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Power from Heat

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.

The ORC 160 kWe power plant

The complete ORC power plant consists of four modules for turnkey delivery:

  • The standard process module, consisting of HTG, recuperator, condenser, storage vessel, pre-feed pump, valves, including connecting piping and instrumentation
  • The heat supply module: an evaporator tuned at the conditions of the available heat, to be connected directly to the heat source
  • The heat rejection module: table coolers for the cooling water which cools the condenser, tuned to the ambient conditions at site. Alternatively, in the presence of a suitable heat consumer, this heat can be delivered up to a temperature of 80°C.
  • The standard electrical cabinets, that contain the power preparation module, which connects the high speed generator directly to the grid to supply the power at 400 V, 3 phase, 50 or 60 Hz, the control module to control the unit and the independent safety system. A typical turnkey supply of an ORC is connected to two biogas engines of 800 kWe each, whereas the process module is centrally positioned. The heat supply module (evaporator) is connected to the flue gas outlet of two gas engines through a set of piping and valves, to allow single engine operation and to isolate the ORC from the running engines in the unlikely event of an ORC failure. All units are to be used as a typical ‘end-of pipe’ solution: the process feeding the ORC is never disturbed by the ORC.

Product for a vast range of applications

Although initially set up for the use of exhaust gas from internal combustion engines, a number of other application possibilities have arisen, such as biomass combustion and residual heat from industrial processes like incinerators. Applications envisaged are:

  • Gas engines running on natural gas, bio-gas, landfill gas,
    mine gas
  • Diesel engines
  • Small gas turbines
  • CHP for green houses, district and residential heating
  • Combustion of biomass, wood, residuals and waste
  • Flares for landfill gas or other industrial flares
  • Process heat from e.g. (petro)chemical, metallurgical, glass or brick production plants

Nearing 200 000 running hours

After having demonstrated more than 4000 hours successful operation of the prototype, Triogen delivered their first commercial ORC in 2007 for a greenhouse cogeneration plant equipped with a 2 MWe Deutz gas engine. After that, 19 plants followed, gathering up to almost 200 000 running hours. Demonstrated availability amounts to more than 97 %. The fleet leader has now run more than 36 000 hours. From the beginning, Vahterus components have been very valuable in setting up the standard and compact process module of the ORC system. Production is now well under way, and continuous product development has already led to several improvements. The first unit was contracted for 145 kWe, now also 165 kWe has been demonstrated. In future, power range will be extended stepwise from the current 60 – 170 kWe towards 1000 kWe.

Jos van Buijtenen