Hydro Lakes Plunge During May

Hydro lake levels plunged during May 2008, a month when inflows were dramatically low and demand consistently high.

New Zealand’s hydro-electric storage lakes stood at 1932 GWh (GigaWatt hours), 64% of average for the time of year at the beginning of May 2008. By month’s end, levels had plunged to 1558 GWh, 54% of average for the time of year. During May last year, storage actually improved rising from 72% to 81% of average.

Storage was consistently below average during the month, climbing slightly to a peak of 66% of average on May 11th before plunging to reach 54% of average at month’s end.

Inflows were below average on 28 days during May, only climbing above average on the 1st, 9th and 10th. Inflows were seriously deficient (less than 50% of average) on 6 days.

Daily demand exceeded last year’s figures on 30 days, peaking at 118.8 GWh on the 28th May. Demand during May 2007 also peaked on the 28th, being 113.6 GWh. The average daily load during May was 110.5 GWh, higher than last May’s average daily load of 104.3 GWh.

The prospect of winter shortages has resulted in some retired plant being brought back online, with generating capacity which had been decommissioned at New Plymouth because of asbestos contamination being partially restored. Pole 1, the aged component of the Cook Strait cables was also recommissioned to improve the transfer of energy between the two main islands, but below its rated maximum capacity.

The South Island storage lakes are the most seriously depleted and were carefully shepherded through May 2008. North to south transfers via the Cook Strait cables exceeded northbound transfers on 31 days. No energy at all was transferred northward on 22 days.

At the end of May, lakes stood at their lowest levels since the 1992 crisis year, when shutdowns occurred, streetlights were switched off and industry was scaled back.

As we approach the coldest months of winter, the possibility of a savings campaign looms. Two key factors will determine whether the country squeaks through without shutdowns, savings campaigns or unplanned blackouts.

An unplanned shutdown of one of the oil- gas- or coal-fired stations could jeopardise the prospects of passing through winter without compulsory blackouts. With hydro at low levels and being kept in reserve, energy is being supplied from these alternative sources to meet much of the base load. If a shutdown was of long duration while repairs were carried out, hydro plant would need to be brought up to speed to maintain supply, drawing down lake levels earlier than planned.

If plant continues running smoothly, and inflows continue to be low as expected, authorities “hope” that voluntary savings will not be required. The margin is thin, however, and the peak daily load period of 6 p.m. to 8 p.m. holds the key. Non-hydro cannot meet peak demand, and additional hydro plant needs to be brought on-line to meet demand during the early evening hours.

There is a fine margin in managing the lake levels downward, as the water flowing out of the depleted lakes to keep the turbines running needs to be balanced against the below average inflows that are expected. Whilst environmental law prevents lakes being run dry, they each have a minimum operating level which must not be reached before the hoped-for spring snowmelt, which begins in October.

If a savings campaign is launched, it will focus on reducing the evening peak load allowing more hydro lakes to be kept in reserve for later in winter.

[Compiled from data supplied by M-co.]

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