Generation of Useful Electrical Power - Literature review Example

SYSTEMS USED TO USE WASTE HEAT FROM ENGINES TO GENERATE USEFUL ELECTRICAL POWER By Name Course Instructor Institution City/State Date Systems Used To Use Waste Heat from Engines to Generate Useful Electrical Power Heat Regeneration Using Steam Technology Generation of electrical power through steam turbines technology encompasses three energy conversions; producing thermal energy from the fuel as well as utilising it for raising steam, changing the steam’s thermal energy into kinetic energy inside the turbine and making use of rotary generator to change the mechanical energy of the turbine into electrical power. According to Electropaedia (2005), steam turbines remain to be the most resourceful and ancient technology that is still commercially viable. Generating heat through steam turbines has been existent for more than one century, when they substituted reciprocating steam engines thanks to their lower costs and higher efficiencies. Presently, traditional power plants that use steam turbine technology produce nearly all electrical power generated in the U.S. Steam turbines’ capacity may range from 50 kilowatts for small utility power plants to hundreds megawatts for bigger utility power plants (EPA, 2008, p.2). Normally, steam turbines technologies are broadly utilised for combined heat and power (CHP) applications. As stated by EPA (2008, p.1), a steam turbine can be defined as a thermodynamic device where energy converted in high-temperature, high-pressure steam into shaft energy, which may consequently be utilised to turn a generator as well as generate electrical energy as heat by-product . Different from reciprocating the engine as well as gas turbine, CHP technologies normally produce electrical energy as a by-product of steam (heat) generation. This form of technology requires a different source of heat and does not convert fuel to electricity in a direct way, rather the energy is conveyed from to the turbine the boiler through steam tat high-pressure, which as a result, powers the turbine together with the generator (EPA, 2008, p.2). In CHP systems, lower pressure steam (heat) is generated by the steam turbine and utilised directly or is transformed to other forms of thermal heat energy. Figure one shows a simple turbine power cycle where electrical energy is generated as a by-product of heat. Fig 1: Simple Steam Turbine Power Cycle (Source: cogeneration.net) As mentioned by Electropaedia (2005), steam turbine technologies are fundamentally heat engines where heat energy is converted into mechanical energy by interchangeably heating as well as condensing a working fluid through Rankine cycle, which is a closed system. The heat recovery steam generator (HRSG) is principally a heat exchanger or a boiler, where steam for the steam turbine is created by passing the hot gas flow of the exhaust from a combustion engine or gas turbine through heat exchanger tubes banks. The HRSG according to Kutz (2007, p.140) may depend on natural circulation or use mandatory circulation through pumps. While the hot exhaust gases run through the tubes of heat exchanger wherein there is circulation of hot water, heat absorption takes place leading to the creation of steam in the tubes. Moreover, the tubes are organised in modules, or sections, all serving a distinct function in the dry superheated steam generation. Such modules are known as preheaters, evaporators, economisers, and reheaters/ superheaters (Electropaedia, 2005). Some of the shortcomings of the HRSG include; it takes more time to warm up from cold conditions as compared to hot conditions, and this consequently affects the start-up time. Thermoelectric generation (TEG) Thermoelectric generators can be defined as devices which convert heat directly into electrical power by means of a process well-known as the Seebeck effect, a type of thermoelectric effect (Ismail & Ahmed, 2009, p.27). Basically, thermoelectric modules function on two distinct basics; first is the Peltier Effect, where power is introduced to the module with a subsequent heating of one side and cooling of the other side. According to TEC (2015), such modules are low amp, normally running at 12 volts as well as in the 6 amp range and are created for low temperature exposure of below 158 Fahrenheit to 176 Fahrenheit in the hot side. This is because, higher temperature exposures may cause the module to either pair joints to liquefy or break apart. Second is the Seebeck Effect, temperature differential is created across the module through cooling the heat removal side and heating the other module side. Such modules according to Electropaedia (2005) have been designed precisely to function at temperatures in the 608° F (bismuth telluride). Furthermore, hybrid modules (combination of lead telluride( PbTe) as well as bismuth telluride (BiTe)) were created to capitalize on hot side temperatures in the range of 260°C to 340°C, as well as Calcium Manganese Oxide (CMO) modules (equal to 800°C) hot side (TEC, 2015). Besides that, CMO Cascade having a BiTe module stacked with CMO is a unique latest TEG module class that can exploit various zones