TWO BREAKTHROUGH INNOVATIONS

Delta Stirling machines are a recent innovation that adapts proven maintenance-free long-life FPS hardware components to a new topology with many advantages. All prior Stirling machines have been categorized as alpha, beta, or gamma configurations. Kinematic Stirling engines, with inherent life and reliability limitations, use all three configurations, but FPS engines were limited to single-acting beta or gamma configurations until a double-acting alpha (DAA) FPS topology was identified in (M. A. White 2005) and (White and et.al. 2006). Multiple protypes have been successfully demonstrated since then, but complex heat exchanger configurations and convoluted interconnecting flow passages that resulted from conventional parallel piston axes and a piston on only one end of the linear alternator, limited performance. What an Infinia 3-cylinder 10-kW DAA FPS engine definitively demonstrated was that the key concern about whether the thermodynamic forces would stably operate with the desired 120-degree phasing between pistons is not an issue. When cooling down after test runs, the proper phasing was locked in as the engine coasted down all the way to the 10-watt range. SI owns the basic delta Stirling machine patents (Emigh 2012, Emigh2014),

Delta Stirling engines are optimized to a level that can’t be fundamentally improved upon

• Offers unprecedented Stirling efficiency and low cost with zero maintenance

• Uses commercially proven extreme life and reliability components

• The first prototype will be close to a pre-production configuration

• Scales to far greater power levels than existing Stirling machines

• Delta Stirling engines are optimized to a level that can’t be fundamentally improved upon

• Offers unprecedented Stirling efficiency and low cost with zero maintenance

• Uses commercially proven extreme life and reliability components

• The first prototype will be close to a pre-production configuration

• Scales to far greater power levels than existing Stirling machines

The 3-cylinder delta Stirling topology shown in Figure 1 reduces what a Stirling engine is to its essential elements. Nothing can be removed without losing basic functionality. Heat exchanger packaging and manifolding cannot be simpler or more direct. These factors make it possible to achieve the lowest cost, highest efficiency, and lightest weight relative to any other Stirling engine configuration. This efficiency assertion was confirmed by engaging Barry Penswick, a master practitioner of the gold standard Sage Stirling analysis code, to conduct a delta engine analysis using Qnergy linear alternator specifications. He evaluated two cases: a generalized optimization and a constrained optimization where both pistons and the regenerator were forced to have the same diameters. The unconstrained case had 62% of ideal Carnot efficiency and the constrained case 58%. This compares with 50% of Carnot efficiency, from a typical Sage prediction and translation to hardware, considered to be an excellent Stirling engine success. Power output is easily modulated and part-load efficiency is exceptional.

The only downsides to delta engines are the three distributed heater heads and relatively large footprint. Also, the degree of piston bias mitigation (present with all FPS machines) that will be needed cannot be determined until a prototype engine is operational. Intuitively, it appears likely that the delta engine will have less bias than other FPS engines because leakage past all pistons is symmetric and is between cyclic pressure variations and the same average buffer pressure. All other DAA topologies have direct cycle-to-cycle pressure drops across the pistons. The delta Stirling configuration is the culmination of many topological iterations that sought to produce more symmetric flow patterns with simple heat exchangers and straightforward manifolding between them for DAA FPS machines. 

As seen in Figure 1 a piston with a clearance seal is deployed on each end of each linear alternator, which has a central moving magnet assembly supported by flexure bearings. This piston/cylinder configuration is essentially identical to the production-line approach used very successfully by Infinia/Qnergy for over a thousand commercial FPS engines, the basic difference being the implementation of a piston on each end of the linear alternator instead of one end.

The cold heat exchanger/regenerator module is directly in line with the compression piston on one end of the alternator, with an expansion piston and hot cap on the other end. The hot heat exchanger (or heater) is deployed between the regenerator associated with one alternator and the expansion piston hot cap from a second alternator. The thin wall hot cap with

A 3-D view of a fully balanced delta Stirling generator is shown in Figure 2. The imbalance in a delta machine does not occur as either a typical linear oscillation or rotational oscillation about an axis. Instead, the entire unit traces a small circle of a magnitude that depends on the moving masses, their amplitude, and the entire system mass. It has been demonstrated that over 90% of this motion can be eliminated with suitable passive balancers. The imbalance is projected to be fully eliminated with two properly coupled delta machines as illustrated in Figure 2. Not shown for this conceptual generator are mechanical couplings between alternator housings, a coolant pump, or a fan-cooled radiator to reject cycle waste heat to the environment. To balance this generator, the upper delta engine is turned “upside down” relative to the lower engine and the heaters are aligned for convenience in connecting the heat source vapor space ducting. The upper and lower engines are rigidly coupled together, and allowance is made to accommodate thermal expansion and contraction that occurs during heatup and cooldown. 

Figure 2. Perspective view of conceptual delta prototype generator with two delta engines integrated for full balancing and a sodium pool boiler with an electric heat source

Figure 2. Perspective view of conceptual delta prototype generator with two delta engines integrated for full balancing and a sodium pool boiler with an electric heat source

It needs to be emphasized that all the delta engine discussion herein is predicated on using existing commercial Qnergy 4-kW and 8-kW linear alternators. Those are indeed what early delta prototype development and demonstration should be based on. Once the proper vetting of the delta engine is completed the engine capacity should be scaled up to further decrease cost, size, and weight per kilowatt in pursuit of Grand Vision objectives. The Qnergy 8-kW linear alternator was produced very economically by essentially integrating two 4-kW alternators in tandem to double the output using the same flexure bearings and pressure vessel diameters. With a modest increase in length the output doubled and the cost per kW decreased substantially. While this is a good approach, an even better one is to increase diameter to enable increasing stroke length, which is flexure diameter limited. A combination of these can greatly increase output while further reducing cost, size, and weight per kilowatt for many of the components. Seven hundred kilowatt linear alternators were conceptualized in (White and et.al. 2007). Success at that capacity may be questionable, but 50-kW and probably 100-kW alternators are relatively straightforward. Using 50-kW alternators results in a 300-kW balanced delta engine. Future assessments will determine the best engine sizes and whether significant scaling up or down from the 1-MW GREAT TES central receiver (CR) module concept described below is most cost effective.