Direct-expansion Ground Source Heat Pump Research Articles

Frost-resistant embankments with a novel ground source heat pump system

This study introduces a novel artificial heating technique aimed at addressing frost heave issues in embankments on frozen ground, offering a sustainable solution. Initial evaluations assessed various heating systems, including boilers, heat pumps, solar heaters, and electric heaters, focusing on their thermal performance and applicability. The study subsequently designed and implemented a direct-expansion ground source heat pump (DX-GSHP) system as the primary heat source for embankment warming. Rigorous testing confirmed the DX-GSHP system's ability to maintain a heat-supply temperature above 40 °C and a heat-absorption temperature below −3.5 °C, effectively extracting geothermal energy for transfer to the topsoil layers. With a demonstrated coefficient of performance (COP) of 3.49, the DX-GSHP system not only proves its energy efficiency but also suggests a potential role in reducing the strain on electricity supply systems. Installation of DX-GSHPs, with heating capacities ranging from 1.0–2.0 kW and spaced at intervals of 2.0–4.0 m, provides a rapid thermal response to frost heave in single-track railway embankments, thereby potentially mitigating frost-induced damage in cold climate regions.

The Development of a Novel Direct-Expansion Ground Source Heat Pump (DE-GSHP) for Embankment Heating in Cold Regions

This study aimed to investigate the practical applications of a ground source heat pump (GSHP) system for heating applications in embankment engineering. A special direct-expansion GSHP (DE-GSHP) was designed and manufactured, and its performance was experimentally assessed. Subsequently, the newly developed DE-GSHP structure demonstrated an excellent heating performance with respect to positive heat-supplying temperatures in cold regions. The temperature could be automatically controlled at 30, 45, 60, and 75; additionally, the heat-absorbing temperatures were maintained below 0 °C, which was extremely lower than that of the deep frost-free stratum. Further, the experimental data were used to evaluate the coefficient of performance of the DE-GSHP, which was more than 3.5. The findings indicated that the GSHP has potential applications for embankment heating in cold regions.

Energy, exergy, and economic analysis of a data center energy system driven by the CO2 ground source heat pump: Prosumer perspective

With increasing reliance on information technology by modern society, the scale and energy consumption of data centers (DCs) are rising rapidly. Approximately 40% of the energy consumed by DCs is associated with refrigeration systems used to maintain optimal operation temperatures. Such systems produce large amounts of waste heat; therefore, the utilization of the waste heat is essential for the optimization of the energy efficiency of DCs, especially with regard to DC prosumers (producers and consumers). However, the low temperature of the waste heat and variation in load demand are two factors limiting the exploitation of waste heat from DCs. In this study, a prosumer DC energy system based on a ground source heat pump (GSHP) is proposed for the recycling of waste heat from DCs for the heating of surrounding buildings. The heat generated in the non-heating period is stored in the soil and made available for building heating using a carbon dioxide (CO2) direct-expansion GSHP. Furthermore, the cooling power consumption, total power consumption, exergy efficiency, and net profit of the prosumer DC waste heat energy recovery system are evaluated. Compared to conventional cooling systems that use air source heat pumps (ASHPs) for building heating, the energy, exergy, and economic (3E) performance of the prosumer DC have the following advantages: 1) the prosumer DC waste heat energy recovery system yields a heat-power ratio of 5.70, with a total power consumption approximately half (52.86%) that of the conventional system; 2) the matching thermodynamic properties are greatly improved; specifically, the exergy efficiency of the prosumer DC waste heat energy recovery system is 3.87 fold that of the conventional system; 3) better economic revenue of the prosumer DC waste heat energy recovery system generates more economic revenue, with an annual net profit approximately 190.34% of that generated by the conventional system. In general, the prosumer system facilitates flexible and effective utilization of the low-temperature waste heat, by coupling the upstream power supplier and downstream thermal users through a GSHP.

