[en] The utilization of unused energy is important because it can afford to offer a chance to increase energy efficiency of a heat pump system. Especially the waste energy recovery by heat pump can be easily utilized in industrial field for processing water supply system. In this article, the efficient way to make a high temperature water by using the waste water as heat source. This article concerns such a cycle that its system making up has the features: a two-stage compression, a two stage expansion and two desorbers, each of which being located after an expansion valve. By comparisons of the performance with those of common two-stage compression and single expansion CAHP systems for four heating cases, the simulation studies showed that the new system has superior performance, under the working conditions of lower compressor outlet temperature and lower maximum system pressure, which is very important for the components of the system to be operated safely. And the performance can be improved further by optimum displacements of the compressors chosen. The simulations also demonstrated how changes of the sink temperature glide, maximum pressure and the performance of the system were adjusted by the circulation rate and the ammonia concentration for a given heat case

[en] Because of the growing interest in supercritical carbon dioxide power cycle technology owing to its potential enhancement in compactness and efficiency, supercritical carbon dioxide cycles have been studied in the fields of nuclear power, concentrated solar power (CSP), and fossil fuel power generation. This study introduces the current status of the research project on the supercritical carbon dioxide power cycle by Korea Institute of Energy Research (KIER). During the first phase of the project, the un-recuperated supercritical Brayton cycle test loop was built and tested. In phase two, researchers are designing and building a supercritical carbon dioxide dual Brayton cycle, which utilizes two turbines and two recuperators. Under the simulation condition considered in this study, it was confirmed that the design parameter has an optimal value for maximizing the net power in the supercritical carbon dioxide dual cycle.

[en] In this study, an organic Rankine-cycle system using HFC-134a, which is a power cycle corresponding to a low temperature heat source, such as that for geothermal power generation, was investigated from the view point of power optimization. In contrast to conventional approaches, the heat transfer and pressure drop characteristics of the working fluid within the heat exchangers were taken into account by using a discretized heat exchanger model. The inlet flow rates and temperatures of both the heat source and the heat sink were fixed. The total heat transfer area was fixed, whereas the heat-exchanger areas of the evaporator and the condenser were allocated to maximize the power output. The power was optimized on the basis of three design parameters. The optimal combination of parameters that can maximize power output was determined on the basis of the results of the study. The results also indicate that the evaporation process has to be optimized to increase the power output

[en] The utilization of unused energy is important because it can afford to offer a chance to increase energy efficiency of a heat pump system. One of the promising unused energy sources is river water. It can be used as a heat source in both heating and cooling effectively with its superior features as a secondary working fluids. In this study, the performance of a 5HP heat pump system using river water as a heat source is investigated by both experiment and simulation. According to system simulation results, performance improvement of condenser seems more effective than that of evaporator for better COPH. The serial connection is also preferred among several methods to improve plate type heat exchanger performance. The experimental results show that the hot water of 50∼60 .deg. C can be acquired from water heat source of 5∼9 .deg. C with COPH of 2.7∼3.5

[en] In this study, a low-grade heat source power generation system using the Kalina cycle was investigated by the simulation method. The Kalina cycle system can be used for the utilization of a low-temperature heat sources such as geothermal and industrial waste heat that are not hot enough to produce steam. The sea/river water can be considered as a cooling media. A steady-state simulation model was developed to analyze and optimize its performance. The model contains a turbine, a pump, an expansion valve and heat exchangers. The turbine and pump were modelled by an isentropic efficiency, while a condenser, an evaporator and a regenerative heat exchanger were modeled by UA-LMTD method with a counter-flow assumption. The effect of the temperature glide in the evaporator on the cycle performance is investigated in detail

