GB2482487A - Hybridisation device - Google Patents

Hybridisation device Download PDF

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Publication number
GB2482487A
GB2482487A GB201012990A GB201012990A GB2482487A GB 2482487 A GB2482487 A GB 2482487A GB 201012990 A GB201012990 A GB 201012990A GB 201012990 A GB201012990 A GB 201012990A GB 2482487 A GB2482487 A GB 2482487A
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Prior art keywords
power source
power
load
voltage
device
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GB201012990A
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GB201012990D0 (en )
Inventor
Shane Slater
Ben Madden
Chris Jones
Neil D Souza Mathew
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ELEMENT ENERGY Ltd
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Element Energy Ltd
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    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01MPROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL INTO ELECTRICAL ENERGY
    • H01M8/00Fuel cells; Manufacture thereof
    • H01M8/04Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
    • H01M8/04298Processes for controlling fuel cells or fuel cell systems
    • H01M8/04313Processes for controlling fuel cells or fuel cell systems characterised by the detection or assessment of variables; characterised by the detection or assessment of failure or abnormal function
    • H01M8/04537Electric variables
    • H01M8/04574Current
    • H01M8/04589Current of fuel cell stacks
    • G01R31/386
    • G01R31/389
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01MPROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL INTO ELECTRICAL ENERGY
    • H01M8/00Fuel cells; Manufacture thereof
    • H01M8/04Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
    • H01M8/04298Processes for controlling fuel cells or fuel cell systems
    • H01M8/04313Processes for controlling fuel cells or fuel cell systems characterised by the detection or assessment of variables; characterised by the detection or assessment of failure or abnormal function
    • H01M8/04537Electric variables
    • H01M8/04544Voltage
    • H01M8/04559Voltage of fuel cell stacks
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01MPROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL INTO ELECTRICAL ENERGY
    • H01M8/00Fuel cells; Manufacture thereof
    • H01M8/04Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
    • H01M8/04298Processes for controlling fuel cells or fuel cell systems
    • H01M8/04313Processes for controlling fuel cells or fuel cell systems characterised by the detection or assessment of variables; characterised by the detection or assessment of failure or abnormal function
    • H01M8/04537Electric variables
    • H01M8/04634Other electric variables, e.g. resistance or impedance
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01MPROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL INTO ELECTRICAL ENERGY
    • H01M8/00Fuel cells; Manufacture thereof
    • H01M8/04Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
    • H01M8/04298Processes for controlling fuel cells or fuel cell systems
    • H01M8/04694Processes for controlling fuel cells or fuel cell systems characterised by variables to be controlled
    • H01M8/04858Electric variables
    • H01M8/04865Voltage
    • H01M8/0488Voltage of fuel cell stacks
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01MPROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL INTO ELECTRICAL ENERGY
    • H01M8/00Fuel cells; Manufacture thereof
    • H01M8/04Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
    • H01M8/04298Processes for controlling fuel cells or fuel cell systems
    • H01M8/04694Processes for controlling fuel cells or fuel cell systems characterised by variables to be controlled
    • H01M8/04858Electric variables
    • H01M8/04895Current
    • H01M8/0491Current of fuel cell stacks

Abstract

A hybridisation device for combining the power outputs of first and second power sources 2, 12 includes a load circuit 22 for applying a load to the first power source 2; and a controller 20 for cycling the applied load, measuring characteristics of the first power source 2, and for thereby determining operating parameters of the first power source 2 for future utilisation of the first power source 2 during hybridisation with the second power source 12. The operating parameter may be the output impedance. The first power source 2 is a high energy density source such as a fuel cell, and the second power source 12 is a lower energy density source such as a secondary cell. A charger circuit may be included to charge the secondary cell 12 from the fuel cell 2.

Description

HYBRIDISATION DEVICE

The present invention relates to a hybridisation device for combining the power outputs of two separate power sources.

Power sources such as fuel cells and renewable energy sources (e.g. solar or wind powered devices) are becoming more widely used. Fuel cells are considered highly advantageous as energy storage devices, due to their high energy density. Renewable energy sources can provide plentiful energy and power for a wide variety of applications without the need for a connection to a wider electrical network or grid.

These types of power sources provide challenges when maiching the power supply to the load. A load will typically work most efficiently when supplied with power within certain range of voltage and current. In addition, power demand at the load may vary and this will inevitably not match the power production characteristics of the power source.

Fuel cells work most efficiently at a constant relatively low power output, which may not match the power demand of the load. The output voltage/current from a fuel cell varies with the load demand, and if the power demand of the load is too great, the fuel cell can be damaged. Renewable energy generation has similar problems, as renewable sources often provide highly variable power output with power characteristics that do not match the requirements of the load. The variety of power sources that are available can also differ greatly in terms of voltage output, voltage polarity and output power capability.

Methods are known to hybridise different types of power sources in order to improve the characteristics of the power supply for a device. For example, US 2009/1 55633 discloses a fuel cell hybrid power supply apparatus that combines power from a fuel cell with power from a secondary battery. This type of system enables the high energy density of the fuel cell to be used even though the power demand from the load could at some points exceed the peak power output of the fuel cell. However, there are problems with the efficiency and effectiveness of existing hybridisation systems.

Viewed from a first aspect the present invention provides a hybridisation device for combining the power outputs of first and second power sources, the device comprising: a load circuit for applying a load to the first power source; and a controller for cycling the applied toad, measuring characteristics of the first power source, and for thereby determining operating parameters of the first power source for future utilisation of the first power source during hybridisation with the second power source.

As a result of the use of a load circuit for automatic set-up, this hybridisation device can advantageously be used with a range of power sources without the need for reconfiguration of the hardware or for a complicated reprogramming process. The preferred hybridisation device should include appropriate features to facilitate hybridisation, for example, the device may include a voltage/current conversion circuit for regulating the output of the power source in such a manner that multiple power sources can be combined and used effectively.

The controller may be a microcontroller or similar.

Preferably, the controller is arranged to use the load circuit to cycle the load from the first power source at various frequencies, whilst simultaneously measuring the input voltage and/or current from the power source. This enables the controller to build up a model of the output impedance of the power source. The controller then preferably uses this information to set a maximum current draw constant for the power source as well as a minimum safe input voltage.

The controller may be arranged to convert output impedance values to a maximum current draw limit by using a blanket rule of the current draw associated with a certain percentage of voltage drop. Alternatively, the controller may be provided with preset output impedance models of types of'power source that could be connected to the hybridisation device in order to permit matching of the measured characteristics with known values. The present output impedance models may be stored at the controller or they may be stored on a data storage device external to the controller.

