Applied Energy 98 (2012) 22–32

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Applied Energy

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Performance analysis and comparison of concentrated evacuated tube heatpipe solar collectors

Dan Nchelatebe Nkwetta a,⇑, Mervyn Smyth b

a Sustainable Building Envelope Centre, Corus Colors, Tata Steel, Shotton Works, Deeside, Flintshire, United Kingdomb Centre for Sustainable Technologies, School of the Built Environment, Faculty of Arts, Design and Built Environment, University of Ulster, Newtownabbey, United Kingdom

a r t i c l e i n f o a b s t r a c t

Article history:Received 21 October 2011Received in revised form 16 January 2012Accepted 21 February 2012Available online 27 April 2012

Keywords:Concentrated evacuated tube heat pipeSingle-sided coated absorberDouble-sided coated absorber and incidentangle modifier

0306-2619/$ - see front matter � 2012 Elsevier Ltd. Adoi:10.1016/j.apenergy.2012.02.059

⇑ Corresponding author. Tel.: +44(0) 77 86013289;E-mail address: Dan.Nkwetta@tatasteel.com (D.N.

Two profiles of concentrated evacuated tube heat pipe solar collectors made of single-sided and double-sided absorber have been analyzed and compared under control conditions and results presented in thispaper. These innovative concentrated evacuated tube heat pipe solar collectors were experimentallytested at a tilt angle of 60� to the horizontal. Using in-door solar simulated experimental conditions tem-perature response, collection efficiency, heat loss coefficients and energy collection rates as well as theincident angle modifier (IAM) were recorded and compared at five different transverse angles (0–40�)at 10� increments.

The use of concentrated single-sided and double-sided absorber evacuated tube heat pipe solar collec-tors is seen to be feasible for integrating solar thermal energy into buildings for heating demands. Theconcentrated double-sided absorber evacuated tube heat pipe proves better compared to the concen-trated single-sided absorber evacuated tube heat pipe solar collector due to higher outlet temperaturewith greater temperature differential and improved thermal performance. The integration of this innova-tive system implies that the number of the evacuated tube heat pipe collectors needed to attain highertemperature is reduced. Furthermore, the size of reflectors and related reflector losses are reduced dueto the truncated nature of the reflectors providing a low concentration ratio.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

World energy demand and CO2 emissions are expected to riseby some 60% by 2030 and the EU energy import dependency isforecasted to increase to about 70% by 2030 [1]. Greenhouse gasemissions resulting from the accumulated use of fossil fuels are aglobal societal issue needing to be addressed. Renewable energytechnologies must make a significant contribution to global energyproduction in order to reduce the accumulation of greenhousegases [2]. Energy related cooling demand is expected to increaserapidly over the century due to global warming, with 1.1–6.4 �Crise in surface temperatures if emissions are not curbed [3]. Med-ium temperature applications (75–120 �C) such as solar cooling orindustrial process heat can be potentially met by using current so-lar technology but at additional cost [4].

In addition, solar cooling is likely to reduced greenhouse gasemission and play a significant role in meeting the demand asthere is good load matching of the available peak solar radiationwith the maximum cooling load [5,6]. In many cases, the applica-tion of solar thermal cooling systems has been conditioned bythe lack of integration between cooling and heating systems [7].

ll rights reserved.

fax: +44(0) 12 44892345.Nkwetta).

Umberto et al. [8] reported that cooling demand has increased inrecent years, due to the increasing demand of higher comfort con-ditions inside buildings. Solar thermal cooling systems are still intheir infancy regarding practical applications, although the tech-nology is sufficiently developed for a number of years [7]. Theincreasing thermal comfort has led to a widespread use of coolingsystems based on compression technology, resulting in significantincrease of electric power peak demand in summer reaching, inmany cases, the capacity limits of network and causing the riskof blackouts [8].

Evacuated tube solar collectors have low heat losses comparedto convectional flat plate collectors due to the vacuum envelopearound the absorber [9–11]. The evacuated tube heat pipe solarcollector operates by transferring thermal energy from the absor-ber fins to the heat transfer fluid (HTF) with only a small temper-ature differential existing between the fin absorber and thecondenser with configuration acting as a thermal diode (prevent-ing reverse flow of heat) during non-operational periods [12–14].Freezing of the heat pipe is unusual and is generally not destruc-tive. The heat pipe system operates independently of gravity, is si-lent with no moving parts and thus needs minimal servicing [15].

The determination and optimisation of the conversion-efficiency of a line-focus parabolic-trough solar-collector, verticalevacuated tubular-collectors utilising solar radiation from all

Nomenclature

A area (m2)Aap aperture area (m2)Qcollected thermal energy collected (MJ)Gave average solar insolation (W/m2)_m mass flow rate (kg/s)

Toutlet collector outlet water temperature (�C)Tinlet collector inlet water temperature (�C)cp specific heat constant of water (kJ/kg/ �C)Dt time interval (minutes)DTinc temperature differential increase (�C)g efficiency (%)goptical optical efficiency (%)gcollector collector efficiency (%)Qincident incident solar radiation (W/m2)Tamb ambient temperature (�C)

Tm mean fluid temperature (�C)

AbbreviationsCOP Coefficient of performanceHTF Heat transfer fluidIAM Incident angle modifierA Heat loss coefficient of collector (W/m2 K)ETHPC Evacuated tube heat pipe collectorDSACPC Double-sided absorber CPCSSACPC Single-sided absorber CPCCPC Compound Parabolic ConcentratorASHRAE American Society of Heating and Air Conditioning

EngineersIPCC Intergovernmental Panel on Climate Change

117.951.1 51.1

19082

.0

45.045.0 130.1

220

276.

