A control method is provided for a cryogenic refrigeration system having an evaporator-heater coil, an electronically controlled valve for regulating the amount of cryogen to the coil, and a vapor motor driven by the cryogen that is coupled to both an alternator for recharging a battery, and a fan for generating a discharge air flow through the coil and into a conditioned space. The shaft rpms of the vapor motor is determined by a rectified output of the alternator, and the electronically controlled valve is incremented or decremented so that the shaft rpms stay above a preselected amount sufficient to both to recharge the battery of the system, and to circulate sufficient air in the conditioned space to avoid undesirable temperature non-uniformities therein. The pressure of the cryogen in the system is further continuously measured, and the electronically controlled valve is incremented or decremented in order to maintain a sufficient pressure to maintain the cryogen in a gaseous state. Finally, the difference between the measured temperature of the return air in the conditioned space and a set point temperature is determined, as well as the rate of change of the measured temperature and the acceleration or deceleration associated with the rate of temperature change. The electronically controlled valve is further incremented or decremented in accordance with a fuzzy logic routine partially on the basis of the temperature difference, the temperature rate of change, and the acceleration or deceleration of the temperature rate of change dependent upon how close the measured temperature is with respect to the set point temperature.