RESIDENTIAL THERMAL STORAGE UNIT
Jassmine Duron, Manuel Monroy, M. A. Rafe Biswas
Mechanical Engineering Department
The University of Texas at Tyler
[email protected], [email protected], [email protected]
Abstract
Increasing demands of electricity have placed a large strain on the electrical grid pushing limits
of peak demand delivery. Significant demand can be attributed to the residential sector where
the majority of electricity consumption occurs between 12 pm and 8 pm. One of the main
drivers of high electricity consumption in the residential area is the standard residential AC
unit, which accounts for more than half of the homeowner’s electricity bill. Due to the evergrowing energy need, there has been a greater development in efficient technologies that
reduce energy consumption. One of these technologies, the Thermal Storage Unit (TSU), is an
alternative technology that is being used by large industrial companies. The TSU works in
conjunction with a standard AC unit as an efficient hybrid unit. However, this system is only
offered in commercial buildings and currently not available for residential houses. Therefore,
a Residential Thermal Storage Unit (RTSU) has been proposed for research and simulation.
The RTSU will generate and store ice during the night when the peak load demand is low.
Once the ice is fully developed, the AC unit will then shut off and the refrigerant line will be
re-routed through the RSTU. The refrigerant line will use the stored ice to remove heat from
the refrigerant without the use of components that use high amount of electricity. Introducing
a hybrid AC unit to the residential area can guarantee a significant drop in electricity
consumption during peak hours.
Introduction
The increasing demand for electricity has become a major concern throughout the world.
Texas is the leading consumer of electricity in the United States being the only state that has
its own stand-alone electric grid1. The peak load demand for Texas, ranges from the hours of
12 pm to 8 pm; this demand results in the electrical grid being constantly challenged to provide
sufficient electricity. During peak hours, especially throughout the summer season, there is a
high use of electricity by residential areas due to great demand for space cooling in homes. The
growing demand for electricity during peak-hours will ultimately result in disruptions in the
electrical grid. This creates a dependence for outside sources of energy by electric companies,
which results in a higher price per kilowatt-hour (kWh) of energy. The increasing demand for
electricity leads to a greater production of energy in power plants which also results in negative
effects to the environment. The use of existing environmentally friendly technologies, like the
use of solar energy, wind power, earth coupled systems, and thermal energy storage systems
can reduce the demand of electricity.
There are several programs currently in place that attempt to assist the energy demand problem.
Electric Reliability Council of Texas (ERCOT) has several agreements with large companies
to reduce their energy consumption if the demand becomes too high1. Peaking plants are also
in use only in the summer time in Texas to compensate for the large increase in power use.
Proceedings of the 2015 ASEE Gulf-Southwest Annual Conference
Organized by The University of Texas at San Antonio
Copyright © 2015, American Society for Engineering Education
There has also been a change from old electric meters to smart meters. These smart meters
allow the consumer to be more aware of the electricity consumption. Many electrical
companies offer incentives for customers like variable rates for time-of-day use, which will
direct customers to consume less power. However, all solutions have minor or temporary effect
and do not address the high electricity consumption problem completely.
Commercial entities in the U.S. have addressed this peak demand problem by installing a
thermal storage system that provides cooling utilizing overnight ice, which is produced during
off-peak period at a lower cost. However, industries make up a relatively small percent of the
grid demand compared to the large percent of electricity used by residential areas2. The large
scale thermal storage units have recently been installed for large residential areas in one of the
states, but no small scale units are available to individual homeowners. Since residential
electricity customers are the most significant drivers of peak-load demand, there are no
programs designed to aid the energy demands for this area. This product is targeted to satisfy
the needs of the average single family household, whose residents spend the majority of time
at home during high energy demand hours. Air conditioning can be provided more efficiently
by using electricity at night to freeze water into ice, then using the ice to cool off the refrigerant
used by the house to provide proper cooling. The idea is to have the Residential Thermal
Storage Unit (RTSU) running in the afternoon to reduce the electricity needed between 12 pm
and 8pm. This technology provides electricity reduction during the peak load hours without
effecting the consumer's lifestyle.
