By Jonathan Skeen
Modern property developers want buildings that are both more comfortable and more energy efficient. Balancing both of these requirements is a difficult challenge for the full project team. In the context of historically low energy prices, architects have placed less emphasis on the energy impacts of their design choices, while engineers have ensured thermal comfort by specifying heating and cooling (HVAC) equipment large enough to meet the worst case demand, regardless of the inherent energy efficiency of the overall building design.
Thus environmental concerns and electricity bills have not typically shaped the design process. As a result, building components – including building fabric, HVAC systems and lighting systems – have been developed compartmentally: with little in-depth interaction amongst the design team on how to improve the combined efficiency of the overall system. Consider, for example, the design of a building’s façade. Typically the realm of the architect, façade design affects daylight penetration into interior spaces. Improved daylight penetration reduces the amount of electric lighting required, and can cut the heating effect of interior light bulbs: reducing the energy use of HVAC systems which must counteract it.
Understanding the impacts of individual design choices requires a means of quantifying a myriad of knock-on effects under the full range of potential operating conditions. Energy modelling allows the design team to model and predict the effect of all design choices: from window sizes, to wall materials, to fan and chiller selections. It enables the development of a more integrated design, where structural elements and electrical and mechanical systems fit together more seamlessly, and are designed as a single energy-using system, rather than multiple parallel systems.
A variety of energy modelling software tools are available to South African design teams. The most effective and useful of these allow the user to simulate both thermal conditions and daylight penetration, under a dynamic range of external weather conditions. Software packages – such as DesignBuilder with EnergyPlus, AutoDesk’s web-based Green Building Design Studio, and Google SketchUp with the EnergyPlus add-in – allow users to visually represent their building designs and understand the manner in which they use energy. It is a field of software development that is rapidly progressing, with the power and usability of the available packages improving quickly.
These tools can provide strong, quantitative evidence on the cost benefit of various design interventions aimed at improving comfort and energy efficiency. Energy modelling during the development of a proposed public safety building in the US town of Raleigh, North Carolina (City of Raleigh, 2011) showed that a high performance façade (with wall U values of 0.391 and window shading coefficients of 0.322) would yield a 14.5% reduction in required cooling air volume, an 8.8% reduction in cooling plant load, and a 17.8% reduction in heating plant load, over an equivalent building meeting ASHREA3 energy standards (Heikin, 2011).
Further analysis found that the financial impact of these changes would add just 0.54% to projected baseline costs. However, it was also found that the knock-on effects of improving the building envelope would ultimately lead to an overall building cost reduction. This is because the improved design would require smaller chillers and air-handling units, smaller heating and cooling pipes and pumping units, less equipment insulation, and would require less of the building space to be dedicated to HVAC plant. Furthermore, annual energy consumption was projected to fall by 5.2% relative to the equivalent ASHRAE building (Heikin, 2011).
Energy modelling and integrated design approaches, as well as the skills required to effectively apply them, are emerging in South Africa. To this end number of project management, architectural, and consulting engineering firms are engaging in projects with goals of sustainability and energy efficiency, while adopting the skills and tools needed to deliver comfortable, cost-effective and energy efficient buildings.
Many of these projects are aiming for accreditation by the Green Building Council of South Africa (GBCSA), an organisation established in 2007 with the aim of developing a more sustainable local built environment. To this end the council has developed a series of ratings tools which address a variety of sustainability criteria, with a particular emphasis on energy use and renewable energy generation. To date the GBCSA has certified nearly twenty new South African buildings under its Green Star SA program, and has a growing list of upcoming projects (GBCSA, 2012).
The Manenberg CiviC CenTre
The new Manenberg Civic Centre was developed in line with the Green Star SA guidelines, and is currently undergoing assessment for a Green Star SA rating. The design team, led by architect Ashley Hemraj of the City of Cape Town – placed a heavy emphasis on sustainable energy use. As such, a number of interlinking design choices were taken in order to improve the overall, integrated energy efficiency of the building. Key interventions included:
- The selection of insulating wall and roof elements, using novel systems such as thick sandbag walls.
- Optimization of daylight penetration while avoiding excessive solar gain.
- The use of efficient HVAC equipment, incorporating heat recovery technology.
- The implementation of lighting controls and occupancy sensors.
- The use of efficient bulbs, and the implementation of a ‘reduced’ lighting scope.
- The use of solar water heaters.
- The incorporation of a hybrid wind and solar PV renewable energy system.
Tasked with modelling the energy use of the building, Emergent Energy – a Cape Town consultancy specialising in renewable energy and energy efficiency – undertook a detailed analysis of these interventions using DesignBuilder with EnergyPlus. By simulating the building’s thermal conditions and electricity demand every ten minutes for a full year, they were able to develop a detailed picture of how each of the energy-using systems in the building would consume electricity under varying conditions.
As a baseline, a notional building model was also developed with the same overall shape and volume of the actual building, but with its building fabric, glazing, HVAC systems and lighting systems set according to the “SANS 204: Energy Efficiency in Buildings” standard (SABS, 2011)4. In parallel, high level modelling of the renewable energy systems was undertaken using the RETScreen software tool (RETScreen, 2012).
Figure 5.1 compares the monthly electricity demand resulting from key energy users5 of three scenarios, namely: the notional SANS 204 compliant building, the actual building with no renewable energy; and the actual building with a 15kW solar PV array. Without the use of renewable energy systems, the building operators could have expected to use around 30% less electricity than the equivalent building meeting SANS 204 standards. With the PV system included, the saving increases to nearly 60% annually. This excludes the effect of the 5kW wind turbine which has also been installed at the centre, which can be expected to further reduce electricity consumption by approximately 5%.
The financial implications of the design interventions are significant. Assuming a standard commercial tariff for small power users in Cape Town, the building operators can expect to save around R50, 000 per annum on their large energy uses – a total reduction of approximately a third. With the introduction of renewable energy systems, this increases to well over R90, 000 per annum – or nearly two thirds of the total. Projected monthly electricity bills for the three scenarios are shown in Figure 5.2.
The results are a testament to the power of integrated design in matching hard engineering goals with the aesthetic, social and economic goals of the architects. Achieving the level of detail and accuracy required to properly assess the different interventions simply cannot be achieved using standard engineering calculations. Energy modelling, by comparison, can provide real economic impetus for more sustainable design choices, especially where the capital costs are high, and payoffs are not clearly understood.
Source: Sustainable Energy Resource Handbook Volume 3
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