GB2508479A

GB2508479A - Transferring Excess Renewable Power to an Energy Storage

Info

Publication number: GB2508479A
Application number: GB1317044.4A
Authority: GB
Inventors: Richard Smith; Stephen Hook
Original assignee: Individual
Current assignee: Individual
Priority date: 2012-09-25
Filing date: 2013-09-25
Publication date: 2014-06-04
Anticipated expiration: 2033-09-25
Also published as: GB2508479B; GB201317044D0; GB201217121D0

Abstract

A system and method for transferring excess renewable power generated in a domestic setting to a secondary load so as to maximize the use of renewable power. Current sensors 11, 12 are used to monitor the amount of power supplied to primary loads, such as appliances, from the renewable source 14 and the maximum instantaneous power generated by the renewable source 14. These values are compared and if the instantaneous power is greater than the demand in the primary loads, excess renewable power is supplied to a secondary load 28, which may be a water heater or battery. A control unit 18 achieves this by operating drive circuitry 19 comprising a pulse width modulator with variable duty cycle as a function of the result of the power comparison; a larger amount of excess power results in a higher duty cycle. Filtering circuitry 15, 29 is provided to reduce electrical noise and absorb harmonic distortions. AC or DC renewable power may be used as the supply.

Description

TRANSFERRING RENEWABLE POWER

The present invention relates to a method and a system for the maximum utilisation of electrical power which can be generated from renewable energy sources in a domestic setting. More specifically, the invention relates to the transfer of excess power (as defined below), generated by a renewable energy source, to a secondary load.

With ever growing concerns of climate change there is an increasing need for sustainable energy sources. It is well known that renewable energy sources, such as solar or wind, can be used to generate power and the use of renewable energy in a domestic setting, to power a household, is becoming increasingly popular.

For maximum benefit, the power demand should be equal to the maximum power generated by the renewable energy source. In reality, this balance is rarely achieved. The difficulty is caused by the continuously varying output from the renewable energy source dependent upon environmental factors and the continuously varying demand, as household appliances and devices are switched on or off. In some cases the renewable energy source is capable of providing more power than required, resulting in an under-utilised renewable energy source. The term excess power" as used herein means the difference between the power being utilised by a primary load (such as a domestic power demand) and the maximum instantaneous power generated by the renewable energy source. Similarly, the maximum instantaneous power generated by the renewable energy source means power which would otherwise be exported to the mains supply, if the set up were configured to do so.

Typically, any renewable power not utilised by the household (excess power) may be exported to the electrical grid in an attempt to utilise the renewable energy source more effectively. In some countries, including the UK, owners of homes with a renewable energy source installed can get paid both for generating electricity and for exporting generated electricity to the electrical grid.

The payment for generated power exported to the electrical grid is significantly less than the cost of power imported from the grid. As a consequence of this relatively low return for power exported to the grid, it would be highly beneficial if all of the power that can be generated by the renewable source could be utilised within the home. Reducing the amount of power that needs to be imported or "drawn" from the grid not only reduces the owner's fuel bills but also their carbon footprint.

One way of utilising the renewable energy source to its full extent is to divert any excess power to an energy store, for example a thermal store comprising an immersion heater in a hot water tank, a storage heater, or a chemical store, such as a battery.

The concept of transferring generated power to energy stores is known, and devices exist for diverting the generated power. A problem with many existing devices is that they do not provide a continuously variable power output and/or are incapable of complying with the appropriate standards relating to harmonic distortion, without the use of expensive and dedicated filtering.

An aim of the present invention is to attempt to address at least some of the problems discussed above by providing a method and system for transferring excess power generated by a renewable energy source to a secondary load (such as an energy store), and which provides a substantially continuously variable output to enable the excess power to be utilised effectively by the secondary load.

According to the present invention there is provided a method of transferring to a secondary load excess power (as defined herein) generated by a renewable energy source, the method comprising the steps of: -monitoring power being supplied to a primary load, generated by the renewable energy source; -monitoring the maximum instantaneous power being generated by the renewable energy source; -comparing the power being supplied to the primary load with said maximum instantaneous power; and -operating a pulse width modulator with a variable duty cycle as a function of the result of the comparison to control the flow of excess power to the secondary load.

According to a second but closely related embodiment there is provided a system for the transfer to a secondary load of excess power (as defined herein), generated by a renewable energy source, the system comprising: -sensing means to monitor power being supplied to a primary load, generated by the renewable energy source; -sensing means to monitor the maximum instantaneous power being generated by the renewable energy source; -a pulse width modulator to control the flow of excess power to the secondary load; and -control means arranged to compare the power being supplied to the primary load with said maximum instantaneous power and configured to operate the pulse width modulator with a variable duty cycle as a function of the result of the comparison to control said flow of excess power to the secondary load.

