Thermodynamic analysis of monomethylamine–water solutions in a single-stage solar absorption refrigeration cycle at low generator temperatures

A theoretical analysis of the coefficient of performance COP was undertaken to examine the efficiency characteristics of the monomethylamine–water solutions for a single-stage absorption refrigeration machine, using low generator temperatures (60–80°C), which allows the use of flat plate solar collectors. The thermodynamic analysis considers both, basic and refined cycles. The refined absorption cycle included a sensible heat recover exchanger (that is a solution heat exchanger). The thermal coefficients of performance COP_h for the basis cycle and COP_SHE for the refined cycle were calculated using the enthalpies at various combinations, at the operating temperatures and concentrations. The flow ratio F_R has been calculated as additional optimization parameter. Due to the relative low pressure and the high coefficients of performance, the monomethylamine–water solutions present interesting properties for their application in solar absorption cycles at moderate condenser and absorber temperatures (25–35°C), with temperatures in the evaporator from −10°C to 10°C which are highly usable for food product preservation and for air conditioning in rural areas.     

Modeling of an intermittent solar absorption refrigeration system operating with ammonia–lithium nitrate mixture

The theoretical performance of an intermittent absorption refrigeration system operating with ammonia–lithium nitrate mixture is presented. The analysis was done for representative days of each season of 2001. Meteorological data were taken from a local meteorological station installed in the Energy Research Centre of the National University of Mexico in Temixco, Morelos, Mexico. The system consists of a generator-absorber, a condenser, a valve and an evaporator. A compound parabolic concentrator (CPC) with a glass cover, operates as the generator-absorber of the cooling system. Since lithium nitrate does not evaporate during the generation, it is not necessary to use a rectifier. The theoretical efficiencies of the CPC varied from 0.78 to 0.33 depending on the time of the day and the season. Also, the results showed that with the proposed system it is possible to produce up to 11.8 kg of ice at generation temperatures around 120°C and condensation temperatures between 40°C and 44°C. These temperatures allow the system to be chilled with air or water. The overall efficiencies of the systems were between 0.15 and 0.4 depending on the generation and condenser temperatures. The efficiencies are satisfactory considering the simplicity of the system.     

Economic performance optimization of an absorption cooling system under uncertainty

Many of the strategies devised so far to address the optimization of energy systems are deterministic approaches that rely on estimated data. However, in real world applications there are many sources of uncertainty that introduce variability into the decision-making problem. Within this general context, we propose a novel approach to address the design of absorption cooling systems under uncertainty in the energy cost. As opposed to other approaches that optimize the expected performance of the system as a single objective, in our method the design task is formulated as a stochastic bi-criteria non-linear optimization problem that simultaneously accounts for the minimization of the expected total cost and the financial risk associated with the investment. The latter criterion is measured by the downside risk, which avoids the need to define binary variables thus improving the computational performance of the model. The capabilities of the proposed modeling framework and solution strategy are illustrated in a case study problem that addresses the design of a typical absorption cooling system. Numerical results demonstrate that the method presented allows to manage the risk level effectively by varying the area of the heat exchangers of the absorption cycle. Specifically, our strategy allows identifying the optimal values of the operating and design variables of the cycle that make it less sensitive to fluctuations in the energy price, thus improving its robustness in the face of uncertainty.     

Performance of solar driven methanol–water combined ejector–absorption cycle in the Athens area

The performance of a solar driven CH₄O-H₂O combined ejector– absorption unit, operating in conjunction with intermediate temperature solar collectors in Athens, is predicted along the five months (May–September) in case of the unit working as heat pump in an industrial area. The operation of the unit and the related thermodynamics are simulated by suitable computer codes and the required local climatological data are determined by statistical processings over a considerable number of years. It is found that the heat gain factor varies in the range from 2.1330 to 2.4481 for the above period of time. The maximum HGF of about 2.4481 is obtained in July at 14.25 hrs with corresponding specific heat gain power 915 W/m². The maximum Q_gain of about 1086 W/m² is obtained in June at 12.54 hrs with corresponding HGF 2.3572. Also the maximum value of HGF was estimated by correlation of three temperatures: generator temperature (85.0°C–97.2°C), condenser temperature (43.3°C–47.6°C) and evaporator temperature (12.6°C–25.4°C).     

Thermal modeling of a greenhouse integrated to an aquifer coupled cavity flow heat exchanger system

A thermal model is developed for heating and cooling of an agricultural greenhouse integrated with an aquifer coupled cavity flow heat exchanger system (ACCFHES). The ACCFHES works on the principal of utilizing deep aquifer water available at the ground surface through an irrigation tube well already installed in every agricultural field at constant year-round temperature of 24 °C. The analysis is based on the energy balance equations for different components of the greenhouse. Using the derived analytical expressions, a computer program is developed in C⁺⁺ for computing the hourly greenhouse plant and room air temperature for various design and climatic parameters. Experimental validation of the developed model is carried out using the measured plant and room air temperature data of the greenhouse (in which capsicum is grown) for the winter and summer conditions of the year 2004–2005 at Chandigarh (31°N and 78°E), Punjab, India. It is observed that the predicted and measured values are in close agreement. Greenhouse room air and plant temperature is maintained 6–7 K and 5–6 K below ambient, respectively for an extreme summer day and 7–8 K and 5–6 K above ambient, respectively for an extreme winter night. Finally, parametric studies are conducted to observe the effect of various operating parameters such as mass of the plant, area of the plant, mass flow rate of the circulating air and area of the ACCFHES on the greenhouse room air and plant temperature.     

Wake modelling of a VAWT in dynamic stall: impact on the prediction of flow and induction fields

Dynamic stall is a relevant phenomenon in the design and operation of a vertical axis wind turbine (VAWT) as it impacts loading, control and wake dynamics. Although streamtube models and single‐wake vortex models are commonly used for VAWT simulation, they either do not explicitly simulate the distribution of vorticity in the wake (streamtube models) or simplify it into a single‐wake release point (single‐wake vortex models). This can lead to inaccurate predictions of the vorticity distribution and wake dynamics, and therefore of the induction field, rotor loading and wake development, including wake mixing and re‐energizing. In this work, we use a double‐wake panel model developed for the simulation of dynamic stall in a VAWT to analyse (i) what is the flow field in dynamic stall, including the induction field, (ii) what is the error due to assuming a simplified wake, in both vorticity distribution and induction and (iii) how an incorrect simulation of the vorticity distribution can affect the prediction of the dynamics of the near and far wake. The results demonstrate that for mild separation (tip speed ratio λ≥3), single‐wake models can produce acceptable results. However, for lower tip speed ratios (λ < 3), the inaccuracy in the prediction of loads, induction field and vorticity distribution becomes significant because of an inadequate representation of the wake dynamics. These results imply that using lower order models can lead to inaccurate estimations of loads, performance and power control requirements at low tip speed ratios. Copyright © 2014 John Wiley & Sons, Ltd.