Experiments

#5 SOLAR PHOTOVOLTAIC (SPV) ENERGY AND APPLICATIONS

Michael Power and Gil Rodgers, Contributions by Bob West and Andrew Pettifor, June 2024

The elements for the PV Experiment are as follows: 

• Provide a U.S. and International energy context for why there are efforts to utilize renewable and sustainable energy technologies, such as PV Collector Systems. 

• Demonstrate available PV charging modules that utilize standard polycrystalline silicone cells. 

• Provide a brief presentation of the research and development efforts in PV technology that have occurred since the Energy Crisis of the 1970s up through the present time. In particular, use the data from the National Renewable Energy Research Laboratory (“NREL”) to identify promising new technologies (e.g., lighter in weight, higher in conversion of light into electricity, more flexible, etc.). 

• Demonstrate the ability of self-contained PV charger modules (with Lithium-ion batteries) to operate a remote-controlled dump truck. 

• Demonstrate the ability of available “thin film” PV collectors to generate an electric current. 

• Illustrate some of the existing and planned transportation vehicles that embody the following possibilities for vehicle propulsion: Internal Combustion Engines (“ICE”); Hybrid Drive Systems; Electric Vehicles (“EVs”) that require external charging; EVs that have PV collectors built into the vehicle. 

Background

The ability for some materials to absorb “light” (i.e., electromagnetic radiation, some of which is within the bands that are visible to the human eye) and to generate a flow of electrons of sufficient quantity that a measurable electric current can be produced has been known for almost a century. The ability to tap such PV capabilities in practical and affordable ways only started to be of commercial interest following the energy price and availability shocks of the early 1970’s; specifically, the so-called “Arab Oil Embargo” in the winter of 1973-1974, which caused massive problems on a worldwide basis. The traditional market basis for allocating supplies of petroleum products became unworkable in the face of huge supply disruptions, and the consequent major shortages of gasoline, and other products. The Federal Government had to directly regulate the creation and allocation of petroleum products. No one who experienced this crisis would ever wish to repeat it. To the contrary, a national energy strategy that fostered “Energy Independence” for the United States was embraced in part through the creation of a new U.S. Department of Energy; however, the specific ways that such a broad concept should be realized, including the proper roles for governmental agencies and for the private sector, have been hotly debated in the ensuing decades. The emphasis has been on promoting research and development across a broad range of potential technologies, and on using commercial firms to commercialize the technology through products that embody it in practical ways. 

In addition to the U.S. concept of “Energy Independence,” in more recent decades there has been a growing worldwide awareness of the dangers posed from “Climate Change,” in part by the emissions of carbon dioxide and other gases, from the combustion of fossil fuels, as well as from leakages of fossil fuels; in particular, of natural gas (“methane”) in the production, transmission, and storage of such fuels. The United Nations (“UN”) has undertaken a sustained program to highlight the worldwide sources of carbon emissions and to coordinate meaningful responses to the threats from Climate Change. In particular, the UN created a Framework Convention for Climate Change (“UNFCC”) that resulted in The Paris Agreement of 2015 [The Paris Agreement | UNFCCC]. Even under this treaty, the UN has no authority to force the national governments to participate, especially to provide accurate data, nor can it force any national government to follow up on their stated plans to curb their emissions. In this context, there has been growing evidence that the effects from Climate Change are real and potentially dire. Thus, any new technologies that can help to reduce carbon gas emissions have added benefits for humanity on a global basis. 

There have been decades of research and development on photovoltaic possibilities, and there have been commitments made to the manufacturing of selected PV products. In particular, the Chinese government has made a massive commitment in manufacturing “single crystal” and “polycrystalline” PV collectors. These PV collector products currently dominate the worldwide marketplaces where solar energy is being utilized for rooftop arrays of PV collectors for the production of electricity. An emerging marketplace for PV collector products, based upon new collector technology, is in the use of widely distributed individual PV collectors that can provide electric power to charge cell phones and other portable electronic devices, and to supplement external chargers for Electric Vehicles (“EVs”). In particular, the possibility exists for “thin film” PV materials to coat the external surfaces for EVs such that there is a greatly reduced need for external charging of EVs.

Summary of Student Experiment 

This demonstration and experiment by the Minerva Action Group (“MAG”) shows students how to use several hand-held PV charging collectors, which were manufactured in China, and are currently available at very reasonable prices, to charge up the students’ cell phones. MAG will also demonstrate one new “thin film” PV collector that in principle could be integrated into a new type of much lighter, and more efficient portable PV charging product. 

Class Experiment Steps: Introduce Minerva Action Group. Give “big picture” explanation of the experiment and what is the main message trying to be communicated. Why is this important? 

