GB2510153A

GB2510153A - Consumption control for grid connected micro-generation system

Info

Publication number: GB2510153A
Application number: GB1301363.6A
Authority: GB
Inventors: Michael Anthony Farr
Original assignee: FARRSIGHT Ltd
Current assignee: FARRSIGHT LIMITED
Priority date: 2013-01-25
Filing date: 2013-01-25
Publication date: 2014-07-30
Also published as: GB201301363D0; WO2014114945A1

Abstract

An AC electrical distribution circuit 6 is connected to receive power from a micro-generation system 4, to provide power to a plurality of loads or consumers 8, including a switchable load, and to receive power from or provide power to a grid or public distribution network 2. The switchable load is switched on to receive excess power from the micro-generation system 4, or switched off to avoid drawing power from the grid 2. The direction of flow of power between the distribution circuit 6 and the grid 2 is determined according to the polarity or phase difference between the current and the voltage at the distribution circuit 6. Signals representing the current and voltage are gated together to generate a control signal for the switchable load. The micro-generation system 4 may use a renewable energy source such as solar power. The switchable load may be a boiler or immersion heater. Additional signals may be used in combination with the first, representing the current and voltage, with inverse polarity to the first signals. Switching may be restricted to when the voltage is close to zero.

Description

Electrical Energy Consumption Controller

BACKGROUND

a. Field of the Invention

This invention relates to a method and apparatus for controlling the consumption of electrical energy in an alternating current (AC) electrical generation and distribution system. In particular the present invention relates to a method and apparatus for controlling the consumption of surplus electrical energy generated by a local electrical energy micro-generation system connected to an electrical energy distribution circuit arranged to provide electrical energy to one or more electrical energy consumers.

b. Related Art It is now increasingly common for households and other premises to generate at least a portion of the electrical energy that they consume by way of a local electrical micro-generation system. These micro-generation systems typically comprise photovoltaic (PV) solar panels fitted to roofs, but may alternatively or additionally comprise wind turbines, water turbines or other similar means of generating electrical energy.

Many countries have introduced financial incentives for the local micro-generation of electrical energy. In the United Kingdom this financial incentive is in the form of tariffs that are paid dependent on the total amount of electrical energy generated by the micro-generation system and the amount of electrical energy that is exported to the National Grid (the public electrical energy transmission network in Great Britain). In many cases the value of the tariff is based on the amount of electrical energy generated, irrespective of whether or not the electrical energy is consumed locally or whether any of that electrical energy is exported to the National Grid. As such, there is a desire to utilise locally as much of the electrical energy generated by the micro-generation system as possible and additionally minimise the amount of electrical energy that is imported from the National Grid.

The amount of electrical energy being generated by a micro-generation system will vary depending upon the available driving energy source. For example, in a PV solar generation system, the intensity of solar radiation falling on the photovoltaic panels will vary throughout the day, as well as according to the season. In particular, the angle of the sun relative to the photovoltaic panels, and the presence of clouds, trees and other obstructions will affect the amount of solar radiation striking the panels. Similarly, the amount of energy generated by wind turbine or water turbine generation systems will be affected by the wind speed or speed of water flow respectively.

In addition, the total electrical energy consuming load within a premises will vary throughout the day, as electrical devices or loads are switched on and off. For example, within a domestic household the electrical loads may include thermostatically controlled loads, such as electric cookers, electric immersion heaters and electric radiators, as well as other varying loads, such as switched-mode supplies for computers and televisions.

It is an object of the present invention to provide a method and apparatus for controlling the consumption of electrical energy by one or more electrical loads so as to utilise substantially all of the electrical energy being generated by a micro-generation system.

