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There is an increasing need to model large networks of inverter-based distributed generation due to the limits of existing simulation tools. A deliberate enforcement of intra-feeder coherency between adjacent droop-controlled inverters is a worthwhile objective, as phase angle coherency will allow for a deliberate partitioning of fast and slow dynamics within the network. This partitioning, in turn, results in simpler and more tractable reduced-order network dynamics that are easier to model. This paper presents the adaptation of the generalized eigenvalue perturbation approach to the identification of coherent groups within grid-forming, droop-controlled inverter networks. For many networks, a rigorously-derived condition of coherency can be difficult to achieve, given the expected range of L-C-L filter and grid-tie impedances. To remedy this limitation, this paper investigates the use of multi-loop voltage regulation and virtual impedance to enforce the conditions that allow for coherency, and thereby more rigorously justify the use of simple nonlinear aggregate inverter models to accurately model large sections of the inverter network.

A droop-controlled, hybrid ac/dc microgrid represents a robust architecture that can coordinate the operation of multiple distributed sources while minimizing the power conversion stages. While the reduced-order nonlinear dynamics of grid-forming, droop-controlled inverter-based microgrids have been rigorously shown to be stable for a wide range of operating parameters, the problem of analyzing the transient stability characteristics of droop-controlled hybrid ac/dc architectures has not been adequately addressed. This work reviews the link between the Virtual Synchronous Machine control concept and grid-forming droop control, and introduces a new droop control strategy for ac/dc hybrid microgrids, termed ‘symmetric droop control’ (SDC). SDC better addresses the dynamic interactions between the ac and dc sub-grids of the hybrid microgrid, by ensuring that the interfacing inverter appropriately represents the dc sub-grid dynamics when interacting with other inverters. SDC can help to ensure that the reduced-order, nonlinear dynamics in both the ac and dc networks remain predictable and well-behaved during large disturbances in the ac network.

This work describes the development of a reduced-order nonlinear dynamic phasor modeling tool for distribution networks containing a large number of P-f and Q-v droop-controlled inverters. Attention is focused on constructing reduced-order models that accurately predict the nonlinear behavior of coherent clusters of droop-controlled inverters in response to a large disturbance, such as an asymmetric fault. In such a scenario, dynamic phasor models are shown to offer valuable insights into how the signal conditioning associated with local power measurement can have a significant effect on the large-signal stability of a droop-controlled inverter or inverter cluster. An algorithm conventionally used to identify coherent groups of synchronous generators is adopted for the investigation of coherency between large numbers of droop-controlled inverters within a distribution network. The coherency results help guide the aggregation of droop-controlled inverters and enable nonlinear reduced-order dynamic phasor modeling. For large disturbances, a reduced-order dynamic phasor simulation of a 50-inverter system is validated against full-order simulation results obtained using an OPAL-RT power system simulator.

Appendix G, the Performance Rating Method in ASHRAE Standard 90.1 has been updated to make two significant changes for the 2016 edition, to be published in October of 2016. First, it allows Appendix G to be used as a third path for compliance with the standard in addition to rating beyond code building performance. This prevents modelers from having to develop separate building models for code compliance and beyond code programs. Using this new version of Appendix G to show compliance with the 2016 edition of the standard, the proposed building design needs to have a performance cost index (PCI) less than targets shown in a new table based on building type and climate zone. The second change is that the baseline design is now fixed at a stable level of performance set approximately equal to the 2004 code. Rather than changing the stringency of the baseline with each subsequent edition of the standard, compliance with new editions will simply require a reduced PCI (a PCI of zero is a net-zero building). Using this approach, buildings of any era can be rated using the same method. The intent is that any building energy code or beyond code program can usemore » this methodology and merely set the appropriate PCI target for their needs. This report discusses the process used to set performance criteria for compliance with ASHRAE Standard 90.1-2016 and suggests a method for demonstrating compliance with other codes and beyond code programs.« less

Moving to the ASHRAE Standard 90.1-2013 (ASHRAE 2013) edition from Standard 90.1-2010 (ASHRAE 2010) is cost-effective for the State of Arkansas. The table below shows the state-wide economic impact of upgrading to Standard 90.1-2013 in terms of the annual energy cost savings in dollars per square foot, additional construction cost per square foot required by the upgrade, and life-cycle cost (LCC) per square foot. These results are weighted averages for all building types in all climate zones in the state, based on weightings shown in Table 4. The methodology used for this analysis is consistent with the methodology used in the national cost-effectiveness analysis. Additional results and details on the methodology are presented in the following sections. The report provides analysis of two LCC scenarios: Scenario 1, representing publicly-owned buildings, considers initial costs, energy costs, maintenance costs, and replacement costs—without borrowing or taxes. Scenario 2, representing privately-owned buildings, adds borrowing costs and tax impacts.

