Debating the electricity generation technologies of the future

Cigre and the SAIEE presented a joint panel discussion recently, which discussed the role of renewable energy technologies in South Africa’s electricity generating future. The panel discussion, chaired by Malusi Maphosa, the director for utilities at Accenture, was held at the University of the Witwatersrand.

In his opening address, Prince Moyo, chairman of Cigre SA’s national committee, said that rounds 1 to 4 of South Africa’s Renewable Energy Independent Power Producers Procurement Programme (REIPPPP), are expected to deliver 5000 MW by 2019 and 7000 MW by 2020, comprising 14 to 20% of the country’s peak demand. This has assisted the country in diversifying power generation sources, leading to lower CO2 and pollutant emissions. The keenly awaited new integrated resource plan (IRP), expected to be released in August 2018 will specify the energy mix and relative share of each technology. Concerns regarding the amount of renewable energy in the IRP have been expressed by some role players, including Eskom, the coal industry and labour unions, he said.

Electricity capacity planning

Garrad Wright said that long term electricity resource planning in South Africa is based on the Department of Energy’s (DoE’s) IRP. Least cost planning principals must meet expected future demand reliably, taking into account all existing and future supply/demand resources with necessary policy adjustments for environmental, social and political goals.

It is common practice globally to have model-based outcomes inform policy, not just in energy. Models and modelling frameworks are useful. The planning process takes inputs from demand forecasts, existing supply, new supply options, and constraints, and uses the IRP modelling framework (Plexos) to produce outputs covering total system costs, capacity expansion, energy share, and other details for each scenario.

After taking policy adjustments into account, the final IRP is promulgated, indicating what to build and when to build it. The IRP process establishes a least-cost base case from technical planning facts from which a range of scenarios can be compared. Scenarios could include constraints such as VRE limits, technology forcing i.e. wind, solar PV, CSP, biogas, hydro, nuclear, coal, gas, etc., and environmental limitations. The range of supply and demand options will meet the long term demand requirement, determined by timing and scale. The modelling framework captures the primary system economic cost drivers and technical characteristics:

Clean coal

Prof. Rosemary Falcon said that coal accounts for 91% of South Africa’s electricity production, and 81% of the region’s energy. Burning coal produces greenhouse gas emissions in the form of CO2, and pollutants in the form of SOx, NOx and particulates, resulting in coal being regarded as a “dirty” fuel. South Africa, however, accounts for only 1,1% of the world’s CO2 emissions.

New technology makes the transition to “clean” coal possible. Emissions reduction in coal fired power plants is a requirement of the climate change mitigation policy. Methods to reduce emissions include increased efficiency of coal combustion through the beneficiation and use of higher grade coals; the adaptation of boiler plant to match the grade of coal supplied to it; the use of advanced high pressure/high temperature plant; co-firing, i.e. burning of carbon neutral renewables (e.g. biomass) with coal in power stations; co-generation, i.e. the use of excess heat from existing industrial processes to supplement power production; and carbon capture and storage/utilisation. Numerous capture methods and storage sites are being considered, as are numerous utilisation processes. “Clean” coal technologies suitable for South Africa include circulating fluidised bed combustion and ultra- Continuous analysis of the fuel flow to the burners to allow better control of the boilers load factor.

Solar technologies

Rashan Arscott said that solar photovoltaics (PV), which contain no moving parts, are expected to last between 20 and 30 years with minimal maintenance. PV modules can be crystalline (mono or poly) or thin film (amorphous silicon, Cadmium teleuride or copper indium gallium selenide). Each of the various PV technologies have unique cost and performance characteristics that drive competition within the industry. PV produces a DC output which must converted to AC by an inverter before supply to the grid.

CSP systems produce electricity by focusing sunlight to heat a thermal fluid. The heated fluid is then used to generate steam that drives a steam turbine generator, and/or to supply heat to a storage medium.

Advantages of CSP include the fact that it is a renewable source, uses a steam cycle similar to conventional generation, has low operating costs, has the ability to provide thermal energy storage, has a high efficiency and is scalable. Disadvantages are variable and intermittent production, water requirements, relatively high construction costs (although costs are expected to reduce significantly as volume increases), space requirements similar to PV, and the fact that efficiency and performance are heavily location dependent.

The viability of PV as a significant portion of the energy mix is heavily dependent on the availability of large scale storage at a price which makes the combination PV and storage an economic dispatchable source of electricity. The cost of storage is falling rapidly and the size of systems available is increasing, making dispatchable PV a possibility for the future.

Storage systems associated with CSP have developed to the point where systems can supply 15 h at full power. CSP is a fully dispatchable solar technology, and prices, including storage are falling to the same as wind or solar PV (without storage).

Wind energy

Dr David Oyedokun said that wind-powered turbines range in size, with the largest being the MHI Vespas 9,5 MW off shore type. Two types exist, he said: fixed speed speed or variable speed. Fixed speed turbines are comparatively cheaper than variable speed, are robust and have a simple construction, but suffer from inadequate control of reactive power consumption, and have relatively high inrush currents. Variable speed wind turbines operate at a speed determined by the wind speed, have a high efficiency, allow active and reactive power control, provide an easy mechanical interface. The technology also decouples active and reactive power control by independently controlling the rotor excitation current using a double fed induction generator.

Challenges posed by wind include noise and aesthetics (visual impacts). The variability of wind results in high intermittency of power generation and the wind resource is difficult to predict on a year-to-year basis. Good wind resources are often located in remote areas, far from demand centres and grid infrastructure.

Economic impact of independent power producers

Eskom’s Matshela Koko said that PPAs bring about R58,5-billion worth of investment and 13 000 construction jobs into the country. Project construction typically takes less than two years and generate R13,2-billion of socio-economic and enterprise development expenditure into surrounding communities over the 20 year lifetime; and distribute R6-billion of dividends to local community shareholders.

Energy storage must be a serious option for South Africa, he said. The World Bank has given a 1440 MWh target to Eskom by 2020, as part of the Medupi loan conditions. The cost of storage is expected to reach US$200/kWh by 2020, 50% of current cost.

Summary

Rob Stephen, Cigre’s international president, said that protection systems will be affected by high penetration of variable renewable energy sources. A low and varying fault level may lead to incorrect protection operation, while the safety of personnel and the public during islanded conditions needs to be addressed.

Power system stability consists of angle stability, frequency stability and voltage stability. All system stability elements must be reviewed. Variability of RE resources, especially wind, is a possible source of system instability.

Low inertia contribution to the power system may lead to incorrect operation of protection such as under-frequency load shedding (UFLS) and out-of-step protection during frequency excursions in the power system. Increased rate of change of frequency, resulting from low system inertia, requires a change in the philosophy of UFLS protection schemes. An increase in the frequency of electromechanical oscillations of machines will require redesign of the power system stabiliser controllers. Increasing amounts of renewables makes conventional power plants become unviable because of low prices, leading to decreasing time of operation of conventional plant, to the point of handling occasional peak loads and exposure to the profitability of energy storage.

Generation reserves are used to balance demand and generation in real time. Renewables will increase the volatility of the system frequency. The effect of this on primary, secondary and tertiary reserves needs to be determined, he said.

 

 

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