Expansion Ground Source Heat Pump Research Articles

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

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

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.

Study on the heat transfer performance of boiling in vertical buried tube of direct expansion ground source heat pump

Two kinds of physical phenomena of phase change heat transfer and gas-liquid two-phase flow co-exist in the buried tube of the direct expansion ground source heat pump system. The performance of boiling heat transfer process in vertical U-tube was discussed in this study. Firstly, the models of the refrigerant flow equation and the phase change heat transfer process in the vertical U-tube, the heat transfer process internal and outside well heat transfer were established. Based on the models, the changes of the refrigerant parameters along the length of tube were simulated and analyzed during boiling heat transfer process. The simulation results showed that the pressure and temperature of refrigerant rise in the downward tube, while the pressure and temperature of refrigerant declined rapidly in the upward tube. So the evaporation kept slow rate in the downward tube but fast in the upward tube. The effect of heat transfer in upward tube is better. At the same time, the influence of the soil initial temperature is simulated. as the soil initial temperature declining, the changes of the refrigerant temperature and pressure in the buried tube are smaller. And the changes of the soil temperature field with the running time and the different depth of the borehole are also simulating studied. The simulation results show that the temperature around the buried tube decreases along with the operating time, and the temperature diffusion radius around the buried tube increases in a descent gradient.

Detailed Theoretical Characterization of a Transcritical CO2 Direct Expansion Ground Source Heat Pump Water Heater

A new avenue in modern heat pump technology is related to the use of natural refrigerants such as carbon dioxide (CO2). The use of CO2 in direct expansion ground source heat pumps (DX-GSHP) has also gained significant interest as it offers opportunities for cost reduction of the ground loop, albeit some challenges remain in their development, design and use. To address these challenges and to characterize CO2-DX-GSHP performance for water heating applications, a detailed theoretical model and a fully-instrumented test apparatus was developed and built at CanmetENERGY Research Laboratory. The theoretical model was validated against a set of experimental results and adopted to investigate the performance of the system over a wide operating range. Validation results showed that the model predicts the experimental results within the measurement uncertainty. A detailed system performance analysis was also performed using the theoretical model to understand the system behavior and explore the actions required for performance improvement in future installations. The results of the analysis showed that improper design and control of some components, such as the gas cooler and ground heat exchanger can degrade the system performance by up to 25%, and the heat pump heating capacity by 7.5%.

Geoexchanger System for Buildings Heating and Cooling

Geothermal heat pumps (GSHPs), or direct expansion (DX) ground source heat pumps, are a highly efficient renewable energy technology, which uses the earth, groundwater or surface water as a heat source when operating in heating mode or as a heat sink when operating in a cooling mode. It is receiving increasing interest because of its potential to reduce primary energy consumption and thus reduce emissions of the greenhouse gases (GHGs). The main concept of this technology is that it utilises the lower temperature of the ground (approximately <32°C), which remains relatively stable throughout the year, to provide space heating, cooling and domestic hot water inside the building area. The main goal of this study is to stimulate the uptake of the GSHPs. Recent attempts to stimulate alternative energy sources for heating and cooling of buildings has emphasised the utilisation of the ambient energy from ground source and other renewable energy sources. The purpose of this study, however, is to examine the means of reduction of energy consumption in buildings, identify GSHPs as an environmental friendly technology able to provide efficient utilisation of energy in the buildings sector, promote using GSHPs applications as an optimum means of heating and cooling, and to present typical applications and recent advances of the DX GSHPs. The study highlighted the potential energy saving that could be achieved through the use of ground energy sources. It also focuses on the optimisation and improvement of the operation conditions of the heat cycle and performance of the DX GSHP. It is concluded that the direct expansion of the GSHP, combined with the ground heat exchanger in foundation piles and the seasonal thermal energy storage from solar thermal collectors, is extendable to more comprehensive applications.

Modeling and Experimental Validation of a Transient Direct Expansion Heat Pump

Geothermal heat pump technology is currently one of the most interesting technologies used to heat buildings. There are two designs used in the industry: geothermal heat pump using a secondary ground loop and Direct Expansion (DX) ground source heat pump. The latter is less used, possibly because less research has been carried out for the design of this kind of heat pump. In this paper, a transient model using the Comsol Multiphysic of a DX ground heat pump is presented in heating mode with R22, and a comparison with experimental results is presented with a 24-hour test. It is shown that the model was adequately validated by our experiment with only a maximum difference of 15%. Following this validation, a parametric analysis was realised on the geometry of the borehole. This study concluded that to have the best heat extraction of the ground, the pipes shank spacing need to be important without increasing the borehole diameter.Article History: Received January 16th 2017; Received in revised form May 28th 2017; Accepted June 6th 2017; Available onlineHow to Cite This Article: Rousseau, C., Fannou, J.L.C., Lamarche, L. and Kajl, S. (2017) Modeling and Experimental Validation of a Transient Direct Expansion Heat Pump. International Journal of Renewable Energy Development, 6(2), 145-155.https://doi.org/10.14710/ijred.6.2.145-155

