Photovoltaic (PV) Systems

• Chapter 2 focuses on Photovoltaic Technology

Solar Energy

• Solar energy comes from the sun in the form of heat and light.
• Photovoltaic (PV) systems utilize sunlight (not heat) to generate electricity through the photovoltaic effect.

PV System Operation

• Sunlight strikes the PV cells of a solar panel and creates direct current (DC) electricity.
• The DC power can be stored in batteries or converted to alternating current (AC) by an inverter for use in buildings.

PV Technology

• A PV cell is composed of layers of semiconductor materials, often silicon.
• One layer is positively charged, and the other is negatively charged.
• Sunlight, when hitting the PV cell, creates an electric field, which then allows electricity to flow.
• Multiple PV cells are connected to form a module.
• Modules can be connected in series to increase voltage, or in parallel to increase current, thus forming an array.

Solar Cell

• The solar cell serves as the foundational unit of a PV system.
• A typical silicon solar cell generates only about 0.5 volt.
• Multiple cells are connected in series and form larger PV modules.
• Thin sheets of EVA (Ethyl Vinyl Acetate) or PVB (Polyvinyl Butyral) bind cells, and protect from the weather.
• Modules are enclosed between a transparent cover (usually glass) and a weatherproof backing sheet (typically made from a thin polymer or glass) for extra mechanical strength and durability.
• Approximately 36 solar cells are typically connected to achieve a voltage around 18V.
• After accounting for heat-related losses reducing voltage to about 17V, the voltage provides enough charge to power a 12V battery.
• 72 cells module produces about 34V (36V - 2V for losses), which can be used to charge a 24V battery.
• A 12V battery usually needs about 14 volts for a chare.
• A 36-cell module is the standard for the solar battery charger industry.
• Common cell sizes are 12.7 x 12.7 cm (5 x 5 inches) or 15 x 15 cm (6 x 6 inches), can produce from 3 to 4.5W.
• Common module size is 1.4 to 1.7 m2, and modules can be found up to 2.5 m2.

PV String

• Connecting individual modules in series, in parallel, or both increases either output voltage or current, thus increasing the output power.
• Connecting multiple modules in series is called a PV string.
• In a series connection, the negative terminal of one module is connected to the positive terminal of the next module.
• In a series connection, voltage adds up, while the current remains constant.
• For example, if 10 modules of 12 V and 3-amp rating are connected to make one string, the voltage of the string will be 120 V and the current will be 3-amp.
• Reversely, when modules are connected in parallel, current adds up, while voltage remains constant.

PV Array

• Multiple PV strings joined in parallel form a solar array.
• A parallel connection increases the current while maintaining the same voltage.
• An inverter converts the DC power produced by the modules into alternating current.
• The alternating current can then plug into the existing infrastructure to power lights, motors, and other loads.
• The modules in a PV array are first connected in series to achieve the desired voltage.
• Individual strings are then connected in parallel to produce more current.
• Solar arrays are measured by the electrical power they produce, often in watts, kilowatts, or megawatts.

PV Materials

• Silicon is the primary material of most PV cells, because it is both abundant and durable.
• The lifespan of silicon PV materials is over 30 years.
• The energy payback period for silicon PV materials is 2-8 years, time to generate the energy used in manufacturing.

PV Types

• There are three main categories: Monocrystalline Silicon, Polycrystalline Silicon, Thin-Film Silicon

Monocrystalline Silicon

• Constructed from a single crystal.
• They have a high efficiency (14-19%).
• They are more expensive but space-efficient.
• Composed of a single crystal ingot of high purity, with dimensions of 12.5 or 15 cm
• They are cut into thin slices that create round, semi-round, or square shapes.
• These cells are the most electrically efficient.
• Monocrystalline cells require less surface area to produce an equivalent amount of power.
• The Disadvantages are high costs, the need for ventilation for efficiency, and a distinctive geometric pattern.
• They are suitable for atrium roofs, partial vision glazing in facades, rooftop installations and commercial sun shading or rooftop retrofits where installation area is limited and maximum electricity generation is desired.
• They have commercial module efficiencies that range around 14-19%.

Polycrystalline Silicon

• Polycrystalline silicon cells are formed by casting in a cuboid form ingot.
• The ingot is cut into bars and sliced into thin wafers that in create the cells.
• The cells are less efficient than monocrystalline.
• Distinctive appearance and lower cost per unit make them popular for large applications.
• They are often used in facade spandrel panels and sun shading elements of commercial buildings.
• Polycrystalline silicon differs from monocrystalline in terms of lower cost and efficiency levels due to grain boundaries.
• To reach the same efficiency levels larger cells (21 x 21cm) are used.
• Commercial module efficiencies for Polycrystalline silicon range around 12-15%.

