Energy Storage and Renewable Hydrogen

Introduction

Renewable electricity from solar photovoltaics and wind is variable, while electrical demand changes continuously. Energy storage provides the time-shifting, balancing, reserve, and power-quality services needed to integrate high shares of renewable generation. Renewable hydrogen extends this function beyond electricity: surplus renewable power can produce hydrogen, which may later be used in industry, transport, heating, or fuel cells.

Storage technologies differ fundamentally in their energy-conversion mechanisms. Batteries store energy electrochemically; pumped hydro and flywheels store it mechanically; thermal systems store internal energy; and hydrogen stores energy chemically. No technology is universally best. The appropriate choice depends on duration, power rating, response time, cycling frequency, efficiency, site conditions, safety requirements, and cost.

This chapter develops methods for comparing storage technologies, sizing battery systems, estimating state of charge and degradation, and evaluating electrolyzers, hydrogen storage, and fuel cells.

Learning Objectives

By the end of this chapter, the reader should be able to:

1. Classify storage technologies as electrochemical, mechanical, thermal, or chemical.
2. Compare storage systems using energy density, power density, efficiency, response time, lifetime, safety, and cost.
3. Size a battery for an energy and power requirement while accounting for depth of discharge and efficiency.
4. Apply state-of-charge and simple degradation models.
5. Explain the purpose and operation of a battery management system.
6. Describe electrolyzer types, hydrogen storage methods, and fuel-cell operation.
7. Evaluate complete renewable-hydrogen pathways and identify their technical and environmental limitations.

1. Storage Services and Performance Metrics

A storage device has two distinct capabilities: power capability and energy capacity. Power capability indicates how rapidly energy can be absorbed or delivered, measured in watts or kilowatts. Energy capacity indicates how much energy can be stored, measured in watt-hours or kilowatt-hours. A device may have high power but low energy, as in a supercapacitor, or high energy but relatively low power, as in a large hydrogen cavern.

Important performance metrics include:

• Round-trip efficiency: energy delivered during discharge divided by energy supplied during charging.
• Response time: time required to begin or complete a requested power change.
• Energy density: stored energy per unit mass or volume, commonly Wh/kg or Wh/L.
• Power density: deliverable power per unit mass or volume, commonly W/kg or W/L.
• Depth of discharge (DoD): fraction of nominal energy capacity removed during a cycle.
• Cycle life: number of specified charge–discharge cycles before capacity or efficiency reaches a defined end-of-life value.
• Calendar life: service life limited mainly by time, temperature, and storage conditions rather than cycling.
• Self-discharge: spontaneous loss of stored energy while the system is idle.
• Levelized cost of storage (LCOS): life-cycle storage cost divided by the total useful energy discharged.

For a storage system, round-trip efficiency is

where η_rt is dimensionless, E_out is useful discharged energy, and E_in is charging energy supplied. The equation applies over the same defined operating period and should include conversion losses if the system boundary includes inverters or power electronics.

Storage duration at rated power is

where t_d is duration in hours, E_useful is usable energy in kWh, and P_rated is rated power in kW. This is an idealized duration; actual duration can change with temperature, state of charge, and power-dependent losses.

2. Classification of Energy Storage

2.1 Electrochemical storage

Batteries convert electrical energy into chemical potential during charging and reverse the reaction during discharge. A cell contains a negative electrode, positive electrode, electrolyte, separator, current collectors, and an external circuit. Cells are connected in series to increase voltage and in parallel to increase capacity and current capability.

Common technologies include lead–acid, lithium-ion, sodium-ion, nickel-metal hydride, sodium–sulfur, and redox-flow batteries. Lithium-ion batteries currently dominate stationary and mobility applications because of their high efficiency, high energy density, fast response, and falling cost. Lithium iron phosphate (LFP) cells generally offer strong thermal stability and long cycle life, whereas nickel-rich chemistries provide higher energy density but require more demanding thermal and safety management.

Flow batteries store active materials in external liquid tanks. Their energy capacity is mainly determined by tank volume and electrolyte concentration, while power is determined by the electrochemical stack. This separation is useful for long-duration applications, although flow batteries typically have lower energy density than lithium-ion systems.

