By definition, a stand-alone Photovoltaic (PV) system is one that is not designed to send power to the utility grid and thus does not require a grid-tie inverter (but it may still use grid power for backup).
Stand-alone systems can range from a simple DC load that can be powered directly from the PV module to ones that include battery storage, an ac inverter, or a backup power supply.
They are typically used for low-power applications and are often used where power is otherwise unavailable, such as in certain rural areas and remote locations where the utility grid is not readily accessible.
Figure 1: A remote traffic sign with warning lights is an ideal application for a stand-alone solar power system.
Basic Stand-Alone PV Solar System
Stand-alone solar electric systems do not supply power to the electric utility grid but can use the grid as an input to back up the system. Solar electrical systems can be used to supplement grid power.
Grid-free systems do not have any input or output to the grid. By definition, all grid-free systems are stand-alone systems.
Stand-alone systems can have a DC or AC output, which is determined based on the load requirements. In general, stand-alone systems make sense for powering equipment that does not require a huge amount of power, such as lights or small appliances.
The most common output voltages for small systems are 12 V or 24 V, with 48 V and higher used with larger systems.
As in the case of any electrical system, national electrical codes and general electrical safety rules, including manufacturer’s recommendations for wire size, grounding, and required environment for the various components, need to be followed.
Depending on the application and the electrical power requirements for the load, most stand-alone PV systems include a battery for supplying power when there is little or no solar input.
For certain basic applications, such as an attic fan or some water-pumping applications, a battery backup is not required, saving the cost and maintenance of batteries.
Stand-Alone Solar PV System Configurations
Table 1 shows five configurations for stand-alone PV systems, in increasing system complexity. Variations of the configurations in Table 1 are common, so the table is the only representative of these systems.
The power output can range from less than 1 W for a small calculator to over 10 kW. Usually, systems that are larger than 10 kW are more effective as grid-tie systems, which is an electrical generating system that is connected to the electric utility grid (discussed in next Section).
Other optional components, such as solar tracking devices or various system-monitoring devices, can be added to any of these systems.
System monitoring can provide basic performance data for the system and may include power, energy and possibly service information or advanced diagnostics.
Poor performance can be attributed to a bad panel, rodent damage, dirt, leaves, or wiring problems; monitoring can pinpoint the problem area or panel.
Table 1 Configurations for Stand-Alone Solar PV Systems
|1||PV module and DC load.||DC ventilation fans, small water pumps such as circulating pumps for solar thermal water heating systems, and other DC loads that do not require electrical storage.|
|2||PV module, DC/DC converter (power conditioning), and DC load.||DC loads that require specific DC voltages but do not require storage, such as a charging station for certain electric vehicles or DC water pumps. This configuration is also useful for miniature applications such as calculators.|
|3||PV module, charge controller/battery storage, and DC load.||DC loads that require power even when there is no solar input, such as small yard lights, traffic warning lights, and buoy power, or mobile and remote power for recreational vehicles (RVs) and communication systems.|
|4||PV module, charge controller/battery storage, inverter, and DC and DC loads.||AC and DC loads, including appliances such as refrigerators and lights.|
|5||PV module, charge controller and battery storage (optional), inverter, supplementary generation, and DC and DC loads.||AC and DC systems where there is large seasonal variation in solar input. The supplementary power can be from any other power-generating system such as a wind generator, a gas generator, or the utility grid.|
Figure 2 shows representative block diagrams of the systems listed in Table 1. These diagrams are meant only as guides to demonstrate how typical system components are connected together.
System 1 represents the simplest system, which is composed of the PV module and a load. A system like this can supply power only when there is solar input, so applications are limited.
System 2 adds a DC/DC converter, which allows the designer to match the electrical load or different voltage requirements for better performance.
System 3 includes battery backup. With this system, a charge controller replaces the DC/DC converter; its main purpose is to regulate and limit charging current to prevent overcharging the batteries.
In a solar PV system, the charge controller also prevents draining the batteries back through the PV modules when they are needed for the load.
System 4 adds an inverter, which converts the DC output to AC for powering small appliances. The inverter is a basic battery-based inverter rather than the more expensive grid-tie inverter, which is required when connecting to a utility grid.
