Transient simulation and parametric study of solar-assisted heating and cooling absorption systems: An energetic, economic and environmental (3E) assessment

Highlights

• A TRNSYS simulation model of SHC absorption chillers was presented.

• An energetic-economic-environmental analysis was performed on each configuration.

• Primary energy saving, CO₂ avoided and total costs were determined.

• Through a parametric study, the optimal size of each configuration was obtained.

• The performance of the alternative configurations was examined and compared.

Abstract

This paper presents energetic, economic, and environmental (3E) analyses of four configurations of solar heating and cooling (SHC) systems based on coupling evacuated tube collectors with a single-effect LiBr–H₂O absorption chiller. In the first configuration (SHC1), a gas-fired heater is used as the back-up system, while a mechanical compression chiller is employed as the auxiliary cooling system in the second configuration (SHC2). The capacity of the absorption chiller is designed based on the maximum building cooling load in these configurations. The third and fourth configurations (SHC3 and SHC4) are similar to SHC2, but the absorption chiller size is reduced to 50% and 20%, respectively. The results show that the highest primary energy saving is achieved by SHC2, leading to a solar fraction of 71.8% and saving 54.51% primary energy as compared to a reference conventional HVAC system. The economic performance of all configurations is still unsatisfactory (without subsidies) due to their high capital costs. However, if a government subsidy of 50% is considered, the results suggest that SHC4 can be economically feasible, achieving a payback period of 4.1 years, net present value of 568,700 AUD and solar fraction of 43%, contributing to 27.16% decrease in the plant primary energy consumption.

Introduction

Air-conditioning demand has been significantly increasing in many parts of the world, mainly due to population growth and higher comfort standards in buildings [1], [2], [3]. Conventional air-conditioners in Australia account for over 40% of the energy use in residential and commercial sectors and consume up to 22% of the total electricity generated in the nation, which contributes to approximately 11% of Australian greenhouse emissions and is the primary cause of peak energy demand on the electricity network [4], [5]. Not only does the large growing air-conditioning demand threaten the stability of electricity grids, it also leads to major environmental problems due to the burning of fossil fuel (mainly coal) which supplies about 90% of Australia's electrical power [6]. Thus, in order to preserve conventional energy resources and reduce greenhouse gas emissions, it is imperative to find environmentally friendly alternatives to meet the cooling and heating requirements of buildings. Solar thermal energy represents one such alternative since the cooling demand of buildings is well correlated to the overall availability of solar resources [7]. Combining solar heating and cooling (SHC) into one system can be used to create a year-round solution, improving the efficiency and economic performance of the system [8]. The east coast of Australia represents a particularly attractive location for these systems since it receives between six to nine hours of sunshine a day, and an annual solar exposure between 1200 and 2400 kW h/m², which is more than sufficient for solar air-conditioning applications [4]. Absorption chillers are considered an attractive option for solar air-conditioning due to the fact that this technology is mature, reliable and can be driven by low grade solar and/or waste heat [9].

There has been a lot of recent research and development in the field of solar-assisted absorption systems [10], [11]. Mazloumi et al. [12] developed a thermodynamic model to simulate a single-effect LiBr–H₂O absorption chiller coupled with a horizontal N–S parabolic trough collector. The plant was designed to supply the cooling demand of a typical house with a maximum cooling load of 17.5 kW. They obtained the minimum required collector area and optimum storage tank capacity for various flow rates within the collector loop. The results showed that the collector mass flow rate has a negligible effect on the minimum required collector area, while it has a significant effect on the optimal capacity of the storage tank. Assilzadeh et al. [13] modeled and investigated a solar-powered absorption chiller designed for a tropical climate in Malaysia using TRNSYS software. Through a sensitivity analysis by varying the collector area and slope, the storage tank volume, and the pump flow rate, the optimal design of the system for Malaysia's climate was determined for continuous reliable operation of the system. Since the units for storage tank volume and collector area were not normalized, it is difficult to compare their approach in sub-optimal conditions. A similar study was carried out by Baniyounes et al. [14] for an office building under subtropical climates in Australia, aiming at improving the system energetic performance. As the study was focused on technical aspects, it is unclear if the systems would be feasible economically. Martinez et al. [8] developed a TRNSYS model for a low-capacity solar absorption system in order to gain new insights into the operation of the system and found the optimal system design parameters (i.e. collector area and storage tank volume) from an energy efficiency perspective. Their simulation results indicated that evacuated tube collectors should be preferred over flat-plate collectors in solar-driven, single-effect absorption chillers. The preceding papers were mainly focused on the thermal analysis of solar absorption chillers, without considering the economic aspects of the system.

