Solar Energy Technology Science Summer Camp Session 7 Thur AM System Components and Configurations

Day 3 Introduction Description of Facilities – if needed Go Over Day 2 and rest of Course Handout Notes Review Safety Rules Safety first Always follow safety rules

Session 7 Topics Overview of System Components Activities: Cells Modules Arrays  Inverters Batteries Charge Controllers  Activities: Consider:  Construct and observe basic galvanic cell TBD 3 3

Shading issues Sun – Radiant Energy PV module 4

Stand Alone PV System Components PV Array/Modules Junction / Combiner Box Charge Controller Inverter Batteries 5

Fundamental Components of the Atom - how the relate to charge

How PV Cells, Modules and Arrays Work 9

Solar Array Components Cells Modules Panels Arrays Reference 1 10

PV Cell A solar cell is a device that converts the energy of sunlight directly into electricity by the photovoltaic effect. Sometimes the term solar cell is reserved for devices intended specifically to capture energy from sunlight such as solar panels and solar cells, while the term photovoltaic cell is used when the light source is unspecified. Assemblies of cells are used to make solar panels, solar modules, or photovoltaic arrays.

PV Module A photovoltaic module or photovoltaic panel is a packaged interconnected assembly of photovoltaic cells, also known as solar cells. The photovoltaic module, known more commonly as the solar panel, is then used as a component in a larger photovoltaic system to offer electricity for commercial and residential applications.

PV Array A photovoltaic array is a linked collection of photovoltaic modules, which are in turn made of multiple interconnected solar cells. By their modularity, they are able to be configured to supply most loads.

How a PV Cell works Structure of PV Cell How Electrons Flow Uses marbles with "marble holder - PV cell" to show simplified movement of electrons in PV cell.

Silicon Atom Four electrons in outer shell Reference 3 16

Crystalline Silicon Models Reference 2 17

Definitions - Electrons and Holes 18

Step 1 – Photoelectric effect When sunlight (photon) hits silicon atom, an electron in its outer shell can be “liberated” and start moving throughout the crystalline structure. A “hole” with a positive charge is “left” behind at the silicon atom that lost its electron. Recombination - Eventually free electron combines with another hole. Reference 3 19

Step 2 – Doping process Doping - Process of adding impurities to prevent free electrons randomly “moving” in PV cell. 20

Addition of Phosphorus Addition of phosphorous creates N-type (negative) semiconductor material 21

Addition of Boron Addition of boron creates P-type (positive) semiconductor material 22

Step 3 – Putting PV cell together 23

Electrical Field at P/N Junction Reference 3 Free electrons from phosphorus atom cross over to fill “holes” in boron atoms. This creates a permanent electric field at p/n junction. 24

Space Charge Zone Depletion Region 25

Step 4 – Sunlight hits PV module and current (electron movement) occurs Reference 3 26

Typical PV Cell Reference 2 27

Crystalline Silicone Polycrystalline Monocrystalline Reference 2 28

Thin Film Cell Examples Reference 2 29

Differences in Cell Type Efficiencies 30

Advantages / Disadvantages of Cell Types Crystalline Silicone Highest cell efficiencies Well established manufacturing technology Durable product Thin Film Cells Con’s Less efficient than crystalline silicon Harder to control / MPPT tracking devices (flatter IV curve) Pro’s Wider spectral response (sunlight wavelengths) More efficient at low irradiance levels Use less energy and material to produce More flexible than crystalline silicone More tolerant of shading issues 31

Typical PV Module Construction Reference 2 32

Typical PV module energy losses Typical Polycrystalline Cell Efficiency PV output = 12 to 15% Solar Irradiance 3% - Reflection and shading by front contacts 23% - Insufficient photon energy of long-wave radiation 32% - Surplus of photo energy of short wave radiation 8.5% - Recombination losses 20% - Electrical gradient in cell, especially in space charge zone 0.5% - Due to serial resistance (electric heat loss) Reference 2 33