1. Have each team go near windows facing sun and experiment with PV solar chargers to recharge student cell phones and I-Pads. Students will need to bring their cell phones or other mobile devices and charging wire typically used at home or in car. [Instructor, Gil, and Bob] 

2. Illustrate power by connecting photocells to the small electronic music application. Play music. [Gil] 

3. Explain a little about the difference between standard dry cells or alkaline batteries used in flashlights and rechargeable batteries (lithium-ion) used in your cell phone or computer. [Gil] 

4. Demonstrate the little red toy dump trump using 6, 1.5 V (DC) AA batteries as the standard out-of-the-box method. Let students play with remote controller for a little while.to steer trucks around. 

5. Introduce thin-film solar cells and connect to little green LED light application to illustrate how this emerging technology may be a thing of the future but isn’t powerful enough to run the truck. [Gil] 

6. Place the thin-film solar cell on truck roof in bright sunny locations and measure the voltage (about 6 - 7 volts DC and power output maximum of about 1000 milliwatts (<1 watt DC.) So, this would be far below what is needed to run the truck. But discuss this as a possible future technology for vehicles on the road with much larger thin film sheets and storage batteries. 

7. Demonstrate operation of truck with the solar cell and battery. Say a little (not too much) about solar PV collector and Buck Booster DC - DC transformer connected directly to truck’s electrical system. Explain how this relates to vehicles on the road and why this is important. [Gil] 

8. Explain that it is necessary to have enough power (watts), correct voltage (volts) and sufficient current (amperes or “amps”) for the truck to operate and this is why we have the Buck-Booster transformer voltage controller. Explain using very basic equation in electrical engineering physics: [Gil]Give each team member the opportunity to drive and steer trucks around the floor in the classroom. [Students, Gil provide any assistance if needed.] 

9. If it’s a sunny day take the class outside to drive trucks around on a smooth surface or even on the grass. 

10. Return inside and discuss cars and trucks now entering the marketplace utilizing latest solar PV technologies (see handout.) [Mike] 

11. If there is time show this 20-minute video of Dutch Lightyear Solar Car. https://www.youtube.com/watch?v=lM6BHvgvrVc 

12. Explain the technological advances in solar cells using updated NREL graph. [Mike]. 

13. Summarize the big “take-aways”? Record on classroom whiteboard and let teams voice their importance and discussion of why? [All] 

Technical note: Power = Voltage x Current. [ P = E x I] Power (P) has to be adequate for running the motors driving the wheels of the truck (≈ 30 watts DC). These must all be Direct Current (DC) for our truck motors. Explain difference between Direct Current – DC and Alternating Current – AC. Similarly, voltage (E) must match the input requirements of the motors (≈ 9 volts - DC). Output from the solar collectors is too low (6-7 volts) and therefore needs to be “boosted” up to 9 volts by the DC to DC “Buck-Booster/Voltage Regulator.” There also must be adequate electrical current supply or flow of electrons (I) (> 4 amperes DC) to provide enough power. Use analogy of a stream of water (molecules) running down a slope (potential energy.) And Energy is work delivered over a period of time when the motors and lights are actually being operated (measured in Watt-hours or Kilowatt hours.)

Here is a simple explanation: https://www.packetpower.com/blog/electricity-basics-for-data-centers 

See video of Dana Hall students’ experiment here: https://youtu.be/HlwHXp8a87w

Graphical Summary of PV Cell Efficiencies [Mike] 

The National Renewable Energy Laboratory (NREL) is one of the 17 laboratories under the US Department of Energy (DOE). NREL has done extensive research on renewable energy and specifically the efficiencies of PV solar technologies. The following link has a summary of the 17 laboratories for additional information: https://www.energy.gov/podcasts/direct-current-energygov-podcast/s2-e4-17-labs-17-minutes . The picture below graphically portrays the evolution of numerous emerging PV collector technologies; it is plotted in years [1975 through 2023] on the horizontal axis and by Cell Efficiency (%) on the vertical axis. This information and demonstration experience hopefully stimulates students to investigate this technology, and to learn from one another and from their teachers in this important emerging field of knowledge. 

Particularly interesting is the rapid and promising increases in efficiency of thin film, organic, and multijunction solar technologies (red, yellow, and purple lines on the right-side of the graph.) 

Explain: Efficiency = electricity output divided by sun energy input (photos) [Mike] 

This link gives an interactive summary of the NREL research and upward progress being made in cell efficiencies. https://www.nrel.gov/pv/cell-efficiency.html 

Electrical Feasibility of Thin Film Solar Power Vehicles (Extra Credit) 

In spite of the VW Buzz described in the appendix with its solar paneled roof, there is reason there are no GM or Tesla solar powered cars or SUVs in the showrooms. That reason is inadequate power density. Also, (until cost competitive long lived thin film panels become widely available) the weight of the solar panels is a major problem. Please see the following calculations. 

