An experimental investigation into the use of water to provide thermal mass in building fabric

The development of renewable energy is essential, however many renewable resources are intermittent. Therefore, developing storage techniques has become a major issue of the energy field. In particular, thermal energy storage can help to manage resources, reducing energy consumption and improving passive buildings. Heavy masonry materials (brick, stone, concrete) have been used for many centuries, and use of phase change materials have been researched recently for this role. But water, which has one of the highest sensible heat capacities known and is free, appears so far to have been almost neglected. This paper presents an experimental laboratory study into the use of water as a sensible thermal storage medium, and a comparison with sand, which has similar properties to masonry. The thermal responses of ‘walls’ containing water or sand have been measured for various dynamic thermal inputs. The experiments were done with an insulated box of length 125 cm, width 60 cm and depth 60 cm, with separate insulated lid. Various ‘walls’ were installed, separating the box into two equal parts. For the first set of experiments, the dividing wall was a box made of 4mm acrylic sheet, internal width 40mm. For the second set of experiments, the wall was made of 6 stackable 5 litre plastic water containers. A heat mat was placed in one half of the box, connected to a DC power supply. Experiments were done with the box top entirely insulated, or with just the heated side insulated, the other side being open to the air, or with the heated side covered but not insulated. The response of the system to various step and cyclic heat inputs, corresponding to heat gains in a room, was investigated. Twelve thermocouples were fixed at different points in the rig, in order to measure the evolution of temperature over time. A heat flux sensor was used to measure heat flow across the wall surfaces. These data were collected with the software LabVIEW and analysed using a spreadsheet. Significant differences in thermal response were observed between water and sand. It was found that the water can store more heat than sand, taking longer to warm up and release heat. Due to convective processes, the heat also transferred more quickly into the water, and across the acrylic box when filled with water compared to sand. These results show that water acts as an effective sensible heat storage medium, and unlike phase change materials will operate across a wide temperature range. Water thermal storage could be applied in buildings, or temporary structures, to provide effective thermal mass at low cost to provide improved comfort and reduced energy consumption.