Solar thermal aided power generation

Applied Energy 87 (2010) 2881–2885 Contents lists available at ScienceDirect Applied Energy journal homepage: www.elsevier.com/locate/apenergy Solar thermal aided power generation Eric Hu a,*, YongPing Yang b, Akira Nishimura c, Ferdi Yilmaz d, Abbas Kouzani d a School of Mechanical Engineering, The University of Adelaide, SA 5005, Australia North China Electric Power University, Beijing 102206, China c Division of Mechanical Engineering, Mie University, Tsu 514-8507, Japan d School of Engineering, Deakin University, Geelong Victoria 3217, Australia b a r t i c l e i n f o Article history: Received 19 March 2009 Received in revised form 25 October 2009 Accepted 26 October 2009 Available online 25 November 2009 Keywords: Solar aided power generation Energy efficiency Power station CO2 emissions a b s t r a c t Fossil fuel based power generation is and will still be the back bone of our world economy, albeit such form of power generation significantly contributes to global CO2 emissions. Solar energy is a clean, environmental friendly energy source for power generation, however solar photovoltaic electricity generation is not practical for large commercial scales due to its cost and high-tech nature. Solar thermal is another way to use solar energy to generate power. Many attempts to establish solar (solo) thermal power stations have been practiced all over the world. Although there are some advantages in solo solar thermal power systems, the efficiencies and costs of these systems are not so attractive. Alternately by modifying, if possible, the existing coal-fired power stations to generate green sustainable power, a much more efficient means of power generation can be reached. This paper presents the concept of solar aided power generation in conventional coal-fired power stations, i.e., integrating solar (thermal) energy into conventional fossil fuelled power generation cycles (termed as solar aided thermal power). The solar aided power generation (SAPG) concept has technically been derived to use the strong points of the two technologies (traditional regenerative Rankine cycle with relatively higher efficiency and solar heating at relatively low temperature range). The SAPG does not only contribute to increase the efficiencies of the conventional power station and reduce its emission of the greenhouse gases, but also provides a better way to use solar heat to generate the power. This paper presents the advantages of the SAPG at conceptual level. Crown Copyright Ó 2009 Published by Elsevier Ltd. All rights reserved. 1. Introduction Nowadays, most power is, and will continue to be, generated by consumption of fossil fuels which has serious negative impacts on our environment. As a clean, free, and non-depleting source, solar energy is getting more and more attention. However, owing to its relatively low intensity, the application of solar energy for power generation purpose is costly, and the efficiencies of the solar thermal power systems having been developed in which solar energy is used as the main heat source are not satisfactory. In addition, solar energy utilisation is subject to the change of seasons and weather. All of these impede the solar energy’s application. How to use solar energy to generate power steadily and efficiently is a problem that needs to be addressed. In this paper a new idea, i.e., solar aided power generation (SAPG) is proposed. The new solar aided concept for the conventional coal-fired power stations, i.e., integrating solar (thermal) energy into conventional power station cycles has the potential to make the conventional coal-fired power station be able to generate * Corresponding author. Tel.: +61 8 83130545; fax: +61 8 83034367. E-mail address: [email protected] (E. Hu). green electricity. The solar aided power concept actually uses the strong points of the two mature technologies (traditional Rankine generation cycle with relatively higher efficiency and solar heating at relatively low temperature range). The efficiencies (the fist law efficiency and the second law efficiency) of the solar aided power generation are higher than that of either solar thermal power systems or the conventional fuel fired power cycles. 2. Solar aided power generation The basis of solar aided power generation (SAPG) technology/ concept, is to use solar thermal energy to replace the bled-off steam in regenerative Rankine power cycle. This extracted bledoff steam is normally used to preheat feed water entering the boiler, it has the effect of increasing the thermal efficiency of the cycle, but at the cost of reducing work output of the turbine due to the reduced steam mass flow. Therefore the SAPG is capable of assisting coal-fired power stations to increase generating capacity (up to 5%) during the peak hours with the same consumption of fuel, or remaining the same generating capacity but reducing its green house gas emissions within the same range. 