Improving the Efficiency Of

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Article

Improving the efficiency of anair conditioning system usinga fire water tank as thermalaccumulator

CJ Renedo, A Ortiz, S Perez, F Delgado, I Fernandezand J Carcedo

Abstract

This paper studies the substitution of air condensation by water condensation in the chiller of a university

building. This building has a surface of about 30,000m2, is located in a city of the Spanish north coast, and

refrigeration is needed for six months a year. In this paper, the novelty lies in the cooling system of the

condensation water, since the typical cooling tower is not going to be considered; instead, the domestic

cold water consumed in the building would be used to cool the chiller condensing water. As the domestic

water consumption in the building is neither constant nor coincident with the cooling requirements of the

chiller, the use of the fire water tank as heat storage was proposed. Thus, the chiller is cooled by the

water contained in the fire tank and this fire water is then cooled by the domestic water. With this

solution, the reduction in the annual energy consumption of the chiller is around 18.6%. The requirements

to implement this solution in buildings are: cool water for air conditioning produced in a centralized way, a

relatively high building domestic water demand and a building fire water tank.

Practical application:The solution proposed in this paper can contribute to primary energy savings by

improving the energy efficiency of HVAC systems. It can be adopted in all buildings that require refriger-

ation, have a significant consumption of domestic water, and have a water storage tank for fire or other

uses. The article includes the preliminary design of a waterwater heat exchanger as a possible solution

for cooling the chiller, but alternative solutions could be adopted. Finally, a cost-benefit analysis has been

performed, showing that the proposed solution is feasible.

Keywords

Air conditioning, chiller, water condensation, energy efficiency, thermal storage

Introduction

The energy consumption in buildings hasincreased considerably in recent years fordeveloping and developed countries. Theimprovement in quality of life and comfort

Electrical and Energy Engineering Department, University of

Cantabria, Santander, Spain

Corresponding author:

CJ Renedo, Electrical and Energy Engineering Department,

University of Cantabria, E.T.S.I.I. y T, Avda. de los Castros,

39005 Santander, Spain.

Email: [email protected]

Building Serv. Eng. Res. Technol.

0(0) 120

! The Chartered Institution of BuildingServices Engineers 2014

DOI: 10.1177/0143624414555812

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requirements of buildings users has led the energyconsumed in the building sector to reach signi-cant values, regarding national energy consump-tion.1 In particular, in the European Union, thisvalue reaches 40%.2 In buildings, energy is con-sumed primarily to power the following systems:lighting, transportation, appliances and com-puters, domestic hot water, heating and air con-ditioning. Depending on the usage prole and theweather, the latter service can represent morethan 25%of the annual energy consumed in com-mercial buildings, where energy consumption inthis system is mainly for3 cold water productionand uid movement through the building (waterand/or air), with the majority of consumption inwater chillers.

The energy consumption of air conditioningsystems in buildings and the percentages overthe national energy consumption vary fromone country to another.1 In Spain, accordingto the Spanish Ministry of Industry,4 theenergy consumption of air conditioning systemsaccount for approximately 2.5% of nationalenergy consumption, Figure 1.

The reduction of the energy consumption ofair conditioning in buildings has been

extensively studied.5,6 Currently, this reductioncan be carried out mainly with three techniques:reducing the heat demand of the building,recovering energy, and using more ecientHVAC systems. In these elds, multiple researchworks have been conducted.

In the eld of thermal demand reduction inbuildings, two main groups of solutions can behighlighted: (1) the use of systems that allow thereduction of the heat input associated to solarradiation: solar shades, venetian or verticalblinds,7 outer vegetation,8 and more recently,smart windows;9 and (2) the use of water evap-oration systems outside the building, so as toremove part of the thermal load before enteringthe building; in this category, several possibili-ties has been studied: spray systems,10 water lmon glazed facades,11 and evaporative walls.12

In the eld of heat recovery, studies consider-ing many types of heat exchangers were per-formed, such as: xed-plate, heat pipes,13 orrotary wheel; a review of these works can befound in Mardiana-Idayu and Riat.14 In add-ition, new techniques such as the use of the heatreleased in the condensation of the chiller toproduce domestic hot water,15 or the use of the

40% Transport 31% Industrial 24% Buildings 5% Agriculture

38% Services

33% Offices

11% Hospitals

22% Commercial

30% Restaurants

4% Educational16% Others

28% Lighting

5% Domestic Hot Water

26% Air conditioning

25% Heating46% Heating

62% Residential

27% Others

7% Lighting

20% Domestic Hot Water

Spanish Energy Consumption

Figure 1. Spanish energy consume in buildings.

2 Journal of Building Services Engineering Research & Technology 0(0)








extraction air to cool the condenser of thechiller,16 have been also explored.

