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SASTELA Overview

Based on CSP’s growing competitiveness, SASTELA is urging the government to increase the allocation of CSP in the Integrated Resource Plan (IRP) and the Renewable Energy Independent Power Producers’ Procurement Programme (REIPPPP). The IRP, which projects South Africa’s electricity requirements until 2030, has allocated 1,200 MW to CSP, making up only 2,1% of the country’s energy mix.
SASTELA’s members include leaders in South Africa’s solar thermal and utility industries. For a full listing, click here.
Membership is open to persons, corporate entities and state owned enterprises or their branches or subsidiaries and/or local branches or subsidiaries of international companies that are active in the solar thermal electricity sector as Business and Industry Members. For more information on becoming a member, click here.

CSP in South Africa

South Africa’s Integrated Resource Plan (IRP) envisages generation of an additional 56,500 MW of electricity by 2030, compared with current capacity of about 38,000 MW, mostly produced by Eskom coal-fueled power stations. Of the new capacity, 21,534, or 38%, is scheduled to be generated through renewable energy technologies, with 1,200 MW allocated to CSP.
Government has introduced a Renewable Energy Independent Power Producers’ Procurement Programme (REIPPPP) through which private developers are able to bid for allocations. Electricity generated by successful bidders will be purchased by the national utility, Eskom, and supplied ot the national grid. Prices paid to renewable energy generators are higher than the Eskom tariff to compensate developers for their initial high installation costs.

However, costs of renewable energy technologies are declining while the Eskom tariff for its coal-based electricity is rising sharply. This suggests that grid parity will be reached within this decade, and within only a few years in the case of some technologies. Although initial capital costs are high, operational and maintenance costs of renewable energy generation plants are much lower than those in a coal-based power station. And unlike fossil fuel plants, the fuel for renewable energy power stations is free.

Additionally, renewable energy is the most effective way in which to reduce South Africa’s excessively high carbon emissions, mainly from coal-based electricity, and to meet the country’s international commitments to combat climate change.

The majority of CSP projects under development are based in the Northern Cape, which has among the world’s highest direct normal irradiation (DNI), the projects are:

Technology

Concentrating solar power (CSP) works on the same principle as a conventional power station, but instead of using coal as the source of fuel to generate electricity, CSP uses the energy from the sun. CSP systems use reflectors to focus sunlight onto solar receivers to heat a working fluid. The heat is then used to create high-temperature steam, which is used either in a conventional turbine to produce electricity or in industrial process applications.

CSP can be hybridized with other energy generation technologies and contribute to lowering the cost of electricity. A CSP unit can supply steam to a fossil fuel power plant and reduce coal consumption and carbon emissions, or work in tandem with industrial steam processing applications.

For more on CSP technologies, click here.

CSP comes to South Africa as a tried and tested technology. The first CSP power station was established in California’s Mojave Desert in 1984. Today, there is more than 1,700 MW of CSP capacity are operational worldwide, with 2,106 MW under construction and plans to develop even more. For a complete listing of solar thermal power projects by country, click here.

Most of the operating plants are in Spain (19) and the United States (8), with Iran have two and Algeria, Egypt, Germany, Italy, Morocco, India and Thailand with one each. New plants are under construction in Spain and the United States, as well as in China, France, India, Israel and the United Arab Emirates.

Growing confidence in CSP and its increasing maturity as a cost competitive and viable technology is illustrated by Saudi Arabia, which recently announced a plan to install solar capacity of 41,000 MW by 2032, of which 25,000 MW will be contributed by CSP.

Concentrating solar thermal power (CSP) technology helps utilities and grid operators address integration challenges by delivering amore firm, reliable and controllable renewable power source compared to other variable generation resources. Because of the plant’s synchronous steam turbine generator, CSP provides important reliability benefits, such as reactive power support, dynamic voltage support, voltage control and some degree of inertia response.

CSP technology compensates for solar resource variability through the ability to increase or decrease the number of mirrors focusing on the receiver. This capability adds stability to the generation profile by allowing facility operators to shape the profile as system needs change. CSP’s operational attributes can also reduce the need for back-up fossil fuel generation to meet grid reliability requirements. CSP systems can also use a small amount of natural gas to achieve quicker morning startup and longer solar generation at the end of each day as well as to produce a less variable, more reliable power output compared to other solar technologies.

