AGENCIAS DE VIAJES

1,300 MW, which is suggested by a previous study based on the Clutha upper and lower discharge bounds (900 m3_s-1 _{and 300 m}3_s-1_{) and based on the results which}

was found the proposed pumped storage scheme to be hydrologically feasible, in terms of balancing water and energy, for stabilising hydropower output from the Waitaki and Clutha hydropower schemes, and for use as standby reserve [72]. The 1,300 MW installed capacity obtained from an average power hydro head of 615 m

with a maximum generating discharge of 240 m3_s-1 _{and a maximum pumping rate}

of 185 m3s-1. The simulated Onslow scheme includes waterway structural elements,

the Onslow Dam and an underground station (Figure 3.12).

The civil engineering components of the 24 km Onslow tunnel include the tunnel itself, as well as the related components of tunnel intake and outlet, surge tank, power house, roading, and site establishment. The diameter of the tunnel is a major consideration as it directly affects the project budget and links to the sizes of the intake, outlet, and surge tank.

The processes of pumping water to elevation and later releasing it create losses as a result of hydraulic friction resistance and turbulence in the headworks, penstock, and tail race, with penstock friction loss representing the major loss component. Computer simulations of hydraulic head loss were carried out using

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standard packages [94] to estimate total head losses contributed by various components, using a range of different trial tunnel diameters and materials, for given pumping and discharge rates.

Two models were used to estimate the head loss due to frictional resistance and transitions or changes in tunnel direction: the Darcy-Weisbach formula [94, 95]. Figure 3.13 shows the calculation sequence for tunnel head loss and operational efficiency.

The benefit of a PHES plant can be maximized by minimizing the cost of civil works, water conduction systems, electro-mechanical equipment and the power evacuation system, which are directly related to the selection of optimum size / capacity of equipment. The selection of optimum diameter is achieved on the basis of friction losses, seepage losses, and cost of construction of the tunnel.

Following the process outlined in Figure 3.13, the method for selecting the optimum diameter of the Onslow tunnel is summarized as follows:

 Draw the layout of the tunnel according to minimum length in a long low-pressure tunnel option (Figure 3.12).

 Select the optimum diameter (D) of the tunnel for several factors (j) such

as friction loss, seepage loss and construction cost including depreciation, interest, operation and maintenance. The construction cost items are the cost of excavation, lining and the cost of grouting.

𝑇𝑜𝑡𝑎𝑙 𝐶𝑜𝑠𝑡 = ∑ 𝐶𝑗(𝐷)

𝑎 𝑗=1

(3.1)

Where D = Diameter of tunnel = f (Q, S, A) S = tunnel gradient

Q = discharge (m3s-1) A = area of the tunnel (m2)

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Start

Input pumping/release flow rate Input length, roughness and kinetic viscosity

Compute tunnel diameter within range minimum and maximum in step of D

Compute mean velocity = f(Q, D) and Reynold number (RN) = f(Q, D, Ki)

If RN <= 2000 then Froude number= 64 / RN Else Froude number = Moody formula

hf = f (L/D)(Sq(v)/2g) hm = K(Sqr(v)/2g)

Compute head loss total (ht) = hf + hm

Power loss (kW) = µ Q ht

Efficiency of conduct % = (1 – Power loss / Power installed) /100

If D < D max

Yes

End

 Optimisation solution can be solved for least cost per kWh by making:

𝑑𝑇𝑜𝑡𝑎𝑙 𝐶𝑜𝑠𝑡

𝑑𝐷 = 0 (3.2)

 Optimal Diameter (D) can be solved: 𝑚𝐷𝑁− 𝐷 − 𝐾 = 0

The most economically efficient design can be obtained by making 𝑑𝑇

𝑑𝐷= 0.

The above procedure was applied and the output shows the optimum internal diameter is 7.5m. Adding an extra 0.5m for future development gives a total of 8.0m as shown in Figure 3.14. Parsons Brinckerhoff Associates Ltd considered a Figure 3.13 System hydraulic head loss and parameter simulation diagram.

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diameter of tunnel 8m in their primary estimation costing of the Onslow scheme. Figure 3.14 shows that the cost to construct a tunnel with a diameter of 8 metres is $NZ 14,276 per metre of tunnel length. This indicates that the cost to construct the tunnel will be $NZ 350M to 400M.

Generally, tunnel constructions in the order of 8 metre diameters have thicknesses of precast concrete segments of 30 – 35 cm. Using 35 cm concrete with a strength of 50 Mpa, the volume of concrete required to construct the Onslow

tunnel is estimated at 220,239 m3_{. Adding 10% for wastage and various components}

gives a concrete lining cost of around $NZ 63M. The amount of steel required for

the precast concrete is estimated at 160 kg/m3 _{of concrete, meaning that the total}

steel requirement is 35,239 tons at a total cost of $NZ 39M. A typical cross sectional dimension of an underground power house is 50 m x 50 m x 40 m, which costs around $NZ 23M including materials.

Although steel has a lower head loss than precast concrete (Figure 3.15), the selection of the optimum diameter of the Onslow tunnel was based on the use of precast concrete as it offers advantages in terms of economics, strength, design flexibility, rapid installation, production controls, long life and durability in comparison with other materials. Therefore precast concrete was selected to construct a lined low pressure tunnel in schist rock. The maximum head loss due to Figure 3.14 Selection of optimum diameter of Onslow tunnel based on the costs of construction, headloss, seepage, operation and maintenance and depreciation and interest.

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friction in the Onslow tunnel is estimated to reach 2% for an 8 m diameter tunnel

with a discharge of 210 m3s-1. In contrast, an unlined rock tunnel of 8 m diameter

would lead to increased head losses amounting to approximately 10% of the operating efficiency. Figure 3.15 shows the operation efficiency head loss due to friction in different materials for tunnels of 8m diameter.