This paper will present and analyze a design of an RTSU for a research house located at the
University of Texas at Tyler3. The unit’s internal components were analyzed through research
and calculations. A model of a final design was created based on 3 design layouts.
Background
The RTSU is a product that is intended for residential use. There are no similar products
currently out in the market. A brand new market is expected to be introduced with the
development of the RTSU. Currently, a large scale commercial design solution is available to
large scale buildings. One unit is the Ice bear system by Ice Energy4. The Ice bear system is an
energy storage system that works with commercial air conditioning systems, 4-20 ton package
rooftop systems common to most small and medium sized commercial buildings. The cooling
cycle only lasts for 6 hours4. Companies like Trane and Evapco also offer commercial thermal
storage units for large scale applications.
There are currently no residential small scale designs on the market. The main problem is the
large electricity consumption produced by residential areas. There is large opportunity for
product growth due to a non-existent residential competitive product. The targeted selling price
is set to be $3000. Incentives may be possible through the electric companies that might offer
discounted rates on electricity. Since the year 2006, United States introduced a tax incentive
for green homes which allowed consumers buying energy saving appliances to be eligible for
tax benefits. The government will issue each customer a tax credit of $300 for purchasing a
high-efficiency central AC unit5. The reduction in electricity consumption will ultimately
reflect on the environment. Through the implementation of the RTSU, residential areas could
potentially decrease the amount of electricity consumed by the AC unit. The overall savings in
electricity would pay off the initial investment and carry on savings through years of usage.
Proceedings of the 2015 ASEE Gulf-Southwest Annual Conference
Organized by The University of Texas at San Antonio
Copyright © 2015, American Society for Engineering Education
To determine the customer feedback regarding the RTSU, a total of 13 people who live in a
house with an average of 4 bedrooms were surveyed. These subjects have a family household
of 3-4 members and 9 of the 13 work full time while the rest stay at home. Table 1 illustrates
the product concerns of the customer with a scale of 1-9, with 9 holding the highest level of
importance. The results shown in Table 1 indicate that the survey responders places high level
of importance on economics. They also seek to maintain the same amount of comfort level that
a standard AC unit provides. Table 2 illustrates that the responders show least amount of
concern with the aesthetics and dimensions of the device.
Table 1: Customer product concerns.
Customer’s concern about the product
Price
How much they can save
Noise
Size
Where it will be installed
Damage to their house structure in the future
Maintenance
Warrantee
#
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
# of customer’s concern
13
13
8
5
9
9
8
10
Table 2: Customer needs summary.
NEED
Lowers the usage of electricity between noon and 8pm
Runs without interruption
Affordable for the average single family household
Easy to install
Provides the user with appropriate cooling needed for comfort
Does not contaminate the air
Allows temperature adjustment by the user
Can be easily accessed for maintenance
Allows easy replacement of worn parts
Does not need significant maintenance throughout the year
Has a long lifespan
Relative dimensions of the standard AC unit
Does not damage the house in the future
Does not make a significant amount of noise or disturbance
Automatic timer
Warranty options
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Organized by The University of Texas at San Antonio
Copyright © 2015, American Society for Engineering Education
Importance
9
7
8
5
9
9
7
6
7
7
8
4
7
5
3
4
Concept Development & Evaluation
The RTSU integrates into the existing AC unit by adding an ice evaporator coil to the unit. No
further changes are needed since RTSU uses the existing ductwork of the existing HVAC
system. Fig. 1 illustrates the proposed layout of the RTSU once it is installed. The existing
refrigerant line that the AC unit used will be extended so that it runs through the RTSU. The
chiller unit, which is included in the RTSU will seek to use existing AC components such as
the compressor to create the ice inside the RTSU. Two types of materials are going to be used
for the pipelines. The glycol pipeline will be made out of galvanized steel and the refrigerant
line will be made out of copper. The two lines have a nominal diameter of 1-1/4 in.
Figure 1: RTSU installation design layout.
