Cleaning up green energy supplies

Nathan Briggs, Managing Director at Advance Electronics, investigates the solution to unreliable power from renewable energy sources.

Statistics show that about 40% of renewable energy generated for sale in the UK is from wind turbines, 30% from tidal sources, 10 to 15% is CHP (combined heat and power) and the remaining percentage from other sources, including geothermal and solar.

No matter what the origin of our renewable energy is, however, there is one thing for certain - a typical renewable energy source will inevitably produce poor quality end power.

The UK Government's stated objective is to achieve 10% renewable power by 2010 and, with planning processes now being implemented to construct offshore wind farms in the North Sea, towards the Orkneys and elsewhere, it is apparent that renewable energy sources will soon become mainstream.

However, one of the major problems facing suppliers is how to get the power back to the mainland, together with the power quality issues that arise from this.

In simple terms, the technology that goes into wind power is highly designed - what can be described as tenth or eleventh generation.

Provided you specify whether your requirements are up or down wind, and you have an accurate measurement of the mast height, the components can be bought off the shelf.

But, because they acknowledge that the power quality is poor, wind turbine designers and manufacturers advise that you can add batteries to their systems, to smooth out the variances in the power generation.

However, this route is not strictly environmentally friendly, as the batteries have their own disposal problems.

Indeed, the advent of the new WEEE (Waste from Electrical and Electronic Engineering) directive is already causing many organisations to assess the likely increased end of life costs associated with the safe disposal of UPS batteries.

Part of the problem is that the National Grid has very tight tolerances on the quality of the electricity that is allowed to be put in or taken out.

This means that, for example, large industrial consumers are charged in kilowatt-hours as well as power factor (harmonic), in addition to being subject to the times of day that they need to draw power.

Although the National Grid criteria can be viewed as unfair, because they are the same for a nuclear installation as for the local wind farm, despite the vastly different amounts of energy that they contribute, the close tolerances placed on the generation of power are entirely for our benefit as consumers.

Of course, if it is intended to sell the power for commercial use, it would be possible to add an inverter, although this is an expensive option.

What is needed, therefore, is a cost-effective solution that is capable of smoothing out the vagaries in the quality of renewable energy production without introducing an element of unreliability or, equally undesirably, material disposal issues.

This latter criterion in particular rules out the use of batteries, while the former questions the advantages posed by an uninterruptible power system (UPS).

After all, a UPS is often too expensive, especially when the cost of environmentally conscious battery disposal is factored in.

Couple this with the need to specify a protected input stage, as well as the costs of routine maintenance, and it's clear that there should be a better solution.

One recent development that is proving ideal for commercial renewable energy suppliers is the universal input constant voltage transformer.

This inexpensive device produces a stable, uncorrupted voltage and frequency output regardless of the quality of the mains supply.

This makes it appropriate for any situation where the quality of the input power is variable, but where computer grade output power is an absolute necessity.

The universal input constant voltage transformer is also perfect for any three phase in, single phase out applications such as rail, military, commercial and medical environments, where the quality of the power supply is of critical importance.

Unlike traditional constant voltage transformer (CVT) technology the universal input CVT can produce a constant frequency output, and is unaffected by frequency variations within the input.

Even the total loss of an input phase will not affect the output.

Because the universal input constant voltage transformer boasts all the key benefits of an uninterruptible power system, but without the costly, bulky and heavy battery technology, it is also ideal for installations where a conventional UPS is simply too large.

In addition the tried and tested technology of the universal input constant voltage transformer has a MTBF (mean time between failure) nearly 10 times that of a conventional UPS.

Renewable energy sources are notorious for suffering from what are known as brownouts, a situation where the mains power is temporarily reduced but does not fail completely.

In these circumstances, a universal input constant voltage transformer can accept an input voltage as low as 30% of nominal, while still producing a stable output voltage, without the need for auxiliary battery power.

All told, therefore, the universal input constant voltage transformer is what the renewable energy generation sector has been looking for - a way of turning 'green', but variable quality, power into something that can be sold to the National Grid as high quality, computer grade mains electricity.

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