Life Cycle Assessment of Novel Heat Exchanger for Dry Cooling of Power
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Applied Energy 271 (2020) 115227 Contents lists available at ScienceDirect Applied Energy journal homepage: www.elsevier.com/locate/apenergy Life cycle assessment of novel heat exchanger for dry cooling of power T plants based on encapsulated phase change materials ⁎ Lige Zhanga, Sabrina Spatarib,c, Ying Suna, a Drexel University, Department of Mechanical Engineering and Mechanics, 3141 Chestnut Street, Philadelphia, PA 19104, USA b Drexel University, Department of Civil, Architectural and Environmental Engineering, 3141 Chestnut Street, Philadelphia, PA 19104, USA c Technion – Israel Institute of Technology, Department of Civil and Environmental Engineering, Haifa 32000, Israel HIGHLIGHTS GRAPHICAL ABSTRACT • An air-cooled heat exchanger with high coefficient of performance (COP) is developed. • The novel heat exchanger (HX) is based on encapsulated phase change material (EPCM). • The EPCM HX is compared with wet cooling tower (WCT) and air-cooled condenser (ACC) • Compared with the WCT, the EPCM HX reduces 13.3% GHG emission and 72% water usage. • The EPCM HX achieves 2.5 × COP compared to ACCs at 11.5% reduced cost. ARTICLE INFO ABSTRACT Keywords: Cooling systems in power plants account for approximately 40% of total freshwater withdrawals in the U.S. Due Power plant cooling to dwindling access to freshwater resources worldwide, continued operation of wet cooling systems poses a Dry cooling technology significant engineering challenge. To reduce water consumption, a novel air-cooled heat exchanger hasbeen Phase change materials developed using encapsulated phase change material (EPCM) for dry cooling of power plants. Compared to Life cycle analysis traditional finned-tube air-cooled condensers, this novel EPCM heat exchanger improves the heat transfer Techno-economic analysis coefficient and power plant efficiency while reducing the pressure drop and cooling system cost.Lifecycle Water-energy nexus assessment (LCA) and techno-economic analysis (TEA) are used to evaluate the environmental and economic performance of EPCM heat exchangers from cradle-to-grave and to compare them to wet cooling and traditional air-cooled condensers. A thermodynamic model is developed to predict the EPCM heat exchanger performance for plant-scale operations. Equipment and construction costs for heat exchangers are estimated based on design parameters obtained from the thermodynamic model. Both process-LCA and economic-input–output LCA are used to simulate and test the sensitivity of EPCM alternatives with commercial wet and dry cooling technologies. We investigate options for EPCM end-of-life management upon retiring the heat exchanger and construct a process-based LCA model to estimate a greenhouse gas (GHG) emissions credit for recycling the EPCM. The life cycle GHG emission of the novel dry cooling technology is 1.16 kg CO2 eq./MWh compared with the 1.1–4.3 kg CO2 eq./MWh reported for commercial dry cooling technologies and consumes 9.5 L/MWhe of water for cradle- to-gate life cycle, which is significantly lower than that of wet cooling systems. The TEA shows many advantages ⁎ Corresponding author. E-mail address: [email protected] (Y. Sun). https://doi.org/10.1016/j.apenergy.2020.115227 Received 13 September 2019; Received in revised form 13 May 2020; Accepted 17 May 2020 0306-2619/ © 2020 Published by Elsevier Ltd. L. Zhang, et al. Applied Energy 271 (2020) 115227 of EPCM cooling technology over the state-of-art dry cooling solutions. Overall, the EPCM heat exchanger provides a better alternative compared to existing dry cooling and wet cooling technologies. 1. Introduction ambient air, of ACCs is usually higher than wet cooling. This higher ITD leads to an increased steam turbine backpressure and hence to reduced Over the last decade, owing to water scarcity and limited water power plant efficiency [23]. Thus, a key to high cooling performance is resources, there has been increasing global demand for reducing the reduction in the steam condensation temperature. freshwater consumption. Power generation is responsible for 25% of To improve the air-side heat transfer performance and minimize global greenhouse gas emissions [1] and additionally a major consumer pressure drop penalties, several heat transfer enhancement techniques, of freshwater resources. Steam-electric power plants account for ap- such as winglet vortex generators extended into the air flow to generate proximately 40% of total freshwater withdrawals in the U.S., over 90% longitudinal vortices along the finned tubes, have been investigated of which is employed for condenser cooling [2,3]. Future water demand [24]. The feasibility of indirect dry cooling in the form of thermosi- for electricity generation is anticipated to increase progressively as phons or heat pipes to replace conventional wet cooling towers [25,26] thermoelectric generation capacity is expected to grow by 6% between has also been demonstrated. 