Direct-expansion Heat Pump Research Articles

Experimental study on a direct-expansion micro-channel PV/T evaporator using water-ZnO nanofluid as the spectral beam splitter

Introducing nanofluid (NF) as the spectral beam splitter (SBS) in photovoltaic/thermal (PV/T) evaporators can significantly enhance their performance and energy flexibility. In this study, a novel direct-expansion micro-channel evaporator using water-ZnO nanofluid as SBS (i.e., NF-MC-PV/T evaporator) was manufactured. Particularly, the stability of low-viscosity water-ZnO nanofluid was first optimized and natural convection within the NF-MC-PV/T evaporator was directly formed. Further, a direct-expansion heat pump system based on the NF-MC-PV/T evaporator was developed and tested. According to the findings, adding sodium hexametaphosphate as a dispersant with the dispersion ratio of 1:5 and ultrasonic treatment for 1.5 h can achieve the optimal stability of water-ZnO nanofluid. The nanofluid's natural convection can reduce the evaporator's operating temperature and improve the PV module's temperature distribution uniformity. Specifically, the NF-MC-PV/T evaporator's operating temperature was lower than the environment temperature and a pure PV module. Besides, the PV module's temperature distribution was uniform in the direction perpendicular to the refrigerant flow with a max temperature gradient of only 4.59 °C. The typical photothermal efficiency of the NF-MC-PV/T evaporator reached 102.55 % while the heat loss coefficient was only 4.37. Moreover, the average photothermal efficiency, photovoltaic efficiency, and COP of the system were 103.58 %, 12.03 %, and 5.09, respectively.

Energy analysis of a direct expansion heat pump assisted by a thermal photovoltaic panel for hot water production in several regions of Brazil

This study focused on evaluating the energy performance and economic viability of a direct expansion photovoltaic-Thermal Heat Pump (PV-T/HP) system for residential heating applications in various regions of Brazil. A detailed mathematical model was implemented to compare the PV-T/HP system with other solar heating technologies, as photovoltaic with solar evacuated tubes collector (PV+TSC) and photovoltaic with electrical heat water (PV+EHW), analyzing key metrics such as electrical efficiency, thermal efficiency, and levelized cost of heat (LCOH). The study revealed variations ranging from 7.5 to 2.5 in the Coefficient of Performance (COP) of the heat pump under different solar irradiation and ambient temperature conditions, emphasizing the necessity for future optimization to enhance the system performance. Results indicated that cities with the lower ambient temperature such as Belo Horizonte and Porto Alegre exhibited favorable conditions for the PV-T/HP system, leading to increased energy generation and positive net electricity production. The study highlighted the need for tailored strategies to control system operations, limit compressor consumption, and improve overall efficiency. These findings contribute to the understanding of PV-T/HP systems in residential heating applications, showcasing their potential for sustainable energy solutions in Brazil.

Study on performance of direct-expansion heat pump water heater integrated uninsulated micro-channel PV/T solar collector

In this paper, a direct-expansion heat pump water heater system integrated uninsulated micro-channel PV/T solar collector (UMC-PVT-SC) is proposed. This system can realize storage for both thermal energy and electricity, ensuring the supply of domestic hot water. The theoretical models of micro-channel PV/T solar collector, hot water storage tank, compressor and electronic expansion valve are established to investigate the performance of the micro-channel PV/T heat pump water heater system. R134a is used as refrigerant. The system performance is numerically simulated using MATLAB for different solar radiation intensity, ambient temperature and wind speed. The results show that solar radiation intensity has the largest influence on the system performance, followed by ambient temperature and wind speed. The coefficient of performance of the heat pump (COPHP) and the overall coefficient of performance (COPpvt) of the integrated system increase while the required operating time reduces, as solar radiation intensity and ambient temperature increase. The uninsulated micro-channel PV/T solar collector (evaporator) can absorb thermal energy from both ambient air and solar radiation and therefore achieve high COPHP, electric efficiency and COPpvt. Even under cold weather conditions, the proposed solar collector and heat pump heater demonstrate high efficiencies, heating and power generation capacities. When ambient temperature is −18.6 °C and solar irradiance is 322 W/m2, the collector efficiency and electric efficiency of UMC-PVT-SC are still at 0.718 and 0.244, respectively, and the COPHP and COPpvt of the heat pump heater are 3.5, 5.63, respectively. The present work is of potential value for delivering net zero.