SUMMARY OF THE INVENTION

According to the present invention there is provided a method of controlling the consumption of electrical energy in an alternating current (AC) electrical energy generation and distribution system, said system comprising a local electrical energy distribution circuit, an electrical energy micro-generation system, one or more electrical energy consumers including a switchable energy consumer, and an electrical energy consumption control circuit, said local electrical energy distribution circuit being operatively connected to receive electrical energy from said micra-generation system, to receive electrical energy from or to provide electrical energy to a public electrical energy distribution network, and to provide electrical energy to said one or more electrical energy consumers, the electrical energy consumption control circuit being operatively connected to control the electrical energy provided to the switchable energy consumer from said local electrical energy distribution circuit, and the electrical energy consumption control circuit being arranged to carry out the method steps of: -generating a first signal representative of the current flowing between the local electrical energy distribution circuit and the public electrical energy distribution network; -generating a second signal representative of the voltage in the local electrical energy distribution circuit; -using the second signal to gate the first signal, said gated signal being representative of the direction of the net flow of electrical energy between the local electrical energy distribution circuit and the public electrical energy distribution network; -generating a switching signal from said gated signal; and -using the switching signal to activate switching means, such that when the direction of the net flow of electrical energy is from the local electrical energy distribution circuit to the public electrical energy distribution network, the switching means are switched to a first state in which electrical energy is provided to the switchable energy consumer, and when the direction of the net flow of electrical energy is from the public electrical energy distribution network to the local electrical energy distribution circuit, the switching means are switched to a second state in which electrical energy is not provided to the switchable energy consumer.

Also according to the invention there is provided an apparatus for controlling the consumption of electrical energy in a local electrical energy distribution circuit, said local electrical energy distribution circuit being operatively connected to receive electrical energy from an electrical energy micro-generation system, to receive electrical energy from or to provide electrical energy to a public electrical energy distribution network, and to provide electrical energy to one or more electrical energy consumers, including a switchable energy consumer, said apparatus comprising: -means for generating a first signal representative of the current flowing between the local electrical energy distribution circuit and the public electrical energy distribution network; -means for generating a second signal representative of the voltage in the local electrical energy distribution circuit; -means for gating the first signal using the second signal, said gated signal being representative of the direction of the net flow of electrical energy between the local electrical energy distribution circuit and the public electrical energy distribution network; -means for generating a switching signal from said gated signal; and -switching means for controlling the supply of electrical energy to said switchable energy consumer using said switching signal, said switching means being arranged such that, in use, when the direction of the net flow of electrical energy is from the local electrical energy distribution circuit to the public electrical energy distribution network, the switching means is switched to a first state in which electrical energy is provided to the switchable energy consumer, and when the direction of the net flow of electrical energy is from the public electrical energy distribution network to the local electrical energy distribution circuit, the switching means is switched to a second state in which electrical energy is not provided to the switchable energy consumer.

In a preferred embodiment the current is an AC current and the first signal is at least substantially synchronised with the AC current. In these embodiments the voltage is an AC voltage and the second signal is at least substantially synchronised with the AC voltage.

It will be appreciated that the AC voltage will have a substantially sinusoidal waveform, and further, that both the AC voltage waveform and particularly the AC current waveform may comprise a plurality of harmonics as understood by a person of skill in the art. Furthermore, when the net flow of electrical energy is in a first direction, the AC current and voltage waveforms will be substantially of the same polarity, while when the net flow of electrical energy is in a second, opposite direction, the AC current and voltage waveforms will be substantially of inverse polarity. As such, when the second, voltage signal is used to gate the first, current signal, the resultant gated signal will comprise a series of positive peaks or negative troughs depending on whether the AC current and AC voltage signals are substantially of similar or of inverse polarity and, therefore, will be indicative of the direction of net flow of electrical energy.

In some embodiments the second signal is a series of pulses. Preferably the gated signal is integrated to produce the switching signal.

In preferred embodiments the method further comprises generating a third signal representative of the current flowing between the local electrical energy distribution circuit and the public electrical energy distribution network, the third signal having an inverse polarity to the first signal, generating a fourth signal representative of the voltage in the local electrical energy distribution circuit, the fourth signal having an inverse polarity to the second signal, using the fourth signal to gate the third signal, combining the gated signals and generating the switching signal from the combined gated signal.

Preferably the times at which the voltage in the local distribution circuit is close to zero Volts is determined and switching means are activated to switch from either the first or the second state to the other state only when the voltage signal is close to zero Volts. In one embodiment of the design a pulsed signal is generated, the pulses coinciding with the times at which the voltage is close to zero Volts. In another preferred embodiment, the switching means comprises a zero-voltage switching means.

