Feasibility research on the novel experimental solar-assisted CO2 based Rankine cycle integrated with absorption refrigeration

Energy Conversion and Management 205 (2020) 112390

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Feasibility research on the novel experimental solar-assisted CO2 based Rankine cycle integrated with absorption refrigeration

T

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Onder Kizilkana, , Hiroshi Yamaguchib a b

Department of Mechanical Engineering, Faculty of Technology, Isparta University of Applied Sciences, Isparta, Turkey Department of Mechanical Engineering, Doshisha University, Kyo-Tanabeshi, Kyoto 610-0321, Japan

A R T I C LE I N FO

A B S T R A C T

Keywords: Solar energy CO2 Power generation Exergy analysis Absorption refrigeration

In this paper, a feasibility analysis of the novel solar-assisted transcritical carbondioxide (tCO2) based Rankine cycle (RC) with an absorption refrigeration system (ARS) is performed. The experimental investigations are conducted on the test rig of the solar-assisted tCO2-RC for a typical day in the spring season. The unique experimental facility consists of evacuated tube type solar collectors, a high-temperature heat exchanger, a lowtemperature heat exchanger, and a pump. In addition, an expansion valve is employed instead of the turbine in order to simulate the turbine operation. The proposed ARS is a single-stage type, and it is supposed to be driven by the rejected thermal energy from the experimental RC cycle. The performance of the integrated system is analyzed in terms of energy and exergy. The ARS analysis is made for water-lithium bromide (H2O-LiBr) and ammonia-water (NH3-H2O) refrigerant pairs for comparison purposes. According to the results, the power generation rate of CO2 based RC reaches up to 0.385 kW. The ARS working with H2O-LiBr presents higher refrigeration capacity with 1.413 kW while the energy and exergy efficiencies of the integrated power-refrigeration system are determined as 29.62% and 5.36% respectively. For the integrated system with NH3-H2O, the energy efficiency is 21.51% and the exergy efficiency is 4.99%. From the results, it is concluded that the ARS working with H2O-LiBr can be successfully integrated with the solar-assisted tCO2-RC for sustainable power generation and refrigeration applications. Future research should, therefore, concentrate on integrating an actual ARS with the experimental power cycle.

1. Introduction The expanding development and modernization of the global society bring about the increasing energy demand which is mainly produced from fossil fuels [1]. However, since fossil fuel sources are being depleted and cause many ecological problems, the utilization of renewable energy resources such as solar energy for power generation has become essential and received increasing attention. For the last three decades, many researchers have worked on evolving new solar-power technologies or improving existing cycles [2]. Within these researches, low-temperature solar energy application has been widely developed. Power systems such as the organic Rankine cycle, tCO2-RC, and Kalina cycle are becoming extensively recognized for energy production purposes utilizing low-grade heat sources [3]. From the outlook of preserving the ozone layer and prohibiting global warming, it became crucial to pay extra attention to ecologically safe ‘natural’ working fluids. One of the significant ways of protecting the environment is decreasing the emissions of greenhouse gases.

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Therefore the relevance of CO2 as a working fluid increased significantly from the 1990s, and several types of research were initiated by the industry and academia [4]. Due to its relatively low critical properties (30.978 °C and 7377 kPa), CO2 has become a promising working fluid for power cycles utilizing the energy from low to/ medium grade heat sources [5]. Moreover, CO2 is harmless, non-flammable, non-toxic, inexpensive and plentiful in the universe with negligible Ozone Depletion Potential and Global Warming Potential [6]. Besides, it is an appropriate working fluid to be employed in thermodynamic cycles with low temperatures from 0 to 250 °C owing to its low critical point [4]. Among the power generation cycles utilizing low-temperature heat sources, there has been an increasing interest in tCO2-RC over the past decades. Many studies have been conducted for tCO2-RC with different cycle configurations utilizing low-grade thermal energy, i.e. waste heat recovery from diesel engines [7], geothermal heat pipe [8], geothermal energy [9] and ambient air [10]. As an eco-friendly and sustainable way of energy production, tCO2-RC assisted by solar energy has also

Corresponding author. E-mail address: [email protected] (O. Kizilkan).

https://doi.org/10.1016/j.enconman.2019.112390 Received 5 September 2019; Received in revised form 4 December 2019; Accepted 6 December 2019 0196-8904/ © 2019 Elsevier Ltd. All rights reserved.

Energy Conversion and Management 205 (2020) 112390

O. Kizilkan and H. Yamaguchi

Nomenclature A CP ex Ėx h L ṁ P Q̇ s Ṡ S T Ẇ X

C coll dest E EV G gen in max min out P Rec Shex T 0

area, m2 specific heat capacity, kJ/kgK specific exergy, kJ/kg exergy rate, kW specific enthalpy, kJ/kg length, m mass flow rate, kg/s pressure, kPa heat rate, kW specific entropy, kJ/kgK entropy rate, kW/K solar radiation, kW/m2 temperature, K or °C work rate, kW concentration

condenser collector destruction evaporator expansion valve generator generation input, inlet maximum minimum output, outlet pump rectifier solution heat exchanger turbine reference state

Abbreviations Greek letters

ε η ηex

ARS CO2 COP H2O-LiBr NH3-H2O RC tCO2

heat exchanger effectiveness energy efficiency exergy efficiency

Subscripts A act

absorber actual

absorption refrigeration system carbon dioxide coefficient of performance water-lithium bromide ammonia-water Rankine cycle transcritical carbon dioxide

