The Design and Development of an Integrated Solar Collector and Thermal Energy Storage Window System for Lightweight Construction

Abstract

Lack of thermal mass is a major barrier to the efficient utilization of window admitted solar thermal energy in traditional lightweight construction in New Zealand. The aim of this project was to design a solar collector and thermal energy storage (SCATES) window system that incorporated thermal mass within the window sill, head, and jambs. Excess daily solar heat gains are stored and can be made available later in the day as space heating becomes a requirement. The performance of the window system has been investigated experimentally and by computer simulation. A pilot window system that used concrete as the thermal storage medium was designed and tested using a pair of matched, room-sized test cells as a proof-of-concept. To reduce weight and provide better control of thermal storage charging and discharging, the window system design was refined and a phase change material (PCM) inside square hollow section aluminium window surrounds was used as the thermal storage medium. A full scale window was fabricated and the window performance was investigated by experiment in controlled conditions using a solar simulator. Full melt of the PCM was easily achieved when the window was exposed to expected insolation levels. Solidification of the PCM however posed a problem meaning that not all of the stored heat was being released from the system in a diurnal cycle. An existing, validated two-dimensional finite-volume numerical CFD heat transfer model was modified to match the geometry and materials of the experimental window. The numerical model was used to simulate a variety of system geometries and insolation values in an attempt to predict temperatures within the PCM, and temperature, velocity and vortex formation in the air entrained within the system. Run-time problems included instability and excessive computational time. There was poor agreement between the CFD numerical model and the experimental results. A final window design was proposed, constructed, and tested experimentally in controlled and realistic conditions. Using a different validated general purpose hour by hour finite difference thermal network analysis program (SUNREL) and realistic climatic data, the effect of the SCATES window system on energy use and indoor air temperature was predicted for three locations in New Zealand (Wellington, Auckland and Christchurch), for three different levels of building insulation. The SUNREL thermal network simulation results are compared with the measured results with good agreement. For a 2400mm x 2400mm single-room building in Wellington, insulated to current Building Code levels, a 1000mm high x 1000mm wide x 300mm deep SCATES window has the potential to store and make available approximately 11.5 GJ of useful energy per annum. With increased insulation levels in the external envelope the overall lifetime energy savings supplied by the thermal energy storage window system decreases. Compared with a typical New Zealand, single glazed, aluminium framed window system of similar dimensions, an energy analysis of the SCATES window system shows an 18.2 GJ increase in embodied energy and a 79 GJ decrease in energy demand, giving an overall predicted 50-year lifetime energy saving of 60.8 GJ. The SCATES window system has a higher initial/capital cost and a higher embodied energy (environmental cost) compared to that of a traditional single glazed window. Using a simple payback calculation and an expected life of 50 years, the SCATES window system shows an overall financial deficit for each level of insulation for Auckland but a financial saving for each insulation level for Wellington and Christchurch. For each location and for each insulation level investigated the SCATES window system shows a net energy saving.