P17487: Kontiki Kiln Heat Recovery System
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Systems Design

Table of Contents

Team Vision for System-Level Design Phase

Our team conducted our first burn, took temperature readings, and brainstormed on heat recovery systems during this phase. Through a long process of concept design and consultations, the team came up with a few key concepts that seemed to be the most feasible.

We found out that modelling flame dynamics would be the most difficult part of computations, though it can be simplified for some rough values that can be perfected by modifying the values based on actual measurements.

Functional Decomposition

public/Problem Definition Documents/functional_decomposition.jpg

Based on Customer Requirements, the functions and subfunctions are as seen in the graphic above. Though creating biochar is not a part of our system, we found it useful to include because the heat recovery system should be in place before beginning the burn. Our main focus for concept design was finding a way to capture heat, though storing the heated water and moving it for use were also brainstormed.

From this graphic, the sub-functions of the system we are required to deliver at the end of the year are:

See the section below for generated concepts.

Benchmarking

Last Year's Project

Image taken from last year's Edge site

Image taken from last year's Edge site

Solar Powered Water Heater

Image from Solar Direct

Image from Solar Direct

Helical Coil Heat Exchanger

Image from Sentry-Equip

Image from Sentry-Equip

Feasibility: Prototyping, Analysis, Simulation

Image from Ithaka Institute presentation

Image from Ithaka Institute presentation

Above is a simple analysis that determines how much excess heat can be used for heating water.[1] Note that the mass of the wood used in this analysis is approximately 4 times what we are actually anticipating using, and also that they are assuming a much higher heat recovery rate than we will likely ever be able to achieve.

Aside from this initial analysis, additional analyses for our entire system are also planned. A CFD model of the passive convection loop design should be fairly easy to create, if simplifying assumptions for the heat generated by the flame are used. A full CFD model of the entire system will be much more difficult to create, as the fluid dynamics associated with combustion are currently impossible to model.

Modelling aside, testing last years project should also yield a large amount of useful data, as their heat transfer system for water heating was also a coil. Taking temperature readings of the outlet temperature over time will let us know if we need to recover more or less heat for our project, and also will help to refine our models.

Overall, the only important parameter for our system is the water temperature at the outlet of the coils. If we want to pasteurize the water, then the water needs to be at least 65 degrees Celsius, and it needs to maintain this temperature for at least 5 minutes to make sure that all harmful bacteria are killed. However, if we want to instead focus on only heating water for showers, then anything about 45 degrees Celsius has a high risk of causing burns.[2] Therefore, we will have to keep in mind which of these two goals our team is aiming for while designing our system.

Morphological Chart and Concept Generation

Morphological Chart

Morphological Chart

After our team created this morphological chart, we each came up with four ideas, and then through discussion narrowed it down to the 10 concepts seen below:

In class concept generation

In class concept generation

Concept Selection

The team decided on 13 different attributes to choose between the different generated concepts.

A Pugh chart was then created to evaluate the twelve different concepts we generated versus last years project used as the datum.

First Pugh Chart analysis

First Pugh Chart analysis

Using this first iteration of the Pugh chart, the concepts were narrowed down to 5 choices. The three main ones were a coil place on the inside of the kiln, a hood over the top of the kiln to collect hot air, and then a "hot tub" design that fills the space in between and the heat shield with water that will be heated via conduction. Additionally, there were two more concepts that were considered to be add-ons, since they could easily be done at the same time as any of the other options. These were a ring around the top of the kiln that heated things up via the heat thrown off by the fire, as well as bricks that were placed in the fire that can be taken out before quenching and used for either heating water or radiant heating. Once the concepts were narrowed down to these choices, two more Pugh charts were created only looking at these designs and last years designs. The datums were changed for each of these Pugh charts, as can be seen below.

Second Pugh Chart analysis

Second Pugh Chart analysis



Third Pugh Chart analysis

Third Pugh Chart analysis

The excel workbook showing all three Pugh charts can be seen here.

After going through these Pugh charts, team discussion led to the possibility of us combining the coil and "hot tub" ideas, since they should likely not interfere with each other. This would allow us to maximize the heat recovered in the cases where the customers have an actual kiln, while still allowing for some heat to be recovered when the customers are using a hand made pit design. Additionally, by combining these two options, this opens up the possibility of using the water in the "hot tub" for showering, while attempting to pasteurize the water that goes through the coils for consumption.

Systems Architecture

Coil design Hot tub design Chamber design

These flowcharts show the process for the top 3 design concepts.

Risk Assessment

Since the previous design review, no new risks have been discovered, as the initial risk assessment was done while at least partially considering last years design, which includes the coil heat exchanger that we ultimately decided would be our primary design going forward.

Design Review Materials

Below are links to:

Plans for next phase

Our team members' individual 3-week plans can be seen below:

References

  1. presentation, Ithaka Institute, Kon Tiki Flame Curtain Pyrolysis
  2. http://www.vba.vic.gov.au/__data/assets/pdf_file/0018/21267/Hot-water-safety.pdf, Hot Water Safety

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