Stand Alone PV System
A Stand Alone Solar System
A free standing or Stand Alone PV System is made up of a number of individual photovoltaic modules (or panels) usually of 12 volts with power outputs of between 50 and 100+ watts each. These PV modules are then combined into a single array to give the desired power output. A simple stand alone PV system is an automatic solar system that produces electrical power to charge banks of batteries during the day for use at night when the suns energy is unavailable. A stand alone small scale PV system employs rechargeable batteries to store the electrical energy supplied by a PV panels or array.
Stand alone PV systems are ideal for remote rural areas and applications where other power sources are either impractical or are unavailable to provide power for lighting, appliances and other uses. In these cases, it is more cost effective to install a single stand alone PV system than pay the costs of having the local electricity company extend their power lines and cables directly to the home.
A stand alone photovoltaic (PV) system is an electrical system consisting of and array of one or more PV modules, conductors, electrical components, and one or more loads. But a small-scale PV system does not have to be attached to a roof top or building structures for domestic applications, they can be used for camper vans, RV’s, boats, tents, camping and any other remote location. Many companies now offer portable solar kits that allow you to provide your own reliable and free solar electricity anywhere you go even in hard to reach locations.
Simplified Stand Alone PV System
While a major component and cost of a stand alone PV system is the solar array, several other components are typically needed. These include:
- Batteries: Batteries are an important element in any stand alone PV system but can be optional depending upon the design. Batteries are used to store the solar-produced electricity for night time or emergency use during the day. Depending upon the solar array configuration, battery banks can be of 12V, 24V or 48V and many hundreds of amperes in total.
- Charge Controller: A charge controller regulates and controls the output from the solar array to prevent the batteries from being over charged (or over discharged) by dissipating the excess power into a load resistance. Charge controllers within a stand alone PV system are optional but it is a good idea to have one for safety reasons.
- Fuses and Isolation Switches: These allow PV installations to be protected from accidental shorting of wires allowing power from the PV modules and system to be turned “OFF” when not required saving energy and improving battery life.
- Inverter: The inverter can be another optional unit in a stand alone system. Inverters are used to convert the 12V, 24V or 48 Volts direct current (DC) power from the solar array and batteries into an alternating current (AC) electricity and power of either 120 VAC or 240 VAC for use in the home to power AC mains appliances such as TV’s, washing machines, freezers, etc.
- Wiring: The final component required in and PV solar system is the electrical wiring. The cables need to be correctly rated for the voltage and power requirements. Thin telephone or bell wire will not work!
Batteries are an important element and the heart of any stand alone solar power system, whether that is one using a large array of panels to power a home or a small pico solar system used to power the garden, shed or fish pond.
Batteries are needed because of the fluctuating nature of the output being delivered by the PV panels or array. They also convert the electrical energy into stored chemical energy for use when the solar array is not producing power. During the hours of sunshine, the PV system is directly fed to the load, with excess electrical energy being stored in the batteries for later use. During the night, or during a period of low solar irradiance, such as a cloudy, rainy days, energy is supplied to the load from the battery.
So battery storage allows a stand alone PV system to be run when the solar panels are not producing enough energy on their own with the battery storage size tied to the electrical usage. There are basically two types of batteries used for solar energy storage: deep cycle batteries and shallow cycle batteries.
Deep cycle lead acid batteries are generally used to store the solar power generated by the PV panels, and then discharge the power when energy is required. Deep cycle batteries are not only rechargeable, but they are designed to be repeatedly discharged almost all the way down to a very low charge.
Comprised of solid lead plates, it’s not uncommon for deep cycle batteries to be emptied to as much as 20% of their total capacity before energy ceases flowing from the battery or the charge controller disconnects them from the solar system. Deep cycle solar batteries are used in most electrically powered vehicles such as golf carts and forklift trucks.
Deep cycle batteries are ideally designed for storing energy generated by a stand alone PV system and then being drawn upon for power on a consistent, daily basis.
A charge controller, also known as a charge regulator, is connected in between the solar panels and the batteries. The charge controller ensures that the maximum output of the solar panels or array is directed to charge the batteries without over charging or damaging them. They operate automatically, with most commercially available charge controllers having a digital display to show how much power has been created at any time, the state of charge of the batteries and programmable settings to discharge the batteries into a resistive dummy load to minimise the chances of sulphation of the battery cells extending the battery life.
