evaluation of hvac system operational strategies for commercial buildings

Drergy Conwrs. Mgw Vol. 38. No. 3. Pp. 225-236. 1997 Coovrieht C’I 1996 Elsevier Science Ltd Pergamon

PII: S0196-8904(96)00039-8 Printed’ik &&Britain. All rights reserved

0196-8904197 $17.00 + 0.00

EVALUATION OF HVAC SYSTEM OPERATIONAL STRATEGIES FOR COMMERCIAL BUILDINGS

MORTEZA M. ARDEHALI and THEODORE F. SMITH* Department of Mechanical Engineering, The University of Iowa, Iowa City, IA 52242, U.S.A.

(Receiryed 25 July 1995)

Abstract-The inherent limitation in the performance of building envelope components and heating, ventilating and air conditioning (HVAC) equipment necessitates examination of operational strategies for improvements in the energy-efficient operation of buildings. Because of the ease of installation and increasing capabilities of electronic controllers, operational strategies that could be programmed with these controllers are of particular interest. The objectives are concerned with the examination of various operational strategies applied to older- and newer-type commercial office buildings utilizing constant-air-volume-reheat and variable-air-volume-reheat HVAC systems, respectively. The operational strategies are night purge (NP), fan optimum start and stop (OSS), condenser water reset (CWR) and chilled water reset (CHWR). The indoor air quality requirements are met and the latest applicable energy rates from local utility companies are used for Des Moines, Iowa. The results show that, in general, NP is nof an effective strategy in buildings with low thermal mass storage, OSS reduces fan energy, and CWR and CHWR could be effective for chillers with multi-stage unloading characteristics. The most energy-efficient operational strategies are the combination of OSS, CWR, and CHWR for the older-type building, and OSS for the newer-type building. Economically, the most effective is the OSS strategy for the older-type building and the CHWR strategy for the newer-type building. Copyright cc 1996 Elsevier Science Ltd

HVAC Operational strategies Air conditioning VAV-RH CAV-RH Night purge Optimum start and stop Condenser water reset Chilled water reset

NOMENCLATURE

R = Thermal resistance (hr ft’ F/Btu) T = Temperature (‘F)

Acronyms BRAD == Building return air dry bulb

CAV-RH = Constant-air-volume with reheat CHWR = Chilled water reset

CWR L Condenser water reset HVAC s Heating, ventilating and air conditioning

IAQ = Indoor air quality NP = Night purge

OCC c Optimum start/stop. condenser water reset. and OSS 5 Optimum start/stop

VAV-RH s Variable-air-volume with reheat

chilled water

INTRODUCTION

The most important objective of heating, ventilating and air-conditioning (HVAC) systems is to provide thermal comfort [l]. The power consumption of building HVAC systems, however, is in excess of one-half of the total energy consumed in the nation [2]. The inherent physical and thermodynamic limitations in the performance of building envelope components and HVAC equipment necessitates examination of operational strategies for improvements in the energy-efficient operation of buildings. The knowledge of how operational strategies affect the building performance can serve as a tool for design professionals and owners. Because of the ease of installation and the increasing capabilities of electronic controllers, operational strategies that

*To whom correspondence should be addressed. Tel: (319) 335-5680. fax: (319) 335-5669, e-mail: tfsmith@ icden.uiowa.edu

225

226 ARDEHALI and SMITH: HVAC OPERATIONAL STRATEGIES

could be programmed for built-up systems or pre-packaged HVAC equipment are of particular interest.

The first part of this investigation focused on the energy consumption and cost effectiveness of HVAC systems [3]. It is found that the variable-air-volume with reheat (VAV-RH) is the most energy efficient system in older-type commercial office buildings. The payback period for conversion from a constant-air-volume with reheat (CAV-RH) system to a VAV-RH system in the older-type buildings is 4.5 yr, where a reduction of 42% in the first year operating cost could be realized. It is also found that, in buildings where the down time is to be kept at a minimum, variable volume and temperature (VVT) systems should be considered as an alternative to CAV-RH systems. The conversion to a VVT system from an existing CAV-RH system results in a 34% lower first year operating cost and a 3-4 yr payback period [3].

