Lupine Publishers | Energy Technologies in the Transition Period between the Fossil and Renewable Eras

 Lupine Publishers | Journal of Environmental & Soil Sciences

Abstract

The scarcity of conventional energy resources rises in the fuel prices and harmful emissions from the burning of fossil fuels have made power generation from conventional energy sources unsustainable and unviable. It is envisaged that the supply–demand gap will continue to rise exponentially unless it is met by some other means of power generation. Inaccessibility of the grid power to the remote places and the lack of rural electrification have prompted for alternative sources of energy until a developed stage of using renewable energy is mature and economical. Until then, a significant research and development is needed in the field of innovative use of hydrocarbons and energy efficiency, such as:

I. Carbon Capture, Storage and Utilization (CCSU) technologies.

II. Conversion of gas to liquids and chemicals (GTL).

III. Compressed Natural Gas (CNG) and Liquefied Natural Gas (LNG) technologies.

IV. Fuel Cell and Hydrogen (FCH).

V. Intelligent Energy Management (energy storage).

VI. Repowering and retrofitting.

VII. Hybrid Power Generating Systems (fossil and renewable sources).

VIII. Green Refinery Processes and Systems.

Keywords: Carbon Capture; Conversion of Gas to Liquid; CNG and LNG; Fuel Cell and Hydrogen; Energy Storage; Repowering and Retrofitting; Hybrid Power; Green Refinery

Abbreviations: CAES: Compressed Air Energy Storage, CCSU: Carbon Capture, Storage and Utilization, CNG: Compressed Natural Gas, CSP: Concentrated Solar Power, DHC: District Heating and Cooling, FCH: Fuel Cell and Hydrogen, FT: Fischer-Tropsch, GTH: Gas to Hydrate, GTL: Gas to Liquids, HRSG: Heat- recovery steam generators, HRES: Hybrid renewable energy system, ISCCs: Integrated Solar Combined Cycle Power Plants, LNG: Liquefied Natural Gas, NGCC: Natural Gas-Fired Combined Cycle, PFBC: Pressurized fluidized-bed combustor

Introduction

Because of the increase in energy demand and the prospective shortage of the fossil fuel with time, there is increasing in the need of sustainable resources. Hence, this is initially handled by using clean fuels [1], utilization of waste heat [2-6] and adopting different configurations [7,8], where resources and environment are conserved. The power generation from traditional energy sources is unsustainable and unviable because of the lack of traditional energy resources, increasing in the fuel prices and harmful emissions from the burning of fossil fuels. It is expected that this supply–demand gap will continue to increase exponentially unless it is met by some other means of power generation. unavailability of the grid power to the remote places and the scarcity of rural electrification have prompted for alternative sources of energy. The renewable energy resources, like sun, wind, water and biomass, have become better alternatives for conventional energy resources [9]. The important feature of hybrid renewable energy system (HRES) is to combine two or more renewable power generation technologies to make use of their operating characteristics is the best and to get efficiencies higher than that could be obtained from a single power source. Hybrid systems can address limitations in terms of fuel flexibility, efficiency, reliability, emissions and economics [10].

Compared to fossil fuels, renewable energy resources display a strong disadvantage: their interrupted nature. However, concentrated solar power (CSP) is a very important electricity generation technology, compared to other renewable electricity generation systems, because of its ability to incorporate a gas turbine. With a gas turbine technology, a hybrid operation guarantees a dispatch able power. while, conventional gas turbine power plants do not have a real ability for the integration of another intermittent energy source [11-13]. Because of the high decrease of fossil fuel reserves and the international commitment for the reduction of pollutant emissions, alternative energy sources are being intensively sought. Solar energy is one of the candidate energies sources to at least partially substitute fossil fueled generation plants. But, up to now generation costs for producing electric energy from pure thermo-solar input are high compared to conventional production [14]. There is a strong problem related to ensure a constant and predictable power delivery to the grid. One possible alternative are solar-fossil hybrid thermo-solar power plants. Among them, those based on a gas-turbine scheme have additional advantages as reliability, quick start-up and shutdown processes, fully dispatch able power, and probably the most important, very low water consumption. This is especially relevant in high solar irradiance areas usually linked to arid conditions [15].

All those reasons have led during the last years to important efforts to develop prototype and experimental plants to investigate the viability of the hybrid solar gas-turbine plant concept. Solar gas-turbine plants work by use of concentrated solar power to heat pressurized air displaying a Brayton cycle before entering the combustion chamber. The combustion chamber adds the demanded energy to fill the gap between the air temperature after receiving the solar input energy and the turbine inlet temperature. Modern recuperated gas-turbines are thus combined with a solar receiver usually located in a central tower [16]. Integrated Solar Combined Cycle Power Plants (ISCCs), contains a Concentrated Solar Power (CSP) plant and a Natural Gas-Fired Combined Cycle (NGCC) power plant, have been recently introduced in the power generation sector as a technology with the potential to simultaneously reduce fossil fuel usage and the costs of integrating solar power in an electricity system. This study quantifies the economic and environmental advantages of an ISCC power plant relative to a stand-alone CSP with energy storage, and a NGCC plant [17].

Carbon Capture, Storage and Utilization (CCSU) Technologies

CCS needs to be transported to safe geological storage, rather than being emitted to the atmosphere. Commercial deployment of CCS is expected to include cost reductions for CO₂ capture technology and improved techniques for monitoring store CO₂ . CCS will always require additional energy compared to projects without CCS [18]. Carbon capture and storage (CCS) is currently considered to be technically feasible at commercial scale using a range of technologies. Such as carbon dioxide (CO₂ ) injection activities in the oil and gas industry. Electricity generation projects with CCS producing 275 MW or more have been proposed around the world [19]. A number of R&D activities to develop technologies that would contribute to CCS implementation can be identified including:

a. actions to reduce costs.

b. Identification, performance and monitoring of appropriate storage sites [20].

CO₂-Capture Technologies

Post-combustion capture:

a. CO₂ removed from the combustion products just before they are vented to atmosphere.

b. Wet scrubbing with aqueous amine solutions and regeneration.

c. The CO₂ removed from the solvent in the regeneration process is dried, compressed and transported to safe geological storage [21] (Figure 1).

Figure 1: Principles of three main CO2 capture options [13].

All types of fossil fuels can be gasified (partially combusted) with sub-stoichiometric amounts of oxygen result in a ‘synthesis gas’ mixture of predominantly CO and H₂ . Additional (steam) is then added and the mixture is passed through a series of catalyst beds for the ‘water–gas shift’ reaction to approach equilibrium: CO+ H₂ O₃ CO₂ + H₂ (This promotes CO conversion to CO₂ ) [22]. The CO₂ can be separated to leave a hydrogen-rich fuel gas. The separation process typically uses a physical solvent. Energy requirements for CO₂ capture and compression in pre- combustion capture systems may be of the order of half that required post-combustion capture. But pre-combustion capture systems have to pay an efficiency penalty for the shift reaction [23]. Three main lines of CCS technologies with respect to efficiency, energy consumption, and technical feasibility, Figure 2 [24]. More gasification and gas turbine design economics are urgently needed for IGCC to capitalize on its emissions and carbon capture advantages [25]. The utility industry generally perceives IGCC power generation as not yet fully commercially proven. Subsequent CO₂ removal is accepted as commercially mature technology. IGCC is 10-20% more expensive [26]. The shift would then be applied to the desulfurized (sweetened) gas bulk CO₂ removal. Tradeoffs between sour vs. sweet shift need to be studied for each gasification technology and feedstock [27].

Figure 2: Schematic phase diagram of pure CO2.

Conversion of Gas to Liquids and Chemicals (GTL)

Chemical conversion of a gas containing hydrogen and carbon monoxide into synthetic crude (syncrude) by the Fischer-Tropsch (FT) synthesis process. The syncrude is then refined by traditional methods to produce liquid transport fuels [28]. Gas (H2 , CO) (Using FT)àSynthetic Crude (refined)àliquid transport fuel. As a feedstock, the process uses synthesis gas (syngas) generated from the gasification of solid, liquid or gaseous carbon rich materials, such as coal, natural gas or biomass [29].

