Shiming Deng

A numerical analysis of heat and mass transfer inside a reversibly used water cooling tower

Abstract In subtropical regions, a desuperheater heat recovery system for service hot-water heating can be applied. However, in colder seasons when building cooling load is reduced, a standard water-cooling tower may be reversibly used to extract free heat from ambient air to make up the reduction of heat source for water heating. Previous related work included developing an analytical method for evaluating the heat and mass transfer characteristics in a reversibly used water-cooling tower (RUWCT), which cannot be used to determine the air and water states at any intermediate horizontal sections along the tower height within an RUWCT. This paper presents a detailed numerical analysis by which the air and water states at any horizontal plane along the tower height within an RUWCT can be determined. The numerical analysis has been partially validated using the experimental data from an installed RUWCT in a hotel building in southern China. The numerical analysis reported in this paper, together with the Analytical Method previously developed, provides a complete method for the analysis of heat and mass transfer characteristics within and at the boundaries of an RUWCT.

Theoretical analysis of low-temperature hot source driven two-stage LiBr/H2O absorption refrigeration system

Abstract A detailed theoretical analysis is presented for a two-stage LiBr/H2O absorption refrigeration system, which consists of an evaporator, a low-pressure absorber, a low-pressure generator, a high-pressure absorber, a high-pressure generator, a condenser, a low-pressure heat exchanger and a high-pressure heat exchanger, driven by a low-temperature hot source. A comparison of results from the theoretical analysis and preliminary experiment indicates that the theoretical analysis developed can represent a real system with a reasonable accuracy, and is useful for future development.

Performance Evaluation of a Nanofluid-Based Direct Absorption Solar Collector with Parabolic Trough Concentrator

Application of solar collectors for hot water supply, space heating, and cooling plays a significant role in reducing building energy consumption. For conventional solar collectors, solar radiation is absorbed by spectral selective coating on the collectors’ tube/plate wall. The poor durability of the coating can lead to an increased manufacturing cost and unreliability for a solar collector operated at a higher temperature. Therefore, a novel nanofluid-based direct absorption solar collector (NDASC) employing uncoated collector tubes has been proposed, and its operating characteristics for medium-temperature solar collection were theoretically and experimentally studied in this paper. CuO/oil nanofluid was prepared and used as working fluid of the NDASC. The heat-transfer mechanism of the NDASC with parabolic trough concentrator was theoretically evaluated and compared with a conventional indirect absorption solar collector (IASC). The theoretical analysis results suggested that the fluid’s temperature distribution in the NDASC was much more uniform than that in the IASC, and an enhanced collection efficiency could be achieved for the NDASC operated within a preferred working temperature range. To demonstrate the feasibility of the proposed NDASC, experimental performances of an NDASC and an IASC with the same parabolic trough concentrator were furthermore evaluated and comparatively discussed.

A simulation study on a water chiller complete with a desuperheater and a reversibly used water cooling tower (RUWCT) for service hot water generation

Abstract In sub-tropical regions, a standard water cooling tower may be reversibly used, as part of a desuperheater heat recovery system for service hot water heating, to extract free heat from ambient air in colder seasons when building cooling load is reduced. Part of chilled water is pumped into a RUWCT where it is heated by warmer ambient moist air. This paper presents a simulation study where a steady-state mathematical model for such a desuperheater heat recovery system complete with a RUWCT has been developed. Simulation results based on the specifications of an actual chiller plant have demonstrated that the model developed is stable and behaves as expected. With the model developed, the operating characteristics of the refrigeration system with a desuperheater and a RUWCT were studied. The required flow rate of chilled water to be pumped into the RUWCT was calculated in order to satisfy a certain heat load. The maximum heating capacity of the system under different operating conditions were also evaluated. The simulation results also indicated that the use of a RUWCT would achieve a higher energy efficiency than the use of electrical heating as backup heat provisions when building space cooling load is reduced.

Parameter optimization for operation of a bed-based task/ambient air conditioning (TAC) system to achieve a thermally neutral environment with minimum energy use

The purpose of this paper is to optimize the operating parameters such as: supply air temperature, supply air flow rate and supply air humidity, for a bed-based task/ambient air conditioning (TAC) system installed in an experimental bedroom with two total insulation values of beddings and bed to obtain a thermally neutral sleeping environment with minimum energy use. A computational fluid dynamics (CFD) method was applied to calculate the values of predicted mean vote (PMV) and energy utilization coefficient (EUC) based on conditions of 16 simulation cases. From the simulation results, the design of experiment method was applied to identify operating parameters, individually or combined, that could significantly affect thermal neutrality and energy use for the bedroom environment to establish linear regression models for PMV and EUC. These models were used to obtain the optimum operating parameters of the bed-based TAC system. These models were validated by comparing the obtained optimization results using the models with the predicted results given by the CFD method. The results suggested that for a bed-based TAC system at a specified total insulation value of beddings and bed, a lower energy use can be achieved through adjusting operating parameters while the thermal comfort is maintained.

