Unlike the case of the off-grid model, when the imposed condition is the balance between required and produced power and heat, in case of the on-grid system the
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imposed condition is the balance between required and produced heat only while the power can be produced in excess.
To exemplify, two alternatives could be considered. In the first alternative, the load sharing strategy could consist in adjusting the nominal area of the PV and ST panels (which are the key variables of the strategy) so that in months with high solar radiation, the power load to be fully covered by PV panels. A second alternative could consist in maximizing the areas of PV panels (respecting the architectural requirements of the building) and adjusting the Stirling engine installed power, so that the maximum load of the system to be covered either only by PV panels or by PV panels and Stirling engine.
In the last alternative, the prosumers can export the electricity to the grid (thermal energy cannot be exported) and the governing strategy for the CHP unit changes so that to supply at any moment the thermal energy needed. Consequently, the additional boiler and thermal solar panels can be discarded. Since the electricity can be exported to the grid, the PV panels area could be A’PV = 0.4 Agfa, for the roof, to which it can be added ApV = 0.15 Agfa, for the fapade (according to the physical and architectural restrictions).
Considering the illustrative exercise and choosing the PV panel on the roof only, ApVpanei = 0.4*100 = 40 m2 (kj, V = 0.4). The numerical simulation results are shown in Figs. 36 and 37. The CHP unit ensures the thermal needs of the residence, while the electricity production of the CHP and PV ensures both the residential consumption and the export to the low-voltage grid.
From the point of view of performance indications, the PES and EFF are changed accordingly having the following expressions:
PES = 1 – ssf S * 100 % <66>
gEref gHref
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^rn q_load q_sys————- Poly. (q_load)—– Poly. (q_sys)
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EFFcchp = esys + egrid+qsys x 100 % (67)
qF + epv + qsT
egpd electricity produced by the CCHP system and exported in grid.
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The obtained performance indicators became PES = 69.6 % and EFF = 94.6 %. The monthly distribution of PES and EFF are shown in Fig. 38.
