GB2541431A - On-grid battery storage system - Google Patents

Abstract

A battery storage system 10 for use with an on-grid micro-generation plant (e.g. photovoltaic panels) to reduce power import and export from the distribution network or grid, and connected to the input of a solar panel emulator that presents one or more selectable voltage-current curves on its output to a Maximum Power point tracking (MPPT) circuit within a PV inverter. The battery storage and emulator arrangement allows for variable battery discharge rate selection and connection to a standard MPPT solar inverter 18a, 18b. The battery storage system further comprises a controller 14 to activate charging when the energy export is above a set threshold, and to adjust the maximum power point presented for discharge rate control when energy import is above a set threshold. A battery control module 26a, 26bmay be provided for each battery or battery bank, the control module having at least one of an over-charge, over-discharge, over-current or temperature protection function or a balancing function.

Description

ON-GRID BATTERY STORAGE SYSTEM

The present invention relates to a battery storage system for storing electrical energy when there is an excess of generation from a microgeneration plant, and for releasing the stored energy when there is an excess of demand.

BACKGROUND TO THE INVENTION

Grid-connected domestic installations of solar photovoltaic cells are now fairly common. A solar array installed on the roof of a small to medium size dwelling can generate typically 3-5kW of power in sunny weather conditions. However, the power output at any given time is inevitably dependent on the available sunlight, which can change unpredictably during the day due to cloud cover. Of course, solar arrays do not generate power at all at night.

Although domestic solar arrays are generally connected to an electrical distribution network, so that power can be exported to the grid when there is an excess of generation and imported from the grid when there is an excess of demand, it is advantageous to store electrical energy, rather than exporting it, so that it can be used locally at times of excess demand to reduce the amount of electricity imported from the grid. From the point-of-view of the owner of the solar array, this is economically advantageous because the cost per kWh of importing energy is much more than the price received per kWh for exporting energy. From the point-of-view of the electrical system as a whole, it is generally better for microgenerated electricity to be used close to the point of generation, since this reduces the demands on a grid which has been designed primarily for efficient transmission of energy from a few large generating stations.

Other types of microgeneration plant, for example wind turbines, can also be connected to a building electrical system and to an electrical distribution network in the same way, so that power can be exported to the distribution network at times of surplus generation, and imported when there is excess demand. Exactly the same advantages of locally storing energy apply, regardless of the microgeneration technology used.

It is known to divert excess generated power to heat hot water, which can be stored efficiently in a well-insulated tank. The Applicant's co-pending British Patent Application 1504191.6 discloses technology in this field. However, as soon as electrical energy is used for heating water, the economic value of that energy is effectively reduced by around two-thirds, since the water could alternatively have been heated by burning gas which is generally much cheaper than electricity.

Battery storage systems are known, but for various reasons have not yet been widely adopted. The high cost of the batteries themselves and their limited lifespan has historically been a significant factor limiting adoption, but battery technology is constantly improving and the cost is falling, making battery storage increasingly viable. In terms of the electrical efficiency of battery storage, around 85% efficiency can currently be realised over the storage-release cycle. This is significantly better than what is effectively around 35% in a thermal storage system.

The remaining barrier to adoption of battery storage systems is that the currently-available electrical and electronic systems for controlling storage and release of energy from batteries do not lend themselves to highly-economic operation. This is because, in terms of releasing energy from the batteries, existing systems are generally either 'on' or 'off', i.e. they are either releasing a maximum amount of power from the batteries or are not releasing any power at all. What this means is that the system will usually be configured to switch on only at times where the excess local demand is at or above the maximum power which can be released from the battery storage system. This may well occur relatively infrequently, and the result is that small amounts of power (below the output of the storage system) will be imported from the grid at various times throughout the day, despite the batteries being charged. The overall amount of energy import is therefore not reduced as much as it could be. A crucial component of any grid-connected battery storage system is the inverter, which changes the DC output from a battery to an AC output which can be synchronised with the electrical grid. A key safety requirement of an 'on-grid' inverter is that it must include 'antiislanding' circuitry to detect any grid outage and immediately switch off, to avoid dangerously feeding power to the grid during a power outage. Inverters connected to the grid in the UK must be accredited to the 'G83/2 or G59/3' standard. Similar accreditation requirements and standards apply in other countries.

