Solar-Assisted HVAC Systems with Integrated Phase Change Materials

Solar-assisted heating, ventilation and air-conditioning (HVAC) systems are receiving increasing attention. This chapter presents the development of HVAC systems with integrated solar photovoltaic-thermal (PVT) collectors and phase change materials (PCMs) to reduce building energy consumption while providing satisfactory indoor thermal comfort. PVT collectors, which can generate both thermal energy and electricity simultaneously, are a promising technology for developing high-performance buildings. As solar energy is intermittent, the integration of phase change materials (PCMs) with PVT-driven HVAC systems can provide an opportunity to effectively utilise solar energy and maximise the performance of HVAC systems. The results showed that the coefficient of performance (COP) of an air source heat pump system with integrated PVT collectors and PCMs was 5.2, which was higher than the use of the air source heat pump only (i.e., 3.06) during the test period investigated

Multi-objective optimisation of thermal energy storage using phase change materials for solar air systems

Thermal energy storage (TES) using phase change materials (PCMs) is being widely considered as one of the alternative solutions for effective use of solar energy. This paper presents a multi-objective optimisation strategy for TES systems using PCMs for solar air systems, in which two performance indicators of average heat transfer effectiveness and effective PCM charging time were used as the conflicting objectives. The influence of the key design variables on the performance of an air-based PCM TES system was first experimentally investigated using Taguchi method, and the results were used to develop two performance models for optimisation. A genetic algorithm was used to search for an optimal Pareto front and a multi-criteria decision-making process was employed to determine the compromise optimal solutions. The results showed that the average heat transfer effectiveness of the PCM TES system can be improved from 44.25 to 59.29% while the effective PCM charging time increased from 4.53 to 6.11 h when using the solutions identified by the proposed strategy with the weighting factors of 0.5/0.5 for both objectives, in comparison to a baseline case. A further comparison showed that the optimal design identified by the proposed strategy outperformed the two designs identified using Taguchi method

Thermal performance evaluation of an integrated photovoltaic thermal-phase change material system using Taguchi method

This paper presents the performance evaluation of an integrated photovoltaic thermal (PVT) collector-phased change material (PCM) thermal energy storage (TES) system. The PVT collectors can generate both electricity and low-grade thermal energy during the daytime, and the thermal energy generated can be temporarily stored in the PCM TES unit and used for space heating during the night-time. Taguchi method and analysis of variance are used for the simulation design and data analysis, respectively. The thermal performance of the proposed system was evaluated in terms of the useful energy stored in the TES system. The results showed that the outlet air temperature of the TES unit remained at least 2°C higher than the inlet air temperature during the discharging process in the selected test day. The PCM type and the PCM charging air flow rate were the most important factors influencing the useful energy stored in the TES system

Investigation on the feasibility and performance of transcritical CO2 heat pump integrated with thermal energy storage for space heating

CO2 heat pumps have drawn a great deal of attention as an economic form of heating under low ambient temperature conditions. However, the system performance is not desirable and shows a lower COP due to the higher inlet water temperature at the gas cooler, which causes a higher refrigerant temperature at the exit of the gas cooler, leading to a large throttle loss when the refrigerant flow through the throttling device. To tackle this issue, a transcritical CO2 heat pump unit integrated with two thermal energy storage (TES) containing phase change materials (PCMs) is proposed in this paper. The objective of this work is to model and simulate the proposed system using TRNSYS based on a typical single family rural house in Beijing (typical cold climate conditions), China. The results showed that the heating capacity and energy consumption decreased by 21 and 24%, respectively, and the heating seasonal performance factor (HSPF) of the proposed system increased by 4% in comparison with the baseline system during the entire heating period. The simulation results demonstrated that TES is helpful to improve CO2 heat pump system performance and monthly energy saving ratio for space heating

Optimal design and size of a desiccant cooling system with onsite energy generation and thermal storage using a multilayer perceptron neural network and a genetic algorithm

A design optimization strategy for rotary desiccant cooling (RDC) systems integrated with a photovoltaic thermal collector-solar air heater (PVT-SAH) and a phase change material based thermal energy storage (TES) (named RDC-PVT-SAH-TES) is presented in this paper. The optimization method was developed using a multilayer perceptron neural network (MPNN) and a genetic algorithm to maximize the specific net electricity generation (SNEG) of RDC-PVT-SAH-TES systems while maintaining the required cooling demand with the assistance of an electric heater. A dimension reduction method was used to determine the main design parameters of the RDC-PVT-SAH-TES system. An RDC-PVT-SAH-TES system was simulated using TRNSYS and the simulation data were utilized for training and validation of the MPNN model and for dimension reduction analysis. A comparison of the design solution identified by this optimization method with a baseline design showed that the SNEG and the solar thermal contribution of this RDC-PVT-SAH-TES system can be increased from 3.77 kWh/m2 to 10.32 kWh/m2 and from 91.5% to 99.4%, respectively. The optimization method developed could be potentially adapted to facilitate optimal design and size of other engineering systems with onsite energy generation and thermal storage

Experimental investigation on thermal characteristics of transcritical CO2 heat pump unit combined with thermal energy storage for residential heating

2019 Elsevier Ltd A preliminary experimental investigation was carried out to analyze the thermal characteristics of a transcritical CO2 air source heat pump unit with thermal energy storage for residential heating. The primary impact factors such as discharge pressure, ambient temperature, supply/return water temperature were considered. Also, a performance comparison of the combined system with the CO2 air source heat pump unit without thermal energy storage was presented. It was found that the maximum coefficient of performance (COP) of the CO2 air source heat pump unit decreased by 18.8%, and the optimum discharge pressure and the throttle loss increased by 14.7% and 47.5%, respectively, as the water temperature at the inlet of the gas cooler increased from 35 to 45 °C. Additionally, the results also indicated that the COP of the combined system was 17% higher than that of the CO2 air source heat pump unit. However, it was worth noted that the water flow rate should be adjusted according to the supply water temperature in the practical application so as to meet the thermal comfort for user. The experimental results demonstrated that thermal energy storage as a sub-cooler is a promising technology for reducing the throttle loss and improving the COP of the combined system, and is helpful to promote the CO2 air source heat pump application for space heating in cold regions