AU2013100349B4

AU2013100349B4 - Solar Energy Capture and Storage System with Revenue Recovery Through Energy Sales

Info

Publication number: AU2013100349B4
Application number: AU2013100349A
Authority: AU
Inventors: Barbara Louise Elliston
Original assignee: ELLISTON BARBARA; PV DIRECT Ltd
Current assignee: PV DIRECT Ltd
Priority date: 2012-03-23
Filing date: 2013-03-22
Publication date: 2016-02-11
Anticipated expiration: 2021-03-22
Also published as: AU2013100349A4; AU2016100264A4; AU2013100349C4; AU2016100264B4

Abstract

Abstract - 2013100349 A solar energy capture and storage system comprising: one or more photovoltaic modules; an inverter electrically coupled to the photovoltaic modules and operable to convert DC generated by the photovoltaic modules to AC, the inverter being further operable to output AC having a time-averaged voltage that is variable between minimum and maximum levels, wherein the inverter is at least operable independently from an electricity supply network; one or more heating elements electrically coupled directly or indirectly to the output of the inverter.

Description

1 SOLAR ENERGY CAPTURE AND STORAGE SYSTEM WITH REVENUE RECOVERY THROUGH ENERGY SALES Field of Invention 5 The present invention relates to a solar energy capture and storage system. More particularly, the invention relates to the use of photovoltaic modules that generate direct current (DC) and an inverter for subsequently generating alternating current (AC) to supply power to a heating element. The inverter operates in electrical isolation from a mains power grid. The heating 0 element heats a working medium which becomes the principal means of storing the solar energy captured. The system provides an energy source independent of existing energy reticulation infrastructure, but can also be used in conjunction with existing infrastructure to achieve total energy cost reductions. 5 Background to the Invention Solar energy is an important source of renewable, clean energy. Increasingly, solar energy systems are being used to harness the sun's energy for our every day needs. The focus of these systems has traditionally been for either direct use (heating domestic hot water, powering 0 machines), or storage in the form of electrical charge within battery systems. Conventional Use of Solar Energy for Heating Fluids It is known that conventional heating of fluids including water and glycol by solar radiation is 25 achieved in non-electrical, solar-thermal collectors, in which the fluid is circulated to the solar collector in order to collect the energy afforded by solar radiation. Thus the working fluid is heated at the solar thermal collector, and these conventional systems require connection of the fluid used as the heat collection medium to be plumbed to the solar thermal collector. 30 Furthermore, the usual purpose of these conventional solar thermal systems is the provision of hot water. From an installation and operational perspective, the shortcomings with these systems include the cost of plumbing, the requirement to meet regulations on allowable materials if the working 35 fluid is household potable water, the complexity of heat control including any pressurisation 2 considerations, the complexity of freeze control, the cost of pumps and the complexity of pump controls, the cost of piping and lagging to address heat loss between the point of collection and storage, the cost of expansion vessels where required, the need for valves and/or vents for the fluids being heated, the costs related to adding the weight of the assembly and working fluid 5 onto rooftops, and the cost of maintenance of equipment with moving parts such as pumps or valves. In addition, as solar radiation varies throughout a day and throughout a year, although systems can be implemented without storage, the usefulness of solar thermal energy is enhanced by 3 storage of the energy collected so that the utilisation of the stored energy is decoupled from the incident radiant energy. The design of a storage vessel or cylinder for a solar thermal collector takes into account particular considerations, including the key concept of solar thermal heating which is the utilisation of a temperature differential between the fluid requiring heating and the temperature of the solar thermal collector. The maintenance of a cooler body of fluid which can 5 be circulated to the collector in order to maximise energy efficiency results in a preference for a solar storage vessel with special characteristics when compared to conventional electrically heated hot water cylinders. Typically, the solar storage vessel must be capable of providing temperature stratification within the stored fluid. In the absence of this stratification, significant efficiency of heat collection may be foregone. The stratification is achieved by the appropriate 3 placement of the solar inlet and outlet ports, the speed of any pumps used to circulate the working fluid, the placement of backup electrical heating elements and thermostats, the selection of materials used for the walls of the storage vessel, and the orientation of the storage vessel. Stratification considerations impact on solar thermal collection systems towards a storage vessel design that is not satisfied by conventional electrically heated hot water 5 cylinders. From a cost of system perspective, a further shortcoming of conventional solar thermal collection systems is that a significant part of the cost of a properly designed system can be attributed to modifications to any existing storage vessel or complete swap out, in order to 0 provide a storage facility that is capable of providing and maintaining stratification. Furthermore it is important to appreciate the various conflicting technical efficiency factors and to tune the configuration of each installed system to best adapt to the unique physical constraints presented by each site. This level of technical expertise and know-how cannot easily 5 be provided by the typical tradesman required for installing these systems.

