Combined Heat and Power System

By

Ryan Christie, Ronald Stepanek, Nathan Duray, Joseph Mudd,

Cory Donavon and Jason Dikes

Team 11

Concept Generation Document

Submitted towards partial fulfillment of the requirements for

Mechanical Engineering Design – Fall 2012

Department of Mechanical Engineering

Northern Arizona University Flagstaff, AZ 86011

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Table of Contents

Overview

3

Problem Statement

3

Concept Generation

3-4

Concepts

4-12

Weighting Factor Tree

13

Decision Matrix and Concept Selection

14

Creative Design

15

Timeline

16

Appendix

17

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Overview This report on concept generation, for our combined heat and power system, includes the

following categories:

- A refined problem statement from Supreme Manufacturing

- Our initial brainstorming of ideas and concepts, compiled and explained in a

morphological chart

- Ten concepts we generated and the reasons why each of them may be a viable solution

- Our decision matrix which helped us decide which concept to further pursue

- The stages of creative designed that will be expressed throughout the design and

manufacturing of our product

- An updated timeline and where we currently stand in the design process

Problem Statement Supreme Manufacturing is dissatisfied with the cost, reliability and overall efficiency of their

current power supply.

Concept Generation During the concept generation process out team prioritized the combined heat and power

system into six main attributes. We then developed several options for each attribute. These

main attributes include: the fuel source (eg. natural gas), the transfer of the fuel’s chemical

energy into mechanical power, the transfer of mechanical power into electrical power and how

we will capture and utilize the waste heat. Along with those criteria, we included broad

configurations are design could take, in addition to the interface between the components and the

operator. With this information we developed a morphological chart (see Table 1) to facilitate

our generation of design concepts.

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Attributes Options

Fuel Natural Gas Diesel Propane Solar

Power Generation Sterling Internal Comb. Turbine Fuel Cell

Capture Heat Buffer Tank Compress/Condition

Exhaust Cold Reservoir

Generator/Inverter Purchase

Tailored Design Off-the-Shelf

Frame/Structure Integrated Stand Alone Versatility/Mobility

Programming GUI Switches Gauges

Table 1: Morphological Chart

Concepts

Alternative Fuel Single-Cylinder Stirling Engine (Ryan Christie)

The Stirling engine will generate both heat and power. To improve overall efficiency,

cold water will be passed across the cold reservoir to maintain a consistent temperature

differential and supply Supreme Manufacturing with hot water. This hot water will be stored in

their hot water tanks; cooled water will be re-circulated back across the cold reservoir. Exhaust

gases will be conditioned and available for heating purposes for the manufacturing facility and/

or the commercial offices. The Stirling engine pistons (displacement and power) will be attached

to a shaft that will in-turn be connected to a generator that will produce electrical power. We are

planning to contact Motor Excellence (Flagstaff, AZ) to see if they would be willing to

manufacture a generator, using their innovative technology, tailored to our combined heat and

power system. This electrical energy will be converted from DC to AC via an inverter. From the

inverter the AC power will be transferred to Supreme Manufacturing’s meter to be used or, if

there is a surplus, sold to the power companies.

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Highlights:

- High efficiency

- Non-invasive

- Low GHG production

- Cost effective

- Steady-state operation

- Highly reliable

Downsides:

- Depending on the type of Stirling engine, there is a potential for low power output

proportional to mass

- May not provide the highest overall efficiency

- Hot water and Heating (heating, aside from for use in ovens) may not be desired,

consistently in Houston

Trigeneration Concept (Ryan Christie) The idea of trigeneration includes using a heat engine to produce electricity, heating and

heating to meet the heat input needs for an absorption refrigeration system. Because Supreme

Manufacturing is located in Houston they will rely on cooling their facility more than heating it.

Therefore, if trigeneration can prove to be economically viable, efficient, reliable and fitting for

this application, it may be the best option for Supreme Manufacturing’s needs.

Highlights:

- Produce electricity and heating for heating the facility and providing (if applicable) heat

input for a absorption refrigeration system

- Effective

- Efficient

Downsides:

- Potentially invasive

- Potentially expensive

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Waste Heat-Powered Ovens (Nathan Duray) This concept is characterized by the use of waste heat to provide hot air for the

manufacturing ovens. A natural gas fuel source is used to create a hot reservoir for a Stirling

power plant. This power plant produces three outputs: hot air, hot water and electricity. The

electricity will be produced by the mechanical energy produced from the Stirling engine while

the hot air and water will be drawn from the cold reservoir of the same engine. This would mean

the cold reservoir would need to operate at approximately 400k while the hot reservoir is held at

800k. This gives us a relatively low max efficiency, ηCarnot = 50%, for mechanical output to the

electric generator. However, due to the use of waste heat to power the ovens, the need for

electrical power is greatly reduced and much of the overall efficiency value can be recovered.

