Technical Field

The present invention relates to electric power control for supplying a load, and in particular though not exclusively, supplying the load from two power sources, being solar panels and an electrical supply grid. Background

Currently solar panels are used to supplement the electrical power requirements of residential commercial and industrial premises which are also supplied by a mains electrical grid fed from power stations, hydro-electric dams and the like. Typically the electrical loads in the premises require power to be supplied according to predetermined characteristics such as alternating current (AC) at 50 or 60 Hertz and at 240 V or 1 10 V for example. As solar panels generate direct current (DC), typically at a lower voltage of 30 - 50 V, some apparatus for conditioning this power supply and integrating with the electrical grid is required in order to supplement power to the various loads. Such power control apparatus are expensive and comparable to the cost of the solar panels which acts as an economic disincentive to installation of this green energy technology.

The amount of power generated by solar panels varies considerably, in response to cloud cover, night time conditions, and relative sun position. In addition many solar panel installations are typically also complimented with a battery storage solution which is charged when the solar panels are generating more power than required for the load. The batteries may then be discharged to supply the load when there is insufficient solar power generation. However, the circuitry required for properly charging and controlling the batteries further adds to the power controller costs.

Alternatively or additionally, excess solar power may be fed back into the electrical grid in order for the solar panel owner to derive some revenue from the electrical grid operator. However typically the electrical grid operator pays significantly less for power supplied in this way than it charges for power delivered to the premises. Electrical power supplied into the electrical grid must also meet compatibility requirements which include synchronizing any converted AC power from the solar panel DC supply to the electrical grid supply and providing anti-islanding isolation in the event of a grid fault. Anti- islanding isolation prevents the section of the grid coupled to the controller from being supplied with power from the solar panels when this section has been disconnected from the rest of the grid, in order to allow maintenance by electrical workers. This isolation process may be complicated by the presence of other solar panel installations coupled to the grid section which can make the grid appear to be still operating. These considerations add to the complexity and cost of a power controller for integrating solar panel generated power with the electrical grid for supplying loads within a premises.

Furthermore, as is well known, the efficiency of solar panels is highly variable and dependent on a complex interaction of factors including solar irradiation, ambient temperature, and the load into which the solar panels are connected. Therefore a further function of a solar power controller is to continuously adjust the input load in order to maximise solar panel efficiency. This is known as maximum power point tracking (MPPT) and this functionality further adds to the complexity and cost of a solar/electrical grid integrating power controller.

Figure 1 is a schematic of a known type of solar power system 100, comprising an array of solar panels 105, coupled to a power controller 1 10 which is also typically coupled to an electrical grid 130 and/or a battery 140. Power received from the solar panels 105, electrical grid 130 and/or battery 140 is provided to one or more loads 120 associated with a premises such as a private house, commercial premises such as a retail park, or an industrial facility for example. The power supplied to the loads is AC typically in the form of a modified sine wave provided by an inverter and sufficiently similar to the electrical grid wave form that standard power consuming appliances can utilise this power. Such appliances may include for example hot water cylinders, televisions, clothes dryers and lighting within a residential premises, and motors, fluorescent lighting and heating appliances in industrial premises. Excess power generated by the solar panels 105 is directed by the power controller into charging the battery 140 or redirected back into the electrical grid in order to supplement that supply. When the solar panel power supply is insufficient for the load the power controller switches to the battery 140 or electrical grid 130. As described previously, this functionality is complex to implement and expensive compared to the cost of the solar panel installation. The reference to any prior art in the specification is not, and should not be taken as, an acknowledgement or any form of suggestion that the prior art forms part of the common knowledge in any country. Summary

It is an object of a preferred embodiment of the invention to provide a power controller which will overcome or ameliorate problems with such at present, or to at least provide the public with the useful choice.

According to one aspect, there is provided a power controller for supplying a load from first and second power supplies. The controller comprises first storage means arranged to store power from the first power supply. Switching means arranged to switch between a second storage means and the first storage means or the second power supply depending on a parameter of the first and second storage means. Power supply means arranged to supply power to the load from the second storage means.

