Comparative performance study of different configurations of organic Rankine cycle using low-grade waste heat for power generation

This paper aims at analyzing the feasibility of a waste heat recovery power generation plant based on parametric optimization and performance analysis using different organic Rankine cycle configurations and heat source temperature conditions with working fluid R-12, R-123, R-134a, and R-717. A parametric optimization of turbine inlet temperature(TIT) was performed to obtain the irreversibility rate, system efficiency, availability ratio, turbine work output, system mass flow rate, second-law efficiency, and turbine outlet quality, along the saturated vapor line and also on superheating at an inlet pressure of 2.50 MP in basic as well as regenerative organic Rankine cycle. The calculated results reveal that selection of a basic organic Rankine cycle using R-123 as working fluid gives the maximum system efficiency, turbine work output, second-law efficiency, availability ratio with minimum system irreversibility rate and system mass flow rate up to a TIT of 150°C and appears to be a choice system for generation of power by utilizing the flue gas waste heat of thermal power plants and above 150°C the regenerative superheat organic Rankine cycle configuration using R 123 as working fluid gives the same results.     

Off-design performance analysis of a transcritical CO2 Rankine cycle with LNG as cold source

The transcritical CO₂ Rankine cycle with liquefied natural gas (LNG) as cold source is a promising power system to utilize mid- and low-temperature heat source. Most previous works focused on thermodynamic and thermoeconomic analysis or optimization for the system. In this article, an off-design performance analysis for the system is conducted. An off-design mathematical model for the system is established to examine the variation of system performance with the variations of heat source mass flow rate and temperature. A modified sliding pressure regulation control strategy, which regulates turbine inlet pressure to keep the temperature difference between heat source temperature and turbine inlet temperature constant, is applied to control the system when off-design conditions happen. The results show that when the mass flow rate or the temperature of heat source is less or lower than that of design condition, both the net power output of system and the system exergy efficiency decrease, whereas when they are more or higher than the values of design condition, the net power output of system increases but the system exergy efficiency still decreases. In addition, both CO₂ turbine and NG turbine could almost keep the designed efficiency values under the applied control strategy.     

Exergy and economic analysis of organic Rankine cycle hybrid system utilizing biogas and solar energy in rural area of China

Due to the existing huge biogas resource in the rural area of China, biogas is widely used for production and living. Cogeneration system provides an opportunity to realize the balanced utilization of the renewable energy such as biogas and solar energy. This article presented a numerical investigation of a hybrid energy-driven organic Rankine cycle (ORC) cogeneration system, involving a solar ORC and a biogas boiler. The biogas boiler with a module of solar parabolic trough collectors (PTCs) is employed to provide heat source to the ORC via two distinct intermediate pressurized circuits. The cogeneration supplied the power to the air-condition in summer condition and hot water, which is heated in the condenser, in winter condition. The system performance under the subcritical pressures has been assessed according to the energy–exergy and economic analysis with the organic working fluid R123. The effects of various parameters such as the evaporation and condensation temperatures on system performance were investigated. The net power generation efficiency of the cogeneration system is 11.17%, which is 25.8% higher than that of the base system at an evaporation temperature 110°C. The exergy efficiency of ORC system increases from 35.2% to 38.2%. Moreover, an economic analysis of the system is carried out. The results demonstrate that the profits generated from the reduction of biogas fuel and electricity consumption can lead to a significant saving, resulting in an approximate annual saving from $1,700 to $3,000. Finally, a case study based on the consideration of typical rural residence was performed, which needs a payback period of 7.8 years under the best case.     

PCM thermal conductivity effect on mechanism of PV/PCM thermal control characteristics

ABSTRACT

According to the structure of photovoltaic/phase change material (PV/PCM), the mechanism of internal heat transfer, transmission, storage, and temperature control is analyzed, and a two-dimensional finite element analysis model of PV/PCM structure is established. This study is carried out on the effect of PCM thermal conductivity on internal temperature distribution characteristics of PV/PCM and temperature control characteristics of solar cells. The results show that the increase in thermal conductivity of PCM can prolong the temperature control time of solar cell in PV/PCM system, for example, when the thermal conductivity is increased from 0.2 W/(m·K) to1.5 W/(m·K) under a thickness of 4 cm, the duration when PV/PCM solar cell temperature is controlled below 40°C and extended from 52 min to 184 min. In addition, PV/PCM experimental prototypes are designed with the LA-SA-EG composite PCM peak melting point of 46°C and thermal conductivity of 0.8 W/(m·K) and 1.1 W/(m·K), respectively. The results indicate that compared with PCM-free solar cells, the maximum temperature of PV/PCM prototype solar cells with thermal conductivity of 0.8 W/(m·K) and 1.1 W/(m·K) is reduced by 10.8°C and 4.6°C, respectively, with average output power increased by 4.1% and 2.2%, respectively, under simulated light sources. Under natural light conditions, the average output power is increased by 6.9% and 4.3%, respectively. The results provide theoretical and experimental basis for the optimization of PV/PCM design by changing the thermal conductivity of PCM.     

