Progress Blog #4

Blog Entry #4:

        During the period between April 9th and 23rd, we accomplished several key aspects of our design for the spring semester. We made a mathematical model of our water loop system and how it interacts with the surface of the chair. Also, we created a 3D-CAD model of our thermoelectric cooler (TEC) Peltier system and how it will fit together.

Figure 1: Model of chair showing pipes and waterloop.

        This model simplifies the system by taking a 2D slice of the chair cushion with the waterloop installed. The pipes are cut across their cross-sections. The layers of the chair and waterloop are shown in green. Nodes are marked in red. The center to center distance of the end pipes is the distance 0 to L along the surface of the chair. We want to maintain a constant comfortable surface temperature of approximately 22°C. When a person rests on the seat, the surface will have a constant heat flux of approximately 33.26 [W/m^2]. This heat must be absorbed by the water in the loop in order to maintain the surface temperature. To determine the temperature of the water, number of straight pipe sections, distance of pipes from the surface, and the inner and outer pipe radii we developed an equation to describe the resistance the system has to heat. The temperature at the surface will follow a parabolic curve with the peak at the center of the cushion. We split the surface into 40, 1[cm^2] sections. The heat transfer from each of the sections to each of the pipes is modeled by calculating the resistance radially and in one dimension. Then, since the same liquid flows through each pipe section at the same temperature and they are all acting on the same point at the surface, the resistance from each pipe can be combined in parallel. The wattage lost by the person resting on the cushion is set equal to the difference in temperature between the liquid in the pipe and the surface divided by the resistance calculated. The temperature is found for each section on the surface to create a graph of the temperature versus position and find the average surface temperature.

Figure 2: Surface Temperature vs. Position graph: Average surface temperature (Black Line) = 22.48°C, Liquid temperature = 9°C, number of pipe sections = 36, heat flux at surface 33.26[W/m^2].

Figure 3 : CAD representation of our thermoelectric system including heat sinks, and mounting brackets.

        This 3D representation is a major step in our design phase that we will use to reference what our system will look like. In the center is the copper waterblock that measures 80x80mm. Surrounding the waterblock are four TEC1-12710 thermoelectric modules that measure 40x40mm each. They will be run at half maximum voltage to minimize heat output and maximize cooling capacity. Two large heat sinks will surround the TEC modules to cool the hot side of the modules. Each component of the thermoelectric system will have a thermal compound to ensure proper thermal contact. Four brackets and four bolts will be mounted to our housing to ensure all components are squeezed together resulting in maximal surface contact for proper heat distribution.
        For the next 2 weeks, until May 3, our team plans to model our waterloop system in its entirety, and provide a bill of materials including all components and costs of our system. We also plan to produce COMSOL simulations of sections of our system to visualize how the system functions, and develop a better understanding for how it will operate in real world conditions. COMSOL in and of itself will be a major challenge as the team hopes to overcome by watching tutorials online and utilizing resources available to us. Another challenge we will face is the electric system and how we actually control the output of the modules. We will use either a potentiometer, or an existing thermoelectric controller. We plan to address these challenges during the summer when we have more time and freedom to plan and test an electric control system. As this is not our area of expertise, we may need to confer with outside sources such as other engineering students, or those with knowledge in the field.