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Energy and Buildings 58 (2013) 27–36

Contents lists available at SciVerse ScienceDirect

Energy and Buildings

j ourna l ho me p age: www.elsev ier .com/ locate /enbui ld

ptimal operation strategy of the hybrid heating system composed of centrifugaleat pumps and gas boilers

eng Li, Guozhong Zheng, Zhe Tian ∗

chool of Environmental Science and Engineering, Tianjin University, Tianjin 300072, China

r t i c l e i n f o

rticle history:eceived 26 April 2012eceived in revised form 6 July 2012ccepted 26 September 2012

eywords:

a b s t r a c t

Centrifugal heat pump has to be coupled with gas boiler to supply high temperature water in radiatorheating system. Regarding to hybrid heating system (HHS), operation strategy has significant impacton its annual energy consumption and cost. In this paper, the optimal operation strategy of the HHScomposed of sewage-source centrifugal heat pumps and gas boilers was analyzed. Firstly, the perfor-mance models of the system components, including terminal radiator, heat pump, gas boiler and water

ybrid heating systemadiatorentrifugal heat pumpperation strategy

pump were established respectively. Secondly, with the aim at minimizing the operating cost of thesystem the optimal operation strategy of the system was analyzed. Finally, the annual operating costand energy consumption of the HHS were compared with these of coal-fired boiler heating system. Theresults indicate that the HHS offers significant reductions in energy consumption (45.2%) and operatingcost (13.5%). Therefore, the HHS has a promising application prospect, the results provide reference forscientific operation of the HHS.

. Introduction

China’s rapid economic growth is inevitably accompanied byerious environmental problems, such as pollution, energy short-ge and climate change [1]. In China, approximately 27.8% of therimary energy is consumed in non-industrial buildings [2]. Inorthern China, 36% of the total building energy consumption istilized for heating in urban areas [3]. Efficient energy use is con-idered to be a solution of addressing fossil fuel depletion, energyecurity, and global warming [4].

Using heat pump to recover waste heat is of great significancen energy saving and environmental protection [5], sewage sourceeat pump (SSHP) heating system is now considered as a viablelternative to conventional heating system. Many studies haveeen conducted on SSHP heating system. Shen et al. [6] stud-

ed the influence of a novel evaporator with defouling functionn the performance of a SSHP. Zhao et al. [7] searched for suit-ble method to improve the performance of a SSHP system. Lit al. [8] studied the feasibility of using low-grade heat for ther-al desalination via a hybrid absorption heat pump system. Baek

t al. [9] investigated the feasibility of the waste water use for heat

ump as heat source. Kahraman et al. [10] analyzed the impactf the heat source temperature on the energy performance of aSHP system. Liu et al. [11] analyzed the advantage of a SSHP

∗ Corresponding author. Tel.: +86 22 27407800; fax: +86 22 27407800.E-mail address: tianzhe@tju.edu.cn (Z. Tian).

378-7788/$ – see front matter © 2012 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.enbuild.2012.09.044

© 2012 Elsevier B.V. All rights reserved.

system used in public shower facilities. Tassou [12] presented asimple methodology for sizing and performance analysis of heatpump systems in sewage effluent heat recovery applications. Liet al. [13] studied the economics and environmental protectionvalue of a SSHP. Funamizu et al. [14] introduced some SSHP sys-tems in Japan. Wu et al. [15–17] put forward a hydraulic reactivemethod to wash the soft-dirt in heat-exchanging pipe, performedthe technical and economic analysis of the increase in heat pumptemperature in the sewage disposal process, and reviewed theutilization progress of intake water heat-exchange technique ofSSHP systems. Qian et al. [18] analyzed the effect of flow rateon energy consumption and economical efficiency of the SSHPsystem.

In order to further improve the energy efficiency and decreasethe initial cost of heat pump heating system, heat pump is oftencoupled with conventional heating equipments, and many stud-ies have been conducted on this type of hybrid heating system(HHS). Scarpa et al. [19] simulated the performance of the HHScomposed of heat pump and gas boiler. Blarke and Dotzauer[20] found the HHS composed of heat pump and CHP (combinedheat and power) offers significant reductions in fuel consump-tion and operational costs. Pardo et al. [21] studied the energyefficiency improvement of the HHS composed of heat pump andthermal storage. Yavuzturk and Spitler [22], Xu [23] and Hackel

and Pertzborn [24] studied the performance of HHSs in reducingthe thermal imbalance of the soil, in these HHSs, ground-sourceheat pump is combined with cooling tower [22,23] or gas boiler[24].

