Highway Energy Harvesting - Roads That Generate Energy

Human energy consumption has kept increasing over the previous century. The rate at which demand for energy has been increasing isn’t nearly matched by the rate at which we have been able to renew our conventional energy sources. Several ominous predictions have been made about a massive energy crunch that we’re going to face in the near future. With an unprecedented number of countries emerging out of crippling poverty to find their own feet in a competitive global economy, there is all the more pressure to find radical solutions to our energy woes. 

It is against this background that energy harvesting technologies have emerged as a potential solution. As things stand, energy harvesting technologies may not be able to deliver a readymade solution that can singlehandedly meet skyrocketing global energy demands; However, taking into consideration just how nascent a technology energy harvesting is and how much progress has been achieved just within the past decade or so, it’s no surprise that massive bets are being made on energy harvesting. 

In previous posts, we have spoken at length about how there are several seemingly “niche” energy harvesting technologies emerging in the world, in addition to the more “run-of-the-mill” energy harvesting technologies such as RF energy harvesting and piezoelectric energy harvesting. 

Highway energy harvesting

Space and cost are two of the biggest challenges with implementing sustainable energy harvesting solutions, be it wind, solar, or any number of other emerging technologies. With an increasing number of nations seeing dramatic increases in their populations, finding and allocating space and resources for sustainable energy projects is proving to be a nightmare. 

All it takes is a little pragmatism to see that when economic interests are pitted against ecological concerns, the former comes out on top, more often than not. This is especially the case in developing economies that are beginning to break open their shackles and make their mark in the new global economy. Bringing millions out of poverty understandably ends up being a bigger concern than reducing emissions or cutting dependence on fossil fuels. 

Reacting with outrage and name-calling isn’t going to be of much help - When you’re barely finding enough food to feed your family, you’ll burn what you have to burn to keep the stove burning. There is an ever-growing understanding amongst the energy gurus of the world that for any push towards sustainable energy to really take off and find widespread acceptance, it has to be aligned with rather than against prevailing economic agendas. 

It is in this context that we look at highways and road systems as a viable source of alternative energy. Harvesting energy from roadways seems to be a genius solution, at least on paper. Roadways and transportation systems form the lifeblood of any nation’s economy and if we could find a way to harness energy from road systems, as they are, it would be nothing short of a masterstroke. 

The Bureau of Transportation Statistics reports that highways account for more than 1/3rd of the United States’ energy consumption. The problem is not just one of scale - it’s also of reach - In addition to requiring enormous amounts of power to be maintained and monitored (to ensure traffic safety, rule compliance etc), supplying power to road systems in far-flung areas presents a major challenge to traditional grid-based power systems. Highways take up huge chunks of a nation’s surface area. 

In this article, however, we are going to examine how this very “problem” might be a solution in disguise. In the following sections, we’ll take a look at some of the more promising solutions to harvest energy from roadways and transportation systems. 

Highway sunshine

The first on any list of renewable energy sources is usually solar. The sun has always been and always will be mankind’s most revered energy source. Solutions to convert highways into solar-energy harvesting systems have been in the works for quite some time already. 

The pros are fairly obvious - Across the globe, there are over 16 million kilometers of sun-exposed asphalt, already in place. That’s a whole lot of prime real estate as far as energy harvesting is concerned. Except for the initial investment in the required infrastructure, this represents, in principle, free energy! Free energy that can be used to power highway infrastructure such as traffic lights and monitoring systems or alternatively, be routed into the main power grid for later use.

The idea of harvesting energy from asphalt surfaces is by no means a recent one. Many such models have been proposed, with some involving pipelines running under major road systems, to convert solar energy into usable electrical or thermal energy and others using a more standard photovoltaic approach. 

While there are many apparent advantages to this kind of setup, it is not without its share of challenges. Firstly, it’s going to be no mean feat to install glass panels on roadways - This one’s a no-brainer - roads need to be grippy enough for tyres and moreover, the glass panels need to be robust and durable enough to be able to withstand heavy weights over long periods of time. Another potential issue could be that shading from buildings, structures and trees could mitigate exposure to the sun. 

In spite of these challenges though, there have been several companies that have come forward with promising solutions. These solutions comply with the international standards developed by the IEC Technical Committee (TC) 47 for semiconductor devices, including those aimed at energy harvesting. 

