INFORMAT ION T ECHNOLOGY

Getting the balance right

By Dr Alan Jones, Managing Director of EDSL/Tas

adequate. If needed the radiosity results can be further enhanced with a ray-tracing overlay. The difference in simulation

time between ray-tracing and radiosity is staggering. A 100 room building would have all external daylight and internal daylight in every room calculated in about an hour. The ray-tracing equivalent would take at least a day for all rooms. Ray-tracing is used in the radiance software, whilst AGi32 offers both ray-tracing and radiosity with the option to combine them. Again, people with memories

T

as is an accredited Dynamic Simulation Model, which is a complete alternative to SBEM. So what does

that actually mean? Basically it all boils down to

the ability to be able to calculate in more detail the true operational performance of a building, its plant and control systems. Whilst SBEM performs an

energy balance for the zones in a building once for each month of the year, Tas will perform an energy balance for each zone for every hour of the year using Typical Reference Year weather data from CIBSE. The energy balance

calculation in Tas is more detailed than that used in SBEM and is based on a response factor energy balance method outlined by ASHRAE and CIBSE. The method also complies with ISO standards for building simulation. What this means is that not only are hourly heating and cooling demands calculated for each space, but also space air and surface temperatures and relative humidity. This means that Tas is not just a CO2 emissions compliance tool, but also a design tool, offering the ability to calculate room heating and cooling sizes and occupant comfort conditions. The more sophisticated heat

balance procedures in Tas are ideal for evaluating the performance of natural ventilation and mixed mode operation, providing a clear insight into required control strategies and checking compliance with a range of

comfort criteria such as BB101 for schools. The question is often asked,

how much additional data is needed compared with SBEM and how long do the calculations take. The answers to these questions often surprise people. The data needed to describe a Tas building model is virtually the same as for SBEM. Tas just does more with it. As for speed of calculation, Tas will perform an hourly simulation for a zone through the year in about one second. So a 100 zone model will simulate a complete year in a few minutes. Those people who remember

the simulation times of twenty years back need to take a fresh look at the latest software procedures and how they perform on current hardware. The need for simplified and approximate

analysis

procedures has passed into history. You can now do it properly and get a far more realistic idea of the true efficiency of your design options.

New regulations

As we move towards the new 2010 Building regulations we can expect a further reduction in CO2 emissions targets of about 25% on average, more for air conditioned buildings. One of the main areas that

will challenge building design professionals is the balance between providing good daylight levels and controlling solar gains. The targets for annual, average,

lighting efficiencies suggested in the

consultation document for the new regulations are such that they are virtually impossible to achieve without good daylight saving controls. Tas is able to calculate

daylight factors for simple window configuration. For more complex scenarios the Tas 3D model may be exported to a number of lighting simulation packages such as AGi32, which provides artificial

comprehensive and daylight

simulation. The reflective properties and surface finishes of the internal and external surfaces of the building are able to be set. Daylight simulations may be undertaken for standards sky specifications compliant with BREEAM and LEED. The result is the calculation of daylight factor contour at the working plane for all spaces. There are two basic methods

used in lighting simulation, ray- tracing and radiosity. The former represents the earliest techniques and produces very clear and realistic images of a chosen view, either external or internal to the building. It is a procedure that is very time consuming and for a building with a number of rooms the calculation needs to be repeated for each room. A more recent development is the radiosity method, which takes a less detailed approach than ray- tracing, calculating the light exchange between small surface patches on the inside and outside of the building. For daylight analysis, to generate daylight factors across a working plane, it is perfectly

22 BUILDING SERVICES & ENVIRONMENTAL ENGINEER MAY 2010

of slow daylight simulations need to take a fresh look at the latest software technology running on modern hardware. Detailed daylight simulation is fast and affordable and a very effective design capability. Having determined the

daylight factor for the various zones in the building, the next stage is to use this to determine how much daylight will enter the zones hourly through the year. This is done by firstly calculating the sky lux hourly through the year. To do this we convert the solar radiation in the design weather data to sky lux. With the sky lux known and the daylight factor for a zone known, the hourly daylight lux level in the room may be determined. The hourly levels of daylight entering the room may be used in a range of lighting control strategies to switch off or dim the artificial lighting. This produces an hourly profile of lighting energy use for each zone which is automatically fed back into the Tas building model. The result is a reduction in lighting heat gain and a reduction in lighting generated CO2 emissions. This reflects on the space temperatures or cooling loads and overall building CO2 emissions in the compliance analysis.

Analyse performance

A further significant difference between Tas and SBEM is in the analysis of plant and controls performance. Whilst SBEM basically multiplies total annual heating and cooling demand by generalised annual plant efficiency ratios such as SCop

VISIT OUR WEBSITE: www.bsee.co.uk

for heating and SEER for cooling, Tas will model the plant at hourly time steps through the year using part

load

characteristics and control logic. The plant modelling is fully coupled with the building model, so any limitations in plant performance will affect building performance. By example, if the climate conditions exceed those used to size cooling capacity, then the amount and frequency of upward drift of internal temperatures will be calculated. The ability to model plant characteristics and control logic means that the true efficiency of the proposed design can be evaluated. This ability also means that manufacturer’s measured

part load

performance data can be used within the software. This adds an additional level of credibility that the predicted performance will be realised in practice and that clients’ return on investment in reductions in CO2 missions and running costs are actually realised.

Energy model

Besides, being able to demonstrate compliance with Part L, the model can also be used to develop what is known as an energy model. Here the NCM database activity schedule are replaced with actual or expected activity schedules and plant operation times are adjusted to match them. Such energy models are used to predict actual energy use and may contain items of energy use not included in the Part L2/EPC procedure, such as small power, lifts, process load, data centres, extended hours of use etc. These models are used for

PFI management contracts, developing realistic energy targets. It is important to

recognise that the energy prediction produced for regulatory

compliance

calculation are based on a fixed, common set of activities, which may not reflect the way a building is actually used. So a DSM is far more than

simply a compliance tool. It offers far more opportunity for building design development and ongoing management. This applies equally to new build and refurbishment. A Part L2/EPC level 5 building

is simply a building which has one or more features or components which cannot be realistically modelled with SBEM and need a more detailed analysis. As mentioned earlier, good

quality DSMs will meet all international standards, giving them a worldwide market of application. ASHRAE Standard 90.1 has

long been the benchmarks for commercial building energy codes in the United States and a key basis for standards in more than 15 countries around the world. Because of this Tas has a new ASHRAE 90.1 studio application, which automates the procedures for baseline building and systems generation. The application is fully interactive allowing easy back tracking for design revisions and task tracking to ensure consistency. The standard also forms the

energy basis of the Leadership in Energy and Environmental Design (LEED) rating system, which is a publicly reviewed and approved standard for environmentally sustainable construction. A building that scores well at LEED can be subject to various benefits, including tax incentives, reduced fees and priority permitting. Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52