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Energetic end exergetic performance of a parabolic trough collector receiver: An experimental study
Accepted Manuscript Energetic end exergetic performance of a parabolic trough collector receiver: An experimental study Mohamed Chafie, Mohamed Fadhel Ben Aissa, Amenallah Guizani PII:
S0959-6526(17)32302-8
DOI:
10.1016/j.jclepro.2017.10.012
Reference:
JCLP 10804
To appear in:
Journal of Cleaner Production
Received Date: 11 May 2017 Revised Date:
20 August 2017
Accepted Date: 1 October 2017
Please cite this article as: Chafie M, Ben Aissa MF, Guizani A, Energetic end exergetic performance of a parabolic trough collector receiver: An experimental study, Journal of Cleaner Production (2017), doi: 10.1016/j.jclepro.2017.10.012. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
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Energetic end exergetic performance of a parabolic trough collector receiver: an
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experimental study
Mohamed Chafie*, Mohamed Fadhel Ben Aissa, Amenallah Guizani
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Research and Technology Center of Energy, Thermal Processes Laboratory, Hammam Lif, B.P. 95,
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2050 Tunis, Tunisia
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* Corresponding author
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E-mail address: [email protected]
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HIGHLIGHTS •
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A parabolic trough collector (PTC) system was designed, manufactured and evaluated.
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An experimental study was conducted to evaluate the thermal behavior.
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A detailed energy and exergy analysis for typical days and for a daily monitoring was performed.
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The energy and exergy efficiency as well as the exergy factor were evaluated.
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The average energy and exergy efficiency are found to be higher under clear
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sky days than the cloudy days.
Abstract
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In this article, an experimental investigation is evaluated with the aim of assessing
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the thermal performance of a receiver tube of a parabolic trough collector. The
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parabolic trough collector is designed, constructed and installed in the Laboratory of
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Thermal Processes, Research and Technology Center of Energy (CRTEn), Borj
ACCEPTED MANUSCRIPT Cedria. The thermal performance was carried out using both first and second law of
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thermodynamics. A detailed energy and exergy analysis was performed to evaluate
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the thermal performance. Throughout the study, the useful energy gain and the
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receiver tube exergy transferred to the heat transfer fluid, the energy and exergy
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efficiency as well as the exergy factor were measured. The measurements were
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conducted during the months of September and October 2015 at Borj Cedria-Tunisia.
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The result has shown that the thermal performance is strongly influenced by climatic
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conditions. The average energy and exergy efficiency are found to be higher under
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clear sky days than the overcast cloudy sky days. The daily energy efficiency varied
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between 19.7% and 52.6%. While the daily exergy efficiency varied from 8.51% to
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16.34%.
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Keywords: Design, Parabolic trough collector, Receiver tube, Energy and exergy
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analysis, Exergy factor, Solar energy Nomenclature
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aperture area (m2)
Aab
effective receiver area (m2)
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Aap
concentration ratio
C
fluid specific heat capacity (J/kg K)
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C p, f
D ex ,ab
outer diameter of absorber tube (m)
D in ,ab
inner diameter of absorber tube (m)
D ex ,en
outer diameter of glass envelope (m)
D in ,en
inner diameter of glass envelope (m)
E
internal energy (J)
ACCEPTED MANUSCRIPT absorbed solar radiation exergy (W)
Ex u
receiver tube exergy rate transferred to HTF (W)
Ex f
Exergy factor
f
focal distance (m)
h
Enthalpy(kJ/kg)
hc ,en − e
convection heat transfer coefficient from receiver tube to glass
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Ex a
h r , ab −en
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envelope (W)
radiation heat transfer coefficient from receiver tube to glass
hr , en -e
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envelope (W)
radiation heat transfer coefficient from glass envelope to environment (W)
lactus rectum of the parabola (m)
ID
direct normal irradiance (W/m2)
k air
thermal conductivity air (W/m K)
K
incidence angle modifier
•
mass flow rate (kg/s)
m
Nusselt number
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Nu Pr
Q
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Hp
Prandtl number heat transfer (kJ)
Qa
heat absorbed by the receiver (W)
Ql
lost heat flux (W)
Qu
useful energy gain (W)
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Reynolds number
S
entropy (kJ/K)
•
entropy generation (kJ/kg K)
S gen ambient temperature (K)
T ab
absorber tube temperature (K)
Ten
glass envelope temperature (K)
T in
fluid temperature at inlet of receiver tube (K)
Tout
fluid temperature at outlet of receiver tube (K)
Ts
temperature of the sun (K)
UL
overall collector heat loss coefficient (W/m2K)
W
work (kJ)
Wa
collector width (m)
wR
total uncertainty in measurement of result
w 1, w 2 , K , w n
uncertainties in independent variables
Y
curve length of the reflector (m)
α
γ σ
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Greek letters
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T amb
Absorptance intercept factor Stefan Boltzmann constant (5.67 10-8 W/m2K4°)
ϕr
rim angle (°)
ρc
specular reflectance of the reflector
ε ab
absorber selective coating emissivity
absorber envelope emissivity
τ
transmittance of the glass
η
thermal efficiency
ηexr
Exergy efficiency
ηopt
optical efficiency
Abbreviations Parabolic Trough Collector
HTF
Heat Transfer Fluid
DNI
Direct Normal Irradiance
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εen
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1. Introduction Since the Industrial Revolution, the international energy production has been
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increased in order to satisfy the tremendous needs of the new manufacturing processes
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In fact, this massive energy production is generally related to the emission of
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greenhouse gases that stand as serious threats to the environment mainly following the
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ozone depletion and global warming. To deal with this situation, several global
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strategies have emphasized that the use of current fuels, which will soon run out,
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should be replaced by other renewable energy resources. This kind of energy is
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inexhaustible (Kalogirou, 2004). It reduces the release of greenhouse gases and
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generates virtually no waste and pollutants. Most renewable energy comes either
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directly or indirectly from the sun. Therefore, the solar energy appears to be as the
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energy of the future. The technology that allows us to use the solar energy is based on
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systems that are called solar collectors. The Parabolic Trough Collector (PTC) is one
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of the advanced solar thermal technologies. The PTC is a solar concentration
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technology that converts solar beam radiation into thermal energy in their receiver.
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The fields of application and use of the PTC are extensive. Based on its aperture area
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and its operating temperature, the fields of application can be divided into two main
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areas (Fernández-García, 2010). The first field of application is intended primarily for
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the concentrated solar power plants when the temperature ranges from 300 °C to 400
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°C (Montes et al., 2011; Al-Soud and Hrayshat, 2009; Nishith and Santanu, 2015).
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The other area of applications requires temperatures between 100 and 250 °C. The
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solar systems using this type of the PTC could be used in Industrial Process Heat
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applications (Fernández-García et al., 2015; Sharma et al., 2016) which include solar
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ACCEPTED MANUSCRIPT water heating (Valan Arasu and Sornakumar, 2007), desalination (Jafari Mosleh et al.,
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2015; Kalogirou, 1998; Palenzuela et al., 2011), solar water disinfection (Bigoni et al.,
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2014), solar cooking (Mussard and Nydal, 2013), solar refrigeration and air-
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conditioning (Abu-Hamdeh et al., 2013; Balghouthi et al., 2012; Cabrera et al., 2013),
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etc.
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Many studies on the energy and exergy analysis of the PTC have been reported
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and discussed in the literature. Kalogirou (2004) presented a detailed inquiry of the
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energy and exergy analysis of different types of solar thermal collectors. (Petela,
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2005) conducted a comprehensive exergy analysis of the PTC. The results show that
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the exergy efficiency is lower compared to energy efficiency. Tyagi et al. (2007)
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presented a parametric study on the variation of the exergetic performance collector
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parabolic trough depending on Hottel–Whillier model and taking into account the
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radiation losses. The parametric study has been done for different concentration ratios
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and mass flow rates for different solar radiation values. Montes et al. (2010) described
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the development and use of a thermofluidynamic model for solar parabolic trough
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collectors system. The aim of this model is to evaluate the performance of the PTC for
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various heat transfer fluids and to analyze their effect on the parameter; receiver
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diameter, collector length, pressure and working fluid temperature. The impact of the
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four parameters has been investigated from the pressure drop, heat loss, exergy and
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energy performance. Reddy et al. (2012) carried out the exergetic analysis and
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performance evaluation of parabolic trough, concentrating solar thermal power plant
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(PTCSTPP) for the locations of Jodhpur and Delhi in India. It was found that the
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Jodhpur location is much better compared to Delhi in terms of the performance. Ilhan
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and Alper (2013) studied and analyzed experimentally a temperature controlled PTC
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system. The system consists of a PTC, storage tank, solenoid valves and process
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ACCEPTED MANUSCRIPT control equipment. The results indicate that the highest energy efficiency of the
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system was calculated as 61.2% for 100 °C and the highest exergy efficiency
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computed as 63% for 70°C. In order to study the effect of environmental and
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operational parameters on the efficiency of the PTC, Padilla et al. (2014) performed a
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detailed exergy balance. Many parameters have been considered in this study such as;
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mass flow rate, wind speed, inlet temperature, direct solar irradiance, etc. The results
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indicate an opposite trend of thermal efficiency and collector exergy efficiency.
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Recently, Bellos et al. (2016) conducted a parametric study on the energetic and
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exergetic performance of a PTC using different gases as working fluids. Six gases are
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investigated for numerous combinations of inlet temperature level and mass flow rate
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in order to analyze their performance in a high area of operating conditions. This
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study is based on a numerical model which was developed with Engineer Equator
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Solver (EES). Optimization of the study shows that the Helium is the best gas up to
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700 K, while CO2 is the best in higher temperatures. The global maximum of the
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exergetic performance is obtained with helium operating to 0.035 kg/s mass flow rate
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and 640 K inlet temperature. In order to study the thermal performance enhancement
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of parabolic trough collector by using nanofluids and converging-diverging absorber
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tube, Bellos et al. (2016) developed a model designed with Solidworks and simulated
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in its flow simulation studio. The model has been validated by data from the literature
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(Dudley et al., 1994). In this context, Jaramillo et al. (2013) developed a
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thermodynamic model framework by using the first and the Second Law of
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Thermodynamics so as to analyze the performance of a PTC with a twisted tape
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insert. In addition to that, there is an exergetic analysis included by the
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thermodynamic model framework with the aim of providing further useful
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information for the exergy efficiency of the PTC. The model has been validated by
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ACCEPTED MANUSCRIPT experimental data. In 2016, an extensive review of exergy investigation of solar
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collectors systems and processes was provided by Kalogirou et al. (2016). It
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comprises Research works on the exergy analysis both different types of solar
114
collectors and different applications of solar collectors systems. Among the different
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types of the solar collectors are flat-plate collector, parabolic dish collectors,
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parabolic-trough collectors, etc. And with regards to their applications there are
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several possibilities including but not limited to drying, heating, cooling, desalination,
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etc.