In order to increase the eciency of the airconditioning system, several concepts have beenstudied, such as free cooling17,18 and free coolingcoupled with heat storage systems;19 nightventilation;20 using evaporative cooling;2123

coupling the buildings with earth heat exchan-gers;22,24,25 employing chillers with: two com-pressors,26 variable-speed compressor,27

electronic expansion valve,27,28 variable refriger-ant ow systems,24,29,30 or with new refriger-ants;31 using absorption or adsorption chillers,either driven by solar energy3234 or wasteheat;35,36 replacement of dry cooling towers byevaporative cooling towers;3739 improvement ofcontrol systems;4044 improvement of chillercondensation using swimming pools as heatlinks,45 or even using aquifers as thermal stor-age.46 In a recent review work,47 dierentaspects that contribute to the energy eciencyon air conditioning systems were analyzed: novelcooling devices, innovative system designs andintegration, and operational management andcontrol.

It is dicult to compare the improvementsachieved with the aforementioned systems; thisis mainly due to three reasons: studies have beenperformed on dierent types of buildings withdierent usage proles, the weather of theareas in which they are located are not similar,and eciency parameters used are not the same.For example, in some cases, improvements inperformance of the chiller or the compressorare proposed, whereas in others the reductionof the thermal load, whether the total buildingor ventilation only, is considered. However,Table 1, in which the improvements obtainedwith the previously discussed systems oer, ispresented.

There are other proposals which, although donot always produce energy savings, contributeto reduce the cost of the energy consumed. Inthis line, thermal storage systems have been stu-died,48 typically associated with cooling nightproduction, where the electric rate is morefavorable.49

This work focuses on improving the eciencyof the system by increasing the chiller perform-ance. This can use water or air condensation, therst of them has better performance.50 This isbecause: (1) the cooling of the machine is carriedout at lower temperatures, as in summer thewater temperature is lower than the air tempera-ture, and (2) the required temperature dieren-tial in the condenser between the refrigerant andheat-absorbing medium can be lower, since thewater convection coecient is several timeshigher than the one of the air.51 Given theabove, the dierence between condensationand evaporation temperatures of coolant islower in water condensing chillers, and thereforethese machines have better coecient of per-formance (COP), equation (1)50

COP Usefull Refrigerating EffectNet Energy Applied

1

The problem with the cooling of condenserswith water is the need for sucient ow of coolwater at the time in which building coolingdemand (CD) occurs. This is usually solved byinstalling a closed circuit for the cooling of con-densation water in cooling towers, and it can beclassied according to: closed or open, andevaporative or dry.37,38 The evaporative typeshows better performance, but has higher main-tenance because of Legionella problems.38 Theinstallation of this auxiliary cooling circuit forthe condenser presents a performance thatreduces the potential energy improvement inthe facility, and in addition represents anextra cost.

This work follows the same line of Woolleyet al.,45 which studies the use of swimmingpools as sinks for the reject heat in the conden-sation; however, public buildings usually do nothave this type of facility. For this goal, air con-densation in the chiller will be substituted bywater condensation without a cooling tower.In this scheme, the domestic water demand(TWD) is used to cool the chiller. This has thedisadvantage that the water consumption ofbuildings is neither constant nor coincident in

Renedo et al. 3









Table 1. Achieved improvements with different proposals applied to HVAC systems.

Studied system Compared with Improvement Reference

Lighting and shading control No control Thirty-one percent reduction in

total secondary energy use

(lighting, heating, and cooling)

7

Electro chromic window Window without and with fully

lowered blinds

Daily cooling loads due to solar

heat gains 8% and 3%

9

Electro chromic window Well-tuned daylighting control by

blinds

Twenty percent of the peak

cooling loads

9

Split-facade electro chromic

window

Shaded reference case Cooling loads due to solar heat

gains by 11%

9

Water film on the glazed facade Conventional system Decreasing indoor temperature

by 2.24.1C

11

Evaporative cooling outside the

building

Conventional system Outdoor surfaces temperatures

are reduce 2C during thenight and 5C during the hot-test hours

12

Heat recovery with heat pipes Conventional air conditioning

system

Effectiveness 3040% 13

Heat recovery with heat pipes Conventional air conditioning

system

Thermal efficiency of heat pipes

is between 45% and 55%

14

Heat recovery with fixed-plate

heat exchanger

Conventional air conditioning

system

Effectiveness was found to

approximately 75% for sens-

ible and 60% for latent

14

Heat recovery with rotary wheel

heat exchanger


system

Temperature efficiencies above

80%

14

Heat recovery with run-around

heat recovery system


system

Thermal efficiency is normally

from 45% to 65%.

14

Heat recovery with a heat pump

in exhaust air

Air conditioning system with

heat recovery system having

80% or 60% energy recovery

efficiency

Energy efficiency in a residential

building can be improved up

to 67% or 41%

14

Heat recovery to a water-cooling

condensing module

Air condensing system with a

COP between 2.1 and 3.18

COP can be as high as about 6.0

(space-cooling and sanitary

water heating mode)

15

Presence of outdoor trees and

vegetation

Wind reduction up to 50% and

air infiltration reduction up to

60%

8

Ventilation demand control with

CO2-sensor combined with

displacement ventilation

Reduction of the total heating

energy demand of up to 21%

6

Stratum ventilation Displacement ventilation and

mixing ventilation

Energy saving can be improved

more than 25% and 44%

47

Night ventilation Energy savings of about 510%

and up to 40% in buildings

optimized for maximum

benefit from night ventilation

22

(continued)




Table 1. Continued.


