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Tale of two low-energy designs: comparison of mechanically and naturally ventilated office buildings in temperate climates.

ABSTRACT

In low-energy building design, energy consumption due to space conditioning is responsible for over 30% of total energy consumed in commercial buildings. In an effort to evaluate low-energy building designs, two commercial office buildings were each monitored for 16 months. A comparison of the two buildings, with different ventilation strategies but located in the same climate, is presented. One is naturally ventilated without mechanical chillers, and the other uses a raised floor system combined with a chilled ceiling for space conditioning. The data were evaluated for thermal comfort, ventilation effectiveness, and energy efficiency. The buildings were of similar configuration: three stories, with a central atrium and an open floor plan, located in office parks outside of London, UK. The energy consumption for the mechanically ventilated building was significantly higher on a kWh per floor area basis (318 kWh/[m.sup.2] [29.6 kWh/[ft.sup.2]] for the mechanically ventilated building and 216 kWh/[m.sup.2] [20.1 kWh/[ft.sup.2]] for the naturally ventilated building), primarily due to the energy required for cooling and fan energy. The temperature of the office space also varied depending on the ventilation used. Mechanically ventilated buildings had much tighter control of the interior temperature when compared with the naturally ventilated building. Both buildings had areas for improvement to further reduce energy consumption.

INTRODUCTION

There is an increased focus on the design and operation of high-performance and low-energy facilities. In the United States, buildings account for over 39% of the energy consumption in the commercial building sector; in developed countries worldwide, buildings account for 26% of total energy consumption, approximately 43 quadrillion Btu annually (EIA 2005). Thirteen percent of the UK total energy consumption is used in the service sector, of which 61 percent is consumed by the private commercial sector (DTI 2004). Commercial office buildings consume almost 8% of total energy used in the UK and 17% in the US (EIA 1995). Although heating makes up most of the energy usage both in the US and UK, the energy required for cooling is still a significant portion, representing almost 10% of all energy usage for commercial buildings (DTI 2004). In the United States, cooling and ventilation comprise 23% of commercial building energy usage (EIA 1995).

Two commercial office buildings in the same general climatic region were selected so that comparisons between decisions on low-energy building design could be evaluated. One building used an underfloor air distribution system to provide uniform ventilation combined with a chilled ceiling to condition the space; the other used natural ventilation to ventilate and passively cool the building without chillers. Monitoring the performance of these buildings in the temperate climate allowed us to assess the indoor environment and the energy required to maintain a comfortable indoor environment. This was a unique opportunity to evaluate similarly configured commercial office buildings to determine the influence of design decisions on the building performance.

In this paper, the performances of one mechanically and one naturally ventilated commercial office building are discussed. First, each building is described in detail, including their ventilation design characteristics. The method used in evaluating the performance of the buildings is presented, including the energy monitoring and ventilation effectiveness. The results are applied to existing benchmarks for commercial buildings. Finally, the indoor environment monitoring results are presented, along with potential areas for improvement of performance.

BUILDING DESCRIPTIONS

As part of a larger study of low-energy building design, two buildings in the United Kingdom were selected for long-term monitoring to evaluate the building performance. This included evaluation of the energy consumption of the building as a whole by selected subsystem and assessment of the indoor environment through temperature, relative humidity, and carbon dioxide levels. Data were logged for a minimum of 12 continuous months to evaluate the buildings during the heating, cooling, and shoulder months.

One building had an innovative mechanical system that used a raised floor system combined with a chilled ceiling to condition the occupied space. A naturally ventilated building with a similar configuration was identified in the same region and monitored for comparison of energy and ventilation performance and occupant comfort. The climate and a detailed description of the buildings are provided below.

Local Climate

Both buildings were located within a 65 km (40mi) radius of London, in a temperate area of the United Kingdom that does not have a significant number of heating and cooling degree-days. On average, the area typically has 1902 heating degree-days (HDD), base 15.5[degrees]C (3424 HDD base 60[degrees]F), and 389 cooling degree-days (CDD), base 15.5[degrees]C (700 CDD base 60[degrees]F). Table 1 provides an overview of the typical weather conditions for the London area. Though the maximum temperature does exceed the average conditions listed in Table 1, in general the maximum temperature is 72[degrees]F (22[degrees]C), with a relative humidity of 59% to 71%. The hourly temperature data for London are shown graphically in Figure 1, separated into occupied and unoccupied hours. Occupied hours are Monday through Friday, 7 a.m. to 6 p.m., throughout the year. The summer of 2003 was an exception due to the record heat wave in Europe.

Mechanically Ventilated Building

Located approximately 20 miles southwest of London, the mechanically ventilated and cooled (MV) building was the first building completed in an office park redevelopment in 2000. The building has three occupied floors that all open onto a central atrium, which runs the entire length of the building on a predominantly north-south axis. The net internal floor area for the building is 5100 [m.sup.2] (55,000 [ft.sup.2]). The atrium level is raised an additional 2.5 m (8 ft) to accommodate roof access and a clerestory, allowing daylight to penetrate the central core of the building. The facade is nearly 100% glazing, with a series of fixed external horizontal louvers that act as shading devices to reduce glare. On the south and west facades, photovoltaics are integrated into the shading device. Each floor has two major zones, which are separated by the atrium and will be referred to as north and south half-floor plates. The floor plan, in general, is very open, except for meeting rooms along the atrium on the lower two floors. Figure 2 shows a typical floor plan and section of the building. Occupants work at clusters of desks with low-height dividers between workstations. The occupancy of the building is approximately 385, though at any given time the building is not fully occupied due to the nature of a customer service and sales office. There is one computer monitor per occupant, though only approximately 60% of the occupants have desktop systems; the other occupants use docking stations for laptops. Virtually all of the office equipment is low-energy with energy-saving modes that become active when not in use for periods of time.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

The MV building was originally designed for a much more dense occupancy--almost twice the current occupancy. Certain low-energy characteristics were incorporated into the design of the building. These design elements include the fixed shading devices on the exterior facade, sensors on all light fixtures, zoned chilled-ceiling system, heat recovery loop, and computer equipment that was Energy Star rated.

Lighting and Plug Loads. High-efficiency fluorescent lamps are used throughout the building. Each fixture has a combination photo and occupancy sensor. A small percentage of light fixtures (approximately 10% based on monitored data) remain on year-round for safety reasons. Also, each meeting room has a manual override switch to operate the lights so that a lack of movement within the room does not cause the lights to turn off. These lights then must be turned off manually as well.

End-use equipment that is plugged into a standard outlet is referred to as plug loads. Common plug loads found in the MV building include desktop and laptop computers, monitors, printers, fax machines, and desktop task lights. Most of the equipment located in the MV building has energy-saving modes that are in use. The wiring for the plug loads is located in the raised floor area (supply plenum).

