FIELD OF INVENTION
The invention relates to a method of maintaining the zone temperature in a
single-zone variable air volume air conditioning (VAVAC). The invention
further relates to a VAVAC system carrying out the method.
BACKGROUND OF THE ART.
In large commercial and residential buildings the core of the building is
generally isolated from external environmental conditions. As a result, it is
essential to cool a building which develops heat due to the heating load of
the lights, machineries and personnel. Thus, in such modern buildings, there
is ordinarily a demand for cooling air to provide and maintain required
temperature and to overcome air stagnation.
The control strategy for Heating, Ventilation and Air Conditioning (HVAC)
system is based on the concept of either a constant air volume (CAV) or a
variable air volume (VAV). In a CAV based system, the volume flow rate of
air to the zone is constant and set points can only be achieved by controlling
the temperature of the flowing air thermostatically. In a VAV based HVAC
system, both the volume flow rate and temperature of the flowing air are
controlled to obtain the set point. The temperature of a space is maintained
at set level by counteracting the cooling load with a certain volume flow rate
of supply air at certain supply air temperature. A small temperature
difference (between zone and supply air) is required at larger volume flow
rates to meet a given cooling load. Small temperature difference requires
lower compressor power and large volume flow rate requires more fan
power. If same load is met by smaller supply air volume flow rate at large
temperature difference, it will require larger compressor power and smaller
fan power. A small supply air flow rate leads to poor temperature
distribution in the zone. Thus the VAV systems have more degrees of
freedom, and comfort requirements can be achieved by varying the fan
speed of a variable speed fan, chilled water temperature, chilled water flow
rate and damper position of supply air VAV box. Therefore, there is a
certain combination of volume flow rate and temperature of supply air at
which total power (comprising of mainly compressor power and fan power)
consumption is minimum.
The usual approaches used to the control of a VAV AC system have the
following features:
Maintaining the chilled water temperature at a fixed value.
- Controlling the flow rate of the chilled water to the air-handling unit
through a three-way valve and /or a variable speed pump.
- Controlling the speed of the air-handling unit using Variable Speed
Drive to control the volume flow rate of cooled air to specified zone.
- Controlling the dampers of the individual spaces to allow the required
amount of cooled air into each zone.
The presently adopted methods of energy management in VAV systems
involve iterative manipulation of the air and water flow rates needed to
meet the comfort requirements. The iterative approach has the
disadvantage as it leads to a slow response. Moreover, heuristics is used
to set the flow and temperature levels at a given cooling load; e.g. the
water flow rate is changed only when the cooling load is too low and the
air flow rates cannot be reduced further. Thus heuristic performs
suboptimally under particular zone and equipment characteristics.
OBJECTS OF THE INVENTION
An object of this invention is to propose a method of maintaining the
zone temperature in a single-zone VAV AC system with optimized power
consumption. Another object of the invention is to propose a VAV AC
system carrying out the method of optimizing the power consumption
while maintaining the zone temperature at a comfort level.
SUMMARY OF THE INVENTION
In accordance with the feature of the invention, there is provided a
method of maintaining the zone temperature in a single-zone VAV AC
system with optimized power consumption wherein the power
consumption of the fan and that of the chiller is determined as a function
of the fan speed and the function of the temperature of the chilled water
at a given cooling load respectively; the minimum of the total power
function identified with repetition of the said activities at different
cooling loads; an objective function for different values of fan speed and
for varying cooling loads determined, minimization of the total power
function being carried out by obtaining an optimal set point through
direct search method, the search commencing from a selected starting
point; a guess point selected by comparing said search result using a
fixed-step size of plurality of different starting points and moving
towards a favourable direction, the total power function evaluated at said
guess points; a new guess point determined by adding said step-size to
said initial guess point and further objective function at said new guess
point evaluated; a minimization step of the total power function
continued in said direction when objective function at new guess point
emerging as lower than the earlier objective function; said searching step
continued until said new guess point showed an increased value of the
objective function; said search being terminated at an optimum point.
The invention further provides a novel cooling only air condition system
which maintains the temperature of the conditioned space with supply of
controlled volumetric air flow of cooled air in the zone with minimum
consumption of energy.
From the total energy function of the VAVAC system it is seen that the
major energy consuming components are the fan ( Pfan) and chiller (PCh).
The formulation of total power function is represented as:
Ptot = Pfan ( N,a ) + PCh ( N,a , Taj, Tamb ) where N= fan speed, a = damper
angle, Taj = supply air temperature, T^b = ambient temperature.
