A unified approach for the thermodynamic comparison of heat pump cycles

The flexible heat pump cycle introduces a heat storage device into the Evans-Perkins cycle to recover, store, and reuse part of the sensible heat carried by the hot liquid refrigerant from the condenser, achieving a higher coefficient of performance than the latter. In this paper, we develop a unified approach, namely cycle superposition to allow comparison of the flexible heat pump cycle with other performance-enhancing cycle layouts including two-stage cycles with intercooling, subcooling, flash gas removal, or their combinations. We show that under ideal conditions, the flexible heat pump cycle is thermodynamically similar to two-stage heat pump cycles with full subcooling or flash gas removal, but no intercooling. From the energy recovery perspective, the two-stage cycles recover and reuse some sensible heat carried by hot liquid refrigerant simultaneously using their high-stage compressor, whereas the flexible heat pump cycle decouples the recovery and reuse of such heat in time using a heat storage. However, the irreversible heat transfer via real heat exchangers during charging and discharging processes will reduce the benefits of the flexible heat pump cycle. The effectiveness of all these performance-enhancing methods strongly depends on the characteristics of refrigerants

Machine learning for sustainable organic waste treatment: a critical review

Data-driven modeling is being increasingly applied in designing and optimizing organic waste management toward greater resource circularity. This study investigates a spectrum of data-driven modeling techniques for organic treatment, encompassing neural networks, support vector machines, decision trees, random forests, Gaussian process regression, and k-nearest neighbors. The application of these techniques is explored in terms of their capacity for optimizing complex processes. Additionally, the study delves into physics-informed neural networks, highlighting the significance of integrating domain knowledge for improved model consistency. Comparative analyses are carried out to provide insights into the strengths and weaknesses of each technique, aiding practitioners in selecting appropriate models for diverse applications. Transfer learning and specialized neural network variants are also discussed, offering avenues for enhancing predictive capabilities. This work contributes valuable insights to the field of data-driven modeling, emphasizing the importance of understanding the nuances of each technique for informed decision-making in various organic waste treatment scenarios

Integration of anaerobic digestion with heat pump: machine learning-based technical and environmental assessment

Anaerobic digestion (AD)-based biogas production mitigates the environmental footprint of organic wastes (e.g., food waste and sewage sludge) and facilitates a circular economy. The work proposed an integrated system where the thermal energy demand of an AD is supplied using an air source heat pump (ASHP). The proposed system is compared to a baseline system, where the thermal energy is supplied by a natural gas-based heating system. Several machine learning models are developed for predicting biogas production, among which the Gaussian Process Regression (GPR) showed a superior performance (R2=0.84 and RMSE=0.0755 L gVS-1 day-1). The GPR model further informed a thermodynamic model of the ASHP, which revealed the maximum biogas yield to be approximately 0.585 L.gVS-1.day-1 at an optimal temperature of 55 °C (thermophilic). Subsequently, life cycle assessment showed that ASHP-based AD heating systems achieved 28.1% (thermophilic) and 36.8% (mesophilic) carbon abatement than the baseline system

Thermo-economic assessment of reverse osmosis desalination system driven by the organic Rankine cycle

A lack of fresh water can be observed in many countries with the increase of the world population. Therefore, waste heat was recovered from a diesel engine and used to run an organic Rankine cycle to supply power to a high-pressure pump, which pumps feed water to a reverse osmosis (RO) system. This study was done to increase the mass flow rate of fresh water production from the seawater. In order to increase the fresh water, a higher value of power is required which increases the total annual cost (TAC) of the system. The fresh water mass flow rate is defined as the efficiency of the system that increases by the increase of the power generated in the turbine. By considering the fresh water mass flow rate and cost as the two objective functions, an enhancement in one function destroys the other function. Due to the conflict between the functions, multi-objective optimization is required to apply to increase the thermal efficiency, and decrease the TAC. For these purposes, seven design parameters that some of the are turbine pressure, condenser pressure were selected, and a set of solutions were obtained for the optimized system parameters to find the effects of system design parameters on the system efficiency, TAC and consequently fresh water production. It was concluded that increasing the turbine pressure enhances the fresh water production, but increasing the condenser pressure decreases the mass flow rate of fresh water. In addition, increasing the feed water temperature and mass flow rate of feed water has positive effects on the RO recovery ratio and the mass flow rate of fresh water. Finally, a single solution is introduced as the final optimum point to evaluated different design parameter’s effects on the system performance and fresh water production. The optimum magnitude for the system thermal efficiency was 37.99% with a TAC of 40,785 $/y as well as 954.67 kg/s of fresh water

