Energy from water cycle

Hydroelectricity, or ‘hydro’, is generated from the energy in the water cycle of the earth. The sun evaporates water on the surface of the earth, causing it to rise up to form clouds. Clouds eventually form droplets, which then rain, snow, or hail down to the surface. Water on the surface flows downhill until it evaporates again. During this time it may become trapped in glaciers, lakes, ponds, puddles, or the ocean. Driven by the sun, the water cycle is a truly renewable resource.

Here we are not considering tidal power because it is not powered by the water cycle. Tides are driven by the gravitational interaction of the earth and moon.

How much electricity is from hydro?

Hydro is the most well-established form of renewable electricity production. In 2010, hydro comprised about 80% of all of the renewable electricity capacity in the world, and accounted for about 20% of global electricity production capacity. In the same year, about 15% of the electricity consumed in the world came from hydro. For more information on how electrical capacity and delivery are different, we suggest you read more about capacity factor.

Installed hydro capacity is only growing at about 3% per year currently compared to solar (53%), wind (32%), and geothermal (4%). However, hydro accounted for 39% of all renewable energy capacity growth in 2009. This difference is due to hydro’s large historical share of the renewable market; so a few percent increase in capacity translates into large increases in gigawatts.

Cost

Hydroelectricity is generally competitive with other forms of power in terms of cost. Sites under current or planned development are generally more remote or difficult to build on than the sites of historical hydro development. This has contributed to a rise in prices for large scale hydro electric projects.

Trends towards both big and small

There are two distinct trends in hydro development today. One trend is towards larger dammed projects. New and planned large-scale hydro projects are designed to produce progressively more electricity. That is, the scale of projects has been increasing throughout the 20th century. This trend is evidenced by recent development of hydroelectric mega-projects such as the Three Gorges Dam in China. This hydro station can produce up to 22.5 GW of power, several times the size of a large coal fired plant of 1 GW, and is in fact the largest electricity producing facility in the world.

The second trend in hydro development is towards small hydro. These systems may have no reservoir, such as with run-of-river hydro. Run-of-river systems generally have a reduced environmental impact as compared to large reservoir techniques. The trade-off is that they are generally not dispatchable, limiting their usefulness. However, if a river flows year-round, then some fraction of a run-of-river hydro system’s power capacity can be considered baseload. Small hydro is being pursued heavily in North America because of its reduced environmental impact. China has led the world in small hydro development recently as part of their efforts to provide renewable electricity in rural areas.

Reservoirs

Most large scale installations use reservoirs to stabilize power production by controlling water levels. The reservoir is a form of energy storage. Energy storage can be very useful for grid management. Reservoirs allow hydro power to be dispatchable, meaning that power is available on demand.

If you are interested in a more in-depth discussion of renewable energy forms that can fulfill the dispatchable role, see our article: How can renewables deliver dispatchable power on demand?

Environmental issues

The construction and use of a water reservoir can have both positive and negative effects. Reservoirs have many uses, such as irrigation, recreation, and flood control, but they also tend to force the migration of people, flood valuable cropland, and damage local ecosystems. Flooded land also contributes to greenhouse gas emissions, since decaying biomass underwater tends to produce a lot of methane, particularly in tropical zones, and particularly when lumber isn’t recovered before flooding.

Somewhat inflexible

There are some limitations on the flexibility of hydro power even with a reservoir. For instance, since rivers are homes to many sensitive species, efforts are generally undertaken to protect species affected by the hydro development. Also, people who own waterfront property on the reservoir, or river users downstream, may lobby for some controls on how the water is used. Seasonal variations in water availability also introduce constraints on the plant’s operations.

Ideal locations built already

In the developed nations, the majority of the ideal reservoir hydroelectric locations have been developed. Newer development is often forced to use locations that are not as ideal as those used for earlier installations. However, advancing technology and societal wealth have also unlocked opportunities that were infeasible in the past. Additional development of hydro will eventually be restricted as a result of us having developed all of the economically viable locations.

Pumped hydroelectric storage

Pumped-hydro storage requires two reservoirs at different elevations. When power demand is high, water is run out of the higher reservoir to generate electricity. When demand is low, water is pumped upwards from the lower reservoir to the higher one, effectively storing it for later use.

Two reservoirs close by

An ideal location would be where you can create two reservoirs of vastly different height, but very close to each other. This greatly limits the possible locations that pumped hydro storage can be employed. Due to the sometimes sudden cycles of these power plants, they can have substantial environmental effects that would need to be studied.

Sometimes the turbines themselves are designed to spin backwards to pump water back up into the upper reservoir. The total efficiency of a pumped hydro system is generally around 65-75%. A pumped-hydro system is generally relatively expensive per installed GW compared to its normal reservoir or run-of-river brethren.

