HOW TO DESIGN A PHOTOVOLTAIC SYSTEM

Every time you hear more talk about renewable energy and more specifically about  solar energy ; if you browse the internet you find many web pages that tell you how to calculate a solar installation and maybe you get to understand something about its elements, but little else.

HOW TO DESIGN A PHOTOVOLTAIC SYSTEM

I have consulted some of them and I have realized that something clearer and simpler can be done  . Whenever I come across one, I want to try to explain it in another way and it is not that I know more than the other, but with a different approach it could be much clearer. Finally I have decided to write something about it, to see if I get it.

To start:

1-Site evaluation in the design of a photovoltaic system

The evaluation of the site is a review of the place where the photovoltaic system will be installed. Information is gathered about local conditions and factors that could affect the performance of the system.

Aspects to evaluate:

Location of the photovoltaic array

The photovoltaic array can be mounted on the ground, on roofs or other suitable support structures, spacing, applicable regulations, wind loads, accessibility for installation and maintenance must be considered, the main considerations to define its location can include the following :

• Area available for PV array.
• Size of the photovoltaic module to be used.
• Type of arrangement of the photovoltaic array.
• Review of the structural condition of roofs and roofs to support the fv.
• Clamping method and type of mounting structure to be used.
• How the system will be maintained (see mounting systems available).
• How close it is to other systems.
• Specific aspects that could compromise the safety and integrity of the user and maintenance personnel.

There are multiple alternatives and possibilities to locate a photovoltaic array, the specialists in design in photovoltaic systems will need to evaluate the conditions of the site with the available information and determine if the installation of the photovoltaic system is feasible.

Existence of objects or buildings that cast shadows on the PV array

Within the cities it is common that a photovoltaic arrangement can receive the projection of some shade and of some object such as water tanks, antennas, equipment or some nearby building, this is not a limitation for the installation of a photovoltaic system however it will be necessary to consider several factors so that the system works in the most efficient way possible to prevent failures and reduce the investment return of the same.

According to the conditions of shading on the site, it is necessary to choose a type of inverter technology that minimizes the effect of shadows in the generation of energy (see types of investors).

Using some tools and software, a shading analysis is performed to calculate with a high degree of confidence the amount of energy that we will receive with the conditions in the place where the photovoltaic array will be placed.

If the effect of the shadows significantly affects the performance of the system it will be necessary to evaluate changing the location, changing the orientation, changing the inclination or raising the height of the photovoltaic array.

To determine the optimal location of the photovoltaic system, it is advisable to carry out multiple analyzes by changing the different variables (location, orientation, inclination, height, type of module, type of inverter).

Orientation of the photovoltaic system

To receive the maximum amount of radiation available on the site, the photovoltaic array must be oriented towards the  solar window.

The more the photovoltaic array is exposed to the sun throughout the year without being shaded, the greater the energy it will produce, and the more cost-effective the photovoltaic system will be.

The angle of inclination of the photovoltaic array is the angle that is formed between the surface of the PV array and the horizontal plane. Generally, the greater the latitude of the site, the greater the inclination of the PV array to maximize energy production.

For Mexico, which is located in the northern hemisphere, the optimal orientation of the photovoltaic array in situations without shadow will be towards the south with an inclination of the arrangement equal to the latitude of the place.

In situations with shadows it will not always be optimal to orientate the photovoltaic array to the south with an inclination equal to the latitude of the site, in these cases it will be necessary to carry out several shading analyzes using multiple variables to determine the optimal orientation and inclinations.

Location of the system balance

The balance of the BOS system   (Balance of System) includes all the mechanical, electrical and hardware components used to assemble and integrate the main components of the photovoltaic system.

The electrical components of the system balance are used to drive, distribute and control the flow of electrical current in the system.

Considerations for BOS system balance locations must provide accessibility to the equipment for installation and maintenance.

When placed outdoors, some elements will be placed inside weatherproof cabinets, some other elements may already be certified to be placed outdoors, some equipment should avoid being exposed to direct sunlight or high temperatures when possible , and provide them with adequate ventilation.

2-We must know what the consumption will be

Letstart with the main points that must define: one is the daily consumption we may have, such as the lights we turn, we see the hours the TV … and another is how much maximum energy we will spend at any given time.

It is essential to clarify and define these two concepts well since they will allow us to establish the capacity of the batteries and calculate the power of our inverter respectively.

But hey, let’s go in parts and start at the beginning.

What elements should have the facility?

The list of the elements necessary to carry out a photovoltaic installation is the following:

1 – Investor.
2 – Solar panels.
4 – Batteries.
3 – Regulator

I’m going to tell you more information about each element:

Investor

To establish the power of the investor that we are going to buy I always ask the following: if you make the contract of light with a company, the first thing they ask you is: how many kilowatts (KW) do you want?

You can only answer this question yourself. What happens if they make a contract for, say, 1.5KW?

Well, as soon as the sum of the consumption that you are having at a certain moment exceeds this value, the automatic of the company will skip you; So if you turn on some light bulbs, plug in a mobile phone charger, music equipment and many other small things, nothing happens, but if you keep adding up, there may come a time when the automatic goes off.

Well, if this concept is clear you will already know what power your investor is going to be.

To finish clarifying the concept we will say the following: the choice of the inverter will cause the device to  jump from time to time, very often, or never  depending on how much power it has.

