Low-temperature Heat Recovery Research Articles

Optimisation of power generation cycles using saturated liquid expansion to maximise heat recovery

The use of two-phase screw expanders in power generation cycles can achieve an increase in the utilisation of available energy from a low-temperature heat source when compared with more conventional single-phase turbines. The efficiency of screw expander machines is sensitive to expansion volume ratio, which, for given inlet and discharge pressures, increases as the expander inlet vapour dryness fraction decreases. For single-stage screw machines with low inlet dryness, this can lead to underexpansion of the working fluid and low isentropic efficiency. The cycle efficiency can potentially be improved by using a two-stage expander, consisting of a machine for low-pressure expansion and a smaller high-pressure machine connected in series. By expanding the working fluid over two stages, the built-in volume ratios of the two machines can be selected to provide a better match with the overall expansion process, thereby increasing the efficiency. The mass flow rate though both stages must be matched, and the compromise between increasing efficiency and maximising power output must also be considered. This study is based on the use of a rigorous thermodynamic screw machine model to compare the performance of single- and two-stage expanders. The model allows optimisation of the required intermediate pressure in the two-stage expander, along with the built-in volume ratio of both screw machine stages. The results allow specification of a two-stage machine, using either two screw machines or a combination of high-pressure screw and low-pressure turbine, in order to achieve maximum efficiency for a particular power output. For the low-temperature heat recovery application considered in this paper, the trilateral flash cycle using a two-stage expander and the Smith cycle using a high-pressure screw and low-pressure turbine are both predicted to achieve a similar overall conversion efficiency to that of a conventional saturated vapour organic Rankine cycle.

Retrofit of low-temperature heat recovery industrial systems using multiobjective exergoeconomic optimization

Reducing the energy consumption of a plant often conflicts with the investment required for heat recovery. This paper presents a design study of shell and tube heat exchanger and direct-contact heat exchanger in three retrofit configurations. Multiobjective optimizations are employed to find optimal solutions that increase exergy efficiency at justifiable costs. A numerical modelization of heat transfer equipements is developed using heat transfer, pressure drop and cost correlations from the open literature. In order to verify the capability of the proposed approach, a case study for heat recovery in a pulp and paper plant is presented. In which multiple structural modifications of existing heat recovery systems are proposed based on an analysis of the Grand Composite Curve pinch targeting method. Each proposed modification is subject to multiobjective optimization based on the fast non-dominant sorting genetic algorithm (NSGA-II). The case study’s results shows significant steam operation cost reduction of up to 89% reducing exergy destruction by 82%. It has also been shown that for some heat recovery modifications the most cost effective solution is close to the minimum exergy destruction solution subject to equipment design constraints.

Comparison of Organic Rankine Cycle Systems under Varying Conditions Using Turbine and Twin-Screw Expanders

A multi-variable optimization program has been developed to investigate the performance of Organic Rankine Cycles (ORCs) for low temperature heat recovery applications using both turbine and twin-screw expanders when account is taken of performance variation due to changes in ambient conditions. The cycle simulation contains thermodynamic models of both types of expander. In the case of the twin-screw machine, the methods used to match the operation of the expander to the requirements of the cycle are described. The performance of turbine expanders in a superheated ORC has been modelled using correlations derived from operational data for single stage reaction turbines to predict the turbine efficiency at “off-design” conditions. Several turbine configurations have been considered including variable nozzle area and variable speed. The capability of the cycle model has been demonstrated for the case of heat recovery from a steady source of pressurized hot water at 120 °C. The system parameters are optimised for a typical operating condition, which determines the required size of heat exchangers and the expander characteristics. Performance at off-design conditions can then be optimized within these constraints. This allows a rigorous investigation of the effect of air temperature variation on the system performance, and the seasonal variation in net power output for the turbine and twin-screw ORC systems. A case study is presented for a low temperature heat recovery application with system electrical power output of around 100 kWe at design conditions. The results indicate that similar overall performance can be achieved for ORC systems using either type of expander.