0306-2619/$ - see front matter Crown Copyright Ó 2009 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2009.10.025 2882 E. Hu et al. / Applied Energy 87 (2010) 2881–2885 The SAPG technology is thought to be the most efficient, economic and low risk solar (thermal) technology to generate power as it possesses the following advantages: Table 1 Data used in the case study. Case study country location 2 (1) The SAPG technology has higher thermodynamic 1st law and 2nd law i.e., exergy efficiencies over the normal coal-fired power station and solar alone power station. Preliminary theoretical studies is presented in the following sections. (2) Utilizing the existing infrastructure (and existing grid) of conventional power stations, while providing a higher solar to electricity conversion than stand alone solar power stations. Therefore a relatively low implementation cost, and high social, environmental and economic benefits become a reality. (3) The SAPG can be applied to not only new built power station but also to modify the existing power station with less or no risk to the operation of the existing power stations. (4) The thermal storage system that at present is still technically immature is not necessary. The SAPG system is not expected to operate clock-round and simplicity is another beauty of the SAPG. The pattern of electricity demand shows that nowadays air conditioning demand has a great impact on the electricity load. Afternoon replaces the evening to be the peak loading period in summer. This means that the extra work generated by this SAPG concept is just at the right time. Namely, the solar contribution and power demand are peak at the same time i.e., during summer day time. (5) The SAPG is flexible in its implement. Depending on the capital a power station has, SAPG can be applied to the power station in stages. (6) The SAPG actively involves the existing/traditional power industry into the renewable technology and assist it to generate ‘‘green” electricity. It is the authors’ belief that without the engagement of existing power industry, any renewable energy (power generating) targets/goals set by governments are difficult or very costly to fulfill. (7) Low temperature range solar collectors e.g. vacuum tubes and flat-plate collectors, can be used in the SAPG. It is a great new market for the solar (collectors) industry. 3. Energy advantages of SAPG To demonstrate the energy advantages of the SAPG, a case study was carried out, using the THERMOSOLV software [1] developed by the authors, on the generation unit 3 of Loy Yang Power station, located in the Latrobe Valley, Victoria Australia. The related data used in the case studies are shown in the Tables 1 and 2, which are an approximate and for demonstration purposes only. It is a typical 500 MWe brown coal-fired power generation unit with one reheater and six feedwater heaters (one of these is an open type i.e., deaerator). Fig. 1 shows the steam cycle structure diagram, which was generated by the ‘‘THERMOSOLV” software for this case. The unaltered unit originally generates 500.353 MWe with the (steam) cycle thermal efficiency of 46.13%. The steam cycle thermal (i.e., the 1st law) efficiency calculated by the ‘‘THERMOSOLV” is defined as: Efficiency ¼ Australia Average insolation (W/m ) Average sunshine hours per day Fuel cost ($/kWh) Electricity peak wholesale price ($/MWh) Cost of Land (for solar collectors) ($/m2) Finance interest rate (%) Insurance, depreciation, maintenance 790 6 0.02 370 Not considered in the case study Not considered in the case study Table 2 Collector data used in the case studies. Australia a Collector Temp. from °C Thermal efficiency Cost, $/m2 Flat plate Vac. Tube CLFRa Concentrator 40 200 250 350 0.4 0.4 0.3 0.4 $75 $110 $165 $200 Compact linear Fresnel reflector. Three scenarios were examined with the THERMOSOLV software: (1) 100% replacement of all the five closed feedwater heaters. (2) 10% replacement of all the five closed feedwater heaters and (3) 100% replacement of IPH2 feedwater heater, which is the first feedwater heater after the reheating process. The summery of the Q and E report are shown in the Table 3, where E report means the benefit of the SAPG comes from the additional power generated with the same fuel consumption, and the Q report means the benefit comes from the reduced coal consumption while remaining the generating capacity. In above case the Q reports show the payback time for the technology is very long comparing with results in the E report. This is because we just considered the return from the saved fuel that is at low prices. If other factors e.g. carbon tax, higher price for ‘‘green” electricity and social/environment benefits etc. were considered, the figures would look better. It should also be noted that the studies were theoretically for demonstrating the energy advantages of SAPG. The scenarios set may be hard to implement in real case. For instance, the 100% replacement for all heaters will increase more than over 72 MWe output (E report), but the real/existing turbine/generator may not have the room to increase such amount. 