Evaluation of Ground-Source Variable Refrigerant Flow System for U . S . Office Buildings

This paper evaluates the energy performance of ground-source variable refrigerant flow (VRF) systems to condition office buildings located in various U.S. climates. Specifically, the performance of the ground-source VRF systems was determined and evaluated against that achieved by conventional space heating and cooling systems, including packaged terminal air-conditioners (PTACs), water-source heat pumps (WSHPs), ground-source heat pumps (GSHPs), and water-source VRF systems. A comparative analysis shows that ground-source VRF systems require significantly lower source energy uses than other heating and cooling systems in all U.S. climates, ranging from 21% to 50% for PTACs, from 36% to 52% for WSHPs, from 22% to 49% for GSHPs, and from 4% to 19% for water-source VRFs. These results indicate that ground-source VRFs can be suitable heating and cooling systems for all U.S. climates when designing high-energy-performance commercial buildings.


Introduction
According to the U.S. Energy Information Administration (EIA), the energy use and electricity consumption by the building sector accounted for 39% and 74% of, respectively, the total U.S. energy use and the total U.S. electricity consumption during 2016 [1].In addition, historical EIA data indicate that the energy consumption attributed to heating, ventilating, and air-conditioning (HVAC) systems can be significant.Indeed, the average energy consumption for HVAC systems accounts for 44% of the total U.S. building energy use [2].In particular, HVAC systems were responsible for, respectively, 18% and 31% of the total national energy and electricity consumption during 2016.Therefore, there is a need to utilize energy-efficient HVAC systems in order to reduce the energy consumption and the carbon footprint of the building sector not only in the United States, but also worldwide.Variable refrigerant flow (VRF) systems have been increasingly considered as energy-efficient alternatives to conventional HVAC systems [3][4][5][6][7][8][9][10][11][12][13].A VRF system is a heat pump system that utilizes a refrigerant as a heat transfer medium between a single condensing unit and multiple air terminal units.Two categories of VRF systems are generally available depending on their condensing medium type: air-source VRFs or water-source VRFs.Indoor air terminal units connected to a single condensing unit can have different capacities and configurations, allowing individual zone control and simultaneous space heating and cooling [8].The documented attractive benefits of VRF systems include the following: -Higher seasonal energy efficiency as a result of the high part-load performance of single or multiple variable speed compressors.
-Reduced ductwork, as indoor units are installed near or inside thermal zones.The short ducts result in lower fan capacities and reduced fan energy use.-Lower energy requirements as a result of using a refrigerant as the heat transfer fluid instead of water or air.-Reduced operation and maintenance costs, as different indoor units can operate simultaneously in heating mode or cooling mode even when connected to a single condensing unit.
Figure 1 illustrates the main components of a typical water-source VRF system connected to a heating source (such as a boiler) and a cooling source (cooling tower) to serve various terminal units in order to heat and cool different thermal zones within a building.In order to improve the energy efficiency of the VRF system, a ground medium can be utilized to provide both heating and cooling sources similarly to ground-source heat pumps (GSHPs) [14].Figure 2 presents the various elements of a ground-source VRF (GS-VRF) system.As noted in Figure 2, the use of boilers and cooling towers required for the conventional water-source VRF systems may not be needed for a GS-VRF system, possibly resulting in lower installation costs.The analysis of the performance of air-and water-source VRF systems has been reported widely in the literature [3][4][5][6][7][8][9][10][11][12][13].For instance, Goetzler discussed through case studies some of the benefits of VRF systems, including ease of installation, low maintenance costs, improved thermal comfort with individual set-point control, and increased energy efficiency [3].Specifically, Goetzler indicated that VRF systems could reduce HVAC energy consumption by 30% to 40% compared to rooftop variable-air-volume (VAV) systems.Similarly, Thornton et al. found that VRF systems could achieve HVAC energy savings ranging from 30% to 60% compared to other conventional HVAC systems [7].In particular, VRF systems have (i) lower cooling energy by a range of 30% to 50% compared to air-cooled chiller and unitary air-conditioning systems; (ii) lower heating energy by up to 75% compared to gas furnaces, boilers, and VAV systems with electric reheats; and (iii) lower fan energy ranging from 25% to 75% compared to conventional constant-air-volume systems.Koh et al. compared the cooling energy consumption and electrical peak