Feature

Hydroelectric Resources

New Zealanders mistakenly believe that we have an overcapacity in electricity generation, and that hydro power stations need not be built or used to the extent they are.

By Iain Murray

It is a widely held belief that New Zealand does not need any further hydro development, that our current and future needs are well served by present facilities.

"Not so, but it's easy to see why people assume this," says Russell Howie, Electricorp Production's senior environmental advisor. "Even with Clyde coming on stream soon, there won't be a surplus of hydro electricity and there isn't likely to be any in the future."

According to Howie, the key to understanding the issue concerns the difference between reserve capacity and overcapacity. New Zealand's geography means that hydro stations have relatively small storage reservoirs, holding just 13-14% of the actual water used. Consequently, the stations have to be built with megawatt generating capacities in excess of the average flow of the river on which they are sited. The capacity to generate electricity over and above the mean flow of a river represents the reserve capacity of that station.

Reserve Capacity

Reserve capacity allows hydro stations to utilise above-average flow when they occur and to meet peak demands. Since little of the extra flow can be stored, New Zealand hydro stations have to either use it or lose it.

"Imagine a hydro station able to generate only as much electricity as the average flow of the river would allow," Howie says. "Then imagine a day when the river is flowing above average. The station wouldn't be able to utilise the additional flow. New Zealand would be wasting a precious resource."

The reserve capacity margin in the New Zealand generating system is not particularly high in comparison with other countries, despite our low storage capacity. It must also meet demands when we have a dry year.

Thermal Supplement

Electricorp Production has 39 power stations, 30 of which are hydro. The latter account for about 75% of Electricorp Production's total generation. As electricity cannot be conveniently stored, it has to be produced as consumers demand it. When hydro stations cannot meet this demand, thermal stations have to be used to supplement generation during peak hours.

"This is more expensive to the consumer, both in terms of money and emission," Howie observes. "By building reserve capacity into the generating system, we are better able to utilise our water resource. This way there is less need for thermal generation, and that is good for everyone."

The reserve capacity of hydro stations puts them above thermal stations in terms of efficiency ranking. When river flows are adequate, the stations can use that reserve capacity rather than relying on supplementary generation from thermal stations. The energy contribution from hydro stations which can be relied on is only 85% of their mean inflows. One year in 20 could see electricity shortages from low levels.

Reserve capacity allows Electricorp Production's integrated generation system to smooth out regional variations in the amount of water available for hydro generation. If some stations are adversely affected by low inflows, other stations can use their reserve capacity to make up the shortfall.

Environmental Effects

Wasting the energy contained in our rivers by not building reserve capacity into hydro stations has a significant effect on the environment. Howie estimates that using thermal power stations to make up for unused hydro power would release up to an extra 100,000 tonnes of carbon dioxide for every 120 gigawatt hours (GWh) of thermal electricity generated.

Despite its efficiency, hydro generation cannot meet the national demand for electricity. Recent figures show a total demand of 29,002 GWh. Total hydro generation in the same period totalled 20,823 GWh. The shortfall had to come from thermal and geothermal plants. Even when Clyde starts generating around 1,900 GWh per annum, thermal plants will still be needed.

New Zealand's need for reserve capacity will always remain, because electricity cannot be stockpiled like coal or other fuels.

"So long as rainfall varies seasonally, annually and geographically, and the demand for electricity continues to vary, it will be impossible to precisely match the generation capacity we install with the amount which is demanded," notes Howie. "We therefore need reserve capacity to cope with these variations."

Iain Murray is a writer for Electricorp Production.