توصیف طیفی و تجزیه و تحلیل عملکرد طولانی مدت از سیالات مختلف تجاری انتقال حرارت (HTF) به عنوان فیلتر مستقیم جذب برای برنامه های کاربردی تقسیم پرتو CPV-T

Hybrid concentrated photovoltaic – thermal systems (CPV-T) provide simultaneous supply of electrical and thermal energy, using solar cells with cooling systems to avoid high cell temperatures that decrease the system electrical conversion efficiency. Heat transfer fluids arranged in front of the cell, acting as selective beam-splitting filters, may represent a feasible alternative to absorb unwanted solar radiation, preventing the cell from overheating and directly generating usable thermal output. The cooling efficiency and the temperature output of the liquid depend on optical transmittance as well as chemical and physical stability. A research study for the most suitable commercial heat transfer fluid for a direct-absorption beam-splitting CPV-T system is conducted in this paper, analysing the effects of high temperature and exposure to UV light on the optical transmittance of the fluid under accelerated lifetime test conditions. Optical transmittance of 18 different commercial heat transfer fluids has been measured. The most promising liquids to serve as Direct-Absorption Filters selected for accelerated tests include Duratherm 600, Duratherm G, industrial Propylene Glycol (PG), pink-dyed PG, and Royco 782. Long-term degradation tests include low temperature test at 75 °C, high temperature test at 150 °C, and UV light exposure. Results from the accelerated tests show that the optimum fluid for our application is the industrial grade Propylene Glycol adapted with a chemically-inert red dye such as Oil Red 235 inorganic dye. Propylene Glycol is a liquid used in food processing, with a cost of A$ 2.50/kg. Graphical abstract Full-size image (21 K)

Concentrated solar energy technologies are well-known and widely used in centralised power plant applications. The basic idea is to collect the sunlight using optical devices comprising mirrors and/or lenses that concentrates the solar energy onto a small area. This leads to a higher solar flux density onto the receiver. The objective of all concentrating systems is to increase the system efficiency and reduce the lifetime energy costs in terms of $/MW h so solar energy systems are competitive on the market. Concentrators can use the incident power for: (a) heat production (domestic solar hot water systems – SHWS); (b) electricity generation converting heat into electricity (thermoelectric systems – concentrating solar power – CSP); (c) electricity generation using photovoltaic solar cells (concentrating photovoltaics – CPV); and (d) heat and electricity generation, using combined thermal-PV hybrid systems (concentrating photovoltaic–thermal – CPV-T) (Fig. 1) [1] and [2]. Full-size image (40 K) Fig. 1. Classification of common technologies and system set-up for concentrated solar irradiance conversion. Figure options Depending on the solar concentration ratio and the design of the thermal application (SHWS/CSP) temperature outputs from a low of around 100 °C to a high temperature around 1000 °C can be provided. Conventional low temperature (SHWS) thermal energy is generally transferred using a suitable liquid circulated through the system to remove the solar energy absorbed by the receiver. The receiver can be a specially coated black tube, for example proprietary black chrome on copper pipe, or evacuated glass tubes using only direct solar power, or tubes situated under a glass plate where they are heated by compound parabolic concentrators (CPCs) with a concentration level of 1.2–2X. These systems are suitable for temperature ranges that cover domestic hot water demand. However high temperature (CSP) is convenient for energy storage, electricity or hydrogen generation, process heat, and some thermo-chemical conversion processes. Operating at this elevated temperature range requires much larger and more sophisticated systems, which are explained in other studies [1]. Concentrated photovoltaic technologies (CPV) aim to reduce the energy generation costs by reducing the cell area, replacing the expensive energy conversion medium with inexpensive collectors and tracking systems. It is important to note that solar cells are capable of converting only a small fraction of the entire solar spectrum into electric power. The fraction depends on the specific type of cell, and is determined by the cell spectral response (SR). Radiation absorbed outside this spectral range is converted to heat in the solar cell, causing stress on the material and a significantly decrease of the electrical conversion efficiency. For mono- and polycrystalline silicon solar cells, this is typically −0.45%/°C in relative terms. The fact that CPV systems operate at an elevated solar concentration level makes the temperature-dependent efficiency loss a crucial issue [3], [4] and [5]. In order to maintain the highest possible performance, the cell needs to be either passively or actively cooled. The type of cooling depends mainly on the concentration level, the size of the cell, and the absorbed irradiance flux. For active cooling, a heat transfer liquid can be circulated across the rear surface to extract the heat. Instead of dumping this absorbed energy, the heat can be utilised in domestic hot water applications, further improving the energy conversion efficiency and economical attractiveness of the system. This simultaneous supply of electrical and thermal energy is a feature of hybrid concentrated photovoltaic – thermal (CPV-T) systems [1] and [4] (Fig. 1). Both CPV and CPV-T systems can obtain additional benefits from spectral-splitting to improve their performance and total efficiency as the cells. The idea is to use bands of the solar spectrum for the applications that are highly efficient in those particular regions. Previous