It will be appreciated that the term "power" is used herein in a general sense to refer to the current which is supplied by the renewable energy source either to the primary load or to the secondary load.

The method and system of the present invention produce a substantially continuously variable power output to the secondary load. That variable output amounts to the excess power generated by the renewable energy source, thus utilising the renewable energy source to the maximum potential. The system and method of use operate instantaneously or near instantaneously, so far as is possible, so that the power transferred to the secondary load is maximised and relates to the "real-time" excess power (as defined herein). In practice, the system may be calibrated so that a small portion of the excess power is directed to the actual components of the system in order to provide sufficient power for operation thereof. That portion will however be minimal compared to the total power. The system may also be calibrated to provide a buffer such that the excess power transferred to the secondary load is slightly less than the maximum available to take into account a sudden decrease in renewable power generated or a sudden increase in the power demands of the household.

When the power demand is greater than the maximum instantaneous power which can be generated by the renewable energy source, there is no excess power and the system will operate in standby mode. In standby mode all of the power generated by the renewable energy source is utilised by the primary load to meet the demand.

Though not forming part of the invention per se, the primary load will in the majority of applications be the electrical requirements of a household, utilising the energy from a renewable energy source -though, of course, the invention need not necessarily be utilised in such a domestic setting.

Preferably, the secondary load is a resistive load. In a preferred arrangement, the secondary load is a thermal store, such as a hot water tank heated by an immersion heater to which the excess power is supplied. Such an arrangement is beneficial to the owner and also to the environment.

Preferably, the control means comprises a microcontroller. The values of monitored power may be supplied to the microcontroller and the microcontroller may then utilise said values to control the pulse width modulator. The sensing means to monitor power may comprise one or more current transformer (often referred to as CT's or clamps) preferably in communication with the microcontroller either directly or indirectly. The values may be supplied to the microcontroller wirelessly.

The supply of excess power to the secondary load may be achieved by electronic control switches (which may comprise one or more components, such as one or more transistors) controlled directly or indirectly by the pulse width modulator. The ratio between switch on time and switch off time (the "duty cycle") is dependent on the comparison between the power being supplied to the primary load and said maximum instantaneous power. The more excess power available the higher the duty cycle.

The excess power may originate from an AC renewable power source.

In this arrangement, the switches may be transistors, and in particular MOSFETs configured in a "buck-boost" arrangement, as typically found in switch mode power supplies or in a push-pull arrangement.

A DC control arrangement may be more stable and reliable and so may be beneficial. In this case, the excess power may be converted to DC before modulation thereof by the pulse width modulator. This may be achieved by providing an AC to DC converter in the system between the power input to the system and the pulse width modulator. The converter may be a bridge rectifier.

Preferably, the system comprises a low pass filter between the switches and the secondary load such that the excess power can be filtered to obtain a smoothed power output to the secondary load. This serves to reduce electrical noise and may be achieved by an LC circuit or other means known in the art.

Particularly in the case of excess power converted to DC, the switching and filtering may serve also to absorb harmonic distortions in the excess power output to the secondary load. This can be beneficial in minimising the harmonic distortions (commonly referred to as "power factor").

The current at the output to the secondary load is preferably monitored.

The monitored current may be compared to a pre-set threshold and, when the threshold is reached, supply of power to the secondary load inhibited. This may be achieved by the use of logic gates configured to operate automatically. Such an arrangement provides a fast response safety mechanism in the event of a fault in the secondary load. Alternatively or additionally, the value of current monitored may be supplied to the microcontroller and the microcontroller is configured to shut down the system if the threshold is reached.

Preferably, thermal overload protection is provided. This may comprise a temperature sensor arranged to monitor the temperature of components in the system and to transfer the temperature values to the microcontroller. In this case, the microcontroller may be configured to inhibit operation of the system if the temperature reaches a pre-set value.

The system may also include display means for displaying information regarding operation of the system. The display means may include a manual override button to switch the pulse width modulator to full duty cycle for a pre-set time to allow the secondary load to be operated even when there is no excess power or when there is no renewable power available by drawing the power from the mains supply.

The system is preferably installed for use with a renewable energy source being used in conjunction with a mains electrical supply source. As is standard practice, a RFI filter may be provided at the input to the system to prevent noise passing back to the mains source.

