BIOMASS AS ENERGY SOURCE -I

Biomass is plentifully available in the rural regions. It is already being used by the rural people as a major source of energy, mainly in cooking food, which constitutes almost 50% of the total energy consumption. Assuming that there are about 140 million households in rural India, and assuming that each family uses annually about 3 tonnes of biomass as fuel, one comes to the figure of about 400 million tonnes of biomass utilised annually only for domestic cooking.

 

Engineers and energy scientists generally think only of the calorific value of fuels and of fuel use efficiency. But there is also a third dimension to fuel use, and that is the pollution arising due to burning of biomass. As cooking is done within the confines of a house, the pollution caused by cooking fires is generally not taken very seriously.

 

But according to statistics published by the World Health Organisation, annually about 500,000 women and children die prematurely in India due to air pollution caused by cooking fires in rural households. Considering the fact that almost 70% of our population is rural, giving the rural women a cleanly burning biofuel is a major task, which is unfortunately not tackled by any of our major research centres.

 

There are many options for providing a clean and economical burning biofuel. The biomass that is currently available to villagers is free of cost.

 

One way of tackling this problem is to redesign the cooking devices in such a way that they burn the biomass more cleanly, so that the pollution caused by them is reduced. This is achieved by providing the fuel with sufficient air, so that it burns completely, reducing automatically the carbon monoxide and the particular matter in the fuel gases. Another strategy is to design a stove in such a way that waste of heat is avoided and a major part of the heat generated by the burning biomass is transferred to the pot. This results in higher fuel use efficiency, requiring the user to burn less fuel. Pollution is naturally reduced if the amount of fuel is reduced. Both the strategies are combined in modern improved cook stoves.

 

However, in practical terms, both the strategies often fail, because the fuel that is used in the laboratory while designing the stove differs from the fuel that the rural housewife actually uses. In a laboratory experiment, one normally uses good quality firewood, that has been properly dried and cut into pieces of adequate size. In contrast to this, the fuel used by the rural housewife consists of stalks of plants like cotton, maize, safflower, arhar, or of bushes growing in the vicinity, maize cobs, dung cakes, rhizomes of sugarcane, etc.

 

The traditional cookstove is designed to burn such material and therefore, the housewife often finds that the improved cookstove emits more smoke and soot than her traditional stove, comparatively. Standardisation of fuel is, therefore, another strategy that is considered in the context of using biomass as cooking fuel. The easiest way of standardising woody biomass is to cut it into  uniform, small pieces called chips. Highly efficient and non-polluting stoves can be designed to burn these chips, but unfortunately not much effort has been made in this direction in India.

 

The second and traditional method of converting a non-standard fuel into standard one is to char it into charcoal. It is the volatile matter in biomass that gives rise to the particulate matter in the flue gases. In the process of charring, the volatiles are removed from the biomass to leave only the carbon and non-combustible matter behind. Therefore, when charcoal burns, it burns cleanly, without producing any smoke or soot. However, the traditional method of producing charcoal is itself highly polluting, because the volatiles are released into the atmosphere in this process. Sophisticated technologies are now available for charring, in which the volatiles are burned in the process of charring itself, to produce the heat required in the process.

 

Agricultural waste is an ideal source of charcoal. When one harvests any crop, one generally harvests only grain, fruits, pods, tubers or rhizomes. This constitutes only about 30 to 40% of the total biomass. This means that about 60 to 70% of the total agricultural biomass, or almost 600 million tonnes, is the waste biomass produced annually in India. A small part of it is used as fodder for cattle, but the rest is just wasted.

 

The standardised Sarai cooker, a stove-and-cooker system, can cook the meal for five persons, using just 100 g of our char briquettes. About 15,000 households in Maharashtra are already using the Sarai cooker.

Applications of Ocean Thermal Energy Conversion -Otec

Aside from the generation of electricity, it has been proposed that OTEC plants could assist ocean based industries, such as aquaculture, refrigeration and air conditioning, desalinated water crop irrigation and consumption as well as mineral extraction through the use of the fresh and chilled water byproducts

 

OTEC has important benefits other than power production.

