WO2012010203A1

WO2012010203A1 - Process to operate continuously a solar array to its maximum power in variable shadowing conditions and device needed to carry it out

Info

Publication number: WO2012010203A1
Application number: PCT/EP2010/060530
Authority: WO
Inventors: Antoine Capel
Original assignee: Mppc Technology
Priority date: 2010-07-21
Filing date: 2010-07-21
Publication date: 2012-01-26

Abstract

The principle of the invention is to get the coordinates of this maximum maximorum point (MMP) of a solar panel composed of n paralleled strings in working conditions, what ever are the string temperatures, their ageing and their sun illumination. The processes are two: The first one is a mathematical process conducting to a rapid, accurate and reproducible result. The second method calls for a voltage sweep. The mathematical process comprises a first step of the identification of the 3 parameters defining the electrical characteristics of each string, a second one the knowledge of the analytical forms of the solar array characteristics, formula (2.14). And finally computation of the MPP of the solar array, which corresponds to the voltage which makes the first derivative of the power function equal to zero: formula (2.15).

Description

PROCESS TO OPERATE CONTINUOUSLY A SOLAR ARRAY TO ITS

MAXIMUM POWER IN VARIABLE SHADOWING CONDITIONS AND

DEVICE NEEDED TO CARRY IT OUT DESCRIPTION OBJECT OF THE INVENTION

The object of the present invention is, as the title of the application states, processes to operate continuously a solar array to its maximum power in variable shadowing condition, it is also another object of the present invention the device needed to carry it out.

The principle of the invention is to get the coordinates of this maximum maximorum point (MMP) of a solar panel composed of n paralleled strings in working conditions, what ever are the string temperatures, their ageing and their sun illumination. The processes to identify the maximum maximorum point MMP, at every change of the environmental conditions, are two: - The first one is a mathematical process conducting to a rapid, accurate and reproducible result.

- The second method calls for a voltage sweep inside the limits defined by the positions of smallest and the highest (Maximum Power Points) MPP of the strings. The power is computed for each point until the voltage sweep is performed. The position of the MPP voltage is automatically available.

The mathematical process involves three operating point measurements and three successive step process. The first step is the identification of the 3 parameters defining the electrical characteristic of each string ISCn, iRn and a_n. To get access to these parameters, it is necessary to solve, as already described, a mathematical system of 3 equations with 3 unknowns.

The second step is the knowledge of the analytical forms of the solar array characteristics, which corresponds to the sum of the currents produced by every string.

The last step is the computation of the Maximum Power Point (MPP) of the solar array. This point corresponds to the voltage which makes the first derivative of the power function equal to zero.

BACKGROUND OF THE INVENTION

Solar arrays are intensively used nowadays in space and terrestrial power systems by their ability to be independent of any electrical distribution network. They supply energy to local or mobile equipments in an autonomous way.

The difficulty rises as these solar arrays cover an important ground area, several tens of square meters, and parts of this area receive a variable illumination. These variable shadowing conditions are the results of clouds passages or specific shadows caused by objects (trees, walls etc.), mobile or static, which intercept the sunlight.

The start of the art involves a tracking process, called MPPT or Maximum Power Point Tracking, which moves the operating point of the solar array on its power characteristics and forces it to remain to the first maximum it finds. This process suffers from the drawback caused by a power characteristics which presents many maxima. The maximum maximorum of such a curve may be not the first maximum found by the MPPT process and where it remains locked. Therefore the solar array does not deliver its maximum power and the efficiency of the solar system is greatly affected.

It is therefore the objective of the present invention to overcome the previously mentioned drawbacks seeking a process to operate a solar array to its maximum power in variable shadowing conditions.

DESCRIPTION OF THE INVENTION

The principle of the invention is the extension of the MPPC process or Maximum Power Point Control, to consider a solar array which presents several maxima of power, which correspond to the maxima of every string, and to force the solar array to operate where the maximum power can be extracted. This point is called the maximum maximorum point (MMP). This extension calls back to the basic MPPC process described in a previous patent and adds a new algorithm especially devoted to the search of this maximum maximorum point.

