Field

This disclosure relates to generating Alternating Current (AC) electrical power from photovoltaic cells (PVCs). More particularly, this disclosure relates to generating utility grade AC electrical power from PVCs without the need for high-power inverters or other transistor- or thyristor-based devices.

Background

Conventional PVCs (also commonly referred to as solar panels) produce direct current (DC) electricity from sunlight. For most high-power applications, however, DC electricity is unsuitable. The majority of grid, domestic and industrial power infrastructure is configured for AC electricity, and stepping DC electricity up to high voltages for transmission is complicated and expensive.

AC power can be transformed easily, stepped up to hundreds of thousands of Volts for efficient long-distance transmission and stepped back down to much lower, safer voltage level for home (single-phase) and industrial use (three-phase), with transformers at power plants and distribution points throughout the power grid.

The power transformers used in this process are cheap and reliable because they are electro-magnetically coupled, passive devices that do not require a separate power source in order to perform their function. Because transformers only operate with time- varying alternating current (AC), global power grids generally deliver AC power.

As a result, utility-grade "grid-tied" photovoltaic (PV) power plants ("solar farms") require the conversion of their electrical output from DC to AC. This conversion from DC to AC is accomplished using inverters, which are the primary point failure for PV power plants because they are the only active devices present. Statement of invention

According to a first aspect there is provided an apparatus comprising: a photovoltaic cell for converting incoming light in to electricity; the photovoltaic cell comprising an arrangement configured to switch between a first state in which the photovoltaic cell is producing electricity and a second state in which the photovoltaic cell is not producing electricity so as to provide a pulse-width-modulated electrical output, in response to a digital binary control signal.

According to an example, the arrangement comprises a liquid crystal panel, the digital control signal configured to switch the liquid crystal panel between an opaque state and a transparent state so as to gate incoming light in to the photovoltaic cell, the photovoltaic cell being in the first state when the liquid crystal panel is in the transparent state and the photovoltaic cell being in the second state when the liquid crystal panel is in the opaque state. According to an example, the arrangement comprises a semiconductor layer, and the arrangement configured to selectively subject the semiconductor layer to an electric field in response to the digital control signal, such that when the electric field is present the arrangement is in the first state, and when the electric field is not present the arrangement is in the second state. According to an example, the digital control signal comprises a high frequency digital control signal.

According to an example, a frequency of the digital control signal is equal to or greater than about 0.5kHz.

According to an example, the digital control signal comprises a clock signal. According to an example, the digital control signal is synchronized with a grid- line frequency of an electrical grid to which the apparatus provides the pulse width modulated electrical output to.

According to an example, the photovoltaic cell comprises one or more cells.

According to a second aspect there is provided a system comprising one or more apparatus according to the first aspect.

According to an example, the system comprises a passive electronic filter downstream of the one or more apparatus.

According to an example, the system comprises a transformer downstream of the passive electronic filter, for voltage matching with a grid-line voltage.

According to an example, the one or more apparatus comprises two or more apparatus arranged in parallel.

According to an example, the system comprises no inverters between the one or more apparatus and an electrical grid to which the system is supplying electricity.

According to a third aspect there is provided a method comprising a control apparatus providing a digital binary control signal to a photovoltaic cell for converting incoming light in to electricity, so as to cause the photovoltaic cell to switch between a first state in which the photovoltaic cell is producing electricity and a second state in which the photovoltaic cell is not producing electricity so as to provide a pulse-width- modulated electrical output.

According to a fourth aspect there is provided a method comprising a control apparatus providing a digital binary control signal to a photovoltaic cell for converting incoming light in to electricity, so as to cause the photovoltaic cell to switch between a first state in which the photovoltaic cell is producing electricity and a second state in which the photovoltaic cell is not producing electricity so as to provide a sine-quantised modulated electrical output. According to an example, the photovoltaic cell comprises a first photovoltaic cell in a first segment of a photovoltaic cell installation, there being one or more further photovoltaic cells in respective one or more further segments of the photovoltaic cell installation, and the method comprises the control apparatus switching each segment on and off independently.

According to an example, the method comprises combining an output of each segment.

According to an example, the method comprises controlling on and off states of each segment so as to approximate a target waveform.

According to a fifth aspect there is provided a control apparatus configured to carry out a method according to the fourth aspect.

Brief Description of Figures

The invention will now be described solely by example and with reference to the accompanying drawings in which:

Figure 1 shows a diagram of a photovoltaic installation that makes use of binary-gated photovoltaic cells, according to an embodiment.

