A novel solar system integrating concentrating photovoltaic thermal collectors and variable effect absorption chiller for flexible co-generation of electricity and cooling

Energy Conversion and Management 206 (2020) 112506

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A novel solar system integrating concentrating photovoltaic thermal collectors and variable effect absorption chiller for flexible co-generation of electricity and cooling ⁎

Li Lin, Yao Tian, Yu Luo , Chongqi Chen, Lilong Jiang

T

⁎

National Engineering Research Center of Chemical Fertilizer Catalyst (NERC-CFC), School of Chemical Engineering, Fuzhou University, Fujian 350002, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Solar cooling Variable effect absorption refrigeration cycle Dynamic simulation Exergy efficiency Concentrating photovoltaic and thermal (CPV/ T) collector

Traditional single effect and double effect absorption chillers have relatively narrow working temperature ranges, which limits their application of solar systems. This study proposes a novel solar system integrating concentrating photovoltaic and thermal collectors, and a variable effect absorption chiller, for more flexible and efficient co-generation of electricity and cooling. In this study, variable effect chiller was optimized, showing that three working modes, combined with optimized control, make variable effect chillers a superior choice to the single effect and double effect types. Then, dynamic simulations of the solar co-generation system were performed, in order to study the effects of temperature control on system performance. The results showed that, a high turn-off temperature for the chiller generally results in higher cooling power, shorter working hours for the chiller, and in some cases, a frequent on–off cycling of chiller. With the increase in working temperature level, the cooling exergy efficiency increases, but total exergy efficiency decreases due to the photovoltaic cell’s degraded performance. The total exergy efficiency is approximately 32%–33%. A larger difference between turnon and turn-off temperatures delays the start time of the chiller while ensuring the full use of solar energy. By adjusting the temperature control strategy, the novel solar co-generation system can offer a cooling-electricity ratio from 1.4 to 2.0, which is capable of meeting the demands in many cases. The proposed system offers flexible co-generation of cooling and electricity.

1. Introduction Solar energy utilization is a promising way to solve the problems of global warming, climate change, and fossil fuel depletion. Researchers around the world have worked on solar energy systems during the past decades, including solar water heating, solar cooling, solar thermal power, solar photovoltaic (PV) systems and various kinds of solar cogeneration systems. The invention of photovoltaic/thermal (PV/T) collectors, which combine the PV cell and solar thermal collector, have provided cogeneration of heat and electricity from solar energy [1]. Different kinds of solar co-generation systems using the thermal energy from PV/T collectors have been studied in the literature, such as solar combined heat and power systems [2], solar desiccant cooling driven by PV/T [3]. Using concentrating systems to increase radiative flux helps offset the low power density of solar energy, which decreases capital costs and increases the operating temperature of thermal systems. Thus concentrating photovoltaic/thermal (CPV/T) collectors have even more

⁎

potential than planar PV/T collectors for solar co-generation systems [4]. Examples include use of the high-temperature heat provided by the CPV/T to drive an Organic Rankine Cycle [5], to support proton exchange membrane electrolyzers [6], or in integration with a heat pump [7]. The lithium bromide/water (LiBr/H2O) absorption chiller, which has been commercially available for decades, is considered as one of the most desirable methods for solar thermal cooling. Traditional single effect and double effect absorption chillers have been extensively studied for use in various solar cooling system [8]. Bellos et al. [9] compared four solar cooling systems with single effect chillers driven by four different solar thermal collectors. Bellos et al. [10] also investigated the viability of solar cooling in various locations worldwide. Shirazi et al. [11] studied the effects of control scenarios and system arrangements on the dynamic performance of solar cooling systems. Shirazi et al. [12] also compared solar cooling systems based on single effect, double effect and triple effect chillers. The results showed that the fraction of direct normal irradiation affects the optimal choice of

Corresponding author. E-mail addresses: [email protected] (Y. Luo), [email protected] (L. Jiang).

https://doi.org/10.1016/j.enconman.2020.112506 Received 13 October 2019; Received in revised form 12 January 2020; Accepted 13 January 2020 0196-8904/ © 2020 Published by Elsevier Ltd.

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Nomenclature

R1 t T w x

Abbreviations 1.n C COP CPV/T E H HA HC HG LA LG1 LG2 LiBr/H2O PV PV/T SCOP SHX

variable effect between single and double effect condenser coefficient of performance concentrating photovoltaic and thermal evaporator daily total irradiation high pressure absorber high pressure condenser high pressure generator low pressure absorber 1st low pressure generator 2nd low pressure generator lithium bromide/water photovoltaic photovoltaic/thermal solar coefficient of performance solution heat exchanger

Greek letter α βT δ δTon-off Δt ε ηex ηopt ηPV λ μ ρPVT ρ σ Φ

English letter a A b cp dch CCPVT DD E h hc H I m M NCPVT ND PPV,net Q

solution distribution ratio time (h) temperature (K) width of receiver in CPV/T collector solution concentration

absorptance temperature coefficient of PV cell solar declination difference between Ton and Toff time step (s) emissivity exergy efficiency optical efficiency electrical efficiency of PV cell thermal conductivity (W m−1 K−1) dynamic viscosity (Pa s) reflectance of PV cell density Stephan-Botzmann constant latitude

