Changing climate and the increasing frequency of drought conditions in many countries could generate renewed interest in external-combustion Brayton-cycle air turbine engines. During the 1950's, gas turbines engines were modified to operate as external combustion engines that burn coal. During the 1960's, Escher-Wyss built a series of closed-cycle, external-combustion air turbine engines able to operate in arid regions and mainly on solid fuel that was otherwise unsuitable for use in internal combustion engines. The design of the heat exchangers that were used in these early external combustion engines and the materials from which they were made restricted peak combustion temperature and in turn restricted peak engine thermal efficiency.
Two recent technical developments may allow external combustion turbine engines to operate at higher temperatures and return higher thermal efficiency than earlier variations of these engines. The Proeschel Group of California developed the annular-counterflow heat exchanging recouperator for gas turbine engines. It showed an effectiveness of near 90% under testing and can raise the thermal efficiency of both internal and external combustion turbine engines. These recouperators can be made from a several high-temperature stainless steels. A competing heat exchanger that can deliver an effectiveness of near 90% is the rotating Ljungstrom design. The steel heat exchange elements used in these heat exchangers has also restricted peak operating temperatures. The development of high temperature ceramic material such as silicon nitride and silicon carbide has resulted materials that maintain constant mechanical and thermal properties up to 1400-degrees C (2500-degrees F).
Combustion chambers of modern jet engines can now be supplied with liners made from silicon nitride and turbine blades made from both ceramics have appeared on turbine engines as well as on engine turbochargers. It is theoretically possible for the heat exchanger elements of rotating Ljungstrom heat exchangers (3 RPM) to be made from material such as silicon nitride and silicon carbide. Silicon nitride occurs in several chemical structures and one has a thermal shock tolerance of 750-degrees C. Thermal elements of rotating Ljungstrom heat exchangers are alternately heated by hot gases then cooled by a cooler air stream. They have been used as exhaust-heat recouperators on turbine engines that use pressure ratios of up to 4 to 1. Ljungstrom heat exchangers using ceramic thermal elements that are mounted in series (to spread thermal shock over many rows of elements) could operate at extreme temperatures and withstand extreme temperature fluctuations over thousands of repeated cycles.
In an external combustion air turbine, the high-temperature heat exchanger(s) would be located between the compressor and power turbine, downstream of which would be a combustion supply fan (driven by the turboshaft) and the combustion chamber. Air that flows through the compressor would pass through one side of the heat exchanger and be heated by hot combustion gases that pass through an adjacent side of the heat exchanger. This fast flowing stream of superheated air would then drive the turbine where it would be cooled and lose pressure, after which it would be partially recompressed before flowing into the combustion chamber. Additional air driven by a turboshaft-driven part-load supply fan (with variable-pitch blades) and heated by exhaust gases would also be supplied to the combustion chamber.
This additional air would bypass the compressor(s) and turbine(s) during part-load operation while air flowing through these components is reduced. Reducing the amount of air entering the combustion chamber during part-load engine operation also reduces combustion temperature along with engine thermal efficiency. Pumping extra air into the combustion chamber ensures higher combustion temperatures and higher velocity superheated air flowing through turbomachinery that is rotating at maximum RPM. The combination of all of these factors ensures higher part-load engine thermal efficiency for single-shaft, non-intercooled, non-reheat turbine engines.
A unique variation of this engine is the Star Rotor engine being developed at Texas A&M University. This engine uses a gerotor-type radial-flow compressor and turbine to combine continuous gas flow with positive displacement, a feature allowing it to operate efficiently over a wide range of engine speed using either internal or external combustion. It can operate efficiently even when built to smaller sizes and with less power output than bladed turbine engines. Members of a group called the Hot Air Engine Society have proposed to build an experimental external-combustion, positive-displacement hot air engine involving a piston air-compressor supplying air (through one-way valves and heat exchangers) to the piston expander that drives it.
Converting an Existing Engine:
For the purpose of generating power in arid regions, it may be possible to convert some existing internal combustion gas turbine engines to external combustion operation. The single-shaft non-intercooled turbine engine that use a centrifugal or radial flow compressor that accelerates air perpendicular to the turboshaft, may be a suitable candidate for conversion. In some small turboprop and turbojet engines, the air may flow through a total angle of 540-degrees from the compressor, through the combustion chamber and through the turbines. For stationary use, this layout allows a longer turboshaft to be used while the centrifugal compressor allows greater flexibility in regard to the installation of a large single heat exchanger. Hot air flowing into axial-flow turbines may need to accelerate through the kind of loop stator used at the entry to the turbine used on Elliot railway turbochargers. Star Rotor’s radial flow turbine and compressor means that the air/gas only flows through an angle of 180-degrees.
Converting a turbine engine using an axial-flow compressor would require the lengthening of the turboshaft in order to allow large heat exchangers to replace the combustion chamber(s). For combustion temperatures of under 1700-deg F, multiple Proeschel annular-counterflow heat exchangers would be radially mounted parallel to the turboshaft and at a radius similar to the outer radius of the turbines. For higher temperatures, 8-rotating Ljungstrom heat exchangers would be mounted between the compressor and turbine and with their axle-centres located at the outer radius of the turbines. The airflow from the compressor to the turbine would be similar to that of an internal-combustion turbine engine. Warm air leaving the turbine would flow through 180-degrees in order to enter the combustion system and be heated before flowing in the opposite direction through the heat exchanger. Additional air would be supplied to the combustion system during part-load operation by a part-load supply fan.
