Life cycle cost analysis of two different refrigeration systems powered by solar energy

Life cycle cost analysis of two different refrigeration systems powered by solar energy

Case Studies in Thermal Engineering 16 (2019) 100559 Contents lists available at ScienceDirect Case Studies in Thermal Engineering journal homepage:...

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Case Studies in Thermal Engineering 16 (2019) 100559

Contents lists available at ScienceDirect

Case Studies in Thermal Engineering journal homepage: http://www.elsevier.com/locate/csite

Life cycle cost analysis of two different refrigeration systems powered by solar energy Saad S. Alrwashdeh *, Handri Ammari Mechanical Engineering Department, Faculty of Engineering, Mutah University, P.O Box 7, Al-Karak, 61710, Jordan

A R T I C L E I N F O

A B S T R A C T

Keywords: Compression refrigeration system Vapor absorption refrigeration system Life-cycle cost analysis

This study provides an economic comparison between a vapor compression refrigeration system powered by a photovoltaic array and a vapor absorption refrigeration system powered by a solar evacuated tubes thermal unit. The comparison between these two technologies was conducted based on life-cycle cost analysis that included the total cost of purchase and operation over their entire service life. The total cost included the costs of acquisition, energy, repair, maintenance and disposal. The results of the life cycle cost analysis have indicated that both complete systems are cost-effective regarding their benefits over their total costs. However, the complete vapor compression refrigeration system is favored over that of the complete vapor absorption refrig­ eration system as it has yielded more benefits, in addition to having more other advantages.

1. Introduction Renewable energy considers as a promising candidate to cover the energy requirement for the different applications [19–42]. Vapor compression systems and absorption refrigeration systems powered by solar energy are used generally to reduce the energy bill as well as to reduce the detrimental effect of greenhouse gases pollution on the environment. In recent years, research has been increased to improve the performance of the refrigeration systems and one way of improving efficiency is through thermodynamic analysis and optimization. Many researchers studied such refrigeration systems that use solar irradiation to cover the electrical or thermal power required to drive the refrigeration cycles. To name only a few; Eicker et al. [1] made economic evaluation of solar thermal and photovoltaic cooling systems through simulation in different climatic conditions. Hartmann et al. [2] made a comparison of solar thermal and photovoltaic options for solar cooling in two different European climates. Aman et al. [3] analyzed an ammonia-water absorption cooling system for energy and exergy. Noro et al. [4] compared between thermal and photovoltaic solar cooling systems economically and on energy analysis basis. Lazzarin [5] made a thermodynamic and economic analysis for solar cooling using ammonia as a refrigerant. Beccali et al. [6] made life cycle performance assessment of small solar thermal cooling systems and conventional plants assisted with photovoltaics. Sarbu et al. [7] demonstrated the solar cooling absorption systems. Weber et al. [8] studied the solar cooling with water-ammonia absorption chillers and concentrating solar collector. Ozgorena et al. [9] studied the hourly performance prediction of ammonia-water solar absorption refrigeration. Sarbu and Sebarchievici [10] provided a detailed review of solar closed sorption (absorption and adsorption) refrigeration systems, which utilized working pairs of fluids. Siddiqui and Said [11], gave a review of recent published work in the field of solar powered absorption refrigeration systems which utilized pairs of working fluid.

* Corresponding author. E-mail address: [email protected] (S.S. Alrwashdeh). https://doi.org/10.1016/j.csite.2019.100559 Received 31 August 2019; Received in revised form 1 October 2019; Accepted 1 November 2019 Available online 6 November 2019 2214-157X/© 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

Case Studies in Thermal Engineering 16 (2019) 100559

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Nomenclature A F G i L P Q T

η

Annual cash flow Future worth Solar irradiation, W/m2 Real interest rate, % Losses, % Power, W, or Present worth Rate of heat transfer, W Temperature, � C Efficiency, %

Fig. 1. Vapor Compression Refrigeration Cycle (VCRC) electrically powered by a PV array system.

