There are obvious reasons for developing a reliable and effective technique for implementing small scale biomass powered cogeneration. In the industrialized countries such a technique would enable optimal use of the biomass potential - in the developing countries it might offer a possibility of production of electricity using the only fuel available.
The steam engine and the Stirling engine offered the first two practical solutions for the conversion of chemically stored energy to mechanical energy.
They formed the basis of the industrial revolution. The latter mainly as stationary installations below 10 kW shaft power.
The Stirling engine works by a very simple principle: A working gas in a closed circuit is moved from the hot section to the cold section and vice versa by a replacement piston. When the gas is being heated a working piston expands the volume and vice versa when the gas is cooled.
It is in fact as simple as it sounds to make a small demonstration model of this process. But to make a reliable engine with good efficiency is something else.
Some of the obstacles are:
x to obtain a good thermodynamic efficiency a large difference between the mean temperature of the gas in the hot section and in the cold section is necessary. In practice it should be above 500 K. This requires a so-called regenerator between the two volumes, where heat can be stored intermediately between the strokes. (The heat is stored in the regenerator as the gas moves from the hot section to the cold and regained when it returns). A very careful design of the regenerator and the heat exchangers in the heating and cooling section is needed to make the process work with the necessary speed and effectiveness.
x to avoid very voluminous engines a high pressure of the working gas is needed. In practice pressures of 50 to 100 Bar are common.
x to avoid losses due to the work involved in moving the gas through tubes and heat exchangers gases with very low viscosity must be used. In practice Helium or Hydrogen.
x the combination of gases with high pressure and small molecules causes leakage problems.
Long-term perspectives for balancing fluctuating renewable energy sources 45 During the last 30 years great efforts have been made at several laboritories to overcome these obstacles using computer aided design and advanced material technology. Phillips, General Motors, United Stirling and Ford should be mentioned. Important activities in Japan and New Zealand have also been reported.
The state of the art is in short that efficiencies above 30 % have been reached, but production costs and leakage problems have so far prevented mass production. A Stirling engine for biomass combustion is produced in India with some success, but this system has a low overall efficiency (10 %).
Stirling engines are produced in limited numbers for military use (low noise) and the German company Solo produces generating sets in the 10 kW range for natural gas in small series.
Particularly for production of electricity by combustion of biomass the Stirling engine has obvious advantages. Due to the closed circuit of the active medium in this engine almost any fuel can be used. Unlike the steam engine it does not require specially trained and certified operators for safety reasons.
However, biomass applications arises new obstacles:
x design of an effective Stirling engine given the special conditions: relatively large heating section, relatively low heater temperature.
x design of a furnace with low ash temperature to avoid clinker on the grate and on the heater and to avoid production of aggressive gases which would corrode the heater.
x design of a furnace with an effective combustion air preheater to enable operation with exhaust gases in the range of 800 qC and input air temperatures in the range of 600 qC.
Some experiments to use high efficient Stirling engines designed for oil or gas combustion with biomass have been reported. All with the result of heat exchangers being clogged up and destroyed in a very short time. With two stage combustion (gasification) and filtration of the gas this might work, but it would effect the simplicity and the costs of the complete system negatively.
In Denmark researchers at the Technical University developed a 10 kW Stirling engine with 30
% efficiency in 1993. /1/ and /2/. Based on this design a complete biomass cogeneration system incorporating a wood chip burner and a 35 kW and a 70 kW Stirling engine is currently being developed in cooperation between the Danish Technical University and a number of private companies. The project receives financial support from the Danish Energy Agency and from the TSO, Energinet.dk.
In the following the design and the operational results are given.
Long-term perspectives for balancing fluctuating renewable energy sources 46 Figure 4-1: Overall design of the system with Sankey diagram of the energy conversion.
Long-term perspectives for balancing fluctuating renewable energy sources 47 Wood chips are fed to a moving grate boiler by screw conveyors. The mechanical parts of this system are standard but the control system is especially developed in order to maintain a very stable temperature on the surface of the heater of the Stirling engine.
On top of the boiler a four cylinder double acting Stirling engine with a specially designed heater is placed 'upside down'. Because of the position of the heater app. 50 % of the heat from the combustion is transferred by radiation. The rest is transferred by convection as the hot flue gas passes the fins of the tubes in the heater. The design temperature of the heater surface is 750 qC. This causes the exhaust to be very hot (770 qC). If this exhaust was entirely used for the production of heat the overall electric efficiency of the system would be rather low despite the high efficiency of the Stirling engine itself. Instead the exhaust it led to an air preheater where it is heat exchanged with the incoming fresh air for the combustion. This causes the combustion air to be 600 qC. The preheated combustion air enables combustion of wet biomass and causes a very fast combustion. In order to keep a constant temperature in the heater a more or less continuous fuel input is used.
The output from the preheater is still 315 qC. In order to obtain a high overall efficiency this output is cooled to app. 110 qC. Further cooling with condensation of the water from the fuel is a possibility.
The Sankey diagram shows that the majority of the heat is produced by the cooling of the Stirling engine. It explains how the electric efficiency of app. 24 % and the heat efficiency of app. 63 % are obtained.
Fig. 0.2 shows the mechanical design of the Stirling engine.
With a four cylinder design no replacement pistons are needed. One cylinder passes the gas on to the next etc. In this design leakage has been avoided by incorporating the generator into the pressurized part of the engine much the same way as the motor is incorporated in a refrigerator compressor.
The engine has a bore of 140 mm and a stroke of 74 mm. The speed of the shaft is 1015 rpm, the working gas is helium and the mean pressure 4 MPa.
Long-term perspectives for balancing fluctuating renewable energy sources 48 Figure 4-2: The 35 kW Stirling engine for solid fuel.
Figure 4-3: 70 kW Stirling engine at boiler. Austia.[Ref. 3]
Long-term perspectives for balancing fluctuating renewable energy sources 49