In designing a geothermal hot water supply system, we have to consider how to correct the unbalance between hot water demand and supply. This unbalance is caused by various random and/or cyclic demand variations. There are two methods to correct this unbalance. One is the control of supply and another is the installation of a buffer tank. The former is used to correct the large and long-term unbalance, and the latter is used to correct the small and short-term unbalance during a period when total demand is nearly balanced with total supply. In this paper we consider a basic system with one demand area and three systems with two demand areas; which have different types of buffer tanks, and present the mathematical models of the system behaviors. The systems are as follows:System I: one supply, one demand and one buffer tank, System II: one supply, two demands and two independent buffer tanks, System III: one supply, two demands and one co-used buffer tank, System IV: one supply, two demands, one co-used buffer tank and two independent buffer tanks.Furthermore, we analyze the effect of buffer capacity on the shortage of hot water supply by a simulation method under the condition that supply is constant and demand has random and/or cyclic variations, and compare System II, III and IV. From the simulation analysis, we have reached the following conclusions, where the random variation is given by the normal distribution with the standard deviation σ
h and the mean of shortage of hot water supply per day is denoted by UD/d:(1) For random variations in System I the decrease of UD/d per unit buffer capacity is large when buffer capacity is small, but it becomes smaller as buffer cap acity increases. When (buffer capacity)/σ
h is constant, UD/d/σ
h is constant.(2) For random variations System III is more economical and effective than System II. Especially whenthe co-used buffer capacity M is large, UD/d/(the number of demand areas) is close to the value which isobtained by installing independent buffer capacity M in each demand area (System II).(3) When buffer tanks are installed in some places, UD/d decreases if it is possible that the surplus hot waterin one area is diverted to another area where the hot water supply is insufficient . When total buffer capacityin System IV is constant and large, the distribution method of buffer capacity has little effect on UD/d .
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