4

Evacuated glasstube

Single-sidedabsorber

Reflector

Full

Hei

ght-

Trun

cate

d H

eigh

t-

Fig. 1. Side section view of the full and truncated SSACPC solar collector.

D.N. Nkwetta, M. Smyth / Applied Energy 98 (2012) 22–32 23

directions and the experimental investigation of the transient ther-mal performance of a bent heat pipe with grooved surface havebeen reported by Bakos et al. [16]; Shah and Furbo [17] and Wang,respectively [18]. Presently, solar cooling and other medium tem-perature applications can be supplied by evacuated tube heat pipesolar collectors. However, the lower fluid output temperatureswith a smaller temperature differential and thus lower thermalperformance in the medium temperature range reduces the effec-tive potential of these systems. Evacuated tube heat pipe solar col-lectors can operate efficiently in their design operationaltemperature range of 75–90 �C to drive a single effect absorptionprocess [19] but this is economically detrimental and thus uneco-nomical. This is also a constraint with the double-effect absorptionrefrigeration systems operating in the temperature range of 90–120 �C. Agyenim et al. [20], demonstrated experimentally thatincreasing the inlet fluid temperature of a solar air-conditioningsystem increases the coefficient of performance (COP), necessitat-ing the combined use of concentration and evacuation of solar col-lectors for such medium temperature applications and beyond.

Medium supply temperatures beneficial for solar cooling can beattained using a combination of concentrating compound parabolicconcentrators and evacuated tube heat pipe solar collectors [5,6].The development and deployment of non-tracking low concen-trated evacuated tube heat pipe solar collectors is necessary as itcan further improve the performance, potentially making themapplicable for the solar heating as well as previously mentionedmedium to high temperature applications [5,6].

In-door testing, evaluation and comparison of temperatureimprovement and thermal performance of solar collectors offeran efficient method in providing collected data compared to out-door test conditions. The in-door laboratory simulated conditionssuch as ambient air temperature and wind speed in addition tothe solar intensity are controllable, less time consuming and avoiddifficulties in changing boundary conditions compared to out-doortesting.

Neither compound parabolic concentrators (CPC) collectors norevacuated tube heat pipe collectors are novel in solar thermal engi-neering terms. However, the integration of CPC reflectors with sin-gle-sided and double-sided absorber evacuated tube heat pipesolar collectors as single collector is seen to be novel in solar ther-mal engineering. Realising a size reduction and improved eco-nomic potential, with higher operating temperature differentialsand a substantial overall improvement in thermal performancethese collectors offer an alternative for medium to high tempera-ture applications. The flat single-sided absorber within a CPC(SSACPC) offers a higher utilisation area with possibility of greater

collection of direct incident solar radiation whereas the double-sided absorber within a CPC (DSACPC) allows collection of incidentsolar radiation on both surfaces of the absorber, eliminating backlosses. This paper focuses on the development of the collector com-ponents and their experimental evaluation and involved;

� Collector design and fabrication.� The techniques and methodology involved in their design and

fabrication.� In-door solar simulated experimental characterisation and anal-

ysis based on temperature response, energy collection rates,collection efficiency, heat loss coefficient and IAM.

2. Collector design and fabrication

The full and truncated SSACPC and DSACPC solar collectors weregenerated using computer based simulation software ‘Eazea’ andTecplot and the co-ordinates used in the design and fabricationof the units at the University of Ulster. Computer generated coor-

289.

6

49.949.9 260.1360.0

152.

1

235.762.2 62.2

Full

Hei

ght-

Trun

cate

d H

eigh

t-Evacuated glasstube

Double-sidedabsorber

Reflector

Fig. 2. Side section view of the full and truncated DSACPC solar collector.

Table 1Geometric characteristics for the full and truncated SSACPC and DSACPC solarcollectors.

Parameter (units) Collectors parameters

DSACPC SSACPC

Aperture area (m2) 0.2004 0.107Absorber area (m2) 0.202 0.102Absorber thickness (m) 0.0002 0.0002Absorber width (m) 0.0637 0.0637Absorber length (m) 1.700 1.700Diameter of heat pipe (m) 0.008 0.008Absorber absorptance 0.95 0.95Emittance (coating) 0.14 0.14Glass tube o.d (m) 0.065 0.065Glass tube length (m) 1.93 1.93Glass tube transmittance 0.88 0.88Heat transfer fluid pressurised water Pressurized waterHalf acceptance angle (�) 30 30Original concentration ratio 2 2Original reflector height (m) 0.289 0.168Truncated concentration ratio 1.85 1.85Truncated aperture width (m) 0.236 0.118Truncated reflector height (m) 0.152 0.82Truncated height to Aperture ratio 0.64 0.69Reflector-absorber gap (mm) 3 3Reflector material AluminiumAluminiumReflectivity of reflector 0.91 0.91

24 D.N. Nkwetta, M. Smyth / Applied Energy 98 (2012) 22–32

dinates for the full and truncated SSACPC and DSACPC solar collec-tors were transferred to ‘AUTOCAD’ formats to draw the profiles ina ‘dxf’ format enabling the profiles to be printed to scale as printingtemplates and used in the construction of the reflector support andprofiles as illustrated in Figs. 1 and 2. The printed ‘AUTOCAD’ de-sign templates were then glued to the wooden supporting plates,which were pre-cut with measuring length and height of360 mm � 360 mm, 360 mm � 210 mm, 220 mm � 260 mm and220 mm � 190 mm for the full and truncated DSACPC and SSACPCprofiles, respectively. The respective profiles were cut out using anelectric jig-saw and assembled in the laboratory. This techniquegave an accurate profile and structure supports.

