WO2015056171A1

WO2015056171A1 - Method and system for renewable energy sources management

Info

Publication number: WO2015056171A1
Application number: PCT/IB2014/065300
Authority: WO
Inventors: Valeria Boscaino; Rosario MICELI; Giuseppe Capponi; Vincenzo Ferraro
Original assignee: Università Degli Studi Di Palermo
Priority date: 2013-10-15
Filing date: 2014-10-14
Publication date: 2015-04-23
Also published as: ITRM20130566A1

Abstract

The present invention places in the field of managing energy, in particular coming from renewable or environmental energy sources, harvester sources, typically not correlated sources and it proposes a system for supplying electrical energy to a load, from a plurality of energy sources, together with a method for managing such system, which manages the power transfer from the plurality of input sources towards the loading device.

Description

METHOD AND SYSTEM FOR RENEWABLE ENERGY SOURCES MANAGEMENT

DESCRIPTION

The present invention places in the field of managing energy, in particular coming from renewable or environmental energy sources, harvester sources, typically not correlated sources.

It is then proposed a system for supplying electrical energy to a load, from a plurality of energy sources, together with a method for managing such system.

A so-called system of power management, based upon the use of several renewable energy sources, manages the power transfer from the plurality of input sources towards the loading device.

Up to now, the solutions in such field can schematically be traced back to two distinct types.

The first solution provides the management of the environmental sources through a function of logic OR. A monitoring system monitors the environmental conditions of each existing source. The system of power management selects the source which is subjected to more favourable environmental conditions (which then delivers higher power) and this selected source is connected to the system of power management. Actually the source selection is wholly asynchronous and only one source at a time can feed the power management system and then it can transfer power towards the load.

More advanced solutions provide the implementation of the function of logic OR by means of diodes and then they presume that, in case of perfect voltage matching between several sources, the sources can be connected in parallel on the load. However, it is to be noted that, with uncorrelated harvester sources, this possibility is particularly remote and then substantially to be excluded.

The solution with logic OR generally provides that the power management system performs to control of the operating point of the single selected source, by not considering the momentaneously inactive other ones to the purpose of the power transfer. The power output of the sources under less favourable conditions then is not used to the purpose of the energy transfer, but it is lost. This is obviously a very serious drawback of the solution with logic OR, in contrast with its own logics of the energy harvesting consisting in the recovery of any form and quantity of environmental energy. Therefore, the logic OR strategy is not effective.

Furthermore, generally, the power management system, directly connected to the connection diodes, performs the control on the maximum input power, by referring to a subsequent conversion stage the control of the output power to be transferred to the load. This happens as there are not sufficient control variables allowing an effective control both in term of output power and in terms of maximum input transfer.

Furthermore, in case of logic OR solution, the algorithm controlling the conversion stage which controls the input power has to re-adapt to the specific source which each time is actually input connected.

A more advanced category of solutions of power management is represented by the so-called 'modular solutions'. In this case, each environmental source is connected to a dedicated power converter, the outputs thereof are adapted on a single common bus. However, sometimes, behind modular solutions, solutions are hidden which actually have logic OR, that is the modules are interfaced by a function of selection with logic OR.

The modular solution allows a single control of the maximum power of the environmental source, as performed by the dedicated converter. In case of adapting on common bus, it avoids the environmental energy dispersion. However, from the common bus to the loading device, a subsequent conversion stage has to be always connected due to the lack of degrees of freedom.

The modular solution has a level of flexibility comparable to the solution with logic OR as it is possible adding easily an additional module to guarantee the system expandability. However, in this case, the adding of an additional module involves adding a whole conversion system and not a simple interface diode such as in case of the logic OR solution.

