Combined heat and power as a platform for clean energy systems

The state of Georgia could dramatically reduce its greenhouse gas emissions, while creating new jobs and a healthier public, if more of its energy-intensive industries and commercial buildings were to utilize combined heat and power (CHP), according to the latest research from Georgia Tech's School of Public Policy.

The paper, digitally available now and in print on December 15 in the journal Applied Energy, finds that CHP -- or cogeneration -- could measurably reduce Georgia's carbon footprint while creating green jobs. Georgia ranks 8th among all 50 states for total net electricity generation and 11th for total carbon dioxide emissions, according to data from the U.S. Energy Information Administration.

"There is an enormous opportunity for CHP to save industries money and make them more competitive, while at the same time reducing air pollution, creating jobs and enhancing public health," said principal investigator Marilyn Brown, Regents and Brook Byers professor of Sustainable Systems at Georgia Tech's School of Public Policy.

Benefiting the Environment, Economy, and Public Health

The research finds that if Georgia added CHP systems to the 9,374 sites that are suitable for cogeneration, it could reduce carbon emissions in Georgia by 13%. Bringing CHP to just 34 of Georgia's industrial plants, each with 25 megawatts of electricity capacity, could reduce greenhouse gas emissions by 2%. The study authors, using modeling tools they developed, note that this "achievable" level of CHP adoption could add 2,000 jobs to the state; full deployment could support 13,000 new jobs.

According to Brown, CHP systems can be 85 to 90% efficient, compared with 45 to 60% efficiency of traditional heat and power systems. CHP has advantages over renewable electricity from solar and wind, which only offers intermittent power.

CHP technologies co-produce electricity useful for heat and cooling, resulting in ultra-high system efficiencies, cleaner air, and more affordable energy. Georgia industries that would profit from CHP include chemical, textile, pulp and paper, and food production. Large commercial buildings, campuses, and military bases also could benefit from CHP. By utilizing both electricity and heat from a single source onsite, the energy system if more reliable, resilient, and efficient.

CHP can meet the same needs at higher efficiency using less overall energy, while reducing peak demand on a region's utility-operated power grid, Brown explained. In addition, if there is an outage or disruption in a community's power grid, companies with their own onsite electricity sources can continue to have power.

Calculating CHP Costs and Benefits per Plant

The research used a database of every Georgia industrial site to determine which facilitates operated or could operate a CHP system. They then identified the appropriate type of CHP system for plants without one. To help assess if a CHP system was a financially sound investment, they developed a model to estimate the benefits and costs of each CHP system, factoring in the cost to install the equipment, operations and maintenance, fuel expenses, and financing. The result was an estimated "net present value" of each system that reflected the present value of future costs and benefits, Brown explained.

The paper also used data analytics to predict economic and health benefits of CHP for Georgians. Plants converting to cogeneration could boost the state's clean energy workforce by 2,000 to 13,000 depending on how widely it's adopted, Brown said. Currently, the state has about 2,600 jobs in electric vehicle manufacturing and less than 5,000 in the solar industry, according to the 11th Annual National Solar Jobs Census 2020.

In addition to job growth, CHP adoption could lead to dramatic health benefits for the state's more vulnerable residents, Brown emphasized. "We're displacing more polluting electricity when companies generate their own from waste heat," she noted.

The study estimates nearly $150 million in reduced health costs and ecological damages in 2030 in the "achievable" scenario for CHP, with nearly $1 billion in health and ecological benefits if every Georgia plant identified in the study adopted CHP.

"The public health improvements are gigantic -- that's a lot of lives saved, as well as childhood asthma and heart problems avoided," Brown said.

Georgia Tech's research was sponsored by Drawdown Georgia, a statewide initiative focused on scaling market-ready, high-impact climate solutions in Georgia this decade. The organization has identified a roadmap of 20 solutions, including electricity solutions such as CHP.

The impact of CHP could be dramatic considering that electricity generation accounts for nearly 37% of Georgia's energy-related carbon dioxide emissions, according to findings Brown and other researchers published earlier this year in the journal, Environmental Management.

Identifying Ideal CHP Sites

Georgia Tech researchers identified numerous different industrial sites in Georgia that could use combined heat and power. Ideal locations include established universities or military bases, and large industrial sites such as paper making, chemical sites, and food processing facilities. Georgia's number-one industry is agriculture, with chemicals and wood products among the state's top manufacturers.

