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Question

Hydro Turbine Installation in Forebay of Cooling Tower in Thermal Power Plant?

Arijit Debroy's picture
Energy Efficiency Analyst Reliance Power

An Energy Efficiency analyst with 9.5 years of experience in the thermal sector in operation, commissioning and efficiency fields. I am also a lead auditor in energy management system ISO...

  • Member since 2020
  • 3 items added with 1,386 views
  • Jan 11, 2021
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Presently we are thinking of installing a hydro turbine in the Forebay of cooling tower with a capacity of 80000m³ per hour flow rate and velocity of 1.8 - 2.0 m/s. The maximum depth is 4 m.

Is it feasible? What can be the maximum power output and what type of hydro turbine can be installed.

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As written here I don’t think this is a feasible project.  It is good you are considering ways to reduce irreversible factors in the plant processes.

Engineers always look at the entire plant process as a whole.  If you are planning to extract energy from this power generation process and export it from the process, it’s a losing proposition.  You’re already exporting energy in an efficient manner at one point in the cycle: the generator step-up transformer.  Consider:

  • One way to look at the basic design of a hydropower turbine is as if it were located at a drop in elevation or waterfall.  Say you wanted to replace an existing vertical 4 m drop discharge with a turbine.  Falling water is converting potential energy to kinetic energy, heat from friction, and vibration in the ground.  By replacing this waterfall with a turbine, you will convert much of that kinetic energy into shaft power.  Some of the other commenters on this post have brought this up, but I really don't see how it's relevant.
  • The forebay of a cooling tower development is not poised on the brink of a 4 m drop.  Rather, your forebay is 4 m deep and water is moving through it at about 1.8 m/s.  Let’s say that all that water smoothly funnels into an opening  4 m wide by 4 m deep.  Some might suggest you could hang some sort of water wheel turbine in that opening, which would spin as the water flowed by.  You’d then take that energy away.  But something has to give:  the water velocity must reduce, or the depth in the forebay must rise.  The amount of energy you can extract is proportional not to the total depth and velocity, but rather to the additional depth (backing up of water), or loss of velocity (restriction of water flow), or both.
  • You still must somehow move 80,000 m³/h water, that’s 500 mgd for those of us in the USA.  You cannot restrict the flow.

If you are going to plow energy generated by this turbine back into your plant process, there is a small possibility of improving the overall plant efficiency.  This is just like the various reheaters and heat recovery units all over power plants.  But if you’re planning on extracting energy from the process and exporting it out of the process for some other use, you’re robbing Peter to pay Paul. Moreover, you’re paying a commission along the way, in the form of friction.

I was asked to reach out on answering this question or at least starting a dialogue. I asked within the specialised team with Siemens Energy

The reply to me was "I would like to introduce myself as sales responsible for Siemens Energy’s small hydro power solutions.

I have attached a small leaflet about our small hydro business/portfolio line to this email for your kind information.

If I understand correctly, there is a request coming out of the EnergyCentral community about the idea of having a water turbine installed in the forebay of a cooling tower in a Thermal power plant.  

From this community request we unfortunately cannot judge who addressed that topic (is it Mr. Arijit Debroy?) and what are the site conditions which need to be considered as well. We have therefore attached our small hydro questionnaire in order to collect necessary project information. 

Maybe it is a good idea to shortcut and talk to the requester directly – therefore, please feel free to forward my email address.

With best regards,
Hubertus Zimmermann

Siemens Gas and Power GmbH & Co. KG
Siemens Energy
SE GP G IC S HS
Freyeslebenstr. 1
91058 Erlangen, Germany
Tel.: +49 9131 17-35151
Mobile: +49 173 3477991

mailto:hubertus.zimmermann@siemens.com
siemens-energy.com

How to calculate output power of a hydroelectric turbine? The simplest formula is :

Where
   P = Mechanical power in kW
   Q = flow rate in (m3/s)
   ρ = density (kg/m3)
   g = Acceleration due to gravity (m/s²)
   H = head (m)
   η = turbine efficiency (typically 0.7 to 0.9)  

 

I would guess the turbine described might generate 300kW with a 2M head, and 600KW when the head is 4M.  Would be interesting if this was a remote site, cant see an interest at a power plant, unless there is a non-economic reason for implementing the turbine.

Tansel  Yılmaz's picture
Tansel Yılmaz on Jan 12, 2021

That formula https://energycentral.com/sites/default/files/users/206324/image-20210112131545-1.jpeg is not used in installing a hydro turbine in the Forebay of cooling tower with a capacity of 80000m³

It is feasible.

Power to be generated if a volume of water turbined with a head is the point in question.

E = (V.H)η / 367

= 80,000m3  4m  0.8 / 367

= 697.55 kW

V : Volume of water turbined (m3)

The peripheral speed of the turbine runner at the entering edge of the runner blades in relation to the spouting velocity of the water affects the efficiency and the cavitation characteristics. Close attention to design of blade angles at inlet and outlet is necessary, as these are major parameters affecting power production

……………………………………………………………………………………………….

                                    P= n QH/11.8      (English units)

where: P-power, kilowatts, kW;

n=efficiency (a number less than one), dimensionless;

Q=flow in cubic meters per second, cms ( cubic feet per second, cfs );

and H =-head in meters (feet). ·

P= n QH/11.8      (English units)

Q=22.22 m3/s = 784,69 cubic feet per second

H= 4m = 13.123 feet

P=0.8     784,69    13.123  /  11.8 = 698.135 kW

…………………………………………………………………

Power available from 1 cubic meter of water falling through 1 m every second

P= Energy per unit of time

=mgh

= 1000 kg x 9.81 m/s2 x 1m/ 1s

= 9800 joules / s

=9800 Watt

=9.8 Kw

So , for every cubic metr of water per meter of drop per second , 9.8 kW of power is available.