Therefore, the modular solution has a worse conversion effectiveness than the logic OR solution as it inserts several power converters. The object of the present invention is then to overcome the above-illustrated problems and this is obtained through a system for supplying electrical energy to a load LD by a plurality of energy sources SPi comprising:

means for acquiring over time electrical parameters at the output Vi, Ai of said energy sources SPi;

means for selecting over time one or more of said sources SPi in function of said electrical parameters Vi, Ai so that each one of the selected sources SSi provides an output power higher than a first predetermined threshold P_min; an energy storage stage ST apt to be fed, alternatively, from one of said selected sources SSi;

an output stage OUT apt to supply an output voltage Vout with respect to a reference point GND for supplying a load LD;

a switching stage SW for connecting said selected sources SSi to said storage stage ST and said stage of storage ST to said output stage OUT; and a control unit CTRL adapted to drive said switching stage SW so that, for at least a fundamental time interval T during which the selection of said sources does not change, each one of the selected sources SSi is alternatively connected for a respective connection time interval Ton, to said storage stage and that said stage of storage, at least for an output time interval Toff, is connected to said output stage OUT, said connection time intervals Ton, being mutually distinct.

An additional subject of the present invention is a method for managing a system for supplying electrical energy to a load LD from a plurality of energy sources SPi comprising:

acquiring over the time electrical parameters at the output Vi, Ai of said energy sources SPi;

selecting over time one or more of said sources SPi in function of said electrical parameters so that each of the selected sources SSi provides an output power higher than a first predetermined threshold P_min; in at least a fundamental time interval T during which the selection of said sources does not change: o alternatively storing energy coming from each one of the selected sources SSi for a respective connection time interval Ton,, and o delivering, at least for an output time interval Toff, the energy stored in said connection time interval Ton,, for supplying an output voltage Vout with respect to a reference point GND to supply the load LD, said connection time intervals Ton, being mutually distinct.

The present invention, by overcoming the problems of known art, involves several and evident advantages.

First of all, the present invention is constituted by a power management system for managing several renewable or environmental energy sources. The created system provides a simultaneous and effective management of the energy coming from the environmental sources.

Furthermore, the present invention proposes a new architecture, new control subsystems, algorithms and power management techniques for systems based upon the use of several uncorrelated harvester sources. Thanks to the present invention a greater effectiveness and lower costs than the solutions known to the inventors are guaranteed.

By means of the proposed power management system, a single compact system with low costs, and innovative control algorithms, it is possible controlling in an effective way the power transfer towards the loading device from several renewable and environmental sources.

They system includes a sub-system called "power conditioning unit" performing the functions for controlling and managing the resting point of the renewable sources for inputting and simultaneously managing the power flow coming form the set of the input renewable energy sources towards the loading device and a subsystem called "power controller" controlling and managing the power conditioning system. The power management system can be directly connected to the loading device or be connected to the load by means of a subsequent sub-system "battery charger" to a storage element, such as for example a battery or a supercondenser.

These and other advantages, together with the features and use modes of the present invention, will result evident from the following detailed description of some preferred embodiments, shown by way of example and not with limitative purposes. The figures of the enclosed drawings will be referred to, wherein:

• figure 1 is a block diagram exemplifying a system according to the present invention;

• figure 2 is a more detailed block diagram of a system according to the present invention;

• figures 3A and 3B are circuit diagrams exemplifying the power conditioning block of the system according to the present invention;

• figures 4A and 4B are circuit diagrams exemplifying a system according to the present invention;

• figures 5A and 5E are graphs showing the course of some quantities measured during the operation of the circuit of figure 4A.

The present invention will be described hereinafter by making reference of the above-mentioned figures.

In particular, figure 1 is a block diagram exemplifying a system according to the present invention.

In very general terms, the system receives as input a plurality of renewable energy sources, designated with SPi ,SP₂,...,SP_n. A sub-system, generally designated as "power conditioning unit" in the diagram of figure 1 , is in charge of managing the power flow from the plurality of input sources towards the load. The sub-system "power conditioning unit", is suitably controlled by a sub-system generally designated in figure as "control unit".

