Sustainable energy systems

Abstract

General There is a general understanding that the so-called “developed countries” have to change their way of life including their energy supply into a more sustainable way. But even in the case of unanimity with regard to the direction, there are still many opinions about the way to follow. This thesis discusses problems and possibilities of more sustainable energy systems first of all for the energy supply of the Netherlands. The “trias energetica” is used to distinguish the steps that have to be taken for this purpose. It considers the following sequence of steps: 1) reduce final energy consumption 2) make use of renewable energy sources 3) improve overall conversion efficiencies. The importance of the first step is obvious, but this thesis focuses on the other two steps with great emphasis on the last one. The consequences of the application of available sources of renewable energy in the Netherlands are discussed in Chapter 3. The application of wind and solar energy will result in serious higher costs of energy supply. These higher costs are not only caused by the primary conversion by wind turbines, photovoltaic cells and so on, but also by the need of energy storage and the corresponding conversions. Biomass is supposed to offer a more affordable alternative, but the biomass available today in the Netherlands for energy conversion is not sufficient to cover the present demand. The required primary energy is determined by the final energy demand as well as the efficiency of the necessary conversions from the primary source to the final demand. Today thermodynamic losses associated with the conversion of primary energy into electricity and heat are very serious. Further reduction of these losses is necessary to arrive at more sustainable energy systems. A rough indication of the effects of higher conversion efficiencies is shown in Chapter 3. Simplified systems for the supply of electricity and heat for the Netherlands have been compared based on the use of one single primary fuel (natural gas). The total heat demand is divided into a low temperature and a high temperature heat demand. A reference system, consisting of power plants for the generation of electricity and boilers for the generation of steam and hot water, was considered first. The calculations in Chapter 3 show fuel savings up to 30% to 40% are achievable by the application of power plants with high efficiencies (80%), improved CHP for the high temperature (industrial) heat demand and improved Heat Pumps for the low temperature heat demand. It is obvious that high power generation efficiencies are in favor of Heat Pump application. Most of this thesis focuses on possibilities to reduce the thermodynamic losses of current energy conversion systems for the generation of electricity and heat. The exergy concept is used comprehensively to quantify these losses and to provide a better understanding of their causes. Fuel conversion processes and thermal power cycles are primarily used today for the generation of power and heat. The limitations of these processes and cycles are discussed respectively in the Chapters 4 and 5. Possibilities to improve the thermodynamic performance of power and heat generation with the use of fuel cells and CHP (Combined Heat and Power) are discussed in Chapter 6 and 7. Exergy The fundamentals of the exergy concept as well as possibilities to use this concept for the analysis of energy conversion systems are presented in Chapter 2. A separate section is dedicated to the determination of the specific exergy of solid and liquid fuels. Because of the complex chemical composition of these fuels it is not possible to calculate the specific exergy only with the use of fundamental equations. Szargut and Styrylska have determined regression equations from the calculated exergy values for a large number of pure substances. These equations are frequently used in particular within the area of chemical engineering. Baehr has described a more fundamental method to calculate the exergy of solid and liquid fuels. With the method of Baehr-I only the entropy of the fuel has to be estimated. For all other parameters exact values are available. Based on this method Baehr presented also a simplified method, called Baehr-II. For this method the mass factions of the fuel in the method of Baehr-I are eliminated by using statistical relations between mass fractions and heating values. Then equations are obtained that enable the calculation of the specific exergy of a fuel if only the heating value is known. A comparison of the three methods is made, assuming that Baehr-I gives the most accurate results. It appears that the method of Szargut and Styrylska agrees good in the case of fuels in the dry and ash free condition. Large deviations might occur in the case of fuels in the as received condition. The results from the Baehr-II method do correspond quite well with the results from Baehr-I for coal in the as received condition. For wood, wood chips and peat the correspondence is good for the fuel in the as received condition as well as in the dry and ash free condition. The chapter further describes various tools for analyzing energy conversion systems like value diagrams, exergy flow diagrams, and exergy efficiencies for apparatuses, plants and thermal power cycles. The application of these tools is evaluated in the other chapters. Fuel conversion The thermal conversion of a primary fuel into heat or into a secondary fuel causes substantial thermodynamic losses. These losses (exergy losses) are usually in the order of 20%-30% or even higher and have a considerable effect on the overall performance of plants using fossil or renewable fuels. The losses are in general higher if the quality of the fuel is lower as is e.g. the case with biomass. In the case of atmospheric combustion the exergy losses are affected