1) Air conditioning

Air conditioning can be a byproduct. Spent cold seawater from an OTEC plant can chill fresh water in a heat exchanger or flow directly into a cooling system. Chilled-

2) Soil agriculture

OTEC technology also supports chilled-soil agriculture. When cold seawater flows through underground pipes, it chills the surrounding soil. The temperature difference between plant roots in the cool soil and plant leaves in the warm air allows many plants that evolved in temperate climates to be grown in the subtropics.

3) Desalination

An OTEC plant that generates 2-MW of net electricity could produce about 4,300 cubic meters (14,118.3 cubic feet) of desalinated water each day.

4) Mineral extraction

An OTEC plant that generates 2-MW of net electricity could produce about 4,300 cubic meters (14,118.3 cubic feet) of desalinated water each day.

 

Some Major Otec Power Plants in World

 

 

Sr.No	LOCATION	YEAR	CAPACITY(KW)
1	Matanzas Bay Hawana Cuba	1930	NA
2	Abidjan Ivory coast	1956	7000
3	Hawaii.U.S.A	1979	50
4	Hawaii.U.S.A	1981	1000
5	Republic of Nauru	1981	100
6	Tokunoshima Japan	1982	52
7	Hawaii.U.S.A	Proposed	49000
8	Bali, Indonesia	Proposed	230
9	Jamaica,West Pacific	Proposed	1580
10	Tahiti,Central Pacific	Proposed	5000
11	Republic of Nauru	Proposed	2500
12	Kalashakharapattanam ,India	Proposed	100000
13	Andhra Pradesh	Proposed	100000

 

India possess excellent thermal gradients and some of the best sites in the world for harnessing OTEC power India has a potential of exploiting 80,000 MW of OTEC based power. Some of the coastal regions of Tamil Nadu and Andhra Pradesh provide excellent sites for OTEC plants. Although the theoretical efficiency of OTEC is small (~2%), there are vast quantities of sea water available for use in power generation. It has been estimated that there could be as much as 10⁷ MW power available worldwide.

Benefits of Ocean Thermal Energy Conversion

OTEC’s economic benefits:

· Helps produce fuels such as hydrogen, ammonia, and methanol

· Produces base load electrical energy

· Produces desalinated water for industrial, agricultural, and residential uses

· Is a resource for on-shore and near-shore mariculture operations

· Provides air-conditioning for buildings

· Provides moderate-temperature refrigeration

· Has significant potential to provide clean, cost-effective electricity for the future.

OTEC’s non economic benefits

· Promotes competitiveness and international trade

· Enhances energy independence and energy security

· Promotes international sociopolitical stability

· Has potential to mitigate greenhouse gas emissions resulting from burning fossil

  fuels.

OCEAN THERMAL ENERGY CONVERSION

Energy is a crucial input in the process of economical, social and industrial development. High energy consumption has traditionally been associated with higher quality of life, which in turn is related to the Gross National Product (GNP). Variation in magnitude of energy resources , differing mix of energy resource profiles , lack of adequate resources of fossil fuels in many nations, dispersed geographical location of energy resources within nations and in the world are some of the complexities that characterize the global energy scene.Sources that are replenished more rapidly are termed as ‘renewable’.These include solar,wind and ocean which are inexhaustible.

 

Significance of Ocean Energy

Oceans cover more than 70% of earth’s surface, making them the world’s largest solar collectors. The sun’s heat warms the surface of water a lot more than the deep ocean water and this temperature difference creates thermal energy.Just a small portion of the heat trapped in the ocean could power the world.

 

Ocean can produce two types of energy: thermal energy from the sun’s heat and mechanical energy from the tides and waves. Ocean thermal energy is used for many applications including electricity generation. There are three types of electricity conversion systems: closed cycle, open cycle and hybrid. Ocean mechanical energy is quite different from ocean thermal energy. Even though the sun affects all ocean activities, tides are driven primarily by the gravitational pull of the moon and waves are driven primarily by the winds. As a result, tides and waves are intermittent sources of energy while ocean thermal energy is fairly constant. Also, unlike thermal energy, the electricity conversion of both tidal and wave energy usually involves mechanical devices.

