Analysis of a gas-driven absorption heat pumping system used for heating and domestic hot water preparation [original]

Chapter 3: Thermally Driven Heat Pumps for Heating

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ANALYSIS OF A GAS-DRIVEN ABSORPTION HEAT PUMPING

SYSTEM USED FOR HEATING AND DOMESTIC HOT WATER

PREPARATION

Harald Moser, Rene Rieberer, Graz University of Technology, Institute of Thermal

Engineering, Inffeldgasse 25 B, A-8010 Graz, Austria, rene.rieberer@tugraz.at

This paper was published in the proceedings of the 10th International Heat Pump Conference 2011

(www.heatpumpcentre.org/en/hppactivities/ieaheatpumpconference).

Abstract: In the building sector the use of thermally driven heat pumps can contribute

substantially to energy conservation and the reduction of end-use energy. Within the IEA

HPP Annex 34 a medium size absorption heat pump application installed in a storehouse in

Graz has been monitored over a period of one year.

The system consists of two directly natural gas-driven ground source ammonia/water

absorption heat pumps (each with ca. 40 kW heating capacity). Via a buffer storage space

heating of an office building and a storage depot as well as domestic hot water preparation is

provided.

During the monitoring period the system showed reliable operation and high energy

performance. Based on the lower heating value of the natural gas the seasonal performance

of the year 2010 was 1.54 which is approx. 60% higher compared to a condensing gas boiler

with a seasonal performance of 96%. However, also room for improvements has been

detected, especially at start/stop operation for domestic hot water preparation in summer.

Key Words: demonstration project, monitoring, IEA HPP Annex 34, ground source

1 INTRODUCTION

In the building sector the use of natural gas driven absorption heat pumps can contribute

substantially to energy conservation and the reduction of CO2-emmisions due to the

reduction of the end-use energy demand. Especially the small-capacity range of absorption

heat pumps for heating purpose are commonly considered as the logical next development

step beyond the condensing gas boiler and thus the market potential is enormous. However,

small-capacity applications still suffer from relatively low efficiencies and/or high investment

cost compared to other technologies.

In the medium capacity range of around 50 kW heating capacity several absorption heat

pumps are available on the market. Large scale AHP for heating purpose are relatively rare

and mainly special designed systems based on chillers are used (Ziegler 2002).

The Annex 34 “Thermally Driven Heat Pumps for Heating and Cooling” within the IEA “Heat

Pump Program” (HPP) aims to the reduction of the environmental impact of heating and

cooling systems by the use of thermally driven heat pumps. One of the main objectives of the

Annex 34 is to quantify the economic, environmental, and energy performance of integrated

thermally driven heat pumps in cooling and heating systems in a range of climates, countries,

and applications. Within this framework at Graz University of Technology (Institute of

Thermal Engineering) a demonstration project has been started. The aim of the project is to

Chapter 3: Thermally Driven Heat Pumps for Heating

evaluate the seasonal performance of two ammonia/water absorption heat pumps for heating

and domestic hot water preparation.

2 APPLICATION DATA

Two “Helioplus 40-S” absorption heat pumps from the company “Helioplus Energy Systems

GmbH” are installed in a storehouse of a brewery in Graz, Austria (compare Figure 1 and

Figure 2). These ammonia/water AHP are directly driven by natural gas, have a maximum

supply temperature of 60°C and a heating capacity of 37.1 kW each. One feature of these

AHP is that they include also a flue gas heat exchanger in order to condense water in the

flue gas by rejecting condensing heat directly to the return flow of the heating circuit.

Figure 1: Storehouse building where the AHPs are

installed

Figure 2: Photo of AHP installation

The AHP supply heat to a 1.2 m³ stratified thermal water storage tank (compare Figure 3).

This storage tank supplies heating water to both the space heating distribution system and

the domestic hot water tank. The outlet nozzle for the space heating system is located at an

intermediate level and for domestic hot water preparation at the very top in order to receive

the required temperature level.

