Measurement and classification of energy efficiency in HVAC systems

Energy and Buildings 130 (2016) 408–419

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Measurement and classification of energy efficiency in HVAC systems Octávio Alves a , Eliseu Monteiro a,b,∗ , Paulo Brito a , Pedro Romano a a b

C3i—Interdisciplinary Center for Research and Innovation, Polytechnic Institute of Portalegre, Portalegre, Portugal INEGI/Faculty of Engineering, University of Porto, Porto, Portugal

a r t i c l e

i n f o

Article history: Received 12 April 2016 Received in revised form 23 August 2016 Accepted 27 August 2016 Available online 28 August 2016 Keywords: HVAC Energy efficiency indicators SCOP SEER Buildings

a b s t r a c t Climate control systems typically account for a substantial part of energy consumption in commercial buildings. The obligations established in global agreements as well as regulations and legislation that limit the energy consumption and greenhouse gases emissions gave a novel importance to the HVAC systems rating. This paper describes various indicators for assessing the energy performance of HVAC systems by defining the concepts involved and presenting calculation procedures. Various methods are used, from the basic indicators (EER, COP) to the most sophisticated (SEER, SCOP), which are due to the need to include more parameters to determine the level of energy efficiency accurately and to adjust it to the actual operation of the equipment. A case study is presented in order to calculate the indicators of actual systems in Europe and to assess developments by geographical location. A difference between the results of the various indicators when changing the geographical location and weather conditions that affect the systems was noticeable, which can lead to misconceptions regarding the energy efficiency. It is therefore important to correctly use the indicators that best fit the location, a task that is not usually performed by the manufacturers. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Heating, ventilation and air conditioning (HVAC) systems play an important role in assuring occupant comfort and are among the largest energy consumers in buildings [1]. Almost 50% of the energy demand in commercial buildings is used to support indoor thermal comfort conditions [2]. Furthermore, as most people spend more than 90% of their time inside [3], the development of energy-efficient HVAC systems that do not rely on fossil fuels will play a key role in reducing energy consumption and greenhouse gas emissions. It was estimated that world energy consumption was increased by 58% between 2001 and 2005 [4] and approximately 80% of the energy usage still comes from fossil fuels [5]. Therefore, performance improvements to traditional HVAC systems can be a remarkable opportunity for significant reductions in energy consumption and greenhouse gas emissions. Measuring energy efficiency requires the definition, assessment and analysis of a set of energy efficiency indicators [6]. The construction of these indices causes some methodological problems:

∗ Corresponding author at: Campus Politécnico, 10, 7300-555 Portalegre, Portugal. E-mail address: [email protected] (E. Monteiro). http://dx.doi.org/10.1016/j.enbuild.2016.08.070 0378-7788/© 2016 Elsevier B.V. All rights reserved.

value judgement, energy quality, boundary definition, energy partitioning and structural effects [7]. Individual air conditioners tend to be installed in an increasing number of buildings and constitute a fast growing electrical enduse in Europe [8]. The energy efficiency of individual heat pumps is characterized by the Coefficient Of Performance (COP), defined as the ratio between the heating power and electric power input. Similarly, in cooling mode, the Energy Efficiency Ratio (EER) is defined as the ratio between the cooling power and the electric power input. The COP and EER are dependent of the climatic conditions (e.g. the EER decreases when the indoor temperature decreases or when the outdoor temperature increases) and on the part load ratio (the ratio between the required cooling load and the maximum cooling load that can be extracted by the appliance). Air conditioner standard performance is evaluated at full capacity with an outside air temperature of 35 ◦ C in cooling mode, and of 7 ◦ C in heating mode [9,10]. The European Standard EN 14511 [11] uses the same testing conditions as the ISO5151 standard [9]. This testing is carried out in design cooling conditions (maximum cooling capacity and maximum summer outdoor temperature). However, over a cooling season, these conditions represents only a few hours of use. Most of the time, the heat pump is working at part load and lower outdoor air temperature. Hence, in order to rate the effi-

O. Alves et al. / Energy and Buildings 130 (2016) 408–419

409

Table 1 Operational hours of air conditioners per functional mode (hrs/year) [14]. Type of function

Season

Cooling mode, if appliances offers only cooling Cooling and heating modes, if appliances offers both modes

