  This paper is concerned with finite dimensional models for the entire term
structure for energy futures. As soon as a finite dimensional set of possible
yield curves is chosen, one likes to estimate the dynamic behaviour of the
yield curve evolution from data. The estimated model should be free of
arbitrage which is known to result in some drift condition. If the yield curve
evolution is modelled by a diffusion, then this leaves the diffusion
coefficient open for estimation. From a practical perspective, this requires
that the chosen set of possible yield curves is compatible with any obtained
diffusion coefficient. In this paper, we show that this compatibility enforces
an affine geometry of the set of possible yield curves.