Subscript or superscript

operation status of CPV/T area of receiver (m2) operation status of variable effect chiller specific heat capacity (J kg−1 K−1) diameter of channel in CPV/T collector concentration ratio number of the day in the year exergy (J) specific enthalpy (J kg−1) convection heat transfer coefficient (W m−2 K−1) daily total irradiation radiation (W m−2) mass flow rate fluid mass in each zone of storage tank number of CPV/T collectors day duration net electricity output from PV cell (W) heat load

b beam col collector in inlet min, max minimum, maximum off turn-off of chiller oil diathermic oil on turn-on of chiller out outlet PVT PV/T module in CPV/T collector rec receiver in CPV/T collector r refrigerant s diathermic oil in HG st storage tank top top layer in CPV/T collector total total w external circuit in chiller

of SHC systems with single effect chillers driven by evacuated tube and CPV/T, demonstrating the potential for high primary energy savings. Buonomano et al. [20] also compared the performance of adsorption chiller systems driven by PV/T and CPV/T. Behzadi et al. [21] performed exergoeconomic analysis and multi-objective optimization of a CPV/T waste heat driven double effect absorption chiller integrated with a geothermal cycle. Behzadi et al. [22] also carried out a multioptimization analysis on a novel solar-based integrated energy system composed of CPV/T, a double effect absorption chiller, a thermoelectric generator and a proton exchange membrane electrolyzer. Despite abundant research and widespread use [23], traditional absorption chillers have some limitations. The most important limitation is a small working range, which makes the absorption chiller hard to integrate with heat sources that run at variable temperatures. For example, when evaporation temperature is 5 °C and cooling temperature is 35 °C, the generation temperature range of LiBr/H2O absorption chiller would be 85–110 °C, and higher than 140 °C for single effect cycle and double effect cycle, respectively [24]. In the optimization research on different solar absorption chillers by Shirazai et al. [25], the hot water inlet temperatures are set to 98 °C and 180 °C for single effect chiller and double effect chiller respectively. Xu [26] compared

chiller, since the concentrating approach cannot use diffuse irradiation. Soussi et al. [13] studied a solar cooling system using a double effect chiller driven by parabolic trough collectors experimentally, and compared the data with dynamic simulations, showing that the integration of an auxiliary heater could increase the chiller’s operating time. Khan et al. [14] studied the dynamic performance of solar cooling systems with single effect chillers driven by evacuated tube collectors, and the results showed that controlling the arrangement of hot working fluid from the chiller’s generator can improve primary energy savings. There has also been research into solar co-generation systems using absorption chillers driven by PV/T [15]. Mittelman et al. [16] investigated a CPV/T system using single effect absorption cooling, finding that under a wide range of economic conditions, the combined solar cooling and power generation plant can be comparable to the conventional alternative. Calise et al. [17] studied a solar heating and cooling (SHC) system with a single effect absorption chiller driven by PV/T collectors, showing that the system could be profitable, provided that an appropriate funding policy was available. Calise et al. [18] also performed dynamic simulations of a SHC system with double effect absorption driven by CPV/T collectors to evaluate the design and operating parameters. Buonomano et al. [19] compared the performance 2

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flexibility, and establish guidelines for temperature control of the overall system.

different solar absorption cooling systems and the cutoff driven temperatures are set to 95 °C and 150 °C for single effect cycle and double effect cycle respectively. Generally, temperatures below working ranges are insufficient to drive the absorption refrigeration cycle, and temperatures above working ranges may cause severe crystallization problems. Thus, when designing absorption chillers in a solar system, measures such as auxiliary heaters have to be included to maintain temperatures suitable for the chiller. Moreover, when combined with CPV/T collectors, the small working ranges for the chillers also put temperature constraints on CPV/T collectors. Thus, the electricity performance of CPV/T collectors is constrained by the temperature chosen for the absorption chiller. Recently, the development of new absorption cooling cycles has helped to enlarge working temperature ranges. Xu et al. [27] introduced a novel LiBr/H2O absorption cycle with an absorber and generator heat exchanger, obtaining 1.n effect performance. Based on these results, they developed a variable effect chiller that covered a wide working range from 85 °C to 150 °C. Xu et al. [26] also studied the application potential of variable effect cycles in solar cooling systems, and the results showed that the variable effect system has a high solar cooling fraction, low auxiliary heat input and medium solar efficiency. This paper proposes a novel solar system integrating CPV/T collectors and a variable effect absorption chiller for co-generation of electricity and cooling. To the authors’ knowledge, there is a lack of this type of solar system in open literature. Considering the high energy efficiency of CPV/T, and the wide adaptability of variable effect chillers, this novel system would seem promising. The optimal performance of a variable effect chiller was studied. Then dynamic simulations of the solar co-generation system were carried out. The effects of the chiller’s turn-on and turn-off temperatures were studied to show the system’s

2. System description The layout of the solar co-generation system under consideration is shown in Fig. 1. It is designed for an office building with CPV/T collectors covering the roof area. Due to the low power density of solar energy, the actual cooling demands of the building is much more than that could be produced by solar system. Electricity-driven air-conditioners are used as complementary system to the solar system to meet the cooling demand. It is assumed that, cooling production from solar energy could be consumed instantly and totally by the building. Thus, cooling load variation on user side is not considered in this study. The examined solar system can be separated to three main parts: the CPV/T collectors, the storage tank and the variable effect chiller. Each subsystem is introduced briefly here.

• The CPV/T collector converts solar radiation into electricity and

•

thermal energy. It consists of a parabolic dish concentrator, equipped with a two-axis tracking system. A planar receiver, as shown in Fig. 1(b), is placed at the focus of the concentrator. The side of the planar receiver facing the dish concentrator is covered by a InGaP/InGaAs/Ge triple-junction solar cell [28], whereas the top side of the receiver is covered with thermal insulation. The receiver includes an inner channel for the working fluid. In this study, diathermic oil [29] was used as a working fluid. The Storage tank with stratification is used in this study. Three mixing isothermal zones are set with the hotter zones in the upper part and the colder zones in the lower part, which could represent a

Fig. 1. Schematic diagram of the novel solar co-generation system: (a) the whole system; (b) subsystem of CPV/T collector. 3

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•

chiller. For 1.n effect mode, all the components are at work. For double effect mode, HA and LG2 do not operate, and the chiller act as a parallel double effect chiller without the high solution heat exchanger. The principles of mass conservation and energy conservation are applied to each component of the variable effect chiller, as summarized in Eqs. (3)–(5) for the brevity. More details about conservation equations of the various components can be found in reference [27].

reasonable degree of stratification [30]. Diathermic oil coming from the CPV/T collectors enters the upper part of storage tank (left side), and is eventually delivered to the chiller’s generator (right side). The colder stream coming from the generator returns to the lower part of the storage tank and is then delivered to the CPV/T collector to absorb heat. The LiBr/H2O variable effect chiller used in this paper was first designed by Xu et al. [27]. It can be adapted for different heat source temperatures by parameter changes instead of construction changes. The variable effect chiller can work in single effect mode, 1.n effect mode and double effect mode under different generation temperature ranges. The 1.n effect mode is shown in Fig. 2. The remarkable feature here is that the refrigerant steam entering the high-pressure condenser (HC) has double effect refrigeration, while the steam entering the high-pressure absorber (HA) has single effect refrigeration. Thus, by adjusting the operating parameters, a 1.n effect can be achieved, permitting a transition between single effect mode and double effect mode.