In some low voltage stand alone systems, the 12 or 24 volts battery power can be used directly, but this necessitates the use of home appliances and lighting designed for low-voltage DC. The use of batteries allows the system to produce usable power even if there is not enough light for the PV cells to operate. A “stand alone PV system” of this type offers independence from the electricity grid and power companies. However, the batteries will eventually discharge if used for long periods or there is no backup power source so stand alone systems include a small gas or diesel generator for extended no-sun periods or to recharge the batteries when they fall below a 60 to 80 percent depth of discharge.
Simple stand along DC systems for camping, camper vans, trailers, tents, etc are generally the cheapest and most popular of solar PV systems as they require no inverter or controller and often have small photovoltaic arrays for direct lighting use. They are often used on locations that have only occasional or light use. They frequently use a small photovoltaic array to charge a small battery only. During periods of infrequent use, the majority of the power is provided by the battery.
Newer low voltage solar technologies have been implemented in a wide variety of lighting applications. Street lights, security lights, solar garden lights and car park lamps can all be designed with small, built-in solar arrays producing a complete stand alone PV system. Exposed to the sun all day, these lights can retain their electrical charge to keep lit all night long. Electric road signs can take advantage of solar panels in the same way, although vital street and traffic signs on major roads and motorway’s also have alternate sources of power as backup.
Stand alone AC systems on the other hand use an inverter (not connected to the electrical grid), charge controller, batteries, fuses for protection and related wiring. Stand alone AC systems are used in remote areas where the electric grid is either non-existent or to expensive to maintain. The batteries provide power to the inverter that produces the required 120 VAC or 240 VAC in order to be utilised by household AC appliances.
This is typical of most domestic or home photovoltaic energy systems. There are two types of inverters: sine wave and non-sine wave inverters. Non-sine wave inverters are cheaper and can be used in stand alone systems for non-critical power requirements like lighting, power tools and pumps for pumping water etc as their output waveform is non-sinusoidal.
Small-scale PV systems are used in many different environments for different situations providing off-grid power to a remote or rural area. Their versatility makes them ideal for any area that receives enough sunlight to make the system feasible. But there are a few factors that may have an effect on the user’s decision to use PV as the power source. The advantages and disadvantages of a stand alone PV system must be taken into account.
First of all, stand alone solar power requires the sun. If the area does not receive a generous amount of direct sunlight each day the photovoltaic system may not be able to produce enough energy when needed or to charge the batteries. Excessive shading from surrounding objects and cloud cover are two things that can affect the amount of direct sunlight that strikes the solar panels so identifying potential shading areas, location and orientation of the PV panels or array are important factors to consider.
Other factors include: sufficient land/area space available, average wind speed, system budget and importantly system efficiency. For example, system efficiency equals (power out)/(power in), the overall system efficiency is the product of component efficiencies, so a solar photovoltaic panel may be capable of delivering 100W peak power into the system, but due to losses in the cabling, inverter, controller etc, the PV system may only provide 60 watts or 60% of its capacity at the output with the rest being lost.
Important factors in having a stand alone PV system
Firstly, you need to become very aware of how and when you use electricity. Solar panels only create electricity while the sun is shining on them so it may be necessary to store enough electricity to get you through one or two days of cloudy weather. In this case solar electricity becomes a valuable resource, you will not want to live without it, but you will not want to waste it, either. Try reducing energy demand through energy efficient measures.
Purchasing energy saving appliances and LED lights, for example, will reduce your electrical demand and allow you to purchase a smaller stand alone PV system to meet your actual energy needs. Energy efficiency allows you to start small and then add on as your energy needs increase.
Secondly, while a stand alone PV system is not a complicated system to install or run compared with other forms of off grid electrification devices, wind turbines, hydro-electric etc, solar PV systems still require regular maintenance that is not normally associated with standard grid connected mains power. You may want to become familiar with how your stand alone solar power system works, and what kind of daily or weekly maintenance is required.
All the systems components have to be checked and cleaned on a regular basis to make sure that the system is running optimally and like many other off grid systems, PV systems require some basic electrical knowledge in order to be able to install and maintain them in an effective manner and to diagnose any problems so become an expert of your system.
There are many advantages of a stand alone PV system some include low maintenance, low upkeep cost, no waste or byproducts, and easy expansion by using multiple solar panels and batteries. The disadvantages include high initial investment, especially for the photovoltaic panels and deep cycle lead acid batteries, reliance on the sun, and the possible danger from battery acid and fumes associated with most forms of renewable energy.
In the next tutorial about solar power we will look at the advantages of a Grid connected PV system compared to an off-grid installation. Grid connected PV systems are permanently connected to the electrical utility grid using a high quality inverter allowing the electric company to pay you if you generate more electricity than you consume.