Thursday, April 23, 2015

Computation of Power in ResSim

This entry will cover the computation of power in ResSim.  For this model, we have a single stream that contains one reservoir.  I put in one outlet, a power plant, at the dam.  In this simulation, I have 5,000 cfs coming into the model at the upstream end (CP1 is upstream end).  The simulation starts with the pool at the top of conservation and maintains that elevation throughout.  In other words, inflow = outflow at each time step.

The figure below shows that model schematic.

The next figure shows the outlets.  You can see that the only outlet is a power plant.

The power equation is as follows:

Power in megawatts (mw) = Q*w*h*e/737,560

where Q is in cfs, w is weight of water in lb/ft^3, h is head differential in feet, e is efficiency

Note that 737,560 ft-lbs/sec = 1 megawatt

Understanding the variables that go into the power equation will help with the understanding of the tabs used to describe the power plant.  

The outlet tab specifies the hydraulic capacity of the power plant.  In this case, the hydraulic capacity is set to 5,000 cfs for all elevations. 

The capacity tab specifies the generating capacity of the power plant.  The purpose of this tab is to limit the computed power generation to no more than the actual generating capacity.  In this case, I set the maximum generating capacity to 10 mw.  

For the efficiency tab, I set the value to a constant 80%.

Station use is for internal use at the dam.  This value is often set to zero; however, I put in 500 cfs to show its impact.  The value that is used for station use is passed through the outlet, but it is not used in the power computation.

The final tab is hydraulic losses.  This reflects the losses associated with the power plant.  The amount used for hydraulic losses is subtracted from the total head difference computation.  For this example, I used a constant loss of 2 feet.

The head differential is computed as the difference between the headwater and tailwater.  The headwater is computed as the pool elevation at each time step.  For this simulation, the top of conservation (50 ft) is held for the entire simulation.  For ease of computation, a constant tailwater elevation of 25 ft is used.  This leads to a head differential of 25 ft.

The figures below show the top of conservation elevation and the tailwater definition.

The next figure shows the pool elevation and outflow results at the reservoir.  Note that the pool elevation is held constant at 50 ft while the outflow is held constant at 5,000 cfs.  Recall that there is only one outlet so all outflow is going through the power plant.  

The power computation at all time steps is as follows:

Q = 5,000 - 500 = 4,500 cfs (note that 500 cfs is subtracted due to station use)
w = 62.4 lb/ft^3
h = 25 - 2 = 23 ft  (2 is subtracted due to the hydraulic losses)
e = .80

Power (mw) = (4,500 * 62.4 * 23 * .80) / 737,560 = 7 mw

Since the generating capacity of 10 mw is not exceeded, the computed generation amount of 7 mw will not be limited.  

The power generation plot is shown below.  The upper plot shows a constant 7 mw of power generation.  The bottom plot shows 5,000 cfs flowing through the power plant with 4,500 cfs of that flow used for power generation.   

Thursday, April 2, 2015

Downstream Control Rules

This example will demonstrate the use of the downstream control function.  This model contains a single stream with one reservoir.  At the upstream end of the model, there is 500 cfs entering.  At the junction named "trib inflow", there is also 500 cfs entering.  "Trib inflow" is highlighted in yellow in the figure below.

The downstream control point is highlighted in the figure below.  Notice that it is below the point "trib inflow".  For the three simulations that I will be showing, the pool begins in the flood pool meaning that ResSim will want to release as much as possible.  To illustrate the use of the downstream control function, a maximum flow is set at the downstream control point.  

In the first simulation, the maximum flow at the downstream control point is set to 400 cfs.

The plot below shows the flow at the downstream control point.  After the lookback period ends, the flow at that point is 500 cfs.  Initially, it may seem that the rule is being violated since the flow exceeds 400 cfs.  However, ResSim is unable to control flow in the uncontrolled areas.  Recall that 500 cfs is coming into the system below the reservoir.

Once we look at the reservoir plot, we can see the ResSim is behaving properly.  In the upper plot, we can see that the pool is rising (green line in upper plot) since we have 500 cfs of inflow and zero outflow (green line in lower plot).  The outflow is set to zero since we are already violating the downstream control point limits.    

In the next example, I set the downstream control point maximum to 800 cfs.

The outflow is then set to 300 cfs since we have 500 cfs coming in downstream.  The pool still rises since inflow exceeds outflow, but not as high as we saw in the first example.  Note that I am using null routing in these examples for simplicity.  

After the lookback period, we can see that the flow at the downstream location is held at 800 cfs.  Also note that the local flow is shown by the line with the triangle symbols.    

The final simulation will use the downstream control function rule from the first example (400 cfs maximum) along with a minimum release rule of 100 cfs.  The minimum release rule is set to a higher priority based on its location in the rule set.  As we saw in the first simulation, the downstream maximum flow of 400 cfs was being violated due to the tributary inflow.  ResSim set the reservoir release to zero because of this.  However, with a minimum release of 100 cfs added to the rule set, the downstream maximum is further violated.  

Rule set with minimum release at higher priority than maximum downstream control function.  

ResSim recognizes that the 400 cfs downstream limit is being violated; however, the minimum release is a higher priority so that release is made (green line in lower plot).

Flow at downstream location consists of the 500 cfs of tributary inflow along with the 100 cfs from the reservoir to give a total of 600 cfs.