Mechanical systems, for the purposes of this discussion of integration, are limited here to HVAC and related thermal comfort components. The other “mechanical” aspects of buildings such as lighting and plumbing are considered in the discussion of interior systems later in this chapter.
There are eight fundamental requirements of mechanical air-conditioning, the last three of which are predominantly for public buildings:
• Temperature control by heating or cooling
• Humidity control by cooling air below its dew point for dehumidification, or by evaporatively adding moisture when humidification is required
• Air motion to occupants by forced air circulation
• Air filtration to remove some level of particulates
• Exhaust of polluted air from indoor sources of heat, odor, moisture, or chemical concentration, such as found in toilet rooms, laboratory vent hoods, and kitchens
• Air change for ventilation, economizer cooling, and nighttime flush cooling
• Air balance for positive indoor pressure to avoid infiltration of untreated outdoor air, dust, and moisture
• Smoke exhaust and fire safety control of indoor air pressure by compartmentalization
Conceptually, the mechanical systems of a building consist of the following functional components:
• Thermal plant where heating and cooling energy are generated
• Distribution of thermal energy to individual zones of the building
• Delivery of comfort to occupants by forced air or radiant temperatures
• Control of mechanical equipment to match HVAC operation to thermal loads
• Thermal energy storage (TES) as an optional HVAC operating scheme
Every mechanical system has at its heart a mechanism whereby source energy is converted into comfort energy. This thermal plant can also be the connection between the building’s indoor conditions and the environment serving as a source of thermal energy or a thermal “sink” for heat being rejected from the building. At the largest scale, a campus of buildings would have a central plant where boilers and chillers produce hot and cold water. This requires the use of utility-provided energy, normally in the form of electricity and natural gas. The hot and cold water is sent from the plant to individual buildings where fans blow room air across coils of circulating water, much like what occurs in the radiator of automobiles. Cooling towers in the central plant reject the space heat captured in chilled water return lines back to the environment by evaporation. This evaporative effect is used to cool the condensing side of the central plant chillers, thus moving the heat from occupied space back to the central plant and then into the environment. Besides using outside air to reject heat, cooling systems can be designed to use below- grade soil as geothermal heat sinks or to reject building heat to large bodies of water such as cooling ponds or even seawater.
The most common sort of thermal plant is the conventional split-system direct-expansion freon cycle machine, which works for one single individual thermal zone of the building. In this arrangement, the outdoor unit of compressor and condensing coil make up the cooling plant and a gas furnace or electrical resistance heat strip is located in the indoor half of the system to provide heating energy. The outdoor compressor and condensing coil are connected by refrigerant lines to the indoor evaporator coil. Heat collected at the evaporator coil is thus rejected to the environment by outdoor air blowing across the evaporator. Split-system heat pumps use a reversing valve to get double-duty cooling and heating from the outdoor unit. They employ a valve to reverse the flow of refrigerant between the indoor and outdoor coils. This allows the unit to either “pump” heat from indoors to out, or from outdoors to in; a feat conventional direct expansion systems can perform only in one direction.
At the smallest scale the thermal plant can be part of a compact package of equipment that contains all the elements of mechanical servicing. A wall unit heat pump, a package rooftop unit, or a window unit air conditioner are examples of systems that not only produce comfort energy by converting source electricity, but also deliver it directly to the occupants.
Integration concerns for incorporating thermal plants into design schemes, or simply in accommodating their functional requirements, entail several givens. The first is that utility services have to be routed to the central plant as a large portion of the building’s power and energy requirements. Second, thermal energy has to be delivered from the plant to the zones in some form. Generally, there is also the necessity of dealing with exhaust flues from combustion equipment and similar functional requirements. Finally, in the vast majority of systems, there must be some outdoor component where heat is rejected to the environment (water source heat pumps are an important exception to this arrangement).
Distribution of Thermal Energy to Zones. In large heating and cooling systems, the distribution of energy from the plant to the fan/coil systems located in zones of a building is a distinct part of the system. These distribution systems are usually composed of all-water systems (also called hydronic systems) in which two, three, or four pipes are used to convey thermal energy from the plant to the individual thermal zones. In medium-sized systems, where direct expansion air-conditioning prevails, delivery from the plant is made by the freon loop between the condenser and evaporator coils.
