A techno-economic study of a pump storage
hydropower system for ultra-deep level mines
applied to Driefontein No. 9 Shaft
DM Steenekamp
orcid.org 0000-0001-7081-6118
Dissertation submitted in fulfilment of the requirements for the
degree
Master of Engineering in Mechanical Engineering
at the
North-West University
Supervisor:
Prof Chris Storm
Graduation May 2018
ABSTRACT
The energy required for mineral recovery is a major operational cost and strategic focus for the global mining sector. Power disruptions result in loss of production throughput and deceased profitability. Therefore, mines are required to review their power sources to ensure the sustainability and viability of their operations. In the South African region, the recent electricity tariff increases have had a major effect on energy-intensive users and subsequently significantly impacted investor decisions.
The business case for alternative energy sources, which include renewable energy sources, has become far more compelling in the current economic climate. However, renewable energy is characterised by an intermittent supply, but with the selection of an appropriate storage mechanism, power variability can be mitigated and the system‟s flexibility is enhanced. Only pumped storage hydropower is capable of meeting the anticipated technological and economic constraints with regard to storage capacity.
High energy recovery has been recorded for energy recovery systems in deep level shafts. More than 50 turbines with a combined capacity of over 65 MW have been installed underground in various mines in South Africa. The use of current mine infrastructure of existing deep level mine shafts for underground pumped hydro-electric storage (UPHES) systems reduces the initial system capital cost and the depths of a potential network of tunnels provides potential hydraulic heads exceeding 1000 m.
Driefontein No. 9 Shaft, a sub-shaft system consisting of a main shaft with shaft diameter 9.15 m and a shaft depth of 2 095 m, has been under care and maintenance for several years. The shaft is fully equipped with a dedicated production winder, dual-purpose production and men/material winder, shaft steelwork, electrical and communication cabling, various pipe columns and main haulages connecting the main shaft to the sub-vertical and ventilation shaft.
A concept UPHES system model was produced in Excel and populated with the Driefontein No. 9 Shaft system parameters. The UPHES system configuration consists of a lower reservoir and pump station on 24 level, 2 095 m below datum (BD), a midshaft reservoir and pump chamber in midshaft -1 050 m BD and a upper reservoir on intermediate pump chamber level, --150 m BD. The turbine chamber is also located on 24 level.
Various assumptions are made, which include operational assumptions, such as when pumps are operational and when turbines are operational and system assumptions, which pertain to the general system components and their associated capacities.
The UPHES system model was verified by using a simulation model developed in Engineering Equation Solver (EES). Unlike the Excel model, the EES simulation allows the pump and generating schedules to be varied for weekdays, Saturdays and Sundays. Therefore, the system was optimised using the EES simulation.
The first priority for optimising the EES simulation model was to reduce the return on investment (ROI) as it is required that the asset investment is returned in the shortest time possible for the mine to benefit from the system‟s energy saving potential. Therefore, in order to optimise the utilisation of
existing infrastructure and lower capital costs, the costs dependent on the system output parameters were optimised along with the output parameters.
The largest energy savings are achieved by maximising the number of hours of generating electricity during periods in which a peak tariff is charged and maximising the number of hours pumped only in periods in which off-peak tariffs are charged. This strategy maximises the number of hours during the day in which the maximum differential cost between tariffs is charged. Strategically scheduled pump and generating schedules are required to achieve the optimal system energy saving.
An optimisation matrix was used to evaluate the pumping and generating schedules and the effect thereof on the number of pumps required per pump chamber, lower reservoir capacity, capital expenditure (CAPEX), electricity savings and ROI.
It was determined that a break-even point exists at which the electricity cost savings are negatively affected by a decreased differential between the higher and lower tariff charged, because the cost of power consumed due to pumping cannot be recovered when energy is generated with the turbine. It was determined that if the percentage differential between peak and off-peak tariffs in the low demand season reaches 64% (off-peak tariff as a percentage of the peak tariff) or higher, the system no longer displays any electricity cost savings. Also, the percentage differential between the high demand season standard and peak tariff for the current 2016/2017 Megaflex tariff structure is 188% (standard tariff as a percentage of the off-peak tariff). The break-even point for the high demand season standard and peak tariff differential was calculated as 157% (standard tariff as a percentage of the off-peak tariff), therefore if the percentage differential reaches 157% or lower, the system no longer displays any electricity cost savings.
The findings described above were applied to the EES simulation and subsequently the optimal and most suitable option was selected for the Driefontein No. 9 Shaft UPHES system. The maximum trim off the electricity cost is given in the table below:
Description Unit Total
Annual total power cost – original system ZAR [RM] 72 350 987
Annual total power cost – UPHES system ZAR [RM] 66 835 000
Annual electricity cost savings ZAR [RM] 6 001 000
ROI Years 5.0
Total system electricity cost trim (% of original power required) 7.63%
In order to validate the EES simulation, operational data of the energy recovery system installed at the Driefontein No. 5 Shaft were used. At the Driefontein No. 5 Shaft, the potential energy of the water in the chilled water shaft column is converted to mechanical energy by the turbine located 1 680 m below surface. The turbine system was retrofitted to the existing system and high energy recovery has been recorded for this installation. The turbine has the capacity to generate 4.6 MW.
When the power calculated with the UPHES model is measured against the Driefontein No. 5 Shaft test data, the UPHES model power calculated is 1.05% lower than the power generated by the Driefontein No. 5 Shaft turbine. The EES simulation was subsequently validated.
KEYWORDS
Pumped hydroelectric storage, PHES, underground pumped hydroelectric storage, UPHES, energy storage, Eskom Megaflex, tariffs, ultra-deep, mines, energy, recovery, optimised custom UPHES system, schedule, capital cost estimate, CAPEX.
ACKNOWLEDGEMENTS
Thanks be to our Lord, Jesus Christ, who was, and is, and is to come. For creating this world, and all in it, with the greatest wisdom of all. Thank you for extending the same wisdom to us and giving us the knowledge perfectly suited to be able to glorify Your Holy Name.
22 “The Lord brought me (wisdom) forth as the first of his works,
before his deeds of old;
23
I (wisdom) was formed long ages ago,at the very beginning, when the world came to be.
24
When there were no watery depths, I was given birth, when there were no springs overflowing with water; 25 before the mountains were settled in place, before the hills, I was given birth, 26 before he made the world or its fields or any of the dust of the earth.
27
I was there when he set the heavens in place, when he marked out the horizon on the face of the deep, 28 when he established the clouds above and fixed securely the fountains of the deep,
29
when he gave the sea its boundary so the waters would not overstep his command, and when he marked out the foundations of the earth.
30
Then I was constantly at his side.I was filled with delight day after day, rejoicing always in his presence, 31 rejoicing in his whole world and delighting in mankind.
