Verification study of a GSHP system Manufacturer data based modeling

GSHP (ground source heat pump) systems have become widely used as a result of the recent increasing demand for new and renewable energy polices in Korea. However, reliability issues have been key issues during installation and operation since they were initially designed. This paper introduces a systematic method for verification of the actual operating performance of a water-to-water GSHP system. The main idea is to compare the actual performance with the manufacturer's data, based on the ISO standard, and then to reduce the gap between the two. The manufacturer's performance data include the EWT (entering water temperature), LWT (leaving water temperature), capacity, flow rate, power, and COP (coefficient of performance). The verification technique was tested using a water-to-water GSHP system designed and installed at the KIER site. The verification study showed that actual performance was lower than that specified by the manufacturer's data. Accordingly, the refrigerant was recharged and the compressor and the expansion valve were replaced, resulting in an increase in the heating and cooling COP by 25.26% and 18.24%, respectively. The new verification method allowed the easy identification of the problems affecting the GSHP system and their subsequent correction.     

Performance characteristics of a high temperature water-to-water heat pump

A simulation model is utilized to predict the performance of a high temperature water-to-water heat pump, running on Refrigerant 11, over a range of evaporator and condenser water temperature (10 to 40°C and 40 to 70°C) and compressor speeds (500 to 3000 r.p.m.). It is shown that heat pump power output can be effectively controlled by varying compressor speed. Effects of compressor speed, heat source and heat sink (end-use) temperature on the heat pump efficiency are presented. Special attention is devoted to the values of predicted refrigerant temperature at the compressor discharge. These are compared with the thermal limit of the refrigerant. Modifications to the system, to reduce refrigerant maximum temperature, are also discussed.     

Performance evaluation of R-22 alternative mixtures in a breadboard heat pump with pure cross-flow condenser and counter-flow evaporator

This paper presents results of performance tests for R-22 and four alternative fluids (R-134a, R-32/134a (30/70%), R-407C, and R-410A) at operating conditions typical for a residential air conditioner. The study was performed in an experimental breadboard water-to-water heat pump in which a water/ethylene glycol mixture was used as the heat transfer fluid. The heat exchangers representing the evaporator and condenser were counter flow and cross flow, respectively. In tests performed at the same capacity, R-410A had the highest coefficient of performance. Test results for the system and data characterizing the performance of the heat exchangers and compressor are presented. The impact of the wide variations in the different alternative fluid properties on the system's operation and performance is particularly noted. The benefit of the liquid-line/suction-line heat exchange cycle is also addressed.     

Cooling performance of a vertical ground-coupled heat pump system installed in a school building

This paper presents the cooling performance of a water-to-refrigerant type ground heat source heat pump system (GSHP) installed in a school building in Korea. The evaluation of the cooling performance has been conducted under the actual operation of GSHP system in the summer of year 2007. Ten heat pump units with the capacity of 10 HP each were installed in the building. Also, a closed vertical typed-ground heat exchanger with 24 boreholes of 175 m in depth was constructed for the GSHP system. To analyze the cooling performance of the GSHP system, we monitored various operating conditions, including the outdoor temperature, the ground temperature, and the water temperature of inlet and outlet of the ground heat exchanger. Simultaneously, the cooling capacity and the input power were evaluated to determine the cooling performance of the GSHP system. The average cooling coefficient of performance (COP) and overall COP of the GSHP system were found to be ～8.3 and ～5.9 at 65% partial load condition, respectively. While the air source heat pump (ASHP) system, which has the same capacity with the GSHP system, was found to have the average COP of ～3.9 and overall COP of ～3.4, implying that the GSHP system is more efficient than the ASHP system due to its lower temperature of condenser.     

