Mike Deed, Managing Director of Geoquip Water Solutions, is the author of Talking Point in the October issue of Groundwater Engineering.
He focuses on the huge benefits of installing a geothermal system and the importance of building in a maintenance programme at the outset to achieve the maximum potential from that system. He goes on to outline some of the problems that could compromise a geothermal system including mineral contamination and its effect on borehole yields and water flow.
Whilst some of the original article was cut due to space we have included the whole article below and the published version can be read in the attached PDF.
TALKING POINT by Mike Deed of Geoquip Water Solutions. It comes as no surprise that geothermal systems are being used increasingly in commercial applications. For every 1btu of electricity used, a geothermal heat pump unit typically produces 3-5 btu’s, making it 300 to 500% more efficient than using electric resistance heat and 20 to 30% more efficient than boiler/tower systems. It’s estimated that installing a geothermal system can produce cost savings of up to 70% and achieve up to 50% reduction in CO2 emissions.
Where suitable hydrogeological conditions are present, open loop geothermal systems are often the most attractive option for larger applications as they are simple to install, the heat transfer loss can be kept to a minimum and both heat pump energy and maintenance costs tend to be lower. However, the effectiveness of these systems depends on borehole yields, and, most importantly, flow rates.A number of 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.
Iron oxide and ironbacteria contamination are estimated to affect about 40% of the world’s water bores and whether caused by naturally occurring bacteria or chemical transference, anecdotal evidence indicates an upward trend.
In a geothermal system, bacterial contamination and its associated residues build up inside the heat exchanger, clogging both the abstraction and recharge wells andincreasing 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.
Ultimately this reduces output, wastes energy, increases operating costs and ultimately compromises the original reason for opting for geothermal energy.
Amongst the growing number of rehabilitation projects Geoquip has been involved with is the geothermal plant at Lodi, Italy, installed in 2008 to service an air conditioning system for the District Council offices. 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 were causing flooding in adjacent areas. The system had to be switched off and, in an attempt to remedy the problem, a second screen was installed and the system cleaned with hydrochloric acid.
Despite these measures the same problem re-occurred the following year when the system was re-started, and ESI, the water management and ground source energy specialists, were brought in. They invited Millars Products, Geoquip’s Italian partners, to propose a solution and their 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 BoreSaver Phased Treatment Programme was recommended, using BoreSaver Ultra C Pro which disrupts and dissolves iron bacteria cells and the associated iron oxide residues. The post-treatment video inspection showed the screen slots virtually clear of contamination and the system worked efficiently through the summer without subsequent borehole clogging.
Towards the end of 2010, levels in the recharge well started to increase again and it was evident that an on-going maintenance programme was needed to control the iron bacteria contamination. This was done in May 2011, again using BoreSaver, and the system was returned to full operational use. It is likely that the continuing iron bacteria contamination in the recharge well occurred because, against recommendation, only the wells, rather than the entire geothermal system were cleaned in 2010.
The Lodi case study demonstrates the importance of implementing a monitoring and maintenance programme, system-wide, when investing in geothermal solutions.
In virtually all studies evaluating the use of geothermal systems, low maintenance costs are identified as one of the key benefits. However these low maintenance costs depend on businesses ensuring maximum flow rates are achieved. Regular proactive maintenance always proves to be the most cost-effective approach in the long run and we would urge consultants to build in a maintenance programme at the project design stage when proposing a renewable energy solution.