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In the present work, an experimental investigation was carried out to evaluate the
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performance of the PTC receiver. The new PTC designed and realized in the Research
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and Technology Center of Energy (CRTEn) in Borj Cedria, Tunisia. Using both first
122
and second law of thermodynamics, energy and exergy analysis is achieved. In section 2, the
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design and the essential component of the experimental device are described. The
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experimental step is described in section 3. In section 4, the energy and exergy of the
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PTC receiver is presented. In section 5 the experimental results are summarized and
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reported. The remarks and outcomes of this work will be reported in the conclusion.
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2. Parabolic trough collector design
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The PTC system in question was designed, manufactured and installed in the
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CRTEn (Research and Technology Center of Energy) in Tunisia based on Eqs (1)-(3).
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The experimental system essentially consists of two parts: (1) a parabolic trough
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collector system, and (2) a thermal pipe. The parabolic trough collector system
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composed of the reflector and the receiver (Fig.1).
ACCEPTED MANUSCRIPT The reflector allows to transmit as much heat as possible to the fluid. For this
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reason, it is necessary to take a mirror or a plate of a cylindro-parabolic shape that
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should be covered by a reflective film. The cross-section of the reflector is a parabola.
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The rays are reflected towards the focal point of the parabola where the receiver is
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located. The receiver is the main component of the PTC. It must absorb as much solar
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radiation as possible. In order to minimize the losses by radiative and convective
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exchanges with the outside, the absorber tube must be covered by a glass envelope.
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The annular space is generally evacuated. In fact, to reduce thermal losses, it is
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important to use a metal absorber tube. In addition, and in order to improve the heat
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collecting efficiency, the absorber tube must have several characteristics such as; good
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conductivity and thermal diffusion, low emissivity (use of selective coating), and high
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absorption factor, etc. the characteristics of the PTC are given in Table 1.
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The equation of the parabola in terms of the coordinate system is given by (Kalogirou, 2009):
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y 2 = 4fx
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where f is the focal distance
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2009):
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W a = 4f tan (ϕ r 2 )
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With ϕr is the rim angle
153
(1)
The aperture of the parabola is given by the following expression (Kalogirou,
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The curve length of the reflector is defined as (Kalogirou, 2009):
(2)
ACCEPTED MANUSCRIPT Hp
sec (ϕ r 2 ) tan (ϕ r 2 ) + ln ( sec (ϕ r 2 ) + tan (ϕ r 2 ) ) 2
Y =
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Where
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H p is the lactus rectum of the parabola
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sec ( x ) = 1 cos ( x
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3. Experimental study
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3.1 Experimental device
(3)
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)
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(4)
A schematic diagram of the experimental methodology and a photograph of the
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PTC system and the related components are shown in Fig. 2. The experimental device
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is mounted in the CRTEn in Borj Cedria-Tunisia: Latitude 36°50’ N and Longitude
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10°44’ E. The experimental device composes of a PTC system with aperture area of
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10.8 m2, oriented east-west and revolves around the horizontal north-south axis
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(Chafie et al., 2016). The solar tracking is manual and uniaxial. The PTC system is
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connected by a thermal pipe. The Heat Transfer Fluid (HTF) was circulated in the
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system by a pump. The HTF is stored in a 46.25 liters storage tank which is thermally
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insulated by a glass wool. The HTF used in this study is the Transcal N thermal oil.
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The thermal oil physical properties are presented in Table 2. The flow rate is settled
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by a pump, a flow-meter and valves integrated at the experimental loop. In order to
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inhibit evaporation of the HTF in the pipe, the experimental loop must be pressurized.
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For safety reasons, the experimental device includes also a manometer pressure
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gauge, an expansion tanks and safety valves. A CR5000 data logger (Campbell
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ScientificInc) was used to record all climatic parameter and measuring instruments
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during the tests (every 5 min). Thus, 2 AA Class RTDs (resistance temperature
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ACCEPTED MANUSCRIPT detectors) are used to measure the inlet and outlet temperature of the receiver. A K-
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type thermocouple was placed in the storage tank to measure the temperature of the
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HTF in the tank. A HMP155A sensor was used to measure the ambient temperature.
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A pyrheliometer CHP1 Kipp and Zonen was used to measure direct normal insolation
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during testing.
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3.2 Uncertainty analysis
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Uncertainty analysis is needed to prove the accuracy of the experiments. In this
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study, errors came from the sensitiveness of equipment and measurements explained
184
previously. The uncertainty analysis of the various measured and calculated
185
parameters are estimated according to Holman (1994). The independent parameters
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measured in the experiments reported here are: ambient temperature, receiver inlet
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and outlet temperature, solar radiation and HTF flow rate. To carry out these
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experiments, the sensitiveness of data acquisition system is about ±0.001 °C, the
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measurement error is ±0.001 °C, the sensitiveness of the AA Class RTDs is ±0.01 °C,
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the sensitiveness of the K-type thermocouples is ±0.01 °C, the HPM155A errors are
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±0.02 °C and the flow meter errors are ±0.3%. A pyrheliometer CHP1 Kipp and
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Zonen with ±1% measurement uncertainties are used. The sensitiveness was obtained
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from a catalog of the instruments.
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The calculated uncertainties of the dependent parameters were estimated by Eq.
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(5). The result R is a given function in terms of the independent variables. Let w
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the uncertainty in the result and w ,w ,K,w be the uncertainties in the independent 1 2 n
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variables. The result R is a given function of the independent variables x , x ,K, x . 1 2 n
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If the uncertainties in the independent variables are all given with the same odds, then
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the uncertainty in the result having these odds is calculated as (Holman, 1994);
R
be
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12 2 2 2 ∂R ∂R ∂R w R = w + w +K + w 2 n ∂x 1 ∂x ∂x 1 2 n
(5)
The total uncertainties associated with the calculated values were found as
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follows: (i) 0.0048 for heat rate, (ii) 0.0059 for thermal efficiency and (iii) 0.0059 for
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exergy efficiencies.
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4. Thermodynamic analysis
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The purpose of the thermodynamic analysis of the energy systems is to determine
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the energy efficiency of the system that has been studied. This efficiency is defined as
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being the ratio between the useful and the incident energy. These energies can be of
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the same type or different ones (chemical, thermal, mechanical, electrical, etc.). This
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analysis derives directly from applications related to the first law of thermodynamics,
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and it only takes into account the quantities of energy without any references to the
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quality associated with it according to the second law. Combining exergy with energy
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in the analysis of energy systems goes back to the association of both quantity and
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quality of energy to their various forms or types. The analysis becomes much more
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substantial than being a mere simple energy analysis.
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4.1 Energy analysis
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The energy analysis of each energetic system can be performed by using the first
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law of thermodynamics. For a control volume flow process in an open system, the
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energy balance is given by the following expression (Borgnakke and Sonntag, 2009):
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dE dt
n
• • • • = Q − W + m h − m ∑ i ∑ i total ,i ∑ 0 htotal ,i CV i = 0 0 i
(6)
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htotal = h +
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Where Q , W , m and h are the heat transfer, work, mass flow rate and the enthalpy.
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In case of steady state condition, dE dt = 0 . The first low efficiency (energetic
+ gz
(7)
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efficiency) of a collector is determined by the ratio of the energy output to the energy
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input of the collector. It is written as follows (Benson, 2013):
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η=
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Desired output energy Input energy
(8)
The useful energy gain transferred to the HTF ( Qu ) is the difference between the
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heat absorbed by the receiver tube ( Q a ) and the lost heat flux ( Ql ). It can be
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estimated as:
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Qu = Qa − Ql
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(9)
The useful energy gain transferred to the HTF is defined as:
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. Qu = m C p ,f
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. Where m and C p ,f are the mass flow rate and the specific heat capacity of the HTF,
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respectively.
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(10)
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(Tout
The heat absorbed by the receiver tube is as follows:
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Q a = ηopt I D A ap
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Where ηopt , I D and Aap are the optical efficiency, Direct Normal Irradiance (DNI)
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and the aperture area.
(11)
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The optical efficiency could be calculated by the following equation (Duffie and
239
Beckman, 2006):
240
ηopt = ρcατγ Kθ
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Where ρc , α , τ , γ and K θ are the reflectance of the reflector, absorbance of the
242
receiver tube, transmittance of the glass envelope, intercept factor and the incidence
243
angle modifier.
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(12)
Since the HTF makes its way through the receiver, its temperature exceeds the
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ambient temperature very quickly, that produces a process of loss of heat from the
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receiver. The modes of heat loss are radiation and convection. They are dependent on
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the difference in temperature between the receiver and the environment and the
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geometry of the receiver and that of the concentrator.
The lost heat flux ( Ql ), includes heat loss by radiation and convection, is given
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by (Kalogirou, 2009):
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Q l = A ab U L (T ab − T amb )
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Where Aab , U L , are the effective receiver tube area ( m 2 ) and the heat loss
253
coefficient (W m 2K ).
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(13)
The heat loss coefficient based on the receiver area is basically influenced by the
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different heat transfer mode. It is given by (Kalogirou, 2009):
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Aab 1 UL = + hc ,en −e + hr , en −e Aen hr , ab −en
(
)
−1 (14)
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258
receiver tube to glass envelope, radiation heat transfer coefficient from glass envelope
259
to environment and the convection heat transfer coefficient from receiver tube to glass
260
envelope, respectively. They are given by the following expressions (Kalogirou,
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2009):
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4 hr ,en _ e = σ εen Ten4 -Tamb
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4 -T 4 σ T ab en hr ,ab _ en = 1 ε ab + (1- εen ) Dex ,ab εen D in ,en
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hc ,en −e =
) )
)
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(
(15)
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(
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Nu air k air Dex ,en
(16)
(17)
For external forced convection flow normal to an isothermal cylinder, the Nusselt
266
number ( Nuair ) is estimated with Zhukauskas’ correlation (Duffie and Beckman,
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2013):
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1/ 4 m n Prair Nu air = C Reair Prair Prog
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With
270
n = 0.37 if Prair ≤ 10 and n = 0.36 if Prair ≥ 10
271
Values of C and m are listed in Table 3.
