Solar Electricity Basics

Although solar electricity producing devices have been around for over 50 years, solar electricity devices, often referred to as photovoltaics or PV, are still considered cutting edge technology. The promise of clean, cheap, and abundant electricity from the sun has been the dream of many scientists and businesses. As a result each year a number of discoveries and advances for this technology have been made.

This primer has been designed to cover some of the basic concepts, components, and uses of PV. Explore each of the sections below to being your enlightening journey.

Current PV Technology

Picture of PV panels at Epcot

Picture of PV panels at Epcot.

Photovoltaics (PV) or solar cells as they are often called, are semiconductor devices that convert sunlight into direct current (DC) electricity. Groups of PV cells are electrically configured into modules and arrays, which can be used to charge batteries, operate motors, and to power any number of electrical loads. With the appropriate power conversion equipment, PV systems can produce alternating current (AC) compatible with any conventional appliances, and can operate in parallel with, and interconnected to, the utility grid.

History of PV

Picture of a thin-film PV cell in a lab.

Picture of a thin-film PV cell in a lab.

The first conventional photovoltaic cells were produced in the late 1950s, and throughout the 1960s were principally used to provide electrical power for earth-orbiting satellites. In the 1970s, improvements in manufacturing, performance and quality of PV modules helped to reduce costs and opened up a number of opportunities for powering remote terrestrial applications, including battery charging for navigational aids, signals, telecommunications equipment and other critical, low-power needs.

In the 1980s, photovoltaics became a popular power source for consumer electronic devices, including calculators, watches, radios, lanterns and other small battery-charging applications. Following the energy crises of the 1970s, significant efforts also began to develop PV power systems for residential and commercial uses, both for stand-alone, remote power as well as for utility-connected applications. During the same period, international applications for PV systems to power rural health clinics, refrigeration, water pumping, telecommunications, and off-grid households increased dramatically, and remain a major portion of the present world market for PV products. Today, the industry’s production of PV modules is growing at approximately 25 percent annually, and major programs in the U.S., Japan and Europe are rapidly accelerating the implementation of PV systems on buildings and interconnection to utility networks.

How PV Cells Work

A typical silicon PV cell is composed of a thin wafer consisting of an ultra-thin layer of phosphorus-doped (N-type) silicon on top of a thicker layer of boron-doped (P-type) silicon. An electrical field is created near the top surface of the cell where these two materials are in contact, called the P-N junction. When sunlight strikes the surface of a PV cell, this electrical field provides momentum and direction to light-stimulated electrons, resulting in a flow of current when the solar cell is connected to an electrical load

A diagram explaining how a PV cell generates electricity.

Figure 1. Diagram of a photovoltaic cell.

Regardless of size, a typical silicon PV cell produces about 0.5 – 0.6 volt DC under open-circuit, no-load conditions. The current (and power) output of a PV cell depends on its efficiency and size (surface area), and is proportional to the intensity of sunlight striking the surface of the cell. For example, under peak sunlight conditions, a typical commercial PV cell with a surface area of 160 cm2 (~25 in2) will produce about 2 watts peak power. If the sunlight intensity were 40 percent of peak, this cell would produce about 0.8 watts.

Cells, Modules, & Arrays

Photovoltaic cells are connected electrically in series and/or parallel circuits to produce higher voltages, currents and power levels. Photovoltaic modules consist of PV cell circuits sealed in an environmentally protective laminate, and are the fundamental building blocks of PV systems. Photovoltaic panels include one or more PV modules assembled as a pre-wired, field-installable unit. A photovoltaic array is the complete power-generating unit, consisting of any number of PV modules and panels.

A diagram of how cells, modules, and arrays are related.

Figure 2. Photovoltaic cells, modules, panels and arrays.

The performance of PV modules and arrays are generally rated according to their maximum DC power output (watts) under Standard Test Conditions (STC). Standard Test Conditions are defined by a module (cell) operating temperature of 25° C (77° F), and incident solar irradiance level of 1000 W/m2 and under Air Mass 1.5 spectral distribution. Since these conditions are not always typical of how PV modules and arrays operate in the field, actual performance is usually 85 to 90 percent of the STC rating.

Today’s photovoltaic modules are extremely safe and reliable products, with minimal failure rates and projected service lifetimes of 20 to 30 years. Most major manufacturers offer warranties of 20 or more years for maintaining a high percentage of initial rated power output. When selecting PV modules, look for the product listing (UL), qualification testing and warranty information in the module manufacturer’s specifications.

