![]() The promise of high conversion efficiency, device simplicity, and robust operation continues to push research and technology development at the cutting edge. The direct conversion of heat to electricity without any intermediate steps or moving parts remains one of the most promising, yet challenging, methods of power production. Recent developments as well as persisting research needs in materials, device design, fundamental understanding, and testing and validation are discussed. This perspective article provides an assessment of the potential of TEC systems for space and terrestrial applications in the twenty-first century, overviewing recent advances in the field and identifying key research challenges. Although the physical mechanism has been known for over a century, it has yet to be consistently realized in a manner practical for large-scale deployment. Thermionic energy conversion (TEC) is the direct conversion of heat into electricity by the mechanism of thermionic emission, the spontaneous ejection of hot electrons from a surface. 6Department of Physics, Arizona State University, Tempe, AZ, United States.5Department of Electrical and Computer Engineering, Quantum Matter Institute, University of British Columbia, Vancouver, BC, Canada.4Max Planck Institute for Solid State Research, Stuttgart, Germany.3Johnson Space Center, National Aeronautics and Space Administration (NASA), Houston, TX, United States.2Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, IN, United States.1Department of Aerospace and Mechanical Engineering, University of Notre Dame, Notre Dame, IN, United States.Haase 1,2 Jeffrey George 3 Jochen Mannhart 4 Robin Wanke 4 Alireza Nojeh 5 Robert Nemanich 6 Order and disorder in biological systems.John R. In animal systems there are also small structures within the cells called mitochondria which use the energy stored in sugar molecules from food to form more highly ordered structures. For animals, this glycogen storage is primarily in the muscles. Used by plants and animals for energy storage. They can be metabolized by humans and other animals for energy.Įven more highly branched chains of glucose, but similar to starches. They are produced by plants and serve as energy resources for the plants. More highly branched chains of glucose molecules. They help maintain the plant structure - the wood of trees is primarily cellulose. Long chains of glucose molecules which are fairly linear. ![]() Some of the common carbohydrates are: Cellulose These carbohydrates may be simple sugars like glucose or complex combinations of sugars. Some of the energy is used for the synthesis of carbohydrates. The process of photosynthesis in plants stores energy in the plants which can be used for accomplishing work. Having a leaf surface area of several hundred square meters, it is capable of making some 2 tons of sugar. ![]() It may have 500 pounds of green leaves employed in the process of photosynthesis. ![]() Using chlorophyll in the process called photosynthesis, they convert the sun's energy into storable form in ordered sugar molecules.Īs an example of the scope of this process, consider a mature maple tree. The leaves use the energy from the sun in tiny energy factories called chloroplasts. In addition to making the sugars, the plants also release oxygen which is essential for animal life. The radiant energy from the Sun gets transferred to the bond energies of the carbons and the other atoms in the glucose molecule. From this disordered beginning, it produces the highly ordered and highly constrained sugar molecules, like glucose. The tree takes in carbon dioxide from the air, water from the earth as well as a small amount from water vapor in the air. The building materials are in a highly disordered state - gases, liquids and vapors. The raw materials for the nutrients for life on the Earth are just carbon dioxide and water! The mechanisms of advanced plant and animal life add nitrogen, phosphorus and sulfur with most of life's operational mechanisms accomplished with the elements summarized in the mnemonic CHONPS. Order can be produced with an expenditure of energy, and the order associated with life on the earth is produced with the aid of energy from the sun. If so, how do biological systems develop and maintain such a high degree of order? Is this a violation of the second law of thermodynamics? The concept of entropy and the second law of thermodynamics suggests that systems naturally progress from order to disorder. Decrease of Entropy in Biological Systems A tree converts disorder to order with a little help from the Sun ![]()
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