 

OCEAN THERMAL ENERGY CONVERSION

Ocean Thermal Energy Conversion (OTEC) utilises the temperature difference between the warm surface sea water and cold deep ocean water to generate electricity. For OTEC to produce a net output of energy, the temperature difference between the surface water and water at a depth of 1000m needs to be about 20oC.

Temperature difference between surface and sub surface (1000m) sea water

 

The concept of OTEC is envisioned by Jacque’s D’Arsonval in 1881. However, D’Arsonval did not live to see his idea to fruition, and the task was completed by his student Georges Claude in 1930. Although the theoretical efficiency of OTEC is small (~2%), there are vast quantities of sea water available for use in power generation. It has been estimated that there could be as much as 10⁷ MW power available worldwide.

 

Otec Systems are Classified into Three Categories

Closed Cycle Otec

D’Arsonval’s original concept used a working fluid with a low boiling point, such as ammonia, which is vapourised using the heat extracted from the warm surface water. The heated working fluid is used to turn a turbine to produce electricity. Cold deep sea water is used to condense the working fluid in a second heat exchanger prior to being recirculated to the first heat exchanger.

Open Cycle Otec

Open cycle OTEC is very similar to the closed cycle one. The only difference is that an open cycle OTEC does not use intermediate fluid with low boiling point but uses the sea water as working fluid that drives the turbine. The warm sea water on the ocean surface is turned into low pressure vapour under a partly vacuumed environment.The steam is then condensed either by a second heat exchanger, as in the closed cycle, or by mixing with the deep cold water.

 

Hybrid Otec System

Hybrid Cycle OTEC is a theoretical method of maximizing the use of ocean thermal energy.

There are two concepts. The first one is to use a closed cycle OTEC to generate electricity to create the necessary low-pressure environment for the open cycle OTEC. The second concept is to integrate two open cycle OTEC (one is used to create the vacuumed environment) so that there will be twice the amount of the original desalinated water.

Leading Wave Energy Technologies

Wave energy is moving off shore. Although a number of successful devices have been installed at shoreline locations, the true potential of wave energy can only be realized in the offshore environment where large developments are conceivable. In terms of power potential, offshore locations offer more than shoreline locations. The negative side is that devices in offshore locations have more difficult conditions to contend with. Shore line technologies have the benefit of easy access for maintenance purposes, whereas offshore technologies are, in most cases more difficult to access. Improving reliability and accessibility are, therefore important in commercialization of wave energy harnessing.

 

Shoreline wave energy is limited by fewer potential sites & high installation cost whereas a 50 MW wave farm is conceivable on offshore locations. No shoreline wave energy converter is able to offer such potential for deployment in this way. Deployment costs for shoreline wave energy devices are high because they are individual projects and economics of scale are, therefore, largely inapplicable. Shoreline devices only account for 8% of forecast capacity between 2004-2008. Offshore represents the most significant wave energy sector, with 58% of all forecast capacity. Offshore is so dominant because devices are typically of a larger capacity than their nearshore compatriots.

 

Ocean Power Delivery Pelamis

In the present time OPD is viewed as market leader which has developed ‘pelamis’ concept. The ‘pelamis’ is made up of five cylindrical segments connected by hinged joints. The wave induced motion on these sections is resisted by hydraulic rams which pump high pressure fluid through hydraulic motors via smoothing accumulators to drive electric generators. The power is fed through a cable to a junction on the sea floor where a single cable carries the electricity to the shore. The

first full-size pelamis has a rated capacity of 750 KW.

 

Wave Dragon A/S – Wave Oragon.

Wave Dragon is the first operational grid connected offshore wave energy device, installed in Denmark. The prototype wave dragon has an output of 20 KW. The device is under study to gain more knowledge & experience. The different models of 7MW, 4 MW & 11 MW capacity are proposed for the different levels of wave resource.

