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Design Impact » Energy

Extreme Energy Efficiency and Robotic Redesign

Earlier this week a presented a conference paper that goes over some work I’ve done recently involving squeezing every bit of energy efficiency out of robotic systems using the natural dynamics of these systems. Any physical system tends to move or vibrate in a specific way (think resonant vibrations of a wine glass), and this is what I’m talking about when I say ‘natural dynamics’. It turns out that we can use natural dynamics to our advantage to reduce dramatically the energy needed to do certain tasks. In this paper the application was robotics, but the idea extends to other domains as well.

You can get all of the details from the conference paper or the MATLAB code, but I thought you might enjoy this video that explains some of the highlights of the paper and includes some animations of the different robot designs compared in the paper:

Posted: April 28th, 2012 | Filed under: Design, Energy, Modeling, Optimization, Publications | No Comments »

Design for Energy Efficiency at ASME iDETC 2011

Last year I announced a conference session very relevant to the theme of Design Impact. We received several submissions, and this year we are soliciting articles again on the topic of Design for Energy Efficiency. If you are working on designing something that helps reduce energy consumption while maintaining or increasing performance or value delivered, please consider submitting a paper describing your work to this year’s ASME iDETC conference. It will be held in Washington D.C. this year from August 28th through 31st.

Abstracts are due by February 11th, and draft papers are due by February 18th. Click here to begin the submission process, and select DAC-7, which is part of the 37th Design Automation Conference (DAC). Articles will be reviewed before acceptance, and authors of accepted papers will have an opportunity to revise their submission after receiving feedback. If you have any questions or suggestions regarding the session or conference, please feel free to contact me, or post your ideas to the comments section below. Read below for more details.

Design engineers have the opportunity to improve quality of life and sustainability simultaneously through better design. One of the most significant areas engineering design has an an impact on is energy use. In addition to reducing consumption, we need to develop and put into service products and systems that use energy more efficiently. By using advanced design techniques, such as design optimization, incorporating more efficient technology, or simplifying systems and processes, engineers can help propel us toward energy sustainability.

Here is a description of the session from the conference website:

Design for Energy Efficiency: DAC-7

The ASME Design Automation Committee invites papers focused on design theory, innovation, or methods that enhance energy efficiency of energy consuming products or systems. Analytical design techniques that reduce energy consumption while maintaining or improving performance are of particular interest. Sample topics of interest include but are not limited to the following:

  • Using optimization to improve energy efficiency
  • Reducing energy consumption through process analysis and redesign
  • Energy recovery and reuse
  • Advanced/intelligent/alternative transportation systems
  • Novel control techniques that reduce energy consumption
  • Efficient energy storage
  • Challenges in transitioning to more efficient technologies
  • Economics of energy efficient technology
  • Energy savings through system simplification

Posted: January 14th, 2011 | Filed under: Design, Energy, Optimization, Sustainability | No Comments »

Designing the Evolution to the Smart Grid

Please welcome Design Impact’s newest guest blogger, Dan Livengood, a Ph.D. candidate from the MIT Engineering Systems Division.

Electricity. In the developed world, electricity is simply interwoven into our lives to the point where most people don’t think about it unless it’s suddenly not there. Although it often gets a bad rap for when blackouts and other disruption events occur, let’s not forget that national and international electricity systems (especially the one in the US and Canada) are often called the largest “machines” in the world. Considering their size and the number of interconnected parts, electricity systems work impressively well. The fact that most people don’t think about electricity except for the moments it’s not there is arguably evidence of that. In recognition of this impressive “machine”, ‘electrification’ was named the top engineering achievement as ranked by the National Academy of Engineering’s ‘Greatest Engineering Achievements of the 20th Century’!

So, is the electrical grid broken? Arguably (which I will not do here), no.

Is there a strong desire to upgrade the system so it operates better and more efficiently? Yes.

The so-called “smart grid” is a hot topic now, with a major influx of investment coming from the 2009 American Recovery and Reinvestment Act’s Smart Grid Investment Grants. However, the grid will not change overnight. In my mind, upgrading the largest “machine” in the world will be a continuous evolutionary process.

