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A Beautiful Source of Energy

 

Here in New Zealand, we have a bountiful source of energy that:

 

Fantasy? No. We are talking about power generation from tidal currents.

 

Let’s take each one of those points in turn:

*  

            where P is the power per unit area of a fluid with density  flowing at a speed V. The density of air (used for wind generation) is about 2.5 kg/m3, but the density of seawater is about 1000 kg/m3, or 400 times greater. Thus, we need tidal current speeds that are 1/7th of wind speeds to get the same power. Furthermore, with tidal currents, there will never be gusty conditions when the turbine must be feathered to protect it from damage.

 

So, where in New Zealand could we locate tidal power stations? Well, there are two different sorts of places:

  1. In Greater Cook Strait, and
  2. At harbour mouths.

 

  1. Greater Cook Strait

At Makara, on the southwestern Wellington coast (Figure 1), the primary lunar semidiurnal tide M2 (which comprises more than 80% of the tidal energy for New Zealand) cancels out, forming what is called an “amphidrome”.

 

Figure 1.  Map showing locations referred to in the text.

 

At some distance from such locations, the gradients of tidal height and current are very large, especially around islands like D’Urville, Arapawa, Mana and even Kapiti (Figure 1), and between these islands and the mainland. And this explains why the tidal currents are so large in French Pass and Tory Channel. Exactly how large are they? Using a tide model, we have calculated the tidal currents at the location in Tory Channel shown in the figure for 100 years from 2001 to 2100, and we present the results in terms of cumulative probabilities in Figure 2. The way to use this plot is as follows: if your turbine can generate only when the current speed is say greater than 2 knots, then you find the point on the curve corresponding to 2 on the abscissa, and its ordinate is the percentage of time you can generate electricity at that site. For the example of 2 knots, it is 48%. This may seem low, but remember that we know exactly when that 48% of time will occur.

Figure 2. Cumulative probability of tidal current speeds exceeding various levels.

 

For example, consider Figure 3 showing a few weeks of currents for the late winter of 2019. Most days, we will be able to generate for 12 hours every day, but between 22 and 25 August, there will be only a few hours each day when generation will be feasible. Therefore, this would be a good time for maintenance. However, we should avoid downtime towards the end of the month because that will be a highly productive generation time.

 

Figure 3. Time record of forecast tidal currents.

 

How much power could we produce from a tidal station at this location? To answer that question, we would need to know the characteristics of the turbine: specifically, its diameter and its efficiency. Table 1 gives the average power and daily energy for a station in Tory Channel with 6 m and 10 m diameter turbines, assuming an efficiency of 30% (i.e., 30% of the tidal energy is converted into electrical energy).

 

Table 1. Output from a typical tidal power station in Tory Channel.

 

Turbine Diameter

 

6 m

10 m

Average Power KW

6

17

Daily Energy KWh

145

402

 

 

ii.         Harbour Mouths

The tidal flow through harbour mouths is huge. In many cases it dwarfs even the largest flood flows in the largest rivers in New Zealand. For example, the Buller River has a 50-year flood flow of 8,000 cumecs, the largest flood flow in New Zealand. This means that the probability of a flood with a maximum flow of 8,000 cumecs occurring in any year is 1 in 50. Yet, at the mouths of the Manukau, Kaipara, and Hokianga Habours, the tidal flow oscillates from roughly –100,000 cumecs to 100,000 cumecs each tidal cycle. That is 12 times larger than the largest flood flow we are likely to experience in any NZ river in a lifetime. And it happens twice every day. Of course, with flows as large as this, huge quantities of sediment are moved backwards and forwards through the mouths, and this will be a significant engineering challenge in harnessing the power. Nevertheless, the scale of the available energy is so large that we believe it is worth investigating.

 

Last Updated: 17 November 2003

For more information contact: d.goring@mulgor.co.nz

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