Paper Conference

Proceedings of SimBuild Conference 2004: 1st conference of IBPSA-USA


Variable Heat Recovery in Double Bundle Electric Chillers

Richard J. Liesen, Rahul J. Chillar
University of Illinois at Urbana Champaign, Dept of Mechanical & Industrial Engineering, 1206 W.Green St, Urbana, IL 61801. USA

Abstract: In programs such as BLAST and DOE-2, the Double Bundle Electric Chiller model uses fixed input values for determining the recoverable heat component. The drawback of this approach is no dependence on system demands, plant, and cooling tower loads. Determining the availability or gradation (quality of heat) of the recoverable heat, is a subjective user input, and independent of the flow rates and temperatures. In a program like EnergyPlus, the plant and heat recovery equipment is now connected with loops having mass flow rates and temperatures, allowing for a more sophisticated simulation. Execution speed still restricts the chiller simulation to empirical models that give the total condenser heat as an output for the cooling tower and heat recovery loops. A new algorithm that bases the recoverable heat fraction on the flow rates and inlet temperatures has been developed. This algorithm uses relatively simple inputs to determine the fraction of the heat recovered and the heat rejected by the cooling tower. The variations in the mass flow rates and temperatures in the simulation are now used to obtain more realistic heat transfer rates of each component. This paper presents an analysis of the new algorithm for determining the heat recovery factor, and presents examples showing its performance and advantages of this model over the simple model. The electric chiller in EnergyPlus now has the option of having its condenser hooked up to a heat recovery loop, or what is commonly known as a double bundled chiller.
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