Optimizing solar energy integration in Tallinn's district heating and cooling systems

Using solar energy is one way to integrate sustainable, clean and non-combustional energy to energy mix. In electricity sector, the share of solar energy has significantly grown over the last decade because of increased public awareness, declining costs and government incentives. However, the adoption of solar energy in heating and cooling sector is relatively new. There is a visible relation between solar energy production curves and cooling energy consumption curves which indicates that using solar energy in cooling sector would be efficient way to use solar energy. Still, the utilisation of solar heat for district cooling remains a grey area. In district heating and cooling sector, the use of solar energy in Estonia has been modest so far, although there is a significant solar energy potential. Hence, Tallinn district heating and cooling system has been chosen as a case study to investigate how solar energy can be used most beneficially and efficiently. In this regard, three main integration scenarios with respect to the different technical configurations and energy transformations are analysed. It was observed that the proposed solar park could generate 27.58 GWh thermal energy per annum. The share of useful solar energy (or solar fraction) reached more than 98.5 %, when TES integration is considered. From the analysed scenarios, it can be concluded that integration of TES is highly important to tap solar heat to the fullest. The seasonal match between load and generation contributed to higher share of solar energy for district cooling than district heating. This study is expected to be useful reference material for project developers, investors and policy makers.