FLOW RATES AND DRAW VARIABILITY IN SOLAR DOMESTIC HOT WATER USAGE

摘要

The performance of some solar domestic hot water systems (SDHWS) depends significantly upon hot-water draw flow rates and upon day-to-day variations in hot-water draw volume. Flow-rate dependence occurs for SDHWS that have immersed load-side heat exchangers, such as with unpressurized integral-collector-storage (UPICS) systems described in (1). Since the immersed heat exchanger effectiveness decreases with flow rate, more energy is left behind to be mostly lost overnight as flow rate increases, reducing annual efficiency. For similar reasons, UPICS performance is also sensitive to day-to-day variation in the daily load volume. Hot-water draw data available from an unrelated project were analyzed to determine appropriate averages for draw flow rate versus volume and for draw volume variability. The analysis showed that most draws occur below 8 liters per minute (1pm) (2 gallons per minute [gpm]), and that the standard deviation of weekday and weekend draw volume distributions is 33% and 40%, respectively.