A recent NPR article covered the link between energy independence and national security. A group of retired generals has pointed out that ‘the way the United States uses energy is jeopardizing national security’ and is ‘challenging the U.S. government and all Americans to reduce energy consumption and look for alternative energy sources.’ In fact, the Pentagon is working to move to more fuel-efficient vehicles for both financial and geopolitical reasons.
In Thomas Friedman’s book, Hot Flat and Crowded, the links between our current energy system, national security, and human rights are discussed at length. Friedman describes a correlation he has observed between decreasing oil prices and improvements in human rights and innovation in oil-rich countries. In essence, he says that when governments in oil-rich nations can derive wealth from selling natural resources, then they don’t have to rely so much on innovation and real value creation. In contrast, when nations run out of oil (or if oil prices were to fall), they need to be more supportive of human rights and creativity to survive. He describes several examples that support this hypothesis, and makes a compelling argument that one of the most effective things Americans can do to improve our own nations security, as well as improve the lives of many others around the world, is to dramatically reduce demand for oil. So even if someone doesn’t support renewable energy development and energy efficiency improvements for environmental reasons (which are in the long term also economic and political reasons as well), then perhaps he or she would support these advancements because of their impact on national security and human rights.
If we can use human power for transportation (e.g., bicycles), what about for electric power generation? Many people have wondered this. When you look at the numbers, it may not make economic sense, and would have very small direct impact on reducing fossil fuel use. For example, an elite cyclist might be able to output about 500 Watts of power continuously (enough to power five 100 W light bulbs). Most of the rest of us could manage only some fraction of that power output (maybe 10%-20% of that). Even at 500 W, and assuming 100% efficiency electricity generation, an elite cyclist could produce only a half kilowatt hour (kWh) of energy per hour, which is worth at most about 10 cents in the U.S.
While the economic incentive is negligible, there are other motivations for human-powered generators that go beyond financial and direct environmental considerations. At the University of Oregon, exercise equipment has been fitted with electric generators. Steve Mital, the University’s sustainability director, explained that while the costs of upgrading the equipment won’t be recouped for 28 years, it does have educational benefit. He goes on to say that “so much of this talk about renewables is fairly abstract. You jump on one of these machines and 30 minutes later you have a deep visceral understanding of what that means.” This effect is definitely valuable, and may have a larger indirect impact on energy consumption. If facilities like this become more universal we will have many more individuals with a deep appreciation for what a kWh really is and how much work is required to produce it. We may not even need to harvest the electricity to get the educational effect: just add sensors and displays to communicate how much energy has been produced. This would cost less than installing working generators (but may not be as satisfying).
What do you think of human-powered generators in gyms (or at rock concerts)? Should we invest in this as an educational campaign?
Researchers at Purdue University and Sandia National Labs have developed a new way of monitoring forces on the blades of wind turbines. In essence, they are collecting real-time information about forces on the turbine blades that will help control the wind turbine to improve power production, as well as help design turbines so that they are more durable and efficient. This is a great example of how better design can improve renewable energy production.
One thing they can do with real-time force data is adjust the blade pitch (how much the blades are angled into the wind) and the generator in the most efficient way for current wind conditions. This can make a big difference in power production. One reason is that electric generators vary in efficiency depending on how fast and how hard the input shaft is turning (torque is a measure of how hard something is being twisted). This is an idea similar to what I described in this post about series hybrid powertrains; gasoline engines vary in efficiency depending on how hard and how fast the output shaft is turning, and this efficiency can be described by an efficiency ‘map‘. An efficiency map is efficiency data that allows us to map torque and speed values to efficiency values. The dependence of generator efficiency on input shaft torque and speed can also be represented using an efficiency map. The drawing below illustrates one way of visualizing a sample generator efficiency map. It is a contour plot, sort of like a topographic map. In a topographic map contour lines give you an idea of elevation changes. In the drawing below the contour lines describe how efficient a generator is depending on the torque and speed of the input shaft driving the generator.
We can use electronic controls to help influence where we are on the generator efficiency map. Another way of controlling where we are is to adjust the turbine blade pitch. There is a tradeoff between torque and speed when controling pitch: pitching them into the wind can help generate more torque, but at lower speed, while pitching them so the flat surface faces the wind more directly will increase speed but will reduce torque. The drawing below helps illustrate this idea.
