PROJECT PROPOSAL &
FEASIBILITY STUDY
NaturaFill: Fuel for Thought
Team 13
Karl Bratt
Jonathan Haines
Andrew Hall
Brandon Koster
NATURAFILL
© 2013, TEAM 13, CALVIN COLLEGE
Last Updated: 12/9/2013
i
© 2013, Karl Bratt, Jonathan Haines, Andrew Hall, Brandon Koster, and Calvin College
Project Proposal & Feasibility Study
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Dedication
For Our Friend and Former Teammate
Eric DeGroot
1992-2013
However great our labors, this project will be forever incomplete.
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Executive Summary
This report outlines the introduction and preliminary development of a home refueling appliance to
successfully fuel a natural gas vehicle (NGV) with compressed natural gas (CNG). As a result of new
shale gas discoveries and new natural gas drilling technology, the supply of natural gas in the United
States has increased. This increase in supply has lowered the price of natural gas, making it a cheaper
fuel alternative compared to gasoline. Team 13, NaturaFill, has explored the many engineering and
business aspects of this project and upon completion of this report, NaturaFill has decided that the
designing and prototyping of this home refueling appliance is feasible and will cost approximately
$3,000.
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Table of Contents
1
Project Overview ............................................................................................................................... 1
1.1
Calvin College Senior Design ......................................................................................................... 1
1.2
Problem Statement ....................................................................................................................... 1
1.3
Industry Overview ......................................................................................................................... 1
1.3.1
Compressed Natural Gas....................................................................................................... 1
1.3.2
Benefits of CNG ..................................................................................................................... 1
1.3.3
Growth Rate Expectations .................................................................................................... 3
1.3.4
Barriers to Entry .................................................................................................................... 3
1.4
Project Proposal ............................................................................................................................ 4
1.4.1
Objective ............................................................................................................................... 4
1.4.2
Target Customers .................................................................................................................. 4
1.4.3
Existing Competitors ............................................................................................................. 4
1.4.4
Competitive Strategy ............................................................................................................ 6
1.5
Team Organization: ....................................................................................................................... 6
1.5.1
Team Members ..................................................................................................................... 6
1.5.2
Team Member Strengths ...................................................................................................... 7
1.5.3
Team Leadership and Management ..................................................................................... 7
1.6
Design Norms ................................................................................................................................ 9
1.6.1
Stewardship: ......................................................................................................................... 9
1.6.2
Trust ...................................................................................................................................... 9
2
Requirements .................................................................................................................................. 10
2.1
Safety .......................................................................................................................................... 10
2.2
Price ............................................................................................................................................ 10
2.3
Refueling Rate ............................................................................................................................. 10
2.4
Heat Loss ..................................................................................................................................... 10
2.5
Serviceability ............................................................................................................................... 10
2.6
Reliability..................................................................................................................................... 10
2.7
Noise ........................................................................................................................................... 10
2.8
Environmental ............................................................................................................................. 10
2.9
Size .............................................................................................................................................. 11
2.10
Weather Proofing........................................................................................................................ 11
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2.11
User Interface ............................................................................................................................. 11
2.11.1
Inputs .................................................................................................................................. 11
2.11.2
Outputs ............................................................................................................................... 11
3
Deliverables..................................................................................................................................... 12
3.1
Project Proposal and Feasibility Study ........................................................................................ 12
3.2
Final Design Report ..................................................................................................................... 12
3.3
Working Prototype...................................................................................................................... 12
3.4
Installation, Operation, and Service Manual .............................................................................. 12
3.5
Team Website ............................................................................................................................. 12
4
Design Considerations..................................................................................................................... 13
4.1
Compression Technology ............................................................................................................ 13
4.1.1
Alternatives ......................................................................................................................... 13
4.1.2
Decision ............................................................................................................................... 14
4.2
Hydraulic Power Units................................................................................................................. 15
4.2.1
Alternatives ......................................................................................................................... 15
4.2.2
Decision ............................................................................................................................... 16
4.3
Natural Gas Compression Cylinder ............................................................................................. 16
4.3.1
Alternatives ......................................................................................................................... 16
4.3.2
Decision ............................................................................................................................... 17
4.4
Piston Material ............................................................................................................................ 17
4.4.1
Alternatives ......................................................................................................................... 17
4.4.2
Decision ............................................................................................................................... 17
4.5
Sealing Technology ..................................................................................................................... 17
4.5.1
Alternatives ......................................................................................................................... 17
4.5.2
Decision ............................................................................................................................... 18
4.6
Piston Ring Layout....................................................................................................................... 18
4.6.1
Alternatives ......................................................................................................................... 18
4.6.2
Decision ............................................................................................................................... 19
4.7
Tubing ......................................................................................................................................... 20
4.7.1
Alternatives ......................................................................................................................... 20
4.7.2
Decision ............................................................................................................................... 21
4.8
Heat Removal .............................................................................................................................. 21
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4.8.1
Alternatives ......................................................................................................................... 21
4.8.2
Decision ............................................................................................................................... 23
4.8.3
Method................................................................................................................................ 23
4.9
Pressure Sensors ......................................................................................................................... 23
4.9.1
Alternatives ......................................................................................................................... 23
4.9.2
Decision ............................................................................................................................... 24
4.10
Development Board .................................................................................................................... 24
4.10.1
Alternatives ......................................................................................................................... 24
4.10.2
Decision ............................................................................................................................... 26
4.11
User Interface ............................................................................................................................. 26
4.11.1
Alternatives ......................................................................................................................... 26
4.11.2
Decision ............................................................................................................................... 28
5
Testing & Calculations ..................................................................................................................... 29
5.1
Preliminary Design Methods ....................................................................................................... 29
5.1.1
Autodesk CFD Simulator ..................................................................................................... 29
5.1.2
Engineering Equation Solver ............................................................................................... 30
5.2
6
Initial Testing ............................................................................................................................... 31
Preliminary Design Decisions .......................................................................................................... 32
6.1
Hydraulic System......................................................................................................................... 32
6.1.1
Overview ............................................................................................................................. 32
6.1.2
Reservoir ............................................................................................................................. 32
6.1.3
Pump ................................................................................................................................... 33
6.1.4
Solenoid Valve ..................................................................................................................... 33
6.1.5
Cylinders.............................................................................................................................. 33
6.2
Natural Gas System ..................................................................................................................... 34
6.2.1
Overview ............................................................................................................................. 34
6.2.2
Residential Natural Gas Supply ........................................................................................... 34
6.2.3
Tubing.................................................................................................................................. 35
6.2.4
Check Valves ....................................................................................................................... 35
6.2.5
Relief Valve.......................................................................................................................... 35
6.2.6
Shut-off Valve...................................................................................................................... 35
6.2.7
Manifolds ............................................................................................................................ 36
6.2.8
Pressure Gauges and Transducers ...................................................................................... 36
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6.2.9
Thermocouples ................................................................................................................... 37
6.2.10
Heat Sinks ............................................................................................................................ 38
6.2.11
Refueling Nozzle.................................................................................................................. 38
6.3
