HOW HYDROPOWER WORKS

HOW HYDROPOWER WORKS

Hydropower plants capture the energy of falling water to generate electricity. A turbine converts the kinetic energy of falling water into mechanical energy. Then a generator converts the mechanical energy from the turbine into electrical energy.

Hydroplants range in size from "micro-hydros" that power only a few homes to giant dams like Hoover Dam that provide electricity for millions of people.

The photo on the right shows the Alexander Hydroelectric Plant on the Wisconsin River, a medium-sized plant that produces enough electricity to serve about 8,000 people.

Parts of a Hydroelectric Plant

Most conventional hydroelectric plants include four major components (see graphic below):

1. Dam. Raises the water level of the river to create falling water. Also controls the flow of water. The reservoir that is formed is, in effect, stored energy.
2. Turbine. The force of falling water pushing against the turbine's blades causes the turbine to spin. A water turbine is much like a windmill, except the energy is provided by falling water instead of wind. The turbine converts the kinetic energy of falling water into mechanical energy.
3. Generator. Connected to the turbine by shafts and possibly gears so when the turbine spins it causes the generator to spin also. Converts the mechanical energy from the turbine into electric energy. Generators in hydropower plants work just like the generators in other types of power plants.
4. Transmission lines. Conduct electricity from the hydropower plant to homes and business.

hydroplant-animate.gif

How Much Electricity Can a Hydroelectric Plant Make?

The amount of electricity a hydropower plant produces depends on two factors:

1. How Far the Water Falls. The farther the water falls, the more power it has. Generally, the distance that the water falls depends on the size of the dam. The higher the dam, the farther the water falls and the more power it has. Scientists would say that the power of falling water is "directly proportional" to the distance it falls. In other words, water falling twice as far has twice as much energy.
2. Amount of Water Falling. More water falling through the turbine will produce more power. The amount of water available depends on the amount of water flowing down the river. Bigger rivers have more flowing water and can produce more energy. Power is also "directly proportional" to river flow. A river with twice the amount of flowing water as another river can produce twice as much energy.

Can I Figure Out How Much Energy a Dam in My Area Can Make?

Sure. It's not that hard.

Let's say that there is a small dam in your area that is not used to produce electricity. Maybe the dam is used to provide water to irrigate farmlands or maybe it was built to make a lake for recreation. As we explained above, you need to know two things:

1. How far the water falls. From talking to the person who operates the dam, we learn that the dam is 10 feet high, so the water falls 10 feet.
2. Amount of water flowing in the river. We contact the United States Geological Survey, the agency in the U.S. that measures river flow, and learn that the average amount of water flowing in our river is 500 cubic feet per second.

Now all we need to do is a little mathematics. Engineers have found that we can calculate the power of a dam using the following formula:

Power = (Height of Dam) x (River Flow) x (Efficiency) / 11.8

HEAD AND FLOW DETAILED REVIEW

If you have a prospective site, it helps to understand the hydropower fundamentals of head and flow.

What is head?

Hydropower all comes down to head and flow. The amount of power, and therefore energy that you can generate is proportional to the head and the flow.

Head is the change in water levels between the hydro intake and the hydro discharge point. It is a vertical height measured in metres. The two diagrams below show how the head would be measured on a typical ‘low head’ and a typical ‘high head’ site. The more head you have the higher the water pressure across the hydro turbine and the more power it will generate. Higher heads are not only better because they generate more power, but also because the higher water pressure means you can force a higher flow rate through a smaller turbine, and because turbine cost is closely related to physical size, higher-head turbines often cost less than their low-head cousins even though they might generate the same power.

                                                                                                                     Diagram of measuring head at high

                                                                                                                                                                               head hydropower site

Higher head also means a faster rotating turbine and generator, which means lower torque.

The cost of drive train is closely related to how much torque it has to transmit, so higher heads = less torque = less cost.

Of course you only have what you have, so if your site only has 2 ½ metres of head you won’t be able to increase this significantly. However, even small increases in head can make a difference.

