Pelton Turbine Design scope

7/30/2019 Pelton Turbine Design scope

1/19

Background

Pelton wheels are a high head, low flow, high velocity turbine. Currently, they are

used in renewable energy by integrating them with existing renewable power systems (wind

turbines, large streams, etc.). For this reason, the Civil Engineering Department at University

of Idaho has a Pelton Wheel Laboratory as part of their Hydraulics curriculum.

Unfortunately, the quality of the two existing Pelton Wheels leaves much to be

desired. First, the current operational assembly is a smaller, integrated package. It is in

working order, but has fewer measurable quantities. This reduces the overall learning value

for students as they are unable to directly measure Energy transfer through the system. In

addition, the systems small size makes it inefficient.

The second Pelton wheel assembly is an older, more robust, system. However, it is

currently inoperable and suffers from several design flaws. The tank is old, top heavy, and

deteriorating. For these reasons, our team designed a new tank, frame, and mounting

assembly to be used with the older three phase motor/pump assembly and turbine.

In this new assembly, students will be able to measure quantities necessary to

analyze power transfer and energy flow through the system. Concerns for the students

safety and learning experience made it necessary to take this ambitious approach to

improving the laboratory setup.


2/19

Problem Definition

At a fundamental level, the goal of the Pelton Wheel Development Project is to

improve the existing implementation of a Pelton wheel inside the Civil Engineering

hydraulics laboratory. To this end, our team has several specific tasks in order to achieve

this overarching goal of improving the laboratory and the learning value that students gain

from it.

First and foremost is the design of a new mobile tank frame. It is to be constructed

with angle iron and metal sheeting that support the tank and additional loading components

on the top of the assembly. The parts for this task have been ordered, designed for, and are

ready for fabrication and welding.

Going forward, the first task that must be tackled in the new semester is fabrication

and the welding of the new assembly. In parallel with this, design and further specification

of the electrical load must be completed. The depth and automation level of this solution is

directly tied in to our budget limitations, as well as time constraints.

The rough budget for the Pelton Wheel Development Project is altogether about

$4,000. The University of Idaho Civil Engineering Department has committed $2,000 to our

project, while the Shell Grant has given an estimate of up to $2,000. We have already

bought some mechanical components for the system. The new tank with shipping cost $380

and framing parts so far have cost $260. Additional framing components will cost another

$100. The most expensive part of the design is the torque transducer, which will cost about

$1,700. The generator will be about $300 and the microcontroller and electrical load

components will be around $100.


3/19

Total spending so far has been $640 taken from the CE Department money. The

torque transducer will be purchased with the Shell Grant money, using $1,700 of their

offered budget. The rest of the materials (generator, frame components, microcontroller,

and electrical load) will cost about $500. This will leave about $860 leftover from the CE

Department budget.


4/19

Project Plan

A list of design parameters for both the tank and torque transducer has been

compiled and verified. Mechanical components have been purchased and the new tank will

be fabricated, primarily due to efforts by Ken and Phil, by February. Meanwhile, Ken, Ian,

and Jenny, will be designing loading logic that allows for fast and precise variations in

induced torque. Simulations of the generators performance will be developed by Ian and

Phil prior to implementation to rate the resistive elements in the array, the wires

transferring the power to the load architecture, and the relays that will be used to

implement the control logic. Once this has been accomplished the loading system can be

purchased and tested. Ken, Ian, and Jenny, will monitor the interactions of the control

system and load architecture and compare the results to expectations. The design will then

be revised as needed to maximize system performance. Once the design has been finalized

the system can be mounted on the assembly and used in demonstrations.

Concepts Considered

Mechanical:


5/19

The two primary options for the pelton wheel lab station were to modify one of the

existing solutions or to create a new solution that would suit our needs. There were two

existing lab stations which were available to be modified: an older and larger model, or a

newer, smaller model.

The older model was a long, shallow, two-part tank mounted on a tubular metal

chassis. The pump was mounted under the upper part of the tank, and the turbine was

solidly mounted on top of the upper tank above the pump. Output power and torque were

obtained by measuring the force from a lever arm on a brake clamped and tightened on the

output shaft. Flow rate was measured by a v-notch gauge. Trundle bearings on the pump

allow the mechanical power produced by the pump motor to be measured. A wattmeter is

available for measuring the electrical power into the motor.

While testing was not possible on this lab station due to a missing force gauge, our

client informed us that in prior tests the overall efficiency of this station was low, mainly due

to poor tank and pump design and placement. This lab station would not have been difficult

to modify as there was a lip on the inside of the top of the tank, allowing for easy mounting

of additional components. The long, narrow shape of this lab station would require that a

generator be mounted axially along the tank, with the electrical load further down the

length of the tank.

The newer lab station is shorter, with a larger cross section. The tank, frame, and

pump compartment form an integrated structure, while the turbine is mounted to a plate

which can be repositioned over the trough the water falls in. As with the older model,

torque is measured with a brake, however this one is a band tightened around the pulley

from both ends rather than clamped to the pulley. Flow measurement is very imprecise as


6/19

it is done by timing how long it takes for a smaller reservoir to fill to a specified level. Power

into and out of the pump cannot be directly measured.

