Renewable energy resources

  Renewable Energy : Basics


Renewable energy is energy which is generated from natural sources i.e. sun, wind, rain, tides and can be generated again and again as and when required. These sources are called renewable because they can be renewed or can be replenished in short duration of time.

They are also known as non-conventional energy resources because they are new to the conventional practice of using fossil fuels.

For eg: Energy that we receive from the sun can be used to generate electricity. Similarly, energy from wind, geothermal, biomass from plants, tides can be used this form of energy to another form.

For example:

  • Wind Energy
  • Solar Energy
  • Hydro-Power
  • Biomass Energy
  • Geothermal Energy etc

 

 

  Wind Energy / Wind Power


Wind is air in motion. Wind is a form of solar energy. This statement could be little weird but its true; reason beings winds are caused by the uneven heating of the atmosphere by the sun, the irregularities of the earth's surface, and rotation of the earth.

The concept has been conventionally used in India for pumping water and grinding grains in windy areas of Punjab; but has evolved with a more technical perspective with increased efficiency of motor in energy conversion  

The terms "wind energy" or "wind power" describe the process by which the wind is used to generate mechanical power or electricity. Wind turbines convert the kinetic energy in the wind into mechanical power. The turbines are placed at great height to get maximum winds. Anemometer (instrument used for measuring wind speed) and wind vane (instrument used for measuring wind direction) are installed in wind mills.

Energy Generation: Total energy generated from the wind turbine/wind mill depends on various parameters; i.e. wind speed, size of blades, density of air etc.

Start-up Speed - the wind turbine need some initial wind speed to rotate its blade by overcoming its heavy weight. This is the speed at which the rotor and blade assembly begins to rotate. But remember this speed is not enough to generate electricity from the wind turbine.

Cut-in Speed - Cut-in speed is the minimum wind speed at which the wind turbine will generate usable power. It is generally 3 – 4.5 m/s.

Cut-out wind speed is a safety feature of wind turbine when the turbine automatically reduces speed to prevent any type of accident in case of strong winds. The speed is generally 30-35 m/s. The turbine shut downs at a wind speed of approximately 40-45m/s.

As the diagram also indicates, the maximum energy output from the wind mill can be attained at a consistent wind speed of 25-30 m/s.

Formula –

P = 13.14 × 10–6 A V 3 KW

  • Where; A is the swept area in sq. meter (area between the blades)
  • V the wind velocity in Km/hr
  • P is the power in Kilo Watt

Parts of Wind Mills

There are four main parts to a wind turbine:

  • the base
  • tower
  • nacelle/Shaft
  • Rotor blades

 The blades capture the wind's energy, spinning a generator in the nacelle. The tower contains the electrical conduits, supports the nacelle, and provides access to the nacelle for maintenance. The base, made of concrete and steel, supports the whole structure.

 

Type of Wind Turbines

  • Horizontal Axis Wind Turbine (HAWT)
  • Vertical Axis Wind Turbine (VAWT)

 

Horizontal Axis Wind Turbine (HAWT)

The horizontal axis wind turbines are like the traditional farm windmills used for pumping water. The horizontal wind turbine is a turbine in which the axis of the rotor's rotation is parallel to the wind stream and the ground

HAWT can be further categorized into :-

  • upwind wind turbine
  • downwind wind turbine

 

 

In Upwind HAWT the wind direction is from the front side of the nacelle; however the direction of wind is from the backside of the nacelle in the downwind HAWT.

 Advantages of HAWT:  

  • most of the HAWTs are self-starting
  • can be cheaper because of higher production volume

Disadvantages of HAWT:

  • it has difficulties operating near the ground
  • the tall towers and long blades are hard to transport
  • high cost of construction and maintenance
  • threat to birds and avian life

 

Vertical Axis Wind Turbine (VAWT)

The vertical axis wind turbine is generally of shape like eggbeater. They are often known as Darrieus model on name of the scientist. The turbines are vertically installed on the ground in small hill top or any other location with winds.

Unlike the HAWT, the rotor of the VAWT rotates vertically around its axis instead of horizontal movement. The VAWT are not as efficient as a HAWT, but it does offer benefits in low wind situations wherein HAWTs have a hard time operating.

VAWT are easier and safer to build, and it can be mounted close to the ground and handle turbulence better than the HAWT. The maximum efficiency that HAWT can attain  is only 30%, therefore it is usually installed for private use.

Advantages of VAWT

  • VAWT components are placed nearer to the ground, it has an easier access to maintenance
  • smaller cost of production, installation, and transport
  • VAWTs are suitable in places like hilltops, ridgelines and passes
  • blades spin at a lower velocity, thus, lessening the chances of bird injury
  • suitable for areas with extreme weather conditions like mountains

Disadvantages of VAWT:

  • efficiency of VAWT is less then HAWT
  • air flow near the ground and other objects can create a turbulent flow, introducing issues of vibration
  • problem of noise when installed near to human settlements

 

Wind Energy: Indian Scenario

The development of wind power in India began in the 1990s, and has significantly increased in the last few years. Although we are relative newcomer to the wind industry compared with Denmark or the United States. India has the fifth largest installed wind power capacity in the world

Wind mill have generally efficiency ranging from 40 – 59%. This is one of the best among other energy resources.

Wind energy potential in India (2015) is estimated to be – 302 GW (Giga watt)

As on 30 Nov 2015 the installed capacity of wind power in India was 24,759 MW

Point to remember:-

  • You need to remember the unit conversion as we will be talking in MW
  • 1 GW = 1000 MW
  • 1 MW = 1000 KW
  • There is always a difference between potential and actual installed capacity. Potential is estimated based on expected energy due to availability of specific location and environmental conditions; however installed capacity is the actual wind turbines already installed which is generating electricity.

 Wind power presently accounts nearly 8.5% of India's total installed power generation capacity.

 

State wise wind power generation in India

Tamilnadu ranks first in wind power generation in India followed by Maharashtra and Gujarat. Rajasthan is also leading towards this energy resource ranking itself at 4th position as per data from MNRE, 2014.

Andhra Pradesh and Madhya Pradesh are also gaining momentum in wind power to reduce their dependence on non-renewable energy and in erg to make clean India by clean energy resources.

 

 


Point to remember:-

Before installing any wind turbine it is very important to conduct detailed meteorological studies at the site. This requires making wind rose diagram for the site.

Wind rose is defined as graphical representation of wind speed and direction of a particular location. Generally data from Indian Meteorological Department (IMD) is used to make a wind rose. The IMD data used should be within 80km vicinity of the site.

The figure will give you a view about how a wind rose looks like. The details of this topic will be studies in air dynamics/EIA tutorial.