Aridhi et al. [12] presented a review of recent studies of improving the energy efficiency and cooling quality of refrigeration systems based on renewable energy. Junaidi and Suwa [13] studied a combination of solar-powered adsorption refrigeration system and thermal storage in which their analytical calculations and transient simulation indicated higher coefficient of performance. Zeyghami et al. [14] reviewed the important developments in thermo-mechanical solar cooling technologies with various system configurations, in which an overview of different solar cooling technologies and a detailed literature survey of published studies on the design and analysis, experimental setups and enhancement approaches for thermo-mechanical solar cooling systems were presented. Almutairi et al. [15] performed regional life cycle cost assessment and economic analysis of residential building air conditioning to evaluate their environmental impacts. The goal of this work was to check using cradle-to-grave life cycle cost assessment, for the same arbitrary chosen refrigeration capacity, which would be more economically cost effective; a standard vapor compression refrigeration system powered electrically by photovoltaic (PV) array system, or a vapor absorption refrigeration system powered thermally by solar evacuated tubes unit. 2. Description of models The Vapor Compression Refrigeration Cycle (VCRC) is described as work operated cycle because the elevation of pressure of the refrigerant is accomplished by a compressor that requires electrical power. The Vapor Absorption Refrigeration Cycle (VARC), on the other hand, is referred to as a heat operated cycle because most of the operating cost is associated with providing the heat that drives off the vapor refrigerant from the high pressure liquid solution in the generator. Indeed there is a requirement for some power in the absorption cycle to drive the absorber solution pump, but its amount of power for a given quantity of refrigeration is negligible compared with that needed to drive the compressor in the vapor compression refrigeration cycle. 2

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Fig. 2. Vapor Absorption Refrigeration Cycle (VARC) thermally powered by evacuated tubes.

Both compressor and absorption refrigerators use refrigerants with low boiling point. The main difference between the two types considered is the way the refrigerant is changed from a gas back into a liquid so that the cycle is repeated. Another difference is the refrigerant used. Compressor refrigerators typically use Ammonia, a HCFC or HFC, while absorption refrigerators typically use inorganic refrigerants, mainly Ammonia or Water. In this work, the refrigerant considered was Ammonia for the VCRC that would be electrically driven by using a PV array system, which are a lot cheaper than they were two or more decades ago, denoted by alternative 1 (see Fig. 1), whereas Ammonia-water pair is used for the VARC with the heat source provided by solar evacuated tubes utilizing the solar radiation thermal energy, denoted by alternative 2 (see Fig. 2). 3. Sizing of the solar systems The refrigeration capacity for either refrigeration system of study was arbitrarily chosen to be 15 kW. The energy demand was limited to 8 operating hours a day with 300 sunny days during a typical year in Jordan. A solar radiation of 1000 W/m2, and 20 � C annual daily average ambient temperature conditions were considered according to typical weather data of Jordan. Both models utilized solar irradiation to power their cycles. 3.1. Photovoltaic system PV is one of energy conversion methods that convert solar radiation into electrical energy directly. Thus, the electricity generated by PV modules is direct current DC, which means that either; one would employ a DC motor driven compressor for the VCRC, or install an inverter to convert the DC produced by the PV modules into AC to power the VCRC compressor. Since the market normally has AC motor driven compressors, this option was considered for the VCRC system. The total peak power of the PV array system required to supply the compressor electrical load depends on, solar radiation, ambient temperature, inverter efficiency and on a safety factor to compensate for the various system losses and temperature effect. The number of PV modules required to cover the VCRC electrical requirement is computed by the following relation (www. retscreen.net), (1)

Number of PV modules ¼ Pinv,kWh / Pmax, actual

where, Pinv,kWh is the daily actual power produced by the PV array which would necessarily represent the power input of the inverter (i. e. the amount of energy required per day in kWh to cover the electrical demand of the VCRC compressor), and can be calculated using the following equation, Pinv,kWh ¼ (Pdemand,max � operation hours/day) / ηinv (kWh/day) where Pdemand,max is the maximum demand in kW required by the VCRC compressor that can be computed from the cycle given specifications (the evaporator temperature at – 5 � C and that of the condenser at 45 � C), the efficiency of the compressor was assumed to be 90%, and ηinv is the efficiency of the inverter. Pmax, actual is the daily actual power produced by a single PV module in kWh taking into account the various losses, which reduce its delivered energy, is given by, (2)