In a preferred embodiment, the device comprises an isolation switch, preferably an automatically and/or electronically controlled isolation switch, to separate the load circuit and first power source from other parts of the device. The controller may be arranged to check that the voltage input from the power source is within the limits of the hybridisation device and to then close the isolation switch. A fuse may be provided after the isolation switch to protect other components of the hybridisation device, for example to protect a converter circuit as used in the hybridisation device described below in relation to the third aspect of the invention.

Advantageously, where the secondary power source is able to supply the power required by the load at start up, the load cycling tests on the first power source may be carried out during continued operation of the hybridisation device with the second power source supplying power to the load. Consequently, the testing and automatic set up of the device need not prevent immediate use of the device to supply power Thus, preferably the hybridisation device is arranged to supply power to the load using the second power source during the process of determining operating parameters of the first power source.

The controller may be arranged to operate the hybridisation device in accordance with the method below.

Viewed from a second aspect, the present invention provides a method of hybridisation of first and second power sources using a hybridisation device in order to supply power to a load, the method comprising: applying a load to the first power source and cycling the applied load; measuring characteristics of the first power source; and based on the load cycle and measured characteristics, determining operating parameters of the first power source for future utitisation of the first power source during hybridisation with the second power source.

Preferably, the load from the first power source is cycled at various frequencies, whilst simultaneously sampling the input voltage and/or current, and the method comprises building a model of the output impedance of the power source. This information may then be used to set operating parameters in the form of a maximum current draw constant for the power source as well as a minimum safe input voltage.

The method may include supplying power to the load using the second power source during the process of determining operating parameters of the first power source.

In a preferred embodiment, after the operating parameters have been determined, during subsequent operation of the hybridisation device power is drawn up to a maximum amount and within the safe limits for voltage and current as determined using the load cycling tests.

Preferably, the method comprises using power from the second power source to start up the device when a voltage is detected from an unrecognised first power source, checking the voltage input from the first power source to ensure it is within the limits of the device, and optionally checking whether it is AC or DC voltage. If the voltages are within preset operational capabilities, an optionally provided isolation switch should be closed to join the first power source to the remainder of the hybridisation device.

One method that may be used to convert output impedance values to a maximum current draw limit is the use of a blanket rule of the current draw associated with a certain percentage of voltage drop. An alternative method that may be used is matching the measured characteristics with preset output impedance models of types of energy source that could be connected to the hybridisation device. The matching process may, for example, be based on a least squares algorithm.

Viewed from a third aspect, the present invention provides a hybridisation device for combining the power outputs of first and second power sources in order to supply power to a load, the first power source having a relatively low maximum output power and/or voltage and the second power source having a relatively high maximum output power and/or voltage, the hybridisation device comprising: a bus having an output for supplying power to the load; a first power source circuit for receiving power from the first power source, the first power source circuit having an output connected to the bus, and the first power source circuit comprising a converter circuit for controlling the voltage and/or current output from the first power source; and a second power source circuit connected to the bus in parallel with the first power source circuit, the second power source circuit being for supplying additional power to the bus from the second power source, wherein the second power source circuit is arranged to connect the output voltage of the second power source to the bus without intervening voltage control.

The features of the above described device and/or the further features described below may be combined with the device of the first aspect as described above.

With the arrangement of the third aspect, the power output from the first power source can be controlled and combined with the power output from the second power source to match the power demand from the load. When a low power is required, the first power source can be used by itself. When a high power is required, both power sources can be used in combination.

By "circuit" it is intended to refer to any assembly of electrical connections and components joined together. The first power source circuit and second power source circuit may be made up of several smaller interconnected circuits. The first power source circuit and second power source circuit may also themselves be considered as being sub-sections of a larger circuit. Voltage control is intended to refer to active control of the voltage such as that provided by a voltage/current converter circuit. Thus, the connection of the second power source to the bus does not include such a converter circuit.

Prior art hybridisation devices such as US 2009/1 55633 require the use of two converter circuits. The device of US 2009/155633 is intended to provide a stable power output control for the fuel cell, and uses power from the battery in combination with power from the fuel cell in order to supply the.Ioad. However, this prior art device requires the use of a voltage conversion unit on the output of the fuel cell and the battery. This greatly reduces the efficiency of the power supply device, and increases the complexity of the control circuitry. In the device of the present invention, the direct connection of the second power source to the load, without voltage conversion circuitry, provides high efficiency and the use of a converter circuit for the first power source enables effective power control for the output power to the load.

Preferably, the converter circuit is a DC/DC converter and the bus is a DC bus. The first power source can hence be connected to the bus and to the load in an efficient manner, with minimal losses.

The converter circuit may be controlled by feedback circuitry so as to provide an appropriate conversion of the output from the first power source. In a preferred embodiment, the hybridisation device comprises a controller, such as a microcontroller, for controlling hybridisation. The controller may be for controlling the feedback circuitry and/or for sensing and monitoring voltage or current levels within the hybridisation device.

Preferably, the first power source is a relatively high energy density power source and the second power source is a relatively low energy density power source. The hybridisation device described above and herein is particularly effective when hybridising power sources with these characteristics. The first power source may, for example, be a fuel cell. The second power source may be an energy storage device such as a secondary cell/battery.

Ideally, in preferred embodiments, the second power source is a secondary cell such as a rechargeable battery. Lithium based cells may be used as the second power source. Where the second power source is a secondary cell, the second power source circuitry preferably comprises a charger circuit. The charger circuit may advantageously be arranged to receive power from the first power source, via the converter circuit. Thus, there may be an input to the charger circuit at the bus.

The second power source circuit preferably comprises a unidirectional flow circuit to prevent flow of power from the bus to the second power source, except via the charger circuit (when present). The unidirectional flow circuit may for example be an active circuit utilising a low loss transistor. This enables the voltage at the second power source to be safely linked with the bus voltage with a minimal voltage drop and maximum efficiency.

In a particularly preferred embodiment a DC/DC boost converter circuit is used in a normal voltage feedback mode at times when only the first power source is delivering power to the load and/or to the charging circuit. The feedback may be a divided down voltage feedback from the output of the converter circuit. The controller senses current/voltage on the bus and it also senses current/voltage at the output of the secondary cell. Preferably, when the controller senses that an increased load is required based on the current through the power source, it sends a signal to a feedback switch of the feedback circuitry, in order to enable feedback of the current signal through a hardware loop. The controller preferably provides a current demand reference to the current feedback circuitry to control the output from the power source. This can be used to regulate the boost converter to output directly about a secondary cell voltage or the bus voltage in order to provide the correct amount of current. With this arrangement of feedback circuitry and control by the controller efficient hybridisation is provided.

Further preferably, at times when a high power is required at the load, the boost converter circuit may be switched into current feedback mode, with the power source delivering its maximum output.