Performance of an effectively integrated biomass multi-stage gasification system and a steel industry heat treatment furnace

The challenges of replacing fossil fuel with renewable energy in steel industry furnaces include not only reducing CO2 emissions but also increasing the system energy efficiency. In this work, a multi-stage gasification system is chosen for the integration with a heat treatment furnace in the steel powder industry to recover different rank/temperature waste heat back to the biomass gasification system, resulting higher system energy efficiency.A system model based on Aspen Plus was developed for the proposed integrated system considering all steps, including biomass drying, pyrolysis, gasification and the combustion of syngas in the furnace. Both low temperature (up to 400°C) and high temperature (up to 700°C) heat recovery possibilities were analysed in terms of energy efficiency by optimizing the biomass pretreatment temperature.The required process conditions of the furnace can be achieved by using syngas. No major changes to the furnace, combustion technology or flue gas handling system are necessary for this fuel switching. Only a slight revamp of the burner system and a new waste heat recovery system from the flue gases are required.Both the furnace efficiency and gasifier system efficiency are improved by integration with the waste heat recovery. The heat recovery from the hot furnace flue gas for biomass drying and steam superheating is the most promising option from an energy efficiency point of view. This option recovers two thirds of the available waste heat, according to the pinch analysis performed. Generally, depending on the extent of flue gas heat recovery, the system can sustain up to 65% feedstock moisture content at the highest pyrolysis temperature studied.

Performance evaluation of an Organic Rankine Cycle fed by waste heat recovered from CO2 capture section

Natural gas-fueled combined cycle (NGCC) allows to reach the best performance among power plants fed by fossil fuels, but causes considerable CO2 emissions. With the aim of reducing greenhouse gases impact, NGCC could be integrated with post-combustion CO2 removal systems, typically based on chemical solvents like amines, that cause very large net efficiency penalties (about 9-12 percentage points at 90% overall CO2 capture). To reduce these high capture penalties, exhaust gas recirculation (EGR) has been studied. To further enhance the overall plant efficiency, the recovery of available low temperature heat from the solvent-based CO2 removal systems could be also performed. Low temperature heat is available in flue gas coolers (80-100°C), in the amine reboiler water cooling (130-140°C) and in the splitter condenser (100-130 °C). This waste thermal energy could be recovered by means of an Organic Rankine Cycle (ORC) that is able to convert heat into electricity efficiently even at comparably low temperatures. N-Butane was found to be as the most promising organic working fluid for the cycle operating temperatures and pressures. ORC produces additional electrical power improving the global performance of the power plant, for example, up to 1-1.5 percentage points in efficiency.

Experimental Study of Low-temperature Organic Rankine Cycle with Axial Flow Turbine

We build a low temperature heat recovery ORC experimental system, in which R245fa is used as working fluids, a single-stage axial flow turbine is used to generate work, and the brazed plate heat exchangers are used as evaporator and condenser. The hot water is used to simulate the low-temperature heat source with temperature in the range of 320-370K, and cooling water at ambient temperature used as heat sink. Overall performance of the low-temperature ORC system is investigated. The results show that the axial flow turbine's isentropic efficiency is near of 0.6 under different evaporating temperature, and the fluctuated characteristics of the turbine appears at evaporating temperature of near 350K, and causes the stable overall performance of the ORC system at that temperature range. Moreover, overall net power output of the ORC system is over zero when evaporating temperature exceeds 350K in this research.