∑ min + ∑ mr,in = ∑ mout + ∑ mr,out

(3)

∑ min xin = ∑ mout xout

(4)

∑ min hin + ∑ mr ,in hr ,in + Q = ∑ mout hout + ∑ mr ,out hr ,out

(5)

The cycle coefficient of performance (COP) is defined by Eq. (6), while the solar coefficient of performance (SCOP) is determined by Eq. (7).

COP =

The pumps P1 and P2 in Fig. 1 are used to control the turn-on and turn-off of the CPV/T collectors and chiller, respectively. In the operations discussed in this paper, when beam radiation was lower than a certain value, pump P1 would be shut down. The temperatures for the chiller’s turn-on and turn-off are chosen as factors which will be discussed later in the details section.

QE QHG

SCOP =

(6)

QE QE = Qsolar NCPVT CCPVT Arec Ib

(7)

In order to analyze the effects of operation control on the chiller’s performance, two independent internal variables are chosen. One is the outlet temperature of the high-pressure condenser (THC), and the other is the solution distribution ratio R1, as shown in Eq. (8), represents the ratio of the solution flow rate at the HG inlet to the solution flow rate at the LA outlet. These two internal variables are easily monitored and controlled from the viewpoint of practical operation, and this is commonly done in parallel double effect chillers.

3. Mathematical model Mathematical models of each part of the system illustrated in Fig. 1 are briefly introduced in this section, and more details can be found in the supplementary document.

R1 =

3.1. Concentrating photovoltaic and thermal collector

mHG, in mLA, in

(8)

The main parameters of the variable effect chiller are given according to the design requirements of 50 kW cooling power at a generation temperature of 125 °C. Thus, the solution mass flow rate at the outlet of the LA is set to 0.269 kg/s in all conditions, and the heat transfer coefficient of the high-pressure generator is 3.426 kW/K. The inlet and outlet temperatures of the external circuits are given in Table 2, which describes typical operating conditions [31].

The model for the CPV/T collector is based on energy balances. The overall energy balance of the entire receiver is as Eq. (1). The terms on the left-hand side of the equation describe the total incident radiation striking the receiver. The terms on the right-hand side refer to the heat absorbed by the diathermic oil, the electricity from the PV cell, the reflective radiation from the PV cell, the radiant loss from the receiver and the convective loss from the receiver, respectively.

3.3. Storage tank

Arec Ib CCPVT ηopt + Atop Itot αtop = mcol (hout − hin) The storage tank is separated into three mixing zones. In each zone

+ CCPVT Arec Ib ηopt ηPV + CCPVT Arec Ib ηopt ρPV 4 4 4 4 + Atop εtop σ (Ttop − Tsky − Tconc ) + APVT εPVT σ (TPVT )

+ APVT hc, PVT (TPVT − Ta) + Atop hc, top (Ttop − Ta)

(1)

The electrical efficiency of the triple-junction PV is given as Eq. (2) [16]:

HC

HA

HG

ηPV = 0.298 + 0.0142 ln CCPVT + βT (TPV − 298) = 0.298 + 0.0142 ln CCPVT + [−0.000715 + 6. 97 × 10−5 ln CCPVT ](TPV C

LG1

LG2

Pressure

− 298) (2) The parameters for the CPV/T collector are shown in Table 1.

SHX

3.2. Variable effect chiller The variable effect chiller has nine components in total as shown in Fig. 2. There are two pairs of thermally coupled components, the highpressure absorber (HA) and 2nd low-pressure generator (LG2), and the high-pressure condenser (HC) and 1st low-pressure generator (LG1). By altering the operation control, the variable effect chiller can work in three different working modes. For single effect mode, HA-LG2 and HCLG1 do not operate, and the chiller functions a traditional single effect

E

LA Solution concentration

Fig. 2. The 1.n effect mode of the variable effect chiller (dashed line: refrigerant loop; solid line: solution loop). 4

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Ton and Toff in this paper, are often set to the same value which is the cut-off temperature of chiller. However, from the view of operation, this is not necessary, and setting different values for Ton and Toff gives the system control an additional degree of freedom, especially for variable effect chiller which has the advantage of large working temperature range. The basis of control logic is that the chiller should be on when Tst1 > Ton and the chiller should be off when Tst1 < Toff. Thus, Ton ≥ Toff is set in this paper, and the difference between Ton and Toff is defined as δTon-off. The whole control logic is shown in Fig. 3(b). Generally, the upper bound of Ton and the lower bound of Toff are related with the temperature demands of CPV/T collectors and absorption chiller. According to Ref. [29], working temperature for CPV/ T collector could be as high as 180 °C, while generation temperature for variable effect chiller could range from 90 °C to 150 °C [27]. Considering the necessary heat transfer temperature difference, three different values for Toff and δTon-off are selected as shown in Table 3.

Table 1 Parameters of CPV/T collectors. parameters Aconc cp,oil dch NCPV/T W αtop

value

unit

12 2980 0.02 30 0.6 0.3

2

m J·(kg·K)-1 m m

parameters

value

unit

εtop, εPVT ηopt λoil μoil ρoil ρPVT

0.2 0.9 0.113 4558 × 10-6 870 0.03

W·(m·K)-1 Pa·s kg·m−3 –

Table 2 Temperatures of external circuits of variable effect chiller.

