At the start of the 20th century, the country faced a serious problem in that domestic coal was expensive, and of low quality.  With high tariff walls, that meant that electricity was costly.  So as the start of the 20th century, there was massive investment in hydroelectric.  This basically covered demand until the 1960s, when the economic miracle took hold.

At that time, Spain turned to petroleum fired plants to cover new demand. At the time petroleum was a cheap alternative to coal. Understandably, they got shellacked come the oil crisis. There had been a a number of nuclear facilities planned at the start of the 1980s, but there was local opposition.  ETA got the idea that this was an issue that they could latch on to.  And....  they killed some of the workers at the planned Lemoniz plant. The government abandoned the expansion shortly thereafter.  At this time, there was a movement towards combined cycle gas plants during the  80s and 90s.  Again, gas was a cheap alternative.  That began to come to an end.  Finally, Spain moved to develop windpower in the late 1990s. The drive to build up renewables has a lot to do with the relative energy poverty of the Iberian Peninsula.  

And I'll give my consent to any government that does not deny a man a living wage-Billy Bragg

• large hydro: 22.0%
  
• nuclear: 81.8%
  
• coal: 40.8%
  
• combined cycle gas: 11.4%
  
• small hydro: 39.4%
  
• wind: 27.1%
  
• solar photovoltaic: 20.5%
  
• solar thermoelectric: ~24.3%
  
• biomass: 58.4%
  
• "non-renewable thermal" (apparent cogeneration): 51.3%

• Even with the big increase vs. last year, water scarcity seems to weigh on large hydro.
  
• Of the thermal plants, it seems biomass is the one where baseload provision is the most significant alongside scheduled variable load (which usually requires load factors around 40%).
  
• Both wind (which is all on-shore) and PV have impressive capacity factors. The latter is twice that in Germany, and only a fifth (5 percentage points) short of what Desertec expected in Africa.
  
• Solar thermal's capacity factor is not much above that of PV although as far as I know, installations are more concentrated on the best spots in southern Spain.

This page describes the "special regime" as

Las instalaciones de producción de energía eléctrica en régimen especial deberán tener potencia instalada igual o inferior a 50 MW y estar en alguno de estos grupos:

a) Instalaciones que utilicen cogeneración u otras formas de producción de energía eléctrica asociadas a la electricidad, con un rendimiento energético elevado.

b) Instalaciones que utilicen energías renovables no consumibles, biomasa, biocombustibles, etc.

c) Instalaciones que utilicen residuos urbanos u otros residuos.

d) Instalaciones de tratamiento y reducción de residuos agrícolas, ganaderos y servicios.

Electical power production facilities in the special regime must have a nominal power no greater than 50 MW and be in one of the following groups:

a) Facilities using cogeneration or other forms of production of high-yield electrical power.

b) Facilities using nonconsumable [?] renewable energies such as biomass, biofuels, etc.

c) Facilities using urban or other waste.

d) Facilities for the treatment and reduction of waste from agriculture, anumal husbandry or services.