Free cooling Conventional air conditioning

unit

Nine percent less electricity

demand

20

Free cooling with PCM Conventional air conditioning

unit

Ventilation load can be reduced

from 46% to 62%

20

Liquid-desiccant

dehumidification

Conventional to extraction of

the moisture of air

COP up to 20% 6

Scroll compressor Reciprocating compressor Ten percent energy efficiency 6

Two-stage vapor-injected scroll

compressor

Scroll compressor COP improves 4% 6

Revolving Vane (RV)

compressor

Current systems on the market Energy reduction as high as 80% 6

Compressor control Onoff control COP rise 0.9 26

Variable speed compressor and

electronic expansion valve

control

PID and Fuzzy controllers Lower power consumption of

8.1% and 6.6%

27

Water cooled VRF Fan-coil plus fresh air About 20% less power 30

Ground Heat Pump VRF Saves 9.4% and 24.1% 24

Evaporative cooling Conventional air conditioning

system operation of a 100%

outdoor air

About 2151% less energy

consumption

21

Evaporative cooling Conventional air conditioning

system

About 625% less annual cooling

coil load

21

Passive cooling dissipation with

earth to air heat exchangers

Compared to the ambient

temperature

Air circulated through the pipes

decreased its temperature

between 5 and 10C

22


indirect evaporative cooling

Conventional system Sixty percent power savings 22


indirect/direct evaporative

cooling

Conventional system About 1625% less annual cool-

ing coil load

22


indirect/direct evaporative

cooling

Mechanical vapor compression

systems

Up to 80% energy savings 22

Central chilling system using

optimal control settings

Existing operation About 4.545.06% daily electri-

city savings

44

Smart chiller sequencing Existing operation Up to 5.34% daily electricity

savings

6,47

System control with artificial

neural networks and genetic

algorithms

Existing operation Electricity savings, 2% during the

warmest days and up to 13%

during the transition period

6

Water pump control strategy Conventional control strategy of

pumping system

Can save about 26.6749.52% of

the secondary chilled water

pump energy. It represents

1232% of total pump energy,

and 1.282.63% of the total

electric energy

42

(continued)

Renedo et al. 5



time with the cooling requirements of the chil-ler.52 In order to harmonize this temporal mis-match (batch process),53 the re water tank isgoing to be used as an intermediate heat storage.This article includes an analysis of the system,showing 18.6% reduction in the annual energyconsumption of the chiller. A sensitivity analysishas been performed that takes into account fouraspects: (1) variation in water demand (WD) ofthe building, (2) variation of the supply watertemperature, (3) change in distribution tempera-ture of domestic water (TW) inside the building,and (4) increased CD.

Case study

The building studied is located in Santander,Spanish city located on the coast ofCantabrian Sea. The prevailing weather in thecity is shown in Table 2,5456 and the buildingneeds cooling from May to October. The

building was built in 1997, belongs to theUniversity of Cantabria, and has a footprint ofapproximately 9500m2, with a total of 30,000m2

distributed over ve oors. It has three blocks,the east and the west are occupied by Law andEconomics faculties, respectively (classrooms,oces, meetings rooms, etc.), and the centralone that is devoted to common services ofboth faculties (cafeteria, restaurant, conferencerooms, library, etc.). Inside the east and westblocks there are small enclosed atriums with askylight, with approximately 80m2 in eachblock. Between the Law and Economics facultiesand the central block, there are two moreenclosed atriums with a skylight of 400m2 eachone. The two lower oors are smaller than theupper ones since the building is adapted to themountainous topography of the terrain,Figure 2. The building is open between 8:00a.m. and 9:00 p.m., and every day 4000 people(teachers, researchers, trainees, support and

Table 1. Continued.


Energy efficient heat/mass

exchangers


system

Energy saving associated with

cooling the outside fresh air

saving spanned 5256%

47

Change refrigerant to R290 R22 chiller Reduction in power consump-

tion by 14%

28

Change refrigerant to R407C Window air conditioner with

R22

COP is improved in the range of

8.0%

31

Combining variable air volume-

based chilled water air condi-

tioning system with thermal

energy storage under demand

controlled combined with the

economizer cycle ventilation

Conventional chilled water-

based A/C system

The system is capable of achiev-

ing 28% and 47%

6

Swimming pools as heat sinks Standard residential air

conditioning

Could save approximately 30% of

overall cooling energy

45

Aquifer thermal storage Conventional system (COP 2.67) COP could get 4.18 46

Variable Refrigerant Flow (VRF) Conventional system Thirty-five percent less energy 30

Multi-split VRF Variable air volume system Saved more than 20% energy 30

Multi-split VRF Fan-coil plus fresh air system Saved more than 10% energy 30

Multi-split VRV system with the

heat recovery ventilation units

Continuous indoor fan operation

without economizer

About 1728% energy savings 30






90 mN

110

m

Facultyof Law

Faculty ofEconomics

Figure 2. Plan and elevation of the building.

Table 2. Santander climate data.

