Results are presented from a monitoring study of the performance of a sample of UK domestic PV systems.Five minutely average climatic and performance data was recorded for 27 PV systems at two sites for up to two years of operation. On average, the annual energy losses due to faults were 3.6% (Site A, first year of operation), 6.6% (Site A, second year of operation) and 18.9% (Site B, first year of operation). Simple empirical models are constructed to describe the performance of the PV systems under 'normal operation' (when no faults occur). New analysis techniques are developed which estimate the energy losses of four different fault categories: sustained zero efficiency faults; brief zero efficiency faults; shading; and non-zero efficiency non-shading faults. The results demonstrate that faults have the potential to cause significant energy losses in domestic PV systems. The benefits of applying the data analysis techniques described in this work to PV system clusters and other urban micro-generation technologies are discussed. KeywordsPhotovoltaic; PV; monitoring; high resolution; analysis; model; fault detection; shading IntroductionOne of the main aims of research in building integrated PV systems is to improve the performance of installed PV systems. A drive for greater efficiency is necessary to increase the amount of electricity generated per Wp of installed PV capacity and so reduce the unit cost of the electricity. An essential part of such work is in understanding the energy losses which occur in installed PV systems and developing methods to minimise these energy losses. Some losses, such as the energy lost in the conversion of sunlight to electricity by the PV cell, are unavoidable and can only be significantly reduced by improvements in the technology of PV system components. Other energy losses, such as shading, are related to the installation of the PV systems and can be minimised through careful planning, design and operation. 06/05/2010-2 -Energy losses in installed PV systems can be detected by monitoring the operational performance and analysing the recorded data. Monitoring of PV systems has been used to develop guides and benchmarks for PV system performance (IEA, 2000). Demonstration PV systems (usually among the first PV systems to be installed in a country) and systems with unique characteristics (such as new cell technologies or novel construction techniques) often have their performance monitored in detail. Such monitoring studies record the performance of the PV systems, analyse the energy losses and provide recommendations to improve the design of future systems (e.g. Roaf and Fuentes, 1999). This becomes an iterative process when the next generation of PV systems incorporating the design improvements are themselves monitored and used to develop further recommendations to raise efficiency (Erge et al., 2001).PV systems are often monitored according to standards, for example the British Standard BS EN 61724 (BSI, 1998) and the recorded data analysed on an annual basis (e.g. ...
The authors used cine magnetic resonance (MR) velocity mapping to study flow in the superior vena cava (SVC) and inferior vena cava (IVC) of 13 healthy control subjects and 13 patients with right-sided cardiac disease. In the control subjects, peaks of flow in systole and diastole were observed, and mean SVC flow was 35% of the cardiac output. Respiratory gating was used in six control subjects to acquire images at end inspiration and end expiration, and although the systolic peak was reduced at end expiration, total flow was unchanged. A reduced systolic peak and retrograde flow in the IVC were observed in patients with tricuspid regurgitation. A reduced diastolic peak was seen in patients with pulmonary hypertension, pericardial constriction, and right ventricular dysplasia, reflecting reduced diastolic compliance of the right ventricle. In the patient with obstruction of the SVC, absence of flow was confirmed, and retrograde flow was seen in the azygos vein. The authors believe that cine MR velocity mapping is a reliable method of studying vena caval flow noninvasively and that it has important potential applications for the investigation of disorders of the right side of the heart.
We established normal ranges for volume flow in the descending aorta and demonstrated abnormalities in patients with aortic coarctation. These abnormalities are likely to be related to resistance to flow imposed by the coarctation and could represent an additional index for monitoring patients before and after intervention.
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