Energy Simulation in Building Design

Joseph Clarke

• Publisher: Routledge
• Year: 2007
• Print ISBN: 9781138133709
• eBook ISBN: 9781136406751
• Edition: 2
• Topic: Technology & Engineering
• Subtopic: Construction & Architectural Engineering
• Index: Technology & Engineering

Since the appearance of the first edition of 'Energy Simulation in Building Design', the use of computer-based appraisal tools to solve energy design problems within buildings has grown rapidly. A leading figure in this field, Professor Joseph Clarke has updated his book throughout to reflect these latest developments. The book now includes material on combined thermal/lighting and CFD simulation, advanced glazings, indoor air quality and photovoltaic components. This thorough revision means that the book remains the key text on simulation for architects, building engineering consultants and students of building engineering and environmental design of buildings. The book's purpose is to help architects, mechanical & environmental engineers and energy & facility managers to understand and apply the emerging computer methods for options appraisal at the individual building, estate, city, region and national levels. This is achieved by interspersing theoretical derivations relating to simulation within an evolving description of the built environment as a complex system. The premise is that the effective application of any simulation tool requires a thorough understanding of the domain it addresses.

1

Introduction

It is not hyperbole to suggest that the better design of new buildings would result in a 50-75% reduction in their energy consumption relative to 2000 levels, and that appropriate intervention in the existing stock would readily yield a 30% reduction. Added together, this would significantly reduce a nation's energy bill, handsomely contribute to environmental impact and climate change mitigation, and help to alleviate the stressful indoor conditions experienced by many citizens. Indeed, energy efficiency may be likened to an untapped, clean energy resource of vast potential.

The barrier to accessing this resource is less to do with technological constraints—much know-how and many approaches already exist—and more to do with ineffective decision-support. This is especially true at the early design stage in the case of new build, and at the remedial options selection stage in the case of existing buildings. It is a strange paradox that we live in an information age and yet information is never in the hands of those who need it to make informed decisions.

It is in response to this deficiency that building simulation has emerged for use to appraise options for change in terms of relevant issues—from human health and comfort, through energy demand reduction, to sustainable practices. Because of the growing acceptance that simulation defines best practice, substantial attempts are being made to transfer the technology into practice. There are two main incentives for this transfer. First, buildings are complex artifacts involving ‘hard’ and ‘soft’ aspects (such as transient energy flows and stochastic occupant interactions respectively). Traditional design methods, by failing to address this complexity, fall short of best practice. Second, there is a need for rapid feedback on the cost and performance of alternative design approaches. The present system of specialist consultants, while adequate for the detailed design and final specification phases, fails to provide this ad hoc advice.

Such incentives provide the impetus for the growth of organisations representing the notion of ‘test driving’ a building using simulation: the International Building Performance Simulation Association (IBPSA 1999), the Energy Design Advice Scheme (McElroy et al 1997) and the Scottish Energy Systems Group (McElroy and Clarke 1999). Such organisations have brought about a better understanding of the potential of a modelling and simulation approach to building design (Howrie 1995).

Notwithstanding the advanced capabilities of contemporary simulation, there remain at least four formidable barriers to its routine and effective application in practice.

First, there are shortcomings in the user interface. These derive predominately from a conflict between the necessity for the underlying model to be comprehensive and rigorous—to adequately represent real world complexity—while also being straightforward and intuitive—to facilitate ease of user interaction. The situation is exacerbated by the divergence of the conceptual frameworks of the design-oriented program users and the technical-oriented program developers. To complete the confusion, there is the subtly different terminology of the architectural, engineering and scientific professions.

Second, there is little agreement on the data model used to define the building and its energy systems. The program specific data models that have emerged serve only to ensure that there is little commonality between the different modelling systems. This frustrates the validation process, forces applications to operate in isolation and presents a formidable barrier to collaborative design.

Third, the absence of agreed performance assessment methods has forced users to devise personalised appraisal strategies and to become expert enough to coordinate a program's operational path accordingly. Clearly, the existence of standard methods would serve to harmonise program use and make the application experience less fraught for the novice.

Fourth, it may be expected that as the rate of uptake of simulation accelerates, user expectation will grow, especially in relation to integrated modelling by which a building's multi-variate state may be appraised. Satisfying this expectation requires the integration of several complex technical domains.

This book has two principal objectives: to establish and integrate sufficiently detailed models for each technical domain comprising a building, and to elaborate and exemplify new work practices aimed at fostering a simulation-based design process. The aim is to remove the mystery surrounding simulation by concisely deriving an integrative theoretical basis and elaborating an apt mode of use.

Chapter 1 commences with an overview of building performance simulation, introduces the underlying energy flowpaths, and introduces the different possible classes of modelling method.

Chapter 2 derives the two main analytical formulations for dynamic building energy modelling—time and frequency domain response function methods—and sets out the elements for a less constrained numerical method based on conservation considerations applied to control volumes.

Chapter 3 undertakes a step-by-step formulation of the numerical method by deriving conservation equations for characteristic control volumes and structuring these equations in a manner that is the topological equivalent of the real building system.

