Research in solar energy in Croatia has experienced rather strong support during the 1970s and 80s. However, starting from late 1980s there are wide gaps in solar radiation measurements on meteorological stations of Croatian State Meteorological Service (Croatian: Drzavni hidrometeoroloski zavod – DHMZ). During 2003 meteorological network of the State meteorological service was upgraded with six instruments, with diffuse solar radiation measured in Zagreb and Split. Energy Institute Hrvoje Pozar initiated several solar radiation measurements projects. In cooperation with the Faculty of Electrical Engineering, Mechanical Engineering and Naval Architecture of the University of Split (FESB) and State Meteorological Service, measurement of solar radiation components with a multipyranometer array has started in Zagreb and Split. In the framework of European Union technical assistance CARDS 2003, a project ‘Solar and Wind Energy Resource Assessment in Croatia’ will initiate complex solar radiation measurements on several locations in County of Zadar, Sibenik-Knin, Split-Dalmatia and Dubrovnik-Neretva. Solar radiation measurements of high quality are in the process of setting-up in Croatia or are just recently established. But for high statistical reliability long time measurement series are needed. Five years data average for a given month can differ for up to 20% to twenty years data average, while the difference can be cut in half if fifteen years average is used . Substantial statistical reliability can be provided with thirty years data series. Lacking data series of satisfactory duration, statistical reliability can be enhanced with correlation to meteorological data measured or observed over longer periods, for example sunshine duration or cloud cover. For this correlation overlapping period of few years is needed.
Solar radiation data of good quality is needed as soon as possible for the design and optimisation of systems for solar energy utilisation. Solar energy in Croatia, in spite of very good solar energy potential, is very much underutilised. Two grid connected installations are brought on-line in last two years, totalling 12,74 MW. It is estimated that 12-15 000 m2 of solar thermal collectors are currently operational and passive solar applications, in form of passive solar houses, are only anecdotal.
In order to provide reliable solar radiation data for the design of solar energy conversion systems, Energy Institute Hrvoje Pozar and the Ministry of Economy, Labour and Entrepreneurship with the cooperation of the State Meteorological Service initiated development of Solar Radiation Atlas of Republic of Croatia based on existing solar radiation data and knowledge. The Atlas, based on sunshine duration measurement and cloud cover observations, covers 43 locations in Croatia (Fig. 1). Algorithmical chain closely follows proposed mathematical models in the European Solar Radiation Atlas, with some modification for measurement situation in Croatia. Expected date of publication is late 2005.
More info at SOLAR SERDAR
Figure 1 : Locations covered in the Solar Radiation Atlas of Croatia
2 SOLAR RADIATION DATA
Starting point of solar energy calculations is monthly mean daily global irradiation on horizontal plane. This data is calculated from sunshine duration measured on 37 locations from 1961 till 1980. We accepted linear correlation between sunshine duration and irradiation, with correlation coefficients calculated from pyranometer measurements from 11 locations, 6 of them in the area of the Republic of Croatia. This data for the coastal area is taken from DHMZ study ‘Meteorological Parameters for Solar Energy Utilization in the Republic of Croatia . Continental locations data are taken from newer study ‘Meteorological Parameters for the Design of Energy Efficiency Buildings’ . However, for Lipik, Topusko and Skrad the irradiation is calculated with correlation to cloud cover, since no sunshine duration measurement took place there.
Climatological data regarding air temperature, humidity, and pressure and precipitation refer to the period from 1960 till 1990. Data series for that period is complete for all stations except Ploce (only since 1969) and Rijeka-Omisalj (since 1970) [3,4]. Heating days and degree-days and standard outdoor design temperature are taken from  for fifteen locations (Brestovac-Belje, Gospic, Hvar, Lipik, Knin, Osijek, Pula, Rijeka, Skrad, Split, Sibenik, Topusko, Vukovar, Varazdin, Zagreb-Gric) while data for other locations were calculated from air temperature data, except the data for standard outdoor design temperature. Data related to sunshine duration measurements are taken from  and data related to cloud cover from .