The controller may be arranged to provide a measure of total energy throughput from the first power source. This could be used for monitoring the performance of the system.

The hybridisation device may include rectification to enable the use of an AC power source for the first power source, and/or a DC/AC converter to supply power to an AC load.

To provide for hybridisation of more than two power sources, the hybridisation device or a wider hybridisation system may include one or more additional power source circuits for additional power sources. The additional power source circuits may have equivalent features to either the first power source circuit or the second power source circuit described above, and are preferably connected to the bus in an identical manner.

Thus, for example, there may be a first power source circuit for connecting a fuel cell to the bus, a second power source circuit for connecting a secondary cell to the bus, and a third power source circuit, with equivalent features to the first power source circuit, for connecting a wind turbine to the bus. In this way, multiple energy sources may be connected and hybridised to provide power via the bus to a single load.

The device of the first aspect may be arranged to operate in accordance with the method described below.

Viewed from a fourth aspect, the present invention provides a method of hybridisation of the power outputs of first and second power sources using a hybridisation device in order to supply power to a load, the first power source having a relatively low maximum output power and/or voltage and the second power source having a relatively high maximum output power and/or voltage, the method comprising: providing a bus having an output for supplying power to the load; providing a first power source circuit for receiving power from the first power source, the first power source circuit having an output connected to the bus, and the first power source circuit comprising a converter circuit for controlling the voltage and/or current output from the first power source; providing a second power source circuit connected to the bus in parallel with the first power source circuit, the second power source circuit being for supplying additional power to the bus from the second power source, wherein the second power source circuit is arranged to connect the output voltage of the second power source to the bus without intervening voltage control; and supplying power to the load via the bus using either the first power source, the second power source or both power sources in combination.

The method may include providing additional features of the hybridisation device as described above. Also, the method of the second aspect may include features of the method of this fourth aspect and/or further features as described below.

In a preferred embodiment the method comprises controlling the supply of power from the first power source and the second power source by means of a controller such as a microcontroller. The controller may control the converter by open loop control to ensure an appropriate power supply to the bus. Thus, the controller may change the converter's setpoint based on certain parameters, for example, the load rising beyond 40A and therefore requiring a change from purely battery operation to hybridised fuel cell I battery operation. Preferably, the changeover point has some hysteresis to prevent erroneous changeover.

Preferably, the method includes a set sequence for use of the first power source and second power source during start-up of the hybridisation process and thereafter. The method may comprise starting either the first power source or the second power source alone, and then only starting the other power source when the demand from the load reaches a set point. One preferred method comprises initiating the supply of power with only the second power source supplying power to the bus, and then starting the first power source when the current through the second power source is greater than a set point.

The preferred method used when the second power source is a secondary cell and a charger circuit is provided may further include a step of starting the first power source when the charge on the secondary cell has dropped below a set point. The first power source may then be used to supply power to the load and/or to recharge the secondary cell.

After it has been started the first power source is preferably run continuously until the hybridisation device is shut-down, e.g. when power is no longer required. Where the second power source is a secondary cell the first power source may continue to run after the load no longer requires power in order to charge the secondary cell. This avoids on/off cycling which is a critical factor in determining the lifespan and maintenance requirements for many types of power source, including fuel cells.

If the load is high then the converter circuit is preferably operated in current feedback mode with the power source delivering its maximum power output. However, if the power source has been started because the secondary cell charge is below the set point, then preferably the current path out of the cell is switched off, the boost converter is used in voltage feedback mode and hence the first power source delivers power to the load, while using any excess to charge the cell.

This method is particularly advantageous when the second power source is a battery such as a secondary cell and when the first power source is a fuel cell or similar.

For power sources like fuel cells, the number of start/stop cycles are a critical factor in the ultimate lifetime of the power source. With these power sources, the charge stored in the secondary cell can initially be used to supply power to the power source whilst avoiding the need to start the fuel cell. When the demand is higher than can efficiently be provided by the secondary cell, the fuel cell is started and hybrid power is supplied. If the charge on the secondary cell has dropped below a set point, then the fuel cell can be used to supply power and/or charge the secondary.celI. The fuel cell them remains on until power is no longer required, which avoids potentially damaging stop-start cycles. Hence, during later operation the fuel cell will supply all power at low power demand. This routine maximises the utilisation of the high energy density fuel cell, whilst minimising the number of start-stop cycles. It also avoids the efficiency loss involved in pushing charge in through the second power source charging circuit and out again from the second power source to the bus.

The above described method of initiating hybridisation provides considerable advantages and therefore, viewed from a fifth aspect, the invention provides a method of hybridisation of a first power source with a relatively high energy density and a second power source with a relatively low energy density using a hybridisation device in order to supply power to a load, the method comprising: initiating the supply of power with only the second power source supplying power; starting the first power source when the current through the second power source is greater than a set point; and thereafter running the first power source until shut down of the hybridisation device.

This method optimises utlisation of the high energy density first power source, whilst avoiding excessive start-stop cycles of the first power source, Once again the features of this method may be combined with the features of the second aspect as set out above.

As above, the preferred method used when the second power source is a secondary cell and a charger circuit is provided may further include a step of starting the first power source when the charge on the secondary cell has dropped below a set point.

The method may include further features as discussed in relation to the method of the fourth aspect above and its optional and preferred features.

Certain preferred embodiments of the present invention will now be described by way of example only and with reference to the accompanying drawings in which: Figure 1 shows a block diagram of a hybridisation device for combining the power outputs of first and second power sources; Figure 2 shows a block diagram of a hybridisation device including a load circuit for automatic characterisation of a connected power source; Figure 3 is a schematic illustration showing an arrangement for hybridisation of more than two power sources; Figure 4 is a diagram of a preferred embodiment of a boost circuit; Figure 5 shows an arrangement producing a divided voltage output; Figure 6 is a diagram of the power elements of the boost circuit; and Figure 7 shows a preferred hybridisation feedback loop.

In the embodiment of Figure 1 the first power source 2 could be a fuel cell as described in the detailed example below. It will of course be appreciated that other power sources could be used as the first power source 2. The first power source 2 is connected to the input of a DC/DC converter circuit 4. This type of circuit is also known as a boost circuit. The output of the converter circuit 4 connects to a bus 6, and via the connection to the bus 6 the output of the converter circuit 4 can supply power directly to the load 8. The DC/DC converter circuit 4 is controlled by feedback circuitry 10, which is described in more detail below.