The following specs for home installation on solar panel units are from the web: 

Size: 26 - 40 inches. = 2/3 x 1 m = 0.67 sq m. 

Power: 100 watts (18v (max) @ 5.56 a), 12 volts. 

Weight: 22 lbs. 

Assuming we would like a 250 hp car (approx. 190 kw), this require 1900 panels with a total area of 1330 sq m. (Or an area of 33 m x 40 m or 100 ft. x 125 ft. and a weight of about 40,000 lbs.! - no wonder the Stanford racing team mentioned below went with custom thin film panels. (They can lift their race car body with four people). 

Clearly these estimates are based on commercial solar panels designed for ruggedness (est. life 20 years) and not pushing the power/area limits very hard, but we are looking for a two orders of magnitude improvement for a self-sustaining solar car on a sunny day. Batteries are going to be part of the solar car deal for a long time! But let’s consider some mitigating circumstances. 

250 hp is nice for bragging rights and freeway entrance ramps on the Merritt Parkway, but for most suburban driving you seldom use more than 50 or 60. That is 20% of what we were calculating, and all the numbers are reduced accordingly. In particular, the area of panels is reduced to 266 sq m, or an area roughly 9 m x 30 m or 30 ft. x 100 ft. - we’re almost in tractor-trailer territory! In other words, a single order of magnitude improvement may enable a game changer down the road (no pun intended). 

Let’s take a different approach: assume that the solar panels described above have 20% efficiency. That is, they convert 20% of the incident sunlight into electrical energy. This is quite a respectable number but as you probably noted from the NREL graph earlier, the last several years have seen significant improvements in PV efficiencies. Most of these data points are from tiny inch square laboratory prototypes, but they blaze the path for commercial products in the future, so that we can envision 50% commercial efficiencies soon. If we imagine 100% efficient panels (the best possible case), what does that do for our calculations of the feasibility of a solar powered car? 

Increasing solar cell efficiency to 100% gives us another half order of magnitude (5x) gain. So, a 50 hp delivery van with these top-of-the-line cells could generate full power from a 50 sq m panel, or 9 ft. x 50 ft, which is only about twice the size of a useful vehicle! Note that the VW Buzz solar roof, which takes up most of the space available, is reported to have a maximum output of 600 watts, which sounds impressive until you convert it and realize that it is less than one horsepower! 

So, the bottom line is that solar cells will be able to generate a useful range extender for an electric vehicle, and power any number of on-board gadgets, but they won’t be able to replace a battery pack no matter how good they get. But if you use it to commute 10 miles a day and leave it in the sun all day, you may not have to plug it in for recharging when you get home for a long time. 

Issues for Discussion and Further Study (If time allows) 

This experiment is nominally about Photovoltaic Collector (“PV”) technologies and how advances in them may affect the ways that our current cell phones, computers, and future portable devices are utilized. In fact, the potential advances in PV technologies will likely affect many aspects of our lives, including the ways that electric power is generated, distributed, and stored; the types of electric vehicles that will comprise our future transportation systems; how buildings and neighborhoods are heated, cooled, and provided with electric power, and the future of our communications networks. In particular, this experiment has highlighted the essential role for some kind of energy storage component (i.e., “battery”) for any realistic ways to utilize solar collectors in transportation applications for the near future. A detailed discussion of existing and promising battery technologies is beyond the scope of this experiment; indeed, it may well be the subject for a future experiment just by itself. The current EV batteries are largely comprised of Lithium (Li) ion technologies using liquid electrolytes. A promising area for enhancing battery performance, including increased energy storage density, safety, faster recharging, and longer lifetimes, is through the use of solid-state electrolytes. 

With this in mind, the following class discussion issues are suggested: 

1. If roofing tiles could be coated with thin film PV materials, how might home energy demand and reliance on the power grid be changed? Tesla currently offers three products: a) thin film coated roof tiles, b) home size storage batteries, and c) home size inverters (to convert DC to AC.) What are your observations from studying the NREL solar PV graph on the future of solar cells and SPVs? 

2. Since inexpensive, portable PV charging devices (from China) are readily available, what should we anticipate on advances in the future technologies for PV? In other words, what features or improvements would make these charging devices even more useful? 

3. How could PV technologies using “thin film materials” alter the way PV collectors are designed into Electric Vehicles (“EVs”)? 

4. How could the availability of thin film PVs, painted onto the surfaces of cars, trucks, or rail vehicles change the needs for charging stations? 

5. How would the electricity grid be affected? 

6. What are the appropriate roles for the Federal, State, and Local governments, universities, and for private (commercial) firms in the pursuit of new, highly efficient PV technologies?  

7. What would be the significance of new high energy density storage batteries be for the design and use of future EVs?