1. Field of the Invention
This invention relates generally to the field of energy conversion, viz., heat to electricity, and, more particularly, to thermophotovoltaic CTPV) energy conversion wherein a heat source radiatively heats a solid-state semiconductor energy conversion device or devices (e.g., photovoltaic cells) which convert the resultant photons into electricity.
2. The Prior Art
Thermophotovoltaic energy conversion of heat to electric power is used as an alternative to classical Rankine steam cycles employed for this general purpose. Thermophotovoltaic systems provide for the conversion of heat into thermal radiation and thereafter into electricity by means of the action of photovoltaic semiconductors.
A high temperature heat source, such as provided by burning of combustion gases or any high temperature fluid, heats an emitter surface which radiatively emits infrared (IR) photons with a spectrum characteristic of the temperature of the heat source. The photons are focused onto a thermophotovoltaic cell which absorbs the photons primarily through electron/hole (e.sup.- h.sup.+) creation. The IR photons must be of an energy greater than the difference between the conduction and valence bands of the lattice--referred to as the "band gap"--in order to create an (e.sup.- h.sup.+ pair). Otherwise, the photon will be parasitically absorbed, producing heat without electrical power. The mobile charge carriers (e.sup.- h.sup.+) drift until they come in contact with the photovoltaic interface (p-n junction field), at which point carriers are accelerated thereby developing a potential difference across the cell which can be used to power an electrical load.
Control of the IR spectrum is required to obtain efficient operation of a thermophotovoltaic system, i.e., the spectrum must be matched to the cell bandgap. Photons with energy below the bandgap energy must be recycled back to the heat source for efficient operation, or never emitted from the heat source. Photons with energies much higher than the bandgap will still produce power, but only the bandgap energy value is recoverable out of the incident energy of the photon, and the remainder is wasted as heat. While high energy photons contribute to the total output power, they decrease the relative efficiency somewhat, and are therefore either suppressed or included depending on the specific application. An ideal system, in terms of spectral matching to the cell bandgap, uses laser-powered photovoltaic energy conversion wherein the incident photon energy is set exactly equal to the cell bandgap. Such systems are being developed and tested for space applications; however, the inefficiency of creating the laser beam (.ltoreq.10%) outweighs the high efficiency with which the cell can convert the spectrally matched incident beam (.about.60%), rendering these systems unattractive compared to more conventional systems of energy conversion. Still, the importance of matching the spectrum to the cell bandgap is clear.
Thermophotovoltaic power systems have classically suffered from the parasitic losses associated with photons of energy less than the target thermophotovoltaic cell bandgap (hereafter referred to as "low energy photons") being radiated off the hot "emitter" surface and absorbed in the cells, producing waste heat equal to their energy. Typically, 60-80% of the energy radiated by the emitter surface is low energy photons, depending on the choice of bandgap. These photons must be efficiently recycled back to the emitter, and therefore not wasted, in order to achieve acceptable system efficiencies. Classically, spectrally selective filters have began used to reflect these photons at the cell surface back to the emitter. However, fundamental limitations in the reflective bandwidth of such filters result in significant parasitic absorption efficiency losses, since large fractions of the spectrum have energies less than the bandgap. The emitter can also be modified to suppress its emission of such low energy photons. However, as the emissivity of the emitter in the low energy photon region decreases, its reflectivity increases in a complementary fashion. Therefore, even if the filter reflects a low energy photon back to the emitter, the photon may be reflected back off the emitter toward the filter, again and again. With each pass, there is a chance the photon will be parasitically absorbed in either the cell or the structurals. Hence, the benefits of modifying the emitter emissivity to preferentially suppress low energy photon emissions are almost completely nullified. This could change if extremely low emitter emissivities for low energy photons are achieved, but this is currently beyond the state-of-the-art.
A key issue with thermophotovoltaic energy conversion using low bandgaps is the minimization of "dark current." Dark current is the electrical current flow in a thermophotovoltaic (or solar) cell that opposes the useful photon-generated electrical current. The photon-generated current must be increased significantly above the dark current in order to produce useful power in the cell. This can be accomplished by maximizing the incident source photon flux, or minimizing the dark current. The incident photon flux is exponentially proportional to the heat source temperature, which affords the system designer a method of overcoming high dark currents. Lower bandgap cells have inherently higher dark currents, necessitating higher incident photon fluxes to achieve comparable efficiencies. However, dark currents can be reduced by various means. These means include, for example, front surface passivation to lower surface velocities, using high purity precursors, using heterostructures, and mirrored photon recycling to mitigate radiative recombination. In the latter, radiative recombination is a loss mechanism where photo-excited charge carriers degenerate with the release of a photon (typically equal to the bandgap energy) prior to being collected at the p-n junction. This dark current contributor can be quantified by time-resolved photoluminescence (TRPL). Mechanisms by which radiative recombination photons are recycled (reabsorbed in the cell) will show increased charge carrier lifetimes relative to methods that are ineffective at photon recycling.
A further important area for consideration is cell costs. The costs associated with the manufacture of thermophotovoltaic (or solar) cells typically are broken down into three major categories: wafer cost; cell growth costs; and processing and array costs. Regarding the former, the cell must be grown on a structure of sufficient strength to allow handling for cell growth and post-growth processing (discussed below). The wafers are typically made of very high purity crystalline compounds with crystal lattice parameters (characteristic unit crystal dimensions) acceptable to the active cell layers to be grown on the wafer. Wafers typically make up between 30-50% of the total cell cost, mainly due to the fabrication and processing tolerances. With respect to cell growth cost, the cells are typically grown on the wafers using some form of chemical vapor deposition. (Atmospheric Pressure Metalorganic Vapor Phase Epitaxy (APMOVPE) and Molecular Beam Epitaxy (MBE) are common methods. ) The cell layers typically have +/-10 nanometer tolerances in thickness, and similarly demanding uniformity, purity and doping tolerances. This step typically accounts for 30-50% of the total cell costs. Development costs are significant for new material systems.
Regarding processing and array costs, once a photovoltaic wafer is grown, electrical contacts must be added (commonly using photolithography), the wafers must be cut into individual cells, and the cells mounted and wired into arrays. This step typically accounts for up to 30% of the cell costs.
Thermophotovoltaic spectral control costs (e.g., filtration, emissivity modification, and the like) can also make up a significant fraction of the total cell costs, especially for thermophotovoltaic systems, wherein spectral control accounts for upwards of a 200-300% relative efficiency difference, and, therefore, warrants state-of-the-art spectral control techniques. However, in a production mode, spectral control costs will not be expected to exceed 30% of cell costs.
The application of thermophotovoltaic direct energy conversion has received relatively little attention for a number of reasons. First, there are limited applications due to high costs, viz., cell, spectral control, structural, and optics costs. Second, there has been lack of viable low bandgap thermophotovoltaic cell materials, at least up until recently. Third, there are problems with energy losses, primarily associated with the absorption of low energy photons that do not produce electricity (poor spectral control). In particular, typical thermophotovoltaic system operating temperatures (&lt;1500.degree. C.) require a low bandgap thermophotovoltaic cell (at or below 0.6 eV) to match the relatively cool emitter spectrum, and thereby achieve both high efficiency conversion (&gt;25%) and high surface power density (&gt;1 amp/cm.sup.2 out of cell). To date, no low bandgap photovoltaic power cells have been produced for these temperature ranges, although several have been made that are close (e.g., Germanium 0.67 eV, National Renewable Energy Lab (NREL) InGaAs 0.75 eV, and Boeing Corporation GaSb 0.7 eV).