**5. Conclusions**

This paper investigated the possibility of adding supplementary electric energy production to a CHP system at a service of a DHN, by converting the prime movers' exhaust heat with an ORC. The inclusion of the ORC allowed operating the prime movers at full-load (thus at their maximum efficiency), regardless of the heat demand, without dissipating not required high enthalpy-heat. Indeed, discharged heat was recovered by the ORC to produce additional electric power at high efficiency. In this work, a specific case study of an existing CHP system was analyzed; however, the same presented approach and similar considerations could be applied to any CHP feeding a DHN as a thermal user.

In the original arrangement, three internal combustion engines covered the heat demand together with the back-up boilers. ICEs load regulation strategy was determined in order to fulfill the DH demand while minimizing the heat dissipation to the ambient. The CHP in its original arrangement was compared to a new arrangement, including an ORC. In the new set-up, the ICEs worked at full-load for the whole year. The ORC was conceived to exploit the residual heat of the internal combustion engine exhaust gases, which has not been used by the district heating in part-load conditions. Three distinct layouts were investigated in this work: i) the ORC placed downstream the heat exchanger feeding the DH network; ii) the ORC supposed to be upstream the DH heat exchanger; iii) a parallel arrangement.

A sensitivity analysis of organic fluid and key cycle parameters was performed in order to identify, for each proposed arrangement, the optimum design of the ORC. Results showed that, for each analyzed configuration, Cyclopentane achieved the best performance compared to other hydrocarbons. The value of the optimal evaporation pressure depended on the considered configuration. As expected, the highest ORC performance was achieved for the highest value of the intermediate heat-transfer fluid circuit design temperature. Maximum power output (2027 kW) was achieved for the second layout solution, with an evaporation pressure of 40 bar and IHTF design temperature equal to 300 ◦C. In this case, the IHTF heat exchanger was fed with the exhaust gas at its highest temperature and mass flow, if compared with the other case studies. Based on sensitivity analysis results, the CHP-ORC hybrid plants were simulated by means of the commercial software *Thermoflex*™ in design and in off-design conditions to account for ORC and DH part-load operation during the year.

The original optimal operation of the CHP plant was compared with the modified one in order to estimate, for each proposed integrated ICEs-ORC arrangement, the amount of additional generated electric power and the corresponding increase in primary energy consumption. The additional electrical energy produced during the year, thanks to the ORC, its operating hours, and total investment costs were quantified and compared among analyzed layouts. Results showed that the performance of the ORC, on the year basis, strongly depended on its part-load behavior and on its regulation limits. Indeed, even if the second layout exhibited the highest ORC power peak value, in design conditions, it did not provide the highest amount of producible electric energy on the year basis because of the limited number of hours in which it operated during the year. The layout that allowed to produce the maximum amount of ORC electric energy per year, 9.77 GWh/year, was the third one, i.e., the parallel arrangement. In the third set-up, the ORC peak power was lower than in the second (1790 kW), but it could operate for the greatest number of hours, namely 7050 h/year. Thus, the actual number of operating hours during the year, determined by the ORC regulation limits, turned out to be fundamental in the plant arrangement decision. An energetic index accounting for the global primary energy saving was introduced, in order to evaluate the convenience in producing the additional electric power with the modified CHP plant rather than with the original plant with the help of the electric

generating mix. The energetic analysis demonstrated that all the proposed solutions granted to reduce the global primary energy consumption by about 18%. The best ICE + ORC solution, in particular, introduced a positive global primary energy saving equal to 19.1%. The inclusion of the ORC entailed significant savings in primary energy consumption because it allowed producing electric energy with an efficiency close to 52%, higher than the electric generating mix efficiency.

Finally, the economic feasibility of each CHP-ORC hybrid system layout was evaluated. The differential cost between the original and the new arrangement was related to the additional primary energy consumption. The differential revenue, instead, was proportional to the additional generated energy. The economic analysis showed that the second layout was the worst solution since it presented the highest investment (due to the highest size) and the lowest yearly earning (due to the lowest yearly energy production). Nevertheless, all the proposed ORC solutions proved to be a good investment since they allowed to return on the investment in barely 5 years, by selling the electric energy at a price equal to 70 EUR/MWh, considering a medium natural gas price equal to 33 EUR/MWh. When the higher natural gas price was considered, instead, the electric energy sell price could be lower in order to return on the investment in the same payback period; when lower natural gas price was considered, on the contrary, the electric energy sell price must increase. Especially for the best ICE+ORC case, in order to return on the investment in 5 years, the electric energy could be sold at a price of 95 EUR/MWh, considering the high gas price, or at a price of even 50 EUR/MWh, considering the low gas price.

In conclusion, results of this work allowed to state that the ORC technology effectively represents an interesting solution to make the plant regulation more flexible, with the purpose of optimizing economic revenue and energetic performance of the CHP plant, while not dissipating high-enthalpy valuable heat to the ambient. Given the promising results of this work, future studies will be dedicated to investigating the dynamic behavior of the DHN, by considering also thermal storage, in order to evaluate the possibility of managing the DHN in a more active and profitable way.

**Author Contributions:** Conceptualization, L.B. and F.M.; Data curation, N.T.; Funding acquisition, F.M.; Methodology, L.B. and F.M.; Project administration, A.D.P.; Software, N.T.; Supervision, L.B., A.D.P., and F.M.; Validation, A.D.P.; Writing—original draft, N.T.; Writing—review and editing, A.D.P. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **Nomenclature**


*Energies* **2020**, *13*, 1314

