GB2487642A

GB2487642A - Renewable energy storage system

Info

Publication number: GB2487642A
Application number: GB1200791.0A
Authority: GB
Inventors: Peter James Duffett-Smith
Original assignee: Individual
Current assignee: Individual
Priority date: 2012-01-18
Filing date: 2012-01-18
Publication date: 2012-08-01
Anticipated expiration: 2032-01-18
Also published as: GB2487642B; WO2013108004A2; EP2805399A2; WO2013108004A3; GB201200791D0

Abstract

Excess electric power generated by a domestic alternative power generation system 101 (such as a solar panel or wind turbine) connected to a domestic electricity wiring network 104 is consumed within a device 110 whose power consumption may be varied using a control signal. The wiring network 104 is also connected to a external mains electric power supply system 108. The net power flowing between the domestic wiring network 104 and the external mains electric power supply system 108 is measured together with its direction of flow. A control signal is created to adjust the power consumed by the power-consuming device 110 in such a fashion that (a) the net power flowing from the domestic wiring network 104 to the external mains electric power supply system 108 is kept near or below a threshold value; (b) it varies in response to changes in the power generated by the domestic alternative power generation system 101, or to changes in the power consumed by the domestic wiring network 104; and (c) it is adjusted to reduce to zero the power consumed by the power-consuming device 110 whenever the direction of net power flow is from the external mains electric power supply system 108 to the domestic wiring network 104, or the net power flowing from the wiring network 104 to the external mains electric power supply system 108 is below the threshold. The power-consuming device 110 may be an electric immersion heater.

Description

Domestic power controller

Description

The invention relates to the use of excess electrical power generated by a domestic alternative power generation system.

Background of the invention

Many homes (individual domestic premises or small groups of houses) have so-called alternative energy generation systems installed in them. These are often wind turbines, or photo-voltaic solar panels on Sun-facing roofs, whose direct-current power is converted to match the alternating-current mains' supply from an external electricity supply system (sometimes known also as the grid, or the National Grid in the UK), using a cycle-matching and voltage-matching inverter. The inverter is connected to the household wiring network so that the generated power is consumed by domestic appliances plugged in' to the domestic wiring network with any residual load being provided via the electricity meter from the grid.

It is often the case that the alternative energy generation system provides more power than is required by the domestic load, for example when the Sun shines directly on to a solar panel array, or when the wind-speed rises and drives a wind-turbine with greater force. In these circumstances, the excess power is supplied to the grid and the net flow of power in the cable connecting the domestic premises to the grid is reversed from its normal direction. However, electricity power supply companies often buy back' this excess power at a small fraction of the charge they make for supplying an equal amount, so that the householder loses out on the transaction. The householder therefore saves money if he can use as much of the locally-generated power as possible, rather than taking it from the grid.

One way to use the excess power is to divert it into an energy storage system, such as a rechargeable battery bank. Whilst attractive in principle, the cost of installing and maintaining batteries on the scale required is usually prohibitive and economically not worthwhile. Another way of using the excess power is to heat the domestic hot water using an electric immersion heater. Many houses in the UK have these fitted to their hot-water cylinders, whether in addition to the main hot-water generation using gas-or oil-fired boilers, or as the sole form of hot-water generation. Excess power used in this way reduces the cost of heating the water. However, such systems often switch the immersion heater in an all-or-nothing fashion, so that the excess power can only be used when sufficient is generated for the immersion heater as well as the normal domestic load. The control is therefore coarse and wasteful of power, and the response time may be slow. An alternative system which provides finer control is needed to overcome these disadvantages and make best use of the excess electric power available.

Summary of the invention

According to the invention there is provided a method of using excess electric power generated by a domestic alternative power generation system connected to a domestic electricity wiring network which has connected to it a power-consuming device whose power consumption may be varied using a control signal, the wiring network also being connected to an external electric power supply system, the method comprising the steps of (a) measuring the net power flowing between the domestic wiring network and the external electric power supply system; (b) determining the direction of flow of the net power between the domestic wiring network and the external electric power supply system; and (c) creating a control signal to adjust the power consumed by the power-consuming device; characterised in that (i) when said determined direction of flow of net power is from the domestic wiring network to the external electric power supply system and the measured net power flow is greater than a pre-determined threshold, the control signal is adjusted to alter the power consumed by the power-consuming device so that the net power flowing between the domestic wiring network and the external electric power supply system approaches the threshold; (ii) the control signal varies in a continuous fashion in response to changes in the power generated by the domestic alternative power generation system, or to changes in the power consumed by the domestic wiring network; and (iii) when the direction of net power flow is from the external electric power supply system to the domestic wiring network or when the net power flowing from the wiring network to the external electric power supply system is below said threshold, the control signal is adjusted to reduce to zero the power consumed by the power-consuming device.

