Conventional Air Source Heat Pump Research Articles

Performance evaluation of underground thermal storage integrated dual-source heat pump systems

The increasing demand for electricity stresses the existing electric grids. Buildings consume 73% of all U.S. electricity and are responsible for 30% of U.S. greenhouse gas emissions. Integrating thermal energy storage (TES) in building heating/cooling systems, which consume considerable electricity, can mitigate the challenges to electric grids. This study reports on a novel thermal energy storage device integrated heat pump system to reshape the building electricity demand profile while maintaining thermal comfort. The annual performance of the proposed system has been evaluated through a dynamic system simulation with high fidelity in the Modelica platform. The dynamic model of the novel hybrid component named ‘dual purpose underground thermal battery’ was developed and validated. It was then incorporated into the system model. Given a time-of-use tariff, a rule-based control strategy was designed to shift the electric demand and switch the heat pump source for a typical single-family house in different climate zones of the United States. The system performance of the new TES-integrated dual-source heat pump was compared with that of a conventional air-source heat pump system. The results indicate that the proposed system can reduce the annual HVAC electricity cost by up to 52% while saving 45.2% on electricity consumption. In the Northern areas, the annual peak load of the HVAC system can be reduced by 64.9%. However, this reduction is less in the Southern areas as the system’s higher efficiency in winter dominates the overall energy-saving potential.

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.

Thermodynamic and economic analyses of modified ejector enhanced solar-air composite dual-source heat pump system in residential buildings

This research utilizes an ejector enhanced solar-air composite dual-source heat pump (EDHP) system for water heating in residential buildings. The main advantage of the EDHP system is the use of an ejector and dual-heat source to efficiently utilize solar energy, thereby enhancing heating performance. This ejector operates by utilizing a refrigerant stream from the solar collector line with higher pressure to elevate the suction pressure of the compressor. This feature enables the EDHP system to maintain stable heating performance even in adverse weather scenarios with low temperatures or limited solar radiation. Thermodynamic modeling using energetic and exergetic analysis methods is employed to theoretically evaluate the conventional air-source heat pump and EDHP systems. The energy analysis of the EDHP system using R134a refrigerant has shown significant improvements in both heating coefficient of performance (COP) and exergy performance (ECOP). Under specific conditions, such as an ambient temperature of −5 °C and an irradiance level of 250 W∙m2, and utilizing a 20 m2 PVT (Photovoltaic and Thermal) collector for a heating supply of 5 kW, the EDHP system exhibits a 23.64 % increase in COP and a 25.83 % increase in ECOP. Furthermore, when the electricity generated by the PV system is incorporated into the EDHP system, the demand for electricity from the grid can be reduced by up to 51.96 % under the same conditions. In the Beijing area and considering the current energy price system, the heating operating cost of the EDHP system is 0.206 Yuan/kWh. Compared to a standard air-source heat pump system, the payback period for EDHP system ranges approximately from 10 to 12 years, taking into account an annual heating period of 5 months. Among the alternative refrigerants to R134a in the EDHP system, R152a is found to be the most suitable. It shows COP improvements of 24.92 %–30.0 % and ECOP improvements of 24.89 %–29.96 %. This innovative approach opens up new possibilities for the application of heat pump technology under adverse weather conditions in residential and commercial buildings.

Performance assessment and working fluid selection for heat recovery ejector heat pump system in cold climatic conditions

The paper aims to analyze the performance and identify suitable working fluids for a new heat recovery ejector heat pump (HREHP) system. This system introduces a flue gas waste exergy recover circuit combined with the traditional ejector heat pump. Considering the factors of performance, environmental impacts, flammability, and fluid type, four working fluids are selected (i.e., R245fa, R365mfc, RE245fa2, and R1233zd(E)). Comparative study initially conducted to demonstrate the advantages of the proposed heat recovery ejector heat pump facility. Subsequently, the effects of the operational parameters on the novel system performance are investigated to derive design and operation criteria through energy and exergy analysis. Finally, sensitive analysis of generation temperature (Tg), flue gas temperature (Tfg,out), and supply water temperature (Tw) on the proposed system is carried out. Compared with traditional air source heat pump, the application of the HREHP shows higher energy efficiency and lower operating costs at lower ambient temperatures. The results indicate that the primary energy efficiency of the proposed system is 27.0 % and 35.56 % higher than that of conventional ejector heat pump cycle and air source heat pump cycle, respectively, at an ambient temperature of −10 °C. Besides, Tg has a stronger effect on the entrainment ratio and heating coefficient of performance; The primary energy efficiency is more sensitive to Tw variation. Lastly, the R365mfc and R1233zd(E) show good potential as the alternative for the R245fa. The design methods, evaluation methods, and operational characteristics obtained in this study can provide reference and guidance for the application of heat recovery ejector heat pump technology in practical engineering.