In preferred embodiments the gated or switching signal is smoothed, a first light emitter is illuminated when said smoothed signal represents a net electrical energy flow from the local electrical energy distribution circuit to the public electrical energy distribution network, and a second light emitter is illuminated when said smoothed signal represents a net electrical energy flow from the public electrical energy distribution network to the local electrical energy distribution circuit. In a further embodiment a third light emitter is illuminated when the control circuit is providing electrical energy to the switchable energy consumer. Preferably the degree of illumination of any light emitter is varied such that the brightness of the light emitter represents the degree to which there is net electrical energy flow either to or from the public electrical energy distribution network, or the amount of electrical energy provided to the switchable energy consumer.

In preferred embodiments the electrical energy consumers comprise a primary switchable energy consumer and a secondary switchable energy consumer, and when the direction of the net flow of electrical energy is from the local electrical energy distribution circuit to the public electrical energy distribution network, a first switching means is activated to provide electrical energy to said primary switchable energy consumer, and if the direction of the net flow of electrical energy is from the local electrical energy distribution circuit to the public electrical energy distribution network after activating the first switching means, a second switching means is subsequently activated to provide electrical energy to said secondary switchable energy consumer.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be further described, by way of example only and with reference to the accompanying drawings, in which: Figure 1 is a block diagram of an electrical energy generation and distribution system including an electrical energy consumption control circuit of the present invention; and Figure 2 is a block diagram of the electrical energy consumption control circuit of Figure 1.

DETAILED DESCRIPTION

Figure 1 shows an electrical energy generation and distribution system 1 comprising means for generating alternating current (AC) electrical energy, means for distributing the electrical energy and means for consuming the electrical energy.

In particular, the system 1 comprises a public AC electrical energy supply and distribution network 2 which includes an external or public source of electrical energy, for example coal tired, gas fired or nuclear power stations, and an electrical energy transmission and distribution network, known as the National Grid in Great Britain, which transmits the electrical energy to a premises which may, for example, be a domestic house, outbuilding or a business premises, such as an office orfactory.

The system 1 also comprises a local electrical energy micro-generation system 4 associated with and arranged to supply AC electrical energy to the premises. This local micro-generation system 4 may include photovoltaic (PV) solar panels, wind turbines, water turbines or other similar means of generating electrical energy.

A local AC electrical energy distribution circuit 6 is located within the premises and is operatively connected to receive AC electrical energy from both the local micro-generation system 4 and the public electrical energy distribution network 2. The local distribution circuit 6 distributes the AC electrical energy to one or more electrical energy consumers 8 operatively connected to the local distribution circuit 6. In domestic premises, the local distribution circuit 6 may comprise the consumer unit and one or more ring mains and other circuits which might include an immersion heater circuit, a cooker circuit and lighting circuits of a house, and the electrical energy consumers 8 may comprise, for example, a television, a computer, a kettle and an electric cooker.

The AC electrical energy being received from the public electrical energy distribution network 2, via one or more mains electrical cables 10, is measured by an electrical energy consumption meter or electricity meter 12. This meter 12 measures the amount of electrical energy being imported to the premises from the public electrical energy distribution network 2, and this measurement is used to charge the owners of the premises an amount dependent on their electrical energy importation and usage.

In situations in which the micro-generation system 4 is generating electrical energy, but not all of that electrical energy is being used by the consumers 8 connected to the local distribution circuit 6, the excess or surplus electrical energy will be exported from the premises to the public electrical energy distribution network 2.

In accordance with the present invention, an electrical energy consumption control circuit 14 is operatively connected to the local distribution circuit 6. The control circuit 14 controls the consumption of the surplus AC electrical energy provided by the micro-generation system 4 so that a minimal amount of AC electrical energy is either exported to or imported from the public electrical energy distribution network 2.

The control circuit 14 requires a current signal input and a voltage signal input.