The present invention leads to a control strategy that leads to at the
optimization of the total energy function. Energy consumption is lowest
for maximum air flow rate for a given speed. Therefore for a single zone,
the damper is kept fully opened. Keeping the chilled water temperature
as high as possible provides major energy savings. The coefficient of
performance ( COP) improves with increased chilled water temperature.
A convenient method of optimizing the chilled water temperature is to set
the chilled water temperature using an automatic control. The most
accurate way of responding to the cooling load is to use the room
thermostats signal. When the signal from any one thermostat indicates
that the airside unit is unable to satisfy the space load, the chilled water
temperature is lowered incrementally. In variable-air-volume (VAV) air
handling unit (AHU), space cooling is controlled by varying the supply
airflow, and the supply air temperature. Raising the chilled water
temperature may raise the supply air temperature, which will cause the
fans to operate at higher speed consuming higher power. Typically, more
energy is saved in the chiller than is lost in the fan, so the best efficiency
is usually obtained by raising the chilled water temperature as much as
possible, or to obtain an optimum compromise between chiller power and
fan power. To achieve an acceptable compromise between compressor
power , fan power and zone temperature control accuracy, the present
invention leads to the minimization of supply air flow rate throttling
range to an upper limit of its admissible range. This contributes to lower
energy costs since the equipment COP rises with increasing air flow rate.
Higher air flow rates with higher supply air temperature gives lower load
to the compressor. Thus compressor requires less power consumption.
Higher flow rates also provides better mixing of cooled supply air and
zone air within the zone that leads to the advantage of improved indoor
air quality (IAQ) since the stagnation zones (regions of low air velocity)
are reduced.
For every cooling load, the minimum of the total function is obtained by
calculating the derivative of the power function and setting the derivative
to zero.
The surface of objective function is plotted for different values of fan
speed with corresponding tank temperatures. From the surface, it is
evident that the total power function is a unimodal function and can be
subjected to minimization. The derived total energy function is a non-
linear one. Though in actual system there are some constraints, the
optimization problem is defined as a non-linear unconstrained
programming problem. Here the optimization problem is subject to the
desired test cell temperature. Classical optimization techniques have
limited scope in practical applications. All the unconstrained
minimization methods are iterative in nature an hence they start from an
initial trial solution and proceed towards the minimum point in a
sequential manner. Several numerical methods are available for solving
an unconstrained minimization problem. These methods are classified
into two broad categories as direct search methods and descent methods.
Assuming that one global minimum exists in the feasible domain, a direct
optimization method is applied to find the optimal set point. A starting
point is chosen at an extreme point, such as minimum value of the
boundary feasible region and the search result is compared to the one
obtained with different starting points. The most elementary approach
for such a problem is to use a fixed step and to move in a favourable
direction.
Minimization is started with an initial guess point. Here initial guess is
the minimum fan speed. The total power function is evaluated at initial
guess point. A new point is obtained by adding a step size to initial
guess. The objective function is evaluated at this point. If the value of
function is less than the previous value then minimization will proceed in
the same direction. The search is continued until a point say X; = Xi + (i-
1)S shows an increase in function value where 'S' is step and T is
number of iteration. The search is terminated at such an Xj . The
efficiency of the process is improved by searching with accelerated step
size. The optimal value of the fan speed (N) is evaluated by applying
search method. Using this search method to optimize the total energy
function, the maximum allowable chilled water temperature and the
optimal fan speed is evaluated for a given space load.
Optimal set points for the fan speed and the chilled water temperature are
determined based on the total cooling load of the system.
Zone temperature is achieved by tank temperature controller in
coordination with system temperature control loop and VAV box control
loop.
The invention is explained in more details hereinbelow with reference to
the accompanying drawings.
In detail:
Fig. 1 shows a typical VAV Air conditioning System showing a single
zone to be conditioned .
Fig. 2 shows an interdependency diagram for a single zone of the system
and formulation of total power, according to the method of the invention,
Fig. 3 shows power consumption for cooling load at 1000 watt, as a
function of fan speed.
Fig. 4 shows optimal fan speed for a given cooling load of the
conditioned space.
Fig. 5 shows power consumption for cooling load at 1000 watt, as a
function of the chilled water tank temperature.
Fig. 6 shows the temperature of the cooling water storage tank as the
function of fan speed and cooling load,
Fig. 7 shows total power consumption as the function of fan speed and
cooling load,
Fig. 8 shows schematic diagram of different control loops of VAVAC
system, according to the present invention.