Wavegen – Limpet

Wavegen is one of the market leaders in wave energy, having installed their Limpet shoreline devices in Scotland in 2000. It is also developing technology which generates power from wave energy, whilest also acting as an artificial reef. The device which rests on the sea bed, could in some cases and coastal protection. The technology is of particular benefit to island communities.

 

 Environmental Impacts

Small-scale wave energy plants are likely to have minimal environmental impacts. However some of the very large-scale projects that have been proposed have the potential of harming the ocean ecosystems covering very large areas of the surface of the oceans with wave energy devices would harm marine life and could have more wide-spread effects. Changes in waves and currents would most directly impact species that spend their lives nearer to the surfaces. The dampening of waves may reduce erosion on the shoreline and may have damaging ecologies effects, that need to be scientifically proved.Wave energy is promising holds huge potential to reduce reliance on fossil fuels. Carefully choosing sites that can withstand the alterations to the environment caused by power plants will be crucial to effectively develop these technologies without harming the ocean.

WAVE ENERGY AS A POTENTIAL RENEWABLE ENERGY RESOURCE

At present most of the energy needs are met through fossil fuels & oil. Therefore developing countries are dependent on oil imports for their energy needs. At present there are about two billion people without access to electricity. Therefore there is an urgent need all over world to tap renewable energy sources. Total renewable energy sources account for 56 EJ of energy, almost 14% of the total world energy i.e. 401 EJ.

Wave energy

Waves in sea are generated by the action of winds blowing over water and can be used as a renewable source of energy. In fact sea could be viewed as a vast collector of energy transferred by wind over large sea. Surface and stored as wave energy. Wave energy potential varies from place to place depending upon its geographic location. Two factors affecting the magnitude of wave energy are wind strength & uninterrupted distance over the sea that the wind can blow.

 

Potential

The total wave power potential of the world is 2 x 10⁶ MW. The tidal energy is wavering with 250 Kw available from December to March, 75 Kw between April to November and has peak value of 150 Kw. The economics of wave energy power, though not yet competitive with fossil fuels, are promising and the situation is improving with more advanced technology. Capital costs for 100 MW installation is $1200 to $1500/Kw with operating costs of 5 cents / Kwh & load factor around 20%. Estimated international cost for power from wave energy is around 9.2 C/KwH.

 

Current Status

The first commercial wave plant in the world, Limpet 500, was installed on the island of Islay, Scotland, in 2000 and has been providing power to the grid for U.K. The Limpet 500 is a 0.5 MW capacity plant for sitting on exposed shores, utilizing an oscillating column. design. In India 150 KW system is set at Thiruvananthpuram. The United Kingdom is said to be the dominant player in wave power with a forecast capacity of 14.7 MW. Portugal, Spain and Denmark are other significant markets but lag far behind U.K.

TIDAL ENERGY – LATEST DEVELOPMENT

The oldest technology to harness tidal power for the generation of electricity involves building a dam, known as a barrage, across a bay or estuary that has large differences in elevation between high and low tides. Water retained behind a dam at high tide generates a power head sufficient to generate electricity as the tide ebbs and water released from within the dam turns conventional turbines. Though the American and Canadian governments considered constructing ocean dams to harness the power of the Atlantic tides in the 1930s, the first commercial scale tidal generating barrage rated at 240 MW was built in La Rance. This plant continues to operate today as does a smaller plant constructed in 1984 with the Annapolis Royal Tidal Generating Station in Nova Scotia, rated at 20 megawatts (enough power for 4,500 homes). Other tidal generating station operating today, is located near Murmansk on the White Sea in Russia, rated at 0.5 megawatts.

 

These first-generation tidal power plants have all withstood the rigours of the marine

environment and been in continuous pollution-free operation for many years. But due to the very high cost of building an ocean dam to harness tidal power, and environmental problems from the accumulation of silt within the catchment area of the dam (which requires regular, expensive dredging), engineers no longer consider barrage-style tidal power feasible for energy generation.

