hik,r = radiative heat transfer coefficient between surfaces i and k, Ti = temperature of interior surface i,
Tk = temperature of interior surface k, and
(6.4) where
hc = convective heat transfer coefficient, and Troom = room air temperature.
Most existing energy simulation programs assume Tair to be uniform in the entire room/building. The assumption is appropriate for a room with a mixing venti- lation system where the room air temperature is relatively uniform. However, a single temperature is not good for displacement ventilation because the non-uniform temperature distribution in displacement ventilation can have a major impact on energy consumption of the HVAC system. The air temperature at the boundary layer of a wall is an important factor for the heat transfer through convection in the air-wall interface. This study uses the air temperature at 4 in. (0.1 m) from a wall surface (Ti,air in Figure 6.2) as the Tair. If the air temperature in the center of the occupied zone is controlled to be Troom, Equation 6.4 becomes
qi,c = hi,c (Ti – Ti,air)
= hi,c(Ti – Troom) – hi,c∆Ti,air , (6.5) qik = hik,r(Ti–Tk) ,
Figure 6.1 Energy balance on the interior surface of a wall, ceiling, floor, roof, or slab.
where
∆Ti,air = Ti,air – Troom .
The Ti,air can be directly obtained from CFD simulation, as shown in Figure 6.2. However, this could be very time consuming if the CFD simulation would be done hourly for a whole year. Hence, the present investigation uses the temperature model developed and used in Chapter 4 to determine ∆Ti,air.
Energy Balance Equation for Window
For the window shown in Figure 6.3, we have the following energy balance equation:
Figure 6.2 Schematic presentation of the coupling between flow and energy programs.
(6.6)
where
qi = conductive heat flux on window i,
qi,s = inward heat flux of the absorbed solar radiation by window i, qi,t = transmitted solar heat flux reabsorbed by window i, and qik = emitted radiative heat flux from window i to room surface k.
The ACCURACY program can calculate qi, qi,s, and qi,t. The qik and qi,c are determined in the same way as those for walls.
Let
qi,in = qi + qi,t for walls, ceiling, floor, etc. qi,in = qi + qi,s + qi,t for windows. Then we have [H] [T] = [q] + [h ∆T] (6.7) where (6.8) (6.9) (6.10) qi+qi,s+qi,t qik+qi,c
k=1 N ∑ = H [ ] h1,c h1k,r k=1 N ∑ + –h12,r … –h1N,r h21,r – h2,c h2k,r k=1 N ∑ + … –h2N,r … … … … hN1,r – … –hNN– r1, hN,c hNk,r k=1 N ∑ + = T [ ] T1 T2 … TN = q [ ] q1,in+h1,cTroom q2,in+h2,cTroom … qN,in+hN,cTroom =
(6.11)
Solving Equations 6.1 to 6.11 together, the surface temperatures of room enclo- sure, Ti, and heat extraction (heating/cooling load) can be obtained.
6.2 SECONDARY SYSTEMS AND PLANTS
Secondary systems simulate mass, energy, and moisture balance at various components (coils, fans, etc.) and junctions (mixing boxes, deviators) in the air- handling systems. Most programs, whole-building simulation programs, provide a number of fixed menus of common secondary systems. A typical program is Ener- gyPlus or DOE-2. Some programs, component-based programs such as TRNSYS (Klein et al. 1994), provide component-based or modular simulators. These programs would allow the user to interconnect freely the components that are pack- aged as algorithms. Generally, those programs are most useful for detailed studies of special systems, such as active solar heating and cooling systems and for control studies.
The whole-building simulation approach seems the best to compare the energy consumption by displacement and mixing ventilation systems, and it is used for the present investigation. Figure 6.4 shows the air-handling systems used. Note that the figure shows only one of many possible systems. Another preferred system is to use 100% outdoor air to achieve better air quality. Then energy in the exhaust air can be conserved by using exchangers.
According to the results shown in Chapter 5, the displacement ventilation system needs a separated heating system for winter heating. The present investiga- tion used a baseboard heater, as shown in Figure 6.4. Further, in order to conserve energy, the air-handling systems use economizers and heat exchangers.
