Personalized Thermoregulation

Spring 2020 MAE 156B Sponsored Project

University of California, San Diego

Background

Dr. Renkun Chen's lab recently developed an energy efficient thermoelectric device (TED) that is able to be cooled or heated to a desired temperature when a current is passed through it using the Peltier effect. This can lead to a much more energy efficient form of climate control for individuals. Rather than cooling the entire room a person is working in, the cooling effort can be directly applied to the skin surface of the individual. Moreover, this device could be used in personalized thermoregulation for individuals who are working in outdoor environments with extremely hot or cold weather, where air conditioning and heating is not accessible.

Rigid TED developed by Dr. Renkun Chen and his lab.

One of the major innovations in Dr. Chen’s TED involves the design of the pillars used in the device. The pillars used in TEDs are made of a material alloy that exhibits the thermoelectric effect. Thus, when a voltage is applied to the device, a temperature difference is generated on both surfaces connected to the pillars. Dr. Chen’s lab group was able to design a TED with low spatial density (6.25%) and high aspect ratio (5:1) for the pillars, both of which reduce the conductive heat transfer between the hot and cold side of the TED. This allows for a higher temperature difference between the two sides of the TED, allowing for more efficient cooling and heating. Dr. Chen's lab group has already proved that the device works in a lab setting and has now tasked the team with designing a portable prototype that utilizes a control system to keep the user's skin at a desired temperature.

(Left) Aspect ratio of a pillar in the TED.

(Right) Spacing between pillars in TED.

Objective

The team's objective was to design and build a personalized thermoregulation device utilizing Dr. Chen's TED that is portable and can maintain a temperature with closed-loop feedback control.

Criteria and Requirements

• • Control system to have the TED maintain an assigned temperature

                Maximum rise time: 60 seconds

                Maximum overshoot: 3oC

                Maximum steady-state error: 1oC

• • Minimum continuous operation time: 1 hour

  • User interface (i.e. ability for user to set temperature)

  • Portability (i.e. wearable armband)

Deliverables

• Control algorithm to maintain target temperature

• Circuit to implement control algorithm

• Mobile app for user interface with circuit

• 3D printed enclosures to hold electrical components

• Clips to attach enclosures to waist

• Wearable garment with TED attached

• Fully documented report with operation manual and bill of materials

Prototype Testing Results

Using the PID controller designed (Kp = 0.25, Ki = 0.00015, Kd = 1), the team managed to wireless control the TED to heat and cool against the heat flux from the user's skin within 5℃ of ambient temperature with the following results:

• • Rise Time: 55 seconds

  • Maximum overshoot: 1.5℃

  • Steady state error: 0.35℃ (heating) and 0.25℃ (cooling)

  • Settling time: 85 seconds

  • Maximum operational time: 2.75 hours

Temperature tracking plots for both cooling (left) and heating (right) with temperature set points 5 degrees above and below ambient temperature (33℃).

With these results, the team was able to design a controller that met all of the sponsor's requirements. One thing to note, however, was that the maximum cooling delivered was 5 below ambient temperature. The team reasoned that the attachment system for the armband most likely limited the natural convective cooling from the atmosphere by reducing the surface area available for the TED to cool. Another aspect of the testing results to note is that the operational time was nearly three times as long as that required by the sponsor. Extrapolating the data to operation of multiple TEDs, the team predicted that control of three TEDs would last 1 hour. This means that in future design iterations of this project, the sponsor can design vests utilizing multiple patches of TEDs to heat and cool the body for at least one hour as the scope and size of the project increase.

Executive Summary

The executive summary can be found here.