How a PV System Works

Simply put, PV systems are like any other electrical power generating systems, just the equipment used is different than that used for conventional electromechanical generating systems. However, the principles of operation and interfacing with other electrical systems remain the same, and are guided by a well-established body of electrical codes and standards.

Although a PV array produces power when exposed to sunlight, a number of other components are required to properly conduct, control, convert, distribute, and store the energy produced by the array.

Depending on the functional and operational requirements of the system, the specific components required may include major components such as a DC-AC power inverter, battery bank, system and battery controller, auxiliary energy sources and sometimes the specified electrical load (appliances). In addition, an assortment of balance of system (BOS) hardware, including wiring, overcurrent, surge protection and disconnect devices, and other power processing equipment. Figure 3 show a basic diagram of a photovoltaic system and the relationship of individual components.

A diagram of how the components of a PV system interact with each other.

Figure 3. Major photovoltaic system components
and how the components interact with each other.

Why Are Batteries Used in Some PV Systems?

Batteries are often used in PV systems for the purpose of storing energy produced by the PV array during the day, and to supply it to electrical loads as needed (during the night and periods of cloudy weather). Other reasons batteries are used in PV systems are to operate the PV array near its maximum power point, to power electrical loads at stable voltages, and to supply surge currents to electrical loads and inverters. In most cases, a battery charge controller is used in these systems to protect the battery from overcharge and overdischarge.

Types of PV Systems

Photovoltaic power systems are generally classified according to their functional and operational requirements, their component configurations, and how the equipment is connected to other power sources and electrical loads. The two principal classifications are grid-connected or utility-interactive systems and stand-alone systems. Photovoltaic systems can be designed to provide DC and/or AC power service, can operate interconnected with or independent of the utility grid, and can be connected with other energy sources and energy storage systems.

Grid-connected or utility-interactive PV systems are designed to operate in parallel with and interconnected with the electric utility grid. The primary component in grid-connected PV systems is the inverter, or power conditioning unit (PCU). The PCU converts the DC power produced by the PV array into AC power consistent with the voltage and power quality requirements of the utility grid, and automatically stops supplying power to the grid when the utility grid is not energized. A bi-directional interface is made between the PV system AC output circuits and the electric utility network, typically at an on-site distribution panel or service entrance. This allows the AC power produced by the PV system to either supply on-site electrical loads, or to back-feed the grid when the PV system output is greater than the on-site load demand. At night and during other periods when the electrical loads are greater than the PV system output, the balance of power required by the loads is received from the electric utility This safety feature is required in all grid-connected PV systems, and ensures that the PV system will not continue to operate and feed back into the utility grid when the grid is down for service or repair.

A diagram of a gird-connected PV system.

Figure 4. Diagram of grid-connected photovoltaic system.

Stand-Alone Photovoltaic Systems

Stand-alone PV systems are designed to operate independent of the electric utility grid, and are generally designed and sized to supply certain DC and/or AC electrical loads. These types of systems may be powered by a PV array only, or may use wind, an engine-generator or utility power as an auxiliary power source in what is called a PV-hybrid system. The simplest type of stand-alone PV system is a direct-coupled system, where the DC output of a PV module or array is directly connected to a DC load (Figure 5). Since there is no electrical energy storage (batteries) in direct-coupled systems, the load only operates during sunlight hours, making these designs suitable for common applications such as ventilation fans, water pumps, and small circulation pumps for solar thermal water heating systems. Matching the impedance of the electrical load to the maximum power output of the PV array is a critical part of designing well-performing direct-coupled system. For certain loads such as positive-displacement water pumps, a type of electronic DC-DC converter, called a maximum power point tracker (MPPT), is used between the array and load to help better utilize the available array maximum power output.

A diagram of a stand-alone photovoltaic system.

Figure 5. Direct-coupled PV system.

In many stand-alone PV systems, batteries are used for energy storage. Figure 6 shows a diagram of a typical stand-alone PV system powering DC and AC loads. Figure 7 shows how a typical PV hybrid system might be configured.

A diagram of a stand-alone PV system with battery storage powering DC and AC loads.

Figure 6. Diagram of stand-alone PV system
with battery storage powering DC and AC loads.

 

A diagram of a photovoltaic hybrid system.

Figure 7. Diagram of photovoltaic hybrid system.

How PV Cells Are Made

An FSEC researcher in the materials laboratory.

An FSEC researcher in the materials laboratory.

The process of fabricating conventional single- and polycrystalline silicon PV cells begins with very pure semiconductor-grade polysilicon - a material processed from quartz and used extensively throughout the electronics industry. The polysilicon is then heated to melting temperature, and trace amounts of boron are added to the melt to create a P-type semiconductor material.

