Energetic and economic investigation of Organic Rankine Cycle applications

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Abstract

The use of organic working fluids for the realization of the so called Organic Rankine Cycle (ORC) has been proven to be a promising solution for decentralized combined heat and power production (CHP). The process allows the use of low temperature heat sources, offering an advantageous efficiency in small-scale applications. This is the reason why the number of geothermal and biomass fired power plants based on this technology have been increased within the last years. The favourable characteristics of ORC make them suitable for being integrated in applications like solar desalination with reverse osmosis system, waste heat recovery from biogas digestion plants or micro-CHP systems. In this paper, the state of the art of ORC applications will be presented together with innovative systems which have been simulated in a process simulation environment using experimental data. The results of the simulation like efficiencies, water production rates or achievable electricity production cost will be presented and discussed.

Introduction

The Organic Rankine Cycle (ORC) is a Clausius–Rankine Cycle in which an organic working fluid is used instead of water–steam. In the last years it became quite popular in energy production processes, due to the fact that it gives the possibility to use exhaust heat of low energy and temperature level. Fig. 1 shows the main components of the ORC process. The organic working fluid is compressed with a pump, which forces the fluid through a regenerator. The regenerator allows the preheating of the liquid working fluid by desuperheating the expanded vapour. The preheated working fluid is then evaporated, superheated and expanded in a turbine, which drives a generator. The desuperheated vapour is condensed in a condenser. If the temperature level of the condensation is high enough, as for example in the case of biomass combustion, the waste heat can be used in a district heating net. If low temperature heat is used for driving the ORC, like geothermal or waste heat, the condenser is cooled back by means of cooling water.

Most of the efforts in the literature include ways of maximizing the efficiency of the thermodynamic cycle for best waste heat recovery [1].

The use of ORC process for waste heat recovery has been applied for many years. An overview of manufacturers together with the appropriate temperature range of the applications is presented in Table 1. In this table, also the expansion machine which is used for the superheated organic fluid vapour is presented.

It can be seen, that in the cases of low power output the use of screw motors is very common for the expansion, because turbines have in small power range high gap losses.

Section snippets

Working fluids

The selection of the working fluid plays a significant role for the use of ORC process and is determined by the application and the waste heat level [2].

Saleh et al. [3] have presented the screening of 31 pure components working fluids for ORC process. In Table 2 some selected fluids and their characteristics are presented.

The fluids are given in the order of rising critical temperature Tc (with the exception of R365mfc) and normal boiling temperature Ts,1 bar, whereas the critical pressure pc

Biomass combustion

The use of the ORC process for CHP production from biomass combustion has been discussed a lot the last decade [9], [10].

Nowadays a lot of biomass combustion plants that work with the ORC process are installed and produce heat and/or power (e.g. Stadtwärme Lienz Austria 1000 kWel, Sauerlach Bavaria 700 kWel, Toblach South Tyrol 1100 kWel, Fußach Austria 1500 kWel) [11], [12].

The main reason why the construction of new ORC plants increases is the fact that it is the only proven technology for

Innovative applications

Due to the great advantage of the ORC to use the waste heat from a process, many innovative concepts coupling the ORC process are being developed and optimized nowadays. For the modeling of these concepts, the process simulation environment IPSEpro was used. IPSEpro is a high flexible software tool used for heat balance analysis of power plants, component design, acceptance test calculations and on-line optimization [19].

Conclusions

Nowadays, the Organic Rankine Cycle can be characterised as the only proved technology that is commonly used in ranges of a few kW up to 1 MW. Despite the fact that it is linked with low efficiencies, new applications of this technology are commonly discussed due to its possibility to utilise the low-level waste heat from other processes. Therefore it’s coupling with waste heat recovery from biogas digestion plants or micro-CHP systems provide very promising solutions for low cost, decentralised

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