[FIGURE 3 OMITTED]

HVAC System and Air Distribution. The MV building is conditioned by an underfloor air displacement ventilation system with chilled ceilings, perimeter trench heaters, and perimeter chilled beams. The system supplies 100% outside air by a constant-volume air-handling unit during occupied hours (typically 8 a.m.-6 p.m.) at a supply temperature of 18[degrees]C (64.4[degrees]F). The chilled ceiling panels provide any additional cooling that may be required due to the internal loads within each zone. Chilled beams provide additional cooling near perimeter windows to prevent overheating due to solar gains, particularly during the summer. The trench heaters are used at the perimeter to overcome heat losses due to conduction through windows and infiltration of outside air during winter months. The central mechanical equipment is located on the upper roof level, above the atrium. The plant equipment includes two natural gas-fired boilers, two air-cooled chillers, and two air-handling units.

The air-handling units deliver fresh air through two independent supply risers. The supply risers both deliver the air to all six supply plenums serving the six half-floor plates. Air is exhausted through the return plenums to two return risers that exhaust air through the air-handling units. The air-handling unit includes supply and return fans, a preheat coil, economizer, cooling coil, and a reheat coil. A diagram of the air-handling unit and components is provided in Figure 3. In the building, the dampers are controlled such that 100% outdoor air is used after 8 a.m., 50% outdoor air from 6 to 8 a.m., and 100% return air before 6 a.m. The entire system is off during the night and weekends. Electrical monitoring showed that the building system was generally turning on at 4 a.m. and off at 7 p.m. on weekdays.

The air is distributed to six supply plenums, one for each half-floor plate. The air enters the space through numerous circular floor diffusers. The air leaves the space and enters the return plenums through the ceiling-mounted, ventilated light fixtures. The air from the six return plenums then returns to the air-handling unit. The air distribution system is shown graphically in Figure 4. The supply flow rate for the entire system and each zone is constant. The air that is conditioned and supplied to the occupied space (100% outside air during occupied hours) is then exhausted outside. Current operation of the HVAC system does not recover any of the heat in the airstream. The heat recovery loop associated with the building air-handling system is not in operation because of problems with the installation.

In each half-floor plate, the entire ceiling of the central zone is composed of chilled ceiling panels. The chilled ceiling consists of metal chilled-water tubing bonded to a thin metal panel. The back side of the ceiling panel is lined with insulation. The chilled ceiling control is based on the zone mean air temperature of 25[degrees]C (77[degrees]F) with a 1[degrees]C (2[degrees]F) throttling range. In the perimeter zone, the chilled ceiling is replaced with chilled beams, which have a higher cooling capacity. The chilled beam, a long and narrow finned tube heat exchanger, is mounted near the ceiling. Hot air rises and passes through the chilled beam, where it is cooled and falls downward. The chilled beams are controlled to maintain a setpoint temperature of 25[degrees]C (77[degrees]F). If the temperature is below the setpoint, there is no flow to the chilled beam. When the temperature reaches the setpoint, flow through the chilled beam is varied to maintain this setpoint.

The perimeter zones also have trench heaters to overcome heat losses through the windows during the winter. The heater is placed in a trench at the base of the window. Air is cooled as it comes in contact with the window and flows downward into the trench where the heater is located. The air then rises after it is heated. The trench heaters are controlled to maintain a setpoint temperature of 20[degrees]C (68[degrees]F). If the temperature is above the setpoint, there is no flow to the trench heater. When the temperature falls to the setpoint, flow through the trench heater is varied to maintain this setpoint due to the three-way valves allowing variable flow.

[FIGURE 4 OMITTED]

Plant. The MV building plant has a hot-water loop, serving the heating coils and trench heaters, and a cold-water loop, serving the cooling coil, chilled ceilings, and chilled beams. The only heat source is the boiler. From the boiler, water can flow through a bypass, either of the heating coils, or into the secondary loop. The secondary loop is maintained at a lower temperature than the primary loop and is fed with water from the primary loop as necessary to maintain this temperature. The secondary loop supplies the trench heaters. The primary loop flow rate is 18.5 L/s (293 gpm); the secondary loop flow rate has a maximum of 7.28 L/s (116 gpm). The primary loop is maintained at a setpoint of 80[degrees]C (176[degrees]F); the secondary loop is maintained at a setpoint of 52[degrees]C (126[degrees]F). The secondary loop in the building has a variable setpoint temperature dependent on the outside air temperature. The secondary loop flow rate varies in the building in order to save pumping energy. The building uses two natural-gas-fired boilers in parallel and has a third boiler as a backup unit.

The only cooling source is the chiller. From the chiller, water can flow through a bypass or the cooling coil. The warmer water exiting the cooling coil and bypass then flows through either another bypass or the secondary loop. The secondary loop is maintained at a higher temperature than the primary loop and is fed with water from the primary loop as necessary to maintain this temperature. The secondary loop supplies the chilled ceilings and beams. As with the primary hot-water loop, the primary chilled-water loop operates at a constant flow rate, which was taken from the building design drawings. The primary loop flow rate is constant at 33.5 L/s (530 gpm); the secondary loop flow rate has a maximum of 31.2 L/s (530 gpm). The primary loop is maintained at a setpoint temperature of 6[degrees]C (43[degrees]F); the secondary loop is maintained at a setpoint temperature of 15[degrees]C (59[degrees]F). The primary loop in the building has a variable setpoint temperature dependent on the need for dehumidification. The setpoint is 6[degrees]C (43[degrees]F) when dehumidification is needed and 10[degrees]C (50[degrees]F) at other times. The secondary loop flow rate varies in the building in order to save pumping energy. The MV building uses two 500 kW air-cooled chillers.

Naturally Ventilated Building

The three-story building is located in an office park with buildings of similar height, just outside of the city of Luton, 60 km (38mi) north-northwest of central London. The buildings in the office park are well spaced, with approximately 20-30 m (65-100 ft) between buildings. To the north of the NV building is an open public access park. The surrounding office buildings in the same office park are all mechanically ventilated and cooled. The building is owner-occupied by a housing assistance organization that decided to consolidate their operations into a central location. They required that the building be low energy and best value, following the "Egan Compliance" doctrine that is based on Sir John Egan's report, "Rethinking Construction" (Egan 1998), even if it was not the lowest in initial cost.

The prototype building is three stories high, has a net floor area of 2600 [m.sup.2] (28,000 [ft.sup.2]), with open plan office space on all three levels: the south side of the first floor, both south and north sides of the second floor, and the north side of the third floor. A typical building floor plan and section through the central atrium are presented in Figure 5. The north side of the first floor is used as a large meeting room that is closed off from the rest of the building and can be mechanically cooled due to the high internal heat gains for the short periods of time that it is used. The south side of the third floor is closed off from the rest of the building and houses the mechanical room, printing room, and file room. The occupied office spaces are completely open onto the central atrium that runs the length of the building along the east-west axis. The atrium extends a floor height above the third floor and is composed of glass panels on both the east and west facades and along the roof. The top of the atrium has five fan-powered ventilation stacks.