A second power source 12 is also connected to the bus 6. This second power source 12 is a secondary cell 12 and could, for example, be a lithium based battery as described in the more detailed example below. The secondary cell 12 is connected to battery protection and fuel gauge circuitry 14 and the via a unidirectional flow circuit 16 to the bus 6. The second power source 12 can thereby supply power directly to the load 8 without any intervening control of the output voltage. Thus, the voltage of the bus 6 is linked to the voltage of the battery 12. The circuitry for the second power source 12also advantageously includes a charging circuit 18 which enables the first power source 2 to recharge the secondary cell 12 at times when the output of the first power source 2 is in excess of that required by the load 8, and/or when the state of charge of the battery falls below a certain level and the first power source 2 has excess power available to recharge the battery. The charging circuit 18 is basically a buck circuit or step down DC/DC converter. This is used to drop the bus voltage at the output of the DC/DC boost converter circuit 4 to an appropriate level for recharging the secondary cell 12.

This basic topology is highly efficient when used for combining a first power source 2 with a relatively low output voltage and relatively low power output, but high energy density, with a second power source 12 such as a secondary cell 12 which has a higher output voltage and higher output power, but lower energy density. For loads in the 300W to 2 kW range this topology enables heat losses to be made sufficiently small and efficiency to be made sufficiently high for the device to be used as an easily portable power pack.

The operation of the hybridisation device will first be described in relatively general terms, and will then be described in more detail with reference to an example utilising a fuel cell and a lithium cell as the first power source 2 and second power source 12. The general operation of the device is as follows, and involves the use of a microcontroller 20 to provide feedback based control of the DC/DC converter circuit 4.

The DC/DC boost converter is used in a normal voltage feedback mode at times when only the first power source 2 is delivering power to the load 8 and/or to the charging circuit 18. This would typically be a divided down voltage feedback from the output of the boost circuit 4. The microcontroller 20 senses currentJvoltage on the bus 6 and the also senses current/voltage at the output of the secondary cell 12. Conventional current/voltage sensing/measurement devices can be used. When the microcontroller 20 senses that an increased load is required based on the current through the first power source 2, it sends a signal to a feedback switch of the feedback circuitry 10, which enables feedback of the current signal through a hardware ioop. The microcontroller 20 provides a current demand reference to the current feedback circuitry 10 to control the output from the first power source 2. This then regulates the boost converter 4 to output directly about a secondary cell voltage or the bus voltage in order to provide the correct amount of current.

At times when a high load is required the boost converter circuit 4 is switched into current feedback mode, with the first power source 2 delivering its maximum output. The output of the first power source 2 can then be augmented with power from the secondary cell 12. This enables high power output to be provided for certain periods of time in accordance with the capabilities of the first power source 2 and the secondary cell 12, provided that the storage capacity of the secondary cell 12 is sufficient.

The microcontroller 20 can provide a measure of total energy throughput from the first power source 2. In normal (voltage feedback) mode, this can be done by sampling the current measurement through the boost converter circuit 4 at regular intervals and integrating the product of this and the constant voltage. In hybridised mode the integrating continues but this time with the voltage on the bus 6 -the other half of the product that is integrated, the current, is already known as it is used as the setpoint. The microcontroller 20 forwards this measurement to a user interface or display, or to an external control system.

To provide for this topology in a highly efficient manner, an active circuit involving a low loss transistor (e.g. MOSFET) is used as the unidirectional flow circuit 16, which is in parallel with the charging circuit 18. This provides a low voltage drop output from the secondary cell 12 onto the bus 6, while preventing current flowing inward to the cell 12 except via the charging circuit 18.

The method of operation of the hybridisation device and the way in which the microcontroller 20 is arranged to operate the device is highly important. For some kinds of power source 2, in particular fuel cells, the number of start/stop cycles are a critical factor in the ultimate lifetime of the power source 2. An effective control by the microcontroller 20 can also ensure that the particular power source 2 is operated at maximum efficiency.

In one preferred mode of the system starts up initially with only the secondary cell 12 on the bus 6. The charge stored in the secondary cell 12 can be used to supply power to the load 8 whilst avoiding the need to start the fuel cell. If the microcontroller 20 detects that the current through the secondary cell 12 is greater than a set point, or that the charge on the secondary cell 12 has dropped below a set point, then the first power source 2, for example a fuel cell, is started up. As noted above, if the load is high then the boost converter circuit 4 is operated in current feedback mode with the power source 2 delivering its maximum power output. However, if the power source 2 has been started because the secondary cell charge is below the set point, then the current path out of the cell 12 is switched off, the boost converter is used in voltage feedback mode and hence the first power source 2 delivers power to the load 8, while using any excess to charge the cell 12.

Operation of the device in this way ensures efficient operation. The power to the load 8 is either drawn from the secondary cell 12 directly without the need for first power source 2 to be started, or the power to the load 8 only goes through one DC/DC converter from the first power source. Similarly, when the first power source 2 is supplying power to the charging circuit 18, the power only goes through the single DC/DC converter. By the use of the current feedback mode when the load is greater than the set point the system is also able to supply power to a load 8 that has a high peak power demand.

In an alternative start-up sequence, the first power source 2 (for example, a fuel cell) is immediately started up and put on the bus 6. The first power source 2 then provides a constant amount of output power continuously until shutdown, assuming that a load is present -either from the load 8 or from the charger circuit 18. The first power source 2 is hybridised with the secondary cell 12 which is also put on the bus 6 to provide any load in excess of what the first power source 2 can power alone. The constant output power from the first power source 2 might be the maximum output from the first power source 2, or a lower power setting to extend the life of the first power source 2. -12-

During subsequent operation, in any hybrid state, the power drawn from the first power source 2 can be dynamically set as a function of: a. the age of the first power source 2, b. the amount of energy that has passed through the first power source 2 since its commissioning, c. the number of stop-start cycles of the first power source 2 since its commissioning, d. according to the average load being drawn, e. the voltage output from the first power source 2, f. the voltage output from the secondary cell 12, g. the secondary cell's state of charge, or h. permutations of the above (e.g. a functional relationship between the current drawn and the number of cycles and the age of the first power source 2, or a relationship between the secondary cell's state of charge and the average load being drawn) Various of these operations can be set depending on the nature of the first power source 2. For example, a., b., and c. have particular benefit with fuel cells, which are sensitive to lifespan and stop-start cycles.

In normal operation, once the first power source 2 has been started up, the boost circuit 4 may switch between constant voltage to power the charger circuit 18 as well as low loads, and current feedback mode where the first power source 2 hybridises with the secondary cell 12. It may swap between these modes as a function of: a. the total load at the output 8 (e.g. if the loads is higher than a set-point it switches to hybridised mode) b. the secondary cell's state of charge (e.g. if the state of charge drops below a set-point it switches to normal voltage boost mode and charges the secondary cell, switching the output off if necessary -certain secondary cell technologies have a lifetime that is critically dependent upon the depth of discharge) c. the variance of the load current (e.g. for highly transient loads a hybridised mode may extend fuel cell lifetime) d. the secondary cell voltage -and thus be capable of using cheap off the shelf inverters for the next power conversion stage Once the first power source 2 has been started, it should generally remain on in order to minimise on/off cycling. The first power source 2 could remain on until shutdown.

