Direct digital control of small and medium-size buildings

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Direct digital control of small and medium-size buildings

Elmahdy, A. H.; Beattie, D. G.

Ser

TK1

DIRECT

DIGITAL CONTROL

OF

SMALL-

AEJn

_MEDIUM-SIZE

_BUILDINGS

b

=k

*+

A,H, Efmahdy and D.G. Beattie

INTRODUCTION

The control of heating, ventilating and air-conditioning (HVAC)

systems is changlng as a result of applying direct d i g i t a l control

(DDC) techniques to

HVAC

_{control, mls report o u t l i n e s the}

_main

features of

DDC

compared with

conventfonal

pneumatic

control

and

shows

t h a t , f o r small- t e medium-size buildings, the

DDC

system can pay for

i t s e l f within two years, after which it effects n e t savings over pneumatic systems.

This report is a summary of a contract report prepared

for

the Natlonal Research Council af Canada by Systemhouse ~ i d t e d [ 11.

CONPARLSDN

BETWEEN

P N E W T L C

CONTROL

AND DDC

Direct digital control of HVAC systems is the dfrect monitoring of

every system input (temperature, flow, pressure) and direct control of

e v e r y system output ( p o s i t i o n , onlaff) from a _{central controller}

_which

i s _{a s i n g l e couputer or combination of computers. DDC is a simple} concept, but i t s significance is n o t obvtous until it is compared

with

traditional

forms of HVAC control.

Traditionally, the control of HVAC systems w a s based on

independent pnewmatic controllers,

which used

compressed air t o operate

the dampers and valve actuators t o control space coaditfans such

as

temperature, humidity and fresh-air

c i r c u l a t i o n .

One b u l l d i n g would

have

several

such systems, which

were

controlled independently. For

example,

an

air-handling system composed of two fans, three dampers and

three valves (Figure

1)

would be c o n t r o l l e d

by

local pneumatic

controllers which operated as independent units. _{Each c o n t r o l l e r had}

a

s i m p l e task: to _{maintain a constant set p o i n t (for example, supply air}

temperature) by monitoring and c o n t r o l l f n g a very l i m i t e d number of variables connected to i t by means of compressed air l i n e s whose

pressures represented t h e values of t h e variables. The c o n t r o l

was

adjusted mechanically by a technician in the field, and, as calibratfon

af _{the pneumatic components was rarely carried out, these systems often}

did not control the b u i l d i n g e f f i c t e n t l y . Because t h e pneumatic

controllers were purely electromechanical devices, their sophistlcatfon

and accuracy of control

were

extremely limited.

A later variant (of pneumatic control) also employed pneumatic

c e n t r a l s , b u t w i t h the a d d l t i o n

of

a c o q m t e s system. This computes

system monitored some

additional

points

(for

example, space

Division of Building Research, National Research Council of Canada,

O t t a w a

temperatures)

and

either c a l c u l a t e d new set p o i n t s f o r each pneumatic c o n t r o l l e r or allowed an o p e r a t o r at a _{computer terminal} to _transmit

manual setpoints to t h e pneumatic controllers. Although this newer

variant aided the b u i l d i n g manager by providing more information about b u t l d i n g conditions

and

performance, o v e r a l l effective control

of

_the b u i l d i n g w a s s t i l l compromised by the local pneumatic controllers. Each controlled polnt was still operated by a pneumatic c o n t r o l l e r wfth very l i m i t e d sophistication and virtually no f l e x i b i l i t y .

These limitations became more important as ways to manage energy

became more sophisticated, Some W A C system, such as variable a i r

volume (VAV) systems, required an accuracy of control not attainable in most c a s e s by pneumatic controllers.

As

_{a r e s u l t , b u i l d i n g energy}

managers were frustrated by their knabilitg t o improve the control strategies wlthout rebuilding the pneumatic c o n t r o l system for each change.

DDC has solved both problems;. Instead of independent local

pneumatic controllers, DDC uses control or monitoring points, each

connected t o a computer (or i n t e r c o n n e c t e d computers) which reads the value of each input and transmits commands to each output ( F i g u r e 2).