Grid Reliability Services

CSP plants using synchronous generators provide the same types of support for the reliable operation of the transmission system as conventional synchronous generators. As a result, CSP plants provide numerous important reliability services, such as reactive power and voltage support, primary and secondary frequency control and some degree of inertia response.

These attributes promote reliable operation of the transmission grid by controlling voltage and frequency within an acceptable band. The primary grid reliability benefits of CSP are described in more detail below.

Reactive Power and Voltage Support

The power system requires reactive power from generators, synchronous condensers, capacitors or other voltage support devices to support power transfer and maintain operating voltage levels under both normal and emergency conditions. On the one hand, inadequate reactive power can result in power transfer reductions and voltage collapse and thus could lead to widespread blackouts. On the other hand, the over-supply of reactive power can increase voltage at points in the system to very high levels and create an unintentional electrical arc that can damage the grid and customer equipment and create unsafe operating conditions.

Power system voltages are affected by a variety of factors, including customer loads, the distance power is transmitted to the loads, and the amount of loading on the power lines. Because the power system conditions are variable and constantly changing, the amount of reactive power needed at various points in the transmission system must include devices capable of constantly and automatically adjusting (injecting and withdrawing) the reactive power supply at specific points in the system. Synchronous generators featured in some CSP plants are capable of automatically adjusting the reactive power supply through the excited/automatic voltage regulator control under normal (all facilities in-service) conditions and under contingency conditions.

During and after sudden changes in grid conditions (e.g., during a fault of following the outage of transmission facilities), fast and automatic injecting and withdrawing of reactive power is crucial to maintain voltage stability and reliable system operations. In addition, if the system voltage begins to collapse, fast automatic increases in reactive power output are required to raise the voltage and prevent a collapse that could cause a blackout. Synchronous generators are capable of providing this grid reliability service and do so in a manner more effectively than other devices such as Static VAR Compensators (SVC) or Static Synchronous Compensators (STATCOM). The reactive power provided by SVC and STATCOM decreases as the voltage drops, making them less effective as the voltage collapses, exactly when reactive power is needed. Synchronous generators will help prevent excessive voltage drop by providing automatic and continuously the same amount of reactive power independent of system voltage levels; thus, better supporting the transmission system as voltage decreases and thus helping to prevent voltage collapse.

Frequency Control

To maintain system frequency in an acceptable band, the system needs to hold resources in reserve to provide frequency control. This is accomplished in two ways – primary frequency control and secondary frequency control. Primary frequency control is the ability to automatically and autonomously adjust output rapidly (within seconds) after the sudden outage of other generators. Secondary frequency control refers to the ability to respond within minutes to changes in system frequency through Automatic Generation Control (AGC) under normal operating conditions. Both primary and secondary frequency control are critical to maintaining overall grid stability and can be provided by synchronous generators. Moreover, since the output of PV is intermittent and PV does not intrinsically offer frequency control services, synchronous generators will serve to promote PV integration by providing the option of a clean source of frequency control needed to maintain grid reliability.

Inertia Response

Inertia on the grid is created by the energy stored in the rotating mass of conventional power plants. Inertia acts as a buffer that helps suppress frequency deviation due to various changes in the system. During and after the sudden loss of a transmission facility or a generator, inertia helps arrest the frequency decay (or overshoot) and allows time for generators in the system to stabilize the system. Since they provide rotating mass, inertia response is provided by the synchronous generators.

Solar Field Multiple and System Inertia

Solar thermal projects typically have collector field mirror areas that allow, during periods of high radiation and the hours near solar noon, more thermal energy collection than can be accommodated by the solar receiver and the steam turbine-generator, commonly referred to as the “solar field multiple.” This combination allows the solar receiver and the steam turbine-generator to operate at maximum continuous duty during the mid-morning hours, the hours near noon, and the mid-afternoon hours. IN effect, solar energy collection is reduced in the hours near noon to allow the steam turbine-generator to operate at maximum continuous duty during the shoulder periods of the day. The ability of the solar field to supply thermal power to the receiver in excess of the receiver rating also allows the steam turbine-generator to operate at high outputs during partially cloudy periods.