The mainstream programs model chillers by curve fits of manufacturers’ data and boilers with a single seasonal efficiency. More detailed models are also avail- able, such as inclusion of combustion calculations in boilers. The present investiga- tion uses a variable-air-volume system with constant air supply temperatures, except in the shoulder season when the supply air temperature fluctuates for maximum use of free cooling. We use two COP values for chillers—one for displacement venti- lation and another for mixing ventilation.
6.3 ENERGY ANALYSIS FOR U.S. CONDITIONS
This section compares energy consumption by displacement ventilation with that by a mixing-type system for an individual office building, a classroom building, and a workshop building. The study is for five climatic regions in the U.S.: hot,
h T∆ [ ] h1,c∆T1,air h2,c∆T2,air … hN,c∆TN,air =
humid (e.g., New Orleans, LA), hot, dry (e.g., Phoenix, AZ), moderate (e.g., Nash- ville, TN), maritime (e.g., Seattle, WA), and cold (e.g., Portland, ME). Table 6.1 lists the building characteristics and thermal conditions used in the study.
In addition, the investigation uses the following assumptions/conditions:
• A fixed fan efficiency = 0.60 • A fixed boiler efficiency = 0.75
• A fixed COP for chiller = 2.9 for mixing ventilation and 3.0 for displacement ventilation
• Pressure drop of the air-handling system: 1900 Pa (7.64 in. of water)
• Supply air temperature for the mixing system = 12.8°C (55°F) for cooling and 40°C (104°F) for heating
• Supply air temperature for the displacement system is determined by Equa- tion (4.19)
• Minimum outdoor air = 10 L/s per person (20 cfm per person) for the mixing ventilation and 7.7 L/s per person (15 cfm per person) for displacement venti- lation to ensure the same indoor air quality
Figure 6.4 The air-handling systems used by the displacement and mixing ventilation.
Table 6.1 Building Characteristics and Thermal Conditions
Space Type Small Office (SO) Classroom (CR) Workshop (WS)
Space Size 5.2 m × 3.7 m × 2.4 m (17 ft × 12 ft × 8 ft) 11.7 m × 9.0 m × 3.3 m (38 ft × 30 ft × 11 ft) 26.2 m × 21 m × 4.5 m (86 ft × 68 ft × 15 ft) Exterior envelope
Wall Seattle & Portland U = 0.72W/m2⋅K (R = 7.9 ft2⋅h⋅°F/Btu) U = 0.72W/m2⋅K (R = 7.9 ft2⋅h⋅°F/Btu) U = 0.72W/m2⋅K (R = 7.9 ft2⋅h°⋅F/Btu) Phoenix, New Orleans & Nashville U = 0.96W/m2⋅K (R = 5.92 ft2⋅h⋅°F/Btu) U=0.96W/m2⋅K (R = 5.92 ft2⋅h⋅°F/Btu) U = 0.96W/m2⋅K (R = 5.92 ft2⋅h°⋅F/Btu)
Glazing 1) Double glazing. U = 4.6 W/m2⋅K (R = 1.24 ft2⋅h⋅°F/Btu)
2) 52% of exterior wall area 3) shading coefficient = 0.5 if facing south & = 0.8 if facing north. 1) Double glazing. U = 4.6 W/m2⋅K (R = 1.24 ft2⋅h⋅°F/Btu) 2) 44% of exterior wall area 3) shading coefficient = 0.5 (facing south) 1) Double glazing. U = 4.6 W/m2⋅K (R = 1.24 ft2⋅h⋅oF/Btu) 2) 61% of exterior wall area 3) shading coefficient = 0.5 (facing south)
Occupancy schedule 8:00 a.m. – 7:00 p.m. Monday – Friday 8:00 a.m. – 7:00 p.m. Monday – Friday 8:00 a.m. – 7:00 p.m. Monday – Friday Internal load
(sensible and latent)
2 persons: 260 W (887 Btu/h) 2 computers: 250 W (853 Btu/h) lights: 204 W (696 Btu/h) 25 persons: 3,250 W (11,088 Btu/h) lights: 1,264 W (4,313 Btu/h) 112 persons: 14,560 W (49,676 Btu/h) equipment: 3,362 W (11,470 Btu/h) lights: 5,502 W (18,772 Btu/h) Room temperature setpoint T = 25°C (77°F) for cooling T = 23°C (73.5°F) for heating T = 25°C (77°F) for cooling T = 23°C (73.5°F) for heating T = 25°C (77°F) for cooling T = 23°C (73.5°F) for heating
Office Building
Figure 6.5 shows the monthly energy consumption of a small office with a south-facing wall and window in Seattle, WA. The displacement ventilation system uses more fan energy than the mixing ventilation system. Although the exhaust air temperature with the displacement ventilation system is higher than that with the mixing ventilation system, the air temperature difference between the air exhaust and supply is smaller. This is because the air supply temperature in the displacement ventilation system is much higher. Typically, for the displacement system,
