Chapter 2: Energy Sources

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The motivations of moving towards small-scale and distribution energy systems are many, such as...

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Nội dung Text: Chapter 2: Energy Sources

Chapter 2 Energy Sources 2.1 Distributed Generation The motivations of moving towards small-scale and distribution energy systems are many, such as • Engine generators providing economic advantages of on-site cogeneration of heat and power, or trigeneration of heat, electric power and cooling, • Small groups of generators being localized and independent of distribution net- work or grid failures, • Reducing the utilisation of distribution networks and the power transmission lines losses by locating energy systems closer to end-user, • Increasing the reliability of the power systems and distribution networks, and • Minimizing the impact of individual generator(s) failure in the distributed gener- ation systems. For the future Australian electricity supply industry, emphasis in the National En- ergy Market (NEM) will be shifted away from the present use of a small number of large centralised generators, such as coal-ﬁred power plants, towards many smaller genera- tors like natural-gas powered microturbines, photovoltaic arrays and wind generators on industrial and commercial sites. 32
2. Energy Sources 33 Figure 2.1: Microturbine power plant. From Cler and Shepard (1996) [1]. 2.1.1 Microturbine A microturbine is a small gas turbine which can generate from 500W to some hundred kW power. Fig. 2.1 displays a simpliﬁed microturbine conﬁguration. It contains a compressor, a turbine and a permanent-magnet generator, all mounting and sharing the same shaft, a recuperator and a combustion chamber. Below is the operation principle: 1. Incoming air is ﬁrst compressed to 3-4 atmosphere pressure and sent through to a heat exchanger called recuperator where the compressed air is heated up by the hot exhaust gases from the turbine. Recall the higher the heat source, the higher the thermal eﬃciency. It is found that with the recuperator the eﬃciency can be improved by up to 15%. 2. The hot and compressed air is burned with the fuel in the combustion chamber. The following fuels can be used such as natural gas, hydrogen, propane and diesel. 3. The hot gases expand and spin the turbine and the generator. Electricity is generated through the permanent-magnet generator. 4. The hot exhaust passes through the recuperator again to heat up the incoming compressed air, then to the atmosphere.
2. Energy Sources 34 AC/DC DC/AC Converter DC Bus Inverter L1 (Rectiﬁer) Gen L2 L3 N DC/DC DC Power Supply Converter &Battery & Controller Figure 2.2: Power conditioner using two-stage AC-AC power electronics converter. The output of the microturbines is of variable frequency AC voltage. It needs a power electronics conditioner to regulate the AC output frequency to 50Hz/60Hz, provides a clean AC output and improves power quality. Fig. 2.2 shows the basic con- ﬁguration of a AC-AC converter. The converter consists of a rectiﬁer and an inverter. The rectiﬁer can be a diode-bridge or thyrister-bridge rectiﬁer. The voltage on the DC Bus contains much low frequency ripple from the generator. The inverter converts this low frequency ripple DC voltage to 50Hz/60Hz sinusoidal AC voltage. Power factor correction technique is usually implemented to ensure the AC quality complies with the internation IEC standard for any kind of load connected to the AC. 2.2 Heating Value (HV) What is a fuel? A fuel is a substance which interacts with oxygen to produce energy and changes to other diﬀerent compounds. For example, the combustion of methanol takes the following chemical reaction: CH4 + 2O2 ⇒ CO2 + 2H2 O + energy (2.1) The heat energy released in this process is the diﬀerence between the chemical energy of carbon dioxide and water, and the chemical energy of fuel and oxygen. The heating value of a fuel is deﬁned as the amount of heat released when a speciﬁc amount of fuel at room temperature is completely burned in the combustion chamber and the combustion products are cooled to the room temperature. As shown in (2.1), water is produced either in the form of vapour or liquid. If the fuel is burned and water leaves the chamber as a vapour, the heating value is called the lower heating value (LHV). If the water in the combustion gases is completely condensed and thus the heat