Because of the popularity of domestic solar arrays, there are now a number of manufacturers of G83/2-accredited solar inverters. In particular, solar 'micro-inverters' which typically have a maximum operating power of around 250W, are widely available. 'Micro-inverters' are usually installed on individual solar panels, instead of connecting a single large inverter to a whole array. Solar inverters, including micro-inverters, have matured as commodity products and as such the price is low. On the other hand, there are relatively few G83/2-accredited battery inverters in the marketplace, and the available products cost more.

It is not possible to use a cheap solar inverter with a battery as a power source, because solar inverters include a feature known as 'Maximum Power Point Tracking' (MPPT). Solar panels have a non-linear output (i.e a curved voltage-current plot). The output characteristics change in a complex relationship with solar irradiance and temperature, and therefore the voltage-current curve which characterises the output from the solar array is constantly changing. MPPT inverters are designed to measure this voltage-current curve, and present an inverter input impedance matched to draw the maximum possible power from the solar array. Connecting an MPPT solar inverter input to a battery would result in an attempt to draw maximum power from the battery, which means a very high current, the dangerous possibility of severe overheating, and damage to the inverter and battery.

Hybrid inverters, which have an MPPT input for use with solar panels and a non-MPPT input for use with batteries, are available. However, they generally suffer from the above mentioned problem that, when used with batteries, they are either 'on' or 'off'. Hybrid devices also suffer from the additional drawback that the battery (non-MPPT) input cannot be used at the same time as the solar (MPPT) input, and so stored energy from the batteries can generally only be used at night. Furthermore, because the same inverter is used for both solar and battery input, some rewiring is required when retrofitting to an existing solar installation. A combination of the high cost of suitable inverters and the fact that the power output cannot be finely controlled means that existing battery storage systems are generally less economic, in that the purchase cost of the system is not justified by the savings in use.

It is an object of the present invention to provide a battery storage system with a low build cost and which can realise greater savings by reducing energy import to a minimum.

STATEMENT OF INVENTION

According to the present invention, there is provided a battery storage system for connection to a building electrical system which in turn is connected to an electrical distribution network and to a microgeneration plant, the battery storage system including: terminals for connecting at least one electrical storage battery; means for detecting and measuring power import and export between the building electrical system and the electrical distribution network; means for charging the storage battery from the building electrical system; an inverter including Maximum Power Point Tracking (MPPT) circuitry and having an output for connection with the building electrical system and an input for connection with a DC power source; a solar panel emulator having an input electrically connected with the battery connection means and an output electrically connected to the inverter input, the solar panel emulator being controllable to present one of at least two different voltage-current curves on its output, each voltage-current curve having a different maximum power point; and a controller adapted to activate the charging means when energy export above an export threshold is detected, and to activate the inverter and solar panel emulator when energy import above an import threshold is detected, the controller being further adapted to adjust the maximum power point presented on the output of the solar panel emulator dependent on the measured energy import or export.

The battery storage system of the invention is advantageous, because the combination of the solar panel emulator and the MPPT 'solar' inverter provides a way of controlling the amount of power released from the batteries. Therefore, the batteries can be used to compensate for relatively small excess demand conditions by setting the solar panel emulator to present a maximum power point which is at a similar level to the excess demand. The MPPT circuitry in the inverter tracks this maximum power point and produces AC output power which is at the right level to compensate for the excess demand and avoid unnecessary importation of energy from the distribution network.

This is in contrast to prior art battery storage systems which generally provide for power output from the battery which is either 'on' or 'off'.

The different voltage-current curves which can be presented on the solar panel emulator output have different maximum power points. The maximum power point is tracked by the inverter and thus the output power of the inverter is controlled by changing the voltage-current curve presented by the solar panel emulator. In the simplest embodiment, the solar panel emulator may be able to present just two different voltage-current curves. For example, one curve with a 250W maximum power point and one curve with a 125W maximum power point. This effectively allows a 250W inverter to operate in a 'half-on' (125W) mode in addition to the 'on' (250W) and 'off' (OW) modes which are possible with all inverters. In more sophisticated embodiments, the solar panel emulator may be able to present many more different maximum power points, allowing power output from the storage batteries to be activated in much finer steps.

