Performance study and comparative analysis of traditional and double-selective-coated parabolic trough receivers

Accepted Manuscript Performance study and comparative analysis of traditional and double-selectivecoated parabolic trough receivers Honglun Yang, Qiliang Wang, Xiaona Huang, Jing Li, Gang Pei PII:

S0360-5442(17)32169-2

DOI:

10.1016/j.energy.2017.12.126

Reference:

EGY 12075

To appear in:

Energy

Received Date: 25 September 2017 Revised Date:

13 December 2017

Accepted Date: 24 December 2017

Please cite this article as: Yang H, Wang Q, Huang X, Li J, Pei G, Performance study and comparative analysis of traditional and double-selective-coated parabolic trough receivers, Energy (2018), doi: 10.1016/j.energy.2017.12.126. 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

Performance study and comparative analysis of traditional and

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double-selective-coated parabolic trough receivers

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Honglun Yang, Qiliang Wang, Xiaona Huang, Jing Li, Gang Pei *

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Department of Thermal Science and Energy Engineering, University of Science and Technology of China, Hefei 230027, China

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_____________________________ * Corresponding author. Tel.: 0551-63601652. E-mail address: [email protected]

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Abstract

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the Soltrace software using the Monte Carlo Ray-Trace Method, an innovative parabolic trough solar

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receiver that employs two solar selective coatings with different properties on the outer surface of the

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absorber is proposed. The concentration ratio and absorber temperature that influence optimal cut-off

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wavelengths of the solar selective coatings are quantitatively analyzed to optimize the property of the

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coating. The optimal cut-off wavelength increases with the concentration ratio, but drops with the

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increasing absorber temperature. The heat transfer process of receivers is numerically simulated to

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predict the thermal performance of evacuated receivers based on spectrum parameters heat transfer

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model. Heat loss simulation results show that: the double-selective-coated receiver can reduce heat

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loss and boost the collecting efficiency significantly compared with PTR70 receiver. When the

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temperature of absorber is 500 °C, the double-selective-coated receiver can reduce heat loss by 157.8

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W/m and increase the collecting efficiency from 64.7% to 68.1%. The System Advisor Model annual

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simulation results indicate that double-selective-coated receivers can decrease the levelized cost of

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electricity of concentrating solar plants by 2.78%–7.34%, and increase electricity production by 2.94%

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– 8.21% compared with traditional PTR70 receivers.

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Keywords

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Based on the simulated non-uniformity solar radiation flux distribution of the absorber by

CSP; PTC; Parabolic Trough Receiver; Solar selective absorbing coating; Heat loss;

ACCEPTED MANUSCRIPT 1. Introduction

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Parabolic trough concentrator (PTC) systems are the most mature and cost-effective technology that

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generate electricity through concentrating solar power (CSP) [1,2]. Unlike photovoltaic systems that

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convert solar radiation directly to electricity, PTC systems concentrate the incident solar radiation

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onto the evacuated receiver where the heat transfer fluid (HTF) is heated to high temperature. Then,

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the heated HTF returns to the power block to generate high temperature and pressures superheated

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steam in the heat exchangers. Finally, the thermal energy is converted to electricity in the steam

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turbine by the Rankine cycle [3]. Photovoltaic systems have undergone considerable development in

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recent years. It is of simple structure, relative lower investment and is easy to realize small scale for

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distribution application. While photovoltaic systems show terrible stability and reliability due to

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instability of the solar radiation. In contrast, CSP has emerged in recent years as a potential solution

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to supply dispatchable baseload and stable electricity, since it can rely on thermal energy storage [4].

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The costs of CSP are expected to decrease significantly to compete with other generation

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technologies and large-scale application.

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Currently, the operating temperature of commercial parabolic trough concentrating solar power

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plants can reach 400 °C [5, 6]. The operating temperature tends to increase further to enhance the

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Carnot efficiency. Archimed 5-MW parabolic trough demo project was the first plant using molten

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salt as HTF, wherein the outlet temperature of the solar field is 550 °C [7]. Moreover, the outer

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surface of the absorber temperature is generally 20°C to 30°C higher than the average temperature of

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the HTF [8]. Thus, the highest temperature of the absorber can reach 580°C.

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The solar parabolic trough receiver is a key component of PTC systems and mainly composed of a

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steel absorber tube with a solar selective absorbing coating on its outer surface, a glass envelope, a

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ACCEPTED MANUSCRIPT glass-to-metal seal, a bellow, and chemical sponges that maintain and indicate the status of

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vacuum[9]. The annulus between the steel absorber tube and the glass envelope is evacuated to

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suppress heat convection and conduction between them. The heat loss in the evacuated receiver

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would be the sum of the absorber tube’s radiation loss and the metal bellow’s conduction loss at the

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end of the receiver. Conduction loss is extremely small compared with the total heat loss of the

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receiver. Conduction loss is only 3% of the total heat loss when thermally well-insulated at an

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absorber temperature of 400 °C [10]. Furthermore, the radiation loss of the receiver increases

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exponentially with the absorber tube temperature. The heat loss of the receiver is significant relative

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to total heat loss of the CSP. Thus, reducing the radiation heat loss of the receiver is an effective

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approach to enhancing CSP efficiency.