The current signal input is obtained from a current detector or sensor 16, which is preferably a current transformer, that senses the AC current flowing in the mains electrical cable 10. In this example, a voltage signal is obtained from the AC low-voltage power supply which powers the control circuit 14, and this voltage signal corresponds to the AC voltage in the local distribution circuit 6.

The control circuit 14 controls the flow of current and thus electrical energy to one or more switchable electrical energy consumers 18. The switchable electrical energy consumer 18 may comprise any suitable electrical energy consumer that can be switched on and off rapidly. In a preferred embodiment the switchable electrical energy consumer 18 is an immersion heater operatively connected directly to the control circuit 14; however, the switchable energy consumer 18 may comprise any other heating device, for example a storage heater, or under-floor heating facility, either directly connected to the control circuit 14, or indirectly connected using a suitable socket and plug arrangement.

The electrical energy consumption control circuit 14 is illustrated in more detail in Figure 2.

A signal from the current detector 16, which corresponds to the AC current flowing in the mains electrical cable 10, is used to generate a pair of signals representative of the current, a second one of these signals being inverted relative to a first one of these signals. The means for generating this pair of signals 20 preferably comprises a differential amplifier for maximum noise immunity, and an inverting amplifier for generating the second inverted current signal. The complementary, inverted and non-inverted, current signals provide inputs to two comparator circuits.

The signal representing the voltage is applied to the two comparator circuits to gate the pair of complementary current signals. The comparator circuits, therefore, generate a gated, and partially inverted, current signal. This is typically achieved using gating means 24 in the form of analogue switches or transmission gates.

Preferably the first signal is at least substantially synchronised with the AC current and has the same frequency as the AC current, and the second signal is at least substantially synchronised with the AC voltage and has the same frequency as the AC voltage. It will be appreciated, however, that the control circuit 14 may comprise means for altering the frequency or the phase of the current and voltage signal, before these signals are input to the comparator circuits.

The gating means 24 preferably comprises two analogue switches or transmission gates. A first analogue switch gates the first, non-inverted signal representative of current using a first signal representative of voltage and a second analogue switch gates the second, inverted signal representative of current using a second signal representative of voltage. When the current and voltage signals are substantially of the same polarity the voltage signals effectively sample the peaks of the pair of current signals. When the current and voltage signals are substantially of opposite polarity, the voltage signals effectively sample troughs of the pair of current signals. The combined signals from the gating means 24, therefore, generate a gated signal which is a partially inverted representation of the sensed current signal.

If the net flow of electrical energy is in a first direction along the cable 10 then the voltage and current signals will be substantially of the same polarity and the gated current signal from the comparator circuits will be substantially positive. If the net flow of electrical energy is in the opposite (second) direction along the cable 10 then the voltage and current will be substantially of inverse polarity and the gated current signal from the comparator circuits will be substantially negative. In this way, the gated current signal comprises pulses having either substantially positive polarity or substantially negative polarity dependent on the net direction of the flow of electrical energy in the mains electrical cable 10. As such, instead of a power measurement being made, only a gated current signal is used to determine the direction of flow of electrical energy in the cable 10, which thus determines whether electrical energy is being either imported from or exported to the public electrical energy distribution network 2.

Integrating means 26 then integrates the gated signal to generate a switching signal representative of the direction of the net flow of electrical energy between the local distribution circuit 6 and the public electrical energy distribution network 2 over time.

In a preferred embodiment, a negative switching signal is representative of a net flow of electrical energy from the public electrical energy distribution network 2 to the local distribution circuit 6, i.e. a net import of electrical energy to the premises, and a positive switching signal is representative of a net flow of electrical energy from the local distribution circuit 6 to the public electrical energy distribution network 2, i.e. a net export of electrical energy from the premises.

The switching signal is used to control switching means 28, in the form of a solid state switching device, either to allow or to prevent electrical energy being supplied to the switchable electrical energy consumer 18. In a preferred embodiment of the invention the switching means 28 comprises a triac.

In another embodiment of the invention the switching signal is used as an output signal to control an external switching means.

