Analysis and Simulation of a Two-Phase Self-Pumping Water Heater

Abstract

IntroductionAlthough self-pumping, two-phase solar heating systems have numerous advantages over mechanically pumped, sensible heat systems, experimental studies indicate that the design could be modified to improve thermal performance, reliability, and economics Neeper, 1985, 1986;. Efforts at optimizing the design and efficiency of these systems have been largely heuristic. Neither the approximate algebraic model of a single accumulator system presented by Neeper (1988) nor a more detailed simulation developed by Gari and Fathalah (1986) applies to the dual accumulator design.System operation is cyclic, progressing through a run or heat collection phase, a pressurizing phase, and a pump phase. During the run phase, shown in Fig. 1, liquid refrigerant (R-113) is gravity fed to the collector from the upper accumulator. Vapor travels downward to the condenser and liquid refrigerant collects in the lower accumulator tank. After a preset quantity of refrigerant (determined by the setting of the lower float switch in the upper accumulator) condenses, solenoid valve No . l closes, solenoid valve No. 2 opens, and the pressurizing phase shown in Fig. 2 begins. Since vapor is prevented from entering the condenser, pressures and temperatures in the collector and lower accumulator increase. When the pressure in the lower accumulator exceeds the pressure in the upper accumulator by an amount equivalent to the hydrostatic pressure of the liquid column in the return pipe, the pump phase

Second-Law Analysis of a Two-Phase Self-Pumping Solar Water Heater

“…The preceding integrals are evaluated numerically for a single day using heat transfer rates and temperatures predicted with a first-law simulation (Walker and Davidson, 1990) of an optimized system operating under SRCC rating conditions (Davidson and Walker, 1991). The simulation time step is ten seconds.…”

“…Prior experimental studies (Hedstrom and Neeper, 1985;DeBeni and Friesen, 1987;Davidson et al, 1989) and theoretical first-law studies (Walker and Davidson, 1990;Davidson and Walker, 1991) have been conducted, but a second-law assessment of entropy generation is not available. Analyses based on the second Jaw have gained acceptance as the proper Contributed by the Solar Energy Division of THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS for publication in the ASME JOIJRNAL OF SOLAR ENERGY ENGINEERING.…”

Design Optimization of a Two-Phase Solar Water Heater Using R-123

“…Daily operation is modeled using a First Law numerical simulation (Walker and Davidson, 1990). The principles of conservation of mass and energy are applied to groups of components assumed to be at the same temperature.…”

“…No mechanical pump, solar controller, nor external power is required and freezing and scaling are eliminated while enhancing heat transfer with two-phase operation. Both single and dual accumulator downward vapor transport solar heating systems have been investigated (DeBeni and Friesen, 1987;Neeper, 1988;Davidson et al, 1989;Walker and Davidson, 1990), but design efforts have not addressed maximizing thermal performance cost effectively. Nor has operation with new environmentally benign refrigerants been considered.…”

“…Prior experimental (Davidson et al, 1989) and analytical studies (Walker and Davidson, 1990) considered a single collector system operating with R-113, an external condenser, and electrical valving. The present optimization analysis is based on an improved design using the alternative refrigerant R-123 Thermal and economic performances are studied for the Solar Rating and Certification Corporation (SRCC) rating procedure (SRCC, 1984) in terms of four design parameters: collector area, in-tank condenser area, water storage volume and refrigerant volume evaporated and condensed in one cycle.…”