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After decades of rigorous studies, researchers and engineers have proposed several solutions to

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improve the solar thermal conversion efficiency of CSP by optimizing the receiver [11]. A

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high-performance selective coating that can absorb as much as possible solar radiation while emit the

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least possible thermal radiation was first developed. Esposito et al. [12] optimized and fabricated

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several solar coatings with excellent photo-thermal conversion efficiency and thermal stability.

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Cespedes et al. [13] proposed a novel Mo-Si3N4 based selective coating for the high-temperature

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concentrating solar power application. The Huiyin Group from China developed a novel

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cermet-selective coating with an absorptivity that can reach as high as 96.5% and its emissivity is

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only 7.2% at 400 °C [14]. Several researchers proposed a novel strategy that employs two different

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types receivers in different sections of a collector loop [15]. The levelized cost of electricity (LCOE)

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of CSP is reduced significantly by using mixed receivers. Schott Group, the leading company in

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glass industry, possesses multitudinous advanced technology and is an experienced maker of

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ACCEPTED MANUSCRIPT glass-to-metal seals and optical materials. They adopted glass-to-metal seals with matching

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coefficients of thermal expansions to improve the durability and reliability of receivers [16]. The

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outer surface of the glass envelope was plated with anti-reflective coating to enhance the

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transmittance of the glass envelope. The chemical sponges that can absorb hydrogen in evacuated

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annuls to suppress convection and conduction heat loss were also investigated [17, 18].

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As mentioned above, many companies have made outstanding contributions to optimize receiver.

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Performance improvements of receiver in materials and manufacturing processes have their limits.

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Nowadays, the receiver is well developed, all manufacturing process and materials have done the

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best. Nevertheless, the efforts of this approach still deserve to be lucubrated. Several researchers in

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relevant fields have attempted to reduce the heat loss of receivers by adjusting their structure and

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geometry. Hany et al. [19, 20] introduced a simple modification in receivers with gas-filled annuli.

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This modification fills the outward-facing half of the gas-filled annulus with a heat-resistant

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insulating material. A comparative analysis of receivers with and without insulating material was

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conducted based on simulation and experiment. Results showed that the receiver with this

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configuration could reduce heat loss by 12% to 15% and increase collector efficiency by 1.8% to

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6.4%. Gee et al. [21] proposed a design with a non-imaging secondary reflector as part of a parabolic

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trough receiver. Their simulation results showed that the design offers approximately 1% of net

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increase in optical efficiency.

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ACCEPTED MANUSCRIPT Nomenclature A area, m2 D diameter, m E emissive power, W/(m2·µm)

HTF LCOE MCRT

Heat transfer fluid Levelized cost of electricity Monte Carlo Ray-Trace

convective heat transfer coefficient, W/(m2·K)

NREL

National Renewable Energy Laboratory

J

radiosity, W/(m2·µm)

PTC

Parabolic trough concentrator

k L Nu

heat conductivity of air, W/(m·K) width of collector, m Nusselt number

SAM Solar advisor model TPV Thermophotovoltaic Subscripts

Pr q Q ,

Prandlt number heat gain of traditional receiver, W heat flux W/m solar irradiance, W/(m2·µm)


a cg cl

spectral ambient between glass envelope and ambient collector

der

optical derate efficiency

dp g gi

dew temperature of ambient glass envelope inner surface of glass envelope

go gsk loss op

outer surface of glass envelope between the glass envelope and sky heat loss optical the solar radiation absorbed by glass

ΔQ



wind speed, m/s view factor increment of heat gain effective emissivity

Greek Symbols α absorptivity β Angle, ° ε emissivity

EP

v0 X

TE D

receiver, W radiation resistance Reynolds number temperature, K

R Re T

rg s sd sg sgd sgu sky ssky sskyd sskyu su

DNI

sun

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η efficiency λ

wavelength, µm  kinematic viscosity of air,m2/s reflectivity

transmittance Abbreviation CSP Concentrating solar power Direct normal irradiance

SC

M AN U

Heat gain of double-selective-coated

q’

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hc

steel absorber tube down side of steel absorber between the absorber and glass envelope between the downside absorber and glass between the upside steel absorber and glass sky between the absorber and sky between the downside absorber and sky between the upside steel absorber and sky up side of steel absorber the solar radiation absorbed by absorber

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Researchers have taken notice of the circumferential non-uniformity heat flux distribution of

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receivers and increased the radiation resistance where there is low heat flux. Traditional parabolic

ACCEPTED MANUSCRIPT trough receivers are coated with one kind of selective absorbing coating on the outer surface of the

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steel absorber tube. Absorptivity and emissivity are key parameters for evaluating selective solar

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coating. Approximately 98% of solar radiation is in the 0.2–2.5 µm range [22], whereas the thermal

95

radiation that contributes to the heat loss of receivers, depending on different working temperatures,

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is in the 1.2–25 µm range. Thus, as the temperature of steel absorber tube increases, the radiation

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spectrum of the receiver shifts to short wavelengths, and the overlap of thermal radiation and solar

98

radiation that bands at the 1.2–2.5 µm range also increases. A coating that possesses high absorptivity

99

for solar radiation bands and low emissivity for thermal radiation bands is an unsolvable

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contradiction. Consequently, the emissivity of the coating and heat loss are inevitably increased at

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high temperature.

