The document discusses combining cogeneration (COGEN) with compressed air energy storage (CAES) to improve energy efficiency. It explains that COGEN captures waste heat from electricity production, while CAES has 70% round-trip efficiency. The combination provides better efficiency than either alone. It also describes using the heat of air compression in CAES, then recovering work from the compressed air. This yields total recovered heat and electricity greater than the input, allowing over 100% efficient energy storage. Diagrams show the thermodynamic processes and potential system design.

1. COGEN
+CAES
>1.0
Combining Compressed Air Energy
Storage with Cogeneration, or Using
Heat of Compression during CAES,
Yields Improved Energy Efficiency
May, 2012
Peter Materna
Peter Materna peter_materna@yahoo.com 1

2. COGEN
+CAES Overview
>1.0 • Cogenerating electricity and • Compressed Air Energy
heat results in utilizing Storage (CAES) has
typically 80%-90% of the “round-trip” efficiency
heating value of fuel, by often described as ~70%
virtue of the capture and (depending on details and
use of “waste” heat. This is on definition of efficiency).
better than the efficiency of This is typical of various
a stand-alone electric forms of energy storage,
generating plant, but is less and is less than unity.
than unity.
• Combining Cogen and CAES
technologies provides a better
efficiency than either one alone.
• Even if not done in conjunction with a
thermodynamic power plant cycle,
capture and use of heat of compression
along with later recovery of work is
beneficial.
Peter Materna peter_materna@yahoo.com 2

3. COGEN
+CAES Overview and Thermodynamics
>1.0
• During compressing of air, Work W is performed and
heat of compression Q is produced (and often is
rejected as waste heat)
W = Work inputted
Atmospheric pressure,
Elevated pressure,
ambient temperature,
ambient temp.,
E=Ezero
E=Ezero
Q = Heat of compression
outputted
• More specifically, if the final temperature of the
compressed air is its temperature at the intake of the
compressor, then this heat of compression Q is
exactly equal to the work of compression W
• How much of this heat Q is useful depends on the
temperature at which heat can be utilized
Peter Materna peter_materna@yahoo.com 3

4. COGEN
Overview and Thermodynamics (cont’d)
+CAES
• The compressed air, at elevated pressure, is able to do work
>1.0 when it is released (and furthermore can be stored so that its
release accomplishes time-shifting for load-leveling).
• The work recovered upon release of the compressed air is in
addition to the heat of compression captured earlier.
• So, heat of compression + recovered work > work of
compression
• This is thermodynamically permissible because the compressed
air released from the work recovery device is cold, which means
that the heat content of the atmosphere after the process is less
than it was before the process.
• The compressed air is actually like the refrigerant in a
refrigeration cycle. This result is analogous to the recognized
fact that a heat pump delivers more heat than the electricity that
it consumes, by virtue of removing heat from the atmosphere or
ground. The performance for a compressed air refrigeration
cycle is not as good as for a typical heat pump refrigeration
cycle involving a phase change of the working fluid, but the
ability to store the compressed air for later use is advantageous.
Peter Materna peter_materna@yahoo.com 4

5. COGEN
+CAES
>1.0 Simplified Energy Budgets
for Power Generation and for Cogeneration
Simple central station Cogeneration
thermal power plants power plants
Mechanical Electricity Mechanical Electricity
Heat work Heat work
value value
Refer- of fuel Refer- of fuel Refer-
ence or ence or ence
Useful
value energy Heat not value energy Heat not value
Rejected heat
source converted source converted
heat
into work into work
Rejected heat
Peter Materna peter_materna@yahoo.com 5

6. COGEN
+CAES Simplified Energy Budget
>1.0 for Using a Fuel for Cogeneration Then Using
the Mechanical Power to Compress a Gas
Mechanical
work Electricity
recoverable
Heat of Useful
Mechanical com- heat
Heat work pression Refer- >100%
value Non-useful heat Useful ence
of fuel heat value Compared to
Refer-
or ordinary co-
ence Useful
energy Heat not generation, this
value heat
source Rejected output contains
converted
heat more heat and
into work
Rejected or less electricity;
non-useful heat nevertheless,
Rejected heat
the total of
heat+electricity
is greater than
Note: sizes of various bars are simply intended as Generation of for ordinary co-
representative values for illustration, and mostly are electricity can generation, and
estimates. Final results are to be viewed also keeping be time- potentially even
in mind that for typical stand-alone energy storage shifted, if greater than
methods, round-trip efficiency is less than unity compressed unity.
typically by several tens of percent. air is stored
Peter Materna peter_materna@yahoo.com 6

7. COGEN
+CAES
>1.0 Simplified Energy Budget
for Direct Generation + Compression
Mechanical
work Electricity
available
Refer- >100%
Useful Useful ence
Refer- Heat of (although
heat heat value
ence Mechanical com- the result is
value work pression a mixture of
Non-Useful Rejected heat heat and
heat electricity,
rather than
being
Generation of completely
electricity can electricity or
be time- mechanical
shifted, if work as was
compressed present at
air is stored the
beginning of
the process)
Peter Materna peter_materna@yahoo.com 7

8. COGEN
+CAES Considerations of work, compressors and turbines
>1.0
Work per unit mass for compressors and turbines
900000
From manufacturers’ data for
800000
commercially available air
Small single-stage compressors. Data usually reported
Work per unit mass (J/kg)
700000 reciprocating as scfm, psig, horsepower has been
600000 converted to this format.
500000 Small two-stage
reciprocating Large recip-
400000
rocating Screw
300000
Centrifugal
Classic thermodynamic formula for
200000
work of isothermal compression
100000
0
0 50 100 150 200 250
Delivery pressure (psig)
Comparison of recovered Turbine, for extracting work, Various small air turbines
turbine work, to compressor performing at 80% or 90% and motors
work, illustrates round-trip of isentropic efficiency
efficiency of CAES
Peter Materna peter_materna@yahoo.com 8

9. COGEN
Thermodynamic states
+CAES
illustrating compression, turbine etc.
>1.0 Compressed air at ambient temperature Air at ambient atmospheric conditions
Illustrated points are for compressing air to about 100 psig (which Recovered work is delta h
corresponds to a depth of water for storage of about 80 meters). Air at discharge from realistic turbine
Storage at greater depths than this is probably better for Air at discharge from ideal turbine
efficiency, but might be less convenient for practical
considerations.
Peter Materna peter_materna@yahoo.com 9

10. COGEN
+CAES
>1.0 Conceptual Designs for Underwater Storage
(Deformable Boundary or Rigid Boundary,
but essentially constant pressure)
(Principles described here
could similarly be used
with compressed gas
storage that is constant
volume variable pressure)
Peter Materna peter_materna@yahoo.com 10

11. COGEN
+CAES Conceptual Design for System
>1.0 PV
Heat
util.
means G
To Grid
Peter Materna peter_materna@yahoo.com 11

12. COGEN
+CAES
>1.0
Is there any other system of energy
storage that can potentially give back
slightly more energy (in total, counting
both heat and electricity) than the energy
that was put into it? Probably not !
Thank You
Peter Materna
peter_materna@yahoo.com
Metuchen, NJ
732-947-2337
Patent pending
Peter Materna peter_materna@yahoo.com 12