Sealing System ............................................................................................................................ 39
6.3.1
PTFE Coated Cylinder Walls ................................................................................................ 39
6.3.2
PTFE Seal Rings .................................................................................................................... 39
6.3.3
PTFE Rider Rings .................................................................................................................. 39
6.4
7
Control System ............................................................................................................................ 40
Financial Estimates.......................................................................................................................... 42
7.1
Cost of Development .................................................................................................................. 42
7.2
Cost of Production ...................................................................................................................... 42
8
Conclusion ....................................................................................................................................... 44
9
Acknowledgements......................................................................................................................... 45
9.1
Professor Ned Nielsen................................................................................................................. 45
9.2
Professor Steve VanderLeest ...................................................................................................... 45
9.3
Professor Matthew Heun ............................................................................................................ 45
9.4
Mr. Jimmy Moerdyk .................................................................................................................... 45
9.5
Mr. Lee Otto ................................................................................................................................ 45
9.6
Mr. Ross Persifull ........................................................................................................................ 45
9.7
Mr. Phil Jasperse ......................................................................................................................... 45
10 Appendices ...................................................................................................................................... 46
10.1
Appendix A. EES Code ................................................................................................................. 46
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Table of Figures
Figure 1. Price Equivalent per Barrel of Oil and Natural Gas .................................................................... 2
Figure 2. Oil Price as a Multiple of Natural Gas Price ............................................................................... 2
Figure 3. Number of Natural Gas Vehicles by Region (1991-2001) .......................................................... 3
Figure 4. BRC FuelMaker Phill® ................................................................................................................. 5
Figure 5. CNG Pump II® ............................................................................................................................. 5
Figure 6. Ross Persifill’s CNG Home Refueling Unit .................................................................................. 8
Figure 7. Hydraulic Cylinder Compression System ................................................................................. 13
Figure 8. Wobble Plate Compression System ......................................................................................... 14
Figure 9. Off-The-Shelf Hydraulic Power Unit Example .......................................................................... 15
Figure 10. Custom Built Hydraulic Power Unit Example ......................................................................... 16
Figure 11. Diagram of Layout 1 ............................................................................................................... 19
Figure 12. Picture of Layout 2 ................................................................................................................. 19
Figure 13. Schedule 80 Steel Pipe ........................................................................................................... 20
Figure 14. Stainless Steel Tubing ............................................................................................................. 20
Figure 15. Stainless Steel Tubing Fittings ................................................................................................ 21
Figure 16. Two Examples of Finned Tubing ............................................................................................ 22
Figure 17. Example of Tubing Loops ....................................................................................................... 23
Figure 18. Pressure Transmitter Example ............................................................................................... 24
Figure 19. Pressure Gage Example .......................................................................................................... 24
Figure 20. Touchscreen Raspberry Pi display.......................................................................................... 26
Figure 21. LCD & Keypad display ............................................................................................................. 27
Figure 22. Mobile User Interface ............................................................................................................ 27
Figure 23. Pressure Results from Simulation .......................................................................................... 29
Figure 24. Plane of Simulation Pressure Results ..................................................................................... 30
Figure 25. Hydraulic System Schematic .................................................................................................. 32
Figure 26. 4-Way, 3-Position Solenoid Valve (Exhaust Center) .............................................................. 33
Figure 27. Natural Gas System Schematic .............................................................................................. 34
Figure 28. High Pressure Natural Gas Check Valves ............................................................................... 35
Figure 29. Adjustable High Pressure Natural Gas Relief Valves .............................................................. 35
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Figure 30. Example of CNG Ball Valve ..................................................................................................... 36
Figure 31. Example of Manifold and Connections at End of CNG Compression Chamber ..................... 36
Figure 32. Pressure Transmitter Example ............................................................................................... 37
Figure 33. Pressure Gage Example .......................................................................................................... 37
Figure 34. High Pressure Pipe-Plug Thermocouple Example .................................................................. 38
Figure 35. Example of Tubing Loops ....................................................................................................... 38
Figure 36. General Purpose CNG Refueling Nozzle for Time-Fill ............................................................ 39
Figure 37. Picture of Ring Layout ............................................................................................................ 40
Figure 38. Control System Schematic ..................................................................................................... 41
Figure 39. Raspberry Pi Size Comparison ................................................................................................ 41
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Table of Tables
Table 1. NGV Reductions in Exhaust Emissions ........................................................................................ 1
Table 2. Compression Technology Decision Matrix ................................................................................ 15
Table 3. Steel Tubing Sizing Chart ........................................................................................................... 21
Table 4. Comparison of Development Boards ........................................................................................ 25
Table 5. Work and Heat Removal Calculation Results ............................................................................ 31
Table 6. Operational Budget ................................................................................................................... 42
Table 7. Cost of Production ..................................................................................................................... 43
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1 Project Overview
1.1 Calvin College Senior Design
Calvin College is a Christian liberal arts college located in Grand Rapids, Michigan. Calvin’s engineering
program is ABET accredited and has a reputation for producing well-rounded engineers by integrating
this liberal arts curriculum with engineering. Senior design is the capstone of the engineering program
in which students conceive, develop, and implement a project of their interest over the course of the
year, combining material learned both inside and outside the classroom.
1.2 Problem Statement
There is scarcity in the residential applications for compressed natural gas (CNG) in the United States
transportation landscape. Currently, only 605 public CNG refueling stations exist, compared to the
168,000 gasoline stations.1 In response, a few small companies, such as BRC Fuelmaker, have
developed home refueling units that run on electricity and connect to existing natural gas lines.2
Unfortunately, these models begin at $5,000 before installation.3 Therefore, the purpose of the project
is to design, build, and test a natural gas home refueling appliance that is lower cost and more reliable
than the appliances currently on the market.
1.3 Industry Overview
1.3.1 Compressed Natural Gas
Natural gas is a colorless, odorless, non-corrosive, and extremely flammable mixture of hydrocarbon
gases. Natural gas, a fossil fuel, originates from the chemically breakdown of organic matter over time
to produce methane, CH4.4 When compressed to 3600psi, natural gas is termed, “CNG,” or compressed
natural gas.
1.3.2 Benefits of CNG
CNG can easily be used as an alternative for gasoline in a dedicated vehicle engine or in conjunction
with gasoline in a “bi-fuel” application. In fact, the octane rating for CNG is higher than that of gasoline,
producing greater power, acceleration, and cruise speed for vehicles running on CNG. Though the
safety concerns are different, CNG’s narrow flammability range makes it inherently safer than gasoline.
In addition, emission comparisons of CNG and gasoline-fueled vehicles have revealed substantial
reductions in greenhouse gases (see Table 1. NGV Reductions in Exhaust Emissions).5
Table 1. NGV Reductions in Exhaust Emissions
NGV Potential Reductions in Exhaust Emissions (%)
Carbon
Monoxide (CO)
Non-Methane Hydrocarbons
(NMHC)
90%
75%
Nitrogen
Oxides
(NOx)
60%
Carbon
Dioxides
(CO2)
25%
1
http://www.afdc.energy.gov/fuels/natural_gas_locations.html
http://www.cngnow.com/vehicles/refueling/Pages/refueling-at-home.aspx
3
Jimmy Moerdyk, Moerdyk Energy Inc. (MEI), 9/27/13, 9:00AM
4
http://www.naturalgas.org/overview/background.asp
5
http://eerc.ra.utk.edu/etcfc/docs/EPAFactSheet-cng.pdf
2
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CNG not only provides a cleaner-burning fuel alternative to gasoline, it is also an economically feasible.
As a result of new shale gas discoveries and new drilling technology the supply of natural gas in the
United States has increased substantially. This increase in supply has forced the price of natural gas
down 70% over the last five years.6 The current pipeline natural gas of about $3.80 per million BTU7
has an energy equivalent price of $0.42 per gasoline gallon equivalent (GGE)8. Over the past twenty
years, this price drop has been reflected in a large price separation between gasoline and natural gas
(see Figure 1. Price Equivalent per Barrel of Oil and Natural Gas).
Figure 1. Price Equivalent per Barrel of Oil and Natural Gas9
In fact, the price of oil is over eight times higher than that of natural gas, as shown in Figure. 2. Oil Price
as a Multiple of Natural Gas Price.
Figure 2. Oil Price as a Multiple of Natural Gas Price10
6
http://www.infomine.com/investment/metal-prices/natural-gas/5-year/
http://www.oil-price.net/
8
http://www.energyalmanac.ca.gov/transportation/gge.html
9
http://pictorial-guide-to-energy.blogspot.com/2012/03/gaseous-emissions.html
10
http://www.infomine.com/investment/metal-prices/natural-gas/5-year/
7
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1.3.3 Growth Rate Expectations
The economic and environmental incentives associated with CNG have already encouraged transit,
garbage, and transportation fleets to convert from diesel and gasoline fuel to CNG.11 Unfortunately,
this has been primarily restricted to vehicles such as taxicabs, transit and school buses, garbage trucks,
and public works vehicles. Because these vehicles are typically maintained and fueled and a central
location, it is has been economical for them to convert to natural gas. In total, the number of CNG
powered vehicles on the road is growing at a staggering rate of 30 percent a year.12 Currently, there
are 135,000 natural gas vehicles in the United States and 15.2 million worldwide13 (see Figure 3. Number
of Natural Gas Vehicles by Region).14
Figure 3. Number of Natural Gas Vehicles by Region (1991-2001)
In the coming years, it is also forecasted that the number of factory built bi-fuel vehicles will also be on
the rise. “Bi-fuel” indicates that a vehicle can run on both gasoline and natural gas fuel sources.
Presently, larger vehicles such as the 2013 Chevrolet Silverado and GMC Sierra 2500 HD are factoryoffered with bi-fuel capability. Ross Pursifull, a research specialist at Ford Motor Company, spoke with
the team15 and confirmed rumors that the 2015 Chevrolet Impala will also be built bi-fuel ready.16
Chevrolet’s decision results from increased market pressure for CNG vehicles in residential applications.
Increased demand for residential natural gas vehicles (NGVs) will correspondingly increase demand for
CNG fueling stations.
1.3.4 Barriers to Entry
The biggest hurdle to widespread consumer adoption of natural gas vehicles is availability to fueling
stations. There are over 270 publically accessible gasoline fuel stations for every one station supplying
11
http://www.cngnow.com/vehicles/fleets/Pages/government.aspx
http://www.ngvamerica.org/media_ctr/fact_ngv.html
13
http://www.ngvc.org/about_ngv/index.html
14
http://www.iangv.org/current-ngv-stats/
15
Pursifull, Ross; Ford Motor Company; 11/9/13, 10:00AM
12
16
http://blog.caranddriver.com/antelope-in-the-gas-2015-chevrolet-impala-to-gain-bi-fuel-gasolinecng-capable-v-6-model/
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CNG.17 The capability to fuel a CNG vehicle at home side-steps waiting for an adequate nationwide
network of fueling stations. Unfortunately, cost has been the major deterrent away from existing CNG
home refueling units.
