Climate-Driven Changes in Photovoltaic and Solar Thermal Operating Conditions in Slovakia: Implications for Solar System Design and Optimisation

Climate change alters not only the availability of solar radiation, but also the thermal, humidity, and cloudiness conditions under which solar energy systems operate. However, limited attention has been paid to the simultaneous comparison of photovoltaic and solar thermal responses using a common hourly climate-based framework under Central European conditions. This study evaluates long-term climate-driven changes in the operating conditions of photovoltaic (PV) panels and solar thermal collectors across five Slovak locations representing contrasting local climatic and topographic settings. Hourly ERA5-Land data for 1985–2024 were used to derive climatic indicators, photovoltaic operating indicators, and solar thermal performance indicators. The analysis combined long-term Mann–Kendall and Sen’s slope trend assessment with a comparison between the reference period 1985–1994 and the recent period 2015–2024. The results show that mean air temperature increased by 1.50–1.69 °C, global horizontal irradiance by 3.24–5.66%, and high-irradiance hours increased substantially across all sites. Photovoltaic yield increased by 2.21–4.52%, but this improvement was accompanied by higher PV cell temperature, more hot operating hours, and increased temperature losses. Solar thermal collectors showed a stronger relative response, with useful thermal gains increasing by 7.27–12.33% at 35 °C and by 9.00–15.73% at 50 °C. The Relative Solar Thermal Gain Advantage was positive at all locations, indicating that recent climatic conditions favored solar thermal gain more strongly than PV yield under the applied assumptions. The findings demonstrate that recent climatic data should be used in solar-system design and that photovoltaic and solar thermal technologies require separate interpretation because they respond differently to warming and changing radiation conditions.