effect of generator temperature on system performance. 330

90.1

7.4

44.0

1164

90.0

310

10.4

mstrg

6.8

mammo

10.2

1.00

E /kW

315

7.0

mdhw

0.96 13.0 340

(a)

360

380

400

420

43.8

exg

32

43.7

31 43.6

10.0

30

mrw

0.94 340

360

380

(b)

PEVP/kPa

Ecool

1161

13.5 0.98

Eheat

-1

mstrg, mammo/kg·s

Wnet

W/kW

10.6

-1

WPUP

1162

mdhw, mrw/kg·s

320

43.9

89.8

/%

WTUB

1163

89.9

7.2

exg

325

400

9.8 420

29 340

360

(c)

PEVP/kPa

380

400

43.5 420

PEVP/kPa

Fig. 10. Effect of evaporation pressure on system performance.

1170

320

9

88

1160

315

8

16

1150

Wnet

25

6

14

mdhw

12

mrw

1.2

10

1.0

20

8

0.8

15

6

0.6

10

0.4 40 41 42 43 44 45 46 47 48

(a)

Xstrg/%

4 40 41 42 43 44 45 46 47 48

(b)

Ecool

44.0

1140 43.5 40 35

43.0

30 25

42.5

20 15

42.0 40 41 42 43 44 45 46 47 48

(c)

Xstrg/%

Fig. 11. Effect of ammonia concentration of ammonia-strong solution on system performance. 12

44.5

exg

Xstrg/%

/%

mammo

E /kW

WPUP

7

mdhw, mrw/kg·s

305

mstrg, mammo/kg·s

WTUB

Eheat

-1

90

-1

10

mstrg

45.0

1180

325

310

W/kW

92

11

exg

330

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process, resulting in an increase from 0.92 kg·s−1 to 1.33 kg·s−1 in the mass flow rate of ammonia vapor generated from the rectifier, but the mass flow rate of the branch of ammonia for refrigeration decreases from 0.82 kg·s−1 to 0.66 kg·s−1 due to the increased reflux ratio. Therefore, the mass flow rate of the obtained refrigerant water in the evaporator drops from 10.84 kg·s−1 to 8.72 kg·s−1 (Fig. 12(b)), i.e. the cooling capacity of system drops. The power consumption of pump decreases from 13.82 kW to 11.12 kW with the increase of the mass flow rate of ammonia-strong solution, so the net power output of system increases from 313.04 kW to 315.74 kW because of the constant power output of turbine, as presented in Fig. 12(a). Although the ammonia vapor releases more heat in the condenser II, the absorption process releases much less heat in the absorber, leading to a decrease from 90.16 kg·s−1 to 89.46 kg·s−1 in the mass flow rate of the obtained domestic hot water (Fig. 12(b)), which indicates the heating capacity of system also drops. In Fig. 12(c), the exergy efficiency of system, as well as the exergy of heating and cooling capacity of system, decreases (from 43.76% to 43.40%) with the increase of the reflux ratio of rectifier. But in general, the impact degree of the reflux ratio of rectifier on the system performance is small.

indicating that the evaporation pressure only has a little influence on the system performance. 4.4.6. Ammonia concentration of ammonia-strong solution Fig. 11 shows the effect of the ammonia concentration of ammoniastrong solution on the system performance. With the increase of the ammonia concentration of ammonia-strong solution from 41% to 47%, the saturated temperature of ammonia-strong solution out of the absorber decreases due to the unchanged absorption pressure, thus the required heat absorption of per unit mass flow rate of ammonia-strong solution for rectification process in the rectification column rises, resulting in a decrease from 9.92 kg·s−1 to 6.39 kg·s−1 in the mass flow rate of ammonia-strong solution pumped into the rectification column, as shown in Fig. 11(b). However, the mass flow rate of ammonia vapor generated in the rectifier increases from 0.56 kg·s−1 to 1.14 kg·s−1 because of the constant rectification pressure and the increased ammonia concentration of ammonia-strong solution. Hence more refrigerant water is obtained from 5.90 kg·s−1 to 11.97 kg·s−1 in the evaporator (Fig. 11(b)), i.e. the cooling capacity of system increases. The power consumption of pump decreases from 18.10 kW to 11.87 kW as the mass flow rate of ammonia-strong solution decreases, so the net power output of system increases from 308.76 kW to 315.00 kW on account of the constant power output of turbine at 326.86 kW, as presented in Fig. 11(a). On one hand, the ammonia vapor releases more heat in the condenser II because of its increased mass flow rate; on the other hand, the absorption process in the absorber also releases more heat due to the unchanged inlet temperature of ammonia-weak solution and ammonia vapor and the decreased outlet temperature of ammoniastrong solution. So more domestic hot water is obtained from 88.59 kg·s−1 to 90.52 kg·s−1 in the system (Fig. 11(b)), which means that the heating capacity of system increases. In Fig. 11(c), both of the exergy of heating and cooling capacity of system increase as the ammonia concentration of ammonia-strong solution increases. Finally the exergy efficiency of system rises from 42.63% to 44.05% apparently.

