Abstract:
A combined heat and power system that may be ON/OFF controlled by the thermal load requirements for the system and/or operated at or below atmospheric pressure.

Description:
PRIORITY INFORMATION 
   This application claims the benefit of U.S. Provisional Application No. 60/513,261 filed Oct. 21, 2003, and U.S. Provisional Application No. 60/499,178 filed Aug. 29, 2003, the entire disclosures of which are incorporated herein by reference. 

   TECHNICAL FIELD 
   This invention relates to the general field of gas turbines and more particularly to gas turbines in a combined heat and power system. 
   BACKGROUND OF THE INVENTION 
   Combined heat and power systems or CHP systems are a well-accepted technology. Also known as cogeneration, many thousands of examples are installed and operating throughout the world. One of the most common versions uses a gas turbine to drive an electrical generator to produce electricity. At the same time, during this electrical energy generation, when available the heat from the gas turbine exhaust can be utilized to produce thermal energy in the form of steam, hot water or hot air. The thermal energy can be used for a myriad of applications, including comfort heating and domestic water heating as well as for air conditioning using an absorption chiller. 
   A typical prior art CHP system is shown in  FIG. 1 . While natural gas is the preferred and most commonly used fuel in CHP systems, and natural gas will be referred to as the fuel in this application, it must be recognized that other fuels may be equally applicable and can be readily used. 
   Ambient air  10  is compressed in compressor  12  to several atmospheres. The compressed air then enters combustor  14 . Natural gas  16  has been compressed by natural gas compressor  18  to a pressure that is high enough to flow through modulating fuel valve  20  and into the elevated pressure in combustor  14  where it burns with the ambient air. The hot combustion gases leave combustor  14  and expand through turbine  22  before going through bypass valve  26  that allows the hot gas turbine exhaust gases to go through heat recovery unit  28  or to bypass it. Bypass valve  26  modulates so that enough hot gases go through heat recovery unit  28  to match the thermal load. The rest of the hot exhaust gases bypass the heat recovery unit  28  and are discharged to atmosphere along with the discharge from heat recovery unit  28 . 
   The turbine  22  drives compressor  12  through shaft  24 . It also drives motor/generator  30  through shaft  24 , compressor  12 , shaft  32 , gearbox  34  and shaft  36 . Motor/generator  30 , such as an induction motor/generator, usually produces 60 Hz. or 50 Hz. power. The electrical output of motor/generator  30  goes through breaker  38  to electric load  40 . 
   Starter motor  42  is powered by battery pack  44  that is charged by battery charger  46 . The modulating fuel valve  20  controls the flow of fuel to regulate the electrical output. 
   In recent years, microturbines have been used in CHP systems. A microturbine is generally defined as a small turbogenerator in which the gas turbine is normally utilized with a permanent magnet rotor rotatably driven within an electrical winding by the gas turbine. 
   The microturbine CHP system concept is essentially the same but with two significant differences which are shown in prior art  FIG. 2 . The gearbox  34 , shaft  36 , generator  30  and starter  42  that were shown in FIG. ( 1 ) have been deleted and replaced with motor/generator  50  that turns at the same speed as compressor  12  and turbine  22 . As motor/generator  50  turns at a very high rate of speed, it puts out high frequency power. Thus rectifier/inverter  52  is added to rectify the output of motor/generator  50  to direct current and then invert it to a more usable frequency such as 60 Hz. or 50 Hz. As motor/generator  50  is a motor as well as being a generator, it is used as a starter motor and receives its energy from rectifier/inverter  52  that receives its power from battery pack  44  which is kept charged by charger  46 . 
   The other significant difference is that most microturbines use recuperators.  FIG. 2  shows recuperator  54  which preheats the air leaving compressor  12  before it goes to combustor  14  by using the heat in the discharge of turbine  22  before it goes to bypass valve  26 . This reduces the fuel consumption of the microturbine but also reduces the available thermal energy. Therefore some microturbines used for CHP systems do not have recuperators. 
   In general, gas turbines are used in those applications where individual units are rated at 500 kW or more, while microturbines are used in applications where individual units are rated from 30 kW to 200 kW. 
   Maximum efficiency in a CHP system occurs when all of the electrical and thermal energies are used beneficially. Thus, it is very hard to achieve efficient operation, and therefore economic operation, when the loads change rapidly and substantially. Residential applications are particularly difficult as electric loads may spike as various appliances are used. Correspondingly, the loads may drop to close to zero during periods when the sole loads are items such as electric clocks. At the same time, the thermal loads change precipitously as comfort heating and domestic water heating units cycle ON and OFF. Many commercial facilities have these same concerns. 
   The cost of a CHP system is generally too high to be considered for small residential or commercial loads. Although the rotor groups could easily be derived from turbochargers, which have annual production rates in the millions and are therefore inexpensive, the need for precision controls, starting mechanisms, recuperators, fuel gas compressors and heat recovery units with bypasses drive up the cost. 
