  The Faddeev-Volkov solution of the star-triangle relation is connected with
the modular double of the quantum group U_q(sl_2). It defines an Ising-type
lattice model with positive Boltzmann weights where the spin variables take
continuous values on the real line. The free energy of the model is exactly
calculated in the thermodynamic limit. The model describes quantum fluctuations
of circle patterns and the associated discrete conformal transformations
connected with the Thurston's discrete analogue of the Riemann mappings
theorem. In particular, in the quasi-classical limit the model precisely
describe the geometry of integrable circle patterns with prescribed
intersection angles.
