Patent Application
20050109386
This invention relates generally to the field of thermophotovoltaic electric generation and more specifically to a high reliability, high efficiency, distributed generation system.
The field of thermophotovoltaic (TPV) generation suffers for a variety of reasons, including: poor energy conversion efficiency, high installation cost, high generation cost per watt hour, high capital cost, variable load, high peak loads, fuel choice, and low manufacturing volume. These factors severely limit public acceptance.
TPV systems suffer from a poor spectral match between the emitter and the photovoltaic (PV) cells. Emissions with wavelengths below the PV bandgap simply heat the PV and emissions with wavelengths above the bandgap heat the PV, less the bandgap energy. Hotter PV cells result in wasted energy and higher recombination losses within the cell. Thus, selecting emissions just slightly above the PV bandgap will optimize efficiency. US 6,583,350 B1 “Thermophotovoltaic Energy Conversion Using Photonic Bandgap Selective Emitters” utilizes a woodpile 3D photonic band gap (PBG) with a complex dielectric constant emitter material, such as Tungsten, to improve the spectral matching over traditional blackbody, rare earth, or micro structured materials, thereby increasing system efficiency. Lower efficiency wavelengths are still emitted, but at lower power than more common emitters, limited by the photonic crystal structure itself and that the surface disruptions of the crystal form an incomplete band gap at the surface. Woodpile and post/hole PBGs are fabricated with multilayer semiconductor processes.
The emitter in a TPV system frequently has peak emissions in the infrared spectrum. This favors the use of low energy electronic band gap PV cells. While it increases the TPV efficiency, many cells must be connected in series to generate a sufficiently high output voltage for efficient power conversion. Some light is lost in the finite area between series PV cells. Low band gap cells also use less popular semiconductors, have extremely low manufacturing volumes, and are more costly.
PV cells have a high internal impedance. A maximum electrical output power point exists as a function of: optical input power, temperature, cell to cell variance, and age. Allowing a voltage or current greater or lesser than this point will decrease the PV efficiency. If operating near the maximum power point, an increased electric load may attempt to draw power in excess of the maximum power available from the PV, causing the voltage to quickly collapse and completely drop the load. This is especially a problem with loads containing a switched mode power supply, such as many florescent lights, computers, and other electronic equipment. A switch mode power supply can present a negative load impedance, if the supplies input voltage drops, it will draw more current from the input to provide a constant output. Electric motors can draw large startup currents, the motor may fail to start and the motor become damaged. Also, a load step increase may simply overload a PV system operating at maximum efficiency.
Solar TPV systems are similar to TPV systems, but with a solar input instead of a fossil fuel input. For useful emitter temperatures, a high grade solar input is required, such as from a dish concentrator. Diurnal storage is available to compensate for nightly input power loss. The thermal storage mass must be at the focus of the dish, limiting the maximum energy storage to the weight the feed arm can structurally withstand. An input shutter can reduce heat loss at night. An output shutter allows the system to be turned off.
All TPV systems benefit from solid state operation and the associated lack of failure modes of moving parts and from low acoustic noise.
Thus a need has arisen for a thermophotovoltaic electrical generation system and method to overcome the limitations of existing systems.
In accordance with the present invention, a system and method for enhanced TPV generation is provided that addresses disadvantages and problems associated with other systems and methods.
A selective emitter is coupled to a PV cell. A thermally stimulated photonic crystal with a PBG is a selective emitter. The photonic crystal has a wide 3D band gap, one material with a complex dielectric constant, and visible emissions. Visible emissions allow use of PV cells with a higher band gap, more mass produced, and lower cost. A filter is interposed between the PV cell and the emitter to limit out of band emissions. The filter is thermally isolated to reduce thermal emissions from the filter and may also have a photonic band gap.
The PV cells have a maximum power point as a function of incident radiation. An iris is interposed between the emitter and PV cells to limit the energy incident on the PV cells to the maximum efficiency point for the given electric load. Applying an electric load beyond the maximum power point will cause the cell voltage to collapse and even less power will be delivered. Some electric energy is stored in an ultracapacitor to support transient events such as load steps, switching power supplies, and motor starts until the iris is adjusted and the system stabilizes. Without the electric energy storage, the system must be backed off of the maximum power point to allow for transient stability, reducing efficiency. The maximum power point is determined by applying a step in incident energy or in electric load, measuring the system response, and adjusting accordingly.
Thermal input may be a fossil fuel, solar, geothermal, waste heat, or any combination of these. A catalytic converter or an afterburner may reduce fossil fueled NOx emissions. A recuperator may increase burner efficiency. Highly concentrated insolation from a parabolic dish collector may be used as is. Lower grade solar heat, geothermal, or waste heat may require a heat pump to increase the temperature to a useful level for a TPV emission, expanding the range of useful energy sources. The heat pump also reduces the re-radiation of collected energy from the solar thermal collector tube. Thermal storage may be implemented. The thermal storage may be sized to compensate for diurnal to seasonal solar variations or batch variations in waste heat. The thermal storage and TPV converter may be placed in an environmentally protected area, thus providing Uninterruptible Power Supply functionality. Thermal energy may be provided for heating. Reflective and vacuum insulation reduce thermal losses. System components may be paralleled or bussed in any combination for increased reliability.