2.2 Mechanical storage

Pumped-storage hydropower stores gravitational potential energy by pumping water to an elevated reservoir. During discharge, water flows through turbines. Its energy is approximately

where ρ is water density in kg/m³, g is gravitational acceleration in m/s², V is stored water volume in m³, h is effective elevation difference in m, and η is the overall charging, hydraulic, and generating efficiency. The result is in joules. Pumped hydro has long life and large capacity, but requires suitable geography and substantial civil infrastructure.

Flywheels store kinetic energy in a rotating mass:

where I is rotational inertia in kg·m² and ω is angular velocity in rad/s. Flywheels respond rapidly and tolerate many cycles, making them useful for frequency regulation and power quality. Their disadvantages include self-discharge, containment requirements, and relatively high cost per stored kWh.

Compressed-air energy storage stores energy by compressing air, usually in underground caverns or pressure vessels. Expansion through turbines produces electricity. Conventional systems may use fuel to reheat expanding air, while adiabatic concepts attempt to retain compression heat and avoid fuel consumption.

2.3 Thermal storage

Thermal energy storage may be sensible, latent, or thermochemical. Sensible storage raises the temperature of a material; latent storage uses a phase change; thermochemical storage uses reversible chemical reactions.

For sensible storage,

where Q is stored thermal energy in joules, m is mass in kg, c_p is specific heat capacity in J/(kg·K), and ∆T is temperature change in K. The equation assumes approximately constant specific heat and negligible phase change.

Molten-salt storage paired with concentrating solar power can shift solar heat from daytime to evening. Hot-water tanks can store heat for district heating or domestic hot-water systems. Thermal storage is often more economical than converting heat to electricity and back when the final demand is thermal. Its electrical round-trip efficiency is less relevant if the stored output is used directly as heat.

2.4 Chemical storage and hydrogen

Chemical storage converts electricity into chemical bonds. Hydrogen produced by electrolysis is the principal renewable chemical-storage pathway. Hydrogen can be compressed, liquefied, stored in underground formations, or converted into derivatives such as ammonia or synthetic fuels.

Chemical storage generally has lower round-trip efficiency than batteries because it includes electrolysis, compression or liquefaction, storage, and reconversion. Its major advantage is duration and versatility: hydrogen can be stored for weeks or seasons and used in sectors that are difficult to electrify directly.

3. Comparative Evaluation

Values are indicative ranges rather than universal specifications. Actual performance depends on system design, operating temperature, power level, maintenance, and the definition of system boundaries.

Technology selection should begin with the service required. Batteries are generally preferred for frequent short-duration cycling and grid services. Pumped hydro is attractive for bulk storage where geography permits. Thermal storage is appropriate when the end use is heat. Hydrogen becomes more compelling as duration increases or when hydrogen is also required as an industrial feedstock or transport fuel.

4. Battery System Architecture and Sizing

A practical battery energy-storage system includes cells, modules, racks, a battery-management system (BMS), a bidirectional inverter, thermal management, protection equipment, enclosures, and an energy-management system. The battery is therefore not equivalent to the cell nameplate capacity.

4.1 Energy sizing

Suppose a load requires E_L kWh of usable energy during an outage. Let η_inv be inverter efficiency, η_bat be battery discharge efficiency, and D be allowable depth of discharge. Required nominal battery capacity is

This expression assumes constant efficiencies and that the load energy is delivered at the design operating condition. It should be increased further if capacity must be guaranteed at low temperature, at end of life, or under high discharge power.

Worked Example: Battery capacity

A remote clinic requires 120 kWh per day from a battery during a two-day autonomy period. The design uses an allowable DoD of 80%, inverter efficiency of 95%, and battery discharge efficiency of 95%. The required load energy is

E_L = 120 × 2 = 240 kWh.

Therefore,

E_nom = 240/(0.80 × 0.95 × 0.95) = 332.4 kWh.

A nominal battery of at least approximately 333 kWh is required before adding temperature, aging, reserve, and manufacturer-sizing margins. If a 20% end-of-life reserve is imposed, the installed initial capacity would be approximately 332.4/0.80 = 415.5 kWh.

4.2 Power sizing

The battery and inverter must also satisfy the maximum simultaneous demand. If the load peak is P_L and inverter efficiency is η_inv, the battery-side discharge power is approximately

The selected battery must meet this power at the relevant state of charge, temperature, and end-of-life condition. Starting currents for motors and compressors may require short-term overload capability.