 

Engineers have recently created two new kinds of devices to harness the energy of tidal currents (AKA ‘tidal streams’) and generate renewable, pollution-free electricity. These new devices may be distinguished as Vertical-axis and Horizontal axis models, determined by the orientation of a subsea, rotating shaft that turns a gearbox linked to a turbine with the help of large, slow-moving rotor blades. Both models can be considered a kind of underwater windmill. While horizontal-axis turbine prototypes are now being tested in northern Europe (the UK and Norway). A vertical-axis turbine has already been successfully tested in Canada. Tidal current energy systems have been endorsed by leading environmental organizations, including Greenpeace, the Sierra Club of British Columbia and the David Suzuki Foundation as having ‘the lightest of environmental footprints,’ compared to other large-scale energy systems.

 

 

 

Vertical-axis tidal turbine – Canadian connection

A Canadian company – Blue Energy Canada Inc. – has completed six successful prototypes of its vertical-axis ‘Davis Hydro Turbine, named after its inventor, the late Barry Davis. Barry Davis got trained as an aerospace engineer, worked on the renowned Canadian Avro ‘Arrow’ project, as well on the equally-remarkable ‘Bras D’Or’ hydrofoil project of the Canadian Navy. Barry, then decided to apply his knowledge of hydrodynamics in creating a tidal energy generator. Barry received support from the Canadian National Research Council and successfully tested 5 turbine prototypes in the St. Lawrence Seaway and on the eastern seaboard. Blue Energy is presently raising funds for a commercial demonstration project of the Davis Hydro Turbine.

COMPARATIVE ENERGY DENSITY (kWh/m²)

SOLAR (PV)          1051 kWh/m²

WIND                   1000 kWh/m²

WAVE                   35-70,000 kWh/m²

BLUE ENERGY       192,720 kWh/m²

 

Horizontal-axis tidal turbine

Although, such tidal turbines were proposed during the oil crisis of the 1970s, the first tidal turbines began operating in the mid-1990s when a 15-kilowatt horizontal-axis tidal turbine was installed in Loch Linnhe on the west coast of Scotland, north of Glasgow. Now, two companies in the United Kingdom are planning to initiate horizontal-axis tidal turbine demonstration projects while another demonstration project has begun off the coast of Norway. A US company has also designed a working prototype. Horizontal-axis tidal turbines closely resemble wind turbines, except  that the turbine and blades are completely submerged in water.

 

Like the ocean dam models of France, Canada and Russia, vertical and horizontalaxis tidal current energy generators are fueled by the renewable and free forces of the tides, and produce no pollution or greenhouse gas emissions. As an improvement on ocean dam models, however, the new models offer many additional advantages:

 

- As the new tidal current models do not require the construction of a dam, they are considered much less costly.

- As the new tidal current models do not require the construction of a dam, they are considered much more environmental-friendly.

 

As the new tidal current models do not require the construction of a dam,further cost reductions are realized from not having to dredge a catchments area.

 

- tidal current generators are also considered more efficient because they can produce electricity while tides are ebbing (going out) and surging (coming in), whereas barrage-style structures only generate electricity while the tide is ebbing.

 

- Vertical-axis tidal generators may be stacked and joined together in series to span a passage of water such as a fiord and offer a transportation corridor (bridge), essentially providing two infrastructure services for the price of one.

 

- Vertical-axis tidal generators may be joined together in series to create a ‘tidalfence’ capable of generating electricity on a scale comparable to the largest existing fossil fuel-based, hydroelectric and nuclear energy generation facilities.

 

- Tidal current energy, though intermittent, is predictable with exceptional accuracy many years in advance.

 

In other words, power suppliers will easily be able to schedule the integration of tidal energy with backup sources well in advance of requirements. Thus, among the emerging renewable energy field, tidal energy represents a much more reliable energy source than wind, solar and wave, which are not predictable.

