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Y Tripanagnostopoulos - One of the best experts on this subject based on the ideXlab platform.

  • Study of the distribution of the Absorbed Solar Radiation on the performance of a CPC-type ICS water heater
    Renewable Energy, 2008
    Co-Authors: Manolis Souliotis, Y Tripanagnostopoulos
    Abstract:

    An Integrated Collector Storage (ICS) Solar water heater was designed, constructed and studied with an emphasis on its optical and thermal performance. The ICS system consists of one cylindrical horizontal tank properly mounted in a stationary symmetrical Compound Parabolic Concentrating (CPC) reflector trough. The main objective was the design and the construction of a low cost Solar system with improved thermal performance based on the exploitation of the non-uniform distribution of the Absorbed Solar Radiation on the cylindrical storage tank surface. A ray-tracing model was developed to gauge the distribution of the incoming Solar Radiation on the absorber surface and the results were compared with those from a theoretical optical model based on the average number of reflections. The variation of the optical efficiency as function of the incident angle of the incoming Solar Radiation along with its dependence on the month during annual operation of ICS system is presented. The ICS device was experimentally tested outdoors during a whole year in order to correlate the observed temperature rise and stratification of the stored water with the non-uniform distribution of the Absorbed Solar Radiation. The results show that the upper part of the tank surface collects the larger fraction of the total Absorbed Solar Radiation for all incident angles throughout the year. This is found to have a significant effect on the overall thermal performance of the ICS unit. In addition, the presented results can be considered important for the design and the operation of ICS systems consisting of cylindrical tank and CPC reflectors.

  • aspects and improvements of hybrid photovoltaic thermal Solar energy systems
    Solar Energy, 2007
    Co-Authors: Y Tripanagnostopoulos
    Abstract:

    Hybrid photovoltaic/thermal (PV/T or PVT) Solar systems consist of PV modules coupled to water or air heat extraction devices, which convert the Absorbed Solar Radiation into electricity and heat. At the University of Patras, an extended research on PV/T systems has been performed aiming at the study of several modifications for system performance improvement. In this paper a new type of PV/T collector with dual heat extraction operation, either with water or with air circulation is presented. This system is simple and suitable for building integration, providing hot water or air depending on the season and the thermal needs of the building. Experiments with dual type PV/T models of alternative arrangement of the water and the air heat exchanging elements were performed. The most effective design was further studied, applying to it low cost modifications for the air heat extraction improvement. These modifications include a thin metallic sheet placed in the middle of the air channel, the mounting of fins on the opposite wall to PV rear surface of the air channel and the placement of the sheet combined with small ribs on the opposite air channel wall. The modified dual PV/T collectors were combined with booster diffuse reflectors, achieving a significant increase in system thermal and electrical energy output. The improved PV/T systems have aesthetic and energy advantages and could be used instead of separate installation of plain PV modules and thermal collectors, mainly if the available building surface is limited and the thermal needs are associated with low temperature water or air heating.

  • industrial application of pv t Solar energy systems
    Applied Thermal Engineering, 2007
    Co-Authors: Soteris A Kalogirou, Y Tripanagnostopoulos
    Abstract:

    Abstract Hybrid photovoltaic/thermal (PV/T) systems consist of PV modules and heat extraction units mounted together. These systems can simultaneously provide electrical and thermal energy, thus achieving a higher energy conversion rate of the Absorbed Solar Radiation than plain photovoltaics. Industries show high demand of energy for both heat and electricity and the hybrid PV/T systems could be used in order to meet this requirement. In this paper the application aspects in the industry of PV/T systems with water heat extraction is presented. The systems are analyzed with TRNSYS program for three locations Nicosia, Athens and Madison that are located at different latitudes. The system comprises 300 m2 of hybrid PV/T collectors producing both electricity and thermal energy and a 10 m3 water storage tank. The work includes the study of an industrial process heat system operated at two load supply temperatures of 60 °C and 80 °C. The results show that the electrical production of the system, employing polycrystalline Solar cells, is more than the amorphous ones but the Solar thermal contribution is slightly lower. A non-hybrid PV system produces about 25% more electrical energy but the present system covers also, depending on the location, a large percentage of the thermal energy requirement of the industry considered. The economic viability of the systems is proven, as positive life cycle savings are obtained in the case of hybrid systems and the savings are increased for higher load temperature applications. Additionally, although amorphous silicon panels are much less efficient than the polycrystalline ones, better economic figures are obtained due to their lower initial cost, i.e., they have better cost/benefit ratio.