This tradeoff can be explained using what’s called conservation of energy, or the first law of thermodynamics. It basically means that the amount of energy in a closed system stays constant; energy can change forms, but cannot be created or destroyed. There is energy in wind, and it’s the wind turbines job to extract as much of that energy as possible and turn it into electrical energy. Power is the rate at which energy is used (Power = Energy/Time), and we can quantify the power going into the generator through the input shaft using a simple model:
Power = Torque x Speed
The power reaching the input shaft does change some depending in the efficiency of the turbine blades, but if for the moment we assume the input shaft power is constant, it helps us understand the tradeoff between torque and speed using conservation of energy. If power is constant, and referring to the equation above, speed must go down if torque goes up; and torque must go down if speed goes up. This explains conceptually why this tradeoff between torque and speed exists when adjusting blade pitch; we cannot increase torque and speed simultaneously without increasing the power coming into the system. For this to happen, either the wind speed needs to go up, or the turbine needs to do a better job of extracting power from the wind (this is where better design and control come in).
We can also draw an efficiency map for a turbine that describes how efficienctly the turbine converts energy from the wind into the mechanical energy of the spinning input shaft going into the turbine. A turbine actuallly has the reverse relationship with torque and speed as the generator does: low-torque/low-speed operation leads to high efficiency turbine blade operation, but low efficiency generator operation. This tradeoff must be managed carefully when designing a wind turbine and figuring out how to control the generator and blade pitch. We can use optimization techniques to investigate these tradeoffs and determine the most efficient design and best way to control pitch and generator for every set of conditions in order to maximize power production for the turbine.
In recent posts I’ve discussed modeling in engineering design. How do you think modeling might help people working on wind turbine design? What larger impacts could better turbine design have?
I just had to share with you a great article from CNN: David MacKay, physics professor from Cambridge University, is urging us to actually look at the numbers when it comes to conserving energy. He explains that ’small actions will not deliver a solution’ to our energy problems, and that‘our failure to talk straight about the numbers is allowing people to persist in wishful thinking, inspired by inane sayings such as “every little bit helps.” ‘. As I discussed in Feeling Green, doing small things to conserve may help us feel better, and are important to pay some attention to, but in many cases have imperceptible impact. We certainly should do everything we can, but we need to keep things in perspective. If we actually look at the numbers, and get a feel for what a kilowatt-hour is, then we can think more critically and focus our efforts on activities that will make a big difference. For example, we need new energy production, energy distribution, transportation, and agricultural systems. Small actions are not going to remake these systems. We need to come together as a society and voice our desire to make these transitions happen. Important policy changes are being debated right now, and we need to urge our representatives to think long-term and instigate policies that will bring real change. We can amplify our voice by talking about these issues with others and sharing our insightful enthusiasm for the solutions. Perhaps we can inspire others to speak up as well.
Looking at the numbers behind issues allows us to make objective comparisons and leads us to more solid decisions. This is part of why I devote a lot of my writing to quantitative modeling and design issues. I try to show how look at things quantitatively (without diving into crazy math). I’m pretty much keeping it to simple algebra and arithmetic. My purpose in writing the more quantitative posts is not to train you to become an engineer, but to help you get a taste of what’s involved, and to understand a little about engineering design so you can see how it is linked to your own life, and to the future of our society. I’m striving to present quantitative posts in a way that is accessible. It’s a real challenge. I want to keep posts from getting too long, but I also want to make sure I explain things in enough detail. If you have ideas on how I can explain things more clearly, I welcome your feedback (you can email me or post a comment). I also aim to give enough context for the quantitative posts so that readers know why these topics are important, in hopes that some readers who might be put off by equations are willing to read and think about them. Let’s become informed about energy and sustainability issues, and do something with this knowledge.
A reader pointed me to a video about the MDI Air Car:
I think it’s fantastic to see exploration of new ideas like this. It has the obvious benefit of no local emissions, as well as freeing us to use renewable power sources to compress the air that powers the car. In addition, this technology is much less expensive than electric vehicles or hybrid electrics. However, when the air is compressed to put into the on board storage cylinders, the air gets hot (think about what happens when you use a bike pump). This heat is lost when the compressor and storage cylinders cool off, meaning that a lot of energy is lost in the process. This reduces the overall efficiency of the system. I have not done any calculations myself yet, and I don’t have any efficiency data, but I’m skeptical that an air car would compare in energy efficiency to an electric vehicle. There may be specific applications where an air-powered car makes great sense, particularly where purchase price is more important than overall efficiency and operating cost throughout the life of the vehicle. We need to strive for both freedom from fossil fuels and lower energy consumption. The air car addresses only the former objective.