28 F. Li et al. / Energy and Buildings 58 (2013) 27–36

Nomenclature

Q heating load of the buildings (kW)K heat transfer coefficient (kW/(m2 K))F area (m2)t temperature (◦C)G volume flow rate (m3/h)cp specific heat of water (kJ/(kg K))x load ratioE input power (kW)q lower heating value (kJ/kg)m the number of gas boilerH pump lift (m)k speed ratio of pumpH0 hydrostatic head (m)S pipe impedance (s2 m−5)abc correction coefficientc operating cost (Yuan)P price (Yuan)n frequency (day)M coal consumption (kg/h)e electricity consumption rate of coal-fired boiler

(kWh/t)

Greek symbols� efficiency� specific weight (9.807 kN/m3)�t delivery efficiency of the power network

Subscriptsin indoora outdoor mean temperatured design conditioni inletR radiatoro outletsys systemDL distribution lossfl full load conditionc condensere evaporatorpl part load conditionh heat pumpgb gas boilerg gasau auxiliary equipmentsp pumpm motorv variable frequency drivercp circulation pumpsp sewage pumpelc electricitymin minimumj different operation modes of the systemmax maximumk different outdoor temperatureco coalb coal boiler

AbbreviationsSSHP sewage source heat pumpHHS hybrid heating system

COP coefficient of performanceOCSR operating cost savings ratioESF energy saving factorREPI relative energy price index

Fig. 1. Flow chart of the hybrid heating system.

From the above literatures, studies on SSHP systems and HHSsmostly focus on system design, energy performance and eco-nomic assessment. To the best of the authors’ knowledge, operationstrategy of HHS has not previously been analyzed. In addition, theliterature mainly focuses on the HHS composed of sewage sourcecentrifugal heat pumps and gas boilers. As the terminal of theheating system is radiator, the design supply water temperatureis higher than the outlet water temperature of condenser, cascademode between heat pumps and gas boilers is an attractive solu-tion to meet the need of radiators. To reduce the operating costand improve the energy efficiency of a HHS, operation strategy isrequired.

For HHS, the mismatch between the heating load of the systemand heat output of each heating equipment at different outdoortemperatures, the outlet water temperature setting for each heat-ing equipment at different outdoor temperatures are the two mainproblems. To solve these problems, operation strategy of the HHSat different outdoor temperatures should be studied. The main pur-pose of this study is to propose a scientific, reasonable and feasibleoptimization control method for the HHS.

2. HHS overview

This HHS is applied in a residential community in Xi’an, China.The residential community is composed of residential buildings (of228,760 m2 floor area with heating load index of 37 W/m2) andpublic buildings (of 11,215 m2 floor area with heating load index of50 W/m2).

In this case study, the terminal of the heating system is radia-tor. Sewage source centrifugal heat pump is adopted to afford thebasic heating load of the system. At the design condition, the supplywater temperature of the system is 57.9 ◦C, the highest outlet watertemperature of the centrifugal heat pump is 51 ◦C, which is lowerthan the design supply water temperature of the system. Therefore,gas boilers are deployed for regulating the supply water tempera-ture and peak load compensation. In order to improve the energyefficiency of the system, the cascade mode is adopted betweensewage source centrifugal heat pumps and gas boilers in the system.The flow chart of the HHS is shown in Fig. 1.

At the design condition, the return water of the HHS is firstheated in the heat pump condenser, then part of the outlet waterfrom the condenser flows into the gas boilers, and the outlet waterfrom the gas boiler is mixed with the rest of the outlet water fromthe condenser, finally, the mixed water is supplied to buildings.For sewage system, in order to prevent freezing in evaporator, theminimum outlet water temperature is set at 5 ◦C.

The major energy consuming equipments in the HHS are heat

pumps, gas boilers, sewage pumps, circulation pumps and electri-cal auxiliary equipments of gas boilers. The parameters of theseequipments are shown in Table 1.

F. Li et al. / Energy and Buildings 58 (2013) 27–36 29

Table 1The parameters of the equipments in the HHS.

Maximal flow rate (m3/h) Heating capacity (kW) Pump head (m H2O) Shaft power (kW) Quantity Design condition

to,sys (◦C) ti,sys (◦C)

Heat pump 390 2800 – – 2 50.4 47.3Gas boiler 100 3500 – 7.5 2 74.8 50.4

25 38.2 2 – –32 44.7 2 5 1012.5 4.5 2 – –

3

tttmtoci

3

ptdo

Q

b

Q

Q

tslTohb

Q

t

I

sta

3

pt

Table 2The effects of the water flow rate of the condenser, the outlet water temperature ofthe condenser and the load ratio of heat pump on the COP.