In addition, solar thermal tower technology, compared to non-thermal solar technologies, possesses an inherent system thermal inertia that results in less immediate and less volatile effects of reduced solar radiation on the electric output of the plant. At the beginning of such a weather event, a significant quantity of saturated water exists in the steam drum of the receiver. Further, the temperatures of the superheater section of the receiver, and the main steam descending piping to the steam turbine, are equal to the normal steam turbine inlet temperature. For the first few minutes of a cloud event, thermal energy can be withdrawn from the steam drum, the superheater section of the receiver, and the descending piping to maintain the output of the steam turbine at a level which is greater than the solar radiation would normally allow. Under certain conditions, the reduction in the electric output can also be minimized by taking advantage of the excess thermal energy capacity of the collector field, as noted above.

Controllability

Concentrating solar thermal tower technology has the particular ability to control the number of heliostats focusing on the receiver to account for variability of insolation in time of day and season, further stabilizing and shaping a plant’s generation profile to meet power system needs. BrightSource’s CSP plants can decrease of “turn down” excess mirrors when available solar energy is greater than can be absorbed by the receiver system and converted to electricity by the turbine. Similarly, toward the end of the day or during times of lesser insolation (*e.g. winter), a BrightSource plant can increase the number of heliostats focused on the receiver to increase production and extend the generating day. These capabilities have the effect of reducing output variability.

Over the long term, one of the compelling attributes of solar thermal power tower technologies is its natural synergy with thermal energy storage, which will provide valuable, clean grid reliability services such as load following and spinning reserves. As described above, CSP plans can be designed to accommodate the addition of a thermal energy storage system.

Increased Solar Energy and Reduced Variability from Natural Gas

CSP plants can be equipped with auxiliary gas boilers. These boilers provide several benefits to the plants, including increased renewable energy production and reduced variability of facility output. The auxiliary boilers are used to aid in system start-up, re-startup, and shutdown. By preheating portions of the solar receiver, the time to reach initial synchronization and solar electricity generation is accelerated. At the end of each day, generation can also be extended after which a minimal amount of gas-fired steam is used to control safe system cool down. In addition, during periods of transient cloud cover, the boilers can reduce the frequency and magnitude of facility output fluctuations that would otherwise need to be balanced by offsite, conventional power sources. In markets where efficient uses of natural gas integrated with a primary renewable generating resource are supported, a CSP plant can supplement its natural gas operations. Even without, auxiliary gas boilers increase the amount of renewable energy on the grid and mitigate imbalances on the system.

Transmission Reliability and Utilization Benefits

As discussed above, synchronous generators can increase the reliability of a system by providing voltage control and inertia response due to the transient and post-transient stability benefits of these generators. The practical effect is that the transfer capability, or maximum line loading, of a transmission corridor can be greater in scenarios where synchronous generators, such as a CSP plant, and asynchronous generators, such as wind or solar PV, are both interconnected, as compared to a corridor with only asynchronous generators.[1]

Solar thermal technology assists utilities and grid operators in addressing integration challenges by delivering a firmer, more reliable, and more controllable renewable power source. CSP promotes broader integration and higher penetration of renewable resources by means of it synchronous generator, providing significant benefits such as grid reliability services, including reactive power, voltage support, frequency control, inertia response, and controllability.

[1] These benefits of synchronous generation general exist where the transfer capability is limited by transient of post-transient stability constraints, rather than by thermal overload constraints.

In a solar power tower system, computer-controlled mirrors track the position of the sun to reflect light onto a ‘central receiver’ or boiler sitting atop a tower. The boiler, containing water, is designed to be heated from the outside to produce superheated pressurized steam. The steam is then transported to a traditional steam turbine generator to produce electricity.

By contrast, parabolic trough systems use synthetic oil as an intermediate ‘heat-transfer fluid’ to absorb heat, which is then pumped through heat-collecting pipes mounted in the focus of parabolic trough-shaped mirrors. The pipes pass through a heat exchanger to generate steam, which drives a turbine generator to produce electricity.