   If the generator is not paralleled with the utility, precision controls are needed to maintain voltage and frequency. If the generator is to be paralleled with the utility, precision controls will be needed to bring the electrical output to the correct voltage, frequency and phase before paralleling. These controls also monitor the loads and match the generator output to the load. A critical need is to control fuel flow during startup so that the generator set will accelerate to its operational speed without overheating. An additional function is to control the bypass on the exhaust heat recovery unit so that the thermal output matches the thermal load. 
   The starting mechanism generally consists of a starter motor, the appropriate electrical and mechanical devices to engage and disengage the motor, and the source of starting energy that is generally batteries. These batteries usually have an associated charger. 
   Recuperators tend to be the most expensive single component in gas turbines and microturbines that are so equipped. They transfer heat from the turbine exhaust into the air entering the combustor to reduce fuel consumption and improve efficiency. 
   The exhaust heat recovery units need bypasses so that the output can be reduced. CHP systems that are not paralleled with the grid operate at electrical power outputs that match the electric load. This also determines the thermal output. When the thermal loads are less than the thermal output, the bypass reduces the output to match the load, thus wasting energy and reducing system efficiency. A typical example of reduced thermal load would be when the thermal output is used for heating or air conditioning and the weather is mild. 
   The combustors of conventional gas turbines operate at several atmospheres of pressure. If the fuel is natural gas, it must be compressed to a pressure higher than that of the combustor or it will not flow into the combustor. These gas compressors are expensive and tend to be inefficient thus imposing a significant parasitic load on the CHP system and further reducing the efficiency. 
   SUMMARY OF THE INVENTION 
   The present invention is a combined heat and power system where the system is controlled to the thermal load so that the system always functions as its greatest efficiency. Another aspect of the present invention is a combined heat and power system which operates with the combustor at atmospheric pressure or below. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic block diagram of a prior art gas turbine CHP system; 
       FIG. 2  is a schematic block diagram of a prior art microturbine CHP system; 
       FIG. 3  is a schematic block diagram of the CHP system of the present invention; 
       FIG. 4  is a schematic block diagram of the CHP system of the present invention having its rotating components integrated; and 
       FIG. 5  is a schematic block diagram of the CHP system of the present invention having a high speed motor/generator. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The preferred thermodynamic cycle for use by the CHP system of the present invention is a subatmospheric Brayton cycle that was developed in the 1970&#39;s. Also referred to as an inverted Brayton cycle, it is described in detail in Section 3.10 “The inverted Brayton cycle” on pages 135-140 of David Gordon Wilson&#39;s book “The Design of High-Efficiency Turbomachinery and Gas Turbines” published by the MIT Press in 1984. 
   The cycle is further described in U.S. Pat. No. 4,280,327 entitled “Solar Powered Turbine System” issued Jul. 28, 1981 to Robin Mackay, and U.S. Pat. No. 4,347,711 entitled “Heat Actuated Space Conditioning Unit with Bottoming Cycle” issued to James C. Noe and David W. Friedman on Sep. 7, 1982, both of which are incorporated herein by reference. 
   The basic concept is to create a heating system that operates in cyclical fashion such as a residential or commercial water heater or a furnace which typically operates in the full-ON or full-OFF mode. Thus, when the heating system is required and turned ON, the system starts and provides the heat using the rejected heat from the gas turbine. Simultaneously, the system produces electricity in parallel with the facility&#39;s electric system and can reduce the amount generated by and purchased from the electric utility. If the power produced is more than the facility is consuming, the additional power can be delivered to the utility for sale or for credit against future or past purchases of electricity. 
     FIG. 3  illustrates the preferred embodiment of the concept. The solid lines terminating in arrowheads show fluid flow, the dashed lines show control wires, and the three solid lines between electric load  40  and breaker  38  and between breaker  38  and motor/generator  30  show power wires. 
   Breaker  38 , which connects the electric load  40  to induction motor/generator  30 , is normally open. As the electric load  40  is connected to the facility&#39;s electrical system and/or the electric utility grid, it becomes a source of power for motor/generator  30  to act as a motor and start the system. Motor/generator  30  is mechanically connected through shaft  36  to the low-speed shaft of gearbox  34 . The high-speed shaft of gearbox  34  is mechanically connected through shaft  32  to compressor  12  which is mechanically connected through shaft  24  to turbine  22 . 
   If breaker  38  was closed and no further actions were taken, motor/generator  30  would accelerate to its synchronous speed less its slip angle. Thus, as an example, if motor/generator  30  had two poles and was operating on 60-Hertz power, the synchronous speed would be 3600 rpm. With a slip angle of five percent, motor/generator  30  would run at 3420 rpm. If gearbox  34  had a step-up gear ratio of twenty, compressor  12  and turbine  22  would run at 68,400 rpm. 
