Evaluation of a Molten Salt Heat Transfer Fluid in a Parabolic Trough Solar Field

ABSTRACT An evaluation was carried out to investigate the feasibility of utilizing a molten salt as the heat transfer fluid (HTF) and for thermal storage in a parabolic trough solar field to improve system performance and to reduce the levelized electricity cost. The operating SEGS 1 plants currently use a high temperature synthetic oil consisting of a eutectic mixture of biphenyl/diphenyl oxide. The scope of this investigation included examination of known critical issues, postulating solutions or possible approaches where potential problems existed, and the quantification of performance and electricity cost using preliminary, but reasonable, cost inputs. The two leading candidates were the so-called solar salt (a binary salt consisting of 60% NaNO 3 and 40% KNO 3 ) and a salt sold commercially as HitecXL (a ternary salt consisting of 48% Ca(NO 3 ) 2 , 7% NaNO 3, and 45% KNO 3 ). INTRODUCTION The use of molten salt HTF in a trough plant has several obvious advantages. With salt, it may be possible to raise the solar field output temperature to 450-500°C, thereby increasing the Rankine cycle efficiency of the power block steam turbine to the 40% range, compared to 393°C with the current hightemperature oil and a cycle efficiency of 37.6%. The HTF temperature rise in the collector field can increase up to a factor of 2.5, reducing the physical size of the thermal storage system for a given capacity. Moreover, molten salt is cheaper and more environmentally benign than the present HTF. In this evaluation, the Solar Two experience [1] with salts was both pertinent and valuable, especially concerning issues related to piping, vessels, valves, and pumps. The major challenge of the molten salt is its high freezing point, leading to complications related to freeze protection in the solar field. The synthetic oil currently used freezes at about 15°C, whereas the ternary and binary molten salts freeze at about 120°C and 220°C, respectively. This demands innovative freeze protection methods and increased operation 1 Solar Electric Generating Systems located in Mojave Desert, California. and maintenance (O&M) requirements. There are also other important considerations related to the use of molten salts. For example, header piping materials and fittings on the hot side of a collector loop will be more expensive, and the desired highside temperature limit may be restricted by the durability and performance of the selective surface of the receivers. On the other hand, thermal-and fluid characteristics of the collector field are improved. Therefore, this evaluation tackled several basic questions, such as: What is the practical upper temperature limit? Is the O&M with salt feasible in a trough field, particularly freeze protection? Do materials, O&M, performance, heat tracing and other factors push the solar system capital cost too high, or in fact will the cost be reduced? Will electricity costs for trough systems be reduced with this approach? Does the integration of thermal storage change the economic results and comparisons? This evaluation addressed all these questions. The result is a comprehensive comparison, on the basis of levelized electricity costs, of a wide range of trough system options using a molten salt HTF, plus an identification of crucial engineering issues. METHODOLOGY The benefits of a molten salt HTF were compared on a basis of Levelized Electricity Cost (LEC) to a reference configuration solar power plant using a synthetic oil HTF. After selection of the power plant parameters and candidate salts, comprehensive parametric calculations were carried out on performance and cost of various power systems, leading to the LEC results. It was determined early in the study that a salt HTF was only attractive for a configuration that includes thermal storage. Along the way, a number of conceptual design analyses were developed to address potential engineering barriers and to arrive at reasonable cost estimates. CANDIDATE SALTS Nitrate salts were selected for Solar Two use because of their favorable properties compared with other candidates. In particular, these nitrate salts have low corrosion rates with common piping materials, are thermally stable in the upper temperature range required by steam Rankine cycles, have very low vapor pressures, are widely available, and are relatively inexpensive. Solar Salt was selected as the most practical salt for molten-salt power tower applications because the upper operating temperature limit (600°C) allows the technology to be used with the most advanced Rankine cycle turbines. In addition, it is one of the lowest cost nitrate salts. However, a major disadvantage with Solar Salt is its relatively high freezing point of 220°C. Hitec salt offers a lower freezing point of about 140°C at a higher cost. The freezing point is of major importance in a trough solar field because of the likely difficulties and cost associated with freeze protection due to the need for extensive heat tracing equipment on piping and collector receivers. Primarily for this reason, a calcium nitrate salt mixture (basis of the commercial product HitecXL), with a lower freezing point of about 120°C, is favored here. Other characteristics, like cost, are important, but in the final analysis were deemed secondary to the risks associated with freezing. The density, viscosity and heat capacity properties are generally similar for the nitrate salts. Calcium nitrate salt has an upper operating temperature limit of about 500°C, but it is expected that the chemical stability of the receiver selective surface, not the salt, will be the limiting operational factor on the maximum operating temperature level. The vapor pressures at these temperatures are very low, typically a fraction of a Pascal. Chemical reactivity and environmental issues are similar