2. Energy Sources 35 Figure 2.3: Schmatic of a single working fuel cell. From Ballard Power Systems [2]. of vapourization is also recovered, the heating value is called the higher heating value (HHV). 2.3 Alternative Energy Sources 2.3.1 Fuel Cell A fuel cell is an electrochemical system that combines hydrogren and oxygen to produce water, heat and electricity. Owing to the absence of conversion step of chemical energy to thermal energy, the fuel cell chemical-electric eﬃciency can reach up to over 60%. Fuel cell also causes less air pollution, favours modular design, has no mechanical vibration and subsequent noises, and produces useful heat through the exothermic reaction. Here we brieﬂy describe the fuel cell operation. Hydrogen is guided by the ﬂow ﬁeld plate to enter the gas diﬀusion plate (anode). It has a thin layer of catalyst coating to accelerate the ionization of hydrogen H2 ⇔ 2H+ + 2e− (2.2) At the proton exchange membrane (PEM), only hydrogen ion can diﬀuse through. The
2. Energy Sources 36 electrons are directed to an external electric circuit connecting the anode and cathode, a ﬂow of electrons as well as ﬂow of current occur. The electrons are then recombined with the hydrogen ions, with again the help of the catalyst coated on the cathode to accelerate the recombination process, to form water and heat. 1 2H+ + O2 + 2e− ⇒ 2H2 O (2.3) 2 To estimate the eﬃciency of fuel cell, that is, how much chemical energy is con- verted to electricity, we need to use three quantities from thermodynamics to ﬁnd out the answer: enthalpy, entropy and free energy. Enthalpy is the measure of energy taken or total energy demand to form a sub- stance by its constituent elements. The reference enthalpy point for a substance is the chemically stable form at 25◦ C and 1 atmosphere pressure or standard temperature and pressure (STP), and is taken as zero. For instance, O2 is a chemically stable and simplest form of molecule of O at STP, so the enthalpy for O2 is zero. In a chemical reaction, if the change of enthalpy is negative, the reaction is exothermic, i.e., heat is released. The change in enthalpy (∆H ) is the diﬀerence between the enthalpy of the substance and the enthalpies of its elements, and it is equal to the sum of Gibb’s free energy (∆G) and the thermal energy (Q = T∆S ). ∆H = ∆G + T ∆S (2.4) Gibb’s free energy refers to as the maximum possible, entropy-free output (in the form of electrical or mechanical energy) from a chemical reaction. Using this we can formulate the maximum eﬃciency as Useful output energy ∆G ∆H − T ∆S T ∆S ηmax = = = =1− (2.5) Total energy input ∆H ∆H ∆H Example 4.9 of [3]: Suppose a fuel cell that operates at 25◦ C (298K) and 1 atm forms liquid water: H2 + 1 O2 ⇒ H2 O(l) ∆H = −258.8kJ/mol of H2 2 (a) Find the minimum amount of heat rejected per mole of H2 . (b) What is the maximum eﬃciency of the fuel cell? Solution: (a) Using Table 4.6 in [3], the loss of entropy by the reactants per mole of H2 is Sreactants = 0.130kJ/mol−K ×1molH2 +0.205kJ/mol−K ×0.5molO2 = 0.2325kJ/K
2. Energy Sources 37 Figure 2.4: Enthalpy of formation, absolute entropy and Gibbs free energy at STP for selected subtances. From Masters [3]. The gain in entropy in the product water is Sproduct = 0.0699kJ/mol − K × 1molH2 O = 0.0699kJ/K The minimum amount of heat is calculated by Qmin = T ( Sreactants − Sproduct) = 298K (0.2325 − 00699)kJ/K = 48.45kJ per mole H2 . (b) From (2.5) we have T ∆S 48.45 ηmax = 1 − =1− = 0.830 ∆H 285.8 Note that we have taken the absolute change of enthalpy, which equals to the high heating value (HHV), so the minus sign of ∆H is removed. The electrical characteristics of a real fuel cell is shown in Fig. 2.5. We can observe that the fuel cell voltage decreases as current density increases. This phoenomenon is explained in Table 2.1 as the resistive losses of the fuel cell. Owing to this phoenomenon, there is a point where fuel cell voltage drops to zero or fuel cell current is zero (open
2. Energy Sources 38 Type of Loss Description Activation loss Energy taken by the catalysts to initiate and speed up the slow reaction at cathode, where oxygen, hydrogen ions and electroncs are combined to form water. Ohmic losses Energy dissipated to resistive contacts of interconnections such as electrolyte membrance and electrodes. Fuel crossover Fuel passes through the electrolyte without releasing its elec- trons to the external circuit. Mass transport losses Hydrogen and oxygen gases have diﬃculty reaching the elec- trodes due to building up of water which clogged the catalyst. Table 2.1: Type of losses within a real fuel cell. From Masters [3]. Figure 2.5: The V-I curve for a typical fuel cell. Also shown is the power delivered, which is the product of V and I. From Masters [3].
