Geothermal heating and cooling has real potential to cut a building’s carbon footprint but there is more to consider than the geological constraints.
It comes as no surprise that geothermal systems are being increasingly used in commercial applications. For every British Thermal Unit (Btu) of electricity used, a geothermal heat pump unit typically produces 3-5 Btu, making it 300-500% more efficient than using electric resistance heat and 20-30% more efficient than boiler/tower systems.
But while the hydrogeological conditions can be ideal for such systems, other factors can impact on the operation and prevent them from delivering their full potential.
Open loop geothermal systems are often the most attractive option for larger applications as they are simple to install. However, the effectiveness of these systems depends on borehole yields, and, most critically, flow rates. Many factors can affect flow rates and one that could be easily managed, and yet is becoming an increasing problem, is the reduction in flow rate due to the build-up of residues and biofilms.
“While the hydrogeological conditions can be ideal for geothermal, other factors can impact on the operation”
Iron oxide and iron bacteria contamination are estimated to affect about 40% of the world’s water bores. In a geothermal system, bacterial contamination and its associated residues build up inside the heat exchanger, clogging both the abstraction and recharge wells and increasing the friction losses in the flow section of the system. This is particularly traumatic for open loop geothermal systems because the resultant biofouling severely reduces the system flow rate and thermal transfer, creating ideal conditions for a vicious cycle of accelerated bacterial growth and continual
re-contamination of the system.
The issues are highlighted by a scheme I worked on in Lodi, Italy, where a geothermal plant was installed to service an office air conditioning system. The system was designed with an overall thermal power of 0.6MW, achieved by one heat pump with heat exchangers, a 42m deep abstraction well, a 29m deep recharge well and 100m of pipework. When the system was commissioned it ran into immediate problems as unexpected blockages in the recharge well caused flooding in adjacent areas.
Despite installing a second screen with a hydrochloric acid cleaning system, the problems re-occurred the following year.
A downhole video inspection revealed that both the abstraction and the recharge wells were completely clogged with iron bacteria and iron oxide residues. The slots in both screens in the recharge well were completely sealed, preventing the discharge of purged water into the ground.
The boreholes were treated to disrupt and dissolve the iron bacteria cells and associated iron oxide residues, but it was evident that an on-going maintenance programme was needed to allow the system to function to its design capacity.
In virtually all geothermal evaluations, low maintenance costs are identified as one of the key benefits, but this depends on ensuring maximum flow rates are achieved. Regular proactive maintenance is the most cost-effective approach in the long run and consultants should build in a maintenance programme at the project design stage.