Processes GTL process comprises three main steps:

a) Gas treatment: To remove impurities from the syngas and, where necessary, correct the hydrogen to carbon ratio (H2/CO) to within the acceptable range for FT synthesis.

b) FT synthesis: To produce syncrude by catalytic conversion.

c) Product work-up and refining: Separation of gases, hydrocarbon liquids and waxes then refining these into final products [30].

Impurities

A. Some impurities act as a poison to downstream catalytic reactions in the GTL process and reduce the efficiency of the process.

B. These impurities include condensed oils, solid particles, arsenic, mercury, oxygen, compounds of Sulphur and carbon dioxide [31].

These originate from the coal itself or through its gasification. Removal of contaminants from the gas can be by catalytic conversion or by adsorption or absorption [32]. On completion of gas treatment, the syngas is free of impurities and is rich in H₂ and CO, which are the basic elements needed to manufacture syn-crude. Gas treatment in commercial scale plants is undertaken using proprietary processes that are readily available and have a long history of development and operation [33]. A significant advantage of the gas treatment processes considered is that a clean CO₂ stream is obtained. The stream of captured CO₂ is ready for sequestration should that option be economically feasible. The purpose of FT synthesis is to combine CO and H₂ into carbon chains by reacting carbon monoxide and H₂ in the presence of a catalyst, heat and pressure. When focusing on diesel fuel production, the preferable carbon chains are long, saturated and paraffinic in nature (linear hydrocarbons containing only single bonds carbon-to carbon or carbon-to-hydrogen) [34]. Both olefins and paraffin’s will adsorb and desorb from the catalyst surface, causing the products to consist of a mixture of paraffin’s (Figure 3). GTL plants are generally significant users of electricity. The power used is for gas compression, heating and cooling. However, there are opportunities to use waste heat and steam recovery options for power generation [34,35] (Figure 4).

Figure 3: Example Hydrocarbon Reactions [33].

Figure 4: Detailed Gas to Liquids Process [35].

Gas to liquids

Thus, gas to liquids (GTL) is a refinery process to convert natural gas or other gaseous hydrocarbons into longer chain hydrocarbons such as gasoline or diesel fuel [36,37] (Figure 5).

Methane to methanol process

Methanol is made from methane (natural gas) in a series of three reactions [38].

Figure 5: GTL process using the Fischer Tropsch method [37].

Steam reforming

CH4 + H₂ O → CO + 3 H₂ ΔrH = +206 kJ mol−1

Water shift reaction

CO + H₂ O → CO₂ + H₂ ΔrH = 41 kJ mol−1

Synthesis

2H₂ + CO → CH₃ OH ΔrH = 92 kJ mol−1

Moreover, making liquid from natural gas could reduce carbon emissions and improve energy security [39].

Compressed Natural Gas (CNG): An Alternative to Liquefied Natural Gas (LNG)

Natural gas is transported to the markets by pipelines as LNG. Transporting the natural gas by pipelines is convenient and economically attractive onshore. For the offshore transport of natural gas, pipe- lines become challenging as the water depth and transporting distance increase. LNG, an effective means of transporting gas for long distances across the seas, constitutes 25% of the world gas movement. But LNG projects need huge investments, along with substantial natural-gas reserves, and are economically applicable for distances of 2,500 miles and beyond [40]. There are a lot of research works have been investigated on combined cycles displaying their proportional advantages such as: high thermal efficiency (47-50%), moderate capital cost, low cooling water requirement, fast loading, high reliability, operational flexibility, and environmental acceptability [41]. As shown in Figure 6, heat is taken from the LNG which is supplied at - 162°C and 53 atm in three stages. The first is through the condenser of a propane Rankine cycle, using sea water as the heat source. Having passed through the condenser, raising the LNG to - 35°C at 50 atm, the LNG is then passed through two expanders to the gas turbine engine, with three sea water heat exchangers being used [42].

Figure 6: Schematic diagram of the combined gas turbine engine and the cryogenic system [42].

The expanders of both circuits drive generators. The waste energy recovered by using the cryogenic plant is expected to improve the power and efficiency of the whole power plant. However, a thermo economic decision must be taken with regard to the feasibility of this combined plant [43]. CNG provides an effective way for shorter-distance transport. The technology is purposed at monetizing offshore reserves that can’t be produced due to the inaccessibility of a pipeline or due to the LNG option is very costly. Technically, CNG is easy to deploy, with lower requirements for facilities and infrastructure [44]. The results show that for distances up to 2,500 miles, natural gas can be transported as CNG at prices ranging from U.S. $0.93 to $2.23 per MMBTU compared to LNG, which can cost anywhere from $1.5 to $2.5 per MMBTU depending on the actual distance. At distances beyond 2,500 miles, the cost of delivering gas as CNG becomes higher than the cost for LNG because of the disparity in the volumes of gas transported [45]. Subsea pipelines, on the other hand, are appropriate for much-shorter-distance natural-gas transportation. The commercial promise for CNG is in shorter distance and smaller volume (and, therefore, smaller required dedicated reserves) for sea-going natural-gas transportation vs. LNG [46].

Converting natural gas to LNG reduces it to one-six-hundredth of its standard-conditions volume, allowing transportation by specialized tanker ships over long distances. The production and storage of LNG are usually conducted in onshore facilities. The major components of the value chain include:

a. natural gas production,

b. the liquefaction process (the “cascade” cycle is the most common technology) in which the pretreated natural gas becomes liquefied at a temperature of approximately (–160°C).

c. transportation.

d. regasification

e. distribution [47].

The equipment involved in the processing and transportation of LNG is very capital intensive and highly specialized. The liquefaction plant is the most expensive unit of LNG production, costing from U.S. $750 million to $1.25 billion. This is nearly 50% of the total investment. Offloading of the LNG requires special facilities, namely a regasification terminal. Regasification facilities cost $500–550 million depending upon terminal capacity. LNG tankers, dedicated specifically to each project, are complex and expensive [48]. Shipping of LNG is a function of distance of transport. Assuming the ships for transporting LNG are newly built, the unit cost of shipping ranges from U.S. $0.41 to $1.5/MMBtu for distances from 500 to 5,000 miles. Overall for LNG, the total investment can range from U.S. $1.5 to $2.5 billion depending on the market needs and number of ships required [49].

Compressed Gas

The gas can be pressured up to 200 atm at ambient temperatures. Another approach to transporting CNG has been named the Votrans (Volume Optimized Transportation and Storage), in which the natural gas is compressed and cooled to lower temperatures. This reduces the volume of the compressed gas, compared to just compressing it at ambient temperatures [50]. At the lower temperatures of -17 to –40°C, the process works at lower pressures than would be required at ambient temperatures. For the Votrans concept, the ships carry the chilled compressed gas in a boxlike structure called the “CNG module”. The design of the module consists of horizontal or vertical stacking of pipes [51].

Gas Compression

The basic concept for CNG is to compress the original natural gas, which is at a certain temperature and pressure, to higher pressures and, in one method, chill it to lower temperatures. Specially designed ships, which have an inclusion system, transport the cold compressed gas. The technology can be divided into three parts: compression, refrigeration, and transportation. The required power by the compressor and compression stage can be calculated on the basis of the compression ratio, volume of gas, temperatures, and pressures [52].

Transportation

The technology part of transportation includes the loading, the voyage using the CNG carriers, and the unloading. The transportation of CNG is capital intensive, requiring 85 to 90% of the total capital requirements for the process. The onshore infrastructure for loading the compressed gas into the ship requires mainly the compressor and accessories. The chillers are on board the transporting ships, which reduced the need for special loading infrastructure to handle chilled fluids. Unloading of the gas is accomplished with a fluid-displacement mechanism. The displacing fluid is a mixture of ethylene glycol and water [53].

Cost of CNG Technology

This transport of natural gas requires less capital to deploy than LNG and is well suited to exploit isolated supply sources and limited consuming markets. A CNG plant with loading facilities including compressors, pipelines, and buoys costs U.S. $30 to $40 million. CNG ships, with chiller and fluid displacement on board, cost from $150 million to $300 million for ship capacities of 400 to 1,000 MMBtu for Veterans. The number of ships required for a certain transport distance depends upon the loading rate, voyage distance, and time required for a ship to make a complete cycle of loading, transporting, unloading, and returning. Therefore, the required CNG ship numbers will increase with the transportationdistance increase [54].

