Direct Expansion Solar Assisted Heat Research Articles

Análisis del rendimiento termodinámico de una bomba de calor asistida por energía solar utilizando un condensador con recirculación

La presente investigación estudia una bomba de calor de expansión directa asistida por energía solar con un sistema de recirculación de agua. Los intercambiadores de calor utilizados fueron un colector/evaporador de cobre de 0.75 m2 y un condensador en forma de serpentín construido en tubería de cobre de 2.49 m de longitud y 8 espiras, inmerso en un tanque con 5 litros de agua.
 Para la recirculación del agua, se utilizó una bomba centrífuga de 20 L/min a 110 V. El compresor instalado es de velocidad variable a 12-24 V DC que utiliza R600a como fluido de trabajo. El dispositivo de expansión utilizado fue una válvula electrónica.
 Las pruebas experimentales se desarrollaron para una configuración con recirculación y sin recirculación de agua, considerando condiciones climáticas similares. De esta manera, se puede establecer cuál es la configuración que brinda un mayor rendimiento en el sistema. Para la obtención de datos de radiación solar, velocidad del viento y temperatura ambiente se empleó una estación meteorológica WS-1201.
 Este sistema proporcionó agua caliente sanitaria, a 45 °C en un tiempo entre 40 a 50 minutos dependiendo de las condiciones climáticas. El COP máximo del sistema con recirculación fue 9.83 un 8.04 % mayor que el sistema sin recirculación. El tiempo de calentamiento tardó 10 minutos menos para el sistema con recirculación, 16.67 % menos que al utilizar el sistema sin recirculación.

Numerical study of a direct-expansion solar-assisted heat pump water heater under frosting conditions based on experiments

A direct-expansion solar-assisted heat pump (DX-SAHP) water heater is studied numerically. A simulation model, which includes the models of components and the frost formation of the collector/evaporator, is established to research the heating performance of the system and the growth of frost. The calculation results conform well to the experiment, which verifies the accurate predictions. On the basis of the model, the influences of the meteorological parameters on the system performance under frosting conditions are analyzed. It is found the formation of frost can be effectively inhibited by decreasing the ambient temperature below a specific value. What’s more, the collector efficiency can also be improved effectively during the incipient growth of frost crystals. Besides, higher solar radiation intensity can significantly improve the heating performance and reduce the frosting rate. By contrast, the enhancement in wind speed will have the same effect, but only slightly. The system coefficient of performance (COP) reaches above 2.75 with the reliable heating performance under typical frosting conditions, and the collector/evaporator presents the superiority in delaying frosting in comparison with the traditional evaporator for air source heat pump.

Experimental study of a direct expansion solar-assisted heat pump (DX-SAHP) with finned-tube evaporator and comparison with conventional DX-SAHP

Direct expansion solar-assisted heat pump system (DX-SAHP) is proven to be an effective energy-saving application. Due to the fact that the traditional bare-plate type direct expansion solar-assisted heat pump occupies large footprint and is inefficient under low solar irradiation, a modified direct expansion solar-assisted heat pump system with finned-tube evaporator is designed and tested in the enthalpy difference lab with a solar simulator. The effects of ambient temperature, relative humidity, solar irradiation, and wind speed on the performance of the finned-tube type system are investigated. Furthermore, the performance of finned-tube type system and bare-plate type system is compared and analyzed under identical experimental conditions. It is revealed that raising ambient temperature, solar irradiation and wind speed can effectively improve the heating performance of the finned-tube type system. In comparative analysis, frosting enhances the overall performance of bare-plate type system, while the performance of finned-tube type system significantly declines due to the heat transfer deterioration caused by the frosting. The COP of finned-tube type system decreases from 1.72 to 1.54, when the relative humidity rises from 50% to 90%. For bare-plate type system, with the relative humidity rising from 70% to 90%, COP increases by 16.3%.