Texhaust – Tsupply = 81°F – 64°F = 17°F or (27°C – 18°C = 9°C) , (6.12) and for the mixing system,
Texhaust – Tsupply = 77°F – 55°F = 22°F or (25°C – 13°C = 12 K) . (6.13) To remove the same amount of cooling load requires a larger amount of supply air with the displacement ventilation. The difference is especially large during summer, when the cooling load is high. However, the fan energy for displacement ventilation in August in Seattle is lower than that for the mixing system. The outdoor temperature in August in Seattle is low, and the outdoor air conditions are good for free cooling. Displacement ventilation has a larger temperature difference between the supply and exhaust air during a free cooling period, which is typically 16°F (9 K), Figure 6.5 Comparison of monthly energy consumption between the
displacement and mixing ventilation systems for an individual office in Seattle, WA (maritime climate).
while the mixing system has only 9°F (5 K). Therefore, the total amount of air for the displacement ventilation is smaller. Consequently, the fan energy consumed is also smaller. Very similar results can be found in other climatic regions, e.g., June, August, and September in Portland, ME, and March and November in Phoenix, AZ (Chen et al. 1999).
In the winter, the heat for the displacement system is mainly supplied via the baseboard heater. The displacement system supplies only fresh air, and the amount of air supply is much lower than that of the mixing system. The fan energy consump- tion also should be lower. Since the office has a high internal heat gain, as do most U.S. office buildings, cooling is required during some office hours even in winter. For these cooling times, the displacement ventilation system uses more fan energy, as explained above. The fan energy consumed during winter is similar between the two ventilation systems due to the combined effect.
Figure 6.5 also indicates that the energy consumed by the chiller in the displace- ment ventilation system is also much less. Since the air supply temperature is higher in the displacement ventilation system than in the mixing ventilation system, this allows the displacement system to use more free cooling during the shoulder seasons. The COP value is slightly higher with displacement ventilation (3.0) than with the mixing ventilation (2.9). On the other hand, the exhaust air temperature in the displacement system is 4°F (2 K) higher than that in the mixing system. More importantly, the mean room air temperature in the displacement ventilation system is higher than that in the mixing system, due to the temperature stratification. The cooling load in the summer months is lower in the displacement ventilation. All of these factors contribute to smaller energy consumption of the chiller.
The energy consumed by the boiler with displacement ventilation is also smaller than that with mixing ventilation. This is especially evident during the winter. With displacement ventilation, the ventilation effectiveness is higher. Maintaining the same air quality, the total amount of fresh air can be reduced. As a result, the energy needed to heat the fresh air becomes less with the displacement ventilation system, as shown in Figure 6.5.
We have further studied the impact of different building orientations on the energy consumption. Figure 6.6 shows the annual energy consumption for the indi- vidual office in three different building zones: one having an exterior wall and window facing south, one having an exterior wall and window facing north, and one having no exterior walls and windows (core region of a building). All of the other thermal and fluid boundary conditions are the same.
