The correct phase relationship to accomplish the desired gas cycle is achieved by tuning the piston and displacer as mechanical spring-mass-damper resonators. This method eliminates the crank mechanism of a kinematic Stirling engine and its associated lubrication, side forces and sealing issues. The piston power is delivered directly to the magnets of a permanent magnet alternator to produce alternating current at any desired voltage (Redlich, 1995; U.S. Patent 4,602,174, 1986a; U.S. Patent 4,623,808, 198 6b). The engine operates at near constant frequency regardless of loading or piston amplitude, and can be directly attached to the grid without intermediaries.

Both piston and displacer float on gas bearings in their cylinders and are resonated by mechanical springs which are arranged to offer no side loads on the bearings. These planar springs (flat plates with spiral slits) also serve to center and support the large radial loads of the permanent magnets, acting in effect as friction-free oscillating bearings. The combination of gas bearings to allow wear-free close fits on the pistons and mechanical springs to give both springing and axial positioning is uniquely advantageous in that it confers high mechanical efficiency, very long life, and the elimination of bearing and seal deterioration. In addition, this configuration is inexpensive and compact. The gas bearings are activated through a restrictor, one-way reed valve and plenum combination connected to the engine compression space. There is no separate pump and the total power consumption is less than 50 Watts in the case of a 3 kWe engine/alternator.

Note that the gas bearings provide precise radial positioning for the oscillating bearings and seals, and the separate function of axial springing and rough location is performed by the flexures. The flexures in this machine are not called upon to provide precise radial centering and as a result of this limited role are much cheaper to make than flexures which are required to perform both springing and highly accurate centering of the pistons to avoid wear and leakage as is required in some free-piston Stirling designs (U.S. Patent 5,525,845, 1996).

In the past, Stirling engines have earned a reputation for slow, complex and inefficient power control. This oft-cited criticism is answered in the new free-piston engines with a power control based on a variable spring between piston and displacer (U.S. Patent 5,502,968, 1996b; U.S. Patent 5,385,021, 1995) . This spring couples the displacer to the piston to greater or lesser degree in proportion to its stiffness. If the spring is at maximum stiffness then the displacer is essentially locked to the pisto n and little or no power is generated. The engine power can rise to maximum in a very few engine cycles if the relative spring stiffness is reduced to zero. Such a variable stiffness spring can be implemented in a number of ways, and is done in this case by a gas spring with a controllable duty cycle. The variable duty cycle is effected by a simple spool valve. The spool valve is operated by coupling to desired output parameters such as voltage or piston amplitude to allow fully automatic and rapid respon se to imposed load. The free-piston engine with this type of power control can operate, for example, as a constant voltage generator with fast response and high efficiency over a wide range of power.

Both the cooler and the heater use folded copper fin internal heat exchangers brazed directly to the pressure vessel wall. This provides an inexpensive and highly effective heat transfer surface. These finned heat exchangers offer comparable performance w ith conventional Stirling tubular heat exchangers but avoid numerous critical braze and/or weld joints. This configuration also allows for good flow conformity of the heater and cooler with the annular gaps of the foil regenerator.

The most unique and distinctive feature of this machine is very long life and low maintenance derived from its non-contact operation and its hermetic containment vessel, which both obviates and prevents any user maintenance or manipulation. Another attractive feature is very easy starting and operation as long as a source of heating and cooling is supplied.

Stirling engines have always had the potential of high efficiency (20% or better) at low power levels. Recent advances in free-piston Stirling engine technology suggest the feasibility of these devices as biomass-fired generators of electricity (Lane and Beale, 1995, 1996a, 1996b). Predicted performance of the biomass-fired free-piston Stirling genset is based on data gathered from tests of 2.5 kW prototypes using propane as fuel (Photographs 1 and 2 ); this engine design currently produces more than 3 kW electric (gross) at an efficiency (heat in to electric out) of greater than 28% with a heater head temperature of 600ƒC. Efficiencies approaching 40% will be possible at higher heater head temperatures. Overall system efficiency, including a two stage biomass combustor at 85% efficiency and all peripherals, is projected at about 22%.

Photograph 1. Stand Alone 2.5 kWe Propane Fired Generator

Photograph 2. Prototype engine with integral linear alternator

In larger sizes (>12 kW), and in applications where a number of engine/alternators operate together to provide a single electrical output, a conventional turbine and rotary generator may be more cost effective than a linear alternator. In this case the en gine piston is also a double acting positive displacement pump providing a supply of high pressure gas to the turbine. In the ganged application a single turbine generator can be supplied with gas from a number of Stirling engines.

