Abstract
This paper presents the optimization of ORCs for recovering waste heat from a hypothetical aluminum production plant to be installed in Norway. The case study is particularly interesting because it features two hot streams at different temperatures (the pot exhaust gases and the cell wall cooling air) which make available about 16 MWth below 250 °C. First, a recently proposed cycle optimization approach is adopted to identify the most promising working fluid and optimize the cycle variables (pressures, temperatures, mass flow rates) for the maximum energy performance. The analysis includes both pure fluids, including recently synthesized refrigerants, and binary zeotropic mixtures assessing in total 102 working fluids. The best pure fluid in terms of exergy efficiency turn out to be HFE-347mcc (which can achieve a target exergy efficiency of 85.28%), followed by neopentane, butane and R114. HFO-1336mzz appears to be one of the most promising non-flammable alternatives with low GWP. The mixture leading to the highest exergy efficiency is isobutane-isopentane, which can increase the net electrical power output by up to 3.3% compared to pure fluids. The systematic techno-economic optimization, repeated for two different electricity prices, shows that RE347mcc is the best option in both low and high electricity prices. The cost of the cycle using HFO-1336mzz is penalized by the larger evaporation heat (negatively influencing the heat integration) and the smaller regenerator.
Highlights
With the current aim of energy saving in order to reduce emission and effort to diminish environmental impact of process and manufacturing plant, energy recovery from industrial processes is becoming a major topic
The two exchangers in series coupled with H1 could be merged in a single larger heat exchanger. This option would lead to some savings in the investment cost, but it cannot be handled by the heat exchanger network (HEN) synthesis methodology that assumes constant specific heat capacity within each heat exchanger
The desuperheater and condenser in series coupled with cooling water could be merged in a single larger heat exchanger, but it cannot be obtained by the adopted HEN synthesis methodology
Summary
With the current aim of energy saving in order to reduce emission and effort to diminish environmental impact of process and manufacturing plant, energy recovery from industrial processes is becoming a major topic. Given the set of available hot and cold streams of the plant, for each candidate working fluid (pure fluids and/or mixtures), the algorithm of Scaccabarozzi et al (2018), which was adopted for this study, can determine the cycle pressure, temperature, and mass flow rates, which maximize a relevant energy performance index. The heat integration between ORC streams and heat sources/heat sinks is optimized with the methodology proposed by Kalitventzeff and Maréchal (1999), which determines the maximum mass flow rate of the working fluid that can be generated (the so-called “maximum heat recovery target”), and the maximum cycle net power, for a given minimum allowed heat transfer temperature difference. Relation between heat exchanger areas, mean logarithmic temperature differences, global heat transfer coefficient, and heat duty of the heat exchanger
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