Unit III: Solar PV and Thermal Systems Study Guide

Introduction to Solar Energy Systems

• Core Principle: Solar Photovoltaic (PV) and Solar Thermal systems both operate by absorbing raw energy from the sun to create usable energy forms.
• System Functions:
  • Solar PV systems create electricity.
  • Solar Thermal systems are used directly for heating water or air.
• Measurement: Solar radiation reaching the earth's surface is measured using instruments such as the Pyrheliometer, Pyranometer, and Photoelectric sunshine recorder.
• Power Plants: Solar thermal power plants generate electricity by collecting and concentrating sunlight to produce high-temperature heat.
• Thermal Energy Storage (TES): This technology stocks thermal energy by heating or cooling a storage medium (such as a Phase Change Material) for later use in heating, cooling, or power generation.
• PV Hierarchy:
  • PV Cell: The basic unit.
  • PV Module: Multiple PV cells connected in series to provide higher voltage.
  • PV Array: Multiple PV modules connected in series or parallel circuits, providing a single Direct Current (DC) output through a combiner box.

Solar Radiation and the Solar Resource

• Definition: Solar radiation (solar resource) is electromagnetic radiation emitted by the sun.
• Sun Characteristics: The sun emits radiation as a black body with a surface temperature of approximately $6000\,K$ due to nuclear reactions (converting Hydrogen to Helium).
• Physical Constants:
  • Sun Radius: $1.39 \times 10^9\,m$.
  • Astronomical Unit ($1\,AU$): The mean distance between the earth and sun ($1.496 \times 10^{11}\,m$).
  • Solar Constant ($SC$): The total radiation power received on a unit area perpendicular to sun rays at $1\,AU$ is $1.353\,kW/m^2$.
• Power Spectral Distribution:
  • The spectrum covers wavelengths from deep ultraviolet ($UV$) to far infrared ($IR$).
  • The maximum spectral irradiance lies at $\lambda = 0.5\,\mu m$.
  • Spectral irradiance $(I(\lambda))$ denotes incident solar power/$m^2/\delta\lambda$ $\left[W/m^2/\mu m\right]$.
  • Irradiance decreases due to atmospheric scattering and absorption by molecules like $O_2$, $H_2O$, and $CO_2$.
• Air Mass ($AM$):
  • $AM0$: Intensity without atmospheric effect ($1\,SC = 135.3\,mW/cm^2$).
  • $AM1$: Intensity after crossing one air mass perpendicular to the earth ($92.5\,mW/cm^2$).
  • Formula: If $\theta$ is the angle of incidence with normal to the earth's surface, the optical path in units of air mass is expressed as: $AM = \frac{1}{\cos(\theta)}$.

Atmospheric Effects and Radiation Components

• Energy Flux Reduction: Solar radiation density is reduced by ~30% on sunny days and up to 90% on cloudy days through absorption, scattering, reflection, and transmission.
• Atmospheric Loss Categories:
  • Absorbed by particles/molecules: $10\text{--}30\%$.
  • Reflected/scattered back to space: $2\text{--}11\%$.
  • Scattered to earth (diffuse radiation): $5\text{--}26\%$.
• Types of Radiation reaching Earth:
  • Beam Radiation: Direct radiation from the solar disk; can be concentrated. It never exceeds 83% of original extraterrestrial flux.
  • Diffuse Radiation: Scattered or reflected radiation from all directions (molecules, particles).
  • Total (Global) Radiation: Sum of beam and diffuse components.
  • Short-wave Radiation: Range from $0.3$ to $3\,\mu m$ (directly from sun).
  • Long-wave Radiation: Range of $3\,\mu m$ or longer, originating from ambient temperature sources (earth surface, atmosphere, collectors).
• Spectrum Filtering: Ionosphere absorbs X-rays; Ozone absorbs UV; and $CO_2$ absorbs long-wave radiation. The main wavelength range for solar applications is $0.29$ to $2.5\,\mu m$.
• Insolation: The measure of incident solar energy on a specified area over a set period.
  • Expressed as $kWh/m^2$ per day (average daily energy).
  • Expressed as $W/m^2$ (average power over a year).