Theoretical study on heat transfer characteristics of a finned tube used in the collector/evaporator under solar radiation

As a key component of energy absorption system, the performance of collector/evaporator directly affects the performance of the direct-expansion solar-assisted heat pump system. However, the existing researches main paid attention to the heat pump system performance using this kind of heat exchanger, while little of them are related to the heat transfer characteristics of this heat exchanger. Therefore, the heat transfer characteristics of a finned tube used in the collector/evaporator under solar radiation are theoretically analyzed in this work, the theoretical expression of finned tube temperature field is obtained, and the accuracy of the theoretical solution is validated by simulation. Theoretical study on how various factors affect heat transfer characteristics of finned tube are carried out. It is found that the heat transfer of fin unexposed to sunlight can be adversely affected by solar radiation, and with the increase of the convective heat transfer coefficient and diameter of copper tube, the negative influence of solar radiation are effectively reduced. Under larger fin height and solar radiation condition, increasing convective heat transfer coefficient on the aluminum fin surface do not increase the air heat absorbed by finned tube, but release more heat into environment. This study can theoretically guide the design and optimization of collector/evaporator.

Feasibility Analysis and Performance Evaluation and Optimization of a DXSAHP Water Heater Based on the Thermal Capacity of the System: A Case Study

The present work conducts an evaluation of the feasibility and the overall performance and consequent optimization of a direct expansion solar assisted heat pump (DXSAHP) employed for domestic water heating. For the study conducted R134a, R404A, R407C and R410A working fluids were evaluated as well as the use of four, six and eight flat-plate solar collectors and a worktime ranging from 1 to 6 h. The case study is based in Mexico City with a 300 L container and a hot water outlet temperature of 51 °C. The paper introduces a new evaluation criterion based on the thermal capacity and all the evaluations conducted throughout this research revolve around this performance metric. The results show that, the system would require at least 4 h of operation to achieve the outlet temperature. Additionally, it was found that the R410A refrigerant has the best heat transfer properties; with an average condensation heat rate of 6.31 kW, followed by the R407C with 5.72 kW, the R404A with 5.42 kW and the R134a with 5.18 kW. Diversely, the R134a refrigerant requires 0.402 kW of compression work, 62% less than the R410A, which requires 1.06 kW. Consequently, R134a delivers the highest COP, which ranges from 7 to 14, followed by the R407C and R404A refrigerants, which present a similar behaviour between them, with COP ranging from 5 to 9 and 4 to 8, respectively, and finally the R410A, achieving the lowest COP, ranging from 3.5 to 6.5. Moreover, it was found that the R134a presents a higher dispersion regarding the energy exchange rate, which reveals that it is the fluid most susceptible to external factors, such as the weather. Contrarily, the remaining refrigerants present a more consistent performance. Finally, the optimization revealed that the R407C refrigerant is the most suitable given that it requires 20% less compression work than the R404A. This provides the heat pump system with a steadier behaviour, a COP ranging from 7 to 8, 30% higher than R410A, a worktime decrease of 1.5 h and heat transfer area of 5.5 flat-plate solar collectors, equivalent to a 31% reduction, both compared to R134a.

Economic analysis and design optimization of a direct expansion solar assisted heat pump

One option to reduce the energy consumption to produce domestic hot water is to replace the electrical heaters by direct expansion solar assisted heat pumps. Although, there are many studies on the performance of this system, there are few studies on economic analysis of the equipment. In this paper, an economic analysis comparing the payback of heat pump over an electrical heater is carried out. The heat pump is modeled using lumped models for heat exchangers and a black box model for the compressor. The results show that the augment of collector size increases the COP, but it decreases the collector efficiency. The economic analysis showed that there is a collector size that minimizes the payback time. Additionally, the influence of the environmental conditions, the refrigerant, the costs and the heating capacity on the payback period and optimum collector size were investigated. The results showed that there is an economically optimum size of collector area for the DX-SAHP, which it is the same for different environmental conditions. A case study was made for three different cities and the payback time found is the range of 1.77–3.24 years.

