An integrated collector/storage solar water heater was designed based on the principles of optics and heat transfer. The system has two horizontal cylindrical tanks made of galvanized — iron (0.8 mm thick), painted matt black on the exterior surface, with a capacity of 61.8 litres each. One of the tanks is located in the upper part while the other tank is located in the lower part of the system. Half of the upper tank was insulated, and a clear glass cover was fitted directly above the lower tank to allow incoming solar radiation reach the tanks. The transparent cover was inclined at 16° to the horizontal to optimize solar radiation collection at the test site, Malawi Polytechnic (15o 48′ S, 35o 02′ E), and the aperture size was 1.1 m2. Further, a stationary parabolic concentrating reflector with focal line along the axis of upper tank, was fitted below the tanks. Hard board and waste cotton were used as insulation materials. The waste cotton was sandwiched between a galvanized-iron sheet case (on the

outside) and hard board inside, with an aluminium foil forming the inner most layer of the bottom and vertical faces of the system. A schematic view of the system cross-section is presented in Fig. 1. The whole system weighed about 65.1 kg, with empty tanks, and the design details are presented in Table 1.

Fig. 1: Schematic presentation of an integrated collector storage solar water heater showing its cross-section (Not drawn to scale).

2. Experimentation

2.1 System mounting

The integrated collector/storage solar water heater was mounted on a horizontal concrete roof top (about 6 m above the ground), and it faced north at the Malawi Polytechnic (15° 48′ S, 35° 02′ E) in Blantyre, Malawi. The tanks were externally connected with insulated 12.7mm — diameter hose pipes : a) parallel to each other (P-connection), b) in series with one insulated

hose pipe from the top part of the lower tank to the bottom part of the upper tank (S1-tank interconnection), and c) in series with 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-interconnection). Outlets from the tanks were bent down to form U-shaped tubes before rising up into the expansion tank (E-tank) to avert back-flow of cold water from the tubes into the collector-storage tanks, during the periods of low insolation or at night. In addition, the arm of the U-tube adjacent to the collector-storage tank was insulated up to the lowest part of the U-section. Details of the experimental set up are shown in Fig. 2.

in the tanks. The ambient temperature was monitored by using a minimum and maximum mercury-in-glass thermometer placed in a room with louvered glass. The louvers were kept open to allow free circulation of air. Wind velocity was measured by using a Casella low — speed air meter (N 1462) while the intensity of global solar radiation was measured by a Kipp & Zonen pyranometer (CM 6B) mounted in the plane of the inclination of the transparent cover, and connected to a Kipp & Zonen solar integrator (CC 14). The water heating process was monitored from 06:00 to 17:00 hrs each day, and hot water was stored from 17:00 to 06:hrs the next day. These experiments were conducted between September and December 2003.

3.3 Data analysis

The overall mean temperatures of water at the beginning (Toi) and end (Toe) of the solar collection processes were used to calculate the total amount of heat (Qw) absorbed by water when the amount solar energy (QR) falls on the aperture of the system surface (Aa), within a period of time t = ti to t = te. In this study, the mean daily collection efficiency (pc) was calculated as follows (Tripanagnostopoulos et al., 2002):

TOC o "1-5" h z Pc = Qw/Qr (1a)

Qw = MCpw (T0e — T0i), (1b)

Qr = Aa (He — Hi) (1c)

where Cpw = specific heat capacity of water at constant pressure,

Hi = total radiation received per unit area from sunrise to t= ti and He = total radiation received per unit area from sunrise to t = te,

M = mass of water, Toe = 0.25 (TLbe + TLte +Tube + Tute), and

T0i = °.25 (TLbi + TLti +Tubi+ Tuti).

The efficiency of heat retention (pr) is given by (Smyth et al., 2003):

hr = (Tof — Tan)/( Toe-Tan) (2)

This is an overall system efficiency of heat retention.