Lighting and Plug Loads. The occupants are located on all three levels of the building: the south half of the ground floor, both north and south halves of the first floor, and the north half of the second floor. Each half-floor has approximately 25 occupants, each with a computer and monitor, and there is a desktop printer for every three people. There are energy-efficient T-8 fluorescent lamps located throughout the building, except for the bathrooms and the "street lighting" in the atrium. The lighting fixtures in the occupied space, each with two lamps, are connected in banks of two to a combined occupancy and photosensor control mechanism. This feature allows the lighting to be shut off when the space is unoccupied. Also, at high daylight levels, the fixtures are continuously dimmed between 100% and 10% to further reduce energy usage when the space is occupied.

[FIGURE 5 OMITTED]

HVAC System. The NV building uses natural ventilation to ventilate and cool the building and a hot-water loop to heat the building during the winter. During the winter, the windows are closed and outside air enters the building only through infiltration or operation of the doors to the exterior. There is no mechanical air conditioning installed in the main office spaces of the NV building. The ventilation is a combination of buoyancy and wind-driven ventilation and fan-assisted ventilation stacks. There are seven sets of two windows, each containing a larger and a smaller window, at each floor level on both the north and south facades. Each larger, occupant-controlled window is located 1 m (3.3 ft) above the floor. It has a horizontal solar shading device located above it on the exterior facade to reduce direct glare and has a corresponding light shelf on the interior side of the window to direct sunlight further into the occupied space. All of the windows are, overall, 1.3 m (4.3 ft) wide, with the lower windows being 1.1 m (3.6 ft) high and the upper windows 0.45 m (1.5 ft) high. All of the windows are manually controlled to keep costs low without sacrificing function.

The building manager controls the operation of the smaller upper windows, determining when to open or shut them for the season. The upper windows are opened in the spring when the internal temperature during an occupied day has risen above 22[degrees]C (72[degrees]F). Ideally, the upper windows would be opened over a period of time, initially opening every other window and eventually having all of the upper windows open until the fall. The building operation and control manual (Rybka 2002) instructs the building manager to close these small windows during the night in the spring so as to not overcool the building. However, these windows are difficult to access due to the height of the handles. In actual practice, the upper windows are usually opened all at once in late spring and left open until the fall. The lower windows are controlled by occupants and opened as needed during the daytime but are always closed at night for security purposes.

Part of the building design included installation of venetian blinds to reduce the amount of glare through the windows, a cause of occupant complaints. The blinds are hung from the top of the upper window frame and are able to hang down and cover both the upper and lower windows on the facade. The blinds are made up of many 2.5 cm (1 in.) slats that can be adjusted as needed by the occupants: they can be drawn up or down as well as tilted to better control the amount of daylight entering the space. Since the blinds are installed on the top frame of the upper window, when the blinds are down, they cover the upper windows, restricting the amount of air that can enter (or exit) the building. Additionally, when these blinds are down, they reduce the effectiveness of the light shelves.

The stack vents at the roof have a series of louvers on each of the four vertical faces of the stacks. These are controlled by orientation (e.g., all of the eastern louvers for all five stacks are either open or closed). The louvers do not modulate and are, therefore, either fully open or fully closed. There also is a rain and wind sensor located on the roof that is tied into the simple control system for the fans in the stacks. If there is any rain, the louvers are closed. If the wind outside exceeds 4 m/s (9 mph) the louvers are closed in the direction of the wind. The louvers are kept open if the fans in the stacks are on. The fans are three-blade, 0.746 kW (1 hp) unidirectional fans. On still days, airflow is driven by buoyancy flow, with the louvers open in the stack vents to enhance the stack effect. On windy days, the airflow can be more similar to single-sided ventilation on the ground and second floors, with the potential for some cross-ventilation on the first floor. In part, this is dependent on the interior loads, number of windows open, and solar radiation. In automated mode, the fans and louvers are controlled by a single temperature point located in the atrium just above the ceiling level of the second floor. If this thermocouple reaches 26[degrees]C (79[degrees]F) or more, the louvers are allowed to open and the fans are turned on. If the temperature drops below 26[degrees]C (79[degrees]F), the fans turn off. Below 22[degrees]C (72[degrees]F), the louvers close.

Performance Evaluation Methods

Long-term monitoring of both buildings was conducted. The internal temperature and relative humidity throughout the occupied spaces was measured along with the energy consumption by subsystem and the external conditions. The equipment was installed at the beginning of the summer season and removed 18 months later. This extended period provided time for troubleshooting equipment during the first summer before recording a complete data set for a 12-month period. Problems were corrected to ensure that data loggers were recording properly and thermocouples remained in position. Through some trial and error and data analysis, after two months the equipment was working properly.

Temperature Sensors. Temperature and relative humidity sensors were used to monitor the climatic conditions of the occupied space for both of the buildings. The sensors had temperature accuracy of [+ or -]0.7[degrees] at 21[degrees]C ([+ or -]1.27[degrees] at 70[degrees]F) and [+ or -]5% accuracy for relative humidity. The same sensors were used for both buildings, but the location of the sensor varied based on suitable locations within the occupied space. These compact, discrete sensors were placed throughout the building. There were six per floor in the MV building. The second floor, south side, had a total of nine sensors to obtain a more accurate horizontal temperature distribution. The sensors were placed at desk level height (1.0 m [3.2 ft]) since most occupants spend much of their day working at a desk. Additional sensors were strung in the atrium, measuring temperature and relative humidity at interstitial spaces. Each end of the atrium had three sensors, for a total of six sensors. Their approximate locations were at 3.9 m (12.8 ft), 7.8 m (25.6 ft), and 11.7 m (38.4 ft) from the ground floor. The sensors had data collectors to record temperature and relative humidity information at 15-minute intervals.

One of the more important parameters monitored was the interior temperature. This is one of the first indications of the performance of any building. If the internal temperature is outside of the comfort range, productivity can deteriorate. The data-logging devices also recorded relative humidity levels--another important characteristic of a naturally ventilated building, since the humidity cannot be directly controlled. For long-term temperature and humidity monitoring in the NV building, compact temperature and relative humidity data loggers were used throughout the building. Most of the data loggers were located at desk height, approximately 1.0 m (3.2 ft) above the floor. An additional series of four data loggers was installed in each occupied floor area at four different heights, at the floor level, 0.76 m (2.5 ft) above the floor, 1.76 m (5.8 ft) above the floor, and 2.54 m (8.3 ft) above the floor, to obtain an initial determination of stratification within the occupied zone. Four other data loggers were placed around the atrium at each floor level to determine the amount of stratification that occurred there. The data loggers were placed between windows, to avoid being in the direct airflow from the window. The data loggers located at the atrium were attached at the floor level at the ledge of the atrium.

