Scholarly article on topic 'Optimization of Standalone Solar Heat Fired Absorption Chiller for Typical Australian Homes'

Optimization of Standalone Solar Heat Fired Absorption Chiller for Typical Australian Homes Academic research paper on "Earth and related environmental sciences"

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{"solar air-conditioning system" / "thermally driven" / "absorption chiller" / "standalone configuration"}

Abstract of research paper on Earth and related environmental sciences, author of scientific article — Gazinga F. Abdullah, Wasim Saman, David Whaley, Martin Belusko

Abstract The increased penetration of residential air-conditioners (AC); specifically vapor compression types, is regarded as one of the foremost causes of a dramatic rise in critical peak electricity demands requiring corresponding upgrades of electricity infrastructures. These upgrades requires heavy investments, consequently, driving up electricity prices. Solar air-conditioning systems can reduce this trend, but current vapor-compression air-conditioners (VCACs) needs very large investments in both photovoltaic system and battery storage. Alternatively solar heat-driven absorption chillers need less expensive solar collectors and thermal storage, drawing only small amounts of electricity to overcome parasitic power. There are ample studies conducted previously on either the technical and/or economic feasibility of solar heat driven absorption chillers. But these studies are only focused on supplementing solar heat energy with an auxiliary heater. This study, examines the option of running the absorption chiller by solely relying on solar heat energy. It focuses on minimizing the life cycle cost of a solar heat driven absorption chiller through optimizing the size of all of its main components. The system is named the standalone solar heat fired absorption chiller (SA-SHF-ABS-CH) sized to sufficiently meet the space conditioning demands, both heating and cooling, of a typical Australian 6 star home. For the aims of this research, TRNSYS17 software was used in modelling and dynamically simulating the integrated system, while GenOpt software was used to carry out the optimization. The economic assessment on the most optimally sized system component configuration shows, firstly, the twenty-year lifecycle cost of the system with the most minimized cost is AU$ 53,387 in Brisbane, AU$ 51,639 in Adelaide and AU$ 32,816 in Melbourne. These investment costs in each of these cities appear higher than those incurred if the householder were to instead install a standard efficient inverter, ducted, reverse cycle air conditioner (IRC-AA-HP) powered by grid electricity; as follows: Brisbane at 77%, Adelaide at 58% and Melbourne at 28%. Secondly, the payback period was found to be longer than the twenty-year system service- life, which is far too long to justify the investment on such solar air-conditioner. However, when compared with IRC-AA-HP, in Adelaide and Melbourne, SA-SHF-ABS-CH consumed at least 50% less power, meaning it offsets half of the carbon dioxide emissions, furthermore, it draws 75% lesser critical peak kWp power, which means it has strong potential to obviate the need for heavy investments in electrical infrastructures, ultimately contributing to mitigating rapid electricity price rises.

Academic research paper on topic "Optimization of Standalone Solar Heat Fired Absorption Chiller for Typical Australian Homes"

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Energy Procedia 91 (2016) 692 - 701

SHC 2015, International Conference on Solar Heating and Cooling for Buildings and Industry

Optimization of standalone solar heat fired absorption chiller for

typical Australian homes

Gazinga F. Abdullaha, Wasim Samana, David Whaleya , Martin Beluskoa

aSchool of Engineering, University of South Australia, 5095, Mawson Lakes, Australia