Gc (m3/h) Ge (m3/h) to,c (◦C) x (%)

100 85 70 50

429 329 50 4.52 4.34 4.02 –268 329 50 4.54 4.35 4.04 –215 329 50 4.56 4.36 4.05 –429 329 45 4.97 4.71 4.41 –268 329 45 4.99 4.73 4.43 –215 329 45 5.00 4.75 4.43 –429 329 40 5.33 5.09 4.78 4.32268 329 40 5.35 5.11 4.79 4.33215 329 40 5.36 5.15 4.80 4.35

heat pump can be expressed as follows:

COPpl =[

0.558to,c + 273.15 + 4

(to,c + 4) − (to,e − 4)+ 1.168

]× (0.393x + 0.612)

(9)

Table 3The effects of the outlet water temperature of the condenser, the load ratio of heatpump, the water flow rate of the condenser and evaporator on the COP.

Gc (m3/h) to,e (◦C) to,c (◦C) x (%)

100 85 70 50

429 5 50 4.52 4.30 3.95 –268 5 50 4.54 4.33 3.96 –215 5 50 4.56 4.34 3.99 –429 5 45 4.97 4.70 4.36 –268 5 45 4.99 4.71 4.38 –

Circulation pumps 1 400 –

Sewage pumps 400 –Circulation pumps 2 100 –

. Methodology

The minimum operating cost of the system at different outdooremperatures is taken as the objective function, which is the func-ion of energy consumption and energy price. In order to analyzehe operation strategy and energy consumption, the mathematical

odels were developed within following steps. Firstly, the sys-em operation performance model including detailed sub-modelsf the main components was established. Secondly, objective andonstrain functions were constructed. Finally, rational evaluationndexes for the HHS were proposed.

.1. Heating parameters model

The heating parameters of the system at different outdoor tem-eratures include two parts: the heating load and the supply wateremperature. The heating load of buildings heavily depends on out-oor temperature, and the relationship between heating load andutdoor temperature can be expressed in Eq. (1) [25].

=(

tin − ta

tin − ta,d

)Qd (1)

For radiators, the heat output of the radiators can be calculatedy Eqs. (2) and (3) [26].

= KF( ti,R + to,R

2− tin

)1.3(2)

= Gcp(to,R − ti,R)3.6

(3)

As the distribution loss of the heating network is unavoidable,he efficiency of heating network should be considered. In order toimplify the calculation process, it is assumed that the distributionoss of the supply water pipe is equal to that of the return water pipe.herefore, based on the characters of the radiator, the efficiencyf the heating network and the heating load of the buildings, theeating load of the system and the supply water temperature cane calculated as follows:

sys = Q

1 − �DL(4)

i,sys = to,R − 1.8Qsys�DL

cpG(5)

n which �DL is assumed to be 6% [27].According to above equations, the heating parameters of the

ystem at different outdoor temperatures can be obtained. Based onhese parameters, energy consumption of the heating equipmentst different outdoor temperatures can be calculated.

.2. Performance model of heat pump

Heat pump has evolved to become a mature technology over theast two decades [5], it is the device that transfers heat from a lowemperature source to a higher temperature sink, whose efficiency

429 329 35 5.68 5.51 5.17 –268 329 35 5.70 5.52 5.19 –215 329 35 5.72 5.53 5.21 –

is mainly influenced by the variation of condensing temperature,evaporating temperature and load ratio.

Under full load condition, the coefficient of performance (COP)of heat pump is the function of the outlet water temperature ofthe condenser and evaporator [28–30]. The relationship can bedescribed as follows:

COPfl = f (to,c, to,e) (6)

Under part load conditions, COP of heat pump varies with theload ratio. The relationship can be described as follows.

COPpl = COPflf (x) (7)

Based on Eq. (7), the electricity consumed by heat pump underdifferent conditions can be calculated as follows:

Eh = Qh

COPpl(8)

Tables 2 and 3 provide the COP of heat pump under differentwater flow rates, outlet water temperatures and load ratios condi-tions. Based on these data provided by manufacturer, the COP of

215 5 45 5.00 4.72 4.38 –429 5 40 5.33 5.08 4.74 4.24268 5 40 5.35 5.10 4.75 4.25215 5 40 5.36 5.11 4.76 4.25

3 Build

tb

(flttuflsdfl

3

pufct

G

E

I

3

awc

E

I

G

�m

t[

E

I

G

i

3

atsf

C

0 F. Li et al. / Energy and

To validate Eq. (9), its calculation results are compared withhe manufacture data shown in Tables 2 and 3, the relative erroretween calculation result and manufacture data is less than 3.6%.

