An experimental exergetic comparison of four different heat pump systems working at same conditions: As air to air, air to water, water to water and water to air

Energy 58 (2013) 210e219

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An experimental exergetic comparison of four different heat pump systems working at same conditions: As air to air, air to water, water to water and water to air
ur Çakır a, *, Kemal Çomaklı b, Ömer Çomaklı b, Süleyman Karslı c Ug a b c

Department of Mechanical Engineering, Bayburt University, 69000 Bayburt, Turkey Department of Mechanical Engineering, Atatürk University, 25240 Erzurum, Turkey Department of Energy Engineering, Atatürk University, 25240 Erzurum, Turkey

a r t i c l e i n f o

a b s t r a c t

Article history: Received 7 July 2012 Received in revised form 4 June 2013 Accepted 7 June 2013 Available online 17 July 2013

In this study, we designed a multifunctional heat pump system using just one scroll compressor and which can be run in four different modes, namely air to air, air to water, water to water and water to air, in order to make an experimental energetic and exergetic performance comparison. Experimental system consists of two condensers and two evaporators and uses R22 as working fluid. One of the evaporators and condensers uses water and the others use air as heat source/sink. Heating capacities of four heat pump types are equal to each other. It is realized by adjusting the mass flow rate and temperature level of external fluid of condenser. Results show that the heat pump unit which has the maximum COP (coefficient of performance) value is water to air type with 3.94 and followed by water to water type with 3.73, air to air type with 3.54 and air to water type with 3.40. Ranking of four heat pump types with respect to their mean exergy efficiency is as follows; water to air type with 30.23%, air to air type with 30.22%, air to water type with 24.77% and water to water type with 24.01%. Exergy destruction rates of the systems were investigated in this study and the results revealed that the heat pump type which has the maximum exergy destruction is air to air type with 2.93 kW. The second highest one is air to water type with 2.84 kW. The third highest one is water to air type with 2.64 kW and last one is water to water type with 2.55 kW. It is understood that the temperature of the evaporator external fluid affects the exergetic efficiency of the system more than the mass flow rate. In contrast to the previous, the dominant parameter which has more important effect on the exergy destruction of the heat pump unit is the mass flow rate of evaporator external fluid. Ó 2013 Elsevier Ltd. All rights reserved.

Keywords: Heat pump Exergy analysis COP

1. Introduction Efficient energy utilization is one of the foremost issues in the world for the environmental and ecological protection in these days. It has become a concern because of environmental and energy problems like global warming, the depletion of conventional energy sources like fossil fuels and the increasing cost of energy [1]. Additionally, the big proportion of energy used in the world is consumed at heating and cooling applications in the residential and other buildings or in industrial plants. According to the recent studies, the energy used for residential and construction activities make up about 40% of the world’s total energy consumption and heating and cooling applications have a key role in it [2]. Efficient

* Corresponding author. Tel.: þ90 458 211 11 53x3211; fax: þ90 458 211 11 72. E-mail addresses: [email protected], [email protected] (U. Çakır). 0360-5442/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.energy.2013.06.014

energy use, including waste heat recovery and applications of renewable energy, can reduce carbon dioxide emission and global warming. The systems of solar water heating and solar source heat pump provide a new and clean way of heating buildings in the world. Therefore, these systems can be used to minimize environmental impacts and air emission. They offer the most energy-efficient way to provide heating and cooling in many applications, as they can use renewable heat sources in our surroundings [3]. One of the heating setups which offer more economic and more efficiently heating applications is heat pump system which promises means of reducing the consumption of fossil energy resources, and hopefully the cost of delivered energy for residential heating/ cooling Heat pumps are advantageous and widely used systems in many applications due to their high utilization efficiencies compared to conventional heating and cooling systems. Those systems first emerged in 1940e1950s and have some advantages

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Nomenclature Hz hertz HP heat pump Q_ conair the heat energy given to the air from air cooled condenser (kW) Q_ conwater the heat energy given to the water from water cooled condenser (kW) Q_ evapair the heat energy given to the refrigerant from air at air heated evaporator (kW) Q_ evapwater the heat energy given to the refrigerant from water at water heated evaporator (kW) _ W Comp elec the electrical energy consumed by compressor (kW) U voltage (V) I current (A) cos (4) power factor COPHP coefficient of performance of heat pump heat energy that given to the space heating fluid from Q_ Cond refrigerant at condenser (kW) E energy (kW) Ex exergy (kW) ex specific exergy (kJ kg1) _E rate of energy inlets to the system (kW) in E_ out rate of energy outlets from the system (kW) _ rate of exergy inlets to the system (kW) Ex in _ out Ex rate of energy outlets from the system (kW) _ rate of the exergy destruction (kW) Ex dest exr;w the specific exergy of refrigerant or water (kJ kg1) exair the specific exergy of air (kJ kg1) h enthalpy (kJ kg1) h0 enthalpy at dead state (kJ kg1) T temperature ( C) Tca in temperature of cooling air inlets to condenser ( C)