  • hybrid pv t Solar systems for domestic hot water and electricity production
    Energy Conversion and Management, 2006
    Co-Authors: Soteris A Kalogirou, Y Tripanagnostopoulos
    Abstract:

    Hybrid photovoltaic/thermal (PV/T) Solar systems can simultaneously provide electricity and heat, achieving a higher conversion rate of the Absorbed Solar Radiation than standard PV modules. When properly designed, PV/T systems can extract heat from PV modules, heating water or air to reduce the operating temperature of the PV modules and keep the electrical efficiency at a sufficient level. In this paper, we present TRNSYS simulation results for hybrid PV/T Solar systems for domestic hot water applications both passive (thermosyphonic) and active. Prototype models made from polycrystalline silicon (pc-Si) and amorphous silicon (a-Si) PV module types combined with water heat extraction units were tested with respect to their electrical and thermal efficiencies, and their performance characteristics were evaluated. The TRNSYS simulation results are based on these PV/T systems and were performed for three locations at different latitudes, Nicosia (35°), Athens (38°) and Madison (43°). In this study, we considered a domestic thermosyphonic system and a larger active system suitable for a block of flats or for small office buildings. The results show that a considerable amount of thermal and electrical energy is produced by the PV/T systems, and the economic viability of the systems is improved. Thus, the PVs have better chances of success especially when both electricity and hot water is required as in domestic applications.

Martin Wild - One of the best experts on this subject based on the ideXlab platform.

  • evaluation of the Radiation budget with a regional climate model over europe and inspection of dimming and brightening
    Journal of Geophysical Research, 2015
    Co-Authors: F Solmon, Marc Chiacchio, Filippo Giorgi, Paul W Stackhouse, Martin Wild
    Abstract:

    Shortwave (SW) and longwave (LW) components of the Radiation budget at the surface and top of atmosphere (TOA) are evaluated in the regional climate model RegCM version 4 driven by European Centre for Medium-Range Weather Forecasts Reanalysis over Europe. The simulated radiative components were evaluated with those from satellite-based products and reanalysis. At the surface the model overestimated the Absorbed Solar Radiation but was compensated by a greater loss of thermal energy while both SW and LW TOA net fluxes were underestimated representing too little Solar energy Absorbed and too little outgoing thermal energy. Averaged biases in radiative parameters were generally within 25 W m−2, were dependent on differences by as much as 0.2 in cloud fraction, surface, and planetary albedo and less dependent on surface temperature associated with the surface longwave parameters, and are in line with other studies. Clear-sky fluxes showed better results when cloud cover differences had no influence. We also found a clear distinction between land versus water with smaller biases over land at the surface and over water at the TOA due to differences in cloud fraction and albedo. Finally, we inspected dimming and brightening for the period 1979–2010 with an indication for dimming early in the time series (i.e., 1979–1987) and brightening after, which agrees with surface-based observations. After 2000, however, a decrease in the brightening by more than 1 order of magnitude was evident which is in contrast to the continued brightening found in surface records and satellite-derived estimates.

  • the global energy balance from a surface perspective
    Climate Dynamics, 2013
    Co-Authors: Martin Wild, Doris Folini, Norman G Loeb, Christoph Schar, Ellsworth G Dutton, Gert Koniglanglo
    Abstract:

