EE535: Renewable Energy:
Systems, Technology &
Economics
Session 3: Energy Conversion 1
Energy Conversion Processes
• A large array of energy conversion
processes occur in nature
• Man is capable of performing a number of
addition processes using a variety of
devices (or processes)
• Usually more than one form of energy will
emerge due to the action of a device
• Many devices perform a number of
conversion processes: e.g. : power plant
Power Plant: Chemical -> heat -> mechanical -> electrical
Energy Conversion Processes
Initial Energy Form
Chemical
Radiant
Electrical
Mechancial
Nuclear
Heat
Reactor
Chemical
Radiant
Photolysis
Electrical
Electrolysis, Battery
Charging
Fuel Cell, Battery
Discharge
Burner, Boiler
Photovoltaic cell
Absorber
Lamp, Laser
Electric Motor
Resistance,
Heat
Pump
Mechanical
Electric Generator,
MHD
Turbines
Friction
Heat
Thermionic &
thermoelectric
generators
Thermodynamic
Engines
Radiator
Thermodynamics
• First Law (Energy Conservation)
– Energy can neither be created or destroyed
– Energy can be utilized, not consumed
• Second Law
– The quality of a particular amount of energy, i.e. the amount of
work or action that it can do, diminishes each time this energy is
used.
– We degrade or randomize energy
• Energy we use is degraded into heat and then radiated
out into space
Entropy
Low Entropy
High Entropy
1kJ of Energy
1kJ of Energy
Use Up
Engine
+ Petrol
Moves
Waste heat
Exhaust gases
Mechanical energy (20%)
Electrical Energy
(Disordered energy has high entropy)
Quality of energy in the universe is constantly diminishing
Engines
• Work can be changed into heat relatively
easily – the reverse is a different challenge
• From the second law of thermodynamics,
we know that :
– it is not possible to change completely into
work, with no other change taking place
• You cannot build a device that violates this
law
Simple Device - Piston
p
pressure
weight
W
Vi
Vf
V
Q
T
volume
Removing weight from the piston permits the gas to expand
The gas remains at constant temperature, absorbing heat Q from the reservoir
The system follows the isotherm and does work W by lifting the weight
The internal energy U (which depends only on temperature for an ideal gas), does
not change during this isothermal expansion
Simple Device - Piston
• From first law of thermodynamics, Q=ΔU + W
• In the piston example, the work W is exactly
equal to the heat Q extracted from the reservoir
– have we not just broken the second law of
thermodynamics?
• No – the system is not in the same state (volume
and pressure changed)
• To restore the system to its original condition,
the piston must operate in a cycle –
• A device that changes heat into work is called a
Heat Engine
Engine Cycle
TH
• During every cycle, heat QH is
extracted from a reservoir at
temperature TH
• A portion is diverted to useful
work W and the rest is
discharged as heat Qc to a
reservoir at temperature Tc
• Because an engine operates in
a cycle, U (gas) returns to its
original value at the end of the
cycle (ΔU = 0)
• So, work done per cycle must
equal net heat transferred per
cycle:
• |W| = | QH | -| Qc |
engine
QH
W
QC
TC
Efficiency
• The purpose of an engine is to transform
as much extracted heat QH into work as
possible
• We measure success of the engine by its
thermal efficiency e – defined as the ratio
of the work done per cycle:
• e = |W| / |QH| = (|QH| - |QC|)/|QH|
• In the case of (an impossibly) perfect
engine, QC = 0
The Ideal Engine
• There are no perfect engines
• When considering gases, we are
assuming an ideal gas – ignoring
complexities of real gases
• Assumed no friction, turbulence, unwanted
heat transfer
• System stays in thermal equilibrium at all
times
• Reversible process
The Carnot Cycle
•
•
Step 1
– Starting at (a), put the cylinder on
a high temperature stand and
remove weight and allow system
to expand to (b)
– Heat QH is absorbed by the
system
– Isothermal process –> U doesn’t
change & added heat appears as
work
p
pa
a
QH
b
pb
Step 2
– From (b), put the piston on an
insulating stand and remove
weight and allow system to
expand to (c),
– This expansion is adiabatic as no
heat enters or leaves the system
– The system does work by moving
the piston further and the
temperature drops to Tc, as the
energy to do the work must come
from internal energy of the system
W
pd
pc
QC
c
d
Va Vd
Vb
Vc
V
The Carnot Cycle
•
Step 3
– From (c), put the cylinder on a
p
colder heat reservoir
– Increase weight to the piston and
compress the gas to point (d)
pa
– Heat Qc is transferred from the
gas to the reservoir
– Compression is isothermal & work
is done on the gas by the
descending piston & its load
a
QH
b
pb
•
Step 4
– From (d), put the piston on an
insulating stand and add weight
and force system to compress
back to (a),
– This compression is adiabatic as
no heat enters or leaves the
system
– Work is done on the gas and its
temperature rises to Tc
W
pd
pc
QC
c
d
Va Vd
Vb
Vc
V
The Carnot Engine
• The special property of the Carnot engine is that its
thermal efficiency can be written as:
 e = (TH – Tc)/ TH [Proof?]
• i.e. the efficiency depends only on the temperatures of
the two reservoirs between which it operates
• No real engine operating between the same 2
temperatures can have an efficiency greater than a
Carnot engine
• The Carnot engine represents the limiting behavior of
real engines
• When run in reverse, the engine can be operated as a
Carnot refrigerator, with a coefficient of performance
given by:
 K = Tc/(TH – Tc) [Proof?]
Efficiency
• A consequence of the second law is that
the perfect heat engine is impossible
The efficiency of an ideal heat engine is: Efficiency e = 1 – T2/T1
Where T1 is the temperature of the heat source (hot gas produced by fuel combustion
Where T2 is the surrounding air temperature
For a car, T1 = 2400K, T2 = 300K
So (Theoretical Efficiency) = 1 – 300/2400
= 0.88
Calorific Value
• The calories of thermal units contained in
one unit of a substance and released
when the substance is burned
• Energy Density
What is the calorific value of petrol: = 10kWh/litre
Note: density of petrol assumed to be 0.84kg/l
Car Example
Energy used per day =
=
Distance travelled per day X Energy per unit of fuel
Distance per unit of fuel
50km/day
12km/litre
X 10kWh/litre
= 41.6 kWh/day
What about the energy cost of producing the car’s fuel?
What about the energy cost of manufacturing the car?
Other Thermodynamic Cycles
Cycle\Process
Compression
Heat Addition
Expansion
Heat Rejection.
Power cycles normally with external combustion - or heat pump cycles
Ericsson (First, 1833)
Brayton
adiabatic
isobaric
adiabatic
isobaric
(Reverse Brayton)
adiabatic
isobaric
adiabatic
isobaric
Carnot
isentropic
isothermal
isentropic
isothermal
Stoddard
adiabatic
isometric
adiabatic
isometric
isothermal
isometric
isothermal
isometric
isothermal
isobaric
isothermal
isobaric
Bell Coleman
Stirling
Ericsson (Second, 1853)
Power cycles normally with internal combustion
Otto (Petrol)
adiabatic
isometric
adiabatic
isometric
Diesel
adiabatic
isobaric
adiabatic
isometric
Brayton (Jet)
adiabatic
isobaric
adiabatic
isobaric
isobaric
isometric
adiabatic
isobaric
Lenoir (pulse jet)
(Note: 3 of the 4 processes are different)
Things to Consider
• How much does a kWh of electricity cost a
domestic user in Ireland?
• Please investigate the use of a Stirling
Engine in Solar – Thermal Applications