Architectural modifications : performance assessment of the solar cooling system for a small size office building
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Across the world, governments, organisations and individuals are seeking to obtain the most effective, cheapest and environmentally clean power sources. During the last decade, air conditioning systems that use renewable energies have undergone significant development. This expansion has been driven, in large part, by successive periods of extreme solar heat and increased demand for the comfort of summer air conditioning in residential and commercial buildings. Most of Australia's greenhouse gas emissions (about 50%) come from the burning of fossil fuels for energy (e.g., for electricity and transport). For this reason, looking for a reduction in the energy used in buildings as well as looking for alternative power sources to fossil fuels should be explored. This project presents approaches to increasing the performance of a solar cooling system by improving the energy efficiency of the building for a typical small-sized Australian office building in Sydney, New South Wales (NSW). Solar cooling systems with a thermally driven LiBr-H2O single-effect absorption chiller that utilises the solar thermal energy provided through evacuated tube solar thermal collectors and thermal back-up is a popular system among solar cooling applications around the world. In this study, the performance of a similar system is evaluated and compared with a conventional cooling system in terms of energy, economic and environmental aspects. The aim of this project was to examine a solar air conditioning system with absorption chiller contributions in a small office building cooled in different thermal efficiency scenarios by demonstrating and evaluating specific architectural improvements to the basic model of the building. These improvements took the form of double-glazed windows, overhanging window shades, and a combination of double-glazed windows and overhanging shades in the environment of Sydney, NSW. TRNSYS and OpenStudio/EnergyPlus software with the graphical interference of Google SketchUp for building energy modelling and simulation were used. To achieve an efficient usage for the building under consideration, the project was divided into two main blocks. The first block included the design and simulation of the building to evaluate a set of proposed architectural improvements, with the aim of achieving optimal intake. For the second block, energy and economic assessments were performed to choose the best alternative application. The building underwent simulation under four different scenarios: the baseline scenario (i.e. the current situation); the introduction of an overhang on the north side of the building; the modification of existing glazing of 4 mm to a double glass and finally a combination of the two modifications: overhang and glazing options. The analysis of the energy and environmental performance of the solar cooling system was based on the system solar fraction and the CO2 foot print. The modelling and simulation of the solar cooling system was carried out by the TRNSYS program in the four different scenarios. The results show that the combination of double glazing with overhang shade is the best option for increasing the thermal efficiency of the building and reducing the cooling load. The favourable assessment of performance of the solar cooling system is indicated by both the equivalent natural gas saved rate (3307 m3 /year) and the solar fraction (0.804). In addition, the environmental performance of the system shows positive results by saving more than 69 Tonnes of CO2/year, which is the optimum value compared to other scenarios. Economically, the window overhang shade is the preferred option as an architectural modification in terms of initial cost, net present value and payback period. However, the economic performance of the solar cooling system studied was not competent when compared to the reference system (vapour compression). The LCC (life cycle cost) calculations show that the solar cooling system values are 40% more than those of the reference system. However, the NPV (net present value) results were found to be negative values after the 25-year life span, indicating that there is no significant profit reward at the end of the term of operation. The PBP (payback period) results, by contrast, show that the solar cooling system will return its value in a lesser period than the proposed life span of 25 years. In conclusion, by applying different architectural modifications for the building envelope, cooling loads can be reduced significantly. However, the window overhang shade is found to be the best option in terms of economic performance. The contribution of the solar cooling system shows that the system is not practical in all four scenarios. This may be due to the high initial cost of the system, even though it is found to be efficient in saving energy and is capable of reducing greenhouse gas emissions.
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