Report 1 – Heat Exchangers, Compressed Air Systems, Heat Sinks and Inline Fans
Report 1 – Heat Exchangers, Compressed Air Systems, Heat
Sinks and Inline Fans
By: Armando Moncada, Certified Energy Manager
(Please Note: The United States Patent Office prohibits patenting work that is already part of published material. If it is not your work you cannot patent it. These reports are brief white papers not patent ideas. They are part of the public domain.
Secondly, these ideas are being shared in an open-source manner with everyone including designers, manufacturers and researchers to discover effective means of reducing energy costs. We encourage individuals, facility owners, businesses and research centers to explore the ideas and develop them even further. This idea pool is available to the public to directly address energy efficiency within product design, in an effort to globally update our technology, reduce CO2 emissions and reduce energy consumption. Good luck, be safe and have fun!)
The following is a presentation of findings associated with
research related to building energy use. Research accomplished so far this year
includes many topics, but this report seeks to compartmentalize the findings
and focus on methods for reutilizing waste heat, optimizing heat sinks,
refrigeration cycle optimization via compressed air, inline fans and energy
recovery devices, a hot side + cold side dual thermal energy storage system
(twin box system) and energy storage with compressed air tanks with a twin box
system.
Each topic will be described below in list form, ten topics
were selected.
1.
Thermal Energy Storage with Twin Boxes:
The twin box system is a thermal energy storage system that
has a hot side and a cold side. It is applicable to large scale facilities that
seek to optimize waste heat or waste cooling. If the refrigeration cycle were
to be summarized as the process of moving heat from one volume to another,
leaving the first volume colder and the second volume warmer, then the twin box
system accomplishes that very idea by using thermal energy storage tanks.
The thermal energy storage tanks in mind are large scale
boxes made of steel and filled with fine grain sand. The sand can be premixed
with a specific percentage of graphite to increase heat transfer rates through
the sand medium. Graphite percents may vary depending on how fine the sand is but
should not exceed 15%. The thermal energy storage tanks interface with the
working fluid for the building such as chilled water or heating hot water via typical
piping connections and heat exchanger coils. It is easier to design the system
to be of the crossflow type as opposed to the counter flow type to maximize the
use of the space. Additionally, there should be three coils not two. The third
is to regulate temperature extremes in case one side gets too hot or too cold,
a third coil returns the temperature to a limited range and interfaces with
outside air as the heat sink or heat source. This is applicable for days that
are extremely hot or cold and the building is mostly calling for either heating
or cooling. Lastly, the boxes are about the size of shipping containers and are
filled with sand and graphite powder on site. Graphite is currently not priced
within an acceptable range but could be manufactured specifically for this
application at a lower price in large quantities at some point in the future by
the manufacturer of the heat exchanger boxes.
Once the system is in operation heat transfer flows steadily
from one box to the other as per building load requirements via compressed air.
Air is compressed to a high pressure to ensure adequate heat transfer. After
the expansion valve, an energy recovery device is installed to capture the
pressure and convert it back to energy. A typical system piping layout between
the boxes would look like a loop that goes directly to and from each box,
allowing air to flow continuously from one heat exchanger coil to the other. At
the entry to the hot box is the compressor and at the entry to the cold box is
an expansion valve. There should also be a way to collect trapped moisture via
a trap or drain down line of some kind.
This description is a standard description and describes
what would be most likely adopted by the average engineer designing the system.
There are methods for optimizing these components which will be described
later. In short, the compressor is traded for an inline fan and the expansion
valve is traded for a valve that can vary an aperture to adjust the pressure
but simultaneously is shaped in a manner where the aperture is circular when
fully open. The energy recovery device is simply a generator built into the
compressed air line, placed after the expansion valve. It will most likely mimic
compressed air storage generators in its design.
In Figure 1 below, one can observe that the standard dimensions
do not exceed that of a shipping container. Systems should be designed to match
acceptable shipping dimensions but not exceed them. One can also observe three
pairs of pipe connections.
Figure 1. Heat Exchanger Boxes in Twin Box System, 1 of 2.
2.
