Erkki Jõgi, Alo Allik, Hardi Hõimoja, Tõnis Peets, Heino Pihlap, Mart Hovi, Eve Aruvee, Janar Kalder, Maido Märss, Jaanus Uiga, Andres Annuk


The current paper addresses energy storage issues in residential buildings with the objective of increasing direct consumption. The building, connected to an utility grid, is supplied by a micro wind turbine and PV panels. The utility grid itself acts as an energy buffer. Only nonshiftable loads (white goods, TV etc.) and electric water heating are taken into account. The studied configuration comprises two cascaded heating boilers, one of them preheating boiler. The annual electricity production of the micro wind turbine and PV panels is chosen to cover the hot water demand and nonshiftable loads inside the building with 70/30 ratio in favour of the wind energy. During the experiments, the generation graphs’ shaving levels vary between 0 and 100 %, with peak energy diverted into a preheating boiler and the remaining part fed into the main boiler. The proposed solution allows increasing locally consumed energy share, as the energy of stochastic peaks is stored and used on later demand. The locally consumed energy is expressed by the cover factor, its increase possibilities are studied in main text. Calculations are based on 5- minute time series. The applied algorithm follows the amount of heat in the main and preheating boiler, including also incoming and outgoing energies. The cover factor cannot be increased without restrictions. Too high shaving levels bring along problem of removing excess heat from the preheating boiler. The allowed drain loss is taken as 10 % of annual boiler energy balance. The presumed growth of the cover factor at preheating boiler volume of  160 l instead of 80 l is at least 8 %. with the main boiler sized as before.

Keywords: cover factor, demand response, domestic hot water, load shifting, solar energy, wind energy

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