Sometimes it is possible to clear silt or re-grade a tailrace or discharge channel to lower the downstream water levels slightly which increases the overall head at the site. Or it may be possible to raise the water level on the upstream side by raising weir crests or sluices, though this must be done carefully to avoid increasing flood risk, and sometimes requires the construction of new spillways or installation of fail-safe tilting weirs to ensure that flood risk isn’t increased during extreme flood events.

Generally speaking the cost of even small increases in head at low-head sites is repaid hundreds of times over from increased energy production for the next few decades, so is always worth the effort.

What is flow?

The flow rate and how it varies over a year is the next and equally important parameter.

The simplest way to characterise the flow in a watercourse is to work out what the long-term annual mean flow is. This is important because it is the overall average flow in a watercourse that is important; it doesn’t matter if it is a raging torrent after heavy rains (all watercourses are…) because in the big scheme of things we only have really heavy rains for a few days or weeks a year, and for the other 50 weeks when it is lightly raining, drizzling or bright summer sunshine you would still want you hydro system to be working and generating energy.

The fundamental piece of information that characterises the flow in a watercourse is the flow duration curve. Although simple once you understand them, they are quite complicated to newcomers. Figure 1 below shows the long-term flow duration curve of a small river in Somerset.

Head and flow - Long-term flow duration curve for the River Yeo in Somerset

The y-axis is the flow rate in m3/s (metres-cubed per second), or sometime in litres per second for smaller watercourses. When the flow duration curve is constructed all of the flow rate data is sorted into descending order, then the highest flow rates are plotted on the left of the curve, then progressively lower flow rates to the right until the very lowest flow is plotted at the extreme left-hand end. The x-axis is the ‘percentage exceedence’. This is normally the difficult part to understand… For a given percentage exceedence it shows the flow rate equalled or exceeded for that percentage of time. For example, if you look at the 50% percentage exceedence on Figure 1 and read off the flow rate at that point you will see that it is 1.1 m3/s. This doesn’t mean that the flow rate in the watercourse is 1.1 m3/s for 50% of the year, it means that the flow rate is 1.1 m3/s or more for 50% of the year. The or more is important because it is clear from the shape of the curve that apart from the instant that the line crosses the 50% mark it is always more than 1.1 m3/s.

The x-axis is always plotted as a percentage exceedence from 0 to 100%. This is so that any data set spanning any interval can be plotted. The data set may span 10 days or more likely several decades of data from a local river gauging station. Generally speaking flow duration curves present long-term annual data, so the flow rates read off them are the annual flow characteristics. The percentage exceedences are often called ‘Q values’, so Q95 is the flow rate exceeded for 95% of a year and Q10 the flow exceeded for 10% of the year. The data set can also be analysed using a spreadsheet to work out the average (arithmetic mean) of all of the flows, and this is called Qmean. In the UK the Qmean is normally somewhere between Q25 and Q30, and in the case of Figure 1 is Q26.5.

Once you understand how a flow duration curve is constructed it can tell you a great deal about the flow characteristics of the water course. Firstly you can work out the Qmean, which is the average flow, and often if you are negotiating with the environmental regulator you will be discussing the Q95 flow, as this is often used as the representative ‘low flow’ that must always flow in any depleted stretches of river while the main volume of water passes through the hydro turbine.

By comparing different Qvalues you can see how ‘flashy’ a watercourse is, or whether it has a high baseflow. The flow duration curve in Figure 1 has a Qmean of 2.54 m3/s and a Q95 of 0.32 m3/s, so the Q95 is 8% of the Qmean. This is typical for a ‘flashy’ river that rises and falls quickly in response to rain because the rain runs straight of the land and into the river without getting stored in bogs or porous rocks before flowing into the river sometime later.

If you make the same comparison using Figure 2, you will see that the Qmean is 11.30 m3/s and the Q95 5.83 m3/s. In this case Q95 is 52% of the Qmean. This would be a ‘high baseflow’ river, typical of the rivers that flow in Hampshire with chalk catchments where the chalk stores the rain in its porous structure and then releases it to the rivers slowly and over a long period of time, rather like a sponge. It is also interesting to note that Qmean is at Q41; much lower on the flow duration curve than a flashy river like in Figure 1.