According to the performance data sheet of heat pumpTables 2 and 3), the COP also varies with the change of the waterow rate. Under the same condition (the outlet water tempera-ure of condenser, the inlet water temperature of evaporator andhe load ratio are the same), the relative error between the COPnder constant flow rate condition and the COP under variableow rate condition is less than 3%, the influence of flow rate iso small that can be ignored in practical engineering. Therefore, toecrease energy consumption of the sewage pumps, variable waterow operation mode is suggested.

.3. Performance model of gas boiler

Gas boiler system includes gas boilers, gas boiler circulationumps and other electrical auxiliary equipments. The energytilized by the system includes natural gas and electricity. In dif-erent operation modes, natural gas consumption and electricityonsumption of gas boilers can be calculated by the following equa-ions:

g = 3600Qgb

qgb�gb(10)

au,sys = m(Ecp,gb + Eau,gb) (11)

n which �gb = 90% [31].

.4. Performance model of pump

In the HHS, two types of pumps are utilized: circulation pumpsnd sewage pumps. The circulation pumps are utilized in closedater pipe system, and the input power of the circulation pump

an be calculated as follows [32].

cp = �HG

3.6�p�m�v(12)

n which:

= 3.6Qsys

cp(to,sys − ti,sys)(13)

p, �m, �v can be obtained by regression of the data provided by theanufacturer.The sewage pumps are utilized in the open water system, and

he input power of the sewage pump can be calculated as follows33].

sp = �Gsp(H0 + SG2sp)

3.6�p�m�v(14)

n which:

sp = 3.6Qh(COPpl − 1)cpCOPpl(ti,e − to,e)

(15)

As described in Eqs. (12)–(15), electricity consumption of pumpsn different conditions can be obtained.

.5. Objective and constraint functions

Under the same heating load, the optimal operation strategy isffected by the flow rate and the price of energy. In order to obtainhe optimal operation strategy, some typical operating conditionshould be analyzed, and the objective function can be described as

ollows:

min = min (Pele(Eh + Esp) + (PgGg + PeleEau,sys) + PeleEcp)j

× (j = 1, 2, . . .) (16)

ings 58 (2013) 27–36

For centrifugal heat pump, in order to prevent compressor surge,the minimum heating capacity of the centrifugal heat pump shouldbe larger than 50% of its rated heating capacity. The continua-tive energy regulation technique is adopted by compressor. Therange of the water flow rate regulation, in terms of evaporator andcondenser, is wider than that of screw heat pump. Therefore, cen-trifugal heat pump can be utilized in large temperature differenceheating system, such as radiator heating system. For gas boiler, themaximum heating capacity should be smaller than its rated heatingcapacity. Reference to performance specification of manufactur-ers, these constrain functions of the objective function are listed asfollows:⎧⎪⎪⎪⎪⎪⎪⎨⎪⎪⎪⎪⎪⎪⎩

to,c ≤ 51

3 ≤ (ti,e − to,e) ≤ 12

3 ≤ (to,c − ti,c) ≤ 12

1400 ≤ Qh ≤ 2800

Qgb ≤ 3500

(17)

The first inequality states that the highest outlet water temper-ature of the condenser is 51 ◦C. The second and third inequalitiesindicate that the water temperature difference of the evaporatorand condenser in design condition should be in the range of 3–12 ◦C.The fourth inequality shows that the heating output of heat pumpin actual conditions should be in the range of 1400–2800 kW. Thefifth inequality represents that the maximum heating output of gasboiler is 3500 kW.

3.6. Evaluation criteria

To carry out the economic performance analysis of HHS, an exist-ing conventional coal-fired heating system, as a reference system,is compared with the HHS system. The operating cost savings ratio(OCSR) is adopted to evaluate the economic performance of theHHS. OCSR is defined as the cost savings ratio of HHS in compari-son with coal-fired heating system, and the OCSR can be describedby the following equations:

OCSR =k=ta,max∑

k=ta,d

((Cco − Cmin) /Cco

)knk (18)

In which,

Cco = PcoMk,co + PeleEcp,b + PeleEau,b (19)

For the coal-fired heating system, coal consumption and elec-tricity consumption at different outdoor temperatures can becalculated as follows:

Mco,k = 3.6Qsys,k

qco�b�DL(20)

Ecp,b = �HQsys,k

cp�p�m�v(to,sys − ti,sys)k

(21)

Eau,b = eQsys,k

qsteam(22)

In which �b is assumed to be 60% [34]; e is assumed to be 10 kWh/t[35]; qsteam is the heat contained in 1 ton steam, 720 kWh/t; forthe coal-fired heating system, the design temperature difference isassumed to be 20 ◦C, and �p�m�v is assumed to be 75%.