1.4 Project Proposal
1.4.1 Objective
The objective of NaturaFill is to design, build, and test a natural gas home refueling appliance with a
production cost of less than $2,000 and refueling rate of 0.35 gasoline gallons equivalent (GGE) per
hour. The system must be capable of compressing natural gas to the standard pressure of 3,600 psi in
a safe and controlled environment.
1.4.2 Target Customers
This increased use of CNG in transportation vehicles has made it seemingly more affordable and feasible
in residential applications. In total, 56 percent of American households are currently using natural gas
for residential heating.18 As a result, NaturaFill’s natural gas home refueling appliance aims to target
customers with existing natural gas lines and fossil fuel dependent vehicles. A potential secondary
market for CNG vehicles and the system would include customers desiring reduced emission large
vehicle options, such as trucks or sports utility vehicles (SUVs). The majority of electric or hybrid
powered vehicles on the market today are small with little hauling capacity. In the realm of trucks and
SVUs, natural gas vehicles are a suitable option for improved environmental impact.
1.4.3 Existing Competitors
BRC FuelMaker
FuelMaker is currently the largest producer of CNG home refueling units. Their most popular unit, the
Phill®, costs $5,000 before installation. The Phill® refuels at an average rate between 1.2-1.5 GGEs per
hour (see Figure 4. BRC Fuel Maker Phill®).
17
18
http://www.afdc.energy.gov/fuels/natural_gas_locations.html
http://www.naturalgas.org/overview/uses_residential.asp
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Figure 4. BRC FuelMaker Phill®
CNGPump
CNGPump, maker of the CNG Pump®, uses hydraulic compression for conversion of natural gas into
CNG. Their cheapest model is the CNG Pump II®, which refuels at a rate of 2 GGEs per hour.
Unfortunately, their upfront cost is $7,000 before installation, an inhibitor for most customers.19
Figure 5. CNG Pump II®
19
http://www.cngpump.com/shoppingcart/products/CNGPUMP%252d2gge-%28Time-Fill%29-HydraulicFueling-Station-for-cars%7B47%7Dtrucks-%282gge-per-hour%29.html
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General Electric
The biggest potential competitor is General Electric (GE). They have announced a partnership with
Chesapeake Energy to develop a natural gas home refueling unit with an upfront cost of $500.20 If
proposition becomes a reality, it would undercut NaturalFill’s suggested retail price and therefore,
undermine its value proposition. From conversations with Ross Persifill, Research Specialist at the Ford
Motor Company, and other industry insiders, the general consensus suggests high unlikeliness of GE’s
proposition becoming a reality. For this reason, the team forecasts the elapse of a substantial amount
of time before GE enters this market.
1.4.4 Competitive Strategy
Existing in-home refueling units are currently in the price range or $5000-$7000+ and fill at a rate of
around 2 GGEs. Today, most systems are currently made in Europe and the United States, however, it
is rumored that a less expensive Asian variety will soon be hitting the market will be released soon.
NaturaFill plans to compete by ensuring quality and safety and competing at a lower price point by
reducing the compression rate of approximately 0.35 GGEs. In actuality, this compression rate would
be comparable to charging a 100% electric vehicle overnight for average daily usage. The team’s
strategy to compete on price and differentiation will make residential use of natural gas vehicles more
feasible in the American transportation landscape.
1.5 Team Organization:
1.5.1 Team Members
Karl Bratt
Bratt is a Mechanical concentration engineering student also in pursuit of a business
minor. Originally from Racine, Wisconsin, Bratt has diverse interests in business
finance, operations, and supply chain. Bratt has had two summer internships
working for General Electric as a manufacturing engineer and continuous
improvement specialist. He is passionate about the potential business opportunities
involved with the NaturaFill project. In his free time, he enjoys playing piano, golf,
and tennis, and is an avid runner in his free time. Last year, he ran the Chicago
Jonathan Haines
Haines is a Mechanical concentration engineering student also in pursuit of a
mathematics minor. Originally from East Brunswick, New Jersey, Haines has
experience with manufacturing and has interests in the auto industry. His
experience with thermodynamics and hands on experience with manufacturing
along with his knowledge of Autodesk CFD simulator will be valuable in the design
and implementation of this design. In his free time, Jon enjoys playing guitar and
cooking.
20
http://oilprice.com/Finance/investing-and-trading-reports/Chesapeake-GE-Get-in-on-Home-RefuelingGame.html
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Andrew Hall
Hall is an Electrical and Computer concentration engineering student. Originally
from McCordsville, Indiana. Hall has obtained a comprehensive array of electrical
engineering experiences and technical skills. He has gained proficient experience
and skills in communications, marketing, research, electrical fabrication, and
component design testing. In his free time, Hall enjoys playing bass guitar in two
of the many local churches Grand Rapids offers. He also enjoys restoring and
refurbishing vintage era electronics as well as playing an occasional round of golf.
Brandon Koster
Koster is a Mechanical concentration engineering student and also pursuing a
business minor. Originally from Visalia, California, Koster has been a natural gas
transportation enthusiast for years. The future of the natural gas transportation
has been personal research interest of his, both from a business and engineering
perspective. Also, growing up as the son of a contractor, Koster gained valuable
fabricating, mechanical design, and project execution skills at a young age. In his
free time he enjoys rock climbing, cheering on the Los Angeles Dodgers, working on
his car, listening to Texas Country music, and taking road trips.
1.5.2 Team Member Strengths
Karl Bratt
Bratt's background in lean manufacturing and ergonomics will be valuable in the design and
manufacture of NaturaFill. In addition, his experience in forecasting and cost analysis will be helpful in
evaluating and marketing the value proposition of the product.
Jonathan Haines
Haines’s experience with thermodynamics and manufacturing, along with his knowledge of Autodesk
CFD simulator, will be valuable in the design and implementation of this design.
Andrew Hall
Hall’s experience in analog and digital electronic system applications will bring a highly distinguishable
attribute to the control and safety systems of NaturaFill.
Brandon Koster
Koster’s industry knowledge will serve to connect the team to companies and individuals in the industry
that will provide valuable guidance and support throughout the project.
1.5.3 Team Leadership and Management
Project Manager
Bratt will act as the Project Manager for NaturaFill. Bratt is primarily responsible for the organizational
components of the project. This includes, but is not limited to, the planning and scheduling of team
meetings, review and on-time delivery of project deliverables, and work breakdown of tasks among the
team member using Microsoft Project.
Webmaster
Hall will act as the team Webmaster. With a concentration in electrical engineering, Hall enjoys HTML
code and has experience in website development. Hall is primarily responsible for maintaining the
team’s website, email address, and Twitter page.
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Budget Manager
Koster will act as the Budget Manager for NaturaFill. Koster is primarily responsible for managing the
team’s operational budget. This includes, but is not limited to, turning in all reimbursement forms,
requesting parts purchases, and communicating with team sponsors.
Senior Design Faculty Advisor
Ned Nielsen is a Mechanical Engineering professor at Calvin College. In addition, Nielsen is the faculty
advisor for mechanically concentrated Senior Design projects. He is responsibility for providing
constructive feedback on the project throughout the course of the year. Nielsen’s extensive
engineering background will be helpful for finalizing the team’s CNG refueling unit design.
Electrical Faculty Consultant
Steve VanderLeest is an Electrical Engineering professor at Calvin College and the faculty advisor for
electrically concentrated Senior Design projects. His electrical engineering background will support Hall
in the design and optimization of NaturaFill’s pressure monitoring systems and safety controls.
Mechanical Faculty Consultant
Matthew Heun is a Mechanical Engineering professor at Calvin College. Heun has agreed to meet with
NaturaFill to address the transfer of heat released by the compression system. Heun has also given the
team advice with the use of Autodesk CFD for compression and temperature modeling.
CNG Industry Consultant
Lee Otto is the Founder and CEO of CNGPump, Inc. in Appleton, WI. Otto offered the team advice on
seal technology, heat dissipation, and control systems. Unfortunately, he was could not provide any
further technical information without compromising confidential intellectual property.
Automotive Industry Consultant
Ross Persifull is a Research Specialist at Ford Motor Company in Dearborn, MI. Persifull, a CNG
enthusiast, agreed to meet with NaturaFill and discuss the CNG industry from an automotive industry
standpoint. In addition, he demonstrated the use of a home refueling unit to fuel his own NGVs (see
Figure 6. Ross Persifull CNG Home Refueling Unit.
Figure 6. Ross Persifill’s CNG Home Refueling Unit
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Industry Mentor
Jimmy Moerdyk is the Vice President of Operations at Moerdyk Energy, Inc. in Grand Rapids, MI.21
Moerdyk has provided support and suggestions to the team based on his experience in the installation
of CNG vehicle conversions and home refueling units. Moerdyk has been enthusiastic of NaturaFill’s
$2,000 production price target.
