A new method to identify the optimal temperature of latent-heat thermal-energy storage systems for power generation from waste heat

The integration of thermal-energy storage (TES) within waste-heat recovery power generation systems has the potential to improve energy-efficiency in many industrial processes with variable and/or intermittent waste-heat streams. The first objective of this paper is to present a novel model of these systems that can be used at an early design stage to provide fast and accurate estimates of performance. More specifically, the method can identify the optimal temperature of latent-heat TES systems for waste-heat recovery applications based only on the known heat-source and heat-sink conditions (i.e., temperature, mass-flow rate and specificheat capacity), and can assess both single-stage and cascaded systems. The model has been validated against optimal organic Rankine cycle systems identified from a thermodynamic cycle optimisation. The second objective is to identify the characteristics of optimal systems for different heat-source profiles. The results indicate that, for a given application, there exists an optimal temperature for the latent-heat TES system that depends primarily on the relative size of the heat sink. Moreover, it is found that, for a heat engine operating with TES, the power rating ranges between 25% and 60% of the corresponding power rating for an optimal heat engine, operating without TES, that adapts instantaneously to heat-source fluctuations, whilst the total energy production is reduced by between 45% and 85% respectively. Finally, a small deviation is observed between the results obtained for the different heat sources considered, which suggests that these findings can be extrapolated to other heat sources not considered within this study

Low-Concentration Solar-Power Systems Based on Organic Rankine Cycles for Distributed-Scale Applications: Overview and Further Developments

This paper is concerned with the emergence and development of low-to-medium-grade thermal-energy-conversion systems for distributed power generation based on thermo- dynamic vapor-phase heat-engine cycles undergone by organic working uids, namely organic Rankine cycles (ORCs). ORC power systems are, to some extent, a relatively established and mature technology that is well-suited to converting low/medium-grade heat (at temperatures up to ~300–400°C) to useful work, at an output power scale from a few kilowatts to 10s of megawatts. Thermal ef ciencies in excess of 25% are achievable at higher temperatures and larger scales, and efforts are currently in progress to improve the overall economic viability and thus uptake of ORC power systems, by focusing on advanced architectures, working- uid selection, heat exchangers and expansion machines. Solar-power systems based on ORC technology have a signi cant potential to be used for distributed power generation, by converting thermal energy from simple and low-cost non-concentrated or low-concentration collectors to mechanical, hydrau- lic, or electrical energy. Current elds of use include mainly geothermal and biomass/ biogas, as well as the recovery and conversion of waste heat, leading to improved energy ef ciency, primary energy (i.e., fuel) use and emission minimization, yet the technology is highly transferable to solar-power generation as an affordable alternative to small-to- medium-scale photovoltaic systems. Solar-ORC systems offer naturally the advantages of providing a simultaneous thermal-energy output for hot water provision and/or space heating, and the particularly interesting possibility of relatively straightforward onsite (thermal) energy storage. Key performance characteristics are presented, and important heat transfer effects that act to limit performance are identi ed as noteworthy directions of future research for the further development of this technology

Potential of Organic Rankine Cycles (ORC) for waste heat recovery on an Electric Arc Furnace (EAF)

The organic Rankine cycle (ORC) is a mature technology to convert low temperature waste heat to electricity. While several energy intensive industries could benefit from the integration of an ORC, their adoption rate is rather low. One important reason is that the prospective end-users find it difficult to recognize and realise the possible energy savings. In more recent years, the electric arc furnaces (EAF) are considered as a major candidate for waste heat recovery. Therefore, in this work, the integration of an ORC coupled to a 100 MWe EAF is investigated. The effect of working with averaged heat profiles, a steam buffer and optimized ORC architectures is investigated. The results show that it is crucial to take into account the heat profile variations for the typical batch process of an EAF. An optimized subcritical ORC (SCORC) can generate an electricity output of 752 kWe with a steam buffer working at 25 bar. However, the use of a steam buffer also impacts the heat transfer to the ORC. A reduction up to 61.5% in net power output is possible due to the additional isothermal plateau of the steam

A dynamic organic Rankine cycle using a zeotropic mixture as the working fluid with composition tuning to match changing ambient conditions

Air-cooled condensers are widely used for Organic Rankine Cycle (ORC) power plants where cooling water is unavailable or too costly, but they are then vulnerable to changing ambient air temperatures especially in continental climates, where the air temperature difference between winter and summer can be over 40 °C. A conventional ORC system using a single component working fluid has to be designed according to the maximum air temperature in summer and thus operates far from optimal design conditions for most of the year, leading to low annual average efficiencies. This research proposes a novel dynamic ORC that uses a binary zeotropic mixture as the working fluid, with mechanisms in place to adjust the mixture composition dynamically during operation in response to changing heat sink conditions, significantly improving the overall efficiency of the plant. The working principle of the dynamic ORC concept is analysed. The case study results show that the annual average thermal efficiency can be improved by up to 23% over a conventional ORC when the heat source is 100 °C, while the evaluated increase of the capital cost is less than 7%. The dynamic ORC power plants are particularly attractive for low temperature applications, delivering shorter payback periods compared to conventional ORC systems

