Victor I. Hanby

Evolutionary Synthesis of HVAC System Configurations: Algorithm Development (RP-1049)

This paper describes the development of a model-based optimization procedure for the synthesis of novel heating, ventilating, and air-conditioning system configurations. The optimization problem can be considered as having three suboptimization problems: the choice of a component set; the design of the topological connections between the components; and the design of a system operating strategy. In an attempt to limit the computational effort required to obtain a design solution, the approach adopted in this research is to solve all three subproblems simultaneously. The computational effort has been further limited by implementing simplified component models and including the system performance evaluation as part of the optimization problem (there being no need, in this respect, to simulate the system performance). The optimization problem has been solved using a Genetic Algorithm (GA) that has data structures and search operators specifically developed for the solution of HVAC system optimization problems. The performance of the algorithm and various search operators has been examined for a two-zone optimization problem, the objective of the optimization being to find a system design that minimizes system energy use. In particular, the performance of the algorithm in finding feasible system designs has been examined. It was concluded that the search was unreliable when the component set was optimized, but if the component set was fixed as a boundary condition on the search, then the algorithm had an 81% probability of finding a feasible system design. The optimality of the solutions is not examined in this paper but is described in an associated publication (Wright and Zhang 2008). It was concluded that, given a candidate set of system components, the algorithm described here provides an effective tool for exploring the design of novel HVAC systems.

Automatic design synthesis and optimization of component-based systems by evolutionary algorithms

A novel approach for automatic design synthesis and optimization using evolutionary algorithms (EA) is introduced in the paper. The approach applies to component-based systems in general and is demonstrated with a heating, ventilating and air-conditioning (HVAC) systems problem. The whole process of the system design, including the initial stages that usually entail significant human involvement, is treated as a constraint satisfaction problem. The formulation of the optimization process realizes the complex nature of the design problem using different types of variables (real and integer) that represent both the physical and the topological properties of the system; the objective is to design a feasible and efficient system. New evolutionary operators tailored to the component-based, spatially distributed system design problem have been developed. The process of design has been fully automated. Interactive supervision of the optimization process by a human-designer is possible using a specialized GUI. An example of automatic design of HVAC system for two-zone buildings is presented.

Energy Aspects of HVAC System Configurations — Problem Definition and Test Cases

This paper reports on the energy implications of HVAC system configuration by analyzing the energy balance and psychrometrics of typical and innovative systems. Three criteria were shown to be significant: (1) the ability to minimize outside air load, (2) the ability to eliminate simultaneous cooling and heating and use mixing effectively, and (3) the availability of interzonal airflow. Configurations that meet these criteria would be able to deliver the desired indoor air quality with reduced energy consumption. The performance of ten two-zone system configurations, including single-duct, dual-duct, fan-coil-based variations, and other specialized systems in the literature, were analyzed for a number of operational conditions. The results confirmed that fan-coil-based configurations with interzonal airflow paths perform better than other configurations. The conclusion of this study may be used as a guideline for multi-zone system designs.

The optimal design for a ground cooling tube in a hot, arid climate:

In many hot, arid climates there is a high level of use of domestic air conditioning which can make heavy demands on local electricity generation. A passive strategy which could mitigate this demand is the deployment of ground cooling tubes in which external air is drawn into the building/plant via a tube buried below ground. Whilst this principle has been known of for a considerable time, and many example installations described, there has been no systematic investigation of the influence of the design parameters on the viability of the system. The paper describes a method for carrying out a systematic parametric optimization for a ground cooling tube by coupling a validated numerical model of the system with a constrained optimization method using an evolutionary strategy. Optimizations were based on a thermo-dynamic objective function (minimizing external energy consumption of the whole system) and an economic function (payback time for the tube). Whilst the results are specific to the hot, arid climate of the state of Kuwait, the methodology has universal applicability and illustrates the functional power of combining a performance model with an optimization method in producing optimized designs of energy systems. Practical application: Building and plant simulation programs are becoming increasingly accepted as a design tool: for confirming the performance of a proposed design or for carrying out ‘what if?’ studies to evaluate the effects of varying design parameters or equipment selection. This paper illustrates that, provided the design problem can be modelled in all relevant aspects, linking the simulation program to a constrained optimization can provide significantly more decision support to the designer.