In the US, a company called Solar Roadways has created solar modules that incorporate LEDs and microprocessing units to enable intelligent communication and signaling. Across the pond in Europe, a French company called Wattway Solar Road is currently being tested across several locations to power toll gates and payment stations on the motorways. There are also several trials around the world, most prominently this one in China, where entire roads have been embedded with energy harvesting modules that can power streetlights and melt snow that has accumulated on the road surface. 

In the Netherlands, a company called “SolaRoad” built an experimental 70 metre long bike lane which was lined with solar panels made of heavy-duty glass that was built to withstand huge loads. The company estimates that the panels could account for well over 3 MWh within the first six months of use. 

Cost is a huge limiting factor that stands in the way of mass adoption of these photovoltaic modules. Some alternative solutions that have been floated include placing photovoltaic panels along roads as opposed to on them or in secondary areas such as parking lots, bike lanes, and driveways. 

The next big challenge is also a pretty obvious one - this type of energy harvesting technology is highly dependent on climatic conditions and may not be a viable proposition globally. Ironically, many countries that can afford to invest in the kind of infrasturcture required don’t get enough sunshine to justify the investment. Moreover, maintaining the structural integrity of the pipelines and underground systems is still something engineers are struggling with, because of the intense heat and constant, heavy loads involved in busy motorways. 

TEGs

Using Thermoelectric generators (TEGs) is another way we can harvest energy from roads. TEGs are based on the Seebeck effect and work by converting geothermal energy into electricity. Geothermal energy refers to the energy generated by the difference in heat between the surface of the road and the layers underneath. Higher the temperature differential, the larger the amount of electrical energy generated. This makes this kind of solution especially suitable for regions that have extremely hot weather year-round. 

All the way back in 2017, a team of researchers led by Samer Dessouky from the University of Texas at San Antonio received funding to conduct research into the viability of using TEGs to generate electricity from pavements and roads. Later that year, the team released a paper that explained the technology in detail.

“Energy is ubiquitous. It is literally all around us. All we have to do is find a way to harness and store it.” 

The paper said and we quote - “The thermal gradient between the surface temperature and the pavement substrata can be used to generate electrical power through thermoelectric generators (TEGs). The proposed prototype collects heat from the pavement surface and transfers it to a TEG embedded into the subgrade at the edge of the pavement shoulder. Early results suggest that the TEG prototype, measuring 64x64mm, is capable of generating an average of 10 mWatt of electric power over a period of 8 hours for weather conditions in Southern Texas. Scaling up the prototype using multiple TEG units could generate sufficient electricity to sustainably power low-watt LED lights and roadway/traffic sensors at offgrid remote areas.”

The team employed drones to scour large swathes of land and identify spots where heat was most densely concentrated. This was done in order to identify the best possible locations to implement this kind of technology. The researchers were especially keen on places like airports and university campuses. 

Harvesting sound energy from passing cars

“Energy is ubiquitous. It is literally all around us. All we have to do is find a way to harness and store it.”

There’s a good chance you’ve heard something along those lines before. Chances are, when you heard or read something like that line, your mind darted towards solar, wind, or even vibrational energy. But what we don’t usually think about in this context is sound. Sounds are all around us and range from the gross to the subtle, from the loud to the muted. 

It’s no surprise then, that when the brighter minds of our world were plugging away at the sustainable energy problem, they stumbled upon the idea of harvesting energy from ambient noise. 

It’s not all that easy though - even the most cacophonic, mind-numbing sounds that we can think of from everyday life, don’t translate to all that much, in terms of energy. The deafening roar of a train engine or the annoying drone of a pneumatic drill only translates to about 1/100 of a watt per square metre. In contrast, sunlight amounts for about 680 watts per square metre, all things considered. That’s nothing short of gargantuan in comparison!

“Harvesting acoustic noise is more about mechanical vibrations than sound itself.The idea is definitely there, and it’s quite promising.” says David Cohen-Tanugi, vice president of the MIT Energy Club. An engineering student who goes by the cybername “Mechanical Attraction” has garnered significant interest on the internet recently with his energy harvesting device. Mechanical Attraction seems to have devised a contraption that transforms noise from passing cars into usable electricity. 

The device consists of a pair of thin piezoelectric wafers which are mounted on beams designed to have a resonant frequency matching that of passing cars. The beams are made to be a very specific length, that was calculated using the Euler-Bernoulli beam theory. 

Without getting too much into the dreary technical details, the device was first tested in the laboratory and then, the setup was tried out on the street. In the video down below, you can take a look at the setup and Mechanical Attraction’s explanation on how it works.