Amen
CONTENTS
DECLARATION...i ABSTRACT...…...ii KEYWORDS…………...iv ACKNOWLEDGEMENTS... ...v CONTENTS……...vi LIST OF TABLES……...x LIST OF FIGURES...xii GLOSSARY OF TERMS...xiv NOMENCLATURE…...xv TECHNICAL UNITS…...xvi1
INTRODUCTION ... 1-1
1.1 BACKGROUND ... 1-1 1.2 PROBLEMSTATEMENT ... 1-2 1.3 OBJECTIVE ... 1-21.4 RESEARCHMETHODOLOGYANDEXPERIMENTALPROCEDURE ... 1-3
1.5 SCOPEANDLIMITSOFTHESTUDY ... 1-3
1.6 DISSERTATIONSTRUCTURE ... 1-4
2
LITERATURE STUDY ... 2-5
2.1 INTRODUCTION ... 2-5
2.2 ENERGYANDMINES ... 2-5 2.2.1 Cost of Energy ... 2-6 2.2.2 Eskom Tariff Increases ... 2-8 2.2.3 Energy Saving Initiatives ... 2-8
2.3 RENEWABLEENERGYANDMINES ... 2-8 2.3.1 What is Renewable Energy? ... 2-9 2.3.2 Renewable Energy in South Africa ... 2-9 2.3.3 Cost of Renewable Energy ... 2-10 2.3.4 Photovoltaic Plants ... 2-11
2.3.5 Storage of Renewable Energy ... 2-12
2.4 UNDERGROUNDPUMPEDHYDROELECTRICENERGYSTORAGE ... 2-13 2.4.1 Recovery of the Energy of Water Going Down Mine Shafts ... 2-13 2.4.2 Concept of UPHES Plants ... 2-14 2.4.3 UPHES generation potential ... 2-14 2.4.4 System storage efficiency ... 2-14 2.4.5 Economics of a UPHES system ... 2-15 2.4.6 Use of existing mine infrastructure ... 2-15 2.4.7 Subsurface construction ... 2-15 2.4.8 Main System Components ... 2-16 2.4.8.1 Pumps and turbines ... 2-16 2.4.8.2 Turbines ... 2-17 2.4.8.3 Pumps ... 2-18 2.4.8.4 Upper and lower reservoirs ... 2-19 2.4.8.5 Emergency storage capacity and pumps ... 2-19 2.4.8.6 Access shaft and pipe column ... 2-19
2.5 TREATINGTHEROOTCAUSEOFACIDMINEDRAINAGE ... 2-20
2.6 RISKSASSOCIATEDWITHUPHES ... 2-21
2.7 STATUTORYREQUIREMENTS ... 2-22
2.8 CONCLUSION ... 2-23
3
TECHNICAL BACKGROUND ... 3-24
3.1 DRIEFONTEINGOLDMINE ... 3-24
3.2 GEOLOGICALCONDITIONS ... 3-24
3.3 DRIEFONTEINNO.9SHAFT ... 3-25
3.4 SHAFTINFRASTRUCTURE ... 3-27 3.4.1 Pelton Turbine ... 3-27 3.4.1.1 Driefontein No. 5 shaft Pelton turbine efficiency ... 3-27 3.4.1.2 System discharge rate to net head ratio ... 3-27 3.4.2 Existing Chilled Water Column ... 3-28 3.4.3 Existing Chilled Water Column Pipe Schedules ... 3-28 3.4.4 Dewatering Pumps ... 3-29
4
UPHES SYSTEM MODEL ... 4-31
4.1 SYSTEMCONFIGURATION ... 4-31 4.1.1 Master UPHES System Model ... 4-31 4.1.2 Pipe Pressure Calculations ... 4-31 4.1.3 CAPEX Model... 4-31
4.2 TYPICALPOD ... 4-33
4.3 MEGAFLEXTARIFFS ... 4-34
4.4 ASSUMPTIONS ... 4-35
4.5 INPUTPARAMETERS ... 4-35
4.6 PIPEPRESSURECALCULATIONS ... 4-36 4.6.1 Pipe Pressure Rating Calculations ... 4-37 4.6.2 Limiting Flow Rate ... 4-38
4.7 CAPITALCOSTMODEL ... 4-40 4.7.1 Basis of Estimate ... 4-40 4.7.2 Bill of Quantities... 4-40 4.7.3 Bill of Quantities – UPHES System: Driefontein No. 9 Shaft ... 4-43 4.8 RESULTS ... 4-44 4.8.1 UPHES System Generating Capacity ... 4-44 4.8.1.1 Instantaneous UPHES system generating capacity... 4-44 4.8.1.2 Power generated during peak tariff periods only ... 4-44 4.8.1.3 Power generated during peak and standard tariff periods ... 4-46 4.8.2 Electricity Cost Savings ... 4-48 4.8.3 Return on Investment ... 4-49
5
EES MODEL ... 5-50
5.1 SIMULATIONINPUTPARAMETERS ... 5-50
5.2 SIMULATIONOUTPUTPARAMETERS ... 5-51
5.3 VERIFICATION ... 5-52 5.3.1 Simulation Results ... 5-52 5.3.1.1 Power generated during peak tariff periods only ... 5-52 5.3.1.2 Power generated during peak and standard tariff periods ... 5-53 5.3.1.3 Evaluation of simulation results ... 5-53
5.4 SYSTEMOPTIMISATION ... 5-54 5.4.1 Optimal Generating Schedules ... 5-54 5.4.2 Optimal Pumping Schedules ... 5-56 5.4.3 Tariff Differential Break-Even Point ... 5-58 5.4.4 Optimised UPHES System for Driefontein No. 9 Shaft ... 5-61
6
MODEL VALIDATION ... 6-63
6.1 DRIEFONTEINNO.5SHAFTENERGYRECOVERYSYSTEM ... 6-63 6.1.1 Layout and Operation ... 6-64 6.1.2 Trips and Alarms ... 6-65
6.2 TESTDATA ... 6-65
6.3 DATAEVALUATION... 6-68
6.4 UPHESMODELVALIDATION ... 6-69
7
CONCLUSION AND RECOMMENDATIONS ... 7-71
REFERENCES AND BIBLIOGRAPHY ... 7-73
REFERENCES ... 7-73
BIBLIOGRAPHY ... 7-76
APPENDICES ... 7-79
LIST OF TABLES
TABLE 2-1:TYPICAL MEGAFLEX STRUCTURE ANTICIPATED FOR A MINE ... 2-7
TABLE 2-2:MOST USED STORAGE TECHNOLOGIES IN RENEWABLE ENERGY SYSTEMS ... 2-13
TABLE 2-3:WATER PUMPED FROM THE WORKINGS AT MINES ON THE WEST WITS LINE ... 2-21
TABLE 3-1:DRIEFONTEIN NO.9SHAFT COMPLEX INFORMATION ... 3-25
TABLE 3-2:DRIEFONTEIN NO.5SHAFT PELTON TURBINE TECHNICAL DESIGN DATA ... 3-27
TABLE 3-3:DRIEFONTEIN NO.5SHAFT PELTON TURBINE EFFICIENCIES ... 3-27
TABLE 3-4:CHILLED WATER SHAFT PIPING DESIGN INPUT PARAMETERS ... 3-28
TABLE 4-1:2015/2016MEGAFLEX TARIFF STRUCTURE USED IN THE MODEL ... 4-34
TABLE 4-2:MODEL INPUT PARAMETERS ... 4-36
TABLE 4-3:SHAFT PIPING PRESSURES ... 4-37
TABLE 4-4:PEAK PIPE PRESSURE AND SELECTED PIPE SCHEDULE BETWEEN BD0.00 M AND BD-698.3 M ... 4-38
TABLE 4-5:PEAK PIPE PRESSURE AND SELECTED PIPE SCHEDULE BETWEEN BD-698.30 M TO BD-1396.6 M ... 4-38
TABLE 4-6:PEAK PIPE PRESSURE AND SELECTED PIPE SCHEDULE BETWEEN BD-698.30 M TO BD-1396.6 M ... 4-38
TABLE 4-7:PEAK PIPE PRESSURE BD0.00 M TO BD-698.3 M AT LIMITING VELOCITY ... 4-39
TABLE 4-8:PEAK PIPE PRESSURE BD-698.30 M TO BD-1396.6 M AT LIMITING VELOCITY ... 4-39
TABLE 4-9:PEAK PIPE PRESSURE BD-1396.6 M TO BD–2095 M AT LIMITING VELOCITY ... 4-39
TABLE 4-10:CAPITAL COST MODEL INPUT PARAMETERS ... 4-40
TABLE 4-11:GENERATING DURING PEAK TARIFF PERIODS ONLY – SYSTEM PARAMETERS ... 4-44
