- The Next Big Thing on the Trash List – Solar?
Solar power is probably the fastest-growing market in the world. According to Solar Energy Industries Association (SEIA), in the past decade, solar power industry experienced an average annual growth rate of ~59%. An estimated 500,000 solar panels were installed globally every day in 2015. If we think of rooftops, a typical American home would require 28 to 34 solar panels to cover its power consumption. The U.S. Department of Energy forecasted that by 2050, the U.S. will have cumulatively installed 700 GW of solar, or hundreds of billions of PV modules [Mulvaney, 2015].
But here is the question: What will happen to the billions of those solar panels now spreading across the globe at the end of their useful lives?
On the average, solar photovoltaic (PV) modules have a useful lifespan of 25-30 years, so with the current growth rates, the first peak of PV waste can be expected around 2030. And there is still some time to plan ahead. Now, as we know how externalities have magnified due to the lack of foresight with fossil fuels, there is an opportunity to do things right with solar.
As the photovoltaic panels contain a variety of valuable metals and materials, which are mined and refined at increasing rates, it is imperative to create recycling methodologies, infrastructure, and policies to maintain the flow of those materials within the industry. This important action would address two problems – waste regulation and resource depletion.
What are the current US domestic programs designed to address the growing PV waste flow? Until recently, the regulations on PV waste did not exist in the USA, except California. However, things have to change soon. In lieu of introduction to this problem, the video below talks about some of the emerging options and initiatives, many of which utilize the successful experience of the European recycling programs:
Solar Basics: How to plan ahead for U.S. solar panel recycling (3:14)
Transcript: Solar Basics: How to plan ahead for U.S. solar panel recycling (3:14)
Welcome to solar basics. I'm Kelsey miss Brenner, senior editor of solar power World. I'm Haley Pickerel, editor-in-chief.
In about 30 years, a wave of thirty-five point three million panels may reach the end of their lifespans. Not counting the hundreds of millions of panels that flooded the US market in the last decade that may need to be disposed of sooner. With no dedicated national program or requirements to safely dispose of solar panels, some unfortunately find their way to landfills. If the system owner is green minded and has the money, panels may get shipped to a recycling facility.
Other industry players are warehousing damaged or old panels until a practical recycling program is established and a few colleagues from consulting company, Solar Cowboys, have started a new recycling program in the U.S. called recycle PV. Modeled after a successful European program called, PV Cycle. The Electric Power Research Institute found system owners recycle their panels in Europe because they are required to. Panel recycling in an unregulated market like the United States, will only work if there is value in the product.
Though there's nothing yet mandated at a national level in the u.s., there are a few states trying to get the required recycling ball moving. In July 2017, Washington became the first state to pass a solar stewardship bill requiring manufacturers, selling solar products into the state, to have end-of-life recycling programs for their own products.
New York passed a similar Senate bill last year that has since been passed to the state assembly. The bill would require solar panel manufacturers to collect end-of-life panels for recycling. In addition to those states, one panel manufacturing company has prioritized recycling. Cad-tel thin-film module manufacturer first solar established a recycling program at the beginning of production to responsibly recycle a manufacturing scrap, warranty returns, and end-of-life panels. This environmental decision also had a financial motivation. But tellurium used in the product is a finite resource.
In any case, First Solar's recycling facilities, attached to its manufacturing plants, have the capacity to recycle two million panels globally on an annual basis. For crystalline silicon modules needing recycling now in the United States, there are a few scattered options. Various glass and electronics recyclers have taken on solar panel recycling but usually not on dedicated lines are on a grand scale.
Industry advocacy groups SIA has begun organizing recycling efforts through its PV recycling working group. The organization will choose preferred recycling partners that offer benefits to SIA members. Time is ticking for panel recycling, the United States has about 15 years before solar panel recycling becomes a major issue. That's plenty of time to figure out the best course of action, but also plenty of time to procrastinate. Here's hoping we set early deadlines.
For more on solar panel recycling, read our story online and stay tuned for the next Solar basics videos.