Figure 2 – Long-term flow duration curve for the River Test in Hampshire.

Head and flow - Long-term flow duration curve for the River Test in Hampshire

In both flow duration curves you can see that the red line is heading steeply upwards at the left-hand end of the graph. This is the ‘extreme flow’ region, which is not much use for energy generation because such extreme flows occur for a relatively small proportion of the year, but they are very important when designing a hydropower systems to ensure that the hydro structures and turbine house don’t get flooded, or even worse, washed away during the next major storm!

What is the minimum head and flow required?

The answer to this depends very much on what return on your investment you want.

For a commercially viable site it would normally need to be at least 25 kW maximum power output. For a low-head micro hydropower system you would need at least 2 metres of gross head and an average flow rate of 2.07 m3/s. To put this in context this would be a small river that was approximately 7 metres wide and around 1 metre deep in the middle.

For a site with 25 metres head a much lower average flow rate of 166 litres / second would be needed. This would be a large stream of 2 – 3 metres width and around 400 mm deep in the middle.

It is technically possible to develop smaller hydropower sites with lower power outputs, but the economics start to get challenging. This is particularly true for low-head sites; when the head drops to 1.5 metres it isn’t normally possible to get any kind of return on investment, though the site could still be technically developed using Archimedean screws or modern waterwheels.

The table below shows the average flow rates needed for a range of heads from 2 metres to 100 metres for system with 100, 50, 25, 10 and 5 kW maximum power outputs. 25 kW would normally be considered the minimum for a commercial project, though a 10 kW system can still produce an acceptable return if the civil engineering works are simple (hence don’t cost much). 5 kW systems are not normally viable, but the figures are shown for interest and may be useful for sites that can generate value from non-tangible benefits such as attracting visitors or positive publicity.

WHAT IS SOLAR ENERGY?

Solar energy is, simply, energy provided by the sun. This energy is in the form of solar radiation, which makes the production of solar electricity possible.

Electricity can be produced directly from photovoltaic, PV, cells. (Photovoltaic literally means “light” and “electric.”) These cells are made from materials which exhibit the “photovoltaic effect” i.e. when sunshine hits the PV cell, the photons of light excite the electrons in the cell and cause them to flow, generating electricity.

Solar energy produces electricity when it is in demand – during the day particularly hot days when air-conditioners drive up electricity demand.

In use, solar energy produces no emissions. One megawatt hour of solar electricity offsets about 0.75 to 1 tonne of CO2.

PV panels are being used increasingly, both in the city and in remote locations, to produce electricity for households, schools and communities, and to supply power for equipment such as telecommunication and water pumps. The majority of solar PV installations in Australia are grid-connected systems.

Also, electricity for remote and regional Australian communities has been supplied by solar energy for many years.

Australia is one of the sunniest countries in the world and there is huge potential for solar PV to make a significant contribution to electricity generation.

But note that; not all radiation reaches the earth's atmosphere

• About 30% is reflected back
• 47% is absorbed during the day by the land sea
• 23% cause evaporation from the oceans and sea in form of vapour
• 0.2% causes conversion current in the air, creating wind power which turns causes wave power
• 0.02% is absorbed by plant during photosynthesis

SOLAR CONSTANT

Solar constant can be defined as the solar energy falling per second per square metre.

SI-Unit for solar constant is kWm²

AMOUNT OF SOLAR ENERGY DEPENDS ON

• The geographical location
• The time or length of a day
• Season of the year( summer, winter e.t.c)
• The altitude - the height above the sea level

PHOTOVOLTAIC DEVICES (PV)

A photovoltaic system, also PV system or solar power system, is a power system designed to supply usable solar power by means of photovoltaics. It consists of an arrangement of several components, including solar panels to absorb and convert sunlight into electricity, a solar inverter to change the electric current from DC to AC, as well as mounting, cabling, and other electrical accessories to set up a working system.

