Improved borehole technology for Geothermal Heat Pumps development (completed)
The implementation of a larger number of geothermal heat pumps (GHP) with better energy efficiency would help contributing to the 40% reduction of domestic greenhouse gases emissions by 2020, which is one of Sweden’s “miljömål” (2030 for the EU according the INDC). The other goals that this project could contribute fulfilling are “God bebyggd miljö”, “Grundvatten av god kvalitet” and “Frisk luft”.
This project aims at investigating innovative technologies (innovative borehole heat exchangers and new methods for thermal response testing) within the field of geothermal heat pumps to provide more efficient and cost effective borehole systems, and to increase the areas where the technology could be used. These actions will provide the industry with tools to further increase the technology penetration into the market place.
Increase in adoption of geothermal heat pump is key for the reduction of the overall energy use for heating and cooling of buildings.
The KTH live-in-lab platform will be utilized as test beds to test innovative heat exchangers design, long boreholes, along with new methods of thermal response testing, i.e. for the assessment of ground properties. Energy performance and economical profitability as well as feasibility of the investigated innovations will be assessed through monitoring and testing.
Geothermal heat pumps are among the most energy efficient technologies for heating and cooling of buildings and have the potential to greatly reduce greenhouse gases emissions. These systems typically have an average Coefficient of Performance in heating mode of approximately 3.5 (Lund, Boyd, 2015). It is reasonable to assume that this figure can be increased to 5.0. According to statistics published in 2015 at the World Geothermal Congress, 20 % of the buildings in Sweden are equipped with a geothermal heat pump (Gehlin et al., 2015). Although the technology is rather mature and has been used for several decades both in the residential and commercial sectors, there is still a great potential to increase its market penetration on both Swedish and European scale. One way to increase the market shares is to develop cost-effective methods to improve the performance and robustness of geothermal heat pumps. In this framework, collaboration between academia and industrial stakeholders plays a key-role role since it contributes to further technical developments of such systems, to highlight opportunities to reduce costs of the technology, to broaden areas for potential implementation, and to demonstrate its performance to the general public.
Swedish Energy Agency
Svensk Energi & Kylanalys
Both industry and researchers have recently invested a lot of resources to introduce innovative high performance ground heat exchanger solutions with very low “borehole thermal resistance”, meaning borehole heat exchangers capable of reducing as much as possible the temperature difference between the fluid circulating in the heat exchanger and the surrounding ground. This fact is crucial for the efficiency of the system since a variation of 1ºC in fluid temperature extraction has an impact of around 3% on the COP of the heat pump. Traditional heat exchanger design such as U-pipes have very limited potential in improving borehole thermal resistance without a significant increase in the pumping power necessary to circulate the secondary fluid in the ground loop.
Relevant improvements can be obtained only by a radical shift in the ground heat exchanger design. Among the proposed designs, coaxial heat exchangers have great potential for very high performance: preliminary tests showed that they can provide very low borehole thermal resistance and lower pressure drops in comparison with current state of the art ground heat exchangers (Acuña, 2013; Cvetkovski et al., 2014; Zarrella et al., 2011; Witte, 2012, Holmberg et al., 2016; Raymond et al., 2015). In coaxial heat exchangers, the fluid is usually circulated in near direct contact with the borehole wall thereby reducing the temperature difference between the ground and the fluid, leading to higher COP. In addition, they usually have a larger volume than traditional U-pipe, potentially allowing higher capacity coverage and thus reducing the need for additional heating systems.
Further research is required to test coaxial heat exchangers, assess their performance and robustness, and evaluate the technical challenges that can arise during the installation process. The study of these aspects is necessary to demonstrate that coaxial heat exchangers are a viable alternative to U-pipes and to promote their adoption in the near future.
Another recent tendency in the industry in Sweden is the utilization of deeper borehole heat exchangers compared to what has been used up to now especially in densely populated areas where the available surface plot for drilling is limited. Detailed studies in this area are still limited and there is a need to provide experimental data to test these systems.
Improved experiments may also help to better characterize the thermal response of the ground, which is a necessary ingredient for both design and control of GHP systems. Developments in this field can be beneficial for reducing operation costs and increase energy efficiency.
