academic research for global optimization (Huang and Lam 1997; Wang and Jin 2000; Chow et
al. 2002; Nassif et al. 2005; Lu et al.
2005b; etc.). GA is a result-based method, and no deriva-
tives are required during the calculation. This feature makes it possible to solve the complicated
and global optimization problems. However, the extensive computational cost and memory
demand may be an obstacle for online application of this technique. Further research of the
robustness and feasibility of this technique for practical application is essentially required.
In practice, optimization techniques should be selected based on the combination of the com-
plicity and characteristics of the system of concern, as well as the number of optimization vari-
ables involved for
a
particular optimization problem. The selected optimization technique
should have less computational cost and memory demand to meet the requirements of practical
application. The convergence should be always guaranteed.
It
should also have a simplified
structure and should be easy to be understood by the practicing engineers.
RESEARCH
AND
APPLICATION OF
OPTIMAL CONTROL STRATEGIES FOR HVAC SYSTEMS
Since control function is one of the major hctions of a BAS, the building society and profes-
sionals have made serious efforts toward the development and application of various control strat-
egies for HVAC systems. Figure
3
is a schematic of the scattered areas of the research and
development of the control studies in HVAC systems. It can be found that most research related to
HVAC system control focused on the local level control (area
l),
while relatively few studies
focused on the supervisory control (area 2). The reason is probably due to the easier implementa-
tion of local controllers in practice. Since sensor faults
and/or component degradations in HVAC
systems may cause significant energy consumption or increase overall system operating cost, the
research on fault detection and diagnosis (area
3)
in HVAC systems has been becoming a large
research area and a hot research topic since the last decade. For a robust control strategy, it should
have the ability of fault-tolerant control, in which the system can be controlled properly even if
some faulty measurements
and/or system component degradations exist. There are numerous stud-
ies that specifically pertain to fault-tolerant control in control engineering. However, it is still in its
infancy in building HVAC field (area
4),
and the practical applications are rare.
Targeted at providing the satisfied indoor thermal comfort and healthy environment with the
least energy input or operating cost under dynamic indoor and outdoor conditions, many super-
visory and optimal control studies related to HVAC systems have been reported (Huang and
Lam 1997; Wang and Jin 2000; Wang and
Bumett 2001; Chow et al. 2002; Nassif et al. 2005;
Lu et al.
2005b; Sun and Reddy 2005; etc.). Chapter
41
of the 2003
ASHRAE Handbook-HVAC
Applications
(ASHRAE 2003) has provided a number of near-optimal strategies for cooling
tower fan control, chilled-water reset with fixed and variable-speed pumps, sequencing and
loading of multiple chillers, strategies for air-handling units, strategies for building zone temper-
ature setpoints, cooling thermal storage control, etc. The implementation issues of most of these
strategies are also presented. The readers are advised to go through this chapter for details, while
a number of strategies will also be addressed in this paper.
In this section, a comprehensive review of the research and development as well as applica-
tion of supervisory
and
optimal control in building
HVAC
field in last two decades is presented,
which intends to summarize most of such studies completed up to date. All studies were
reviewed according to the classification schematic of supervisory control methods presented in
the "Supervisory Control Methods" section.
Physical Model-Based Supervisory Control Strategies
A
few studies related to the supervisory control in building HVAC systems use dynamic
and/or static governing equations and detailed andlor simplified physical models to construct the
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