Pmax, actual ¼ (1-LT) (1-LC) (1-LM) (1-LD) � Pnominal � PSH (kWh/day) 3

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where Pnominal is the maximum nominal power produced by one PV module of the selected type in Wp at standard conditions. LT, LC, LM, and LD are the loss factors due to temperature losses, cable losses, module mismatching losses, and dust accumulation losses, consequently. PSH is the number of peak sun hours per day, which is merely an estimation of the amount of time hours per day that the irradiance is equal to a peak sun of 1000 W/m2, and since PV modules are rated for their output under peak sun conditions, the number of daily peak sun hours indicates how many hours of each day the PV array will operate at its full nominal power output. The selected PV modules were SolarWorld 300Wp Mono-crystalline. The inverter chosen was SMA STP15000TL-4 which has a maximum efficiency of 98.1%. The PV module losses assumed were; temperature loss of 5%, cable loss of 4%, mismatching loss of 4%, and dust accumulation loss of 5%, which were adequate for this study. Furthermore, an annual average daily PSH of 4.5 h is normally assumed for Jordan region. Based on the data given above and Eq. (2), the number of PV modules can be computed. The work required by the VCRC compressor was determined to be equal to 3.59 kW. The PV calculations performed using Eqs. (1) and (2) showed that we needed 26 modules of 300 Watt capacity each to power the VCRC compressor. 3.2. Solar evacuated tubes system Solar evacuated tubes are a solar collector that converts solar radiation into thermal energy. Thus, it is used as the heat source to supply the VARC generator with a thermal energy needed to separate the refrigerant and absorbent mixture in the VARC. The performance of solar evacuated tubes can be represented as a graph or set of three performance variables, values of which may be provided based on gross area of tubes array, aperture area, or absorber area. More often the aperture or absorber area is used. In this work, however, the absorber area is used. Thus, the solar evacuated tubes performance can be represented by the following equation (www.apricus.com), (3)

η(x) ¼ η0 - a1 � (x) – a2 � G � (x) 2

here, x ¼ (Tm Ta)/G. where, η(x) is the evacuated tubes efficiency of heating up the water from the solar thermal energy, η0 is a conversion fac­ tor ¼ 0.717, a1 is a loss coefficient ¼ 1.52 W/(K2. m2), and a2 is another loss coefficient ¼ 0.0085 W/(K2. m2). Tm is the average inletoutlet water temperature to the solar evacuated tubes assumed from experience to be about 70 � C, and Ta is the ambient temperature in the location of the system ¼ 20 � C annual daily average, G is the solar irradiation assumed to be ¼ 1000 W/m2 for all cases, The actual peak output of each evacuated tube, Qevac, tube, is then determined from, Qevac,

tube ¼ Area

(4)

of each pipe � G � η(x) (W)

The VARC temperatures of the major components were assumed as follows; the evaporator at – 5 C, the condenser at 45 C, the generator at 90 � C, and the absorber at 25 � C. The heat exchanger in the absorption cycle is assumed to have an efficiency of 75%. First, the heating load at the generator, Qgen, in kW thermal was estimated using the data specified for the absorption cycle. Second, the efficiency of the solar evacuated tubes η(x) was estimated from Eq. (3), then the peak output of each evacuated tube collector was estimated using Eq. (4), upon which, the number of evacuated tubes that would cover the thermal power needed at the generator was determined by, �



(5)