   Thermostat  74  is located where it can monitor the temperature of the fluid to be heated. When thermostat  74  senses that heat is needed, it sends a signal to ON/OFF fuel valve  60  enabling it but not opening it. Simultaneously, thermostat  74  sends a signal to breaker  38  closing it which allows current from electric load  40  to energize motor/generator  30  causing it to rotate and in turn causing shaft  36 , gearbox  34 , shaft  32 , compressor  12 , shaft  24  and turbine  22  all to rotate. Compressor  12  pulls a partial vacuum in pressure sensor  72  and heat recovery unit  66 . Ambient air  10  flows through preheater  62 , combustor  14  and turbine  22  into this partial vacuum. When the pressure sensed by pressure sensor  72  drops to a predetermined pressure, pressure sensor  72  sends a signal to igniter  15  tuning it on. Simultaneously, it sends a signal to ON/OFF fuel valve  60  that has already been enabled by thermostat  74  and allows it to open after a predetermined delay to ensure that igniter  15  has had time to function. Igniter  15  incorporates a timer that shuts off ignition after a predetermined period. 
   Natural gas  16  can now flow through ON/OFF fuel valve  60  to combustor  14  where it is mixed with ambient air  10  which has been preheated in preheater  62  and the mixture is then burned. The hot gases from combustor  14  are expanded through turbine  22  into the partial vacuum in heat recovery unit  66 . The gases then flow through pressure sensor  72  and are compressed back to close to atmospheric pressure by compressor  12 . The gases are then discharged to atmosphere by going through preheater  62  where they preheat the incoming ambient air  10 . 
   With the increased temperature, turbine  22  now produces more power than that being absorbed by compressor  12 . This surplus power accelerates the rotating components through and above the synchronous speed of motor/generator  30  which now produces power that it sends through breaker  38  into the electric load  40  where it is used by the facility and/or delivered to the electric utility for sale or for credit against electricity purchased in the past or the future. 
   Heat recovery unit  66  is shown as a heat exchanger where cool fluid  68  is heated and comes out as hot fluid  69 . However, heat recovery unit  66  may also store the heated product inside it as in a domestic water heater. 
   When the thermostat  74  senses that no more heat is required, it sends signals to ON/OFF fuel valve  60  closing it, and to breaker  38  opening it. The system then shuts down. 
   All of the components in the system are low cost and either off-the-shelf items or are similar in design to off-the-shelf components. Breaker  38 , motor/generator  30 , pressure sensor  72  and thermostat  74  should be off-the-shelf items while the gearbox  34  uses conventional technology. Compressor  12 , shaft  24 , and turbine  22 , are directly derived from turbochargers that are currently produced in quantities of several million per year. If heat recovery unit  66  is designed to produce hot water, it will be similar to residential or commercial water heaters without the burner. If heat recovery unit  66  is designed to produce hot air, it will be similar to residential or commercial furnaces but without the burner. ON/OFF fuel valve  60 , combustor  14 , and igniter  15  should be similar to those used in residential or commercial clothes dryers, water heaters or furnaces as they operate with low-pressure fuel and ambient pressure air and do not modulate. Shafts  36  and  32  are easily fabricated. A wide variety of control systems can be used. As an example, an rpm sensor could be used in lieu of pressure sensor  72 . 
   The CHP system of the present invention only operates at full efficiency as it is designed to replace heating systems such as hot air furnaces or domestic water heaters that operate at maximum heating or are OFF. Therefore it only operates at full thermal load. It also operates at full electrical output as the electricity produced is either used in the facility, or sent to the electric utility. 
   The preferred embodiment of the CHP system of the present invention is shown in  FIG. 3  and described above. As illustrated in  FIG. 4 , an alternative would be to integrate motor/generator  30 , gearbox  34 , compressor  12 , and turbine  22  so that motor/generator  30 , compressor  12 , and turbine  22  are close-coupled to and suspended off gearbox  34 . 
   As shown in  FIG. 5 , another alternative would be to replace the motor/generator  30 , shaft  36 , and gearbox  34  of  FIG. 3  with a bi-directional rectifier/inverter  80  and high-speed motor/generator  50 . The operation is identical to that described with respect to  FIG. 3  except that the high speed motor/generator operates at the same speed as compressor  12  and turbine  22 . It does, however, require the bi-directional inverter  80  to match the frequency of the electric utility to that of high speed motor/generator  50 . 
   The CHP system of the present invention defines a system that does not require precision controls, a starter motor, starting batteries, battery charger, recuperator, heat recovery bypass or fuel gas compressor. It is very efficient as it only operates with full electrical and thermal output. It is suitable for residential, commercial and other applications. In general, it uses either readily available, inexpensive components, or components that are derived from ones that are currently in mass production. 
   While specific embodiments of the invention have been illustrated and described, it is to be understood that these are provided by way of example only and that the invention is not to be construed as being limited thereto but only by the proper scope of the following claims.