indirectly connected to the collectors and water was used as the heat transfer fluid. In another study, they conducted an optimization procedure with exergy analysis for the similar cycle with hydrogen production [19]. All the above mentioned recent literature on the solar-assisted tCO2RC are theoretical studies based on assumptions and mathematical modelings. The first and probably the unique experimental setup for solar-assisted tCO2-RC in the literature was designed and constructed in 2004, by Yamaguchi and his colleagues at Doshisha University, Kyoto, Japan. From that time, numerous theoretical and experimental studies were done for the investigation of the CO2 based power generation cycle. Since the present study is basically dealing with the same experimental setup, detailed information about the experimental facility will be given in the following sections. Zhang et al. theoretically investigated the tCO2-RC in terms of annual performance [20] and seasonal performance [21]. Another theoretical study was about the seasonal thermodynamic analyses of the cycle for different working fluids: CO2, NH3, H2O, C3H8, R134a [22]. The first experimental results of the tCO2 power cycle were published in 2005 [23]. It was about the feasibility study of tCO2-RC focusing on the hourly performance of the system. The experiments were performed in August 2004 and the maximum solar radiation was recorded as 0.83 kW/m2. According to the results, the cycle efficiency was estimated as 16%. In the following experimental study, the performance of the solar-assisted cycle was evaluated with economic estimations and emission reduction calculations [24]. Later on, Zhang et al. [4] reported the result of the analysis which was about the basic performance of the tCO2-RC for winter and summer conditions. In the study, it was noticed that the measured average radiation and ambient temperature were 0.58 kW m2 and 36.38 °C in summer, and 0.30 kW m2 and 17.08 °C in winter, respectively. The cycle efficiency for the summer season was calculated as 6.3% and it was 5.3% for the winter season. Zhang and Yamaguchi [25] examined the system, particularly for the basic collector characteristics. The experimental data from August 2004 to July 2005 was used in the analysis. According to the results of a specific day, the CO2 temperature

become appealing by many scientists. Pan et al. [11] conducted research on the solar-assisted tCO2-RC theoretically with a molten salt energy storage medium. They investigated the effects of operation parameters on system performance. However, no detail was given about the specifications of the solar collector. According to the results, the maximum thermal efficiency was found to be 24.2% with an optimal cooling pressure of 11.0 MPa. Al-Zahrani and Dincer [12] carried out energy and exergy analysis of a solar-assisted reheat tCO2-RC. The system was integrated with the thermal energy storage facility and an ARS. A parabolic trough collector was utilized to meet the heat energy demand of the cycle. They investigated the system performance theoretically and performed parametric analyses. The energy and exergy efficiency of the tCO2-RC were calculated as 34% and 82%. Furthermore, the COP of the NH3-H2O based ARS was found to be 0.7 and where exergy efficiency was estimated as 27%. Sarmiento et al. [13] proposed a parabolic trough solar collector assisted tCO2-RC in order to investigate the system performance for a constant solar radiation rate of 1 kW/m2. In the study, the efforts were mainly paid for evaluating the effect of the regenerator on the system performance and an optimization procedure was applied to find the optimum working conditions. Another theoretical study was carried out by Naseri et al. [14] about hydrogen and water production utilizing solar-assisted tCO2-RC in which a Stirling engine employed for the condenser. In the next study [15], they modified the system by replacing the Stirling engine with the condenser which was connected to an LNG tank for utilizing cryogenic energy. Ahmadi et al. [16] applied thermo-economic and multi-objective optimization procedure for a solar-assisted tCO2-RC to maximize the efficiency of the cycle. The system consisted of flat plate solar collectors, a storage tank and, an axillary heater for maintaining continuous operation during nigh-time. Another theoretical study was performed by Xia et al. [17] utilizing solar energy to drive a tCO2-RC integrated with a desalination system. The authors proposed an LNG system for the condensation of CO2 in the condenser. Song et al. [18] investigated the performance of the solar-assisted tCO2-RC by employing flat-plate collectors and an auxiliary heater. The cycle was 2

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supercritical state. The supercritical CO2 flows through the turbine, where thermal energy is converted to electrical energy. Following this, The CO2 exiting the turbine passes from inside the two heat exchangers where it is cooled down to a saturated state. In the system, two heat exchangers are utilized in order to cool down the CO2. The first one is the high-temperature heat exchanger which is employed for heat recovery purposes, and the second one is the condenser. In the present study, a generator takes place instead of the high-temperature heat exchanger in order to simulate the ARS. In the generator, CO2 rejects some amount of its thermal energy to the refrigerant couple. After leaving the generator, CO2 reaches condenser and becomes saturated liquid before the pumping process. The liquid CO2 is then pumped by the feed pump, back into the higher pressure condition, and the cycle recommences. The proposed ARS is schematically illustrated in the right-hand side of Fig. 1. The NH3-H2O and H2O-LiBr cycles are similar to each other with the substantial difference: A rectifier is supplemented in the ammonia cycle to inhibit the water flow in the ammonia vapor leaving the generator [38], which causes a decrease in the coefficient of performance [39]. In the figure, the rectifier is indicated with dashed lines and it is valid only for the NH3-H2O cycle. In the case of H2O-LiBr absorption cycle, there is no rectifier, thus the refrigerant directly passes through the generator and enters the condenser. Another difference between the two cycles is in the H2O-LiBr absorption system, water is the natural refrigerant, and LiBr is the absorbent while in the NH3-H2O system, ammonia is the refrigerant and water is the absorbent [37]. Since the NH3-H2O refrigeration system operates at relatively high pressure and high vapor fraction, it is necessary to employ a distillation unit, which results in more capital costs and energy consumption. On the other hand, the H2O-LiBr system is restricted with refrigeration temperature due to the freezing temperature of water [40]. In addition, crystallization of LiBr may occur if the solution concentration is too high or the solution temperature is too low. In this situation, the crystallization of the solution blocks some parts of the system and stops the operation. For maintaining a continual process, the cycle should be operated above the crystallization line [41]. Thus, in this feasibility study, the analyses are performed taking into account the above-mentioned concerns.