TABLE 4-12:GENERATING DURING PEAK AND STANDARD TARIFF PERIODS – SYSTEM PARAMETERS ... 4-46
TABLE 4-13:GENERATING DURING PEAK TARIFF PERIODS ONLY – TRIM ... 4-48
TABLE 4-14:GENERATING DURING PEAK AND STANDARD TARIFF PERIODS – TRIM ... 4-48
TABLE 4-15:ROI ASSUMING A TARIFF INCREASE OF 8% PA ... 4-49
TABLE 5-1:EES SIMULATION MODEL INPUT PARAMETERS ... 5-50
TABLE 5-2:EES SIMULATION MODEL OUTPUT PARAMETERS ... 5-51
TABLE 5-3:GENERATING DURING PEAK TARIFF PERIODS ONLY,EES SIMULATION RESULTS ... 5-52
TABLE 5-4: GENERATING DURING PEAK AND STANDARD TARIFF PERIODS,EES SIMULATION RESULTS ... 5-53
TABLE 5-5:OPTIMISATION EVALUATION MATRIX – OPTION 1 TO 9 ... 5-56
TABLE5-7:TARIFF DIFFERENTIAL BREAK-EVEN POINT FOR LOW DEMAND SEASONS ... 5-59
TABLE5-8:TARIFF DIFFERENTIAL BREAK-EVEN POINT FOR HIGH DEMAND SEASONS ... 5-60
TABLE 5-9:OPTIMISED UPHES SYSTEM EES SIMULATION RESULTS ... 5-61
TABLE 6-1:DRIEFONTEIN NO.5SHAFT SYSTEM OPERATING PARAMETERS ... 6-64
TABLE 6-2:DRIEFONTEIN NO.5SHAFT SYSTEM OPERATING PARAMETERS DURING DATA COLLECTION ... 6-65
TABLE 6-3:MODEL INPUT PARAMETERS ... 6-69
TABLE 6-4:MODEL OUTPUT PARAMETERS ... 6-69
TABLE 7-1:OPTIMISED DRIEFONTEIN NO.9SHAFT UPHES SYSTEM – TRIM ... 7-72
______________________________
LIST OF FIGURES
FIGURE 2-1: ELECTRICITY CONSUMPTION BREAKDOWN (GOLDFIELDS,2011) ... 2-6
FIGURE 2-2:TARIFFS AND CHARGES BOOKLET 2013/14(ESKOM,2013) ... 2-6
FIGURE 2-3:REIPPPP TENDER OUTCOME – AVERAGE POWER TARIFFS ... 2-10
FIGURE 2-4:AVERAGE ESKOM TARIFFS VERSUS UTILITY–SCALE RENEWABLE ENERGY TARIFFS (ESKOM,2016) ... 2-11
FIGURE 2-5:POWER GENERATION FACTOR ... 2-11
FIGURE 2-6:TYPICAL POD LOAD PROFILE ... 2-12
FIGURE 2-7:PELTON TURBINE MANUFACTURED AT VOITH IN GERMANY (VOITH,2015)... 2-16
FIGURE 2-8:APPLICATION RANGE FOR PELTON TURBINES (VOITH,2015) ... 2-17
FIGURE 2-9:TURBINE EFFICIENCY FOR 2,4 AND 6 NEEDLE OPERATION AT STANISHOUS POWERHOUSE (ROBERTS ET AL.,2011) ... 2-18
FIGURE 2-10:MULTISTAGE CLEAR WATER PUMPS’ PERFORMANCE RANGE (SULZER,2015) ... 2-18
FIGURE 2-11:CROSS-SECTION OF A SHAFT WITH TWO PIPE COLUMNS (WINDE ET AL.,2017) ... 2-20
FIGURE 2-12:WATER FROM THE INRUSH OVERFLOWING INTO NO.4SHAFT ... 2-22
FIGURE 3-1:LOCATION OF THE DRIEFONTEIN GOLD MINE (GOLDFIELDS,2009) ... 3-24
FIGURE 3-2:GEOLOGICAL CONDITIONS AT THE DRIEFONTEIN GOLD MINE (WINDE EL AL,2017) ... 3-25
FIGURE 3-3:DRIEFONTEIN NO.9SHAFT COMPLEX LAYOUT (SATELLITE VIEW) ... 3-26
FIGURE 3-4:SHAFT INFRASTRUCTURE DIAGRAM ... 3-26
FIGURE 3-5:TYPICAL HAULAGE CROSS-SECTION ... 3-30
FIGURE 4-1:UPHES SYSTEM CONFIGURATION ... 4-32
FIGURE 4-2:TYPICAL POD DURING LOW DEMAND SEASON ... 4-33
FIGURE 4-3:TYPICAL POD DURING HIGH DEMAND SEASON ... 4-33
FIGURE 4-4:POD LOAD PROFILE – POWER GENERATED DURING PEAK TARIFF PERIODS ONLY ... 4-45
FIGURE 4-5:DAILY ENERGY COST PROFILE – POWER GENERATED DURING PEAK TARIFF PERIODS ONLY ... 4-45
FIGURE 4-6:POD LOAD PROFILE – POWER GENERATED DURING PEAK AND STANDARD TARIFF PERIODS ... 4-47
FIGURE 4-7:DAILY ENERGY COST PROFILE – POWER GENERATED DURING PEAK AND STANDARD TARIFF PERIODS ... 4-47
FIGURE 4-8:ROI(ASSUMING AN 8% PA TARIFF INCREASE) ... 4-49
FIGURE 5-1:LOAD AND COST PROFILE VERSUS TIME (WEEKDAYS)... 5-55
FIGURE 5-2:LOAD AND COST PROFILE VS TIME (SATURDAYS) ... 5-55
FIGURE 5-4:LOAD AND COST PROFILE VERSUS TIME (WEEKDAYS) OPTION 3,9 AND 10 ... 5-57
FIGURE 5-5:LOAD AND COST PROFILE VERSUS TIME (SATURDAYS) OPTION 3,10 AND 11 ... 5-57
FIGURE 5-6:LOAD AND COST PROFILE VERSUS TIME (SUNDAYS) OPTION 3,10 AND 11 ... 5-57
FIGURE 5-7:OPTIMISED LOAD AND COST PROFILE VERSUS TIME (WEEKDAYS) ... 5-62
FIGURE 5-8:OPTIMISED LOAD AND COST PROFILE VERSUS TIME (SATURDAYS) ... 5-62
FIGURE 5-9:OPTIMISED LOAD AND COST PROFILE VERSUS TIME (SUNDAYS) ... 5-62
FIGURE 6-1:DRIEFONTEIN NO.5SHAFT TURBINE ... 6-63
FIGURE 6-2:DRIEFONTEIN NO.5SHAFT TURBINE BYPASS LINE AND LUBRICATION ... 6-64
FIGURE 6-3:DRIEFONTEIN NO.5SHAFT TURBINE TRIPS AND ALARMS ... 6-65
FIGURE 6-4:TURBINE MONITORING SCREEN ... 6-66
FIGURE 6-5:TURBINE TEST DATA DATED 27JANUARY 2014 ... 6-67
FIGURE 6-6:DATA EVALUATION – ACTUAL AND THEORETICAL POWER VERSUS TIME ... 6-68
FIGURE 6-7:DATA EVALUATION – ACTUAL POWER (BLACK), THEORETICAL POWER (BLUE) AND DISCHARGE RATE TO NET HEAD RATIO (RED) VERSUS FLOW RATE ... 6-69
FIGURE 6-8:VALIDATION ... 6-70
GLOSSARY OF TERMS
Chilled water Water cooled by refrigeration plants and used for underground cooling purposes
Generating schedules A schedule with time allocated to generating electricity with the turbine within a 24-hour cycle
Life of mine The number of years that the mine is expected to be productively operated
Pilot calculation Model calculation using low demand season or high demand season tariffs charged
Pumping schedules A schedule with time allocated to pumping water to the upper reservoir within a 24-hour cycle
Shaft column Piping installed in the shaft; includes the shaft dewatering column and the turbine feed column
Trim Term used to describe the reduction in the total electricity costs
NOMENCLATURE
AMD Acid Mine Drainage
ASME American Society of Material/Mechanical Engineers ASTM American Society for Testing and Materials
B.Eng. Baccalaureus in Engineering
BD Below Datum, ground elevation is at 0 m BD CAPEX Capital Expenditure
DoE Department of Energy
DSM Demand Side Management
EES Engineering Equation Solver. Computer program used in cycle design and optimisation in this project.
