## 5 Conclusions  
The MILP model presented in this paper has been used to optimize the configuration and operation for a case study of a distributed generation system for heating and cooling in an urban area. The model allows us to obtain the optimal configuration of the system, satisfying the energy requirements of the users, that minimizes the total annual cost and the total CO2 emissions during operation. The aim of the work is to compare on a common basis, different district heating integration options. In more detail, five cases have been considered:  
* conventional solution;
* isolated solution;
* distributed generation solution without central unit and district cooling network;
* distributed generation solution with central unit but without cooling network; and
* complete distributed generation solution. Starting from the conventional solution, the further cases consider, at each step, additional components and a more thorough district heating/cooling integration. The best solution, both from the economic or environmental points of view can be achieved with the complete distributed generation solution, which includes various kinds of energy components, a DHCN, a solar field and a seasonal thermal storage. The best economic solution allows a 43% reduction of the total annual cost compared with the conventional solution, while the best environmental solution allows about a 50% reduction of the total annual emissions (see Table 16). This result can be achieved only with the optimal size and/or multiplicity of the components and with the optimal operation of the energy system. The economic optimization of the Isolated Solution, which comprises only the distributed cogenerators and absorption machines, without any district integration, permits a consistent reduction of the total cost compared with the conventional solution (37%), together with a reduction of the CO2 emissions (16%). In this case, the adoption of local thermal storages is suggested from both the economic and environmental points of view). This performance is similar to the one obtained for the economic optimization of the complete distributed generation; the isolated solution also implies a much lower investment cost (Table 7). On the other hand, the complete distributed generation solution is cheaper in the long term view (over 20 years) and allows also a greater reduction of the annual emissions. In fact, important reductions of the annual emissions can be obtained only with the adoption of the solar field, of the seasonal storage and of the district heating network. The distributed generation solution, with heating district integration, but without the solar field and the seasonal storage, permits us to slightly improve both the economic and the environmental objective functions (less than 2%) compared with the isolated solution. The further integrations of the DG system allow incremental benefit of similar magnitude for the economic optimal solutions (1-3% each), reaching the higher reduction with the complete distribution generation solution, as highlighted before. From the analysis of the Pareto fronts obtained, it can be inferred that a very high environmental benefit (greater than 40%) can be obtained by complete distribution generation solution, without giving up a large long-term cost reduction of about 32% (see Table 18). At the same time, it is evident that the advantages obtained by introducing the cooling district network are very small, at least when the optimal synthesis and operation of the system is considered. When the central solar field with central TS can be chosen, the optimal compromise solution adopts a central TS of about 172,000 kWh and a solar field of about 22,000 m\({}^{2}\). The optimal operations identified in this paper show that the central thermal storage is operated with seasonal charging/discharging cycles only when the environmental objective function is considered in the optimizations (Figure 13). The heat produced by the solar field during warmer months is used during the first colder months.