Abstract
Nearly half of Europe’s total energy consumption is contributed to buildings. Heating and
cooling consist of a significant part of this consumption. Considering climate change and the
countries’ measures towards greenhouse gas emissions (i.e., European energy and climate
targets of achieving net-zero greenhouse gas emissions by 2050), it is crucial to implement
renewable energy sources such as geothermal energy based heating and cooling systems at
district to city scale. Many district-level heating and cooling systems have been established in
Europe. However, just a limited number of them are geothermal energy based. One of the most
efficient ways of heating and cooling supply is to use geothermal energy stored in ground water.
Groundwater heat pumps (GWHP) are highly efficient, environmentally friendly and low-carbon
technology that can supply heating and cooling to buildings on small or large scales by
benefitting from low-temperature sources between 0°C and 30°C.
GWHPs have been extensively researched in recent years to understand the impact of thermally
affected zones (TAZ) using numerical and experimental methods. However, studies on the
utilisation of GWHP at a district scale, particularly in chalk aquifers, are relatively rare. The
implementation of district-scale geothermal heat pump (GWHP) systems poses several
challenges, including dealing with the scale and complexity of the systems, addressing
geological variability, managing high initial investments, balancing energy demand and
supply, ensuring proper maintenance and monitoring, and mitigating potential environmental
impacts. These challenges require careful consideration and strategic planning to ensure the
successful deployment and sustainable operation of these systems. The development of TAZ
and system performance depend heavily on the characteristics of the aquifer. This study focuses
on one of the largest GWHP systems in a confined chalk aquifer, and examines thermal plume
development, system performance, and sustainability.
This study evaluates various factors that affect GWHP system design and performance, such
as groundwater flow, injection, and extraction rates. The effects of variations in these factors
on TAZ development and system performance have not been fully examined and have not been
reported in the literature. Therefore, this study investigates their impact through laboratory-based
experimental examination and numerical investigation, considering various scenarios.
The numerical investigation has been carried out considering a field study called Northern
Gateway Heat Network, located in Colchester, UK. The results indicate that the space heating
operation, cold water injection, creates a thermal plume that primarily develops around the
injection wells. Additionally, the results show that the thermal energy gained from the
groundwater is about 11% lower than the designed value. The numerical study suggests that
the system can also supply direct cooling for two months or longer every year if required, which
would extend thermal feedback time. The study also suggests that incorporating an aquifer
thermal energy storage (ATES) system would improve system efficiency and long-term
sustainability. The numerical study further reveals that the injection and extraction create a
significant drawdown around the wells, which should be carefully analysed as the water level
might drop below the submersible pump. The groundwater flow has a significant impact on
thermal plume development, although the hydraulic gradient at the site is relatively small.
The experimental study results demonstrate that incorporating the ATES system significantly
increases efficiency. The increase in the injection and abstraction rates increases the thermal
plume dimension, leading to a greater change in abstraction temperature. The study also
highlights that the groundwater flow rate has a significant impact on thermal plume
development, as it spreads the thermal plume in the direction of the groundwater flow. The
research suggests that the groundwater flow, injection, and extraction rates must be taken into
account during the design stage of a GWHP system.