Cooling mode Heating mode

Heating mode, if appliances offers heating only

Average Warmer Colder

Average Warmer Colder

ciency of heat pumps, two seasonal performance indicators have been defined; they combine several test conditions (different cooling loads and different outdoor temperatures) and their respective hours of occurrence in a typical climate for a given load curve. The Seasonal Coefficient Of Performance (SCOP) in heating mode and the Seasonal Energy Efficiency Ratio (SEER) in cooling mode. They are the ratio between the seasonal cooling needs (or heating for SCOP) and the seasonal electricity consumption. A comprehensive calculation method, described in [12], allows the assessment of these efficiency metrics, which takes into account the effects of temperature and energy load variations on performance. In both cooling and heating modes, auxiliary power consumption (oil heater, thermostat off, standby and off mode) is also included. European seasonal efficiencies are used to evaluate the average energy efficiency of heat pumps. This paper aims to highlight and describe some existing indicators to determine and classify the level of energy efficiency of the HVAC systems, specifically air conditioners and heat pumps in the European regulation framework. Applicable calculation procedures and scales for qualitative evaluation will be identified. A case study is presented in which some of the indicators often used in Europe related to actual heat pumps will be determined, in order to establish a comparison between them and examine how they vary with the geographical location of the system. 2. Energy performance of HVAC systems Two main mechanisms were implemented in Europe in order to improve the energy efficiency of the HVAC market. The first mechanism consists of voluntary minimum energy efficiency requirements. By participating in the Eurovent (European Committee of Air Handling and Refrigeration Equipment Manufacturers) certification scheme, manufacturers are allowed to include their products in the annual Eurovent-Certification product directory. The second consists of energy labelling of household air conditioners that has been applied since 2002 through the Commission Directive 2002/31/EC [13]. This directive was repealed by the Delegated Regulation N◦ 626/2011 with regard to energy labelling of air conditioners applied from 26 July, 2011 [12]. This regulation establishes requirements for the labelling and the provision of supplementary product information for electric mains-operated air conditioners with a rated capacity of ≤12 kW for cooling, or heating, if the product has no cooling function. The 12 kW level is the

On mode

Thermostat-off mode

Standby mode

Off mode

Crankcase heater mode

350 350 1400 1400 2100 1400 1400 2100

221 221 179 755 131 179 755 131

2142 2142 0 0 0 0 0 0

5088 0 0 0 0 3672 4345 2189

7760 2672 179 755 131 3851 4476 2944

generally agreed limit between small (mainly domestic) and bigger (mainly commercial) air conditioning appliances. The Commission Regulation N◦ 206/2012 of 6 March 2012 [14] presents a new efficiency calculation method establishing minimum energy efficiency requirements higher than the level A. Consequently, split, window and wall air conditioners should have a new A-G energy efficiency class scale with a “+” added on the top of the scale every two years until the A+++ class has been reached. Some room air conditioners have a heating mode where the internal systems can be reversed for use in winter to produce warm air for the room and to expel cool air outside. Room air conditioners with this function must quote a second A-G rating for the heating mode, based upon the COP. This mechanism is comprehensively described in the following section. 2.1. European commission classification The European Commission communication regarding the implementation of Regulation No. 206/2012 (ecodesign of air conditioners and comfort fans) [14] and Commission Delegated Regulation No. 626/2011 (energy labelling of air conditioners) [12] are applicable to the following categories of air conditioning apparatus: • • • • •

Heat rating below 12 kW, either in heating or cooling mode; Air conditioning systems without air ducts; Single-duct air conditioning systems; Double-duct air conditioning systems; Air cooling fans with a power rating below 125 W.

For sake of simplicity only the most common air conditioners in Europe (air conditioners without air ducts) will be described. This type of system does not have any connection to the outside air, which is taken from and rejected in the same space that is to be conditioned. Common examples are mono-split and multi-split air conditioners [15]. Energy efficiency levels for these systems are determined independently for the cooling and heating seasons through the indicators SEER and SCOP, respectively. These indicators are determined according to various parameters, such as: • Operating time of the apparatus when the outside temperature assumes several values established for each season;

Table 2 Reference design conditions (temperature in ◦ C dry bulb/wet bulb) [14]. Function/season

Indoor air temperature (Tin )

Outdoor air temperature Tdesignc /Tdesignh

Bivalent Temperature Tbiv

Operating limit temperature Tol

Cooling Heating/Average Heating/Warmer Heating/Colder

27/19 20 ◦ C/ /max. 15

Tdesignc = 35/24 Tdesignh = −10/-11 Tdesignh = 2/1 Tdesignh = −22/-23

n.a. Max. 2 ◦ C Max. 7 ◦ C Max. −7 ◦ C

n.a. Max. −7 ◦ C Max. 2 ◦ C Max. −15 ◦ C

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tj (h)

250

Table 3 Testing temperatures used to calculate EERd (Tj ) and Pdc (Tj ) [14].

200

Variablea

Outdoor temperature (◦ C)

Indoor temperature (◦ C)

150

EER(35 ◦ C) and Pdc (35 ◦ C) EER(30 ◦ C) and Pdc (30 ◦ C) EER(25 ◦ C) and Pdc (25 ◦ C) EER(20 ◦ C) and Pdc (20 ◦ C)

35 30 25 20

27

100

a In the case of compressors that operate in stages, at each testing temperature Tj two values (maximum and minimum) must be reported for EERd (Tj ) and Pdc (Tj ) when the apparatus is set to maximum or minimum capacity, respectively. Assum¨ minimum and hi ¨ for ¨ maximum, the above quantities ing the use of the indices ¨lofor may now be denoted as EERd (Tj )hi , Pdc (Tj )hi , EERd (Tj )lo and Pdc (Tj )lo . In compressors with variable power, if it is possible to regulate Pdc (Tj ) to the value resulting from the P (Tj )×(Tj −16) (within a 10% tolerance range) at each outdoor temperexpression designc T −16

50 0 17

19

21

23

25

27

29

31

33

35

37

39

Tj (ºC)

designc

Fig. 1. Distribution of occurrence times of reference outdoor temperatures in the cooling season [14].