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(18)
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Substituting Eqs. 10, 11 and 12 into Eq.9 yields
. m C p ,f
(Tout −T in ) = ηopt
I D Aap − Aab U L (T ab −T amb )
(19)
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The instantaneous efficiency η of a PTC is defined as the ratio of the useful
275
energy gain by the incident radiation of the aperture area of the PTC. It is given as
276
follows:
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. m C p .f (T out − T in ) Qu η= = Aap I D A ap I D
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By injecting Eq. 19 in Eq. 20, we find that the equation of the thermal efficiency can also be written in the following form:
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η = ηopt − A abU L
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4.2 Exergy analysis
T ab − T am b I D A ap
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(20)
(21)
Exergy analysis is a method based on the second law of thermodynamics for the
283
analysis and thermodynamic evaluation of systems. Its interest is that it provides a
284
very powerful calculation methodology to quantify the thermodynamic quality of any
285
process or system. The exergy analysis is based on the comparison of the system to be
286
evaluated with respect to an idealized system where the transformations of energy are
287
reversible, without production of entropy. For non-steady condition the entropy
288
generation is given by (Gakkhar et al. 2016):
•
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n
•
• • Q − ∑ i − ∑ m i si + ∑ m 0 s0 ≥ 0 CV i = 0 T i i 0
289
dS S gen = dt
290
where S , Q , T and m are entropy, heat transfer, temperature and mass flow rate.
291 292
(22)
The receiver tube exergy rate transferred to HTF with reference to the surroundings is written as it follows (MacPhee and Dincer, 2009):
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. Ex u = m C p ,f
(Tout -T in )
. T - m C p ,f T amb ln out T in
(23)
By injecting Eq. 10 in Eq. 23, the receiver tube exergy rate transferred to HTF can
295
also be written in the following equation:
296
. T Ex u = m C p ,f (Tout −T in ) - T amb ln out T in
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(24)
The absorbed solar radiation exergy by the system (reflector and receiver tube),
298
assuming the sun as an infinite thermal source is given by the Petela expression
299
(Petela, 2003) and is defined as:
300
4 Tamb 4 T amb 1 Ex a = Aap I D 1 + 3 Ts 3 Ts
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Where T s = 5762 K is the apparent sun temperature. It represents 75% of blackbody
302
temperature of the sun (Dutta Gupta and Saha, 1990).
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The exergy efficiency, analogous to the energy case, is defined as the ratio of the
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receiver tube exergy rate by the absorbed solar radiation exergy. It is written as:
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. T m C p ,f (Tout −T in ) - T amb ln out Ex u T in ηexr = = 4 Ex a T amb 4 T amb 1 Aap I D 1 + 3 Ts 3 Ts
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(26)
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The exergy factor is defined as the ratio of the receiver tube exergy rate by the
307
useful energy gain transferred to the HTF. It is proposed for quantifying the thermal
ACCEPTED MANUSCRIPT 308
performance and it is used as a measure of the performance of the receiver. It is
309
evaluated using the following definition:
310
. T m C p , f (T s −Te ) - T amb ln s Ex u Te Ex f = = . Qu m C p ,f (T s −Te )
311
5. Results and discussion
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(27)
In this work, In order to evaluate the energy and exergy performance of our PTC,
313
a thermodynamic analysis was carried out. The measurements were performed at the
314
Research and Technology Centre of Energy (CRTEn) in Borj Cedria, Tunisia during
315
the period between September and October 2015.
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5.1 Performance of the PTC: typical days
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In order to guarantee the energy and exergy performance of the PTC and the
318
ability to compare the performance with other collectors, experiments were conducted
319
on typical days: September 19th, 2015 characterized by a severely solar radiation
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fluctuation and September 29th, 2015 characterized by a clear sky day. During the
321
experiments, the inlet and outlet HTF temperature, ambient temperature, DNI, energy
322
and exergy rate, energy and exergy efficiency and the exergy factor were recorded
323
(Fig. 3-5). The experimental tests are taken from 9:00 h to 16:00 h with an average
324
speed wind wind speed at 3 m/s and a same flow rate of 0.2 Kg/s.
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The inlet and outlet HTF temperatures and the ambient temperature of the typical
326
days were presented in Fig. 3. During the cloudy day (September 19th, 2015) (Fig. 3a),
327
we notice that the ambient temperature varies slightly. It is changed between 26 °C
328
and 31.5 ° C. We also find in Fig. 3a that the evolution of HTF inlet and outlet
ACCEPTED MANUSCRIPT temperatures is irregular throughout the day. This fluctuation is due to the variability
330
of the solar radiation caused by the cloudy passages. This disruption of the evolution
331
of the HTF inlet and outlet temperatures is clearly observed during the interval of time
332
from 11:00 h to 13:40 h. The HTF inlet and outlet have a temperature of 60 °C and 65
333
°C, respectively at 11:00 h, decreases until 59 °C and 59.7 °C, respectively at 12:30 h
334
and then increases again till reaching 72.7 °C and 77.65 °C at 13:40 h.
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Plot b of Fig. 3 (clear sky day) shows that the ambient temperature ranges between 29 °C and 36 °C with an average daily about 33 °C.
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On the same figure (Fig.3b), it can be seen that the evolution of the inlet and
338
outlet HTF temperatures increases progressively with time and reaches a maximum at
339
noon. The maximum outlet HTF temperature obtained by the PTC is 105 °C at 12:20
340
h. Pursuant to the HTF temperature evolution in the PTC, the HTF temperature
341
difference between outlet and inlet of the PTC increases with time until noon, and
342
then decreases. The maximum HTF temperature difference reaches to 12 °C.
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In order to evaluate the experimental energy gain and the exergy rate of our
344
system, Eqs. (10) and (23) have been respectively used. The evolution of the DNI,
345
useful energy gain and the exergy rate are presented in Fig. 4. The experimental data
346
were recorded on 19th and 29th September, 2015.
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We found out that during September 19th, 2015 the evolution of the DNI has a
348
very clear fluctuation. The fluctuation of solar irradiance is caused by passing clouds.
349
During that cloudy day, the DNI varied irregularly from 220 to 814 W/m2 (Fig. 4a).
350
The evolution of the energy gain and the exergy rate during the cloudy day follows
351
the same trend as the DNI. This similarity is very clear in the period of time between
ACCEPTED MANUSCRIPT 12:00 to12:40 h when the solar radiation presents low values, and also in the time
353
between 13:00 to 14: 00 h during which the solar radiation has high values. In the
354
same figure (Fig. 4a), we find that the DNI undergoes a fluctuation around 12:30 h.
355
Moreover, in the same period the evolution of the energy gain and the exergy rate
356
undergoes also a simple fluctuation. The receiver tube exergy rate transferred to HTF
357
during the experiment is much lower than the energy rate. Its average value only
358
reaches 115.1 W while the average useful power is 1.2 KW.
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Plot b of Fig.4 (clear sky day) shows that the DNI is 540 W/m2 at 9:15 h and it
360
increases up to 750 W/m2 at noon. Beyond this time, the DNI decreases until the end
361
of the experiment, and it is 270 W/m2. Since the measured solar radiation is the only
362
the direct component, it increases up to midday, and then starts decreasing.
363
The results obtained during the both sunny and cloudy days (Fig 4) confirm that the
364
evolution of the energy gain and the exergy rate are strongly influenced by direct solar
365
radiation.
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The evolution of the energy gain with respect to time during the day (September
367
29th, 2015) of the experiment increases and reaches a maximum of 4820 W at 12:00 h
368
which the solar radiation reaches its peak of 750 W/m2. After noon the sun starts its
369
retreat and the solar radiation starts decreasing which leads to the reduction of the
370
useful heat gain. This is owed to the fact matter that the energy gain is forcefully
371
influenced by the DNI, and therefore follows its variation. During the experiment, the
372
energy rate was much greater than the exergy rates with its maximum value being
373
around 4820 W as opposed to around only 890 W for the exergy rate. It can be
374
concluded that the quality of energy from the receiver tube is very low perhaps due to
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a combination of a broad bulk of irreversible energy changes such as heat losses and
376
the transfer of high quality radiative energy to the HTF. Therefore, to evaluate the thermal efficiency, exergy efficiency and the exergy
378
factor, Eqs. (20), (26) and (27) are used respectively. Fig. 5 presente the evolutions of
379
the energy efficiency, exergy efficiency and the exergy factor during to typical days;
380
September 19th, 2015 and September 29th, 2015.
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A general view of plot 5a and according to the evolution of the inlet and outlet
382
HTF temperatures (Fig. 3) and the evolution of the DNI and energy and exergy rate
383
(Fig. 4) during September 19th, 2015, shows a fluctuating and irregular variation with
384
respect to the time of the evolution of the energy and exergy efficiency as well as the
385
exergy factor. We noted that the latter is highly affected by the severely solar
386
radiation fluctuation. This disruption is precisely detected during the interval of time
387
from 10:00 h to 13:30 h. The energy and exergy efficiency are of 6.6 % and 7.1 % at
388
12:30 h respectively, this low energy and exergy efficiency values coincides with a
389
low value of DNI as well as low useful power (411 W/m2 for direct solar radiation
390
and 265.77 W for useful power). On the other hand, at 13:30 h, the thermal the energy
391
and exergy efficiency are of 29.3% and 10.85%, respectively. The exergy factor can
392
also be used as a parameter for measuring the performance of the receiver tube. The
393
exergy factor appears to be relatively similar to the evolution of the solar radiation,
394
high under high solar radiation conditions and it declines when the solar radiation is
395
small.
