Applied Thermal Engineering 25 (2005) 1459–1471 www.elsevier.com/locate/apthermeng

Exergoeconomic analysis of a solar assisted ground-source heat pump greenhouse heating system Onder Ozgener

a,*

, Arif Hepbasli

b,1

a

b

Solar Energy Institute, Ege University, 35100 Bornova, Izmir, Turkey Department of Mechanical Engineering, Faculty of Engineering, Ege University, 35100 Bornova, Izmir, Turkey Received 1 June 2004; accepted 25 September 2004 Available online 7 December 2004

Abstract EXCEM analysis may prove useful to investigators in engineering and other disciplines due to the methodology are being based on the quantities exergy, cost, energy and mass. The main objective of the present study is to investigate between capital costs and thermodynamic losses for devices in solar assisted groundsource heat pump greenhouse heating system (SAGSHPGHS) with a 50 m vertical 32 mm nominal diameter U-bend ground heat exchanger. This system was designed and installed at the Solar Energy Institute, Ege University, Izmir, Turkey. Thermodynamic loss rate-to-capital cost ratios are used to show that, for components and the overall system, a systematic correlation appears to exist between capital cost and exergy loss (total or internal), but not between capital cost and energy loss or external exergy loss. This correlation may imply that devices in successful air conditioning are configured so as to achieve an overall optimal design, by appropriately balancing the thermodynamic (exergy-based) and economic characteristics of the overall system and its devices. The results may (i) provide useful insights into the relations between thermodynamics and economics, both in general and for SAGSHPGHS, (ii) help demonstrate the merits of second-law analysis. It is observed from the results that the maximum exergy destructions in the system particularly occur due to the electrical, mechanical and isentropic efficiencies and emphasize the need for paying close attention to the selection of this type of equipment, since components of inferior performance can considerably reduce the overall performance of the system. In conjunction with this, the

*

Corresponding author. Tel.: +90 232 388 4000/1242; fax: +90 232 388 6027. E-mail address: [email protected] (O. Ozgener). 1 Currently as a visiting professor in Faculty of Engineering and Applied Science, University of Ontario Institute of Technology (UOIT), 2000 Simcoe Street North, Oshawa, ON, L1H 7K4 Canada. 1359-4311/$ - see front matter
2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2004.09.015

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total exergy losses values are obtained to be from 0.010 kW to 0.480 kW for the system. As expected, the largest energy and exergy losses occur in the greenhouse and compressor. The ratio of thermodynamic loss rate to capital cost values are obtained for a range from 0.035 to 1.125.
2004 Elsevier Ltd. All rights reserved. Keywords: Heat pump; Exergy; Energy; Exergoeconomic; Renewable energy

1. Introduction Greenhouses provide a shelter in which a suitable environment is maintained for plants. Solar energy from the sun provides sunlight and some heat, but you must provide a system to regulate the environment in your greenhouse. This is done by using heaters, fans, thermostats, and other equipment [1]. The heating requirements of a greenhouse depend on the desired temperature for plants grown, the location and construction of a greenhouse, and the total outside exposed area of the structure. As much as 25% of the daily heat requirement may come from the sun, but a lightly insulated greenhouse structure will need a great deal of heat on a cold winter night. The heating system must be adequate to maintain the desired day or night temperature. Solar-heater greenhouses were popular briefly during the energy crisis, but did not prove to be economical to use. Separate solar collection and storage systems are large and require much space. However, greenhouse owners can experiment with heat-collecting methods to reduce fossil-fuel consumption. One method is to paint containers black to attract heat, and fill them with water to retain it. However, because the greenhouse air temperature must be kept at plant growing temperatures, the greenhouse itself is not a good solar heat collector. There are many systems available that may be used for cooling and/or raising the relative humidity in the greenhouse. Relative humidity (RH) is a measure of how much water is dissolved in the air at a particular temperature expressed as a percentage. Generally, growth of many plants is relatively unaffected by RH between 45% and 85%. Plants growing at RH below 45% may grow slowly, have smaller leaves, require water more frequently, or develop burned leaf margins or leaf tips. Plants growing at RH above 85% are susceptible to fungal pathogens, especially if water condenses on the foliage. Several conditions can occur in a greenhouse that result in problems caused by high or low RH. During the summer, high light, high temperature, and rapid air movement from fans can reduce RH to unacceptable levels. Shading to reduce light and temperature and using misters or evaporative cooling are the best solutions. It is also advisable to keep the greenhouse full of plants because plants generate a lot of RH [2]. Greenhouses also have important economical potential in TurkeyÕs agricultural. In addition to solar energy gain, greenhouses should be heated during nights and cold days. In order to establish optimum growth conditions in greenhouses, renewable energy sources should be utilized as much as possible. Effective uses of heat pump with suitable technology in the modern greenhouses play a leading role in Turkey in the future [3]. In the analysis and design of energy systems, techniques are often used which combine scientific disciplines (mainly thermodynamics) with economic disciplines (mainly cost accounting) to achieve optimum design. For energy conversion devices, cost accounting conventionally considers relative cost per an unit based on energy [4].

