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Solar Energy 122 (2015) 1080–1090

Experimental study on a coiled tube solar receiver under variablesolar radiation condition

K. Wang a, H. Wu b,⇑, D. Wang a, Y. Wang a, Z. Tong a, F. Lin a, A.G. Olabi c

a Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190, ChinabDepartment of Mechanical and Construction Engineering, Faculty of Engineering and Environment, Northumbria University, Newcastle upon Tyne,

NE1 8ST, United Kingdomc Institute of Engineering and Energy Technologies, School of Engineering and Computing, University of the West of Scotland, Paisley PA1 2BE,

United Kingdom

Received 25 March 2015; received in revised form 7 August 2015; accepted 6 October 2015

Communicated by: Associate Editor C. Estrada-Gasca

Abstract

This article presents an experimental investigation of the heat transfer characteristics as well as energy and exergy performance for acoil type solar dish receiver under variable solar radiation conditions. During a 4.5 h period of continuous operation in the morning, thesolar irradiance fluctuates in the range between 160 W/m2 and 650 W/m2, and the concentrated solar flux is approximately 1000 kW/m2

at aperture. Experimental results show that: (1) the efficiency of the solar receiver is found to be over 58% and the highest efficiency canbe reached to 64%. Generally, the efficiency is maintained at around 60% under steady state condition. (2) A very low value of the heatloss factor (0.035 kW/K) could be achieved during steady state operation. (3) The highest value of the exergy rate is approximately7.3 kW, whereas the maximum energy rate can reach 16.2 kW. (4) The highest exergy efficiency is approximately 23%, whereas the high-est energy efficiency is around 64%.� 2015 Elsevier Ltd. All rights reserved.

Keywords: Solar receiver; Exergy; Energy efficiency; Heat transfer; Radiation

1. Introduction

With rapidly increasing energy prices and globalisation,process industries seek opportunities to reduce productioncosts and improve energy efficiency. Among the energy-efficient technologies, Concentrated Solar Power (CSP) sys-tem is considered as one of the most attractive ways tosolve the energy crisis in the future (Steinfeld, 2005; LeRoux et al., 2011). Many developed countries like the Uni-ted State and the European Commission have been devotedto the solarised Brayton micro-turbines over the past

http://dx.doi.org/10.1016/j.solener.2015.10.004

0038-092X/� 2015 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +44 (0) 1913495365.E-mail address: hongwei.wu@northumbria.ac.uk (H. Wu).

decades. In the authors’ previous work (Wang et al., 2013),a solar dish–Brayton system with micro-turbines has beendesigned, as illustrated in Fig. 1. The air was supplied bya compressor and regulated to the required pressure of4 atm, then the air was preheated by the turbine exhaustedgas in a heat exchanger. The heated air was driven to thesolar receiver which will be heated to a higher temperatureby solar energy. After that the air was mixed with naturalgas and burned in the combustor. Finally, the high temper-ature gas drives a two-stage turbine to generate electricity.Previous work was aimed at designing and optimizing acentrifugal compressor using computational fluid dynamic(CFD) method. It was also identified that both the energyefficiency and total pressure loss of the receiver would

Nomenclature

Aap effective aperture area of dish (m2)Ap project area (m2)cav average specific heat capacity (J/kg K)Df focus point diameter (m)ED concentrated solar radiation power (kW)EL heat loss (kW)ER receiver power (kW)ES solar radiation power on the dish (kW)ExD rate of dish exergy concentrated (kW)Exf exergy factor (–)ExR receiver exergy (kW)

Exs rate of solar exergy delivery (kW)G direct solar radiation (W/m2)_m mass flow rate (kg/s)nd parabolic dish combined optical efficiency (–)rc concentration ratioTin inlet temperature of the air (K)Tout outlet temperature of the air (K)Tave average temperature of the air (K)UL heat loss coefficient (kW=m2 K)gth.R energy efficiency of the receiver (–)gex.R exergy efficiency of the receiver (–)

K. Wang et al. / Solar Energy 122 (2015) 1080–1090 1081

directly affect the system power generation efficiency andcost. For this reason, it is therefore imperative to designa low cost solar receiver with improved performance.

