energy materna
TRANSCRIPT
Peter Materna [email protected] 1
COGENCOGEN+CAES+CAES>1.0>1.0
Peter Materna
May, 2012
Combining Compressed Air Energy Storage with Cogeneration, or Using Heat of Compression during CAES, Yields Improved Energy Efficiency
Peter Materna [email protected] 2
COGENCOGEN+CAES+CAES>1.0>1.0
Overview
• Cogenerating electricity and heat results in utilizing typically 80%-90% of the heating value of fuel, by virtue of the capture and use of “waste” heat. This is better than the efficiency of a stand-alone electric generating plant, but is less than unity.
• Compressed Air Energy Storage (CAES) has “round-trip” efficiency often described as ~70% (depending on details and on definition of efficiency). This is typical of various forms of energy storage, and is less 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 [email protected] 3
COGENCOGEN+CAES+CAES>1.0>1.0
Overview and Thermodynamics
• During compressing of air, Work W is performed and heat of compression Q is produced (and often is rejected as waste heat)
• 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
Elevated pressure, ambient temp., E=Ezero
W = Work inputted
Q = Heat of compression outputted
Atmospheric pressure, ambient temperature, E=Ezero
Peter Materna [email protected] 4
COGENCOGEN+CAES+CAES>1.0>1.0
Overview and Thermodynamics (cont’d)• The compressed air, at elevated pressure, is able to do work
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 [email protected] 5
COGENCOGEN+CAES+CAES>1.0>1.0 Simplified Energy Budgets
for Power Generation and for Cogeneration
Simple central station
thermal power plants
Cogeneration
power plants
Heat value of fuel or energy source
Mechanical work
Heat not converted into work
Electricity
Rejected heat
Refer-ence value
Heat value of fuel or energy source
Mechanical work
Heat not converted into work
Electricity
Useful heat
Rejected heat
Refer-ence value
Refer-ence value
Peter Materna [email protected] 6
COGENCOGEN+CAES+CAES>1.0>1.0
Heat value of fuel or energy source
Mechanical work
Heat not converted into work
Heat of com-pression
Rejected heat
Useful heat
Useful heat
Mechanical work recoverable
Useful heat
Electricity
Non-useful heat
Rejected heat
Rejected or non-useful heat
>100%
Simplified Energy Budget for Using a Fuel for Cogeneration Then Using
the Mechanical Power to Compress a Gas
Generation of electricity can be time-shifted, if compressed air is stored
Refer-ence value
Refer-ence value
Note: sizes of various bars are simply intended as representative values for illustration, and mostly are estimates. Final results are to be viewed also keeping in mind that for typical stand-alone energy storage methods, round-trip efficiency is less than unity typically by several tens of percent.
Compared to ordinary co-generation, this output contains more heat and less electricity; nevertheless, the total of heat+electricity is greater than for ordinary co-generation, and potentially even greater than unity.
Peter Materna [email protected] 7
COGENCOGEN+CAES+CAES>1.0>1.0
Mechanical work
Heat of com-pression
Useful heat
Mechanical work available
Electricity
Useful heat
Rejected heat
>100%(although the result is a mixture of heat and electricity, rather than being completely electricity or mechanical work as was present at the beginning of the process)
Non-Useful heat
Simplified Energy Budget for Direct Generation + Compression
Generation of electricity can be time-shifted, if compressed air is stored
Refer-ence value
Refer-ence value
Peter Materna [email protected] 8
COGENCOGEN+CAES+CAES>1.0>1.0
Work per unit mass for compressors and turbines
0
100000
200000
300000
400000
500000
600000
700000
800000
900000
0 50 100 150 200 250
Delivery pressure (psig)
Wo
rk p
er u
nit
mas
s (J
/kg
)
Considerations of work, compressors and turbines
Small single-stage reciprocating
Small two-stage reciprocating Large recip-
rocating Screw
Centrifugal
Turbine, for extracting work, performing at 80% or 90% of isentropic efficiency
Classic thermodynamic formula for work of isothermal compression
From manufacturers’ data for commercially available air compressors. Data usually reported as scfm, psig, horsepower has been converted to this format.
Comparison of recovered turbine work, to compressor work, illustrates round-trip efficiency of CAES
Various small air turbines and motors
Peter Materna [email protected] 9
COGENCOGEN+CAES+CAES>1.0>1.0
Thermodynamic states illustrating compression, turbine etc.
Illustrated points are for compressing air to about 100 psig (which corresponds to a depth of water for storage of about 80 meters). Storage at greater depths than this is probably better for efficiency, but might be less convenient for practical considerations.
Air at ambient atmospheric conditions
Compressed air at ambient temperature
Air at discharge from realistic turbineAir at discharge from ideal turbine
Recovered work is delta h
Peter Materna [email protected] 10
COGENCOGEN+CAES+CAES>1.0>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 [email protected] 11
COGENCOGEN+CAES+CAES>1.0>1.0
Conceptual Design for System
G
To Grid
PV
Heatutil.means
Peter Materna [email protected] 12
COGENCOGEN+CAES+CAES>1.0>1.0
Thank You
Peter [email protected], NJ
732-947-2337
Patent pending
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 !