Many commercial Concentrated Solar Power (CSP) plants are operating in various places around the world, the technological development of storage capacity is advancing with each new CSP plant that comes online. The 50 MW Bokpoort CSP plant is capable of storing more than nine hours of thermal energy. At its recent inauguration, EE Publishers sought to find out more about its thermal storage system to shed light on why solar developers are going this route.
ACWA Power’s Solafrica Bokpoort CSP project is located just outside Groblershoop in the Northern Cape. The 50 MW plant, which cost R5-billion to construct, reached full capacity in mid-December 2015 and provides 220 000 MWh to the Eskom grid per year. What makes this plant unique is its thermal storage system (often referred to as a “rechargeable battery”), which stores nine hours and 20 minutes of heat energy and keeps the plant supplying electricity at night.
Solafrica Bokpoort CSP has signed a 20-year power purchase agreement with Eskom, after which the financial viability will be re-evaluated to establish whether a new agreement will be signed. All electricity generated at Bokpoort CSP is fed to the Eskom power grid via the Garona substation and distributed by the utility. The electricity produced at Bokpoort CSP must meet Eskom’s required specifications for quality before leaving the site.
How the plant works
Bokpoort CSP is a parabolic trough system that consists of eight solar fields with 241 920 mirrors which provide a reflective surface area of 658 000 m2. There are 8460 solar collector elements and 25 920 heat collector elements (HCEs). This parabolic trough technology uses precise parabolic-shaped, sun-tracking mirrors to concentrate and collect sunlight on thermally-efficient receiver tubes (HCEs or “collectors”), running through the optical focal line of the parabolic mirror troughs. The heat transfer fluid (HTF) inside the collectors is heated to 393°C as it circulates through the receivers and returns to a series of heat exchangers that convert water drawn from the Orange river into steam, known as the steam generation system, which is in the power island where the HTF is used to generate high pressure superheated steam at 380°C and 105 bar. This steam is then used by a high-pressure steam turbine to generate electricity. A stream of the same HTF is also used to heat a molten salt mixture (for the rechargeable battery), which is stored in large insulated tanks.
The high-pressure steam is directed to a conventional Rankine-cycle steam turbine/generator where the steam provides the energy to rotate the turbine and drive the generator to produce electricity. The remaining steam is transported to a condenser where it is returned to a liquid state. The low-pressure steam from the turbine is condensed by a cooling system as it flows through the cooling loop. After being cooled and condensed, the condensate is returned to the HTF heat exchangers by pumps to be turned into steam again for electricity generation. The main components of the plant are the solar collector assembly, heat exchanger system, molten salt energy storage system, cooling towers and generator. Associated infrastructure includes the auxiliary power plant, sub-transmission power line and the water pipeline.
As mentioned above, the plant is equipped with more than nine hours of thermal storage capacity, making it operate like a rechargeable battery. This unique utility-scale storage system allows this power plant to continue feeding electricity to the grid after sunset. The molten salt (made up of sodium nitrate and potassium) retains heat for long enough to be released into the steam cycle during the night or when the mirrors are obscured by clouds.
The molten salt will not need replenishing for the lifetime of the storage units, which is composed of two tanks – a hot one and a cold one. Each tank is 14 m high and 40 m in diameter and has the capacity to hold the 38 000 t of molten salt. Between the two storage tanks is a set of heat exchangers that convert the thermal heat from the salt in the hot tank to energy, and transfer the heat from the collectors to the salt from the cold tank on its way to the hot tank for storage.
The temperature of the salts in the cold tank is 290°C and the temperature in the hot tank is 386°C. The system is sized precisely to ensure that the hot tank is full by the time the sun sets at the end of the day and heat energy can be drawn from the thermally-heated molten salt. The battery “discharges” during the night as the molten salt moves from the hot tank, through the heat exchangers, to the cold tank, and then it “charges” during the day when moving from the cold tank back through the heat exchangers to the hot tank.
Bokpoort CSP’s thermal capacity of nine hours and 20 minutes is significantly greater than other CSPs built in South Africa. Kaxu Solar One, near Pofadder, Northern Cape, was the first solar thermal energy (STE) storage plant to operate commercially in the country, having come online in early 2015. It has an installed capacity of 100 MW plus two-and-a-half hours of storage capacity in molten salt.
