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comparison of energy and exergy analysis of fossil plant ground and air source heat pump building he


Renewable Energy 35 (2010) 1275–1282

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Renewable Energy
journal homepage: www.elsevier.com/locate/renene

Comparison of energy and exergy analysis of fossil plant, ground and air source heat pump building heating system
S.P. Lohani a, *, D. Schmidt b
a b

University of Oldenburg, Oldenburg, Germany Fraunhofer Institute of Building Physics, Kassel group, Germany

a r t i c l e i n f o
Article history: Received 2 May 2009 Accepted 5 October 2009 Available online 31 October 2009 Keywords: Ground source heat pump system Air source heat pump system Conventional system Energy analysis Exergy analysis

a b s t r a c t
The energy and exergy ow for a space heating systems of a typical residential building of natural ventilation system with different heat generation plants have been modeled and compared. The aim of this comparison is to demonstrate which system leads to an efcient conversion and supply of energy/ exergy within a building system. The analysis of a fossil plant heating system has been done with a typical building simulation software IDA–ICE. A zone model of a building with natural ventilation is considered and heat is being supplied by condensing boiler. The same zone model is applied for other cases of building heating systems where power generation plants are considered as ground and air source heat pumps at different operating conditions. Since there is no inbuilt simulation model for heat pumps in IDA–ICE, different COP curves of the earlier studies of heat pumps are taken into account for the evaluation of the heat pump input and output energy. The outcome of the energy and exergy ow analysis revealed that the ground source heat pump heating system is better than air source heat pump or conventional heating system. The realistic and efcient system in this study ‘‘ground source heat pump with condenser inlet temperature 30 C and varying evaporator inlet temperature’’ has roughly 25% less demand of absolute primary energy and exergy whereas about 50% high overall primary coefcient of performance and overall primary exergy efciency than base case (conventional system). The consequence of low absolute energy and exergy demands and high efciencies lead to a sustainable building heating system. 2009 Elsevier Ltd. All rights reserved.

1. Introduction The use of energy in the building sector for heating and cooling is nearly one third of the total energy consumed in the world [16]. As there is growing concern in the use of fossil fuels that is being depleted soon and because of the sustainability issue, an alternative source of energy must be found to meet energy supply of high energy consumption sector. The building sector is one of the prominent sectors, which could save tremendous amount of fossil fuels if renewable energy source like ground coupled heat pumps (GCHPs) substituted them. The use of GCHPs is growing signicantly in commercial and residential sectors and has numerous advantages over air source heat pumps as described by [11]. The increase in interest to the heat pumps is due to their high utilization efciency over conventional heating and cooling systems. More or less constant temperature over the year is an important feature of the
* Corresponding author. Tel.: 4791004758. E-mail address: splohani@daad-alumni.de (S.P. Lohani). 0960-1481/$ – see front matter 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2009.10.002

ground coupled heat pump over air source heat pump that has extreme low outside temperatures at severe weather conditions and lead to high operational energy consumption. Capital cost of GCHPs is 30–50% more expensive than air source heat pumps, which is a major hurdle in gaining overwhelming demand despite having several advantages. Nevertheless, the annual operation cost is less making the unit justiable over the life time operation [12]. As heat pumps are advantageous from the energy, environment and sustainability point of view, efforts should focus on to show scientic evidence to the knowledge body of the society. Thermodynamic analysis of the system would produce scientic results that help convince scientic knowledge body to propel the system at large. Thermal analysis of a system focuses on rst law and second law of thermodynamics. First law deals with energy balance of a system whereas second law address energy and entropy of a system, it gives in depth of the system operations. Combining rst law and second law of thermodynamics is necessary for exergy analysis of the system that gives detail know how of the performance evaluation and optimization of the system. Exergy analysis is the basis