There are a number of recyclable components included in PV module – some of those are rare, and some of those are toxic and thus require a proactive plan for recycling. Crystalline Si PV modules, in addition to silicon, contain materials such copper, aluminum, silver, and glass. CdTe PV modules contain cadmium, steel, and copper. Metal components are usually much more expensive than non-metal materials, and extracting them during recycling process and reusing in manufacturing brings sensible economic benefits. Materials such as silicon wafers are critical to recycle, as a substantial amount of energy is spent to purify them for use in PV modules. Thin-film modules contain such elements as tellurium, indium, gallium, and molybdenum, which are in limited supply in the Earth’s crust. Indium is the element that will face resource use competition between solar and flat-screen displays. [Williams, B., 2016]
In the News:
Here are a few short articles outlining options for PV recycling available in the US and in Europe. For example, “PV Cycle, a European solar panel recycling association, developed a mechanical and thermal treatment process that achieves 96% recovery rate for silicon-based photovoltaic panels.” This sounds quite impressive! “The remaining 4 percent is utilized in an energy recovery process, using a waste-to-energy technology.” The more recent release reports on PV Circonomy campny in California which adopted a high-efficiency automated process for disassembling cSi panels.
Web Article: Lozanova, S., Are Solar Panels Recyclable, Earth 911, 2018. URL
Web Article: Marsh, J., Recycling Solar Panels in 2018, EnergySage, 2018. URL
Web Article: Thompson, V., U.S. Startup Unveils Highly Automated Low-Waste Solar Panel Recycling Tech, PV Magazine, 2025. URL
More education on this topic - the following webinar (by International Solar Energy Society - ISES) presents an extended overview of PV recycling practices, policies, and current research innovations around the world. The first talk is more on the legal background and policies existing in different countries. The second presentation explores the way to incorporate PV panel reuse practice in circular economy. The last presentation in the webinar goes deeper into the weeds of the recycling process itself. You will see the actual equipment used for the mechanical, chemical, and thermal extraction of materials from the discarded panels.
ISES Webinar: PV Recycling and End of life Processing (1:22:01)
Transcript: ISES Webinar: PV Recycling and End of life Processing (1:22:01)
All right, welcome everybody to today's webinar hosted by the International Solar Energy Society (ISES). We are very pleased to have all of you here, and we are especially happy to welcome our speakers and you, the audience.
Recycling and end-of-life processes are crucial topics when discussing successful renewable energy transformation, and we are very happy to present this webinar to you today.
My name is Arabella. I am the Communications and Outreach Officer here at the ISES headquarters in Freiburg, Germany, and I will give you a short introduction to ISES and the work we do.
As we have many new participants joining us on the webinar today, the International Solar Energy Society (ISES) is a non-profit UN-accredited membership NGO. Our vision is 100% renewable energy for all, used efficiently and wisely.
ISES represents a diverse membership of academics, researchers, energy practitioners, consultants, students, businesses, and advocates. ISES works together with like-minded organizations from countries all around the world to advance the renewable energy transformation.
There are many benefits to joining ISES as a member, and you can find out more on our homepage. Some of the benefits are the exclusive access to presentations and webinar recordings, such as today's, in the ISES webinar archive. ISES members can get discounts and even free registrations to ISES events and partner events.
Every month ISES publishes a newsletter for our members where you can follow our progress and share your news. Members can also subscribe to our academic journal, Solar Energy, our flagship publication, at a reduced price.
In the ISES online bookshop, ISES members qualify for reduced prices on different publications. We welcome those who are not yet members to join today to support our work. For those who are already members, we thank you.
There is also a special offer on this webinar today. All attendees of today's webinar will be offered a 20% discount code as a follow-up to this webinar, so join us today and benefit from this discount. We are very happy to welcome you to the global solar community.
Now for some brief information on the Q&A section before we start.
For the Q&A section, we invite you to send in your questions, and we are looking forward to your participation. When sending in a question, please write who the question is for and keep your questions short and precise. Please feel free to start sending in your questions anytime throughout the webinar via the chat box in your GoToWebinar panel.
I am now happy to introduce our moderator for today, Dave Bruning.
Dave will introduce our speakers and guide us through the Q&A session. Dave served as President of the International Solar Energy Society from 2010 to 2013. He is currently a member of the ISES Board of Directors as Immediate Past President.
From 1991 until his retirement, Dave managed solar resource assessment activities at the U.S. National Renewable Energy Laboratory (NREL). He is dedicated to the concept of urgently achieving 100% renewable energy to meet all of our end-use energy requirements as the best solution to the climate crisis.
Now I'm happy to hand over to Dave. Dave, thank you for joining us. The floor is yours.
