Heat pumps are not a new technology. However, much is still unknown when it comes to the exact nature of heat transfer and its interactions in the ground. Ground Source Heat Pump (GSHP) systems tend to be designed on a ‘rule of thumb’ basis, with generally little ground investigation and analysis of the heat balance and solar recharge.
Consequently, there is a danger that a failure of these systems will occur due to a mis-estimating of the ground parameters. The performance of the ground is also the dominant factor affecting the capital and running costs of GSHP systems.
Field measurement and numerical model simulations help to inform a solid base for the design and performance evaluation of GSHP systems. Data collected from field measurements will improve our understanding of the interactions between soil, heat exchange and the above-ground environment. Effective soil parameters for optimised system performance will open up for exploration.
Under the scope of the work described, Seren’s ground source heat team is currently undertaking the full-scale monitoring of a horizontal ground loop system. This will provide reliable data and a sound knowledge base for GSHP design and installation.
This field study is conducted at Lower Pentre Farm in Ffynnon Gynydd, Powys. A GSHP system has been designed and constructed by WDS Green Energy, who are one of Seren’s industrial partners on the project. The system consists of a heat pump with a rated heating capacity of 14 kW, capable of delivering both space heating (SH) and domestic hot water (DHW). The heat pump is connected to the under-floor heating system of the building. The 750 meter long horizontal system loops are placed in five trenches which spread approximately 40 meters wide.
A network of thermal sensors was set up to allow the monitoring of changes in ground temperature, fluid temperature and the effects of climate parameters on the system performance. In total, 112 ground temperature sensors have been installed underground. These will monitor the ground temperature distribution immediately adjacent to the ground loop and in the vicinity of the heat exchanger trench. Three dimensional positions of sensor and sampling points are located using a Global Positioning System (GPS) and these are illustrated in the figure below. Data from this sensor network is collected at intervals of 15 minutes by a remotely logged acquisition unit. The data set obtained from this full scale monitoring site is also used to experimentally validate a simulation model.
It is envisaged that the outcomes of this analysis will help designers to take full advantage of the energy contained in shallow ground, and will lead to a better appreciation of system behaviour.