According to an embodiment the power conditioning unit is connected to a storage element, such as for example a battery or supercondenser, and in parallel to the load. This represents one of the preferred embodiments including the possibility of managing the power flow in three distinct modes:

- Battery charging through sources and PMU (charge): the phase provides the power transfer from the set of input sources which are subjected to favourable environmental conditions towards the loading device. During this phase the working point of the single input sources can be optimized with the purpose of maximizing the power transfer and the maximum loading status on the storage element connected as output is restored. - Load fed only by battery (discharge): the mode appears in case the environmental conditions are not favourable for all input sources. During this phase the correct load feeding is assigned to the output storage element.

- Load fed by battery and sources (hybrid mode): the mode appears when the environmental conditions of the input sources are not sufficient, based upon the instantaneous request of the loading device. In this phase, both the output storage element and the power conditioning unit and therefore the input sources contribute to the correct feeding of the loading device.

More in details, the diagram of figure 2 illustrates a system according to the present invention. The system has to be managed through a corresponding managing method, according to the following description.

The system first of all comprises means for acquiring over time electrical parameters at the output Vi, Ai of the energy sources SPi (i = 1 ... n). This allows implementing a system for sensing and monitoring the environmental conditions.

Such electrical parameters (for example, voltage, current and/or output power), which then in a different way for each source, will depend upon the environmental conditions, are used by means for selecting, over time, one or more of the SPi sources in function of the electrical parameters Vi, Ai. The selection is performed so that each of the selected sources, designated with SSi (i=1..n), provides an output power higher than a first predetermined threshold P_min.

Of course, such threshold value could be chosen in function of the specific system application, and the determination thereof of course is to be considered within the comprehension of a person skilled in the art. For example, the threshold could set equal to zero.

In other words, the selection means selects only the sources SSi which can participate actively in the electric energy supply, by switching-off from the system the ones which instead, in that moment, cannot contribute to the electric energy supply as, due to contrary environmental conditions, do not produce enough power.

The system furthermore comprises an energy storage stage ST and an output stage OUT. The storage stage ST is so as to be fed, alternatively, by one of the selected sources SSi and the output stage is so as to be able to provide an output voltage Vout, measured with respect to a reference point GND, for supplying a load LD.

For connecting the selected sources SSi to the storage stage ST and the storage stage ST to the output stage OUT, the system comprises a switching stage SW, controlled and driven by a control unit CTRL.

Such control unit is arranged so as to be suitable to drive the switching stage SW so that, for at least a fundamental time interval T, during which the selection of the sources is not modified, each one of the selected sources SSi is alternatively connected for a respective connection time interval Ton, to the storage stage. In particular, all connection time intervals Ton, have to result mutually distinct.

Furthermore, the switching stage has to be driven even so that the storage stage, at least for an output time interval Toff, is connected to the output stage OUT.

Advantageously, the control unit can be so as to adjust even the voltage Vout, and consequently the input power, based upon a predetermined value VT, still chosen depending upon the specific application.

The subsequent figures 3A and 3B are circuit diagrams exemplifying the power conditioning block of a system according to the present invention.

It is presumed that the input sources have been already selected. The power conditioning unit is represented in figure 3A in case of two selected input sources, and in figure 3B in case of a generic number N of selected input sources.

By referring to figure 3A, the system according to such embodiment proposes as a conversion system of multi-input, single-output, single inductor type. The input- output ratio is of buckboost-buckboost type not-inverting with respect to both sources.

The topology includes a switching stage comprising three switches Sw1 , Sw2 and Swout, an energy storage stage comprising an inductive element L and an output stage comprising a capacitor element.

It is clear that the output capacitor element could be replaced by a battery or a supercondeser.

Furthermore two input capacitor buffers C1 e C2 can be provided, each one connected in parallel to a respective selected renewable source. The node NS common to the three switches will be designated hereinafter as switching node.

Between the selected sources SS1 and SS2, one will be designated hereinafter as primary source or master, the other one will be defined and identified as secondary or slave source.

The master source carries out a fundamental role in the operation of the conversion topology as it is the only one among the sources which is connected between the inductive storage element L and a reference point of the GND circuit, in particular, in the example, the system mass.

The switch Sw2 has to be connected directly to the slave source SS2. The switch Sw1 has to be connected between the switching node and the common mass. The switch Swout has to be connected between the switching node and the output capacitive node.