when compared with conventional or traditional heating systems. Heat pump systems do not produce exhaust gases while heating any space and use less energy than other systems. In addition, heat pumps are capable to use the abundant natural resources such as air source, geothermal source, waste heat and the heat of the soil. Heat pumps are widely used not only for air conditioning and heating applications, but also cooling, producing hot water and preheating feed water in various types of facilities including office buildings, public buildings, computer centers, restaurants. In addition to plants, heat pump applications have a great variation in type of drive energy, size of systems, operating conditions, variable heat sources and sinks, and application type. The temperature degree of heat source has a big importance for using heat pumps efficiently. The heat pump units are designed for the various thermal applications which have specific properties in general and therefore they are unique setups. In short, they provide with high levels of comfort, make significant reductions in electrical energy consuming and they are friendly systems for the nature [4]. Although a simple heat pump system consists of four main components, which are compressor, two heat exchangers (condenser and evaporator) and expansion valve, many auxiliary components may be used on heat pumps such as valves, thermostats, some measurement tools, pumps, fans, or extra heaters. The main structure and the type of new heat pumps have changed very much due to the improving technology and changing thermal demands all over the world. The properties of the components used on heat pumps are effective on the performance and on the thermodynamic behavior of heat pumps. In addition, thermal characteristics and the types of the heat sources and heat sinks are very

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temperature of cooling air outlets from condenser ( C) temperature of cooling water inlets to condenser ( C) temperature of cooling water outlets from condenser ( C) Tea in temperature of heating air inlets to evaporator ( C) Tea out temperature of heating air outlets from evaporator ( C) Tew in temperature of heating water inlets to evaporator ( C) Tea in temperature of heating water outlets from evaporator ( C) T0 temperature at dead state ( C) s entropy (kJ kg1) s0 entropy at dead state (kJ kg1) Cpa specific heat of air (kJ kg1 K1) Cpvapor specific heat of vapor (kJ kg1 K1) u specific humidity ratio of air u0 specific humidity ratio of air at dead state Rair gas constant of air (kJ K1 kg1) P pressure (kPa) P0 pressure at dead state (kPa) _a m mass flow rate of cooling or heating air at condenser or evaporator (kg s1) _w m mass flow rate of cooling or heating water at condenser or evaporator (kg s1) _ vapor m mass rate of vapor in air (kg s1) _ air m mass rate of dry air (kg s1) hex;HP exergy efficiency of heat pump _ exergy of heat energy given to the cooling fluid at Ex heat condenser (kW) _ Ex in cond rate of total exergy inlets to the condenser (kW) _ Ex out cond rate of total exergy outlets from the condenser (kW) _ Ex dest HP rate of total exergy destructions at the heat pump (kW) _ rate of total exergy inlets to the heat pump (kW) Ex in HP _ out HP rate of total exergy outlets from the heat pump (kW) Ex Tca out Tcw in Tcw out

important for the performance of heat pumps. Commonly used heat sources and heat sinks are ambient air, exhaust air, lake water, river water, ground water, earth, rock, wastewater and effluent. Most used heat sources and heat sinks on heat pump systems are ambient air and water around the world. Ambient air is a widely available and a free heat source for heat pumps. However, the thermodynamic performance of air source heat pump systems decreases depending on a decrease in the air temperature during the heating seasons and increase in the air temperature in cooling seasons. Because of the heat transfer properties of water, water source heat pump systems offer some performance advantages over heat pump systems which use air as heat source. Heat Pump systems are available in array of types and combinations that can suit almost any application. For heating purposes, they can be divided into basic types, determined by the source and the destination of the heat and the medium that the heat pump uses either to absorb or reject the heat in each of these locations. For both of the heat exchangers, the heat transfer appliance can be either liquid (water, or often a glycol mixture) or air; sometimes it is a combination of these two. While describing the type of heat pump, generally the heat source is provided first, followed by the destination or heat sink. The main variants in common use are air to air, water to water, water to air, air to water, ground to water, ground to air types. Because of the fact that utilization of heat pump has been spread all over the world as a result of the improvements and developments on it, the structure of heat pumps, types and the sizes of the components used on heat pump systems have changed very much and in the light of these developments, modern and more