Figure 2 shows a cutaway view of the layers that comprise a photovoltaic cell in which a liquid crystal panel gates the incoming photons, according to an embodiment.

Figure 3 shows a cutaway view of the layers that comprise a screening-engineered field- effect photovoltaic cell that uses ultrathin geometry for its top contact.

Figure 4 shows a cutaway view of the layers that comprise a screening-engineered field- effect photovoltaic cell that uses narrow geometry for its top contact.

Figure 5 shows a diagram of the waveforms at each stage of the grid tying process for sinusoidal pulse width modulation. Figure 6 shows a diagram of a photovoltaic installation that makes use of binary-gated photovoltaic cells in a manner that allows strings of PVCs to be controlled separately, according to an embodiment.

Detailed description

As will be discussed in more detail below, the present invention derives sinusoidal, high- voltage grid-quality AC power from DC PV sources, without the use of inverters, or any associated high-power semiconductor switches. This results in a much more reliable and simplified power plant topology.

As discussed above, grid-tied photovoltaic installations require the conversion of PV electrical output from DC to AC, a process referred to as "inversion".

This conversion from DC to AC is nominally performed using an inverter. Depending on the size of the installation, inverters can be distributed as "micro-inverters" integrated into the solar panels themselves, or as large "string inverters", separate units within the installation that handle the conversion of DC power from their respective strings of panels.

Inverters contain complex and expensive buck/boost switched-mode power supply (SMPS) circuits. SMPS circuits require a power source to run and generate a lot of heat, wasting much of the energy that is being converted. The heat causes additional problems. It degrades the semiconductors from which the circuit is made, dramatically reducing the mean time between failures. SMPS circuits therefore require cooling, which expends more power, adds another failure point, and increases the amount of necessary preventative maintenance. Furthermore, it makes it much more challenging to locate large solar farms in the hot and arid regions in which they would generate the most value. In order to alleviate some of the preventative maintenance overhead, inverters are networked with sensors and tied-in to an installation-wide intra-network, in which computers constantly monitor the equipment. This adds yet more cost, uses more power, and complicates the topology of the installation.

Another problem is synchronization. Any photovoltaic installation supplying more than a single inverter's maximum rated power output, for example, must synchronize multiple inverters, else the inverters will contend with each other. There is an even more pressing need for synchronization if the PV installation intends to inject electrical power directly into the grid. Because the grid is subject to many unpredictable and fluctuating loads at any one time, its waveform is constantly changing. This means that any power plant feeding into the grid needs to be tightly synchronized with all AC parameters of the grid's fluctuating power and line frequency (i.e. the frequency of the AC signal travelling over the grid power line) in real time, as well as with the voltage, amplitude, phase angle, and any other perturbations the grid may be experiencing. A loss of line frequency synchronization or an imprecise voltage level can cause power surges that can lead to extended downtime, permanent inverter damage, or explosive fire hazard. This further complicates a distributed field of large, high-power string inverters. The unreliability that results, when compared to the high reliability of traditional power plants, is a barrier to adoption of green power sources.

This synchronization, known as output power regulation, is conventionally performed by inverters. The regulation technique is called "closed-loop regu lation", a practice that is standard within the industry. The AC waveform of the grid is continuously sampled and compared with the waveform of the solar farm AC power output waveform, and the differences measured result in appropriate feedback being sent to the solar farm inverters so that the solar farm waveform can be coerced to change its shape accordingly. The closed loop results when the compared differences of the two waveforms are zero, and this is what the regulating circuitry aims to achieve at all times.

Inverters are responsible for most of the downtime in photovoltaic installations. They are power hungry active devices, expensive and difficult to maintain, in contrast to the relative efficiency, low cost and reliability of the other, generally passive, components in the installation. Through their need for constant maintenance and cooling, inverters also restrict the geographical range of large photovoltaic installations to cooler, less hostile and more accessible locations. Through the electrical proximity requirements of thyristors that mean long inductive paths between the thyristors and the magnetics they switch need to be avoided, they complicate the layout and topological makeup of a PV power plant.

Given all the drawbacks detailed above, inverters represent a significant barrier to the global adoption of clean energy. Prior approaches to generating AC electricity from solar cells have been analogue in their approach, meaning that a sinusoidal electrical output behavior is achieved by the sinusoidal modulation of specialized or hybrid PV devices. Instead of being fully on or fully off, these devices spend their time in a varying state between fully on or fully off. This makes the electrical characteristics of these PV devices unstable. Given unpredictable grid loads, reflected as a variable power grid line or transformer impedance, this resulting output can behave as a voltage source with a variable Effective Series Resistance (ESR).