The invention includes control apparatus for enabling the use of excess electric power generated by a domestic alternative power generation system which is connected to a domestic electricity wiring network which has a power-consuming device whose power consumption may be varied using a control signal, the wiring network also being connected to a external electric power supply system, the apparatus comprising means for measuring in use the net power flowing between the domestic wiring network and the external electric power supply system; means for determining in use the direction of flow of the net power between the domestic wiring network and the external electric power supply system; means for creating in use a control signal to adjust the power consumed by the power-consuming device; means for adjusting in use the control signal so that the net power flowing between the domestic wiring network and the external electric power supply system approaches a pre-determined threshold value when said determined direction of flow of net power is from the domestic wiring network to the external electric power supply system, and the measured net power flow is greater than the threshold; means for varying in use the control signal in response to changes in the power generated by the domestic alternative power generation system, or to changes in the power consumed by the domestic wiring network; and means for adjusting in use the control signal so as to reduce to zero the power consumed by the power-consuming device when the direction of net power flow is from the external electric power supply system to the domestic wiring network or when the net power flowing from the wiring network to the external electric power supply system is below said threshold.

The invention also includes a domestic electricity wiring network which has a power-consuming device whose power consumption may be varied using a control signal, the wiring network also being connected to a external electric power supply system, and control apparatus as defined above.

Domestic premises fitted with a system according to the invention may also incorporate a by-pass switch so that the control of the power-consuming device may be made inoperative. For example, in a house having an electric immersion heater as the sole means of water heating, the householder can activate the by-pass switch and connect the immersion heater directly to the house wiring so that the external electric power supply system supplies the power. This may be useful at night when there is a demand for hot water and no power is being generated by the alternative power generation system.

The means for creating the control signal may be implemented using a digital microcontroller executing a program of instructions or using analogue devices acting on continuously-variable voltages.

The control signal may be a variable voltage or current, or a light signal.

Domestic premises in the UK incorporate a domestic electric wiring network, often known as the "house wiring" or the "mains wiring". An example network consists of a consumer unit which carries fuses or circuit breakers protecting a number of wiring loops (so-called ring mains) and spurs to which electric sockets, electric lights, and other fixed devices, such as electric showers, are connected. In other countries, the architecture may be different, for example adopting a spoke and hub arrangement rather than a ring main. The consumer unit is also connected to an external electric power supply system via an electric energy meter which measures the total accumulated electric energy consumed within the domestic premises.

The electrical energy supplied externally to the domestic premises usually comes from a regional or national supplier. In the United Kingdom, there are several companies which generate and supply electricity via a single supply system known as the National Grid. The householder buys electricity from one of these companies, paying periodically according to the reading of the electric energy meter. Different arrangements may apply in other countries.

Alternative power generation systems include arrays of solar photo-voltaic cells fitted to the roofs of houses and garages which face towards the Sun for part of the day, or wind turbines. However, the invention is not confined to systems of these particular types but may be applied to electric power generation systems of any sort which provide power to the domestic wiring network in addition to power supplied from the external electric power supply system.

The power-consuming device of the invention may consist of any electrical device such as a heater, battery charger, or electric motor, whose power consumption may be varied using a control signal such as an electric voltage or current, or a light signal.

For example, an electric heater may be connected via a thyristor throttle which governs the net power consumed by the heater in response to a control voltage by limiting the electrical power to bursts whose average power is in proportion to the control voltage.