Unfortunately the company overextends itself in the claims made in this ad. Early on they claim ZERO cost to fill up this car, conveniently neglecting the cost of compressing air. MDI acknowledges this cost only at the end of the ad. When they discuss an on-board compressor, they flaunt the term ‘perpetual motion‘, which is a huge red flag.
Innovation is essential to our progress, and I commend exploration of new ideas. But we need to complement exploration with healthy debate that keeps us honest and helps drive us to seek the very best solutions. I’m concerned that wild claims may hurt the transition to clean energy and transportation. Let’s put our ideas out there, but let’s be up front about what we have actually accomplished and what the real potential is. We can’t afford to be deceptive; even the appearance of snake oil marketing could damage the movement.
The Automotive Research Center, in conjunction with industry and government partners, has investigated a similar technology that does indicate some practical benefit: hydraulic hybrid powertrains. Unlike air, hydraulic fluid doesn’t change volume when compressed, so doesn’t heat up much when compressed. Hydraulic regenerative braking systems have been installed on heavy trucks (and even bikes). A hydraulic pump connected to drive wheels pumps fluid into a pressure accumulator while providing some braking force. When the driver is ready to move again the pressure accumulator feeds the hydraulic motor, which assists the conventional engine. This system has been shown to have significant energy benefit, at a low level of cost and complexity when compared to hybrid electric systems (although all energy still comes from fossil fuels). This may be a good short term patch for current vehicles, but we also need to be working on long-term solutions that don’t involve fossil fuels.
I would be especially interested in hearing from anyone who has experience with compressed air powertrains to learn more about both their benefits and problems. Has anyone found data on the overall efficiency (grid to wheels) of an air-fueled powertrain system so we can compare then quantitatively to EVs? Or, has anyone done any calculations to estimate the overall efficiency?
This is the second post in a series on engineering modeling. In the first post, Introduction to Modeling I: Overview, I showed you a simple example system, a basic pendulum, that will be used to talk about some modeling concepts.
Before we start constructing any model, we need to create a list of questions the model is supposed to answer. In this pendulum example, we might want to know things like:
if the rod will be strong enough to support the weight
how the pendulum will move in a variety of conditions
what kind of force would be required to get the pendulum to move in a certain way
Here we will look at a model that addresses the first question. We will explore the other questions in later posts.
Much of the coursework engineers go through in college is dedicated to learning how to create models for a variety of systems that predict behavior to answer questions. One interesting activity engineers engage in is gathering knowledge about a system, whether from past school work, research publications, commercial software, or other resources, and integrating it all together into a model that answers important questions about a system, which in turn helps engineers make decisions. To answer our first question about the pendulum, we will look at material that engineering students might learn in classes on statics (the study of forces in things that are not moving) and solid mechanics (the study of what happens to solid objects when you apply force to them).
Every model is an approximation of a real system. Approximate models are based on assumptions about a system and its environment; if these assumptions were all completely true, then the model would be 100% accurate. In reality, assumptions are only partially correct. Adding more assumptions can simplify a model, but can also make it less accurate. An engineer must manage the tradeoff between model accuracy and simplicity. Many assumptions are reasonable to make, but engineers need to be careful or they might get very unexpected results from the real system. Have a look at what happened when bridge engineers assumed that vibrations caused by wind blowing across a bridge had no effect:
Let’s start off with a very simple model for our pendulum, and assume the following:
The 2mm thick rod is made of an aluminum alloy with a yield stress (explained below) of 20 MPa
The rod is much less massive than the weight at the bottom, so we can neglect the mass of the rod
The aluminum is homogeneous, that is, it has the same properties everywhere inside the rod. It has no spots in the rod that are weaker than others.
The pendulum is not moving
A solid object can break apart when the stress inside gets too high. You can think of stress as a type of internal pressure; it has the same units: force per area (PSI in U.S. customary units), just like pressure in a liquid or gas. In metric the unit for pressure is a Pascal (Pa), which is defined to be one Newton per square meter. The symbol normally used to represent a stress value is sigma (). Stress is a little more complicated concept than pressure in a fluid. Stress can be positive (compressive stress) or negative (tensile stress), and the direction of the stress matters. In the case of the pendulum, we will focus on just one type of stress: axial stress (), stress that occurs due to a force along the length of an object. The drawing below illustrates the idea of axial stress.
Because of gravity pulling down on the weight with a force of Newtons (where is the acceleration of gravity), there is an internal axial force along the length of the rod of 49.05 N. In the drawing above I show an imaginary cut through the rod. Imagine yourself being in the middle of the cut. To keep the weight from falling, you would have to pull the two halfs of the rod together with a force of 49.05 N. Each cross section of the rod must also resist of force of 49.05 N. The cross-sectional area of the rod is , where , and is the diameter of the rod, which is 2 millimeters (mm), or 0.002 meters (m). This square meters of aluminum must resist 49.05 N of force without breaking at every cross-section of the rod.