Single-sided and double-sided absorber evacuated tube heatpipes were integrated into two external CPC reflectors both witha design half-acceptance angle of 30� and a geometrical concentra-tion ratio of 2 and 1.85 upon truncation. A 30� design acceptancehalf-angle (ha) (in a north–south orientation) was selected, as itwould allow optimal daily collection at a northern latitude withno tilt adjustment. The single-sided absorber evacuated tube heatpipe compound parabolic concentrator (SSACPC) had an effectiveabsorber width of 63.7 mm with an aperture width of 130.1 mm.The reflector profile for the full single-sided absorber evacuatedtube heat pipe collector had a total height of 168.4 mm.

Truncation of the upper part of the reflector was conducted,reducing the reflector to a height of 92.8 mm with an aperturewidth of 121 mm and concentration ratio of 1.9. The reflector areawas reduced to 55.1% of the original surface area saving 44.9% of thereflector material. The height to aperture ratio decreased from 0.77to 0.69. A further truncation to a total height of 82 mm resulted in areduction of concentration ratio to 1.85 with the reflecting materialfurther reduced to 48.7% of the original reflector surface area withan overall saving of 51.3% of the reflector material. The height toaperture ratio of the full and truncated single-sided absorberevacuated tube heat pipe with a compound parabolic concentratordecreased from 1.29 to 0.77 and to 0.69, respectively with a totalaperture area of 0.107 m2.

The upper reflector of the DSACPC solar collector was truncatedfrom a total height of 289.6 mm to 168.9 mm with the concentra-tion ratio reduced slightly from 2.04 to 1.9. The reflector area wasreduced to 58.3% of the original reflector surface area, thus saving41.7% of reflecting material. A further truncation to a total height of

152.1 mm resulted in the concentration ratio being reduced to 1.85while the reflecting material was further reduced to 52.5% of theoriginal reflector surface area with a saving of 47.5% of the originalreflecting material. The height to aperture ratio of the full and trun-cated DSACPC solar collector decreased from 1.11 to 0.69 and to0.64, respectively with a total aperture area of 0.2004 m2. Trunca-tion of the reflectors resulted in 48.7–52.5% reduction in reflectingmaterial for the SSACPC and DSACPC, respectively but with only7.5% reduction in concentration ratio, which is of cost advantageand provides compactness of the solar collector.

The selection of a 1.85 concentration ratio compared to 1.9 wasdue to the fact that a further truncation of the reflective materialfrom 1.9 to 1.85 resulted in a 6.4% savings in reflecting materialwith only a 2.63% reduction in concentration ratio. This cost savingin reflective material is also benefited by the compactness of thesolar collectors and the good reflectivity of the aluminium sheet(0.91) employed minimises optical losses.

Figs. 1 and 2 present the side sectional view of the full and trun-cated single-sided and double-sided absorber evacuated tube heatpipe with a compound parabolic concentrating solar collectors,respectively (all dimension in mm) and the specification (physicaland geometric characteristics) for the solar collectors compared isdetailed in Table 1.

In each design configuration, a 3 mm gap was created betweenthe evacuated heat pipe tube and the bottom of the reflector toavoid thermal conduction between the hot absorber and the con-centrator. The criteria for the selection and design of the single-sided and double-sided absorber evacuated tube heat pipe with acompound parabolic concentrator was based on their theoreticallycalculated higher concentrations ratios, their lower material andfabrication costs and the suitability for Northern European Mari-times climates.

3. In-door experimental test facility and description of the in-door experimental set-up

The in-door experimental characterisation of the single-sidedand double-sided absorber evacuated tube heat pipe solar collec-tors was carried out using a state-of-the-art solar simulator test

D.N. Nkwetta, M. Smyth / Applied Energy 98 (2012) 22–32 25

facility located at the University of Ulster. The merits of using a so-lar simulator, compared to the actual solar conditions is based onthe fact that a controlled solar intensity can be created, steadystate conditions can be attained, allowing a range of repeatablescenarios/parameters to be simulated [18]. However, the unifor-mity of the intensity on the illuminated area, collimation and thelight spectral distribution from the solar simulator all require par-ticular attention and thus the need for accurate pre-measurementof flux distribution before testing. A schematic detail of the in-doorexperimental test facility using a pressurised water heat exchangerloop is illustrated in Fig. 3.

The single-sided and double-sided absorber evacuated tubeheat pipe solar collectors were mounted on a movable testingframe and positioned at the centre of the simulator test plane. Eachconcentrating collector was positioned such that the absorberswere aligned in a north–south orientation, facing the simulatorat a tilt angle of 60� to simulate a typical operation, but permittingtesting at different transverse angles (0–40�).

The lamps of the solar simulator had lenses fitted over themwith an overall light collimation of 80–85% and were also at aninclination of 60� from the horizontal facing parallel to the solarcollectors [21]. A 60� inclination was chosen for the optimum col-lection of solar radiation at a latitude of 52�N. A distance of 1.8 mfrom the centre of the incline of the solar collectors to the centre oftilted solar simulator was selected as the optimal length to ensureuniformity of radiation distribution on the aperture of the solarcollectors. To determine the levels of insolation incident on theaperture of the solar collectors, a series of measurements of inci-dent solar radiation were performed across the aperture of the col-lectors using a Kipp and Zonen pyranometer (first class) [22]. Two

Pr

Flowmeter

Sight glass

Pyranometer

Manifold

Data logger

Solar collector

Pump

Outlet fluid temperature Sensor (PT100)

Inlet fluid temperature Sensor (PT100)

Ambitempe

Automvent (

Fig. 3. Schematic detail of the in-door solar simulated experiment

pyranometers were used to measure the simulated solar irradianceon the aperture of each solar collector prior to testing. One of thepyranometer was the feedback pyranometer (Kipp and ZonenCM4) for the solar simulator and aided in regulating and stabilisingthe solar radiation of the lamps to the set-up value of 800 W/m2.The second pyranometer; (the Kipp & Zonen CM 11) with sensitiv-ity of 4.39 � 10�6 Vm2/W and an uncertainty of ±2% [22] was fixedon the plane of the solar collector aperture at mid-height to mea-sure solar radiation thought out the experimental period. Both pyr-anometers were free from obstructions and shading.

