تجزیه و تحلیل شبیه سازی تمرکز انرژی خورشیدی برای ذخیره سازی حرارتی

The aim of this study was to evaluate the capacity and analyze the performance of thermal storage required for solar thermal electric power plants in order to increase their capability to supply base load power with less need for back up from fossil fuels. For this purpose, a mathematical-statistical model of hybrid solar-fossil power cycles was developed, which is based on energy balance equations and historical hourly data of direct normal irradiance and load profiles available in the literature. As follows from the computations performed for base load operations, an extremely large storage capacity equivalent to near a thousand full load operating hours should be available to a power plant to achieve continuous electricity production entirely on solar energy (solar fraction equal 1.0) during an annual operating cycle. For state of the art thermal storage technologies having a potential capacity of 10–14 full load operating hours for large-scale parabolic through solar power plants, the assessed solar fraction was 0.4–0.5 respectively, with relation to the specific conditions of calculations. The performance characteristics of thermal storage presented in the paper cover the whole extent of solar fractions from 0.2 (no storage applied) to 1.0 (pure solar operation of a power plant).

In the last two decades, the concept of Concentrating Solar Power (CSP) has successfully demonstrated its capability of producing high-temperature steam to power the conventional Rankine cycle for electricity generation. Today, the CSP technology is under a wide deployment of large-scale solar power plants for 50 MW and more electrical power capacity in various sun rich regions around the globe [1], [2], [3] and [4]. The greatest technical challenge of producing electricity from the sun is the high intermittency of solar power supply that makes it incompatible with common types of electrical load profiles, such as domestic, commercial or industrial [5] and [6]. In order to stabilize power delivery and prolong daily operating hours, solar thermal power plants have the options of using either or both solar thermal storage and fossil fuel combustion. Depending on the installed backup power capacity, the solar plant can be run continuously at full load during the day and several hours in the nighttime [7], [8] and [9]. The share of solar energy in the annual electricity production capacity of hybrid solar-fossil power plants is called the solar fraction or annual solar capacity factor. In fact, it is a primary indicator of the sustainability of solar thermal electricity generation technology. The capability of hybrid solar-fossil power systems without solar energy storage to match the typical grid load demands is limited to a solar fraction α = 0.13–0.25 [4], [6] and [8]. Consequently, a major part of the thermal power consumption, 75% or more, must come from conventional fossil energy sources, such as coal, natural gas, etc. In terms of thermal storage capacity, it is customary in addition to units of energy to use units of time, e.g. hours, as an operating duration of a power plant when the full load demand is provided solely from the energy storage. Simulations performed for a parabolic trough power plant including a thermal storage capacity for the typically considered 6 full load operating hours yielded α = 0.4 versus 0.25 without storage [10]. To reduce significantly the fossil fuel dependency of hybrid power plants by making the most of solar energy, CSP systems should have the ability to accumulate a large amount of solar energy during sunlight hours in order to retrieve the storage on a seasonal basis. For the purpose of storage, the CSP system must be increased so that part of the available solar power can be used to charge the thermal storage simultaneously with operating the power block, whenever the solar flux is sufficiently high. In principle, adequately sized seasonal thermal storage should permit uninterrupted electric power generation during on- and off-sun hours, 24 h a day, all year round, to a so-called pure solar power plant, for which the parameter α = 1.0. Although the subject of thermal storage has received considerable attention in the literature during the past few decades, basic researches and developments concerning CSP applications have been mainly focused on short-term storage systems capable to provide a few full load operating hours [1] and [7]. Until now, there are only a few credible studies concerning the impact of large-scale storage systems on the operating efficiency of solar power plants. As follows from simulations carried out in [11] for solar thermal electric power plants, solar fractions as large as α = 0.75–0.9 are achievable with modest storage capacities ranging from φ = 10 to 50 full load operating hours respectively, for the site of Albuquerque, USA. According to the estimates of storage capacity presented in [12], a solar fraction value resulting from φ = 12 h is α = 0.53, and due to [13], a larger storage capacity φ = 15 h leads to α = 0.65–0.71. As a whole, the capability of thermal storage to replace fossil fuels in backing solar operating cycles has not yet been thoroughly explored and deserves to be studied in depth. The objective of this work was to analyze the general case of hybrid solar thermal power systems combining thermal storage and fossil fuel backup facilities to produce electric power required by grid and to explore the ways of reducing the fossil fuel consumption by increasing the capacity and operating efficiency of energy storage. For this purpose, an energy balance model of yearlong solar operating cycles comprising historical hourly data of direct normal irradiance and load demand was developed and applied to base load power plants located at different geographic sites. The computations performed allowed quantifying the amount of stored energy as function of the solar fraction in the whole range of storage capacities, from a few to more than a thousand full load operating hours.

The process characteristics of thermal storage presented in this study were computed over a wide range of thermal storage capacities varied from a few hours to the order of magnitude of a thousand full load operating hours. The corresponding solar fraction values of hybrid solar-fossil power plants supported by energy storage ranged between 0.2 (no storage applied) and 1.0 (the pure solar operation). Although the modeling was specifically developed to simulate parabolic trough systems operated as base load plants, the obtained results can qualitatively be extended to most other CSP systems and also to different continuous load profiles (except peaking power generation), since the dominating factor affecting system performance is the intermittent solar power source. Based on the statistical analysis of thermal energy variations in time throughout an annual operating cycle, four distinctive storage scales were identified, referred to as diurnal (φ < 14 h), intermediate (14–102 h), seasonal (102–103 h) and yearly (∼103 h) storage capacities. The operational performance of the diurnal-scale storage was shown to be strongly influenced by hourly and daily solar power conditions so that thermal storage efficiency with respect to month-mean solar fractions is considerably smaller in the low-sun season (winter) than in the high-sun season (summer). Correspondingly, the annual solar fraction of hybrid power plants employing thermal storage with a nominal capacity within 14 full load operating hours is limited to below 0.5, so that half or more of the energy input must be provided by burning fossil fuels. An improved diurnal storage efficiency resulting in a solar fraction of 0.6 can be achieved by increasing the solar field size along with losing some a few percent of the total annual amount of solar energy collected. The results of simulations show that a pure solar power plant (solar fraction equal 1.0) should include a storage capacity almost 2 orders of magnitude greater than what is available with the most advanced thermal storage systems. The state of the art solar thermal storage technologies utilizing either sensible or latent heat of molten salts or various other materials are limited to the diurnal range of capacities as they suffer primarily from a relatively low energy density of the exploiting storage materials and also from significant heat losses from the system during lengthy operating cycles. Hence, the further development of thermal storage technology should be particularly aimed at vastly more powerful energy storage concepts extending the storage capability into the intermediate and seasonal scale ranges, in order to achieve a higher level of sustainability for solar thermal electric systems.