demand of VRF systems to those obtained for conventional chiller-based VAV systems and packaged VAV (PVAV) systems for a typical light commercial building [8].Koh et al. concluded that VRF systems can have lower peak electrical demand than VAV and PVAV systems by respectively 40% and 30% and can save operating energy use by 47% compared to VAV systems and by 40% compared to PVAV systems.Kim et al. also compared VRF systems to VAV systems for a prototypical medium-office-building model developed by the U.S. Department of Energy (DOE) for various U.S. climate conditions [9].Kim et al. found that VRF systems saved between 15% and 42% in terms of site energy and between 18% and 33% in terms of source energy associated with HVAC equipment, compared to those achieved for VAV systems for all climate conditions.Zhou et al. performed energy analysis of VRF systems for office buildings using a whole-building energy simulation program and compared the energy consumption of VRF systems to VAV systems and fan-coil plus fresh air systems (FPFAs) [10].On the basis of their analysis results, Zhou et al. found that VRF systems saved 22.2% and 11.7% in building energy consumption relative to, respectively, VAV and FPFA systems.Using an experimental testing approach, Im et al. evaluated the energy performance of a VRF system and a rooftop VAV unit for a multi-zone building with emulated office occupancy [11].The testing analysis showed that the VRF system could reduce cooling energy consumption compared to the rooftop VAV unit by 29%, 36%, and 46% under 100%, 75%, and 50% thermal load conditions, respectively.Liu et al. assessed the energy-efficiency levels of both air-source VRF systems and GSHPs to heat and cool small office buildings located in Chicago and Miami [12].Unlike other reviewed studies, Liu et al. found that GSHPs were more energy efficient than air-source VRF systems for both U.S. climates (Miami and Chicago).Finally, Aynur et al. used experimental data as well as simulation analyses to compare the energy performance of VRF systems to VAV systems to air condition an existing office building [13].The analysis results indicated that the VRF systems provide energy-use savings ranging from 27.1% to 57.9% compared to the VAV systems.
Compared to air-source VRF systems, limited studies have been published to evaluate the performance of GS-VRF systems.Thornton et al. [7] mentioned that a ground source connected to a water-source VRF system would provide large energy savings, although it would require additional costs for ground-source borehole wells without detailed analysis.Karr [15] evaluated the energy performance of GS-VRF systems as well as other HVAV systems to maintain thermal comfort in a single-story assisted living building under various climatic conditions [15].Specifically, Karr found that GS-VRF systems could achieve energy savings that ranged from 32% to 41% relative to air-source heat pumps.In addition to the fact that the selected structure is not representative of commercial buildings, the modeling details for GS-VRF systems and other HVAC systems are not provided in Karr's analysis, including energy-efficiency indicators and performance curves.A more recent study by Im et al. used measured data and simulation analysis to evaluate the performance of a GS-VRF system to heat and cool a university building located in Rochester, MI [16].The field-testing data indicated that pumps and indoor units accounted for, respectively, 16% and 33% of the total energy consumed by the GS-VRF system.The simulation analysis compared the energy performance of the GS-VRF system to that achieved by two configurations of VAV system with hot water reheat: (i) the first configuration consisted of an air-cooled chiller and a gas-fired boiler that were rated on the basis of the minimum energy-efficiency levels of the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Standard 90.1-2010; (ii) the second configuration had a chiller and a boiler that had the highest efficiency then available on the market.The analysis results showed that the GS-VRF system achieved source energy savings of 33% and 29% compared to VAV system configurations (i) and (ii), respectively.However, the study of Im et al. was limited to one U.S. climate and lacked modeling details for GS-VRF systems.
The study presented in this paper fills the need for a comprehensive evaluation of the energy performance of GS-VRF systems compared to other types of VRF systems, GSHPs, and VAV systems for a wide range of climates to assess its suitability as an energy-efficient HVAC system for U.S. office buildings.First, the analysis methodology is described, including the details for the office-building model, the performance curves, and energy-efficiency parameters for the VRF systems and GSHPs.Then, the analysis results are summarized in terms of both total energy consumption and electrical peak demand for all HVAC systems and two office-building design configurations.Finally, general guidelines are provided to assess the suitability of GS-VRF systems to heat and cool U.S. office buildings.

