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Energy and Buildings 118 (2016) 291–300

Contents lists available at ScienceDirect

Energy and Buildings

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

lectricity consumption and economic analyses of district heatingystem with distributed variable speed pumps

ianjie Sheng, Lin Duanmu ∗

chool of Civil Engineering Faculty of Infrastructure Engineering, Dalian University of Technology, Liaoning, China

r t i c l e i n f o

rticle history:eceived 12 April 2015eceived in revised form 2 March 2016ccepted 2 March 2016vailable online 4 March 2016

eywords:istrict heating

a b s t r a c t

The increasing of energy efficiency and decreasing the transportation power consumption of DistrictHeating (DH) systems have been increasingly focused due to the need to save energy. In this paper a newdistrict heating network with distributed variable-frequency speed pumps (DVFSP) is presented andanalyzed. The approach for minimum of the capital costs and energy consumption in a district heatingnetwork is presented using a case study based on a district heating network with DVFSP in Dalian, China.The computational tool is a commercial software package, which is used to design and analyze DH net-works. Increasing temperature difference between supply and return pipes, the annual total electricity

istributed variable-frequency speed pumpranched networknergy saving

consumption and the equivalent annual cost are reduced. The energy efficiency in the DH system with theDVFSP was compared with the one in the DH system with conventional central circulating pump (CCCP).Analysis results show that the average electrical energy saved by the DH system with the DVFSP is 49.41%of the one saved by the DH system with conventional central circulating pump. The average pumps powerconsumption of heating area per square meter in primary heat supply network is 0.325 kWh/m2.

© 2016 Elsevier B.V. All rights reserved.

. Introduction

District heating system (DHS) supplies and distributes heat,enerated from heating sources, for water heating and tech-ological needs in heating buildings and industrial enterprises.

t includes 3 main parts: (1) heating water source, (2) heat-ng supply and distribution piping, and (3) consumer systems

hich use heat [1]. By far, DHS is an integral part of infras-ructure in Cold Regions, such as China, Russia, Denmark,inland, Sweden, Switzerland, etc. According to statistics, 7.5illion square meters are heated in Northern China’s urbannd rural areas in winter, with energy utilization equivalento 0.143 billion tons of standard coal per year, accounting for0% of the total operational cost of buildings in urban and ruralreas. As urbanization accelerates, the heating networks in urban

nd rural areas will also expand. It is expected that heating costsill increase to 0.3 billion tons of standard coal per year by 2020

ased on the current rate of development [2]. It is an important

Abbreviations: DH, district heating; DHS, district heating system; CCCP, con-entional central circulating pump; DVFSP, application of distributed variablerequency.∗ Corresponding author.

E-mail addresses: shengxianjie@yeah.net (X. Sheng), duanmu@dult.edu.cnL. Duanmu).

ttp://dx.doi.org/10.1016/j.enbuild.2016.03.005378-7788/© 2016 Elsevier B.V. All rights reserved.

element of the economy too. With increasing energy demands,improving the efficiency of energy systems is an important issue.Today more attention is being paid to energy saving and efficiencyimprovement. Conventionally, district heating (DH) networks builtas branched networks for which circulating pump is designed inthe heat source (Fig. 1). Circulation pumps are another compo-nent which should be selected to ensure sufficient flow circulationin the network. Traditionally, pumps are selected using the maxi-mum pressure difference for the most remote consumer. But somedisadvantages exist in this topology. The control system is slow,which reduces the energy efficiency of the system. Besides, thepipe lengths are different between the heating source and the con-sumers, which cause some special needs to the network. The waterflow has a tendency to flow via the shortest routes, where thepipes have the lowest flow resistance. This leads to large differ-ences and losses in local pressure, thereby complicating the use ofthe network. Additionally, the main circulation pump in the net-work is dimensioned according to the pressure difference neededfor the most distant consumer. However, as for conventional DHS,most of them are now run in the large flow mode, resulting in adramatic increase in transport electricity consumption [3]. Espe-cially, in large conventional district heating systems, supply and

return water temperature difference (�T) diminution plays anever increasing role. Floss [4] pointed out that condensing boilerdemanded a low return water temperature to the boiler to achieve