Advantages of wind power:

  • Eco- friendly resource
  • Wind turbines take up less space than the average thermal power station
  • The wind is free, and we are able to cash in on this free source of energy
  • Wind turbines are a great resource to generate energy in remote locations, such as mountain communities and remote countryside
  • When combined with solar electricity, this energy source is great for developed and developing countries to provide a steady, reliable supply of electricity

Disadvantages of wind power:

  • Unreliability factor. In many areas, the winds strength is too low to support a wind turbine or wind farm
  • Wind turbines generally produce allot less electricity than the average fossil fuelled power station, requiring multiple wind turbines to be built in order to make an impact
  • Wind turbine construction can be very expensive and costly to surrounding wildlife during the build process
  • The noise pollution from commercial wind turbines is sometimes similar to a small jet engine.

 

  Solar Energy / Solar Power


We discussed in previous unit that why sun is said to be the ultimate and major source of energy. Apart from originating many other sources of energy for us, sun itself has enormous amount of energy which it generate from nuclear fusion reaction taking place in it. The picture clearly explains that sun can annually produce approximately 23,000 TW of energy if we can capture its all energy; this is much higher than the world annual energy requirement. However, it is practically impossible to do that therefore we look for technologies that can trap the maximum amount of sun’s energy.

Solar energy harnessing potential depends on geographical location on earth which decides the amount of sunlight that reaches and also on the technology and its efficiency. Various technologies are available to convert solar energy into usable forms.

The technologies can be broadly divided into two main categories depending on the way they capture and distribute solar energy or convert it into solar power:

Passive Solar Energy: technology is said to be passive when we don’t use any addition instrument/sensor/device to capture solar radiation, however make the best use of available radiation. For example solar energy can be utilized passively by orienting a building to the Sun, selecting favourable materials, light dispersing properties, and designing spaces that naturally circulate air.

Active Solar Energy: the technology is said to be active when we use some device/sensor to collect the solar radiation for use. For example photovoltaic systemsconcentrated solar power and solar water heating.

Point to remember:-

The term active and passive is variedly used in science. Any technology is said to be passive when we don’t give any addition device/instrument/sensor to capture energy from source and active when we use some device. The best example to understand this is remote sensing, if the sensor of your satellite give radiation to target to capture some image it is called active sensor; however when it do not give radiation and use the background light to capture the image it is called passive sensor.

Components of Solar Energy Devices

Due to the nature of solar energy, two components are required to have a functional solar energy generator; collector and storage unit

Solar Collector

The collector simply collects the radiation that falls on it and converts a fraction of it to other forms of energy (either electricity and heat or heat alone). Collectors can be of different types :

  1. Flat plate collector
  2. Solar panel
  3. Parabolic collector
  4. Solar heating tubes

 

Storage unit

The storage unit can hold the excess energy produced during the periods of maximum productivity (day time), and release it when the productivity drops (night hours).

Solar Energy Technologies

Solar energy technologies can be broadly divided into four main categories which are generally used to capture solar power:

  • Solar Photovoltaic Technology - These technologies convert sunlight directly into electricity to power homes and businesses
  • Concentrating Solar Power - These technologies harness heat from the sun to provide electricity for large power stations
  • Solar water heaters - These technologies harness heat from the sun to provide hot water for homes and businesses
  • Passive Solar Technology - These technologies harness heat from the sun to warm our homes and businesses in winter

 

  Solar Photovoltaic Technology


Photovoltaic (PV) materials and devices convert sunlight into electrical energy. The devices are called PV cell (PVC) and are commonly known as solar cells.

Working of Solar Cell

The energy of the absorbed sunlight is transferred to electrons in the atoms of the PV cell which are made up of semiconductor material  example Silicon (Si). With their newfound energy, these electrons escape from their normal positions in the atoms and become part of the electrical flow, or current, in an electrical circuit. A special electrical property of the PV cell or we must say semiconductor—what is called a "built-in electric field"—provides the force, or voltage, needed to drive the current through an external load, such as a light bulb.

A typical home will use about 10 to 20 solar panels to power the home.

The panels are mounted at a fixed angle facing south, or they can be mounted on a tracking device that follows the sun, allowing them to capture most of the sunlight. Many solar cells combine to form a solar module; and module combine together to create one system called a solar array.

The efficiency of solar panel is less than 14%

 

Research & Advancement in solar cells

Traditional solar cells were made from silicon, usually flat-plate, and generally are the most efficient.

Second-generation solar cells are called thin-film solar cells because they are made from amorphous silicon or non-silicon materials such as cadmium telluride. Thin film solar cells use layers of semiconductor materials only a few micrometers thick. Because of their flexibility, thin film solar cells can double as rooftop shingles and tiles, building facades, or the glazing for skylights.

Third-generation solar cells are being made from a variety of new materials besides silicon, including solar inks using conventional printing press technologies, solar dyes, and conductive plastics. Some new solar cells use plastic lenses or mirrors to concentrate sunlight onto a very small piece of high efficiency PV material.

Concentrating Solar Power (CSP)

As the name indicates, the solar power is concentrated at one place for generating electricity. Remember the game or experiment we all use to do in childhood; burning a paper by focussing sunlight with the help of a lens. This clearly indicates the concentration of energy at one point.

CSP uses mirrors and lenses to reflect and concentrate sunlight onto receivers that collect solar energy and convert it to heat. This thermal energy can then be used to produce electricity via a steam turbine or heat engine that drives a generator.

CSP plants produce power by first using mirrors to focus sunlight to heat a working fluid (working fluid are better than water due to high heat exchange capacity; anyhow water can also be used as heat exchanger). Ultimately, this high-temperature fluid is used to spin a turbine or power an engine that drives a generator. The final product is electricity.

CSP efficiency is around 20%

Types of CSP

  • Linear concentrator
  • Dish Sterling/ Dish Engine
  • Solar Power tower systems

Linear concentrator

Linear concentrator systems collect the sun's energy using long rectangular, curved (U-shaped) mirrors. The mirrors are tilted toward the sun, focusing sunlight on tubes (or receivers) that run with the length of the mirrors.

The reflected sunlight heats a fluid flowing through the tubes. The hot fluid then is used to boil water in a conventional steam-turbine generator to produce electricity.

There are two major types of linear concentrator systems: parabolic trough systems, where receiver tubes are positioned along the focal line of each parabolic mirror; and Linear Fresnel reflector systems, where one receiver tube is positioned above several mirrors to allow the mirrors greater mobility in tracking the sun

Dish Sterling/ Dish Engine

A dish/engine system uses a mirrored dish similar to a very large satellite dish, although to minimize costs, the mirrored dish is usually composed of many smaller flat mirrors formed into a dish shape.

The dish-shaped surface directs and concentrates sunlight onto a thermal receiver, which absorbs and collects the heat and transfers it to the engine generator.

The most common type of heat engine used today in dish/engine systems is the Stirling engine. This system uses the fluid heated by the receiver to move pistons and create mechanical power. The mechanical power is then used to run a generator or alternator to produce electricity

Solar Power Tower systems

A power tower system uses a large field of flat, sun-tracking mirrors known as heliostats to focus and concentrate sunlight onto a receiver on the top of a tower. A heat-transfer fluid heated in the receiver is used to generate steam, which, in turn, is used in a conventional turbine generator to produce electricity. Some power towers use water as the heat-transfer fluid. Other advanced designs are experimenting with working fluids like molten nitrate salt because of its superior heat-transfer and energy-storage capabilities. (liquid sodium, 40% potassium nitrate, 60% sodium nitrate as the working fluids)

The energy-storage capability, or thermal storage, allows the system to continue to dispatch electricity during cloudy weather or at night.