Energetic, economic and environmental analyses of frost-free air-source heat pump in multi-type buildings and different locations

Frost-free air-source heat pumps (FFASHPs) have gained increasing attention as an alternative to conventional air-source heat pumps (ASHPs), chillers and boilers (CBs), in the context of low-carbon space heating and cooling. Conducting energetic, economic, and environmental (3E) analyses on these systems in various building types and locations is crucial for system selection, yet relevant research is scarce. To address this issue, this paper presents a comprehensive framework for 3E analyses, including location selection, building assessment, load calculation, system sizing and modeling, annual simulations, and subsequent data visualization. The study includes seven building types and 353 locations with specific climate conditions. The results indicate that FFASHPs outperform ASHPs in terms of energy efficiency, lifecycle costs, and carbon emissions. Compared to CBs, FFASHPs exhibit superior energy efficiency across most regions and building types, except for severe cold, hot summer and warm winter regions. In central and coastal areas, the advantages are more prominent due to higher natural gas prices, making FFASHPs commercially viable. In hot summer and cold winter region with electricity carbon emission factors ranging from 0.3574 to 0.6829 kgCO2/kW, the FFASHP exhibits significant potential for decarbonization due to its high efficiency. In renewable energy-rich areas with low emission factors (0.1031–0.2602 kgCO2/kWh), FFASHPs can achieve significant carbon reductions, up to 90 %. This paper, for the first time, achieves a comprehensive 3E analyses of FFASHPs, ASHPs, and CBs. It offers a general methodology, extensive datasets, and visualized graphs for system selection, promoting low-carbon space heating and cooling.

Feasibility analysis of thermal storage defrosting method for air source heat pump: From energetic and economic viewpoints

Frosting issue of air source heat pump (ASHP) is a continuing concern. The conventional reverse cycle defrosting (RCD) method absorbs heat from indoor heat exchanger, thus greatly impacts the indoor thermal comfort. To solve the problem of insufficient defrosting heat supply for RCD method, thermal energy storage (TES) based defrosting method has gained increasing attention over the past years. Yet existing studies have only focused on specific prototype tests. There lacks general analysis on system characteristics and design considerations. To better address application concerns of TES-based defrosting method, this study carried out feasibility analysis for the TES-based ASHP system from both energetic and economic aspects. The energetic analysis reveals that the stable operation of TES-based ASHP relies on proper system design and control. Feasibility conditions concerned with key operating parameters are defined and deduced. Impact factors on average heating efficiency are also analyzed. Theoretical and numerical results indicate that energy saving potential of TES-based ASHP over conventional ASHP is conditional on the defrosting discount factor of conventional RCD method. TES-based ASHP enhances heating efficiency of conventional ASHP by 3.0% to 19.5% at ambient from −2°C to 2 °C due to relatively large defrosting discount factor of conventional RCD method. With respect to the economic aspects, the capital cost of TES-based ASHP is 26% higher than that of the conventional ASHP. But the total life cycle cost can be reduced by higher heating efficiency. The condensing heat storage ASHP exhibits cost advantages to the conventional ASHP when defrosting discount factor of conventional ASHP is higher than 1.13. Further prototype test with the TES-based ASHP could shed more light on the feasibility characteristics as revealed by the theoretical analysis.

Use of a residential building’s foundation as Energy Geo-Structures: A case study in the Mediterranean environment