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complicated heat pump systems have come out recently. Therefore, it is necessary to make a comprehensive thermodynamic performance and economic comparison among different types of heat pumps to determine which of them is better in terms of their heat sources and sinks as water or air. Many studies and investigations have been reported in the open literature on different types of heat pumps and on comparing them experimentally or by simulations, according to their heat sources, heat sinks, and the place they are being used, the refrigerant used as working fluid, structures and capacities of components i.e. For example; Swardt and Meyer [5] reported that the performance of a reversible ground-source heat pump coupled to a municipality water reticulation system was compared experimentally with simulations of a conventional air-source heat pump for space cooling and heating. The experimental and simulated comparisons of the ground source system with the air source system were conducted in both cooling and heating cycles. The results showed that utilization of municipality water reticulation system for a heat source/sink is a viable method of optimizing energy usage in the air conditioning industry, especially when used in the heating mode. Especially at low ambient air temperatures, ground-source heat pumps have significant capacity (24%) and efficiency improvement (20%) over air-source heat pumps. Petit and Meyer [6] made a comparison of the economic viability in South Africa of horizontalground source systems and air-source systems. In order to realize this aim, monthly heating and cooling capacities and coefficients of performance for both systems were determined. The payback period, net present value, and internal rate of return of systems were calculated. It was concluded that ground source systems are more viable than air source systems. Urchueguia et al. [7] made an experimental comparison between a ground coupled heat pump system and a conventional air to water heat pump system, focusing on the heating and cooling energy performance. For the whole climatic season the results obtained showed that the geothermal system saves in terms of primary energy consumption a 43  17% of the energy consumed by the conventional one when the system is working in heating mode and a 37  18% win cooling mode when the system is working in cooling mode. The results also demonstrated that a ground source heat pump system is a viable energy efficient alternative to conventional system for heating and cooling applications in the South European regions. Kavak Akpınar and Hepbas¸lı [8] reported that they made a comparative study on exergetic assessment of two ground-source (geothermal) heat pump systems for residential applications. The study dealt with the exergetic performance evaluation of two types of ground source heat pump systems installed in Turkey based on the actual operational data. The first one is a ground source heat pump system designed and constructed for investigating geothermal resources with low temperatures, while the other one is a ground source heat pump with a vertical ground heat exchanger. Esen et al. [9] made a study which reported a techno-economic comparison between a ground-couple heat pumps system (GCHP) and air coupled heat pump (ACHP) system. The experimental results were obtained from June to September in cooling season in 2004. The test results indicated that system parameters can have an important effect on performance and that GCHP systems are economically preferable to ACHP systems for the purpose of space cooling. Heat pump systems are compared according to their thermodynamic performances depending on refrigerant type used in most of the comparative studies. Venkataramanamurthy and Kumar [10] made a study which presents an experimental comparison of energy, exergy flow, and second law efficiency of R22 and is substitute R436b (hydrocarbon mixture of 52% of propane (R290) 48% of isobutene (R600a)), vapor compression refrigeration cycles. The

exergy flow of various points on refrigeration cycle, efficiency and second law efficiency for both R22 and R436b refrigeration cycles were compared. The results showing the location of inefficiencies were presented graphically. The study made by He et al. [11] describes the investigation in a single-evaporator domestic refrigerator of an R22/R142b refrigerant mixture as a substitute for R12. Notes that a steady-state thermodynamic cycle analysis indicated an increase in the coefficient of performance 3%e5% with R22/ R142b in the composition range of 0.3e0.6 mass fraction R22 compared to R12. Sopha [12] et al. studied on factors that influence the choice of heating system based on Norwegian households’ perceptions. Electric heating, heat pump and wood pellet heating were compared with a special focus on wood pellet heating. This study was conducted as questionnaire survey on two independent samples. The first sample consisted of 188 randomly chosen Norwegian households, mainly using electric heating and the second sample consisted of 461 households using wood pellet heating. Their results showed that socio-demographic factors, communications among household, the perceived importance of heating system attributes and the applied decision strategy all influence the Norwegian homeowners. Strategies for possible interventions and policy initiatives are discussed. Bakirci et al. [13] made a study in order to investigate the performance of the solar-ground source heat pump system in the province of Erzurum having cold climate. The COP of the heat pump and the system were found to be in range of 3.0e3.4 and 2.7 to 3.0, respectively. It is also claimed that the system investigated could be used for residential heating in the province of Erzurum, a cold climate region of Turkey. S.P. Lohani [14] made a study which dealt with the energy and exergy analysis of a fossil plant and ground and air source heat pump building heating system at two different dead-state temperatures. The outcome of the energy and exergy flow analysis at two different dead-state temperatures revealed that the ground source heat pumps with ambient reference have better performance than other ground reference systems as well as fossil plant (conventional system) and air source heat pumps with ambient reference. In a study which is made by Jiangfeng Guo and Xiulan Huai [15] a multi-parameter optimization approach of Isopropanol Acetone Hydrogen (IAH) chemical heat pump is developed based on the entransy theory. In the optimization process, the total lowtemperature heat consumed by the heat pump system generally decreases while the high temperature heat recovered by the heat pump increases remarkably. In another study it is made by Meggers et al. [16] presents implementations of LowEx technologies in prototypes, pilots and simulations, including experimental evaluation of our new hybrid PV-thermal (PV/T) panel, operation of integrated systems in an ongoing pilot building project, and cost and performance models along with dynamic simulation of our systems based on our current office renovation project. LowEx systems provide many heating and cooling methods for buildings using moderate supply temperatures and heat pumps that exploit more valuable energy sources. In the study which is presented by Li et al. [17] a direct expansion solar-assisted heat pump water heater (DXSAHPWH) with rated input power 750 W was tested and analyzed. Through experimental research in spring and thermodynamics analysis about the system performance, some suggestions for the system optimization are proposed. Then, a small-type DX-SAHPWH with rated input power 400W was built, tested and analyzed. Through exergy analysis for each component of DX-SAHPWH (A) and (B), it can be seen that the highest exergy loss occurs in the compressor and collector/evaporator, followed by the condenser and expansion valve, respectively. A theoretical and experimental exergy analysis of a solar-assisted heat pump for air heating is presented by Torres Reyes et al. [18]. An experimental prototype