HOW IT WORKS

Photovoltaic cells are made of special materials called semiconductors such as silicon, which is currently used most commonly. Basically, when light strikes the cell, a certain portion of it is absorbed within the semiconductor material. This means that the energy of the absorbed light is transferred to the semiconductor. The energy knocks electrons loose, allowing them to flow freely.

PV cells also all have one or more electric field that acts to force electrons freed by light absorption to flow in a certain direction. This flow of electrons is a current, and by placing metal contacts on the top and bottom of the PV cell, we can draw that current off for external use, say, to power a calculator. This current, together with the cell's voltage (which is a result of its built-in electric field or fields), defines the power (or wattage) that the solar cell can produce.

Advantages and disadvantages of photovoltaics

Advantages

1. Electricity produced by solar cells is clean and silent. Because they do not use fuel other than sunshine, PV systems do not release any harmful air or water pollution into the environment, deplete natural resources, or endanger animal or human health.
2. Photovoltaic systems are quiet and visually unobtrusive.
3. Small-scale solar plants can take advantage of unused space on rooftops of existing buildings.
4. PV cells were originally developed for use in space, where repair is extremely expensive, if not impossible. PV still powers nearly every satellite circling the earth because it operates reliably for long periods of time with virtually no maintenance.
5. Solar energy is a locally available renewable resource. It does not need to be imported from other regions of the country or across the world. This reduces environmental impacts associated with transportation and also reduces our dependence on imported oil. And, unlike fuels that are mined and harvested, when we use solar energy to produce electricity we do not deplete or alter the resource.
6. A PV system can be constructed to any size based on energy requirements. Furthermore, the owner of a PV system can enlarge or move it if his or her energy needs change. For instance, homeowners can add modules every few years as their energy usage and financial resources grow. Ranchers can use mobile trailer-mounted pumping systems to water cattle as the cattle are rotated to different fields.

Disadvantages

1. Some toxic chemicals, like cadmium and arsenic, are used in the PV production process. These environmental impacts are minor and can be easily controlled through recycling and proper disposal.
2. Solar energy is somewhat more expensive to produce than conventional sources of energy due in part to the cost of manufacturing PV devices and in part to the conversion efficiencies of the equipment. As the conversion efficiencies continue to increase and the manufacturing costs continue to come down, PV will become increasingly cost competitive with conventional fuels.
3. Solar power is a variable energy source, with energy production dependent on the sun. Solar facilities may produce no power at all some of the time, which could lead to an energy shortage if too much of a region's power comes from solar power.

WIND ENERGY

Wind power is the use of air flow through wind turbines to mechanically power generators for electricity.

Wind Turbines

Introduction

Wind is caused by uneven heating of the earth from the sun making wind a renewable and free source of energy. Wind turbines are an alternate source of energy that harnesses this renewable wind power to make electricity. Since wind turbines run solely on wind, they cause no pollution making them environmentally friendly. Basically, wind turns blades that are connected to a generator, the generator then makes electricity (more on this later). There are two main types of wind turbines, horizontal and vertical axis. A wind turbine applicable for urban settings was also studied. All three types of wind turbines have varying designs, and different advantages and disadvantages.

Horizontal axis

Horizontal axis wind turbines are the most common type used (see figure 1). All of the components (blades, shaft, generator) are on top of a tall tower, and the blades face into the wind. The shaft is horizontal to the ground. The wind hits the blades of the turbine that are connected to a shaft causing rotation. The shaft has a gear on the end which turns a generator. The generator produces electricity and sends the electricity into the power grid. The wind turbine also has some key elements that adds to efficiency. Inside the Nacelle (or head) is an anemometer, wind vane, and controller that read the speed and direction of the wind. As the wind changes direction, a motor (yaw motor) turns the nacelle so the blades are always facing the wind. The power source also comes with a safety feature. Incase of extreme winds the turbine has a break that can slow the shaft speed. This is to inhibit any damage to the turbine in extreme conditions.

