A field study of the feasibility and the performance of a sustainable drainage technique combined with a renewable energy device to provide heating in a domestic setting was carried out from March 2008 to November 2010 to acquire practical data about the system’s operation.
Among all the sustainable drainage techniques, permeable pavement system (PPS) was selected to be applied in this project since this particular technique can be used for driveways and car-parking hard standings, but more specially they can be designed as a tanked system whereby an impermeable membrane is installed at the bottom of the tank in order to hold the rainwater collected as runoff from hard areas and roofs before releasing it in a controlled manner. The renewable energy device applied in this study is a ground source heat pump system (GSHP), which has been found in previous studies to provide a better performance when installed in wet conditions. Based on this, the PPS and the GSHP with horizontal ground heat exchanger (GHE) were integrated in a 350mm deep reservoir under ‘real life’ conditions. The combined system operated in heating mode in a family–sized, three bedrooms detached EcoHouseat the Building Research Establishment Innovation Park, Watford, UK. Monitoring the combined system included taking measurements of the temperature of the conditioned space, the ground around the PPS/GSHP system, and of the ambient air every 10 minutes.
Assessing the performance of the PPS/GSHP system involved investigating the effect of extracting heat via the GHE on the ground temperature, the impact of the PPS/GSHP on the thermal profile of the air above the surface of the reservoir, and computing the PPS/GSHP coefficient of performance (CoP).
The thesis includes information about the design of the PPS/GSHP system including the structure of the sub-base, types and size of the used aggregate and stone, the depth of the excavated reservoir amongst others, also the technical problems that materialized, largely due to the fact that the PPS/GSHP was installed and operating under real-life circumstances. Results obtained from the study provided evidence for the workability of the combined system in regards of stormwater management and of providing heat to the EcoHouse. However, monitoring the rainwater stored in the reservoir showed that, due to leakage, the top part of the buried coil was not covered with water. The monitoring also revealed that the rainwater surrounding parts of the coil was, in severe weather, frozen. Moreover, highly significant correlations (p<0.01) were calculated for the ambient air and the ground temperature relationships with the CoP. All of these factors resulted in a 1.8 coefficient of performance being obtained. This low figure was related to the shallow depth of the reservoir since it became clear that its ground temperature was greatly influenced by the ambient air temperature. The study also revealed that the evaporation process was prevented from occurring due to the Inbitex™ composite layer, as a result there was no significant effect on cooling the thermal profile of the air near the surface of the pavement. Furthermore, it was concluded that continuous heat extraction from the ground contributed to an underground temperature drop.