"I find Georgia's potential to take advantage of existing industrial and commercial facilities to build CHP plants very interesting," said study co-author Valentina Sanmiguel, a 2020 master's graduate of the School of Public Policy in sustainable energy and environmental management. "I hope both industries and policymakers in Georgia realize the benefits that cogeneration has on the environment, the economy and society and take action to implement CHP in the state at a greater scale."

Dissecting Hurdles to Adoption

Despite the advantages of CHP, there remains hurdles to its adoption -- for one, establishing these facilities is capital-intensive, ranging from tens of millions for a campus CHP plant to hundreds of millions for a large plant at an industrial site. Once built, these facilities require their own workforce to operate, explained Brown.

"The cost-competitiveness of CHP systems depends significantly on two factors -- whether they are customer or utility-owned, and the type of rate tariff they operate under," said Brown.

In the paper, Georgia Tech cited three ways to improve the business case for CHP: clean energy portfolio standards, regulatory reform, and financial incentives such as tax credits.

Those approaches have worked well in North Carolina, noted Isaac Panzarella, director of the Department of Energy Southeast CHP Technical Assistance Partnership, and the assistant director for Technical Services for the North Carolina's Clean Energy Technology Center at North Carolina State University. North Carolina State University, where Panzarella is based, recently installed its second CHP facility on campus.

North Carolina, he added, has a policy that supports the use of renewable energy. Along with solar and wind, North Carolina embraced converting waste from swine and poultry-feeding operations into renewable energy.

"It's taken a long time, but finally there are more and more of these digester or biomass operations, using CHP to generate electricity and thermal energy from those waste resources," he said.

While Georgia Tech is not yet operating a CHP system, the Campus Sustainability Committee is currently examining options for lessening their energy footprint.

"Georgia Tech seeks to leverage Dr. Brown's important research, and the deep faculty expertise at Georgia Tech in climate solutions, as we advance the development of a campus-wide Carbon Neutrality Plan and Campus Master Plan in 2022," said Anne Rogers, associate director, Office of Campus Sustainability. "The Campus Master and Carbon Neutrality Plan will provide a roadmap to implementing sustainable infrastructure solutions to advance Georgia Tech's strategic goals."

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Static electricity could charge our electronics

These images show how the surfaces of magnesia (top block) and barium titanate (bottom block) respond when they come into contact. The resulting lattice deformations in each object contributes to the driving force behind the electric charge transfer during friction.

Unhappy with the life of your smartphone battery?

Thought so.

Help could be on the way from one of the most common, yet poorly understand, forms of power generation: static electricity.

"Nearly everyone has zapped their finger on a doorknob or seen child's hair stick to a balloon. To incorporate this energy into our electronics, we must better understand the driving forces behind it," says James Chen, PhD, assistant professor in the Department of Mechanical and Aerospace Engineering in the School of Engineering and Applied Sciences at the University at Buffalo.

Chen is a co-author of a study in the December issue of the Journal of Electrostatics that suggests the cause of this hair-raising phenomenon is tiny structural changes that occur at the surface of materials when they come into contact with each other.

The finding could ultimately help technology companies create more sustainable and longer-lasting power sources for small electronic devices.

Supported by a $400,000 National Science Foundation grant, Chen and Zayd Leseman, PhD, associate professor of mechanical and nuclear engineering at Kansas State University, are conducting research on the triboelectric effect, a phenomenon wherein one material becomes electrically charged after it contacts a different material through friction.

The triboelectric effect has been known since ancient times, but the tools for understanding and applying it have only become available recently due to the advent of nanotechnology.

"The idea our study presents directly answers this ancient mystery, and it has the potential to unify the existing theory. The numerical results are consistent with the published experimental observations," says Chen.

The research Chen and Leseman conduct is a mix of disciplines, including contact mechanics, solid mechanics, materials science, electrical engineering and manufacturing. With computer models and physical experiments, they are engineering triboelectric nanogenerators (TENGs), which are capable of controlling and harvesting static electricity.

"The friction between your fingers and your smartphone screen. The friction between your wrist and smartwatch. Even the friction between your shoe and the ground. These are great potential sources of energy that we can to tap into," Chen says. "Ultimately, this research can increase our economic security and help society by reducing our need for conventional sources of power."

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Generating electrical power from waste heat

This tiny silicon-based device developed at Sandia National Laboratories can catch and convert waste heat into electrical power. The rectenna, short for rectifying antenna, is made of common aluminum, silicon and silicon dioxide using standard processes from the integrated circuit industry.

