The 24-hour life cycle of a solar PV park can be characterized as a daytime opportunity and a night-time waste. When the sun rises, its time to make money by selling zero-emission power. When it sets, tens of millions of dollars in capital assets sit idle waiting once again for the crack of dawn. But selling megawatts into the grid isnt the only revenue-generating role a solar PV facility can play. Rajiv Varma, Associate Professor of electrical engineering at Canadas University of Western Ontario, has come up with an inexpensive way to modify solar inverters to behave as static synchronous compensators (STATCOMs), making it possible in the future for solar operators to offer services during the evening that help increase and stabilize distribution and transmission capacity on the grid. And its not just a night-time opportunity. When the sun is less intense or clouds are persistent, solar inverters with idle capacity can still function partially as STATCOMs during the day. It opens up new possibilities for the power system and revenue-making opportunity for the solar farm, in addition to what one gets selling power during the day, says Varma.
STATCOMs are a type of flexible AC transmission system (FACTS) device used by utilities to provide voltage control that can improve grid capacity where distribution and transmission constraints emerge. In addition to voltage control it can reduce power system oscillations that cause system instabilities. Such constraints and instabilities are becoming more common as utilities move to integrate more large-scale renewable systems, such as solar and wind farms, into the grid. The intermittent energy provision of wind farms, for example, has posed significant challenges for European transmission system operators which must comply with grid codes designed to assure a minimum level of power quality and stability.
This is a worldwide problem, says Varma. There are two main options. The most reliable option is to construct a new transmission line for hundreds of millions of dollars. The next option is to add FACTS devices at a cost of $50 million (U.S. dollars) or more, meaning you dont have to construct a new line or can delay constructing it.
Sun and wind
Varma, who has spent the past 20 years conducting research on FACTS devices, was driving home one day from a meeting when a random thought struck him: inverters lie at the heart of every FACTS device. Why couldnt the inverters used in a solar farm be programmed to do the same thing? he thought. That Varma lives and works in Ontario, Canadas largest province, likely inspired his thinking.
Ontario is one of the largest and fastest-growing solar markets in North America, due largely to the introduction two years ago of a Green Energy Act and Economy Act and, months later, the continents most comprehensive and generous feed-in-tariff program. The program offers payment of between 44.3 and 80.2 Canadian cents per kilowatt-hour (equivalent to roughly 0.32 to 0.58 euros) for solar electricity. Within that range, the lowest tariff rates go to the largest projects.
In less than two years the provinces power authority has received applications totalling about 6,000 megawatts, and so far 1,600 megawatts have been offered contracts. At the same time, thousands of megawatts of wind projects are also vying for system access. This has led to major transmission constraints, effectively limiting the potential of the program. Increasingly, around the world, wind and solar are being co-located in certain regions, and were seeing that in Ontario itself, says Varma. Our lines do not have enough capacity to add this new generation. Varma confirmed with at least one wind-turbine manufacturer that this was becoming a serious issue, forcing developers to purchase FACTS devices as a way to justify gaining access to the grid. The manufacturer said it had at least 30 installations across North America where STATCOMs were needed, and that the number will continue to grow. If there are large solar farms close to them, I thought, maybe the STATCOM investment isnt needed.
Testing the theory
To test out his theory, Varma assembled and led a research team that worked closely with Ontarios transmission system operator, Hydro One, and the Canadian division of First Solar, the US thin-film giant. First Solar has been active in Ontario, having already built a solar PV farm in Sarnia, Ontario, that at 80 megawatts (AC output) ranks as the largest operational PV facility in the world. Ironically, its located near a town called Petrolia, the recognized birthplace of North Americas commercial oil industry.
The seven-million Canadian dollar research initiative proved through sophisticated computer models that solar PV inverters could be modified to work as STATCOMs, and Varmas team developed software to easily upgrade the physical hardware. Their next step is to develop ten kilowatt prototype inverter systems, which will be tested first in the lab and then taken into the field. Two project co-funders in the area local utilities London Hydro and Bluewater Power will deploy the modified inverters on their distribution networks. A new company, tentatively named SMG Night Solar, is also being spun out of the university this year to commercialize the technology, likely through a licensing model.
The potential is quite exciting, says Dan McGillivray, managing director of the Ontario Centres of Excellence, a provincial research and development agency that provided the bulk of funding for the project. That view is echoed by Peter Carrie, Vice-President of Canadian business development for First Solar, which also contributed funding. The concept is clearly interesting. It has a lot of promise.