In an analogue light-modulated approach whereby, for example, the light level is modulated as a sinusoid of constantly changing light intensity transmitted to the photovoltaic elements, the light is gradually obstructed until none hits the photovoltaic elements, and is then gradually reintroduced. This results in a proportionally sinusoidal output current but a constant cell output voltage. The electrical resistance of the PVCs, specifically the source impedance, therefore increases dramatically at lower light levels (because E=IR), the effect known as ESR (Effective Series Resistance). When connected in strings, the impact of ESR increases exponentially and the string is prevented from delivering a constant current. While the output power, measured in Watts, is therefore proportional to the amount of light hitting the photovoltaic elements (because P=IV), the voltage can only be so if the output current is applied to a known and constant resistive load that is linear and non-reactive. A utility power grid is not such a load, and as a result, the output power is stunted. Even with closed-loop regulation, the transfer characteristics of the device may not regulate with adequate settling or response time, and may exhibit over- or under-shoot, ringing, or even oscillation.

The same effect comes into play when the field effect in the depletion layer of the p-n junction of a screening-engineered field-effect PVC is modulated in an analogue manner, rather than the incoming photons. This is akin to combining a photovoltaic

semiconductor structure with a field-effect transistor structure, aiming at combining a solar cell and transistor into a single monolithic semiconductor device that shares the same substrate silicon. These field-effect-integrated pass elements require a field supporting current in order to maintain a photovoltaic effect, since it is the current that creates a useful p-n junction within a field-effect PVC. Operation of a PVC at less than full capacity places it in a de-rated operational area, where it performs poorly as an electrical generator. This again results in a non-linear change in ESR.

Since the PVCs are only ever fully on or fully off under the approach used in this invention, their electrical resistance remains low throughout and strings of PVCs are always operating with near-minimum ESR. By employing strategically defined strings, the need for DC-to-DC converters as well as Maximum Power Point Tracking subsystems can be eliminated as well.

The second problem with analogue approaches is that the frequency at which they are capable of modulating is limited to the line frequency of the grid, generally between 50Hz and 60Hz. The present inventors have noted that even if digital modulation is used, so that the photoelectric effect is only ever "fully on" or "fully off" and thereby avoiding the drawback discussed above, the PVCs would produce a square wave at this frequency. Although this square wave could be made to match the line voltage and frequency of the grid it is feeding into, a square wave would create an enormous amount of harmonic distortion compared to the grid's fundamental and harmonic-free sinusoidal AC waveform. As a result, although the output could be harnessed for local lighting, there is no possibility of it producing high-quality AC power suitable for a utility grid. Even reactive loads locally, such as motors, would overheat due to the excess power being supplied for the entirety of the AC cycle.

With these drawbacks in mind, the present invention relates to a PVC, PVCs or groups of PVCs whose outputs can be gated digitally and in a binary fashion (binary-gated), and at frequencies that can be above 500Hz. This enables the PVCs to produce pulsed direct current (PDC) at their maximum power point (MPP) under available light conditions. This results in an efficiently modulated PVC electrical output capable of Pulse Width

Modulation (PWM).

PWM is a well-understood way of creating an analogue effect with binary or digital means, in this case a sine wave from square waves. It is a form of switched mode operation, in which the regulation point is constantly changing since it is an AC signal. Normally, an SMPS AC-to-DC or DC-to-DC converter regulates its DC output using a reference voltage. The closed-loop regulation circuit maintains a zero difference between the output DC level and that reference level by taking the difference between the actual output and the reference voltage and using it to modulate the PWM, or pulse train, such that the duty-cycle of that pulse train increases to raise the voltage, decreases to lower the voltage. This way, even under varying loads, an SMPS source can maintain the voltage at a specified level. The reflexor response time under transient loads is directly proportional to the pulse train frequency.

By having a sufficiently high ratio between the carrier frequency and the line frequency, a SMPS DC supply can actually become a unipolar arbitrary waveform supply simply by presenting the desired output waveform as the reference voltage. If the SMPS has a bipolar output capability, an AC reference waveform can produce an AC output. This is the concept that drives a modern inverter, as well as a DC-to-DC power converter.

When feeding power into a utility grid, as discussed above, the power output must be synchronized with it. The ability to track and match unpredictable fluctuations caused by the loads imposed upon the grid is a critical requirement. Sinusoidal Pulse Wave

Modulation (SPWM) is the standard for grid tying renewable energy through an inverter, and the present invention accomplishes the same effect but through digital or binary modulation of the PVC output itself rather than afterwards with inverters.