Chapter 4 demonstrates conservation equation-set formulation for a simple building example and derives matrix partitioning protocols by which fast, variable frequency (time step), simultaneous solutions can be achieved.

Chapter 5 derives complementary approaches to the modelling of inter- and intra-room air movement and moisture flow within the building fabric. A technique for linking the flow and building models is then elaborated.

Chapter 6 applies the theory of chapter 3 to heating, ventilating and air conditioning (HVAC), renewable energy conversion (REC) and control systems and shows how the equation-sets to emerge can be solved simultaneously with the building/flow models. To support REC simulation, a numerical model of electrical power flow is introduced.

Chapter 7 introduces models for the technical sub-systems that impact upon the parameters of the conservation equations, the boundary conditions under which they must be solved and the interpretation of results: weather, non-orthogonal geometry, shading and insolation, shortwave and longwave radiation exchange, surface convection, casual heat sources, daylight illuminance distribution and mould growth.

Chapter 8 addresses use in practice with the emphasis on practical advice aimed at those readers who seek to apply simulation in the real-time, real-scale context of design practice.

Finally, chapter 9 places building energy simulation in the future context of virtual design whereby the different disciplines may collaborate in real time to ensure that buildings are acceptable in terms of their multi-variate performance and impact.

In order to retain a definite focus throughout, the book intersperses theoretical derivations relating to the different technical domains within an evolving description of the building as a complex energy system. In this way an integrated modelling system is arrived at by the book's end. This modelling system is similar in its form and content to the ESP-r system <http://www.esru.strath.ac.uk/> which, since 1974, has evolved in accordance with the software development process as espoused by Maver and Ellis (1982):

Research into model needs, methods, algorithms and organisation. This leads to a research prototype embodying the fundamental laws governing energy flow.

Development of a pilot program based on the research findings and which offers a reasonable platform for testing.

Validation of the program to test the underlying models, the in-built assumptions and the various numerical schemes.

Implementation trials to test the robustness, relevance and efficacy of the program when applied to practical problems.

Improvement of the software and documentation with respect to commercial standards and the incorporation of the lessons learned through the validation and trial implementation studies.

Commercial exploitation and the development of user training and support procedures.

It is instructive to note that the resource required at any stage is typically greater than the accumulated resource required for the preceding stages. Thus, it may be expected that the validation and implementation trial stages will be significantly more costly than the resource required to produce the initial pilot program; and that the commercial exploitation and user support stage will require a more substantial investment again.

1.1 A brief history of simulation

Design tools have traditionally been constructed by reducing the complexity of the underlying system equations in an attempt to lessen the computational load and the corresponding input burden placed on the user. Some portion of the system may be neglected (e.g. longwave radiation exchange), time invariant values may be assigned to some system parameters (e.g. material thermal properties) or simple boundary conditions may be imposed (e.g. steady state or steady cyclic). Within a simulation program such assumptions are heresy. Instead, a mathematical model is constructed to represent each possible energy flowpath and their interactions. In this sense simulation is an attempt to emulate the reality. The evolution of design tools, from traditional manual methods to contemporary simulators, is summarised in table 1.1

Table 1.1: Evolution of design tools.

Generation	Characteristics	Consequences
1	handbook orienteda simplified and piecemeal familiar to practitioners	easy to use, difficult to translate to real world, non-integrative, application limited, deficiencies hidden
2	building dynamics stressed less simplified, still piecemeal based on standard theories	I I I
3	field problem approach shift to numerical methods integrated modelling stressed graphical user interface partial interoperability enabled	increasing integrity vis-à-vis the real world I I I
4 and beyond	good match with reality intelligent knowledge-based fully integrated network compatible/interoperable	deficiencies overt, easy to use and interpret, predictive and multi-variate, ubiquitous and accessible

Traditionally, designers have relied on a range of disparate calculation techniques to quantify and assess building performance at the design stage. The approach is piecemeal in that, at best, only a weak coupling is evident between the various calculation steps. These calculations are based on analytical formulations that embody many simplifying assumptions to permit their formulation in the first instance. Significantly, there is no attempt to faithfully represent the energy and mass flowpaths that occur in real buildings. The intention is only to provide users with an indication of performance: a 1st generation program is consequently easy to apply but difficult to interpret since the user is required to appreciate its limitations and make appropriate allowances.

I...

Table of contents

• Front Cover
• Half Title
• Title Page
• Copyright
• Contents
• Preface
• 1 Introduction
• 2 Integrative modelling methods
• 3 Building simulation
• 4 Processing the building energy equations
• 5 Fluid flow
• 6 HVAC, renewable energy conversion and control systems
• 7 Energy-related sub-systems
• 8 Use in practice
• 9 Future trends
• Appendix A Thermophysical properties
• Appendix B Deficiencies of simplified methods
• Appendix C Fourier heat equation and construction time constant
• Appendix D Admittance method: worked example
• Appendix E Point containment algorithm
• Appendix F Radiosity based lighting simulation
• Appendix G The ESP-r system
• Index