Auxiliary power unit

Details Auxiliary power unit Auxiliary power unit

2000-03-02

2003-03-25

Nguyen, Hoang (Department: 3748)

Power plants

Combustion products used as motive fluid

External-combustion engine type

C060S517000, C060S524000

Reexamination Certificate

active

06536207

ABSTRACT:

TECHNICAL FIELD
The present invention pertains to auxiliary power units for the co-generation of heat and power for indoor use wherein the auxiliary power unit includes an external combustion engine and in particular, a Stirling cycle engine.
BACKGROUND OF THE INVENTION
An auxiliary power unit (“APU”) consists of an engine and an electric generator. Thermal energy of a burning fuel is converted to mechanical energy in the engine of the APU and mechanical energy is converted to electrical energy in the generator of the APU. One advantage of an APU is that it is a portable size such that it can be easily transported and used in a remote location, such as a construction site, cell tower or cabin, that is not connected to the local power grid. APU's are also important for providing emergency backup power for businesses and homes during a power outage.
Small and portable APU's using an internal combustion engine are widely available. For example, a 350 W APU weighs as little as 20 lbs while a 1 kW APU weighs around 70 lbs. However, APU's which use an internal combustion engine cannot be used in a closed environment because of the toxic emissions generated by the internal combustion engine. Even if the exhaust fumes were vented to the outside air, the noise generated by the internal combustion engine makes it very unappealing to a user. The venting of the exhaust fumes also reduces the overall efficiency of the system by about 35% due to the loss of the thermal energy carried away by the exhaust gases. Internal combustion engines are further disadvantaged by their high maintenance costs and short lifetimes of the order of 100 operating hours.
Also known in the prior art are co-generation units and heat pumps which use external combustion engines, such as Stirling cycle engines. However, these co-generation units are typically quite large (and therefore not portable) as dictated by the size of the external combustion engine. In addition, the exhaust fumes must still be vented to the outside air. As discussed above, venting of the exhaust fumes reduces the overall efficiency of the system due to the loss of the thermal energy carried away by the exhaust gases and requires additional hardware.
One type of external combustion engine which may be used to power an APU is a Stirling cycle engine. A Stirling cycle engine produces both mechanical energy and heat energy. The history of Stirling cycle engines is described in detail in Walker,
Stirling Engines
, Oxford University Press (1980), herein incorporated by reference. The principle of operation of a Stirling engine is well known in the art.
One disadvantage of a Stirling cycle engine in comparison to an internal combustion engine is the longer response time of a Stirling cycle engine to sudden changes in the load placed on the engine. The response time of a Stirling cycle engine is limited by the heat transfer rates between the external combustion gases and the internal working fluid of the engine and may be on the order of 30 seconds. The response time of an internal combustion engine, on the other hand, is very short because the combustion gas is the working fluid and can be directly controlled by the fuel flow rate. Prior attempts to increase the responsiveness of a Stirling cycle engine provided a variable dead space for the working fluid as described in U.S. Pat. No. 3,940,933 to Nystrom and U.S. Pat. No. 4,996,841 to Meijer or controlled the pressure of the working fluid as described in U.S. Pat. No. 5,755,100 to Lamos. The foregoing references are hereby incorporated by reference in their entirety. However, both these approaches tend to increase the complexity, size, and weight of the engine design.
SUMMARY OF THE INVENTION
In accordance with one aspect of the invention, in one of its embodiments, a method for providing auxiliary electrical power and heat to an indoor area of a house includes generating mechanical energy and thermal energy using an external combustion engine, the external combustion engine burning a fuel and having substantially complete combustion and converting the mechanical energy generated by the external combustion engine into electrical power using a generator coupled to the external combustion engine. The external combustion engine and generator are placed in the indoor area such that the thermal energy generated by the external combustion engine heats an area surrounding the external combustion engine. The external combustion engine and generator may be contained within a portable housing. In a preferred embodiment, the external combustion engine is a Stirling cycle engine. In other embodiments, the fuel burned by the external combustion engine may be propane or natural gas. In accordance with another embodiment of the invention, the electrical power may direct current power or alternating current power.
In accordance with another aspect of the invention, in one of its embodiments, an auxiliary power system for providing electrical power and heat to an indoor area of a house includes an external combustion engine for generating mechanical energy and thermal energy, the external combustion engine burning a fuel and having substantially complete combustion and a generator, coupled to the external combustion engine, the generator for converting the mechanical energy of the external combustion engine to electrical power. The system further includes a first power output for providing electrical power and a portable housing containing the external combustion engine and the generator. The thermal energy generated by the external combustion engine heats the an area surrounding the portable housing. In a preferred embodiment, the external combustion engine is a Stirling cycle engine. The housing may also be mounted in a window or on a wall of the indoor area.
The auxiliary power system may further include a battery for providing starting power to the external combustion engine and for providing power to the first power output. A sensor is coupled to the battery to produce an output signal. The charge level of the battery may be determined based in part on the output signal of the sensor. In one embodiment, the output signal represents the battery voltage and current. In another embodiment, the auxiliary power system further includes an inverter coupled to the first power output for converting direct current power to alternating current power and a second power output for providing alternating current power. In yet another embodiment, the auxiliary power system further includes an air conditioning module for cooling the atmosphere surrounding the housing.
In accordance with yet another aspect of the present invention, a system for controlling the power output of a thermal engine having a heater head, includes a burner for delivering heat to the heater head of the engine and having an exhaust gas product, a fuel supply regulator for delivering fuel to a burner at a specified rate of fuel delivery and a blower for delivering air to the burner. In one embodiment, the system further includes an input for receiving a signal related to a specified temperature of operation of the burner, a sensor for monitoring an oxygen concentration in the exhaust gas product of the burner and a controller for governing the rate of fuel and air delivery based at least on the input related to the specified temperature of operation and the oxygen concentration in the exhaust gas product. The input for receiving a signal may include a slew rate limiter.
In another embodiment, the system for controlling the power output of a thermal engine further includes a head temperature sensor for measuring the temperature of the heater head and a controller for governing the rate of fuel and air delivery based at least on the temperature of the heater head. The system may further include a sensor for monitoring an oxygen concentration in the exhaust gas where the controller includes a controller based at least on the temperature of the heater had and the oxygen concentration in the exhaust gas product.
In a further embodiment, the h

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