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that operates as a solar-assisted or as a conventional heat pump was tested to determine exergetic efficiency, total system irreversibility and component irreversibilities. A methodology for determination of the optimum temperature of the working fluid in the evaporation and condensation steps is proposed. A numerical simulation of a plate contact-type isothermal heat pump dryer (HPD) is used to examine the energy efficiency improvement obtainable from this system compared with a conventional HPD by Catton et al. [19]. The simulation incorporates a detailed plate, product and air flow model, solving the mass, momentum and energy balances within the drier, into a pre-existing model of the remaining HPD components. There are various studies on comparison of different heat pump types. Most of them were made between two heat pump systems. Some of the comparisons were made between the systems, which were installed at different regions and which have different environmental conditions or which were used for different aims at different places or which uses different refrigerants as working fluid. Additionally, some comparison studies were made between the heat pumps which were run and tested under different conditions or which have different heating capacities or different properties. For instance, Venkataramanamurthy and Kumar [10] made a performance comparison study between two heat pumps; one of them uses R22 and the other one uses R436b as refrigerant. For another example, Petit and Meyer [6] compared the performances of two heat pump systems as they are reversible ground-source heat pump coupled to a municipality water reticulation system and a conventional air-source heat pump. In a comparative study presented by Kavak, Akpınar and Hepbas¸lı [8], exergetic performance evaluation of two types of ground source heat pump systems which are installed in Turkey and have different properties was determined. In this study, unlike the others, a comprehensive energetic and exergetic comparative investigation is made for four different heat pump types which are used for heating application. An experimental multifunctional heat pump setup was designed and established to achieve this aim. Four different heat pump systems can run on one system using only one compressor and same piping lines. The multifunctional heat pump experimental setup that can be operated in four different modes, including air to air, air to water, water to air and water to water modes, was installed in the laboratories of Mechanical Engineering Department of Atatürk University in Erzurum, Turkey (Fig. 1). The system uses R22 as refrigerant and heating capacity of four systems are equal to each other. Experimental system consists of two condensers including one water-cooled and one air-cooled and two evaporators, including one water-heated and one air-heated, two expansion valves and one compressor. The direction of the refrigerant used in the system can be easily changed between the condensers and evaporators with the help of valves. In this way, the designed heat pump experimental setup can be operated in different modes. Heating capacity of each heat pump mode is equal under the same conditions and that can be achieved by changing the flow rate and temperature levels of the fluid used as heat source and sink. Only one measurement equipment was used to get data for all heat pump types and all experiments. 2. Experimental setup and measurement procedure A schematic diagram of the experimental apparatus is shown in Fig. 1. The system was originally designed for operating with R22. The main components of the system are a scroll compressor, an air cooled evaporator, a water cooled evaporator, an air cooled condenser, a water cooled condenser, two thermostatic expansion valves, and the other auxiliary elements like measurement and

213

control equipment. In addition, electrical air and water heaters are used in order to keep the temperature of air and water passing into the evaporators and condensers at the desired levels. The compressor used on the system is a hermetic scroll type, which is powered by 2.8 HP and driven by three-phase electricity 380 V 50 Hz current and which can be operated with R22, R407C and R134a refrigerants. The air cooled evaporator and condenser used on the system are plate-fin types heat exchangers. The capacity and heat transfer area of air cooled condenser and evaporator are 6.35 kW15.45 m2 and 4.35 kW-15.7 m2. The water cooled heat exchangers which are used as condenser and evaporator on the system are shell and tube types. Water cooled condenser has a 6000 kCal/h capacity with 0.38 m2 and the evaporator has 5400 kCal/h capacity with 0.44 m2. The expansion valves are put on the inlets of the evaporators. Two slight glasses are installed at the outlets of the evaporators to provide visual confirmation of the phase state of the refrigerant and other auxiliary fluids. Two electrical speed adjustable fans are used to circulate the air on the evaporator and condenser which are air cooled and tap water is used directly for the water cooled evaporator and condenser by using no pump. Temperature and pressure of working fluid were measured at several points of interests, as shown in Fig. 1. K type nickel thermocouples were used to measure the temperatures of the working fluid and the thermocouples were calibrated with a digital temperature controller. The working fluid temperatures are measured at the inlets and outlets of the evaporators, condensers and compressor. The temperatures of the water used on the water cooled condenser and evaporator were measured at the inlets and outlets of the heat exchangers and storage tanks. In addition, four thermocouples were used at different points of every entrance and exit of air channels to determine air temperatures correctly at the inlet and outlet of the air cooled evaporator and air cooled condenser. Measurement of the temperature and humidity of outdoor air was recorded for every test. Only one data logger is used to determine and record all the temperature measurements obtained from the system. Six bourdon type manometers were installed at the inlets and outlets of the condensers, evaporators, and compressor to measure the pressures for working fluid. Measurement of flow rate of the water was made by using two rotameters, flow rate of working fluid used in the cycle was determined with a flow meter, and air flow was measured by using an anemometer. Compressor input current and voltage were obtained by using a multifunctional amperemeter and wattmeter which can show cos (4) value additionally. The system was charged only once for all tests with 15 Bar R22 while all the valves were in opened position under the temperature of outdoor air. When the system is wanted to be run in any heat pump mode, it is needed to close required valves. If it is needed to change the running mode of the system, it is enough to open some relevant valves and close others. The experiments were made and completed for air to air mode first and followed by air to water, water to air and finally water to water. When the tests were completed for any mode and it was required to convert the system to another mode, we opened all of the valves on the system and waited for one day. Then we closed the required valves for new mode and made the new tests. Temperature and pressure values in the key-point of the plant, as shown in Fig. 1, were continuously monitored in order to check the achievement of steady-state conditions. Mostly, the start-up time required about 1 h. After each experimental run, the raw data was recorded, including temperatures from the thermocouples, pressures from the manometers, air flow rates from the anemometer, and refrigerant flow rate from the flow meters, water flow rates from the rotameters, compressor input current and voltage from the amperemeter.

