Solar thermal collectors are crucial for decarbonizing thermal energy needs in both residential and industrial sectors. However, improving thermal efficiency remains a key challenge for this technology, as it is significantly affected by the conductive and convective thermal resistances between the working fluid and the absorber. Additionally, reducing investment costs is necessary for widespread adoption. To overcome these issues, Direct Absorption Solar Collectors (DASCs) using nanofluids with tailored optical properties have been proposed. In DASCs, the working fluid directly absorbs the solar radiation and converts it into heat, which simplifies the system's design and improves the temperature distribution within the fluid, therefore enhancing the overall thermal efficiency. The present study involves numerical simulations in ANSYS Fluent to evaluate the thermal performance of two DASCs: a flat rectangular and an evacuated tube configuration. Both systems operate with carbon nanofluids, specifically Single-Wall-Carbon-NanoHorns (SWCNHs) suspended in deionized water. The impact of nanofluid temperature and mass flow rate, nanoparticles’ concentration, glass properties and geometrical features on thermal efficiency is thoroughly analyzed. The optimization of DASC geometry, proper material selection and tuning of nanoparticles’ concentration are found to be crucial for the future deployment of DASCs in the building sector, ensuring higher performance and cost-effectiveness.

Industrial high-temperature waste heat remains a largely underused high-grade energy source due to the limited techno-economic performance of conventional thermal energy storage (TES) systems. This paper proposes a novel multi-layered hybrid TES concept that combines phase change materials (PCMs) and molten salts to achieve high energy storage density while maintaining adequate heat transfer performance and stable charging-discharging behavior. The design incorporates two concentric layers filled with different melting temperature PCMs and a central core filled with molten salt, which serves as the heat transfer fluid and a high-temperature thermal buffer. The paper analyzes the thermal response, cycling behavior, and techno-economic performance of the hybrid TES for a given PCM pair. It benchmarks it against the two-tank TES across three scenarios: electricity generation with a power block, heat supply to an industrial process, and standalone energy storage. The optimized hybrid design reduces the levelized cost of electricity (LCOE), heat (LCOH), and storage (LCOS) by up to 2.6%, 12.4%, and 33.2%, respectively. The optimization also provides guidelines for the internal tube configuration, tube pitch, and energy storage allocation between the PCM layers. This article demonstrates the strong potential of our hybrid TES approach for cost-effective high-temperature energy storage.

Latent thermal energy storage (LTES) employing phase change materials (PCMs) offers a promising solution for thermal management in various applications, compensating for the intermittent and unstable characteristics of several thermal energy sources, such as solar energy. However, the inherently low thermal conductivity of PCMs hinders their heat transfer efficiency, resulting in extended charging and discharging times. This limitation can be addressed either by enhancing the thermal conductivity of the PCM or by optimizing the storage system geometry. In this study, two LTES configurations, finned and finless units based on bar-and-plate technology, were tested under different conditions of mass flow rate (100, 150, 200 kg h⁻¹) and heat transfer fluid (HTF) inlet temperature (46, 49, 52 °C), corresponding to temperature difference (∆T_thermal) of 3, 6 and 9 °C. To the best of the authors' knowledge, the bar-and-plate technology has been only marginally addressed in the context of LTES systems, and no comprehensive experimental investigations are currently available in the literature. The PCM employed, a paraffin wax (RT42), has a melting temperature ra nge between 38.2 °C and 42.5 °C. Results demonstrated that the finned unit reduced the melting time by up to 84 % compared to the finless configuration. At ∆T_thermal = 9 °C and a mass flow rate of 200 kg h⁻¹, the charging process was completed within 2 hours for the finned unit versus about 8 hours for the finless unit. Moreover, for the finned unit, increasing ∆T_thermal from 3 °C to 9 °C resulted in a 28–50 % decrease in melting time, while an increase in the mass flow rate from 100 to 200 kg h⁻¹ shortened melting time by about 35 %. As a further step, the experimental data were used to validate a resistance-capacitance numerical model of the LTES unit, providing a valuable tool for LTES optimization and design according to specific application requirements. Unlike other available calculation methods, the developed model accounts for the explicit incorporation of fin geometry and PCM material in equivalent conductivities (PCM-fin composite) to capture the directional heat transfer pathways. Moreover, a parametric study was carried out to analyze the effect of fin parameters on melting time and energy storage.