The parameters may be voltage levels associated with the first and second storage means. The first and second storage means may comprise capacitors. The power supply means may comprise an inverter for converting the voltage across the second storage means into an AC output for driving the or each load.

In embodiments the switching means may be further arranged to switch between the first storage means and the second storage means dependent on a level of the other storage means. The switching means may comprise a first switch coupled between the first and second storage means, and a second switch coupled between the second power supply and the second storage means.

In embodiments the first power supply comprises solar panels and the second power supply may be an electrical grid, the controller having a rectifier coupled between the grid and the switching means. In other embodiments the first power supply may be generating from another renewable source such as wind energy.

In an embodiment a microprocessor is arranged to receive signals indicative of the voltage across the first and second storage means, and to control the switching means in order to maintain the second storage means within a predetermined voltage range by topping up with power from the first storage means when available, or the second power supply. The inverter is then able to supply a steady AC output to the load using power from the solar panels topped up with power from the grid when necessary. The first storage means is supplied by the solar panels and when above a predetermined voltage is used to supply the second storage means.

These arrangements provide a number of advantages over known systems, including a relatively simple and inexpensive power controller for combining solar and electrical grid power to a load. The power supplied from the solar panels can be simply switched using voltage levels of the respective storage devices such as capacitors, thereby reducing the complexity and cost of the power controller arrangement. Similarly because the solar power is not fed back into an electrical grid or a battery, the corresponding circuitry for managing these processes is also not required. It is estimated that the cost of such a simplified solar power controller can be reduced to one tenth of the normal "complex" solar power controllers currently available and described above. Although the arrangement may result in some loss of excess solar generated power, which is not fed back into the grid for example, this is offset by the large cost reduction of the power controller. It is anticipated that this may be a significant factor in encouraging up take up of residential, commercial and industrial solar power installation.

In another aspect there is provided a power controller for supplying a load from first and second power supplies. The controller comprises a first storage element coupled to the first power supply. A first switch arranged to couple the first storage element to a second storage element dependent on a voltage associated with the first storage element. A second switch arranged to couple the second power supply and the second storage element dependent on a voltage associated with the second storage element. An inverter coupled between the second storage element and the load.

In an embodiment the first power supply comprises one or more solar panels. The storage elements may be capacitors, including for example super capacitors. The first and second switches may be comprise solid state devices which are controlled by a microprocessor and responsive to the respective storage element voltages. The first and second switches are arranged such that the first storage element and the second power supply are not coupled to the second storage element at the same time.

In an embodiment the second power supply is an electrical grid, and the controller further comprises a rectifier coupled between the second power supply and the second switch. There is also provided a solar panel installation comprising a number of solar panels coupled to a power controller as defined above. The power controller in use is also coupled to an electrical grid and a number of loads. In another aspect there is provided a solar panel power controller comprising:

an inverter arranged to supply power to a load,

a first storage device arranged to store power from one or more solar panels,

a second storage device arranged to supply power to the inverter, and

a switch operable to transfer power from the first storage device to the second storage device depending on a parameter of the first or second storage devices.

Unless the context clearly requires otherwise, throughout the description and the claims, the word "comprise", "comprising", and the like, are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense, that is to say, in the sense of "including, but limited to".

Brief Description of the Drawings

Embodiments of the invention are described with respect to the following drawings, by way of example only and without intending to be limiting, in which:

Figure 1 shows a schematic of a known solar panel system;

Figure 2 is a schematic of a solar panel system according to an embodiment;

Figure 3 is a schematic of a power controller for the system of Figure 2;

Figure 4 is a circuit diagram for a power controller according to another embodiment.