By way of example only, one specific embodiment of system of this invention will now be described in detail, reference being made to the accompanying drawings in which:-Figure 1 is a block diagram of the system of the present invention; Figure 2 is a waveform showing the duty cycle modulated signal; Figures 3a, 4a, 5a and 6a are pulse width modulation signals applied to the power supplying the system; Figure 3b shows the modulated signal of Figure 3a applied to an AC excess power waveform at a duty cycle of 25%; Figure 4b shows the modulated signal of Figure 4a applied to an AC excess power waveform at a duty cycle of 75%; Figure 5b shows the modulated signal of Figure 5a applied to an DC excess power waveform at a duty cycle of 25%; Figure 6b shows the modulated signal of Figure 6a applied to an DC excess power waveform at a duty cycle of 75%; and Figures 3c, 4c, 5c and 6c show the excess power waveform after modulation and filtering.

Throughout this description the term "excess power" shall be taken to mean the difference between the power being utilised by the primary load and the maximum instantaneous power generated by the renewable energy source.

Similarly, the word "power" is used in a general sense to refer to the current, which is supplied by the renewable energy source either to the primary load or to the secondary load.

Referring to Figure 1, there is shown a block diagram showing the main features of the system 10. The system 10 includes two power sensors 11, 12 in the form of standard clip on' current transformers (CT's or clamps). One power sensor 11 is configured to monitor the maximum power that is generated by a renewable source (not shown) and the other power sensor 12 is arranged to monitor the power being utilised by a primary load (also not shown). The sensors 11, 12 are operatively connected to a sensor unit 13 serving as a communication interface.

The system 10 includes an electrical power input 14 for excess power drawn into the system 10. A FIF filter 15 is provided at the input 14 and is arranged to suppress electrical noise. A low voltage power supply 16 is configured to reduce a small portion of the excess power drawn into the system for operating components of the system, specifically in order to power control 18 and drive circuitry 19.

The control circuitry 18 comprises a microcontroller 20 programmed to control operation of the system 10. The microcontroller 20 receives power values from the sensor unit 13 and compares these values to determine the excess power available to the system 10. The values are received by the microcontroller either wirelessly 22 or by way of a physical connection 23.

Where the values are transmitted wirelessly to the microcontroller, the sensor unit 13 is configured to pass the values to a transmitter 24. The transmitter 24 transmits the values to a wireless link 25 which is connected to the microcontroller 20 of the control circuitry 18 and which is arranged to transfer the received values to the microcontroller 20. Alternatively a physical connection 26 may be provided between the sensor unit 13 and the microcontroller 20.

The drive circuitry 19 includes a pulse width modulator (not shown) and associated components. The microcontroller 20 operates the pulse width modulator with a variable duty cycle as a function of the excess power. The pulse width modulator is configured to operate switches (not shown) in accordance with the variable duty cycle to control the flow of excess power to a secondary load 28.

A low pass filter 29 is arranged towards the output 30 of the system 10 to filter the excess power to the secondary load 28 thus reducing electrical noise.

The system includes temperature sensors (not shown) to measure the temperature at various points in the system 10. The temperature values are transmitted to a safety unit 31, which is operatively connected to the microcontroller 20. Thermal overload protection 32 is provided to ensure that the microcontroller 20 inhibits operation of the system 10 if the temperature reaches a pre-set value. A current sensor (not shown) is provided at the output of the system 10 to measure the current to the secondary load 28. The current values are transmitted to the safety unit 31 and the microcontroller 20 is configured to compare the current with a pre-set threshold and when the threshold is reached the microcontroller 20 inhibits the supply of power to the secondary load 28.

A user configurable display 33 is also provided and this is wirelessly connected to the microcontroller 20. The microcontroller 20 is programmed to transmit information concerning the status and operation of the system 10 to the display 33. The display 33 also includes a manual override button 34 to switch the pulse width modulator (described in more detail below) to full duty cycle for a pre-set time to allow power to flow to the secondary load 28 regardless as to whether there is sufficient excess power or indeed renewable power.

The pulse width modulation drive circuitry 19 is controlled by the control circuitry 20 and may comprise more than one modulated signal applied to the excess power simultaneously.

Figure 2 shows two pulse width modulated signals representing the duty cycle. The waveform on the left 35 represents a high duty cycle and the waveform on the right 36 represents a low duty cycle. The duty cycle represents the ratio of the duration of switch on time (referred to as t) to the total period of the signal p. The lower the excess power available to the secondary load 28, the shorter the switch on period t. Thus, a low duty cycle has a shorter ON period t relative to the OFF period (referred to as x).

Figures 3a and 4a show two modulation duty cycle signals 37, 38 and Figures 3b and 4b show these signals as applied to an AC power waveform 33.