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Thus, only a small room exists for decreasing the heat loss of receiver further by relying on material

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research. In this study, we propose a novel double-selective-coated receiver with two kinds of solar

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selective absorbing coatings based on the spectrum parameter model of radiation heat transfer. Two

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coatings with different spectral properties are used to plate corresponding surfaces of the receiver

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with and without concentrated light spots. Generally, the maximum heat flux of the absorber surface

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with a concentrated light spot tends to be 70 times higher than that without a concentrated light spot.

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The light path of the parabolic trough concentrator system is shown in Fig. 1. The coating that plates

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the side with concentrated light spot prioritizes high solar radiation absorptivity. By contrast, the

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coating that plates the side without concentrated light spot prioritizes the emissivity of thermal

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radiation. In this paper, the spectrum parameters heat transfer model is presented. The solar radiation

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flux distribution of the steel absorber tube is also simulated using Soltrace software based on the

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Monte Carlo Ray-Trace (MCRT) method. The optimal cut-off wavelength changing with the optical

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ACCEPTED MANUSCRIPT concentration and absorber temperature is analyzed. In addition, the heat transfer processes of the

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receivers and the performance of CSP are simulated. Traditional and double-selective-coated

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receivers are compared and evaluated.

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Fig. 1. Light path diagram of parabolic trough concentrator system

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2. Simulation model

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2.1 Assumptions

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As previously mentioned, a parabolic trough receiver with two kinds of solar selective absorbing

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coatings is proposed. The simplified 1D heat transfer models of the traditional and

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double-selective-coated receivers are shown in Fig. 2. As shown in Fig. 2a, the outside surface of the

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absorber tube of the traditional receiver is coated with one kind of solar selective coating. To

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describe the radiation heat transfer process accurately, the double-selective-coated receiver is divided

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into two parts, as shown in Fig. 2b. I-region is an ultra-low emissivity coated area, while II-region is

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a high solar radiation absorptivity coated area. The heat transfer processes of the two types of

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receivers are similar. Only the spectral parameters of the solar selective coating are different. The

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ACCEPTED MANUSCRIPT central angle of I-region isβ, which changes within the 120°–180° range in accordance to the type

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of collector to ensure low emissivity coating is outside of the concentrated light spot. The following

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assumptions are made to simplify the model and establish a unidimensional, steady-state model [23].

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1) The heat loss of the two ends of the receiver and metal bellow is ignored.

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2) The conduction resistances of the glass envelope and the steel absorber tube are ignored.

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3) All the involved surfaces in this study are diffusing surfaces.

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4) The heat convection and conduction in the vacuum annuls are also ignored.

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5) The axial and circumferential temperatures of the glass envelope and the steel absorber tube are

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uniform.

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6) This model simulates the heat loss of the evacuated receivers equipped with EuroTrough ET150

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collector.

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Heat Transfer Fluid

Steel absorber Evacuated annulus

Glass envelope

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(a) Heat transfer process of traditional receiver

ACCEPTED MANUSCRIPT

I β～

Glass envelope

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Heat Transfer Fluid

～II

Steel absorber

Evacuated annulus

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(b) Heat transfer process of double-selective-coated receiver

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Fig. 2. 1D heat transfer model

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2.2 Radiation heat transfer model of parabolic trough receivers

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The spectrum parameter of the radiation heat transfer model is established based on a formula

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derivation of the radiation heat transfer by Holman [24]. Fig. 3 shows the radiation resistance

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network of the traditional receiver. The thermal resistances of this receiver are presented in Table 1.

149 150 151 152 153 154

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Fig. 3. Radiation resistance network of traditional receiver

ACCEPTED MANUSCRIPT Table 1. Values of radiation resistances of the traditional receiver

155 Resistance

Function

Remarks

Rλ ,s

(1− ελ,s ) / (ελ,s As )

Surface resistance of absorber

Rλ,sg

1/[As Xsg (1−τλ,g )]

Space resistance between absorber and glass

Rλ,gsky

1/ [As Xgsky (1−τλ,g )]

Rλ,ssky

1/ ( As Xsskyτλ,g )

Rλ,gi

ρλ,g /[ελ,g Agi (1−τλ,g )]

Surface resistance of inner side glass

Rλ,go

ρλ,g /[ελ,g Ago (1−τλ,g )]

Surface resistance of outer side glass

Space resistance between glass and sky

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Space resistance between absorber and sky

The radiation resistance network of the double-selective-coated receiver is shown in Fig. 4. Table 2

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presents the corresponding thermal resistances of the double-selective-coated evacuated receiver.