It is clear that if we want to use a dryer it is not convenient for us to have less than 1.5KW, since any dryer will easily be of this power or more (unless we use it without hot air).

The fact that we need to turn on the lights many hours a day, that we want to have the music always on or things like that do not interest us when it comes to establishing the power of the investor.

Naturally, if we buy a 5KW everything is solved …
But do you really think it’s the best?

There are a number of reasons why, although we can afford it, it is not convenient to do this. In our courses this point is of utmost importance since it is not a matter of “big horse walk or not walk”, rather to correctly size the entire installation. But power, it can be installed.

There are different types of investors, square wave or pure sine wave; In the  following article , we see the differences between one and the other. In any case, the second one is more advisable.

Batteries

For an inverter to work in principle, one or more batteries are necessary.
The inverter makes that we have the 220V in our house and in our plugs, and for this the capacity of the battery is not decisive: a small battery can operate perfectly a 5 KW inverter without any problem.

How much autonomy do I want to have in my installation?

I will try to explain it better.
We know well that  while there is sun there will be energy , and therefore our inverter can work even without a battery, yes, I said well, without a battery.

The point is that when the sun goes down, if we do not have a battery in which the energy accumulates, we would be left without light.

Consequently, if we have a small accumulator we will be able to extract energy for a short time, while if we have large accumulators we can have energy weeks and weeks even without sun …

Although then we have to fill the extracted energy, but we will see this in the following section of the solar panels.

The important thing is that the concept is clear,  little battery capacity equal to little autonomy  when there is no sun, and otherwise  much more capacity autonomy .

However,

How do we choose how many batteries do we need?

Simple, we add all the consumptions that we plan to spend in a day and we establish how many days of autonomy we want; This also depends on how much sunlight there is in your region, since the sun does not burn batteries. We give an example:

• 4 hours of led bulbs (20W) 4×20 = 80W / h
• 2 hours of music equipment (50W) 2×50 = 100W / h
• 3 hours of computer (100W more or less) 3×100 = 300Wh

Following your other needs without taking sporadic uses into account, such as the dryer, the toaster …, the final sum will be a certain number, for example 480Wh.

Now we have a small problem, and that is that the capacity of a battery is measured in Ah and not in Wh, what to do?

Using the ohm law we can perform the conversion and know how many Ah we would need to supply our installation for a day if there is no sun.

I (A) = P (W) / V (battery voltage, we consider 12V)

480/12 = 40Ah

Now we have to make a series of considerations, the most important is that using a battery that has only 40Ah would make it stay at zero in a day, which is not convenient.

Always try to ensure that the capacity of the battery does not drop more than 30-40% , otherwise, its life would be greatly reduced.

So, what to do? We simply double the amps. Well, we have 80Ah, a day without sun and the battery is at 50%, and if there is another day without sun? Well we would meet again with the same problem …

In short, it is about increasing this value until we get the autonomy we want and avoid the capacity of the battery to go down a lot.

For all this we are saying is very good to take into account a number of details, such as, for example, that  we use the energy when there is sun and avoid, as far as possible, use large night consumption , because while sun will suck current directly from the board, at night we will do it from the battery.

Regarding the choice between 12-24 or more volts, the general criterion is that for low consumption facilities, such as a weekend house, 12V is sufficient.

The higher the consumption, the higher the voltage; For the habitual residence 24V can be adequate and for a company where there is a lot of consumption it is even advisable 48V.

Obviously the inverter will be of the same input voltage.

Photovoltaic solar panels 

Nor is it the best solution. For this reason, solar panels have to be well dimensioned.

 If we have a low battery it is necessary to charge it as fast as possible, but you can not exceed it either, since it is not advisable to spend more than 20% of the battery capacity and ideally around 10%. 

The  comparison that I always make is that of a tank (accumulator) and a tap (photovoltaic plate). Although it is possible to fill a large water tank with a small tap, it would take weeks; The same can be said for a small plate and a large battery.

 And automatically the question arises: what if we put a lot of panels?

According to my experience, it can be 100W of plate per 100Ah of battery.

An important point is that the plates and batteries can be added on the fly according to the need.

For this reason it will be convenient to dominate the  connections in series and in parallel , depending on the installation.

Regulator

As the sun hits the solar panels, the battery is charged, but it usually happens that it reaches a load limit and does not want more amps and if there is no “regulating” device, never better, it deteriorates.

So it is not convenient to go beyond your maximum capacity.
The regulator does not only do this, but also gives more or less current depending on the situation, since the load is supplied with an electronic control of precision, all this to favor the useful life of the accumulator.

There are several types of regulators,  the minimum that you need to know are two things: that  the voltage  must be the same as the system and that  the amperage  should not be less than the total current given by the board to the battery.
If, for example, the plate gives 20A, the regulator should never be of less amperage.

Lately investors are being built that have it already incorporated, it is much more comfortable, of course.

Summing up

To summarize, on the one hand I have focused more on explaining concepts and not just giving data, and on the other I have to trust that a person who wants to make their own solar installation will be well advised and our workshops are designed for this.

In addition to delving into all the fields related to renewable energies, we explain everything you need to understand electricity and we focus on raising awareness of real needs, putting a touch of awareness in relation to the resources of our planet, that of being considered unlimited are becoming limited or even scarce.

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