Abstract

The increased penetration of residential air-conditioners (AC); specifically vapor compression types, is regarded as one of the foremost causes of a dramatic rise in critical peak electricity demands requiring corresponding upgrades of electricity infrastructures. These upgrades requires heavy investments, consequently, driving up electricity prices. Solar air-conditioning systems can reduce this trend, but current vapor-compression air-conditioners (VCACs) needs very large investments in both photovoltaic system and battery storage. Alternatively solar heat-driven absorption chillers need less expensive solar collectors and thermal storage, drawing only small amounts of electricity to overcome parasitic power. There are ample studies conducted previously on either the technical and/or economic feasibility of solar heat driven absorption chillers. But these studies are only focused on supplementing solar heat energy with an auxiliary heater. This study, examines the option of running the absorption chiller by solely relying on solar heat energy. It focuses on minimizing the life cycle cost of a solar heat driven absorption chiller through optimizing the size of all of its main components. The system is named the standalone solar heat fired absorption chiller (SA-SHF-ABS-CH) sized to sufficiently meet the space conditioning demands, both heating and cooling, of a typical Australian 6 star home. For the aims of this research, TRNSYS17 software was used in modelling and dynamically simulating the integrated system, while GenOpt software was used to carry out the optimization. The economic assessment on the most optimally sized system component configuration shows, firstly, the twenty-year lifecycle cost of the system with the most minimized cost is AU$ 53,387 in Brisbane, AU$ 51,639 in Adelaide and AU$ 32,816 in Melbourne. These investment costs in each of these cities appear higher than those incurred if the householder were to instead install a standard efficient inverter, ducted, reverse cycle air conditioner (IRC-AA-HP) powered by grid electricity; as follows: Brisbane at 77%, Adelaide at 58% and Melbourne at 28%. Secondly, the payback period was found to be longer than the twenty-year system service- life, which is far too long to justify the investment on such solar air-conditioner. However, when compared with IRC-AA-HP, in Adelaide and Melbourne, SA-SHF-ABS-CH consumed at least 50% less power, meaning it offsets half of the carbon dioxide emissions, furthermore, it draws 75% lesser critical peak kWp power, which means it has strong potential to obviate the need for heavy investments in electrical infrastructures, ultimately contributing to mitigating rapid electricity price rises.

© 2016 The Authors.Publishedby Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-review by the scientific conference committee of SHC 2015 under responsibility of PSE AG

1876-6102 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Peer-review by the scientific conference committee of SHC 2015 under responsibility of PSE AG doi:10.1016/j.egypro.2016.06.232

Gazinga F. Abdullah et al. / Energy Procedia 91 (2016) 692 - 701 Keywords: solar air-conditioning system, thermally driven, absorption chiller, standalone configuration

1. Introduction

Space heating and cooling in Australia account on average for about 40% of total residential energy consumption [1]. Although only around 3% of this total energy is used in space cooling, energy for meeting space cooling demands remains significant since the energy demanded needs to be in the form of electricity, a need which is projected to grow with the global average temperature increase. Despite the fact that 5% of critical space air-conditioner demands for electricity occurs for only around 40 hours per year in most Australian states , reliability standard mandates that the entire electricity infrastructure needs to be sized for handling critical peak electricity demand [2]. Since the number of residential air-conditioner installations is growing, an ongoing heavy investment in extending the capacity of electricity infrastructures is needed. Since 2005, increased peak demands for air-conditioning electricity has been the main cause behind Australia's rapid electricity price rises [3]. Solar air-conditioning systems have the potential to decrease both electricity consumption and peak electricity demand, thereby, reducing greenhouse gas emissions and mitigating the investments needed to extend the capacity of electrical infrastructures, consequently, mitigating the cost of electricity price rises [4]. However, since powering conventional VCACs by solar requires a very large investment in both the photovoltaic (PV) system and the battery for storage [5] [4]; it is worthwhile bearing in mind the advantages of solar thermally driven air-conditioners. Sorption thermally driven chillers, specifically a single-effect, hot-water driven, lithium bromide/water absorption chiller, needs hot water at 75oC and above to operate, a temperature which can be produced by low temperature solar collectors. The chillers needs only small amount of electricity to operate several pumps and a re-cooler. Of late, several manufacturers have presented small scale, highly pre-engineered chiller suitable for operation by solar energy in residential buildings. Although the market for such a small cooling capacity chiller exists mostly in European countries [6] [7], the technology has the potential to spread to other countries such as Australia that have more sunshine. Previous studies conducted on this kind of chiller focused mostly on the solar-heat assisted types, meaning that auxiliary heat was required to back-up the chiller operation whenever available solar-heat energy was deficient. But these studies did not place enough focus on optimizing all the main system-components; namely, the chiller and the cold storage beside thermal collector and the hot buffer storage. In this study, our focus is on optimizing the size of these four main components to a level of operate the absorption chiller only by solar heat energy. The sizing criteria applied is the annual hour's loss in meeting building thermal load probability, sensible and latent cooling in summer and sensible heating in winter. These criteria enable matching the thermal capacity of the system well with building specific on-demand space conditioning. The yearly average hourly loss of thermal load probabilities considered as an optimization constraint, while the optimization objective was set to minimizing the life-cycle cost of the system. The system components sized will be optimized for a typical Australian house model and for three different climatic conditions. In order to justify the high investment in a solar system, in winter the collected solar-heat energy will be used to provide space heating. To rationalize the investment, the life-cycle cost will be compared with a basic conventional type of air-conditioner, the model chosen is a ducted inverter-driven reverse cycle air-air heat pump (IRC-AA-HP) sized for space conditioning the same house model.

