IntroductionExperimental (Davis and Bartera, 1975;Lavan and Thompson, 1977;Fanney and Klein, 1988) and analytical (Sharp and Loehrke, 1979;Wuestling, 1983 ;Jesch and Braun, 1984;Wuestling et al., 1985) investigations show that thermal stratification of the solar storage tank enhances the performance of solar domestic hot water (SDHW) systems . Maximum possible thermal performance is obtained when no mixing occurs in the solar storage tank. In this case, the coldest stored fluid is circulated through the collector(s). Heated water is then returned to the tank without inducing mixing of fluid layers with different temperatures. During water draws to the load, the hottest water in the tank is withdrawn from the tank and replaced (again without mixing) with cold water from the mains water supply. Minimum performance is obtained when fluid streams entering the storage tank mix completely (and instantaneously) with water in the tank . In this case, water circulated to the collector is at the same temperature as that supplied to the load.Loss of stratification of the thermal storage tank results from convective mixing, both forced and natural , and, to a lesser extent, from conduction between hot and cold fluid layers. Maintaining thermal stratification requires inhibition of mixing in the tank. Mixing depends on the design of the tank and the operating conditions (flow rate and temperature of incoming fluid streams and the temperature distribution in the tank). Forced convection mixing is due to the momentum of the fluid streams entering the storage tank and depends on the flow rate of the entering stream and the design of the inlet. Momentum diffusers have been employed to reduce mixing due to jet entrainment.
The level of thermal stratification that can be maintained in forced-flow, direct solar water-heating systems using a fabric manifold is studied in a 372-liter tank with an inlet flow rate of 0.07 1/s. A rib-knit, lightweight, spun-orlon acrylic is the most effective manifold material in a comparative study of 13 synthetic and natural fabrics. Thermal stratification (or more appropriately mixing) in the tank equipped with this acrylic manifold is compared to the level of stratification achieved using a rigid, porous manifold and a conventional drop-tube inlet. Initial tank temperature profile, temperature of the water entering the tank, and test duration are varied in three testing schemes. Comparison of vertical temperature profiles and height-weighted energy stored in the tank indicate that under realistic operating conditions, the fabric manifold is 4 percent more effective than the rigid manifold, and 48 percent more effective than the conventional drop-tube inlet.
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