Hassani-Iran doc

Hydropower'15 Stavanger, Norway 15-16 June 2015

Towards Optimization of the Required Capacity of Re-regulating Dams: results of an actual experiment

Hooshang Hassani Project Manager, Mahab Ghodss Consulting Engineering Co. –Iran

([email protected] ) Alireza Zia ([email protected])

Assistant Professor at Qom University of Technology - IranNasser Kamjou

Head of Dam, Hydropower and Infrastructures Dept. of Mahab Ghodss Consulting Engineering Co. ([email protected])

ABSTRACT

Hydropower plants generally run for peak period of power demands and stop operating during off-peak hours. Re-regulating dams are usually constructed downstream of these plants to guarantee uniform release during non-operating hours. The required capacity of these dams is calculated based on the difference between power plant outflow and re-regulating dam release during plant operation. By modifying this common procedure and applying it to Gotvand re-regulating dam, an optimization was achieved. Gotvand re-regulating dam is located downstream of a 2000 MW hydropower plant in southern Iran. This optimization, which was obtained by taking into account the storing capacity of the river reach at downstream of the dam axis, led to considerable savings in project costs. In this method, the geometry of downstream plays a critical role in determining the required capacity.This paper describes this modified procedure and presents results of its application to a real case.

Keywords: re-regulating dam, reservoir capacity, routing, peak hours

1. INTRUDUCTION

In most cases, any change to an already constructed hydraulic structure such as heightening of a dam might be more difficult than the construction of the same dam from the beginning. The reason is the constraints resulting from the operation of the foregoing structure and the work conditions around that hydraulic structure. Gotvand Re-regulating/ Diversion Dam is one of those previously constructed hydraulic structures that in view of the change in its upstream conditions, i.e. the construction of the large Storage Dam of Upper Gotvand with the power generation capacity of 2000 MW, is now in need of heightening. Gotvand Re-regulating/ Diversion Dam has been operational for 40 years and its aim has been to provide the communities living downstream of the River Karun with the agricultural, drinking, and industrial water. So as to avoid any interruption in the operation of the aforementioned re-regulating dam and to provide adequate volume for regulation, a new approach was used to determine the required capacity.

2. PROBLEM STATEMENT

The operation of the 2000 MW powerhouse of Upper Gotvand Dam within the peak power consumption hours gives raise to considerable fluctuation in the river flow. The conventional solution for this problem is to heighten the existing downstream re-regulating dam to obtain the volume required for re-regulating the downstream flow completely. Based on the existing situation, an additional required volume of 20 MCM, corresponding to about 6 meter increase in the height of the re-regulating dam is needed. However, there are structural and operational limitations as follow:

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The tailrace of the powerhouse of the main dam is located in the reservoir of the re-regulating dam. Therefore any rise in the re-regulating dam height leads to a decrease in the generation of power. As per the estimations made, 1 m of increase in the re-regulating dam height is equal to 30 GWh decrease in the annual generated power.

From the stability point of view, the foundation of the existing re-regulating dam is able to bear maximum of 2 m of dam heightening. So, any further increase in the dam height entails reinforcing the dam foundation and proceeding with huge and costly structural activities.

It is necessary to incessantly have the irrigation canals located in both sides of the regulation dam operational. So, it makes problems for any structural activities on the dam body.

In addition, with regard to the topographic conditions of the region downstream of the existing diversion dam, the construction of a new re-regulating dam with enough capacity entails having a long dam crest which entails a significant cost apart from having a vast area of farmlands inundated.

Seeing the problem encountered in the conventional method, several assessments were made to find better solutions. The most appropriate method was to make use of the present structure and efficiently control/manage the reservoir to attain flow regulation.

Layout of Upper Gotvand Storage Dam and Gotvand Re-regulating/ Diversion Dam is shown in figure 1.

Figure 1-Layout of Gotvand Dam and river in downstream

3. OPTIMIZED SOLUTIONIn order for having an efficient management, the parameters that were studied in more detail include the real operation of the powerhouse, the current function of the existing reservoir, and the topography of the River Karun downstream".

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3.1 The Real Operation of the Powerhouse

The investigations made in this regard indicated that the powerhouse outflow is affected by the following factors:

As prioritized, the total volume of the diurnal water release is determined by the downstream agricultural demand.

The operation hours of the powerhouse units are determined by the national dispatching center in accordance with the demands of the national grid network.

Normally, power units start/stop to operate in a given intervals, so it is not practical to start and stop all units simultaneously.

In some days, the national grid network demand reaches its maximum amount in two peak hours of midday and evening.

The national grid network demands in different days of the week are different. They reach their minimum amount during the holidays.

In order to increase the scope for the maneuver of hydroelectric power plants of the national grid network, in some hours, one of the powerhouse units starts giving service to the network with minimum power generation capacity.