Number of evacuated tubes ¼ Qgen / Qevac, tube

It was found that the heating load needed for the generator in the VARC system was equal to 28.1 kW, and upon using Eq. (3) – (5), the number of evacuated tubes required to cover the generator heating load was determined to be 160. 3.3. Energy savings cost calculations Both of the refrigeration systems were required to provide the refrigeration capacity of 15 kW using solar power, and consuming neither; electrical power or conventional thermal energy. Therefore, the energy savings would be assumed to be the same for both systems in providing the required refrigeration capacity. The annual energy savings could be assumed as the energy required (3.59 kW electric power) to operate the standard refrigeration system with the specifications given to provide the 15 kW refrigeration capacity, running for 8 h a day and for 300 days a year. This would amount to 8616 kWh/year. Based on a fixed energy rate cost for industrial organizations of 0.25 ¢/kWh electrical, it would turn out to be $2,154/year. It was assessed that the PV system that was tied to the national electrical network would provide the required electrical power for the VCRC all over the year, as there would be an over generation of power in some months and less generations in other months of a typical year. In this work, a complete economic analysis using the Life Cycle Costing method (LCC) was conducted for each of the two refrig­ eration technologies in order to determine the most cost - effective system.

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Table 1 Macroeconomic statistics of Jordan 2004–2015. Actual Values (%) Average Nominal Interest Rate in Inflation Rate if Average Real Interest Rate i

2004

2005

2006

2007

2008

2009

2010

2011

2012

2013

2014

2015

3.2 3.1 0.1

3.7 2.4 1.4

4.2 8.8 -4.6

5.0 5.2 -0.2

4.6 24.7 -20.1

4.6 8.1 -3.5

4.5 4.7 -0.2

4.5 4.8 -0.3

5.0 4.5 0.5

4.5 4.8 -0.3

4.25 2.9 1.35

3.75 -0.9 4.65

Source: 2010 Staff Report and Public Information Notice prepared by the International Monetary Fund IMF, Jordan Department of Statistics, and Central Bank of Jordan, 2017.

4. Life cycle cost analysis 4.1. Introduction The life cycle cost (LCC) for a project or a piece of equipment is its total cost of purchase and operation over its entire service life. This total cost includes the costs of acquisition, operation including energy costs, maintenance and disposal. Most of these costs occur at some future time beyond the purchase date, and must be analyzed using the time value of money. The Simple Payback Period (SPP) method is commonly used by some businesses and other organizations, but it does not consider the operating and maintenance costs at all. The LCC can, therefore, lead to more rational purchase decisions, and can often lead businesses to higher profits. Several methodologies exist in LLC analysis, including present worth, future worth, annual worth, internal rate of return, and benefit/cost. However, this study uses the present worth method for the comparison since it’s considered the standard against which other methods are judged. The LCC method using present worth, when applied properly, allows the analyst to compare projects with different cash flows occurring at different times. In this study, the present worth of the total costs of both systems was calculated over a time line, and the system with the highest benefits or lowest LCC would be preferred. A common “study period” of 20 years, which is suitable for both systems, was chosen as both alternatives must be considered over the same time line. Moreover, the interest rate used that is applied to future costs to equate them with present day costs must be the same. In this work, two refrigeration systems were assessed in terms of life cycle cost analysis. They were, Alternative 1: Vapor Compression Refrigeration Cycle (VCRC) electrically powered by grid-tie PV system. Alternative 2: Vapor Absorption Refrigeration Cycle (VARC) thermally powered by solar evacuated tubes unit. 4.2. Real interest rate (inflation-adjusted interest rate) of Jordan The Nominal Interest Rate in is defined as the rate at which interest is paid by a borrower for the use of money that they borrow from a lender. It is normally used to describe the time-value of money which would decide the life-time cost, profits or loss. The Inflation Rate if on the other hand is a measure of Inflation, which is the rise in the general level of prices of goods and services in an economy over a period of time. Inflation rate represents the rate of increase of a price index, in other words, it is the percentage rate of change in the price level over time. Following a decade of strong growth in the 90s, the Jordanian economy has slowed considerably due to the global and regional downturn and the economic crisis. That caused the inflation rate to continuously fluctuate with a tendency to propagate sometimes, especially in 2008. Meanwhile, the interest rate had a consistent behavior as can be seen from Table 1. The Real Interest Rate, i, or Inflation-Adjusted Interest Rate, is approximately equal to the nominal interest rate, in, minus the expected inflation rate, if. The stochastic variation, as shown in Table 1, resulted in a decreasing real interest rate up to 2008 and then increasing near the present time. The decline in market real interest rates is consistent with a collapse of investment spending, which is occurring recently in the region. Fortunately, Jordan’s future does look a little bright as it normally does at the end of a recession. Therefore, the selection of a properly averaged real interest rate to represent the next 20 years (study period) was difficult. In this work, however, an optimistic value of 5% real interest rate could be adequate and close to the values in the late past few years of the Jordanian economy. 4.3. Present worth (present value of money) The Present Worth (PW) is the value on a given date of a future payment or series of future payments, discounted to reflect the time value of money and other factors such as investment risk and inflation rate. The present value of an Annuity (A) (periodic cash flows) can be calculated by taking each cash flow during the total life n and discounting it back to the present using the real interest rate i, and adding up the present values as indicated by Equation (6); [that is to find P-present worth given A-annuity, (P/A,i,n)], � � � � � ð1 þ iÞn 1 PW ​ of ​ Annuity; ​ P ​ ¼ P A; i; n ¼ ​ A (6) n ið1 þ iÞ 5