The system leaves aside the specific type of the two sources. The sources can be indifferently solar panels, fuel cells, piezo-electric devices, wind generators, thermoelectric generators or other harvester devices.

The switching stage, furthermore comprises two auxiliary switches SA1 and SA2. These switch will be controlled so that between the selected sources there is always one, and possibly only one, connected as primary or master source, connected to the reference point GND.

Figure 3B repeats exactly the same topology, with the difference of having selected several input sources.

From the comparison of the circuits it is easy highlighting that the insertion of an additional energy source is possible by means of inserting:

- an additional auxiliary switch (SAN)

- an additional input capacitive buffer (CN)

- an additional primary switch (SwN)

Therefore, it is evident that, by means of inserting a small number of components, it is possible including easily an additional module, by guaranteeing wide flexibility to the designer.

The system topology, as described up to now, allows inserting input harvester sources wholly uncorrelated therebetween (different environmental sources). In this optics the input sources are not to be considered banally as ideal voltage generators considering that the harvester sources supply a current depending upon the environmental conditions thereto they are subjected. The considered environmental sources can be wholly uncorrelated therebetween and therefore the sources cannot be connected directly in series or in parallel.

This particular proposed topology allows to connect uncorrelated harvesters thanks to the connection of a central node directly connected to an inducer, node whereon switches are not applied. In this way, the current differences has a recirculation path outside the two sources. It is to be noted that for this the connections in parallel with the input capacitive buffers are not sufficient, as the current, by circulating on the energy buffers, would alter the operation point of harvesters, according to the peculiar feature V-l of the element.

Therefore, the input configuration is particularly suitable for using such topology with uncorrelated harvester sources.

The operation of the conversion topology is based upon the principle of power transfer simultaneously from several sources towards the loading unit, within a control and multiplexing strategy suitable to drive the switches so that, for at least a fundamental time interval T, during which the selection of the sources is not modified, each one of the selected sources SSi is alternatively connected for a respective connection time interval Ton, to the storage stage. In particular, all connection time intervals Ton, have to result mutually distinct.

The multiplexing strategy guarantees that the average power in the period T transferred to the load is equal to the sum of the powers output by the single sources involved in the multiplexing strategy.

Advantageously, the period T has to be chosen sufficiently small, so as to be able to consider constant the environmental conditions, and not to lead again to an asynchronous operation.

Let's consider by simplicity the case of two sources SS1 , master source, and SS2 slave source (illustrated in figure 3A). The system operation is described in case both sources are under the conditions of participating actively to the power transfer. Under these conditions the configuration of the auxiliary switches is the following one: - SA1 ON

- SA2 OFF

- Sw1 and Sw2 partecipate to the multiplexing strategy

It is to be noted that in the calculation of the dissipated power and therefore in the system effectiveness, the switches driven in asynchronous way (in this case SA1 and SA2) provide a negligible contribution to the power on the whole dissipated by the power conditioning unit.

Three time sub-intervals are distinguished inside the fundamental period T.

Sub-interval Ton1 :

- Sw1 ON

- Sw2 OFF

- Swout OFF

The inductive storage element stores energy from the source SS1 , being connected in parallel to the source SS1 for the whole duration of the sub-interval Tonl

Sub-interval Ton2:

- Sw2 ON

- Sw1 OFF

- Swout OFF

The inductive storage element stores energy from the source SS2, being connected in parallel to the source SS2 for the whole duration of the sub-interval Ton2.

Sub-interval Toff:

- Sw1 OFF

- Sw2 OFF

- Swout ON

The inductive storage element is directly connected between the master source and the output capacitive buffer and it provides the energy previously stored in the loading device.

Preferably, the control unit has to guarantee the presence of a dead time between the sub-interval Ton1 and the sub-interval Ton2 to avoid the connection in parallel of the two energy sources. During the dead time Toff the system is in the configuration:

- Sw1 OFF

- Sw2 OFF

- Swout ON

Therefore the interval Toff can be, and in general is, physically divided into even not consecutive time sub-intervals. This does not damage the generality of the description and, to the purpose of the energy exchange, the physical arrangement of the distinct time sub-intervals is irrelevant, but the overall duration is exclusively important wherein the system is in the three just-described configurations.