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Nomenclature CV coefficient of variation K capital cost L thermodynamic loss n number of values in a set R ratio of thermodynamic loss rate-to-capital cost K RH relative humidity SD standard deviation GRP glass reinforced plastics GSHP ground-source heat pump SAGSHPGHS solar assisted ground-source heat pump greenhouse heating system Subscripts en energy loss ex exergy loss ex-e external exergy loss ex-i internal exergy loss j jth value in a set m mean max maximum min minimum s overall station

Many researchers e.g., [5–37] have recommended that costs are better distributed among outputs if cost accounting is based on the thermodynamic quantity exergy and [4]. One rationale for this statement is that exergy, but not energy, is often a consistent measure of economic value. Exergy analysis is important for all energy resource utilization, because exergy is the part of the energy analysis. The theory of exergy analysis is essentially that of available energy analysis. The concepts of exergy, available energy, and availability are essentially similar. The concepts of exergy destruction, exergy consumption, irreversibility, and lost work are also essentially similar. Terminology does not appear to have been standardized [35]. Exergy is a measure of the maximum useful work that can be done by a system interacting with an environment which is at a constant pressure P0 and a temperature T0. The simplest case to consider is that of a reservoir with heat source of infinite capacity and invariable temperature T0. It has been considered that maximum efficiency of heat withdrawal from a reservoir that can be converted in to work is the Carnot efficiency [36,37]. In addition, exergy-based economic analysis methodologies exist (e.g., exergoeconomics, thermoeconomics) [4]. Many of these researchers have developed methods of performing economic analyses based on exergy, which are referred to by a variety of names (e.g., thermoeconomics, second-law costing, cost accounting and exergoeconomics) [4–7].

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The approach involved examining data for devices in SAGSHPGHS, and showing that correlations exist between capital costs and specific second-law-based thermodynamic losses (i.e., total and internal exergy losses). The existence of such correlations likely implies that designers, knowingly or unknowingly, incorporate into their work the recommendations of exergy analysis. In this paper, the relations between thermodynamic losses and capital costs for devices is examined, and suggests possible generalizations in the relation between thermodynamic losses and capital costs. This paper (i) provides useful insights into the relations between thermodynamics and economics, both in general and for SAGSHPGHS, (ii) helps demonstrate the merits of second-law analysis.