at the outlet of the collector was varied between 85.0 °C and 187.5 °C. The mean CO2 temperature was measured as 162.5 °C. Furthermore, the research group were analyzed the solar-powered tCO2 based system for hydrogen production [26], heat transfer characteristics of CO2 [27,28], exergy analysis [29,30], employing a thermally driven pump instead of mechanical pump [31], different design arrangements of solar collector [32], utilizing an actual prototype turbine [33] and integration of photovoltaic solar panel to improve the system performance [34]. The key problem is that solar thermal applications for energy production have higher investment costs and lower thermal efficiency. For this reason, these systems have not been commercialized yet to the scale of power plants that utilize fossil fuels [35]. In order to make full use of solar thermal energy and to improve the overall system performance, integration of absorption refrigeration for cooling purposes is one of the most common ways [36]. On the other hand, the integration of an ARS with the novel solar-assisted experimental tCO2 cycle has never been investigated previously by the research group. To fill this gap, and to analyze the applicability of an actual ARS as a base for future studies, this paper presents a feasibility study for the system integration of ARS with the unique experimental power generation system. For this aim, energetic and exergetic performance assessment of the ARS, as well as the tCO2-RC is performed for the two most common fluid pairs used in absorption cycles; H2O-LiBr and NH3-H2O. For the analyses, the experimental data for tCO2-RC are utilized in order to evaluate the system characteristics with the proposed ARS. In addition, the exergy destruction rates are determined for each system component in order to identify the system’s deficits. Herewith, the present work differs from the earliest studies as follows: i) Energetic and exergetic performance evaluation of the novel solar assisted system is performed using the latest experimental data. ii) A feasibility analysis of the ARS integration with the experimental system is carried out in order to improve the overall system efficiency. iii) The analysis is carried out for two different refrigerant couples for determining the best system performance for future research. 2. System description 2.1. Solar-Assisted tCO2 Rankine cycle with absorption refrigeration

2.2. Experimental setup and procedure

A schematic representation of the solar-assisted power system integrated with ARS is shown in Fig. 1. The tCO2 based power generation system consists of an evacuated solar collector array, turbine, heat exchangers and CO2 pump. The actual experimental system with the system components will be described in detail in the next sub-section. The working principle of the cycle is as follows: The evacuated solar collector is used as a heat generator for heating the CO2 to a

The basic experimental tCO2-RC was initially constructed and installed on the roof of the Energy Conversion Research Center at Doshisha University, Kyoto, Japan. In Fig. 2 the illustrative representation of the experimental facility is shown. In the original experimental setup, a mechanical piston pump with a flow rate of

Fig. 1. Solar-assisted tCO2-RC integrated with ARS. 3

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Fig. 2. Schematic representation of the experimental facility.

high solar absorbance 0.927 and a low emissivity 0.193 is applied on the vacuum side of the inner glass tube. The absorbed heat is conducted through the inner glass tube wall and then removed by CO2 in a metal U-tube. The U-tube is located in the inner glass tube with an aluminum fin connecting the outlet arm of the tube to the inner glass. The total opening area of the evacuated solar collectors is 29.55 m2 and its effective area is reported as 9.6 m2 by the manufacturer. The collector used is a commercial product modified and constructed to have a maximum allowable working pressure of 12 MPa and have a maximum working temperature of 250 °C. At the time of the first establishment of the experimental setup, there was no turbine available for CO2. For this reason, an expansion valve was employed instead of the turbine in the primary setup, for simulating the turbine operation without generating electricity. The expansion valve enables to adjust the CO2 mass flow rate during the experiments besides providing practical turbine behavior by simulating the pressure drop. In 2015, Yamaguchi and his research group designed

0.03 kg/s and a maximum operating pressure of 12 MPa, is used for feeding the CO2. In order to remove heat from the tCO2–RC, and refrigerate the CO2 to the liquid phase, two shell and tube heat exchangers are utilized: a high-temperature heat exchanger and a lowtemperature heat exchanger. The total area of the shell and tube heat exchangers are approximately 0.76 m2. Also, for removing the heat recovered from the tCO2-RC to the environment, a water cooling tower which is 22 kW in cooling capacity, is used as a heat sink. The solar collector is the most critical part of the solar-powered tCO2–RC. Hence, for heating CO2 to a higher-temperature supercritical state effectively, U-tube evacuated solar collector is used in the system (Fig. 3). This type of collector has got many advantages such as costeffectiveness, simplicity, commercially availability for over 30 years [42] and exhibits better performance than flat plate solar collectors especially for low and moderate temperature applications [43]. As seen from Fig. 3, the collector consists of a glass envelope over an inner glass tube coated with a selective solar absorber coating. This coating with a

Fig. 3. The picture of solar collectors and cross-section of the evacuated tube. 4

Energy Conversion and Management 205 (2020) 112390

O. Kizilkan and H. Yamaguchi

Q̇ +

and manufactured a centrifugal contra-rotating turbine for the supercritical CO2. They investigated the turbine characteristics and performance experimentally and analytically [33]. However, since the turbine was a hand-made experimental one that was not at all optimized to give high-performance industrial turbine alike, the experimental research presented here were made using the expansion valve for better simulation. A further study could assess the performance improvement and the optimization of the specially designed turbine. For the measurement of the CO2 temperature and pressure, five Ttype thermocouples and five pressure transmitters are attached at several locations of the setup as shown in Fig. 2. A Coriolis Effect mass flow meter is mounted on the discharge of the pump for the measurement of the mass flow rate of CO2. It has a measuring range of 0.09–2.0 kg/min and a maximum allowable operating pressure of 12 MPa. For measuring the cooling water temperature at the inlet and outlet of the heat exchangers, four platinum resistor sensors are utilized. Two mass flow meters are located at the outlet of heat exchangers in order to observe and record the mass flow rate of water streams. Furthermore, a sun radiation sensor and an air temperature gauge are employed for the measurement of solar radiation and environment temperature. The technical characteristics of the measurement instruments are tabulated in Table 1. In a typical experimental practice, the water pumps are primarily started and the mass flow rates of the water streams in the high and low-temperature heat exchangers are adjusted. The water pumps circulate the water through the cooling system with a stationary cooling temperature of 10 °C and with the speed range of 106–180 rpm. Later on, the CO2 feed pump is switched on with a mass flow range between 0.005 and 0.013 kg/s, and the expansion valve is opened in order to simulate the turbine operation. After the completion of the experiments, the CO2 pump is switched off first followed by turning off the water pumps. During the experiments, mass flow rate, pressure, temperature, and solar radiation are measured from specified locations of the experimental facility. The measured data is transmitted to the computer via a data-acquisition system where it is monitored and recorded every two seconds. In the course of the test, there are two essential causes of the measurement uncertainties. The first one is on the account of the accuracy of temperature, pressure, and mass flow measuring devices and the other one is due to the computer faults occurred during the data logging and reading operation. In order to measure, achieve and commit the real-time data, sensitive measurement and data-acquirement devices are utilized. Considering the accuracy of the measurement devices listed in Table 1, the highest uncertainty in the performance parameters is lower than ± 3.0% which is admissible for the current research.