Moving to the ASHRAE Standard 90.1-2013 (ASHRAE 2013) edition from Standard 90.1-2010 (ASHRAE 2010) is cost-effective for the State of Oklahoma. The table below shows the state-wide economic impact of upgrading to Standard 90.1-2013 in terms of the annual energy cost savings in dollars per square foot, additional construction cost per square foot required by the upgrade, and life-cycle cost (LCC) per square foot. These results are weighted averages for all building types in all climate zones in the state, based on weightings shown in Table 4. The methodology used for this analysis is consistent with the methodology used in the national cost-effectiveness analysis. Additional results and details on the methodology are presented in the following sections. The report provides analysis of two LCC scenarios: Scenario 1, representing publicly-owned buildings, considers initial costs, energy costs, maintenance costs, and replacement costs—without borrowing or taxes. Scenario 2, representing privately-owned buildings, adds borrowing costs and tax impacts.

Moving to the ASHRAE Standard 90.1-2013 (ASHRAE 2013) edition from Standard 90.1-2010 (ASHRAE 2010) is cost-effective for the State of South Carolina. The table below shows the state-wide economic impact of upgrading to Standard 90.1-2013 in terms of the annual energy cost savings in dollars per square foot, additional construction cost per square foot required by the upgrade, and life-cycle cost (LCC) per square foot. These results are weighted averages for all building types in all climate zones in the state, based on weightings shown in Table 4. The methodology used for this analysis is consistent with the methodology used in the national cost-effectiveness analysis. Additional results and details on the methodology are presented in the following sections. The report provides analysis of two LCC scenarios: Scenario 1, representing publicly-owned buildings, considers initial costs, energy costs, maintenance costs, and replacement costs—without borrowing or taxes. Scenario 2, representing privately-owned buildings, adds borrowing costs and tax impacts.

Moving to the ASHRAE Standard 90.1-2013 (ASHRAE 2013) edition from Standard 90.1-2010 (ASHRAE 2010) is cost-effective for the State of Delaware. The table below shows the state-wide economic impact of upgrading to Standard 90.1-2013 in terms of the annual energy cost savings in dollars per square foot, additional construction cost per square foot required by the upgrade, and life-cycle cost (LCC) per square foot. These results are weighted averages for all building types in all climate zones in the state, based on weightings shown in Table 4. The methodology used for this analysis is consistent with the methodology used in the national cost-effectiveness analysis. Additional results and details on the methodology are presented in the following sections. The report provides analysis of two LCC scenarios: Scenario 1, representing publicly-owned buildings, considers initial costs, energy costs, maintenance costs, and replacement costs—without borrowing or taxes. Scenario 2, representing privately-owned buildings, adds borrowing costs and tax impacts.

Moving to the ASHRAE Standard 90.1-2013 (ASHRAE 2013) edition from Standard 90.1-2010 (ASHRAE 2010) is cost-effective for the State of Pennsylvania. The table below shows the state-wide economic impact of upgrading to Standard 90.1-2013 in terms of the annual energy cost savings in dollars per square foot, additional construction cost per square foot required by the upgrade, and life-cycle cost (LCC) per square foot. These results are weighted averages for all building types in all climate zones in the state, based on weightings shown in Table 4. The methodology used for this analysis is consistent with the methodology used in the national cost-effectiveness analysis. Additional results and details on the methodology are presented in the following sections. The report provides analysis of two LCC scenarios: Scenario 1, representing publicly-owned buildings, considers initial costs, energy costs, maintenance costs, and replacement costs—without borrowing or taxes. Scenario 2, representing privately-owned buildings, adds borrowing costs and tax impacts.

In order to ensure resilient and high power quality microgrid operation, P-f and Q-V microgrid droop control algorithms rely on the calculation of active and reactive power outputs based on locally-measured voltage and current. A low-pass filter (LPF) on the calculated power signal is often incorporated by design into the droop controller, but the complete effect of the LPF on microgrid dynamics has not been adequately studied in the literature. This paper analytically and experimentally investigates the impact of the power measurement LPF bandwidth on several important aspects of microgrid performance. These include the dynamics of the interaction between network swing modes, voltage regulator dynamics, and filter dynamics as the filter bandwidth is swept across a large range. Three different voltage regulators are studied. Additionally, the role of the controller's LPF characteristics on the reduced-order large-signal nonlinear dynamics of inverter-based microgrids is investigated using an equivalent electromechanical system. Using a Lyapunov energy function, a family of linear power measurement filters is identified that ensures stability for a grid-tied frequency-droop-controlled inverter.