FWR Far West Rand
HDS High Demand Season
ID Internal diameter
IPC Intermediate Pump Chamber IRP National Integrated Resources Plan
LDS Low Demand Season
NB Nominal Bore (pipe specification) NWU North-West University
PA Per Annum
PHES Pumped Hydro Electric Storage POD Point of Distribution
PV Photovoltaic
REA Renewable Energy Advisors
REIPPPP Renewable Energy Independent Power Producer Procurement Programme ROI Return on Investment
SCH Schedule (pipe specification)
TOU Time of Use
UPHES Underground Pumped Hydro Electric Storage
U/G Underground
TECHNICAL UNITS
kPa Kilopascals
kW Kilowatts
kWh Kilowatt-hour
GWh Gigawatt-hour
%VAR Percentage variation
Hydraulic efficiency
Pump efficiency
Turbine efficiency h Effective head
c/kWh Cents per kilowatt-hour
kg/(ms) Fluid dynamic viscosity, kilogram per second
kg/m3 Fluid density, kilogram per cubic metre
kV Kilovolt
l Litres
l/s Litres per second
m Metre
m/s2 Metre per second squared
m3 Cubic metre
MR Million rand
MWh Megawatt-hour
NB Nominal bore (pipe diameter)
Nm Newton-metre
R System discharge rate to net head ratio
R/kVA Rand per kilovolt-ampere
Rpm Rotations per minute
ZAR South African rand
1
INTRODUCTION
1.1 BACKGROUND
The energy required for mining and beneficiation is a key cost driver amounting to between 18% and 20% of the operational cost of mines. Energy is a major operational cost and therefore a strategic focus for the mining sector today.
The South African national grid, supplied by the public utility provider, Eskom, has been under strain since 2007. Between 2012 and 2015, infrastructure failure, maintenance backlogs and failure to bring new generating capacity online as planned resulted in load shedding, the impact of which affected economic growth negatively. Furthermore, although South Africa‟s tariff prices are ranked 10th
most expensive out of 18 countries surveyed in a 2016 Energy Market Survey conducted by NUS Consulting, they also displayed the second biggest jump behind Belgium‟s 9.9% increase (Writer, 2015). Tariff price hikes are constantly under South African media scrutiny and remain a controversial matter.
In South Africa, the majority of mining operations are entirely tied into the grid and electricity costs charged by Eskom are structured according to the Megaflex tariff structure. In 2016 the off-peak and peak tariffs charged during the high demand season (June to August) was 44.10 c/kWh and 268.06 c/kWh respectively (Eskom, 2016). The peak and off-peak tariffs increased at an approved 8% per annum since July 2014 until the end of June 2018. In June 2017, Moneyweb reported to have viewed an undisclosed confidential draft submitted by Eskom for comment to National Treasury proposing an increase of 19.9% for the year 2018/2019 (Slabbert, 2017).
Mines must therefore consider alternative energy sources because it is strategically imperative for the sustainability of their operations. The business case for alternative energy sources has consequently become far more compelling as renewable energy is an alternative to conventional sources of energy. However, the sustainability of renewable energy is hampered by a characteristic intermitted supply. With the selection of an appropriate storage mechanism, power variations can be mitigated and system flexibility enhanced. Only pumped hydroelectric storage (PHES) supplies high amounts of power for significantly long periods and is capable of producing the quantum of energy required by mining operations.
PHES is used to store energy in the form of potential energy. Water is pumped from a lower reservoir during periods when off-peak tariffs are charged to an upper reservoir. During peak tariff charge periods, electricity is generated by turbines converting this potential energy into electrical power. Traditionally a PHES system is applied to take advantage of the difference between a peak and off-peak tariff. Electricity generated by the water flowing through the turbine fed from the upper reservoir is sold back to the grid, during periods in which the peak tariff is charged.
A PHES system requires either a very large body of water or large variation in height. Ultra-deep level gold mines provide large variations in head since their depths typically exceed 1 000 m. Furthermore,
the infrastructure to support great volumes of water is created by the underground excavations made to access the ore body. The use of ultra-deep level South African mines for underground pumped hydroelectric storage (UPHES) systems can therefore possibly have great economic potential. UPHES not only provides a solution to the issue encountered with regard to storing renewable energy, but also to the tariff increases threatening the sustainability of mining operations in South Africa.
Mines that have reached the end of life of mine have been or are in the process of being decommissioned. These mines are normally flooded, all entries are sealed and the site is secured. In some cases, mines are placed under care and maintenance, because of trepidations about profitability. Vertical shafts placed under care and maintenance are maintained in so far as is necessary to allow the shaft to be used for possible future operations by the current or future owner. An opportunity is presented in both cases: equip the shaft with a UPHES system. Not only can the existing shaft infrastructure be used to reduce the capital costs and gain schedule advantages, but electricity can be generated in sufficient quantities and returned to the grid in periods in which peak tariffs are charged.
There is therefore an opportunity to capitalise on decommissioned mines and mines placed under care and maintenance, in South Africa. If proven technologically feasible, as has already been proven from experience with similar systems, the system can be implemented at Driefontein No. 9 Shaft.
Driefontein No. 9 Shaft, a sub-shaft system consisting of a main shaft with shaft diameter 9.15 m and a shaft depth of 2 095 m, has been under care and maintenance for several years. The shaft is fully equipped with a dedicated production winder, dual purpose production and men/material winder, shaft steelwork, electrical cables, communication cables, various pipe columns and main haulages on 21, 21.5, 22, 23 and 24 level connecting the main shaft to the sub-vertical and ventilation shaft. The opportunity has been identified to install a UPHES system in this shaft.
1.2 PROBLEM STATEMENT
There is no existing techno-economic study for a UPHES system, customised to suit the Driefontein No. 9 Shaft infrastructure, to evaluate the feasibility for implementation of this identified opportunity.
1.3 OBJECTIVE
This study endeavours to evaluate a custom UPHES system for possible implementation at the Driefontein No. 9 Shaft by:
(1) Developing a concept UPHES model to evaluate: Pumping and turbine generating efficiencies
Peak- and off-peak tariff structures (for generating and pumping) Capital expenditure (CAPEX)
Calculation of return on investment (ROI) (2) Verifying the concept model
(4) Validating the simulated UPHES system with actual operational data from a similar installation.
Furthermore, the study will endeavour to identify the limitations of the UPHES system applied to ultra-deep level gold mines and give special consideration to the laws and regulations that govern the safe operation of mines in South Africa.
1.4 RESEARCH METHODOLOGY AND EXPERIMENTAL PROCEDURE
The methodology to evaluate a custom UPHES system applied to ultra-deep level gold mines will consist of:
(1) A literature survey to determine what existing technology is available
(2) Development of a concept UPHES model by using Excel. The model must consist of: Principal input and output
Off-peak, standard and peak-tariff structure values Summer season pilot calculation
Winter season pilot calculation
Pipe pressure calculation sheets (linked externally to the principal input and output sheets), for the feed to the turbine and water pumped to the upper reservoir
CAPEX model
Return on investment (ROI)
(3) Population of the concept UPHES model with real component parameters of the Driefontein No. 9 Shaft
(4) Verification of the custom system UPHES model with the Engineering Equations Solver (EES) program
(5) Evaluation of the verified, optimised and custom system model and investigation of optimisation opportunities by considering:
Reducing capital costs by using existing infrastructure
Optimal generation schedules to optimise electricity savings and ROI Optimal pumping schedules to optimise electricity savings and ROI
Determination of the break-even point with regard to the Eskom tariff differential Energy and overall system efficiency implication
Optimum efficiency
(6) Validation of the Driefontein No. 9 Shaft UPHES system model with actual test data from the Driefontein No. 5 Shaft UPHES system
(7) Investigation of the legacy of the Driefontein No. 9 Shaft as a future community asset (8) Derivation of a conclusion and formulation of recommendations from the results.
