Isolated PV systems do not need a connection to an electrical network and their operation is independent or autonomous from said network.

Applications that are currently being implemented the most are small installations for lighting houses that are not reached by the general network, pumping, various agricultural facilities, signaling, hostels, campsites, shelters, summer and weekend chalets.

The criterion followed in isolated PV systems sizing is not so much to produce maximum energy but rather the concept of reliability appears (to ensure the proper functioning of the system, ensuring that failures are minimal).

Sizing an isolated photovoltaic system requires 7 steps:

1. Estimation of electrical load (electrical consumption)

We must know power of each element of consumption and the estimated time of use. Normally the calculation is made using W / h as the unit of energy.

To estimate these values we can consult following link

2. Estimation of solar energy available

Hm is the energy in kWh that affects a square meter of horizontal surface on an average day of month m. From the corresponding table the value is obtained in MJ / m2 (mega joules / m2).

The conversion must be carried out and expressed in Wh / m2 or kWh / m2. Being 1 MJ at 277.77 Wh or 0.277 kWh.

To estimate these values we can consult following link

3. Battery sizing

To define accumulator size, you must set N (Days of autonomy). It is the number of consecutive days that in the absence of Sun, accumulation system is able to meet consumption, without exceeding maximum discharge depth of the battery.

Having identified N and knowing the total energy required Et (final electricity consumption) in a period of 24 hours, we are going to calculate the real energy Er that the modules must contribute to the chosen battery (which will have a maximum admissible discharge depth pd).

The daily energy Er must take into account the different losses that exist:

Er = Et / R

Where R is a global factor of installation performance, whose value will be:

R = 1 – [(1-kb-kc-kv) ka. N / pd] – kb – kc – kv

kb: coefficient of battery performance. It varies between 0.05 (if there are no intense discharges) and 0.1 (for more unfavorable cases).
ka: self-discharge coefficient. If the data does not appear on the battery’s technical sheet, it can be estimated at 0.005 (0.5% daily).
kc: loss coefficient in the converter. If the system does not incorporate an inverter, it is zero. It ranges from 0.2 for sine wave inverters to 0.1 for square wave inverters.
kv: coefficient of other losses. It is usually estimated at 0.15 and 0.05 if we have already considered the performance of each device when calculating consumption.

Once R is calculated and Er obtained, we proceed to determine the useful capacity Cu of the battery. The battery must be able to accumulate the energy to be supplied throughout this period:

Cu = Er. N

To go from Wh to Ah, we will divide Cu by the nominal battery voltage (usually 12 V or 24 V).

Now we calculate the maximum nominal capacity C assigned by the battery manufacturer. These capacities will be assigned for temperatures between 20º and 25º C.

C = Cu / pd

With these data, the batteries offered on the market will be selected that most closely approximates the nominal capacity C obtained.

To estimate these values we can consult following link

4. Dimensioning of modules area

Energy originating in modules that must reach the accumulator (Er) suffers losses originated by the regulator, which are estimated at approximately 10%; therefore the daily amount of energy to be produced by the Ep modules is:

Ep = Er / 0.9

From the following formula we will calculate the HSP (hours of peak sun or hours of sun at an intensity of 1000 W / m2), starting from H expressed in MJ (1 kWh = 3.6 MJ):

HSP = 1 / 3.6k. H (MJ) = 0.2778 k. H

k is the correction factor for modules inclination according to the latitude of installation location.
H is the average daily radiation of each month expressed in MJ / m2.

To access these values we can consult following link

As we have already said, we must base ourselves on the most unfavorable month and also correct according to area climatological factors (clean atmosphere or mountain area = 1.05; area with pollution = 0.95; area with fog = 0, 92).

The ideal orientation is always towards equator and to determine the inclination we can follow recommendations in PV modules support structure post.

To calculate modules number we will use the following formula:

NM = Ep / 0.9. Pp. HSP

Pp is nominal (peak) power of chosen modules. The most suitable modules combination for the installation will be selected (price, available space, load to satisfy, etc.).

It is multiplied by 0.9 to consider possible additional losses that can cause modules dirt, reflection, etc.