This algorithm requires the knowledge of all parameters defining each string of the solar array. In a first step, the MPPC algorithm which allows the computation of the electrical parameters of a string is recalled, and repeated for each string. The electrical characteristics v(i) of a solar array in its working conditions, has been developed by Tada and Carter in the eighties. It corresponds to the relationship tying the coordinates of the running point:

In this equation, n is the number of cells in series in each string of the m strings of the solar panel. The parameter A is called the form factor of the characteristics, isc and /^'R are the short circuit and the dark current of the cell and kT/q is a physical coefficient dependent on the temperature T and the material of the cell.

These parameters have to be known permanently in order to get accesses to the analytical form of the electrical characteristic (2.1 ) ruling the operation of the solar array at time t with the existing environmental conditions.

The coordinates of the MPP (VMPP JMPP) are the coordinates of the maximum of the power function P_SA(t). As the condition to have a maximum of a function corresponds to find the zero of the first derivative of this function, the following equation has to be solved:

It conducts to the identification of the MPP voltage VMPP. The MPP current IMPP is found by doing:

The knowledge of the MPP is available as soon as the parameters A, m isc et /^'R are known. In order to compute these three parameters, a mathematical system of 3 equations involving 3 unknowns has to be set. To define such a system, it is necessary to measure 3 operating points of the solar array electrical characteristics. They are

respectively at 0.6nvoc ,0.7nvoc and 0.8nvoc displayed as shown on fig 1 .

The 3 equations are, by letting a=q/nAT and neglecting 1 in front of exp(av)

If the points are selected such as:

Then

The steps needed to carry out computation of isc are: handling equations (2.4) it can be written (2.7):

To calculate a and /?the steps needed are: Handling equations (2.4) it can be written also

which gives

and finally

Now that the parameters of every string are identified, it is proceeded to carry out the identification of the maximum maximorum point (MMP) There are as many MPP as there are strings with different illumination and temperature conditions. The objective of this patent is to describe the process which allows the identification of the coordinates of the point which permits the extraction of the maximum power (MMP) of the solar array submitted to these variable environmental conditions. This point is the maximum of all maxima. It corresponds to the MPP of the electrical characteristics of the shadowed solar array. The process involved in the identification of the MPP of the solar array implies 3 steps. The first one is the identification of the 3 parameters defining the electrical characteristics of each string. They are, if there are n strings in the solar array, iscn, iRn and θη. To get access to these parameters, it is necessary to solve, as already described, a mathematical system of 3 equations with 3 unknown. This is possible by performing the measurements of 3 operating points for each string. Fortunately several measurements may be common for several strings, as shown on fig 2.

The second step is the knowledge of the analytical forms of the solar array characteristics. They have the following forms:

The last step is the computation of the MPP of the solar array. This point corresponds to the voltage which makes the first derivative of the power function equal to zero. It comes:

Finally the MPP satisfies the following relationship:

The computation of the MPP of the solar array may call for 2 procedures, as it has been already mentioned. The first one has been already developed. It is a mathematical process conducting to a rapid, accurate and reproducible result. The second method calls for a voltage sweep inside the limits defined by the positions of smallest and the highest MPP of the strings. The power is computed for each point until the voltage sweep is performed. The position of the MPP voltage is automatically available.

EXPLANATION OF THE FIGURES

Further characteristics and advantages of the invention will be explained in greater detail in the following detailed description of an embodiment thereof which is given by way of non-limiting example with reference to the appended drawings, in which: fig 1 : Measurement of 3 operating points of the i(v) characteristics fig 2: Electrical characteristics of the 4 strings of the solar array, fig 3: Electrical characteristics of the shadowed solar array fig 4: Circuit block diagram of a S3R power conditioning unit

PREFERRED EMBODIMENT OF THE INVENTION

When a solar panel is shadowed, every string receives a different illumination and presents different electrical characteristics i(v) and P(v). Let suppose a solar panel with 4 strings. Receiving a different illumination affects not only the short circuit string current isc but also the string temperature that are parameters a and IR. These electrical characteristics are represented on fig 2. It can be noticed that each string has a different maximum power.

