Suitable for retrofitting in urban areas due to low required drilling depth
State of the art
Currently the majority of ground source heat pump systems in Europe use so-called vertical borehole heat exchangers. These are installed to depths ranging between 100 and 300 meters and offer a high quality, robust source for heat pumps with a life span of 50 years or more. Currently this is a proven technology, used for more than 30 years in hundreds of thousands systems worldwide.
Disadvantages of vertical borehole heat exchangers are relatively high installation cost due to drilling and environmental concerns as different water bearing formations are traversed, that may be used for drinkwater supply. In certain areas drilling depth may be limited: in some areas of the Netherlands, for instance, it is not permitted to drill more than 30 m below ground surface. This, together with large required infrastructures (and related costs), makes these mainstreamed solutions not-applicable when considering retrofitting projects, where geothermal solutions are more expensive than in new buildings. Furthermore, with typical depths of 100 to 300 m2, this technology is almost inapplicable in relevant market environments – typically urban environments – due to pre-existing and complex underground city services infrastructures such as utilities, foundations, etc. and increases the complexity of logistics and operations.
One of the limiting factors is the non-existence of standardized Thermal Resistance Test (TRT) methods specifically designed for non-standard and highly flexible configurations. The lack of specific TRT methods makes contractors and installers choose other types of heat exchangers with standardized TRT methods, since the use of non-standard configurations is an extra cost due to the fact that each time a specific TRT method has to be designed.
The GEOFIT solution
To allow geothermal systems to be realized in retrofitting works, and furthermore in areas where drilling is too expensive or difficult (due to complex geology – e.g. loose overburden of gravel and sand on top of hard rock), to reduce overall cost, alternatives such as compact geometries based on the Helix, slinky heat exchangers or “earth baskets” have been considered and will be optimized in GEOFIT. The non-standard heat exchangers for limited spaces are essential in retrofit applications, for reducing costs and allowing systems to be realized where deep drilling is not possible.
The GEOFIT contribution
Currently Thermal Resistance Test methods and robust design methods for these systems do not exist. Also, the optimal heat exchanger configurations (depending on required thermal capacity) are not exactly know. This reduces the acceptance of the technology due to perceived risks and also leads to higher costs due to over-sizing.
The state of art will be advanced through field-test campaigns and by developing test methods. Such new methods will allow designers and engineers to verify project site conditions and design parameters. From the point of view of the client the risk of choosing geothermal technology will be greatly reduced, thus increasing acceptance.
The state of art will be further significantly advanced when validated models of heat exchangers are available, as these will enable optimizing the configuration of individual heat exchangers. Moreover, engineering design tools derived from the detailed models will ensure that the installation provides the optimal thermal and economical design required for the heating/cooling capacity at the intended efficiency, without either undersizing or oversizing.
GEOFIT will also target the development and standardization of special TRT methods for the novel shallow heat exchanger design (basket and helix configurations) deployed in this project. This novel method, oriented to geothermal retrofitting, will permit to shorten by about 60% the TRT time duration.
Technology readiness level improvement: From TRL6 to TRL7