In the framework of the French PEGASE project (Pro-duction of Electricity by gas turbine and Solar Energy);CNRS/PROMES laboratory is developing a 4 MW pres-surized air solar receiver with a surface absorber basedon a compact heat exchanger technology (Wang andSiddiqui, 2010). ETH Zurich and Paul Scherrer Institutehave done a field testing of a 42 m-long full-scale solarreceiver prototype installed on a 9 m-aperture solar troughconcentrator, the receiver efficiency ranges from 45% to29% for a solar power input of 280 kW (Grange et al.,2011; Bader, 2012; Bader et al., 2010). Comparing to theparabolic trough system with the concentration ratio of100 suns and the central tower system with the concentra-tion ratio of 1000 suns, the parabolic dish system canachieve concentration ratio of 10,000 suns. According tothis, very high efficiency can be achieved in a parabolic dishsolar system due to its high concentration ratio (Baderet al., 2011). The solar concentration part which is usedto provide high temperature air is very crucial for the entiresolar power system. The system efficiency and the cost of

Fig. 1. Solar dish–B

power generation are highly depended on the solar concen-tration part conversion efficiency from solar radiation tothermal fluid. Thus, the solar concentration part has tobe well designed in order to achieve high efficiency andlow pressure loss.

Most of the published work on the solar concentrationsystems for various applications mainly focused on theconcentrator and the receiver. Lovegrove et al. (2011)introduced the design development and construction stepsof a 500 m2 paraboloidal dish solar concentrator in Aus-tralian National University. Samanes and Barberena(2014) presented a transient numerical model of solar cav-ity receivers for concentrating solar power plants. The pro-posed model aims to consider all the major phenomenainfluencing the performance of a cavity receiver, includingradiation, convection and conduction heat transfer mecha-nisms. Li et al. (2011) utilized the Monte-Carlo ray-tracingmethod to predict the radiation flux distributions of theconcentrator–receiver system. The radiation flux profilesof a faceted real concentrator and an ideal paraboloidalconcentrator for different aperture positions and receivershapes were analyzed, respectively. Wang et al. (2014)developed an inverse design method in order to quickly

rayton system.

1082 K. Wang et al. / Solar Energy 122 (2015) 1080–1090

find possible cavity receiver with relative uniform cavitywall surface temperature for a solar dish cavity receiver.A heat transfer model of the absorber wall was establishedto analyze the heat transfer process between the cavity wallouter surface, the inner surface and the working fluid.

Compared to the traditional gas turbine, solarised Bray-ton turbines use solar receiver to replace the combustionchamber in the traditional gas turbine (Chen et al., 2007).Many studies have been devoted to the designs and perfor-mance of the receiver. A heat transfer model of the absor-ber wall was established to analyze the heat transferprocess between the cavity wall outer surface, the inner sur-face and the working fluid. Most recently, Zhu et al. (2015)carried out experimental investigation to examine the heattransfer characteristics of a coil type solar dish receiverunder actual concentrate solar radiation conditions.Energy and exergy performance of the solar receiver wasanalyzed. Fernandez and Miller (2015) presented a multi-disciplinary design optimization of a 5 MW Small ParticleHeat Exchange Receiver (SPHER) for central receiversolar plants. The new developed solar receiver aims to heatthe air to temperatures that in excess of 1300 K and use theobtained high-temperature energy to drive a Brayton cycleor a combined Brayton/Rankine cycle. It was found thatthe receiver efficiency can be increased by 6% with respectto the previous baseline design from the same author(Fernandez, 2013). Wang and Siddiqui (2010) developeda three-dimensional model of parabolic dish-receiver sys-tem with argon gas as the working fluid to simulate thethermal performance of a dish-type concentrated solarenergy system. It was concluded that the aperture sizeand different inlet/outlet configuration have a considerableimpact on the receiver wall and gas temperatures, but therim angle of the parabolic dish has negligible influence.Lu et al. (2012) proposed the heat and mass transfer mod-els of solar heat receiver, and the heat absorption and exer-getic performance were further investigated by consideringthe heat loss outside the receiver and fluid viscous dissipa-tion inside the receiver. It was found that the exergetic effi-ciency of the solar heat receiver will approach to themaximum at proper inlet temperature due to the couplingeffects of heat absorption efficiency and exergetic efficiencyfrom fluid internal energy. Buck et al. (2001) introduced areceiver module consisting of a secondary concentrator anda volumetric receiver unit which heated the compressed airof the gas turbine before it entered the combustor. Thereceiver unit was closed with a domed quartz window totransmit the concentrated solar radiation. Then the sec-ondary concentrator was redesigned and rebuilt withimproved efficiency. Flesch et al. (2014) numerically ana-lyzed the impact of head-on and side-on wind on large cav-ity receivers with inclination angles ranges from 0�(horizontal cavity) to 90� (vertical cavity) and comparedwith the data published in the open literature. The numer-ical results were in accordance with published results forhorizontal cavities. It was also pointed out that an experi-mental analysis of large scale cavity receivers for power