This document lays out the U.S. Department of Energy’s (DOE’s) method for evaluating the cost-effectiveness of energy code proposals and editions. The evaluation is applied to provisions or editions of the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Standard 90.1 and the International Energy Conservation Code (IECC). The method follows standard life-cycle cost (LCC) economic analysis procedures. Cost-effectiveness evaluation requires three steps: 1) evaluating the energy and energy cost savings of code changes, 2) evaluating the incremental and replacement costs related to the changes, and 3) determining the cost-effectiveness of energy code changes based on those costs and savings over time.

Building energy codes have significantly increased building efficiency over the last 38 years, since the first national energy code was published in 1975. The most commonly used path in energy codes, the prescriptive path, appears to be reaching a point of diminishing returns. The current focus on prescriptive codes has limitations including significant variation in actual energy performance depending on which prescriptive options are chosen, a lack of flexibility for designers and developers, the inability to handle optimization that is specific to building type and use, the inability to account for project-specific energy costs, and the lack of follow-through or accountability after a certificate of occupancy is granted. It is likely that an approach that considers the building as an integrated system will be necessary to achieve the next real gains in building efficiency. This report provides a high-level review of different formats for commercial building energy codes, including prescriptive, prescriptive packages, capacity constrained, outcome based, and predictive performance approaches. This report also explores a next generation commercial energy code approach that places a greater emphasis on performance-based criteria. For commercial building energy codes to continue to progress as they have over the last 40 years, the next generation of more » building codes will need to provide a path that is led by energy performance, ensuring a measurable trajectory toward net zero energy buildings. This report outlines a vision to serve as a roadmap for future commercial code development. That vision is based on code development being led by a specific approach to predictive energy performance combined with building-specific prescriptive packages that are designed both to be cost-effective and to achieve a desired level of performance. Compliance with this new approach can be achieved by either meeting the performance target, as demonstrated by whole building energy modeling, or by choosing one of the prescriptive packages. This review of the possible code formats (further described in Section 2.1) arrives at the following conclusions: • Predictive performance with energy use index (EUI) targets falls short as a code mechanism, since it is difficult to match individual building use to broad EUI targets. • Outcome-based codes–while an essential approach that should be applied to all buildings–are not a substitute for design and construction energy codes that focus on compliance at occupancy. • For a design and construction code, a differential predictive performance method with a stable and independent baseline provides the best accuracy and potential for a highly automated approach that could eventually be applied to most buildings. • Current performance codes that have a dependent and time-variable baseline should be replaced by a differential predictive performance method with a stable and independent baseline. • At some point in the future, tools that demonstrate predictive performance compliance may become so simple that there will no longer be a need for any prescriptive path. • As a bridge, prescriptive packages can provide a transition from the current component prescriptive approach to a performance only code, while providing flexibility and improved energy equivalency. « less

This report and an accompanying spreadsheet (PNNL 2014a) compile the end use building simulation results for prototype buildings throughout the United States. The results represent he energy use of each edition of ASHRAE Standard 90.1, Energy Standard for Buildings Except Low-Rise Residential Buildings (ASHRAE 2004, 2007, 2010, 2013). PNNL examined the simulation results to determine how the remaining energy was used.

This report provides a final quantitative analysis to assess whether buildings constructed according to the requirements of ANSI/ASHRAE/IES Standard 90.1-2013 would result in improved energy efficiency in commercial buildings. The final analysis considered each of the 110 addenda to Standard 90.1-2010 that were included in Standard 90.1-2013. PNNL reviewed all addenda included by ASHRAE in creating Standard 90.1-2013 from Standard 90.1-2010, and considered their combined impact on a suite of prototype building models across all U.S. climate zones. Most addenda were deemed to have little quantifiable impact on building efficiency for the purpose of DOE’s final determination. However, out of the 110 total addenda, 30 were identified as having a measureable and quantifiable impact.