An Investigation of Flashing-Driven Natural Circulation

Natural circulation driven boiling systems are extensively used for passive heat removal, such as in nuclear power plants. The flashing phenomenon and the associated flow instabilities in a natural circulation loop are reported here. An experimental facility is constructed with a heater, an adiabatic riser, a condenser and an adiabatic down comer to study the instability regions. The experimental geometry is modeled using RELAP5. At high power (i.e., at 6 kW) flashing instability is observed when the inlet sub cooling is within the range of 7° C to 12° C. It is also found that the presence of an inlet restriction does not help to suppress the flashing induced flow oscillation. A good comparison between the experimental and numerical results is observed and this loop can be used for validation exercises of commercial codes.     

Optimum Heat Conductance Distribution for Power Optimization of a Regenerated Closed Brayton Cycle

In this paper, the power output of the cycle is taken as objective for performance optimization of an irreversible regenerated closed Brayton cycle coupled to constant-temperature thermal energy reservoirs in the viewpoint of finite time thermodynamics (FTT) or entropy generation minimization (EGM). The analytical formulae about the relations between power output and pressure ratio are derived with the heat resistance losses in the hot- and cold-side heat exchangers and the regenerator, the irreversible compression and expansion losses in the compressor and turbine, and the pressure drop loss in the piping. The maximum power output optimization is performed by searching the optimum heat conductance distribution corresponding to the optimum power output among the hot- and cold-side heat exchangers and the regenerator for the fixed total heat exchanger inventory. The influence of some design parameters, including the temperature ratio of the heat reservoirs, the total heat exchanger inventory, the efficiencies of the compressor and the turbine, and the pressure recovery coefficient, on the optimum heat conductance distribution and the maximum power output are provided. The power plant design with optimization leads to smaller size including the compressor, turbine, and the hot- and cold-side heat exchangers and the regenerator.     

A Novel Solar Thermal Cycle with Chemical Looping Combustion

In this paper, we have proposed a thermal cycle with the integration of chemical-looping combustion and solar thermal energy with the temperature of about 500-600°C. Chemical-looping combustion may be carried out in two successive reactions between a reduction of hydrocarbon fuel with metal oxides and a reduced metal with oxygen in the air. This loop of chemical reactions is substituted for conventional combustion of fuel. Methane as a fuel and nickel oxides as an oxygen carrier were employed in this cycle. Collected high-temperature solar thermal energy is provided for the endothermic reduction reaction. The feature of the proposed cycle is investigated through Energy-Utilization Diagram methodology. As a result, at the turbine inlet temperature of 1200°C, the exergy efficiency of the proposed cycle would be expected to be about 4 percentage points higher than that of a conventional gas turbine combined cycle. Compared to the previous study of chemical-looping combustion energy systems, the proposed cycle with the integration of green energy and traditional hydrocarbon fuels will offer the possibility of both greenhouse gas mitigation, with green energy, and a new approach to the efficient use of solar energy.     

Performance of a small-scale solar-powered adsorption cooling system

In this study, an experimental investigation on the performance of a small-scale residential-size solar-driven adsorption (silica gel-water) cooling system that was constructed at Assiut University campus, Egypt is carried out. As Assiut area is considered as hot, arid climate, field tests for performance assessment of the system operation during the summer season are performed under different environmental operating conditions. The system consists of an evacuated tube with a reflective concentration parabolic surface solar-collector field with a total area of 36 m², a silica gel-water adsorption chiller of 8 kW nominal cooling capacity, and hot and cold water thermal storage tanks of 1.8 and 1.2 m³ in volume, respectively. The results of summer season field test show that under daily solar insolation varying from 21 to 27 MJ/m², the solar collectors employed in the system had high and almost constant thermal efficiency. The daily solar-collector efficiency during the period of system operation ranged from about 50% to 78%. The adsorption chiller performance shows that the chiller average daily coefficient of performance (COP) was 0.41 with the average cooling capacity of 4.4 kW when the cooling-water and chilled-water temperatures were about 31°C and 19°C, respectively. As the chiller cooling water is cooled by the cooling tower in the hot arid area, the cooling water is at a higher temperature than the design point of the chiller. Therefore, an experiment was carried out using the city water for cooling. The results show that an enhancement in the chiller COP by 40% and the chilling power by 17% has been achieved when the city water was 27.7°C.     