The energy saving factor (ESF) is adopted to evaluate the energyperformance of the HHS. ESF is defined as the energy saving ratio

of HHS system in comparison with coal-fired heating system. In aheating system, some kind of energy (e.g. electricity, coal, naturalgas) would be utilized, in order to calculate energy consumption ofdifferent heating systems, the electricity utilized in heating system

F. Li et al. / Energy and Buildings 58 (2013) 27–36 31

Input outdoor

temperature

Heating load of the

system

Input flow rate of

the system and

performance models

of pump

Supply water

temperature

Electricity

consumption of

circulation pumps

Input operation mode of heating

equipments, performance models of

pump, heat pump and gas boiler

Energy consumption of heating

equipments and sewage pumps

Input the price of

energy utilized inthe system

The operating cost (Cj) of the system

at different combined working

conditions

jmin CC min=

The optimal

operation strategy

End

al ope

so

EEcp)/

[

Fig. 2. Flow chart for the optim

hould be converted to primary energy, and the ESF at differentutdoor temperatures can be described as follows.

SFsys.k = (Qsys/�b + (Ecp,b + Eau,b)/(�g�t))k− ((Eh + Esp + Eau,sys +

(Qsys/�b + (Ecp,b + Eau,b)/(�g�t))k

In which �g is assumed to be 33% [30], �t is assumed to be 93.2%36].

ration strategy determination.

(�g�t) + Qgb/�gb)k (23)

According to Eq. (23) and the frequency distribution of the out-door temperature in the heating period, the average ESF in thewhole heating period can be calculated as follows:

ESFsys =∑k=ta,max

k=ta,dnkESFsys,k∑k=ta,max

k=ta,dnk

(24)

32 F. Li et al. / Energy and Buildings 58 (2013) 27–36

5

2

5

8

4

88

10

15

12

16

11

8

7

11

-5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10

0

2

4

6

8

10

12

14

16

18

Outdoor temperature (ºC)

Fre

qu

en

cy

(d

ay

)

3

4

5

6

7

8

9

10

H

eati

ng

lo

ad (

MW

h)

Fo

4

a

4

Htifibotd

4

1fFhttst

4

tttcf

(ir

-6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11

35

40

45

50

55

60

65

Wate

r te

mp

era

ture

(ºC

)

Outdoor temperature ( C)

Δt=20ºC

Δt=18ºC

Δt=16ºC

Δt=14ºC

Δt=12ºC

Δt=10ºC

Average water

temperature of the radiator

Fig. 4. The relationship between supply water temperature (average temperature

ig. 3. The heating load and the frequency distribution are with the change ofutdoor temperature.

. Optimal operation strategy analysis of the HHS

In order to simplify the calculation of the system, some physicalssumptions are applied:

(1) The sewage water temperature is constant;(2) The efficiency of gas boiler is constant;(3) The distribution loss of the heating network is constant.

.1. Optimization method

Based on the above detailed analysis of each component of theHS and the input data of each equation, the input data are outdoor

emperature, operation mode, flow rate and energy price. The heat-ng load and the average water temperature of the radiator wererst calculated, then the optimal flow rate of the HHS was analyzedased on the price of energy utilized in the HHS, finally the optimalperation strategy at different outdoor temperatures is obtained onhe basis of the objective function. The optimal operation strategyetermination flow is described in Fig. 2.

.2. Heating demand of the system

The heating period in Xi’an is from November 15th to March5th, and the frequency distribution of the outdoor temperaturerom the typical meteorological year weather files [37] is shown inig. 3. On the basis of the characters of the building and radiator, theeating load of the system and the average water temperature ofhe terminals at different outdoor temperatures were calculated,he results are shown in Figs. 3 and 4. There is a liner relation-hip between average water temperature of radiator and outdooremperature.

.3. Effect of energy price on the optimal flow rate

The supply water temperature increases with the decrease ofhe water flow rate. The decrease of the water flow rate will lead tohe decrease of energy consumption of the circulation pumps andhe decrease of the COP, and further lead to the increase of energyonsumption of heat pump. Therefore, the flow rate is a decisiveactor determining energy consumption of the HHS.

