This report is the phase second phase of the CNG pressure vessel where the analysis for designing all composite CNG pressure vessels for trucks will be continued. This report is a feasibility study that will analyze the use of the all composite compressed natural gas pressure vessels in a truck, in which it will be concluding if the project is doable or not from various aspects and standpoints. These aspects are based on the cost of the project, the effect on the performance of the truck, the ability to manufacture the all composite CNG-pressure vessel and the benefits of this type of pressure vessel. Moreover, the manufacturing processes of composite materials will be studied and the applicable processes for all composite CNG pressure vessels will be discussed as well.
Introduction
Composite materials are used in many applications recently because of recent scientific discoveries due to its unique specifications that help in playing a big role in improving application performance. CNG-PV used in vehicles is one of the recent applications which need to be improved in order to be applicable in CNG powered vehicles. For this purpose, this design project is of the utmost importance as it challenges this problem by first dividing it into five phases. This report is a continuation for the previous report (phase1) where we discussed different types of pressure vessels, design methodology, effects of CNG on vehicles, available standards for CNG pressure vessels and the failure modes of CNG pressure vessels. The feasibility in this report will be analyzed through different aspects, which are, the possible manufacturing processes for manufacturing all composite CNG-PV, comparison between steel and composite pressure vessels, cost analysis and the effects of the all composite CNG-PV on the truck performance. After doing all these analysis, it will be easier to judge if the project is feasible or not.
Discussion
Manufacturing methods for composite materials
There are several methods to manufacture composite materials in the industry:
The wet lay up or (hand layup) method:
This method is commonly used for small scale production. The mold used in this method is being coated with special gel to have better surface finish in the final object. The next step is laying down the fabric with resin layers one by one using brush or roller until having the preferred thickness needed for the application. Then the laminates will be left to dry out in the room temperature condition.This unhealthy method needs high skilled labor and much time in order to be accomplished as it's considered the most time consuming process. On the other hand, it is universally used as it could manufacture complex shapes while having it also uncomplicated and low priced. Moreover any resin and any fiber can be used in this method and it has many applications such as wind turbine blades and boats [1] [2] [6]
Figure 1: hand layout method (2)
Spray layup method:
This method is more like the hand layup method but here the fiber will be chopped into small parts and fed to a spray gun in which it will mix the fiber with the resin and spray the mixture to the mold. Then the mold will be kept to dry under room temperature conditions. This method is applicable with less hand labor than the hand layup method, its tools are cheaper, and complex shapes can also be achieved using this method. On the other hand this method strikes some health hazards, also it has very heavy laminates because of the rich low viscosity resin, and it is limited into short fibers. The resin used for this method is commonly polyester and the fibers used are glass roving. This method is applicable for uncomplicated enclosures and light panels. [1][2]
Figure 2: spray layout method (2)
Vacuum bagging method:
This method can be considered as an advanced hand layup method because the reinforcement and the resin will be laid manually but pressure will be applied, in order to have better finishing results by vacuuming process. The vacuuming process will be done by covering the mold with a plastic cover and emptying the air inside using an air pump. The resin is used commonly in this method are epoxy and phenolic, and the fibers used are heavy fibers. This method's advantages are: having more fiber in the laminate, it's healthy and safe, the low unneeded content in the laminate and the fiber will wet out easily because of the applied pressure. On the other hand this method needs highly skilled labor and it has extra cost for bagging and air exhausting. [1][6]
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Figure 3: vacuum bagging (1)
Filament Winding method:
Continuous fibers are used in many applications which makes filament winding one of the most important methods in such applications. This method is generally used to create composite circular shape components by pulling the fiber and passing them through the resin bath then wrapping fibers over a rotating mandrel which is rotating by a motor, and fiber t can be rotated in different orientations to satisfy the need of the application. Then the mold will be inserted into an oven to make the composite solidify and create the final shape demanded. Any resin or fiber can be used in this method. This method is not costly, fast and it has good structural properties. On the other hand, it has some disadvantages such as the limitation to rounded shapes, laying the fiber may cause some problems because of inaccuracy and its resin has low viscosity. [1][2][6]
Figure 4: Filament Winding method (2)
Pultrusion method:
In this method the fiber will be a form of roving, tow, mat or fabric which will be pulled using a puller, and then it will pass through a resin bath to be wetted out. After, it will pass through heated die in order to cure the resin. This process is one of the processes which can be used with many resins and fibers types with the most common resins used are polyester, epoxy, phenolic and vinyl ester. The final shape will be cut into certain length using a cut off saw. This method is economic, fast, and easy to control. Moreover, it has good structural specifications of laminates. On the other hand, it is limited to constant cross sectional shapes and the die may cost more. This method is applicable for beams and girders. [1][2] [6]
Figure 5: Pultrusion method (2)
Resin Transfer Molding (RTM):
In this method the fabrics will be laid up in to the mold, and then another mold will be inserted on the top of the first one. Then the resin will be injected in the space between the two molds to wet the fabric. Later on the laminate will be left to dry out. This process is applicable for aircraft and automotive components. [1]
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Figure 6: RTM method (1)
VARTM
In this method the fabric will be laid on the mold then it will be covered with non structural fabric. Then vacuum bag will be inserted on the mold and the resin will be injected to wet the fabric inside.