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Fig. 4. Radiation resistance network of double-selective-coated receiver

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Table 2. Value of Radiation resistances of double-selective-coated receiver Expression

Remarks

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Resistance

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1-ελ,su / (ελ,su Asu

Surface resistance of upside absorber

Rλ,sd

(1−ελ,s ) / (ελ,s Asd )

Surface resistance of downside absorber

Rλ,sgu

1/[Asu Xsg (1−τλ,g )]

Rλ,sgd

1/ Asd Xsg (1−τλ,g )

Rλ,sskyu

1/ (As Xsskyuτ λ,g )

Space resistance between upside absorber and sky

Rλ,sskyd

1/ (As Xsskydτ λ,g )

Space resistance between downside absorber and sky

Rλ,gsky

1 / [Ag Xgsky (1−τ λ,g )]

Rλ,gi

ρλ,g / [ελ,g Agi (1−τλ,g )]

Rλ,go

ρλ,g / [ελ,g Agi (1−τλ,g )]

Rλ,su

Space resistance between upside absorber and glass Space resistance between downside absorber and glass

Same as Table 1

ACCEPTED MANUSCRIPT All property parameters of materials, namely, ε λ,g ,

αλ,g , ρλ,g , τ λ,g , ε λ ,s , and ελ,su as well as the

162

radiation resistances and emissive power of the blackbody are functions of wavelengths. The thermal

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emittance energy of the steel absorber tube is mainly considered in the 0.3–25 µm range, and solar

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radiation is in the 0.305–4.045 µm range [25]. Given the ambient temperature, sky temperature, wind

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speed, and solar direct normal irradiance, the value of heat transfer between all parts of the receiver

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can be obtained by solving the radiation resistance network. Tables 3 and 4 list the radiation heat

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transfer between all parts of the receiver.

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Expression

J λ ,s − E λ ,g

25

Qsg

∫

0.3

J λ ,s − E λ ,sky

25

Qssky

Rλ ,sg + Rλ ,gi

∫

Rλ ,ssky

0.3

Qrgsky

∫

0.3

dλ

Heat flux between absorber and glass

dλ

Heat flux between absorber and sky

Rλ ,gsky + Rλ ,go

4.045

∫Q

λ,dir

dλ

Heat flux between glass and sky

Dsε λ,s τ λ ,g d λ

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Qsun

E λ ,g − E λ ,sky

25

Remarks

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Heat flux

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Table 3. Radiation heat transfer between all parts of traditional receiver

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Solar radiation absorbed by absorber

0.305

Qrg

 ( L − Dg )η op + Dg   

4.045

∫Q

170 171 172 173 174 175 176 177 178 179

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EP

0.305

λ ,dir

α λ ,g d λ

Solar radiation absorbed by glass

ACCEPTED MANUSCRIPT 180 Table 4. Radiation heat transfer between all parts of double-selective-coated evacuated receiver Expression

∫

Qsgu

Rλ,sgu

0.3

J λ,sd − J λ,gi

25

∫

Qsgd

Rλ,sgd

0.3

∫ ∫

∫

0.3

' sun

Q

2Qλ,dir Ds 1 − s in

4.045

∫

Heat flux between I-region of absorber and sky

dλ

Heat flux between II-region of absorber and sky

dλ

Heat flux between glass and sky

Rλ ,gsky + Rλ ,go

0.305

+

dλ

E λ ,s − E λ ,sky

4.045

∫

Heat flux between II-region of absorber and glass

Rλ ,sskyd

0.3 25

dλ

J λ ,sd − Eλ,sky

25

' Qrgsky

Heat flux between I-region of absorber and glass

Rλ ,sskyu

0.3

Qsskyd

dλ

J λ ,su − Eλ,sky

25

Qsskyu

Remarks

2Qλ,dir Ds sin

0.305

β 2

β 2

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J λ,su − J λ,gi

25

SC

Heat flux

ε λ ,sτ λ , g d λ

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Solar radiation absorbed by absorber

ε λ,suτ λ , g d λ

2.3 Spectral emissive power

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The spectral emissive power of each part of the system plays an important role in linking the

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calculation of models and is solved using Eq. (1), which was established by Planck [26].

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Ebλ =



C1  C2    − 1 λ T  b 

λ 5 exp 

EP



(1)

where Tb is the absolute temperature of the blackbody in K. The first and second radiation constants

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are C1= 3.742 × 108 W·µm4/m and C2= 1.439 × 104 µm·K, respectively. λ is the wavelength in

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µm.

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As shown in Eq. (1), emissive power is the function of wavelength and absolute temperature. Thus,

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the spectral radiated power in full bands, including , , , and , , can be obtained with

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several related temperatures.

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2.4 Convection heat transfer between glass envelope and ambient

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The heat transfer between the glass envelope and ambient mainly consists of the heat convection

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ACCEPTED MANUSCRIPT 193

between glass envelope and air and the radiation heat transfer between glass envelop and sky that has

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been obtained above. The heat convection between glass and air can be expressed as Qcg = hc (Tg − Ta ) Ago .

(2)

The heat convection between the glass tube and the environment can be regarded as the forced heat

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convection of a single tube. The Nusselt number is calculated as follows [26]:

hc Dg k

1

= CRen Pr 3 ,

Re =

v0 Dg

ν

,

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Nu =

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(3)

(4)

Where hc is the convective heat transfer coefficient in W/(m2·K); k is the heat conductivity of

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in W/(m·K); Re is the Reynolds number of the air; and

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kinematic viscosity, and Prandtl number of air, respectively. The values of C and n in Eq. (3) are

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listed in Table 5[26].