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Nomenclature avail 0 D, dis electr f FH H irrev p q COP Q cp T Available Reference state Distribution Electricity Fuel Floor Heating Heat Irreversible primary quality Coefcient of Performance Heat ow, [W, kWh/a] Specic heat capacity at constant pressure, [J/KgK] Temperature, [K] Energy efciency Exergy efciency Carbon dioxide

out des evp exp aux ret gen env cond Ex En S h

Outlet Destruction Evaporator Expansion Auxiliary energy return generation subsystem Envelope subsystem Condenser Exergy ow, [W, kWh/a] Energy, [J] Entropy, [J/K] Floor average temperature

h j
CO2 Indices in

Abbreviation GCHP Ground coupled heat pump GSHP Ground Source Heat Pump ASHP Air source heat pump COP Coefcient of Performance

Inlet The environment or system surrounding is considered as large thermal reservoir and has no inuence of local activity of source or sink. Reference temperature is taken as dynamic environment temperature and atmospheric pressure.

for identifying irreversibility of the system and is helpful to minimize entropy generation in a process where heat and work interaction takes place [1]. Exergy analysis is an important tool to determine how efcient thermal systems can be designed or in other words it can reveal unavoidable thermal inefciencies of the system [4]. Few numbers of experimental investigations of ground coupled heat pumps in building application based on exergetic analysis has been published [5–8,13]. However, all analysis has been performed with steady state condition. Theoretical investigation of similar system has not yet been published to the knowledge of the author. This study intends to carry out dynamic energy analysis and quasi static exergy analysis of the system on theoretical basis.

2.1. Balance equation for building system When the abovementioned assumption is observed, the exergy is basically divided into four different subcomponents, physical exergy ExPH, chemical exergy ExCH, kinetic exergy ExKN and potential exergy ExPT [1]. However, physical exergy is considered as important while dealing with heat and mass interactions of the systems. Total exergy of a system can be written as following equations.

2. Thermal analysis This paper focus on the analysis of the energy and exergy ow of the fossil plant and air or ground coupled heat pump building heating systems, which takes into account of basic governing equations of rst and second law of thermodynamics for the analysis. However, both laws alone cannot investigate quality of energy ow in any systems. Thus, combination of both laws which gives the concept of exergy analysis will be imperative for the quality analysis. Moreover analysis has been based on simulation work on IDA-ICE where energy and exergy model has been developed and implemented in to the simulation environment. While developing model following basic assumption has been made. There is no kinetic and potential energy effects and is no chemical or nuclear reactions. All processes are steady state and steady ow.
1 3

Ex ExPH ExKN ExPT ExCH
CH KN

(1)

Neglecting chemical exergy Ex , kinetic exergy Ex and potential exergy ExPT, the exergy balance can be expressed as:

Exin Exconsumed Exout

(2)

Since this building heating system is a oor heating system this circulating water heats or cools the oor followed by heating or cooling of the room with some response time depending on the heavy or light oor heating systems. In the process exergy is transferred to the room air through oor surface, since the comfort temperature demand of the room is not high, the exergy demand of the room is therefore low. Some exergy is consumed in the oor heating system with entropy generation. The mathematical formulation depicting the exergy ow in the process can be stated as follows.
4 5

2 z}|{ z}|{ z}|{ z}|{ z}|{ ! ! ! Ti _ gen ,T Q , 1 T0 m,c, Tret T T ,ln Tret m,c,1 h T Tret T ,ln Tin _ _ _ m,c, Ti T0 T0 ,ln S 0 0 0 in h F T0 Th T0 Tret

(4)

S.P. Lohani, D. Schmidt / Renewable Energy 35 (2010) 1275–1282

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Exergyin;floor Exergyconsumption Exergysupply;floor Exergyret;floor Exergyloss;floor (3)

_ _ Q ghx mw ,cp;w ,Tout Tin

(11)

An amount of exergy extracted by the ground heat exchanger from the ground surface is

In the above equation (3), Exergyin,oor (1) refers an amount of exergy supplied to the oor heating system. An Exergyconsumption (2), represents exergy consumption in a oor heating system solely because of the irreversibility in process of the system. The other term Exergyloss,oor (5), is also an exergy losses term but it has nothing to do with entropy generation. It is a loss due to inefcient oor heating system that is expressed as efciency factor of the oor heating system, hF. Exergysupply,oor (3) stands for transported exergy from the oor heating system to the oor construction, which eventually transferred to the room air and Exergyret,oor (4) is an amount of returned exergy through returned water of the oor heating systems. The efciency factor of a oor heating system can be calculated with the given equations.

