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First presented at the Southern Alberta Chapter of
ASHRAE. Want to see this presentation?
Contact us.
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.
The illustration below
sheds some light on the principle, and the slides show
below it will explain its over all importance to
sustainability. A high exergy efficient schematic is
presented at the end to illustrate a typical mechanical
eXergy based system.
Above: eXergy 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: Slides from ASHRAE Southern Alberta Chapter
meeting.
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
with
high performance buildings (< 10 Btu/hr/sf), using
conductive floors (tile, slate, concrete) and
tight tube
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.
Suggested Reading
Factor E5 = Energy • Efficiency • Entropy
• Exergy • Efficacy
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