Yieng Wei Tham

An examination of UKCIP02 and UKCP09 solar radiation data sets for the UK climate related to their use in building design

The climate is changing, both globally and in the UK. To adapt effectively, engineers and planners need as much information as possible on how the climate will evolve. The UK Climate Impacts Programme (UKCIP) provided this in 2002 with UKCIP02 and the latest data UKCP09, which provides data to a resolution of 5 km square grids over the UK. Data sets from these were used in this study along with the historical measured data for three locations — Bracknell (London), Manchester and Edinburgh — to analyse critically the likely changes that may occur in the key climate variables, that is temperature, sunshine duration and solar irradiation. These parameters have an important bearing on the design and function of buildings and building services. Sunshine duration is the main variable that is used to obtain solar radiation in the UKCP09 5 km grid data. For the grids containing Bracknell, Manchester and Edinburgh, most of the UKCP09 data sets for the years 2050 and 2080 showed abnormally elongated sunshine duration, that is from sunrise to sunset, for clear days. In contrast, the latest historic measured data sets indicate only a third of the above sunshine duration. Note that the latter data are used in cooling load design calculations and for the generation of sol-air temperatures.1 Of particular note was the anomalous occurrence in UKCP09 of late evening sunshine duration. For Bracknell and Edinburgh, the sunshine duration at hour ending 20 and beyond showed substantial amount of predicted sunshine. As a result of this work, corrective action has been proposed for UKCP09 data. Furthermore, a very significant increase was also noted in solar irradiation for UKCP09. For the historic measured data for Bracknell, the clear day noon irradiation is 818 Wh/m 2. For the UKCP09 grid containing Bracknell the 2080 High Emission scenario data gives an average value of 1002 Wh/m2, an increase of 23%. The same trend occurs for Edinburgh, (a present value of 789 Wh/m 2 and the predicted value of 948 Wh/m2, an increase of 20%). Note that compounded with presently found increase of 4—5°C increase for the above locations, the substantial increase in irradiation will have a much more pronounced increase in the cooling load of buildings. An evaluation of the change in the character of solar radiation was also undertaken. This was done by noting the change in the diffuse fraction of global irradiation. For Bracknell and Edinburgh historic data and UKCP09 data 2080 High Emission data set show a drastic decrease, respectively from 0.37 to 0.13 and from 0.33 to 0.14. Diffuse fraction may be used as an indicator of the prevailing sky clarity. If the predictions come true a drastic decrease in the diffuse fraction of this magnitude signifies a radical shift in the character of solar climate for the future. The current solar climate of Bracknell is known for its above average turbidity, the latter stemming from the following factors: inland location, high-density housing, proximity to Heathrow airport and M25 London orbital motorway. Whether such an extreme shift in the sky clarity will occur within a matter of 60—70 years is open to discussion. Practical applications: To adapt effectively against the challenge posed by climate change engineers need to know the extent to which the basic climate variables such as temperature and solar radiation will change. This work has used basic data from the UKCP09 project to analyse the extent of the above change with respect to the basic and other derived data. It was shown that for Scottish and English locations a temperature rise of up to 4—5°C may occur between the present age and the year 2080 for High Emission scenario. It was also shown that the corresponding irradiation strength may increase around 22%. Furthermore, it was also found that if these predictions come true then a drastic decrease in the diffuse fraction of irradiation will produce a radical shift in the character of solar climate. The resulting higher proportion of beam irradiation will have to be handled with care in design of overhangs and other shading contraptions to prevent an excessive increase in cooling load of buildings.

Estimation of hourly averaged solar irradiation: evaluation of models:

Hourly solar radiation data are required in many building services applications. These are also reported in the Chartered Institution of Building Services Engineers Guides A & J. Data from 16 locations in the UK were used to evaluate the so called Liu and Jordan model1 for monthly averaged hourly solar irradiation. Individual data sets spanned periods from 12 to 26 years between 1968 and 1994, and overall, provided data from practically the full range of latitude of the UK (50.22°N—58.13°N). For hourly estimation, the model only slightly underestimated both global and diffuse radiation before noon and overestimated, again only slightly, after noon. In addition, a discrepancy was observed between the measured data and the model’s predictions at low sunset angles. Following earlier research work, an attempt was made to further improve the Liu and Jordan model. However, it was found that at least for the UK data set, any such attempts were futile. This behaviour was attributed to the highly random nature of UK’s solar climate. Practical applications: Most meteorological stations report solar radiation data on a daily averaged basis. However, most building energy simulation software requires hourly radiation. Research studies have confirmed that the well-known Liu and Jordan model, which enables the above conversion, performs well for locations in the US. This paper evaluates the above model for locations in the UK and compares it with previously studied Indian locations. According to the evaluation, the average accuracy of the model to estimate hourly radiation from its daily counterpart is 85%.

Inter-relationship between mean-daily irradiation and temperature, and decomposition models for hourly irradiation and temperature

Terrestrial temperature records have existed for centuries. These records are available for very many locations. Temperature is indeed the most widely measured meteorological parameter. In contrast, solar radiation being a parameter of secondary importance and also in view of the excessive measurement cost and the associated due care, it is recorded very infrequently. This article presents evaluation of a new type of model for mean-daily and hourly solar radiation based on temperature. The proposed model is of a very simple constitution and does not require any secondary meteorological parameters as required by other group of models that are available in literature. Furthermore, hourly temperature models are also presented that only require mean-daily temperature data. A comparison was undertaken regarding the performance of the presently proposed and previous models. It was found that the present models are able to provide reliable irradiation and hourly temperature estimates with a good accuracy. Copyright The Author 2010. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com, Oxford University Press.