mainly by the temperature of the combustion air and the air factor. A comprehensive evaluation of these effects for various fuels is presented in Chapter 4. Air preheating appears to be the most effective way to reduce the exergy loss of combustion. Usually the full flue gas flow is used to preheat the combustion air. Then the maximum achievable temperature of the combustion air is limited because of the difference in thermal strength of the air flow and the flue gas flow. Higher air preheat temperatures are conceivable by splitting the flue gas flow. But even in the case of the combustion of natural gas with an air preheat temperature of 1000°C the exergy loss is still higher than 20% of the fuel exergy. Pressurized combustion is usually associated with gas turbine units. In the case of simple gas turbine cycles the exergy loss of combustion is determined primarily by the pressure ratio and the TIT (Turbine Inlet Temperature). It is shown that even in the case of a pressure ratio of 50 and a turbine inlet temperature of 1900°C the exergy loss of combustion is higher than 20%. Alternative gas turbine cycles, like recuperated cycles and cycles with reheat and intercooling, have been proposed in the past. Such cycles can reduce the exergy loss of combustion. But even with these cycles it is unlikely that exergy losses of combustion seriously lower than 20% can be realized. For the thermal conversion of primary fuels into secondary fuels examples of three processes are discussed: coal gasification, biomass gasification and reforming of natural gas. In general the exergy losses of the gasifier or reformer can be significant lower than the exergy loss of combustion. But the conversion of a primary fuel into an appropriate fuel for power systems will require various auxiliaries (e.g. the generation of steam) and also further processing of the gas. The total exergy loss of the fuel conversion system depends also on the quality of integration into the power plant but is usually higher than 20%. This means that the highest efficiencies achievable for power plants using solid fuels are roughly 20 to 25% lower than the efficiencies of natural gas fuelled systems using similar power cycles. Thermal power cycles Thermal power cycles are primarily used today for the generation of power. High overall plant efficiencies do require an appropriate match between combustion process and power cycle. Large conventional power plants consisting e.g. of a coal fired boiler and steam turbine cycle have total exergy losses of around 60% . The application of advanced steam conditions is an option to reduce these losses. Net thermal efficiencies higher than 45% are supposed to be feasible today and higher efficiencies up to 50% (LHV) are expected to become feasible in the future. At present IGCC plants have not serious higher efficiencies and without CO2 capture IGCC is not economic competitive with PC or CFBC plants. But the IGCC might benefit in the future from the further development of the CC (combined cycle) and more rigorous requirements with regard to CCS (carbon capture and storage). The application of alternative topping cycles like e.g. potassium topping cycles will not enable significant higher efficiencies. Natural gas fired CC (combined cycle) plants can reach net thermal efficiencies up to 60% today. Determining for these efficiencies are primarily the thermodynamic equivalent temperature of heat transfer to the combined cycle ( ) and the internal exergy efficiency of the combined cycle ( ). Heat transfer to the combined cycle occurs only in the gas turbine, thus the thermodynamic equivalent temperature of heat transfer to the combined cycle is the same as the thermodynamic equivalent temperature of heat transfer to the gas turbine cycle. During a period of 25 years, from 1983 to 2008, the temperature of large heavy duty gas turbines has been raised from around 650°C to around 800°C because of the application of higher turbine inlet temperatures and pressure ratios. The internal exergy efficiency has been increased during this period from around 0.7 to 0.8. Thermal efficiencies of 0.62 to 0.71 are achievable with CC plants if is increased to 1140°C and the internal exergy efficiencies are in between 0.8 and 0.9. Considering the developments in the past, it is expected that thermal efficiencies around 0.7 might be achievable after serious development efforts during several decades. Fuel cell systems The strive for higher power plant efficiencies is hindered primarily by the high thermodynamic losses of thermal combustion. These fuel conversion losses can be reduced seriously by replacing the thermal conversion of fuel by the electrochemical conversion as is the case in fuel cells. In particular with high temperature fuel cells (MCFC or SOFC) the exergy loss is less than 2% of the total exergy that enters the cell, or less than 4% of the chemical exergy that enters the cell. The exergy losses in low temperature fuel cells like the PEMFC are significantly higher . Overall system efficiencies of fuel cell systems will depend highly on the system design and in particular on the overall design of the fuel conversion system. Natural gas is initially the most likely primary fuel for fuel cell systems. With low temperature fuel cells the fuel has to be converted first into almost pure hydrogen. CO removal at ppm level is required to avoid poisoning of the electrodes. Because of the higher exergy loss of the cell and the additional losses of fuel conversion and purification the efficiency of low temperature fuel cell systems will always be much lower than the efficiencies achievable with high temperature fuel cell systems. High temperature fuel cells offer the opportunity to raise conversion efficiencies of power generation based on natural gas to values around 80%. This has been confirmed by system calculations for a target system, a so-called