Carbon Dioxide Sensor. One method to determine the ventilation rate of the buildings was monitoring carbon dioxide (C[O.sub.2]) levels in the occupied spaces, combined with a data recorder. Measuring C[O.sub.2] can be used to determine air exchange rates and to evaluate indoor air quality. Several groups have defined maximum acceptable levels of C[O.sub.2] for office spaces. Levels above 1,000 ppm can lead to lethargy and headaches (EMS 2004). However, both the US Occupational Safety and Health Administration (OSHA) and the UK Building Services Research and Information Association (BSRIA) have defined maximum exposure limits to be 800 ppm over an eight-hour period for office areas. The C[O.sub.2] level depends on the ventilation distribution, occupant density, and amount of outside air being introduced into the space (ASHRAE 2001). When evaluating the indoor environment with respect to occupant health, ASHRAE suggests that an indoor C[O.sub.2] level of 650 ppm above the outside level is representative of an air exchange rate of 20 [ft.sup.3]/min per person, with an occupant density of 100 [ft.sup.2] per person (ASHRAE 1997). Occupant comfort is also affected by higher C[O.sub.2] levels; 20% of people are dissatisfied at C[O.sub.2] concentrations of 650 ppm above the outdoor level (Liddament 1996). In offices, carbon dioxide levels are primarily due to the respiration of the occupants.

Carbon dioxide sensors were used along with measurements of fresh air flow introduced into the occupant space to determine ventilation effectiveness of the building. The accuracy of the carbon dioxide sensor was [+ or -]50 ppm or 5% of the reading (whichever is greater) and a [+ or -]20 ppm repeatability. There was some minimal temperature dependence of [+ or -]0.1% of reading per [degrees]C or [+ or -]2 ppm per [degrees]C, whichever is greater, referenced at 25[degrees]C. Initially the C[O.sub.2] and temperature monitor was placed outside, away from the building, in order to record the external conditions as a baseline. Then the C[O.sub.2] sensor was placed at desk level, away from direct exposure from occupants, in the second floor office area, and data were recorded every 15 minutes over the 12-month monitoring period. On site visits, the number of people in each office area was logged over the period of the day and compared to the data recorded for that day. The meeting rooms on each floor of the MV building use transfer fans in order to introduce fresh air into these spaces. The design engineers claimed that it was the UK fresh air requirement for the meeting rooms and the minimum 8 L/s per person that determined the ventilation rate for the building. Therefore, measurements of the carbon dioxide levels were used to determine ventilation effectiveness for these spaces as well as the general office area. The carbon dioxide sensor had an unobtrusive data collector attached to it and was set up to log the carbon dioxide levels every 10 minutes. The sensor was placed in a meeting room, left in place for a week, and then relocated to another meeting room. This provided information on meeting rooms throughout the building, for a week-long period.

Hot Wire Anemometer. To measure the airflow for the building, both at a local level and an overall airflow rate for the building, a hot wire anemometer was used. For the MV building, air velocity measurements were recorded around several diffusers, at various radial positions along the floor plane, as well as at distances above the floor plane. Velocity measurements were taken at the floor level and at 0.5 m (1.6 ft), 1 m (3.2 ft), and 2 m (6.5 ft) heights in a radial pattern to determine the horizontal dispersion of supply air into the occupied space. The measurements were taken at the center, at the midpoint, and at the edge of the supply diffuser as well as 0.25 m (0.8 ft) away at eight locations along the circumference. Duct traverses for the supply and return ducts at each floor level were taken, using existing holes drilled into the ductwork. A grid pattern was set up to get an accurate airflow velocity profile at each branch location. A minimum of eight measurements were recorded within the measurement plane for each location. These data were recorded in order to determine the average airflow rate at that particular plane in the supply or return duct.

In the NV building, air velocity measurements were recorded at the windows, the stack vents, and within the occupied space to determine the penetration of fresh air into the office area and determine airflow patterns. The measurements recorded at the window were taken in the horizontal plane created by the awning-type window. At the stack vents, measurements were recorded between the louvers when the fans were on and off and on the underside of the fan housing when the fans were on. Air velocity measurements and smoke pencils were used to determine the airflows within the occupied space. Measurements were taken 1.5, 4.5, and 7.5 m from the window and at the atrium for three heights: floor level, 1.0 m (1.6 ft) from the floor, and 2.0 m (6.5 ft) from the floor. These measurements in the occupied space then were combined to create a map of typical airflow patterns within the occupied space, while monitoring the outside wind conditions.

Electrical Metering Equipment. As part of the overall building performance, electrical submetering equipment was installed to determine the respective contributions to the overall electrical usage and to verify the schedule of these systems. The occupancy schedule and energy usage profile can be determined by monitoring the energy consumption and usage patterns. The more detailed the metering of the electric energy within the building, the more thorough is the understanding of the building energy performance. Current transducers (CTs) in a variety of sizes were used in measuring the energy usage of system locations throughout the building. The CTs were connected to data loggers, set up to record the electrical usage every 15 minutes. CTs were installed on each of the three phases for each subsystem, when appropriate. Monitoring equipment was placed to monitor the building mechanical systems, including the chillers, air handlers, elevators, photovoltaics, and landlord services. This included five data loggers and CTs ranging from 50 to 500 amps to capture energy usage data.

To obtain accurate information for lighting and plug load usage in the building, additional electrical monitoring equipment was installed at each half-floor plate in the MV building (it was not feasible in the NV building). This enabled recording energy use by the lighting systems and plug loads by orientation, for a total of six data logging points. In the NV building, the detailed monitoring included data collection on each floor level by orientation, miscellaneous building services, lifts, atrium fans, and external/outside lights to determine their energy consumption. CTs were also installed on the actuators for the boiler in the NV building so that there was a measure of how often and on what schedule the boilers were in operation. Though the data were recorded over an 18-month period, there was still a 10% margin of error between the monitored data and the monthly energy bills. The total energy consumption of each building was not recorded due to the limitation in size of CTs available and the location of the incoming power supply. As a validation for the recorded data, 12 months of utility bills were collected and a walk-through assessment was conducted to inventory the energy-consuming equipment and systems of each building for comparison.

Weather Station. Accurate external conditions were collected using a portable, external weather station combined with a data recorder, mounted on the roof of each building. This provided data both on the conditions to which the building was exposed and on the entering air properties. Temperature, relative humidity, barometric pressure, and wind speeds were monitored. Because there was a substantial amount of glazing on the facades of the buildings (nearly 100% of the MV building and 46% of the NV building), solar radiation was an important condition to be monitored. Two pyranometers were used to measure both global and diffuse solar radiation. The weather stations were located on the roof of each building, away from any obstructive points, including boiler exhaust and items that could produce shadows. The outside temperature gauge was housed in a white plastic device so that it was not subject to erroneous readings due to solar gain, rain, or wind. The rotating vane cup anemometer was located 1 m (3 ft) above the atrium roofline, approximately 1.5 m (5 ft) away from any vent. The gusting or peak wind speed over the 15-minute interval as well as the average value over the same interval were recorded. Data were recorded every 15 minutes, and the shadow band shading the pyranometer was adjusted once every four months to ensure that it had not been moved by high winds or birds.

MEASURED PERFORMANCE RESULTS

The data recorded for each of the buildings are presented and compared below. It is important to examine not only the energy consumption of these two low-energy building designs but also the interior conditions in which the occupants work. Mechanically ventilated buildings are known for their good control of the indoor environment, filtering outside air, cooling or heating the air as needed, and also humidifying the air. However, naturally ventilated buildings rely almost completely on the external environment for ventilating the occupied space, with the external environment also influencing the interior temperature conditions during the summer because there is no mechanical cooling present. Results of the building performance monitoring for these two low-energy buildings are presented here.