5. Conclusion This paper proposed a new geothermal combined cooling, heating and power (Geo-CCHP) system, which couples flash power cycle and ammonia-water absorption refrigeration cycle, to supply electricity, refrigerant water and domestic hot water simultaneously to users. This system has a prominent feature that all waste heat of the power and refrigeration cycles is recovered in a certain sequence to provide users with thermal energy, which significantly improves the energy conversion efficiency of whole system. The thermodynamic mathematical models of the new system are established in detail, and a preliminary system design condition of the system is given through numerical simulation calculation. Based on the design condition, the exergy destruction/loss distribution in the system is obtained by exergy loss analysis. Finally a thermodynamic parameter analysis is performed, which investigates the effects of several key thermodynamic parameters on the exergy efficiency of system. Some main conclusions obtained from the research are summarized as follows.

4.4.7. Reflux ratio of rectifier Fig. 12 shows the effect of the reflux ratio of rectifier on the system performance. As the reflux ratio increases from 10% to 50%, more condensed ammonia liquid is sent back to the rectification column, which means that the required heat absorption of per mass flow rate of fluid in the rectification column rises. Whereas the heat release of the geothermal water in the rectification column is constant, so the mass flow rate of ammonia-strong solution pumped into the rectification column decreases from 7.49 kg·s−1 to 6.03 kg·s−1, as shown in Fig. 12(b). However, more reflux fluid promotes the rectification 8.0 7.5

W/kW

315 310

6.5

mammo

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mdhw

1160 89

mrw

11

5.5 1.4

20

43.6

1155

/%

mstrg, mammo/kg·s

-1

Wnet

43.8

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E /kW

WPUP

320

1165

90

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-1

WTUB

mdhw, mrw/kg·s

325

44.0

1170

91

exg

330

(1) An ammonia-water absorption refrigeration cycle is employed to recover the waste heat of the liquid geothermal water discharged from the flasher of the flash power cycle, and meanwhile the waste heat that should have been exhausted to the environment from the condenser and absorber of the absorption refrigeration cycle is

35

10

30

9

25

8 60

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43.4

Eheat

1.2 15

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Ecool exg

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60

0

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RR/%

Fig. 12. Effect of reflux ratio of rectifier on system performance. 13

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50

43.0 60

Energy Conversion and Management 208 (2020) 112591

J. Wang, et al.

recovered to preheat the domestic hot water, thus considerably improving the system performance. According to the preliminary design condition results, the exergy efficiency of Geo-CCHP system could achieve 43.69% under the condition of 170 ℃ geothermal water. (2) The exergy loss analysis results reveal that the maximal exergy destruction, which reaches 48.53% of the total exergy destructions of the system, exists in the condenser of flash power cycle, due to the large heat transfer temperature difference and the large mass flow rate of fluids during the heat exchange process. Additionally, the exergy destructions in the rectification column, absorber and flasher, also make up a large portion of the total exergy destructions of Geo-CCHP system, accounting for 17.68%, 9.02% and 8.30% respectively, which indicates that the components used for separating or mixing fluids are prone to generate exergy destructions. (3) Based on the design condition, the thermodynamic parameter analysis results show that an optimal flash pressure about 300 kPa and an optimal generator temperature about 120 ℃ exist, which could yield the maximal exergy efficiency of system respectively. While as for other parameters, within some scopes, lower turbine back pressure and rectification column pressure, higher ammonia concentration of ammonia-strong solution, bring about higher exergy efficiency of system; the evaporation pressure and the reflux ratio of rectifier just make little difference on the exergy efficiency of system.

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CRediT authorship contribution statement

[21]

Jianyong Wang: Writing - original draft. Chenxing Ren: Investigation. Yaonan Gao: Methodology. Haifeng Chen: Project administration. Jixian Dong: Supervision.

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Declaration of Competing Interest [24]

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Acknowledgements

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The authors gratefully acknowledge the financial support by National Natural Science Foundation of China (Grant No. 51906131), Natural Science Research Start-up Fund of Shaanxi University of Science and Technology (Grant No. 2018GBJ-09) and the Youth Innovation Team of Shaanxi Universities.

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