    In the framework of the global energy balance, the radiative energy exchanges between Sun, Earth and space are now accurately quantified from new satellite missions. Much less is known about the magnitude of the energy flows within the climate system and at the Earth surface, which cannot be directly measured by satellites. In addition to satellite observations, here we make extensive use of the growing number of surface observations to constrain the global energy balance not only from space, but also from the surface. We combine these observations with the latest modeling efforts performed for the 5th IPCC assessment report to infer best estimates for the global mean surface radiative components. Our analyses favor global mean downward surface Solar and thermal Radiation values near 185 and 342 Wm−2, respectively, which are most compatible with surface observations. Combined with an estimated surface Absorbed Solar Radiation and thermal emission of 161 and 397 Wm−2, respectively, this leaves 106 Wm−2 of surface net Radiation available globally for distribution amongst the non-radiative surface energy balance components. The climate models overestimate the downward Solar and underestimate the downward thermal Radiation, thereby simulating nevertheless an adequate global mean surface net Radiation by error compensation. This also suggests that, globally, the simulated surface sensible and latent heat fluxes, around 20 and 85 Wm−2 on average, state realistic values. The findings of this study are compiled into a new global energy balance diagram, which may be able to reconcile currently disputed inconsistencies between energy and water cycle estimates.

John T Fasullo - One of the best experts on this subject based on the ideXlab platform.

  • relationships among top of atmosphere Radiation and atmospheric state variables in observations and cesm
    Journal of Geophysical Research, 2015
    Co-Authors: Kevin E Trenberth, Yongxin Zhang, John T Fasullo
    Abstract:

    A detailed examination is made in both observations and the Community Earth System Model (CESM) of relationships among top-of-atmosphere Radiation, water vapor, temperatures, and precipitation for 2000–2014 to assess the origins of radiative perturbations and climate feedbacks empirically. The 30-member large ensemble coupled runs are analyzed along with one run with specified sea surface temperatures for 1994 to 2005 (to avoid volcanic eruptions). The vertical structure of the CESM temperature profile tends to be top heavy in the model, with too much deep convection and not enough lower stratospheric cooling as part of the response to tropospheric heating. There is too much Absorbed Solar Radiation (ASR) over the Southern Oceans and not enough in the tropics, and El Nino–Southern Oscillation (ENSO) is too large in amplitude in this version of the model. However, the covariability of monthly mean anomalies produces remarkably good replication of most of the observed relationships. There is a lot more high-frequency variability in radiative fluxes than in temperature, highlighting the role of clouds and transient weather systems in the Radiation statistics. Over the Warm Pool in the tropical western Pacific and Indian Oceans, where nonlocal effects from the Walker circulation driven by the ENSO events are important, several related biases emerge: in response to high SST anomalies there is more precipitation, water vapor, and cloud and less ASR and outgoing longwave Radiation in the model than observed. Different model global mean trends are evident, however, possibly hinting at too much positive cloud feedback in the model.

  • global warming due to increasing Absorbed Solar Radiation
    Geophysical Research Letters, 2009
    Co-Authors: Kevin E Trenberth, John T Fasullo
    Abstract:

    [1] Global climate models used in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) are examined for the top-of-atmosphere Radiation changes as carbon dioxide and other greenhouse gases build up from 1950 to 2100. There is an increase in net Radiation Absorbed, but not in ways commonly assumed. While there is a large increase in the greenhouse effect from increasing greenhouse gases and water vapor (as a feedback), this is offset to a large degree by a decreasing greenhouse effect from reducing cloud cover and increasing radiative emissions from higher temperatures. Instead the main warming from an energy budget standpoint comes from increases in Absorbed Solar Radiation that stem directly from the decreasing cloud amounts. These findings underscore the need to ascertain the credibility of the model changes, especially insofar as changes in clouds are concerned.

Meeko Oishi - One of the best experts on this subject based on the ideXlab platform.

  • estimation of Solar heat gain using illumination sensor measurements
    Solar Energy, 2018
    Co-Authors: M Toufiq H Imam, Joseph D Gleason, Sandipan Mishra, Meeko Oishi
    Abstract:

    Abstract Solar Radiation is an important but unpredictable source of thermal energy in an indoor space. The incident and Absorbed Solar Radiation, and consequently Solar heat gain, is difficult to model accurately even when detailed information about the building design, orientation, and material properties is available. This article presents a novel approach to estimate radiant Solar heat gain using measurements from ceiling mounted illumination sensors. This proposed approach captures the effect of directional Solar Radiation on Solar heat gain of an indoor space that cannot be captured (or estimated) by local weather station measurements. Measured illumination data from day-long experiments for several (cloudy and sunny) days is first compared with Solar heat gain to demonstrate strong correlation between them irrespective of sky condition (with average correlation coefficients of 0.84 and 0.77 for cloudy and sunny days respectively). Next, a linear model to estimate radiant heat gain from illumination sensor readings is proposed and validated against calculated Solar heat gain values using the well-known Perez model. For further validation, similar experiments are performed on another testbed with different geographical location and orientation. Finally, we demonstrate that illumination sensors can also provide spatial distribution of Solar heat gain inside an indoor space.