Fig. 2 illustrutes preliminary design 1. The height of the unit is 4 ft, the width and length are
both 5 ft. The casing of the RTSU has a total thickness of 1/4 in. This unit was designed to go
2 ft underground. A 1/16 in. thick layer of corrosion resistant paint is necessary to mantain the
walls rust free. A total depth of 8 ft is needed to properly seal the unit underground. This design
also required an additional component that would lift the device once maintanance is needed.
This design includes one 1/2 in. nominal diameter copper line and three 1/2 in. nominal
diamerter galvanised steel pipel. The 3D renderings of this design are illustrated in Fig. 3 and
4.
Proceedings of the 2015 ASEE Gulf-Southwest Annual Conference
Organized by The University of Texas at San Antonio
Copyright © 2015, American Society for Engineering Education
Figure 2: Preliminary design concept 1 dimension layout
Figure 3: Preliminary design concept 1 top view
Proceedings of the 2015 ASEE Gulf-Southwest Annual Conference
Organized by The University of Texas at San Antonio
Copyright © 2015, American Society for Engineering Education
Figure 4: Preliminary design concept 1 angled view
The second design concept was designed with six 1-1/4 in. nominal diameter U-bend pipes.
Each U-bend pipe would then be placed inside a spiral 1-1/4 in. nominal diameter copper tube.
Each spiral tube and U-bend pipe had have clearance of 2 in. The casing of the RTSU
maintained a thickness of 1/4 in. Preliminary design concept 2 was designed to be above the
ground next to the existing AC unit. Fig. 5-7 illustrate the design concept in more detail.
The third and final design concept took the strongest features of each previous design. The
refrigerant line was designed with an extra U-bend. This allowed the refrigerant line to gain a
greater surface area inside the RTSU. Fig. 8 illustrates the 1-1/4 in. nominal diameter
refrigerant line. The casing was modified to include a total thickness of 3/4 in. In Fig. 8, both
the 1-1/4 in. nominal diameter copper line and the refrigerant line are shown. A flush hole was
added for maintenance purposes. The casing insulation is designed to be vacuumed sealed to
eliminate conduction to the environment. Fig. 10 and 11 demonstrate the 3D model of the final
design.
Proceedings of the 2015 ASEE Gulf-Southwest Annual Conference
Organized by The University of Texas at San Antonio
Copyright © 2015, American Society for Engineering Education
Figure 5: Preliminary design concept 2 dimension layout
Figure 6: Preliminary design concept 2 side view
Proceedings of the 2015 ASEE Gulf-Southwest Annual Conference
Organized by The University of Texas at San Antonio
Copyright © 2015, American Society for Engineering Education
Figure 7: Preliminary design concept 2 angled view
Figure 8: Final design concept dimension layout
Proceedings of the 2015 ASEE Gulf-Southwest Annual Conference
Organized by The University of Texas at San Antonio
Copyright © 2015, American Society for Engineering Education
Figure 9: Final design concept with spiral tubes
Figure 10: Final design concept side view
Proceedings of the 2015 ASEE Gulf-Southwest Annual Conference
Organized by The University of Texas at San Antonio
Copyright © 2015, American Society for Engineering Education
Fig 11. Final design concept angled view
Feasibility Study
Given the 3 different designs, the layout of the pipes, the construction material, the type of
refrigerant, and the system dimensions will all be analyzed and evaluated through simulation
and optimization. Based on the analysis, an optimized design is developed and compared for
feasibility to a commercial system like the Ice bear system.
Ice bear system in Fig. 12 has a commercial design that was considered for a possible design
layout of the system. This design has multiple copper coils that are placed vertically all
throughout the system. These coils run a refrigerant solution that freezes the water surrounding
them. Once the Ice bear system freezes the ice, it then runs hot refrigerant coming from the AC
unit through a series of copper coils. The ice will cool the refrigerant flowing through the
copper coils which is then transferred back into the AC unit. This system splits the lines where
the refrigerant flows through. The refrigerant then flows through the copper coils which are
spread evenly through the ice. This results in a greater heat transfer because of the longer
residence time of the refrigerant, which is in contact with the copper coils that run through the
ice. Fig. 13 and 14 illustrate the layout of the ice bear system4.