2. System description and modeling 2.1. System description 2.1.1. Experimental set-up An experimental system was installed for investigating thermal performance of a SAGSHPS for greenhouse heating. It has been satisfactorily operated without any serious defects in the (2003/2004) heating season. The thermal performance results obtained during the heating period of 20th of January till 31st of March 2004 were given and discussed. At the same period, cucumus sativus cv. pandora F1 was raised, and product quality was improved with the climatic conditions in the designed SAGSHPGHS. The effects of climatic conditions and operating parameters on the system performance parameters were also investigated. The tests were conducted on the SAGSHPGHS under steady state conditions in the heating mode over the period from the 20th of January till the 31st of March 2004. Daily average values of 37 measurements from 8.30 a.m. to 4.00 p.m. with an interval of 15 minutes are given detail in authorsÕ previous works [31,34,38]. Fig. 1 illustrates a schematic diagram of the constructed experimental system, while Fig. 2 shows the main components of the system, namely a GSHP, flat plate solar collector, and solar greenhouse [31,34,38]. This system of three separate circuits: (i) the ground coupling circuit with solar collector (brine circuit or water–antifreeze solution circuit), (ii) the refrigerant circuit (or a reversible vapour compression cycle) and (iii) the fan coil circuit for greenhouse heating (water circuit). The main characteristics of the elements of the SAGSHPGHS system are given in Table 1, where the numbers in parentheses correspond to these elements as depicted in Fig. 1. Conversion from the heating cycle to the cooling cycle is obtained by means of a four way valve. To avoid freezing, a 10% by weight ethylene glycol water mixture was prepared. The refrigerant circuit was built of as closed loop copper tubing. The working fluid is R-22. The studied SAGSHPGHS was installed at Solar Energy Institute of Ege University (latitude 3824 0 N, longitude 2750 0 E), Izmir, Turkey. Solar greenhouse was positioned towards the south along a south-north axis. The greenhouse will be conditioned according to the type of the agricultural products to be raised in it. The exergoeconomic methodology is described by considering the balance equations for appropriate quantities, from [4].

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Sun Expansion tank VII

I

IV

GSHP unit

Solar collector VIII

II X

Greenhouse XI

IIII Pump I IX

Pump II VI

Valve

50 m

Ground level

Ground heat exchanger V

Fig. 1. The main components and schematic of the SAGSHPGHS.

Fig. 2. Various views of SAGSHPGHS: (a) solar greenhouse (consisting of glass fiber reinforced plastics) and flat plate collector; (b) a view of solar greenhouse inside; (c) solar assisted geothermal heat pump system.

2.2. Modeling and fundamental relationships 2.2.1. The general balance equation A general balance for a quantity in a system may be written as Input þ Generation  Output Consumption ¼ Accumulation

ð1Þ

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Table 1 Main characteristics of the solar assisted ground-source heat pump greenhouse heating system [31,34,38] Element

Technical specification

Refrigerant circuit

Compressor (I)

Type: hermetic; reciprocating; manufacturer: Tecumseh; model: TFH 4524 F; volumetric flow rate: 7.5 m3/h; speed: 2900 rpm; the rated power of electric motor driving: 2 HP (1.4 kW); refrigerant: R-22; capacity: 4.134 kW (at evaporating/condensing temperatures of 0/45 C) Manufacturer: Alfa Laval; model: CB 26-24; capacity: 6.66 kW; heat transfer surface: 0.55 m2

Heat exchanger (II) • condenser for heating • evaporator for cooling Capillary tube (III) Heat exchanger (IV)

Copper capillary tube; 1.5 m long; inside diameter: 1.5 mm Manufacturer: Alfa Laval; model: CB 26-34; capacity: 8.2 kW; heat transfer surface: 0.80 m2

• evaporator for heating • condenser for cooling Ground coupling circuit

Ground heat exchanger (V)

Brine circulating pump (VI)

Expansion tank (VII) Solar collector (VIII) Fan-coil circuit

Water circulating pump (IX)

Fan-coil unit (X) Greenhouse (XI)

Vertical-single U-bend type; bore diameter: 105 mm; Diameter of U-bends: 32 mm; of a bore diameter with a boring depth of 50 m; boring depth: 50 m; material: polyethylene Manufacturer: Marina; type: KPM 50; range of volumetric flow rate: 0.36–2.4 m3/h; pressure head: 41-8 m of water column, power: 0.37 kW; speed: 2800 rpm Manufacturer: Zimmet; type: 541/L; capacity: 12 litre; precharge: 1 bar 1.82 m2, flat-type Manufacturer: Marina; type: KPM 50; range of volumetric flow rate: 0.36–2.4 m3/h; pressure head: 41-8 m of water column; power: 0.37 kW; speed: 2800 rpm Manufacturer: Aldag; type: SAS 28; Cooling/heating capacities: 3.25/9.3 kW, air flow rate: 600 m3/h GRP surface area: 48.51 m2

