New possibilities for reducing the cost of pumped storage backup for intermittent wind and solar power by Prof. Dawid E. Serfontein, School of Mechanical and Nuclear Engineering, North-West University
History The North-West University recently conducted an academic study on feasible deployment dates of nuclear units in South Africa, which is also available from us. This study devoted a large amount of focus on the instabilities that intermittent wind and solar-pv creates on the power grid. To our great surprise we found that a number of new technologies now offer affordable solutions to these grid instability problems.
History (continued) The list of solutions to the grid instability problem includes: The price of imported Liquefied Natural Gas (LNG) has dropped sharply and sustainably, due to new cheaper technologies that resulted in an overproduction of gas in the US. Since the output of gas turbines are very flexible, they offer a natural solution to the intermittency of wind and solar-pv. CSIR wind map discovered reasonable wind cites all over the country. If wind farmes were to be distributed optimally, this would substantially alleviate intermittency.
Trends for US natural gas prices
History (continued) The list of grid instability solutions (cont.): Almost the whole of the Northern Cape, which currently is viewed as SA s solar power paradise, is also suitable for wind power generation. Addition of wind farms would mean that the current solar transmission cable can double up as a wind power carrier during the times of day when the sun is not shining. If wind and solar farms were to be distributed optimally, this could actually alleviate the overload on the transmission network.
History (continued) The list of solutions to the grid instability problem includes: The possibilities for maintaining grid stability by optimal deployment of Dynamic Demand Response (DDR) should be studied: Time-of-use meters and pricing. Frequency sensitive trip switches, with compensation Deploying variable speed turbine-pumps for hydro pumped storage.
History (continued) Reasons for lack of deployment of strategies to combat grid instability: The capacity expansion computer model of the IRP represents the international state of the art. However its model input assumptions contains some shortcomings, including: Ignoring external cost of imported energy sources, including natural gas. Underestimating the external cost of coal. Focusing on fixing mismatches between supply and demand by increasing supply, rather than by decreasing or shifting demand by means of aggressive time-of-use pricing and Dynamic Demand Response.
Shortcomings of IRP model input assumptions (cont.): Assuming that the role players (producers and consumers) will automatically do what s good for the country, instead of creating incentives to drive these changes.
Reasons for lack of deployment of strategies to combat grid instability: Roll-out of renewables were primarily controlled by the REIPPPP (who apparently thought that combating the intermittency of renewables was unnecessary) and by Eskom who have publicly stated that intermittent renewables are unwelcome.
History (continued) Therefore no one actively targeted the necessary strategies to combat intermittency: The REIPPPP did not implement time-ofproduction or location-of-production prices. Therefore almost all wind turbines are in the Western Cape, which greatly increased intermittency and congestion of the transmission network.
Eskom did not deploy additional gas turbines, or DDR. Ingula received constant speed pump-turbines, which cannot vary it input/output continuously in order to combat the continuous variations of the power output of wind and solar.
Potential of Pumped Storage The need for pumped hydro storage: Traditionally pumped storage has been used for peak shaving:
Normalised Power Demand vs. Supply ( kw ) SA time-of-day power demand Normalised avg. time-of-day power demand vs. PV-solar and wind power output curves 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0 3 6 9 12 15 18 21 24 Time of day (h) SA Power Demand (1 kw Av.) Avg. PV-Solar Power Output (1.03 kwp) Avg. Wind Power Output (1.25 kwp)
Potential of Pumped Storage The need for pumped hydro storage: Introduction of intermittent wind and solar PV power is creating the need to short-term pumping when the wind is blowing or the sun is shining: Wind and PV-solar will create multiple opportunities to empty and refill the dam in a typical day, which will reduce capital cost Pumped storage has virtually zero variable costs: multiple extra pumping and power production sessions will come for free.
Constant vs. variable speed Pumped Storage With price out of the way, there remain only one stumbling block to large scale deployment of wind and solar-pv power: Intermittency In the US the current glut of cheap shale gas is used to fuel gas turbines as cheap back-up for wind and solar-pv. South Africa has not yet been able to confirm large amounts of shale gas. Gas imports also create strategic risks for South Africa.
Constant vs. variable speed Pumped Storage These problems create scope for a local clean, sustainable and virtually CO2-free storage technology: Pumped hydro storage Variable speed pump-turbines adds the possibility to do ultra-short-term frequency control:
Normalised Power Demand vs. Supply ( kw ) SA time-of-day power demand Normalised avg. time-of-day power demand vs. PV-solar and wind power output curves 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0 3 6 9 12 15 18 21 24 Time of day (h) SA Power Demand (1 kw Av.) Avg. PV-Solar Power Output (1.03 kwp) Avg. Wind Power Output (1.25 kwp)
Technical aspects of variable speed pumped storage: Control pumping power via varying power frequency. Control turbine power via water by-pass. More expensive civil works and pumps, less efficient Twice as profitable
Conclusions and Recommendations Variable-speed pumped storage may present a cheaper, cleaner, and more sustainable form of back-up for intermittent wind and solar-pv power than the currently planned fleet of LNG fuelled open cycle gas turbines. Variable-speed pumped storage can replace OCGT with the fuel efficient LNG-fuelled CCGT. The combination could provide backup that is affordable, clean and sustainable.
Conclusions and Recommendations Such backup could substantially increase the scope for deploying large generation capacities of cheap new intermittent wind and solar-pv power. This could be to substantially lower South Africa s greenhouse gas emissions at an affordable cost.
Thank you! Any questions?