The net power flowing in a wire carrying direct current (DC) is equal to the voltage multiplied by the current. Thus, a 100 V system supplying 20 amps provides 2 KW of power. However, the calculation is more complicated when the supply carries alternating current (AC). In Europe, the frequency of the AC supply is 50 Hz whilst 60 Hz is commonly used in other parts of the world such as the USA. During the course of one cycle of the AC supply, the voltage and the current reverse direction. The current spends half the cycle flowing in one direction, and the other half of the cycle flowing in the reverse direction. The voltage reverses sign every half cycle and there may also be a difference in the phase, 0, of the current, I, with respect to the voltage, V. If the frequency of the AC supply is f, then the voltage and the current can be expressed approximately as V = V sin(2tft), and I =I0sin(2tft+�), where V and 1 are the amplitudes of the voltage and current respectively, and t represents the time. The instantaneous power, P, is given by P = VI = V010 sin(2mft) sin(2icft + 0), and the net power flowing is the average of this over one cycle. Using c> braces to represent the average, the average power is given by the expression J sin(2mft)sin(2mft +0) dt (r)=(vi)=vi VI0cos(�) 00 T 2 where T= 1/f is the period of the AC.

Thus, the net power flowing may be found by measuring the phase difference and amplitudes of the voltage and current and using the equation above. Alternatively, the instantaneous values of the voltage and the current may be measured at a rate of many times per cycle, and the product averaged over an interval many cycles in length.

The instantaneous current can be measured using any convenient technique. For example, the voltage drop across a known resistance or reactance in series with the current can be measured, or a clip-on current probe can be used. One example of such a probe is the so-called Rogowski coil. A probe may behave like a transformer with a single turn in its primary circuit and a large number of turns in its secondary.

The alternating magnetic field produced by the current induces a voltage in the secondary, and this can be measured to deduce the value of the current. Another design makes use of the Hall effect, and can be used to measure DC current.

Whichever technique is used, the current flowing between the domestic wiring network and the external electric power supply system must be determined. The voltage, however, can be measured anywhere convenient in the domestic wiring network since its phase is virtually the same everywhere.

The phase difference, � between the voltage and the current changes by 180 degrees when there is a reversal of direction of the net power flowing between the domestic wiring network and the external electric power supply system. This changes the sign of the cos(�) term in the above equations, and thus also the sign of the measured power. Hence the measurement of the net power flowing from the product of voltage and current automatically provides an indication of which way the net power flows.

The control signal is created in proportion to the difference between the measured net power flowing and a pre-determined threshold value. For example, the threshold may be set at zero, or a low value of the power, say 100 W. A system according to the invention preferably increases the power consumed by the power-consuming device to maintain the net power flowing as close as possible to the threshold value.

Whenever the direction of net power flow is from the domestic wiring network towards the external electric power supply system, and the net power flow is above the threshold, the difference between the two is filtered and amplified and applied to increase the power consumed by the power consuming device. In principle, the amplification factor should be made as large as possible, and the filter bandwidth (proportional to the inverse of the time constant" of the system) should be made as wide as possible, so that the tracking error between the net power flowing and the threshold should be as small as possible, and the response to changes in power generated or consumed should be as fast as possible. However, in practice the system may become unstable if the values of either quantity are made too large because of other small delays in the system. This characteristic is well-known in the

field of servo control systems.

A system according to the invention having well-chosen values of the amplification factor and the filter bandwidth will respond quickly to changes in the power generated by the domestic alternative power generation system, or to changes in the power consumed by the domestic wiring network. These changes may come about if, for example, the Sun's radiance on a solar panel is modulated by changing cloud cover, or the wind speed changes on a gusty day so that a wind turbine's blades are driven with greater or lesser force. Even when the domestic alternative power generation system is providing a steady output, the net power flowing can be altered by the changing power consumption within the wiring network, such as when an electric kettle is plugged in and turned on, or a thermostatically-controlled heating device turns on or off. The inventive system will then operate to reduce substantially the fluctuations in the net power flowing from the wiring network to the external electric power supply system. However, whenever the direction of net power flow is from the external electric power supply system to the domestic wiring network, or the net power flowing from the wiring network to the external electric power supply system is below the threshold value, it is important to turn off the power-consuming device as quickly as possible in order not to incur unnecessary charges on the electricity bill.

This is achieved by adjusting the control signal so that the power consuming device is turned off.

The threshold may be set at any value. For example, a system according to the invention which responds instantaneously to a change in the power flowing from the wiring network to the external electric power supply system may use zero as the threshold value. However, in practice it is likely that the response will not be instantaneous and that there may be an overshoot or a lag in the control signal so that the system takes several moments to adjust itself to the change. It may then be an advantage to set the threshold value at a small positive value, say 100 W, in order to make sure that that significant amounts of power are not inadvertently taken from the external supplier when, for example, the Sun is coming out' and going in' on a cloudysunnyday.