The following pre-test procedure was carried out each time be-fore the testing of each solar collector. The in-line pump wasswitched on and the fluid flow rate stabilised via a variable speeddrive (VSD) inverter. The solar collector was covered with an opa-que sheet to prevent any solar gain during the solar simulatorwarm-up prior to testing. Pre-test stabilisation lasted for 30 min,allowing fluid to circulate through the system ensuring uniformtest conditions.

Air movement across the solar collectors was maintainedthrough a fan, producing an average air speed across the apertureof the solar collectors of 2.2 m/s, verified through measurementfrom an anemometer. The uniformity of the solar radiation inci-dent on the aperture of the solar collectors was measured usinga grid measurement system. This task was carried out as follows;A pyranometer was set at four pre-marked positions and four mea-surement noted and averaged against a fixed datum pyranometerthat was use to measure the solar radiation incident on the aper-ture for each solar collector throughout the duration of the exper-imental period. The 30-min pre-test stabilisation allowed thelamps to warm up, achieving uniformity of intensity along with

essure Vessel

Drain valve

In-lineimmersion heater

Solasyphon heat exchanger

PID temperature regulator

Cooling circuit

PC

Isolation valve

ent airrature sensor

atic air AAV)

al test facility using a pressurised water heat exchanger loop.

26 D.N. Nkwetta, M. Smyth / Applied Energy 98 (2012) 22–32

the constant fluid flow. After the initial 30-min warm up period,with the solar intensity and distribution on the aperture of the so-lar collector measured, determined and recorded, the test wasready to begin. The readings from the fixed pyranometer on theplane of the solar collector aperture were used to monitor any var-iation in insolation values during the experiment and to determinethe correction factor in order to compensate any non-uniformity inthe simulated solar radiation. The solar collector units were evalu-ated under the solar simulator for a period of 6 h at an insolation of800 ± 15 W/m2 over a range of transverse angles.

In total ten experimental tests covering different transverse an-gles (0�, 10�, 20�, 30�, 40�) with 10� increment were conducted forthe truncated single-sided and double-sided absorber evacuatedtube heat pipe with compound parabolic concentrator solar collec-tors. Water, pressurised to 3.5 bar to avoid boiling above 100 �C andto permit performance evaluation at temperatures of up to about140 �C was used as the heat transfer fluid (HTF). The fluid flow ratewas maintained at a constant rate of 0.0038 ± 0.0002 kg/s through-out each test period using an Oval Gear Wheel Nixon flow meters(OG4-SU5-VHT-B).

The oval gear wheel nixon flow meter (OG4-SU5-VHT-B) wascharacterised by a flow range 0.25–50 lpm, NPN pulse output of100 pulses per litre and an accuracy of ±0.5%, FDS. The test se-quence was repeated for each solar collector and can be summa-rised as;

� All sensors checked.� Data logging sequence started. The inlet and outlet fluid tem-

peratures and ambient air temperatures, fluid flow rates andsimulated solar radiation values are automatically sampledevery half minute and averaged every 1 min and recorded viathe Delta-T logger to the computer.� Removal of the opaque cover.� Six hour test period. During this period the test set-up was con-

tinually monitored to ensure the test was operating as required.� At the end of the 6th hour the test ended and the opaque cover

replaced.� Lamps and pump switched off, valves closed and the logger

stopped and the data retrieved.

The results from each experimental investigation period at dif-ferent transverse angles have been reported using temperatureprofiles (inlet and outlet fluid temperature variations, mean fluidand ambient air temperature changes), energy collection rate, opti-cal efficiency and heat loss coefficient and the incident angle mod-ifier (IAM).

4. Results and discussion

Experimental results and analysis for the single-sided and dou-ble-sided coated evacuated tube heat pipe absorber integrated intoa compound parabolic concentrator solar collector were evaluatedusing ANSI/ASHRAE Standard 93-2003 [23]. Critical analysis in-volve determining the changes in fluid temperature profiles overtime (inlet and outlet fluid temperatures, outlet and inlet fluidtemperature differential, mean fluid collector and ambient air tem-peratures changes), energy collection rates, optical efficiency andthermal performance (heat loss coefficient) and the incident anglemodifier as a function of transverse angles.

4.1. Temperature variation for the solar collectors

The temperature profiles are presented in the form of the inletand outlet fluid temperatures, fluid temperature differential across

the solar collectors and mean fluid temperature gain across the so-lar collector.

4.1.1. Inlet and outlet fluid temperatureThe variation in the inlet and outlet fluid temperatures for the

single-sided and double-sided coated evacuated tube heat pipe ab-sorber integrated into compound parabolic concentrating solar col-lectors over the 6 h experimental test period with solar radiationperpendicular to the aperture of the solar collector using wateras the HTF is shown in Fig. 4.