The Importance of Monitoring Renewable Energy Plants: Three Case Histories

Many renewable energy plants are put into operation without providing a monitoring system to evaluate their performance over time. Then if is often difficult to realise the bad working of the system and the loss of efficiency results in an economic loss. In the Author\u2019s experience as designer or supervisor of such plants, he came across various examples that pointed out the advantages of having installed a monitoring system, of course with a careful data analysis. Problems sometimes arose from poorer performance than anticipated in the design, but more often from inefficient plant operations after some months or years from the starting. Three quite different examples, derived from the Author\u2019s direct experience, are reported to illustrate how real performance can be lower than designed due respectively: 1. To bad settings of the parameters; 2. To a hurried commissioning that did not reveal the mistakes in the design of the plant; 3. To a failure of a single component over time

A Survey of Hypersonic-Ramjet Concepts

A brief discussion is presented of the major problem areas involved in the development of a hypersonic ramjet engine. Keeping the structural temperature to an acceptably low level is the severest problem expected. A rapid survey is made of some of the relatively unconventional concepts that may find application in the hypersonic region. These include supersonic combustors, underwing burning, atmospheric-recombination, engine installation, nuclear power, variable geometry, and fuel-rich operation

Case study of an Organic Rankine Cycle (ORC) for waste heat recovery from an Electric Arc Furnace (EAF)

The organic Rankine cycle (ORC) is a mature technology for the conversion of waste heat to electricity. Although many energy intensive industries could benefit significantly from the integration of ORC technology, its current adoption rate is limited. One important reason for this arises from the difficulty of prospective investors and end-users to recognize and, ultimately, realise the potential energy savings from such deployment. In recent years, electric arc furnaces (EAF) have been identified as particularly interesting candidates for the implementation of waste heat recovery projects. Therefore, in this work, the integration of an ORC system into a 100 MWe EAF is investigated. The effect of evaluations based on averaged heat profiles, a steam buffer and optimized ORC architectures is investigated. The results show that it is crucial to take into account the heat profile variations for the typical batch process of an EAF. An optimized subcritical ORC system is found capable of generating a net electrical output of 752 kWe with a steam buffer working at 25 bar. If combined heating is considered, the ORC system can be optimized to generate 521 kWe of electricity, while also delivering 4.52 MW of heat. Finally, an increased power output (by 26% with combined heating, and by 39% without combined heating) can be achieved by using high temperature thermal oil for buffering instead of a steam loop; however, the use of thermal oil in these applications has been until now typically discouraged due to flammability concerns

Extended temperature range ACPS thruster investigation

The successful hot fire demonstration of a pulsing liquid hydrogen/liquid oxygen and gaseous hydrogen/liquid oxygen attitude control propulsion system thruster is described. The test was the result of research to develop a simple, lightweight, and high performance reaction control system without the traditional requirements for extensive periods of engine thermal conditioning, or the use of complex equipment to convert both liquid propellants to gas prior to delivery to the engine. Significant departures from conventional injector design practice were employed to achieve an operable design. The work discussed includes thermal and injector manifold priming analyses, subscale injector chilldown tests, and 168 full scale and 550 N (1250 lbF) rocket engine tests. Ignition experiments, at propellant temperatures ranging from cryogenic to ambient, led to the generation of a universal spark ignition system which can reliably ignite an engine when supplied with liquid, two phase, or gaseous propellants. Electrical power requirements for spark igniter are very low

Thermal design and analysis of a hydrogen-burning wind tunnel model of an airframe-integrated scramjet

An aerodynamic model of a hydrogen burning, airframe integrated scramjet engine has been designed, fabricated, and instrumented. This model is to be tested in an electric arc heated wind tunnel at an altitude of 35.39 km (116,094 ft.) but with an inlet Mach number of 6 simulating precompression on an aircraft undersurface. The scramjet model is constructed from oxygen free, high conductivity copper and is a heat sink design except for water cooling in some critical locations. The model is instrumented for pressure, surface temperature, heat transfer rate, and thrust measurements. Calculated flow properties, heat transfer rates, and surface temperature distributions along the various engine components are included for the conditions stated above. For some components, estimates of thermal strain are presented which indicate significant reductions in plastic strain by selective cooling of the model. These results show that the 100 thermal cycle life of the engine was met with minimum distortion while staying within the 2669 N (600 lbf) engine weight limitation and while cooling the engine only in critical locations