Application of Hopfield Neural Network to the optimal chilled water supply temperature calculation of air-conditioning systems for saving energy

The values of chilled water supply temperatures of chillers indicate their load distributions due to the chilled water return temperatures of all chillers being the same in a decoupled air-conditioning system. This study employs Hopfield Neural Network (HNN) to find out the chilled water supply temperatures of chillers for solving Optimal Chiller Loading (OCL) problem. HNN overcomes the flaw that Lagrangian method is not adaptable for solving OCL as the power consumption models include non-convex functions. The chilled water supply temperatures are used as the variables to be solved for the decoupled air-conditioning system and solves the problem by using HNN method to improve this defect. After analysis and comparison of the case study, it has been concluded that this method not only solves the problem of Lagrangian method, but also produces results with high accuracy. It can be perfectly applied to the operation of air-conditioning systems.     

Optimal chilled water temperature calculation of multiple chiller systems using Hopfield neural network for saving energy

The values of chilled water supply temperatures in chillers indicate the load distributions as the chilled water return temperatures in all chillers are the same in a decoupled air-conditioning system. This study employs the Hopfield neural network (HNN) to determine the chilled water supply temperatures in chillers, which are used to solve the optimal chiller loading (OCL) problem. A linear input–output model is utilized as a substitute for the sigmoid function, which eliminates the shortcoming of the conventional HNN method. Notably, HNN overcomes the flaw in the Lagrangian method in that the latter cannot be utilized for solving the OCL problem as its power-consumption models include non-convex functions. The chilled water supply temperatures are used as variables to be solved for a decoupled air-conditioning system and solve the problem using the HNN method to overcome the defect in the Lagrangian method. After analysis of the case study and comparison of results using these two methods, we conclude that the HNN method solves the problem of the Lagrangian method, and produces highly accurate results. The HNN method can be applied to the operation of air-conditioning systems.     

Genetic algorithm based optimal chiller loading for energy conservation

This study employs genetic algorithm (GA) to solve optimal chiller loading (OCL) problem. GA overcomes the flaw that Lagrangian method is not suitable as there is non-convex kW-PLR function in a system. This study uses the part load ratios (PLR) of chiller units to binary code chromosomes, and execute reproduction, crossover and mutation operation. Since the semiconductor plant is the largest a/c load for power consumption, it is used as an example in this paper. After analysis and comparison of the case study, we are confident to say that this method not only solves the problem of Lagrangian method, but also produces results with high accuracy within a rapid timeframe. It can be perfectly applied to the operation of air conditioning systems.     

An innovative approach for demand side management—optimal chiller loading by simulated annealing

This investigation attempts to solve the optimal chiller loading (OCL) problem by utilizing simulated annealing (SA). SA eliminates the limitation that the Lagrangian method cannot solve OCL because the kW-PLR curves simultaneously include convex functions and non-convex functions. This study fully addresses the load balance constraint and chiller operating limits. The case study analysis demonstrates that this method solves the Lagrangian problem and generates highly accurate results. The proposed approach can be applied perfectly to the operation of air conditioning systems.     

Experimental study with operational solar‐sorption cooling

Building integrated solar systems have been considered as a reasonable system for building heating, cooling and hot water supply. Various types of solar collectors, such as plate type, evacuated tube type and solar air collector, have been used as the heat source, whereas adsorption chillers, absorption chillers and desiccant dehumidification systems have been considered to match the above solar heat sources. Now, such sorption chillers are more matured, but their coupling with suitable solar heat source is not well researched. Experimental study has been done in this paper to analyse four kinds of typical solar air‐conditioning system with different sorption chillers and solar collectors. Copyright © 2012 John Wiley & Sons, Ltd.     

