Development of 20 integrated energy efficiency ratio rooftop units—system modeling and building energy simulations

Abstract

Based on a detailed steady-state system and component modeling, a rooftop unit system design was developed that is can achieve an integrated energy efficiency rating higher than 20. Fin-and-tube and microchannel heat exchangers were modeled using a segment-to-segment approach, and an AHRI 10-coefficient compressor map used to simulate compressor performance. The system modeling is based on a component-based modeling approach, which facilitates flexible simulation of complicated system configurations. Starting with a baseline system having integrated energy efficiency rating of 16.6, numerous technical options were extensively investigated, i.e., varying compressor sizes, heat exchanger fin densities, fin-and-tube or microchannel heat exchanger, suction line heat exchanger, desiccant wheel, tandem compressor (TD), variable-speed compressor (VS), and condenser evaporative pre-cooling; an innovative system configuration was developed by combining a tandem compression system with a variable-speed compression system. The combined system can achieve a high integrated energy efficiency ratio as well as process the outdoor ventilation air over an extensive range. The design concept for a 20-ton (70.4-kW) unit, as well as a 10-ton (35.2-kW) unit was successfully evaluated. All selected components are readily accessible on the market, and performance predictions were validated against existing rooftop unit products at the rating condition. This article illustrates a potentially cost-effective high integrated energy efficiency ratio rooftop unit design. In addtion, extensive building energy simulations were conducted using EnergyPlus to predict seasonal energy saving potentials and peak power reductions using the high integrated energy efficiency ratio rooftop unit in 16 U.S. cities in comparison to a rooftop unit with a minimum efficiency.