_ Exghx

! _ Q ghx Tout Tout Tin T0 ln Tout Tin Tin

(12)

The exergy transferred from ow of stream to the evaporator can be calculated using a following equation.

" Qevap Tout;evap;r Tin;evap;r Exevap Tout;evap;r Tin;evap;r # Tout;evap;r T0 ln Tin;evap;r

(13)

hF

Qh Qin Qret

(5)

The electrical energy supplied to the compressor is taken as energy supplied to the uid system that is also considered as exergy supplied to the system, since the quality factor of electrical energy is 1. The exergy destruction in a compressor can be determined using the equation below.

The exergy demand of the room is, therefore, subtraction of exergy input to exergy return in the oor heating system. The fossil plant or ground source heat pump is responsible to meet the difference, in other words, heat generator of the systems.

Exdes;comp T0 Sin;comp Sout;comp

(14)

Exdemand Exin Exret Exsupply;floor Exloss;floor Exconsumption _ T Q T Tret T0 ln in Tret Tin Tret in ! (6)

It can be referred as total generation energy from ground coupled heat pump is being delivered through condenser, Exergy transfer through a condenser is investigated with this formula;

Excond Exdemand (7)

" Qcond Tin;cond;r Tout;cond;r Tin;cond;r Tout;cond;r # Tin;cond;r T0 ln Tout;cond;r

(15)

Where the term Q is introduced, which is nothing but an amount of heat energy delivered by the oor heating system to the oor construction can be expressed as:

The working uid is returned to its original states and enters to the evaporator to complete the cycle. The exergy destruction in an expansion device can be evaluated using the following equation.

_ _ Q m,c,Tin Tret

(8)

Exdes;exp T0 Sout;exp Sin;exp

(16)

Exergy supplied to the oor can be represented in terms of heat and temperature;

3. System description Since this study is focus on comparing energy and exergy ow in the building system supplied with fossil fuel plant, ground source and air source heat pump system, an exact and detail simulation of the whole building is not imperative at the moment. In addition this analysis is focus on evaluating one representing gure that can be extrapolated for in general all building system with reasonable accuracy and simplicity. The zone model is found as a best model to meet all criteria required for this analysis, therefore, is selected in the building system. A simple building zone model ‘‘reference zone’’ has been created for the energy and exergy analysis that represents the thermal behavior of the whole multi-family building. The reference zone should be considered at a height that has no inuenced from the ground and the roof [9]. Hauser further claim that the corner room will likely mimic the thermal behavior of the building. In this study we conform to the ndings of [9] and adopt the reference zone accordingly. However, the inuence of the orientation of the zone has also signicant role in evaluating the thermal behavior and performance of the system. The simulation results of energy consumption in the north/east and south/west orientation differs

Exsupply;floor Qh 1

T0 Th

(9)

Exergy consumption in a system due to irreversibility can be evaluated with an equation,

Exconsumption

! _ T Q ,hF Tin Tret T0 ,ln in Tin Tret Tret (10)

T Qh 1 0 Th
2.2. Balance equation for heat pump system

Heat extracted by the ground heat exchanger from the ground surface is calculated with the general equation 11, Where mass ow rate of the antifreeze water solution, specic heat capacity and temperature difference of the water gives an amount of heat taken out by the system.

