re
:
think
Winter 2013
4
THE summer drought hit farmers hard and
that will impact the whole economy. The dry
weather also prompts us to consider how we
might better store and distribute water and
prepare for extreme weather events.
At the University of Waikato, with
support from Meridian Energy and the
Ministry of Business, Innovation and
Employment, doctoral student Varvara
Vetrova is trying to find out the degree to
which dry periods in the southern hydro
lakes can be forecast in advance.
Unlike the weather forecast, such “climate
forecasts” look at large-scale factors like El
Nino and water temperature and pressure
variations over the Pacific Ocean. If we can
work out how different wind patterns and
ocean circulation influence rainfall we will be
able to develop statistical models to predict
river inflows to the main hydro-storage lakes
in the Waitaki hydro-scheme. We’re also
studying historical data to run a series of
analytical tests over different seasons. Then
we’ll measure results against existing data in
the region.
Climate forecasting for hydro power is
an inexact science and there is not a lot we
can do about an extended dry period, even
if it could be forecast well in advance. This
is where the second PhD project comes in,
being undertaken by Mohammed Majeed,
as an engineering solution to our limited
hydro storage.
Finding hydro solutions
For engineering, it needs to be
remembered that by far the greatest amount
of hydro storage in New Zealand is from
controlling the levels of significant natural
lakes like Tekapo, Pukaki, Hawea and Taupo,
with smaller amounts stored behind valley
dams like Benmore. In principle, more storage
could be gained by raising other lakes like
Wakatipu or Wanaka. However, such schemes
would never be considered seriously because
of the large shoreline impact. This raises the
question of where new hydro storage could
come from.
There is also the special South Island
issue of seasonal water competition. Left
to themselves, the main South Island rivers
would flow high in summer and low in winter
when the mountain precipitation falls as snow.
Summer high flows are good for irrigation and
recreation but the opposite is required for hydro
power which needs to store summer water in
headwater hydro lakes for release later in winter
when power demand is high. For example,
the Waitaki River now has less summer flow
because of the Waitaki Power Scheme.
Mohammed’s project seeks to solve both
these issues at the same time. His thesis is
simulating the operation of a large but still
hypothetical pumped storage scheme in
Central Otago, which could more than double
the national hydro storage capacity.
The idea is that instead of holding back
the summer flows in the southern hydro
lakes, the water is released out of the lakes at
almost the same rate as it flows in from rivers.
This gives stable lakes with natural-looking
shorelines at average lake level. The new
summer river water is available for irrigation
and recreation downstream, with the extra
summer power generated now used to pump
water up from the Clutha River, during high
flows, to the sparsely-populated Onslow basin
near Roxburgh.
In winter, the water runs back down
through the pumps in reverse to generate
1,000 MW of electricity. In this way hydro
storage is shifted from the scenic lakes to the
Onslow basin. At the same time, holding the
hydro lakes at average level leaves room for
any summer flood, as in January this year,
so the reduced spill loss serves to offset the
inefficiencies of pumping. The Onslow scheme
is not on the horizon of any power company
because although it could generate profitably,
it would not give a commercial rate of return.
Its construction would need to be government
funded as a recognised national good, similar to
the Transmission Gully highway project and for
roughly similar cost. And look how long we’ve
been waiting for that.
web@waikato.ac.nz
ASSOCIATE PROFESSOR EARL BARDSLEY:
Recent dry spells should promote
new thinking.
SOUTHERN WATERS: Waikato doctoral students are looking for smarter ways to store and
distribute South Island water.
By ASSOC PROF EARL BARDSLEY
COMMENT