Energy Consumption

The energy usage data recorded by the monitoring equipment were compared to total monthly energy usage data (in the form of utility bills) provided by each facility. The total annual usage data from our monitoring was within 10% of the actual usage numbers. It provided important information on the energy usage of the subsystems with in the building. For the MV building, both chillers and the cooling motor control panel (fans and pumps associated with mechanical cooling) data were categorized as cooling, while the landlord services, lifts (elevators), and atrium lights were put under the title of services. The ventilation category includes the humidifiers and the HVAC motor control panel, while the tenant lighting and plug loads each include all six half-floor plates of data. It was found that during the winter and part of the fall and spring, heat was required at the perimeter of the building, while cooling was still required to control the internal loads. This resulted in both the chillers and boilers being in operation at the same period of time. From detailed submetering of the building, the photovoltaics' contribution to the building's energy consumption was determined. The photovoltaics were integrated with the exterior shading devices on the southern and western facades. The monthly electrical energy contribution of the photovoltaics ranged from 885 kWh in July 2002 to 126 kWh in December 2002. The annual contribution of the solar cells to the building energy, as measured during the monitoring period, was almost 9000 kWh, or 1.7% of the annual energy for the MV building.

The overall energy demand for each building type also varied based on the type of building and, more specifically, the occupant and equipment density. The MV building was able to accommodate more people and their equipment per floor area comfortably. Based on the values presented in Table 2, the energy demand as monitored per occupant is, in general, twice as high for the MV building as it is for the NV building during the summer. The occupied hours for the buildings were similar, so we would expect the energy consumption per occupant to be similar as well.

In the MV building, each light fixture has an occupancy and photo sensor so that the lights will automatically turn off after a period of time when the space under the fixture is unoccupied or if there is enough daylight from the windows. Detailed submetering of energy consumption was measured for each half-floor plate, and due to the orientation of the building, the effectiveness of the lighting sensors was determined. The energy usage per floor area is provided in Figure 6. On the ground and second floors, there is some reduced energy usage by the lighting system on the south versus the north half-floor plate of the building. However, the first floor had very similar usage patterns for the south versus the north side of the building. The position of the window blinds influenced the lighting energy. On the first floor, the blinds on the south side remained closed most of the time because of glare. On the other floors, the blinds were adjusted for different daylighting conditions.

[FIGURE 6 OMITTED]

Ventilation Effectiveness. Measurements of airflow rates within both buildings were important in determining the effectiveness of the ventilation system. For the MV building, there were two supply risers that serve the entire building, with branches in the ducts occurring at floor levels. Duct traverse measurements were taken during a typical weekday in January 2002, and the data are presented in Table 3. The data are for a typical work day, when the HVAC equipment was in operation supplying 100% outside air.

The overall airflow rate for the building, based on the occupancy and duct traverse measurements, was determined to be 40 L/s (85 cfm) per person. This is four times higher than the US and UK minimum required fresh air rates, which are 8-10 L/s (7-21 cfm) per person. The building was originally designed to accommodate more than twice the current number of people in the building. Thus, higher airflow rates were included in the building. Additionally, the building was not intended to use 100% outside air during occupied hours. However, in the first several years of operation, that was the operating condition.

In comparing the MV and NV buildings, the air change rates were substantially different. The air change rates were determined by using the ventilation rates (duct traverse measurements and window air velocities) and the volume of the occupied space. The MV building had over 30 air changes per hour (ach) due to the overventilation. Even if the ventilation were reduced to 10 L/s per person (21 cfm per person), the ach would be approximately 8 ach. With the NV building, the highest ventilation rates were reached during the summer, when all of the windows were open. Then the ventilation rate reached 3 ach. As mentioned previously, the use of blinds over the windows, the external wind conditions, and the decision by the occupants as to window openings influences the air change rate of the building.

Indoor Environment

The internal conditions of the occupied space were evaluated using temperature, relative humidity, and carbon dioxide levels. Data recorded for both the MV and NV buildings are presented and compared below.

Temperature and Relative Humidity. Using the sensors that recorded both temperature and relative humidity throughout the occupied space, a comparison of the conditions in the MV and NV building was completed. For the NV building, it is important to examine the internal conditions by season, as the internal environment is heavily dependent on the external conditions. However, with the MV building, the occupied space temperature is tightly controlled and, therefore, an average for the entire year can be used in the evaluation. Table 4 presents the average, minimum, and maximum temperature and relative humidity for both the MV and NV buildings over the 12-month monitoring period. The table does not include internal temperature data from the record heat wave during the summer of 2003 in the NV building. For the NV building, even during the warm summer months, the interior temperature was 3[degrees]C(5[degrees]F) lower than the outside air temperature. The internal temperatures in the MV building were not dependent on the outside temperature. The annual temperature for the occupied space was maintained during occupied hours around 21.5[degrees]C (71[degrees]F). The data in Table 4 are for the occupied periods only and do not include data from weekend periods when any ventilation, heating, or cooling equipment was turned off.

Carbon Dioxide. Both the United States and the United Kingdom have maximum allowable levels of carbon dioxide and corresponding exposure times for people in the workplace. It is recommended by EMS (2004), a UK group, that levels above 1000 ppm are not to occur for longer than eight hours before problems with lethargy and headaches can occur. ASHRAE Standard 62-2004 contains a guideline value for carbon dioxide of 1000 ppm. However, it is based on the association of higher levels of carbon dioxide concentrations with unacceptable levels of body odor, not on any health or comfort effects of the carbon dioxide itself.

After determining from duct traverse measurements that the building was providing too much fresh air, the engineering design team for the MV building was consulted. The meeting rooms are supplied with fresh air through underfloor ducts using transfer fans. The fans turn on when the room is occupied and turn off when it is unoccupied. When in use, the doors to the meeting rooms were almost always closed, thereby relying on the transfer fans to provide fresh air into the space. The meeting rooms were the limiting factor with the desired airflow rate due to the size of the transfer fans and the amount of air that is able to be supplied. Therefore, carbon dioxide (C[O.sub.2]) measurements were recorded in these meeting rooms. The C[O.sub.2] sensor was placed outside, on the roof of the MV building, to obtain accurate data on the outside C[O.sub.2] level. This was measured at 415 parts per million (ppm). The small C[O.sub.2] sensor was then placed in each of four meeting rooms for a week at a time, over the period of a month. The meeting rooms in the MV building are well below the allowable limits, even when the meeting rooms are fully occupied.

Building Performance Benchmark

During evaluation of these buildings, two benchmarks were used for comparison. These benchmarks are the UK Energy Consumption Guide (ECG 019) for office buildings (Action Energy 2003) and the 2005 Buildings Energy Data Book (Kelso and Patterson 2005). A summary of each of these methods is provided in the following sections.