1.5 SCOPE AND LIMITS OF THE STUDY
• UPHES with Pelton wheel turbine(s) (not Francis and Kaplan turbines) • Only on custom system development and plant is used for validation
• Only RSA/Eskom tariff structure and local CAPEX • Only RSA laws and regulations
1.6 DISSERTATION STRUCTURE
The dissertation consists of seven chapters. Each chapter follows the research methodology described in section 1.4. In summary, this dissertation consists of:
Chapter 1: The background, problem statement, study objective, research methodology listed in points, the scope and limitations of the study
Chapter 2: Literature survey considering similar studies and suitable technology available in UPHES systems
Chapter 3: Technical background of Driefontein No. 9 Shaft and the available shaft infrastructure
Chapter 4: Concept UPHES model on Excel, as well as the Driefontein No. 9 Shaft model populated with real component parameters
Chapter 5: EES model with verification of the Driefontein No. 9 Shaft model and model optimisation
Chapter 6: Validation of the Diefontein No. 9 Shaft UPHES system model with actual test data from the Driefontein No. 5 Shaft UPHES system
Chapter 7: Conclusions and recommendations.
2
LITERATURE STUDY
2.1 INTRODUCTION
The focus of the global mining sector is productivity and increased profits. Energy has become a major operational cost and strategic focus for the African mining sector due to power disruptions causing major operational issues, loss of production throughput and therefore deceased profitability. The challenge however is not limited to disruptions, but also concerns energy price increases. Mines are therefore required to review their power options to ensure the sustainability and viability of their operations, whether their operations are locally tied to the grid or self-generating in remote locations.
This study commences with an investigation to establish the relationship between energy and mines and then broadly reviews challenges currently faced in the mining sector. Various energy-saving initiatives have been implemented with the purpose of reducing energy consumption by using energy-efficient equipment or simply by means of managing energy demand in order to shift loads to periods when peak tariffs are not charged. This may solve some of the issues discussed in this investigation, but not the issue of infrastructure constraints, i.e. energy supply constraints and power disruptions.
During the Renewables and Mining Summit in July 2015, mining leaders responsible for decisions on power for operations stressed that these energy constraints had strengthened the business case for renewable energy as a suitable alternative source, especially for mines that are required to curtail production because of supply constraints (Baker, 2015). However, intermittent supply is a characteristic of renewable energy sources such as solar power and wind power. Current storage technologies above 10 MWh are restricted to batteries, compressed air and pumped storage hydropower (Pickard, 2009). Only pumped storage hydropower supplies high volumes of power for significantly long periods in which renewable energy, such as solar and wind power, cannot be generated.
In the case of ultra-deep level mine operations, systems for recovering the potential energy of water flowing down the production shafts have been successfully implemented. During the shaft‟s production life, chilled water is piped underground in shaft columns for refrigeration purposes and in the process the water‟s potential energy is recovered. Because of the success of the current energy recovery systems, various economic studies have been conducted to determine the feasibility of applying these system principles to develop the concept of UPHES.
2.2 ENERGY AND MINES
South African mining operations consume approximately 15% to 20% of the electricity generated at coal-powered and hydroelectric plants by the national energy provider, Eskom. Gold mines account for 47% of the mining industry‟s power, platinum mines consume 33% and all other mines the remaining 20% (Eskom, 2016).
Energy consumption is directly related to the depth of the mineral reserve mined. Gold mines are generally deeper owing to the depth at which the gold deposits are located. Electricity demand increases with an increase in mine depth because approximately 48% of energy consumed is for ventilation, pumping and refrigeration purposes (Goldfields, 2011). The chart below shows a typical breakdown of energy consumption for major equipment in deep level mining operations:
FIGURE 2-1: Electricity consumption breakdown (Goldfields, 2011)
In 2013, the Southern African Power Pool region lacked 2 131 MW of generating capacity (SAPP, 2013). Of the 300 000 GWh demanded in 2013, the national energy provider, Eskom, was able to generate just over 232 000 GWh. After the commissioning of Madupi Unit 6 and synchronisation of the Ingula Units 3 and 4 in 2016, the power utility was able to generate only 239 000 GWh of the 400 000 GWh demand forecast for 2016.
2.2.1 Cost of Energy
The Megaflex costing structure is used to determine the time of use (TOU) electricity tariff for mines, which are grouped with urban customers with a notified maximum demand of greater than 1 MVA capable of shifting load. Charges included in this costing structure include three TOU periods, namely peak, standard and off-peak (Eskom, 2013). The TOU periods defined by Eskom are illustrated in Figure 2-2:
Notwithstanding the above electricity charge, the following charges are also included: Access charges associated with the distribution network
Transmission network charges based on voltage and length of the transmission zone Affordability subsidy charges applied to the total active energy purchased
Reliability charges based on the voltage of the supply Service charges based on the monthly utilised capacity
Network demand charges based on electricity demand on the network
Electrification and rural subsidy charge, which is applied to the total active energy measured at the point of distribution (POD)
Reactive energy charge supplied in excess of 30% (0,96 power factor or less) of the kWh recorded during the peak and standard periods
Administration charges based on the monthly utilised capacity of each POD.
Table 2-1 shows the urban tariffs based on the Megaflex structure for a typical mine of which the supply network is rated between 500 V and 66 kV and of which the transmission line is no longer than 300 km. Mines are typically categorised as key customers, who are charged higher service and administration charges than consumers using less than 1 MVA.
Table 2-1: Typical Megaflex structure anticipated for a mine
U ni t E s k om t a ri ff 2 0 1 3 /2 0 1 4 * E s k om t a ri ff 2 0 1 4 /2 0 1 5 ** E s k om t a ri ff 2 0 1 5 /2 0 1 6 *** E s k om t a ri ff 2 0 1 6 /2 0 1 7 **** E s k om t a ri ff 2 0 1 7 /2 0 1 8 **** *
Low-demand season (September – May)
Peak (c/kWh) [c/kWh] 65.68 70.93 79.93 87.44 89.36
Standard (c/kWh) [c/kWh] 45.20 48.82 55.02 60.19 61.51
Off peak (c/kWh) [c/kWh] 28.68 30.97 34.90 38.18 39.02
High-demand season (June – August)
Peak (c/kWh) [c/kWh] 201.33 217.44 245.03 268.06 273.96
Standard (c/kWh) [c/kWh] 60.99 65.87 74.23 81.21 83.00
Off peak (c/kWh) [c/kWh] 33.12 35.77 40.31 44.10 45.07
Network Access Charge (NAC) [R/kVA] 10.67 11.52 12.45 13.44 14.52 Transmission Network Charge (TNC) [R/kVA] 5.35 5.78 6.51 7.12 7.28
NAC and TNC [R/kVA] 16.02 17.30 18.96 20.56 21.80
Affordability Subsidy Charge [c/kWh] 2.07 2.24 2.44 2.65 2.87 Reliability Service Charge [c/kWh] 0.26 0.28 0.32 0.35 0.36
Combined Affordability & Reliability Charge [c/kWh] 2.33 2.52 2.76 3.00 3.23
Network Demand [R/kVA] 20.23 21.85 24.62 26.93 27.52
Electrification & Rural Network Subsidy Charge [c/kWh] 5.20 5.62 6.33 7.90 7.08 Service Charge [R/day/account] 2616.1 2825.3 3183.9 3483.2 3559.8 Administration Charge [R/day/POD] 83.55 90.23 101.68 111.24 113.69 Reactive Energy Charge [c/kVArh] 9.40 10.15 11.44 12.52 12.80 * Eskom (2013), ** Eskom (2014), ***Eskom (2015), **** Eskom (2016 (2)), ***** Eskom (2017)
2.2.2 Eskom Tariff Increases
On 28 February 2013, the National Energy Regulator of South Africa approved an annual 8% price increase in Eskom‟s tariffs for the period 2013/14 to 2017/18 (Eskom, 2015), as shown in Table 2-1. Media groups have reported that this tariff increase was not enough to ensure Eskom‟s sustainability. Nevertheless, according to an Energy Market Survey conducted by NUS Consulting, South Africa‟s tariff prices still remain 10th most expensive out of 18 countries surveyed, but displayed the second biggest jump behind Belgium‟s 9.9% increase (Writer, 2015).