Performance analysis of different orc power plant configurations using solar and geothermal heat sources

ABSTRACT

Energy optimization is performed on hybrid solar-geothermal power plant working according to Organic Rankine Cycle and installed in southern Tunisia. The performances of four different configurations of the power plant are studied. Mass and energy balances are established for the different compounds. The effects of the main operating parameters such as the geothermal water temperature, ambient temperature and direct normal irradiation on the power plant performances are analyzed. A code is established using Engineering Equation Solver software (EES) to perform the required calculations. Obtained results show that the hybrid solar-geothermal power plant with a heat recovery system is the most suitable configuration design giving a better overall energy efficiency of 15.77 and13.11% and a maximum net power of 1089 and 1882 kW in winter and summer, respectively. However, in the summer season, using a heat recovery system can valuable only when the water temperature is higher than 66°C. For air-cooled condenser, the suitable condensing pressure is 1 bar in winter and 1.9 bars in summer.     

Experimental investigation on the phase change material-based modular heat exchanger for thermal management of a building

A phase change material (PCM)-based flat plate modular heat exchanger for free cooling application, suitable for the diurnal temperature variation that prevails during the summer months of Bangalore city, India is designed and experimentally investigated. The flow and other parameters selected in the present study are meant to suit the accelerated charging of the PCM in the modular heat exchanger during the early morning hours, and to provide cool energy to the room during the daytime, by circulating the ambient air through the heat exchanger at a lower velocity. It is observed from the charging experiments that the decrease in the inlet air temperature has more influence in reducing the solidification time than the increase in the inlet air velocity. The heat exchanger designed in the present investigation is capable of maintaining the room temperature around 30°C for a longer duration of 8 hr when the heat load is 0.5 kW. It is suggested to design the modular heat exchanger with a surface area proportionate to the present heat exchanger size when the room heat load increases beyond 0.5 kW, in order to maintain a minimum comfortable temperature of 30°C in the room.     

Ignition advancement study for optimized characteristics of a raw biogas operated spark ignition engine

In the current investigation, raw biogas obtained from rural sectors was used as the alternative to gasoline fuel in the spark ignition (SI) engine. The performance and efficiency are mainly dependent on the combustion phasing for which “ignition timing” is an effective tool in a SI engine. Hence, the objective of the present work is to understand the effect of “variable ignition timing” for a biogas-fueled SI engine. For this purpose, a single cylinder, 4-stroke, SI engine of rated power 4.5 kW was operated with raw biogas at a compression ratio (CR) of 10. By maintaining a speed of 1650 rpm, the engine was operated in wide open (WOT) and part throttle (PT) mode with an equivalence ratio of 0.81 and 0.83, respectively. It was observed that the biogas fueled SI engine was found to be operative only within the ignition advance (IA) range of 33–47° CA bTDC both in WOT and PT conditions. The results showed optimal brake power (BP), brake thermal efficiency (BTE) and brake specific fuel consumption (BSFC) are achieved at 45° CA bTDC. The average peak cylinder pressure, neat heat release rate (NHRR) and mean gas temperature (MGT) are also observed to be maximum while CO and HC emission at this point of IA were found to be minimum. Due to controlled and complete combustion, CO₂ and NO_x concentration in the exhaust emission were found to be higher at this point of ignition timing.     

Comparative study on performances of a heat-pipe PV/T system and a heat-pipe solar water heating system

The heat-pipe solar water heating (HP-SWH) system and the heat-pipe photovoltaic/thermal (HP-PV/T) system are two practical solar systems, both of which use heat pipes to transfer heat. By selecting appropriate working fluid of the heat-pipes, these systems can be used in the cold region without being frozen. However, performances of these two solar systems are different because the HP-PV/T system can simultaneously provide electricity and heat, whereas the HP-SWH system provides heat only. In order to understand these two systems, this work presents a mathematical model for each system to study their one-day and annual performances. One-day simulation results showed that the HP-SWH system obtained more thermal energy and total energy than the HP-PV/T system while the HP-PV/T system achieved higher exergy efficiency than the HP-SWH system. Annual simulation results indicated that the HP-SWH system can heat the water to the available temperature (45°C) solely by solar energy for more than 121 days per year in typical climate regions of China, Hong Kong, Lhasa, and Beijing, while the HP-PV/T system can only work for not more than 102 days. The HP-PV/T system, however, can provide an additional electricity output of 73.019 kWh/m², 129.472 kWh/m², and 90.309 kWh/m² per unit collector area in the three regions, respectively.     