Nomenclature

AC Air-conditioner

Acoll Thermal collector area

Cchiller Chiller capacity

IRC-AA-HP Inverter-driven reverse cycle air-air heat pump

NatHERS Nationwide House Energy Rating Scheme

PV Photovoltaic

SA-SHF-ABS-CH Standalone solar heat powered absorption chiller

VCAC Vapor compression air-conditioner

Vcold Cold tank volume

Vhot Hot tank volume

2. Research methodology

2.1. Locations and house model

The building's thermal loads and the intensity of solar energy available are both dependent upon the climatic nature of its location. Therefore, three locations with vastly different climates around Australia were chosen these are Brisbane, Adelaide and Melbourne. In each of these climates, the space conditioning thermal load was determined for a typical Australian house model. The house is a detached two story single family with 180 m2 conditioned area [8]. The house under consideration is assumed to be constructed of brick veneer, and then its thermal fabric energy efficiency was further adjusted to comply with the Australian mandatory minimum 6 stars energy rating criteria stipulated by Nationwide House Energy Rating Scheme (NatHERS), which is a national framework for the purpose of regulating how Australian homes are rated for their thermal performance. That criteria mandates that the minimum thermal energy in MJ/m2 per annum required to maintain the house at a thermally comfortable level should not exceed 43 in Brisbane, 96 in Adelaide and 114 in Melbourne [9]. That rating was achieved by adding and removing energy efficiency enhancements to the fabric in term of insulation and glazing (without affecting the essential house-plan) until the required minimum rating was achieved.

2.2. System description

A simplified schematic diagram illustrating the SA-SHF-ABD-CH main component configuration and auxiliary components is given in figure (1). The main components in this study are: solar thermal collector, hot storage tank, absorption chiller and a cold storage tank. Auxiliary components are five circulating pumps, a dry re-cooler, a ducted fan coil, and controllers.

Fig. 1 simplified schematic diagram shows main and auxiliary components configuration

The major specifications characterizing the thermal performance of components, and consequently, the system are as follows: the thermal collector is evacuated tube, direct flow type, with collectors configured in parallel array, installed facing north and tilted to an angle equal to the local latitude. The collector has a thermal efficiency of 70%, and first and second loss coefficients are 1.59 W/m2.K, and 0.085 W/m2.K. Both the hot and chilled thermal storages are stratified and modelled with 20 nodes, insulated thermally to level R2. The selected chiller is hot water fired absorption with Lithium bromide/water as a working pair. The chiller has a nominal coefficient of performance of 0.6, and can be operated with hot water at a temperature above 85oC and below 100oC. The selected re-cooler is a dry type, having a capacity matched with the required heat rejection needed from the chiller, and equipped with a variable speed fan that is set to cool the return-water to 35oC. The circulating pumps are all of a constant speed

flowrate type, and their flowrate in each circulation loop is adjusted with the capacity of the optimized size matched components. The parasitic power found from the pressure drop at nominal flowrate is based on specifications in several catalogues; it was then further adjusted with other non-nominal flowrate.

The system control strategy is as follows. All year round, the solar circulation pump is switched on whenever the temperature at the top of the collector was higher than that at the top of the storage by 5oC, this was accomplished by a differential controller. In summer, the absorption chiller, the dry cooler and the three circulation pumps are all switched on whenever the temperature at the top of the chilled water storage tank rises above 10oC. But, a further operating-constraint occurs; the temperature of the hot water at the top of the hot water tank must be above 75oC for the chiller to start operating. The fan-coil pump and fan switch on whenever the temperature and humidity inside the building, in summer, rises above the space cooling setting temperature and humidity suggested in [9], and in winter, if the temperature drops below the suggested setting for space heating.

The basic system used to compare and justify the investment is an IRC-AA-HP which is modelled on the normalised performance data taken from one manufacturer's full performance data for an entire series of this type of air-conditioner. The coefficients of performance of most heat pumps detailed in that series specifications were around 3.5. The conditioner control function was adjusted to represent variable frequency drive, i.e. inverter technology, by using an iterative feedback controller which was set to the setting temperature for that season and the control variable, which tracked the house average temperature.