Refrigeration Cycle Optimization:
The refrigeration cycle typically uses refrigerants at its
core process. Even chillers that work off chilled water have a heat exchanger
adjacent to the compressor where chilled water drops off its heat and the
refrigerant picks up the heat. Compressed air is strongly recommended as a
substitute for refrigerant for multiple reasons. Air is abundant, it is free,
the greenhouse gas warming potential is zero, as a working fluid the heat
transfer conductivity is competitive with refrigerants in their gaseous state. If
enhanced conductivity is desired, one can add helium to the compressed air in a
ratio that is acceptable to the design engineer. See Table 1 on page 9 for
thermal conductivity values associated with potential refrigerants. The central
reason for switching to air is because common refrigerants become liquid at
higher pressures, air can achieve higher pressures without liquefying which in
turn means the delta T (difference in temperature) that is achievable is much
greater. This implies that by operating at higher delta T’s the equipment can
move heat much faster.
Working at higher pressures often implies consuming more
energy. There is a strong need for an energy recovery device to be placed
within the piping system directly after the expansion valve. By placing an
energy recovery device in the system, the system can work at a much higher
pressure and larger delta T, allowing heat transfer to occur more efficiently without
paying the cost. The COP, (Coefficient of Performance), increases when one
includes the energy recovery device. The energy returned to the system is
subtracted from the energy input into the system and the net energy is much
lower than expected. For example non-adiabatic compressed air storage has
recorded efficiencies near 60%. The energy recovery device could be very
sophisticated, compact and mounted inline. A bulge in the piping and the
whirring of the machine would be the only way to know that it is there. It
could be as simple as a spiral that spins with the air flow.
The scenario of compressed air with a higher working
pressure for the refrigeration cycle implies that one would only need three
pieces of equipment – a compressor, an expansion valve, and an energy recovery
device. In a typical system a compressor would build up the pressure, the heat
would be transferred to a medium, the enthalpy of the working fluid drops, then
the expansion valve returns the working fluid to a lower pressure and the
piping in that part of the system is ready to absorb heat and the cycle begins
again. In an optimized system with an inline fan there is a constant flow that
is defined by the fan RPM. As the fan spins the mass flow rate increases then
the aperture on the opposite side, which in a typical system would be the
expansion valve, modulates its cross-sectional area to provide the required
pressure difference between both sides. The two systems are similar, but it is predictable
that a compressor is less efficient than an inline fan if we count the number
of reciprocating moving parts, these represent continuous losses of kinetic
energy as they represent inertial losses every time the part accelerates and
decelerates.
The argument for multiple inline fans is also considered. Inline
fans may have different designs, but they always have a blade angle. The blade
angle in an ideal scenario is mathematically predictable as a composite of two
vectors – mass flow rate velocity and circumferential velocity associated with
the spin of the blade about its axis. In a condenser fan for example the mass
flow rate of the air moving through the condenser would be considered constant
and if the RPM is constant then the blade angle would only vary with radius. The
further out on the blade we look the flatter the angle of the blade would need
to be to move through the air in a streamlined manner. Installing multiple
inline fans is an attempt to direct flow along different paths in the piping of
the compressed air system in a manner where the fan blade angle more closely
matches the flow rate + angular velocity composite vector. Briefly stated, as
the fan speed increases, the fan blade angle should be steeper relative to the
plane of spin. In summary, this strategy is represented with a layout where
approximately three inline fans exist in parallel to each other and the system
can choose which fan to operate via valves and automated controls. Each fan
matches a different flow rate with its blade angle.
4.
Condenser Fan Blade Angle Optimization:
Condenser fans are often made of very thin sheet metal that
is stamped flat with little variance in the blade angle and then added onto the
condenser equipment with a few points of connection, the angle of the blade
does not change with a change in the distance from the central axis. Examples
where the blade angle is calculated very precisely include wind turbine blades or
propellors for planes. If we were to calculate the measured air velocity moving
vertically through the condenser and measure the velocity along the plane of rotation,
we would realize that the angle should change with distance from the central
axis and be a composite vector of the two velocities as described in the
previous strategy.
5.
Heat Sink Optimization and Thermal Energy
Storage Systems:
The best heat sinks are the coldest ones. Compared to a water-cooling
tower, or an outside air condenser, where the difference in temperature might
be 20 - 30 degrees F, a very cold heat sink changes the electrical consumption
of upstream equipment such as pumps and fans. This scenario predicts that pumps
and fans can operate at a slower speed consuming less electricity, because the
heat transfer rate is dependent upon the mass flow rate times the difference in
temperature.