towers should be pursued. Hischier et al. (2009) proposeda novel design of a high-temperature pressurized solar airreceiver for power generation via combined Brayton–Rankine cycles. It consists of an annular reticulate porousceramic bounded by two concentric cylinders. The heattransfer mechanism was analyzed by the finite volume tech-nique and by using the Rosseland diffusion, P1, andMonte-Carlo radiation method. It was found that, for asolar concentration ratio of 3000 suns, the outlet air tem-perature can reach 1000 �C at 10 bars, yielding a thermalefficiency of 78%. Tu et al. (2015) proposed a modifiedcombined method to simulate the thermal performance ofa saturated water/steam solar cavity receiver. The modifiedone can describe the whole steady process of the receiverand accurately estimate the steady-state efficiency and massflow rate of the generated steam in the boiling tubes. It wasfound that the uniformity of surface heat flux wasimproved effectively and the maximum mean square devia-tion of the external tube temperature could drop from32.4 �C to 23.6 �C. Capeillere et al. (2014) numericallystudied the thermomechanical behavior of a plate solarreceiver with asymmetric heating. The numerical resultsshowed that the choice of the shape and levels of the solarirradiance map is crucial. The distribution of the most rel-evant incident solar flux and the geometry compromisewere determined. Wang et al. (2014) conducted a numericalstudy focusing on the thermal performance of porous med-ium receiver with quartz window. Their results indicatedthat the pressure distribution and temperature distributionfor the condition of fluid inlet located at the side wall wasdifferent from that for the condition of fluid inlet located atthe front surface.

It appears from the previous investigation that the keypoint for the solarised Brayton micro-turbines is to developsolar receivers which have terrific performance on the pres-sure loss and heat transfer. To the best of the authors’knowledge, there is a lack of available experimental dataunder real concentrate solar conditions especially for thecases of extremely high heat flux and high temperature.To this end, the present research is aimed to experimentallyanalyze both the efficiency and heat loss of a coiled tubeunder real solar radiation conditions in more detail.

2. Experimental apparatus and method

2.1. Experimental apparatus

The experimental study was conducted at a locationwith the geographical position of 38�230 latitude and 109�90 longitude, 15 km South of Yinchuan, China. The wholesystem, shown schematically in Fig. 2a, mainly consists ofthree components: dish, compressor and receiver. The dishused for the experimental tests of the developed solar heatreceiver was illustrated in Fig. 2b. All 33 trapezoidal, pre-bent mirrors are resin molded and laminated. The reflectivesurface is applied as an adhesive foil. At the bottom of thedish a cut out is made for the tower. The main dish

Fig. 2a. Schematic drawing of experimental test rig.

Fig. 2b. Parabolic dish.

Table 1Dish design parameters.

Parameters Value (unit)

Effective aperture area of dish (Aap) 56.8 m2

Project area (Ap) 54.4 m2

Focus length (Ld) 5.46 mRim angle (a) 45�Focus point diameter (Df) 0.2 mConcentration ratio (rc) 1731Parabolic dish combined optical efficiency (nd) 73%Mirror number 33Slop error 0.5 mrad

Fig. 3a. Dish in operation during morning.

K. Wang et al. / Solar Energy 122 (2015) 1080–1090 1083

parameters utilized in the current study are listed in Table 1.These parameters are provided by the dish reflector manu-facturer. To make sure the light reflected by the mirrorfocus on the aperture of the receiver, each mirror wasadjusted individually.

The dish is controlled by a solar tracker which is embed-ded in the inner program to make the dish face the Sunautomatically. The inner program could accurately calcu-late the attitude angle in terms of the dish location of theearth and the local time. As illustrated in Fig. 2a, the

attitude angel is formed between the boom line and horizonline. A stepping motor can be well controlled to changethe dish attitude angle slowly. Fig. 3a shows the dish in

Fig. 3b. Dish in operation during noon.

