Case study: slurry power
23 Feb 2016
A biogas plant is generating electricity from low-temperature waste heat.
In the village of Trechwitz in Germany, a modern biogas plant is exploiting the energy potential of manure.
The development of the plant began when the local farmers Hergen Wessels and his son Timo Wessels sought a biogas plant for their farm and realised that there was no suitable technology.
The plant was built in 2008 at Wessels’ farm, to capitalise on the manure and slurry generated by a number of agricultural operations in and around the village.
Among these operations are Wessels’ own cow barns in Damsdorf, a riding stable in Trechwitz and a chicken farm in Damsdorf.
In the summer of 2012, another cow barn was added on the site of the biogas plant itself, holding 120 cattle.
When the biogas plant started operating in January 2012, corn and sugar beets were used in addition to manure and slurry.
“After only a few months, we started to operate the plant completely without additional plant mass,” says Radko Doldzhev, facility manager.
“Every day, we process 45 tons of manure and slurry. The logging of the supplied material is done automatically: the trucks drive up to the remotecontrolled weighing machine and the data is immediately logged in the computer.
“We generate about 500 kilowatt-hours of power per hour. Since the plant has a total capacity of 800 kilowatt-hours, there is still potential to generate more electricity.”
Power generation
From the beginning, Doldzhev says there was a strong focus on operating the plant with maximum energy efficiency.
In cooperation with companies Etalon and ElectraTherm, the farm put into operation a whole new concept of an Organic Rankine Cycle (ORC) plant – which is a procedure to drive steam turbines with a working fluid other than steam.
This process is used to generate electricity with the help of combined heat and power generation.
ElectraTherm developed a compact ORC machine called the Green Machine, able to generate electricity from lowtemperature waste heat by using an organic working fluid.
To do this, it uses low-temperature waste heat (77 – 116°C) to excite a working fluid into pressurised vapour. As the vapour expands, it drives a twin screw power block, which spins an electric generator to produce fuel-free, emissionfree electricity.
Coil condenser
The working fluid used in the ORC process of the Green Machine is cooled by a coil condenser from the Güntner GVD series, which has been developed especially for the needs of the energy industry.
This V-shape coil condenser has a very small footprint compared to its power density and can be adapted to any application due to its modular design, says Güntner.
The Green Machine ORC can increase engine electrical efficiency up to 8% and uses heat that would otherwise go to waste, says Doldzhev.
In addition to biogas, ElectraTherm’s ORC technology works with satellite combined heat and power (CHP), district heating systems and geothermal applications.
On top of the machine room, there are two Güntner GFH series drycoolers; a unit with one fan serving as oil-mixture cooler and a unit with four fans serving as emergency cooler.
Before the Green Machine was installed, the entire waste heat of the motor was dissipated into the ambient air by the GFH drycooler with four fans, says Doldzhev.
Since the installation of the Green Machine, which uses the waste heat to generate electrical energy, this emergency cooler is now only used during maintenance work on the block heating station.