292 X. Sheng, L. Duanmu / Energy and B

Nomenclature

G volume flow rate m3/hQ volume flow of the heat source, m3/hH pressure difference, mH2O�P the resistance of each branch, PaN power of pump, kW�T temperature difference, ◦C� friction factortw outdoor temperature ◦Ctn outdoor temperature ◦Cn the total number of the each heating sectionl pipe length, mli. i-th pipe length, m�R pressure difference, Pad pipe diameter, mK the equivalent of absolute roughness for pipe wall,

m� the density of hot water, kg/m3

q heat load, kW� local drag coefficient,� pump efficiencyS the resistance friction factorld the local resistance equivalent lengthc specific heat capacity, J/kg K

aBaflthrgthdoanaiait�

z power saving rate

significant condensing effect and the expected high efficiency.ased on practical experience of the authors, the mass flow was notdjusted downward as the loads drop, leading to excessive waterow rates and �T diminution. For district heating systems in par-icular, �T diminution has three efficiency detriments, which areigher pumping electricity consumption, higher heat losses in theeturn pipes, and lower primary energy efficiency in heat and powereneration. Moreover, the high mass flow rates at low �T limithe maximum heating capacity that can be provided by the districteating system, preventing the opportunity to grow the reach of theistrict heating network to new customer. While the existing bodyf literature identifies the causes of �T diminution qualitatively,

quantitative analysis, especially for hydronic heating systems, isot yet available. What is missing and what this work provides, isn investigation into the relative contribution of hydronic networkmbalance, control valve oversizing, incorrect valve characteristic,

nd control loop parameters to the overall �T diminution symptomn order to rank the individual faults and formulate recommenda-ions. Besides the objectives described above, an increased system

T also contributes to reduce the energy used in the distribu-

Fig. 1. Conventional Distri

uildings 118 (2016) 291–300

tion pump, as less volume is needed to obtain the same amountof energy. The distribution losses from the return pipes will alsobe reduced as the temperature gradient to the surroundings isdecreased. Persson stated that an increase in the system �T of10 ◦C will result in a 55% reduction in required pumping power, and,depending on the heat-production method, the total primary fuel-source savings can vary between 0.1% and 14% [5]. Similar numberscan also be found in other reports [6]. Thus, there is great interestin �T maximization within the district heating industry. How toreduce the heat source electricity consumption and pump powerconsumption have been become a concerned topic to researchersand heating enterprises. Therefore, DH system has the potential tocontribute to energy targets.

There have been many studies that aim at optimizing the over-all performance of the DH system [7–18]. However, these studiesare based on a CCCP heating system, which cannot come to funda-mentally deal with the throttling loss of pipe network. In order toincrease the efficiency of distribution system, a number of scholarspointed out some improvements that could be made. It is possi-ble to use distributed variable speed pumps as control valves tosave energy; and therefore, this attracts more and more atten-tions [19–25]. Gamberi et al. simulated a heating system withmulti-pumps using Newton-Raphson algorithm [19]. Vogelesangquantitatively discussed the improvement in energy saving poten-tial by applying variable speed pumps [20–22] . Jiang discussed thetechnical feasibility of replacing control valves with variable speedpumps [23]. Wang and Li studied the optimal configuration of a DHsystem with distributed variable speed pumps [24]. Di and Yuanconducted economic assessment and the results showed a largeprofit from the use of DVFSP, especially for part-load operations[25]. Deng and Fu theoretically proved the possibility of using vari-able speed fans and variable speed pumps to adjust wind valvesand water valves. They pointed out that the hydraulic stability andcontrollability of the new system were greatly improved in additionto the obvious energy saving effect [26] . Nowadays, energy savingefforts demands the search for new technical and scientific exper-tise in the field of heating techniques [27–29]. A new system withdistributed variable- frequency speed pumps (DVFSP), as a multi-pumps system, is of quite a difference on principles and hydrauliccharacteristics with conventional central circulating pump (CCCP)DH systems. In this paper, the new system we only study is theprimary side. The whole district heating system is an indirect heat-ing system. In this study, we only study the primary pipe network(heat plant to substations). The main idea of mass flow control isto increase the supply temperature and reduce the return tem-

perature. Increased temperature cooling makes it possible to havesmaller mass flows and increase the efficiency of the heat produc-tion. Smaller mass flow means reduced pressure losses in the DHnetwork and therefore a reduced need for pumping power. The DH

ct Heating Network.