Geoexchanger System for Buildings Heating and Cooling

Geothermal heat pumps (GSHPs), or direct expansion (DX) ground source heat pumps, are a highly efficient renewable energy technology, which uses the earth, groundwater or surface water as a heat source when operating in heating mode or as a heat sink when operating in a cooling mode. It is receiving increasing interest because of its potential to reduce primary energy consumption and thus reduce emissions of the greenhouse gases (GHGs). The main concept of this technology is that it utilises the lower temperature of the ground (approximately <32°C), which remains relatively stable throughout the year, to provide space heating, cooling and domestic hot water inside the building area. The main goal of this study is to stimulate the uptake of the GSHPs. Recent attempts to stimulate alternative energy sources for heating and cooling of buildings has emphasised the utilisation of the ambient energy from ground source and other renewable energy sources. The purpose of this study, however, is to examine the means of reduction of energy consumption in buildings, identify GSHPs as an environmental friendly technology able to provide efficient utilisation of energy in the buildings sector, promote using GSHPs applications as an optimum means of heating and cooling, and to present typical applications and recent advances of the DX GSHPs. The study highlighted the potential energy saving that could be achieved through the use of ground energy sources. It also focuses on the optimisation and improvement of the operation conditions of the heat cycle and performance of the DX GSHP. It is concluded that the direct expansion of the GSHP, combined with the ground heat exchanger in foundation piles and the seasonal thermal energy storage from solar thermal collectors, is extendable to more comprehensive applications.

Heat Transfer Performance of the Underground CO2 Pipe in the Direct Expansion Ground Source Heat Pump

Under the refrigerating condition, the pipe circulating CO2 is directly buried underground. It acts as the gas cooler releasing heat from the refrigerant to the soil directly. The heat transfer performance of the CO2 gas cooler is very important to the performance of the direct expansion ground source heat pump. The heat transfer process at the gas cooler is analyzed. Based on that, the mathematic model of the buried pipe is established using energy balance method. Then the influence factors including shank spacing, thermal conductivity of soil and backfill material, inlet temperature and mass flow rate of refrigerant are studied. The results show that the heat transfer performance of the underground CO2 pipe has a tendency to increase with the factors. The results also show that thermal short circuiting exists between the shanks of CO2 gas cooler.

Energy saving potentials from the application of heat pipes on geothermal heat pump system

A ground-loop heat exchanger is one of the key components to collect low grade ground heat for a ground source heat pump system. A typical ground source heat pump consists of a refrigerant loop operated by a compressor and a geothermal brine loop operated by a circulation pump. Although ground source heat pumps are known as one of the most efficient HVAC devices, their performance still need to be improved since the energy consumption by the peripherals like the circulation pump are considerable and estimated more than 10% of the total electricity input. In order to reduce the ground circulation energy, a variety of researches such as direct expansion geothermal heat pumps are studied, but no apparent performance improvement was found due to other negative effects like the increase of pressure drop in the ground loop. Geothermal heat pipe can be a good alternative to replace ground-loop heat exchanger for heating purpose. The energy saving potential using heat pipe as ground heat exchanger was investigated by comparing the energy consumption and COP of three types ground source heat pump. As a result, energy consumption for heating in Seoul by 10.3% than direct expansion ground source heat pump and 21.1% than secondary loop ground source heat pump. Therefore, ground source heat pump with geothermal heat pipe can be a suitable choice to save energy in a cold region.

Experimental investigation of the performance of a ground-source heat pump system for buildings heating and cooling

Geothermal heat pumps (GSHPs), or direct expansion (DX) ground source heat pumps, are a highly efficient renewable energy technology, which uses the earth, groundwater or surface water as a heat source when operating in heating mode or as a heat sink when operating in a cooling mode. It is receiving increasing interest because of its potential to reduce primary energy consumption and thus reduce emissions of the greenhouse gases (GHGs). The main concept of this technology is that it utilises the lower temperature of the ground (approximately <32°C), which remains relatively stable throughout the year, to provide space heating, cooling and domestic hot water inside the building area. The main goal of this study is to stimulate the uptake of the GSHPs. Recent attempts to stimulate alternative energy sources for heating and cooling of buildings has emphasised the utilisation of the ambient energy from ground source and other renewable energy sources. The purpose of this study, however, is to examine the means of reduction of energy consumption in buildings, identify GSHPs as an environmental friendly technology able to provide efficient utilisation of energy in the buildings sector, promote using GSHPs applications as an optimum means of heating and cooling, and to present typical applications and recent advances of the DX GSHPs. The study highlighted the potential energy saving that could be achieved through the use of ground energy sources. It also focuses on the optimisation and improvement of the operation conditions of the heat cycle and performance of the DX GSHP. It is concluded that the direct expansion of the GSHP, combined with the ground heat exchanger in foundation piles and the seasonal thermal energy storage from solar thermal collectors, is extendable to more comprehensive applications.

