29. Nov 2010 - Discussion: “Prospects for CHP technology”

Abstract

What is happening in the area of micro-CHP? How can we coordinate the wide range of technologies for decentralised power plants? What requirements are demanded of cogeneration (CHP)? In Stuttgart, 50 researchers came together to network and exchange current research findings over the course of 15 technical presentations. Everyone is trying to achieve higher electrical efficiencies and lower cost structures – from micro gas turbines right through to heat exchangers.

more details

Against the background of the 6th Energy Research Programme currently being prepared, the German Federal Ministry of Economics and Technology, Project Management Jülich and the German Aerospace Center organised a status seminar on the topic of the "Outlook for CHP technology".

Potential offered by flexibility

Micro-CHP in single-family houses is financially viable if thermal and electrical storage is used. Professor Münch stated that this was true for the energy supplier EnBW. An increase of 3 percent in the efficiency of an electricity-generating heating system would result in additional income of around 1,000 euros per annum. The Managing Director of Stadtwerke Rosenheim, Dr. Götz Brühl, emphasised the potential offered by district heating as part of a commercial market approach. He stated that CHP could make a financially viable contribution to balancing energy if it were managed not only based on heat, but also in accordance with electricity prices on the spot market. On behalf of the contractor company KWA, Anton Lutz requested that researchers develop ORC technology for new capacity ranges.

Micro gas turbines have been well proven in practice

Scientists are using experience gathered from numerous service applications in their research on micro gas turbines (MGTs). Axel Widenhorn from the German Aerospace Center in Stuttgart plans to use ceramic components designed for contact with hot gases to increase temperatures and efficiencies. "MGTs are very well suited to biogas," said Dr. Bernd Krautkremer from the Fraunhofer Institute in Kassel. He added that the heating concept was critical given the electrical efficiency of only 25 percent, however. David Eyler from the European Institute for Energy Research in Karlsruhe was able to benefit from considerable project experience gathered in France. He said that micro gas turbines could be used effectively together with motors, and that coupled design would ensure optimal operation.

Motor technology using unconventional approaches

New motors are demonstrating potential for improved efficiencies both in initial design concepts and also in field testing. At the development company Meta in Herzogenrath, Ralf Bey is constructing a motor with 33 piston discs that will increase efficiencies to up to 50 percent using overexpansion. Professor Peter Eilts from the Technical University of Braunschweig is investigating concepts that employ alternative piston guides to achieve good fuel flexibility. Dr. Markus Gräf from UMC Stuttgart is aiming to achieve better efficiencies in the partial-load range by 2013 using a free-piston linear generator that operates with variable piston displacement and variable compression and without a crankshaft. The Stirling motor from the heating engineering company Viessmann is close to being ready for market launch. Jaenette Liehr quotes an overall efficiency of 96 percent for this maintenance-free micro-CHP device. It combines a free-piston Stirling motor with a peak-load burner in a compact design suitable for single-family houses.

Flexible organic working fluids

The Organic Rankine Cycle (ORC) is becoming increasingly popular for electricity generation at low temperature levels (100 to 400 °C). "Biomass CHP plants have triggered a boom," says Jochen Fink from the mechanical engineering company Dürr. He added that it is not only in industry that low specific plant costs are more important than the efficiency of between 10 and 20 percent when waste heat is harnessed by using various organic working fluids. Michael Schmidt is developing a motor for steam expansion in the ORC process for Devetec in Saarbrücken that will have double the efficiency of an ORC turbine. Dr. Dariusz Szablinski from Pfalzwerke in Ludwigshafen believes that the correct sizing of the heat exchanger is important in other areas, too, outside of geothermal systems in order to determine the optimal return temperature independently of the working fluid used to transfer heat.

New materials of construction for heat exchangers

Optimal heat exchange is the key to effective heat storage. Heat storage systems will be an essential element in ensuring the economic viability and grid integration of CHP electricity in the future. CHP will thus move from being a grid problem to being a grid solution, according to Dr. Stefan Zunft (German Aerospace Center). He is developing ceramic heat exchangers for high-temperature use at up to 1,250 °C – for example, in efficient electricity generation from biomass in gas turbine processes. Heat exchangers are always one-off solutions, in the opinion of Professor Reiner Numrich of the University of Paderborn. Even tried-and-tested tube bundles still have to be designed specifically for each particular plant, he added. He has measured heat transfer in newly developed corrugated tubes that is 20 percent better than with smooth tubes. This alone could increase the efficiency of a power plant by 0.1 percent. Over a power plant's service life of many decades, this increase in efficiency could save huge amounts of carbon dioxide emissions by increasing efficiency.

Research needs identified

There are no standard solutions for the wide range of CHP plants – this opinion is shared by researchers and those responsible for funding at Project Management Jülich and at the German Federal Ministry of Economics and Technology. The common goals of CHP research are lower plant costs and higher electrical efficiencies. The aim here is to balance out the deficits compared to large combined cycle power plants. On the other hand, the high availability levels, absence of maintenance and fuel flexibility will be features of CHP in the future. Financial support from electricity feed-in has distracted attention from the harnessing of heat up to now. This is now a challenge for scientists and policy-makers.