The approach is potentially a win-win-win, with benefits extending to solar operators, wind operators and utilities. Varma estimates that the inverters used within a 4.5-megawatt solar park, for example, would expand distribution line capacity enough in the night to allow an additional seven megawatts of wind power in an otherwise congested area of a network. In the Ontario market, where some wind operators are required to cur tail production in the evening because of grid constraints, that additional seven megawatts could increase annual revenue for an operator by 2.1 million Canadian dollars.
To achieve the same benefit with a STATCOM (or D-STATCOM, when applied to a distribution network) would require an additional investment of between five and eight million Canadian dollars, Varma estimates. The cost of modifying the circuit board and software in solar inverters to do the same job, by comparison, would amount to only 100,000 Canadian dollars. We basically add another card to the inverter that provides entirely new functionality, he says, adding that it can be a built-in feature of new inverters or added retroactively to existing inverters.
The only catch is that, to be most effective at the distribution level, the solar and wind facilities need to be located within five kilometers of each other, beyond which the benefits begin to decrease. Varma envisions a scenario in which a wind operator enters into a revenue-sharing agreement with a nearby solar operator. For every kilowatt-hour of extra wind power the solar operator can help squeeze onto the grid in the evening, it would get a portion of wind revenue earned.
The opportunity is even greater at the transmission system level that is, as an alternative to building a new high-voltage transmission line or spending 50 million Canadian dollars or more on new FACTS devices that can ease congestion on an existing line. Assuming enough solar PV capacity existed in a given area, the less expensive route would be to spend about 500,000 Canadian dollars upgrading solar PV inverters, says Varma.
Take, for example, a 100-megawatt solar park that has had all of its inverters modified and lies in the middle of a 400 -kilovolt transmission line. It can create new capacity of 200 megawatts in the night by working like a STATCOM, says Varma, who recently presented his findings at the IEEE Power and Energy Society conference in Phoenix, Arizona.
But the benefits also extend into daytime, since even then solar PV facilities often generate less than their rated output. Varma found that if a solar facility is, for example, producing 94 megawatts the remaining inverter capacity still has the potential of increasing transmission limits by 97 megawatts except, perhaps, between eleven a.m. and one p.m., when, in the case of Ontario, the sun shines brightest. Except for those two hours when you have peak time, you still have at least 22 hours when you can go up from 94 megawatts to 200 megawatts of new transmission capacity, says Varma. That capacity can accommodate more nuclear, hydroelectric, but more importantly, more output from huge wind farms. It all looks great on paper. Now Varma has to prove it works in the field. And even if it does, many questions remain. How do you monetize it and who pays to do it? asks Peter Carrie of First Solar. The fundamental challenge is how do assign value to such a service. Every situation is different, and utility operations are complex when it comes to power quality. So, commercially, there are a lot of issues that need to be worked through. Coming up with a business model that suits all interested parties will be crucial.
The grid code poses another challenge. According to existing IEEE rules, a solar PV facility can operate as a STATCOM during the night as long as it is dedicated to that function. During the day, however, the IEEE 1547 standard forbids such a facility serving dual or multiple purposes.
Varma is optimistic. As we speak theyre coming up with a new rule that will allow this, he says, referring to IEEE 1547.8. It would expand on the existing standard by giving permission to distributed generators such as solar farms to perform voltage regulation and other functions. He expects similar changes at the transmission level.
Inverter manufacturers are watching closely, including Germanys KACO new energy Inc., which has been fully briefed on Varmas research. Indeed, KACOs Canadian manufacturing operation in London, Ontario, has had preliminary talks with Varma about a potential commercial partnership.
Major solar developers are also taking notice, including SunEdison, the largest solar energy services provider in North America. Varma presented his research to the company earlier this year. We look forward to learning the results from the testing taking place in regions across the province, as they will help us better understand how this innovation might be leveraged to support solar growth across Ontario, and indeed, globally, says Oleg Popovsky, Business Development Manager at SunEdison.
Varma is delighted that his research has been so well-received. Another sign that hes on the right track: General Electric has filed a technology patent on a similar approach. But were ahead of them, says Varma, with detectable relief in his voice. Our patent was filed five months earlier, he adds with a smile.
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