Control techniques in Heating, Ventilating and Air Conditioning systems
Journal of Computer Science, Sept, 2008 by Hossein Mirinejad, Seyed Hossein Sadati, Maryam Ghasemian, Hamid Torab
INTRODUCTION
The primary goals of control strategies for the Heating, ventilating and Air Conditioning (HVAC) systems are to maintain occupants’ thermal comfort and energy efficiency (1). Thermal comfort is a vague and subjective concept and varies from one person to another. Research done by American Society of Heating, Refrigerating and Air-conditioning Engineers (ASHARE) during last years identified the most important parameters that influence thermal comfort as Temperature, Relative Humidity, Air Velocity and Radiant Temperature. However, activity level and clothing insulation of occupants are effective in thermal comfort, but they are variable and usually not measurable.
On one hand, HVAC systems are used for comfort purposes, hence categorized in Comfort System, On the other hand, it’s been well established that the consumption of energy by the HVAC equipment in commercial and industrial buildings constitutes 50% of the world energy consumption (2-5). Therefore, HVAC systems are also classified in Energy Management Systems (EMS).
The HVAC system is a typical nonlinear time-variable multivariate system with disturbances and uncertainties (6), so it is very difficult to find a mathematical model to accurately describe the process over a wide operating range.
MATERIALS AND METHODS
The design of controller for HVAC systems is a big challenge for practical engineers (6). Many control methods have been proposed in HVAC systems from traditional controllers to advanced and recently intelligent controllers during the last years (1-3), (5-8). The present research constitutes a thorough review of the variety of the control methodologies as used in the area of HVAC systems. Such a review will help pave the way for an HVAC engineer to better design the system.
Thermal comfort model: This model was represented by Fanger and includes two sets of parameters: environmental and personal (9). Environmental parameters are air temperature, radiant temperature, air velocity and relative humidity, whereas personal parameters are activity level and clothing insulation. First, these six factors must be measured or estimated and then the Predicted Mean Vote (PMV) index will be calculated. PMV was proposed by Fanger in 1970, used to predict the mean thermal sensation vote on a standard scale for a large group of people (8). This index is a real number and comfort conditions are achieved if the PMV belongs to the [-0.5 0.5] range (9). Figure 1 shows the Fanger model structure (10).
[FIGURE 1 OMITTED]
However, since the human sensation of thermal comfort is a subjective evaluation that changes according to personal preferences, the development of floor furnace an HVAC control system on the basis of the PMV model had proven to be impossible (11-13). In practice, in the majority of HVAC control systems instead of comfort level control through PMV index, two main climatic parameters temperature and relative humidity are controlled through Comfort Field in the ISO h,x diagram as the set point (9). A temperature set point of 22[degrees]C and a relative humidity set point of 45% with a deviation of [ or -] 2[degrees]C and [ or -] 15% RH are common for rooms and workplaces (14). The ISO h,x diagram is shown in Fig. 2.
[FIGURE 2 OMITTED]
HVAC systems characteristics: The most important specifications of HVAC systems -which are as inherent part of all thermal systems – are Time lags. There are several types of time lags in HVAC systems such as distance-velocity lag, exponential lag and capacity lag. The distance-velocity lag is the time between a signal being sent to an element and the element starting to respond, arising from the finite speed of propagation of the signal. An exponential lag occurs when the change with time in the output from an element or system (resulting from the application of a step change in the Input signal to that element or system), is of simple exponential Form. This lag may be defined as a capacity for storing energy which may be on the demand side of the process such as heated water in a tank, or on the supply side such as the hot water in the primary heating coils (15).
HVAC process has several nonlinear components like temperature and humidity which are nonlinear and extremely interrelated (16), (17)
Oct 22, 2009
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