Renewable energy, sustainable energy, must eventually supplant burning fossil fuels. Solar and wind energy have already made their presence felt in the energy industry. But renewable energy can’t simply rest on its laurels. Here are ten emerging innovations in renewable energy technologies. Some, at least, will probably become commercially viable.

Solar and wind must both overcome technological and environmental hurdles before they can be truly sustainable energy.

Several of these technologies represent new ways of producing and saving solar energy. Others focus on hydropower, biofuels, or geothermal energy. I say nothing about innovations in wind energy technologies here, but I have recently written about progress in building wind turbines for residential use.

Most of these emerging technologies are still experimental and not commercially available. Some of them appear to have serious environmental problems of their own that might prevent them from ever becoming truly sustainable energy.

But let’s start out with a functioning system that can probably become commonplace relatively quickly.

In-pipe hydropower

The city of Hillsboro, Oregon and its industrial partners have just completed an innovatie project. It harvests electricity from the pressure in city water mains. Water utilities must manage the pressure in their pipelines. Maintaining correct pressure delivers water to customers with safe pressure and protects the pipes from leaking. The pressure exerts kinetic energy, which essentially goes to waste.

InPipe Energy, one of Hillsboro’s partners, has designed valves to convert the pressure into electricity instead. The concept, called in-pipe hydropower, has been around for a while, but InPipe’s valve marks the first time it has been adapted at utility scale. This control technology can be installed quickly and turned over to the utility operate.

It will generate up to 200,000 kWh of electricity every year. The city will use the electricity to provide power to its Gordon Faber Recreation Center for lighting, concessions, and charging stations for electric vehicles.

Because the system integrates so well with existing pipelines, other cities can take advantage of it. The electricity produced can offset the costs of upgrading their infrastructure.

This technology feeds off energy that would otherwise be wasted in a process that has to be done anyway, so it is truly sustainable energy.

Marine energy

Oceans cover about 75% of the Earth’s surface, They store much more energy than humanity is ever likely to need. Therefore, they make very attractive renewable energy sources. So far, however, oceans haven’t provided a large share of the world’s energy supply. Marine energy presents enormous challenges.

Marine energy comprises five emerging renewable energy technologies: wave energy, tidal energy, tidal currents, and gradients of salinity and temperature.

Waves form when the wind transfers its energy to the water. They can travel long distances without significant loss of power—or can even increase in power. Wave energy converters make wave power into electricity.

Tides form as a result of the interaction of gravitational forces among the Earth, moon, and sun. The timing of high and low tides is highly predictable. The behavior of the water as the tides move is so far little understood. Even so, tidal power provides the largest amount of energy of these marine technologies.

As the tides rise and fall, they create a tidal current flow. Again, this flow is highly predictable in timing, but not in water’s behavior.  Tidal current energy uses different kinds of turbines from other tidal energy.

The difference in salinity between two liquids likewise creates energy. It occurs, for example, whenever a freshwater river flows into the salty ocean. Techniques such as electrodialysis and pressure-retarded osmosis can harness it. Extracting salinity gradient energy can either happen as a standalone process or as a hybrid with desalination or wastewater treatment projects.

Finally, ocean thermal energy conversion takes advantage of the difference in temperature between the sun-warmed surface and the colder subsurface layers.

Marine energy must overcome serious technological, environmental, and economic challenges before it can live up to its potential as sustainable energy.

Concentrated solar power

Unlike the familiar solar panels, concentrated solar power uses mirrors or lenses to reflect sunlight into a receiver. The heat can boil water to operate a turbine. When a concentrated solar power plant is paired with a heat storage system, it can generate electricity with the stored heat even after dark or on cloudy days.

As with marine energy, there are several different methods of concentrating the sun’s rays: parabolic troughs, parabolic dishes, linear Fresnel reflectors, and solar towers.

Most projects now under development or construction use parabolic troughs. In fact, come parabolic trough projects have been operating for almost a decade. To build them, it was necessary to clear large tracts of land of all vegetation. The whole construction process required burning a lot of fossil fuels.

And then operating the plants requires boilers and therefore a lot of water. They are far from major population centers. Therefore, most of them required building long transmission lines. (The one in Andalusia ready had a nearby transmission line.) I don’t see the lifetime benefits of this technology as outweighing the environmental costs of building it.

Fresnel systems and solar towers, more recent developments, have the potential to improve performance and reduce capital costs. Like parabolic troughs, Fresnel systems require large tracts of land. Solar towers need less land, and parabolic dishes the least of all.

Even with storage systems, however, concentrated solar power must have backup systems operated by conventional fuels. It certainly counts among emerging renewable energy technologies, but existing projects hardly seem like sustainable energy.

Flexible all-carbon solar cells

Standard solar panels are made of silicon. It’s a very abundant element, but making solar cells and panels requires a lot of steps. Scientists at Stanford University, led by Zhenan Bao, invented a solar cell made entirely of carbon using nanotechnology. Unlike parabolic troughs, this emerging technology appears more likely to become truly sustainable energy.