SOFC-GT hybrid system. The assumed design data of the fuel cell stack of the target system are based on the application of existing materials and an operation temperature of 700°C. The high efficiency can be achieved at plant power levels around 30 MWe or even lower and without the need for cooling water. Further improvement of the system efficiency appears to be conceivable by using the residual heat. The application of a bottoming cycle (e.g. ORC) will raise the exergy efficiency even above 80%. It is obvious that lifetime and costs of HT fuel cell stacks have to be at the appropriate levels before these systems will be considered seriously for commercial application. Combined heat and power During the generation of heat in natural gas fired hot water boilers or industrial steam boilers, most of the fuel exergy is lost. Also the highly recommended high efficiency hot water boilers have exergy efficiencies below 15%. A substantial reduction of these losses is possible only by the application of alternative heat generation processes like heat pumps and combined heat and power plants (CHP plants). The fuel savings achievable with these technologies (roughly 30% to 40%) are discussed in Chapter 3. In Chapter 7 a more comprehensive discussion of various concepts for CHP plants is presented. In the literature on CHP plants it is often assumed that the thermodynamic advantages are obvious. If merit indicators are presented, thermal efficiencies (electrical, thermal and overall efficiencies) are frequently used. Exergy efficiencies are only mentioned occasionally. Besides a general description of the thermodynamic aspects of CHP and the thermodynamic concepts for the evaluation of these plants Chapter 7 focuses on the definition and application of true merit indicators. It appeared that both, thermal efficiencies and exergy efficiencies, don’t indicate the merits of CHP in the right way. Finally, the true merit indicators are used to discuss the characteristics of different CHP plants like industrial CHP plants, CC plants for district heating and micro-CHP units. The evaluations in Chapter 7 are made for design conditions of the considered plants. An evaluation of off-design conditions is discussed in Appendix 7.1 but is useful only for the optimization of specific CHP plants. Fuel savings up to 20% are achievable with CHP plants in operation today (see Table 7.4.1). For the future higher savings are conceivable depending on the development of the electrical efficiencies of CHP plants and power stations. A literature evaluation shows that determining the merits of CHP is a serious problem. Useful indicators will require any kind of comparison with separate generation of heat and power. If there is no need to consider external requirements, the fuel savings factor of a CHP plant in comparison with separate generation is a true indicator of the thermodynamic quality of the CHP plant. In the case of a government or an owner of an industrial site who is looking for minimum fuel consumption of a wider system, it has to be checked first how the overall energy demand affects the maximum installed power of CHP. If the maximum installed CHP power is limited by the power demand, the fuel energy savings factor per unit electricity is the true criterion to achieve the maximum benefit from combined generation. If the heat demand is limiting the maximum installed CHP power, the fuel exergy savings factor per unit heat exergy is more appropriate. The application of different fuels will complicate the discussion about merit indicators. True values of the merit indicators can be obtained only if CHP and reference system use the same fuel. If for the generation of power always other primary fuels are used than for the generation of heat, additional (arbitrary) criteria are needed to define the merits of CHP. With CHP plants for the generation of low temperature heat in general higher fuel savings can be achieved than with CHP plants for the generation of high temperature heat. Thus, the application of CHP plants for the generation of low temperature heat is more attractive than the application of industrial CHP. For the generation of low temperature heat, however, the Heat Pump is an alternative option to reduce the thermodynamic losses of heat generation. A general conclusion with regard to the preferred technology is impossible. Useful conclusions can be drawn only for specific cases and require more detailed evaluation of the alternatives. Micro-CHP units with high electrical efficiencies will enable high fuel savings as well as high fuel energy savings per unit electricity. The application of units with electrical efficiencies lower than 20% is not really beneficial with regard to fuel savings, but might be useful for the implementation of micro-CHP into the market. Natural gas fuelled fuel cell systems with high electrical efficiencies are very attractive. The development of reliable and cheap micro-CHP fuel cell systems with electrical efficiencies of 40% or higher seems to be attractive. Combined Cycle plants with heat extraction for district heating show high values for the relevant merit indicators, in particular the fuel energy savings per unit electricity. In the evaluations the heat losses of the district heating system due to transport and distribution are ignored. These losses can be substantial and have a serious effect of the thermodynamic merits of the CHP plant. They have to be considered in particular if e.g. the application of small scale units has to be compared with the application of large scale plants. However, the selection of true values of these losses is in general not easy. Detailed evaluation of the losses of transport and distribution for a specific case is necessary to come to the right conclusions. Suggestions that an optimum value for the heat to power ratio of CHP units does exist are obviously false.