Micro-combustor system for the production of electrical energy

A system for the production of electrical energy, comprising: a combustion chamber (14) made of material that is able to withstand high temperatures, an injection device (16) connected to said combustion chamber (14) by means of an injection conduit (15), means (17) for supplying combustion support substance into the combustion chamber (14) and means (18) for the removal of gaseous combustion products, means (26) for the selective emission of radiation onto the outer surface of the combustion chamber (14). The combustion chamber (14) is enclosed in a conversion chamber (20) within which are maintained sub-atmospheric pressure conditions, so that a substantial part of the heat developed by the combustion reaction is converted into electromagnetic radiation.

This application is the US national phase of international application PCT/IB2003/004908 filed 3 Nov. 2003 which designated the U.S. and claims benefit of IT TO2002A001083, dated 13 Dec. 2002, the entire content of which is hereby incorporated by reference.

The present invention relates to a micro-combustor system for the production of electrical energy.

The present invention is based on the physical principle whereby thermal energy produced by a combustion is transformed into electromagnetic energy, which in turn is converted into electrical energy, for instance by means oF photovoltaic cells made of semiconductor material.

The object of the present invention is to provide a micro-combustor system for the production of electrical energy with high efficiency of conversion of the thermal energy into electrical energy.

According to the present invention, this object is achieved by a system having the characteristics set out in the main claim.

With reference toFIG. 1, the number10schematically designates a micro-combustor system for the production of electrical energy. The system10comprises a plurality of conversion devices11, electrically connected to each other in series or in parallel, each of which is constructed as described hereafter. The system10comprises a pipeline of conduits12to supply fuel and combustion supporter to the individual conversion devices11, a pipeline13of exhaust conduits for the removal of gaseous combustion products from the conversion devices11and a network of electrical connections, for regulating generated power, for the electrical ignition of the combustors and for transporting the current from the combustor to the load resistance.

With reference toFIG. 2, each conversion device11comprises a combustion chamber14made of material that is able to withstand high temperature. Preferably, the combustion chamber has spherical shape and is constituted by such material as to withstand temperatures in the order of 1500-2000 K.

The combustion chamber is preferably provided with means for the selective emission of electromagnetic radiation, preferably made in the shape of a lining26applied onto the outer surface of the combustion chamber14. The combustion chamber is preferably constituted by a material with high heat conductivity (for instance tungsten or molybdenum), to allow the heat generated by combustion to reach the outer surface26. At least a part of the inner surface of the combustion chamber14is preferably coated with a material with low heat conductivity of the meso-porous or nano-porous type with porosity coated by catalysing agents, having the function of lowering the combustion activation temperature and reducing emissions of polluting reaction products (for instance nitrogen oxides). The material with low heat conductivity can be interleaved with the material with high heat conductivity in the form of a composite.

The lining26preferably has a selective emissivity in a wavelength band of a few hundredths of nanometres. The lining26can for instance be constituted by a micro-structure obtained directly on the outer surface of the combustion chamber, or a thin layer of oxide having a highly selective spectral emission (oxide of yttrium, thorium, cerium, europium, erbium, terbium, ytterbium or other rare earth).

The combustion chamber14communicates with a fuel injection conduit15, with a conduit17for supplying the combustion support and with a conduit18for the exhaust of gaseous reaction products. The conduit15preferably has cylindrical shape with a conical terminal segment, in proximity to the micro-injection system16, with a section that increases outwardly; the purpose of the conical terminal section is to assure that the combustion support substance is aspirated by Venturi effect. The conduits15,18are preferably constituted by ceramic material, or other material with low heat conductivity, to prevent the heat of the combustion chamber to propagate by thermal conduction to the exterior. The outermost part of the exhaust conduit18is preferably metallic to allow exhaust gases to release their residual heat before leaving the conversion chamber. The conduit15may have an articulated shape, for instance a spiral or a coil, to prevent the combustion products from returning towards the micro-injector. Similarly, the exhaust conduit18can have articulated shape to favour the cooling of the combustion products. The supply conduit17is preferably connected to the injection conduit15; alternatively, it can be connected directly to the combustion chamber. The conduit17for supplying the combustion supporting substance can be eliminated if a mixture of fuel pre-mixed with liquid or gaseous combustion supporting substance is injected into the injection conduit15.

The combustion chamber14is closed and it does not exchange gaseous products with the exterior except through the conduits15,17and18.

