La energía solar y la eólica son recursos limpios que reducen el consumo de petróleo.

Spanish often omits a repeated noun and leaves just the article + adjective as shorthand.
Here energía is understood: la energía solar y la (energía) eólica.
This is the same pattern as la falda roja y la azul (the red skirt and the blue one) where falda is omitted the second time.

Yes: La energía solar y eólica son recursos limpios… is also correct.
Not repeating la makes solar and eólica just two adjectives of the same noun energía.
Repeating the article (la solar y la eólica) slightly highlights them as two distinct types, but in practice both versions are fine.

In Spanish, descriptive adjectives normally come after the noun: energía solar, coche eléctrico, casa grande.
Putting solar before energía (solar energía) sounds wrong in standard Spanish, except maybe in very marked or poetic language.
So you should generally remember: noun first, then adjective.

Spanish uses ser for permanent or defining characteristics and estar for temporary states.
Being clean resources is a defining property of solar and wind energy, so son recursos limpios (they are clean resources) uses ser.
Estar would be used for something like están sucios (they are dirty right now), which is a temporary condition.

Recursos is simply a different noun from energía; it’s not an adjective agreeing with it.
The structure is: La energía solar y la eólica (subject) son recursos limpios (predicate).
The predicate noun recursos has its own gender (masculine) and number (plural) and does not have to match the gender of energía.

Limpios agrees with recursos, not with energía.
In son recursos limpios, the adjective limpios must match recursos (masculine plural), so limpios is masculine plural.
If the sentence were La energía solar es limpia, then limpia would be feminine singular to agree with energía.

Que is a relative pronoun here and refers back to recursos limpios.
So recursos limpios que reducen el consumo de petróleo means clean resources that reduce the consumption of oil.
Because que refers to a plural noun (recursos), the verb is plural: reducen, not reduce.

No, not with this subject.
The subject of reducen is recursos limpios, which is plural, so the verb must be plural too: que reducen.
Que reduce would only work if the antecedent were singular, e.g. una tecnología limpia que reduce….

After consumo, Spanish normally uses de to show what is being consumed: consumo de agua, consumo de energía, consumo de carne.
So consumo de petróleo literally means consumption of oil.
Using a here would be incorrect; a is not used for this kind of noun–noun relationship.

El consumo de petróleo talks about oil in general, as a type of resource, so Spanish often omits the article in this noun + de + mass noun pattern.
If you say el consumo del petróleo, it tends to sound like you are talking about the consumption of that particular oil (more specific or already known oil), or about the oil sector in a more definite way.
For general, generic reference to a material (oil, water, energy), de petróleo without article is very common.

Petróleo is crude oil (and by extension, oil as a fossil fuel resource).
Gasolina is gasoline/petrol, a refined product made from petróleo.
In environmental or energy contexts in Spanish from Spain, petróleo is the usual word when talking about dependence on oil as a resource.

You actually can say Las energías solar y eólica son recursos limpios…, and it is grammatically correct.
The original sentence chooses to coordinate them as two specific instances: La energía solar y la eólica….
Both structures are acceptable; the version with la … y la … sounds very natural because it treats each energy type as a separate item in a list.

Descriptive adjectives normally follow the noun in Spanish, so recursos limpios is the neutral order.
Limpios recursos is grammatically possible but would sound unusual and very marked (more like literary or rhetorical style), and it could change the nuance.
For standard, neutral speech, keep adjectives like limpios after the noun: recursos limpios.

Spanish uses the present tense to state general truths and habitual facts, just like English: Los perros son mamíferos, El agua hierve a 100 grados.
Here, son recursos limpios and reducen el consumo de petróleo express general facts about solar and wind energy, so the simple present is the natural choice.