A flexible heat pump cycle for heat recovery

Heat pumps will play a key role in transitioning domestic heating to fossil-free sources. However, improvement in energy efficiency and cost reduction are still needed. Current vapour-compression heat pumps are built upon the Evans-Perkins cycle which was originally designed for refrigeration applications. Once hot liquid refrigerant has transferred energy to the central heating system, it leaves the condenser with sensible heat which can be utilized. Here we report a modified and flexible Evans-Perkins heat pump cycle integrating heat recovery and storage which is then used as an ancillary heat source for the heat pump’s operation. It operates in a quasi-two-stage mode to theoretically save up to 20% in compressor power consumption compared with single-stage cycles. We build a prototype with off-the-shelf parts and demonstrate a practical 3.7% power saving at a heat production temperature of 35 °C. Power saving will further increase with heat supply temperature. We also qualitatively show that hot refrigerant exiting the condenser can be directly used for defrosting the evaporator, providing additional energy saving

Review of heat pump integrated energy systems for future zero-emission vehicles

Climate action is essential if global warming is to be limited to 1.5 °C and, and consequently, the transportation sector aims to phase out fossil fuel vehicles, to ensure that carbon net-zero can be achieved by 2050. It is expected that batteries or hydrogen fuel cells will most likely be the main driver of future zero-emission vehicles in order to achieve the zero-emission target for transport. One of the key research challenges in fully electric vehicles is the space heating/cooling in the cabin, which consumes a huge amount of electricity through conventional methods. Moreover, batteries and fuel cells both require properly designed thermal management systems to ensure the operational function of the systems. This work aims to provide a comprehensive summary of various advanced thermal management strategies/systems for future zero-emission electric vehicles. First, the latest battery thermal management systems are described, in terms of different operating conditions. Second, novel heat pump systems designed for Electric vehicles (EV) to achieve sufficient cabin space heating/cooling production and to address existing cabin issues are discussed. Finally, the heat pump-assisted integrated thermal management system, including cabin and battery thermal management, is reviewed regarding performance and intelligent control logic. This literature review not only addresses the research gaps but also identifies potential solutions to tackle the heating/cooling of cabin space for future zero-emission vehicles

An evaluation of wind turbine waste heat recovery using organic Rankine cycle

Wind turbine size has increased to megawatt capacity, and the related technologies and facilities have improved, including the cooling systems. Currently, all of the heat generated by wind turbine components is wasted to the environment. This study presents a conceptual design of a novel method for waste heat recovery of a wind turbine using an organic Rankine cycle (ORC). An organic Rankine cycle is implemented to the wind turbine as a part of the cooling system. The proposed system is thermodynamically modeled to evaluate the amount of recovered energy. Seven working fluids are chosen and investigated in the simulations to estimate the working fluid effect. The results revealed that the organic Rankine cycle can be a suitable choice for cooling wind turbines while simultaneously produce the power. Both exergy and energy analysis are conducted. A maximum power of 7.1 kW is provided by the proposed system using R134 when 20 K superheat is utilized. While R600a and SE36 stand on the next place. Furthermore, a minimum of 6.25 kW power is achievable. The generated work can be used as pump driving or electricity generation. In the next step, the thermal efficiency and total annual cost are optimized simultaneously using a multi-objective optimization for R134a as best working fluid in the thermodynamic viewpoint. It is found that the ideal efficiency of the system is obtained to be 14.7% with the total price of 7050 $/year. Finally, the effects of design parameters on the efficiency are obtained