ECG 019. This set of benchmarks was derived from energy consumption measurements in a wide range of occupied office buildings. The buildings surveyed were organized into four categories based on the manner in which the building was ventilated and the energy density of the building. Data were presented by end use and total energy consumption in kilowatt-hours per square meter and carbon emissions in kilograms of carbon per square meter for each of the building types described below. The four categories are defined as follows:

* Naturally ventilated cellular office buildings: These buildings have individual lighting and heating controls, and operable windows, allowing occupants to have more control over their environment. There are few common facilities such as vending in this category.

* Naturally ventilated open-plan office buildings: Buildings in this group have higher illuminance levels, lighting power densities, and occupied hours than the cellular type. The controls often serve a larger area and tend to operate for longer periods of time. This building type tends to have more office equipment and vending equipment than cellular offices.

* Air-conditioned, standard office buildings: These buildings are based on a variable-air-volume ventilation system with air-cooled chillers. This category of buildings has a deeper floor plan than do the naturally ventilated buildings and often has tinted glazing to reduce glare, which, in turn, reduces natural daylighting.

* Air-conditioned, prestige office buildings: This class of buildings has operation and systems similar to the standard air-conditioned office building but with longer operating hours, conditioned central computer rooms, and more energy-intensive amenities, such as food preparation areas.

Buildings Energy Data Book. This publication by the US Department of Energy provides the typical characteristics for the building stock in the United States. From building surveys, a typical commercial building profile is compiled, including energy intensity by building type and by end-use category. The data are gathered from government documents, models, and analysis. Though developed from US data, it provides a comparison of a typical commercial office building energy use. The Buildings Energy Data Book does not distinguish between building types, as in the ECG019, but rather lumps all commercial building types into a category.

COMPARISON OF NATURALLY AND MECHANICALLY VENTILATED OFFICE BUILDINGS

The building data obtained from both the MV and NV buildings were compared with the appropriate reference buildings from the ECG 019 standard. The MV building was compared to the standard and air-conditioned building, and the NV building was compared to the open plan, naturally ventilated building. The benchmark data are presented in Table 5. The range from standard practice (Std.) to what is considered good practice (GP) for energy consumption by category is presented for reference. The categories in common (lighting, office equipment, and heating/hot water) are presented for both the MV and NV buildings for comparison.

Table 5 shows the connected load in watts per floor area for the two building ventilation types and the range (Std. and GP) prescribed for UK office buildings by the ECG019. Energy use in kilowatt-hours per floor area is dependent on the number of hours that the energy-using system operates. For the ECG019, this varies by both building and ventilation type due to the configuration of the building and the incorporation of energy-conserving technologies and practices. Naturally ventilated buildings have lower lighting, office equipment loads, and occupant density than mechanically ventilated buildings. This leads to the slightly lower numbers when comparing the standard MV and NV energy end-use numbers. When comparing the MV building, NV building, and the US commercial building benchmark, the NV building performs better than the US average, but the MV building has a higher consumption per floor area, as shown in Table 6. This is the combined natural gas and electrical energy site usage. The data for both countries are the average over the entire region. For the United States, this encompasses a wide variety of climate types, whereas for the United Kingdom in general the climate is temperate overall.

Energy usage data for both buildings by type of energy and end use per floor area are provided in Table 7. As the buildings are in the same climate, they can be more directly compared. The data presented were recorded over a minimum 12-month period for each building by energy-using equipment for the electrical energy. Natural gas usage was determined from utility bills provided by the building manager.

The NV building uses 13.6% less natural gas than the monitored MV building, but of more significance is the reduction of over 50% in electric energy usage per floor area. Both buildings were "un-conditioned" on the weekends and both incorporated energy-saving features on both lighting fixtures and office equipment. The overall energy consumption per floor area for lighting and office equipment is quite similar for the two buildings. The mechanically ventilated building was designed to have three times the current occupancy, which was approximately 120 people, whereas the NV building had 100 occupants, which was close to the maximum designed occupancy level. However, the energy required for mechanical cooling, the chiller and fan categories, makes a substantial difference. A substantial amount of additional energy usage for the mechanically ventilated office building is required for operating the mechanical cooling equipment, which required the chillers to operate year-round to meet the cooling needs for the office building. By relying on the environment for not only cooling but also air movement, the NV building has much less energy consumption for both chiller (and associated pumps) and fan energy usage. Overall, the mechanically ventilated system used 354.5 kWh/[m.sup.2] (32.7 kWh/[ft.sup.2]), compared to the naturally ventilated building with 216.1 kWh/[m.sup.2] (20.1 kWh/[ft.sup.2]). Both buildings provided at least the minimum requirements for outside air as required by standards of 8-10 L/s (7-21 cfm) per person of outside air. This was based on duct traverse measurements in the mechanically ventilated building and hot-wire anemometer measurements at the windows for the naturally ventilated building. The NV building was designed to be a low-cost building, owner-occupied, while the mechanically ventilated building was designed to be a commercially viable and rentable space. In a previous portion of the Massachusetts Institute of Technology Institute (CMI) program, Olsen et al. (2003) simulated the temperature conditions in the mechanically ventilated building if it were converted to cross-flow natural ventilation. Using the measured internal loads, along with CFD to predict the air flow and temperature conditions in the cross section of the MV building, they found that the interior temperature did not exceed 28[degrees]C (82[degrees]F) for more than 40 hours per year using the London historic weather data.

An issue of some concern in naturally ventilated buildings is temperature conditions and variation within the occupied space. In the NV building, there was temperature variation throughout the occupied hours, with personal fans used to provide additional breezes on particularly warm days, such as during the heat wave that occurred at the beginning of the monitoring period. The vertical temperature within an occupied space was observed to vary by as much as 3[degrees]C (5.4[degrees]F), with further variation from floor to floor. The thermal mass at the ceiling level helped to temper the overall temperature variation within an individual floor. Some temperature stratification was observed between floor levels during the summer, when the top floor would be several degrees warmer than the ground floor. On the other hand, in the MV building, the temperature never varied by more than 2[degrees]C (3.6[degrees]F) anywhere in the building. The chilled-ceiling system, controlled by zones, maintained the setpoint temperature, while the primary cooling load was provided by the cool supply air at the floor level. The minimum, maximum, and average temperature for each building for a typical summer weekday is presented in Figure 7. The MV building temperature difference between the minimum and maximum is much smaller than that for the NV building during occupied hours.

[FIGURE 7 OMITTED]

DISCUSSION AND RECOMMENDATIONS

Several issues in terms of building energy usage and thermal comfort were observed in the MV building. Some of these issues are straightforward, while others require some additional investigation. Many of the identified issues are presented below. These measures would help reduce the overall energy consumption of the buildings, thereby bringing their energy usage per floor area closer to the "Good Practice" values.

Repairing the heat recovery loop in the MV building has the greatest rate of return. Because the building uses 100% outside air, there is great potential, particularly during the winter, to recover some of the waste heat. As discussed previously, the building is overventilated, providing 40 L/s per person (85 cfm per person) of outside air for the occupants. Two options are available: using less than 100% outside air or reducing the airflow rate. Reducing the airflow rate could save additional fan energy. The specifications for the heat recovery loop were available, resulting in a calculated heat recovery efficiency of 43%. With the measured airflow rate of 15.7 [m.sup.3]/s (33,260 cfm) and 36,000 heating degree-hours based on the temperature bin analysis, the resulting energy savings using the heat recovery loop is estimated to be over 290,000 kWh annually. Reducing the volume of air by half in addition to repairing the heat recovery loop could reduce the energy consumption by an additional 78,000 kWh annually.