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With regards to the day, September 29th, 2015 (sunny day) the evolution of the
397
energy and exergy efficiency and the exergy factor with time has the same trends. The
398
energy efficiency, exergy efficiencies and the exergy factor vary in a similar way to
ACCEPTED MANUSCRIPT DNI and thus it can be concluded that it is the most influential factor affecting these
400
parameters. We noted that the energy and exergy increases until it reaches a
401
maximum value of 61.54% and 17.48% at midday, respectively where the DNI has a
402
maximum value of 750 W/m2. Afternoon the energy and exergy efficiency begins to
403
decrease until the end of the experiment. The exergy factor is lower than the energy
404
efficiency and peaks a maximum around 0.17 at noon and then decreases until to 0.09
405
at 16:00 h.
406
5.2 Daily energy and exergy efficiency
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In order to study the energy and exergy efficiency of our receiver tube, we
408
conducted a test campaign during the period between September 16th, 2015 and
409
October 19th, 2015. During working days, there are days that correspond to favorable
410
climatic conditions and other days more severe.
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Fig. 6 illustrates the values of the daily energy and exergy rate as function of days
412
of experiments. The daily average energy rate varied between 1202.31 W and 3670,75
413
W. The daily average value of the energy rate is around 2496.96 W. During the period
414
of the experiment, the exergy rate changed between 115.1 W and 536.2 W. Their
415
average value is about 317.53 W.
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The values energy and exergy efficiency as well as the exergy factor with respect
417
to days are presented in Fig. 7. It is clear from this figure that the higher daily energy
418
and exergy efficiencies recorded on19th October, 2015. The daily average energy
419
efficiency changes from 19.7% to 52.6% with an average of 36.10% (during the
420
period between 16
421
daily average exergy efficiency, it can be seen that it is comprised between 8.51% and
422
16.34%. We noted that the average exergy efficiency and exergy factor are about
th
September 2015 and 19
th
October 2015). With regards to the
ACCEPTED MANUSCRIPT 11.85 % and 0.11, respectively. This value is enough in terms of efficieny exergy and
424
factor exergy. When values are compared with the daily average energy efficiency,
425
the exergy efficiency and exergy factor daily average of the receiver tube was lower
426
than the daily energy efficiency for all days. The results of the energy and energy
427
performance study show the influence of climatic factors on the performance of the
428
system. Table 4 summarizes the energy and exergy performance of our system.
429
6. Conclusion
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In this work, an experimental study to evaluate the thermal behavior of a
431
parabolic trough collector was conducted. The experimental device in question was
432
designed and realized in the Research and Technologies Centre of Energy in Tunisia
433
(CRTEn). The thermal behavior is evaluated using energy and exergy analysis. The
434
useful energy gain transferred to the HTF and the exergy efficiency are proved to be
435
smaller than the useful exergy rate and the exergy efficiency. The exergy factor
436
parameter is also used to evaluate the thermal performance. The exergy factor is
437
proved to be heavily influenced by the DNI as well as the operating temperature. The
438
average energy efficiceny, exergy efficiceny and the exergy rate varied between
439
19.72%, 8.51% and 0.08 to 36.1%, 11.72% and 0.12 for the cloudy day and sunny
440
day, respectively. The daily average energy and exergy efficiceny as well as the
441
exergy factor obtaining during the experimental period are about 36.10%, 11.85% and
442
0.11 respectively.
443
Acknowledgment: We are deeply grateful for the cooperation and assistance we got
444
from all the staff at the Thermal Processes Laboratory of Research and Technology
445
Center of Energy-Borj Cedria, Tunisia.
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ACCEPTED MANUSCRIPT List of tables Table 1: Characteristics of the PTC Table 2: Thermal properties of Transcal N oil
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Table 4: Summary of daily performance PTC receiver.
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Table 3: Constants for Equation (18)
ACCEPTED MANUSCRIPT Table 1: Characteristics of the PTC Value
Length (mm)
4000
Aperture (mm)
2700
Aperture area (m2)
10.66
Focal distance (mm)
835
C
12.02
Rim angle (°)
76.3
Reflected surface Reflectivity
0.93
Type of glass cover
High borosilicate glass
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Type of receiver
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Parameter
Evacuated Glass-steel coated tube
Type of steel
SUS304
Diameter outer tube (mm)
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Thickness outer tube (mm)
120
3
70
Thickness inner tube (mm)
1.5
Length of receiver (mm)
4000
Emittance
<8%
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Diameter inner tube (mm)
Absorbtivity
>93%
Glass transmissivity
0.95
Vacuum rate (pa)
P<5.10-3
ACCEPTED MANUSCRIPT Table 2: Thermal properties of Transcal N oil Data
Units
Density (15°C)
875
Kg/m3
fire point
243
°C
flash point
221
Viscosity (100°C)
5.2
Viscosity (40°C)
31
Specific heat
1925.9
Coefficient of Thermal Expansion
0.00077
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Transcal N
°C
mm2/s
mm2/s
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J/kg °C per °C
ACCEPTED MANUSCRIPT
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m
1-40
0.75
0.4
40-1000
0.51
0.5
1000-200000
0.26
0.6
200000-1000000
0.076
0.7
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Table 3 : Constants for Equation (18)
ACCEPTED MANUSCRIPT Table 4. Summary of daily performance PTC receiver. Standard deviation
Maximum Minimum
Energy rate (W)
2496,96
678,65
3670,75
1202.31
Exergy rate (W)
317,53
126,24
536,2
115,1
Energy efficiency (%)
36,10
8,8
52,6
19,7
Exergy efficeincy (%)
11,85
1,84
16,34
8,51
Exergy factor
0.11
0,01
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Average
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0,08
ACCEPTED MANUSCRIPT List of figures Fig. 1: Photograph of the experimental system: (1) reflector, (2) receiver Fig. 2: (a) A schematic view of the experimental methodology; (b) the photograph of experimental set-up. Fig. 3: Evolutions of the ambient temperature and the inlet and outlet HTF
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temperatures: (a) September 19th, 2015 (b) September 29th, 2015
Fig. 4: Evolutions of the solar radiation, useful energy gain and the exergy rate: (a) September 19th, 2015 (b) September 29th, 2015 (a) September 19th, 2015 (b) September 29th, 2015
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Fig. 5: Evolutions of the energy efficiency, exergy efficiency and the exergy factor:
Fig. 6: Daily energy and exergy rate as a function of days
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(b) Fig. 2. (a) A schematic view of the experimental methodology; (b) the photograph of experimental set-up
ACCEPTED MANUSCRIPT 50
90
Tamb Tin Tout
45
80
70
60
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Temperature (°C)
Tamb (°C)
40
50
30
40
25
20 09:00
10:00
11:00
12:00
13:00
14:00
15:00
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45
35
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80 70 60 50
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100 90
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40
110
Tamb Tin Tout
Temperature (°C)
50
40 30
20
09:00
10:00
11:00
12:00
13:00
14:00
15:00
16:00
Local time (hh)
(b)
Fig. 3. Evolutions of the ambient temperature and the inlet and outlet HTF temperatures: (a) September 19th, 2015 (b) September 29th, 2015
ACCEPTED MANUSCRIPT 4500
900
DNI Qu
4000
Exu
3500 3000
600 2500 500
2000
Energy and exergy rate (W)
700
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Direct Normal Irradiance (W/m2)
800
400
1500 1000
300
500
200
10:00
11:00
12:00
13:00
14:00
15:00
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16:00
(a)
900
TE D
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500 400
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3000
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700
5000
1000
Energy and exergy rate (W)
Direct Normal Irradiance (W/m )
800
DNI Exu Qu
0
100
09:00
10:00
11:00
12:00
13:00
14:00
15:00
16:00
Local time (hh)
(b)
Fig. 4. Evolutions of the solar radiation, useful energy gain and the exergy rate: (a) September 19th, 2015 (b) September 29th, 2015
ACCEPTED MANUSCRIPT 0,6 0,12
0,5
0,10
0,08 0,3
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Exergy efficiency
0,4
Energy efficiency and exergy factor
Exrgy efficiency Energy efficiency Exergy factor
0,06
0,2
0,04
0,02 09:00
10:00
11:00
12:00
13:00
14:00
15:00
0,0
16:00
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0,1
(a)
Exergy efficiency Energy efficiency Exergy factor
0,18 0,16
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0,12 0,10 0,08 0,06
AC C
0,04
0,5
0,4
0,3
0,2
0,1
0,02
09:00
0,7
0,6
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Exergy efficiency
0,14
0,8
Energy efficiency and exergy factor
0,20
0,0 10:00
11:00
12:00
13:00
14:00
15:00
16:00
Local time (hh)
(b)
Fig. 5. Evolutions of the energy efficiency, exergy efficiency and the exergy factor: (a) September 19th, 2015 (b) September 29th, 2015
ACCEPTED MANUSCRIPT 800
4000
Energy rate (W) Exergy rate (W)
700
3500 600 500 2500
400
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300
2000
200
1500
100
19 Oct
18 Oct
17 Oct
13 Oct
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11 Oct
9 Oct
10 Oct
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8 Oct
5 Oct
3 Oct
1 Oct
29 Sept
29 Sept
29 Sept
29 Sept
19 Sept
17 Sept
0
16 Sept
1000
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Fig. 6. Daily energy and exergy rate as a function of days
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Exergy rate (W)
Energy rate (W)
3000
ACCEPTED MANUSCRIPT exergy efficiency (%) Energy efficiency (%) Exergy factor
0,55 0,50 0,45 0,40 0,35
0,12 0,30
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Exergy efficiency
0,14
0,25
0,10
0,20 0,15
0,08
Energy efficiency, exergy factor
0,16
19 Oct
18 Oct
17 Oct
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13 Oct
12 Oct
11 Oct
9 Oct
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10 Oct
8 Oct
5 Oct
3 Oct
1 Oct
29 Sept
29 Sept
29 Sept
29 Sept
19 Sept
17 Sept
16 Sept
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Fig. 7. Daily energy and exergy efficiency as well as exergy factor as a function of
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days
S0959-6526(17)32302-8
DOI:
10.1016/j.jclepro.2017.10.012
Reference:
JCLP 10804
To appear in:
Journal of Cleaner Production
Received Date: 11 May 2017 Revised Date:
20 August 2017
Accepted Date: 1 October 2017
Please cite this article as: Chafie M, Ben Aissa MF, Guizani A, Energetic end exergetic performance of a parabolic trough collector receiver: An experimental study, Journal of Cleaner Production (2017), doi: 10.1016/j.jclepro.2017.10.012. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
1
Energetic end exergetic performance of a parabolic trough collector receiver: an
2
experimental study
Mohamed Chafie*, Mohamed Fadhel Ben Aissa, Amenallah Guizani
4
Research and Technology Center of Energy, Thermal Processes Laboratory, Hammam Lif, B.P. 95,
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2050 Tunis, Tunisia
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* Corresponding author
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E-mail address: [email protected]
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HIGHLIGHTS •
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A parabolic trough collector (PTC) system was designed, manufactured and evaluated.