A cost‐effective operating strategy to reduce energy consumption in a HVAC system

The operation of the building heating, ventilating, and air conditioning (HVAC) system is a critical activity in terms of optimizing the building's energy consumption, ensuring the occupants' comfort, and preserving air quality. The performance of HVAC systems can be improved through optimized supervisory control strategies. Set points can be adjusted by the optimized supervisor to improve the operating efficiency. This paper presents a cost‐effective building operating strategy to reduce energy costs associated with the operation of the HVAC system. The strategy determines the set points of local‐loop controllers used in a multi‐zone HVAC system. The controller set points include the supply air temperature, the supply duct static pressure, and the chilled water supply temperature. The variation of zone air temperatures around the set point is also considered. The strategy provides proper set points to controllers for minimum energy use while maintaining the required thermal comfort. The proposed technology is computationally simple and suitable for online implementation; it requires access to some data that are already measured and therefore available in most existing building energy management and control systems. The strategy is evaluated for a case study in an existing variable air volume system. The results show that the proposed strategy may be an excellent means of reducing utility costs associated with maintaining or improving indoor environmental conditions. It may reduce energy consumption by about 11% when compared with the actual strategy applied on the investigated existing system. Copyright © 2007 John Wiley & Sons, Ltd.     

Chillers energy consumption, energy savings and emission analysis in an institutional buildings

Chillers consume more than 40% of the total energy used in the commercial and industrial buildings for space conditioning. In this paper, energy consumption by chillers and chilled water pumps, condenser pumps and fan motors has been estimated using data collected by a walkthrough energy audit for the 16 faculties of the University of Malaya. It has been estimated that chillers and motors and pumps used in chillers consume 10,737 MWh (i.e. 51% of total energy consumption) of electric energy for different percentage of loadings. As chillers are major energy users, variable speed drives are applied in chillers to reduce their energy consumption. It has been estimated that about 8368 MWh annual energy can be saved by using efficient chillers at different loadings. It has also been found that about 23,532 MWh annual energy can be saved for chilled water supply pumps, condenser pumps and cooling tower fan motors by matching required speeds using variable speed drives for 60% of speed reduction. About 1,274,692 kg of CO₂ emission could be avoided for using energy efficient chillers at 50% load. It has been also found that about 2,426,769 kg CO₂ emission can be reduced by using variable speed drives for 60% speed reductions. Payback periods found to be only few months for using variable speed drives in chilled water pumps, condensers and fan motors.     

Low‐energy cooling of rooms with chilled ceilings and ceiling‐mounted devices

Energy‐efficient and comfortable cooling systems have recently gained considerable attention in removal of heat loads from large office spaces and industrial buildings. Here in this paper, somewhat similar heat load removal analysis is made for rooms with not so large dimensions. The ceiling of the room is chilled. The supply velocity is kept in small values. In order to enhance the circulation in the room, a wing‐like structure is mounted to the ceiling. A transient heat transfer problem involving conduction and mixed turbulent convection and relative humidity is studied with solving the 3‐D unsteady full Navier–Stokes equations numerically. CFD is resorted to as a tool for the flow analysis. The results show that using wing‐like structure in the room with chilled ceiling provides better ventilation and this approach does not need any chemical substances such as CFC. Copyright © 2005 John Wiley & Sons, Ltd.     

Economic dispatch of chiller plant by gradient method for saving energy

This study employs gradient method (GM) to solve economic dispatch of chiller plant (EDCP) problem. GM overcomes the flaw that with the Lagrangian multiplier (LM) method the system may not converge at low demand. In this study, the load balance constraint and the operating limit constraints of the chillers are fully accounted for. After analysis and comparison of the two cases studies, we are confident to say that this method not only solves the problem of convergence, but also produces results with high accuracy within a rapid timeframe. It can be perfectly applied to the operation of air-conditioning systems.     

Energy evaluation of optimal control strategies for central VWV chiller systems

Under various conditions, the actual load of the heating, ventilation and air conditioning (HVAC) systems is less than it is originally designed in most operation periods. To save energy and to optimize the controls for chilling systems, the performance of variable water volume (VWV) systems and characteristics of control systems are analyzed, and three strategies are presented and tested based on simulation in this paper. Energy evaluation for the three strategies shows that they can save energy to some extent, and there is potential remained. To minimize the energy consumption of chilling system, the setpoints of controls of supply chilled water temperature and supply head of secondary pump should be optimized simultaneously.     