The quantitative evaluation of design parameter's effects on a ground source heat pump system

The main solution for the reduction of energy consumption in the field of HVAC is the development of new and renewable energy technologies. Among the various renewable energy systems, ground source heat pump (GSHP) systems have been spotlighted as efficient building energy systems because of their great potentials for energy reduction in building air conditioning and reducing CO₂ emissions. However, higher initial cost works as a barrier to the promotion of their use. Therefore, it is critical to reduce the initial costs by optimizing the design of the system. In this paper, parameters that affect the performance of the GSHP system and the size of ground loop heat exchanger (GLHX) have been investigated. Ratio of GLHX length to unit capacity (L/Q) decreased according to increasing value of thermal conductivity, but L/Q increased according to increasing value of borehole heat transfer resistance. In cooling mode, L/Q decreased according to increasing EWT of underground circulating water and borehole distance but increased in heating mode. The value of L/Q tended to increase according to increasing underground initial temperature in cooling mode, but decreased in heating mode. L/Q decreased according to increasing U-tube separation distance and decreasing underground circulating water flow rate, because the thermal interference effect of underground circulating water and heat absorption and emission rate from the ground decreased. The reduction of the size of GLHX is very important in the aspect of saving total installation cost of a GSHP system. Therefore, the size of GLHX and the performance of GSHP system should be considered together for optimum design of the GSHP system.     

Extremum seeking control for efficient operation of hybrid ground source heat pump system

The Hybrid Ground Source Heat Pump (GSHP) systems combine the renewable geothermal energy and cooling tower for rejecting the cooling load, which is often adopted for high cooling demand. Model based control can be limited due to variations in ambient conditions, ground-loop heat exchanger (GHE) and equipment characteristics, cost and reliability of sensors. A self-optimizing control scheme is proposed for efficient operation of the hybrid GSHP based on Extremum Seeking Control (ESC), with feedback of the total power consumption and the control inputs of the relative flow rate of cooling tower and the water pump speed. The cooling capacity of the heat pump regulates the evaporator leaving water at 7 °C. A Modelica based dynamic simulation model is developed for a Hybrid GSHP system, with the vertical GHE model adopted from Modelica Buildings Library. The transient heat transfer is implemented with a finite volume method inside and outside the borehole. The proposed ESC scheme is evaluated under the scenarios of fixed cooling load, ramp change in the evaporator inlet water temperature, diurnal sinusoidal cycle of air wet-bulb temperature, and realistic ambient and cooling load condition. Simulation results show the proposed ESC strategy effectively achieves nearly optimal efficiency without the need for plant model.     

A distributed model of a space heat pump under transient conditions

A prototype heat pump was designed and tested, as means of active thermal management for electronics packages to be used on stratospheric balloon missions. The evaporator worked as a cold plate to absorb heat dissipated by the electronics, while the condenser rejected heat primarily by radiation to the rarified environment. To predict the transient performance of the heat pump under varying environmental temperature and cooling load conditions, a dynamic model of the heat pump is created with a graphical user interface (GUI). The simulation of the evaporator and condenser are fully transient and the components are segmented, whereas the compressor and expansion device are lumped models and assumed to be at quasi-steady state. A detailed model for the mass and energy conservation in the two heat exchangers is presented. The spatial and temporal variation of temperature and mass flow rate in the heat exchangers are predicted. Several types of transient conditions such as step changes of the space temperature and cooling load, system start-up, shutdown, and cycling, are studied. The space temperature, cooling load, compressor power, mass flow rates of the compressor and expansion device, pressures and refrigerant charges of the condenser and evaporator, and temperature distribution in the heat exchangers are dynamically displayed on the GUI. The simulation results are compared with experimental data for step changes in the cooling load and show good agreement in terms of trends. Copyright © 2002 John Wiley & Sons, Ltd.     

Performance and economic feasibility of ground source heat pumps in cold climate

Ground source heat pumps (GSHP) are attractive alternatives to conventional heating and cooling systems owing to their higher energy utilization efficiency. In this paper, the effect of various system parameters on GSHP performance is studied using a computer model. Also, a comparative economic evaluation is carried out to assess the feasibility of using a GSHP in place of conventional heating/cooling systems and an air source heat pump. The results indicate that system parameters can have a significant effect on performance, and that GSHP is economically preferable to conventional systems. © 1997 John Wiley & Sons, Ltd.     

