Monodraught uses software to assess ventilation

Benjamin Jones, research engineer, Monodraught, takes a look at the benefits of natural ventilation strategies and the use of DSM software to assess their performance.

Unlike mechanical air conditioning, Natural Ventilation consumes no electrical energy, relying instead on two forces to drive air flow: the action of the wind, and the buoyancy of hot air.

At night, cool external air can be drawn into a building, which helps to dissipate heat absorbed during the day, and reduce peak temperatures and the day/night temperature swing.

During power cuts the ventilation rate remains unchanged because the strategy does not require mechanical input to function, but on abnormally hot days fans can be used to maintain the flow of air out of the stacks, thus allowing the building to remain operational during extreme events.

Tests show that expected CO2 emissions could be half those of an equivalent mechanically ventilated building.

The life expectancy of the building depends upon its internal heat gains, so if heat gains are kept to a minimum, the building is less likely to overheat and will have greater longevity.

All of these conclusions can be made using computers simulations, but the use of computer based modelling is not exclusive to the realms of academia.

The use of computer software to assess the performance of a building is de rigueur within the building services industry.

Building energy and environmental modelling is widely used to show compliance with building regulations, generate energy performance certificates, support planning applications, estimate the operating costs of plants, perform life cycle assessment, determine carbon footprints, investigate the feasibility of integrating low carbon technologies into a building, and model the behaviour of occupants.

Models are a simplified view of the real world and are intended to highlight a trend or predict an outcome within a range of certainty.

They can be used to examine a number of alternatives and test the sensitivity of a design to uncertainty.

Weather data based on average patterns over the last twenty years are used to simulate the performance of a building throughout the year.

The data can be amended to estimate the effects of climate change on a building, thus projecting that a new building could stand for the next 30 years or more.

The Energy Performance of Buildings Directive (EPBD) has made the simulation and analysis of buildings using building simulation software compulsory, and requires using building simulation software according to an approved National Calculation Methodology (NCM).

Because this has become mandatory, the government provides a free simple tool that is able to model the NCM and show a building's compliance with regulations.

The tool is known as the Simplified Building Energy Model (SBEM), and provides a monthly analysis to show compliance and generate Energy Performance Certificates (EPCs), but is unsuitable for detailed design purposes.

Although SBEM is constantly updated, it still has difficulties modelling some advanced features such as night purge ventilation, demand controlled ventilation, adaptive set points, and the detailed effects of thermal mass.

For a detailed analysis, dynamic (or transient) simulation modelling (DSM) software provides a more realistic prediction of the performance of a building.

It uses hourly time steps (instead of monthly) and is better able to model complicated design strategies, plants, renewable energy technologies, shading and glazing, and flexible environment control.

Precisely for these reasons, Monodraught has adopted DSM to investigate natural ventilation strategies that incorporate its products, particularly in large and complex buildings such as hospitals, schools, etc.

This enables the performance of its Windcatchers (and Sunpipes) to be quantified over a wide range of environmental conditions, and to confirm that a building meets government requirements.

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