In a preferred embodiment, a comparator is used to control the solid state switching device such that when the integrated switching signal indicates a net export of electrical energy, which in this example is a positive going signal, the switching means 28 is switched to a first state in which current and electrical energy is supplied to the switchable electrical energy consumer 18. This electrical consumer 18, therefore, consumes electrical energy and the amount of electrical energy being exported is either reduced or, if the new total energy consumption or load is greater than the electrical energy being generated locally by the micro-generation system, then electrical energy ceases being exported and electrical energy is imported to the premises. The reversal of electrical energy flow from export to import from the public electrical energy distribution network drives the integrated signal in the reverse direction so that with time it will change polarity. In this example the switching signal becomes negative and the switching means 28 is switched to a second state in which current and, therefore, electrical energy is prevented from being supplied to the switchable electrical energy consumer 18.

An indicator in the form of a light emitter may be used to indicate when electrical energy is being provided to the switchable energy consumer or additional load 18.

The light emitter is preferably a light emitting diode (LED).

In preferred embodiments the gated signal is smoothed and this smoothed signal is used to illuminate one or other of a pair of light emitters, which are preferably LEDs, to indicate whether there is a net import or export of electrical energy.

Preferably the degree of illumination of any light emitter is varied such that the brightness of the light emitter represents the degree to which there is net energy flow either to or from the public electrical energy distribution network, or the amount of electrical energy provided to the switchable energy consumer.

In further particularly preferred embodiments, the control circuit 14 comprises a zero voltage switching circuit (not shown). The zero voltage switching circuit is used to control the switching means 28 such that the switching means is only enabled to switch on or off the supply of electrical energy to the additional load 18 close to a zero voltage crossing of the mains voltage. This prevents unnecessary electromagnetic emissions and electrical interference associated with switching high voltages or large currents.

In some embodiments of the invention, the switching signal may be used as an output signal to control an external switching module for the control of the supply of electrical energy to a switchable energy consumer. The external switching module may comprise a burst tiring trigger module.

Additionally, in some embodiments of the present invention, a timer or other override device may be provided in parallel with the solid state switch to enable an override function, so that electrical energy may be supplied to the additional load 18 at will, irrespective of the state of the switching means 28 in the control circuit 14.

Additionally, an arrangement may be incorporated whereby a primary, master switchable energy consumer and a secondary, slave switchable energy consumer may be connected to the control circuit 14. In this arrangement the switching means 28 comprises a first switching means for controlling the supply of electrical energy to the primary switchable energy consumer and a second switching means for controlling the supply of electrical energy to the secondary switchable energy consumer. In these embodiments the control circuit 14 further comprises prioritising means for prioritising the activation of the first switching means such that when the direction of the net flow of electrical energy is from the local distribution circuit 6 to the public electrical energy distribution network 2, the first switching means is activated to provide electrical energy to the primary switchable energy consumer and if, after activating the first switching means, the direction of the net flow of electrical energy is still from the local distribution circuit 6 to the public electrical energy distribution network 2, i.e. not all of the surplus electrical energy is being consumed, the second switching means is activated to subsequently provide electrical energy to the secondary switchable energy consumer.

Whereas the foregoing description of the control circuit 14 uses a combination of analogue and digital electronic components to control the electrical energy supplied to a switchable energy consumer 18, it will be appreciated that the same functions may be implemented using a microcontroller or other such similar computer based device, including but not limited to devices such as Raspberry Pi TM, PlC TM or Arduino TM for example.

It will additionally be appreciated that the signals described in the foregoing description may be of any suitable type, for example the signals may be in the form of voltages or currents where appropriate, or the signals may be optical or wireless signals.

The present invention, therefore, provides a method and apparatus for controlling the consumption of electrical energy by one or more electrical devices or loads so as to utilise substantially all of the electrical energy being generated by an electrical energy micro-generation system.

