By using this cycle, larger aerodynamic components with optimum performance can be used to achieve good fuel efficiency. In addition, a simple gravity fuel system can be used.  The SBC concept is designed to provide power at near peak fuel-to-electricity thermal efficiencies of around 30%. The engine includes a recuperator, which results in high efficiencies, even at small scales.

This stands in contrast to conventional simple cycle gas turbines which reach efficiencies approximately half as good as the SBC.

Electrical output is controlled by modulating the turbine nozzle area from full load (2 kW) down to 50% (1 kW) load while holding the turbine inlet temperature constant and the turbocompressor speed nearly constant. To match lesser loads, the turbine inlet temperature is lowered.

The net result is a fairly flat thermal efficiency curve down to about 1 kW, corresponding to the range in which output is controlled by turbine inlet nozzle area modulation. For lesser loads where the turbine inlet temperature is decreased, thermal efficiency steadily declines to 17.5% at 500 Watt output. By conventional power plant standards, these are high part load efficiencies. Even in high ambient temperatures of 120ºF at 4,000 feet, the efficiency at 2 kW output is still a respectable 26%. In operations at higher ambient temperatures, up to 140ºF, which is now fashionably sited as the Iraq Hot Day, the maximum available power will drop slightly below 2,000 Watts.

System Description:

The unit runs at approximately 200,000 RPM, weighs less than 45 lb, uses a quarter of a gallon of JP-8 per hour at 2 kW output, half that amount at 1 kW and approximately the same at 500 Watts.

This design is intended for front-line use to recharge the soldiers' batteries and can be carried by a soldier with a day's worth of fuel.

These characteristics fall within the range of current U.S. Army requirements for future mobile force portable power systems and is consistent with the Department of Defense Tactical Mobile Generator Program.

Small Mobile Battlefield Generators:

An attractive solution for mobile power in the 2 to 15 kW size range is an external combustion, closed Brayton cycle ("CBC") system which offers virtually constant system efficiency from 2 to 15 kW output and could  cover the whole range with a single model, thereby doing away with the current 3, 5, 10 and 15 kW TMG's. This argument is particularly persuasive because the 130 lb weight of the 15 kW CBC system is less than the expected improved weight of the existing 2 kW Diesel TMG and ten times lighter than the current 15 kW Diesel TMG!

This comes about partly because of the profound reduction in alternator weights resulting from the high rotor speeds employed in a CBC system; in this case 240,000 RPM, which also reduces the size and weight of the power inversion and control system as opposed to the 1,800 RPM of the Diesel equivalent.

For example the 40 kW alternator on the Diesel engine shown on the right is 30" in diameter by 61" long and weighs approximately 900 lb; almost as much as the Diesel engine.

In contrast, the high frequency permanent magnet alternator on an equivalent capacity CBC engine driven by its low pressure turbocompressor at 120,000 RPM only weighs 21 lb and measures 9.6″ long with a stator diameter of 3.6″.

This difference is enough to raise eyebrows and to erase any doubt about the veracity of these figures, the cross sectional drawing of the production Capstone micro turbine on the left is presented to reveal the small size of its 30 kW alternator which is 6.6″ long and 3.2″ in diameter.

Overall, the proposed CBC system design reduces the specific weight of a 15 kW MTG to less than 10 lb/kW from 163  lb/kW for the present Diesel 15 kW TMG.

Added to the this weight saving, the weight of fuel storage for a mission must also be considered. The fuel economy of the CBC, especially at part load, is much better than that of the comparable Diesel as shown in the comparison table. For a 24 hour mission the fuel containment weights are also shown

System Description:

Starting at the inlet to the compressor stage¹ of the low pressure turbocompressor, air is compressed about 4 times² and then cooled in an intercooler to near ambient temperature³. The air is compressed an additional 4 times by the compressor stage of the high pressure turbocompressor⁴. The result is an overall pressure ratio of 16. The high pressure air is heated further by the recuperator⁵ followed by the combustion heat exchanger to 1800°F⁶. This hot air is expanded through the turbine stage of the high pressure turbocompressor, providing the energy required to drive its compressor stage⁷. The air leaving the turbine is reheated to 1800°F by passing it around the outside finned surface of the externally fired combustor⁸, before expanding further through the turbine of the low pressure turbocompressor⁹ which drives its compressor and the system alternator. The hot air leaving the turbine is sent through the recuperator¹⁰ where it is cooled and then finally cooled to near ambient temperature by the sink heat exchanger. The cycle is completed by sending the cool air through the alternator before entering the compressor stage of the low pressure turbocompressor to start the cycle over.