1.6 Design Norms
1.6.1 Stewardship:
As the concentration of CO2 in the atmosphere approaches the 400 ppm mark, the political, social, and
economic pressures for alternative energies has risen drastically22. Compressed natural gas, which
burns cleaner than coal or oil, is a solution to this environmental and economic challenge.
Environmental Sustainability
It has been proven that natural gas vehicles produce less emissions than traditional gasoline powered
vehicles (see Table 1. NGV Reductions in Exhaust Emissions). As worries rise on the negative effects of
greenhouse gases on the environment, the team believes that the transition from gasoline fueled
transportation to natural gas exemplifies upright stewardship over the environment.
Economic and Political Sustainability
Natural gas is widely available in the United States. In fact, 98% of the natural gas consumed in the
United States is produced in either the US or Canada.23 In addition, the price of natural gas has declined
tremendously in the past five year, especially in comparison to oil (see Figure 1. Price Equivalent per
Barrel of Oil and Natural Gas). The economic feasibility of this project promotes the team’s role as
stewards over limited financial resources.
1.6.2 Trust
The team’s mission is to provide a fully-functional natural gas home refueling unit that is safe and made
easy-to-use. Safety is the team’s most important design requirement. Since a prototype malfunction
could be highly dangerous, the team will conduct all preliminary testing of the devise with a noncombustible gaseous materials. In addition, by integrating easy-to-use controls into the device’s user
interface, the team hopes to create a level of trust with the end user. All in all, these considerations
align with our role as engineers to have integrity with the design of safe and high-quality products.
21
http://www.moerdykenergy.com/Home.php
http://www.gasnaturally.eu/uploads/Modules/Publications/the-role-of-natural-gas-in-a-sustainable-energymarket-final.pdf
23
http://anga.us/why-natural-gas/clean#.UmCKfPmsgyo
22
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2 Requirements
2.1 Safety
Due to high pressures and the flammable nature of natural gas, the most important design requirement
for the home refueling appliance is safety. The system must come equipped with gas leak detector
sensors, pressure controls, and a monitoring system to ensure that in the case of a gas leak, an
unexpected jump in pressure, or any other malfunction that the machine will be turned off in a safe
manor.
2.2 Price
Since the majority of commercially available CNG home refueling units cost more than $5000, the teams
aims to design a unit that could be sold at a the $3000 price range.
2.3 Refueling Rate
Nearly all of the units on the market operate with a faster fueling rate ranging from 1.2-2+ GGEs. These
are considered fast filling units since you would be able to reach near full capacity in the matter of a
few hours. The team aims to build a slow filling unit. A slow filling unit can be compared to plugging in
an electric car overnight. With a fuel rate of 0.35 GGEs and having it fill over night for an average of 8
hours a night would be able to replace nearly 20 gallons of gasoline. Therefore, the team as chosen to
design the unit to have a fueling rate of 0.35 GGEs.
2.4 Heat Loss
During compression, the temperature of the natural gas increases due to work being done on the gas
and the conservation of energy. When compressing to 3600 psi without any sort of cooling the
temperature of the gas reaches a temperature of over 350 °C. This is clearly too hot to be putting into
a vehicle. Because of this, the team will design the system to make sure that the gas is around room
temperature before it goes into the vehicle.
2.5 Serviceability
Many CNG enthusiasts are hands on people. Because of this, the team will design the unit to be easily
serviced by those who are knowledgeable in the industry.
2.6 Reliability
Because this unit is being designed with the thought of being people’s primary source of fuel for
transportation, the unit must be reliable. This will be accomplished by stress testing the unit to insure
that it runs continuously for long periods of time.
2.7 Noise
Since this unit is being designed to be used in people’s garages overnight, it only makes sense for the
unit to operate nearly silent. To achieve this, the team plans to design the unit to operate at a noise
level of under 50 dB from a distance of 10 ft. away.
2.8 Environmental
Since the team values stewardship as a core value, the team plans to design the unit to be
environmentally friendly by having it run efficiently and making sure that the unit is free of leaks. Also
in the case of a leak, the unit would be designed to shut off.
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2.9 Size
Units like the one being designed are typically stored in garages. Because of this the team will design
the total size of the unit to fit reasonably in an averaged sized garage.
2.10 Weather Proofing
Since not all users of this refueling unit will necessarily be putting it in their garage, the exterior casing
must be designed to be weather proof.
2.11 User Interface
The user interface brings together the entire refueling unit into one easy to use screen and keypad
push button system. Designing a simple user interface is key, but will also diminish any chance for the
user to become confused on how to operate the unit.
2.11.1 Inputs
Cutting down on user interaction for the sake of simplicity, there will be a simple on/off button and a
button to begin fueling. This simple user input design in turn places the leftover burden of the complex
decisions of the system to then be decided by the Raspberry Pi.
2.11.2 Outputs
Following the few inputs necessary to power on and begin fueling, the user will receive a few phrases
of confirmation to ensure that the unit is fully operating. There also will be a few LED lights indicating
that the system is on, fueling, and or malfunctioning, which would throw an error message to the
display screen.
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3 Deliverables
3.1 Project Proposal and Feasibility Study
NaturaFill shall submit a complete project proposal and feasibility study detailing a background on the
project, a proposed solution, and the feasibility of this solution. This report will be submitted no later
than December 9, 2013 at 3:30pm E.S.T.
3.2 Final Design Report
NaturaFill shall submit a final report at the completion of the project to explain the project background,
research and data analysis, design alternatives and decisions, prototype design and testing, and final
design conclusions. This final design report will be submitted no later than May 14, 2014 at 3:30pm
E.S.T.
3.3 Working Prototype
The team will demonstrate a working prototype of the refueling unit at the Engineering Department
Senior Design Night on May 10, 2014. The team hopes to demonstrate, on-stage, the refueling of an
actual natural gas vehicle.
3.4 Installation, Operation, and Service Manual
In conjunction with the working prototype, NaturaFill shall deliver an installation, operation, and service
manual by May 10, 2014 at 7:00pm E.S.T. This manual shall outline all necessary instructions for
installation, operation, and corrective and preventive maintenance.
3.5 Team Website
The team shall maintain a website as a means of updating the college, sponsors, and the public on the
project status. These updates shall be executed by Andrew Hall and will occur at milestones throughout
the project lifespan. All final updates to the website, including published final versions of the PPFS and
Final Design Report, must be made by May 12, 2014 at 3:30pm E.S.T.
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4 Design Considerations
4.1 Compression Technology
4.1.1 Alternatives
Hydraulic Cylinder Compression
A hydraulic cylinder compression system would use multiple double rod hydraulic cylinders to compress
the gas at a low cycle rate, compared to most other technologies. This is a technology that is slowly
being introduced as an alternative to the motor driven reciprocating compressor. Many companies and
individuals in the industry believe that it has the potential to become the industry standard technology
for compressing natural gas in the future because of the higher reliability it offers and the fact that a
unit can be assembled using off-the-shelf hydraulic components. Currently, this technology is more
developed in China than in the United States. See Figure 7. Hydraulic Cylinder Compression System, for
a diagram of this compression method.
Figure 7. Hydraulic Cylinder Compression System24
Pneumatic Cylinder Compression
A pneumatic cylinder compression system would use multiple double rod pneumatic cylinders to
compress the gas at a low cycle rate compared to most other technologies. It technology is almost
identical to hydraulic cylinder compression, the only difference being the use of pneumatic cylinders
instead of hydraulic cylinders. According to the team’s extensive research, this has technique not yet
been used to compress natural gas.
24
http://www.hydropac.com/GRAPHICS/hydrogen3b.jpg
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Wobble Plate Compression
Figure 8. Wobble Plate Compression System25
A wobble plate compressor is the type of compressor typically used in air conditioning units in cars.
This design option was suggested by Lee Otto since very little work has been done to use this type of
compressor for CNG. See Figure 8. Wobble Plate Compression System, for a diagram of this
compression alternative. The team did extensive research on the potential of using wobble plate
technology to compress natural gas. After this research the team came to the conclusion that using
this technology for a senior design project was not feasible. The complexity of the linkages and the
potential for unbalance issues due to the different sizes of each compression stage was determined to
be too much for the team to execute in a nine-month timeframe.
Motor Driven Reciprocating Compression
A motor driven reciprocating compressor is the industry standard for compressing natural gas. This
option was ruled out early in the process since so much work has already been done with these types
of compressors. This compression technology as also has had many reliability issues due to its high cycle
rate.
4.1.2 Decision
To make this decision, a decision matrix was created be weighting a list of desired outcomes and rating
each design alternative on a scale of one to ten. The totals were then totaled up and the design
alternative with the highest score was chosen since it accomplishes the tasks that the team as
designated as important. See the decision matrix in Table 2 for the criteria and weights used to judge
alternatives and make a decision.