The operation of energy-efficient buildings requires a study of operational strategies that can be integrated into the design of the building HVAC system [4]. Application of these strategies may not prove to be beneficial if they are applied to existing buildings. Computer simulations of a particular operational strategy, in advance, provide insights that are instrumental in realizing the expected energy efficiency from a particular operational strategy and justify making the additional investment. In an experimental investigation, application of the night purge (NP) operational strategy resulted in an 18% reduction in cooling energy consumption with no influence on the peak energy demand for a 14,500 ft’ office building in Jacksonville, FL [5]. In that study, the effects of the NP strategy on the utility costs is not given. In a study by Braun 161, it is shown that, unless motivated by time-of-day utility rates, the reduction in energy cost is insignificant when the NP strategy is applied, and the building is pre-cooled below comfort-level temperatures. Because of the limited studies conducted, the second part of the investigation, as covered in this study, is concerned with the energy efficiency and the economic impacts of various operational strategies, namely, NP, fan optimum start and stop (OSS), condenser water reset (CWR), and chilled water reset (CHWR) applied to older- and newer-type commercial office buildings.

To enhance the performance of HVAC equipment further, the operational strategies must attain dynamic characteristics and provide instructions for operating equipment so that energy efficiency is achieved [7]. For example, knowledge of the full-speed operation schedule ahead of time, which is provided to the controller in advance, can result in lower power consumption by the main supply fan. The implementation of operational strategies that depend on, and provide, control for various building and equipment components requires controllers that have multi-input and multi-output capabilities. Direct-digital controllers (DDC) are on-line controllers that apply pre-programmed strategies and do not perform any optimization of the control variables [8]. The pre-programmed instructions consist of an array of choices and conditional loops for addressing a particular event with a proper response. This feature enables DDC controllers to operate in real time. Thus, the operational strategies must be defined in advance of the future events and categorizing these events is a necessity.

For this study, various operational strategies are examined so that their applicability and features can be identified. The objectives are concerned with the examination of various operational strategies applied to older- and newer-type commercial office buildings utilizing constant-air-volume-reheat and variable-air-volume-reheat HVAC systems, respectively. The operational strategies are night purge (NP), fan optimum start and stop (OSS), condenser water reset (CWR), and chilled water reset (CHWR).

Energy simulation and life-cycle cost analysis of operational strategies for commercial buildings can assist in identifying the advantages and disadvantages, and the costs can be quantified. Realistic simulation modeling of a building requires the application of indoor air quality (IAQ) requirements [9] and related utility rates, such as electricity and gas costs, from the local service companies. The latest energy rates from utility companies for Des Moines, Iowa are used for the cost analyses.

BUILDING DESCRIPTION

The building type, and utilization schedules selected for evaluation of the operational strategies are based on the commercial building studied by Ardehali and Smith [3]. However, in this study,

ARDEHALI and SMITH: HVAC OPERATIONAL STRATEGIES 227

some of the older-type building envelope features are modified to reflect the latest efficiency measures taken in design of newer buildings. Therefore, the two building types are referred to as

the older and newer buildings. The older building is an office building with 30,000 ft’ of conditioned area, located in Des Moines, Iowa, and with features of three floors; 12 ft floor-to-floor height; glazing with an area equal to 15% of the total exterior wall area, a thermal resistance of R-2.04, and a shading coefficient of 0.51; wall insulation of R-5; and roof insulation of R-l 1. The ceiling space is chosen as 2 ft to allow needed clearance for recessed type light fixtures, a fire suppression system, communications conduit and wiring, sanitary piping, and the necessary HVAC system and ductwork. The occupancy-type is office, without the need for any special exhaust or make-up HVAC requirements. There are no shading devices associated with the building envelope, nor are there any adjacent tall buildings that would result in daytime shading. There is no roof glazing or skylights in the building. The color of the building walls and roof is assumed to be dark to account for an envelope solar absorptivity of 0.9. The specific mass of the building is 100 lb/fP, which is considered as light- to medium-weight construction.

Each floor of the building is divided into five thermal zones, corresponding to four outside exposures (north, east, south, west) and the interior, as shown in Fig. 1. Each perimeter zone has a conditioned area of 900 ft’ and the interior zone of each floor has 6400 ft’ of conditioned area. The geographical and weather data, orientation, infiltration, and other parameters related to building load calculations are fixed and are the same for all the HVAC systems simulated.