Residual waste heat can be utilized to create hot water for the commercial facility. Though this

design reduces the water need for cooling, it requires an invasive integration process. The current

electric ovens would need to be adapted to function from an external hot air heat source. Also,

the hot air lines to convey this hot air would need to be installed into the facility—increasing the

cost of the installation for our customer.

Fuel Source

(Natural Gas)

Power Production

(Stirling Engine)

Hot Air

(From Cold Reservoir)

Hot Water Electricity

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Combine Cycle Generation (Nathan Duray) This concept is characterized by the use of waste heat to power a second power cycle.

Fuel in the form of natural gas will be used to power an internal combustion engine. This

internal combustion engine will produce the mechanical energy to provide electric energy

generation. To boost efficiency, the exhaust gasses from the internal combustion engine will

then be used to produce the hot reservoir for a Stirling engine. This Stirling engine will then

produce additional electric energy which will contribute to the total electric output. Residual

thermal energy can then be absorbed through the cold reservoir of the Stirling engine to produce

hot water for the commercial facility.

Fuel Source

(Natural Gas)

Internal

Combustion Engine

Exhaust Gases

Electric Power

Sterling Engine Electric Power

Hot Water

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Horizontally Opposed “Boxer” Stirling Engine (Joseph Mudd) By combining two beta-type Stirling Engines out of phase, a large flywheel is no longer

needed. This is important to cut costs, weight, and complexity. This design also reduces

vibration, increases power output, and makes a smoother, more reliable design.

Figure 1: Horizontally Opposed Stirling Engines

Fuel Cells (Cory Donavon)

Background:

- Uses fuels to convert hydrogen and oxygen into water, creating electricity in the

process.

- No pollution, because the only by-products are heat and water.

- Can run on fuels such as: methane, gasoline, and hydrogen. Hydrogen works the

best.

- Converts chemical energy and produces electrical energy.

- Durable; good for cyclic/transient loading

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- Is flexible in terms of fuels

- Some fuel cells generate a lot of heat, which can be captured and used to do work

o These fuel cells consist primarily of molten carbonate fuel cells (MCFC),

phosphoric acid fuel cells (PAFC), and solid oxide fuel cells (SOFC).

- Very clean and energy efficient

- Quiet

- Very few moving parts, so integration would be minimally invasive

- Potential for renewable energy – excess heat and power can be used to generate

hydrogen, which can then be reused by the CHP system or can even power a hydrogen

fuel cell vehicle!

Cons/Difficulties:

- Although hydrogen works best, it is difficult to acquire.

- Costly

- Needs to be more efficient in order to be competitive with other methods in cost

Six Stroke Engine (Cory Donavon)

Background:

- Traditional four-stroke engines have one stroke dedicated to powering the vehicle or

device. The six stroke engine adds an additional power stroke in order to greatly increase

efficiency.

- Once the exhaust cycles out of the chamber, rather than introducing more fuel right away,

water is instead injected. Upon doing this, the heat of the chamber expands the water to

1600 times it original volume, creating enough pressure that forces the piston down

again.

o This water injection also allows cooling of the engine, rendering the heavy

radiator, coolant, and fans useless.

- Allows the engine to be more efficient – using less fuel – yet delivers just as much power

as standard ICEs. All friction losses to the fan and radiator are gone, since the extra

components are not necessary anymore.

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Cons/Difficulties

- Concept still in its infancy. There are still a lot of bugs to be worked out, including:

o Needing distilled water so no residue forms inside chambers

o Water freezing (unlikely in Houston, Texas, but still noteworthy)

Organic Rankine Power Cycle (Jason Dikes) This concept is based on using an Organic Rankine power cycle to turn waste heat into

electricity. The term organic comes from the use of an organic material, with a high molecular

mass, as the working fluid of the system. The boiling point of the organic material is lower than

that of water and therefore the system does not require as much heat to produce electricity when

compared to a normal Rankine cycle. This is a proven concept in industry and offers great

reliability. Additionally, alternative fuels can be used to power the system.

The system would also use a Gerotor instead of a turbine. The Gerotor is cheaper to

produce and creates very little friction. The reduction in friction means that the working fluid can

exert less force to generate electricity. This will also lower the temperature needed to power the

system.