The system identification discipline (Ljiung 1987) provides tools to characterize the response of a system from collected data. Yet, to date, the only methodology utilized for identification of borehole systems is the Thermal Response Test (TRT). In this project the standard TRT which uses constant heat injection will be improved by applying advanced system identification approaches using variable heat injection to excite the system. The method proposed has the potential of addressing some limitations and drawbacks of traditional TRT such as difficulties in maintaining constant power during the test duration (3 to 4 days) and the lack of information that the test provides regarding the thermal response in time ranges of minutes to hours (short-term response).
This new TRT method as well as innovative / longer BHEs will be experimented in the KTH live-in-lab site. In this installation, a 100 m borehole dedicated to research will be accessible freely, meaning that it will be possible to remove and replace the BHE used in this well. Twelve other boreholes of lengths ranging from 225 to 350 m will also be monitored with a limited control over the operation. Instrumentation with fiber optic cables as well as measurements of the borefield geometry have already been done.
Data produced within the project is intended to be made public, within the framework of a data management plan that needs to be agreed upon with all involved parties. The idea is to create an data infrastructure accessible to the community so that data can be harnessed by other organisations according to a predefined schedule.
Aim and objectives
The project aims at investigating innovative borehole system solutions (namely deep borehole heat exchangers and coaxial borehole heat exchangers) and data analysis methodologies for improving design and operation of shallow geothermal systems.
Specific goals are:
- achieve at least 5 % reduction of the electricity consumed by the compressor and the circulation pump of a geothermal heat pump system;
- 10 GHP systems should be equipped with innovative BHEs by 2020;
- investigate the potential use of the technology proposed for retrofitting application
- decrease the required size and energy consumption of additional heating systems used for peak-load coverage by 15% and 30%, respectively;
- reduce environmental risks by easing the use of water as secondary fluid instead of ethanol or glycols;
- reduce life-cycle environmental impact;
- determine the increase in capacity (power and energy) per square meter of land that can be reached by using longer BHE;
- investigate the possibility of implementing the findings of this project in the Albano campus installation.
The ASHRAE criterion for heat load stability under thermal response tests might be insufficient to ensure proper estimation of the ground thermal conductivity via the Infinite Line Source (ILS) model (linear regression approximation). Deviations as high as 26% were observed for some cases, which may have a big impact on the design and, thus, investment and operating costs of GSHP installations.
Similarly, white noise on both heat rate and temperature measurements may lead to bias in the estimation of the ground thermal conductivity if time-superposition is applied. Deviations as high as 18% were observed for some cases. On the other hand, if the ILS approximation (linear regression) is used, the estimation bias is not as high (max. ca 7%).
Fast optimization algorithm for parameter estimation of thermal response tests using time-superposed ILS as interpretation model (to be made freely available).
Design, construction and instrumentation of a Ground-Source Heat Pump lab facility for development of new testing methods.
Thermal and hydraulic testing of coaxial borehole heat exchangers.
Performance mapping of a heat pump unit under different conditions.
Lazzarotto, A., Mazzotti Pallard, W., 2019. Thermal response test performance evaluation with drifting heat rate and noisy measurements, in: Proceedings. Presented at the European Geothermal Congress, The Hague, p.9
Mazzotti, W., Acuña, J., Lazzarotto, A., Palm, B., 2018a. Deep Boreholes for Ground-Source Heat Pump : Final report (Project report). KTH Royal Institute of Technology, Stockholm, Sweden.
Mazzotti, W., Firmansyah, H., Acuña, J., Stokuca, M., Palm, B., 2018b. Newton-Raphson method applied to the time-superposed ILS for parameter estimation in Thermal Response Tests, in: Research Conference Proceedings. Presented at the International Ground Source Heat Pump Association Research Conference, Stockholm, Sweden. Newton-Raphson method applied to the time-superposed ILS for parameter estimation in Thermal Response Tests (shareok.org)
Olausson, L., 2018. Construction and test of a new compact TRT equipment (Master thesis). KTH Royal Institute of Technology, Stockholm, Sweden.
Ovall, T., 2018. Borehole Heat Exchanger Test Rig Design (Internship report). University of Aberdeen, Stockholm, Sweden.
Vautrin, A., 2019. Development of a new test method for the thermal characterization of borehole heat exchangers (Master thesis). INSA Strasbourg, Stockholm, Sweden.
Project contact persons