25
http://www.jaguar-swansea.co.uk/aircon/systems.htm
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Table 2. Compression Technology Decision Matrix
Criteria
Score
Weight
Motor Driven
Reciprocating
Compressor
Wobble
Plate
Compressor
Hydraulic Cylinder
Driven Reciprocating
Compressor
Pneumatic Cylinder
Driven Reciprocating
Compressor
Cost to Manufacture
10
14%
8
6
7
9
Reliability
9
13%
5
6
9
7
Safety
9
13%
8
8
8
8
Ease of Use
7
10%
8
8
8
8
Serviceability
7
10%
4
3
10
10
Efficiency
6
9%
8
8
8
3
Prototype Cost
5
7%
5
8
8
10
Innovation
5
7%
1
10
8
10
Marketability
5
7%
5
5
7
6
Ability to Prototype
4
6%
5
3
8
10
Noise Pollution
3
4%
3
6
10
6
5.9
6.5
8.2
8.0
It can be seen from this matrix that the design alternative with the highest score is hydraulic
compression. Because of this, the team has decided to pursue using hydraulic compression for the unit.
4.2 Hydraulic Power Units
The team discussed their needs for a hydraulic power unit with Ryan Anderson, a hydraulic power unit
technician with Bond Fluidaire, during a visit. The team discuss many of their requirements with Anderson
and they discussed ballpark pricing for their budget. The low-noise and very high run time (often >12
hours) makes the team’s hydraulic power unit specifications different than most units used in the industry.
4.2.1 Alternatives
Custom Built
These units are relatively inexpensive and easy to find. They also come in sizes that fit well with the
team’s requirements. A concern is the fact that these units are not typically designed to run for
extremely long periods of time reliably. See Figure 9 for an example of this type of power unit.
Figure 9. Off-The-Shelf Hydraulic Power Unit Example26
26
http://www.northerntool.com/images/product/2000x2000/107/1077_2000x2000.jpg
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Off-The-Shelf
These units are designed with the specific application of the unit in mind. They are constructed out of
a reservoir, hydraulic pump, electric motor, manifold, valves, and piping and put together by local
custom hydraulics companies like Bond Fluidaire. This alternative would give the team the flexibility to
select the individual components for the unit in such a way that the final unit meets all of their needs.
This would also give the team more control over the footprint of the power unit and their final product
because the reservoir could be custom made to fit in available space and the components could be
mounted in a manner that maximizes available space. The team could also save labor be possibly
building the hydraulic power unit themselves out of purchased components. See Figure 10 for an
example of this type of power unit.
Figure 10. Custom Built Hydraulic Power Unit Example27
4.2.2 Decision
The team decided to use a custom-built unit. They came to this decision because they were concerned
about the off-the-shelf units not being able to deliver on the noise and run time requirements. They
would also like to give themselves the option of building the unit themselves to save cost.
4.3 Natural Gas Compression Cylinder
4.3.1 Alternatives
Thick-walled Steel Pipe
Thick-walled Steel pipe is reliable, easy to purchase, and relatively inexpensive. However, it only comes
in a limited variety of diameters. Furthermore, some machining work might need to be done to get the
inside wall to the specified exact diameter and tolerance.
27
http://www.gshydraulics.com/images/products/large/HydraulicPowerUnit01_CustomCompact.jpg
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Machined Steel Rod
While this method is more labor intensive, it allows the team to maintain control of wall thickness, inner
diameter, and tolerances.
Machined Steel Block
Another alternative is to machine a pair of compression blocks. This method would require a lot of time
to machine. Another concern is that if there is an error made on one of the last steps of machining the
part would need to be scrapped and restarted which would consume a lot of time.
Reinforcement Rods
When constructing compression cylinders that are not in a block, reinforcement rods are typically used
to carry the pressure load and to hold the manifold and the cylinders together.
4.3.2 Decision
The team has decided to pursue the method of thick walled steel pipe to create compression cylinders.
Doing so will allow the team to design the cylinder size to any specified diameter with minimal machine
work. This choice also reduces risk in manufacturing, in the unfortunate case there is a mistake made,
the time required to make a new cylinder would be small. Reinforcement rods will also be used to attach
the manifold to the cylinder.
4.4 Piston Material
4.4.1 Alternatives
Steel
Steel pistons could be constructed out of steel rod stock. Steel is relatively inexpensive for its strength.
Steel is the heaviest material that was considered, there were concerns about the heavier steel pistons
leading to unnecessary unbalance issues. Steel also does not conduct heat as fast as aluminum.
Aluminum
Aluminum pistons are most common in both compressors and internal combustion engines. This is a
result of their light weight and high thermal conductivity.
PTFE
There were thoughts of possibly making the pistons out of PTFE (Teflon). This would eliminate the need
for seal rings because the entire piston would be made out of an extremely low-friction material.
Disadvantages of using PTFE include the very high cost of the raw material and fact that using PTFE as
a piston material is unproven.
4.4.2 Decision
The team decided to use aluminum pistons. The decision came in light of aluminum’s low weight and
high heat transfer coefficient. The team also wanted to work with a material that is commonly used in
this application and steel and PTFE did not meet that requirement.
4.5 Sealing Technology
4.5.1 Alternatives
Oil Seals
Using oil seals is the conventional method for sealing air and natural gas compressors. Oil provides
excellent wear protection and blow-back elimination. Disadvantages to using oil seals include the
added complexity involved with preventing leaks and having to check oil levels on a regular basis. Also,
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natural gas compressor with oil seals have to have additional filters to ensure that oil does not get into
the natural gas going into the vehicle.
PTFE Coatings and Rings
Oil-less compressor technology uses low-friction cylinder wall coatings, such as PTFE (Teflon), and seal
rings made out of low friction polymers, such as PTFE or PEEK, to protect against wear and excessive
heat generation while still maintaining at tight seal. A disadvantage of this alternative is the high cost
of low-friction coatings and polymers.
Bronze Sleeves and Seals
In discussions with mentor Lee Otto, using a bronze sleeve inside the cylinder wall along with bronze
bushings as seals was discussed. This alternative would be simple and inexpensive. A disadvantage of
this alternative is the relatively high coefficient of friction of bronze compared to oil seals and PTFE.
4.5.2 Decision
The team decided to use PTFE cylinder wall coatings and PTFE rings. This decision was based on the
team’s requirement for a very low friction seal, as well as the need to avoid contaminating the natural
gas with oil. Many natural gas compressors are now using this technology for the above reasons.
4.6 Piston Ring Layout
4.6.1 Alternatives
Ring Layout 1: Two Outside Rider Rings and Two Inside Seal Rings
This was the ring layout that the team came across most often in their research. It stabilizes the piston
with the rider rings on the outside. Gas can pass through grooves in the rider rings because the function
of the rider ring is only to stabilize the piston and not to seal out gas. A diagram of Layout 1 can be seen
below in Figure 11.
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Figure 11. Diagram of Layout 128
Ring Layout Two Outside Seal Rings and Two Inside Rider Rings
The team also came across this ring layout in their research, but it was far less common than Layout 1.
Gas is sealed using the two outside seal rings and the piston is stabilized using the two inside rider rings.
An example of Layout 2 can be seen below in Figure 12.
Figure 12. Picture of Layout 2
4.6.2 Decision
The team has decided to pursue Layout 1: two outside rider rings and two inside seal rings. This design
is far more common in the industry and the team hypothesizes that the gas being allowed to pass
through the rider rings freely will lead to better stabilization for the piston. If the sealing from this
layout proves insufficient the team will modify it by adding additional seal rings.
28
http://www.ihi.co.jp/compressor/en/products/process-gas/gas-recipro/images/fea_photo_01.jpg
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4.7 Tubing
4.7.1 Alternatives
Thick-walled steel pipe
Thick-walled steel pipe was proposed as an alternative for the piping system. Steel pipe is readily
available and offers good sealing using NPT threads with Teflon tape. Disadvantages to this alternative
include not being able to bend it and possible corrosion of the steel because it is not a naturally
corrosion resistant alloy. A picture Schedule 80 thick-walled steel pipe is shown below in Figure 13.
Figure 13. Schedule 80 Steel Pipe29
Stainless Steel Tubing
Stainless steel tubing was also considered as an alternative for the piping system. Stainless steel tubing
is the most common method of piping compressed natural gas. An example of stainless steel tubing is
shown below in Figure 14. To change direction, the stainless steel tubing can be bent or compression
fitting can be used. Examples of these compression fittings is shown below in Figure 15. Disadvantages
include the fact that it must be ordered from a specialty supplier and adapters and fittings are more
expensive than for standard steel pipe. A chart showing stainless steel tubing sizes and wall thicknesses
and their corresponding pressure rating can be seen below in Table 3.
Figure 14. Stainless Steel Tubing30
29
30
http://www.amazon.com/Anvil-Fitting-Schedule-Seamless-Nipple/dp/B006N2VXCI
http://www.swagelok.com
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Figure 15. Stainless Steel Tubing Fittings31
Table 3. Steel Tubing Sizing Chart32
4.7.2 Decision
The team decided to use stainless steel tubing for the piping system of their compressor unit. The team
is doing this to conform to industry standards for compressed natural gas piping and for the design
flexibility that will come with being able to bend the tubing. Being able to bend the tubing is also very
critical to the team’s heat sink design.