∑ ṁ in hin = Ẇ + ∑ ṁ out hout

(3)

where Q̇ is the heat transfer rate, Ẇ is the work and h is the specific enthalpy. For analyzing the absorption cycle, heat exchanger effectiveness is a useful way to define the performance of the heat exchanger. If the effectiveness is specified, other thermodynamic properties can be found easily. The effectiveness of a heat exchanger is defined as [38];

ε=

Q̇act Q̇ max

(4)

In Eq. (4), Q̇ act is the actual heat transfer and Q̇ max is the maximum possible heat transfer for the given inlet conditions. Q̇ max can be calculated by:

Q̇ max = (ṁ Cp )min ΔTmax

(5)

where (ṁ Cp)min is the smaller value calculated for hot and cold streams of the heat exchanger and ΔTmax is the temperature difference between the two inlet streams. For the exergy analysis of the experimental system, general exergy balance equation for a control volume is given below [46]:

Ėx Q − ĖxW =

∑ ṁ out ex out − ∑ ṁ in exin + Eẋ dest

(6)

In the above equation, EẋW and Eẋ Q are exergy of work and heat, respectively, ex is the specific flow exergy of the stream, Eẋ dest is the exergy destruction rate. Exergy destruction rate can also be written as follows:

Eẋ dest = T0 Ṡgen

(7)

where T0 is the environment temperature and Ṡgen is the entropy generation. The exergy transfer accompanying heat, work and fluid flow are defined as [45]:

T − T0 ⎞ Eẋ Q = Q̇ ⎛ ⎝ T ⎠

(8)

EẋW = Ẇ

(9)

Eẋ = ṁ ex

(10)

Specific flow exergy is defined as the difference of the flow availability of a stream and that of the same stream at its restricted dead state, which is given by [45]:

ex = (h − h 0) − T0 (s − s 0)

(11)

For the exergy analysis of a solar system, the solar exergy is calculated from Eq. (12) [47]: 4

1 T 4 T Eẋ solar = SA ⎜⎛1 + ⎛ 0 ⎞ − ⎛ 0 ⎞ ⎞⎟ 3 ⎝ Tsun ⎠ 3 ⎝ Tsun ⎠ ⎠ ⎝ ⎜

3. Thermodynamic analysis

∑ (ṁ X)in = ∑ (ṁ X)out

⎜

⎟

(12)

where S is the solar radiation and A is the area of the solar collectors. From this equation, it is clear that the exergy of solar energy related to the sun’s surface temperature, and in the present study, it was taken as 5739 K [48]. Using the above-mentioned balance equations, the energy capacities

This section deals with thermodynamic balance equations for the performance evaluation of the tCO2-RC integrated ARS. For the thermodynamic analysis of the integrated system, the principles of conservation of mass, first and second laws of thermodynamics are applied to each component. The governing equations of mass for a steady-state and steady flow system are [38,44]:

∑ ṁ in = ∑ ṁ out

⎟

Table 1 Technical characteristics of the measurement instruments.

(1) (2)

In the above equations, ṁ is the mass flow rate, X is the mass concentration of LiBr or NH3 in the ARS, and the subscripts in and out represent inlet and outlet flows, respectively. According to the first law thermodynamics, the energy balance equation can be written as [45]: 5

Measurement device

Measurement range

Accuracy

Sun radiation sensor Air temperature gauge Thermocouples (CO2) Pressure transmitters (CO2) Temperature sensors (water) Mass flow meter (CO2) Mass flow meter (water)

50–210 MJ 0–100 °C 0–250 °C 0–12 MPa 0–100 °C 0.09–2.0 kg/min 0.0–3.0 kg/s

± ± ± ± ± ± ±

0.3% 0.15 + 0.0002 |T| °C 0.1 °C 0.2% 0.15 + 0.0002 |T| °C 0.1% 0.5%

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and exergy destruction rate for each component are tabulated in Table 2 referring to the state points indicated in Fig. 1. The thermal efficiency of the tCO2-RC is expressed as:

ηRC =

ẆT ̇ Qcoll + ẆP,RC

(13)

The coefficient of performance of the ARS can be determined using the equation below:

COP =

Q̇E Q̇G + ẆP,ARS

(14)

where Q̇E is the evaporator (refrigeration capacity) and Q̇G is the generator capacity. The overall energy efficiency of the integrated system is:

ẆT + Q̇E Q̇coll + ẆP,RC + ẆP,ARS

ηoverall =

Fig. 4. Variation of solar radiation and ambient temperature with time.

(15)

• The heat transfer to/from ambient and pressure drops in the pipes are neglected. • The temperatures of the generator, absorber, condenser, and evaporator are taken as 90 °C, 40 °C, 40 °C and 4 °C, respectively [38]. • Heat exchanger effectiveness of the Shex is taken as 0.75. [50]

For the exergy efficiency of the integrated system and sub-cycles, the following equations are used [49]:

ηex,RC =

ẆT Eẋ solar + ẆP,RC

ηex,ARS =

(16)

(Eẋ22 − Eẋ23) ̇ (Ex2 − Eẋ3) + ẆP,ARS

ηex,overall =

In Fig. 4, from the representative measurement, the variation of solar radiation and ambient temperature with time are given. After the sunshine, the solar radiation starts to increase from 6 am and reaches to about 0.6 kW/m2 at 9:00. At 12:20, it attains a maximum value of 0.96 kW/m2 and then starts to decrease. At about 17:00, the solar radiation was measured as 0.2 kW/m2. The ambient temperature is approximately 17 °C at 9:00 in the morning, and it remains nearly constant at about 24 °C between 12:00 and 16:00. Fig. 5 shows the CO2 temperature at the exit of the solar collector (T1) and the exit of the turbine (T2). It must be noted that referring to Fig. 1, T2 is also the CO2 inlet temperature of the heat recovery system in which the heat energy demand of the absorption cycle is supplied. T1 and T2 temperatures start to increase with the sunshine early in the morning and reach 145 °C and 115 °C, respectively at 09:00. These temperatures are adequate for starting the operation thanks to the low critical temperature of CO2. In addition, Pumaneratkul et al. [8] assumed the CO2 temperature at the turbine inlet < 90 °C while Wu et al. [9] assumed it between 120 and 130 °C and it was 65 °C in the study of Song et al. [18]. After starting the experimental tests, the CO2 temperature at the exit of the collector increases to a maximum value of

