To fulfill its commitment to optimize the use of electricity and comply with the Clean Air Charter, the Water Supplies Department of the Government of Hong Kong has taken various measures to keep reducing the electricity consumption of its fresh water and sea water facilities, including the introduction of advanced energy-saving technologies to improve energy efficiency. In December 2008, the Water Supplies Department selected one pump house (BY) with relatively higher operating efficiency from its 185 water pumping stations / pump houses and invited EnergyWin to conduct an energy efficiency assessment.
Project Background
Located on Hong Kong Island, BY Pump House is a middle-level pump house among the three levels of pump houses for pumping water to the high-altitude water reservoir on the mountain top of Tin Ping Mountain. With its retrofit completed in 2005, BY Pump House provides fresh water to the neighboring residents and commercial buildings, with annual water supply of 2,634,590 tons and electricity consumption of 884,901 kWh.
The assessment revealed that the actual operating efficiency of the centrifugal pumps at BY Pump House is 73.5%, close to the maximum efficiency achievable by the pumps of the particular model. However, by utilizing ENERGYWIN’s energy-saving technology, the operating efficiency of the pump house as a system can still be improved by 9.6% while the operating efficiency of individual pumps remains the same.
Project Overview
BY Pump House is installed with three horizontal single-stage dual-suction centrifugal pumps driven by electric motors. The pump house stands at an altitude of 98.15 meters and the difference between the water levels of the high-water-level reservoir and the low-water-level reservoir is 80 meters. The nameplate ratings of the pumps: rated flow rate: 95 l/s; rated head: 100 meters; and rated RPM: 1,490 r/m. Each pump is directly driven by an 185KW induction motor. Among the three pumps, one would be running and the rest two would stand by. The motor runs at the rated RPM at power frequency. The start/stop of the pumps and the number of running pumps is controlled by the water level at the high-water-level reservoir. The pump house is installed with an SCADA system and monitored remotely.
On the day when the assessment was conducted, the operating point of the pump was at a flow rate of 105 l/s and a head of 83 meter. In that December, the pump house supplied 7,500 tons of fresh water per day on average, consuming 2,536 kWh of electricity per day, which means a daily electricity cost of HK$3,220.
Project Team
The project team consisted of ENERGYWIN’s energy efficiency assessors and the engineers from the Water Supplies Department, who jointly surveyed the site, collected and analyzed the operating data, and written the energy efficiency assessment report.
Conclusion of the Assessment
Based on the collected data, ENERGYWIN established a mathematical model of this pump system and ran a simulation. The assessment revealed that the operating efficiency of the pumps at BY Pump House is 73.5%, close to the maximum efficiency achievable by the pumps of the particular model. Therefore, if an “individual equipment (pump) approach” is adopted to assess this pump house, the conclusion would be that there is no room for improvement of energy efficiency.
However, if a “system approach” is adopted to assess this pump house, we would find out through mathematical simulation that by utilizing ENERGYWIN’s energy-saving technology, the operating efficiency of the pump house as a system can still be improved by 9.6% although the operating efficiency of individual pumps remains the same, which means the electricity consumption will be reduced by 84,951 kWh, the power bill by HK$108,000 and the CO2 emissions by 71 tons every year. With the investment in the energy-saving retrofit of the system estimated at HK$350,000, the simple payback period would be 3.3 years.
Summary of the Experience
The experience gained through the energy efficiency assessment of BY Pump House can be summed up as follows:
Energy efficiency can be significantly improved by adopting advanced control strategy and methodology;
The motor speed regulation technology still has potential for application in water supply systems with high hydrostatic head because it can adapt to the fluctuation of the load and reduce energy consumption; and
By adopting a “system approach” instead of an “individual equipment approach” to the assessment of energy efficiency, we can find unexpected room for energy saving.