Energy storage in the form of a cooled volume of water is a
common method of storing cooling for assistance in reducing kW demand charges
during peak hours. Furthermore, the electricity that is used at night helps the
local electric utility reduce distribution line losses when delivering power. The
demand charges can be such a significant part of a facility owner’s bill that
thermal energy storage systems have been developed to offset electricity used during
peak hours. Two methods for optimizing heat sinks will be discussed - Radiant
Panels or Night Sky Cooling Coupled with Heat Pumps and Nighttime Outside Air
Cooling of a Sand Bank using Heat Pumps.
6.
Radiant Sky Night Cooling Coupled with
Heat Pumps:
Radiant Panels or Night Sky Cooling is a method for cooling
a medium using radiation from space. The panels radiate heat out into space,
but also the night sky radiates the coldness of space back to the panels, and
therefore temperatures can be obtained that are colder than the surrounding
air. Radiation panels function because of the temperature difference between
space and the medium to be cooled. Water or another working fluid is circulated
via heat exchanger tubing or some other conductive medium to the panels and
then back to the cold reservoir. In an optimized scenario, a heat pump would be
added to the system to increase the temperature difference, allowing for the
radiation to space to happen at an increased rate. This enables the thermal
energy storage medium, for example a cold-water tank or a pool that is insulated
with a cover to avoid water loss and heat gain, to reach colder temperatures.
Without the heat pump, the system is almost entirely passive, the user pays
energy costs only to recirculate the heat transfer fluid, in this system the user
pays to operate a heat pump system with compressed air as the working medium.
Secondly an expansion valve is required and lastly an energy recovery device is
installed after the expansion valve in order to operate at higher pressures and
also to achieve higher temperatures. Although more energy is invested to move
the heat, more heat is being removed from the reservoir. It is important to
note because radiant panels can achieve very cold temperatures but only for an
intermittent period of time. It should also be noted that radiant panels have
been developed that operate during the day by blocking the wavelengths
associated with daylight, enabling cooling to occur even with the sun directly
overhead.
Volumes filled with fine grain sand stand as a good argument
for thermal energy storage type systems. The heat transfer rate of moist fine
grain sand can be as high as 1.7 W/m-K, while the value for water rests at 0.6
W/m-K. Graphite is much higher and can easily achieve 200 W/m-K. In an energy
storage system that utilizes sand, water corrosion is no longer an issue. Also,
graphite when added to the fine grain sand can increase the conduction rate
significantly even if added in only small percentages.
Other benefits include access to cooler nighttime temperatures,
the use of night sky cooling radiant panels, reduced electrical demand charges,
ability to size cooling equipment to a smaller scale if the amount of cooling
is no longer 100% on demand, rather some of the cooling is stored from the
nighttime cooling process at the sand bank heat sink.
The benefit of compressed air is that one can choose the
operating delta T and achieve colder temperatures in a shorter amount of time.
If an energy recovery device is used, the majority of the energy used for
compression is recovered. Most refrigerants phase change to liquid at higher
pressures which places a ceiling on the maximum delta T achievable.
The sand bank system is almost self-explanatory. It is a
heat sink connected to the working fluid of the HVAC system, for example
chilled water, with two heat exchanger coils. One for the working fluid and one
for the compressed air. The compressed air piping moves heat to the outside
environment via a condenser typically, but a night sky cooling radiant panel
could also be utilized. Cooling takes place at night in an effort to make use
of cooler outside air temperatures. Lastly, high working pressure is utilized
on the hot side of the compressed air loop to further increase the delta T. An
energy recovery device is also utilized to conserve energy.
A side note worth mentioning: In a system that is only
attempting to store cooling via sand at a cold temperature and expels waste
heat to the environment, waste heat cannot be utilized later, although obvious
it allows for a comparison to be drawn between a singular system and twin box
system.
8.