Latitude Longitude Elevation

Heating Cooling

Dry bulb Dry bulb Wet bulb Dry bulb Wet bulb Dry bulb Wet bulb

43.47N 3.82 W 64m 99.6% 99% 0.4% 1% 2%

2.3 4 26.5 19.5 24.7 19.4 23.7 19

Mean monthly air temperature (C)May June July August September October

14.6 17.1 19.4 19.9 18.3 15.4

Daily mean maximum temperature (C)18.5 20.8 23.1 23.7 22.5 19.6

Daily mean minimum temperature (C)10.7 13.4 15.6 16.1 14.1 11.3

Monthly rainfall (mm)

89 62 52 72 85 135

Relative humidity (%)

75 76 78 78 78 77

Sun hours per month

169 174 189 182 157 127

Diurnal temperature variation (C)7.8 7.4 7.5 7.6 8.4 8.3

Renedo et al. 7



services sta, and students) can be insideintermittently.

In the blocks corresponding to the two facul-ties, there are classes both in the morning and inthe afternoon. Annually, classrooms are highlyoccupied during both terms, starting from 1September and nishing 15 July, and they arevirtually empty the rest of the year, becausethe teaching is reduced only to some summercourses.

In the faculties, there are some areas, such ascomputer rooms, that need cooling the wholeyear, and they are equipped with unitary air con-ditioners. This part of the system, because of itslow power and its dispersion in the building, hasnot been considered in this work.

In the central block, the air conditioning isutilized primarily to condition the library. Thisspace is occupied throughout the morning, suf-fering a decline in occupancy around lunchtime(around 2:00 p.m. in Spain), which againincreases from 3:30 p.m. The annual occupancyis greatest just before and during examinationperiods (15 January to 28 February, 15 May to30 June and from 20 August to 15 September),and it is reduced the following month to theseperiods.

The cooling system of the building demandscold water at 712C, which is produced in achiller. The main characteristics of the chillerare: the refrigerant is 407C; the compressor isdriven by a natural gas (NG) engine; the evap-orator is a tubular and shell heat exchanger; andthe air-cooled condenser is comprised of vefans consuming 11 kW of power.57

The water consumption in the building meetsthe demands of toilets, general cleaning services,and cleaning of cafeteria and restaurant. Itoccurs mainly on working days, and coincidingwith certain demand peak periods where thereare rest times for students between the changesof the dierent classes.

Table 3 provides the data supplied by theFacilities Service of the University with the aver-age natural gas consumption (NGC) of the chil-ler compressor over the past ve years, andTWD.58 This includes the working days of themonth and the TW inlet temperature in thebuilding from the city network (TTW in).

59

The building has a re water tank constructedof concrete, with a capacity of approximately50m3 (7.5 4.5 1.5m). This is open, and ishoused in a room with exterior openings for nat-ural ventilation of the room. The water enteringthe tank is controlled by a oat, which is respon-sible for maintaining a constant level of water inthe tank.

Proposed facility

For the design of the facility, the substitution ofair condensation by water condensation in thechiller has been studied; water condensationbeing energetically more favorable since it pro-vides a higher COP by lowering the condensa-tion temperature of the refrigerant uid.

The scheme of the new system is shown inFigure 3. In order to undertake this design, itis necessary (1) to install in the chiller a watercondenser in parallel with the current one of air,

Table 3. Natural gas consumed by the chiller, water demand, and water inlet temperature in the period of building

cooling.

May June July August September October

Working days 22 21 22 22 21 22

NGC (Nm3) Month 459.8 907.7 1755.3 1429.7 1592.7 770.3

Day 20.9 43.2 79.8 65.0 75.8 35.0

TWD (m3) Month 2140 2179 2320 1940 2498 2113

Day 97.3 103.8 105.5 88.2 119.0 96.0

TTW.in (C) 13.0 15.0 16.0 16.0 16.0 14.0





(2) to install a closed-loop water system (CLW)for cooling the condenser, and this loop con-nects the condenser with the re water tankand consists of pipes, pump, valves, lters, etc.,(3) to build and place a waterwater heatexchanger to cool the water in the tank withthe domestic water consumed by the building,

and (4) to install a water cooler to serve, if neces-sary, as auxiliary cooling of the tank.

The new waterwater exchanger could be asimple bundle of tubes immersed in the tankand for these tubes circulate the water consumedby the building, Figure 4. The urban networksupplies water from a reservoir located about

Fire Water Tank (50 m3)C.L.W.

C.L.W. C.L.W.

C.L.W.

C.L.W.

7.5 m

1.5

m

4.5 m

C.L.W.

Net Water to Building


Net Water to BuildingNet Water to Building

Figure 4. Scheme of the immersion heat exchanger in the fire water tank.

Current SystemNew Condensation System

Compressor

Expansor

FireWaterTank

(50 m3)

Evap

orat

or

Chille

d W

ater

Clos

ed-L

oop

Wat

er

Bypa

ss

Pump

AirCondenser

WaterCondenser

Water distributedinto the Building

AuxiliaryWatercooler


Figure 3. Design scheme of the proposed system.

Renedo et al. 9



40m above the level of the building, and there-fore the incoming water has sucient pressureto overcome the pressure losses caused by theinstallation of the heat exchanger before of thebuilding water network. In the case that a crackappears in the heat exchanger, the urban net-work pressure would cause leakage from the net-work water into the tank, therefore the domesticwater distributed in the building will be neverpolluted.

