Long read: Solar + wind, the benefits of co-location


From the September edition of pv magazine

When subsidies for renewable energy generators were more generous, wind and solar fiercely competed for the attention of policymakers. This rivalry resulted in a fervent debate over the relative merits of each technology. In a way, it became the renewable energy version of Monty Python’s Judeans People’s Front – or was it the People’s Front of Judea?

In recent years, however, a number of companies have started to venture into wind and solar co-location, and the trend is now picking up serious momentum. In February, U.S.-based NextEra Energy Resources signed a deal with Portland General for a 300 MW wind farm to be paired with 50 MW of solar and 30 MW of storage. Just a few months later, the company made headlines again with a new deal in Oklahoma. The project will include 250 MW of wind, 250 MW of solar and a 200 MW storage system – the largest of its kind in the United States, the company claims.

In early August, Adani Electricity Mumbai also launched a tender for 350 MW of wind and solar in India. Around the same time, Canberra-based developer Windlab connected 43 MW of wind and 15 MW of solar to the grid at the Kennedy Energy Hub in the Australian state of Queensland, backed by 2 MW/4 MWh of battery storage capacity. The project followed the successful integration of
165 MW of wind at Australia’s 10 MW Gullen solar farm in early 2018.

The reason for this shift is primarily due to cost reductions, although developers can enjoy many technical advantages from the pairing of PV and wind. The competitive landscape has also been stirred up, as the technical barriers to entry for wind are generally higher than those of solar. Therefore, it is more likely that wind developers will venture into solar, rather than vice versa.

New environment

Hannah Staab, principal of solar and energy storage for renewable energy consultancy Natural Power, points to financial factors as the main reason for the growing interest in wind-PV projects. A small number of U.K. companies started looking at co-location a few years ago, she says. “But in a subsidy environment there weren’t really any commercial drivers to seriously follow up with that,” she adds.

Companies need to get creative about where to cut costs to survive in a subsidy-free market. Co-locating wind and solar capacity means that the two generators can share a grid connection, land, substation and power electronics, as well as permitting procedures and even some operations and maintenance work. All the while, the two energy sources maximize the capacity factor of the grid connection.

“Just by sharing the infrastructure you immediately have a lower capex and then we arBae moving toward a world where a firmer generation profile can protect you in a merchant environment, where as a pure wind or solar farm you could be more exposed to electricity price fluctuations,” Staab elaborates.

The savings resulting from shared grid and civil infrastructure could translate into a 10% discount per MW of solar PV, depending on the characteristics of a project. And in a capex-heavy industry such as solar, a 10% savings on infrastructure-related outlay is no small change. “Grid-connection costs have largely remained stable or even increased over the last years, while module and other component costs have come down significantly,” Staab says.

The costs of the grid connection can be shared, but overall development costs can also be driven further down because on-site roads have already been established. O&M crews can also be trained to handle both wind and solar inspections, so the additional PV is often worth the effort, despite the expectation of higher curtailment losses.

Over the past 18 months, for example, renewable energy developer BayWa r.e. has been looking at co-locating wind and solar, says Philipp Kunze, the head of global hybrid and project development for the company in Greece. “What we are trying to do is get out of government support schemes,” Kunze explains. One way to do this is to enter into power purchase agreements, often with industrial off-takers. When a power plant fails to deliver the promised power due to a lack of wind or sun, the power must be bought from the spot market, which can drive up balancing costs and damage the economics of a PPA. By co-locating wind and solar, the power supply becomes firmer, as the two energy sources are complementary in many markets – that is, the sun shines when the wind doesn’t blow, and vice versa.

Kunze points to Wind Europe’s recent “Renewable Hybrid Power Plants – Exploring the Benefits and Market Opportunities” report, which says that wind and solar are almost perfectly aligned in some markets. In parts of China, Argentina, Somalia, Australia, the United States and India, for example, such hybrid power plants can exceed 7,000 full-load hours. In particularly ideal locations, analysts have calculated the possibility of 8,000 full-load hours per year. As there are only 8,760 hours in a year, hybrid power plants in such locations almost match the capacity factor of thermal generators. But in most parts of Europe, Asia and Africa, it’s easy to achieve 4,000 to 6,000 full-load hours per year. According to Kunze, solar alone would achieve little more than 1,400 full-load hours in Europe.

Sweet spot

Capturing these full-load hours is not easy, however. From a broader regional perspective, wind and solar resources tend to be complementary, but this is not always the case in a site-specific context, Staab explains. “In the U.K., wind is often installed on hilly terrain, which is often covered in forests,” she says. “This is not so ideal for solar. In Australia, on the other hand, there are huge open spaces that are ideal for both solar and wind.”

Sizing a plant is very dependent on the type of project and its location. “There are really two kinds of projects in this field,” Staab says. “One in which developers plan the co-location from scratch, which is what we see in Australia, India, or the U.S. In the other one, existing wind farms are retrofitted with additional solar arrays, which is something we see a lot more in the U.K., Germany or France.”

The team at Natural Power modeled the ideal sizing ratio between solar and wind for the United Kingdom, for a wind park set to be retrofitted with solar. The two generation profiles are complementary, but there will still be curtailment losses, mainly due to the added capacity that goes beyond the grid export limit. When a wind generator produces at 100% of its grid export limit, anything that the solar array provides will be curtailed. Based on wind and solar resource profiles that are typical for the United Kingdom, if you had a 10 MW wind farm and added about 5 MW of solar capacity, curtailment losses would reach around 5%.

Kunze points to another layer of complication in sizing the generators, as one can add seasonal changes to production and consumption. He says that on many Greek islands, the production of both wind and solar peaks during the summer months. This aligns itself with the tourist season, making such hybrid plants a suitable candidate for the power supply on these islands. Yet still, this alignment has to be planned and modeled correctly. “This is considerably more complex than having a 10 MW solar plant with a fixed feed-in tariff,” Kunze says.

Virtue out of necessity

Beyond the economics, co-location offers more stable, predictable and dispatchable power output. Both wind and solar are naturally subject to cloud cover or gust changes, which alternates the power output. If the two work in tandem, they level out the effect to a certain degree. He explains that by co-locating wind and solar, one can reduce the grid capacity, because the power is more likely to be produced where it is needed. Today, power still needs to be distributed from windswept coastal lines further inland, for example.

“This is why we see markets like India or Australia with a relatively weak grid picking up on the trend,” explains Kunze.