The new building has the same features as the older building with the exception of applying (1) low-E glazing with R-2.6 and shading coefficient of 0.58, (2) wall insulation of R-l 1 and (3) roof insulation of R-30.

The ventilation rate for office occupancy varies from 15 to 20 ft’/min-person, as suggested by the standards [9]. The minimum ventilation air is 20 fQ/min-person. For both building types, the occupancy is from 7 a.m. to 5 p.m. on weekdays. The floors are carpeted, and the indoor design condition is 75°F dry bulb with relative humidity of 50%. The cooling thermostat driftpoint is 95”F, and the heating thermostat driftpoint is 60°F for the unoccupied periods during weekdays and weekends. The building is thermally zoned with sufficient core area such that one or several tenants may occupy the space; however, for multiple occupancy, the central system serves the entire building and the cost of the HVAC system is applied to individual tenants on the basis of $/ft2 for conditioned space. The occupancy of the building is considered to be typical office use at the rate of 100 ft’/person, and 2.5 Wjft’ for standard lighting and office automation electrical load.

The schedules for people, lights, and outdoor air are shown in Fig. 2. The schedules are considered as multipliers for utilization of input design values. For example, the lighting schedule of 5% at night describes that fraction of the load due to lighting that is employed in computing the cooling load at night. There are various schedules, such as people, lighting, and outdoor air, that must be defined. During the hours of 10 p.m. to 5 a.m., there are no people, lights, or outdoor air scheduled for the occupancy of the building.

Fig. 1. Thermal zones of a typical building floor.


100 -

90-

80-

70 -

* 60-

f so-

a 40-

-

0 6 12

Hour of day

I8 24

Fig. 2. Building operational schedules.

HVAC SYSTEMS

It is desirable to improve the inefficient energy consumption of CAV-RH systems [3] in existing older-type buildings by means of NP, OSS, CWR, and CHWR operational strategies. It is also of interest to explore possibilities to enhance further the energy-efficient performance of VAV-RH systems [3] in newer buildings by means of operational strategies. The simulation program, TRACE [lo], performs cooling and heating load calculations, energy simulation and economic analyses and is utilized to study the effects of operational strategies on the performance of CAV-RH and VAV-RH systems. The following sections are intended to provide a broad overview and a brief description of each HVAC system simulated. The major differences are also noted for each system. The reader is referred to Stanford [l l] for complete and detailed information.

Constant-air-volume system The CAV-RH system is a multi-zone system and uses a central supply fan to provide a constant

volume of conditioned air to the reheat coils of multiple zones. Terminal heating coils are controlled by the individual zone thermostats to provide reheating for the purposes of maintaining the zone temperatures, as shown in Fig. 3. The main supply fan continues operation during all scheduled occupied hours. Because the demand for most zones is cooling rather than heating, a constant volume of air at the design cooling supply temperature is distributed to all zones. The building load computation for this system is based on the peak load. The supply air temperature is fixed by the user, and the design air flows are computed, based on the peak load for each zone. During

Supply Air

Fan -

Damper -Es

Zone

Return Air I

Fig. 3. Constant-air-volume reheat system schematic diagram.


Supply Air

Damper 63

Terminal Damper

Reheat Coil -

-@ Zone

Exhaust m Return Air

-4 1

Fig. 4. Variable-air-volume reheat system schematic diagram.

occupied hours and when a zone cooling load is not at a peak, the required zone temperature is maintained by means of reheat coils. During unoccupied hours, a night set-back temperature is specified, and the system operates in the same manner.

Variable-air-volume with reheat A VAV-RH system, as shown in Fig. 4, is a multi-zone type system, where the supply air

temperature is kept constant, and the volume of air delivered to each zone is varied according to the load fluctuation in the zone. The thermostat in each zone modulates the terminal damper, allowing conditioned air into the zone to maintain the setpoint temperature. The static pressure build-up in the distribution ductwork as the terminal damper closes is used to modulate the VAV supply fan inlet vanes. If the modulating terminal damper is pre-set to allow for a minimum amount of cooled air into the zone, for the purposes of ventilation requirements, some means of reheating is required to avoid over-cooling. The reheating can be accomplished by means of a coil placed down stream of the VAV damper (VAV-RH). In the next section, the operational strategies of energy-efficiency measures and methods to simulate them are discussed.

