Download Energy: Forms and Conversions

Energy Forms and Energy
Conversion
Energy
• The capacity of vigorous activity
• The ability to act
• The capacity of a body or a system to do work
(and heat).
Mechanical Energy: Kinetic + Potential
mv
E  U  K  mgh 
2
Use directly for hydro
energy storage
2
h
Chemical Energy:
Microscopic Version of the Potential Energy
Potential energy of a bond between atoms Perhaps the most important way to store
forming a molecule
energy to be released via chemical reaction,
such as methane burning
3
CH 4  O2  CO2  2H 2O
2
or fuel cell reaction
1
H 2  O2  H 2O
2


Thermal Energy:
Microscopic Version of the Mechanical Energy
Use for solar thermal power generation
Ethermal  CT
C – heat capacity
For a gas the thermal energy is
just the kinetic energy
In general thermal energy has
kinetic and potential components
(potential energy stored in the
vibrating bonds)
Electrical Energy: Potential Energy of
Electrostatic Interactions
q1
q1
r
Eelectrostatic
q1 q2

r
In a capacitor
1
2
E  CV
2

C

 0 A
d
Electromagnetic Energy
Energy in oscillating electromagnetic waves
These waves can be considered as massless particles called
photons traveling with the speed of light
Photon energy hv (h –Planck constant, v frequency)
Power generator turns
chemical into mechanical
into electromagnetic, into
electric current
Nuclear Energy
Potential energy of
bonded protons and
neutrons forming
nucleus
The energy amount is
so high that during
nuclear energy
release it can be
observed that mass is
reduced according to
E=mc2
Energy Conservation
Energy can not be created or destroyed – it can be only
converted
Energy of the system can be changed by the flow of energy
form or to the outside in the form of work, W, and heat, Q
ΔE = W + Q
Work
Work = force times displacement
W = Fd
The unit of work is the same as the unit of energy
m
kg 2
s
The unit of force
thus the unit of energy
This unit is called Joule


m2
kg 2
s
Heat
Heat, as work is also a process – it is essentially work done
by molecules thus it can change the energy of the system
Energy and Power
Power is the energy change rate
W = ΔE/Δt
The unit of power is Watt (W)
The World uses about 15 Terra Watts of power, with about
2.5 TW of electricity
Energy Units - I
Joule = Watt x second
Kilowatt-hour = 3.6 Megajoules– on your electric bill
Therm = 105 Megajoules = 100,000 British thermal units
(BTU) – about 100 cubic feet natural gas burning equivalent
BTU – energy needed to heat one pound of water by one °F
Energy Units - II
Calorie - heating 1 g of water by 1 °C = 4.2 Joules
Electronovolt – energy gained by moving one electron across
potential difference of one Volt = 1.6 x 10-19 Joules (one Volt
= 1J/Coulomb)
Energy = Volt x Coulomb = Volt x Ampere x time
Power = Volt x Ampere
Energy Conversion
Useful output is generally
one that can produce work
(mechanical, electrical)
Sometimes heat can be
useful e.g., to heat a house
Chemical Energy Conversion
Most important is combustion – potential chemical energy is
transformed to kinetic energy of the gas (high temp and pressure).
For example,
Yields about 240 kJ/mole energy leading to a high pressure gas.
This energy shows as the kinetic energy
This energy can be used directly as mechanical energy (car) or
converted to other useful energy, such as electrical energy
Alternator
Converts mechanical energy to AC electricity
Stationary wire
Rotating magnet
Transformer
Can easily change the voltage of an alternating current
Why AC won over DC
AC transformers can easily increase and decrease voltage
The input power is P=VI (voltage times current)
The voltage on a transmission wire is V = IR
The power loss on the transmission wire is Ploss = I2R, and since I=P/V,
Ploss =R P2/V2
Higher voltage means much lower transmission loss
This was a rare time that Thomas Edison lost big time to Tesla and
Westinghouse (War of Currents).
AC vs. DC in the future
Distributed power generation, e.g., by solar, allows to avoid
transmission power losses due to short lines.
Transformation losses are eliminated
Many devices work on DC only
So Edison might be right in the future
Solar to Electricity
Photon excites negatively charged
electrons and separates them from
positively charged holes – this generates
electrostatic energy and associated
voltage (DC)
Limits of efficiency is mostly due to two
factors
Not all photons lead to excitation
Electrons and holes recombine and just
generate heat
Efficiency – Carnot Cycle
Efficiency = Work/Input heat
Hot heat is useful
Efficiency and Thermal Energy Storage
Carnot conversion efficiency
(Thot-Tcold)/Thot*100%
Increases with increasing Thot
But stored heat radiates at the rate ~ (Thot)4
thus the loss increases with increasing Thot
Efficiency in General
Efficiency = Useful Energy/Energy Input
Typical fossil fuel power plant up to 40%
Hydro – about 90%
Solar 6-40%, 15% typical
Combustion engine ~ 30%
Electric engine 70-90%
Photosynthesis few percent
Energy Returned on Energy Invested
EROEI = Usable Energy/Energy Expended
Sankey Diagram – Electric Plant
Sankey Diagram – USA