It is to be noted that during the sub-intervals Ton1 and Ton2, during which the inducer stores energy from the sources, the inductance L is subjected to the maximum possible voltages inside the circuit and coming from the input, that is of type VL= Vin. This is a peculiarity of the proposed system particularly useful to the purpose of the system operation. In fact, under these conditions, being the single sources equal, the inducer stores energy in the shortest time possible as it is subjected directly to the voltage of the corresponding source. In other multi-input topologies this does not happen and it makes slower the power transfer towards the load, that is the same average power is transferred in a longer time.

If V1 is the voltage value at the ends of the master source, if V2 is the voltage value at the ends of the considered slave source and Vout the value of the output voltage.

Having designated with d1 the duty-cycle of the switch Sw1 and with d2 the duty- cycle of the switch Sw2, the input-output relation of the proposed converter under operating conditions is:

wherein d1=Ton1/T e d2=Ton2/T.

All what precedes can be generalized in case of N input sources. By referring to figure 3B, the operation of the proposed conversion sub-system is described. In relation to the generic source SSi, with i=2,...,N the following operating modes are distinguished:

- Cooperation mode: the source Si contributes to the power transfer towards the load. The system for sensing and monitoring the environmental conditions of the source SSi does not detect particularly adverse environmental conditions. The source SSi partecipates to the power transfer towards the load and the corresponding switch Swi intervenes in the time-multiplexing strategy.

- Exclusion mode: the source SSi is excluded by the power transfer towards the load. If the sensing and monitoring system detects that the environmental conditions thereto Si is subjected are wholly unfavourable (absence of sun for photovoltaic panels, lack of hydrogen for fuel cells, etc.), the system excludes the corresponding source from the multiplexing strategy. In this case, the power controller unit disables, that is drives under interdiction conditions, the corresponding switch Swi, which will not participate in the multiplexing strategy until a subsequent command of the sensing and monitoring unit.

The previous strategy is valid for any slave source inserted in the system. The master source, instead, carries out a fundamental role in the conversion topology as it is the only one among the sources which is connected between the inductive storage element and the system mass. With referent to the master source, the two provided operation modes are the following ones:

- Cooperation mode: the master source SS1 contributes to the power transfer towards the load. The system for sensing and monitoring the environmental conditions of the source Si does not detect particularly unfavourable environmental conditions. The source Si participates to the power transfer towards the load and the corresponding switch Sw1 intervenes in the time-multiplexing strategy, the switch SA1 in constantly under conduction, the auxiliary switches SA2,...,SAN are constantly driven under interdiction.

- Exclusion mode: the source SS1 is excluded from the power transfer towards the load. If the sensing and monitoring system detects that the environmental conditions thereto SS1 is subjected are absolutely unfavourable (absence of sun for photovoltaic panels, lack of hydrogen for fuel cells, etc.), the system excludes the master source from the multiplexing strategy, by raising to master source a slave source, chosen among the slave sources in that moment included in the multiplexing strategy. The power controller unit, based upon the information provided by the sensing and monitoring unit, inserts as master source the one that among the slave sources is under most favourable environmental conditions with respect to the other ones. Let's consider for example the case wherein the generic source SSj with j=2,...,N is risen to the master source role. In such case the auxiliary switch SA1 and the remaining auxiliary switches, except for SAj, are driven under interdiction conditions, the switch Swj is driven under interdiction by not participating to the multiplexing strategy and the switch SAj is driven under conduction. The described configuration remains until subsequent control of the sensing and monitoring unit.

In such case, the input-output ratio becomes the following one:

Vou^-^d^ ---^d" V_{l +} ½ V_{2 +}

\-d_l -d₂ -...-d_n \-d_l -d₂ -...-d_n \-d_l -d ^₂ -...-d_n r₃ +.....+ \-d_l -d ½₂ -...-d_{n Vn} wherein:

Vj is the output voltage of the selected primary source;

Vi is the output voltage of the selected ^~th source; and

di is calculated as Toni/T.