Financial viability of storage capacity
In the case of this CSP, it is financially feasible to include the storage component in the project. The site is exposed to year-round sunshine, which provides enough radiation to store the thermal heat. This, combined with the tariff received from Eskom for the electricity, makes the storage system worthwhile and profitable. The storage capacity gives Eskom an added benefit because it allows the plant to distribute to the utility on demand: if Eskom has a shortage, Bokpoort CSP can provide it.
So, an obvious advantage of a solar field with a thermal storage unit, compared to other renewable energy technologies, such as a wind farm, is the ability to store thermal energy and extend the electricity supply past sun down.
Turbine and generator
The steam turbine is a tandem-compound reheat condensing unit with its high-speed/high-pressure section connected by means of a speed reduction gearbox to a single-flow single-casing low pressure section. A single turbine with two rotors (high and low pressure) are connected to one another through a speed reduction gearbox and to the generator rotor via a solid bolted coupling. The steam turbine is connected to a high-pressure steam inlet (supplying working steam from the steam generator) and a steam outlet transferring spent steam to the cooling system. The turbine is also connected via a common axial shaft to a single generator to which it supplies the input power which is converted to electricity.
There are three weather stations installed at Bokpoort CSP. These calculate the solar energy input at any time and the aggregation sensors measure the maximum amount of energy available.
Additional sensors measure wind speed to calculate thermal losses and help protect the solar field. If high wind speeds are detected, the safety system activates and the mirrors and the collectors are moved into park positions. From the data acquired by the weather stations, the plant’s efficiency can be monitored at all times.
Each collector works with a control system which receives three inputs: solar position, individual collector’s position and the ambient temperature. An algorithm implemented in the control box calculates the solar position in relation to the exact location of the individual collector. The collector moves the mirrors into the optimum position to collect the maximum amount of thermal radiation.
The HCE has a glass-to-metal seal which reduces heat loss. It is coated with a material that has high solar radiation absorbance (that filters out infrared rays) and low thermal remittance that attracts visible light.
A liquid fuel-fired boiler provides heat directly to the HTF to prevent crystallisation of the fluid during cold nights. These heaters are used to maintain the temperature of the HTF above its freezing point of 12°C. The operator will start up the auxiliary heaters when it is impossible for the system to maintain its minimum operational temperature.
An intake from the Orange river through a pipeline supports the water requirements of the CSP facility. Water use is minimised by the increase of the recirculation cycles within the cooling towers. Pre-treated wastewater is collected into evaporation ponds on site.
The locally-sourced components and services included the manufacturing and assembly of the solar field collector steel structures, and the manufacturing, site erection, installation and testing of the water storage and molten salt tanks. The supply of piping and cables was sourced locally, as well as site preparation works including earth moving, foundation work and all ancillary civil works. The mechanical and electrical erection and assembly of the turbine, heat exchangers, transformer, pumps, piping and cable installation was also locally sourced. Construction began in August 2013 and the site reached full capacity in just over two years.
R1,6-billion worth of locally sourced components and services have been used in the construction of this plant and created 1300 construction jobs. Of these, 400 were from the !Kheis Municipality and most of the workers were previously unemployed and unskilled. They received training which provided them with skills for future employment. Bokpoort CSP also created more than 60 permanent jobs for the ongoing operating and maintenance of the plant for the next 20 years. The maintenance schedule includes five to ten days a year of outages for maintenance work on the steam turbines. The main power block and the thermal storage system are maintained at the same time as the turbines, while the mirror repairs and cleaning, as well as the maintenance of the solar field piping, is done every night where necessary.
ACWA Power donated 5% of the ownership of this business venture to the local community via a community trust and a further 5% ownership goes to the national non-profit organisation, LoveLife. The company has also committed R5-million in providing education and training for people from the local community and in improving social infrastructure in the area.
The project is said to contribute significantly to the government’s initiative to augment much-needed power capacity, attract foreign direct investment, and create jobs while also stimulating the country’s economy. The tariff offered by ACWA Power at the second procurement window was 12% lower than the cap set by government for CSP technology at that round.
The project was awarded the title of African Community Project of the Year at the African Utility Week Industry Awards in 2015 as well as Project Finance “Deal of the Year” by Project Finance International in 2013, and World Finance’s Solar Deal of the Year in 2013.
by Joanne Taylor, EE Publishers