Neuro-optimal operation of a variable air volume HVAC&R system

Low operational efficiency especially under partial load conditions and poor control are some reasons for high energy consumption of heating, ventilation, air conditioning and refrigeration (HVAC&R) systems. To improve energy efficiency, HVAC&R systems should be efficiently operated to maintain a desired indoor environment under dynamic ambient and indoor conditions. This study proposes a neural network based optimal supervisory operation strategy to find the optimal set points for chilled water supply temperature, discharge air temperature and VAV system fan static pressure such that the indoor environment is maintained with the least chiller and fan energy consumption. To achieve this objective, a dynamic system model is developed first to simulate the system behavior under different control schemes and operating conditions. A multi-layer feed forward neural network is constructed and trained in unsupervised mode to minimize the cost function which is comprised of overall energy cost and penalty cost when one or more constraints are violated. After training, the network is implemented as a supervisory controller to compute the optimal settings for the system. Simulation results show that compared to the conventional night reset operation scheme, the optimal operation scheme saves around 10% energy under full load condition and 19% energy under partial load conditions.     

螺杆式冷水机组群控策略

叙述了螺杆式冷水机组在冷冻/冷却水系统定流量情况下,低负荷或高负荷时,能效比均较低的特点,提出了一种在保证机组稳定、可靠运行的基础上,以系统总能效比为目标的螺杆式冷水机组群优化控制策略。     

余热回收吸收式制冷集中空调方案研究

为回收大型炼油装置丰富的低温余热用于生活区的夏季集中空调,本文对余热资源状况和生活区集中空调建筑物供冷负荷分别进行了综合标定和统计计算,在分析比较的基础上确定采用余热热水型溴化锂吸收式制冷冷水机组和半集中式风机盘管全水型空调系统,并利用现有双管制热水集中供暖管网和泵站输送空调冷水,可以显著地节约投资。技术经济分析表明,该方案在技术上可行,节能效果和经济效益十分显著,而且可以大大改善厂区的热污染和居民的生活条件。     

An experimental study on multi‐purpose desiccant integrated vapor‐compression air‐conditioning system

In this paper, a multi‐purpose hybrid desiccant integrated vapor‐compression air‐conditioning system of a small capacity is experimentally investigated. The system, referred as hybrid desiccant‐assisted air conditioner (HDAC), is designed to meet the cooling load of spaces having large latent heat portions and at the same time to extract water from atmospheric air. The system is mainly consisted of a liquid‐desiccant dehumidification unit integrated with a vapor‐compression system (VCS). The dehumidification unit uses lithium chloride (LiCl) solution as the working material. The effect of different parameters, such as desiccant solution flow rate, process airflow rate, evaporator and condenser temperatures, strong solution concentration and regeneration temperature on the performance of the system, is studied. This system has a water recovery rate of 6.7 l/h TR (1.91 l/h kW) of pure water at typical north Egyptian climate (20–30°C dry bulb and 35–45% relative humidity). The HDAC system has a COP as high as 3.8 (an improvement of about 68% over the conventional VCS). The system offers a total cooling capacity of about 1.75 TR (6.15 kW) using a 0.75 TR (2.6 kW) VCS unit. Finally, the proposed system is found to have a payback time of about 10 months without any considerable extra capital cost compared with the known split air‐conditioning system. The results emphasize the potential benefits of the HDAC system. Copyright © 2010 John Wiley & Sons, Ltd.     

Optimal chiller sequencing by branch and bound method for saving energy

This paper proposes a method for using the branch and bound (B&B) method to solve the optimal chiller sequencing (OCS) problem and to eliminate the deficiencies of conventional methods. The coefficient of performance (COP) of the chiller is adopted as the objective function because it is concave. The Lagrangian method determines the optimal chiller loading (OCL) in each feasible state. The potential performance of the proposed method is examined with reference to an example system. The proposed method consumes much less power than the conventional method and is very appropriate for application in air conditioning systems.     

