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Fall2008Projects

Page history last edited by JDD 14 years, 6 months ago

Current Project from Fall, 2008:

Students: An Son, Max, Matt

Project Description: The goal of the project is to utilize a low temperature difference to generate electricity.

Introduction
Background
1. Stirling Background Information
2. Peltier Backgound Information
  1. Research Sources
Proposed solution
1. First Stirling Prototype
2. First Peltier Prototype

Introduction

The Tiverton project is an ongoing project to build a building that will leave a small environmental footprint. Multiple groups have been involved with the many different parts of the project, most notably the conduction of a feasibility study on the proposed site. There are currently two (known) groups actively contributing to the overall Tiverton project:

1) Brown University Engineering1000 group - finding the best solution of generating electricity from a low temperature differential

2) Rhode Island School of Design group - designing the architecture of the building, and acquiring funds

One of the ways that the Tiverton structure will reduce its environmental impact, is to store heat from sunlight in order to: heat the interior during winter, and generate electricity to power necessities such as light. The proposed method of storing heat is to use a material with the property of changing phase at a specific temperature. This property allows large amounts of energy to be stored at a constant temperature that is optimal with respect to the source of energy input.

The Tiverton structure is primarily being used as a teaching center. As such, the problem that Brown's ENGN1000 group faces is to design a solution that will generate sufficient electrical energy from the extra energy available in the heat store to power at least some of the lighting for the structure.

Tiverton Solar Model.xls

Background

Stirling Background Information

Stirling engines are driven by temperature differences to move a piston by causing a permanently contained fluid to alternately expand and contract. They can theoretically operate at the carnot efficiency, but this requires the use of a 100% efficient regenerator. Regenerators conserve energy, and thus increase the engine efficiency, by acting as a preliminary heating or cooling stage: it absorbs heat from the fluid as the fluid is pumped to the cold side of the engine, and releases heat back to the fluid as the fluid transitions back to the hot side of the engine. Compared to internal combustion engines, stirling engines are potentially quieter, more efficient, more reliable, and lower maintenance. However, they have a lower power output and a higher material cost. Stirling engines are therefore primarily used as cryogenic coolers.

How Stirling Engines Work.doc

Upload force diagrams

Types of Stirling engines

Alpha - Two power pistons, where one is heated and the other is cooled.

Pros: Highest power to volume ratio. Higher compression ratios or more easily achieved.

Cons: Requires seals on both pistons, which creates a problem, in most applications, of the hot piston seals breaking down.

Image from Wikipedia

Beta - One cylinder with one power piston on the cold side, and one loosely-fitted displacer. One end of the cylinder is heated, and the other end is cooled.

Pros: No hot moving seals, thus longer life time, less friction, and easier to fabricate.

Cons: Lower power to volume ratio that alpha-type.

Image from Wikipedia

Gamma - One power piston offset from one loosely-fitted displacer. The cylinder with the displacer is heated on one end and cooled on the other.

Pros: Easier to construct than beta-type.

Cons: More dead space and thus lower efficiency and compression ratio.

Image from http://www.ent.ohiou.edu/~urieli/stirling/me422.html

Regenerator

Regenerators are notoriously difficult to theoretically model, due to the large number of variables involved. Among the most prominent of these are:

1) Dead space. A large regenerator introduces spaces that limit the piston's ability to compress the working fluid. However, a large regenerator allows a larger surface area for the regenerative material to contact the fluid.

2) Friction/pumping losses. A tightly packed regenerator forces the fluid to pass through the regenerative material, but impedes the flow and reduces the pressure.

3) Turbulent fluid flow. A fast flowing, turbulent fluid allows more of the fluid to come into contact with the regenerator, but also complicates the processes of modeling and analyzing.

Research Sources:

The Stirling Engine Manual, Volumes I and II, by James G. Rizzo

Enter name of Matt's stirling manual

http://en.wikipedia.org/wiki/Stirling_engine

http://www.howstuffworks.com/stirling-engine.htm

http://www.sesusa.org/

http://stirlingenergy.com/default.asp

http://www.ent.ohiou.edu/~urieli/stirling/me422.html

http://www.airmaticcompressor.com/STM%20Power%20New%20Brochure.pdf

http://www.infiniacorp.com/technology/product_opportunities.php

http://www.grc.nasa.gov/WWW/TECB/RPS_ASRG_%20Handout.pdf

http://www.thermalengines.com/

http://peswiki.com/index.php/Directory:Stirling_Engines

Peltier Backgound Information

The Peltier effect occurs when a current is passed through a junction of two "dissimilairly electrically conductive materials" and causes one junction to release heat, and the other to absorb heat. "Current is propogated by electrons in n-type materials, and by holes (traveling in the opposite direction) in p-type materials". Applying a properly directed current across a junction causes holes and electrons to be created and flow to the other side where they recombine. The process of creating the holes and electrons removes energy from the junction, thus cooling it, while the process of recombining the holes and electrons releases energy, thus heating it. Just as stirling engines can be used as a heat displacer, the junction can be run in reverse to generate electricity, in a process called the Seebeck effect.

Research Sources

TE_rev.pdf

http://en.wikipedia.org/wiki/Peltier_effect

Peltier Vendor: http://search.ebay.com/_W0QQsassZthermalenterprises

Proposed solution

The feasibility study projects the allowance of 11.6 kW of power for 9 hours to be used for electricity generation without exhausting the store of heat. The two most promising solutions that will be focused on are:

1) Stirling engine

2) Peltier junctions

For a group consisting of three mechanical engineering students, the stirling engine will be of primary interest. It presents a very open-ended problem to solve and will require the design of a unique engine. The peltier solution only entails the purchase of a number of junctions and the design of a simple set-up to place it under a temperature differential. Therefore, the stirling engine will provide the benchmark from which the potential of peltier junctions will be judged.

For the given circumstances, a high-efficiency stirling engine can in theory produce 660W of power for the required duration of time. Over the span of one semester, a 30W stirling engine model is intended to be constructed, and information of the difficulties present in scaling up the engine to 300W is intended to be provided.

Thermoelectric cooling junctions are generally 5-10% as efficient as an ideal Carnot cycle refridgerator but can be obtained much more easily than a Stirling engine can be built.

First Stirling Prototype

After considering the different manufactured parts available, a 2-cylinder alpha-type stirling engine was decided upon. The primary drawbacks of an alpha engine of having the hot cylinder seal degrade due to the high temperature was not present for this project, because the engine would be working under relatively low temperatures

It was also thought that an external regenerator would allow easy testing of various regenerators - a key variable in creating an efficient stirling engine.

It is hoped that this initial model can produce an output of 3W, from which the larger 30W model can be built upon.