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As it is, the Biomass isn't a source of energy - it's a raw material.
The hive ships themselves, IIRC, never actually land - that would be inefficient, and I'm entirely sure that the Hive Mind would be more inclined to simply engineer a creature to do the heavy lifting for it. Further, it isn't just Biomass that the Tyranids take - worlds consumed by the Tyranids are reduced to cold, lifeless, airless rocks, as all biomass, all natural resources. the water, and the atmosphere itself are all taken for later use.
When it comes to generating energy, I'm sure there are plenty of biological and chemical methods - using the oxygen taken from atmospheres and surface water, and the starlight it'll get as it passes through a system, to photosynthesise, or 'recharging' biological batteries by taking geothermal energy from the planets they devour. Chemical reactions generated by specialised symbiote-creatures that equate to bioplasma 'generators' or something similar.
Given that a Tyranid invasion takes everything it can from a planet - even rendering its own armies to the constituent biomass (few Tyranid soldier-organisms have much in the way of internal organs, and most lack a dedicated digestive system, so their natural lifespan will be a matter of weeks anyway) - there are planety of potential ways for a Tyranid Hivefleet to gather the energy and materials needed to sustain itself between 'meals'.
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. Also Tyranids have slightly different needs for support and logistics. Few Dominatrixes and/or Synapse nodes with the army and they can get their new troops without any external support.