Final results from IEA HPT Annex 52 - Long-term performance of large GSHP systems

Annex 52 - the international collaboration project on long-term performance of larger ground-source heat pump (GSHP) systems initiated through the Heat Pumping Technologies (HPT) technical collaboration program (TCP) of the International Energy Agency (IEA) in 2018 - was completed in December 2021. The aim of this IEA Annex was to analyze monitored long-term performance data from a variety of GSHP systems serving commercial, institutional, and multi-family buildings in the seven participating countries. To account for the variation and complexity of large GSHP systems, an extended system boundary schema, based on the SEPEMO schema, was developed, and used within Annex 52. 29 large GSHP performance-monitoring case studies, located in Sweden, Norway, Finland, Germany, the Netherlands, the UK, and the USA have been analyzed and reported. Performance factors and other system efficiency indicators for relevant time frames and system boundaries were determined. The experience from the included case studies have resulted in guideline documents for instrumentation and monitoring, as well as for uncertainty calculations and performance analysis and reporting. These documents will be of help for future GSHP projects, system optimization and fault detection. This paper provides a summary of the outcomes and results from Annex 52.Mechanical and Aerospace Engineerin

Approximate g-functions for selection of borehole field configurations used with ground-source heat pump systems

The arrangement of boreholes in ground heat exchangers used with ground-source heat pump systems is commonly based on pre-computed libraries of g-functions with standard configurations, e.g. placing the boreholes on a uniformly-spaced rectangular grid. Particularly for larger fields with many boreholes in situations with significant annual heat rejection/extraction imbalance, these configurations may be far from optimal. That is, depending on the space constraints, it may be possible to reduce the number of boreholes and amount of drilling required by shifting the positions of the boreholes to make better use of the available space. These configurations of boreholes are unlikely to be found in any library. Furthermore, manual arrangement of boreholes in complex-shaped fields is tedious and time-consuming for the engineer. Therefore, tools are needed that can automatically arrange boreholes in candidate configurations to fit the available land area, calculate the g-function for these configurations, select the best configuration, and determine the required depth for the best configuration. These tools need to be reasonably fast in order to be practical for the design engineer. This paper reports on a fast method for calculating approximate g-functions using non-uniform segments and pre-computed integral tables. Despite being “approximate” g-functions, the difference between a g-function calculated with a more detailed method and the approximate g-function is usually under 1% RMSE. The g-functions for borehole fields with 300, 500, and 1000 boreholes can be calculated in about 2, 6, and 30 seconds on a run-of-the-mill desktop PC. The paper presents the methodology, quantifies the computational time requirements and accuracy of both the g-function and the resulting designs.Mechanical and Aerospace Engineerin

Calculation Tool for Effective Borehole Thermal Resistance

This paper presents a tool for computing thermal resistance of single U-tube ground heat exchangers placed in vertical boreholes. The tool is complete in the sense that it can compute both local and effective thermal resistances for either grouted or groundwater-filled boreholes. For grouted boreholes, it utilizes the highly accurate multipole method. For groundwater-filled boreholes, it utilizes recently-published convection correlations. Thermal property routines for water and water-antifreeze mixtures allow calculation of the interior convective thermal resistance for a wide range of cases

Three years’ performance monitoring of a mixed-use ground source heat pump system in Stockholm

The student center, Studenthuset, at Stockholm University in Stockholm, completed in the fall of 2013, is a thoroughly instrumented mixed-use 6300 m2 four-story building. Space heating and hot water are provided by a ground source heat pump (GSHP) system consisting of five 40 kW off-the-shelf water-to-water heat pumps connected to 20 boreholes in hard rock, drilled to a depth of 200 m. Space cooling is provided by direct cooling from the boreholes. The Studenthuset building monitoring project is part of the IEA HPT Annex 52 – Long-term performance measurement of GSHP systems serving commercial, institutional and multi-family buildings. This paper presents results from three years of measured performance data to calculate the long-term performance of the Studenthuset GSHP system. A number of performance indices are calculated and presented to describe the short-term and long-term system performance for selected system boundaries. Seasonal, monthly and binned performance coefficients for both heating and cooling operation are presented and discussed. The Legionella protection system, hot water continuous circulation system, and internal heating/cooling distribution system reduce the system energy performance.Peer reviewedMechanical and Aerospace Engineerin

Half-term results from IEA HPT Annex 52: Long-term performance monitoring of large GSHP systems

IEA HPT Annex 52, Long-term performance measurement of GSHP systems serving commercial, institutional and multi-family buildings, started in January 2018 and will close in December 2021. Within the annex, a large number of larger ground source heat pump (GSHP) systems in seven countries are monitored and analyzed from a long-term performance perspective. By the end of 2019, 40 GSHP performance-monitoring case studies, located in Sweden, the Netherlands, the UK, Finland, Germany, Norway and the USA, form part of the Annex 52 work. These case studies cover a range of building types, system applications and ground sources. Annex 52 offers unique experience and information on GSHP system performance, which will result in guidance on instrumentation, monitoring, uncertainty analysis, data analytics, performance analysis and suitable performance indices based on international experience. This paper gives an overview of Annex 52, including the active monitoring projects and the work and findings so far.Peer reviewedMechanical and Aerospace Engineerin