3 Solar heating systems using photovoltaics Therefore, there has previously been identified a need for a robust and durable solar energy 5 capture and storage system which is able to provide a similar hot water service to these conventional systems, but which is less complex, which would not require technical modifications to existing heating systems where present, and which would reduce installation costs and have fewer auxiliaries to maintain, thereby shortening payback times and improving overall system cost effectiveness. D An electrical solar energy capture and storage system utilising photovoltaic (PV) modules provides a method to capture and store solar energy independent of existing electrical infrastructure, while at the same time addressing a number of the difficulties and complexities of conventional solar thermal systems. 5 Such a system is also capable of being used for space heating as well, for example, to heat a solid energy storage medium such as bricks in "night store" heaters which have been traditionally heated using off-peak electricity. D An electrical solar energy capture and storage system that provides electricity in some form to the heating elements of conventional storage vessels or cylinders removes the need for any solar working fluid to be circulated to the solar collection device. This approach removes the cost of new plumbing, and at the same time removes the need for stratification of the working fluid in the storage vessel or cylinder. It also has no requirement for heat control or frost control .5 at the solar collector, or special plumbing or pressure control devices. In addition, the use of a photovoltaic source which does not connect to any power grid may facilitate extensive adoption of household photovoltaic systems unencumbered by stability issues associated with grid-tie photovoltaic systems, thereby lowering the cost and operational 30 complexity for both utilities and households. US 5,293,447 (Fanney) discloses an electrical solar heating system operating on photovoltaic arrays configured to adjust either the resistive load or the connection of cells within the photovoltaic array, in order to meet the objective of applying the maximum power output under 35 various irradiance and ambient temperatures to the solar heating system. Where the load 4 resistance is altered, switching circuitry connects the plurality of heating elements in various combinations in order to present a discrete but changeable resistive load to the photovoltaic array, in an attempt to ensure the array is operated at a current and voltage which would approximate the maximum power point for the conditions. US 5,293,447 discloses that the 5 output of the photovoltaic array is a monitored direct current electrical output and the power input to the resistive elements operates within the voltage and current curves of the incumbent photovoltaic array. One shortcoming with the system disclosed in US 5,293,447 is that the manufactured 0 parameters of the photovoltaic modules and the operational characteristic of the photovoltaic array limits the voltage and current able to be applied to the resistive heating elements of the system to that of the array's intrinsic current and voltage curves. However, a different operating voltage may be more desirable, including higher voltages such as, for example, 240V DC. 5 WO 2011/073938 (Ashkenazy) discloses an electrical solar heating system operating on photovoltaic cell arrays configured to supply power from a photovoltaic cell array at maximum power point to the electric heating element of a storage vessel in order to maximize power transfer efficiency from a photovoltaic cell array to the electric element. Load resistance of the heating element is not altered. The disclosed electrical solar heating system is proposed to D operate at the maximum power point of the photovoltaic cell array through the use of a maximum power point tracking circuit. One shortcoming is that the MPPT circuit concept presented in Fig 4 of WO 2011/073938 may not work effectively as intended. It presents the DC power from the photovoltaic cell array to the .5 input of a transformer (which only effectively transforms AC). In this configuration the transformer core may saturate and the system may not function. The systems of both Fanney & Ashkenazy provide DC electrical power to a resistive heating element. A key problem with providing power as DC is that available elements and associated 0 thermostats and Over Temperature Thermal Cut-out (OTTC) devices are designed to work with AC. DC is problematic as it is prone to arcing and burning of contacts as circuit contactors open. DC also causes accelerated electrolytic corrosion of metallic parts. Some solar energy systems are known that comprise inverters for converting DC into AC. 5 These inverters are classified either as grid-tie inverters or stand-alone inverters. Both need to 5 operate along with an energy storage device if they are to operate efficiently and exploit the maximum power from the photovoltaic modules. Grid-tied inverters typically use the mains grid power supply as the means for storage while stand-alone inverters use a DC battery and deliver AC at a fixed voltage (typically 230 Vac) at varying power. Both arrangements involve 5 complexity or the cost of additional componentry that it would be desirable to minimise. Object of the Invention It is an object of the invention to provide an improved solar energy capture and storage system, D an improved inverter for use with a solar energy capture and storage system and an improved method of controlling such an inverter. Alternatively, it is an object to address the disadvantages with existing solar thermal heating systems, such as discussed above. Alternatively, it is an object to address the disadvantages with existing inverters, such as discussed above. Alternatively, it is an object of the invention to at least provide the public with a useful choice. 5 Summary of the Invention According to a first aspect of the invention, there is provided a solar energy capture and storage system comprising: D one or more photovoltaic modules; an inverter electrically coupled to the photovoltaic modules and operable to convert DC generated by the photovoltaic modules to AC, the inverter being further operable to output AC having a time-averaged voltage that is variable between minimum and maximum levels, wherein the inverter is at least operable independently from an electricity supply network; 5 one or more heating elements electrically coupled directly or indirectly to the output of the inverter. It will be understood that a variable time-averaged voltage level of AC, as described herein, will refer to a measure of the overall voltage level of the AC signal, rather than the inherent regular 0 wave-like fluctuations of AC. For example, the variable AC voltage level may be measured by a time-averaged value such as root mean square or any other suitable measure. The amplitude of the AC voltage may also be varied, which will consequently vary the time-averaged value of the voltage.

6 Preferably, the outputted AC voltage level is variable to adjust the power outputted by the inverter to the heating elements, so that it substantially matches the maximum amount of power available from the photovoltaic modules. In preferred embodiments of the invention, varying the outputted AC voltage level from the inverter allows the maximum power to be applied to heating 5 elements with fixed resistances. Preferably, the inverter comprises: an inverter controller; and means for sensing the power received from the photovoltaic modules and for sending a D first signal indicative of the power received to the inverter controller, wherein the inverter controller is operable to vary the AC voltage level outputted by the inverter based on the first signal. More preferably, the inverter comprises: 5 means for sensing the voltage outputted by the inverter and for sending a second signal indicative of the voltage outputted to the inverter controller, wherein the inverter controller is operable to vary the AC voltage level outputted by the inverter based on the second signal. D More preferably, the inverter comprises: first switching means operable to receive control signals from the inverter controller that cause the first switching means to vary a rate of switching the received DC; and a transformer coupled to the first switching means, the transformer being arranged to generate AC from the switching output of the first switching means. 5 The inverter controller may comprise a maximum power point tracking (MPPT) means and be operable to control the inverter to output AC to maximise the power outputted by the inverter. More preferably, the MPPT means may comprise a MPPT circuit or processor. 0 Preferably, the minimum level of AC root mean square voltage outputted by the inverter is 0 V. Preferably, the maximum level of AC root mean square voltage outputted by the inverter is the nominal mains voltage, for example, 230 Vac. More preferably, where the heating elements have a higher voltage rating than the nominal mains voltage, the maximum level of AC root 7 mean square voltage outputted by the inverter is able to match the higher voltage rating of the heating elements. In preferred embodiments of the invention, the solar energy capture and storage system comprises a vessel in which are housed the heating elements, the vessel further containing a 5 working medium able to be heated by the heating elements. In some embodiments, the solar energy capture and storage system comprises an auxiliary electrical power source for supplying power to the heating elements. D Preferably, the solar energy capture and storage system comprises a second switching means for selectively electrically coupling the heating elements to the inverter and/or an auxiliary electrical power source. More preferably, the second switching means is operable to electrically isolate the inverter from the auxiliary electrical power source. 5 In embodiments of the invention, the second switching means is operable to select between the inverter and/or the auxiliary electrical power source based on one or more of the following parameters: time of day; power produced by the photovoltaic modules; irradiance of the photovoltaic modules; physical characteristics or arrangement of the photovoltaic modules; temperature of the working medium heated by the heating elements; and historical power 3 consumption data. For example, the solar energy system may comprise a timer operable to control the second switching means based on the time of day. The solar energy system may also comprise one or more temperature sensors to measure the temperature of the working medium. For example, 5 the system may comprise a thermostat to monitor the temperature of the working medium and control the second switching means accordingly. In one embodiment of the invention, the inverter is decoupled from any other electrical power supplied to the heating elements, e.g. the auxiliary electrical power source. For example, the o inverter may be decoupled from a mains power grid. Such an inverter may be referred to as an "islanded" or stand-alone inverter. In another embodiment of the invention, the solar energy capture and storage system comprises third switching means operable to selectively couple the inverter to the auxiliary electrical power 5 source and/or the heating elements to supply energy thereto.