Radiation Measurement Instruments

• Pyrheliometer: Measures direct beam radiation at normal incidence.
  • Structure: Looks like a long telescope tube pointed at the sun.
  • Operation: Radiation falls on a black object at the bottom. Heat exciters atoms, increasing temperature recorded by Thermocouple junction 'A'. A current loop flows to a galvanometer. The deviation is neutralized using a rheostat to calculate the heat absorbed (linearly proportional to radiation).
• Pyranometer: Measures total hemispherical radiation (beam plus diffuse).
  • Structure: Saucer-shaped with a glass dome protecting a central blackbody.
  • Operation: Radiation heats the blackbody, sensed by a thermocouple module/chain. Temperature difference creates output voltage proportional to total radiation. Using a shade allows it to measure diffuse radiation only, from which beam radiation can be calculated (Total - Diffuse).
• Quantum Sensors: Measure Photosynthetically Active Radiation ($PAR$) in a specific band of the visible spectrum.
  • Specifically measure Photosynthetic Photon Flux Density ($PPFD$).
  • Applications: Agriculture (land selection, growhouses) and Oceanography (sunlight zone boundaries). Often built with waterproof housing and use photovoltaic technology.

Solar Thermal Power Plants

• Energy Conversion Path: Solar Energy $\rightarrow$ Thermal Energy (Heat) $\rightarrow$ Mechanical Energy (Turbine) $\rightarrow$ Electrical Energy (Generator).
• Cycles by Temperature:
  • Low Temperature: Limited to ~${100^\circ C}$ (uses Rankine cycle).
  • Medium Temperature: Range of ${150^\circ C}$ to ${300^\circ C}$ (uses Rankine cycle).
  • High Temperature: Above ${300^\circ C}$ (uses Brayton cycle).
• Major Components:
  • Solar pond/Collectors.
  • Working fluid (brine or organic fluids with low evaporation points).
  • Evaporator/Boiler (adds latent heat of vaporization).
  • Turbine and Generator.
  • Condenser and Cooling Tower.

Solar Ponds (Solar Salt Ponds)

• Definition: Artificially designed pond filled with salty water maintaining a non-uniform vertical concentration gradient to reduce heat loss.
• Vertical Zones:
  1. Surface Convective Zone (SCZ): Homogeneous, $10\text{--}20\,cm$ thick, low concentration, serves as a buffer.
  2. Lower Convective Zone (LCZ): Bottom layer, highest salt concentration, builds up high temperatures.
  3. Concentration/Intermediate Gradient Zone (CGZ): Insulation layer that prevents convection, transmitting solar radiation while keeping SCZ and LCZ apart.
• Operations: Salt transported via molecular diffusion to the SCZ must be replaced in the LCZ; fresh water is added to the SCG and brine removed. Brine can be recycled via solar distillation.
• Heat Loss Prevention: Spreading a plastic grid over the surface prevents wind disturbance and evaporative loss.
• Ground Protection: High-concentration LCZ requires a plastic liner or impermeable soil to prevent groundwater infiltration.
• Limitations: Requires sunny climate, large land area, and availability of salt/water. Efficiency averages ~20% but can reach 50%.

Solar Energy Collectors

• Non-Concentrating (Flat-Plate): Collecting area equals absorbing area.
  • Components: Black absorber surface, transparent glazing cover, fluid tubes, support structure, and insulation.
  • Evacuated-Tube Collector: Absorber strips in pressure-proof vacuum glass tubes. Reduces heat loss substantially; ~25-year lifespan.
• Concentrating (Focusing) Type:
  • Parabolic Trough Collector: Line-focusing type. Radiation concentrates on an absorber pipe containing synthetic oil/water. Requires tracking sun's elevation.
  • Power Tower Receiver (Central Receiver): Thousands of tracking mirrors (heliostats) focus light onto a receiver at the top of a tower (~$500\,m$ height). Generates steam at ${600^\circ C}$ to ${700^\circ C}$.
  • Parabolic Dish Collector: Point-focusing. Receiver at focal point of a $6\,m$ diameter dish. Two-axis tracking required. Yields temperatures up to ${3000^\circ C}$.
  • Fresnel Lens Collector: Uses fine linear grooves on a refracting material to act like a common lens, focusing light on an absorber tube.