Fig. 4. Protecting shield under the concentrated solar radiation.

Fig. 5a. 3D model of the solar receiver.

1084 K. Wang et al. / Solar Energy 122 (2015) 1080–1090

operation during the morning and the attitude angle islower than 40� for the sun just rising over the horizontalline. The dish in operation at noon is shown in Fig. 3b,the attitude angle is approximately 80�. It should be recog-nized that the initial setting for the location and accuratetime is very crucial during the test. An uninterruptiblepower supply (UPS) system is also adopted to assist thedish off the solar direction in some emergency to furtherprotect the receiver.

During the experiments, a piston compressor is drivenby a diesel engine with the power of 25 kW. The pressur-ized air is compressed at environmental temperature andpressure. After the filter, the pressurized air is supplied intothe receiver. A valve is installed at the pipe outlet to ensurethe receiver works under high pressure. By adjusting thisvalve, the pressure of the whole system as well as the por-tion of the receiver can be well controlled. The output massflow rate is 0.03 kg/s. Thermocouple and pressure sensorare placed at the inlet and outlet of the pipe respectivelyto obtain the receiver efficiency and heat loss. The receiveritself is mounted onto the cantilever arm. In the currentstudy, the heat flux of the focus power could achieve1000 kW/m2 for the dish concentrator has the concentrateratio of 2000. It can be expected that, except for the recei-ver and protecting panel, other components of the systemwould be burned in a short time. To protect other partof the receiver from misaligned radiation an additionalprotecting panel is mounted circumferentially to the recei-ver. As shown in Fig. 4, the protecting panel is made ofstainless steel with 4 mm in thickness. The diameter of

the aperture in the protecting panel is 180 mm. Four K-type thermocouples with an accuracy of 0.5 �C are fixedon the back to monitor the temperature of the protectingpanel. When the temperature is over 850 �C, it means thatthe concentrate solar spot is not located into the aperture.As a result, the inner program has to be reset to adjust theattitude angle in order to prevent fatal damage.

2.2. Solar receiver model

For the current experimental evaluation, the solar recei-ver is designed as a type of coiled tube, as shown in Fig. 5.The advantage of the coiled tube type receiver is its lowcost with easy installation. It needs to be stressed here thatthe key point for designing the receiver is the mass flow ratewithin the 12 small tubes should be equally distributed. Inother words, one can imagine that the small tube would beeasily burned for the case of small mass flow rate. For thisreason, a big inlet volute is used with the diameter of

Fig. 5b. Cross sectional view.

K. Wang et al. / Solar Energy 122 (2015) 1080–1090 1085

80 mm. The pressured air is injected into the inlet volute,and then, it is divided into 12 small tubes with the diameterof 14 mm. Thus, the area of the inlet volute is 2.7 times ofthe total area of the 12 small tubes. The total pressure lossin the inlet tube is very small and the inlet pressure of the12 small tubes is always the same, which guarantees thatthe mass flow is equally distributed. The 12 small tubesare welded into a cylinder which forms the main part ofthe solar receiver. The diameter and the height of the mainpart of the receiver are 250 mm and 456 mm, respectively.The concentrate solar radiation (CSR) passes through theaperture and heat the inner surface of the core. To avoidthe stress induced by thermal expansion, the 12 small tubesare arranged with a 5 mm clearance. All the small tubescombine together in the cone-type collector, and then con-nect with the outlet tube. A high-temperature alloy Inconel718 with density of 8.24 g/cm3 and melting point tempera-ture of 1320 �C is selected for making the receiver becauseof its strength at high temperature 900 �C. To increase thesolar absorb ability, the tube surface is treated by a black-ing process. Between the tube and the case, it is stuffed withchamotte whose heat conductivity coefficient is 0.6 W/m2 K�1. The chamotte can be used to fix the tube and alsoacted as the thermal insulator to minimize the heat loss.

2.3. Energy and exergy analysis

Experimental energy and exergy parameters to charac-terize the thermal performance of the receiver are presentedin this section (Mawire and Taole, 2014; Macphee andDincer, 2009).