Next, an ingot, or block of silicon is formed, commonly using one of two methods:

  1. by growing a pure crystalline silicon ingot from a seed crystal drawn from the molten polysilicon or

  2. by casting the molten polysilicon in a block, creating a polycrystalline silicon material. Individual wafers are then sliced from the ingots using wire saws and then subjected to a surface etching process.

After the wafers are cleaned, they are placed in a phosphorus diffusion furnace, creating a thin N-type semiconductor layer around the entire outer surface of the cell.

Next, an anti-reflective coating is applied to the top surface of the cell, and electrical contacts are imprinted on the top (negative) surface of the cell. An aluminized conductive material is deposited on the back (positive) surface of each cell, restoring the P-type properties of the back surface by displacing the diffused phosphorus layer.

Each cell is then electrically tested, sorted based on current output, and electrically connected to other cells to form cell circuits for assembly in PV modules.

Another FSEC researcher in the materials laboratory.

Another FSEC researcher in the materials laboratory.

Thin Film PV

Thin-film photovoltaic modules are manufactured by depositing ultra-thin layers of semiconductor material on a glass or thin stainless-steel substrate in a vacuum chamber. A laser-scribing process is used to separate and weld the electrical connections between individual cells in a module. Thin-film photovoltaic materials offer great promise for reducing the materials requirements and manufacturing costs of PV modules and systems.

Pros and Cons of PV

An FSEC researcher giving an explantion of how PV works to visitors.

An FSEC researcher giving an explantion of how PV works to visitors.

Photovoltaic systems have a number of merits and unique advantages over conventional power-generating technologies. PV systems can be designed for a variety of applications and operational requirements, and can be used for either centralized or distributed power generation. PV systems have no moving parts, are modular, easily expandable and even transportable in some cases. Energy independence and environmental compatibility are two attractive features of PV systems. The fuel (sunlight) is free, and no noise or pollution is created from operating PV systems. In general, PV systems that are well designed and properly installed require minimal maintenance and have long service lifetimes.

At present, the high cost of PV modules and equipment (as compared to conventional energy sources) is the primary limiting factor for the technology. Consequently, the economic value of PV systems is realized over many years. In some cases, the surface area requirements for PV arrays may be a limiting factor. Due to the diffuse nature of sunlight and the existing sunlight to electrical energy conversion efficiencies of photovoltaic devices, surface area requirements for PV array installations are on the order of 8 to 12 m2 (86 to 129 ft2) per kilowatt of installed peak array capacity.

Q: Can photovoltaic systems operate normally in grid-connected mode and still operate critical loads when utility service is disrupted?

A: Yes, however battery storage must be used. This type of system is extremely popular for homeowners and small businesses where a critical backup power supply is required for critical loads such as refrigeration, water pumps, lighting and other necessities. Under normal circumstances, the system operates in grid-connected mode, serving the on-site loads or sending excess power back onto the grid while keeping the battery fully charged. In the event the grid becomes de-energized, control circuitry in the inverter opens the connection with the utility through a bus transfer mechanism, and operates the inverter from the battery to supply power to the dedicated loads only. In this configuration, the critical loads must be supplied from a dedicated sub panel. Figure 8 shows how a PV system might be configured to operate normally in grid-connected mode and also power critical loads from a battery bank when the grid is de-energized.

A diagram of a grid-connected critical power supply system.

Figure 8. Diagram of a grid-connected critical power supply system.

Glossary

An open dictionary.

There are always new words and terminology that are used to describe new technologies and solar electricity is no exception. Use this section to better understand some of the terms used by this primer.

Activation Voltage(s):

The voltage(s) at which the controller will take action to protect the batteries.

AIC:

Amperage interrupt capability. DC fuses should be rated with a sufficient AIC to interrupt the highest possible load.

Alternating Current (AC):

Electrical current (flow of electrons) in which the direction of flow is reversed at constant intervals, such as 60 cycles per second.

Array:

A collection of photovoltaic modules electrically wired together in one structure to produce a specific amount of power.

British Thermal Unit (Btu):

The quantity of heat needed to raise the temperature of 1 pound of water by 1°F at or near 39.2°F.

Centrifugal Pump (rotating pump):

A water pump using a rotating element or screw to move water. The faster the rotation, the greater the flow.

Charge Controller:

A component of a photovoltaic system that controls the flow of current to and from the battery subsystem to protect batteries from overcharge, over discharge, or other control functions. The charge controller may also monitor system operational status.

Cogeneration:

The production of electricity and another form of useful energy (such as heat or steam) used for industrial, commercial, heating, or cooling purposes.