Central Receiver Power Plants

• Concentration Ratio: Sunlight is concentrated 600 to 1000 times, achieving temperatures from ${800^\circ C}$ to over ${1000^\circ C}$.
• Efficiency: Up to 35% peak and 25% annual efficiency when used in combined cycle plants.
• Heliostat Field: Accounts for 36% of total plant cost. Mirrors are typically thin, second-surface, low-iron glass supported by a substrate as a concave surface.
• Storage Media: Molten nitrate salt mixtures ($565^\circ C$ max, freezepoint $140\text{--}220^\circ C$), Liquid sodium ($600^\circ C$ max, freezepoint $98^\circ C$), or pressurized Air/Helium ($850^\circ C$).
• Receiver Types:
  • External: Panels of vertical tubes ($20\text{--}56\,mm$) welded side-by-side.
  • Cavity: Absorbing surface inside an insulated cavity with an aperture to reduce convective loss.

Thermal Energy Storage (TES) with Phase Change Materials (PCM)

• Principle: PCMs undergo solid-liquid phase transformation (melting-solidification) to store latent heat.
• Latent Heat of Fusion: Energy absorbed during melting without a temperature increase. Expressed in $J/g$ or $kJ/kg$.
• Comparison: Latent heat of vaporization involves higher energy but significant density change and volume requirements (boilers/condensers). Solid-liquid transition has small density changes.
• Advantages: Maintains constant temperature during melt, high energy density (smaller tanks than sensible storage), and no external pumps required.
• Limitations: Low thermal conductivity/diffusivity slows transition rates; corrosive nature of inorganic salts.
• Concentrating Solar Power (CSP) Application: Excess thermal energy during high insolation is stored in PCM tanks. Direct Steam Generation ($DSG$) uses HTF to power turbines or store heat.
• Solar Salt: A commonly used PCM mixture consisting of $60\%\,NaNO_3$ and $40\%\,KNO_3$.
• Domestic Uses: Flat plate collectors use PCM in tanks to store water heat for overnight use.

Solar Photovoltaic (PV) Systems: Principles and Components

• Photovoltaic Effect: Sunlight incident on a p-n junction transfers energy to electrons, moving them to the conduction band (excited state) and creating current.
• Cell Construction:
  • Semiconductor Layers: p-type and n-type joined.
  • Conducting Layers: Backside fully covered; front side sparingly covered to allow light entrance.
  • Anti-reflection Coating: Reduces significant reflection loss from semiconductor surface.
• System Components:
  1. PV Module: Converts sunlight to DC electricity.
  2. Solar Charge Controller: Regulates voltage/current and prevents battery overcharging/deep-discharging.
  3. Battery: Stores energy for night use.
  4. Inverter: Converts DC to Alternating Current ($AC$) for appliances.
  5. Lightning Protection: Essential for large systems, includes grounding.

Classification and Operation of PV Systems

• PV Direct System: Simplest type; only panels and load (e.g., water pumping).
• Off-Grid (Stand-alone): Independent of grid; requires battery maintenance; expensive but cost-effective for remote areas.
• Grid-Tied without Battery: Most common; routes extra energy to the grid via net metering; provides no outage protection during grid failure.
• Grid-Tied with Battery: Utility grid + battery backup for designated loads during failure; complex and expensive.
• Hybrid: Sources power from multiple origins (sun, wind, diesel) to ensure availability.

Types of Solar Photovoltaic Cells

• Crystalline Silicon (c-Si): ~90% of the market.
  • Mono-crystalline: Uniform single-crystal structure. Efficiency: $15\text{--}20\%$. Reliable but expensive.
  • Poly-crystalline: Multi-crystal structure with grain boundaries that restrict electron flow. Efficiency: $10\text{--}14\%$. Cheaper to produce.
• Thin-Film Solar Cells: Faster, cheaper manufacturing; sprayed on substrate.
  • Cadmium Telluride (CdTe): Minimal material needed; toxic cadmium; Efficiency: $< 10\%$.
  • Amorphous Silicon (a-Si): High light absorption; used in calculators; degrades quickly to $< 5\%$ efficiency.
  • Copper Indium diSelenide (CIS): High absorption (90%); Efficiency: $~10\%$. Complex/toxic (Hydrogen Selenide gas).
  • Copper Indium Gallium diSelenide (CIGS): Liquid Gallium matches solar spectrum; Efficiency: $~12\%$.
• 3rd Generation / Emerging:
  • Multijunction: Layered semiconductors to extract energy from different spectrum portions.
  • Dye-Sensitive (DSSC): Photosensitive dye releases electrons; can be screen-printed onto surfaces.
  • 3D Photovoltaic: Miniature molecular structures capture light from all directions.