X. Sheng, L. Duanmu / Energy and Buildings 118 (2016) 291–300 293

Fig. 2. Schematic Diagram 1 of DVFSP DHS.

mal s

sqstwsairastos

Fig. 3. Location of ther

ystem using mass flow control is meant for the concept of a fre-uency conversion technology where mass flow rates in consumerubstations are controlled by pumps with inverters to improve heatransfer. It will replace the traditional DH network and control inhich water flow is throttled by control valves. The new control

ystem will enable temperature difference to be adopted for supplynd return temperatures and more significant temperature cool-ng. This new network and the method used to control the flowate of the primary supply water and its temperature are changeds the outdoor temperature. The correct temperature level of the

upply temperature is fixed as functions of the outdoor tempera-ure. The target is to keep the value of �T. The design and operationf a minimum-cost DHS also depend on the environmental andocial-political contexts. The objective of this paper is to research

tations in DVFSP DHS.

minimise the annual total electricity consumption and investmentcosts compared with the CCCP DHS

2. Electrical energy saved analysis on DVFSP DHS

For the district heating system with the conventional centralcirculating pump (CCCP), the output power of pump is

N1 = GHn

367�1(1)

where G is the total flow rate (m3/h), Hn is the resistance loss of themost adverse loop pipe network (m H2O), and �1 is the operatingefficiency of pump.

294 X. Sheng, L. Duanmu / Energy and Buildings 118 (2016) 291–300

Fig. 4. Topology of DVFSP DHS.

curve

cn

N

wHwo

Fig. 5. Delay

The end-user needs to add a booster pump in case the mainirculating pump is laid out to the second most adverse loop pipeetwork. So the output power of pump is

2 = GHn−1

367�2+ Gn(Hn − Hn−1)

367�n(2)

here Gn is the flow of the most adverse loop pipe network (m3/h),n-1 is the resistance loss of second most adverse loop pipe net-ork (m H2O), �2 is the operating efficiency of pump, and �n is the

perating efficiency of booster pump.

of heat load.

A method of energy saving with respect to the conventionalmethod is

�N = N1 − N2

N1= 1 − �nHn−1

�1Hn− Gn(Hn − Hn−1)�2

�nGHn−1(3)

Different pump models are different in design efficiency and opera-tional efficiency; however, it can be ignored as a program of energyassessment and analysis.

�N = (1 − Gn

G)(1 − Hn−1

Hn) (4)

It is concluded that the booster pump added in DHS is energy-efficient. The higher the saving rate, the greater the ratio of the

X. Sheng, L. Duanmu / Energy and Buildings 118 (2016) 291–300 295

Fig. 6. Each thermal station annu

0

2000000

4000000

6000000

8000000

10000000

Ann

ual e

lect

rici

ty c

onsu

mpt

ion

(kW

h)

rttaslp

�

wbosb

ptbsl

CCCP DVFSP

Fig. 7. Annual electricity consumption.

esistance loss of the second most adverse loop pipe network tohe resistance loss of the most adverse loop pipe network is. Buthe higher the saving rate, the less the ratio of the flow of the mostdverse loop pipe network to the total flow of system is. When theelection of main circulating pump only meets the needs of the i-thoop pipe network, the users of the (i + 1)-th, (i + 2)-th,. . ., n-th loopipe network need to add a booster pump. So, the saving rate is

N∗ =n∑

j=i+1

(1 − Gj

G)(1 − Hi

Hj) (5)

In other words, the value of the saving rate is related to the ratehich is the resistance loss of a loop pipe network added with a

ooster pump to the i-th loop pipe network and which is the flowf the loop pipe network to the total flow of the system. This fullyhows that the energy- saving rate of the DH system with addedooster pumps as pipelines can be improved.

