The combination of an existing heat source, a solar thermal plant and a thermal storage tank makes it necessary to develop an ‘energy management’ as a fundamental component (Fig. 3). This energy coordination has to guarantee, that

• The overall system operates in economic efficient way, i. e. a minimum of fuel is used to meet the demand of the district heating grid

• Boundary conditions will be fulfilled. These general conditions are e. g. physical limits of the storage tank or the combined gas and steam cogeneration plant.

The necessary heat supply of the cogeneration plant can be calculated at any time instant from the heat demand of the heating grid, reduced by the actual power of the solar thermal plant and the contribution of the storage unit (which can be either positive or negative — depending on charging or discharging operation of the storage tank). Changing the contribution of the storage unit results in a changed operating point of the cogeneration plant and it associated degree of efficiency.

Fig. 3: Structure of the solar assisted district heating grid

The energy management has then to decide, if — at a certain heat demand — the storage tank has to be charged or discharged. This decision is based not only on actual values of the overall system but can also depend on future assumption of the plant behaviour. That means e. g., that the storage tank is not

charged, if the solar thermal plant can provide enough energy to the district heating grid in the next hours. To predict such an event, a weather forecast has to be implemented in the energy management tool. Since the heat demand is correlated strongly with the weather conditions, the weather forecast can also be used to predict the heat demand.

The concept of such an energy coordination unit is typically based on an enormous number of ‘lingustic’ rules, like

• “… if the heat demand is low, the ‘state of charge’ of the storage tank is low and the temperature forecast predicts 15°C in the next hour, then storage unit is charged with 780 kW heat power…”

Within this project, a different approach for the design of the energy management was used. It is based on an analytical procedure [2, 3], where the development of the energy coordination unit is the result of an optimization task

T

min [ ф(и(т),…)ёт = min^ф(ик,…)

u (t) uk

s. t. boundary conditions are fulfilled,

where ф(-) is the instantaneous fuel consumption of the cogeneration plant and u(t) represents the charging/discharging level of the thermal storage tank. Boundary conditions have to guarantee, that the storage tank is not overcharged of discharged completely.

The basic principle of the ‘load management’ and the development of the underlying mathematical methods have been shown in [2]. The used optimization horizon was set to 1 year. Since one can not know exactly the weather conditions and solar radiation for such a long forecasting horizon, the optimization problem was split up into two tasks

• The long-term scheduling problem (with assumed average values within the optimization horizon) and

• The short-term scheduling problem (works as a correction of the long-term problem with actual values).