Performance characteristics of mobile heat pump for a large passenger electric vehicle

In this study, the performance of a mobile heat pump for an electric bus, which uses the wasted heat of electric devices for a heating and air source for a cooling, was evaluated. Both cooling and heating performances of the mobile heat pump were tested under various experimental conditions, and then optimized by varying the refrigerant charge and the compressor frequency. The cooling capacity at all compressor frequencies was over 23.0 kW, which is sufficient for the cooling loads of an electric bus. The heating COP decreased but the heating capacity increased with the rise of outdoor temperature and the compressor frequency. The heating COP was 2.4 at an outdoor temperature of 10.0 °C. The observed heating and cooling performance characteristics of the mobile heat pump means it could be used for cabin heating and air conditioning of an electric vehicle with a short driving range.     

Theoretical performance analysis of heat pump water heaters using carbon dioxide as refrigerant

The aim of this paper is to simulate the performance of an air source heat pump water heater using carbon dioxide (CO₂) as a working fluid. The heat pump water heating system consists of a compressor, a gas cooler, an expansion device and an evaporator. The computer simulation model has been developed by using the heat transfer data and the thermodynamic properties of CO₂. The effects on the heat pump performance by the operating parameters such as the compressor rotational speed, the inlet water temperature at the gas cooler, the inlet air temperature at the evaporator and the mass flow rate ratio of water to refrigerant were presented. For rated capacities of a 4 kW compressor with a 10 kW gas cooler and a 6 kW evaporator, the coefficient of performance is found to be between 2.0 and 3.0. The mass flow rate ratio of water and CO₂ between 1.2 and 2.2 is the most suitable value for generating hot water temperature above 60°C at 15–25°C ambient air temperature. Copyright © 2007 John Wiley & Sons, Ltd.     

Current status of ground source heat pumps in Korea

In Korea, ground source heat pumps (GSHP) have been gaining popularity for space heating and cooling. Because there are few sources of high-temperature geothermal energy in the country, public baths (25–40 °C) and geothermal heat pumps (～15 °C) using low-temperature groundwater or ground are the most dominant direct geothermal uses. The Promotion Law of the New and Renewable Energy Development, Use and Dissemination, enacted in 2004, imposed an obligatory installation of space heating and cooling systems using new and renewable energy sources including geothermal energy for newly constructed public buildings (more than 5% of total construction cost). Between 2004 and 2007, ground source heat pump systems occupied about 60% of the total public installation of new and renewable energy equipment. Starting with 35.2 kW of two facilities in 2000, systems with the capacity of over 127.1 MWt have been installed in 551 buildings (facilities) as of August 2008. The vertical closed heat pump system (closed loop) and the groundwater heat pump system (standing column well type; SCW) occupied 65.1% and 29.3%, respectively, among the total GSHP systems installed. The depth of the vertical loops ranged between 65 and 250 m (average 159 m) and the well depth of the SCW system ranged between 150 and 600 m (average 391 m). The number of geothermal energy companies, installing the GSHP systems, that are officially registered in the relevant authority increased from 5 in 2000 to 397 in July 2008. This paper presents details of the current status of ground source heat pumps in Korea.     

Calculation algorithm of the temperatures for pipe arrangement of multiple ground heat exchangers

The authors introduce calculation algorithm of the temperatures of the ground and heat carrier fluid in multiple ground heat exchangers for pipe arrangement of ground source heat pump (GSHP) systems. First, the outline is explained. Next, in order to investigate possibility for the operation of the GSHP system with steel foundation piles and validate reproducibility of the value calculated by the design tool including the calculation algorithm, field tests of heating and heat extraction were conducted with a residential GSHP system using 25 steel foundation piles of 8 m long as ground heat exchangers. From a result of comparison between temperatures of the measurement in the test and calculation by using the design tool, it was confirmed that the tool could predict the temperatures with acceptable precision and speed for utilizing as a design tool. In addition, performance of GSHP systems with steel foundation piles in long term is predicted with the design tool. In moderate climate region, since the GSHP systems using multiple ground heat exchangers with short length can operate with high efficiency as well as the GSHP system using a single ground heat exchanger with long length, the GSHP systems with steel foundation piles have possibility to become popular.     