Once thermal energy is delivered to a zone, there are a host of choices as to how the system will distribute comfort to individual areas of the zone or to separately controlled subzones. Aside from the common “single zone, single duct” system, there are variable air volume, multizone, and dual duct options.
Delivery of Comfort to Occupants. Delivery of comfort to the occupant of each space is generally accomplished by forcing supply air through a tapering network of insulated ducts. Air delivery registers then direct the air into rooms to promote complete circulation. A return-air system serves the complementary function of extracting the room’s thermal load and excess humidity by recycling room air though ducts or open plenums and removing it. Air is returned to the distribution fan for filtering and reconditioning, and the cycle is repeated. Exhaust fans are located in kitchens and toilet rooms and other sources of air pollution and excess humidity. Outdoor air is provided in public buildings to mechanically replace the polluted air and keep buildings positively pressured relative to the outdoor environment. The prime components of the delivery system are ducts, registers, and grilles.
Aside from air delivery systems, the designer can use radiant systems to provide or ensure thermal comfort. The mean radiant temperature of surrounding surfaces is some 10 percent more important to comfort than air temperature at normal room and activity conditions. Controlling surface temperatures is sometimes accomplished by artificially heating an overhead “radiant panel” or a floor slab with hot water or electricity. Frank Lloyd Wright used hot water coils in the floor slab of the Solar Hemicycle Jacobs House (Madison, Wisconsin, 1944) to replace upright radiators hidden behind intricate wood grilles of his earlier Prairie Houses. This was one of many innovations that defined the course of his Usonian Houses.
Passive solar heating of the thermal mass of a floor, Trombe wall, or other interior room surface is another way of accomplishing the radiant effect. Passive systems are slower and rely on subtle temperature differences than does active radiant heating. Further, passive systems use greater mass and therefore have greater thermal inertia, resistance to temperature swings, and are more difficult to control as compared with the on/off of active heating. Recently “air walls” have been used to circulate return air from a room through a glass cavity exterior wall as part of the return air path. This eliminates the effects of cold window walls during cold weather. Richard Rogers and Partners’ headquarters for Lloyd’s of London is an early example of this comfort delivery strategy (see case study #24).
Control of Mechanical System to Match Thermal Load.
Thermal loads in a building vary both in magnitude of heat transfer and in mode of operation between heating and cooling. The match between thermal loads and mechanical systems operation is intentionally controlled to maintain a fairly constant indoor condition. This is accomplished by modulating the heating and cooling energy expended to offset thermal loads. Better controls mean both lower energy consumption and superior comfort conditions, because controls replace indiscriminate energy use with some level of building intelligence. Each 1°F of error in a cooling thermostat, for example, results in about a 5 percent error in energy use.
This match between the operation of mechanical equipment and the opposing variations in environmental conditions amounts to what has been termed “using information to replace power” (Lyle, 1994). More exact information about needs and conditions always allows for better design and operation of systems. Basic control strategies can vary from on/off switching to one-way sensing like that found in a thermostat. More sophisticated systems include feedback loops of two-way sensing, whereby mechanical equipment can provide operating information back to the controls. At the extreme, computerized controls result in intelligent building systems that can anticipate indoor and outdoor environmental changes and learn to accommodate such things as the building’s thermal response time.
Control strategies are often used in combination to fit the required degree of critical control to the appropriate sophistication of hardware. The most common of these are as follows:
• On/off switching—such as may be used on a ceiling fan
• Clock timer—providing a daily or weekly on/off equipment schedule
• Thermostat—sensing the dry-bulb temperature of the return air stream
• Humidistat—sensing the moisture content of the return air stream
• Setback thermostat—a programmable thermostat with a schedule of selectable temperature setpoints
• Occupant sensor—motion or infrared detection of the presence of room occupants
• Direct digital control — automated control of mechanical systems by computer program and potential integration of controls with lighting, security, fire safety, and other systems
• Intelligent buildings—robotic intelligence of building control decisions; sensing of outdoor, indoor, and mechanical systems conditions; may incorporate the ability to learn and anticipate changes; may integrate control of all building systems; can interface with building management personnel
For hardware controls to be most effective, it is essential that the thermal scheming that underlies the design of a building’s mechanical system be integrated with the interior systems. These schematic strategies include the following:
^ Zoning of interior into distinct areas of servicing, usually under the control of an individual thermostat;
divisions are made according to exposure, orientation, occupancy type, and use schedule.