2. Energy Sources 39 Figure 2.6: Schematic of a direct methanol fuel cell. From Jet Propulsion Laboratory. circuit), maximum power point of a fuel cell exists. From Fig. 2.5, the relationship between current and voltage in a fuel cell is given by y = mx + c (2.6) 0.7−0.4 I V= 0.6−1.8 J + 0.85 = 0.85 − 0.25J = 0.85 − 0.25 A Hydrogen Generation and Storage As hydrogen is an extrememly ﬂammable gas, storage and delivery of hydrogen is there- fore diﬃcult and dangerous. The present most likely choice is compressed hydrogen gas (CHG), solid metal hydrides such as sodium allumium hydride [6] and other solid (or liquid) bonding agents making hydrogen storage more attractive, economical and safe are under research and development. Methanol, natural gas, gasoline, diesel fuel, aviation jet fuel, and ethanol are the possible fuels for hydrogen generation. They are investigated with considerations of the processing temperature, amount of oxides of carbon and nitrogen generated, operation life, etc. [7]-[8]. Example of direct methanol fuel cell is shown in Fig. 2.6. Another example is the use of renewable energy sources to assist electrolysis of water to generate hydrogen, as seen from Fig. 4.32 of [1].
2. Energy Sources 40 Fuel Cell Applications Among the many fuel cell applications, fuel cell vehicles such as scooters, vans, buses and automobiles are under vast research and development. For instance, Honda plans to begin leasing a hydrogen fuel-cell vehicle based on its FCX Concept in Japan and the United States in 2008, and possibly in mass production in 2018 [9]-[10]. 2.3.2 Wind Energy Generation of electricity with wind energy is achieved by atmospheric wind power rotating a rotor-blade propeller on a wind tower generator shaft that turns a wind turbine. Wind turbines can be generalized into two catagories: vertical-axis type and horizontal-axis type. Some examples are shown in Fig. 2.7. Wind Power Characteristics The kinetic energy of the wind is given by 1 mv 2 K. E. = ˙ (2.7) 2 where m is the mass ﬂow rate of air (kg/s) and is deﬁned as ˙ Mass passing through surface A m= ˙ = ρAv (2.8) Time taken Substituting (2.8) into (2.7) we obtain the wind power (Pwind ) as 1 Pwind = ρAv 3 (2.9) 2 A is the cross-sectional swept area of the turbine rotor and A = π ·radius2 = Diameter 2 /(4π ). ρ is the desity of dry air at 15◦ C (or 288K) and is equal to kg 288 × B ρ = 1.225 (2.10) 3 × 760 × T m where B = barometric pressure (in mm Hg) and T = ambient air temperature at rotor height (in K). Wind Speed versus Energy Example: Compare the wind energy at 15◦ C, 760 mmHg of pressure, contained in 1m2 of the following wind regimes:
2. Energy Sources 41 Figure 2.7: Some examples of the machines that have been proposed for wind energy conversion. Source: Eldridge, 1975.