If result is not a whole number, it will be rounded to the higher unit if decimal is equal to or greater than 0.5 and lower if it is less than 0.5.

Knowing the total modules number of PV generator and battery nominal voltage, which coincides with installation nominal voltage, it is possible to determine if it is necessary to group the modules in series and in parallel. The number of modules to be connected in series is calculated as follows:

Ns = VBat / Vm

Where:
Ns modules number in series per branch
VBat nominal battery voltage (V)
Vm nominal voltage of the modules (V)

And the number of branches in parallel to connect to supply the necessary power is given by:

Np = NM / Ns

Where Np is the number of modules to be connected in parallel branches.

5. Specify the controller or regulator

For sizing we can consult Solar charge controller post.

The installation will be dimensioned in such a way that the safety factor corresponds to a minimum of 10% between maximum power produced and that of regulator. The minimum possible number of regulators will be used.

To find the number of regulators Nr we will use the following equation:

Nr = Npp. ip / go

Being:
Npp the number of modules in parallel.
ip the peak intensity of the selected module.
go the maximum intensity that the regulator is capable of dissipating.

6. Sizing of the inverter

When sizing the inverter, the power demanded by the load made up of AC devices will be taken into account, so that an inverter will be chosen whose nominal power is slightly higher than the maximum demanded by the load.

For inverter sizing if PV systems has AC devices we can consult Solar converter post.

7. Choice of cable section

To select the cable section, the recommendations in the section Other elements (Wiring) post will be taken into account.

The sizing of the wiring constitutes one of the tasks in which special attention must be paid, since whenever there is consumption there will be losses due to voltage drops in the cables.

We can consult Solar wiring post.

This is an extract of contents included in Technical-Commercial Photovoltaic Solar Energy Manual and Sopelia e-learning training.

All you need is Sun. All you need is Sopelia.

There are two modes of on-grid connection:

– User continues to buy the electricity they consume from distributor at the established price and also owns an electricity generating system that can bill the kWh produced at a higher price.

– In Self-consumption or “Net Metering” the system will be able to inject energy into the network when its production exceeds self-consumption, and extract energy from it otherwise.

A 1.5 kWp system occupies about 22 m2 of roof (12 m2 of modules net surface) and will feed as much energy to the grid as that consumed by a small house throughout the year.

Estimation of energy produced by an on-grid PV system we will carry out is a simple prediction that consists of mere multiplication of an irradiation value by another of peak power that usually leads to estimates that are far from system real behavior.

An approach to more exact calculations should consider different factors that influence the useful energy generation process (PV generator location, temperature variations, shadows, maximum available power, second-order phenomena, inverter characteristics, etc.).

Whatever procedure adopted, we should try to combine simplicity with precision.

When calculating an on-grid PV system, following conditions must be taken into account:

1- System nominal power (kWp)

In practice, it will be established based on available surface area, investment to be made and amount of solar electricity to be generated.

Once module power to be used is determined, Wm, we multiply it by modules number to be installed Nm to obtain system peak nominal power Pmp:

Wm. Nm = Pmp

2- Electric energy to generate

The energy that could be obtained for each month can be calculated using the following expression:

Em = km. Hm. Pmp. PR. nm / GCEM

Where:

Em is solar energy production of month m in kWh.

km is correction factor to be applied due to modules inclination for month m (its values for northern hemisphere can be accessed in Censolar tables and at http://www.cleanergysolar.com/2011/09/15/tutorial-tables-correction-factor-of-k/) according to latitude of system location.

Hm is energy in kWh that affects a square meter of horizontal surface on an average day of month m. From the corresponding table the value in MJ / m2 (mega joules / m2) is obtained. The conversion must be carried out and expressed in kWh / m2.

To obtain the average daily radiation of each month expressed in MJ / m2 anywhere in the world, we can consult Opensolar DB.

The monthly mean daily irradiation can also be obtained from renowned databases such as NASA http://eosweb.larc.nasa.gov/sse or Joint Research Center [JRC], http://sunbird.jrc.it/pvgis /pv/imaps/imaps.htm Institute for Environment and Sustainable Renewable Energies, Ispra (Italy).