Figure 5 shows the relevant waveforms at each stage of the grid tying process for sinusoidal PWM. A high-frequency sawtooth wave 43 is generated and determines the on and off times of the resulting carrier wave 44 emitted by the PVC by comparing the source waveform 42 (ie. the grid's waveform) with the sampling wave. The pulse train 44 is the SPWM carrier, encoding the grid's waveform in pulses and reflecting in real time fluctuations in the grid. A passive electronic filter such as an LC circuit then decodes the sinusoid 45 and removes the carrier frequency. Any distortions 46 are removed by further filtering. The higher the carrier frequency, the smaller the filter components need to be and the more accurate the resulting grid synchronization.

Embodiments can therefore provide the same high power, high quality and precisely controllable PW-Modulated photovoltaic DC output as a standard inverter front end. Every pulse delivers a constant voltage at a constant current and for a programmable time, governed by a gating modulation signal.

As mentioned, the present invention achieves this effect without thyristor devices. Instead, material properties of a PVC's physical structure enable the PVC to be electrically gated. In other words, a PVC of the present invention may be considered inherently or innately gated. In some examples the terms innately gated or inherently gated photovoltaic cells (IGPVCs) are used to distinguish the PVCs of the present invention from the prior art PVCs which require downstream inverters.

In some examples, the gating of the present invention can be electric or photonic in nature. Through applying one or more small electrical control signals such that the photoelectric conversion itself is switched on or off, this enables or disables the photonic (light) to electronic (electricity) conversion process that is fundamental to the production of PV electricity within the cell. That is in some examples "gating" may be considered to mean switching a PVC between a first state and a second state, wherein in the first state the photoelectric capability of the PVC is enabled and in the second state the photoelectric capability of the PVC is disabled (or in other words the PV effect is binary).

These control signals (i.e. the signals used to control the gating effect) can be timed. The timing may depend on the application. For example, in an SPWM application the control signal will carry the SPWM pulse train that encodes the grid's waveform. In some examples the timing is controlled using an installation-broadcast low power electrical modulation clock signal. This approach is ideal for generating multi-phase AC power from arrays of indirectly binary-gated PVCs and feeding the AC power into the grid in compliance with the stringent utility specifications, since the present invention allows the PVC strings themselves to provide Sinusoidal PWM.

In some examples the control signal comprises a digital binary control signal. It will be understood that there could be a number of different control signals active within a same installation. For example on a basic grid-tied solar farm there would be three different timing signals, one for each phase of the three-phase output. If a sine- quantized approach is taken (discussed in more detail below) there could be more, and perhaps several dozen control signals.

As will be discussed in more detail below, one embodiment of the present invention comprises new hybrid PVC structures which stack electrically operated solid-state optical shutter technology onto, or integrate them into, the PVC itself. These hybrid PVCs gate the incoming light itself, which starts or stops (i.e. gates) the photoelectric output current.

Another embodiment employs existing PV structures that exploit specifically targeted semi-conducting properties of their materials. These structures rely on an electrically- enabled field effect to create the p-n junction within the semiconductors. In prior art PV structures the p-n junction is intended to be self-powered and always on. The present invention employs the same physical structures but in a deliberate and different manner to their original design intent to display modified behaviour. In such embodiments, external timed gating signals are applied instead of the self-generated DC bias currently exploited. The PV device's physical characteristics change accordingly such that the photoelectric effect itself is gated. This in turn switches the output current on or off.

Figure 1 shows a SPWM-based photovoltaic installation 40. It contains gated

photovoltaic cells or groups of cells 12 according to the invention, rather than conventional photovoltaic cells that output DC electrical power which is then converted to AC by inverters. Although in Figure 1 four PVC units 30, 32, 34 and 36 are shown by way of example, it will be understood that in other examples more or fewer PVC units may be provided. Each PVC unit 30, 32, 34, 36 may comprise a solar panel. It may generally be considered that the installation 40 comprises one or more innately gated PVCs 12 as described below. Also schematically shown in Figure 1 is cabling 8, clock 9, grid entry point 10, transformer 11, power cables 14, and passive electronic filter stage 15. It should be noted that Figure 1 is a simplified diagram, and does not show some elements, such as blocker diodes, that are standard elements of a photovoltaic installation and would also be included in a photovoltaic installation that makes use of the current invention.