of temperature where such materiel generate the most resourceful energy conversions. CMO CASCADE as pointed out by Electropaedia (2005) is the only cascade that has ever been manufactured commercially or accessible. Figure two exhibits TEG based on heat applied and heat rejected. Fig 2: TEG System Source: mpoweruk.com) Basically, the Seebeck Module generates power from a heat differential power generator) and the Peltier Module is for cooling. Still, the Peltier module can be used as a generator, but cannot generate adequate power since the materials utilised for bonding together the device are low temperature, and so, if exposed to high temperature the module will be destroyed. The key issue of making use of TEG is rooted in its low thermal efficiency. According to Saidur et al. (2012, p.5652), the efficiency of thermoelectric materials relies on the thermoelectric figure of merit, Z. Future thermoelectric materials according to Saidur et al. (2012, p.5652) display the potential of getting considerably higher thermoelectric figure of merit, Z values, and as a result, higher power densities as well as efficiencies may be achievable. Materials such as BiTe, tin tell- uride (SnTe), zinc–beryllium (ZnBe), skutterudite (CeFeSb), silicon–germanium (SiGe, as well as new nano-wire or nano-crystalline thermoelectric materials are presently in the development phase with the intention of improving the TEGs’ conversion efficiency. TEGs may be utilised in various applications (Saidur et al., 2012, p.5652). Regularly, TEGs are utilised for applications that uses lower energy or where massive heat engines that are more efficient like Stirling engines are possible. Different from heat engines, Winder and Ellis (1996, p.940) posit that the electrical modules (solid state) normally utilised for performing conversion of thermal to electric power has no moving parts. In this case, the conversion of thermal to electric power may be carried out through modules that need no maintenance, have integrally high consistency, and may be utilised for constructing generators with long lifespans of free service. So, this makes TEGs much suitable for systems with modest to lower needs of power in remote unoccupied or unreachable locations like Deep Ocean and mountaintops. Rankine bottoming cycle technique These days, the global problem concerning the rapid development of the economy as well as the energy shortage, source from the ICE (internal combustion engine) exhausted waste heat as well as environmental pollution (premkumar et al., 2014, p.81). In ICE, roughly, 30 to 40 per cent of the overall heat supplied to the engine in fuel form is transformed into beneficial mechanical work while the residual heat is rejected to the air through engine cooling systems as well as exhaust gases; therefore, it is imperative to use waste heat beneficially. Waste heat use does not just save fuel but as well lessens the amount of greenhouse gases as well as waste heat released to the air (premkumar et al., 2014, p.81). Waste heat according to premkumar et al. (2014, p.81) is the heat produced in a process through fuel combustion or reaction of chemicals, and afterwards rejected to the atmosphere despite the fact that it may still be reutilised for a number of beneficial work as well as profitable purpose. The heat rejected relies partly on the waste heat gases’ temperature as well as exhaust gas mass flow rate. For instance, consider in ICE almost 40 per cent is renewed into valuable mechanical work, but the other 60 per cent is dumped into the atmosphere through engine cooling systems as well as exhaust gases. When exhaust gases leave the engine they may have temperatures beyond 455 to 621°C; so, such gases have high content of heat (Khan & Siddiqui, 2014, p.97). Figure four shows a Rankine cycle system. Fig 3: Rankine cycle system (Saidur et al., 2012) Therefore, Rankine bottoming cycle system’s working fluid contains an isentropic, dry, or wet fluid, a pump for circulating the working fluid, an boiler/evaporator for absorbing exhausted heat energy, an expander for releasing energy through reduction of the fluid pressure to a lower level, a condenser for releasing the fluid’s heat and liquidising the fluid prior to starting over the entire cycle. As mentioned by (Saidur et al., 2012, p.5653), the working fluids’ T-s diagram may have positive slope of vertical slope¸ negative slope or saturation curve. The exhaust heat low-grade temperature cannot resourcefully be transformed to electrical energy through traditional techniques as observed in industrial systems waste heat recovery. Rather, there are scores of other thermodynamic cycles suggested for generating electrical energy from exhaust heat. Such are supercritical Rankine, Kalina, Goswami, trilateral flash, and organic Rankine cycles. Fascinatingly, organic Rankine as well as Kalina cycles have been contrasted in scores of studies such as Saadatfar et al. (2014) and Tchanche (2010). As noted by Saidur et al. (2012, p.5653), although there are claims of almost 50 per cent of more power output for similar input for Kalina cycles than organic Rankine cycles, real operations’ data exhibits a variance of