CNG vs. LNG

In comparing CNG with LNG, the same transporting-ship realvolumetric capacity is used. However, in making the comparison, it is worth remembering the disparity in the actual standard volume of the gas transported. For the same ship capacity, LNG transports 2.1 Bcf of natural gas compared to a maximum volume of 1.2 Bcf transported as CNG [55]. To compare CNG with LNG, a review of the costs involved with both is indicated. These do not include the transportation costs of ships and other facilities required for loading or unloading. For CNG, the ships cost approximately U.S. $230 million, while for LNG, the ships cost approximately $160 million. The simplicity of the CNG operations provides an added advantage over LNG. The equipment required is easily available, with little or no detail to be customized. Both technologies share the common need for specially built ships carrying the respective cargoes of LNG or CNG. For LNG, the main consideration during its transportation is maintaining the state of the liquid, which in the case of CNG is simply maintaining the pressure and temperature without concern for changes of phase [56] (Figure 7).

Figure 7: Cost components for a CNG project [57].

H2 and Fuel Cells

Figure 8: Schematic drawing of a fuel cell.

The key criteria for an ideal alternative fuel are inexhaustibility, cleanliness, convenience, and independence from foreign control. H₂ is considered as one of the most promising fuels for generalized use in the future. Mainly because it is versatile, energy- efficient, low polluting, and a renewable. Hydrogen is environmentally favorable replacement for gasoline, heating oil, natural gas, and other fuels in both transportation and industrial applications. In nature, mostly the hydrogen is engaged to either oxygen or carbon atoms. So, to get hydrogen from natural compounds, there will be energy expenditure. So, hydrogen is considered as an energy carrier- a means to store and transmit energy derived from a primary energy source [58]. Presently hydrogen is mainly used in production of gasoline, fertilizers and metals. However, hydrogen requires energy to produce, store and distribute. Hence, hydrogen technologies need to be developed to reach the stage of competing with fossil fuels and other alternatives to produce power [59]. These technologies should emphasize efficient systems to reduce energy losses, and emissions. Among high efficiency technologies, fuel cells appear to be the most promising with high efficiency and very low environmental impact. Fuel cells are able to convert the fuel chemical energy into electricity, heat and water by reverse electrolysis, Figure 8. This leads to a much higher conversion efficiency. Both considerable primary energies saving and pollutant reduction, are achieved by upgrading conventional systems to fuel cell hybrid plants, Figures 9 & 10. Oil is essential in the transport sector while natural gas will become a more dominant fuel in power generation. Hydrogen economy is expected to offer considerable opportunities. Fuel cell development is an important step to the efficient use of hydrogen hence, research must continue in this area.

Figure 9: Combined SOFC and micro Gas Turbine.

Figure 10: Vehicle with Fuel cell and hydrogen gas.

Figure 11 shows an experiment was done in Jordan University of Science and Technology for testing fuel cells and nanotechnology. The preferable solution is producing H₂ from sustainable sources: wind energy, solar energy, waterpower or biomass. However, these energies will not be able to provide a massive contribution to meeting the energy demand for many decades to come.

Figure 11: Fuel Cell + Nano technology testing in Jordan University of Science and Technology [60].

i. Environmental reasons (large scale tolerance of wind energy).

ii. Practical reasons (availability of surfaces).

iii.

Economic reasons (cost of photovoltaic energy).

iv. Technological reasons (storage of intermittent energies).

1. Hence, the fuel cell is seen to be the most efficient energy converter in the near future, using H₂.

However, it still has major problems, such as:

A. Reducing the cost of fuel cells by a factor of 90%.

B. Enhancing the performance and durability of fuel cell systems by factor of 2.

C. Reducing the H₂ production and distribution costs by a factor of 3.

Intelligent Energy Management

Energy Efficiency

Energy efficiency is a convenient technology to be adopted by the developing countries. Energy conservation implies reduction in the consumption of energy, such as (turning thermostats down) [61]. Consuming less energy results in protection of the environment and preventing climate change through forcing people to make sacrifices in comfort, pleasure and convenience. Efficiency implies obtaining more useful heat, or work from each unit of energy supplied, either by technological improvements or reducing waste. Hence, Energy efficiency could be described in three ways:

a) Less energy for the same benefit (conservation).

b) The same energy for a greater benefit.

c) More energy for an even greater benefit.

Only the first description of energy efficiency (conservation) is sustainable. The second doesn’t lower gross energy use, and the third increases it.

If promoting energy efficiency, enhances the benefits of the end users, and doesn’t reduce the impact of energy and environmental costs. This is not sustainable. Improved energy efficiency must lead to measurably less gross energy use (reduced use of fossil fuels) and polluting emissions. Improved “energy efficiency technology” involves much more efficient: motors, air conditioners, furnaces, direct and indirect water heaters and computers [62]. Variable speed drives and variable volume HVAC with direct digital control, and energy management. Cogeneration and air-to-air heat pumps reduce the use of electricity and fuel in commercial buildings. Engines, for transportation, to have lighter aluminum blocks, fuel injection, turbo charging, overhead cams, automatic speed controls, and using unleaded fuel with catalytic converters to reduce emissions [63]. The bodies and frames of the cars to become lighter, smaller and more unified. They are made lighter with plastics and fiberglass shaped into aerodynamic forms. steel belted radial tires, front wheel drive, disk brakes with anti-lock features, light emitting diodes [64]. Interstate highway systems, speed limits legalized, carpooling, and most recently, the internet, email, and telecommuting reduced gasoline consumption and improved energy efficiency.

In power industry

Combined cycles and cogeneration systems. Trigeneration of power, heating and cooling. One-third of the oil used in most countries is used in transportation, by passenger cars and light trucks. The overall fuel efficiency of vehicles could be increased by improvements primarily in aerodynamics, materials, and electronic control [65].

The most fuel-efficient cars are:

a. Compact with small engines, manual transmission, low frontal area, front wheel drive and reduced vehicle weight.

b. Radial tires usually reduce the fuel consumption by 5 to 10 percent by reducing the rolling resistance.

Compressed Air Energy Storage CAES

Many countries devise their energy plans based on a preplanned energy mix option. Some do not have hydraulic systems and decided to exclude nuclear energy due to obvious reasons. Solar and wind, although the most mentioned renewable energy as important and clean, they have many drawbacks not the least being costly, intermittency, energy recovery and intensity. They do not constitute a high percentage of the world energy consumption. Hydrogen and fuel cells are nearer to the research and development stage. Some countries have some experience in utilizing gas turbines as a mature technology, with available caverns from oil fields, underground tunnels or especially designed and excavated, in a CAES system [66].

CAES has excellent load shaving characteristics, utilizing excess available energy at night, otherwise engines remain running in a spinning reserve non-efficient mode. Relatively much higher power (about 65%) when desperately needed for industry at a higher efficiency during the day becomes available instead of the nominal power [67]. CAES technology can help in increasing the viability of renewable sources. The integration of a wind farm and a PV system with CAES technology has been analyzed on a daily cycle. Integration of CAES and heat storage enables trigeneration of electrical power, heating and cooling to be produced. CAES has the common advantages of peak load gas turbine power plants and that of the pumped storage scheme [68]. The primary energy efficiency ƞpe, amounts to 47%, which could increase with higher air discharge rates. Thereby, CAES plants enjoy economic superiority at part loads versus the conventional gas turbine engines [69].

Repowering and retrofitting of Fossil Fuel Power Generation

Repowering

Figure 12: Schematic diagram of compressed air energy storage system for dry region [66].