The control strategies are implemented by computer programs, which can be changed by the operator at

will.

Also, each strategy has available to it t h e value of every system knput instead of a very l i m i t e d l o c a l s e t .

In short,

under the DDC concept, t h e entire b u i l d i n g operates as

one integrated system rather t h a n as independent srrrall systems.

Four main results accrue: 1) c o n t r o l can be as simple or

sophisticated as desired, and can be changed e a s i l y ; 2) the system is more reliable because

fewer

electromechanical components are needed;

3) control is more accurate because of the inherent greater accuracy

of DDC electronic components; and 4 ) energy is saved because an over- all strategy elidnates energy waste resulting

from

simltaneous

h e a t i n g and cooling, which usually occurs

in

pneumatic systems.

The a b i l i t y of DDC to accommodate v i r t u a l l y any c o n t r o l strategy has had a dramatic impact on mechanical design. Some new mechanical

systems can operate in many different modes, depending on external

conditions, space temperatures, season, condition of storage tanks, and utilitylprfcing structures. DDC allows such systems to be operated

continuously

in

t h e i r optimum modes, a _{s t a n d a r d which simply cannot be}

a t t a i n e d by o r d i n a r y pneumatic systems or even pneumatic systems with

cornput er monitoring. Consequently

,

mechanical designers are now free to d e s i g n t h e best energy system for a particular building

with

the assurance t h a t whatever c o n t r o l strategies they s p e c i f y can

be

carried

out.

Each loop at the remote processors can activate Itself independent of the others; however, the most efficient use of energy is achieved by controlling

all

the loops through the central processor. Scheduling air-conditioning and h e a t i n g l o a d s and selectively dropping e l e c t r i c a l l o a d s if the total b u i l d i n g power approaches the demand limir are t w o common energy o p t i m i z a t i o n features available.

Other features, such as optimal stop/stast, whtch calculates the ovtimrn starting and stopping times of heating/cooling units to p r e p a r e

spaces

f o r

occupancy without wasting

e n e s w ,

are a l s o used as part of

an over-all strategy. Most of these a p t i d z a t i o n routines do n o t

require any additional hardware since they are implemented

by

simply adding programs that sense existfng inputs and change the strategy f o r controllZng e x i s t i n g output actuators.

The building owner

or

manager

who

uses

DDC

effectively

needs

feedback

to

e v a l u a t e h i s

strategies

f o r

o p t i d z i n g

building

performance. DDC simplifies t h i s process because it contrnually

monitors each input d i r e c t l y and has storage capacfty to keep files

of

the h i s t o r i c a l data thus obtained. These historical data can be p l o t t e d in colour on a TV screen or summarized and printed i n report

format for management review. The most advanced DDC systems (Figure 3 )

i n c l u d e a generalized report generator which can produce nee types of

reports at any time r a t h e r than limit the user to t h e reports endsaged when the system was procured* This feature of

DDC

i s p a r t i c u l a r l y important s i n c e the owner's

power

to change h i s energy strategy generally creates a need for

new

repests on energy-sensitive

areas

identified by continued use o f t h e system.

A n a n c i l l a r y b e n e f i t i s the ability of the

DDG

system to include

f a c i l i t i e s other than WAC. With little increase

in cost,

factors such

as control of security and l i g h t i n g can be added ta t h e system, thereby enabling greater energy savings and eliminating the need t o purchase separate systems for badge reading and door-lock

control.

There is

no

doubt t h a t DDC offers more e f f e c t i v e energy management than

conventional controls but,

until

very

recently,

i t s application to HVAC

installations has been limited to large _{building complexes. Many}

small- and m e d i u r s i z e building installations

do

not use

DDC mainly

because of i t s high c c a s t [ ~ ] .

In

the f o l l o w i n g sections a t y p i c a l

small

b u i l d i n g is _{analyzed and DDC is compared}

_with

_{pneumatic control on a}

cost and payback b a s f s.