Impact of shading on PV array performance 34

PV Inverter A Solar inverter or PV inverter is a type of electrical inverter that is made to change the direct current (DC) electricity from a photovoltaic array into alternating current (AC) for use with home appliances and possibly a utility grid. Solar inverters may be classified into three broad types: * Stand-alone inverters, used in isolated systems where the inverter draws its DC energy from batteries charged by photovoltaic arrays and/or other sources, such as wind turbines, hydro turbines, or engine generators. Many stand-alone inverters also incorporate integral battery chargers to replenish the battery from an AC source, when available. Normally these do not interface in any way with the utility grid, and as such, are not required to have anti-islanding protection. * Grid tie inverters, which match phase with a utility-supplied sine wave. Grid-tie inverters are designed to shut down automatically upon loss of utility supply, for safety reasons. They do not provide backup power during utility outages. * Battery backup inverters. These are special inverters which are designed to draw energy from a battery, manage the battery charge via an onboard charger, and export excess energy to the utility grid. These inverters are capable of supplying AC energy to selected loads during a utility outage, and are required to have anti-islanding protection Solar inverters use special procedures to deal with the PV array, including maximum power point tracking and anti-islanding protection.

PV Inverter

Inverter Functions Convert DC power from solar array into AC household power Change voltage levels A) Grid-tied systems Up to 600 VDC to 240 VAC, B) Grid-tied with battery backup From 12, 24, 48 VDC to 120 VAC Provide maximum power point tracking (MPPT) according to IV curve. 37

Types of Inverters 1. Grid-Dependent – Inverter provides power to household loads or grid during daylight hours. No storage 2. Bi-Directional – Performs like grid-dependent inverter but also has ability to charge stand-by batteries. Can charge batteries from PV modules or utility grid. 3. Stand-Alone – Inverter designed to operate independently of the grid. Energy stored in batteries. Remote sites. 38

Energy Conversion and Storage Shading issues Sun – Radiant Energy PV module 39

Batteries A voltaic cell for demonstration purposes. In this example the two half-cells are linked by a salt bridge separator that permits the transfer of ions, but not water molecules

Battery System Components Grid-Tied with Battery Backup (Hybrid) Stand Alone PV Systems PV Systems 41

Batteries Function Store energy from PV array for when it is needed by household loads. Provide stable power supply to loads upon demand (stable voltages / suppress transients) Provide surge currents to electrical loads (motors, etc). Reference 2 42

Electron Flow from Negative plate to positive plate Battery Cell Components Active materials PbO2 (positive electrode) Pb (negative electrode) Grid Plate (grid and active material) Separator Electrolyte (diluted sulfuric acid, H2SO4) Case Vent Chemical reaction occurs between the active materials on the plate and the electrolyte) 43

Lead Acid Cell Chemical Reaction (reversible) Electrons used “up” at positive plate Electrons “released” at negative plate 44

Batteries will deteriorate over time Sulphation – Small quantities of lead sulphate (PbSO4) do not redissolve when battery is charged The greater the “depth of discharge”, the greater the loss of capacity due to sulphation. Stratification – Electrolyte concentration stratifies due to insufficient charging and gassing Leads to severe negative grid corrosion and, hence loss of battery capacity. Overcharging can lead to water (H20) being hydrolyzed into hydrogen (H2) and oxygen (O2) which escape from the battery in the form of gas. Leads to H2SO4 concentration and stratification 45

Charging Terminology Battery Capacity – Measure in Amp-hours. Usually given at C20 discharge rate (battery total discharged over 20 hours) Autonomy – Days of battery storage without additional PV array input State of Charge (SOC) – Percentage of energy stored in battery. Depth of Discharge (DOD) – Percentage of capacity that has been drawn down from full battery Battery cycles – Number of times that a battery has been drawn down to a specified level of discharge. 46

Battery capacity (Amp-Hrs) changes with time to discharge 47

Depth of discharge vs. battery life Gel Wet 48

charge controller A charge controller, charge regulator or battery regulator limits the rate at which electric current is added to or drawn from electric batteries. It prevents overcharging and may prevent against overvoltage, which can reduce battery performance or lifespan, and may pose a safety risk. It may also prevent completely draining ("deep discharging") a battery, or perform controlled discharges, depending on the battery technology, to protect battery life. The terms "charge controller" or "charge regulator" may refer to either a stand-alone device, or to control circuitry integrated within a battery pack, battery-powered device, or battery recharger.