For me, this is the connection to the Design Impact blog, and I’d like to thank Dr. James Allison for inviting me to write a guest entry about the smart grid. The smart grid will ultimately have many levels of design. How should we design the smart grid? How should we design the consumer products that will interact with the smart grid? How do we design the evolution to the smart grid while continuing to operate the grid in whatever state it is currently in? With apologies to whoever said this originally (as I have forgotten), an analogy I particularly like is that upgrading the current grid to the smart grid on the fly is effectively equivalent to changing the engine on a commercial jet while it’s flying.

Designing and managing the smart grid evolution will be a huge challenge, although not insurmountable. Ultimately, designing the underlying enabling infrastructure for the smart grid will be key. At the moment, we simply aren’t sure which technologies or systems will work best for the smart grid. To address this, I am a firm believer in experimenting and trying new technologies in demonstration projects, which is precisely the point made recently by Patricia Hoffman, DOE’s assistant secretary for electricity delivery and reliability. The Smart Grid Investment Grants are certainly a solid start at funding some experimental smart grid designs. Some ideas will work, some won’t. As these demonstration projects progress, there will be a desire to keep what works and jettison what doesn’t on the fly, meaning that the smart grid will always be in a state of transition. So, how do we design the smart grid to continuously operate under continuous change?

I return to my point earlier that the underlying enabling infrastructure will be key. One effort to help support this goal is the monumental task being spearheaded by NIST to establish communication standards for the smart grid. Among other things, smart meters, utility energy management systems, home energy management systems, and even appliances will need to be able to ‘talk’ with one another. The full spectrum of devices that will connect to the smart grid will almost certainly come from more than one manufacturer, much like a multitude of devices connects seamlessly to the Internet. Establishing communication and interoperability standards is thus critically important for innovation to flourish on the smart grid just as it has on the Internet.

Smart meters are also undoubtedly a key enabling piece of the smart grid’s evolution. Electricity usage is read off of older meters at a frequency of at most once a month, whereas these smart meters will be read on the order of a few minutes to hourly. With this more frequent feedback of electricity usage, electricity customers will have a better understanding of how much electricity they use and at what times they use it. However, smart meters are just a starting point, and as a few utilities have found out, there will be some growing pains along the way as we transition into the smart grid.

These growing pains are likely part of what was behind a recent announcement that had the smart grid world buzzing: the Maryland Public Service Commission (PSC) turned down Baltimore Gas and Electricity’s smart meter rollout proposal. Personally, I think the Maryland PSC made the right call for reasons along the lines of what Chris King discusses in an article for SmartGridNews (which is a smart grid newsletter that I recommend perusing for anyone interested in easy reading and quick introductions to the many movers and shakers in the smart grid space). It’s not that the Maryland PSC doesn’t support the smart grid. Quite the opposite, I believe. My interpretation of their reasoning is simply that ‘we like where you’re going, but we think your smart grid system design should be better.’ Designing these systems is, frankly, going to be hard. Some pieces, like smart meters, are necessary enablers of the smart grid, but there is much more to truly make the system work. There are many questions to answer as well. Among them, how will customers react in the long run to smart meters, real-time electricity information and possibly time-varying pricing? Will the new smart grid system truly operate more efficiently than the old system? Again, one of the best ways to find this out in my mind is to try out some ideas through demonstration projects, just as Patricia Hoffman suggested.

I’ll stop here for this entry and return at a later date with some thoughts on one or more of the other pieces of the smart grid. I welcome any comments, questions or suggestions of which topic or topics to discuss next.
Once again, many thanks to Dr. James Allison for providing me the opportunity to write this guest entry for his Design Impact blog. Have a great day, everyone!

Posted: July 20th, 2010 | Filed under: Design, Energy, Policy, Sustainability | No Comments »

Virtual Energy Conference Tomorrow

A major theme of Design Impact is how better engineering analysis and design can improve the sustainability of our society. One important way this can be realized is through advancing renewable energy sources, as well as improving energy efficiency. For example, advanced design techniques can help us make wind turbines and solar arrays more effective, as well as bring them online faster. In addition to addressing the resource side of the energy issue, advanced design techniques, such as design optimization, can help engineers develop transportation systems, buildings, and other engineered products that consume far less energy, while still meeting performance demands.

