An economic perspective on the optimisation of a small-scale cogeneration system for the Portuguese scenario

By Ferreira, A.C.M.; Nunes, M.L.; Teixeira, S.F.C.F.; Leão, C.P&pe

Energy

2012

Abstract

A  numerical  optimisation  method,  previously  applied  to  a  large  industrial  cogeneration  plant   has  been  adapted  for  a  small-­‐scale  unit  delivering  125  kW  of  useful  heat  and  85  to  105  kW  of   electricity,  suitable  for  medium-­‐size  buildings.  The  unit  is  based  on  a  micro  gas-­‐turbine  and  includes  an   internal  pre-­‐heater  (typical  of  these  types  of  small-­‐scale  units)  and  a  Water  Heater.  The  mathematical   model  yields  a  non-­‐linear  objective  function,  subject  to  physical  constraints  of  system  operation.  The   electrical  power  production  is  one  of  the  six  decision  variables  that  were  selected  for  the  optimisation   algorithm  where  the  objective  function  is  defined  as  the  maximization  of  annual  worth  of  the  CHP   system.  A  sensitivity  analysis  was  carried  out  and  showed  that  the  relationship  between  the  electricity   feed-­‐in  tariff  and  the  natural  gas  price  significantly  affects  the  heat  to  power  ratio  and  the  annual  worth   of  the  system.  Regarding  the  physical  aspects  of  the  major  system  components,  the  analysis  disclosed   that  the  most  relevant  and  sensitive  variables  are  the  compressor  pressure  ratio,  the  internal  pre-­‐ heater  effectiveness  and  the  turbine  inlet  temperature.

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