Heat Rejection Rate Research Articles

A Numerical Model for the Simulation of a Vertical U-Bend Ground Heat Exchanger Used in a Ground-Coupled Heat Pump

Heat transfer between the ground heat exchanger (GHE) and surrounding soil is a common problem for the optimum design and simulation of ground-coupled heat pump (GCHP). In this article, a quasi-three-dimensional numerical model of vertical U-bend GHE was proposed and developed by coupling the one-dimensional model of fluid in the depth direction with the two-dimensional heat transfer model of soil in the level direction but considering the soil temperature variation along depth. In the model, the soil in depth direction was divided into saturated and unsaturated regions by water table, and the influence of groundwater advection on the heat transfer performance of GHE was considered for the saturated soil region. Based on the model, the variations of heat rejection and fluid outlet temperature of GHE with time were discussed, and the influences of soil thermal properties and groundwater advection on the heat transfer performance of GHE were analyzed. The results show that the heat rejection in cooling mode will decrease with time, and an intermittent heat rejection mode can improve the heat transfer performance. Meanwhile, the increases of soil thermal conductivity and heat capacity can all enhance the heat transfer characteristics, and the exist of groundwater advection can improve the heat exchange between the GHE and surrounding soil. Furthermore, an experimental validation was performed in a solar-geothermal multifunctional heat pump experiment system. The results indicate the predicted values of heat rejection rate of GHE are agreed well with the corresponding measured data, and the guess average relative error for the daily heat rejection is about 5.5%. This means the numerical model proposed in this article is feasible and thus can be used to simulate the heat transfer of GHE.

Performance optimization of a hybrid ground source heat pump with the parallel configuration of a ground heat exchanger and a supplemental heat rejecter in the cooling mode

A hybrid ground source heat pump (HGSHP) combining a ground flow loop and a supplemental heat rejecter loop in parallel is proposed to overcome the degradation of the ground thermal condition. The objective of this study is to optimize the performance of the HGSHP with the parallel configuration by varying the amount of refrigerant charge, the secondary fluid flow rate of the ground loop, and the secondary fluid flow rate of the supplemental loop. The coefficient of performance (COP) of the HGSHP with the parallel configuration was 21% higher than that of the conventional ground source heat pump (GSHP), and the heat rejection rate of the ground flow loop was 42% lower than that of the GSHP at the entering fluid temperature (EFT) of 40 °C in the ground heat exchanger and the EFT of 28 °C in the supplemental heat exchanger.

Effects of Wind on Hot Air Recirculation (HAR) Behavior

The air-cooled condenser system is impacted greatly by many factors such as the wind speed, the direction of wind and arrangement of site equipments. This paper concentrates on the hot air recirculation phenomenon and its dependence on ambient winds are numerically simulated by using the computational fluid dynamics code, FLUENT. In this paper, two different wind directions (case A and case B) and wind speeds are considered: Results show that the hot air recirculation increases with the increment of velocity speed. Case A has a critical wind direction angle. Wind causes an air temperature increase at the fan inlet due to hot air recirculation, resulting in the deterioration of the heat transfer performance. The hot air recirculation is the main factor responsible for the reduction of heat rejection rate. In case A, fan inlet temperature is higher than case B. The peak value of the HAR occurs at 9 m/s in case A.

Wet and dry cooling systems optimization applied to a modern waste-to-energy cogeneration heat and power plant

In Brescia, Italy, heat is delivered to 70% of 200.000 city inhabitants by means of a district heating system, mainly supplied by a waste to energy plant, utilizing the non recyclable fraction of municipal and industrial solid waste (800,000 tons/year, otherwise landfilled), thus saving annually over 150,000 tons of oil equivalent and over 400,000 tons of CO 2 emissions. This study shows how the performance of the waste-to-energy cogeneration plant can be improved by optimising the condensation system, with particular focus on the combination of wet and dry cooling systems. The analysis has been carried out using two subsequent steps: in the first one a schematic model of the steam cycle was accomplished in order to acquire a knowledge base about the variables that would be most influential on the performance. In the second step the electric power output for different operating conditions was predicted and optimized in a homemade program. In more details, a thermodynamic analysis of the steam cycle, according to the design operating condition, was performed by means of a commercial code (Thermoflex ©) dedicated to power plant modelling. Then the off-design behaviour was investigated by varying not only the ambient conditions but also several parameters connected to the heat rejection rate, like the heat required from district heating and the auxiliaries load. Each of these parameters has been addressed and considered in determining the overall performance of the thermal cycle. After that, a complete prediction of the cycle behaviour was performed by simultaneously varying different operating conditions. Finally, a Matlab © computer code was developed in order to optimize the net electric power as a function of the way in which the condensation is operated. The result is an optimum set of variables allowing the wet and dry cooling system to be regulated in such a way that the maximum power is achieved. The best strategy consists in using the maximum amount of heat rejection in the wet cooling system to reduce the operational cost of the dry one.