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Table 2 Initial PV system component costs. Component

Qty

Unit price $

Total $

PV modules Inverter Mounting system Cables Total

26 1 – –

160 900

4160 900 850 – 5910

The present value of any money to be received in the future can be computed by Equation (7), [that is to find P-present worth given F-future worth, (P/A,i,n)], PW ​ of ​ Future; ​ P ​ ¼ ðP=F; i; nÞ ¼ ​ Fð1 þ iÞ

(7)

n

4.4. Initial system costs Costs that will be incurred prior to the running of the system are initial investment costs. All initial costs are to be added to the LCC analysis total at their full value. 4.4.1. VCRC initial costs The purchase of a regular VCRC, with a 3.59 kW of compressor power was estimated based on prior experience with the market of such systems. An estimated price (shipping and customs included) of $2000 was set for the VCRC system. 4.4.1.1. PV array systems initial costs. For the PV array system, the cost of each major component in terms of user specified variables was to be determined. The major components, which were the PV modules, inverter, mounting system, and cables, with their quantities, and unit price (Obtained from latest pricing of each product) and the total cost are listed in Table 2. The estimated cost of one PV module, SolarWorld 300Wp Mono-crystalline (German brand) was $160, so that the cost of 26 modules amounted to $4160, and that of the inverter, SMA STP15000TL-4 (German brand), was $900. The cost of the PV modules mounting system was estimated to be $850. It was assumed that the cables costs were negligible compared to other component costs. An installation cost of 20% of the total PV system’s price was assumed, which amounted to $1182. The total estimated cost of the PV system was therefore ¼ $7092. 4.4.2. VARC initial costs The purchase of a regular VARC, with a 28.07 kW of thermal heating requirement was estimated based on prior experience with the market of such systems. An estimated price (shipping and customs included) of $3000 was set for the VARC system. 4.4.2.1. Solar evacuated tubes system initial costs. For the solar evacuated tubes system, the cost of each major component in terms of user specified variables was to be determined. The major components, which were the 160 evacuated tubes, cylinder, pipes, and mounting system, with their quantities, and unit price (obtained from latest pricing of each product) reached an estimated cost of the system of about $4500. An installation cost of 20% of the total evacuated tubes system’s price was assumed, which amounted to $900. The total estimated cost of the solar tubes’ system was therefore ¼ $5400. 4.5. Operation costs The second step in the completion of the LCCA was to define all future operation costs of each system. The operation costs were annual costs of manpower, overhead and fuel involved in the operation of the facility. It excluded maintenance, transport and repair costs. All operation costs were to be discounted to their present value prior to addition to the LCCA total. 4.5.1. VCRC operation costs The main source of cost for the VCRC system was manpower and overhead costs for this system which would be relatively small and amounted to about 3% per year of initial system cost, which would be $60/year. 4.5.1.1. PV system operation costs. Most of PV system operational costs are related to overheads and labors. It included labors for cleaning the modules and regular component checks. PV systems would require higher cost of personnel for the level of skill that they should possess; this cost could be three times the usual overheads of electrical systems, Goodrich et al. [16]. According to Goodrich et al. [16], life cycle labor and overhead costs can be estimated to equal 0.49 $/Wp for ground-mounted panels. In this system, at present, the life-cycle cost of only 0.2 $/Wp was considered to compensate for additional manpower and skill needed. Accordingly, for the total number of 26 modules of 300 Wp each needed, a total operational costs for the PV system would amount to $78/year.