research analysed different methods of spectral splitting, focusing mainly on selective mirrors, combinations of PV cells, and physical films [6] and [7]. For instance, CPV systems use multi-junction cell technology, which consists of a stack of different cell types which are mechanically and electrically connected. With multi-junction structures, each junction generates electricity within a specific spectral range, successively passing through the unused wavelengths for the cell underneath. In this way, a much larger fraction of the available spectrum can be electrically converted, reaching efficiencies over 41% instead of about 25% for single-junction silicon cells [6], [8] and [9]. The complexity of these structures, their spectral sensitivity, and the need for them to be integrated in CPV with very high concentration levels, currently makes them expensive and less competitive. CPV-T systems can also use spectral-splitting technologies. One method involves the use of a liquid in front of the cell, acting as a selective spectral absorption filter, and directly generating a usable thermal output from the absorbed energy [3], [10] and [11]. This technology is called ‘Direct-Absorption Filters for CPV-T’. Simulations conducted by Sabry et al. have shown that a spectral matching fluid drastically reduces the operating temperature of the cell, leading to a higher Voc and to an efficiency increase of about 30%. In addition, about 40% of the incident irradiation was calculated to be converted to thermal energy [3]. Two studies, one from 1986 and one from 2011, reported testing a small number of heat transfer liquids for their optical properties, in particular the transmittance in the original state and again after long-term lifetime tests. However, there is insufficient experimental information on which to select a suitable liquid, and the few available results are either old or not comparable in scope [10], [11], [12] and [13]. One example of a small scale CPV-T system for domestic, commercial and industrial rooftops is the hybrid photovoltaic thermal micro-concentrator (PV-T MCT) developed at the Australian National University (ANU) [14]. Using a planar Fresnel mirror array, sunlight is concentrated and directed towards the silicon solar cell mounted within a receiver, as shown schematically in Fig. 2. To ensure the system is running at optimal temperature a liquid is channelled on the back side of the receiver, cooling it and supplying low-temperature heat (≈80 °C) for hot water. Full-size image (24 K) Fig. 2. Using a traditional hybrid photovoltaic thermal micro-concentrator (PV-T MCT), electricity and hot water can be generated simultaneously. Figure options The aim of this paper is to investigate grounds for support for an adaptation of this hybrid micro-concentrator by integrating the spectral splitting technology to facilitate high-temperature heat production (≈135 °C) in a modified MCT system. The idea is that the same heat transfer liquid, which initially cools the cell, flows then through a glass channel located in the pathway of the concentrated beam in front of the cell (Fig. 3a and b). By absorbing the spectral components of the concentrated radiation with the unwanted wavelengths (UV, Vis, FIR) a fluid temperature output of up to 135 °C might be achievable, while keeping the cell operating temperature much lower. With this configuration, the entire solar spectrum is used for high temperature thermal output as well as electricity generation, with overall conversion efficiencies exceeding 70% expected [13] and [15]. Full-size image (31 K) Fig. 3. (a) Schematic of the heat transfer liquid flow in the adapted system. For heat generation, the fluid cools first the cell before splitting the incident sun light and absorbing the unwanted wavelengths [13] and [15] and (b) new concept of a photovoltaic thermal micro-concentrator system (PV-T MCT) incorporating spectral splitting technology with a liquid filter (red) in front of the solar cell. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) Figure options A liquid with temperature in the range of 135–150 °C is valuable as process heat in industries, such as hotels and hospitals, because of their high demand on air conditioning and hot water. As solar air conditioning needs high temperature but reduces the liquid temperature by only about 20 °C, the fluid after the first energy extraction is still about 110 °C. This allows driving a second system, say for hot water generation, to make use of the remaining thermal energy. Similar coupled systems could also be used for offices and food processing industries, even though their hot water demand is much smaller. To reach this high temperature, both the thermal and optical properties of the heat transfer fluid are important for determining the liquid selection. Among other factors, the concentration ratio, the collection and system area, the dimensions of the system (for example, the storage tank size), and the fluid flow rate are central to the design. The fluid flow rate and the tank size could be used to compensate fluctuations in irradiance intensity to smooth irregularities between energy supply and actual heat consumption. In this paper, several different commonly-used heat transfer liquids have been analysed in terms of their optical transmittance and an examination for the most suitable liquid has been conducted. To ensure a relatively constant energetic output of this beam-splitting system concept over the system lifetime – in both electrical and thermal terms, the durability and reliability of the liquids has been tested. This study is structured in three parts: • Selection of possible liquids, • Optical characterisation, measuring the liquid optical transmittance, and • Degradation tests, conducting accelerated tests at low and high temperature, as well as UV light exposure.

