Concentrated solar power (CSP) is enjoying a boost of renewed attention worldwide as the renewable industry moves into the storage phase. There are currently a number of CSP plants operating and under construction in South Africa, and we went to look at one.
The CSP sector is receiving renewed attention after a lull in interest, and there are several large plants under construction or on order worldwide. For instance the Mohammed bin Rashid Al Maktoum solar power complex in Dubai will consist of 100 MW CSP tower and 600 MW of parabolic trough collectors, as well as several other projects of several hundred MW capacity.
The renewed interest in CSP has probably been brought about by the realisation that the problem of variability and intermittency of renewable energy sources can best be solved by storage, and the development of suitable bulk electricity storage is still quite far down the road. The major advantage of CSP is thermal storage, which is now an integral part of all CSP plant. The requirement for storage has led to the use of molten salt both as a heat transfer and storage medium.
The region around Upington has the highest level of solar radiation in Africa, as well as an abundance of available land, most of it flat, which makes this area ideal for the application of CSP technology. South Africa has both solar tower (ST) and parabolic trough (PT) systems installed and under construction. Table 1 shows the field up until window 3 of the Renewable Energy Independent Power Producers Procurement Programme (REIPPPP). There was no CSP allocation in window 4.
|Project name||Technology||Capacity (MW)||REIPPPP Window||Nearest town||Status|
|Bokpoort CSP||Parabolic trough||50||2||Groblershoop||Operational|
|Ilanga CSP 1||Parabolic trough||100||3||Kimberley||Construction|
|Kathu Solar Park||Parabolic trough||100||3||Kuruman||Operational|
|Kaxu solar 1||Parabolic trough||100||1||Pofadder||Operational|
|Khi solar 1||Tower||50||1||Upington||Operational|
|Xina CSP||Parabolic trough||100||3||Pofadder||Operational|
Parabolic troughs have dominated the concentrated solar thermal power industry for the last two decades, winning the confidence of utilities and investors, but ST technology is experiencing a new interest, especially in African applications.
Parabolic trough systems
The parabolic trough system consists of rows of parabolic mirrors using single axis tracking . The 600 MWth, 100 MWe Xina Solar one system near Pofadder is an example where the rows are orientated on a north/south axis and use east west horizontal tracking (Fig. 1).
Oil is currently used as the heat transfer fluid in PT systems and this limits the temperature to <400°C , which limits the efficiency of the system. The oil temperature in this system is limited to 393°C. The mirrors focus the sun on an absorber tube enclosed in an evacuated glass tube. The vacuum between the absorber tube and the glass envelope prevents conduction and convection heat losses. The outer surface of the glass tube receives a non-reflective coating, which increases the transmittance of radiation through the glass onto the absorber tube. Furthermore, the glass is treated with a hydrophobic coating, which increases its resistance to atmospheric conditions, which might negatively affect the cleanliness of the glass (Fig. 2).
The heated oil passes follows two different paths, one to the heat exchanger to drive the turbines, and the other to the thermal storage, which consists of molten salt. The storage system consists of two tanks, the hot tank at 390°C, and the cold tank at 290°C. Molten salt solidifies at about 230°C and the cold tank is always maintained above this temperature.
The possibility of using molten salt as a heat transfer fluid and a direct storage medium in a parabolic trough plant has been investigated. This allows for higher temperatures, which in turn increases the efficiency of the steam cycle, and the molten salt can be stored directly, which eliminates the need for an oil-to-salt heat exchanger. The use of solar salt at a maximum operating temperature of 450°C could reduce the LCOE of the parabolic trough technology by 14,2%, and it has been suggested that costs could be further reduced if higher temperatures were attained .
The problem however is material and mechanical one. More than 13 different variations of ball joints, flexible hoses and rotary joints were tested by a developer but a solution could not be found that could perform under the high temperatures and pressures associated with the molten salt in the solar field .
Solar tower systems
The tower system is a fairly new entrant to the commercial market although demonstration and pilot systems have been in operation all over the place. There are still very few tower systems of a significant capacity in operation. The tower system uses higher temperatures than the trough system and achieves higher efficiencies. There are two types of solar tower in use in South Africa: Direct steam and molten salt.
The tower system uses a solar field of individual mirrors with both horizontal and vertical tracking, focused on a central tower. The solar field for the SA systems is circular with the tower in the centre, and the tower uses a cylindrical receiver. Earlier systems located in the northern hemisphere used a single solar field located on one side of the tower, but the high solar elevation, and the flatness of the land of the South African sites allows a circular field to be used, making more use of the available radiation. The cylindrical receiver also allows the concentrated solar flux to be spread over a larger area, reducing the temperature of the receiver surface. The ST system has a concentration ratio of 800 suns and can reach receiver temperatures of 1000°C
Direct steam system
Khi solar one began operation in South Africa in 2015. This 50 MWe plant uses direct steam generation together with steam accumulators for a small amount of thermal storage. In a direct steam generation thermal solar plant, the solar receiver directly generates steam for the turbine, without resorting to other heat transport fluids. As they do not require separated steam generators, such plants are economical and allow the highest efficiency. However, given the necessarily high pressures involved, steam storage tanks are thick, heavy and expensive. Direct steam generation is, therefore, not the best adapted technology for a large energy storage. In this case, a molten salts plant will be more efficient. Khi Solar One highlights one of the drawbacks of using direct steam generation – a high level of thermal storage is not feasible.