This heat exchanger can be sized using someof the numerous methods for calculating theheat transfer coecient. In this case, the owinside the pipes will lead to forced convectionwhile the external ow will lead to naturalconvection.

The internal heat transfer coecient can bedetermined from Dittus-Boelter correlation.Taking into account de value of the Reynoldsnumber and the Prandtl number, the Nusseltnumber can be calculated and then the convect-ive coecient. In this case, hci is about 5103 W/(m2K). The external heat transfer coecient canbe estimated from the Rayleigh number; in thiscase, hce is about 5.5102 W/(m2K). Consideringthe thinness of the pipes (1mm) and the highthermal conductivity of the copper (395 W/(mK)), its thermal resistance is negligible com-pared to convection one.

Finally, the LMTD method (logarithmicmean temperature dierence) can be used todetermine the eective surface of the heatexchanger. In summary, the heat exchangershown in Figure 4 would consist of two layersof 120 copper pipes 6/8mm diameter (inside/outside) and 4m long. These results show thetechnical feasibility of the proposed solutionsince such a heat exchanger could be placedinto the existing re tank.

It could be also possible to use a plate heatexchanger, through which circulate the domesticwater and the tank water. However, the purposeof this paper is not the mechanical calculation ofthis element, but studying the technical feasibil-ity of the proposed system.

The water temperature in the tank must below enough in order to be available in case of

re without risk of cavitation in pumps, and alsoto prevent eventual Legionella problems. Forthis, three conditions must be fullled: (1) itshould not suer daily increments, so that ifthe heat accumulated by the condensation ofthe chiller exceeds heat that can be dailyremoved by the domestic water of the building,it would be necessary to cool the water on thetank, and this can be performed at night with thewater cooler, (2) it must not be exceed a certainvalue at any time of the day, in this case 30C isset, and if this value exceeds, the water conden-sation should be stopped and it should bereturned to condense with air, and (3) in theunlikely case of a re in the building, the chillercould work using the pre-existing condensationsystem.

Mathematical modeling

In this section, the hourly consumption of NG ismodeled considering the chiller is cooled usingthe re water tank. This requires knowing threeaspects:

. The hourly demand of air conditioning. It hasbeen determined taking into account: thehourly demand prole, the NG consumptionand the chiller COP in the current air-cooledsystem.

. The operating parameters of the chiller,which have been obtained from the technicaldata sheet of the machine.

. The re water temperature, that has been cal-culated taking into account the three follow-ing factors: The initial re water temperature,the heat supplied by the chiller condenser andthe heat removed by the domestic water usedin the building.

The inuence of gains and losses to the environ-ment has not been considered due to the factthat gains are negligible compared to losseswhen calculating the re water temperature.According to Table 2 and taking into accountthat the maximum expected re water tempera-ture is about 21C, gains from the environment




would be about 0.35 kW, whereas losses wouldbe about 2.08 kW. Gains are mainly due to con-vective heat transfer from air to water, whilelosses are mainly due to water evaporation,whose amount can be determined from Bernierequation. Therefore, not considering theseeects leads to a more conservative design.Finally, knowing the chiller new operating con-ditions with water condensation, the COP andthe NGC could be calculated.

Schedules of cooling demand (% CD(i)) andTWD must be rst established (Figure 5). Forthis reason, the daily data of NGC and TW havebeen collected, Table 2, and typical hourly pro-les of this type of buildings have been con-sidered,60,61 with the following exceptions: (1)in Spain it is usual to stop for lunch between2:00 p.m. and 3:30 p.m., and therefore at thistime heated spaces are relatively empty, signi-cantly lowering the CD, (2) the occupation ofthe building in the afternoon and evenings issignicantly lower than in the morning, and (3)at the end of the day the building is empty anddoes not require cooling; however, there is a

small water consumption demanded by thebuilding cleaning service.

The daily cooling demand, CD(d), has beendetermined considering: the hourly COP of theair-cooled chiller, COPa(i), the gas engine per-formance, NGC, Table 3, and NG net caloricvalue (NCVNG), equation (2); in which it hasbeen considered a value of NCVNG providedby the company, 10.66 kWh/Nm3,62 and a per-formance of the chiller gas engine, 0.335%.63

CDi COPai Gas Engine EffiencyNGCi NCVNG

CDd X24h1

CDi

2

The COPa(i) simulation has been performedwith the software DUPREX Version 3.2 con-sidering: (1) outside air temperature, which wascalculated with the average monthly tempera-ture and the average daily oscillation (Table 2),(2) temperature variations between the refriger-ant and the air in the condenser of 15C,64 and

15,0%

% Water

% Refrigeration

Porc

en

tage

of d

aily

dem

and

12,5%

10,0%

7,5%

5,0%

2,5%

0,0%

Hour of the Day

8 to

9

9 to

10

10 to

11

11 to

12

12 to

13

13 to

14

14 to

15

15 to

16

16 to

17

17 to

18

18 to

19

19 to

20

20 to

21

21 to

22

22 to

23

23 to

24

Figure 5. Profiles of daily demands for cooling and domestic water.