[en] In this study, the potential performance enhancement in a dual heat pump system through series operation was investigated by a comparison between the performance for parallel and series operation for a heating supply temperature of 60 .deg. C. To compare the performance of each configuration fairly, the heat transfer surface area of the heat exchangers was fixed. The inlet temperatures and the flow rates of the heat source and the load were also fixed. In addition, the heat transfer and pressure drop characteristics of the working fluids were considered to achieve a more realistic comparison. The results show that the heating coefficient of performance (COP) of the series configuration is approximately 5% higher than that of the parallel configuration under the simulation conditions considered in the present study

[en] This research aims to develop a compression/absorption hybrid heat pump system using an NH₃/H₂O as working fluid. The heat pump cycle is based on a combination of compression and absorption cycles. The cycle consists of two-stage compressors, absorbers, a de superheater, solution heat exchangers, a solution pump, a rectifier, and a liquid/vapor separator. The compression/absorption hybrid heat pump was designed to produce hot water above 90 .deg. C using high-temperature glide during a two-phase heat transfer. Distinct characteristics of the nonlinear temperature profile should be considered to maximize the performance of the absorber. In this study, the performance of the absorber was investigated depending on the capacity, shape, and arrangement of the plate heat exchangers with regard to the concentration and distribution at the inlet of the absorber

[en] This research concerns the development of a compression/absorption high-temperature hybrid heat pump that uses a natural refrigerant mixture. Heat pumps based on the compression/absorption cycle offer various advantages over conventional heat pumps based on the vapor compression cycle, such as large temperature glide, temperature lift, flexible operating range, and capacity control. In this study, a lab-scale prototype hybrid heat pump was constructed with a two-stage compressor, absorber, desorber, desuperheater, solution heat exchanger, solution pump, liquid/vapor separator, and rectifier as the main components. The hybrid heat pump system operated at 10-kW-class heating capacity producing hot water whose temperature was more than 90 .deg. C when the heat source and sink temperatures were 50 .deg. C. Experiments with various NH₃/H₂O mass fractions and compressor/pump circulation ratios were performed on the system. From the study, the system performance was optimized at a specific NH₃ concentration

[en] A Sub-kWe small-scale experimental test loop was manufactured to investigate characteristics of the supercritical carbon dioxide power cycle. A high-speed turbo-generator was also designed and manufactured. The designed rotational speed of this turbo-generator was 200,000 rpm. Because of the low expansion ratio through the turbine and low mass flowrate, the rotational speed of the turbo-generator was high. Therefore, it was difficult to select the rotating parts and design the turbine wheel, axial force balance and rotor dynamics in the lab-scale experimental test loop. Using only one channel of the nozzle, the partial admission method was adapted to reduce the rotational speed of the rotor. This was the world’s first approach to the supercritical carbon dioxide turbo-generator. A cold-run test using nitrogen gas under an atmospheric condition was conducted to observe the effect of the partial admission nozzle on the rotor dynamics. The vibration level of the rotor was obtained using a gap sensor, and the results showed that the effect of the partial admission nozzle on the rotor dynamics was allowable.

[en] Highlights: • Chimney-based radial heat sinks are studied subject to natural convection. • The effect of various parameters on the dimensionless temperature is investigated. • The orientation effects of different chimney-based radial heat sinks are investigated. • The heat sink with optimal structure enhances the thermal performance by up to 20%. - Abstract: In this study, the natural convection heat transfer from chimney-based radial heat sinks was investigated numerically and experimentally. The thermal performance of a radial heat sink with chimney was estimated and compared with that of a radial heat sink without a chimney. The effects on the heat sinks’ thermal performance were investigated in regard to the orientation angle with respect to gravity (0° ⩽ θ ⩽ 180°), the fin number (15 ⩽ N ⩽ 30), and the distance between the base plate and chimney (5 mm ⩽ H_c ⩽ 25 mm). The results show that there exist optimal values for H_c and N. The orientation angles 0° and 45° result in slightly better thermal performance than 135° and 180°, while 90° results in the worst thermal performance. The chimney-based heat sink with optimal structure enhances the thermal performance by up to 20% compared with the radial heat sink without a chimney.