Let consider a solar array with four identical strings. Let suppose all strings receive a different illuminations. String 1 is fully in sunshine and receives 1000W/m². Strings 2 and 3 and 4 receive respectively 600W/m², 400W/m² and

2

200W/m . Their parameters are respectively computed by solving a mathematical system of 3 equations as a result of 3 operating points measurements:

The power characteristics are given by (2.17):

On fig 3 are represented the electrical characteristics i(v) and P(v). The first one presents a discontinuity at 15.5V and the power characteristics presents 2 maxima, one at 17.5V for 155W and a second at 13.2V for 205W. A minimum point is available between both maxima at 15.5V for 145W. The process is able to identify the number of maxima and computes the position of the maximum maximorum point (MMP), that is the MPP of the solar array.The MPP is given by solving:

The solution of is performed using EXCEL tool. It comes vMPP:13.140 V iMPP: 15.725 A

dP/dv: 1 .05E-05 W/V PMPP: 206.63 W

Let's consider a situation of cloud shadowing. The most common case occurs when a cloud shadows part of the solar array which was normally illuminated

2

just before, let say with a solar flux of 1000W/m . All strings had the same parameters before the cloud passage, let say the ones indicated for Stringl from table 1 . All strings have the same MPP at 12.0V and DC current at 7.13A. This voltage is imposed by the power regulator and the current value is kept in the microprocessor memory.

As soon as the cloud reduces the sun flux, the string currents are affected. They are measured and these measurements define the first operating point Mi over the three necessary points to apply the MPPC procedure. The power regulator increases the operating voltage by steps of 5% amplitude until a 5% change of one string current is detected. The voltage and string current measurements define point M2. Another move of the operating point to detect a 5% change of a string current is executed. The measurements constitute the coordinates of the third point M3. The following table can be raised:

The parameters of each string are defined. They are detailed on table 1 hen the MPP coordinates of each string are computed. They are:

The computation of the MPP of the solar array may call for 2 procedures. The first one has been already developed. It is a mathematical process conducting to a rapid, accurate and reproducible result. The second method calls for a voltage sweep inside the limits defined by the positions of the smallest and the highest MPP of the strings. The power is computed for each point until the voltage sweep is performed. The position of the MPP voltage is automatically available.

It must be noticed that the solar array presents 2 maxima, one at 155W, the highest at 206.6W. The addition of all maxima would have given 240.2W.

After the clouds are vanished, and the sunlight is recovered, two cases are possible. Total illumination is returned and all strings are again with identical currents. The basic MPPC procedure applies using the 3 points procedure. The second case concerns a partial recovery of sunlight on some strings. The procedure which has been detailed in the previous chapter applies.

In order to carry out the processes describe it is necessary to use a power device which is a Sequential Switching Shunt (or Series) Regulator (S3R). It involves a non dissipative power cell connected to each string to force it to operate at a regulated voltage (the MPP is this application). This power cell insulates the strings from the users during a part of the switching period. On fig 4 is represented the block diagram schematic of an S3R, shunt type topology, involving 3 modules. The basic principle of such a shunt is to get an electronic switch shunting a solar panel module, in this case a FET, and to operate this switch in only two modes: open circuit or short circuit. The advantage is to eliminate power dissipation on all switches.

As these switches have only two operating states, the string module is, or in short circuit and the parameter isc is directly available, or in open circuit and automatically delivering power to the users via the series diode. In that case the coordinates of point Mi are also directly available. This unit is driven by a microprocessor which acts as the reference voltage VR for the regulator. This microprocessor drives also the inverter connected between the S3R and the Grid, indicating the amount of power to be transferred to the Grid.

When all the strings are fully illuminated, all current flowing through the diodes Dn are identical, except for the last channel which is submitted to a duty cycle around 5%. When a cloud is passing, the available power decreases and the S3R cannot regulate any longer the string voltage according to the reference voltage VR. The microprocessor measures each string current and the solar array voltage. These measurements constitute the data for point Mi. The microprocessor will reduce the power to be transferred to the Grid and decreases the reference voltage VR by voltage steps of 5% until the last channel of the S3R starts to regulate again. If one string current has decreases down to 5%, all string currents and regulator voltage are recorded and constitutes the second point M2. The same procedure applies to get the data for the third point M3. Then the procedure described in the previous chapter is developed to find the string parameters, compute their MPP and find the solar array MPP.