Performance prediction of an improved air-cooled steam condenser with deflector under strong wind

For an air-cooled steam condenser (ACSC), environmental wind can cause a large flow rate reduction in the axial fans mainly near the windward side of the air-cooled platform due to cross-flow effects, resulting in a heat transfer reduction. This leads to an increase of turbine back pressure, and occasional turbine trips occur under extremely gusty conditions. A new method is proposed in this paper to remove the strong wind effect by adding deflecting plates under the air-cooled platform, which contributes to forming a uniform air mass flow rate in the axial fans by leading enough cooling air to the fans in the upwind region. Numerical simulation is made of the thermal-flow characteristics and heat transfer performance of the improved ACSC with deflectors. A heat exchanger model is used for simulating the flow and heat transfer in the ACSC, in which the heat exchanger is simplified to a porous medium and all flow losses are taken into account by a viscous and an inertial loss coefficient. A fan model is used for reaching the flow condition at the heat exchanger inlet with the actual performance curves of the fan. It is found that the improved ACSC with deflector shows a significant enhancement in both the cooling air mass flow rate and the heat rejection rate compared with the conventional ACSC. The higher the wind speed is, the larger the heat transfer enhancement of the improved ACSC is. The effect of the plate inclination is also investigated, and the inclination angle of 45° is found to be the optimum value for the arrangement of the deflector.

Experimental study of a ground sink direct cooling system in cold areas

A conception with ground as a heat sink directly for cooling of the buildings in cold areas is presented. A corresponding experimental system was constructed in Harbin area and operated in summer in 2006. Some important performance parameters, such as the cooling seasonal performance factor (CSPF) and the average heat rejection rate unit depth of borehole, were obtained and some analyses for the system characteristics are made. The experimental results show that the ground sink direct cooling system (GSDCS) presented has great potentialities in energy saving within a specified region.

Comparative analysis of an evaporative condenser using artificial neural network and adaptive neuro-fuzzy inference system

This study deals with predicting the performance of an evaporative condenser using both artificial neural network (ANN) and adaptive neuro-fuzzy inference system (ANFIS) techniques. For this aim, an experimental evaporative condenser consisting of a copper tube condensing coil along with air and water circuit elements was developed and equipped with instruments used for temperature, pressure and flow rate measurements. After the condenser was connected to an R134a vapour-compression refrigeration circuit, it was operated at steady state conditions, while varying both dry and wet bulb temperatures of the air stream entering the condenser, air and water flow rates as well as pressure, temperature and flow rate of the entering refrigerant. Using some of the experimental data for training, ANN and ANFIS models for the evaporative condenser were developed. These models were used for predicting the condenser heat rejection rate, refrigerant temperature leaving the condenser along with dry and wet bulb temperatures of the leaving air stream. Although it was observed that both ANN and ANFIS models yielded a good statistical prediction performance in terms of correlation coefficient, mean relative error, root mean square error and absolute fraction of variance, the accuracies of ANFIS predictions were usually slightly better than those of ANN predictions. This study reveals that, having an extended prediction capability compared to ANN, the ANFIS technique can also be used for predicting the performance of evaporative condensers.