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S.P. Lohani, D. Schmidt / Renewable Energy 35 (2010) 1275–1282 Table 1 Overview of the most important parameters dening the base case building model. Parameters External Wall, U Value Internal Wall, U Value Floor and Ceiling, U Value Windows Value 0.48 (W/m2 K) 1.072 (W/m2 K) 0.41 (W/m2 K) U-glass: 2 W/m2 K, U-frame: 1.5 W/m2 K Frame fraction to the total window: 20% Solar heat gain coefcient: 0.76 Internal emissivity: 0.9 Solar Transmittance: 0.6764 21 C–23 C (Heating Mode) 40 C 0.6 h1 No Ventilation System Tsupp (30 C)–45 C Tsupp (20 C)–22 C ASHRAE IWEC Weather File

about 20%, hence the average of both could be of best representative gure to evaluate the energy demand of the system [15]. The reference zone has two outer walls with length of 8 m each and the zone area of 64 m2 (8 m 8 m). Height of the zone is 2.6 m is at height of 5.2 m and the total height of the building is considered as 13 m so that no ground and roof inuence has occurred in the zone model. The zone model is shown in Fig. 1. The Fig. 1a shows that the whole surface of the building, the size, and the location of the zone under investigation. Fig. 1b depicts the exterior walls and windows placed on them. In the gure it has been seen that there is two interior walls that is considered as adiabatic meaning no heat and work transfer through the walls between zone and the surroundings. The oor and ceiling of the zone are adjacent to each other, which are involved in heat interaction between zone and the surroundings along with two exterior walls. Floor heating system is incorporated here in the simulation of the zone model of the building. The mechanical ventilation system is not considered and the air exchange rate with inltration is taken as 0.6 h1 that is supposed to represent a typical modern tight German building. The value is taken as representing gure for all European buildings analysis. The internal loads of a building including equipments, occupants are taken as 5 W/m2 year around value according to German standard [3]. The climate data for the analysis of a building is taken from ASHRAE IWEC Weather le for Hamburg, since analysis is being done for heating in this study. The various inputs of the construction details of the building are given in the Table 1. The reference zone model is an inbuilt function of IDA-ICE building simulation and has been modeled in detail using the CEDETZONE model. In this model the long wave and short wave heat interaction are taken in to account using the net radiation method [2]. IDA-ICE has also provision for two different zone models, Climate model and Energy model. The climate model is in detail and calculates vertical temperature gradient, includes long wave radiation calculations, mean radiant and operative temperatures, and comfort index and daylight level. The energy model is simple and has a more conventional precision level and based on a mean radiant temperature. Both models evaluate CO2, air mass, humidity and energy of the system. Selecting climate model in the simulation does not complicate and lengthen the simulation process and time instead it assures better accuracy and precision of the results. Therefore, this study takes in to account of climate model for the simulation of the building system. Table 2 shows different cases that have been studied. 4. Model analysis and simulation procedure Energy analysis model developed by Schmidt [14] is quite explicit and has detail investigation of energy chain from primary

Set Point Temperature Maximum supply inlet temperature for oor heating system Inltration rate Boiler with proportional controller Weather data

energy source via building to the sink. However, this analysis has been carried out taking consideration of high grade energy sources, fossil fuels as a source of primary energy. Here, primary energy source is taken as renewable energy (ground source heat pump, and air source heat pump), therefore, the model is slightly changed to incorporate the modication. Moreover, the model from distribution to building envelope system is used in accordance with Schmidt [14], Fig. 2. The energy ow in a system is divided into thermal energy and auxiliary energy ow. Thermal energy ow at each subsystem provides an information of energy demand at each modules whereas auxiliary energy ow is an amount of electricity required to operate the system. In this analysis, however, primary and generation energy is substituted by ground and/or air source heat pump. Since there was not a typical simulation module developed for a heat pump integrating heating or cooling system, an earlier studies of representative heat pumps characteristic curves (COP curves) are used for the purpose of analysis and in this approach of evaluation heat pumps are usually deemed as black box that is to say no detail analysis of component basis has been done, but input and output from the heat pumps are obtained. Output of the heat pump is generation energy which then ows through distribution subsystem to nally envelope subsystem. Storage is not considered in this analysis. A characteristics curve from Ito [10] is used in this study. Brief descriptions of these curves are presented herein. Fig. 3 shows the COP curve that is used in this analysis. The relation between COP and the temperature of the heat sources is shown in Fig. 3. Generally, a water source heat pump gives a higher COP than an air source heat pump if both temperatures are the same, which in above gure can also be seen. While obtaining the typical characteristics curves, few operating

Fig. 1. Location of the reference zone and its boundary walls.