In the example of Figure 1 the gating is controlled by the central modulation clock shown schematically at 9. The clock 9 is arranged to broadcast a low power timing signal to each cell 30, 32, 34 and 36. The clock signal is generated centrally (as schematically shown by clock 9) and synchronized with the oscillation of the grid power to which the power generated by the installation shall be fed. Grid tying is itself a known technique and for conciseness is not explained in further detail here, but it will be understood that in examples one or more controllers may be used in order to tie the AC output of the installation 40 with the wider grid. The wider grid is schematically shown at 13. In some examples the electrical signal carrying the modulation clock is fed into the photovoltaic cells through separate cabling 8. The clock signal can alternatively be carried along the power cables 14 (i.e. the power cables carrying the electricity to the grid 13, reducing the need for extra cabling across the installation 40). A wireless approach to carrying the clock signal may also be used in some examples. In some examples the modulation clock 9 is used to employ pulse-width modulation (PWM) to excite the transformer 11 (i.e. the clock signal itself is pulse width modulated, and the string DC power is pulsing at the rate of the SPWM on the binary-gate). The clock signal is timed according to the exact grid line frequency at that instant, as described above and in Figure 5, using SPWM. The clock signal may also be timed to match the line potential (i.e. voltage) of the grid in order to provide a sinusoidally alternating electrical current that is in-phase to the grid line voltage. This is

accomplished by providing a divide-down sample of the actual grid power line (meaning the waveform is exact, only the high voltage of the line power has been divided down from many thousands of volts to 10 volts, for example).

In the example of Figure 1 the electrical power from the photovoltaic cells 12 is then combined in to power cables 14 to give a single PWM output that can go through an LC filter stage 15 to produce the grid-tied sinusoidal waveform. The output from the LC filter stage 15 can then be stepped up by a transformer 11. From the transformer 11, the electricity is then fed into the grid 13 at the grid entry point 10. In summary, the strings (i.e. cells 30, 32, 34, 36) generate SPWM, which is fed through LC filter(s) 15; those LC filter(s) 15 turn the pulse trains into sinusoidal waves. The properties of those waves can be adjusted to a fine degree; for example, if it is 50Hz, then each half-wave (hump) is 10ms in duration, but if we have a PWM carrier frequency of 2kHz, then there is a pulse coming every 500 microseconds, (20 pulses per half-wave of line AC) so the shape of the sinusoid can be tweaked to match the grid sinusoid exactly, such that the grid is helped to react to the loads rather than appear as a reactive load by not matching it exactly.

The circuit topology is simplified in Figure 1, but may exploit both series and parallel circuits of individual PV cells or groups of PV cells in a similar manner to existing solar panel topologies to achieve specific voltage and power characteristics. In some examples, groups or strings of cells can be organised as dedicated phase generators with pairs for single-phase PVC-AC installations and triplets for 3-phase industrial and utility poly-phase AC power generating PVC-AC plants. A "string" of cells may be considered a group of cells wired or connected together, in series, parallel or a mixture of series and parallel. For single phase, just two strings 180 degrees out of phase may be required.

For 3 phase systems a minimum of 3 strings is needed, each 120 degrees out of phase. However, in some examples it may be more efficient to run six strings as three single phase systems 120 degrees out of phase from each other. With higher string voltages, megawatts can be bumped much more efficiently and cheaply than a set of inverters can.

Multiple installation-broadcast modulating control signals can then be used to control segments (e.g. one or more strings of cells) of the system topology in a precise, deliberately timed fashion to create pulsed DC electrical power or grid-tied sinusoidal pulse width modulation. In some examples each string belongs to one of up to 6 SPWM phases. This results in the array of photovoltaic cells 12 generating any desired output AC electrical characteristic in terms of voltage, power and waveform without needing the back-end magnetics used in inverters. The control signals can also modulate the output of the PVCs for other purposes, such as turning off sections of a PV installation for maintenance.

In examples having an anti-parallel or other topology that generates the positive and negative segments of the AC cycle from separate strings, only half of the PV cells are in use at a given time - cells can provide Quadrant I (Q1 - the positive half of the AC waveform) or Quadrant III (Q3 - the negative half of the AC waveform), but not both, and so you need two strings of PVCs to provide the entire waveform. The present invention can use PV string DC outputs on a per-phase basis, only running in Ql. Each phase can be generated by the phase angle of the sinusoidal pulse width modulation of the phase-associated PV arrays. With all 3 phases in Ql, but 120-degrees phase shifted, reverse currents equivalent to Q3 are incurred in a 3-phase Delta-primary transformer. Using this unipolar full-wave approach, all cells in a PV installation are engaged full-time in generating each half-cycle of plant power output. They are fully on 70% of the time as a result, rather than the 35% of the time they would be on in an anti-parallel topology. In some examples, simpler, non-grid-tied installations may use a simpler form of solar farm architecture. For example the architecture may employ the gated PV cells of the invention, but do away with line synchronization. Some examples may even do away with matching transformers. In such examples solar panels can be wired in series to create very high voltages at very low currents. This may enable electric fences to be electrified directly, without requiring a fence electrification unit, because a string of the gated PV cells can be arranged in any topology to derive the electrical output characteristics required for any application. Electric fences generally have a voltage of around 8,000 Volts and a current of around 120 milliamps. For night-time usage, a capacitive bank could store enough charge as well as meter the discharges; this would be recharged during daylight hours.

