Planning a Stand Alone PV System

Stand alone or autonomous systems are not connected to the grid. Some stand alone systems known as PV-hybrid systems or island system, may also have another source of power, wind turbine, bio-fuel or diesel generator, etc.

A stand alone system varies in size and type, but 20Wp - 1KWp are quite common. The stand alone system is also known as an off grid system.

Off-Grid systems (standalone) use a photovoltaic system to supply electricity to a consumer unit directly or via a 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).

Stand-Alone System Design Procedure

This example deals with the design of a stand-alone PV system for powering a remote CCTV transmission system.

Tare a few steps that need to be taken when designing a stand-alone system and we recommend the following:

1. Determine the Load
LOAD: - CCTV camera. It draws an average of 25W for 24 Hrs per day at 24V DC. The current draw is 2.09A.

- Load calculation can be down in two ways: either calculated on a daily basis or weekly. Regardless of which you choose you need to be as accurate as possible.

1.Decide Battery Storage
To be able to handle the CCTV load and allowing for 5 days of battery storage, we require a battery capacity of:

Battery Storage is usually expressed in Amp hours. However, it can be given in Watt hours.

For the above load the battery storage (Ah) would be calculated as: 2.09 A x 24 h x 5 days = 250.8 Ah

• Watt-Hours = Volts x Amphours
• Example: 12V @ 250.8Ah = 12 x 250.8 = 3009 Watt hours or (3.9kWh)

2.Determine solar radiation for the site location

Let's assume the chosen site is in Jersey. The table shows the average monthly readings of direct solar radiation falling on a horizontal plane in Jersey. It should be noted that for locations north of the tropics, above 23 degrees north) is usually used. Take into account the design month - the month when there is the least sun and the most demand.

From the table we can see that for January the total insolation falling on the array is therefore 841 Wh/m2. For July it is 5648 Wh/m2 and for Dec 699 Wh/m2.

3. Approximation of Array Size - If the system is going to be used all year round and the energy requirement is fairly constant then the design month will be December or January, that is when the weather is at its worst.

Array Size (Wp) = Daily energy requirement in Watt Hours [Wp] / Average Daily PEAK SUN HOURS in the design month / System Effeciency.

Array Size = [25W x 24h] / 2.5 / 0.65

Array Size [Wp] = 600Wh / 2.5 / 0.65 = 369.2Wp

Components of a stand alone solar PV system.

Solar Panels (PV) Modules

The DC electricity produced by the solar panel or module(s) is used to charge batteries via a solar charge controller. Any DC appliances that are connected to the battery will need to be fused.  DC lights are normally connected to the charge controller. Any AC appliances are powered via an inverter connected directly to the batteries. NOTE: inverters used in grid tie and stand alone systems are different and should not be interchanged.

Most stand alone pv systems need to be managed properly. Users need to know the limitations of a system and tailor energy consumption according to how sunny it is and the state of charge (SOC) of the battery.

Configuration

The solar panels need to be configured to match the system DC voltage, which is determined by the battery. System voltages are typically, 12V DC and 24V DC, larger systems will operate at 48V DC.

The operating voltage of a solar panel in a stand-alone system must be high enough to charge the batteries. For example, a 12V battery will require 14.4V to charge it. The solar panel must be able to deliver this voltage to the battery after power losses and voltage drop in the cables and charge controller and in conditions in which the solar cells operate at a high temperature. A solar panel with a Voc of about 20V is required to reliably charge a 12V battery.

Charge Controllers

A charge controller is designed to protect the battery and ensure it has a long working life without impairing the system efficiency. Batteries should not be overcharged and the function of the charge controller is to ensure that the battery is not over charged.

• Charge controllers are designed to function as follows:
• protect the battery from over-discharge, normally referred to as low voltage disconnect (LVD) that disconnects the battery from the load when the battery reaches a certain depth of discharge (DOD).
• protect the battery from over-charging by limiting the charging voltage - this is important with sealed batteries - it is usually referred to as high voltage disconnect (HVD).
• prevent current flowing back into the solar panel during the night, so called reverse current.

NOTE: controllers with MPP tracking will ensure that the solar modules operate at optimal rating and can increase output by 10% or more.

Batteries

The power requirements of stand alone pv systems are rarely in sync with the battery charging. Appliances and loads need to be powered when there is sufficient solar radiation, during overcast weather and during the night. Bad weather may last for several days and the daily charging and discharging of the batteries takes its toll on them. Batteries that are able to handle the constant charging and discharging are known as deep cycle batteries. Batteries need to have a good charging efficiency, low charging currents and low self-discharge.

Battery Ah Efficiency

The Ah efficiency of a battery describes the relationship between Ah that are put into the battery and the Ah that are taken out. Under ideal conditions a new deep-cycle battery would be 90% efficient.

Choosing the most appropriate battery

The important characteristics to look for are:

• capacity
• cycle life
• price / performance
• size and space requirements
• Ah efficiency
• self-discharge rate
• installation - vertical or horizontal
• environmental - will batteries be placed near water supplies or in wildlife parks etc

Cables and Accessories

Cables need to be UV resistant and suitable for outdoor applications. It is very important to keep power losses and voltage drop in the cable to a minimum. It is recommended that this be less than 3% between the the array and the batteries and less than 5% between the battery and DC loads.

Components of a stand alone solar pv system

Solar Panels		Charge Controller		Power Inverter		Mains Electricity
	Battery Bank