Description of Embodiments

Figure 2 illustrates a solar power installation or system 200 according to an embodiment, and which comprises an array of solar panels 205. In this embodiment the solar panels are connected in series to the power controller 210, unlike the parallel connection illustrated in Figure 1 . Because solar panels are arranged to supply DC power, known installations are typically limited by safety regulations to a maximum DC voltage of 50 V. As a typical solar panel will operate up to 30-40V this requires the solar panels to be connected in parallel in order to stay under this maximum voltage requirement. However, as will be described below, the power controller of this embodiment is arranged to draw power from the solar panels in pulses or bursts and may therefore be considered an AC installation. Although current does not change direction, the periodic or intermittent variation in voltage means that the maximum voltage safety limitation can be relaxed allowing series connection of the solar array to the power controller. The series connection arrangement significantly reduces the length of cabling required and in addition, because of the higher voltage, a lower current flow is required for the same power provision. This allows for the installation of reduced size cabling, further reducing cabling costs. Whilst series cabling connection is described with respect to this embodiment, a parallel connected solar panel installation could alternatively be used with the power controller 210. The power controller 210 is also coupled to an electrical grid 130, and therefore receives power from both the solar panel array 205 and the electrical grid 130. No surplus solar power is delivered to the grid 130 and the power controller supplies power to the load 120 in the form of an AC voltage such as 240V at 50 Hz or 1 10V at 60 Hz. These power flows are illustrated by the arrows shown in Figure 2.

Figure 3 shows a schematic diagram of the power controller 210 of Figure 2 which comprises a first storage means 315, a second storage means 330, switching means comprising a first switch 325 and a second switch 335, a rectifier 340, an inverter 345, and a microprocessor 350. The first and second storage means 315, 330 are typically large capacitors which facilitate cost reduction, however other storage means may be employed such as super capacitors and batteries. The switches 325, 335 may be MOSFET or any suitable power switches. The rectifier 340 may be a diode bridge, for example, although any suitable rectifier arrangement for converting the AC power input from the electrical grid 130 to a DC voltage could be used.

Similarly the inverter 345 may be any suitable arrangement such as an H-bridge circuit for example. The microprocessor 350 may be relatively simple and low cost.

The power controller 210 receives solar generated power from the solar panels 205 at the first storage means 315. Electrical grid power is received at the rectifier 340. The inverter 345 supplies AC power to the load 120.

The solar panels 205 supply DC power into the first capacitor 315, charging this and increasing its voltage V1 . Depending on whether the solar panels are connected in series or parallel, the voltage V1 may range up to 40V (parallel) when the sun is shining, and perhaps up to 150V when connected in series and under the same solar energy generating conditions. The microprocessor 350 is arranged to monitor the voltage V1 across the first capacitor 315 or a signal indicative of this voltage V1 , and allows the first switch 325 to close when this voltage V1 is above a pre-determined threshold which is also greater than V2. Upon closing the first switch 325, charge from the first capacitor 315 is conducted to the second capacitor 330. The resulting reduction in charge on the first capacitor 315 causes its voltage V1 to fall below a threshold, resulting in the microprocessor 350 opening the first switch 325. Because of this arrangement, power will be transferred from the first capacitor 315 to the second capacitor 330 in a series of pulses as the first switch 325 opens and closes depending on the voltage level V1 across the first capacitor 315.

This pulsing pattern is illustrated generally at 360 which initially indicates sunny conditions resulting in fast charging of the first capacitor 315 This in turn results in more frequent switch closing to the second capacitor 330, and hence a series of relatively high density pulses or bursts (A). When the solar collection conditions are less optimal, for example under cloudy conditions, the first capacitor 315 will charge less rapidly resulting in less frequent pulsing of power to the second capacitor 330 (B). This pulsing pattern is also superimposed on the power drawn from the solar panels 205 to recharge the first capacitor 315. This pulsing pattern of the current drawn from the solar panels is a varying current implementation, which as described previously allows for higher voltages across the first capacitor 315 and some cabling

advantages. In an example parallel connected implementation, the maximum voltage of the solar panels may be 30V, and the threshold voltage for V1 set at 27V, such that the switch 325 is closed whenever the voltage level V1 across the first capacitor 315 is above 27V. In an example series connected implementation, the maximum voltage may be 1 10V and the threshold set at 100V. Under cloudy or night time conditions the power supplied by the solar panels will fall and hence the voltage V1 may fall for extended periods below the threshold required to close switch 325. The thresholds referred to may be varied in some embodiments depending on factors such as a load demand and available energy