Point to remember:-

Any energy/heat when need to convert into electricity; needs to move a turbine means needs to do some mechanical work. Most widely used technology is steam engine; thus the heat trapped in working fluid is used to heat water which makes steam that runs turbine to do mechanical work to generate electricity (as in coal fired thermal power plants). However, working fluid can also be directly used to spin a turbine, but the technique is rarely used due to technical difficulties.

Solar water heaters

Solar water heaters use the sun’s heat to provide hot water for a home or building.  Most solar water heating systems have two main parts: a solar collector and a well-insulated storage tank.

Solar collectors absorb solar radiation (heat) and transfer the heat to potable water.  There are two main types of collectors for residential and commercial applications: flat plate and evacuated-tube. 

Flat Plate collection devices provide a large flat surface area (black in colour to absorb maximum sunlight) to capture the sunlight which heats water inside the collector; this hot water is then stored in an insulated storage tank for its usage.

 

Evacuated Tubes are long hollow tubes black in colour with minimum energy loss to heat the water in the tube. This water can be stored in insulated tanks for usage in residence or work place.

Passive solar technologies

Passive solar design uses a building’s windows, walls, and floors to collect, store, and distribute solar energy in the form of heat.  Conversely, in summer, passive solar design rejects heat.  For example, buildings designed for passive solar heating usually have large south-facing windows (the south face receives the most sunlight).  Materials that absorb and store the sun’s heat can be built into the sunlit floors and walls, which will then absorb heat during the day and slowly release the heat at night.

Space Heating

Solar ventilation system can preheat the air, saving both energy and money. This type of system typically uses a transpired collector, which consists of a thin, black metal panel mounted on a south-facing wall to absorb the sun's heat. Air passes through the many small holes in the panel. A space behind the perforated wall allows the air streams from the holes to mix together. The heated air is then sucked out from the top of the space into the ventilation system

Space Cooling

Space cooling can be accomplished using thermally activated cooling systems (TACS) driven by solar energy. Because of a high initial cost, TACS are not widespread.

Solar absorption systems - use thermal energy to evaporate a refrigerant fluid to cool the air

Solar desiccant systems - use thermal energy to regenerate desiccants that dry the air, thereby cooling the air.

  Solar Water Treatment


Solar stillSolar water disinfectionSolar desalination and Solar Powered Desalination Unit

Solar distillation (Solar Still) can be used to make saline or brackish water potable.

  • Solar water disinfection (SODIS) involves exposing water-filled plastic polyethylene tetraphathalate (PET) bottles to sunlight for several hours. Exposure times vary depending on weather and climate from a minimum of six hours to two days during fully overcast conditions. It is recommended by the World Health Organizationas a viable method for household water treatment and safe storage.
  • Exposure to sunlight has been shown to deactivate diarrhea-causing organisms in polluted drinking water. Three effects:
  • UV-A interferes directly with the metabolism and destroys cell structures of bacteria.
  • UV-A (wavelength 320–400  nm) reacts with oxygen dissolved in the water and produces highly reactive forms of oxygen (oxygen free radicals and hydrogen peroxides) that are believed to also damage pathogens.
  • Cumulative solar energy (including the infrared radiationcomponent) heats the water. If the water temperatures rises above 50 °C (122 °F), the disinfection process is three times faster

Solar energy may be used in a water stabilisation pond to treat waste water without chemicals or electricity.

  Solar Ponds


A solar pond is a pool of saltwater which acts as a large-scale solar thermal energy collector with integral heat storage for supplying thermal energy. The saltwater naturally forms a vertical salinity gradient also known as a "halocline", in which low-salinity water floats on top of high-salinity water. The layers of salt solutions increase in concentration (and therefore density) with depth. Below a certain depth, the solution has a uniformly high salt concentration.

There are 3 distinct layers of water in the pond:

  • The top layer, which has a low salt content
  • An intermediate insulating layer with a salt gradient, which establishes a density gradient that prevents heat exchange by natural convection
  • The bottom layer, which has a high salt content

 

When solar energy is absorbed in the water, its temperature increases, causing thermal expansion and reduced density. If the water were fresh, the low-density warm water would float to the surface, causing a convection current. The temperature gradient alone causes a density gradient that decreases with depth. However the salinity gradient forms a density gradient that increases with depth, and this counteracts the temperature gradient, thus preventing heat in the lower layers from moving upwards by convection and leaving the pond. This means that the temperature at the bottom of the pond will rise to over 90 °C while the temperature at the top of the pond is usually around 30 °C.

This variation in temperature is used to harness energy, the hotwater can be used for generating electricity or for other heating purposes.

Advantages & Disadvantages of solar pond

  • Solar ponds are attractive for rural areas in developing countries. Very large area collectors can be set up for just the cost of the clay or plastic pond liner
  • The evaporated surface water needs to be constantly replenished
  • The accumulating saltcrystals have to be removed and can be both a valuable by-product and a maintenance expense
  • No need of a separate collector for this thermal storage system
  • Efficiency 17-20%

 

  Solar Power: Indian Scenario


National Institute of Solar Energy, India determined India’s solar potential to be 750 GW (MNRE). India has presently 1.4 GW of installed capacity and has a target to have 20 GW by 2022.  Among the Indian States, Gujarat is leading in solar energy potential followed by Rajasthan and Madhya Pradesh.

 

The map shows the range of MW solar energy production in India among various States. The subsidy given by Indian Government in solar energy sector is attracting many entrepreneurs and commercial operators in the area.

 

  Geothermal Energy


Geothermal energy means the heat (thermal) from the earth (geo). The geothermal energy of the Earth's crust originates from the original formation of the planet i.e. crust, mantle and hot core (20%) and from radioactive decay of materials (80%)

Geothermal energy is generated in the earth's core. Temperatures hotter than the sun's surface are continuously produced inside the earth by the slow decay of radioactive particles, a process that happens in all rocks.

The important role here is played by geothermal gradient. The geothermal gradient is the difference in temperature between the core of the planet and its surface; this drives a continuous conduction of thermal energy in the form of heat from the core to the surface.

Geothermal energy finds its way to the earth's surface in three ways:

  • Volcanoes and fumaroles (holes where volcanic gases are released)
  • Hot springs
  • Geysers

 Energy resources:

The geothermal energy is exploited by us in two forms; hot water and steam reservoirs. They are present deep in the earth that is accessed by drilling wells known as geothermal wells/reservoir.

Based on the availability of resource the reservoir is drilled in the site and technology is decided. Most of the wells provide very hot water which is generally used for energy conversions. Geothermal wells are drilled into a geothermal reservoir; the wells bring the geothermal hot water to the surface, where its heat energy is converted into electricity at a geothermal power plant which is generally closely located to the reservoir to avoid the loss of heat.

