Transport and chiller energy minimization for air conditioning systems

A control system for minimizing the total energy consumed by an air conditioning system of a building is disclosed having a sensor for sensing the energy expended by the fan of the air conditioning system, a sensor for sensing the energy expended by the chiller of the air conditioning system, and a controller responsive to the fan energy sensor and the chiller energy sensor for controlling the air conditioning system so that the total energy consumed by the fan and chiller systems is minimized.

BACKGROUND OF THE INVENTION 
The present invention relates to a control system for the air conditioning 
plants of a building and, more particularly, to a control system to 
minimize the energy consumed by the transport and chiller systems of a 
building in delivering cool air to the building zones. 
Typical commercial and other large present day buildings or building 
complexes comprise a plurality of floors each floor having exterior zones 
having at least one wall exposed to the outdoors and interior zones having 
no walls exposed to the outdoors. A zone is defined as a room or group of 
rooms or an area of a floor or floors. Each floor of the building may have 
a plurality of fan systems for delivering treated air to the interior and 
exterior zones depending upon the size of the floor or floors of the 
building. For example, the interior zones of a building represent a 
cooling load both in winter and summer whereas the exterior zones of a 
building represent a cooling load in summer and a heating load in winter. 
Thus, it is typical to connect the interior zones to one fan system and 
the exterior zones to a second fan system. Of course, if the size of the 
floor is sufficient, the interior and exterior zones may each be supplied 
by more than one fan system. 
There have been a number of efforts to minimize the energy consumed by the 
air conditioning systems of a building. For example, the prior art has 
devised load cycling systems for cycling on and off fans, radiators and 
other energy consuming equipment in air conditioning systems, outdoor air 
damper control systems for most effectively utilizing outdoor air, load 
shedding when energy consumption in the building approaches a peak limit, 
and the like. All of these prior art systems have substantially reduced 
the energy consumed by present day buildings. 
The present invention increases these energy savings by recognizing that 
there is a tradeoff between the amount of energy required to transport 
cooled air to the building zones and the amount of energy consumed by the 
chiller in chilling the water supplied to the cooling coils of the fan 
systems to a sufficient degree to satisfy the needs of the zones connected 
to the fan system. The transport energy may be defined simply as the 
energy required by the fan system to deliver cool air to the zones or may 
be defined as a combination of the fan energy plus the pump energy 
expended by the pump associated with the chiller in supplying chilled 
water from the chiller to the cooling coils of the fan systems. Thus, at a 
particular building load it may be cheaper to use more fan rather than 
more chiller whereas at a different load it may be cheaper to use more 
chiller and less fan. These savings cannot be recognized by those systems 
which merely select a chilled water supply temperature and adjust a fan 
accordingly. According to the present invention, savings can be increased 
by selecting a chilled water supply temperature and a cooperating fan 
speed which will satisfy the zone conditions at a minimum expenditure of 
energy. 
SUMMARY OF THE INVENTION 
These savings are realized by providing an air conditioning control system 
having a fan energy sensor for sensing the energy expended by a fan in 
transporting treated air through a duct, a chiller energy sensor for 
sensing the energy expended by the chiller the building, and a controller 
responsive to the sensors for controlling the air conditioning system in 
such a fashion as to minimize the total energy consumed by the transport 
and chiller systems of the building.

DETAILED DESCRIPTION 
Typical air conditioning systems in buildings comprise the ducts running 
throughout each floor in the building for the supply of air to the zones. 
Air is driven through the ducts, from both return air ducts returning air 
from the zones and from outdoor air ducts bringing in fresh air, by a fan 
which drives the air over various heat exchange coils. These heat exchange 
coils usually take two forms, either a cooling coil supplied with a 
cooling fluid such as water cooled by a chiller or a heating coil supplied 
with hot water from a boiler or furnace. The air is then supplied to other 
ducts which have dampers located therein operated by thermostats located 
in the zone supplied by the duct to regulate the temperature within a 
zone. The fan system may have a static pressure sensor for sensing the 
static pressure in the main supply duct so that as the zone dampers are 
being regulated to control the temperatures of their associated zones, the 
fan can be adjusted for supplying only the requisite amount of air instead 
of wasting energy by operating the fan system at its maximum capacity 
continuously. Thus, when controlling an air conditioning system to operate 
efficiently, not only should the energy supplied to the chiller be taken 
into account, but also the energy supplied to the fan. 