For district heating system with distributed variable-frequencyumps (DVFSP DHS), refer to the most favorable circulatory sys-

em functioning with booster pumps. The degree of utilization ofooster pumps varies widely within distribution networks. In thetudy, we considered using booster pumps to increase the headift, designed to be utilized as much as possible in the optimization

al electricity consumption.

method employed, with the aim of eliminating the valve throttleresistance. Decreasing the pressure difference between the supplyand the return lines by means of the extra head lift that the boosterpumps provided made it possible to reduce the energy consump-tion in DHS. Relative to the CCCP DHS, the main circulating pumpfor the most adverse loop pipe network is replaced by the mostfavorable circulatory system in DVFSP DHS. Some booster pumpinstallations substituting for regulating valves are added in otheruser segments. Compared with the conventional heating pipe net-work of centralized power system, the district heating system withdistributed variable-frequency pumps is provided with an appro-priate node in the pipe network as the pressure difference controlpoint; therefore, the main circulating pump head only overcomesthe resistance from the heat source to the pressure difference con-trol point, while the respective pumps take charge of the pressurehead from respective users. The pipeline network differential pres-sure control point is the position where the pressure differencebetween supply water and return water is zero. With the contin-uous improvement of the new regulating equipment and controls,variable- speed booster pump is used in DHS. In theory, regulat-ing equipment can be eliminated from pipe network. This pointis fully reflected in DVFSP DHS. Thanks to the hydraulic connec-tor, a pipe connecting supply-water pipe and return-water pipe,the heat source loop is hydraulically independent. Under full-loadoperation, the flow rate in the hydraulic connector is nearly 0; whileunder part-load operation, the hydraulic connector is working as abypass pipe. A variable speed pump is installed at each substationand is used to provide the required hydraulic head for each loop.As there is a pump at each substation, no central circulating pumpsand control valves are required (Fig. 2).

In other words, a pressure difference control point which is setto be in network of an appropriate node, to which the heads ofmain circulating pumps only meet the needs of heat source. Andeach of the user’s depends on their respective booster pump. Thepressure difference control point is the position where the pressuredifference between supply water and return water is zero. It is alsocalled almost zero pressure difference. DVFSP DHS is implementedby multi-pump system. Whether design or operation, the flow and

head of booster pump must be accurately calculated and controlled.Otherwise, it is difficult to achieve ideal conditions. To this pur-pose, various forms of circulating pump must be designed for the

296 X. Sheng, L. Duanmu / Energy and Buildings 118 (2016) 291–300

t cost

fs

N

wkhtf�am

rbwpa

N

wf

Dh

z

e

z

Fig. 8. The investmen

requency conversion pumps. To CCCP district heating system, thehaft power of the heat cycle pump is:

= 2.72QH

�= 2.72

�(

n−1∑

i=1

qyi+qyn)(�Rs + �Ryn)2n∑

j=1

�Rpj) (6)

here N is the shaft power of traditional heat source cycle pump,W, Q is the flow of the heat source cycle pump, m3/h, H is the lift ofeat source cycle pump, n is the total number of heat users; qyn ishe most unfavorable design flow, m3/h; qyi is the i-th connectionrom the flow, m3/h; �Rs is heat source internal pressure loss, m,

Ryi is the i-th user’s working head, m, �Ryn is the most unfavor-ble working head, m, �Rpj is the j-th section of the pressure drop,.In DVFSP DHS, the influences caused by the ranges of the internal

esistance of the main heat pipe, heat pressure drop, user num-ers,heat user’s working pressure and the rules followed by whichill be provide as a reference for energy saving design. The totalower of the distributed variable frequency pump can be writtens:

0 = 2.72�

[n∑

i=1

qyi�Rs+n∑

qyi

i=1

(2i∑

j=1

Rpj + �Ryi)] (7)

here N0 is the total power consumption of distributed variablerequency pump, kW;

Respecting the Eqs. (6) and (7), the energy saving rate of theVFSP district heating system distinguished from the CCCP districteating system can be written as:

= N − N0N

=

n−1∑

i=1

qyi(�Ryn − �Ryi)+2n∑

j=1

�Rpj

j-i∑

i=1

qyi)

n∑

i=1

qyi(�Rs + �Ryn)2n∑

j=1

�Rpj)

(8)

We assume that the quantity of heat for every consumer is

quivalent. The Eq. (8) can be simplified as

= N − N0

N= (n − 1)�Rpj

(�Rs + �Ryi+2n�Rpj)(9)

s of pressure pumps.