 

- present tidal current, or tidal stream technologies are capable of exploiting and generating renewable energy in many marine environments that exist worldwide. Canada and the US, by virtue of the very significant tidal current regimes on its Atlantic and Pacific coastlines

 

– proximal to existing, significant electro transportation infrastructure – is blessed with exceptional opportunities to generate large-scale, renewable energy for domestic use and export.

 

Tidal energy power systems are expected to be very competitive with other conventional energy sources, and excellent cost advantages arise from there being no pollution or environmental expenses to remediate nor are their fuel expenses (the kinetic energy of tidal currents is free). Further, ongoing maintenance costs are expected to be modest, as they are with other large-scale marine infrastructures, e.g. bridges, ships, etc., and a non-polluting tidal energy regime will qualify for valuable carbon offset credits. A 2006 feasibility report on tidal current energy in British Columbia by Triton Consultants for BC Hydro stated, “Future energy costs are expected to reduce considerably as both existing and new technologies are developed over the next few years.

 

Assuming that maximum currents larger than 3.5 m/s can be exploited and present design developments continue, it is estimated that future tidal current energy costs between 5C / kWh and 7C / kWh are achievable

Existing Hydrogen Transport and Storage Methods

Hydrogen is currently stored in tanks as a compressed gas or cryogenic liquid. The tanks can be transported by truck or the compressed gas can be sent across distances of less than 50 miles by pipeline

 

Safety is essential in the entire energy conversion process. This begins with production, storage, transport, distribution and utilization. Each energy form poses its own specific risk, which should be taken care. The safety of combustible energy carriers in their ignition, combustion, explosion and detonation behaviour when mixed with air is still under study.

 

Applications

Hydrogen is high in energy, yet an engine that burns pure hydrogen produces almost no pollution. NASA has used liquid hydrogen since the 1970s to propel the space shuttle and other rockets into orbit. Hydrogen fuel cells power the shuttle’s electrical systems, producing a clean byproduct—pure water, which the crew drinks. You can think of a fuel cell as a battery that is constantly replenished by adding fuel to it—it never loses its charge. A device has been designed to generate hydrogen to drive a cellular phone.

 

Fuel cells are a promising technology for use as a source of heat and electricity for buildings, and as an electrical power source for electric vehicles. Although these applications would ideally run off pure hydrogen, in the near future they are likely to be fueled with natural gas, methanol, or even gasoline. Reforming these fuels to create hydrogen will allow the use of much of our current energy infrastructure—gas stations, natural gas pipelines, etc.—while fuel cells are phased in.

 

In the future, hydrogen could also join electricity as an important energy carrier. An energy carrier stores, moves, and delivers energy in a usable form to consumers. Renewable energy sources, like the sun, can’t produce energy all the time. The sun doesn’t always shine. But hydrogen can store this energy until it is needed and can be transported to where it is needed.

 

Some experts think that hydrogen will form the basic energy infrastructure that will power future societies, replacing today’s natural gas, oil, coal, and electricity infrastructures. They see a new hydrogen economy to replace our current energy economies, although that vision probably won’t happen until far in the future.

Storage Of Hydrogen In Carbon Nanotube

Many new methods use carbon as a storage medium and bring us a step closer to the widespread use of hydrogen as a fuel source. Scientists are using various approaches to shape carbon into microscopic cylindrical structures known as nanotubes.

 

The first method of producing nanotubes uses an electric arc to vaporize a metal-impregnated carbon electrode.

 

The second method uses a laser to vaporize a heated carbon target that has been treated with a metal such as nickel, cobalt or iron.

 

The third method is known as catalytic chemical vapor deposition (CCVD), and researchers at Washington University in St. Louis believe this is the most promising approach. In the CCVD technique, a heated metal element breaks down a hydrocarbon gas (such as methane, ethylene, acetylene, etc.) into carbon and hydrogen. The hydrogen gas is released while the carbon is extruded as a nanofiber. The advantage of CCVD is that it is a low-temperature technique and is suitable for large-scale production.