The graph shown in FIG. 1 shows typical fan and chiller curves. The amount 
of energy per ton for the chiller and fan have been charted as a function 
of chilled water supply temperature. It should be realized that, for a 
given load in a typical air conditioning system, either the chilled water 
supply temperature of the chiller supplied water can be decreased for 
allowing the supply air temperature to decrease which causes the amount of 
air supplied by a fan to decrease or the chilled water supply temperature 
can be increased which raises the supplied air temperature for causing the 
amount of air supplied by the fan to be increased to maintain desired 
conditions. Thus there is a trade off between fan and chiller energy to 
satisfy any given load. The fan curve and the chiller curve have been 
added together and is represented by the total energy curve which shows 
that at any given load condition there is a chilled water supply 
temperature which results in a minimum total energy expenditure between 
fan and chiller systems to satisfy the needs of the building. The graph of 
FIG. 1 is shown for a load condition at which a chilled water supply 
temperature of 50.degree. results in a minimum total energy expenditure. 
However, as the load on the building changes, the chilled water supply 
temperature which results in this lowest total energy expenditure will 
also change. 
FIG. 2 shows one type of system which may take into account the advantages 
demonstrated by FIG. 1. In FIG. 2, fan system 11 comprises duct 12 for the 
supply of treated or conditioned air to one or more zones. Fan system 11 
includes fan 13 and cooling coil 14 located within duct 12. Cooling coil 
14 is supplied with chilled water by pump 16 from chiller 15 with the 
amount controlled by valve 17. A second fan system 21 includes duct 22 in 
which is located fan 23 and cooling coil 24. Cooling coil 24 is supplied 
with chilled water by pump 16 from chiller 15 with the amount controlled 
by valve 27. Chilled water exiting cooling coils 14 and 24 is returned to 
the input side of chiller 15. As mentioned previously, a fan system within 
a building may be used to supply a plurality of smaller ducts each having 
a damper located therein. Each damper in these smaller ducts is controlled 
by a corresponding thermostat in the zone supplied with air by the smaller 
duct. As the temperature within the zones changes, the thermostats change 
the positions of the dampers which affects the static pressure within the 
fan system. Thus, fan system 11 has static pressure sensor 18 therein for 
controlling fan 13 so that fan 13 supplies only the necessary amount of 
air to the smaller ducts which may be connected to main supply duct 12. 
Likewise, fan system 21 has static pressure sensor 28 located therein for 
controlling fan 23 so that fan 23 need only supply the necessary amount of 
air to satisfy the smaller ducts which may be connected to main supply 
duct 22. 
In order to minimize the total transport and chiller energy requirements of 
the air conditioning system, it is preferable to measure transport energy 
and chiller energy. One way to derive a representation of fan energy is to 
sense the volume of air supplied by the fan. That is, the higher the 
volume of air moved by the fan, the greater the fan energy requirement. 
Moreover, a chiller requires increased energy to increase refrigerant 
head. Thus, fan energy can be indirectly sensed by a volume sensor and 
chiller energy can be indirectly sensed by temperature sensors for sensing 
refrigerant head. Accordingly, volume sensor 42 senses the volume of air 
moved through duct 12 by fan 13. Volume sensor 43 senses the volume of air 
moved through duct 22 by fan 23. And, temperature sensors 51 and 52 
together with difference element 53 senses the refrigerant head across 
chiller 15. 
The control means utilizes these sensors for then minimizing transport and 
chiller energy. In order to control valve 17 and thus control the amount 
of cooling supplied by cooling coil 14, thermostat 31 is located within 
duct 12 and provides an input to discharge air temperature controller 32 
which receives a setpoint input at 33. Discharge air temperature 
controller 32 compares the signal received from thermostat 31 to the 
setpoint established at input 33 and provides an output at 34 for 
controlling valve 17. Likewise, the temperature of the air supplied by 
duct 22 is controlled by the valve 27 which receives an input from 
discharge air temperature controller 35 which receives an input from 
thermostat 36 to be compared to a setpoint at input 37. Discharge air 
temperature controller 35 then provides an output at 38 to control valve 
27. Each of the discharge air temperature controllers 32 and 35 may be a 
Honeywell RP908A. 
