Renewable and Sustainable Energy Reviews 15

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statistics to determine the share of each application. Authors in the present paper report and analyze different organic Rankine cycle applications with respect to the nature of the heat source and technology maturity. 2. OrganicRankinecycleapplications 2.1. Binarygeothermalpowerplants The Earth is increasingly warmer the deeper one goes. This underground energy emitted from the center of the earth and usu- ally called geothermal energy can be used for heating processes and/or electricity generation. The Earth’s heat flow – i.e. the amount of heat released into space from the interior through a unit area in a unit of time, varies from place to place on the surface and with the time at a particular location. The Earth’s total output is estimated at about 4 × 1013 W, which is more than threefold the world total energy consumption [20]. The average geothermal gradient near the Earth’s surface is about 300 K/km and is not equally distributed, allowing some locations to be more suitable for geothermal appli- cations than others. First use of geothermal energy for electricity generation started in Italy with experimental work by Prince Ginori Conti in 1904–1905. And first commercial plant of 250 kWe is reported to have been erected in 1913 at Larderello, Italy [21]. Currently, there are 504 geothermal power plants in operation in 27 countries with a total installed capacity of about 10 GW [22]. Major types of geother- mal power plants are [22,23]: dry steam, single-flash, double-flash and binary-cycle plants. Comparison between available options is summarized in Table 2. Flash systems are used for moderate and liquid-dominated resources, dry steam plants for dry-steam resources and binary cycles are well adapted for low-temperature liquid-dominated resources. In the geothermal binary plant depicted in Fig. 2, the thermal energy of the geothermal fluid is transferred to a secondary work- ing fluid via heat exchangers for use in a conventional Rankine cycle. The organic working fluid receives heat, evaporates and expands in the turbine before being condensed and returned back to the evap- orator by the feed pump. Cooling of the condenser is assured by air coolers, surface water cooling systems, wet-type cooling towers or dry-type cooling towers. The first binary geothermal plant was put into operation at Paratunka, Russia in 1967 [21,24]. It was rated proved the feasibility of the binary concept. For low-temperature geothermal fluids below 150 ◦ C, it is difficult to implement cost effective flash steam plants and the binary option is the sole solu- tion. Today, binary power plants are the most widely used type of geothermal power plant with 162 units, generating 373 MW of power. They constitute 32.14% of all geothermal units in operation but generate only 4% of the total power [22]. Few of these plants are given in Table 3. The technology has been developed and com- mercialized since the 1980s by Ormat Technology Inc. [25,26]. In the MW power range, ORC modules incorporate conventional tur- bines and are cost-effective, while at lower power outputs the lack of cheap turbines renders the technology hardly applicable. Brasz et al. [27] suggested to use HVAC components. By applying this con- cept, they turned a standard 350 ton air-conditioning system into a 200 kW ORC power plant. The product is commercialized under the brand name PureCycle®280 by United Technologies Corpora- tion (UTC). Plants based on this technology are: East Hartford (CT), Austin (TX), Danville (IL) and Chena (Alaska) to quote just a few [27–29]. Similar developments have been carried out by Smith and Stosic at City University, UK [30–32] who successfully converted screw compressors into screw expanders. Electratherm [33] and BEP Europe [34] are companies commercializing screw expanders based ORCs. In geothermal plants, the constant preoccupation is the opti- mal resource utilization. This is measured in terms of energy and exergy efficiencies. First law efficiencies are found in the range 5–15% while second law efficiencies are typically in the range 20–54% [35,36]. A large number of studies define criteria and guidelines for the optimal design of binary cycle power plants. According to Borsukiewicz-Gozdur and Nowak [37], geothermal water mass flow should be appropriately chosen for power maxi- mization. Kanoglu and Bolatturk [38] assessed the thermodynamic performance of the Reno (Nevada, USA) binary plant. This plant uses geothermal fluid at 158◦C and isobutane as working fluid. Exergy and energy efficiency obtained were 21% and 10.20%, respectively. The brine re-injected at relatively high temperature (90◦C), accounts for 35.3% of exergy losses and 55.7% of energy losses and could be used for district heating to increase the over- all efficiency of the plant. Numerous studies are dedicated to the selection of adequate fluids using very different optimum criteria [10,36,39–46]. Heberle and Brüggermann [39] compared second law efficiencies of organic Rankine cycle in series and parallel cir- cuits. Gawlik and Hassani [40] used the levelized electricity cost (LEC) to select among isobutane and propane based mixtures, in addition to pure fluids which ones were suitable for resources in the range 129.44–190.55◦C. Madhawa Hettiarachchi et al. [41] used the ratio of the total heat exchanger area to net power out- put as an objective function and implemented the steepest descent method for function minimization. Other authors choose a set of criteria during their investigation. Screening criteria used by Guo et al. [46] include net power output per mass flow rate, the ratio of total heat transfer area to net power output and electricity produc- tion cost (epc). Shengjun et al. [45] conducted similar study, where they added two more indicators: thermal and exergy efficiencies. Till now, No single criterion has been found as most important for optimal design – optimized fluids always vary with the objec- tive function and the plant optimum operating parameters as well. Although, theoretical studies point out that fluids mixtures owing to their temperature glide during the evaporation process have the advantage of reducing the system exergy destruction, and increase the plant efficiency through better matching with the heat source and cold sink profiles, no installation to our knowledge operates at the moment on this concept. The thermal/chemical stability or the variations of the components fractions during cycle oper- ation require investigations. Transcritical cycles were investigated [45,47–49] as way of increasing the cycle performance but this con- at 680 kWe using water at a temperature of Table 2 Comparison of different types of geothermal plants [22]. 81 ◦ C and this plant Plant cost and complexity Moderate 􏴫high Low-moderate Moderate Moderate 􏴫high Type Double-flash Dry-steam Single-flash Basic binary Resource temperature (◦C) 240–320 180–300 200–260 125–165 Utilization efficiency (%) 35–45 50–65 30–35 25–45 B.F. Tchanche et al. / Renewable and Sustainable Energy Reviews 15 (2011) 3963–3979 3965 Fig. 2. Flow diagram for a binary geothermal power plant.

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