S.P. Lohani, D. Schmidt / Renewable Energy 35 (2010) 1275–1282 Table 2 Describes the different cases that have been studied during analysis. Cases Case 1: Conventional system (base case) Case 2: Ground coupled heat pump integration system Description Building with condensation boiler (hboiler 0.95) and oor heating system. Inltration rate 0.6 h–1. Building system same as base case and ground coupled heat pump with constant condensation temperature 30 C with varying inlet evaporator temperature. Building system same as base case and ground coupled heat pump with constant condensation temperature 40 C with varying inlet evaporator temperature. Building system same as base case and Air source heat pump with constant condensation temperature 30 C with varying ambient temperature. Building system same as base case and Air source heat pump with constant condensation temperature 40 C with varying ambient temperature.

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Case 3: Ground coupled heat pump integration system

Case 4: Air source heat pump integration system

Case 5: Air source heat pump integration system

conditions were imposed during experimentation, condenser inlet temperature was maintained constant at 30 C and 40 C respectively, while varying the evaporator source temperatures. Working uid of the heat pump in all cases was R22. This operating condition of the heat pump with air and water source temperature resembles with our analysis for the heat pump integrated building heating system. As the building heating system is low temperature oor heating system, Working uid R22 is quite applicable for low and medium temperature oor heating system that has been used in getting the above curves. Similarly, from the simulation result of conventional heating systems, we obtained inlet temperature of the generation plant lies in the range of 26–32 C ensuing condenser inlet temperatures constant at 30 C is quite reasonable for our investigations. However, condenser inlet temperature 40 C is also rational to be examined because in many other cases this situation might occur for low/medium oor heating systems. The source temperature was varying in air source heat pump as ambient temperature uctuates with day and time, but in case of ground source heat pump we consider a system with vertical heat exchanger of 50 m depth and took constant temperature of 10 C, thus inlet water to evaporator approximates around 8 C. COP in ground source heat pump is, therefore, constant in this analysis.

Fig. 3. Relation between COP and the temperature at the evaporator using air or water as a heat source (Ito et al., 2000).

4.1. Simulation procedure As the aim of this study is to compare energy and exergy ow of a system supplied with fossil fuel plant and ground and air source heat pump for the same building zone model. Hence there is no need of changing anything in the IDA-ICE building zone model for both systems. The self developed exergy analysis module that uses dynamic values from IDA-ICE is implemented on each system and starts simulation of the system. The simulation is performed dynamically with periodic start up for 1st January, 2007 a representative day for winter climate using ASHRE IWEC climate le for Hamburg. The self developed exergy model with steady state exergy equations has used dynamic values of energy from the simulation and calculates quasi steady state exergy for the each subsystem. While in the case of heat pumps dynamic values of energy from IDA-ICE is taken from distribution subsystems to envelope subsystems. For the primary energy transformation and generation part, COP curves of earlier studies has taken and t into the analysis of this study. Energy and exergy input output of the

Fig. 2. Energy ow at different modules of a building system (Schmidt, 2004).

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S.P. Lohani, D. Schmidt / Renewable Energy 35 (2010) 1275–1282 Table 4 Overall primary exergy efciencies for the building in all cases. Description Case 1 5 67.1 Case 2 7.1 106 Case 3 5.4 81 Case 4 3.7 55.6 Case 5 3.47 51.7

heat pump are calculated with the help of spread sheet using similar steady state equations of the exergy.

