A valve and pipeline solution, used in the cooling of deep- level mines, may be the answer to the current energy crisis sweeping the country, hydropower specialist company Hydro Power Equipment (HPE) CEO André Swart says.
“With mine pumping contributing about 2 300 MW to the 36 000 MW electricity peak demand in the country, it is critical to investigate new demand-side management opportunities, technologies and methods to reduce the electricity consumption in the industry,” Dr Danie le Roux, energy specialist from Optimine and subcontractor to HPE states.
The three-chamber pipe system (3CPS), which is currently being implemented by hydropower equipment company HPE, in conjunction with State-owned electricity utility Eskom’s demand-side management (DSM) programme at three mining projects, has the potential to save about 157 GWh a year, which means a considerable electricity cost reduction for the mines.
Eskom agreed to fund 50% of all three projects, resulting in a payback period of less than 2,5 years for HPE’s clients, while the total combined electricity cost saving for the three 3CPS projects is estimated at R15,8-million a year. Full implementation of the 3CPS at the mines is expected to be complete by September 2008, and May 2009.
SAVINGS ON ENERGY
Besides the total combined electricity cost saving of about R15,8-million a year, the 3CPS has other positive effects on the South African economy.
Le Roux says that Eskom DSM announced in 2007 that it has a target of achieving 4 563 GWh electricity savings by 2012.
With the 3CPS installed and commissioned, savings on the systems alone would amount to 630 GWh by 2012, a saving of about 14% of Eskom DSM’s total target. The energy saved in this way could then be used elsewhere without the need for Eskom to generate additional supply. In this way, the electricity savings from the 3CPS can be seen as a virtual power station.
“The new projects save about 17,9 MW a day or 157 GWh a year. This is equivalent to power used by about 47 800 low-income houses or 4 500 high-income houses, or almost 10 000 midincome houses,” Le Roux explains.
This means that by implementing the 3CPS, electricity is saved in the industrial sector and applied in the residential sector, without influencing production results.
It is calculated that Eskom will emit about 7 800 t of greenhouse-gas emissions a year from its coal-fired power stations for every 1 MW generated. The electricity saving resulting from the 3CPS, should the system save 157 GWh of electricity a year, will mitigate the utility’s greenhouse-gas emissions by 141 000 t/y.
“The effect of the electricity savings due to the 3CPS is significant to South Africa. Almost 20 3CPS projects will achieve Eskom DSM’s electricity saving target of 4 563 GWh by 2012. There is large potential to roll out the technology in mines using existing excavations in South Africa,” Le Roux explains.
Currently, Eskom DSM will not fund 3CPS projects on greenfield projects, but Le Roux says that if this could be altered, energy-efficiency technology could be planned for new sites.
3CPS IN OPERATION
Currently, only the Tshepong mine, in the Free State, uses 3CPS technology. Swart says that a 3CPS was installed at the mine in the early 1990s. The valves of the system were costly to maintain, forcing the mine to use electricity- intensive pumps.
HPE became involved five years ago and replaced all the valves in the system with HPE valves, which resulted in the system availability exceeding 95%.
HOW 3CPS WORKS
Swart says that the system works on the basis of recovering the energy from incoming water, to pump water out of the mine.
“Chilled water from the refrigeration plant is piped down the shaft to the chambers of the 3CPS. The three pipe-system eliminates water pulses in the same way that multiple cylinders in a conventional car’s engine makes it run smoothly.
The pipes are 450 mm in diameter and can be up to 100 m long. They are about 120º out of synchronisation, meaning that while one of the chambers is under high pressure, the other is on low pressure, while the last chamber is in the phase of either going up in pressure or down,” Swart says.
Incoming chilled water is then used to displace the outgoing warm water, with a small booster pump used to overcome friction in the system. A U-tube is formed on the one side by the high-pressure chilled water feed column, the 3CPS at the bottom of the shaft, and the outgoing warm water delivery column on the other side. A small filling pump fills the chambers with warm water.
The three chambers are each fitted with HPE’s valves at either end, which are actuated in a sequence to achieve a continuous and steady flow in and out of the system.
“The 300NB valves have to withstand enormous pressure. In some of the cases, the valves have to withstand up to 20 000 kPa of water pressure, while opening and closing about 15 000 times a month,” Swart points out.
Le Roux says that a programmable logic controller (PLC), which integrates start-up, operation, and shutdown sequences, and the necessary safety interlocks control the valves. The PLC and the instrumentation are connected through a fibre network to the mine’s supervisory control and data acquisition system in the control room on the surface, and a full view of the status and performance of the system is available, and can be controlled from the control room.
“The valves are hydro-actuated, meaning that although a PLC controls the solenoids to open and close, the water pressures in the columns are used to open and close the main valves. Every valve also has a displacement transducer relaying in analogue how much the valve is opened or closed and how long it takes the valve to open or close. In this way, the system can be upgraded to relay to the operator when a valve needs maintenance,” Le Roux states.
He says that the PLC is constantly monitoring the high-pressure and low-pressure chambers, synchronising the system.
3CPS was originally developed with the displacement of abrasive slurries in mind. The slurries are displaced from a pressurised chamber using high-pressure water. This had the advantage that the mechanical pump would pump only water, and not the abrasive slurry.
To convert the process into a continuous process, at least two chambers are needed – one chamber to be pumped, while another is being filled for the next pumping cycle. A third chamber was added to assist the change, and to smooth out the flow process.