almost three per cent supportive of Kalina cycle in contrast to organic Rankine cycle under same circumstances. Basically, Rankine bottoming cycle derives from the Rankine cycle, and due to the inferior heat sources, the cycle efficiency relies on the chosen working fluids as well as the system’s operating conditions. In Chen et al. (2010) study, they reviewed different type of working fluid under varying conditions of operation, and they established that the best working fluids whose cycles have highest efficiency cannot be similar for other conditions of operations as well as for different working fluids. As mentioned before, working fluid has the most important role in determining the efficiency of the cycle. In Rankine bottoming cycle technique, the selected fluid can have an effect on various facets of the entire system, which includes the whole efficiency of the system, conditions for operating, economic feasibility as well as environmental effect because of the working fluids’ chemical nature. Generation of power through the Rankine cycle is a technology that has widely been adopted. Mostly, water or wet working fluid is utilised in the cycle’s closed circuit as the working fluid. Because of the steam thermal stability it may be utilised in applications where the temperatures of heat source are exceedingly high deprived of the panic of thermal disintegration. Nevertheless, in applications capturing heat from substandard sources as well as where output capacity offered is smaller than 1 Megawatts as seen in motorised engines, organic working fluid turbines normally have upper efficiency as compared to steam turbines owing to considerations of design with lesser working fluids’ molecular weight as well as improved economics. There exists differing views on the impact of latent densities, heats, in addition to specific working fluids heats on the Rankine cycle. For instance, Dai et al. (2009) points out that high latent density and heat with smaller specific heat liquid are more preferred because it can absorb more energy evaporator source; thus leading to lesser pump consumption and realisation of the needed rate of flow. Differently, Saidur et al. (2012, p.5654) posits that a fluid with low latent heat may offer the greatest condition for operating because of the saturated vapour at the inlet of the turbine. An organic Rankine cycle (ORC) uses isentropic or dry fluid as working fluid (organic fluid) rather than water, and according to Dincer et al. (2014, p.48) the cycle efficiency is significantly reliant on the working fluid selection. Generally, an organic Rankine cycle make use of isentropic organic fluids because of their low vaporisation heat as well as they do not require to be superheated so as to heighten their efficiencies of recovery as required for water. A number of studies talks about the working fluids’ ideal operating conditions as well as properties for a Rankine cycle. For instance, in Badr and his colleagues as cited by Saidur et al. (2012, p.5654) study, different organic fluids properties as regenerative Rankine-cycle units’ candidates were studied through BASIC computer programs so as to calculate ideal working fluid, operating conditions, and design of the recommended system. In this case, Badr et al (1990) established that the working fluids’ cycle efficiency was exceedingly sensitive to pressure of evaporation, but not sensitive to inlet temperature of the expander. Saidur et al. (2012, p.5654) further established that systems with lesser irreversibility could generate an improved power output, but this varies based on various types of heat source as well as working fluid. Hung (2001) analysed the ORC system performance that used dissimilar working fluids, and they established that the cycles when using organic working fluids generated exceedingly high energy efficiency as compared to the cycle when using wet working fluids. Moreover, Hung (2001) noted that the crucial features of working fluid utilised in ORC is the saturation vapour curve’s slope whereby the efficiency is exceedingly impacted by the working fluid evaporating temperature. According to Quoilin (2011, p.76), in a specific temperature gradient for work output optimisation, the evaporation enthalpy of the working fluid must be exceedingly high. Moreover, by making use of a suitable organic fluid to replace the water, maximum power output as well as superlative efficiency may be attained when wasted heat energy at adequate temperature is used as the source of heat at the inlet. This according to Quoilin (2011) is because of the inferior irreversibility between the heat source as well as the working fluid. In their study about Rankine bottoming cycle system used in a hybrid vehicle, they established that the main waste energies sources may be found in the cooling system and exhaust gases of a car. Recently, the increased scholarly attention towards Rankine bottoming cycle technique has incited a number of automakers to examine its capacities. Scores of researchers such as Saidur et al. (2012, p.5655) have reported that BMW and Honda correspondingly attained a reduction in fuel consumption equal to or over 10 per cent for