Long story short, the device was able to produce voltages ranging from 0.225 volts to 0.275 volts. Now, in case you're wondering - that’s not nearly enough to solve any real-world problems. But you'd have to concede that it does look like a neat experiment.

Piezo - It’s all about the vibes man!

Harvesting sound energy from passing cars may not be the most practical solution out there but all hope isn’t lost - The loud noises we are likely to encounter around us, like an airplane taking off or a rock concert thumping away at full-tilt, are not just loud - they also make their surroundings vibrate. 

“There’s a strong interplay between vibrations through the medium that you hear through — air or water — and the physical objects around you,” quips Cohen-Tanugi. “It’s perfectly conceivable to absorb that movement and glean usable energy. You’re not going to power a city with it, but you can power small devices.”

That’s the basic principle underlying piezoelectric energy harvesting. Piezoelectric-based harvesting technologies are another promising way to harvest usable amounts of energy from roadways. 

“What is piezoelectricity?”, you might find yourself asking. It’s a physical property some materials have, due to which they accumulate an electrical charge on their surface when they are subjected to an external force or deformation. These substances are called “piezoelectric substances” and include a variety of crystals and polymers. 

Using piezoelectric technology to harvest energy from traffic, is a concept that is already gaining some serious traction (ha!) - Piezoelectric pavements have already been in use in parts of Europe and Japan for quite some time now. These are aimed at using footfalls to generate electricity. 

A pilot research project conducted on a remote motorway in a rural part of the Netherlands has shown that harvesting vibrational energy from road traffic can be practical and viable. The project was conducted on a road that has a 100kmph speed limit. The project found that depending on the total number of vehicles, the average speed, and the number of piezoelectric devices placed, a small but significant amount of energy could be produced. They found that although the amount of power they could harvest was not enough to totally power traffic signals, streetlights, control units, etc, it was significant enough to drastically reduce these devices’ dependence on traditional sources of power. 

Moreover, the amount of power generated from roads using piezoelectricity could be sufficient to power small devices such as motion sensors, which are increasingly becoming indispensable parts of road infrastructure. California has invested heavily in two projects aimed at embedding piezoelectric harvesters under asphalt. There is also a trail along the same lines being conducted at Lancaster University.

However, much like the previous technologies that we saw, this one’s not without its challenges either. We are yet to see if the costs involved in installing piezoelectric harvesters under road systems justifies the power that can be collected. As such, Photovoltaic modules and roadside turbine-based solutions, like this one in Turkey, might make more economic sense. 

The Big Battery Problem

Wireless sensors are an integral part of all traffic monitoring systems. Any smart highway system is only made possible by wireless sensors that facilitate the free flow of information that is essential for these systems to function. 

One of the biggest challenges in this context is the battery problem. For instance, SenSys vehicle detection sensors, which have received notable acclaim in the IoT community, are primarily powered by batteries. In fact, most vehicle detection sensors, which are now becoming quite common in road systems around the world, are dependent on batteries for their power. The claimed battery life ranges from 2-10 years (nodes and repeaters). And how this number could change with increased traffic volumes and extreme weather conditions remains to be seen. 

Over large highway systems containing tens of thousands of wireless sensors, millions of battery replacements might be required. This poses a logistical nightmare of herculean proportions. Add to this the maintenance costs associated with the batteries and all of a sudden, the picture doesn’t look quite so rosy anymore. Factoring in the terrifying environmental and humanitarian cost of batteries only makes the picture look grimmer.

It is in this context that road-based energy harvesting solutions like the ones we looked at in the previous sections assume great significance. Especially in remote areas, using technologies that harvest energy from moving vehicles could be truly revolutionary. It could dramatically reduce dependence on grid power which then could translate to lower expenditure in terms of both time and resources. Moreover, the costs associated with maintaining, replacing, and disposing of batteries could well be eliminated altogether. 

Researchers also postulate that this could lead to a drop in total energy consumption over an entire transportation system.  

Just the beginning

Using energy harvesting solutions in roadways may not be quite the fully formed solution as of today. These solutions are still in their infancy and are by no means mature or cost-efficient enough for large scale adoption. 

Investment in these technologies is not where it could be, mostly due to the limited availability of data and high costs. But we are sure that these technologies are going to find increasing acceptance in the years to come. For one, there is a tremendous amount of enthusiasm and optimism about these solutions, in the right places. 

We’ll have to wait and watch to see how big a role these technologies will play in our pursuit of a more sustainable future.