1. Introduction
Over recent years, a large number (over 200) of organic rankine cycles (ORCs) based on siloxanes as the working fluids has begun operating all across Europe [
1]. Siloxanes are advertised as uncomplicated and long-lasting working fluids. Many siloxanes are classified as inflammable, and they are more stable than their carbo-organic equivalents. The bond energy of a siloxy group is significantly higher than the bond energy of a carbon chain [
2]. There are only a few sources in the literature describing the decomposition of siloxanes in high-temperature applications (250–350 °C) in detail, such as [
3]. Most studies are related to the use of poly-siloxanes as solids under oxygen and nitrogen atmospheres [
2]. The siloxy-bonds in siloxanes are very strong (809
), but compared to the methyl-silane group, they are the weaker link. The methyl-silane decomposition takes place above 400 °C. In general, siloxanes have a tendency to polymerize or oligomerize above 300 °C. The equilibrium of the polymerization and the product configuration depend on pressure and educt configuration. Formic acid may be produced during thermal decomposition. In thermal conversion, siloxanes react to SiO
2, CO
2 and hydrocarbon residues [
4]. Some authors claim that siloxane working fluids last throughout the lifespan of the ORC unit, approximately 20–25 years [
5], without an aging process.
However, over longer periods of operation, cycle fluids degrade through decomposition or contamination. Strong oxidants, catalytic alloys or sulfur can trigger or accelerate the decomposition processes. Contaminants and decomposition products of cycle fluids can lead to severe disturbances of the process. High boilers, which are still liquid while the main working fluid is fully evaporated, can damage the turbine or heat exchangers by droplet impingement. Furthermore, the liquid is transported across the turbine section without contributing to expansion and, consequently, to power generation. Vaporous low boilers can lead to cavitation in the feed-pump, galloping in the pre-heater, excessive super-heating in the evaporator and low heat transfer coefficients. The pressure level in the condenser and recuperator rises with a larger fraction of low boilers in the working fluid. The pressure differences across the turbine decrease accordingly. The volume distribution across the cycle can significantly differ from the original design. As a consequence, lower electric efficiencies and higher maintenance costs have to be expected.
Over the last few years, the authors have monitored an ORC power plant in detail (see Case Study Power Plant #8 in
Section 2.2) [
6,
7]. The efficiency, as a well as the cycle characteristics of the unit have changed during this period: the annual thermal efficiency of the system decreased significantly from 11.5% (in 2008) to 7.6% (in 2013) and 5.04% (in 2014). A first working fluid test in 2013 showed degradation. The purchase of a new filling of working fluid would have been a reinvestment of € 140,000. In addition, between 2006 and 2012, the prices of wood chips doubled [
8]. The confluence of the aforementioned points seriously threatened the economical feasibility of this biomass-fueled ORC power plant. Consequently, the authors decided to develop a method to recycle old working fluid and to test it on an existing power plant. The following
Section 2 describes the method of working fluid analyses. In
Section 3, working fluid analyses of several power plants are presented in order to prove that the observed effects are not an isolated incident, but rather a common problem. The procedures of recycling in the laboratory and
in situ are shown in
Section 4. A working fluid recycling system prototype that is integrated into the existing power cycle is presented in
Section 5. It can be easily applied to similar power plants of this type.
5. Cycle Modifications
The modifications that were made in order to conduct the recycling procedures are depicted in
Figure 9a,b and the scheme in
Figure 8. Besides the original vacuum pump, a membrane pump was installed (
Figure 9, highlighted in red). Vapor from the condenser vessel is collected in a reservoir outside the turbine house where it partially condenses (
Figure 8b). As the tests show (
Table 3), the fluid mixture from the vacuum pump contains a great share of MM. In order to remove the MM from the mixture, a pressure between 10
and 43
is necessary (ambient temperature in a range of 0–20 °C). This system is operated batch-wise; as soon as the capacity of the vessel is reached (200
), the membrane pump is started. After the end of the procedure, the remaining MDM is returned to the cycle. The removal of working fluid with a high fraction of high boilers is done via a drain (Tap 3,
Figure 9) in the recuperator vessel. With the original design of the cycle, this was not possible. The liquid from this tap is evaporated in the distilling apparatus depicted in
Figure 8. With the modifications of the cycle, the costs for the recovery of one liter of MDM, with a purity of more than 95%, are roughly € 8. After more than one year of operational experience with regular working fluid checks and replacement, the following observations were made.
