This study describes the impact of widened system boundaries in the choice of heat generating plants, the total cost of the defined energy system and the use of combined heat and power (CHP) production plants. The economic and environmental benefits of co­ operation between public utilities and industries and the importance of enlarged system boundaries are shown in the different case-studies.

The analysis was based on computer modelling of municipal and industrial energy systems. A user-friendly graphical interface program based on the linear programming model, MODEST, was developed as part of this study. The majority of the case-studies presented in this thesis were then performed with this application program.

The various case-studies clearly indicate the cost reduction potential of co-operation. Both utilities and heat supplying industries benefit from this co-operation concerning heat supply. The cost saving potentials change from one case to another and depend on several factors.

New plants within a sub-system appear to be attractive when this system is connected with other sub-systems within the extended system boundaries. In general, we can say that an extended system boundary leads normally to reduced total costs, more rational utilisation of plants, increased heat and electricity generation through the CHP plant and declining use of fossil fuels.

The net present value of the maximum allowable investment cost for district-heating pipeline connections is roughly estimated when the cost for connection is not given or known. This cost can be used as a guideline to indicate whether connections should be considered or not.