their passenger vehicles. Moreover, for commercial trucks, it was reported by Saidur et al. (2012, p.5655) that almost 10 per cent of fuel consumption is improved by Cummins for their trucks through utilisation of ORC. Lots of other research studies concerning Rankine bottoming cycles have been performed, with some such as Saidur et al. (2012) proposing the use of organic Rankine bottoming cycle that has been integrated to thermoelectric generator. Importantly, the Rankine bottoming cycle technique is crucial for automakers because it improves fuel economy through conversion of exhaust heat from the prime mover into valuable energy. In summary, evaporative system for engine cooling is used so as to get high simplicity as well as thermal efficiency of the Rankine bottoming system. Turbocharger The naturally aspirated ICE according to Saidur et al. (2012, p.5655) generates enormous amount of waste heat, and the process of fuel combustion in the cylinder discharges heat energy, which is exhausted through the exhaust manifold and eventually to the atmosphere. So, this exhaust energy that is wasted to the environment may be recovered through using a turbocharger. Essentially, a turbocharger can be defined as a form of supercharger, which is driven by exhaust power/energy. Belt-driven supercharger is a different type of automotive supercharger. As stated by Saidur et al. (2012, p.5656), a turbocharger is a form of gas turbine where pressure as well as heat in the exhaust gas that is increasing is utilised for boosting the engine power through air compression that goes into the combustion chambers of the engine. The pump’s turbine blades will be rotated by the high temperature exhaust gases from the cylinders. Normally, a turbocharger component constitutes of (1) shaft, (2) turbine, (3) actuator, (4) waste gate valve, (5) compressor as well as (6) CHRA (centre housing and rotating assembly). Figure four demonstrated a normal turbocharger. As indicated by Saidur et al. (2012, p.5656), turbocharger technology allows for engine downsizing by means of work reduction in spark-ignition engines. Moreover, its capability to heighten power density significantly impacts the revitalisation of diesel engines whereby majority of diesel engines these days are furnished with turbochargers. Fig 4: Typical turbocharger (Saidur et al., 2012) Turbocharging according to Challen et al. (2005, p.69) heightens the flow rate of the air mass into the engine which considerably lessens diesel engines’ particulates that are discharged to the environment. Additionally, it has been accounted that diesel engine that are turbocharged may enhance the passenger vehicles’ fuel economy by almost 50 per cent while that for downsized gasoline engine by almost 20 per cent . Essentially, turbocharging was mostly used in diesel engines and latest inspiration for more economic, fuel efficient, as well as high-performance engines. Besides that, turbocharging has gradually been used in gasoline engines even though the demands are not the same as that of diesel engines. Turbocharger has experienced scores of challenges; for instance, the first creation of exhaust-driven turbocharger generated no interests. According to Saidur et al. (2012, p.5656), the first exhaust-driven turbocharger was recorded in Switzerland early in 20th century, and in 1915 the proposal by Dr. Alfred for first turbocharged diesel engine was ignored by the community. So, in its initial stages, turbochargers according to Saidur et al. (2012, p.5656) were utilised largely in heavy-duty applications. By tradition, turbochargers have crucial issues that justify their low acceptance by automakers. Turbochargers experience turbo lag (specifically, transient or hesitation response) when acceleration of the car is low and also heated bearings have been a key concern. As mentioned by Saidur et al. (2012, p.5656), turbo lag may inadequately affect the performance of the engine well as its drivability. In Park et al. (2010, p.1) study concerning response delay mechanism of a turbocharger, they established that boost pressure response interruption is because of a permutation of issues. The key reason is because of the turbocharger systems’ physical properties; that is to say, the compressor and the turbine weight. Another problem that sources from the first issue is largely because of the decrease turbine power that takes place because of operating mechanism disturbances. In order to boost the transient response, by Saidur et al. (2012, p.5656) posit that the weights of compressor as well as turbine wheel should be reduced by utilising new materials. Furthermore, reducing exhaust and intake system volume can as well improve turbocharger transient response. Combined ORC and VCC (Organic Rankine Cycle and Vapour Compression Cycle) Technique Heat activated cooling as argued by Wang et al. (2011, p.1) can use the thermal sources that presently go unexploited like industrial waste heat as well as engine exhaust heat. Utilising such heat sources may offer improved energy use and reduced usage of fuel in applications that need cooling. The idea in Wang et al. (2011, p.1) study was to use waste heat from mobile