2.3.1. Energy analysis

The energy that the whole system receives comes fromthe solar radiation. The solar radiation power on the para-bolic dish reflector can be expressed as:

ES ¼ AapG ð1Þwhere ES is the solar radiation power on the dish, Aap is theeffective aperture area of the parabolic dish, and G is thedirect solar irradiation from the Sun to the dish. G is

measured with a normal incidence pyrheliometer (NIP)Hukseflux DR01 attached to the solar tracker.

The solar radiation is concentrated and delivered to thereceiver by the parabolic dish. The concentrated solar radi-ation power (ED) can be expressed as:

ED ¼ ndES ¼ ndAapG ð2Þwhere ED is the concentrated solar radiation power fromparabolic dish to the receiver, nd is the parabolic dish com-bined with optical efficiency described in Table 1.

The concentrated solar radiation on the receiver isabsorbed by the heat-transfer fluid flowing in the tubes ofthe receiver. The energy rate that air absorbs or receivespower is given by:

ER ¼ _mcavðT out � T inÞ ð3Þwhere _m is the mass flow rate of the air, cav is the averagespecific heat capacity of the air which is a function of theaverage air inlet temperature (Tin) and air outlet tempera-ture (Tout). The average temperature of the receiver (Tave)can be defined by:

T ave ¼ ðT in þ T outÞ=2 ð4ÞThus, the relation between the average specific heat

capacity of the air and the average temperature can beobtained as:

cav ¼ 0:9956þ 0:000093T ave ð5ÞBased on the energy conservation, the receiver power is

the difference between the concentrated solar radiationpower and the overall heat losses. The receiver power canalso be described as

ER ¼ ED � EL ð6Þwhere EL is the rate of the heat loss from the receiver to thesurroundings, which contains the convective heat losses,conductive heat losses and radioactive heat losses. EL canbe expressed as

EL ¼ ULARðT ave � T ambÞ ð7Þ

1086 K. Wang et al. / Solar Energy 122 (2015) 1080–1090

where UL is the total heat loss coefficient determined, AR isthe effective receiver area, and T amb is the ambient temper-ature. The product ULAR is referred as the heat loss factorgiven by

U 0L ¼ ULAR ð8ÞTherefore, combination of Eqs. (2), (3), (6) and (7) yield

_mcavðT out � T inÞ ¼ ndAapG� U 0LðT ave � T ambÞ ð9Þ

The thermal energy efficiency of the receiver is defined asthe ratio of the receiver power to the concentrated solarradiation power from the parabolic dish to the receiverwhich is expressed as:

gth:R ¼ ER

ED

¼ _mcavðT out � T inÞndAapG

ð10Þ

By dividing AapG on both side of Eq. (9) and combinewith Eq. (10) leads to

gth:Rnd ¼ nd � U 0LðT ave � T ambÞ

AapGð11Þ

2.3.2. Exergy analysis

The exergy rate of the receiver or the quality of theenergy delivered to the circulating fluid with reference tothe surroundings can be expressed as

ExR ¼ ER � _mcavT amb lnT out

T in

� �ð12Þ

Substituting Eq. (3) into Eq. (12) yields

ExR ¼ _mcav ðT out � T inÞ � T amb lnT out

T in

� �� �ð13Þ

The rate of the solar exergy delivery by the Sun to thedish and then to the concentrator is given by the Petelaexpression (Mawire and Taole, 2014) and is expressed as

Exs ¼ GAap 1þ 1

3

T amb

T s

� �4

� 4T amb

3T s

" #ð14Þ

where T s is the surface temperature of the Sun, which isapproximately 5762 K.

The exergy efficiency is defined as the ratio of the recei-ver exergy rate to the rate of the solar exergy and can bedetermined as follows:

gex:R ¼ ExRExs

¼_mcav ðT out � T inÞ � T amb ln

T out

T in

� �h iGAap 1þ 1

3T amb

T s

� �4

� 4T amb

3T s

� � ð15Þ

The exergy factor is defined as the ratio of the receiverexergy rate to the receiver energy rate and can be repre-sented by equation:

Exf ¼ ExRER

¼_mcav ðT out � T inÞ � T amb ln

T out

T in

� �h i_mcavðT out � T inÞ ð16Þ

3. Uncertainty analysis

The uncertainties of the measurement in the experimentare dependent on the experimental conditions and the mea-surement instruments. An uncertainty analysis is per-formed on the receiver power ER and the receiver exergyExR, which is the most important derived quantities fromthe measurements of using the propagation of errormethod described by Moffat (1988). The uncertainty ofthe receiver power is calculated by the following equation:

dER¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffidER

d _m

� �2

ðd _mÞ2þ dER

dT out

� �2

ðdT outÞ2þ dER

dT in

� �2

ðdT inÞ2s

ð17Þand the uncertainty of the receiver exergy rate is given by

dExR ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffidExRd _m

� �2

ðd _mÞ2 þ dExRdT out

� �2

ðdT outÞ2þ dExRdT in

� �2

ðdT inÞ2 þ dExRdT amb

� �2

ðdT ambÞ2s

ð18ÞIn the current study, the main uncertainty parameters

are the mass flow rate ( _m), the inlet temperature (Tin) andthe outlet temperature (Tout). The relative uncertainty ofthe mass flow rate is given by the float flowmeter, whichis 2%. Therefore, d _m ¼ 2%� _m ¼ �0:0006 kg=s. Theuncertainty of the temperature is given by the K-type ther-mocouple, which is dT out ¼ dT in ¼ �0:5 K.

The maximum experimental values for the receiverpower and exergy rate are around 16.2 kW and 7.3 kW,respectively. The uncertainty of the receiver power is0.325 kW, and the uncertainty of the receiver exergy rateis 0.147 kW. Therefore, the overall uncertainty of the recei-ver power and exergy rate are 2.01% and 2.02%,respectively.

4. Results and discussion

Fig. 6 shows the measured solar irradiance (G) plottedwith respect to the testing time. The experimental data werecollected on April 15th, 2014. For the purpose of compar-ison, experiments are done for two different time slots:morning (see Fig. 6a) and afternoon (see Fig. 6b). Fig. 6ashows the variation of the solar irradiance from 7:00 amto 11:30 am. According to Fig. 6a, it is shown that the solarirradiance increases monotonously with time except a fastdrop of the solar irradiance is observed at around 7:15and lasts for 15 min. This could be explained by the factthat a passing cloud. Afterwards, there is a quick increasein the solar irradiance. In general, the solar irradiance risesslowly from 160 W/m2 at 7:00 am to around 650 W/m2 at11:30 am. Fig. 6b shows the variation of the solar irradi-ance from 12:45 pm to 14:35 pm. The results presented inFig. 6b also clearly show that there exist two low pointsof the solar irradiance. The reason for this is due to the factthat two short period of passing cloud occurred. From thisfigure, it can be seen clearly that the solar irradiance isalmost unchanged and maintained at around 650 W/m2

07:00 08:00 09:00 10:00 11:00 12:000

100

200

300

400

500

600

700

Sola

r irr

adia

cne (

W/m

2 )

Time

G

Fig. 6a. Solar irradiance profile during the morning.

12:45 13:00 13:15 13:30 13:45 14:00 14:15 14:300

100

200

300

400

500

600

700

Sola

r ir

radi

ance

(W/m

2 )

Time

G

Fig. 6b. Solar irradiance profile in the afternoon.

06:43 07:12 07:40 08:09 08:38 09:07 09:36 10:04 10:33 11:02 11:31 12:0002468

10121416182022242628

Pow

er (k

w)

Time

ED

ER

Fig. 7a. Time series of air temperatures at the inlet and outlet of thereceiver.

06:43 07:12 07:40 08:09 08:38 09:07 09:36 10:04 10:33 11:02 11:31 12:00

Time

TinTout

0

100

200

300

400

500

600

Tem

pera

ture

(o C

)

Fig. 7b. Inlet and outlet temperature profile during the test period.

K. Wang et al. / Solar Energy 122 (2015) 1080–1090 1087

in the afternoon. It is noted that the solar receiver is work-ing under variable solar irradiance condition during themorning. The variable solar irradiance distribution profilewould be crucial to analyze the energy and exergy perfor-mance of the solar receiver. Accordingly, a test period ofcontinuous 4.5 h in the morning is selected. Dynamicacquisition system is used to record the parameters auto-matically during the test. The ambient temperature is main-tained at around 10 �C during the experiment.

