Chapter 2 - Forms of Energy Flashcards
Give examples of two energy storage systems that uses mechanical potential energy?
Storage of water in basins and compressed air systems.
Calculate the energy stored of 1 tonne of water brought up 100 meters. How much coal would be needed to achieve this?
E_pot = mgh.
Then convert from J to kWh to kgCE. (see notes)
Derive the energy storage of compressed gases.
See notes.
What is the expression for rotational energy?
E_rot = 1/2 I w^2 (inertial momentum of sphere is 2/5 mr^2).
What is the energy density of an infinite plane wave of electromagnetic radiation?
E/V = 1/2 eps eps_0 E(t)^2 + 1/2 µ µ_0 H(t)^2 = 1/2 ( ED + HB).
(see slides)
What is the energy in a capacitor?
E = 1/2 CU^2, with C = eps eps_0 * A/d. (see slides)
What can be done to increase the amount of energy stored in a capacitor?
We have to increase the capacitance. This can be done by increasing the area (through nanostructures), decreasing the electrode distance (through use of electrolytes which form a Helmholtz layer) or use an isolation medium with a large permittivity.
How can one describe the energy stored in a coil?
E = 1/2 L I^2 = 1/2 PHI I Coil with n windings, length L and area A: L = µµ_0 * n^2 A/l PHI = n B A E = 1/2 B^2 * lA/µµ_0 = 1/2 µ µ_0 H^2 V.
(see slides)
How does the energy density in a supermagnetic storage medium compare to Li-ion batteries?
About one order of magnitude less. (for µ = 1 and B_max = 20T)
How does the energy density of a commerical supercapacitor with C = 30 F compare to a Li-ion battery?
About two orders of magnitude less.
What is latent heat? Give an example of a technical realization of latent heat as energy storage.
The inner potential energy of condensed matter related to a phase change.
One example is phase change salt. Energy density of around 10^9 J / m^3 (comparable to Li-ion batteries). Used for buffering of solar thermal power plants.
Describe schematically how chemical energy is stored.
When molecules form, the interatomic potential energy is lowered, and thus the excess energy is released (exothermic reaction).
Give examples of the volumetric energy densities for chemical energy.
For hydrogen stored at 700 bar this is about 5 * 10^9 J/m^3.
For gas this is an order of magnitude larger.
Give examples of how chemical energy can be transformed to other forms of energy.
Heat in turbines.
Pressure in explosion engines.
Light in chemo-luminescence.
Electrical energy in batteries and fuel cells.
What are primary, secondary and tertiary elements when talking about transformation of chemical energy into electrical energy?
Primary: non-rechargable.
Secondary: rechargable.
Tertiary: consume external masses.
Explain the basics of a battery.
Defined by the voltage row, that is the “willingness” of an element to undergo reduction. In an electrolyte, the metal is dissolved into metal ions. The countercharge remains in electrode (electrons), which makes a voltage between electrode and electrolyte build up to prevent further dissolution. When connected to another electrode of a different reduction potential, there is a flow of electrons.
What is the Nernst equation?
It gives the relationship between differences of ion concentration and the resulting voltage.
c = c_0 exp (-zeU/kT) U = -kT/Ze ln c/c_0
Which atom has the highest binding energy per nucleon?
Iron.
What is the energy of 1 kg U in terms of kilogram coal equivalent?
Approx. 19 000 kgCE.
Give an example of a fusion reaction and the associated energy released.
2H + 3H -> 4He + n + 17.6 MeV
Give an example of a fission reaction and the associated energy released.
n + 235U -> T_1 + T_2 + 2.43 n + 200MeV.
What is exergy?
The part of energy which can be transformed into work.
What is anergy?
The part of energy which can not be transformed into work.
What happens to the anergy and exergy in a reversible process?
Both quantities are conserved.
What happens to the anergy and exergy in an irreversible process?
Exergy is tranformed into anergy.
How is anergy related to the increase in entropy?
E_an = T_c ∆S = T_c * Q_c/T_c = Q_c
where T_c is the temperature of a the cold reservoir in a heat engine and Q_c is the heat transfered to the cold reservoir.
How is the Carnot efficieny defined?
n_C = |W| / |Q_w| = |Q_w| - |Q_c| / |Q_w|
