Solar Energy Vol. 5 I, No, 5, pp. 339-347, 1993 0038-092X/93 $6.00 + .00 Printed in the U.S.A. Copyright © 1993 Pergamon Press Ltd. MEASURED PERFORMANCE OF A SOLAR WATER HEATER WITH A PARALLEL TUBE POLYMER ABSORBER W. M. K. VAN NIEKERK* and T. B. SCHEFFLER* University of Pretoria, 0002 Pretoria, South Africa Abstract--We performed calibrated side-by-sidemeasurements of the thermal performance of solar water heaters with spaced polypropylene tube absorbers in identical glazed and insulated collector boxes. This was to determine the influence of tube spacing and tube height above the back insulation. A low flow rate (4.32 g s-' m -2) was used. As the number of tubes (spaced over the same aperture) is increased by 76.3% (from 38 to 67), the mean mid day efficiencyincreases by only 12.9% (7.4 percentage points), from 57.2% to 64.6%. It may therefore be more cost effective to use fewer, more widely spaced tubes. As tube height h above the insulation increased from the lowest value (3 ram) to the next (8 mm), the thermal efficiency dropped 1.4 percentage points. From h = 8 to 28 mm, the efficiencywas almost independent ofh. 1. INTRODUCTION Copper is a popular construction material for absorbers for domestic solar water heaters because of its high thermal conductivity, mechanical strength, and resis- tance against corrosion. Other metals also used are stainless steel, aluminium, and mild steel [l]. Metal absorbers are, however, prone to scaling in areas with hard water, and (in many designs) to damage by freez- ing, and (depending on the design) may be rather heavy. Absorbers made out of polymers dominate in ap- plications where the operating temperatures are lower: the heating of swimming pools and heat pump appli- cations. Polyolefin absorbers are not prone to scaling, for three reasons: a large difference in the thermal ex- pansion coefficients of scale and polyolefins, the well- known nonstick property of polyolefins, and a much larger "wetted surface" over which formation of scale is distributed. In practice, a limy sludge, rather than scale, forms. This is usually easily rinsed out. (Scale, should it ever form, can also be dissolved with hydro- chloric (pool) acid, which does not attack polyolefins-- provided that the absorbers have been isolated or re- moved from the system). Polymers are light and their use can reduce the cost of the absorber[2]. The lower strength of polymers (especially at elevated temperatures) can be a disad- vantage. This dictates a low operatingpressure. It makes this type of absorber especially suited to variable vol- ume storage [ 3,4 ] (usually with nonpressurized tanks) or for application in third world environments where dwellings are often not equipped with piped water. The most serious disadvantage of black polyolefin absorbers inside glazed insulated collector boxes is their embrittlement by prolonged exposure to high temper- atures. The stabilizers compounded into the polyolefin play a key role in determining what temperatures are acceptable, and for how long. (Carbon black, the most common and effective UV stabilizer, interferes with * ISES member. stability against high temperatures. A glass cover ab- sorbs the "harder" or shorter wave UV, so that UV resistance is of less importance than thermal stabili- zation). An apparent drawback of polymers is the poor thermal conductivity as compared to metals. By using a design where all or most of the irradiated surface of the absorber is in contact with the water, the efficiency of a polymer absorber can match that of a metal ab- sorber of similar design [ 5 ], and surpass that of con- ventional metal absorbers [ 2,6]. The polymer absorbers are usually made from rib connected parallel plates as in Fig. 1 or from extruded sections with tubes joined at the sides (Figs. 2 and 3). 2. THE COLLECTOR: ANTICIPATED EFFECTS OF TUBE SPACING AND HEIGHT ON THERMAL PERFORMANCE The present study (and Heliocol and Suntap ab- sorbers, shown in cross section in Fig. 4, and as pho- tographs in Fig. 5 ) involves an absorber with separate parallel black polypropylene tubes (of diameter d and intertube spacing s) with spacer ribs at intervals. The tubes are at a small distance (or height h) from the matt black upper surface of the thermally insulated back of the collector box. We assume that the tubes slope upward (at the collector tilt angle) between hor- izontal manifolds. In South Africa unglazed Heliocol and Suntap ab- sorbers are used in the pool heating industry, and glazed insulated Heliocol absorbers are used for producing hot water. We now consider the possible effects of the spacing s and height h on the thermal performance of the col- lector. For the sake of definiteness (in terms of angle of incidence), we consider only direct solar radiation. (Our measurements were also restricted to clear cloudless days, which are frequent in the local climate). (a) The flow velocity of the water increases with inter- tube spacing. However, in the laminar flow regime, which clearly applies to this single pass system with its closely spaced multitude of parallel riser tubes, 339