Performance of multi junction photovoltaic cells with high concentration ratio

AbstractConcentrating solar radiation on Photovoltaic (PV) has the potential to replace the expensive PV material with cheaper optical elements which also enhance the overall electrical output. The use of high solar concentration ratios with the triple junction III-V solar cells offers potential of high solar cell efficiency and power output. However, using high concentration ratios will increase the solar cell surface temperature which is inversely proportional to the PV electrical efficiency. This work investigates the effect of active cooling on the performance of triple junction PV cells with high solar concentration (up to 500X) in the harsh environment of Saudi Arabia where ambient temperatures can reach to 50o C in summer time, but with good clearance index of 0.6 and high yearly solar radiation of up to 2200 kWh/m2. Simulation results showed that as the concentration ratio increases, the effect of cooling on the PV efficiency increases

Effects of contact resistance and metal additives in finned-tube adsorbent beds on the performance of silica gel/water adsorption chiller

Recently interest in adsorption cooling systems has increased due to their capability to utilise low grade heat sources and environmentally friendly refrigerants. Currently, most of the commercially available adsorption cooling systems utilise granular packed adsorbent beds. Enhancing the heat transfer process inside the adsorbent bed will improve the overall efficiency of the adsorption system. Using recently developed empirical lumped analytical simulation model for a 450 kW two-bed silica gel/water adsorption chiller, this paper theoretically investigates the effects of various adsorbent bed heat transfer enhancement techniques on the adsorption system cooling capacity. Firstly, coating the first adsorbent layer to the metal part and packing the rest of adsorbent granules to eliminate the thermal contact resistance between heat exchanger metal and granules while keeping the same level of permeability. Secondly, adding metal particles to the adsorbent in order to enhance the granules thermal conductivity. The effective thermal conductivity of adsorbent/metal mixtures were determined and validated by comparing it with published experimental data. Also, the combined effect of using both techniques simultaneously was investigated. All these investigations were carried out at various adsorption bed fin spacing. Results of the combined techniques showed that the enhancement in the cooling capacity and system coefficient of performance (COP) increased with increasing the fin spacing ratio to reach maximum of 25% and 10% respectively at fin spacing ratio of 2

Thermal analysis of stirling engine to power automotive alternator using heat from exhaust gases

AbstractThis paper investigates the development of small scale beta type Stirling engine to recover the exhaust heat from the main engine and to drive the alternator (decouple it from the main engine), thus providing the required electrical power for onboard devices. The ideal adiabatic model was used to predict the thermodynamic performance of the engine. CFD investigation was also carried out to optimise the heater and the cooler geometry of the Stirling engine. The results showed that it is possible to generate a power output of 1.5-2kWe at an ideal thermal efficiency of 40% and engine overall weight of 11-14kg

Usefulness and limitations of transthoracic echocardiography in heart transplantation recipients

Transthoracic echocardiography is a primary non-invasive modality for investigation of heart transplant recipients. It is a versatile tool which provides comprehensive information about cardiac structure and function. Echocardiographic examinations can be easily performed at the bedside and serially repeated without any patient's discomfort. This review highlights the usefulness of Doppler echocardiography in the assessment of left ventricular and right ventricular systolic and diastolic function, of left ventricular mass, valvular heart disease, pulmonary arterial hypertension and pericardial effusion in heart transplant recipients. The main experiences performed by either standard Doppler echocardiography and new high-tech ultrasound technologies are summarised, pointing out advantages and limitations of the described techniques in diagnosing acute allograft rejection and cardiac graft vasculopathy. Despite the sustained efforts of echocardiographic technique in predicting the biopsy state, endocardial myocardial biopsies are still regarded as the gold standard for detection of acute allograft rejection. Conversely, stress echocardiography is able to identify accurately cardiac graft vasculopathy and has a recognised prognostic in this clinical setting. A normal stress-echo justifies postponement of invasive studies. Another use of transthoracic echocardiography is the monitorisation and the visualisation of the catheter during the performance of endomyocardial biopsy. Bedside stress echocardiography is even useful to select appropriately heart donors with brain death. The ultrasound monitoring is simple and effective for monitoring a safe performance of biopsy procedures

Simulation of Cascaded Thermoelectric Devices for Cryogenic Medical Treatment

Abstract: This study is focused on using a thermoelectric device (TED) as an alternative to the cryogenic liquid for cooling cryosurgical probe used for cancerous tissue ablation. Thermoelectric device, namely Peltier, is a solid state device which converts electric current to thermal gradient. In past years thermoelectric devices have been successfully utilized in refrigeration and air conditioning industry for generating low temperature. Using TEDs offers advantages over conventional method for cooling the cryogenic medical device, including compact in size, light in weight and its capability to control temperature precisely. This paper presents a comprehensive analysis of the single stage and multi-stage thermoelectric device using COMSOL Multiphysics. The simulation results show the thermal performance of the single stage and cascaded Peltier modules. Simulation results proved the possibility of generating very low temperature of -70 °C for cancerous tissue ablation, by implementing multi-stage Peltier

Effect of Acceptance angle on the design and performance of a heat pipe based compound parabolic collector at Kano, Nigeria

Abstract. The performance of any solar collector depends on the solar radiation available, its design and orientation. The aim of this work is to develop a compound parabolic solar collector (CPC) and optimize its performance for domestic use under climatic condition of Kano, Nigeria (Φ =12.05 o N). The acceptance angle plays an important role in determining the solar radiation acceptance of the CPC collector. Therefore this paper investigates the effect of the acceptance angle on the CPC design, solar radiation collection and absorption by the heat pipe receiver installed inside the CPC collector. From the radiation data and the analysis used, this study shows that as the acceptance angle increases, the collector height decreases and the concentration ratio increases