Tw,LA,in Tw,LA,out Tw,C,in Tw,C,out Tw,E,in Tw,E,out THG,out TLA,out TC TE

State point

Temperature (oC)

cooling water at LA inlet cooling water at LA outlet cooling water at C inlet cooling water at C outlet chilled water at E inlet chilled water at E outlet generation temperature of HG solution temperature at LA outlet refrigerant temperature at C outlet refrigerant temperature at E outlet

TLA,out-8 TLA,out-3 TC-8 TC-3 TE+8 TE+3 90–150 35 40 5

3.4. Weather data For this study, weathaer data for Fuzhou in the southeast of China was selected for modeling purposes. Specifically, the minimum daily temperature, maximum daily temperature and solar irradiation for Fuzhou are taken from the EnergyPlus database [33]. The ambient temperature distribution can be approximated by Eq. (12) [34], where tmax has been selected to be 14:00 to present the time of highest daytime ambient temperatures.

an energy balance is made as shown in Eqs. (9)–(11) [9]. The ratio of total aperture area to the storage tank volume is set to 0.03, which is in the typical range of solar thermal systems [32]. The ratio of diameter to height is set to 2, as done in Ref. [9]. The mass flow rate of diathermic oil in the solar collectors is equal to the flow rate used to drive the chiller’s generator.

Mcp

Mcp

Mcp

ND =

(9)

∂Tst 2 = aNCPVT mcol cp (Tst1 − Tst 2) + bms cp (Tst 3 − Tst 2) − hc, st Ast 2 ∂t (Tst 2 − Ta)

2 arccos( −tan Φ·tan δ ) 15

δ = 23.45 sin ⎛2π ⎝ (10)

284 + DD ⎞ 365 ⎠

(13)

(14)

The total incident solar irradiation in the collector surface is calculated from Eq. (15) [35], where H is the daily total irradiation in the collector level. The ratio of beam radiation to total incident radiation is set to 0.6 for all cases, estimated based on actual radiation data from EnergyPlus [33].

∂Tst 3 = aNCPVT mcol cp (Tst 2 − Tst 3) + bms cp (Tsout − Tst 3) − hc, st Ast 3 ∂t (Tst 3 − Ta)

(12)

The day duration in the region can be calculated from the Eq. (13) [30], for a given latitude Φ. The solar declination (δ) is given by Eq. (14), where DD is the number of the day in the year, from1 to 365.

∂Tst1 out = aNCPVT mcol cp (Tcol − Tst1) + bms cp (Tst 2 − Tst1) − hc, st Ast1 ∂t (Tst1 − Ta)

Tmax + Tmin T − Tmin t − tmax ⎞ + max cos ⎛2π 2 2 24 ⎠ ⎝

Tam =

(11)

The coefficients a and b are used to control the on and off states of collector and chiller, respectively. 1 refers to the status of turn-on, and 0 to turn-off status. The operation status of CPV/T collectors is determined by beam irradiation level as shown Fig. 3(a). The operation status of chiller is determined by temperature in tank and the setting temperatures for chiller control. It should be noticed that, in many researches, the start-up and shut-down temperatures of absorption chiller,

Itotal =

πH π (t − 12) ⎞ cos ⎛ 2ND ND ⎠ ⎝ ⎜

⎟

(15)

Three typical days are selected to represent the different weather conditions that would appear during the summer, as shown in Table 4. The hourly distribution of beam radiation and ambient temperature are shown as Fig. 4.

Fig. 3. Control strategy for the novel solar system: (a) control of CPV/T collectors; (b) control of variable effect chiller. 5

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and electricity exergy efficiency, respectively.

Table 3 Turn-on and turn-off temperatures for chiller control.

∫ EE dt + ∫ PPV , net dt

Ton (δTon-off=0.1)

Ton (δTon-off=3)

Ton (δTon-off=6)

110.1 125.1 140.1

113 128 143

116 131 146

Toff=110 Toff=125 Toff=140

ηex , total =

Tmax (°C)

H (kWh/m2)

DD (-)

Day 1

30

37

5.5

182

Day 2

28

35

4.8

212

Day 3

25

32

4

252

(19)

The chiller’s single effect and double effect modes are almost the same as traditional absorption chillers, so only the 1.n effect mode was analyzed in detail in this study. Fig. 5 shows the performance maps of the chiller’s COP when THG,out is assigned different values(100 °C, 110 °C, 120 °C, 130 °C, 140 °C), with the two internal variables THC and R1 varying accordingly. Notably, the available ranges for THC and R1 change with the generation temperature. The relevant constraints include heat transfer demands, as shown in assumptions in section 3.3, as well as crystallization constraints. From the point of single variable analysis, the cycle COP does not present the same trends in all cases as a function of increases in R1 or THC. This suggests the complexity of the 1.n effect mode. A more useful way to analyze the results illustrated in Fig. 5 is to look for guidelines for operation adjustment of a real machine. Reduction of THC can be realized by improving the heat transfer condition between HA and LG2, as discussed in Xu et al. [27]. From Fig. 5, it is clear that decreasing THC does not always increase the cycle COP. The maximum COP under a given THG,out can only be obtained when the independent variables (THC, R1) are optimized. The optimized values of THC and R1 for different THG,out values in 1.n effect mode can be determined from calculation results like Fig. 5, and it is shown in Fig. 6(a). It can be seen that the optimal THC increases with THG,out, and the optimal R1 decreases with THG,out. The optimized

4

⎟

(17)

The exergy of electricity equals the electricity output, and the exergy of the cold energy used for cooling purposes is calculated as Eq. (18). ⎜

∫ Esolar dt day

4.1. Performance map and optimal performance curve of variable effect chiller

A time step of 60 s was selected, after a sensitivity analysis. Dynamic simulations were carried out with all components at an initial temperature of 100 °C. By solving the system with the same weather data for 4 consecutive days, it was demonstrated that the system behavior eventually converged. Exergy analysis was used to evaluate the performance of the new proposed solar co-generation system. The exergy flow of solar energy can be calculated as Eq. (17) [36]. Here, the sun temperature and the reference temperature are equal to 4350 K and 298.15 K, respectively.