Directly converting electrical power to heat is easy. It regularly happens in your toaster, that is, if you make toast regularly. The opposite, converting heat into electrical power, isn't so easy.

Researchers from Sandia National Laboratories have developed a tiny silicon-based device that can harness what was previously called waste heat and turn it into DC power. Their advance was recently published in Physical Review Applied.

"We have developed a new method for essentially recovering energy from waste heat. Car engines produce a lot of heat and that heat is just waste, right? So imagine if you could convert that engine heat into electrical power for a hybrid car. This is the first step in that direction, but much more work needs to be done," said Paul Davids, a physicist and the principal investigator for the study.

"In the short term we're looking to make a compact infrared power supply, perhaps to replace radioisotope thermoelectric generators." Called RTGs, the generators are used for such tasks as powering sensors for space missions that don't get enough direct sunlight to power solar panels.

Davids' device is made of common and abundant materials, such as aluminum, silicon and silicon dioxide -- or glass -- combined in very uncommon ways.

Silicon device catches, channels and converts heat into power

Smaller than a pinkie nail, the device is about 1/8 inch by 1/8 inch, half as thick as a dime and metallically shiny. The top is aluminum that is etched with stripes roughly 20 times smaller than the width of a human hair. This pattern, though far too small to be seen by eye, serves as an antenna to catch the infrared radiation.

Between the aluminum top and the silicon bottom is a very thin layer of silicon dioxide. This layer is about 20 silicon atoms thick, or 16,000 times thinner than a human hair. The patterned and etched aluminum antenna channels the infrared radiation into this thin layer.

The infrared radiation trapped in the silicon dioxide creates very fast electrical oscillations, about 50 trillion times a second. This pushes electrons back and forth between the aluminum and the silicon in an asymmetric manner. This process, called rectification, generates net DC electrical current.

The team calls its device an infrared rectenna, a portmanteau of rectifying antenna. It is a solid-state device with no moving parts to jam, bend or break, and doesn't have to directly touch the heat source, which can cause thermal stress.

Infrared rectenna production uses common, scalable processes

Because the team makes the infrared rectenna with the same processes used by the integrated circuit industry, it's readily scalable, said Joshua Shank, electrical engineer and the paper's first author, who tested the devices and modeled the underlying physics while he was a Sandia postdoctoral fellow.

He added, "We've deliberately focused on common materials and processes that are scalable. In theory, any commercial integrated circuit fabrication facility could make these rectennas."

That isn't to say creating the current device was easy. Rob Jarecki, the fabrication engineer who led process development, said, "There's immense complexity under the hood and the devices require all kinds of processing tricks to build them."

One of the biggest fabrication challenges was inserting small amounts of other elements into the silicon, or doping it, so that it would reflect infrared light like a metal, said Jarecki. "Typically you don't dope silicon to death, you don't try to turn it into a metal, because you have metals for that. In this case we needed it doped as much as possible without wrecking the material."

The devices were made at Sandia's Microsystems Engineering, Science and Applications Complex. The team has been issued a patent for the infrared rectenna and have filed several additional patents.

The version of the infrared rectenna the team reported in Physical Review Applied produces 8 nanowatts of power per square centimeter from a specialized heat lamp at 840 degrees. For context, a typical solar-powered calculator uses about 5 microwatts, so they would need a sheet of infrared rectennas slightly larger than a standard piece of paper to power a calculator. So, the team has many ideas for future improvements to make the infrared rectenna more efficient.

Future work to improve infrared rectenna efficiency

These ideas include making the rectenna's top pattern 2D x's instead of 1D stripes, in order to absorb infrared light over all polarizations; redesigning the rectifying layer to be a full-wave rectifier instead of the current half-wave rectifier; and making the infrared rectenna on a thinner silicon wafer to minimize power loss due to resistance.

Through improved design and greater conversion efficiency, the power output per unit area will increase. Davids thinks that within five years, the infrared rectenna may be a good alternative to RTGs for compact power supplies.

Shank said, "We need to continue to improve in order to be comparable to RTGs, but the rectennas will be useful for any application where you need something to work reliably for a long time and where you can't go in and just change the battery. However, we're not going to be an alternative for solar panels as a source of grid-scale power, at least not in the near term."

Davids added, "We've been whittling away at the problem and now we're beginning to get to the point where we're seeing relatively large gains in power conversion, and I think that there's a path forward as an alternative to thermoelectrics. It feels good to get to this point. It would be great if we could scale it up and change the world."

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