• Operating mode (active, thermostat-off, compressor heating (to prevent condensation of the refrigerant and its mixture with oil), stand-by and off); • Power variation regimen of the compressor (fixed, staged (discrete power consumptions) or variable (continuous change of power)); • Degradation factor of the efficiency of the equipment, which represents the performance depletion with the passage of time and amount of use; • The use of the electric heater for backup purposes, when the refrigerant circuit is insufficient to provide the thermal energy needed for the space. The SEER is calculated as: SEER =

Qc Ec

 

EERbin Tj is the bin-specific energy efficiency ratio that applies to bin j, calculated according to the flowcharts below for either fixed, staged or variable capacity units (Figs. 2–4). The meaning of variables used in the flowcharts is the following: EERd (Tj ) is the declared energy efficiency ratio at the specified outdoor temperature Tj , as declared by the manufacturer; Pc (Tj ) is the part load for cooling at bin temperature Tj , calculated as follows:

Qc = Pdesignc × tc

(2)

where: Pdesignc is the design load for cooling (kW), equal to the declared capacity for cooling at Tj = Tdesignc outdoor temperature; tc is the equivalent active mode hours for cooling, as provided in Table 1. Ec is the annual electricity consumption for cooling (kWh/year), calculated as:

24  j=1

tj ×Pdesignc ×(Tj −16)

(T

−16)×EER



(T )

designc bin j 24  tj ×Pdesignc ×(Tj −16) 

j=1

Tdesignc −16

+tTO × PTO + tCK × PCK + tOFF × POFF + tSB × PSB

(3)

where: Tj is the bin temperature assigned to bin with index j, from Fig. 1; tj is the number of hours assigned to bin with index j, from Fig. 1; Tdesignc is the cooling season reference design outdoor temperature, from Table 2; tTO , tCK , tOFF , tSB are the annual number of hours the unit is considered to be in respectively thermostat-off mode, crankcase heater operation mode, off-mode, and stand-by mode, the value of which depends on the designated season and function. PTO , PCK , POFF , PSB is the power consumption of the unit (kW) while in thermostat-off mode, crankcase heater operation mode, off-mode, and stand-by mode.



 

Pc Tj =



Pdesignc × Tj − 16

(4)

Tdesignc − 16

Pcmin is the minimum part load for cooling at the specified outdoor temperature; Cdc is the degradation factor for cooling, due to age and usage of the equipment, which is either the default value 0.25 or determined by tests and calculated for Tj = 20 ◦ C as:



(1)

Where Qc is the reference cooling demand (kWh/year), calculated as:

Ec = Qc ×

ature Tj referred in Table 3 only one value for Pdc (Tj ) and EERd (Tj ) should be obtained; otherwise, the procedure is the same as for staged capacity units.

Cdc =

 

1−

EERcyc

 

EERd Tj

/

1−



Pcycc

 

Pdc Tj

(5)

Where: EERcyc is the average energy efficiency ratio over the cycling test interval (active + off mode) calculated as the integrated cooling capacity over the interval (kWh) divided by the integrated electric power input over that same interval (kWh); Pcycc is the (time-weighted) average cooling capacity output (kW) over the cycling test interval (active + off mode); Pdc (Tj ) is the declared cooling capacity at the specified outdoor temperature Tj , as declared by the manufacturer; Pdc (Tj )lo , Pdc (Tj )hi is the declared cooling capacity at the specified outdoor temperature Tj , when the apparatus is set at minimum and maximum capacities, respectively; EERd (Tj )lo , EERd (Tj )hi are the EER values stated by the manufacturer at temperature Tj , when the apparatus is set to minimum and maximum capacities, respectively.   The determination of EERbin Tj is effected by using the default EER and cooling capacity values declared by the manufacturer EERd (Tj ) and Pdc (Tj ), respectively. These default values are obtained in tests carried out at pre-established temperatures, which are shown in Table 3. The SCOP is the seasonal coefficient of performance for heating. The calculation of the SCOP shall be specific for a designated heating season (colder, warmer and average), being mandatory its calculation for the average heating season. The inclusion of the other types of seasons on the datasheet of the equipment is optional. The SCOP is calculated as: SCOP =

Pdesignh × th Qh = Eh Eh

(6)

O. Alves et al. / Energy and Buildings 130 (2016) 408–419

Fig. 2. Calculation procedure of EERbin (Tj ) for fixed capacity units.

Fig. 3. Calculation procedure of EERbin (Tj ) for staged capacity units.

Fig. 4. Calculation procedure of EERbin (Tj ) for variable capacity units.

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Table 4 Part load test conditions [14]. Outdoor air temperature for designated season (◦ C)

Variable

Pdh (−7 ◦ C) Pdh (2 ◦ C) Pdh (7 ◦ C) Pdh (12 ◦ C) Pdh (Tbiv )

Average

Warmer

Colder

−7 2 7 12 n.a

n.a 2 7 12 n.a

−7 2 7 12 −15

Where: Qh is the reference annual heating demand (kWh/year), calculated as: Qh = Pdesignh × th

(7)

where: Pdesignh is the design load for heating (kW), which is calculated from the declared bivalent point Tbiv and the declared capacity Pdh (Tj ) at Tj = Tbiv ; th is the equivalent active mode hours for heating, as provided in Table 1. Eh is the seasonal electricity consumption for heating (kWh/year), calculated as: 46

Eh = Qh ×

j=1







    



tj × Pdesignh × Tj − 16 − Pb Tj × Tdesignh − 16



Indoor air temperature (◦ C)