To convert from MJ to Wh or kWh we use the following equivalence:

1 MJ = 106 J = 0.277 kWh = 277.77 Wh

Pmp is the peak power of the generating field expressed in Kwp.

PR is the system energy performance factor or performance ratio defined as system efficiency in real working conditions. In practice, PR = 0.8 is usually taken

nm is number of days in month considered.

GCEM = 1kW / m2 CEM means Standard Measurement Conditions universally used to characterize solar generators, which as we have already seen are equivalent to: Solar irradiance: 1000 W / m2; Spectral distribution: AM 1.5 G; Cell temperature: 25 ° C.

Estimate of the energy injected annually into network will be obtained by adding the energy values Em for each of the twelve months of the year.

The key element in a grid-connected system is the inverter, which ensures that the circuit-module-grid coupling is perfect, safe and efficient.

This content was extracted from the Commercial Technical Manual of Photovoltaic Solar Energy and is part of Sopelia Solar e-learning.

All you need is Sun. All you need is Sopelia.

Coupling of two or more modules in series produces a voltage equal to the sum of individual voltages of each module, keeping the intensity unchanged.

In parallel connection, it is the current that increases while voltage remains the same.

The most common is to select modules of desired voltage (those of 12 V are the most used) and combine them in parallel so that the total intensity (and therefore resulting power) is necessary to satisfy the electrical demand.

Interconnecting modules must have the same i-V curve to avoid decompensation.

If in a group of modules connected in series one of them fails (due to failure or shade), this module becomes a resistive load that will hinder or prevent the passage of the current generated by the other modules in the series. The module in question could be totally damaged.

To prevent this situation, modules connected in series are equipped with a by-pass or bypass diode, connected in parallel between their terminals. This element provides an alternative path to current generated by the other modules in the series.

There are different types of configurations that respond to systems characteristics and especially to load type. Most common ones are detailed below:

• Modules directly connected to a load
It is the simplest system. Photovoltaic generator connects directly to the load, normally a direct current motor. It is used for example in pumping water. In absence of batteries or electronic components, reliability increases but it is difficult to maintain efficient performance throughout the day.

• Modules and battery
This setting can be used to replenish self-discharge of a battery or in small power rural electrification systems. One or two modules connected in parallel are usually used to achieve the desired power.

• Modules, battery and regulator
In this configuration, photovoltaic generator is connected to a battery through a regulator so that it is not overcharged or reaches an undesired depth of discharge. Batteries supply loads in direct current.

• Modules, battery, regulator and inverter
When AC power is required, an inverter will be incorporated into the scheme of previous configuration. Power generated in the photovoltaic system can be completely transformed into AC or DC and AC loads can be simultaneously supplied.

• Network connected systems
Grid-connected photovoltaic systems are made up of a photovoltaic generator that is connected to the conventional electrical grid through an inverter.

There may be two cases:

– The system injects energy into the network when its production exceeds self-consumption, and extracts energy from it otherwise.
– The system only injects energy into the network.

Fundamental difference between an isolated photovoltaic system and those connected to the grid consists in the absence, in the latter, of the battery and charge regulation.

The inverter, in grid-connected systems, must be in phase with the grid voltage.

Here are some examples of photovoltaic systems:
– Centrals connected to network with subsidy production.
– Microwave and radio repeater stations.
– Villages electrification in remote areas (rural electrification).
– Medical facilities in rural areas.
– Electric current for country houses.
– Emergency communication systems.
– Environmental data and water quality surveillance systems.
– Lighthouses, buoys and maritime navigation beacons.
– Pumping for irrigation systems, drinking water in rural areas and watering holes for livestock.
– Beaconing for aeronautical protection.
– Cathodic protection systems.
– Desalination systems.
– Recreational vehicles.
– Railway signaling.
– Systems for charging ship accumulators.
– Power for spaceships.
– SOS posts (road emergency telephones).
– Parking meters.
– Recharge of scooters and electric vehicles.

This content was extracted from the Commercial Technical Manual of Photovoltaic Solar Energy and is part of Sopelia Solar e-learning.

All you need is Sun. All you need is Sopelia.