Refrigerants selection for a direct expansion solar assisted heat pump for domestic hot water

An important step during a project of a heat pump system is choosing a more suitable refrigerant. This paper presents a comparative study among refrigerants for a small direct expansion solar assisted heat pump (DX-SAHP). The mathematical model used in this study is presented in detail and validated from an experimental setup. The R134a is the reference and the refrigerants with low GWP are R290, R600a, R744 and R1234yf. The results show that R290 has better COP than others refrigerants for solar radiation between 300 W/m2 and 700 W/m2, as well as for environment temperature between 10 °C and 35 °C. On the other hand, for solar radiation less than 50 W/m2, the R134a has better COP than R290. TEWI (Total Equivalent Warming Impact) analysis indicates that the indirect emission is the most important effect, and then, the TEWI results almost followed the COP outcome. A parametric analysis was conducted to evaluate the influence of the CO2 emission factor for producing electricity. In countries with higher emission factor, the refrigerant with the best COP has the best TEWI. The influence of wind speed and ambient temperature in COP of a DX-SAHP using R290 were more relevant in low solar radiation.

Performance improvement of solar photovoltaic/thermal heat pump system in winter by employing vapor injection cycle

Direct-expansion solar assisted heat pump (DX-SAHP) is a promising technology for saving energy. In this paper, vapor injection (VI) cycle system to DX-SAHP system is studied. Thermodynamic comparative analyses of ideal VI cycle and ideal conventional vapor compression cycle was carried out for verifying the advantages by applying VI cycle into DX-SAHP. Additionally, photovoltaic/thermal (PVT) modules were used in the system to harvest thermal energy and electric power. A PVT based solar heat pump system employing VI cycle (PVT-VISHP) was proposed, fabricated and investigated experimentally. Performance of the experimental prototype was subsequently evaluated for seven days in winter. The system’s performance was evaluated based on the average ambient temperature of −1.13 °C and solar irradiation of 164.03 W/m2. Results showed that the total generated electricity, heat, and consumed power of the system were 0.51 kWh, 23.68 kWh, and 7.24 kWh, respectively. Moreover, average heating coefficient of performance (COPth) and advanced coefficient of performance (COPPVT) of the PVT-VISHP system were 3.27 and 3.45, respectively. In the study, the ambient-evaporating temperature difference ranged from 9.2 °C to 17.1 °C. Furthermore, variations of injected vapor’s pressure, mass flow rate, mass flow rate ratio, and subcooling effects were analyzed in detail. Performance of the PVT modules during a sunny day was tested. It was found that the PVT modules’ average thermal collecting and PV efficiencies were 49.9% and 7.51%, respectively.

Numerical study and experimental validation of a direct-expansion solar-assisted heat pump for space heating under frosting conditions

Frosting problem restricts the wide application of heat pump, and direct expansion solar-assisted heat pump system (DX-SAHP) can be a solution. However, research on DX-SAHP for space heating under frosting conditions is rare in the published literatures, especially on numerical level. A dynamic model for a DX-SAHP system for space heating is proposed and validated in this paper. The root mean square deviations of the simulation results from the experimental data are no more than 6%, which validates the model. Based on the model, the frost characteristics are investigated and parametric analysis is conducted. Compared to the widely used air source heat pumps with fin-and-tube heat exchangers, the frost pattern of the evaporator-collector is nearly steady and the frosting rate is less. The analyzed parameters include ambient temperature, solar irradiation and relative humidity. Solar irradiation can retard frosting and improve the heating performance of the DX-SAHP system. Solar irradiance of 100 W/m2 can totally prevent frost formation as the ambient temperature and the relative humidity is −1 °C and 70%, respectively. Under the condition that the relative humidity and solar irradiance are 70% and 0 W/m2, frost is the most serious with the ambient temperature of −1 °C, with higher heating capacity and COP comparing to that under other ambient temperatures. No frost forms when the ambient temperature and solar irradiance are 1 °C and 0 W/m2, and the relative humidity is lower than 60%. When the relative humidity is over 60%, the frost thickness and the heating performance increase with increasing relative humidity. The frosting process is found to be beneficial for the heating performance of the system. The results indicate that the DX-SAHP system is applicable under frosting conditions.