There are different peak hours in different seasons of the year.

A more precise study on the above data indicates that the real operation conditions are different from the initial presumption pertaining to the 6 hours of uniform and non-stop operation. So as to obtain the optimum volume of the re-regulating dam reservoir, it would be necessary to study different operation scenarios for different seasons of the year. Figure 2 depicts a sample of different water release patterns required for power dispatching.

Figure 2- Some of the patterns used as powerhouse daily power generation

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3.2 Reservoir Conditions of Existing Re-regulating Dam

Taking into consideration the problems articulated in Section 2, the computations showed that in case of 1.5 m increase in the height gates of the existing-regulating dam, by adding a simple steel beam on top of gates as shown in the figure 3, the corresponding reservoir volume would be equal to 8.0 MCM. Figure 4 shows the Elevation –Volume Curve of the reservoir under new the condition.

Figure 3-Adding fix steel beam on top of gates

Figure 4-Elevation – Volume curve of the re-regulating reservoir

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So as to investigate the effects of the re-regulating dam reservoir on the powerhouse outflow, 100 cross sections of the river were extracted from the area under consideration making use of the re-regulating dam reservoir hydrographic results. Having entered the above sections data into HEC-RAS software, the routing of the outflow of the Upper Gotvand Dam Powerhouse was computed for each power generation patterns of the powerhouse starting in the powerhouse and ending in the re-regulating dam reservoir.

The results are shown in Figure 5.

Figure 5– Result of intermediate model

The following results were obtained in accordance with the above computation:

The powerhouse outflow goes 10 km inside the re-regulating dam reservoir with delay and a portion of it is routed (the green graph of Curve No. 1)

By foreseeing the total volume of the diurnal powerhouse outflow, it would be possible to release water downstream in two steps in 24 hours (the blue graph of the Curve No. 1 depicts the steps 250 and 600 m3/ sec).

3.3 - Study of the River Downstream of Dam

At the second step of flow modeling, surveying of Karun cross sections was carried out up to 300 km downstream of the re-regulating dam. River location and general layout is shown in figure 6.

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Figure6: Downstream river plan

Subsequently, a mathematical model of the River Karun was developed making use of HEC-RAS software (750 sections were resorted to in creating the foregoing model). Besides, the boundary conditions have also been applied for different water release scenarios. For implementing upstream boundary condition of this model, the results presented in figure 5 are used. By running the model for different operation conditions of the powerhouse, the velocity, water surface level and discharge at each point of the river were calculated. The results thus obtained are presented in Figures 7-9.

Figure7: Flow in 30 km downstream

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Figure No. 8: Flow conditions in 60 km downstream (middle of Shotayt River)

Figure 9: Flow in 120 km downstream (Ahwaz city)

Figures 7, 8, and 9 illustrate that the non-uniform discharge released from the re-regulating dam gradually becomes uniform at downstream due to the temporary storage capacity of the river reach. This makes it possible to divide the required capacity behind the re-regulating dam into two parts. The first part is normally provided by the reservoir of the re-regulating dam and the second part is provided by the river reach at downstream of the re-regulating dam. In case of no objections regarding the morphological, environmental and social conditions at the downstream reach of the re-regulating

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dam for the non-uniform release of the water, the project would yield a considerable reduction in the re-regulating dam costs.

4. CONCLUSION

The results of this experiment show that the conventional approach requires 20 million cubic meter extra regulating capacity, which can be provided by heightening the existing dam by 6 meter. There would be some time years interruption in the irrigation water supply canals, an annual 100 GWh reduction of electrical energy due to the corresponding tail water rise of the power plant, and significant construction costs.

Our new optimized approach, however, can fulfill the regulation requirement, employing only a simple steel structure for 1.5 meter rise in water level which provides only 4 million cubic meter additional volume, making use of routing capacity of the river downstream of the dam. There would be a limited construction cost, negligible annual energy loss, and no interruption in water supply system.

It should be noted that the conventional approach described in this paper was originally used for detail design of this project. However, in 2012 the subject was revisited, and based on the results of the investigation presented here, the new optimized approach was adopted. The dam was therefore modified accordingly and has successfully been operating since then.

In general, for all hydropower development projects with peak power generation, this method of discharge regulation can be compared with that of the classical approach. Of course, specific environmental and social aspects of each project must also be considered in order to achieve an optimized solution.

5. REFFERNCES: Mahab Ghodss consulting Engineer, 1999, Gotvand 2000 MW Power Plant Design report. Mahab Ghodss consulting Engineer, 2007, Gotvand re-regulating dam Design report. Hydrologic Engineering Centers River Analysis System (HEC-RAS),

http://www.hec.usace.army.mil/software/hec-ras

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