A Humid Air Turbine–Organic Rankine Cycle combined cycle for distributed microgeneration

This paper analyses the integration of Humid Air microturbine and an Organic Rankine Cycle in a combined cycle for distributed generation. This integration concept maximizes heat recovery at the exhaust of the micro gas turbine combining the capacities for medium temperature heat recovery of the Organic Rankine Cycle and low temperature heat recovery of the Humid Air Turbine.The integration analysis based on different sets of organic fluids and different recovery temperatures showed that the optimal combination was achieved using toluene and R245fa for medium and low temperature Organic Rankine Cycles respectively. Their combination with the low temperature heat recovery at the Humid Air Turbine saturator maximizes the heat recovery profile from the exhaust gases.The calculations carried out reveal a 25% improvement in net power output of the dual cycle compared to the standalone Humid Air microturbine. Such improvement also affects to global efficiency, increasing it up to 52% in the optimal configuration, above the 41% estimated for the standalone Humid Air Turbine and the 33% rated efficiency of the state of art microturbine Capstone C200. The economic analysis shows that these increases in efficiency and power values make the proposed concept highly competitive, with a 15% reduction in the Levelized Cost of Electricity compared with the standalone Humid Air Turbine and a reduction of 30% compared with the micro gas turbine base case.The presented results show this novel integration as a very promising solution for distributed generation applications at power range under 200kW, due to its relative simplicity and cost.

Thermodynamic and Environmental Analysis of Low-Grade Waste Heat Recovery System for a Ship Power Plant

Shipping is a growing sector in the global economy and its contributions to global greenhouse gas (GHG) emissions are expected to increase. Energy efficiency has always been an important factor to minimize GHG emissions. Waste heat recovery (WHR) systems can be used for both improving energy efficiency and reducing energy consumption and GHG emissions. Organic Rankine Cycles (ORC) systems are the most widely used technology for low temperature heat recovery from exhaust gases in the industrial applications. But this system is not widely used in ship applications. In this study thermodynamic performance criteria of ORC system for a container ship are developed and thermodynamic, environmental and economic effects of the system are analyzed. It can be achieved additionally 2% reduction for the ship fuel consumption by utilizing the ORC system.

Low GWP alternatives to HFC-245fa in Organic Rankine Cycles for low temperature heat recovery: HCFO-1233zd-E and HFO-1336mzz-Z

HFC-245fa is a common working fluid used in Organic Rankine Cycles generating mechanical power from low temperature heat. This paper compares the predicted ORC performance of two novel low GWP working fluids, HCFO-1233zd-E and HFO-1336mzz-Z, to HFC-245fa over a wide range of evaporating temperatures, condensing temperatures and vapour superheat values. Expander power output, required pump power input, net cycle efficiencies, mass flow rates and turbine size parameters with HCFO-1233zd-E, HFO-1336mzz-Z and HFC-245fa were compared for a given thermal power input. HCFO-1233zd-E and HFO-1336mzz-Z are predicted to have attractive thermodynamic Rankine power cycle performance. HCFO-1233zd-E would require 10.3%–17.3% lower pump power and would enable up to 10.6% higher net cycle efficiencies than HFC-245fa over the range of cycle conditions examined in this paper. The turbine size required with HCFO-1233zd-E would be up to about 7.5%–10.2% larger than with HFC-245fa. HFO-1336mzz-Z would require 36.5%–41% lower pump power and would enable up to 17% higher net cycle efficiencies than HFC-245fa over the range of cycle conditions examined in this paper. The turbine size required with HFO-1336mzz-Z would be up to about 30.9%–41.5% larger than with HFC-245fa. HFO-1336mzz-Z cycle efficiency is benefitted substantially by a recuperator. The net cycle efficiency increases and the required turbine size decreases relative to HFC-245fa for HCFO-1233zd-E and for HFO-1336mzz-Z at higher evaporating and condensing temperatures.

Study on power plants arrangements for integration

The three power cycles viz. gas cycle (Brayton cycle), steam cycle (Rankine cycle) and organic Rankine cycle (ORC) are arranged in two possible methods to form a triple cycle and to increase its power generating performance. In one approach, all the three cycles are arranged in series, the heat rejection of gas cycle is supplied to steam power plant having back pressure turbine and the heat rejection of steam plant is supplied to ORC plant similar to a cascading. In second option, the two bottoming cycles i.e. steam plant and ORC plant are arranged in parallel to gas turbine exhaust. In this connection, steam cycle works with high temperature heat recovery and ORC plant with low temperature heat recovery. Thermal efficiency and specific work of two triple cycles are compared with combined cycle (CC) power plant to draw the relative merits of two choices with a focus on compressor pressure ratio and gas turbine inlet temperature (GTIT). The results showed that the parallel arrangement in triple cycle offers greater benefit over the series arrangement. Therefore the wok is focused on parallel configuration triple cycle results.