As shown in figure 1, fresh air from outside is drawn into the system at
fresh air inlet junction (FAIJ). The fresh air is mixed in proportional
amount with return air from the return air duct system and some return
air is rejected from the return air duct via FAIJ thereby providing a
mixed supply air to the cooling coil (CC). Cooling and dehumidification
of moist supply air are achieved by circulating chilled water through the
coil tubing. The cooled supply air is forced through the supply air duct
system by means of a blower. Supply air duct system further comprises
Variable Air Volume (VAV) box. The supply air is communicated via
supply air duct system to the VAV box. VAV box consists of a damper
assembly secured within a housing fitted with suitable actuators for
moving the damper for selectively controlling the flow rate of supply air
into the zone. The VAV boxes used in the system are of pressure
independent type and employ an Air flow sensing device.
The VAV boxes receive an electric signal from the thermostat located
within the zone. The signal indicates the amount of airflow required for
the VAV box, usually to satisfy the temperature of the system cell. The
flow sensor in the box ensures that the desired airflow will be maintained
even as the pressure within the main supply duct changes. The VAV box
can control the air flow to any level between the present minimum and
maximum airflow (cfin). Closer room control can be achieved by
varying the position of the damper to compensate for changes in supply
duct static pressure brought about by the variable speed motor.
The chilled water storage tank (TANK) is the link between the chiller
compressor and the cooling coil in the AHU. The water from the storage
tank is pumped to the chiller for cooling. The large thermal capacity of
the chilled water stored in the tank helps to supply chilled water without
sudden temperature changes. The temperature sensor is kept inside the
tank to maintain the chilled water temperature at a constant level. The
chiller contains a refrigerating unit and a compressor unit, which cool the
chilled water with the help of a liquid/liquid heat exchanger. The
temperature of the chilled water can be varied by switching the
compressor on and off. The chilled water is circulated between the
chiller and tank by the chilled water pump at a constant volume flow rate.
The Chilled water is supplied at a temperature Twi by the secondary
chilled water pump to the cooling coil and returns at a temperature Twr to
the water storage tank. The supply chilled water temperature is the
feedback signal to the tank controller which controls the input energy of
the compressor. By changing the 3-way valve (3-WV) position the water
flow rate to cooling coil can be controlled.
Figure 2 illustrate the formulation of total power function and
dependency of P^n and PCh . In practice, the chiller consumes much more
energy than the fan. An optimum compromise between chiller power and
fan power is determined.
The power consumed by the fan and the compressor has been evaluated,
as well as the summation of these two powers. It is seen that the power
consumption is minimal for a fan speed of 440rpm. Doing this for every
cooling load between 100 and 3000 W leads to the result given in Figure
4, which gives the optimal fan speed for a given cooling load of the
conditioned space.
In Figure 5, the power consumption for Cooling load = 1000 W is shown
as a function of the chilled water tank temperature . The minimum
power consumption is obtained for that particular cooling load at a tank
temperature of about 16.5° C.
Figure 6 shows the surface of objective function determined for different
values of fan speed with corresponding tank temperatures. This surface
shows an increase in fan speed correspondingly increases the tank
temperature. The surface depicts the optimal value of fan speed in the
range of 100 rpm to 500 rpm with varying cooling load for minimum
power consumption.
Figure 7, illustrates the surface of the objective function ascertained for
different values of fan speed, with varying cooling load.
Figure 8, shows that the chilled water temperature set point being given
to the tank controller. The chilled water temperature set point is
maintained by switching ON/OFF the chiller plant. The fan is set to the
optimal fan speed. For a particular chilled water temperature the space
cooling load requires a certain airflow rate. The total airflow rate
depends on the fan speed, as the damper is kept fully opened for the
single zone.
WE CLAIM :
1. A method of maintaining the zone temperature in a variable air volume
air conditioning (VAVAC) system with optimized power consumption
comprising the steps of:
(a) determining the power consumption of the fan of an Air handling
unit (AHU) of said VAVAC system as a function of said fan speed at
a given cooling load;
(b) determining the power consumption of a chiller of said VAVAC
system as a function of the temperature of a chilled water storage
tank at said cooling load, total power consumption at said cooling
load being derived by summation of said two powers for feeding to
a control means;
(c) identifying the minimum of said total power function by
ascertaining the derivative of the power function and setting the
derivative to zero;
(d) repeating steps at (a), (b), & (c) for every cooling load between
100 watt to 3000 watt to ascertain the optimal fan speed and the
minimum power consumption as a function of the temperature of
said chilled water storage tank;
(e) determining an objective function in said control means for
different values of fan speed with varying cooling load, the
objective function being subjected to minimization being an
unimodal function;
(f) selecting a starting point at the minimum value of a feasible
boundary region and applying a direct search technique in said
control means to obtain an optimal set point;
(g) continuing said search technique using a fixed step-size of a
plurality of different starting point and moving towards a
favourable direction;
(h) selecting a guess point being the minimum fan speed and
evaluating the objective function at said guess point;
(i) determining a new guess point by adding said step-size to said
initial guess point and further evaluating the objective function at
said new guess point;
(j) proceeding with a minimization step of the objective function in
said direction in case the determined value of said objective
function emerging as lower than the value of earlier said objective
function;
(k) continuing said searching step until said new guess point shows an
increased value of said objective function determined at step (j)
the search being continued up to a point Xj, the value of Xs being
equal to Xi + (i-l)S, where 'S' represents the step-size and i
representing the number of iterations;
(I) terminating said search at said point X( where the increase in the
objective function is indicated.