Claims

CLAIMS1. A method of controlling the consumption of electrical energy in an alternating current (AC) electrical energy generation and distribution system, said system comprising a local electrical energy distribution circuit, an electrical energy micro-generation system, one or more electrical energy consumers including a switchable energy consumer, and an electrical energy consumption control circuit, said local electrical energy distribution circuit being operatively connected to receive electrical energy from said micro-generation system, to receive electrical energy from or to provide electrical energy to a public electrical energy distribution network, and to provide electrical energy to said one or more electrical energy consumers, the electrical energy consumption control circuit being operatively connected to control the electrical energy provided to the switchable energy consumer trom said local electrical energy distribution circuit, and the electrical energy consumption control circuit being arranged to carry out the method steps of: -generating a first signal representative of the current flowing between the local electrical energy distribution circuit and the public electrical energy distribution network; -generating a second signal representative of the voltage in the local electrical energy distribution circuit; -using the second signal to gate the first signal, said gated signal being representative of the direction of the net flow of electrical energy between the local electrical energy distribution circuit and the public electrical energy distribution network; -generating a switching signal from said gated signal; and -using the switching signal to activate switching means, such that when the direction of the net flow of electrical energy is from the local electrical energy distribution circuit to the public electrical energy distribution network, the switching means are switched to a first state in which electrical energy is provided to the switchable energy consumer, and when the direction of the net flow of electrical energy is from the public electrical energy distribution network to the local electrical energy distribution circuit, the switching means are switched to a second state in which electrical energy is not provided to the switchable energy consumer.
2. A method as claimed in Claim 1, wherein the current is an AC current and the first signal is synchronised with the AC current, and the voltage is an AC voltage and the second signal is synchronised with the AC voltage.
3. A method as claimed in any preceding claim, wherein the method comprises integrating said gated signal to generate the switching signal.
4. A method as claimed in any preceding claim, wherein the second signal is a series of pulses.
5. A method as claimed in any preceding claim, wherein the method comprises: -generating a third signal representative of the current flowing between the local electrical energy distribution circuit and the public electrical energy distribution network, said third signal having an inverse polarity to said first signal; -generating a fourth signal representative of the voltage in the local electrical energy distribution circuit, said fourth signal having an inverse polarity to said second signal; -using the fourth signal to gate the third signal; -combining said gated signals; -generating the switching signal from said combined gated signal.
6. A method as claimed in any preceding claim, wherein the method turther comprises: -determining the times at which the voltage in the local electrical energy distribution circuit is close to zero Volts; and -activating the switching means to switch from either the first or the second state to the other of the first and second states only when the voltage signal is close to zero Volts.
7. A method as claimed in Claim 6, wherein the step of determining the times at which the voltage is close to zero Volts comprises generating a pulsed signal, the pulses coinciding with the times at which the voltage is close to zero Volts.
8. A method as claimed in any preceding claim, wherein the method further comprises: -smoothing the gated signal; -illuminating a first light emitter when said smoothed signal represents a net electrical energy flow from the local electrical energy distribution circuit to the public electrical energy distribution network; -illuminating a second light emitter when said smoothed signal represents a net electrical energy flow from the public electrical energy distribution network to the local electrical energy distribution circuit; and -illuminating a third light emitter when the control circuit is providing electrical energy to the switchable energy consumer.
9. A method as claimed in Claim 8, wherein the method further comprises varying the degree of illumination of any light emitter such that the brightness of the light emitter represents the degree to which there is net electrical energy flow either to or from the public electrical energy distribution network.
10. A method as claimed in any preceding claim, wherein the electrical energy consumers comprise a primary switchable energy consumer and a secondary switchable energy consumer, and wherein the step of using the switching signal to activate switching means comprises: -when the direction of the net flow of electrical energy is from the local electrical energy distribution circuit to the public electrical energy distribution network, activating a first switching means to provide electrical energy to said primary switchable energy consumer; and -if the direction of the net flow of electrical energy is from the local distribution circuit to the public electrical energy distribution network after activating the first switching means, subsequently activating a second switching means to provide electrical energy to said secondary switchable energy consumer.
11. An apparatus for controlling the consumption of electrical energy in a local electrical energy distribution circuit, said local electrical energy distribution circuit being operatively connected to receive electrical energy from an electrical energy micro-generation system, to receive electrical energy from or to provide electrical energy to a public electrical energy distribution network, and to provide electrical energy to one or more electrical energy consumers, including a switchable energy consumer, said apparatus comprising: -means for generating a first signal representative of the current flowing between the local electrical energy distribution circuit and the public electrical energy distribution network; -means for generating a second signal representative of the voltage in the local electrical energy distribution circuit; -means for gating the first signal using the second signal, said gated signal being representative of the direction of the net flow of electrical energy between the local electrical energy distribution circuit and the public electrical energy distribution network; -means for generating a switching signal from said gated signal; and -switching means for controlling the supply of electrical energy to said switchable energy consumer using said switching signal, said switching means being arranged such that, in use, when the direction of the net flow of electrical energy is from the local electrical energy distribution circuit to the public electrical energy distribution network, the switching means is switched to a first state in which electrical energy is provided to the switchable energy consumer, and when the direction of the net flow of electrical energy is from the public electrical energy distribution network to the local electrical energy distribution circuit, the switching means is switched to a second state in which electrical energy is not provided to the switchable energy consumer.
12. An apparatus as claimed in Claim 11, wherein the current is an AC current and the first signal is synchronised with the AC current; and the voltage is an AC voltage and the second signal is synchronised with the AC voltage.
13. An apparatus as claimed in Claim 11 or Claim 12, further comprising: -means for generating a third signal representative of the current flowing between the local electrical energy distribution circuit and the public electrical energy distribution network, said third signal having an inverse polarity to said first signal; -means for generating a fourth signal representative of the voltage in the local electrical energy distribution circuit, said fourth signal having an inverse polarity to said second signal; -means for gating the third signal using the fourth signal; and -means for combining said gated signals.
14. An apparatus as claimed in any of Claims 11 to 13, wherein the apparatus comprises means for integrating said gated signal.
15. An apparatus as claimed in any of Claims 11 to 14, wherein the switching means comprises a triac.
16. An apparatus as claimed in any of Claims 11 to 15, wherein the apparatus further comprises: -means for determining the times at which the voltage in the local electrical energy distribution circuit is close to zero Volts; and -means for activating the switching means to switch from the either the first or second state to the other of the first and second states only when the voltage signal is close to zero Volts.
17. An apparatus as claimed in Claim 16, wherein the means for determining the times at which the voltage is close to zero Volts comprises means for generating a pulsed signal, the pulses coinciding with the times at which the voltage is close to zero Volts.