The essential nature of this invention is not altered by variations in materials, form, size and arrangement of the component elements, described in a non- restrictive manner, sufficient for an expert to proceed to the reproduction of thereof.

Claims

1 .- Process to operate continuously a solar array to its maximunn power in variable shadowing conditions wherein the electrical characteristics v(i) of a solar array in its working conditions, corresponds to the relationship tying the coordinates of the running point:

The coordinates of the Maximum Power Point (MPP) (VMPP JMPP) are the coordinates of the maximum of the power function P_SA(t) which corresponds to find the zero of the first derivative of this function, the following equation has to be solved:

It conducts to the identification of the MPP voltage VMPP. The MPP current ΪΜΡΡ is found by doing:

Characterized in that the process comprises three steps:

- The first one is the identification of the 3 parameters defining the electrical characteristics of each string, ISCn, iRn and a_n. being necessary to solve, a mathematical system of 3 equations with 3 unknown parameters. This is possible by performing the measurements of 3 operating points for each string.

- The second step is the knowledge of the analytical forms of the solar array characteristics. They have the following forms:

- The last step is the computation of the MPP of the solar array. This point corresponds to the voltage which makes the first derivative of the power function equal to zero. It comes:

Finally the MPP satisfies the following relationship

2.- Process to operate continuously a solar array to its maximum power in variable shadowing conditions according to claim 1 , characterized in that, in order to solve the system of three equations with three unknown parameters the 3 equations are, by letting a=q/nAT and neglecting 1 in front of exp(av)

If the points are selected such as:

Then

3.- Process to operate continuously a solar array to its maximum power in variable shadowing conditions according to claim 2, characterized in that the steps needed to carry out computation of isc are: handling equations (2.4) it can be written (2.7):

Finally

4.- Process to operate continuously a solar array to its maximum power in variable shadowing conditions according to claim 2, characterized in that to calculate a and /?the steps needed are:

Handling equations (2.4) it can be written also:

which gives

and finally

5.- Process to operate continuously a solar array to its maximum power in variable shadowing conditions characterized in that when a cloud shadowing process starts the string currents are measured defining the first operating point Mi over the three necessary points to apply the MPPC procedure, then a power regulator increases the operating voltage by steps of 5% amplitude until a 5% change of one string current is detected; the voltage and string current measurements define point M2, another move of the operating point to detect a 5% change of a string current is executed. The measurements constitute the coordinates of the third point M3.

6.- Process to operate continuously a solar array to its maximum power in variable shadowing conditions wherein the electrical characteristics v(i) of a solar array in its working conditions, corresponds to the relationship tying the coordinates of the running point:

Characterized in that, the process comprises the steps of:

- identification of the 3 parameters defining the electrical characteristics of each string, ISCn, iRn and a_n. being necessary to solve, a mathematical system of 3 equations with 3 unknown parameters. This is possible by performing the measurements of 3 operating points for each string.

- calls for a voltage sweep inside the limits defined by the positions of smallest and the highest MPP of the strings. The power is computed for each point until the voltage sweep is performed. The position of the MPP voltage is automatically available

7.- Device to carry out a process to operate a solar array to its maximum power in variable shadowing conditions according to the process claims previously claimed characterized in that the device is a Sequential Switching Shunt (or Series) Regulator (S3R) which involves a non dissipative power cell connected to the strings to force them to operate at a regulated voltage wherein the power cell insulates the strings from the users during a part of the switching period, being an electronic switch shunting a solar panel module, and to operate this switch in only two modes: open circuit or short circuit, and additionally the unit is driven by a microprocessor which acts as the reference voltage VR for the regulator, the microprocessor drives also the inverter connected between the S3R and the Grid, indicating the amount of power to be transferred to the Grid.