Let’s think conceptually about axial stress in a rod. If the force on the rod goes up, then the stress inside goes up proportionately. If we want to reduce the stress, we can make the rod thicker. This line of thinking is reflected in a very simple model for axial stress: , where is the tension in the rod. We can rewrite this equation, our model for stress, in terms of the rod diameter and the mass at the bottom of the pendulum: . We can see from this equation that increasing mass increases axial stress, while increasing the rod diameter reduces stress.
Different materials have different tolerance for stress. We say that a material that can handle more stress than another is stronger. This material property can be quantified using something called yield stress. A material will yield, or stretch past the point in can return back to its normal shape, when the stress inside exceeds its yield stress. A material will break when the stress reaches an even higher level, called the ultimate strength or rupture stress of the material. We are assuming that the rod here is made of a type of aluminum that will yield when the stress exceed 20 MPa (mega-Pascals: one mega-Pascal is one million Pascals). We can determine the yield stress and rupture stress of a material using a machine like the one in the video below (something happens at 53 seconds):
By plugging numbers for mass and diameter into our equation for stress, we can calculate that the axial stress in the rod (when the pendulum is not moving) is 15.6 MPa, which is less than the yield stress of 20 MPa, so our model predicts that the rod will in fact be strong enough to support the weight. Congratulations! We have answered the first question using an engineering model.
When might our model for whether the rod will be strong enough break down? What conditions can you think of that would cause some of the assumptions not to hold? What things does the model not account for?
As you may know, May is National Bike Month, and this week is National Bike to Work Week. If you have been thinking about testing out what it’s like to bike to work, there is no better time than tomorrow: it’s National Bike to Work Day. For helpful tips on pedaling to work, head on over to the League of American Bicyclists, or the Bike Commuters Blog (which currently has a great story about someone who switched to bike commuting after car problems). One of the important take-aways is you don’t need to be in great shape to get started. Just ride at a pace that is comfortable for you, and you might be pleasantly surprised at how far you can ride.
Shai Agassi is someone who has taken a vision and is seeing it through to reality on a very large scale. He is working to make electric vehicles (EVs) a reality for many people in many nations. Shai founded Better Place to develop a complete solution to making battery electric vehicles a practical transportation option. Better place ‘aims to reduce global dependency on petroleum through the creation of a market-based transportation infrastructure that supports electric vehicles, providing consumers with a cheaper, cleaner, sustainable, personal transportation alternative’.
There are many obstacles standing in the way of widespread adoption of EVs, and Better Place seems to be addressing all of them head on. One of their solutions is a quick, automated system for switching out depleted batteries with fresh ones, sort of like refueling for EVs. Better Place demonstrated this system in Japan yesterday. Better place has also proposed a new ownership model for batteries to help improve the affordability and practicality of EVs. Better Place has partnered with the Renault-Nissan Alliance; Renault and Nissan are developing the vehicles, and Better Place is addressing infrastructure. Momentum is building, and agreements are being made worldwide to deploy this system. There are even a few agreements in the U.S. You can hear more from Shai by watching his inspiring TED video below, or by checking out his blog.
It’s my pleasure to introduce Design Impact’s first guest blogger: Greg Kushmerek. Greg is a colleague of mine at the MathWorks, where we have not only software for super computers, but employees who are ’super commuters’. Greg is an avid cyclist who commutes in by bike year round, and will be sharing his insights into bicycle commuting in a series of posts.
This is a time of transition as we explore options that will help us develop a renewed, sustainable transportation system. It’s my opinion that human-powered transportation should become an important component of such a system. Greg’s insights will help us understand what we can do to move toward greater adoption of cycling and it’s acceptance as a mainstream mode of transportation. This topic clearly is related to sustainability, but is an important design issue as well; better design of cities, policy, and of course bicycles can all help stimulate progress toward a cleaner and healthier way of getting around.
Why don’t more people bike to work?
I’m a regular bike commuter, traveling 30 miles a day (round-trip) most days of the week, most days of the year. I’m clearly an outlier, but only if you consider the distance.
Clearly in the United States I’m just an outlier, period. But in The Netherlands, a country of 16 million people, a lot of people bike. If you’ve been to The Netherlands, then you also know that people pretty much expect to get rained on when they bike (my wife would say “It’s not whether it will rain today, but how many times?”). People bike to work, a few or more miles, and it’s just not a big deal regardless of the weather.