Modeling and simulating the transcritical CO2 heat pump system

In this paper, a mathematical model for steady-state simulation of transcritical CO₂ water-to-water heat pump system with an expander has been developed. It is used to simulate the performance of transcritical CO₂ system with CO₂ expander prototype. Simulated results are compared with experimental data to verify the accuracy of the simulation model. The comparison results show the average deviation of about 15% for COP_c(cooling coefficient of performance) and COP_h(heating coefficient of performance), about 17% for cooling and heating capacity at experimental high pressure ranges. With this model, which has been validated in a limited high pressure range, the influence of water mass flow rate and water inlet temperature of both evaporator and gas cooler on the performance of transcritical CO₂ expander system is analyzed. The results show that decreasing inlet temperature and increasing mass flow rate of cooling water cannot only increase the system performance but also reduce the optimal heat rejection pressure, at which the maximum COP (coefficient of performance) can be obtained. For chilling water, increasing its inlet temperature and mass flow rate is favorable for increasing the system performance, while the optimal heat rejection pressure does not vary very much.     

Experimental investigation of heat transfer enhancement in a double pipe heat exchanger with a twisted inner pipe

In the present work, an experimental investigation is conducted to address the influence of inner pipe twisting on the overall performance of a double pipe heat exchanger. With the fluid to fluid heat exchange, both parallel and counter flow directions are examined as well. In addition to the original elliptical pipe, three pipes with different numbers of twisting (3, 5, and 7 twists per unit length) constructed from the elliptical pipe are considered where the heat transfer rate and pressure drop are addressed. All tests are carried out in the turbulent flow regime where the Reynolds number (Re) ranging from 5000 to 26,000 and water is used as the working medium. The obtained outcomes show that for both flow directions, there is an enhancement in the heat exchanger overall performance with all considered twisting pipes. The maximum enhancement in the Nusselt number is found to be 1.8 for the parallel flow and around 2.2 for the counter flow compared with the original pipe. The inner pipe with 7 twists, however, improves the overall performance the most, where a maximum performance enhancement factor of 1.63 and 1.9 are observed at Reynolds number of 26,000 in the parallel and counter flow configurations, respectively.     

Transcritical CO2 heat pump systems: exergy analysis including heat transfer and fluid flow effects

This paper presents the exergetic analysis and optimization of a transcritical carbon dioxide based heat pump cycle for simultaneous heating and cooling applications. A computer model has been developed first to simulate the system at steady state for different operating conditions and then to evaluate the system performance based on COP as well as exergetic efficiency, including component wise irreversibility. The chosen system includes the secondary fluids to supply the heating and cooling services, and the analyses also comprise heat transfer and fluid flow effects in detail. The optimal COP and the exergetic efficiency were found to be functions of compressor speed, ambient temperature and secondary fluid temperature at the inlets to the evaporator and gas cooler and the compressor discharge pressure. An optimization study for the best allocation of the fixed total heat exchanger inventory between the evaporator and the gas cooler based on heat transfer area has been conducted. The exergy flow diagram (Grassmann diagram) shows that all the components except the internal heat exchanger contribute significantly to the irreversibilities of the system. Unlike a conventional system, the expansion device contributes significantly to system irreversibility. Finally, suggestions for various improvement measures with resulting gains have been presented to attain superior system performance through reduced component irreversibilities. This study is expected to offer useful guidelines for system design and its optimisation and help toward energy conservation in heat pump systems based on transcritical CO₂ cycles.     

Water-to-water heat transfer in tube–tube heat exchanger: Experimental and analytical study

Tube–tube heat exchanger (TTHE) is a low cost, vented double wall heat exchanger which increases reliability by avoiding mixing of fluids exchanging heat. It can be potentially used for heat recovery from engine cooling circuit, oil cooling, desuperheating in refrigeration and air conditioning, dairy, and pharmaceutical industry, chemical industry, refinery, etc. These tube–tube heat exchangers are successfully demonstrated for superheat recovery water heating applications, condenser and evaporator in heat pumps, lube oil cooler for shipboard gas turbines, milk chilling and pasteurizing application. This paper presents an experimental study on various layouts of TTHE for water-to-water heat transfer. The theoretical and experimental results on this type of heat exchanger configuration could not be located in literature. Overall heat transfer coefficient and pumping power were experimentally determined for a fixed tube length and surface area of serpentine layouts with different number of bends and results are compared with straight tube TTHE. In the case investigated, serpentine layout TTHE with seven bends has shown optimum performance, with overall heat transfer coefficient 17% higher than straight tube TTHE. Two out of five serpentine layout TTHE have shown poor heat transfer performance than straight tube TTHE. The experimental results also indicate that there is a definite optimum for a number of bends in serpentine layout TTHE. An analytical model for prediction of thermo-hydraulic performance of straight layout has been developed and validated experimentally.     

