Patent Publication Number: US-2016245123-A1

Title: Steam Micro Turbine Engine

Description:
BACKGROUND 
     1. Field of the Invention 
     This disclosure relates to improved gas micro turbine engine, wherein water is injected into the combustion chamber to maintain preset exhaust temperature for maximized efficiency with minimum waste heat. The burning air-fuel mixture evaporates the injection water into steam, thereby creating extra pressure to drive the turbine for useful work. 
     2. Description of Related Art 
     Over the past one hundred years, gas turbine engine has been the major power source for aviation apparatus, ships, locomotives, helicopters, and power plants. Industrial gas turbines usually have fewer moving part than internal combustion reciprocating engines found in most of the motorized vehicles. As the result, gas turbines are more reliable and more efficient. Their efficiency ranges from 35% to 90% in co-generation configuration. However, internal combustion engines (ICE) have been dominated in the small application area, like motorized vehicles, emergency electric generators, lawn mowers etc because an ICE can be designed and built less expensive and more compact in size although the average ICE thermal efficiency is lower at about 20%. 
     A typical gas turbine engine consists of a compressor section, a combustion section and a turbine section. Gas turbines are described thermodynamically by the Brayton cycle, in which air is compressed isotropically, combustion occurs at constant pressure, and expansion over the turbine occurs isotropically back to the starting pressure. It uses atmospheric air as the working medium. Flammable fuel burns and heats the atmospheric air to very high temperature (usually in 1000° C.) to achieve differential pressure between turbine inlet and exit. During this process, a huge amount of waste heat is generated and released into the atmosphere. To recover this waste heat in exhaust gas for higher efficiency, manufacturers spend a major portion of the resources into heat exchange systems and multiple stage turbine systems. It increases the size of the generator system and the total cost of the gas turbine. 
     Although micro turbine engines have a great power-to-weight ratio and are small in size compared to internal combustion reciprocating engines, they have not been used in small applications like motorized vehicles, household emergency electric generators except in large applications like jet airplanes and power plants. The prior arts are not able to address the tough problem in designing and manufacturing gas turbines from both the engineering and materials standpoint because of the high operating temperatures. 
     High combustion temperature in a gas turbine engine also causes Nitric Oxides emission and reduces turbine blade working life. Some common practices, like injecting steam and cool air into the engine have applied to slightly reduce the temperature at the turbine blade surface. Strict material requirements to withstand higher temperature and pressure contribute to higher design and fabrication cost. 
     In most of the prior arts, a compressor is driven by the turbine, which establishes a fixed input and output air ratio. In order to adjust the output power to match the load, the controller can only change the fuel injection amount. It takes significant amount of time (in a few seconds) to have the effect triggered down to the turbine. When the turbine engine is used in automotive applications, it is harder to control the dynamic of the overall output to follow fluctuating load. In this embodiment, the compressor is controlled by an electrical motor, which can be controlled rapidly and precisely by a computer system to adjust output according to the load on top of the adjustment of the fuel injection amount. 
     Some prior arts address the waste heat issue by adding an intake-exhaust air recuperator as shown in  FIG. 7 . It does make some improvement of the thermal efficiency of the engine. 
     Some prior arts address the NOx emission issue by adding steam injection feature to control the combustion temperature as shown in  FIG. 6 . 
     SUMMARY 
     One aspect of this disclosure is to improve gas micro turbine efficiency and reduce complexity by using steam as the primary working medium instead of atmospheric air. The primary expansion pressure in the combustion chamber is caused by evaporation of the preheated injection water instead of high temperature of exhaust gases. The waste heat is recovered by evaporating the water into pressurized steam mass to generate mechanical work. The exhaust steam mixture temperature at the turbine intake is controlled at much lower level (like 300° C. instead of 1000° C. in most gas turbines) so that the waste heat is minimized. The amount of heat exchange equipment is reduced significantly. 
     It is also an object of this invention to significantly reduce NOx pollution by controlling fuel combustion temperature well below 1500° C., at which temperature NOx emissions form in significant amount. 
     It is also an object of this invention to significantly reduce unburned fuel, CO by maintaining richer air to fuel ratio so that the fuel is completely burned in the combustion chamber. At least 20% of the oxygen in the air remains after burned with fuel. 
     In embodiment disclosed in the present disclosure, the temperature at the turbine inlet is reduced from 700-1000° C. to 350-500° C. The loss of thermal volume (about 50%) is replaced by volume gained in water-steam conversion (1,600 times of the volume of liquid water). The lower operating temperature reduces NOx formation and more importantly reduces waste heat. The exhaust condenser converts steam back into water, reduces mass volume and creates low pressure at the exit of the turbine section. Lower operating temperature also reduces the requirements for turbine blades and bearing materials, which are big obstacles, in terms of cost, of applying turbine technology in small application such as motorized vehicles and portable electric generators. 
     In certain embodiments disclosed in this disclosure, injection water heat exchange occurs through conduits in the combustion chamber wall to simplify manufacturing process and further more prevent thermal energy from releasing into surrounding environment. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram of a steam micro turbine engine constructed in accordance with an aspect of the present disclosure. 
         FIG. 2  is a diagram of a traditional micro turbine engine. 
         FIG. 3  is a T-S diagram for simple Brayton cycle of a traditional micro turbine engine. 
         FIG. 4  is a T-S diagram for a steam micro turbine engine constructed in accordance with an aspect of the present disclosure. 
         FIG. 5  is the flow diagram for a steam micro turbine engine constructed in accordance with an aspect of the present disclosure. 
         FIG. 6  is the flow diagram for a prior art micro turbine engine with steam injection. 
         FIG. 7  is the flow diagram for a prior art micro turbine engine with intake air recuperator. 
     