(17)

ẆT + (Eẋ22 − Eẋ23) Eẋ solar + ẆP,RC + ẆP,ARS

(18)

4. Results and discussion The solar supercritical CO2 Rankine cycle was driven in a typical spring day in March, in Kyoto City, Japan (34°47′58.1″N 135°46′04.6″E). During the experiments, high-temperature water was supplied to the heat recovery system to simulate the high-temperature heat recovery for simulating the thermal energy demand of the ARS. Also, for the condenser, low-temperature water was used for cooling CO2. For the thermodynamic modeling of the experimental system, the actual data obtained from the measurements are used. Since the integrated ARS is a theoretical model, the following assumptions are made for the simulation:

• All processes are of steady-state and steady flow. • The changes in potential and kinetic energies are negligible. Table 2 Energy capacity and exergy destruction of each system component. Energy capacity

Exergy destruction

Solar collector

Q̇solar = SA Q̇coll = ṁ 1 (h1 − h5) Q̇ loss = Q̇solar − Q̇coll

Eẋ dest,coll = Eẋ solar + Eẋ5 − Eẋ1

Turbine

ẆT = ṁ 2 (h2 − h1) Q̇C,RC = ṁ 3 (h3 − h 4) = ṁ 18 (h19 − h18) ẆP,RC = ṁ 4 (h5 − h 4)

Eẋ dest,T = Eẋ1 − Eẋ2 − ẆT Eẋ dest,C,RC = Eẋ3 − Eẋ 4 + Eẋ18 − Eẋ19 Eẋ dest,P,RC = Eẋ 4 − Eẋ5 + ẆP,RC

Q̇G = ṁ 2 (h2 − h3) = ṁ 9 h9 − ṁ 8h8 + ṁ 12 h12 − ṁ 13h13 Q̇Rec = ṁ 26 (h26 − h27) = ṁ 12 h12 − ṁ 13h13 − ṁ 14 h14 Q̇G = ṁ 2 (h2 − h3) = ṁ 9 h9 − ṁ 8h8 + ṁ 14 h14

Eẋ dest,G = Eẋ2 − Eẋ3 + Eẋ 8 − Eẋ 9 + Eẋ13 − Eẋ12 Eẋ dest,Rec = Eẋ12 − Eẋ13 − Eẋ14 + Eẋ27 − Eẋ26

Condenser (RC) Pump (RC) Generator (for NH3-H2O) Rectifier (for NH3-H2O) Generator (for H2O-LiBr) Condenser (ARS)

Eẋ dest,G = Eẋ2 − Eẋ3 + Eẋ 8 − Eẋ 9 − Eẋ14 Eẋ dest,C,ARS = Eẋ14 − Eẋ15 + Eẋ24 − Eẋ25

Q̇C,ARS = ṁ 14 (h14 − h15) = ṁ 24 (h25 − h24) Q̇E = ṁ 16 (h17 − h16) = ṁ 22 (h22 − h23) Q̇ A = ṁ 21 (h21 − h20) = ṁ 11h11 + ṁ 17 h17 − ṁ 16 h16 ẆP,ARS = ṁ 6 (h7 − h6)

Eẋ dest,E = Eẋ16 − Eẋ17 + Eẋ22 − Eẋ23 Eẋ dest,A = Eẋ11 − Eẋ 6 + Eẋ17 + Eẋ20 − Eẋ21 Eẋ dest,P,ARS = Eẋ 6 − Eẋ7 + ẆP,ARS

Expansion valve-1

Q̇Shex = ṁ 7 (h8 − h7) = ṁ 9 (h9 − h10) h10 = h11

Eẋ dest,Shex = Eẋ7 − Eẋ 8 + Eẋ 9 − Eẋ10 Eẋ dest,EV1 = Eẋ10 − Eẋ11

Expansion valve-2

h15 = h16

Eẋ dest,EV2 = Eẋ15 − Eẋ16

Evaporator Absorber Pump (ARS) Shex

6

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two cycles remains nearly constant during the whole operation time, which is approximately 0.4 kW. This is mainly due to the constant heat capacity of the generator for both cases which is provided by the rejected heat from the CO2 at the exit of the turbine. At the beginning of the operation time, the H2O-LiBr cycle capacity is 0.62 kW while it is 0.27 kW for the NH3-H2O cycle. The maximum refrigeration capacity occurs during noontime, 1.413 kW for H2O-LiBr, and 0.978 kW for NH3H2O. These values are thought to be very well for a small scale air conditioning unit. Fig. 8 presents the energy efficiency of the tCO2 Rankine cycle alone and the overall system. The energy efficiency shows a slight increase between 9:00 and 12:00, where it is almost 6.5%. It is worth recalling that, an expansion valve was employed instead of the turbine and this relatively small efficiency rate is due to the utilization of the expansion valve. If a turbine is used, the actual power output should be much larger than the present value which results in higher energy efficiency [23]. After 12:00, the efficiency starts to decrease and it is estimated as 5% at 15:00. For the same experimental system, Pumaneratkul [52] reported an energy efficiency of 3.31% for winter conditions. Besides, Zhang et al. [53] made similar experiments for the time interval between April and June of 2005. They reported that the average power generation efficiency was 8.20%. The difference between the energy efficiency of the three studies is mainly because of the date of the experiments performed. The first study was done in the winter season, the second one was for the summer season while the current analysis was carried out for the spring season (March). In the theoretical analysis of Song et al. [18], maximum power generation efficiency was found to be 6.51% for the solar-assisted tCO2-RC. With the same thermodynamic properties of this study, the efficiency value was reported as 11.03% by Naseri et al. [14]. However, in both studies, turbine inlet temperature and pressure were taken as 65 °C and 10 MPa, respectively. The overall efficiency of the system involving absorption refrigeration can also be seen in the upper part of the figure. The overall efficiency is relatively high when compared to the power generation efficiency, thanks to the refrigeration effect of the absorption cycle. It is clear from the figure that, the system integrated with the H2O-LiBr cycle has got higher energy efficiency than that of the NH3-H2O cycle. During the operation hours, the maximum COP for the ARS working with H2O-LiBr found to be 0.75, while it is estimated as 0.52 for the ARS working with NH3-H2O. In general, for the same operating conditions, the H2O-LiBr cycle is more efficient than the NH3-H2O cycle. One of the main reasons for this is the rectifier, which usually required for NH3-H2O cycles. Furthermore, the specific heat of the NH3-H2O solution is about two times bigger than that of the H2O-LiBr, which causes larger negative effects for any inefficiency of the solution heat exchanger. Besides, the latent heat of NH3 is about double times than that of H2O. For the same refrigeration capacity, the NH3-H2O cycles

