Thermal decomposition of ORC working fluids is normally avoided by using an intermediate heat transfer fluid to separate the working fluid from the high temperature exhaust streams. This has the disadvantage of reducing the attainable conversion efficiency by reducing the maximum temperature of the cycle. It also increases capital cost by requiring an additional heat exchanger. Even when the maximum temperature is limited by use of a secondary heat exchanger and thermal oil, decomposition of pentane has been noted in some facilities where low winter operating temperatures cause vacuum conditions in part of the system (Sweetser and Leslie 2007). Vacuum conditions can result in air infiltration into the system, particularly if it was designed for positive pressure operation. Entrained air will significantly lower decomposition temperature.
In addition to the concern of loss of the working fluid itself to decomposition, there is the concern of potential safety hazards and equipment damage depending on the nature of decomposition products. In the case of alkanes as working fluids, products of decomposition will likely be smaller alkane chains, hydrogen gas, and eventually carbon (Marek and McCluer 1931; Paul and Marek 1934; Morgan et al. 1935; Frey and Hepp 1933; Pease 1928). These products are not particularly concerning unless the working fluid is diluted to the point that system performance is affected or carbon is deposited as a solid. Solid carbon can reduce the efficiency of heat exchangers as well as eventually clogging them. Also, the turbine could be damaged by carbon particulates. Fluorinated hydrocarbons, on the other hand, may form HF or F during 2 decomposition in addition to possibly forming carbon and hydrogen. There is a significantly greater safety concern in this case. This is an interesting point considering that safety, by virtue of not having a flash point, is frequently a reason given for using these refrigerants as working fluids as opposed to other hydrocarbons. Any air or water leaks into the system provide for the possibility of still other reaction productions such as carbon dioxide, hydrofluoric acid, or carbon monoxide.
The current state of understanding of working fluid decomposition is a barrier to adoption of ORC based waste heat to power applications. Advances in the understanding of both the rates of decomposition and the reaction products of the decomposition could facilitate more widespread use. Even if decomposition of working fluids cannot be eliminated, understanding it may make it possible to produce decomposition products that are largely benign and to predict when maintenance is necessary. The effect of decomposition of organic working fluids needs to be taken into account in the calculation of recoverable power as it will affect the efficiency of the conversion of the waste heat into electric power.
EFFICIENCY ASSUMPTIONS
Carnot efficiency was calculated using a cold source temperature of 120°F (49°C) and a heat source temperature either equal to the exhaust gas temperature or limited at some lower temperature to avoid working fluid decomposition. It is estimated that efficiency of approximately half of the Carnot ideal is achievable in most waste heat to power applications due to loss in heat exchange, non-isentropic heating, pumping, and expansion and in the generation equipment and gearbox. This estimate appears to be reasonable based on a recent technology assessment performed by the authors that evaluated the current state of ORC technology. This assessment found that under the assumptions described above the overall waste heat to electric power efficiency was roughly half of the calculated Carnot efficiency. The results of the calculated efficiency compared to measured and quoted values from various suppliers are shown in Figure 5 (Leslie 2009). Clearly the actual efficiency will be influenced by a wide variety of factors in addition to working fluid minimum and maximum temperature including the thermodynamic properties of the fluid, overall system design and optimization, and ambient temperature conditions. However, for the purpose of the present analysis the assumptions given here should be sufficient to estimate the overall opportunity and the impacts of working fluid temperature limitations.
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