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Fig. 1. Schematic view of the experimental setup.

The tests were performed under laboratory conditions, during the air temperature was about 20e22  C, and the relative humidity was about 30e40%. The experiments were made under the following order for four running mode; first air to air mode, second air to water mode, third water to air and finally water to water mode tests were completed. Each experiment was repeated at least three times under the same conditions at different times. When all the tests were made and completed for any running mode, the experimental setup was closed and waited about one day by opening all valves on it before starting the tests for another running mode. A lot of tests were made by changing the temperature and

mass flow rates of the cooling fluid of evaporator, under constant heating capacity of the condenser for each of the four heat pump modes. First, the temperature degree and mass flow rate of the condenser fluid were adjusted to the required values; for instance, 20  C-1.2 kg/s for air, 20  C-0.15 kg/s for water and experiments were performed at six different temperature levels for a constant mass flow rate of the evaporator fluid. And then the same process was applied for other four different mass flow rates of the evaporator fluid. For example, for air to air mode; first, the mass flow rate of the evaporator air was adjusted to a constant value for level 1 (0.41 kg/s) and the tests were performed for five different

U. Çakır et al. / Energy 58 (2013) 210e219

evaporator air temperatures, they were 20e22e26e28e30  C. The same experiment procedure was applied for the second mass flow degree (0.71 kg/s) of the evaporator air. The numerical values of the evaporator fluid temperatures and flow rates are presented in Table 1 for four heat pump experiments. As seen on the Table 1, mass flow rates and temperature degrees of the evaporator fluid are different for heat pump modes if it is air or water. It is necessary to bring the capacity of heat pump types to an equal value. 3. Data reduction

215

3.2. System exergy analysis Energy and exergy balances can be written as;

E_ in ¼ E_ out

(7)

_ ¼ m,ex _ Ex

(8)

_ _ _ Ex in  Exout ¼ Exdest

(9)

The specific flow exergy of the refrigerant and water is evaluated Since this study focuses on comparing exergy treatments of four different heat pump systems, air to air, air to water, water to water and water to air, an exact and detailed exergy analysis of four heat pump mode is required at that moment. In addition, this analysis focuses on evaluating some representing figures which in general can be extrapolated for all heat pump modes with reasonable accuracy and simplicity.

as

exr;w ¼ ðh  h0 Þ  T0 ðs  s0 Þ

where h is enthalpy, s is entropy and the subscript zero indicates properties at the reference (dead) state. The total flow exergy of air is determined as [21];

exair ¼

3.1. Determination of heat-pump characteristics All of the measured values which consist of temperatures from the thermocouples, pressures from the manometers, compressor input power from the wattmeter, and flow rates of the fluids used on the system from the flow meters, rotameters and anemometers were used to determine the exergetic performances of the four heat pump modes to achieve the aim mentioned above. If you want to make a healthy exergy analysis, first it is necessary to make an energy analysis. Mass, energy and exergy balances are employed to determine the heat input, the rate of exergy destruction, and energy and exergy efficiencies. From the measured parameters: General energy and exergy calculations of the systems are made as follows [20,21]. The heat delivered by the condenser to the air or water is calculated by

_ a Cpa ðTca out  Tca in Þ Q_ conair ¼ m

(1)

_ w Cpw ðTcw out  Tcw in Þ Q_ conwater ¼ m

(2)

    T  T  1  ln Cpa þ uCpvapor T0 T0 T0   þ ð1 þ 1:6078uÞRair T0 ln P=P p 0 þ Rair T0 ð1 þ 1:6078uÞln½ð1 þ 1:6078u0 Þ= 

 ð1 þ 1:6078uÞ þ 1:6078u ln u=u 

where the specific humidity ratio is;

u¼

mvapor mair

(12)

Exergy efficiency and exergy destruction of the heat pumps are as follow;

_ Ex

hex;HP ¼ _ heat ¼ W in;elec

_ _ Ex in cond  Exout cond pffiffiffi I,U,cosð4Þ, 3

_ _ _ W in elec ¼ W comp elec þ W fans elec

_ a Cpa ðTea in  Tea out Þ Q_ evapair ¼ m

(3)

