Energy Materna

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



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.



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



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.


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


Mechanical work


Electricity

Useful heat

Rejected heat

Refer-ence value

Refer-ence value




Mechanical 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.



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



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



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


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)



Conceptual Design for System

G

To Grid

PV

Heatutil.means



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 !

Energy Materna

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