DESCRIPTION OF THE BIOMASS BURNER

Burners have been designed and operated with prototype engines which give promise of good service in biomass fuelled small Stirling machines of the type discussed here. These burners designs accept any biomass fuel in a hopper, which is then gravity fed to the primary burn zone in which a rich gas is generated by the impingement of primary air on the base of the fuel bed. This rich gas is then directed to a ring surrounding the engine heater head, mixed with secondary air and burned at a temperature of approximately 1200ƒC. It then flows over the heater head external heat exchanger and to the recuperator, from which it is drawn by an ejector pump and exhausted. Any remaining heat from this exhaust can be recovered in downstream heat exchangers. Experience with small burners of approximately 3 to 10 kW thermal output has shown good performance. No difficulty has been encountered from ash or alkali accumulation, and the recuperator is designed to be very easily disassembled and cleaned.

In use, the correct heat rate and fuel/air ratio is maintained by controlling the flow of primary and secondary air. The entire burner is kept below atmospheric pressure by the exhaust pump, so that there are no vapor leaks.

Fuel can be fed by gravity or by controlled auger feed if appropriate fuels like pellets or other uniform shapes are used. While the manufacture of pellets and similar formed fuels require some energy and additional equipment they may be the best choice for some situations, or for convenience.

Since biomass is not energy dense, it is important to minimize the distance between production and use, and small engines lend themselves to this requirement. With the burner described, unprocessed local fuel may be used directly, with minimal drying and dividing into small sizes. In agricultural sites, it may be best to use locally available fuel with minimal pretreatment, such as cutting to length or dicing solids for use in a gravity feed hopper. In urban situations, it is far more likely that some pre treatment such as pelletizing may be the preferred method because of its convenience and high energy density. Fuel sizes such as spheres or cubes of 30 mm or so may require less energy to form than pelletizing and still provide an easily handled fuel.

In locations where sawdust is available, it may be readily burned in a cyclone burner with tangential entry of an appropriate ratio of air and sawdust.

STIRLING MICRO-BIOMASS POWER AS AN ALTERNATIVE TO CENTRALLY GENERATED BIOMASS POWER

The biomass fired electric generation industry faces two major difficulties in competing with fossil fuels. Firstly, the advantageous power purchase agreements that were negotiated under PURPA in the 1980s are no longer available, and secondly there has been large feedstock cost instability over the last 17 years. Of the nearly 1000 wood-fired plants in the U.S., only a third offer electricity for sale and more than 70% of biomass power is cogenerated with heat (Craig et al., 1995).

Craig et al show that direct steam generation produced electric power at 5.43¢/kWh during the early 1980s but that under current conditions direct steam can only produce power at 8.30¢/kWh with an overall 20% efficiency (fuel in to electric out). They argue that by moving to IGCC systems, with lower capital cost and high efficiency (45%), biomass power generation could become competitive with fossil fuel systems and produce power at between 5.1¢/kWh and 5.6¢/kWh. IGCC systems do not scale down to low power levels (less than 5 MWe) and therefore the cost of building engineering demonstration units is very high, ranging from $25-30 million for a 8 MWe plant to over $70 million for a 20 Mwe plant (Craig et al., 1995). Further, by going to large IGCC systems cogeneration is not possible, but this is how 70% of the biomass power is generated.

There is another way. Rather than competing with large fossil fuel plants a substantial opportunity exists to generate micro-biomass electric power, at power levels from fractions of a kilowatts through to tens or hundreds of kilowatts, at the point of en d use. At these power levels neither small internal combustion engines, which cannot use biomass directly, nor reciprocating steam engines, with low efficiency and limited life, can offer the end user economic electric power. Free-piston Stirling engines are an economic alternative.

Stirling offers the following advantages over significantly larger systems:

• Stirling machines have reasonable overall efficiencies at moderate heater head temperatures (~600ƒC)
  
• machines are being tested with heat in to electric out efficiency of 28%. Overall efficiency (fuel in to electric out) of 22% is expected
  
• placing machines at the user's site avoids the electric distribution cost. For example, 1 mile of 60 kV power line on wooden poles delivering 32 MW cost $120,000 (Fuldner, 1997)
  
• sensitivity to fuel feedstock prices is reduced since a large single stream of feedstock is not required. In fact, end-users may have access to fuel which is free or equates to a negative cost (such as sawmill waste products which would otherwise have to be removed at a cost)
  
• cogeneration is simple
  
• large amounts of capital do not have to be raised to build a single evaluation plant with its associated technical and economic risks
  
• A large fraction of the value of the engine alternator can be reused at the end of its life
  
• Stirling systems can be ganged with multiple units operating in parallel.

Figure 2 shows a range of lifetime costs of electric power depending upon the fuel used, the fraction of heat recovered and the fraction of purchase price recovered by factory rebuilding the generator. The analysis is based on test data from a 2.5 kWe system, with a target purchase price of $6,000, an expected life of 40,000 hours and an expected total maintenance cost of $2,000. In cases where some of the rejected heat is used, the value of this heat is used to discount the total fuel cost (that is, the heat is assigned the value of the fuel which would be required to produce it).