Renedo et al. 11



(3) temperature variations between the refriger-ant and the water cooled in the evaporatorof 7C.64

The adjustment of simulation parameters forCOP calculation, such as pressure losses in con-denser, evaporator and suction line, superheat inevaporator and in suction line, condenser sub-cooling, and compressor isoentropic eciency,was performed considering a COP in the chillerof 2.93, in nominal working conditions.57 Theseare: 251 kW of cooling production, a NGC of24Nm3/h, an air temperature in the condenserof 35C, and 12 and 7C as input and outputtemperatures of the cooled water. This agreeswith equation (3).

COPjNominal Cooling Capacity kWh

NGCNm3 NCVNGkWhNm3Gas Engine Efficiency

3

To calculate the CD(d) value, the %CD(i)value, Figure 5 has been taken into account.This is an iterative process that, consideringthe COPa(i), is conducted to verify that theNGC(d) obtained coincides with that of Table 3.

Then, the hourly cooling demand was estab-lished for the building, CD(i), equation (4). Forthis, CD(d) and %CD(i) have been considered,Figure 5.

CDi CDd %CDi 4

The NGC with air condensation has been cal-culated each hour, NGCa(i), consideringNGCa(d), Table 2; the %CD(i), Figure 5; andthe COP simulated hourly, COPa(i), equation (5).

NGCai NGCad %CDi COPaiP24i1%CDi COPai

5With the proposed facility, condensation is

performed with water, and in order to simulatethe new COP, COPw (i), the only variations

considered respect to the air condensationscheme have been: a temperature dierence inthe condenser between the water and therefrigerant of 7C,62 and a temperature dier-ence of 7C between the water in the tank andthe water inlet in the supply network of thebuilding, Table 3.

In this case, the condensation water is thecontent of the tank, and its temperature, TTank,is variable throughout the day, as it depends on:the initial temperature of the tank, the tempera-ture increment provided in the tank by the chil-ler condensation, TTCond, and the decrease intemperature experienced by the tank due to theheat removed by TWD, TTTW. Therefore, theCOPw (i), with the proposed installation dependson TTank at the end of the previous hour.

With regard to the tank water temperature atthe start of the day, TTank 0, it has been con-sidered equal to the one of the city watersupply network, Table 3, and this is because:(1) the room in which the tank is located haslarge openings to the outside, which favors theevaporation of the water contained in the tank,so there is natural evaporative cooling in thetank, (2) the water consumption performednightly by the cleaning service, and (3) naturalcooling occurs on non-working days and week-ends. The thermal losses in the water distribu-tion pipeline between the condenser and thestorage tank have not been considered, andthis eect contributes to reduce the heat inputto the tank, and hence to reduce TTank.

The TTank at the end of each hour, TTank (i), iscalculated considering: the temperature at theend of the previous hour, TTank (i1), the tempera-ture increment suered by the heat input of con-denser at that time,TTCond (i), and the decreasein temperature experienced in the tank, due to theheat removed by thewater consumed in the build-ing at that time, TTTW (i), equation (6).

TTank i TTank i1 TTCond i TTTW i6

The TTCond (i) must take into account theheat provided by the water condenser of the




chiller, QCond w (i), the volume of water in thetank, MW Tank, and the specic heat of water, cp,equation (7).

TTCond i C QCond wikJMw Tank kg cpkJ=kgC

7From the First Law for Thermodynamics

applied to a steam compression refrigerationcycle, equation (8), and the denition of COP,equation (1), it can be deduced that QCond w (i)depends on: CD(i) and the COPw (i); equation (9).

QCond CD Power 8

QCond Wi CDi 11

COPw i

9

The TTTW is calculated taking intoaccount: TWD(i), the temperature incrementallowed in the cold water supplied to the build-ing, when passing through the heat exchangerlocated in the tank, TTW, MWTank, and thewater density, w, equation (10).

TTTW i C

TWD:i m3 TTW C W kg=m3

MwrTank kg10

In this study, the maximum supply tempera-ture of TW inside the building has been limitedto 25C, so the TTW is calculated based on(TTW in) as equation (11)

TTWC 25C TTW inC 11

Regarding the water temperature of the tankat the end of each time period, TTank (i) (equa-tion (6)), the following must be taken intoaccount: it cannot drop below TTW in becauseit is cooled with it, Table 3, and 25C has beenset as maximum limit.

NG consumption with water condensation athour i, NGCW (i), can be calculated with: CD(i),

the COP obtained each hour with water conden-sation, COPW (i), NCVNG, and the eciency ofNG engine, equation (12), the daily NGC,NGCW (d), is calculated with the correspondingsum, equation (13).

NGC: wi CDiCOPwi NCVNG Engine Efficiency

12

NGCwd Xi

NGCwi 13

In summary, the calculation of NGC(d) fol-lows several steps: (1) to estimate CD(1) andTWD(1), (2) to know TTank (0), (3) to calculateCOPW (1), (4) to calculate QCond W (1), (5) to cal-culateTT_Cond (i), (6) to calculateTT TW, (7) tocalculate TTank (1), (8) to calculate NGC(1), (9) torepeat the process for the following hours, and(10) to make the sum of all the NGC(i) values.