8 Preferably, the inverter is selectively operable to output AC having a time-averaged voltage that is variable between minimum and maximum levels when the inverter supplies energy to the heating elements, and to operate in a grid tied mode when the inverter supplies energy to the 5 auxiliary electrical power source. It will be understood that an inverter operating in a grid tied mode operates such that the frequency of its AC output is synchronised to that of the grid to which it is connected, and that its output voltage is limited to a prescribed range around a nominal value for the particular electricity grid to which it is electrically coupled. D In another embodiment of the invention, the solar energy capture and storage system comprises a second inverter electrically coupled between the photovoltaic modules and the auxiliary electrical power source and a further switching means operable to selectively connect the second inverter to the auxiliary electrical power source to supply energy thereto. 5 In another embodiment, the heating elements comprise at least two heating elements with at least one of the heating elements able to be coupled to the inverter and at least one of the heating elements able to be coupled to the auxiliary electrical power source. Where the working medium is water, the system may comprise an auxiliary heating element D thermostat operable to cause the heating element coupled to the auxiliary electrical power source to cut out at a predetermined temperature of the working medium based on legionella bacteria or other control considerations. In some embodiments of the invention, the system comprises a first meter for measuring the .5 power delivered to the heating elements from the photovoltaic modules. More preferably, the first meter is an AC power meter. The system may additionally or alternatively comprise a second meter for measuring the power delivered to the heating elements from the auxiliary electrical power source. 0 In some embodiments of the invention, the system comprises a fluid meter for measuring the energy delivered from the system in the form of heated fluid. More preferably, the fluid meter is a hot water meter which measures the flow rate and temperature of the hot water flowing from the system. 35 9 In some embodiments, the power delivered by the solar energy capture and storage system may be ascertained by the difference over a particular time period between the energy supplied in the heated fluid, as measured by the fluid meter, and the energy supplied by the auxiliary power source, as measured by the second meter. 5 In one embodiment, the solar energy capture and storage system may comprise a heat exchanger housed inside the vessel and in thermal contact with the working medium, the heat exchanger containing a further medium able to be heated in the heat exchanger. D Preferably, the system comprises a system controller operable to control the switching means and/or the inverter. The system controller may be operable to control the switching means and/or the inverter to maximise photovoltaic energy supplied to the heating elements. In some embodiments of the invention, one or more of the inverter, switching means and 5 system controller may be housed in the same physical device. Preferably, the system controller is configured to control the switching means and/or inverter based on user-defined settings, including desired temperature of the working medium. More preferably, the system controller is configured to receive the user-defined settings through a D wireless interface. According to a second aspect of the invention, there is provided an inverter for use in a solar energy capture and storage system, the inverter comprising: means for electrically coupling the inverter to one or more photovoltaic modules to 5 receive DC generated by the photovoltaic modules; means for converting the received DC into AC; means for outputting AC at a time-averaged voltage that is variable between minimum and maximum levels; and means for coupling the output of the inverter directly or indirectly to one or more heating 0 elements, wherein the inverter is at least operable independently from an electricity supply network. Preferably, the outputted AC voltage is variable to maximise the power delivered to the heating 5 elements.

10 Preferably, the inverter comprises: a controller; and means for sensing the power received from the photovoltaic modules and for sending a 5 first signal indicative of the power received to the controller, wherein the controller is operable to vary the AC voltage level outputted by the inverter based on the first signal. More preferably, the inverter comprises: D means for sensing the voltage outputted by the inverter and for sending a second signal indicative of the voltage outputted to the controller, wherein the controller is operable to vary the AC voltage level outputted by the inverter based on the second signal. 5 More preferably, the inverter comprises: switching means operable to receive control signals from the controller that cause the switching means to vary a rate of switching the received DC; and a transformer coupled to the switching means, the transformer being arranged to generate AC from the switching output of the switching means. The controller may comprise a maximum power point tracking (MPPT) means and be operable to control the inverter to output AC to maximise the power outputted by the inverter. More preferably, the MPPT means may comprise a MPPT circuit or processor. 5 Preferably, the minimum level of AC root mean square voltage outputted by the inverter is 0 V. Preferably, the maximum level of AC root mean square voltage outputted by the inverter is the nominal mains voltage, for example, 230 Vac. More preferably, where the heating elements have a higher voltage rating than the nominal mains voltage, the maximum level of AC root 0 mean square voltage outputted by the inverter is able to match the higher voltage rating of the heating elements. According to a third aspect of the invention, there is provided a method of controlling an inverter in a solar energy capture and storage system, the method comprising: 11 receiving a first signal indicative of power received by the inverter from one or more photovoltaic modules; receiving a second signal indicative of an AC voltage outputted by the inverter to one or more heating elements; and 5 controlling the inverter to output AC at a time-averaged voltage that is variable between minimum and maximum levels based on the first and second signals, wherein the inverter is at least operable independently from an electricity supply network. Preferably, the method comprises controlling the inverter in order to maximise the power D delivered to the heating elements from the photovoltaic modules. Preferably, the method comprises: sending control signals to a switching means, wherein the control signals cause the switching means to vary a rate or duty cycle of switching DC received by the inverter from the 5 photovoltaic modules. In one embodiment, the method comprises executing a maximum power point tracking (MPPT) algorithm in a processor, the MPPT algorithm being used to control the inverter to maximise the power delivered to the heating elements. Preferably, the method comprises varying the AC root mean square voltage outputted between a minimum level of 0 V and/or a maximum level of the nominal mains voltage, for example 230 Vac. More preferably, where the heating elements have a higher voltage rating than the nominal mains voltage, the maximum level of AC root mean square voltage outputted by the inverter is 5 able to match the higher voltage rating of the heating elements. According to a fourth aspect of the invention, there is provided a method of charging for energy generation in a solar energy capture and storage system according to the first aspect of the invention, the method comprising receiving data indicative of energy generated by the solar 0 energy capture and storage system and calculating energy charging rates on the basis of said data. Further aspects of the invention, which should be considered in all its novel aspects, will become apparent to those skilled in the art upon reading of the following description which 5 provides at least one example of a practical application of the invention.