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4.5.2. VARC operation costs Manpower and overhead costs for this system would be relatively small which amounts to about 3% per year of initial system cost, which would be $90/year. The power required to pump the refrigerant solution from the absorber to the generator was assumed negligible. 4.5.2.1. Solar evacuated tubes system operation costs. Most of the solar evacuated tubes system operational costs were related to overheads and labors. It included labors regular component checks. In this system, the life-cycle cost of only 0.1 $/W thermal of heating load generated, was considered to compensate for additional manpower and skill needed during the system’s life span of operation. That amounted to $140/year. 4.6. Maintenance and repair costs For simplicity, maintenance costs and repair costs have been combined in the LCCA; however, it should be comprehended that there is a distinction between those two costs. Maintenance Costs are scheduled costs associated with the upkeep of the facility, while Repair Costs are unanticipated expenditures that are required to prolong the life of equipment system without replacing them. 4.6.1. VCRC maintenance and repair costs For durability of the VCRC system, frequent preventive maintenance is also required that includes general inspection of system and leaks of refrigerant, especially in harsh weather conditions. In general, the system was mainly comprised of a compressor and heat exchangers, so that repairs would be normally for providing makeup of loss of refrigerant in the system and replacement of the compressor once over the lifespan of the system. Therefore, it was assumed that maintenance and repair costs amounted to about 3% per year of initial system cost, which would be $60/year. 4.6.1.1. PV system maintenance and repair costs. Grid-connected photovoltaic systems are more reliable nowadays, and once a system is running properly, it should require very little maintenance. Maintenance and repair costs for a PV system can usually be estimated as 10–15% of the operational costs for the whole life of the PV system, Hansen et al. [17]. Accordingly, it was conveniently taken as 10%, which would almost be equal to $8/year. Further maintenance cost also would include cleaning of dust accumulation on the modules, which was assumed to take place once monthly at a cost of $20 for such a small system, and with a total of $240/year. 4.6.2. VARC maintenance and repair costs Frequent preventive maintenance of the VARC system is required that includes general inspection of system and leaks of working fluids. In general, the system was mainly comprised of heat exchangers and solution pump, so that repairs would be normally for providing makeup of loss of working fluids in the system and replacement of the pump twice over the lifespan of the system. Therefore, it was assumed that maintenance and repair costs amounted to about 3% per year of initial system cost, which would be $90/year. 4.6.2.1. Solar evacuated tubes maintenance and repair costs. Maintenance and repair costs for the solar evacuated tubes system can usually be estimated as 10% of the operational costs for the whole life of the system. Accordingly, it would be equal to $9/year. Additional maintenance cost includes cleaning of dust accumulation on the tubes which reduces their absorption efficiency of solar radiation. The cleaning was assumed to take place once monthly at a cost of $20 for such a small system, and with a total cost of $240/ year. 4.7. Equipment replacement and addition costs Replacement and Addition Costs are anticipated expenditures to major system components that are required to maintain the operation of a facility. 4.7.1. VCRC component replacement and addition costs The VCRC system would need replacement and addition costs as follows; that for Ammonia refrigerant in replacement of that leaked; and that of the compressor one time over the lifespan of the system. This would be $200 for the refrigerant replacement and $400 for the compressor. 4.7.1.1. PV system component replacement and addition costs. Major PV system components that could be considered for the replacement and addition costs include PV modules and inverter. Life of inverters is usually taken as 10–15 years, Hansen et al. [17], which requires the system inverter to be replaced one time at the 15th year at a cost of $900. The photovoltaic modules will not require replacement during the system lifetime since their expected life is 25 years. However, due to and decreased module efficiency with time; additional PV modules of about 15% would be needed, and that would be at a cost of $640. 7

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Table 3 Cash flows of the two refrigeration technologies. Component Initial system costs, $ Operation costs, $/year Maintenance and repair costs, $/year Equipment replacement and addition costs, $ Residual value at the 20th year, $ Energy savings, $/year