Utilities have been adopting repowering as an option in their plants to meet the growth in loading. Repowering of older and marginally efficient steam plant by conversion to combined-cycle operation is carried out by the addition of a twin-shaft gas-turbine engine and a pressurized fluidized-bed combustor (PFBC). The lowpressure compressor rotates at variable speed to provide the PFBC with the required mass of air satisfy various load conditions [70]. Four arrangements were studied as shown in Figure 12, namely the basic System 1 with a heat exchanger interposed between the gas turbine and the PFBC, in addition to circulating the condensate of the steam plant through the gas turbine intercooler; System 2 circulates direct cold water; System 3 cancels the heat exchanger; System 4 cancels the heat exchanger and raises the turbine inlet temperature to 1250 C to anticipate the future performance of advanced systems. Results show the system 2 achieves marginal improvement, whereas system 3 achieves 7.3% higher power and 4% higher efficiency relative to System 1. System 4. achieves figures of 33.4 and 13.2% respectively [71]. In their continuous planning for load growth, utilities search for the most economic generation, subject to a number of constraints. One of the options open to many utilities with existing steam units is re-powering, which involves the installation of gas turbines and heat- recovery steam generators (HRSG) to convert older and marginally efficient steam plant to combined- cycle operation. One obvious benefit of re-powering relative to new construction is the saving in capital investment, mainly because of the re-use of existing equipment and structures, the use of owned land, existing transmission capacity and the sharing of infrastructure with other units [72].

Other important advantages are lower operation and maintenance costs, accruing from the sharing of existing support facilities and staff, and less complicated relicensing by the use of the station’s thermal discharge permits and intake structure. The combined cycle efficiency that comes of using large modern gas and steam turbo-sets is higher than that of current supercriticalpressure reheat steam power plants [73]. Furthermore, this efficiency is achieved with lower capital investment for standardized gas turbines and matching highly conventional sub-critical pressure steam conditions, and shorter periods for delivery and installation. Generally, the capital investment for such conversions, which improve overall station heat rate and increase station output, can yield high rates of return especially where high fuel costs prevail. If natural gas is available economically, gasturbine re-powering can offer many benefits besides increasing plant output and efficiency. These include lower specific emissions, improved operation and maintenance costs, improved capacity factor for large existing fossil-fuel units, higher reliability and improved operational capabilities such as cycling, load following and peaking [74]. The main disadvantages re-powering includes the complexity that may result when new high-efficiency units are matched to old equipment. There is also a potential for increased maintenance frequency of refurbished equipment. However, the repowering option is believed to be economically competitive overall. The major re-powering methods based on gas-turbines, involving fairly conventional technology and cycle configurations, call for addition of new equipment to an existing boiler/steam turbine installation such as substituting HRSG, adding supplemental HRSG, and gas turbine exhausting into the hot wind-box or feed-water heater. There could also be consideration of station re-powering where generating plant is replaced by a new combined cycle, retaining some existing plant auxiliaries and systems [75] (Figure 13).

Figure 13: The PFBC combined system.

Retrofitting

Gas turbines are emerging as a superior economic and environmental alternative to electric and diesel drives for refrigeration chillers- for retrofit and new application. Until recently, many of the facilities have used electric motor driven chillers or, in some cases, reciprocating engines. However, for economic and environmental reasons, there is growing interest in gas turbine drivers and gen-sets for both retrofit and new DHC operations [76]. In general, gas turbines have become serious competitors for refrigeration chiller applications. Key factors in favor of going to natural gas fired gas turbines as a driver for future applications are the high cost and uncertainty of electricity under peak demand conditions. Peak electric demand will only continue to rise in coming years. This raises the possibility of outages that would be less likely to occur in natural gas fueled turbines drive units [77]. There’s also question of cost: in a typical large building requiring 1500 to 2000 tons of refrigeration, the chiller system can account for 50% or more of electric demand and typically cost in range of $14000 to $ 15000 per month. With gas, you can achieve significant saving in fuel prices. Chillers loads are very expensive loads to satisfy with electric power, because they usually occur at a time of day when energy costs are highest. In addition to the fact that gas is less expensive than electricity at peak conditions, the demand for natural gas is lowest in the summer months which is when chiller loads are highest. This means that refrigeration operators can purchase gas at the lowest spot market prices available during the year. For these reasons, we feel you will see more and more DHC plants providing steam in winter months and chiller loads during the summer, Figure 14 [78].

Figure 14: Gas Turbine/ Absorption Chilling System.

Hybrid Power Generation Systems

Figure 15: Diagram of ISCC plant [79].

It has become essential for the power and energy engineers to search for the renewable energy sources like wind, sun, geothermal, ocean and biomass as prospective and cost-effective alternatives for traditional energy sources. However, because of these renewable energy resources not available all the time during the year that has led to investigate in the area of hybrid renewable energy systems. In the past few years, a lot of research has taken place in the design, optimization, operation and control of the renewable hybrid energy systems. It is indeed evident that this area is still emerging and vast in scope [79] (Figure 15). The unprecedented increase in the demand for energy and in particularly electricity has led to a huge supply-demand gap in the power sector. The scarcity of conventional energy resources rise in the fuel prices and harmful emissions from the burning of fossil fuels has made power generation from conventional energy sources unsustainable and unviable [80]. It is envisaged that this supply-demand gap will continue to rise exponentially unless it is met by some other means of power generation. Inaccessibility of the grid power to the remote places and the lack of rural electrification have prompted for alternative sources of energy. The renewable energy resources, such as wind, sun, water, sea and biomass, have become better alternatives for conventional energy resources [81].

HRES is becoming popular for stand-alone power generation in isolated sites due to the advances in renewable energy technologies and power electronic converters which are used to convert the unregulated power generated from renewable sources into useful power at the load end. The important feature of HRES is to combine two or more renewable power generation technologies to make best use of their operating characteristics and to obtain efficiencies higher than that could be obtained from a single power source. Hybrid systems can address limitations in terms of fuel flexibility, efficiency, reliability, emissions and economics [82]. Solar PV and wind systems are the primary sources of energy. This energy is available in abundance but is intermittent in nature and site specific. To overcome this drawback, back up energy devices (secondary sources) are introduced into the system to supply the deficit power and to take care of transient load demands. Renewable energy sources essentially have random behavior and cannot have accurate prediction [83]. To provide a trusted and economic supply from the new technologies of renewable energy, need a huge scale, effective cost, efficient energy storage system to keep the lights on when there is no blowing for the wind or no shining for the sun. the needed storage technology is not existing and is unlikely to emerge in the next 20 years [84].

Modern power systems are huge, complex and inherently unstable. If the changes are too much, the lights will go out. The discontinuous energy technologies like wind power and solar power give a new source of main changes and increase the difficulties of keeping the light on [85]. Most power systems have a capacity factor of between 50% and 70%. In some countries the peak demand occurs in winter evenings (heating loads) and in the middle of summer (air-conditioning loads) in other countries. Power systems have a need for “frequency keeping” to guarantee that the amount of generated electricity completely matches the demand. Such as, the beginning and end of a popular TV program [86]. If there is an unexpected loss of a huge amount of generating capacity, then it is occasionally needful to shed part of the load to avoid a system collapse (blackout). Unlike traditional power stations, a change in the output of large wind farms of 50% in short time is not strange. The efforts to expect the output of wind farms more than an hour or so ahead have not been successful [87]. Capacity factors vary from 18% to 37%. At a 25% capacity factor wind power costs about 12c/ kWh at the station gate. In the US, the costs of power generation of coal and nuclear are 2–4c/kWh. A new Finnish study calculated a cost for nuclear of 3.5 Euro cents (5.25 US c) and for coal of 4.5 Euro cents (6.75 US c). The solar power capacity factor is about 20%. The output changes predictably every day and unpredictably every time a cloud passes over the sun and drops the output by as much as 60%. The capital cost is in excess of $5000 per peak kW. The unit cost is around 40cents [88].

Most new renewable energy technologies increase the cost and problems of operating a power system because they are discontinuous and/or unexpected and their output varies quickly. For a power system operator, there is no difference between a large load change and a quick change in the output of a large wind farm or solar power plant. None of these renewable energy technologies would exist without grants, tax breaks and massive subsidies. In addition, consumers, taxpayers and ratepayers, not the generators, pay for the cost of transmission and backup power stations [89]. Modern economies can make a massive reduction in the consumption of energy without any effect on the economy. A lot of these demands come from organizations participated in carbon trading or the already heavily subsidized renewable energy industry. Renewable Energy Council (Europe’s largest renewable energy trading association) said that it was possible to exchange all United States power generation from coal, nuclear and oil-fired power stations with low cost renewable energy. However, most of this renewable energy-wind and some solar and ocean would be discontinuous and migrant [90].