Commercial Building:

A building with more than 50 percent of its floor space used for commercial activities. Commercial buildings include stores, offices, schools, churches, gymnasiums, libraries, museums, hospitals, clinics, warehouses, and jails. Government buildings are also included, except buildings on military bases or reservations.

Commercial Sector:

Business establishments that are not engaged in transportation or in manufacturing or other types of industrial activity (agriculture, mining, or construction). Commercial establishments include hotels, motels, restaurants, wholesale businesses, retail stores, laundries, and other service enterprises; religious and nonprofit organizations; health, social, and educational institutions; and federal, state, and local governments. Street lights, pumps, bridges, and public services are also included if the establishment operating them is considered commercial.

Conversion Factor:

A number that translates units of one system into corresponding values of another system. Conversion factors can be used to translate physical units of measure for various fuels into Btu equivalents.

Direct Current (DC):

Electric current (flow of electrons) in which the flow is in only one direction.

Displaced or Volumetric Pump:

A type of water pump that utilizes a piston, cylinder and stop valves to move packets of water.

District Heat:

Steam or hot water from an outside source used as an energy source in a building. The steam or hot water is produced in a central plant and is piped into the building. District heat may be purchased from a utility or provided by a physical plant in a separate building that is part of the same facility (for example, a hospital complex or university).

Duty Rating:

The amount of time an inverter (power conditioning unit) can produce at full rated power.

Dynamic Head:

The vertical distance from the center of the pump to the point of free discharge of the water. Pipe friction is included.

Electric Power Plant:

A station containing prime movers, electric generators, and auxiliary equipment for converting mechanical, chemical, and/or fission energy into electric energy.

Electricity Generation:

The process of producing electric energy or transforming other forms of energy into electric energy. Also, the amount of electric energy produced or ex pressed in watt-hours (Wh).

End-Use Sectors:

The residential, commercial, industrial, and transportation sectors of the economy.

Energy:

The capacity for doing work as measured by the capability of doing work (potential energy), or the conversion of this capability to motion (kinetic energy). Energy has several forms, some of which are easily convertible and can be changed to another form useful for work. Most of the world’s convertible energy comes from fossil fuels that are burned to produce heat that is then used as a transfer medium to mechanical or other means in order to accomplish tasks. Electrical energy is usually measured in kilowatt-hours, while heat energy is usually measured in British thermal units.

Energy Consumption:

The use of energy as a source of heat or power or as an input in the manufacturing process.

Energy Density:

The ratio of the energy available from a battery to its volume (wh/m^3) or weight (wh/kg).

Energy Source:

A substance, such as oil, natural gas, or coal, that supplies heat or power. Electricity and renewable forms of energy, such as wood, waste, geothermal, wind, and solar, are considered to be energy sources.

Exports:

Shipments of goods from the 50 states and the District of Columbia to foreign countries and to Puerto Rico, the Virgin Islands, and other U.S. possessions and territories.

Federal Energy Regulatory Commission (FERC):

The federal agency with jurisdiction over interstate electricity sales, wholesale electric rates, hydroelectric licensing, natural gas pricing, oil pipeline rates, and gas pipeline certification. FERC is an independent regulatory agency within the Department of Energy and is the successor to the Federal Power Commission.

Friction Head:

The energy that must be overcome by the pump to offset the friction losses of the water moving through a pipe.

High Voltage Disconnect Hysteresis:

The voltage difference between the high voltage disconnect set point and the voltage at which the full PV array current will be reapplied.

Independent Power Producer:

Wholesale electricity producers (other than qualifying facilities under the Public Utilities Regulatory Policies Act of 1978) that are unaffiliated with franchised utilities in the area in which the independent power producers are selling power and that lack significant marketing power. Unlike traditional electric utilities, independent power producers do not possess transmission facilities that are essential to their customers and do not sell power in any retail service territory where they have a franchise.

Industrial Sector:

Manufacturing industries, which make up the largest part of the sector, along with mining, construction, agriculture, fisheries, and forestry. Establishments in this sector range from steel mills, to small farms, to companies assembling electronic components.

Insolation:

The amount of energy in sunlight reaching an area. Usually expressed in watts per square meter (W/m^2), but also expressed on a daily basis as watts per square meter per day (W/m^2/day).

Load:

Any device or appliance in an electrical circuit that uses power, such as a light bulb.

Low Voltage Disconnect Hysteresis:

The voltage difference between the low voltage disconnect set point and the voltage at which the load will be reconnected.

Low Voltage Warning:

A warning buzzer or light that indicates the low battery voltage set point has been reached.