When the first switch 325 is closed, this allows the second capacitor 330 to be charged from the first capacitor 315. This charging increases the voltage level V2 across the second capacitor 330. However, when the first switch 325 is open, the charge from the second capacitor 330 will be drawn off by the inverter 345 to power loads 120 - thereby reducing the voltage V2 across the second capacitor. As will be appreciated by those skilled in the art, the inverter input 345 will have an optimal voltage range, and in order to ensure the voltage V2 of the second capacitor 330 remains within this range, the microprocessor 350 is arranged to close the second switch 335 depending on the voltage level V2 of the second capacitor 330. Thus if the voltage level V2 or a signal indicative of this voltage V2 falls below a pre-determined threshold due to opening of the first switch 325 then the second switch 335 will be closed by the microprocessor in order to allow the second capacitor 330 to be charged from the electrical grid input to the power controller 210. Electrical power from the electrical grid is available at the rectifier 340, which converts the AC power to DC power. Current then flows through the closed second switch 335 to charge the second capacitor 330 thereby increasing its voltage V2. This arrangement ensures that the inverter 345 is operating within a desired input voltage range.

In this embodiment the microprocessor 350 is configured to open the second switch 335 whenever the first switch 325 is closed. This ensures the solar panel installation is isolated from the electrical grid.

The embodiment automatically and cheaply adapts to changes in the power supplied by the solar panels 205, as well as the power consumed by the load 120. For example in sunny conditions and with a light to medium load, all of the power supplied to the second capacitor 330 may come from the solar panel array. However when cloudy conditions materialise, an increasing proportion of the power supplied to the second capacitor 330 may come from the electrical grid. Similarly as the load demand increases, supplemental power may be drawn from the electrical grid resulting in an increasing proportion of time when the second switch 335 is closed compared to the first switch 325.

As discussed, the microprocessor or microcontroller is configured to close this first switch when the voltage V1 across the first storage means or capacitor 315 falls is above a predetermined threshold, and to open the switch when the voltage V1 falls below a threshold. These thresholds may be the same although for stability the switch open thresholds may be lower than the switch close threshold. Similarly the microprocessor 350 is also configured to close the second switch 335 when the voltage V2 across the second storage means or capacitor 330 falls below a predetermined threshold, and to open the second switch 335 when this voltage V2 is above a threshold. This ensures that the second capacitor is sufficiently charged to supply power to the inverter 345. Power is drawn first from the solar panels 205, and only when this is insufficient to supply the load is supplementary power drawn from the electrical grid 130.

It will be appreciated that alternative switch control strategies could be employed to deliver the same effect for example control of the switches may depend only on the voltage V1 of the first capacitor such that whenever the first switch opens the second switch closes. This power controller architecture has a relatively simple structure and reduced component count compared with the known power controllers previously described. There is no

requirement for battery charging or grid integration control, and furthermore there is no requirement for maximum power tracking control as the solar panels are only required to drive into the first capacitor 315. Because of the relatively rapid switching, and the relatively slow response time of the panels, these tend to see the capacitor as having high average

impedance. This arrangement therefore provides for sufficiently efficient performance of the solar panels without the need for complex MPPT control. The simple switching approach also allows for the use of a simple inexpensive microprocessor. Such an arrangement is

considerably cheaper than known solar power controllers.

Figure 4 shows a second embodiment solar power system 400 in which the solar panels 405 x-y are connected in parallel to the power controller 410. One or each of the solar panels includes an integrated capacitor 415x - y connected in parallel. This effectively relocates and distributes the first storage means function of Figure 3 - the first capacitor 315.

These solar panels 405 x - 415 x-y are fed into the power controller 410 which includes a voltage divider 470 for providing a signal Va indicative of a voltage level of the first storage means (415 x-y). The voltage divider also provides a high impedance input which again reduces the need for MPPT. The power controller 410 also includes a modified multi pole switch 425, a second storage means or capacitor 430, a second voltage divider 475 arranged to provide a signal Vb indicative of a voltage level across the second capacitor 430. The controller also comprises an inverter 445 arranged to drive loads 420a - 420c, as well as a diode bridge 440 and transformer 460 forming a rectifier for AC power received from an electrical grid.