Fig. 7a presents a comparison of the power for theconcentrated solar radiation and receiver power. For thecase of the variable solar irradiance from 150 W/m2 to650 W/m2, the concentrated solar radiation power (ED)increases from 6.5 kW to 26.5 kW. In an alternative way,the above observed one low point is lower than 1 kW. Inaddition, the accurate control system can make sure thereflection focus located at the aperture of the receiver.The red line shown in Fig. 7a is the receiver power (ER)during the testing period. It is obvious that the variationtendency of ER is very similar to that of ED. At the firsttwo hours from 7:00 am to 9:00 am, for both ER and ED,the plot shows a rapid increase. From there till the end,

it is observed that the gradient of ER is obviously lowerthan that of ED. This can be explained by the fact thatthe average temperature of the receiver continues toincrease with the increase of the concentrated solar irradi-ation. Since the ambient temperature is maintained ataround 10 �C, the difference between ED and ER, whichmeans the heat loss of the receiver would become largerfor the difference between the average temperature of thereceiver and the ambient air increases. It is very clear thatthe decrease of the receiver power is very quick, whereasthe recovery of the receiver power is very slow. The timefor the dish shadowed by the cloud is about 4 min whilethe recovery time for the receiver is approximately12 min, which is three times of the shadowed time. Thelow point is negative for the receiver, this would be a dam-age to the solar generate system. Thus, the site selection ofthe solar power station should be in some place with strongsunshine and little cloud. After the low point, the receiverpower (ER) rises slowly and achieves the maximum valueof 16.2 kW at the end of the experimental.

Fig. 7b presents the time series of air temperatures at theinlet and outlet of the receiver. The inlet temperature rises

06:43 07:12 07:40 08:09 08:38 09:07 09:36 10:04 10:33 11:02 11:31 12:000.0

0.1

0.2

0.3

0.4

0.5

Hea

t los

s fac

tor U

' L(kw/

K)

Time

U'L

Fig. 8. Heat loss factor profile during the test period.

1088 K. Wang et al. / Solar Energy 122 (2015) 1080–1090

slowly during the period of preheating the inlet air. Fig. 7cpresents the energy efficiency of the receiver. From Fig. 7c,it takes the solar power about one hour to achieve steadystate. Although the solar irradiation condition increaseswith time, the solar receiver efficiency is maintained ataround 60% after 8:00 am. The first hour is used for pre-heating. It is due to the fact that the receiver tubes are sur-rounded by the chamotte with high specific heat capability(Cp). At the starting point, the temperature of the receiveris approximately 10 �C and the entire receiver needs to bepreheated prior to the real test. It is noteworthy that therising speed of the receiver efficiency is very high withinthe first 30 min. This phenomenon is very important andshould be stress here since the sunshine is limited in theday time, quick start up will make the overall solar powergeneration system to generate more electricity. Therefore,the cost of the power generation will be lower and theinvestment recovery period could be shorter.

Since the ED is rising slowly during the entire test per-iod, the profile of the receiver energy efficiency shows thesame trend as the profile of ER, as shown in Fig. 7a. It isfound that when the solar receiver turns into steady state,the efficiency of the solar receiver can be above 58%. Thepeak value of the efficiency is 64%, and finally, the effi-ciency is maintained at around 60%.

Fig. 8 demonstrates the evolution of the heat loss factorðU 0

L). At the starting point, U 0L is very high (0.46 kW/K)

because of the receiver preheating, and then it drops veryquickly within the first 30 min. The peak point is becauseof the weather, as previously mentioned. When the receiverworks at steady state, the heat loss becomes lower and U 0

L

achieves the minimum value of 0.035 kW/K. The heat lossis mainly including the conduction heat loss, convictionheat loss and radioactive heat loss. Conduction heat losscould be reduced by using material with low thermal con-ductivity. In the current experiment, the chamotte withlow thermal conductivity of 0.6 W/m2 K�1 is selected asthe insulation material to minimize the heat loss. The ther-mal convection between the receiver and ambient is the

06:43 07:12 07:40 08:09 08:38 09:07 09:36 10:04 10:33 11:02 11:31 12:000.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Ene

rgy

effic

ienc

y

Time

Energy efficiency

Fig. 7c. Energy efficiency profile during the test period.

main heat loss. Since the wind speed is very low duringthe experimental test, as a result, the heat loss of the natu-ral convection is mainly considered. It can also be con-cluded that the wind is a crucial factor for the siteselection of the solar power station. The radioactive heatloss is also an important part of the heat loss, which canbe reduced by using small aperture, as shown in Fig. 5b.In terms of the design of the solar receiver, choosing appro-priate aperture diameter is important for achieving betterperformance of the receiver.