Investigation on Operating Processes for a New Solar Cooling Cogeneration Plant

Some commercial units and industries need more amount of cooling than the power such as cold storage, shopping complex, etc. In this work, a new cooling cogeneration cycle (Srinivas cycle) has been proposed and solved to generate more cooling with adequate power generation from single source of heat at hot climatic conditions with ammonia–water mixture as a working fluid. The operational processes conditions for the proposed cooling cogeneration plant are different compared to the power-only (Kalina cycle system) system and cooling-only (vapor absorption refrigeration) system. This work focused to generate the optimum working conditions by parametric analysis from thermodynamic point of view. An increase in cycle maximum temperature is only supporting the power generation but not the cooling output. Cooling output is also 15 times more than power generation. So, it has been recommended to operate the integrated plant with low temperature heat recovery. The resulted cycle thermal efficiency, plant thermal efficiency, specific power, specific cooling, cycle power efficiency, cycle coefficient of performance (COP), and solar collector's specific area are 27%, 10%, 15 kW, 220 kW, 1.8%, 0.25, and 10 m2/kW, respectively.

Thermal Optimization of a Solar Thermal Cooling Cogeneration Plant at Low Temperature Heat Recovery

A simple cooling cogeneration has been developed by coupling a Kalina cycle system (KCS) with a vapor absorption refrigeration (VAR) system. The working fluid used in this theoretical thermodynamic evaluation is ammonia water mixture. A low temperature heat recovery (150 °C–200 °C) from engine exhaust gas, solar collectors, or similar can be used to operate the plant. A controlling facility is provided to set the required amount of power or cooling to meet the variable demand. In this proposed plant, the liquid refrigerant absorbs more amount of heat from evaporator surroundings with a flow control located in between power and cooling cycles. The extra included components are condenser, heat exchanger and throttling device over KCS plant. Due to possibility of more cooling, it offers high energy utilization factor (EUF). The coupled plant characteristics are studied with changes in mass split ratio, separator vapor fraction, separator temperature, and turbine concentration to develop efficient working conditions. The power mass split ratio is varied from 80% to 100% to run the coupled plant at nearly full load conditions. The separator vapor fraction and temperature are optimized at 45% and 150 °C, respectively. It is recommended to maintain the turbine concentration above 0.85 for optimum power and cooling. The maximum cycle EUF and plant EUF are 0.15 and 0.06, respectively, at 80% power mass split ratio. The specific power and specific cooling at these conditions are 62 kW/kg and 72 kW/kg, respectively.

Experimental Study on Energy Saving between Heat Pump Unit of Heat Recovery Fresh Air and Heat Pipe Heat Pump Low Temperature Heat Recovery Unit in the Cold and Severe Cold Regions

In the cold and severe cold regions of our country, the average temperature in winter outdoor usually below -20°C. Air conditioning units was limited and damaged when it running under such conditions. In order to solve the the problems pointed out previously, the paper put forward solutions of the fresh air handling units in combination with heat pump unit of heat recovery fresh air and heat pipe heat pump low temperature heat recovery unit. Comparison energy-saving and recovery period of the two method through experimental research.