Traditional heat pump systems typically rely on a single low-temperature source, such as air, ground, or solar energy, each with intrinsic limitations. Air-source heat pumps are highly sensitive to temperature fluctuations across seasons and daily cycles, while the efficiency of solar-source heat pumps is constrained by the intermittent availability of solar radiation. Ground-source heat pumps, in contrast, deliver consistent seasonal performance but involve higher costs for the installation of borehole heat exchangers. Multisource heat pumps offer a promising technology to overcome these challenges and maximize the use of renewable energy. This paper presents a numerical investigation of an innovative multisource heat pump using CO₂ as a low GWP refrigerant, that can exploit three different thermal sources through dedicated evaporators: air source with a finned coil heat exchanger, solar source with photovoltaic-thermal (PV-T) collectors, and ground source with a U-tube borehole heat exchanger (BHE). Two modes of operation are foreseen: solar-air mode (SA-mode) and ground-air mode (GA-mode). The novelty of this system lies in the concept of a multisource direct expansion heat pump in which the CO₂ directly vaporizes in flooded mode in the solar or ground evaporators, while the finned coil works in dry expansion mode. Differently from all other systems, the solar and ground evaporators operate simultaneously with the air evaporator. Simulations are performed to assess the performance of the multisource heat pump under varying environmental conditions: air temperature, solar irradiance, and soil temperature. The results demonstrate that while air temperature influences the performance of both SA-mode and GA-mode, each mode exhibits distinct sensitivities to the other environmental parameters. SA-mode performance is significantly affected by solar irradiance, with a 100 W/m² increase in irradiance corresponding to a 2.8 % enhancement in the coefficient of performance (COP). Conversely, GA-mode performance shows a notable response to soil temperature variations, where a 1 K increase in soil temperature results in a 0.9 % improvement in COP. The results compare SA-mode and GA-mode with air-source heat pump mode under varying thermal loads for space heating (SH) and domestic hot water (DHW) production, showing up to 22 % COP increase for GA-mode and SA-mode. As a further step, the study investigates the effect of varying the number of PV-T modules and borehole heat exchangers on the heat pump performance. The simultaneous use of two energy sources always results in improved system performance even with limited PV-T or BHE heat transfer areas.

This study numerically compares the seasonal heating performance (SCOP) of three 15 kW heat pumps using natural refrigerants: two with carbon dioxide and one with propane. The propane system is an air-source heat pump (R290-AHP). The carbon dioxide systems include an air-source heat pump (R744-AHP) and a dual-source solar-air heat pump (R744-SAHP) equipped with finned-coil and photovoltaic-thermal evaporators that can work simultaneously. In the case of a transcritical carbon dioxide cycle, the use of several low-temperature sources is a promising solution to improve the performance of the system by enhancing the exploitation of renewable energy sources. Since the efficiency of air and solar-based systems is related to weather conditions and location, there is the need for accurate models to evaluate the SCOP. In this work, a numerical model has been developed to design the three 15 kW heat pumps and assess the SCOPs under variable space heating and domestic hot water demand profiles, using climatic data for Rome and Strasbourg. The results indicate that the R290-AHP consistently achieves the highest SCOP, while the R744-AHP performs the lowest. The R744-SAHP overperforms the R744-AHP by approximately 4 % regardless of heating demand characteristics. In particular, the results show that the performance of the three heat pumps is significantly influenced by the distribution of the thermal load throughout the day. Specifically, when the thermal load is concentrated during daylight hours, the heat pumps can operate at a higher SCOP, especially for the R744-SAHP, and also increase, up to 44 %, the self-consumed photovoltaic energy produced.

In this work, numerical simulations are performed to predict two-phase annular flow of refrigerant R245fa inside a 3.4 mm diameter vertical channel. The VOF (Volume of Fluid) method implemented in an OpenFOAM solver is used to accurately track the vapor-liquid interface. A 2D axisymmetric domain is considered and the Adaptive Mesh Refinement (AMR) method is applied to the cells near the liquid/vapor interface. The Reynolds-Averaged Navier Stokes (RANS) equations are solved and the k-ω SST model is adopted for turbulence modelling in both the liquid and vapor phase. Simulations are used to calculate instantaneous and mean values of the liquid film thickness at mass flux G = 100 kg m^-2 s^-1 and vapor quality ranging between 0.2 and 0.85. Numerical results are compared against measurements of the liquid film thickness taken during vertical annular downflow. Previous works from the literature and the deviations observed between present numerical and experimental results suggest the need for turbulence damping at the vapor-liquid interface by adding a source term in the ω equation. The simulations show that a low value of the turbulence damping parameter (e.g. 1) causes the average liquid film thickness to increase by 25 %–52 % compared to the non-damped scenario. The interface presents large amplitude disturbance waves in the non-damped case, whereas small ripple waves are predicted when turbulence damping is introduced. Furthermore, the difference between the application of a symmetric and asymmetric treatment for the source term is analysed. From the comparison between experimental data and numerical simulations, it emerges that the value of the correct damping source term to be applied is strictly dependent on the vapor quality.