The converter behaves like a boost against the master source and like a buck- boost not inverting against any slave source connected to the circuit, by leaving the designer a wide margin of flexibility in using and positioning the harvester sources. This input-output relationship, basically for the correct use in the finalized applications, exclusively depends upon the specific position of the components of the proposed conversion topology.

The input-output relationship reveals an important feature: the relationship includes the sum of two terms, each one correlated to the voltage of one of the input sources and both values of duty-cycle. This important feature allows controlling this single conversion system both to implement a control of the output power according to the load request, the charging state of the possible output storage element, and to implement separately a control of the resting point of the single sources. The control unit implements an algorithm apt to explicit both these functions. It is to be noted that the algorithm is made possible by the particular input-output expression and therefore it is to be considered peculiar of the proposed invention. It is also to be noted that the system according to the present invention, with one single conversion stage, then with the use of a small number of components and then by guaranteeing the maximum effectiveness, allows all the functions guaranteed by solutions known in literature.

In terms of effectiveness, by considering the case of two sources, inside one period T there are:

- two switches of the switch Sw1 (ON->OFF, OFF->ON)

- two switches of the switch Sw2 (ON->OFF, OFF->ON)

- four switches of the switch Swout

The number of switches is minimum with respect to the already known topologies of multi-input type. In terms of effectiveness, the system then has a higher effectiveness for the subject topology than the known multi-input topologies, the sources and energy transmitted to the load being equal.

The control unit has to provide the controlling signals for the switches of the conversion topology. The proposed control algorithm provides a combined control of the duty-cycle d1 and d2 to guarantee the correct transfer of output power, that is the control of the output voltage, and an independent control of the duty-cycle d1 and d2 in function of the electric parameters acquired by the input sources to adjust the resting point of the single source and to optimize the power transfer.

The subsequent figures 4A and 4B are circuit diagrams exemplifying a system according to the present invention, in case of two uncorrelated sources in cooperation mode.

The control unit implements a hysteretic control (control with variable frequency) to guarantee the optimization of the power transfer. It is provided to implement a control with constant frequency and to show the obtained results.

In particular, as exemplified in figure 4B, the control unit, in this case, includes for each one of the energy sources, a respective input hysteretic comparator configured to compare an output voltage of the respective energy source with its own reference value of said source, and to drive as output a corresponding switch of the switching stage to connect/disconnect the energy source from the storage stage. Preferably, the control unit further includes an output hysteretic comparator configured to compare the output voltage Vout with said predetermined value, and to drive as output the switching stage so that when the output voltage Vout exceeds a higher threshold of the comparator, all selected sources SSi are disconnected from the storage stage, for a discharge time period Ts distinct from the fundamental time interval T.

In particular, the control unit comprises:

- a hysteretic comparator Hyst_out connected to the output node

- a hysteretic comparator Hyst_in1 connected to the master source

- a hysteretic comparator Hyst_in2 connected to the slave source

- Control logic ports (1 port AND with three inputs, one port AND with two inputs and one port NOT)

The control system is implemented so as to guarantee the control of the output voltage (nominal voltage in the specific case equal to VT=5.2V and the control of the maximum power point of the two sources. The MPPT algorithm implemented for both sources is a fractional control, typical and characteristic of applications with reduced power. For applications with high power other types of MPPT control are suggested such as the incremental conductance. It is to be noted that the system according to the present invention does not preclude the implementation of a specific and different MPPT algorithm.

The system maximizes the conversion effectiveness by means of a burst operation of the created converter. The burst can be divided into two time intervals: a charge period Tc and a discharge period Ts. During the discharge period Ts, the load supply is guaranteed by the output storage element, the switching of all inner components is inhibited by the control unit and the power output of the input sources is stored in the corresponding input capacitive buffer.

During the charge period Tc, the power output of the sources is transferred as output by means of a control of the input power and the charging state of the output storage element is restored.