12 Brief Description of the Drawings One or more embodiments of the invention will be described below by way of example only, and 5 without intending to be limiting, with reference to the following drawings, in which: Figure 1 is a schematic view illustration of a solar energy capture and storage system according to an embodiment of the invention; o Figure 2 is a schematic block diagram of an inverter according to an embodiment of the invention; Figure 3 is a schematic illustration of part of a solar energy capture and storage system according to another embodiment of the invention; 5 Figure 4 is a schematic illustration of part of a solar energy capture and storage system according to a further embodiment of the invention; Figure 5 is a schematic view illustration of a solar energy capture and storage system 0 according to another embodiment of the invention; Figure 6 is a schematic view illustration of a solar energy capture and storage system according to another embodiment of the invention; 5 Figure 7 is a schematic view illustration of a solar energy capture and storage system according to yet another embodiment of the invention; and Figure 8 is a graph of current and power output plotted against voltage for an array of photovoltaic modules in a solar energy capture and storage system according to 0 an embodiment of the invention. 5 13 Detailed Description of Preferred Embodiments of the Invention An exemplary solar energy capture and storage system 5 Figure 1 is a schematic view illustration of a solar energy capture and storage system 10 according to an embodiment of the invention. Heating system 10 comprises one or more photovoltaic modules 11 which may be configured in an array as is known in the art and positioned appropriately to receive solar radiation. The position or orientation of the photovoltaic modules 11 may be able to be altered in some embodiments, in any known manner, to enable J the modules to be orientated at any given moment to maximise the level of irradiance and hence the power able to be generated. In this document, the term photovoltaic (PV) "modules" is used to refer to any component or group of components able to produce electrical energy from solar energy. Many conventional 5 PV modules have a cellular structure and are therefore known as PV cells. PV modules may be arranged in an array and have a panel-like form. However, the invention is not limited to any type, structure or arrangement of photovoltaic devices. The photovoltaic modules 11 use incident solar radiation to generate electrical energy in the D form of direct current (DC). The direct current is supplied to an inverter 12 by means of an electrical coupling between the photovoltaic modules 11 and inverter 12. Inverter 12 functions to convert the DC into alternating current (AC). The AC outputted by inverter 12 has a voltage level that is able to be varied between a minimum and maximum level. 5 That is, the overall time-averaged voltage level, such as the root mean square voltage of the AC signal, can be varied between the minimum and maximum levels, as opposed to the mere inherent variability of an AC signal. The inverter is operable to enable unconstrained variation in output voltage over the full minimum and maximum voltage range. 0 One example of such an inverter is described below in relation to Figure 2. Referring again to Figure 1, the inverter 12 is electrically coupled to one or more heating elements 13, which receive electrical power in the form of AC and produce heat. Any appropriate form of heating element that converts electrical energy into thermal energy may be 5 used.

14 In the embodiment shown in Figure 1, the heating elements 13 are used to heat a working medium stored in a vessel 14. The heating elements 13 may be housed in the vessel 14 and are in thermal contact with the working medium. Vessel 14 may be, for example, a conventional 5 hot water cylinder in which water acts as the working medium, storing the energy captured from the sun in the form of heat. The water may be able to be ducted into a plumbing system, for example to be used as hot water or in a water circulation heating system. An exemplary inverter D Figure 2 is a schematic block diagram of an inverter 20 according to an embodiment of the invention. Inverter 20 is an exemplary inverter that is suitable for use in a solar energy capture and storage system such as described with reference to Figure 1. Inverter 20 generates an AC output that has a voltage that can be varied to maximise the power output to the load R to which 5 the inverter is connected. Load R represents the impedance (in this case resistance) of the heating elements of the solar water heating system or solar energy storage system utilising a working fluid other than water. Inverter 20 receives DC from one or more photovoltaic modules at an input that is electrically D coupled to the photovoltaic modules. A power sensing means 21 senses the power received from the photovoltaic modules, for example by sensing the input voltage and current and calculating the power as the product of these values. Power sensing means 21 sends a signal indicative of the calculated power to a controller 27, described further below. The signal may be sent in any appropriate form, including wired and wireless signals. In an alternative 5 embodiment, the power sensing means 21 sends separate signals indicative of the input current and voltage (which collectively are indicative of the power received from the photovoltaic modules) and the controller 27 uses these signals to calculate the power received. Any signal indicative of the power received from the modules may be sent to the controller 27. o An input filter 22 acts on the received DC to smooth the fluctuating power requirements of the inverter that result from the varied AC output signal, thus ensuring a smooth and continuous DC consumption from the photovoltaic modules. In one embodiment, input filter 22 comprises a conventional L-C (inductor capacitor) filter. In other embodiments, input filter 22 comprises a DC-DC boost converter thereby allowing the use of a smaller smoothing capacitor. 5 15 Inverter 20 further comprises a switching means 23 for switching the received DC in the inverter circuitry. The switching means may be a high frequency switching device comprising sold state switching units. These may comprise MOSFETs, bi-polar transistors and/or IGBTs, for example, as is known in the art. Switching means 23 generates a high frequency (for example 20 kHz to 5 100 kHz) AC signal to drive a primary coil of a transformer 24. Switching means 23 may receive control signals from the controller 27 to vary the rate of switching, as will be described below. Transformer 24 comprises a primary coil carrying the high frequency switched current from the switching means 23 and a secondary coil in which AC is generated as a result of the switched D current in the primary coil. The high frequency transformer 24 provides electrical isolation which allows the photovoltaic modules to be earthed to meet the requirements of some types of photovoltaic modules for which earthing is desirable (e.g. amorphous panels). A high frequency may be used to allow a smaller and cheaper transformer to be used compared with a 50 Hz transformer that requires a large inductance iron core design. The secondary coil of the 5 transformer 24 provides 50 Hz AC electrical power, however it also contains the high frequency switching signal. An output filter 25 removes the high frequency signal from the output of transformer 24 and allows a 50 Hz signal to pass. In one embodiment, a conventional 230 V AC line filter may be D used, for example. A voltage sensing means 26 is coupled to output filter 25 to sense the AC voltage outputted by inverter 20 to load R. Sensing means 26 may detect any aspect of the AC voltage, including its waveform and/or parameters such as time-averaged voltage level (e.g. root mean square 5 voltage) or amplitude. Sensing means 26 sends a signal indicative of the voltage outputted by inverter 20 to controller 27, in any appropriate known manner. Controller 27 receives the signal(s) indicative of the power received by the inverter from the photovoltaic modules from the sensing means 21 and the signal from the sensing means 26 0 indicative of the AC voltage outputted by the inverter. Using these signals the controller 27 controls the switching means 23 in order to vary the AC output voltage of the inverter 20 to the desired level to maximise the power delivered to load R, i.e. the heating elements in the solar energy capture and storage system.