Energy, exergy, economic, and environmental (4E) analysis of SAHP water heaters in very cold climatic conditions

Solar-assisted heat pumps (SAHPs) are emerging as promising solutions for space/water heating. Compared to conventional air-source heat pumps (ASHPs), SAHPs show superior thermal performance in cold climates where outdoor air temperatures are below freezing most of the heating season. However, given the inherent variability of solar energy, SAHPs have poor thermal performance to meet the load requirements during periods of weak solar radiation. To enhance the reliability of SAHPs, an air-source evaporator can be integrated with a SAHP, forming a dual SAHP – ASHP system with better performance. In this research, the energy, exergy, environmental, and economic performance of a dual-source solar/air source heat pump, which provides hot water for a typical Canadian household, is investigated. The performance of the system is compared with a solar-assisted direct-expansion heat pump (DX-SAHP), an ASHP, and conventional gas/electric water heaters. A thoroughly validated mathematical model based on heat transfer and thermodynamics fundamentals is developed and implemented in MATLAB® coupled with the CoolProp® library, which gives the refrigerant's properties. Results highlight the superior performance of the dual-source heat pump, particularly in the summer, where it surpasses the DX-SAHP. Among the examined heat pump configurations, the dual-source heat pump demonstrates the lowest annual power consumption. The system attains the maximum monthly average COP of 3.94 and achieves a minimum of 2.4 during the severe winter season in Calgary, Alberta. Compared to a conventional ASHP, the energy consumption of the system was reduced by 25%. The second-law analysis shows that the ASHP obtains the highest monthly average exergy efficiencies, followed by the DX-SAHP and the dual-source heat pump. Furthermore, results show that the solar/air source heat pump water heater can reduce CO2 emissions by 1450 kg/year, while the system's payback period is 19 years.

A dynamic model of a vertical direct expansion ground heat exchanger

The paper presents a general distributed model of a vertical U-tube direct expansion heat exchanger coupled with the ground. This model is developed for studying the dynamic thermal behavior of a buried heat exchanger (the evaporator or condenser) which is an integral part of the so-called direct expansion heat pump. The transient conservative equations of mass and energy considering the two-phase flow of refrigerant are derived and presented. The heat exchange with the ground is treated using an analytical approach to treat short time response of vertical boreholes based on an imposed heat flux. The thermal interference between the two pipes of the heat exchanger is also considered. The mathematical equations of the model are numerically presented using a finite difference scheme and the solution of the system of equations is obtained using a built-in iterative solver function of MatLab®. The dynamic behavior of a condenser and an evaporator are first obtained by imposing a change of the inlet conditions. Experimental data from a test rig are used to demonstrate the validity of the dynamic model

Simulation analysis of a photovoltaic/thermal-air dual heat source direct-expansion heat pump

Simulation analysis of a photovoltaic/thermal-air dual heat source direct-expansion heat pump

CFD Analysis of Heat Transfer Enhancement in a Flat-Plate Solar Collector/Evaporator with Different Geometric Variations in the Cross Section

There is a growing demand from the industrial sector and the population to cover the need for water temperature increases that can be covered with systems such as heat pumps. The present research aims to increase the heat transfer to the working fluid in a collector/evaporator, part of a solar-assisted direct expansion heat pump. This research was developed using a numerical analysis and by applying computational fluid dynamics; different simulations were performed to compare the performances of collector/evaporators with models exhibiting variations in the cross-section profile under similar conditions. An average incident solar radiation of 464.1 W·m−2 was considered during the analysis. For the comparison, profiles with hexagon-, four-leaf clover-, and circular-shaped sections with floral shapes, among others, were analysed, resulting in a temperature increase at the outlet of the working fluid of 1.3 °C. In comparison, the collector/evaporator surface temperature varied between 4 and 13.8 °C, while the internal temperature of the fluid reached 11.21 °C. Finally, it is indicated that the best results were presented by analysing the profile corresponding to the circular section with the flower shape.