• Staging of successive levels of cooling capacity to accommodate different modes of occupancy (such as entertaining large crowds) or environmental loads (such as morning versus afternoon).
• Diversity of independent package units for individual zones versus economy of central plant, which can shift capacity from zone to zone.
• Modulation of cooling capacity in very large buildings or campus central plants by use of variable speed equipment.
• Thermal energy storage systems (TES).
Because most mechanical cooling systems are sized to the peak hourly load of a building, their full capacity is utilized only about 5 percent of the year. And because heating systems operate at a much higher temperature difference from room air than do cooling systems, the need for air circulation in heating is much less than that for cooling. Consequently, heating is generally sized to match the fan and delivery systems required for the cooling system. Heating systems usually use the same fan and ducts as the cooling system by operating less frequently and more intermittently than the cooling. At any rate, most of the heating and cooling capacity of a conventional system is greatly underutilized.
The economics of this oversizing does not end with the excessive capacity of the cooling equipment that must be installed and maintained to meet infrequent peak conditions. The real penalty comes in the way large buildings pay for electrical power to operate their cooling systems. Not only do they pay for energy in kilowatt hours (kWh), they also pay for their peak demand in watts, or in utility engineering language, in kilovolt-amperes (kva). This is where thermal energy storage can be beneficial.
TES systems use smaller cooling plants that run continuously under full load to produce the same amount of ton-hours of cooling that a peak-load-sized plant would produce running intermittently at part load, or on/off cycling, over the same period of time. Instead of providing cooling energy directly to the building load, however, TES systems produce chilled water or ice and store it for later use. The building then draws from the thermal storage as needed. The time period for equalizing the storage capacity and the fluctuating building load may be as short as a day, but the aim is frequently a one-week interval in order to save up cooling energy over the weekend when the building is used more sparingly. Thus, instead of operating a large cooling plant to relieve small off-peak cooling loads, TES systems use smaller plants to produce ice or chilled water for later use.
TES systems require less peak power (kva) at any one given time than conventional systems because they are significantly smaller in cooling capacity. They do use about the same amount of total energy (kWh) over the course of a month as conventional systems, as they ultimately produce the same amount of cooling. Because the constant power load of TES matches the way utility companies have to continuously produce power in case it is needed by a customer somewhere on their electrical grid, there are significant savings in avoiding the cost of having to construct new generating capacity. The utility companies can pass these savings on to the consumer in the form of lower demand charges as an incentive to control peak loads. Many utilities even rebate some portion of the initial cost of TES installations.
The designer should remember that TES requires a significant increase in equipment and physical storage area for ice or chilled water, as compared with conventional systems. The first costs of a conventional system and a thermal energy storage system are frequently about the same. The benefit of thermal energy storage results from the savings in peak load demand from the utility, measured as the highest power requirement at the building meter for any one time during a month.
Another consideration in favor of TES, more specifically for ice storage systems, is the ability to supply conditioned air at very low temperatures. Whereas conventional systems deliver cooling at about 55°F supply air temperature, ice storage systems can approach 35°F. This enables the use of much smaller, high-velocity duct systems for delivering the cooling energy to occupied spaces. The smaller ducts require less space and allow for lower floor- to-floor heights.
• Basic controls—temperature, humidity, air motion, air filtration, ventilation.
• Size and placement—HVAC requirements range from 10 to 50% of a building’s area, volume, and cost.
• First cost—capacity in installed tons and associated distribution, delivery, and control. There is also an associated cost in size of required electrical service.
• Energy and power—efficiency in Seasonal Energy Efficiency Ratio (SEER) and Coefficient of Performance (COP), energy use (kWh), and power demand (kva).
• Servicing—maintenance, replacement, and repair of equipment.