Experimental analysis and performance of an asymmetric inverted absorber compound parabolic concentrating solar collector at various absorber gap configurations

Renewable Energy, Vol. 10, No. 213, pp. 235-238,

Copyright

Pergamon

PII: s0960-1481(96)ooo71-7

1997 0 1996 Published by Elsevier Science Ltd Printed in Great Britain. All rights reserved 096&1481/97 $15.00+0.00

EXPERIMENTAL ANALYSlS AND PERFORMANCE OF AN ASYMMETRIC INVERTED ABSORBER COMPOUND PARABOLtC CONCENTRATING SOLAR COLLECTOR AT VARIOUS ABSORBER GAP CONRGURATlONS A. FAROUK KOTHDtWALA, P.C. EAMES and 6. NORTON PROBE: centre for Performance Research On the Built Environment, School of the Built Environment, Faculty of Engineering, University of Ulster, Newtownabbey, Co. Antrim, BT37 OQB, NJreland.

ABSTRACT The performance of an asymmetric inverted absorber line axis compound parabolic concentrating collector (IACPC) was evaluated under a solar simulator. The lACPC had a concentration ratio of 2 and a maximum length-to-width ratio of 2.5 with provision to adapt from an untruncated collector to a half height truncated version. The selectively surfaced absorber/receiver was copper sheeting onto which was reverse-bonded tubing along the long axis of the IACPC. High reflectance film covered the aluminium substrate of the collector. Water was the heat transfer fluid flowing in the system. Stagnation and efficiency tests were undertaken for the full and truncated versions of the IACPC at various absorber gap heights to determine the optimum performance and absorber gap configuration. Overall performance is marginally better for the higher gap height absorber configurations than for the lower at the range of coUector water inlet temperatures of 20 - 70 “C. Copyright 0 1996 Published

by Elsevier Science Ltd. KEYWORDS Asymmetric inverted absorber compound parabolic concentrating solar collector: experimental analysis and performance; absorber gap height. INTRODUCTION Despite their often identified potential advantages (Mills and Giutronich, 1978), few practical realisations of asymmetric line-axis CPCs exist and few studies of their experimental performance have been undertaken. Results from an experimental investigation employing an IACPC design, tested under laboratory solar simulator conditions, is presented. EXPERIMENTATION The IACPC tested was a CPC with an augmented reverse flat plate collector (circle version) possessing an extended parallel section to house an adjustable absorber (Fig. 1). The characteristics of lACPC I.0 are given in Table 1. Tests were undertaken indoors under a solar simulator (Lo et al., 1993). This allowed ambient temperatures, flow rates, wind speed and insolation levels to be controlled. Radiation was incident at 37” to the aperture cover. The main components of the IACPC system consist of the solar simulator, the IACPC and the storage tank as shown in Fig. 2. The collector circuit is also illustrated along with the auxiliary heating circuit and the connected pumps and valves. When water is being heated in the absorber (collector circuit), valves 1 and 2 are open and pump A is in operation. During heating of the water tank, valves 3 and 4 are open and pump B is operating. Absorber configuration 1 (Fig. 1) is 25 mm above the base of the parallel section. Absorber configurations 2, 3, and 4 are a further 50, 100 and 150 mm respectively, above 1. The configuration chosen is held in place by two threaded bars fixed at each end of the wooden absorber frame. 235

A. FAROUK

236

KOTHDIWALA

Fig. 1 A schematic diagram of the inverted absorber under testing and analysis

compound

et al.

parabolic

concentrating

solar cotl%tOr

Table 1 Charactc istics of IACPC 1.0 IACPC 1.0 Concentration ratio, C Acceptance angle, OmuP 50% of full height

