Abstract
In this paper, we investigate the adoption of working-fluid mixtures in ORC systems operating in combined heat and power (CHP) mode, with a power output provided by the expanding working fluid in the ORC turbine and a thermal energy output provided by the cooling water exiting (as a hot-water supply) the ORC condenser. We present a methodology for selecting optimal working-fluids in ORC systems with optimal CHP heat-to-electricity ratio and heat-supply temperature settings to match the seasonal variation in heat demand (temperature and intermittency of the load) of different end-users. A number of representative industrial waste-heat sources are considered by varying the ORC heat-source temperature over the range 150–330°C. It is found that, a higher hot-water outlet temperature increases the exergy of the heat-sink stream but decreases the power output of the expander. Conversely, a low outlet temperature (~30°C) allows for a high power-output, but a low cooling-stream exergy and hence a low potential to heat buildings or to cover other industrial thermal-energy demands. The results demonstrate that the optimal ORC shaft-power outputs vary considerably, from 9MW up to 26MW, while up to 10MW of heating exergy is provided, with fuel savings in excess of 10%. It also emerges that single-component working fluids such as n-pentane appear to be optimal for fulfilling low-temperature heat demands, while working-fluid mixtures become optimal at higher heat-demand temperatures. In particular, the working-fluid mixture of 70% n-octane+30% n-pentane results in an ORC-CHP system with the highest ORC exergy efficiency of 63% when utilizing 330°C waste heat and delivering 90°C hot water. The results of this research indicate that, when optimizing the global performance of ORC-CHP systems fed by industrial waste-heat sources, the temperature and load pattern of the cogenerated heat demand are crucial factors affecting the selection of the working fluid.
Highlights
The rising global demand for energy and increasing desire for sustainable, secure energy provision are major drivers for enhancing the efficiency of energy processes and systems
Results relating to working-fluid selection and subsequent organic Rankine cycles (ORCs)-combined heat and power (CHP) system performance are presented in Sections 4.1 and 4.2 for the aforementioned high-temperature heat source (330 °C, 560 kg/s) and in Section 4.3 we investigate the impact of the heat-source temperature (250 °C, 120 kg/s and 150 °C, 30 kg/s)
The ORC-CHP model was optimized with different working fluids while considering heat recovery and conversion from the high-temperature heat source (Ths = 330 °C, m_hs = 560 kg/s), by maximizing the ORC exergy efficiency at different hot-water supply temperatures and assuming xCHP = 1
Summary
The rising global demand for energy and increasing desire for sustainable, secure energy provision are major drivers for enhancing the efficiency of energy processes and systems. The particular, the utilization of the considerable quantities of wasted heat that are available in large quantities from numerous sources in the industrial, tertiary, residential and transportation sectors is a promising way of achieving an increase in overall system efficiency [1]. Low- and medium-grade waste or renewable heat can be converted into useful power such as electricity, or recovered. The Kalina cycle, for example, uses a mixture of ammonia and water, whereas organic Rankine cycles (ORCs) can employ different organic working fluids such as hydrocarbons, refrigerants or siloxanes [3,4,5,6], or mixtures thereof. A significant effort has been placed on the development and improvement of ORC power systems in different applications [7,8,9,10,11,12,13], usually at plant scales in the $1–10 MW range
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