For a cell or module arrangement, if the required DC voltage is V_sys, the number of series-connected units is approximately

and the number of parallel strings is determined from the required ampere-hour capacity and the capacity of each series string. In practice, integer rounding and voltage operating limits must be checked rather than simply using fractional values.

5. State of Charge and Degradation

State of charge (SoC) is the remaining available charge expressed as a fraction or percentage of a defined reference capacity. The simplest coulomb-counting model is

where SoC_k is the present state of charge, I_c and I_d are charging and discharging currents in amperes, ∆t is the time interval in hours, Q_n is nominal capacity in ampere-hours, and η_c and η_d are charge and discharge efficiencies. Charging and discharging should not be active simultaneously in this simplified model. Current direction conventions must be defined consistently.

Coulomb counting accumulates sensor bias and cannot alone determine the true SoC. Practical BMSs correct it using voltage–SoC relationships, open-circuit-voltage measurements during rest, impedance estimates, and state observers such as Kalman filters.

A simple capacity-fade model is

where Q₀ is initial capacity, N_eq is equivalent full cycles, k_c is a cycling-fade coefficient, k_t is a calendar-aging coefficient, and t is time. This is an empirical approximation, not a universal battery law. Real degradation depends on temperature, average SoC, C-rate, DoD, chemistry, mechanical stress, and time.

Equivalent full cycles can be estimated by summing absolute energy throughput and dividing by twice nominal energy:

The factor of two represents one full charge and one full discharge. This metric is useful for reporting, but it does not imply that partial cycles cause exactly linear degradation.

6. Battery Management and Safety

A BMS performs measurement, estimation, protection, balancing, communication, and thermal control. It monitors cell voltage, pack current, temperature, insulation resistance, contactors, and fault conditions. Its protection functions may include overcharge, over-discharge, overcurrent, short circuit, overtemperature, undertemperature, and isolation faults.

Cell balancing prevents divergence among series cells. Passive balancing dissipates excess energy through resistors and is simple but inefficient. Active balancing transfers energy between cells and can improve efficiency, though it adds complexity and cost.

Thermal management is essential because elevated temperature accelerates aging and can increase the probability of thermal runaway. Protection is based on prevention, detection, containment, and emergency response. A professionally designed installation requires appropriate spacing, ventilation where applicable, fire detection, fault isolation, maintenance procedures, and compliance with applicable electrical and fire codes.

Safety is not determined solely by chemistry. It also depends on manufacturing quality, enclosure design, charging controls, mechanical damage, installation, and emergency procedures. Recycling and end-of-life management should be included in procurement decisions, especially for systems containing critical minerals or hazardous electrolytes.

7. Electrolyzers and Renewable Hydrogen

Electrolysis splits water into hydrogen and oxygen using electricity:

2H₂O → 2H₂ + O₂.

The minimum reversible electrical work is related to the Gibbs free-energy change, while the thermoneutral voltage includes the enthalpy change. Actual electrolyzers require additional voltage because of activation, ohmic, and mass-transfer losses.

The principal electrolyzer types are:

• Alkaline electrolyzers: use an aqueous alkaline electrolyte, commonly potassium hydroxide. They are mature and relatively economical but generally have slower dynamic response and lower current density than some alternatives.
• Proton-exchange-membrane (PEM) electrolyzers: use a solid polymer membrane, respond rapidly, and can operate at high current density. They may require costly catalyst materials and high-purity water.
• Solid-oxide electrolyzers: operate at high temperature and can use electrical energy plus process heat. They offer potentially high efficiency but have demanding thermal management and durability requirements.

Hydrogen production can be estimated from specific electricity consumption:

where m_H2 is hydrogen mass in kg, E_el is electricity input in kWh, and e_spec is specific electricity consumption in kWh/kg. The value of e_spec must include the chosen system boundary, such as stack auxiliaries, water treatment, and compression.

Worked Example: Electrolyzer output

An electrolyzer consumes 52 kWh per kg of hydrogen, including balance-of-plant consumption. With 10,400 kWh of renewable electricity,

m_H2 = 10,400/52 = 200 kg.

If the hydrogen lower heating value is approximately 33.3 kWh/kg, its chemical energy is about 200 × 33.3 = 6,660 kWh. The electricity-to-hydrogen energy efficiency on an LHV basis is therefore approximately 6,660/10,400 = 64.0%. This calculation excludes any later compression, transport, storage, or reconversion losses.