In order to perform a comprehensive assessment of a solar powered air-conditioning plant, the economic aspects of the system should also be taken into account as this determines whether or not it makes sense to actually build such plants [15]. Boopathi Raja and Shanmugam [16] reviewed past theoretical and experimental investigations on solar absorption cooling systems and proposed a wide variety of ideas to minimize the capital and operational costs and to increase the COP of absorption chillers. Desideri et al. [17] evaluated the technical and economic feasibility of solar absorption cooling systems designed for industrial refrigeration and air-conditioning applications in order to determine technical solutions for higher energy efficiency. Al-Alili et al. [18] performed a thermal and economic analysis of solar driven absorption cycles in Abu Dhabi. The results showed that the proposed solar cooling plant reduced the electricity consumption by 47% compared to conventional cooling systems and the collector area was found to be the key parameter influencing payback period of the initial investment. The authors did not take into account the cost associated with equipment installation, integration, and piping (which can be even higher than the purchased cost of equipment) while performing the economic analysis of the system. Recently, Eicker et al. [19] carried out a primary energy analysis and economic evaluation of solar thermal and photovoltaic cooling systems for an office building in three locations in Europe. The annual results revealed that the primary energy consumption of such systems can be up to 50% less than a reference conventional cooling system. A similar study was conducted by Mokhtar et al. [20] for buildings in UAE. They concluded that the cost of solar collectors and the performance of the refrigeration technologies are the two most significant parameters effecting solar cooling plant costs.

In recent years, due to increasing environmental concerns, the emission produced by cooling and heating systems has become a crucial issue [21], [22]. Bukoski et al. [23] investigated the life cycle emissions of solar-assisted absorption chillers compared to conventional mechanical vapor compression chillers in a stadium in Bangkok, Thailand. The results showed that solar absorption chillers are environmentally advantageous due to significantly lower electricity consumption relative to conventional chillers. Ghaith and Abusitta [24] evaluated the thermal and environmental performance of a solar single-effect absorption chiller using a bio-mass heater as an auxiliary heat source. The results revealed that annual savings of 176 kWh energy consumption and 140 tons of carbon emissions were achieved with the solar-assisted system. A similar study was carried out by Tsoutsos et al. [25] for a hospital in Greece, leading to an annual solar fraction of 74.2%, representing 58.7% saving in the primary energy use of the plant as compared to a conventional reference HVAC system. Furthermore, Hang et al. [26] conducted a systematic energetic, economic, and environmental assessment on a 150 kW solar absorption cooling system for a medium-sized office building in Los Angeles, California. The results showed that there is a trade-off between the economic performance and the energetic/environmental performance of the system. Using the cost of CO₂ emission reduction as an economic indicator, the optimized solar cooling system configuration had a solar collector area of 280 m² and an 11 m³ hot water storage tank, thereby providing 100,014 kWh annual cooling through solar energy and achieving a solar fraction of 83%.

Most of the cited papers analyzed the operating behavior of solar absorption chillers only in their cooling mode, without taking into account the integration of cooling and heating modes for all year round operation. They also typically considered an auxiliary heater as the sole backup system, while a mechanical vapor compression chiller can also be implemented as an auxiliary cooling device for the solar-assisted plant. Furthermore, most of studies in the literature have utilized a simple economic model which only includes the purchased equipment cost, operating and maintenance costs, without taking into account the installation, integration, and piping costs of the system, escalation rate of expenditures and fuel price, and carbon tax for CO₂ emissions. They also did not present a decision making process to find an optimum design point of the system.

Taken together, to the best of the authors' knowledge, very little emphasis was placed on different backup options for solar heating and cooling (SHC) absorption chillers from primary energy saving standpoint [27]. Furthermore, the effects of (i) solar collector field and (ii) absorption chiller size relative to building load have not yet been studied with respect to these options. Furthermore, the above literature summary reveals that the published literature rarely includes an environmental (emissions cost) analysis as a critical part of their solar cooling system analysis.

Motivated by these research gaps, the present work aims to evaluate the potential of solar-assisted single-effect absorption chillers for combined cooling and heating applications in Australian buildings from a comprehensive (energetic, economic, and environmental (3E)) standpoint. Thus, in this paper, a complete dynamic simulation model for four SHC absorption system layouts is developed within the TRNSYS software environment in order to perform a comprehensive parametric study.

First, an energy analysis was performed to compare the proposed systems by calculating their solar fraction and primary energy saving (PES) compared to a reference conventional HVAC system. Next, we applied a detailed economic analysis, which calculated the levelized annual cost of the entire system, comprising the capital investment, operating and maintenance costs, the fuel cost, and the pollution cost of CO₂ emissions. As a part of this calculation, the amount of CO₂ emission reduction was also assessed for all cases.

A comprehensive evaluation of alternative backup options (i.e. an auxiliary gas-fired heater or a mechanical vapor compression chiller) and the size of the absorption chiller was performed in order to determine the most energy efficient and economically profitable design. Using the LINMAP decision-making method, the final optimal design point of each configuration was determined from energetic, economic, and environmental viewpoints. Finally, a sensitivity analysis on the key economic indicators of the plant (payback period, net present values, and internal rate of return) was conducted to evaluate the effect of the absorption chiller size, fossil fuel costs, and subsidies and public financing in order to find the break-even point with conventional HVAC systems.