A Developed Method for Energy Saving Prediction of Heat-and Energy Recovery Units

Available online at www.sciencedirect.com

ScienceDirect Energy Procedia 85 (2016) 311 – 319

Sustainable Solutions for Energy and Environment, EENVIRO - YRC 2015, 18-20 November 2015, Bucharest, Romania

A developed method for energy saving prediction of heat-and energy recovery units Miklos Kassaia* a

Budapest University of Technology and Economics, Faculty of Mechanical Engineering, Department of Building Service Engineering and Process Engineering, Budapest, H-1111, Hungary

Abstract In heat and energy exchangers frosting decreases the effectiveness of the exchangers. In a previous research effectiveness data were investigated experimentally considering the frosting effect [1]. Using these tested effectiveness data and ambient temperature/enthalpy duration curves annual heating energy consumption of ventilation systems were predicted in three different climate European locations. During this investigation two common frosting protection techniques were also considered: outdoor air preheating and outdoor airflow bypassing (supply side closing). The evaluation shows the outdoor air preheating technique results higher energy saved than the supply side closing. Furthermore if the indoor air quality demands are lower using a sensible heat recovery unit is satisfactory and the investment cost is lower. ©2015 TheAuthors. Authors. Published by Elsevier Ltd. is an open access article under the CC BY-NC-ND license © 2016 The Published by Elsevier Ltd. This Peer-review under responsibility of the organizing committee EENVIRO 2015. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee EENVIRO 2015 Keywords: Ventilation; defrosting; energy consumption; ambient temperature-enthalpy curves

1. Introduction Energy savings and energy efficiency are important parameters in the energy policy in EU countries [2]. Reduction of energy consumption of ventilation systems has been studied intensively [3-5]. Without energy recovery energy consumption of the ventilation system and also the size of heating and cooling facilities is higher to provide the required indoor thermal comfort conditions [6-8]. Heat and energy recoveries have been used for many years in ventilation systems and are now becoming increasingly common in cold climates regions [9-10]. Proper

* Corresponding author. Tel.: +36/1 463-2024; fax: +36-1-463-17-08 E-mail address: [email protected]

1876-6102 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee EENVIRO 2015 doi:10.1016/j.egypro.2015.12.257

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dimensioning of ventilation systems can reduce the energy demand of fans and result in lower running costs. This is a complicated procedure particularly when the unit recovers both heat and moisture exchange especially during frosting periods [11-13]. Based on the research conducted by Mohammad [1] in the Thermal Science Laboratory at the University of Saskatchewan, the decreasing effect of defrosting cycle on sensible and total effectivenesses of heat and energy recoveries were investigated by experimental tests during frosting time. In this research using these tested effectiveness data and ambient temperature/enthalpy duration curves annual heating energy consumption of ventilation systems were predicted in three different climate European locations. Nomenclature cP ER HR OTR Q PH RH T t W

specific heat capacity (kJ/ kg K) energy recovery h enthalpy (kJ/ kg) heat recovery operation time ratio m mass flow rate (kg/s) heat or energy consumption (kJ) pre-heater re-heater (-); relative humidity (%) temperature (oC) operation time (hr) humidity ratio (kg/kg)

Greek letters ε effectiveness Subscripts air air flow aux auxiliary avg average bp bypass DF defrosting ex energy exchanger F frosting Frost frosting limit hx heat exchanger I indoor if ignoring frosting o outdoor PH pre-heater ph pre-heating Ref reference RH re-heater rec recovered Sup supply s sensible t total total total

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2. Heat recovery and energy recovery calculations During our research three European cities were selected based on their different climate locations: Helsinki as cold climate region, Krakow as North Temperate climate and Palermo as Mediterranean climate location. The meteorological data were generated by TRNSYS 17 energy simulation software in hourly period. 2.1. Pre-heating technique A common frost-protection technique for heat/energy exchangers is to preheat the outdoor supply air above the frosting limit before the air enters to the exchangers’ cores. When the outdoor air temperature goes below the frosting limit, the heating elements are activated. Preheating the cold outdoor air consumes auxiliary energy (i.e. electricity or gas), but the exchangers’ operation is not interrupted by the frost-prevention system. The actual effectiveness of heat/energy exchangers is essentially equal to the nominal effectiveness without frosting. During this research it was assumed that the heat recovery and also the energy recovery are controlled based on temperature. Based on performed tests the frosting starts to appear when the ambient temperature is lower than the frosting temperature limit (TFrost). 2.1.1. Sensible energy recovery The air temperature calculated by Eq. 1 after the heat recovery depends on the temperature on that the frost occurs ( TFrost ) in the heat recovery, the exhaust air temperature ( TEI = 22°C) and also on the measured effectiveness ( H s ):