Tdesignh − 16 × COPbin Tj

46





20

PTO , PCK , POFF , PSB is the electric power input (kW) in respectively thermostat-off, crankcase heater operation, off- and stand-by mode. The reference outdoor temperatures Tj for each designated heating season is shown in Fig. 5.   The calculation of COPbin Tj is performed according to the flowcharts illustrated in Figs. 6–8, corresponding to the situations in which the capacity of the compressor is fixed, staged2 or variable.3 Where: Ph (Tj ) is the part load in kW at the specified outdoor temperature Tj , calculated as:

 



+ Pb Tj



+ tTO × PTO +

tj × Pdesignh × Tj − 16

(8)

Tdesignh − 16

j=1

+tCK × PCK + tOFF × POFF + tSB × PSB where: tj is the number of hours assigned to bin temperature with index j, from Fig. 5. Tj is the bin temperature assigned to bin with index j, from Fig. 5;

 

Pb Tj is the capacity of a back up heater (kW) for bin j, needed to meet the heating part load if the declared capacity does not suffice, calculated as:

    ⎧ P × Tj − 16     Pdesignh × Tj − 16   ⎨ designh − Pdh Tj , ifPdh Tj < Tdesignh − 16 Tdesignh − 16 Pb Tj = ⎩ 0, else

(9) Where Pdh (Tj ) (in kW) is the declared heating capacity applicable to bin j, to be calculated using the declared values of Pdh (Tj ) at testing points defined in Table 4 and/or Tbiv ,1 the availability of which depends on which heating season is designated. Pdh (Tj ) for other bins than specified shall be calculated through linear interpolation of declared capacities Pdh (Tj ) at the nearest outdoor temperatures. Tdesignh is the heating season reference design temperature, from Table 2, which   is determined by the designated heating season; COPbin Tj is the bin-specific coefficient of performance that applies to bin j; tTO , tCK , tOFF , tSB are the number of seasonal operating hours (hrs/year) for heating in respectively thermostat-off, crankcase heater operation, off-mode and stand-by mode;



 

Ph Tj =



Pdesignh × Tj − 16

(10)

Tdesignh − 16

Ph min is the minimum part load at the specified outdoor temperature; COPd (Tj ) is the coefficient of performance at the specified outdoor temperature Tj , as declared by the manufacturer; Cdh is the degradation factor for heating either taken as default value 0.25 or equal to Cdc (for cooling) or determined by tests and calculated for Tj = 12 ◦ C as: Cdh =





1−

1−

COPcyc COPd (Tj )

Pcych





(11)

Pdh (Tj )

Where: COPcyc is the average coefficient of performance over the cycling test interval (active + off mode) calculated as the integrated heating capacity over the interval [kWh] divided by the integrated electric power input over that same interval [kWh]; Pcych is the (time-weighted) average heating capacity output [kW] over the cycling test interval (active + off mode);

2 For devices with staged power compressors, two values for Pdh (Tj ) and COPd (Tj ) must be declared at each testing outside temperature Tj , as defined in Table 4. One of ¨ index) and the values is obtained when the apparatus is set to maximum capacity (hi” the other at minimum capacity (“lo” index), as a result of which they are designated respectively by Pdh (Tj )hi , COPd (Tj )hi , Pdh (Tj )lo and COPd (Tj )lo . P (Tj )×(Tj −16) 3 If it is possible to regulate Pdh (Tj ) to the result of the expression designh T −16 designh

1

Bivalent temperature – means the outdoor temperature declared by the manufacturer for heating at which the declared capacity equals the part load and below which the declared capacity must be supplemented with electric back up heater capacity in order to meet the part load for heating.

(with a tolerance of 10%) at each outside temperature Tj shown in Table 4, only one value for Pdh (Tj ) and COPd (Tj ) should be calculated; otherwise, two values (maximum and minimum) should be obtained for both quantities, in the same way as happened with staged power compressors.

O. Alves et al. / Energy and Buildings 130 (2016) 408–419

Fig. 5. Occurrence times of the reference outdoor temperatures for the designated heating season.

Fig. 6. Calculation procedure of COPbin (Tj ) for fixed capacity units.

Fig. 7. Calculation procedure of COPbin (Tj ) for staged capacity units.

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Fig. 8. Calculation procedure of COPbin (Tj ) for variable capacity units.

Table 5 Energy efficiency classes for air conditioners [12].

3. Comparative case study of energy efficiency indicators used in Europe

Class

SEER

SCOP

A+++ A++ A+ A B C D E F G

SEER ≥ 8.5 6.1 ≤ SEER< 8.5 5.6 ≤ SEER< 6.1 5.1 ≤ SEER< 5.6 4.6 ≤ SEER< 5.1 4.1 ≤ SEER< 4.6 3.6 ≤ SEER< 4.1 3.1 ≤ SEER< 3.6 2.6 ≤ SEER< 3.1 SEER < 2.6

SCOP ≥ 5.1 4.6 ≤ SCOP< 5.1 4.0 ≤ SCOP< 4.6 3.4 ≤ SCOP< 4.0 3.1 ≤ SCOP< 3.4 2.8 ≤ SCOP< 3.1 2.5 ≤ SCOP< 2.8 2.2 ≤ SCOP< 2.5 1.9 ≤ SCOP< 2.2 SCOP < 1.9

Pdesignh is the design load for heating [kW] which is calculated from the declared bivalent point Tbiv and the declared capacity Pdh (Tj ) at Tj = Tbiv . Pdh (Tj )lo , Pdh (Tj )hi − thermal power for heating (kW) at temperature Tj , relating to the regulation of the apparatus at minimum and maximum capacity; Tdesignh is the heating season reference design temperature, which is determined by the designated heating season; COPd (Tj )lo and COPd (Tj )hi are declared coefficient of performance values at temperature Tj , with the apparatus set to minimum and maximum capacity, respectively.