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18. An apparatus as claimed in any of Claims 11 to 17, wherein the apparatus further comprises: -a first light emitter, a second light emitter and a third light emitter; -means for smoothing the gated signal; -means for illuminating the first light emitter when said smoothed signal represents a net flow of electrical energy from the local electrical energy distribution circuit to the public electrical energy distribution network; -means for illuminating the second light emitter when said smoothed signal represents a net flow of electrical energy from the public electrical energy distribution network to the local electrical energy distribution circuit; and -means for illuminating the third light emitter when the apparatus is providing electrical energy to the switchable energy consumer.
19. An apparatus as claimed in Claim 18, wherein the apparatus further comprises means for varying the degree of illumination of any light emitter such that the brightness of the light emitter represents the degree to which there is net electrical energy flow either to or from the public electrical energy distribution network.
20. An apparatus as claimed in any of Claims 11 to 19, wherein the electrical energy consumers comprise a primary switchable energy consumer and a secondary switchable energy consumer, and wherein the switching means comprises: -a first switching means for controlling the supply of electrical energy to said primary switchable energy consumer; -a second switching means for controlling the supply of electrical energy to said secondary switchable energy consumer; and -prioritising means for prioritising the activation of the first switching means, such that, in use, when the direction of the net flow of electrical energy is from the local electrical energy distribution circuit to the public electrical energy distribution network, said first switching means is activated to provide electrical energy to said -21 -primary switchable energy consumer, and if, after activating the first switching means, the direction of the net tiow of electrical energy is still from the local electrical energy distribution circuit to the public electrical energy distribution network, the second switching means is activated to subsequently provide energy to said secondary switchable energy consumer.
21. A method substantially as herein described with reference to the accompanying drawings.
22. An apparatus substantially as herein described with reference to the accompanying drawings.