KaXu Solar One, South Africa’s first parabolic trough CSP plant

Credit: Abengoa

Two projects in Africa are showcases for the energy storage potential of molten salt, writes Javier Relancio

South Africa is abundantly sunny, so it makes sense to capture this natural resource and use it to turn the lights on.

In recent years the South African government has taken action to stimulate delivery of renewable power generation capacity by awarding project agreements to developers through the Renewable Energy Independent Power Producer Procurement (REIPPP) programme.

Mott MacDonald was the lead consultant to the government on the development of REIPPP. Projects have been awarded under five bidding windows so far, and many of these have achieved financial close. A sixth bidding window is expected to commence soon.

The majority of projects selected for development under the programme are wind farms and solar photovoltaic (PV) plants. However, a small and slowly growing proportion has focused on an increasingly important player in renewable energy: concentrated solar power (CSP).

Compared to PV, which is becoming ubiquitous, CSP is still nascent as a commercial-scale solar generation technology. But it has an advantage over PV in that it is easier to incorporate energy storage into a large-scale CSP plant.

This is a valuable benefit for South Africa, given its power demand profile. A sharp peak in demand consistently occurs around sunset, when solar generation yield is tailing off. With energy storage, state-owned utility Eskom can deal with this peak in demand by calling on reserves of solar energy stored earlier in the day by CSP plants.

Sub-Saharan Africa’s first large-scale operational CSP plant, KaXu Solar One in South Africa, was commissioned in early 2015 after project owner and developer Abengoa successfully steered the scheme through REIPPP. Mott MacDonald was the lenders’ technical advisor (LTA) during its financing and construction, and is now providing operational monitoring services on behalf of the lenders.

It is easier to incorporate energy storage into a large-scale CSP plant than a PV project.

Credit: Abengoa

As South Africa’s first parabolic trough CSP plant, KaXu is a major achievement in itself. But it is about to be overshadowed by a ‘big brother’ being constructed next door: Xina Solar One.

Mott MacDonald is also the LTA for Xina, which also progressed through REIPPP and is being developed by Abengoa.

CSP uses giant curved mirrors – parabolic troughs – to focus the sun’s heat onto horizontal absorber tubes filled with oil-based heat transfer fluid, which convey the thermal energy to a main heat exchanger where water is converted to steam. The steam drives a turbine, which produces power for the grid. Xina features a new generation of the technology based on a larger version of the trough collectors, meaning it can achieve the same yield as KaXu with fewer collectors.

The new collector size required some of the components therein to also be redesigned. Mott MacDonald undertook technical due diligence to review the engineering and design of the modified components, including the supporting structure, the hydraulic mechanism, the mirrors and the enlarged absorber tube.

As part of this exercise, the consultancy also engaged with certain critical suppliers to assess whether they were capable of producing the larger components as required by the project’s construction schedule.

Salt is used for storage because it has exceptional heat retention properties.

Credit: Abengoa

KaXu and Xina are notable for their use of innovative molten salt energy storage technology to retain thermal energy captured during the day. Salt is used because it has exceptional heat retention properties.

To capture the energy, a proportion of the heat transfer fluid is diverted into a second heat exchanger, where it heats molten salts which are circulated from a cold salt storage tank to a hot tank.

The molten salts are heated to approximately 400oC, then discharged into a separate tank for storage. After sunset, the system reverses to maintain power generation.

Both KaXu and Xina use molten salt storage technology, but once again Xina has the advantage of size because its larger salt tanks mean it can store twice as much energy as KaXu.

Xina’s storage capacity is equivalent to the energy that would be produced by the plant operating for five hours at full capacity.

Mott MacDonald’s due diligence played an essential role in the lenders’ and investors’ assessment of these new technologies, and in enabling the project to go ahead.

The consultancy is currently monitoring Xina’s construction. When Xina enters operation in mid-2017, the two plants will collectively produce enough energy annually to serve 175,000 households and prevent the emission of 648,000 tonnes of CO2 while providing a valuable source of ‘dispatchable’ energy production. Moreover, Xina’s pioneering collector design will pave the way for use of bigger, better, more efficient CSP technology in the future.

Javier Relancio is Solar Programme Leader, Africa, at Mott MacDonald