4.8 Heat Removal
4.8.1 Alternatives
Stepped Heat Removal
This approach to heat removal would remove heat in between compressions stages. The benefits of
this form of heat removal is that the highest temperature reached is lower, resulting in overall less
power used since natural gas takes less work to compress at lower temperatures. The down side to this
method is that takes up slightly more space since additional tubing and fans are required.
31
32
http://www.swagelok.com
http://www.swagelok.com/downloads/webcatalogs/EN/MS-01-107.PDF
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Bulk Heat Removal
The bulk method of heat removal would let the natural gas compress to 3600 psi before cooling. The
benefit of this method is that all the heat transfer is done at once which saves on space used. The
downsides are that waiting until the end of the cycle to cool allows the temperature of the natural gas
to reach temperatures around 350 °C which is a safety and material cost concern. Secondly this method
is less efficient since natural gas required more work to compress natural gas at higher temperatures.
Finned Tubing
Finned tubing is a very common way to dissipate heat from a working fluid in many different
thermodynamic systems. The fins provide increased surface area for heat transfer to the air to occur
and therefore increase heat transfer rates. A disadvantage to this alternative is the fact that most
finned tubing is designed for low pressure applications and fins cannot simply be added to the team’s
high pressure tubing because separation between the tube and the fins will occur with thermal
expansion and contraction due to temperature changes. Custom finned tubes designed for high
pressure applications, where the fins and the tubes have been bonded together in a heat welding
process, are available. However, these custom, high pressure finned tube are very expensive.
Figure 16. Two Examples of Finned Tubing33
Tubing Loops
Tubing loops are also a very common way to dissipate heat from a working fluid. Adding loops to the
tubing allows for increased surface area for heat transfer to take place. There will be a higher volume
of gas in the heat sink using this method due to the increased tube length. This will allow the gas to
have more time to cool while it passes between the stages. Disadvantages to this alternative include
the additional cost of buying extra tubing and the additional labor cost involved in bending the tubing
to create the heat sinks.
Fans
Fans are a very effective way to dissipate heat from a working fluid into the surrounding air.
Disadvantages to using fans include increased noise pollution and the added potential maintenance
that come along with additional moving parts.
33
http://www.lpspa.it/UserFiles/1/Image/aleteados/hiresolution/CRW_6068hr.jpg
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Figure 17. Example of Tubing Loops34
4.8.2 Decision
Since the increase of size is relatively small and size is not a major concern for the design, and due to
the safety and cost concerns with bulk heat removal, the team has decided to use a stepped heat
removal method to cool the natural gas.
4.8.3 Method
The method to be used for the stepped heat removal is forced convection by fans and coiled tubing.
This method of heat removal is proven and is used widely in industry. Adding fins to increase surface
area to improve heat transfer was considered but for a single tube application adding a finned surface
would not be cost effective.
4.9 Pressure Sensors
4.9.1 Alternatives
Pressure Transmitter
Pressure transmitters measure pressure using a strain gauge and output a low-voltage signal that
corresponds to a certain pressure. An example of a pressure transmitter can be seen below in Figure
18. This low-voltage signal must be converted to a digital signal using a DC to AC converter chip to be
of any use to a computer-based control system. These sensors would be used in conjunction with
thermocouples to determine the thermodynamic state of the compressed natural gas. This is critical
to creating accurate heat transfer and fluid flow models. The team needs a minimum of one pressure
transmitter, to be used on the final stage to determine when the final stage pressure has reached 3600
psi, meaning the natural gas vehicle tank is full. The disadvantage of using pressure transmitters their
relatively high cost compared to simple pressure gages.
34
http://www.lmcompressor.com/images/Web%20Page%20CNG_3021%20_.jpg
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Figure 18. Pressure Transmitter Example
Pressure Gages
Pressure gages are a very inexpensive way to measure the pressure throughout the various stages of
the team’s natural gas compressor. An example of a pressure gage can be seen below in Figure 19.
They are also very robust and reliable compared to pressure transmitters. The major disadvantage of
using pressure gages is the fact that data can only be recorded at the speed it can be recorded manually
by a human being. Also, using pressure gages would limit the number of real-time computer modeling
that could be done for the system because pressure data would not be constantly being feed into the
computer.
Figure 19. Pressure Gage Example
4.9.2 Decision
The team has decided to use a pressure transmitter for the final compression stage and used pressure
gages for the other stages. This one pressure transmitter is necessary to tell the computer when the
natural gas vehicle tank is full. The team will do much of their testing and thermodynamic computer
modeling using a one cylinder model with a pressure transmitter, however using pressure transmitters
at every stage on the prototype was deemed to be too costly.
4.10 Development Board
4.10.1 Alternatives
A table showing a variety of potential development boards is shown below in Table 4.
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Table 4. Comparison of Development Boards35
Development
Board
Price
Raspberry Pi
BeagleBone Black
CubieBoard
$25-$30
$45-$55
$50-$60
700 MHz Low Power
ARM1176JZ-F
AM335x
1GHz ARM
Cortex-A8
Arm Cortex A8
1GHz
8 GPIO pin
I²C
SPI
UART
5V, 1A
Two 46 pin
configurable bus
Two 48 pin
configurable bus
5V, 700mA
5V, 2A
Memory
512MB SDRAM
(Model B)
2GB embedded
MMC
Dimensions
8.6x5.4x1.7(cm)
10×6 (cm)
NAND
(max 64GB) SATA II,
SD Card 3.0
90x40x13(mm)
CPU
GPIO
Power
Consumption
Application



Ease of Use
Learning
Embedded
apps
Robotics
Easy



Basic
Learning
Embedded
apps
Robotics
Easy (but closed




Advanced
Learning
Embedded
apps
Video
Robotics
I/O and peripherals
are not so easy to
manage
The Raspberry Pi not only bridges the overall mechanical and electrical processes, it also provides
extremely necessary safety precautions as well as the ability to offer a robust user interface. The only
downside to the limited input/outputs onboard the Raspberry Pi. Likewise, the onboard I/O is limited
in connection pins, as well zero tolerance to anything above 0 or 3.3V. Consequently, the onboard
GPIO pins are connected directly to the connection pins of the processor, in turn causing potential for
the entire board to fry. With no surge protector or general protection, developers began deriving
additional designs to be added. The Slice of PI/O is just one option in the efforts to protecting the
RasPi’s unprotected general-purpose input output pins. The Slice of PI/O ad-on is a small addition to
the Raspberry Pi providing a buffer, conversion levels, analog I/O and protection devices, in order to
avoid the risk of damaging it. There are many breakout boards that have been developed and can be
easily plugged into the Raspberry Pi GPIO pins.
35
http://www.open-electronics.org/a-comprehensive-comparison-of-linux-development-boards/
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4.10.2 Decision
The team has decided to use the Raspberry Pi development board because of its compact size, ease of
use, and its vast online community presence available to us for referencing as the team begins to
prototype. The BeagleBone Black and the CubieBoard both provide a few aspects that the Raspberry Pi
does not. However these development boards are more expensive, more expensive development
boards could be helpful, the team believes the Raspberry Pi is a good development board to start off
with without having to put forth a considerable amount of funds.
4.11 User Interface
4.11.1 Alternatives
Touchscreen
The touch screen user interface opens the realm of possibilities, but it can still be simplified for ease of
use purposes. A detailed representation of the touchscreen can be seen in Figure 20. Having a full
interactive screen allow the Raspberry Pi to provide the user with a more detailed response or
diagnostics error if necessary. The touchscreen user interface is interchangeable depending on the
user’s desire for a complex and or simple interface interaction.
Figure 20. Touchscreen Raspberry Pi display36
LCD & Keypad
The 16x2 LCD display and keypad is simple and easy to use all in one, which is depicted in Figure 21.
This option cuts out any possible confusion between user and unit interaction. Given LCD’s small
compact characteristics, it will be limited to what information it can output to the display. Though the
display is limited, the team has also looked into providing real time system operations analysis that the
Raspberry Pi could upload to via Wi-Fi.
36
http://learn.adafruit.com/adafruit-pitft-28-inch-resistive-touchscreen-display-raspberry-pi
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Figure 21. LCD & Keypad display37
Remote Access
Incorporating the world of smartphones to the refueling unit would be quite exciting, but as a team this
option is not necessarily what the team is looking for as an interface for other non-team member users.
Incorporating an interface via a smartphone, illustrated in Figure 22, would most likely be an option
strictly for the team to utilize as it provides a convenient means for us to test and analyze the
functionality of the Raspberry Pi.
Figure 22. Mobile User Interface38
By integrating these specialized safety precautions, the refueling unit will not only be self-governing
during operations, but it will autonomously provide the ability for users to remotely survey the current
37
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http://blog.davidsingleton.org/introducing-piui/
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environmental readings of their unit. As the team plans to allocate the necessary time to provide not
only a reliable product, but also a reliable safety system.