Geothermal energy, a type of renewable energy obtained from the Earth, can be used by Ground Source Heat Pumps (GSHPs) for space heating and cooling. GSHP systems extract/reject heat from/into the earth via a network of pipes or tubes known as Ground Heat Exchangers (GHEs),. GHEs can be horizontal or vertical and come in a variety of forms, including slinky loops and vertical spiral loops for horizontal configurations, and single U-tube, double U-tube in series or parallel, coaxial pipe, and spiral/helical pipe for vertical configurations. Although GSHPs exhibit a higher performance than conventional Air Source Heat Pumps (ASHPs), the systems are less appealing as investments due to their longer payback times and greater upfront costs. A competent design of the GSHP system with the appropriate GHE sizing, including the case of employing the GHEs in the structures foundations, can minimize the initial cost. Such structures are referred to as Thermo-Active Structure systems or Energy Geo-Structures (EGS), which have already found applications as energy piles, barrette piles, diaphragm walls, shallow foundations, retaining walls, embankments, and tunnel linings [1, 2, 3].
 Residential buildings in Cyprus have a higher cooling than heating demand, with an extended payback period for the investment, especially if the system is solely utilized for heating [4]. EGS coupled with a GSHP could offer a substitute to reduce the system’s initial expenses and make it more attractive as an investment. EGS have not yet seen applications in Cyprus, therefore an initial computational analysis utilizing the COMSOL software and relevant data is taken into consideration. To determine the heating and cooling loads, a three-bedroom, two-storey house with a total floor space of 190 m2, with nearly Zero Energy Building (nZEB) characteristics, is used as the typical case for a single detached residential building in Cyprus. The TRNSYS software was used to calculate the heating and cooling loads. In order to analyze the rejected/absorbed energy from the earth, typical foundations were constructed as full sized models in COMSOL (Figure 1). With the exception of pipes, where a 1D solution is employed, the convection diffusion equation for heat transfer is utilized with 3D conservation of heat transfer for an incompressible fluid. A detailed methodology can be found in Aresti et al. [5].
 For the COMSOL compuations, the site ground temperature characteristics and the local ambient temperature were taken into account. Based on the provided loads, the inlet and outlet temperatures were hourly calculated for the highest demand months: February for winter – heating, and July for summer – cooling.
 To assess the potential of using the foundation bed as an alternative to energy piles, a comparison on the performance of the two cases is presented in Figure 2 for both summer and winter conditions. The foundation bed is seen to work in a steady manner, which is highly desired in a system. However, it performs less well, with a lower COP than the energy piles system, which exhibits greater COP values than the foundation bed system both in winter and summer. It remains that both systems exhibit high enough COPs and nearly steady conditions. The COPs vary between 4.4 and 4.8. A yearly simulation (not shown here) yielded a lowest COP value of 4.5.
 The systems were also assessed economically by being compared to a convectional ASHP system. The economic study demonstrated that the suggested foundation systems had relatively short payback times, making them a plausible alternative. More detailed discussion on the results can be found in Aresti et al. [5]. As a future objective, an environmental and Life Cycle Analysis investigation, similar to the one performed by Aresti et al. [6], could provide a deeper insight of the plausibility of using the building foundation as EGS.

Study on the Performance of a New Ultra-Low Temperature Air Source Heat Pump (ASHP) Unit in Cold Regions

Low air temperature and frosting have been reported as the critical factors that greatly attenuate the efficiency and performance of the ASHP in cold regions. In order to ensure the potential prevalence of the ASHP in cold regions of China, a new ultra-low temperature ASHP unit was developed, and the field measurement was carried out in an office building where these ASHP units were installed in Shanxi Province. Results showed that a coefficient of performance (COP) of 1.83 was obtained at the ultra-low environmental temperature of −25 °C. Meanwhile, measured results indicated significant frosting suppression and improved heating performance under three typical frosting conditions. In addition, long-term measurement results revealed that the mean COP and COPsys reached up to 3.34 and 2.63, respectively, indicating a higher performance in the cold regions of China. Consequently, the corresponding CO2 emission reached 11.3 kg per year and per square meter, and the annual total cost on the unit reduced by 15.8% compared with the conventional ASHP, which meant that the total investment could be covered in the second year. The reduced CO2 emission and the annual cost implied that the ASHP unit could produce better environmental and economic benefits. Findings of this study revealed that this ultra-low temperature ASHP unit had a better performance under cold environment, which offered a possibility for the prevailing of the ASHP in cold or extremely cold regions, as well as could contribute to the carbon peaking and neutralization.

