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Session: New applications: Process Integration
Session starts: Tuesday 08 October, 10:00
Presentation starts: 10:40
Room: Willem Burger Zaal

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Doris Weiss (Devon Canada)
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/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 ORC expander 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. Another challenge, for larger (>1MW) ORC units is the large quantity of waste heat fluid required and thus the larger waste heat exchanger in comparison to smaller low grade heat ORC units as well as those using high grade waste heat. Again, non-traditional heat exchangers should be evaluated and considered instead of the traditional shell and tube heat exchangers which are very large, costly and take up a large area. The design as well as operation of low grade waste heat ORC units, in cold climates using air cooling, are very challenging and pose unique problems compared to water cooled operation. The authors are familiar with small units in the 50kW range that operate with wet cooling where only small amounts of water are required. However, for plants in excess of one megawatt air cooling becomes the only option due to the combination of sub-freezing ambient temperatures and scarcity of water. Where the source temperature and corresponding ORC thermal efficiency are low the air coolers represent approximately 30-40 %1 of the cost of the unit as the air exchanger requires a much larger surface area than one utilizing cooling water. The design of air cooled condensers (ACC) is more crucial where source temperatures are low and ACC fan power consumption represents a much larger proportion of generator output. Because of this the ACC design becomes more critical in achieving favorable economics. Greater emphasis must be placed on reducing surface area and reducing fan power consumption. However, the design of the ACC must be done with due regard to ambient temperature and power output. An ACC designed for winter ambient will likely be substantially too small for summer ambient. Thus, the designer must conduct a trade study to reach an optimum solution. For example, if the maximum surface area is used for peak power production at the maximum anticipated ambient temperature (usually 30-35°C), then the unit becomes too expensive and uneconomical. On the other hand, if the cooler/condenser is kept smaller by designing for a lower ambient temperature (0-10°C in Northern climates), then the power production will drop off considerably during the spring, summer and fall months where temperatures can reach 15 – 30 ° for many weeks/months. Common practice has shown that transient behaviour of air cooled condensers are often ignored in pursuit of optimizing the turbine design and cycle performance for steady state operation. Frequently, these result in operational problems (forced outages) that require expensive design modifications and/or repair of ORC units installed in northern climates. For example, temperature swings on the order of 15-20 °C per day are frequently encountered. In the morning it is common to have temperatures on the order of 0-5°C and then a peak temperature of 20 -25°C 6-8 hours later. If the condenser is allowed to float on pressure, then there can be a point where complete condensation is not achieved causing insufficient suction head to the pump resulting in plant shut down. . Thus, in order for the condenser to operate within these temperature swings, a condensing pressure should be chosen (usually at the higher pressure) and this becomes an explicit control parameter. (Refrigeration or Dew point control units operate in this manner in temperate climates and those above the 49th parallel in order to insure complete condensation of the working fluid or refrigerant). Further, the heat exchanger design itself can also be challenge at very low ambient temperature. This will be explored in detail to show how fluid velocity inside the tubes increase with higher volume flow at lower temperatures. High fluid velocity increases pressure drop and potential for tube erosion. This is an important factor as these occur where plant output reaches its peak. There is a lack of discussion of these constraints in current literature for larger units in high grade waste heat recovery. Figure 1 shows the limitations that can be expected for a two bay air cooler/condenser for a nominal 1 MW ORC unit. The ambient design temperature is 5°C. As can be seen, the tube erosional velocity is exceeded at -5°C. Thus the expected increase in power production is limited also by the air cooler design. As previously mentioned the air cooler is one of the largest cost items and increasing its surface area can impede the success or implementation of the project as the payback period is high and more than the life of some projects. 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. Very low ambient results in excessive volume flow, choking the expander nozzle, and over expansion causing lower expander efficiency, both combined to limit ORC output. This paper will explore the effects of low ambient temperature on the expander performance in conjunction with the waste heat and ACC exchangers. Figure 1: Graph of Erosional Velocity vs.Outlet Expander Pressure and Power Production 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. In addition, this paper serves to provide realistic options that seek to reduce not only the cost of the ORC unit, but also reduce its footprint which is especially necessary in existing facilities. REFERENCES [1] Franco,A. Villani, M., Optimal design of binary cycle power plants for water dominated, medium-temperature geothermal fields, Geothermics 38(2009) 379-391. [2] G. Holdman, “The Chena Hot Springs 400 kW Geothermal Plant: Experience Gained During the First Year of Operation”, Chena Hot Springs/Chena Power. Fairbanks, Alaska, 2007. [3] W. Gu and Y. Weng et al., “Theoretical and Experimental Investigation of an organic Rankine Cycle for Waste Heat Recovery System,” Proc IMechE., Vol.223, pp.523-531, (2008). [4] M.Hettiarachchi and H.D. Golubovic et al., “ Optimum design criteria for an organic Rankine cycle using low-temperature geothermal heat sources,” Energy , Vol 9:32(9), pp.1698-706, (2007).