System Components and Configurations: Unit Summary, Definitions, Quiz / Answers (Guide to Photovoltaic Systems)

Summary

• The primary component common to all PV systems is the PV array.
• PV systems usually require means to store or condition PV power so that it can be used efficiently by loads.
• Energy storage systems balance energy production and demand.
• Batteries, particularly lead-acid types, are by far the most common means of energy storage in PV systems.
• Most PV systems require more battery capacity than can be supplied by a single battery, so batteries in PV systems are often connected together to form battery banks.
• Power conditioning components may be individual devices or may be combined into a single power conditioning unit.
• Inverters convert DC power from battery systems or arrays to AC power for AC loads or export to the utility grid.
• Charge controllers manage the charging of batteries from a DC power source, typically a PV array.
• Battery chargers are used when an AC power source, such as the utility grid or an engine generator, is avail able to provide supplemental battery charging.
• Since a PV array produces DC power, DC loads are used in many PV applications to avoid having to change the power to AC, simplifying the system.
• Balance-of-system (BOS) components are all of the remaining electrical and mechanical components needed to integrate and assemble the major components in a PV system.
• Besides the PV array, the electric utility grid is the most common source of electricity connected to PV systems.
• Other electrical systems may be interfaced with PV systems as additional sources of electricity, depending on the type of system and application requirements.
• Stand-alone PV systems are most popular for meeting small- to medium-sized electrical loads and are extensively used in remote off-grid areas or where extending the utility service is cost-prohibitive or impossible.
• In direct-coupled PV systems, the output of a PV module or array is directly connected to a DC load.
• A self-regulated PV system consists of only an array, battery, and load.
• Whenever loads are variable or uncontrolled, charge control is required to prevent damage to the battery from overcharge or over-discharge.
• Utility-interactive systems operate in parallel with and are connected to the electric utility grid.
• Bimodal systems can operate in either utility-interactive or stand-alone mode and use battery storage.
• Hybrid systems include an energy source other than an array and do not interact with the utility.
• Common energy sources used in hybrid systems include engine generators, wind turbines, and micro-hydroelectric generators.

Definitions

A battery bank is a group of batteries connected together with series and parallel connections to provide a specific voltage and capacity.

A power conditioning unit (PCU) is a device that includes more than one power conditioning function.

An inverter is a device that converts DC power to AC power.

A charge controller is a device that regulates battery charge by controlling the charging voltage and /or cur rent from a DC power source, such as a PV array.

A rectifier is a device that converts AC power to DC power.

A charger is a device that combines a rectifier with filters, transformers, and other components to condition DC power for the purpose of battery charging.

A DC-DC converter is a device that converts DC power from one voltage to another.

A maximum power point tracker (MPPT) is a device or circuit that uses electronics to continually adjust the load on a PV device under changing temperature and irradiance conditions to keep it operating at its maximum power point.

An engine generator is a combination of an internal combustion engine and a generator mounted together to produce electricity.

A generator is a device that converts mechanical energy into electricity by means of electromagnetic induction.

A gas turbine is a device that compresses and bums a fuel-air mixture, which expands and spins a turbine.

A turbine is a bladed shaft that converts fluid flow into rotating mechanical energy.

An uninterruptible power supply (UPS) is a battery-based system that includes all the additional power conditioning equipment, such as inverters and chargers, to make a complete, self-contained power source.

A wind turbine is a device that harnesses wind power to produce electricity.

A micro-hydroelectric turbine is a device that produces electricity from the flow and pressure of water.

A fuel cell is an electrochemical device that uses hydrogen and oxygen to produce DC electricity, with water and heat as byproducts.

An electrolyzer is an electrochemical device that uses electricity to split water into hydrogen and oxygen.

A stand-alone PV system is a type of PV system that operates autonomously and supplies power to electrical loads independently of the electric utility.

A direct-coupled PV system is a type of stand-alone system where the output of a PV module or array is directly connected to a DC load.

