Grantee Research Project Results

 The goal of this project is to develop safer, reliable, and user-friendly solar ovens to be generally used for food cooking and heating purpose at outdoor and indoor locations. It is the objective of phase I project to demonstrate a prototype and obtain preliminary results to proof that the proposed solar-energy-based cooking and heating device is highly safe, reliable, low cost, sustainable, and capable of providing heat with sufficiently high temperatures for all cooking purpose including boiling, baking, and frying at outdoor and indoor locations. Upon a successful development of a matured product on top of the phase I prototype, the technology will greatly benefit a large population of the world. Particularly in the rural area of developing world, solar cooking will dramatically decrease the use of biofuel that people usually get from wood and trees, which is one of the reasons that many third world countries get deforested. In developed world, widely use of solar heat for cooking/heating will help to reduce the use of fossil fuels and natural gas, and thus to protect the environment. The PI and his team hope that this project will significantly benefit people and planet, and is helpful to the sustainability and prosperity of human society.

 

As a key novelty, the current project avoided using the old-fashioned parabolic reflectors for concentration of sunlight; instead, Fresnel lenses were used to form a concentrated sunlight beam going forward after the lens. The Fresnel lens in the system is controlled properly that it can cast the focused sunlight always on a designated receiving surface which is connected to a cooking plate. A complete shielding of the sunlight-receiving surface from the cooking plate is made possible. People who cooks using the cooking plate is fully protected from any exposure to concentrated sunlight. The solar cooking based on this concept is intrinsically safe. Large Fresnel lenses, typically used for flat TV screens, are made of plastic at relatively low-cost. The projected product will be featured with low-cost and better sustainability, which benefits people and planet on a global scale.

 

While the project will greatly benefit developing world, it will also create a great benefit to the US. Using solar energy for daily cooking and heating especially in southwest of US will help to save a tremendous amount of electrical power. The project meets the EPA’s P3 objective for sustainable development of environment and economy based on renewable energy. Disciplines involved in the project include, but not limited to, optical system design, radiation, heat conduction and protection, convective heat transfer, solar tracking and control, and industrial manufacturing. The project is multi-disciplinary involved. Students from Departments of Optical Science and Engineering, and Mechanical Engineering at the University of Arizona have been and will participate in the analysis, design, fabrication, and test of the prototype solar stoves.

 

The team has built a first-version prototype system and demonstrated the concept very successfully. Test shows that the system is able to boil water, fry and bake meat. It is believed that a fully developed commercial grade prototype will be ready for production and commercialization in the future through the research and development in phase II project, if granted.

This project was proposed to demonstrate a small scale solar cooking and heating system more applicable to home-scale, fast food or restaurant commercial use at low cost while considering the user’s safety. The phase I project is largely research oriented with the goal of illustrating and demonstrating such a possibility.

 

In the originally proposed design, a Fresnel lens is used to concentrate and cast sunlight on a receiving surface which is connected to a metal plate (serving as a hot plate), as shown in Fig. 1-1. The focus point of sunlight on the receiving surface is largely fixed. Except the small focus area the entire sunlight receiving surface and the hot plate can be thermally protected to prevent heat loss. Since heat loss is minimized and also the sunlight will not be interrupted by the person who cooks, it is expected that high safety and energy efficiency will be achieved in this system.

 

The hot surface as shown in Fig. 1 serves as an outdoor heating plate. When indoor heating is needed, optical fiber bundle was proposed to transmit concentrated sunlight into a house over a distance. The optical fiber bundle receives concentrated sunlight from one end, and at the other end the light comes out and cast on a target surface for heating.

 

 

The project is incorporated as a senior design project. Students in the team applied standard engineering design procedures in the project. This resulted in a list of the functional requirements and constraints defined in the design stage. As shown in Table 1-1, the team gave a considerable attention about how the project progress and how the developed prototype meet the requirement of benefiting people, planet, and the prosperity of a sustainable economy and environment.

 

In the design, the team applied a two-freedom control scheme for the system to track the sunlight. First, the entire lens can be controlled to rotate in zenith direction about a pivot, as shown in Fig. 1-2. The distance between the lens and the pivot is just one focal length of the lens. Sunlight focus point is therefore kept always at the same height on the sunlight receiving plate. Seating on a rotating tray (Lazy Suzan), the box that holds the lens rotating system and the sunlight-receiving plate can rotate in azimuth direction. The combined control in azimuth and zenith directions can maintain the lens always facing the sun and focuses sunlight always on a point area on the receiving plate. The adjustment for tracking the sun can be manually controlled every few minutes by referring to a sundial attached to the arms which hold the lens. A sundial is an in-house-made simple device to indicate if a surface is right facing the sun. This low-cost, single-person manual control is particularly suitable to customers in developing countries. For customers in developed countries, both active and passive automatic controls for sun tracking are available for installation to the system at extra cost. A solar panel is considered to provide the electricity for this control.

 

In the design stage, cost-effective and sustainability is always incorporated as an important principle. Less-welding and more standard connecting and fitting parts were planned to be used in the system. This is designated to give convenience to people in less developed area for them to install and assemble the solar stove.

 

Figure 1-2 Design-stage image of the system (A protection shield between focus point and stovetop is not shown in the image.)

