Low Firing Temperature Absorption Chiller System

Abstract

Two types of the absorption chillers, the single and half effect cycles, can operate using low temperature hot water. The advantage of the single effect over the half effect cycle is the higher COP but the single effect has a narrower temperature operating range. The half effect has the capability of using lower hot water temperature but the lower COP increases operating cost and requires investing a larger cooling tower. A detailed computer model was written for the single and half effect cycles based on heat transfer coefficients for the inside and outside tubes of each component [generator, absorber, condenser, and evaporator], energy, mass, salt balances, and rate equations. The single effect component model was calibrated with known data from a US absorption chiller manufacturer. The cooling tower was modeled using the analogy approach, calibrated and validated with performance data from a cooling tower manufacturer. Capacity and dollars per ton were used to determine lower limits on the firing temperature. The results show that at 225oF (107 C) at 2000 gpm (7570 L/min) of hot water, the cost to maintain capacity starts to change for the single effect cycle. At 205oF (99 C) the cost to maintain capacity increases rapidly. The half effect cycle can maintain capacity at temperatures as low as 185oF (82 C) hot water at 2000 gpm (7570 L/min) without a large increase in capital investment. The capital cost for the half effect chiller system is 200 $/ton (57 $/kW) more than the single effect, using hot water temperatures above 200 oF (93 C). The single effect cycle can only be competitive with an electric centrifugal chiller if the heat source is free or a combination of high electrical cost with a low cost of heat. The half effect cycle can be competitive with the single effect if the waste heat is free and the temperature is below 200oF (93 C) or has a low flow rate in the range of 1000 gpm (3785 L/min).