Hybrid PV/T Heat Pump System with PCM for Combined Heating, Cooling and Power Provision in Buildings

Hybrid photovoltaic-thermal heat pump (PV/T-HP) solar energy systems are promising since they can achieve a system total efficiency greater than 80%. By maximizing the output of a PV/T system for simultaneous heating and cooling, this strategy can meet over 60% of urban households’ heating needs and around 40% of their cooling needs. In this work, a novel PV/T evaporator was designed, fabricated, and an aluminium foil encapsulated hydrated salt (HS36) PCM was integrated with the PV/T evaporator of the PV/T direct expansion heat pump system (PV/T-DXHP). Energy analysis was carried out on the PV/T-DXHP system with PCM in tropical climate regions of India for achieving net zero energy buildings. The experimental study revealed that the average PV electricity efficiency was 14.17%, which is near the PV panel’s STC value. The average thermal efficiency of the system was 104.38%, and the PV/T system’s average overall efficiency was 117.58%. The heating and cooling COPs of the system were 5.73 and 4.62, respectively. It was concluded that net-zero energy buildings are possible with the help of photovoltaic heat pump systems that use PCM and solar energy to make electricity, cool spaces, and heat water.

Experimental research on direct expansion heat pump flue gas waste heat recovery and humidification nitrogen reduction system

In order to solve the heat loss and the pollution of gas boilers, a synergistic system consisting of waste heat recovery tower and air humidification tower is proposed. Increasing in moisture content of the air can inhibit the generation of nitrogen oxides and increase the dew point of the flue gas, which is beneficial to the utilization of the waste heat from the flue gas. More heat is absorbed by the humidification water at the fin heat exchanger after the flue gas enters the waste heat recovery tower, which enhances the heat and mass exchange process in the air humidification tower. But the performance of the heat pump was found to show a downward trend, indicating a competitive relationship between nitrogen oxides reduction and waste heat recovery. Under the optimal condition, the experimental system can reduce nitrogen oxides emissions by 62.35%, and the exhaust gas temperature can be reduced to 24.46 °C. The heat pump can recover 6.94% heat while maintaining a minimum nitrogen oxides emission of 39.66 mg/m3. The heat pump makes more heat from the fuel to enter the heating network, which improves the heating efficiency of the boiler system. The system can meet the high-efficiency and clean production requirements of the energy system at the same time.

Annual analysis of the photovoltaic direct-expansion heat pump assisted by double condensing equipment for secondary power generation

Thermoelectric generation (TEG) converts heat directly into electricity based on temperature difference. Many studies on combining TEG with photovoltaics to convert waste heat into electricity to increase overall power generation have been conducted. However, through previous research, if TEG was installed between the cooling and PV, it will hinder PV heat dissipation and cause electricity deterioration, which could not be compensated by TEG output. To ensure the PV cooling and meanwhile realize thermoelectric conversion, a photovoltaic direct-expansion double-condensing heat pump system based on TEG assisted by micro-channel heat pipes and water-cooling condenser is proposed. Experiments & mathematical model are carried out & verified. The regional weather conditions at different latitudes and altitudes are adopted to predict the system performance on a year basis. From the results, the electrical efficiency is improved due to the additional TEG power output. With a higher ambient temperature in Hongkong, the heating capacity is higher, but the compressor consumption also increases. With better irradiation and a lower ambient temperature, Garze has brilliant electrical performance, the COPPVT & NEER are the highest at 9.9 monthly & 8.0 annually. Furthermore, the operating costs and CO2emissions of this system are just 1/4–1/3 and 1/3–1/2 of gas boiler, indicating significant potential for energy-savings & emission reduction.