Taking n as a independent variables of the function, other influ-encing factors constant, we have the partial derivative form of theEq. (9) as follows:

∂z

∂n= �Rpj�Rs+�Ryi+2�Rpj)

(�Rs + �Ryi+2n�Rpj)2

(10)

∂z/∂n > 0 Since the partial derivative of the function larger thanzero, the energy-saving rate will go higher following the increasingof n, the heat user number, but its increasing amplitude will be inthe decline.

Taking �Rpj as an independent variables of the function, otherinfluencing factors constant, the partial derivative form of the Eq.(9) is as follows:

∂z

∂�Rpj= (n − 1)(�Rs+�Ryi)

(�Rs + �Ryi+2n�Rpj)2

(11)

∂z/∂�Rpj > 0 Since the partial derivative of the function larger thanzero, the energy-saving rate will go higher following the increas-ing of Rpj, the pressure drop of the main pipe segment, but itsincreasing amplitude will be in the decline.

It is concluded that the CCCP district heating system with moreheat users and larger specific frictional resistance of the networkmain line is for better energy saving to turn into the DVFSP districtheating system.

3. Method

A branched network layout is used in the distribution networksof DH systems as well as in draining and irrigation systems. Suchlayouts are also observable in natural objects such as blood vesselsand trees [30,31]. Branched (also known as tree- like) DH networksare formed in layouts permitting a unidirectional flow from theheat source to the end-consumers (Fig. 4). In a layout of this typethere are only two pipe segments connected to each interior nodeand a unidirectional flow from the root node (the node without anypreceding nodes—i.e., the heat source) toward the leaf nodes (thenodes without any successor nodes—i.e. the end consumers) [32].This enables there to be a descending succession of pipe diame-ters from the heat source to the end consumers, larger diameters

being followed by smaller diameters, therefore in such a branchednetwork layout, the topographical configuration of the DH networkis determined in Dalian, China. The meteorological parameters inDalian, China are that the outdoor temperature for heating is −11 ◦C

and B

apeotssai(hn

a

wamvid

�

S

wrKw

l

l

�

l

w

[

P

wv

)(n)

]

Pm−

Pm−

m) ][

X. Sheng, L. Duanmu / Energy

nd the average outdoor temperature is −1.5 ◦C during the heatingeriod of l52 days. At a heating zone in Dalian, heating area cov-rs 14 km2, including the floor area of 13,974,190 m2 and a totalf 42 thermal stations. To obtain design cases, maximum supplyemperature at the heat source and return temperature at the con-umer’s thermal stations were assumed to be known [33]. Heatource provides 120/70 ◦C for the supply and return water temper-tures. Heat load forecasting method is used for the area index thats 50 W/m2 satisfying the “Code for Design of Urban Heat Network”GJJ34-2002). Pump efficiency (�) is 70%. The internal resistance ofeat source is 20 m H2O and the substation is 10 m H2O. The pipeetwork diagram is shown in Figs. 3 and 4.

Where solid yellow square is heat source, solid purple squaresre thermal stations.

The computational tool is a commercial software package,hich is used to design and analyze DH networks in many energy

nd design companies. It computes temperatures, pressures, andass flows at nodal points of the network as a function of input

alues such as the outdoor temperature, heating demands of build-ngs, and the network structure. The fluid flow state of DHS is in therag square area. So, in DVFSP DHS,

P = SG2 (12)

= 7.02 × 10−3 K0.25

�d5.25(l + ld) (13)

here �P is pressure drop in pipe sections of supply and returnespectively, G is mass flow rate, �P is the resistance of pipe,

= 0.005m, S is the resistance friction factor, � is the density ofater, l is the pipe length, ld is the local resistance equivalent

ength (m),

d = �d

�(14)

= 0.11(K

d) (15)

d = 9.1d1.25

K0.25

∑� (16)

here � is local drag coefficient, � is the friction factor.The pump power in kW was calculated using the following Eq.