Tref EE = QE ⎛ − 1⎞ ⎠ ⎝ TE

day

The variable effect chiller’s performance under different conditions was analyzed to find the effects of operation control and to obtain the optimal performance curve. Then, the optimal performance curve of the variable effect cycle was used for the dynamic simulation of the cogeneration solar system. In addition to presenting these results, this study discusses the chiller’s turn-on and turn-off temperatures to explore the flexibility of the new proposed solar system.

(16)

⎜

∫ Esolar dt day

∫ PPV , net dt +

4. Results and analysis

The mathematical models of this system mostly consist of sets of algebraic equations, except the model of storage tank, which includes differential terms with respect to time, in order to handle the dynamic performance of the system. The differential terms are discretized according to Eq. (16).

⎟

day

These mathematical models were solved with Engineering Equation Solver (EES), which provides built-in thermo-physical property functions useful for engineering calculations and can automatically solve a set of algebraic equations [37].

3.5. Dynamic simulation and system performance

⎜

∫ Esolar dt

∫ EE dt =

= ηex , cooling + ηex , elec

Tmin (°C)

4 ⎛ Tref ⎞ 1 ⎛ Tref ⎞ ⎤ Esolar = A conc NCPVT Ib ⎡ ⎢1 − 3 Tsun + 3 Tsun ⎥ ⎝ ⎠ ⎝ ⎠⎦ ⎣

day day

Table 4 Weather data for three typical days in Fuzhou in summer.

∂T T NEW − T OLD = ∂t Δt

day

⎟

(18)

The exergy efficiency of the solar co-generation system can be calculated by integration throughout the whole day, as shown in Eq. (19). The two terms on the right-hand side are the cooling exergy efficiency

Fig. 4. Weather conditions of the three typical days: (a) beam radiation; (b) environment temperature. 6

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Fig. 5. Performance maps of the variable effect chiller in 1.n effect mode: (a) THG,out = 100 °C; (b) THG,out = 110 °C; (c) THG,out = 120 °C; (d) THG,out = 130 °C; (e) THG,out = 140 °C;

adaptability of variable effect chiller, the whole system can work at widely different temperature levels with varied performance. The electricity output power of the CPV/T collectors did not differ much due to the low temperature coefficient of the multi-junction PV cell as shown in Eq. (2). However, the cooling power of the chiller did change significantly when the whole system operated at different temperatures. A higher temperature level leads to greater cooling power, because the COP of the variable effect chiller will tend to increase with the generation temperature, as shown in Fig. 6. A higher temperature also leads to shorter working hours for chiller. Moreover, when the turn-off temperature for chiller was set to 140 °C, the chiller was switched off twice in the morning. This situation did not happen at turn-off

COP in the 1.n effect mode increases with THG,out, presenting a transition region between the single effect mode and double effect mode, as shown in Fig. 6(b). Thus, the availability of three working modes does enlarge the working temperature range of the chiller, especially in the 95 °C–140 °C range, where the 1.n effect mode prevails. The performance curve obtained by the means discussed in this section was later used in the dynamic simulation of the solar co-generation system. 4.2. Effects of chiller turn-off temperature: Hourly performance The hourly outputs of the solar co-generation system on Day 1 are shown in Fig. 7(a) and (b). It can be seen that, because of the wide 7

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Fig. 6. Performance of variable effect chiller: (a) Optimized R1, THC and COP in 1.n effect mode; (b) COP in three working modes.

Fig. 7. Effects of chiller turn-off temperature on hourly performance of the solar co-generation system: (a) electricity output in Day 1; (b) cooling output in Day 1; (c) electricity output in Day 3; (d) cooling output in Day 3;

temperature of 140 °C. However, working at lower turn-off temperatures such as 120 °C and 110 °C, this effect was still not observed.

temperature of 125 °C or 110 °C. The reason is when the system works at a high enough temperature, the heat consumption rate of chiller’s generator can exceed the heat input rate from CPV/T, resulting in the decrease of the temperature in the storage tank. Fig. 7(c) and (d) shows the hourly output of the solar co-generation system on Day 3. Since the radiation on Day 3 is less than on Day 1, the electricity output of the CPV/T collectors is likewise lower on Day 3. The decrease of solar radiation also leads to shorter chiller working hours for Day 3, compared with Day 1. However, the cooling power level in Day 3 was nearly the same as that in Day 1, because cooling power is mainly determined by the temperature level in the tank. Again, different turn-off temperatures had limited effect on electricity output. It can be seen that, when solar radiation decreased, the frequent on–off cycling of the chiller became more pronounced at a turn-off

4.3. Effects of chiller turn-off temperature: Daily performance Fig. 8 shows the total outputs of the solar co-generation system on Day 1, Day 2 and Day 3. The electricity and cooling outputs increase with beam radiation. As the chiller’s turn-off temperature increases, the electricity output decreases, while cooling output increases. Quantitatively, when the turn-off temperature for chiller decreases by 30 °C from 140 °C to 110 °C, the electricity output rises about 4% and the cooling output would decreases by 14%. As shown in Fig. 8(b), the ratio of cooling output to electricity output increases with radiation and with increasing chiller turn-off temperature, ranging from 1.4 to 2.0. 8

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Fig. 8. Effects of chiller turn-of temperature on daily performance of solar co-generation system: (a) cooling output and chiller working hours; (b) electricity output and cooling/electricity ratio; (c) SCOP and ηex,cooling; (d) ηex and ηex,elec.