The heating area comprises of an approx. 2000 m² storage depot and an office area with

approx. 600 m². The storage depot has a nominal heating capacity of ca. 57 kW and is

heated to 18°C. The office has a heating capacity of ca. 19 kW and is heated to 22°C room

temperature. For domestic hot water (DHW) preparation a 0.5 m³ storage tank is used. It is

charged by the heat from the heating water storage tank via a plate heat exchanger.

For space heating the heating water storage is controlled with respect to the ambient

temperature and heating curve in order to achieve a temperature level in the storage approx.

2 K in excess of the required supply temperature (ca. 40°C at -12°C ambient temperature) for

the heat distribution system.

If the temperature level of the DHW tank drops below a certain threshold the control of the

AHP is switched to the DHW temperature set point in order to heat the upper part of the

heating water storage to the required temperature level of more than 50°C. Using this

heating water the domestic hot water tank is charged via a plate heat exchanger. After the

temperature level of the domestic hot water tank has reached the temperature set point of

50°C the control of the AHP is switched back to the current space heating temperature set

point.

Chapter 3: Thermally Driven Heat Pumps for Heating

The low temperature energy source for the AHP is provided by 7 ground probes each with a

length of 100 m and a mixture of water and propylene glycol (ca. 20 wt%) is used as heat

carrier. During summer the ground probes are used for free cooling of the office area.

Therefore a plate heat exchanger is used in order to reject the heat from the heating water to

the heat carrier of the ground probes. However, free cooling operation is not discussed in this

paper and active cooling by the AHP is not provided.

GAS

AHP 1

GAS

AHP 2

Gas

Ground

Domestic

Hot Water

0,5 m³

Heating

Water

1,2 m³

WAHP

VAHP

DpESC

Ground

Probes

Free Cooling-

Secondary

Circuit

VESC

Free Cooling

Primary Circuit

TESC,in

TESC,out

TAHP,in

TDHW,out

TDHW,in

TAHP,out

WDHW

WFRC

TGAS,in

THW2,in

THW2,out

THW1,in

THW1,out

TFLG,2

TFLG,1

TENV TROM

VGAS

Heating Storage Depot

57,2 kW nom. capacity

Heating Office: 19,2 kW

Free Cooling Office: 9,5 kW

TDWS,out

TDWS,in

pENV

Figure 3: Schematic drawing of the heating system including measurement instrumentation

(for legend symbols see Nomenclature)

3 MONITORING CONCEPT

The system has been monitored from 28.12.2009 until 03.01.2011 using a data logger (type

“e.reader”) and all relevant data have been measured, processed and recorded within a

measurement interval of 10 seconds. Due to some problems with the data transfer within the

time period of week 17 to 19 and 23 to 25 the data sets are incomplete for these weeks.

However, all available data have been used for system evaluation.

3.1 Measurement Instrumentation

The measurement instrumentation contains all relevant temperatures, the energy input of the

heat source, natural gas and the electricity consumption as well as the delivered heat to the

heating water storage tank. The measurement instrumentation is also indicated in the

schematic drawing in Figure 3.

Chapter 3: Thermally Driven Heat Pumps for Heating

For the natural gas measurement a bellows-type gas flow meter is installed outside the

building. In order to calculate the energy input of natural gas Eq. 1 and Eq. 2 have been

used. For this the average upper heating value (HvGAS) was assumed to be 11.176 kWh/Nm³

(AGGM, 2011) as derived from the gas supplier (average HvGAS value for the period Jan. to

Apr. and Sep. to Dec 2010). The ambient pressure has been measured and the gas pressure

(pGAS) has been assumed to be 22 mbar above the ambient pressure as controlled by the

pressure reducing regulator upstream the gas flow meter. The standard pressure (pNORM) is

1.013 bara, the standard temperature (TNORM) is 0°C. Because exact measurement of the gas

temperature was not possible a temperature of 6°C has been assumed as it is also used for

the gas billing procedure.

NORM,GASGASGAS VHvQ

(1)

NORM

GAS

NORM

GASNORM,GAS p

(2)

The heat coming from the ground probes have been calculated using Eq. 3 and Eq. 4. Due to

the fact that the system was already installed and in operation when monitoring was decided

within the IEA Annex 34 it was not permitted to insert flow meters in the hydraulic lines. That

is why the volume flow has been measured indirectly by measuring the pressure loss via a

balancing valve (compare Eq. 4).