TFrost  H s ˜ (TEI  TFrost ) [qC ]

TO [°C]

TSO _ PH

(1)

ARH_s [ ͦC h]

330 TEI = 22 [°C] = constant 220

TSR =20 [°C] = constant

Hs

10

Hs

TSO 0 0

-10

2000

4000

Tbefore RH = 13.26 [[°C] C] = con C] constant onnnsstaannntt TFrost = -1 [°C] 6000 00

Arec_s [ ͦC h]

88000 80 000 00 Time [h]

-20

-30

APH_s [ ͦC h]

Fig. 1. The areas on the ambient temperature duration curve that proportional with the pre-heater, re-heater energy consumption and heat recovery energy saved considering also frosting time.

The energy saved by the heat recovery (Eq. 2) and energy consumption of the pre-heater (Eq. 3) and re-heater (Eq. 4) can be also calculated.

Qrec _ s

cPair ˜ m air ˜ ( Arec _ s ) cPair ˜ U air ˜Vair ˜ ( Arec _ s ) [kJ ]

(2)

QPH _ s

cPair ˜ m air ˜ ( APH _ s ) cPair ˜ U air ˜Vair ˜ ( APH _ s ) [kJ ]

(3)

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cPair ˜ m air ˜ ( ARH _ s ) cPair ˜ U air ˜Vair ˜ ( ARH _ s ) [kJ ]

QRH _ s

(4)

2.1.2. Total energy recovery Using pre-heater, the air handling process is similar than with heat recovery. The ambient air is pre-heated to the frosting enthalpy ( hFrost ) (Eq. 5) that is calculated from the air enthalpy equation with constant air frosting temperature limit ( TFrost ). After the pre-heater, the energy recovery increases the enthalpy more ( hBefore _ RH ) (Eq. 6) and finally the re-heater heats up the air to the required supply room enthalpy value ( hSR ) (Fig. 2).

hFrost

hAfter PH

hBefore _ RH

1.006 ˜ TFrost  Wo ˜ (2501  1.86 ˜ TFrost )

(5)

hFrost  H t (hEI  hFrost )

(6)

ho [kJ/kg]

Enthalpy - Temperature correlation

hEI (22 ͦC, 40% RH) hS (20 ͦC, 35% RH) hbefore_RH

70 60

'hRH _ t

50

40 30 20 10

hFrost = h After_PH -30

-20

0 -10

-10

0

-20

'hrec _ t

'hPH _ t

10

ho hS h20 EI 30 hBefore_RH T [°C] o hFrost

-30 Fig. 2. The enthalpy differences on the ambient temperature-enthalpy diagram that proportional with the annual pre-heater, re-heater energy consumption and total energy saved by energy recovery.

The areas that represent the energy saved and consumptions can be calculated by Eqs. 7-9. 6'hPH _ t ˜W

8760

APH _ t

6'hRH _ t ˜W

8760

ARH _ t Arec _ t

6'hrec _ t ˜W

61 (hFrost  hO _ sel ) 61 (hSR  hBefore _ RH )

[kJ h / kg ] [kJ h / kg ]

(7) (8)

8760

61 (hBefore _ RH  hFrost )

[kJ h / kg ]

(9)

The equations for pre-heater, re-heater energy consumption and energy saved by energy recovery are the followings (Eqs. 10-12).

QPH _ t

m air ˜ ( APH _ t ) U air ˜Vair ˜ ( APH _ t ) [kJ ] (10)

QRH _ t

m air ˜ ( ARH _ t ) U air ˜Vair ˜ ( ARH _ t ) [kJ ] (11)

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Qrec _ t

m air ˜ ( Arec _ t ) U air ˜Vair ˜ ( Arec _ t ) [kJ ] (12)

2.2. Supply side closing In this technique, ventilation is stopped temporarily, and indoor air is recirculated through the exchanger to melt the frost. To implement the measured effectivenesses for energy saved calculations in frosting period (Fig. 3), operation time ratio (OTR) were defined (Eq. 13) using the previously developed method and measured data. H [%]

tF1 F

tF2 F

tF3 F

t [h]

tDF2 DF F

tDF1 DF F

Fig. 3. The frosting and defrosting periods.

OTR (t F1  t F 2  t F 3 ) / ttot []

(13)

Frosting test duration was around 180 min for heat recovery, 250 min for energy recovery. Using the membrane based energy recovery would reduce the frosting limit results in lower frost formation than heat recovery under similar test condition [15]. The heat recovery core was installed in a horizontal surface though, that caused the accumulated water stays inside the core. If the orientation is changed appropriately, it is expected to observe much lower defrosting time. However, the test facility in this project was limited to horizontal surface. By this way the defrosting duration was 650 min. Using the measured OTR, the sensible and total effectivenesses could be calculated by (Eqs. 14-15) which takes the frosting effect into consideration.