Fig. 5. Variation of turbine inlet and outlet temperatures and pressures with time.

209 °C at 12:25. As seen from the figure, the CO2 temperature remains over 170 °C between the hours 9:40–15:10. Besides, the CO2 temperature at the exit of the turbine is higher than 140 °C between the mentioned time intervals, which is sufficient for supporting the absorption refrigeration. The right-hand side of the figure shows the turbine inlet and outlet pressures during the operation hours. The pressure valves were opened after 6:00 as seen from the figure and CO2 reaches supercritical pressure after 8:15. The high-pressure reaches a maximum value of 9400 kPa at about 12:15 while the low-pressure is approximately 4900 kPa at the same moment. As mentioned previously, the CO2 properties throughout the Rankine cycle are sufficient enough for power generation after 09:00, thus the analyses are performed using the experimental data acquired between 09:00–15:00. It must be noted that tCO2-RC is still capable of generating power after 15:00, however, the ARS cannot be operated after this time due to the deficient thermal energy supplied from the tCO2 cycle. According to the actual measured data from the tCO2 Rankine cycle, the P-h diagram was plotted for different operation hours (Fig. 6). As seen from the figure, the turbine inlet pressure (highpressure side) is above critical pressure which is 7377 kPa for all operating hours. The high-pressure is very close to the critical pressure of CO2 at about 15:00, and still, it is above the critical point with a pressure value of 7425 kPa. It is noteworthy that the figure was plotted by using hourly-average data. On the other hand, the CO2 pressure after exiting the turbine (a low-pressure side) remains in the sub-critical region for all operating hours. Fig. 7 shows the power generation rate of the experimental tCO2 Rankine cycle and refrigeration capacity for H2O-LiBr and NH3-H2O cycles for operating hours. The calculations were performed using the data obtained from the experimental measurements of pressure and temperature. The thermodynamic properties of CO2, and H2O-LiBr and NH3-H2O refrigerant couples were determined using EES software [51]. According to the results, turbine power generation is 0.275 kW at the beginning of the operation time, i.e. 9:00. It increases with the time and reaches a maximum value of 0.385 kW at noon due to the increment of the CO2 temperature with solar radiation. After 12:00 the power generation tends to decrease prominently till to 15:00. According to the results of the Pumaneratkul et al. [52] which is one of the latest research on the experimental tCO2 power generation system, the calculated turbine power was reported as 0.238 kW. They performed the analysis using the experimental data for December 2017 and January 2018. In the theoretical calculations of Zhang et al. [22], the turbine capacity was estimated to be 0.650 kW. This higher capacity is due to the assumptions made during the theoretical calculations. The right-hand side of the figure shows the refrigeration capacity for both absorption cycles. It is obvious from the figure that, the refrigeration capacity of the H2O-LiBr cycle is higher than the ammoniawater cycle. The difference between the refrigeration capacities of the

Fig. 6. P-h diagram tCO2 Rankine cycle for different operation hours. 7

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from the figure that the exergy destruction rate increases with the operation hours, i.e., solar radiation, and reaches to a maximum value of 12.63 kW owing to the increasing solar radiation. The exergy destruction rates of the ARSs exhibit the same course for the Rankine cycle. However, the destruction rates for refrigeration cycles are relatively lower than the Rankine cycle. Additionally, the exergy destruction of the H2O-LiBr cycle is found to be slightly higher than that of the NH3H2O cycle. Fig. 12 shows the exergy efficiencies as a function of operation hours. The lower part of the figure demonstrates the exergy efficiency of the Rankine cycle alone and with absorption refrigeration system for H2O-LiBr and NH3-H2O. The upper part of the figure is plotted for the exergy efficiency of ARS due to the difference in the values. The exergy efficiency of the Rankine cycle alone and the integrated system increase with the operation time owing to the increase of solar radiation and then they start to decrease after noontime. The exergy efficiency of the absorption cycles alone increases with the operation time as well. Also from the figure, it can be seen that H2O-LiBr ARS has got higher exergetic efficiency than NH3-H2O ARS, as for energy efficiency. The properties of the working fluid for the state points are listed in Table 3 for the integrated system with H2O-LiBr and Table 4 for the integrated system with NH3-H2O referring to Fig. 1. The tables present the measured values of pressure, temperature, and mass flow rate of CO2 for the highest solar radiation rate at noontime. In addition, the state point properties of the ARSs were provided according to the assumed simulation parameters. The corresponding enthalpy and entropy values were determined using EES software [51] while the specific exergy and exergy rate of each state were calculated using Eq. (10) and Eq. (11). According to the results of the calculations, the overall energy and exergy efficiencies for the H2O-LiBr ARS integrated tCO2-RC are found to be 29.62% and 5.36% respectively. For the tCO2 based power cycle with the NH3-H2O refrigeration system, the energy efficiency is calculated as 21.51% while the exergy efficiency is determined as 4.99%. It was reported in a previous study on the current experimental setup [30] that the energy efficiency of the system is about 60% and the exergy efficiency is about 5%. The study was conducted for the performance investigation of the tCO2 cycle with a heat recovery system. A comparison between the results of this study and the current study shows that the exergy efficiency values are close to each other. However, there is a significant difference between energy efficiency results. In Ref. [30], the capacity of the heat recovery system was taken as the energy output which corresponds to the generator capacity in the present study. On the other hand, in the present study, an ARS is integrated into the system for refrigeration purposes and the output is the evaporator capacity which is nearly two-thirds of the generator capacity. This may be the reason for the difference. The results of exergy destruction calculations for the data provided in Tables 3 and 4 show that the total exergy destruction for tCO2-RC

Fig. 7. Power generation and refrigeration capacities as a function of operation hours.