The previously described system was modelled using TRNSYS 17 which is a component-based system simulation sftware [10]. TRNSYS software was selected as it includes pre-validated components of most of our solar collection and air-conditioning related main, auxiliary and controller components. The major components that formed the TRNSYS project model are listed in the table (1).

Table 1. TRNSYS component types used to model the system

TRNSYS type Function

Type (71) Standard library Evacuated tube collector

Type (107) Standard library Absorption chiller

Type (534) TESS Thermal storage

Type (740) TESS Pump

Type (996) TESS Performance map fan coil

Type (2) Standard library Controller

Type (11) Standard library Diverter

Type (511) TESS Dry cooler

Type (23 ) Standard library iterative feedback controller

Type (583) TESS TRNOPT

Type (24) Standard library Integrator

Type (25) Standard library Printer

Type (65) Standard library Plotter

TRNSYS type Function

Type (9) Standard library Data reader

Type (16) Standard library Weather processor

Type (69) Standard library Sky effective temperature

Type (33) Standard library Psychometric

Type (14) Standard library Forced function

Type (661 ) TESS Delay output device

Type (223) Author developed [11] IRC-AA-HP

System operation was simulated incrementally in 7 minute steps, and evaluated upon the completion of each full year's simulation. Furthermore, the system was evaluated with technical and economic criteria written in Boolean language inside several equation cards.

2.3. Technical and economic criteria

The technical criteria used to evaluate the size of components required by the standalone solar-heat powered absorption chiller, also had to account not only for the availability of sufficient solar thermal energy to the chiller generator, but also the building specific 'on-demand' reliability, in terms of when the energy was needed. Hence, the annual hourly loss of load probability is used as sizing criteria. This criteria was used to assess and count the percentage number of hours throughout the year when the capacity of the system was deficient in meeting the building thermal space conditioning demand. In the residential sectors, air-conditioning systems should cover 95%

of the hours comfort load, i.e. tolerating 5% of annual hours infrequent peak load. The demand was considered to be unmet, whenever the indoor design conditions did not match those specified by NatHERS [9],

The economic criterion used to evaluate the system is the twenty years lifecycle cost. The prices of components, listed in table (2) are compiled from several online retail pricelist with suppliers based in Australia. Justifying the decision to invest, supplemented by a payback period which is calculated by finding the difference over a twenty-year life cycle cost between a SA-SHF-ABS-CH and IRC-AA-HP option, then dividing that difference by the annuity charge. All cash flow occurring during the simulated period was inflated with 2.2% inflation rate, and discounted back using 7% real discount rate.

Table 2. Components economic parameters used to find the life cycle cost

Component Initial cost Installation Future cost reduction Maintenance Life years

Absorption chiller (3700*C"0-45)*C*1.5 AU$ (C in kWc) [12] 3750 AU$ - 4% initial cost 20

Re-cooler 173.14*C/3600 + 2697.1 (C in kWC 0 0 10% 20

Evacuated tube 550 AU$/m2 (each meter contain 10 15% initial cost 30% by 2035 0 15

collector tubes)

Pump 269+239.6 *m (m in kg/s) $AU100 per pump 0 0 15

Storage tank 680*V + 720 (V in m3) $AU 100 per m3 0 0 15

Air-handler unit 35.95*C+680.69 (C in kWo) 0 0 0 20

Revrese cycle sir-conditioner and variable (324.42*C/3600+1500)+(135.6* Cc/3600+93) (C in kWc) 2500 1 6% 0 15

frequency drive

The tariff used in each climate to find the running cost of conventional system and the parasitic power of absorption chiller. The tariff was based on fixed charge considering a standing offer mentioned in [13], which are 34 c/kWh for Brisbane, 35 c/kWh for Adelaide and 33 c/kWh for Melbourne.

2.4. System optimization

After the TRSNSY model control strategy was tuned and the results monitored on several plotters to ensure correct system operation, it was coupled with GenOpt software via a TRNOPT component to carry out system optimization. The optimization was performed by a Hybrid algorithm because it combines the speed and accuracy essential to match that required by the current study. Optimization was performed utilizing the main component sizes (thermal collector area, hot water tank volume, chiller cooling capacity, cold water tank volume) as design variables. The system's twenty year life-cycle cost was an objective, and the 5% annual hour loss of comfort load probability was used as the constraint.