Because of the design and simplicity of this system, the electrical grid power may be provided by a simple wall socket 465 as no special isolation measures are required. The diode bridge 440 will prevent current flow from the power controller back into the grid. The multi pole switch 425 is arranged to switch between the rectified grid power and the solar panel located first storage means (415 x-y) depending on a voltage level V2 of the second storage means 430. The switch may also include an intermediate OFF position in which neither of the two power supplies is coupled to the second storage means 430, which may allow for system isolation for safety or maintenance purposes. An H-bridge inverter 445 is shown which comprises four transistors with parallel coupled diodes and which will be well known to those skilled in the art. The transistors are switched in known manner in order to generate an AC output from the DC input supplied by the second storage capacitor 430. The invertor 445 may supply one or more loads 420A - C as shown.

The system 400 operates in substantially the same way as the system 300 of Figure 3. Solar energy collected by the panels 405 x-y is used to charge their associated first storage means 415 x- y which will typically be in the form of large capacitors. The voltage divider 470 provides an indicative voltage Va of the voltage V1 across the parallel connected solar panels. When this voltage level is above a pre-determined threshold, the switch 425 is arranged to couple the first storage means 415 x-y to the second storage means 430. When the voltage level V1 across the first storage capacitors 415 x-y falls below the threshold, the switch 425 disconnects the first storage means and instead connects the second storage means 430 to the second power supply or electrical grid supplied power input. As described this will be a rectified DC power output from the diode bridge 440.

In an alternative arrangement, the switch may only couple the second power supply (465, 460, 440) when the voltage level V2 across the second capacitor 430 falls below a threshold, which may be indicated by voltage Vb from the voltage divider 475. In this case the switch 425 may move to the OFF position when the first indicative voltage Va falls below a threshold, and only connect to the diode bridge 440, when the second indicative voltage Vb falls below a threshold corresponding to the voltage input required for the inverter 445.

The switching may be controlled by a microprocessor or any suitable controlling arrangement, in which the coupling or switching of the second storage means 430 to the first storage means 415x-y or the electrical grid 465 is dependent on parameter V1 , V2 of the first and/or second storage means.

Again this embodiment provides a relatively simple and low cost solar power controller and system. Whilst some specific components have been described in order to simplify explanation, various alternative arrangements could be used. For example the inverter may comprise a transformer or inductive element. The determined parameter of the first and second storage means 415, 430, 315, 330 may be a parameter other than voltage, although voltage is simple to measure. The first and second storage means may comprise any type of capacitive element, including for example super capacitors, although any electrical storage mechanism could alternatively be used including relatively small batteries. The switches 325, 335, 425 may be any suitable solid state switching devices, however it is possible that a simple electromechanical relay or equivalent arrangements could be used. Similarly although a diode bridge 440 and transformer 460 have been described, any suitable rectifier arrangement for converting the AC electrical grid power to DC electrical power for use by the power controllers 310, 410 could alternatively be used.

In a further arrangement the solar panels with integrated capacitors may be connected in series, as illustrated in figure 2. The integrated capacitors (not shown) in this series connected arrangement, are connected across respective solar panels in parallel, with the panels then being connected in series. As with the arrangement of figure 4, this allows the input capacitor function (315) of figure 3 to be distributed to the panels and allows for smaller and cheaper individual capacitors. A minor software change is all that is required for the power controller to accommodate series or parallel connected solar panels, with the threshold voltage merely requiring appropriate modification as previously described.

Where in the foregoing description, reference has been made to specific components or integers of the invention having known equivalents, then such equivalents are herein

incorporated as if individually set forth. Although this invention has been described by way of example and with reference to possible embodiments thereof, it is to be understood that modifications or improvements may be made thereto without departing from the scope of the invention.