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What Is The Head And Flow Rate In A Hydropower Project, And How To Calculate The Power Generation

November 20, 2020

Design Flow

Even though your Flow may be very high after exceptionally rainy periods, it probably won’t be cost effective to design your turbine system to handle all that water for just a few days of the year. Instead, it makes sense to build a system that uses Flow you can count on for much of the year. This is called Design Flow, and it is the maximum Flow your hydro system is designed to accommodate.Design Flow, along with Net Head, determines everything about your hydro system, from pipeline size to power output.

 

How the Penstock Affects Head Pressure

Measuring Pipeline (Penstock) Length

The length of your pipeline (also known as the penstock) has a major influence on both the cost and efficiency of your system, as we'll discuss below. The measurement is easy, though. Simply run a tape measure between your intake and turbine locations, following the route you'll use for your pipeline.

 

Computing Net Head

In the section Measuring Head you measured Gross Head – the true vertical distance from intake to turbine – and the resulting pressure at the bottom when no water is flowing. Net Head, on the other hand,is the pressure at the bottom of your pipeline when water is actually flowing to your turbine, and will always be less than the Gross Head you measured due to energy losses within the pipeline. Longer pipelines and smaller diameters create greater friction.Net Head is a far more useful measurement than Gross Head and, along with Design Flow, is used to determine hydro system components and power output. This section will show you the basics for determining pipeline size and Net Head, but we suggest you work with your turbine supplier to finalize your pipeline specifications.

 

Head Loss

Head Loss refers to the loss of water power due to friction within the pipeline (also known as the penstock). Although a given pipe diameter may be sufficient to carry all the Design Flow, the sides, joints and bends of the pipe create drag as the water passes by, slowing it down. The effect is the same as lowering the Head; there will be less water pressure at the turbine. Note that the effects of Head Loss cannot be measured unless the water is flowing. A pressure meter at the bottom of even the smallest pipe will read full PSI when the water is static in the pipe. But as the water flows, the friction within the pipe reduces the velocity of the water coming out the bottom. Greater water flow increases friction further. Larger pipes create less friction, delivering more power to the turbine. But larger pipelines are also more expensive, so there is invariably a tradeoff between Head Loss and system cost. A good rule of thumb is to size your pipe so that not more than 10% to 15% of the Gross (total) Head is lost as pipeline friction.

 

Example Site Characteristics:

Gross Head = 100 feet,Pipeline length = 400 feet,Acceptable Head Loss = 10% -15% (10-15 feet),Design Flow = 200 gallons per minute

To determine what size pipe would be best, look up your Design Flow (200 GPM) in the Head Loss Chart above. Our maximum acceptable Head Loss is 15 feet (15% of our 100-foot Gross Head),which means we cannot exceed 3.75 feet loss for every 100 feet of our 400-foot pipeline. Reading down the column under 200 GPM, we find that a four-inch pipe would cause a loss of 2.02 feet per 100 feet – within our limits. Using a four-inch pipeline, Head Loss for this example would be Head Loss = 2.02 feet (per 100 feet) x 4 = 8.08 feet, Therefore, Net Head for this example would be, Net Head = 100 feet – 8.08 feet = 91.92 feet, Note the significant difference in Head Loss between 3-inch and 4-inch pipes. Likewise, a 6” or 8” pipe would cause even less Head Loss and deliver more power to the turbine, but the performance improvement may not be sufficient to justify the added cost. Keep in mind that these Head Loss computations assume a straight pipe; they do not take into

account bends in your pipeline that can rob significant power from your water. Your turbine manufacturer should be well versed in measuring head losses, and can be an excellent resource for pipe diameter recommendations.

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Calculating the Power in Your Stream

Computing Water Power

Once you’ve determined Net Head and Design Flow, you can begin to estimate the power output from your hydro system. These computations are only rough estimates; consult with your turbine supplier for more accurate projections. We begin by computing the theoretical power output from your water, before taking into consideration any efficiency losses in the turbine, drive system, and generator. You can compute the Theoretical Power of your water supply as either Horsepower or Kilowatts using one of these formulas:

Theoretical Horsepower* (HP) = HEAD (feet) x FLOW (cfs)

8.8

Theoretical Kilowatts* (kW) = HEAD (feet) x FLOW (cfs)

11.81

* Note that these are Theoretical Power equations, which do not account for the inevitable efficiency losses that will occur at various points within your hydro system. The actual power output of your generator will be less, as we’ll discuss later.

Hypothetical Example:

A stream in New Zealand has 100 feet of HEAD, with 2 cubic feet per second (cfs) of FLOW. Applying our formula, we find that we should have about 17 theoretical kW available:

kW = 100 (feet) x 2 (cfs) = 16.93 Theoretical Kilowatts*

11.81

As you can see, both HEAD and FLOW have a linear effect on power. Double the head, and power doubles. Double the flow, and power doubles. Also keep in mind that HEAD will remain constant once your system is installed; you can count on it year-round. It is also the least expensive way to increase power generation because it has minimal affect on turbine size. In contrast, FLOW will likely change over the course of a year, and it may not be cost-effective to size your hydro system for maximum, flood-level Flow. Always maximize Head, and work with your turbine supplier to determine the most practical Design Flow. Accuracy is important! The design of your turbine revolves around your measurements of Head and Flow, and errors will directly impact the efficiency of your system. Take the time to measure Head and Flow accurately before you begin to evaluate hydro system components.

Adjusting for Efficiency Losses

As noted earlier, the Theoretical Power calculations shown above represent a theoretical maximum, and the actual power output from your hydro system will be substantially less. In addition to the pipeline losses discussed earlier, small amounts of power will be lost through friction within the turbine, drive system, generator, and transmission lines. Although some efficiency losses are inevitable, don’t underestimate the importance of good design. Efficient systems produce greater power output, often at a lower cost-per-watt. For example, a turbine system that is carefully matched to your Head and Flow may not cost any more than a less suitable design, but produce much greater efficiency. Other improvements, such as larger pipeline diameter or a better drive system, may yield enough added power to justify their higher cost. Because of the many variables in system design, it is impossible to estimate efficiency without first knowing your Head and Flow. As a general guideline, however, you can expect a home-sized system generating direct AC power to operate at about 60% - 70% “water-to-wire” efficiency (measured between turbine input and generator output). Larger utility systems offer much better efficiencies.

Smaller DC

systems generally have lower efficiencies.

If you have accurate measurements for your Head and Flow, your turbine supplier should be able to provide some preliminary estimates of efficiency, as well as ideas for optimization.

 

Measuring Transmission Line Length

The last important measurement is the length of your transmission line between your generator and the point of electrical usage. As with your pipeline, you can simply measure the distance along the route you plan to run your wiring.Transmission lines are a lot like pipelines. Instead of moving water, they move electrical current, but the same fundamentals of friction losses apply. Longer transmission lines, smaller wires, and higher current all contribute to power loss through friction. You can minimize these losses, but the power you can actually use will always be somewhat less than what your generator is producing.Power loss over transmission lines is most evident by a drop in voltage. As you use more power, you'll see the voltage drop and lights glow dimmer.There are three ways to reduce, or compensate for, transmission line losses

1. Shorten the transmission line

2. Use a larger wire

3. Increase the voltage on the transmission line

Shorter lines and larger wires will reduce line losses for any system. For very long runs, it may be appropriate to boost the voltage (via a transformer) for transmission, and then reduce it back to normal (via another transformer) at the point of usage. Boosting the voltage reduces the current necessary to produce the same amount of power, allowing the use of smaller wires. Your turbine supplier can help you determine the best approach.