for the nitrate salts and are acceptable for this application. Because thermal storage is an important issue for a trough system, the cost effectiveness of nitrate salts in a trough solar field was initially evaluated in terms of cost per unit thermal energy stored. That is, the costs were analyzed taking into account not only the raw costs of the salt constituents, but also the effective heat capacities of the salt solutions. Raw costs were based on dry industrial grade costs of the appropriate constituents or costs of commercial pre-mixed products. The temperature rise in the solar field was varied from 100°C to 200°C. The cost of the SEGS HTF (Therminol VP-1) was used for comparison at the 100°C point. The comparison is shown in ENGINEERING ISSUES Preliminary conceptual design work defined the system requirements and estimated costs of changes in the solar steam system design and equipment necessary for operation with a molten salt HTF. While the detailed engineering results cannot be elucidated in this short paper, the following list highlights the main issues taken into account: • Operation and durability of the heat collection element, particularly the selective surface, at higher operating, • Temperatures, this includes increased radiation heat losses at higher fluid temperatures and the potential exacerbation of the asymmetric temperature distribution around the circumference due to a lower salt flow rate, • Solar field HTF flow rate, piping layout and parasitic pumping power, which are affected by salt properties and 2 Copyright © 2002 by ASME Copyright © 2002 by ASME fluid temperature rise across the solar field, and the selection of more expensive steels for the headers operating at higher temperatures, • Freeze protection of the solar field piping and heat collection elements, including the ball joints used between collectors, • Detailed thermal storage system analysis using either twotank or thermocline systems, with use of the same or different fluids in the solar field and the storage system. For example, a VP-1 solar field is configured to use molten salt for thermal storage by installing an oil-to-salt heat exchanger between the two systems. These choices have large effects on power cycle operation and costs. • Enhanced operation of the power block at higher steam conditions, taking into account the detailed effects of the storage system, and • Selection of valves, fittings and pumps for molten salt application. Many detailed design and cost evaluations were carried out on the areas outlined above in order to develop reasonable information for the performance and cost analyses. Particular attention was placed on the design of the thermal storage systems, major heat exchangers, and power cycle performance Furthermore, issues associated with freeze protection methods, costing, and operation were identified, evaluated; and resolved, at least at a preliminary stage. These included freeze protection operating scenarios for nighttime (low-flow circulation of hot salt from thermal storage tanks throughout the solar field); routine loop maintenance that requires HTF removal; freeze protection methods for piping, fittings, HCEs, and ball joints; and recovery from freeze incidents. For example, an innovative approach using impedance heating for freeze protection of the HCE, in contrast to an external heating coil, was deemed to be feasible. Ball joint freeze protection, on the other hand, was left unresolved and requires further investigation. LEC COST COMPARISON The purpose of this step was to evaluate the economics of the proposed salt HTF concept and compare it with the state-ofthe-art parabolic trough power plant. The evaluation is based on a LEC calculation. The following tasks need to be carried out to estimate the LEC of a power plant: • Plant design, • Annual performance calculation, • Estimation of O&M cost, • Estimation of investment cost, and • Determination of economic boundary conditions and LEC calculation. The best concept design can only be determined by comparing performance and cost of different approaches. Both performance and cost are reflected in the LEC. An optimization of the concept requires several iterative steps and re-definition of input parameters and assumptions within the evaluation process. Investment costs and O&M costs were estimated based on past work [4] and the conceptual design work carried out for this study. Cost Sensitivity: Sensitivity analyses determined the required accuracy for cost estimates for this comparative evaluation. LEC runs were carried out to evaluate the sensitivity of the LEC to 10% variations in several key components in a molten salt HTF system. The results showed that a 10% variation had the following impacts on LEC: • Investment cost 8% • O&M cost 2% • Performance 10-12%. This leads to the issue of the magnitude of additional costs resulting from the use of a molten salt HTF compared to the total investment costs. It was found that for a 20% uncertainty in most cost adders the effect on LEC would be less than 0.5%. For the salt inventory cost, a 20% uncertainty can have an effect on LEC on the order of 1-1.5%. Based on this analysis, it was concluded that the cost bases for the present cost evaluation are adequate for making comparisons. Nevertheless, for some factors such as salt inventory cost and selective surface emissivity, specific sensitivity runs were carried out to quantify the effect of uncertainties on LEC. The sensitivity of the results to the emissivity coefficient was examined by calculating the LEC for several cases at a value of 0.15 (at 350°C) in contrast to the reference emissivity of 0.1. This 50% increase in the emissivity coefficient lowered the solar field efficiency and resulted in an LEC increase of 0.6 cents/kWh for the salt cases, which corresponds to an increase of about 5%. By no means insignificant, this points to the importance of