5. Results and discussion The considered system has two parts: building part and energy supply part that is fossil plant, ground source and air source heat pump systems. While illustrating energy and exergy performance of the system; basically energy and exergy demand, energy and exergy losses and energy and exergy efciencies of each subsystems and overall system are being derived. Table 3 presents energy and exergy demand for all cases studied here, a glimpse of comparison that is followed by energy and exergy efciency and/or COP in Table 4. Above table gives energy and exergy demands status in number for each subsystem of different cases analysed in this study. Just to give a bit detail overview total demands are separated in to thermal and electrical parts. From case 2 to case 6, primary energy and exergy is taken a sum of compressor and evaporator. However, an external supplied energy and exergy is only compressor part and evaporator energy/exergy is free but monitored during analysis, which is extracted from ground or air sources. The thermal energy and exergy demand of the building envelope subsystem depicts overall losses through the building envelope that is being supplied by active as well as passive heating system of the building. Above Table 4 presents overall primary energy efciency and coefcient of performance; and overall primary exergy efciency for all cases investigated and found quite different values for each case. Since case 1 is conventional fossil plant system the term efciency is used whereas cases 2–4 are heat pump systems the term coefcient of performance is used to designate overall input/ output energy ratio. The term exergy efciency is used for both systems. This overall efciency and/or coefcient of performance indicate the extent of matching of energy/exergy levels of the supply and demand sides in the building. The overall primary exergy efciency is of important to compare total quality of energy required to produce the same amount of exergy that is being supplied at the building envelope. In case 1, the overall primary exergy efciency is 5% that means an amount of total exergy needed to produce exergy for building envelope is 20

Overall primary exergy efciency [%] Overall primary energy efciency [%] & COP

times. In ground source heat pump system overall primary exergy efciency is higher than that of air source heat pump with same condensation temperature that is obvious and complies with basic heat pump theory. From above primary exergy efciency comparison gives that fossil plant need more exergy at primary level than to ground source heat pump whereas air source demand is highest to all to supply the same amount of exergy at an envelope system. The output of the energy and exergy analysis which were presented on the above table are compared graphically and shown in Figs. 4 and 5 below. The above graph compares the energy ow from primary subsystem to envelop subsystem, where the dashed line at primary energy is energy extracted from evaporator in the case of heat pumps. In the envelope sub system, the total energy demand is what the total energy dissipated from the building shell to the environment. Moreover, the active energy demand is about 32% of the total energy dissipated through the building shell, which is delivered by active heating system like condensing boiler and ground or air source heat pumps respectively via oor heating system, the rest of the energy demand in a building envelope is met with internal loads and solar gains. The generation sub system in cases 2, 3, 4 and 5 have around 6% less energy demand against case 1. But, at the primary sub system reduction of energy demand for cases 2 and 3, are about 36.6%, and 17%, whereas for cases 4 and 5 energy demand increases at 22.6% and 31.8% respectively against case 1. This proves that the ground source heat pumps have better performance against fossil plant (conventional system) and air source heat pumps in terms of energetic point of view that lead to reduction of overall primary energy demand following reduction in environmental impacts and uphold sustainability.

Table 3 Energy and exergy demands for the different subsystems of all cases. Primary Energy Energy [kWh/a] Case 1 Th. Elec. Total Th. Elec. Total Th. Elec. Total Th. Elec. Total Th. Elec. Total 3365 51 3415 2175 2166 4341 1952 2832 4784 1521 4127 5648 1418 4436 5854 Exergy [kWh/a] 3028 51 3078 44 2166 2210 33 2832 2865 6 4127 4133 5 4436 4441 Generation Energy [kWh/a] 3059 15 3074 2899 0 2899 2899 0 2899 2899 0 2899 2899 0 2899 Exergy [kWh/a] 2753 15 2768 267 0 267 267 0 267 267 0 267 267 0 267 Distribution Energy [kWh/a] 2896.7 2 2899 2896.7 2.3 2899 2896.7 2.3 2899 2896.7 2.3 2899 2896.7 2.3 2899 Exergy [kWh/a] 265 2 267 264.7 2.3 267 264.7 2.3 123.6 264.7 2.3 267 264.7 2.3 267 Emission Energy [kWh/a] 2503 0 2503 2503 0 2503 2503 0 2503 2503 0 2503 2503 0 2503 Exergy [kWh/a] 227 0 227 227 0 227 227 0 227 227 0 227 227 0 227 Room-Air Energy [kWh/a] 2292 0 2292 2292 0 2292 2292 0 2292 2292 0 2292 2292 0 2292 Exergy [kWh/a] 200 0 200 200 0 200 200 0 200 200 0 200 200 0 200 Envelope Energy [kWh/a] 7147 0 7147 7174 0 7174 7174 0 7174 7174 0 7174 7174 0 7174 Exergy [kWh/a] 450 0 450 450 0 450 450 0 450 450 0 450 450 0 450

Case 2

Case 3

Case 4

Case 5

Th.: thermal energy/exergy; Elec.: electrical energy/exergy ow.