The pump motor on this lab station is 580W, according to the nameplate, however

the pelton wheel produces approximately 38W of power at most, for an overall system

efficiency of less than 10%. Modification of this assembly would be more difficult due to the

integrated nature of the tank and frame. The generator and electrical load could be placed

in a number of configurations as there is more free space on the top of the assembly.

In addition to modifying the two existing solutions, we considered building a new

tank/chassis assembly that existing components, as well as future components, could be

integrated into.

The tank for the new assembly had multiple options, each with their own advantages

and disadvantages. A short, wide tank would provide more stability to the final design, as it

would have a wider footprint and a lower center of gravity. However, a taller tank would

provide better net positive suction head to the pump, as well as reduce the chance of a

vortex forming which could allow introduction of air into the pump inlet. A custom tank

design was briefly considered as it could allow for inclusion of the advantages of both,

however it was quickly ruled out due to cost.

The main options for the material for the frame were flat metal strips, metal tubing,

and angle iron. The metal strips were the cheapest option but provided the least support.

The metal tubing was more expensive than the flat metal, however provided more support.

The round sides would present challenges in mounting the tank. The angle iron provided


7/19

the greatest deal of support and ease of mounting the tank; however it was the most

expensive.

There were two main geometric arrangements considered for the chassis. The first

had the pump mounted sideways next to the tank at the bottom of the assembly, with a

metal frame holding the tank and the turbine/generator assembly in place. This option

reduces the height to improve stability while maintaining the pump near the bottom of the

assembly. Alternately, a two-level frame was considered which would allow the pump to be

underneath the tank. This option provides more space for the motor controller, as well as

allowing for pump to be positioned so a longer run of straight piping could be provided to

the pump inlet.

Electrical:

One of the primary concerns of our client is coupling an electrical torque transducer

between the turbine and load. Knowing that this would a necessary component, we had to

constrain our design decisions (as well as budget) to match these needs. Since the electrical

rotary torque transducer will have an output, we knew having some control logic, to handle

inputs like torque and possibly a venture flow-meter, would be desirable.

Moreover, this torque transducer must be coupled to an electrical generator. The

purpose of this generator is to act as the loading device on the turbine. This is in place of the

manual braking arm which was discussed earlier. The generator will be acting as an

electrical load on the turbine; a way to manually adjust load in real-time is a necessary

solution.


8/19

Since our project had a wide operating range of speeds, it was desirable to select a

machine with a large speed bandwidth, as well as a linear torque-versus speed output

characteristic. This is desirable since the lab requires students to vary load, and the

relationship between torque and speed of the pelton turbine is also linear. With these

considerations in mind, the two main machine types considered were a synchronous

induction machine, as well as a direct current permanent magnet machine. The

consideration for each machine is further discussed in the following section.


9/19

Concept Selection

Mechanical:

The main decision made for the project was that a new tank and chassis assembly

was wanted. Both of the existing solutions had significant disadvantages, and they would

have given a final product that both our team and our client would likely be unsatisfied with.

The older lab station had more learning potential. More quantities could be measured, but

the tank was deteriorating and required repairs and extensive cleaning. The new lab station

was in good condition but was very limited in the measureable quantities. Thus it was

decided to design a new assembly despite the additional work that would be required.

The tank selected for the final design was the taller one. Although it was not ideal for

stability, the increased pump efficiency was deemed to be more important. As stated

earlier, the custom tank design was not considered due to the significant increase in price.

A rectangular tank was chosen as it would be much easier to mount than a cylindrical tank.

In addition to the shape, a thicker tank was chosen to meet client specifications.

A two-level frame was chosen for the final design. While this was counterproductive

to our goal of increased stability, it was necessary as our client strongly desired increased

pump efficiency, including a longer run of straight pipe leading into the inlet, necessitating

that the pump be mounted below the tank. Angle iron was chosen for the portion of the

frame holding the tank as it could be arranged in such a way as to easily hold the tank with

minimal amounts of material. It was decided that the lower frame would be made out of

the metal tubing as it is cheaper than the angle iron and did not need to interface with a flat

surface.


10/19

Electrical:

As discussed previously, it was necessary for students to operate in a wide speed

range (from 0 rpm non-moving turbine to no load conditions). For this reason, an optimal

solution would have a linear torque speed characteristic. This is desirable since the lab

requires students to vary load, and the relationship between torque and speed of the pelton

turbine is also linear. For these reasons, a direct current permanent magnet brush machine

was selected. They are very robust machines with all the characteristics desired. A

synchronous induction machine would have been a better simulation of the large pelton

wheels used in industry. However, this solution was not practical for cost and size

considerations (small scale) of our project.

In order to control the load level of our project, as well as the output current, a

microcontroller will be used in conjunction with relays. Do to availability, low cost, and

familiarity with the PIC32MX7 Cerebot, our team plans to do initial load testing and design

with this board using the MPLab Integrated Development Environment.

