A. Madhlopa

Department of Physics & Biochemical Sciences, Malawi Polytechnic,

P/Bag 303, Blantyre 3, Malawi
E-mail: amadhlopa@poly. ac. mw

Abstract

Integrated collector-storage solar water (ICSSW) heaters are generally more cost effective than systems with separate collector and storage units. However, ICS solar water heaters lose a substantial proportion of the captured heat during periods of low insolation or non­collection. In this study, an ICS solar water heater with two horizontal cylindrical tanks (made of galvanized steel, with a capacity of 61.8 litres each) was designed, constructed and tested. The two tanks were parallel to each other, and horizontally and vertically spaced out, with the lower tank fitted directly below a glass cover, and half of the upper tank insulated. In addition, a truncated stationary parabolic concentrator was fitted below the tanks, with its focal line along the axis of the upper tank. The system was installed outdoor (facing north) on top of a horizontal flat concrete roof at the Malawi Polytechnic (15° 48′ S, 35° 02′ E) in Malawi. It was tested with the two tanks aligned east-west, and in parallel (P) and series (S) connections.

For the series-tank interconnection, the two tanks were connected with: a) one insulated hose pipe (12.7 mm diameter) from the top part of the lower tank to the bottom part of the upper tank (S1-tank interconnection) and b) two insulated hose pipes of which one pipe linked the bottom part of the lower tank to the bottom part of the upper tank while the other pipe linked the top part of the lower tank to the top part of the upper tank (S2-tank interconnection). The solar collection process was monitored from 06:00 to 17:00 hrs local time, and hot water was stored from 17:00 to 06:00 hrs the next day, without any draw-off for a sequence of 4 days. Meteorological measurements were taken during the day (06:00 to 17:00 hrs). Results show that the S2-tank interconnection yielded the most satisfactory results. In this connection configuration, the system stored 28.7 to 39.7 % of the collected thermal energy for use the next morning, comparable with results obtained from previous studies conducted elsewhere. Other results are presented and discussed.

1. Introduction

Integrated collector storage solar water (ICSSW) heaters combine solar collector and water storage tank in one unit and are cost effective (Garg et al., 1997). Nevertheless, ICSSW systems have higher top heat loss during the night when ambient temperature is relatively low. So, previous ICSSW heater designs focused on reducing top heat loss by including heat retention mechanisms either at the aperture, within the collector cavity or on the ICSSW heater vessel surface.

An insulated opaque lid that was removed in the morning and replaced at night was very effective but required manual removal and replacement every day (Garg, 1975; Baer, 1975). Selective coating materials (Stickney and Nagy, 1980), transparent insulating materials (Schmidt et al., 1988) and multi-glazed units (Bishop, 1983) have been used with varying
extents of success, but unfortunately they lead to an increase in the cost of the unit and in some cases to a reduction in the capture of solar radiation. Kalogirou (1999) introduced a primary cylinder between the main cylindrical tank and the glass cover, with cold water introduced directly into the primary tank which fed the main tank. It was concluded that this modification greatly improved the system draw-off characteristics. Tripanagnostopoulos et al. (1999) designed ICSSW heaters with two cylindrical storage tanks connected in series and incorporated in a stationary asymmetric compound parabolic concentrator. They found that the systems operated efficiently and were suitable for practical applications. Later, Tripanagnostopoulos et al. (2002) developed four ICS solar water heaters with stationary compound parabolic concentrating (CPC) reflectors. The systems consisted of single and double cylindrical tanks placed in symmetric and truncated CPC troughs. These authors used two cylindrical tanks, connected in series from the top part of the lower tank to the bottom part of the upper tank, to increase temperature stratification. They found that asymmetric CPC reflectors contributed to lower thermal losses and that the two connected in series cylindrical tanks resulted in effective water temperature stratification. Moreover, the water temperature in the top part of the lower tank was higher than the water temperature in the bottom part of the upper tank (as shown in Figs 17 and 18 of this reference). For their double-tank system models, natural convection of heat from the lower tank to the upper tank would increase the efficiency of thermal storage because a larger proportion of the top part of the upper tanks was thermally insulated. In addition, hotter water from the top part of the lower tank would mix with colder water from the bottom part of the upper tank during periods of charging and discharging, resulting in loss of stratification.

The objective of this study was to assess the performance of a simple ICSSW heater with two cylindrical horizontal tanks incorporated in a stationary parabolic concentrating reflector. Half of the top part of the upper tank was thermally insulated while the lower tank was bare. The system was tested with tanks connected in parallel (P-connection) and series configurations, without draw-off. For the series-tank interconnection, the two tanks were connected with: a) one insulated hose pipe (12.7 mm diameter) from the top part of the lower tank to the bottom part of the upper tank (S1-tank interconnection) and b) two insulated hose pipes (12.7 mm diameter) of which one pipe linked the bottom part of the lower tank to the bottom part of the upper tank while the other pipe linked the top part of the lower tank to the top part of the upper tank (S2-tank interconnection). The S2-tank interconnection yielded the best performance results.