2.2. Definition of energy class of air conditioners The information regarding the level of energy efficiency is included on standard labels affixed on the apparatus, indicating the values for SEER, SCOP, EERd and COPd as well as the corresponding energy class according to the criteria defined in Table 5.

3.1. Description of the case study and the methodology applied The comparative case study is based on the determination of various energy efficiency indicators (EER, SEER, COP, SCOPaverage and SCOPwarmer ) for three inverter-type heat pump models.4 These devices are identified in Table 6. The energy efficiency indicators were calculated using spreadsheets considering the methodologies explained above and the information contained in the datasheets of the systems [16]. In the specific case of EER and COP, thermal and electrical powers at temperatures Tdesignc and Tdesignh established for the calculation of the SEER and SCOPaverage were taken into account. The energy classes were also evaluated. The value of SCOP in the optional colder season was not calculated because none of the devices operate at the reference outdoor temperature of −22 ◦ C [14]. In a next phase EER, COP, SEER and SCOP were recalculated taking into account the climatic conditions of two cities, one in southern Europe (Portalegre, Portugal) and the other in northern Europe (Brize Norton, England). For the EER and COP it was considered the average temperatures of cooling and heating seasons observed in both cities (25 ◦ C and 11 ◦ C for Portalegre, and 16 ◦ C and 9 ◦ C for Brize Norton, respectively). To calculate SEER and SCOP, reference outdoor temperatures Tj and corresponding periods of occurrence tj were gathered from climatological databases relative to both cities [17,18]. The cooling season occurs between June and September

4 Inverter heat pump: a HVAC system that incorporates a control circuit that adjusts the thermal load required for the space through precise and continuous variation of compressor speed, depending on the instantaneous indoor and outdoor temperatures.

Table 6 Models of heat pumps and characteristics used in the study [16]. Code

Mark and model

Indoor unit model

Outdoor unit model

A B C

LG H09AK LG H12AK LG P24EL

ASUW096MU3 ASNW126MMS3 P24EL NS2

ASNW096MMS3 ASUW126MUF3 P24EL UL2

a

Rated thermal powera [W] Cooling

Heating

2500 3500 6800

3200 4000 8000

Determined under the following conditions: indoor temperature of 27 ◦ C (cooling) and 20 ◦ C (heating), and outdoor temperature of 35 ◦ C (cooling) and −10 ◦ C (heating).

O. Alves et al. / Energy and Buildings 130 (2016) 408–419

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Table 7 Data about each heat pump for the cooling season [16]. System model

A

B

C

Nominal thermal power, Pdesignc [kW] Nominal electric power, Pec [kW] Electric power at indoor/outdoor temperatures of 27 ◦ C/25 ◦ C, Pec (25 ◦ C) [kW]a Electric power at indoor/outdoor temperatures of 27 ◦ C/16 ◦ C, Pec (16 ◦ C) [kW]a Minimum thermal power, Pcmín [kW] Degradation factor, Cdc Thermostat-off mode power consumption, PTO [kW] Crankcase heater operation mode power consumption, PCK [kW] Off-mode power consumption, POFF [kW] Standby mode power consumption, PSB [kW] EER at indoor/outdoor temperatures of 27 ◦ C/20 ◦ C, EER(20 ◦ C) EER at indoor/outdoor temperatures of 27 ◦ C/25 ◦ C, EER(25 ◦ C) EER at indoor/outdoor temperatures of 27 ◦ C/30 ◦ C, EER(30 ◦ C) EER at indoor/outdoor temperatures of 27 ◦ C/35 ◦ C, EER(35 ◦ C) Thermal power at indoor/outdoor temperatures of 27 ◦ C/20 ◦ C, Pdc (20 ◦ C) [kW] Thermal power at indoor/outdoor temperatures of 27 ◦ C/16 ◦ C, Pdc (16 ◦ C) [kW]a Thermal power at indoor/outdoor temperatures of 27 ◦ C/25 ◦ C, Pdc (25 ◦ C) [kW] Thermal power at indoor/outdoor temperatures of 27 ◦ C/25 ◦ C, Pdc (25 ◦ C) [kW]a Thermal power at indoor/outdoor temperatures of 27 ◦ C/30 ◦ C, Pdc (30 ◦ C) [kW] Thermal power at indoor/outdoor temperature of 27 ◦ C/35 ◦ C, Pdc (35 ◦ C) [kW]

2.500 0.463 0.4 0.4 0.300 0.25 0.013 0.000 0.0018 0.0018 14.9 11.2 7.1 5.2 1.0 2.9 1.2 2.7 1.8 2.5

3.500 0.780 0.6 0.6 0.300 0.25 0.013 0.000 0.002 0.002 15.5 10.5 6.8 4.4 1.1 4.0 1.5 3.8 2.6 3.5

6.8 2.193 1.8 1.8 0.300 0.25 0.035 0.000 0.008 0.008 12.0 7.3 4.6 3.0 1.4 7.7 3.2 7.4 5.0 6.8

a

Assuming a length for the fluid refrigerant piping of 7.5 m and that the indoor and outdoor units of the system are at the same height.