TruncaMn Full aperture width /m

Truncated aperture width Im Apelture length /m Absorber width /m

0.195

Gap height. d /mm

o-175

Reflector substrate

0.3mm thick aluminium sheeting

Reflecting surface

3M solar reflecting film, ECP-305, reftectance = 0.94 at air mass 2

Absorber substrate

0.5mm thick copper sheeting

Absorbing surface

Maxorb solar selective surface absorptance = 0.97, emfltance = 0.10

Absorber tube diameter

22mmoD.19mmiD

Absotier

80 mm thick polystyrene

insulation

Aperture material

3 mm thick transparent glass

Heat transfer medium

Water

Maximum axial length-to-aperture

width ratio

2.5 with provision to change from a fuil height untruncated collector to a half height truncated var.&on via a reflecting detachable section

Analysis and performance of an IACPC

237

*

Fig. 2 A schematic diagram of the experimental

1-1

IACPC System

Average insolation = 713.8 Wm.*

Fig. 3 Truncated IACPC absorber stagnation

I....

I

0.020

.

.

tet?IperStUmS

..I

., 0.030

.,

with and without water in the absorber

I ., 0.040

(T(- Lpu

, I,, 0.050

I.Mo

mw

Fig. 4 Experimental IACPC efficiencies for various absorber configurations with and without truncation

A. FAROUK KOTHDIWALA

238

et al.

EXPERIMENTAL TESTS AND RESULTS (i) Stagnation tests A 90 minute period was allowed so that steady state was attained under either of the experimental scenarios below at a particular absorber configuration (Fig.l):stagnant water was in the absorber the absorber was empty of water Fig. 3 shows the stagnation temperatures attained for each absorber configuration for scenario (a) and (b). Absorber temperatures vary from 144.2 to 157.1”C as the configuration is changed from 1 to 4 for scenario (a). For scenario (b), the temperature increases from 108.6”C (absorber configuration 1) to 109.2”C (absorber configuration 4). When water is present, it conducts heat from the absorber to the fittings and the ambient. Additionally, convection occurs within the water. (ii) Efficiency tests By employing a range of inlet water temperatures, the collector experimentally and the Hottel-Whillier-Bliss efficiency curves generated.

efficiency

was determined

Fig. 4 shows the performance of a truncated IACPC at the four absorber configurations and that of the full IACPC at absorber confiiuration 4. It can be seen that in general the efficiency of the IACPC increases with increasing gap height. Configurations 3 and 4 (truncated) are the more efficient configurations. Above 0.035 Km2W’, absorber configuration 3 is preferred over 4 suggesting that 3 is possibly at or near the optlmum gap height for this collector and its characteristics. The optimum gap height is a trade-off between optimum opt&f efficiency and‘rer&ed, prfmarffy convective, heat losses. CONCLUSIONS A number of conclusions may be drawn from the study:-

(i) (ii)

(iii)

Maximum stagnation temperatures of 109 and 157°C are attainable for this IACPC system with water and without water respectively in the absorber. In general, at water inlet temperatures in the range 20 - 7o”C, the efficiency of the IACPC increases with increasing gap height. Truncated configurations 4 and 3 are the more efficient configurations with one being preferred to the other depending on operating conditions. The most efficient full profile IACPC is that with absorber configuration 4.

ACKNOWLEDGEMENT - A University of Ulster Research Scholarship is acknowledged by A Farouk Kothdiwala and this work is supported by the International Scientific Cooperation Programme of the European Union. NOMENCLATURE C d iD I Z Tvnb T,

Concentration ratio Absorber gap height (m) Tube internal diameter (m) Average total insolation (Wm.‘) Tube external diameter (m) Ambient temperature (“C) Collector inlet temperature (’C)

REFERENCES Lo, S.N.G., P.C Eames, B. Norton and K.D. McHugh, (1993). The design and construction of line-axis and planar solar simulators, In: Budapest, Hungary, Vol2, pp. Mills, D.R. and J.E. Giutronich (1978). Asymmetrical non-imaging cylindrical solar concentrators. $c& Enemy, 20, 45-55.