Frequency Control Characteristics of a Direct-Mode Untreated Sewage Source Heat Pump in the Heating Mode

An untreated sewage source heat pump system directly makes use of the urban raw sewage instead of that treated by a sewage plant. At present In China, most systems adopt the indirect mode for avoiding the harmful effect of the sewage on the heat pump unit. In this article a direct-mode untreated sewage source heat pump system, which shows less theoretical analysis of the mathematical model, is theoretically designed and analyzed to simulate the system dynamic characteristics in the heating mode. The results show that the system COP changes from 4.1 to 3.4 and the heating capacity from 9.5 to 15.3 kW when the sewage inlet temperature is 12°C and the frequency increases from 18 to 32 HZ. The condenser heat-transfer coefficient increases with the frequency reducing while the change trend of evaporator heat-transfer coefficient is the opposite. The highest values of them are 303 and 1617 W.m^?2?K^?1, respectively. The frequency control simulation supplies the operation adjustment with theoretical instructions and some reference values.     

Multi-stage chemical looping combustion (CLC) for combined cycles with CO2 capture

This paper presents application of the chemical looping combustion (CLC) method in natural gas-fired combined cycles for power generation with CO₂ capture. A CLC combined cycle consisting of single CLC-reactor system, an air turbine, a CO₂-turbine and a steam cycle has been designated as the base-case cycle. The base-case cycle can achieve net plant efficiency of about 52% at an oxidation temperature of 1200 °C. In order to achieve a reasonable efficiency at lower oxidation temperatures, reheat is introduced into the air turbine by employing multi CLC-reactors. The results show that the single reheat CLC-combined cycle can achieve net plant efficiency of above 51% at oxidation temperature of 1000 °C and above 53% at the oxidation temperature of 1200 °C including CO₂ compression to 110 bar. The double reheat cycle results in marginal efficiency improvement as compared to the single reheat cycle. The CLC-cycles are also compared with a conventional combined cycle with and without post-combustion capture in amine solution. All the CLC-cycles show higher net plant efficiencies with close to 100% CO₂ capture as compared to a conventional combined cycle with post-combustion capture, which is very promising.     

Numerical studies on the thermal regimes of the horizontal tube falling film evaporation under varying feeder height

ABSTRACT

This paper discusses about the effect of feeder height and heat flux on the heat transfer characteristics of horizontal tube falling film evaporation in the thermal regimes. In order to investigate this, a two- dimensional CFD model was developed to perform simulation and results were compared and validated with published data available in the literature. Heat transfer co-efficients in the thermal regimes were determined from the CFD simulation and the results were recorded, analyzed and validated with the mathematical models available in the literature. The novelty of the current study is to predict the commencement of the fully developed thermal region over the tube from the simulation model under varying feeder height and heat flux. An effort was also made to measure the liquid film thickness around the tube from the CFD model in the thermal regimes. It is observed that angle of thermally developing region contracts and fully developed thermal region extends with the increase of the feeder height and heat flux. It is observed from the study that increase of heat flux by 10 kW/m² resulted in increase of heat transfer co-efficient value by 10–12% average in thermally developing region and 12–15% average in fully developed region. Thinnest liquid film thickness observed between 85 and 127°angle. Shifting of thinnest region of liquid film upward from the mid tube with the increase of the feeder height and heat flux is noted.     

From exogenous to endogenous development in Scottish forestry: the feasibility of small-scale wood energy enterprise

This paper presents a change in the Scottish forestry policy from the exogenous to the endogenous development approach, and feasibility of heat entrepreneurship based on locally produced woodfuels in the Highlands of Scotland. The cost structure and heat pricing in the case of an 800 kW solid fuel boiler is presented, and the profitability of local heat entrepreneurship is analysed with scenarios of different investment costs and fuel prices. The results indicate that a district heating (DH) system, using locally produced woodchips, could produce heat at a lower price than single-house heating systems using light fuel oil. The profitability of replacing existing heating units by investing in a new district heating (DH) scheme is very dependent on the available investment support, price level of woodchips and substituted light fuel oil, and the amount of sold energy. In the case of an 800 kW DH scheme, and woodchip prices of 14 and 22 €/MWh, investments should remain under break-even points of 280 and 420 €/kW of heating power (230,000–335,000€).     