The conversion of an intercooled, reheat turbine to external combustion would require that the turboshaft by lengthened between compressors and turbines, as well as between the high-pressure and low (intermediate) pressure turbines. The front part of the engine (intercooler) will remain unchanged while heat exchangers replace the combustion chambers. Air leaving the low (intermediate) pressure turbine would flow through 180-degrees in order to enter the external combustion system that will supply both primary and secondary heat exchangers. Air from the intercooler fan would be further heated by exhaust heat before entering the combustion system. This additional air would raise the air mass flow through the combustion system at double or higher than the air mass flow through the engine turbomachinery. The overall engine thermal efficiency of the intercooled/reheated engine would be marginally higher than that of the non-reheated/non-intercooled single-shaft engine.
The table below gives an idea as to the range of temperature expected from non-intercooled, non-reheat, single shaft turbine engines. The centrifugal compressor has an isentropic efficiency of 80% and a pressure ratio of 4 to 1 while the axial-flow turbine has an isentropic efficiency of 85% and a pressure ratio of 3.8 to 1. Ambient air temperature is spec’d at 32-degrees C or 90-degrees while the compressor outlet temperature has been calculated at 424-degrees F. The part-load fan and combustion induction fan are spec’d at pressure ratios of up to 1.1 to 1.
The range of combustion temperatures reflects the range of fuel being used, from gasified biomass having high moisture content to anthracite coal. Gasified coal typically combusts near 2100-deg F. The engine exhaust temperature at maximum power is high enough to convert water into saturated steam, reducing the overall energy required to generate superheated steam needed to operate a bottom cycle stream engine. In regions where water is available, the overall combined energy efficiency of an external Brayton and Rankine combination could go as high as 30% burning high moisture biomass or as high as 50% burning anthracite coal. At an ocean-side power station where an ample heat sink would exist, heat rejected from the steam engine could energise a downstream Brayton engine using R-34 as its working fluid, thereby further raising overall engine efficiency. In dry and arid regions, exhaust heat from small externally heated Brayton (turbine) engines could energise thermoacoustic engines and raise overall engine thermal efficiency to over 50%.
Internal-Combustion Solid Fuel Engine:
The advent of solid-fuel gasifiers that liberate combustible gases from solid fuel can allow certain types of internal-combustion gas turbine engines to be fueled by biomass. Many gasifier combustors have a feature that continually transfers ash into an ashpan during engine operation, allowing only the combustible gases to be burned in the combustion chambers. The main drawback of this system is that single-shaft non-reheat turbine engines only deliver peak efficiency at maximum engine speed and at maximum power output. The turbine may be used to generate power at stationary locations while its hot exhaust partly energises a bottom-cycle steam engine.
Solid fuel gasifiers may be applied to intercooled/reheat turbine engines, one design of which is known as a complex-cycle engine. It is an internal combustion 3-shaft gas turbine that delivers a competitive efficiency when operating at over 20% of its maximum power output. The high-pressure compressor and turbine are located on one turboshaft, the intermediate-pressure power turbine that drives the load is on a second shaft while the low-pressure compressor and turbine are located on the third turboshaft. Fuel combustors are located upstream of both the high and intermediate pressure turbines. There would ideally sufficient oxygen in the high-pressure turbine exhaust to support combustion to reheat gas entering the power turbine. Historically, the complex cycle turbine was a temperamental and problematic engine.
Modern computerised engine control technology using exhaust-system oxygen-sensors can regulate the combustion in both the high-pressure and low-pressure combustors and alleviate the operation problems of the 3-shaft complex turbine engine. In this engine, an intercooler is located between the low and high-pressure compressors while exhaust recouperators would be located downstream of the high-pressure compressor and ahead of the primary combustor. Biomass may be the main solid fuels being gasified in this type of engine. Dell-Point Energy (Montreal) has developed a line of solid fuel biomass gasifiers that can be upscaled and adapted for use with turbine engines. Auger mechanisms would remove ash produced by the gasifier during engine operation to reduce carbon deposit formation on turbine blades.
Solar Energy & Energy Storage:
Highly concentrated solar thermal energy could be used to energise an air turbine engine during daylight hours. The same technology could also be used to energise a thermal energy storage system that will enable the air turbine to generate electric power after sunset. Heat gently applied to calcium carbonate (CaCO3) will decompose it into calcium oxide (CaO) and carbon dioxide (CO2) that may be sent into storage. Research undertaken at Nagoya University revealed that at a pressure of 5-atm (73.5-psia), calcium oxide would react with carbon dioxide and release 168 to 239-BTU/lb of heat at 900 to 1000-degrees C (1650-F to 1830-F). This heat could energise and air turbine engines driving electrical equipment and the turbine exhaust heat could either drive thermoacoustic engines or preheat water intended for use in a steam engine that will generate electric power. Several other metallic carbonates may also be similarly used as thermal energy storage material for air turbine engines.
Nuclear Powered Air Turbines:
Pebble-bed modular reactor (PBMR) technology can be scaled to operate in mini- and micro power stations. The flexibility of this technology to rapidly shut down and re-adjust heat levels allows it to be used in conjunction with air turbines. Pebble-bed fuel spheres are coated with silicon carbide, the same material from which some turbine heat exchange components can be made. Pebble-bed nuclear air turbines may be used to generate commercial electric power in arid regions of the Southwestern United States, parts of South Africa and parts of Australia. The technology may also be adapted for use in large future azipod-propelled intermodal freight ships that are 250-ft wide by over 4,000-ft long and have rail lines on most of its levels. Container trains may be ferried across oceans among nations using the 4’ 8.5” railway gauge.
There is future potential for waterless coal, nuclear and biomass thermal power stations to appear in the future. Externally heated air turbine engines as well as solid-fueled internal-combustion air turbine engines could be used to generate electric power in the future.