Energy Materna

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Energy Materna

  1. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 11. COGEN+CAES Conceptual Design for System>1.0 PV Heat util. means G To Grid Peter Materna peter_materna@yahoo.com 11
  12. 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

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