8. Hydrogen Storage and Transport

Hydrogen may be stored as compressed gas, cryogenic liquid, in metal hydrides, in chemical carriers, or in underground formations.

Compressed gas is common for distributed systems and vehicle refueling. Increasing pressure reduces volume but raises compression energy, vessel cost, and safety requirements. Liquid hydrogen has higher volumetric density than compressed gas but requires temperatures near 20 K and incurs liquefaction energy and boil-off losses. Salt caverns can provide very large seasonal storage where suitable geological formations exist. Ammonia and liquid organic hydrogen carriers simplify some transport functions but require additional conversion steps and may introduce toxicity or energy penalties.

Hydrogen has low density and a wide flammability range in air. It can leak through small openings and has a low ignition energy. Design therefore requires leak detection, ventilation, separation distances, pressure relief, compatible materials, grounding, hazardous-area classification, and controlled ignition sources. Hydrogen embrittlement can affect some metals and must be addressed through material selection.

9. Fuel Cells and the Complete Hydrogen Pathway

A fuel cell converts chemical energy directly into electricity through electrochemical reactions. In a proton-exchange-membrane fuel cell, hydrogen is oxidized at the anode and oxygen is reduced at the cathode. Protons cross the membrane, while electrons travel through the external circuit.

The overall reaction is

2H₂ + O₂ → 2H₂O + electricity + heat.

Fuel-cell efficiency is

where E_electric is electrical output and E_H2 is the chemical energy of consumed hydrogen on a stated heating-value basis. Fuel cells produce heat as well as electricity; combined heat and power can substantially improve total useful-energy efficiency.

A renewable-hydrogen-to-electricity chain may include renewable generation, rectification, electrolysis, drying, compression, storage, fuel-cell conversion, and power conditioning. If component efficiencies are η₁, η₂, …, η_n, the approximate overall efficiency is

This assumes sequential independent conversion stages and clearly defined system boundaries.

For example, with electrolyzer efficiency 65%, compression and storage efficiency 90%, fuel-cell efficiency 55%, and inverter efficiency 95%,

η_total = 0.65 × 0.90 × 0.55 × 0.95 ≈ 0.306.

Thus approximately 31% of the original renewable electricity returns as electricity. Despite this low round-trip value compared with batteries, hydrogen may be justified for seasonal storage, backup power, remote logistics, or industrial applications where batteries are impractical.

10. Applied Case: Renewable Microgrid for an Island Clinic

Background. An illustrative island clinic has a 150 kW peak load and consumes 600 kWh per day. Solar generation is abundant during daylight but unavailable at night, and fuel delivery is expensive and vulnerable to disruption.

Problem. The operator requires overnight energy shifting, short-duration power quality, and resilience during several cloudy days. Space is limited, and local personnel have limited technical training.

Analysis. A battery can efficiently handle daily cycling and fast load changes. A hydrogen system can provide longer-duration backup without installing an extremely large battery. Thermal storage may reduce electricity consumption by shifting water-heating demand.

Approach. The design uses a 400 kWh usable LFP battery, a 200 kW bidirectional inverter, solar PV, efficient refrigeration, and a small electrolyzer supplied only after the battery reaches a high SoC. Hydrogen is stored in certified pressure vessels and used in a fuel-cell generator during extended low-solar periods. The control hierarchy gives priority to critical clinical loads, then refrigeration and communications, and finally noncritical loads.

Assume the battery must supply 300 kWh during a night, with DoD 80%, battery efficiency 95%, and inverter efficiency 95%. The nominal energy requirement is

E_nom = 300/(0.80 × 0.95 × 0.95) = 415.8 kWh.

A 400 kWh nominal battery would therefore be marginal and would not meet the stated requirement with aging and temperature margins. The design should either increase battery capacity or reduce the required night energy. This illustrates why usable capacity, rather than nameplate capacity, must guide engineering decisions.

Outcome. The battery handles normal daily operation, while hydrogen reduces the amount of battery capacity needed for multi-day resilience. The project remains dependent on reliable maintenance, water purification, hydrogen safety training, and replacement planning.

Learning points. Hybrid storage is often more economical than forcing one technology to perform every service. Critical-load definition, end-of-life sizing, and operational training are as important as nominal equipment ratings.