Economic evaluation and annual performance analysis of a novel series-coupled PV/T and solar TC with solar direct expansion heat pump system: An experimental and numerical study

Hybrid photovoltaic/thermal (PV/T) systems are proposed to harvest the two renewable forms of energy, thermal and electrical. The PV/T system generates low-temperature thermal and electrical energy, which is usually not much and might meet the demand for residential building heating and hot water. This research presents the economic evaluation and annual energy performance of a novel series-coupled PV/T and a solar thermal collector (TC) with a solar direct expansion heat pump system. A comprehensive approach was developed and utilized to evaluate the quantitative and qualitative performance of the hybrid refrigerant-based PV/T-TC system using the 1st (energy) and 2nd (exergy) laws of thermodynamics. A 0.5234 kW rating heat pump prototype system with a PV/T-TC area of 3.5 m2 was built and tested under the real conditions of Karachi, Pakistan, and a simulation model was developed via MATLAB to predict the annual energy performance. The experimental results show that the PV/T-TC system's mean daily electrical, thermal, overall energy and exergy efficiencies were 14.08%, 60.12%, 74.20%, and 18.12%, respectively. The numerical results reveal that a PV/T-TC assisted solar heat pump system can generate 303.51 kWh of electricity and 3213.12 kWh of thermal energy annually. The annual average COP of the hybrid refrigerant-based PV/T-TC system is 5.68. According to the comparison of economic performance between the PV/T assisted direct-expansion heat pump system and PV/T-TC assisted direct-expansion heat pump system, the hybrid refrigerant-based PV/T-TC assisted direct-expansion heat pump system has a payback period of 5.20 years, whereas the refrigerant-based PV/T assisted direct-expansion heat pump system has a payback period of 7.15 years. The experimental data is consistent with the simulation outcomes, with an average relative error of 3.30%.

Experimental designs applied to a direct expansion heat pump with solar energy

The direct expansion heat pump with solar energy is an energy conversion system used for water heating applications, air heating for air conditioning buildings, water desalination, solar drying, among others. This paper reviews the main designs and analysis of experiments in order to identify the fundamental objectives of any experiment which may be: to determine the factors that have a significant influence, to obtain a mathematical model and/or to optimize performance. To achieve this task, the basic and advanced configuration of this system is described in detail in order to characterize its thermal performance by means of energy analysis and/or exergy-based analysis. This review identifies possible lines of research in the area of design and analysis of experiments to develop this water heating technology for industrial applications.

Research on drying Lentinus edodes in a direct expansion heat pump assisted solar drying system and performance of different operating modes

In this study, a direct expansion heat pump assisted solar drying system was proposed and applied for drying Lentinus edodes. The operation modes of this system were divided into three types according to the purpose of reducing drying energy consumption. Under different environment conditions, the operation performance of direct expansion heat pump assisted solar drying system was investigated. In addition, the system was compared with open solar drying to analyze the drying characteristics of Lentinus edodes. The results showed that the direct expansion heat pump assisted solar drying system under solar collector drying operation mode could increase the outdoor air temperature by 9.8 °C, and the air temperature of the drying chamber could be maintained above 40 °C. The average performance coefficient of the direct expansion heat pump continuous drying was 2.56, and the duration of the drying chamber temperature above 40 °C was 63.8% of the total drying time. The average performance coefficient of the direct expansion heat pump assisted solar energy intermittent drying can reach a maximum value of 6.01. High compressor operating frequency and solar radiation intensity are the main factors affecting the coefficient of performance improvement of the direct expansion heat pump assisted solar drying system. Furthermore, the moisture content of Lentinus edodes under open sun drying was 0.274 g water/g dry matter higher than that under direct expansion heat pump assisted solar drying system. The Page model with R2 value of 0.996 and RMSE value of 0.0211 was considered the best drying kinetics model for direct expansion heat pump assisted solar drying for Lentinus edodes.