3. Results and Discussion

3.1. Optimal component sizes

The optimized sizes of the components that formed the most minimized life cycle cost SA-SHF-ABS-CH in each of the three climates considered were found via the optimization algorithm search and are given in table (3). These values were found after performing more than 500 year round simulations, with five optimization attempts for each climate. The discrepancy between the resultant findings was less than 2%, which confirmed the convergence for optimization. The practical validity of optimal SA-SHF-ABS-CH component size configurations listed in table (3) is supported by the following justification. Firstly, in Brisbane, the capacity of both the chiller and the cold storage tank were greater than the other two climates. This is because of the need to remove large amounts of latent load in Brisbane required continually maintaining a larger quantity of water chilled at around 10oC. In Adelaide, clearly, the solar collector and the hot storage tank sizes are both larger than other two climates since to overcome the peak sensible cooling load in summer required, beside sufficient chilled buffer water storage, that the hot water is ready to

operate the chiller whenever the cold water storage is depleted (mostly after sunset). In Melbourne, the sizes of the four main system components are comparably smaller than those for the other two climates due to a lower sensible space cooling load. Besides meeting the space cooling loads, the sufficiency of the system capacity, i.e solar collector and hot water storage tank, for handling the entire house space heating load in winter was found to be more than adequate in all three climates considered.

Table 3. The optimal size of SA-SHF-ABS-CH component configuration.

Location Acoll (m2) VHot (m3) CChiller (kWc) Vcold (m3)

Brisbane 18.5 0.5 6 4.1

Adelaide 23 3.8 3.2 2.8

Melbourne 10 1.5 2 1.5

Previous literature [14] [15] [16] [17] on solar-heat powered absorption chillers professed encountering technical difficulties in the attempts to increase the solar fraction to reach 100%. Such difficulties were: when using a large solar thermal collector, the collector efficiency degraded due the temperature increase. For large hot water storage, there was a morning delay before the buffered hot water reached the minimum temperature needed to operate the chiller. But these studies did not focus on optimizing the size of all four of the system's main components concurrently, i.e. the chiller and cold storage capacity to be included as well. In our current study, we reveal that with a considered control strategy and adjusted flowrate, chilled storage enables decoupling the absorption chiller cooling capacity from the spike variation in building loads, and the hot water tank decouples the chiller operation from fluctuations in the availability of solar energy. Furthermore, both together with a correct sized chiller between the two, facilitated operating the SA-SHF-ABS-CH without the need of an auxiliary heater.

Although the large size of the storages (which reached 4,100 litres) seems far too large for a residential air-conditioner, when the buffer size is compared with some previous related literature it becomes more acceptable. For example, [18] in their research focus on demand side management strategy for Northern Ireland (UK) detached houses insulated to 1990 UK building regulation standards, show that shifting a 1 hour space heating demand, while switching off the air-water heat pump during the on-peak period, required a buffer of 800 litres of hot water. Also, [19] in his experimental investigations when sizing an absorption chiller to meet a space cooling demand of 170 m2 classrooms in a tropical climate, stated that coupling the chiller with a 1,500 litre hot water tank and a 1,000 litre chilled water tank provided only 45 minutes autonomy. Hence, as a fully thermally driven standalone system meeting an entire space conditioning of a 180 m2 conditioned area, the physical size of the storage was deemed justifiable. Should more volumetrically efficient thermal buffering be desired, then latent energy storage with appropriately selected phase change material can be used, but this in turn requires a higher investment in the storage. In terms of the area required to mount the collector, the house under consideration has 45 m2 pitched roof facing north, which offers more room than the collector area stipulated for either of the three climates,

It is important to mention that the above listed optimal size combinations does not represent the one that provides the best technical operation, but rather those that in view of their estimated market price listed in table (2) provide the most minimised life cycle cost system. Should the system be sized with different purchase-costs or building thermal load; the economic parameters in our TRNSYS project deck file for the SA-SHF-ABS-CH can be updated and run directly from GenOpt software to find the size of system components for the system that provides the most minimized life-cycle cost.

3.2. Economic evaluation

The twenty years life cycle cost of SA-SHF-ABS-CH considering component sizes listed in the table (3) and prices estimated in table (2) found to be AU$ 53,387 in Brisbane, AU$ 51,639 in Adelaide and AU$ 32,816 in Melbourne. In figure (2), the three systems life cycle costs are broken-down to a percentage contributions to cost of

each component. Obviously, the sum of component costs attributed to the air conditioner subsystem, i.e. chiller, dry cooler, cold storage, pumps and fan coil, formed more than 50% of the overall system cost, and the rest were attributed to the solar water heating subsystem, i.e. evacuated tube collector and the hot storage tank. More precisely, the system cost is dominated by the price of the thermal collector and the chiller unit, clearly if the cost of these two units reduces in the future, it will favour the wide uptake of the current system.

