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Useful Energy From Wasted Heat

Sustainable energy – a global grand challenge

Recent decades have seen an ever-increasing interest in the utilization of renewable and sustainable energy sources such as wind, solar, and biomass amongst other solutions, as well as in improving energy efficiency in the residential and commercial sectors through a wide variety of solutions.

In both cases, this has been backed by substantial levels of investment, new policies, and regulation, driven by an overriding desire to reduce fossil-fuel consumption and its associated impact on the environment, e.g., by reducing the associated emissions. On the other hand, less has been done to address the significant amount of “wasted” heat in industries, which is currently being disposed of, or “rejected” to the atmosphere from a wide range of sources, which is an important energy resource in its own right.

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In particular, a significant amount of surplus heat is available at temperatures between 100 °C and 400 °C, which are considered low relative to the range of temperatures available in industry but high relative to what is available naturally. The rejection of this surplus heat to the environment upsets local ecosystems in water bodies and the atmosphere in general, while presenting noteworthy opportunities as an energy resource if it can be recovered and utilized economically.

Organic Rankine cycle technology

Although the potential for displacing and thereby minimizing overall primary-energy consumption through waste-heat recovery and utilization is widely acknowledged, currently there remains a lack of a widespread deployment of technologies that are capable of harnessing this important energy resource in commercial and industrial settings. Low-cost options include the recovery and re-use of waste heat directly as a useful source of thermal energy; however, such options are severely constrained due to the frequent lack of suitable onsite uses for this heat.

An interesting alternative is presented by so-called “over-the-fence” heat-sharing options, but these present added complexity (e.g., the amount and temperature-level of the energy demand needs to match that of the energy supply) and significant risk (e.g., arising from the lack of certainty of supply and possible impact interruptions may have to end-users). A particularly promising over-the-fence heat-sharing option is that of district-heating networks, in which heat is delivered to a wide range of end-users via a suitable network of pipes, yet the aforementioned concerns have led to few examples of successful implementation. On the other hand, the conversion of this wasted heat to electricity bypasses many of these issues as electricity is a more fungible (usable or exchangeable) form of energy than heat, and since it can be readily exported to the grid, but this avenue is associated with its own set of challenges.

Of the many technologies that can be considered suitable for the conversion of recovered waste-heat to electricity, the organic Rankine cycle (ORC) is one of the most promising given its technical maturity and demonstrated performance. By deploying suitably-designed organic “working” fluids, ORC systems can extract heat from low-/medium-grade heat sources and generate useful power by expansion of the fluid in an expander or a turbine. Nevertheless, as with other technologies, the widespread uptake of this technology is at present hindered by the high investment costs of waste-heat recovery projects.

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Figure 1: ORC system recovering waste heat from an industrial flue-gas stream for power generation. Adapted from https://doi.org/10.1016/j.energy.2012.11.010

The wider adoption of ORC technology for power generation or cogeneration from the conversion of recovered waste-heat, or renewable heat, in many applications can be facilitated by improved thermodynamic performance, but also by reduced capital costs. These challenges have been serving to motivate on-going research into ORC technology, including at the Clean Energy Processes (CEP) Laboratory in the Department of Chemical Engineering at Imperial College London.

The research being conducted at the CEP Laboratory is guided by the principle that further development of ORC power systems should be driven by a combination of thermodynamic, thermal, but also, crucially, economic assessments that can directly capture the trade-offs between technology performance and cost, with the global aim of proposing solutions with high resource-use efficiency and, importantly, improved economic viability.

Economical working fluids and system designs

The selection, or indeed design, of a working fluid is crucial to this goal of improving the economic viability of ORC systems and enabling their widespread uptake for distributed power generation, while also meeting increasingly restrictive environmental legislation. This is being addressed by researchers in the CEP Laboratory who have developed a design and optimization framework based on computer-aided molecular and process design (CAMPD) techniques, where thermodynamic models of suitable ORC systems are integrated with component sizing and cost models, and also coupled with molecular descriptions of fluids, in this case the “statistical associated fluid theory,” or SAFT.

The resulting framework presents a novel and powerful approach with extended capabilities that allows the simultaneous thermodynamic and economic optimization of ORC systems and working fluids to be performed in a single step, thus removing subjective and pre-emptive screening criteria that exist in conventional two-step approaches.