In annular downward flow, an annular liquid film flows at the perimeter of the channel pushed down by the gravity force and by the shear stress that the vapor core exerts on it. Depending on the working conditions, the vapor-liquid interface can be flat or rippled by waves. The knowledge of the liquid film thickness is very important for the study of annular flow condensation because the thermal resistance of the liquid is often the most important parameter controlling the heat transfer. A new approach for the simulation of annular flow is here proposed using an in-house developed transient solver based on the Volume of Fluid (VOF) adiabatic solver interIsoFoam available in OpenFOAM. With the VOF method, in addition to the standard set of equations (continuity and momentum), a transport equation related to the advection of the volume fraction scalar field has to be solved. The numerical setup consists of 2D axisymmetric domain. An adaptive mesh refinement (AMR) method is added to the solver to better capture the interface position. The k-ω SST model is used for turbulence modelling in both the liquid and vapor phases and a source term (whose magnitude is controlled by a model parameter named B) is included in the ω equation to damp the turbulence at the interface.

Dual-source solar-air heat pumps represent a promising solution for overcoming the limitations associated with single-source utilization, thereby enhancing heat pump performance. However, running the heat pump by alternatively employing the more advantageous source requires the integration of a controller capable of continuously monitoring and predicting the heat pump's performance in response to dynamic environmental and operational variables. Even so, a selective alternate operation does not allow to get the maximum possible performance from the use of the two heat sources. A different approach to address this challenge is the simultaneous utilization of the two sources, by properly combining two evaporators in the CO₂ circuit. This paper presents an experimental investigation of a dual-source heat pump using CO₂ as refrigerant, which can operate in three different evaporation modes: air-mode (using a finned-coil evaporator), solar-mode (using a photovoltaic-thermal PV-T evaporator), and simultaneous-mode (using both the evaporators simultaneously). The novel solution presented here does not require to split the refrigerant flow rate between the two evaporators and at the same time it solves the problem of possible maldistribution at the inlet of the evaporators. Experimental data indicate that the heat pump operating in simultaneous-mode allows to increase the evaporation pressure and the coefficient of performance compared to operation in air-mode or solar-mode. The measurements have been employed for validating a model of the system, capable of predicting steady-state and dynamic performance under various environmental and operational conditions. Simulation results show that the simultaneous-mode operation can be outperformed by the solar-mode only at high irradiance and low air temperature, when the evaporation temperature gets higher than the air temperature. Finally, the impact of the number of PV-T collectors and solar irradiance on the heat pump performance has been simulated and discussed. On this regard, the simultaneous use of the two heat sources adds more flexibility to the system and its design, because even the availability of a small solar area can contribute enhancing the performance over the mere air source heat pump.

To overcome the limitations of conventional solar thermal collectors (high conductive and convective thermal resistances between the absorber and the fluid), a promising technology is represented by direct absorption solar collectors working with nanofluids, where the incoming solar irradiance is absorbed directly within the volume of fluid. The main issue hindering the diffusion of such technology is related to its reliability, since nanofluids can lose their chemical stability due to nanoparticles sedimentation. Thus, the present work aims at investigating the stability and absorption capability of two nanofluids made of Single-Wall-Carbon-NanoHorns in a volumetric solar receiver. The present investigation covers the study of material compatibility, the laboratory measurements of nanofluid absorbance and the field simultaneous measurement of thermal and optical efficiency. Since the final performance of direct absorption solar collectors strongly depends on the nanofluid stability, the double efficiency measurement allows to better verify any possible instability effect. Furthermore, field measurements during nanofluid circulation are rare in the literature. The efficiency of the volumetric solar collector is between 88 % and 92 % at null reduced temperature difference. Finally, tests are performed at high flow rate leading to an evident performance degradation, due to nanoparticles deposition, that can be reversed with sonication.