by Mike Rycroft, Now Media –
Large hybrid systems (LHS) are common as power sources for remote mines and other installations. This has been brought about by the unreliability of the national grid, and the all-too-frequent events of load shedding. The decreasing cost of renewable energy systems and storage also plays a part. LHS have evolved from simple combinations of two generation sources to overly complex systems.
Hybrid systems consisting of gensets, solar photovoltaic (PV) systems, and storage are quite common but up to now have been confined to sizes in the region of a few MW and below. Recent developments, however, have seen the appearance of large multi-megawatt systems.
A hybrid system is basically the combination of renewable energy with other generation sources which complement one another to provide a cost-effective reliable source of electricity.
Modern LHS are similar in function to microgrids, with the exceptions that the LHS is not grid connected and usually serves a single customer or site. A microgrid can consist of many distributed energy sources, where the LHS consists of a few dedicated sources.
LHS used in mines evolved from plant at mine sites powered by generators running on diesel or heavy fuel oil (HFO). The first step towards a LHS was the incorporation of solar PV to reduce the fuel usage during daylight hours. Next came the use of batteries to cater for short drops in PV output, and handle load variations. The final stage has been the inclusion of wind turbines as a source of energy. In future, fuel cells could replace generator sets.
As the prices of batteries and renewable energy (RE) equipment continue to fall and the penetration of variable wind and solar generation rises, renewable energy systems combined with storage will become even more attractive as additions to generator sets. Hybrid wind/PV/genset systems have been used to provide power to island communities for many years, but the inclusion of wind-power in onshore and inland systems is a recent development.
A feature of early systems was that gensets were the main source of power, with PV displacing some of this during daylight hours. The situation is changing to the extent that one mine expects to derive 70% of its energy requirements from RE with its LHS. The genset component has nonetheless been retained in most cases to ensure security of supply.
Price and availability of battery storage was initially a barrier to usage of storage. The decrease in price and increase in availability of large capacity batteries have led to more adoption of these, initially to smooth out short term variations in PV output – which causes spurious cycling (start and stop) operation of the genset – but have more recently become important components of the system.
The most basic configuration consists of a PV system and genset as shown in Figure 1. During daylight hours, generation from a genset is displaced by PV generation, meaning that several gensets will not be running during daylight hours. The disadvantage of this system is that as PV output drops for short periods when a cloud covers the panels, “cycling” of one or more gensets occurs, where the generator starts up and runs for a short period and then stops.
This can be compensated for by continuously running one or more gensets at a low load. Neither of these situations is desirable. Cycling can be a problem for certain cloud patterns, with numerous short drops in output. The impact of cycling depends on the size of the PV array and the size and number of gensets. The output of the PV array can drop by 50% under passing clouds. Generally, PV would have been added to an existing genset fleet and the impact on the existing installation would have to be considered when sizing the PV array. The varying PV output also causes problems in balancing load between gensets and PV. This configuration may include a controller.
A second configuration involves adding a battery storage system to the PV array as shown in Figure 2. The primary purpose of the battery is to bridge short drops in PV output and prevent genset cycling. The size of the battery needs to be sufficient to cover the power drop in the PV system for the duration of the cloud transit. This configuration may include a controller.
Figure 3 shows a further development made possible by both the reduced cost of battery storage and the availability of larger capacity batteries. The battery now serves as storage for the whole installation and would have a longer duration than in the previous case. This extends the use of solar power by storing power from the PV array, caters for sudden load changes, provides spinning reserve displacement, and allows optimum balancing of generation between the PV and the gensets. The system requires an advanced controller to perform these functions.
Figure 4 shows the latest development which incorporates wind generation. The battery now performs a crucial role in balancing generation and consumption and needs to be large enough to smooth the output of the wind turbines, as well as that of the PV array, and provide for bulk storage. A complex control system is required for this configuration.
Although it is technically possible to design hybrid systems consisting entirely of RE components, including storage, it is not yet possible to produce a RE hybrid which would provide the required security of supply and reliability at a competitive price.
The problem with wind turbines is one of modularity, with most turbines producing 3 MW or more. One advantage of wind is that the resource is generally available at night, and for a mine operating a 24 h shift this can be advantageous but will depend on site characteristics. The addition of wind adds a continuous source of generation but adds complications to the control functions, especially frequency control.
Current and planned systems
LHS are installed at several mines around the world. Table 1 provides the configuration of several existing and planned mine hybrid power systems. Most are upgrades or modifications of existing diesel or HFO generator powered systems.
Relative sizes of plant give an indication of the reliance on the different technologies. The largest hybrid system is that powering the Agnew goldmine in Australia, which consists of an 18 MW wind farm, a 4 MW solar farm, a 13 MW/4 MWh battery system and a 21 MW gas/diesel engine power plant. In favourable weather conditions, the project can supply up to 70% of the mine’s power requirements from renewable energy.
As can be seen from Table 1, designs which retained the ability to run completely on gensets can be modified by adding solar PV, batteries, and wind to reduce the use of fuel. In the case of the Agnew mine shown in Table 1, the full demand can be met from the wind and solar resources operating at maximum output simultaneously, and under normal conditions the wind and solar would be expected to supply an average power of 8 MW, about half of the demand.
Solar PV and wind-powered generation use inverters and all inverters require a reference frequency. The simplest solution is for the generator to provide the frequency reference. This would be simpler than having one of the inverters, which is receiving a varying supply from the PV or wind turbine, provide the frequency reference. This does mean however, that at least one generator will always be operational, and this limits the contribution from renewable energy sources in terms of fuel saving. However, by adding a battery, the battery inverter could provide the frequency reference. This would allow operation on wind and solar power alone.
Project lifetime, costs, and technology choices
To achieve economic feasibility, the cost of generating electricity using a hybrid system must be lower than any other method. (If we take loss of production due to power outages into account). Mines can have a lifetime ranging from 5 to 20 years or more. Both wind and solar PV systems have expected lifetimes of about 20 years, while batteries are expected to last for 10 years. The lifetime cost of projects is highly dependent on the initial capital cost of these systems, and the unit cost of electricity will depend on the annual capital amortisation costs.
For a project where the lifetime is annual capital amortisation costs. For a project where the lifetime is significantly less than the expected power system lifetime, the unit cost of electricity can make the hybrid project uneconomical.
A common arrangement today is an on-site power purchase or system lease agreement, where the mining company does not own the power system but purchases power from an on-site IPP at an agreed price for an agreed period.
This has the advantage that the mining company does not need power generation expertise, and that the IPP owns the equipment, and can move equipment to different sites it serves as power demands change. This also allows the full lifetime of equipment to be exploited, although in the case of wind it may be difficult to relocate turbines at the end of a mine’s life.
Power rental companies have been active in the mine genset rental field for many years. The easiest item to move around are gensets, followed by storage, and PV. Wind turbines, with the huge infrastructure, may not be as easily moved.
South African situation
Several mining houses have applied for approval to produce their own power as shown in Table 2. All planned systems appear to be solar PV based, but must include some energy storage system. None are capable of off-grid operation.
Connection to the grid is available at all existing mine sites. The decreasing cost of solar PV and storage, as well as the increasing unreliability of grid power, is likely to result in more and more large, hybrid systems in the network.
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