16 In some embodiments, the controller 27 operates by only receiving the signal(s) indicative of power received from sensing means 21, and does not necessarily need to receive any signal indicative of the output voltage from sensing means 26. Controller 27 can adjust the high frequency switching performed by switching means 23 to achieve the desired power 5 consumption from the photovoltaic modules. The output voltage would then be a function of this power level P and the load resistance R (explicitly output voltage = (PR)). However, in such embodiments, if an open circuit occurred across the output of the inverter, the output voltage would be dangerously high. Therefore, in embodiments in which the output voltage is sensed and fed back to the controller, the controller is able to limit or reduce to zero the output voltage o where necessary. In one embodiment, the controller comprises a means for performing maximum power point tracking (MPPT) on the received power signal from sensing means 21 to determine the optimum voltage and current at which to operate the photovoltaic modules. The MPPT performing means 5 may comprise a circuit for performing MPPT or a processor configured to execute a MPPT algorithm. The controller 27 may send control signals to the switching means 23 to adjust its switching rate using pulse width modulation to vary the AC voltage outputted by the inverter 20 and preferably 0 to ensure the inverter functions at the maximum power point which would allow the maximum amount of power available from the photovoltaic modules to pass through. The controller 27 may use the signal received from the sensing means 26 indicative of the outputted voltage to adjust the switching rate of the switching means 23 accordingly. The AC output voltage is a consequence of the maximum power received from the photovoltaic modules and varies .5 dynamically as the irradiance on the photovoltaic modules varies. In some embodiments, the controller ensures that the inverter output voltage never exceeds a maximum level, for example 230V ac, even in fault conditions such as an open circuit at the output of the inverter 20. The feedback signal from the sensing means 26 monitors this, allowing 30 the controller to regulate the output voltage accordingly. This ensures the output voltage never exceeds the voltage rating of the heating elements and/or the standard mains voltage for which conventional heating elements are suitable for use. It is noted that the standard mains voltage is nominated by local jurisdictions and may consequently be different from 230V ac.

17 The controller 27 may comprise a switch mode power supply (SMPS) control circuit or may comprise a purpose programmed microcontroller circuit, which may also perform other system management, user control and parameter setting functions. 5 Above there has been described one example of an inverter that is operable to vary the AC voltage output supplied to the heating elements in a solar energy capture and storage system. Other forms of inverter may be apparent to one of skill in the art upon reading the description herein that also operate in the necessary way to provide the invention with its inherent advantages. When installing a solar energy capture and storage system according to the invention, the number of photovoltaic modules may be selected to match the size of the resistive load, i.e. the impedance of the heating elements, to which power will be supplied in use. For example, the maximum power able to be generated by the photovoltaic modules (during conditions of optimal 5 irradiance) should match or closely match (within an allowable degree of tolerance) the power rating of the resistive heating elements. This enables a maximum power available from the photovoltaic modules at any given time to be delivered to the heating elements through the inverter because, if the resistive load of the heating elements is too high, the output voltage may exceed the maximum allowed voltage (e.g. 230V ac), meaning the controller will limit the D voltage output and efficiency will be lost. As discussed above, the output of the inverter 12 is varied by the controller between minimum and maximum voltage levels. This allows the varying output current and voltage characteristics of the photovoltaic modules to match the impedance of the heating elements so that maximum 5 power is transferred to the heating elements, thereby increasing the efficiency of the system. Ideally, the load impedance R is matched to the ratio of the voltage of the photovoltaic modules to the output current of the photovoltaic modules at the maximum power point (VMppflMpp). The inverter effectively transforms the load impedance "seen" by the photovoltaic modules so that it matches this ratio VMpp/lMpp. 0 In a typical embodiment, the inverter is configured to output AC at a root mean square voltage variable between 0 V and 230 V, although other minimum and maximum levels may be used depending on the requirements of each storage system installation.

18 Figure 8 is a graph of current and power output plotted against voltage for an array of photovoltaic modules in a solar energy capture and storage system according to an embodiment of the invention. Four different sets of current and power curves have been plotted, which indicate some different irradiance levels of the modules (250, 500, 750 and 1000 Wm- 2 ). 5 For a given irradiance level, there is a particular DC voltage value that provides maximum power output. As the irradiance of the modules changes, the voltage output of the inverter is dynamically varied, for example through the operation of the inverter controller described with reference to Figure 2, to maximise the power output from the photovoltaic modules, or at least to provide a power output within a predetermined tolerated range of the maximum 0 instantaneous power available. The table below shows the relationship between maximum power available from the photovoltaic modules, inverter output voltage and inverter output current for a typical heating element in a hot water cylinder having a power rating of 2kW and a fixed resistance of 26.45 0 5 for a 230 V nominal supply voltage: Maximum power (W) Inverter output voltage (V) Inverter output current (A) 2000 230 8.7 1410 193 7.3 860 151 5.7 360 98 3.7 The maximum power values correspond to the maximum powers for the irradiance levels illustrated in Figure 8. As the maximum power varies, the AC output voltage from the inverter 0 also varies, in order to vary the output current from the inverter. In the example shown in the table, the inverter restricts the maximum output voltage to 230 V AC. Further features of an exemplary solar energy capture and storage system 25 Referring again to Figure 1, solar energy capture and storage system 10 may comprise a thermostat 19 adapted to measure the temperature of the working medium in vessel 14 and to disconnect the heating elements 13 from the power supply when a predetermined temperature is reached, as is known in the art.

