Drives help power plant produce more electricity 07 June 2010

Västerås, Sweden-based power and district heating provider Mälarenergi, says it has increased the amount of energy it can sell by 35 GWh/year, following installation of medium voltage drives.

Sven Olof Kindstedt, system engineer at Mälarenergi, says that, because of issues affecting the industry – and in particular EU directives, electricity certificates and the Kyoto protocol – Mälarenergi was looking for ways to be more environmentally friendly and efficient.

He explains that the organisation had been using resistors connected to slip-ring motors to control speed, and hence the flow, in its district heating pumps. Heat from those resistors was used in the production of district heat, but it was an expensive method.

ABB carried out an energy appraisal, which revealed that a lot of energy could be saved by upgrading the pump and fan applications with variable speed control technology, high efficiency motors and transformers, in place of the resistors and slip-ring motors on the district heating pumps.

That system now involves seven ACS 1000s and one ACS 6000 controlling four district heating pumps (4 x 1,765kW), a boiler feed pump (5,750kW), an accumulator pump (800kW) and a fan and pump for a new bio-fuelled boiler.

Kindstedt reports that, since installing the equipment, losses have been reduced considerably and, although it has also cut district heat production from the resistors, higher electrical energy output more than compensates.

The losses were removed from the district heating system, he says, which increased the cooling water temperature difference across its heat exchangers. That has resulted in an increase in saleable electricity of about 35GWh/year, along with comparable CO2 emissions reductions.

Also, with differential pressure in the district heating pumps now controlled automatically, system stability has improved and the temperature of the return water dropped – allowing better utilisation of heat by consumers

"[At first] it was difficult for us to understand how much the overall efficiency would improve with ABB's variable speed drive systems," says Kindstedt. "But ABB managed to visualise this very well. Thanks to ABB's technical competence we have a more efficient operation, improved heat rate and better balance in the district heating network."

Brian Tinham

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