Weather

3.1 Weather variations and the influence of weather variations

The weather is unique for different locations around the world. But, the weather also varies for specific locations. Both the ambient temperature and the solar radiation can vary much from one year to another and also the distribution of temperature and solar radiation varies during different years. With measured weather data from the Solar Radiation Measurement station placed on the top of building 119 at the Technical University of Denmark the weather variations in the period from 1990 to 2002 are studied. As a reference to the measured weather data, the Danish Design Reference Year, DRY data file is used (Skertveit et al. 1994). The file contains measured weather data at climate stations in Tåstrup and Værløse from the period 1975 to 1989, which is a time period just prior to the weather data period investigated. Usually the annual thermal performance of a solar heating system is estimated with DRY weather data and fixed consumption and consumption pattern. The consumption and the consumption pattern have a great influence of the thermal performance of solar heating systems, and so has the weather. In years with weather different from DRY weather data, measurements of the thermal performance of solar heating systems in practice will most likely be different from the estimated thermal performance with DRY weather data.

Figure 3.1 shows the measured yearly global and diffuse solar radiation on horizontal from the period 1990 – 2002 and DRY. It is clear, that there are large variations from one year to another.

In Figure 3.2, the monthly average global radiations from the period 1990 – 2002 and from DRY data file are shown. Also the measured monthly radiation variations are shown. It is clear, that the average global radiation from 1990 – 2002 is similar to the global radiation from DRY data file and that the largest radiation variations take place in the summer period April – September.

In Figure 3.3, the monthly average day temperatures from the period 1990 – 2002 and from DRY data file are shown. Also the measured monthly temperature variations are shown. It is clear, that the average day temperature from 1990 – 2002 is higher than in DRY data file, especially in the period January – April and in July and August. The temperature variations are in average ± 3 K, except in February where the temperature variations are larger and in April where they are smaller. In Denmark, February can be a gray and rainy month with rather high temperatures or a beautiful and cold winter month with snow. In April, the weather is mostly cloudy and rainy which prevents large temperature variations.

Solar irradiance data are usually measured on horizontal. The total irradiance on a tilted surface is then calculated from the horizontal irradiance by means of solar radiation processing models. Figure 3.4 show the relative yearly total solar radiation on a south facing 45°-tilted surface as a function of the relative yearly global radiation. The relative values are relative to the same values from DRY weather data.

The dotted line indicates a linear relationship between the relative total global

the dotted line have more solar radiation during spring/fall than DRY weather data while years situated below the dotted line have less solar radiation during spring/fall than DRY weather data. The figure shows that there is no linear relationship.

Figure 3.1 Global and diffuse solar radiation on horizontal.

Figure 3.2 Monthly average global radiation and the deviation interval from the average global radiation from the period 1990 – 2002 and DRY weather data.

0 50 100 150 200 250

Janu ary

Febru ary

March April

May

June July Aug

ust September

Oktober No

vem ber De

cem ber

Monthly global radiation on horizontal [kWh/m2 ] 1990-2002 DRY

0 200 400 600 800 1000 1200

DRY

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999

2000 2001 2002 1990-20

02 Annual Radiation on horizontal [kWh/m2 ]

Global Radiation Diffuse Radiation

Figure 3.3 Monthly average day temperature and the deviation from the average day temperature from the period 1990 – 2002 and DRY weather data.

Figure 3.4 The relative annual total solar radiation on a south facing 45°-tilted surface as a function of the relative annual global radiation. The relative values are relative to the same values in DRY weather data file.

The investigations on the influence of weather variations on the thermal performance of solar heating systems are based on the measured weather data from 1990 – 2002 and DRY data file. Solar collectors as well as solar combi systems are included in the investigations.

The annual thermal performance of solar collectors and of solar combi systems increases for increasing annual solar radiation.

-5 0 5 10 15 20 25

Januar y Febr

uar y

Ma rch

Apri l

May

June July August

Septem ber

Octobe r Nov

ember December

Monthly day temperature [°C] 1990-2002 DRY

0.8 0.85 0.9 0.95 1 1.05 1.1

0.85 0.9 0.95 1 1.05 1.1

Relative annual global solar radiation [-]

Relative annual total solar radiation on south 45° [-] DRY weather data

The relationship between the yearly thermal performance and the yearly solar radiation can for all types of solar collectors be fitted to a linear relationship, with a good approximation for the yearly radiation on the solar collectors and with a reasonable approximation for the yearly global radiation.

It is not possible to fit the relationship between the yearly thermal performance of solar combi systems and the yearly global solar radiation or the yearly solar radiation on the collector to a linear equation.

The annual utilization of the solar radiation for all types of solar collectors is increasing for increasing annual solar radiation on the collector.

The annual utilization of solar radiation for solar combi systems is not significantly influenced by the annual solar radiation on the collector, regardless of the collector efficiency, the heating demand and the size of the solar heating system. However, the annual utilization of solar radiation is higher and varies more for solar combi systems with high efficient solar collectors than for systems with low efficient solar collectors Finally the investigations show that the investigated evacuated tubular solar collector utilizes less sunny years with large parts of diffuse radiation relatively better than the flat plate solar collectors.

Further details in Paper II.

3.2 Solar radiation processing models

Measured solar irradiance data are normally available as global irradiance, which is the solar irradiance on horizontal. Solar radiation processing models are used to calculate the solar radiation from horizontal to a certain collector tilt and orientation.

The transformation of beam radiation incident on horizontal to a tilted surface can be done exactly whereas the transformation of diffuse radiation incident on horizontal to a tilted surface depends on the assumptions of the distribution of the diffuse radiation.

Transient simulation programmes such as TrnSys offer the possibility of four different solar radiation processing models. The different models offered are the isotropic diffuse model (Liu and Jordan 1963) that assume that the diffuse radiation is uniformly distributed over the entire sky dome, the anisotropic diffuse model (Hay and Davis 1980) that divide the diffuse radiation into contribution from uniformly distributed radiation and circumsolar radiation from the area around the sun disc and two models (Reindl et al. 1990b) and (Perez et al. 1987, 1988) that apart from the isotropic and circumsolar contribution also uses horizon brightening.

A comparison between measured solar radiation on differently tilted and orientated surfaces and solar radiation calculated with the four different models show that the anisotropic models are best suited for predicting the solar radiation on tilted surfaces.

Also the influence on the energy from a solar collector calculated with the four different solar radiation processing models is investigated. The results show that the energy output from a collector using the isotropic model is lower than the energy output using the anisotropic models. However, the calculated energy output from the collector, regardless of the solar radiation model used lies within the accuracy of the calculation.

Further details in Paper III.