OPERATIONAL STRATEGIES AND SIMULATION

Night purge

The NP operational strategy is designed to reduce building utility costs by means of exploiting the thermal capacity of the building mass, and to improve the IAQ in buildings by removing odors and other air-borne pollutants caused by daytime building usage. During the unoccupied periods, when maintaining thermal comfort is not required, outdoor air is brought into the building to cool the building mass for offsetting the cooling load at the beginning of the occupied hours. As a prerequisite for applying this energy-efficiency measure, therefore, the controller must be capable of monitoring the outdoor and indoor air temperatures, as well as controlling the supply fan. Furthermore, the outdoor air schedule must allow the outdoor air damper to open fully when the controller activates the NP strategy. When the outdoor air reaches a pre-defined temperature, the controller activates the main supply fan to introduce outdoor air into the building. The supply of cool air continues until the outdoor air temperature falls to a specified temperature, say 3°F above the building return air dry bulb (BRAD) temperature, as depicted in Fig. 5. In the simulation program, first the NP schedule is defined (Fig. 5a) and as the BRAD is tracked (Fig. 5b) the effective damper opening schedule is determined. Though not used for this study, it is possible to introduce another constraint for regulating the operation of NP in humid climates where the zone relative humidity may not exceed a specified value to avoid introducing a large latent load by bringing moisture-laden air directly into the building. The application of NP in climates where it may cool the building far below an acceptable temperature, thereby requiring heating in the following morning, must be identified. Thus, a simulation of this operational strategy is necessary to avoid consuming additional heating energy while attempting to provide more energy-efficient cooling.


Optimum start/stop

The OSS operational strategy is devised to achieve higher energy efficiency and to reduce building utility costs by decreasing the time duration of operation of the HVAC system supply fan. Conceptually, without sacrificing thermal comfort, a delay in start-up and an early shut-down of the equipment can potentially increase energy efficiency. The thermal capacity of the building mass allows for reduction in supply fan operation, therefore it is expected that more massive buildings have higher potential for benefiting from this strategy. The control system could have the capability to record the ambient conditions and equipment settings for future use. The recorded data would provide an array of options, or a “look-up” table, so that the equipment can be started and stopped without causing discomfort for the building occupants. In this manner, the OSS strategy minimizes the supply-fan run time.

In the simulation program, the desired OSS operation schedule is specified by means of two time-windows during which the optimum start and stop of the fan is to be applied. This schedule must be in accord with a previously-defined building operation schedule. The normal time of operation for the supply fan is from 5 a.m. to 6 p.m., Fig. 6a. The OSS schedule controls the supply fan operation and determines how much later than 6 a.m. the fan can be started within the optimum

(a) Scheduled NP operation period.

IW period 4 c

10 9 8 7

B 6 ^ g 5

4 3 2 1 0

0 6 12 18 24

(b) BRAD and outdoor air temperature difference. r

I_ _ AT > 3*F

0 6 12 18 24

(c) Damper schedule.

80

E 60

8 b a 40

0 6 12 18 24

Time, hr

Fig. 5. NP strategy and outdoor air damper schedule.

90

60


(a) Operation schedules.

0 6 12 18 24

(b) BRAD and setpoint temperature difference

AT-3’F

0 6 12 18 24

0 6 12 18 24

Time, hr Fig. 6. OSS strategy and supply fan operation.

start window (2 hr), and how much earlier than 6 p.m. the fan can be stopped within the optimum stop window (2 hr). Because the supply fan can only be stopped and not started by the OSS controller, the main supply fan operation time must include the time-windows specified for the OSS operation. For the hourly simulations, the number of minutes needed for the supply fan to bring the BRAD temperature up or down to the setpoint temperature is estimated by the OSS controller based on recorded performances from previous days. If this value is greater than or equal to the number of minutes remaining in the optimum start period, the fan is started. If the number of minutes is less than 1 hr, the fan is turned on for a fraction of the hour to maintain the setpoint temperature. Because the optimum start controller estimates the time needed for startup, the accuracy of the estimates decreases if longer optimum start time-windows are specified by the user. Therefore, it is recommended that the optimum start windows do not exceed 3 hours.

