Modernizing the Physics Curriculum by Being Less Modern

Philip Gleckman

Introduction

I was invited to the 2010 APS March Meeting to offer suggestions from an industrial point of view on the development of new academic programs in renewable energy. As APS is a physics society, I took it as an opportunity to place the program we already have - the undergraduate physics curriculum—under the spotlight. The standard physics curriculum begins with an introduction to two pillars, mechanics and electricity & magnetism, and then, following an intermission when these are combined in vibrations & waves, veers off into quantum mechanics where it stays for the next two terms. Most, if not all, of the students’ advanced laboratory consists of repeating famous experiments in modern physics such as Rutherford and Compton Scattering. Is there anything wrong with that? Yes. The critically important subjects of classical thermodynamics, heat transfer, and fluid mechanics ("THF" for short) have been hijacked by the Department of Mechanical Engineering. THF are the sciences that underlie not only renewable energy but also conventional electrical production as I will show through examples.

Energy’s past and future

Most people have no idea that 90% of the electricity in the United States comes from steam. It is a triumph of elementary thermodynamics that we can calculate the efficiency of this process, known as the Rankine Cycle, just by applying the First and Second Laws of Thermodynamics and using steam tables for entropy and enthalpy. In the first step of the idealized four-step process, water is pumped adiabatically to a state of higher pressure and temperature represented by an increase in enthalpy. In the second step, heat generated by burning coal (or through a nuclear reaction) is added to convert the water to superheated steam at constant pressure resulting in a further enthalpy increase. The steam expands through a turbine in the third, work-producing step. The final step that closes the cycle uses a cooling tower to condense the steam back into water. Cycle efficiency is calculated by writing the laws of thermodynamics for the control volumes in each step and solving for the net work [1].

Solar thermal plants are being built in the desert for utility-scale electric power (above 100 MW). These new Rankine Cycle plants replace the heat generated by burning fossil fuel with renewable concentrated sunlight and do not exhaust any greenhouse gases. SEGS1, the first solar thermal plant, was built in the Mojave Desert in 1984. Therminol oil is heated inside vacuum-insulated steel tubes at the focal line of parabolic mirrors and the heat is transferred to steam in a heat exchanger. By expressing the enthalpy increase in terms of a change in fluid temperature, the First Law can easily be used to determine the length of the oil piping needed to reach the desired outlet oil temperature of 400°C for a given flux, flow rate, and tube diameter. For example, in SEGS1 where the flow rate is about 400 lpm, if you assume a flux of 70 suns on a 70 mm-diameter tube, you can show that the tube must be 1/3 of a kilometer long! What if you wanted to know the outlet steel temperature to make sure it does not overheat? You cannot calculate that from first principles! The problem is that the flow is turbulent. Newton’s Law of Cooling tells us that the known heat flux q into the fluid is related to the difference between the absorber and fluid temperatures by q = hDT where h is the heat transfer coefficient. All the physics of convection is captured in the non-dimensional heat transfer characteristic known as the Nusselt number which in turn is a function of the non-dimensional Reynolds and Prandtl numbers as well as the friction factor. The idea of using completely empirical correlations is foreign to most physics students.

The Rankine Cycle does have its shortcomings for solar power: water is obviously not abundant in the desert where the sun is bright, and steam plant equipment is expensive to build and maintain. There have been ongoing research efforts to instead use air as the working fluid in a Brayton Cycle. Rather than parabolic trough mirrors, an array of heliostats produces much higher fluxes (hundreds of suns) into a receiver atop a tower. Air flows through tubes in the receiver and is heated by convection to meet the conditions of gas turbines. The challenge of designing an air receiver can be appreciated by considering that the heat transfer coefficient is typically 1/3 that of oil while the flux can be an order of magnitude higher.

My final example comes from the wind power industry. A director of a national lab has said that the "technologies needed for wind power aren’t rocket science." The engineers at a company called FloDesign may disagree with this characterization since they have used an important part of rocket science - aerodynamics—to achieve a breakthrough in the efficiency of wind turbines through the use of a shrouded rotor. Interestingly, undergraduate-level fluid mechanics is all that is necessary to derive the maximum possible efficiency (59%) of a conventional wind turbine. It is only necessary to express familiar mechanical laws such as energy and momentum conservation in unfamiliar forms for fluids passing through control volumes [2].

Recommendations

While the preceding examples highlight the central role of THF in energy, these classical disciplines are also critical to a scientific understanding of many natural phenomena that people experience. Physics students should be encouraged to take courses in these areas if they are offered in the mechanical engineering curriculum, and at liberal arts schools these subjects should be included in the physics curriculum. Heat transfer cannot be understood without fluid mechanics, so fluids should be taught first. Fluid mechanics can be used to introduce vector fields and can serve as a conceptual bridge between mechanics and E&M. New laboratory experiments should be designed so that students can experience the science of fluids and heat.

A physics graduate student approached me after my talk and told me that while he agreed that these were important subjects he had learned them well as an undergraduate. Impressed, I asked him where he studied. "China," he replied.

References

[1]For example see R. Sonntag and C. Borgnakke, "Fundamentals of Engineering Thermodynamics," 2nd ed. (Wiley, Hoboken NJ, 2007).
[2] M. Hansen, "Aerodynamics of Wind Turbines," 2nd ed. (Earthscan, London UK, 2008).

Philip Gleckman received his Ph.D. in physics from the University of Chicago and his B.S. in physics from M.I.T. He is the Chief Scientist at eSolar, Inc.

Disclaimer - The articles and opinion pieces found in this issue of the APS Forum on Education Newsletter are not peer refereed and represent solely the views of the authors and not necessarily the views of APS.