Module:

A number of photovoltaic cells wired together to form a unit, usually in a sealed frame of convenient size for handling and assembling into arrays. Also called a panel.

Movistor:

Metal Oxide Varistor. Used to protect electronic circuits from surge currents such as produced by lighting.

NEMA:

National Electrical Manufacturers Association. This organization sets standards for some non-electronic products like junctions boxes.

North American Electric Reliability Council (NERC):

A council formed in 1968 by the electric utility industry to promote the reliability and adequacy of bulk power supply in the electric utility systems of North America. The NERC consists of ten regional reliability councils and encompasses essentially all the power systems of the contiguous United States and Canada.

Organization for Economic Cooperation and Development (OECD):

Current members are Australia, Austria, Belgium, Canada, Czech Republic, Denmark and its territories (Faroe Islands and Greenland), Finland, France, Germany, Greece, Greenland, Hungary, Iceland, Ireland, Italy, Japan, Luxembourg, Mexico, the Netherlands, New Zealand, Norway, Poland, Portugal, South Korea, Spain, Sweden, Switzerland, Turkey, United Kingdom, and United States and its territories (Guam, Puerto Rico, and Virgin Islands).

Organization of Petroleum Exporting Countries (OPEC):

Countries that have organized for the purpose of negotiating with oil companies on matters of oil production, prices, and future concession rights. Current members are Algeria, Indonesia, Iran, Iraq, Kuwait, Libya, Nigeria, Qatar, Saudi Arabia, United Arab Emirates, and Venezuela.

Outgas (gassing):

Gas by-products, primarily hydrogen, produced when charging a battery. Also, termed out-gassing.

Peak Sun Hours:

The equivalent number of hours when solar insolation averages 1000 watts per square meter and produces the same total insolation as actual sun conditions.

Photovoltaic Energy:

Direct-current electricity generated from sunlight through solid-state semiconductor devices that have no moving parts.

Photovoltaic (PV) System:

A complete set of interconnect components for converting sunlight into electricity by the photovoltaic process, including array, balance-of-system components, and the load.

Pyrheliometer:

An instrument used for measuring direct beams of solar irradiance. Uses an aperture of 5.7 deg. to transcribe the solar disc.

Renewable Energy:

Energy obtained from sources that are essentially inexhaustible (unlike, for example, fossil fuels, of which there is a finite supply). Renewable sources of energy include conventional hydroelectric power, wood, waste, geothermal, wind, photovoltaic, and solar thermal energy.

Root Mean Square (RMS):

The square root of the average square of the instantaneous values of an AC output. For a sine wave the RMS value is 0.707 times the peak value. The equivalent value of AC current, I, that will produce the same heating in a conductor with resistance, R, as a DC current of value I.the corrosion of the protected structure.

Static Head:

The vertical distance from the water level to the point of free discharge of the water. It is measured when the pump is not operating.

Sacrificial Anode:

A piece of metal buried near a structure that is to be protected from corrosion. The metal of the sacrificial anode is intended to corrode and reduce the corrosion of the protected structure.

Solar Insolation (Insolation):

The solar radiation incident on an area over time. Equivalent to energy and usually expressed in kilowatt-hours per square meter.

Stocks:

Supplies of fuel or other energy source(s) stored for future use. Stocks are reported as of the end of the reporting period.

Storage:

This term has dual meaning for water pump systems. Storage can be achieved by pumping water to a storage tank, or storing energy in a battery subsystem.

Suction Head:

The vertical distance from the surface of the water source to the center of the pump (when the pump is located above the water level).

TC, TW, THHN, UF, USE (Wire Types):

See article 300 of National Electric Code for more information.

Transportation Sector:

Private and public vehicles that move people and commodities. Included are automobiles, trucks, buses, motorcycles, railroads, and railways (including streetcars), aircraft, ships, barges, and natural gas pipelines.

Varistor:

A voltage-dependant variable resistor. Normally used to protect sensitive equipment from power spikes or lightning strikes by shunting the energy to ground.

Voltage (V):

A measure of the force or "push" given the electrons in an electrical circuit; a measure of electrical potential. One volt produces one amp of current when acting against a resistance of one ohm.

Watt (W):

A measure of electrical power or amount of work done in a unit of time and equal to the rate of current flow (amps) multiplied by the voltage of that flow (volts). One amp of current flowing at a potential of one volt produces one watt of power.

Wind Energy:

The kinetic energy of wind converted into mechanical energy by wind turbines (i.e., blades rotating from a hub) that drive generators to produce electricity.


Source: Florida Solar Energy Center

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