By incorporating the spectral splitting technology in a hybrid photovoltaic thermal micro-concentrator system (PV-T MCT), electrical and thermal energy output fluxes are obtained. A thermodynamic model of such a beam-splitting system has been developed at ANU and shows that temperatures around 135 °C are achievable [15]. In order to use a common and inexpensive heat transfer fluid it is critically important to have extensive data regarding its optical properties, such as spectral transmission, UV stability, physical and chemical stability, as well as thermal stability. Optical transmittance of 18 different commercial heat transfer fluids in their original state has been measured and presented within this study. Depending on their spectral transmittance properties and its correlation with the pre-defined ideal filter for this application, the most promising liquids that have been selected for further analysis include Duratherm 600, Duratherm G, industrial Propylene Glycol (PG), pink-dyed PG, and Royco 782. These fluids have been subjected to accelerated lifetime tests to study their long-term degradation, including low temperature test at 75 °C, high temperature test at 150 °C, and UV light exposure. Results from the accelerated tests show that the optimum fluid for our application is the industrial grade Propylene Glycol, followed by the Duratherm G. In order to obtain the spectral transmittance to matching the preferred filter design, we need to adapt the liquid with a chemically-inert red colour – preferably using inorganic dyes such as Oil Red 235. Organic dyes degrade rapidly under high temperature and exposure to UV light. Propylene Glycol is a liquid used in food processing, which makes it readily available and much more convenient than other heat transfer fluids. It is known from the manufacturer of these liquids, shown in Table 1 and Table 2, that the cost is about A$ 2.50/kg (A$ 538/215 kg). On the other hand, the price for 55 gallon drums of Royco 782 and the Duratherm G are about A$ 7.0/kg and A$ 7.70/kg, respectively. Other advantages include the specific heat and the thermal conductivity of the PG, which are much better than the mineral oils; whereas a drawback is the higher density and viscosity that introduces additional requirements for a stronger hydraulic system. In applications where an outlet temperature of only about 80 °C needs to be achieved – for domestic hot water supply for instance – the PG and the Royco 782 are both very suitable. Colouring the PG red also elevates it to the highest performing liquid. In general, liquids under low temperature exposure tend to change their spectral transmission properties only marginally, but with respect to the service life-time and bulk consumption of a liquid in economic and ecological aspects, the best performing fluid should be used. Even though the liquids analysed in this study are non-toxic and non-hazardous, they should not be released into the environment because of their high mobility in soil, and the danger they present for aquatic organisms. The accelerated lifetime tests have been conducted for up to 700 h under constant elevated temperature which poses a much higher stress on the liquid than under fluctuating operating conditions. For an accurate prediction of their real life durability, corresponding to these 700 h, it will be necessary to model the PV-T MCT system with a focus on the number of heating cycles, and integrated heat exposure times, within a certain period. For further studies, the remaining liquids from the list and more red heat transfer fluids should be tested. Furthermore, a similar analysis for different inorganic red dye materials needs to be conducted to identify dyes with excellent stability and good spectral response. Calculations of monetary performance under different market and energy price forecasts will allow a first assessment on the future prospectus of this concept.