SMALL-BUILDING

SYSTEMS

The cast of an WAC controls Installation is generally related to the number of " p o i n t s " t o be monitored

or

controlled, where each point is _defined as _{an analog o r digital i n p u t ( e . g . , temperature}

_sensor,

_fan s t a t u s switch) or analog or d i g i t a l output ( e . g , , damper

position

or pump onloff c o n t r o l .

Each

building system,

such

as air handling,

domestic h o t water, or c h i l l e d water, includes a certain number of points. A recent study

[I]

which included d e t a t l e d a n a l y s i s of a series of b u i l d i n g HVAC system, shorwed t h a t a small- to medium-size b u i l d i n g of about

37,175

m;! (400,000 sq. ft .) would c o n t a i n about

180

p o i n t s , of which 35% would be analog inputs, 19% analog outputs, 25% d i g i t a l inputs and 21% d i g i t a l o u t p u t s . Although different building c o n f i g m a t i o n s and mechanical designs would a f f e c t

t h e

distribution of

p o i n t types, t h e total number of points for a building of this size

would usually be close to

180.

DESIGNING

A DDC

SYSTEM

Given the building layout and the number of points fn WAC equipment, t h e single greatest design trade-off is that between

c e n t r a l i z e d extrem, a s i n g l e central computer c o n t r o l s a l l functions

directly and all points are wired to it. A t the o t h e r extreme ( f u l l y -

d i s t r i b u t e d ) , a

smaller

central computer is connected t o a myriad of other

small

computers, each

of which

is wired to 10 to 20 nearby

p o i n t s ,

In

t h i s second i n s t a n c e t h e c e n t r a l machine presides aver the

whole system

and

controls t h e points through t h e intermediary of the remote processors. Each remte processor can control a s i n g l e WAC s y s t e m ( e

.g.

,

air-handling unit, chiller) independently. A median

approach is to employ a moderate number of remote units each of which

is wfred to

50

t o 120 points.

Although all these approaches utilize the benefits of DDC, the three l e v e l s of centralization/distribution involve three factors that musr be weighed against one another. The f i r s t factor is _{the cost: of}

computer hardware. The f u l l y - c e n t r a l i z e d approach employs a s i n g l e processor,

which

is the least expensive since it combines all the

computing power

in

one place w i t h one e n c l o s u r e and no duplication of functions. The fully-distributed approach requfres the heaviest c a p f t a l c o s t for computer hardware.

The second factor is electrical installation cost. _{The f u l l y -} d i s t r i b u t e d arrangement y i e l d s the lowest _{installation c o s t because}

each remote processor can be located very close to its points and thus

wiring runs are short. The fully-centralized arrangement may be quite expensive unless all points are in one mechanical room. The median

arrangement ( F i g u r e 4 ) may be the most economtcal o v e r - a l l because f o u r remote processors can be u s e d , one in a penthouse, one in some other logical l o c a t i o n such as a basement mechanical room, and others on various floors of the b u i l d i n g .

The

third

factor is reliability* The fully-centralized scheme is

most s e n s i t i v e to failure since f a i l u r e of t h e single computer causes t h e e n t i r e system to fail. Although t h e system can be made to fail safely

,

a system failure is inconvenient. The fully-dis t r i b u t e d scheme is least sensitive s i n c e any component computer can fail

while

still

l e a v i n g all the others running, but, as previously mentioned, the cost

of the computing equipment is highest.

A median approach for s m a l l buildings makes good sense, A compromise o n all _{factors is established by d e s i g n i n g a system}

c o n s i s r i n g of _{a c e n t r a l computer and four remote u n i t s .}

COST ANALYSIS: DDC VERSUS

PNEUMATIC

CONTROL

The

i n s t a l l e d cast _{of DDC} systems has traditionally been h i g h e r than f o r p n e u m a t i c sys tens, especially in small installations, where

the cost o f the

DDC

control processor is spread over fewer points. The c o s t o f a pneumatic system t e n d s to rise linearly with the number of p o i n t s , as a large system requires more independent l o c a l controllers, whereas with DDC a central processor is required even f o r system w i t h very f e w points. However, the r a p i d l y f a l l i n g cost of computing

hardware has eroded the historical prfce difference between DDC and

pneumatic installations. F o r a specific building [ I ] of 37,175 n? ( 4 0 0 , 0 0 0 s q . ft.), the installed c o s t of a pneumatic system is a b o u t