Each conversion device11is provided with a micro-injection device16preferably constituted by an ink-jet injector, of the “bubble” type or of the piezoelectric type, able to inject drops of fuel or a combustion-support substance mixture of a volume of a few picolitres and with a frequency which can be controlled by means of a controller (30) from a few kHz to hundreds of kHz. Alternatively, if a gaseous fuel is used, the injection system can be constituted by a miniaturised Bunsen burner. The fuel injected by the injection system16penetrates inside the combustion chamber14through the injection conduit15. Preferably, the gaseous fuel injected by the injection device16is selected within the group comprising: methane, propane, butane, hydrogen, natural gas or other fuels including the possibility of adding metallic particles to the fuel.

Each conversion device11comprises a hollow structure19forming a sealed conversion chamber20, within which is obtained a vacuum or is contained an inert gas at low pressure. The combustion chamber14is located inside the conversion chamber20and the conduits15,18extend through the walls of the hollow structure19. The walls of the hollow structure19defining the conversion chamber20can be made of metal, if a vacuum is obtained in the hollow structure19, or of ceramic material coated with a high reflectance layer, in all other cases.

The hollow structure19comprises an elliptical wall21and a planar wall22, so the conversion chamber20has the shape of a rotational semi-ellipsoid with half-axes A and B. The dimensions of the axes of the conversion chamber20may vary from a minimum of 3 to 50 times the diameter of the combustion chamber14. The combustion chamber14is preferably positioned in the first focus of the elliptical surface. The inner surface of the elliptical wall21is preferably provided with a lining23having high reflectance over the entire emission spectrum of the source of radiation.

The planar wall22of the hollow structure19bears means for converting electromagnetic energy into electrical energy, schematically designated by the reference number24. Said conversion means are preferably constituted by photovoltaic cells made of semiconductor material, preferably with a band gap in the order of 0.5-0.8 eV in order to maximise the conversion efficiency by Planck radiation with colour temperature of 1500-2000 K. In a preferred embodiment, the photovoltaic cell is of the Schottky type and the active junction is constituted by silicon and aluminium. In the case of the selective electromagnetic energy the material of the cells24constituting the conversion means is selected in such a way that the band gap energy is slightly greater than the energy of the photons corresponding to the wavelength of maximum emission, in order to maximise the conversion efficiency at that wavelength.

The exterior face of the conversion means24is preferably coated by a reflective metal layer. The inner wall of the conversion means24can be coated by a layer operating on the electromagnetic radiation as a band pass filter. Said layer can be a multi-layered dielectric coating, a metallic coating at the percolation state, an anti-reflection micro-structure (for instance a grid with sub-wavelength period) or a photonic crystal.

The conversion means24are positioned in correspondence with the plane that is perpendicular to the greater axis of the ellipsoid and passing through the centre of the ellipsoid, in such a way that the radiation emitted by the combustion chamber4reaches the photovoltaic means uniformly. Moreover, also by means of the selected geometry, the radiation not absorbed by the conversion means24is reflected by the reflecting rear face or by the front surface of the photovoltaic cell24and falls back onto the combustion chamber14where it is absorbed.

The particular geometry of the conversion chamber20causes both the radiation emitted by the combustion chamber and reflected by the photovoltaic chamber24, and the radiation emitted by the combustion chamber14and reflected by the inner walls of the semi-ellipsoid to be concentrated on the combustion chamber14. This assures a maximum recycling of the electromagnetic energy within the conversion chamber and hence a minimisation of fuel consumption and a maximisation of overall conversion efficiency. The radiation reflected by the inner surface of the semi-ellipsoid or by the photovoltaic cell24is re-absorbed by the lining26with the same efficiency with which it is emitted thereby.

The heat developed by the fuel-support substance reaction warms the surfaces of the combustion chamber and is wholly converted into electromagnetic radiation. The dimension of the conduits15,18extending within the conversion chamber20is such as to minimise the transfer of thermal energy by conduction between the combustion chamber14and the hollow structure19. The radiation emitted inside the conversion chamber20impacts on the conversion means24which convert electromagnetic radiation into electric energy. The electrical power generated by each conversion device11can vary from a few watts to some tens of watts. Each device11is provided with electrical contacts (not shown herein) which collect electrical energy produced by the semiconductor cells24.

Maintaining a vacuum or sub-atmospheric pressure conditions inside the combustion chamber20allows to reduce the quantity of thermal energy dispersed by convection. Consequently, nearly all the heat developed by the combustion reaction is converted into electromagnetic radiation which in turn is converted into electrical energy by the conversion means24. To obtain a vacuum or low pressure conditions within the conversion chamber20, various known techniques for assembling components in a vacuum may be used.