Other technical advantages are readily apparent to one skilled in the art from the following figures, descriptions, and claims.
For a more complete understanding of the invention, and for further features and advantages, reference is now made to the following description, taken in conjunction with the accompanying drawings, in which:
Embodiments of the present invention and their advantages are best understood by referring to
PV 150 has a maximum electric power output for a given optical power input. Power limiter 140 is a reflective iris operable to limit incident energy on PV 150 and reflecting unwanted energy back to emitter 120 for re-absorption. The reflective iris is perforated as to maintain approximately even illumination of PV 150 as to not create significant partial shadowing losses. Power limiter 140 is operable to track electric power output 180 to maintain PV 150 at or slightly above the maximum power point. If electric power output 180 requires power beyond the maximum power point of PV 150, the voltage output of PV 150 will collapse. Controller 168 periodically applies a transient to power limiter 140 and monitors the electric output of PV 150 to determine how close PV 150 is operating to its maximum power point and to optimize the steady state condition of power limiter 140. In an alternate embodiment, a load pulse is applied, via transient storage 170 or via a load resistor. Transient storage 170 may first source power then sink power, providing a power transient on PV cells 150 of twice the peak flow from transient storage 170. Alternatively, a single sided pulse may be used.
Voltage collapse can be exacerbated if electric power output 180 is connected to a switching power supply, as a switching power supply will drop its input impedance in attempt to deliver its expected output, causing further voltage collapse of PV 150. Electric motors frequently require a high starting current followed by a lower running current. Failure to provide adequate starting current can cause premature motor failure. Traditionally, a PV cell without significant design margin is not suitable to start a motor due to the PV cell's high impedance. Traditionally, a TPV device must have excess incident energy to have a low enough impedance to allow a motor start, at the expense of efficiency. Load steps at electric power output 180 may also cause voltage collapse if PV 150 is operated close to its maximum power point. Transient storage 170 is an array of ultracapacitors coupled to a bi-directional power supply to provide a low impedance at electric output 180 while operating at the maximum power point of PV 150 and thus mitigate user concerns over motor starts, load steps, switch mode power supplies, and tuning pulses from controller 168. In an alternate embodiment, transient storage may be a capacitor, a battery, or a flywheel.
Emitter 120 is generally operated at a high temperature, between 500K and 1500K. Values beyond this range can be used, but are less desirable. Thermal conduction and convection through air from emitter 120 can be a significant heat loss causing PV 150 to operate at higher temperatures. Hot plate 110, cold plate 114, and bellows 112 form a vacuum can, significantly limiting non-radiative energy coupling. Bellows 112 provides a long thermal path between hot plate 110 and cold plate 114 and allows for relief of thermal stresses. Hot plate 110 and cold plate 114 may be ceramic, metal, or glass. Alternatively, the system may be rearranged to operate within a tank, reducing mechanical stresses from vacuum pressures. Optional getter 118 helps reduce vacuum degradation with time. Optionally, if emitter 120 contains Tungsten, the vacuum may be backfilled with a halogen gas, well known to reduce metal deposition on cold surfaces. Alternatively, after providing appropriate component spacing, backfilling the vacuum with a high Knudsen number gas reduces the mechanical stresses on the vacuum can. Heat sink 116 provides a cooling mechanism of PV 150.
Typical voltages and currents from PV 150 must be converted to levels useful for electric power output 180. A wide input range power supply consists of: primary side switches 162, transformer 164, and secondary side switches 166. Primary side switches 162 and one half of transformer 164 are placed within the vacuum can. Energy is magnetically coupled through non-metallic cold plate 114. No feed through penetrations of the vacuum can are required, improving the long term leak rate of the can. In an alternate embodiment, an electric vacuum feed through is utilized and all of the power conversion is done outside of the vacuum can. If 60 Hz single phase AC source is desired at electric power output 180, power limiter 140 may provide modulation. Power limiter 140 may also include an optical chopper wheel in addition to an iris. Modulating the optical power incident on PV 150 allows smaller bulk capacitors, as ripple currents may be reduced and the energy associated with less than peak voltage output does not need to be electrically stored to maintain the higher efficiency associated with operation at the maximum power point. If a PV cell 150 fails and only a single PV cell 150 is present in TPV power conversion system 100, power limiter 140 is completely closed to minimize lost efficiency. If multiple PV cells 150 are present, a relay or FET may short the underperforming cell. Alternatively, a diode may be substituted for a relay at the expense of higher losses.
TPV power conversion system 100 may be paralleled for increased capacity or increased redundancy. Emitter 120, filter 130, may be segmented for manufacturability. PV 150 may be segmented and series or parallel stacked for increased output voltage or for use of standard size cells. TPV power conversion system may be sized for outputs of anywhere from sub milliwatt to parallel combinations of multiple megawatts.