System 5 is a hybrid system that uses more than one module in parallel, so the outputs are combined in a combiner box (not shown).
A combiner box is a double-insulated box that allows several strings from modules to be connected together in parallel; it also houses fuses for the strings and includes surge and overvoltage protection from potential lightning strikes.
It may have load switches that allow system serviceability. A switch that disconnects the output to the inverter allows it to be disconnected from the DC side.
Note that a combiner box can be used in any of the previous systems, or the modules can be connected in series.
System 5 also adds a backup power source that can be switched in when the power from the solar system is low; a combiner box may be used to connect modules in parallel.
In this system, the battery backup may be reduced or eliminated depending on the requirements. The backup power can be a wind generator, an engine generator, or utility power.
Figure 2 Block Diagrams of Typical Stand-Alone PV Systems. The systems here are representative of different types; other configurations are possible.
Stand Alone Solar PV System Costs
Solar systems are generally evaluated on the basis of the cost of the system compared to conventional systems.
Frequently, the cost analysis of a system is done as a life-cycle cost, which means that all costs over the expected life of a system, including purchase price, operating costs, maintenance costs, any supplemental energy costs, and recycling costs, are factored into the total system cost.
Solar systems frequently have a high initial cost but lower maintenance cost and are almost free of energy costs, so the life-cycle cost can be more or less than conventional systems, depending on factors such as resource availability, government subsidies, and interest cost.
Lifetime costs are less if the product is designed to last in order to achieve the expected yield. Product reliability and safety is ensured with universally accepted standards and certifications; certification of modules and components is mandatory in some parts of the world.
The goal of certification from standard testing labs is to help manufacturers identify potential problems with modules and to avoid common failures such as moisture ingress, cracking, ground faults, hot spots, and other problems that can lead to premature failure in the field.
Safety testing and certification is viewed as a complementary component to product certification and addresses issues of preventing electrical shock, fire hazards, or personal injury.
In the United States, the National Renewable Energy Lab (NREL) has a program to accelerate stress testing in order to predict how and when a test unit might fail. Another program by the US Department of Energy is designed to gather data using simulated environmental stresses to determine reliability.
Stand-Alone Solar PV System Components
The heart of a solar electrical system is the PV module, which needs to be able to provide power for the loads in the system and to charge batteries when they are used for backup power.
The module selected depends on the load requirements and the batteries used. For a 12 V system, the PV module needs to provide about 20 V to charge batteries reliably. For a 24 V system, the PV module should provide 40 V.
When battery backup is used, a charge controller is needed. It protects the batteries from overcharging and switches to the battery backup when the PV module power is too low for the load.
In cases where there is reliable utility power, it may be used as a backup rather than batteries. For ac loads, an inverter is needed that changes the dc to ac.
Photovoltaic (PV) wire is a special type of stranded copper wire that is sunlight-resistant and dedicated for the interconnection of PV modules. In addition, it is designed to operate at elevated temperatures and is rated for wet locations.
Ground faults occur when current finds an alternate return path to the source; they can pose a serious safety hazard. When a ground fault occurs, normally grounded parts can become energized and present a shock hazard to anyone working on them.
When troubleshooting a ground fault, it is important to wear protective equipment, including safety glasses and insulating gloves rated for the highest possible voltage in the system. Safety boots are also recommended.
- What are three advantages of a stand-alone system?
- Would you use a backup battery to supply power for an attic ventilation fan? Explain your answer.
- What are some goals for certification of modules from standard testing labs?
- What are the advantages to evaluating a system with a computer code like HOMER?
- What is the purpose of the combiner box?
- What does GFPD stand for?
- Advantages are that they (1) can provide power in remote locations where the power grid is unavailable, (2) are less expensive in some cases than bringing in power from the grid, (3) can reduce the need to burn limited wood supplies in developing countries.
- An attic fan is needed to cool the attic primarily when the sun is shining, so a battery backup would be unneeded.
- The goal of certification is to help manufacturers avoid common failures with modules by identifying potential problems.
- The code enables the evaluation of multiple systems and options and provides a cost analysis of these systems.
- A combiner box is a junction box that allows several strings of modules to be connected together in parallel; it also includes surge and overvoltage protection from potential lightning strikes.
- Ground fault protection device