Direct Expansion Geothermal Heat Pump Research Articles

Modeling of a Condenser in Dynamic Operation Using Comsol Multiphysics Software

This paper is about modelling in dynamic operation the heat exchanger system using as condenser in one dimension characterized by two coaxial tubes with ribbed inner tube by using the equations of mass, momentum and energy conservation. The Comsol PDE interface is used to simulate the monophasic and biphasic flows of refrigerant. Heat transfer in water and inner wall of the condenser are modeled with two Heat Transfer Interfaces (solid, fluid) in Comsol software.
 The model has been validated by comparing the numerical and experimental results obtained with the direct expansion geothermal heat pump. The analysis of the comparative results shows that the obtained model adequately fits the experimental data with an average deviation of less than 5%. Therefore, it proves that it's a good model which can be used for simulation purposes. This developed numerical model was used to simulate the superheating, condensing and subcooling phases in the condenser. Vapor quality, pressure, enthalpy of the refrigerant and water temperature are also simulated.

A comparative exergo-economic analysis of four configurations of carbon dioxide direct-expansion geothermal heat pump

The present paper deals with a comparative exergo-economic analysis among four different horizontal Direct-Expansion Geothermal Heat Pumps (DX-GHP) with Carbon Dioxide (CO2) refrigerant in transcritical cycle. The studied configurations include the cycle with (a) expansion valve (EVC), (b) with expander (EC), (c) with expansion valve and Internal Heat Exchanger (EVC + IHE), and (d) with expander and IHE (EC + IHE). The unit cost of heat ($/kWhex) for each configuration was determined under two scenarios of a) constant Space Heating (SH) and b) constant Domestic Hot Water (DHW) loads with the peak of 8 kW. The results showed that the lowest unit cost of heat (cq) and the highest total exergy efficiency (ηex) are associated with the (EVC + IHE) and (EC + IHE) configurations, respectively. Also, it was shown that for the SH (or DHW) load, 20% (or 14%) increase in the ηex results in 21.5% (or 20%) increase in the cq considering the electricity cost of 0.025 $/kWh. For the electricity cost of 0.25 $/kWh, however, 11% (or 20%) increase in ηex results in increasing the cq by about 7% (or 13%). It was also concluded that the configuration with the highest ηex is not that with the lowest cq.

Cooling mode experimental analysis of a direct expansion geothermal heat pump with a control depending on the discharge pressure

This work presents an experimental study of a direct expansion geothermal heat pump in cooling mode. The three geothermal loops are installed in parallel. To improve the system performance, a specific control depending on the discharge pressure was implemented to select the geothermal loops to serve as condensers. The fundamental question is whether this particular heat pump can operate continuously? So the goal was to determine the operating limits in order to develop the best strategies for the most efficient use of this recent geothermal heat pump.The analysis of experimental results enabled us to reach a maximum of 12 h continuous for this heat pump with the implemented control. During this optimal period of operation, the coefficient of performance reaches an average of 2.60 with maximum values of up to 3.40. The average cooling capacity is 9.60 kW for an average ground heat rejection rate of about 290.8 W m−1.Beyond this period, the performance of the DX GHP decreases and some phenomena like the oscillation loop, soil saturation, were identified and presented.The experimental results allowed also generally reveal that the controls implemented promotes geothermal loops cooling and helps increase the performance of the DX GHP.This article represents a logical continuation of our previous published study on heating mode.

Effects of using expander and internal heat exchanger on carbon dioxide direct-expansion geothermal heat pump

In this article, the effects of using expander and Internal Heat Exchanger (IHE) have been investigated numerically in a horizontal Direct-Expansion Geothermal Heat Pump (DX-GHP), based on energy and exergy analyses. Carbon dioxide (CO2) in transcritical cycle has been used as the refrigerant, and the effects of the refrigerant pressure drop on heat exchangers, including gas cooler, evaporator, and IHE have been considered. A comparative study has been conducted among four cycles: (a) the cycle with expansion valve (EVC), (b) the cycle with expander (EC), (c) the cycle with expansion valve and IHE (EVC + IHE), and (d) the cycle with expander and IHE (EC + IHE). In the numerical model, the energy and exergy analyses have been performed for four cycles in different operational conditions and two different scenarios of (1) specific heating load and (2) constant Evaporator Loop Length (ELL). The model evaluates the performance of four cycles based on the coefficient of performance (COP), heating capacity, exergy efficiency, exergy destruction rates, and ELL. As a result, the COP and the exergy efficiency of the ECs are more than that of the EVCs. Using an additional IHE in the EVC always leads to a slight increase in the COP and the exergy efficiency. However, the use of an additional IHE in the EC has no significant effect on the COP and the exergy efficiency, although, it results in a significant decrease in the ELL. Also, the increase of IHE length in the EC causes a significant reduction in the ELL.