Table 8 Data about each heat pump for the heating season [16]. System model

A

B

C

Nominal thermal power—average season, Pdesignhaverage [kW] Nominal thermal power—warmer season, Pdesignhwarmer [kW] Nominal electric power—average season, Pehaverage [kW] Nominal electric power—warmer season, Pehwarmer [kW] Electric power at indoor/outdoor temperatures of 20 ◦ C/9 ◦ C, Pec (9 ◦ C) [kW]a Electric power at indoor/outdoor temperatures of 20 ◦ C/11 ◦ C, Pec (11 ◦ C) [kW]a Minimum thermal power, Pmính [kW] Degradation factor, Cdh Bivalent temperature, Tbiv [◦ C] Minimum temperature of operation, Tol [◦ C] Thermostat-off mode power consumption, PTO [kW] Crankcase heater operation mode power consumption, PCK [kW] Off-mode power consumption, POFF [kW] Standby mode power consumption, PSB [kW] COP at indoor/outdoor temperatures of 20 ◦ C/−7 ◦ C, COP(−7 ◦ C) COP at indoor/outdoor temperatures of 20 ◦ C/2 ◦ C, COP(2 ◦ C) COP at indoor/outdoor temperatures of 20 ◦ C/7 ◦ C, COP(7 ◦ C) COP at indoor/outdoor temperatures of 20 ◦ C/12 ◦ C, COP(12 ◦ C) COP at indoor/outdoor temperatures of 20 ◦ C/Tbiv , COP(Tbiv ) COP at indoor/outdoor temperatures of 20 ◦ C/Tol , COP(Tol ) Thermal power at indoor/outdoor temperatures of 20 ◦ C/−7 ◦ C, Pdh (−7 ◦ C) [kW] Thermal power at indoor/outdoor temperatures of 20 ◦ C/2 ◦ C, Pdh (2 ◦ C) [kW] Thermal power at indoor/outdoor temperatures of 20 ◦ C/7 ◦ C, Pdh (7 ◦ C) [kW] Thermal power at indoor/outdoor temperatures of 20 ◦ C/7 ◦ C, Pdh (9 ◦ C) [kW] Thermal power at indoor/outdoor temperatures of 20 ◦ C/11 ◦ C, Pdc (11 ◦ C) [kW]a Thermal power at indoor/outdoor temperatures of 20 ◦ C/12 ◦ C, Pdc (12 ◦ C) [kW] Thermal power at indoor/outdoor temperatures of 20 ◦ C/Tbiv , Pdh (Tbiv ) [kW] Thermal power at indoor/outdoor temperatures of 20 ◦ C/Tol , Pdh (Tol ) [kW]

3.200 3.01 0.570 0.673 0.7 0.7 0.300 0.25 −8 −10 0.013 0.000 0.0018 0.0018 3.4 5.0 6.5 7.8 3.0 3.3 3.1 1.7 1.2 3.3 3.4 0.9 3.4 3.3

4.000 3.8 0.755 0.727 0.8 0.8 0.300 0.25 −8 −10 0.013 0.000 0.002 0.002 3.1 4.9 6.6 7.3 3.3 3.1 3.5 2.0 1.4 4.2 4.3 0.9 3.7 3.9

8.0 7.5 2.285 2.203 2.4 2.4 0.300 0.25 −7 −10 0.035 0.000 0.008 0.008 2.6 3.8 4.5 5.5 2.6 2.4 5.5 3.3 2.1 8.3 8.6 0.9 5.5 6.2

a

Assuming a length for the refrigerant piping of 7.5 m and that the indoor and outdoor units of the system are at the same height.

[17,18], and the heating season between October 31st–April 23rd (for Portalegre) [17] and from October 4th–May 30th (for Brize Norton) [18].

3.2. Input data to calculate energy efficiency indicators Tables 7 and 8 indicate the parameters required to determine the energy efficiency indicators for each heat pump in the cooling and heating seasons, collected from the LG datasheet [16]. Table 9 shows the outside temperature values (Tj ) and periods of occurrence (tj ) in cooling and heating seasons, registered in the two cities concerned.

4. Results and discussion Table 10 and Fig. 9 show the results of the calculated energy efficiency indicators throughout Europe. Fig. 9 shows a generalized decline in all indicators as the thermal power of systems increases, due to a shrinking ratio among the thermal power (Pdesignc , Pdesignhaverage and Pdesignhwarmer ) and electric power (Pec , Pehaverage e Pehwarmer ), as well as to an increase in electric power in the remaining operating modes (thermostat-off, standby and inactive). The decline observed eroded the energy classes of all systems, which fell from A+++ to A++ in the case of SEER and from A+++ to A in the case of SCOPaverage (Table 10). This means that energy efficiency decreases with the increase in thermal power.