General Analysis Approach
The analysis carried out in this study utilized a whole-building energy simulation tool to model and evaluate the energy performance of various HVAC systems, including GS-VRF systems, to maintain thermal comfort within a medium-size office building.Specifically, and in order to account for the dynamic impact of various building systems on heating and cooling thermal loads, an hourly whole-building energy modeling (BEM) tool, the DOE-2 energy simulation engine, was utilized to assess the performance of the various HVAC systems considered in the study [17].Energy consumption data obtained from a U.S. office building were first used to calibrate the energy model considered in the simulation analysis.Then, the calibrated energy model was utilized to develop office-building models for various U.S. climates in order to assess the performance of GS-VRF compared to other HVAC systems.Figure 3 illustrates the various data, simulation tools, and analysis techniques used in the study.For the whole-building energy analysis, the DOE-2 simulation engine was utilized because it can model various HVAC systems, including VAV systems, GSHPs, water-source VRFs, and GS-VRF systems.In particular, DOE-2 utilizes the G-function technique to model the thermal response of the ground medium associated with GSHPs and GS-VRF systems depending on the design characteristics and thermal properties of boreholes [18].For this study, Table 1 summarizes the data used to model the ground-source heat exchangers and boreholes for both GSHPs and GS-VRF systems.

Building Energy Models
The building energy models considered in the study are specific to medium-size office buildings.As an initial step in the development of the energy models, an existing office building located in Barberton, Ohio was selected to establish a calibrated building energy model using the DOE-2 simulation analysis tool.The existing office building was initially built as an education building in 1956 and later in 2015 was renovated and converted to an office building.Table 2 presents a summary of the main features of the baseline office-building model considered in this study.Figure 4 illustrates a three-dimensional (3D) rendering of the building energy model.As a baseline system, the office building uses packaged terminal air-conditioners (PTACs) for space cooling and unit ventilators served by hot-water boilers for space heating.Figures 5-7 illustrate the hourly occupancy schedules, lighting schedules, and equipment schedules used for the office-building modeling analysis.Using 1 years' worth of electricity and natural gas consumption data, the building energy model was calibrated by comparing the utility data to the predicted energy-use results obtained from the hourly simulation energy modeling.The calibration process was based on criteria set by the ASHRAE Guideline 14 using the coefficient of variance of the root-mean-square error (CV-RMSE) method as described by Equation (1) [19].The calibrated model was obtained by adjusting operating input variables for the energy model, including occupancy, lighting, and equipment schedules.After the calibration, CV-RMSE values of 8% for electricity use and 15% for natural gas consumption were reached.These resulting CV-RMSE values were within the criteria set by ASHRAE [19].Figures 8  and 9 compare the monthly electricity use and natural gas consumption on the basis of the utility data and those predicted by the hourly simulation tool. (1)

Evaluation of GS-VRF System
As noted earlier, one of the benefits of VRF systems is their high energy performance under part-load conditions.Figure 10 illustrates the part-load performance curves for a water-source VRF system considered in this study obtained under both heating and cooling modes [20].As indicated in Figure 10, the performance curves were similar for both the heating and cooling modes.Moreover, the impact of part-load was rather small on the energy efficiency of the VRF system, particularly for high-part-load ratios [20].Figures 11 and 12 provide, respectively, the energy efficiency and the capacity curves for the same water-source VRF system of Figure 10 as a function of condensing medium temperature [20].The system capacity and energy input were normalized by the rated size and the rated energy efficiency as defined by ASHRAE Standard 90.1-2010 as well as by Air-Conditioning, Heating, & Refrigeration Institute (AHRI) Standards 1230-2010, 320-98, and 325 [21][22][23][24].The rated conditions set by the AHRI standards for water-source VRF and GS-VRF systems are summarized in Table 3.In particular, the rated conditions for the closed-loop GS-VRF systems consisted of an entering water temperature (EWT) of 25 • C for the cooling operation and 0 • C for the heating operation, while the EWT was set at 30 • C during the cooling operation and at 20 • C during the heating operation for the water-source VRF systems.A dedicated outside air system (DOAS) is considered to provide the fresh air requirements.Table 3. Summary of performance data for water-source variable refrigerant flow (VRF) and ground-source variable refrigerant flow (GS-VRF) systems [20].

System Water-Source VRF (WS-VRF) Ground-Source VRF (GS-VRF)
Cooling efficiency 5.20 COP at EWT of 30 In order to evaluate the performance of GS-VRF and water-source VRF systems to heat and cool office buildings in the United States, other HVAC systems were considered, including the baseline heating and cooling systems defined by ASHRAE Standard 90.1, as summarized in Table 4 [21].