Field experimental investigation on electricity and thermal performances of a large scale photovoltaic solar-thermal direct expansion heat pump system

The comprehensive utilization of photovoltaic and solar-thermal (PVT) is an important approach to achieve the carbon peaking and carbon neutrality goals, and can realize the generation of electricity and thermal energy cleanly and efficiently. In order to satisfy the basic living demands including electricity, space heating and domestic hot water of Chinese rural residents, a large scale direct expansion PVT heat pump (DX-PVT-HP) system used plate-tube evaporator has been proposed in the present paper. An experimental platform with a solar panel installation area of 32.47 m2 is installed on the roof of a dormitory in Hefei, and its operation characteristics including electricity and thermal performance are field tested and analyzed in winter. The data indicates that during the operation of the heat pump, the cooling effect of the plate-tube evaporator on the solar panel can maximum increase the photoelectric conversion efficiency by 25.0% in comparison with that of the conventional solar panel. In the process of heating 1500 L water from 5.5 °C to 45 °C on a sunny day, the average coefficient of performance (COP) of the system reaches 4.96, and the power generation of the system is greater than the power consumption. Finally, the concept of equivalent net thermal energy is proposed to unify the output of electricity and thermal energy to evaluate the comprehensive performance of the system. The results demonstrate that it is feasible to adopt the designed PVT heat pump system to meet the basic living needs of rural residents.

Experimental and numerical investigation on a solar direct-expansion heat pump system employing PV/T & solar thermal collector as evaporator

The objective of the research is to present an investigation on the operational performance of the integrated photovoltaic/thermal system (PV/T) and solar thermal collector (TC) with a heat pump system (PVT-TC) and its potential use for generating high thermal energy and electricity simultaneously. The numerical simulation model and testing rig of the PVT-TC heat pump system have been developed to analyze the electrical, thermal, and total efficiency of the system. An experimental test was carried out under real weather conditions. The experimental results revealed that the average electrical, thermal efficiency, and coefficient of performance (COP) of the PVT-TC heat pump system are 14.08%, 66.71%, and 6.11, respectively. According to comparative analysis with the similar systems, the PVT-TC heat pump system exhibits a higher electrical and thermal efficiency, thus showing a better application potential in residential buildings. The PVT-TC system occupies 3.3278 m2 and generates an average of 0.86 kWh/day of electricity and 6.35 kWh/day of thermal energy, as a result of 86.1% of primary energy saving efficiency being obtained. In addition, the PVT-TC heat pump system is simulated using COMSOL Multiphysics® to examine the temperature distribution of the solar TC and PV/T systems. The numerical model is also verified by the experimental data, with a root mean square deviation with less than 1.53%.

A Solar Dually PVT Driven Direct Expansion Heat Pump One-Year Field Operation Results at Continental Climate

The high energy-consuming building sector needs to meet both electricity and heat demands. In a nearly zero energy building scenario, most of the consumed energy would be generated locally by means of renewable solutions that nowadays seem not to provide an attractive performance or cost-competitiveness. Solar-based technologies tend to be the most promising ones, but for high densely populated areas, the usual photovoltaic or thermal single approaches may not be efficient enough. The current work is focused on the analysis of the dual use of the solar resource by means of hybrid PVT collectors and their smart combination with direct expansion heat pumps through predictive control strategies. To that end, a system was developed, installed in a real-use single-family house at a continental climate for domestic hot water application, operated and monitored for one entire year. The average day indicator results show 83% renewable energy share, 220% self-sufficiency ratio, 41% heat pump self-consumption and 46% of the solar fraction.