Complete VCRC System (Alternative 1)

Complete VARC System (Alternative 2)

VCRC

PV

Total

VARC

Solar Evac. Tubes

Total

–2000 –60 –60

–7092 –78 –248

–9092 –138 –308

–3000 –90 –90

–5400 –140 –249

–8400 –230 –339

–600 in the 12th year 200 2154

–1540 in the 15th year 191

–190 in 7th year and –190 in the 14th year 300 2154

–4000 in the 12th year 450

750

391

4.7.2. VARC component replacement and addition costs In general, the system would need replacement and addition costs as follows; for Ammonia refrigerant in replacement of that leaked; and that of the solution pump two times over the lifespan of the system. This would be $200 for the refrigerant replacement and $180 for the solution pumps. 4.7.2.1. Solar evacuated tubes system component replacement and addition costs. Major solar evacuated tubes system components that could be considered for the replacement and addition costs include evacuated tubes. Life of these tubes is usually taken as 10–15 years, which requires the system tubes to be all replaced once during the lifespan of the system. This would amount to about $4000 at the 12th year for the replacement of all of the evacuated tubes. 4.8. Residual value (salvage value) Residual value, as defined earlier, is the net worth of a system at the end of its Life Cycle period. This is the only cost category in an LCCA that reduces cost. 4.8.1. VCRC residual value The residual value of the VCRC system components was estimated as that for the VARC system components with end-of-life price of about 10% of the initial costs, and this would be $200. 4.8.1.1. PV component residual value. The PV system components are expected to return a salvage value through selling it at the end of the project to various types of recycling industries. PV panel recycling is currently not viable because waste volumes generated are too small. However, significant volumes of end-oflife modules will only begin to appear in many years later. While the number of treatment and recycling processes are under devel­ opment globally for photovoltaic modules, there are currently only two treatment and recycling methods developed specifically for PV modules, both have been tested and put into operation: The Deutsche Solar’s process for crystalline silicon panels, and The First Solar’s process (Operational in USA, Germany and Malaysia) for CdTe panels.”, Bio Intelligence Service [18]. Based on PV cycle’s reported experiences in the 2010 collection and recycling activities, a price of $190 was agreed per ton of endof-life silicon PV panels, Bio Intelligence Service [18]. It should be noted that salvage logistics’ costs can vary based on the collection and transport system chosen, and the distance between the collection point and the recycling center of PV modules. Referring to the mechanical data of the SolarWorld 300Wp Mono-crystalline modules, the weight of one module is about 21.5 kg. Based on the price of $190/ton for the end-life PV panels, the salvage value of end-of-life of PV modules, can be obtained through using Equation (8), � No: of modules � WeightðkgÞ=module PV Costsalvage ¼ � Price $ ton (8) 1000 This would yield an amount of $106. Other salvage value would be the mounting materials for the PV system components and cables. Its estimated salvage value can be assumed to equal 10% of its initial cost at about $85. 4.8.2. VARC replacement residual value VARC system components at the end of its lifespan are usually sold for interested workshops or junk yards. The price of an end-oflife of VARC system depends totally on its condition. But in all cases it won’t exceed 10–15% of the initial cost referring to the local market trends. Therefore in this study, the residual value of the VARC was taken as 10% of the initial costs. Accordingly, that would be $300. 4.8.2.1. Solar evacuated tubes system residual value. Major solar evacuated tubes system components can hardly be reused at the end of their life span. However, it was assumed that about 10% would be reused, and this would give revenue of about $450. 8

Case Studies in Thermal Engineering 16 (2019) 100559

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Fig. 3. Percentages of cash flows for both refrigeration systems. Table 4 Comparison between VCRC and VARC systems. Parameter

VCRC

VARC

Availability Components Maintenance Volume needed Weight Vibration and noise Losses

