Solar electricity generation in hybrid thermal power plants

The EU Directive on renewable sources, Directive 2009/28/EU, requires European countries to increase renewable energy uses until they reach 20% of gross final energy consumption. This target has been distributed among the member states which have put incentives on renewable energy uses and renewable electricity generation. Among renewable sources, solar energy is one of the most interesting but it presents a key issue: it is a non-dispatchable renewable energy source. For this reason, when solar energy is used to produce electricity, concentrated solar thermal power plants are often integrated in conventional fossil power plants (hybrid power plants); in this case it is very important to distinguish the amount of electricity produced by this renewable source from the one produced by fossil source, since the two heat inputs can contribute to electricity production in different measure. It is possible to elaborate numerical models able to quantify performance of hybrid power plants and to allocate the total electricity for each energy source: for example, assuming a constant fuel consumption and adding solar heat, these numerical simulations allow evaluating the extra electricity generated, that is really the electricity from solar energy. However, such an evaluation is impossible during power plant operation. This paper presents a simple, but effective, methodology able to distinguish electricity generation of each energy source during hybrid power plants operation: it is necessary to know (by measures) only heat input from fuel and solar energy, initial and final temperatures of working fluid during heat addition and rejection, and the factor of internal losses. This last parameter is very important and this paper not only demonstrates that it doesn't depend on solar energy share but it also proposes its evaluation. Finally the paper compares results of this methodology with those of numerical models here elaborated showing their perfect correspondence in solar electricity evaluation.