Hybrid Unitary HVAC System Model Integration with EnergyPlus

Cooling and ventilation are responsible for nearly 30% of the electricity consumption in California commercial buildings (CEUS 2006). In some climates, cooling and ventilation can account for more than 70% of the primary fuel consumption associated with commercial buildings (Chua 2013). Furthermore, daily and seasonal variations in outdoor temperature impose shifting demands upon the electricity grid, which are challenging and costly to manage. During peak periods of electricity use, air conditioning can account for more than half of the total demand (CEUS 2006). Hybrid air conditioners offer a solution for reducing cooling and ventilation electricity consumption. Hybrid air conditioners are those which include various subcomponents to improve performance for particular climates, and building conditioning needs. In hot and dry climates like California, hybrid air conditioners generally include some form of evaporative cooling, providing a low-energy supplement to standard vapor-compression cooling. Evaporative cooling has the most benefit when temperatures are hot, so these systems are most beneficial during peak periods. The goal of this project was to develop modeling tools for appropriately simulating hybrid air conditioning equipment, and demonstrate the tools by evaluating an evaporative retrofit package (Retrofit A) for Roof-Top Units (RTUs) on multiple building types and in various climate zones. Specific project objectives were: -Develop an EnergyPlus object for simulating a hybrid air conditioner’s capacity, energy use, and operating mode. -Simulate Retrofit A using the EnergyPlus model on multiple building types and in various climate zones. -Determine the overall energy and peak demand savings of Retrofit A compared to the same unit without the retrofit. We developed a model that uses detailed performance data from an example unitary system as an input to EnergyPlus. This “black-box” approach was determined to be most suitable, based on the number of possible configurations and component arrangements hybrid systems may have. While there are some advantages to a component-based model, each part of a hybrid system would need to be independently tested, or have a first-principles model developed to accurately predict performance. To avoid this limitation, a black-box method was implemented. The National Renewable Energy Laboratory (NREL) has not yet officially released our model for public use. At this point, some errors still occur when simulating hybrid equipment, so we could not validate the model’s evaporative retrofit system performance data. However, this project achieved several accomplishments that advanced the capability of modeling hybrid equipment. This project has largely completed development of the EnergyPlus object for simulating hybrid HVAC systems. The remaining errors the model produces appear to be caused by the EnergyPlus interpolation method, and NREL must correct the error. This project also developed hybrid equipment resources for the Technology Performance Exchange (TPEX), including a new technology class, “Hybrid Unitary HVAC,” which modelers can use to access hybrid equipment data. TPEX allows equipment manufacturers and third-party evaluators to submit the necessary input data to model their hybrid systems. Once the hybrid model is fully integrated into EnergyPlus and released to the public, WCEC recommends hybrid systems with available performance data be simulated in SCE territory, to help develop appropriate programs around this equipment. The simulations will provide guidance on what type of systems perform better in particular circumstances, or in specific climate regions. There should also be an effort to encourage manufacturers to provide system performance maps. Since the model tool relies on detailed performance data, systems must be available in TPEX, if modeling is desired. There is a significant effort involved in obtaining the necessary data to describe system performance appropriately, so manufacturers may need incentives to do so.