How to heat a city...by Ashley Patton & David Boon

Ashley Patton and Gareth Farr measuring groundwater temperatures
at the famous Brains Brewery using a specially adapted water
level ‘dipper’ equipped with a thermometer and conductivity meter.
No beer was consumed during the survey, honest! 

Ashley Patton and David Boon from BGS Wales explain how an exciting new project in Wales is helping to tackle fuel poverty through urban geology.   

For the last year BGS scientists have been monitoring shallow groundwater temperatures across the city of Cardiff, and surprisingly they found that the ground beneath the city is significantly warmer than expected. The heat lost from buildings and sewers in cities is naturally stored in the ground, as well as released to the atmosphere, in a process referred to as the ‘Urban Heat Island’ effect.  Our work has shown that, in Cardiff at least, this anthropogenic effect has increased the groundwater temperature from 11 to 14^o Celsius in many places.  So why not use this abundant source of free, low-carbon heat to warm poverty stricken homes in the city?

To learn more about our project read on...

A flooded basement in the city centre shows
how shallow the water table is under the city. 

Cardiff, a city of some 350,000 people, was once the largest exporter of coal in the world, however the majority of its docks are now infilled and there has been significant urban redevelopment over the last 20 years. Cardiff is underlain by geologically young ‘superficial’ deposits such as estuarine and river alluvium deposited by rivers and marine waters, and sands and gravels deposited from glacial melt waters at the end of the last ice age.  The shallow sand and gravel deposits hold significant quantities of groundwater which can be readily accessed by drilling boreholes into the ground. Using 168 existing groundwater level monitoring boreholes distributed throughout the city we measured the seasonal temperature of the groundwater at 1m depth intervals using a thermometer attached to a long wire. In Cardiff groundwater is often encountered just 3-4m below the surface making GSHP systems more cost effective to deploy compared to standard deep borehole systems that require larger water pumps. The image on the right provides an interesting illustration of depth to groundwater below the city. We found that the groundwater in Cardiff is several degrees warmer than expected (Patton et al., 2015) making it an attractive prospect for the development of ground source heat systems. To help planners and developers make the most of this resource we produced a city-wide map showing the distribution of the groundwater temperature and it caused developers to reconsider their future renewable energy strategy for city.

A shallow groundwater temperature map for the city of Cardiff (average 2014 spring time temperatures, in degrees Celsius) (after Patton et al., 2015). 

A local partnership formed between the BGS, City of Cardiff Council, Cardiff Harbour Authority and WDS Green Energy Ltd led to a follow-on project funded by Energy Catalyst InnovateUK in 2015. As proof of concept for a city-scale underground heat capture system, we are installing an open-loop ground source heating system in a local nursery school and monitoring the sustainability of the system.  Boreholes have been drilled to abstract water from the sand and gravel aquifer. The video below shows a cable percussion (or shell and auger) drilling rig installing one of the boreholes to be used for the ground source heating system.  Groundwater will be pumped from the aquifer so it can be passed through a heat exchanger, then returned into the ground via a second borehole. This type of system is called an ‘open-loop’ ground source heat pump. The heat we remove will be used to generate hot water to keep the school warm during the winter, whilst also helping to increase energy security, as well as reducing CO₂ and our reliance on conventional fossil fuels.

Future work in 2015/16 will involve analysis of groundwater temperature and chemistry data, and installation of the first telemetered urban groundwater temperature network in the UK, which you will shortly be able to view live via a BGS Webportal. We are also creating a 3D geological model to support subsurface planning and sustainable integration of future systems.  Watch this space...