Introduction - Problem domain and scope

Key concepts

• need for integral approach;
• the building as an integration of energy systems;
• quality of the indoor environment; environmental impact of buildings;
• integrated energy performance; building energy and mass flow paths and interactions;
• energy and mass balances;
• importance of integrity.

Lecture structure

Here we will address:

• problem;
• context;
• aims.

"Everything is in balance and influences other things."

For example, the speed, direction and other characteristics of the wind influence:

• distribution of pressure coefficient (c.p.) -> air infiltration rate
• convection heat transfer coefficient (hc) -> rate of heat transfer
• wind turbine performance -> generated power for auxiliary systems

Similarly, changing the size of windows in a building design affects:

• heating
• cooling
• lighting

These interactions are happening at any point in time but also integrated over time.

Access to other facilities

You can access more information under a variety of topics:

Course material (initial):

The Building as an Integrated Dynamic System

First published as:
`On the thermal interaction of building structure and heating and ventilating system'.
Doctorate thesis by Jan L. M. Hensen, 1991.

The need for an integral approach

The dynamic thermal interaction, under the influence of occupant behaviour and outdoor climate, between building and heating / cooling and ventilating system is still difficult to predict. In practice this often results in non-optimal, malfunctioning, or even "wrong" building / system combinations. Examples of problematic issues in this context are: sick building syndrome, the higher (comfort) requirements we have nowadays, systems in low-energy buildings (especially housing), industrial ventilation (including cleanroom problems), the `rules' used in building energy management systems, application of passive solar energy, HVAC system and control development and testing, design of integrated systems (eg floor heating, ice rink, swimming pool), and unusual building / system combinations which may occur for instance when a historical building finds a new destination (eg a church being converted into a multi-purpose centre) or in case of relatively new developments like atria, climate facades, time variant transparent systems, double-skin, ground coupled heat exchangers, etc.

The above mentioned problems and the need for an integral approach of the complete `building system' (consisting of building structure, occupants, heating, ventilating, and air-conditioning (HVAC) systems, and prevailing outdoor conditions) is becoming more and more important due to a number of inter-related economical, technical, political and social developments:

• environmental issues;
• increasing complexity of buildings;
• need for future flexibility;
• increasing awareness of quality;
• internationalization & industrialisation of the construction industry;
• `chaining' of liabilities in the building process.

Up to now the building design process is more or less sequential; first the building is designed and subsequently the heating / cooling / ventilating system. The dynamic thermal interaction is usually left out of consideration completely. Thermal comfort requirements are commonly reduced to required air temperature, neglecting other important thermophysiological environmental parameters like radiant temperature and air velocity. For system design, usually only extreme internal and ambient conditions are considered.

It is obvious that this cannot be the right approach for either thermal comfort or for energy consumption. So it is clear that there is definitively need for tools which enable an integral approach of the building and its plant system as a whole.

The building as an integrated, dynamic system

One could argue that the main objective of a building is to provide an environment which is acceptable to the building users. Whether or not the indoor climate is acceptable, depends mainly on the tasks which have to be performed in case of commercial buildings, whereas in domestic buildings acceptability is more related to user expectation.

As illustrated in Figure 1 a building's indoor environment is determined by a number of sources acting via various energy and mass transfer paths. The main sources may be identified as:

• outdoor climate which, in the present context, the main variables are: air temperature, radiant temperature, humidity, solar radiation, wind speed, and wind direction
• occupants who cause casual heat gains by their metabolism,usage of various household or office appliances, lighting, etc.
• auxiliary system which may perform heating, cooling, and / or ventilating duties.

These sources act upon the indoor climate via various energy and mass transfer processes:

• conduction through the building envelope and partition walls
• radiation in the form of solar transmission through transparent parts of the building envelope, and in the form of long wave radiation exchange between surfaces
• convection causing heat exchange between surfaces and the air, and for instance heat exchange inside plant components
• air flow through the building envelope, inside the building, and within the heating, cooling, and / or ventilating system
• flow of fluids encapsulated within the plant system.

The indoor climate may be controlled by the occupants basically via two mechanisms:

• altering the building envelope or inner partitions by for example opening doors, windows, or vents, or by closing curtains, lowering blinds, etc.
• scheduling or adjusting the set point of some controller device which may act upon the auxiliary system or upon the building by automating tasks exemplified above.

Within the overall configuration as sketched in Figure 1, several sub-systems may be identified each with their own dynamic thermal characteristics:

• the occupants, who may be regarded as very complicated dynamic systems themselves
• the building structure which incorporates elements with relatively large time constants, although some building related elements may have fairly small time constants (eg the enclosed air volume, furniture, etc.)
• the auxiliary system which embodies components having time constants varying by several orders of magnitude (eg from a few seconds up to many hours in case of, for instance, a hot water storage tank).

The cycle periods of the exitations acting upon the system are also highly diverse. They range from something in the order of seconds for the plant, via say minutes in case of the occupants, to hours, days and years for the outdoor climate. From the above it will be apparent that we are indeed addressing a very complicated dynamic system.

Building energy flowpaths

Without attempting to be complete, the main energy flowpaths in buildings include:

Conduction	- k (W/mK) function of temperature & moisture - real and apparent - isotropic and anisotropic - related parameters rho, Cp - derived parameters eg thermal diffusivity (k/rho Cp)
Convection	- hc (W/m2K) - usually evaluated from empirical expressions - these typically depend on: heat flow direction characteristic dimension local air speed surface-to-air temperature difference
Long-wave Radiation	- hr (W/m2K) - black and grey body - emissivity & shape factors - specular and diffuse reflections - non-absorbing media - surface-to-surface temperature difference
Short-wave Radiation	- sky short-wave distribution - direct and diffuse components - surface transmittance, absorptance and reflectance - geometrical relationships (eg shading devices) - non-absorbing media
Air Flow	- pressure distribution causes pressure differences - affected by terrain roughness and local obstructions - stack effect - leakage distribution - temperature distribution - incompressible fluid
Casual Gains	- stochastic - sensible & latent components - convective/ radiant split - electrical power component
Control	- sensed condition/location - signal lag - actuated condition/ location - regulation `law' (on/off, PID, etc.)
Climate	- Idn & Igh, Ifh, Tdb,Twb, Wd & Ws, RH - Tsky, CC, Tgrd, Rainfall - severity and typicallity
HVAC & Other Plant	- components - connections - steady-state or dynamic - building link

Objective and outline of the course

Having identified the need for tools which enable an integral approach of the complex dynamic system incorporating the building and its HVAC system, we are now able to state the aim of this course: introduction of building performance evaluation tools which treat the building and plant as an integrated, dynamic system.

There may be several alternative ways to achieve the objective identified above. However, one of the most powerful tools currently available for the analysis and design of complex systems, is computer simulation. As Aburdene (1988) points out:

"Simulation is the process of developing a simplified model of a complex system and using the model to analyze and predict the behaviour of the original system. Why simulate ? The key reasons are that real-life systems are often difficult or impossible to analyze in all their complexity, and it is usually unnecessary to do so anyway. By carefully extracting from the real system the elements relevant to the stated requirements and ignoring the relatively insignificant ones (which is not as easy as it sounds), it is generally possible to develop a model that can be used to predict the behaviour of the real system accurately."

Given the (increasing) complexity of energy/ environmental systems, computer modelling and simulation is emerging as a viable approach to design and performance evaluation. This class aims to give an understanding of the theoretical and operational principles underlying this new technology.

References

Aburdene, M.F. 1988. Computer simulation of dynamic systems,
Wm. C. Brown Publishers, Dubuque, IA.