Solar PV water pumping study - FINAL REPORT ... - UNDP, Namibia

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Feasibility Assessment for the Replacement of Diesel Pumps with Solar Pumps 3.1.2 Operating range FINAL REPORT: September 2006 The study covers water supply systems that are able to deliver over a head of up to 200m with a maximum daily delivery of 60m 3 /day for the PVP option and a maximum hourly delivery of 12m 3 /h for the diesel pumping option. The head used in the calculations refers to the total dynamic head, and therefore includes the possible drawdown in the borehole. Comparing pumps is slightly more complex than comparing energy systems since a pump delivers a flowrate over a head depending on the array size whereas an energy system delivers energy (kWh/day) for a particular array size. A pump therefore has two parameters, flowrate and vertical lift. A useful comparison basis for pumps is therefore given by the value referred to as the daily hydraulic load. This is stated as: Daily hydraulic load [m 4 /day] = daily flowrate [m 3 /day] x head [m] In the ideal case this relationship is a constant. For example the hydraulic load at 50m head and 20m 3 /day is the same as the hydraulic load at 20m and 50m 3 /day, i.e. 1,000m 4 /day. Every PV pump reaches a particular hydraulic limit but that value depends on the maximum allowable array size and on the pump’s ability to meet the various heads at more or less constant efficiency. Examples of different constant hydraulic load are shown in Figure 3.2. Head [m] 210 180 150 120 90 60 30 0 0 50 100 150 200 250 300 Daily flow rate [m3/day] Figure 3.2: Example of hydraulic load graphs 1,000m4/day 2,000m4/day 3,000m4/day In reality a PVP of fixed array size will not follow the line exactly because of increasing friction head (more losses) with deeper boreholes and due to variations in the characteristics of the specific pump used. Refer to Figure 2.1. The results of the costing analysis will make use of the constant hydraulic load line to indicate thresholds for different viabilities of PVPs and diesel pumping ranges. 3.1.3 Sizing In order to create a basis for comparison between PVPs and DPs the performance of the pumping options are linked to dedicated delivery heads. These are: Page 16 of 76
Feasibility Assessment for the Replacement of Diesel Pumps with Solar Pumps FINAL REPORT: September 2006 Table 3.1: Operating heads for pumping comparison Operating point Head [m] Operating point Head [m] 1 10 8 100 2 20 9 120 3 30 10 140 4 40 11 160 5 50 12 180 6 60 13 200 7 80 For each of the above heads a water delivery range from 3 to 60 m 3 per day with changing array size is considered. Throughout the study the delivery of the PV pump system is based on the daily delivery whereas the diesel pump system is based on the hourly flowrate. To compare PVP and DP at a particular head a daily delivery is used. While PVP delivery is daily, DPs operate at higher delivery rates for less hours so the daily flowrate is converted to an hourly flowrate for DPs (refer to section 3.1.3.2). 3.1.3.1 Solar PV Pump The sizing of the PVP systems is based on the performance charts provided by the manufacturers. The performance charts for irradiation levels of 6kWh/m 2 /day have been selected, which is a representative average of Namibia’s solar resource. For each of the heads listed in Table 3.1 a corresponding list of daily delivery in conjunction with array size (Watt peak) has been compiled. That information has been entered into the spreadsheet costing tool. It is possible to change the irradiation levels between 5 and 7kWh/m 2 /day and to introduce a tracking array for modelling specific locations as well as the use of a passive or active tracking array. The cost model increases/decreases the array size proportionally for decreased/increased irradiance levels. This is therefore a linear adjustment and the error introduced through the non-linear efficiency changes of the PVP is minor since only a 16% variation is allowed. The use of a tracker similarly reduces the required array size to pump the daily water requirements since the tracker has extended operating hours at peak sun conditions. In reality higher irradiance levels and tracking would increase the daily flowrate however, the daily flowrate and the head form the basis of the comparison between PVP and DP and can therefore not be changed. 3.1.3.2 Diesel Pump The diesel pump is sized by calculating the actual power required to lift water, i.e.: Hydraulic power [W] = ρ x g x head x flow, where ρ = density of water [kg/m 3 ] g = gravitational acceleration [m/s 2 ] head = total dynamic head [meters] flow = flowrate [m 3 /s] Page 17 of 76
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