Analysis of two heat storage integrations for an Organic Rankine Cycle Parabolic trough solar power plant

Highlights

• Thermal storage Solar Parabolic Trough integrated with Organic Rankine Cycle.

• Small Parabolic Trough Direct/Indirect thermal storage layouts integration.

• Full system performance analysis at design and off design conditions.

• Best result for 10 h of storage with Levelized Cost of Electricity 16.3 c€/kW.

• Application in off-grid and distributed generation (as mines).

Abstract

Among the concentrated solar power technologies, those based on Organic Rankine Cycles have a very low market presence. However they have favorable characteristics for applications with low temperature and small/medium size (<10 MW), such as off-grid applications or distributed power generation.

In this paper is analyzed a 5 MW parabolic trough plant integrated with an Organic Rankine cycle power block and thermal storage. On this purpose, two different thermal storage integrations are analyzed. They are based on two different heat storage layouts: direct system using Hitec XL both as Heat Transfer Fluid and as storage medium; indirect system using Therminol VP-1 as Heat Transfer Fluid and Hitec XL as storage medium.

Full system performance at rated and off-design conditions is presented operating with different organic working fluids. Its potential application and main challenges for its development are discussed in terms of performance and costs. Among the analyzed working fluids, the best results were obtained for the cycle working with Toluene with an efficiency at the power block of 31.5% and an estimated power block cost of 825 €/kW. The indirect storage layout was the most interesting from the point of view of Levelized Electricity Cost (16.19 c€/kW) and productivity (28.2 GW h/y for a 5 MW_el plant) for 10 h of storage However, it results in a storage tanks volume 26% greater than the obtained for the equivalent direct storage layout. The results show the competitiveness and the potential of the proposed integrated small size parabolic trough designs for isolated applications as mines or for some distributed generation uses where grid capacity is limited.

Introduction

Concentrated Solar Power (CSP) technologies are among the renewable technologies with greater potential for mitigating climate change [1]. They combine flexibility for energy generation and capacity for thermal energy storage (TES) to enhance energy supply security [2]. CSP plants with thermal energy storage can support power generation and provide ancillary services including voltage support, frequency response, regulation and spinning reserves, and ramping serves [3]. In addition, to increase flexibility, CSP plants can also be equipped with backup power from combustible fuels to cover demand when solar resource is not available [4]. In 2012 the global power in CSP plants was 2.7 GW whereas the projects in development or under construction will increase the installed CSP capacity in the world up to 15 GW [5].

Among CSP technologies, parabolic trough collector technology (PTC) has reached the highest level of commercial maturity and accounts for the largest share of the current CSP market, but other technologies at different stages of technology maturity will increase their presence in the future [6]. A parabolic solar collector consists of a parabolic trough concentrator reflecting direct radiation on the focal line of the parabola, where the absorber tube is located. Different commercial collectors are available e.g. LS-3, EuroTrough and Solar Genix Collector, SENER [7]. It is the most mature CSP technology due to continuous advances improving the characteristics and performance of the parabolic trough solar collector and its parabolic-shaped reflectors [8]. Different models have been developed to evaluate the cost evolution of the PTC CSP-solar electricity as function of different parameters [9] and to analyze the effects of the economic strategies on CSP plants viability and sizing [10]. The main cost of CSP parabolic trough plants is solar field, with a contribution above 50% of the total investment cost [11]. Current investment costs are estimated in the range between 6600 and 8688 $/kWe [11], [12] but it is expected that in next decade they will go to values below 6000 $/kWe by 2020 [11], [13]. Levelized Cost of Electricity (LCE) of parabolic trough plants is in the range 0.20–0.33 c$/kW h at present, depending on their location, whether they include energy storage and the particulars of the project [11], [12]. Perspectives are that PTC-CSP plants will evolve to LCE values below 13 c$/kW h in future large scale plants [11], [13]. Advances for reduction costs in PTC plants are associated to: technology scaling up; solar field massive production with associated cost reduction; high temperature heat transfer fluids; high efficiency power blocks; advances in energy storage [12], [14].