The use of MPPT 'solar' inverters in the battery storage system also reduces the cost of the system, since it takes advantage of the low cost and diversity of supply in G83/2-accredited MPPT 'solar' inverters, avoiding the use of G83/2-accredited 'battery' inverters which cost more.

The inverter is preferably a 'micro-inverter', that is an inverter having a maximum power rating of no more than 500W. Preferably, the power rating of the inverter is less than 300W, for example around 250W.

In some embodiments, multiple inverters, specifically multiple micro-inverters, may be provided. A solar panel emulator may be provided for each inverter and each inverter and solar panel emulator set may have its input connected to a different battery, or set of batteries. In these embodiments, the controller may be adapted to control the total amount of power released from each battery or each battery bank independently at any given time. As an example, in an embodiment where there are two battery banks (which may each include for example three batteries), the total capacity of each battery bank may be around 66Ah at a nominal voltage of 38.4V, equivalent to around 2.5kWh. The microinverter and solar panel emulator set may be adapted for a maximum output of for example 250W, but controllable in 125W steps. The controller, depending on the measured import/ export from the grid can therefore release power from the two battery banks either at 125W, 250W, 375W or 500W. In addition, the controller may be adapted to select battery banks for power release based on the charging state of the battery banks, and stored information regarding the number of charge/discharge cycles which each battery bank has carried out. In this way, the demand on the battery banks over their lifetime can be managed to ensure approximately even wear over the life of the batteries. For example, if one battery bank is nearly discharged then the controller may release 250W by taking 250W from one battery bank, rather than 125W from each of the two battery banks. On the other hand, if both battery banks are charged then it may be preferable to take 125W from each, to ensure even wear.

Preferably, the batteries are LiFeP04 (Lithium Iron Phosphate) batteries. LiFeP04 batteries having a nominal voltage of 12.8V are commonly available. Preferably, each battery bank includes multiple batteries wired in series, for example, three LiFeP04 batteries wired in series, which together gives a battery bank having a nominal voltage of 38.4V. Because the MPPT micro-inverters are generally designed for use with a single solar panel having an output voltage range of around 22V to 55V, a 38.4V battery pack is ideal and well within the range both when fully charged and fully discharged. Preferably, each battery or each battery pack includes circuitry for over-charge protection, over-discharge protection, overcurrent protection, and temperature protection. A cell balancing function may also be provided in each battery, to ensure that individual cells in the battery maintain the same charge. This is important for Lithium-based batteries, as the efficiency of the battery pack is determined by the weakest cell.

Preferably, over-charge protection, over-discharge protection, over-current protection and a balancing function is provided as part of a battery control module for the battery pack as a χα/ΚλΙλ I r\ -j rl rl i + iω-ϊ /-> k kri-H-Λ rv/ ic r\ r/-»v/irl Λ/-Ι i iv/irl ι ι η 11 v/ vA/i+k cirv^ilrir γχγλ+λγ'+ιλιλ r'irr'iii+rv/

Preferably, the battery terminals allow the batteries to be removed and replaced. However, it is envisaged that the battery storage system will generally be supplied as a complete unit, including batteries.

Other battery chemistries, for example Lithium Titanate, Lithium Sulphur, or types of Lead Acid batteries, for example Absorbed Gas Mat (AGM) batteries, may also be used in embodiments. Each battery chemistry has its own advantages and drawbacks and the skilled person will be able to choose an appropriate type of battery for a given type of installation. The minimum and maximum battery pack voltage is within the MPPT range of the inverter. Each type of battery will have its own considerations in terms of, for example, maximum current draw. This can be taken into account by never presenting a maximum power point at the output of the solar panel emulator over a predetermined limit. In some embodiments, this predetermined limit may be configurable for use with different types of batteries, or to take into account different installation factors, for example the readiness with which heat can dissipate from the batteries in a given installation. A battery control module may be provided for each battery, or alternatively one battery control module may be provided for each battery pack.