Experimental study on the direct/indirect contact energy storage container in mobilized thermal energy system (M-TES)

A mobilized thermal energy storage (TES) system has been proposed to recover and use industrial waste or excess heat for distributed users. In this paper, lab-scale test facilities have been built to understand the mechanisms of heat charging and discharging processes. The facilities consist of a direct/indirect-contact thermal energy storage container, heat transfer oil (HTO)/water tanks, an electrical boiler, HTO/water pumps and a plate heat exchanger. The organic phase change material (PCM), erythritol, which is sugar alcohol, was chosen as the working material due to its large heat density (330kJ/kg) and suitable melting point (118°C) for industrial low-temperature heat recovery, as well as non toxic and corrosive. Although differential scanning calorimetry tests have shown that a large temperature range exists during the phase change of erythritol, it did not affect the heat discharging during the tests of system performance. Heat charging/discharging results show that for the direct-contact storage container, heat discharging process is much faster than charging process. At the initial stage of heat charging, heat transfer oil is blocked to enter the container, resulting in a slow charging rate. Meanwhile, the PCM attached on the container wall on the bottom always melts last. It has been found that increasing the flow rate of HTO can effectively enhance the charging/discharging processes. For the indirect-contact storage container, heat charging and discharging take almost the same time; and the flow rate of HTO does not show an obvious effect on the charging and discharging processes due to the weak thermal conductivity of the solid phase change material. Comparatively, using the direct-contact storage container may achieve shorter charging/discharging processes than using the indirect-contact storage container.

Techno-economic Assessment of Recovery and Reuse of Low Temperature Heat (T

This article presents a techno-economic evaluation of various options for recovery and reuse of low temperature heat (LTH) with temperatures below 350°C. A process integration approach has been applied to illustrate the investment strategies for a typical European integrated steel plant towards positive energy and environmental effects. The modelling results indicate a CO2 reduction potential of 0.44-1.80% of the total CO2 emissions by recovering and reuse of LTH from the flue gas in various process units. The pay-back time depends on reuse pathways and waste heat temperature, varying from 0.5 to 7.6 years. The results are only valid for the simulated generic site. For the results implementation at real steel plants, local boundary conditions should be considered.

Experimental investigation on heat recovery from condensation of thermal power plant exhaust steam by a CO2vapor compression cycle

SUMMARY In this paper, a method that utilizes CO2 vapor compression thermodynamic cycle to recover low-temperature heat from exhausted water steam of fossil fuel thermal power plants is reported. Experimental investigation was carried out to study the characteristics of low-temperature heat recovery by liquid CO2 evaporation process from vacuum exhausted steam condensation occurring at the turbine exit. Furthermore, measured heat recovery performances over one whole year are presented and discussed. Experimental results show that the present heat recovery process by CO2 vapor compression cycle is able to operate stably. The yearly averaged water temperature at the CO2 condenser outlet was measured at 87.5 °C with a COP value above 5.0. This high energy efficiency ratio is found to be mainly due to two factors: the transcritical CO2 vapor compression and steam condensation phase change occurring on the CO2 evaporator. The findings from this paper provide helpful guidelines for low-temperature heat recovery system design and improving fossil fuel utilization efficiency. Copyright © 2013 John Wiley & Sons, Ltd.

Recovery of low temperature heat in oil mills

Energy consumption in oil mills is a major item of costs and a sensitive point in the production of biofuels. To improve their performance, industrials can recover lowtemperature heat thanks to a new technology of heat exchangers suitable for treating granular solid materials. Information about the energy requirements of the rapeseed crushing being not readily available, the article gives a detailed assessment of consumption items (per ton of seed: 263 MJ for preparation operations and 284 MJ for solvent extraction). These exchangers used as pre-conditioners saves about 55 MJ.t<sup>−1<sup/> of heat by use of steam condensates. We could go further in use of these devices on the one hand to recover heat from press cake and meal, and secondly to use recovered energy to dry and warm up the seeds before pre-pressing. In this configuration, the energy savings could reach 38% of current needs.