Geothermal heat is used directly for heating purposes, without involving a power plant or a heat pump, for a variety of applications such as space heating and cooling, food preparation, hot spring etc.

Geothermal power is cost effective, reliable, sustainable, and environmentally friendly, but has historically been limited to areas near tectonic plate boundaries. On exploration, geothermal wells release greenhouse gases trapped deep within the earth, but these emissions are much lower per energy unit than those of fossil fuels.

 Types Geothermal Power Plant

There are three types of geothermal power plants:

  • Dry steam geothermal power plant
  • Flash steam geothermal power plant
  • Binary cycle geothermal power plant
  • Hybrid geothermal power plant

 

Dry steam geothermal power plants

The name can give you a clear indication; the geothermal plant which only give steam but no hot water. Usually geysers are the main source of dry steam geothermal power plants. Such geothermal reservoirs which mostly produce steam and little water are used in electricity production systems.

As steam from the reservoir shoots out, it is used to rotate a turbine, after sending the steam through a rock-catcher. The rock-catcher protects the turbine from rocks which come along with the steam. The rotation of turbine thus rotates a conductor in magnetic field which in turns generates electricity.

 

The hot water after rotating a turbine can either be used for heating purposes or can be send to cooling tower to reduce temperature and can further be injected to the reservoir through injection well.

 Flash steam geothermal power plants

 Hot stream of water is availed by this type of reservoir thus it is called a flash steam power plant. The extremely hot water from drill holes when released from the deep reservoirs in high pressure steam (termed as flashed steam) is released. This force of steam is used to rotate turbines. The steam gets condensed and is converted into water again, which is returned to the reservoir. Flashed steam plants are widely distributed throughout the world. They are the most common geothermal power plants. They use geothermal reservoirs of water with temperatures greater than 182°C.

 Binary cycle geothermal power plants

 The binary cycle geothermal power plants operate on water at lower temperatures of about 107°-182°C. As the temperature of water is lower than flash steam plants; thus working fluids is used for efficient utilization of heat rather than water. These plants use the heat from the hot water to boil a working fluid i.e. isobutene, iso-pentane or ammonia–water mixture present in an adjacent, separate pipe. Due to this double-liquid heat exchanger system, it is called a binary power plant.

 

The working fluid is vaporized in a heat exchanger and used to turn a turbine. The water is then injected back into the ground to be reheated. The water and the working fluid are kept separated during the whole process, so there are little or no air emissions.

 Hybrid geothermal power plant

These plants are not much used commercially and are in infant stages of research and development. These systems use flash and binary system together to harness energy.  Therefore, the system uses working fluids for water even at high temperature to harness maximum output from the plant.

The efficiency of geothermal power plants ranges from 10 – 13% in electricity production.

Point to remember:-

Working fluids used for heat exchange are generally chemicals which have low heat of vaporization than water; it means they get converted into vapours at lower temperature than water. If water boils at 1000C than iso-pentane boils at 280C thus will get converted into vapour earlier than water and can spin the turbine for energy generation. However, water is always preferred over other chemicals if at higher temperature.

 Geothermal Energy: Potential in India

 India has reasonably good potential for geothermal energy. As we discussed earlier that geothermal energy resources are only limited to places which tectonic plate boundaries; thus the potential sites for this energy are limited.

In India, the potential geothermal provinces can produce around 10,600 MW of power but experts are practically confident only to the extent of 100 MW of energy.

Seven potential sites (located in map) for geothermal energy marked in India are:

  • Puga Valley, Jammu & Kashmir
  • Tatapani, Chhattisgarh
  • Godavari Basin Manikaran, Himachal Pradesh
  • Bakreshwar, West Bengal
  • Tuwa, Gujarat
  • Unai, Maharashtra and
  • Jalgaon, Maharashtra

  Biomass Energy: Basics


Biomass energy refers to the energy generated from plant biomass. The energy can be used in various forms like solid fuel like firewood; liquid fuel like bio ethanol/biodiesel; and gases fuel as biogas.

Plants produce their food by the process of photosynthesis. Through photosynthesis plants convert sunlight (solar energy) into biomass (chemical energy). This biomass is stored in plant body and can be converted into:

  • Fuel
  • Electricity
  • Heat
  • Fertilizer

Biomass is biological material derived from living, or recently living organisms. In the context of biomass for energy this is often used to mean plant based material, but biomass can equally apply to both animal and vegetable derived material.

Biomass resources include any plant-derived organic matter that is available on a renewable basis. These materials are commonly referred to as feedstocks. Biomass is available everywhere in the world. It is considered to be a good biomass if it produce high dry yield in minimum land use. Biomass is considered to be renewable and carbon neutral source of energy (if harvested sustainably)

Types of Feedstock

Virgin wood

Virgin wood consists of wood and other products such as bark and sawdust which have had no chemical treatments or finishes applied

Energy crops

Energy crops are grown specifically for use as fuel and offer high output per hectare with low inputs.

Classes of energy crops

Short rotation woody energy crops - Short-rotation woody cropsare fast-growing hardwood trees that are harvested within 5 to 8 years of planting. These include hybrid poplar, hybrid willows, salix etc.


Short rotation coppice- Some fast growing tree species can be cut down to a low stump (or stool) when they are dormant in winter and go on to produce many new stems in the following growing season.  Example Poplar, willow etc

Short rotation forestry- Short rotation forestry (SRF) consists of planting a site and then felling the trees when they have reached a size of typically 10-20cm diameter at breast height

Grasses and non-woody energy crops - Herbaceous energy crops are perennials that are harvested annually after taking 2 to 3 years to reach full productivity. Grasses, bamboo, Miscanthus , Phragmitis etc.


Agricultural energy crops - They can be used either simply as biomass or to provide a specific product for a particular energy application. However, these plants require more intensive management than other energy crops. The basic three component that make crops a good energy source are:

 

Aquatics (hydroponics) - Aquatic plants offer a number of potential advantages over land based crops. Seaweeds and micro/macro algae. They offer lot of benefit over land based plant because of their fast growth and minimum management. As the water provides support, they can also usually take in nutrients and carbon dioxide from the surrounding water and consequently may not need to develop roots.  Many, therefore, can display very high photosynthetic efficiencies. 

As they do not require soil, they can be grown in areas unsuitable for conventional agriculture. 

Agricultural residues - Agricultural residues are of a wide variety of types, and the most appropriate energy conversion technologies and handling protocols vary from type to type.  The most significant division is between those residues that are predominantly dry (such as straw) and those that are wet (such as animal slurry). Arable crop residues such as straw or husks, Animal manures and slurries, Animal bedding such as poultry litter, Most organic material from excess production or insufficient market, such as grass silage.

Food waste - There are residues and waste at all points in the food supply chain from initial production, through processing, handling and distributions to post-consumer waste from hotels, restaurants and individual houses.