In some embodiments, over-charge protection, over-discharge protection, over-current protection, temperature protection and cell-balancing circuity are provided as part of the or each battery control module, which may also include the charging means for the battery. Preferably, a controlled constant-current constant-voltage charger is provided as part of the battery control module.

The solar panel emulator may be integrated into the battery control module. In a simple embodiment, the controller may also be integrated into a battery control module, but it is preferable to provide a 'master controller' which reads the input from the import / export measuring means and determines which batteries or battery packs are to be charged or discharged and by how much, and then a battery control module on each battery or each battery pack to control the battery-specific functions. A single master controller may send signals to multiple battery control modules. This allows the system to be expanded in a modular way, by providing a single master controller and a control bus. The system can thpn hp pxnandpd hv addin? pxtra hattprv control modiilps iwith assoriatpH invprtprs solar panel emulators and batteries) to the control bus. Initially a system may have, for example, just one battery control module, one battery bank, one inverter and one solar panel emulator all controlled by a master controller. However, if the capacity of the system needs to be upgraded, then additional sets of control module, battery bank, inverter and solar panel emulator can be added and connected to the existing master controller.

The control bus may be a local interconnect network conforming to the RS-485 standard.

Preferably, the battery control module includes a microcontroller.

Preferably, the means for detecting and measuring power import and export is a current clamp, which may be quickly and easily installed around the incoming supply cable between the building electrical system and the distribution grid. However, other types of measuring means will work with the system. For example, a bi-directional electricity meter with a suitable output could be used.

In an example installation, the battery storage system is connected to a building electrical system which includes a solar array. When the sun is shining brightly, the solar array may produce for example 4kW. This is likely to be more than the electricity demand in a small house, especially if it is unoccupied during the day. There will therefore be a surplus of generation which initially will result in export to the distribution network. The export can be detected, and in response the controller will activate the charging means to charge the batteries. Preferably, the charging power of the charging means is adjustable so that low-power charging can occur when there is a small generation surplus, and higher power charging can occur when there is a larger generating surplus. In this way, energy which would otherwise be exported to the distribution network is instead stored in the batteries.

An excess demand is the opposite condition to a generating surplus, and can occur for a number of reasons, often for a relatively short period of time. For example, an electric kettle is typically rated at around BkW, and so turning on a kettle may easily create an excess demand where there was surplus generation, though typically it will only be switched on for a few minutes. At the same time, a thick cloud may significantly reduce the power output of a solar array, but again may only shade the array for a matter of minutes.

Where there is excess demand, initially there will be power import from the distribution network. The import is detected, and in response the controller will activate the inverter and solar panel emulator to draw power from the storage batteries to reduce the amount of imported power. In one embodiment, the controller may initially set the output of the solar panel emulator to the lowest maximum power point, and then increase it in steps until any measured import is below a certain threshold.

In different embodiments, the controller may be adapted to increase power draw from the batteries until the measured import from the grid is below a certain threshold, or alternatively until there is just a small amount of energy export. In some embodiments, the solar panel emulator may be able to present many different maximum power points at its output, in small steps, and so the system will be able to reduce energy import to close to zero as long as the batteries are charged and the maximum power output of the system is sufficient. In other embodiments, the steps may be larger. For example, if the step size is 125W then the system will not be able to reduce a 100W import without exporting energy. Whether it is better to export 25W or import 100W depends on the currently-available import and export prices, the expected net generation or demand from the building as a whole over a longer period, and the capacity of the batteries. If the microgeneration plant is large compared with the power demand, such that a net generating surplus is expected over time, and the batteries are of sufficient capacity that they are generally able to cover any transient excess of demand, then it may be preferable to allow a small export in order to avoid import. In most embodiments the relevant logic is in software in the controller, and therefore can easily be adjusted to suit different installations or to take account of changes in prices.