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O. Alves et al. / Energy and Buildings 130 (2016) 408–419

Table 9 Reference outdoor temperatures Tj and periods of occurrence tj observed in Portalegre and Brize Norton [17,18]. Temperature Tj [◦ C]

Annual time of occurrence, tj [h] Portalegre

−4 −3 −2 −1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39

Brize Norton

Cooling season

Heating season

Cooling season

Heating season

0 0 0 0 0 0 0 0 0 0 0 0 0 0 4 8 7 21 53 89 83 133 145 144 151 179 165 166 156 149 150 127 141 139 149 121 130 101 90 62 32 19 10 4

0 0 0 0 2 9 27 50 116 204 306 337 340 378 340 358 326 311 276 207 160 151 128 92 64 14 4 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0 0 3 14 17 19 34 72 128 177 219 235 302 322 280 242 201 207 152 98 104 37 25 19 19 2 0 0 0 0 0 0 0 0 0 0 0

6 15 38 54 78 89 176 260 267 355 426 600 534 511 456 473 414 295 234 159 99 80 59 16 11 14 8 2 6 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

It is not possible to establish a consistent mathematical relationship between the simplest indicators (EER and COP) and the most complex (SEER, SCOPaverage and SCOPwarmer ), as shown in Fig. 9b), where COP is higher than SCOPaverage in the systems A and B but lower in the system C. The simplest indicators have smaller values than of the most complex (except SCOPaverage ), where differences are well-marked between EER and SEER (average of 86.9%) and between COP and SCOPwarmer (average of 28.0%). The difference between SCOPaverage and SCOPwarmer varies from 29.0% in the system C to 35.3% in the system A, reflecting the great variability between indicators depending on the type of heating season. Given

that SCOPwarmer is always higher than SCOPaverage (Fig. 9b), conclusion could be drawn that the efficiency of equipment is higher in warmer climates (southern Europe). This is due to the lower difference between indoor and outdoor temperatures, which in turns reduces the power consumption of the compressor. All these observations, combined with the rise of the efficiency class of the system C (from A to A++, Table 10), suggest the importance of selecting the most appropriate type of SCOP (SCOPcolder , SCOPaverage or SCOPwarmer ), in line with the geographic location of deployment of the system. This helps to define a more realistic level of efficiency.

Table 10 Results of indicators throughout Europe. Season

Indicator

System A

System B

System C

Value

Class

Value

Class

Value

Class

Cooling

EER SEER

5.4 9.1

– A+++

4.5 8.8

– A+++

3.1 6.1

– A++

Heating

COP SCOPaverage SCOPwarmer

5.6 5.1 6.9

– A+++ A+++

5.3 5.0 6.7

– A++ A+++

3.5 3.8 4.9

– A A++

O. Alves et al. / Energy and Buildings 130 (2016) 408–419

10

10 9.1

8.8

6

6.1

5.4 4.5

4

3.1 2

EER

8

Indicator value

8 Indicator value

417

6

6.9

6.7

5.6 5.1

5.3 5.0

4 2

SEER

COP

SCOPaverage

4.9 3.8 3.5 SCOPwarmer

0

0 A

B System

A

C

B System

C

(a)

400

tj [h]

tj [h]

Fig. 9. Comparison between the indicators calculated for Europe in the (a) cooling and (b) heating seasons.

300

(b)

600

400

200 200 100 0

0 17

22

27

32

-10

37

Portalegre

0

5

10

15

Temperature (ºC) EU - average EU - warmer SCOP average SCOP warmer Portalegre Brize Norton

Temperature (ºC) SEER EU

-5

Brize Norton

Fig. 10. Temperature occurrence profiles used in the calculation of the indicators for: (a) cooling season; (b) heating season [14,17,18].

Table 11 Results of EER, COP, SEER and SCOP in Portalegre and Brize Norton. Location

Indicator

System A Value

Class

Value

Class

Value

Class

Portalegre

EER COP SEER SCOP

7.2 4.7 7.4 6.6

– – A++ A+++

6.0 5.5 7.0 6.5

– – A++ A+++

4.1 3.6 4.8 4.6

– – B A++

Brize Norton

EER COP SEER SCOP

7.5 4.7 12.1 6.1

– – A+++ A+++

6.2 5.4 12.4 6.0

– – A+++ A+++

4.3 3.6 9.1 4.4

– – A+++ A+

Fig. 10 shows the occurrence profiles of temperatures (tj ) in entire EU (as an average), Portalegre and Brize Norton, that were used to calculate SEER, SCOPaverage and SCOPwarmer . In the cooling season (Fig. 10a), the temperature profile of Portalegre is more close to the EU’s temperature profile than that of Brize Norton. Such temperature profiles allow predicting that the difference between SEER calculated for EU and Portalegre should be less than that between the EU and Brize Norton. In the heating season (Fig. 10b), the temperature profile of Portalegre is more close to the SCOPwarmer (absolute mean error: 51 ◦ C) than to the SCOPaverage (absolute mean error: 116 ◦ C), leading to the conclusion that the former is the most appropriate indicator for the city. In the case of Brize Norton, it is expected that either of the two indicators will be appropriate because the absolute mean errors (111 ◦ C and 117 ◦ C) are similar. Table 11 shows the results for EER, COP, SEER and SCOP in Portalegre and Brize Norton, as well as the corresponding efficiency classes for each system. Fig. 11 summarizes the same results for EU and the two cities. Fig. 11a shows that there is a significant deviation between the EER in EU and those determined for the two cities (average difference of 35.7%), showing that the indicator did not accurately represent the efficiency of the apparatus in each city. The conditions