Chiller 35,4%

Cold tank 9,0%

Hot tank 2,6%

Collector 25,2%

Brsibane (AUS 53,387)

Re-cooler

11,3% pumps

4,9% Fan-coil 1,7%

Parastic power 9,8%

Chiller 28,1%

Hot tank 8,8%

Cold tank 6,9%

Collector 32,4%

Adelaide (AU$ 51,639)

Re-cooler

8,8% pumps

Fan-coil 2,4% arastic power 7,7%

Hot tank

Chiller 7,1% Re-cooler

Collector power

22,2% 3,9%

Melbourne (AU$ 32,816)

Fig. 2 break-down the SA-SHF-ABS-CH life cycle cost to its main forming components.

Looking again at figure (2), the twenty years discounted grid electricity purchased to operate the pumps and fans, formed a small share of the system's overall cost in the three climates. In Melbourne, the very small share, 4%, purchased electricity is attributed to two circulating pumps, i.e., the solar collector to a hot storage loop and the hot water tank to a fan-coil loop, whilst the chiller pumps and the dry cooler fan operate only for short period of time due to Melbourne's smaller summer cooling load.

3.3. Comparison with conventional AC system

In order to quantify whether the above mentioned twenty years life-cycle cost of investment in a SA-SHF-ABS-CH is justifiable or not, it was compared with the life cycle cost of IRC-AA-HP. The optimized cooling capacity of an IRC-AA-HP needed for the same house and the same annual hourly loss of thermal load probability for consistency, i.e. 5%, was found to be 5 kWC in Brisbane, 10.5 kWC in Adelaide, and 9 kWC in Melbourne. Technically, these cooling capacities are larger than the cooling capacity of the absorption chillers used in Adelaide and Melbourne since the undersized chillers meet the peak space cooling load with supplement from the integrated

cold storage. But for Brisbane, the smaller capacity IRC-AA-HP meets the space conditioning demand via exposing the passing air directly to direct expansion evaporator coil, since there is no need to maintain an excessively large amount of constantly chilled water to remove humidity, which is case with the SA-SHF-ABS-CH.

The economic comparison is revealed in terms of the overall twenty years life cycle cost (including running costs) that the investment in SA-SHF-ABS-CH is higher than IRC-AA-HP by 77 % in Brisbane, 58 % in Adelaide and 28 % in Melbourne, see figure (3). Consequently, the net present value if counted as the difference between IRC-AA-HP and SA-SHF-ABS-CH is negative, signifying the investment in the former system is worthless.

100.000 80.000

\A 60.000

40.000 -

20.000 12.909 0

53.387 51.639

I! 32.816

21.210 23.563

■ I ■ 1

Brisbane Adelaide Melbourne

■ IRC_AA_HP ■ SA_SHF_ABS_CH

Fig. 3 comparison between SA-SHF-ABS-CH and IRC-AA-HP in term of complete life cycle cost.

The economic comparison in terms of the twenty years discounted purchased electricity, see figure (4), revealed that in Brisbane the SA-SHF-ABS-CH needs more purchased electricity than conventional IRC-AA-HP by 42.6 % rendering any consideration of the SA-SHF-ABS-CH for Brisbane's climate unfavourable. This occurred in Brisbane because the latent load is very high; requiring a chiller to run extensively in order to maintain the water at 10oC throughout summer, also, the space heating demand in Brisbane is very little in winter, hence, we exclude SA-SHF-ABS-CH as unsuitable for Brisbane, and hence from any of our further economic evaluation. But opposite is the case in both Adelaide and Melbourne, where the running cost of SA-SHF-ABS-CH is lesser by 50.8% and 89.3%, respectively, from case of IRC-AA-HP. Thus, SA-SHF-ABS-CH offers considerable electricity charge saving in both Adelaide and Melbourne, but it is unsuitable for Brisbane due to its high latent load.

15.000

11.920

10.000

^ 5.000

□ U I LI1280

Brisbane Adelaide Melbourne

■ IRC AA HP HSA SHF ABS CH

Fig. 4 comparison between SA-SHF-ABS-CH and IRC-AA-HP in term of the twenty years discounted running cost.