Storage

One of the critical factors in nanotubes’ usefulness as a hydrogen storage medium is the ratio of stored hydrogen to carbon. According to the US Department of Energy, a carbon material needs to store 6.5% of its own weight in hydrogen to make fuel cells practical in cars. Such fuel cell cars could then travel 300 miles between refueling stops.

 

Researchers at MIT claim to have produced nanotube clusters with the ability to store 4.2% of their own weight in hydrogen. In recent months, scientists from the National University of Singapore have released figures for nanotubes and nanofibers that can store 10-20% of their weight in hydrogen. These results, when combined with new car manufacturing technologies have the potential of transforming our transportation industries.Single-walled carbon nanotubes are remarkable forms of elemental carbon. Their unique properties have stimulated the imaginations of many scientists and engineers to propose a wide range of applications.

 

Nanotubes do have a dramatic visual Impact. If beauty rests on symmetry, nanotubes have inherent beauty. Further, their cylindrical structures led to suggestions that they would be ideal gas storage materials. The appearance of these potential storage materials conveniently coincided with the revivification of interest in the hydrogen economy. The potential for coupling carbon-based storage materials to supply pure hydrogen to automotive fuel cell power plants was quickly seen.

 

Initial reports of experiments showing high levels of hydrogen storage were encouraging. Theoreticians were then quick to calculate the possible amounts of hydrogen that could be stored using arrays of tubes of various sizes and packing parameters. Since the appearance of the initial reports, the results have been varied and controversial. Some are higher, some lower; some imply physisorption, and some chemisorption. It is clear that storage is a complex issue, partly because the, materials are more far complex than the visual comprehension of the single ideal nanotube would allow.

 

Studies have been conducted and it has been found that purified Multi walled carbon nanotubes (MWNT) can be used for bulk storage of hydrogen. Multi walled carbon nanotubes have been synthesised by catalytic decomposition of hydrocarbon using a floating catalyst method. The mean diameter of the MWNTs was found to be 5.1 nm.

 

The MWNTs are then purified and hydrogen storage techniques are used. It is found that the gravimetric hydrogen storage capacity of purified MWNTs is much higher than that of as-prepared one which means that purification process is very important for hydrogen storage. This could be attributed to the fact that there is more exposure to more surfaces of the multiwalled nanotubes. The ends were seen to be opened up. This allowed hydrogen to more easily move into the hollow core of MWNTs. XPS spectra of C1s of the purified sample is narrower and has no notable peak in the range of high electron binding energy. This indicates that the sample is in simple chemical state. This simple chemical state of C and lower oxygen contained groups correspond higher hydrogen storage capacity of carbon nanotubes.

 

There are many questions that must still be answered regarding nanotube hydrogen storage: How do we make process more efficient at lower temperatures in order to increase supply and decrease cost? What is the capacity loss with each storage cycle? Can other forms of carbon produce the same results just as effectively? What additional applications can increase demand and research into nanotubes?

Storage of Hydrogen

Various technologies are available for the storage of hydrogen.

High pressure tanks : Hydrogen gas can be compressed and stored in storage tanks at high pressure. These tanks must be strong, durable Liquid Hydrogen: It can be stored as liquid but has to be kept at cold.

Hydrogen combines with some metals, which can result in higher storage capacity compared to high pressure gas or liquid.

Carbon Nanotubes can store hydrogen.

Challenges

For transportation, the overloading technical challenge for hydrogen storage is how to store the amount of hydrogen required for a conventional driving range, within the vehicular constraints of weight, volume efficiency, safety and cost. The performance lifetime durability of these systems must also be verified and validated. The main challenges are:

• Weight & Volume: – The weight and volume of hydrogen storage systems are 

   presently too high.

• Efficiency: – Energy efficiency is a challenge for all hydrogen storage approaches.

• Durability: – Materials and components are needed that allow hydrogen storage

   systems with a lifetime of 1500 cycles.

• Refueling Time: – There is a need to develop hydrogen storage systems with

   refueling times being very low.

• Codes and Standards: – Codes and Standards for hydrogen storage systems and

   interface technologies which will help commercialization and implementation on a

   large scale and assure safety, have not been established.