Experimental performance investigation of a horizontal ground source compression refrigeration machine

In this paper, the effect of various system parameters on horizontal ground source compression refrigeration machine (CRM) performance is studied experimentally in Bursa, Turkey. A ground heat exchanger (GHE) system connected to CRM in a test room in the air conditioning and refrigeration laboratory of Uludag University has been designed and constructed. This system was tested for space cooling in August and September 2004 and performance tests were performed during this period. Overall, system mainly consists of the GHE (GHE–water circuit) and CRM (CRM–refrigerant circuit). Refrigerant is R134a. Hourly variations of inlet and outlet water temperatures, extracted heat from test room, rejected heat to ground, compression ratio, total power consumption, and coefficient of performance (COP) values for both whole system and only CRM are obtained. Thermal properties of soil are also estimated by using experimental data and theory. Cooling load of test room and rejected heat to ground, and all COP values are also presented. Finally, heat rejection rate to ground with respect to leaving water temperature from GHE is given. The COP of the overall system changes between 2 and 2.5, also CRM COP values are in the range of 3 and 3.86. Copyright © 2007 John Wiley & Sons, Ltd.

Performance prediction of a cooling tower using artificial neural network

Abstract This paper describes an application of artificial neural networks (ANNs) to predict the performance of a cooling tower under a broad range of operating conditions. In order to gather data for training and testing the proposed ANN model, an experimental counter flow cooling tower was operated at steady state conditions while varying the dry bulb temperature and relative humidity of the air entering the tower and the temperature of the incoming hot water along with the flow rates of the air and water streams. Utilizing some of the experimental data for training, an ANN model based on a standard back propagation algorithm was developed. The model was used for predicting various performance parameters of the system, namely the heat rejection rate at the tower, the rate of water evaporated into the air stream, the temperature of the outgoing water stream and the dry bulb temperature and relative humidity of the outgoing air stream. The performances of the ANN predictions were tested using experimental data not employed in the training process. The predictions usually agreed well with the experimental values with correlation coefficients in the range of 0.975–0.994, mean relative errors in the range of 0.89–4.64% and very low root mean square errors. Furthermore, the ANN yielded agreeable results when it was used for predicting the system performance outside the range of the experiments. The results show that the ANN approach can be applied successfully and can provide high accuracy and reliability for predicting the performance of cooling towers.

Artificial neural network analysis of an automobile air conditioning system

Abstract This study deals with the applicability of artificial neural networks (ANNs) to predict the performance of automotive air conditioning (AAC) systems using HFC134a as the refrigerant. For this aim, an experimental plant consisting of original components from the air conditioning system of a compact size passenger vehicle was developed. The experimental system was operated at steady state conditions while varying the compressor speed, cooling capacity and condensing temperature. Then, with the use of some experimental data for training, an ANN model for the system, based on the standard back propagation algorithm was developed. The model was used for predicting various performance parameters of the system, namely the compressor power, heat rejection rate in the condenser, refrigerant mass flow rate, compressor discharge temperature and coefficient of performance. The ANN predictions for these parameters usually agreed well with the experimental values with correlation coefficients in the range of 0.968–0.999, mean relative errors in the range of 1.52–2.51% and very low root mean square errors. This study shows that air conditioning systems, even those employing a variable speed compressor, such as AAC systems, can alternatively be modelled using ANNs with a high degree of accuracy.

Artificial neural network analysis of a refrigeration system with an evaporative condenser

This paper describes an application of artificial neural networks (ANNs) to predict the performance of a refrigeration system with an evaporative condenser. In order to gather data for training and testing the proposed ANN, an experimental refrigeration system with an evaporative condenser was set up. Then, steady-state test runs were conducted varying the evaporator load, air and water flow rates passing through the condenser and both dry and wet bulb temperatures of the air stream entering the condenser. Utilizing some of the experimental data, an ANN model for the system based on standard backpropagation algorithm was developed. The ANN was used for predicting various performance parameters of the system, namely the condenser heat rejection rate, refrigerant mass flow rate, compressor power, electric power input to the compressor motor and the coefficient of performance. The ANN predictions usually agree well with the experimental values with correlation coefficients in the range of 0.933–1.000, mean relative errors in the range of 1.90–4.18% and very low root mean square errors. Results show that refrigeration systems, even complex ones involving concurrent heat and mass transfer such as systems with an evaporative condenser, can alternatively be modelled using ANNs within a high degree of accuracy.