Non-conventional combined cycle for intermediate temperature systems

SUMMARY This work presents a non-conventional combined cycle comprising a topping supercritical Brayton cycle working with carbon dioxide (S-CO2) and a bottoming organic Rankine cycle (ORC). As shown by previous authors in the field of nuclear energy, carbon dioxide cycles are characterised by lower turbine inlet temperatures (around 1100 K) than conventional gas turbines, and they usually incorporate a highly efficient recuperative heat exchanger in order to increase system efficiency. Accordingly, temperatures of around 200 °C are achieved at the outlet of the recuperator, hence leaving a certain amount of heat at the disposal of a bottoming cycle. Presently, ORCs are the most appropriate commercially available technology for such low temperature heat recovery. In this paper, an analysis of ORCs with different organic fluids is presented. Initially, only pure substances are considered. Then, the performance of the same cycle working with mixtures of hydrocarbons is assessed finding a 7 percentage point improvement in global efficiency with respect to the stand-alone topping cycle when the bottoming system (heat recovery unit and ORC) is incorporated. Following this interesting result, the performance of a S-CO2-ORC, a conventional gas turbine and ORC and an ORC–ORC combined cycles are evaluated for a tower-type central receiver Concentrating Solar Plant. Isopentane (the optimum pure substance obtained in the analysis mentioned earlier) is used in all cases for the bottoming system, showing favourable results for the proposed combined cycle configuration. Additionally, a discussion about the most relevant techno-economic concerns is presented at the end of the work showing that complementary economic benefits must be expected because of the lower footprint of the heat transfer equipments when carbon dioxide is used. These positive features derive from the more interesting transport properties of carbon dioxide when it is used at very high pressure, what also contributes to reducing the size of turbomachinery by decreasing their volumetric flows. In summary, the work presents a very interesting proposal, both thermodynamically and economically, to be used in intermediate temperature systems even if this technology is not mature yet. Copyright © 2012 John Wiley & Sons, Ltd.

Off-design analysis of ORC and CO2 power production cycles for low-temperature surplus heat recovery

In process industry large amounts of energy are rejected to the ambient. Recovery of this surplus energy is a wide topic. Among the strategies for energy recovery, production of electricity is very interesting, due to the versatility of this form of energy. For the relatively low temperature heat, which is most commonly encountered for industrial surplus heat, the Organic Rankine Cycle is a well established technology. The transcritical Rankine Cycle recently received special attention due to its performances for energy recovery from low temperature sources. CO2 is a natural candidate as working fluid for this technology. It combines high performance, low cost, low toxicity, is non-flammable and has no environmental impact. This paper focuses on the off-design operation of Rankine cycles and compares the behaviour of transcritical CO2 cycles and an ORC cycle with R-123 as working fluid. The main observation is that the ORC is very sensitive to reduction in available heat, and will with only small changes get droplets in the inlet of the expander. This shows that it is reasonable to operate the ORC with some degrees superheat, to have a buffer. Superheating the outlet 5 to 10 K has only a small effect on the cycle performance. However the gained robustness is also relatively small. With small increments in the available heat source, the CO2 cycle also seem to have a marginally better response without control of the process, which indicates that it is more robust and less in need of detailed control.

Part-load analysis of gas turbine &amp; ORC combined cycles

Abstract Organic Rankine Cycles (ORC) are considered to be effective solutions for low and medium temperature heat recovery for power production. In such applications where the temperature of waste heat is in the 300–450 °C range, they stand as a more interesting option to enhance system performance than the more complex steam cycles typically used in heavy duty combined cycle plants. In this paper, the rated and part-load performances of combined cycles formed by a topping gas turbine and a bottoming ORC are analysed. To this aim, five commercial gas turbines are used in combination with an ORC bottoming cycle in lieu of a conventional steam cycle, with the goal of maximising the overall efficiency of the plant at part-load. These operating conditions are typical of power stations operating in load-following mode. During the analysis of part-load operation, different control strategies are studied in order to recuperate as much heat as possible from the gas turbine exhaust. It is observed that keeping the live vapour conditions in the bottoming cycle as constant as possible produces maximum power at any operating conditions, compensating for the poor performance of the gas turbine engine at part-load. The interest of this work is twofold. First, it confirms the interest of merging gas turbines and ORC units for efficient power generation under variable operating conditions. Second, the work is based on commercial gas turbines, thus trying to analyse the real potential of state-of-the-art technology in the application proposed.