We have an opportunity tomorrow to learn from an impressive array of speakers at the MathWorks Virtual Energy Conference. Anyone can register (free) to watch and listen to the speakers, or to network with other participants. Many of you have probably already heard of or participated in virtual conferences, an emerging trend. If not, the basic idea is to capture many of the benefits of attending a conference in person, but via a virtual environment. You can participate from your home or office computer. I hope to see you at tomorrow’s conference!

Posted: March 24th, 2010 | Filed under: Design, Energy, Sustainability | No Comments »

Engineering Week 2010

This week is National Engineering Week, and today is Introduce a Girl to Engineering Day. There are events across the U.S. throughout this week focused on both encouraging students to consider engineering as a profession, and to help everyone deepen their understanding of what the profession of engineering is about. In Boston we had a two-day long program with design competitions, career guidance, and a career fair. What are some e-week events happening near you?

While I’m certainly an advocate of encouraging more students to consider engineering as a profession, I’m especially interested in e-week as an opportunity for the public to learn what engineering is about: what engineering has done in the past to help humanity, and the potential it has to address some of society’s most pressing present challenges. In fact, emphasis on the role of engineering in society could stimulate more interest in engineering as an attractive career choice. Senator Ted Kaufman (D-Del.), the only engineer in the Senate, explained recently that one of the road blocks in encouraging more students to pursue science and engineering careers is that they “don’t view engineering and science as the way to make a difference”, but then points out several critical issues that depend on a strong engineering workforce, including energy and economic recovery.

A clear theme throughout Design Impact articles is the positive impact engineers have on humanity. What do you see as the most important issues today that call for engineering solutions? How can we communicate best to students that a career in engineering is an opportunity to make an important difference?

Posted: February 18th, 2010 | Filed under: Education, Energy, Policy | 1 Comment »

Heliostats, Prisms, and Platinum

Genzyme Center[Image Credit: TreeHugger]

Earlier this week I had the grand opportunity to tour the Genzyme Center in Cambridge MA with a group of engineers. This building is a little out of the ordinary; it was designed using a whole-systems approach to dramatically reduce its environmental impact, while providing an exceptional environment for those working inside. A system of heliostats on the roof track the sun throughout the day, aiming natural light downward through the expansive atrium. Gently swaying prisms suspended in the atrium then scatter this light throughout the rest of the building. Interior gardens, terraces, and pools not only add to the aesthetics, but contribute inviting areas for employee collaboration, and provide ecological services that help maintain quality of the interior environment.

Among the several distinctions awarded to the Genzyme Center is the vaunted Platinum LEED (Leadership in Energy and Environmental Design) Certification. The building was completed in November 2003, and is now on the elite list of 80 buildings worldwide with the highest LEED certification. You can read more about the Genzyme Center’s LEED profile here. What does it mean for a building to be LEED certified? While reducing fossil fuel consumption is an important consideration, there are several other characteristics evaluated, including:

  • Sustainability of site location
  • Water efficiency
  • Energy and atmosphere (building sector energy use is 48% of the U.S. total: substantial opportunity for improvement here)
  • Materials and resources
  • Indoor environmental quality (low VOCs, occupants can control their environment)
  • Location and linkages (how people will commute to this building: support for bicycle commuting, access to public transportation)
  • Awareness and education (helping building users get the most from building features)
  • Innovation and design process (designing the building as a whole system, not a collection of parts)
  • Regional priority

In addition to capitalizing on natural light, the Genzyme Center utilizes a variety of other innovative techniques to cut down on energy consumption. The building is designed to exploit natural convection currents for heating and cooling. About a third of the exterior has a ‘ventilated double-facade that blocks solar gains in summer and captures solar gains in the winter.’ What was most impressive to me was the flexibility of the systems in the building. The building manager can try out new strategies for heating, cooling, lighting, etc. to adapt to changing conditions. This kind of flexibility makes possible the discovery of synergies between systems that enable even better energy efficiency than what was expected during building design. Building management for the Genzyme Center seems to be an ongoing optimization process. Right now people control the adaptation, but it seems to a be a perfect application for machine learning. Does anyone know of building systems that use machine learning or other algorithms to fine tune operations?