Performance evaluation of an integrated automotive air conditioning and heat pump system

This study deals with the performance characteristics of an R134a automotive air conditioning system capable of operating as an air-to-air heat pump using ambient air as a heat source. For this aim, an experimental analysis has been performed on a plant made up of original components from an automobile air conditioning system and some extra equipment employed to operate the system in the reverse direction. The system has been tested in the air conditioning and heat pump modes by varying the compressor speed and air temperatures at the inlets of the indoor and outdoor coils. Evaluation of the data gathered in steady state test runs has shown the effects of the operating conditions on the capacity, coefficient of performance, compressor discharge temperature and the rate of exergy destroyed by each component of the system for both operation modes. It has been observed that the heat pump operation provides adequate heating only in mild weather conditions, and the heating capacity drops sharply with decreasing outdoor temperature. However, compared with the air conditioning operation, the heat pump operation usually yields a higher coefficient of performance and a lower rate of exergy destruction per unit capacity. It is also possible to improve the heating mode performance of the system by redesigning the indoor coil, using another refrigerant with a higher heat rejection rate in the condenser and employing a better heat source such as the engine coolant or exhaust gases.

Development of a cryogen-free continuous ADR for the constellation-X mission

Constellation-X is a multi-satellite X-ray astronomy mission presently being planned for launch in the 2010 time frame. Each of four identical satellites will contain a telescope and instruments for imaging and spectroscopic analysis of hard and soft X-rays. The X-ray Microcalorimeter Spectrometer (XMS) instrument will use arrays of microcalorimeters to detect X-rays with energies from 0.2 to 6 keV. To achieve the desired energy resolution of 2 eV, the detectors will be cooled to 50 mK. The projected cooling power requirement is approximately 5 μW. In order to meet the mission lifetime requirement of 6 years (with a goal of 10 years), a mechanical cryocooler will be used to provide a heat sink for the low temperature cooler. In this paper we discuss the development of a continuous adiabatic demagnetization refrigerator (CADR) that meets the cooling requirements of the XMS instrument. The four-stage CADR presently being tested can operate continuously at 50 mK using a 4.2 K helium bath as a heat sink. Its cooling power of 6 μW at 50 mK exceeds the XMS requirement, and its peak heat rejection rate is less than 7 mW. Details of the CADR's design and operation, as well as the development program leading up to a flight-qualified instrument, are discussed.

Modeling and experimental evaluation of parallel flow micro channel condensers

This paper reports results obtained in a theoretical and experimental study involving micro channel/louvered fin condensers for automotive applications. A simulation model has been developed based upon three zones related to the thermodynamic states of the refrigerant in the condenser. Experiments with condensers have been performed in a set up developed for thermal performance evaluations of automotive air conditioning systems. Comparisons between simulation model and experimental results for refrigerant HFC-134a have been performed with respect to three important condenser parameters: heat rejection rate, refrigerant pressure drop and overall heat transfer coefficient. It has been shown that model results for the particular tested condenser compare very well with the experimental data, with deviations being within an acceptable range.

Performance evaluation of a vertical ground-source heat pump system in Izmir, Turkey

This study summarizes the cooling performance of a ground-source heat pump system which was installed in a 65-m2 room in the Solar Energy Institute, Ege University, Izmir (568 degree-days cooling, base: 22°C; 1226 degree-days heating, base: 18°C) Turkey. The institute, built in 1986, has a liveable floor area of 3000 m2, and uses passive solar techniques. The heating and cooling loads of the room were, respectively, 3.8 and 4.2 kW at design conditions. The system was commissioned in May 2000 and the performance tests have been carried out since then. Based upon the measurements, the heat rejection rate to the soil with an average thermal diffusivity of 0.00375 m2h−1 in the cooling mode was found to be in average 51 W m−1 of bore depth, while the maximum entering water temperature to the unit was recorded as 35.9°C. The cooling coefficient of performance of the heat pump and the whole system was relatively low when compared to other heat pumps operating under conditions at or near design values. The primary reasons for this were discussed in detail and the potential for performance improvements was also suggested. Copyright © 2002 John Wiley & Sons, Ltd.

Effect of the Shape of the Tower Supports and Walls on the Performance of a Dry-Cooling Tower Subjected to Cross Winds

Cross winds generally reduce the heat rejection rates of natural draft dry-cooling towers. The influence of different tower support geometries and a windbreak wall on the performance of a dry-cooling tower with a horizontal heat exchanger arrangement, during windy periods, is investigated. Based on experimental results, expressions for the tangential and radial pressure loss coefficients for an array of supports in an inclined-flow situation are found. A numerical analysis reveals that the wind effect on the heat rejection rate is strongly dependent on both the shape and the pressure loss coefficients of the supports. Significant reductions in the wind effect on the tower can be achieved under certain conditions.