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Main circuit

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Here, input and output refer respectively to quantities entering and exiting through system boundaries, generation and consumption refer respectively to quantities produced and consumed within the system, and accumulation refers to build-up (either positive or negative) of the quantity within the system. Differential and integral forms of the general balance equation may be written. The terms in Eq. (1) are written as rates, in the differential form [4]: Input rate þ Generation rate  Output rate  Consumption rate ¼ Accumulation rate

ð2Þ

and as amounts, in the integral form: Amount input þ Amount generated  Amount output  Amount consumed ¼ Amount accumulated

ð3Þ

The differential balance describes what is happening in a system at a given instant of time, and the integral balance describes what happens in a system between two instants of time. Differential balances are usually applied to continuous processes, and integral balances to batch processes. For steady-state processes, the accumulation rate term in the differential balance is zero. 2.2.2. Thermodynamic balance equations Energy, being subject to a conservation law (neglecting nuclear reactions), can be neither generated nor consumed. Exergy is consumed during a process due to irreversibilities, and is therefore subject to a nonconservation law. Consequently, the general balance equation (Eq. (1)) can be written for these quantities as [4] Energy input  Energy output ¼ Energy accumulation

ð4Þ

Exergy input  Exergy output  Exergy consumption ¼ Exergy accumulation

ð5Þ

and The output terms in Eqs. (4) and (5) can be separated into product and waste components. That is, Energy output ¼ Product energy output þ Waste energy output

ð6Þ

Exergy output ¼ Product exergy output þ Waste exergy output

ð7Þ

and

2.2.3. Economic balance equations Cost is an increasing, nonconserved quantity. The general balance equation (Eq. (1)) can be written for cost as [4] Cost input þ Cost generation  Cost output ¼ Cost accumulation

ð8Þ

Cost input, output and accumulation represent respectively the cost associated with all inputs, outputs and accumulations for the system. Cost generation corresponds to the appropriate capital and other costs associated with the creation and maintenance of a system. That is,

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Cost generation ¼ Capital cost of equipment þ All other creation and maintenance costs ð9Þ Other costs include, for example, interest and insurance costs. The ‘‘cost generation rate’’ term in a differential cost balance represents the total cost generation levelized over the operating life of the system. The ‘‘amount of cost created’’ term in an integral cost balance represents the portion of the total cost generation accounted for in the time interval under consideration. 3. Definitions of key terms Two types of thermodynamic losses are considered in the present paper. These are defined here, with the aid of differential forms of the thermodynamic balances already described (see Eqs. (4)–(7)). For simplicity, it is assumed that no losses are associated with the accumulation terms in the energy and exergy balances (Eqs. (4) and (5)). Energy losses can be identified directly from the energy balances in Eqs. (4) and (6). For convenience, the energy loss rate for a system is denoted in the present analysis as Len (loss rate based on energy). As there is only one loss term, the ‘‘waste energy output,’’ in Eq. (6) [4], Len ¼ Waste energy output rate

ð10Þ

Exergy losses can be identified from the exergy balances in Eqs. (5) and (7). There are two types of exergy losses: the ‘‘waste exergy output’’ in Eq. (7), which represents the loss associated with exergy that is emitted from the system, and the ‘‘exergy consumption’’ in Eq. (5), which represents the internal exergy loss due to process irreversibilities. These two exergy losses sum to the total exergy loss. Hence, the loss rate based on exergy, Lex, is defined as [4] Lex ¼ Exergy consumption rate þ Waste exergy output rate

ð11Þ

The capital cost is defined here using the cost balances in Eqs. (8) and (9) and is denoted by K. Capital cost is simply that part of the cost generation attributable to the cost of equipment: K ¼ Capital cost of equipment