PV Module I-V Characteristics and Performance

• I-V Curve: Graphical representation of the current (I) vs. voltage (V) relationship.
• Key Parameters:
  • $V_{OC}$ (Open Circuit Voltage): Maximum voltage at zero current.
  • $I_{SC}$ (Short Circuit Current): Maximum current at zero voltage.
  • MPP (Maximum Power Point): Optimal point where $P = I \times V$ is maximized ($V_{mp}$ and $I_{mp}$).
  • Vmp Estimation: $V_{mp} \cong (0.8\text{--}0.90) V_{OC}$.
  • Imp Estimation: $I_{mp} \cong (0.85\text{--}0.95) I_{SC}$.
• Fill Factor ($FF$): Quality measure comparing max power to theoretical power ($V_{OC} \times I_{SC}$).
  • $FF = \frac{P_{max}}{V_{OC} \times I_{SC}}$. Typical values: $0.7\text{--}0.8$.
• Efficiency ($\eta$): Ratio of energy output to incident energy.
  • $\eta = \frac{P_{max}}{P_{incident}} = \frac{V_{OC} \times I_{SC} \times FF}{P_{incident}}$.
  • Tested under $AM1.5$ conditions at ${25^\circ C}$ for terrestrial use ($AM0$ for space).
• Factors Limiting Efficiency: Wavelength mismatch, indirect recombination (impurities/defects), high temperature (decreases voltage), and reflection.

Series and Parallel Connections

• Wiring in Series: Panels connected in a "string."
  • Total Voltage: Sum of individual panel voltages.
  • Current (Amps): Remains constant across the string.
• Wiring in Parallel: Panels connected to a centralized wire.
  • Total Current: Sum of individual panel amps.
  • Voltage: Remains constant across the system.

Maximum Power Point Tracking (MPPT)

• Definition: Algorithm in charge controllers to extract maximum power by forcing cells to operate at the peak power voltage ($V_{pp}$).
• Operation: MPPT checks PV output, compares to battery voltage, and fixes the best power point. It is essentially a DC-to-DC converter.
• Converter Types:
  • Boost Converter: PV input voltage < battery voltage.
  • Buck Converter: PV input voltage > battery voltage. Useful for systems under $48\,V$.
• Effectiveness: Most effective in cold weather (higher $V_{pp}$) or when batteries are deeply discharged. Gain decreases in very hot weather as $V_{mp}$ drops.
• Example (Cold Day): Outside ${20^\circ F}$, $V_{pp} = 18\,V$, Battery $= 12\,V$. Theoretical current increase is 50%; actual observed increase is $20\text{--}30\%$.

Applications of Solar Energy

• Residential: Water heating, electricity generation, cutting fuel expenditure.
• Industrial: Powering radio/TV stations, lighthouses, warning lights for aircraft.
• Agricultural/Animal: Crop drying (potatoes, maize, ginger, cashew-nuts), timber drying, tobacco curing, spray drying milk, fish drying.
• Greenhouses: Structures covered with transparent material to act as solar collectors for plants.
• Cooking: Solar cookers prevent charring and preserve vitamins; require no fuel.
• Furnace: Concentrating radiation via heliostats for high-temperature testing.
• Other: Solar-pumping (irrigation), Solar-distillation (converting saline water to potable water), and Public Transportation (buses, trolleys).

Market Data and Industry Information

• India Statistics:
  • Installed Capacity: $34.404\,GW$ (as of 29 February 2020).
  • Installation Cost: Rs 70,000 to 1,20,000 per kW (Rooftop); ~Rs 42,000 to 49,000 after 30% subsidy.
• Top 5 Largest Solar Power Plants in India:
  1. Pavagada Solar Park (Karnataka)
  2. Kurnool Ultra Mega Solar Park (Andhra Pradesh)
  3. Kamuthi Solar Power Project (Tamil Nadu)
  4. Bhadla Solar Park (Rajasthan)
  5. Charanka Solar Park (Gujarat)
• Top 10 Solar Companies in India: Adani Power, Tata Solar, Jinko Solar, Trina Solar, ACME Solar, Vikram Solar, Waaree Energies, EMMVEE, Goldi Solar, Canadian Solar.
• Global Facts:
  • China: World's largest manufacturer of solar panels.
  • Germany: Cumulative capacity reached $47.72\,GW$ (May 2019).
  • Noor Complex (Sahara Desert): World's largest concentrated solar power (CSP) plant ($580\,MW$).