These claims all ignore:

A. The big and complicated problem of storing the energy when it is available so that it can be used when it is needed;

B. The serious problems of coping with the unpredictable and rapidly fluctuating output of the wind farms;

C. The very high real cost of wind power.

D. The cost and problems of the large-scale long-distance power transmission that would be needed [91].

To clarify the problems listed a demand of 10,000 MW with a capacity factor around 60% giving a yearly energy demand of about 55,000 GWh pa. was studied the total cost of supplying this system, from:

a) The well proven combination of nuclear power and hydro pumped storage.

b) A combination of wind power and pumped storage [92].

Nuclear power is not well suited to following variations in load it is normal practice to associate nuclear power stations with hydro pumped storage. This allows the nuclear power stations to run efficiently and economically at a steady output. The pumped storage schemes pump water into an upper reservoir when the load is low and uses this water to generate extra electricity when the load is high. For a nuclear-based system these pumped storage schemes need to have sufficient storage for about 10 hours’ operation at maximum output. lines would be heavily utilized. Assuming a typical wind power capacity factor of 25% and allowing for pumping losses, the system supplied by wind power needed 31,000MW of wind power to generate the required 68,000 GWh pa. It is well established from aggregating existing wind farms that wind power output would seldom exceed 80% of installed capacity (25,000MW) so a pumped storage capacity of 20,000MW was needed for long periods [93]. The next step was to calculate the cost of generation from the two systems. a cost of $4000 per kW for the nuclear power stations was used. For wind power a cost of $2250 per kW was used based on investigations carried out into the cost of recent wind farms worldwide. A cost of $1500 per kW for the pumped storage schemes, is reasonable for schemes with 6-10 hours storage. It is assumed that the transmission cost would be directly proportional to the installed capacity. This is conservative because the wind farms would be widely scattered around the country and the transmission system would have to be able to transmit large amounts of power from regions where the wind was blowing-which could vary from hour to hour-to the load centers and the pumped storage schemes.

Green Refinery Processes and Systems

With 300MW cogent-merchant plant operating on refinery residue gas. Cogen plant is sized to supply up to 2.7 million kg/h process steam and 90 MW of electric power surplus 240 MW of capacity is being marketed as merchant plant power. Process steam production is the main driver for the entire cogent project [94]. By contract, steam must be delivered continuously 365 days of the year to the Complex refinery. There is little room for error and stiff penalties if there are major outages beyond certain exceptions [95].

Residue gas fuel:

a) The refinery provides both the primary fuel which is high hydrogen residue gas and the supplemental and backup fuel which is natural gas.

b) Preferred operation is 75 % residue gas and 25 % natural gas for max plant economics.

Figure 16: Schematic diagram of the cogeneration system [97].

c) Residue gas varies in hydrogen content from around 20 % up to 80 % by volume, depending on what the refinery is producing, with the remainder being methane, with traces of ethane, butane, etc. [96] (Figure 16).

IGCC plant is fully integrated with refinery (Figure 17). A new refinery in Germany has tremendous demands for steam, electricity and process water. A power plant has been built at the refinery which in addition to meeting these needs also burns residue oil from the refinery process [98].

Figure 17: Overall material/energy balance of the IGCC plant.

Water Treatment Plant

The water treatment plant handles some 2000t/h of raw water to produce demineralized water. The other part is provided by purification of return condensates (400t/h). The heat contained in the return condensate is also used for heating boiler feed water [99]. The special challenges and complexities of the industrial power plant are well illustrated by the IPP facility that is supplying electricity steam water and compressed air to the new MIDER refinery in eastern Germany. The plant burns very heavy and awkward oil residues, must achieve very high levels of reliability and must meet rigorous flue gas cleaning requirements. In addition, it takes its raw water from a polluted river necessitating construction of one of Europe’s largest reverse osmosis water treatment plants [100]. Oil refinery IGCCs turning waste feedstock into product and power. The C-rich syngas properties of many gasification products coal, petroleum, coke, residual oil, wastes or other hydrocarbon compounds, when added to an IGCC, the process converts part of the CO-rich syngas to methanol, and the unconverted gas is used to fuel the combined cycle system [101] Table 1.

Table 1:

Conclusion

Until renewable energy has a great portion of energy pie, it is recommended to work on a wide front of innovative energy systems with high efficiencies and reduced harmful effects on the environment. This front could include some or all of the following technologies:

a. Carbon Capture, Storage and Utilization (CCSU) technologies.

b. Conversion of gas to liquids and chemicals (GTL).

c. Compressed Natural Gas (CNG) and Liquefied Natural Gas (LNG) technologies.

d. Fuel Cell and Hydrogen (FCH).

e. Intelligent Energy Management (energy storage).

f. Repowering and retrofitting.

g. Hybrid Power Generating Systems (fossil and renewable sources).

h. Green Refinery Processes and Systems

Acknowledgement

The author would like to thank Eng. Ahmad Abu-baker and Duaa M.H. Kharouf for the valuable help.

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Lupine Publishers | Electricity Generation using Fossil Fuel, Renewable and Nuclear Energy: A Review

 Lupine Publishers | Journal of Environmental & Soil Sciences

Abstract

Electricity makes our lives better, brighter, and cleaner. But before it is transmitted on high-voltage power lines and then distributed to our homes and businesses, it needs to be generated by a power plant. Electricity is vital to modern life. It powers our lights and appliances at home. It powers many industry processes. It is used to power trains and to charge electric vehicles. Globally, electricity use is rising rapidly as new major economies develop in giant countries in population. This need for electricity drives a growing demand for electricity generation, with thousands of new power plants needed across the world over the coming decades. The fundamentals of electricity generation haven’t changed much over time, but the associated technologies have radically changed over time and are constantly evolving. Politics also plays a large role and influences the types of fuels we use for electricity, and politics will continue to drive major changes in the future. Energy sources are either fossil fuels, nuclear, renewable energy. Electricity generation is a key contributor to global emissions of greenhouse gases (GHG), NOx and SO₂ and their related environmental impact.

Keywords: Electricity; Electricity production; Fossil Fuel; Nuclear Power; Renewable energy

Abbreviations: AC: Alternating Current; CCGT: Combined Cycle Gas Turbines; CCS: Carbon dioxide Capture and Storage; CH₄ : Methane; CO: Carbon Monoxide; CO₂ : Carbon Dioxide; CSP: Concentrated Solar Power; GHG: Green House Gases; HVDC: High Voltage Direct Current; IGCC: Integrated Coal Gasification Combined Cycle; LCA: Life Cycle Assessment; LCI: Life Cycle Inventory; NH₃ : Ammonia; N₂ O: Nitrous Oxide; PV: Solar Photovoltaic; SO₂ : Sulfur Dioxide; UF₆ : Uranium Hexafluoride; UO₂ :Uranium Dioxide

Introduction

Electricity plays an important role, especially in developing economies because the efficient usage of different resources depends directly on the electricity. Almost all sectors, whether it is agriculture sector, industrial sector, or household, all depend directly on electricity [1]. But Electricity generation is a key contributor to global emissions of greenhouse gases (GHG), NOx and SO₂ and their related environmental impact. There are a lot of case studies including the life cycle assessment (LCA) of electricity generation depending on hard coal, lignite, natural gas, oil, nuclear, biomass, hydroelectric, solar photovoltaic (PV) and wind was studied to know the ranges of emission data for GHG, NOx and SO₂ related to individual technologies [2]. With the increase in energy demand and the expected shortage of the fossil fuel with time the need for sustainable resources increases. Hence, this is initially handled by using clean fuels [3], utilization of waste heat [4-8] and adopting different configurations [9-11], where resources and environment are conserved. Between 1990 and 2008, world energy consumption increased by 40%. nowadays 68% of the energy used in the world originates from fossil fuels, with electricity generation being responsible for 40% of global CO₂ emissions [12]. Between 1973 and 2007 the worldwide electricity generation increased threefold as the world became more and more dependent on electricity to meet its energy needs as shown in Figure 1.