2010 and 2035 [4,5]. Among the available condenser cooling systems, Solid-liquid phase change materials (PCMs), owing to their large wet cooling is predominantly used in existing power plants. Once- energy densities, small density variations, and relatively low cost, have through wet cooling systems, which exploit and disrupt water resources been considered for short- and long-term energy storage systems and threaten the survival of local ecosystems, account for nearly 43% of [27,28]. A PCM-based technology using heat transfer fluids comprising the U.S. thermoelectric generating capacity [5,6]. These systems ad- encapsulated PCM particles suspended in a base fluid has been in- ditionally have the disadvantage of returning water at elevated tem- vestigated with the potential to improve the thermal performance of peratures (typically ~ 10 °C above intake temperatures) making it heat exchangers [29,30]. Most recently, we have employed PCMs to difficult to site power plants near available water at sources [7–9]. decouple the steam condensation and heat rejection to air in air-cooled Compared to once-through condensers, evaporative wet cooling sys- condensers where the PCM particles are directly suspended in their own tems in the form of cooling towers have lower water withdrawal re- melt to provide a short-term thermal storage between the steam-con- quirements, and are currently installed in 42% of U.S. thermoelectric densing (hot) and air (cold) sides [31]. In comparison to conventional power plants [4,5]. Wet cooling towers draw ambient air that cools ACCs, this novel PCM ACC sprays millimeter-sized liquid PCM droplets down the sprayed hot water exiting the condenser. The counter flow of to take advantage of crossflow of air over small PCM droplets and hence ambient air and hot water removes most of the process heat and 2–3% provides up to 5 times larger air-side heat transfer coefficient. For steam of water is lost by evaporation while the remainder is returned to the condensation, melting of PCM particles in a two-phase PCM slurry bath condenser and cooled to ~ 4.4 °C (8°F) above the ambient wet-bulb anchors the condensation temperature close to the PCM melting point temperature [3,10]. Thus, wet cooling towers provide higher cooling regardless of the ambient air conditions. One potential challenge of the rates and improved system performance in arid climates with low wet PCM ACC is the particulate losses to the environment as well as the bulb temperatures. Although, evaporative wet cooling systems have inevitable contamination of PCM exposed to the air. much lower water withdrawal rates than the once-through systems, To overcome particle loss and contamination, we have converted they tend to have appreciably higher water consumption because of the open system for PCMs [31,32] into a closed loop system by en- evaporative water loss. capsulating the PCMs with thin polymer tubes [33,34]. The resulting In comparison to wet cooling systems, dry cooling systems such as highly porous heat exchanger with encapsulated phase change material air-cooled condensers (ACCs) use air instead of water to condense the (EPCM) is used downstream from a water-cooled steam surface con- steam and consequently reduce the overall power plant water con- denser. Hot water exiting the condenser is cooled by melting the PCMs sumption by more than 90% [11–13]. ACCs have not been widely embedded in the thin polymer tubes. As ambient air passes across the employed in the U.S., with only ~ 1% of existing U.S. plants using melted EPCM tubes, the PCM solidifies and rejects the heat to air. The them, but are expected to see increased adoption due to competing porous EPCM heat exchanger has a high heat capacity, high heat water demands and water conservation regulations. Among the existing transfer coefficient, low pressure drop, and low cost. It also eliminates ACCs’, direct-coupled mechanical-draft ACCs have been utilized in most all drift and particulate losses to the environment [35]. of the dry cooling systems in the U.S. [4,14,15]. In these systems, steam Our objective is to compare the life cycle environmental and eco- exiting the turbine is routed through a series of large horizontal steam nomic performance of the novel EPCM heat exchanger relative to the pipes running along the top of ACC cells. A typical ACC cell has con- state-of-the-art ACC and evaporative cooling technologies using life densate carrying finned tubes along the inclined walls that form anA- cycle assessment (LCA) and techno-economic analysis (TEA). Most prior frame. Air is driven through the inclined finned-tube arrays by a large LCA research on power generation has evaluated multiple environ- axial-flow fan approximately 9–10 m in diameter [16,17]. The benefits mental metrics related to renewable energy sources such as solar [11], of ACC systems however come with some drawbacks such as sub- wind [36–38] and biomass [39–42] including LCA/TEA approaches to stantially higher capital investment (~10 times) [17] compared to guide investment in electricity [40] and combined heat and power [43], evaporative wet cooling systems due to their larger footprint area, use to address climate change.