The battery storage system is suitable for use with many types of microgeneration plant, for example, solar, wind and hydro systems. Solar panel systems are used as the primary example throughout, but it will be understood that the system has advantages with any type of system where generation or demand is variable with external factors. For example, just as solar panels output less power in cloudy weather than in bright sun, a wind turbine will output less power when the wind speed is below an optimal level. Note that, in most embodiments, there is no connection between the battery storage system and the κν>ίη*Λ(,τΛΛΛΐ*Ί + ίΛΐΛ λ+Ιλλι* tk-ίΐΛ \/i-> 1-4 i i I /*1 i 1-4 rt λΙλγ+ι*Ιγ->Ι r v / r +/¾ i-4-4 ΤΙλλ i 1-4 r + -411 -4 + 1 r\ 1-4 nf +1-4/¾ battery storage system can therefore be carried out completely independently of the microgeneration plant, without requiring any rewiring. The microgeneration plant may be changed or upgraded, without interfering with the battery storage system, and likewise the battery storage system may be changed or upgraded without interfering with the microgeneration plant. Since embodiments require only a single connection to the building electrical system at a consumer unit, plus connection of the measuring means (which is typically a current clamp and is straightforward to install), installation is cheap and easy.

DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, and to show more clearly how it may be carried into effect, an embodiment will now be described, by way of example only, with reference to the appended drawings in which:

Figure 1 shows a schematic of a battery storage system according to the present invention; and

Figure 2 shows an example of a voltage-current curve measured at an output of a solar panel emulator, part of the battery storage system of Figure 1.

DESCRIPTION OF AN EMBODIMENT

Referring firstly to Figure 1, a schematic of a battery storage system according to the present invention is shown. The system comprises a master unit 10, and a slave unit 12. The master unit 10 on its own is a battery storage system according to the invention, and the slave unit 12 serves to add capacity to the battery storage system with additional batteries and associated components.

The master unit 10 comprises a controller 14, and two power modules 16a, 16b. Each of the power modules 16a, 16b comprises a micro-inverter 18a, 18b, a battery bank comprising three batteries 20a, 20b, 22a, 22b, 24a, 24b, and a battery control module 26a, 26b. A current clamp 28 is provided for measuring power import / export between the building electrical system 32 and the electrical distribution network (not shown in the Figure). The current clamp 28 is connected to an input on the controller 14. A control bus 30, in the form of a local interconnect network to the RS-485 standard, is provided for transmitting control signals between the controller 14 and each battery control module 26a, 26b.

The batteries 20a, 22a, 24a are connected in series. Each of the batteries is a 12.8V LiFePC>4 battery with a nominal voltage of 12.8V, and so the battery bank has a nominal voltage of 38.4V. The battery bank provides the input power for a solar panel emulator, which is integrated within the battery control module 26a. The output of the solar panel emulator is connected to the DC input of the micro-inverter 18a, and the AC output of the microinverter 18a is connected to the building electrical system 32.

As shown in Figure 1, the slave unit 12 includes a further pair of power modules 16c, 16d. The power modules 16c, 16d are identical to power modules 16a, 16b. The only difference between the slave unit 12 and the master unit 10 is that the slave unit does not include a controller (14). Each power module 16c, 16d includes batteries 20c, 22c, 24c, 20d, 22d, 24d, a battery control module 26c, 26d and a micro-inverter 18c, 18d.

The micro-inverters 18c, 18d are connected to the same building electrical system 32 as the micro-inverters 18a, 18b, and the battery control modules 26c, 26d are connected to the same control bus 30 as the battery control modules 26a, 26b and the controller 14. The controller 14 sends signals, via the control bus 30, to each of the battery control modules 26a, 26b, 26c, 26d to determine which batteries are charged and discharged, by how much, and when.

Each power module 16a, 16b, 16c, 16d can be individually controlled by the controller 14. Any number of power modules can be added to the system, limited only by the capabilities of the controller 14 and control bus 30. Each power module is identical, and although in this embodiment the power modules are shown combined into units of two power modules, individual power modules may be added. Adding a power module to an existing system expands the capacity of the system.

