Month: December 2017

1.6 Power of water: Hydropower

Hydropower is the oldest source of renewable energy that the mankind has been using to generate electricity in large-scale. It has a long history as a source of energy. It was used by ancient people for irrigation and to power some mechanical systems such as sawmills and flour mills. In the late 19^thcentury, people realized that the kinetic energy of falling water could be coupled with a turbine and a generator to generate electricity. Thereby, world’s first hydropower plant was opened in 1882 on the Fox River in Appleton in the United States [1].

Unlike other renewable energy sources we discussed in previous articles, hydropower or power of water is widely being used around the globe and is the second largest contributor to the today’s renewable energy industry (Largest contributor is traditional biomass). It is a proven way of replacing the role of fossil fuels in electricity generation. One clear example would be Bhutan which is a carbon negative country thanks to its vast amount of hydropower available. Norway is another green country generating 98.9% of its electricity using hydropower [2]. In addition, many other countries are generating substantial amount of hydroelectricity.

Origin of hydropower

It is needless to say that hydropower entirely relies on continuous supply of water from an elevated position. Water falling from an elevated position turns a turbine installed at a lower position. The electrical generator attached to the turbine thereby generates electricity to the rhythm of the water flows through the turbine.

Water enters the system at a higher position, deliver its kinetic energy to a turbine and exits the system at a lower position.

To keep the plant running (generating electricity), water should be continuously running through the turbine.  In other words, water should be directed back to the higher position from the lower level. This is naturally done by the Sun in a process called “water cycle” or “Hydrologic cycle”.

How does it work?

The Sun’s heat evaporates water from the oceans, rivers, lakes, trees, soil and all the water bodies. Evaporated water molecules or Water vapor moves up and form clouds in the sky. Water molecules in clouds then cool and condense into raindrops or snow once the temperature reaches the dew point and fall back to the ground as precipitation. This process is a never-ending, cyclic natural process and therefore, is referred to as “water cycle”.

During the complete process, water is brought from lower levels to higher levels and thereby water stores Sun’s energy in the form of potential energy. Therefore, hydropower is an indirect source solar energy and is a renewable energy source.

Water cycle is the sole driving force of hydropower and ensures the continuous supply of water by which water is repeatedly cycled through the turbines.

Hydropower: General background

A typical hydropower plant consists of a dam (not necessary for some cases), a turbine, an electrical generator and transmission lines.

First, the kinetic energy of falling water is converted into mechanical energy of turbines. An electrical generator attached to the turbine converts the mechanical energy of the turbine into electricity which is then distributed through power transmission lines.

Hydropower plants are often classified according to their operational principle as below.

Storage hydropower

Run of river

Run of river (ROR) represents a type of hydropower which typically requires no or little water storage. ROR hydropower plants are extremely intermittent as they are not regulated by large storage or dams. However, they could be built with no or minimal alteration to the surrounding.  In particular, environmental impacts of ROR hydropower plants are insignificant compared to conventional hydropower plants which require large-scale water storage, dams, onsite and nearby alterations.

Pumped storage hydropower

Pumped-storage hydropower as its name suggests is a type of energy storage. These hydro power plants pump water to a higher elevation from a lower elevation during off-peak hours and thereby store energy surplus in the form of potential energy. During the peak hours, pumped water is released through a turbine to generate electricity.

The capacity of hydropower plants may range from few kilo watts to several Giga Watts. Depending on the size of power capacity, hydropower plants are categorized into several groups as mentioned below [3, 4].

Large hydro

Plants having capacity of 100 MW or more.

The worldwide technically accessible hydropower capacity has been estimated to be 2200 GW. However, only 25% of this figure is currently being used for electricity generation.

Medium Hydro

Plants having the capacity in the range of 25 MW-100 MW

Small Hydro

Plants having the capacity in the range of 1 MW-25MW

Mini Hydro

Plants having the capacity in the range of 100 kW-MW

Micro Hydro

Plants having the capacity in the range of 5 kW-100 kW

Pico Hydro

Plants having the capacity of 5 kW or less

Some important facts

• Hydropower provides 2.2% of the world’s primary energy demand [5]
• Hydropower represents the third largest single contributor to the global electricity generation and it accounted for 17.9% in 2007 [5]
• World’s cumulative hydropower capacity reached 970 GW in 2011 [6]
• Paraguay, Venezuela, Brazil, China, Canada, United States of America, and India are some of the countries having large-scale hydropower plants.
• Norway, Brazil, Bhutan and Venezuela heavily rely on hydropower to generate electricity
• World’s largest hydroelectric project, Three Gorges Dam in China has been designed to generate 22500 MW of power [7] which is 5 times higher than the total installed capacity in Sri Lanka (The cumulative power generation capacity of grid-connected power plants in Sri Lanka was 3,887 MW in 2016 according to the public utilities commission of Sri Lanka) [7].
• World’s second largest hydroelectric project, Itaipu (Paraguay and Brazil) generates 14,000 MW of power [8]

Unlike other energy sources we discussed in previous articles, hydropower can play a vital role in the renewable energy industry, especially in supply-demand management. In next article, we will further discuss hydropower in detail.

