Concentrated Solar Cooking and Heating System

The second law of thermodynamics tells that energy, in many different modes, has quality, which measures the usefulness of the energy. It is easy to understand that electrical power is the energy of the highest quality, since electricity is often generated by concentrating larger supplies. Using electricity for heating is a poor and inefficient use of dense energy since it is used in an inefficient way, generally dissipating largely unused to the environment. Using solar energy for heating, however, is a highly economic use of energy. Solar radiation travels to the Earth as heat and may be directly used for heating purposes in extremely efficient energy life-cycles.

Although solar heating has long been recognized as a cheap, effective method, there are not many well-studied and designed products available to the public. One typical example is the traditional solar cooker or solar oven. To cook in front of a reflective solar collector, people are often subject to strong and concentrated sunlight, causing considerable safety issues involving both burns and blinding. Furthermore, traditional solar cookers do not allow for much control of surface temperatures or sun location tracking. Finally, parabolic and box solar cookers can only be used outside due to their inability to store and transfer the heat indoors.

A student team was assembled according to the system developed by the instructional staff for the University of Arizona's engineering senior design course. The team consisted of one optical and five undergraduate mechanical engineering students. In addition to the design and development tasks required for the P3 timeline, the course demanded several presentations, reports, design processes, and other minor deliverables. The fall semester was dedicated to design considerations while spring was reserved for prototyping and testing.

It is expected that a safe and versatile solar cooking and heating device which remedies these issues will garner significant interest worldwide. A user-friendly, safe, controllable system could have many potential applications. The opportunities of marketing such a sustainable technology are enormous both inside and outside of the US. Phase I of this project was to develop a method of cooking and heating using only the sun, in order to improve upon common solar cooking technology.

The research is in early stages and only one prototype has been completed. The prototype is a proof of concept for both outdoor and indoor applications.

Figure 1 - Thermal loop schematic

The system principles are relatively simple to understand. Incident sunlight is concentrated by a large Fresnel lens by a factor of about 750. This focal point is small and stationary throughout the day. A fixed focal point allows for a small collection area to maximize efficiency. This is achieved by dual-axis, manual sun tracking about the focal point.

The beam of light is of very high intensity and can be dangerous to users. However, the system can be made very safe by isolating the concentrated light with a curtain or shield.

The focal point heats a small, blackened collection surface adjacent to the outdoor cooking surface. The surface is part of a chamber filled with mineral oil, the working fluid. For outdoor cooking purposes, no mass flow is enforced, keeping the heat in the chamber. The oil circulates passively, transferring the heat to the cooking surface evenly. Alternatively, for indoor use, the oil is pumped through a piping loop using an internal gear pump. The oil is transported 6 feet to an indoor heat exchanger embedded in a secondary cooking surface, representing the potential of indoor application. The pump is powered by a DC motor, but could be easily powered with photovoltaic conversion.

Figure 2 - EPA P3 conference and testing

Due to time constraints of the students, only several tests were conducted, but the results are promising. The outdoor surface reached 290°C, as hot as a kitchen stove. At this point the oil reached its boiling point and a load had to be added so that it did not boil and break down. It is expected that the outdoor surface temperature could go higher if oil were not present in the chamber, restricting the maximum temperature.

At 140 minutes flow through the loop was initiated to test the indoor performance. With throttling of the flow rate the indoor surface temperature reached 150°C.

The team was able to pan-fry potatoes and boil broccoli on the outdoor surface. The indoor surface got hot enough to cook hot dogs to a safe temperature (170°F) directly on the metal. Insulating covers to enclose both the cookware and cooking surface could help trap heat. A simple glass cover could suffice. These could be used to create a baking environment.

Figure 3 - Full performance curve

The first full testing yielded results that could be used to calculate both the indoor and outdoor efficiencies. Unfortunately, testing was performed on a windy and partly cloudy day. As such, it is difficult to determine the solar radiation flux without proper equipment. Also, the free convection was enhanced by the wind. It was estimated that the solar radiation flux on this day was 600 W.

Using heat transfer methods, the power output at the indoor surface was calculated to be 106 W. This results in an estimated efficiency of 18%. The power output from the outdoor surface testing was calculated as 465-500 W. This results in a range of estimated efficiencies of 77-83%. This efficiency range matches intuition. For indoor cooking to be viable, the indoor efficiency must reach this higher range.

Time will be needed for development into a marketable, fully-implementable product. Already, several organizations, both commercial and academic, have expressed interest in collaboration and a provisional patent has been filed.

The ideas presented here could be applied in many ways. Especially intriguing are commercial cooking operations, outdoor models, and small, residential scale solar thermal energy applications. Modular systems of multiple lenses could provide heat for a large cooking operation by heating large griddles or cooking oil. Outdoor models could be applied in developing countries, roadside vendor operations, and backyard cooking. If cost effective, such stoves could reduce the need for cutting of trees for fuel in rural regions where electricity and gas are not available.

Residential scale solar thermal systems could provide heat for cooking, air conditioning, or water heating. In concert with a small thermal energy storage unit, the system could provide usable heat for nighttime use. Along with automatic tracking, the unit could be installed on rooftops for full sun exposure and aesthetics. Piping could be installed within walls by professional contractors just as water pipes are. The system could run all day while the owners are at work, providing heat when they need it.

An economic study has not been done, but it is projected that the cost would be about $400 for an outdoor unit and about $2000 for an indoor unit. The cost of an indoor unit could be included in home construction.

The prototype is not ideal, as is the case of most first-generation models. Shortcomings include excessive weight and indoor surface temperature outputs. Redesign processes should take some considerations into account.

Different outdoor and indoor surface designs could be numerically determined, fabricated, and tested. In this way, volumes, channel design, collection surface geometry, and materials could be optimized. One issue that needs work is the transfer of heat from the collection surface to the oil. No computational modeling of the heating chamber was performed, so there is much room for improvement. By optimizing the chamber geometries, the oil flowing through can be heated more efficiently. Fins within the chamber could be part of this solution.

The efficiency loss due to the length of piping could also be minimized. As the oil flows through the piping to and from the secondary surface, heat is lost. Insulation choices were not optimal and much of the insulation is just taped on. Perhaps using the same type of insulation used in solar thermal power plants could be beneficial.

Perhaps, if more energy supply is needed, a larger lens or multiple lenses could be implemented. An aperture control of the lens could help regulate the energy input for enhanced temperature control. A control system could be implemented to regulate the temperatures based on desired user temperature inputs.

Continuous, automatic control using small, solar powered motors can enhance the system by eliminating the need for manual tracking. A control system using a light sensor or sun-angle database could accomplish this.

The global demand for energy has led the world to a momentous time in history, yet an opportunity of unprecedented magnitude exists. New, sustainable energy technologies must be integrated into the current system, and this project could be one of those many important possibilities. Technologies such as this concentrated solar cooking system are needed for the continued prosperity and health of the planet and its peoples.