Climate Zones
In order to evaluate the impact of climate conditions on GS-VRF and water-source VRF systems, various representative U.S. climate zones were considered [25].Table 5 summarizes the design heating and cooling temperatures and average deep-ground temperatures for 11 locations representing different U.S. climate zones.Hourly weather data of each city listed in Table 5 were obtained for BEM using the typical meteorological year 2 (TMY2) data format [26].For each climate zone, the envelope characteristics of the baseline office-building model of Table 2 were adjusted on the basis of the minimum requirements set by ASHRAE Standard 90.1, as indicated in Table 6 [21].

High-Performance Buildings
In addition to the baseline models for the office buildings set to meet the requirements of ASHRAE Standard 90.1 as outlined in Tables 2 and 6, this study evaluates the performance of VRF systems for high-performance office buildings designed on the basis of ASHRAE Standard 189.1 for various representative U.S. climate zones.In particular, Tables 7 and 8 summarize the building envelope characteristics and the HVAC energy-efficiency settings considered for the high-performance office-building models used in the analysis for various climate zones [27].In addition, the high-performance models had a lighting power density of 9.26 W/m 2 and were equipped with daylighting controls and occupancy sensors [27].

Energy Performance of GS-VRF Systems Compared to Other HVAC Systems
For the baseline office-building energy models developed for each climate zone established from the calibrated model of the existing office building as discussed in Section 2.2, a series of simulation analyses was carried out to compare the energy performances of water-source VRF and GS-VRF systems to those obtained for the other HVAC systems listed in Table 4. Figures 13 and 14 provide the monthly energy consumption for electricity and natural gas, respectively, for all the HVAC systems considered in the analysis.Typical hourly building electricity-use profiles for a peak cooling day are shown in Figure 15 for all the HVAC systems.Table 9 lists the annual energy end-uses for the various HVAC systems considered to heat and cool the existing office building located in Barberton, OH (climate zone 5A).As clearly shown in Figures 13-15 and Table 9, GS-VRF systems had the lowest energy consumption when compared to all HVAC systems.In particular, GS-VRF systems achieved the highest energy-use savings of 58.3% relative to the PTAC units.Moreover, the GS-VRF system reduced the annual HVAC energy consumption by 22.1% relative to GSHPs.As noted earlier, GS-VRF as well as water-source VRF systems have better energy performance at low loads, particularly when compared to PTAC units that utilize electric direct expansion (DX) cooling coils and natural-gas-fired boilers.In addition, ground-source systems (i.e., GSHPs and GS-VRF systems) required less energy to heat and cool the office building when compared to water-source systems (i.e., WSHPs and WS-VRFs).Indeed, while water-source systems need to use cooling towers for the heat sink and hot-water boilers for the heat source, ground-source systems utilize the ground medium as both the heat source and sink.Moreover, the condensing water temperature of ground-source systems is set to be lower than that of water-source systems during space cooling mode.Table 10 summarizes the annual HVAC energy costs of the existing office building in Barberton, OH.The cost analysis shown in Table 10 is based on 2016 electricity and natural gas price rates provided for Ohio [28,29]: specifically, an electricity rate of $26.864 per GJ and a natural gas rate of $4.932 per GJ.As indicated in Table 10, the HVAC energy-cost savings by the GS-VRF system to heat and cool the existing office building in Barberton, OH were 23% relative to the PTAC, 39% relative to the WSHP, 29% relative to the GSHP, and 7% relative to the water-source VRF.These energy-cost savings were generally due to the different energy prices for the heating source (i.e., natural gas) and cooling source (i.e., electricity).The energy costs associated to heating were significantly lower than those associated to cooling, resulting in lower overall energy-cost savings than site energy-use savings.Table 11 summarizes the peak energy demand incurred by all the HVAC systems to maintain indoor thermal comfort for the office building in Barberton, OH.As shown in Table 11, both the water-source VRF and GS-VRF systems had a lower peak energy demand than the other HVAC systems as a result of their high energy performance, including lower power requirements for VRF condensing units and air terminal fans.However, because of low heating efficiency of the GS-VRF system at the rated conditions, as shown in Table 3, the peak energy demand of the GS-VRF system occurred during heating mode (i.e., natural gas use rate during winter time), while the peak energy demand of the other HVAC systems occurred during cooling mode (i.e., electrical power demand during summer time).