•
An experimental study was conducted to evaluate the thermal behavior.
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•
A detailed energy and exergy analysis for typical days and for a daily monitoring was performed.
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The energy and exergy efficiency as well as the exergy factor were evaluated.
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•
The average energy and exergy efficiency are found to be higher under clear
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sky days than the cloudy days.
Abstract
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In this article, an experimental investigation is evaluated with the aim of assessing
19
the thermal performance of a receiver tube of a parabolic trough collector. The
20
parabolic trough collector is designed, constructed and installed in the Laboratory of
21
Thermal Processes, Research and Technology Center of Energy (CRTEn), Borj
ACCEPTED MANUSCRIPT Cedria. The thermal performance was carried out using both first and second law of
23
thermodynamics. A detailed energy and exergy analysis was performed to evaluate
24
the thermal performance. Throughout the study, the useful energy gain and the
25
receiver tube exergy transferred to the heat transfer fluid, the energy and exergy
26
efficiency as well as the exergy factor were measured. The measurements were
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conducted during the months of September and October 2015 at Borj Cedria-Tunisia.
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The result has shown that the thermal performance is strongly influenced by climatic
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conditions. The average energy and exergy efficiency are found to be higher under
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clear sky days than the overcast cloudy sky days. The daily energy efficiency varied
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between 19.7% and 52.6%. While the daily exergy efficiency varied from 8.51% to
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16.34%.
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Keywords: Design, Parabolic trough collector, Receiver tube, Energy and exergy
34
analysis, Exergy factor, Solar energy Nomenclature
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aperture area (m2)
Aab
effective receiver area (m2)
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Aap
concentration ratio
C
fluid specific heat capacity (J/kg K)
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C p, f
D ex ,ab
outer diameter of absorber tube (m)
D in ,ab
inner diameter of absorber tube (m)
D ex ,en
outer diameter of glass envelope (m)
D in ,en
inner diameter of glass envelope (m)
E
internal energy (J)
ACCEPTED MANUSCRIPT absorbed solar radiation exergy (W)
Ex u
receiver tube exergy rate transferred to HTF (W)
Ex f
Exergy factor
f
focal distance (m)
h
Enthalpy(kJ/kg)
hc ,en − e
convection heat transfer coefficient from receiver tube to glass
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Ex a
h r , ab −en
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envelope (W)
radiation heat transfer coefficient from receiver tube to glass
hr , en -e
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envelope (W)
radiation heat transfer coefficient from glass envelope to environment (W)
lactus rectum of the parabola (m)
ID
direct normal irradiance (W/m2)
k air
thermal conductivity air (W/m K)
K
incidence angle modifier
•
mass flow rate (kg/s)
m
Nusselt number
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Nu Pr
Q
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Hp
Prandtl number heat transfer (kJ)
Qa
heat absorbed by the receiver (W)
Ql
lost heat flux (W)
Qu
useful energy gain (W)
ACCEPTED MANUSCRIPT Re
Reynolds number
S
entropy (kJ/K)
•
entropy generation (kJ/kg K)
S gen ambient temperature (K)
T ab
absorber tube temperature (K)
Ten
glass envelope temperature (K)
T in
fluid temperature at inlet of receiver tube (K)
Tout
fluid temperature at outlet of receiver tube (K)
Ts
temperature of the sun (K)
UL
overall collector heat loss coefficient (W/m2K)
W
work (kJ)
Wa
collector width (m)
wR
total uncertainty in measurement of result
w 1, w 2 , K , w n
uncertainties in independent variables
Y
curve length of the reflector (m)
α
γ σ
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Greek letters
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T amb
Absorptance intercept factor Stefan Boltzmann constant (5.67 10-8 W/m2K4°)
ϕr
rim angle (°)
ρc
specular reflectance of the reflector
ε ab
absorber selective coating emissivity
absorber envelope emissivity
τ
transmittance of the glass
η
thermal efficiency
ηexr
Exergy efficiency
ηopt
optical efficiency
Abbreviations Parabolic Trough Collector
HTF
Heat Transfer Fluid
DNI
Direct Normal Irradiance
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εen
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1. Introduction Since the Industrial Revolution, the international energy production has been
40
increased in order to satisfy the tremendous needs of the new manufacturing processes
41
In fact, this massive energy production is generally related to the emission of
42
greenhouse gases that stand as serious threats to the environment mainly following the
43
ozone depletion and global warming. To deal with this situation, several global
44
strategies have emphasized that the use of current fuels, which will soon run out,
45
should be replaced by other renewable energy resources. This kind of energy is
46
inexhaustible (Kalogirou, 2004). It reduces the release of greenhouse gases and
47
generates virtually no waste and pollutants. Most renewable energy comes either
48
directly or indirectly from the sun. Therefore, the solar energy appears to be as the
49
energy of the future. The technology that allows us to use the solar energy is based on
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systems that are called solar collectors. The Parabolic Trough Collector (PTC) is one
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of the advanced solar thermal technologies. The PTC is a solar concentration
52
technology that converts solar beam radiation into thermal energy in their receiver.
53
The fields of application and use of the PTC are extensive. Based on its aperture area
54
and its operating temperature, the fields of application can be divided into two main
55
areas (Fernández-García, 2010). The first field of application is intended primarily for
56
the concentrated solar power plants when the temperature ranges from 300 °C to 400
57
°C (Montes et al., 2011; Al-Soud and Hrayshat, 2009; Nishith and Santanu, 2015).
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The other area of applications requires temperatures between 100 and 250 °C. The
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solar systems using this type of the PTC could be used in Industrial Process Heat
60
applications (Fernández-García et al., 2015; Sharma et al., 2016) which include solar
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ACCEPTED MANUSCRIPT water heating (Valan Arasu and Sornakumar, 2007), desalination (Jafari Mosleh et al.,
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2015; Kalogirou, 1998; Palenzuela et al., 2011), solar water disinfection (Bigoni et al.,
63
2014), solar cooking (Mussard and Nydal, 2013), solar refrigeration and air-
64
conditioning (Abu-Hamdeh et al., 2013; Balghouthi et al., 2012; Cabrera et al., 2013),
65
etc.
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Many studies on the energy and exergy analysis of the PTC have been reported
67
and discussed in the literature. Kalogirou (2004) presented a detailed inquiry of the
68
energy and exergy analysis of different types of solar thermal collectors. (Petela,
69
2005) conducted a comprehensive exergy analysis of the PTC. The results show that
70
the exergy efficiency is lower compared to energy efficiency. Tyagi et al. (2007)
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presented a parametric study on the variation of the exergetic performance collector
72
parabolic trough depending on Hottel–Whillier model and taking into account the
73
radiation losses. The parametric study has been done for different concentration ratios
74
and mass flow rates for different solar radiation values. Montes et al. (2010) described
75
the development and use of a thermofluidynamic model for solar parabolic trough
76
collectors system. The aim of this model is to evaluate the performance of the PTC for
77
various heat transfer fluids and to analyze their effect on the parameter; receiver
78
diameter, collector length, pressure and working fluid temperature. The impact of the
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four parameters has been investigated from the pressure drop, heat loss, exergy and
80
energy performance. Reddy et al. (2012) carried out the exergetic analysis and
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performance evaluation of parabolic trough, concentrating solar thermal power plant
82
(PTCSTPP) for the locations of Jodhpur and Delhi in India. It was found that the
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Jodhpur location is much better compared to Delhi in terms of the performance. Ilhan
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and Alper (2013) studied and analyzed experimentally a temperature controlled PTC
85
system. The system consists of a PTC, storage tank, solenoid valves and process
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ACCEPTED MANUSCRIPT control equipment. The results indicate that the highest energy efficiency of the
87
system was calculated as 61.2% for 100 °C and the highest exergy efficiency
88
computed as 63% for 70°C. In order to study the effect of environmental and
89
operational parameters on the efficiency of the PTC, Padilla et al. (2014) performed a
90
detailed exergy balance. Many parameters have been considered in this study such as;
91
mass flow rate, wind speed, inlet temperature, direct solar irradiance, etc. The results
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indicate an opposite trend of thermal efficiency and collector exergy efficiency.
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Recently, Bellos et al. (2016) conducted a parametric study on the energetic and
94
exergetic performance of a PTC using different gases as working fluids. Six gases are
95
investigated for numerous combinations of inlet temperature level and mass flow rate
96
in order to analyze their performance in a high area of operating conditions. This
97
study is based on a numerical model which was developed with Engineer Equator
98
Solver (EES). Optimization of the study shows that the Helium is the best gas up to
99
700 K, while CO2 is the best in higher temperatures. The global maximum of the
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exergetic performance is obtained with helium operating to 0.035 kg/s mass flow rate
101
and 640 K inlet temperature. In order to study the thermal performance enhancement
102
of parabolic trough collector by using nanofluids and converging-diverging absorber
103
tube, Bellos et al. (2016) developed a model designed with Solidworks and simulated
104
in its flow simulation studio. The model has been validated by data from the literature
105
(Dudley et al., 1994). In this context, Jaramillo et al. (2013) developed a
106
thermodynamic model framework by using the first and the Second Law of
107
Thermodynamics so as to analyze the performance of a PTC with a twisted tape
108
insert. In addition to that, there is an exergetic analysis included by the
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thermodynamic model framework with the aim of providing further useful
110
information for the exergy efficiency of the PTC. The model has been validated by
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ACCEPTED MANUSCRIPT experimental data. In 2016, an extensive review of exergy investigation of solar
112
collectors systems and processes was provided by Kalogirou et al. (2016). It
113
comprises Research works on the exergy analysis both different types of solar
114
collectors and different applications of solar collectors systems. Among the different
115
types of the solar collectors are flat-plate collector, parabolic dish collectors,
116
parabolic-trough collectors, etc. And with regards to their applications there are
117
several possibilities including but not limited to drying, heating, cooling, desalination,
118
etc.
