Modelling, design, and control of hybrid ground source heat pump system coupled with district heating in an educational building complex

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School of Engineering | Doctoral thesis (article-based) | Defence date: 2025-01-31
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Date

2025

Department

Energia- ja konetekniikan laitos
Department of Energy and Mechanical Engineering

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Mcode

Degree programme

Language

en

Pages

101 + app. 107

Series

Aalto University publication series Doctoral Theses, 17/2025

Abstract

Ground source heat pumps (GSHPs) are an efficient solution for the decarbonization of building heating and cooling systems. They utilize borehole fields to extract heat from the ground for heating. The borehole fields can also supply free cooling energy for buildings in cold climates. Since buildings are heating-dominated in cold climates, conventional GSHPs face significant underground thermal imbalances. This often requires larger and more costly borehole fields to sustain high operating performance. An effective way to improve operational viability and financial sustainability is to integrate GSHPs with backup heating sources, such as district heating (DH), creating a hybrid GSHP system. This thesis investigated the modeling, design, and control of a hybrid GSHP system coupled with DH in an educational building complex based on simulation studies. The simplified modeling of an asymmetric-layout borehole field was validated using onsite brine temperature measurements. High-performing design methods for the hybrid GSHP system were explored by adjusting key design parameters, such as air handling unit (AHU) cooling water temperature level, indoor air temperature heating and cooling setpoints, GSHP design heating power, borehole number, and borehole depth. Regarding control, a cost-effective control strategy was developed to reduce system energy costs. The effects of power limitations, the input coefficient of performance (COP) value, and the control time horizon within the control algorithm were analyzed. Furthermore, a demand response control strategy was implemented to utilize the thermal storage of building thermal mass, investigating various DR control algorithms and parameters. Results indicate the simplified-geometry borehole field models can predict average inlet and outlet brine temperatures within a deviation of 1 °C from measured data and can also reduce computational time by up to 72% compared to the detailed models. Compared to the reference hybrid GSHP system design, increasing AHU cooling water temperature level and using lower indoor air temperature heating and cooling setpoints can raise the minimum outlet brine temperature from –6 °C to –3 °C over a 25-year lifetime, while also slightly improving the average heat pump COP in the last heating season. To ensure non-freezing boreholes, it is still necessary to increase the overall borehole length or reduce the GSHP design heating power. Compared to a GSHP-prioritized control strategy, the cost-effective control strategy, which incorporates power limitations and optimizes input COP values for control, achieves a 6.4% reduction in annual energy costs with hourly electricity and DH pricing. Implementing the DR control strategy to space heating enhances energy flexibilities for both electricity and DH networks without compromising indoor thermal comfort. Among studied DR control algorithms, the dual-price DR algorithm yields the highest cost savings. When it is combined with the cost-effective control, the hybrid GSHP system can achieve by up to a 10.8% reduction in annual energy costs with hourly electricity and DH pricing.

Description

Supervising professor

Kosonen, Risto, Prof., Aalto University, Department of Energy and Mechanical Engineering, Finland

Thesis advisor

Jokisalo, Juha, Dr., Aalto University, Department of Energy and Mechanical Engineering, Finland

Keywords

hybrid GSHP system, district heating, borehole field, modelling of hybrid systems, energy efficiency, energy flexibility, cost-optimal control, demand response

Other note

Parts

  • [Publication 1]: Xue, T.; Jokisalo, J.; Kosonen, R.; Vuolle, M.; Marongiu, F.; Vallin, S.; Leppäharju, N.; Arola, T. (2022). Experimental evaluation of IDA ICE and COMSOL models for an asymmetric borehole thermal energy storage field in Nordic climate. Applied Thermal Engineering, 217, 119261.
    Full text in Acris/Aaltodoc: https://urn.fi/URN:NBN:fi:aalto-202209215660
    DOI: 10.1016/j.applthermaleng.2022.119261 View at publisher
  • [Publication 2]: Xue, T.; Jokisalo, J.; Kosonen, R.; Ju, Y. (2023). Design of High-Performing Hybrid Ground Source Heat Pump. Buildings, 13(7), 1825.
    Full text in Acris/Aaltodoc: https://urn.fi/URN:NBN:fi:aalto-202308164798
    DOI: 10.3390/buildings13071825 View at publisher
  • [Publication 3]: Xue, T.; Jokisalo, J.; Kosonen, R. (2024). Cost-Effective Control of Hybrid Ground Source Heat Pump (GSHP) System Coupled with District Heating. Buildings, 14(6), 1724.
    Full text in Acris/Aaltodoc: https://urn.fi/URN:NBN:fi:aalto-202408065288
    DOI: 10.3390/buildings14061724 View at publisher
  • [Publication 4]: Xue, T.; Jokisalo, J.; Kosonen, R. (2024). Demand Response Potential of an Educational Building Heated by a Hybrid Ground Source Heat Pump System. Energies, 17(21), 5428.
    Full text in Acris/Aaltodoc: https://urn.fi/URN:NBN:fi:aalto-202411217439
    DOI: 10.3390/en17215428 View at publisher

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https://urn.fi/URN:ISBN:978-952-64-2370-8

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[diss] Insinööritieteiden korkeakoulu / ENG