The increasing inlet fluid temperature over the experimentaltime period illustrated in Fig. 4 resulted from the use of a closedloop where the circulating heat transfer fluid is heated continu-ously. The maximum outlet fluid temperature for the truncatedSSACPC was 55.9 �C with an accompanying inlet fluid temperatureof 51.2 �C compared to 69.8 �C and 60.4 �C maximum outlet and in-let fluid temperatures, respectively recorded by the truncatedDSACPC after 6 h of testing (Fig. 4). The maximum fluid tempera-ture increase for the solar collector is determined from the meaninlet and outlet fluid temperatures and the ambient air tempera-ture using Eq. (1). The mean fluid temperature from Eq. (2) is usedrather than the inlet fluid temperature as it takes into account theheating effect across the solar collector. The maximum fluid tem-perature increase was determined to be 28.5 �C and 41 �C for thetruncated SSACPC and DSACPC solar collectors, respectively atthe end of the test. This represents an overall improvement of12.4 �C or 30.4% in mean fluid and ambient air temperature differ-ential realised by the truncated DSACPC solar collector comparedto the truncated SSACPC solar collector.

T inc ¼ Tm � Tamb ð1Þ

where

Tm ¼ ðT inlet þ ToutletÞ=2 ð2Þ

From the fluid temperature profiles of the solar collectors, it isevident that the truncated DSACPC solar collectors showed a morerapid thermal response when initially exposed to incident solarradiation compared to the truncated SSACPC solar collector. Thisrapid thermal response is ultimately due to increase collection ofincident solar radiation resulting from the larger aperture collect-ing area and the collection of more incident solar radiation on bothsides of the absorber, thus increasing the energy flux absorbed.

4.1.2. Outlet and inlet fluid temperature differentialThe outlet and inlet fluid temperature differential for the trun-

cated SSACPC and DSACPC solar collectors over 6 h of testing at dif-ferent transverse angles was calculated using Eq. (3). Thetemperature differential between the outlet and inlet of the solarcollectors were based on experimental data collected with a timeinterval of 30 min as illustrated in Fig. 5. The average outlet and in-let fluid temperature differential with respect to different trans-verse angles for all solar collectors are presented in Figs. 5 and 6,respectively. The average outlet and inlet fluid temperature differ-ential for all the solar collectors decreases with increasing trans-verse angle with some minor variation.

DT inc ¼ Toutlet � T inlet ð3Þ

The maximum outlet and inlet fluid temperature differential forthe truncated SSACPC over the 6 h of testing was calculated to be5.2 �C whereas 4.7 �C was determined as the minimum outletand inlet fluid temperature differential. The maximum outlet andinlet fluid temperature differential for the truncated DSACPC solarcollector was determined to be 9.4 �C whereas 8.1 �C was realisedas the minimum outlet and inlet fluid temperature differential,respectively. Maximum and minimum outlet and inlet fluid tem-perature differential improvements of 4.2 �C (44.5%) and 3.4 �C

0

10

20

30

40

50

60

70

80

0 30 60 90 120 150 180 210 240 270 300 330 360Time (minute)

Tem

pera

ture

(°C

)

Toutlet-Truncated DSACPC [0°] Tinlet-Truncated DSACPC [0°] Ambient temperature

Toutlet-Truncated SSACPC [0°] Tinlet-Truncated SSACPC [0°]

Fig. 4. Inlet and outlet fluid temperature profile for the truncated SSACPC and DSACPC solar collectors with solar radiation perpendicular to the aperture.

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

30 60 90 120 150 180 210 240 270 300 330 360Time (minutes)

Tem

pera

ture

diff

eren

tial,T

outle

t-Tin

let (

o C)

Truncated SSACPC [0°] Truncated SSACPC [10°]Truncated SSACPC [20°] Truncated SSACPC [30°]Truncated SSACPC [40°] Linear (Truncated SSACPC [0°])Linear (Truncated SSACPC [10°]) Linear (Truncated SSACPC [20°])Linear (Truncated SSACPC [30°]) Linear (Truncated SSACPC [40°])

Fig. 5. Outlet and inlet fluid temperature differential for the truncated SSACPC over the test periods at different transverse angles.

D.N. Nkwetta, M. Smyth / Applied Energy 98 (2012) 22–32 27

(42.3%) were realised by the truncated DSACPC solar collector com-pared to the truncated SSACPC solar collector. The overall averagedoutlet and inlet fluid temperature differential at a transverse angleof 0� was determined to be 8.5 �C for the truncated DSACPC solarcollector compared to 4.9 �C for the truncated SSACPC, represent-ing a 3.6 �C (42.4%) overall averaged outlet and inlet fluid temper-ature differential improvement for the truncated DSACPC solarcollector compared to the truncated SSACPC.

It is evident that the outlet and inlet fluid temperature differen-tial, that the overall average outlet and inlet fluid temperature dif-ferential decreased with time and with the increasing transverseangle. This decreasing fluid temperature differential can be attrib-uted to the increasing inlet and outlet fluid temperatures resultingin increased heat losses and lower insolation on the absorber due tothe increasing path length of the incident solar radiation and the re-duced projected absorber area with increasing transverse angles.

Based on the technical design guide reported by Kingspan Renew-ables [24], the mean fluid and ambient temperature differentialfor the truncated SSACPC and DSACPC solar collectors experimen-tally investigated with simulated solar radiation perpendicular toaperture of the concentrating collectors were evaluated andmatched to various operation areas.