Existing plants

Pumped-hydro can come in a wide range of sizes, depending on factors such as economics, need for storage, ecological and cultural considerations. Some nations have invested substantially in pumped hydro storage because of the tremendous value of dispatchable sources in creating reliable power. Here are two case studies among the many existing pumped hydro plants in the world.

The UK has Dinorwig Power Station, which has a max power output of 1.8 GW and a storage capacity of about 9.1 GWh. Dinorwig can dispatch from zero output to max power in 16 seconds, making it one of the fastest-responding power plants in the world. Dispatchability of this much power can be extremely advantageous for peak-matching during the high demand times of the day when demand is also often quite volatile. See our article on energy storage for more information.

In 1993 China commissioned the Tianhuangping Pumped-Storage Hydro Plant. Completed in 2001, it can generate a max power of 1.83 GW, making it comparable to Dinorwig. Its storage capacity is a bit higher however, at 13 GWh. The difference between the heights of the two reservoirs is an impressive 600 meters. The overall cycle efficiency is around 70%.

Dam uprating

This is a term that we only ran into recently. However, we have been studying the concept for some time. Dam uprating is the process of upgrading the power output of a reservoir-based hydroelectric system. This can be accomplished by adding turbines, or installing higher capacity ones. This concept can be applied to both traditional dams and pumped-hydro storage systems.

We are very interested in the possible synergy of wind and hydro. To this end we have been researching what is possible on the Canadian prairies in this direction. This led us to examine the quantity of dispatchable hydro on the prairies, as well as whether this hydro power was already allocated to other tasks. For instance, since reservoir hydro is such a fast-responding power source, it is often used to cover the lead-in times for slower forms of generation. This is necessary if demand rises sharply, and the slower forms of generation cannot keep up.

We have inquired about dispatchable hydro issues with one of the prairie power crown corporations, but not received any response. One of our core ideas for innovating the power grid of the prairies is that we can up-rate our existing reservoir-based hydroelectric capacity so that we can use it to leverage more wind power.

Run of river hydro

Run-of-river hydro power does not have a large reservoir, if any. If there is a dam across a river, it will create power from the power that collects behind the dam. In other forms, the run-of-river plant is one large part of the river flow, while another part flows on unobstructed. Using this second method it is possible to minimize the environmental effects of the power production.

Some even smaller run-of-river systems simply use a turbine placed in the middle of the flow of the river, or something similar to a classic water wheel. These systems are generally much smaller, perhaps only a few kilowatts in size.

Induction generators

Induction generators are a useful technology for run-of-river hydro when it will be connected to a power grid. Induction generators need connection to a central power source (or some electrical energy storage) because they require some energy to run. They produce more energy than they use as long as the turbine is being spun above a certain minimum speed. They are designed to be able to utilize turbines spinning at any speed. The have been used extensively in wind turbines because of the variable speed of the wind and thus the rotation speed of the blades. These turbines are generally simpler and longer-lived than other generator types.

Baseload to some extent

Run-of-river systems can be intermittent depending on many factors. Variation can exist day to day or even hour to hour in flowing water sources. Significant variation also exists on the scale of seasons in most places. During the rainy season a run-of-river plant is likely to be running near full capacity. Dry season power production can be drastically lower than capacity in some cases.

If a river flows all year, as most rivers in the world do, some amount of the power production from a run-of-river plant can be considered baseload. Run-of-river hydro is somewhat intermittent, but not as intermittent as wind power for instance. Wind power is very hard to predict more than a few minutes or hours in advance sometimes, but run-of-river hydro can be predicted rather well.

Less intermittent is better

Intermittent sources require dispatchable sources to compensate for them. We go into more detail on this subject in our post on leveraging dispatchable hydro to use wind. What is important to understand is that the predictability and size of your intermittent power sources is very closely related with how easy or difficult it is to manage your grid. Since run-of-river hydro is more predictable than wind power, it should introduce less grid management costs. Uncertainty about power generation can end up being expensive, because a larger spinning reserve is required.

Call for submissions

This concludes the third installment of the Renewable Energy Review Blog Carnival. For a complete list of all publications in this series, see our post regarding the launch of the carnival. If you are interested in submitting an blog post or article to this carnival, see our submission page on the Blog Carnival website. This carnival is published weekly, and we are always interested in seeing new material.

The intent of this publication is an ongoing investigation of the progress and potential of renewable energy in our world. Our goal is to collect the best writing and news on the subject of renewable energy projects and policies. We have observed that humanity is innovating rapidly as the energy security of the future becomes a global priority.