Alternatively, on shutdown, the system may keep the first power source 2 on for a further period of time in order to charge the secondary cell 12 until fully charged. This ensures that after use the device has a fully charged secondary cell 12. A further alternative is for the first power source 2 to remain on after shutdown or load removal, charging the battery at a lowered rate, and therefore stays on as long as possible in order to avoid a stop/start cycle.

These steps are taken is in order to minimise on/off cycling which is a critical factor for many types of power source 2, especially fuel cells. The first, high energy density, power source 2 outputs its maximum power whenever possible in order to best utilise the high energy density. This power is either used to supply the load 8 whilst charging the secondary cell 12, if possible, or it is hybridised with the secondary cell output when the load demand is higher than the maximum power output of the first power source 2.

A preferred implementation of the hybridisation system shown in the block diagram of Figure 1 involves the use of three circuit boards. Two of these circuit boards are power electronics boards and the third board is a microcontroller board.

The first power electronics board is the boost board which includes the DC/DC boost converter circuit 4 shown in Figure 1 along with the current feedback circuitry 10.

The input to the boost board is the terminals of the first power source, e.g. outputs of a fuel cell stack. The boost board performs power conditioning and also hybridisation functions, and its output is connected to the second power electronics board.

This second circuit board, considered in the context of the block diagram of Figure 1, includes the bus 6, charging circuit 18, unidirectional flow circuit 16 and also the battery protection and fuel gauge circuitry 14. The inputs to the second power electronic circuit are the second power source 12 (e.g. a secondary cell 12) and the output from the boost board. This second circuit charges the secondary cell 12, measures its state of charge and protects it. The bus 6 on this second circuit provides the main power output which is connected to the load 8 or to a DC/AC converter.

As will be evident from Figure 1 this second circuit board comprises two main paths. One path is from the boosted first power source 2 input straight to the output and hence forms the bus 6. The other path passes through the buck charger circuit to the battery protection and monitoring circuitry 16. There is also a return path from the battery directly from the charge protection circuit to the output (i.e. onto the bus 6) via an active diode circuit, which is a unidirectional flow circuit 16. The preferred implementation of this active unidirectiOnal flow circuit 16 uses four parallel MOSFETs to allow charge to flow to the output from the battery and not in reverse. It should be noted that whilst this embodiment uses four parallel MOSFETs other arrangements of unidirectional flow circuit 16 could also be used, for example using a different number of MOSFETs. The use of parallel MOSFETs is preferred as this reduces heating losses.

The possible operation of a preferred embodiment will now be described in the context of an exemplary system where the first power source 2 is an 8V, 400W proton exchange membrane (PEM) hydrogen fuel cell 2 and the second power source 12 is a secondary cell 12 in the form of a LiFePO4 battery with a nominal open circuit voltage of I 3V, a maximum power output of 1.2 kW and a capacity of 1 O0Wh. The LiFePO4 battery, like other lithium based cells, has a high discharge and charge capability and energy density, and it also has a good operational temperature range, which is important when the power hybridisation device is intended for portable use. It is also one of the safest Lithium-based technologies.

With a 400W fuel cell 2 and 1.2kW secondary cell 12, a combined 1.6kW output is provided. If the usable capacity of the secondary cell 12 is lO0Wh and it can be charged at 400W then the system can deliver 1.6kW for five minutes, after which it must go into charge mode for approximately 15 minutes. The charging capacity of the secondary cell 12 is advantageously selected to be a peak power output of the fuel cell 2 in order to allow maximum utilisation of the fuel cell 2 during the charging cycle. Depending on the requirements of the load 8, other power supply and recharge profiles are possible. For* example 1.6kW could be supplied for two minutes, followed by a six minute recharging period. The ratio of time supplying maximum power output to recharge time is a function of the maximum charging power and the battery capacity. It will be appreciated that the system can also supply a low power to the load 8 for a much larger period of time.

A device utilising a fuel cell 2 and battery 12 of the type set out above can be used in many off grid power tool applications with a duty cycle involving short to medium duration bursts of readily high power output demand followed by periods where no power is required. Examples might be angle grinding, drilling, needle scaling and so on. The device may also find use for remote expeditions, for instance to power a kettle for boiling water.

Obviously, depending on the requirements of the load 8, the device can include a DC/AC converter on the output of the bus 6 to enable AC power devices to be used as the load 8.

In another example, the same nominally 8V, 400W PEM fuel cell 2 could be combined with a nominally 48V, 1.2kW secondary cell 12. This would provide a high energy density and higher power battery replacement for off-grid power tool applications requiring higher power, such as portable welding. The system could directly replace a conventional battery of the portable welder and sit before the constant current or constant voltage supply to the welder. In some portable MIG welding applications where the welder is connected directly to the battery terminals, the hybridisation device would function as the entire power supply unit. Assuming a similar usable capacity for the battery 12 to the above example, it would be possible to operate the welder on a 20% duty cycle with much higher energy density than conventional battery powered systems. This would therefore provide much greater portability and/or a higher amount of welding time.

Another example of an advantageous use for the hybridisation device would be as power source for a device which has a relatively low ongoing power requirement (e.g. 400W) and a higher peak power requirement (e.g. 1.6kW). For example, remote control vehicles, such as those intended for military surveillance use or bomb disposal, may have a 400W traction power requirement and a 1.6kW peak power requirement for manipulators, sensors and communications. Once again, there are major advantages over a battery-only solution due to the higher energy density that is possible with a hybrid energy source having an equivalent peak power capability. There are also advantages over internal combustion engines, especially for military applications, asthe power source would have a considerably lower heat signature and would be barely audible in operation.

Figure 2 shows a similar hybridisation device to that of Figure 1, with generally similar components. The hybridisation device of Figure 2 differs to that in Figure 1 by the addition of an on-board load circuit 22 that is used to automatically detect the characteristics of the first power source 2. This enables a single hybridisation device to be used with a range of power sources 2 without the need for reconfiguration of the hardware or for a complicated reprogramming process.

As with the hybridisation device of Figure 1 the device of Figure 2 is arranged to operate with a first power source 2 connected via a DC/DC boost converter 4 to a bus 6.