A Study of High-Temperature Heat Pipes With Multiple Heat Sources and Sinks: Part I—Experimental Methodology and Frozen Startup Profiles

A high-temperature sodium/stainless steel heat pipe with multiple heat sources and sinks was fabricated, processed, and tested. Experimental results from tests performed both under vacuum and in air are presented. The startup behavior of the heat pipe from the frozen state was investigated for various heat loads and input locations, with both low and high heat rejection rates at the condenser. No startup evaporator dryout failures were found, although the heat pipe was sonic-limited during startup in air.

Superfluid transport and its applications in space

Various transport modes in superfluid helium are discussed in this paper. They include zero net mass flow (ZNMF) and finite mass flow (FMF) for pure superfluid and normal fluid flow. An attempt is made to characterize these transport modes in a common frame of reference. Two dimensionless numbers are used, namely the dimensionless heat flux number and the dimensionless driving force number. The equations are generalized by the use of a characteristic length so that they can be applied to the transport of He II in any geometry. The theories are then extended to applications in space. In particular, fountain effect pumps (FEPs) and superfluid management at zero g by vapour—liquid phase separators (VLPSs) will be discussed in detail. While transport in a phase separator is close to that of ZNMF, the flow in a FEP belongs to the FMF mode. The transport modes of the above systems using porous media are found to be strongly size dependent. For VLPSs, the heat rejection rate is proportional to the square root of plug permeability, K P. As in the case of FEPs, the volumetric flow rate is inversely proportional to K P. These findings are supported by ZNMF data in capillaries and FMF (gravitational flow) data in millipores, respectively.

Waste heat disposal from U.S. geothermal power plants—An update

Dissipation of the heat rejected from geothermal power plants is a major concern because the inherently low efficiencies result in heat rejection rates that are three to four times greater per kW of installed capacity than is typical of fossil- or nuclear-fueled stations. The most cost-effective methods of waste heat dissipation involve the evaporation of water, yet most of the important hydrothermal resources of the U.S. are located in areas where cooling tower makeup water for power plants is in short supply. Flashed-steam power cycles can use condensate derived from the geofluid for tower makeup unless reinjection is necessary, as is already required at some sites. Condensate is not available from binary cycles because the geofluid is reinjected. Geothermal station makeup water requirements have been estimated at 50–100 m 3/yr per kW of electrical capacity. Some of the more interesting and significant methods that are currently being studied in the U.S. for reducing waste heat dissipation system costs and water consumption are (1) allowing plant power output to vary with ambient conditions, (2) use of ammonia to transport waste heat from the turbine condenser to air-cooled coils, (3) development of a plastic-membrane type wet/dry tower, (4) marketing of steam turbines that can tolerate a wider range of back pressures, (5) use of circulating water storage to delay heat dissipation until more favourable ambient conditions exist, (6) development of tubes with enhanced heat transfer surfaces to reduce condenser capital costs, and (7) use of evaporative condensers to reduce costs in binary cycles. Many of these projects involve large-scale tests that are now installed and producing some preliminary data. Definitive results from some of the tests may not be available until mid-1982 or later.

Analysis of evaporative cooling and enhancement of condenser efficiency and of coefficient of performance

The graphical method to determine with the aid of a Mollier i - x diagram (psychrometric chart) combined heat and mass transfer is simulated by a computer program. Heat rejection rates from a plate-fin tube type condenser are determined for various flow rates and inlet state conditions of air and for different degrees of wetting of the heat transfer surfaces. The presence of water and the cooling by latent heat makes it possible to exchange more heat than the unwetted exchanger would even for idealized conditions of infinite heat transfer coefficient of the air. The evaporative cooled condenser also can exchange heat with ambient air which has much higher temperature than the condensing fluid. Evaporative cooling increases heat transfer by a factor of more than three for saturated inlet air and greater than five for lower inlet humidities. Wetted heat exchangers require less extended surfaces and can operate effectively with bare tubes only. Wetting the condenser of a refrigeration or heat pump system makes it possible to exchange the condenser load at lower temperatures. This yields an increase of COP of the order of 30 to 60% and therefore a substantial decrease in compressor power and its energy consumption.