In some embodiments, it is possible that the power modules are not identical. E.g. some power modules may have greater energy storage than others, or a greater power output capability. In any case, even when power modules are identical when installed, it is expected that the capabilities of the batteries will not remain identical as they wear over time. Each power module may therefore have different characteristics, which may be measured by the battery control modules 26 and communicated to the controller 14.

The operation of the system will now be described primarily in terms of a single power module, but of course the other power modules are arranged in a similar way.

Each power module provides effectively two functions - energy storage (charging) and energy release (discharging). Each power module has the capability to operate either in charging mode or in discharging mode, or remain switched off altogether. In discharging mode, the power module can operate at at least two different power output levels, as described in more detail below. In this embodiment, the power module has different power settings in charging mode as well. The power module is controlled (i.e. it is turned on and off, put into charging or discharging mode, and the power level selected) by signals from the controller 14.

In charging mode, a constant-voltage constant-current charger, which is integrated into the battery control module, is activated to charge the batteries. The charger draws power from the building electrical system 32. Suitable chargers for charging batteries from the mains electricity at selectable power outputs are widely available, and will not be described in detail here.

The battery control module also includes over-charge protection circuitry, over-discharge protection circuitry, over-current protection, temperature protection and a balancing function. The battery control module may also include means for measuring the condition of the batteries, and for reporting their condition and capabilities to the controller 14 so that the controller may select charge and discharge modes to optimise performance and battery life.

The batteries are connected to the input of a solar panel emulator (which in this embodiment is integrated into the battery control module 26). The output of the solar panel emulator is connected to the DC input of the micro-inverter 18. The micro-inverter 18 includes Maximum Power Point Tracking (MPPT) circuitry. These inverters are widely available and are sometimes known as "solar inverters". The term 'micro-inverter' means an inverter with a relatively low maximum power rating, for example 250W. MPPT micro-inverters are designed to present an input impedance to draw the maximum power from a connected solar panel. The output characteristics of a typical solar panel are illustrated in the graph of Figure 2. A solar panel operates substantially as a current-source at a range of low load impedances, as shown by the substantially horizontal current/voltage plot in the left-hand half of the graph. In this region, a lower load impedance will generally mean a greater power supply from the solar panel. However, at a certain point the panel's ability to supply current begins to drop dramatically as the load impedance continues to increase. At around this point, the maximum power is being drawn from the panel. Either a lower or higher load impedance will result in a lower power supply from the panel. The maximum power point is the peak of the power/voltage plot in the Figure.

In Figure 2,

Ucis the short-circuit current;

Imp is the current at the Maximum Power Point (MPP);

Voc is the open circuit voltage;

Vmp is the voltage at the Maximum Power Point (MPP); and

PmP is the power at the MPP, i.e. the maximum power.

The position and shape of the plot, and the current and voltage at the maximum power point, varies with solar irradiance and temperature. For this reason, MPPT inverters are designed to 'track' the maximum power point, constantly adjusting their input impedance to draw maximum power from the panel.

In the battery storage system of the invention, the inverters 18 draw their input power from batteries 20, 22, 24 and not from solar panels. However, the MPPT circuitry is used as a way of varying the power drawn from the batteries 20, 22, 24 by the inverter 18 when a power module is in discharging mode. The solar panel emulator, which forms part of the battery control module 26, presents output characteristics which if measured would result in a current/voltage and power/voltage plot similar to that shown in Figure 2. The solar panel emulator can in fact present at least two different current/voltage curves on its output, and therefore can present at least two different maximum power points to the inverter. The inverter will track the maximum power point, and therefore the power drawn from the batteries and fed into the building electrical system can be controlled by switching the solar panel emulator to a selected output characteristic. In this embodiment, the solar panel emulator can present either a 125W or a 250W maximum power point, so that the 250W micro-inverter can be either 'off' (OW), 'half on' (125W) or 'full on' (250W).

Solar panel emulators are already known. They are generally expensive laboratory devices, used for testing MPPT inverters. Existing solar panel emulators are designed to accurately replicate the properties of particular solar panels under certain conditions. They are generally not electrically efficient. In the battery storage system of the invention, the solar panel emulator does not need to accurately emulate any particular solar panel, as long as it can provide output curves with the required maximum power points. There are fewer constraints and so electrically efficient solar panel emulators can be produced at a relatively low cost.

