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Solar radiation in Africa: An overview
Renewable Ener~l) Vo[. 3, No. 4/5, pp, 399M-01, 1993 Printed in Great Britain.
0960 1481'93 $6.00+.00 Pergamon Press Ltd
SOLAR RADIATION IN AFRICA: AN OVERVIEW JOHN E. HAY Environmental Science, University of Auckland, Auckland, New Zealand Abstract--This paper presents summary comments and an overview arising from the presentation of the preceding six papers and the ensuing discussions at the workshop. A number of recommendations are made as to where effort should be focused in the coming years.
INTRODUCTION Previous papers have presented material related to determining the solar irradiances for horizontal and inclined surfaces using a variety of approaches and input data. The procedures for quality controlling observed solar radiation data were also reviewed. The current paper presents summary comments and an overview arising from the presentation of the preceding six papers and the ensuing discussions at the workshop. A number of recommendations are made as to where effort should be focused in the coming years. QUALITY CONTROL Emphasis should be placed on ensuring appropriate exposure for the radiation sensors to avoid shading, obstructions to the natural horizon and reflection from nearby objects. Sensors and the associated data logging equipment (e.g. chart recorder, digital data Jogger, integrator or computer) should be inspected at frequent intervals (at least daily) to ensure that performance is not impaired. Sensors should be inspected and cleaned so that dust and other materials do not degrade the quality of the data. Calibration of sensors and the monitoring system is an important part of any solar radiation monitoring programme. Sensor calibrations provided by manufacturers should be verified by independent means. Calibrations should be confirmed or changes noted at approximately annual intervals. The performance of the recording equipment should also be checked to determine whether it is introducing errors into the monitoring programme. Quality assurance of the observed data should be undertaken soon after its acquisition. This procedure will be assisted if a logbook is kept and the results of the regular inspections and equipment servicing are
all recorded in the book. Quality control procedures are time consuming and often frustrating but they must be undertaken if the validity of the data is to be assured.
CALCULATING GLOBAL SOLAR I R R A D I A N C E S FOR A H O R I Z O N T A L SURFACE Simple models based on bright sunshine data provide adequate estimates for monthly time intervals, or longer. However, regionally representative values of the empirical coefficients are currently lacking and there is a real need to provide such information for the use of various projects that require estimates of the global solar radiation. Similarly, there is an urgent need for independent regionally-based validations of the various methods (e.g. sunshine, cloud and satellite based models) which can be used to estimate solar radiation inputs. The use of satellite based methods for estimating solar irradiances in data sparse regions is recommended. Such methods can be implemented without great difficulty since some require access to hard copy satellite imagery only. These have reasonable availability, often from facsimile transmissions to local weather (meteorological) offices. But again, there is the need to determine regionally appropriate coefficients if simpler and more common empirical methods are employed. In addition, regional validation of the estimates provided by such models will be required before the data can be used with any confidence.
CALCULATING SPECTRAL IRRADIANCES Numerical methods are available for clear skies and results have been obtained for Nigeria. There is urgent need for similar work to be undertaken in other parts
399
400
J. E. HAY
of Africa if the matching of solar photovoltaic systems and meteorological conditions (specifically the spectral distribution of the solar radiation) is to produce optimized photovoltaic systems. Similarly, the work must be extended to cloudy conditions (especially partly cloudy skies) in order to be able to design more efficient and effective photovoltaic systems. CALCULATING SOLAR 1RRADIANCES FOR INCLINED SURFACES Simple methods which produce reliable estimates of the solar radiation incident on an inclined surface are now available. They require little data other than the direct and diffuse solar irradiances for the equivalent horizontal surface and an estimate of the surface albedo. While some efforts have already been made to provide regional validation of such models and their estimates of inclined surface irradiances, further work is required. Once this has been achieved and a reliable mode] chosen, there is an urgent need
GLobaL solar irradiance for horizontal surface
- - Direct
Latitude
declination t me
Isotropic
J
No parameters--
--Diffuse Anisotropic ,, , Jkatitude
(e.g. Hay]
J
I i 'e'n ti°n[
- - RefLected - - Tsotropic
GLobaL irradiation for ncL ned sfc
Fig. 2. Procedure for calculating solar irradiance for an inclined surface. I SoLar constant I
GraphicaL, sunshine, theoretical
Theoretical
[
for calculations of monthly mean solar irradiances for a range (in both slope angle and aspect) of inclined surfaces at various locations in Africa. Such information is critical to a rational evaluation, assessment and development of the continent's solar resources.