Axel Kleidon - One of the best experts on this subject based on the ideXlab platform.

  • Physical limits of wind energy within the atmosphere and its use as renewable energy: From the theoretical basis to practical implications
    'Schweizerbart', 2021
    Co-Authors: Axel Kleidon
    Abstract:

    How much wind energy does the atmosphere generate, and how much of it can at best be used as renewable energy? This review aims to give physically-based answers to both questions, providing first-order estimates and sensitivities that are consistent with those obtained from numerical simulation models. The first part describes how thermodynamics determines how much wind energy the atmosphere is physically capable of generating at large scales from the Solar radiative forcing. The work done to generate and maintain large-scale atmospheric motion can be seen as the consequence of an atmospheric heat engine, which is driven by the difference in Solar radiative heating between the tropics and the poles. The resulting motion transports heat, which depletes this differential Solar heating and the associated, large-scale temperature difference, which drives this energy conversion in the first place. This interaction between the thermodynamic driver (temperature difference) and the resulting dynamics (heat transport) is critical for determining the maximum power that can be generated. It leads to a maximum in the global mean generation rate of kinetic energy of about 1.7 W m−2 and matches rates inferred from observations of about 2.1–2.5 W m−2 very well. This represents less than 1 % of the total Absorbed Solar Radiation that is converted into kinetic energy. Although it would seem that the atmosphere is extremely inefficient in generating motion, thermodynamics shows that the atmosphere works as hard as it can to generate the energy contained in the winds. The second part focuses on the limits of converting the kinetic energy of the atmosphere into renewable energy. Considering the momentum balance of the lower atmosphere shows that at large-scales, only a fraction of about 26 % of the kinetic energy can at most be converted to renewable energy, consistent with insights from climate model simulations. This yields a typical resource potential in the order of 0.5 W m−2 per surface area in the global mean. The apparent discrepancy with much higher yields of single wind turbines and small wind farms can be explained by the spatial scale of about 100 km at which kinetic energy near the surface is being dissipated and replenished. I close with a discussion of how these insights are compatible to established meteorological concepts, inform practical applications for wind resource estimations, and, more generally, how such physical concepts, particularly limits regarding energy conversion, can set the basis for doing climate science in a simple, analytical, and transparent way

  • Imprints of evaporative conditions and vegetation type in diurnal temperature variations
    Hydrology and Earth System Sciences, 2020
    Co-Authors: Annu Panwar, Maik Renner, Axel Kleidon
    Abstract:

    Abstract. Diurnal temperature variations are strongly shaped by the absorption of Solar Radiation, but evaporation, or the latent heat flux, also plays an important role. Generally, evaporation cools. Its relation to diurnal temperature variations, however, is unclear. This study investigates the diurnal response of surface and air temperatures to evaporative conditions for different vegetation types. We use the warming rate, defined as the increase in temperature in response to Absorbed Solar Radiation in the morning, and evaluate how it changes with evaporative fraction, which is an indicator of the evaporative conditions. Results for 51 FLUXNET sites show that the warming rate of air temperature carries very weak imprints of evaporative fraction across all vegetation types. However, the warming rate of surface temperature is highly sensitive to evaporative fraction with a value of ∼ 23 × 10 - 3  K (W m - 2 ) - 1 , indicating stronger evaporative cooling for moister conditions. Contrarily, the warming rates of surface and air temperatures are similar at forest sites and carry literally no imprints of evaporative fraction. We explain these contrasting patterns with an analytical surface energy balance model. The derived expressions reproduce the observed warming rates and their sensitivity to evaporative fraction in all vegetation types. Multiplying the warming rate with daily maximum Solar Radiation gives an approximation for the diurnal surface temperature range (DT s R). We use our model to compare the individual contributions of Solar Radiation, evaporative conditions, and vegetation (by its aerodynamic conductance) in shaping DT s R and show that the high aerodynamic conductance of forests reduces DT s R substantially more ( −56  %) than evaporative cooling ( −22  %). We further show that the strong diurnal variation in aerodynamic conductance ( ∼2.5  times of the mean across vegetation types) reduces DT s R by ∼35  % in short vegetation and savanna but only by ∼22  % in forests. We conclude that diurnal temperature variations may be useful for predicting evaporation for short vegetation. In forests, however, the diurnal variations in temperatures are mainly governed by their high aerodynamic conductance, resulting in negligible imprints of evaporative conditions.

