Schematics of a hybrid system

A stand-alone power system (SAPS or SPS), also known as remote area power supply (RAPS), is an off-the-grid electricity system for locations that are not fitted with an electricity distribution system. Typical SAPS include one or more methods of electricity generation, energy storage, and regulation.

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Electricity is typically generated by one or more of the following methods:

Storage is typically implemented as a battery bank, but other solutions exist including fuel cells. Power drawn directly from the battery will be direct current extra-low voltage (DC ELV), and this is used especially for lighting as well as for DC appliances. An inverter is used to generate AC low voltage, which more typical appliances can be used with.

Stand-alone photovoltaic power systems are independent of the utility grid and may use solar panels only or may be used in conjunction with a diesel generator, a wind turbine or batteries.[1][2]

Types

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The two types of stand-alone photovoltaic power systems are direct-coupled system without batteries and stand alone system with batteries.

Direct-coupled system

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The basic model of a direct coupled system consists of a solar panel connected directly to a dc load. As there are no battery banks in this setup, energy is not stored and hence it is capable of powering common appliances like fans, pumps etc. only during the day. MPPTs are generally used to efficiently utilize the Sun's energy especially for electrical loads like positive-displacement water pumps. Impedance matching is also considered as a design criterion in direct-coupled systems.[1][3]

Stand alone system with batteries

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Schematic of a stand-alone PV system with battery and charger

In stand-alone photovoltaic power systems, the electrical energy produced by the photovoltaic panels cannot always be used directly. As the demand from the load does not always equal the solar panel capacity, battery banks are generally used. The primary functions of a storage battery in a stand-alone PV system are:

• Energy Storage Capacity and Autonomy: To store energy when there is an excess available and to provide it when required.
• Voltage and Current Stabilization: To provide stable current and voltage by eradicating transients.
• Supply Surge Currents: to provide surge currents to loads like motors when required.[4]

Hybrid system

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The hybrid power plant is a complete electrical power supply system that can be easily configured to meet a broad range of remote power needs. There are three basic elements to the system - the power source, the battery, and the power management center. Sources for hybrid power include wind turbines, diesel engine generators, thermoelectric generators and solar PV systems. The battery allows autonomous operation by compensating for the difference between power production and use. The power management center regulates power production from each of the sources, controls power use by classifying loads, and protects the battery from service extremes.[5][6]

System monitoring

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Monitoring photovoltaic systems can provide useful information about their operation and what should be done to improve performance, but if the data are not reported properly, the effort is wasted. To be helpful, a monitoring report must provide information on the relevant aspects of the operation in terms that are easily understood by a third party. Appropriate performance parameters need to be selected, and their values consistently updated with each new issue of the report. In some cases it may be beneficial to monitor the performance of individual components in order to refine and improve system performance, or be alerted to loss of performance in time for preventative action. For example, monitoring battery charge/discharge profiles will signal when replacement is due before downtime from system failure is experienced.[7]

IEC standard 61724

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IEC has provided a set of monitoring standards called the "Standard for Photovoltaic system performance monitoring" (IEC 61724). It focusses on the photovoltaic system's electrical performance and it does not address hybrids or prescribe a method for ensuring that performance assessments are equitable.[8]

Performance assessment

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Performance assessment involves:

• Data collection, which is a straightforward process of measuring parameters.
• Evaluation of that data in a manner that provides useful information.
• Dissemination of useful information to the end user.[7]

Load related problems

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The wide range of load related problems identified are classified into the following types:

• Wrong selection: Some loads cannot be used with stand-alone PV systems.
• House wiring: Inadequate or low quality wiring and protection devices can affect the system's response.
• Low efficiency: Low efficiency loads may increase energy consumption.
• Stand-by loads: Stand-by mode of some loads waste energy.
• Start-up: High current drawn by some loads during start-up Current spikes during the start-up can overload the system temporarily.
• Reactive power: The circulating current can differ from the current consumed when capacitive or inductive loads are used.
• Harmonic distortion: Non-linear loads may create distortion of the inverter waveform.
• Mismatch between load and inverter size: When a higher rated inverter is used for a lower capacity load, overall efficiency is reduced.[9]

Gallery

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• Solar powered parking meter

See also

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References

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stand-alone inverter

A stand-alone inverter is an electrical inverter that converts DC power stored in batteries to AC power that can be used as needed. Selecting an inverter for your power system based on the maximum load you will be powering, the maximum surge required, output voltage required, input battery voltage and optional features needed. High quality stand-alone inverters are available in sizes from 100 watts, for powering notebook computers and fax machines from your car, to 8000 watts, for powering an entire house or small commercial operation. The size of an inverter is measured by its maximum continuous output in watts. This rating must be larger than the total wattage of all of the AC loads you plan to run at one time. The size of the inverter can be minimized if the number and size of the AC loads is kept under control. Wattage of most AC loads can be determined from a tag or label on the appliance, usually located near where the power cord enters, or from the owner's manual. If the inverter is expected to run induction motors, like the ones found in automatic washers, dryers, dishwashers and large power tools, it must be designed to surge, or deliver power many times its rating for short periods of time while these motors start.

 

Stand-alone inverters are available with three basic power output waveforms: square wave, modified square wave (sometimes called modified sine wave), and pure sine wave. Synchronous inverters and utility companies deliver a pure sine wave.

 

Square wave inverters have the lowest cost and efficiency. Modified square wave output are an economical choice in power systems where waveform is not critical. Their high surge capacity allows them to start large motors while their high efficiency makes them economical with power when running small loads like a stereo or a small light. They can power most lighting, televisions, appliances and computers very well. However, this type of inverter may destroy some low cost rechargeable tools and flashlights, and their waveform will not allow many laser printers, copiers, light dimmers, and some variable speed tools to operate. Some audio equipment will have a background buzz that may be annoying to music connoisseurs.

 

Sine wave inverters have a slightly higher cost, but they can operate almost anything that can be operated on utility power. They range in size from 150 watts for small applications to 200,000 watts that can run a small village.

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