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Analysis | February 2012

Supply systems for rural communities

The option of stand-alone or off-grid based renewable energy for electrification of rural or remote areas should be looked into, says Narayan Bhat.

More than 2 billion of the world's population lives without access to electricity and 85 per cent of them live in rural areas. In India over 50 per cent of the population is yet to have access to electricity. Some approaches like extending the electricity grid, installation of stand-alone power system or mini-grid at the local level can bring electricity to remote areas. Extending the transmission lines and grid can be costly and unfeasible, being far away from the grid or due to difficult terrain. Installation of a stand-alone power system can be one of the options which can be close to the load at individual households and can provide an easily accessible and relatively inexpensive solution. Mini-grids using diesel or use of renewable energy resources like solar photovoltaic, wind or small hydro power can be one of the most powerful approaches for accelerated rural electrification. These solutions, in particular can be feasible with renewable energy and the modern technologies of renewable energy can play a crucial role.

Indian rural electrification can be explored with non-grid based system at remote locations with renewable energy sources. May be exploring all sources of wind, solar and mini-hydro could be a means to fulfill the 'Electricity for All' objective. Offshore wind could be a resource that needs to be looked into. The efficient power evacuation and storage technologies can connect the energy generated to a grid or stand-alone power system.

Typical power systems of renewable sources

A solar home system which normally has a PV module with solar charge controller and battery storage can be directly used for DC appliances with DC/AC inverters that supply to AC loads. Large stand-alone PV systems used for residential or commercial areas usually adopt DC/AC inverters to supply to the AC network of loads. In case of small wind turbines, power ranging from 1 kW to 50 kW, the permanent magnet generators are directly connected to the rotor, produce AC current with rectification and can be stored in batteries to make effective use of energy.

The power conversion unit features of wind consist of a generator, a rectifier, a buck-boost DC/ DC converter, a battery bank, and a DC/AC inverter. One of the most promising renewable energy generations lies in the development of power supply systems that make it feasible to connect loads to grid or standalone off-grid power supply.

Wind turbine generators, inverters, storage systems

Wind turbines convert kinetic energy into rotational or mechanical energy which is converted into electrical energy using a generator in windmills. Small wind turbines are equipped with DC generators of up to a few kilowatts in capacity. Modern wind turbine systems use three-phase AC generators. Different types of generators such as induction or synchronous generators with excitation or permanent magnets are used in wind turbines. Induction generators (IG), such as squirrel cage or wound rotor, have been extensively used in commercial wind turbine units.

Asynchronous generators are considered advantageous due to flexibility when wind speeds fluctuate. Synchronous generators have the capability of direct connection to wind turbines, with no gearbox. Generally, turbines equipped with doubly-fed induction generator systems for variable speed are one of the most efficient configurations for wind energy conversion.
Inverters that convert DC power from batteries into AC power are complex electronic devices and mostly installed in dust-free environments. They are commonly a part of battery-based and grid-connected systems.

Storage technologies play a vital role for sustainable operation of wind-based off-grid systems. If wind conditions are favourable, a system can be without batteries and provide electricity at a lower cost. However, often batteries are included in the system, so surplus energy can be stored because the available wind does not always produce the required quantities of power or if wind power exceeds the load demand. Lead-based batteries are most popular and an efficient type of storage in off-grid systems with very low or no maintenance. There are international regulations and IEC standards to validate the quality of batteries. New technologies like Lithium-ion batteries are being adapted to off-grid renewable energy.

Nickel (Ni-Cd) based batteries are another option for further developments in the storage technology, especially for locations wherein extreme temperature conditions range from -25°C to +50°C. Slow ageing at high temperatures and low capacity loss below 0°C and less maintenance requirements are an important factor in selection of Ni-Cd batteries.

For over two decades, our group has been working with renewable energy companies to help solve the demanding technical, regulatory and commercial challenges they face, improve the safety and reliability of assets and people, and systems and processes involved.

Industry-first experimental offshore wind farm, Beatrice, UK, was supported by our company, which also played a central role in creating certification guidelines for the European Marine Energy Centre (EMEC) standards. To build robust, reliable and safe generation technology, certification and due-diligence reviews of devices are necessary and knowledge and expertise in turbines, generators, electrical, civil and structural, environmental, geo-technical, geo-physical, experience in installation, placement of substations, assessment of speed and turbulence intensity is essential and will help designers, manufacturers and project developers.

The way forward

The world has recognised the long-term importance of harnessing natural energy resources like wind, solar etc., in the energy mix to cope with sustainable energy security. In the wake of Fukushima, there is a noticeable shift world-wide towards renewable resources. Mostly the driver for renewable energy has been environmental issues, especially climate change and improvements in technology. Globally, government polices and subsidies encouraged renewable developments, including wind power. China has shown remarkable growth in wind power. Most countries including India, followed onshore wind technology, but growing trends of offshore wind are seen in China, European Union and United States. India has so far tapped only 13-14 GW of wind potential and as per various estimates of C-WET and WISE, greater potential (in the range of 50-100 GW) exists. May be India needs to re-look and assess its potential and feasibility of offshore wind, which could be another source of energy, with its long coastline. Overcoming challenges to integrate renewable energy to the grid can bring in more growth in renewables. So India should also look at the option of stand-alone or off-grid based renewable energy for electrification of rural or remote areas.

The author is Head Power Asia, Lloyd's Register. Views are personal.
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