During the discharge period Ts the power dissipations are annulled as the power conditioning unit is disabled and therefore there are no switchings of switches. During the charge period Tc, incisive interval on power dissipation, it has to guarantee the transfer of the maximum power available from any input source in the shortest time possible.

The output hysteretic control compares the output voltage with a reference value equal to the nominal value wished for the output voltage, by means of a hysteresis comparator. The hysteresis comparator band determines the tolerance admitted on the voltage control and then it is a sizing parameter in the planning phase.

The implemented output algorithm is the following one:

- In the intersection instant between the output voltage and the maximum hysteresis threshold, the output comparator disenables the switches Sw1 and Sw2. In this phase the discharge of the output storage element takes place and the energy storage by the inductive storage element of the power conditioning unit is inhibited.

- In the intersection instant between the output voltage and the minimum hysteresis threshold, the output comparator enables the switching of the switches Sw1 and Sw2. In this phase, the power output of the active sources is transferred as output to restore the charge on the output storage element. During this phase, the control unit manages the power conditioning unit so as to optimize singularly the resting point of the active sources, by guaranteeing the maximum power transfer.

In case the signal 'Enable' coming from the output hysteresis comparator enables the switchings of the switches Sw1 and Sw2, the control signal of the two switching elements is established by the input hysteresis of the corresponding source. In this way, the switch Sw1 (or Sw2) is driven so as to guarantee the operation of the corresponding solar panel S1 (or S2) in the point of maximum power compatible with the actual environmental conditions.

The hysteresis comparator of the master source compares the instantaneous voltage at the ends of the master solar panel with the reference, equal to the MPP voltage of the panel. The hysteresis band of the hysteresis comparator of master input source determines the precision of the implemented MPPT algorithm. If the instantaneous input voltage exceeds the upper threshold of the corresponding hysteresis comparator, the comparator action has to be so as to make the input voltage to decrease. Therefore under these conditions, the switch Sw1 is driven under conduction. In this way the average current supplied by the master source increases by causing a decrease in the voltage at the ends of the same according to the panel feature. When the instantaneous voltage value of the master source tends to assume lower values than the minimum hysteresis threshold of the master hysteresis comparator, the controller action has to be so as to cause a voltage increase. Under these conditions, the switch Sw1 is driven under interdiction, the supplied current decreases and the voltage increases according to the feature of the solar panel. The same control logics is implemented for the hysteresis comparator of the slave source.

Definitely, the output comparator guarantees the control of the output voltage, whereas the input comparators manage the MPPT algorithm that is they guarantee that the maximum power is transferred in the minimum time possible.

The present invention has been sofar described with reference to the preferred embodiments thereof. It is to be meant that each one of the technical solutions implemented in the preferred embodiments described herein by way of example could advantageously be combined differently therebetween, to create other embodiments, belonging to the same inventive core and however all within the protection scope of the herebelow reported claims.

Claims

1. A system for supplying electrical energy to a load (LD) from a plurality of energy sources (SPi) comprising:

- means for acquiring over time electrical parameters at the output (Vi, Ai) of such sources (SPi);

- means for selecting over time one or more of said sources (SPi) in function of said electrical parameters (Vi , Ai ) so that each of the selected sources (SSi) provides an output power higher than a first predetermined threshold (Pmin);

- an energy storage stage (ST) apt to be fed, alternatively, from one of said selected sources (SSi) ;

- an output stage (OUT) apt to supply an output voltage (Vout) with respect to a reference point (GND) for supplying a load (LD) ;

- a switching stage (SW) for connecting said selected sources (SSi) to said storage stage (ST) and said storage stage (ST) to said output stage (OUT), and

- a control unit (CTRL) adapted to drive said switching stage (SW) so that, for at least a fundamental time interval (T) during which the selection of said sources does not change, each of the selected sources (SSi) is alternatively connected to said stage of storage for a respective connection time interval (Toni) and that said stage of storage is, at least for an output time interval (Toff), connected to said output stage (OUT), said connection time intervals (Toni) being mutually distinct.