Performance of GS-VRF Systems for Various U.S. Climates
The simulation analysis was extended to consider the impact of the HVAC selection on the energy consumption, peak demand, and energy cost for baseline office-building models in the U.S. climate zones outlined in Tables 5 and 6.Figures 16 and 17 provide the analysis results for site and source energy consumption specific to the HVAC systems obtained for the various U.S. climate zones.Figures 18 and 19 compare the performance of GS-VRF to other HVAC systems for all U.S. climate zones.It is clear that GS-VRF systems could save both site and source HVAC energy consumption by averages of, respectively, 56.5% and 40.9% relative to PTACs, 55.0% and 46.6% relative to WSHPs, 31.4% and 35.3% relative to GSHPs, and 41.5% and 18.8% relative to water-source VRFs.In order to estimate the annual energy costs for various HVAC systems, utility rates were obtained as indicated in Table 12 for locations representative of U.S. climate zones (refer to Table 5).Table 13 provides the annual HVAC energy costs to heat and cool the baseline office buildings in various U.S. climates.Figure 20 shows the annual HVAC energy-cost savings obtained for GS-VRF systems for the office buildings located in various U.S. climates when compared to other HVAC systems.As shown in Figure 20, higher energy-cost savings could be achieved by GS-VRF systems in climate zone 3B compared to most other HVAC systems.Typically, GS-VRF systems provided greater energy-cost savings when compared to WSHPs in all climates.In order to consider the impact of the building design specifications on the performance and suitability of GS-VRF systems, an analysis was carried out for the high-performance office-building energy models defined in Tables 7 and 8.The results of the analysis are summarized in Figure 21, which lists the source energy savings achieved by the high-performance design compared to the baseline design for the office buildings in various U.S. climates.As indicated in Figure 21, the high-performance buildings (designed on the basis of ASHRAE Standard 189.1 recommendations) had lower source energy consumption relative to the baseline buildings (designed based on the basis of ASHRAE Standard 90.1 requirements) by a range from 20% to 29% depending on the HVAC system type and the U.S. climate.the performance of GS-VRF systems to other HVAC systems using, respectively, site and source energy savings.It is clear that GS-VRF systems could save both site and source HVAC energy consumption, when considering all U.S. climates, by averages of 54.7% and 36.7% relative to PTACs, 52.0% and 41.9% relative to WSHPs, 29.2% and 33.0%relative to GSHPs, and 41.1% and 17.5% relative to water-source VRFs.Therefore, the relative energy savings achieved by the GS-VRF systems for the high-performance office buildings were lower than those obtained for GS-VRF systems for the baseline office buildings.There reduced relative savings were due to the lower space cooling and heating thermal loads associated with high-performance office buildings that have higher thermal resistance envelope components, lower lighting power density, and higher-energy-efficiency HVAC systems.

Figure 1 .
Figure 1.Typical components of a water-source variable refrigerant flow (VRF) system.

Figure 3 .
Figure 3. Flowchart for the simulation environment for the ground-source variable refrigerant flow (GS-VRF) evaluation analysis.

Figure 5 .
Figure 5. Occupancy schedules used for the office-building model.

Figure 6 .
Figure 6.Lighting schedules used for the office-building model.

Figure 7 .
Figure 7. Equipment schedules used for the office-building model.

Figure 8 .
Figure 8. Calibration results of electricity use for the office-building energy model.

Figure 9 .
Figure 9. Calibration results of natural gas consumption for the office-building energy model.

Figure 10 .
Figure 10.Part-load performance curves of a water-source variable refrigerant flow (VRF) system for both heating and cooling modes.

Figure 11 .
Figure 11.Energy efficiency curves of a water-source variable refrigerant flow (VRF) system as a function of condensing medium temperature for both heating and cooling modes.

Figure 12 .
Figure 12.Capacity curves of a water-source variable refrigerant flow (VRF) system as a function of condensing medium temperature for both heating and cooling modes.

Figure 13 .
Figure 13.Monthly electricity consumption of various heating, ventilating, and air-conditioning (HVAC) systems for the office building located in Barberton, OH.

Figure 14 .
Figure 14.Monthly natural gas consumption of various heating, ventilating, and air-conditioning (HVAC) systems for the office building in Barberton, OH.

Figure 15 .
Figure 15.Hourly electricity use for various heating, ventilating, and air-conditioning (HVAC) systems during the cooling design day (July 21) for the office building in Barberton, OH.

Figure 16 .
Figure 16.Site heating, ventilating, and air-conditioning (HVAC) energy use for baseline office-building models in various U.S. climate zones.

Figure 17 .
Figure 17.Source heating, ventilating, and air-conditioning (HVAC) energy use for baseline office-building models in various U.S. climate zones.