Volume sensor 42 and volume sensor 43 are connected to high pressure 
selector 44 which supplies the highest of its input pressures to ratio 
relay 45. Ratio relay 45 may be a Honeywell RP971 to convert the high 
pressure signal into a 3-13 psi output having a selected relationship 
(ratio) to the input signal. Because the output from ratio relay 45 is a 
non-linear function, it is supplied through square root extractor 46 and 
then operates as one input to fan system controller 41. Such a square root 
extractor may be shown in U.S. Pat. No. 4,201,336. 
The other input to fan system controller 41, which may be a Honeywell 
RP908B, is derived from the refrigerant head sensing system. Temperature 
sensor 51 senses the temperature of the chilled water supply in the pipe 
leading to pump 16. Temperature sensor 52 senses the temperature of the 
water being returned to chiller 15. Together, the difference between the 
temperature sensed by sensors 51 and 52 represent the refrigerant head of 
chiller 15. This difference is provided by element 53 which may be any 
suitable element for supplying a pneumatic pressure proportional to the 
difference between the outputs from sensors 51 and 52. The output from 
element 53 is supplied through another ratio relay 54 and forms the second 
input to fan system controller 41. 
The output from fan system controller 41 then provides the setpoint inputs 
33 and 37 to corresponding discharge air temperature controllers 32 and 
35. This control system essentially uses the fan system having the 
greatest amount of air moving through it as representative of the entire 
fan system of the building. Fan system controller 41 will thus in effect 
compare the energy required by this representative fan system to the 
amount of energy required by the chiller and control both the fan system 
and the chiller at a point where the slopes of their response curves are 
equal but opposite thus resulting in the lowest total energy expenditure 
of the combined fan and chiller systems to maintain the conditions of the 
building. 
The temperature of the chilled water exiting chiller 15 and supplied to 
valves 17 and 27 by pump 16 is controlled by chilled water temperature 
controller 61 which receives an input from temperature sensor 51 and which 
receives another input from high pressure selector 62. High pressure 
selector 62 selects the higher of the pressures in lines 34 and 38. 
In order to understand how the control of valves 17 and 27 results in 
minimizing total energy, the entire air conditioning system must be 
considered. If there is an increase in the load to a zone of the building, 
the thermostat within the zone will respond by causing the damper 
regulating the amount of air supplied to that zone to open more for 
supplying more air to the zone. The static pressure sensor located in the 
fan system of the main duct supplying air to that zone, such as static 
pressure sensor 18 of fan system 11, will sense a decrease in the static 
pressure and will control fan 13 to increase the volume of air moved by 
fan 13 through duct 12. Volume sensor 42 will thus provide an increasing 
output signal based upon the increased volume of air moving through duct 
12 which, if it is the highest of the signals connected to high pressure 
selector 44, will operate through ratio relay 45, square root extractor 
46, and fan system controller 41 for adjusting the setpoint 33 to 
discharge air temperature controller 32 for opening valve 17 in an 
increasing manner to supply more cold water to cooling coil 14. Thus, the 
air moving through duct 12 will have a lower temperature. At the same 
time, if the branch line pressure supplied by discharge air temperature 
controller 32 to valve 17 is or becomes the higher of the pressure 
supplied to high pressure selector 62, the output of high pressure 
selector 62 will increase to adjust the setpoint input to chilled water 
temperature controller 61 for supplying an output to control the inlet 
vane of centrifugal chiller 15 in such a manner as to reduce the chilled 
water temperature of the water supplied by pump 16 to the cooling coils of 
the fan systems. As both the amount of water supplied to cooling coil 14 
increases and the temperature of that water decreases, the air exiting 
cooling coil 14 will be colder. The thermostat located in the zone which 
experienced the load change will then receive more air at a lower 
temperature. As it becomes satisfied, it begins driving its damper in a 
closing manner which then increases the static pressure in duct 12 and 
static pressure sensor 18 will then cause fan 13 to slow down. Thus, a 
balance is reached between the amount of air delivered by fan 13 and the 
temperature of the chilled water supply which balance will result in the 
lowest total energy input into the system to maintain desired conditions. 