Experimental performance analysis of a multiple-source and multiple-use heat pump system: a predictive ANN model of sky-source heat pump

In this study, an artificial neural network (ANN) was used to model the thermal performance of a novel direct-expansion solar-assisted sky-source heat pump (SSHP) during winter. The input parameters of the ANN take into account the weather conditions, water loop characteristics, and the compressor characteristics of the SSHP. The following four output parameters were adopted to evaluate the SSHP performance: the outlet water temperature of the water loop, electricity consumption, heat production, and the coefficient of performance. To increase the accuracy of the ANN and simultaneously investigate the effects of each of the input parameters on the performance of the SSHP, the combination of input parameters for the validation data set was varied in multiple case studies. Additionally, learning curves were introduced to clarify the relationship between the training data size and the generalization performance of the ANN. Finally, the ANNs with the best performance were selected and evaluated based on the test data set by using metrics such as the root mean square error. The reported results demonstrated that the ANN model has comparatively high SSHP winter performance prediction accuracy.

Modelling & Simulation of a Direct-Expansion Solar Assisted Heat Pump for Water Heating Application in South Africa

Hot water heating accounts for about 7% of the energy demand in South Africa and about 30% in the residential sector. The most common heating method electrical resistance water heaters (geysers) are inherently fossil-fuel driven and thus responsible for carbon emissions. In this paper, the potential application of a direct-expansion solar-assisted heat pump water heater (DX-SAHPWH) system in South Africa was investigated. The DX-SAHPWH combines the beneficial characteristics of both the solar thermal collector and heat pump, however, the performance of a DX- SAHPWH system is significantly influenced by the changes in meteorological conditions. A quasi-steady state mathematical model was developed and theoretical simulations were conducted using meteorological data of Cape Town. The results indicated an hourly average COP of was 4.9, collector efficiency of 50 %, solar fraction and heating times of 68 % and 192 minutes for average temperature, solar radiation and wind speeds of 23 °C , 700 W/ m2 and 3.3 m/s. The annual average COP was 5.3, collector efficiency 48 %, solar fraction 66% and heating time of 127 minutes. Furthermore, a parametric study indicated that the ambient temperature, solar radiation and wind speeds influence the system performance.

Dynamic modelling and performance evaluation of a direct-expansion solar-assisted heat pump for LPG vaporisation applications

This study investigated a novel LPG vaporisation system utilising direct-expansion solar-assisted heat pump (DX-SAHPV)1DX-SAHPV is the abbreviation of “LPG vaporisation system utilizing direct-expansion solar-assisted heat pump”.1 for supplying residential gas. The DX-SAHPV applies a collector-evaporator to obtain heat energy from both solar radiation and ambient air to produce hot water to vaporise LPG liquid. It can operate in six operating modes depending on the weather and gas load conditions. In this study, a dynamic model and control strategies realising the mode switching were developed to carry out the operating performance evaluation of the system. By applying the typical meteorological year data and the gas load data of a community in Beijing, China, the performance of the DX-SAHPV was evaluated and analysed for the whole year and two selected days. The simulation results show that the system can vaporise adequate LPG liquid for the community throughout the year. The solar energy contribution accounts for approximately 68% of the total heat energy for the water heated vaporisation. It was also found that the DX-SAHP obtains average monthly values of COP ranging from 2.72 to 3.37 and solar collector efficiency varying between 93 and 152%. In addition, which mode the system operates at any time on selected days was predicted and the running time of each mode in each month was discussed.

