Table of Contents
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Solar Cooker ExperimentSolar Cooker 3-Day Experiment Spreadsheet - Was performed using a Sun Oven provided by Dr. Thorn from the Industrial and Systems Engineering Dept.
Design ConceptsThe current concept is slightly changed from the concept proposed in the Systems Level Design Review. The selected design includes a cylindrical container for waste handling inside another cylindrical container, likely a 5-gallon polypropylene bucket, using air as insulation. The waste is heated from direct sunlight through the acrylic lid and from sunlight reflect from the aluminum foil covered flaps. The image shows only three flaps so that the top of the bucket can be seen, but in actuality the design is proposed with four. The lid and inner container are removable. The inner bucket rests on a stand made from some combination of concrete and rebar.
The above design features a stand with four legs to support the inner bucket. This is likely to change in MSDII because with such a stand the center of gravity of the entire assembly is higher in the bucket which creates instability and a higher likelihood of the waste pasteurizer being knocked over. Different stand strategies will be explored, including:
- Solid concrete filling the bucket to a certain point
- A cinder block placed inside the outer bucket
- A combination of cement and rebar created using a mold and placed inside the outer bucket
These three ideas will lower the center of gravity of the assembly, increasing stability. Simulations will be run to estimate the amount of heat loss due to each type of stand (see Simulations section in Detailed Design node). The type of stand to be used in the final concept will be decided on based on the thermal analysis results and the level of improvement of stability.
The temperature indicator is a specialized tool based on the WaPI (Water Pasteurization Indicator). It is different from the WaPI in that the wax does not need to be visible for the temperature to be indicated. The user will press a rod down along a fixed track and if a indication notch on the rod goes beneath a designated level, the wax is melted and the waste has reached temperature. No touching parts are submerged in the waste.
Preliminary Prototype Assembly Drawings
These are some of the Solidworks Assembly Drawings of the Prototype at the end of MSD 1.
SimulationsThe detailed design was modeled and simulated using ANSYS. To achieve this the thermal properties of the materials used were determined, including an effective thermal conductivity of human waste in Haiti.
The waste pasteurizer was modeled in two dimensions, taking advantage of the center axis of symmetry. A heat load was applied across the top line of the modeled lid, based on preliminary calculations done. The assembly was initially modeled assuming that the stand used to support the inner pot is thermally negligible and so the inner bucket is modeled completely surrounded by air.
Additional analysis tested the feasibility of filling the the outer bucket partially with concrete to act as a stand for the inner bucket.
Allowing the inner bucket to rest on solid concrete allows some of the heat held within the waste to be lost to the concrete stand. With less concrete and more air filling the space between the outer and inner buckets the waste within the inner bucket is better insulated. Two additional concrete stands were modeled, as shown below.
With the same heat load applied to each model, the waste in all concrete models does not reach as high of a temperature. Therefore, Use of a concrete stand in the model would require a longer heat time. An additional model was created using a PVC pipe stand because PVC is less thermally conductive than concrete.
Using the PVC pipe stand yields the best results of the stand options modeled. In this instance, the PVC pipe is modeled as a solid, whereas in the final product the PVC pipe would be hollow, therefore the temperatures obtained from the model with the PVC pipe stand are relatively conservative. A problem posed by using PVC however, is that the center of gravity of the assembly is higher from the ground, resulting in instability. Because of this a combination of concrete and PVC will be the best way to hold up the inner bucket. The model below shows the same PVC configuration with an inch of concrete in the bottom of the bucket.
The addition of concrete to the PVC stand only causes the temperature of the waste to drop by about 1 degC and would increase stability and durability of the assembly significantly. A summary of additional analyses is shown in the table below.
|Stand Type||Applied Heat||Max Temp of Waste||Min Temp of Waste|
|None||50W||75.4 degC||73.5 degC|
|None||60W||85.5 degC||82.8 degC|
|None||70W||95.1 degC||52.1 degC|
|Solid Concrete||50W||37.3 degC||32.4 degC|
|Solid Concrete||60W||39.4 degC||33.5 degC|
|Solid Concrete||70W||41.4 degC||34.5 degC|
|Hollow Concrete||50W||41.3 degC||35.9 degC|
|Hollow Concrete||60W||44.2 degC||37.7 degC|
|Hollow Concrete||70W||47.1 degC||39.5 degC|
|Concrete Legs||50W||48.9 degC||44.0 degC|
|Concrete Legs||60W||53.3 degC||47.4 degC|
|Concrete Legs||70W||57.6 degC||50.8 degC|
|PVC Pipe||50W||64.3 degC||60.4 degC|
|PVC Pipe||60W||71.1 degC||67.1 degC|
|PVC Pipe||70W||79.2 degC||73.8 degC|
|PVC Pipe with Concrete Layer||50W||63.6 degC||59.8 degC|
|PVC Pipe with Concrete Layer||60W||71.0 degC||66.3 degC|
|PVC Pipe with Concrete Layer||70W||78.3 degC||72.9 degC|