Ratio relays 45 and 54 and fan system controller 41 are thus arranged to 
insure that the fan and chiller are operated along their curves at a point 
wherein their slopes are equal but opposite. 
The circuit shown in FIG. 3 is an alternative arrangement to that shown in 
FIG. 2. The system shown in FIG. 2 is concerned only with the energy 
required by the fans and the chiller to maintain building conditions. The 
system of FIG. 3 also recognizes that the pump for supplying chilled water 
to the cooling coils from the chiller also requires energy. The system 
shown in FIG. 3 takes into account the total transport energy, i.e. the 
energy required by the fans to deliver conditioned air to the zones, and 
the energy required for the pump to deliver chilled water to the cooling 
coils. 
In FIG. 3, fan system 101 includes main duct 102 in which fan 103 and 
cooling coil 104 are located. Also located within duct 102 are thermostat 
105, volume sensor 106 and static pressure sensor 107. Fan system 111 
includes duct 112 in which are located fan 113 and cooling coil 114. Also 
located within duct 112 are thermostat 115, volume sensor 116 and static 
pressure 117. Cooling coil 104 is supplied with chilled water through 
valve 108 and cooling coil 114 is supplied with chilled water through 
valve 118. Both valves are supplied with water by pump 121 which derives 
its supply of chilled water from chiller 122. 
The control system for controlling the air conditioning system is centered 
around central processing unit 131 which may be, for example, part of one 
of the Delta systems manufactured by Honeywell. CPU 131 is connected by 
communication bus 132 to data gathering panel 133 which controls fan 
system 101 and to data gathering panel 134 which controls fan system 111. 
CPU 131 is also connected over communication bus 132 to data gathering 
panel 135 which controls the chiller system. 
Data gathering panel 133 has inputs from volume sensor 106 and static 
pressure sensor 107 and has outputs for controlling discharge air 
temperature controller 136 and fan 103. Data gathering panel 134 has 
inputs from volume sensor 116 and static pressure 117 as well as outputs 
to discharge air temperature controller 137 and to fan 113. Additionally, 
thermostats 105 and 115 may also be connected to data gathering panel 133 
and 134 respectively so that CPU 131 can read the discharge air 
temperatures within ducts 102 and 112. 
In order for CPU 131 to determine which zone has the greatest demand, 
pressure-to-electric switches 141 and 142 are connected to the output of 
discharge air temperature controller 136 and are set to switch at 
different pressures to provide inputs to data gathering panel 133. For 
example, P/E switch 141 may be set at a pressure representing a full open 
condition of valve 108 and P/E switch 142 may be set for a slightly less 
open valve condition. Likewise, P/E switches 151 and 152 may be similarly 
arranged. 
In the chiller system, the temperature of the chilled water is controlled 
by chilled water temperature controller 161 which has a setpoint input 
from data gathering panel 135 and a feedback input from thermostat 162 
located to sense the temperature of the water leaving chiller 122. Flow 
sensor 163 is located to sense the flow at the output of pump 121 and 
thermostat 164 is located to sense the temperature of the water being 
returned to chiller 122. By utilizing thermostats 162 and 164, the 
refrigerant head of chiller 122 may be determined. By using flow sensor 
163, the transport energy of the pump 121 may be determined. By utilizing 
flow sensors 106 and 116, the amount of air and thus the transport energy 
represented by fans 103 and 113 may be determined. Data gathering panels 
133, 134 and 135 are also part of the Delta systems and the discharge air 
temperature controllers 136 and 137 and the chilled water temperature 
controller 161 may be substantially the same as those shown in FIG. 2. 
