Water Journal : Water Journal May 2011
demand management technical features 74 MAY 2011 water Table 3 shows the shutdown energy consumption for each pump and controller combination tested in the study, for each event. Pumping energy consumption For domestic rainwater harvesting systems with typical end use profiles (toilet flushing, washing machines, outdoor taps), the pumping energy is the most significant component of energy consumption. The pumping energy consumption (expressed in terms of specific energy) over a range of flow rates for a selection of the pumps included in the study is shown in Figure 4. The lower the specific energy, the more energy efficient a pump is. The specific pumping energy consumption was found to vary according to the flow rate. The greater the flow of water, the more efficient the pump becomes. This is because pump motors were found to have a very similar power draw regardless of the flow rate. Surprisingly, it was also noted that varying the pressure within practical limits (20 to 45kPa) had little impact on pumping energy. The graph in Figure 5 compares the specific pumping energy for two flow rates typically seen in domestic rainwater systems for the systems tested. Figure 5 shows a general trend in terms of a pump-by-pump comparison -- smaller pumps (in terms of pump motor power) typically have a lower specific energy use for a given flow rate, though there are some exceptions to this. This is essentially because smaller pumps with a lower power rating use less energy to achieve a given (low) flow rate than larger pumps with a higher power rating. The higher energy consumption to deliver a given volume of water at a lower flow rate leads to interesting observations related to filling toilet cisterns. Some cistern valves allow a fast filling at a higher flow, while others permit a slow filling at much smaller flow rates only. To quantify the impact of this, toilets with two different cistern filling valves were tested. The first toilet had an older float arm valve. The other had a newer sinker valve (Figures 6 and 7). Figures 8 and 9 (both of which are on the same time scale on the x-axis) show the total water supply (in black) and the instantaneous power draw (in blue) for a full (6L) toilet flush. Each was filled using a Davey HP45-05 with a Torium pressure control switch. Note that the second chart shows two toilet flush events. The time scale is the same though. These figures clearly show that the time taken to fill the toilet with the sinker valve (fast-filling, Figure 9) was far less than the Table 3: System shutdown energy per event for various pump-switch/controller combinations. System (Pump and Controller) System Shutdown Energy Cons. (kWh) Davey 42A/B (submersible) with Rainbank 0.00038 Davey HM60-10 with Rainbank 0.00054 Davey HM60-10 with Speedman @ 4bar 0.00074 Davey HM60-10 with Speedman @ 6bar 0.00095 Davey HP45-05 with PressControl 0.00157 Davey HP45-05 with Torium 0.00200 Leader with Pressure Switch 0.00184 Onga SMH35 with WaterSwitch 0.00132 Onga SMH55 with WaterSwitch 0.00184 Onga TankBuddy (sub.) with WaterSwitch 0.00189 SilverStorm 800W with PS-01B/1 PS 0.00174 Figure 4: Specific pumping energy for different flow rates. Figure 5: Specific pumping energy comparison for 8 and 17L/min. Figure 7: Toilet with sinker valve. Figure 6: Toilet with float arm valve. Figure 8: Water supply and power consumption profile: Slow-filling cistern. Figure 9: Water supply and power consumption profile: Fast-filling cistern.
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