2. Energy Sources 42 1. 50 hours of 8m/s winds, 2. 25 hours of 12m/s winds followed by 25 hours of 4ms/ winds (average wind speed is 8m/s for 50 hours) Solution: 1. 1 1 Energy (8m/s) = ρAv 3 ∆t = ×1.225kg/m3 ×1m2 ×(8m/s)3 ×50h = 15, 680W h 2 2 2. Energy (4m/s) + Energy (12m/s) 1 × 1.225kg/m3 × 1m2 × [(12m/s)3 × 25h + (4m/s)3 × 25h] = 2 = 27, 440W h From the above example, it is obvious that high wind speed produces much more energy than low wind speed (75% in this case) though the average wind speed of two cases are the same. The reason is due to Pwind ∝ v 3 . Wind Density versus Temperature and Pressure It can be seen from (2.10) that the wind density is aﬀected by both temperature and pressure, which is inﬂuenced by altitude as well. Since the air density in (2.10) is under 1 atm and 15◦ C, we need to obtain two correction factors (KT and KA ) on variations of temperature and pressure respectively. A more accurate deﬁnition of air desity is expressed as P × M.W. × 10−3 ρ(kg/m3 ) = (2.11) RT where P is the absolute pressure (atm), M.W. is the molecular weight of the gas (g/mol), R is the ideal gas constant = 8.0256×10−5 m3 · atm · K−1 · mol−1 , and T is the absolute temperature (K). The molecular weight of air is approximately 28.31 g/mol. Example: Find the density of air at 1 atm and 15◦ C. Solution: From (2.11), 1 atm × 28.31g/mol × 10−3 = 1.225kg/m3 ρ = ρ◦ = 8.0256 × 10−5 · atm/(K· mol) × (273 + 15)K
2. Energy Sources 43 Altitude 6 . . . ? . . . .. . . . . . . . . .. . ...... .. dz .... .. . .. . . 6 . . . .. . . . . . . . . . . .. . . . .... ........ .. . .. . .. .. . . . .. . . . . . Area A . ... . .. ...... . . . . . . .... . . z Figure 2.8: A column of air in static equilibrium for determination of air pressure according to height. If we set ρ◦ , which is at 1 atm and 15◦ C, as the normalized value, any change of air density due to temperature-only eﬀect will be modeled as a density ratio KT (same symbol and meaning as Masters [1] on page 316) which is given by ρ T◦ KT = = (2.12) ρ◦ T where T◦ = (273+15)K = 288K. The pressure of air decreases as sites above sea level is increased. To obtain the correction factor of pressure, KA , we consider a static column of air with cross sectional area A, as shown in Fig. 2.8. A horizontal slice of air with thickness dz and density ρ will have a mass m of m = ρ × V = ρ × (A · dz ) (2.13) The change of pressure, dP , from bottom to top of the air slice is equal to the increase of weight of air over the cross sectional area A. It is given by F orce −gρAdz dP = = = −gρdz (2.14) Area A The change of pressure with respect to increment of height will become dP = −gρ (2.15) dz Putting (2.11) into (2.15) and assuming 15◦ C and constant temperature throughout the air column, we obtain the relationship of pressure P and height H , −4 H P = P0 · e−1.185×10 = KA (2.16) where P0 is the reference pressure and equal to 1 atm and H is measured in meters.
