Long-Term Changes And Assessment Models Of Solar Energy Over China

Solar energy is almost the unique energy source of the earth system,its long-term change and spatial distribution have always been the most important research subject of climatology, and also been the most important precondition for the development of renewable energy industry.Based on ground observed data sets of solar radiation,sunshine duration and related meteorological and environmental factors of China,this doctoral dissertation discussed the causes of the long-term change of solar energy deeply and systematically.Further more,a series of models for computing solar radiation arriving at earth surface were established by using satellite data,in which,the effect of cardinal meteorological and environmental factors on radiative transfer process were considered.The main achievements of this project were concluded as follows:1.In recent 50 years,the time series of solar energy over China exhibits obvious decreasing trend and with significant characters of 'dimming in prior period and brightening in later period',but the change has apparent regional and seasonal features.On regional basis, the dimming is universal all over the country,especially in Eastern and Southern parts,but the brightening only occurred in some certain areas,namely North Xinjiang,North Area of Northeast China and most parts of Southern China.On seasonal basis,the decrease trend occurred almost in all seasons only with difference in change rate during dimming period, but the brightening only presented in some certain seasons,mainly in spring and summer, that is to say.different season has different contribution to the brightening in later period.2.Factors that lead to the decrease of solar energy are notably discrepant in different regions.Speaking generally,in east part wherein the human activity is more drastic,the decrease trend is mainly induced by the increase of aerosol particles,which primarily presented as the increase of haze and mist days.In west part,however,the decrease trend occurred mainly because the climate changes from dry to wet,which primarily presented as the increase of low cloud amount and water vapor.3.By establishing mathematical models,the impact of cloud and environmental factors upon the solar energy change is detected separately,wherefrom the different effects of the two types of factors were discussed.The result recommend that,both the cloud and environmental factors have important impacts on long-term solar energy change,but the dominant factor varies with the region in space and the stage in time domain.The enhancement of weakening effect of environmental factors on solar radiation(i.e., environmental deterioration) is the mutual reason that causes the decrease of solar energy in most areas,besides,the decreasing of effective cloud fraction is also a crucial cause for Xinjiang and southeast China(the contribution rate is about 13%-35%).From early 1990's, the decline trend ceased in most regions,even followed by a sight increase in some certain areas,but the causes are not the same.For the regions of North Xinjiang,and Yunnan,the brightening is mainly induced by the improvement of environmental quality(the contribution rate is about 100%).For Central South China and Sichuan Basin,the decrease of effective cloud fraction is the dominant reason(the contribution rate is about I00%).For North Area of Northeast China and Southeast China,the brightening is resulted from the combined impact of environmental factors and effective cloud fraction,and the former is more important(the contribution rate is about 60%and 80%for the two regions,respectively). However,due to the influence of the increase of effective cloud fraction,the solar energy still shows remarkable declining trend in North China Plain,central part of Northwest China and most part of Northeast China,among which,continuous deterioration of environment also plays a certain role in North China Plain(the contribution rate is about 15.9%).Furthermore, there exists a prominent and widespread signature that the descending rate of solar energy is faster in summer and winter than that in spring and autumn,and the study indicate that the feature hereof is ascribed mostly to the change of cloud.In one word,the long-term change of solar energy(i.e.,dimming,brightening) in China can be explained fairly well by means of cloud cover and environmental factors.4.By expressing environmental factors as various weather phenomenons which can lead to obstruction to vision,the dominate environmental factors were confirmed.The study comes to the results as follows:In West China,the environment status presents a feature of 'getting worse in prior period and getting better in later period',and the deterioration (improvement) is closely related to the increase(decrease) of smokescreen and sand-dust weather days.Whereas in East China,the environment quality changes with little improvement for the sustaining increase of mist days and(or) haze days,the results hereof also suggest that the effect of environmental factors should have been considered in computing solar energy arriving at the earth surface.5.Change of Solar energy usually accompanies corresponding change of radiation components,i.e.,the proportion of direct radiation decreases in dimming stage and increases in brightening stage.6.Various current mainstream solar radiation models are simply recommended contrastively,on the basis of which,an assessment of solar energy over China was conducted based on dataset of monthly radiation and sunshine duration from 1961 to 2007 by means of most commonly used sunshine based models.The results recommend that:solar energy resource over plateau and west arid regions is richer than which over plain and east humid regions,and the energy is more stable over resource-richer places on the basis of annual change.The national averaged time series of solar direct and global radiation retrieved from sunshine duration showed an obvious decreasing trend from 1960s to 1980s,and at the begging of 1990s,the decline ceased,followed by a slight increase to some extent.The resources have been continuously getting stronger over some certain areas of energy-rich part during the past several decades.Compared with some European countries where the solar energy industry is well developed,China presents innate advantages of solar energy, even for Sichuan Basin.The distribution of the proportion of direct radiation is similar with resource itself,in high-elevation areas or dry areas,direct radiation is the predominant composition,whereas diffuse radiation is main composition in low-elevation areas or wet areas,this property should be considered in solar energy industry development.The practice has proved that even though the empirical models can easily turn sunshine duration to radiation by simple regression,but it requires the existence of radiation and sunshine duration data in the first place to produce the prediction equation,and the uncertainty increases with a distance between the location of interest and the measuring stations, especially in mountainous regions or areas with atmospheric environment changing violently in space.In view of this,it is necessary to take topography,meteorological and environmental factors into account.In order to achieve this goal,we propose an optimized calculation scheme on the basis of satellite data.7.By virtue of Bird Clear Sky Model,with the input of environmental factors detected by satellites,the solar radiation under clear sky is computed,the output from which indicates that the distribution of solar radiation is different from zonal distribution to a great extent in general cases due to the effects of atmospheric factors,among which aerosol is the most important one.8.A series of mathematical models for computing solar radiation arriving at the earth surface are established by using satellite data,in which,the effect of cardinal meteorological and environmental factors are considered.The MAPE(RMSE) value of direct radiation model,diffuse radiation model and global radiation model is 22.1%(13.20kWh.m^-2),18.3% (12.85 kWh.m^-2) and 11.6%(16.39 kWh.m^-2),separately.The calculated solar energy can reflect the influence of astronomical factors,geographical factors,meteorological factors and environmental factors fairly well.Utilizing satellite data absolutely and by means of the new models established above,the annual and seasonal value of solar radiation is mapped for China,which provides an effective method for solving the discontinuity problems of solar radiation data.