Dispatchable Solar Runs 24/7 For 14 Days In South Africa

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ACWA Power has now demonstrated fully dispatchable solar for when the sun doesn’t shine, at Bokpoort in South Africa.
ACWA Power day and night dispatchable solar
Image Credit: ACWA Power

Running around the clock for two weeks, ACWA Power’s 50 MW Bokpoort project is an example of Concentrated Solar Power (CSP), the solar that can be called on day or night, because when it has integrated thermal energy storage in molten salts, it is dispatchable solar.

A “trough” CSP project, Bokpoort was built in just two years, and includes 450 MWh/day of molten salt storage.

“Although Bokpoort is not the first CSP plant to provide round the clock generation, ACWA has achieved this milestone in record time,” said Belén Gallego, who now heads up strategy and business development at ATA (Astrom Technical Advisors) Renewables, the 11-year old Madrid-based technical consultancy that has provided technical expertise to many of the CSP projects built to date.

“Going forward this milestone will pave the way for the next generation of CSP projects,” added Gallego, who founded CSP Today and has been an industry expert for ten years.

In South Africa, the value of dispatchable solar, which can be tapped when the sun doesn’t shine, is understood by regulators. 

Instead of having to compete on price with PV (an already successful daytime renewable) in South Africa, CSP is incentivized to generate after dark, for “on demand” solar. Under the REIPPPP energy policy, CSP is given its own carve-out within the renewable target, like PV and wind.

As a result, all of the CSP awarded in South Africa has included integrated thermal storage.

In the first three rounds, South Africa awarded 400 MW of CSP, and storage capacity has steadily increased over these three bidding windows. In the first window, storage was only for 2 hours (Khi Solar One). In the latest award, Redstone has 12.5 hours.

Because of the certainty that such a carve-out provides for CSP developers, in these first three bidding windows prices have dropped in half from an initial 33 cents (US) for the very first project to 16.6 cents in Round 3.

The fact that Bokpoort was completed so fast suggests that South Africa’s sped-up deployment program for CSP also shortens construction time.

PV has “reached escape velocity,” not just because of its low prices, but also because it is generally the fastest way to get any electric power online. All the traditional power technologies — nuclear, coal, gas, geothermal, and hydro — require turbines and power blocks, and they take longer to build.

But some fossil plants are taking even longer than most traditional plants. South Africa’s state-owned utility, Eskom, has several coal plants underway that have been so beset by delays that newly awarded PV plants have been doing the job for them in the meantime.

That solar-fueled Bokpoort, which also generates from a power block and steam turbine like traditional electric power station, was so much faster than fossil energy to build, was remarkable.

According to Nandu Bhula, CEO of ACWA Power SolAfrica, all the CSP projects located in the excellent solar resource of South Africa have proven to be less prone to blackouts due to sudden load losses from the nation’s increasingly frail coal plants.

To test how long it could run on its stored solar energy each day, ACWA Power operated Bokpoort at 66 percent of its capacity. When operating at the full 50 MW, the storage is enough for 9.3 hours each day. Operating it at 66 percent, ACWA was able to spin out quite a bit longer, with round-the-clock generation from solar for 14 days.

Bhula said that Bokpoort’s 14-day record indicates that South Africa could replace coal generation after dark with CSP. South Africa needs to expand its CSP allocation to allow the technology to reach its full potential as the replacement for fossil fuel after dark or on demand, he said.

He suggested that the carve-out should be increased to utilize more CSP, which would lead to lower prices with increased deployment. South Africa needs to use CSP with storage to replace Eskom’s ageing coal-fired power stations.

The CSP allocated in the first three rounds has been a fifth of the capacity allocated for PV, which is nearly 2 GW. Wind, with 1.5 GW awarded, is more than three times the 400 MW of CSP awarded.

Until recently, Eskom relied on coal power, and not very well: blackouts

South Africa’s coal-dependency and years of blackouts – coupled with a new and widely shared political commitment on climate – is opening a real path for CSP to replace nighttime power.

“The quality of the coal that our power stations are getting is really poor. They are not getting the right grade of coal, it is a really bad grade,” said Davin Chown, who was Managing Director at Mainstream Renewable Power when it was awarded several wind contracts in South Africa.

“They are already having some difficulties in meeting the load now. In 2008 it had everything to do with the lack of build in South Africa. The economy simply grew faster than the government predicted. So they need to add new capacity with renewables. But also the distribution networks and transmission systems had not been adequately maintained for years, and the whole system collapsed. So a combination of not enough supply, increased demand, and neglected infrastructure.”

There are policies coming into place to have an independent system operator, but until now the grid has been run by the state-owned utility, Eskom.

“The whole balancing of supply and demand comes through the national planning center and that’s run by Eskom, so their task and mandate is just to keep the lights on,” Chown said.

Eskom will have to begin decommissioning coal power stations by 2020.

South Africa’s renewable procurement policy emphasizes domestic industry growth from a jobs perspective. Points are assigned based on meeting these requirements, and are part of what determines which projects get awarded.

Much of a CSP plant involves parts available domestically, so it is not likely to get embroiled in the fierce trade wars of the PV industry.

“South Africa has a lot of the infrastructure already with important major components like mirrors, steel, and concrete structures,” said Vikesh Rajpaul, Renewables Unit CSP Program Manager at Eskom, which is building its own 100 MW CSP, in addition to contracting for another 400 MW of CSP (to date) as the off-taker.

“Roughly 40 percent of the plant could be local,” aid Vikesh. “And as we get more CSP plants online, we anticipate that the percent of local input would increase.”

How trough CSP works

Trough CSP – as in Bokpoort – is the older, tried and tested form of CSP technology. It works quite differently from the newer tower CSP.

In trough projects, straight rows of trough shaped mirrors reflect and focus sun onto tubes running immediately in front of each row, heating a HTF (heat transfer fluid) within the pipe to 400° C, then sending the heated fluid through larger pipes at the end of each row to a central power block to generate electricity, either by running it through a heat exchanger to use immediately to generate steam, or by first sending it to thermal storage in molten salt.

Molten salt is a more mature storage technology at utility scale than batteries

Image Credit: Deloitte

Although tower technology is newer, and storage is more typically associated with tower CSP, molten salt storage has been proven at utility-scale for storage since 2011.

SENER’s Gemasolar tower CSP in Spain, at only 19 MW,  but “top heavy” with storage for 15 hours, is a very storage-dense project – almost literally a battery – with 285 MWh/day.

Bokpoort, though a trough project, uses molten salt to generate 450 MWh/day of thermal energy storage.

Not all trough CSP has included storage, but Bokpoort includes enough to run at full load for 9 hours daily on stored solar energy when the sun doesn’t shine.

At Bokpoort, when the HTF is piped to a central power block, it can be either used directly right away, or run through a heat exchanger to transfer its heat into molten salt for storage in a huge tank.

Whenever needed to deliver power, the heat stored in the molten salt can be called upon to boil water to steam to drive the steam turbine. Bhula said that Bokpoort can switch from direct generation to generating from the stored molten salt in about 30 minutes.

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