In a study conducted by Boonzaier et al., a model of where and when tipping points are breached with regard to rapidly rising electricity tariffs was created and used to determine its effect on sector and company level. The study found that for energy-intensive users, rapid increases in electricity tariffs will have an impact on investor decisions with regard to cutting costs, implementation of energy efficiency measures, closing parts of their operations, moving plants to other countries and investing in other sectors (Boonzaaier et al., 2013).
In June 2017 Moneyweb reported having viewed an undisclosed confidential draft submitted by Eskom for comment to National Treasury proposing an increase of 19.9% (Slabbert, 2017).
2.2.3 Energy Saving Initiatives
Various energy saving initiatives have been deployed by many, if not all mining operations, with the intention of lowering the electricity bill without negatively affecting production and mineral recovery rates. These energy saving initiatives, termed demand side management, are aimed at either shifting loads during periods in which peak tariffs are charged or reducing the quantity of electricity consumed by installing energy-efficient equipment (Pelzer et al. 2008).
Measures implemented at many South African mining operations to lower energy consumption include the use of variable speed drives, energy-efficient lighting and improving the efficiency of heating, cooling, ventilation and air-conditioning systems. Furthermore, because dewatering systems contribute to more than 6% of the 36 937 MW electricity peak in South Africa, pumping cycles are optimised to reduce the pumping requirements during peak tariff periods by using real-time energy management systems. Savings of up to 596 MW have been recorded subsequent to the implementation of 202 energy-saving initiatives in South African mines, contributing 19% of the savings accumulated in the mining, commercial, agriculture and residential sectors (Etzinger, 2012).
2.3 RENEWABLE ENERGY AND MINES
The business case for alternative energy sources has become far more compelling in the current economic climate. Renewable energy is an alternative to the conventional sources of energy for both grid-tied and off-grid mining operations. These renewable energy sources make a compelling case, given the significant challenges faced (Baker, 2015).
Akram Elhenawy, global mining key accounts manager from the mechanised mining forerunners Caterpillar, stressed this point during the Renewable and Mining Summit in 2015, stating that in response to diminishing concerns about reliability, investment in the renewable energy market has increased, which reflects better understanding of the benefits of integrated renewable energy solutions (Elhenaway et al., 2015).
In South Africa, the mining sector questioned the reliability of renewable energy for many years, having been content with the supply from Eskom, the sole provider of electricity to the South African electricity grid. Eskom CEO Brian Danes suggested in a statement made in 2014 that state-owned Eskom‟s reluctance to react to the growth of electricity demand after the decision to not pursue increasing the generating capacity of the national grid in 1998, had caused the problem of reliability to shift from renewable energy sources to the once resolute supply.
2.3.1 What is Renewable Energy?
Renewable energy refers to a source of energy that cannot expire or become depleted and is therefore sustainable. There are only two primary sources of renewable energy, which have an impact on the earth, i.e. gravitation and nuclear power (Pickard et al. 2009):
1) The nuclear source naturally emanates from nuclear fusion reactions of the sun and the nuclear decay within the earth. The effects of heat radiation can be harnessed with chemical-based conversion methods or mechanical methods capturing the effects of the induced pressure differentials. These methods are also used today to harness the effects of deep terrestrial fission reactions that produce geothermal resources. The secondary source emanating from this primary source is therefore solar, wind and geothermal renewable energy sources.
2) The second primary source of renewable energy is the earth‟s gravitational force arising from the earth-moon interaction, transferred to the seas to produce the tides. Human-mediated harnessing of the earth‟s gravitational power source is either by wave energy converters or by converting the potential energy from water flowing from a higher to a lower elevation. Therefore, the secondary sources emanating from this primary source are hydropower and ocean energy.
It must be noted that human-mediated nuclear processes, such as controlled fission or fusion, are viewed as a sustainable source but not a renewable source of energy. This is because the fuel used to generate energy is diminished and cannot be used again in the same process.
2.3.2 Renewable Energy in South Africa
Renewable energy has gained momentum globally owing to sustainability and energy security concerns. This global shift has resulted in cheaper renewable energy technologies, government policy support and procurement programmes.
In order to encourage investment and develop socio-economic and environmentally sustainable growth in South Africa, the Renewable Energy Independent Power Producer Procurement Programme (REIPPPP) was developed and implemented by the South African Department of Energy (DoE). International investors view the sunshine and wind throughout the year, as well as the large open tracts of land in South Africa, as huge potential for investment in renewable energy projects. Projects covered by the programme are onshore wind, solar photovoltaic (PV), concentrated solar power, landfill mass, small hydropower and biogas.
2.3.3 Cost of Renewable Energy
The South African national government uses the bidding process facilitated by the REIPPPP to procure renewable energy generation capacity in line with the National Integrated Resources Plan (IRP) for electricity 2010 – 2030. The first four bid windows of the REIPPPP for solar PV and wind are summarised by Bischof-Niemz et al. (2016) in Figure 2-3.
Figure 2-3: REIPPPP tender outcome – average power tariffs
The tariffs offered by utility scale solar PV and wind energy in South Africa, shown in Figure 2-4, are falling below the 1 R/kWh mark. In comparison, the tariff charged by Eskom is expected to increase above this mark.
Decreases in the tariffs from renewable energy technologies are a result of increases in the global renewable energy generation capacity (GreenCape, 2017). Renewable Energy Advisors (REA) suggested in 2013 that the cost of renewable energy technology has decreased in the past two years. Further to the decrease in the cost of the technology, tax incentives are also now available and assist reducing the system ROI (Sola, 2013).
Figure 2-4: Average Eskom tariffs versus utility–scale renewable energy tariffs
(Eskom, 2016)
While renewables such as PV plants and wind turbines are capable of lowering the consumption of fuel to power large generators in off-grid mining operations in the range of 30% to 70%, the use of these renewable resources are hampered by their characteristic intermitted supply.
2.3.4 Photovoltaic Plants
Some sectors of the renewable energy market, such as solar power, have developed rapidly to the extent where it has reached price parody with regard to industrial loads. This has been achieved because of the possibility of storing energy in lead-acid and sodium-sulphur batteries, but is limited to 10 MWh (Pickard et al, 2009). Therefore, global mining and renewable energy sectors focus their attention mainly on driving the solutions associated with the effects of the first primary source of renewable energy: solar radiation. In 2015, Sibanye Gold, the largest gold mining house in South Africa, announced plans to build a PV solar plant with an installed capacity of 150 MW by 2017 (Wallington, 2016). The plant is expected to provide approximately 30% and 10% of their peak and total operational power requirements respectively.
The generating capacity of such a PV plant is limited to the surface area available for the erection of the solar panels and can essentially only convert radiation energy to electricity when these panels are exposed to the sun‟s rays. This means that a power-generating factor exists that can reflect the energy-generating capacity of a PV plant at any given time, as is illustrated in Figure 2-5 (Ma, 2014).
