Figure 6-4. Hydraulic equivalent load limits at annual average solar radiation energy levels of 3, 5, and 7 kWh/m2/d.
Economic comparisons were made between the Bergey 1500 type wind pumps, the AC and DC PV pumps, the diesel-driven mechanical pumps, a diesel generator pump, and a gasoline pump. The results are presented in Figures 6-5, 6-6, and 6-7. The Figure 6-5 evaluation is based on the average wind speed of 5.65 m/s for the wind pump, using fuel prices of $0.35/L for the diesel-driven mechanical pumps (in Ethiopia), and $0.51/L and 0.47/L for diesel genset and gasoline pumps (in Mexico), respectively. The average annual solar radiation used for this evaluation in Ethiopia is about 5.5 kWh/m2/d and in Mexico is about 5.8 kWh/m2/d. The Figure 6-6 evaluation is based on the fuel prices of $0.35/L for the diesel-driven mechanical pumps (in Ethiopia), and $0.51/L and 0.47/L for diesel genset and gasoline pumps (in Mexico), respectively. The average annual solar radiation used for this evaluation in Ethiopia is about 5.5 kWh/m2/d and in Mexico is about 5.8 kWh/m2/d.
The economic comparison shows that small DC PV pumps are the cheapest option for small applications. Such systems are ideal for low hydraulic equivalent loads (to about 600 m4/d), as shown in Figure 6-6. The Bergey 1500 wind pump is the second cheapest pumping option, which is competitive with diesel-driven mechanical pumps to about 60 meters total pumping head, based on the 5.65 m/s 5 years' wind data (1992-1997) from Bushland, Texas. These wind pumps are also competitive with the DC PV pumps to about 25 meters total pumping head.
The comparison shows that the mechanical engine-driven pumps are cheaper than the genset pump. The economic evaluation of diesel engines for mechanical and electrical water pumping applications shows that competitiveness depends on several factors. Generally, diesel-driven mechanical pumps are more competitive than genset pumps if direct coupling (rack and pinion or bolt and flange) is used between the engine and the pump because the power transmission efficiency is very high, near 98%. However, they may not be competitive if other power transmission methods (such as gear, pulley and belt drives, and feed screw transmissions) are used because such methods have lower power transmission efficiencies. The efficiency of the driving mechanism depends on the coupling ratio, which is the ratio of the driver torque (engine, motor, or generator) to the load torque. The transmission power loss is high when speed decreases substantially: power loss between the engine and the pump can be as low as 40%. The power transmission efficiency between the engine and electrical alternator is 85%-90%.
Mechanical diesel pump
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