II
[CLear sky irradiation I
Ii Sunshine, cloud
II Theoretical
theoretical sateLLite It
JCLoudysky irradiation [ q
Ii DaiLy
e.g. dain
tl HourLy
e.g. Erbs (Liu and Jordan)
II
JDirect, diffuse 1
I
Isotropic
e.g. Hay, Perez
II
JInclined surface
[
Fig. 1. Schematic of methods to compute given solar irradiances.
SUGGESTED PRIORITIES FOR FUTURE STUDIES The following is a preliminary listing of priority activities which should be undertaken in Africa to ensure that a valid data archive is available to those who wish to make assessments of the solar resources of the continent. (i) Calibration of existing and new solar radiation sensors and associated monitoring equipment. (ii) Quality control of both existing (i.e. current archives) and new (i.e. data now being acquired) solar radiation and bright sunshine data. (iii) An increase in the number of stations measuring bright sunshine and the use of the resulting data and appropriate models to provide estimates of the solar energy available at these locations. (iv) The undertaking of comprehensive (i.e, global, direct, diffuse, inclined surface, spectral) solar radiation monitoring programmes at a few locations in the continent. These will provide high quality and
Solar radiation in Africa : an overview extended data sets with which to calibrate and/or verify the various models to be used at other locations with more restricted (e.g. bright sunshine alone) observational programmes. (v) The undertaking of the regional validation of various models (e.g. sunshine and satellite data based models for estimating horizontal surface irradiances and empirical models for estimating inclined surface irradiances). (vi) The undertaking of both spectral measurements and model calculations and, after verifying the resulting estimtes at selected locations using observed data, produce a data base of regionally representative spectral distributions of the solar irradiance for design applications. S U M M A R Y OF M E T H O D S FOR C O M P U T I N G SOLAR IRRADIANCES
Figure 1 summarizes the various procedures which can be used to estimate the solar irradiance for a
401
horizontal or inclined surface. The methods and input data requirements for the inclined surface calculations are given in more detail in Fig. 2. The reader is referred to the relevant section of the preceding papers for further details. Copies of computer programmes which undertake these calculations and provide estimates of the solar radiation for horizontal and inclined surfaces are available from the author or from Professor P. Jain of the University of Zambia.
CONCLUSIONS While considerable progress has been made in developing methods and acquiring the data with which to assess the solar energy resources of the African continent further effort is required. These include use of currently available data and methods and the development of new procedures and the acquisition of additional data with which to provide further refinements to the assessment process.
0960 1481'93 $6.00+.00 Pergamon Press Ltd
SOLAR RADIATION IN AFRICA: AN OVERVIEW JOHN E. HAY Environmental Science, University of Auckland, Auckland, New Zealand Abstract--This paper presents summary comments and an overview arising from the presentation of the preceding six papers and the ensuing discussions at the workshop. A number of recommendations are made as to where effort should be focused in the coming years.
INTRODUCTION Previous papers have presented material related to determining the solar irradiances for horizontal and inclined surfaces using a variety of approaches and input data. The procedures for quality controlling observed solar radiation data were also reviewed. The current paper presents summary comments and an overview arising from the presentation of the preceding six papers and the ensuing discussions at the workshop. A number of recommendations are made as to where effort should be focused in the coming years. QUALITY CONTROL Emphasis should be placed on ensuring appropriate exposure for the radiation sensors to avoid shading, obstructions to the natural horizon and reflection from nearby objects. Sensors and the associated data logging equipment (e.g. chart recorder, digital data Jogger, integrator or computer) should be inspected at frequent intervals (at least daily) to ensure that performance is not impaired. Sensors should be inspected and cleaned so that dust and other materials do not degrade the quality of the data. Calibration of sensors and the monitoring system is an important part of any solar radiation monitoring programme. Sensor calibrations provided by manufacturers should be verified by independent means. Calibrations should be confirmed or changes noted at approximately annual intervals. The performance of the recording equipment should also be checked to determine whether it is introducing errors into the monitoring programme. Quality assurance of the observed data should be undertaken soon after its acquisition. This procedure will be assisted if a logbook is kept and the results of the regular inspections and equipment servicing are
all recorded in the book. Quality control procedures are time consuming and often frustrating but they must be undertaken if the validity of the data is to be assured.