Performance of a direct expansion ground-source heat pump for air conditioning

Performance of a direct expansion ground-source heat pump for air conditioning

The application of thermoelectric and ejector in a CO2 direct-expansion ground source heat pump; energy and exergy analysis

The thermodynamic analysis of a horizontal CO2 direct-expansion ground source heat pump in transcritical cycle was performed to investigate the influence of using thermoelectric sub-cooler and the ejector on the performance parameters of the cycle. A comparative analysis was conducted between the Base cycle and the cycles with ejector and thermoelectric sub-cooler considering the pressure drops in the gas cooler and the ground heat exchanger. The effects of compressor inlet temperature, soil temperature, thermoelectric sub-cooler length, thermoelectric sub-cooler cooling water mass flow rate, and ground heat exchanger outlet temperature on different performance parameters of the cycles were studied. The results demonstrated that the application of ejector exhibits a more stable operation of the cycle and increasing its COP by 16.5% as compared to the Base cycle considering the same ground heat exchanger length. Also, the length of ground heat exchanger is considerably decreased by using the ejector considering the same COP for the Base and EJR cycles. Besides, thermoelectric sub-cooler decreases the irreversibilities of the cycle by generation more power, thereby COP is boosted by 16% as compared to the Base cycle. Furthermore, the major irreversibility in the cycle with thermoelectric and ejector is occurred in the compressor and followed by ejector, ground heat exchanger, gas cooler, thermoelectric sub-cooler, and the expansion valve, respectively.

Experimental characterization of a transcritical CO2 direct expansion ground source heat pump for heating applications

Space heating and domestic hot water account for a large portion of energy consumption in buildings. Direct expansion ground source heat pump (DX-GSHP) is a promising renewable energy technology to provide heat efficiently in a wide range of applications in buildings. Moreover, there is a renewed interest on CO2, which is a viable option to replace synthetic refrigerants with an environmentally friendly alternative, yet limited studies have examined the performance of a CO2 DX-GSHP for different heating applications in buildings. A transcritical DX-CO2 GSHP had been built, and a number of tests were carried out to evaluate the performance of such a system and the associated ground heat exchanger (GHE) for three heating applications in buildings. Those applications represent low to high water temperature demand. DX-CO2 GSHP demonstrated a better performance and a higher heating capacity in applications with lower temperature demand. The highest COP was around 4 when DX_GSHP provides heat to radiant floor heating systems. Moreover, the performance of the system was examined with different number of active boreholes. The results implied that there could be an optimum number of boreholes to provide the highest heating capacity.

Influence of an internal heat exchanger on the operation of a CO2 direct expansion ground source heat pump

In this work, the influence of an internal heat exchanger (IHE) on the operation of a CO2 direct expansion ground source heat pump (DX-GSHP) was assessed through transient energy simulations. A previous theoretical model was extended to account for an IHE, and was used to evaluate the performance of DX-GSHP systems under various controls, ground conditions and building loads. A showcase of the benefit of integrating an IHE to the DX-GSHP was carried out, through 4-month heating energy simulations period of a small residential building located in Canada, under 4 different cases. Results show that adding an IHE to the system effectively counters the effect of thermal short-circuiting in the borehole as well as ground surface temperature variations, and thereby increases its efficiency.

A transient coupled model of a variable speed transcritical CO2 direct expansion ground source heat pump for space heating and cooling

A transient coupled model for a variable speed CO2 direct expansion ground source heat pump (DX-GSHP) is developed. An annual simulation for integrated space heating and cooling of a small residential building located in a Canadian climate is conducted, during which numerous operation parameters are evaluated. The model includes a control strategy that regulates pressures of the fluid, thereby allowing the compression cycle to go from subcritical to transcritical for optimal heat rejection. The model can be used to simulate DX-GSHP systems under various loads and evaluate the effect of dynamic thermal behaviour of the ground on the system performance. The results show that there is a strong thermal short-circuiting within BHE when the system is operating at low mass flow, which hinders the system performance.

Experimental study of a carbon dioxide direct-expansion ground source heat pump (CO2-DX-GSHP)

Recently, the use of natural refrigerants in air conditioners and heat pumps has raised special interest from industry and the scientific community. It is seen as one of the most promising alternatives to replace conventional refrigerants, especially in ground source heat pumps for many applications. This paper presents the results of an experimental study of a transcritical carbon dioxide direct-expansion ground coupled heat pump, in heating mode. A test facility of this system was instrumented and tested for different operating conditions. Overall system performance, evaporation phase change of CO2 inside the ground heat exchanger and effect of active borehole number on system performance were investigated. In the particular case of this work, the coefficient of performance of DX-GSHP using CO2 as refrigerant could reach 3.5.