Performance analysis of combined microgas turbines and gas fired water/LiBr absorption chillers with post-combustion

The integration of microgas turbines (MGT) and absorption chillers is an emerging technology that uses a wide range of fuels to produce electricity, cooling and heating simultaneously for small scale distributed generation in grid connected or isolated locations.This paper studies the performance of MGTs of different power capacities directly coupled to double-effect water–LiBr absorption chillers. In these systems the MGT exhaust gas is the heating medium to drive the chiller. Also post-combustion natural gas is used to increase the cooling capacity of the system. The paper analyses the effect of the post-combustion degree on the integrated system performance of four MGT power sizes. Two cases are considered. In the first, fresh air is added together with the post-combustion natural gas and in the second it is not added. In the latter case the oxygen necessary for the combustion reaction is extracted from the MGT exhaust gas stream. For the sake of comparison a study is also made of the performance of a more conventional system consisting of an MGT and a hot water heat exchanger to drive an absorption chiller. The main advantages of the new technology over this conventional system are that the COP of the chillers is higher because they are driven by higher temperatures, the production of electricity and chilled water is decoupled and there is a wider range of chilled water production capacity.     

Applications of Thermal Energy Storage in Saudi Arabia

In Saudi Arabia, the heating, ventilating and air conditioning (HVAC) system typically accounts for 65% of the total electrical energy consumption in buildings. This is due to a very high ambient temperature which persists for a long period of time in a summer season. Moreover, gas turbines efficiency decrease also with the high ambient temperatures. In the HVAC industry cool storage, or commonly known as Thermal Energy Storage (TES) is the most preferred demand side management (DSM) technology for shifting cooling electrical demand from peak daytime periods to off‐peak night‐time. The most popular and well‐suited TES concept for Saudi Arabia is either chilled water or ice storage system, depending upon the applications and the required storage capacity. This paper shows how TES offers a means of reducing the electrical demand in large commercial buildings. Additionally, it is seen that efficiencies of the air cooled chillers are increased if they run overnight. Similarly efficiencies of gas turbine is also increased when a TES based pre‐cooled air is used as an inlet to the turbine. This paper also discusses favouring conditions and other aspects of cool storage applications in Saudi Arabia. TES economics are considered and a cost analysis is presented to illustrate the potential savings that can be achieved by the use of TES in Saudi Arabia. Copyright © 1999 John Wiley & Sons, Ltd.     

Improved energy performance of air cooled centrifugal chillers with variable chilled water flow

This paper considers how to apply optimum condensing temperature control and variable chilled water flow to increase the coefficient of performance (COP) of air cooled centrifugal chillers. A thermodynamic model for the chillers was developed and validated using a wide range of operating data and specifications. The model considers real process phenomena, including capacity control by the inlet guide vanes of the compressor and an algorithm to determine the number and speed of condenser fans staged based on a set point of condensing temperature. Based on the validated model, it was found that optimizing the control of condensing temperature and varying the evaporator’s chilled water flow rate enable the COP to increase by 0.8–191.7%, depending on the load and ambient conditions. A cooling load profile of an office building in a subtropical climate was considered to assess the potential electricity savings resulting from the increased chiller COP and optimum staging of chillers and pumps. There is 16.3–21.0% reduction in the annual electricity consumption of the building’s chiller plant. The results of this paper provide useful information on how to implement a low energy chiller plant.     

Solar-powered systems for cooling, dehumidification and air-conditioning

This paper describes current trends in solar-powered air conditioning, which has seen renewed interest in recent years due to the growing awareness of global warming and other environmental problems. Closed-cycle heat-powered cooling devices are based mainly on absorption chillers, a proven technology employing LiBr–water as the working fluid pair. Recent developments in gas-fired systems of this type make available double- and triple-effect chillers with considerably higher COP than their single-effect counterparts, which makes it possible to reduce the amount of solar heat required per kW of cooling. These systems require, however, high-temperature solar collectors. The principles of multi-staging absorption systems are described. An economic comparison is provided which shows the total system cost to be dominated by the solar part of the system. At current prices, the high COP, high temperature alternative is still more costly than the low temperature one. Open-cycle desiccant systems employing either solid or liquid sorbents are described. While the main thrust in research on novel closed-cycle absorption systems has been toward increasing the operating temperature in order to improve efficiency through multi-staging, open-cycle absorption and desiccant systems have been developed for use with low temperature heat sources such as flat plate solar collectors. A novel open-cycle (DER) system is described, which makes it possible to use the solar heat at relatively low temperatures, for producing both chilled water and cold, dehumidified air in variable quantities, as required by the load.