Industrial waste and co-products from manufacturing and industrial processes - Many industrial processes and manufacturing operations produce residues, waste or co-products that can potentially be used or converted to biomass fuel.  These can be divided into woody materials and non-woody materials. Paper pulp waste, textile waste etc

  Biomass Energy Conversion Technology


Conversion Technologies

There are a number of technological options available to make use of a wide variety of biomass types as a renewable energy source.

Conversion technologies may release the energy directly, in the form of heat or electricity, or may convert it to another form, such as liquid biofuel or combustible biogas. While for some classes of biomass resource there may be a number of usage options, for others there may only one appropriate technology. Three basic categorization of technology are:

  • Thermal conversion
  • Chemical conversion
  • Biochemical conversion

Thermal Conversion

These are processes in which heat is the dominant mechanism to convert the biomass into another chemical form. The basic alternatives are separated principally by the extent to which the chemical reactions involved are allowed to proceed:

  • Combustion
  • Gasification
  • Pyrolysis

Chemical Conversion

Chemical conversion involves use of chemical agents to convert biomass into liquid fuels. A range of chemical processes may be used to convert biomass into other forms, such as to produce a fuel that is more conveniently used, transported or stored, or to exploit some property of the process itself. We will be taking this section in brief just to focus on common technologies used in India.

Biochemical Conversion

As biomass is a natural material, many highly efficient biochemical processes have developed in nature to break down the molecules of which biomass is composed, and many of these biochemical conversion processes can be harnessed. Biochemical conversion makes use of the enzymes of bacteria and other micro-organisms to break down biomass. In most cases micro-organisms are used to perform the conversion process:

Thermal Conversion Process

There are various application of thermal conversion process like:

Combined heat and power (CHP) - Cogeneration

Combined heat and power (CHP), or co-generation, is the simultaneous generation of electricity and heat. Tri-generation is a further extension to include a refrigeration process for air conditioning as well.

Co-firing

Co-firing is the process of replacing part of the fossil fuel supplied to a power station or boiler with a 'carbon lean' renewable alternative. Usually it is used to refer to the use of solid biomass within coal fired power stations.

Combustion

Combustion is the process with which everyone is familiar by which flammable materials are allowed to burn in the presence of air or oxygen with the release of heat. Combustion is the simplest method by which biomass can be used for energy, and has been used for millennia to provide heat.  This heat can itself be used in a number of ways:

  • Space heating
  • Water (or other fluid) heating for central or district heating or process heat
  • Steam raising for electricity generation or motive force.

All carbohydrates, such as cellulose, also contain oxygen in the molecular structure. Other atoms potentially found in biomass includeNitrogen (N), Phosphorus (P), Potassium (K), Silicon (Si) and Sulphur (S). Other trace elements, such as some heavy and alkali metals may also be present in both virgin biomass and in wastes and co-products either from uptake from the soil or air during growth, or as a result of treatments, finishes or contamination. 

Gasification

Gasification is a partial oxidation process whereby a carbon source such as biomass, is broken down into carbon monoxide (CO) and hydrogen (H2), plus carbon dioxide (CO2) and possibly hydrocarbon molecules such as methane (CH4).

This mixture of gases is known as 'producer gas' or product gas (or wood gas or coal gas, depending on the feedstock), and the precise characteristics of the gas will depend on the gasification parameters, such as temperature, and also the oxidizer used.  The oxidizer may be air, in which case the producer gas will also contain nitrogen (N2), or steam or oxygen.

Gasification technology can be used for:

  • Heating water in central heating, district heating or process heating applications
  • Steam for electricity generation or motive force 
  • As part of systems producing electricity or motive force 
  • Transport using an internal combustion engine

Gasification can be carried out in two conditions:

Low temperature gasification

If the gasification takes place at a relatively low temperature, such as 7000C to 10000C, the product gas will have a relatively high level of hydrocarbons compared to high temperature gasification. A gasification system may be closely integrated with a combined cycle gas turbine for electricity generation (IGCC - integrated gasification combined cycle)

High temperature gasification

Higher temperature gasification (12000C to 16000C) leads to few hydrocarbons in the product gas, and a higher proportion of CO and H2. This is known as synthesis gas (syngas or biosyngas) as it can be used to synthesize longer chain hydrocarbons using techniques such as Fischer-Tropsch (FT) synthesis.

 If the ratio of H2 to CO is correct (2:1) FT synthesis can be used to convert syngas into high quality synthetic diesel biofuel which is completely compatible with conventional fossil diesel and diesel engines.

Pyrolysis

Pyrolysis is the precursor to gasification, and takes place as part of both gasification and combustion.  It consists of thermal decomposition in the absence of oxygen. It is essentially based on a long established process, being the basis of charcoal burning.

The products of pyrolysis include gas, liquid and a solid char (Biochar), with the proportions of each depending upon the parameters of the process.

Applications for pyrolysis include:

  • Biomass energy densification for transport or storage
  • Co-firing for heat or power
  • Feedstock for gasification.

Lower versus higher temperature Pyrolysis

Lower temperatures (around 4000C) tend to produce more solid char (slow pyrolysis), whereas somewhat

higher temperatures (around 5000C) produce a much higher proportion of liquid (bio-oil), provided the vapour residence time is kept down to around 1s or less.

After this, secondary reactions take place and increase the gas yield.

The bio-oil produced by fast (higher temperature) pyrolysis is described as a dark brown, mobile liquid with a heating value about half that of conventional fuel oil.  It can be burned directly, co-fired, upgraded to other fuels and gasified. It can therefore be used as an energy vector, effectively increasing the energy density of biomass for transportation and storage.

Chemical Conversion Technologies

Chemical conversion of biomass involves use of chemical interactions to transform biomass into other forms of useable energy.

Trans-esterification is the most common form of chemical-based conversion. Trans-esterification is a chemical reaction through which fatty acids from oils, fats and greases are bonded to alcohol. This process reduces the viscosity of the fatty acids and makes them combustible.

Trans-esterification

This chemical conversion process can be used to convert straight and waste vegetable oils into biodiesel

  • Vegetable oil, animal fat, or grease into biodiesel – fatty acid methyl ester
  • Base catalyzed of the oil with alcohol, direct acid catalyzed, and conversion of the oil to fatty acids and then to alkyl esters with acid catalysts

Biodiesel is a common end-product of trans-esterification. Common example of product manufactured by this reaction aree glycerin and soaps. Almost any bio-oil (such as soybean oil), animal fat or tallow, or tree oil can be converted to biodiesel.

Biochemical Conversion Technologies

As we discussed; it involves use of enzymes, bacteria or other microorganisms to break down biomass into liquid fuels, and includes anaerobic digestion, and fermentation.

Anaerobic digestion

Anaerobic digestion (AD) is the process whereby bacteria break down organic material in the absence of air, yielding a biogas containing methane. The products of this process are:

  • Biogas (principally methane (CH4) and carbon dioxide (CO2))
  • A solid residue (fibre or digestate) that is similar, but not identical, to compost 
  • A liquid liquor that can be used as a fertilizer.