4.2. Useful energy collection

The amount of simulated radiation falling on the collecting sur-face area of the truncated SSACPC and DSACPC solar collectors overeach test period at different transverse angles was calculated usingin the following equation

Q incident ¼ GaveAapDt ð4Þ

where

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

30 60 90 120 150 180 210 240 270 300 330 360

Time (minutes)

Tem

pera

ture

diff

eren

tial,T

outle

t-Tin

let (

o C)

Truncated DSACPC [0°] Truncated DSACPC [10°]Truncated DSACPC [20°] Truncated DSACPC [30°]Truncated DSACPC [40°] Linear (Truncated DSACPC [0°])Linear (Truncated DSACPC [10°]) Linear (Truncated DSACPC [20°])Linear (Truncated DSACPC [30°]) Linear (Truncated DSACPC [40°])

Fig. 6. Outlet and inlet fluid temperature differential for the truncated DSACPC solar collector as a function of transverse angles.

28 D.N. Nkwetta, M. Smyth / Applied Energy 98 (2012) 22–32

Gave ¼

R tstart

tendGðtÞdt

� �Dt

ð5Þ

The amount of useful energy collected by each solar collectorduring the experimental test period was determined using in thefollowing equation

Q collected ¼ _mcpðToutlet � T inletÞ ð6Þ

The energy collection rate for the truncated SSACPC andDSACPC over the test period at different transverse angles usingwater as the HTF is illustrated in Figs. 7 and 8, respectively. The en-ergy collection rates for the truncated SSACPC and DSACPC withincident solar radiation perpendicular to the collector apertureranges from 88.82 W to 76.46 W and 171.13 W and 137.49 W at

20

30

40

50

60

70

80

90

100

30 60 90 120 150 180Time (m

Aver

age

ener

gy c

olle

cted

(W)

Truncated SSACPC [0°] Truncated STruncated SSACPC [30°] Truncated SLinear (Truncated SSACPC [10°]) Linear (TrunLinear (Truncated SSACPC [40°])

Fig. 7. Useful energy collection rate for the truncated SSAC

the start and end of the investigation period, respectively. Theoverall averaged energy collection rate was determined to be82.11 W and 150.67 W over the 6 h experimental test period forthe truncated SSACPC and DSACPC, respectively with the incidentsolar radiation perpendicular to the collector aperture.

This represents 68.56 J (45.50%) more energy on average col-lected by the truncated DSACPC solar collector compared to thetruncated SSACPC solar collector over the 6 h experimental testperiod. The decrease in the energy collection rate and overall aver-age energy collection rate with increasing test time was similar atdifferent transverse angles. As expected, the energy collection rateat the start of the experiment was higher than the energy collec-tion rate after 6 h of testing and the overall averaged daily energycollected. This is due to increased heat losses from increasing inlet

210 240 270 300 330 360inutes)

SACPC [10°] Truncated SSACPC [20°]SACPC [40°] Linear (Truncated SSACPC [0°])cated SSACPC [20°]) Linear (Truncated SSACPC [30°])

PC over the test period at different transverse angles.

20

40

60

80

100

120

140

160

180

30 60 90 120 150 180 210 240 270 300 330 360Time (minutes)

Aver

age

ener

gy c

olle

ctio

n ra

te (W

)

Truncated DSACPC [0°] Truncated DSACPC [10°] Truncated DSACPC [20°]Truncated DSACPC [30°] Truncated DSACPC [40°] Linear (Truncated DSACPC [0°])Linear (Truncated DSACPC [10°]) Linear (Truncated DSACPC [20°]) Linear (Truncated DSACPC [30°])Linear (Truncated DSACPC [40°])

Fig. 8. Energy collection rate for the truncated DSACPC solar collector as a function of transverse angles.

D.N. Nkwetta, M. Smyth / Applied Energy 98 (2012) 22–32 29

and outlet fluid temperatures. Energy collection rates at differenttransverse angles also decreased with increasing transverse angles.One exception, the truncated SSACPC realised a higher energy col-lection rate at ten degree (10�) than at zero (0�) as illustrated inFig. 7.

The total energy collected by the truncated SSACPC and DSACPCsolar collectors over the test period at different transverse angles ispresented in Table 2. The total energy collected by the truncatedSSACPC solar collectors increased from 1.77 MJ to a maximum of1.92 MJ from transverse angles of 0–10� but dropped to 0.77 MJat a transverse angle of 40�. The total energy collected by the trun-cated DSACPC solar collector decreased from 3.26 MJ to 1.03 MJ attransverse angles of 0–40�, respectively. The decrease in energycollection rates with increasing transverse angle is as a result of re-duced projected absorber area and increasing path length throughwhich the incident rays travel before getting to the absorber. It wasdetermined that the truncated DSACPC solar collector collected1.5 MJ (46%) and 0.48 MJ (46.6%) more energy compared to thetruncated SSACPC solar collector at transverse angles of 0�and40�, respectively. This resulted from incident rays hitting bothsides of the absorber and increase collection of diffuse radiations.

The total energy collected by the non-concentrated evacuatedtube heat pipe called the control ETHPC solar collector decreasedfrom 1.32 MJ to 0.23 MJ at transverse angles of 0–40�, respectively.The decrease in energy collection rates with increasing transverseangle is as a result of reduced projected absorber area. The compar-ison in energy collection for three different collectors at five differ-ent transverse angles is illustrated in Table 2. The truncatedDSACPC and SSACPC solar collectors collected 1.94 MJ (59.51%)and 0.80 MJ (77.67%) and 0.44 MJ (25%) and 0.32 MJ (58.18%) moreenergy compared to the control ETHPC solar collector at transverseangles of 0�and 40�, respectively.

4.3. Instantaneous efficiencies and thermal characterisation of thesolar collectors

Experimental efficiencies and thermal performance were deter-mined using the experimental test data to generate efficiencycurves. These linear ‘best curves’ were obtained by calculatingthe instantaneous efficiencies from the solar inputs, flow rates,ambient air temperature and inlet and outlet fluid temperatures.

