Optimal Operation of Heat Storage Systems withVariable Temperature Tanks for District HeatingNetwork

The optimal operation of heat storage systems for district heating networks has been usually determined by means of MILP algorithms. In fact, simplifying hypotheses are introduced to linearize model equations and constraints: one of these hypothesis regards heat storage temperature, which is considered constant. This paper presents the non-linear model of a heat storage system featuring the actual off-design characteristic curve of a CHP system and a variable temperature heat storage. Optimal operation for a fixed time-dependent demand is searched by varying CHP system loads. The objective is the maximization of management profit in a deregulated electricity market, taking into account the investment costs (CHP and heat storage systems) and the operating costs. The nature of the problem is investigated and a decomposition of the objective function is proposed to simplify the solution procedure. The impact of different fuel costs and average electricity prices on the results is also analyzed.