Commissioning is well underway at DEWA IV, aka fourth phase of the Mohammed bin Rashid Al Maktoum (MBR) solar park (and, somewhat confusingly, also called Noor Energy 1), in Dubai, a 950MW hybrid project consisting of 100MW of central-tower CSP (concentrated solar power), 600MW of parabolic-trough CSP (in three tranches of 200MW) and 250MW of bifacial PV
CSP at the Mohammed bin Rashid Al Maktoum Solar Park. Credit: Dubai Electricity & Water Authority (PJSC)
Noor Energy 1, jointly owned by DEWA (Dubai Electricity and Water Authority), Saudi Arabia-based ACWA Power and Silk Road Fund (state-owned investment fund of the Chinese government), is the project company established to implement this fourth phase, under a 35-year power purchase agreement.
The 700MW of CSP is thought to make it the world’s largest single-site concentrated solar power facility, while the 262.44m high central solar tower is believed to be the world’s highest such tower.
The DEWA IV project, which will occupy an area of 44 square kilometres, was awarded to an ACWA Power-led consortium in 2017.
Shanghai Electric is the EPC contractor and NOMAC (First National Operation & Maintenance Company, a wholly-owned subsidiary of ACWA Power) is the O&M contractor.
Thanks to significant molten salt thermal energy storage capacity – 15 hours of operation – the plant has the ability to deliver reliable and dispatchable solar-generated power round the clock.
The 950MW 4th phase is based on the independent power producer (IPP) model with investments totalling AED 15.78 billion.
The levelised cost of electricity (LCOE) is USD 7.3 cents per kWh for the 700MW of CSP capacity and USD 2.4 cents per kWh for the 250 MW of PV.
The plant will support the Dubai clean energy strategy goal of increasing the share of clean energy in Dubai to 25% by 2030.
Central-tower CSP
The basic principle of the central-tower CSP plant is to convert primary solar energy into electrical energy using a field of 70 000 heliostats (flat mirrors on mounting poles with tracking capability), a solar receiver located at the top of a high tower in the centre of the solar field, a system for transferring and storing the thermal energy, based on molten nitrate salts, and a steam cycle with a turbo-generator, rated capacity 100MW.
The John Cockerill designed and supplied solar thermal receiver includes a wall of pipes, which acts as a heat exchanger. The solar radiation directed to the receiver heats the working fluid, molten nitrate salts, from 300°C to 565°C. John Cockerill signed a contract in 2018 for the design and supply of the receiver with Shanghai Electric Brightsource Solar Energy Limited for the owner Noor Energy 1.
The ‘cold’ salt is held in a storage tank at around 300°C. The receiver’s circulation pump pumps the salts from the cold salt storage tank to the receiver, passing through the Inlet vessel.
At the receiver outlet, the hot salts (at 565°C) enter the outlet tank, which acts as a buffer for the hot salt system.
From the receiver’s outlet tank, the hot salts descend from the tower to the hot salt tank, where they are stored. From this tank, the circulation pump pumps the flow required for steam generation, which consists of a group of salt/water-steam heat exchangers.
On cloudy days, and at nights, the operation of the solar field stops, but the discharge of the storage system can begin. The receiver’s salt circulation pump stops, the receiver empties, and the salt circulation pump supplies hot salt to the steam generator.
To keep the salt from solidifying, the plant includes electric resistance elements submerged in the salt storage tanks.
A major milestone for the project was achieved in July 2022, when the first salts, at around 290°C, were injected into the receiver. The heating of the salts was then progressively increased by focusing more and more of the heliostats on the receiver in order to reach the required 565°C at the receiver outlet.
The solar thermal receiver is “impressive both in terms of its dimensions and its capabilities”, says John Cockerill, “a giant interlacing of pipes, tanks and heat exchangers.” It weighs around 1500 tons and is itself about forty meters high, located within the 262 m central tower.
John Cockerill describes itself as “the world’s leading supplier of molten salt solar receivers”, with five reference projects, including DEWA IV and a 2021 contract for the design and supply of a receiver for a CSP project in South Africa.
Parabolic-trough CSP
The parabolic trough collectors (arranged in three groups, 200MW each) track the sun from east to west to maximise electricity generation. At their focus, they have absorber tubes through which the heat transfer fluid (HTF) circulates. This fluid consists of a eutectic mixture of diphenyl and biphenyl oxide (type Therminol VP1, Dowtherm A or equivalent) with a freezing temperature of around 12°C.
The parabolic trough collectors raise the temperature of the heat transfer fluid up to 393°C. The energy contained in this fluid is transferred directly to a steam generator or it can be sent to a thermal storage system (two molten salt tanks) where it is kept for later use.
The parabolic trough power generation system can be operated in different modes. In the direct operation mode, the heat transfer fluid flows from the solar field to the solar steam generation system where main steam is produced at a temperature of around of 380°C and a pressure of 100 bar, passing the fluid through different (or just one) parallel trains of heat exchangers, each one composed of three heat exchangers connected in series (preheater, evaporator and superheater).
A train of heat exchangers in parallel is used to produce reheat steam. The heat exchangers cool the HTF and send it to the solar field to be reheated. Surplus energy is sent to the thermal energy storage system.
The steam-water cycle is a classical Rankine regenerative cycle with reheat.
An auxiliary PV is connected to the CSP electrical auxiliaries busbar to supply auxiliary power during the day.
Photovoltaics dominant
The 250MW of PV included in Phase IV is split into two PV plants. One is adjacent to the heliostat field of the central tower. This has a net export capacity of 217MWac.
The second, a smaller PV installation, with a net export capacity of 33MWac, is adjacent to the parabolic trough area.
The 13MW photovoltaic first phase of the solar park became operational in 2013. The 200MW second phase, also PV, was commissioned in March 2017. The photovoltaic third phase has a capacity of 800MW. A power purchase agreement was signed in March 2020 for the 900MW photovoltaic fifth phase.
In October 2022, expressions of interest were invited by DEWA from international developers for phase 6, a further 900MW of PV, due to become operational in phases, starting from Q3 of 2025.
Overall, the Mohammed bin Rashid Al Maktoum facility is the largest single-site solar park in the world based on the IPP model, with a planned capacity of 5000MW by 2030 and total investments of up to AED 50 billion.
This article first appeared in Modern Power Systems magazine.