The combustion air is preheated by the combustion recuperator before being admitted to the liquid fuel burning, external combustor. The hot gas is partially cooled by the surface heat exchanger which helps to reduce the NO_x emissions and then the hot gas is sent through the combustion heat exchanger to provide the heat for the high pressure turbine stage and a second heat exchange stage to heat the high pressure turbine inlet air. The hot gas leaving the combustion heat exchanger passes through the combustion recuperator and the low temperature gas is exhausted to atmosphere.

System Benefits:

System efficiencies:

As described, the CBC system has important thermal efficiency advantages over the full load range from 2 to 15 kilowatts, which allows a 15 kW capacity system to be used for lower capacity applications, without incurring fuel consumption penalties.

Critics of the CBC concept express reservations about the validity of these efficiency claims at the scale of 15 kW and compare them with conventional simple cycle gas turbines, when in fact a comparison with automotive superchargers would be more appropriate. Compressor maps of small production, turbochargers reflect achievement of 75% compressor efficiency without diffusers or particularly tight clearances. The CBC concept builds on automotive turbocharger production technology, augmented with the use of  proven high performance aerodynamics in small centrifugal compressor wheel designs along with the addition of nozzles and diffusers plus holding tighter clearances substantially aided by the use of low thermal expansion Silicon Nitride material. This combination of cost effective production designs with more refined aerodynamics, provides a confident basis for achievement of compressor efficiencies above 80%. The high pressure turbocompressor in the 15 kW unit is of a size very much like the turbochargers used in high performance motor cycles as held in a hand in the photo to the left.

Advantages of Ceramics:

Silicon Nitride turbocompressor spools take advantage of the very low coefficient of thermal expansion of ceramics as compared with steel (3 vs 13/K), which permits the use of significantly tighter clearances to reduce turbocompressor internal leakage losses and so improve efficiency by several points.

Ceramic turbine wheels are commonly used in turbochargers. The durability of Silicon Nitride (Si3N4) turbine wheels has thus been proven conclusively and offers the added advantage of lower inertia, since Si3N4 weighs about as much as aluminum (3.1 vs 2.7 g/cc), while offering tensile strengths similar to that of stainless steel (0.38 vs 0.5 Gpa). However, the most important advantage of ceramic turbine wheels is that they endure much higher inlet temperatures than high temperature steels (melting point 3,452ºF vs 2,782ºF), thereby providing a significant future cycle efficiency growth margin in CBC power plants

System Endurance:

Since the CBC power plant operates at constant turbine inlet temperature and nearly constant speed, it offers built-in long endurance and reliability. For instance, Diesel engine turbochargers in heavy duty earth moving equipment generally outlast the Diesel engine by several overhaul cycles. In those terms, the reliability of the CBC system would be an order of magnitude higher than the modest maximum 600 hour MTBF being achieved with current Diesel Tactical Quiet Generators (“TQGs”). Almost identical turbocompressors regularly come back to Thermo Mechanical Systems ("TMS") for teardown inspection from field trials with 10,000 hours on them and they are usually just cleaned and put back into service.

Production systems equipped with air bearings require no lubrication and since there are no cooling liquids in the systems they require no scheduled maintenance, other than filling the fuel tank. The air bearings are self pumping and require no external sources. They are custom designed for each specific application, but make good economic sense even in modest production volumes.

Members of the company were instrumental in pioneering the use of air bearings into cooling turbines in the DC-10 environmental control system and also in the design and developed the Capstone micro turbine which marked the first use of air bearings in gas turbines.

Low Noise:

Since the CBC is a closed cycle operating at high rotational speeds, the high frequency noise generated in the loop is contained within the engine, making it inherently quiet. With reasonable attention to noise control on the combustion and exhaust systems, the CBC can  be brought into compliance with much more stringent noise requirements than the 70 dBA specified for current military TQGs. The objective is to make CBC generators compatible with power generation during silent watch.

Agile Response:

Power output in a CBC is controlled by adjusting the air pressure in the closed loop. The turbine inlet temperatures remain constant and the speeds of the turbocompressors remain virtually constant too. The breathing necessary to increase or decrease loop air pressure is brought about very quickly, making the system more agile in following output demand changes than Diesel engines. There is significant thermal mass in the system, including the recuperator and the combustion system, which act as a thermal flywheel, allowing the combustor time to catch up with sudden, large load changes.