4.11.2 Decision
The team has decided to go with the 16x2 LCD display & keypad user interface option given its overall
ease of use and simple connectivity to the Raspberry Pi. This decision was also made in light of concerns
about environmental conditions and how those might negatively affect reliable touchscreen operation.
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5 Testing & Calculations
5.1 Preliminary Design Methods
In order to determine the size requirements for the natural gas compression cylinders along with the
tubing connecting the cylinders, the team has decided to use a combination of Engineering Equation
Solver (EES) and Autodesk CFD simulator. EES will be used to perform the calculations required, it was
chosen for its ease of use, integration of thermodynamic properties, and potential for optimization.
Autodesk CFD simulator was chosen for its ability to simulate fluid flow, compression, and heat transfer
simultaneously.
5.1.1 Autodesk CFD Simulator
Overview
Due to the complexity of this project and the difficulty and expense in implementing design changes
after production begins, the team will use Autodesk CFD to test initial designs. A variety of design
changes such as length of the cylinders, diameters of the cylinders, and wall thickness of the cylinders
will be altered and simulated in order to optimize our design.
Goals
The end goal of using Autodesk CFD is to have a full working model of the system that simulates and
shows the changes in pressure and temperature as the natural gas goes through it.
Status
Currently a simple model of compression has been successfully simulated. The purpose of creating this
model was to gain understanding of how to simulate compression in CFD simulator.
Results
Figure 23 shows the trend of the data obtained from the simulation modified to account for calibration
error.
Figure 23. Pressure Results from Simulation
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Figure 24 shows a plane from the simulation pressure results showing the higher pressure after
compression.
Figure 24. Plane of Simulation Pressure Results
Next Steps
The next steps for CFD are to first of all simulate the cooling of the natural gas between compression
chambers. The second step is then to put the two parts together to create a full working model of the
system.
Obstacles
Some obstacles that still need to be resolved with Autodesk CFD Simulator are a few calibration issues,
this will be resolved once a new model with information from EES is implemented.
5.1.2 Engineering Equation Solver
Overview
In conjunction with simulating the compressor, calculations are being done in EES in order to ensure
that the results obtained from the simulation are accurate as well as create the frame work for the
initial design.
Status
A basic model has been created in EES in order to calculate a preliminary piston size, work required
from the hydraulic compressors, and heat removal needed in order to compress most effectively.
Results
Table 5 shows the results for work required from the pistons and preliminary heat removal
requirements assuming that the temperature will be brought down to about room temperature in
between compression stages. Full results and calculations can be found in Appendix A.
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Table 5. Work and Heat Removal Calculation Results
Compression Stage
1
2
3
4
Work Required [W]
135.6
127.7
135.6
140.8
Heat Removal Required [W]
0
153
110.3
123.5
Next Steps
The next step to research methods of heat removal in order to obtain a coefficient of heat transfer so
that the amount of tubing needed to reach the temperature required along with pressure drop
associated with the tubing can be found. Also, once a model of compression is simulated in CFD, the
results can be used in an iterative manner to obtain the optimal compression ratios, compression areas,
and stroke lengths of each of the four compression cylinders.
5.2 Initial Testing
In order to increase understanding of seal technology and test basic forms of compression the team
plans to borrow a hydraulic pump from the physical plant. The pump would be used along with a
hydraulic cylinder to compress and monitor a gas with similar properties but without the safety concern
associated with natural gas. From this testing the team hopes to gain information about what type of
seal should be used, experience with manufacturing and assembling a compression cylinder, and
experience working with a hydraulic pump from this testing.
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6 Preliminary Design Decisions
6.1 Hydraulic System
6.1.1 Overview
A hydraulic system will be used to convert electrical energy into mechanical work that will drive the
pistons a therefore compress the gas. An overview schematic of the proposed hydraulic system is
shown below in Figure 25.
Figure 25. Hydraulic System Schematic
6.1.2 Reservoir
After discussing their needs with a hydraulic power unit technician, the team determined that they
would be best suited with a two gallon hydraulic reservoir. This reservoir is a common size and will
have all the storage capacity required to meet the team’s needs. This reservoir will be part of a custom
built hydraulic power unit.
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6.1.3 Pump
After discussion with the technician mentioned above, the team also determined the type of hydraulic
pump that will be best serve the team’s needs. A submersible hydraulic pump will be used. This pump
is very standard in industry and provides better cooling and noise reduction compared to an external
pump. This pump will be part of a custom built hydraulic power unit. Sizing the pump will take place
in the spring after initial tests using a hydraulic pump lent to the team by Calvin Physical Plant.
6.1.4 Solenoid Valve
After researching different valve options, the team determined that they will be using one solenoidoperated hydraulic control valve. The chosen valve is a 4-way, 3-position, exhaust center, double
solenoid valve. A schematic of this type of solenoid operated valve is shown below it Figure 26. When
either of the two solenoids are charged, the valve will be pulled to the left or right to let fluid into one
side of the cylinders, or vice-versa when the other solenoid is charged. When neither solenoid is
charged the valve will center and exhaust fluid to the reservoir.
Figure 26. 4-Way, 3-Position Solenoid Valve (Exhaust Center)
6.1.5 Cylinders
The team will be using two double-rod hydraulic cylinders to power their four-stage compressor. Exact
sizing for these cylinders will be determined after initial tests using a used, inexpensive, small single
rod, double acting cylinder.
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6.2 Natural Gas System
6.2.1 Overview
The natural gas system will move the gas from a residential natural gas line, through each of the four
compression cylinders, and ultimately into the tank of a natural gas vehicle. It also is designed to
prevent backflow at every possible point and monitor the thermodynamic state of the natural gas
using thermocouples, pressure gages, and a pressure transmitter. A schematic of the natural gas
system is shown below in Figure 27.
Figure 27. Natural Gas System Schematic
6.2.2 Residential Natural Gas Supply
The team’s unit will be optimized to operate on a residential natural gas system with a pressure of 2
psi. This is the industry standard for larger homes, while 0.3 psi is standard for average homes. Natural
gas utility companies will usually install a 2 psi meter/regulator on a house if the customer has a special
reason to have one. Since the team’s system is an “intensifier” type of compressor, the initial pressure
into the first stage of the compressor is very important. To protect older appliances not design to used
2 psi natural gas, the team will recommend that customers who have to upgrade to a 2 psi
meter/regulator add an additional step down regulator to their residential natural gas system so that
their other appliances can continue to operate on 0.3 psi.
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6.2.3 Tubing
The team will be using stainless steel tubing to pipe the natural gas between compressor stages. The
team has selected ¼ in. OD tubing, with a wall thickness of 0.049 in. According to Table 3, this will give
the team the capability to handle 3600 psi with a safety factor of 2.
6.2.4 Check Valves
As displayed in in Figure 28, the team’s design uses check valves at every point where backflow could
possibly occur.
Figure 28. High Pressure Natural Gas Check Valves39
6.2.5 Relief Valve
The system utilizes an adjustable pressure relief valve after the final stage. Gas is always free to flow
in the forward direction, therefore if over-pressurization occurred anywhere in the system it will be
relieved through this relief valve. An examples of adjustable relief values made for high pressure
natural gas applications are shown below in Figure 29.
Figure 29. Adjustable High Pressure Natural Gas Relief Valves40
6.2.6 Shut-off Valve
A manual shut-off valve will be utilized between the residential natural gas system and the first stage
of the compressor. The will allow the compressor to be shut of manually during servicing. An example
of a ball valve made for compressed natural gas applications is shown below in Figure 30.
39
40
http://www.hylokusa.com/products/valves/check-relief-valves.aspx
http://www.hylokusa.com/products/valves/check-relief-valves.aspx
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Figure 30. Example of CNG Ball Valve41
6.2.7 Manifolds
Manifolds will be used to distribute the compressed natural gas at the end of each compression
chamber. The piping system will be attached to the manifolds using NPT treaded adapters and Teflon
tape to seal the connection. These manifolds will be constructed out of steel. An example of a
manifold used to distribute gas at the end of a hydraulic natural gas compression system is shown
below in Figure 31.
Figure 31. Example of Manifold and Connections at End of CNG Compression Chamber42
6.2.8 Pressure Gauges and Transducers
The team knew that they would have to monitor pressure and temperature inside each stage to test
and optimize their design. Temperature and pressure will be known so that the thermodynamic state
can be determined. This will allow the team to perform accurate heat transfer and fluid flow analyses
on the system.
Pressure transducers are very costly, however the team needed at least one to monitor the pressure
after the final stage. An example of a pressure transmitter is shown below in Figure 32. This is
necessary in order to tell the control system when the vehicle tank is full (pressure has reached 3600
psi). Regular pressure gages will be used to monitor the pressure of all other stages, these an example
of one of these gages is shown below in Figure 33.
