--- title: "Mechanical and thermal energy storage" subject: "Power Plant Design" chapter: 1 key: "semester_8/power_plant_design:energy-storage-technologies" source: https://www.bctengineer.com/q/semester_8/power_plant_design/energy-storage-technologies site: BCT Engineering --- # Mechanical and thermal energy storage Power Plant Design · Chapter 1: Energy Sources and Electric Power Generation - Asked in: 2079 Jestha, 8 marks - Video: [Thermal Energy Storage](https://www.youtube.com/watch?v=3zjSoY7BNUU) — MEP Academy - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/power_plant_design/energy-storage-technologies - Whole chapter: https://www.bctengineer.com/md/semester_8/power_plant_design/chapter-1 ## Question Explain about mechanical and thermal energy storages technologies. ## Short answer Mechanical storage includes pumped storage (70-80% efficiency, hours), CAES (40-70%), flywheels (fast, short duration), and gravity. Thermal storage includes sensible heat (molten salt, water), latent heat using PCM, thermochemical, and cold storage. These store surplus energy and support renewables, peak shaving, and frequency regulation. ## Answer **Energy storage** stores surplus energy when generation is more than demand and returns it when demand is high. It supports variable renewables (solar, wind), peak shaving, frequency regulation and reliability. #### Mechanical energy storage **1. Pumped storage hydropower (PSH)** - Two reservoirs at different levels. Off-peak power pumps water up; at peak, water flows down through the turbine. - Energy stored: $E = \rho g V H$. - Round-trip efficiency about 70–80%; large capacity (hundreds of MW, many hours); long life. - Needs suitable topography; high capital cost. Nepal has good sites in the hills. ```text Upper reservoir | ^ gen v | pump (reversible pump-turbine) Lower reservoir ``` **2. Compressed air energy storage (CAES)** - Off-peak power compresses air into underground caverns; at peak, air is heated and expanded in a turbine. - Efficiency about 40–70%; large scale; needs suitable caverns. **3. Flywheel energy storage** - Energy stored in a rotating mass: $E = \frac{1}{2}J\omega^2$. - Fast response (milliseconds), high power, many cycles; short duration (seconds to minutes); self-discharge by friction. - Used for UPS, power quality and frequency regulation. **4. Gravity storage** – lifting heavy blocks and lowering them to generate power (emerging). #### Thermal energy storage **1. Sensible heat storage** – heat stored by raising temperature of water, rock, sand, concrete or molten salt: $Q = mc\Delta T$. Example: hot water tanks, molten salt in concentrated solar power plants (allows generation after sunset). **2. Latent heat storage** – uses phase change materials (PCM) such as paraffin wax or salt hydrates; stores large heat at nearly constant temperature: $Q = mL$. **3. Thermochemical storage** – heat stored in reversible chemical reactions; highest energy density, still developing. **4. Cold storage** – ice or chilled water made at night for daytime air conditioning, shifting load to off-peak. #### Comparison | Technology | Scale | Duration | Efficiency | Main use | |---|---|---|---|---| | Pumped storage | 100–3000 MW | Hours | 70–80% | Peak shaving, reserve | | CAES | 100–300 MW | Hours | 40–70% | Bulk storage | | Flywheel | kW–MW | Seconds–minutes | 85–95% | Frequency, UPS | | Molten salt | 10–200 MW | Hours | 90%+ (thermal) | Solar thermal plants | | Ice/chilled water | Building | Hours | High | Load shifting | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How does pumped storage hydropower store energy? A. Compressing air in caverns B. Pumping water to an upper reservoir off-peak and releasing it through a turbine at peak C. Heating molten salt D. Spinning a flywheel Answer: B. Energy stored is E = rho g V H between two reservoirs. (Key point: Pumped storage) 2. Which describes a flywheel? A. Hours of storage at large scale B. Fast response and high power but short duration (seconds to minutes) C. Needs underground caverns D. Uses phase change materials Answer: B. Flywheels store 1/2 J w^2 and suit UPS and frequency regulation. (Key point: Flywheel characteristics) 3. Latent heat storage differs from sensible heat storage because it A. raises temperature of water only B. uses phase change at nearly constant temperature, Q = mL C. uses chemical reactions D. stores cold only Answer: B. PCMs absorb heat during phase change; sensible storage uses Q = m c deltaT. (Key point: Latent vs sensible heat) 4. Molten salt storage in CSP plants allows A. generation after sunset B. pumping water uphill C. air compression D. instant millisecond response Answer: A. It is sensible heat storage kept hot for later use. (Key point: Molten salt use) 5. Compressed air storage has efficiency of about A. 85-95% B. 70-80% C. 40-70% D. 10-20% Answer: C. CAES is 40-70%, lower than pumped storage's 70-80%. (Key point: CAES efficiency) ## Related in Power Plant Design - [Geothermal energy uses and plant types in Nepal](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Ageothermal-use-nepal) — Power Plant Design, chapter 1 ## Related in other subjects - [Energy storage technologies and challenges](https://www.bctengineer.com/question.md?key=semester_7%2Fenergy_environment_and_society%3Aenergy-storage-technologies-challenge) — Energy Environment and Society, chapter 5, asked 3× - [Faraday's laws; mechanical to heat energy](https://www.bctengineer.com/question.md?key=semester_1%2Fengineering_physics%3Afaraday-law-mechanical-to-heat) — Engineering Physics, chapter 8 - [Forms of energy storage](https://www.bctengineer.com/question.md?key=semester_7%2Fenergy_environment_and_society%3Aenergy-storage-forms-four) — Energy Environment and Society, chapter 5 - [Short note: Flywheel energy storage](https://www.bctengineer.com/question.md?key=semester_7%2Fenergy_environment_and_society%3Aflywheel-storage) — Energy Environment and Society, chapter 5