The energy consumed by the lighting system could be reduced in two ways. In both the MV and NV buildings, the blinds can remain open more often. There should be more significant difference in the lighting energy usage between the north and south sides of the building than was measured in the MV building. Second, for the MV building in the clerestory area, the blinds were closed, preventing natural light from penetrating the central atrium. This would have triggered some of the lights on the perimeter of the atrium to either dim or turn completely off during certain times of the day. The atrium street lights were often on during the day in the MV building when no additional lighting was needed. These lights are controlled by a manual switch that requires an occupant to determine when the lights should be on or off.

The temperature of the supply air is 18[degrees]C (64[degrees]F) in the MV building, which can feel very cold in the winter on people's feet, particularly with the high ventilation rate. Raising the supply air temperature would assist with this issue; however, further studies would be needed to determine if raising the supply air temperature would adversely affect the chilled-ceiling operation. Also, there were some complaints by occupants, particularly on the ground floor, south floor plate, who experienced large drafts of cold air in the winter time. As most of the occupants of the building leave and return for lunch around the same time, there are long periods when both sets of entry doors are open, allowing cold air to enter the building. To address this issue, a revolving door or having one set of doors offset to open on a different, protected side of the vestibule would be necessary.

In the NV building, the occupants better adapted to their environment, as the supply air was not at a controlled temperature but was a function of the outside conditions. The building has areas of improvement, including better precooling of the thermal mass and use of the fans and window blinds to control the interior temperature. The window blinds affected not only the airflow into the building but also the performance of the photosensors on the overhead lights and light shelves that were part of the low-energy building design. By better placement and use of the blinds, the NV building can further reduce its energy consumption while maintaining occupant comfort in the indoor environment.

SUMMARY AND CONCLUSIONS

In general, long-term detailed monitoring of the MV and NV buildings allowed the unique opportunity to study in depth the energy, ventilation, and overall performance of two different commercial office buildings. The level of detail that was completed during the 12 months of monitoring of the buildings is unusual. Most commercial facilities have building energy management systems, which assist in operating the building, but submetering provides an additional level of information. An accurate representation of the energy usage by subsystems over one year can determine the distribution of energy use throughout the building and track energy savings due to the implementation of energy-efficient measures. Occupant comfort issues can better be addressed with space temperature reading, and systems can be evaluated for their efficiency and effectiveness.

The MV building was designed and built to be a commercially viable property that could be leased as needed. Therefore, the ability of the space to meet the needs of any future occupant was important while minimizing the energy consumption of the building. The MV building had energy-efficient design components that were not used optimally to maximize their impact on the building energy consumption. The NV building was owner-occupied, designed to be a permanent location for the occupants, with the potential to expand on the current property as needed. These two views on the design intent of the low-energy building design significantly influenced the level of flexibility in the range of acceptable indoor environmental conditions for the building space. Had the MV building been owner-occupied and not built with the intent of leasing it to other tenants in the future, other design considerations may have been implemented.

REFERENCES

Action Energy. 2003. Energy Consumption Guide-ECG019: Energy Use in Offices. Best Practices Programme, Crown, London.

ASHRAE. 2004. ANSI/ASHRAE Standard 62. 1-2004, Ventilation for Acceptable Indoor Air Quality. Atlanta: American Society for Heating, Refrigerating and Air-Conditioning Engineers, Inc.

ASHRAE. 2001. 2001 ASHRAE Handbook--Fundamentals, chapters 12, 9. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.

ASHRAE. 1997. 1997 ASHRAE Handbook--Fundamentals, chapter 9. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.

BBC Weather Center. 2006. Average conditions: London, United Kingdom. London: Helicon Publishing. www.bbc.co.uk/weather/world/city_guides/results.shtml?tt=TT003790.

DTI. 2004. The Digest of the United Kingdom Energy Statistics 2004. Norwich: The Stationary Office. Department of Trade and Industry, UK.

Egan, J. 1998. Rethinking Construction. Department of Trade and Industry. Office of Deputy Prime Minister, London.

EIA. 2005. Energy Consumption by End-Use Sector. International Energy Outlook 2005. Washington, DC: US Department of Energy, Energy Information Agency.

EIA. 1995. Commercial Buildings Energy Consumption Survey. Washington, DC: US Department of Energy, Energy Information Agency.

EMS. 2004. Environmental Monitoring Services. www.emsonline.co.uk.

Kelso, J.D., and P.D. Patterson. 2005. 2005 Buildings Energy Data Book. Washington, DC: US Department of Energy, Office of Planning, Budget Formulation, and Analysis. http://buildingsdatabook.eere.energy.gov.

Liddament, M. 1996. Why C[O.sub.2]? Air Infiltration Review, vol. 18 (December).

Rybka. 2002 (October). Aldwyck housing association: Building operational philosophy. Rybka Consulting Engineers.

BIBLIOGRAPHY

Awbi, H. 2003. Ventilation of Buildings, 2d ed., chapters 6, 7, 9. London: Spon Press.

Etheridge, D., and M. Sandberg. 1996. Building Ventilation--Theory and Measurement, chapter 14. New York: John Wiley & Sons.

Levermore, G.J. 2000. Building Energy Management Systems: Applications to Low-Energy HVAC and Natural Ventilation Control. London: Spon Press.

MacDonald, J.M., and D.M. Wasserman. 1989 (May). Investigation of metered data analysis methods for commercial and related buildings. ORNL/CON-279. US Department of Energy, Oak Ridge National Laboratory, Oak Ridge, TN.

Walker, C.E., S. Manchanda, H.C. Spindler, and L.K. Norford. 2004. Building performance: Analysis of naturally ventilated UK office building. RoomVent 2004 Conference Proceedings, Coimbra, Portugal.

Christine E. Walker, PhD

Associate Member ASHRAE

Leon R. Glicksman, PhD

Member ASHRAE

Leslie K. Norford, PhD

Member ASHRAE

Christine E. Walker is a senior research engineer in the Energy Resources Center, Department of Mechanical and Industrial Engineering, University of Illinois at Chicago. Leon R. Glicksman and Leslie K. Norford are professors in the Building Technology Program, Massachusetts Institute of Technology, Cambridge, MA.
Table 1. Average Climatic Conditions for London, UK (BBC 2006)

 Temperature
 Min Max
Month [degrees]C [degrees]F [degrees]C [degrees]F

Jan 2 36 6 43
Feb 2 36 7 45
Mar 3 37 10 50
Apr 6 43 13 55
May 8 46 17 63
Jun 12 54 20 68
Jul 14 57 22 72
Aug 13 55 21 70
Sep 11 52 19 66
Oct 8 46 14 57
Nov 5 41 10 50
Dec 4 39 7 45
Avg Year 7.3 45 13.8 57

 Relative Humidity Average Precipitation
Month AM PM mm in.