An obvious question LEED certification, and sustainable building design in general, is whether the additional cost for alternative building approaches is worth the investment. According to a report for the California Sustainable Building Task Force, an initial investment of an extra 2% of the building cost will yield more than ten times the initial investment of the life cycle of the building. In addition to energy, water, and other resource savings, companies that invest in LEED certified or other sustainable building practices reap the benefits of increased worker productivity. My tour of Genzyme Center this week convinced me of the reality of this last point.

Sustainable building practices may cost more up front, but are sound business decisions when a long-term perspective is maintained. Greener building design is not only the right thing to do for humanity and our world, but also for businesses.

Posted: November 14th, 2009 | Filed under: Design, Energy, Sustainability | No Comments »

Design for Energy Efficiency at ASME DETC 2010

A central theme of Design Impact is how design engineers can improve quality of life and sustainability simultaneously through better design. Design engineers make decisions about how things work and how they are made, and these decisions have profound impact on our society. One of the most significant areas engineering design has an an impact on is energy use. In addition to reducing consumption, we need to develop and put into service products and systems that use energy more efficiently. By using advanced design techniques, such as design optimization, incorporating more efficient technology, or simplifying systems and processes, engineers can help propel us toward energy sustainability. It’s important to recognize that efficiency alone won’t solve our energy challenges. Without incentive to consume less, energy consumption may not go down. Motorists, for example, tend to drive more miles as fuel efficiency rises. We need policy changes that stimulate energy conservation, which in turn will drive demand for energy efficient products and improved engineering design.

To provide a forum to discuss recent advances in energy efficiency research, I’m organizing a new session (DAC-9) at 2010 ASME iDETC, an engineering design conference organized by the American Society of Mechanical Engineers. The conference will be held August 15-18, 2010 in Montreal. The topic of the session I’m organizing is Design for Energy Efficiency, and I’m hoping to get the word out early about this session to stimulate interest in the topic and encourage strong participation. If you are working on any projects that involve improving energy efficiency through design, please consider sharing what you have learned by contributing to this session. Draft papers are due by January 29th, 2010. If you have any questions or suggestions regarding the session or conference, please feel free to contact me, or post your ideas to the comments section below. Here is a description of the session from the conference website:

Design for Energy Efficiency: DAC-9

The ASME Design Automation Committee invites papers focused on design theory, innovation, or methods that enhance energy efficiency of energy consuming products or systems. Analytical design techniques that reduce energy consumption while maintaining or improving performance are of particular interest. Sample topics of interest include but are not limited to the following:

  • Using optimization to improve energy efficiency
  • Reducing energy consumption through process analysis and redesign
  • Energy recovery and reuse
  • Advanced/intelligent/alternative transportation systems
  • Novel control techniques that reduce energy consumption
  • Efficient energy storage
  • Challenges in transitioning to more efficient technologies
  • Economics of energy efficient technology
  • Energy savings through system simplification

Posted: October 13th, 2009 | Filed under: Design, Education, Energy, Optimization, Sustainability | No Comments »

Chapter 18: The Great Disruption, and the Case for Design Optimization

Thomas Friedman, the author of Hot, Flat, and Crowded, has invited readers to contribute ideas for a final chapter for the second version of the book. He wants to hear our thoughts on how we might ‘grow people’s living standards in a more sustainable and regenerative way’. (If you haven’t yet read HFC, I highly recommend it.) Here is my response to Friedman’s invitation:

In Hot, Flat, and Crowded you discuss the importance of ’smarter’ design; by changing how things are built, how they work, and are retired, we can reduce energy consumption and environmental impact dramatically, as well as improve quality of life and national security. I believe better design is at the core of a green revolution, and we need increased efforts to help others solidify mental links between design improvements and a vision for a sustainable future. In addition to helping citizens deepen their appreciation for the role of design, we must address this issue on two other fronts: public policy and engineering expertise. We need the right policy and incentives to set the stage for a transition to sustainability, as well as the technical expertise to implement the transition rapidly. I would like to address the latter issue.

