Water Journal : Water Journal May 2011
demand management water MAY 2011 75 time for the toilet with the float arm valve. The characteristics of each of these events are described in Table 4. Thus the float arm valve causes the cistern to fill 3.5 times slower than the sinker valve, leading to an energy consumption almost three times higher (Table 4). This demonstrates the importance of flow rates. Figure 5 appears to suggest a clear order from least to most energy-efficient pumps. Since the pump size in terms of power rating in Watts (W) generally increases from left to right, Figure 5 seems to suggest that smaller pumps are more energy efficient than larger pumps for domestic rainwater harvesting purposes. However, further testing indicated that this was not necessarily the case. Figure 10 shows the total pumping energy for a standard toilet flush for each of the pumps studied. The specific energy use of each pump at a flow rate of 8L/min is also shown. Figure 10: Specific pumping energy comparison -- toilet flushing. Figure 10 suggests a quite different pump energy efficiency hierarchy than Figure 5. It shows that while some larger pumps, such as the Davey HM60-10 @6bar, have a higher specific energy for a given flow rate compared to smaller pumps such as the Onga SMH35, the total amount of pumping energy they use to fill a toilet cistern is actually lower. This is because the larger pump is capable of pumping water into the toilet cistern at a higher flow rate than the smaller pump. The Davey HM60-10 filled the toilet cistern at a flow rate of 13.5L/ min in under 30s while the Onga SMH35 filled it at only 5.1L/min, taking 70s. Thus a larger pump can fill the toilet cistern in a shorter time meaning that the pump, although its power draw is much greater, runs for much less time and consumes less total energy. This can be further demonstrated by comparing the duty points on the pump curves for the two pumps (Figure 11). Initially, Figure 11 seems to confirm that the Onga SMH35 uses less energy than the Davey HM60-10. However, when considering the actual operating points for the filling of the cistern (green and blue dots), the results reverse. The Onga SMH35 operates at a specific energy use close to 1.25kWh/kL while the Davey HM60-10 runs at just below 1.1kWh/kL. Figure 11: Pump operating points for toilet cistern filling. Thus the specific energy use for filling a toilet cistern is some 10% lower for the larger Davey HMN60-10 than for the smaller Onga SMH35. Hence the simple conclusion that smaller pumps are more energy efficient in the domestic rainwater harvesting scenario is not always true. This suggests the following conclusion: For a pump to be judged as energy efficient for the purposes of domestic rainwater supply, it must be energy efficient both in terms of specific energy for any given flow rate and for transferring a discrete volume of water such as when filling a toilet cistern. This will allow it to operate efficiently for both steady flow rate applications (such garden irrigation or hosing) and for applications where a given volume of water is transferred (such as filling a toilet cistern or a washing machine). Pressure vessels Pressure vessels containing 5L--10L of pressurised water potentially have the ability to supply water to small-volume (1L--2L) end uses without the need for the pump to turn on, thus reducing pump start-up and system shutdown energy consumption. However, the tests undertaken with a pressure vessel suggested that this did not occur. When opening a tap for only a short period of time, the pressure vessel did not prevent the pump from turning on. Discussions with the pressure vessel's supplier (Davey) indicated that this was due to the large drop in system pressure caused by opening the tap, making the pressure switch turn the pump on regardless of the presence of the vessel. Filtration Many domestic rainwater harvesting systems are equipped with filtration systems. Testing of system responses both with and without these filters showed that when the filter cartridges are dirty, they can reduce pumping flow rates and thus push specific pumping energy consumption up -- in the study, a filter which had been used to only a low-mid range level reduced energy efficiency by 3%. Thus the cartridges Table 4: Toilet valve comparison. Float Arm Valve Toilet Sinker Valve Toilet Volume of water supplied by pump 6.3L 6.2L Cistern filling time 139s 40s Average water flow rate 2.7L/min 9.4L/min Total energy used 0.0270kWh 0.0092kWh Overall system specific energy use 4.3kWh/kL 1.5kWh/kL Figure 12: Energy use breakdown by component -- toilet flush. Figure 13: Energy use breakdown by component -- full day's use.
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