11. Economic, Environmental, and Governance Considerations

Capital cost should not be evaluated without life-cycle cost. A simplified LCOS expression is

where C_cap is capital cost, C_fixed is discounted fixed operating cost, C_variable is variable operating cost, C_replacement is replacement cost, V_residual is residual value, and E_lifetime is total useful energy discharged. A rigorous financial analysis discounts future cash flows and treats degradation explicitly.

Environmental assessment should include materials extraction, manufacturing, transport, operating emissions, water use, recycling, and disposal. Hydrogen is not automatically renewable: its environmental quality depends on the electricity source, production timing, water supply, methane leakage if natural gas is involved, and accounting method.

Governance issues include transparent carbon accounting, worker safety, emergency planning, land and water rights, responsible mineral sourcing, recycling obligations, and clear rules for grid interconnection. AI-based forecasting and control can improve renewable-storage dispatch, but models must be validated against extreme weather, protected from cyberattack, and supervised by qualified operators. Automated decisions should not bypass safety interlocks or regulatory requirements.

12. Contemporary Developments and Best Practices

Current development trends include sodium-ion batteries, improved LFP systems, flow batteries for longer duration, solid-state cells, high-temperature electrolysis, reversible solid-oxide systems, and hydrogen derivatives such as ammonia. These technologies remain at different stages of commercial maturity; claims should be assessed using independent performance data and full system boundaries.

Best practice includes:

1. Define the required service before selecting technology.
2. Specify both power and usable energy, not only nominal capacity.
3. Include efficiency, temperature, aging, reserve, and end-of-life margins.
4. Use independent measurements and consistent system boundaries.
5. Design BMS, thermal management, fire protection, and cybersecurity together.
6. Assess water, materials, recycling, and community impacts.
7. Maintain operational records for SoC, temperature, alarms, energy throughput, and degradation.
8. Use hybrid systems when different technologies have complementary strengths.

Chapter Summary

Energy storage enables renewable electricity to provide dependable energy and grid services. Electrochemical batteries are efficient and fast; mechanical systems can deliver large-scale or high-cycle storage; thermal systems are highly effective when the final demand is heat; and chemical storage, especially hydrogen, supports long-duration storage and sector coupling. Meaningful comparison requires power, energy, efficiency, response time, density, lifetime, safety, cost, and site constraints.

Battery sizing must distinguish usable from nominal capacity and account for depth of discharge, conversion efficiency, temperature, aging, and reserve requirements. State-of-charge estimation commonly combines coulomb counting with voltage and model-based correction. A BMS protects cells, balances them, manages temperature, and communicates operating conditions.

Electrolyzers convert renewable electricity into hydrogen, while compression, liquefaction, geological storage, or chemical carriers determine how hydrogen is stored and transported. Fuel cells convert hydrogen back to electricity, but the complete electricity-to-electricity pathway has substantially lower efficiency than direct battery storage. Hydrogen is nevertheless valuable for seasonal storage, industrial feedstock, transport, and resilient backup. Sound engineering selects technologies according to the service required and includes safety, environmental, economic, and governance considerations.

Key Terms

Energy density: Stored energy per unit mass or volume.

Power density: Deliverable power per unit mass or volume.

Round-trip efficiency: Useful discharge energy divided by charging energy.

Depth of discharge: Fraction of nominal capacity removed during a discharge cycle.

State of charge: Remaining available charge expressed relative to a defined capacity.

Equivalent full cycle: Normalized measure of cumulative charge–discharge throughput.

Calendar aging: Degradation caused primarily by time and storage conditions.

Battery-management system: Electronics and software that monitor, protect, balance, and control a battery.

Thermal runaway: Self-accelerating exothermic failure that can occur in a battery cell.

Pumped-storage hydropower: Mechanical storage using elevated water and turbines.

Flywheel: Rotating mechanical storage device that stores kinetic energy.

Sensible thermal storage: Storage based on temperature change without phase change.

Electrolyzer: Device that uses electricity to split water into hydrogen and oxygen.

Alkaline electrolyzer: Electrolyzer using an aqueous alkaline electrolyte.

PEM electrolyzer: Electrolyzer using a proton-exchange membrane.

Fuel cell: Electrochemical device that converts fuel chemical energy directly into electricity and heat.

Hydrogen embrittlement: Loss of material ductility or strength caused by hydrogen interaction.

Levelized cost of storage: Life-cycle storage cost per unit of useful discharged energy.

Usable capacity: Energy available within specified operating limits and efficiency boundaries.

Sector coupling: Use of one energy carrier, such as hydrogen or electricity, across multiple sectors.