A quasi-transient model of a transcritical carbon dioxide direct-expansion ground source heat pump for space and water heating

In this study, a theoretical quasi-transient model is developed for detailed simulations of a carbon dioxide (CO2) direct-expansion ground source heat pump (DX-GSHP). This model combines a transient analytical model for the ground, steady-state numerical models for the borehole and the gas cooler, as well as several thermodynamic models for the remaining components of a conventional heat pump, organized in interacting subroutines to form a powerful simulation tool. Extensive validation combining experimental data and CFD-generated results was performed for the borehole before the tool was used to simulate a practical application case.Performance is investigated for a system satisfying both space heating and domestic hot water requirements of a typical single family detached home in a cold climate region. The variation of different system parameters is also evaluated in this study. It is shown that CO2 DX-GSHPs can offer relatively efficient and stable performance for integrated water and space heating applications. Furthermore, the importance of an accurate geothermal borehole sizing is highlighted for the DX-CO2 heat pump systems. It is shown that, due to changes in the system working conditions, the total borehole length is not linearly correlated with the heat pump energy consumption and other parameters such as heat pump coefficient of performance and pressure drop in ground heat exchangers. Results showed that increasing the total borehole length of an optimum design (reference case study) by 25% decreases the total annual energy consumption by only 6%. However, reducing total borehole length of the reference case by 25% increases the total annual energy consumption by 10%.

A techno-economic comparison of a direct expansion ground-source and an air-source heat pump system in Canadian cold climates

This study aims to compare two commonly used ASHP (air-source heat pump) and DX-GSHP (direct-expansion ground-source heat pump). There have been many debates on energy efficiency, system costs and relative payback period of DX-GSHP against ASHP systems over the past few years. In this context, and with the aim of enriching this debate, a detailed screening heat pump model previously developed is modified and used to compare the seasonal performance of ASHP vs DX-GSHP in a residential building in the cold climate city of Montreal. Further, a life cycle cost analysis is performed to account for the difference between initial and 10-year operating costs of the two systems based on the current prices in Quebec. The obtained results show that by proper sizing, energy consumption of the DX-GSHP system can be reduced by 50%. Moreover, with current borehole installation prices, the relative payback period of the GSHP (ground source heat pump) compared to ASHP is more than 15 years. However, if the borehole installation price reduced by 50% the payback period would be reduced to just a few years. Such results highlight the importance of further investigations in the area of DX-GSHPs, in order to reduce the borehole installation cost and increase its performance.

Performance Evaluation of Geothermal Heat Pump With Direct Expansion Type Vertical Ground Heat Exchanger

Vertical closed-loop groun-source heat pump systems have been installed widely in Korea since they can extract a moderate temperature level of geothermal heat in a relatively small area. For a ground heat exchanger, a vertical closed-loop type with brine circulation is mostly preferred since it is simpler and less harmful to the ground environment. However, it requires a secondary loop between the refrigerant in a heat pump and the brine. By adding a geothermal heat exchanger in the secondary heat exchange loop, circulation pumps should be attached and the temperature difference between refrigerant and ground would be increased, which causes performance degradation. In this paper, performances of direct expansion ground-source heat pump were evaluated mathematically as an alternative to the conventional secondary-loop ground-source heat pump.

Experimental performance analysis of a direct-expansion ground source heat pump in Xiangtan, China

The DX GSHP (direct-expansion ground source heat pump), which uses a buried copper piping network through which refrigerant is circulated, is one type of GSHP (ground source heat pump). This study investigates the performance characteristics of a vertical U-bend direct-expansion ground source (geothermal) heat pump system (DX GSHPS) for both heating and cooling. Compared with the conventional GCHP (ground coupled heat pump) system, the DX GSHP system is more efficient, with lower thermal resistance in the GHE (ground heat exchanger) and a lower (higher) condensing (evaporating) temperature in the cooling (heating) mode. In addition, the system performance of the whole DX GSHP system is also higher than that of the conventional GCHP system. A DX GSHP system in Xiangtan, China with a U-bend ground heat exchanger 42 m deep with a nominal outside diameter of 12.7 mm buried in a water well was tested and analysed. The results showed that the performance of this system is very high. The maximum (average) COPs of the system were found to be 6.08 (4.73) and 6.32 (5.03) in the heating and cooling modes, respectively.

