Transcript comfort and health

Systems Control
Design considerations for:
Control systems and building energy
management systems
Control?
• Control can be defined as:
“the modification of the behaviour of a system
so that it acts in a pre determined manner”
Example
Example
• Tossing a caber requires strength but
balancing a caber requires control!
• For caber to remain upright continual
adjustments to the position of the caber
tosser’s body are required
• e.g. if the caber falls to the left then the
caber tosser moves to the left to regain
control
• The upright position is known as the
“reference”
• The difference between the reference
and the actual angle of the caber is
known as the “error”
reference
error
Example
• The body (legs) is commanded by
the brain
• Command based on the error that is
seen by the eyes
 The brain is therefore the
“controller”
 The body is the “actuator”
 The eyes are the “sensor”
 The caber is the “controlled
system”
• These are the basic components of
ALL control systems!
caber
reference
eyes
error
body
brain
Block Diagram
• We can represent the “caber+tosser” control system using a
“block diagram” of the bio-mechanical control system
Desired
angle
Actual angle
Error
Actual angle
Observed
angle
Example
We can see the same basic control elements
in the function of a heating or air conditioning
system, where the aim is to maintain
comfortable conditions within a space for a
specified period of time.
This is achieved through the use of a electromechanical control system.
A basic room temperature control system
heating coil
Qo
room
flow control
valve
T
s
valve
position
controller
+/sp
error (e)
set
point
A basic room temperature control system
heating
coil
Qo
flow
control
valve
T
s
valve
posit
ion
control
ler
roo
m
error
(e)
+
/
set
poi
nt
This type of control
mechanism is commonly
termed feedback control,
where the controlled
variable (temperature) is
fed back to the control
system
sp
A sensor (thermostat) sends a temperature (usually a
mix of air and radiant temperature) back to a controller.
This is usually passed in the form of an electrical voltage,
where the voltage magnitude is proportional to the
temperature.
The controller compares this temperature to a set-point
(desired) temperature and generates an error value. The
error is the difference between the set-point temperature
and the sensed temperature.
Depending on the magnitude of the error, the controller
will adjust the output of the heating system up or down.
In practice, this will involve operating a mechanical
component such as a valve to increase or decrease the
flow of hot water through a heating coil or radiator.
The controller employs an algorithm to determine the
heat output as a function of the error.
Components of control
• We can represent the “HVAC+room” control system using a
“block diagram” of the electro-mechanical control system
desired
temperature
actual
Error
CPU
measured
temperature
valve + heating coil
temp
ON/OFF control
This is the simplest type of control used in buildings:
• If the sensed temperature is below the set-point then the heating system is
fully ON.
• If the sensed temperature rises above the set-point then the heating
system is OFF.
o
C
heat output
set point temp.
room air temperature
Q max
This shows the temperature and heat
output in a room controlled by an
ON/OFF controller. In practice the use
of ON/OFF control can cause problems.
As can be seen, the heating system
rapidly switches ON an OFF leading to
inefficient system operation and
increased mechanical wear.
ON/OFF control with deadband
To address this deficiency a 'dead band' may be introduced, i.e an upper and lower set-point.
• if the sensed temperature is below the lower set-point then the heating system is ON;
• if the sensed temperature rises above the lower set-point but is still below the upper set-point then
the heating system is ON;
• if the sensed temperature is above the upper set-point then the heating system is OFF; and
• if the sensed temperature falls below the upper set-point but is still above the lower set-point then the
heating system is OFF.
o
C
r oom
h eat
u pper
temp.
lower
temp.
air temperature
output
The addition of the upper and lower set-points acts to reduce the frequency of the plant
switching at the expense of poorer control of the controlled variable (temperature).
Q max
Proportional control
This is a more advanced control algorithm, where the control action is proportional to the
size of the error:
Q o (t )  Ke(t )
where K is known as the gain of the controller.
o
C
heat output
room air temperature
upper temp.
Qmax
lower temp.
offset error
Taking the example of room
temperature control:
•if the temperature is below the
set-point then the heating is ON
and the output is proportional to
the difference between the
sensed temperature and the
desired temperature; and
•if the temperature is above the
set-point then the heating is OFF.
As the sensed temperature gets
closer to the set-point temperature,
so the output of the heating system
is reduced.
Proportional control (continued)
In practice the operation of a proportional
controller is often limited as the output of the
heating system is limited (i.e. it has a
maximum capacity). This is achieved by
introducing a 'proportional band' or 'throttling
range'—this is similar to a dead band in that a
single set-point is replaced by an upper and
lower limit. The control action now follows the
following rules:
• if the temperature lies above the
throttling range then the heating system
is OFF;
• if the temperature lies below the
throttling range then the heating system
is ON at full power; and
• if the sensed temperature is within the
throttling range then the output is a
function of the error.
u & set
point
oC
heat output
throttling
range
l
0
0
Qm
ax
Proportional control (continued)
Within the throttling range, the output of the heating system is:
 u   s  