34]

[

�P1 − H1

�P2 − H2

· · ·�Pm−n − Hk−n

] = −[

Bf 1(m−n+3) Bf 1(m−n+4) · · · Bf 1(n)

Bf 2(m−n+3) Bf 2(m−n+4) · · · Bf 2(n)

· · · · · · · · · · · ·Bf(m−n)(m−n+3) Bf (m−n)(p−n+4) · · · Bf (m−n

{

H1 = �P1 + [ Bf 1(m−n+3) Bf 1(m−n+4) · · · Bf 1(m) ][ �Pm−n+3 �

H2 = �P2 + [ Bf 2(m−n+3) Bf 2(m−n+4) · · · Bf 2(m) ][ �Pm−n+3 �

· · ·Hm−n = �Pm−n + [ Bf (m−n)(m−n+3) Bf (m−n)(m−n+4) · · · Bf (m−n)(

i = Gi × Hi

367�(17)

here Pi, Gi, Hi are the power, flow, lift of i-th pump, 367 is con-ersion coefficient.

uildings 118 (2016) 291–300 297

District heating network is a kind of fluid network which similarto electric network. According to the logical network diagram andthe second Kirchhoff’s law, the equation can be written

Bf × �H = 0 (18)

where Bf is the basic circuit matrix of the heat-supply network;Bf is an (m − n) × m order matrix. H is the differential pressurecolumn vector of each pipe in the heat-supply network, [�H1,�H2,. . .�Hm] T

Partition the basic loop pressure balance equation (Bf × �H = 0),and get �H = [�Hl, �Ht], Bf = [Bfl, Bft] = [I, Bft]. In the equation,�Hl corresponds to the column matrix of the pressure loss of theremainder of the tree; �Ht corresponds to the column matrix of thepressure loss of the branches; Bfl corresponds to the column matrixof remainder of the tree, an m-n order unit matrix; Bfl correspondsto the column matrix of the branches, an (m − n) × n order matrix.

So,Bf × �H = [Bfl,Bft][�Hl, �Ht]T

= [I,Bft][�Hl,�Ht]T = �Hl + Bft�Ht = 0

And Eq. (19) is got:

�Hl = −Bft�Ht (19)

Presume�H = �P − H

where �P is the resistance of each branch, an m order columnmatrix; H is the lift provided by power plant of each branch, anm order column matrix

[�H = �P − H]1 = −Bft[�H = �P − H]t(20)

It is got

[

�Pm−n+3 − Hm−n+3

�Pm−n+4 − Hm−n+4

· · ·�Pn − Hn

] (21)

By Eq. (21), in order to make pipeline run under the lowestenergy consumption condition, circulating pump is installed onlyin thermal station and heat source branch pipe or only in supplyand return main pipe. The pumps installed onto the pipe of heatsource and onto each thermal station branch are adopted. So, Hi = 0(i = m − n + 3∼n) Eq. (21) is to be written in the following Eq. (22)

n+4 · · · �Pn ]T

n+4 · · · �Pn ]T

�Pm−n+3 �Pm−n+4 · · · �Pm ]T

(22)

P =

m−n+2∑

i=1

Gi×Hi

367�(23)

4. Result

The calculated parameters of pumps are shown in Table 1The delay curve of heat load is analyzed based on the outdoor

temperature for heating during nearly a decade in Dalian (Fig. 5).The conclusion is obtained that during the heating period of

3684 h, the duration for outdoor temperature below −10 ◦C is 178 h,the duration for −10 ◦C and −5 ◦C is 533 h, the duration for −5 ◦C

298 X. Sheng, L. Duanmu / Energy and Buildings 118 (2016) 291–300

Table 1Configuration of circulating pumps in DVFSP DHS.

