The two principal sources of musical energy are dissonance and sequence. On a large scale dissonance is by far the more powerful. To keep a piece going the early eighteenth century relied chiefly on sequence--a harmonic movement with the propulsive force of rhythmic repetition. The extension of dissonance to the level of the large structure, however, is largely the invention of sonata style and it is the dramatic tension of the prolongation of this dissonance with its balancing resolution that is the quality common to all the various sonata forms. [Sonata Forms, p. 244]Which sparks the question: so how the heck do you drive a piece forward in the absence of either of those elements? To clarify: it is dissonance and the resolution of dissonance that is a source of musical energy. As for sequence, this is the technique of setting up an harmonic pattern that repeats at different levels. Here is the Wikipedia article for reference. This technique is extremely common in Baroque music and if you want an example, just about any piece by Vivaldi will provide one. There is a nice sequence starting at the 12 second mark that is repeated at the 18 second mark. Another one starts around the 25 second mark.
Now the thing is that with the advent of 12-tone or serial music, the whole idea of dissonance and resolution was declared to be obsolete. The slogan was the emancipation of the dissonance as if dissonances were just some oppressed minority! Now we could still have sequence, though its harmonic foundation was eliminated so all that remained was melodic sequence in the form of repetition of the 12-note series in different forms. Since the level of dissonance remained constant and the "melodies" were characterless series of pitches, music largely lost any feeling of direction or energy--except that provided by texture and rhythm. Steve Reich showed how rhythm alone could almost compensate for the loss of the use of dissonance and sequence.
But the problem remains, how can musical energy be generated in the absence of dissonance and sequence? It is a bit like, how can you replace all the gas and coal-fired power generation with solar and wind power? And the answer is, you can't: not cheaply and not conveniently. But the "narrative" in both cases says, "of course we can."
This explains, of course, why so much contemporary music seems to lack any kind of direction. It is remarkable how subtle textures, orchestrations and rhythmic structures have enabled the creation of music that seems lacking in the dissonance/consonance aspect and the sequence aspect, but still is very successful. I am thinking of music by Stravinsky, Schoenberg and others. But it remains the case that, in the absence of any kind of narrative underpinning (as in the Rite of Spring) or other non-musical framework, longer pieces are extremely difficult to construct given these limitations. We are still searching for alternatives to dissonance and sequence. That is not surprising: the principles and techniques of dissonance and sequence took hundreds of years to develop and it was only a hundred years ago that they were tossed aside. It may be a while yet before we have discovered new ones.
No, writing a fifteen minute long piece for orchestra that consists of drones, flurries of rhythmic activity, changes of orchestral colour and crescendos does not actually have much in the way of real musical energy, though this kind of thing has won a few Pulitzer prizes lately.
And in the meantime, hey, we can always listen to hip-hop. Yeah, that's it!