For the optimum stop, the supply fan is stopped if the BRAD temperature does not fall below the heating setpoint or rise above the cooling setpoint. The optimum stop time-window is usually less than an hour because minimum IAQ requirements must be met during the occupied time.

In a similar way to the NP strategy, it is possible to constrain the OSS strategy by means of an offset between the BRAD and the setpoint temperatures, as shown in Fig. 6b. The actual fan


operation is reduced, Fig. 6c, as a result of applying the OSS strategy, so that better energy efficiency is achieved.

In applying the OSS strategy, the building construction type must also be considered. For example, light buildings (30-70 lb/ft-‘) cool quickly and heavier buildings (13fL200 lb/ft’) require longer times for cooling at start-up, hence, shorter and longer cooling time periods are needed for the optimum start. At the end of the day, a light building warms up quickly and heavier buildings take longer to warm, resulting in shorter and longer time periods, respectively, for optimum stop of equipment.

Condenser water reset The CWR operational strategy enhances the chiller operational efficiency as it reduces overall

energy consumption by the HVAC system. The source of energy consumption in a chiller is the work required by the compressor where the refrigerant gas is elevated from a lower pressure (evaporator) to a higher pressure (condenser). The effects of an increase in the refrigerant-pressure differential between the condenser and evaporator increases the work of the compressor. Because the capacity of a cooling tower is a function of the outdoor conditions, lower outdoor temperatures allow for additional cooling of the condenser water. The condenser water temperature decrease is a means to lower the pressure differential, thereby reducing the compressor work. Reducing the condenser entering water (or the cooling-tower leaving water) temperature by means of resetting increases the operational efficiency (kW/ton) of a chiller. Prerequisites for this operational strategy are a chiller with multi-stage unloading characteristics and a controller with monitoring and regulating capabilities. As a constraint to the CWR strategy, a minimum pressure differential must be maintained between the condenser and evaporator for proper oil movement [12]. The chiller efficiency may suffer if the cooling tower temperature is lowered to the extent that the return of refrigerant to the evaporator is impeded. Further, the cooler condenser water temperature could be obtained at higher cost of the fan energy consumed by the cooling tower fan.

In the simulation program, the cooling tower controls are originally set so that 85°F condenser entering water is produced. Depending on the type of chiller specified and the CWR range specified by the user, the outdoor wet-bulb temperature is monitored continuously for opportunities to unload the compressors. The condenser entering water temperature is allowed to be reset downward by as much as 30°F (i.e. 55°F cooling-tower leaving water) depending on the chiller type and operating conditions.

Chilled water reset The operational efficiency realized by the CHWR strategy is by means of increasing the chilled

water supply temperature to the cooling coil according to a preset plan. The increase in the evaporator discharge (or the cooling-coil inlet) temperature due to the decrease in the building load causes a decrease in the pressure difference between the condenser and evaporator. Similar to the case of the CWR strategy, this reduction in the pressure difference results in reduced compressor work, and lower overall system energy consumption is achieved. The procedure is implemented in the simulation program as described by the following example. A chilled water system with 44°F and 54°F supply and return chilled water, respectively, is considered. It is desirable to make the operation of the system more energy efficient by means of the CHWR operation strategy with a reset range of 8°F. The operation of the chiller at 60% cooling load implies that the temperature of the supply chilled water leaving the evaporator is

Tevap.?.ncw = Tevap.~, + (% cooling load)(reset range) (1)

where the new reset temperature for the chilled water leaving the evaporator is Tevap.2.neW and the old reset temperature for the chilled water leaving the evaporator is TFvap.2.01d = 44°F. Then, T CVilp.2.MV = 48.8”F. The maximum chilled water reset value is set by the user. Also, the selection of the chiller must be such that the part-loading can be accomplished by unloading the compressors as necessary. For this study, a maximum range of 10°F and a 3-stage centrifugal chiller is selected to allow for 33 and 66% part-load operation. There are other strategies that could accomplish the energy-efficient operation task at part-load, namely, variable speed pumping. Because the CHWR


and variable speed pumping strategies conflict, their simultaneous application is avoided in practice.

