Dottorato in ENERGIA E AMBIENTE

Titolo della tesi: Advanced modeling of a micro solar CHP system for residential applications

Nowadays, thanks to the Renewable Energy Resources (RES), the energy supply is shifting towards a hypothetical nearly zero emissions society. Distributed Energy Resources are the most promising way to integrate RES into the thermal and electrical grid, realizing the so-called 4 th generation District Heating networks. In this dissertation a possible solution to the energy supply issues is shown, it is based on the most abundant resource, namely solar energy. The micro solar plant here analyzed was conceived and tested both in cogeneration and trigeneration, because this is one of the optimal configurations to efficiently convert a thermal power source. The design specifications concerning the system size agrees with other scientific research, thus remarking the feasibility of these choices. In this regard, although medium and large plants are available in this small market segmentation, many efforts have to be spent in order to reduce the investment cost and the electric efficiency of this kind of power plants. This dissertation investigates the current potential of such technology at this small scale trying to find rooms for improvement. Nevertheless, few scientists investigate micro solar CHP plants, even less with latent heat thermal energy storage placed between the solar field and the power block. The basic plant solution consists of: (i) a Linear Fresnel Reflector (LFR of 80 kWth), (ii) a latent heat thermal storage (TES of 4h) and (iii) a micro Organic Rankine Cycle (ORC of 2 kWe). The system was designed for 250-300 °C, namely a reasonable temperature derived from technical and economical constraints. The contribution in innovation of such micro CHP plant, leads to face up with many open research lines: (i) Phase Change Materials selection considering the toxicity, corrosion, stability after several thermal cycles, phase change temperature close to 250-280 °C, cost, thermal conductivity; (ii) heat pipes designed for withstand at mid-high temperature, ability to transfer heat into the storage medium; (iii) the maximization of the electric output of the micro ORC keeping a feasible cost; (iv) trying to find out novel rules to manage the energy system efficiently, maintaining high levels of reliability. Modeling solar ORC plants encompasses all the heat transfer problems, for example the most complicated ones regard the optical characterization of the LFR (far more complex than parabolic trough), or phase change: in the Heat Pipes, plate heat exchanger (ORC), phase change material etc. A trade-off between CPU burden and quality of the output results was found. For this purpose,ad-hoc subroutines for each subsystem were written in MATLAB and the numerical investigations accomplished to characterize the plant performance with the specific degree of accuracy. Particular attention was paid for modeling the dynamic behavior of most critical subsystems in order to increase the dynamic response fidelity of the system in light of the subsequent testing of the control system. This dissertation shows the potential production of such kind of plants, for example with a mirror ground area of about 246 m 2 , in the city of Lerida (Spain), the annual electric and thermal energy production is about 6500 kWh and 58,800 kWh respectively. Preliminary numerical investigations showed high thermal losses of the pipelines, thus new mathematical models were developed to increase the precision of such results. Moreover, to reduce the wasted heat of the thermal storage, due to its low State of Charge level (unexploitable by the ORC) a novel strategy for the thermal management was presented, it was able to increase 5 % the annual energy production. Coupling the simulation system with a building is it possible to assess the influence of the user demand on the system performance. More precisely, an additional low temperature storage was added in bottoming to the ORC and the heat source was used for space heating and space cooling. Moreover, an absorption chiller was added to recover the large wasted heat in summer seasons. The new set-points of the ORC cooling water and the non-contemporaneity between the thermal source and the user demand was taken into account to quantify the coverage level from solar energy, and hence the extra cooling and heating power required from the vapor compression pump and boiler respectively. Basically, the optimal number of apartments was found to cover as much as possible the required thermal and electrical demand with RES. A further investigation dealt with the inner wall temperature prediction of the receiver tube in the solar field, by its knowledge the plant can be operated in safer conditions avoiding thermal stress and getting a more accurate thermal efficiency. Finally, a communication framework for testing the plant control Hardware with the in the Loop approach was developed. An external control board was connected to the simulated plant to optimize preliminary control strategies. This study allowed to significantly reduce the time spent in designing the control architecture. Concluding, the numerical investigations here accomplished underlined to what extent the thermal losses are critical in such kinds of plants and how they can be accounted for with more accuracy. Despite the high plant complexity and cost, in the trigeneration configuration it can be a valuable alternative to integrate conventional space heating, cooling and domestic hot water systems. The simulated plant can considerably reduce the control system design by means of Hardware in the Loop technique.