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Figure 7: VARTAM method (1)
Compression molding method:
This method compresses a mixture of fiber and resin using male molds and a female mold in which the mixture will be inserted between them and then compressed using a hydraulic device. [1][2]
Figure 8: compression molding method (2)
Possible manufacturing processes to be used in all-composite CNG-PVs
After researching among different methods to manufacture composite shapes for different applications, I discovered that the most appropriate methods to create circular cross sectional shapes are the Pultrusion and Filament processes. The Pultrusion method however can only create circular shapes with small diameter, thus it is not applicable for pressure vessels which sometimes might have huge diameter specially the ones used in CNG vehicles. Nonetheless, the filament method could create huge shapes thus it's the most appropriate method for all composite CNG-PVs. Moreover, it's fast, low in cost and it produces good structural properties for objects. Additionally, this process can be automated which will reduce the labor cost in manufacturing. While researching through internet, it was obvious that most professional companies such as Lincoln Company are producing all composite CNG-PVs using this process as well, so this is the best process used for this purpose.[9]
Comparison of all-steel and all-composite CNG-PVs
Figure 9: composite PVs vs steel PVs (7)
All composite CNG-PVs: [7]
It has long life (it can resist static and cyclic fatigue)
Low weight (50% reduction when compared to steel CNG-PVs)
It operates smoothly at high pressure
It can resist impact
Flexible in design
It can resist corrosion
High thermal isolation
It has good performance in high temperature
expensive
All steel CNG-PVs: [7]
Needs to be maintained from time to time
Heavy weight
Inflexible in design
Sensitive toward corrosion
Can't perform properly under high pressure or temperature
Low price
Thus, the all composite is better than all metal CNG-PVs
Cost analysis
Table 1: comparison between the 4 types of CNG-PVs (3)
Cost estimation is one of the most essential elements for this design, because it will make the company more interested in the proposed design. The process which is going to be used in this design will be the filament method so the cost will be divided as following as Sanjay K. Mazumdar mentioned: [8] (in US dollar)
Part cost is from 4$ to 30$/lb so average = 17$
Material cost from 2$ to 10$/lb so average = 6$
Labor rate is 15$ to 25$/hr so average = 20$
Overhead is 2 to 4 times the labor cost so average = 3 times
Equipment cost for filament winding will be 250,000 to 500,000$ so average= 375000$
Mandrel cost is 50$ to 500$/ft^2 so average cost= 550$/ft^2
Calculations: (1 pound = 0.45359237 KG) , (1 mm = 0.0032808399 ft)
The weight of the steel PV= 61 kg, the length = 815 mm = 2.67 ft, specific weight = 0.3
The net weight for one of the all composite PV= 0.3*61kg= 18.3 kg (from figure 10)
The net weight in lb = 40.344
The cost to make one PV: (17*40.344) + (6*40.344) + (20*1) + (3*20)= 1007.912 $
The Min cost = 161.4 + 80.688 + 15 + 30= 287.088 $
So the minimum cost for the design in our project according to Sanjay will be 287.088 US dollars per one pressure vessel.
Effect of using type IV CNG-PVs on the truck performance
Using type IV CNG-PVs in the truck will increase the truck's performance positively because of the following standards:
Low weight: will help in increasing the capacity that the truck can carry, reducing the fuel consumption slightly, reducing the footprint produced by the truck engine, and increasing the distance of refueling.[3]
Corrosion resistance: will help in improving the safety and life of the truck that the cylinder wont fail due to corrosion, increasing the life, and reducing the maintenance of the truck.[3]
Flexibility in design: will help in improving the integrated fuel system, and make it more efficient and less consuming from the design aspect.[3]
It operates smoothly at high pressure: so the all composite CNG-PV can contain more gas than the steel PV [7].
Figure 10: CNG truck (3)
Conclusions
After doing the feasibility study for the all composite CNG-PVs in truck, it proved that the all composite CNG-PVs is feasible and worthy to be used in the trucks because of the following:
It is very safe
It is recyclable and environment friendly
It is durable
It increases the truck performance positively
It doesn't need maintaining
Although the all steel CNG-PVs are much cheaper, but they are much less durable and they need to be maintained from time to time which will end up costing more than the cost of the all composite CNG-PVs. The extra cost for using the all composite CNG-PVs in the truck will be recovered in the long term period. The next phase of this project will be the preliminary design of the all composite CNG-PV.