)TT(cpVQ in,ESCout,ESCESCESCESCESC



(3)

1000

3600

VESC

ESC



(4)

The flow coefficient (KvESC = 19.31 m³/h) of the balancing value has been determined on site

using an ultrasonic flow meter. For calculation of the density and specific isobaric heat

capacity a temperature depended polynomial curve fitting of the thermodynamic data has

been used.

Electricity meters have been used for the electrical energy input to the AHP (which

comprises of the heating water pumps, energy source pumps and AHP control), the pump for

domestic hot water preparation and the pump for the primary free cooling circuit. The

electrically consumption of the pumps for the heat distribution system have not been

measured.

In the return flow line of the AHP heating water circuit a heat meter is installed which gives

the possibility to use it as volume flow counter. Using this flow counter the heat output of the

AHP has been calculated acc. to Eq. 5 for each measurement interval.

)TT(cpVQ in,AHPout,AHPAHPAHPAHPAHP

(5)

All temperatures have been measured using PT100 sensors or thermocouples. In order to

analyse the efficiency of the absorption heat pump the weekly Coefficient of Performance for

heating (COPH) has been calculated as indicated in Eq. 6. In order to analyse the absorbed

energy from the energy source additional the COP for cooling (COPC) has been calculated

using Eq. 7.

AHPGAS

AHP

HWQ

COP

(6)

Chapter 3: Thermally Driven Heat Pumps for Heating

AHPGAS

ESC

CWQ

COP

(7)

3.2 Estimation of Measurement Uncertainties

In order to evaluate the combined standard uncertainties of the calculated heat flows and

COP values the individual uncertainties of the measurement instrumentation have been

estimated and combined by Gaussian error propagation as shown in Eq. 8 (ISO 1995).

(8)

For the natural gas energy consumption the measurement uncertainty of approx.

0.3 kWh/Nm³ has been estimated which correspond to the individual uncertainties given in

Table 1.

Table 1: Estimated uncertainty for calculation of u(QGAS)

u(HvGAS)

u(VGAS)

u(TGAS)

u(pENV)

[MJ/Nm³]

[%]

[K]

[bar]

0.54

0.02

The measurement uncertainty for the low temperature heat source (QESC), the delivered heat

of the AHP (QAHP) and the electrical energy input (WAHP) has been calculated for each

measurement interval using the individual measurement uncertainties as given in Table 2.

Table 2: Estimated measurement uncertainty for calculation of u(QESC), u(QAHP) and u(WAHP)

u(cpESC)

u(mESC)

u(TESC,out)

u(TESC,in)

u(cpAHP)

u(mAHP)

u( TAHP)

u(WAHP)

[kJ/kg K]

[kg/h]

[K]

[kJ/kg K]

[%]

[K]

[%]

0.05

350

0.1

0.02

1.5

0.4

It should be noted, that the temperature sensors of the low temperature heat source have

been wet installed through drain nozzles, thus the process temperatures could be measured

quite accurately. Unfortunately this was not possible at the heating water pipes of the AHP,

thus these temperature sensors have been installed on the outside surface of the pipes.

Subsequently these piping sections have been well insulated in order to limit the effect of the

ambient air temperature. However, the measured temperature possibly deviates from the

process temperature, especially at transient operating conditions. Because the temperature

difference ( TAHP=TAHP,out-TAHP,in) which is relevant for calculation of the capacity can be

measured with higher accuracy this difference has been used for uncertainty propagation.

For all data sets of every week the uncertainties of the COP have been calculated acc. to Eq. 9

and Eq.10.

AHPGASAHP

AHP,H W

AHPGAS

AHP

AHPGAS

AHP

AHPGAS

COP u

)WQ(

(9)

AHPGASESC

AHP,C W

AHPGAS

ESC

AHPGAS

ESC

AHPGAS

COP u

)WQ(

(10)

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