System optimization and experimental research on air source heat pump water heater

This paper deals with the system optimization of air source heat pump water heater (ASHPWH), including calculating and testing. The ASHPWH system consists of a heat pump, a water tank and connecting pipes. Air energy is absorbed at the evaporator and pumped to storage tank via a Rankine cycle. The coil pipe/condenser releases condensing heat of the refrigerant to the water side. An ASHPWH using a rotary compressor heated the water from initial temperature to the set temperature (55 °C). The capillary tube length, the filling quantity of refrigerant, the condenser coil tube length and system matching are discussed accordingly. From the testing results, it could be seen that the system performance COP could be improved obviously.     

Experimental investigation of heat transfer in a triple tube heat exchanger with coiled spring inserts

Experiments have been performed to investigate the effect of coiled spring inserts on heat transfer, pressure drop, and performance parameters of a triple tube heat exchanger (TTHX). Three different spring inserts having a pitch of 5, 10, and 15 mm are used and the diameter of the spring wire is taken as 1 mm. The experiments were carried out under a turbulent flow regime, with water as a working medium in parallel and counter flow configurations. The variation in different performance characteristics like heat transfer coefficient, Nusselt number, and effectiveness have been compared at various Reynolds numbers ranging between 4000 and 16,000 in the considered flow patterns. The Nusselt number of TTHX with the lowest pitch spring is found to be higher than that of the plain TTHX by 57.27% at Re = 4000 for the counter flow configuration. Both the thermal performance factor and effectiveness increased as the pitch of the spring insert was decreased. The effectiveness of TTHX with the lowest pitch spring insert is found higher than that of the plain TTHX by 43.84% in the counter flow pattern.     

Experimental measurement and numerical simulation of horizontal-coupled slinky ground source heat exchangers

Results from both experimental measurements and 3D numerical simulations of Ground Source Heat Pump systems (GSHP) at a UK climate are presented. Experimental measurements of a horizontal-coupled slinky GSHP were undertaken in Talbot Cottage at Drayton St Leonard site, Oxfordshire, UK. The measured thermophysical properties of in situ soil were used in the CFD model. The thermal performance of slinky heat exchangers for the horizontal-coupled GSHP system for different coil diameters and slinky interval distances was investigated using a validated 3D model. Results from a two month period of monitoring the performance of the GSHP system showed that the COP decreased with the running time. The average COP of the horizontal-coupled GSHP was 2.5. The numerical prediction showed that there was no significant difference in the specific heat extraction of the slinky heat exchanger at different coil diameters. However, the larger the diameter of coil, the higher the heat extraction per meter length of soil. The specific heat extraction also increased, but the heat extraction per meter length of soil decreased with the increase of coil central interval distance.     

Comprehensive exergy analysis of a ground-source heat pump system for both building heating and cooling modes

This paper presents a comprehensive exergy analysis of three circuits and whole system of a ground-source heat pump (GSHP) for both building heating and cooling modes. The purpose is to search out the key potential energy saving components. The analytical formulae of exergy loss, exergy efficiency, exergy loss ratio, exergy loss coefficient and thermodynamic perfect degree are derived, respectively. The results show that these exergy indexes should be used integratively, and in the whole system the location of maximum exergy loss ratio is the compressor, while the location of minimum exergy efficiency and thermodynamic perfect degree is the ground heat exchanger, so that the compressor and the ground heat exchanger should be primarily improved. The results also indicate that the exergy loss of a GSHP system for building heating mode is bigger than that of cooling mode, and the exergy efficiency of a whole GSHP system is obviously lower than those of its components for both building heating and cooling modes. Therefore, a comprehensive exergy analysis of a GSHP should be paid more attention to. The results may provide guidelines for the design and optimization of GSHP systems.