June 28, 2008
by Daniel Hensel, Director of Product Research

As of June 28, the construction of a test solar cooker for researching and testing our solar autoclave designs is complete. The solar cooker is built as a wooden frame covered with fiberboard, filled with sawdust (or, more accurately, wood curls created by planing wood), the outside lined with zinc and the inside lined with printing laminate reflecting material. An elaborate construction manual for the standard solar cooker has just been completed, and is available for download (see link on the right of this page).

The test unit is somewhat larger than a standard solar cooker (see photo at right). The inside clearance height is 10 inches, rather than 6 inches, to allow room for testing purposes. The extra space allows for a pressure cooker with a height of 8 inches and for varying the distance of the focal point of a Fresnel Lens. The pressure cooker has been painted black to retain heat rather than reflect it. Another unique aspect of the modified solar cooker is the addition of angled inserts. These are triangular prisms made of the same materials as a solar cooker.

Our current testing challenge is to bring the test unit inside temperature up to the required level of 170°C. For dry heat sterilization, a temperature of 170°C must be maintained for at least 60 minutes. Usually, a solar cooker can reach about 140°C. Achieving an incremental 30°C is not much of a problem, but sustaining it for an hour may prove challeging.

If a pressure vessel is used for sterilization, at 15 psi, a temperature of 121°C must be held for at least 15 minutes. While this is feasible, it requires the use of a pressure cooker every time an autoclave is purchased. It is unknown whether it is practical to locally purchase a pressure cooker (which can be expensive outside the US), or whether the people will have the capability to manufacture a safety-standardized pressure vessel. We are currently researching options for a type of pressure vessel other than a commercially available pressure cooker.

A significant safety concern is using water for the pressure vessel. The local water source is not santitary, usually gotten from wells. Purified water could be purchased, but water is a somewhat valuable resource. However, using wax as a substitute has been suggested; when wax melts, it releases significant energy. Wax is available throughout world and very inexpensive and abundant. In a closed container, the energy cannot escape and heats the contents considerably. One property of wax is that it releases energy very quickly, but accepts energy very slowly. This means that the container will stay hot longer. Therefore, wax seems to be a plausible design element that is worthy of further investigation.

A vessel designed to be made most anywhere in the world, contain pressure, and hold heat would be extremely useful for many purposes. Not only can it be used in the autoclave to sterilize surgical instruments, but it can even be placed on a hot tin roof to cook beans or even purify water. (See www.sodis.ch, Meierhofer and Wegelin, ISBN 3-906484-24-6 for more on the topic of water disinfection.)

Another aspect of the autoclave being investigated is tracking the sun. This means utilizing the fact that the sun moves across the sky as the day progresses. This has been a long standing problem. If we can utilize the heat of the sun without moving the cooker/autoclave, then we will maximize its effectiveness. When using a Fresnel lens, the problem is that th lens achieves a focal point at only one angle to the sun. At other times, the lens may actually be blocking the sun from entering the cooker by diffusing the light. However, if the Fresnel lens can be moved with the sun such that the focal point is always located on the surgical instruments, then we believe the necessary temperatures can be achieved and sustained.

The only way for the focal point to be maintained on the same point of the instrument container is for the lens to move in an arch with the focal point being the radial center. That is, the lens needs to move in a circular arch just over the cooker, moving in precise time with the sun. Our thought is to attach a wheel (such as a bicycle wheel) to the side of the cooker and attach the lens at the proper distance and height with a simple arm set-up extending from the wheel. A weight on one side of the wheel would pull it down the wheel, turning it, and a controller mechanism on the other would ensure that it moves at the speed at which the sun moves across the sky (in other words, at the speed which the Earth rotates).

What can serve as the controller mechanism? We have not yet started this research but have some ideas to share. It could involve a spring, or a pneumatic device of some sort. It is important that it be relatively simple, inexpensive, and locally available to Nicaragua (or other areas whereautoclaves will be built). Another consideration that must be tested is whether a tracking mechanism constructed from a wheel, weight, and controller will work when there is considerable wind. I believe that if the temperature is consistent throughout the day – if it is a sunny, hot day – then there will be minimal wind; this is the type of day that is desirable for autoclaving. On a windy day, it is usually cooler and clouds tend to be present, which make it a less than optimal day for using the autoclave.

Another issue is the degree of accuracy with which this controller must work. Intuitively, this tracking mechanism is something that has to be trusted to work properly by moving at the correct speed all day, assuming it is started at the correct position. This is something that I plan to look into more towards the end of the summer, when I have finished collecting raw data.

Our current testing involves taking temperature measurements using a DAQPro data acquisition system and several thermocouples strategically located throughout the autoclave (see photo at left). Locations include just below the glass, on top of the pressure vessel (a standard pressure cooker painted black), within the vessel, within the walls of the autoclave, and next to the oven thermometer placed within the autoclave on the floor between the door and the pressure vessel. This is for comparison purposes. Several other thermocouples are used outside of the DAQPro, which are placed either next to one of the four testing thermocouples, or placed in other strategic locations for information gathering.

It seems to me that we are constructing, with data, what a heat gun used on buildings could easily tell us quickly. However, we are also, of course, determined to ensure that the temperatures desired really can be held for the time needed. Furthermore, these temperatures will depend on the day. With enough information, I have a long-term objective to construct an ¨ok go¨ table that depends on the outside ambient temperature, amount of available light or absence of clouds, and possibly other factors such as humidity or distance from the equator or the angle of the sun depending on location. This is a longer-term objective, and I don´t expect to be completed in the near future.

This report is a brief description of the stages of our June 2008 research in Nicaragua, and what we hope to explore in the future. Some of our research is only at the idea stage. To date, other people have come forward and provided support in the form of advice and willingness to help. Salud del Sol is an appropriate technology research and assistance company and we welcome whatever outside help others may provide to improve our research and resuls. If you have a comment, see a fault, or have a brilliant suggestion, please share it with us at info@saluddelsol.org. We are grateful for whatever you share.

• More about Salud del Sol

  • The News & Events page has postings to Salud del Sol news coverage. You can read about the Business Plan Competition that we won, and background in our product research for solar autoclaves.

  New! Solar Cooker Construction Manual, 2008 by CJ Colavito, Grupo Fenix volunteer

  • The Solar Cooker Construction Manual, complete with photos and detailed diagrams for building your own solar box cooker, is now available. Download the English version (4Mb PDF) here:
    
  
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