AD is typically performed on biological material in an aqueous slurry. However there are an increasing number of 'dry' digesters.

Anaerobic digestion processes

There are two basic AD processes, which take place over different temperature ranges.

  • Mesophilic digestion that takes place between 200C and 400C and can take a month or two to complete
  • Thermophilic digestion takes place from 50-650C and is faster, but the bacteria are more sensitive.

Fermentation

Fermentation is the process used in brewing and wine making for the conversion of sugars to alcohol (ethanol CH3CH2OH). The same process, followed by distillation, can be used to obtain pure ethanol (bioethanol) for use as a transport biofuel.

 

Bio-ethanol / Bio-fuel

This should not be confused with bio-diesel that we studied in chemical conversion process. It’s a pure product of fermentation. It can be readily added to conventional petrol in concentrations up to 10%, but most European manufacturers' vehicle warranties only cover up to a 5% bioethanol/95% petrol blend. Higher concentrations are also possible, however modifications are required to vehicles to use them.  Flex fuel vehicles (FFVs) can run on either high bio-ethanol blends or 100% fossil fuels and many of the major vehicle manufacturers are developing them.

There are also fuel handling issues associated with higher ethanol concentrations concerning its vapour pressure and affinity for water (it vaporizes very fast and has good solubility with water). Biobutanol, derived from bioethanol, has been proposed as an alternative that does not suffer from the above fuel handling issues.

Fermentation advancements

  1. First generation biofuel - Conventional fermentation processes for the production of bioethanol make use of the starch and sugar components of typically cereal or sugar (beet or cane) crops.
  2. Second generation biofuel - bioethanol precedes this with acid and/or enzymatic hydrolysis of hemicellulose and cellulose into fermentable saccharides to make use of a much larger proportion of available biomass.
  3. Third generation biofuel- has only recently enter the mainstream it refers to biofuel derived from algae. 
  4. Fourth Generation biofuels are aimed at not only producing sustainable energy but also a way of capturing and storing CO2. This carbon capture makes fourth generation biofuel production carbon negative rather then simply carbon neutral.

 

Composting

This technique is primarily used in waste treatment rather than energy generation. Similar to anaerobic digestion, though making use of different bacteria, composting is the aerobic decomposition of organic matter by micro organisms. It is however typically performed on relatively dry material rather than slurry. Using composting for heat and power
Instead of, or in addition to, collecting the flammable biogas emitted, the exothermic nature of the composting process can be exploited and the heat produced used, usually using a heat pump.

Points to remember

  Hydro Power


Hydro Power deals with all renewable sources of energy that are generated from water. Therefore, this section will deal not only with hydro-electric power but with wave energy, tidal energy, marine currents and OTEC in brief.

 

Hydro-electric Power (HEP)

Hydropower energy is ultimately derived from the sun, which drives the water cycle. In the water cycle, rivers are recharged in a continuous cycle. Because of the force of gravity, water flows from high points to low points. There is kinetic energy embodied in the flow of water which is utilized to spin a turbine for energy generation from a hydro electric power station.

 

How a Hydroelectric Power System Works?

As we have discussed in previous units; the spinning of turbine is the last step of any engine is where any form of energy is converted into mechanical energy which in turns get converted into electrical energy.

In case of hydro electric dams this mechanical energy comes from the kinetic energy of water which falls on turbine from great height rather than steam in other resources. Flowing water is directed at a turbine (remember turbines are just advanced waterwheels). The flowing water causes the turbine to rotate, converting the water’s kinetic energy into mechanical energy.

The mechanical energy produced by the turbine is converted into electric energy using a turbine generator. Inside the generator, the shaft of the turbine spins a magnet inside coils of copper wire. It is a fact of nature that moving a magnet near a conductor causes an electric current (Faraday’s Law of Induction). The picture given below will explain the concept.

 

How much electricity can be generated by HEP?

The amount of electricity that can be generated by a hydropower plant depends on two factors:

  • Flow rate - the quantity of water flowing in a given time; and
  • Head - the height from which the water falls

The greater the flow and head, the more electricity produced. When more water flows through a turbine, more electricity can be produced. The flow rate depends on the size of the river and the amount of water flowing in it. Power production is considered to be directly proportional to river flow. You might have heard in summers; the dams are shut down due to lack of water in river. This generally takes place because of drying of rivers in summers.

Head also plays a very important role; the farther the water falls, the more power it has. Power production is also directly proportional to head. While determining head, hydrologists take into account the pressure behind the water. Water behind the dam also puts lot of pressure on the falling water.

Calculating electricity generated by HEP

The following general formula is used to find the power/energy produced from HEP :

 

Where

  • P is Power in watts,
  • p is the density of water (~1000 kg/m3),
  • h is height in meters,
  • r is flow rate in cubic meters per second,
  • g is acceleration due to gravityof 9.8 m/s2,
  • k is a coefficient of efficiency ranging from 0 to 1.

A standard equation for calculating energy production:

Power      =  (Head) x (Flow) x (Efficiency) / 11.8

  • Power = the electric power in kilowatts or kW
  • Head = the distance from which the water falls (measured in feet)
  • Flow = the amount of water flowing (measured in cubic feet per second or cfs)

Efficiency = How well the turbine and generator convert the power of  falling water into electric power. This can range from 60%  (0.60) for older, poorly maintained hydro plants to 90%  (0.90) for newer, well maintained plants.

11.8 = Index that converts units of feet and seconds into kilowatts

Types of Dams based on height

High-head Hydropower

Tall dams are sometimes referred to as “high-head” hydropower systems. That is, the height from which water falls is relatively high; more than 20 feet.

 

Low-head Hydropower

Many smaller hydropower systems are considered “low-head” because the height from which the water falls is fairly low. Low-head hydropower systems are generally less than 20 feet high.

 Types of HEP

The Hydro electric dams are categorized mainly into three categories:-

Their are also other small dams used for recreation, stock/farm ponds, flood control, water supply, and irrigation.

Impoundment HEP

The most common type of hydroelectric power plant is an impoundment dam. An impoundment facility is a  typical large hydropower system, uses a dam to store river water in a reservoir. Water released from the reservoir flows through a turbine, spinning it, which in turn activates a generator to produce electricity.

The water may be released either to meet changing electricity needs or to maintain a constant reservoir level.

 The dams are always under the lens due to controversies related to negative impacts of reservoir on biological as well as social environment. Few of the environmental impacts are on water quality due to sinking of forest area as well as animals, fish spawning and migration, the loss of biodiversity, resettlement and rehabilitation of people whose assets comes under the submergence area of dam and reservoir; threat of subsidence, floods, landslide and earthquakes etc

Diversion HEP

Diversion HEP are a substitute to the large HEP as they don’t have more negative impacts on environment. They are also called run-of-the-river system; it uses the river’s natural flow and requires little or no impoundment. Due to least storage of water or reservoir the dam do not cause all negative impacts that we discussed in impoundment systems.