Thermodynamic analysis of a direct expansion solar-assisted heat pump system working with R290 as a drop-in substitute for R22

The thermodynamic analysis of direct expansion solar-assisted heat pump system working with R290 as a drop-in substitute for R22 was carried out under the metrological conditions of Calicut, India. A prototype of a DXSAHP system consists of a compressor, an air-cooled condenser with evaporator–collector and thermostatic expansion valve. The experiments were carried during the winter months of 2016. The artificial intelligence technique artificial neural network integrated with genetic algorithm model was presented to predict energy and exergy performance of a system. The energy performance ratio of a system was found to be 5.75% higher and reduced heating capacity of about 6.8% when compared to R22. Similarly, the second law analysis (exergy analysis) of a total system working with R290 was found to be better when compared to baseline refrigerant. The selected alternative working fluid is a hydrocarbon and has zero ozone depletion and negligible global warming potential. Hence, R290 can be used as a drop-in substitute for R22 in DXSAHP systems.

Subcooling control method for the adjustable ejector in the direct expansion solar assisted ejector-compression heat pump water heater

A subcooling control method is proposed in this paper, aiming to regulate the adjustable ejector in the presented direct expansion solar assisted ejector-compression heat pump water heater. The proposed subcooling control method adopts the PID (Proportion Integration Differentiation) algorithm and aims to help the presented system track the given subcooling degree through automatically adjusting the nozzle throat area of the adjustable ejector. The proposed subcooling control method is firstly examined by dynamic simulation method. The simulation results show that the proposed subcooling control method can meet the purpose of controlling subcooling degree and reduce the power consumption by 7.95% under the considered condition. The experimental setup with and without the subcooling controller are tested for comparison, and the test results confirm the simulation conclusions. The experimental system with the subcooling controller maintains the subccoling degree in a range of 4–7 °C, and reduces the power consumption by 6.24% and 6.99% compared with those with two different ejector throat areas. Further analysis shows that the subcooling controller brings the highest ejector entrainment ratio but not evident improvement in the pressure lift ratio aspect. In brief, the proposed subcooling control method achieves satisfactory subcooling degree control precision and further improves the system performance.

Modeling and experimental analysis of the solar radiation in a CO2 direct-expansion solar-assisted heat pump

In the present work is presented the dynamic model of an evaporator of a Direct Expansion Solar Assisted Heat Pump (DX-SAHP), charged with CO2. This dynamic model was used to analyze the evaporator response to sudden variations in the solar radiation. Two strategies are used to make the system reach the steady state after the heat pump start-up. The first one is the usual balances of mass, energy and momentum. The second strategy consisted in impose an equal refrigerant mass flow rate at the evaporator inlet and outlet. Both strategies were able to conduct the system to a steady state, however, the second one required less computational effort. The mathematical model was validated using experimental data and employed to perform several simulations. The results obtained with the mathematical model revealed that a small variation of the solar radiation leads to a significant variation in the superheat, therefore requiring an immediate action of the expansion device. It was concluded that an Electronic Expansion Valve (EEV) would be better suited to meet the needs of rapid interventions on the mass flow rate at the evaporator inlet, and also because the DX-SAHP could operate in a continuous transient condition in some seasons.

A variable frequency control method and experiments of a direct-expansion solar-assisted heat pump system

A direct-expansion solar-assisted heat pump hot water system using variable frequency control was studied experimentally. The system mainly consisted of a frequency conversion compressor, a condenser using microchannel heat exchanger, a bare plate solar collector/evaporator, and an electronic expansion valve as the throttling device. Based on experimental data and the opening control method of the electronic expansion valve presented previously, a variable frequency control method was developed, including the initial setting and real-time regulation of the compressor speed. Then the control method was tested under the year-round environmental conditions, which confirmed that the system could obtain a higher coefficient of performance (COP) and control the heating time effectively. It was found that the compressor speed was negatively correlation affected by the solar radiation intensity, and the electronic expansion valve opening was negatively correlation influenced by the compressor speed. Experiments showed that most of the average system COP was above 3.0 throughout the year. Even in the spring for solar radiation intensity of 124 W m−2 with environmental temperature of 8.5 °C and in the winter for solar radiation intensity of 323 W m−2 with environmental temperature of −3.0 °C, the average system COP still reached 2.81 and 2.58, respectively. Meanwhile, the heating time expectations were found to agree well with experimental data within 20 min accuracy.