Fig. 8. Energy efficiencies as a function of operation hours.

require higher mass flow rates [38]. For a better understanding of the tendency of COP with the experimental operation, the variation of COP is plotted against the generator temperature as shown in Fig. 9. As seen from the figure, with the increase of generator temperature, COP values increase for both cycles. The increment for the H2O-LiBr cycle (upperpart of Fig. 9) is small, and it starts to decrease after 100 °C slightly. As seen from the lower part of the figure, COP increases for the NH3-H2O cycle evidently until 98 °C of the generator temperature and hereafter it starts to decrease. This is due to the higher specific heat capacity of NH3-H2O, as explained before. Similarly in Fig. 10, the heat energy capacity of the generator and evaporator were plotted versus generator temperature. From the figure, it is apparent that the generator and evaporator capacities for both refrigerant couples increase with the generator temperature. Also, it is important to stress that the slope of the lines indicates the trend of the COP as described by Eq. (14). According to the results of exergy analysis, the variations of the exergy destruction rate of the cycles with the operation time are given in Fig. 11. Since the difference of exergy destruction rates between the Rankine cycle (including solar collectors) and absorption cycles were high, they were plotted with different scales for better observation. At the beginning of the operation time, the exergy destruction rate of the Rankine cycle was calculated as 10.48 kW. The exergy destruction of tCO2-RC is the sum of the exergy destructions of the cycle components involving the turbine, CO2 condenser, CO2 pump, and solar collector. This high destruction rate is mainly due to the solar collector. The main reason is relying on the collector heat losses to the ambient air. The second reason is probably due to the large difference between the ambient temperature and the sun’s temperature. Also, it is determined

Fig. 9. Variation of COP as a function of generator temperature. 8

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destruction rate is due to solar collectors. A similar result was reported by Li and Zhang [29]. In their study, the relative irreversibility of the solar collectors is 91.08% for the experimental data of August 2004. Eliminating this amount from the total destruction rate, the blue colored part of the figure shows the exergy destruction rate of the rest of the system components. In Fig. 13, the total exergy destruction rate is 0.661 kW for the H2O-LiBr integrated system excluding solar collectors. The highest destruction rate is calculated for the turbine operation due to utilizing an expansion valve instead of it. In order to reduce this amount, a possible solution is replacing it with an actual turbine. In addition, the second highest destruction rate occurs in the absorber followed by pump and condensers. Similar to the previous result, for the NH3-H2O integrated system, the highest exergy destruction rate belongs to the turbine followed by the absorber. The exergy destruction rate, in this case, is calculated to be 0.686 kW, excluding the solar collectors again. Fig. 10. Variation of generator and evaporator capacities as a function of generator temperature.

5. Conclusions In this study, the aim was to assess the feasibility of the ARS with the solar-assisted tCO2-RC. The analyses were performed using the experimental data from the solar-based novel power generation system. The experimental system was analyzed energetically and exergetically with the integration of the proposed absorption refrigeration for H2OLiBr and NH3-H2O refrigerant couples. The experiments were started at 06:00 in the morning and the analyses were performed using the experimental data for the time interval between 9:00–15:00 due to the solar radiation intensity. Based on the investigations, the following conclusions can be drawn from the present study: 1. From the experimental measurements, the maximum solar radiation was recorded as 0.96 kW/m2, and the maximum CO2 temperature at the exit of the solar collector was measured as 209 °C. 2. The time-average turbine power generation was calculated as 0.342 kW. The maximum power generation was determined as 0.385 kW at noon. 3. The maximum energy efficiency of the tCO2-RC alone was found to 6.5%. The COP values of the ARS were calculated as 0.75 and 0.52 for H2O-LiBr and NH3-H2O, respectively. The overall energy efficiency of the integrated system was determined as 29.62% with H2O-LiBr ARS, and it was found to be 21.51% with NH3-H2O ARS. 4. The overall exergetic efficiency of tCO2-RC with H2O-LiBr ARS was found to be 5.36% while it is calculated as 4.99% for tCO2-RC with NH3-H2O ARS. 5. The highest exergy destruction was observed in solar collectors with an amount of 90% of the total destruction.

Fig. 11. Exergy destruction rates as a function of operation hours.

Based on the bullet points reported above, it was concluded that the ARS working with H2O-LiBr can be successfully incorporated with the experimental solar-assisted tCO2-RC for cleaner energy production and air-conditioning purposes. Therefore, the future work will concentrate on the construction of an experimental ARS working with H2O-LiBr and integrating it with the solar-assisted tCO2-RC. Another important practical implication is that more experiments should be performed with the actual turbine in order to fully develop the potential of the experimental device. Accordingly, another future study on the CO2 turbine would be very interesting that contains an investigation of the flow characteristics inside the turbine, examination of the possible ways for the performance improvement of the turbine and optimization of the turbine for the specific operating conditions.

Fig. 12. Exergy efficiencies as a function of operation hours.

with H2O-LiBr ARS is found to be 6.63 kW and for tCO2-RC integrated NH3-H2O ARS it is calculated as 6.66 kW. The relative irreversibility rate of each system component is illustrated in Fig. 13 for the integrated system with H2O-LiBr and in Fig. 14 for the integrated system with NH3-H2O. It is clear from the figures that, approximately 90% of this

CRediT authorship contribution statement Onder Kizilkan: Conceptualization, Methodology, Formal analysis, Investigation, Software, Writing - original draft, Writing - review & editing, Visualization. Hiroshi Yamaguchi: Conceptualization, Data 9

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Table 3 Thermodynamic property data of tCO2–RC with H2O-LiBr ARS.