Hs_ f

H s ˜ (t F1  t F 2  t F 3 ) / ttot

H s ˜ OTR [%] (14)

Ht _ f

H t ˜ (t F1  t F 2  t F 3 ) / ttot

H t ˜ OTR [%] (15)

The results for a heat recovery and an energy recovery are listed in Tables 3 and 4. (Table 1-2). Table 1. Decreased sensible effectiveness values by frosting effect TO [°C]

OTR

-5.3

0.42

-10.5

0.36

-16.3

0.30

Hs [-]

Hs_f = OTR x Hs [-] 0.26

0.62

0.22 0.19

Table 2. Decreased total effectiveness values by frosting effect TO [°C]

OTR [-]

-10.2

0.82

-15.1

0.70

-18.7

0.62

Ht [-]

Ht_f = OTR x Ht [-] 0.47

0.57

0.40 0.35

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Based on the tested average effectiveness values and the measured outdoor air temperatures, exponential equations (Fig. 4) could be simply fitted to evaluate the average effectiveness of heat/energy exchangers at different outdoor air temperatures when they are lower than the frosting limits of the exchangers.

0,45

H [-] 0,40 0,35 0,30 0,25 0,20 Hs_ext = 0.0068*To + 0.2954 0,15 R² = 0.9945 0,10 0,05 0,00 -15 -10 -5 T [° C] 0 O Ht_ext = 0.0111*To + 0.4879 R2 = 0.9999

-20

Fig. 4. The exponential extrapolation for effectivenesses

Using the extrapolated effectiveness values the temperature (T SO_ext) and enthalpy (hSO_ext) in frosting time are calculated by (Eqs. 16-17).

TSO _ ext

TO  H s _ ext ˜ (TEI  TO )

hSO _ ext

hO _ sel .  H t _ ext ˜ (hEI  hO _ sel )

[qC]

(16)

[kJ / kg ] (17)

2.2.1. Sensible energy recovery Two separated areas could be determined on the ambient temperature duration curve. A RH_s represents the area that proportional the re-heater energy consumption, Arec_s represents the energy saved by the heat recovery unit (Fig. 5). ARH_s [ ͦC h]

TO [°C]

30 3 0 TEI =22 [°C] = constant

20

TSR =20 [°C] = constant

10 TSO 0 0

2000

4000

6000

-10 -20

8000 000 0 Time [h]

Arec_s [ ͦC h] TSO_ext

-30

Fig. 5. The areas on the ambient temperature duration curve that proportional with the annual pre-heater energy consumption and heat recovery energy saved considering the frosting effect.

Using the defined areas, the energy saved by the heat recovery (Eq. 18) and energy consumption of the re-heater (Eq. 19) can be also calculated.

Qrec _ s

cPair ˜ m air ˜ ( Arec _ s ) cPair ˜ U air ˜Vair ˜ ( Arec _ s ) [kJ ]

(18)

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QRH _ s

cPair ˜ m air ˜ ( ARH _ s ) cPair ˜ U air ˜Vair ˜ ( ARH _ s ) [kJ ]

(19)

If the frosting effect is taken into consideration, based on the area sizes, and their proportion it is obvious that the energy consumption will increase and the energy saved will decrease. 2.2.2. Total energy recovery The areas that represent the areas that proportional with energy consumption of the re-heater ( ARH _ t ) and total energy saved by energy recovery (

Arec _ t _ f ) could be created (Eqs. 20-21) based on the related enthalpy differences

on the correlated ambient temperature-enthalpy diagram in frosting time (Fig. 6). ho [kJ/kg]

Enthalpy - Temperature correlation

'hRH _ t

hEI (22 C ͦ , 40% RH) hS (20 ͦC, 35% RH)

70 60

50 40 30 hSO

ho hS hEI hSO/SO_ext

20 10

0 -30

-20

-10

-10 -20

0

10

'hrec _ t

20

30 To [°C]

-30

Fig. 6. The enthalpy differences on the ambient temperature-enthalpy diagram that proportional with the annual re-heater energy consumption and total energy saved by energy recovery considering the frosting effect.

Arec _ t

6'hrec _ t ˜W

ARH _ t

6'hRH _ t ˜W

8760

61 (hso / so _ ext  ho _ sel ) [kJ h / kg ]

(20)

8760

61 (hS  hSO _ ext ) [kJ h / kg ]

(21)

The total energy saved by the energy recovery (Eq. 22) and energy consumption of the re-heater recovery (Eq. 23) can be also calculated.