ð12Þ

The principal reason that capital costs are used here is that the use of the cost generation term increases significantly the complexity of the analysis, since numerous other economic details (interest rates, component lifetimes, etc.) must be fully known. There are two main justifications for this simple cation [4]: • Capital costs are often the most significant component of the total cost generation. Hence, the consideration of only capital costs closely approximates the results when cost generation is considered. • Cost generation components other than capital costs often are proportional to capital costs. Hence, the trends identified in the present work will likely be in qualitative agreement with those identified when the entire cost generation term is considered. For a thermal system operating normally in a continuous steady-state steady-flow process mode, the accumulation terms in Eqs. (1)–(5) and (8) are zero. Hence all losses are associated with

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the already discussed terms Len and Lex. The energy and exergy loss rates can be obtained through the following equations [4]: X X Energy flux rates  Energy flux rates ð13Þ Len ¼ products

inputs

and Lex ¼

X

Exergy flux rates 

inputs

X

Exergy flux rates

ð14Þ

products

where the summations are over all input streams and all product output streams. Eqs. (13) and (14) are obtained by rearranging Eqs. (4)–(7), (10) and (11) [4]. 3.1. Ratio of thermodynamic loss rate-to-capital cost A new parameter, R, is defined as the ratio of thermodynamic loss rate L to capital cost K as follows [4]: R ¼ L=K

ð15Þ

The value of R generally depends on whether it is based on energy loss rate (in which case it is denoted Ren), or exergy loss rate (Rex), as follows: Ren ¼ Len =K

ð16Þ

Rex ¼ Lex =K

ð17Þ

and

Values of the parameter R based on energy loss rate, and on total, internal and external exergy loss rates are considered. In investigating sets of R values, maximum (Rmax), minimum (Rmin), mean (Rm), standard deviation (SD(R)) and coefficient of variation (CV(R)), which is the ratio of standard deviation to mean, are considered. Thermodynamic and economic data for SAGSHPGHS are used in the work. The following system sections are considered: the greenhouse coverings; condenser, evaporator, heat exchanger pipe, compressor, solar collector, and pumps. Based on values for K, L and R for the station devices, and statistical data for the four sets of R values, the following assessments can be made [4]: (i) the ratio of thermodynamic loss rate-to-capital cost is a significant parameter; (ii) since the relative spread in values for Ren and Rex-e is large and for Rex and for Rex-i is small, a systematic correlation may exist between total or internal exergy loss rate and capital cost, but likely not between energy loss rate and capital cost; (iii) the ratios Rex-i and Rex exhibit similar characteristics, likely since for real systems exergy losses are associated mainly with internal consumptions and only minimally with external emissions; (iv) the exergy-related parameters Len and Rex are less sensitive than the corresponding energy-related parameters Len and Ren to changes in the thermodynamic loss definition used, where multiple device cases are considered, while values of Len-i and Rex-i are independent of thermodynamic loss

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definition, since all cases involve changes in the contribution of external losses to total losses; and (v) values for devices in SAGSHPGHS of Rex and Rex-i appear to conform approximately to particular values, denoted Rex and Rex-i which may reflect the ‘‘appropriate’’ trade-off between exergy losses and capital costs practised in ‘‘successful’’ system designs.

4. Results and discussion Values for devices of K and several L and R parameters are listed in Table 2 for the SAGSHPGHS as given in Table 1. Statistical data for R values are presented for the system. The costs shown in Table 2 are in New Turkish Liras (YTL). The work discussed in this paper can likely be extended to marginal costs. Here, the marginal cost would be the cost increase resulting from saving one unit of energy or exergy (i.e., from reducing the energy or exergy loss by one unit). The results would be expected to indicate that marginal costs based on exergy for many devices have similar values, while marginal costs based on energy vary widely. The ratio of thermodynamic loss rate to capital cost values are obtained for a range from 0.035 to 1.125. As can be seen from Table 2, the most potential for optimisation is probably in the compressor, pump II, condenser, followed by the pump I, and the greenhouse. In conjunction with this, the total exergy losses values are obtained to be from 0.010 kW to 0.480 kW for the system. As expected, the largest energy and exergy losses occur in the greenhouse and compressor [34]. Greenhouse heating can be improved by using conduction heating systems; soil heating, floor heating and some branch heating. These heating systems can be installed as near to the plant heating systems, so floor heating can provide a good micro climate near to plant and result in important energy savings compared to air space heating. Since compressor power depends strongly on the inlet and outlet pressures, any heat exchanger improvements that reduce the temperature difference will reduce compressor power by bringing the condensing and evaporating temperatures closer together. From a design standpoint, compressor irreversibility can be attacked independently. In recent years, it has been reduced substantially by improving motors, valves, lubrication, etc. [33].