 

The system may involve a diversion of a portion of the stream through a canal or penstock, or it may involve placement of a turbine right in the stream channel. Run-of-the-river systems are often low-head and produce very less electricity/power in comparison to impoundment system

Pumped Storage HEP

Another type of hydropower called pumped storage works like a battery, storing the electricity generated by other power sources like solar, wind, and nuclear for later use. It stores energy by pumping water uphill to a reservoir at higher elevation from a second reservoir at a lower elevation. When the demand for electricity is low, a pumped storage facility stores energy by pumping water from a lower reservoir to an upper reservoir. During periods of high electrical demand, the water is released back to the lower reservoir and turns a turbine, generating electricity.

The dams are practically not efficient as lot of power is required to pump water to top, which makes the enrgy production neutral in terms of cost benefit analysis.

 Types of HEP : Size

  • Large HEP
  • Small HEP
  • Micor HEP

Large Hydropower

Although definitions vary in different countries and places, as per Department of Energy, India (DOE) large hydropower are the facilities that have a capacity of more than 30 megawatts i.e. 30,000 Kilo Watt.

Small Hydropower

Small hydropower are the facilities that have a capacity of 100 kilowatts to 30 megawatts as per DOE, India.

Micro Hydropower

A micro hydropower plant has a capacity of up to 100 kilowatts. A small or micro-hydroelectric power system can produce enough electricity for a home, farm, ranch, or village.

Turbines

Another very important part of HEP is turbine, the movement and efficiency of turbine becomes the sole important factor when we have equali head and flow from rivers. The turbines are generally of two types:

  • Reaction Turbinesuction/pressure based. Example PELTON and CROSS FLOW
  • Impulse Turbinemomentum/impulse based. Example PROPELLER (Bulb, tube, kaplan,straflo) FRANCIS, KINETIC

Generally, for high heads HEP we use Pelton turbines; whereas Francis turbines are used to exploit medium heads. For low heads, Kaplan and Bulb turbines are applied.

The classification of what ‘high head’ and ‘low head’ varies widely from country to country, and no generally accepted scales are found in literature.

 

  Dam Types : Technology


The dams are divided technically into many categories based on its construction. Taking examples from India;

Gravity Dams - Dams built of concrete, stone, or other masonry are called gravity dams. Example arch dams or buttress dams

Embankment dams - Dams built of earth or rocks are called embankment dams

Arch Dams

Arch dams are concrete dams that curve upstream toward the flow of water. They are generally built in narrow canyons, where the arch can transfer the water's force to the canyon wall. Arch dams require much less concrete than gravity dams of the same length, but they require a solid rock foundation to support their weight.

  • Arch shape gives strength
  • Less material (cheaper)
  • Narrow sites
  • Need strong abutments

Example - Idukki dam, Kerala

Concrete Gravity Dams

Gravity dams depend entirely on their own weight to resist the tremendous force of stored water. In the earlier times, some dams have been constructed with masonry blocks and concrete. Today, gravity dams are constructed by mass concrete or roller compacted concrete.

  • Weight holds dam in place
  • Lots of concrete (expensive)

Example - Bhakra Dam across river Sutlej in Himachal Pradesh

Buttress Dams

Buttress dams depend for support on a series of vertical supports called buttresses, which run along the downstream face

  • Face is held up by a series of supports
  • Flat or curved face

Example - Srisailam Dam, Telangana

Embankment Dams

Embankment dams are constructed of either earth fill or a combination of earth and rock fill. Therefore, embankment dams are generally built in areas where large amount of earth or rocks are available. They represent 75% of all dams in the world.

  • Earth or rock
  • Weight resists flow of water

Example - Tehri Dam, Uttarakhand

Coffer Dam

The name may be confused with the types of dam; but it is a temporary dam constructed for facilitating construction. It is an enclosure constructed around a site to exclude water so that the construction can be done in dry.

 

  Biggest HEPs in India


The Tehri Dam –

Highest dam in India, its a multipurpose dam on Bhagirathi river near Tehri, Uttarakhand.

  • Height – 260.5m
  • Length – 575 m
  • Total capacity from THPC (Tehri hydro power complex) – 2400 MW

The Koyna HE Dam

Second largest HEP in India on Koyna river, Maharashtra.

  • Height – 103 m
  • Length – 807 m
  • Capacity 1960 MW

The Srisailam Dam –

The dam is constructed across Krishna river in Kurnool district of Andhra Pradesh. Third largest capacity HEO in India.

  • Height – 145 m
  • Length – 512 m
  • Capacity 1670 MW

 

The Sardar Sarovar Dam –

Its is a gravity dam on Narmada River near Navagam, Gujarat.

  • Height – 163 m
  • Length – 1210 m
  • Capacity - 1450 MW

The Bhakra Nangal Dam

The dam is built across Sutlej river in Bilaspur, Himachal Pradesh. Its reservoir is known as Gobind Sagar (second largest reservoir in India)

  • Height – 226 m
  • Length – 520 m
  • Capacity - 1325 MW

 

The world's largest HEP : Three Gorges Dam

Three Gorges dam in China is the world’s largest dam with a generation capacity of 22,500 MW replacing Itaipu HEP, Brazil (14,000MW) in 2012. The dam has a height of 181 m and length of about 2335m.


Hydro Electric Project and EIA

Hydro electric dams are considered to be one of the most controvertial development activity because it affects social as well as environmental well being. Environment Impact Assessment (EIA) is very stringent in case of HEPs.

The project get environmental clearance (EC) from Center (MoEFCC) if it is a big HEP with power generation capacity more than 50 MW and get EC from State if the power generation is less than 50 MW and more than 25 MW.

The projects also decides their threshold based on culturable command area (The area which can be irrigated from a scheme and is fit for cultivation) of the project. If te area is more than 10,000 hactares the project get EC from Center and if less than that and more than 2000 hactares; it get clearance from State.

  Wave Energy


The second form of hydro power that we are going to discuss is the wave energy i.e. the energy generated from the movement of waves in water. Wave power is the transport of energy by ocean surface waves, and the capture of that energy to do useful work – for example,electricity generationwater desalination, or the pumping of water (into reservoirs). A machine able to exploit wave power is generally known as a wave energy converter (WEC).

The major problem with the wave power is that it is not concentrated at a place.

Formula for Wave Energy

We can roughly estimate the Power from a wave by using formula

E (per m2) =  1/2 ρ ga2

where ρ is density of sea water, g is acceleration due to gravity and a(H = 2a) is the amplitude of the wave. It is assumed that a typical wave measures 2 to 3 metres in height throughout the year.

Mechanism for collecting wave energy

The wave energy is captured by installing devices which can continuously float on the surface of sea/ocean to capture the kinetic energy of wave. These devices are generally connected to the seabad / shore. Some of such devices are:

Point absorber Buoy

This device floats on the surface of the water, held in place by cables connected to the seabed. Buoys use the rise and fall of swells to drive hydraulic pumps and generate electricity. The device move up and down on confrontation with wave; which spins the turbine connected to it leading to energy generation though small in amount but continuous.

