Henry Price

Two-tank molten salt storage for parabolic trough solar power plants

The most advanced thermal energy storage for solar thermal power plants is a two-tank storage system where the heat transfer fluid (HTF) also serves as storage medium. This concept was successfully demonstrated in a commercial trough plant (13.8 MWe SEGS I plant; 120 MWht storage capacity) and a demonstration tower plant (10 MWe Solar Two; 105 MWht storage capacity). However, the HTF used in state-of-the-art parabolic trough power plants (30–80 MWe) is expensive, dramatically increasing the cost of larger HTF storage systems. An engineering study was carried out to evaluate a concept, where another (less expensive) liquid medium such as molten salt is utilized as storage medium rather than the HTF itself. Detailed performance and cost analyses were conducted to evaluate the economic value of this concept. The analyses are mainly based on the operation experience from the SEGS plants and the Solar Two project. The study concluded that the specific cost for a two-tank molten salt storage is in the range of US

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

30–40/kWhth depending on storage size. Since the salt storage was operated successfully in the Solar Two project, no major barriers were identified to realize this concept in the first commercial parabolic trough power plant.

Engineering aspects of a molten salt heat transfer fluid in a trough solar field

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 (Solar Electric Generating Systems located in Mojave Desert, California) 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 exist, and the quantification of performance and electricity cost using preliminary 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 ). Assuming a two-tank storage system and a maximum operation temperature of 450°C, the evaluation showed that the levelized electricity cost can be reduced by 14.2% compared to a state-of-the-art parabolic trough plant such as the SEGS plants. If higher temperatures are possible, the improvement may be as high as 17.6%. Thermocline salt storage systems offer even greater benefits.

Trough Integration into Power Plants - a Study on the Performance and Economy of Integrated Solar Combined Cycle Systems

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 large-scale solar parabolic trough plants in the USA currently use a high temperature synthetic oil in the solar field consisting of a eutectic mixture of biphenyl/diphenyl oxide. The scope of the overall 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% NaNO3 and 40% KNO3) and a salt sold commercially as HitecXL (a ternary salt consisting of 48% Ca(NO3)2, 7% NaNO3, and 45% KNO3). Operation and maintenance (O&M) becomes an important concern with molten salt in the solar field. This paper addresses that concern, focusing on design and O&M issues associated with routine freeze protection, solar field preheat methods, collector loop maintenance and the selection of appropriate materials for piping and fittings.

A Parabolic Trough Solar Power Plant Simulation Model

Parabolic trough solar technology has been proven at nine commercial Solar Electric Generating Systems (SEGS) power plants that are operating in the California Mojave desert. These plants utilize steam Rankine cycle power plants, and as a result, most people associate parabolic trough solar technology with steam Rankine cycle power plant technology. Although these plants are clearly optimized for their particular application, other power cycle designs may be appropriate in other situations. Of particular interest is the integration of parabolic trough solar technology with combined cycle power plant technology. This configuration is referred to as integrated solar combined cycle systems (ISCCS). Four potential projects in India, Egypt, Morocco, and Mexico are considering the ISCCS type solar power cycle configurations. The key questions are when is the ISCCS configuration preferred over the SEGS power cycle configuration and how is the ISCCS plant designed to optimize the integration of the solar field and the power cycle. This paper reviews the results of a collaborative effort under the International Energy Agency SolarPACES organization to address these questions and it shows the potential environmental and economic benefits of each configuration.

Experimental Analysis of Overall Thermal Properties of Parabolic Trough Receivers

As interest for clean renewable electric power technologies grows, a number of parabolic trough power plants of various configurations are being considered for deployment around the globe. It is essential that plant designs be optimized for each specific application. The optimum design must consider the capital cost, operations and maintenance cost, annual generation, financial requirements, and time-of-use value of the power generated. Developers require the tools for evaluating tradeoffs between these various project elements. This paper provides an overview of a computer model that is being used by scientists and developers to evaluate the tradeoff between cost, performance, and economic parameters for parabolic trough solar power plant technologies. An example is included which shows how this model has been used for a thermal storage design optimization.Copyright

Reducing the Cost of Energy From Parabolic Trough Solar Power Plants

The heat loss of a receiver in a parabolic trough collector plays an important role in collector performance. A number of methods have been used to measure the thermal loss of a receiver tube depending on its operating temperature. This paper presents methods for measuring receiver heat losses including field measurements and laboratory set-ups both based on energy balances from the hot inside of the receiver tube to the ambient. Further approaches are presented to measure and analyze the temperature of the glass envelope of evacuated receivers and to model overall heat losses and emissivity coefficients of the receiver. Good agreement can be found between very different approaches and independent installations. For solar parabolic trough plants operating in the usual 390°C temperature range, the thermal loss is around 300W/m receiver length.

Field Survey of Parabolic Trough Receiver Thermal Performance

Parabolic trough solar technology is the most proven and lowest cost large-scale solar power technology available today, primarily because of the nine large commercial-scale solar power plants that are operating in the California Mojave Desert. However, no new plants have been built during the past ten years because the cost of power from these plants is more expensive than power from conventional fossil fuel power plants. This paper reviews the current cost of energy and the potential for reducing the cost of energy from parabolic trough solar power plant technology based on the latest technological advancements and projected improvements from industry and sponsored R&D. The paper also looks at the impact of project financing and incentives on the cost of energy.Copyright

Design and Construction of the APS 1-MWe Parabolic Trough Power Plant

This paper describes a technique that uses an infrared (IR) camera to evaluate the in-situ thermal performance of parabolic trough receivers at operating solar power plants. The paper includes results to show how the glass temperature measured with the IR camera correlates with modeled thermal losses from the receiver. Finally, the paper presents results of a field survey that used this technique to quickly sample a large number of receivers to develop a better understanding of how both original and replacement receivers are performing after up to 17 years of operational service.

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

Arizona Public Service (APS) is currently installing and operating new power facilities to generate a portion of its electricity from solar resources that will satisfy its obligation under the Arizona Environmental Portfolio Standard. During FY04, APS began construction on a 1-MWe parabolic trough concentrating solar power plant. Site preparation and construction activities continued throughout FY05 and early FY06. Construction was completed and initial startup occurred by the end of December 2005. Full power production and initial performance monitoring and evaluation will occur early in 2006. This project is the first commercial deployment of parabolic trough collector technology developed by Solargenix Energy, Inc. of Raleigh, North Carolina. The plant, located near Red Rock, Arizona, uses an organic Rankine cycle power plant by Ormat, which is much simpler than conventional steam Rankine cycles and allows unattended operation of the facility. APS has teamed with the National Renewable Energy Laboratory and Sandia National Laboratories (collectively called SunLab), along with Nexant, Inc. to support design and startup activities and performance assessment. SunLab has developed TRNSYS models of the plant and will utilize initial performance baseline data to validate the models. Eventually, those models will be used to determine whether a proposed thermocline energy storage system designed by Nexant, Inc. is technically and economically feasible for this plant. SunLab has also assisted APS with development of an O&M database using the Maximo system to track solar plant costs and component failure modes.Copyright