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air

v0 , ν , and Pr are the wind speed,

Table 5. Value of C and n

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Re

EP

0.4 4 4 40 40 4000

4000

40000

C

n

0.989 0.911 0.683

0.330 0.385 0.466

0.193

0.618

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can be derived using the heat transfer model. The total heat loss of traditional and

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double-selective-coated receivers can be expressed as follows:

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AC C

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40000 400000 0.0266 0.805 The values of heat transfer between all parts of the receiver, including radiation and convective heat,

where

Qloss and

Q lo' s s

Qloss = Qsg + Qssky ,

(5)

' Qloss = Qsgu + Qsgd + Qsskyu + Qsskyd ,

(6)

the total heat loss of traditional and double-selective-coated receivers,

ACCEPTED MANUSCRIPT 206

respectively.

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To assess the thermal performance of receivers objectively, the total solar radiation absorbed by the

208

receiver, the heat-collecting efficiency, and the increment of heat gain of the double-selective-coated

209

receiver are defined as: 4.045

4.045

0.305

0.305

ηcl =

Qλ,dirτ λ,gε λ,su d λ − Qloss ,

Q Acl ∫

4.045

Qλ,dir dλ

,

0.305

∆Q = Q' − Q

210

(7)

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Qλ,dirε λ,sτ λ,g dλ + Dsηder ∫

SC

Q = Aηopηder ∫

(8)

(9)

ηder

where  and  are the optical efficiency and aperture area of the collector, respectively.

212

the optical derate efficiency of the receiver. Q and

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double-selective-coated receivers, respectively. The parameters of EuroTrough ET150 collector are

214

used to calculate the heat transfer model. The optical efficiency and aperture area of the collector are

215

87.11% and 5.77 m [27], respectively. The effects of the cosine loss of the collector are ignored.

216

3. Simulation parameters of the receivers

217

The geometrical parameters of receivers in this paper are same to those of the Schott 2008 PTR 70

218

receiver which is a high temperature parabolic trough receiver designed and manufactured by Schott

219

[16]. The outer diameter of the steel absorber tube, and the inner and outer diameters of the glass

220

envelope are 70 mm, 115 mm, and 120 mm, respectively. The spectral emissivity of the coating,

221

which is applied in Schott 2008 PTR 70, is shown in Fig. 5a. The solar radiation absorptivity of this

222

coating can reach 96.5% and its emissivity is 9.4% at 400 °C [10]. Fig. 5b shows the curve of the

223

spectral emissivity of the coating adopted in solar thermophotovoltaic (TPV) systems. The

224

absorptivity and emissivity of this absorbing coating are 0.868 and 0.073 at 727

225

The TPV coating has lower absorptivity and emissivity than those of the coating applied in Schott

226

PTR 70, which indicates it is suitable for high temperatures to decrease the heat loss from thermal

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the heat gain of the traditional and

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Q ' are

is

, respectively [28].

ACCEPTED MANUSCRIPT 227

radiation.

228

As shown in Fig. 5c, the emissivity curve of the ideal coating is a step function from a high drop to a

229

low value.

230

low value [29]. The cut-off wavelength directly determines the coating absorptivity and emissivity,

231

and can be used to design and optimize the performance of realistic coatings. The solar and 400 °C

232

blackbody spectral radiation powers are also shown in Fig. 5c. The overlap of solar radiation and

233

thermal radiation is obvious.

234

The ambient temperature and wind speed are set to 15 °C and 2.5 m/s, respectively. The temperature

235

of the sky can be calculated by the following empirical formula [30]:

RI PT

is the cut-off wavelength where the absorptivity/emissivity changes from a high to a

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λc

0.25 Tsky =κsky ⋅ Ta

236

where

κsky can be expressed as:

κ sk y = 0.711 + 0.56 (Tdp / 100 ) + 0.73 (Td p / 100 )

2

(11)

where Tdp is the dew temperature of ambient in °C. 1.0

TE D

237

(10)

EP

0.6

AC C

Emissivity

0.8

0.4

0.2

0.0 1

238 239

10

Wavelength（ µm） (a) Emissivity of the selective coating of PTR70

40

SC

RI PT

ACCEPTED MANUSCRIPT

240

(b) Emissivity of TPV coating[28]

Solar direct normal irradiance of AM=1.5 Ideal selective absorbing coating Emissive power of 400℃ blackbody

cut-off wavelength

1000

600 400

0.2

242 243 244

AC C

0

1

0.8

0.4

EP

200

1.0

0.6

TE D

800

1.2

Emissivity and Eλ/Eλmax

Radiation intensity (W·m-2·µm-1)

1200

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0.0

λc Wavelength (µm)

10

(c) Emissive power of the sun and blackbody, and ideal coating Fig. 5. Spectral parameters of receiver

245

4. Results and discussions

246

The heat transfer between all parts of the receiver, including the radiation, convective heat transfer,

247

and simulation parameters, are presented above. Based on the above model and parameters, the

ACCEPTED MANUSCRIPT simulation process is implemented in a MATLAB program.

249

4.1 Analysis and discussion of the ideal coating receivers

250

Optical concentration is used in high and large-scale solar thermal systems to obtain high-quality

251

energy and enhance the efficiency of systems. The optimal cut-off wavelength of the ideal coating

252

for receivers is obtained. The optimization process is based on the spectrum parameter of the

253

radiation heat transfer model of the receiver. The process aims to determine the optimal cut-off

254

wavelength to maximize the heat collecting efficiency of the receiver. In this paper, the spectral

255

absorptivity/emissivity for the ideal coating receiver is given by:

(12)

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ε λ =α λ = 

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The optimal cut-off wavelength variation with the optical concentration ratio and temperature of the

257

steel absorber tube is shown in Fig. 6. The optimal cut-off wavelength rises with the optical

258

concentration ratio increases, but decreases as the absorber temperature rises. This phenomenon

259

shows the correlation between solar radiation and blackbody emissivity. On the one hand, at a given

260

temperature and with the increase of optical concentration ratio, the solar radiation flux increases.