Swimming Pool Heating using Geothermal Source VRF Heat-Pump in Morocco

Abstract— Heating swimming pools requires a great deal of energy, especially outdoor pools. The swimming pools must be constantly maintained at temperature - generally 28 °C - the most used means in Morocco is the heating by boiler. As part of this research, and in order to develop ecofriendly and lowimpact solutions on the environment, we choose to study an outdoor swimming pool heated by gas boiler in Rabat. The objective of this article is to size a swimming pool heating installation, using a direct expansion heat pump type VRF geothermal on groundwater. Keywords— Modeling, Swimming pool heating, Air Conditioning, Heating, Dynamic Modeling, Geothermal, Electricity consumption, COP

Study on Performance of Direct-Expansion Heat Pump Water Heater Integrating Uninsulated Micro-Channel Pv/T Solar Collector

Study on Performance of Direct-Expansion Heat Pump Water Heater Integrating Uninsulated Micro-Channel Pv/T Solar Collector

Comparative study on dual-source direct-expansion heat pumps based on different composite concentrating photovoltaic/fin evaporators

Dislike the electrical performance, the heat collection capacity is always disappointing in the photovoltaic evaporators. It is always handled by adding additional finned-tube evaporators, which will increase space occupation and pipeline complexity. In this paper, a novel direct-expansion heat pump based on linear Fresnel photovoltaic/fin evaporator is designed. By integrating concentrating photovoltaic and fins into one composite photovoltaic-air evaporator, the above shortcomings can be solved and the solar & air energy can be fully utilized to maximize the output of the evaporator with a limited area. Compared with the direct-expansion heat pump with compound parabolic concentrator/fin evaporator, this system owns a higher concentration ratio, smaller photovoltaic area (making it possible to be equipped with GaAs cells), and larger finned-tube structure to enhance the heat capacity under the same receiving area. Dynamic models are established in MATLAB and comparative analysis is carried out under different wind speeds, irradiance, and ambient temperature. The influence of running time and the system performance with 40 °C water temperature (in actual application) are also discussed. The average electrical efficiency, COPPVT, and exergy efficiency of the heat pump system based on linear Fresnel photovoltaic/fin evaporator could be 28.21%, 3.55, and 27.07% under 900 W/m2, which are all higher than 16.02%, 3.15, and 20.09% in the system with compound parabolic concentrator/fin evaporator. Therefore, the newly designed system could enhance the performance of the hybrid system.

Parametric study and sensitivity analysis of a PV/microchannel direct-expansion CO2 heat pump

The work numerically investigates the effect of operating and ambient conditions on the performance of a new PV-heat pump configuration: a PV/microchannel evaporator, direct–expansion heat pump (DXHP) which uses CO2 in a transcritical cycle. A steady-state model of the heat pump is used to conduct a parametric study and sensitivity analysis. The variables considered are the evaporation temperature, CO2 and water mass flow rates, insolation, and ambient temperature. The parametric study shows that the system can achieve a COP of 5.16 with an outlet water temperature of 45 °C or an outlet water temperature of 95 °C at a COP of 3.34. At the baseline operating conditions, the maximum increase in PV efficiency due to the evaporator is ~1.20%-points. The PV efficiency increases with a reduction in evaporation temperature. It is insensitive to CO2 and water mass flow rates. The sensitivity analysis shows that the COP has a strong dependency on the CO2 and water mass flow rates. The water outlet temperature has a strong dependency on the water mass flow rate followed by the insolation, ambient temperature, and evaporation temperature. The results of the study can help guide how to operate the system. For example, for the average ambient conditions of Fargo, ND, an evaporation temperature between 0 and 2 °C, a CO2 mass flow rate of 0.017–0.024 kg∙s−1, and water mass flow rate of 0.020–0.026 kg∙s−1 should be used to maintain a relatively high COP (e.g. 3.8–4.8) while producing hot water over 55 °C.