Solar energy Study Guide

📖 Core Concepts Solar Energy – Radiant light & heat from the Sun that can be harvested by active (PV, CSP, solar water heating) or passive (building orientation, thermal‑mass, natural ventilation) technologies. Insolation – Solar radiation reaching Earth’s surface; at the upper atmosphere the Earth receives ≈ 174 PW, about 122 PW is absorbed after 30 % is reflected. Photovoltaic (PV) Effect – Direct conversion of photons to electric current in a semiconductor cell. Concentrated Solar Power (CSP) – Mirrors/lenses focus sunlight onto a receiver, producing high‑temperature heat for electricity (usually via a Rankine cycle). Thermal‑Mass Storage – Materials with high specific heat (water, stone, earth) store solar heat for later use, smoothing daily temperature swings. Molten‑Salt Storage – High‑temperature salts retain heat (99 % annual efficiency) and drive steam turbines when sunlight is unavailable. Passive Solar Design – Building geometry, orientation, shading, and thermal mass that capture and store heat without mechanical equipment. Active Solar Enhancements – Pumps, fans, switchable glazing, or hybrid PV‑thermal panels that augment passive designs. --- 📌 Must Remember Incoming Solar Power: $174\ \text{PW}$ at the top of the atmosphere; $122\ \text{PW}$ absorbed. Global Solar Potential: $3.85\times10^{6}\ \text{EJ yr}^{-1}$; only ≈ 0.3 % of land ($450{,}000\ \text{km}^2$) needed to meet all 2021 human energy demand. PV Share (2024): 7 % of global electricity, >1 % of primary energy; 75 % of new electricity capacity is solar. Levelised Cost (2021 Lazard): <$37 / MWh for utility‑scale solar – cheaper than new coal or gas. Solar Water Heating: Provides 60–70 % of domestic hot‑water demand; water heated up to $60^\circ\text{C}$. Molten‑Salt Efficiency: ≈ 99 % storage efficiency, can supply electricity for hours‑to‑days. Floating PV Benefits: Cooler panels → higher efficiency; reduces water evaporation and algae growth. --- 🔄 Key Processes Photovoltaic Generation Photon absorption → electron‑hole pair → separation by built‑in electric field → DC current. CSP Power Cycle Sun‑tracking mirrors focus light → receiver heats fluid → hot fluid stores heat (often molten salt) → steam generator → turbine → electricity (Rankine cycle). Solar‑Assisted Heat Pump Solar thermal collector heats evaporator → refrigerant vaporizes → compressor raises pressure/temperature → condenser releases heat to space → expansion valve returns low‑pressure vapor. Molten‑Salt Storage Operation Day: solar heat → hot salt (≈ 560 °C) pumped through heat‑exchanger to generate steam. Night: salt cooled, stored in insulated tanks; when needed, hot salt recirculated. Passive Solar Building Heating Sunlight enters south‑facing glazing → absorbed by thermal‑mass (e.g., concrete) → mass releases heat when outdoor temps drop. --- 🔍 Key Comparisons PV vs. CSP – PV: Direct electricity, low‑temp, modular; CSP: Indirect electricity via heat, high‑temp, needs storage, better for large‑scale, dispatchable power. Passive vs. Active Solar – Passive: No moving parts, relies on design; Active: Requires pumps, fans, controls, higher cost but more flexibility. Molten‑Salt vs. Thermal‑Mass Storage – Molten salt: High temperature, long‑duration (hours‑days), high efficiency; Thermal mass: Low‑temp, daily cycle, simple, cheaper. Floating PV vs. Land‑Based PV – Floating: Higher efficiency (cooling), water‑evap reduction; Land: Easier installation, no water‑related constraints. --- ⚠️ Common Misunderstandings “Solar energy is endless” – The flux is finite; land/area, storage, and intermittency limit usable power. “PV works at night” – PV produces no electricity without sunlight; storage or grid‑interaction is required. “Higher solar panel temperature improves output” – Efficiency actually decreases with temperature; cooling (e.g., floating PV) raises performance. “All solar water heaters reach 60 °C” – Only in mid‑latitude regions with adequate insolation; colder climates achieve lower temperatures. --- 🧠 Mental Models / Intuition Sun‑Tracking = Concentration Multiplier – Every degree of accurate tracking adds a proportional boost to heat/energy captured. Thermal‑Mass = “Solar Battery” – Think of concrete or water as a battery that stores heat during the day and discharges at night. CSP = “Solar Boiler” – Mirrors act like giant lenses focusing sunlight onto a boiler, producing steam just like a conventional power plant. --- 🚩 Exceptions & Edge Cases High‑Latitude Sites – Limited winter insolation; passive solar gains drop dramatically, requiring supplemental heating. Dust & Weather – CSP mirrors lose 1 % efficiency per 10 % surface soiling; regular cleaning is essential. Albedo Effects – In snowy regions, high surface reflectivity can reduce net absorbed solar energy unless low‑albedo surfaces are used. --- 📍 When to Use Which Choose PV when: Need distributed, modular electricity; space is limited; budget favors lower upfront cost. Choose CSP with molten‑salt when: Large‑scale, dispatchable power is required; high‑temperature process heat is a bonus; site has strong direct normal irradiance (DNI). Use Passive Solar Design for: New building projects in climates with strong seasonal sun angle differences and where low‑maintenance solutions are preferred. Add Active Solar (pumps, fans, hybrid PV‑T) when: Passive measures alone cannot meet heating/cooling loads or when precise temperature control is needed. Deploy Floating PV if: Water surface is available, land cost is high, and evaporation reduction is a priority. --- 👀 Patterns to Recognize High DNI → CSP suitability – Look for clear, low‑cloud regions (e.g., deserts). Seasonal shading patterns – Overhangs that block high summer sun but admit low winter sun indicate good passive design. Temperature‑dependent efficiency drop – Panel temperature > 25 °C → efficiency loss ≈ 0.4–0.5 % per °C. Energy‑to‑area ratio – Rough rule: 1 MW of utility‑scale PV needs 4–5 acres; CSP with storage needs 10 acres per MW. --- 🗂️ Exam Traps “Solar water heating can meet 100 % of hot‑water demand” – In reality, 60–70 % is typical for mid‑latitude climates. Confusing “solar constant” with surface insolation – The solar constant (1361 W m⁻²) applies at the top of the atmosphere; surface values are lower due to atmosphere and albedo. Assuming all PV is “grid‑tied” – Off‑grid systems require battery storage; net‑metering only applies to grid‑connected setups. Misreading “levelised cost” – LCOE values are for unsubsidized utility‑scale projects; residential or small‑scale PV can have higher effective costs. Over‑estimating CSP storage duration – Molten‑salt tanks typically store heat for hours to a few days, not weeks. ---