Some other practical examples include (but are not limited to):

1. Rural farm or residence, single phase, charging batteries not tied to any grid other than to know when to switch to battery power.

2. Community single phase 240V AC e.g. equivalent to one windmill per rural village

3. Substation linked pre-distribution, three 12kV phase used for assisting the grid during peak demands like hot summer days or business hours running industrial machinery.

4. Long distanced transmission lines (megawatt class plants) e.g. lOOkV, 300kV or upwards of a million volts.

5. Generation of a military- or aviation-grade 400Hz power source, replacing the expensive motor-generator sets currently used for this purpose in a way that would be particularly useful for applications such as temporary hospitals.

It is to be noted that option 3 above is accommodated with present day PV plants, by generating 480V 3phase, and stepping that up to 12kV distribution (telephone pole) suburban power levels. This results in about 10% average power loss, due to limitations of the thyristors used in MPPT and final AC generation, mostly lost in heat, some at the transformer. High frequency binary gating of PVCs also enables a second method of generating AC without an inverter, namely sine-quantised modulation (SQM). This can be used with cheaper components with slower or asymmetrical responses than are necessary for SPWM, while still making full use of the fundamental innovation of binary switching, the high granularity of timing made possible by the approach that can also generate gating at frequencies above 500Hz, and the ability to do this without thyristor switches. Using this approach, the PVCs in the PV installation are divided into segments. Each segment is switched on and off independently, and the output of these segments is combined to create the desired output voltage. If all the segments are switched "on", the voltage output from the installation is at its peak, and if they are all switched "off" the output is zero. Voltages between the peak and zero can be achieved by having varying numbers of segments switched "on" at any one time, and fine control of these segments across an entire PV installation can create a serviceable approximation of a target waveform.

Figure 6 depicts a PV installation 40 that generates a grid-tied output signal in this manner. Apart from the differences described below, it is otherwise similar to the installations as described above and in Figure 1.

In this example, the PVCs have been partitioned into segments 30, 32, 34 that are independently controlled by control signals distributed by control cables 8 or by other means from the installation's central processor 9. Although three segments are illustrated in this diagram, there can be as many as are necessary to achieve the desired resolution in the output waveform. For example, a total of 36 segments would allow the full wave AC cycle of 360 degrees to be created in 10 degree steps.

The first segment 34 is connected to an earth 49, and the others 30, 32 are all wired in parallel with a bypass diode 47, 48. These bypass diodes allow the current to bypass segments when they are in "off" mode, allowing the outputs of the segments in "on" mode to stack on top of each other without being affected by the resistance of the "off" segments or needing thyristor-based switches to reroute the current around the "off" segments. This allows the central control processor to combine the capacity of any number of the available segments at any one time in order to produce a desired output voltage.

At its most simple, the processor can perform segment gating as a chase of 1 through n followed by n through 1 backwards to create a sinusoidal output. A more complex processor, however, could use predictive algorithms to produce a closer approximation to the grid's waveform.

As discussed above, the gating of the PVCs of the present invention can be photonic (e.g. optically switched PWM) or electric (e.g. field effect PWM) in nature. These are discussed in turn below.

Optically switched PWM

The PVCs 12 themselves may use an optical shuttering system to gate light (e.g. sunlight) entering the cell 12. This requires optical shuttering systems capable of high frequency shuttering (for example in excess of 0.5kHz). In some examples "off the shelf" enhanced liquid crystal 'pi cells' can be used for this purpose (see for example

http://boldervision.com/liquid-crvstals/pi-cell-shutters/ ).

Figure 2 illustrates a way of achieving optical gating using a liquid crystal pi-cell whose transparency is modulated either fully on or fully off by a control signal.