The framework can be used to design and to propose operating strategies for customized, optimal ORC systems for a range of targeted industrial waste-heat recovery applications. In recent work, fluids such as isoheptane, 2-pentene and 2-heptene have been identified as particularly promising candidates worthy of further investigation, with levelized electrical costs (LECs) down to $20-40 per generated MWh based on a 25-year lifetime and 50% capacity factor for small-scale systems generating a few hundred kW of power that are suitable for industrial use. By comparison, conventional, centralized power-generation plants have LECs from about $40 to above $150 per MWh, and alternative energy generation technologies for LECs from about $30 to $200 per MWh, also depending on whether energy storage is included in the price of the plant.

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Outlook

These recent findings suggest that although the recovery of industrial waste-heat for power generation purposes can be more expensive than other direct onsite uses of this heat from the plant’s perspective, from a whole-energy-system angle it appears that this capability can provide electricity at an overall cost that is lower than the construction of conventional power plants and renewable energy technologies, especially if the cost of energy storage is considered.

Of interest is the fact that most whole-energy system frameworks that are being used currently to propose decarbonization pathways towards a clean and sustainable energy future do not account for the role that power generation from waste-heat sources can have in various scenarios. In the context of a national strategy that is evolving towards decarbonized energy provision, it appears important that this pathway should be considered as part of the overall energy strategy.

These findings are described in the article entitled Computer-aided working-fluid design, thermodynamic optimisation and thermoeconomic assessment of ORC systems for waste-heat recovery, recently published in the journal Energy. This work was conducted by M.T. White from City University of London, O.A. Oyewunmi, M.A. Chatzopoulou, A.J. Haslam, and C.N. Markides from Imperial College London, and A.M. Pantaleo from Imperial College London and the University of Bari.

Further reading:

The interested reader can find further information on ORC technology and on the development and application of CAMPD and SAFT methodologies to such systems in the following articles:

  1. Computer-aided working-fluid design, thermodynamic optimisation and thermoeconomic assessment of ORC systems for waste-heat recovery; Energy: http://doi.org/10.1016/j.energy.2018.07.098
  2. Thermodynamic optimisation of a high-electrical efficiency integrated internal combustion engine–organic Rankine cycle combined heat and power system; Applied Energy: http://doi.org/10.1016/j.apenergy.2018.06.022
  3. Integrating cogeneration and intermittent waste-heat recovery in food processing: Microturbines vs. ORC systems in the coffee roasting industry; Applied Energy: http://doi.org/10.1016/j.apenergy.2018.04.097
  4. Hybrid solar-biomass combined Brayton/organic Rankine-cycle plants integrated with thermal storage: Techno-economic feasibility in selected Mediterranean areas; Renewable Energy: http://doi.org/10.1016/j.renene.2018.08.022
  5. Industrial waste-heat recovery through integrated computer-aided working-fluid and ORC system optimisation using SAFT-γ Mie; Energy Conversion and Management: http://doi.org/10.1016/j.enconman.2017.03.048
  6. Performance of working-fluid mixtures in ORC-CHP systems for different heat-demand segments and heat-recovery temperature levels; Energy Conversion and Management: http://doi.org/10.1016/j.enconman.2017.05.078
  7. Case study of an organic Rankine cycle (ORC) for waste heat recovery from an electric arc furnace (EAF); Energies: http://doi.org/10.3390/en10050649
  8. Thermo-economic and heat transfer optimization of working-fluid mixtures in a low-temperature organic Rankine cycle system; Energies: http://doi.org/10.3390/en9060448
  9. On the use of SAFT-VR Mie for assessing large-glide fluorocarbon working-fluid mixtures in organic Rankine cycles; Applied Energy: http://doi.org/10.1016/j.apenergy.2015.10.040
  10. Low-concentration solar-power systems based on organic Rankine cycles for distributed-scale applications: Overview and further developments; Frontiers in Energy Research: http://doi.org/10.3389/fenrg.2015.00047
  11. An assessment of working-fluid mixtures using SAFT-VR Mie for use in organic Rankine cycle systems for waste-heat recovery; Computational Thermal Sciences: http://doi.org/10.1615/.2014011116
  12. Integrated computer-aided working-fluid design and power system optimisation: Beyond thermodynamic modelling; http://hdl.handle.net/10044/1/51841
  13. Exploring optimal working fluids and cycle architectures for organic Rankine cycle systems using advanced computer-aided molecular design methodologies. http://hdl.handle.net/2263/62470

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