Serial number Substation andheat source

Heating area(104 m2)

Power ofpumps (kW)

Needed flow ofpumps (t/h)

Needed head ofpumps (m)

The number ofpumps(quantity)

The annualelectricity ofpumps (kWh)

1 T1 31 9 293 8 2 183842 T2 26.62791 12 251 13 2 220133 T3 23.76744 20 224 24 2 272594 T4 25.23256 19 238 22 2 274915 T5 19.7093 17 186 25 2 232096 T6 29.38372 29 278 29 2 381257 T7 38.06977 45 360 34 2 549328 T8 29.13953 39 275 39 2 461869 T9 34.06977 56 322 48 2 63035

10 T10 25.40698 42 240 48 2 4700411 T11 48.83721 89 460 53 2 9719812 T12 50 99 470 58 2 10657313 T13 37.63953 18 356 14 2 3232114 T14 35.46512 29 335 24 2 4076615 T15 48.37209 65 457 39 2 7679416 T16 51.62791 80 488 42 2 8852717 T17 30.93023 48 292 45 2 5448118 T18 18.37209 25 173 37 2 2800219 T19 27.7907 40 263 39 2 4419520 T20 28.32558 45 268 43 2 4834721 T21 39.53488 60 374 41 2 6515822 T22 24.93023 6 235 7 2 1290123 T23 29.4186 18 278 15 2 2609924 T24 21.9186 16 206 19 2 2188525 T25 35.0814 31 330 23 2 3913826 T26 25.69767 24 246 24 2 2993627 T27 27.03488 34 255 34 2 3891028 T28 23.22093 33 220 38 2 3628929 T29 20.23256 27 191 36 2 3032530 T30 31.74419 46 300 39 2 5041231 T31 21.33721 12 212 15 2 1990332 T32 22.90698 19 217 22 2 2506633 T33 49.12791 45 464 25 2 5789834 T34 44.94186 45 425 27 2 5565835 T35 46.10465 56 436 33 2 6518236 T36 37.55814 53 355 40 2 6075137 T37 44.30233 62 419 40 2 7170338 T38 48.95349 57 421 35 2 7198539 T39 38.83721 79 462 44 2 9324940 T40 53.72093 66 334 51 2 75361

af

w(d

G

Ttsn

5

i

41 T41 23.37209 41

42 T42 27.67442 53

43 Heat source circulating pump 1397.419 1637

nd 0 ◦C is 1035 h, the duration for 0–5 ◦C is 1,178 h and the durationor outdoor temperature above 5 ◦C is 760 h.

Variable flow regulated in DVFSP DHS, According to Eq. (24)

qj

qn= tn − tw

tn − t,w

(24)

here qj is operated heat load (kW/m2), tw is outdoor temperature◦C), and qn is design operated heat load (kW/m2). t,

w is outdooresign temperature that is calculated (◦C).

Table 2 is got

i = qj

c�T(25)

he annual electricity consumption of pumps in DVFSP DHS is equalo 4,550,931 kWh. It is got that the average pumps power con-umption of heating area per square meter in primary heat supplyetwork is 0.325 kWh/m2 (Fig. 6).

. Economic analysis

The capital costs include pipe investment costs and pumpnvestment costs. Pipe investment costs consist of:

201 53 2 46178238 54 2 56127

12017.8 35 4 2515975

• The price of pre-insulated steel pipes including fittings, site jointsand termination seals: this is based on a price list obtained fromthe market price in Dalian;

• The cost of civil works: this depends on the pipe size, groundcondition and method of digging. The ground condition and dig-ging type were assumed to be the same for all pipes. Civil workcosts were assumed according to the pipe size obtained from themarket price in Dalian.

• Pump investment costs: these consist of costs for 88 pumps. Itwas assumed that the pumps were equipped with variable speeddrives. Prices of the pumps and variable speed drives were takenfrom the market price in Dalian.

Operating costs of a DH network: pumping costs (costs of elec-tricity consumption) were taken into consideration.

In conventional scheme, the flow of heat circulating pump is12,017.8 m3 and the pressure head is 100 m H2O (Table 3).

The annual electricity of the circulating pump is

P =∑ GiHi × n (25)

367� i

where Gi is the flow of the total system (m3/h), Hi is the head ofcirculating pump (m H2O), and ni is the duration of heating (h).

X. Sheng, L. Duanmu / Energy and Buildings 118 (2016) 291–300 299

Table 2Unit heat load with outdoor temperature.

Serial number Outdoor temperature (◦C) Indoor temperature (◦C) Unit heat load (W/m2) Relative heat load The total heat load (MW)

1 −11 18 50 1 698.72 −10 18 48.3 0.966 674.93 −5 18 39.65 0.793 554.14 0 18 31.05 0.621 433.95 5 18 22.4 0.448 313

Table 3Variable water flow regulated in stages.