2. A VAVAC system with an integrally packaged PC based control means
for carrying out the method as claimed in claim 1, comprising:
a chilled water storage tank (Tank) arranged between a
chiller compressor and a cooling coil (CC) placed in an
air handling unit of said system, the temperature of the
chilled water being made variable through said chiller
compressor;
a temperature sensor interposed in said chilled water
storage tank to measure the temperature of the chilled
water at a constant level and sending data to the
control means;
a fresh air inlet junction (FAD) drawing fresh air into
the system for mixing with the return air from a return
air duct (RAD), the mixing proportion being determined
by the input from said control means and balance
return air rejected to atmosphere;
a blower for supplying cooled air after said mixed air is
dehumified and cooled by circulating chilled water
through said cooling coil to a supply air duct, and to the
zone to be maintained, the temperature of chilled water
being regulated based on data supplied by said control
means;
a variable air volume box (VAV) provided in a supply air
duct for receiving the supply of said cooled air forced
through said blower, the optimum air flow required
being determined by said control means and the flow
being correspondingly controlled by regulating the fan
speed of said blower;
3. The system as claimed in claim 2, wherein said chiller comprises a
refrigeration device having a compressor unit, and a liquid heat
exchanger for cooling the chilled water.
4. The system as claimed in claim 2, wherein said chiller compressor is
provided with a solenoid coil for varying the temperature of the chilled
water on receiving signals from said control means.
5. The system as claimed in claim 2, wherein said VAV comprises a
rotatable damper assembly accommodated in a housing, and attached
to a plurality of actuators provided with a sensing device, the sensing
device transmitting to and receiving data from said control means.
6. The system as claimed in claim 5, wherein said actuators imparting
movement to said rotatable damper assembly for selectively controlling
the flow rate of cooled supply air to the zone to be maintained;
7. The system as claimed in claim 5 or 6, wherein said sensing device
receives an electric signal from a thermostat provided in the zone, the
signal indicating the amount of required flow of cooled air in the zone.
8. The system as claimed in claim 2, wherein said control means
comprises an integrally controlled computer operated software.

The invention relates to A method of maintaining the zone
temperature in a variable air volume air conditioning (VAVAC) system
with optimized power consumption comprising the steps of
determining the power consumption of the fan of an Air handling unit
(AHU) of said VAVAC system as a function of said fan speed at a given
cooling load; determining the power consumption of a chiller of said
VAVAC system as a function of the temperature of a chilled water
storage tank at said cooling load, total power consumption at said
cooling load being derived by summation of said two powers for
feeding to a control means; identifying the minimum of said total
power function by ascertaining the derivative of the power function
and setting the derivative to zero; repeating steps at (a), (b), & (c) for
every cooling load between 100 watt to 3000 watt to ascertain the
optimal fan speed and the minimum power consumption as a function
of the temperature of said chilled water storage tank; determining an
objective function in said control means for different values of fan
speed with varying cooling load, the objective function being subjected
to minimization being an unimodal function; selecting a starting point
at the minimum value of a feasible boundary region and applying a
direct search technique in said control means to obtain an optimal set
point; continuing said search technique using a fixed step-size of a
plurality of different starting point and moving towards a favourable

direction; selecting a guess point being the minimum fan speed and
evaluating the objective function at said guess point; determining a
new guess point by adding said step-size to said initial guess point and
further evaluating the objective function at said new guess point;
proceeding with a minimization step of the objective function in said
direction in case the determined value of said objective function
emerging as lower than the value of earlier said objective function;
continuing said searching step until said new guess point shows an
increased value of said objective function determined at step (j) the
search being continued up to a point Xi, the value of Xi being equal to
Xi + (i-l)S, where 'S' represents the step-size and i representing the
number of iterations; terminating said search at said point Xi where the
increase in the objective function is indicated.