An advantage of the system of the invention is that it provides fine control of the power consumed by the power-consuming device to make best use of the excess electric power available. This is in contrast to a system in which the power-consuming device is simply switched on or off.

Brief description of the drawings

Examples of the method and system according to the present invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a schematic representation of a domestic power control system; Figure 2 is a schematic representation of power controller unit; and Figure 3 shows the principle steps in a microcontroller program.

Detailed description of the invention

A block diagram of one embodiment of a system 100 according to the invention is shown in Figure 1. A domestic alternative power generation system 101 feeds electrical power into an inverter/controller device 102 which converts the power into cycle-matched and voltage-matched AC. This is fed via an energy-generation meter 103 into the domestic wiring network consisting of a ring mains 104, several spurs 105, and a consumer unit 106. Power from an external electricity supply system 108 is provided to the domestic wiring network via an electricity energy consumption meter 107 and a wire 115. In normal use, when the domestic alternative power generation system 101 is not generating any power, the electric energy consumption meter 107 records the accumulated amount of energy (usually measured in KWh, or units) dissipated in the various domestic electric appliances connected to the domestic wiring network. When the domestic alternative power generation system 101 is generating power, the power taken from the external electricity supply system 108 is reduced. If the power generated is more than that required by all of the domestic appliances connected to the domestic wiring network, the excess power is passed to the external electricity supply system and the power flow through the meter 107 and the wire 115 is reversed. However, standard domestic digital meters do not usually record reverse power flow, so the reading remains static in this case. Some electricity suppliers install a two-way meter which does record the accumulated energy flowing in both directions. Others make the coarse assumption that a constant fraction, say one half, of the total energy generated by the alternative system, as measured by the meter 103, is supplied to the external electric power supply system.

An electric water (immersion) heater 110 is also connected to the domestic wiring network via a thyristor power throttle 109, which is controlled by a control voltage on line 111. The control voltage varies smoothly between a lower limit, which may be 0 V, and an upper limit, which may be 5 V. The system is set up so that the power throttle operates between 0 % and 100 % as the control voltage varies between its limits. In a common embodiment, 0 V may set the throttle at 0 %, so that no power is passed to the heater 110, and 5 V may set the throttle at 100 %, so that the heater operates at full power. There may also be an over-ride switch 118 to allow the immersion heater 110 to be connected directly to the house wiring. Controller unit 112 generates the control voltage on line 111 using measurements of the current flowing in wire 115 from current sensor unit 113 supplied via line 117, the voltage of the domestic wiring network supplied via line 116, and a power threshold, shown schematically in Figure 1 at 114. The heater 110 and power throttle 109 constitute the power-consuming device of the invention, whose power consumption is varied using the control signal (a voltage in this case) on line 111.

The controller unit 112 is shown in more detail in Figure 2. The current sensor unit 113 provides an analogue voltage, via line 117, which is proportional to the current flowing in wire 115 passing through it. This wire connects the consumer unit 106 to the power consumption meter 107, as shown in Figure 1. The analogue voltage is converted to a digital representation in the analogue-to-digital (ADC) converter 201, which is then made available to the digital microcontroller 203. Similarly, the voltage on wire 115 is connected via line 116 to an ADC unit 202 which converts it to a digital representation also made available to the microcontroller. The microcontroller has a program memory 204 containing a program to carry out the multiplication, averaging and other function of the controller unit 112, and also some dynamic memory 205 for the temporary storage of variables etc. A digital representation of the control signal is converted to an analogue control voltage by the digital-to-analogue (DAC) converter 206 and this appears on line 111.

The main steps in the program running on the microcontroller 203 are shown in Figure 3. The program runs in an endless loop from the start (step 301), reading the current (302) and voltage (303), taking the product to obtain the instantaneous power (step 304), and combining the product with previously accumulated values in step 305. This step, in effect, performs the function of a low-pass filter and it defines the bandwidth of the system. On the first pass, the accumulated values are zero, but gradually increase as more products are combined so that after a time the accumulated values represent the average net power flowing in the wire between the consumer unit 106 and the meter 107.