Frost suppression performance of an air source heat pump using sensible heat from indoor air to preheat outdoor air

Frost forms on the surfaces of air source heat pump (ASHP) outdoor heat exchangers under heating mode in winter, when the surface temperature of the heat exchanger is lower than both 0°C and the ambient dew point temperature. This can reduce the coefficient of performance (COP) and heating capacity of the ASHP unit. A novel ASHP was examined using two rooms with the same size, orientation, and structure. To test the frost suppression performance of the novel system, two ASHPs of the same size and specifications, but one including a sensible-heat preheating device, were placed in the test platform rooms. By comparing and analyzing field-measured parameters such as COP, heat loss, and frost weight of the two ASHP units, the frost suppression performance of the novel ASHP was evaluated in severe, moderate, and mild frost areas. The results show that when the air conditions were −3.9°C and 98.7% in the moderate frost area and 3.2°C and 96.7% in the severe frost area, the novel ASHP showed significantly improved performance. The frost formation time increased from 25 min to 38.5 min, frost-defrost heat loss decreased by 31.6%–43.7%, frost weight decreased 11.8%–17.4%, and COP increased 7.4%–13.1% compared with those of the conventional ASHP. In the low temperature and mild frost area, when the air temperature was −11.8°C and the relative humidity was 88.8%, the frost suppression effect of the novel ASHP was unapparent. Affected by the climate, the operation performance of ASHP will be different. Different ASHP can be used to adapt to the weather in different regions. The novel ASHP can significantly suppress frost and improve the heating performance of the unit in the mild climate and frost serious areas.

TRNSYS simulation study of the operational energy characteristics of a hot water supply system for the integrated design of solar coupled air source heat pumps

To address the issues with solar water heating systems taking up a lot of space, unstable hot water supply, air source heat pumps susceptible to frost in the winter, and low energy efficiency. The TRNSYS tool is employed in this work to simulate a solar-coupled air source heat pump system. The heat pump operation is first investigated using the inverse Carnot cycle. The performance coefficient is then calculated by the second law of thermodynamics without considering the pipeline's pressure drop and heat loss. The output temperature of the hot water that the heat pump circulates is then determined. The daily hot water needs can be estimated roughly based on information about solar radiation. The heat balance equation for flat plate solar collectors was used to compute the intensity of solar diffused radiation. The Berlage calculation was used to determine the solar radiation received on the collector's surface. After a qualitative analysis of the heat from the heat source, the efficiency of the linked heat pump and the conventional air source heat pump was compared. Analyzing the water temperature change graph for each month's data reveals that the system can achieve 50 °C during the water supply time each month. The heat pump's annual energy consumption is 6252.01 kWh, while the system's annual energy consumption is 9100.47 kWh. The study findings may be used as a guide to improving the design and management of the whole system. In addition, they may improve the solar water supply system's performance.

Is Ground Source Heat Pump the Future of Space Cooling in Southeast Asia?

The rapid economic growth of Southeast Asian countries is followed by accelerated population and energy demand growth. On the other hand, the region still relies heavily on fossil fuels as the primary source of electricity. As a result, there is a growing concern about future regional energy sustainability. Space cooling has become one of the fastest-growing energy sectors. Introducing a new space cooling technology capable of providing a higher thermal efficiency can be an alternative solution. The Ground Source Heat Pump (GSHP) is a proven technology to reduce the required energy for space cooling and heating in four-season countries. However, its performance in the tropical climate is still questionable. This paper describes the challenges of GSHP application in tropical climates, such as in Southeast Asia. The research conducted to investigate the suitability of GSHP system application in Southeast Asia is also discussed. While more data are being gathered from several GSHP pilot tests in Vietnam and Thailand, the short to medium-term performance data analysis shows its advantages over the conventional Air Source Heat Pump (ASHP) or standard air conditioner. However, extensive research is still required to evaluate its long-term performance and the financial advantages over the ASHP.

Finned coil-type energy storage unit using composite inorganic hydrated salt for efficient air source heat pumps

The imbalance between the variable power load during day and night and the energy supply of office building heating can supplement a staggering electricity economy. In this study, a novel composite inorganic hydrated salt phase change material (PCM) was fabricated with a melting temperature of 50.3 °C using CH3COONa·3H2O (the primary material), KNO3 (the eutectic material), Na2HPO4·12H2O (the nucleating agent), and carboxymethyl cellulose (CMC, the thickener) in a mass ratio of 89 %:8 %:1.5 %:1.5 %. Moreover, we developed a modular finned coil-type energy storage unit (ESU) with a PCM charging capacity of 1200 kg and a theoretical heat storage capacity of 315 MJ. Subsequently, we created an ESU test system for an air source heat pump (ASHP) operated at the valley electricity period from 23:00 to 7:00. The heat storage, heat release, and defrosting functions were tested, and the operational cost of heating for the office building was evaluated. The results reveal that the composite PCM exhibited stable thermal properties, but more accurate working medium flow matching is required to improve the heat transfer tail phenomenon. Furthermore, the ASHP-ESU reduces the defrosting time by 63 % and improves the efficiency by 6–9 % compared to conventional ASHP. The operating costs are the lowest at 4.66 CNY/month/m2 with valley electricity operation compared with the other three heating modes. Therefore, the ASHP-ESU system for office building heating in off-peak operation is an economical and sustainable approach for the power system.