_ Ex dest HP ¼

_ w Cpw ðTew in  Tew out Þ Q_ evapwater ¼ m

(4)

(5)

where U, I and cos (4) are voltage (V), current (A) and power factor, respectively. The coefficient of performance (COP) for any heat pump;

COPHP ¼

Q_ Cond _ _ W comp elec þ W fan elec

X

_ Ex in HP 

X

(13)

(14)

_ out HP Ex

(15)

3.3. Experimental uncertainties

The power input to the compressor is calculated by

pffiffiffi 3,cosð4Þ,U,I

By using the estimation method of Kline and McClintock [22,23], maximum uncertainties of the COP and exergy efficiency are found as follows: COP, 3.53%; exergy efficiency, 3.53%. The individual contributions to the uncertainties of the COP and exergy efficiency for each of the measured physical properties are summarized in Table 2.

(6) Table 2 Uncertainties in the values of the relevant variables.

Table 1 Levels of parameters. Parameter

Fluid

Level 1

Level 2

Level 3

Level 4

Level 5

Mass flow rate (kg/s)

Air Water Air Water

0.41 0.100 20 12

0.71 0.110 22 14

0.83 0.125 26 16

1.1 0.139 28 18

1.2 0.150 30 20

Temperature ( C)

(11)

0

The heat extracted by the evaporator from the air or water is calculated by

_ W Comp elec ¼

(10)

Variables

Uncertainty (%)

Temperature (T) Pressure (P) Voltage (U) Current (I) Power factor (cos (4)) Mass flow rate (mr)

2.5 1.6 1.7 1.7 1.7 1,3

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4. Results and discussion Four different heat pump systems which are run on the same experimental setup have the same heating capacity, use just one compressor, are controlled by the same control equipment. The same measurement system is used. They are compared experimentally and exergetically with each other by using second law of thermodynamics in this study. All of the experiments were performed in the laboratory under the same conditions such as the air temperature and air humidity ratio. Within the scope of the study presented here; COP, exergy efficiency and rate of exergy destruction of heat pump types are evaluated, discussed and compared as the results of this paper. Energetic and exergetic performance differences of four different heat pump types according to the increasing rate of temperature and according to flow rate of the fluid used as heat source are expressed and compared. The results obtained from the main study may be presented and compared in three groups, namely COP values, exergy efficiencies and the rate of exergy destructions. Figs. 2 and 3 shows the change in COP values of four heat pump units according to the increasing rate of temperature and mass flow rate levels of the fluid which is used as heat source. As seen on the Fig. 2, the heat pump unit which has the maximum COP value is water to air type (3.94), second one is water to water type (3.73), and third one is air to air type (3.54). The heat pump that has the lowest COP value is air to water type heat pump system (3.40). As it is known, heat pumps are the systems which transfer the heat from a low-temperature medium to a high temperature medium. When the heat sink temperature rises by 12  C COP of water to air type heat pump increases from 3.78 to 3.94. The COP of water to water type heat pump increases from 3.71 to 3.77; COP of air to air type heat pump increases from 3.42 to 3.53 and COP of air to water type increases from 3.34 to 3.80. When the heat pumps analyzed here are compared with each other according to their mean COP values, we get the following results. COP of the water to air type is 4.68% higher than COP of water to water type, 11.66% higher than COP of the air to air type and 16.71% higher than COP of air to water type. COP of the water to water heat pump system which is the second in ranking is 6.66% higher than COP of air to air type and 11.49% higher than COP of air to water type. Additionally COP of the air to air heat pump is 4.53% higher than COP of the air to water heat pump. Total energy of the refrigerant that enters the condenser consists of the heat energy absorbed from the cooling fluid at the evaporator and of the energy that the refrigerant gains in the compressor. As mentioned above, heating capacity of the heat pumps are equal to each other and that means Qcond is constant in this study. Additionally, as it is known, Qcond consists of the sum of Qevap and Wcomp.

Fig. 2. COP change of heat pumps versus increasing of the evaporator fluid temperature.

Fig. 3. COP change of heat pumps versus increasing of mass flow rate of the evaporator fluid.