S.P. Lohani, D. Schmidt / Renewable Energy 35 (2010) 1275–1282

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Fig. 4. Comparison of energy ows in cases 1, 2, 3, 4 and 5.

The exergy curve gives completely different picture from the energy curve; exergy demand of envelope sub system is very low down despite having the highest energy demand for the same. The discrepancy between two analyses substantiates the need of exergy analysis in getting more insight of source energy requirement. Since the energy dissipates from the envelope sub system to the environment is at low temperature between 21 C and 23 C, the exergy content of these massive energy ows is then very small and in principle could be supplied through low quality energy sources say low exergy energy. As it is evident from the curve that in the fossil plant system (case 1) biggest exergy losses take place at primary and generation sub systems, which is the point of high quality energy in the form of fuels or electricity is being fed to the building system. The exergy ow fed to the building envelope is zero when it nally gets to the outside air (reference). Thus, all exergy provided to the building envelope is at last consumed. While in cases 2, and 3, the absolute amount of primary exergy needed for maintaining the same temperature level of the zone 21–23 C in the heating mode, are 30%, and 8%

less than that of case 1, but for cases 4 and 5 exergy demand is 34% and 44% higher as compared to case 1. While generation exergy demand is 91% less for all heat pumps. This clearly substantiate that ground coupled heat pump building heating system is far more sustainable approach than conventional building heating system. It needs less absolute energy and exergy supply at primary level and it also make clear that high grade energy is not a requirement for the building heating energy supply. 6. Prioritizing the systems from energetic/exergetic viewpoint The performance of the different cases has been presented mostly with energetic/exergetic standpoint. The reason behind a typical results for cases, pro and cons of it with regard of sustainability issue that is utmost important in days to come to keep up balance between energy supply and climate change have been unveiled. A nal conclusion of the system, ranking of the cases

Fig. 5. Comparison of exergy ows in cases 1, 2, 3, 4 and 5.

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Table 5 Ranking of all cases from best to worst with energetic and exergetic perspective. Cases Case1 – Base case Case 2 – GSHP @ condenser inlet temperature 30 C Case 3 – GSHP @ condenser inlet temperature 40 C Case 4 – ASHP @ condenser inlet temperature 30 C Case 5 – ASHP @ condenser inlet temperature 40 C Energetic perspective Reference case Better Good Worse Worst Exergetic perspective Reference case Better Good Worse Worst

analysis gives case 2 is one of the most realistic systems that have around 50% high overall primary coefcient of performance and overall primary exergy efciency and about 25% less primary energy and exergy demand than the base case ‘‘case 1’’.

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studied have been presented primarily based on sustainability viewpoint in Table 5 below. 7. Conclusion The analysis of the several cases under investigations has revealed that exergy analysis is very important to get more insight of the processes than that of sheer energy analysis. The discrepancy over overall primary energy and exergy efciencies has divulged the fact that exergy demand of a building heating system is very low, however energy demand is not. Exergy efciency is in the range of 0.035–0.09 whereas energy efciency is from 0.5 to 1.06. The energy efciency exceeds over 1 just because extraction of evaporator energy (which is free) is ignored in the calculation of the efciency but is monitored. The efciencies alone do not reveal more insight of the absolute demand, though overall losses proportion in the system can be determined, hence absolute energy and exergy demands are crucial to determine heat generation plant. The biggest exergy losses occur in all cases studied, by far in the energy conversion process namely: in conventional heating system at generation subsystem and in heat pumps at primary subsystems. Nevertheless, energy losses are negligible in conventional system. The exergy losses in generation subsystem is in the range of 90% while in heat pumps exergy losses in primary subsystems is in the range of 85–95%. Exergy losses in each subsystem are basically due to intrinsic thermodynamic irreversibility in the processes. The comparison of the


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