Experimental determination of the efficiencies of the SSACPC andDSACPC solar collectors were based on the collector aperture area(Aapt) under constant incident solar radiation and were presentedusing plots of efficiency against ((Tm � Tamb)/Gave).

Eq. (7) permitted the determination of the instantaneous collec-tion efficiencies for these solar collectors over the investigatedperiod.

gcollected ¼_mcpðToutlet � T inletÞ

GaveAapð7Þ

Eq. (7) was used to determine the collection efficiencies forthese solar collectors based on operational values, which can befurther expressed using the Hottel-Whillier-Bliss equation for solarenergy collection represented in Eq. (8) [25].

gcollector ¼ goptical � aTm � Tamb

GnAap

� �ð8Þ

Fig. 9 illustrates the experimentally determined efficiencies forthe SSACPC and DSACPC solar collectors, respectively. Graphicalrepresentation of efficiencies against ((Tm � Tamb)/Gave) permittedthe heat removal-factor (intercept on the Y-axis) and the heat losscoefficient or slope of best-fit line to be identified and aids in sys-tem characterisation as well as long-term performance prediction.The experimental optical efficiencies for these solar collectors weredetermined from the intercept on the Y-axis. These experimentaloptical efficiencies with the incident solar radiation perpendicularto the collector aperture are 59.8% and 57.2% for the SSACPC andDSACPC solar collectors, respectively.

Table 3 details the characteristic representation (optical effi-ciencies, heat loss coefficients and statistical uncertainties) forthe solar collectors based on the aperture area over the test periodswith incident solar radiation perpendicular to the collectoraperture.

The truncated DSACPC solar collector had a lower optical effi-ciency due to greater optical losses resulting from the larger reflec-tor and aperture area but had lower heat loss coefficients asdemonstrated by the shallow drop in efficiency as the collector in-let and outlet fluid temperature increases. The illumination onboth sides of the absorber for the DSACPC solar collector also re-duces heat losses (no back losses) compared to the back lossesfor the SSACPC solar collector. The maximum mean fluid and ambi-

Table 2Total energy collected by the control ETHPC, truncated SSACPC and DSACPC solarcollectors as a function of transverse angle over the test period.

Transverseangles (�)

Total energycollected by thecontrol ETHPC(MJ)

Total energycollected by thetruncated SSACPC(MJ)

Total energycollected by thetruncated DSACPC(MJ)

0 1.32 1.76 3.2610 1.26 1.92 3.0720 1.24 1.37 2.4130 0.83 1.09 1.7240 0.23 0.55 1.03

30 D.N. Nkwetta, M. Smyth / Applied Energy 98 (2012) 22–32

ent temperature differential realisable by the truncated SSACPCwas up to 89 �C with abscissa values ranging from 0.06 to 0.11(medium temperature range). This allowed it to be efficient for so-lar heated DHW with solar assisted central heating and processapplication including solar assisted cooling, space heating anddesalination. The use of the truncated DSACPC solar collector at ab-scissa values ranging from 0.06 Km2 W�1 to 0.11 Km2 W�1 (med-ium temperature range) is slightly less efficient compared to thetruncated SSACPC solar collector.

However, at abscissa values equal to and greater than 0.11Km2W�1and up to 0.18 Km2 W�1 (much higher medium tempera-ture range), the truncated DSACPC solar collector can attain a po-tential maximum mean fluid and ambient temperaturedifferential of up to 146 �C. This makes the DSACPC much moreefficient for process application including solar assisted cooling,space heating, desalination and other higher temperature applica-

y = -3.553x + 0.5981R2 = 0.8922

y = -2.306x + 0.57R2 = 0.901

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.0000 0.0200 0.0400 0.0600 0.0800 0.1(Tm-Ta

Effic

ienc

y (-)

Efficiency-Truncated SSACPC [0°]

Linear (Efficiency-Truncated SSACPC [0°])

Fig. 9. Experimental efficiencies and best fit linear characterisation curves for the trunccollector aperture.

Table 3Characterisation of the truncated SSACPC and DSACPC solar collectors based on the apertu

Solar collector Characteristic equation based on aperture area Performan

Optical effi

Truncated SSACPC g = 0.598–3.55 (Tm � Tamb)/Gave 59.8Truncated DSACPC g = 0.572–2.31 (Tm � Tamb)/Gave 57.2

tions compared to the truncated SSACPC solar collectors. The use ofthe truncated SSACPC and DSACPC solar collectors for solar heatedDHW, solar assisted central heating and process application im-plies that the truncated DSACPC solar collector had 57 �C (39.1%)better fluid temperature operation compared to the truncatedSSACPC solar collector at higher abscissa values ranging from0.11 Km2 W�1and to 0.18 Km2 W�1.

In the medium temperature range with abscissa values greaterthan 0.065 Km2W�1, the truncated SSACPC solar collector wasmore efficient than the truncated DSACPC solar collector at abscis-sa values up to 0.09 Km2 W�1. However, at abscissa values greaterthan 0.09 Km2 W�1, the truncated DSACPC solar collector slightlyoutperforms the truncated SSACPC solar collector. Thus at a muchhigher medium temperature range, the truncated DSACPC solarcollector shows better thermal performance and would be a moresuitable choice compared to truncated SSACPC solar collectors forpowering double-effect solar absorption refrigeration systems,needing much higher input temperatures (above 90 �C) and otherprocess heat applications. The heat losses coefficients realised bythe truncated SSACPC solar collector was 1.3 Wm�2 K�1 (34.9%)higher than that attained by the truncated DSACPC solar collector.The lower thermal losses of the DSACPC solar collector resultedfrom the effective use of the absorber (illumination and collectionof incident solar radiations on both sides of the absorber).