For that reason, one of the main factors that determines the COP of the system is amount of the heat energy that refrigerant obtains in the evaporator from the cooling fluid, and the other one is the amount of the energy that refrigerant gains in the compressor. For example, when the refrigerant takes more energy in the evaporator that means it will take less energy in the compressor, thereby, COP of the system will be higher. Based on this knowledge we can say that when we use water as cooling fluid at the evaporator, COP of the systems becomes better because of the heat transfer and thermal properties of water. When water is used as external fluid at the evaporator, COP of the system becomes higher than others. It is well known that COP of heat pumps increases depending on the increase in the heat source temperature. When four heat pump types are compared, there is a further increase in COP value of the heat pumps which use air as condenser fluid (water to air and air to air type) than COP of the other heat pumps which are water to water and air to water types. It is observed that COP of the air to air heat pump becomes higher than COP of water to water mode when the temperature of the heat source rises over 12  C, namely when the temperature of evaporator air exceeds 30  C and the temperature of evaporator water exceeds 20  C. This situation may occur since specific heat of air increases with temperature increase and specific heat of water does not change. Mass flow rate of the fluid used as heat source has a big importance for the performance of heat pumps. It is known that heat transfer is improved with the increasing flow rate. It is understood from Fig. 3 that COP of the heat pumps which use air as evaporator fluid (air to air and air to water) are decreasing while COP of the other types (water to water and water to air types) are increasing with the increasing of the mass flow rates of the fluid used as heat source. For example COP of the water to air and water to water heat pump increase from 3.87 to 3.94 and from 3.6 to 3.70 when mass flow rate of the heat source increases from level 1 to level 5. On the contrary to this COP of the air to air and air to water heat pumps decrease from 3.67 to 3.55 and from 3.51 to 3.41. This situation may be derived from the high air velocity in the evaporator. The duration which is required for heat transfer decreases with an increase in air velocity. At the same time this is mainly due to absence of the energy consumption of fans. Additionally, the highest increase rate is actualized on the COP of the water to water heat pump with the increase in mass flow rate of the evaporator water. Exergy efficiencies of water to air and air to air heat pumps are higher than the exergy efficiencies of water to water and air to water type heat pumps as presented on Fig. 4. Ranking of the heat pump types according to their exergy efficiency from high to low is as follows; water to air (30.23%), air to air (30.22%), air to water (24.77%) and water to water (24.01%). Heating capacities of four

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Fig. 4. Exergy efficiency change of heat pumps versus increasing of heat source temperature.

Fig. 5. Exergy efficiency change of heat pumps versus increasing of mass flow rate of the evaporator fluid.

heat pump types can be synchronized easily by adjusting the mass flow rate and temperature of the condenser fluid even if the temperature of refrigerant enters the condenser with different levels. The amount of the heat energy given to the condenser fluid from the heat pumps is controlled by this adjustment but the amount of the exergy given to the condenser fluid from the heat pump cannot be controlled since the exergy is not the only property of the systems. It is a property of systemesurroundings combination. The exergy of a fluid is related to the properties of the surroundings as dead state temperature. The difference between the temperature of surroundings and the temperature of fluid determines the exergy of fluid. Mass flow rate and temperature of the air and water which are used as condenser cooler must not be equal if it is wanted to synchronize the heating capacity of the heat pump systems. For that reason, the amount of exergy given to the air from condenser is higher than the exergy given to water. That is why the exergy efficiency of the heat pumps using air as external fluid in the condenser is higher than the others’. Exergy efficiencies of the four heat pump types increase depending on an increase in the heat source temperature as expected. On the other hand, exergy efficiencies of the heat pumps, which are air to air and water to water types, increase more than others. Exergy efficiency of air to air heat pump becomes higher than the exergy efficiency of water to air mode. Likewise, exergy efficiency of water to water heat pump becomes higher than exergy efficiency of air to water mode when the temperature of the heat source fluid increases up to the higher levels over 20  C for water and over 30  C for air. Exergy efficiency of heat pumps changes as follows; depending on the increase of 12  C in the temperature of the fluid which is used as heat source. As a result of this exergy efficiency of water to air system increases from 29.86% to 30.26; air to air system from 29.78% to 30.29%; air to water system from 24.31% to 24.78% and water to water system from 22.92% to 24.14%. Variations of the heat pump exergy efficiencies depending on the increases in mass flow rates of the heat source fluid can be seen on Fig. 5. When mass flow rate of the evaporator fluid increases from level 1 to level 5, exergy efficiency of the air to air heat pump decreases from 30.83% to 30.20%; water to air heat pump from 30.30% to 30.20%; water to water heat pump from 24.17% to 24.50% and air to water heat pump from 24.83% to 24.76%. The exergy input of the compressor depends on the temperature of the refrigerant which comes from evaporator. When the refrigerant enters to the compressor with higher temperature, it becomes much more difficult to transfer exergy and energy to the refrigerant from compressor and thus the compressor causes to increase the

temperature of refrigerant to the higher levels. Increasing the mass flow rate of evaporator fluid makes the compressor use more energy and that situation causes more exergy input to the system. After the mass flow rate of the evaporator fluid increases to the second level or more, the compressor consumes much more energy but simultaneously it causes much more rise in refrigerant temperature at its exit. For that reason, more exergy is given to the condenser fluid and exergy efficiency increases. According to mean values of the exergy efficiency of each heat pump system; exergy efficiency of the water to air heat pump is 0.07% higher than exergy efficiency of air to air heat pump, 22.10% higher than exergy efficiency of air to water heat pump, 26.02% higher than water to water heat pump. Additionally exergy efficiency of the air to air heat pump is 22.01% higher than exergy efficiency of air to water heat pump, 25.93% higher than exergy efficiency of water to water heat pump. Exergy efficiency of air to water heat pump is 3.21% higher than exergy efficiency of water to water heat pump. Fig. 6 shows the exergy destruction rate changes of four systems according to the conditions of evaporator fluid. As seen on the figure, ranking of the exergy destruction rates of the four heat pump types from high to low is as follows; air to air (2.93 kW), air to water (2.84 kW), water to air (2.64 kW) and water to water (2.55 kW). As seen on the figure, the rate of exergy destruction of the air to water heat pump is being affected more than others

Fig. 6. Exergy destruction rate changing of heat pumps versus increasing of the evaporator fluid temperature.

