In recent years, the urge for utilizing all forms of available energy has increased significantly. One of these sources is waste or residual heat, found in engine exhausts, process heat, flare gas burners, combustion of wood and other residuals. A remarkable way to utilize waste or residual heat is to convert it into electricity using a so-called Organic Rankine Cycle (ORC) system, marketed by Triogen B.V. of the Netherlands. The development work of the ORC system started in the early nineties at the Lappeenranta University of Technology in Finland, and the system was industrialized and commercialized by Triogen in 2002. Deployment took place since 2006, resulting in 21 units built so far, 15 operational units and 10 units on order or being commissioned.
System Design of ORC
Figure 1 shows the cycle scheme and the process of the ORC system in a T-s diagram. Liquid is pumped from the storage vessel into the main pump, which is mounted on to the same shaft as the turbine and the high-speed generator. There the liquid will get the maximum working pressure at which it will enter the recuperator and subsequently the evaporator. Residual or waste heat is led through the evaporator, where the working fluid is heated up to the boiling point, evaporated and superheated. Expansion takes place in the turbine, which drives the high-speed generator and the main pump. After expansion, the sensible heat in the fluid is re-used in the recuperator, to preheat the liquid before it enters the evaporator. The condenser is situated below the recuperator, where the vapour is condensed using a liquid coolant, before it flows back into the storage vessel.
ORC plant design
As can be seen from the cycle scheme, the main components of the ORC system include an evaporator, a turbine-generator-main pump assembly called High Speed Turbo Generator (HTG), a recuperator, a condenser and a storage vessel. Thanks to the high-speed generator and turbine, the HTG is a very energy-dense compact unit. The other components of the ORC process are heat exchangers with relatively low temperature differences.
One of the engineering tasks of Triogen was to make a compact and, as much as possible, standardized unit for different applications. As the evaporator is determined by the properties of the residual heat offered, the module can be sized accordingly. However, the internal cycle heat transfer in the recuperator and the condenser can be standardized for all applications, keeping the system very compact. For the system, Vahterus offered a compact solution based on their standard plate and shell type heat exchangers. The recuperator exchanges heat between vapour leaving the turbine and liquid working fluid before it is entering the evaporator, thus serving as a preheater. In the condenser, the working fluid is condensed from vapour to liquid at sub-atmospheric pressure, using a coolant, which rejects its heat to any available heat sink or to the atmosphere by external table coolers. As the turbine exhaust is vertical, both Vahterus heat exchangers are positioned below the turbine. After condensing, the working fluid is stored in an appropriate quantity in the storage vessel, at its turn positioned below the condenser. The three vessels, recuperator, condenser and storage tank, are factory assembled by Vahterus, and form the central element in the ORC process module. During assembly of the ORC, this Vahterus delivered unit serves as the starting point, as shown in Figure 2. The three blue vessels are the mentioned Vahterus components, with Triogen turbo- generator HTG on top. The special connection flange is delivered by Triogen and welded to the recuperator top by Vahterus.