ASHRAE Journal - August 2022 - 22

TECHNICAL FEATURE
FIGURE 1 Diminishing returns of increases in discharge air to the critical zone(s).
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2,300 2,500
2,700
2,900
Vps (cfm)
perfect effi ciency, the system primary airfl ow increases
by 1,000 cfm (492 L/s), while the outdoor air requirement
only drops by a further 39 cfm (18 L/s), in contrast
with the much better rate of reduction we saw before.
This process is illustrated in Figure 1, with a distinctive
kink that occurs once the system moves from having just
a single critical zone to having two. In an example with
more zones, there would be additional kinks, with one
occurring each time a new zone is added to the collection
of critical zones.
Putting It Together
Supplying more air to the critical zone beyond what is
necessary to satisfy the sensible load requires that the
discharge air be reheated to prevent the zone from getting
too cold. This newly required reheat constitutes an
energy penalty. At the same time, according to the above
analysis, this extra discharge air increases the ventilation
effi ciency of the system and allows for a reduction
in the required outdoor air volume, which reduces the
cooling load at the air handler. Starting from the system
baseline, this savings generally exceeds the penalty,
resulting in a net reduction in energy consumption.
(This was recognized and acted upon in RP-1747.8) Due
to the diminishing returns that were just discussed,
though, this is not always the case. At some point while
increasing airfl ow to the critical zone(s), the additional
savings that come from the reduction in the outdoor
air requirement will be eclipsed by the rise in reheat
consumption, fan power for moving the extra airfl ow,
and cooling of the additional return air. Figure 2 shows
how the cost curve might look for the simple example
system. In this case, the minimum cost occurs when
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ASHRAE JOURNAL ashrae.o rg A U G U S T 2 0 2 2
the overall airfl ow is 2,800 cfm (1321 L/s), with 720 cfm,
1,080 cfm and 1,000 cfm (340 L/s, 510 L/s and 492 L/s)
being discharged to Zones A, B, and C, respectively. The
precise location of the minimum depends on many realtime
system conditions, such as utility rates and each
airstream's temperature and enthalpy. As sensible loads
and air parameters change over time, the minimum
value will shift somewhere else.
The challenge, then, is to be able to determine where
this minimum cost point is for any possible VAV system
state. Since all states cannot be known in advance, the
solution is to create a mathematical model that uses
various system parameters and current conditions to calculate
the energy cost of a collection of setpoints in real
time. These setpoints should include discharge volumes
for each of the zones, as well as the supply air temperature,
since, in essence, changing it adjusts the shape of
the curve in Figure 2. Optimizing this model yields the collection
of setpoints that achieves the minimum cost while
still satisfying all ventilation and thermal requirements.
The next section gives an overview of this process.
Optimization Process
For both real systems and modeled ones, operating
costs in terms of dollars per hour can be calculated by
summing the energy used at the reheat coils (hot water
or electricity), cooling coils (chilled water) and supply
and return fans (electricity), multiplied by their respective
utility rates. Hot and chilled water consumption are
estimated by multiplying each airstream's mass fl ow
rate by its change in enthalpy across the coils, while fan
electricity consumption is a function of the total amount
of air being pushed through the system. This overall cost
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Vot
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FIGURE 2 Example operating energy cost as a function of overall system airflow.
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Vps (cfm)
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3,300
3,500
Evz (%)
Vot (cfm)
Cost ($/h)
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ASHRAE Journal - August 2022

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