Qrec _ t

m air ˜ ( Arec _ t ) U air ˜Vair ( Arec _ t ) [kJ ] (22)

QRH _ t

m air ˜ ( ARH _ t ) U air ˜Vair ( ARH _ t ) [kJ ] (23)

3. Results and discussion During our research previously measured effectiveness values were implemented to calculate the energy saved by heat recovery and energy recovery and energy consumption of heaters: pre-heater and re-heater. The results are presented separately based on the analyzed techniques: supply side closing, pre-heating and frosting effect ignoring. All the calculations were performed to different weather European countries: in Helsinki (HEL.) in Finland as cold climate, in Krakow (KRAK.) in Poland as North Temperate climate and in Palermo (PAL.) in Italy as Mediterranean climate. During the energy investigation it was assumed that the heat recovery and also the energy recovery are controlled by temperature. Fig. 7 shows the amount of energy saved by heat recovery (HR) with sensible heat exchange (S), energy recovery (ER) with sensible (S) and total energy exchange (T), plus the energy consumption of the necessary auxiliary heaters

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(pre-heater, re-heater) in each analyzed case. The energy saved and consumption data are presented in [MJ] units. The sensible and total energy saved is also calculated by the amount of recovered energy saved and total auxiliary energy needed ratio ( Qrec _ s / Qtotal _ aux_ s ; Qrec _ t / Qtotal _ aux_ t ) in [%] unit (Fig. 8). Ignoring Frosting

Suppy Side Closing

Pre-Heating

14000

[MJ]

12000 10000

RH PH HR(S) ER(S) ER(T)

8000

6000 4000 2000 0

HEL. KRAK. PAL.

HEL. KRAK. HEL. KRAK.

Fig. 7. The amount of the energy saved and energy consumption of each analyzed cases in MJ unit.

Ignoring Frosting

Suppy Side Closing

Pre-Heating

90 80

[%]

70 60 HR(S) ER(S) ER(T)

50 40 30 20 10 0

HEL. KRAK. PAL.

HEL. KRAK. HEL. KRAK.

Fig. 8. The amount of the recovered energy saving in each analyzed cases in % unit.

Taking into consideration the frosting effect that decreases the amount of effectiveness is necessary in frosting time. If the frosting effect is ignored (ideal, theoretical case), the amount of the energy saved can be 21% oversized comparing the results with supply side closing and 13% oversized with pre-heating technique (Helsinki) when frosting effect was considered during our investigation. Based on the results the highest energy saved (85%) can be reached in Palermo. Due to the Italian Mediterranean climate freeze-protection is not required. The energy saved in Palermo can be 37% higher than in Helsinki (H) in Finland where the climate is cold. According to the results shown in Fig. 8, it is found outdoor air preheating technique results higher energy saved than the supply side closing and if the indoor air quality requirements are lower, then using a sensible heat recovery unit is suitable for heat recovery and the investment cost is lower. As shown in Fig. 8, the amount of energy saved by the heat recovery using preheating technique is 63-68%, which results 19-31 % higher energy saving than operation with supply side closing technique (48-57%). It is because the heat recovery is more sensitive (comparing with energy recovery) to frosting and its average effectiveness value is very low using the supply side closing technique (i.e. <26% in Table. 1), which significantly degrades the heat recovery performance. For the energy recovery, the preheating technique results also higher amount of energy saved than the supply side closing method, but the energy saving difference between these two methods (i.e. 1-2%) is much lower than that of heat recovery (i.e. 19-31%). It is because the energy exchanger has much lower possibility to undergo frost and therefore the

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decreased total effectiveness values considering frosting effect are higher comparing the results with heat recovery (Table. 1-2). 4. Conclusion Based on the results it is obvious if the indoor air quality requirements are lower, then using a sensible heat recovery unit is suitable for heat recovery and the investment cost is lower. If the total energy exchange (heat and moisture exchange) is important, then using an energy recovery unit is required. In this case the investment cost is higher, but the amount of sensible heat energy saved is also higher (max. 14%) than using a sensible heat recovery unit based on the present study. Using the pre-heating method and comparing the energy consumption sharing on pre-heater and re-heater, the amount of pre-heater energy needed is much higher correlated to the re-heater energy needed in a colder climate (Helsinki), because the pre-heater has to pre-heat a colder ambient air to the frosting temperature. By way of continuing of this research study the cooling energy saving will be also investigated.

Acknowledgements This research was financially supported by the National Research, Development and Innovation Office of Hungary – NKFIH PD 115614 – Budapest, Hungary.

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