Fig. 9. Effects of δTon-off on the system’s cooling performance: (a) cooling power on Day 1 with Toff 110 °C; (b) cooling power on Day 1 with Toff 125 °C; (c) cooling power on Day 3 with Toff 110 °C; (d) daily cooling output and chiller working hours; (e) cooling power on Day 3 with δTon-off 15 °C.

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delayed for higher values of δTon-off, due to the higher temperature demand for the chiller. The shut-down time for chiller is likewise delayed with the increase of δTon-off. Fig. 9(d) shows the total cooling output during each day and the effective working hours for different δTon-off. This shows that increasing δTon-off slightly increases the cooling output for a given day, and slightly shorten the chiller’s working hours. This is similar to the effects of raising turn-off temperatures. These results suggest the experiment of testing a much greater value of δTon-off to see the effects of different combinations of turn-on and turn-off temperatures. Fig. 9(e) shows the hourly cooling power for a turn-on temperature of 125 °C and a turn-off temperature of 110 °C. The results of turn-off temperatures of 110 °C and 125 °C with default δTonoff values are also given for comparison. Adjusting the temperature control yielded a cooling distribution of 11:00–19:00, compared to the distribution of 8:30–17:00 in the other two cases. Thus, for places where cooling in the morning is not needed, a higher value of δTon-off is recommended to appropriately match the hourly cooling distribution. A more critical reason for recommending higher values of δTon in these cases is that, while the chiller does not operate in the morning, the CPV/T collectors keep working to harvest heat from solar radiation to make up the difference between the turn-on and turn-off temperatures. In this way, the morning’s solar energy is fully utilized despite the chiller being turned off.

Table 5 Reference solar cooling systems for comparison. Systems

Configuration

Ref-system-1 [26]

concentrated collectors + storage tank + variable effect chiller CPV/T collectors + storage tank + double effect chiller

Ref-system-2 [18]

Notably, the effects of the chiller’s turn-off temperature on electricity performance mainly depend on the cell’s temperature coefficient. Thus, the chiller turn-off temperature also has an impact on the cooling/ electricity ratio, and more discussions about that can be found in the supplementary document. Fig. 8(a) also shows the effective working hours of the chiller in the solar co-generation system, and the frequent on–off cycling were considered. The working hours of the chiller decreased with the increase of the turn-off temperature. Cases with a turn-off temperature 140 °C have approximately two-thirds as many working hours as those set to 110 °C. However, too short working hours and frequent on–off cycling of the chiller are not practical. An auxiliary heater could be introduced or more CPV/T collectors could be used to drive the chiller if high temperature operation or a high cooling power from variable effect chiller are needed. One can thus choose the working temperature level of the solar co-generation system according to the demands on cooling power and working hours, which illustrates the flexibility of this new proposed solar system. Fig. 8(c) gives the SCOP and exergy efficiencies in various cases. As discussed, increased solar radiation and higher chiller turn-off temperatures increase the working temperature of the system, which increases the SCOP and cooling exergy efficiency, but decreases the electrical efficiency of the system. Notably, the total exergy efficiency of the system decreases as the turn-off temperature decreases, indicating that the change of electricity exergy dominates the change of total exergy. Fig. 8(d) further shows that the solar radiation only has a minor effect on the total exergy efficiency of the system, when considering the effects of solar radiation on cooling exergy efficiency and on electricity exergy efficiency together. This indicates that the proposed co-generation system can maintain high efficiency under diverse weather conditions.

4.5. Comparison with reference system To further demonstrate the outstanding performance of the proposed solar system, comparisons with two reference solar cooling systems were presented in this section. The reference systems had configurations similar to that in Fig. 1, composed of solar collectors, storage tank and absorption chiller, as shown in Table 5. Parameters of concentrated solar collectors in Ref-system-1 are the same as those of CPV/T collector in section 3.1, except that no PV cells were used. Series flow double effect chiller [38] with solution heat exchanger efficiency of 0.7 was used in Ref-system-2, and the cutoff temperature is set to 140 °C. Other parameters of reference systems were the same as those in section 3. Fig. 10(a) showed the exergy efficiency of reference systems on Day 1. It can be seen that, due to the electricity production from CPV/T collectors, the novel solar system and Ref-system-2 both had higher exergy efficiency than Ref-system-1, despite that Ref-system-1 produced more cooling output. Fig. 10(b) showed the hourly cooling output during Day 3. Since solar radiation is Day 3 is relatively low, Ref-system-2 encountered frequent on–off cycling. However, the novel solar system could use a much lower turn-off temperature for chiller to make continuous operation. Furthermore, flexible operation as those in Fig. 9(e) cannot be realized by Ref-system-2. Thus, the new proposed solar system has a better flexibility than Ref-system-2.

4.4. Effects of chiller turn-on temperature The effects of the chiller’s turn-on temperature are studied by adjusting δTon-off, the difference between Ton and Toff, while Toff is kept constant. Fig. 9(a)–(c) shows the hourly cooling power distribution for Day 1 and Day 3 when δTon-off is set to different values. It can be seen that the increase of δTon-off raises the temperature of the whole system leading to increased cooling power. The start-up time for the chiller is

Fig. 10. Performance comparison with two reference systems: (a) system exergy efficiency; (b) hourly cooling output. 10

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5. Conclusions [4]

This paper proposes a new design for a solar system integrating concentrating photovoltaic and thermal (CPV/T) collectors and a variable effect absorption chiller for co-generation of electricity and cooling. This novel system has the advantages of high solar energy efficiency and high adaptability of working temperature. Dynamic simulations and parametric analysis of the solar co-generation system were carried out in this study, and the main conclusions are as follows.