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In the present work, an experimental investigation was carried out to evaluate the
120
performance of the PTC receiver. The new PTC designed and realized in the Research
121
and Technology Center of Energy (CRTEn) in Borj Cedria, Tunisia. Using both first
122
and second law of thermodynamics, energy and exergy analysis is achieved. In section 2, the
123
design and the essential component of the experimental device are described. The
124
experimental step is described in section 3. In section 4, the energy and exergy of the
125
PTC receiver is presented. In section 5 the experimental results are summarized and
126
reported. The remarks and outcomes of this work will be reported in the conclusion.
127
2. Parabolic trough collector design
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The PTC system in question was designed, manufactured and installed in the
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CRTEn (Research and Technology Center of Energy) in Tunisia based on Eqs (1)-(3).
130
The experimental system essentially consists of two parts: (1) a parabolic trough
131
collector system, and (2) a thermal pipe. The parabolic trough collector system
132
composed of the reflector and the receiver (Fig.1).
ACCEPTED MANUSCRIPT The reflector allows to transmit as much heat as possible to the fluid. For this
134
reason, it is necessary to take a mirror or a plate of a cylindro-parabolic shape that
135
should be covered by a reflective film. The cross-section of the reflector is a parabola.
136
The rays are reflected towards the focal point of the parabola where the receiver is
137
located. The receiver is the main component of the PTC. It must absorb as much solar
138
radiation as possible. In order to minimize the losses by radiative and convective
139
exchanges with the outside, the absorber tube must be covered by a glass envelope.
140
The annular space is generally evacuated. In fact, to reduce thermal losses, it is
141
important to use a metal absorber tube. In addition, and in order to improve the heat
142
collecting efficiency, the absorber tube must have several characteristics such as; good
143
conductivity and thermal diffusion, low emissivity (use of selective coating), and high
144
absorption factor, etc. the characteristics of the PTC are given in Table 1.
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The equation of the parabola in terms of the coordinate system is given by (Kalogirou, 2009):
147
y 2 = 4fx
148
where f is the focal distance
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2009):
151
W a = 4f tan (ϕ r 2 )
152
With ϕr is the rim angle
153
(1)
The aperture of the parabola is given by the following expression (Kalogirou,
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The curve length of the reflector is defined as (Kalogirou, 2009):
(2)
ACCEPTED MANUSCRIPT Hp
sec (ϕ r 2 ) tan (ϕ r 2 ) + ln ( sec (ϕ r 2 ) + tan (ϕ r 2 ) ) 2
Y =
155
Where
156
H p is the lactus rectum of the parabola
157
sec ( x ) = 1 cos ( x
158
3. Experimental study
159
3.1 Experimental device
(3)
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)
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(4)
A schematic diagram of the experimental methodology and a photograph of the
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PTC system and the related components are shown in Fig. 2. The experimental device
162
is mounted in the CRTEn in Borj Cedria-Tunisia: Latitude 36°50’ N and Longitude
163
10°44’ E. The experimental device composes of a PTC system with aperture area of
164
10.8 m2, oriented east-west and revolves around the horizontal north-south axis
165
(Chafie et al., 2016). The solar tracking is manual and uniaxial. The PTC system is
166
connected by a thermal pipe. The Heat Transfer Fluid (HTF) was circulated in the
167
system by a pump. The HTF is stored in a 46.25 liters storage tank which is thermally
168
insulated by a glass wool. The HTF used in this study is the Transcal N thermal oil.
169
The thermal oil physical properties are presented in Table 2. The flow rate is settled
170
by a pump, a flow-meter and valves integrated at the experimental loop. In order to
171
inhibit evaporation of the HTF in the pipe, the experimental loop must be pressurized.
172
For safety reasons, the experimental device includes also a manometer pressure
173
gauge, an expansion tanks and safety valves. A CR5000 data logger (Campbell
174
ScientificInc) was used to record all climatic parameter and measuring instruments
175
during the tests (every 5 min). Thus, 2 AA Class RTDs (resistance temperature
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ACCEPTED MANUSCRIPT detectors) are used to measure the inlet and outlet temperature of the receiver. A K-
177
type thermocouple was placed in the storage tank to measure the temperature of the
178
HTF in the tank. A HMP155A sensor was used to measure the ambient temperature.
179
A pyrheliometer CHP1 Kipp and Zonen was used to measure direct normal insolation
180
during testing.
181
3.2 Uncertainty analysis
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Uncertainty analysis is needed to prove the accuracy of the experiments. In this
183
study, errors came from the sensitiveness of equipment and measurements explained
184
previously. The uncertainty analysis of the various measured and calculated
185
parameters are estimated according to Holman (1994). The independent parameters
186
measured in the experiments reported here are: ambient temperature, receiver inlet
187
and outlet temperature, solar radiation and HTF flow rate. To carry out these
188
experiments, the sensitiveness of data acquisition system is about ±0.001 °C, the
189
measurement error is ±0.001 °C, the sensitiveness of the AA Class RTDs is ±0.01 °C,
190
the sensitiveness of the K-type thermocouples is ±0.01 °C, the HPM155A errors are
191
±0.02 °C and the flow meter errors are ±0.3%. A pyrheliometer CHP1 Kipp and
192
Zonen with ±1% measurement uncertainties are used. The sensitiveness was obtained
193
from a catalog of the instruments.
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The calculated uncertainties of the dependent parameters were estimated by Eq.
195
(5). The result R is a given function in terms of the independent variables. Let w
196
the uncertainty in the result and w ,w ,K,w be the uncertainties in the independent 1 2 n
197
variables. The result R is a given function of the independent variables x , x ,K, x . 1 2 n
198
If the uncertainties in the independent variables are all given with the same odds, then
199
the uncertainty in the result having these odds is calculated as (Holman, 1994);
R
be
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200
12 2 2 2 ∂R ∂R ∂R w R = w + w +K + w 2 n ∂x 1 ∂x ∂x 1 2 n
(5)
The total uncertainties associated with the calculated values were found as
202
follows: (i) 0.0048 for heat rate, (ii) 0.0059 for thermal efficiency and (iii) 0.0059 for
203
exergy efficiencies.
204
4. Thermodynamic analysis
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The purpose of the thermodynamic analysis of the energy systems is to determine
206
the energy efficiency of the system that has been studied. This efficiency is defined as
207
being the ratio between the useful and the incident energy. These energies can be of
208
the same type or different ones (chemical, thermal, mechanical, electrical, etc.). This
209
analysis derives directly from applications related to the first law of thermodynamics,
210
and it only takes into account the quantities of energy without any references to the
211
quality associated with it according to the second law. Combining exergy with energy
212
in the analysis of energy systems goes back to the association of both quantity and
213
quality of energy to their various forms or types. The analysis becomes much more
214
substantial than being a mere simple energy analysis.
215
4.1 Energy analysis
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The energy analysis of each energetic system can be performed by using the first
217
law of thermodynamics. For a control volume flow process in an open system, the
218
energy balance is given by the following expression (Borgnakke and Sonntag, 2009):
219
dE dt
n
• • • • = Q − W + m h − m ∑ i ∑ i total ,i ∑ 0 htotal ,i CV i = 0 0 i
(6)
ACCEPTED MANUSCRIPT ν2
220
htotal = h +
221
Where Q , W , m and h are the heat transfer, work, mass flow rate and the enthalpy.
222
In case of steady state condition, dE dt = 0 . The first low efficiency (energetic
+ gz
(7)
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223
efficiency) of a collector is determined by the ratio of the energy output to the energy
224
input of the collector. It is written as follows (Benson, 2013):
225
η=
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Desired output energy Input energy
(8)
The useful energy gain transferred to the HTF ( Qu ) is the difference between the
227
heat absorbed by the receiver tube ( Q a ) and the lost heat flux ( Ql ). It can be
228
estimated as:
229
Qu = Qa − Ql
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(9)
The useful energy gain transferred to the HTF is defined as:
231
. Qu = m C p ,f
232
. Where m and C p ,f are the mass flow rate and the specific heat capacity of the HTF,
233
respectively.
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(10)
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(Tout
The heat absorbed by the receiver tube is as follows:
235
Q a = ηopt I D A ap
236
Where ηopt , I D and Aap are the optical efficiency, Direct Normal Irradiance (DNI)
237
and the aperture area.
(11)
ACCEPTED MANUSCRIPT 238
The optical efficiency could be calculated by the following equation (Duffie and
239
Beckman, 2006):
240
ηopt = ρcατγ Kθ
241
Where ρc , α , τ , γ and K θ are the reflectance of the reflector, absorbance of the
242
receiver tube, transmittance of the glass envelope, intercept factor and the incidence
243
angle modifier.
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(12)
Since the HTF makes its way through the receiver, its temperature exceeds the
245
ambient temperature very quickly, that produces a process of loss of heat from the
246
receiver. The modes of heat loss are radiation and convection. They are dependent on
247
the difference in temperature between the receiver and the environment and the
248
geometry of the receiver and that of the concentrator.
The lost heat flux ( Ql ), includes heat loss by radiation and convection, is given
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by (Kalogirou, 2009):
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Q l = A ab U L (T ab − T amb )
252
Where Aab , U L , are the effective receiver tube area ( m 2 ) and the heat loss
253
coefficient (W m 2K ).
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(13)
The heat loss coefficient based on the receiver area is basically influenced by the
255
different heat transfer mode. It is given by (Kalogirou, 2009):
256
Aab 1 UL = + hc ,en −e + hr , en −e Aen hr , ab −en
(
)
−1 (14)
ACCEPTED MANUSCRIPT Where h r , ab −en , hr , en -e and hc ,en − e the radiation heat transfer coefficient from
258
receiver tube to glass envelope, radiation heat transfer coefficient from glass envelope
259
to environment and the convection heat transfer coefficient from receiver tube to glass
260
envelope, respectively. They are given by the following expressions (Kalogirou,
261
2009):
262
4 hr ,en _ e = σ εen Ten4 -Tamb
263
4 -T 4 σ T ab en hr ,ab _ en = 1 ε ab + (1- εen ) Dex ,ab εen D in ,en
264
hc ,en −e =
) )
)
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(
(
(15)
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(
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Nu air k air Dex ,en
(16)
(17)
For external forced convection flow normal to an isothermal cylinder, the Nusselt
266
number ( Nuair ) is estimated with Zhukauskas’ correlation (Duffie and Beckman,
267
2013):
268
1/ 4 m n Prair Nu air = C Reair Prair Prog
269
With
270
n = 0.37 if Prair ≤ 10 and n = 0.36 if Prair ≥ 10
271
Values of C and m are listed in Table 3.