Table 2 Device parameter values for SAGSHPGHS (using New Turkish Liras (YTL)*) Device

K (103 YTL)

Len (kW)

Ren (kW/YTL)

Lex (kW)

Lex-i (kW)

Rex-i (kW/YTL)

Lex-ei (kW)

Rex-e (kW/YTL)

Greenhouse Condenser Evaporator Heat exchanger pipe Compressor Solar collector Pump_I Pump_II

2.6500 0.7308 0.7308 1.1360 0.4000 0.2300 0.1218 0.1218

3.977 0 0 0 0 0 0 0

1.5 0 0 0 0 0 0 0

0.480 0.220 0.140 0.040 0.450 0.010 0.029 0.049

0.480 0.220 0.140 0.040 0.450 0.010 0.029 0.049

0.181 0.301 0.191 0.035 1.125 0.043 0.238 0.402

0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0

*

1USD = 1.45 YTL (for 19 November 2004).

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5. Conclusions The present study, which uses the methodology proposed by Rosen and Dincer [4], is intended to provide insights into the relations between energyetic and exergyetic losses and capital costs for SAGSHPGHS, in particular, and for energy systems, in general. The results appear to be useful to those involved in the development of analysis and design methodologies that integrate thermodynamics and economics, and in the SAGSHPGHS. A systematic correlation appears to exist between exergy loss rate (total or internal) and capital cost for SAGSHPGHS. Furthermore, a correlation appears to exist between the mean thermodynamic loss rate-to-capital cost ratios for all of the devices in a SAGSHPGHS and the ratios for the overall system, when the ratio is based on total or internal exergy losses, but not when it is based on energy losses. This correlation may imply that devices in successful SAGSHPGHSs are configured so as to achieve an overall optimal design, by appropriately balancing the thermodynamic (exergy-based) and economic characteristics of the overall system and its devices. Particular attention must be given to improving energy savings of SAGSHPGHS due to the their increasing influence in global energy consumption. In the paper, an exergoeconomic analysis has been used to search for the optimal design of a compressor condenser to be used in a conventional vapour-compression heat pump. The causes of exergy destruction in the system include the compressor, greenhouse, heat exchanger (ground heat exchanger, condenser and evaporator), circulating pumps and solar collector losses. It is obvious from Table 2 that the highest irreversibility occurs in sub-regions I and V for the GSHP unit and the whole system, respectively. Energy and exergy analysis as partly described in this paper and more detail in a series of companion authorsÕ papers [31,34,38], has been applied to the design optimization of a SAGSHPGHS, using realistic cost estimates. The exergetic equivalents of the capital and labour costs have been computed based on global data available for Turkey in 2004. The losses in the motor–compressor subassembly are due to the electrical, mechanical and isentropic efficiencies and emphasize the need for paying close attention to the selection of this type of equipment, since components of inferior performance can considerably reduce the overall performance of the system. The second largest irreversibility in the GSHP unit is due to the condenser. This is partly due to the large degree of superheat achieved at the end of the compression process, leading to large temperature differences associated with the initial phase of heat transfer. The third highest irreversibility is in the capillary tube due to the pressure drop of the refrigerant passing through it. Besides this, the evaporator has the lowest irreversibility on the basis of the heat pump cycle. This example shows how the use of simple thermoeconomic optimisation methodologies in SAGSHPGHS could contribute to determining the a correct design of new equipment.

Acknowledgement The authors gratefully acknowledge financial support from the Ege University Research Fund, State Planning Organization of Turkey (DPT), Arges Inc., Entes Inc., Guz Technical Service Co., and Golden Seed Inc., Turkey.

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