CALCULATING GLOBAL SOLAR I R R A D I A N C E S FOR A H O R I Z O N T A L SURFACE Simple models based on bright sunshine data provide adequate estimates for monthly time intervals, or longer. However, regionally representative values of the empirical coefficients are currently lacking and there is a real need to provide such information for the use of various projects that require estimates of the global solar radiation. Similarly, there is an urgent need for independent regionally-based validations of the various methods (e.g. sunshine, cloud and satellite based models) which can be used to estimate solar radiation inputs. The use of satellite based methods for estimating solar irradiances in data sparse regions is recommended. Such methods can be implemented without great difficulty since some require access to hard copy satellite imagery only. These have reasonable availability, often from facsimile transmissions to local weather (meteorological) offices. But again, there is the need to determine regionally appropriate coefficients if simpler and more common empirical methods are employed. In addition, regional validation of the estimates provided by such models will be required before the data can be used with any confidence.
CALCULATING SPECTRAL IRRADIANCES Numerical methods are available for clear skies and results have been obtained for Nigeria. There is urgent need for similar work to be undertaken in other parts
399
400
J. E. HAY
of Africa if the matching of solar photovoltaic systems and meteorological conditions (specifically the spectral distribution of the solar radiation) is to produce optimized photovoltaic systems. Similarly, the work must be extended to cloudy conditions (especially partly cloudy skies) in order to be able to design more efficient and effective photovoltaic systems. CALCULATING SOLAR 1RRADIANCES FOR INCLINED SURFACES Simple methods which produce reliable estimates of the solar radiation incident on an inclined surface are now available. They require little data other than the direct and diffuse solar irradiances for the equivalent horizontal surface and an estimate of the surface albedo. While some efforts have already been made to provide regional validation of such models and their estimates of inclined surface irradiances, further work is required. Once this has been achieved and a reliable mode] chosen, there is an urgent need
GLobaL solar irradiance for horizontal surface
- - Direct
Latitude
declination t me
Isotropic
J
No parameters--
--Diffuse Anisotropic ,, , Jkatitude
(e.g. Hay]
J
I i 'e'n ti°n[
- - RefLected - - Tsotropic
GLobaL irradiation for ncL ned sfc
Fig. 2. Procedure for calculating solar irradiance for an inclined surface. I SoLar constant I
GraphicaL, sunshine, theoretical
Theoretical
[
for calculations of monthly mean solar irradiances for a range (in both slope angle and aspect) of inclined surfaces at various locations in Africa. Such information is critical to a rational evaluation, assessment and development of the continent's solar resources.
II
[CLear sky irradiation I
Ii Sunshine, cloud
II Theoretical
theoretical sateLLite It
JCLoudysky irradiation [ q
Ii DaiLy
e.g. dain
tl HourLy
e.g. Erbs (Liu and Jordan)
II
JDirect, diffuse 1
I
Isotropic
e.g. Hay, Perez
II
JInclined surface
[
Fig. 1. Schematic of methods to compute given solar irradiances.
SUGGESTED PRIORITIES FOR FUTURE STUDIES The following is a preliminary listing of priority activities which should be undertaken in Africa to ensure that a valid data archive is available to those who wish to make assessments of the solar resources of the continent. (i) Calibration of existing and new solar radiation sensors and associated monitoring equipment. (ii) Quality control of both existing (i.e. current archives) and new (i.e. data now being acquired) solar radiation and bright sunshine data. (iii) An increase in the number of stations measuring bright sunshine and the use of the resulting data and appropriate models to provide estimates of the solar energy available at these locations. (iv) The undertaking of comprehensive (i.e, global, direct, diffuse, inclined surface, spectral) solar radiation monitoring programmes at a few locations in the continent. These will provide high quality and
Solar radiation in Africa : an overview extended data sets with which to calibrate and/or verify the various models to be used at other locations with more restricted (e.g. bright sunshine alone) observational programmes. (v) The undertaking of the regional validation of various models (e.g. sunshine and satellite data based models for estimating horizontal surface irradiances and empirical models for estimating inclined surface irradiances). (vi) The undertaking of both spectral measurements and model calculations and, after verifying the resulting estimtes at selected locations using observed data, produce a data base of regionally representative spectral distributions of the solar irradiance for design applications. S U M M A R Y OF M E T H O D S FOR C O M P U T I N G SOLAR IRRADIANCES
Figure 1 summarizes the various procedures which can be used to estimate the solar irradiance for a
401
horizontal or inclined surface. The methods and input data requirements for the inclined surface calculations are given in more detail in Fig. 2. The reader is referred to the relevant section of the preceding papers for further details. Copies of computer programmes which undertake these calculations and provide estimates of the solar radiation for horizontal and inclined surfaces are available from the author or from Professor P. Jain of the University of Zambia.
CONCLUSIONS While considerable progress has been made in developing methods and acquiring the data with which to assess the solar energy resources of the African continent further effort is required. These include use of currently available data and methods and the development of new procedures and the acquisition of additional data with which to provide further refinements to the assessment process.