Proceedings of the 2015 ASEE Gulf-Southwest Annual Conference
Organized by The University of Texas at San Antonio
Copyright © 2015, American Society for Engineering Education
Fig 12. Ice Bear system layout with ice2
Fig 13. Ice Bear system layout copper line2
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Fig 14. Ice Bear system copper coils2.
For the final design, a Pugh Selection Chart was used as a method to choose the best design
concept6. The Pugh chart compares each design concept to a reference or datum concept and,
for each selection criterion, a sign is assigned for each comparison to the datum concept. Table
3 shows the datum concept to be the Ice Bear system 4. The first column shows the selection
criteria used to compare the datum concept to each of the 3 design concepts. A "+" sign in each
cell of a concept dictates that the design concept is better than the datum concept for the
corresponding criteria. A "-" sign shows that the design concept is poorer than the datum
concept. An "S" on the concept columns shows that it is about the same as the datum concept.
The column with the most "+" signs and least "-" signs is chosen to be the best design concept
from the group. Analysis of Table 3 results in elimination the first and second designs. The
third design was chosen as an optimal design of the RTSU because it has the highest positive
rating and the lowest negative rating. Once testing and simulations have been complete, the
final concept will change.
For the final design concept, a piece of each design is incorporated into the system. This design
will use 6 galvanized steel coils and distribute them horizontally throughout the system. The
galvanized steel coils will run the Ethylene Glycol through the system and freeze the water.
Each steel coil will enclose two S-loops of the copper pipe. The refrigerant will run through
each of the 6 copper pipes. This will evenly space out both the galvanized steel coils and the
Proceedings of the 2015 ASEE Gulf-Southwest Annual Conference
Organized by The University of Texas at San Antonio
Copyright © 2015, American Society for Engineering Education
copper pipes for a uniform thermal distribution. This design incorporates more coils in the
system for higher heat transfer. The copper pipes loop back so the refrigerant flow will be
slower therefore increasing the heat transfer residence time. The flush system was also added
to the final design.
Table 3: Pugh Selection Chart
Pugh Selection Chart for RTSU
Selection Criteria
Cost
Weight
Size
Length of material
Heat Transfer
Refrigerant flow rate
Coils
Noise Pollution
Future maintenance
# Number of Pluses
# Number of Minuses
Ice Bear
Design
Concept
1
+
+
+
+
+
5
4
Datum
2
+
+
+
+
S
+
5
3
3
+
+
+
+
+
S
+
6
2
The size of the Residential Thermal Storage unit is 5 in x 5 in x 4 in. Table 4 displays the
dimensions and thermal conductivities of each of the materials that were used for the
Residential Thermal Storage unit.
Table 4: Materials Specifications
Component
Material
Thermal
Conductivity
14.3 W/mK
Thickness
Unit Interior
Stainless Steel
Unit Exterior Carbon Steel
43 W/mK
0.5 in
-
-
Piping
Glycol
Galvanized Steel
coil
50 W/mK
-
1.25 in
M
Piping
Refrigerant
Copper pipe
401 W/mK
-
1.25 in
K
0.25 in
Nominal Schedule
Diameter 40 Type
-
Force Balance
Internal forces act within the piping system that need to be considered for the final design. In
Fig. 15, the forces due to bends in the copper pipes are shown. To calculate the resulting force
in pipe bends, a pipe bend resulting force calculator was used. The density of the refrigerant,
Proceedings of the 2015 ASEE Gulf-Southwest Annual Conference
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Copyright © 2015, American Society for Engineering Education
the internal diameter of the pipe, and velocity of the fluid, along with the gauge pressure and
turning angle of the bend are used to calculate the resulting forces in the copper pipes7. Fig. 16
shows the inputs that were used to calculate the forces in the bends. Table 3 displays the
calculated forces due to pressure.
Figure 15: Forces of bends7
Table 3: Inputs for calculations
Input term
Density of Fluid
Internal Diameter of Pipe
Velocity of fluid
Turning Angle of Bend
Gauge Pressure
Value
1410 kg/m^3
0.035 m
0.00739 m/s
90°
78 kPa
Units
Table 4: Results of Forces
Forces due to pressure
Force in x-direction:
75 N
Force in y-direction:
75 N
Resulting force:
75 N
Concerns and Recommendations
The temperature of glycol increases as it flows through ice coils during the ice build cycle.
