Power from small amounts of waste process gas

Amberg, Germany – Generating energy from process gasses that currently escape into the atmosphere is the target of a promising German government-backed project, which is set to start production at Amberg-based Deprag Schulz GmbH & Co.

The aim is to deliver turbines that can generate energy from relatively small amounts of waste process gases, explains Dr. Eng. Rolf Pfeiffer, managing director of Deprag – a 600-employee company specialises in the field of air motors.

“What is new with our development is that it enables the conversion of even small amounts of residual energy in the range of 5 to 20 kilowatts into electricity using a small, compact and decentralised energy recovery unit,” said Pfeiffer.

Deprag, he said, has already ’successfully concluded” first prototypes of a new, patent-pending turbine generator, which it now wants to bring into production. The company is now looking for partners who want to use this new system for energy recovery in their facilities.

“Our recovery system can be used in a variety of applications to convert process gases into electricity or even to utilise unused surplus heat,” said Pfeiffer.

The energy recovery system was designed be a small, simple, and robust system for the 5 to 20 kW range.

Use of gears was to be avoided based upon costs and maintenance, though this proved to be a major challenge: the physical characteristics and the small diameter of the turbine rotors result in a relatively high speed for the turbine and thus for the generator.

The strength properties of the suitable materials set tight limitations. No standard generator was small enough and constructed from suitable materials to meet the demands for fatigue strength at the calculated rotational speed of around 40,000rpm.

Deprag’s team, therefore, had to develop a suitable electric generator themselves, focusing particularly on the fatigue strength of the rotor.

The result prototype is based on a permanent magnet synchronous induction machine for the generation of electricity: a micro-expansion turbine with an electrical generator which produces electricity from gas.

As it does not include an electrical control box, the turbine generator is not much bigger than a shoebox. Following a ’plug & earn’ principle, it can be used locally where gas is either released unused by the industrial process or where a high level of pressure is reduced to a lower value. In operation, said a Deprag statement.

In operation, gas flows into the turbine, is pressed through jets, and is accelerated. When it meets the blades of the turbine and is diverted, it releases its energy. The kinetic energy is converted to electrical energy in the generator.

The turbine and electric generator have one shared drive shaft: when the turbine rotates, the generator’s rotor rotates at the same time - electrical energy is generated.

Among its target applications, Deprag cites how the turbine generator could be used in in the smelting of metals, for example, where the melting tanks are cooled by compressed air.

In this process, the compressed air flows through cooling channels and absorbs heat. Normally, it is then released into the atmosphere without being used.

With the new turbine generator, the energy absorbed from the heat can be converted to electricity by the micro-expansion turbine and the integrated generator and fed into the power grid.

To date in large biogas plants and co-generation units, residual energy has been converted to electricity based on the ORC (organic Rankine cycle) process. But, notes Deprag, the power range of these installations is between 200-1,500kW.

However, the current trend is to build smaller biogas plants and co-generation units. The electrical efficiency of such plants can be effectively optimised with the new turbine generator by also allowing smaller amounts of waste heat to be used efficiently in an ORC recovery process.

Thus, the overall efficiency of these plants can potentially be increased to more than 45%. In accordance with the German Renewable Energy Act (EEG), a technology bonus of Euro0.02/kWh will be earned for the whole plant.

Natural gas is pumped thousands of kilometres at high pressure from producing countries to the consumer. In order to introduce it into the regional networks, which operate at a lower pressure, the pressure must be reduced, and the gas must be decompressed.

The utility companies reduce the gas pressure once again before the natural gas reaches private homes. In the transformation of pressures in the gas pipelines, energy is lost in gas distribution technology which could, Deprag engineers believe, be converted to electrical energy without major effort using the turbine generator.

“Even smaller amounts of residual energy can be converted to electricity at a reasonable price with the compact recovery system,”Pfeiffer summarises. “The ecological benefits of this sort of energy recovery also pay off financially. The feed-in tariff in accordance with the Renewable Energy Act (EEG) offers an additional incentive. The investment costs should also remain affordable.”

The MD also envisages Deprag offering the turbine generator unit for 10 kW for less than Euro10,000 at a corresponding volume, at some stage in the future.