Feedback circuitry 10 and a microcontroller 20 are provided and the feedback circuitry 10 operates in a similar manner to that described above. The hybridisation device also connects to a second power source 12 in the form of a rechargeable secondary cell 12 in the same way as described above in connection with Figure 1. Thus, there is a buck charging circuit 18 connecting to the battery protection and monitoring circuitry 16, and the second cell connects through the battery protection and monitoring circuitry 16 directly to the bus 6 via a unidirectional flow circuit 16. Similarly, the connection to the load 8 may once again be via a DC/AC converter. The method of operation of the hybridisation device is also broadly similar, with the option for minimising start/stop cycles and maximising efficiency using the microcontroller 20 to control the operation along the lines set out above.

Where the embodiment of Figure 2 differs is in the connection of the power source to the converter circuit 4. At the input end of the converter circuit 4 a load circuit 22 is provided which is controlled by the microcontroller 20. The microcontroller 20 also senses the input voltage. An isolation switch 24 and a fuse F are provided to protect the DC/DC boost converter circuit 4.

In relation to the device of Figure 1 it was assumed that the power source would be a DC power source. With the device of Figure 2 it is contemplated that an additional high -16-efficiency rectifier module (not shown in the figure) could be attached between the power source and the converter circuit 4 to allow for AC input energy sources. Alternatively, a specific device adapted for AC use could be provided, with an on-board rectifier. These features could of course be added to the embodiment of Figure 1 in a similar manner.

The hybridisation device of Figure 2 operates with an additional sub-routine occurring upon start up when a power source 2 is initially connected to the device at the DC/DC converter 4 input end. When a voltage is detected on the input to the boost converter circuit 4 the device starts up using power from the second power source 12. The microcontroller 20 checks that the voltage input is within the limits of the device, and checks whether it is AC or DC voltage. If the power source is supplying an AC voltage, the microcontroller 20 checks if a rectifier is present. If the voltages are within preset operational capabilities, the microcontroller 20 closes the isolation switch 24 and joins the power source 2 to the converter circuit 4.

A series of tests are then run using the on-board load circuit 22. The load from the first power source 2 is cycled at various frequencies, whilst simultaneously sampling the input voltage and current using the microcontroller 20. This builds up a model of the output impedance of the power source 2. The microcontroller 20 then uses this information to set a maximum current draw constant for the power source 2 as weli as a minimum safe input voltage. During subsequent operation of the hybridisation device, the microcontroller 20 allows power to be drawn up to a maximum amount and within the safe limits for voltage and current as determined using the load cycling tests. The hybridisation device can then be used in the same manner as the device of Figure 1, as described above.

The load circuit may consist of parallel switched load resistorsin a logarithmic configuration in order to quickly sample the impedance space. For example, with base 10, loads of 1, 10, 100 can very quickly give an approximation to the impedance characteristic of the power source.

If using a very basic algorithm to decide on the power draw, the voltage which is also being measured can be used to compute peak power point as a function of source voltage. One method that could be used to enable the microcontroller 20 to convert output impedance values to a maximum current draw limit is the use of a blanket rule of the current draw associated with a certain percentage of voflage drop. An alternative method is for the microcontroller 20 to be provided with preset output impedance models of all the main types of energy source 2 that could be connected to the hybridisation device. The microcontroller 20 could then perform the matching process, for example based on a least squares algorithm, to identify what the energy source 2 is from the collected data. This second method would enable a more precise maximum current draw to be identified. -17-

Although the hybridisation device has been described above in the context of Figure 1 and Figure 2 in relation to hybridisation of two power sources 2, 12, one typically being a secondary cell 12 with relatively high output power, and the other being, for example, a fuel cell 2 or renewable energy source 2 with a relatively low maximum power output, the advantages of the hybridisation device are not limited to the use of just two power sources 2, 12 and it may be desirable to enable hybridisation of multiple power sources, especially first power sources 2 of different types.

Figure 3 shows a schematic of an arrangement for hybridisation of multiple power sources 2. As illustrated by the dashed lines in Figure 2, the hybridisation device of Figures 1 and 2 can be considered to consist of two main elements. The first element is a power module 26 for attaching the first power source 2, and the second element is a secondary cell module 28 for attaching the second power source 12 (in these embodiments taking the form of a secondary cell 12). In the hybridisation device of Figure 3 multiple power modules 26, 28 are attached to a single bus 6 for powering a single load 8, including the use of an optional inverter 30 to enable an AC load to be powered.

Thus, a first DC power module 26 including DC/DC boost converter circuit 4 and current feedback circuit as described above is used to connect a wind turbine 2 to the bus 6. A second DC power module 262 with essentially the same circuitry is used to connect a PEM fuel cell 22 to the bus 6. An AC power module 263 is also shown, ie. including an on-board rectifier as discussed above, and is used to connect an AC power source 23 such as an AC generator to the bus 6. A secondary cell module 28 completes the hybridisation device, and connects a rechargeable battery 12 to the bus 6.

It will be appreciated that of course multiple AC power modules 263 could be used, as well as greater or fewer DC power modules 26, 262, as required. It will be appreciated that, with the preferred circuit arrangement, it will only be possible to have one uncontrolled power source being hybridised, and so multiple secondary cell modules 28 would not generally be used. However, additional secondary cell modules 28 could be included for redundancy and/or extra flexibility. For example, it may be of benefit to be able to switch operation to a back-up secondary cell module for maintenance or repair to the main secondary cell module.

As with the hybridisation devices shown in Figure 1 and Figure 2 the bus 6 of Figure 3 has an output to the load 8. With this arrangement multiple power sources 2, 1.2 and different types can be used to power a single load 8. The microcontroller 20 can be arranged to select a power source 2 in accordance with the user's requirements. For example, the user may indicate that wherever possible wind power should primarily be used in order to avoid using fuel. The fuel cell 22 could then be used in hybrid with the -18-wind turbine 2 to provide increased power output, or could be used instead of the wind turbine 2 at times when weather conditions do not permit the required supply of power from the wind turbine 2. To avoid using fuel at the fuel cell 22 or the AC generator 23, the wind turbine 2 could be selected as the primary mechanism for recharging the secondary cell 12. In this way an increased energy density can be realised by intelligent hybridisation of multiple power sources 2.

The power modules shown in Figure 3 could of course be as set out in Figure 1 or as set out in Figure 2. If the power module of Figure 2 was used, then advantageously different power sources 2 could be connected as and when available. The hybridisation device can then determine the nature of the new/additional power source 2 without the need for reconfiguration of the hardware or complicated reprogramming of the microcontroller 20.

Figures 4 to 7 are diagrams of preferred circuits for the hybridisation device.

Conventional terminology is used in identifying the components of the circuit and their functions.