Nodal network and CFD simulation of airflow and heat transfer in double skin facades with blinds

The paper describes a modelling study of heat transfer and buoyancy-driven airflow in double skin facades consisting of a glass outer layer, a control device (venetian blind) and a double-glazed inner skin. The modelling study was based on two approaches — a component-based, lumped parameter simulation which used a public domain, open source differential/algebraic equation solver and a detailed, CFD calculation which included air flow, conduction, convection and radiation. The primary objective of the work was to compare the performance of the simplified model with the output of a rigorous CFD calculation.

A Robust Evolutionary Algorithm for HVAC Engineering Optimization

It is common to make use of suitable plant and energy simulation programs, like TRNSYS (SEL 2000) or EnergyPlus (DOE 2008), to build up heating, ventilating, and air-conditioning (HVAC) problems for in-depth study. Optimization is a useful approach to find out the suitable parameters for system design and energy management of HVAC systems. It is ineffective to use traditional optimization methods if the derivative information is not available. Among a variety of heuristic optimization methods, the evolutionary algorithm is suitable to handle such problems. In order to determine reliable results with a limited number of function calls, a robust evolutionary algorithm (REA) based on the paradigm of evolution strategy instead of the popular genetic algorithm can effectively and efficiently search the optimal solution for HVAC problems. The REA can be applied to other engineering problems of a similar nature, as well.

Modeling the dynamic response of conduits

A method for the dynamic modeling of a fluid conduit is developed, based on its discretization into a sequence of well-mixed flow nodes. This enables the time delay produced by the fluid flow to be simply modeled in any time domain simulation. An optimal level of discretization, based on the residence time distribution produced by fully developed turbulent flow, is presented. The model is capable of calculating the response to changes in flow rate, fluid inlet temperature, and species concentration. The thermal response is based on a second-order model for each node, taking account of the thermal capacitance of the fluid and of the conduit inner wall. It is demonstrated that the model predicts a time delay, then a rapid initial response due to flow effects, followed by slower dynamics controlled by the thermal inertia of the walls. An intermodel comparison of output for a prototype duct is made with three published models, and an empirical validation is reported.

Optimization of MVAC systems for energy management by evolutionary algorithm

Energy management in existing building services installations is an essential focus of contemporary facilities management. The subway company that is one of the major utilities services in Hong Kong Special Administrative Region has considered better energy management schemes in its subway stations to reduce the running cost. In the past few years some feasible measures in the mechanical ventilation and air conditioning (MVAC) systems were implemented, however the engineering decisions were supported by trial‐and‐error or imprecise estimation. Improvement to this process would be possible if numerical optimization methods were to be used. An evolutionary algorithm coupled with an external plant simulation programme was applied to determine the optimum conditions of the essential parameters of the MVAC systems. For the centralized MVAC systems under study, the developed optimization and simulation model was found useful in appraising the energy management opportunities for effective design and facilities management.

CFD modelling of naturally ventilated double-skin facades with venetian blinds.

This study describes computational fluid dynamics (CFD) modelling of naturally ventilated double-skin facades (DSFs) with Venetian blinds inside the facade cavity. The 2D modelling work investigates the coupled convective, conductive and radiative heat transfer through the DSF system. The angles of the Venetian blind can be adjusted and a series of angles (0°, 30°, 45°, 60° and 80°) have been modelled. The modelling results are compared with the measurements from a section of a prototype-facade testing facility and with predictions from a component-based nodal model. Agreement between the three methods is generally good. It is thought that discrepancies in the results are caused by the simplification of the CFD model resulting in less turbulence mixing within the facade cavity. The CFD simulation output suggests that the presence of the Venetian blinds is able to enhance the natural ventilation flow within the facade cavity and significantly reduce the heat gains to the internal environment. It was also found that the convective heat transfer coefficients on the glazing surfaces are insensitive to the blind angles. The work demonstrated the capability of CFD for modelling complicated heat transfer processes through the DSF system and offered some guidance for CFD practitioners who wish to model similar type of flow.

Simulation of the operation of a climatic wind tunnel

Abstract The climatic wind tunnel (CWT) is a facility which is widely used in vehicle and component manufacturing to reduce product development times. In view of the wide range of temperatures, humidities and wind speeds required for testing, the energy cost can be a significant part of the overall cost of testing. The paper describes a project which was primarily concerned with the development of a detailed, component-based model of a climatic wind tunnel, its associated plant and the test vehicle. The model was tested against measured plant performance and shown to give a satisfactory representation of plant performance. The potential uses of the model are illustrated by the exploration of alternative control strategies and plant configurations.