As CSP technology, parabolic trough technology allows the possibility to store thermal energy. It involves over-sizing the solar field and increasing the annual utilization factor of the plant. Different technologies [15] and materials [16] for energy storage can be used with different levels of maturity and costs. Most recent commercial CSP plants use molten salts as storage commercial technology [17].

Current large-scale systems rely on traditional well-established steam-based Rankine cycles for power production. Organic Rankine Cycle power plants are an interesting alternative for small-medium plants and relative low temperature heat sources. Fluid characteristics make ORC favorable for applications with medium–low temperature heat recovery [18] (normally less than 400 °C), as in the case of parabolic trough solar energy applications. In relative small power plant sizes (<5 MW), adequate for modular solutions, the use of organic working fluids results in a more compact and less costly plant than traditional steam cycle power plants. Steam Rankine cycle operating at moderate temperature and small power ranges would have to use a simple Rankine cycle layout because a regenerative steam turbine would not be not viable for this power size. The simple Rankine cycle would have a similar (or lower) power block efficiency than the obtained with the equivalent ORC but without the advantages of ORC: modularity and high efficiency at partial load operation.

The use of ORC in CSP plants have been studied in different applications as hybrid systems combined with biomass [19], integrated in CSP tower plants [20] or in demonstration pilot plants for analyzing the integrated operation [21]. ORC plants have good performance at partial loads. Off-design and partial operation of ORC power block has been studied by different authors. In [22] is studied the effect of heat source temperature in cycle’s performance, meanwhile in [23] is analyzed the off-design operation of a CSP-ORC with compound parabolic collector. In [24] off-design performance analysis of a geothermal ORC is presented based on the preliminary design of turbines and heat exchangers.

Application to mid and small size CSP plants is found in [25] where partial load operation for 5 MW_el indirect ORC cycle coupled to linear solar collectors with different technologies is assessed evaluating the effect of two control strategies on annual electricity output. In this analysis Levelized Electricity Cost value (LEC) was 180–175 €/MW h with sliding pressure off-design control strategy. In [26] is presented a thermodynamic analysis of a micro ORC CSP plant working with different organic fluids. In [27] a dynamic simulation is given for the coupling of solar thermal collectors with a 6 kW_el ORC, simultaneously producing electric energy and low temperature heat. Besides integration of both technologies is shared with applications for refrigeration as solar heat pumps [28] and combined cooling, heating and power systems (CCHP) [29], combined with geothermal [30], in high efficiency systems integration for trigeneration [31] or double-stage expansion solar organic Rankine cycle [32]. In [33] is studied the potential for small-scale combined solar heat and power (CSHP) system based on an Organic Rankine Cycle (ORC) for the combined provision of heating and power for domestic use in the UK. Main challenges for this integration of technologies are the reduction of investment costs for the whole system and the main components: solar field, heat transfer fluid, heat storage system and power block [34].

The aim of this paper is the characterization of an ORC parabolic trough power plant of 5 MWe with two different active storage systems: (a) two tanks indirect storage system and (b) two tanks direct storage system. For the analysis of the integration, three organic fluids have been selected by considering the temperature range, cycle efficiency, critical temperature and thermal stability: Toluene, Cyclohexane and siloxane D4. Two Heat Transfer Fluids (HTF) have been used: Oil Therminol VP-1 and molten salt Hitec XL [35]. Therminol VP1, used as HTF fluid in the indirect system, can work in a wider range of temperatures and with slightly higher maximum temperatures than other commercial heating oils. For the direct storage system layout, the use of a molten salt as HTF and storage media allows operating at higher temperatures (up to 500 °C) and eliminates the need for the oil-to-salt heat exchanger. It allows a substantial reduction in the costs of TES system, improving the performance of the plant and reducing the levelized electrical cost [36]. Hitec XL has been selected because its relatively low freezing point (120 °C).

The structure of this paper is the following. First, the plant layouts and thermodynamic models of the components are described. Then, these models are used to analyze the effect of main design parameters in the two plant configurations and to set the rated operation parameter criteria. Once the design conditions are established and equipment sized, off-design operation is analyzed. Finally, the economic models are described and, with the defined equipment and operation parameters, economic analyses are developed under the operation criteria. The results of the study show the interest of the two storage configurations for small and mid-size power plants (<5 MW).