Step 306 causes the program to loop back to step 302 until it is time to output a new value of the control voltage. The predetermined threshold 114 is subtracted from the accumulated values representing the average net power flow (step 307) and the difference is compared with zero (step 308). If the difference is less than zero, the control voltage is set to zero (step 309) and is output on line 111 (step 311). If the difference is greater than zero, it is multiplied by the amplification factor in step 310 before being output in step 311. Both the filter bandwidth (step 305) and the amplification factor (step 310) need to be chosen so that the control system is stable but responds as quickly as possible to changes. The threshold shown schematically in Figures 1 and 2 at 114 may be a constant value within the program, or it may be an externally adjustable value which can be read by the microcontroller every time the program passes through step 307.

The above embodiment of the invention may be described as partly digital, since the function of the controller unit 11 2 is implemented in the digital microcontroller unit 203 running a computer program. It will be clear to one skilled in the art that the controller function can also be implemented using analogue devices for multiplication, averaging and the other functions of the controller unit 112. In this case, there is no need for the ADCs 201 and 202, or for DAC 206, since the analogue circuits in the controller act on analogue inputs (i.e. voltages which vary continuously between limits) and provide an analogue output directly on line 111.

Claims

Claims 1. A method of using excess electric power generated by a domestic alternative power generation system connected to a domestic electricity wiring network which has connected to it a power-consuming device whose power consumption may be varied using a control signal, the wiring network also being connected to an external electric power supply system, the method comprising the steps of (a) measuring the net power flowing between the domestic wiring network and the external electric power supply system; (b) determining the direction of flow of the net power between the domestic wiring network and the external electric power supply system; and (c) creating a control signal to adjust the power consumed by the power-consuming device; characterised in that (i) when said determined direction of flow of net power is from the domestic wiring network to the external electric power supply system and the measured net power flow is greater than a pre-determined threshold, the control signal is adjusted to alter the power consumed by the power-consuming device so that the net power flowing between the domestic wiring network and the external electric power supply system approaches the threshold; (ii) the control signal varies in a continuous fashion in response to changes in the power generated by the domestic alternative power generation system, or to changes in the power consumed by the domestic wiring network; and (iii) when the direction of net power flow is from the external electric power supply system to the domestic wiring network or when the net power flowing from the wiring network to the external electric power supply system is below said threshold, the control signal is adjusted to reduce to zero the power consumed by the power-consuming device.
2. Control apparatus for enabling the use of excess electric power generated by a domestic alternative power generation system which is connected to a domestic electricity wiring network which has a power-consuming device whose power consumption may be varied using a control signal, the wiring network also being connected to a external electric power supply system, the apparatus comprising means for measuring in use the net power flowing between the domestic wiring network and the external electric power supply system; means for determining in use the direction of flow of the net power between the domestic wiring network and the external electric power supply system; means for creating in use a control signal to adjust the power consumed by the power-consuming device; means for adjusting in use the control signal so that the net power flowing between the domestic wiring network and the external electric power supply system approaches a pre-determined threshold value when said determined direction of flow of net power is from the domestic wiring network to the external electric power supply system, and the measured net power flow is greater than the threshold; means for varying in use the control signal in response to changes in the power generated by the domestic alternative power generation system, or to changes in the power consumed by the domestic wiring network; and means for adjusting in use the control signal so as to reduce to zero the power consumed by the power-consuming device when the direction of net power flow is from the external electric power supply system to the domestic wiring network or when the net power flowing from the wiring network to the external electric power supply system is below said threshold.
3. Apparatus according to claim 2, in which the means for creating the control signal is implemented using a digital microcontroller executing a program of instructions.
4. Apparatus according to claim 2, in which the means to create the control signal is implemented using analogue devices acting on continuously-variable voltages.
5. Apparatus according to any of claims 2 to 4, in which the control signal is a variable voltage or current.
6. Apparatus according to any of claims 2 to 4, in which the control signal is a light signal.
7. A domestic electricity wiring network including a power-consuming device whose power consumption may be varied using a control signal, the wiring network also being connected to an external electric power supply system, and control apparatus according to any of claims 2 to 6.
8. A domestic electricity wiring network according to claim 7, further including a by-pass switch to enable a user to supply power to the power-consuming device regardless of the state of operation of said control apparatus.