improvements in the selective surface. However, even with the increased emittance the analysis favors a salt HTF system over the VP-1 system with storage. Performance Model: A comprehensive parabolic trough model developed at FLABEG was used for performance and economic analyses. This computer code simulates the performance of entire solar power plants. Such a tool is indispensable when the daily, monthly, and annual output of a certain solar power plant configuration is to be estimated, the output of an existing plant is to be recalculated, or the potential of improvements is to be assessed. The model accommodates normal quasi-steady state conditions, daily start-up and shutdown, or changing weather conditions during operation. The model was developed based on experience gained from similar programs such as SOLERGY and the LUZ model for plants of the SEGS type. It has been significantly extended to include power plant configurations with combined cycles, thermal energy storage and dry cooling. The computer model output has been validated with measured data from actual performance reports of SEGS plants From the given meteorological input values of insolation and ambient temperature, the performance model calculates hourly performance values of HTF mass flow and temperatures, collected solar thermal energy, thermal energy Copyright © 2002 by ASME fed into the storage, thermal energy taken from the storage, heat losses of solar field, piping and storage, dumped energy, and electric gross and net power. The model also considers thermal inertia of the solar field, storage, and the HTF system under transient insolation conditions. The following modifications of the performance model were necessary to properly consider the system changes for a molten salt HTF: • Ability to use a fluid other than VP-1, • Allowing operation temperatures higher than 400°C, • Modeling a direct 2-tank storage system and new operation strategy, • Improvement of heat loss model, and • Change of freeze protection mode. IMPACTS OF SALT HTF ON PERFORMANCE The use of salt as HTF in the solar field has the following main effects on the performance of the plant: • Molten salt can operate at higher temperatures than the synthetic oil used in the current SEGS plants in California. Consequently, higher steam temperatures can be achieved in the Rankine cycle leading to higher cycle efficiency. • The mass flow in the solar field is considerable lower with molten salt, which leads to a lower pressure loss in the piping. Both effects combined -low mass flow and low pressure loss -lead to relatively low pumping parasitics compared to a VP-1 solar field. • Because of the higher outlet temperature the average temperature in the solar field also increases. Consequently, the heat losses of the solar field are higher, and the solar field efficiency decreases. • The freezing point of HitecXL is rather high (about 120°C). Therefore, more thermal energy is consumed in freeze protection operation. The solar field temperature must be kept well above 120°C throughout the night. That also leads to additional heat losses. The impact of these four effects is illustrated in The improvements in performance are significantly higher than the penalties due to the higher temperature and freezing point. The largest improvement is caused by the lower parasitics in the solar field, an effect that was not initially expected in this evaluation. It is also important to note that the higher heat losses cause only a slight decay of the performance. The biggest penalty resulted from the freeze protection operation. IMPACTS OF SALT HTF ON ECONOMICS The reduction of investment cost is just 2.2% with a correspondingly small effect on the LEC. The most important effect is the performance improvement. The annual electricity output increases by 8.7%, leading to a reduction of the LEC of $10 /MWh. Less than half of this improvement is caused by the better performance of the Rankine cycle at higher temperatures while the other portion comes from the lower parasitic consumption of the solar field. The higher solar field and piping thermal losses are, of course, included in the analysis. These improvements are partly diminished by higher O&M costs, due to more costly maintenance associated with freeze protection equipment and salt-driven maintenance of valves and ball joints. These results project that the potential reduction in levelized electricity cost by switching from VP-1 to a ternary salt HTF at 450°C in a trough plant with 6 hours storage is slightly over 1 cent/kWh. This would be a very significant gain for a trough power plant, and can be realized over and above cost reductions owing to collector field cost reductions. If the higher temperature of 500°C proves to be possible, the potential cost reduction could be more than 1.5 cents/kWh. These relative gains are generally true for either 2-tank salt storage or thermocline systems. FINAL OBSERVATIONS 4 Copyright © 2002 by ASME Copyright © 2002 by ASME From a technology viewpoint, R&D is required in several areas. A few of the more important needs are: • Thermocline storage offers an important potential for cost reduction in trough plants with storage, even with a VP-1 HTF system. • In the solar field, a significant challenge is the simplification and cost reduction of the heat tracing and sealing of ball joints and HCEs. • Selective surface development is required for durability and good performance at the temperature levels needed for use of a salt HTF. • Prototype testing at small commercial-level capacities will be required for validation of both thermocline storage and a salt HTF solar field loop. Though not directly associated with a molten salt HTF system, but of significant importance to current oil-based HTF systems, is the observation that 2-tank molten salt storage systems appear ready for commercial application in trough plants. ACKNOWLEDGMENT