Research and application of park multi-source synergistic and complementary heating system and its coordinated control method

The integration of solar thermal collectors (STC) and air source heat pump (ASHP) heating systems has been widely used for building heating. However, these systems have the problems that the STC cannot fully utilize the solar energy at weak radiation, and the ASHP has insufficient output as well as low coefficient of performance (COP) under extreme cold conditions. In this study, a multi-source synergistic and complementary heating system (MSSCHS) with a heating mode selection model based on belief rule base and evidential reasoning (BRB-ER) method is designed. First, the MSSCHS composed of the STC and the ASHP as the main heat sources, and the water source heat pump as the auxiliary heat source, along with its three heating modes under different operating conditions were proposed. Then, a system heating mode selection model based on the BRB-ER method was establish to achieve mode switching. Finally, the MSSCHS with BRB-ER heating mode selection model was tested experimentally. The results show that compared with the conventional ASHP + STC heating system, the solar energy utilization rate of the MSSCHS had increased by 19.3 %, 16.6 % and 8.2 % respectively in three typical days, and the ASHP's COP had increased by 37.7 %- 55.6 % under extreme cold conditions. The high efficiency and stability of the MSSCHS were verified through operational tests during the heating period. Furthermore, by applying in different types of buildings, it is verified that the MSSCHS can meet different heating requirements and has a good applicability. It was demonstrated that the proposed MSSCHS has more efficient heating mode and more accurate mode selection method, which can provide a new research idea for building heating.

The frost restraining effect of solar air collector applied to air source heat pump

The air source heat pump has been demonstrated to be an efficient clean space heating technology, but the frosting on the exterior surface of the evaporator will largely decrease its performance. In this paper, the triangular solar air collector is adopted for evaporator frost restraint of the air source heat pump, and the dynamic heat transfer model of the triangular solar air collector and quasi-steady-state frosting model of the evaporator were established and coupled. The effect of frost layer thickness variation on evaporator air flow is considered based on the resistance factor to improve the applicability of the model. The heat flux and water vapor diffusion flux at different frost time and frost thickness were calculated, and the frosting characteristics of the air source heat pump with the triangular solar air collector and the conventional air source heat pump were compared on varying working conditions and typical daily meteorological parameters. Results show that triangular solar air collector can effectively restrain the frosting of the air source heat pump, and when the solar irradiance is 500 W/m2, the triangular solar air collector can reduce the frost thickness by more than 15 %. The triangular solar air collector increases the total heat flux by 101.4 W/m2 under frosting conditions, which reduces half of the defrosting times and increases the heat exchange of the evaporator by 36.6 % during a typical day, indicating that triangular solar air collector significantly improved the energy efficiency of air source heat pump on frosting condition.