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Fig. 7. Exergy destruction rate changing of heat pumps versus increasing of the mass flow rates of evaporator fluid.

depending on the increase in the heat source temperature. It is understood from Fig. 7 that mass flow rate of the fluid used as heat source is more effective on the exergy destruction rates of the heat pumps which use air as heat source fluid. When the temperature of the evaporator fluid increases by 12  C, exergy destruction of the air to air heat pump increases from 2.83 kW to 3.10 kW; air to water heat pump from 2.73 kW to 2.95 kW; water to air heat pump from 2.643 kW to 2.645 kW and water to water from 2.44 kW to 2.68 kW. According to the mean values of the amount of exergy destruction of each heat pump system; exergy destruction of air to air heat pump is 3.20% higher than exergy destruction of air to water heat pump, 11.14% higher than exergy destruction of water to air heat pump and 14.97% higher than exergy destruction of water to water heat pump. Exergy destruction of air to water heat pump is 7.69% higher than water to air heat pump and 11.41% higher than exergy destruction of water to water type. Additionally exergy destruction of water to air heat pump is 3.45% higher than exergy destruction of water to water heat pump type. The exergy destruction in any component of the heat pump is not caused only by the inefficiencies of it. That means the exergy destruction of a component depends on the inefficiencies of other system components. We can clearly see the change of exergy destruction rates of the systems on the figures which were formed by using the results of tests but the contributions of the components to the total exergy destruction of the system must be taken into account to comment these figures. The largest exergy destruction of the heat pump systems occurred in condenser and was followed by compressor, evaporator and expansion valve. When air is used as evaporator fluid, exergy destruction of systems becomes higher than other types due to the increase of exergy destruction in the compressor and condenser. As mentioned before, using air at the evaporator causes to increase the refrigerant temperature in the compressor and in the condenser. Additionally, the effect of the exergy destruction rate of air fans must be taken into account. As seen on Fig. 7, exergy destruction of the heat pump systems which use air at the evaporator increases with the increase in mass flow rate of air due to the increase in exergy destruction rate of evaporator and fans.

The coefficient of performance of the heat pumps primarily depends on the temperature of the heat source. In addition, it depends on the mass flow rate of the heat source fluid. This analysis shows that the heat pump unit which has the maximum COP value is water to air type with 3.94, second one is water to water type with 3.73, and the third one is air to air type with 3.54. The heat pump that has the lowest COP value is determined as water to air type heat pump system with 3.40. The results presented in this study differ from some studies because four heat pump types were combined on one system with constant heating capacity in this study. Additionally, the specifications of the system components (like compressor) were determined to be able to work in all running modes. The results on energetic performance of the heat pump types are nearly similar to the results of other researches like Büyükalaca et al. [24], Chen et al. [25], Lee et al. [26] etc. The results of the study with reference number 24, which includes a comparison of air source and water source heat pumps and uses air cooled condenser, are very compatible to our results for air to air and air to water heat pump types. Additionally, in a study made by Urchuegia et al. [7] and which includes a comparison between air to water and water to water heat pumps, some results that supports our results are presented. The ranking of the four heat pump types from high to low according to the mean exergy efficiencies of each one is as follows; water to air type (30.23%), air to air type (30.22%), air to water type (24.77%) and water to water type (24.01%). Additionally the ranking of the four heat pump types from high to low according to the exergy destruction rates of them is as follows; air to air (2.93 kW), air to water (2.84 kW), water to air (2.64 kW) and water to water (2.55 kW). It is difficult to compare this study with others according to exergetic results since the heat pump comparison studies in the literature focus on energy performance in general. Additionally, exergy performance of the systems can differ depending on so many parameters. The systems compared in this study use just one compressor. For instance, the exergy destruction in any component of any system is not caused only by the inefficiencies of it. That means the exergy destruction of a component depends on the inefficiencies of other system components. When we compare some results of this study with other studies’ results partially, it is seen that they are similar to some studies [24e27]. It is understood that exergy efficiency of water to air and air to air heat pump systems are higher than the exergy efficiency of water to water and air to water type heat pumps. Variations of the exergy efficiency and exergy destruction of the systems according to the evaporator fluid temperature and according to mass flow rate differ from each other. When air is used at the evaporator, change in mass flow rate causes a serious increase in exergy destruction of the system. Acknowledgments This work was supported by The Turkish Scientific and Technological Research Council of Turkey (TUBITAK Project No. 105M030) and Ataturk University, Research Project Foundation (Project No. BAP-2005/16). The Authors wish to thank to TUBITAK and Ataturk University. References

5. Conclusion In this paper, energy and exergy analysis of four different heat pump types (air to air, air water, water to water and water to air) have been investigated; regarding temperature and mass flow rate changes of the evaporator fluid which is used as heat source.

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