    
    
     DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     With reference to  FIG. 2 , a traditional micro turbine engine  30  comprises of air compressor  33 , combustion chamber  34 , turbine  35 , spark plug  34  and fuel nozzle  31 . A fresh air steam is compressed by the compressor  33  and enters into combustion chamber  34 . It is mixed with the fuel injected by fuel nozzle  31 . The spark plug  34  ignites the air-fuel mixture. The mixture is heated by energy released by the chemical reaction in the burn process and expanding with higher volume and velocity. This exhaust gas flows through turbine to extract energy for useful mechanical work. 
     With reference to  FIG. 1 , a steam micro turbine engine  1  in accordance to an aspect of the present disclosure comprises of air compressor  3 , combustion steam chamber  4 , turbine  5 , controller  6 , water heat exchange  7 , water pump  9 , spark plug  14 , water solenoid  15  and fuel nozzle  13 . 
     A fresh air stream is heated by a condenser  11  and flows through into the compressor  3 . The compressed heated air enters the combustion chamber and is mixed with the fuel that is injected by a fuel nozzle  13 . The spark plug ignites the air fuel mixture. Water injection nozzles spray high pressurized heated water droplets at the flame front. The exhaust gas quickly evaporates the water into super-heated steam. It creates 1600 time mass volume of the water in the liquid form. The steam becomes the primary working fluid with over 50% of the mass volume in the mixture. While the exhaust steam gas mixture is flowing through the combustion chamber, the injection water is heated in water heat exchange  7  along the combustion chamber outer wall. The waste heat that would have been released outside the combustion chamber is greatly reduced. 
     The exhaust steam gas mixture enters into turbine  5 . The expansion of this mixture, caused by the differential pressure between the inlet and the ambient pressure, rotates the turbine machine. Useful mechanical work is extracted from this process. It can be used for driving a motorized vehicle or generating electricity through a rotor. 
     The gas steam mixture enters the condenser  11  and the water is recycled for injection in the next cycles. The thermal energy is transferred to the air flow stream to the compressor in this heat exchange process. The flow diagram is shown in  FIG. 5 . 
     The water solenoid  15  is turned on and lasts for duration to provide adequate water injection amount, which is calculated based on the turbine exit temperature and fuel injection amount. The more fuel is injected into the combustion chamber, the more water is needed. Typically, the higher the turbine exit temperature, the more water is needed, and vice versa. In one embodiment, the fuel injection amount is approximately the same as the water injection amount by volume, but other fuel/water ratios can be used depending on the engine configuration and size. Since the steam micro turbine engine has higher efficiency (about 100% improvement), the fuel injection amount is reduced by 50% replaced by 50% water in the illustrative embodiment. In other words, instead of injecting two units of fuel, the steam micro turbine engine described in this embodiment injects 1 unit of fuel and 1 unit of water. 
     The water injection amount can be expressed as the following formula. 
       water volume= a×f+b ×( Te−Tp )
 
     where f is fuel volume, Te is turbine exit temperature, Tp is desired turbine exit temperature, a and b is the coefficient based on engine size and thermal dynamics. The target temperature of Tp is less than 500 degree Fahrenheit to minimize waste heat. 
     With reference to  FIG. 3 , it is a basic Brayton cycle diagram; describes the characteristics of a traditional gas turbine engine. Point S 4  indicates the air working fluid at the compressor inlet position. Point SI indicates the air working fluid entry point in the combustion chamber. The transition from point S 4  to point S 1  represents the adiabatic compression process. Point S 2  indicates the air working fluid at the turbine inlet position. The transition from point S 1  to point S 2  represents the constant pressure heat addition process. Point S 3  indicates the air working fluid at the turbine exit position. The transition from point S 2  to point S 3  represents adiabatic expansion process where the thermal energy is extracted and becomes mechanical work. The transition from point S 3  to point S 4  represents the constant pressure heat extraction process at which the major deficiency occurs. As the diagram indicates, the temperatures at S 2  and S 3  are relative high comparing to the outside environment for traditional gas turbine engine. It causes more heat loss during the process. 
     With reference to  FIG. 4 , it is an improved Brayton Cycle T-S Diagram in accordance with an aspect of the present disclosure. The diagram is similar to  FIG. 3  in terms of processes. The significant difference is that the temperatures at point S 2  and point S 3  are much lower than the ones in traditional gas turbine engine. The other difference is that the entropy property value of steam air mixture working fluid is twice as the one of the air working fluid found in traditional gas turbine engine. The steam air mixture carries more energy per degree temperature. This property makes it a better working fluid than pure air. 
     The above specification, examples and data provide a complete description of the manufacture and use of the composition of the invention. Since many embodiments of the invention can be made without departing from the spirit and scope of the invention, the invention resides in the claims hereinafter appended.