1.5 Osmosis power

We have already discussed four major forms of ocean energy. All of them were well-known sources. Except ocean thermal energy, other three types represent kinetic energy of water in the ocean.  In this article, we will focus on another form of ocean energy: It is osmosis power!

It does not represent kinetic energy of water in the ocean and therefore, nobody can identify its existence with the naked eyes.

It is sort of a latent energy and majority of the people have never heard of it!

What is osmosis power?

Osmosis power relies on the difference in salt concentrations (salinity gradient) between two water bodies which meet at the same place. Usually, brackish or brine water and fresh river water are the two resources of choice.

The places where rivers meet the ocean are ideal for osmosis power plants.

As we know it, brine water is extremely saltier than fresh river water and therefore, there is a sizeable difference in the salinity of water where a river meets the ocean. When fresh river water mixes up with brine water it loses its osmotic pressure where a significant amount of energy is lost. This untapped energy could be harnessed to generate electricity. Osmosis power plants are sometimes known as salinity gradient power plants since this technology relies on the difference in the salinity (salinity gradient) of two water bodies.

Osmosis power attracted much attention during the energy crisis in the 1970s. But the perseverance largely waned with the falling oil price that happened in the early 1980s.

Osmosis power (blue energy) is still under its development stage and is an infant among all other forms of oceanic energy.

A typical osmosis power plant makes use of the osmosis pressure difference between two water bodies having different salt concentrations. They transform the osmosis pressure into a hydraulic pressure which is then used to turn a turbine attached to a generator.

Several technologies such as pressure retarded osmosis, reversed electrolysis and capacitive method have been already developed to generate electricity from osmosis pressure. However, none of the technology is uneconomically viable, unfortunately. It is still at its development stage.

Technology

Usually, an osmosis power plant employs semi permeable membrane by which water having two different salt concentrations are separated. The semi permeable membrane allows water molecules to pass through it. But it does not allow ions to go through it. It is, of course, an unfair treatment!!!

But plays an essential role in osmosis power harvesting.

Say, you have fixed a semi permeable membrane in a storage tank as shown in the above figures. The membrane separates the tank into two parts.

We know that chemistry of materials always tries to equalize the ion concentrations of two materials in contact via a natural process called diffusion. Ions flow from the high-concentrated area to the low-concentrated area and eventually, ion concentrations in both sides become equal. But as mentioned earlier, semipermeable membranes used in osmosis power plants do not allow ions to pass through them but allow water molecules.

The consequence is amazing!

Say, you fill both chambers of the tank with the same liquid as shown in figure 1. You would see no change in the liquid levels with time.

But… What will happen if we fill two chambers with two different liquids (Liquids with two different concentrations)?

You may probably expect no change in the liquid levels with time though concentrations are different. If so, you are wrong.

The actual result definitely would surprise you!

Take a look at figure 2. It illustrates what happens if two partitions are filled with liquids having two different concentrations.

Isn’t it astonishing to see?

How does it happen?

This unusual effect is a result of the diffusion. As mentioned earlier, diffusion of matter always tries to balance the ion concentrations of two liquids in contact. This is done by transferring the ions from high concentrated are to the low-concentrated area. But now we have installed a semi permeable membrane. It prevents the transition of ions from one chamber to the other. However, the same semi permeable membrane permits the water molecules to pass through it. Consequently, the membrane promotes the transition of water molecules from low-concentrated to high concentrated. This process reduces ion concentration in the high concentrated area while increasing concentration in the low-concentrated area. Eventually, the ion concentrations in both areas become equal.

You cannot observe the changes in ion concentrations in either side with your naked eye. But there is a curious, physically observable effect. It is nothing but what you can see in figure 2.

An increase in the water level at the high-concentrated area and a drop in the water level at             low-concentrated area!

Obviously, it must occur since the semi membrane prevents the transition of ions but not water molecules. This process ultimately leads to a gap between the water levels (See figure 2).

A gap between the water (liquid) levels means a hydraulic pressure!

Osmosis power plants use this hydraulic pressure to turn a turbine and generate electricity.

Some important facts

However, none of the present-day technologies offers economically practicable opportunity to harness energy from this renewable source that has concealed in water bodies.

Hopefully, it would become an economically feasible in near future at least in highly saltier areas such as the Dead Sea, red sea and Atlantic Ocean [1, 2].

The worldwide osmosis power potential is approximately 2.6 TW [3]. It is higher than the worldwide wave power potential (2 TW) [4]. But it represents only 15% of the current global energy demand even at its full capacity and would further reduce to 4.1% by 2100 (Note that the global power demand would be 63 TW by 2100) [5].