The external statistical uncertainty (based on measurementequipment) and internal statistical uncertainty (based on experi-mental data) for optical efficiency and heat loss coefficient werecalculated based on the formulas reported by Shukla et al. [26]and an overall statistical uncertainty of 5.42% was released. Thedetermination of the incident angle modifier as a function of trans-

20

000 0.1200 0.1400 0.1600 0.1800 0.2000

mb)/Gave

Efficiency-Truncated DSACPC [0°]

Linear (Efficiency-Truncated DSACPC [0°])

ated SSACPC and DSACPC solar collectors with solar radiation perpendicular to the

re area.

ce

ciency (%) Heat loss coefficient (Wm�2 K�1) Statistical uncertainties (%)

3.55 5.422.31

y = -0.0004x 2 + 4E-17x + 1.0809R2 = 0.9782

y = -0.0004x 2 - 1E-17x + 0.9463R2 = 0.9774

0

0.2

0.4

0.6

0.8

1

1.2

-40 -30 -20 -10 0 10 20 30 40

Transverse angle (o)

IAM

Experimental IAM-Truncated SSACPC Experimental IAM-Truncated DSACPCPoly. (Experimental IAM-Truncated SSACPC) Poly. (Experimental IAM-Truncated DSACPC)

Fig. 10. Incident angle modifier (IAM) as a function of transverse angle for the truncated SSACPC and DSACPC solar collectors.

D.N. Nkwetta, M. Smyth / Applied Energy 98 (2012) 22–32 31

verse angle is illustrated in Fig. 10. Both collectors showed veryclose IAM values of 1 at a transverse angle of 0� but decreased withan increase in transverse angles with a minor variation recorded bythe truncated SSACPC at 10� transverse angle. The truncatedSSACPC shows an increase in IAM from transverse angle of 0–10�and then drops off whereas the truncated DSACPC shows a contin-uous drop off in IAM with an increase in transverse angle.

The decrease in the IAM is more noticeable in the DSACPC com-pared to SSACPC solar collector dropping off from 1 to 0.32 than 1to 0.44, respectively. These peak IAM values are in agreement withthe energy collection values recorded by both the SSACPC andDSACPC solar collectors. The decrease in incident angle modifierfor both solar collectors can be attributed to an increase in pathlength of the incoming rays with resultant increase multiple reflec-tions, reduced projected exposed absorber area and increase inrays missing the absorber.

Lower than expected efficiencies realised by the two systemsresulted from the partial vacuum loss suffered by the modifieddouble-sided absorber evacuated tube heat pipe solar collector.In addition, the prototype DSACPC solar collector suffered moreheat losses with resulting lower efficiencies accounted for by thepoor nature of the MAXORB foil coating on the modified double-sided absorber evacuated tube heat pipe compared to the proto-type SSACPC solar collector where only one side was used. Theadditional reflection of solar radiation between adjacent tubes(mutual scattering between adjacent tubes) was not possible dueto the use of the concentrators. Optical efficiencies experimentallydetermined were also lower compared to those theoretically pred-icated from ray trace simulation.

This resulted from slight reflector and absorber fabrication er-rors, light spectral distribution (5–10%) and non-collimation effect(15–20%) from the solar simulator. However, the efficiency pat-terns with respect to system profile are the same as those deter-mined optically.

5. Conclusion

By designing, fabricating, installing and experimentally investi-gating and comparing the temperatures, energy collection rates

and collection efficiencies for the SSACPC and DSACPC solar collec-tors with a concentration ratio of 1.85, it was found out that anoverall improvement of 3.6 �C (42.4%) in average outlet and inletfluid temperature differential was realised by the truncatedDSACPC compared to the truncated SSACPC at a transverse angleof 0�.

In total, up to 1.5 MJ (46%) and 0.48 MJ (46.6%) more energy wascollected by the truncated DSACPC collector over the truncatedSSACPC solar collector at transverse angles of 0�and 40�, respec-tively. Optical efficiency and overall heat loss coefficients of59.8% and 3.55 Wm�2 K�1 and 57.2% and 2.31 Wm�2 K�1 weredetermined for the SSACPC and DSACPC, respectively.

The 1.24 Wm�2 K�1 (34.9%) higher heat loss coefficient for theSSACPC collector is primarily due to the back losses from the absor-ber. Reflector truncation reduces the size of the reflector and re-lated reflector losses whilst concentration reduces the number ofthe evacuated tube heat pipe collectors needed to attain highertemperatures.

The use of SSACPC and DSACPC solar collectors is seen to be fea-sible for integrating solar thermal energy into buildings for heatingdemands as well as greater potential for solar cooling due to bettertemperature improvement, higher outlet and inlet fluid tempera-ture differential and substantial improvement in thermal perfor-mance (lower heat loss coefficient).

The result trends shows that testing these systems at mediumto high temperature range, the SSACPC solar collector array willbe potentially more suitable and an economical option for power-ing single-effect solar absorption air-conditioning systems whilstthe DSACPC solar collector array will be potentially suited for dou-ble-effect solar absorption air-conditioning systems needing high-er generator inlet temperatures.

Acknowledgements

The authors wish to thank Charles Parson Energy ResearchAward for the financial assistance/scholarship awarded to the firstauthor for his PhD research period during which this work wasconducted.

32 D.N. Nkwetta, M. Smyth / Applied Energy 98 (2012) 22–32

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