261

The cut-off wavelength lengthens in order to absorb as much as possible solar radiation. On the other

262

hand, the emissive power of absorber increases rapidly as the absorber temperature rises. The cut-off

263

wavelength tends to shorten to reduce the thermal radiation of the steel absorber tube. The curves of

264

the optimal cut-off wavelength are not a continuous function because the solar spectrum reaches the

265

earth drop to zero around 1.4 µm and 1.8 µm due to atmospheric absorption [31].

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ACCEPTED MANUSCRIPT

2.4 2.2 2.0 1.8

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Optimal cut-off wavelength (µm)

2.6

1.6

Ts=200℃ Ts=300℃ Ts=400℃ Ts=500℃ Ts=600℃

1.4

1.0 0.1

1

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1.2

10

100

Optical concentration ratio

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Fig. 6. Optimal cut-off wavelength variation with the optical concentration ratio and temperature

268

Many researchers have discovered the non-uniform heat flux distribution of receivers and established

269

mathematic models to characterize these distributions [32-37]. As shown in Fig. 7, the diagram

270

models of parabolic trough reflector and receiver with incident solar rays are built in Soltrace

271

software based on MCRT method. The methodology of MCRT is the stochastic trajectories of a large

272

number of rays as the rays intersecting with surfaces. The rays come from the sun firstly encounter

273

the absorber and reflector probabilistic, and the rays that encounter to the reflector are reflected to

274

and absorbed by the absorber tube. The yellow lines represent the sun’s rays and each dot stands for

275

a ray intersection. The solar radiation flux distribution on the outer surface of the steel absorber tube

276

equipped with EuroTrough ET150 collector is plotted in Fig. 8. The sun-shaped parameter is

277

supposed to be a Pillbox type and the number of traced rays is 1×106. For results showing simplicity,

278

we use simple parameter value:  = 0.94,  = 0.96, DNI=1000W/m2, 

279

'()*+, = 1.5mrad. The non-uniformity of solar radiation flux distribution of the absorber tubes is

280

conspicuous. The max heat flux of the absorber surfaces is approximately 50–60 times higher than

281

that without concentrated light. Fig. 8 shows that the angle of the absorber tube without a

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!

= 4.65mrad,

ACCEPTED MANUSCRIPT concentrated light spot is approximately 160 ° in the 0 °–80 ° and 280 °–360 ° ranges.

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283

Fig. 7. The geometry model and ray paths of the collector

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Heat flux(kW/m2)

50

Direction of incident ray

0°(360°)

20

270°

90°

10

Absorber

0

0

285

40

80

120

160

200

240

280

320

360

θ(°)

286

Fig. 8. Heat flux distribution of absorber tube

287

The analysis above shows that different optical concentrations and operating temperatures

288

correspond to different optimal cut-off wavelengths. Therefore, a parabolic trough receiver plated by

ACCEPTED MANUSCRIPT 289

a coating with single spectral property is unreasonable. A receiver with two kinds of solar selective

290

absorbing coating is proposed to enhance the performance of PTC systems. The central angle of

291

I-region . is set to 150 °, as shown in Fig. 2b. 3.0

Traditional receiver I-region of double-selective-coated receiver II-region of double-selective-coated receiver

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2.6 2.4 2.2 2.0

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1.8 1.6 1.4 1.2 1.0 200

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300

400

500

600

Absorber temperature (℃ )

292

Fig. 9. Optimal cut-off wavelength variation with temperature

294

Fig. 9 shows the optimal cut-off wavelength variation with the absorber temperature. The optimal

295

cut-off wavelengths for both traditional and double-selective-coated receivers decrease with the

296

increase of absorber temperature. The optimal cut-off wavelength of the I-region of the

297

double-selective-coated receiver is shorter than that of the traditional receiver at a given temperature,

298

whereas the optimal cut-off wavelength of II-region is longer than that of the traditional receiver. The

299

results are explained by the following: for the traditional receiver, heat gain and loss must be taken

300

into account. The optimal cut-off wavelength corresponds to the highest efficiency of the traditional

301

receiver. For double-selective-coated receiver, the solar radiation flux of I-region is low, we should

302

take more attention on heat loss and shorten the cut-off wavelength that can reduce thermal radiation

303

to decrease heat loss of I-region. Similarly, the cut-off wavelength of II-region should be reasonably

304

extended to obtain maximum solar radiation and ensure the heat loss doesn’t increase rapidly.

305

The analysis above shows that the optimal optical properties of I and II receiver regions are different.

306

In addition, ultralow emissivity selective coating by varying the thickness and filling factors of

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ACCEPTED MANUSCRIPT absorbing layer has been fabricated [13, 28].The design of the double-selective-coated of receiver

308

may be an effective approach to enhancing the collecting efficiency of the receiver and deserves

309

further study. The validity of this idea will be verified in the following simulation of typical coating.