Because the MPPT inverter is controlled via the solar panel emulator, the power module can operate at many different power outputs in discharging mode. This allows for the best local utilisation of power from a microgeneration plant, because the 'step size' can be small and the amount of power released from the batteries to the electrical system can be closely controlled. Exported energy can be reduced to close to zero, and imported energy can also be reduced to close to zero, subject of course to the net surplus generation or excess demand of the building as a whole, and the capacity of the batteries. This is done without a high build cost, since readily available (and already accredited) 'solar' (MPPT) microinverters are used, rather than requiring specialist devices for battery connection. At the same time, no special interface to a solar panel array or other microgeneration plant is required. Although the system is designed to work in conjunction with a microgeneration plant, for example a solar array, there is no direct connection between the battery storage system and the microgeneration plant. The battery storage system is simply connected to the building electrical system, and a current clamp installed at the interface between the

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Claims (17)

1. A battery storage system for connection to a building electrical system which in turn is connected to an electrical distribution network and to a microgeneration plant, the battery storage system including: terminals for connecting at least one electrical storage battery; means for detecting and measuring power import and export between the building electrical system and the electrical distribution network; means for charging the storage battery from the building electrical system; an inverter including Maximum Power Point Tracking (MPPT) circuitry and having an output for connection with the building electrical system and an input for connection with a DC power source; a solar panel emulator having an input electrically connected with the battery terminals and an output electrically connected to the inverter input, the solar panel emulator being controllable to present a selected one of at least two different voltage-current curves on its output, each voltage-current curve having a different maximum power point; and a controller adapted to activate the charging means when energy export above an export threshold is detected, and to activate the inverter and solar panel emulator when energy import above an import threshold is detected, the controller being further adapted to adjust the maximum power point presented on the output of the solar panel emulator dependent on the measured energy import or export.

2. A battery storage system as claimed in claim 1, in which one of the voltage-current curves has a maximum power point of 125W, and one of the voltage-current curves has a maximum power point of 250W.

3. A battery storage system as claimed in claim 1 or claim 2, in which the inverter is a micro inverter.

4. A battery storage system as claimed in any preceding claim, in which multiple inverters are provided.

5. A battery storage system as claimed in claim 4, in which a solar panel emulator is connected to the input of each inverter.

6. A battery storage system as claimed in claim 5, in which terminals are provided for connecting a different battery or battery bank to the input of each solar panel emulator.

7. A battery storage system as claimed in claim 6, in which the controller is adapted to control the amount of power released from each battery or each battery bank independently.

8. A battery storage system as claimed in any of claims 4 to 7, in which a battery control module is provided for each battery or battery bank, each battery control module having at least one of an over-charge protection function, an over-discharge protection function, an over-current protection function, a temperature protection function and a balancing function.

9. A battery storage system as claimed in claim 8, in which the or each solar panel emulator is integrated into the or each battery control module.

10. A battery storage system as claimed in claim 8 or claim 9, in which the or each battery control module receives control signals from the controller.

11. A battery storage system as claimed in claim 10, in which a control bus is provided, the control bus being adapted to transmit signals between the controller and the or each battery control module.

12. A battery storage system as claimed in claim 11, in which the control bus is a local interconnect network conforming to the RS-485 standard.

13. A battery storage system as claimed in any preceding claim, in which the measuring means is a current clamp.

14. A battery storage system as claimed in any of the preceding claims, in which a battery or battery bank is connected with the battery terminals.

15. A battery storage system as claimed in claim 14, in which the battery or batteries are LiFePC>4 (Lithium Iron Phosphate) batteries.

16. A battery storage system as claimed in claim 15, in which a battery bank is connected with the battery terminals, the battery bank comprising three 12.8V LiFeP04 batteries wired in series.

17. A battery storage system substantially as described herein, with reference to and as illustrated in Figures 1 and 2 of the accompanying drawings.