19 In the embodiment of the invention illustrated in Figure 1, the solar energy capture and storage system 10 also comprises a revenue meter 15 electrically coupled between the inverter 12 and the heating elements 13. The revenue meter 15 may be any device able to measure the amount of AC electrical power delivered to the heating elements 13, such as conventional meters known 5 in the art. Existing solar heating systems using photovoltaic modules that generate and supply electrical power in the form of DC to heating elements lack the ability to meter the energy produced and supplied. Certified DC meters are not readily available and this poses a barrier to the D widespread adoption of photovoltaic modules for energy generation. The use of inverter 13 to generate a variable AC power output enables solar heating system 10 to use conventional AC power meters already certified by relevant authorities for revenue collection. This use of AC metering provides a commercial benefit, as it enables a business model based 5 on leasing, or consumption based revenue collection, which makes these systems more accessible to a broader range of consumers. More specifically, the initial investment required to procure and install the equipment for solar power generation is often beyond the means of a domestic consumer, even if the payback 0 period is reasonable (5 to 7 yrs). In contrast, a consumption-based leasing model, where a commercial organisation or utility company bears the equipment and installation cost, provides a much more attractive option for the domestic user. Furthermore, existing metered heating systems are based on grid-tied configurations which, 5 because of their connection to the mains power grid, are subject to a broad range of legislative and regulatory requirements. These requirements increase costs and moreover provide grid power utility companies with high levels of control over the technical installation requirements and its associated revenue model. 30 As a result, in some embodiments of the invention, the energy generated by the solar energy capture and storage system, or the energy consumed by its use, is measured by appropriate meters and data indicative of the energy generated or consumed is received by an appropriate body, such as a utility company, which calculates energy charges based on the received data. The user of the system is therefore charged on a consumption basis as discussed. 35 20 The solar energy capture and storage system described herein generates useful power from solar radiation independently from the grid (a so-called "islanded" or stand-alone arrangement such as inverter 12 in the embodiment shown in Figure 1, which is electrically decoupled or isolated from other electrical power supplies, such as the mains grid) and meters power 5 consumption, thereby supporting a leasing model (with consumption based revenue collection). This enables open business competition, ultimately leading to more cost effective and efficient energy systems. In some embodiments, the solar energy capture and storage system 10 may comprise a source D of auxiliary electrical power 16 connected to the heating elements 13. For example, the auxiliary power source 16 may be the mains grid or a back-up generator. Solar energy capture and storage system 10 may also comprise a further revenue meter 111 to measure the power delivered to the heating elements 13 from the auxiliary power supply 16, 5 thus enabling a utility company or the like to charge based on consumption of the part of the energy used by heating elements 13 that is supplied by the utility company. The auxiliary power source 16 may be connected to the heating elements 13 via a switching means, for example in the form of a switch 17. Switch 17 is operable to selectively couple one D or both power sources to the heating elements 13, i.e. power may be provided by just the photovoltaic modules 11, just the auxiliary power source 16, or both (if two elements are installed). In other embodiments, further power sources may be provided and such a system may comprise a switch able to connect any number of the power sources in any combination. The switch 17 ensures that the photovoltaic modules 11 and inverter 12 are decoupled from the 5 auxiliary power supply. Switch 17 may be operable to select between the solar or auxiliary power sources by manual or automated means. Any number of factors may be used to determine which connection state switch 17 is in, dependent on the power needs and abilities of the power sources to meet those 0 needs at particular times. The following parameters are examples of factors used to determine which power source switch 17 connects to provide power to the heating elements 13: * Time of day - photovoltaic modules 11 are active under irradiance conditions which occur largely during daylight hours. During these times, the auxiliary power source 16 5 may be disconnected to any heating elements 13 and the photovoltaic modules 11 are 21 electrically connected to the elements. In some embodiments, the solar energy capture and storage system comprises a timer 18 operable to control the switch 17 based on the time of day. * Power produced by the photovoltaic modules 11 - the amount of power produced by the 5 modules may be used to determine the switching state of the switch 17. If the photovoltaic modules 11 are able to produce sufficient power to fulfil the needs to the heating elements then no auxiliary power supply is required. However, if there is a shortfall in supply then the auxiliary power supply can be connected to augment the photovoltaic modules. Suitable means for measuring the power outputted by the D photovoltaic modules may be provided and operably connected to the switch 17. * Irradiance of photovoltaic modules 11 - the amount of sunlight falling on the photovoltaic modules may vary based on time of day, time of year and atmospheric conditions (e.g. cloud cover). The amount of electrical energy produced by the photovoltaic modules will vary accordingly. While the photovoltaic modules can be arranged to maximise power 5 production, for example, using apparatus to alter the orientation of the modules to face the sun, and the inverter ensures maximum power is delivered to the heating elements, insufficient power may still be available. The switch 17 may operate based on the detected power being generated by the photovoltaic modules and a switch control mechanism may be provided accordingly. D 0 Temperature of the working medium in the storage vessel - if the temperature of the water in storage vessel 14 falls below a desired level, and the photovoltaic modules are unable to supply enough power to increase its temperature, then the auxiliary power source may be connected. Thermostat 19 may be operably connected to switch 17 and in thermal contact with the working medium to monitor its temperature and operate 5 switch 17 accordingly. * Historical daily power consumption data - trends of energy consumption may be recorded and analysed by a processing apparatus to predict energy consumption patterns. In such embodiments, the processing apparatus also controls switch 17 to ensure sufficient energy is provided according to consumption predictions. 0 Alternative embodiments of the invention In the embodiment of the invention shown in Figure 1, the inverter 12 is used to deliver a variable output voltage and switch 17 is used to selectively disconnect the inverter 12 and 5 connect the heating element 13 to the auxiliary power source 16. This occurs when a 'boost' is 22 required to supplement low levels of power available from the photovoltaic array 11. While in 'boost' mode the inverter 12 does not deliver any power to the heating element 13 and any small amounts of power available from the photovoltaic array 11 will not be captured. 5 Figure 5 is a schematic view illustration of a solar energy capture and storage system 50 according to another embodiment of the invention. Photovoltaic modules in a photovoltaic array 51 use incident solar radiation to generate electrical energy as DC, which is supplied to a variable output voltage (VOV) inverter 52, which may be similar to that described with reference to Figure 2. The VOV inverter 52 is able to operate in a variable output voltage mode as well as 0 a conventional grid tied (GT) mode, in which the frequency of its AC output is synchronised to that of the grid to which it is connected, and that its output voltage is limited to a prescribed range around a nominal value for the particular electricity grid to which it is electrically coupled. The output of the inverter 52 is connected to two switch units or contactors 57 and 58. Contactor 5 57 is operable to connect the inverter 52 to an auxiliary power source, for example a utility power grid 55, and contactor 58 is operable to connect the inverter 52 to the heating element 513. In the embodiment shown in Figure 5, the inverter 52 is connected between the two contactors 57 and 58, with the contactors being connected in series between the utility power grid 55 and the heating element 513. 0 A system controller 54 operates to control the contactors 57 and 58, as well as control the mode of operation of the inverter 52 (either VOV or GT). The controller 54 also receives input signals from temperature sensors 511 located on the working medium storage vessel 512. The controller 54 may be operable to control the mode of operation of the inverter 52 dependent on .5 tank temperature, power generated from the photovoltaic array 51 and/or user requirements for hot water usage in order to maximise the overall capture of photovoltaic energy while ensuring that hot water is available when required. By selectively turning the two contactors 57 and 58 off and on, four system states can be 30 achieved by controller 54. These four system states are summarised in the following table: 35 23 State Contactor 57 Contactor 58 Description 1 - 'Off' OFF OFF The output of inverter 52 is open circuited and no power is transferred to heating element 513. 2 - 'Grid tied (GT)' ON OFF The inverter 52 is connected to the utility power grid 55 and operates in grid tied (GT) mode. 3 - 'Variable output OFF ON The inverter 52 is connected to the voltage (VOV)' heating element 513 and operates in variable output voltage (VOV) mode. 4 - 'Boost' ON ON The inverter 52 is connected to both the utility power grid 55 and the heating element 513 and operates in grid tied (GT) mode. In an example of operation of the system shown in Figure 5, the four system states may be used as follows. As a starting point in the example, the water in the storage vessel 512 may have been cooled by user consumption of hot water. The cooled water temperature is detected 5 by the system controller 54, which selects the 'VOV' state to heat water from power available from the PV array 51. When the controller 54 detects that the water has reached a predetermined maximum temperature the controller switches the contactors to operate in 'GT' state. As a result, further energy generated by the PV array 51 is delivered to the utility grid 55 rather than being lost. 0 If, during heating in 'VOV' state, there is insufficient PV power to heat the water in the storage vessel 512 to a desired temperature in a desired time (parameters which can be configured by the user), then the system controller 54 switches the contactors 57 and 58 to operate in 'Boost' state. The utility power grid 55 then provides power to the electric heating element 513 in 5 addition to the power generated by the PV array 51, boosting the supply of energy to achieve the desired temperature of water. If, during the boost period, there is PV power available then it contributes to the heating of water in the storage vessel 512 rather than being wasted. When the water in the storage vessel 512 reaches the predetermined desired temperature the system switches to 'GT' state, and any remaining PV power is supplied directly to the utility grid. ?0 24 Dependent on time of day and user settings (for example in the early evening) the controller 54 may switch the system to the 'Off' state. Further hot water consumption will not trigger the 'Boost' state, but will progressively cool the tank, and the system will be ready to capture PV energy the next day. 5 This embodiment of the invention shown in Figure 5 provides a number of efficiencies because the use of multiple system states allows all of the PV energy to be captured, regardless of hot water consumption variations, while at the same time increasing the use of PV power for hot water heating in preference to utility grid power. D The system controller 54 is able to be controlled via a user interface, for example enabling the user to set temperature and time of day parameters (reflecting their typical hot water requirements). These parameters are used by the controller to maximise usage of PV energy for hot water heating in preference to other auxiliary power sources. 5 System 50 includes revenue meters 53 and 56 to measure the supply of power from the power inverter 52 and the utility power grid 55 respectively to enable the user to be charged according to the energy consumed. Revenue meter 56 may be operable to measure the power supplied to and from the utility power grid 55 so the user can be charged or reimbursed based on whether D they are taking power from, or supplying power back to, the grid. Figure 6 is a schematic view illustration of a solar energy capture and storage system 60 according to another embodiment of the invention. System 60 is similar in structure and operation to system 50, although in system 60 the inverter 52, revenue meter 53, contactors 57 5 and 58, and the system controller 54 of system 50 are incorporated into one physical device, inverter 62. The inverter 62 operates in a similar way to the equivalent group of components illustrated in Figure 5 and described above. In some circumstances the use of a single device that 0 incorporates the functions of the separate components illustrated in Figure 5 may be advantageous. For example: 0 The hardware requirements of the system are reduced, leading to efficiencies in installation and componentry, and allowing factory pre-wiring of components. 5 0 Only one mechanical enclosure is required.