Instead of rigid panels, the carbon nanotubes in solution can be applied as a coating. It can be applied to roofs, walls, and windows to generate electricity. And not just of buildings, either.  Carbon nanotubes can be applied to cars and potentially generate electricity whether the car is moving or parked in the sun.

The coating technique can potentially reduce manufacturing costs. It does not require as many steps as processing silicon-based cells. Nor does it need expensive specialized machinery or tools.

Flexible solar cells have existed for a while. They comprise a photoactive layer between two electrodes. But the current technology requires both silver and scarce and expensive iridium. Previous carbon solar cells have used a carbon photoactive layer, but Bao’s team also made its electrodes from carbon in the form of graphene. A new substance, graphene comprises sheets of carbon only one atom thick.

So far, Stanford’s cells primarily work on near-infrared light. They have only 1% efficiency. To improve efficiency, they need to develop materials that will absorb visible light. They also need to make the coatings very smooth.

Once the team figures out how to boost efficiency, carbon solar cells will have other advantages over current technology besides price and ease of manufacture. Carbon structures remain chemically stable in extreme heat. They can also operate under great physical stress.

Liquid sunlight

Batteries can store solar energy, but we still don’t have efficient long-term storage at a reasonable cost. Liquid sunlight might solve the problem in another ten years.

Swedish scientists have developed an innovative renewable energy technology they call solar thermal fuel. It’s a specialized fluid that can store sunlight at room temperature for as much as 18 years! Unlike a battery, which stores electricity, this fluid, norbornadiene, stores heat.

Norbornadiene is made of carbon, hydrogen, and nitrogen. When exposed to sunlight, the atomic bonds rearrange themselves to become the isomer quadricyclane. Or, to express it in more ordinary language, the new form of the molecule traps heat between its chemical bonds. Even when it cools to room temperature, the heat energy stays there.

Drawing the isomer through a catalyst turns it back into norbornadiene and releases the heat.

In the current state of the technology, the difference between the quadricyclane, which holds heat, and the norbornadiene, which releases it, is 63º C (113º F). The scientists think they can manage a temperature difference of 110º C (230º  F).

If all works out, this stored heat can operate appliances that require heat: furnaces, hot water heaters, clothes dryers, and dishwashers.

The current prototype sits on the roof of a university building. A concave reflector tracks the sun, as a satellite dish would do. The norbornadiene travels through transparent pipes, where sunlight converts it to its isomer. Then, to extract the heat, the isomer flows through a special catalyst that returns it to its original form.

Researchers have subjected the fluid to this cycle more than 125 times. The molecule hasn’t suffered significant damage. It can hold 250 watt-hours of energy per kilogram. Tesla’s Powerwall batteries hold only half that. The emerging technology might be providing sustainable energy before the end of this decade.

Tires that generate electricity

Goodyear has announced very different innovative renewable energy technology. It’s a tire that generates electricity, which can be used to recharge batteries in either plugin electric vehicles or hybrids. It uses a combination of piezoelectric and thermoelectric technology. It is a tricky engineering challenge.

Piezoelectric materials generate electricity when stress is applied to them. Most are brittle. One function of a tire is to reduce stress to make a smooth ride. One challenge, then, would be to find a piezoelectric material that’s both efficient enough and suitably pliable.

Thermoelectric materials rely on semiconductors and a temperature gradient. Most tires are designed to dissipate heat. Goodyear’s tire absorbs it. So it would appear at first glance that using tires to generate electricity calls for major rethinking of tire-building principles.

My source for this section is five years old. When I tried to find something published within the past year, I found one very skimpy article, from which it appears that nothing much has changed. I also found lots of scholarly literature. It appears that electricity-generating tires are a good idea that is still very much experimental. Someday engineers may find something that works commercially.

I have also read of piezoelectric materials mounted on the suspension, which may be a more logical place for them. The idea of a car generating some of the energy it needs just in the process of moving down the road has generated real excitement.

Artificial photosynthesis

Plants naturally convert the energy from sunlight into chemical energy they can store or use. In the process, they take in carbon dioxide and emit oxygen.

Artificial photosynthesis aims to perform the same conversion using manmade materials that imitate the natural process. Possible applications include hydrogen fuel cells that don’t require applying high energy to petroleum in order to produce the hydrogen.

Artificial photosynthesis entails using sunlight to harvest chlorophyll, using tyrosine as a catalyst, and using a manganese complex to send electrons to the chlorophyll. It splits water into hydrogen and oxygen using semiconductors.

The catalytic step is the major bottleneck in the way of large-scale use of artificial photosynthesis. For one thing, oxygen can degrade the performance of the catalyst. Other concerns include developing an inexpensive electrode, corrosion of materials, and safe handling of the hydrogen.