The maximum flow rate is the design value of the HHS�t = 10 ◦C, G = 780 m3/h). The choice of the minimum flow rate isnfluenced by the efficiency of circulation pumps. When the flowate of a pump drops to less than 50–60% of its rated condition,

of radiator) and outdoor temperature.

the efficiency of pump diminishes rapidly [38,39]. Therefore, theminimum flow rate is set at 50% of the design value (�t = 20 ◦C,G = 390 m3/h). In this paper, the flow rate of the HHS is controlledby frequency converter. To improve system stability and simplifythe control of the frequency converter, frequency converter is setfor six-speed shifting [40], the frequency of the frequency con-verter at different speeds are 50 Hz, 45 Hz, 40 Hz, 35 Hz, 30 Hz and25 Hz respectively. In different frequencies, the design tempera-ture difference of the radiator is 10 ◦C, 12 ◦C, 14 ◦C, 16 ◦C, 18 ◦Cand 20 ◦C respectively. Therefore, 6 water flow rates (G = 390 m3/hwhen �t = 20 ◦C, G = 433 m3/h when �t = 18 ◦C, G = 487 m3/hwhen �t = 16 ◦C, G = 557 m3/h when �t = 14 ◦C, G = 650 m3/h when�t = 12 ◦C, G = 780 m3/h when �t = 10 ◦C) are chosen as the typicalconditions.

Under different flow rates, the relationship between supplywater temperature and outdoor temperature is shown in Fig. 4.Under the same flow rate condition, the supply water temperatureincreases with the decrease of the outdoor temperature; under thesame outdoor temperature condition, the supply water tempera-ture increases with the decrease of the flow rate; the average watertemperature of the radiator is only affected by outdoor tempera-ture, and it increases with the decrease of outdoor temperature.Based on these temperatures and flow rates, energy consumptionof the HHS in different outdoor temperatures can be calculated.

The optimal flow rate is analyzed on the basis of the objectivefunction, and the objective function is the function of energy con-sumption and energy price. Therefore, the optimal flow rate shouldbe analyzed on the basis of energy consumption and energy price.In order to eliminate the effect of energy price fluctuation (the pricefluctuation of the natural gas and electricity) on the analysis of theoptimal flow rate, relative energy price index (REPI) is adopted.REPI is defined as the price ratio between the price of natural gas(Yuan/Nm3) and the price of electricity (Yuan/kWh).

In this paper, the value of REPI is set at between 2 and 4. In orderto obtain the optimal flow rate at different outdoor temperaturesand different REPIs, 54 groups of typical data are obtained at differ-ent combined working conditions involving 6 water flow rates at390 m3/h, 433 m3/h, 487 m3/h, 557 m3/h, 650 m3/h, 780 m3/h and9 REPIs at 2, 2.1, 2.2, 2.3, 2.4, 2.5, 3, 3.5, 4. As described above, whenthe REPI is at the range of 2–2.5, the quantity of the value of the REPIis added, the reason is that when the value of the REPI is located in

this range, the optimal operation strategy of heating equipments atdifferent outdoor temperatures changes remarkably.

F. Li et al. / Energy and Buildings 58 (2013) 27–36 33

10 12 14 16 18 20

3.6

3.8

4.0

4.2

4.4

4.6

4.8

5.0

5.2

5.4

2.0

2.1

2.2

2.3

2.4

2.5

3.0

3.5

4.0Op

erat

ing

co

st (

Mil

lio

n Y

uan

)

The design temperature difference of the radiator (ºC)

Ft

itatttc

eiaid(flfl

ioic

tamsw

atalvtoH

4

eot

t

2.0 2.5 3.0 3.5 4.0

3400000

3600000

3800000

4000000

4200000

4400000

Op

erat

ing

co

st (

Yu

an)

REPI

Fig. 6. The minimum annual operating cost of the HHS at different value of the REPI.

10 12 14 16 18 20

1.00

1.01

1.02

1.03

1.04

1.05

Rel

ativ

e v

alu

e

The design temperature difference of the radiator (ºC)

2.0

2.1

2.2

2.3

2.4

2.5

3.0

3.5

4.0

and −1.3 ◦C, two heat pumps and two gas boilers work in part loadcondition, the outlet water temperature of the condenser is 51 ◦C,and the gas boilers are deployed for regulating the supply water

Table 4The optimal operating mode under different outdoor temperatures of the system.

Flow rate (m3/h) Outdoor temperature (◦C) Heat pump Gas boiler

487 (8.8, 10) 1 1

ig. 5. The minimum operating cost of the system in 54 groups of working condi-ions.