Figure 1: Horizontal axis wind turbine

Advantages

• Blades are to the side of the turbines center of gravity, helping stability

• Ability to wing warp, which gives the turbine blades the best angle of attack

• Ability to pitch the rotor blades in a storm to minimize damage

• Tall tower allows access to stronger wind in sites with wind shear

• Tall tower allows placement on uneven land or in offshore locations

• Can be sited in forest above tree-line

• Most are self-starting

Disadvantages

• Difficulty operating in near ground winds

• Difficult to transport (20% of equipment costs)

• Difficult to install (require tall cranes and skilled operators)

• Effect radar in proximity

• Local opposition to aesthetics

• Difficult maintenance

Vertical axis

In vertical axis turbines the shaft the blades are connected to is vertical to the ground (see figure 2). All of the main components are close to the ground. Also, the wind turbine itself is near the ground, unlike horizontal where everything is on a tower. There are two types of vertical axis wind turbines; lift based and drag based. Lift based designs are generally much more efficient than drag, or ‘paddle’ designs.

Figure 2 : Vertical axis wind turbine (lift type)

Advantages

• Easy to maintain

• Lower construction and transportation costs

• Not directional

• Most effective at mesas, hilltops, ridgelines and passes

Disadvantages

• Blades constantly spinning back into the wind causing drag

• Less efficient

• Operate in lower, more turbulent wind

• Low starting torque and may require energy to start turning

Ducted Wind Turbines

Ducted wind turbines are positioned at the edge of the roof of a building and utilize the airflow along a building’s side. The air flows upwards, hugging the building wall then enters the front of the duct. Turbine blade diameter is usually around 600 mm. The devices are relatively small leaving little visual impact to the building. They are positioned on a building as shown in figure 3 below.

Figure 3: Ducted Wind Turbine

Advantages

• Less visual impact on buildings architecture than traditional HAWT or VAWT turbines

• Make use of unused roof space in cities

• Allows energy need to be met on-site avoiding transmission losses associated with centralized energy generation

Disadvantages

• Suitable for urban environments, but not households (only effective on urban high-rise buildings)

• Uni-directional. Fixed position and are dependent upon wind blowing in the correct direction

• Much more research and development is needed. Research in this field is growing as people become more interested in urban wind generation.

• Research has to be done to determine energy production potential

Advantages of Wind Energy

Wind energy has numerous benefits in helping to provide a source of clean and renewable electricity for countries all over the world. This section takes a look at the many different advantages of wind energy.

1. Renewable & Sustainable

Wind energy itself is both renewable and sustainable. The wind will never run out, unlike the earth’s fossil fuel reserves (such as coal, oil and gas), making it the ideal energy source for a sustainable power supply.

2. Environmentally Friendly

Wind energy is one of the most environmentally friendly energy sources available today. After the manufacture and installation of wind turbines, there will be little to no pollution generated as a result of the wind turbines themselves.

Wind turbines produce no greenhouse gases such as carbon dioxide (CO2) or methane (CH4) which are both known to contribute towards global warming.

It should be noted that noise and visual pollution are both environmental factors, but they don’t have a negative effect on the earth, water table or the quality of the air we breathe.

3. Reduces Fossil Fuel Consumption

Generating electricity from wind energy reduces the need to burn fossil fuel alternatives such as coal, oil and gas. This can help to conserve dwindling supplies of the earth’s natural resources, allowing them to last longer and help to support future generations.

4. Wind Energy is Free

Unlike some other energy sources, wind energy is completely free. There’s no market for the supply and demand of wind energy, it’s there to be used by anyone and will never run out. This makes wind energy a viable option for generating cheap electricity.

5. Small Footprint

Wind turbines have a relatively small land footprint. Although they can tower high above the ground, the impact on the land at the base is minimal. The area around the base of a wind turbine can often be used for other purposes such as agriculture.

6. Industrial & Domestic Installations

Wind turbines aren’t just limited to industrial-scale installations such as wind farms. They can also be installed on a domestic scale, with many landowners opting to install smaller, less powerful wind turbines in order to provide part of a domestic electricity supply. Domestic wind turbines are often coupled with other renewable energy technologies such as solar panels or geothermal heating systems.

7. Remote Power Solution

Wind turbines can play a key role in helping to bring power to remote locations. This can help to benefit everything from a small off-grid village to a remote research station.

