A simulation evaluation is presented of the seasonal operation a possible 1,300 MW pumped storage scheme in New Zealand. The simulations are with respect to a site in Central Otago, where the existing Lake Onslow is expanded to serve as the upper reservoir. The lower reservoir would be Lake Roxburgh on the Clutha River.
The simulations are based on permitting a large operating range (720 to 780 masl) for Lake Onslow, increasing New Zealand’s hydro storage capacity from 4,000 GWh to 11,000 GWh. However, this range does not represent seasonal variation of a typical year. Other operating options are possible as well, including a smaller operating range or maintaining a raised lake Onslow at constant level as an energy reserve against dry years, avoiding the need for stand-by thermal stations.
In addition to providing a buffer against future dry periods, the simulations (based on past river flow records) indicate that active seasonal operation of Onslow pumped storage could allow more efficient use of the main South Island hydro lakes (Hawea, Tekapo, and Pukaki). Specifically, the possibility arises to maintain water levels of those lakes at their present mid-ranges. This essentially removes the active hydro storage role of the South Island scenic lakes, with lake outflows reverting to their natural seasonal regime of high flows in spring and summer and low flows in winter. In the simulations, the surplus power from the higher summer flows in the Clutha and Waitaki rivers is used for pumping water up to Lake Onslow, to be released later for winter power.
The simulations using past hydro lake inflows indicate that this mode of operation would significantly reduce spill losses, particularly at Waitaki power stations. Although hydro spill is intermittent, the net effect appears to be that spill reduction is sufficient to offset operating inefficiencies in pumped storage, with a small power surplus left over. In addition, reversion to higher summer lake outflows would result in a considerable increase in mean Waitaki River summer discharge, enabling additional river water diversions for irrigation developments.
Additional advantages of the pumped storage scheme are identified as:
• Reduced need for sending power from the North to South Islands during times of low South Island hydro inflows, reducing carbon dioxide emissions from North Island fossil fuel thermal stations.
• The new 1,300 MW capacity could be used for frequency keeping and also buffer the short-time variability of wind power, enabling wind power expansion without risking grid instability. The additional installed capacity could also provide peaking capacity generally, including offsetting plant outages.
• There will be some degree of flood peak reduction in the lower Waitaki River, as a consequence of reduced spill magnitudes from lakes Tekapo and Pukaki. At the same time, more stable lake levels should result in reduced lake shore erosion.
• The large increment of energy storage capacity may have the effect of stabilising electricity price fluctuations in the wholesale market, reducing the need to take out hedging contracts.
Taking all advantages into account, the scheme appears economically viable.