Serial number Outdoor temperature (◦C) Indoor temperature (◦C) Relative flow The duration of heating hour

w8

orsCtetDAepFtcDCsDbpsw

6

Diftflvtrtdg

rtfdetfi

1 Below −5 18

2 −5 to 0 18

3 Above 0 18

The annual electricity consumption of the circulation pumpith variable flow regulated in stages in CCCP DHS is equal to

,996,296 kWh.In DVFSP DHS, it was shown that for the design cases based

n DVFSP DHS max temperature difference between supply andeturn pipes: 120/70 ◦C, the minimum annual total energy con-umption when using the variable flow rate compared with theCCP DHS methods. From the results given in Fig. 7 it can be seenhat the design case was the design with minimum annual totalnergy consumption for investigated type. Similarly, from Fig. 8he design case was found to be the increased most cost effectiveH design when pumps were considered with the DH network.

comparison of results shows that the major difference betweennergy efficient and cost effective DH design cases can be shownumps. For the DVFSP DHS and CCCP DHS, the difference seen inig. 8 is due to the pumps investment costs (260 × 104RMB) andhe cost of pump saving electricity in Fig. 7 (444 × 104RMB). Whenonducting an economic evaluation of the DVFSP DHS and CCCPHS, it was found that the optimal solution corresponds to pumps.omparison of DH design cases with DVFSP and different flow rateshowed that in order to achieve energy efficient and cost effectiveH design, it was more advantageous to reduce system flow ratey increasing temperature difference between supply and returnipes. By reducing system flow rate, the annual total energy con-umption was reduced. The investment increased in DVFSP DHSill be paid back within a year, compared with CCCP DHS.

. Conclusion

In this study, the energy efficiency of a new DH system calledVFSP is examined. By analyzing the pressure pump configuration,

t can be concluded that each heat user which has installed its ownrequency pressure pump has its own loop. A topology of DH sys-em in Dalian region was studied. The DVFSP DHS using variableow regulated is meant for the concept of pump technology. Here,ariable flow regulated of DH water in consumer substations is con-rolled by pumps with inverters to improve heat transfer. It willeplace the conventional network and control where water flow ishrottled by control valves. The new control system will enable out-oor temperature curves to be adopted for water flow. This methodives clear benefits for the DH systems considered hereof.

For the heat users, it will increase the comfort of living as aesult of the improved usability of the network. It allows consumerso adjust electricity consumption more easily with rapid feedbackrom outdoor temperatures. Variable frequency control can more

irectly reflect the electricity consumption of the building and beasier to manage. Meanwhile, it can be conducive to avoiding thehermal imbalance system and greatly improve economic bene-t in heating companies. On the producer’s side, the benefits are

1 12440.75 10350.5 1938

mainly economic. Variable frequency control allows smaller pres-sure drops in the network, thus reducing the pumping power. Theaim of this study is to determine the most energy-efficient DHvariable frequency control in the network. If the DVFSP design isutilized, it will be easier to achieve the variable frequency control.As a result, the total heat consumption during the heating season isreduced compared to CCCP DH systems. On the basis of the results,the variable frequency control is significantly more energy-efficientin the examined DVFSP compared with the variable flow regulatedin CCCP. The average electrical energy saved for the case in Dalianwithin the pumping energy is 49.41% compared with conventionalcentral circulating pump. The average pumps power consumptionof heating area per square meter in primary heat supply networkis 0.325 kWh/m2. Increasing temperature difference between sup-ply and return pipes by frequency conversion control, the annualtotal electricity consumption and the equivalent annual cost willbe further reduced. The investment increased costs in DVFSP DHSwill be paid back within a year, compared with CCCP DHS. Thisfully demonstrates that network used DVFSP technology has greatpotential in energy efficient.

Acknowledgments

This research has been conducted under The National 12thFive-Year Plan of Science and Technology Support Program “Theresearch and demonstration of comprehensive building energy sys-tem based on heat pump”(2014BAJ01B04)supported by the Scienceand Technology Ministry of China.

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