2. Energy Sources 44 Figure 2.9: Friction coeﬃcient for various terrain characteristics. Source: Masters, 2004 [1]. Example: Find the air density at 0◦ C, at an elevation of 2000m. −4 ×2000 Solution: From (2.16), P = 1 atm ×e−1.185×10 = 0.789 atm. From (2.11), 0.789 atm × 28.31g/mol × 10−3 kg/g = 1.043kg/m3 ρ= 8.0256 × 10−5 · atm/(K· mol) × (273 + 0)K Impact of Tower Height The wind speed in the ﬁrst few hundred meters about ground is greatly aﬀected by the the friction that the air experiences as it moves across the earth’s surface. Smooth surface such as calm sea has little resistance while high irregularities such as forests and buildings impose high resistance to wind. To express the roughness of the earth’s surface, we can related the wind speed and height by the following equation: v H = ( )α (2.17) v0 H0 where v is the windspeed at height H , v0 is the windspeed at height H0 (often a reference height of 10m), and α is the friction coeﬃcient. Fig. 2.9 shows a table of friction coeﬃcient for various terrain characteristics. Example: An anemometer mounted at a height of 10m above a surface with tall grass on level ground shows a windspeed of 4m/s. Estimate the windspeed and the
2. Energy Sources 45 speciﬁc power in the wind at a height of 45m. Assume 15◦ C and 1 atm of pressure. Solution: From Fig. 2.9, the friction coeﬃcient α for ground with tall grass is estimated to be 0.15. From the 15◦ C and 1 atm conditions, the air density is ρ = 1.225 kg/m3 . Using (2.17), the windspeed at 45m will be 45 0.15 v45 = 4 · ( ) = 5.01m/s 10 Speciﬁc power will be 1 P45 = ρv 3 = 0.5 × 1.225 × 5.013 = 77.0W/m2 2 Comparing to the speciﬁc power at 10m, which is equal to 39.2W/m2 , an increase of 25% in wind speed brings about 2 times as much power as available at 10m. From (2.17), we can further relate speciﬁc power with height by the following 1/2ρAv 3 P v3 H 3α = 3 = (v ) = (H ) (2.18) P0 1/2ρAv0 0 0 Maximum Rotor Eﬃciency By Betz eﬃciency or Betz’s Law, an ideal wind turbine would decrease the wind speed to one-third of its original speed. Fig. 2.10 shows how the wind ﬂows through the blades with diﬀerent velocities. The power extracted by the blades Pb is given by 1 Pb = m(v 2 − vd ) 2 ˙ (2.19) 2 where m is the mass ﬂow rate, v and vd are the upwind and downwind velocities ˙ respectively. As deﬁned in (2.8), the mass ﬂow rate at the plane of the rotor is given by m = ρAvb ˙ (2.20) By assuming the velocity vb is just the average velocity between v and vd , and substi- tuting (2.20) into (2.19), we have 1 v + vd 2 2 Pb = ρA( )(v − vd ) (2.21) 2 2 Letting vd λ= (2.22) v and substituing (2.22) into (2.21) we obtain 1 1 Pb = ρAv 3 [( (1 + λ)(1 − λ2 )] (2.23) 2 2
2. Energy Sources 46 Figure 2.10: Wind velocities around the blades and rotor area. From Masters page 324 [1]. Diﬀerentiate both sides of (2.23) with respect to λ and equal zero, we obtain 1 λ= (2.24) 3 Putting (2.24) back to (2.23) we have the maximum rotor eﬃciency Pb,max 1 16 1 Pb,max = ρAv 3 · = 0.593 ρAv 3 · Pwind (2.25) 2 27 2 Recall that Pwind is the wind energy and the maximum rotor eﬃciency will be at one-third of it. Environment Impact The good sides of wind power generation are that the wind turbines does not involve the release of carbon dioxide or pollutants that cause acid rain or smog and they does not require water for power generation. The latter is important as drought in Australia becomes more severe and water shortage occurs frequently around the country. The possible environment impacts of wind turbine are mechanical and aerodynamic noises, electromagnetic interference (EMI) and visual impacts such as strobe light caused by low rotary speed of wind blades. Research and development to reduce the impacts are underway. Further information can be found in [5].
