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

 

 

  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 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.

 

  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

  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.

 

  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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