So what’s the big difference between here and there? Some factors to consider:
Infrastructure — The Netherlands has dedicated bike lanes, roads, stop lights, directional signs, and parking spaces all through the country just for bikes.
Money — Gas is more expensive.
Safety — Anyone with a license is trained to be on the lookout for cyclists. Further, by law a car driver is automatically at fault and his/her insurance must pay if a car gets in an accident with a cyclist.
Enforcement — The Police ticket wayard cyclists who violate the rules of the road.
You could argue that the flat terrain makes it easier, but I counter that the heavy winds make up for hills. It’s a different kind of physical challenge, but it’s a physical challenge that people put up with nonetheless.
Consider this: The Netherlands only became a bike-oriented nation in the 60’s. Most of that supporting infrastructure just didn’t exist until the state (they have a centralized system, then again the place is small) decided to commit to it.
Could it happen here? What do you think it would take? What would it take to get you to bike commute?
A couple weeks ago I mentioned that May is National Bike Month. It turns out that May is also National EcoDriving Month, at least according to the EcoDriving Program of the Auto Alliance. Driving habits can have tremendous impact on fuel efficiency. Did you hear about the expert hypermilers who drove a stock Ford Fusion Hybrid almost 1500 miles on a single tank of gasoline late last month? You can check out more tips on squeezing the most out of your gasoline on Ecomodder. I especially like the advise they give that has nothing to do with how you drive a car, but on what you can do instead of driving (walk, ride a bike, drive fewer miles). These are all great ideas, but we would be hard pressed to see big gains through better driving habits without the right incentives. Economic incentives would magnify the impact of EcoDriving. Hypermiling became popular last summer when gasoline broke the $4/gallon threshold, but how many EcoDrivers do we have now with $2/gallon fuel? Stable, moderately high fuel prices would not only motivate better driving habits, but reduce total miles driven (by petroleum fueled vehicles), create a market for more efficient vehicles, and provide the impetus for more Americans to consider cycling as a viable transportation option. We would probably even see a lot more creative activity in creating bicycle designs that make cycling practical for more people in more situations; for example, accommodating cargo and passengers, providing weather protection, and utilizing electric assist when human power alone is not sufficient. For a good example of this, have a look at the new FedEx delivery vehicles in Paris.
Electric-assist delivery bicycles used by FedEx in Paris
May is already National Bike and EcoDriving month, perhaps we should just declare it National Sustainable Transportation month! We could promote sustainable transportation on many fronts: driving habits (hypermiling), vehicle choice (appropriate size vehicles, bicycles), vehicle design (electric/hybrid electric vehicles, practical bicycles), infrastructure design (advanced/adaptive traffic light timing, roundabouts), and transportation policy. In fact, the time between now and this fall is a great opportunity to get politically active and have an important influence on transportation reform. This fall several federal transportation programs are up for reauthorization by Congress. Let’s ensure our representatives understand that we do not support continuing the transportation system status quo. Reforms need to come sooner than later to provide a foundation for a vibrant economy and healthy ecosystems that provide invaluable services to humanity. Changes are being debated right now. If you want to make a big difference, more than switching to CFLs or buying a plug-in hybrid for yourself, please consider learning about these proposed changes and voice your support for a renewed transportation system that can serve our society sustainably.
). Stress is a little more complicated concept than pressure in a fluid. Stress can be positive (compressive stress) or negative (tensile stress), and the direction of the stress matters. In the case of the pendulum, we will focus on just one type of stress: axial stress (
), stress that occurs due to a force along the length of an object. The drawing below illustrates the idea of axial stress.
Newtons (where 
is the acceleration of gravity), there is an internal axial force along the length of the rod of 49.05 N. In the drawing above I show an imaginary cut through the rod. Imagine yourself being in the middle of the cut. To keep the weight from falling, you would have to pull the two halfs of the rod together with a force of 49.05 N. Each cross section of the rod must also resist of force of 49.05 N. The cross-sectional area of the rod is 
, where 
, and 
is the diameter of the rod, which is 2 millimeters (mm), or 0.002 meters (m). This 
square meters of aluminum must resist 49.05 N of force without breaking at every cross-section of the rod.
, where 
is the tension in the rod. We can rewrite this equation, our model for stress, in terms of the rod diameter and the mass at the bottom of the pendulum: 
. We can see from this equation that increasing mass increases axial stress, while increasing the rod diameter reduces stress.