A PVC is generally shown at 12. A light polarizing film 1 and a liquid crystal panel 2 are provided in the PVC 12. That is the liquid crystal panel 2 forms one of the layers of the PVC 12. The liquid crystal panel 2 in this example is a liquid crystal pi-cell. In the example of Figure 2 the light polarizing film 1 and the liquid crystal panel 2 are laminated on top what is otherwise a conventional photovoltaic cell consisting of electrical contacts 3, an n-type silicon wafer 4, a p-type silicon wafer 5, a backing sheet 6 and a frame 7. In the example of Figure 2 the assembly is protected by glass 19 on a front face 21 of the PVC 12. The backing sheet 6, frame 7 and glass 19 are not a fundamental part of this technology. The liquid crystal panel 2 is controlled by the control signal. When the liquid crystal panel 2 is transparent the PVC 12 operates as a conventional PVC (i.e. is "on" and generating electricity), but when the liquid crystal panel 2 is opaque the light is prevented from reaching the photovoltaic elements 4,5 and as a result the PVC 12 stops generating electricity (i.e. is "off"). During PWM operation the liquid panel 12 switches between opaque and transparent at a sufficient frequency for effective SPWM, which can involve frequencies between 0.5kHz and 15kHz. Thus it may be considered that the PVC is either on or off in a binary fashion, in response to a binary control signal.

Field effect switched PWM

An alternative approach makes use of screening-engineered field-effect photovoltaics (SFPV). These cells, which have already been developed (Berkeley U. California, USA, 2011) as a way of avoiding the need to dope expensive silicon wafers with toxic materials like arsenic and phosphorus, create a p-type and an n-type interface using cheaper metal oxides and sulfides as semiconductors in order to introduce a field effect. In this particular semiconductor chemistry, the field must be electrically stimulated in order for the cell to exhibit a photovoltaic effect.

It should be noted that cost reduction and ease of manufacturing were the design intention of this prior art.

Figures 3 and 4 respectively illustrate two distinct contact geometries that can be used to achieve a field effect within a cell 12. In some examples any semiconductor can be used (e.g. silicon, germanium, sapphire, diamond). The glass protection 19, backing panel 6 and frame 7 can remain as per Figure 2. The bottom electrical contact 3 may also remain. Instead of a p-n junction created by chemical doping as it is in conventional silicon-based PVCs, however, the p-n junction is induced within a semiconductor 23 by a field effect (which may be a very small field effect) generated by a gate contact 20 and dielectric 21. When this field effect is present, the p-n junction remains and the photovoltaic cell 12 produces electricity (i.e. the PVC is "on"). When this field effect is not present, there is no p-n junction and the photovoltaic cell does not produce electricity (i.e. the PVC is "off"). A challenge is that a contact above the semiconductor 23 is needed in order to deliver the electric field. There are two approaches to solving this. One is the narrow geometry shown in Figure 4, in which a series of narrow, fin-like contacts 24 made from a low resistance material such as aluminium are used. The other is an ultrathin geometry, shown in Figure 3, in which an ultrathin top contact 22 made from a material like graphene is used. Where an ultrathin top contact 22 is used, scintillation may occur on a side of the contact 22 opposite from that impacted by incoming photons.

The present invention employs a previously unexplored application of the structures of Figures 3 and 4, by using the modulating control signal (e.g. of clock 9) to gate the field effect that is created, such that it turns the photoelectric effect on and off at the desired frequency. A reason is that like a giant MOSFET (Metal Oxide Field Effect Transistor) the gate capacitance can be reduced, such that it can be switched much faster than a Pi Cell. Two reasons to seek a higher PWM carrier or pulse train frequency are:

1) The higher the PWM pulse frequency, the easier it is to filter out of the line sinusoid because it is much further away from the line frequency. Also because high frequencies can be filtered by smaller capacitances and inductances, and so that makes a difference in the cost of the LCL filters, their reliability and size.

2) There is a lot less THD (Total Harmonic Distortion) when synthesizing or integrating a sinusoid (or morphing to the same waveshape as the grid) and this makes a much more efficient energy transfer to the grid.

For grid-tied SPWM, as discussed above, in some examples the frequency of the sampling wave would be 0.5-15kHz. Since only a small current is needed to create the field effect, the control signal itself can be used to create the field effect within the semiconductor 23. When the control signal is on, therefore, the PVC will work as normal and will output electricity, and when the control signal is off there will be no p-n junction and therefore the PVC will not output electricity.

SFPV technology has hitherto had no known application requiring a timed control voltage. The existing implementation of SFPV specifically uses part of the solar cell output to self-generate a constant DC bias voltage to maintain the p-n junction region, and the photoelectric effect is always enabled. Therefore, embodiments of the present invention employ a modified SFPV structure in order to eliminate that permanent DC bias voltage, and instead supply the low power timing signal externally.