2. Energy Sources 47 Wind Farms in NSW Fig. 2.11 shows the wind resources in Australia. Some costal regions receive wind exceeding 8m/s [11]. At present there are more than 42 wind farms and around 563 wind turbines are in operation. They can produce up to 817 MW power (=2500GWh) to power 384,000 homes. This equivalent to a saving of almost 3,256,000 tonnes of CO2 every year [12]. There are four wind farms in NSW: Blayney Wind Farm, in the central tablelands of NSW, has 15 wind turbines, each with a capacity of 660 kW. It was commissioned in October 2000, and will produce enough electricity annually to power 3,500 homes; Crookwell Wind Farm was the ﬁrst grid-connected wind farm in Australia when in- stalled by Paciﬁc Power in 1998. It consists of eight 600kW turbines giving a total capacity of 4.8MW. Now owned by Eraring Energy, the wind farm supplies electricity to Country Energy’s GreenPower customers. Hampton Wind Park is the newest wind farm in NSW and is a 2 hour drive from Sydney, past the Blue Mountains. Power from two 660kW wind turbines enhances the quality of supply in the surrounding ru- ral electrical grid. This wind farm supplies electricity to Integral Energy’s GreenPower customers; Kooragang Island’s single 600kW wind turbine has been operated by Energy Australia since its installation in 1997. It provides GreenPower for Energy Australia’s Pure Energy customers [13].
2. Energy Sources 48 Figure 2.11: Wind resources in Australia. Source: Zahedi, 2006 [11].
Bibliography [1] G. I. Cler, and M. Shepard, Distributed Generation: Good Things Are Coming in Small Packages, Esource Tech Update, TU-96-12, Esource, Boulder, CO. [2] Ballard Power Sytems Inc., Fuel cell technology - how the technology works, online material, http://www.ballard.com/be informed/fuel cell technology/ how the technology works. [3] G. M. Masters, Renewable and Eﬃcient Electric Power Systems, John Wiley & Sons, 2004. [4] P. Kruger, Alternative Energy Resources, John Wiley & Sons, 2004. [5] G. Boyle, Renewable Energy: Power for a Sustainable Future, Oxford University Press, 2004. [6] C. M. Jensen, K. J. Gross, “Development of catalytically enhanced sodium alu- minum hydride as a hydrogen-storage material”, Applied Physics A: Materials Science & Processing, vol. 72, no. 2, pp. 213-219, 2001. [7] T. Gilchrist, “Fuel cells to the fore [electric vehicles]”, IEEE Spectrum, vol. 35, issue 11, pp. 35-40, Nov. 1998. [8] Mazumder, S.K.; Acharya, K.; Pradhan, S.K.; Hartvigsen, J.; von Spakovsky, M.R.; Haynes, C., “Energy buﬀering techniques for load transient mitigation of solid-oxide fuel cell (SOFC) power conditioning system (PCS)”, in Proceedings, IEEE Power Electronics Specialists Conference, vol. 6, pp. 20-25, June 2004. [9] Honda Australia Pty Ltd, “Honda Unveils Next-Generation Technologies”, online material, http://www.honda.com.au/wps/wcm/connect/Honda.com.au/Home/ News/Honda+Unveils+Next-Generation+Technologies, 2006. 49
2. Energy Sources 50 [10] Takeo Fukui, “Honda Sees Mass Production of Fuel-Cell Cars Possible by 2018”, Congress, online material, Green Car http://www.greencarcongress.com/2006/12/honda sees mass.html, 2006. [11] A. Zahedi, “Hydrogen as storage option for intermittent renewable technologies such as solar and wind”, Proceedings, Australasian Universities Power Engineer- ing Conference, 2006. [12] Auswind, “Wind Energy in Australia”, online material, http://www.auswind.org. [13] NSW Department of Energy, Utilities and Sustainability - Wind, online material, http://www.deus.nsw.gov.au/energy/Renewable%20Energy/ Renew- able%20Energy.asp/.