Energy and exergy investigation of a carbon dioxide direct-expansion geothermal heat pump

In this paper, a numerical simulation model has been extended for analyzing the energy and exergy of direct-expansion geothermal heat pumps (DX-GHP) in domestic hot water and heating applications. Carbon dioxide (CO2) in transcritical cycle has been used as the refrigerant and the effects of the refrigerant pressure drop in the heat exchangers have been considered. The present numerical investigation gives the energy and exergy analysis under different operational conditions in two separate cases. In the first case, the ground heat exchanger loop length is considered to be constant and in the second case, the heating load is considered to be fix. With the aim of evaluation the system work performance, a detailed parametric investigation has been carried out on the effect of different parameters including the difference between soil temperature and the outlet temperature of the evaporator, the compressor speed, the temperature and the mass flow rate of inlet water to the gas cooler, the gas cooler length, the number of ground heat exchanger loops, and soil temperature. The results of this study include heating capacity, coefficient of performance (COP), exergy efficiency, ground heat exchanger loop length can be used for design and optimization of a DX-GHP.

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

Modeling of a direct expansion geothermal heat pump using artificial neural networks

The real potential for energy savings exists in heating, ventilation, and air conditioning systems in general, and especially in geothermal heat pumps systems. Recent studies indicate that a mere 1% improvement in the efficiency of such systems generates millions of dollars in savings at the national level. This efficiency can be optimized when better control strategies are implemented. A first step in the control and optimization process is to establish a model that describes the system's behavior. In this study, artificial neural networks were selected for modeling a particular type of heat pump called direct expansion geothermal heat pump because the ground heat exchanger is a component of the heat pump, and thus directly plays the role of condenser or evaporator according to the operation mode. The data collection methodology and the algorithms used for training are presented. Of the four algorithms tested in this study with variable numbers of neurons in the hidden layers, the model obtained using the Levenberg–Marquardt (LM) algorithm with 28 neurons in the hidden layer appears to be the best, with an average coefficient of multiple determinations of about 0.9991, an average RMS of 0.16330, and an average COV of 2.9319.

Experimental analysis of a direct expansion geothermal heat pump in heating mode

Abstract In this study, we present an experimental analysis of a direct expansion (DX) geothermal heat pump (GHP) installed in the Thermal Technology Center (TTC) of the Ecole de technologie superieure in Montreal. The residential heating system studied consists of three geothermal 30 m deep wells, which use R22 as refrigerant. During the test campaign, which ran over a one-month period in early spring, the coefficient of performance of the heat pump varied between 2.70 and 3.44, with a daily average of 2.87. The heating capacity reached a daily average of 8.04 kW, for a cooling water constant volumetric flow rate of 0.38 L s −1 . The mean the ground heat extraction rate from was 58.2 W m −1 . The tests performed helped to highlight a pressure drop coupled with a relatively large superheating revealing a flow rate mal-distribution in geothermal evaporators. The effects of some factors (condenser cooling water inlet temperature, condensing temperature, pressure drop in the evaporator, thermal properties of soil and grout) that affect DX system performance are also presented. Finally, a comparative study between the use of electricity and the DX heat pump as home heating source shows that the DX heat pump provides savings of approximately 70% over the electricity.

주거용 직접순환식 수직밀폐형 지열원 열펌프의 연간 운전성능 평가

Vertical closed-loop ground source heat pump systems(GSHP) have been installed widely in Korea since it can extract moderate temperature level of geothermal heat in a small area. As a ground heat exchanger, a vertical closed -loop type with brine circulation is mostly preferred since it is simple and less harmful to ground environment. However, it requires a secondary heat exchange 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 is increased, which are important parts of performance degradation. In this paper, annual and seasonal performances of direct expansion(DX) geothermal heat pump were estimated mathematically as an alternative of classical indirect geothermal heat pump based on the annual performance evaluation. As results, DX geothermal heat pump showed 43% higher annual performance than the classical U-tube geothermal heat pump.

Parametric study on the performance of a direct-expansion geothermal heat pump using carbon dioxide

Abstract A numerical model has been developed to analyze the steady state performance of a direct-expansion geothermal heat pump using carbon dioxide in a transcritical cycle. The system consists of a compressor, counter-flow gas cooler/water heater, expansion device and the ground-coupled evaporator coils. The simulation results are found to be in good agreement with the results available in the literature. To analyze the performance characteristics of carbon dioxide in the ground-coupled evaporator, the parametric study focused on evaporator parameters including coil length, number of ground circuits and mean evaporation temperature. Also investigated is the relationship between gas cooler and evaporator sizing. Results indicate that there exists an optimum number of ground circuits which results in higher heating capacity and coefficient of performance. Performance can be further improved by optimizing the mean evaporation temperature with the surrounding soil temperature. With optimization, the system under study could achieve a coefficient of performance of 2.58, representing an 18% improvement compared to the baseline system.

5564282 Variable capacity staged cooling direct expansion geothermal heat pump

An analysis based on the laws of thermodynamics has been used to establish the amount of net work, efficiency, and the temperature of exhaust gases of a diesel engine (ideal cycles) as a function of pressure ratio and temperature ratio of the cycle. As a result the magnitude of the thermal energy available and the energy recoverable from the exhaust gases to drive an absorption refrigeration unit have been calculated. The variations of efficiency and net work of the diesel engine at different configurations as a function of cycle pressure ratio are given, a diagram has also been plotted for the amount of cooling capacity available for air-conditioning purposes.