Thank you, Arabella, for this opportunity to moderate this important webinar on this very important topic.
I would also like to welcome all of our many attendees who are joining us today. Given the significant growth we've seen in solar PV installations around the world over this past decade, the need to address end-of-life scenarios and recycling options for aging PV systems has become ever more important in our ongoing energy transformation.
Currently, there are well over 650 gigawatts of installed PV capacity around the world, and already we are seeing older systems being dismantled and replaced with more modern equipment.
Traditionally, PV systems have demonstrated long lifetime cycles. For example, there are systems at the U.S. National Renewable Energy Laboratory's PV test facility that have been operational with little degradation of performance for nearly 40 years.
But as technologies continue to advance and efficiencies increase, there will be many older systems that will be going through upgrades to more modern panels. This will create a significant opportunity for companies and organizations to find meaningful solutions for recycling, refurbishing, or otherwise disposing of these systems.
I am now going to start introducing our speakers.
Before we start, I want to mention that we had planned to have a representative from Jinko Solar also participate as a panelist in this webinar, but unfortunately they had to cancel at the last minute.
Let's get started.
I should first note that the International Solar Energy Society has a strong partnership with, and is a founding member of, the Global Solar Council, of which our first speaker, Jan Clyncke, currently serves as Treasurer on its Board of Directors.
Jan and I have been working together on the Council's Board, and it's an honor to work with him in developing this important webinar.
Thank you, Dave, for the nice introduction.
You should now be able to see my screen.
I'm here to introduce the legal settings of recycling PV panels around the world, and I'm happy to do this.
At the start, I'll show you the agenda for my presentation. I will try to limit it to 15, maximum 20 minutes.
Maybe surprisingly, the first point will be a kind of glossary or dictionary covering some words you will hear not only from me but also from my colleagues and next speakers, Carsten and Esther.
Then I will tell you very briefly about PV Cycle and the milestones we achieved so far.
After that, I'll move to the main topic: where we are on the legislative front around the globe. Where is there already legislation in place regarding end-of-life management of PV panels, where is there not, and where are draft regulations currently published?
Finally, I will briefly highlight the European Union in two slides. Most of you may have heard terms such as WEEE, and I will explain them because in Europe there is clear legislation for electronic waste, including photovoltaic panels.
I will not read through all the definitions on these slides. They simply provide a flavor of terminology you might hear from the other speakers as well.
For example, reuse is a term you will certainly hear several times when Esther speaks. Mr. Bombach will discuss waste, waste treatment, and related concepts.
I also point out that these definitions are currently applicable in the European Union, and your local jurisdiction may define disposal, recycling, or material recovery differently.
At least in some cases, the European definitions may guide other jurisdictions around the world.
For this webinar, I think reuse is an important concept. On the next slide you will also see recycling, recovery, and preparation for reuse, which is actually a step before the reuse stage itself.
In Europe, another very important definition is the definition of waste. This is not just a technical but also a politically sensitive topic because the definition has remained essentially unchanged since 1975.
Waste management and the waste hierarchy are also important concepts. The first objective is, of course, to prevent waste. Only in the final phase, in the worst-case scenario, do you dispose of or landfill material. In Europe, disposal generally refers to operations such as landfilling.
At the bottom you see WEEE, which we will discuss in the next section.
Who is PV Cycle?
PV Cycle is a not-for-profit association. Since 2014, when relevant legislation came into effect in Europe, we have functioned as a Producer Responsibility Organization.
We were founded in July 2007.
The organizations involved in founding PV Cycle included private companies in the solar industry as well as industry associations such as the European and German sector organizations. They came together and said that they wanted a take-back program for PV panels in Europe.
At that time there was no legislation, so participation was purely voluntary.
Why is PV Cycle a not-for-profit organization, and why is this common in many European situations?
It is important to have an independent role because, in the end, you represent the interests of those placing products on the market. Those parties must be able to organize the take-back, recycling, and financing of end-of-life products.
By being not-for-profit, you remain independent from potential suppliers, whether they are in logistics, recycling, or treatment services. This allows you to focus on one objective: striving for a zero-cost operation for the industry under this extended producer responsibility system.
During the 13 years we have been active, we can clearly state that we were the first collective take-back and recycling scheme for household and discarded PV panels anywhere in the world.
