Page - 54 - in Clean Water Using Solar and Wind - Outside the Power Grid

54 Clean Water Using Solar and Wind: Outside the Power Grid 4.4 PUMP EFFICIENCY The pumping efficiency depends on the flow rate as well as on the design of the pump. • The electric power delivered to the electric motor connected to the pump is called the incoming power. • The power transferred to the pump shaft is the mechanical power (the shaft power) and is slightly smaller than the incoming power and depends on power losses in the motor. The rated power of a motor is the mechanical power at the normal operating point and is slightly smaller than the consumed electric power. Typically, the motor efficiency is higher than 90%. • The mechanical power is transferred to the hydraulic power Phydr – the power that the pump transfers to the liquid in the shape of flow. This efficiency depends on the flow rate and is illustrated in Figure 4.3. The figure illustrates that the pump should be designed so that it has a maximum efficiency at the most common flow rates. For the most common pump types, the term power rating normally refers to the shaft power and is measured in W or kW. The efficiency of a large-scale pump may be around 89%. High- performance, small-scale pumps can reach an efficiency of 85%, but the performance of less expensive small pumps may be significantly lower. The hydraulic power to lift water H metres is derived from: P Q H ghydr = ⋅ ⋅ ⋅ρ (4.2) where the hydraulic power is expressed in watts (W). Q = flow rate (m3/s), H = head (m), ρ = liquid density (kg/m3), and g = acceleration of gravity (m/s2). Obviously, the required electric power must be higher than the hydraulic power. Naturally the total pump efficiencies depend on the actual equipment, and here we assume a typical value of 80%. Example 4.1: Power to Pump Groundwater Assume that the groundwater source is located 10 m below ground (H = 10). We need to supply 1 m3 per hour (Q = 1/3600 m3/s). The water density is 1,000 kg/m3 and g = 9.81 m/s2. Downloaded from https://iwaponline.com/ebooks/book-pdf/520710/wio9781780409443.pdf by IWA Publishing user