Figure 2-5: Power generation factor
0 0.2 0.4 0.6 0.8 1 0: 30 1: 30 2: 30 3: 30 4: 30 5: 30 6: 30 7: 30 8: 30 9: 30 10 :3 0 11 :3 0 12 :3 0 13 :3 0 14 :30 15 :3 0 16 :3 0 17 :3 0 18 :3 0 19 :3 0 20 :3 0 21 :3 0 22 :3 0 23 :3 0
Matching the potential energy that can be harvested from solar radiation to a POD load profile can become rather complicated. Ideally the energy produced should be applied to the periods in which the load profile peaks, especially periods when electricity is charged at a peak tariff. Figure 2-6 shows a POD load profile for a typical operational shaft with the power generating factor overlain.
Figure 2-6: Typical POD load profile
The excess energy generated that is not used instantaneously must therefore be stored. The means for doing this must be capable of storing vast quantities, as is evident from the 150 MW PV plant envisioned by Sibanye Gold and variations in a typical load profile.
2.3.5 Storage of Renewable Energy
With the selection of an appropriate storage mechanism, power variations can be mitigated and system flexibility can be enhanced. Studies considering strategies for storing energy of input power provided by intermitted resources have demonstrated that only a few options are feasible, considering the current and future predicted technological capabilities. In a study conducted by Amrouche et al., various renewable energy storage methods are investigated and the most frequently used technologies and their associated efficiencies are listed as follows:
0.000 0.100 0.200 0.300 0.400 0.500 0.600 0.700 0.800 0.900 1.000 10000 11000 12000 13000 14000 15000 16000 17000 18000 19000 20000 0: 30 1: 30 2: 30 3: 30 4: 30 5: 30 6: 30 7: 30 8: 30 9: 30 10 :30 11 :30 12 :30 13 :30 14 :30 1 5 :3 0 1 6 :3 0 17 :30 18 :30 19 :30 20 :30 21 :30 22 :30 23 :30 P o w er gen er atio n f ac to r kW h
TYPICAL OPERATING MINE POD LOAD PROFILE
Standard tarrif charges Peak tarrif charges
Table 2-2: Most used storage technologies in renewable energy systems
Energy Storage Method Technologies Time Scale Efficiency
[%]
Electrochemical Batteries Medium (minutes) 90 - 95
Flow batteries storage Medium (hour) 75 - 80
Hydrogen (HES) Hydrogen Long 65 - 75
Mechanical
Flywheel energy storage Short (seconds) 80 - 90
Pumped hydro energy storage Long (hours) 70 - 85
Compressed air energy storage Long (hours) 64 - 75 Electrical Super capacitor energy storage Short (seconds) 90 - 98
Electromagnetic Superconducting magnetic
energy storage Short (seconds) 90 - 99
Thermal Thermal Medium 80 - 90
Current storage technologies above 10 MWh are restricted to lead-acid and sodium-sulphur batteries, compressed air and pumped storage hydropower. Only pumped storage hydropower supplies high values of power for significantly long periods, capable of producing 1000 MWh (Montero et al. 2015). Pumped storage hydropower has therefore been identified in these studies to be capable of meeting the anticipated technological and economic constraints (Pickard et al. 2009). It can be expected that pumped hydro storage technology will advance in order to meet the demand.
2.4 UNDERGROUND PUMPED HYDROELECTRIC ENERGY STORAGE
2.4.1 Recovery of the Energy of Water Going Down Mine Shafts
In South Africa, deep level mining operations make use of large cooling systems to reduce underground ambient temperatures to conditions conducive to a healthy work environment as regulated by the Mine Health and Safety Act 29 of 1996 and Minerals Act 50 of 1991. These systems comprise one or more central refrigeration plants located on surface. Chilled water is distributed to underground heat-exchanging systems via one or multiple shaft columns to working levels of up to 3 000 m below surface.
The implication of surface-located refrigeration plants is the rise in the temperature of chilled water resulting from the dissipation of pipe friction into thermal energy. Whillier (1977) suggests that the rise in temperature is approximately 2.33˚C per 1000 m shaft depth. The increase in temperature adversely affects the temperature of cooled air circulated at the mining face. If the water were passed through a turbine at the bottom of the shaft, the water would return to the ambient pressure under constant entropy conditions, therefore more than halving the rise in temperature. The potential energy of the water in the shaft column is converted to mechanical energy in the turbine, therefore recovering a part of the energy required to pump the water back to surface.
High energy recovery has been recorded for many of these types of energy recovery systems over a wide capacity range of differential heads. More than 50 turbines with a combined capacity of over 65 MW have been installed underground in various mines in South Africa (Karanitsch, 2010).
2.4.2 Concept of UPHES Plants
PHES refers to a system in which electric power is used to pump water to a reservoir, located at a height above a lower reservoir. In the upper reservoir the water is stored until there is a shortage of electrical power. Water from the upper reservoir is then gravity-fed through a turbine converting the gravitational potential energy into electrical power, thereby ensuring grid stability.
The use of the potential head created in mine shafts for the storage of energy is not a new concept. A UPHES system refers to a PSHP system of which all system power-generating components are underground, specifically applied to deep level mines. Fessenden (1917) patented a “system of storing power” underground and suggested that one horse power hour of energy may be stored for every cubic yard of water content of the lower reservoir in a mine shaft, for which a shaft depth of 1200 feet would be sufficient.
2.4.3 UPHES generation potential
The feasibility of a UPHES system must consider the amount of energy that can be generated. The parameters determining the energy that can potentially be generated by a UPHES system are shown in Equation 1:
where the volume of water in the upper reservoir is given in cubic metres ( , water density in kilograms per cubic metre ( ), gravitational acceleration ( and distance between the upper and lower reservoir in metres ( .
The electrical power ( ) generated in Megawatt is determined by taking into consideration the turbine efficiency ( ) and the period in hours in which the turbine is used to generate electricity ( ):
.
2.4.4 System storage efficiency
The system storage efficiency of conventional pumped hydroelectric storage systems varies in practice between 70% and 80%, with some claiming 87% (Rehman et al., 2015). Losses are attributed to pipe friction, turbulence and heat loss. The storage efficiency ( ) of a UPHES system can be determined by Equation 1:
.
2.4.5 Economics of a UPHES system
The basic principle of a UPHES system is applied to take advantage of the Megaflex tariff structure. The Megaflex tariff structure, discussed in section 2.2.1, consists of peak, standard and off-peak tariffs. In low and high demand seasons, the off-peak tariff is 56% and 17% of the peak tariff charged respectively. Water is therefore pumped to the upper reservoir during the period in which off-peak tariffs are charged, and allowed to flow to the lower reservoir through the turbine to generate electricity during periods in which peak tariffs are charged. The power generated is then sold back to the grid, at a lower price than sold by Eskom.
All PHES systems are net consumers of energy and therefore not independent sources of power. They are therefore conventionally used as:
Storage systems for conventional power plants (coal, oil, gas or nuclear) Storage of energy generated by renewable energy plants (solar and wind) Energy input during peak demand periods.
Therefore a PHES or UPHES system is only economically viable if the difference between the peak and off-peak tariff can account for the efficiency related additional energy required from Eskom (Winde, Kaiser & Erasmus, 2017).
2.4.6 Use of existing mine infrastructure
The main advantage of using existing mine infrastructure is not only the reduction in construction costs due to the largest excavation component already existing, but also the depths at which a potential network of tunnels exists, providing potential heads exceeding 1000 m and more.
Montero et al. (2015) conducted a study to analyse various aspects of using the existing mine infrastructure of the Prosper-Haniel mine in Germany for the development of a UPHES system. The underground tunnel network envisioned as the lower reservoir can hold approximately 600 000 m3 at a potential head of 600 to 1000 m.