25 " Allows dual use of output contactors already required for GT inverter isolation purposes. " Allows use of a common controller to perform both the inverter mode control function, and the additional system state control functions described above. * Allows use of a common wireless user interface to manage the inverter configuration 5 parameters and the additional hot water control functions described above. * Use of a common controller also allows the easy synchronisation of inverter mode switching with system state switching (it is important that the inverter is in the correct operating mode for each system state). D It will be appreciated that one or more of the above-identified advantages of the system shown in Figure 6 may also be realised in a system that incorporates other combinations of the components shown in Figure 5 into a single device. Figure 7 is a schematic view illustration of a solar energy capture and storage system 70 5 according to yet another embodiment of the invention. Many of the components of the system 70 shown in Figure 7 are similar to those shown in previously described embodiments. Only the features that differ from the other embodiments will be described. In system 70, the output of the photovoltaic array 71 is connected in parallel to two power 9 inverters 72 and 73. Power inverter 72 is configured to operate in a grid-tied (GT) mode while the other power inverter 73 is configured to operate in a variable-output-voltage (VOV) mode, in a similar manner to the inverters described above. System 70 includes revenue meters 721 and 731 to measure the supply of power from inverters 72 and 73. Revenue meters 721 and 731 may be incorporated within the inverters 72 and 73. 5 The GT power inverter 72 is connected via contactor 75 and revenue meter 78 to the utility power grid 77. The output of the VOV power inverter 73 is connected via a switch 76 to the electric heating element 713. Switch 76 also allows the electric heating element 713 to be connected to the utility power grid 77 via the revenue meter 78. For example, one of the inputs 30 to switch 76 may be connected between the contactor 75 and revenue meter 78. System controller 74 controls the operation of both power inverters 72 and 73, contactor 75, and switch 76. These components are preferably operated by the controller to seek to maximise the use of PV power generated by the photovoltaic array 71 for heating the water stored in the 26 storage vessel 712, and to supply excess power to the utility power grid 77 when it is not needed for hot water heating. Compared to the embodiments of the invention shown in Figures 5 and 6, the system 70 shown 5 in Figure 7 functions in a similar manner. Depending on the configuration of components, system 70 may involve additional hardware to the embodiments of Figures 5 and 6. However in some circumstances it may have the advantage that the hardware items are simpler, for example each of the inverters 72 and 73 in system 70 only needs to operate in one mode. D Figure 3 is a schematic illustration of part of a solar energy capture and storage system according to another embodiment of the invention. A vessel 30 contains a working medium that is heated by two heating means 31 and 32, both housed in the vessel 30 in thermal contact with the working medium. Each heating means 31 and 32 may comprise one or more individual heating elements. Heating means 31 is coupled to an auxiliary power source 33 and heating 5 means 32 is coupled to a photovoltaic power source 34 via an inverter of the type previously described. In the embodiment of Figure 3, no switching between the photovoltaic power source 34 and the auxiliary power source 33 is necessary, allowing completely separate operation of the two D power sources, which can be controlled through separate or integrated controllers. In this embodiment, solar energy capture may be maximised if the placement of the heating element 31 powered by the auxiliary power source 33 is located towards the upper third of the storage vessel, together with a thermostat also placed in that region, so that the auxiliary power .5 source 33 acts as a short term energy boost rather than a steady state heat source. The thermostat for the auxiliary power source 33 may be configured to cut the supply of energy from the auxiliary power source at a temperature that ensures compliance with the relevant legionella bacteria control regulations, taking into account the height or position of the heating element 31 powered by the auxiliary power source 33 within the vessel 30. 30 Figure 4 is a schematic illustration of part of a solar energy capture and storage system according to another embodiment of the invention. A vessel 40 contains a working medium 42 that is heated by one or more heating elements 41 coupled to at least a photovoltaic power source (not shown) as discussed in relation to Figure 1. Inside vessel 40 is housed a heat 35 exchanger such as heat exchange coil 43, through which a further medium is able to circulate.