272
273
(18)
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Substituting Eqs. 10, 11 and 12 into Eq.9 yields
. m C p ,f
(Tout −T in ) = ηopt
I D Aap − Aab U L (T ab −T amb )
(19)
ACCEPTED MANUSCRIPT 274
The instantaneous efficiency η of a PTC is defined as the ratio of the useful
275
energy gain by the incident radiation of the aperture area of the PTC. It is given as
276
follows:
277
. m C p .f (T out − T in ) Qu η= = Aap I D A ap I D
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By injecting Eq. 19 in Eq. 20, we find that the equation of the thermal efficiency can also be written in the following form:
280
η = ηopt − A abU L
281
4.2 Exergy analysis
T ab − T am b I D A ap
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(20)
(21)
Exergy analysis is a method based on the second law of thermodynamics for the
283
analysis and thermodynamic evaluation of systems. Its interest is that it provides a
284
very powerful calculation methodology to quantify the thermodynamic quality of any
285
process or system. The exergy analysis is based on the comparison of the system to be
286
evaluated with respect to an idealized system where the transformations of energy are
287
reversible, without production of entropy. For non-steady condition the entropy
288
generation is given by (Gakkhar et al. 2016):
•
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n
•
• • Q − ∑ i − ∑ m i si + ∑ m 0 s0 ≥ 0 CV i = 0 T i i 0
289
dS S gen = dt
290
where S , Q , T and m are entropy, heat transfer, temperature and mass flow rate.
291 292
(22)
The receiver tube exergy rate transferred to HTF with reference to the surroundings is written as it follows (MacPhee and Dincer, 2009):
ACCEPTED MANUSCRIPT
294
. Ex u = m C p ,f
(Tout -T in )
. T - m C p ,f T amb ln out T in
(23)
By injecting Eq. 10 in Eq. 23, the receiver tube exergy rate transferred to HTF can
295
also be written in the following equation:
296
. T Ex u = m C p ,f (Tout −T in ) - T amb ln out T in
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293
(24)
The absorbed solar radiation exergy by the system (reflector and receiver tube),
298
assuming the sun as an infinite thermal source is given by the Petela expression
299
(Petela, 2003) and is defined as:
300
4 Tamb 4 T amb 1 Ex a = Aap I D 1 + 3 Ts 3 Ts
301
Where T s = 5762 K is the apparent sun temperature. It represents 75% of blackbody
302
temperature of the sun (Dutta Gupta and Saha, 1990).
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The exergy efficiency, analogous to the energy case, is defined as the ratio of the
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receiver tube exergy rate by the absorbed solar radiation exergy. It is written as:
305
. T m C p ,f (Tout −T in ) - T amb ln out Ex u T in ηexr = = 4 Ex a T amb 4 T amb 1 Aap I D 1 + 3 Ts 3 Ts
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(26)
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The exergy factor is defined as the ratio of the receiver tube exergy rate by the
307
useful energy gain transferred to the HTF. It is proposed for quantifying the thermal
ACCEPTED MANUSCRIPT 308
performance and it is used as a measure of the performance of the receiver. It is
309
evaluated using the following definition:
310
. T m C p , f (T s −Te ) - T amb ln s Ex u Te Ex f = = . Qu m C p ,f (T s −Te )
311
5. Results and discussion
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(27)
In this work, In order to evaluate the energy and exergy performance of our PTC,
313
a thermodynamic analysis was carried out. The measurements were performed at the
314
Research and Technology Centre of Energy (CRTEn) in Borj Cedria, Tunisia during
315
the period between September and October 2015.
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5.1 Performance of the PTC: typical days
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In order to guarantee the energy and exergy performance of the PTC and the
318
ability to compare the performance with other collectors, experiments were conducted
319
on typical days: September 19th, 2015 characterized by a severely solar radiation
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fluctuation and September 29th, 2015 characterized by a clear sky day. During the
321
experiments, the inlet and outlet HTF temperature, ambient temperature, DNI, energy
322
and exergy rate, energy and exergy efficiency and the exergy factor were recorded
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(Fig. 3-5). The experimental tests are taken from 9:00 h to 16:00 h with an average
324
speed wind wind speed at 3 m/s and a same flow rate of 0.2 Kg/s.
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The inlet and outlet HTF temperatures and the ambient temperature of the typical
326
days were presented in Fig. 3. During the cloudy day (September 19th, 2015) (Fig. 3a),
327
we notice that the ambient temperature varies slightly. It is changed between 26 °C
328
and 31.5 ° C. We also find in Fig. 3a that the evolution of HTF inlet and outlet
ACCEPTED MANUSCRIPT temperatures is irregular throughout the day. This fluctuation is due to the variability
330
of the solar radiation caused by the cloudy passages. This disruption of the evolution
331
of the HTF inlet and outlet temperatures is clearly observed during the interval of time
332
from 11:00 h to 13:40 h. The HTF inlet and outlet have a temperature of 60 °C and 65
333
°C, respectively at 11:00 h, decreases until 59 °C and 59.7 °C, respectively at 12:30 h
334
and then increases again till reaching 72.7 °C and 77.65 °C at 13:40 h.
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Plot b of Fig. 3 (clear sky day) shows that the ambient temperature ranges between 29 °C and 36 °C with an average daily about 33 °C.
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On the same figure (Fig.3b), it can be seen that the evolution of the inlet and
338
outlet HTF temperatures increases progressively with time and reaches a maximum at
339
noon. The maximum outlet HTF temperature obtained by the PTC is 105 °C at 12:20
340
h. Pursuant to the HTF temperature evolution in the PTC, the HTF temperature
341
difference between outlet and inlet of the PTC increases with time until noon, and
342
then decreases. The maximum HTF temperature difference reaches to 12 °C.
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In order to evaluate the experimental energy gain and the exergy rate of our
344
system, Eqs. (10) and (23) have been respectively used. The evolution of the DNI,
345
useful energy gain and the exergy rate are presented in Fig. 4. The experimental data
346
were recorded on 19th and 29th September, 2015.
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We found out that during September 19th, 2015 the evolution of the DNI has a
348
very clear fluctuation. The fluctuation of solar irradiance is caused by passing clouds.
349
During that cloudy day, the DNI varied irregularly from 220 to 814 W/m2 (Fig. 4a).
350
The evolution of the energy gain and the exergy rate during the cloudy day follows
351
the same trend as the DNI. This similarity is very clear in the period of time between
ACCEPTED MANUSCRIPT 12:00 to12:40 h when the solar radiation presents low values, and also in the time
353
between 13:00 to 14: 00 h during which the solar radiation has high values. In the
354
same figure (Fig. 4a), we find that the DNI undergoes a fluctuation around 12:30 h.
355
Moreover, in the same period the evolution of the energy gain and the exergy rate
356
undergoes also a simple fluctuation. The receiver tube exergy rate transferred to HTF
357
during the experiment is much lower than the energy rate. Its average value only
358
reaches 115.1 W while the average useful power is 1.2 KW.
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Plot b of Fig.4 (clear sky day) shows that the DNI is 540 W/m2 at 9:15 h and it
360
increases up to 750 W/m2 at noon. Beyond this time, the DNI decreases until the end
361
of the experiment, and it is 270 W/m2. Since the measured solar radiation is the only
362
the direct component, it increases up to midday, and then starts decreasing.
363
The results obtained during the both sunny and cloudy days (Fig 4) confirm that the
364
evolution of the energy gain and the exergy rate are strongly influenced by direct solar
365
radiation.
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The evolution of the energy gain with respect to time during the day (September
367
29th, 2015) of the experiment increases and reaches a maximum of 4820 W at 12:00 h
368
which the solar radiation reaches its peak of 750 W/m2. After noon the sun starts its
369
retreat and the solar radiation starts decreasing which leads to the reduction of the
370
useful heat gain. This is owed to the fact matter that the energy gain is forcefully
371
influenced by the DNI, and therefore follows its variation. During the experiment, the
372
energy rate was much greater than the exergy rates with its maximum value being
373
around 4820 W as opposed to around only 890 W for the exergy rate. It can be
374
concluded that the quality of energy from the receiver tube is very low perhaps due to
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a combination of a broad bulk of irreversible energy changes such as heat losses and
376
the transfer of high quality radiative energy to the HTF. Therefore, to evaluate the thermal efficiency, exergy efficiency and the exergy
378
factor, Eqs. (20), (26) and (27) are used respectively. Fig. 5 presente the evolutions of
379
the energy efficiency, exergy efficiency and the exergy factor during to typical days;
380
September 19th, 2015 and September 29th, 2015.
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A general view of plot 5a and according to the evolution of the inlet and outlet
382
HTF temperatures (Fig. 3) and the evolution of the DNI and energy and exergy rate
383
(Fig. 4) during September 19th, 2015, shows a fluctuating and irregular variation with
384
respect to the time of the evolution of the energy and exergy efficiency as well as the
385
exergy factor. We noted that the latter is highly affected by the severely solar
386
radiation fluctuation. This disruption is precisely detected during the interval of time
387
from 10:00 h to 13:30 h. The energy and exergy efficiency are of 6.6 % and 7.1 % at
388
12:30 h respectively, this low energy and exergy efficiency values coincides with a
389
low value of DNI as well as low useful power (411 W/m2 for direct solar radiation
390
and 265.77 W for useful power). On the other hand, at 13:30 h, the thermal the energy
391
and exergy efficiency are of 29.3% and 10.85%, respectively. The exergy factor can
392
also be used as a parameter for measuring the performance of the receiver tube. The
393
exergy factor appears to be relatively similar to the evolution of the solar radiation,
394
high under high solar radiation conditions and it declines when the solar radiation is
395
small.