Qo  
Qmax
 u   l 
where, in this case, u = sp and so u - s = e
 Qmax 

Qo  
e

 u   l 
where K 
Qmax
u  l
As with ON/OFF control, the throttling range affects the operation of the system: a
narrow throttling range gives close control (a small error) at the expense of the system
switching ON/OFF frequently; and a wide throttling range reduces the ON/OFF
switching off the system (cycling) at the expense of poorer control.
PID control
It is impossible to completely eliminate the
error between the desired temperature and
the sensed temperature using only
proportional control. There is always an offset
error, where the controlled temperature never
quite reaches the desired temperature.
o
C
heat output
set point
temp.
room air
temperature
Qmax
A PID controller incorporates a mix of
proportional, integral and derivative control
action. In this case the control output is a
function of the size of the error, the rate of
change of the error with time, and the integral
of the error over time.
t


de
(
t
)
1

Qo  K e(t )  Td
  e(t )dt
dt
Ti o


where Td is the derivative action time (s),
Ti the integral action time (s) and K the
gain
Building Energy Management Systems
We’ve looked at a single controller – in reality a large building will have hundreds of
different controllers operating – BEMS is means of monitoring and coordinating their
operation
ventilation
cooling
heating
BEMS
fire
humidity
security
lighting
Building Energy Management Systems - facilities
Monitoring
They allow plant status, environmental conditions and energy to be monitored, providing
the building operator with a real-time understanding of how the building is operating. This
can often lead to problems being identified which may have gone unnoticed, e.g. high
energy usage. Energy meters can easily be connected to a BEMS, providing real-time
consumption patterns and ultimately a historical record of the building's energy
performance. BEMS can therefore improve management information by trend logging
performance, benefiting forward planning and costing. This can also encourage greater
awareness of energy efficiency amongst occupants and management.
Alarms
They allow alarms to indicate that plant has shut down or requires maintenance, or that
environmental conditions are outside limits. This is particularly important when applied to
geographically remote buildings.
Integrated Control
They allow close integrated control of building services equipment, saving energy and
improving comfort levels for the occupants.
BEMS – layout of centralised system
Centralized systems control all connected site services from a single computer
unit, and are most appropriate for large commercial buildings, such as hospitals
with over 500 beds, motor factories and airport terminals.
BEMS – layout of distributed system
Distributed systems comprise a number of local "intelligent" outstations which
each control a small building, part of a large building, or a particular service. The
outstations feed data back to a central unit for data collation and perhaps
supervisory control (where, for example, set points can be changed by the
central station). This type of system is generally used for a group of small to
medium-sized buildings under common ownership (e.g. schools, hotels).
BEMS – communications
BEMS – typical control functions - optimiser
An optimiser (or optimum start controller) is basically a device incorporating a time
switch, which switches the plant on at such a time that the room temperature reaches the
required value at or just before the predetermined occupancy start time. Most optimisers
calculate the start-up time from a combination of space and ambient temperature sensor
measurements. Similar functionality is used for optimum stop, so that heating systems can
be switched off before the end of the working day.
BEMS – typical control functions - compensator
For buildings other than dwellings, the Building Regulations require that the
temperature of the heating system is regulated according to outside
temperature. This can be achieved by using a compensator which adjusts the
flow temperature in the heating circuit as the outside temperature rises or falls.
Adaptive compensators are self-learning and can adjust the flow temperature
based on the flow temperature/outside temperature relationship of previous
days. Similarly, some optimisers are self-adapting.
BEMS – typical control functions - compensator
The 3-way valve reduces the flow water temperature, according to the
heating load, by reducing the water flow from the boiler while increasing the
return water flow. The flow water temperature is set in proportion to the
outside temperature measured on an external north-facing wall. Control of
the valve is with a PI control loop.
BEMS – typical control functions - others
Sequence control: boiler sequence control enables only the number of boilers
that are actually required to meet system demand, and avoids the low
efficiencies that can result from boilers being used at low part-loads.
Frost and fabric protection by switching on heating at night to prevent
condensation and freezing
Lighting control based on measured workplane illuminance levels
Supply and return fan control in air-conditioning systems
Duty cycling: regular on-off switching to achieve required temperatures
Mixing of re-circulated and ventilation air: e.g. to use outside air to
reduce cooling requirements.
BEMS - disadvantages
Capital cost
This is high - a large building BEMS could be £50-100k, although smaller systems
with intelligent outstations can be effective. Paybacks in energy terms can be 510 years, but this excludes monitoring and alarm benefits.
Training
An important consideration - poor operation of BEMS can negate any energy
benefits.
Over-sophistication
There is a danger in installing a high-tech control system that is difficult to
understand.
“Intelligent” buildings
This term is often used to describe buildings with a high level of computerised
control, with integration of different control systems. For example, energy
management, security, office automation systems and external communication
systems may be linked.
Fully integrated systems are estimated to cost up to 50% more than for a
conventional building, but potentially offer energy savings, efficiency and
adaptability.
Conclusions
Most of the examples have involved control applied to room temperatures. Many
other control targets may be considered within a building design context. For
example:
•
control of blinds located on the façade;
•
control of fan speed;
•
control of damper/valve positions;
•
control of hot water temperature; and
•
control of re-circulated air
Good control is vital to acceptable performance in terms of comfort and energy
efficiency. The control algorithms such as ON/OFF, proportional and PID must be
configured for optimum performance:
•
selection of set-points;
•
selection of proportional/dead band values; and
•
selection of integral/derivative action times.
Conclusions
Inappropriate control parameters lead to a poorly configured
control system, which in turn may give rise to uncomfortable
conditions, energy waste and a reduction in the lifetime of
system components.
It is important that a building's control system is well
designed, commissioned and maintained.