The capacity factor of a power plant is simply the number of hours of electricity it produces divided by the number of hours in a year.
Parasitic energy is how much electricity the plant itself uses. For example, the pumps and motors of a solar field or receiver are examples of parasitic energy. The biggest use of parasitic energy in a parabolic trough plant is to pump the synthetic oil throughout the heat-collecting pipes throughout the field.

Tower systems have less parasitic energy loss because they do not circulate fluid – water- in the solar field. The water/steam circulation pump in a central receiver requires less electricity, and as a result total parasitic energy usage in a tower system is approximately 50% less than in a comparable trough plant.

Typical parasitic energy values (including all solar field and heat exchange systems, the power block and balance of plant) are 12-14% of electricity for parabolic trough systems and 5-6% for a solar power tower plant.

Cost and Value

While the cost of electricity from a CSP plant will vary based on the technology used, CSP generally costs more to install than other forms of renewable energy and the price of the electricity it will supply to the national grid is higher. However, CSP costs more because it delivers more value to utilities. Unlike other forms of renewable energy, CSP can produce electricity during peak times, when it is needed most by Eskom and the country.

Eskom supplements its peak-time generation by using emergency Open Cycle Gas Turbine (OCGT) power stations that consume expensive, imported diesel fuel. According to information given to Parliament by the Minister of Public Enterprises, the OCGT’s generated electricity at a cost of R2,47 per kilowatt hour (kWh) and consumed R1,55 billion worth of diesel fuel over a seventh month period from 1 August 2011.

The first CSP power stations in South Africa will generate electricity at a cost between R2,68 and R2,51/kWh, on par with the OCGT power stations. And instead of using diesel as fuel, CSP power stations harness the energy of the sun.

Therefore, CSP with storage is a cost-effective and environmentally friendly alternative for peak-time electricity generation. Despite this, government’s Integrated Resource Plan has allocated an additional 5,000MW to OCGT over the next 18 years, whereas CSP has been allocated only 1,200MW.

CSP prices per kilowatt hour are still relatively high compared with PV and wind energy, but they are already becoming lower internationally and in South Africa. The average rand per MW price for CSP has dropped from R2 686 in the first round of bidding to R2 512 in the second round.

The CSP industry has a history of significant cost reductions when introducing new technologies. The SEGS plants reduced costs by 50% over nine plants. With next generation technologies being deployed today, similar cost reductions can be achieved with as well.

Based on research by the National Renewable Energy Laboratory, the Academies Science Advisory Council, Stellenbosch University and the International Renewable Energy Agency, given a longer-term roadmap with more MW allocations for deployment of CSP technologies, cost will come down significantly due to economies of scale, volume production and technology innovation. In the U.S., for example, many CSP projects are contracted to deliver energy at prices at or below similar utility-scale PV projects.

Towers have a unit capital cost advantage over troughs, which can be broken down into four distinct elements:

  1. Glass: Flat glass mirrors are less expensive than curved glass mirrors.
  2. Structural steel: Tower heliostats are mounted singly or in pairs, creating a low wind load and therefore requiring far less structural steel per square meter of mirror.
  3. Pipes: A tower system contains far fewer heat-collecting pipes in its boiler because of the higher sunlight concentration ratios. Furthermore, tower piping is installed only at the central tower and not distributed throughout the field. IN addition, trough systems require kilometers of header pipes for distribution of cold and hot oil to and from the working collector assemblies.
  4. Civil works: Trough assemblies require sizable concrete foundations, and trenching and cabling throughout the solar field to bring power to the drive motors. The compact heliostats in a tower system to not require foundations and use minimal cabling.

Storage

Because heat can be stored more cost-efficiently than electricity, CSP technology also provides the foundation for a thermal energy storage system that can support plant operations according to market and power system needs, rather than depending on the immediate availability of sunlight.

In a solar thermal system with thermal energy storage, heat is transferred to a thermal storage medium, usually molten salt, in an insulated reservoir or tank during the day, and withdrawn for power generation at night. This allows the use of solar power for base-load and peak-time generation, when it is needed most, and gives it the potential to match coal- and natural gas-fired power plants.