Figure 4 shows a preferred boost circuit, which, in the preferred embodiment, uses the LTC3862EGN boost controller. Normal operation of the boost converter involves feeding back a divided down output voltage signal to the switching IC (to pin 8, FB), which is then internally compared with a I.225V bandgap reference and the output gate drive from the IC is then appropriately adjusted to maintain the desired voltage at output.

The resistors R218 and R223 shown in the schematic of Figure 5 are used to provide an output voltage of 14.7V. The divided output does not however go directly to the feedback input of the boost switching IC -it goes into the Normally Closed input of an analogue switch IC U303 instead (see Figure 7).

Figure 6 shows the power elements of the boost circuit. The output of the boost converter goes through a couple of small paralleled power resistors, R213 and R216, before leaving the board. The (high side) differential voltage across these resistors goes through an amplification stage that uses an op-amp infeedback to eliminate the need for matched resistors.

The hybridisation feedback loop is shown in Figure 7.

Software on the microcontroller communicates with a Digital to Analogue Converter over 12C protocol, in order to set a current demand signal. The output from the current amplification stage is then subtracted from this current demand signal using an op-amp configured in differential amplification mode. The gain is < 1. in this stage in order to achieve feedback stability in the current implementation. -19-

The output of that stage is similarly subtracted from a 1.225V bandgap reference voltage signal in order to normalise it about the voltage that this particular IC (and many switch-mode lCs) expect to receive.

The output from this is then fed into the Normally Open output of the analogue switch IC U303.

The microcontroller then has the ability to set the feedback to the boost IC to be either in normal voltage boost mode, or alternatively it may dynamically set the hybridisation current through its DAC and switch U303 to provide current feedback in the appropriate form for the boost IC in order to control the power draw from the fuel cell stack.

This allows the same set of power switching components (inductors / MOSFETs / diodes I capacitors) to be used for both hybridisation with a secondary cell, as well as in normal voltage boost to power a charger of the same secondary cell.

In a possible alternative arrangement of the current feedback path, it may be advantageous for the purpose of enhancing closed loop stability to incorporate integrator and I or derivate stages to the output of U302A. This would be done to achieve P1, PD or PID control of the feedback loop and use standard tuning techniques to improve feedback stability. This error' signal from U302A would then have to be split of multiple branches (depending on the choice between P1, PD or PID) performing integration of the error signal and multiplying by a modifiable constant, and / or taking the derivative of the error and multiplying it by a separate modifiable constant and in all cases involving a proportional term, Alternatively, an easy circuit addition to reduce the controller bandwidth would be to add the same value of capacitor parallel to R301 and R310. This would create a single pole at 1/(2*pi*R*C) Hertz. For example a lOnF capacitor across R301 and R310 would give a pole at 10kHz.

Turning back to applications for the hybridisation system, it will be appreciated that exemplary arrangement set out above in connection with Figure 1, using a 400 W fuel cell and 1.2 kW secondary cell, can be applied analogously to the embodiments of Figure 2 and Figure 3. Some possible uses for the hybridisation device are mentioned above, and a further selection of potential applications are set out below, in relatively general terms. It.

will be understood that the hybridisation device can be easily adapted to provide for the specific power requirements of each application.

1) Golf Buggies and similar small electric vehicles: There is a growing need to replace small electric vehicles batteries with a fuel based system that does create noise or pollution. Typically this would be a device like a golf cart where the intermittent nature of the duty cycle lends itself to hybridisation as discussed herein. Advantageously the proposed system is very quiet which is also a requirement.

Batteries versions of the golf buggy are less common than the internal combustion engine powered versions. Generally the engines tend to output approximately 8kW. The engine being small is relatively polluting for its size and rarely operates at an optimum temperature and speed.

If electric, then the batteries tend to be lead acid. These do need replacing at regular intervals and cannot withstand large numbers of deep cycles (-200) before their performance is reduced. While replacing the batteries this is not prohibitively expensive, lead acid batteries do have other well-known limitations such as maintenance and inability to measure the energy within the battery.

A fuel cell powered golf buggy using the system as described will have several advantages. It will maintain its performance for the life of the buggy. It can be refuelled quickly compared to the speed of electrical charging and therefore there is little risk that the golf buggy will either run out of charge and players will be stranded. A fuel cell powered buggy will be very quiet unlike the internal combustion engine equivalent. If using hydrogen from renewable sources, the buggies will be effectively pollution free, which is desirable.

2) Welding: The traditional oxygen -acetylene welding has long been a useful and is the main method for portable cutting and welding metal. This is a two cylinder system where the gases are controlled burnt in a torch. This market is on the decline because of concerns of' acetylene safety and the method produces a relatively crude weld.

Electric arc welding has developed technically. Initially the arc was open to air and oxygen and as a result produced a relatively crude weld through oxidation. MIG and TIG welders shroud the arc in inert gas and the as a result the metal is not oxidised. These systems are largely limited by being connected to a mains supply of power and are therefore not portable.

The key observation is that welding generally needs cylinders of gas in order to operate. Therefore the operators are familiar with using gases and the delivery channels are developed. This hybridisation system therefore is advantageous because it creates a new way of creating a portable welder. The system as described can output high currents for intermittent periods. This power output fits well with the operating duty required for electrical welding where high amperages are required for short periods, If required inert gas can be provided by a second cylinder containing argon or another gas mixture. The resultant system is a two cylinder electrical system.

3) Uninterruptable power supply (UPS) systems -21 -Uninterruptable power supplies require immediate power so that systems do not fail. Even a short period will cause an electronic system to trip. Typically internal combustion engines require a few seconds to start and produce power and often they rely on other technologies such as batteries to instantaneously produce power. Even when internal combustion engines are running they will struggle to instantaneously match peak loads.

Batteries can be used but often the amount of energy held in the battery cannot be determined. In the case of lead acid batteries this situation can be acute where when required, a seemingly charged battery delivers only a very small amount of power. The factors that affect this are ambient temperature and the number of deep cycles. The present system, when used with a fuel cell, has the advantage of using hydrogen and as long as there is pressure in the cylinder then there is potential power available. This is a measureable variable.

The system as described, because it is a hybrid system has the advantage that it will immediately provide power the battery which allows the rest of the system time to normalise start the fuel cell system. This can be achieved in approximately 5 seconds.

Once operating the system will effectively operate on the fuel cell and the battery is reserved to meet peak intermittent loads.

An example of its use would be a telecommunications system where it is important for the systems to beep running at all times. The power range can range between I kW to 10kW. Part of that system might be a cooling system where the additional load only comes on intermittently.

Once the system has been activated, it is relatively easy to measure how much hydrogen has been consumed and deliver more hydrogen to the unit in cylinders. Battery systems or compressed air storage may take days to recharge and during that time the system is vulnerable to power outages.