Investigation of the integrated fuel cell, battery, and heat pump energy systems

The integration of an advanced fuel cell battery electric vehicle was proposed to extend the driving range, increase the overall energy efficiency, and produce an additional cabin space heating service for on-board vehicle application in winter. The adopted heat pump absorbs heat from the fuel cell and battery during charging and discharging scenarios at the heat pump's cold end and transfers it to the cabin supply air for cabin heating. A numerical model for the proposed system was developed and validated by published experimental results. A comprehensive evaluation of the influence of fuel cell output current, battery discharging rate, ambient air temperature, evaporator coolant inlet temperature and refrigerant subcooling degree on the Coefficient of Performance (COP), outlet air temperature, and Equivalent Effective Battery Capacity (EEBC) were conducted. In addition, in this study, a detailed comparison of proposed system to previously published existing systems was carried out in terms of COP, heating capacity, EEBC, and operating price in order to examine the advantages of the proposed system. Results showed that the proposed system was not sensitive to ambient air temperature, but evaporator coolant inlet temperature had the greatest impact on COP, compared to other defined independent variables. A maximum COP of 6.07 could be observed when the evaporator coolant inlet temperature was 32 °C. Although increasing fuel cell output current and battery discharging rate could increase heating capacity, it would decrease outlet air temperature as well. The price of a single charging cycle for running for a 4-hour period was estimated. Compared to conventional Electric Vehicle Air Source Heat Pump (EVASHP) system and Positive Temperature Coefficient (PTC) heaters, the proposed system has the lowest cost which was £10.40per charging cycle while the others are £13.8 and £19.2 respectively when providing the same EEBC and heating capacity. Additionally, the payback period was 10,500 charging cycles. Overall, this study provides a potential solution for efficient cabin heating of electric vehicles in extremely cold weather with twice the EEBC of the existing EVASHP system and with 2.8 times of a PTC system both at −20 °C and the operating price is also 1/3 lower than the existing EVASHP. Meanwhile, the COP of the proposed system is 2.3 times that of the existing systems. It can sufficiently increase the driving range of electrical vehicles in winter and eliminate driving range anxiety.

Sequential coupled numerical simulations of an air/ground-source heat pump: Validation of the model and results of yearly simulations

Numerical simulations are important tools for the assessment of exploiting geothermal energy in heat pump applications. They can be used to evaluate the performance of the system, the long-term production scenarios and the sustainability of the geothermal reservoir. The present work introduces and describes a numerical model, in which a dedicated Matlab® script has been realized to allow sequentially coupled simulations of a shallow geothermal reservoir and of a heat pump system. A mathematical model of a dual-source heat pump, working alternatively with the ground or the air as heat source/sink, has been developed in Matlab® environment. The heat exchangers of the heat pump have been modelled considering the equations that govern the physical phenomena. The dynamic numerical simulator FEFLOW®, based on the finite element method, has been used to simulate the behaviour of the geothermal reservoir, subjected to heat extraction/reinjection by a closed loop vertical heat exchangers field. This methodological approach is useful to evaluate the performance of the coupled system in the long term, and it is important for understanding the advantages and limits of the dual-source heat pump in assuring sustainability over time avoiding the depletion of geothermal resources. The models and their coupling have been calibrated and validated with experimental data from a shallow geothermal plant located in Tribano (Padova, IT). It consists of eight coaxial borehole heat exchangers 30 m deep, connected to a 16 kW dual-source heat pump prototype. The heat pump system provides heating and cooling to an office area. The coupled model has been used to compare the performance of the heat pump when working in air-mode only or in ground-mode only. This allowed the development of a switching control strategy between the two thermal sources. Yearly simulations with the switching strategy have shown that the seasonal performance factor of the dual-source heat pump during the heating mode can be 13.8 % higher compared to the one obtained with a conventional air source heat pump and 3.8 % higher with respect to a ground source heat pump.

Performance evaluation of a heat pump-driven liquid desiccant dehumidification system integrated with fresh air supply

With the higher demand for indoor thermal comfort and the accurate control of air temperature and humidity, energy consumption in air conditioning systems is increasing over these years, especially for buildings with high moisture load such as natatoriums. However, freezing dehumidification is still widely adopted in eliminating moisture load, which requires lower evaporating temperature and causes excessive energy consumption. To improve energy utilization and efficiency, a heat pump-driven liquid desiccant dehumidification system is proposed in this research, the surface condenser and liquid desiccant dehumidifier are adopted to realize two-stage air dehumidification. In the summer condition, the solution in regenerator is heated by the condenser of heat pump and the residual heat is to heat the regenerated air, and the cooling capacity of heat pump is applied to the surface condenser and terminal heat exchanger, simultaneously the solution in dehumidifier is cooled by cooling tower. Under the meteorological conditions in Nanjing, the coefficient of performance could reach 6.148 in this proposed system, and compared to conventional air source heat pump dehumidification air conditioning system, the rising ratio of energy efficiency is between 11.4% and 80.7%. Eight cities in China are selected to analyze the energy-saving effects, and energy consumption could be reduced by 31.9%-63.2%, the result demonstrates the superior energy-saving potential and application prospect of the heat pump-driven liquid desiccant dehumidification system.