4. Exergy advantages of SAPG Normally the temperatures of the heat carrier attained by solar collectors are low. The flat-plate collector can supply hot liquid at up to 110 °C; the evacuated-tube collectors can supply fluid (liquid or vapour) at a bit higher temperature and perform better. Recent advances in the performance of selective surfaces will allow high temperature (350 °C) to be achieved in evacuated tubular sum of ðsteam specific enthalpy change cross each stage of turbine stages multiplied by mass flow through each stageÞ: ðsteam specific enthalpy change cross boiler for heating and reheating multiplied by boiler steam mass flowsÞ 2883 E. Hu et al. / Applied Energy 87 (2010) 2881–2885 Fig. 1. Steam cycle structure diagram, generated by THERMOSOLV, for unit 3, Loy Yang Power. Table 3 Results of the LoyYang power station study. Type of replacement Gen. output Cycle eff. increasea Add. gen. cap Add. income CO2 reduction Fuel savings Collector area Collector cost Payback a 100% replacement of all closed feed heaters 10% replacement of all closed feed heaters 100% replacement of IPH2 feedwater heater E report Q report E report Q report E report Q report 572.5 MWe 6.65% 72.18 Mwe $58M/year 0.0%/year $0/year 977,896 m2 $116.4 M 1.99 years 500 MWe 6.65% 0.0 MWe $0.61 M 3.15/year% $2.76 M/year 854,613 m2 $101.7 M 36.8 years 507.34 Mwe 0.64% 6.99 Mwe $5.6 M/year 0.0%/year $0/year 93,792 m2 $11.3 M 2 years 501.11 Mwe 0.64% 0.75 MWe $0.61 M/ 0.31%/year $268,253/year 92,640 m2 $11.1 M 12.7 years 521.5 Mwe 2.03% 21.115 Mwe $17.1 M/ 0.04%/year 38,164/year 227,892 m2 $25 M 1.46 years 500.97 Mwe 2.03% 0.615 Mwe $ 0.5 M 1.02%/year $0.9 M/year 219,020 m2 $24 M 17.25 years The percentage point. collectors with moderate concentration ratio [2,3]. At these temperature levels, it is difficult to use it to generate power efficiently and the power output is not stable all year-round either. The SAPG is to use solar energy to replace some bleed steam in the regenerative Rankine cycle where the temperatures of the solar fluid and the working fluid are matched. In the conventional regenerative Rankine cycle, the feedwater is at a relatively low temperature and is heated by the steam extracted from the turbine. Since in the heaters the temperature profiles of feedwater and the extracted steam do not match well, there is a large loss of exergy. The exergy loss in the heaters can be reduced by increasing the number of extraction stages (and feedwater heaters), but it can never be eliminated; Moreover, it is not practical to have a large number of extraction stages. This is the place where the solar energy can fit in. The solar energy collected by the collectors is hot enough to replace the extracted steam to heat the feedwater in low temperature range, or even in all, regenerative stages in the regenerative Rankine cycle. So, using the heat carrier heated by solar energy to replace the extracted steam to heat the feedwater where the temperature profiles of the two fluids are matched, and using the saved steam to generate work sound perfect. To illustrate the exergy advantages of the SAPE, let us examine a single-stage regenerative Rankine cycle with open feedwater heater (Fig. 2). In energy system analysis, not only the quantity, but also the quality of energy should be assessed. The quality of an energy stream depends on the work (or work potential) available from Turbine Generator Boiler and superheater Condenser TH Feed water heater TL Pump Fig. 2. Single-stage regenerative Rankine cycle with open feedwater heater. 2884 E. Hu et al. / Applied Energy 87 (2010) 2881–2885 that stream. The capacity for the stream to do work depends on its potential difference with its environment. If a unit of heat flows from a source at a constant temperature TH to its environment at temperature Ta, with a reversible heat engine, the maximum work the heat energy can do, is called the availability and also called exergy of the heat at the temperature TH In the case of using solar energy (heat), the exergy in the solar irradiation, Exs, is [7]:   4T a Exs ¼ 1  ð1  0:28 ln f Þ Q s 3Ts ð1Þ where Ta is the ambient temperature and the Ts is the temperature of the sun, f is the dilution factor which equals 1.3  105, and Qs is the solar heat. In SAPG, the solar heat is used to replace the bled-off steam and heat feed water, so that the solar heat Qs equals: Q s ¼ m  Dh ¼ m  cðT H  T L Þ ð2Þ where m is the mass (or flow rate) of the feedwater in the feedwater heater, c is the mean specific heat capacity of the feed water, Dh is the specific enthalpy change of the feed water cross the feedheater. The net solar exergy efficiency of the SAPG system is then: gsex ¼ DW Exs ð3Þ where DW is the extra work generated by the turbine due to the saved bled-off steam. Owing to the irreversibility, the exergy efficiency of using the solar energy in the regenerative Rankine cycle is certainly less than the above value. The study of the following case demonstrates the exergy advantage of the concept in practice. 5. An example—a multi-stage regenerative system Here is an example of using the solar energy in a three-stage regenerative Rankine cycle. Assuming that the state of the working fluid at every point of the system does not change with or without solar-aided feedwater heating, only the flow rate changes (with the solar energy aided, the flow rate will increase in the turbine). The pattern is shown in Fig. 3. Some important properties are listed in Table 4. Without the solar energy aided, the conventional regenerative Rankine cycle yields work: W 0 ¼ h1  h4 þ ð1  m1 Þðh4  h6 Þ þ ð1  m1  m2 Þðh6  h8 Þ þ ð1  m1  m2  m3 Þðh8  h2 Þ ¼ 1084:96 kJ=ðkg steam in boilerÞ Table 4 Some properties of the cycle. Point in Fig. 3 P (kPa, absolute) t (°C) h (kJ/kg) 1. 2. 3. 4. 5. 6. 7. 8. 9. 