Figure 18 .
Figure 18.Site heating, ventilating, and air-conditioning (HVAC) energy savings of ground-source variable refrigerant flow (GS-VRF) systems compared to other HVAC systems for baseline office-building models in various U.S. climate zones.

Figure 19 .
Figure 19.Source heating, ventilating, and air-conditioning (HVAC) energy savings of ground-source variable refrigerant flow (GS-VRF) systems compared to other HVAC systems for baseline office-building models in various U.S. climate zones.

Figure 21 .
Figure 21.Source energy-use savings achieved by the high-performance designs compared to the baseline designs for office buildings located in various U.S. climates.

Figures 22 and 23
Figures 22 and 23  provide the analysis results for, respectively, site and source HVAC energy consumption specific to the high-performance office buildings obtained for various U.S. climates considered in the study.Figures24 and 25compare the performance of GS-VRF systems to other HVAC systems using, respectively, site and source energy savings.It is clear that GS-VRF systems could save both site and source HVAC energy consumption, when considering all U.S. climates, by averages of 54.7% and 36.7% relative to PTACs, 52.0% and 41.9% relative to WSHPs, 29.2% and 33.0%relative to GSHPs, and 41.1% and 17.5% relative to water-source VRFs.Therefore, the relative energy savings achieved by the GS-VRF systems for the high-performance office buildings were lower than those obtained for GS-VRF systems for the baseline office buildings.There reduced relative savings were due to the lower space cooling and heating thermal loads associated with high-performance office buildings that have higher thermal resistance envelope components, lower lighting power density, and higher-energy-efficiency HVAC systems.
Figures 22 and 23  provide the analysis results for, respectively, site and source HVAC energy consumption specific to the high-performance office buildings obtained for various U.S. climates considered in the study.Figures24 and 25compare the performance of GS-VRF systems to other HVAC systems using, respectively, site and source energy savings.It is clear that GS-VRF systems could save both site and source HVAC energy consumption, when considering all U.S. climates, by averages of 54.7% and 36.7% relative to PTACs, 52.0% and 41.9% relative to WSHPs, 29.2% and 33.0%relative to GSHPs, and 41.1% and 17.5% relative to water-source VRFs.Therefore, the relative energy savings achieved by the GS-VRF systems for the high-performance office buildings were lower than those obtained for GS-VRF systems for the baseline office buildings.There reduced relative savings were due to the lower space cooling and heating thermal loads associated with high-performance office buildings that have higher thermal resistance envelope components, lower lighting power density, and higher-energy-efficiency HVAC systems.

Figure 22 .
Figure 22.Site heating, ventilating, and air-conditioning (HVAC) energy use for high-performance office buildings in various U.S. climates.

Figure 23 .
Figure 23.Source heating, ventilating, and air-conditioning (HVAC) energy use for high-performance office buildings in various U.S. climates.

Figure 24 .
Figure 24.Site energy savings achieved by ground-source variable refrigerant flow (GS-VRF) systems compared to other HVAC systems for high-performance office buildings in various U.S. climates.

Figure 25 .
Figure 25.Source energy savings achieved by ground-source variable refrigerant flow (GS-VRF) systems compared to other heating, ventilating, and air-conditioning (HVAC) systems for high-performance office buildings in various U.S. climates.

Table 1 .
Characteristics of ground heat exchangers used for ground-source heat pumps (GSHPs) and ground-source variable refrigerant flow (GS-VRF) systems.

Table 2 .
Characteristics of the baseline office building.

Table 5 .
Main climatic characteristics for 11 selected U.S. locations.

Table 6 .
Envelope characteristics for the baseline office-building models for U.S. climate zones considered in the analysis.

Table 7 .
Envelope characteristics for the high-performance office-building models for U.S. climate zones considered in the analysis.

Table 8 .
Energy-efficiency levels for heating, ventilating, and air-conditioning (HVAC) systems specific to high-performance office-building models.

Table 9 .
Annual site energy end-uses for various heating, ventilating, and air-conditioning (HVAC) systems for the office building in Barberton, OH.

Table 10 .
Annual heating, ventilating, and air-conditioning (HVAC) energy costs of the existing office building in Barberton, OH.

Table 11 .
Peak energy demand of heating, ventilating, and air-conditioning (HVAC) systems for the office building in Barberton, OH.

Table 13 .
Annual heating, ventilating, and air-conditioning (HVAC) energy costs of the medium-size office building in various climate zones.Performance of GS-VRF Systems for High-Performance Buildings in Various U.S. Climates