As I mentioned, we started as a voluntary initiative since no legislation existed at the time. In June 2010, we organized the first collection of PV panels in Europe.
Over those 13 years, many developments took place, including legislative changes. For example, Europe has battery directives that impose producer responsibility obligations on battery manufacturers. These have become increasingly important as energy storage grows.
Since 2014, the revised European directive has explicitly included photovoltaic panels within its scope, so since then we have offered compliance solutions for companies selling PV panels in Germany, Italy, Spain, and the other European Union member states.
Over time, PV Cycle expanded its scope. Europe remains our core market, but since 2017 we have become increasingly active globally.
The slide also highlights some achievements.
As I mentioned, the first PV panels were collected in June 2010. Two years later, we had already collected our first 1,000 tons. With the introduction of the WEEE Directive, we also expanded our solutions to other types of electronic equipment in countries such as Germany and Italy.
In 2015, we achieved ISO 9001 and ISO 14001 certifications. As the directive was implemented, we opened offices and local operations in Belgium, France, Italy, Germany, and elsewhere. In Italy, we have also become quite active in the battery sector.
Looking at the first nine years, from 2010 through the end of 2019, we collected roughly 37,000 tons of PV modules. In terms of power capacity, this corresponds to approximately 0.5 gigawatts.
That figure also demonstrates how small end-of-life volumes remain compared with the roughly 100 gigawatts installed in Europe.
Now let us move into the main topic: the global legal framework.
I will start with the European Union, where I have already mentioned the WEEE Directive several times.
This directive was adopted in 2012, and from February 2014 onward, each EU country was required to transpose and apply it within its national legislation. We now have almost seven years of practical experience with it.
Outside Europe, Washington State in the United States is one of the few jurisdictions with similar legislation already in place. Its requirements began in January 2022 and include a PV panel stewardship and take-back program for those selling panels for the first time within Washington State.
In the final section of my presentation, I will look at the European Union in more detail.
What is key in Europe? Where does responsibility start?
To understand who is responsible for organizing take-back, recycling, and financing when PV panels reach end of life, the legislation first defines who is considered a producer.
A company established in an EU country is considered a producer if:
- It manufactures and sells PV panels within that country.
- It imports and distributes PV panels from another brand.
- It purchases OEM panels, rebrands them, and sells them under its own name.
- It sells panels directly through online channels into that country.
The online sales provision was particularly important because it addressed situations where consumers purchase products through webshops operated by foreign companies.
For example, if a consumer in France buys PV panels online from a company based outside France, that foreign company must appoint an authorized representative within France to fulfill WEEE obligations.
The key principle is simple: the first entity placing the PV panel or inverter onto the market in a given European country is considered the producer.
It is important to understand that “producer” is a legal term. It refers to much more than manufacturing.
Once you determine who the producer is, the next question is: what are their obligations?
There are seven major obligations under WEEE legislation:
- Organize take-back and treatment of products.
- Register in the national WEEE registry.
- Report quantities placed on the market.
- Inform users how products should be properly disposed of.
- Label products appropriately.
- Finance current and future waste management operations.
- Provide treatment facilities with information regarding product composition and hazardous substances.
Companies may fulfill these obligations individually or hand them over to a producer compliance scheme such as PV Cycle.
Thank you for your time, and I am looking forward to hearing from the next speakers.
Thank you very much, Jan, for your excellent overview of this important topic.
Just a reminder to all of our attendees that if you have questions for any of the speakers, please enter them into the question area on your screen. Some questions will be answered online, and others we will try to address at the end of the webinar after all speakers have presented.
Our next speaker is Esther Voroshazi.
Dr. Esther Voroshazi is the R&D Manager of IMEC’s photovoltaic activities, covering thin-film silicon and tandem cells, innovative module technologies, and PV system simulations.
In parallel, she serves as the photovoltaics activity leader at EnergyVille.
She holds a Ph.D. in Engineering from KU Leuven and has extensive experience with device and module technologies, reliability, characterization, and sustainability.
Esther, the floor is yours.
Thank you for the introduction.
It is my pleasure to speak with you today. Welcome, PV colleagues, to my presentation.
Let me share my screen.
Welcome, PV colleagues.
Today I would like to discuss how we at IMEC are working to make photovoltaics more circular. In particular, I would like to explore how reuse can be integrated into the PV sector.