Figure 2. Cost of electricity using biomass fired free-piston Stirling

Note that in the most optimistic scenario, free fuel with heat recovery and 35% of the purchase price recovered, the cost of electricity is very similar to what could be achieved with IGCC systems; however once the distribution cost and efficiency are included it is likely that the micro-biomass power will be cheaper. Also note that even with the most expensive likely fuel, hybrid poplar (a biomass crop) the micro-biomass system produces power at prices comparable to those which the domestic end-user currently pays in the United States and less than the user pays in Europe. These prices would be very attractive throughout the developing world where raising the capital to build large plants and distribution grids is difficult.

Figure 3. Cost of Energy ((¢/kWh) vs. Plant Size (data from Craig and Mann,) 1996

Another illustration of the potential market for micro-biomass power generation is shown in Figure 3. Here data for the cost of electricity (¢/kWh) versus the plant size (Craig and Mann, 1996) is extrapolated and shown on log axes. It is apparent, , given the unfavorable scaling characteristics of gas turbine systems when moving down in size, that there is an opportunity for micro-biomass power. For a large power range, from ~100W through to several hundred kilowatts, Stirling, either as single units or in ganged systems, is both possible and more economic than gasifier gas turbine systems.

SUMMARY AND CONCLUSIONS

Stirling powered micro-biomass systems offer a number of advantages over centralized power plants. They can be placed at the end-user location taking advantage of local fuel prices and do not require a distribution grid. They can directly provide electric al output with integral linear alternators, or where power requirements are larger they can be ganged and drive a conventional rotary turbine. They are hermetically sealed and offer long lives through their non-contact operation.

Given the characteristics of biomass fuels-their low energy density, wide distribution and expense when derived from biomass crops-and given the significant financial hurdles to implement IGCC plants, there is a significant market for biomass energy that can be served by Stirling powered micro-biomass systems. In the U.S. where local biomass fuel may be free and in other countries where the electrical distribution grid is not in place, micro-biomass systems are the solution to cost effectively developing power from ~100W through to several hundred kilowatts.

Engines at the 2.5-5 kW range are currently being tested which would be ideal candidates for micro-biomass power.

ACKNOWLEDGMENTS

The authors would like to acknowledge the support of Wood-Mizer Products, Inc., and in particular of Wood-Mizer Chairman Donald Laskowski, in the writing of this paper.

REFERENCES

Beale, W. T. (1995). Free-Piston Stirling Engines for Domestic Cogeneration and Biomass Energy Conversion. Symposium on Greenhouse Gas Emissions and Mitigation Research, sponsored by U.S. EPA and Acurex Environmental Corporation, Washington, DC, June 27-2 9, 1995.

Craig, K.R., Bain, R.L., and Overend, R.P. (1995). Biomass Power Systems-Where are We, Where are We Going, and How Do We Get There? The Role of Gasification. EPRI Conference on New Power Generation Technology, October 25-27, 1995, San Francisco, Californi a.

Craig K. R. and Mann M. K. (1996). Cost and Performance Analysis of Three Integrated Biomass Gasification Combined Cycle Power Systems. http://www.eren.doe.gov/biopower/snowpapr.html, August 1996.

Fuldner, A. H. (1997). Upgrading Transmission Capacity for Wholesale Electric Power Trade. http://www.eia.doe.gov/fuelelectric.html, April 1997

Lane, N. W. and Beale, W.T. (1995). A 5 kW Electric Free-Piston Stirling Engine. Proc 7th International Conference on Stirling Cycle Machines, Tokyo, Japan, November 5-8, 1995.

Lane, N. W. and Beale, W.T. (1996a). Stirling Engines for Micro-Cogen and Cooling. Proc Strategic Gas Forum, Detroit, MI, June 19-20, 1996.

Lane, N. W. and Beale, W.T. (1996b). A Free-Piston Stirling Engine-Alternator for Solar Electric Power", 8th International Symposium on Solar Thermal Concentrating Technologies, October 6-11, K–ln, Germany, 1996.

Redlich, R. (1995). A Summary of Twenty Years Experience with Linear Motors and Alternators. Report distributed at Linear Drives for Industry Applications, Nagasaki, Japan, May 31-June 2, 1995; available from Sunpower, Inc., Athens, Ohio, USA.

U.S. Patent 4,602,174 (1986a) Electromechanical Transducer Particularly Suitable for a Linear Alternator Driven by a Free-Piston Stirling Engine. Germany Patent 0,218,682, UK Patent 0,218,682; Japan Patent 1,966,438.

U.S. Patent 4,623,808 (1986b) Electromechanical Transducer Particularly Suitable for a Linear Alternator Driven by a Free-Piston Stirling Engine.

U.S. Patent 5,525,845 (1996a) Fluid Bearing with Compliant Linkage for Centering Reciprocating Bodies. Foreign patents pending.

U.S. Patent 5,502,968 (1996b) Free Piston Stirling Machine Having a Controllably Switchable Work Transmitting Linkage Between Displacer and Piston.

U.S. Patent 5,385,021 (1995) Free Piston Stirling Machine Having Variable Spring Between Displacer and Piston for Power Control and Stroke Limiting. Foreign patents pending.

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