Other Sources and Energy Flexibility

Comprehensive practice flashcards covering hydroelectric, wave, tidal, geothermal, bioenergy, and grid flexibility topics from lectures RB1 through RB6.

What percentage of the sun's incoming energy is used for evaporating water, and what is hydropower's share of the global electricity supply?

Approximately $25\%$ of the sun's incoming energy goes into evaporating water into the atmosphere. Hydropower provides about $20\%$ of the world's electricity supply.

What is the equation for theoretical hydraulic power ($P$) in a water flow, and which factors determine the choice of turbine?

$P = \rho \cdot g \cdot Q \cdot h$ (where $\rho$ is water density, $g$ is gravity, $Q$ is volume flow rate, and $h$ is head height). The turbine choice depends on head and flow: Pelton (Impulse) for high head, low flow; Francis (Reaction) for medium head, medium flow; and Kaplan (Propeller) for low head, high flow.

How can tidal basin energy extraction be boosted during high tide, and what is the formula for the basic potential energy ($E$)?

Energy can be boosted by pumping water into the basin at high tide when the sea and basin levels match. The basic formula is $E = \frac{1}{2} \times \rho \cdot g \cdot A \cdot R^2$, where $A$ is the basin surface area and $R$ is the tidal range.

How does a Wells turbine operate and why is it used in Oscillating Water Column (OWC) systems?

A Wells turbine has symmetrical blades that always spin in the same direction regardless of air flow direction. This is ideal for the bidirectional, oscillatory airflow generated by waves in shoreline OWC chambers.

What is the primary distinction of deep geothermal energy compared to other renewables, and what are the typical heat flow values?

Deep geothermal is the only renewable source completely independent of the sun. Global average heat flow is approximately $60\,mW/m^2$, while tectonic plate boundaries can reach approximately $300\,mW/m^2$.

Why is deep geothermal extraction characterized as "mining"?

Local extraction rates typically exceed natural thermal recharge from the Earth's core by $10$ to $100$ times. This causes the localized rock temperature to cool, requiring a rest period to recover.

How do Ground Source Heat Pumps (GSHP) utilize solar energy and what is the sizing equation for the ground loop array?

GSHPs use the earth's surface layer as a solar thermal accumulator. To prevent soil freezing, the area is sized using: $\text{Area } (m^2) = \frac{\text{Heat Evaporator Capacity (W)}}{\text{Average Solar Energy Absorbed by Soil (W/m}^2\text{)}}$.

What is the balanced chemical equation for the photosynthesis process in biomass formation?

$$6\text{CO}_2 + 6\text{H}_2\text{O} + \text{light energy} \rightarrow \text{C}_6\text{H}_{12} \text{O}_6 + 6\text{O}_2$$

Name the three primary pathways for converting raw biomass into modern secondary energy carriers.

1. Thermal conversion (e.g., direct combustion or gasification); 2. Chemical conversion (e.g., processing vegetable oils into biodiesel); 3. Biochemical conversion (e.g., anaerobic digestion for biogas or fermentation for bioethanol).

What is the contribution of biomass to UK electricity and what is the related land-use concern?

Biomass provided approximately $11\%$ of UK electricity in 2022. The concern is the displacement of food production, as roughly $2.2\%$ of arable UK land ($133,000\text{ hectares}$) is used for bioenergy crops.

Why does a decarbonized grid increase the need for energy flexibility?

1. Intermittency: Generation is weather-dependent and un-dispatchable, mismatching demand. 2. Reduced Inertia: The loss of heavy rotating mass from fossil-fuel synchronous generators causes grid frequency to fluctuate much faster during power changes.

What are the four temporal scales of grid balancing and their associated challenges?

1. Short duration (~$1\text{ second}$): Frequency balancing and stability. 2. Diurnal (Within day): Shaving morning/evening peak demand. 3. Longer duration (Days to weeks): Managing "wind droughts." 4. Interseasonal: Compensating for differences between summer generation and winter heating demand.

What is the key difference between Sealed Batteries and Flow Batteries?

Sealed Batteries (e.g., Li-ion) have fixed internal electrodes where scaling power also scales capacity. Flow Batteries (e.g., Vanadium Redox) store liquid electrolytes in external tanks, allowing independent sizing of energy capacity (tank size) and power capacity (cell stack size).

Distinguish between Diabatic and Adiabatic Compressed Air Energy Storage (CAES).

In Diabatic CAES, compression heat is wasted, and air must be reheated by natural gas during discharge ($\text{RTE} < 50\%$). In Adiabatic CAES, compression heat is captured and stored to reheat the air during discharge ($\text{RTE} > 70\%$).

What are the characteristics and typical efficiency of Hydrogen energy storage?

Advantages include high chemical energy density and cheap long-duration storage utilizing existing gas networks. Disadvantages include high capital costs, high pressure requirements, and safety hazards. Typical Power-to-Gas-to-Power (P2G2P) Round Trip Efficiency is very low, around $~40\%$.

Define Levelised Cost of Storage (LCOS) and provide its formula for a $0\%$ discount rate ($r = 0\%$).

LCOS is the total cost per unit of discharged electricity over the facility's lifetime. For $r = 0\%$: $\text{LCOS} = \frac{\text{CAPEX} + \sum \text{OPEX}_n + \text{End-of-Life Costs}}{\sum \text{Discharged Energy Out}_n}$.

How does charging efficiency affect the operational cost of storage and how is the Effective Charging Cost calculated?

Lower Round Trip Efficiency ($\eta$) requires buying more electrical energy than can be sold back. The calculation is: $\text{Effective Charging Cost} = \frac{\text{Electricity Purchase Price}}{\text{Round Trip Efficiency } (\eta)}$.