First, let me introduce IMEC.
IMEC is a non-profit international research center focusing on nanotechnology, digital technologies, and energy technologies.
Our mission is to look ahead and explore, validate, and scale technologies three to ten years ahead of industrial needs.
In addition, IMEC is part of EnergyVille, a partnership between KU Leuven, UHasselt, VITO, and IMEC.
EnergyVille focuses on accelerating the energy transition by bringing together research on materials and technology, system integration, electrical and thermal networks, buildings, cities, districts, and policy requirements.
Today I will specifically focus on circular approaches within photovoltaics.
As we enter the terawatt era, many of us are excited that the one-terawatt milestone is now within reach. Within two to five years we expect to achieve one terawatt of cumulative installed PV capacity worldwide.
But this is only the beginning.
Looking a decade ahead, we anticipate more than 10 terawatts of cumulative PV capacity deployed globally.
That is excellent news. However, as Jan already mentioned, PV waste will follow.
Already today, we are roughly at one million tons of PV waste worldwide. That figure is expected to increase tenfold during the coming decade.
If we examine the geographic distribution, no continent will be spared. This is a truly global issue that requires a global solution.
That is why we believe circular economy principles represent the only viable solution.
A circular economy is one in which the value of products and materials is maintained as long as possible while minimizing waste and resource consumption. When a product reaches the end of its life, it is used again to create future value.
In a circular economy, rather than extracting materials, producing products, using them, and discarding them, we focus on reuse, repair, remanufacturing, and recycling at every stage.
Today, I specifically want to focus on reuse.
I will provide an overview of the current PV reuse sector, discuss recommendations based on technical assessments, and address economic, environmental, and social aspects of reuse.
I will also give an overview of market participants and present several examples of reuse projects.
To understand the PV reuse sector, we first examined the origins of PV modules that can potentially be reused.
Where do reusable PV modules come from?
One important source is commercial and utility-scale PV systems that have been partially damaged by extreme weather events.
For example, a hailstorm may severely damage some modules in an array while leaving many neighboring modules fully functional. Those undamaged modules represent a valuable reuse opportunity.
This source is expected to become increasingly important during the coming years.
Insurance industry data already show a clear increase in weather-related damage to PV systems over the past five years.
A second source consists of repairable modules. In many situations, modules with relatively minor defects are fully replaced, even though they could be restored with straightforward repairs.
Examples include:
- Junction box failures
- Frame damage
- External cable failures
A third source arises from repowering projects.
Many commercial and utility-scale PV plants are upgraded after 10 to 15 years of operation. Even though the original modules remain functional, economic considerations may justify replacing them with newer, higher-efficiency units. These removed modules can enter the reuse market.
When discussing circularity, environmental considerations are obviously very important.
Does reuse actually make environmental sense?
Our assessment clearly showed that reuse has a very positive environmental impact.
Extending module lifetime distributes the original material and energy investment over a longer operational period, reducing the overall environmental footprint per unit of electricity generated.
In addition, reuse can increase access to solar power in lower-income countries and regions with weak electric grids where diesel generators remain common.
In such settings, photovoltaic systems can improve air quality and reduce emissions.
We also examined environmental feasibility.
Can reuse remain environmentally beneficial once transportation and logistics are considered?
Our analysis indicates that repair and reuse can indeed be environmentally favorable.
However, one major concern remains.
Even reused modules will eventually reach end of life. In countries without robust waste legislation or recycling infrastructure, there is a risk that those modules may ultimately be landfilled.
Because of this, we strongly recommend the development of accredited reuse centers that can ensure modules are properly recycled when they finally reach the end of their useful lives.
We also considered social impacts.
Reuse creates jobs.
Employment opportunities arise both during decommissioning and at reuse facilities. These jobs are local and generally cannot be outsourced.
If we compare with other electronic waste sectors, studies estimate that approximately 60 jobs can be created for every 1,000 tons of electronic waste managed.
Across all perspectives, one issue stands out as critically important: trust.
Second-hand electronic products must remain affordable while still inspiring confidence among buyers.
Factors such as:
- Product information
- Product insurance
- Warranties
- Vendor reputation
- Reuse center reputation
all play crucial roles.
This is why we strongly advocate for standards, guidelines, and quality criteria specifically addressing second-hand PV modules.