A reduced-order model is used to derive a candidate energy function for a system that includes an infinite bus connected with a single frequency-droop-controlled inverter (a ‘MicroSource’) whose frequency command is actuated according to its low-pass-filtered active power measurement. The analysis of the large-signal stability of such a system is important in understanding the duration that a frequency-droop-controlled inverter can withstand a particular grid transient before islanding or tripping offline becomes mandatory. It is shown that a large-signal instability for this system can occur under abnormal grid conditions. Additionally, for poor selections of droop gain and low-pass cutoff frequency, these control parameters can potentially have a significant effect on the ability of the system to maintain synchronism after a fault on the main grid is cleared. The proposed energy function is shown through simulation to provide insight on the impact of the frequency droop gain and filter time constant on this instability during and after faulted conditions, particularly for high levels of droop gain and/or measurement filter time constant.

Usability is an important topic in the field of telerehabilitation research. Older users with disabilities in particular, present age-related and disability-related challenges that should be accommodated for in the design of a user interface for a telerehabilitation system. This paper describes the design, implementation, and assessment of a telerehabilitation system user interface that tries to maximize usability for an elderly user who has experienced a stroke. An Internet-connected Nintendo(®) Wii™ gaming system is selected as a hardware platform, and a server and website are implemented to process and display the feedback information. The usability of the interface is assessed with a trial consisting of 18 subjects: 10 healthy Doctor of Physical Therapy students and 8 people with a stroke. Results show similar levels of usability and high satisfaction with the gaming system interface from both groups of subjects.

This report focuses on enhancements to prototype building models used to determine the energy impact of various versions of ANSI/ASHRAE/IES Standard 90.1. Since the last publication of the prototype building models, PNNL has made numerous enhancements to the original prototype models compliant with the 2004, 2007, and 2010 editions of Standard 90.1. Those enhancements are described here and were made for several reasons: (1) to change or improve prototype design assumptions; (2) to improve the simulation accuracy; (3) to improve the simulation infrastructure; and (4) to add additional detail to the models needed to capture certain energy impacts from Standard 90.1 improvements. These enhancements impact simulated prototype energy use, and consequently impact the savings estimated from edition to edition of Standard 90.1.

A second-life battery is an electric vehicle battery pack that has reached an end-of-life condition for its vehicular use, yet retains enough performance to be re-purposed for another application. One promising application of a second-life battery is stationary energy storage within a CERTS microgrid. This paper investigates the modeling of multiple paralleled traction battery packs within a CERTS microgrid, examining the impacts of elevated internal pack impedance on microgrid system operation. Impedance spectroscopy and hybrid-pulse power characterization are used to model vehicular Li-ion cells under a range of conditions that include second-life aging. The ac bus dynamics of the microgrid model are validated experimentally. Second-life battery models are incorporated into two CERTS microgrid architectures and system-level effects of changing the battery impedance are explored. Simulation results indicate that the modeled EV second-life batteries deliver promising performance characteristics in both CERTS microgrid architectures that were investigated.

The purpose of this analysis is to assess the relative energy and energy cost performance of commercial buildings designed to meet the requirements found in the commercial energy efficiency provisions of the International Energy Conservation Code (IECC). Section 304(b) of the Energy Conservation and Production Act (ECPA), as amended, requires the Secretary of Energy to make a determination each time a revised version of ASHRAE Standard 90.1 is published with respect to whether the revised standard would improve energy efficiency in commercial buildings. As many states have historically adopted the IECC for both residential and commercial buildings, PNNL has evaluated the impacts of the commercial provisions of the 2006, 2009, and 2012 editions of the IECC. PNNL also compared energy performance with corresponding editions of ANSI/ASHRAE/IES Standard 90.1 to help states and local jurisdictions make informed decisions regarding model code adoption.

A large Illinois oil field implemented the use of a new low cost manual tubing rotator in 25 marginal oil wells to reduce wear failures. Wear failures were indicated by "tubing splits" (Mechanical Wear1) or "corrosion occurring along a straight line due to corrosion inhibitor being wiped off from wear (Corrosion Due to Wear)". The failure rate was lowered from 203 repairs/year to 49 repairs/year (a 76% reduction) for "all failure mechanisms". Failures due to "Mechanical Wear or Corrosion Wear" were reduced from 108 repairs/year to 18 repairs/year (an 83% reduction). This success yielded over a 100% rate of return since the tubing rotator will normally pay out after eliminating the first pulling job and requires only 15 minutes per year of extra labor. Economics suggest that 100% to 20% investment rate of returns are achievable in wells failing once every 24 months or more often (not considering downtime losses). Thus, the manual rotator is not just for "problem" wells and significantly reduces wear failures.