Although the initial c o s t of a DDC system is higher than f o r a pneumatic system, it can be recovered in a s u r p r i s i n g l y s h o r t t i m e . It is realistic to assume t h a t a DDC system will yield a 10X energ?T sawing

over and above conventional pneumatic control, due s i m p l y to its more

accurate and sophisticated c o n t r a l , and t o its _{ability to p r o v i d e t h e}

b ~ i i l d i n g owner w i t h i n f o r m a t i o n about building performance and areas

where energy should be better controlled. Features such as load shed

and flexible s c h e d u l i n g alone will produce large energy savings, and

t h e s e s a v i n g s

will

increase as the owner becomes more f a m i l i a r ~ 5 t h t h e

operation of the building. If we assume yearly maintenance c o s t s of $12,000 and $10,000 f o r the DDC and pneumatic systems respectively, and an energy usage of 322 equivalent kb7h/dyr. (30 kWh/sq.ft./yr.) at

$0.0275

per kwh f o r both systems, it will take 1.4 _{years more for the}

DDC to pay f o r i t s e l f than it will for the pneumatic system when used in t h e b u t l d i n g under consideration. After that t i m e the

DDC

system

will save money compared with the pneumatic controls. Another simple c a l c u l a t i o n shows t h a t for a three-year payback the

DDC

energy saving need be only 5.J%, an e a s i l y attainable figure.

CONCLUSIONS

Direct d i g i t a l c o n t r a l is now c o s t competitive w i t h pneumatic control for W A C controls in small- to medium-size b u i l d i n g s . Given

the o t h e r advantages of DDC, particularly i t s a b i l i t y t o accommodate

changes t o c o n t r a l strategy and to provide d e t a i l e d r e p o r t s of b u i l d i n g performance,

DDC

should become the dominant technology for b u i l d i n g s in t h i s s i z e range.

Additional development is _{needed to lower t h e} c a s t further. I n the exarnple presented

in

this paper, instrumentation is the h i g h e s t s i n g l e c o s t , l a r g e l y because all s p e c i f i e d instrumentation is

i n d u s t r i a l grade. The development of commercial-grade sensors and actuators, particularly all-electronic types, which would o b v i a t e the need f o r an instrument air s u p p l y , will c o n r s i b u t e greatly to the acceptance of

_DDC.

_{A second area of development lies in d e v i s i n g} computer p r o g r a m for t h e central and remote computers. As energy managers demand more sophistication and t h e l a b o u r cost for custom

development r i s e s , comprehensive and f l e x i b l e software packages will

dominate the

DDC/HVAC

market.

REFERENCES

1. Feasibility Study of Small Building Direct Digital Control.

Prepared by Sytemhouse L t d . , for National Research Council of Canada Contract No. 1SX80-00031, Uec. 1980.

Note: I n q u i r i e s concerning t h e _{a v a i l a b i l i t y o f Contract Report} 1SX80-00031 should be sent to the Canadian Institute f o r Scientific

and Technical Informtion, B u i l d i n g

M-55,

Montreal R d . , Ottawa,

KIA

OR6.

2. Elmahdy, A , , An Overview of Central Control and I l o n i t o r i n g Systems f o r Large B u i l d i n g s and Building Complexes, Building Research Note

No. 159, D i v i s i o n of _{Building Research, N a t i o n a l} Research C o u n c i l

SENSORS 8 ACTUATORS

OPTIONAL

CONSOLE PROCESSING PROCESSING

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P R O C E S S I N S REDUNDANT

OPTIONAL ALARM OPERATOR COLOUR Dl SPLAY LOGGER CONSOLE

TIGIJRE 3

C E N T R A L COMPUTER F A C l L l T Y

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