Molten salt system
In modern molten salt power tower plants, molten salt (MS) is pumped up the tower, through the receiver where it is heated, and back down the tower after which it is stored directly in large tanks. The MS is therefore used as heat transfer fluid (HTF) and a storage medium. The piping layout of an MS system is short and vertical, allowing fast and easy drainage, and central towers are the safest solution for direct heating of molten salts in the receiver.
A 100 MWe molten salt power tower , Redstone, is currently in development in South Africa. The plant will implement 12 hours of storage and use a dry-cooled power cycle due to water availability concerns in South Africa. In a typical liquid salt solar tower (LSST) the liquid salt flows through the receiver at a rate of 22 000 l/min. Liquid salt is heated from 288 to 566°C. This high temperature differential stores more energy in less salt compared to other technologies. Fig. 3 shows the layout of a molten salt solar tower (MSST) system.
Storage and the solar multiple
The first requirement for storage is that there must be surplus energy to store, and this is achieved by oversizing the collector field. The characterizing factor by how much a solar field is oversized is called the solar multiple (SM). In a plant without storage it is typically about 1,4. A plant with storage needs an SM of >2. Typical values are 2,4 to 3.
Oversizing has another advantage. The direct normal irradiance (DNI) is obviously not constant throughout the day but reaches its maximum around noon. The turbine, however, is aimed to run at constant maximum capacity. For this to be possible the solar field needs to deliver the equivalent capacity long before noon and must therefore be oversized by a certain factor. As a result, the solar field provides extra capacity early in the cycle and late in the afternoon. In a CSP plant without storage the surplus energy at the solar peak needs to be dumped. Oversizing the plant gives the advantage that the plant can start operating at full output very early in the solar cycle and can continue operating at full output on solar energy until late in the solar cycle.
The CSP system, even without separate storage, has advantage over PV in the form of thermal inertia. Storage of heat in the thermal transfer fluid in trough system should cater for passing clouds and short variations in irradiance. No short interruptions in output. The same applies to the tower system, the thermal inertia of the transfer fluid will carry the system through short interruptions in clouds cover, etc. The effect of thermal inertia can be as much as 15 minutes of storage .
The relatively high cost of electricity from CSP plants has been one if the barriers to wider acceptance of CSP. The cost of CSP electricity in round 3 of the REIPPPP is R1,69/kWh. The technology is still in its early stages, compared to PV and wind which have had 20 years or more to develop, and prices are expected to decrease considerably as the technology matures. World prices have decreased dramatically in the past year. Here are some examples :
This kind of fall in price is astonishing for an innovative technology that has barely started, and for bids to be half what they were just six months ago is unprecedented. It seems that 2017 was a turning point for CSP. In many cases the low prices were made possible by the extremely high irradiance at the sites selected, and system developers and suppliers caution that it may not be possible to achieve the same low prices in other countries and in other localities.
Previous South African studies have forecasted that prices would drop by 50% or even lower by 2030, although this point may already have been reached . Fig. 5 shows the projected decrease in costs until 2050.
Comparison with PV bulk storage
One of the great advantages of thermal storage is the large amount of energy storage ( kWh) possible. A 100 MW plant with 12 h of storage at maximum capacity stores 1200 MWh. The largest battery storage of 100 MW only delivers 129 MWh of energy. The critical issue here is the cost per kWh of storage.
In addition to the large utility scale plants, the industry has seen the development of MicroCSP plant ranging in size from 100 kW to 5 MW for electricity generation. The technology is usually based on downsized trough, although small tower versions have emerged. Micro CSP is most often used for industrial heat generation and several plants are running in South Africa
 O Craig: “The current and future energy economics of concentrating solar power (CSP) in South Africa”, SAJIE, Nov. 2017.
 S Kraemer: “Solar thermal power prices have dropped an astonishing 50% in six months”, SolarPACES, October, 2017.
 F Dinter: “A review of Andasol 3 and perspective for parabolic trough CSP plants in South Africa”, SolarPACES, 2015.
 W Scanlon: “Thermal storage gets more solar on the grid”, NREL news, 2012.
 V Rajpaul: “CSP in South Africa”, AfDB and WB workshop, Tunisia, June, 2012.
 I Poole: “Concentrating solar power in South Africa – a comparison between parabolic trough and power tower technologies with molten salt as heat transfer fluid”, Msc Thesis, SU, 2017.
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