Improving the effectiveness of solar pumping systems by using modular centrifugal pumps with variable rotational speed Daniele Fiaschi * , Roberto Graniglia, Giampaolo Manfrida Dipartimento di Energetica ‘‘Sergio Stecco’’, Universita ` di Firenze, Via Santa Marta, 3-50139 Firenze, Italy Received 30 September 2002; received in revised form 3 November 2004; accepted 24 November 2004 Available online 30 December 2004 Communicated by: Associate Editor Charles Kutscher Abstract The possibility of improving the performance of deep well solar pumping systems by using centrifugal pumps with variable rotational speed and modular number of working stages (i.e. Divided Shaft Pump, DSP) was investigated and compared with traditional systems equipped with pumps having a fixed number of stages (i.e Standard Centrifugal Pump, SCP). Starting from commercially available pumps with a given head–mass flow characteristic, a visual simula- tion tool for the evaluation of the modified DSP pump performance and costs was developed. In principle, it would be possible to use the desired number of modular stages, thus achieving the highest efficiency of the system for all condi- tions of radiation. In practice, in order to reduce the DSP pump costs to an affordable level, only one shaft breakpoint is suggested (and then two modular blocks of stages), whose optimised position is determined by the simulation program on the basis of insolation curve during the daylight and required head and shaft speed. Referring to a 30 m 2 PV system (about 3000 W peak power) and to a well depth of 100 m and considering a com- mercial 46-stage submersible pump, it was found that a breakpoint at the 31st impeller produced an increase close to 9% of the yearly pumped water yield with respect to a conventional, non-modular pump. For the above system, assuming that the cost of a modified modular pump is 1.5 times higher than that of a standard pump, the payback time varies from 0.5 to 2.5 years when the water sale price ranges between 1.1 and 0.6 €/m 3 . Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Photovoltaic; Water pumping; Head; Peak power; Well depth 1. Introduction The use of solar pumps to provide water to commu- nities living in remote areas has undergone a consistent increase in recent years (Hamza and Azmi, 1995). One of the first tasks in designing a PV-driven water supply system is the choice of the pump, which depends on the well depth and diameter, the flow rate of water, the presence of abrasive suspended solid particles en- trained by the flow and the daily radiation peak power. In the field of small installations, interesting perfor- mance levels are achieved by the ‘‘second generation’’ PV pumping systems with positive displacement or dia- phragm pumps (Protogeropoulos and Pearce, 2000). 0038-092X/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2004.11.005 * Corresponding author. Tel.: +39 055 4796436; fax: +39 055 4796342. E-mail address: danif@de.unifi.it (D. Fiaschi). Solar Energy 79 (2005) 234–244 www.elsevier.com/locate/solener