0 0 0 1 2 3 4 5 6 7 8 9 10 11 14 15 16 17 18 19 20 21 22 23 24 25

Fluid

P (kPa)

T (°C)

H2O-LiBr water CO2 CO2 CO2 CO2 CO2 CO2 H2O-LiBr H2O-LiBr H2O-LiBr H2O-LiBr H2O-LiBr H2O-LiBr water water water water water water water water water water water water

101.3 101.3 101.3 8769 4859 4803 4703 8709 0.9353 7.385 7.385 7.385 7.385 0.9353 7.385 7.385 0.9353 0.9353 101.3 101.3 101.3 101.3 101.3 101.3 101.3 101.3

23.08 23.08 23.08 196.7 159.6 41.1 10.77 17.44 40 40 73.1 95 53.75 53.75 95 40 6 6 9.311 13.23 19.31 27.62 21 16 19.31 26.36

m (kg/s)

h (kJ/kg)

s (kJ/kgK)

e (kJ/kg)

Ėx (kW)

0.01347 0.01347 0.01347 0.01347 0.01347 0.005306 0.005306 0.005306 0.004719 0.004719 0.004719 0.000587 0.000587 0.000587 0.000587 0.2 0.2 0.05 0.05 0.06574 0.06574 0.05 0.05

46.75 96.9 −2.568 126.3 100.8 −35.39 −279.2 −268.6 103.4 103.4 169.4 246.2 172 172 2678 167.5 167.5 2512 39.23 55.64 81.12 115.9 88.19 67.26 81.12 110.6

0.1637 0.3402 −0.00768 −0.4786 −0.4314 −0.7996 −1.645 −1.624 0.2334 0.2334 0.4338 0.5027 0.2886 0.2886 8.564 0.5724 0.6011 8.999 0.1408 0.1986 0.2866 0.4037 0.3107 0.239 0.2866 0.3862

268.4 228.9 201.8 208.4 212.8 38.12 38.12 44.72 101.1 90.37 90.37 145.3 1.851 −6.661 −150.1 1.385 0.7022 0.1013 0.1437 0.03076 0.3601 0.1013 0.07506

3.614 3.083 2.717 2.806 2.866 0.2023 0.2023 0.2373 0.4772 0.4264 0.4264 0.08525 0.001086 −0.00391 −0.0881 0.2769 0.1404 0.005066 0.007183 0.002022 0.02367 0.005066 0.003753

X%

0.5723 0.5723 0.5723 0.6435 0.6435 0.6435 0 0 0 0

Acknowledgments

curation, Methodology, Investigation, Validation, Writing - review & editing, Resources.

The Authors gratefully acknowledge the financial support from the Scientific and Technological Research Council of Turkey (TUBITAK) and the assistance from Doshisha University Energy Conversion Research Center.

Declaration of Competing Interest 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. Table 4 Thermodynamic property data of tCO2–RC with NH3-H2O ARS.

0 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

Fluid

P (kPa)

T (°C)

NH3-H2O water CO2 CO2 CO2 CO2 CO2 CO2 NH3-H2O NH3-H2O NH3-H2O NH3-H2O NH3-H2O NH3-H2O NH3 water NH3 NH3 NH3 NH3 water water water water water water water water water water

101.3 101.3 101.3 8769 4859 4803 4703 8709 526.6 1556 1556 1556 1556 526.6 1556 1556 1556 1556 526.6 526.6 101.3 101.3 101.3 101.3 101.3 101.3 101.3 101.3 101.3 101.3

23.08 23.08 23.08 196.7 159.6 41.1 10.77 17.44 40 40.1 75.47 95 53.82 53.28 95 85 45.11 40 5.593 6 9.311 13.23 19.31 26.73 21 16 19.31 24.15 19.31 20.41

m (kg/s)

h (kJ/kg)

s (kJ/kgK)

e (kJ/kg)

Ėx (kW)

0.01346 0.01346 0.01346 0.01346 0.01346 0.00652 0.00652 0.00652 0.005615 0.005615 0.005615 0.000958 0.000053 0.000905 0.000905 0.000905 0.000905 0.2 0.2 0.05 0.05 0.04551 0.04551 0.05 0.05 0.05 0.05

268.5 96.9 −2.568 126.3 100.8 −35.39 −279.2 −268.6 −60.22 −58.95 102.7 193.8 6.14 6.14 1485 145.7 1310 190.7 190.7 1244 39.23 55.64 81.12 112.2 88.19 67.26 81.12 101.4 81.12 85.73

1.621 0.3402 −0.00768 −0.4786 −0.4314 −0.7996 −1.645 −1.624 0.4372 0.4372 0.926 1.181 0.6407 0.6444 4.731 1.051 4.223 0.6578 0.6954 4.455 0.1408 0.1986 0.2866 0.3914 0.3107 0.239 0.2866 0.3553 0.2866 0.3023

268.4 228.9 201.8 208.4 212.8 120.2 121.5 138.3 153.9 126.3 125.2 393.9 144.5 369.3 305.8 294.7 233.9 1.385 0.7024 0.1013 0.09322 0.03076 0.3601 0.1013 0.008055 0.1013 0.05067

3.613 3.082 2.717 2.805 2.866 0.7838 0.792 0.902 0.8643 0.7092 0.703 0.3774 0.007766 0.3341 0.2766 0.2665 0.2116 0.2769 0.1405 0.005066 0.004661 0.0014 0.01639 0.005066 0.000403 0.005066 0.002534

10

X%

0.5092 0.5092 0.5092 0.4302 0.4302 0.4302 0.9706 0.483 0.9996 0.9996 0.9996 0.9996

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Fig. 13. Relative exergy destruction rates for tCO2-RC with H2O-LiBr ARS.

Fig. 14. Relative exergy destruction rates for tCO2-RC with NH3-H2O ARS.

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