To realize a green revolution, we can’t settle for products that are ‘good enough’, or green technology that evolves slowly. Instead, we must seek to develop the very best, most efficient designs, and do so quickly. Instead of taking small steps each year with slightly more efficient cars, slightly better wind turbines, let’s make giant leaps! We need the backing of citizens, the support of policy makers, and boldness from engineers and engineering educators to advance our ability to create sustainable systems and products. Researchers have developed impressive new engineering design methods the last few decades that can help us create products and systems that use less energy and other resources, while making leaps forward in performance. Some of these methods are mature and proven, but unfortunately are not yet used widely by engineers. First, let’s have a look at the conventional design process.

Suppose we were designing a car to be very energy efficient, but still performs well at a reasonable cost. Using a conventional design process, engineers would generate design ideas, test these candidate designs, propose new designs, and iterate until they converge on a design that meets (or comes close to) design targets. In the past, engineers relied heavily on expensive physical prototypes for testing. More firms now use computer models that predict how something will perform without having to build it. While this saves time and money, design refinements often are still made by engineers based on test results, experience, and expertise. Managing all these often conflicting design decisions is often overwhelming, particularly as products evolve and become more complicated; engineers stop when they find a design that meets basic requirements, instead of pursuing the best possible, or optimal, design.

One prominent method developed by researchers is design optimization. Other readers have also described optimization as an important solution; I hope to strengthen this position and clarify the link between optimization and engineering design. When using design optimization, engineers work to minimize or maximize some important aspect of a product, in addition to seeking to meet design requirements. In the car example, we might seek to maximize fuel economy, while meeting acceleration, handling, comfort, cost, safety, and other constraints. Framing a design problem in this way allows engineers to use computer models and powerful optimization algorithms together to help generate the best possible design. In this process design candidates to be tested are chosen analytically using mathematical techniques, reducing the number of tests and time to market. It can help engineers learn what is really achievable, opening our eyes to new possibilities. Design optimization also accelerates design evolution by enabling engineers to make more substantial design changes between product generations, instead of just small perturbations of the last version (as is usually the case now).

The design optimization approach is actually a pretty natural fit for how engineers already go about designing things; using formal design optimization is an enhancement that produces better results in less time, and leverages investments many firms have already made in computer modeling. It’s not a push-button solution; it automates some aspects of design, but requires engineering expertise and experience to implement successfully. (In the parlance of The World is Flat, design optimization is a high-level, ‘icing’ activity). Awareness is perhaps the biggest hindrance to the adoption of design optimization. It needs to be taught in undergraduate (not just graduate) engineering courses, as well as in industry training programs.

In summary, design engineers make a lot of important decisions that have tremendous impact on our world. Moving beyond status quo design processes can help engineers deliver sustainable products and systems while improving living standards; these changes in engineering design are essential to a successful green revolution. Right now there is a lot of low-hanging fruit; there are many opportunities to improve our world through better design. Design optimization can help us put new technology into production faster, as well as refine systems that use existing technology. This can help us bring energy efficient designs into production more quickly, and accelerate the transition to renewable energy systems. We have the technical tools, but we need the societal impetus to put them to broad use.

James T. Allison, Ph.D.

Posted: September 24th, 2009 | Filed under: Design, Education, Energy, Optimization, Sustainability | 1 Comment »

The Oil Age

Frank Wicks gives a nice history of the “Oil Age” in this month’s issue of Mechanical Engineering magazine. His article traces the rise of petroleum in modern society, and discusses challenges we face today. He describes early medicinal uses of petroleum by Seneca Indians, the first commercial drilling, and the transition to ubiquitous petroleum use. In the early stages of the oil age, kerosene for lighting was a dominant petroleum product, and natural gas and gasoline were wasted byproducts. In 1879, “Thomas Edison predicted the end of oil when he invented the light bulb”,  but this was of course on the heels of internal combustion engines and the phenomenal expansion of petroleum consumption that helps fuel our modern economy.