Review Questions

Short Answer Questions

1. Distinguish energy capacity from power rating in an energy-storage system.
2. Why is usable battery capacity smaller than nominal battery capacity?
3. State two functions performed by a battery-management system.
4. Name three methods for storing hydrogen.
5. Why can hydrogen be attractive despite having lower electricity-to-electricity efficiency than batteries?

Long Answer Questions

1. Compare lithium-ion, pumped hydro, flywheel, thermal, and hydrogen storage using efficiency, response time, lifetime, safety, and application duration.
2. Explain coulomb counting for SoC estimation and discuss its principal sources of error.
3. Describe the operating principles and main engineering characteristics of alkaline, PEM, and solid-oxide electrolyzers.
4. Explain the safety requirements for a compressed-hydrogen installation.
5. Discuss why hybrid battery–hydrogen systems may be preferable for renewable microgrids.

Analytical / Application Questions

1. A facility requires 180 kWh of usable energy. If allowable DoD is 75%, battery efficiency is 92%, and inverter efficiency is 96%, calculate the minimum nominal battery capacity. State two additional design margins that should be considered.
2. A renewable-hydrogen system has electrolyzer efficiency 68%, storage and compression efficiency 88%, fuel-cell efficiency 52%, and inverter efficiency 94%. Calculate its approximate electricity-to-electricity efficiency and compare the result qualitatively with a battery system.
3. A remote site has frequent one-second power disturbances, daily energy shifting, and occasional ten-day renewable shortages. Recommend a combination of storage technologies and justify the choice using power, duration, efficiency, and lifecycle considerations.

Multiple Choice Questions with Answers

Question 1

Which metric describes the fraction of charging energy that can be recovered during discharge?

A. Power density

B. Round-trip efficiency

C. Depth of discharge

D. Calendar life

Correct Answer: B. Round-trip efficiency

Question 2

A load requires 100 kWh of usable energy. With allowable depth of discharge 80%, battery efficiency 95%, and inverter efficiency 95%, what nominal battery capacity is required?

A. 80.0 kWh

B. 90.3 kWh

C. 138.5 kWh

D. 100.0 kWh

Correct Answer: B. 90.3 kWh

Question 3

Which storage technology stores energy as gravitational potential energy?

A. Flywheel

B. Lithium-ion battery

C. Pumped-storage hydropower

D. PEM electrolyzer

Correct Answer: C. Pumped-storage hydropower

Question 4

In the simplified SoC model, which measurement is directly accumulated over time?

A. Current

B. Ambient humidity

C. Hydrogen pressure only

D. Reservoir elevation

Correct Answer: A. Current

Question 5

What is a principal function of a battery-management system?

A. Generate hydrogen from water

B. Balance cells and protect them from unsafe conditions

C. Increase the gravitational head of a reservoir

D. Remove all battery conversion losses

Correct Answer: B. Balance cells and protect them from unsafe conditions

Question 6

An electrolyzer consumes 50 kWh per kilogram of hydrogen. How much hydrogen is produced from 5,000 kWh, ignoring other constraints?

A. 10 kg

B. 50 kg

C. 100 kg

D. 250 kg

Correct Answer: C. 100 kg

Question 7

Which electrolyzer type is known for rapid dynamic response and a proton-exchange membrane?

A. PEM electrolyzer

B. Conventional pumped hydro

C. Molten-salt tank

D. Compressed-air cavern

Correct Answer: A. PEM electrolyzer

Question 8

Why is hydrogen generally suitable for seasonal storage?

A. It has zero conversion losses

B. It can be stored chemically for long periods and used in multiple sectors

C. It always has higher round-trip efficiency than batteries

D. It requires no safety controls

Correct Answer: B. It can be stored chemically for long periods and used in multiple sectors

Question 9

For a sensible thermal-storage medium, which equation estimates stored heat under constant specific heat?

A. E = 1/2 Iω²

B. Q = mc_p∆T

C. η = E_out/E_in

D. P = V/I

Correct Answer: B. Q = mc_p∆T

Question 10

A system has electrolyzer efficiency 0.60, storage efficiency 0.90, fuel-cell efficiency 0.50, and inverter efficiency 0.95. What is its approximate electricity-to-electricity efficiency?

A. 0.26

B. 0.41

C. 0.95

D. 2.05

Correct Answer: A. 0.26