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Cultivation facilities, due to irrigation and resulting
transpiration, can have a sensible heat ratio around
0.50. That means the “off-the-shelf” 20-ton unit will be
7 tons SHORT on latent capacity. In order to properly
take care of these facilities a more complex design must
be used.
Latent
heat is the most difficult and energy intensive to
remove. The most widely accepted way to remove that
moisture is by brute force…cooling the air down below
its dewpoint. This is a function of the temperature of
the coil and coil depth. Physics places certain limits
on the coil temperature, so coil depth (number of rows)
needs to be increased in order to properly dehumidify
the grow space.
This
leaves two real options regarding system selection. The
first is a purpose-built, packaged, direct-expansion
unit and the other is a chilled water system. Both
systems have their pros and cons that facilities
operators need to understand, but in the end, both will
maintain the environment necessary for plants the
thrive.
System
Optimization
Once you have your system properly sized and selected,
it is time to look at what further measures can be taken
to reduce the energy consumption of the HVAC system.
Items like high efficiency compressors and ECM fans are
easy places to start but have a small impact on these
systems that run 24/7. Much like in the commercial HVAC
world, CEA facilities need to look towards economization
to try and remove as much mechanical cooling and
dehumidification as possible, thus saving energy.
In
commercial HVAC design, we are already familiar with
this concept. Not only does bringing in outside air
dilute the toxins and contaminants being produced in a
commercial building, on suitable days the outside air
can even be used to condition the space itself. However,
with CEA facilities we have moved the crop inside to try
to limit exposure to the pests and pathogens commonly
found outside, and to keep CO2 levels high. Traditional
outside air economization works against the fundamental
goals of a CEA facility, putting the predictability of
the product at risk. As a result, alternative
economization methods need to be considered.
To
maintain the integrity of the environment, indirect
economization is the best approach. For those not
familiar, indirect economization utilizes a heat
exchanger to do the economizing in the system. For a CEA
facility, a recirculated air stream (in this case from
the growing environment) flows across one side of the
heat exchanger and the other side using the outside air
as a heat sink running it, single pass, across the heat
exchanger and exhausting it
back into the atmosphere. Heat is transferred from the
recirculated air stream into/thru the heat exchanger and
then passed on to the outside air.
This
approach is common and well understood when transferring
sensible heat, however the latent heat of cultivation
spaces further complicates indirect economization,
limiting available options. Most experts are familiar
with heat pipes, run around coils, or fixed plate heat
exchangers. Application of these products transfer
sensible heat but allow the latent heat in the space to
build. The answer for indirect economization of a CEA
facility is a heat exchanger
that can transfer both sensible and latent heat.
There are
two main types of energy recovery devices that
accomplish this: total energy wheels and enthalpy cores.
Both are commonly used in commercial buildings for
recovering energy from relief air before being exhausted
and transferring that energy to the outside air being
brought into the space. When used as an indirect
economizer, the energy recovery device sits between the
return air and outside air streams. On one side of the
device, return air is recirculated to the space and on
the other outside air is pulled through and exhausted.
Sensible
heat is absorbed and released by the aluminum frame of
the total energy wheel or enthalpy core. Latent heat is
transferred at a molecular level, water vapor, and only
happens on the surface of the wheel. The vapor pressure
difference between the opposing air streams is what
drives the water vapor transfer. This transfer of
moisture is achieved without the energy intensive
process of mechanically cooling the air down to dewpoint.
This means dehumidification can be achieved indirectly
with simple fans and either a total energy wheel or an
enthalpy core. The effectiveness of the heat exchanger
maximizes the number of hours throughout the year
capable of indirect economization. Depending on your
location, you can expect to turn your compressors OFF up
to 40% of the year.
Summary
Controlled environment agriculture is a rapidly growing
segment of the agriculture market and is here to stay.
The ability to control all aspects of the growing
environment translates into predictability of yield and,
by extension, predicable profitability. But controlling
these variables comes at a significant energy cost that
is beginning to be noticed by regulators.
LED
lighting was the initial low-hanging fruit of energy
optimization and has largely been played out. The next
energy revolution in the CEA industry will focus on the
HVAC systems. The industry needs to provide
purpose-built equipment that can handle the unique
environments presented in a CEA facility, but do so with
energy efficiency in mind. The days of brute-force
cooling to achieve proper dehumidification are no longer
an option for these 24/7 facilities.
Increasing
the efficiency of the compressors and fans only works to
a point. It will take creativity to drastically reduce
the energy efficiency of these HVAC systems while not
affecting the growing environment. Indirect
economization uses the outside air to cool the space,
without introducing contaminants into the space. This
allows the compressors
to stay off and can save up to 75% on the energy
expended by the HVAC unit. These savings not only please
regulators but will add to the bottom-line profitability
of the companies implementing the technology.
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