The risk of investing in such SA-SHF-ABS-CH for long run saving can be measured via the payback period. The payback period here needs to be comprehended in the following terms, theoretically, if a householder were to invest more in installing an SA-SHF-ABS-CH instead of investing less on an IRC-AA-HP, how many years would it take their savings from paying less for electricity to recoup their extra investment. Based on this, the payback period in Adelaide was found to be 89 years. Such a long payback period is an unacceptable but nevertheless predictable outcome. On one hand, in Adelaide a SA-SHF-ABS-CH necessitates an excessive life cycle cost investment of AU$ 51,639. On the other hand, the system offsets only a small amount of electricity which would otherwise need to be purchased from the grid. The electricity potentially saved in terms of the equivalent annual annuity was AU$390 out of AU$ 767 if the householder installed an IRC-AA-HP instead. In Melbourne, the payback period was found to be 20 years which is shorter when compared with Adelaide since, on one hand, the life cycle cost was relatively less, i.e. AU$ 32,816, on the other hand, the savings gained by avoiding purchasing electricity was higher, the annuity avoided is AU$ 1,005 out of AU$ 1,126 if an IRC-AA-HP was running instead. Still, in both climates, the payback periods were extremely long making it difficult to justify investment in a SA-SHF-ABS-CH. In order to shorten the payback period, the system may be used in a combined configuration, i.e, to provide sanitary hot water as well. With this configuration purchasing electricity or gas needed for domestic hot water preparation will be avoided, but the solar hot water subsystem will then need to be oversized as well.

Both the life cycle cost and payback period evaluation quantified the SA-SHF-ABS-CH as too unappealing for householder uptake. But the peak kWp drawn by a SA-SHF-ABS-CH is lower than that of the IRC-AA-HP by about 75% as a rough estimate, signifying that the SA-SHF-ABS-CH has potential to mitigate 75% heavy investment in augmenting electricity infrastructure, consequently, mitigating rapid price rise of electricity originated from the need to upgrade network infrastructures. Also, since in both Adelaide and Melbourne, during twenty years study period the system consumed less electrical power by 22,771 kWh and 62,206 kWh which is interpreted as offsetting 16.4 ton carbon dioxide in Adelaide and 83.4 ton in Melbourne during the study period. Further on, such a system may have attractive potential for use in remote houses where instead of investing heavily in augmenting electricity networks or in installing large expensive standalone PV systems to power an IRC-AA-HP. But for remote houses, small standalone PV systems are then needed which upgrade SA-SHF-ABS-CH to a standalone capacity, electrically and thermally. Energetic, economic, infrastructural and environmental figures for optimized component configuration of such system will be our focus in a follow up publication.

4. Conclusion

In this study, SA-SHF-ABS-CH was techno-economically optimized for meeting the entire space heating and cooling demand of a typical Australian 6-star rated home. The optimization was conceived in terms of finding the required thermal collector area, the absorption chiller cooling capacity, the hot and the cold storage tank volume that facilitates operation of the chiller solely from solar heat energy. Sizing was constrained with condition of covering 95 % total annual yearly hours space heating or cooling demand of the building. The twenty years life cycle cost of such optimized system for typical Australian house model found to be AU$ 53,387 in Brisbane, AU$ 51,639 in Adelaide, and AU$ 32,816 in Melbourne. These life cycle costs were quantified to be un-justifiable, since they are more costly than if householder were to install an IRC-AA-HP and purchase its electricity from the grid. The cost increase was found to be 77 % for Brisbane, 58 % for Adelaide and 28 % for Melbourne. Further to this, the payback period to recoup the extra investment in an SA-SHF-ABS-CH in order to save money by purchasing less electricity (when compared with the IRC-AA-HP) was found to exceed a 20 year period, a period too long to justify the investment. However, the IRC-AA-HP when compared with the SA-SHF-ABS-CH, in both Adelaide and Melbourne consumed much less electricity, meaning that it offset a large amount of carbon dioxide emissions. Besides, the SA-SHF-ABS-CH draws a lower peak kWp meaning that it is able to mitigate investment in upgrading electricity infrastructure capacity to support an increased uptake of air-conditioners, consequently, having the potential to mitigate rapid electricity price hikes.

Acknowledgment

The first author would like to thank the Higher Committee for Education Development (HCED) in Iraq for sponsoring her PhD study at University of South Australia.

References

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