冷冻水系统优化控制方法

介绍了确定冷冻水系统优化控制的两种方法:基于设备的优化和基于系统的优化。处理优化问题的手段,如二次费用和线性输出,独立控制变量的确定等,对于冷冻水系统的优化控制具有一定的借鉴意义。     

An entransy dissipation-based optimization principle for building central chilled water systems

The recently developed entransy theory is introduced in this paper to tackle the heat transfer processes in building central chilled water systems so as to improve their energy efficiency. We first divide the irreversible heat transfer processes into four categories: (1) air mixing processes; (2) heat transfer processes between chilled water and air; (3) chilled water mixing processes; and (4) heat transfer processes between chilled water and refrigerant. The formulas of entransy dissipation rates for each irreversible process are derived, and then the total entransy dissipation rate in the whole chilled water systems is obtained, which connects the geometrical structures of each heat exchanger and the operating parameters of each fluid directly to the demands of users and the supply of refrigerating unit. Based on the formula of entransy dissipation rate together with the conditional extremum method in mathematics, two optimization equation groups are deduced theoretically. Simultaneously solving such equation groups will easily find the optimal central chilled water system with the highest energy efficiency. Finally, a simple building central chilled water system with two users is taken as an example to illustrate the applications of the newly proposed optimization principle.     

An imperialist competitive algorithm for optimal design of plate-fin heat exchangers

This paper presents the application of imperialist competitive algorithm (ICA) for optimization of a cross-flow plate fin heat exchanger. Minimization of total weight and total annual cost are considered as objectives. Seven design parameters, namely, heat exchanger length at hot and cold sides, fin height, fin frequency, fin thickness, fin-strip length and number of hot side layers are selected as optimization variables. A case study from literature is presented to show the effectiveness of the proposed algorithm. The numerical results reveal that ICA can find optimum configuration with higher accuracy in less computational time when compared to conventional genetic algorithm (GA).     

Improved model control strategy with dynamic adaptation for heat exchangers in energy system

We propose a novel model control with dynamic adaptation to improve the control accuracy and speed of heat exchangers. The proposed method first applies an accurate mathematical model of heat exchanger as a motion model to create the inverse problem algorithm of manipulated variable. Subsequently, it improves the transient performance using a dynamic adaption of manipulated variable. Compared the control performance to feedback control method, the dynamic adaptation of manipulated variable fed in an exponential function resulted in both approximately a 100% reduction in overshoot and settling time elimination in response to inlet temperature and flow rate disturbances of objective fluid.     

Potential of geothermal heat exchangers for office building climatisation

Low depth geothermal heat exchangers can be efficiently used as a heat sink for building energy produced during summer. If annual average ambient temperatures are low enough, direct cooling of a building is possible. Alternatively the heat exchangers can replace cooling towers in combination with active cooling systems. In the current work, the performance of vertical and horizontal geothermal heat exchangers implemented in two office building climatisation projects is evaluated.A main result of the performance analysis is that the ground coupled heat exchangers have good coefficients of performance ranging from 13 to 20 as average annual ratios of cold produced to electricity used. Best performance is reached, if the ground cooling system is used to cool down high temperature ambient air. The maximum heat dissipation per meter of ground heat exchanger measured was lower than planned and varied between 8 W m^?1 for the low depth horizontal heat exchangers up to 25 W m^?1 for the vertical heat exchangers.The experimental results were used to validate a numerical simulation model, which was then used to study the influence of soil parameters and inlet temperatures to the ground heat exchangers. The power dissipation varies by ±30% depending on the soil conductivity. The heat conductivity of vertical tube filling material influences performance by another ±30% for different materials. Depending on the inlet temperature level to the ground heat exchanger, the dissipated power increases from 2 W m^?1 for direct cooling applications at 20 °C up to 52 W m^?1 for cooling tower substitutions at 40 °C. This directly influences the cooling costs, which vary between 0.12 and 2.8€ kW h^?1.As a result of the work, planning and operation recommendations for the optimal choice of ground coupled heat exchangers for office building cooling can be given.     

Analysis of horizontal mantle heat exchangers in solar water heating systems

The characteristics of horizontal mantle heat exchangers are investigated for application in thermosyphon solar water heaters. An experimental model of a horizontal mantle heat exchanger was used to evaluate the flow patterns in the annular passageways and the heat transfer into the inner tank. Flow visualisation was used to investigate the flow structure, and the heat transfer was measured for isothermal inner tank conditions. A numerical model of the flow and heat transfer in the annular passageway was developed and used to evaluate the heat flux distribution over the surface of the inner tank. The numerical results indicate that configurations of mantle heat exchangers used in current solar water heater applications degrade thermal stratification in the inner tank. The effects of inlet flow rate, temperature and connecting port location are quantified.     