Currently, and in the past, the supply is almost entirely based on conventional fossil fuels: they amount to worldwide over 80 percent of the generated 19,771 TWh in 2007 [13-14]. Emissions of GHG, NOx and SO₂ were selected depending on their contribution to many critical LCA influence categories and on their importance in decision making and strategic planning. in all the world, the energy sector contributes 19% and 56% of overall NOx and SO₂ emissions [15], respectively, however contributions to GHG emissions amount to 40%. In addition to NH₃ , emissions of NOx and SO₂ are largely responsible for acidification. Because NH₃ is primarily emitted from animal waste in agriculture [16], NOx and SO₂ emissions provide a reasonable approximation for contributions to acidification due to electricity generation. The only technology available to mitigate GHG emissions from large-scale fossil fuel usage is carbon dioxide capture and storage (CCS). CCS is a process whereby CO₂ shall be captured from gases that would otherwise be emitted via smokestacks into the atmosphere, and then be injected into deep geologic formations for permanent storage [17]. However, CCS already faces many challenges that are not only related to issues such as financing demonstration projects and integration of adequate infrastructures, but also to efficiency. For example, capturing and compressing CO₂ would increase the fuel needs of a coal-fired power plant by 25 to 40 percent [18-20].

Figure 1: Worldwide Electricity Generation, Fuel Shares Comparison 1973 to 2007 [14].

Electricity is a primary energy carrier, and the emission data that are related to electricity generation are utilized extensively for accounting and reporting purposes. Datasets and emission factors for electricity generation are utilized often when performing LCA and/or GHG accounting of products. However, even though the importance of data reliability and the large number of studies that value electricity generation, significant discrepancies can be found among LCI datasets for similar electricity technologies [21-23]. Policies to decrease climate change are driving the decarburization of electricity generation worldwide and may be tackled by a combination of technologies, from renewables like hydro, wind and solar, to fossil fuels with carbon capture and storage (CCS) and nuclear power [24-26]. For example, In the UK, 90% of electricity generation generated by thermoelectric power stations, whilst electricity sector abstractions make up approximately half of all water abstractions in England and Wales [27].

Figure 2: The UK Electricity Sector in 2010.

For UK the electricity mix is dominated by thermoelectric generation capacity which contributes to 90% of the roughly 380 TWh generated each year. Of electricity supplied to the grid, in 2010 conventional thermal contributed 124 TWh (34%), combined cycle gas turbines (CCGT) 168 TWh (46%), nuclear 56 TWh (15%) and the remaining 17 TWh (5%) was renewables as shown in Figure 2 [28-29]. Energy has become a crucial element for sustainable development and well-being of any country in modern era. Thermoelectric generation contributes to 80% of global electricity production. Cooling of thermoelectric plants is often achieved by water abstractions from the natural environment. Globally, 80% of electricity generation generated from thermoelectric power stations such as fossil fuels and nuclear, all of it requires cooling for efficient and safe operation [30]. Most of the power in the United States produced from thermal generation-using heat to create highpressure steam and drive turbines [31]. By this coal, nuclear, and most natural gas plants produce electricity. In 2013, coal, natural gas, and nuclear power accounted for a combined 86% of total electricity generation in the U.S. (39.1% coal, 27.4% natural gas, and 19.4% nuclear) as shown in Figure 3 [32]. There are 4 main types of cooling used by the electricity sector which use different amounts of water and energy as shown in Table 1 [33]. Along many decades almost all the consumed electricity in the world has been generated from three different forms of power plant - fossil, hydro and nuclear. Renewables currently generate a relatively small share of the world’s electricity, although that share is growing fast [34].

Figure 3: Electricity Generation in the U.S. for 2013.

Table 1: Characteristics of different power generation cooling systems.

Fossil Fuel Power Plants

Fossil fuels are an important part of global energy portfolio and play a significant role in current and future domestic energy security. A sustainable, low-carbon future will need to extract carbon dioxide from major coal and natural gas plants. However, carbon dioxide emissions from fossil fuel generation are usually very variable every day, with daily side variations between plants. We consider this change is critical to determining the appropriate carbon price as well as determining whether the CO₂ power plant will capture [35-36]. Natural gas is playing an increasingly important role in the current and future world energy portfolio. For example, the ratio of the US’ electricity generated by natural gas increased from under 18% in 2002 to almost 25% in June 2012 [37], with increase of around 40% (coal use fell from 50% to 42% and non-hydro renewables rose from 2% to 5% in the same time period).More than 65% of the world’s electrical energy used today is generated by steam turbine generators burning fossil fuels as their source of energy and large scale fossil fueled plants provide most of the world’s base load generating capacity. Fossil fueled plants use either coal (60%), oil (10%) or gas (30%) in purpose designed combustion chambers to raise steam. These are all nonrenewable resources whose supply will ultimately be exhausted [38-40]. Figure 4 shows the fossil fuel powered steam turbine electricity generation.

Figure 4: The Fossil Fuel Powered Steam Turbine Electricity Generation.

Fossil fuel power plants burn carbon fuels such coal, oil or gas to generate steam that drives large turbines that produce electricity. These plants can generate electricity reliably over long periods of time. However, by burning carbon fuels they produce large amounts carbon dioxide, which causes climate change. They can also produce other pollutants, such as sulfurous oxides, which cause acid rain [41,42]. Fossil fuel plants require huge quantities of coal, oil or gas. These fuels may need to be transported over long distances. The price of fuels can rise sharply at times of shortage, leading to unstable generation costs [43]. In fossil-fuel power plants, water is heated by burning fossil fuels in a furnace to produce steam which drives a turbine as shown in Figure 5. The turbine is attached to an electric generator that converts mechanical energy into electricity. During this process, about 60 percent of the energy input is “lost” irreversibly [44,45]. The losses occur within:

Figure 5: The Process of Electricity Generation [44].

a. The burning process (flaring losses).

b. In the conversion of liquid water to steam.

c. In heat radiation.

d. The mechanical losses in the turbine and the generator and in cooling the steam after it has been used in the turbine.

Many Other Places

The electricity generated at the power plants is alternating current (AC)at relatively low voltage, because of insolation inside the generators. The electricity is then stepped up to high voltage using transformers, before being transmitted to substations located hundreds of kilometers away [46]. Finally, the electricity enters, again via transformers, the low voltage distribution system for its use in homes, offices and factories. As an unavoidable consequence, at least another 10 percent of the electricity vanishes in transmission losses. Moreover, many of today’s electric appliances are extremely inefficient, wasting much of the power they consume as heat. Approximately 95 percent of the energy consumed by an incandescent bulb is emitted as heat, only the remaining 5 percent are given off as light [47,48]. When the entire cycle of generating, transporting and using electricity is considered, the electricity is delivered to the user of the incandescent bulb with an overall efficiency of just 2 percent.

All these losses are simply accepted because the user, be it industry or households, are paying all the losses with their utility bills [49]. If the consumers were to be shown that they are not only paying the kWh logged by their meter, but in fact the fuel which is delivered into the power plants, they might start thinking about their energy-consuming behavior [50]. Coal-fired power plants are also by far the largest source of the carbon dioxide (CO₂ ) and other global-warming pollutants such as methane (CH₄ ) and nitrous oxide (N₂ O). The efficiency of fossil fired power plants range from 30 to 38 percent depending on the age of the plant and the technologies used [51,52]. Electricity generation from natural gas power plants changes depending on an hourly basis. Usually this is because it is easy to ramp generation up and down and send natural gas power at short notice for changing demand throughout the day unlike, say, nuclear power that tends to fulfill steady base load requirements. Figure 6 shows electricity generation (columns, primary y-axis) and capturable CO₂ (solid areas, secondary y-axis) over a 24 h period for the average (median) natural gas power plant peaking at 1000 MW h [53,54].