[5]

[6]

(1) The variable effect chiller’s temperature adaptability is enhanced by its three working modes, and the optimal performance of the 1.n effect mode can only be obtained when the solution distribution ratio and high pressure condenser temperature are optimized. (2) Temperature control has a small effect on the electricity production of CPV/T collectors due to the low temperature coefficient of multijunction PV cells, but is vital to cooling performance in the cogeneration system. Generally, a high temperature level results in high cooling power and short working hours for chiller. The ratio of cooling output to electricity output increases with radiation and temperature, ranging from about 1.4 to 2.0 in this study. (3) With higher working temperatures, the cooling exergy efficiency increases, but the total exergy efficiency decreases due to degrade PV cell performance. Generally, the total exergy efficiency is in the range of 32–33% under a variety of weather conditions. (4) A greater difference between turn-on and turn-off temperatures would delay the start time of chiller while ensuring the full use of solar energy, and it would be advisable for locations where the cooling is not needed in the morning. (5) Comparison with other two reference systems confirmed both the high exergy efficiency and good flexibility of the new proposed solar system. The wide working temperature range provides the system with flexible working performance, enabling it to adjust temperature control to meet varying demands for electrical output, cooling power, cooling working hours and cooling start time.

[7]

[8]

[9]

[10]

[11]

[12]

[13]

[14]

[15]

[16]

CRediT authorship contribution statement [17]

Li Lin: Methodology, Formal analysis, Writing - original draft. Yao Tian: Validation, Software. Yu Luo: Conceptualization, Supervision, Writing - review & editing. Chongqi Chen: Resources, Data curation. Lilong Jiang: Supervision, Project administration, Funding acquisition.

[18]

Declaration of Competing Interest

[19]

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.

[20]

Acknowledgement

[21]

The study is financially supported by the Distinguished Young Scientist Foundation from the National Natural Science Foundation of China (No. 21825801), the National Natural Science Foundation of China, NSFC (No. 21908028), Center-guided Local Science and Technology Development Projects (2018L3010) and Research Start-up Funding of Fuzhou University (XRC-18087).

[22]

[23]

[24]

References [25]

[1] Chow TT. A review on photovoltaic/thermal hybrid solar technology. Appl Energy 2010;87:365–79. https://doi.org/10.1016/j.apenergy.2009.06.037. [2] Kasaeian A, Nouri G, Ranjbaran P, Wen D. Solar collectors and photovoltaics as combined heat and power systems: a critical review. Energy Convers Manag 2018;156:688–705. https://doi.org/10.1016/j.enconman.2017.11.064. [3] Guo J, Lin S, Bilbao JI, White SD, Sproul AB. A review of photovoltaic thermal (PV/ T) heat utilisation with low temperature desiccant cooling and dehumidification.

[26]

[27]

11

Renew Sustain Energy Rev 2017;67:1–14. https://doi.org/10.1016/j.rser.2016.08. 056. Ju X, Xu C, Liao Z, Du X, Wei G, Wang Z, et al. A review of concentrated photovoltaic-thermal (CPVT) hybrid solar systems with waste heat recovery (WHR). Sci Bull 2017;62:1388–426. https://doi.org/10.1016/j.scib.2017.10.002. Rahbar K, Riasi A, Khatam Bolouri Sangjoeei H, Razmjoo N. Heat recovery of nanofluid based concentrating Photovoltaic Thermal (CPV/T) Collector with Organic Rankine Cycle. Energy Convers Manag 2019;179:373–96. https://doi.org/10.1016/ j.enconman.2018.10.066. Wang H, Li W, Liu T, Liu X, Hu X. Thermodynamic analysis and optimization of photovoltaic/thermal hybrid hydrogen generation system based on complementary combination of photovoltaic cells and proton exchange membrane electrolyzer. Energy Convers Manag 2019;183:97–108. https://doi.org/10.1016/j.enconman. 2018.12.106. Ramos A, Chatzopoulou MA, Guarracino I, Freeman J, Markides CN. Hybrid photovoltaic-thermal solar systems for combined heating, cooling and power provision in the urban environment. Energy Convers Manag 2017;150:838–50. https://doi. org/10.1016/j.enconman.2017.03.024. Shirazi A, Taylor RA, Morrison GL, White SD. Solar-powered absorption chillers: a comprehensive and critical review. Energy Convers Manag 2018;171:59–81. https://doi.org/10.1016/j.enconman.2018.05.091. Bellos E, Tzivanidis C, Antonopoulos KA. Exergetic, energetic and financial evaluation of a solar driven absorption cooling system with various collector types. Appl Therm Eng 2016;102:749–59. https://doi.org/10.1016/j.applthermaleng. 2016.04.032. Bellos E, Tzivanidis C. Energetic and financial analysis of solar cooling systems with single effect absorption chiller in various climates. Appl Therm Eng 2017;126:809–21. https://doi.org/10.1016/j.applthermaleng.2017.08.005. Shirazi A, Pintaldi S, White SD, Morrison GL, Rosengarten G, Taylor RA. Solarassisted absorption air-conditioning systems in buildings: control strategies and operational modes. Appl Therm Eng 2016;92:246–60. https://doi.org/10.1016/j. applthermaleng.2015.09.081. Shirazi A, Taylor RA, White SD, Morrison GL. A systematic parametric study and feasibility assessment of solar-assisted single-effect, double-effect, and triple-effect absorption chillers for heating and cooling applications. Energy Convers Manag 2016;114:258–77. https://doi.org/10.1016/j.enconman.2016.01.070. Soussi M, Balghouthi M, Guizani AA, Bouden C. Model performance assessment and experimental analysis of a solar assisted cooling system. Sol Energy 2017;143:43–62. https://doi.org/10.1016/j.solener.2016.12.046. Khan MSA, Badar AW, Talha T, Khan MW, Butt FS. Configuration based modeling and performance analysis of single effect solar absorption cooling system in TRNSYS. Energy Convers Manag 2018;157:351–63. https://doi.org/10.1016/j. enconman.2017.12.024. Alobaid M, Hughes B, Calautit JK, O’Connor D, Heyes A. A review of solar driven absorption cooling with photovoltaic thermal systems. Renew Sustain Energy Rev 2017;76:728–42. https://doi.org/10.1016/j.rser.2017.03.081. Mittelman G, Kribus A, Dayan A. Solar cooling with concentrating photovoltaic/ thermal (CPVT) systems. Energy Convers Manag 2007;48:2481–90. https://doi. org/10.1016/j.enconman.2007.04.004. Calise F, D’Accadia MD, Vanoli L. Design and dynamic simulation of a novel solar trigeneration system based on hybrid photovoltaic/thermal collectors (PVT). Energy Convers Manag 2012;60:214–25. https://doi.org/10.1016/j.enconman. 2012.01.025. Calise F, Dentice d’Accadia M, Palombo A, Vanoli L. Dynamic simulation of a novel high-temperature solar trigeneration system based on concentrating photovoltaic/ thermal collectors. Energy 2013;61:72–86. https://doi.org/10.1016/j.energy.2012. 10.008. Buonomano A, Calise F, Palombo A. Solar heating and cooling systems by CPVT and ET solar collectors: a novel transient simulation model. Appl Energy 2013;103:588–606. https://doi.org/10.1016/j.apenergy.2012.10.023. Buonomano A, Calise F, Palombo A. Solar heating and cooling systems by absorption and adsorption chillers driven by stationary and concentrating photovoltaic/ thermal solar collectors: modelling and simulation. Renew Sustain Energy Rev 2018;82:1874–908. https://doi.org/10.1016/j.rser.2017.10.059. Behzadi A, Gholamian E, Ahmadi P, Habibollahzade A, Ashjaee M. Energy, exergy and exergoeconomic (3E) analyses and multi-objective optimization of a solar and geothermal based integrated energy system. Appl Therm Eng 2018;143:1011–22. https://doi.org/10.1016/j.applthermaleng.2018.08.034. Behzadi A, Habibollahzade A, Ahmadi P, Gholamian E, Houshfar E. Multi-objective design optimization of a solar based system for electricity, cooling, and hydrogen production. Energy 2019;169:696–709. https://doi.org/10.1016/j.energy.2018.12. 047. Xu ZY, Wang RZ. Absorption refrigeration cycles: categorized based on the cycle construction. Int J Refrig 2016;62:114–36. https://doi.org/10.1016/j.ijrefrig.2015. 10.007. Kang YT, Kunugi Y, Kashiwagi T. Review of advanced absorption cycles: performance improvement and temperature lift enhancement. Int J Refrig 2000;23:388–401. https://doi.org/10.1016/S0140-7007(99)00064-X. Shirazi A, Taylor RA, Morrison GL, White SD. A comprehensive, multi-objective optimization of solar-powered absorption chiller systems for air-conditioning applications. Energy Convers Manag 2017;132:281–306. https://doi.org/10.1016/j. enconman.2016.11.039. Xu ZY, Wang RZ. Comparison of CPC driven solar absorption cooling systems with single, double and variable effect absorption chillers. Sol Energy 2017;158:511–9. https://doi.org/10.1016/j.solener.2017.10.014. Xu ZY, Wang RZ, Xia ZZ. A novel variable effect LiBr-water absorption refrigeration

