The role of Electric Drives in Renewable Energy

Renewable energy is on the rise. Long regarded as an ‘alternative’ power source, the stuff of backyards and backwaters, it is at last headed for the mainstream. Grid parity – that point when the cost of renewable energy becomes equal to that of the conventional kind – is expected to arrive some time in the 2020s. In the solar photovoltaic (PV) sphere, some countries, such as Germany, are already there.

New technologies, more than policies or pressure groups, are thought to be the main impetus behind the change. As renewable energy systems have become more efficient, prices have fallen dramatically, a trend set – with concepts like smart metering – to continue. Clean energy is becoming as much an affordable reality as it is the popular aspiration of those wishing to decarbonise their footprint.

That these systems are becoming just as sophisticated as their more established equivalents is amply demonstrated by the growing role electrical drives are playing in their configuration. Recent years have seen drives bring viability to a variety of renewable energy projects: from solar-powered irrigation systems to wind turbines, and from wave power stations to off-grid, sustainable power generation networks.

Types of renewable energy technology

Small scale applications of renewable energy technology, of course, continue to have strong relevance to remote and underdeveloped parts of the world. Motorised water pumps, for example, vital for irrigation purposes or supplying drinking water in isolated, rural communities, are traditionally vulnerable to power cuts or undependable supplies of diesel. Solar PV panels have been developed to provide an effective solution.

A modern solar water pump consists of the PV panels, the pump itself, a motor and a drive to ensure that the motor runs on the correct voltage and at the desired speed. As the sun’s radiation increases during the day, the current from the panel cells rises and the drive softly starts the motor; correspondingly, as night falls, the system automatically shuts off.

The drive, which would normally be taking an AC input from the mains, here must conduct the DC power from the PV cells straight into the DC bus capacitors before converting it to AC power at the right frequency. And in order to manage the natural fluctuations characteristic of solar radiation, the algorithmic function known as Maximum Power Point Tracking (MPPT) optimises the power available to the system from the PV array.

The most challenging feature of renewable energy sources is that their strength levels are not steady: just as sunlight may be more or less powerful depending on weather conditions, so wind power, even more so, may be intermittently strong or weak.

Renewable energy from the wind

While an electrical current that ebbs and flows with the weather might represent no great impediment to the efficiency of a local watering system, it is obviously unsuitable for larger power networks requiring standardised voltage levels and a constant frequency. As pioneering as they were in their day, early stand-alone wind turbines, used to generate domestic electricity, had no wider social usefulness than the windmills of centuries before.

In order to develop modern relevance – as contributors to the national grid – wind turbines had to become fixed speed machines, running at one rotational velocity irrespective of wind conditions. Built-in gearboxes would then generate electricity of the correct frequency for the grid.

Now, thanks to advances in electrical engineering, wind turbines are again becoming variable speed machines. Industrial drives, in particular, are being used in regenerative mode to convert the energy obtained from motors turned by the turbines and to direct it into the grid – efficiently capturing surges in speed such as those caused by wind gusts.

The energy available for harnessing from the turbulence of the planet’s natural kinetic forces is abundant; the challenge lies in converting it into a standardised, usable format. Today’s electrical drive systems are playing a leading role in meeting this challenge.

Renewable energy from the ocean

Wave power furnishes a further example. Islay LIMPET (Land Installed Marine Power Energy Transmitter) was the world’s first grid-connected power station to draw energy from the movement of ocean surface waves. It did so by means of an Oscillating Water Column, a device in which a body of air is repeatedly pushed and pulled through a chamber positioned over seawater that rises and falls with the tides; this airflow drives an energy-generating turbine.

During its development and demonstration phase, LIMPET was fitted with an experimental design of turbo-generator in which drives were used both to motor the turbine to optimum speed and to run in regenerative mode, feeding AC to the grid. Maximum turbine speed was continuously adjusted in accordance with the on-board programming and monitoring of the chamber air pressure in real-time.

As a commercial entity LIMPET did not survive. One of the reasons cited for its decommissioning was uncertainty over the future of a subsea power cable linking its remote Hebridean location to the mainland. This, in its way, is illustrative of the uphill climb many renewable energy projects still face if they are to find ways of connecting – in this case, literally – to the mainstream.

What is the future of renewable energy?

The mainstream, however, may itself face an uncertain future. The essentially twentieth-century model of a monolithic, centralised power supply is increasingly being challenged by more flexible, distributed schemes. Energy harvested from a diversity of local sources, now that it can be properly coordinated, is looking more affordable, more efficient and, of course, greener than the old grid.

West Beacon Farm in Leicestershire gives a taste of the idea. This 50-acre homestead uses a combination of PV arrays, wind and water turbines to generate its own power without fossil fuels or, much of the time, electricity from the mains. No retro throwback, the project, on the contrary, relies on advanced electronics, most importantly a suite of programmable drives to monitor and analyse every aspect of the system and distribute power around the farm at controlled levels.

From domestic arrangements to industrial dimensions of the future grid, intelligent monitoring and control will be key concepts when it comes to making a success of the next generation of renewable energy projects.  Across the board, appropriately programmed drive systems are at the front line of this effort to make of natural forces the precisely regulated energy supply of the future.