3 SOLAR RADIATION MODELLING
In the course of preparation of the Solar Energy Atlas of Croatia care has been taken to follow methodology proposed in The European Solar Radiation Atlas  as closely as possible. However, proposed Muneer model  for solar radiation on inclined planes, developed in the region of Northern Europe has yet to be verified in Croatian conditions. Isotropic Klein (Andersen) model  was chosen for estimating irradiation on inclined surfaces. Optimal tilt angles for maximization of monthly, seasonal and annual irradiation are calculated according to Balouktisus [?10?]. Daily profile of horizontal and vertical plane irradiation was modeled according to Liu-Jordan [??11] and Collares-Pereira and Rabl [??12].
4 LAYOUT OF THE ATLAS
Solar Radiation Atlas of Croatia covers data for 43 locations (Figure 1.). For each location data are organized in eight pages, enabling a user to easily find and use the data for a particular location.
Data content for each location:
1. General information:
Geographic latitude [N]
Geographic longitude [E]
Heating degree days (10, 12 and 15°C thresholds)
Heating days (10, 12 and 15°C thresholds)
Standard outdoor design temperature [°C]
2. Climatological information, monthly and annual
Mean monthly sunshine duration (1961-80)
Relative sunshine duration (1961-80)
Mean number of days with no sunshine (1961-80)
Mean number of days with continuous sunshine (1961-80)
Mean number of cloudy days, by decades (1961-80)
Mean number of cloudless days, by decades (1961-80)
Mean monthly air temperature (1961-90)
Mean monthly precipitation (1961-90)
Maximal daily precipitation (1961-90)
Mean monthly relative humidity (1961-90)
Mean monthly water pressure (1961-90)
3. Global, direct, diffuse and reflected solar radiation on southerly oriented tilted planes
Tilted plane mean daily irradiation for monthly, seasonal and annual optimal tilt angle
Tilted plane mean daily irradiation for angles of tilt ranging from 0 to 90° in 10° steps
4. Global solar radiation on tilted planes
Tilted plane daily irradiation for orientation offset from South ± 15°, ± 30°, ± 45°, ± 60°, ± 75° and ± 90°, and with angles of tilt ranging from 0 to 90° in 10° steps
5. Global, direct and diffuse irradiance on a horizontal plane for every hour of a characteristic day in a month
6. Vertical global irradiance on a horizontal plane for every hour of a characteristic day in a month and for main and sub compass orientations
7. Sunrise and sunset times in local and true solar time, duration of a day
8. Sunpath diagram
5 SOLAR RADIATION MAPS
Solar radiation maps are very useful tool for visualization of spatial distribution of solar radiation. The maps of horizontal plane global irradiation, made by the State meteorological service [2,13], are based on irradiation measurements (6 locations), sunshine duration measurements (37 locations) and cloud cover observations (94 locations). The maps are updated and modified for GIS (geographic information system) applications. One annual and twelve monthly maps are made. Monthly maps have the gradient of 200 Wh/m2. In some particular areas the gradient is even smaller, 100 Wh/m2.
Closely following the variations with latitude, global irradiation is generally decreasing from northwest towards southeast of the country. Major influence of latitude is modified on the ground due to a complex terrain orography. Area around Krizevci, Brinje and Licka Pljesivica have global annual irradiation between 1,15 and 1,20 MWh/m2. Irradiation values are increasing with a small gradient over Psunj area towards Ucka Mountain, and trough Velebit towards Bosnia border (Drvar). Going west from that region, the contours are very dense and oriented almost parallel to the coastline because of the orientation of mountain ranges on the coast and islands .
Irradiation is the greatest on open-sea islands of the South Adriatic (1,65 MWh/m 2). Irradiation is generally the strongest in July (only somewhere June) with values ranging from 7,43 kWh/m2 (Komiza) to 5,89 kWh/m2 (Brinje). December is the month with the smallest values, ranging from 0,62 kWh/m2 (Krizevci) to 1,50 kWh/m2 (Komiza).
Solar energy is very valuable natural resource. The applications of solar energy differ from very simple black painted hot water tanks to photovoltaic concentrators with two axis solar tracking. However, designers of the systems for conversions of solar energy need accurate knowledge of this resource. The Solar Radiation Atlas of Croatia presents existing solar radiation data and knowledge. Upgrade of the measuring systems and investigation of new modelling approaches will increase accuracy of this data.