In these calculations, the power consumptioncaused by the new pumps installed in the watercondensation system was not considered,because as initial estimate, it is assumed thatthis is compensated by the reduction in con-sumption that occurs when fans of the chillerair condensation system stop.

Results and discussion

Figure 6(a) and (b) gives the results of COPa (i),COPW (i), NGCa (i), and NGCW (i) for the twomonths of minimum and maximum coolingdemand (May and July) and Figure 7 showsTTank (i) over the six months in which the build-ing demands cooling (May to October).

From the analysis of Figure 6 it can bededuced that the COPW is always higher toCOPa, the NGCW is lower at all hours of theday to NGDa, and the operating regime ofthe chiller is more constant with the newcondensing system, as it provides a moreconstant COP.

It seems that the COPa is variable along theday in every month; this is due to the variationof outside air temperature. However, the COPW

Renedo et al. 13



is constant in the coolest months (May, June,and October), and variable in the hottest (July,August, and September), this is because in therst case the TW is capable of absorbing allQCond w (the condensation temperature doesnot change), but not in the second case, inwhich TTank increases throughout the day (seeFigure 7). It can be seen that the maximumdaily increase in TTank occurs in July, thisvalue being lower than 5C. In addition, at anytime of the day TTank is lower than 25

C, that

has been established as the maximum acceptablevalue for TTank. In the warmer months, if thistemperature rises over this value, it is possible touse air condensation, and make use of auxiliarynight cooling of the tank.

Table 4 presents the results of dailyand monthly calculations of building CD,NGCa, and NGCw. Similarly, it is deducedthat with the proposed new system of water con-densation, the annual NGC is reducedby 18.6%.

5,8

(a)

(b)

COP

COPa COPw NGCa (Nm3) NGCw (Nm3)

COPa COPw NGCa (Nm3) NGCw (Nm3)

COP

8,5

NG

(Nm3

)N

G (N

m3)

7,0

4,0

5,5

2,5

1,0

10,0

8,5

7,0

4,0

5,5

2,5

1,0

10,0

5,4

5,0

4,6

4,2

3,8

MAY

JULY

5,8

5,4

5,0

4,6

4,2

3,8

8 to

9

9 to

10

10 to

11

11 to

12

12 to

13

13 to

14

14 to

15

15 to

16

16 to

17

17 to

18

18 to

19

19 to

20

20 to

21

8 to

9

9 to

10

10 to

11

11 to

12

12 to

13

13 to

14

14 to

15

15 to

16

16 to

17

17 to

18

18 to

19

19 to

20

20 to

21

Hour of the Day

Hour of the Day

Figure 6. Hourly COP and NGC of air and water condensation for the two months of minimum and maximum

cooling demand.




Cost analysis

Considering that the cost of NG is about 0.095E/kWh, the annual cost saving would be near1300 E. Furthermore, it must be taken intoaccount that the cost of the NG has beenincreasing along the years and is expected togo on rising.

On the other hand, if this proposal wouldhave been taken into account during the designphase of the building, the re water tank and the

chiller could have been placed closer to eachother. In addition, the chiller would have beena water-cooled unit instead of an air-cooled one.And due to the 50m3 re water tank, the auxil-iary water cooler could be noticeably smallerthan a typical cooling tower. As a result, con-sidering this proposal during the design phase ofthe building, the extra cost could be estimated at6800 E, including pumps, piping, accessories,heat exchanger and auxiliary water cooler.

22,0

May June JulyOctoberSeptemberAugust

17,0

12,0

19,5

T Ta

nk(i

) (C)

14,5

Hour of the Day

8 to

9

9 to

10

10 to

11

11 to

12

12 to

13

13 to

14

14 to

15

15 to

16

16 to

17

17 to

18

18 to

19

19 to

20

20 to

21

21 to

22

22 to

23

Figure 7. Hourly temperature of the fire water tank for the six month with cooling demand.

Table 4. Results of air and water systems condensation systems.

May June July Aug. Sep. Oct.

Building CD (kWthh)

Monthly 7566 14,256 25,251 20,209 23,309 12,264

Daily 344 679 1148 919 1110 557

Chiller NGC (Nm3) Annual

Air cond. Monthly 459.8 907.7 1755.3 1429.7 1592.7 770.3 6915.4

Daily 20.9 43.2 79.7 64.9 75.8 35.0

Water cond. Monthly 375 753.5 1446.9 1134.2 1291.1 628.1 5628.4

Daily 17.0 35.8 65.7 51.5 61.4 28.5

Renedo et al. 15



According to these considerations, the returnperiod would be about ve years.

In this case, due to the building and facilitiesprevious design, an expensive piping systemwould be necessary since the chiller and there tank are 90m far from each other. In add-ition to this, new elements such as a water con-denser should be installed. Thus, the investmentcould be about 20,000E, that is, the returnperiod would be longer than 15 years.

Finally, maintaining this system would not bedierent from maintaining a typical water-cooled chiller and a typical re water tank. Ifanything, cleaning the heat exchangers shouldbe thoroughly done.