Figure 6: Electricity generation (columns, primary y-axis) and capturable CO² (solid areas, secondary y-axis) over a 24 h period for the average (median) natural gas power plant peaking at 1000 MW h [53].

New technologies like the (ultra-) supercritical pulverized coal combustion or the process of integrated coal gasification combined cycle (IGCC) can raise the efficiency for electricity generation to 45 percent and to more than 50 percent respectively. However, the environmental damage remains the same, inducing further even unknown changes in the global climate system [55]. Fossil fuel power will continue to play an important role in local energy policy. This will be driven by relatively low-cost and abundant gas supply and demand for a clean, reliable fuel that can respond quickly to changes in the electric grid. In addition to increased use, renewable energy integration will require natural gas to generate electricity to balance fluctuating supply, increasing gas trend to be used in an uneven and peaking fashion [56].

Nuclear Power Plants

The future of nuclear power will depend on whether it can meet several objectives simultaneously economics, operating safety, proliferation safeguards and effective solutions to waste disposal. Nuclear power plants use the heat produced by nuclear fission to generate steam that drives turbines, like what happens in fossil fuel plants. However, no greenhouse gases are produced in this fission process, and only small amounts of greenhouse gases are produced across the whole fuel cycle. Nuclear power plants can run for many months without interruption, providing reliable and predictable supplies of electricity [57-60]. Some consider nuclear power plants to be a “clean” electricity source, since the plants themselves do not directly emit CO₂ and other GHGs. Nevertheless, the operation of nuclear power plants results in the immense environmental impacts which are displayed in Figure 7. After a cost intensive exploration process, uranium ore is recovered from the earth’s crust under quite difficult conditions [61]. Nuclear power generation is an effective GHG mitigation option, especially by way of investments to extend the lifetime of existing plants. Whether or not building more nuclear power plants will be accepted depending on new designs becoming economically competitive, and on the industry’s ability to restore public confidence in its safe use [62].

Figure 7: Electricity from Nuclear Energy

It must be extracted from the mined ore using strong acids and bases, and then be converted into either uranium dioxide (UO₂ ) for heavy water reactors or gaseous uranium hexafluoride (UF₆ ) for light water reactors. Most reactors require uranium fuel to have a U-235 (an isotope of uranium) content of 3 to 5 percent. For this step, large amounts of electricity, mostly provided by fossil fuel plants, are needed to increase the actual concentration of 0.7 percent to 3 to 5 percent [63]. Afterwards, the uranium is manufactured into fuel pellets by pressing powdered UO₂ or UF₆ into cylindrical shapes and baking them at high temperatures, usually between 1,600 and 1,700⁰C. Finally, energy is released in a reactor by controlled nuclear fission reactions just to:

a. boil water.

b. produce steam.

c. drive a turbine that generates electricity.

This process alone has an efficiency of only 35 percent [64]. For steam production and for cooling, approximately 2.5 times more water is needed for nuclear than is required for fossil fuel plants. This is the reason why nuclear power plants are located at rivers or lakes. After the nuclear fuel is consumed in the reaction process, it is removed from the reactor and stored on site in large water-filled pools for about five years. Later, the radioactive waste is transferred to underground caverns for medium-term storage [65]. Nuclear fuel can be used in a reactor for several years. The used fuel that remains after this time must be stored and then either recycled to make new fuel or carefully disposed of. However, because the amount of fuel used to generate electricity is so much less than that used in fossil fuel plants it is much more practical to do this with used nuclear fuel than with the wastes and emissions from fossil fuels [66].

At present, there are no safe disposal facilities in operation anywhere in the world which can accept radioactive waste for permanent storage. A radioactive waste disposal facility since the seventies, its storage has recently been found to be unstable. According to World Nuclear News, roughly 126,000 barrels filled with low-level radioactive waste including contaminated clothes, paper and equipment need to be brought to the surface for alternative storage [67-69]. A challenge involves approximately Euro 3.7 billion, and a rather gracious heritage for future generation(s). Can a process like this that poses health risks exceeding that of any other process of electricity generation be called “clean”? We always need to keep in mind that already a minor failure in a nuclear power plant can create severe consequences for all forms of life on earth [70].

Renewable Energy

The negative influences of fossil fuels forced scientists to call attention to cleaner energy sources that are both friendly and renewable most suitable environment [71]. The fast depleting conventional energy sources and today’s continuously increasing energy demand in the context of environmental issues, have encouraged intensive research for new, more efficient, and green power plants with advanced technology. Since environmental protection concerns are increasing in the whole world today, both new energy and clean fuel technologies are being intensively pursued and investigated. Most of the renewable energy from wind, micro-hydro, tidal, geothermal, biomass, and solar are converted into electrical energy to be delivered either to the utility grid directly or isolated loads [72-75]. Natural energy flows vary from location to location and make the techno-economic performance of renewable energy conversion highly site specific. Intermittent sources such as wind, solar, tidal and wave energy, require backup if not grid connected, while high penetration into grids may eventually require storage and/or back-up to guarantee reliable power supply [76]. Renewable electricity has come to dominate the debate over the development of the European electricity market. Among European countries. Most wind turbines and solar panels are installed in Germany where renewable electricity has become even more important since the March 2011 decision regarding the nuclear phase-out [77]. In 2011, wind electricity accounted for 8 per cent of gross electricity production in Germany, whereas solar PV for 3 per cent. All renewable sources combined made up 20 per cent of gross electricity production in 2011 and are Germany’s second most important source of electricity generation after lignite [78]. Renewables such as wind, solar and small-scale hydro produce electricity with no greenhouse gas emissions at the point of generation and very low amounts of greenhouse gas emissions across their entire lifecycle [79]. Figure 8 shows the installed capacity and generated electricity in Germany.

Figure 8: Installed Capacity and Generated Electricity in Germany [80].

Many renewables do not produce electricity predictably or consistently. Electricity generation from wind turbines varies with the wind speed. The output of solar panels is reliant on the strength of the sunshine, which depends on the time of day and the amount of cloud cover. This means that renewables have to be backed up by other forms of electricity generation, often fossil fuel generation with their resultant greenhouse gas emissions [81-83]. System and market operators in the world face two main challenges as more renewable power generation is added:

a. Electricity generated by wind turbines and photovoltaic panels is intermittent and hardly adjustable to electricity demand. Therefore, variable electricity generation is not a perfect substitute for conventional energy sources.

a. World’s renewable energy policy grants priority dispatch and fixed feed-in tariffs for renewable electricity generation. Renewable electricity can be fed into the grid whenever it is produced, regardless of energy demand, and in-feed can be switched off only if grid stability is at risk [84-85].

b. The cost of electricity generation from many renewables tends to be higher than other forms of generation. Often requiring subsidies to compete with other forms of generation, although these costs are coming down.

Wind

The global market for renewable energy sources including wind energy is growing rapidly, especially during the last two decades. The turbine has a power source fuel-free and does not cause any inexhaustible amount of pollution during the production of electricity. In addition, it can produce wind turbine energy close to load centers eliminated a transmission loss in the lines in rural and urban landscapes [86]. Wind energy is the most efficient renewable energy source for electricity generation in modern power systems. In the past decades, wind power has experienced a rapid growth worldwide. Wind power can supply up to 20% of the electricity consumption in many countries. Wind power brings many more uncertainties than conventional generation. Accurate and reliable wind power forecasting becomes extremely important to optimize the operation cost and improve the reliability of the power system with increased wind penetration [87].

While it is true that renewable energy sources are environmentally friendly, or “green”, one has also to consider their feedstock. Solar, wind, hydro and geothermal energies are “free” at first glance, although they require huge land-use investments with environmental unfriendly footprints especially biomass [88]. It is also obvious that electricity from renewable energies has considerable disadvantages in the way they are deployed today. First, and foremost, they are dependent on certain conditions (availability of wind, water and sunshine) [89]. Due to their intermitted nature, this deployment method is overstraining the grid, which is additionally rather inefficient. This fact is used by the grid operators as an argument to ask for governmental help to not only improve the grid, but to make it “smart”. At the end of 2008, the worldwide nameplate capacity of wind-powered generators added up to 121 GW, a mere of 1.5 percent of the world’s electricity usage. But the rapid growth continues, with China doubling its wind power capacity for the fifth consecutive year since 2004 [90-91].

