The economic and ecological balance of a combined heat and power plant can be substantially improved by using energy-efficient drive systems. The Stadtwerke public utility in Chemnitz therefore operates the cooling water and district heating pumps in its combined heat and power plant in variable speed mode.
This has cut energy requirements to a mere fraction of their former level.
The Chemnitz combined heat and power plant supplies the city s 240,000 residents with reliable and environmentally acceptable electricity and district heating. It is an ecologically acceptable solution because the combined heat and power cycle allows the steam used to generate electricity to be used again in a downstream phase for heating water. The hot water then passes through an extensive network of pipes to provide district heating to a large number of residents who use it for heating and hot water. There is considerable potential for making savings in the pumps which transport the district heating water or, to put it more precisely, in the drive systems of the pumps.
As the demand for district heating fluctuates very widely, the Stadtwerke Chemnitz decided to convert the flow control system for the hot water circulation pumps from a hydrokinetic coupling to speed control with frequency converters. The objective was to optimize the power plant s internal energy balance, because the pump drives had always run at the speed producing the maximum delivery rate, although this was not really necessary.
The use of the right motor-converter system enables the speed of the hot water circulation pumps to be adjusted to provide exactly the required delivery rate. In this way, the frequency converter only consumes the amount of energy that it actually requires at any given moment, which can reduce the energy requirement by some 35 percent. In Chemnitz, the drive system uses Siemens type N 690 kW low-voltage motors and Sinamics G150 frequency converters. The latter are especially reliable, quiet and ideal for single drives, such as pumps, which do not require line regeneration.
The Stadtwerke Chemnitz exploits the advantages of frequency conversion for its district heating pumps and its cooling water pumps and benefits from an energy-efficient, precise operating mode that places less stress on plant components. As the drive system operates on medium voltage due to the higher power ratings involved, Siemens type H compact high-voltage motors are used. The motor speed is controlled by Siemens Robicon Perfect Harmony medium-voltage converters which can be brought into operation on-site quite simply on the plug-and-play principle, without needing assembly units.
Variable speed operation has other advantages in addition to saving energy. Delivery rates can be adjusted considerably more quickly and precisely to current demands. The mechanical systems along the entire drive train start and stop softly, thus reducing wear and maintenance expenditure and extending the service life of the components. And the Stadtwerke Chemnitz no longer has to worry about pressure waves in the pipe network.