Visualization experiment and numerical simulation on behaviors of gas-liquid falling film flow on the air-side of two-row plain finned-tube heat exchanger

The frost-free air-source heat pump (ASHP) with integrated liquid desiccant dehumidification is a more promising energy-saving alternative to the conventional ASHP unit. The performance of spray-type finned-tube heat exchanger (FTHE), as one of the key equipment of this frost-free ASHP system, is affected by a combination of various factors including wall wettability, liquid film flow pattern, liquid film thickness, velocity distribution in a liquid film, interface wave, interface velocity and so on. To provide a theoretical basis for the operation, design and optimization of high-performance spray-type FTHE, a visualization experiment system and a three-dimensional (3D) numerical model are employed to study the behaviors of counter-current gas-liquid falling film flow on the air-side of two-row plain FTHE under different parameters in the present paper. According to the experimental results, the flow pattern of liquid film on the finned-tube surface is a mixture flow composed of the rivulet flow and the thin falling film flow, demonstrating that the assumption of the uniform distribution for liquid film thickness proposed in previous studies is not completely consistent with the actual situation. Meanwhile, the numerical results indicate that the transient-average film thickness on the surface of two-row plain finned-tube is significantly 18.3% – 32.5% higher than that on the vertical plate calculated by Nusselt's model. After that, the critical gas and liquid Reynolds numbers of the flow pattern transition for the liquid film are obtained, and it is pointed out that the wall contact angle of 10° is more conducive to the formation of complete film flow on the air-side of the two-row plain FTHE within the fin pitch range of 4.8 – 6.0 mm. The changes of the falling liquid film and gas-liquid interface characteristics are fundamentally attributed to the drag effect of the interfacial shear force. Furthermore, there is a cubic curve relationship between average interface shear force and gas Reg. A quadratic curve relation exists between average wall shear stress and gas Reg. At the same time, the velocity of falling film fluid at the bottom region of laminar flow is reduced, resulting from the inhibiting effect of the interfacial shear force.

Performance and optimization of a novel solar-air source heat pump building energy supply system with energy storage

In order to solve the problem that the traditional heat pump system in the cold area of North China cannot supply heat efficiently and stably, a novel solar-air source heat pump system is proposed to meet the needs of building energy consumption. In winter, the latent heat of water in the ice tank is used as the main low-temperature heat source for the heat pump, and the outdoor air is used as the auxiliary heat source. During the day, the solar energy is used to restore the heat of the ice tank, so as to realize the complementary utilization of renewable energy. The air source heat pump can be used for cooling in summer. In addition, combined with night energy storage (cold storage and heat storage), the “peak load shifting” and the minimization of building operation energy consumption and cost are realized. The experimental platform and simulation model of the system are established, and the operation and design scheme of the system are optimized with the optimal economy. The optimization results show that the system can still maintain efficient operation under continuous extreme weather in winter and summer, and the SPF of the system in winter and summer are 2.93 and 2.6, respectively. Solar photovoltaic power generation meets part of the power demand of the system, which can save about 1.85 t of standard coal compared with thermal power generation. Compared with a conventional air source heat pump system, the novel system has better economy and a dynamic investment payback period of 3.86 years. The proposed system presents a way to meet building energy supply that is worthy of popularization and application in cold areas.

Environmental Impact of Ground Source Heat Pump Systems: A Comparative Investigation From South to North Europe

Ground Heat Exchangers (GHEs), buried in the ground either horizontally or vertically (in a borehole), are coupled with a heat pump to form a Ground Source Heat Pump (GSHP) system, which is a type of Renewable Energy System that exploits geothermal energy for space heating and cooling. GSHP systems are proposed as an alternative to conventional Air Source Heat Pumps (ASHPs) as they exhibit a higher efficiency. In this study, this difference in efficiency is tested in order to determine how the systems perform in terms of environmental impact. Three types of GSHP systems (with different GHE configuration), each compared to ASHPs, undergo a Life Cycle Analysis using the ReCiPe method from both mid-point and end-point perspectives. The heating and cooling loads required for a single residential building of area 220 m2, with nearly Zero Energy Building technical characteristics, is used as a Functional Unit, for seven cases (locations/countries) from South to North Europe. Additionally, a Simple Payback Period method is employed to investigate the CO2 payback time for the GSHPs. It is concluded that the use of GSHP systems in residential buildings, even with nZEB (nearly Zero Energy Buildings) characteristics of low heating/cooling demand, can be a more environmentally friendly solution than that of an ASHP system, depending on the factors affecting the system, namely the ground thermal characteristics, the heating/cooling demand, the heating/cooling peak loads and electricity mix.