Cost factors and system construction

The evaluation of HVAC system operational strategies is concerned with equipment cost, operation cost, and other related cost factors. The cost for the construction of the building is not considered because it is the same for all strategies examined in this investigation. The economic evaluation and life-cycle cost analysis of the HVAC systems operated with the considered operational strategies are simulated for the older- and newer-type buildings. Certain financial parameters, such as system installed cost, maintenance cost, monthly building energy consumption, utility rate structures, interest rate, and inflation rate are required. The interest and inflation rates are assumed as 10 and 3%, respectively. The mortgage life and life-cycle periods are taken as 20 years.

For the mechanical HVAC system construction costs and related analyses, the $/ft’ method is used [13]. The HVAC system cost is inclusive of the cost for heating, ventilating, and cooling equipment, as well as the necessary cost for system controls and power wiring. The capital cost for each installed system and the associated maintenance costs are shown in Table 1. The first column displays the simulated system. In the next column, the cooling load capacity of each system is tabulated. The term “peak”, abbreviated as P following the capacity in tons of refrigeration, is used for the load computations of the systems where the building load is equal to the sum of the peak loads of all zones. If the design-day building peak load is used, then the term “block” is used, abbreviated as B, in describing the type of load modeling for the particular system.

The available figures from the noted reference are given in terms of $/ton for each system, as they are used to compute the $/ft’ for the corresponding system. The basic maintenance cost for all systems is taken as $0.26/ft’.

Because the existing older-type buildings operate with chillers without unloading capabilities, the implementation of CWR and CHWR strategies in an older building requires accounting for the additional cost of a new 80 ton 3-stage chiller with an installed cost of $55,000. Because older chillers operating with environmentally-unsafe refrigerants need to be replaced, the replacement cost is further justified.

RESULTS AND DISCUSSION

The results of the simulations for the purposes of evaluating the operational strategies are presented in categories that describe the energy consumption index (Btu/ft’-yr), first-year utility cost ($), and life-cycle cost ($). As noted, the operational strategies are simulated for the older building with CAV-RH and the newer building with VAV-RH. The results are reported in Figs 7 and 8.

The simulation results for the older building (Fig. 7) indicate that OSS is the most energy-efficient strategy among all four strategies examined. However, if the OSS, CWR, and CHWR (OCC) strategies are simultaneously applied, the least amount of energy is consumed. The NP strategy is not useful, as the examined building with a density of 100 lb/ft-’ does not allow for reduction of the peak cooling load by means of mass thermal storage. For all strategies, except for NP as shown in Fig. 7b, the first-year operating costs remain within 3% of those of the CAV-RH system. The additional cost for a three-stage centrifugal chiller required for implementing the CWR, CHWR, and OCC strategies creates an unrecoverable offset in the life-cycle costs, Fig. 7c. Neglecting the cost for the new chiller, however, the life-cycle costs for application of these

Table 1. System construction and maintenance costs

System Maintenance Load cost cost

System (Tons) ($/ft’) ($/fF-yr)

CAV-RH SO/P 8.83 0.26 VAV-RH 13/B 8.10 0.26


172,572

CAV-RH hT oss CWR CHWR OCC

(a) Annual energy consumption (BtulA*-yr).

I CAV-RH hT oss CWR CHWR OCC

(b) First year utility cost ($).

839,285 837.382 835,538

CAV-RH w oss CWR CHWR OCC

(c) Life-cycle cost (ts). Fig. 7. Energy and economics of HVAC systems: Older-type building.

strategies are $755,013 for CWR, $753,110 for CHWR, and $751,266 for OCC, which places these strategies in a favorable position.

For the newer building, the OSS is shown as the most effective strategy among all others examined, Fig. 8a, where the advantages are due to the modulation of the VAV supply fan. The highest energy consumption occurs when the CWR strategy is employed because of the higher cooling-tower-fan operation rates. As compared to all others and similarly to the older building results, the NP strategy results in the highest operational cost due to the lack of sufficient building thermal mass storage. In comparing all strategies, Fig. 8b, the largest possible first-year utility cost savings is $329/yr compared to the VAV-RH system, and it is due to utilization of the CHWR strategy. Because of the lower utility cost, therefore, the life-cycle cost of the VAV-RH system with


the CHWR strategy remains the lowest, as shown in Fig. 8c. Also, because the cost of the VAV-RH system includes the cost of the three-stage centrifugal chiller, the CWR and CHWR life-cycle costs are more favorable as compared to those of the same strategies applied to CAV-RH system.