STATUS REPORT ON HEAT STORAGE WELLS1

ABSTRACT. The Heat Storage Well concept evolved at TEMPO in 1972. Technical, economic, institutional, and legal aspects of implementing the concept are being studied. Heat Storage Wells offer the possibility of conserving a substantial part of the heat energy now wasted in generating electricity and of reducing thermal pollution caused by the waste heat. Large thermal electric power plants would produce heat at useful temperatures such as 300–400°F. Combined electric- and heat-generating systems will require low-cost, low-loss storage of large amounts of hot water for periods of 90 to 180 days to serve both electric loads and seasonally-varying heat loads. Compared to conventional electricity-only systems, combined electricity-heat systems can save more than 20 percent in energy, reduce the cost of both electricity and heat, and eliminate the need for cooling water or towers. Possibilities for changes in legal and institutional practices are suggested, such as making resource-allocation decisions on the basis of energy units rather than dollars.     

Designing a supercritical steam cycle to integrate the energy requirements of CO2 amine scrubbing

Absorption by chemical solvents combined with CO₂ long-term storage appears to offer interesting and commercial applicable CO₂ capture technology. However one of the main disadvantages is related to the large quantities of heat required to regenerate the amine solvent that means an important power plant efficiency penalty. Different studies have analyzed alternatives to reduce the heat duty on the reboiler and the thermal integration requirements on existing power cycles. In these studies integration principles have been well set up, but there is a lack of information about how to achieve an integrated design and the thermal balances of the modified cycle flowsheet. This paper proposes and provides details about a set of modifications of a supercritical steam cycle to overcome the energy requirements through energetic integration with the aim of reducing the efficiency and power output penalty associated with CO₂ capture process. Modifications include a new designed low-pressure heater flowsheet to take advantage of the CO₂ compression cooling for postcombustion systems and integration of amine reboiler into a steam cycle. It has been carried out several simulations in order to obtain power plant performance depending on sorbent regeneration requirements.     

Chemical looping combustion for power generation—Concept study for a 10 MWth demonstration plant

A semi-commercial 10 MW_th chemical looping combustion (CLC) plant for power production is proposed as a next scale demonstration plant after successful operation of a 120 kW CLC pilot rig. The design criteria for the CLC boiler are derived from the experience obtained from the CLC pilot rig at Vienna University of Technology. The IPSEpro simulation environment is chosen for implementation of the process flow sheet of the CLC power plant. A single pressure steam cycle is suggested for this small scale demonstration plant. Heat exchangers and a five-stage steam turbine are arranged. Basic design parameters of the power plant are derived from detailed mass and energy investigations and discussed. It turns out that the net electric efficiency of such a small scale plant can be expected to be in the range of 32.5–35.8%. However, a demonstration of CLC at such a scale is necessary in order to gain confidence in more sophisticated CLC power generation concepts at larger scale.     

PREDICTING INFLUENCES OF URBAN DEVELOPMENT ON THERMAL HABITAT IN A WARM WATER STREAM1

ABSTRACT: Watershed and aquatic ecosystem management requires methods to predict and understand thermal impacts on stream habitat from urbanization. This study evaluates thermal effects of projected urbanization using a modeling framework and considers the biological implications to the fish community. The Stream Network Temperature Model (SNTEMP) was used in combination with the Hydrologic Simulation Program Fortran (HSPF) to assess changes in stream thermal habitat under altered stream‐ flow, shade, and channel width associated with low, medium, and high density urban developments in the Back Creek watershed (Roanoke County, Virginia). Flow alteration by the high density development scenario alone caused minimal heating of mean daily summer base flow (mean +0.1°C). However, when flow changes were modeled concurrently with reduced shade and increased channel width, mean daily temperature increased 1°C. Maximum daily temperatures exceeding the state standard (31°C) increased from 1.1 to 7.6 percent of the time using summer 2000 climatic conditions. Model results suggest that additional urban development will alter stream temperature, potentially limiting thermal habitat and shifting the fish community structure from intolerant to tolerant fish species in Back Creek. More research is needed on the sub‐lethal or chronic effects of increased stream temperature regimes on fish, particularly for those species already living in habitats near their upper limits.