8. Wind Technology Becoming Cheaper

The first ever electricity-generating wind turbine was invented in 1888. Since then, wind turbines have improved significantly and nowadays the technology is beginning to come down in price, making it much more accessible.

Government subsidies are also helping to reduce the cost of a wind turbine installation, with many governments across the world providing incentives for not only the installation of such technologies, but also for the ongoing supply of environmentally friendly electricity.

9. Low Maintenance

Wind turbines are considered relatively low maintenance. A new wind turbine can be expected to last some time prior to any maintenance work needing to be carried out. Although older wind turbines can come up against reliability issues, each new generation of wind turbine is helping to improve reliability.

10. Low Running Costs

As wind energy is free, running costs are considered to be low. The only ongoing cost associated with wind energy is for the maintenance of wind turbines, which are considered low maintenance in nature anyway.

11. Huge Potential

Wind energy has huge potential. It’s both renewable and sustainable and is present in a wide variety of places. Although a significant level of wind energy is required to make a wind turbine installation cost effective, the technology isn’t limited to just a handful of locations such as is the case for geothermal power stations.

12. Increases Energy Security

By using wind energy to generate electricity, we are helping to reduce our dependency on fossil fuel alternatives such as coal, oil and gas. In many cases, these natural resources are often sourced from other countries.

War, politics and overall demand often dictate the price for natural resources, which can fluctuate and cause serious economic problems or supply shortages for some countries. By using renewable energy sources a country can help to reduce its dependency on global markets and thus increase its energy security.

13. Job Creation

The wind energy industry has boomed since wind turbines first became available on the market. This has helped to create jobs all over the world. Jobs have been created for the manufacture of wind turbines, the installation and maintenance of wind turbines and also in wind energy consulting, where specialist consultants will determine whether or not a wind turbine installation will provide a return on investment.

Disadvantages of Wind Energy

So, we’ve seen the advantages, now it’s time to take a look at the main disadvantages of wind energy. Wind energy has a number of drawbacks, with the NIMBY (not in my back yard) factor playing a large role.

1. The Wind Fluctuates

Wind energy has a similar drawback to solar energy in that it is not a constant energy source. Although wind energy is sustainable and will never run out, the wind isn’t always blowing. This can cause serious problems for wind turbine developers who will often spend significant time and money investigating whether or not a particular site is suitable for the generation of wind power.

For a wind turbine to be efficient, the location where it is built needs to have an adequate supply of wind energy. This is why we often see wind turbines built on top of hills or out at sea, where there are less land obstacles to reduce the intensity of wind energy.

2. Installation is Expensive

Although costs are reducing over time, the installation of a wind turbine is considered expensive. First, a site survey will need to be carried out which may involve having to erect a sample turbine to measure wind speeds over a significant period of time. If deemed adequate, the wind turbine will need to be manufactured, transported and erected on top of a pre-built foundation. All of these processes contribute to the overall cost of installing a wind turbine.

When the above is taken into account for offshore wind farms, costs become much greater. It’s much harder to install wind turbines out at sea than it is on land, and some companies have even commissioned bespoke ships capable of transporting and installing wind turbines at sea.

3. Threat to Wildlife

It’s widely reported that wind turbines pose a threat to wildlife, primarily birds and bats. It is however believed that wind turbines pose less of a threat to wildlife than other manmade structures such as cell phone masts and radio towers. Nevertheless, wind turbines are contributing to mortality rates among bird and bat populations.

4. Noise Pollution

One of the most popular disadvantages of wind turbines is the noise pollution that they generate. A single wind turbine can be heard from hundreds of meters away. Combine multiple wind turbines and the audible effects can be much greater.

Noise pollution from wind turbines has ruined the lives of some homeowners. Although steps are often taken to site wind turbines away from dwellings, they do sometimes get built too close to where people live and this is why new wind farms often come up against strong public objection.

5. Visual Pollution

Another widely reported disadvantage of wind turbines is visual pollution. Although many people actually like the look of wind turbines, others do not and see them as a blot on the landscape. This tends to come down to personal opinion, and as more wind farms are built, public acceptance is becoming commonplace.