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Optimising WHR systems with direct heat exchange

The worlds electrical consumption is rising, with up to 40% of the fossil fuel energy lost as waste heat in the exhaust gas [1], by using Waste Heat recovery (WHR) technologies there is an abundance of thermal energy for conversion to low carbon electricity. Following the recent Paris COP21 climate change agreement, industry will need to move towards supporting the needed action to decarbonise their processes. Realising that the thermal power available in the waste heat from their industrial processes can be used to generate their own electricity, helps them to take the lead on achieving decarbonisation targets.

At the heart of WHR system development is Organic Rankine Cycle (ORC) technology, with many successfully proven systems installed in the cement, lime, glass and steel sectors around the world. With more WHR systems using ORC technology being rolled out, to increase technology integration, innovation is needed to improve the economic benefit. Optimising ORC technology with direct heat exchange┬Ł technology provides a step change for reducing system costs and increasing the electrical generating power.

Optimising an ORC generator with direct heat exchange is achieved when the thermodynamic working fluid is directly heated by the exhaust gas from the industrial process. To date the majority of ORC projects have installed an intermediate loop to transfer the thermal energy from the exhaust gas to the working fluid. The two most common intermediate loops use pressurised hot water circuit or a thermal oil circuit. The thermal energy from the industrial exhaust is first transferred to the intermediate loop, which is then transferred to the working fluid circuit of an ORC as shown in the Figure 1. The gross electrical efficiency of the ORC system with an intermediate loop is estimated between 8-17%, depending on temperatures of the exhaust gas.

Figure 1: Configuration of an ORC system with an intermediate loop

With the advancement in the ORC technology control and the testing of working fluids at varying temperatures, it is now possible to transfer the thermal energy from the industrial exhaust gas directly to the ORC generator. In this arrangement the ORC working fluid is directly passed through the exhaust gas heat exchanger and then through the turbine as shown in the Figure 2. The avoidance of intermediate loop leads to improved performances, lower investment costs from simpler systems. The gross electrical efficiency of the ORC system with direct exchange solutions can increase to between 14-25% or more, depending on temperatures of the exhaust gas and other variations.

The direct heat exchange ORC solution can recover medium temperature exhaust gases up to 600 C (2). One of the leading ORC manufacturers Turboden has been developing direct heat exchange solutions since 2009. The first direct exchange system developed by Turboden was installed on the exhaust of 7 MW Diesel engine generating 500 kWe and subsequently installed 4 other ORC systems with a direct exchange solution. Other ORC manufacturers also recognise this as a key advancement to optimising their systems, when the following main challenges can be overcome.

    The selection of working fluids suitable for the exhaust gas conditions.
    Rapid variations in exhaust gas temperatures need optimal control.
    Site layout constraints of distance between heat exchanger and ORC generator.

These challenges are being overcome through innovative design, the result is in an optimised WHR system generating more low carbon electricity and providing a better return on investment for industry.

 

Bibliography

W. Pu, C. Yue, D. Han, W. He, X. Liu, Q. Zhang and Y. Chen, “Experimental study on Organic Rankine cycle for low grade thermal energy recovery,” Applied Thermal Engineering, vol. 1, no. 94, pp. 221-227, 2016.

Altenergymag, “altenergymag.com,” 09 September 2015. [Online]. Available: http://www.altenergymag.com/news/2015/09/29/turboden-provides-the-first-direct-heat-exchange-organic-rankine-cycle-solution-from-multiple-heat-sources-/21537/. [Accessed 09 December 2015].

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