Wicks discusses how oil supplies are bounded: “Although oil has been found … at many locations, it should always be recognized to be a finite resource because we can burn it far faster than nature can replace it.” In fact, this issue was recognized very early on. Wicks explains that “Henry Ford feared that gasoline from oil would not last long enough to sustain a rapidly growing auto industry, and started research for alternatives.” While there has been enough gasoline to fuel a booming auto industry for more than a century, it will not last forever. Some predict that we are near peak oil, evidenced by the current production rates and the declining rate of discovery. Estimates of how much longer petroleum supplies will last vary widely. Wicks cites one estimate that postulates that:

…the world started the Oil Age with about two trillion barrels of recoverable oil. About half of that has been extracted. The remaining trillion barrels represent about a 30-year supply at the current rate of consumption and will be much more difficult to recover. The fundamental problem is that oil is too good. It is required for most things that we do. The alternatives are mostly inferior or less acceptable. Adapting to the next half and the end of the Oil Age may be the greatest challenge our civilization has ever had to face.

Regardless of how much is actually left, the amount is finite and irreplaceable. It will be increasingly difficult (and damaging) to recover, meaning that we will not be able to keep up current rates of consumption. In addition, we rely on petroleum for far more than fuel. It is feedstock for countless products (think of how many things are made using petroleum-derived plastics and chemicals). It may not happen tomorrow, or perhaps not even in some of our lifetimes, but at some point petroleum will become scarce and very expensive. How are we going to transition to alternatives? Clearly, the earlier we start, the easier the transition will be. And if we curtail petroleum use for fuel sooner than later, then perhaps we can prolong the transition to alternatives for petroleum-derived plastics and chemicals.

Wicks’ article focused on the issue of petroleum finiteness, which is only one factor compelling us to curb consumption. When we combine finiteness with national security, climate change, and other relevant issues, it’s clear we need to take action and make rapid progress. We’ve grown accustomed to the ease of oil, and change to something different can intimidating, but these changes can also be exciting opportunities to create a cleaner, more sustainable world to live in. These changes could even be liberating, leading to better quality of life for more people.

One interesting aspect of this article is its audience: engineers. It’s essential that this audience recognizes the importance of moving (quickly) toward a sustainable way of living. Engineers are the folks who can develop the  alternatives we need. But creating alternatives won’t automatically make society sustainable; alternatives need to be implemented and used widely. The rest of us need to support efforts to create complete solutions that combine technical advances with the right public policy, the right incentives, and enough popular support to help wean us off petroleum (and other unsustainable practices). Supporting these efforts is an important way to amplify our individual impact.

Posted: August 19th, 2009 | Filed under: Energy, Policy, Sustainability | No Comments »

Extreme Efficiency: Secrets Behind Miserly Solar Cars Part III (Tires)

Solar cars designed specifically to race in competitions such as the North American Solar Challenge or the Green Global Challenge (previously the World Solar Challenge) must somehow move at freeway speeds with less power than a typical hairdryer. Building these vehicles is a grand exercise in energy efficient design, and demonstrates what’s possible when engineers focus on producing maximum results with very limited power consumption. This is the third installment in a series that discusses several strategies solar car designers use to squeeze the most performance out of a vehicle-sized solar array. As with the larger energy system that powers our homes, vehicles, and factories, the best solution is not necessarily to focus only on producing more (ideally renewable) power, but also to identify and eliminate waste in systems that use energy. Designing for energy efficiency is a very cost-effective strategy to addressing energy problems.

One significant source of energy consumption in cars is tire rolling resistance. Basically, it takes some amount of force to roll a tire forward, even if you are not accelerating or going uphill. A simplified model of rolling resistance is:


This equation describes how much force is required to roll a tire forward at a constant speed on a flat road; this force is called rolling resistance F_r. It depends on two things: the vertical load supported by the tire (i.e., the normal force F_n), and the coefficient of rolling resistance C_{rr}. The normal force depends on how weight is distributed in your car, and the rolling resistance is a function of tire design (and is also influenced by things like temperature, speed, and tire slip). Rolling resistance goes up proportionately with both normal force and with C_{rr}. We would like to reduce rolling resistance in order to reduce the energy consumed while driving. It’s easy to see that one way to do this is to reduce how much a car weighs, which reduces F_n. Suppose we’ve eliminated as much vehicle mass as possible, and still want to reduce rolling resistance further. How do we reduce C_{rr}? To understand this, let’s have a look at where rolling resistance comes from.