South Africa’s first three CSP power stations will include storage ranging from two to nine hours, giving them the ability to supply electricity to the national grid at peak times, when it is needed most.

There are five main benefits associated with integrating thermal energy storage:

  1. Reducing the total energy costs by increasing a plant’s capacity factor – the amount of hours that a plant runs annually
  2. Shifting electricity production to periods of highest demand
  3. Providing firm capacity to the power system; replacing the need for conventional power plants as opposed to just supplementing their output
  4. Providing ancillary services such as spinning reserves to help support a reliable grid
  5. Avoiding the variability and integration costs that other renewable resources like photovoltaics (PV) and wind create the utilities and the grid operator; reducing the need for additional fossil fuel units required to back up intermittent renewables that put a hidden financial burden on ratepayers
Utilities value solar thermal plants as a reliable power source, much like a conventional power plant. Large solar thermal plants will continue to be highly attractive to utility customers because they can produce significant amounts of reliable, dispatchable electricity. Solar thermal with storage avoids the intermittency issues associated with photovoltaic solar and wind generation, which is a necessary power characteristic to maintain a stable grid for utilities and their customers. Solar thermal integrates more seamlessly into the transmission grid because it has similar flexibility as a conventional thermal plant.

As thermal energy storage is added to solar thermal power plants, solar thermal becomes even more valuable because stored energy can be used to accommodate the variability of other non-dispatchable renewable, including wind and PV. Stored energy can also be used to meet electricity demand in the late afternoon and early evening hours, after the sun has gone down and PV output drops. This extra generation is highly valued by utilities.

Recent studies have shown CSP with thermal energy storage is much more competitive when the comprehensive net grid system costs of the CSP plant are compared to wind or solar photovoltaics (PV), These net costs include the long-term energy, ancillary service and capacity benefits and have been shown to provide an additional $30-60/MWh, or even higher, of benefits when compared to a PV plant with equal annual energy production in high renewable penetration scenarios.

For more information on the value of solar thermal with thermal energy storage, download our new report The Economic and Reliability Benefits of CSP with Thermal Energy Storage here.

Dry Cooling

In thermal steam systems, the super-heated steam inside the boiler pipes must be cooled and condensed back into water in a closed loop system. Dry cooling, or air cooling, uses an air-cooled condenser comprised of many large fans to circulate air over the pipes to cool and condense the steam. By comparison, a wet cooling system will circulate water across the pipes to cool and condense the steam.
There are a number of benefits to using a dry cooling system. The primary motivation to use dry cooling is to conserve scarce water resources in the arid desert climates where plants are often built. Overly taxing an area’s water resources can very seriously damage the biological ecosystem of the area, negatively impacting both animal and plant species. Dry cooling requires 0- percent less water than competing wet-cooled or hybrid dry-wet cooled systems. By using dry cooling, we are also able to eliminate the need for evaporation ponds and extensive water treatment facilities, which provides greater flexibility on where solar power plants can be sited.
The primary impact of using a dry cooling system are the slightly increased cost and the loss of efficiency. Power tower is the most cost-effective dry cooling plant because it produces more power, offsetting the additional cost per unit of electricity and has the ability to produce higher temperature steam, which results in a smaller efficiency loss when interfacing with dry cooling technologies.

Dry cooling requires large fans to cool the water, which adds no more than a couple percentage points to a plant’s capital costs and efficiency loss that varies depending on the ambient air temperature. By nature, dry cooling requires large fans to cool and condense the steam, and these fans require electricity to operate. The electricity required to operate the fans takes away from the total amount of electricity that can be sold to a customer, referred to as ‘parasitic loss.’ The warmer the ambient temperature, the larger the parasitic loss.

An air-cooled condenser (ACC) condenses the steam by forcing ambient air over tubes that contain the steam that exits the turbine. ACCs are typically comprised of modules arranged in parallel rows, with each module containing a number of finned tube bundles. An axial flow, forced-draft fan located under each module forces the cooling air across the heat exchange area of the finned tubes.