In different combined working conditions, based on the heat-ng parameters shown in Figs. 3 and 4 and the objective function,he minimum operating cost at different outdoor temperaturesre calculated. On the basis of the minimum operating cost andhe frequency distribution of the outdoor temperature in Fig. 3,he annual operating cost (the price of the electricity is assumedo 1 Yuan/kWh) under different combined working conditions arealculated, and these results are shown in Fig. 5.

As shown in Fig. 5, in the same design temperature differ-nce operation mode, the annual operating cost increases with thencrease of the REPI. At the same value of the REPI, the annual oper-ting cost fluctuates with the flow rate of the HHS. When the REPIs smaller than 2.4, the annual operating cost increases with theecrease of the flow rate, and the optimal flow rate is 390 m3/h�t = 20 ◦C). When the REPI is between 2.5 and 3.5, the optimalow rate is 487 m3/h (�t = 18 ◦C). When the REPI is 4.0, the optimalow rate is 557 m3/h (�t = 16 ◦C).

As described above, the optimal flow rate increases with thencrease of the REPI, the main reason is that electricity consumptionf the centrifugal heat pump decrease with the decrease of the REPI,n order to improve the economic performance of the HHS, energyonsumption of the circulation pump should be reduced.

Based on the analysis of the 54 combined working conditions,he optimal operation strategy of the HHS can be obtained in vari-ble flow rate condition. In the optimal variable flow rate operationode, the minimum annual operating cost in different REPIs is

hown in Fig. 6, the results indicate that the operating cost increaseith the decrease of the REPI.

In order to determine the flow rate at different values of the REPInd different outdoor temperatures, the relative values betweenhe annual operating cost in constant flow rate operating modend that in optimal variable flow rate operating mode are calcu-ated, and these relative value are shown in Fig. 7. At the samealue of the REPI, the smallest relative difference is less than 4‰,he relative difference is so small that it can be ignored in actualperating mode. In order to simplify the control strategy of theHS, the optimal constant flow rate operation mode is suggested.

.4. Optimal operation strategy of the HHS

In Xi’an, the price of natural gas is 2.21 Yuan/Nm3, the price oflectricity is 0.8323 Yuan/kWh. The value of the REPI is 2.66. Based

n the results described above, the optimal constant flow rate ofhe HHS is 487 m3/h (�t = 16 ◦C).

On the basis of the heating parameters (Figs. 3 and 4) andhe objective function, the optimal operation mode and energy

Fig. 7. The relative value of the operating cost between the optimal constant flowrate and optimal variable flow rate.

consumption of the system at different outdoor temperatures canbe obtained, and the results are shown in Table 4 and Figs. 8 and 9.

As shown in Figs. 8 and 9 and Table 4, the operation strategy ofthe HHS can be illustrated as follows. When outdoor temperatureis between 8.8 ◦C and 10 ◦C, one heat pump works in full load con-dition (two heat pumps work in part load condition will lead to adrastically decrease of the COP) and one gas boiler is deployed forpeak load compensation. When outdoor temperature is between4.6 ◦C and 8.8 ◦C, two heat pumps work in part load condition. Whenoutdoor temperature between −1.3 and 4.6 ◦C, two heat pumpswork in full load condition and one gas boiler is deployed for peakload compensation. When outdoor temperature is between −3.7

(4.6, 8.8) 2 –(−2.5, 4.6) 2 1(−3.7, −2.5) 2 2(−5, −3.7) 1 2

34 F. Li et al. / Energy and Buildings 58 (2013) 27–36

-5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10

500

600

700

800

900

1000

1100

1200

1300

1400

1500

a b

Inp

ut

po

wer

(k

W)

Sewage pump

Circulati on pump

Auxili ary equ ipment of the gas boil er

Hea t pump

-5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10

0

100

200

300

400

500

600

700

800

Gas

co

nsu

mp

tio

n (

m3/h

)

tures

tal

ohfotb

t

ttf

t

wt

Fi

Outdoor temperature (ºC)

Fig. 8. Energy consumption of the equipments at different outdoor tempera

emperature and peak load compensation. When outdoor temper-ture is between −5 ◦C and −3.7 ◦C, one heat pump works in fulload condition and two gas boilers work in part load condition.