Energy Conversion and Management 206 (2020) 112506

L. Lin, et al.

11.081. [33] Weather Data by Region | EnergyPlus n.d. https://energyplus.net/weather-region/ asia_wmo_region_2/CHN [accessed December 10, 2019]. [34] Chekirou W, Chikouche A, Boukheit N, Karaali A, Phalippou S. Dynamic modelling and simulation of the tubular adsorber of a solid adsorption machine powered by solar energy. Int J Refrig 2014;39:137–51. https://doi.org/10.1016/j.ijrefrig.2013. 11.019. [35] Belessiotis V, Mathioulakis E, Papanicolaou E. Theoretical formulation and experimental validation of the input-output modeling approach for large solar thermal systems. Sol Energy 2010;84:245–55. https://doi.org/10.1016/j.solener.2009.10. 024. [36] Enteria N, Akbarzadeh A, editors. Solar energy sciences and engineering applications. 1st ed.Boca Raton: CRC Press; 2013. [37] EES: Engineering Equation Solver | F-Chart Software: Engineering Software n.d. http://fchartsoftware.com/ees/mastering-ees.php [accessed December 10, 2019]. [38] Herold KE, Radermacher R, Klein SA, Radermacher R, Klein SA. Absorption chillers and heat pumps CRC Press; 2016. https://doi.org/10.1201/b19625.

cycle. Energy 2013;60:457–63. https://doi.org/10.1016/j.energy.2013.08.033. [28] Nishioka K, Takamoto T, Agui T, Kaneiwa M, Uraoka Y, Fuyuki T. Annual output estimation of concentrator photovoltaic systems using high-efficiency InGaP/ InGaAs/Ge triple-junction solar cells based on experimental solar cell’s characteristics and field-test meteorological data. Sol Energy Mater Sol Cells 2006;90:57–67. https://doi.org/10.1016/j.solmat.2005.01.011. [29] Buonomano A, Calise F, Dentice d’Accadia M, Vanoli L. A novel solar trigeneration system based on concentrating photovoltaic/thermal collectors. Part 1: design and simulation model. Energy 2013;61:59–71. https://doi.org/10.1016/j.energy.2013. 02.009. [30] Duffie JA, Beckman WA. Solar engineering of thermal processes. 4th ed. Hoboken: Wiley; 2013. [31] Maryami R, Dehghan AA. An exergy based comparative study between LiBr/water absorption refrigeration systems from half effect to triple effect. Appl Therm Eng 2017;124:103–23. https://doi.org/10.1016/j.applthermaleng.2017.05.174. [32] Cabrera FJ, Fernández-García A, Silva RMP, Pérez-García M. Use of parabolic trough solar collectors for solar refrigeration and air-conditioning applications. Renew Sustain Energy Rev 2013;20:103–18. https://doi.org/10.1016/j.rser.2012.

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