Ageing study of thermal heat transfer fluids used in solar thermal power plants based on Fresnel Collector technology Hind Grirate1,2, Nadia Zari1*, Abdellah Elmchaouri2 , Mohamed Maaroufi3,Sophie Molina4, Raphael Couturier4 1
Moroccan foundation for Advanced Science Innovation and Research MAScIR, Avenue Mohamed El Jazouli,Rabat, Morocco 2 Faculty of Science and Technology, Laboratory of Physical Chemistry and Bioorganic Chemistry, Mohammedia, Morocco 3 Mohammadia School of engineering, Dept. of Electrical Engineering, Rabat, Morocco 4 CEA LITEN, Storage Systems Laboratory,Grenoble,France E-mail*:
[email protected]
Abstract— Solar energy has experienced growth in these last decades due to financial supports delivered by governments and technological enhancements. Reducing cost and maintaining good performance is the main object of current studies. Fresnel system seems to be a good choice, as this technology is cheaper to implement due to its straightforward design. However, the Fresnel power plant efficiency depends on many criterions such as the appropriate heat transfer fluid (HTF). The present paper reports the ageing behavior of two synthetic oils (dibenzyletoluène (DBT) and hydrogenated terphenyl (HT)) at 250°C for 500 h. The aim is to choose the best HTF to use with Fresnel Collector technology which corresponds to a temperature range of 250-450°C. Keywords- Heat transfer fluid; Concentrated solar power; thermal oil; Solar energy.
I.
INTRODUCTION
The production of electricity is provided by various sources including crude oil, coal and natural gas. These forms of fossil fuel are considered as the most non-renewable resources, threatened by depletion and price increase. Furthermore, these resources contribute to massive greenhouse gas emissions leading to global warming. To address the issue, great attention is paid to renewable energy and especially solar energy [1]. Solar energy is the most abundant of renewable resources. The effective solar irradiance reaching the earth surface is ranging from about 0.06kW/m² at the highest latitudes to 0.25kW/m² at low latitudes [2]. According to expectations of the International Energy Agency (IEA), solar technology will represent 20% of global electricity generation in 2050 [3]. In fact, Solar energy is harnessed using different technologies such as: solar thermal, concentrating solar power (CSP) and photovoltaic (PV). CSP and PV are promising options for electricity production and could deeply contribute to reducing greenhouse gas emissions reduction [4]. These categories are available with differing performance characteristics. PV
generates electrical power by converting solar radiation into direct electric current via the photovoltaic effect occurred by semiconductors. CSP systems use mirrors to concentrate sunlight onto a receiver in which a heat transfer fluid (HTF) is heated to a high temperature. The thermal energy is then converted into electricity using steam turbines, gas turbines or stirling engines [5]. The main advantage of CSP over PV is the power dispatchability after sunset or during intermittent cloudy weather conditions using Thermal Energy Storage (TES). It can enhance the capacity factor of CSP. Furthermore, concentrating solar power can be used as part of a hybrid energy source into the electricity production mix, improving grid integration and economic competitiveness [6, 7]. Also, CSP technologies can present a significant potential to supply several processes, such as heat for industry, cooling and power of heating, co-generation and water desalination [3]. CSP is divided into four plant variants, namely: Parabolic Trough, Fresnel Collector, Solar Tower and Solar Dish. The technologies differ by reflectors design, receiver configuration and heat transfer fluid used. According to the Renewable Energy Association (REA), parabolic trough power plants dominate the CSP installations at a large scale by up to 96% compared to other types of power plants [8]. This is due to the maturity of the technology. However, the main disadvantage of parabolic trough is mirrors high cost. Concave mirrors used in parabolic trough are more expensive than flat mirrors of linear Fresnel [9]. This conception can reduce mechanical constraints caused by the wind movement. Also, a lower land is needed in a location close to the ground. In addition, risk of HTF leakage and resulting maintenance labor are reduced by the fixed receiver assembly [10]. This indicates that the Fresnel system is very promising. Based on Fresnel technology, most developed systems are using steam as HTF in order to avoid heat exchangers. Water is the cheapest HTF that can be used in solar power plant. Some
prototypes using Fresnel technology with direct steam generation (DSG) have been built in Australia, Spain and California [11, 12]. However, employing water as HTF required high pressure to maintain it on liquid phase and thus the two phases flows (liquid and steam) in the same tube absorber. Moreover, high purity water for steam generation is primary criterion to be used in solar thermal power plant which would cause the price level to rise. Other fluids studies are in progress in order to be used in a linear Fresnel reflector power plant such as: molten salts [13]. Molten salts (mixture NaNO3/KNO3 (60/40% wt)) can reach a high temperature of about 550°C. Nevertheless, their high freezing temperature adds complexity to the solar power plant [14]. Using air as HTF in Fresnel reflectors is not a good choice because, its volumetric density is significantly low. As a result, the pipes size should be extended [15]. Otherwise, thermal oil offers low freezing point (