 

Following what was proposed of having an indoor cooking/heating function, a low-cost optical fiber bundle was tested prior to the design and fabrication for the optical system. This was also designated to verify the capability of optical fiber bundle in transmitting concentrated sunlight. Unfortunately the temperature of the plastic optical fiber bundle at the end that receives concentrated sunlight rose fast and even started melting very quickly. Although some glass optical fibers may tolerate high temperature, the cost is hardly acceptable in a solar cooking device targeting at home-scale and fast-food, or restaurant application. Due to the pursuit of a sustainable and cost-effective system, the original proposed indoor cooking using optical fiber bundle was not adopted in the final design. However, the pursuit of indoor cooking using solar energy was not given up by the team, instead, the new design considered pumping heat transfer fluid to transport heat from outdoor stove to indoor stove. This resulted in the special features of the outdoor and indoor stoves, as shown in Fig. 1-3. Both the outdoor and indoor stoves are essentially designed as fluid chambers (walls made of aluminum) that heat transfer fluid (at temperatures as high as 300 oC) can flow through. Heat transfer fluid from outdoor stove can be pumped to flow through the indoor stove when needed, which transfers heat for the purpose of indoor use. A small pump powered by electricity from a solar panel was considered. The heat transfer fluid in the chamber also helps to increase the heat capacity of the stove, which can run for about 10 minutes longer at sufficient high temperature if sun is temporarily hidden.

 

 

Fig. 1-3 The design-stage image of outdoor and indoor stoves

 

Mobility of the solar stove for outdoor use is considered by mounting casters on the bottom of the plate which holds the whole system. Light-weight and reduced use of expensive material in the system is also an important principle. Except the bearing and control for the rotating of lens, all other parts are made of aluminum. Students in the team carried out majority of the fabrication work based on engineering design and drawings made by the team. The partly-assembled system is shown in Fig. 1-4, which is a picture taken during a preliminary test. In the picture a shield between the cooking surface and the sunlight receiving surface was not shown.

 

Fig. 1-4 Partly-assembled solar cooking

 

Experimental work was conducted to test the temperature of the hot surface of outdoor stove and thus estimate the temperature of the heat transfer fluid in both the outdoor and indoor stove chambers. The outdoor stovetop takes about 18 minutes to reach a temperature of 100 oC (212 F) after started. In the morning, every 6 to 10 minutes the lens needs to be adjusted to track the sunlight. From noontime to around 3:00 pm, the time interval between two adjustments of the lens can be as long as 15 minutes. After that, it will be 10 minutes between two adjustments. The adjustment of lens can be easily done by referring to the sundial attached to the arm of the lens assembly.

 

The team directly used the outdoor stovetop and baked shrimps and meat successfully. A saucepot was also put on top of the stove surface, and it boiled water and vegetables. The feeling of using the solar stove is quite the same as like using a regular gas stove. This is an indication that the solar stove receives sufficiency energy for heating. The temperature of the stovetop was also measured during preliminary test. The temperature could reach 168 oC (334 F) at one of the side walls of chamber most far from the focus point. After implementation of a better thermal protection and black painting of the light-receiving surface, the temperature is expected to be 20-60 oC higher, and that will be sufficiently high for even deep frying.

The conceptual design and experimental test was successfully carried out in this phase I project so far. Student team from multidisciplinary areas was formed for the project. The faculty advisor emphasized and distilled P3 concept to students in the project analysis, design, and fabrication process. A prototype solar stove was built and tested successfully. The solar cooking device has the feature of highly safe, reliable, cost-effective, and user-friendly. Experimental test was conducted and the proposed function of cooking food has been achieved using solar energy safely and efficiently. On the basis of the prototype from this current research and development, commercial grade prototype can be readily developed in the near future. Such a product will promote the use of solar energy for better living and environment protection, particularly in developing area where protection of forest and environment is urgent to the earth. This project will surely benefit people, planet, and the prosperity of human society. The team is really grateful to the financial support from EPA.

Thanks to the phase I grant, the team have successfully fabricated a prototype device which demonstrates the concept of a safe, reliable, low-cost, and sustainable solar cooking and heating for both outdoor and indoor application. The objective of the phase II project is to finalize and develop a commercial-grade prototype for production. There are three major tasks to fulfill this technology and to introduce to worldwide, in order to completely reach the goal of benefiting P3. The first task of the research is to improve and modify the prototype device and fulfill the development of a commercial-grade prototype. The second task is to introduce and showcase the technology to worldwide, particularly developing area, so as to get a large population as potential customers in the future. The third task is to study and investigate issues about scaling up for variety level of energy need and adding more functions required by different market for commercial products.

 

In the first 18 months of the two-year phase II project, the team will focus on modification of the system. The modification will include three major tasks: 1) to development a pan-type cooking surface, 2) to add the system with an automatic solar tracking function properly, 3) to integrate  the system with a heat storage device and provide high temperature heat to home-scale heating application as desired. In the last 10 months of the phase II project, issues of scaling-up and adding multiple functions will be fully studied. Production cost estimation and documentation for commercial production will also be completed.

 

The newly developed cooking surface in phase II will be able to serve as a pan that customer can directly use it for cooking and frying. The hot surface on the stove may also be in a pan-style shape so that pans and saucepots can fit in for better heat conduction. Depending on the market and application (in developing or developed areas), it may be manual controlled, or automatic controlled for sunlight tracking in the solar stove. The current prototype can be easily integrated with control devices to have automatic control function, or stand as it is in manual control. Two micro stepmotors may be installed and controlled by a micro controller. The micro controller can be programmed according to the location of the system on the earth, and can control the azimuth angle of the box and the zenith angles of the lens at any time. Adding a heat storage capacity to the system has the advantage to supply heat to customers at any desired time. According to a preliminary study by the research team, heat storage at the capacity of 20 (kW hr) will not increase the cost of the system significantly, if low cost heat storage material, such like sand, is used. For restaurant and commercial use, both the size and number of lens and heat storage capacity need to be scaled up.

 

The research team will actively showcase the project and the prototype by participating in renewable energy shows and Expos in the US and worldwide. Particularly the team will give people from developing countries better access to this affordable renewable energy technology.