16500 7 7 6000 6000 1000 1000 101.3 101.3 538 38.83 38.83 369.82 275.6 179.9 179.9 100 100 3404.78 1993.92 162.7 3097.15 1213.4 2701.53 762.81 2326.44 419.04 Turbine inlet Turbine exhaust Condensed water High pressure extracted steam High-stage heater outlet Medium pressure extracted steam Medium-stage heater outlet Low pressure extracted steam Low-stage heater outlet Note: the weight fraction of the extracted steam m1 = 0.193 m2 = 0.1215, m3 = 0.0812. Assuming the ambient temperature is 25 °C (298 K), and the temperature difference for heat transfer in the condenser is 10 °C. When aided heat is used, assuming the average temperature difference for heat transfer in heaters is 10 °C for liquid heat carrier, let us investigate the following cases. From above cases, it can be seen, in Table 5, that using the low temperature thermal energy to heat the feedwater in the regenerative Rankine cycle, the values of exergy efficiency is quite high, comparing to other solar thermal power generation systems [4–7]. From the thermodynamic point of view, generally using liquid as the heat carrier for solar energy in these systems is better than using vapour. With SAPG, we can use water (liquid) rather than other low-boiling point substance as working fluid and do not need to use the more sophisticated vapour-generating collectors. With a little advanced collector, the medium and even high temperature fluid can be made easily. When high temperature heat carrier of the solar energy can be provided, it is suggested to install the multi-stage collectors with different temperature levels to heat the feedwater serially in the multi-stage heaters (see also Fig. 3). One advantage of this multi-stage design is the system can be made more flexible so particular stage(s) of extracted steam can be closed according to the load demand in practice. If the vapour/steam can be generated by the (solar) collectors, the pattern of multi-stage collectors with different temperature levels is preferable as the solar net exergy efficiencies of the multi-stage systems are much higher than that of the one-stage system, and the more the stages, the higher the efficiencies. In the calculations leading to the Table 5, we adopted the point that the exergy of the heat source, the solar energy, should be identical regardless of the temperatures of the heat carrier the collector can yield, i.e., Eq. (1) is used. Fig. 3. A three-stage regenerative condensing-steam Rankine cycle. E. Hu et al. / Applied Energy 87 (2010) 2881–2885 Table 5 Analyses on solar aided systems. Phase of the heat carrier of the aided energy Highest temperature of the aided energy, °C The stage(s) closed Extra work done by the saved steam DW, kJ/(kg steam generated in boiler) The aided solar heat input Qs, kJ/(kg steam generated in boiler) Thermal efficiency of the solar energy in the aided system gI (gI = DW/Q), % Exergy contained in the aided solar energy Exs, kJ/(kg steam generated in boiler), using Eq. (1): Net solar exergy efficiency in the SAPG, %, gsex in Eq. (3) Work increased (comparing with the conventional regenerative Rankine cycle) (DW/W0), % Case 1 Case 2 Liquid 110 Stage 3 only 27 Liquid 286 All stages 325.9 175.72 1050.7 15.37 30.12 163.1 975.4 16.6 33.4 2.5 30.04 2885 thetical cases studies. By using solar energy to replace the extracted steam in order to heat the feedwater in a regenerative Rankine plant cycle, the energy and exergy efficiencies of the power station can be improved. The higher the temperature aided heat source is, the more beneficial the system can generate. It can be seen that the low-grade energy, e.g. solar heat nd other possible waste heat, is a valuable source of work if it can be used properly. This ‘‘aided” concept has special meanings for solar energy. For in summer weather, both the solar radiation and the electrical load demand peak, and it is easy to make heat carrier in different temperatures with different type of collectors. So the increased solar radiation can supply the increased energy to meet the increased power demand. In addition, the solar aided system can also eliminate the variability in power output when the power is generated by other cycles heated by solar energy alone. The concept of the solar aided power system is really a superior energy system and is a new approach for solar energy power generation. Acknowledgement If there is other waste heat sources available, instead of the solar heat, then the exergy input i.e., Exs or Exwasteheat should be calculated, instead of Eq. (1), using: Exwasteheat ¼ Z TH TL   Z TH Ta cdT TH dq ¼ q  T a 1 ¼ q  T 0 c ln T T TL TL ¼ cðT H  T L Þ  T a c ln TH TL Or in a simplified form: Ta Exwasteheat  q 1  T L þT H 2 ! where TH and TL are high and low temperatures of the waste heat stream before and after the feedheater, and q is the waste heat. The exergy benefit can be calculated using so-called Exergy Merit Index (EMI) [8], instead of the net solar exergy efficiency. 6. Conclusions The paper shows the advantages of solar aided power generation concept in the aspects of its energy and exergy, through hypo- The authors would like to thank Dr. Ying You of Quantum Energy for his original contribution to this topic. References [1] Hu Eric. THERMOSOLV—a tool to assess the solar aided power generation concept for coal fired power stations. 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