Design and In-Situ Performance of Vapor Compression System Project

Space-conditioning equipment in buildings accounts for 37% of the Nation’s primary energy consumption in buildings and 15% of the Nation’s overall primary energy consumption. For this reason, reaching the goal of net-zero energy buildings requires high-efficiency air conditioner and heat pump designs and ensuring that the equipment operates properly. This project facilitates efficient operation over the lifetime of space-conditioning equipment by providing a comprehensive evaluation of the effects of improper installation on energy consumption of heat pumps for improving installation practices.  This project will also provide industry with simulation based design tools for vapor compression systems, the dominant technology for comfort space conditioning and refrigeration. This effort will include the development of measurement science for determining air flow distributions through finned-tube heat exchangers and optimizing their designs to ensure efficient operation.

Objective:  By FY2013, develop and deploy the measurement science to (1) support and improve commissioning methods for residential space-conditioning systems, and (2) create novel simulation and optimization tools for designing highly efficient vapor-compression cooling and heating equipment in a cost effective manner.

What is the new technical idea?  NIST will perform a sensitivity study of quality installation parameters for space-conditioning systems. Proper commissioning of this equipment is essential to ensure that it operates efficiently and with minimal maintenance cost. Previous investigations indicated common installation errors (faults) and documented considerable increase in energy use as a result of these faults. However, so far these effects were studied independently. It is unclear whether these inefficiencies are additive, whether small variances within a given installation parameter are significant, and which installation parameters in various applications and geographical locations have a larger impact than others. The new idea in the proposed study is to combine building effects, equipment effects, and climate effects in a comprehensive evaluation of the effects of improper installation on energy consumption of space-conditioning equipment. Combining these effects will provide more realistic information on electrical energy consumption and will allow prioritization of different commissioning steps for different climates.

NIST will also develop a methodology and simulation tool for designing high-efficiency air-conditioning and heat pump systems with optimized finned-tube, air-to-refrigerant heat exchangers. The novelty of the NIST approach is based on using particle image velocimetry (PIV) to accurately characterize the air velocity distribution through a finned-tube heat exchanger and applying computation intelligence-based optimization methods to maximize heat exchanger’s performance. This effort will include extensive laboratory PIV and thermal measurement, development of the novel computational algorithms, and development of component and system level simulation models. The developed design tools will allow achieving cost-neutral efficiency improvements for the space-conditioning equipment, directly supporting the National Science and Technology Council’s R&D Agenda for High-Performance Green Buildings.^[1]

What is the research plan?  The objective of this project is supported by three tasks.

Task 1 examines the performance sensitivity of the installation of an air-to-air heat pump providing indoor comfort over a full calendar year. In the FY2011 laboratory effort, NIST characterized heat pump performance degradation in cooling and heating modes due to single installation faults. The FY2012 effort started with building a model for a residential heat pump system and extensive simulations of yearly energy use by improper installation. Heat pump faults and air distribution faults (leaky ducts, airflow misbalance) were considered at different individual fault levels. Energy use simulations were performed for four U.S. climates. The FY2013 effort will entail characterization of heat pump performance under multiple faults, annual simulations for multiple fault scenarios, and an extensive data analysis. The ultimate aim of the sensitivity study is to verify requirements in the Air Conditioning Contractors of America (ACCA) Standard 5^[2] and to provide recommendations for possible modifications. This study of quality installation issues is aligned with the measurement science priorities for Net-Zero Energy Buildings (NZEB)^[3], and its results will be the U.S. contribution to the International Energy Agency’s Annex 36^[4].

Task 2 is the simulation aspect of this project centered on EVAP-COND, a simulation model for finned-tube heat exchangers. In FY2011, EVAP-COND was equipped with an Artificial Intelligence-based module ISHED (Intelligent System for Heat Exchanger Design) which maximizes heat exchanger effectiveness by optimizing the refrigeration circuitry, and was placed on the NIST website. In FY2012, the ISHED module underwent modifications to improve the robustness and the manufacturability of automatically generated refrigerant circuitry designs. The option of the “hair pin” designs was finalized in the 2012 version of EVAP-COND. In FY2012, different approaches to automated simplification of complex refrigerant circuitries were also investigated. In FY2013, upgraded evaporator and condenser models from the EVAP-COND package will be implemented in the HPSIM heat pump model.

Task 3 involves an extensive laboratory test effort to demonstrate efficiency improvements realized by optimizing refrigerant circuitry with ISHED. The current case study is a collaborative effort with an equipment manufacturer of a 7.5 ton rooftop unit (RTU). In FY2012, the performance of this RTU was tested and its evaporator air distribution was measured using PIV. The air flow distribution data were used to redesign the evaporator circuitry using ISHED, the resulting design was prototyped by the manufacturer, and the unit with the optimized evaporator will be tested by the end of FY2012. A similar study using a window air conditioner was started in FY2012 and will continue during FY 2013. Efficiency improvements obtained through EVAP-COND/ISHED are cost neutral; hence they directly support the goal of net-zero energy buildings^[5].

 

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[1] National Science and Technology Council, Committee on Technology, Federal Research and Development Agenda for Net-Zero Energy, High Performance Green Buildings, October 2008. http://www.whitehouse.gov/sites/default/files/microsites/ostp/nstc-netzero-2008.pdf

[2] ANSI/ACCA Standard 5 QI-2010, “HVAC Quality Installation Specifications; Residential and Commercial Heating, Ventilating and  Air Conditioning (HVAC) Applications”, Air Conditioning Contractors of America, Arlington (ACCA), VA

[3] Measurement Science Roadmap for Net-Zero Energy Buildings, Workshop Summary Report,  NIST Technical Note 1660, National Institute of Standards and Technology, March 2010, Exhibit VII.2 and VII.3

[4] International Energy Agency Heat Pump Program, Annex 36: Quality Installation/Quality Maintenance Sensitivity Studies. 15 December 2010.

[5] Griffith, B., Long, N., Torcellini, P., Judkoff, R., Crawley, D., Ryan, J., 2007. Assessment of the Technical Potential for Achieving Net Zero-Energy Buildings in the Commercial Sector; NREL Report No. TP-550-41957.

 

Recent Results:

Outputs:

The outputs from the previous three years of the project include: two peer reviewed journal publications, including one invited paper for a special issue, five conference papers, one NIST publication, and one publicly released software package.

Outcome:

Manufacturers provided with means to improve heat exchanger capacity by 5 % and a corresponding increase in equipment efficiency without additional manufacturing cost using detailed air flow knowledge and AI design software.

Impact:

HVAC&R equipment manufacturers use EVAP-COND/ISHED to design high-efficiency space-conditioning systems (over 6000 downloads to date).

Standards and Codes:

The work is expected to result in recommendations for improved heat pump installation practices and a revision of ANSI/ACCA Standard 5. This standard is included in EPA’s specifications for the quality installation requirement to obtain the Energy Star designation for residential buildings and is planned to be included in the California Title 24.