The condenser pressure has been held stable at a low level. As a consequence, the electric output and efficiency increased significantly.
The availability of enough fluid and an appropriate filling level of the evaporator leads to less super-heating, due to that fact hot spots in the evaporation section are avoided. The thermal efficiency of the system rises.
The turbine and the recuperator are suffering less from droplet impingement. A qualitative estimation is not possible in this case. However, the noise emissions and vibrations were reduced significantly.
6. Discussion
The presented research with a field analysis of eight power plants shows that the degradation and aging of working fluids is a problem. In order to turn an ORC power plant into a good investment for a long period, the working fluid properties have to be observed. ORC power plants are often said to have low personnel and maintenance costs. As the results here show, this is not entirely true. Quality checks and the distilling process demand manpower on site. In most cases, where ORC power plants are biomass fueled, the personnel already exist. The recycling procedure can be integrated in the weekly and monthly routines of the operation of such a power plant. Due to design and concept reasons, the usage of a separated lubricating system is state-of-the art in many large-scale ORC modules. If designed properly, the contamination of working fluid by turbine oil can be mostly avoided. However, gaskets do fail, and oil pumps do fail. Taking a look at the analyses of the different case studies, this happens regularly. The data in
Figure 4 indicate a correlation between the contamination and the degradation of MDM. The reaction kinetics are not clear to the authors, as the interplay of the different factors, such as contamination, composition, time and temperature, are too complex. As the fluid aging process seems to be unavoidable, countermeasures are necessary. Both, the laboratory experiments, as well as the
in situ experiments have shown that the distilling of old working fluid is a technical and economical alternative to the refilling of the cycle with fresh fluid. By doing so, lubricants can be completely removed, and the MDM fraction can be significantly increased. While the replacement of the working fluid takes time to shut down the facility and clean the cycle, recycling can be done while the engine is running. Furthermore, the old working fluid does not have to be disposed. The modifications shown in
Figure 8a,b and
Figure 9 are relatively straight forward. The actual process is not affected nor disturbed by this method. The results of operation of the power plant in Case Study #8 for more than one year with this strategy prove the effectiveness of the proposed methods. The annual electric efficiency increased to a value of 13.22% (7.6% in 2013 and 5.04% in 2014).
7. Conclusions
The following conclusions can be drawn from the observations in this paper:
The data indicate that the degradation of MDM in ORCs is related to the amount of petroleum-based lubricants. If this is related to the main component of the lubricant, to its additives or to the decomposition products of the lubricant is not entirely clear.
The decomposition process under the observed conditions leads to a large amount of high boilers in the working fluid.
The fluid composition throughout the cycle differs significantly. A large fraction of high boilers precipitate after the turbine in the recuperator. This offers the opportunity to remove these high boilers from the system if the recuperator has a closed reservoir.
Low boilers can be removed from the cycle with the vacuum system by adding an additional membrane pump.
The distilling of highly contaminated working fluid can deliver 95% of pure MDM after the first treatment. Lubricants can be removed from the fluid reliably under laboratory conditions.
The laboratory distilling tests have shown that 64–80% of MDM in the working fluid can be recovered. This means that 54–64% of the aged working fluid can be reused. The rest has to be replaced by working fluid from another source.
The economical effort of distilling contaminated fluid is far below the effort of a reinvestment in new working fluid. The costs (investment and running costs) for the distillation for one liter of pure working fluid are around € 8. Over the last few years, the price for new working fluid ranged between € 15 and € 22 per liter. The personnel expenses for the working fluid recycling are negligible, as the power plant at hand requires attendance of at least one mechanic. The necessary work steps can be included in the day-to-day work flow.
The reduction of low boilers led to lower condensing pressures and higher electric outputs. The removal of high boilers reduced droplet impingement and resulted in smoother operation of the engine. With higher working fluid purity and appropriate filling level, the super-heating could be reduced.
After cleaning the working fluid and regular working fluid checks and replacements, in 2015, the annual electric efficiency of Case Study #8 increased significantly to 13.22% (7.6% in 2013 and 5.04% in 2014).
Even though the number of analyzed power plants is rather low, the data show the relevance of this topic and the necessity for further investigation. The detailed chemical processes, especially the interaction of hydrocarbons from the lubricant oil with MDM, could not be analyzed in detail. Further chemical analyses, under laboratory conditions, are necessary to determine the reaction kinetics of this complex process. However, the implementation of the recycling procedure in an existing power plant proves the potential of the applied methods. The replication of this method could improve the economical situation of many power plants of this type.