 

 Surface attenuator

These devices act similarly to point absorber buoys, with multiple floating segments connected to one another and are oriented perpendicular to incoming waves. 

 

Oscillating water column

Oscillating water column devices can be located on shore or in deeper waters offshore. With an air chamber integrated into the device, swells compress air in the chambers forcing air through an air turbine to create electricity

 

 Overtopping device

Overtopping devices are long structures that use wave velocity to fill a reservoir to  a greater water level than the surrounding ocean. The potential energy in the reservoir height is then captured with low-head turbines. They are somehow working on the principle of HEPs; they create a temporary reservoir when wave comes and let it go out through the way connected to turbine. The outgoing water spins the turbine generating electricity.

Oscillating wave surge converter

These devices typically have one end fixed to a structure or the seabed while the other end is free to move. Energy is collected from the relative motion of the body compared to the fixed point. Oscillating wave surge converters often come in the form of floats, flaps, or membranes. In the picture you can see the paddle that actually moves when wave hits it, the paddle is connected to generator for energy capture.

  Tidal Energy


Very similar and close to wave energy in functioning is the Tidal Energy. Contrary to waves which are generated continuously in sea/ocean tides are caused by the gravitational pull of the moon and sun, and the rotation of the earth.

Near the shore, water levels can vary up to 40 feet as a result of tides. The movement of water as a result of tidal forces can be used to produce energy. Tidal power is more predictable than wind energy and solar power. A tidal range of 10 feet is needed to produce tidal energy economically.

Tidal barrages

Similar to HEP; we need tidal barrage to capture the tidal energy. Tidal barrages make use of the potential energy in the difference in height (hydraulic head) between high and low tides.

When using tidal barrages to generate power, the potential energy from a tide is seized through strategic placement of specialized dams. When the sea level rises and the tide begins to come in, the temporary increase in tidal power is channeled into a large basin behind the dam, holding a large amount of potential energy. (Reservoir flooding)

With the receding tide, this energy is then converted into mechanical energy as the water is released through large turbines that create electrical power through the use of generators.(Ebb Generation)

Barrages are essentially dams across the full width of a tidal estuary. A tidal stream generator, often referred to as a tidal energy converter (TEC) is a machine that extracts energy from moving masses of water, in particular tides.

 

  Marine current energy


Marine current is caused by tidal effects; thermal & salinity differences of sea water

Energy can be extracted from ocean currents by using submerged water turbines similar to wind turbines. These turbines would have rotor blades, a generator for converting the rotational energy into electricity, and a means of transporting the electrical current to shore for incorporation into the electrical grid.

 

Unlike wind, because water is much denser than air, the size of turbine needed to extract energy underwater can be much smaller than a wind turbine. The velocities of the currents are lower than those of the wind. They are often not accepted due to threat to aquatic biodiversity.

  Ocean Thermal Energy Conversion (OTEC)


Ocean thermal energy conversion(OTEC) uses the temperature difference between cooler deep and warmer shallow or surface ocean waters to run a heat engine and produce electricity. The system works on the rankine cycle principle by creating a hot and cold reservoir.

OTEC works best when the temperature difference between the warmer, top layer of the ocean and the colder, deep ocean water is about 36°F (20°C). These conditions exist in tropical coastal areas, roughly between the Tropic of Capricorn and the Tropic of Cancer.

OTEC Principle

The Ocean is the largest solar collector in the world and due to its mass, there is little temperature difference between day and night. 

The surface layer, the Euphotic zone, receives all the solar energy and extends from the ocean surface to a depth of 200 meters. It is the warmest layer, and depending on geographical location, can reach temperatures of over 30°C

The deep layer, the Disphotic zone, occurs at depths from 200m to 1,000m and is sometimes referred to as the twilight zone since the sunlight is very faint. Due to the lack of solar energy, the water temperature decreases rapidly with increasing depth.

This temperature stratification in the oceans is referred to as the thermocline.

The temperature differences below depths of 1000m are small and therefore not considered for OTEC.

Rankine Cycle

For the working of the rankine cycle; two pumps are installed in the seawater. The warm seawater is pumped to the evaporator while the cold see water from depth is pumped to the condenser. Cold working fluid (we discussed its property in previous units) is pumped to evaporator; the fluid get vaporized due to warm sea water. The vapours rotate the turbine which generates electricity. The vapours than eneter the condenser to get cooled by the cold sea water and again returned to the evaporator. The working fluid is thus recycled, generating continous electricity. 

Types of OTEC

The OTEC are generally categorized into:

  • Closed Cycle OTEC
  • Open Cycle OTEC
  • Hybrid OTEC

Closed-Cycle OTEC

Closed-cycle systems use working fluids with a low boiling point, such as ammonia, to rotate a turbine to generate electricity.

Warm surface seawater is pumped through a heat exchanger, where the low-boiling-point fluid is vaporized. The expanding vapor turns the turbo-generator. Cold deep seawater—which is pumped through a second heat exchanger—then condenses the vapor back into a liquid that is then recycled through the system.

Open-Cycle OTEC

In open-cycle OTEC, the sea water is itself used to generate heat without any kind of intermediate working fluid.

At the surface of the ocean, hot sea water is turned to steam by reducing its pressure (remember that a liquid can be made to change state, into a gas, either by increasing its temperature or reducing its pressure). The steam drives a turbine and generates electricity (as in closed-cycle OTEC), before being condensed back to water using cold water piped up from the ocean depths.

One of the very interesting byproducts of this method is that heating and condensing sea water removes its salt and other impurities, so the water that leaves the OTEC plant is pure and salt-free. That means open-cycle OTEC plants can double-up as desalination plants, purifying water either for drinking supplies or for irrigating crops.

Hybrid OTEC

Hybrid systems combine the features of closed- and open-cycle systems. In a hybrid system, warm seawater enters a vacuum chamber, where it is flash-evaporated into steam, similar to the open-cycle evaporation process. The steam vaporizes a low-boiling-point fluid (in a closed-cycle loop) that drives a turbine to produce electricity.

Advantages of OTEC system :

  • Power from OTEC is continuous, renewable and pollution free
  • Unlike other forms of solar energy, output of OTEC shows very little daily or seasonal variation
  • Drawing of warm and cold sea water and returning of the sea water, close to the thermocline, could be accomplished with minimum environment impact
  • Electric power generated by OTEC could be used to produce hydrogen
  • OTEC system might help in enrichment of fishing grounds due to the nutrients from the unproductive deep waters to the warmer surface waters
  • A floating OTEC plant can generate power even at mid sea
  • Fresh water from open cycle system

Limitations of OTEC system:

  • Capital investment is very high
  • Due to small temperature difference in between the surface water and deep water, conversion efficiency is very low about 3-4%
  • Low efficiency of these plants coupled with high capital cost and maintenance cost makes them uneconomical for small plants

 

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