Optimisation and financial analysis of an organic Rankine cycle cooling system driven by facade integrated solar collectors
Introduction
Currently, air-conditioners are normally powered by electricity [1]. Most electricity is generated by coal in Australia, which is the single largest contributor to GHG emissions (199.5 Mt of CO2 or 36.9% of national GHG emissions in 2007) [2]. Meanwhile, the working fluids applied in conventional chillers have negative impacts on the environment and high GHG emission factors [3].
In general, cooling dominant localities receive abundant solar radiation and the availability of this radiation matches the cooling demand. In other words, the building’s cooling load is in phase with the solar radiation. Therefore, the use of solar energy to drive space cooling systems is an attractive and logical approach for reducing the peak electricity demand in hot sunny climates. Solar cooling encompasses a wide variety of cooling technologies driven by either photovoltaic (PV) modules or solar thermal collectors. Cooling can be achieved through solar electrical, thermal cooling process and thermal mechanical. Among these, the solar organic Rankine cycle (ORC) cooling system has been proven to be the most technically and financially feasible solar cooling technology in façade integration applications [4].
Solar ORC cooling systems were investigated widely in the 1970s and 1980s. However, this research area did not attract much attention over the past two decades until the recent development of the ORC cooling system and the introduction of environmentally friendly working fluids. Prigmore and Barber [5] conducted an experimental study on a solar ORC cooling system to produce 10.55 kWr cooling effect or 1 kW of electricity for residential use. In this system, R113 and R22 were employed for the ORC and VCC chiller, respectively. They concluded that the overall coefficient of performance (COP) of the solar ORC cooling system under off-design conditions can surpass the overall COP of the solar absorption cycle. Wang et al. [6] developed a prototype cooling system with a separate ORC and VCC chiller, with working fluids R245fa and R134a, respectively. A scroll compressor was used to achieve 5 kWr cooling capacity and 0.5 COP. Demierre et al. [7] also built a prototype solar-driven ORC heat pump for both heating and cooling with R134a. In this prototype, a radial in-flow turbine and a centrifugal compressor were directly coupled. Bu et al. [8] developed an ice maker driven by a solar ORC-VCC system with R123 as the most suitable working fluid. The optimal evaporating and condensing temperature were also identified to achieve maximum overall system efficiency. Hu et al. [9] developed a thermodynamic model to study the effect of evaporating temperature, condensing temperature and working fluid types on an ORC-VCC system. As one of the most important components, ORC has been studied comprehensively in terms of both theory and experiment [10], [11], [12], [13], [14], [15]. Meanwhile, several studies focused on ORC optimisation. Quoilin et al. [16] used a single-objective function to optimise a high temperature ORC. The specific investment cost was selected as the objective function. They found that a thermo-economic optimisation can lead to different outcomes compared with a pure thermodynamic optimisation. Sun et al. [17] optimised the exergy efficiency and the system net power output of an ORC for ocean thermal energy conversion in order to select the most suitable working fluid, and evaporating and condensing temperatures. Wang et al. [18] developed a float-point coding scheme based on the genetic algorithm optimisation method to optimise a solar driven regenerative ORC. They used the daily average efficiency as the objective function and minimised the turbine inlet pressure, pinch temperature difference and approach temperature difference. They also studied the effects of the key thermodynamic parameters on the system’s performance. Pierobon et al. [19] aimed to find the optimal design for medium size ORCs by employing multi-objective optimisation with the genetic algorithm as the optimiser. Three objective functions were considered: thermal efficiency, the total volume and the net present value of the system. The working fluid, turbine inlet temperature and pressure, condensing temperature, pinch points and mass flow rate were selected as optimisation variables. Liu et al. [20] developed a generic model of a scroll expander in MATLAB/Simulink for a waste heat recovery ORC. The model takes into account the geometric characteristics of the expander, and this geometry can be optimised to avoid under- or over-expansion for any given operating conditions or fluids. Freeman et al. [13] maximised solar ORC electrical power generation by optimising the working fluid, selecting a solar collector module and utilising a recuperator. However, none of these studies considered the optimal design parameters for ORC in cooling generation applications. Meanwhile, the effects of the façade integrated ETCs on the ORC system also have not been considered.
Therefore, the aim of this study is to optimise the heat exchanger area of an ORC to minimise the unit cooling cost (UCC) of the façade integrated ETC-ORC-VCC cooling system. Unlike other previous studies that mainly focused on the optimisation of operating conditions rather than the system design, this paper tends to study the effect of each heat exchange area on the net present value (NPV) of the ORC system.
Section snippets
System description and initial design
The solar cooling system comprises four main components: ETCs, ORCs, water-cooled VCC chillers and electric generators. ETCs coupled with the building facade are known as façade integrated ETCs. To maximise the installation areas, ETC modules are also placed on the building rooftop. In addition, several water pumps and buffer tanks are used to obtain the required constant heat transfer fluid flow rate. Gas-fired auxiliary heaters are required to ensure that the heat source for the ORC maintains
Optimisation
The main reason for integrating the building façade with the ETCs and ORC-VCC chiller is to minimise the cooling energy consumption of a building. This can be achieved by increasing the area of the heat exchangers in the ORCs when heat input from the solar collectors is fixed. However, this is associated with a high investment cost. Therefore, there is a financial trade-off leading to a restriction such that the area of the heat exchangers cannot be increased for financial reasons. Based on the
Financial analysis of the optimised system
Based on the results of the simulation model developed in Section 2 and the outcomes of the optimisation in Section 3, a financial analysis of the façade integrated ETC-ORC-VCC system was conducted.
Conclusions
A façade integrated ETC-ORC-VCC system for a typical office building in a tropical climate (Darwin) was optimised to maximise the electricity savings from the ORC separately. For the ORC, the financial performance depends on the heat exchanger area. The optimisation was conducted for a 20-year project life. The ORC optimisation results in an increased heat exchanger area, but the electricity savings are offset by the cost increase from adding more plates to the heat exchangers once a certain
Acknowledgements
This work was supported by the Australian Research Council Linkage Project funding [ARC-LP 110100429]. The advice and funding support provided by Permasteelisa Group are also acknowledged.
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