Fig. 9a shows the comparison between the receiverexergy (ExR) and the concentrated solar energy as well asthe receiver energy. From this figure, the exergy rate andenergy rate vary in a similar manner, the passing cloudcan also result in the drop of the exergy rate. It is notedthat the highest value of the exergy rate during the testperiod is around 7.3 kW, whereas the maximum energyrate can reach 16.2 kW. It can be concluded that thequality of the energy from the receiver is low due to a largeamount of irreversible energy changes such as heat lossesand the transfer of high quality solar energy to a fluid that

06:43 07:12 07:40 08:09 08:38 09:07 09:36 10:04 10:33 11:02 11:31 12:000

5

10

15

20

25

30

Pow

er (k

w)

Time

Es

ER

ExR

Fig. 9a. Variation of the power of the receiver energy and exergy duringthe test period.

K. Wang et al. / Solar Energy 122 (2015) 1080–1090 1089

circulating at a relatively low temperature. From Eq. (13),it can be concluded that under the same temperature differ-ence (Tout � Tin) and the same energy rate _mcavðT out � T inÞcondition, increasing the receiver inlet temperature (Tin)can get higher exergy rate (ExR). It will be very helpfulfor the design of the solar power system. Therefore somerecuperator or heat exchanger should be used in the inletof the solar receiver to recover the waste heat and increasethe solar inlet temperature.

Fig. 9b presents the comparison between the energy effi-ciency and exergy efficiency. It is shown from Fig. 9b thatsimilar trends in the exergy efficiency and the energy effi-ciency are obtained. The highest exergy efficiency isapproximately 23%, whereas the highest energy efficiencyis around 64%. This suggests low quality energy obtainedfrom the receiver. It is because the inlet temperature ofthe receiver is lower than 303 K, whereas the outlet temper-ature is very high and with the maximum value of 800 K.The temperature ratio (Tout/Tin) is very high so that toomuch exergy loss is caused. Therefore, increasing the inlettemperature could be a potential way to increase the exergyefficiency.

06:43 07:12 07:40 08:09 08:38 09:07 09:36 10:04 10:33 11:02 11:31 12:000.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Eff

icie

ncy

Time

Energy efficiencyExergy efficiency

Fig. 9b. Energy and exergy efficiency profile during the test period.

0 100 200 300 400 500 6000.0

0.1

0.2

0.3

0.4

0.5

Exe

rgy

fact

or

Tout-Tin

Exergy factorEx=0.08956+0.00072(Tout -Tin)

Fig. 10. The effect of temperature difference on the exergy factor.

Fig. 10 shows the exergy factor plotted as a function ofthe temperature difference between the outlet and inlet tem-perature of the receiver with a linear fitting equation. Theexergy factor can also be used as a measure of the perfor-mance of the receiver. Obviously, as the temperature differ-ence increases, the exergy factor also increases. This plotsuggests that the higher exergy factor can be obtained whenhigh temperature difference is available. As seen from thisfigure, a lower value of exergy factor (0.32) is obtained eventhough the temperature difference is high (300 K). Thehighest exergy factor is 0.45 during the entire test.

5. Conclusions

This paper performs an experimental study to investi-gate the thermal performance of a coiled tube solar receiverunder real solar radiation condition. A parabolic dish withsolar tracker system is used in the experiment. The experi-mental data are analyzed using energy and exergy analysemethod. Some major conclusions can be drawn as below:

(1) The solar irradiance rises slowly from 160 W/m2 toaround 650 W/m2 in the morning, and keep almostconstant at around 650 W/m2 in the afternoon.

(2) The efficiency of the solar receiver can be above 58%.The peak point of the efficiency is 64%, and generallythe efficiency is maintained at about 60%.

(3) During the steady state, the heat loss becomes lowerand U 0

L achieves the minimum value of 0.035 kW/K.The highest exergy efficiency is approximately 23%,whereas the highest energy efficiency is around 64%.As the temperature difference increases, the impactof the exergy factor increases.

(4) A lower value of exergy factor (0.32) is obtained eventhough the temperature difference is high (300 K).The highest exergy factor is 0.45 during the entiretest.

Acknowledgement

The authors would like to acknowledge the financialsupport from National Natural Science Foundation ofChina (No. 51206164).

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