In 2016, wind turbines in the United States were the source of nearly 6% of total U.S. utility-scale electricity generation. The amount of electricity generated from wind has grown significantly since 2000. Electricity generation from wind in the United States increased from about 6 billion (kWh) in 2000 to about 226 billion kWh in 2016. This turns out to be a big problem for the gridoperating utilities, because electricity must be used as soon as it is produced. But how easily can you forecast when and where the wind will blow? You can’t simply start a wind mill up when you need it most [92]. Thus, at least as the electricity grids are operated today, the intermittency of wind always requires backup systems with an equal amount of dispatch able generation capacity. Unfortunately, now these back-up systems are mostly conventional power plants that do not have short run-up times [93].

In addition to the unpredictability of wind, wind farms usually need high investments to be built, and are also very expensive to properly maintain. At least 20 percent of the windmills are shut off for maintenance or repairs. What is even worse, they are often taken off the grid, because their electricity is not needed at that given moment. What a system, what a waste of resources. There are no commercially viable ways to store wind energy at this time, other than pumping up water electrically in water reservoirs, but this only makes sense when wind farm and water reservoir are close to each other [94,95].

Solar

The various forms of solar energy, solar heat, solar photovoltaic, solar thermal electricity, and solar fuels offer a clean, climatefriendly, very abundant and in-exhaustive energy resource to mankind. Solar power is the conversion of sunlight into electricity, either directly using photovoltaic (PV), or indirectly using concentrated solar power (CSP) [96]. The amount of energy that comes from the sun is phenomenal: If we could somehow gather all the energy that reaches the earth on one day and store it, it would supply the energy needs of the whole world for almost 30 years [97]. Moreover, solar radiation is the sole source for fossil or renewable energy that we use today. Electricity from sunlight can be generated directly using photovoltaic, solar cells, or indirectly as with concentrating solar power. Photovoltaic systems and some other renewable energy systems are, therefore, an excellent choice in remote areas for low to medium power levels, because of easy scaling of the input power source [98,99].

Consider the interesting aspect: PV solar cells convert the sun’s radiation into DC power on which most of our appliances actually run. But this power is converted into AC power by inverters and fed into the inefficient grid, only to be inverted again to DC [100]. The efficiency of energy conversion depends mainly on the PV panels that generate power. The practical systems have low overall efficiency. This is the result of the cascaded product of several efficiencies, as the energy is converted from the sun through the PV array, the regulators, the battery, cabling and through an inverter to supply the ac load [101,102]. At this point, the most cost-effective and efficient technology for converting solar power into electricity are huge solar-thermal power plants (Figure 9). Here, sunlight is gathered by a large solar-collecting field with parabolic mirrors, so called troughs [103]. These collectors track the sun over the course of the day and concentrate the sunlight onto absorber pipes where the radiation is converted into heat. A heat transfer fluid which is circulating through the pipes is heated up to temperatures of almost 400^o C [104].

Figure 9: Scheme of a Solar Thermal Power Plant (STPP).

The heat is used to generate steam with which electricity is then produced by conventional steam turbines. The process water is then cooled and returned to the cycle. The surplus heat could be used for heating, desalination, cooling, air conditioning, and other applications, but in most cases, it is currently rejected to the atmosphere. Solar-thermal power plants have been in commercial use for several decades since (1982). Thermal molten salt storage enables electricity production even during the night, or on cloudy days. The storage time, however, is calculated to be seven hours [105]. Water is mainly used for cooling the steam circuit, i.e. from the vaporization of water in the cooling towers. So, the plant operators not only have to capture the power of the sun, but also need immense amounts of water for cooling of the heat transfer media. As most solar power plants today is located in deserts, this physical necessity may be an obstacle to development on the long run [106].

Biomass Gasification

Gasification is a thermochemical partial oxidation process in which carbonaceous substances (biomass, coal, and plastics) are converted into gas in the presence of a gasifying agent (air, steam, oxygen, CO₂ or a mixture of these). The gas generated, commonly referred to as syngas (synthesis gas), consists mainly of H2 , CO, CO₂ , N₂ , small particles of char (solid carbonaceous residue), ashes, tars and oils [107,108]. Gasification technology has been investigated to effectively and economically convert low value and highly distributed solid biomass to a uniform gaseous mixture mainly including hydrogen (H2 ), carbon monoxide (CO), methane (CH₄ ) and carbon dioxide (CO₂ ) [109]. The main target from biomass gasification process is the best possible conversion of solid biomass fuels into a high calorific product gas. Thereby, biomass reacts with air, oxygen, steam or CO₂ , which provide oxygen for the process. Due to the thermal cracking and the partial oxidation a product gas is formed. The composition of the product gas depends on the biomass fuel, the reaction conditions and the fumigator and consists of different concentrations of hydrogen (H2 ), carbon monoxide (CO), steam (H2 O) and methane (CH₄ ). In case of air the product gas includes nitrogen (N₂ ) as well. Char coal, ash with varying carbon contents and condensable low molecular hydrocarbons are produced besides the product gas.

The char coal and the hydrocarbons (summarized as tar) are the products of an incomplete gasification [110]. Biomass is any material which once was living, and which can be utilized for energy production. The use of biomass for energy production is on rise worldwide [111]. Biomass like forest, agricultural and organic processing residues can be converted to commercial products by either biological or thermochemical processes [112,113]:

a. Biological conversion of low-value lignocellulosic biomass is facing challenges in low economy and efficiency till now [114].

b. Combustion, pyrolysis and gasification are three main thermochemical conversion methods. Biomass is traditionally burnt to supply heat and power in the process industry. The net efficiency for electricity generation from biomass combustion is usually very low, ranging from 20% to 40% [115]. Gasification provides a competitive way to convert diverse, highly distributed and low-value lignocellulosic biomass to syngas for combined heat and power generation, synthesis of liquid fuels and production of hydrogen (H2) [116-119]. But gasification is a complicated technology that’s not flexible, less competitive than others, and till now not mature so it’s exposed to certain risks plus It is not a perfect alternative for generating electricity [120].

The reasons of that:

a. It is complicated to choose the right gasifier for a given plant size, and one that is suitable for the biomass to be used.

b. There is a very wide range of designs and set-ups, many of which are still at the research stage, and biomass does not have the same steady behavior as fossil fuels.

c. Plant operation is more complex than with combustion, and it is sensitive to numerous parameters, which means that it may incur unwanted operating instabilities. Furthermore, a plant’s operating regime depends on variables that cannot always be controlled, mainly the uniformity and availability of the biomass [121-123].

Conclusion

The usage of alternating current (AC) was mainly used because AC was, at least with the technology of that time, easier to transform up to high voltage and down again. This was done in order to reduce the energy losses during transportation, especially over long distances. The latest trend is to implement high voltage direct current long distance lines (HVDC). Some of them are in daily use in India, China and Europe already, and are planned for the DESERTEC project between North Africa and Europe in 2015 or later. Daily energy losses take place in external chargers for batteries in cellular phones, laptops, cameras, navigation devices, game consoles, and electronic weather stations. In the year 2009, for example, more than 1.21 billion new mobile phones were sold worldwide and ALL of them were equipped with a new AC/DC converter to charge their batteries.

That means there are 1.21 billion of them around, only from the new cellular phones, sold in only one year. Existing AC/DC chargers are wasting our limited resources because they have to rely on an old-fashioned process of electricity production and transformation, and, in most cases also rely on an antique grid network. It seems not to be of much interest how much AC power these devices ultimate use. In fact, most of them will only deliver efficiencies in the 80 to 85 percent range. A better alternative may be to substitute DC power as a replacement for conventional AC power and thus skipping the whole power conversion process. It seems that as long as electricity remains cheap, the inefficiency of every AC/DC converter can be easily afforded. Fuel cells powered by hydrogen have the potential to do this job and thus end our reliance on global oil companies and utilities.

Acknowledgement

The author likes to acknowledge the help offered from Engineer Duaa MH Kharouf for the help during the work.

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