310

4.2 Analysis and discussions of receivers with typical coatings

311

The coating developed by Peter Bermel [28] is used in the double-selective-coated receiver, as

312

shown in Fig. 5b. For the double-selective-coated receiver, I-region is plated with the coating

313

developed by Peter Bermel, whereas the II-region is plated with the coating applied in Schott PTR 70.

314

In this study, we investigate two kinds of traditional receivers with PTR 70 and TPV coating. The

315

performance of the receiver, including heat loss and collecting efficiency, was investigated using the

316

heat transfer model. The heat loss and collecting efficiency of receiver variation with absorber

317

temperature are presented in Table 6 and Fig. 10. The validity of the heat transfer model is verified

318

by the experiment date of Schott PTR 70 receiver published by National Renewable Energy

319

Laboratory (NREL) of America. As the temperature of steel absorber tube rises, the heat loss of all

320

receivers grows rapidly, while the collecting efficiency drops significantly. As expected, the heat loss

321

of the double-selective-coated receiver is lower than that of the traditional receiver with PTR 70

322

coating

323

double-selective-coated receiver is higher than that of the traditional receiver with PTR 70 coating.

324

The higher the absorber temperature, the reduction of heat loss and the increase in efficiency become

325

more apparent.

327 328 329 330 331 332

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Similarly,

the

collecting

efficiency

of

the

ACCEPTED MANUSCRIPT Table 6. Simulation of heat loss

333

Traditional receiver with TPV coating

Heat loss /W·m-1

Heat loss / W·m-1

Heat loss / W·m-1

24.9 41.5 66.1 101.8 152.4 222.6 317.9 445.1 611.7 826.7 1100.2

3.8 6.3 10.3 16.6 26.8 42.7 67.8 105.5 161.1 240.8 352.5

16.1 26.8 42.8 66.3 100.1 147.8 213.9 303.8 424.3 583.1 789.3

1200

EP

200

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Heat loss(W·m-1)

800

400

Percentage of heat loss reduction 35.01% 35.30% 35.15% 34.80% 34.27% 33.58% 32.71% 31.73% 30.63% 29.47% 28.25%

Traditional receiver with PTR 70 coating Traditional receiver with TPV coating Double-selective-coating receiver Experimental data of NREL

1000

600

SC

200 240 280 320 360 400 440 480 520 560 600

Double-selective-coated receiver . = 150°

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Traditional receiver with PTR 70 coating

0

334 335

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200

250

300

350

400

450

500

550

Absorber temperature(℃ )

(a) Heat loss of various receiver with absorber temperature

600

ACCEPTED MANUSCRIPT 85

Traditional receiver with PTR 70 coating Traditional receiver with TPV coating Double-selective-coating receiver

75 70 65

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Collecting efficiency (%)

80

60

CSP operation temperature

55

45 150

200

250

350

400

450

500

550

600

Absorber temperature(℃ )

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(b) Collecting efficiency variation with absorber temperature (DNI=800W/m2)

Fig. 10. Simulation result of all kinds of receivers

338

Compared with the PTR 70 receiver, the percentage of heat loss reduction and the increment of heat

340

gain of the double-selective-coated receiver are 31.1% and 157.8 W/m when the absorber

341

temperature is 500 °C. The heat loss in the traditional receiver with TPV coating is significantly

342

lower than those in the other two types of receivers, but its heat gain is lower due to the low solar

343

radiation absorptivity. That is why the collecting efficiency is defined to evaluate performance of

344

receiver. The efficiency of traditional receiver with PTR70 coating and the double-selective-coated

345

receiver are 64.7% and 68.1%, respectively, at the absorber temperature of 500 °C. The efficiency of

346

the double-selective-coated receiver is 3.4% higher than that of the PTR70 receiver. The reason is

347

that the II-region of the double-selective-coated receiver has a high absorptivity coating that ensures

348

that heat gain does not decrease significantly, and the I-region of the receiver is coated with low

349

emissivity coating that decreases heat loss. This design becomes more efficient at mitigating heat

350

loss at higher temperatures. The efficiency of the traditional receiver with TPV coating is higher than

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ACCEPTED MANUSCRIPT that of the double-selective-coated receiver when the temperature of the absorber is higher than

352

560 °C. The reason for this phenomenon is that heat loss plays a more important role with the

353

increase of the temperature. The traditional receiver with TPV coating can significantly decrease heat

354

loss and compensate for the decrement of heat gain through low solar radiation absorptivity.

355

However, the operating temperature of the receiver applied in the CSP is generally in the range of

356

290 °C to 550 °C. Consequently, the double-selective-coated receiver can improve the performance

357

of CSP at the operating temperature of CSP compared to the other two traditional receivers.

358

4.3 Annual performance analysis of CSP

359

The System Advisor Model (SAM) is a commercial software developed by NREL to evaluate the

360

annual performance and economic value of CSP [27]. Using the value of heat loss of typically

361

coating receivers, SAM can calculate the effect of receiver heat loss on the power plant performance.

362

The main plant configuration data for system simulation are shown in Table 7. The LCOE and

363

annual electricity production are simulated for different CSP configurations, with an assumption that

364

the different types of receivers have the same cost. The simulation results are presented in Fig. 11

365

and Table 8.