A self-regulating PV system is a type of stand-alone system that uses no active control systems to protect the battery, except through careful design and component sizing.

A utility-interactive system is a PV system that operates in parallel with and is connected to the electric utility grid.

Net metering is a metering arrangement where any excess energy exported to the utility is subtracted from the amount of energy imported from it.

Dual metering is the arrangement that measures energy exported to and imported from the utility grid separately.

Islanding is the undesirable condition where a distributed-generation power source, such as a PV system, continues to transfer power to the utility grid during a utility outage.

A bimodal system is a PV system that can operate in either utility-interactive or stand-alone mode and uses battery storage.

A hybrid system is a stand-alone system that includes two or more distributed energy sources.

A DC bus hybrid system is a hybrid system that combines DC power output from all energy sources, including the PV array, for charging the battery bank.

An AC bus hybrid system is a hybrid system that supplies the loads with AC power from multiple energy sources.

Review Quiz:

1. Why is energy storage needed in most PV systems?

2. Besides energy storage, what advantages do battery systems provide?

3. What is the difference between an inverter and a power conditioning unit?

4. Explain the difference between a charge controller and a charger.

5. How does a maximum power point tracker maximize array output?

6. Which types of electrical energy sources are typically integrated with PV systems?

7. Why is system sizing critically important for stand-alone PV systems?

8. Compare the different types of stand-alone PV systems.

9. Explain the ways in which interactive inverters interface with the utility grid.

10. How can bimodal systems be used to lower electricity costs when utility rates vary by time of day?

11. Compare the advantages and disadvantages of hybrid systems.

Answers to Odd-Numbered Questions:

1. Electrical power demand usually fluctuates considerably but PV power is produced in a relatively steady process. Therefore, the electricity supply does not always coincide with when it is needed. Excess energy must be stored for later use, and then recovered when it is needed at high demand. Energy-storage systems level out energy production and demand.

3. Inverters convert DC power from battery systems or arrays to utility- grade AC power for AC loads or export to the utility grid. Inverters may be individual devices or may be combined with other devices into a single power conditioning unit (PCU). The other devices included in power conditioning units may include charge controllers, rectifiers, or maximum power point trackers. However, since an inverter is commonly the major component in a multifunction PCU, the unit is still often called an “inverter.”

5. The voltage and current output from an array can vary with temperature, irradiance, and load, and produce power outputs anywhere up to the rated (maximum) power level. For any combination of temperature and irradiance, there is a maximum possible power output that corresponds to a certain voltage and current. A maximum power point tracker (MPPT) uses electronics to continually adjust the load on a PV device under changing temperature and irradiance conditions to keep it operating at its maximum power point, maximizing the array output.

7. Stand-alone PV systems are sized and designed to power a specific electrical load using the solar radiation resource at a given location. Stand-alone systems, since they have no other power source to rely on, are often sized for the worst-case scenario: highest load demand with lowest average insolation. If the system includes a battery bank, the array must be sized to both charge the battery bank and operate the load.

9. Utility-interactive systems make a bi-directional interface with the utility at the distribution panel or electrical service entrance. When the PV system does not produce enough power to meet system loads, additional power is imported from the utility. If there is an excess of PV power, the excess power is fed back (exported) to the grid. For exporting, the inverter must monitor the utility’s power to match the voltage, phase, and other parameters. If the inverter is bimodal, then it also senses utility outages and then switches to back-up mode.

11. Hybrid systems offer greater system reliability and flexibility in meeting variable loads because they use multiple energy sources. Common energy sources used in hybrid systems include engine generators, wind turbines, and micro-hydroelectric generators. Since the other energy sources may be active at night or on demand, unlike PV arrays, hybrid systems are better able to meet varying electricity demands.

Next: Cells, Modules, and Arrays

Prev: PV System Configurations

Top of page      More Articles    Home