Progress towards an all-renewable electricity supply

To the Editor — We agree with your editorial1 on the need for more funding for renewable energy research. However, we would like to make a few points. First of all, on the issue of the commercial competitiveness of photovoltaic (PV) electricity generation we would like to note that PV is already cutting the price of electricity on German and Italian grids. Figure 1a shows the cumulative installed PV capacity in Germany, Italy and the UK2, 3. Figure 1b shows that the difference between the peak price of electricity and the base-load price on the German grid has fallen significantly4, while PV installations in Germany have risen exponentially. Electricity prices are highest at the time of peak demand. Data from the Kombikraftwerk project5 show that PV supply in Germany is at its maximum within an hour or so of the peak daytime demand on most days in summer and winter6. The PV contribution is smaller in winter than in summer. Hence the cost reduction is less in winter (Fig. 1b). Figure 1a shows Italy catching up with Germany. In the early afternoon of 2 May 2012 the price of electricity on the southern Italian grid dropped to zero7. Indeed, expressed as price per kilowatt-hour, a measure which favours conventional electricity generation, PV electricity in southern Italy is already below grid parity6. We also agree that 'wind and other clean technologies need to be developed in parallel'. However, the Kombikraftwerk project has shown that an all-renewable electricity supply is possible with existing technologies5. Throughout 2006 the project matched 1/10,000 of the real-time demand on the German grid to the real-time output of PV and wind generators, which together supplied 78% of the power. Biogas generators provided the back-up capacity (17%). Only a small pumped-water storage capacity (5%) was necessary. In ref. 6, we discuss evidence of the complementarity of wind and solar contributions to supply. We also note that the German grid has coped with supply peaks above 30% from both PV and wind energy. Kombikraftwerk ignored contributions from other renewables such as hydropower, solar thermal, geothermal and marine power. We therefore feel confident in calling for a moratorium on all forms of electricity generation except on the renewables as the quickest way to lower carbon emissions6. Your criticism of the German feed-in tariff (FIT) seems to be based on out-of-date figures for the guaranteed price paid to PV producers. Furthermore, the FIT has not inhibited German support for research on renewable energies. The dedicated Fraunhofer PV and wind laboratories are excellent examples the rest of Europe should follow. In our opinion Germany's FIT has produced the steady, exponential growth in the PV market (Fig. 1a) and the steep reduction in PV costs. Ending the subsidies for fossil and nuclear power would be a better source of funding for research on renewables. Our proposed moratorium would reduce the need for these subsidies. Italy and the UK have severely cut their FITs. If they were to introduce FITs like Germany they should follow the dashed lines (blue and black, respectively) in Fig. 1a. If the blue dashed line is extended, Italy would then achieve 55 GW of PV by 2015. Results from the Kombikraftwerk project suggest Germany needs 55 GW of PV for an all-renewable electricity supply5. The UK monthly electricity demand variation matches the average monthly wind and PV supply. Hence the all-renewable requirement for the UK is only 37 GW of PV (ref. 6). Extrapolation of the black dashed line suggests this should be achieved in 2020. Last but not least: we agree that there will be benefits from higher PV cell efficiency. Two of us (K.B. and M.M.) co-founded a concentrator PV (CPV) company (QuantaSol) that produced 40%-efficient cells8. This third-generation technology has yet to take off, not because of cell efficiency or manufacturability, but because CPV system prices are still above the rapidly falling first- and second-generation prices. We suggest CPV and engineering companies collaborate to develop multi-functional applications of 40%-efficient technology. Realizing such efficiencies involves different dynamics than undercutting flat panel prices. Examples are the direct powering of ventilation, air conditioning and dehumidification systems, solar combined heat and power for buildings supplying electricity and hot water, for example 'smart windows'9, and finally electric cars. A domestic roof-top PV system with double the first-generation efficiency would additionally power the family electric car for the average annual mileage; even in the UK8. Download references