System B

System C

for measuring thermal and electrical power (according to European standards EN 14825) or those used by the manufacturer (see footnote of Table 8) may constitute a justification for the discrepancies found. EER is slightly higher in Brize Norton than in Portalegre due to the smaller difference of the indoor and outdoor temperatures. It is apparent that by decreasing the difference in temperatures in the case of Brize Norton, the compressor power consumption is lower, leading to an improvement in the efficiency of the apparatus. Fig. 11b shows that the COP obtained for EU is very near the obtained for the two cities considered, particularly in systems with higher power (B and C, mean difference of 7.3%). The similarity between the temperature profiles of both cities in the heating season can explain the analogous COP values. Fig. 11c shows that the SEER calculated for EU remains between the results obtained for both cities, demonstrating its average nature and an appropriate indicator to measure efficiency in the cooling season. Portalegre achieves the lowest values with deviations between 18.7% and 21.3% in relation to EU, due to the prevalence of higher outdoor temperatures (see Fig. 10a). This variation of the outdoor and indoor temperatures gives rise to an abrupt drop in efficiency class from A++ to B (Table 11).

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O. Alves et al. / Energy and Buildings 130 (2016) 408–419

a) 7.2 7.5

8 6

6.0 6.2

5.4

4.1 4.3

5.6

5.3 5.5 5.4

4.7 4.7

3.5 3.6 3.6

4

3.1

2

2

0 A

B System

UE

Portalegre

0

C

A

Brize Norton

UE

12.4

12.1

12 9.1

7.4

8.8

9.1

7.0

6.1

6

4.8

3

SCOP

c)

15 SEER

8 6

4.5

4

9

b)

10 COP

EER

10

B C System Portalegre Brize Norton

d)

15 12 9 6

6.9 6.6 6.1 5.1

6.7 6.5 6.0 5.0

3.8

4.9

4.6

4.4

3

0 A

B

C

System UE

Portalegre

Brize Norton

0 A UE (average) Portalegre

B System

C UE (warmer) Brize Norton

Fig. 11. Comparison of the indicators EER, COP, SEER and SCOP for EU, Portalegre and Brize Norton.

Conversely, the SEER in Brize Norton is significantly higher than in the EU (an increase between 33.0% and 49.2%), showing that systems are more efficient when installed at that city, as can be seen by the maximum energy class achieved for all systems (Table 11). The occurrence of cooler outdoor temperatures reduces the compressor power consumption required to maintain the same indoor temperature. Fig. 11d shows that the SCOPwarmer is more appropriate to the weather conditions in Portalegre than SCOPaverage , due to the approximation to the SCOP measured in the city (difference of 4.5% for the first and 26.8% for the second). SCOPwarmer maintain the energy class obtained by the SCOP calculated for Portalegre, while SCOPaverage dropped the class of system C in two levels from A++ to A (Table 11). It was also found that SCOPwarmer is slightly higher than the SCOP of Portalegre due the higher temperatures considered (see Fig. 10b), which approach the indoor temperature of 20 ◦ C and improves the system performance. The SCOP of Brize Norton stood at an intermediate state between SCOPaverage and SCOPwarmer . Therefore, whichever of the indicators may be used to describe the efficiency of the systems in the city (Fig. 11d). Nevertheless, SCOPwarmer has a small advantage over SCOPaverage due to the deviation to the SCOP of the city (10.7% versus 18.5%) and because of the improvement in the energy class of the system C. In summary, COP and SEER are the energy efficiency indicators that best suit, respectively, the heating and cooling seasons, for the cities under study.

5. Conclusions This paper presented some methods for determining the level of energy efficiency of HVAC systems, which are responsible for a significant proportion of the energy consumption in service build-

ings. The methods vary from one another in many ways, such as the geographical area of application, the weightings allocated to the operating conditions (partial load and season) and the technology used (compressor power control). There has been an evolution in the complexity of the methods used, from the basic indicators (EER and COP) to the most sophisticated (SEER and SCOP), in order to accurately determine the level of energy efficiency adequate to the real operation conditions of the equipment. Through the case study, a difference was noticeable between the results of the various indicators when changing the geographical location and weather conditions, which can lead to misconceptions regarding the energy efficiency levels, especially in the heating season. It is, therefore important, to correctly use the indicators that best fit the location, a task that is not usually performed by the manufacturers. The application of the methods described enables a proper assessment of the performance of the systems in terms of energy consumption, which encourages manufacturers and users to design or acquire more efficient equipment with lower levels of consumption. This will contribute to the reduction of the energy consumption, which allows decreasing the use of fossil fuels and consequently the greenhouse gas emissions.

Acknowledgements This paper was prepared under the project “MITTIC – Modernizac¸ão e Inovac¸ão Tecnológica com base TIC em setores estratégicos e tradicionais”, sub-project “Aplicac¸ão de TIC a poupanc¸a e eficiência energética de edifícios de servic¸os e desenvolvimento do setor”, that was co-financed by the Fundo Europeu de Desenvolvimento Regional (FEDER) through the programme Programa Operacional de Cooperac¸ão Transfronteiric¸a (POCTEP) 2007–2013.

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