Calcium Carbonate Scaling in Solar Domestic Hot Water Systems

The buildup of scale inside the potable water pipes in active solar domestic hot water (SDHW) systems located in regions with hard water has caused numerous problems, including disastrous collector freezing in open-loop systems. In order to understand the key variables involved and evaluate solutions, the scaling process has been modeled and applied to SDHW systems in a southwest climate withhard water. The model is structured in three steps: (1) collector temperature simulation; (2) equilibrium chemistry solution, and (3) crystallization rate model. The model predicts that destructive scale buildup rates of as much as 0.5 cm/yr for very hard water will occur in hot, sunny climates with hard water, consistent with field reports. For this extreme cse, the collector of a recirculationsystem or the heat exchanger of a drainback system would be totally plugged with scale in less than a year. Design guidelines deduced for open-loop systems include: avoid oversizing the collectors, undersizing th tank or installing one-tank systems. Lastly, general prevention measures are discussed.