Without clear standards, confidence in the reuse market remains limited.
What does the current market look like today?
Through interviews and market research, we identified approximately 15 companies worldwide actively trading second-hand PV modules.
Within Europe, we found around five companies operating in this space, all of them headquartered in Germany.
Most activity, however, occurs outside Europe, particularly in China and regions where waste legislation is more limited.
The current market is still relatively small.
Based on available data, we estimate that approximately 500 to 600 megawatts of second-hand PV modules are traded annually worldwide.
Although this estimate likely underestimates the true market size, it provides at least a baseline understanding.
The majority of module flows are from the United States and China toward Africa and South Asia.
I would now like to share a few practical examples.
One example is a co-housing project in Belgium. The building was equipped with second-hand PV modules, allowing residents to access solar power despite budget limitations that would otherwise have prevented installation.
Another example involves communities located in remote regions experiencing energy poverty.
In these projects, reused PV modules are deployed in low-voltage and even DC-based systems, giving perfectly usable modules a second life while bringing significant benefits to local communities.
To summarize:
PV module reuse is clearly beginning to grow and mature.
Opportunities for reuse are increasing, and we already see an active market of approximately 500 to 600 megawatts per year.
From a technical perspective, quality testing and low-cost repairs are feasible. However, we strongly advocate for detailed recommendations and standards because those are still lacking today.
This absence of standards creates uncertainty and limits confidence in reused modules.
From an economic standpoint, reuse is already highly attractive in lower-income countries. In other regions, circularity policies or carbon-related regulations may be needed to further encourage adoption.
From both environmental and social perspectives, PV module reuse is highly desirable.
Additional legislative development is still required, and we intend to continue working with policymakers both within and outside Europe to help make reuse a more integral component of the solar economy.
As a final thought, I would like to share an updated vision of the circular economy.
Circularity should not only focus on reuse, repair, and recycling after products are manufactured.
We must also begin at the design stage.
Future PV modules should increasingly be designed for repairability and recyclability from the outset.
We need a new generation of PV products created with circularity in mind from day one.
Thank you very much for your attention.
Thank you very much, Esther.
That was a very interesting presentation and clearly highlighted the market opportunities for reusing older PV systems.
As we move from one terawatt of installed capacity toward potentially eight to ten terawatts during the coming decades, this will become an increasingly important topic.
The opportunities you presented are very compelling, and I also appreciated your observations regarding the effects of climate change and the increased risk of system damage.
Thank you again for your presentation.
I would once again remind our attendees that you can submit questions through the question panel. We will try to answer as many as possible at the end of the session.
Our third speaker today is Carsten Bombach.
Carsten has more than 30 years of experience in photovoltaics and environmental technologies. He has worked with Bayer, Pilkington, SolarWorld, Bombach Consulting, and BIFA Environmental Institute.
His work focuses on circular economy practices, waste classification, and resource recovery.
He is active internationally through organizations and initiatives including EPIA, BSW, PV Cycle, and IEA PVPS Task 12.
He was also one of the initiators and co-founders of PV Cycle and served as its first president.
Carsten, the floor is yours.
If you want more insight in the process of recovering of specific elements and design of the material flow, this article provides a comparative analysis of recycling of two types of PV panels - Deutsche Solar and First Solar - including LCA considerations and cost analysis.
Supplemental Reading:
Journal publication: Kim, S., Jeong, B., Closed-Loop Supply Chain Planning Model for a Photovoltaic System Manufacturer with Internal and External Recycling, Sustainability 2016, 8(7), 596.
URL: https://www.mdpi.com/2071-1050/8/7/596
The presented analysis and modeling shows that using the external recycling facilities as material source, the PV manufacturers can save on some costs. Joining a recycling association decreases the total cost of c-Si panels by 55.28% and CdTe panels by 2.28%.
Probing Question
Do you know what programs and policies for electronic and PV recycling exist in your town, city, or area? Do residents and business choose to use them? Why yes or why not?
References:
Mulvaney, D., Act Now To Handle The Coming Wave Of Toxic PV Waste, Solar Industry Mag 2015. Accessible from URL: https://solarindustrymag.com/
Williams, B., Photovoltaic (PV) Recycling, Final Project, EME 807 Technologies for Sustainability Systems, Renewable Energy and Sustainability Systems (RESS) Program, Penn State University, 2016.