An energy assessment of the water pumping Systems at the Gigiri pumping station

Abstract

The current energy situation in the country is characterized by high demand for energy and escalating costs. As a result, peak electricity demand has risen to 1)07 megawatts (MW)against an effective supply of 1)35 MW, leaving a reserve margin of 4%against the desired 15%necessary for grid system stability [1]. Since energy demand is growing faster than the energy supply growth, it has become necessary for the country to manage its existing energy supply efficiently in order to prevent the imminent collapse of the country's electric supply system. Municipal water pumping is an energy-intensive process. The cost per unit of electricity has continued to rise at the Nairobi City Water and Sewerage Company's Gigiri pumping station, with an increase of 54% from May 2010 to February 2012. The pumping station had a maximum electricity demand range of 1,314kVA and 1,664kVA during the period 2010-2011and 2011-2012respectively. The Gigiri pumping station operates on a 24 hour schedule. There are four pumping units each comprising of an electric motor connected to a centrifugal pump, a capacitor, surge vessel, piping system and flow control accessories. The only source of energy at the pumping station is electricity. The pumping station's average monthly electricity consumption was 882,753kWh in the period 2010-2011and 847,894kWh in the period 2011-2012with observed specific energy intensities of 0.667 kWh/m3, and 0.629 kWh/m3, respectively. The pumping station's average monthly flow discharge was 1,338,533m3 during the period 2010-2011 and 1,348,568m3 during the period 2011-2012. The pumping station's average system efficiency stood at 49%, with the highest pumping system efficiency of 70%being attained when pump No.4 was running alone. The lowest pumping system efficiency of 34%was observed when running the parallel combination of the pumps No.1 and No.2. iii The pump with the highest pump efficiency was No.4 with an efficiency of 74%. The lowest pump efficiency was recorded in the pump No.1 with an efficiency of 46%. Areas in the pumping system that were identified for potential energy cost savings included: Efficient single pump operation in place of inefficient and mismatched parallel pump operation - with potential annual energy savings of KES4,904,254with an immediate and at no cost savings benefit, the adjustment of the best efficiency point (BEP)of the Pump No.1 through an overhaul with potential annual energy savings of KES7,220,495with a payback period of approximately 1.5 years and recovery of cooling water losses from the pump bearings with potential annual water and energy cost savings of KES1,110,798with a payback period of approximately 1 month.