Thin-Film Silicon

• PV material is applied as a thin layer to glass, metal, or plastic.
• These are flexible and less expensive, though with a lower efficiency (6-10%).
• Thin film photovoltaics are made by printing or spraying a thin layer of PV material onto a glass, metal or plastic foil substrate.
• Lowering the overall thickness of each photovoltaic cell makes it substantially smaller than a cut crystalline cell hence the name "thin film".
• Manufacturing is faster and cheaper because PV materials are sprayed directly onto a glass or metal substrate.

Thin-Film Silicon Light Absorption

• Exhibits high light absorption, but lower conversion efficiency due to crystal structure.
• This requires larger cells.
• Semiconductor materials used include Cadmium telluride (CdTe), Copper indium diselenide (CIS), Amorphous silicon (a-Si), and Thin film silicon (thin film-Si).
• Amorphous silicon is in commercial production, while the other three technologies are becoming increasingly available.
• Amorphous silicon can be deposited on flexible substrates like polymers, thin metals and plastics.
• It suffers from very low conversion efficiency, ranging from 6 to 8% when new.
• Light absorption for amorphous silicon can be over 40 times higher than crystalline silicon.
• Amorphous silicon requires a thinner layer, therefore reducing manufacturing costs and price.

PV Module Rating

• The peak power rating of a panel is frequently abbreviated as kWp.
• kWp is the peak power of a PV module or system that shows the energy output of a system under full solar radiation, i.e. full irradiance.
• The module temperature must be 25°C, the solar spectrum must have air mass of 1.5, and the solar radiation must be 1,000 W/m2 under Standard Test Conditions (STC).
• Less than full sun will proportionally reduce the cell's current output.

STC Conditions

• Specify the power output of a module under Standard Test Conditions (STC).
• Standard Test Conditions include a 1000 W/m² sunlight intensity, 25°C module temperature., and Air mass of 1.5.

Efficiency

• Efficiency (%) = (Electrical Output Power / Solar Input Power) × 100.
• Higher efficiency means less space is needed for installation.

Module Efficiency

• Efficiencies range from as low as 5% to as high as 15%-19%, which is specified by the manufacturer.
• A technology's conversion efficiency rate determines a commercial PV product’s electricity output.
• Although thinfilm amorphous silicon PV modules need less semiconductor material and can be less expensive to manufacture than crystalline silicon modules, thinfilm amorphous silicon PV modules have lower conversion efficiency rates.
• Because of their lower efficiency, they will need close to twice the space of a crystalline silicon PV array for the same nominal capacity under Standard Test Conditions (STC).

PV System Components

• Key parts of a PV System are PV Array, Inverter, Batteries (optional), and Balance of System (BOS)

PV Subsystems

• PV Array collects the sunlight.
• Inverter converts DC to AC electricity.
• Batteries are optional, but stores energy for later use.
• Balance of System (BOS) includes wiring, mounting, and other necessary components.
• Components excluding the PV modules are Balance of System.

PV Array

• PV Array is a group of PV modules, which are environmentally collections of PV Cells that convert sunlight to electricity.
• Common PV modules dimensions: 5 - to 25 square feet and weighs around 3-4 lbs/ft2.
• Set often made of four or more smaller modules framed or attached together by struts.
• The panel is typically around 20-35 square feet in area for ease of handling on a roof.

Batteries

• The battery stores electric power for operation during nighttime, cloudy, or overcast weather.
• This weather is when the PV array cannot supply enough power.
• The amount of days from the battery storage providing power to the load is called days of "autonomy".
• Standard autonomy periods are between two and six days for less critical PV applications.
• Public safety and critical applications may see autonomy periods of greater that ten days.
• Lead-acid or Lithium-ion batteries are typically used

Inverter

• Purpose is to convert the DC electricity to AC electricity.
• The photovoltaic array and battery produce DC current and voltage.
• AC electricity is used by electrical appliances/exported into the AC grid
• The typical Low Voltage (LV) supply is residential or small commercial buildings.
• Typical Low Voltage comes in either will be 220V AC single phase or 415V AC three phase.
• Ranging from a few hundred watts to 2000kW central inverters.

Charge Control

• A charge controller connects The Battery to the PV array.
• Controller is meant to protect the battery from overcharging/discharging, providing system information or enable metering and payment.

Balance of Systems

• PV modules, battery, inverter, and charger, have other components often required in PV solar micro grid system as The Balance of Systems (BoS) equipment.
• Most common components are mounting structures, tracking systems, electricity meters, cables, power optimizers, protection devices, transformers, combiner boxes, and switches.

Explore the basics of photovoltaic (PV) systems, including electricity generation, solar cell interconnection, and material usage. Learn about voltage optimization and module configurations for efficient power supply in solar technology.