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First presented at the Southern Alberta Chapter of
ASHRAE. Want to see this presentation?
Above: Slides from ASHRAE Southern Alberta Chapter
In the context of using energy for heating and cooling,
the word 'eXergy' describes the usefulness of the
highest generated temperature and also reveals the wastefulness
when supply temperatures are grossly mismatched to load
temperatures. In reality, we must direct our
efforts at eXergy since it can be destroyed
whereas energy is conserved. To put this into practice,
a high efficiency furnace or boiler may achieve a
combustion efficiency of 97% but only 3% exergy
efficiency. Exergy will get you to look at energy in a
drastically different way.
efficiency increases as the source temperature
approaches the load temperature. In the case above,
2800°F at the burner of a furnace or boiler (upper
right corner of illustration) is overkill for the 140°F
needed to deliver 70°F space temperatures as such it
only delvers 3% eXergy efficiency resulting in a
destruction of otherwise potential useful work. It's
like using a sledgehammer to drive in a finishing nail.
If the 140°F were supplied by a solar system or
geothermal system the eXergy and energy efficiency would
be in line and more sustainable.
Above: The higher exergy efficiency of a hydro
powered ground source water to water heat pump is
obtained by using it with low temperature radiant
heating and high temperature radiant cooling which occurs
high performance buildings (< 10 Btu/hr/sf), using
conductive floors (tile, slate, concrete) and
densities (6" to 8" o.c.). Typical fluid
temperatures for heating are 80°F to 120°F and for
cooling 55°F to 70°F.
Factor E5 = Energy • Efficiency • Entropy
• Exergy • Efficacy