3D printed solar trees

Scientists at Finland’s VTT Technical Research Centre may have found another innovative way to use sunlight the way nature does. Besides advanced solar technology, they use 3D printing to make what look like leaves. They combine the leaves to make trees, which can operate either indoors or outdoors. The trunks also come from 3D printing using waste wood from the lumber industry.

Since solar trees come from standard techniques of mass production, scaling up for commercial production will be no problem. Already it’s possible to make 100 meters of leaf rolls every minute.

So far, these photovoltaic leaves can power mobile devices and other small appliances. Each one has its own power converter, which can collect energy not only from the sun but also wind and temperature changes.

Imagine a whole forest of solar energy trees. It will be able to produce a lot more energy than what it takes to charge a cell phone. And it needn’t be as ugly as current solar or wind farms or use up as much land.

Each leaf has a useful life of two or three years. VTT claims that they are recyclable. That is, spent leaves can be the material used to make new ones. I have no estimate of when commercial manufacture will happen or what problems must be solved first.

Cellulosic ethanol

Cellulosic ethanol has been an emerging innovative renewable energy technology for most of the century. It faces several hurdles before it can emerge as a commercially viable industry.

Most ethanol comes either from corn starch or sugar cane. That is, it uses sugars from edible materials used for human food. It actually takes more energy to grow and transport the crops than burning the ethanol produces.

Cellulosic ethanol comes from more complex carbohydrates, cellulose, that humans and most livestock cannot digest.

These can be found in several different sources.

• Harvesting corn, sugar cane, and other food crops leave behind agricultural waste with a lot of cellulose.
• Industries that use a lot of wood, such as paper and furniture, likewise leave behind usable wastes.
• Cellulose-rich non-food crops such as switchgrass can grow on land not suitable for growing food crops.
• Municipal solid waste also includes cellulosic material.

The chemistry and economics of producing alcohol from such complex molecules are complicated, however.  The biological approach requires converting cellulose to simple sugars. The thermochemical approach converts the carbon in the feedstock to carbon dioxide, carbon monoxide, and hydrogen.

You can see some of the challenges in reviewing research into gribbles. These marine pests eat wooden fishing vessels. More than 10 years of research has yielded important understanding of the chemistry of the gribble’s digestive system. If the team learns how to replicate it at industrial scale, cellulosic ethanol will become economically viable. Research also continues on other completely different natural processes to imitate.

Once science solves the technological and economic barriers, cellulosic ethanol can potentially lower greenhouse gases by 90% compared to gasoline. Cellulosic ethanol from some feedstocks may even be carbon negative. That is, over its lifetime, it takes more carbon dioxide from the atmosphere than burning it adds. Talk about sustainable energy!

Engineered geothermal energy

The Earth’s crust likewise has temperature gradients. Home heat pumps work on geothermal energy.

Unlike most renewable energy technologies, geothermal energy can serve as the baseload source of generating electricity. But large-scale geothermal energy has been limited to places that have reservoirs of naturally occurring hot water and steam. Engineered geothermal energy creates its own reservoirs. It takes four steps:

1. Dig an injection well deep in the earth into hot basement rock. The rock layer must have low fluid content and limited permeability.
2. Inject water to create or open fractures in the rock.
3. Continue to pump water to extend the fractures.
4. Drill production wells into the fracture system to circulate the water and extract heat.

Engineered geothermal energy comes with all the environmental risks of fracking. These include land use, water use, water pollution, induced seismicity, and the possibility of inducing landslides. Even so, it may carry less environmental risk than fossil fuels or nuclear power. What’s more, the technology can mitigate carbon emissions.

So emerging renewable energy technologies, or many of them anyway, have the promise of making our energy usage more nearly sustainable. Scientific research hardly ever works as quickly as non-scientists might hope, but it does work diligently until it finds what it’s looking for.

Sources:

3D printed trees harvest energy from sun, wind, & temperature / Steve Hanley, Clean Technica. December 17, 2016
3D printed solar energy trees / Alternative Energy News. February 28, 2020
Electricity generating tires: good concept or just crazy? / Mark Atwater, engineering.com. March 11, 2015
Emerging renewable and sustainable energy technologies: state of the art / Aktar Hussain et al. Renewable and Sustainable Energy Reviews 71 (May 2017): 12-28 via Science Direct
Scientists develop liquid fuel that can store the sun’s energy for up to 18 years / Carly Cassella, Science Alert. November 6, 2018
Stanford scientists build the first all-carbon solar cell / Mark Schwartz, Stanford News. October 31, 2012
US city now generating renewable energy from an underground water pipeline / Nicholas Nhede, Smart Energy International. October 18, 2020

Photo credits:
Floating tidal energy platform. Wikimedia Commons
Parabolic mirror. Some rights reserved by Tom Raftery
Carbon nanotube. Some rights reserved by IBM Research
Green leaf. Some rights reserved by NSiddhu
Corn stover harvest. Image courtesy of Idaho National Laboratory (INL) Bioenergy Program, via Flickr