Under the practical operating conditions, the operation controlf the HHS is realized by setting the outlet water temperatures ofeat pump and gas boiler at different outdoor temperatures. As

or the heat pump, the outlet water temperature can be calculatedn the basis of the operation strategy in Table 4. The outlet wateremperature of heat pump at different outdoor temperatures cane expressed as follows:

o,c =

⎧⎪⎪⎪⎪⎪⎪⎨⎪⎪⎪⎪⎪⎪⎩

ti,c + 3.6Qs

cpG(4.6 < ta ≤ 8.8 ◦C)

ti,c + 3.6 × 2800cpG

(8.8 < ta < 10 ◦C or − 5 < ta < −3.5 ◦C)

ti,c + 3.6 × 5600cpG

(−1.3 ◦C < ta < 4.6 ◦C)

51 (−3.7 ◦C < ta < −1.3 ◦C)

(25)

The supply water temperature of the system at different outdooremperatures is shown in Fig. 4. The relationship between outdooremperature and supply water temperature can be expressed asollows:

s = −1.64ta + 53.14 (26)

As for gas boiler, the outlet water from the gas boiler is mixedith the rest of outlet water of the condenser, and the mixed water

emperature equals to supply water temperature of the system.

-7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11 12

400

600

800

1000

1200

1400

1600

1800

2000

2200

2400

Operating cost

Water temperature

Outdoor temperature(ºC)

Op

erat

ing

co

st (

Yu

an)

34

36

38

40

42

44

46

48

50

52

Ou

tlet

wat

er t

em

pera

ture

of

hea

t p

um

ps

(ºC

)

ig. 9. The operating cost of the HHS and outlet water temperature of heat pumpsn different outdoor temperatures.

Outdoor temperature (ºC)

(a: input power of electrical equipment; b: gas consumption of gas boilers).

Therefore, the outlet water temperature of gas boiler can be con-trolled by supply water temperature.

5. Comparison of the HHS against coal-fired boiler heatingsystem

5.1. Economic performance analysis

Energy consumption constitutes a decisive factor influencingthe operating cost of two different heating systems. For a coal-firedheating system, energy consumption at different outdoor temper-atures is calculated, and the results are shown in Fig. 10.

At present, the price of coal is 847 Yuan/t (the lower heat-ing value of this kind of coal is 5500 kcal/kg). Based on the datadescribed in Figs. 3, 8 and 10, the price of the energy utilized inthe heating system and Eq. (18), the OCSR can be obtained, and theresult is 13.5%.

5.2. Energy performance analysis

Based on the results shown in Figs. 8, 10 and Eq. (23), the ESF atdifferent outdoor temperatures can be calculated, and the resultsare shown in Fig. 11. The energy consumption of the HHS is muchlower than that of a coal-fired boiler heating system. Based on the

frequency distribution of outdoor temperature in Figs. 3, 11 andEq. (24), the average ESF in the heating period can be calculated,and the result is 45.2%. Compared with the conventional coal-firedboiler heating system, HHS has huge energy saving potential.

-6 -4 -2 0 2 4 6 8 10 12

40

60

80

100

120

140 Circulation pump

Auxiliary equipments

Coal-fired boiler

Outdoor temperature(ºC)

Inp

ut

po

wer

(kW

h)

800

1000

1200

1400

1600

1800

2000

2200

2400

2600

Co

al

co

nsu

mp

tio

n(k

g)

Fig. 10. Energy consumption of coal-fired boiler heating system at different outdoortemperatures.

F. Li et al. / Energy and Build

-5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10

0.0

0.1

0.2

0.3

0.4

0.5

0.6

ES

F

6

aces

cIpp

A

P

R

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

Outdoor temperature(ºC)

Fig. 11. The relationship between ESF and outdoor temperature.

. Conclusions

This paper presents a method for analyzing the optimal oper-tion mode of a HHS. The operation strategy analysis for a HHSan provide simple and reasonable technical support for improvingnergy efficiency and decreasing operating cost. The main conclu-ions of this study can be summarized as follows.

(1) COP is the function of the load ratio, the outlet water temper-ature of the condenser and evaporator. Based on the function,energy consumption of heat pump in different operatingconditions can be obtained easily.

(2) To reduce energy consumption of the heating equipments,variable supply water temperature is adopted for heat pumpsin different outdoor temperatures.

(3) Constant flow rate operation mode is suggested for the HHS,and the optimal flow rate increase with a decrease of theREPI.

(4) Compared with a conventional coal-fired boiler heating sys-tem, the OCSR is 13.5% and the ESF is 45.2%.

(5) When the design supply water temperature of the terminalheating equipments is lower than 60 ◦C and the secondarywastewater is easy to obtain, this type of HHS should be givenpriority.

For this HHS is still in its construction stage, the study resultsannot be compared with the measured data in actual operation.n the future, the operation strategy in this study should be accom-anied by measurements so that it can be adjusted for the realroject.

cknowledgment

This research has been supported by Tianjin Municipal Researchrojects (Grant No. 11ZCGHHZ00900).

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