Jan 86 77 54 2.1
Feb 85 72 40 1.6
Mar 81 64 37 1.5
Apr 71 56 37 1.5
May 70 57 46 1.8
Jun 70 58 45 1.8
Jul 71 59 57 2.2
Aug 76 62 59 2.3
Sep 80 65 49 1.9
Oct 85 70 57 2.2
Nov 85 78 64 2.5
Dec 87 81 48 1.9
Avg Year 78.9 66.6 49.4 1.9

Table 2. Comparison of Energy Usage and Number of Occupants for Summer
Months

 Mechanically Naturally
 Ventilated Ventilated
 Peak Peak
 Energy, No. of Energy, No. of
 kW Occup. kW Occup.

Ground Floor North 13.67 56 N/A --
Ground Floor South 11.98 55 3.6 30
First Floor North 11.58 50 3.1 27
First Floor South 11.02 54 3.7 30
Second Floor North 12.88 85 2.7 18
Second Floor South 12.87 85 N/A --

Table 3. MV Building Duct Traverse Measurements

 South North South North South North
 Ground Ground First First Second Second

Air supply rate, 2.23 2.23 2.41 2.49 3.67 2.77
 [m.sup.3]/s
Air supply rate, 4725 4725 5106 5276 7776 5869
 cfm

Table 4. Internal and External Temperatures

 NV Building Over 24 Hours by Season
 Spring, Summer, Fall,
 [degrees]C [degrees]C [degrees]C
Temperature ([degrees]F) ([degrees]F) ([degrees]F)

Building average 20.8 (69.4) 22.5 (72.5) 21.2 (70.2)
Building minimum 18.5 (65.3) 16.9 (62.4) 16.0 (60.8)
Building maximum 23.5 (74.3) 25.2 (77.4) 25.0 (77.0)
Outside average 11.3 (52.3) 17.4 (63.3) 13.3 (55.9)
Outside minimum 2.9 (37.2) 11.0 (51.8) 2.0 (35.6)
Outside maximum 24.4 (75.9) 28.3 (82.9) 27.4 (81.3)

Relative Humidity Spring Summer Fall
RH average 35.9 47.2 47.7
RH minimum 23.7 29.8 29.7
RH maximum 52.4 72.1 53.2

 NV Building Over 24 Hours by Season MV Bldg
 Winter, Annual
 [degrees]C [degrees]C
Temperature ([degrees]F) ([degrees]F)

Building average 20.2 (68.4) 21.5 (70.7)
Building minimum 13.6 (56.5) 20.2 (68.4)
Building maximum 22.5 (72.4) 22.3 (72.1)
Outside average 5.1 (41.2)
Outside minimum -2.8 (27.0)
Outside maximum 11.8 (53.2)

Relative Humidity Winter MV Bldg
RH average 38.0 39.9
RH minimum 25.7 30.9
RH maximum 55.4 43.0

Table 5. Energy Usage Comparison to Benchmarks--Energy Consumption Guide
019 Benchmark Data for UK (Action Energy 2003)

Building Type Configuration
Mechanically ventilated (MV) General
Naturally ventilated (NV) Open plan
Energy End Use MV Std. MV GP

Interior lighting, W/[m.sup.2] 20 (1.9) 12 (1.1)
 (W/[ft.sup.2])
Office equipment, W/[m.sup.2] 16-18 (1.5-1.7) 14-15 (1.3-1.4)
 (W/[ft.sup.2])
Interior lighting, kWh/[m.sup.2] 54-60 (5.0-5.6) 27-29 (2.5-2.7)
 (kWh/[ft.sup.2])
Office equipment, kWh/[m.sup.2] 31-32 (2.9-3.0) 23 (2.1)
 (kWh/[ft.sup.2])
Heating/hot water, kWh/[m.sup.2] 178-201 (16.5-18.7) 97-107 (9.0-9.9)
 (kWh/[ft.sup.2])
Fans, pumps, & controls, 60 (5.6) 30 (2.8)
 kWh/[m.sup.2] (kWh/[ft.sup.2])
Cooling, kWh/[m.sup.2] 31 (2.9) 14 (1.3)
 (kWh/[ft.sup.2])
Whole Building, kWh/[m.sup.2] 300-330 (28-31) 173-186 (16-17)
 (kWh/[ft.sup.2])

 Floor Area
 2000-20,000 [m.sup.2] (21,500-215,000
Building Type [ft.sup.2])
Mechanically ventilated (MV) 500-4000 [m.sup.2] (5000-43,000
Naturally ventilated (NV) [ft.sup.2])
Energy End Use NV Std. NV GP

Interior lighting, W/[m.sup.2] 15-18 (1.4-1.7) 12 (1.1)
 (W/[ft.sup.2])
Office equipment, W/[m.sup.2] 12-14 (1.1-1.3) 10-12 (0.9-1.1)
 (W/[ft.sup.2])
Interior lighting, kWh/[m.sup.2] 23-38 (2.1-2.6) 14-22 (1.3-2.0)
 (kWh/[ft.sup.2])
Office equipment, kWh/[m.sup.2] 18-27 (1.7-2.5) 12-20 (1.1-1.9)
 (kWh/[ft.sup.2])
Heating/hot water, kWh/[m.sup.2] 151 (14.0) 79 (7.3)
 (kWh/[ft.sup.2])
Fans, pumps, & controls, 6 (0.6) 2 (0.2)
 kWh/[m.sup.2] (kWh/[ft.sup.2])
Cooling, kWh/[m.sup.2] -- --
 (kWh/[ft.sup.2])
Whole Building, kWh/[m.sup.2] 219-248 (20-22) 127-145 (12-13)
 (kWh/[ft.sup.2])

Table 6. Comparison of Building Energy-Intensity Benchmarks

 Total Energy Use
 x [10.sup.3] x [10.sup.3]
 MBtu/[m.sup.2] MBtu/[ft.sup.2]

MV commercial building, UK 1087.1 101.0
NV commercial building, UK 738.4 68.6
Typical commercial building 974.1 90.5
 (MV), US

Table 7. Mechanically Ventilated (MV) and Naturally Ventilated (NV)

 Raised NV
 Floor MV Building

Total natural gas, kWh/[m.sup.2] (kWh/[ft.sup.2]) 162 (15.1) 140 (13)
Total electricity, kWh/[m.sup.2] (kWh/[ft.sup.2]) 156 (14.5) 76 (7.1)
Lighting & office, kWh/[m.sup.2] (kWh/[ft.sup.2]) 55 (5.1) 51 (4.7)
Chiller, kWh/[m.sup.2] (kWh/[ft.sup.2]) 29 (2.7) 0
Fans & controls, kWh/[m.sup.2] (kWh/[ft.sup.2]) 50 (4.6) 5 (0.5)
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Author:Walker, Christine E.; Glicksman, Leon R.; Norford, Leslie K.
Publication:ASHRAE Transactions
Geographic Code:4EUUK
Date:Jan 1, 2007
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