Figures 3a to 3c show the application of a low duty cycle of 25%. This is represented by areas 39 which correspond to a period of switch-on time.

Figures 4a to 4c show the application of a high duty cycle of 75%. This is represented by areas 40 which correspond to a period of switch-on time. In both cases, the output power waveform to the secondary load is shown in Figures 3c and 4c as a smooth sinusoidal wave which corresponds to the excess power available. The waveform 41 of Figure 4c represents a transfer of high excess power from the renewable source whereas the waveform 42 of Figure 3c represents a transfer of low excess power from the renewable source.

Figures 5a and 6a show two modulation duty cycle signals 45, 46 and Figures 5b and 6b show these signals as applied to a DC power waveform 47.

Figures 5a to 5c show the application of a low duty cycle of 25%. This is represented by areas 48 which correspond to a period of switch-on time.

Figures 6a to 6c show the application of a high duty cycle of 75%. This is represented by areas 49 which correspond to a period of switch-on time. In both cases, the output power waveform to the secondary load is shown in Figures 5c and 6c as a smooth wave which corresponds to the excess power available.

The waveform 50 of Figure 6c represents a transfer of high excess power from the renewable source whereas the waveform 51 of Figure Sc represents a transfer of low excess power from the renewable source.

The output signals have been filtered to produce a smooth output to the secondary load 28. The filtering also serves to reduce phase shift and harmonic distortions.

Claims

CLAIMS1. A method of transferring to a secondary load excess power (as defined herein) generated by a renewable energy source, the method comprising the steps of: -monitoring power being supplied to a primary load, generated by the renewable energy source; -monitoring the maximum instantaneous power generated by the renewable energy source; -comparing the power being supplied to the primary load with said maximum instantaneous power; and -operating a pulse width modulator with a variable duty cycle as a function of the result of the comparison to control the flow of excess power to the secondary load.
2. A method as claimed in claim 1, wherein the pulse width modulator is operated to control switches to control the supply of excess power to the secondary load.
3. A method as claimed in claim 2, further comprising the step of filtering the excess power from the switches to obtain a smoothed power output to the secondary load.
4. A method as claimed in claim 3, wherein the excess power originates from an AC power source.
5. A method as claimed in claim 4, wherein the excess power is converted to DC before modulation thereof by the pulse width modulator.
6. A method as claimed in claim 5, wherein the switching and filtering serve also to absorb harmonic distortions in the excess power output to the secondary load.
7. A method as claimed in any of the preceding claims, further comprising the step of monitoring the current at the output to the secondary load.
8. A method as claimed in claim 7, wherein the current is compared to a pre-set threshold and when the threshold is reached supply of power to the secondary load is inhibited.
9. A method as claimed in any of the preceding claims wherein the values of monitored power are supplied to a microcontroller and the microcontroller utilises said values to control the pulse width modulator.
10. A method as claimed in claim 9, wherein the values are supplied wireless ly.
11. A method as claimed in any of the preceding claims, wherein the method includes providing thermal overload protection.
12. A system for the transfer, to a secondary load, of excess power (as defined herein), generated by a renewable energy source, the system comprising: -sensing means to monitor power being supplied to a primary load, generated by the renewable energy source; -sensing means to monitor the maximum instantaneous power generated by the renewable energy source; -a pulse width modulator to control the flow of excess power to the secondary load; and -control means arranged to compare the power being supplied to the primary load with said maximum instantaneous power and configured to operate the pulse width modulator with a variable duty cycle as a function of the result of the comparison to control said flow of excess power to the secondary load.
13. A system as claimed in claim 12, wherein the control means comprises a microcontroller.
14. A system as claimed in claim 13, wherein the sensing means to monitor power comprises one or more current transformer.
15. A system as claimed in claim 14, wherein the one or more current transformer is in communication with the microcontroller and is arranged to supply values of monitored power thereto.
16. A system as claimed in any of claims 12 to 15, further comprising electronic control switches controlled by the pulse width modulator to control the flow of excess power to the secondary load.
17. A system as claimed in claim 16, further comprising a low pass filter between the switches and the secondary load to filter the excess power from the switches to obtain a smoothed power output to the secondary load.
18. A system as claimed in claim 17, wherein the switches are MOSFETs configured in a push-pull arrangement.
19. A system as claimed in any of claims 12 to 18, wherein a converter is provided to convert excess power originating from an AC source into DC before modulation by the pulse width modulator.
20. A system as claimed in claim 19, wherein the converter is a bridge rectifier.
21. A system as claimed in claim 12 and substantially as hereinbefore described, with reference to and as illustrated in the accompanying drawings.