and stationary engine cycles so as to produce cooling for vehicles as well as structures. It integrates an organic Rankine cycle (ORC) and vapour compression cycle (VCC). For this reason, a nominal prototype system capacity for cooling was created with regard to this concept and analysed in the laboratory. So as to retain high system performance while still reducing weight as well as size for moveable applications, scroll based compression as well as expansion and micro-channel components for heat transfer were utilised. Even though the system was examined during its design point, the combined ORC and VCC technique performed remarkably getting a 4.4 kW of cooling. As stated by Bing Hu et al. (2014), both the second law and conversion efficiencies were nigh on the results of the model, ascertaining it to be a productive technology. However, the organic Rankine/vapour-compression cycle technique is impacted by a number of input parameters: waste heat source temperature, heat rejection temperature, efficiencies of compressor and turbine, and air flow rates of the components (condenser as well as evaporator). Basically, in the ORC/VCC system, waste heat is converted into a cooling effect, and this is achieved at installation site through utilisation of the ORC so as to produce the shaft work needed to drive a VCC. Figure five presents an ORC/VCC Schematic: Fig 5: ORC/VCC Schematic (Source: smartech.gatech.edu) In hot springs, ORC/VCC system according to Bu et al. (2013) may by driven by waste energy for air conditioning at hotels and homes that largely has an outlet and inlet for geothermal water, generator, hot water pump, compressor, expander, working fluid pump, water pump for cooling, condenser, throttle valve, in addition to evaporator. In ORC/VCC, heats from the waste are collected from hot springs, and offer energy for heating and vaporising a working fluid that has low boiling point. In the expansion engine, Bu et al. (2013) posit that energy is extracted from this vapour then utilised for driving an air conditioning vapour compressor. Later, the fluid leaving the expander is condensed as well as pumped back into the generator for vaporisation. In this case, a vertical generator is utilised and counter current is used in the generator in order to enhance the system efficiency. Specifically, from top to bottom the hot water flows into the generator while the working fluid flows into the generator from the bottom to top. In this system as mentioned by Bu et al. (2013) a water-cooled condenser is utilised for making the system more powerful and compact than the air cooled system. The liquid level sensor, frequency converter as well as programmable logic controller are used in the in the In ORC/VCC system so as to control the working fluid liquid level in the generator automatically and make certain the generator has high efficiency in heat exchange. The waste heat temperature from the coolant an s well as hot spring, normally differs together with the ambient variation. So as to enhance the drive efficiency, the expander and the compressor should be directly coupled on one shaft with no coupling and gear. References Bing Hu, Bu, X. & Ma, a.W., 2014. Thermodynamic Analysis of a Rankine Cycle Powered Vapor Compression Ice Maker Using Solar Energy. [Online] Available at: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4151356/ [Accessed 8 March 2015]. Bu, X., Wang, L. & Li, H., 2013. Performance analysis and working fluid selection for geothermal energy-powered organic Rankine-vapor compression air conditioning. [Online] Available at: http://www.geothermal-energy-journal.com/content/1/1/2 [Accessed 8 March 2015]. Challen, B., Baranescu, R. & DESIGN, D., 2005. Diesel Engine Reference Book By Bernard Challen and Rodica Baranescu: Diesel Engine Reference Book. Oklahoma City: Digital Designs. Chen, H., Goswami, Y. & Stefanakos, E.K., 2010. A review of thermodynamic cycles and working fluids for the conversion of low-grade heat. 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Saadatfar, B., Fakhrai, R. & Fransson, T., 2014. Thermodynamic Vapor Cycles for Converting Low-to Medium-grade Heat to Power: A State-of-the-art Review and Future Research Pathways. Journal of Materials and Environmental Science, vol. 21, no. 1, pp.1-25. Saidur, R. et al., 2012. Technologies to recover exhaust heat from internal combustion engines. Renewable and Sustainable Energy Reviews, vol. 16, pp.5649–59. Tchanche, B.F., 2010. Low - Grade Heat Conversion into Power Using Small Scale Organic Rankine Cycles. Doctoral Thesis. Athens: Bertrand Fankam Tchanche Agricultural University of Athens. TEC, 2015. Thermoelectric modules work on two different principals. [Online] Available at: http://thermoelectric-generator.com/how-thermoelectric-teg-generators-work/ [Accessed 8 March 2015]. Wang, H. et al., 2011. Performance of a Combined Organic Rankine Cycle and Vapor Compression Cycle for Heat Activated Cooling. Corvallis, OR: Oregon State University Oregon State University. Winder, E.J. & Ellis, A.B., 1996. Thermoelectric Devices: Solid-State Refrigerators and Electrical. Journal of Chemical Education, vol. 73, no. 10, pp.940-46. Read More