Modeling and experimental validation of a transient direct expansion geothermal heat exchanger

Geothermal heat pump technology is currently one of the most interesting technologies used to heat buildings. There are two designs used in the industry: geothermal heat pump using a secondary ground loop and direct expansion (DX) ground source heat pump. The latter is less used, with one of the possibly reasons being that less research has carried out into the design of this sort of heat pump. In this paper, a model of a ground heat exchanger of a DX geothermal heat pump is presented in heating mode and a comparison with experimental results is presented. It is shown that the model is adequately validated by our experiment. After this validation, an analysis of the effect of the mass flow rate, the length and the angle of the borehole on the heat flux rate is presented. To conclude, an optimum configuration for the experiment is proposed.

SUSTAINABLE ENERGY SOURCES FOR THE FUTURE: A NEW GEOTHERMAL HEAT PUMP SYSTEM FOR THE BUILT ENVIRONMENT

In the recent attempts to stimulate alternative energy sources for heating and cooling of buildings, emphasise has been put on utilisation of the ambient energy from ground source heat pump systems (GSHPs) and other renewable energy sources. Exploitation of renewable energy sources and particularly ground heat in buildings can significantly contribute towards reducing dependency on fossil fuels. Geothermal heat pumps (GSHPs), or direct expansion (DX) ground source heat pumps, are a highly efficient renewable energy technology, which uses the earth, groundwater or surface water as a heat source when operating in heating mode or as a heat sink when operating in a cooling mode. It is receiving increasing interest because of its potential to reduce primary energy consumption and thus reduce emissions of the greenhouse gases (GHGs). The main concept of this technology is that it utilises the lower temperature of the ground (approximately &lt;32°C), which remains relatively stable throughout the year, to provide space heating, cooling and domestic hot water inside the building area. The main goal of this study is to stimulate the uptake of the GSHPs. Recent attempts to stimulate alternative energy sources for heating and cooling of buildings has emphasised the utilisation of the ambient energy from ground source and other renewable energy sources. The purpose of this study, however, is to examine the means of reduction of energy consumption in buildings, identify GSHPs as an environmental friendly technology able to provide efficient utilisation of energy in the buildings sector, promote using GSHPs applications as an optimum means of heating and cooling, and to present typical applications and recent advances of the DX GSHPs. The study highlighted the potential energy saving that could be achieved through the use of ground energy sources. It also focuses on the optimisation and improvement of the operation conditions of the heat cycle and performance of the DX GSHP. It is concluded that the direct expansion of the GSHP, combined with the ground heat exchanger in foundation piles and the seasonal thermal energy storage from solar thermal collectors, is extendable to more comprehensive applications.

Performance of heat pumps with direct expansion in vertical ground heat exchangers in heating mode

Ground source heat pump systems represent an interesting example of renewable energy technology for heating and cooling of buildings. The connection with the ground is usually done by means of a closed loop where a heat-carrier fluid (pure water or a solution of antifreeze and water) flows and, in heating mode, moves heat from ground to refrigerant fluid of heat pump. A new solution is the direct expansion heat pump. In this case, the heat-carrier fluid inside the ground loop is the same refrigerant fluid of heat pump.This paper focuses on the energy performance of direct expansion ground source heat pump with borehole heat exchangers in heating mode, looking at residential building installations. For this purpose, the evaporating process of the refrigerant fluid inside vertical tubes is investigated in order to analyze the influence of the convective heat transfer coefficient on the global heat transfer with the surrounding ground. Then, an analytical model reported in literature for the design of common borehole heat exchangers has been modified for direct expansion systems. Finally, the direct expansion and common ground source heat pumps have been compared in terms of both total borehole length and thermal performance. Results indicate that the direct expansion system has higher energy performance and requires lower total borehole length compared to the common system. However, when the two systems are compared with the same mean fluid evaporating temperature, the overall length of the ground heat exchanger of the direct expansion heat pump is greater than that of the common system.

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 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.

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.

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.

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.