We need a mix

French consultant and sustainable energy expert Bernard Chabot analyzes Germany's PV and wind power installations performance “from power to energy and to percentage of electricity production” and draws some useful conclusions for optimal balance between RE technologies.

Figure 1: Installed power and electricity production from RE in Germany. Source of all images: Bernard Chabot

From detailed data published on the German EEX transparency platform web site, it is possible to analyze the electricity production from PV, wind and conventional power systems (nuclear, fossil fuels based and hydropower plants larger than 100 MW). A six-month period from December 2011 to June 2012 is covered here to see whether PV and wind production complement each other in different seasons.

The large increase in GWs of variable (but predictable) RE technologies as wind and solar PV deserve to be analyzed to see whether they complement each other and if the short to midterm balance between them is optimal or not.

Figure 2: Electricity production in Germany for six months in GWh and as percentage of total

Figure 2 provides the first interesting answer. The contribution of wind power (including both onshore and offshore) slightly decreased from December 2011 (a very windy month) to May 2012, with a low production in January due to exceptional cold conditions in Europe. Of course, on the contrary, solar PV production continuously increased from December to May.

In these 6 months, wind power contribution varies from 8.7 % to 21.3 %; PV, from 0.8 % to 12.3 %. The total [wind + PV] range is between 11.6 % in January to 22.2 % in December, so the combination between two large installed capacities of wind (29.075 GW at the end of 2011) and PV (24.8 GW registered end of 2011, 26.5 GW end of April 2012) provides clearly a “smoothing effect” of great interest for the management of the electrical system.

Figure 3: Hourly percentage of production from PV, wind, [Wind + PV] in December 2011

“Zooming” at hourly and daily periods gives complementary useful information. Examples are given for two months: December 2011 and May 2012. Figure 3 indicates the percentage of total hourly electricity production from PV, from wind and from [PV + wind] in December 2011. Maximum [wind + PV] hourly percentage of production reached 41.6 % on Friday December 9th between 3 and 4 am, and hence only from wind.

As seen in figure 4, on December 9th, the [wind + PV] daily percentage of production was 15.9 % (14.2 % from wind and 1.7 % from PV).  On Thursday December 29, the maximum daily contribution from wind and PV was 36.4 %, of which 35.6 % from wind and 0.8 % from PV.

Figure 4: Daily contribution to electricity production from wind, PV and [wind + PV] in December 2011

Figure 5 indicates the percentage of total hourly electricity production from PV, from wind and from [PV + wind] in May 2012. Maximum [wind + PV] hourly percentage of production reached 48.7 % on Saturday May 12nd between 3 to 4 pm (of which 21.7 % from PV and 27 % from wind).

On May 12th, the maximum daily absolute and relative contribution from wind and PV was 33.6 %, of which 24.2 % from wind and 9.4 % from PV (from 238 and 92 GWh/day, respectively).

Some conclusions can be drawn from an analysis of the German PV experience. First, diversity in RE technologies, mainly resulting from a wise feed-in tariff policy and straightforward regulations for project authorization and implementation, is clearly an advantage.

Figure 5: Hourly percentage of production from PV, Wind, [Wind + PV] in May 2012

Second, by combining variable (wind, PV, run of river hydropower) and dispatchable RE (mainly biomass and biogas, hydropower with seasonal reservoirs) allows a higher and easier RE power penetration level than with only or mainly variable ones.

Third, an optimal mix within those RE technologies must be based more on monthly and annual electricity production than on installed power, taking also into account the relative levels of kWh production cost.

Fourth, diversity within variable RE technologies is critical: as shown in the German case, wind and PV production are complementary, resulting in a “smoothing effect” of their combined contribution during the year. Relative optimal levels between those technologies must also take into account their relative kWh production cost as differences within onshore wind, offshore wind and solar PV are still significant.

Figure 6: Daily contribution to electricity production from wind, PV and (wind + PV) in May 2012

Finally, performance analysis must encompass hourly, daily, monthly and annual electricity production. Emphasis must not be only on “maximum instantaneous power” from only one technology, but on combined power, production and percentage of RE penetration.

As more and more countries are aiming at a significant part of their electricity consumption coming from renewables, these conclusions should be taken into consideration for their energy policies, and adequate detailed data and statistics on RE installations and their performance should be open to the public, as is the case in Germany. 

Renewable energy (RE) for electricity production started in Germany mainly after the first feed-in tariffs in 1991 and those defined by the RE law (EEG) in 2000, leading to a sharp increase both in terms of installed power and annual electricity production as shown in figure 1. The total RE production was 121.9 TWh in 2011, representing 20.1 % of Germany’s electricity demand in 2011, compared to 6.4 % in 2000 and compared to the “Energiewende” (energy transition) target of at least 80 % in 2050. (Bernard Chabot / Craig Morris)