27 The medium in the coil 43 is heated by the working fluid 42 and conveys the heat energy to where it is required. A tempering valve 44 may be connected to the heat exchange coil 43 as is known in the art. 5 In the embodiment described in relation to Figure 1, the system comprises water as the working medium in the storage vessel. In other embodiments, the system may comprise other working mediums, of which the following are examples: glycol, alcohol, salt solutions, metals, composite materials, a refrigerant in a solid, liquid or gaseous phase. D An independent solar energy capture and storage system that allows storage of energy in the form of heat of higher quality than is obtainable by heating water (e.g. by achieving high temperatures in a working medium of significant heat capacity) enables other non-electric applications, other than the provision of hot water. For example, absorption cooling technologies and phase change applications are examples of two advanced technologies which require high 5 grade heat, and which would benefit from "free" solar energy stored in the form of a high temperature working fluid. In some embodiments, the solar energy capture and storage system may comprise a fluid meter for measuring the energy obtained from the system in the form of heated fluid, i.e. the "useful" ) energy obtained from the system after all losses. A hot water meter of conventional design may be provided to measure the flow rate and temperature of the hot water flowing from the system and the energy obtained can be calculated accordingly. In systems in which a working medium other than water is used, a flow meter suitable for measuring the properties of the relevant medium is used. 5 In some embodiments, the power delivered by the solar energy capture and storage system of the present invention may be calculated from the difference between the energy in the form of heated fluid, as measured by a suitable meter such as the hot water meter discussed above, and any energy supplied from an auxiliary power source, as measured by a suitable meter such 0 as a conventional mains grid meter. This enables the supply of energy from the photovoltaic cells to be measured without the need for an electricity meter in the solar energy capture system, which reduces the complexity of this system but still enables revenue metering, which can provide new commercial ways of making expensive systems available to consumers, as discussed above. 5 28 Unless the context clearly requires otherwise, throughout the description and the claims, the words "comprise", "comprising", and the like, are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense, that is to say, in the sense of "including, but not limited to". 5 The entire disclosures of all applications, patents and publications cited above and below, if any, are herein incorporated by reference. Reference to any prior art in this specification is not, and should not be taken as, an D acknowledgement or any form of suggestion that that prior art forms part of the common general knowledge in the field of endeavour in any country in the world. The invention may also be said broadly to consist in the parts, elements and features referred to or indicated in the specification of the application, individually or collectively, in any or all 5 combinations of two or more of said parts, elements or features. Where in the foregoing description reference has been made to integers or components having known equivalents thereof, those integers are herein incorporated as if individually set forth. D It should be noted that various changes and modifications to the presently preferred embodiments described herein will be apparent to those skilled in the art. Such changes and modifications may be made without departing from the spirit and scope of the invention and without diminishing its attendant advantages. It is therefore intended that such changes and modifications be included within the present invention.

Claims

1. An inverter for use in a solar energy capture and storage system, the inverter comprising: means for electrically coupling the inverter to one or more photovoltaic modules to receive DC generated by the photovoltaic modules; means for converting the received DC into AC; means for outputting AC at a time-averaged voltage that is variable between minimum and maximum levels; and means for coupling the output of the inverter directly or indirectly to one or more heating elements, wherein the inverter is operable in a first mode in which the inverter operates independently from an electricity supply network.

2. An inverter as claimed in claim 1, wherein the inverter is operable in a second mode in which the inverter operates electrically coupled to an electricity supply network.

3. An inverter as claimed in claim 1 or claim 2, wherein the inverter comprises a controller and means for maximum power point tracking, and wherein the outputted AC voltage level is adjusted by the inverter so that the output power substantially matches the maximum amount of power available from the photovoltaic modules.

4. A solar energy capture and storage system comprising: one or more photovoltaic modules; an inverter as claimed in any one of claims 1-3, the inverter being electrically coupled to the photovoltaic modules; and one or more heating elements electrically coupled directly or indirectly to the output of the inverter; wherein the outputted AC voltage level is variable to adjust the power outputted by the inverter to the heating elements.

5. A solar energy capture and storage system as claimed in claim 4, wherein the system comprises at least one further heating element coupled to an auxiliary electrical power source.