The results show that the energy consumption trend is the same for all three building zones. The energy consumption by the boiler is the highest for north-facing zones because of the high heat loss through the exterior wall and window during the heating period. The chiller does not use much more energy in the south-facing zones because the heat gain due to solar radiation is comparable to the heat loss to the moderate outdoor air temperature in Seattle, WA. In the core region where no exte-
rior windows and walls exist, no heating is needed. Therefore, the separated heating system with baseboard heater can be eliminated in the core region.
In most cases, the sum of the energy consumed by displacement ventilation is slightly smaller than that with mixing ventilation. The fan uses higher energy in the displacement ventilation system because of the high cooling loads found in U.S. buildings. The study for the five climatic regions—Seattle, WA (maritime); Port- land, ME (cold); Phoenix, AZ (hot and dry); New Orleans, LA (hot and humid); and Nashville, TN (moderate)—shows very similar results (Chen et al. 1999).
Classroom and Workshop
The investigation also compares annual energy consumption by the displace- ment and mixing ventilation systems for a classroom and a workshop in the five climatic regions. Figure 6.7 shows the results for Seattle, WA. The results of the classroom facing south are also used for the comparison. The energy consumed is normalized by the floor area.
A classroom has less heated equipment than an individual office; the occupants are the major heat sources. Each occupant has a smaller floor area in a classroom than in an individual office. Therefore, the amount of fresh air per square foot floor area is higher in a classroom. The fresh air consists of a large portion of cooling and Figure 6.6 Annual energy consumption of the displacement and mixing
ventilation systems for an individual office at different locations of a building in Seattle, WA (maritime climate).
heating load. Hence, the energy saving in winter becomes more significant in the classroom with displacement ventilation than in the individual office, as shown in Figure 6.7.
The workshop used in the present study has a population density similar to the classroom but with more heated equipment. As a result, the workshop needs little auxiliary heating in winter.
This investigation has studied an individual office, classroom, and workshop for all five climatic regions (Chen et al. 1999). Generally, the displacement ventilation system uses more fan energy and less chiller and boiler energy than the mixing venti- lation system. The overall energy used by the displacement ventilation system is slightly less.
6.4 FIRST COST ANALYSIS FOR U.S. CONDITIONS
The first cost analysis has been performed for the individual office building. This study has assumed that the building has 100 identical individual offices, as listed in Table 6.1. This would allow us to select reasonably sized chillers, boilers, and air-handling units and to distinguish the difference between the displacement and mixing ventilation systems. We have further assumed that one-third of the Figure 6.7 Annual energy consumption per unit floor area of the
displacement and mixing ventilation systems for three different types of rooms in Seattle, WA (maritime climate).
offices are facing south, one-third are facing north, and the other one-third are in the core region.
The equipment capacity is selected to simultaneously handle the maximum load in the three zones of the building. With the equipment capacity, the first costs of the air-handling units, chillers, and boilers can be estimated by using the 1998 R.S. Means building construction cost data. Figure 6.8 shows results for the five climatic regions. The costs are for materials and labor but do not include project overhead.
Figure 6.8a shows that the first costs of the air-handling units are higher for the displacement ventilation system than for the mixing ventilation system. This is because the displacement ventilation system needs to handle a larger amount of air. In contrast, the displacement ventilation system needs a smaller chiller, as shown in Figure 6.8b, due to a higher air supply temperature and smaller cooling load. However, the boiler size is almost the same between the two ventilation systems (Figure 6.8c). Although the displacement ventilation system needs a slightly lower boiler capacity, it falls to the same category by using the building construction data. The overall first costs, including the costs for the air-handling unit, chiller, and boiler, are shown in Figure 6.9a. The costs do not include those for air distribution, such as ducts. The displacement ventilation system supplies air at floor level and returns at ceiling level. It may be more desirable to use wall cavities for the ducts. However, the mixing ventilation system should use a false ceiling, which will defi- nitely have an impact on construction costs. Though the first cost analysis provides an insight on the costs of major units, the results show only a rough estimation. Figure 6.9a shows that the overall first cost for displacement ventilation is smaller than that for mixing ventilation. This is mainly due to a small chiller, as illustrated in Figure 6.8b.