The application of feedforward control in a direct expansion (DX) air conditioning plant

A computerized feedforward controller for a DX air conditioning plant has been developed to avoid the use of problematic measurement of air conditions immediately downstream of a cooling coil for control action. The feedforward controller was based on a mathematical model for capacity control using the principle of energy conservation. A safety control procedure was also included as part of the control algorithms to ensure the safe operation of the plant. Experimental tests on the controllability of the controller confirmed that both control accuracy and sensitivity could be achieved. The control algorithms involved is expected to have wide application for controlling DX air conditioning systems.

A new capacity controller for a direct expansion air conditioning system for operational safety and efficiency

For achieving a balance between the operational safety and efficiency of a variable speed direct expansion air conditioning system, a new capacity controller that can not only simultaneously control indoor air temperature and humidity, but also select an optimized degree of superheat setting to properly balance the operational safety and efficiency has been developed, by adding a control module to a previously developed capacity controller. The core of the control module was an artificial neural network based model representing the known relationship between the inherent operational characteristics and operational stability of a variable speed direct expansion air conditioning system. Using the control module developed, the degree of superheat setting could be varied in accordance with the variation in operating conditions for ensuring a safe and an efficient operation. The performance of the new capacity controller was experimentally tested using an experimental variable speed direct expansion air conditioning system. Controllability test results showed that the system hunting was mitigated when a larger degree of superheat setting was selected by the developed control module and an improvement in COP at about 3.4% was achieved when a smaller one was selected when using the new capacity controller for simultaneously controlling indoor air temperature and humidity. Practical application : This paper reports a new capacity for a VS DX A/C system, which can not only control indoor air temperature and humidity through varying compressor and supply fan speeds, but also select an optimized DS setting for ensuring both a safe and efficient system operation. The controllability test results showed that the new capacity controller could be applied to DX A/C systems for indoor thermal control with a higher operational safety and energy efficiency.

Performance of a Solar-Air Source Heat Pump System for Water Heating on Different Weather Conditions

A new type of solar-air source heat pump water heating system was developed for space heating and residential hot water supply. The specially designed multi-source collector/evaporator (C/E) using spiral-finned tubes could absorb heat from both solar radiation and ambient air. A prototype was designed and tested on different weather conditions in October in Nanjing, China. The experimental results showed that the prototype could heat water from 40degC up to 55degC efficiently both in the daytime and at night. It operated in the solar-source mode with a high averaged COP of 5.1 in the daytime when the solar radiation was 731 W/m 2 . Meanwhile, it operated in the air-source mode stably with a averaged COP of 3.5 at night, avoiding using electricity as a backup energy source.

An alternative general method to evaluate the atmospheric down-welling radiation

The importance of atmospheric down-welling radiation in studying building thermal environments and energy performance has been well identified, and empirical formulae to evaluate atmospheric down-welling radiation values developed with their inadequacies. In this paper, a study of developing an alternative general method for evaluating atmospheric down-welling radiation values to the water pond-based empirical formula by Clark and Allen is reported. The validity of the alternative general method has been demonstrated by comparing the atmospheric down-welling radiation values evaluated using the alternative general method and that using the Clark and Allen’s formula. Since the alternative general method developed is based on a building roof system, and no water is involved, the alternative general method developed in this paper appears to have more advantages. On one hand, this could eliminate any potential impacts on prediction accuracy when water is used in different climates. On the other hand, the application of the alternative general method is relatively easier as only a suitable existing roof system is required. Therefore, the use of alternative general method can provide a more reliable and economic alternative in evaluating atmospheric down-welling radiation, when compared to using Clark and Allen’s empirical formula. Practical application The alternative general method developed for evaluating atmospheric down-welling radiation values in this paper could be used as an alternative to the empirical formula proposed by Clark and Allen and is valid for all climate conditions and easy to be implemented to evaluate alternative general method values. It can be used when evaluating building night sky cooling, building passive cooling system, etc.

Optimum Network Sizing for Delivering Different Media

This paper aims at fundamentally finding optimal shape of networks used in delivering different distribution networks from one supplier plant to a number of end-users in order to minimize power loss in delivery process under a constraint of total flow volume. A general equation is developed for evaluating the power loss for different shape networks. Based on the equation, a parameter, γ, is deduced which can be applied to define the volume flow into three classifications. For each flow classification, the optimal shape network will be discussed. Results indicate that the radial shape will be optimum when γ ≥ 1. When γ<1, an optimal tree shape network is deduced for evenly distributed end-users on a square.Copyright