Faster computation of g-functions used for modeling of ground heat exchangers with reduced memory consumption

This is a preprint of a paper presented at conference whose final version has been published in the Proceedings of Building Simulation, International Building Performance Simulation Association, September 2021.Temperature response functions, known as g-functions, are a computationally efficient method for simulating ground heat exchangers (GHEs), used with ground-source heat pump (GSHP) systems or direct ground cooling systems as part of a whole-building energy simulation. In fact, at present, there are no other methods that have sufficient accuracy and are fast enough to simulate a ground-source heat pump system in a whole-building energy simulation.The concept, mathematical derivation and an implementation of a g-function calculation program were originally developed by Claesson and Eskilson (1985). More recently (Cimmino 2018a, Cimmino 2018b, Cimmino 2019) developed an open-source g-function calculation tool known as pygfunction. This tool offers great flexibility for the user to compute g-functions for specific configurations of boreholes. However, for large borehole configurations (with ~1000 boreholes), the required time to compute a single g-function can take several hours, and the required RAM can be on the order of 100 GB, greatly exceeding most desktop PCs. In order to develop libraries of g-functions and training sets for machine learning approaches, we are computing hundreds of thousands of g-functions. This paper describes further development of Cimmino's methodology to speed the computation and reduce the memory requirements.Mechanical and Aerospace Engineerin

Long-term GSHP system performance measurement in the USA and Europe

This paper presents an overview of the International Energy Agency (IEA) technology collaboration program Heat Pumping Technologies (HPT) Annex 52, "Long term performance measurement of ground source heat pump (GSHP) systems serving commercial, institutional and multi-family buildings." This project, which ran from 2018 through 2021, focused on measuring the performance of larger GSHP systems, going beyond energy use intensities which commingle the performance of the building envelope, occupancy effects and the system performance. Instead, performance factors were calculated, similar to coefficients of performance, but measured over various time intervals and system boundaries. The primary objectives of the Annex were refining and extending methodologies to better characterize system performance in larger buildings, creating a library of quality long-term measurements in the form of case studies, and providing guidelines for instrumentation, uncertainty analysis, key performance indicators, data management and quality assurance.This paper summarizes the Annex outcome and illustrates use of the Annex boundary levels with a comparison between typical European and US GSHP systems. It is anticipated that this experience and the guidelines produced as a result of this experience will lower the cost and improve consistency and quality for future performance measurements

Ground heat exchanger design tool with rowwise placement of boreholes

Simulation-based design tools have been used since the late 1980s for designing ground heat exchangers (GHE) used with ground source heat pump (GSHP) systems. The ground heat exchanger simulations used in these tools rely on thermal response functions known as g-functions. Because of the significant computational burden in computing g-functions for even a single configuration, the design tools have relied on libraries of pre-computed g-functions. These g-functions were available for standard configuration shapes, such as lines, rectangles, open rectangles, L-shapes, and U-shapes. Standard shapes are often sub-optimal. For any building on a site, the available land may preclude use of a standard shape. For large GSHP systems with significantly imbalanced annual heat rejection and extraction loads, large rectangular fields may experience significant heat build-up (or heat draw-down) in the interior of the field. This paper describes a new ground heat exchanger design tool capable of automatically selecting and sizing both standard and irregular configurations. The focus of this paper is a method for creating, selecting and sizing irregular configurations where the available land area and "no-go" zones are described as irregular polygons

Use of cross g-functions to calculate interference between ground heat exchangers used in ground-source heat pump systems

Ground-source heat pump systems are increasingly popular for providing single-family home heating in Nordic countries. As the density of installations increase, questions sometimes arise as to the influence of new systems on existing systems. These questions cannot be readily answered, as design and simulation techniques developed over the last 35 years have focused on analysis of individual systems without regard to the influences of other systems.Response factor models of ground heat exchangers utilize pre-computed response functions known as g-functions. These g-functions give the response of the ground heat exchanger (non-dimensionalized temperature) to the past and current heat rejection or extraction of the ground heat exchanger. We might call this a “self-g-function.”In this paper, we define a “cross g-function” that gives the response of one ground heat exchanger to heat rejection or extraction of another ground heat exchanger. With this formulation, it is possible to determine the impact of a neighboring ground heat exchanger with a different load profile and history. This has many possible applications, but we demonstrate its use to study the sensitivity of nearby residential ground heat exchangers upon one another.Peer reviewedMechanical and Aerospace Engineerin