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With regards to the day, September 29th, 2015 (sunny day) the evolution of the
397
energy and exergy efficiency and the exergy factor with time has the same trends. The
398
energy efficiency, exergy efficiencies and the exergy factor vary in a similar way to
ACCEPTED MANUSCRIPT DNI and thus it can be concluded that it is the most influential factor affecting these
400
parameters. We noted that the energy and exergy increases until it reaches a
401
maximum value of 61.54% and 17.48% at midday, respectively where the DNI has a
402
maximum value of 750 W/m2. Afternoon the energy and exergy efficiency begins to
403
decrease until the end of the experiment. The exergy factor is lower than the energy
404
efficiency and peaks a maximum around 0.17 at noon and then decreases until to 0.09
405
at 16:00 h.
406
5.2 Daily energy and exergy efficiency
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In order to study the energy and exergy efficiency of our receiver tube, we
408
conducted a test campaign during the period between September 16th, 2015 and
409
October 19th, 2015. During working days, there are days that correspond to favorable
410
climatic conditions and other days more severe.
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Fig. 6 illustrates the values of the daily energy and exergy rate as function of days
412
of experiments. The daily average energy rate varied between 1202.31 W and 3670,75
413
W. The daily average value of the energy rate is around 2496.96 W. During the period
414
of the experiment, the exergy rate changed between 115.1 W and 536.2 W. Their
415
average value is about 317.53 W.
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The values energy and exergy efficiency as well as the exergy factor with respect
417
to days are presented in Fig. 7. It is clear from this figure that the higher daily energy
418
and exergy efficiencies recorded on19th October, 2015. The daily average energy
419
efficiency changes from 19.7% to 52.6% with an average of 36.10% (during the
420
period between 16
421
daily average exergy efficiency, it can be seen that it is comprised between 8.51% and
422
16.34%. We noted that the average exergy efficiency and exergy factor are about
th
September 2015 and 19
th
October 2015). With regards to the
ACCEPTED MANUSCRIPT 11.85 % and 0.11, respectively. This value is enough in terms of efficieny exergy and
424
factor exergy. When values are compared with the daily average energy efficiency,
425
the exergy efficiency and exergy factor daily average of the receiver tube was lower
426
than the daily energy efficiency for all days. The results of the energy and energy
427
performance study show the influence of climatic factors on the performance of the
428
system. Table 4 summarizes the energy and exergy performance of our system.
429
6. Conclusion
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In this work, an experimental study to evaluate the thermal behavior of a
431
parabolic trough collector was conducted. The experimental device in question was
432
designed and realized in the Research and Technologies Centre of Energy in Tunisia
433
(CRTEn). The thermal behavior is evaluated using energy and exergy analysis. The
434
useful energy gain transferred to the HTF and the exergy efficiency are proved to be
435
smaller than the useful exergy rate and the exergy efficiency. The exergy factor
436
parameter is also used to evaluate the thermal performance. The exergy factor is
437
proved to be heavily influenced by the DNI as well as the operating temperature. The
438
average energy efficiceny, exergy efficiceny and the exergy rate varied between
439
19.72%, 8.51% and 0.08 to 36.1%, 11.72% and 0.12 for the cloudy day and sunny
440
day, respectively. The daily average energy and exergy efficiceny as well as the
441
exergy factor obtaining during the experimental period are about 36.10%, 11.85% and
442
0.11 respectively.
443
Acknowledgment: We are deeply grateful for the cooperation and assistance we got
444
from all the staff at the Thermal Processes Laboratory of Research and Technology
445
Center of Energy-Borj Cedria, Tunisia.
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ACCEPTED MANUSCRIPT List of tables Table 1: Characteristics of the PTC Table 2: Thermal properties of Transcal N oil
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Table 4: Summary of daily performance PTC receiver.
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Table 3: Constants for Equation (18)
ACCEPTED MANUSCRIPT Table 1: Characteristics of the PTC Value
Length (mm)
4000
Aperture (mm)
2700
Aperture area (m2)
10.66
Focal distance (mm)
835
C
12.02
Rim angle (°)
76.3
Reflected surface Reflectivity
0.93
Type of glass cover
High borosilicate glass
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Type of receiver
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Parameter
Evacuated Glass-steel coated tube
Type of steel
SUS304
Diameter outer tube (mm)
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Thickness outer tube (mm)
120
3
70
Thickness inner tube (mm)
1.5
Length of receiver (mm)
4000
Emittance
<8%
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Diameter inner tube (mm)
Absorbtivity
>93%
Glass transmissivity
0.95
Vacuum rate (pa)
P<5.10-3
ACCEPTED MANUSCRIPT Table 2: Thermal properties of Transcal N oil Data
Units
Density (15°C)
875
Kg/m3
fire point
243
°C
flash point
221
Viscosity (100°C)
5.2
Viscosity (40°C)
31
Specific heat
1925.9
Coefficient of Thermal Expansion
0.00077
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Transcal N
°C
mm2/s
mm2/s
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J/kg °C per °C
ACCEPTED MANUSCRIPT
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m
1-40
0.75
0.4
40-1000
0.51
0.5
1000-200000
0.26
0.6
200000-1000000
0.076
0.7
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Table 3 : Constants for Equation (18)
ACCEPTED MANUSCRIPT Table 4. Summary of daily performance PTC receiver. Standard deviation
Maximum Minimum
Energy rate (W)
2496,96
678,65
3670,75
1202.31
Exergy rate (W)
317,53
126,24
536,2
115,1
Energy efficiency (%)
36,10
8,8
52,6
19,7
Exergy efficeincy (%)
11,85
1,84
16,34
8,51
Exergy factor
0.11
0,01
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Average
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0,08
ACCEPTED MANUSCRIPT List of figures Fig. 1: Photograph of the experimental system: (1) reflector, (2) receiver Fig. 2: (a) A schematic view of the experimental methodology; (b) the photograph of experimental set-up. Fig. 3: Evolutions of the ambient temperature and the inlet and outlet HTF
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temperatures: (a) September 19th, 2015 (b) September 29th, 2015
Fig. 4: Evolutions of the solar radiation, useful energy gain and the exergy rate: (a) September 19th, 2015 (b) September 29th, 2015 (a) September 19th, 2015 (b) September 29th, 2015
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Fig. 5: Evolutions of the energy efficiency, exergy efficiency and the exergy factor:
Fig. 6: Daily energy and exergy rate as a function of days
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(b) Fig. 2. (a) A schematic view of the experimental methodology; (b) the photograph of experimental set-up
ACCEPTED MANUSCRIPT 50
90
Tamb Tin Tout
45
80
70
60
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Temperature (°C)
Tamb (°C)
40
50
30
40
25
20 09:00
10:00
11:00
12:00
13:00
14:00
15:00
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(a)
45
35
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80 70 60 50
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100 90
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Tamb (°C)
40
110
Tamb Tin Tout
Temperature (°C)
50
40 30
20
09:00
10:00
11:00
12:00
13:00
14:00
15:00
16:00
Local time (hh)
(b)
Fig. 3. Evolutions of the ambient temperature and the inlet and outlet HTF temperatures: (a) September 19th, 2015 (b) September 29th, 2015
ACCEPTED MANUSCRIPT 4500
900
DNI Qu
4000
Exu
3500 3000
600 2500 500
2000
Energy and exergy rate (W)
700
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Direct Normal Irradiance (W/m2)
800
400
1500 1000
300
500
200
10:00
11:00
12:00
13:00
14:00
15:00
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16:00
(a)
900
TE D
600
500 400
300
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4000
3000
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2
700
5000
1000
Energy and exergy rate (W)
Direct Normal Irradiance (W/m )
800
DNI Exu Qu
0
100
09:00
10:00
11:00
12:00
13:00
14:00
15:00
16:00
Local time (hh)
(b)
Fig. 4. Evolutions of the solar radiation, useful energy gain and the exergy rate: (a) September 19th, 2015 (b) September 29th, 2015
ACCEPTED MANUSCRIPT 0,6 0,12
0,5
0,10
0,08 0,3
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Exergy efficiency
0,4
Energy efficiency and exergy factor
Exrgy efficiency Energy efficiency Exergy factor
0,06
0,2
0,04
0,02 09:00
10:00
11:00
12:00
13:00
14:00
15:00
0,0
16:00
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0,1
(a)
Exergy efficiency Energy efficiency Exergy factor
0,18 0,16
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0,12 0,10 0,08 0,06
AC C
0,04
0,5
0,4
0,3
0,2
0,1
0,02
09:00
0,7
0,6
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Exergy efficiency
0,14
0,8
Energy efficiency and exergy factor
0,20
0,0 10:00
11:00
12:00
13:00
14:00
15:00
16:00
Local time (hh)
(b)
Fig. 5. Evolutions of the energy efficiency, exergy efficiency and the exergy factor: (a) September 19th, 2015 (b) September 29th, 2015
ACCEPTED MANUSCRIPT 800
4000
Energy rate (W) Exergy rate (W)
700
3500 600 500 2500
400
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300
2000
200
1500
100
19 Oct
18 Oct
17 Oct
13 Oct
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11 Oct
9 Oct
10 Oct
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8 Oct
5 Oct
3 Oct
1 Oct
29 Sept
29 Sept
29 Sept
29 Sept
19 Sept
17 Sept
0
16 Sept
1000
EP
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Fig. 6. Daily energy and exergy rate as a function of days
AC C
Exergy rate (W)
Energy rate (W)
3000
ACCEPTED MANUSCRIPT exergy efficiency (%) Energy efficiency (%) Exergy factor
0,55 0,50 0,45 0,40 0,35
0,12 0,30
RI PT
Exergy efficiency
0,14
0,25
0,10
0,20 0,15
0,08
Energy efficiency, exergy factor
0,16
19 Oct
18 Oct
17 Oct
SC
13 Oct
12 Oct
11 Oct
9 Oct
M AN U
Date
10 Oct
8 Oct
5 Oct
3 Oct
1 Oct
29 Sept
29 Sept
29 Sept
29 Sept
19 Sept
17 Sept
16 Sept
0,10
Fig. 7. Daily energy and exergy efficiency as well as exergy factor as a function of
AC C
EP
TE D
days