Computer simulation of borehole ground heat exchangers for geothermal heat pump systems

Computer simulation of borehole ground heat exchangers used in geothermal heat pump systems was conducted using three-dimensional implicit finite difference method with rectangular coordinate system. Each borehole was approximated by a square column circumscribed by the borehole radius. Borehole loading profile calculated numerically based on the prescribed borehole temperature profile under quasi-steady state conditions was used to determine the ground temperature and the borehole temperature profile. The two coupled solutions were solved iteratively at each time step. The simulated ground temperature was calibrated using a cylindrical source model by adjusting the grid spacing and adopting a load factor of 1.047 in the difference equation. With constant load applied to a single borehole, neither the borehole temperature nor the borehole loading was constant along the borehole. The ground temperature profiles were not similar at different distances from the borehole. This meant that a single finite difference scheme was not sufficient to estimate the performance of a borefield by superposition. The entire borefield should be discretized simultaneously. Comparison was made between the present method and the finite line source model with superposition. The discrepancies between the results from the two methods increased with the scale of borefield. The introduction of time schedule revealed a discrepancy between the load applied to the ground heat exchanger and that transferred from the borehole to the ground, which was usually assumed to be the same when using analytical models. Hence, in designing a large borefield, the present method should give more precise results in dynamic simulation.     

海水制冷热泵系统的海水换热方案分析

对海水制冷热泵系统的原理与特点进行了简要的概述,给出了三种海水制冷热泵系统的海水换热方案,并对三种方案进行了初步技术经济分析,为海水制冷热泵系统的设计与应用提供参考。     

Natural convection heat exchangers in solar water heating systems: Theory and experiment

Natural convection heat exchangers can be used in solar hot water systems to replace the pump on the tank side of the exchanger. There is currently no experimentally verified way of designing this type of heat exchanger. An experimental apparatus to test natural convection heat exchangers was built and an extensive set of measured data obtained on two different exchangers sized for low-flow stratified tank system. Two theoretical models for the exchanger are presented: a finite-volume primitive variable numerical solution of the fundamental laminar equations of fluid motion and a laminar forced-convection-based solution method. Comparison of the model predictions with the experimental data showed good agreement when the modified Rayleigh number is less than about 400. The poor agreement under other conditions was attributed to turbulence and recirculation neither of which was accounted for in the models.     

Improved control strategy for forced flow solar water heating systems

The solar water heating system functioning on a fixed temperature control (FTC) mode, rather than a differential temperature control (DTC) mode (continuous water flow rate), has appreciable advantages for saying parasitic power consumption and reducing the maintenance cost. A computer simulation model, using a numerical analysis method, has been developed to study a forced flow solar water heating system taking into account the realistic conditions of operations. It is noted that the most critical parameter in designing such types of system is the water flow rate. The results are obtained, corresponding to a solar water heating system of 4000 1/day capacity working at an average temperature of 60°C.     

The chilled water storage analysis for a university building cooling system

In this paper, a methodology is presented to determine the optimal chilled water storage (CWS) capacity and corresponding operating strategy for the air conditioning loads for different electricity tariff. Several scenarios were studied for the KMUTNB air conditioning system. It was concluded that the CWS with a chiller of 450 RT (2 units) running continuously, thermal-energy storage of 9413 RT-h and 5175 m³ volume was most suitable. The mechanical chiller (MAC) capacity and peak demand could be decreased by over 2 times and 31.2% respectively. It can move the energy consumption from the on peak to the off peak periods by 35.7%. The economics result showed PB, IRR and NPV are 10 y, 21% and 0.834 MUS$ compared with the existing system.     

Reduction in the surface tension of water due to physical water treatment for fouling control in heat exchangers

The purpose of the present study was to investigate whether or not a physical water treatment (PWT) reduced the surface tension of hard water. Two different PWT devices were used: a permanent magnet—Drexel University (PMDU) and a solenoid coil electronic device (SCED). The effects of the treatment number of the PWT on the surface tension were studied. Two separate experiments were conducted: one was the measurement of surface tension, and the other was a flow-visualization of dye behavior in water samples. As the number of treatments of the PWT increased, the surface tension of the sample water decreased, a phenomenon that was consistent with the results in the dye flow-visualization experiment.     

A generalized multifluid optimal pressure for heat exchangers operating with supercritical fluid

Distinct thermophysical properties encountered in supercritical fluids operating near the critical point have made them strong candidates for working fluids in various engineering applications. Particularly due to the existence of heat capacity maxima near the critical point, heat transfer involving supercritical fluids and their employment in power generation systems have received special attention. In this paper, the existence of optimal operating pressures that maximize the global conductance of supercritical heat exchangers is demonstrated. Analysis of the behavior of the isobaric specific heat along the heat transfer process shows that optimal performance is achieved when the average isobaric specific heat is maximized. Consequently, optimal pressure maps can be created to assist heat exchanger design for various combinations of inlet temperatures and heat transfer rates. Furthermore, it is demonstrated that simple dimensionless groups can correlate—with a mean absolute error (MAE) of 0.0332—the optimal operating pressures of up to 122 different fluids. In addition, it is shown that the correlation is even stronger closer to the critical point and for separate classes of fluids, where MAE can be as low as 0.0103 for triatomic substances.     