Correction to: Exergy analysis of direct-expansion solar-assisted heat pumps working with R22 and R433A

The article Exergy analysis of direct-expansion solar-assisted heat pumps working with R22 and R433A, written by L. Paradeshi, M. Mohanraj, M. Srinivas, S. Jayaraj, was originally published electronically on the publisher’s internet portal (currently SpringerLink) on 13, June 2018 with open access.

Performance of hydrocarbon mixture in a direct expansion solar assisted heat pump system

The energy performance of a direct expansion solar assisted heat pump system, working with a hydrocarbon mixture (HCM) of R290/R1270, 70:30 (by mass ratio) was investigated. The experiments were carried out at a metrological condition existing near Calicut, India. The experimental system mainly consists of a compressor, condenser, thermostatic expansion valve and evaporator collector. A system simulation technique was used for modelling the system. The experimental results obtained are compared with simulation output of the system working with R22 and a mixture of R290/R1270. The selected alternative working fluid mixture showed better energy performance ratio about 5.8% higher. The parametric analysis of the system working with R22 and a mixture of R290/R1270 is investigated with the help of an established system simulation model. The effect of an evaporator (solar collector) area and solar insolation are found to be the significant influencing parameters on the system performance. Moreover, the new hydrocarbon mixture is found to be a good energy efficient and environmental friendly working fluid to phase out R22 in the solar assisted heat pump systems.

Energy and exergy analysis of a new direct-expansion solar assisted vapor injection heat pump cycle with subcooler for water heater

A new direct-expansion solar assisted vapor injection heat pump cycle with subcooler for water heater is proposed in this paper which aims to improve the performance of traditional subcooler vapor injection heat pump cycle by utilizing the solar energy. The new cycle has two working modes (SVIC mode and DX-SVIC mode) to be switched according to the solar radiation intensity. The performance enhancement potential of the new cycle is investigated by comparing with the traditional cycle through theoretical method. Under the considered conditions, the new cycle yields an average of 14.6% and 42.9% improvement in the heating coefficient of performance and heating capacity aspects compared with the traditional cycle, respectively. Both the new cycle and the traditional cycle have optimum injection pressures to obtain the maximum heating coefficient of performance. The simulation results indicate that the DX-SVIC mode outperforms the SVIC mode on most solar radiation condition, while the latter one is suitable for the extremely low and no solar radiation condition. The energy performance and heat exergy of the new cycle are improved with enhancing the solar radiation, while the variation of exergy efficiency shows contrary tendency. As increasing the injection pressure, the exergy input decreases more rapidly than the heat exergy, and thus the exergy efficiency is improved. The increase of the solar collector area always leads to the energy performance improvement of the new cycle.

Mathematical Thermal Modelling of a Direct-Expansion Solar-Assisted Heat Pump Using Multi-Objective Optimization Based on the Energy Demand

An analytical model is proposed to evaluate the performance of a Direct Expansion Solar-Assisted Heat Pump under a given environmental condition. These thermal machines commonly employ uncovered flat plate solar collectors, and given this, the convection phenomenon is taken into account as well as the effect of the diffuse and reflected solar radiation in addition to the normal beam radiation absorbed by the tilted surface of the collectors. The heat pump cycle is modelled through a first law of thermodynamics approach in order to compute the heat yielded through condensation and the minimum heat required by the volume of water in the thermal storage unit. Consequently, the thermal capacity of the heat pump, the ratio at which the system yields heat to a given load of water, is calculated and discussed. The results of the model proposed are compared with the experimental data provided by three research papers with experiments conducted in different geographic coordinates and test rigs operating during diverse atmospheric conditions. A maximum relative error of 20% was obtained and furthermore, a statistical analysis of the data was conducted having found that there is no significant statistical difference between the analytical and experimental data samples within the 95% confidence interval. Finally, based on the thermal capacity, the performance of the heat pump is evaluated and a multi-objective optimization technique is implemented to obtain the best possible combination of factors to further enhance the performance of the heat pump.