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Session: System design and optimization V
Session starts: Tuesday 08 October, 14:00
Presentation starts: 14:20
Room: Ruys & Rijckenvorsel Zaal

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Doris Weiss (Numbers Unlimited Inc)
Hank Leibowitz (Waste Heat Solutions)

Abstract:
There is increasing global interest in the recovery of energy from low temperature streams from which low-grade waste heat (<100°C) is captured in order to increase the energy efficiency and reduce emissions of various oil and gas and energy facilities. While many sources of this waste heat exist, the challenge to utilize the heat economically still remains, especially in northern climates (above the 49th parallel) where water cooling is not an option. Currently the Organic Rankine Cycle (ORC) technology is the most promising and much emphasis is placed on developing efficient expanders to operate with new organic refrigerants i.e. R134a and R245fa. This is necessary to allow for larger units that are more economically viable. However, even the most efficient expander and ORC unit will encounter operational challenges if certain aspects of the process are overlooked and not taken into consideration. Of prime importance in Canadian oil sands operations is the effect of ambient temperatures on air cooler/condenser design as well as the process control of the unit, both of which differ from traditional processes that use cooling water. Common practice has shown that these parameters are often overlooked in pursuit of optimizing the turbine design and cycle performance. Frequently, this results in ORC units that are not operational in northern climates and costly to modify to get them working. For example, temperature swings on the order of 15-20 °C per day are frequently encountered in the spring and fall months. The typical control scheme of a water cooled condenser does not lend itself to these types of temperature swings and many condensers have failed using the cooling water control scheme model. Another issue is the potential to increase power production at low ambient temperatures. While this can be done, it is challenging to design a system that is optimized at both high and low ambient temperatures as the working fluid can create problems within the expander if the temperature is too cold. Furthermore, heat exchanger design can be challenged when the ambient temperature falls below 0°C and the potential for higher power production is realized. This paper will discuss the various aspects and challenges of the design of low-grade ORC units in order to provide mechanical and process engineers with design considerations to produce workable ORC units in challenging environments.