2. The system according to the preceding claim, wherein said control unit is adapted to adjust each connection time interval (Toni) of each selected source (SSi) so to maximize the power output.

3. The system according to the preceding claim, wherein said control unit is such to adjust the output voltage (Vout) on the basis of a predetermined value (VT).

4. The system according to one of the preceding claims, wherein said means for selecting is adapted to control said switching stage so that one selected primary source (SS1), among the selected sources (SSi), is connected to said reference point (GND).

5. The system according to claim 4, wherein said storage stage (ST) comprises an inductive element (L) .

6. The system according to claim 5, wherein the output voltage (Vout) is determined by the relationship:

Vout= ^{~dl ~d}^ ^~ -^~dn V_x + ½ V₂ + ^ V₃ + +

\-d_l -d₂ -...-d_n \-d_l -d₂ -...-d_n \-d_l -d₂ -...-d_n \-d_l -d₂ - wherein:

Vj is the output voltage of the selected source primary;

Vi is the output voltage of the ^~th selected source; and

di is calculated as Toni/T.

7. The system according to one of the claims 2 to 6, wherein said control unit comprises, for each of said energy sources, a respective input hysteretic comparator configured to compare an output voltage of the respective energy source with a reference value of said source, and to drive a corresponding switch of said switching stage for the connection/disconnection of the energy source from the storage stage.

8. The system according to one of the claims 3 to 7, wherein said control unit includes an output hysteretic comparator configured to compare the output voltage (Vout) with said predetermined value (VT), and to drive said switching stage so that when the output voltage (Vout) exceeds an upper threshold of the comparator, all selected sources (SSi) are disconnected from said storage stage (ST), for a discharge time period (Ts) distinct from said at least one fundamental time interval (T) .

9. A method for the management of a system for supplying electrical energy to a load (LD) from a plurality of energy sources (SPi) comprising:

- acquiring over the time electrical parameters at the output (Vi, Ai) of these sources (SPi) ; - selecting over the time one or more of said sources (SPi) in function of said electrical parameters so that each of the selected sources (SSi) provides an output power higher than a first predetermined threshold (Pmin) ;

- at least in a fundamental time interval (T) during which the selection of these sources does not change:

* alternatively storing energy from each of the selected sources (SSi) for a respective connection time interval (Toni) , and

* delivering, at least for an output time interval (Toff), the energy stored in said connection time interval (Toni), for supplying an output voltage (Vout) with respect to a reference point (GND) to supply the load (LD),

said connection time intervals (Toni) being mutually distinct.

10. Method according to the preceding claim, comprising a step of determining each connection time interval (Toni) of each selected source (SSi) so to maximize the power output.

11. Method according to the preceding claim, comprising a step of adjusting the output voltage (Vout) on the basis of a predetermined value (VT).

12. Method according to one of claims 9 to 11 , wherein said step of selecting is so that one selected primary source (SS1), among the selected sources ( SSi ), is connected to said reference point (GND).

13. Method according to claim 12, wherein said step of storing energy comprises the connection of a selected source (SSi) to an inductive element (L) .

14. Method according to claim 13, wherein the output voltage (Vout) is determined b the relationship:

wherein: Vi is the output voltage of the selected source primary;

V₂ ... Vn are the output voltages of the other selected sources; and

d1 ... dn are calculated as Toni/T.

15. Method according to one of claims 10 to 14, wherein said step of determining each connection time interval (Toni) comprises, for each of said energy sources, an input hysteretic control phase for comparing an output voltage of the respective energy source with a reference value of said source, and allowing/ not allowing the storage of energy by the energy source.

16. Method according to one of claims 1 1 to 15, wherein said step of adjusting the output voltage (Vout) comprises an output hysteretic phase control for comparing the output voltage (Vout) with said predetermined value (VT), so that when the output voltage (Vout) exceeds an upper threshold of the hysteretic control storage of energy by any of the sources selected (SSi) is permitted, for a discharge time period (Ts) distinct from said at least one fundamental time interval (T).