The results for the displacement ventilation system shown in Figure 6.9a do not include the first costs of a separated heating system needed in the perimeter zones of the building. If we include the first cost of the separated heating system, the total first cost for the displacement ventilation become slightly higher. Therefore, the displacement ventilation system is more desirable for the core region of a building where no heating is required.
6.5 CONCLUSIONS
This investigation has studied energy consumption of an individual office, a classroom, and a workshop for five U.S. climatic regions. The study has been done for three building zones in an individual office building: one having a south-facing exterior wall and window, one having a north-facing exterior wall and window, and one having no exterior walls and windows.
The study uses the energy balance method to calculate hourly cooling load. The load calculation considers the non-uniform temperature distribution in room air. The secondary systems are analyzed by a whole-building simulation program. The energy analysis shows that displacement ventilation may use more fan energy and less chiller and boiler energy than mixing ventilation. The total energy used is
Figure 6.8 Comparison of first costs of air-handling units, chillers, and boilers for the displacement and mixing ventilation systems.
Figure 6.9 Comparison of the total first costs between the displacement and mixing ventilation systems.
b) With the separated heating system for displacement ventilation. a) Without the separated heating system for displacement ventilation.
slightly less for displacement ventilation, although the ventilation rate is increased to handle the high cooling loads found in U.S. buildings.
The study has also analyzed first costs of the displacement and mixing venti- lation systems for the office building. The displacement ventilation system needs a larger air-handling unit, a smaller chiller, and a boiler similar to that for the mixing ventilation system. The overall costs are lower for displacement ventilation if the system is applied for the core region of a building. In the perimeter zones, the displacement ventilation system needs a separated heating system, which will slightly increase the first and maintenance costs.
Design Guidelines
Since U.S. buildings have a high cooling load and complex geometry, the design guidelines available from the literature need revision and further development for U.S. buildings. This chapter presents a ten-step design procedure for displacement ventilation systems. The design guidelines are for the determination of the key parameters in the displacement ventilation system, such as ventilation rate, location and type of supply diffuser, and supply air temperature.
Step (1): Judge the Applicability of Displacement Ventilation
Displacement ventilation is suitable when the contaminant sources are associ- ated with heat sources and the ceiling height is no less than eight feet. There is also a limitation on the cooling load that can be handled by displacement ventilation. This study shows that the maximum can be as high 38 Btu/(h⋅ft2) (120 W/m2) if the venti- lation rate is increased and if there is sufficient space for installing large diffusers. When the cooling load is high, the energy consumption with displacement ventila- tion will increase significantly. Displacement ventilation is especially effective in the premises with high ceilings (open atriums, cinemas, theaters, and industrial buildings). In addition, the displacement ventilation system may require a separated heating system for perimeter zones of a building to prevent downflow from cold windows/walls in winter. This will increase first and maintenance costs. Hence, the displacement ventilation system is the best for the core region where no heating is needed.
Step (2): Calculate Summer Design Cooling Load
Use a cooling load program or the ASHRAE manual method to calculate the design cooling load of the space in summer. If possible, assume a 1°F/ft (2°C/m) vertical temperature gradient in the space in the computer calculation because the air temperature in a room with displacement ventilation is not uniform.
Itemize the cooling load into
• the occupants, desk lamps, and equipment, Qoe (Btu/h, W); • the overhead lighting, Ql (Btu/h, W); and
• the heat conduction through the room envelope and transmitted solar radia- tion, Qex (Btu/h, W).
Step (3): Determine the Required Flow Rate of the Supply Air for Summer Cooling
Displacement ventilation creates a thermal stratification. To maintain a comfort level, the design air temperature difference between the head and foot level of a sedentary occupant, ∆Thf, should be less than 3.6°F (2 K). The required ventilation rate, n, can be determined according to Equation 4.25:
(7.1a) (I-P)
(7.1b) (SI)
where
n = ventilation rate (ach)
∆Thf = 3.6°F (2 K)
ρ = air density (lb/ft3, kg/m3)
Cp = specific heat of air (Btu/lb⋅°F, J/kg⋅K)