Experimental investigation on the operation performance of a direct expansion ground source heat pump system for space heating

This paper describes a small-scale experimental direct-expansion ground source heat pump system in typical clay soils for space heating of a building in Jinzhou, China. It uses R22 as the refrigerant, consisting of four single U-shaped copper ground heat exchangers with a uniform depth of 20m. A special circular bend is designed on the copper tube to enhance the lubricant oil return of the scroll compressor. The operation performance and energy efficiency of the DX-GSHP system is tested in the heating mode. Results show that the present DX-GSHP system is reliable and effective. During the heating period, when the heating capacity of the indoor fan coil is 6.41kW, the room temperature varies from 18 to 20°C even if the door is opened or closed frequently. The average COP of the heat pump unit and the whole DX-GSHP system are 3.12 and 2.88, respectively, and the average heat-transfer rate of GHEs per depth is 54.4W/m. In addition, the influence of the opening of the electronic expansion valve on the operation performance is analyzed. The present results can provide useful guides for the design and further improvement of DX-GSHP systems under similar geological conditions.

Analysis of Thermodynamic Characteristic Changes in Direct Expansion Ground Source Heat Pump Using Hydrofluoroolefins (HFOs) Substituting for HFC-134a

HFO-1234yf and HFO-1234ze[E] have low global warming potential and zero ozone depletion potential. If they are used in the direct expansion ground source heat pump system substituting for HFC-134a, the system will be beneficial to mitigating climate change. This study aims to find out the thermodynamic characteristics of the direct expansion ground source heat pump system using HFO-1234yf or HFO-1234ze[E] by theoretical calculation. The results indicate that HFO-1234yf system in an actual cycle has the highest COP. HFO-1234yf and HFO-1234ze[E] have such smaller capacity per unit of swept volume that they need larger compression capacity if providing the same heating or cooling loads. For a given unit when HFC-134a is replaced with HFO-1234yf or HFO-1234ze[E], the capacity will decrease. More refrigerant charge is required in the HFO-1234yf or HFO-1234ze[E] system. The results also present that more refrigerant charge is required in the cooling mode than in the heating mode.

Comprehensive Benefit Analysis of Direct Expansion Ground Source Heat Pump System

Direct Expansion Ground Source Heat Pump (DXGSHP) system directly extracts heat or cold energy from ground by consuming electricity to provide for space conditioning. Compared with the currently widely-used secondary loop Ground Couple Heat Pump (GCHP) system, it has higher energy efficiency, lower operating costs, and less environmental impact. A case study is carried out in this paper. The subject is a residential building located in Beijing, China. It is assumed that the building adopts the DXGSHP system and the GCHP system respectively. Annual loads and energy consumption are simulated and computed. Then the initial cost, operating cost and CO2 emission are calculated. The economic benefit is analyzed with the Payback Time method and the Dynamic Annual Cost Value method. The environmental benefit is discussed mainly by comparing the CO2 emission savings. The results show that the DXGSHP system has higher initial costs, but lower operating costs, and less greenhouse gas emissions. The DXGSHP system has better comprehensive benefits than the GCHP system.

A techno-economic comparison of a direct expansion ground-source and a secondary loop ground-coupled heat pump system for cooling in a residential building

Abstract This paper reports a techno-economic comparison between a direct expansion ground-source heat pump system (DX-GSHP) and a secondary loop ground-coupled heat pump system (SL-GCHP). For this purpose, a DX-GSHP and an SL-GCHP system are designed and installed in parallel for same space cooling load in a demonstrating building in Hunan province, China. The structures of the two systems are described. The performances of the two systems were experimentally determined from June to September in cooling season of 2009. The average cooling performance coefficients (PFsys) of the DX-GSHP system was obtained to be 6.03 and that of the SL-GCHP system was determined to be 5.64. The average input power values for the DX-GSHP system and the SL-GCHP system are found to be 1.39 and 1.715 kWh, respectively, which means that DX-GSHP has a 23.8% higher efficiency than SL-GCHP in cooling mode. The paper studied the initial investment, the annual cost (AC) and the present worth (PW) of the two systems. It is concluded that the DX-GSHP is more economic than the SL-GCHP. Further more, the advantage and disadvantage of the DX-GSHP are analyzed. The minimal refrigerant velocity in order to ensure oil return is derived. It is shown that DX-GSHP could be vigorously pushed when oil return problem is solved satisfactorily.