This causes thicker ice to form near the inlets of the coils and thinner ice to form on the outlets.
This can cause the cylinders of ice to taper. Since tube spacing is dependent upon the design
ice build thickness, the useful volume for the ice to build is affected as well. The tapering of
ice can lead to wasted volume, which can result in the ice melting more quickly. The amount
of time that the ice lasts will be greatly reduced which will reduce the overall efficiency of the
Proceedings of the 2015 ASEE Gulf-Southwest Annual Conference
Organized by The University of Texas at San Antonio
Copyright © 2015, American Society for Engineering Education
system. This will also reduce the RTSU operation time which is not ideal, since it must operate
for 8 hours during peak demand. Ice tapering is shown in Fig.16 8.
Several concerns have been identified that can potentially lead to problems with the RTSU.
One of the possible risks was that the water level in the system would lower due to losses. A
solution to this problem would be to include a sensor that would alert the consumer that the
water level in the system is too low for ice production. Several tests also must be done to make
sure that there are no breaks in the system that could cause leakage. Another potential problem
that may arise could be that ice would get trapped in the coils and bends of piping causing
tapering. Coil circuiting could be used instead of parallel circuiting to mitigate that occurrence.
Air agitation piping could also be included in the system to cause turbulence that would melt
the ice. Residue buildup from water impurities along the pipes will reduce heat transfer rate
and lower the system efficiency. A seal flush system could be included to drain the system
every 6 months to reduce the risk of fouling. Also, other chemical could be added to the water
to keep fouling from affecting the system. Another possible risk is what happens when water
expands in the system. It could possibly crack or bend the piping. The pressure could also
cause the internal piping to collapse. The buoyant force of the ice could tear the coil assemblies
loose from their attachments. The material of the pipes need to be strong enough to prevent
the internal piping from bending. Several tests need to be done on different materials to see
which one can withstand deformation. A stress analysis needs to be done on the material. The
internal storage system material holding the compartment together also could explode in the
system if the ice is not given enough room to expand given the inner layer is too thick. Tests
and calculations need to be done to choose the best material suited and the thickness.
Fig 16. Tapered Ice
Conclusion
The residential thermal storage unit seeks to compromise the demand of electricity during peak
hours. With the proposed design, consumers who seek to lower electricity consumption will
look no further. The RTSU will replace the functions of the standard AC unit during peak hours
but with a lower electricity consumption rate. The RTSU design incorporates the functions of
existing commercial hybrid AC units but at smaller scale suitable for residential use. The
RTSU will build ice during the night so that during peak hours, the refrigerant line can then
use this ice to cool off the hot refrigerant. The RTSU is composed of internal stainless steel
walls insulated with galvanized metal sheets to provide protection from the environment. Two
separate pipe lines have been proposed for the current design. One line includes a coiled setup
Proceedings of the 2015 ASEE Gulf-Southwest Annual Conference
Organized by The University of Texas at San Antonio
Copyright © 2015, American Society for Engineering Education
refrigerant line and the other is the exiting Glycol line. The final design proposed setup has the
potential to provide cooling needs at a lower price due to its reduced use of high electricity
components found in standard AC units.
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JASSMINE DURON
Jassmine will graduate with a Bachelor’s of Science in Mechanical Engineering from the
University of Texas at Tyler in May, 2015.
MANUEL MONROY
Manuel will graduate with a Bachelor’s of Science in Mechanical Engineering from the
University of Texas at Tyler in May, 2015.
M. A. RAFE BISWAS
Dr. Biswas currently serves as an Assistant Professor of the Mechanical Engineering
Department at the University of Texas at Tyler. His research interests are process model and
control development of alternative energy systems and prediction of residential building
energy.
Proceedings of the 2015 ASEE Gulf-Southwest Annual Conference
Organized by The University of Texas at San Antonio
Copyright © 2015, American Society for Engineering Education