4) Power integration There is an increasing desire to integrate a wide number of disparate power systems to gather small amounts of energy available. Micro generation in the built environment is only useful if it can be fed into a system along with other power sources.

Ideally one or more of those power inlets or outlets is a power storage device.

As an example, the system as described, could be part of the power system of a building where there are different forms of power generation. Examples might be wind arid photovoltaic generation. Within the building there will be intermittent power demands and the system would enable an efficient way of connecting those systems so that ppwer is -22 -stored and recovered when required. The system is fully scalable and can be expanded to connect to multiple sources.

Equally, the system as described could be mobile and part of a car or bike for instance. This is commercially attractive because it is an expensive asset when not being used when parked up. Energy generating devices along with the mains electrical system would be plugged in for charging the vehicle. It would then form part of the total energy system of the building where the fuel cell and battery is called upon to produce power when additional resources are required. This solution is ideal because a parked vehicle indicates there is occupancy of a building, and this occupancy normally creates intermittent demand for additional power. For example heating, cooking and washing.

5) Quiet modular power The use of internal combustion engine powered generators is common. However, there are circumstances where a quiet and high power source is required. Examples might be outside or inside residential areas where people are sleeping or working. Luxury accommodation such as yachts, mobile homes, holiday camps and island accommodation.

Other applications might be truck APU where there is a sleeping cabin or cooling units.

Street cleaning and other temporary street furniture such as lighting or traffic lights. In military applications the noise signature is also critical. In these circumstances there is little choice other than to use an internal combustion engine with significant noise reduction equipment.

The system described herein provides a portable generation system that can be used as either a single unit or as module. The advantage is its ability to load follow high power demands that are often operating in the circUmstances described above.

Other types of applications are where portable power tools are used intermittently in remote locations.

Claims (20)

  1. CLAIMS: 1. A hybridisation device for combining the power outputs of first and second power sources, the device comprising: a load circuit for applying a load to the first power source; and a controller for cycling the applied load, measuring characteristics of the first power source, and for thereby determining operating parameters of the first power source for future utilisation of the first power source during hybridisation with the second power source.
  2. 2. A device as claimed in claim 1, wherein the controller is arranged to use the load circuit to cycle the load from the first power source at various frequencies, whilst simultaneously measuring the input voltage and/or current from the power source, and to thereby build up a model of the output impedance of the first power source.
  3. 3. A device as claimed in claim 2, wherein the controller is arranged to use the model to set a maximum current draw constant for the first power source and a minimum safe input voltage.
  4. 4. A device as claimed in claim 2 or 3, wherein the controller is arranged to convert output impedance values to a maximum current draw limit by using a blanket rule of the current draw associated with a certain percentage of voltage drop.
  5. 5. A device as claimed in claim 2 or 3, wherein the controller is provided with preset output impedance models of types of first power source that could be connected to the hybridisation device, and the controller is arranged to match the measured characteristics with known values to thereby identify the first power source.
  6. 6. A device as claimed in any preceding claim, wherein the device is arranged to supply power to the load using the second power source during the process of determining operating parameters of the first power source.
  7. 7. A device as claimed in any preceding claim, the first power source having a relatively low maximum output power and/or voltage and the second power source having a relatively high maximum output power and/or voltage, the hybridisation device further comprising: -24 -a bus having an output for supplying power to the load; a first power source circuit for receiving power from the first power source, the first power source circuit having an output connected to the bus, and the first power source circuit comprising a converter circuit for controlling the voltage and/or current output from the first power source; and a second power source circuit connected to the bus in parallel with the first power source circuit, the second power source circuit being for supplying additional power to the bus from the second power source, wherein the second power source circuit is arranged to connect the output voltage of the second power source to the bus without intervening voltage control.
  8. 8. A device as claimed in any preceding claim further comprising one or more additional load circuits for additional first power sources in order to provide for hybridisation of more than two power sources.
  9. 9. A method of hybridisation of first and second power sources using a hybridisation device in order to supply power to a load, the method comprising: applying a load to the first power source and cycling the applied load; measuring characteristics of the first power source; and based on the load cycle and measured characteristics, determining operating parameters of the first power source for future utilisation of the first power source during hybridisation with the second power source.
  10. 10. A method as claimed in claim 9, wherein the load from the first power source is cycled at various frequencies, whilst simultaneously sampling the input voltage and/or current, and the method comprises building a model of the output impedance of the power source.
  11. 11. A method as claimed in claim 10, wherein the model is used to set operating parameters in the form of a maximum current draw constant for the power source as well as a minimum safe input voltage.
  12. 12. A method as claimed in claim 10 or 11, wherein output impedance values are converted to a maximum current draw limit using a blanket rule of the current draw associated with a certain percentage of voltage drop.
    -25 -
  13. 13. A method as claimed in claim 10 or 11, comprising matching the measured characteristics of the first power source with preset output impedance models of types of energy source that could be connected to the hybridisation device.
    S
  14. 14. A method as claimed in any of claims 9 to 13, wherein after the operating parameters have been determined, during subsequent operation of the hybridisation device power is drawn up to a maximum amount and within the safe limits for voltage and current as determined using the load cycling tests.
  15. 15. A method as claimed in any of claims 9 to 14 comprising supplying power to the load using the second power source during the process of determining operating parameters of the first power source.
  16. 16. A method as claimed in any of claims 9 to 15 comprising using power from the second power source to start up the device when a voltage is detected from an unrecognised first power source, checking the voltage input from the first power source to ensure it is within the limits of the device, and thereafter cycling the load to determine the operating parameters of the first power source.
  17. 17. A method as claimed in any of claims 9 to 16, the first power source having a relatively low maximum output power and/or voltage and the second power source having a relatively high maximum output power and/or voltage, the method further comprising: providing a bus having an output for supplying power to the load; providing a first power source circuit for receiving power from the first power source, the first power source circuit having an output connected to the bus, and the first power source circuit comprising a converter circuit for controlling the voltage and/or current output from the first power source; providing a second power source circuit connected to the bus in parallel with the first power source circuit, the second power source circuit being for supplying additional power to the bus from the second power source, wherein the second power source circuit is arranged to connect the output voltage of the second power source to the bus without intervening voltage control; and supplying power to the load via the bus using either the first power source, the second power source or both power sources in combination.
  18. 18. A method as claimed in any of claims 9 to 17, comprising: -26 -initiating the supply of power to the load with only the second power source supplying power; starting the first power source when the current through the second power source is greater than a set point; and thereafter running the first power source until shut down of the hybridisation device.
  19. 19. A hybridisation device substantially as hereinbefore described with reference to Figure 2 or 3.
  20. 20. A method of hybridisation substantially as hereinbefore described with reference to Figure 2 or 3.
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