Initially, the system is shown operating at maximum power, with a minimum energy loss between PV incident power 410 and PV output power 420. A first tuning pulse is illustrated in PV incident power 410. For the negative portion of the pulse, PV cells 150 cannot produce enough energy to supply the load on PV cells 150 and PV output power 420 drops by significantly more than the drop in PV incident power 410. For the positive portion of the pulse, PV cells 150 produce more energy than can be delivered to the given load. The ratio of input to output power of PV cells 150 is measured. From the slope of measured efficiency, it is determined that the PV cells were already operating at the maximum power point for the given load. Two or more power measurements are taken to make this determination. This tuning provides closed loop efficiency optimization. Transient storage 170 delivers or consumes the excess power shown in transient storage power 430 so that system output power 440 does not become unstable with a switch mode power supply load, and has no transient.
The load is increased at a first load step. Stored energy is supplied from transient storage 170 until the incident power 410 is increased to supply the new system output power 440 plus additional power to recharge transient storage 170. Once transient storage 170 is recharged, incident power 410 is returned to a nominal value. The value of desired incident power 410 may be determined, open loop, from a look up table given the desired PV output power 420. The desired PV output power 420 is the system load 440 plus, if the PV cells are not overloaded, power to return to or maintain transient storage 170 in a fully charged state. The lookup table values are not always accurate due to temperature, component aging, dirt or deposits on PV cells 150 or filter 130, inaccurate opening of iris 140, or manufacturing variance in any of these components. Thus, a slight deficiency in output power is shown after the load step, and is supplied from transient storage power 430. Net power flows may be monitored to determine this deficiency and a second tuning pulse applied to determine if incident power 410 should be increased or decreased to re-optimize efficiency. The look up table is updated with the optimized incident energy 410, closing the open loop, replacing default values with learned values. If TPV power conversion system 100 is connected to a load management system, a load may request a change in supplied power before presenting the demand. In this case, incident energy 410 may be adjusted preemptively, thereby reducing transient storage power 430. Again, transient storage 170 delivers the energy deficit or stores the excess energy so that system output power 440 has no transient.
The load is decreased at a second load step. Transient storage 170 is maintained at capacity. The excess power produced by PV cells 150 cannot be consumed and represents a temporary efficiency loss. Iris 140 is adjusted to reduce incident power 410 and regain efficiency.
Thermal energy is converted to optical energy in step 530. Step 530 employs a photonic crystal emitter with some or all of the following properties: a high refractive index contrast, one material has a complex dielectric constant, a full 3D PBG, a wide band gap, and inverse opal structure. In alternate embodiments, these parameters may vary in degree or be absent. In yet another embodiment, a micro structured, rare earth, or blackbody emitter is employed. Optical emissions are spectrally matched to PV cells in step 532 to maximize PV conversion efficiency. The spectral matching may be accomplished with a dielectric filter, a PBG filter, a phosphor layer, a quantum dot layer, or other optical filter. Optionally, the filter may have a high emissivity coating on a portion of the filter to re-direct dissipated thermal energy back to be re-emitted in step 530. Emitted power is optimized to the maximum power point of the PV cells in step 534. A reflective iris may be used to limit power. PV cells convert spectrally shaped radiation to electricity in step 536. Electricity is stored in step 540 to compensate for: transients, load steps, switch mode power supplies, and optimization of incident radiation by tracking the maximum power point of PV cells in step 550, thereby increasing efficiency. Electricity may be stored in series or parallel combinations of ultracapacitors, capacitors, or batteries. A bidirectional switch mode power supply also in step 540 is operable to maintain output voltage regulation and ultracapacitor charge. Step 550 applies an impulse in the power incident on the PV cells while measuring the slope of the change in input to output power of the PV cells. If the iris is slightly closed and the PV conversion efficiency increases, the incident energy may be decreased. If the iris is slightly closed and the PV conversion decreases, the PV is already operating at the maximum power point and the iris is immediately returned to the original opening, the reduction in incident energy having resulted in operation past the maximum power point and a small collapse in output power. Alternatively or additionally, step 550 may use a lookup table to determine the desired incident power based on the load. The lookup table may be updated with optimized values. Alternatively, step 550 may use bidirectional power supply to create a load step by decreasing and increasing the state of charge of ultracapacitors. Step 550 may also control the burn rate of the heat source. Step 542 provides a DC electric output and step 544 provides an AC electric output. Steps 540, 542, and 544 may be combined to optimize power conversion electronics.
Any step may be combined with itself in a parallel fashion, or any group of steps may be combined in a series or parallel fashion to achieve the desired power flows or desired reliability.
Although embodiments of the system and method of the present invention have been illustrated in the accompanying drawings and described in the foregoing detailed description, it will be understood that the invention is not limited to the embodiment disclosed, but is capable of numerous rearrangements, modifications, and substitutions without departing from the spirit of the invention as set forth and defined by the following claims.
This application claims the priority benefit of U.S. Provisional Application Ser. No. 60/518,488, entitiled “System and Method for Thermal to Electric Energy Conversion”, filed Nov. 10, 2003.
| Number | Date | Country | |
|---|---|---|---|
| 60518488 | Nov 2003 | US |