2.4.7 Subsurface construction
Traditionally only certain parts of PHES plants are situated underground. Turbines are placed in subsurface caverns to protect the landscape and other aesthetics. The first combination of a PHES plant with a subsurface cavern for water was put into operation in Austria in 2006 with the expansion of the Nassfeld plant. The expansion of the plant was inundated with various technical, landscape-related and legal implications, leading to the establishment of a subsurface pipe system. After six months of construction, 160 000 m3 was excavated to construct a 1950 m tunnel. The cost of excavation and support of the surrounding rock was exorbitant and the expansion project amounted to 13 MЄ (Tschernutter, 2010).
In Italy, Austrian energy provider KELAG AG studied an entirely subsurface PHES to be built into a mountain. Water from an upper cavern was planned to flow to a cavern 900 m lower and the plant
would have an installed capacity of 250 MW. The upper and lower reservoirs were to be excavated caverns 0.6 Mm3 in volume. The investment costs were estimated at 300 MЄ. Protests and subsequent negative public relations effectively caused the project to be abandoned (Madlener et al., 2013).
The use of UPHES has the added advantage that the construction is not topographically restricted to sites where above-ground upper and lower reservoirs can be placed together because the lower reservoir can be excavated from the subterranean rock almost anywhere immediately above the upper reservoir (Montero, 2015).
Goldisthal, one of the largest PSHP plants in Germany, has both upper and lower reservoirs located on surface, while the generator, turbine and transformers are built into a cavern in a mountain. Around 152 000 m3 and 32 000 m3 had to be excavated for the turbine house and entry tunnel respectively (Heiland et al., 2013). The building of the upper reservoir on the hilltop required removal of the peak, which raised great aesthetic and ecological concerns, eliciting serious protest from the local community and nature protection groups.
2.4.8 Main System Components
2.4.8.1 Pumps and turbines
The study conducted by Winde et al. (2017) suggests the use of one of two options:
Option 1: Combined pump-turbine unit (reversible Francis turbine) Option 2: Pelton turbine.
The combined pump-turbine can pump water to heads of up to approximately 800 m, which restricts the system‟s potential head and is therefore not suitable for ultra-deep shafts (Winde et al., 2017). The Pelton turbine (an impulse type water turbine) is suitable for heads exceeding 1000 m. A German manufacturer manufactured a Pelton turbine capable of generating electricity as an hydraulic head of 1 220 m in 2008 and installed it at Akkoy II, Turkey (Voith, 2015).
2.4.8.2 Turbines
The following formulae are used (Gupta et al., 2016) for determining the theoretical power input in Watts ( the actual output power in Watts ( and the hydraulic efficiency ( :
.
The application range of standard and special application Pelton turbines manufactured by Voith is shown in Figure 2-8:
Figure 2-8: Application range for Pelton turbines (Voith, 2015)
The Pelton runner typically operates in atmospheric pressure with one to six jets of water through needle nozzels impinging tangentially on the runner. The function of the needle nozzle is to regulate the flow of water to the runner. The needle jet is regulated by the governor via mechanical- or electric hydraulic controls. The shape of the needle nozzle is designed for rapid acceleration at the exit end and for assuring a uniform water jet at all the openings. The needle valve/nozzle assembly is placed as close to the runner as possible to avoid the jet spreading due to air friction (HAP, 2012).
Pelton turbines with multi-needles are among the most efficient designs of hydroelectric turbines. The Pelton turbine used at the Stanislaus powerhouse in California, commissioned since 1963, which has a nameplate rating of 113 000 hp at 1 525 ft hydrauluic head, and generator nameplate rating of 91 MW, 91 MVA, 0.9 power factor at 13.8 kV, is operated with six nozzles (Roberts & Nunnelly, 2011). The efficiency of the turbine at Stanislaus is shown in Figure 2-9:
Figure 2-9: Turbine efficiency for 2, 4 and 6 needle operation at Stanishous
powerhouse (Roberts
et al.
, 2011)
2.4.8.3 Pumps
Multistage clear water pumps are used for high volume and high lift applications at gold mines. The quality of the water is clear or slightly polluted with abrasive particles. These pumps are therefore particularly fitting for mine dewatering. Heads of between 120 and 1 800 m can be pumped at capacities between 130 and 1 000 m3/h, as shown in Figure 2-10:
2.4.8.4 Upper and lower reservoirs
Winde et al. (2017) suggest that the upper and lower reservoirs be constructed by using existing mine haulages and tunnels. Main haulages, developed from the shaft stations, are about 5 m wide by 3 m high in typical South African gold mines. Reef cross-cuts connecting the main haulages to reef drive are on average 180 m long, 3 m wide and 3 m high. These cross-cuts and main haulages canbe used to construct underground reservoirs by simply constructing dam walls. The quartzite walls are sufficiently inert and will therefore not adversely affect water quality
.
2.4.8.5 Emergency storage capacity and pumps
Winde et al. (2017) further suggest that a number of levels below the lower reservoir be kept available for emergency storage capacity. The emergency storage volume must be sufficient to accommodate the entire volume of the circulating system, including a buffering capacity to accommodate additional water egress; 1 Mm3 is suggested to be a sufficient buffering capacity.
Furthermore, submersible pumps must be provided to dewater the emergency storage excavations (Winde et al. 2017)
2.4.8.6 Access shaft and pipe column
The shaft, connecting the surface and underground workings, equipped with shaft steelwork, conveyance guides, man riding conveyances, electrical cables, instrumentation cables, communication cables and pipe columns, will be used as is. Ventilation of the mine is also routed through the shaft (upcast/downcast configuration).
Winde et al. (2017) note that the shaft diameter must be able to accommodate the size of equipment required to be transported in the shaft during construction of the UPHES system. It is also important to note that the winders must be able to accommodate the mass of the equipment to be transported.
The pipe size required for feeding water to the turbine to accommodate the required flow rate is dependent on the required generation capacity of the system. The generation capacity is linked to the storage capacity of the upper and lower reservoir. The system contemplated by Wilde et al. (2017) assumes a hypothetical storage volume of 1 Mm3 used to generate power for four hours per day. This would require a flow rate through the turbine of 70 m3/s. To accommodate this flow rate in a pipe, the pipe‟s inner diameter would have to be 2.5 m with a flow velocity of 14.2 m/s. In order to reduce the losses attributed to turbulence and friction, it is suggested that two pipes be installed, as shown in Figure 2-11.
The use of 2.5 m diameter pipes would be a very costly to manufacture and install. Winde et al. (2017) suggest that a 5 m raisebored shaft be considered. Such constructions have been used in conventional PHES systems, for example at Goldisthal, Germany.
.
Figure 2-11: Cross-section of a shaft with two pipe columns (Winde
et al.
, 2017)
2.5 TREATING THE ROOT CAUSE OF ACID MINE DRAINAGE
The excavations required to access ore bodies such as the shaft excavation, development of the main haulages and eventually access to the reef horizon is a major constituent of capital cost of the shaft. After the ore body has been mined, these excavations are abandoned and flooded after decommissioning the shaft. The oxidisation of the sulphides contained in mined ore and waste in the water creates highly acidic and polluted water, eventually emerging from these flooded mines, known as acid mine drainage (AMD).
The largest groundwater resource in South Africa, located above the gold reefs in the Far West Rand (FWR), is held by karstified dolomites supporting a range of high-yielding karst springs (Schrader & Winde, 2015). Flooding of some of the already decommissioned mines has occurred because of the egress of groundwater and lack of proper management and planning, the result of which is AMD draining into the environment in and around Johannesburg. The cost of addressing these issues has been estimated at R10 billion, which involves decanting AMD, but not treating the root cause. An estimated total volume of 130 million litres per day will be treated and potentially an additional 200 million litres per day once the other mines in the FWR are decommissioned (Kolver, 2014).
In a study conducted by Winde, Kaiser and Erasmus (2017), the viability of using deep level gold mines on the FWR was considered. The study suggests that current and future decommissioned mines can be kept from becoming sources of AMD by using an open UPHES system. The continuous ingress of fissure water is balanced in an open UPHES system by cleaning and discharging excess
