KEY POINTS:
Much importance is being put on developing renewable energy sources in New Zealand, almost invariably referring to wind, geothermal and even solar.
What is usually overlooked is that New Zealand's electricity supply is predominantly renewable and sustainable in broad terms, principally from hydroelectric plants.
In an average rainfall-snow melt year, about 65 per cent of generation comes from the big dam stations in the South Island and along the Tongariro-Waikato system in the North Island, together with a number of smaller stations run by local power generators such as TrustPower.
It is evident that hydro is often damned because of the "dam" syndrome, with its attendant adverse perceptions of drowned lands and scenic rivers. What appears to be incorrectly presumed is that hydro is fully exploited and cannot be increased.
In fact, there are many potential options for more run-of-river stations, of which there are many examples in both islands. Some make use of existing lakes, others include more modest storage to give at least daily or weekly flow regulation to meet peak power demands.
In the 1970s and early 80s the Government funded evaluations of smaller hydroelectric potential - less than 50 megawatts (MW) - throughout the North Island and much of the South Island.
The regional studies in the North Island identified 300-350MW of technically feasible schemes, reasonably economically viable and with modest environmental effects, and a further 150MW of less economic and environmentally acceptable schemes.
The regions with the most potential were Wanganui-King Country with 85MW; Hawkes Bay 70MW; and Bay of Plenty 60MW.
Comparable and potentially more MW were partly identified in the South Island, but some of it too distant from load centres.
In most cases in the North Island, the hydropower scheme sites are not far from existing transmission lines. Such local generation could reduce distribution losses on rural lines and increase security of supply in extreme events, as happened in the Edgecumbe earthquake in the Bay of Plenty, with generation continuing from the Aniwhenua hydropower station.
Run-of-river schemes usually involve relatively modest headworks such as a concrete weir or barrage, and a conduit intake feeding a canal, pipeline or penstock.
Several of the existing schemes in the Bay of Plenty and Taupo areas are good examples of such power stations. In fact in the North Island most schemes other than the Waikato dams and Matahina and Patea are of the diversion type, embodying combinations of canals, tunnels and pipelines and not involving large dams.
The potential impact of dewatering a stretch of river and the effect on wildlife can be at least partially addressed by ensuring residual base flows, sometimes reinforced by periodic "freshet" releases. The modified river course (for example, a canal) often provides a more productive fishery with better access.
Other benefits which may accrue from such developments include aquatic recreation, improved rural access and employment suited to local businesses.
One of the key impediments to the development of sustainable smaller hydro stations is the unbalanced influence of aspects of the Resource Management Act, and the Conservation Act, with its preoccupation with exotic fish species to the detriment of wider community needs.
In the context of controlling carbon emissions, which is more important - clean renewable energy or absolute protection of every sports fishery?
The technology of hydroelectricity is well-established and proven. The economics of local generation is at present constrained by low energy prices, but how long before we face up to realistic pricing for a sustainable energy policy?
Greater recognition is warranted of the role of hydro as the backbone of our generation system and the significant potential for local hydro development to meet regional demands economically, without long transmission distances, and by mitigation of environmental issues under realistic legislation.
* John Duder is a consulting engineer.