Identifying optimal cleaning cycles for heat exchangers subject to fouling and ageing

Fouling of heat exchangers causes reduced heat transfer and other penalties. Regular cleaning represents one widely used fouling mitigation strategy, where the schedule of cleaning actions can be optimised to minimise the cost of fouling. This paper investigates, for the first time, the situation where there are two cleaning methods available so that the mode of cleaning has to be selected as well as the cleaning interval. Ageing is assumed to convert the initial deposit, labelled ‘gel’, into a harder and more conductive form, labelled ‘coke’, which cannot be removed by one of the cleaning methods. The second method can remove both the gel layer and the coke layer, but costs more and requires the unit to be off-line longer for cleaning. Experimental data demonstrating the effects of ageing are presented. The industrial application is the comparison of cleaning-in-place methods with off-line mechanical cleaning. A process model is constructed for an isolated counter-current heat exchanger subject to fouling, where ageing is described by a simple two-layer model. Solutions generated by an NLP-based approach prove to be superior to a simpler heuristic. A series of case studies demonstrate that combinations of chemical and mechanical cleaning can be superior to mechanical cleaning alone for certain combinations of parameters.     

Irreversibility minimization of heat exchangers for transcritical CO2 systems

Irreversibility analyses of both evaporator and gas cooler of a CO₂ based transcritical heat pump for combined cooling and heating, employing water as the secondary fluid, have been reported. The analysis includes both operational and material associated irreversibilities. Optimization of heat exchanger tube diameter and length and effect of design parameters on overall system performance is also presented. Results clearly show that higher heat transfer coefficient can be achieved by reducing the diameter only to a limited extent due to rapid increase in pressure drop. The minimum possible diameter depends on mass flow rate (capacity) and division of flow path. The right combination of optimum diameter and length depends on the number of passes, capacity and operating parameters. It is noteworthy that due to higher pressure drop occurring in the evaporator compared to the gas cooler, zero temperature approach is attained before the optimum length is reached in case of the evaporator. Presented results are expected to help choose effective heat exchanger size in terms of diameter, length and number of passes.     

An adaptive feedback linearization strategy for variable speed wind energy conversion systems

This paper presents a control strategy based on adaptive feedback linearization intended for variable speed grid‐connected wind energy conversion systems (WECS). The proposed adaptive control law accomplishes energy capture maximization by tracking the wind speed fluctuations. In addition, it linearizes the system even in the presence of turbine model uncertainties, allowing the closed‐loop dynamic behaviour to be determined by a simple tuning of the controller parameters. Particularly, the attention is focused on WECS with slip power recovery, which use a power conversion stage as a rotor‐controlled double‐output induction generator. However, the concepts behind the proposed control strategy are general and can be easily extended to other WECS configurations. Copyright © 2000 John Wiley & Sons, Ltd.     

An optimal algorithm for drilling strategy

A mathematical model is constructed to determine the optimum drilling strategy once a geophysical anomaly has been detected and the decision to drill has been made. The model deals essentially with an orebody of massive structure in two dimensions. A numerical example is presented to illustrate the impact of parameters on optimal drilling depth.     

Mantle heat exchangers for horizontal tank thermosyphon solar water heaters

This paper describes the characteristics of horizontal mantle heat exchangers for application in thermosyphon solar water heaters. A new correlation for heat transfer in horizontal mantle heat exchangers with bottom entry and exit ports was used to predict the overall heat transfer and stratification conditions in horizontal tanks with mantle heat exchangers. The model of a mantle heat exchanger tank was combined with the thermosyphon solar collector loop model in TRNSYS to develop a model of a thermosyphon solar water heater with collector loop heat exchanger. Predictions of stratification conditions in a horizontal mantle tank are compared with transient charging tests in a laboratory test rig. Predictions of daily energy gain in solar preheaters and in systems with in-tank auxiliary boosters are compared with extensive outdoor measurements and the model is found to give reliable results for both daily and long-term performance analysis.     

An approach for the optimum design of heat exchangers

In this paper, an approach for the optimum design of heat exchangers has been presented. Traditional design method of heat exchangers involves many trials in order to meet design specifications. This can be avoided through the present design method, which takes the minimization of annual total cost as a design objective. In alternative optimum design methods, such as Lagrange multiplier method, by changing one variable at a time and using a trial–error or a graphical method, optimum results are obtained in a long time. In the present design optimization problem, the total annual cost has been taken as the objective function and heat balance, and rate equation have been taken as equal constraint. The method using the penalty function transforms the constrained problem into a single unconstrained problem. To solve the optimal problem, the method of steepest descent has been used. Initial design variables include the tube‐inside coefficient of heat transfer, tube‐outside coefficient of heat transfer, temperature difference and outside tube area of heat transfer. The changes in variables are considered simultaneously to reach an optimum solution. The results show that the present approach is a powerful tool for optimum design of heat exchangers and is expected to be beneficial to energy industry. Copyright © 2004 John Wiley & Sons, Ltd.