In this section a new formulation of the definition of virtual sensor is proposed, limited to the case of building applications. The need of making the subject clear is introduced in the literature review, section 2.4.1 (“The need for a new definition of virtual sensor”). The new definition intends to highlight key characteristics that every model should comply with in order to be named virtual sensor (or virtual meter). In process control the term virtual sensor is used for models which allow the measurement of process parameters “where direct measurement is too
expensive or even not possible” (Dementjev et al. 2010). Similarly, within the field of HVAC
monitoring, the additional cost due to purchasing a new physical sensor, and practical issues related to physical sensors installation, maintenance and accuracy, appear to be the two most recurrent reasons for which new virtual sensors are proposed:
“A virtual sensor uses low-cost measurements and mathematical models to estimate a difficult to measure or expensive quantity.” (Li et al. 2011)
“… measuring and monitoring SCFM [supply air flow] in rooftop air-conditioning units
(RTUs) by using the conventional SCFM metering devices are very costly and more than often problematic.” (Yu et al. 2011)
“Direct refrigerant pressure measuring practices are expensive and more than often problematic.” (Li and Braun 2009a)
“Virtual sensors are usually used when the targeted monitoring or control value is not directly or only expensively measureable (e.g. hostile environments), or only measureable with large delays (e.g. dead-time processes).” (Ploennigs et al. 2011)
“A model correcting a single-point mixed-air temperature measurement for inaccuracy caused by
thermal stratification and non-uniform velocity distributions […]” (Hjortland and Braun 2016)
“[…] the non-intrusive virtual flow meter introduced in this paper provides a solution to one of
the measurement barriers and challenges: a low cost, reliable energy metering system at the AHU level.” (Swamy et al. 2012)
123 Twenty-eight scientific papers have been reviewed (Table 8.1) that explicitly use the term
virtual sensors (or virtual meter) to refer to models used for indirectly measuring variables of
interest in HVAC. Twenty papers out of twenty-eight explicitly state that a virtual sensor allows to overcome practical issues, as sensor installation, maintenance, recalibration, and uncertainty. Common practical issues are, for instance, the air stratification in AHU ducts, which makes difficult to measure the air temperature with a single-point sensor (Lee and Dexter 2005 ; Wichman and Braun 2009), or the compact structure of HVAC installation which may not guarantee optimal locations for sensors installation (Mcdonald et al. 2014). Moreover, nineteen of the twenty-eight reviewed papers identified the additional cost of a new physical sensor as one of the main reasons to implement virtual sensors. Only four out of the twenty-eight reviewed publications do not mention the need to overcome the increase of costs and the rise of practical issues behind the use of virtual sensors. Those two identified key aspects (practical issues and additional costs) are here used to formulate a new definition of virtual sensor.
In HVAC monitoring, thus, virtual sensor applications are considered to be a valid alternative to physical sensors, which can be expensive and/or difficult to be implemented. It is essential here to remark that those physical sensors exist, and the reasons why a virtual sensor should be preferred to them, are strictly correlated to the features of the specific products currently present on the market: i) the cost, and ii) the installation and maintenance requirements. Thus, the new virtual sensor definition should include this aspect.
The proposed new definition of virtual sensor is the following:
A virtual sensor uses a mathematical model along with measurements from other installed sensors to derive the value of a variable of interest, which could be directly measured with a physical sensor available on the market if the cost and practical issues do not prevent the use of such a physical sensor.
124 Table 8.1 – List of reviewed papers which use the term virtual sensor (or virtual meter).
Reference Additional cost Practical issues description of the practical issue
Alhashme and Ashgriz 2016
Andiroglu et al. 2016 X lack of optimal location for sensors installation
Fernandez et al. 2009
Hjortland and Braun 2016 X air stratification
Kusiak et al. 2010 X sensor failure, improper installation, poor maintenance
Li and Braun 2007a X
Li and Braun 2009a X X improper practice
Li and Braun 2009b X X
Li et al. 2011 X X
McDonald and Zmeureanu 2014 X
McDonald et al. 2014 X X lack of optimal location for sensors installation
Mishukov and Horyna, 2015 X X
Padilla et al. 2015
Ploennings et al. 2011 X X
Reppa et al. 2014 X X invasive action required for sensor installation
125
Song et al. 2013 X uncertainty
Swamy et al. 2012 X X
Tahmasebi and Mahdavi 2013
Wang et al. 2014 X X
Wichman and Braun 2009 X Compact structure, air stratification
Yu et al. 2011a X
Yu et al. 2011b X X
Yu et al. 2011c X X
Yu et al. 2011d X X
Zhao et al. 2012 X
Zhao et al. 2012b X X maintenance and calibration
Zmeureanu and Vanderbrook 2015 X
126 According to the above proposed definition, a model which estimate the mixed air temperature using measurements of other variables would be said to be a virtual sensor, as physical sensors for the measurements of air temperature in AHU ducts are currently available on the market. The chiller COP formulation, as function of the thermal load at the evaporator and the electrical power input to the chiller (eq. 8.1), should not be named virtual sensor, as a physical sensor capable to directly measure the chiller COP does not exist.
COP = 𝑄𝐸𝑉
𝐸 eq. 8.1
where QEV is the thermal load at the chiller evaporator, and E is the chiller power input.
Zhao et al. (2012) proposed a method to diagnose fouling in chiller condensers using measurements of temperature and pressure of the refrigerant, and named it virtual condenser
fouling sensor. If the new definition of virtual sensor given above is accepted, a method which use
temperature and pressure measurements to diagnostic condenser fouling should not be named virtual sensor. On the other hands, because physical sensors for fluids temperature within pipes exist, the non-invasive model proposed by Gorman et al. (2013) to derive the fluid temperature using measurements of the pipe wall surface temperate, should be referred to as virtual fluid
127