Solar Powered Autoclave

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Project Summary

This project aims to create an affordable, easy to operate autoclave for sterilizing surgical equipments. In third world countries, where there is a lack of adequate infrastructure, unsanitary equipments can pose a huge risk to patient health. The Solar Powered Autoclave is an off the grid solution, that is both environmentally friendly and portable.    

Intro

Sterilization of medical equipments is extremely important for preventing infections and transmission of viruses or bacteria from user to user. By destroying the microorganisms on the surface of equipments, sterilization and disinfection have shown to lower the surgery infections rates significantly.

Specifically, steam sterilization at 121 °C and 15 psi is about 6 times more effective at preventing infections that simple boiling water sterilization procedures. The moist heat renders the microorganisms dead by denaturing essential proteins and enzymes needed for survival. More information about the Center for Disease Control guidelines for proper disinfection and sterilization can be found here. CDC 2008 Guidelines for Disinfection and Sterilization in Healthcare Facilities. The diagram below outlines the process by which a commercial autoclave would be used to disinfect lab equipment.

We hope that by constructing a solar powered, portable, inexpensive, efficient autoclave, we can reduce infections in locations not equipped with proper sterilization equipment. Our project is related to many recent designs for solar powered autoclaves, intended to provide sterilization of surgical instruments for medical clinics where electricity is unreliable or unavailable. Since this project's scope does not include actual partnership with intended users or consideration of the specific needs of any one community, it is instead motivated by the common requirements that we found from researching existing designs. Different types of solar autoclaves are currently in development - please refer to the research page for more information.

Cho 2012 - "The Case of a Low Cost Autoclave for Primary Health Clinics"  From the "Ottoclave" project  

Intellectual Property

Both relevant patents to our design, and patents and documentation for existing solar autoclaves will be discussed further in our research page.

Patents:

Scharmer 1991 - "Medical Sterilizer" details separate solar-steam generator with pressure regulating valve, connecting to the autoclave vessel. 

Essig 2006 - "Inflatable Parabolic Reflector" constructs a parabolic mirror from a flexible reflective material.

Proposed Solution

Our design will be in unique in achieving portability and low materials cost. There are three main components to our proposed solution: vessel, solar collector, and electronics and sensors. The solar collector will concentrate sun light on the vessel, causing the water inside to heat and transform into pressurized steam. The sensors fitted on the vessel will monitor the pressure and temperature of the steam to ensure proper sterilization. The block diagram and schematic below illustrate our approach for providing off-the-grid sterilization in third world countries.

                   Schematic                                                               Block Diagram

Vessel

The pressure vessel must be able to withstand the minimum operation levels of the autoclave, mainly temperatures around 121 C and pressures up to 15 psi. The shape of the vessel will depend on the geometry of the solar collector, to make sure that there will no external power needed for sterilization processes. In addition, we will explore various ways to minimize heat losses through conduction and convection to maximize efficiency of the autoclave. 

Solar Collector

Previous iterations of solar autoclaves have large, heavy solar collectors as the heating source. We have made portability the main constraint of our solar collector design because it will allow visiting doctors to easily transport solar autoclaves with them when they are faced with rural conditions with no electricity. Instead of mirrors (the material used in almost all previous designs), we will utilize Mylar to minimize costs and weight. Mylar is the trade name for biaxially-oriented polyethylene terephthalate, coated with metals to make the sheets more reflective. With an extremely low density of 1.38 g/cm³, we can create a large surface for light collection without affecting the portability of the collector. The low weight of the solar collector does pose a problem against wind, but we hope to make a strong stand for the collector and perhaps reinforce the Mylar sheets with a stiffer fabric to prevent it from falling over. 

The solar collector can take many different shapes, as long as it allows for the concentration of solar energy. In general, there is 1 kW/m² of solar power available. Further research will be conducted on the power our steam sterilizers needs to heat up and maintain the steam at the optimal pressure and temperature for a minimum of 20 minutes.

Electronics and Sensors

Monitoring the autoclave is crucial for ensuring that proper sterilization takes place. We will explore different ways of determining the temperature and pressure inside the vessel that best suit our needs. With this component of the solar autoclave, we must consider the limitations of cost, power requirements, and vessel fixturing. 

Requirements + Results

The most important requirement for this project is operation at levels in accord with the CDC sterilization standards: the vessel must maintain a temperature of 121 °C and a pressure of 15 psi (which is the pressure of saturated steam at 121 °C) for 30 minutes. To meet this operation level, the solar collector must be able to deliver at least 0.25 kW of incident radiation to the vessel during direct sunlight exposure (see Solar Autoclave - Third Prototype for thermodynamic calculations).

Because the vessel will be pressurized, it's important to think of safety measures in case of overpressurization. A pressure safety release valve needs to be incorporate and set to release at a pressure that ensures both proper sterilization - by not prematurely releasing steam - and safety of users and people nearby. Therefore the vessel will be designed to handle ~20 psi for prolonged periods of time, with a failure pressure of 10 times that amount (250 psi).

Our design also aims for low cost and portability.  In accordance with this goal, we have established that the total cost should not exceed $600, the total weight should not exceed 50 pounds and the length in any dimension should not exceed 4 feet. The cost requirement was chosen based on an analysis of the prices of solar autoclave made by other groups (see Autoclave Research) and extensive research on making the most efficient and useful autoclave. The portability requirements were chosen subjectively to reflect what an average, healthy person can carry over short distances.

** See Solar Autoclave - Third Prototype for why this number is a simple calculation that leads us to believe that we are still far away from achieving our desired power output from the solar collector.

Besides the power input, we met all our other requirements by the third prototype. The 3 iterations of the solar powered autoclave are clearly outlined in the prototype pages.

Solar Autoclave - First Prototype

Solar Autoclave - Second Prototype

Solar Autoclave - Third Prototype

Future Work

Our work over the semester has led us to the fabrication of a pressure vessel, with appropriate sensors and solar collector capable of focusing light. However we were not able to fully integrate the two systems and are currently dependent on non-solar power sources to properly operate the autoclave. Our future work will focus on improving the solar collector to deliver sufficient power to the pressure vessel by further testing why the last iteration of the solar collector was incapable to do so. While we believe that the chosen geometry is optimal for collecting and focusing light, we can focus on other parts of the collector like the fabrication process and the materials. In future iterations, we think it would be valuable to use a stiffer reflective material that will not wrinkle and deflect light away from the focus. At the same time we need to develop better testing protocols to test how much power is captured by the collector. In terms of the chamber, there is a need for incorporation of more electronics and in particular a timer to determine when the the autoclave reached the operating temperature and pressure.