In general, the results confirm that utilization of these operational strategies is not recommended for operation of commercial buildings in the midwest region of the country. These operational strategies are motivated and devised to be used for thermal storage HVAC systems in larger cities where on-peak demand charges are high and maintaining the lowest possible on-peak demand remains as an important factor in operating such systems.

79,856

16,562 77,128

. ,

VAV-RI4 NP oss CWR CHWR OCC

(a) Annual energy consumption (Btu/f?*-yr).

18,622

18,312

I 1

VAV-RH NP oss CWR CHWR OCC

(b) First year utility cost ($).

679.915

677,642

VAV-RI4 NP oss CWR CHWR OCC

(c) Life-cycle cost (%). Fig. 8. Energy and economics of HVAC systems: Newer-type building.


CONCLUSIONS AND RECOMMENDATIONS

The assessment of HVAC system operational strategies is made for the older-type building with a CAV-RH system and the newer-type building with a VAV-RH system. The advantages and disadvantages of various operational strategies are presented in terms of yearly energy consumption per unit area, first-year utility costs, and life-cycle costs, which could be important factors in selecting, designing and owning these systems. It is expected that life-cycle costs due to employing these operational strategies will vary for different locations because of the dependency on location-specific utility cost factors. Also, because the climates and outdoor conditions influence the operational efficiencies, every operational strategy must be examined for each locale on an individual basis. The results show that, for midwestern states, such as Iowa, where utility cost factors are heavily-based on usage and motivation for operation during off-peak hours of the day is limited, these operational strategies are not effective. Of the strategies considered, OSS for the older-type buildings and CHWR for the newer-type buildings are the most effective operational strategies. From an energy-efficiency point of view, the most operationally efficient strategy is OCC for the older-type building and 0% for the newer-type building. For the life of the system, economically, the most effective is the OSS strategy for the older-type building and the CHWR strategy for the newer-type building. In the case of the need for replacement of chillers with environmentally-unsafe refrigerants in older-type buildings, the application of the OCC strategy is considered to be the most effective.

Actual tests of these operational strategies are recommended as part of the future extension of this work, so that the findings may be verified. Also, future work may include examination of various forms of economizer cycle, fan cycling, demand limiting, and super-cold air distribution. Further, it would be of interest to examine the effects of variations in the building design parameters, such as daylighting, envelope insulation, glazing, and mass, on the HVAC system performance.

Acknol~ledgpnzmr-The authors acknowledge the support of the Iowa Energy Center: Project 93-16-01.

REFERENCES

1. EPRI, Space-Condiiioning S~sten~ Selection Guide. Research Project 2983-13, Palo Alto, CA (1993). 2. D. P. Metha and A. Thumann, Handbook ?fEnergy Engineering. Fairmont Press, Lilburn. GA (1991). 3. M. M. Ardehali and T. F. Smith, Evaluation of variable volume and temperature HVAC system for commercial and

residential buildings. Energy Conversion and Management 37(9), 1469-1479 (1996). 4. J. Y. Kao, ASHRAE Trunsucrions 91(2B). 810-824 (1985). 5. M. D. Rudd, J. W. Mitchell and S. A. Klein. ASHRAE Trunsuctions 96(2), 820-829 (1990). 6. J. E. Braun, ASHRAE Trunsacrions 96(2). 876888 (1990). 7. J. M. House and T. F. Smith, A Generul Pwpose Sinwlution Code,for Optimal Controi,ftir Builditlg und HVAC S~.s/ems.

Technical Report, ME-TFS-94-008. Department of Mechanical Engineering. The University of lowa, Iowa City (1994). 8. M. J. Coffin, Direct Digitul Control,fbr Building HVAC Systems. Van Nostrand Reinhold. NY (1992). 9. ASHRAE IAQ, Ventilation ,for ucceptable indoor air quuliy, ASHRAE Standards 62-1989. American Society of

Heating, Refrigerating, and Air-Conditioning Engineers, Inc., Atlanta, GA (1989). IO. TRACE, TRACE ” 600 Loud Design und Economics Simulo/iorl Program. The Trace Company, La Cross, WI (1993). I I. H. W. Stanford, III, Anulwis and Design of’ Heuring, Ventilating. and Air Conditioning Sys.tems. Prentice Hall.

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evaluation of hvac system operational strategies for commercial buildings

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