right hand page wing letters: renewables

1.

I disagree with the contentions of Professors Younger and McInnes (Letters, March 10) that it is impossible to generate electricity without the continued use of fossil fuels to provide base load. The source of this assertion lies in the need, in our traditional grid system, to actively intervene to balance supply and demand, otherwise frequency stability is degraded and this "balancing act" is deemed to be severely compromised by the introduction of "inflexible" and "intermittent" renewable sources.

Surprisingly, perhaps, in general electrical engineering practice outside the power supply industry, this deliberate need for predictive balancing of supply and demand is not an issue. For example, any stand-alone complex electronic device, such a mobile phone, laptop computer or electric car, is a small but self-contained power system. It exhibits varying power demand from a multiplicity of electrical circuits and components influenced by changing operational modes, while the electrical supply comes from an energy storage device (usually a battery). Significantly, this energy source delivers power only when it is needed. Thus supply and demand are automatically balanced. If the battery is rechargeable, which it usually is, then balanced demand/supply is maintained as long as the battery contains sufficient charge.

Importantly, it does not matter how erratic or intermittent the charging process is as long as on average over a defined time period, the charging power supplied to the battery is equal to or greater than the power demanded over the same interval, by the electrical circuits of the device in question.

The inference is that an electrical power supply system, at the national grid level, can function perfectly adequately with intermittent renewables acting as the primary power sources, if the supply side is buffered from the demand side by massive energy storage (MES) plants such as pumped hydroelectric schemes. A few of these storage facilities are already in place in Scotland. There are many other massive storage technologies which could be implemented, for example the Nordhaven "green island" storage scheme near Copenhagen but, tragically, their development is almost non-existent at the present time.

Alan J. Sangster,

37 Craigmount Terrace,

Edinburgh.

2.

I refer to Lang Bank's Agenda article ("Way wind is blowing, it is time to ride wave of renewables revolution", The Herald, March 11). Electrical energy, delivered by the national grid, is a unique product. Supply must instantly match demand at all times. Perturbations challenge system stability and, if not managed in real time, a mismatch will result in catastrophic failure.

Network stability relies on the fact that large rotating synchronous machines such as those at Longannet, Hunterston and Torness introduce sufficient inertia to enable control mechanisms to cope with the inevitable unexpected changes in supply and demand before the dispatch of pump storage and gas fired generation is available to meet the shortfall.

A generation resource which is unpredictable and intermittent imposes enormous challenges for network stability. At present, system control is possibly just manageable because we still have the benefit of a generation mix. As the proportion of renewables (excluding biomass) increases, within the generation resource, the risk of total grid failure rises disproportionately.

In the event of a catastrophic grid failure, a substantial synchronous generation resource is required to re-establish the grid. Wind turbines are not synchronous machines and could not be used to re-energise the grid. If the whole resource is renewable we will be in a real predicament.

I would not like to consider how long it would take to restore the grid without Longannet or our nuclear stations. It could be many days and the consequences of an extended electrical blackout are truly awful to contemplate. The risk exists and can only increase.

The picture Lang Bank's describes is seriously misleading. While Longannet's days are numbered and, without carbon capture, coal has a limited future , we need to plan for a genuine energy mix at least containing fossil, nuclear and renewables.

It is not installed capacity that matters. It is what can be delivered when it is needed. There are frequent occasions when barely five per cent of the total UK wind capacity is available. This can extend for several days and coincide with empty hydro reservoirs. Such events are not unusual, even in the UK, and are sometimes Europe-wide.

Storage of electrical energy is viewed as the panacea. It might be in the distant future, albeit at huge capital cost, if our requirement for electrical energy is reduced to a fraction of what it is at present.

Electricity generation's contribution to carbon emissions is less than 20 per cent of the total.

Norman McNab,

14 Branziert Road North,

Killearn.

3.

Further to the excellent letter from Professors Younger and McInnes regarding a future Scotland without significant non-renewable power, may I add the following "political" question for consideration by the SNP Government. What exactly would be the relevance of the word "independence" if much of our electricity, or on occasion most of it, was being supplied from "foreign" England?

Alastair Runcie,

5 Vivian Avenue,

Milgavie