I reccommend Ossberger Crossflow turbines.

The following hydraulic design criteria are recommended:

a) The live storage volume of the forebay tank should be determined according to the response characteristics of the turbine governors. Normally a volume of Qp×120 m3 (or two minutes at (Qp= maximum plant flow) will be satisfactory for mechanical governors. For digital governors the control volume can be further reduced. In this case the engineer should contact the turbine manufacturer to define the control parameters in order to calculate the control volume needed.

Water Level Control

A water level control system requires that real time water level measurements in the forebay tank and tailrace canal be transmitted to the turbine governor. In the water level control mode the governor will estimate the inflow to the forebay tank and adjust the wicket gates to correct for difference between turbine and canal flows so as to maintain forebay tank levels within a prescribed range. A float type water level gauge with electronic data transmitter is recommended.

                                                              APPENDIX

What are the three main factors for power output of hydroelectric plant?

This depends on a lot of factors like

- Capacity of the Water Conductor System,

-Water Storage Capacity of the reservoir etc.

  • Available head.
  • Discharge.
  • Efficiency
  • rpm  i.e speed of the turbine
  • Capacity factor

……………………………………………………………………………………………………………………………………….

For low heads, below 15 m Kaplan is used.

For low head installations, the diameter of a penstock must be quite large to accomodate the large discharges necessary for a given power output. Its size is a compromise between head loss and cost.

Advantages of a Kaplan turbine  include:

• Smooth operation to low flows;

• Higher specific speed and higher rotational speed and, therefore, smaller generator;

• Higher efficiency is realized over a wide range of head and flow conditions. This is the principal advantage of the Kaplan turbine over a propeller or Francis turbine.

•. May result in a single unit instead of two Francis or propeller units.

LOW HEAD TURBINES

There are a large number of turbines suitable for operation in a head range from about 4 to about 30 m. These include:

– open flume Kaplan

– axial flow turbines with and without movable guide vanes

– tubular turbines

– vertical Kaplan with and without movable guide vanes

– bulb turbines.

Each has a particular range of application and it is often difficult to decide which is the best option.

The runner of a Kaplan turbine is similar to a ship’s propeller except that each blade rotates on its axis over a fairly wide angle. Kaplan turbines can have between three and five runner blades. A three bladed Kaplan turbine is used for unusually low heads and a five bladed turbine would be used at the highest head. Turbine flow is controlled by conventional guide vanes.

A very high efficiency over a wide range of flows and heads is achieved by varying the runner blade angle according to the position of the guide vanes and the head on the turbine. The runner blades are set to move at a slower rate than the guide vanes.

Until recently the relationship between the guide vanes and the runner blades was set by a cam attached to the guide vanes that controls the opening of the runner blades.Where there was a large head variation cams with a three-dimensional shape were used and moved transversely along guides so that, at any particular head, the blade angle was optimum. All this is now done by electronics but the reference to “3-D cam’’ remains.

As already mentioned, Kaplan turbines have a high runaway speed of between 2.5 and 3.3 times the rated speed. There is an “on cam’’ runaway speed and a higher “off cam’’ runaway speed. Because the runner blades move slowly compared to the guide vanes and because, if the turbine overspeeds, there must be something wrong with the governing system, it is normal to design the generator to survive “off cam’’ runaway speed. This has serious implications for the generator manufacturer so it is something that must be considered carefully during the specification stage. Some manufacturers simply assume that the generator will never be exposed to runway speed, but such a philosophy should be accepted with extreme caution and only where there is little or no chance of anyone being injured if the generator disintegrates. As the highest runaway speed is reached when the blades are near to the closed position, there is an argument for using only the “on cam’’ runaway speed in cases where action of the water on the blades would always cause them to open if the control system lost oil pressure.

A double regulated Kaplan turbine with a 3 dimensional cam maintains high efficiency over a wide range of heads and flows while a turbine with adjustable blades and fixed guide vanes does not have good efficiency at low outputs. A turbine with fixed runner blades and adjustable guide vanes has a very peaky efficiency curve and should be avoided unless it operates at a variable speed. Most Kaplan turbines will operate safely over a range of heads from about 60% to about 125% of rated head. Turbines with fixed guide vanes or with fixed runner blades are more sensitive to head variations.

Deciding whether or not movable guide vanes are needed can be difficult. Dispensing with them reduces the cost by 10 to 15% and can be considered if the cost needs to be minimised, the operating range does not include low outputs and the operating head is fairly constant.

………………………………………………………………………………………………………………..............................

In isolated status , the load curve will decide the size and number of units.

Load Demand Investigations

Load demand and market survey is required to assess the energy sale and financial analysis of the scheme. These may of two types.

Grid Connected: The investigations will involve the details on the nearest sub station for trouble free power evacuation, its capacity, the length of transmission line required, land required etc.

Isolated Status: The daily, seasonal and annual consumption of domestic/commercial/ community/industrial and irrigation needs are to be worked. The load curve will decide the size and number of units

………………………………………………………………………………………………………………..............................

For the high head Francis turbine approximately 50% of the energy is converted into kinetic energy at the runner inlet, and there is a pressure drop through the runner of approximately 50% of the total energy.

For a Kaplan turbine the drop in pressure energy from inlet of the runner is relatively larger than for a Francis turbine.

……………………………………………………………………………………………………………………

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