41
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http://www.gonaturalcng.com/wp-content/uploads/2012/11/cng_compressors.png
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Figure 32. Pressure Transmitter Example
Figure 33. Pressure Gage Example
6.2.9 Thermocouples
As stated above, the team needs to monitor the pressure and temperature of the gas at each stage for
accurate heat transfer and fluid flow modeling. Monitoring the temperature at every stage is more
important than monitoring the pressure at each stage because it is impossible for over-pressurization
to occur during the first three stages because gas is always free to flow in the forward direction. Overheating, however, can theoretically occur at any stage in the system. Therefore, thermocouples will
monitor the temperature inside the natural gas compression chamber at each stage. The team has
researched possible thermocouple models that are designed to operate in a high pressure environment.
An example of a thermocouple made to operate in a high pressure environment is shown below in
Figure 34. The team will test each possible thermocouple model at high pressures to ensure that they
will operate reliably.
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Figure 34. High Pressure Pipe-Plug Thermocouple Example43
6.2.10 Heat Sinks
The compressor design will utilize tubing loop heat sinks and a fan to being the compressed gas back to
room temperature following each compression stage. An example of utilizing tubing links as a heat sink
in conjunction with a fan is shown below in Figure 35. This will improve the overall isentropic efficiency
of the system and result in safer operation that is less prone to overheating.
Figure 35. Example of Tubing Loops44
6.2.11 Refueling Nozzle
Fuel will be dispensed using a purchased compressed natural gas fueling nozzle. There is an accepted
industry standard for these nozzles. Any nozzle purchased will fuel any compressed natural gas vehicle.
More complex and expensive nozzles are available, but because they are design for fast-fill units.
Because the team’s compressor is a time-fill unit, the simplest nozzles will be sufficient. An example of
a general purpose compressed natural gas refueling nozzle is shown below in Figure 36.
43
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http://www.novatech-usa.com/Products/Specialty-Thermocouple-Probes/WD-08516-74
http://www.lmcompressor.com/images/Web%20Page%20CNG_3021%20_.jpg
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Figure 36. General Purpose CNG Refueling Nozzle for Time-Fill45
6.3 Sealing System
6.3.1 PTFE Coated Cylinder Walls
PTFE (Teflon) coated cylinder walls will provide an excellent low-friction surface for the piston rings to
move along. This coating will protect against provide an excellent low-friction seal and protection
against excessive wear on both the cylinder wall surface and the rings. This coating is most common in
oil-less compressor designs.
6.3.2 PTFE Seal Rings
Two PTFE (Teflon) seal will be used to seal the gas inside the compression chamber. These rings provide
an excellent low fiction seal. These rings will be located to the inside of the two rider rings, as displayed
below in Figure 37.
6.3.3 PTFE Rider Rings
Two PTFE (Teflon) rings will be used to stabilize the piston as it moves up and down the cylinder walls.
These rings will be located to the outside of the two seal rings, as displayed below in Figure 37.
45
http://www.opwglobal.com/Product.aspx?pid=138
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Figure 37. Picture of Ring Layout46
6.4 Control System
A Raspberry Pi will control the brains of our refueling unit. It will provide a unique crossover between
the mechanical and electrical operations to the CNG unit. The Raspberry Pi, depicted in Figure 39, is a
small, compact, yet powerful micro-processing development board. The basic idea of the Raspberry Pi
is its capability of becoming an embedded Linux computer, but at the fraction of the price. There are
two models of Raspberry Pi boards, but the team has chosen the model B board due to its overall
specifications upgrade from the first generation (model A) board.
A schematic of the proposed control system is shown below in Figure 38.
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Figure 38. Control System Schematic
Figure 39. Raspberry Pi Size Comparison47
47
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7 Financial Estimates
7.1 Cost of Development
The estimated budget for the development of one, full-functional home refueling unit is listed in Table
6. Cost of Development. Initial research into the component costs for the preliminary Bill of Materials
resulted in a total operational budget of $2,950. All values listed on Table 6 are all subject to change
based on the purchase price for each component.
Table 6. Operational Budget
Vendor
Bond Fluidaire
Best Metal Products
DECC Company
Parker
Alro Steel
Aavolyn
Mouser
Parker
Kendall Electric
Parker
McMaster Carr
N/A
N/A
Total
Part
Cost
Hydraulic Power Unit
Hydraulic cylinders (2)
Teflon coating of compression cylinders
Hydraulic solenoid valve
Steel pipe for compression cylinders
Rings
Electronic components
Check valves
Electronics enclosure
CNG filling hose nozzle
Compression Tube Adaptors
Materials for enclosure contraction
Miscellaneous
$
$
$
$
$
$
$
$
$
$
$
$
$
$
600.00
350.00
300.00
300.00
200.00
200.00
200.00
200.00
100.00
100.00
100.00
100.00
200.00
2,950.00
7.2 Cost of Production
The estimated budget for the production of 1,000, full-functional home refueling unit is listed in Table
7. Cost of Production. Using component costs listed in the operational budget (see Table 6. Cost of
Development), a preliminary production budget was calculated by assuming industry standards in
economics of scale. The team analyzed whether to sell NaturaFill preassembled or as a kit. Selling the
product unassembled and as a kit would reduce production costs, but have less desirable implications
to the end consumer. On the flip side, selling the unit assembled would increase the cost of production
and have greater ease of accessibility for the end user. The team concluded to sell the product
preassembled in order to align with the design norm of “trust.” This resulted in a total production
budget of $1,995, still in range of the $2,000 desired budget. All values listed on Table 7 are all subject
to change based on the purchase price for each component.
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Table 7. Cost of Production
One Unit
Cost
Vendor
Part
Bond Fluidaire
Best Metal Products
DECC Company
Parker
Alro Steel
Aavolyn
Mouser
Parker
Kendall Electric
Parker
McMaster Carr
N/A
N/A
N/A
N/A
Total
Hydraulic Power Unit
Hydraulic cylinders (2)
Teflon coating of compression cylinders
Hydraulic solenoid valve
Steel pipe for compression cylinders
Rings
Electronic components
Check valves
Electronics enclosure
CNG filling hose nozzle
Compression Tube Adaptors
Materials for enclosure contraction
Miscellaneous
Direct Labor
Manufacturing Overhead
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$
$
$
$
$
$
$
$
$
$
$
$
$
$
$
600.00
350.00
300.00
300.00
200.00
200.00
200.00
200.00
100.00
100.00
100.00
100.00
200.00
100.00
100.00
Economies of
Scale Savings
55%
50%
50%
25%
30%
30%
30%
25%
30%
40%
25%
40%
30%
0%
0%
Production
Cost
$
$
$
$
$
$
$
$
$
$
$
$
$
$
$
270.00
175.00
150.00
225.00
140.00
140.00
140.00
150.00
70.00
60.00
75.00
60.00
140.00
100.00
100.00
$
1,995.00
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8 Conclusion
The preliminary calculation and design research confirms that the NaturaFill project is feasible. The
preliminary design includes a hydraulic system to move the compression pistons, a natural gas system
to pipe the gas between stages and prevent backflow, and a control system to operate the hydraulic
system and monitor the safety system components. Moving forward, major obstacles include
adequate heat dissipation, ensuring safe prototype operation, and properly sealing the hydraulic
cylinders to allow for compression up to 3600psi. The team’s upcoming plans for next semester begin
with the purchase of materials and components to begin assembling a prototype of the NaturaFill CNG
home refueling unit.
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9 Acknowledgements
9.1 Professor Ned Nielsen
Professor Ned Nielsen is the faculty advisor for NaturaFill. The team is grateful to Professor Nielsen for
his insights and constructive criticism throughout the preliminary design process.
9.2 Professor Steve VanderLeest
Professor Steve VanderLeest is an adjunct advisor for NaturaFill. The team is grateful to Professor
Nielsen for his presentation feedback and advice on the project’s electronic control systems.
9.3 Professor Matthew Heun
Professor Matthew Heun is a Mechanical Engineering professor with a wide range of knowledge in
thermodynamics. The team is grateful to Professor Heun for providing feedback on the AutoCAD CFD
simulation and supporting the team’s efforts to model heat generation by the compression system.
9.4 Mr. Jimmy Moerdyk
Mr. Jimmy Moerdyk is the VP of Operations of Moerdyk Energy, Inc. The team is grateful to Mr.
Moerdyk for his mentorship and industry insights.
9.5 Mr. Lee Otto
Mr. Lee Otto is the CEO and founder of CNGPump, Inc. The team is grateful to Mr. Lee for his advice to
the team regarding hydraulic cylinder seal technology and safety controls.
9.6 Mr. Ross Persifull
Mr. Ross Persifull Otto is a Research Specialist at the Ford Motor Company. The team is grateful to Mr.
Lee for his industry insight on the future of CNG and willingness to demonstrate the use of his CNG
home refueling unit.
9.7 Mr. Phil Jasperse
Mr. Phil Jaspers is the Calvin College metal and wood shop supervisor. The team is grateful to Mr.
Jaspers for his training in metal cutting and fabrication. The team is also grateful to Mr. Jaspers for his
advice on the fabrication of components.
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10 Appendices
10.1 Appendix A. EES Code
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