Compressed Air Energy Storage using a
Twin Box System, and a Waste Heat Recovery Coil:
Compressed Air is a very attractive form of energy storage
because it is so simple, tanks are easy to manufacture, and the specialty parts
that require more precise manufacturing are few or off the shelf. Compressed
Air Energy Storage can be brought to very high pressures and store considerable
amounts of energy. The drawbacks are that there is a space requirement for the
large tanks and the round-trip efficiency may be close to 60% for the best
systems. Engineers have investigated recycling the waste heat to achieve higher
efficiency. This is very possible but has yet to be commercialized. The only
thing that eternally remains true for compressed air storage is when the energy
is generated and the compressed air is discharged, the decompressed air is very
cold and the cooling is free.
A compressed air system that interfaces well with the ideas described
so far would require utilizing the free cooling effectively, but also making
use of the waste heat is a straightforward method for getting a discount on the
energy used to compress the air.
A compressed air energy storage system is by default a heat
generation device, as well as a cooling device during decompression. During
peak hours clients can make use of energy that is generated via the
decompression of the air, which can then be applied to HVAC cooling loads. At
nighttime when the compression of the air begins, heat is generated, and that
heat can be brought into either the HVAC system for heating or a thermal energy
system that stores heat. In addition, if the air being generated doesn’t meet
heating specifications, it can be compressed and brought to a higher
temperature.
9.
Compressed Air with Hydraulics in
Exchange of a Typical Compressor for Higher Pressures:
Compressors, when analyzed mechanically, are slightly
inefficient because the main piston reciprocates back and forth linearly losing
inertia constantly. Energy is required to accelerate the part, then accelerate
the part in the opposite direction. One of the ways in which compressed air
systems could be optimized is with an inline fan. An inline fan cannot achieve
higher pressures as easily because it doesn’t form a complete pressure seal. In
this optimization strategy, the concept of having a threaded cylinder spinning
slowly and pumping hydraulic fluid without any reciprocating parts can function
as a device for increasing the efficiency of the air compression. A compression
cycle could occur where hydraulics are added to the system using a hydraulic
fluid pump that is essentially a large threaded cylinder with the capacity to
create a good seal. Potential materials for the cylinder might be stainless
steel, while potential materials for the seal since it will be under high
pressure might be nylon or another rigid plastic with a low coefficient of
friction.
10. Shutter
Valves as a substitute for other types of Valves.
Although minor energy saving could be calculated, this
measure is intended to optimize valves that regulate air flow in compressed air
systems with precision and quick reaction time.
A shutter valve is a valve that operates like a hexagonal camera
shutter, enabling calibration to occur with precision in a short amount of
time.
See Figure 2 below. A shutter valve will be in demand if
compressed air systems that modulate in speed become popular, for the reason
that as the inline fan increases its RPM, the shutter valve would work in
tandem with the inline fan and create the required pressure differential in a
brief period of time using a PID (Proportion Integral Derivative Controller) loop
and a pressure sensor.
It may be in demand from a controls engineer’s perspective
for hydronic flow as well but would be redesigned more robustly for a denser
fluid. Control engineers often spend a large amount of time coordinating valve
position with GPM flow rate. To reduce calibration time, points are marked out
ahead of time in 10% increments on the outer wheel, which controls the aperture
area. For incompressible fluids, aperture area is expected to be proportional
to GPM, for a similar pumping head pressure.
Fig. 2. Shutter Valve
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Thermal Conductivity of Common Materials |
|
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|
Material |
Thermal Conductivity |
||
|
Watts/ meter - Kelvin |
|||
|
|
|
||
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Graphite |
200-500 |
||
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Sand Course |
0.25 |
||
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Sand Fine Dry |
0.34 |
||
|
Sand Fine Moist |
1.7-2.3 |
||
|
Water |
0.6 |
||
|
|
|
||
|
Helium |
0.1513 |
||
|
Nitrogen |
0.02598 |
||
|
Oxygen |
0.02674 |
||
|
Air |
0.025969 |
||
|
R-32 (gas) |
0.0124 |
||
|
R-32 (liquid) |
0.135 |
||
|
R-454B (gas) |
0.013 |
||
|
R-454B (liquid) |
0.1054 |
||
|
|
|
||
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Aluminum |
237 |
||
|
Copper |
401 |
||
|
Steel |
45 |
||
|
Stainless Steel |
15-25 |
||
|
Titanium |
21.6-21.9 |
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