An on-grid PV hybrid system has to show a lower FCR than that of the fossil firing power plants which feed the grid. Hence, full annual load of PV hybrid systems should not be recommended in general, unless generators with improved efficiencies are employed, which will permit extended hours of operation. Shown in the figure are three less steep, light dashed lines, which represent the use of CC of 60% (instead of a diesel or a Rankine cycle) for the PV hybrid. As expected, much better FCR and GREF values come out, which provide continued environmental benefits for extended hours. A 50% efficient combined diesel — Rankine system will exhibit intermediate lines (between the SEGS and CC lines). For each mode and strategy the related environmental benefits (as a function of the length of operation time) can be directly evaluated according to the system FCR and GREF by use of Figure 1.

The 60% level seems to offer a worthwhile standard for industrial countries with advanced power systems and grids and availability of gas. Tentatively, however, it seems that standards may be partially relaxed for particular locations due to specific conditions such as system size, availability of gas and distance from grid. Thence the secondary standard of 40% may come to play. It represents many power plants which exist to day and which continue to spread. Whatever GREF standards taken, they should always be transparent. It is obvious that a GREF related to a standard of 40% is numerically different from one related to a standard of 60%, which means also a difference in environmental contribution. The equations below will quantify the differences as a function of the specified standards.

If a PV system is going to be added to an existing, off-grid diesel or Rankine-cycle generator at an isolated, remote site, it seems be reasonable to consider the new PV output as green energy (GREF=1), independently of the conversion efficiency of the existing generator. However, when we plan to install a new hybrid system, which includes a new fuel firing subsystem, it is mandatory to raise the question of GREF and standards, in view of the global strategy for abating the global GHG (greenhouse gas) menace. Implementation of the strategy requires frequent scrutiny of on-going decisions about variety of ways in using fuel. These require the application of the yardsticks of FCR, GREF and cCa (cost of carbon avoidance, see below). Global strategy should encourage system refurbishment and installation of mini-grids at remote sites in order to enable improvement of generators efficiency. Fig. 1 and CCA (Equation 3) are useful for modelling off-grid PV hybrid systems as well.