Designing Solar PV Power Systems

There are two types of solar installation used for generating electricity:

Off-Grid systems (standalone) use a photovoltaic system to supply electricity to a consumer unit directly or via a buffer storage unit (e.g. battery) independently of other energy sources. These systems are suitable for small devices and equipment not close to an electricity supply (e.g. street lighting, water pumps, radio and signal equipment).

Grid Connect systems are used when access to a mains electricity supply is available, but a photovoltaic electricity system supplements the electricity provided. The energy generated is either fed into the grid or consumed directly. thus reducing dependence on a mains electricity provider.

To build a grid-connected PV system, you will need the following components:

Solar Photovoltaic (PV) Modules
To achieve the required output from a photovoltaic system, you need to connect the solar (PV) modules to a solar generator. The solar modules are equipped with connecting boxes or prefabricated connector systems to connect to the electrical circuit. The PV modules are available in a range of sizes and output. They are usually framed with a protective aluminium profile to ensure stability.

DC connecting cables:
When solar modules are connected and working, direct currents of several hundred volts can be generated. For this reason, when DC currents exceed a protective low voltage of 120V you must use touch-proof wiring protected against short circuiting and accidental earthing. For outdoor PV installations, it is also very important that the materials used have high UV, ozone and temperature resistance. Accordingly, special solar cables with double insulation and excellent temperature/UV resistance are used for PV systems.

Inverters:
Inverters convert the direct current (DC) created by the solar modules into alternating current (AC) suitable for the grid. At the same time, they monitor and regulate the feeding of the current into the grid, automatically disconnecting the PV generator from the grid in the event of a fault or grid failure.

Off-grid standalone system

Solar Panels		Charge Controller		Power Inverter		Mains Electricity
	Battery Bank

Grid-connected PV installation

Solar Panels		Inverter		Mains Electricity
	Export Meter				Import Meter
			Low voltage mains power supply 230V

Variables and configuration

Location:
In the UK, global radiation varies between xxxx and xxx kWH/m2a depending on location. Solar radiation is also subject to extreme fluctuations with around two thirds energy supply occurring between April and September.

Orientation (azimuth angle):
The orientation of the roof is described using the azimuth angle, which is the angle of the roof surface measured from due south. The ideal roof orientation for solar installations is due south. If your roof faces in other directions you can achieve the same percentage solar contribution by enlarging the collector array.

Angle of inclination:
This is the angle of the tilted collector measured against the horizontal. It is determined by the pitch of the roof or the bracket used in flat roof installations. The ideal angle is between 25 and 35 degrees.

Energy Requirements:
The energy requirements are defined by the consumption of electrical energy during a particular period of time. It is described in kWh/a. personal energy requirements can be worked out from a utility bill. For a 4-person household in the UK, the average requirements is approx. 4000kWh/a.

Shade:
Avoid installing modules on roof areas that are likely to be shaded. Shade cast by aerials, cables, railings, chimneys etc, can significantly impact yield and should be avoided.

System Losses:
The solar modules generate direct current (DC), the amount of which depends on solar radiation levels. The energy generated by the solar modules is converted by an inverter and fed into the grid as alternating current (AC). During the process of transportation and conversion, some energy losses occur (e.g. in the inverter, on the clip connectors and on the cables). Compared to the rated output of the solar modules, you can expect the system capacity utilisation to be around 60-90%, depending on the system design and relevant conditions.

Solar generator output:
The size and output of the solar module or array is determined by various influencing factors and the energy yield required. With grid-connected systems, you can choose any solar generator output, as the energy generated is fed directly into the grid. The solar module or array output (rated output) should be approx, 10-20% above the AC rated output to ensure that the inverters are running at their ideal level even if the levels of solar radiation are low (e.g. diffuse light, cloudy conditions).

Inverter output:
The inverter output depends on the defined solar module or array output. It is established after consideration of the various factors (e.g. The size of the collector array is determined by various factors, the type of use and the required percentage solar contribution. The area on which the collectors are to be installed should not be to large.

Choosing a PV system
You can use two different criteria to hep you select a PV system:

• Configuration based upon energy requirements
• Configuration based on a surface area

You can find out how much solar radiation is received at your site location on the basis of the location zone. Call us for mode details on your location zone values.

Configuration based upon energy requirements
If your system configuration is based on the energy requirements, you work out the number of PV modules you need using the location, the angle of inclination and the desired energy yield. There are guide values available for some solar modules. Please call for more information.

You do need to know the type of solar module you want, orientation, location zone and Angle of inclination and then apply the formula below.

Example:

A customer would like to generate approx. 2000kWh of energy a year from a solar photovoltaic array

n modules (Qty) = Vtarget (kWh/annum)
Vmodule (kWh/annum)

Selecting the inverters
With the inverters, the modules are connected in series in a string. The number of modules per inverter depends on the input voltage range and transferable power. You can only connect a certain number of modules in series; the voltage of the solar modules (array) must not exceed or be less than that permitted for the inverter within the full possible temperature range. If the solar modules output is too great for the inverter output, the inverter limits how much energy is converted. This means energy is lost. If the solar modules output is too low, the inverter works inefficiently and does not convert the available solar output as well it should.

Circuit connection of solar modules
The solar modules can be connected in series or parallel. The voltage and current of the solar modules is added up accordingly. You can also use a combination of series and parallel connections.