In summary, the present invention provides a novel apparatus, system and method for providing AC to the grid. By using a PWM control signal, the PVC(s) are either operating at full power (the value of which may vary throughout the day e.g. due to level of sunlight) and therefore can easily be compensated-for, or the PVC(s) are in an off state. Through high-frequency switching of the control signal, time-variant pulses that are on and off at continuously changing intervals that follow a sinusoidal function are created. As such, the effective voltage can be dynamically adjusted to match the grid's sinusoidal voltage exactly, many times per 50Hz sinusoidal period, thus the voltage can be regulated with high accuracy for variable loads as well as reactive loads.

It may also be noted that a grid's voltage is a function of the lead/lag of the current, which in turn, is a function of the reactive and resistive loading on the grid from utility customers. Thus when the grid's voltage is tracked, it is required to inject the available current aligned with the grid voltage in frequency and amplitude, but also in phase - meaning no lead or lag between our voltage/current, and the voltage/current of the grid. This is known as a power factor of 1. PV cells themselves are unipolar devices This is addressed in some examples by using magnetics (e.g. transformers) to create true AC (i.e. current reversing back and forth) even though the sinusoidal PWM (SPWM) is unipolar, meaning current is not reversing. Thus in some examples two unipolar SPWM PV strings are provided, that are 180-degrees out of phase, which effectively results in true AC (one is heading toward zero, the other toward the peak voltage, then they reverse roles, thus true AC when measured between the two SPWM sources). The purpose of the transformer is to provide the interface to the grid at the proper stepped- up line voltage. Therefore an entire plant can be tightly integrated, eliminating some elements (e.g. inverters) compared to current PV plant topologies. With respect to known PV plant topologies, prior to pumping any inverter (which are eliminated in the present invention), present day PV plants have to employ DC-to-DC front-ends, because of MPPT (Maximum Power Point Tracking) which results from ESR issues incurred by partial shading from clouds, trees, or as the daylight comes and goes throughout the day. Those DC-to-DC converters are essentially the same thing as inverters; they just rectify the inverter AC output to DC. Then that DC is fed to another inverter, to produce predictable AC. One reason the present invention does not require DC-to-DC MPPT conversion is because of "massively paralleled strings" (i.e. PV cells arranged in parallel) which renders the ESR negligible, even under low-light conditions (since putting resistors in parallel reduces the effective total resistance). Present day PV plants do not do this, because the size of the transistors is limited; if they used massively paralleled PV strings, then the current from a string would be so high, that they would need commensurately paralleled thyristors to share that current between them. The more thyristors, the higher the cost and failure rate. Moreover, the present invention can also have much higher voltages from the PV strings for the same reason; the present-day PV plants employ about 400VDC maximum out of any PV string, because of the material science behind transistors. They would have to stack them in series to handle higher voltages, therefore again incurring higher failure rates and more cost.

Thus in the present invention the PV cells can be arranged in parallel to eliminate the low or partial lighting problem that requires MPPT hardware, at least in part because there is no need for thyristors. The present invention can also use higher voltages, in order to reduce the size and complexity and cost of step-up transformer(s) used to interface to the grid transmission lines. This is due to eliminating thyristors and using binary-gated PVCs instead.

A result is that topology is greatly simplified, and in examples re-purposes multiple elements present in standard plant topologies. The reliability and cost advantages are in avoiding thyristor expense and failures, as well as overall parts reduction across the entire plant's design or topology. Hence, the present invention derives sinusoidal, high-voltage grid-quality AC power from DC PV sources, without the use of inverters, or any associated high-power semiconductor switches. This results in a much more reliable and simplified power plant topology. Thus it will be understood that embodiments of the present invention may substantially reduce the cost of a photovoltaic installation and increase its reliability and efficiency. This makes (potentially very large) photovoltaic installations feasible in hostile and remote environments, and can be achieved using technologies and approaches that lend themselves to mass manufacture. By not using any thyristors, the voltages of the strings of PVCs also have no theoretical limit since the panels incorporate bypass diodes which can stack. As a result, it is possible to increase the string voltages to line levels. llkV could be achieved without the need even for a transformer.

The examples described herein are to be understood as illustrative examples of embodiments of the invention. Further embodiments and examples are envisaged. Any feature described in relation to any one example or embodiment may be used alone or in combination with other features. In addition, any feature described in relation to any one example or embodiment may also be used in combination with one or more features of any other of the examples or embodiments, or any combination of any other of the examples or embodiments. Furthermore, equivalents and modifications not described herein may also be employed within the scope of the invention, which is defined in the claims.