More available Simple Easy 0.79 m3 180 kg Noisy Low

less available Complex Complex 1.3 m3 730 kg Quiet and vibration-free High

4.9. Final LCCA The total life cycle cost for the two refrigeration systems are summarized as inflow (þve) and outflow (-ve) cash flows in Table 3. All pertinent costs have now been established and were brought to their present value. The costs can be summed up to generate the total life cycle cost of each of the two refrigeration technologies as LCCAlternative 1 (or LCCAlt 1) and LCCAlternative 2, (or LCCAlt 2) as in Eq. (9), LCCAlternative ¼ Energy savings – CostInitial – CostOperation – CostMaintenance

and Repair

– CostReplacement and addþCostResidual ($)

(9)

The present worth of both projects using the P/A factor to find the equivalent cash flows of a series of annual savings and costs for the “study period” of 20 years with a 5% real interest rate, and the P/F factor to discount the salvage value in year 20 to the present were used. Each project LCC is brought to present value using Eqns. (6) and (7) since it must be considered at the start of the project, and is as follows, PW 1 ¼ LCCAlt 1 ¼ $2154(P/A5, 20) – $9,092 – $446(P/A5, 20) – $600(P/F5, 12) – $1,540(P/F5, 15) þ $391(P/F5,20). PW 1 ¼ LCCAlt 1 ¼ $2154(12.4622) – $9,092 – $446(12.4622) – $600(0.5568) – $1,540(0.4810) þ $391(0.3769) ¼ $11,266. PW 2 ¼ LCCAlt 2 ¼ $2154(P/A5, 20) – $8,400 – $569(P/A5, 20) – $190(P/F5, 7) – $190(P/F5, 14) – $4,000(P/F5, 12) þ $750(P/F5,20). PW 2 ¼ LCCAlt 2 ¼ $2154(12.4622) – $8,400 – $569(12.4622) – $190(0.7107) – $190(0.5051) – $4,000(0.5568) þ $750(0.3769) ¼ $9,178. Fig. 3 displays an insight on the percentages of each parameter of cash flows, excluding energy savings, for both alternatives. Since the present value of the benefits of both projects exceed the present value of the cost by $11,266 for alternative 1, and by $9,178 for alternative 2, either project is a highly cost-effective one. However, the results have shown that alternative 1 has more benefits and thus can be favored over alternative 2. Note that, if the energy savings were not included in the LCC calculations, the decision rule for the LCC analysis would be to choose 9

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Table 5 Comparison between PV array and Evacuated Tubes unit. Parameter

PV

Evacuated Tubes

Availability Breakage Area needed Energy storing Losses Freeze Lifespan

Available Low 41 m2 Power to the grid Medium losses No 25 yrs

Available High 30 m2 Thermal Low losses Yes 20 yrs

the alternative with the lowest LCC. That is one would prefer alternative 1 with $15,578 in cost compared to $17,666 for alternative 2. It’s worth noting that the simple payback period (initial cost, $/savings, $/year) of alternative 1, is 4.2 years, whereas, that of alternative 2 is 3.9 years, which is reasonable for both systems. 5. Other differences A summary of other differences between the two systems using various parameters is presented in Tables 4 and 5. Table 4 shows a comparison between the VCRC and VARC systems, and Table 5 shows a comparison between the PV and solar evacuated tubes systems. It can be seen that each system has some advantages and disadvantages over the other. The VCRC would require more land for its PV array system than would the VARC for its solar evacuated tubes, but the latter is more bulky and complex regarding components. Nevertheless, the tables clearly show that the complete VCRC system has more advantages. 6. Conclusions An economic comparison is made between a vapor compression refrigeration system VCRC powered by photovoltaic array and a vapor absorption refrigeration system VARC powered by solar evacuated tubes unit. The comparison between these two technologies is carried out based on life cycle cost analysis. The results have indicated that either complete system, VCRC or VARC, is cost effective regarding their benefits over their total costs, although the VCRC yielded more benefits. After examination of all of the parameters considered, one would prefer to opt for the VCRC system, as first of all, it has yielded more benefits, and besides of being less bulky and easily available in the market, simpler and requires easy maintenance. Moreover, VCRC systems have wider applications in industry and commercial buildings. VARC systems, on the other hand, can also be used with waste heat recovery systems, in addition to the possibility of being combined with domestic hot water systems. 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