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Table 7. Main plant configuration data

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Plant design gross power Solar field HTF Solar field inlet temperature Solar field outlet temperature SCA type Number of SCAs per loop TES full load hour Rankine cycle efficiency Freeze protection temperature Other parameters

MW °C °C

h % °C

Synthetic Oil

Molten Salt

111 VP-1 293 391 EuroTrough ET150 8 7.5 37.7 60 SAM 2017.1.17 Defaults

111 Hitec Solar Salt 293 550 EuroTrough ET150 8 7.5 43.3 260 SAM 2017.1.17 Defaults

ACCEPTED MANUSCRIPT 35

Traditional receiver with PTR 70 coating Double-selective-coating receiver

30

28.7 27.27

LCOE(￠·kWh-1）

25 19.43 18.89

20

16.63 15.41

10 5 0 Lhasa VP-1 391 ℃

Daggett VP-1 391 ℃

367

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15

Daggett Solar Salt 550 ℃

Fig. 11. LCOE of the parabolic trough power plants

369

As shown in Fig. 11, two SAM simulations were carried out for each configuration. One simulation

370

used the traditional receivers with PTR 70 coating, that is, Schott 2008 PTR 70, and the other used

371

the double-selective-coated receiver. It is observed that the LCOE of the plant in Lhasa is higher than

372

that in Daggett with the same configuration, and the LCOE of the plant that employed molten salt as

373

heat transfer fluid is lower than the plant that used oil in Daggett. It can explained by the facts that

374

the DNI of Daggett is extremely higher than that of Lhasa, and higher Rankine cycle efficiency due

375

to higher operating temperature. With the same configuration, higher DNI and higher Rankine cycle

376

efficiency mean higher electricity production in the plant and lower LCOE.

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Table 8. LCOE reduction and electricity production increase using double-selective-coated receivers Location Lhasa Daggett

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Annual DNI

1778

2792

kWh/m2

Operation temperature 291~391°C 291~391°C 291~550°C

LCOE reduction 4.98% 2.78% 7.34%

Electricity production increase 5.32% 2.94% 8.21%

378

The relative improvement of double-selective-coated receivers is illustrated in Table 8. It can be

379

observed that the use of double-selective-coated receivers instead of traditional receivers in parabolic

380

trough CSP has tremendous potential in enhancing the performance of the CSP. The lower DNI and

381

the higher temperature of CSP tend to more apparent improvement. When the CSP operates at

ACCEPTED MANUSCRIPT 291 °C – 550 °C in Daggett, the LCOE reduction and electricity production increase can reach 7.34%

383

and 8.21%, respectively. The reason is that heat loss plays an important role in heat gain of CSP at

384

high temperatures and low DNI area, indicating that the performance of double-selective-coated

385

receivers are more excellent at this conditions.

386

5. Conclusion

387

In this study, a novel parabolic trough receiver with two kinds of solar selective absorbing coating to

388

enhance the performance of CSP is proposed. The heat transfer models based on the spectrum

389

parameters of the traditional and double-selective-coated receivers are presented. The heat transfer

390

processes of receivers are simulated, and solar radiation flux, optimal cut-off wavelength, heat loss of

391

receivers, and the performance of CSP were analyzed. According to the numerical simulation results,

392

the following conclusions are summarized:

393

1. The non-uniformity of solar radiation flux distribution of absorber tube is conspicuous. The

394

optimal cut-off wavelength rises with the growing optical concentration ratio, but drops with the

395

elevated absorber temperature. The optimized results of the cut-off wavelength indicate that the

396

optimal optical properties of I and II-region of receiver are different.

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2. The typical coating simulation results indicate that double-selective-coated evacuated receiver

398

can reduce heat loss and enhance collecting efficiency significantly relative to Schott PTR 70,

399

and the heat loss reduction and improvement of collecting efficiency increase with the rising

400

absorber temperature in the entire simulation. When the temperature of the absorber is 500 °C

401

the double-selective-coated receiver can reduce heat loss by 157.8 W/m, and the percentage of

402

heat loss reduction is 31.1%. The collecting efficiency of double-selective-coated receiver and

403

Schott PTR 70 are 68.1% and 64.7%, respectively.

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3. The SAM annual simulation results indicate that double-selective-coated receivers decrease the

405

LCOE of CSP by 2.78%–7.34% and increase electricity production by 2.94%–8.21% compared

406

with traditional receivers. Double-selective-coated receivers perform better at high working

ACCEPTED MANUSCRIPT 407

temperatures and low DNI areas. Acknowledgements

409

This study was sponsored by (1) the National Science Foundation of China (NSFC 51476159,

410

51776193), (2) the Dongguan Innovative Research Team Program (No. 2014607101008), (3) and the

411

International Science and Technology Cooperation Project of Science and Technology Department of

412

Anhui Province (BJ2090130038).

413

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ACCEPTED MANUSCRIPT

Highlights 1. A novel parabolic trough receiver with double selective coating was proposed. 2. A heat transfer model was established and verified by NREL experimental data.

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3. The cut-off wavelength and heat flux distribution of receivers were studied. 4. The heat loss and collecting efficiency of receivers were analyzed and compared.

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5. The annual simulation was conducted to evaluate solar power plant performance.