When rubber tires roll over the road they deform. The spot that touches the road (the contact patch) is flattened just a little due to the force of the car pushing down. Imagine what happens to one piece of rubber in your tire as the tire rolls on the ground. Looking at the drawing below, at position 1, the piece of rubber is slightly curved. As the tire rolls, the piece of rubber moves into position 2, and it starts to deform. By the time it gets to position 3, it’s pretty much flat, and then as it moves through position 4 to position 5, it returns to its original shape.


All the rubber in the tire tread and sidewalls goes through some type of deformation with each revolution of the tire. It takes energy to deform rubber. We get most of that energy back when the rubber ’springs’ back into shape. But rubber is not exactly like a spring; you don’t get back all the energy you put into it. Rubber is what we call viscoelastic. The elastic part of viscoelastic is the springy part. Something has elastic behavior if it springs back into shape after being deformed. The viscous part means that when something is deformed, energy is lost, and resistance to deformation increases with how fast you try to deform it. Think of stirring a pot of honey; if you stir it slowly it doesn’t take much effort, but if you try to stir it fast the viscosity of the honey makes it harder to stir. Where does all the energy go from stirring? The honey doesn’t ’spring back’, so you can’t recover the energy like you can with a spring. The energy from stirring was converted to heat; the honey became a little bit warmer.

Tires exhibit both viscous and elastic behavior. Some of the energy is recovered when the rubber springs back into shape after rolling through the contact patch (point 3). Due to the viscous nature of rubber, there is extra resistance to deformation, as well as resistance to returning to its normal shape. The energy used to overcome this extra resistance is converted to heat; bending rubber back and forth makes it heat up (sort of like stirring the honey). Have you ever noticed how tires get warm after driving? The energy that warms your tires is energy lost. How can we minimize this lost energy (and reduce rolling resistance)? There are three main approaches:

  • Reduce tire deformation: if tire rubber is deformed less, then less energy will be consumed. This can be accomplished by increasing tire pressure (one important reason to make sure your tires are inflated properly). It’s important not to over-inflate tires, however, as this could degrade handling and ride quality, compromise safety, and accelerate tire wear. Tire deformation can also be reduced by adjusting tire design, that is, changing its shape and what it’s made of.
  • Reduce how much tire is deformed: narrower tires and tires with thinner tread have less rubber that moves in and out of the contact patch, reducing how much energy is lost from tire deformation. There are tradeoffs, however. Narrow tires may not handle as well, and thinner tread reduces durability.
  • Reduce rubber ‘viscosity’: using a harder rubber compound can help shift tire behavior closer to purely elastic, meaning that a greater proportion of energy that goes into deforming rubber is elastically recovered. Again, there is a tradeoff. Harder rubber compounds may not grip the road as well as softer compounds.

Some tire manufactures have created tire specifically for solar cars, with emphasis on ultra-low rolling resistance. Solar car tires are thin, high-pressure tires with hard rubber compounds. They have rolling resistance coefficients as low as 0.0025, whereas high efficiency passenger car tires have coefficients near 0.006, and typical passenger car and light truck tires have coefficients much higher than that. To give you a sense of the legendary efficiency of solar car tires, I was contacted by engineers interested in using solar car tires on bicycles they were developing for breaking human-powered speed records. Solar car tires are more efficient than racing bicycle tires.

Below is a photo of a solar car tire along with a view of the suspension (this is a photo of the Stanford solar car from several years ago). Notice the electric hub motor just to the right of the wheel. There is a direct connection between electric motor and wheel; no drive shafts, gears, belts or chains to sap energy.


The next photo shows a pile of solar car tires. Since these tires are optimized for energy efficiency, they don’t last very long. They must be replaced frequently, and it takes a large pile of tires to make it through a long cross-country solar car race.


Solar car tires are intended for specialized racing vehicles, and are obviously impractical for passenger vehicle applications. Nevertheless, we can take lessons from their design to help improve efficiency of production vehicles. Maybe we could move toward higher pressure tires, and use more advanced suspension design to help counteract the harsher ride from stiffer, high pressure tires. As we make other vehicle aspects more efficient (such as aerodynamics or powertrain design), the energy lost through rolling resistance will become an increasingly important factor, and is an opportunity for improvement.

Posted: August 11th, 2009 | Filed under: Design, Energy, Sustainability, Transportation | 5 Comments »