  • Imprints of evaporation and vegetation type in diurnal temperature variations
    2020
    Co-Authors: Annu Panwar, Maik Renner, Axel Kleidon
    Abstract:

    Abstract. Diurnal temperature variations are strongly shaped by the absorption of Solar Radiation, but evaporation, or the latent heat flux, also plays an important role. Generally, evaporation cools. Its relation to diurnal temperature variations, however, is unclear. This study investigates the diurnal response of surface and air temperatures to evaporation for different vegetation types. We used the warming rate of temperature to Absorbed Solar Radiation in the morning under clear-sky conditions and evaluated how the warming rates change for different evaporative fractions. Results for 51 FLUXNET sites show that the diurnal variation of air temperature carries very weak imprints of evaporation across all vegetation types. However, surface temperature warming rates of short vegetation decrease significantly by ~ 23 × 10−3 K/W m−2 from dry to wet conditions. Contrarily, warming rates of surface and air temperatures are similar at forest sites and carry literally no imprints of evaporation. We explain these contrasting patterns with a surface energy balance model. The model reveals a strong sensitivity of the warming rates to evaporative fraction and aerodynamic conductance. However, for forests the sensitivity to evaporative fraction is strongly reduced by 74 % due to their large aerodynamic conductance. The remaining imprint is reduced further by ~ 50 % through their enhanced aerodynamic conductance under dry conditions. Our model then compares the individual contributions of Solar Radiation, evaporation and vegetation types in shaping the diurnal temperature range. These findings have implications for the interpretation of land-atmosphere interactions and the influences of water limitation and vegetation on diurnal temperatures, which is of key importance for ecological functioning. We conclude that diurnal temperature variations may be useful to predict evaporation for short vegetation. In forests, however, the diurnal variations in temperatures are mainly governed by their aerodynamic properties resulting in no imprint of evaporation in diurnal temperature variations.

  • Diurnal land surface energy balance partitioning estimated from the thermodynamic limit of a cold heat engine
    Earth System Dynamics, 2018
    Co-Authors: Axel Kleidon, Maik Renner
    Abstract:

    Abstract. Turbulent fluxes strongly shape the conditions at the land surface, yet they are typically formulated in terms of semiempirical parameterizations that make it difficult to derive theoretical estimates of how global change impacts land surface functioning. Here, we describe these turbulent fluxes as the result of a thermodynamic process that generates work to sustain convective motion and thus maintains the turbulent exchange between the land surface and the atmosphere. We first derive a limit from the second law of thermodynamics that is equivalent to the Carnot limit but which explicitly accounts for diurnal heat storage changes in the lower atmosphere. We call this the limit of a “cold” heat engine and use it together with the surface energy balance to infer the maximum power that can be derived from the turbulent fluxes for a given Solar radiative forcing. The surface energy balance partitioning estimated from this thermodynamic limit requires no empirical parameters and compares very well with the observed partitioning of Absorbed Solar Radiation into radiative and turbulent heat fluxes across a range of climates, with correlation coefficients r2≥95  % and slopes near 1. These results suggest that turbulent heat fluxes on land operate near their thermodynamic limit on how much convection can be generated from the local radiative forcing. It implies that this type of approach can be used to derive general estimates of global change that are solely based on physical principles.