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152 V. Minea / Applied Thermal Engineering 69 (2014) 143e154 methods for lowering the condensing temperatures during the hottest periods of the year locally. 5. Potential applications Today, as well as in the future, the challenge is to develop appropriate, performing, and economically feasible industrial ap- plications for ORC machines, especially in countries or markets with high fuel prices and utility capital costs, and/or scarce natural resources. Leibowitz et al. [13] noted that rising fuel prices, the wide availability of industrial waste heat, and renewable energies (e.g. solar and biomass) open a significant market for ORC machines in small (30e65 electrical kW) and medium (750e1500 kWe) power output ranges, if they could be built and installed at economically competitive prices. Today, the growing need to recover thermal power from industrial low-grade waste heat (or renewable en- ergies) has led to developing systems with initial costs ranging from US$1500 to US$2000 per net electrical kW produced, without any incentives, when parasitic power consumption is ignored [13]. With potential financial incentives and/or for higher output power capacities, the return on the initial investment may accelerate. The economic competitiveness of ORC machines strongly de- pends on local energy costs, and therefore their market would be more attractive in those regions of the world with above average electricity prices. Moreover, because turbines may constitute 40e 50% of the total cost of conventional steam power plants, inversed compressors and/or expanders have been developed to reduce the cost of this component by up to 90%. The main applications of ORC machines include heat recovery from high-temperature industrial processes, internal combustion and reciprocating engines (from both jacket cooling water and combustion gas exhaust, up to 1500 kWe/unit), medium size gas turbines, deep geothermal, oil and gas coproduced fluid wells, solar energy sources, biomass boilers (up to 3 MWe/unit), combined heat and power plants, and gas compression stations. Some high-temperature industrial processes (e.g. aluminium) lose about 50% of the energy used for the electrolytic process, about 2/3 of which is lost through the walls of hundreds of furnaces in each plant and 1/3 from the combustion gases [1]. The energy lost through the walls is difficult to recover, but the heat from elec- trolysis gases at about 100e130 C is easier to recover. After filtering and purifying the exhaust gases, the waste heat could be recovered and used for power generation, thus enhancing process efficiency and reducing greenhouse emissions. The power generated may avoid the need to purchase extra electricity to increase aluminium production, and the energy intensity of the process can be reduced and competitiveness improved. In most of internal combustion engines, only 30e40% of the total fossil thermal heat input is turned into mechanical work, while the remaining 60e70% leaves the engine as waste heat, mainly through the jacket water cooling system and the exhaust pipe. The waste heat from the engine jacket water cooling process (at about 90 C) is combined with the waste heat from the exhaust gases (at temperatures above 300 C). Heat is added to the water inside the engine, raising the temperature. The heated water then passes through an exhaust gas heat exchanger, where heat is transferred from the high temperature exhaust into the heated water, increasing the temperature even more. About 10% of these wasted heat sources can be converted into electricity with ORC machines in addition to fulfilling other heating requirements. When fuel costs are high and internal combustion engines are used for electrical production, ORC machines will save fuel costs by allowing the engine to operate at a lower fuel input rate for the same electrical output (Fig. 11a) [9]. Such a heat recovery system, designed and built the exhaust gases of a truck engine, demon- strated its technical feasibility and economical relevance by reducing by 12.5% the fuel consumption [28]. Oomori and Ogino [29] developed an ORC system to recover heat only from the jacket cooling system of an internal combustion engine and Endo et al. [30] proposed a 2.5 kWe ORC machine to recover heat only from the exhaust gases of a 19.2 kWe automotive engine moving at 100 km/h. The heat-to-electricity conversion efficiency was of 13%, while the overall thermal efficiency of the engine increased from 28.9% to 32.7%. These authors suggested that the heat source inlet temperature be controlled by varying the water flow rate through the ORC evaporator with a variable speed pump and that the expander inlet pressure be controlled by varying the rotational speed. Freymann et al. [31] reported on an ORC system recovering heat from both the exhaust gases and jacket cooling fluid. ORC units can also produce electricity from deep geothermal heat sources at low or medium temperatures ranging between 90 C and 180 C. For low-temperature geothermal sources (i.e. below 100 C) efficiency is relatively low and depends strongly on heat sink temperature that is the ambient air in many applications. Deep geothermal heat sources at temperatures >150 C enable combined heat and power generation if condensing temperatures are set at higher values (e.g. 60 C), allowing the cooling water to be used for industrial or space heating. In the case of ORC machines that recover waste heat at temperatures higher than 150 C, condensing temperatures as high as 60 C could be achieved. Such temperatures may be efficiently used, for example, for drying biomass and/or preheating the intake air or feed water of biomass boilers, preheating the make-up air of building ventilation systems and/or domestic hot water, and directly using the heat for building radiant floor and swimming pool heating, roadway ice melting and aquaculture of tropical fish species [9]. The heat harnessed by solar collectors can also be converted into electricity through ORC ma- chines. However, solar thermal arrays need thermal storage and have limited periods of operation depending on daylight hours. On the other hand, higher efficiencies can be achieved using parabolic solar collectors combined with Stirling engines. According to Nguyen et al. [32], a solar energy-to-electricity conversion effi- ciency rate of up to 29.4% has been achieved with a 25 kWe system where the collector temperature reached 750 C. However, because ORC cycles work at lower temperatures, Fresnel linear concentra- tors can be used to reduce power plant size and cost. Such an ORC solar plant (1 MWe) installed in Arizona in 2006, using n-pentane as a working fluid, has achieved a 12.1% global solar-to-electricity efficiency rate. Biomass is another important renewable energy source widely available in a number of industrial processes, such as in the waste and wood industries, that can be used in combined heat and power plants. Biomass is mainly used locally because of its low energy density, which increases transportation costs, and for on-site power demand, which makes biomass suitable for off-grid areas. Biomass boilers are fed with virgin wood chips and thermal oil (mineral or synthetic) at 300 C within a closed circuit. The single-stage biomass-fired boiler represented in Fig. 11b, the ORC machines can be used because of the lower operating pressure and less stringent legislation compared to steam cycles [27]. The ORC condenser heat can be used as a biomass dryer or for district heating. The boiler cools the thermal oil down to 250 C, and the turbo-generator produces electrical power with a net efficiency rate of about 18% at full load. At 50% partial load, the net efficiency rate is about 16.5%. Biomass and bio-gases are renewable energy sources for power generation with a net efficiency rate above 16% and high annual operating rates. Biomass is available all over the world and can be used for the production of electricity with small to medium sized scaled power plants. The problem with high specificPDF Image | Power generation with ORC machines low-grade waste heat
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