System Architecture

Mechanical systems:


11/19

Once the tank shape and frame components had been selected, the main

mechanical considerations were providing adequate suction head, improving stability,

ensuring the frame can support the assembly, providing adequate area to mount

components, and preventing vortex formation and air introduction.

No data was available for the required net positive suction head for the pump, so a

direct calculation was not possible. Therefore, the tank height was selected based on the

height of the existing tanks. Our client informed us that the older tank, which was

approximately 1ft tall, was not sufficient for this purpose. The newer lab station,

approximately 30 inches tall, had no evidence of cavitation when in use. Due to the lack of

data on the pump, the minimum tank height was unknown; however a tank height of 30

inches was selected, as although this may have not been the minimum, it was sufficient to

prevent cavitation.

In order to improve the stability, two things were done. First, the base of the

assembly was made wider. The goal was to make it larger than the existing solution while

still being narrow enough to be maneuverable through doors. An upper limit of 2 feet

was established for the entire assembly, and the width of the main frame was decided to be

24 inches. Second, the center of gravity was lowered by making the tank as close to the

ground as possible. The lower bound for this was determined by providing sufficient space

for the piping to run under the tank for improved pump efficiency. The wheels and lower

frame are approximately 14 inches high, which is a significant improvement over the

existing solution, which had the tank more than 2 feet above the ground.

A statics analysis was conducted to determine the stresses on the main frame

beams. While there were smaller sizes that could support the weight, 1.5 inch angle iron


12/19

was selected as smaller sizes would have caused difficulties in providing enough space for

the tank to rest on. The corner posts on the tank will extend 2 inches above the tank to

attach the upper assembly consisting of the turbine, generator, and electrical load. The

lower frame is made of 1.5 inch steel tubing (sized to match the angle iron), and extends 1

foot to the rear of the tank to provide a place to support the motor controller and pump.

The lower frame is 8 inches high to provide room for the pipe to pass under the tank.

The final mechanical concern was preventing air introduction into the pump inlet.

There are two main mechanisms that would cause air introduction are entrained air from

the water plunging into the tank, and formation of an air core vortex in the tank. In order to

counter this, a flow spreader and a vortex breaker were included. The flow spreader is a

slanted plate mounted below the turbine, extending into the water into the tank. Water

falls onto the plate and runs down into the water in the tank rather than plunging directly

into the tank from the turbine. The vortex breaker is a pair of crossed thin metal plates

positioned on top of the pipe inlet which prevents the circular water motion that would lead

to a vortex.

Electrical systems:

At a rudimentary level, adding an electrical generator will have increased value to

students by allowing them to see energy transfer and power flow through the entire system

(electricalmechanical hydraulicmechanical electrical). On top of this, being able

to manually adjust mechanical loading by varying the electrical load on the generator should

give a better applications understanding from a students view.


13/19

The specific torque transducer selected for the project was justified based on the

worst case possible scenario. This was used assuming maximum efficiency in all

components, as well as maximum torque. This worst case scenario is shown in graphical

format in Figure 4 of appendices. Adding this torque transducer, as well as the electrical

generator, satisfies to basic requirements of the project directly from our client. The

quantitative analysis of the torque requirements is shown in the calculations on Figure 3a &

b.


14/19

Future Work

Next Semester

Going forward, the team is prepared to begin fabricating the new tank frame and

chassis. In addition to construction the frame, the new tank will need to be slightly modified

from its manufactured dimensions. This will incorporate how we are going to remount and

reassemble the three phase induction motor and pump assembly.

Ideally, mechanical construction and mounting will not extend far into the semester.

With that in mind, the team plans for the mechanical aspect of the new assembly to be

completed by early February of the New Year. This leaves the remainder of the semester to

begin debugging and testing the electrical load on the physical assembly.

At a minimum, we would like to have a working assembly with an adjustable load. A

baseline assembly would be a manually adjustable rheostat configuration to electrically load

the machine. If team ideals are realized, well have implemented a microcontroller to sense

and measure voltage and current (with the load being adjustable using pushbuttons). This

setup would incorporate the use of relays and an LED display.

Future Phases of Work

There will still be plenty of work for future teams to improve upon after our team is

finished next semester. Namely, this includes further automation and implementation in

regards to real time data acquisition.

More specifically, our client had requested a change-order at preliminary design

review. This involved integrating real-time data acquisition into a program called LabView.


15/19

Due to the lateness of this request and the number of team members qualified to

incorporate this solution; it is a tertiary item. For this reason, this would be a great project

for future Computer Engineers or Computer Scientists who could improve upon the data

acquisition and interfacing with a PC lab station.

Appendices

Figure 1: Moment and Force calculations


16/19

Figure 2: Moment and Force diagrams

Figure 3a: Torque and Power calculations


17/19

Figure 3b: Torque and Power calculations


18/19

Figure 4: Power and Torque as a function of Speed

Figure 5: New Tank Design


19/19

Figure 6: Team Schedule

Pelton Turbine Design scope

Documents

Transcript of Pelton Turbine Design scope