P15441: Mini-Air
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Systems Design

Table of Contents

All documents listed on this page can be found in the Systems Level Design Documents directory.

Functional Decomposition

Because our device is very user dependent and will require user input, the best way to express the functions of our system is through a process-flow diagram. This diagram shows the matter, energy, and information that is input into the system and after processing, has only matter, energy, and information leaving. The diagram is shown below, the blue square representing the device, and the file can be found here
Process Flow Diagram

Process Flow Diagram

Benchmarking

Similar personal cooling devices on the market that we plan to use as benchmarks include the Handy-Cooler and ClimaWare Scarf. The Handy-Cooler uses forced air, while the ClimaWare uses thermoelectrics. Taking parameters from both, we will use their performance values to help us develop test plans. The current state of the market for these devices is shown below.

Market State

Market State


Below is a table of ambient temperature versus relative humidity. It is populated with the cooling difference provided by the Handy Cooler. The Handy Cooler also states it can generate 15 knots of air while only producing 30dB of sound.

Table of temperature versus humidity courtesy of Amazon.com

Table of temperature versus humidity courtesy of Amazon.com

Concept Development

This portion includes three phases:

Concept Generation: Using the process flow diagram above, we were able to determine what functions of the device are possible to brainstorm ideas on. The main functions that we are not constrained on include the cooling of the air, heating of the waste air, and intake/movement of the air. We are constrained on the use of the thermoelectric but not on how it transfers heat to the surroundings. Some of our concepts are listed below.

Brainstoming Ideas

Brainstoming Ideas


The movement of the air is realistically limited to the fan as we won't be able to provide the size needed for all the other options. Using these ideas, we generated a morphological chart which lists them all for the functions we deemed important in the process flow diagram. The gray-highlighted boxes are eventually what we selected as our concept which is discussed further below.

Morphological Chart

Morphological Chart


Using the initial morph chart and brainstorming ideas, we were able to generate a few concepts that have some plausibility. They consisted of the same overall picture of the device with tiny changes to include variations of the brainstorming ideas. Those concepts are shown below.
public/Photo Gallery/Concept1.JPG public/Photo Gallery/Concept2.JPG public/Photo Gallery/Concept3.JPGpublic/Photo Gallery/Concept4.JPG

Concept Improvement & Selection: Using our requirements and functions, we were able to identify eight selection criteria for our device. These criterion are independent of the concept and necessary for our project and device to meet what we have set out to do. Using a Pugh Matrix, we are able to compare the concepts to a common datum (one of the concepts or an existing model if one exists) to generate and easily view the advantages and disadvantages of each concept. To avoid any bias, we ran our matrix twice as can be seen below. A '+' indicates the concept is better regarding that criteria, while a '-' indicates it is worse than the datum.

First Trial through with Concept A as Datum

First Trial through with Concept A as Datum


Second Trial through with Concept B as Datum

Second Trial through with Concept B as Datum


From the Pugh Matrix, we can see that some concepts have advantages that may be better but with trade-offs to power or cost. These are things that we will need to keep in mind going forward as possible options if issues appear. From the matrix, we can also see that concept 2 proves to be the overall best option. This is the concept that we plan to move forward with, run our feasibility analysis on, and eventually our in-depth sub-system analysis.

Systems Architecture

Much of what we plan to design is extremely dependent on the user. Since no one person is alike, we plan to give the user control of the fan speed and a slight change in temperature. A model of how the device will function this way is shown below.


System Architecture

System Architecture

Feasibility

To gain some idea of the feasibility of the project, we wanted to focus on the cooling aspect of the device. Our general model and assumed variables used are:

public/Photo Gallery/EarlyFeasibility.jpg public/Photo Gallery/InitialAssumedVariables.jpg

These variables and parameters are assumed constant and some are subject to change as we later will fine tune our analysis. Overall, we can find our necessary cooling by Equation 1 (See bulk equations at end of section). This is a thermodynamic equations and looks at the input and output neglecting the internal workings. With 8.33 Celsius change in temperature, our cooling for our selected flow rate is about 16 Watts. We can then use this cooling rate to find out our surface temperature of our thermoelectric using Equation 2. With this equation, we are also assuming a lumped thermal resistance equal to 1/hA (the convection coefficient times the surface area of the heat sink). For our analysis, we are assuming this to be ~1 K/W although lower than 1 is desirable. These parameters we can control and adjust to reduce our surface temperature needed and eventually power consumption of the thermoelectric.

After finding our surface temperature, T1, we can estimate the dT across the thermoelectric which can be used with spec. sheet data to find an approximate current and voltage needed for those values. Our estimated dT is around 30-35. Looking at the spec. sheets, this correlates to about 3.5A of current. Below is sample spec. sheet data from one of our potential thermoelectrics.
Left: Characteristics for T2 = 25C. Right: Characteristics for T2 = 50C

Left: Characteristics for T2 = 25C. Right: Characteristics for T2 = 50C



From the data sheet, we can see that 3-3.5A is needed for around 16W of cooling at a dT of 30-35. From another graph provided on the spec. sheet, we can take the current and find voltage. The corresponding voltage for the current is ~10V for each T2. This means our total power consumption is 30-35W. To figure out the heat going out on the other side of the thermoelectric, we combine the cooling done (as each Watt of cooling produces a Watt of heat) and the power provided to the device as seen in Equation 3. This would be the heat needed to be dissipated to prevent our device from overheating or burning the user.

To obtain a better dT for the same cooling rate, we would need to drive our thermal resistance down. That again is mostly dependent on the surface area of the fins. As we drive our surface area of the fins up, our dT should go down assuming the efficiency doesn't decrease for the heat sink. Changing the flow rate will affect numerous parameters such as Reynolds and Nusselt number which affects our convective coefficient.

Essentially, our project is feasible but requires a very good heat sink with enough surface area to reduce our power consumption and generated heat. More in-depth analysis is necessary to find the trade-offs with flow rate, opening area to the cold side, heat sink size, convective coefficient, and the type of thermoelectric that we choose.
Preliminary Equations for Analysis

Preliminary Equations for Analysis


Risk Assessment

Our risk assessment at this stage in the design process is shown below. We plan on using this to watch our progress, as well as update it as we go further into analysis and the rest of MSD I.
Risk Assessment

Risk Assessment

Systems Design Review


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