Figure 2: Rankine Cycle

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Figure 3: Gerotor

Shape Memory Alloy (Jason Dikes) One design concept is possibly using a shape memory alloy (SMA) heat engine to turn waste heat

into usable electricity. There are a lot of small scale SMA heat engines on the market currently, with a

wide range of designs. Our job for the capstone project would be either taking one of these small scale

designs and scaling it up for an industrial application or coming up with our own original design that will

meet our client’s needs. One major consideration for this design concept would be the price of SMA. The

price can vary anywhere from $10 per kilogram to $200 per kilogram. One possible design is shown

below.

Figure 4: Shape Memory Alloy

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Quasiturbine Engine (Ronald Stepanek) For our combined heat and power system we have considered many different options for

engines to power a generator. One of these options is the Quasiturbine engine. There have been

several instances in which the Quasiturbine engine has been considered as an alternative to a

standard type Stirling Engine design. This engine design has shown potential as an internal

combustion type engine, as well as a steam powered engine and pneumatic engine. There are

already Quasiturbine engines in production that utilize steam and pneumatic capabilities to

produce up to 12 kW of power from a single engine.

Production units that produce 12 kW while utilizing steam or compressed air are readily

available for $8900 per unit. The capabilities of these units are dependent on the pressures at

which they operate. The 12 kW model requires a pressure of 4 bar.

In order to integrate a Quasiturbine engine into our design we must first find a way to

create pressures upwards of 4 bar. Potential ideas for this include using waste heat from the

ovens to aid in steam generation, using compressed air from a holding tank, or perhaps using the

Quasiturbine engine in conjunction with another engine type that produces high pressure

differentials or waste heat.

Figure 5: Quasiturbine schematic

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Weighting Factor Tree

this space intentionally left blank

Combined Heat and

Power System

K = 1.0 w = 1.0

Cost

K = 0.15 w

= 0.15

Geometry

K = 0.1 w = 0.1

Efficiency

K = 0.2 w = 0.2

Power Output

K = 0.2 w = 0.2

Reliability

K = 0.35 w = 0.35

Initial Cost

K = 0.2 w = 0.03

Operating Cost

K = 0.3 w = 0.045

Rate of Return

K = 0.5 w = 0.075

Volume

K = 0.3 w = 0.03

Integration

K = 0.7 w = 0.07

Energy Efficiency

K = 0.8 w = 0.16

Alternative Energy

K = 0.2 w = 0.04

Water

K = 0.1 w = 0.02

Use Life

K = 0.3 w = 0.105

On Demand Power

Production

K = 0.7 w = 0.245

Electricity

K = 0.8 w = 0.16

Heating

K = 0.1 w = 0.02

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Decision Matrix and Concept Selection Shown below is the decision matrix we implemented to help us determine the best concept.

Table 2: Decision Matrix

In this decision matrix we compare six of our most likely generated concepts against our

five primary weighted criteria from the weighting factor tree. As is clearly seen, our Internal

Combustion Engine (ICE) plus Heat Recovery system scored highest in both the weighted and

non-weighted totals, distinguishing it as our most viable option thus far. The ICE + Heat

Recovery system attained scores that matched or exceeded all other scores in three of the five

categories for evaluation. One may note that while the ICE + Heat Recovery tied the Beta-type

Stirling Engine in reliability, the low scores under power output for the Beta-type Stirling make

the ICE + Heat recovery a much better option. Again, while the Rankine Cycle received the same

score for power output as the ICE + Heat recovery system, the Rankine Cycle’s low scores in

cost and geometry further solidify our decision. Lastly, it is worth noting the effects of the

weights of the criteria. Despite being assigned equal non-weighted totals, the 6-stroke ICE, Beta-

type Stirling Engine, and Horizontally opposed Stirling Engines differ significantly in their

weighted totals column.

Cost (15%) Geometry (10%) Efficiency (20%) Reliability (35%) Power Output (20%) TotalsWeighted

Totals

ICE + Heat Recovery 4 3 3 5 4 19 4.05

6-Stroke I.C.E 2 4 3 3 3 15 2.95

Trigeneration 2 1 4 2 3 12 2.5

Rankine Cycle 1 1 2 4 4 12 2.85

Beta-type

Stirling Engine3 3 3 5 1 15 3.3

Horizontally Opposed

Stirling Engines3 3 3 4 2 15 3.15

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Creative Design

Selection Design

- Alternative Fuel, Internal Combustion Engine and Electricity Generator

Configuration Design

- Ancillary Engine and/ or Heat Recovery System

Parameter Design

- Engine Displacement and Geometry of System

Original Design

- Control System/ Graphical User Interface (GUI), Frame and Substructure, Heat Recovery

System, Shaft from engine to generator

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Timeline

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Appendix

Table 1

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Table 2

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Figure 1

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Figure 2

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Figure 3

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Figure 4

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Figure 5

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