The maximum expected re water tempera-ture, about 21C, would require a detailedstudy about biological activity and its eectson the system performance. Growth rates of spe-cies of algae depend on the temperature,having been observed several dierencesbetween species for growth at low and hightemperature.65

Sensitivity analysis

This study was completed by performing a sen-sitivity analysis that takes into account threeaspects, which are:

. change in building WD (10%);

. variation in urban water supply temperature,TTW in, of (1C); and

. variation of the maximum temperature of thedomestic water supply inside the building,TTW, (1C).

The results obtained are shown in Table 5.The result of this analysis shows that: (1) with

all the changes considered, the current energyconsumption of the facility with air condensa-tion is improved, (2) in the months in whichthe TW is capable of cooling the entire QCondW, May, June, and October, the variations inWD in the building and in TTW in do notchange the energy consumption, (3) changes of10% in WD make performance improvementto change by approximately 1.0%, (4) changesof 1C in TTW in make performance improve-ment to change by approximately 3.85%, and(5) changes of 1C TTW make performanceimprovement to change by approximately1.22%.

Finally, a 10% increase in the CD of thebuilding has been considered. As previouslymentioned, the average data shown in Table 3have been obtained from the Facilities Service ofthe University. The deviation of these data sug-gests the study of this proposal considering a10% hotter summer. In this case, in additionto calculate the NGD of the system with watercondensation, NGDW, it is necessary to recalcu-late the NGD that would occur with air conden-sation, NGDa. The results are shown inFigure 8. NGCa would be 7607Nm

3, whileNGCw would be 6304Nm

3.

Table 5. Sensitivity analysis of monthly NGC (Nm3) with varying: water demand (WD), inlet temperature of tap

water in the building (TTW in), and temperature of distribution tap water in the building (TTW db).

May June July Aug. Sep. Oct. Annual

Air condensation 459.8 907.7 1755.3 1429.7 1592.7 770.3 6915.4

Water condensation 374.5 753.5 1446.9 1134.2 1291.1 628.1 5628.4

Water demand (10%) 374.5 753.5 1484.2 1158.6 1309.7 628.1 5708.7Water demand (+10%) 374.5 753.5 1406.4 1117.4 1286.0 628.1 5565.9

TTW in (+1C) 387.5 778.5 1539.8 1201.9 1362.6 648.2 5918.6

TTW in (1C) 362.3 730.1 1361.2 1081.1 1245.0 607.0 5386.8TTW db (1C) 374.5 753.5 1492.4 1163.9 1319.2 628.1 5731.7TTW db (+1

C) 374.5 753.5 1404.3 1116.6 1285.6 628.1 5562.5




The analysis of these results shows that a10% increase in CD reduces the energy improve-ment of the proposed system to 17.1%, while netsavings increase. This is because in the warmermonths, the water condensation system isalready saturated, and therefore this demandincrease should be reached with the air conden-ser. The net saving occurs in the months coolestmonths, in which the water condensation canabsorb more heat than the one would be elimi-nated by the chiller condensation.

Conclusions

The study presented in this paper has consideredthe use of the TW consumed in a universitybuilding to cool the condenser of the chiller,which provides the cooling used for air condi-tioning in the building. Due to the dierencebetween cooling needs and the water consump-tion, the use of the re water tank has beenproposed for heat storage.

The results reveal that performing the con-densation at low temperatures, the COP of thechiller increases, which produces several bene-cial eects: (1) the annual energy consumptionof the chiller is reduced, in this case 18.6%, (2)

the heat removed in the condenser is reduced, (3)the chiller operation becomes more stable, and(4) the chiller is operated under lower condenserpressure conditions. These two eects contributeto extend the useful life of the chiller.

The sensitivity analysis performed in thiswork considered aspects such as: (1) change inbuilding WD, 10%, (2) variation in urbanwater supply temperature, 1C, (3) variationof the maximum temperature of the domesticwater supply inside the building, 1C, andnally (4) increase in building cooling demandof 10%.

Conflict of interest

None declared.

Funding

This research received no specic grant from anyfunding agency in the public, commercial, or not-

for-prot sectors.

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2.000

1.500

NG

C (N

m3)

NGCa (100% CD)NGCa (110% CD)

NGCw (100% CD)NGCw (110% CD)

1.000

500

0

May

June

July

Aug.

Sep.

Oct

.

Month

Figure 8. Monthly natural gas demand when considering a 10% increase in building thermal demand.

Renedo et al. 17



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Appendix 1

Notation

a air or air condensationcp specific heat

CD cooling demand%CD schedule of daily CD in %

(d) dailydb distributed in the building(i) at hour i

Renedo et al. 19



in inlet from city networkMW Tank water mass in fire water tankQCond w heat provided to the Tank

through the water condenserT temperature

Tank fire water tankTW tap water (domestic water)

TTank temperature of fire water tankTTW db temperature of TW distributed in

the buildingTTW in inlet temperature of TW in the

building from city networkw water or water condensation

TTCond temperature Increment in the tankas a result of the heat supplied bythe condenser

TTTW temperature Increment in the tankas a result of heat removed by theTW

TTW temperature increment allowed inTW to pass through the heatexchanger located in the tank

W density of water



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