The operation of the system shown in FIG. 3 may be understood with respect 
to FIG. 4. If a zone located within a building experiences a load change, 
its thermostat will modulate the damper controlling the supply of air to 
that zone. Thus, one of the static pressure sensors located in one of the 
fan systems will experience a change in static pressure. This static 
pressure is maintained by controllers 165 or 166 for changing fan 
capacity. The volume of air moving through the duct will change and is 
sensed by the central processing unit through the associated data 
gathering panel and sensor 106 or 116. Alternatively, fan capacity can be 
indicated by control signals from controllers 165 or 166 which are 
transmitted to the CPU's through pneumatic-to-electric switches 167 or 168 
and DGPs 133 or 134. When the routine represented by the flow chart shown 
in FIG. 4 is entered, central processing unit 131 will run through its 
series of operations for the first fan system. Its first decision is to 
determine whether or not the selected fan system is the greatest demand 
system. The central processing unit will know whether or not this system 
is the greatest demand system by monitoring the action of the 
pressure-to-electric switches through the data gathering panel. If this 
fan system is the greatest demand fan system, then it is most efficient to 
reset the chilled water temperature upward to a point where the valve 
controlling the supply of chilled water to the cooling coil in that fan 
system is wide open and to modulate back all of the other valves in all of 
the other fan systems. However, the chilled water supply temperature 
should not be set higher than a temperature which will result in an equal 
but opposite slope between the energy required by the chiller to chill the 
water and the transport energy which for this fan system is defined as the 
energy required by the fan to deliver air to the zones to which the fan 
system is attached and the energy required by the pump to supply chilled 
water to the cooling coils located in the fan systems. 
Thus, if the selected fan system is the greatest demand fan system, then 
the CPU will assume an incremental change of, for example, 1.degree. in 
chilled water supply temperature in a direction to satisfy a load change. 
Thus, assuming an incremental change of 1.degree., CPU 131 determines 
whether or not the resulting incremental change in transport energy is 
greater than or equal to the resulting incremental change in chiller 
energy. It the incremental change in transport energy is greater than or 
equal to the incremental change in reset (chiller) energy, then the 
chilled water temperature is decreased (reset) thus requiring more chiller 
energy and less fan energy. If the incremental transport energy is not 
greater than or equal to the incremental chiller energy, then a decision 
is made whether or not the temperature of the zone experiencing the load 
change is above or below the upper edge of the comfort range established 
for that zone. The comfort range is a range of temperatures such that, if 
the actual temperature is within the range, typically no change in heating 
or cooling is made. In the present system, however, if the temperature in 
the zone is above comfort range, then the chilled water temperature is 
decreased so that the air supplied to that zone can be made colder. If the 
temperature is within the comfort range then the chilled water temperature 
is increased. The upper edge of this range, therefore, represents the 
temperature requiring the lowest energy input to the chiller to still 
maintain the temperature within the range. Thus, the chilled water 
temperature is continually being either increased or decreased around the 
lower of two points, one which represents equal but opposite slopes 
between transport energy and chiller energy or one which just satisfies 
comfort zone needs. 
In any event, if this system is not the last system to be monitored by CPU 
131, the next system is obtained. For systems other than the greatest 
demand systems, the CPU determines what effect a 1.degree. change in 
chilled water supply temperature in a direction determined by the load 
change will have on the system. Thus, a determination must be made whether 
or not the resulting incremental change in fan energy is greater than or 
equal to the resulting incremental change in pump energy. Specifically, a 
determination is made whether or not the fan should change its speed to 
deliver a modulated amount of air or if the condition can be satisfied 
merely by changing the amount of water supplied by valve 108 to cooling 
coil 104. Less water allowed to flow through valve 108 requires less 
energy by pump 121 to deliver that water. Again, it is desirable to 
control fan 103 and pump 121 at a point to minimize the total transport 
energy of fan and pump in attempting to meet zone conditions. Thus, if the 
incremental change in fan energy is greater than or equal to the 
incremental change in pump energy, the discharge air temperature is 
lowered causing the valve to supply more chilled water to cooling coil 104 
through valve 108. This will require less air flow and fan energy and thus 
the total transport energy is lowered. If the incremental change in fan 
energy is not greater than or equal to the incremental change in pump 
energy, than again it is determined whether or not the zone temperature is 
within or above the comfort range. If the zone temperature is above the 
comfort range, again the discharge air temperature is lowered in an 
attempt to bring the temperature back within the comfort range. If the 
temperature is within the comfort range, then the discharge air 
temperature is increased causing reduced water flow through the valve to 
the cooling coil and reduced pumping energy and requiring an increase in 
air flow and fan energy, but reducing the total transport energy. 
Once the last system has been adjusted in this manner, the system exits and 
awaits the time when it again periodically enters.